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The dynamic processes present in ligand-benzylated formazanate boron and aluminium complexes are investigated using variable temperature NMR experiments and lineshape analyses. The observed difference in activation parameters for complexes containing either organic countercations (NBu4+) or alkali cations is rationalized on the basis of a different degree of ion-pairing in the ground state, and the data are in all cases consistent with a mechanism that involves pyramidal inversion at the nitrogens in the heterocyclic ring rather than homolytic N–C(benzyl) bond cleavage. [ABSTRACT FROM AUTHOR]

Formazans (Ar1-NH-N＝CR3-N＝N-Ar5), a class of nitrogen-rich and highly colored compounds, have been known since the late 1800s and studied more closely since the early 1940s. Their intense color has led to their widespread use as dyes, especially in cell biology where they are most often used to quantitatively assess cell-viability. Despite structural similarities to well-known ligand classes such as β-diketiminates, the deprotonated form of formazans, formazanates, have received relatively little attention in the transition metal and main group coordination chemistry arenas. Formazanate ligands benefit from tunable properties via structural variation, rich optoelectronic properties owing to their highly delocalized π-systems, low-lying frontier orbitals that stabilize otherwise highly reactive species such as radicals, and redox activity and coordinative flexibility that may have significant implications in their future use in catalysis. Here, we review progress in the coordination chemistry of formazanate ligands over the past two decades, with emphasis on the reactivity and applications of the subsequent complexes. [ABSTRACT FROM AUTHOR]

Group 13 complexes of the type [(PhNNC(p-tol)NNPh)ZPh2]2− (Z = B, Al) containing a highly reduced, trianionic formazanate-derived ligand were studied and the differences in the structure, bonding and reactivity between the B and Al compounds were investigated. The increased ionic character in the bonding of the Al complex is evident from the enhanced charge delocalization onto the peripheral ligand substituents (N–Ph) via the π-framework, as shown by the rotation barrier around the N–C(Ph) bond. The electron-rich nature of these compounds allows facile benzylation at the ligand, and the structures of the products were analysed by X-ray crystallography. The products are inorganic analogues of 1-alkylated 1,2,3,4-tetrahydro-1,2,4,5-tetrazines ('leucoverdazyls'). The six-membered heterocyclic cores of the B and Al compounds are shown to be different, having envelope- and boat-type conformations, respectively. Homolysis of the N–C(benzyl) bond in these compounds was studied by NMR spectroscopy under conditions that trap the organic radical as TEMPO-Bn. Analysis of the reaction kinetics affords activation parameters that approximate the N–C(benzyl) bond strength. The ionic Al compound has one of the weakest N–C bonds reported so far in this type of inorganic leucoverdazyl analogues. [ABSTRACT FROM AUTHOR]

The influence of the spin state of the metal centre in spin crossover compounds on the aromaticity of the ligands has been investigated for iron(II)tris-bipyridine (Fe(bpy)32+), and Fe(II)(formazanate)2 (as a truncated model and the full phenyl substituted compound). It was found that the aromaticity of the bipyridine ligands is unaffected by changing the spin state of the central iron atom, but that of the formazanate ligands is reduced upon transition to the high-spin state. This change in aromaticity is rationalized using the symmetry selection rules for aromaticity in terms of virtual excitations from occupied to empty orbitals. A further consequence of this loss in aromaticity is a shift to higher energy in the ring vibrations of the formazanate compounds that can be observed in either its IR or Raman spectrum; this prediction has been confirmed here. This change in aromaticity as a consequence of change in spin state can be regarded as an indication for non-innocent ligands. [ABSTRACT FROM AUTHOR]

The solution-phase thermolysis of (formazanate)boron dihydrides (LBH2; 2) results in the formation of aminoborane compounds (4) via a series of boron-to-ligand intramolecular hydride transfer reactions. Monitoring the reactions by NMR spectroscopy allowed identification of several intermediates, and a reaction mechanism is proposed. In the case of a ligand with an N-Mes substituent it was possible to characterize an intermediate (7b-i) during this transformation that shows an unexpected cyclohexadiene moiety, which results from hydride transfer to the ortho-position of the mesityl substituent. Two consecutive boron-to-ligand hydride transfers eventually result in reductive N-N bond cleavage to give triazaboroles as the final product. [ABSTRACT FROM AUTHOR]

The solution-phase thermolysis of (formazanate)boron dihydrides (LBH2; 2) results in the formation of aminoborane compounds (4) via a series of boron-to-ligand intramolecular hydride transfer reactions. Monitoring the reactions by NMR spectroscopy allowed identification of several intermediates, and a reaction mechanism is proposed. In the case of a ligand with an N-Mes substituent it was possible to characterize an intermediate (7b-i) during this transformation that shows an unexpