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Inspired by the enzyme acetylene hydratase, we investigated the reactivity of acetylene with tungsten(II) pyrazole complexes. Our research revealed that the complex [WBr 2 (pz-NHCCH 3 )(CO) 3 ] (pz = 3,5-dimethyl-pyrazolate) facilitates the stochiometric reaction between pzH and acetylene to give N -vinyl-pz. This vinyl compound readily hydrolyzes to acetaldehyde, mirroring the product of acetylene hydration in the enzymatic process. The formation of the vinyl compound likely involves a reactive intermediate complex where acetylene acts as a two-electron donor, in contrast to isolable acetylene complexes that are inert to nucleophilic attack by water. Results suggest an alternative mechanism for the enzyme, including vinylation of a neighboring amino acid by acetylene in the active site prior to hydration.

Inspired by the first shell mechanism proposed for the tungstoenzyme acetylene hydratase, the electrophilic reactivity of tungsten-acetylene complexes [W(CO)(C 2 H 2 )(6-MePyS) 2 ] ( 1 ) and [WO(C 2 H 2 )(6-MePyS) 2 ] ( 2 ) was investigated. The biological nucleophile water/hydroxide and tert -butyl isocyanide were employed. Our findings consistently show that, regardless of the nucleophile used, both tungsten centers W(II) and W(IV), respectively, are the preferred targets over the coordinated acetylene. Treatment of 2 with aqueous NaOH led to protonation of coordinated acetylene to ethylene, pointing toward the Brønsted basic character of the coordinated alkyne instead of the anticipated electrophilic behavior. In cases involving isocyanides as nucleophiles, the attack on the W(II) center of 1 took place first, whereas the W(IV) complex 2 remained unchanged. These experiments indicate that the direct nucleophilic attack of W-coordinated acetylene by water, as some computational studies of acetylene hydratase propose, is unlikely to occur.

Bioinspired tungsten acetylene complexes containing pyridine-2-selenolato (PySe) or 6-methyl-pyridine-2-selenolato (6-MePySe) ligands were synthesized. 77 Se NMR spectroscopy allowed for an assessment of the resonance structures in the pyridine-2-selenolato ligands and the rationalization of chemoselectivity observed in regard to 1,2 migratory insertion of HC≡CH. [W(CO)(C 2 H 2 )(CHCH-PySe)(PySe)] is formed exclusively via insertion of HC≡CH into the W-N bond, while the use of bulkier 6-MePySe allows for the isolation of [W(CO)(C 2 H 2 )(6-MePySe) 2 ], which only partially reacts with excess HC≡CH to give [W(CO)(C 2 H 2 )(CHCH-6-MePySe)(6-MePySe)]. Oxidation of [W(CO)(C 2 H 2 )(6-MePySe) 2 ] with pyridine- N -oxide gave the tungsten(IV) complex [WO(C 2 H 2 )(6-MePySe) 2 ]. Complexes [W(CO)(C 2 H 2 )(6-MePySe) 2 ] and [WO(C 2 H 2 )(6-MePySe) 2 ] react with trimethyl phosphine to carbyne complex [W(CO)(CCH 2 PMe 3 )(PMe 3 ) 2 (6-MePySe)]Cl and alkylidene complex [WO(CHCHPMe 3 )(PMe 3 ) 2 (6-MePySe)]Cl, respectively. The addition of substituted alkynes to [W(CO) 3 (PySe) 2 ] via thermal decarbonylation gave complexes [W(CO)(MeC≡CMe)(PySe) 2 ] and [W(CO)(HC≡C t -Bu)(PySe) 2 ], respectively. The here presented complexes are relevant for the modeling of the active site of acetylene hydratase from Pelobacter acetylenicus , in which a tungsten atom is enclosed in a sulfur-rich coordination sphere. A recently published theoretical study concluded that the exchange of sulfur for selenium would increase the activity of the enzyme. Our findings contrast this claim as comparative analysis concludes negligible structural and electronic differences between the selenium-based and previously published sulfur-based complexes.

Poly(phenylene methylene) (PPM) is a multifunctional polymer that is also active as an anticorrosion fluorescent coating material. Although this polymer was synthesized already more than 100 years ago, a versatile synthetic route to obtain soluble high molar mass polymers based on PPM has yet to be achieved. In this article, the influence of bifunctional bis-chloromethyl durene (BCMD) as a branching agent in the synthesis of PPM is reported. The progress of the reaction was followed by gel permeation chromatography (GPC) and NMR analysis. PPM-based copolymers with the highest molar mass reported so far for this class of materials (up to M n of 205,300 g mol -1 ) were isolated. The versatile approach of using BCMD was confirmed by employing different catalysts. Interestingly, thermal and optical characterization established that the branching process does not affect the thermoplastic behavior and the fluorescence of the material, thus opening up PPM-based compounds with high molar mass for applications.

An increasing number of discovered tungstoenzymes raises interest in the biomimetic chemistry of tungsten complexes in oxidation states +IV, +V, and +VI. Bioinspired (sulfur-rich) tungsten(VI) dioxido complexes are relatively prevalent in literature. Still, their energetically demanding reduction directly correlates with a small number of known tungsten(IV) oxido complexes, whose chemistry is not well explored. In this paper, a reduction of the [WO 2 (6-MePyS) 2 ] (6-MePyS = 6-methylpyridine-2-thiolate) complex with PMe 3 to a phosphine-stabilized tungsten(IV) oxido complex [WO(6-MePyS) 2 (PMe 3 ) 2 ] is described. This tungsten(IV) complex partially releases one PMe 3 ligand in solution, creating a vacant coordination site capable of activating dioxygen to form [WO 2 (6-MePyS) 2 ] and OPMe 3 . Therefore, [WO 2 (6-MePyS) 2 ] can be used as a catalyst for the aerobic oxidation of PMe 3 , rendering this complex a rare example of a tungsten system utilizing dioxygen in homogeneous catalysis. Additionally, the investigation of the reactivity of the tungsten(IV) oxido complex with acetylene, substrate of a tungstoenzyme acetylene hydratase (AH), revealed the formation of the tungsten(IV) acetylene adduct. Although this adduct was previously reported as an oxidation product of the tungsten(II) acetylene carbonyl complex, here it is obtained via substitution at the sulfur-rich tungsten(IV) center, mimicking the initial step of the first shell mechanism for AH as suggested by computational studies.

Molybdenum(VI) bis(imido) complexes [Mo(NtBu) 2 (L R ) 2 ] (R=H 1 a; R=CF 3 1 b) combined with B(C 6 F 5 ) 3 (1 a/B(C 6 F 5 ) 3 , 1 b/B(C 6 F 5 ) 3 ) exhibit a frustrated Lewis pair (FLP) character that can heterolytically split H-H, Si-H and O-H bonds. Cleavage of H 2 and Et 3 SiH affords ion pairs [Mo(NtBu)(NHtBu)(L R ) 2 ][HB(C 6 F 5 ) 3 ] (R=H 2 a; R=CF 3 2 b) composed of a Mo(VI) amido imido cation and a hydridoborate anion, while reaction with H 2 O leads to [Mo(NtBu)(NHtBu)(L R ) 2 ][(HO)B(C 6 F 5 ) 3 ] (R=H 3 a; R=CF 3 3 b). Ion pairs 2 a and 2 b are catalysts for the hydrosilylation of aldehydes with triethylsilane, with 2 b being more active than 2 a. Mechanistic elucidation revealed insertion of the aldehyde into the B-H bond of [HB(C 6 F 5 ) 3 ] - . We were able to isolate and fully characterize, including by single-crystal X-ray diffraction analysis, the inserted products Mo(NtBu)(NHtBu)(L R ) 2 ][{PhCH 2 O}B(C 6 F 5 ) 3 ] (R=H 4 a; R=CF 3 4 b). Catalysis occurs at [HB(C 6 F 5 ) 3 ] - while [Mo(NtBu)(NHtBu)(L R ) 2 ] + (R=H or CF 3 ) act as the cationic counterions. However, the striking difference in reactivity gives ample evidence that molybdenum cations behave as weakly coordinating cations (WCC)., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Upon replacement of molybdenum by tungsten in DMSO reductase isolated from the Rhodobacteraceae family, the derived enzyme catalyzes DMSO reduction faster. To better understand this behavior, we synthesized two tungsten(VI) dioxido complexes [W VI O 2 L 2 ] with pyridine- (PyS) and pyrimidine-2-thiolate (PymS) ligands, isostructural to analogous molybdenum complexes we reported recently. Higher oxygen atom transfer (OAT) catalytic activity was observed with [WO 2 (PyS) 2 ] compared to the Mo species, independent of whether PMe 3 or PPh 3 was used as the oxygen acceptor. [W VI O 2 L 2 ] complexes undergo reduction with an excess of PMe 3 , yielding the tungsten(IV) oxido species [WOL 2 (PMe 3 ) 2 ], while with PPh 3 , no reactions are observed. Although OAT reactions from DMSO to phosphines are known for tungsten complexes, [WOL 2 (PMe 3 ) 2 ] are the first fully characterized phosphine-stabilized intermediates. By following the reaction of these reduced species with excess DMSO via UV-vis spectroscopy, we observed that tungsten compounds directly react to W VI O 2 complexes while the Mo analogues first form μ-oxo Mo(V) dimers [Mo 2 O 3 L 4 ]. Density functional theory calculations confirm that the oxygen atom abstraction from W VI O 2 is an endergonic process contrasting the respective reaction with molybdenum. Here, we suggest that depending on the sacrificial oxygen acceptor, the tungsten complex may participate in catalysis either via a redox reaction or as an electrophile.

Multinuclear tungsten complexes are intriguing candidates for new contrast media that can provide substantial improvements in CT imaging diagnostics. Herein, we present a ligand strategy, based on amino acids, and mono- and disubstituted EDTA derivatives, that enables the development of stable complexes with high tungsten content and reasonably low osmolality. Accordingly, a series of neutral and monoanionic di-μ-sulfido W(V) dimers have been synthesized via a convenient procedure utilizing microwave heating in combination with ion-pair HPLC reaction monitoring. The compounds were characterized in detail by various techniques, including ESI-HRMS, NMR spectroscopy, HPLC, elemental analysis, and X-ray crystallography. The aqueous stability of the complexes under physiologically relevant conditions, and during heat sterilization was also examined as an initial assessment of their potential applicability as radiocontrast agents. Monoanionic complexes featuring monosubstituted EDTA derivatives have demonstrated high stability, while producing a lower number of ions in solution (resulting in lower osmolality) in comparison to their bis-anionic EDTA counterparts. Nevertheless, they exhibited insufficient water solubility for application as intravascular contrast agents. However, our study showed that aqueous solubility of this type of complexes can be tuned by small modifications in the ligand structure.

The synthesis and structural determination of two isomers of the molybdenum(II) complex (η 2 -but-2-yne)carbonylbis[2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)benzenethiolato-κ 2 N,S]molybdenum(II), [Mo(C 11 H 12 NOS) 2 (C 4 H 6 )(CO)] or Mo(CO)(C 2 Me 2 )(S-Phoz) 2 , are presented. The N,N-cis-S,S-trans isomer 1 shows quite different bond lengths to the metal atom [Mo-N = 2.4715 (10) versus 2.3404 (11) Å; Mo-S = 2.4673 (3) versus 2.3665 (3) Å]. In the N,N-trans-S,S-cis isomer 2, which is isotypic with the corresponding W complex, the Mo-N bond lengths [2.236 (2) and 2.203 (2) Å], as well as the Mo-S bond lengths [2.5254 (8) and 2.5297 (8) Å], are almost the same., (open access.)

Intending to deepen our understanding of tungsten acetylene (C 2 H 2 ) chemistry, with regard to the tungstoenzyme acetylene hydratase, here we explore the structure and reactivity of a series of tungsten acetylene complexes, stabilized with pyridine-2-thiolate ligands featuring tungsten in both +II and +IV oxidation states. By varying the substitution of the pyridine-2-thiolate moiety with respect to steric and electronic properties, we examined the details and limits of the previously reported intramolecular nucleophilic attack on acetylene followed by the formation of acetylene inserted complexes. Here, we demonstrate that only the combination of high steric demand and electron-withdrawing features prevents acetylene insertion. Nevertheless, although variable synthetic approaches are necessary for their synthesis, tungsten acetylene complexes can be stabilized predictably with a variety of pyridine-2-thiolate ligands., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

The isolation of a molybdenum(IV) acetylene (C 2 H 2 ) complex containing two bioinspired 6-methylpyridine-2-thiolate ligands is reported. The synthesis can be performed either by oxidation of a molybdenum(II) C 2 H 2 complex or by substitution of a coordinated PMe 3 by C 2 H 2 on a molybdenum(IV) center. Both C 2 H 2 complexes were characterized by spectroscopic means as well as by single-crystal X-ray diffraction. Furthermore, the reactivity of the coordinated C 2 H 2 was investigated with regard to acetylene hydratase, one of two enzymes that accept C 2 H 2 as a substrate. While the reaction with water resulted in the vinylation of the pyridine-2-thiolate ligands, an intermolecular nucleophilic attack on the coordinated C 2 H 2 with the soft nucleophile PMe 3 was observed to give a cationic ethenyl complex. A comparison with the tungsten analogues revealed less tightly bound C 2 H 2 in the molybdenum variant, which, however, shows a higher reactivity toward nucleophiles., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

Inspired by the proposed inner-sphere mechanism of the tungstoenzyme acetylene hydratase, we have designed tungsten acetylene complexes and investigated their reactivity. Here, we report the first intermolecular nucleophilic attack on a tungsten-bound acetylene (C 2 H 2 ) in bioinspired complexes employing 6-methylpyridine-2-thiolate ligands. By using PMe 3 as a nucleophile, we isolated cationic carbyne and alkenyl complexes.

Reaction of [NiCl 2 (PnH) 4 ] (1) (PnH=6-tert-butyl-pyridazine-3-thione) with NiCl 2 affords the binuclear paddlewheel (PW) complex [Ni 2 (Pn) 4 ] (2). Diamagnetic complex 2 is the first example of a PW complex capable of reversibly binding and releasing NH 3 . The NH 3 ligand in [Ni 2 (Pn) 4 (NH 3 )] (2⋅NH 3 ) enforces major spectroscopic and magnetic susceptibility changes, thus displaying vapochromic properties (λ max (2)=532 nm, λ max (2⋅NH 3 )=518 nm) and magnetochemical switching (2: S=0; 2⋅NH 3 : S=1). Upon repeated adsorption/desorption cycles of NH 3 the PW core remains intact. Compound 2 can be embedded into thin polyurethane films (2 P ) under retention of its sensing abilities. Therefore, 2 qualifies as reversible optical probe for ammonia. The magnetochemical switching of 2 and 2⋅NH 3 was studied in detail by SQUID measurements showing that in 2⋅NH 3 , solely the Ni atom coordinated the NH 3 molecule is responsible for the paramagnetic behavior., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The structure of the title hydrated mol-ecular salt, C 10 H 8 NS + ·Cl - ·H 2 O, obtained by the reaction of sodium quinoline-8-thiolate Na(Quin-8-S) with CH 2 Cl 2 and an aqueous solution of [Bu 4 N]Cl, contains π-stacked cations [plane-to-plane separation = 3.338 (4)-3.356 (4) Å] and features chains built by alternating Cl - anions and H 2 O mol-ecules connected by O-H⋯O hydrogen bonds. The cation shows whole-mol-ecule disorder over two flipped orientations in a 0.853 (3):0.147 (3) ratio., (© Ehweiner et al. 2020.)

Four dioxidomolybdenum(VI) complexes of the general structure [MoO 2 L 2 ] employing the S,N-bidentate ligands pyrimidine-2-thiolate (PymS, 1 ), pyridine-2-thiolate (PyS, 2 ), 4-methylpyridine-2-thiolate (4-MePyS, 3 ) and 6-methylpyridine-2-thiolate (6-MePyS, 4 ) were synthesized and characterized by spectroscopic means and single-crystal X-ray diffraction analysis ( 2 - 4 ). Complexes 1 - 4 were reacted with PPh 3 and PMe 3 , respectively, to investigate their oxygen atom transfer (OAT) reactivity and catalytic applicability. Reduction with PPh 3 leads to symmetric molybdenum(V) dimers of the general structure [Mo 2 O 3 L 4 ] ( 6 - 9 ). Kinetic studies showed that the OAT from [MoO 2 L 2 ] to PPh 3 is 5 times faster for the PymS system than for the PyS and 4-MePyS systems. The reaction of complexes 1 - 3 with PMe 3 gives stable molybdenum(IV) complexes of the structure [MoOL 2 (PMe 3 ) 2 ] ( 10 - 12 ), while reduction of [MoO 2 (6-MePyS) 2 ] ( 4 ) yields [MoO(6-MePyS) 2 (PMe 3 )] ( 13 ) with only one PMe 3 coordinated to the metal center. The activity of complexes 1 - 4 in catalytic OAT reactions involving Me 2 SO and Ph 2 SO as oxygen donors and PPh 3 as an oxygen acceptor has been investigated to assess the influence of the varied ligand frameworks on the OAT reaction rates. It was found that [MoO 2 (PymS) 2 ] ( 1 ) and [MoO 2 (6-MePyS) 2 ] ( 4 ) are similarly efficient catalysts, while complexes 2 and 3 are only moderately active. In the catalytic oxidation of PMe 3 with Me 2 SO, complex 4 is the only efficient catalyst. Complexes 1 - 4 were also found to catalytically reduce NO 3 - with PPh 3 , although their reactivity is inhibited by further reduced species such as NO, as exemplified by the formation of the nitrosyl complex [Mo(NO)(PymS) 3 ] ( 14 ), which was identified by single-crystal X-ray diffraction analysis. Computed Δ G ⧧ values for the very first step of the OAT were found to be lower for complexes 1 and 4 than for 2 and 3 , explaining the difference in catalytic reactivity between the two pairs and revealing the requirement for an electron-deficient ligand system.

A series of W IV alkyne complexes with the sulfur-rich ligand hydridotris(2-mercapto-1-methylimidazolyl) borate) (Tm Me ) are presented as bio-inspired models to elucidate the mechanism of the tungstoenzyme acetylene hydratase (AH). The mono- and/or bis-alkyne precursors were reacted with NaTm Me and the resulting complexes [W(CO)(C 2 R 2 )(Tm Me )Br] (R=H 1, Me 2) oxidized to the target [WE(C 2 R 2 )(Tm Me )Br] (E=O, R=H 4, Me 5; E=S, R=H 6, Me 7) using pyridine-N-oxide and methylthiirane. Halide abstraction with TlOTf in MeCN gave the cationic complexes [WE(C 2 R 2 )(MeCN)(Tm Me )](OTf) (E=CO, R=H 10, Me 11; E=O, R=H 12, Me 13; E=S, R=H 14, Me 15). Without MeCN, dinuclear complexes [W 2 O(μ-O)(C 2 Me 2 ) 2 (Tm Me ) 2 ](OTf) 2 (8) and [W 2 (μ-S) 2 (C 2 Me 2 )(Tm Me ) 2 ](OTf) 2 (9) could be isolated showing distinct differences between the oxido and sulfido system with the latter exhibiting only one molecule of C 2 Me 2 . This provides evidence that a fine balance of the softness at W is important for acetylene coordination. Upon dissolving complex 8 in acetonitrile complex 13 is reconstituted in contrast to 9. All complexes exhibit the desired stability toward water and the observed effective coordination of the scorpionate ligand avoids decomposition to disulfide, an often-occurring reaction in sulfur ligand chemistry. Hence, the data presented here point toward a mechanism with a direct coordination of acetylene in the active site and provide the basis for further model chemistry for acetylene hydratase., (© 2020 The Authors. Published by Wiley-VCH GmbH.)

The syntheses and characterizations of oxidorhenium(v) complexes trans-dichlorido [ReOCl2(PPh3)(L1a)] (trans-2a), cis-dichlorido [ReOCl2(PPh3)(L1b)] (cis-2b) and ethoxido-complex [ReO(OEt)(L1b)2] (4b), ligated with the dimethyloxazoline-phenol ligands HL1a and HL1b are described. The bidentate ligand HL1a (2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-phenol) is unsubstituted on the phenol ring; ligand HL1b (2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-4-nitrophenol) contains a nitro group in para-position to the hydroxy group. In the reaction of precursor complex [ReOCl3(PPh3)2] and HL1a the two stereoisomers cis/trans-2a, with respect to chlorido ligands, are formed. The solid state structures of both isomers cis- and trans-2a were determined by single crystal X-ray diffraction analysis. In contrast, with ligand HL1b, only the cis-isomer cis-2b was obtained. Ethoxido-complex 4b is exclusively obtained when precursor [ReOCl3(OPPh3)(SMe2)] is reacted with 2 equiv. of HL1b in ethanol in the presence of the base 2,6-dimethylpyridine (lutidine). If no lutidine is added, chlorido-complex [ReOCl(L1b)2] (3b) is obtained. Complexes [ReOCl2(PPh3)(L1a)] (cis/trans-2a), [ReOCl2(PPh3)(L1b)] (cis-2b), [ReO(OMe)(L1a)2] (4a) and [ReO(OEt)(L1b)2] (4b) were tested as homogeneous catalysts in the benchmark reaction of cyclooctene epoxidation. The influence of isomerism and effects of ligand substitutions on catalytic activity was investigated. Based on the time-conversion plots it can be concluded that cis/trans-isomerism does not influence catalytic activity, but electron-withdrawing substituents, as in cis-2b, 3b and 4b, show a beneficial effect.

Three molybdenum(VI) dioxido complexes [MoO 2 (L) 2 ] bearing Schiff base ligands were reacted with B(C 6 F 5 ) 3 to afford the corresponding adducts [MoO{OB(C 6 F 5 ) 3 }(L) 2 ], which were fully characterized. They exhibit Frustrated Lewis-Pairs reactivity when reacting with silanes. Especially, the [MoO{OB(C 6 F 5 ) 3 }(L) 2 ] complex with L=2,4-dimethyl-6-((phenylimino)methyl)phenol proved to be active as catalyst for the hydroalkylation of aryl alkenes with organohalides and for the Atom-Transfer Radical Addition (ATRA) of organohalides to aliphatic alkenes. A series of gem -dichloride and gem -dibromide compounds with potential for further derivatization were synthesized from simple alkenes and organohalides, like chloroform or bromoform, using low catalyst loading., (© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Bioinspired complexes employing the ligands 6-tert-butylpyridazine-3-thione (SPn) and pyridine-2-thione (SPy) were synthesized and fully characterized to mimic the tungstoenzyme acetylene hydratase (AH). The complexes [W(CO)(C 2 H 2 )(CHCH-SPy)(SPy)] (4) and [W(CO)(C 2 H 2 )(CHCH-SPn)(SPn)] (5) were formed by intramolecular nucleophilic attack of the nitrogen donors of the ligand on the coordinated C 2 H 2 molecule. Labelling experiments using C 2 D 2 with the SPy system revealed the insertion reaction proceeding via a bis-acetylene intermediate. The starting complex [W(CO)(C 2 H 2 )(SPy) 2 ] (6) for these studies was accessed by the new acetylene precursor mixture [W(CO)(C 2 H 2 ) n (MeCN) 3-n Br 2 ] (n=1 and 2; 7). All complexes represent rare examples in the field of W-C 2 H 2 chemistry with 4 and 5 being the first of their kind. In the ongoing debate on the enzymatic mechanism, the findings support activation of acetylene by the tungsten center., (© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Bioinspired models for contrasting the electronic nature of neutral tris-histidine with the anionic 2-histidine-1-carboxylate facial motif and their subsequent impact on catalysis are reported. Herewith, iron(ii) complexes [Fe(L)(CH 3 CN) 3 ](SO 3 CF 3 ) 2 1-3 of tris(2-pyridyl)-based ligands (L) have been synthesized and characterized as accurate structural models for the neutral 3-histidine triad of the enzyme diketone dioxygenase (DKDO). The molecular structure of one of the complexes exhibits octahedral coordination geometry and Fe-N11 py bond lengths [1.952(4) to 1.959(4) Å] close to the Fe-N His bond distances (1.98 Å) of the 3-His triad in the resting state of the enzyme, as obtained by EXAFS studies. The diketonate substrate-adduct complexes [Fe(L)(acac R )](SO 3 CF 3 ) (R = Me, Ph) of 1-3 have been obtained using Na(acac R ) in acetonitrile. The Fe 2+/3+ redox potentials of the complexes (1.05 to 1.2 V vs. Fc/Fc + ) and their substrate adducts (1.02 to 1.19 V vs. Fc/Fc + ) appeared at almost the same redox barrier. All diketonate adducts exhibit two Fe(ii) → acac MLCT bands around 338 to 348 and 430 to 490 nm. Exposure of these adducts to O 2 results in the decay of both MLCT bands with a rate of (k O 2 ) 5.37 to 9.41 × 10 -3 M -1 s -1 . The k O 2 values were concomitantly accelerated 20 to 50 fold by the addition of H + (acetic acid), which nicely models the rate enhancement in the enzyme kinetics by the glutamate residue (Glu98). The oxygenation of the phenyl-substituted adducts yielded benzoin and benzoic acid (40% to 71%) as cleavage products in the presence of H + ions. Isotope-labeling experiments using 18 O 2 showed 47% incorporation of 18 O in benzoic acid, which reveals that the oxygen originates from dioxygen. Thus, the present model complexes exhibit very similar chemical surroundings to the active site of DKDO and mimic its functions elegantly. On the basis of these results, the C-C bond cleavage reaction mechanism is discussed.

The syntheses of oxidorhenium(v) complexes [ReOCl(L1a-c) 2 ] (3a-c), equipped with the bidentate, mono-anionic phenol-dimethyloxazoline ligands HL1a-c are described. Ligands HL1b-c contain functional groups on the phenol ring, compared to parent ligand 2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-phenol H1a; namely a methoxy group ortho to the hydroxyl position (2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-6-methoxyphenol, H1b), or a nitro group para to the hydroxyl position (2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-4-nitrophenol, H1c). Furthermore, oxidorhenate(v) complexes (NBu 4 )[ReOCl 3 (L1a-b)] (2a-b) were synthesized for solid state structural comparisons to 3a-b. All novel complexes are fully characterized including NMR, IR and UV-Vis spectroscopy, MS spectrometry, X-ray crystallography, elemental analysis as well as cyclic voltammetry. The influence of functional groups (R = -H, -OMe and -NO 2 ) on the catalytic activity of 3a-c was investigated in two benchmark catalytic reactions, namely cyclooctene epoxidation and perchlorate reduction. In addition, the previously described oxidorhenium(v) complex [ReOCl(oz) 2 ] (4), employing the phenol-oxazoline ligand 2-(4,5-dihydro-2-oxazolyl)phenol Hoz, was included in these catalysis studies. Complex 4 is a rare case in oxidorhenium(v) chemistry where two stereoisomers could be separated and fully characterized. With respect to the position of the oxazoline nitrogen atoms on the rhenium atom, these two stereoisomers are referred to as N,N-cis and N,N-trans isomer. A potential correlation between spectroscopic and structural data to catalytic activity was evaluated.

The synthesis and characterization of a series of novel tungsten complexes employing the bioinspired, sulfur-rich scorpionate ligand [PhTt] (phenyltris((methylthio)methyl)borate) are reported. Starting from the previously published tungsten precursor [WBr 2 (CO) 3 (NCMe) 2 ], a salt metathesis reaction with 1 equiv of Cs[PhTt] led to the desired complex [WBr(CO) 3 (PhTt)] (1), making it the first tungsten complex employing a poly(thioether)borate ligand. Surprisingly, the reaction of [WBr 2 (CO) 3 (NCMe) 2 ] with an excess of the ligand gave complex [W(CO) 2 (η 2 -CH 2 SMe)(PhTt)] (2) with a bidentate (methylthio)methanide ligand as the major product. Thereby, phenyldi((methylthio)methyl)borane is formed, which was isolated and characterized by NMR spectroscopy. The bromido ligand in [WBr(CO) 3 (PhTt)] was further substituted by the S,N-bidentate methimazole in order to make the first coordination sphere more sulfur-rich forming [W(CO) 2 (mt)(PhTt)] (3). Alkyne tungsten complexes employing the sulfur-rich scorpionate ligand were accessible by reaction of [WBr 2 (CO)(C 2 R 2 ) 2 (NCMe)] (R = Me, Ph) with Cs[PhTt] forming [WBr(CO)(C 2 R 2 ) 2 (PhTt- S, S')] (R = Me 4, Ph 5), with the potentially tridentate ligand coordinated only via two sulfur atoms. In the case of 4, the higher flexibility of the bidentate coordination leads to the formation of two isomers with respect to the six-membered ring formed by the tungsten center and the two coordinated sulfur atoms of the ligand. All complexes 1-5 were characterized by single-crystal X-ray diffraction analysis.

Two novel iminophenolate ligands with amidopropyl side chains ( HL2 and HL3 ) on the imine functionality have been synthesized in order to prepare dioxidomolybdenum(VI) complexes of the general structure [MoO 2 L 2 ] featuring pendant internal hydrogen bond donors. For reasons of comparison, a previously published complex featuring n -butyl side chains ( L1 ) was included in the investigation. Three complexes ( 1 - 3 ) obtained using these ligands ( HL1 - HL3 ) were able to activate dioxygen in an in situ approach: The intermediate molybdenum(IV) species [MoO(PMe 3 ) L 2 ] is first generated by treatment with an excess of PMe 3 . Subsequent reaction with dioxygen leads to oxido peroxido complexes of the structure [MoO(O 2 ) L 2 ]. For the complex employing the ligand with the n -butyl side chain, the isolation of the oxidomolybdenum(IV) phosphino complex [MoO(PMe 3 )( L1 ) 2 ] ( 4 ) was successful, whereas the respective Mo(IV) species employing the ligands with the amidopropyl side chains were found to be not stable enough to be isolated. The three oxido peroxido complexes of the structure [MoO(O 2 ) L 2 ] ( 9 - 11 ) were systematically compared to assess the influence of internal hydrogen bonds on the geometry as well as the catalytic activity in aerobic oxidation. All complexes were characterized by spectroscopic means. Furthermore, molecular structures were determined by single-crystal X-ray diffraction analyses of HL3 , 1 - 3 , 9 - 11 together with three polynuclear products {[MoO( L2 ) 2 ] 2 (µ-O)} ( 7 ), {[MoO( L2 )] 4 (µ-O) 6 } ( 8 ) and [C 9 H 13 N 2 O] 4 [Mo 8 O 26 ]·6OPMe 3 ( 12 ) which were obtained during the synthesis of reduced complexes of the type [MoO(PMe 3 ) L 2 ] ( 4 - 6 ).

Lewis acid-base pair chemistry has been placed on a new level with the discovery that adduct formation between an electron donor (Lewis base) and acceptor (Lewis acid) can be inhibited by the introduction of steric demand, thus preserving the reactivity of both Lewis centers, resulting in highly unusual chemistry. Some of these highly versatile frustrated Lewis pairs (FLP) are capable of splitting a variety of small molecules, such as dihydrogen, in a heterolytic and even catalytic manner. This is in sharp contrast to classical reactions where the inert substrate must be activated by a metal-based catalyst. Very recently, research has emerged combining the two concepts, namely the formation of FLPs in which a metal compound represents the Lewis base, allowing for novel chemistry by using the heterolytic splitting power of both together with the redox reactivity of the metal. Such reactivity is not restricted to the metal center itself being a Lewis acid or base, also ancillary ligands can be used as part of the Lewis pair, still with the benefit of the redox-active metal center nearby. This Minireview is designed to highlight the novel reactions arising from the combination of metal oxido transition-metal or rare-earth-metal compounds with the Lewis acid B(C 6 F 5 ) 3 . It covers a wide area of chemistry including small molecule activation, hydrogenation and hydrosilylation catalysis, and olefin metathesis, substantiating the broad influence of the novel concept. Future goals of this young and exciting area are briefly discussed., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The synthesis and structural determination of four tungsten alkyne complexes coordinated by the bio-inspired S,N-donor ligand 2-(4',4'-dimethyloxazoline-2'-yl)thiophenolate (S-Phoz) is presented. A previously established protocol that involved the reaction of the respective alkyne with the bis-carbonyl precursor [W(CO) 2 (S-Phoz) 2 ] was used for the complexes [W(CO)(C 2 R 2 )(S-Phoz) 2 ] (R=H, 1 a; Me, 1 b; Ph, 1 c). Oxidation with pyridine-N-oxide gave the corresponding W-oxo species [WO(C 2 R 2 )(S-Phoz) 2 ] (R=H, 2 a; Me, 2 b; Ph, 2 c). All W-oxo-alkyne complexes (2 a, b, c) were found to be capable of alkyne release upon light irradiation to afford five-coordinate [WO(S-Phoz) 2 ] (3). The photoinduced release of the alkyne ligand was studied in detail by in situ 1 H NMR measurements, which revealed correlation of the photodissociation rate constant (2 b>2 a>2 c) with the elongation of the alkyne C≡C bond in the molecular structures. Oxidation of [WO(S-Phoz) 2 ] (3) with pyridine-N-oxide yielded [WO 2 (S-Phoz) 2 ] (4), which shows highly fluxional behavior in solution. Variable-temperature 1 H NMR spectroscopy revealed three isomeric forms with respect to the ligand arrangement versus each other. Furthermore, compound 4 rearranges to tetranuclear oxo compound [W 4 O 4 (μ-O) 6 (S-Phoz) 4 ] (5) and dinuclear [{WO(μ-O)(S-Phoz)} 2 ] (6) over time. The latter two were identified by single-crystal X-ray diffraction analyses., (© 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Poly(phenylene methylene)s (PPMs) with high molar masses were isolated by polymerization of benzyl chloride catalyzed with tungsten(II) compounds and subsequent fractionation. Four different tungsten(II) catalysts were successfully exploited for the polymerization, for which a strict temperature profile was developed. The PPMs possessed roughly a trimodal molar mass distribution. Simple fractionation by phase separation of 2-butanone solutions allowed to effectively segregate the products primarily into PPM of low molar mass ( M n = 1600 g mol -1 ) and high molar mass ( M n = 167,900 g mol -1 ); the latter can be obtained in large quantities up to 50 g. The evolution of the trimodal distribution and the monomer conversion was monitored by gel permeation chromatography (GPC) and ¹H NMR spectroscopy, respectively, over the course of the polymerization. The results revealed that polymerization proceeded via a chain-growth mechanism. This study illustrates a new approach to synthesize PPM with hitherto unknown high molar masses which opens the possibility to explore new applications, e.g., for temperature-resistant coatings, fluorescent coatings, barrier materials or optical materials.

Palladium and platinum boratrane complexes of the type [M{B(Pn Me, tBu ) 3 }(PPh 3 )] (M = Pd 1, Pt 2b) have been prepared via the reaction of the soft scorpionate ligand potassium tris(4-methyl-6- tert-butyl-3-thiopyridazinyl)borate KTn Me, tBu with bis(triphenylphosphine)metal(II) dichloride. While reaction with the Pd precursor allowed direct isolation of a symmetric boratrane complex, the Pt analogue led to the hydrido compound [Pt{B(Pn Me, tBu ) 3 }(PPh 3 )H]Cl (2a), which after reaction with a base gave 2b. Subsequent oxidation with Br 2 and I 2 , respectively, led to the dihalide compounds of the molecular formula [M{B(Pn Me, tBu ) 3 }X 2 ] (3a,b-4a,b). Halide abstraction with Ag(SbF 6 ) further gave interesting cationic compounds of either dimeric [Pd{B(Pn Me, tBu ) 3 }X] 2 (SbF 6 ) 2 (5a,b) or monomeric [Pd{B(Pn Me, tBu ) 3 }(NCMe) 2 ](SbF 6 ) (6) nature. All compounds were spectroscopically and X-ray crystallographically characterized revealing strong metal to boron interactions. DFT calculations of 1, 2a, and 2b confirm the strong M-B interaction and a high positive charge on the metal centers.

The terminal zinc hydride complex [Tntm]ZnH (2; Tntm=tris(6-tert-butyl-3-thiopyridazinyl)methanide) is an efficient hydrosilylation catalyst of CO 2 at room temperature without the need of Lewis acidic additives. The inherent electrophilicity of the system leads to selective formation of the monosilylated product (MeO) 3 SiO 2 CH (at room temperature with a TOF of 22.2 h -1 and at 45 °C with a TOF of 66.7 h -1 ). In absence of silanes, the intermediate formate complex [Tntm]Zn(O 2 CH) (3) is quantitatively formed within 5 min. All complexes were fully characterized by 1 H and 13 C NMR spectroscopy and single-crystal X-ray diffraction analyses. Density functional theory (DFT) calculations reveal a high positive charge on zinc and the increased preference of the ligand to adopt a κ 3 -coordination mode., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The reaction of a molybdenum(VI) oxido imido complex with the strong Lewis acid B(C 6 F 5 ) 3 gave access to the Lewis adduct [Mo{OB(C 6 F 5 ) 3 }(NtBu)L 2 ] featuring reversible B-O bonding in solution. The resulting frustrated Lewis pair (FLP)-like reactivity is reflected by the compound's ability to heterolytically cleave Si-H bonds, leading to a clean formation of the novel cationic Mo VI species 3 a (R=Et) and 3 b (R=Ph) of the general formula [Mo(OSiR 3 )(NtBu)L 2 ][HB(C 6 F 5 ) 3 ]. These compounds possess properties highly unusual for molybdenum d 0 species such as an intensive, charge-transfer-based color as well as a reversible redox couple at very low potentials, both dependent on the silane used. Single-crystal X-ray diffraction analyses of 2 and 4 b, a derivative of 3 b featuring the [FB(C 6 F 5 ) 3 ] - anion, picture the stepwise elongation of the Mo=O bond, leading to a large increase in the electrophilicity of the metal center. The reaction of 3 a and 3 b with benzaldehyde allowed for the regeneration of compound 2 by hydrosilylation of the benzaldehyde. NMR spectroscopy suggested an unusual mechanism for the transformation, involving a substrate insertion in the B-H bond of the borohydride anion., (© 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

The bidentate N,O-ligands phenol-pyrazole (HL1), naphthol-pyrazole (HL2) and the commercially available ligand 5-methylphenol-benzotriazole (HL3) were used for the synthesis of novel iron(iii) complexes. The mononuclear iron complexes [FeCl(L1)2] (1), [FeCl(L2)2] (2) and [FeCl(L3)2] (3) are stable to air and moisture, both in the solid state as well as in solution, while the dinuclear, μ-oxido bridged complex [{Fe(L1)2}2(μ-O)] (1a) is air sensitive. All four complexes 1, 2, 3 and 1a were investigated for their catalytic activity in the direct one-pot oxidation of primary alcohols to carbonic acids with 30% aq. hydrogen peroxide (H2O2) as the oxidation agent. The activity in oxidation reactions of the isolated, mononuclear complexes 1-3 was further compared to their in situ prepared analogues IS1-3. Experimentally obtained results indicate a tendency of higher activity for the oxidation of primary alcohols for the in situ prepared complexes. In conclusion, the oxidation of aromatic primary alcohols to carboxylic acids using isolated iron(iii) complexes and in situ generated complexes in the presence of H2O2 results in good to high yields. The reaction is straight-forward, clean and generates water as the only by-product.

A novel selenium-containing pyridazinyl-based soft scorpionate ligand (KTnse) was synthesized. It reacts with CoCl 2 and NiCl 2 , yielding the first metallaboratrane complexes with selenium in their donor positions. Further substitution with Ag(OTf) or NaN 3 allows isolation of the respective triflate or azide complexes. Reaction with Ag(OTf) leads in the case of nickel to a dinuclear, dicationic complex with a short Ni-Ni distance, while cobalt gave a mononuclear cationic species. Substitution of the chloride by azide yields with both metals the respective azide complexes. All compounds were characterized via single-crystal X-ray diffraction analysis. Density functional theory calculations on the chloride species point to oxidized cobalt(III) and nickel(III) centers.

Activation of molecular dioxygen at a molybdenum(IV) imido compound led to the isolation and full characterization of a remarkably stable transition-metal imidoperoxido complex.

The reaction of iron(II) triflate with 6-tert-butyl-3-thiopyridazine (PnH) and 4-methyl-6-tert-butyl-3-thiopyridazine ( Me PnH) respectively led to iron bis(diorganotrisulfide) complexes [Fe( R PnS 3 Pn R ) 2 ](OTf) 2 [R = H (1a) and Me (2a)]. The corresponding perchlorate complexes were prepared by using the iron(II) chloride precursor and the subsequent addition of 2 equiv of NaClO 4 , giving [Fe( R PnS 3 Pn R ) 2 ](ClO 4 ) 2 [R = H (1b) and Me (2b)]. The compounds were fully characterized including single-crystal X-ray diffraction analysis. All four compounds exhibit nearly perfect octahedral geometries with an iron center coordinated by four nitrogen atoms from two R PnS 3 Pn R ligands and by two sulfur atoms of the central atom in the S 3 unit. The diamagnetic complexes exhibit unusually high redox potentials for the Fe 2+/3+ couple at E 1/2 = 1.15 V (for 1a and 1b) and 1.08 V (for 2a and 2b) versus Fc/Fc + , respectively, as determined by cyclic voltammetry. Furthermore, the source of the extra sulfur atom within the S 3 unit was elucidated by isolation of C-N-coupled pyridazinylthiopyridazine products.

Synthesis of molybdenum(vi) dioxido complexes 1-3, coordinated by one or two functionalized iminophenolate ligands HL1 or HL2, bearing a donor atom side chain or a phenyl substituent, respectively, allowed for systematic investigation of the oxygen atom transfer (OAT) reactivity of such complexes towards phosphanes. Depending on stoichiometry and employed phosphane (PMe3 or PPh3), different molybdenum(iv) and molybdenum(v) complexes 4-7 were obtained. Whereas molybdenum(iv) complexes 4 and 5, bearing a terminal PMe3 ligand, readily reacted with molecular O2 to form oxido peroxido complexes 8 and 9, phosphane free μ-oxido bridged dinuclear molybdenum(v) complexes 6 and 7 proved to be stable towards oxidation with molecular O2 under ambient conditions. Single-crystal X-ray diffraction analyses revealed different isomeric structures in the solid state for dioxido complexes 1 and 2 in comparison with oxido phosphane complex 5, dinuclear oxido μ-oxido complex 6 and oxido peroxido complexes 8 and 9, pointing towards an isomeric rearrangement during OAT. Compounds 1 and 2 were furthermore tested for their ability to catalyze the aerobic oxidation of PMe3 and PPh3. A significant difference in catalytic activity has been observed in the oxidation of PMe3, where complex 1 bearing donor atom functionalized ligands led to higher conversion and selectivity than complex 2 coordinated by phenyl iminophenolate ligands. In the oxidation of PPh3, complex 2 leads to higher conversion compared to 1. In a control experiment, phenyl-based dinuclear μ-oxido complex 7, derived from complex 2, was found to be catalytically active, which suggests a lower energy barrier for disproportionation into [MoO(L)2] and [MoO2(L)2] in comparison with methoxypropylene based compound 6, a prerequisite for subsequent reactivity toward molecular O2.

The synthesis of oxidorhenium(V) complexes 1-3 coordinated by tetradentate iminophenolate ligands H2L1-H2L3 bearing backbones of different rigidity (alkyl, cycloalkyl, and phenyl bridges) allows for the formation of distinct geometric isomers, including a symmetric trans-oxidochlorido coordination motif in complex 3. The complex employing a cycloalkyl-bridged ligand (2) of intermediate rigidity exhibits an interesting solvent- and temperature-dependent equilibrium between a symmetric (trans) isomer and an asymmetric (cis) isomer in solution. The occurrence of a symmetric isomer for 2 and 3 is confirmed by single-crystal X-ray diffraction analysis. Chlorido abstraction from 2 with AgOTf yields the corresponding cationic complex 2a, which does not exhibit an isomeric equilibrium in solution but adopts the isomeric form predominant for 2 in a given solvent. All complexes were, furthermore, employed in three benchmark oxygen-atom-transfer (OAT) reactions, namely, the reduction of perchlorate, the epoxidation of cyclooctene, and OAT from dimethyl sulfoxide (DMSO) to triphenylphosphane (PPh3), to assess the influence of the isomeric structure on the reactivity in these reactions. In perchlorate reduction, a clear structural influence was observed, where the trans arrangement in 3 led to the complete absence of activity. In the epoxidation reaction, all complexes led to comparable epoxide yields, albeit higher catalytic activity but lower overall stability of the catalysts with a trans arrangement was observed. In OAT from DMSO to PPh3, also a clear structural dependence was observed, where the trans complex 3 led to full phosphane conversion with an excess of oxidant, while the cis compound 1 was completely inactive.

In Z-type ligands the electrons for the coordination bond are formally provided by the metal. They represent an important addition to the much more extensively used L- and X-type σ-donor ligands for the development of transition metal complexes with new reactivities. We report here a new boron Z-type ligand with three tethering thiopyridazinyl donors forming exclusively complexes that feature a metal boron bond. Rational substitution pattern in the backbone of the pyridazinyl heterocycle led to a well-behaved ligand system that allowed preparation of a series of copper boratrane complexes in high yields. They are found to be more soluble in common organic solvents allowing reactivity studies in contrast to previous complexes with this type of ligand. Thus, copper complexes [Cu{B(Pn(Me,tBu))3}X] with X = Cl, OTf, N3, and κN-NCS are reported. Solution behavior was explored, and the molecular structures were determined by single-crystal X-ray diffraction analyses. The thiocyanate ligand is found to coordinate via its nitrogen atom pointing to a high oxidation state of the copper. Density functional theory calculations indicate a high positive charge on copper and a strong copper-boron interaction. Thus, here reported complexes deliver synthetic evidence for the Z-type nature of the ligand. These findings are important for further dissemination of these types of ligands in coordination chemistry.

Preparation of molybdenum dioxido complexes with novel iminophenolate ligands bearing pendant secondary amide functionalities led to unprecedented C-C and C-N coupling reactions of two α-iminoamides upon coordination. The diastereoselective cyclization to asymmetric imidazolidines occurs at the metal center in two consecutive steps via a monocoupled intermediate. A meaningful mechanism is proposed on the basis of full characterization of intermediate and final molybdenum-containing products by spectroscopic means and by single-crystal X-ray diffraction analyses. This process constitutes the first example of a diastereoselective self-cyclization of two α-iminoamides.

A sterically demanding molybdenum(VI) dioxo complex was found to catalytically activate molecular oxygen and to transfer its oxygen atoms to phosphines. Intermediate peroxo as well as reduced mono-oxo complexes were isolated and fully characterized. Monomeric Mo(IV) monooxo species proved to be of an unusual nature with the coordinated phosphine trans to the oxo group. The reduced molybdenum centers can activate O2 to form a stable Mo(VI) oxo-peroxo complex unambiguously characterized by single crystal X-ray diffraction analysis. NMR experiments demonstrate that both oxygen atoms of the peroxo unit are transferred to an accepting substrate, generating the Mo(IV) intermediate and restarting the catalytic cycle.

The synthesis and characterization of a biomimetic system that can reversibly bind acetylene (ethyne) is reported. The system has been designed to mimic catalytic intermediates of the tungstoenzyme acetylene hydratase. The thiophenyloxazoline ligand S-Phoz (2-(4',4'-dimethyloxazolin-2'-yl)thiophenolate) is used to generate a bioinspired donor environment around the W center, facilitating the stabilization of W-acetylene adducts. The featured complexes [W(C2 H2 )(CO)(S-Phoz)2 ] (2) and [WO(C2 H2 )(S-Phoz)2 ] (3) are extremely rare from a synthetic and structural point of view as very little is known about W-C2 H2 adducts. Upon exposure to visible light, 3 can release C2 H2 from its coordination sphere to yield the 14-electron species [WO(S-Phoz)2 ] (4). Under light-exclusion 4 re-activates C2 H2 making this the first fully characterized system for the reversible activation of acetylene., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The soft scorpionate ligand hydrotris(6-tert-butyl-3-thiopyridazinyl)borate (Tn) was found to exhibit pronounced photoreactivity. Full elucidation of this process revealed the formation of 6-tert-butylpyridazine-3-thione (PnH) and 4,5-dihydro-6-tert-butylpyridazine-3-thione (H2PnH). Under exclusion of light, no solvolytic reactions occur, allowing the development of high-yield preparation protocols for the sodium, potassium, and thallium salts and improving the yield for their derived copper boratrane complex. The photoreactivity is relevant for all future studies with electron-deficient scorpionate ligands.

Oxorhenium(V) complexes of the general formula [ReOCl2(PPh3)(L)] (2a-c) and [ReOCl(L)2] (3a-c) with L being monoanionic, bidentate phenolate-pyrazole ligands 1a-c that bear substituents with various electronic features on the phenol ring (1a Br, 1b NO2, 1c OMe) were prepared. The compounds are stable toward moisture and air, allowing them to be handled in a normal lab atmosphere. All complexes were fully characterized by spectroscopic means and, in the case of 2b, 2c, 3b, and 3c, also by single-crystal X-ray diffraction analyses. Electrochemical investigations by cyclic voltammetry of complexes 3a-c showed a shift to more positive potentials for the Re(V)/Re(VI) redox couple in the order of 3b > 3a > 3c (R = NO2 > Br > OMe), reflecting the higher electrophilic character of the Re atom caused by the ligands 1a-c. Complexes 2a-c and 3a-c display excellent catalytic activity in the epoxidation of cyclooctene, where all six complexes give quantitative conversions to the epoxide within 3 h if tert-butylhydroperoxide (TBHP) is employed as oxidant. Moreover, they represent rare examples of oxorhenium(V) catalysts capable of using the green oxidant hydrogen peroxide, leading to high yields up to 74%. Also, green solvents such as diethylcarbonate can be used successfully in epoxidation reactions, albeit resulting in lower yields (up to 30%).

The bidentate phenolate-oxazoline ligands 2-(2'-hydroxyphenyl)-2-oxazoline (1a, Hoz) and 2-(4',4'-dimethyl-3',4'-dihydrooxazol-2'-yl)phenol (1b, Hdmoz) were used to synthesize two sets of oxorhenium(V) complexes, namely, [ReOCl2(L)(PPh3)] [L = oz (2a) and dmoz (2b)] and [ReOX(L)2] [X = Cl, L = oz (3a or 3a'); X = Cl, L = dmoz (3b); X = OMe, L = dmoz (4)]. Complex 3a' is a coordination isomer (N,N-cis isomer) with respect to the orientation of the phenolate-oxazoline ligands of the previously published complex 3a (N,N-trans isomer). The reaction of 3a' with silver triflate in acetonitrile led to the cationic compound [ReO(oz)2(NCCH3)](OTf) ([3a'](OTf)). Compound 4 is a rarely observed isomer with a trans-O═Re-OMe unit. Complexes 3a, 3a', [3a'](OTf), and 4 were tested as catalysts in the reduction of a perchlorate salt with an organic sulfide as the O acceptor and found to be active, in contrast to 2a and 2b. A comparison of the two isomeric complexes 3a and 3a' showed significant differences in activity: 87% 3a vs 16% 3a' sulfoxide yield. When complex [3a'](OTf) was used, the yield was 57%. Density functional theory calculations circumstantiate all of the proposed intermediates with N,N-trans configurations to be lower in energy compared to the respective compounds with N,N-cis configurations. Also, no interconversions between N,N-trans and N,N-cis configurations are predicted, which is in accordance with experimental data. This is interesting because it contradicts previous mechanistic views. Kinetic analyses determined by UV-vis spectroscopy on the rate-determining oxidation steps of 3a, 3a', and [3a'](OTf) proved the N,N-cis complexes 3a' and [3a'](OTf) to be slower by a factor of ∼4.

Six novel oxorhenium(V) complexes incorporating quinoline and isoquinoline carboxylic acid derivatives were prepared in good yields. Relying on the experimental conditions, compounds with two chelate ligands [ReOCl(iqc)2]·MeOH (1), [ReO(OMe)(iqc)2] (2), [ReO(OMe)(mqc)2] (3) and [ReO(OMe)(8-qc)2] (4) and compounds incorporating one bidentate ligand [ReOCl2(iqc)(PPh3)] (5) and [ReOCl2(mqc)(PPh3)] (6) were synthesized (iqcH = isoquinoline-1-carboxylic acid, mqcH = 4-methoxy-2-quinolinecarboxylic acid and 8-qcH = 8-quinolinecarboxylic acid). The reported compounds were characterized by spectroscopic methods and single crystal X-ray diffraction analysis. In compounds 1 and 2, one chelate ligand occupies an axial and an equatorial position, while the other one occupies two equatorial positions, forming a cis-(N,N) isomer. In turn, complexes 3 and 4 show a rare ligand arrangement with two trans-N, trans-O chelate ligands in the equatorial plane and a linear axial [O=Re-OMe] unit. The complexes with one chelate ligand 5 and 6 are cis-(Cl,Cl)-isomers. All compounds were tested as potential catalysts in the epoxidation of cyclooctene with 3 equiv. of tert-butylhydroperoxide. The yield of cyclooctane oxide varies between 16 and 68% (50 °C, 24 h), and the catalytic competency of compounds 1-6 was discussed with regard to the structure of each complex.

Reaction of [MoO(2)(η(2)-tBu(2)pz)(2)] with Schiff base ligands HL(X) (X = 1-5) gave molybdenum(VI) dioxo complexes of the type cis-[MoO(2)(L(X))(2)] as yellow to light brown solids in moderate to good yields. All ligands coordinate via its phenolic O atom and the imine N atom in a bidentate manner to the metal center. The third donor atom (R(2) = OMe or NMe(2)) in the side chain in complexes 1-4 is not involved in coordination and remains pendant. This was confirmed by X-ray diffraction analyses of complexes 1 and 3. Complexes 1, 3, and 5 exist as a mixture of two isomers in solution, whereas complexes 2 and 4 with sterically less demanding substituents on the aromatics only show one isomer in solution. All complexes are active catalysts in the epoxidation of various internal and terminal alkenes, and epoxides in moderate to good yields with high selectivities are obtained. In the challenging epoxidation of styrene, complexes 1 and 2 prove to be very active and selective. The selectivity seems to be influenced by the pendant donor arm, as complex 5 without additional donor in the side chain is less selective. Experiments prove that the addition of n-butyl methyl ether as intermolecular donor per se has no influence on the selectivity. The basic conditions induced by the NMe(2) groups in complexes 3 and 4 lead to lower activity.

Synthesis, characterization, and epoxidation chemistry of four new dioxomolybdenum(VI) complexes [MoO(2)(L)(2)] (1-4) with aryloxide-pyrazole ligands L = L1-L4 is described. Catalysts 1-4 are air and moisture stable and easy to synthesize in only three steps in good yields. All four complexes are coordinated by the two bidentate ligands in an asymmetric fashion with one phenoxide and one pyrazole being trans to oxo atoms, respectively. This is in contrast to the structure found for the related aryloxide-oxazoline coordinated Mo(VI) dioxo complex 5. This was confirmed by the determination of the molecular structures of complexes 1-3 by X-ray diffraction analyses. Compounds 1-4 show high catalytic activities in the epoxidation of various olefins. Cyclooctene (S1) is converted to its epoxide with high activity, whereas the epoxidation of styrene (S2) is unselective. Internal olefins (S3 and S4) are also acceptable substrates, as well as the very challenging olefin 1-octene (S5). Catalyst loading can be reduced to 0.02 mol % and the catalyst can be recycled up to ten times without significant loss of activity. Supportive DFT calculations have been carried out in order to obtain deeper insights into the electronic situation around the Mo atom.

Substitution of a methyl by a trifluoromethyl moiety in well-known β-ketimines afforded the ligands (Ar)NC(Me)CH(2)CO(CF(3)) (HL(H), Ar = C(6)H(5); HL(Me), A r= 2,6-Me(2)C(6)H(3); HL(iPr), Ar = 2,6-(i)Pr(2)C(6)H(3)). Subsequent complexation to the [MoO(2)](2+) core leads to the formation of novel complexes of general formula [MoO(2)(L(R))(2)] (R = H, 1; R = Me, 2; R = iPr, 3). For reasons of comparison the oxo-imido complex [MoO(N(t)Bu)(L(Me))(2)] (4) has also been synthesized. Complexes 1-4 were investigated in oxygen atom transfer (OAT) reactions using the substrate trimethylphosphine. The respective products after OAT, the reduced Mo(IV) complexes [MoO(PMe(3))(L(R))(2)] (R = H, 5; R = Me, 6; R = iPr, 7) and [Mo(N(t)Bu)(PMe(3))(L(Me))(2)] (8), were isolated. All complexes have been characterized by NMR spectroscopy, and 1-4 also by cyclic voltammetry. A positive shift of the Mo(VI)-Mo(V) reduction wave upon fluorination was observed. Furthermore, molecular structures of complexes 2, 4, 5, and 8 have been determined via single crystal X-ray diffraction analysis. Complex 8 represents a rare example of a Mo(IV) phosphino-imido complex. Kinetic measurements by UV-vis spectroscopy of the OAT reactions from complexes 1-4 to PMe(3) showed them to be more efficient than previously reported nonfluorinated ones, with ligand L' = (Ar)NC(Me)CH(2)CO(CH(3)) [MoO(2)(L')(2)] (9) and [MoO(N(t)Bu)(L')(2)] (10), respectively. Thermodynamic activation parameters ΔH(‡) and ΔS(‡) of the OAT reactions for complexes 2 and 4 have been determined. The activation enthalpy for the reaction employing 2 is significantly smaller (12.3 kJ/mol) compared to the reaction with the nonfluorinated complex 9 (60.8 kJ/mol). The change of the entropic term ΔS(‡) is small. The reaction of the oxo-imido complex 4 to 8 revealed a significant electron-donating contribution of the imido substituent.

The preparation and oxygen-atom-transfer (OAT) reactivity of oxoimido complexes [MoO(N-t-Bu)(t-Bu(2)-4-Rpz)(2)] [where R = H (1), Br (2), and Me (3); t-Bu(2)pz = 3,5-di-tert-butylpyrazolate] are reported. The reaction of the potassium salt of the respective pyrazolate ligands and the molybdenum oxoimido precursor, [MoO(N-t-Bu)Cl(2)(dme)] (dme = dimethoxyethane), in toluene afforded complexes 1-3 in good yields. The complexes were fully characterized by (1)H and (13)C NMR and IR spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray crystallography. The solid-state structures reveal that, in each case, the molybdenum center is coordinated by one oxo, one N-t-Bu group, and two sterically demanding pyrazolate ligands via their two adjacent nitrogen atoms in an η(2) fashion. Coordination around the metal center is severely distorted from octahedral and might be seen as closely approaching a distorted trigonal-prismatic geometry, which is relevant to the active site of dimethyl sulfoxide reductase in its oxidized form. The potential utility of all of the complexes 1-3 for OAT reactivity toward PMe(3) at room temperature is examined, and plausible mechanistic pathways are explored by density functional theory calculations. Furthermore, the complexes reported here open a new and convenient entry into mixed oxoimidomolybdenum complexes.

Reaction of potassium tris(mercapto-tert-butylpyridazinyl)borate K[Tn(tBu)] with copper(II) chloride in dichloromethane at room temperature led to the diamagnetic copper boratrane compound [Cu{B(Pn(tBu))(3)}Cl] (Pn = pyridazine-3-thionyl) (1) under activation of the B-H bond and formation of a Cu-B dative bond. In contrast to this, stirring of the same ligand with copper(I) chloride in tetrahydrofuran (THF) gave the dimeric compound [Cu{Tn(tBu)}](2) (2) where one copper atom is coordinated by two sulfur atoms and one hydrogen atom of one ligand and one sulfur of the other ligand. Hereby, no activation of the B-H bond occurred but a 3-center-2-electron B-H···Cu bond is formed. The reaction of copper(II) chloride with K[Tn(tBu)] in water gave the same product 2, but a formal reduction of the metal center from Cu(II) to Cu(I) occurred. When adding tricyclohexyl phosphine to the reaction mixture of K[Tn(R)] (R = tBu, Me) and copper(I) chloride in MeOH, the distorted tetrahedral Cu complexes [Cu{Tn(R)}(PCy(3))] (R = tBu 3, Me 4) were formed. Compound 4 is exhibiting an "inverted" κ(3)-H,S,S, coordination mode. The copper boratrane 1 was further investigated by density functional theory (DFT) calculations for a better understanding of the M→B interaction involving the d(8) electron configuration of Cu., (© 2011 American Chemical Society)

The synthesis and characterization of a series of molybdenum ([MoO(2)Cl(L(n))]; L(1) (1), L(2) (3)) and tungsten ([WO(2)Cl(L(n))]; L(1) (2), L(2) (4)) dioxo complexes (L(1) = 1-methyl-4-(2-hydroxybenzyl)-1,4-diazepane and L(2) = 1-methyl-4-(2-hydroxy-3,5-di-tert-butylbenzyl)-1,4-diazepane) of tridentate aminomonophenolate ligands HL(1) and HL(2) are reported. The ligands were obtained by reductive amination of 1-methyl-1,4-diazepane with the corresponding aldehyde. Complexes 3 and 4 were obtained by the reaction of [MO(2)Cl(2)(dme)(n)] (M = Mo, n = 0; W, n = 1) with the corresponding ligand in presence of a base, whereas for the preparation of 1 and 2 the ligands were deprotonated by KH prior to the addition to the metal. They were characterized by NMR and IR spectroscopy, by cyclic voltammetry, mass spectrometry, elemental analysis and by single-crystal X-ray diffraction analysis. Solid-state structures of the molybdenum and tungsten cis-dioxo complexes reveal hexa-coordinate metal centers surrounded by two oxo groups, a chloride ligand and by the tridentate monophenolate ligand which coordinates meridionally through its [ONN] donor set. In the series of compounds 1-4, complexes 3 and 4 have been used as catalysts for the oxygen atom transfer reaction between dimethyl sulfoxide (DMSO) and trimethyl phosphine (PMe(3)). Surprisingly, faster oxygen atom transfer (OAT) reactivity has been observed for the tungsten complex [WO(2)Cl(L(2))] (4) in comparison to its molybdenum analog [MoO(2)Cl(L(2))] (3) at room temperature. The kinetic results are discussed and compared in terms of their reactivity., (This journal is © The Royal Society of Chemistry 2011)

A series of first-row transition metal complexes with the unsymmetrically disubstituted pyridazine ligand picolinaldehyde (6-chloro-3-pyridazinyl)hydrazone (PIPYH), featuring an easily abstractable proton in the backbone, was prepared. Ligand design was inspired by literature-known picolinaldehyde 2-pyridylhydrazone (PAPYH). Reaction of PIPYH with divalent nickel, copper, and zinc nitrates in ethanol led to complexes of the type [Cu(II)(PIPYH)(NO(3))(2)] (1) or [M(PIPYH)(2)](NO(3))(2) [M = Ni(II) (2) or Zn(II) (3)]. Complex synthesis in the presence of triethylamine yielded fully- or semideprotonated complexes [Cu(II)(PIPY)(NO(3))] (4), [Ni(II)(PIPYH)(PIPY)](NO(3)) (5), and [Zn(II)(PIPY)(2)] (6), respectively. Cobalt(II) nitrate is quantitatively oxidized under the reaction conditions to [Co(III)(PIPY)(2)](NO(3)) (7) in both neutral and basic media. X-ray diffraction analyses reveal a penta- (1) or hexa-coordinated (2, 3, and 7) metal center surrounded by one or two tridentate ligands and, eventually, κ-O,O' nitrate ions. The solid-state stoichiometry was confirmed by electron impact (EI) and electrospray ionization (ESI) mass spectrometry. The diamagnetic complexes 5 and 6 were subjected to (1)H NMR spectroscopy, suggesting that the ligand to metal ratio remains constant in solution. Electronic properties were analyzed by means of cyclic voltammetry and, in case of copper complexes 1 and 4, also by electron paramagnetic resonance (EPR) spectroscopy, showing increased symmetry upon deprotonation for the latter, which is in accordance with the proposed stoichiometry [Cu(II)(PIPY)(NO(3))]. Protic behavior of the nickel complexes 2 and 5 was investigated by UV/vis spectroscopy, revealing high π-backbonding ability of the PIPYH ligand resulting in an unexpected low acidity of the hydrazone proton in nickel complex 2., (© 2011 American Chemical Society)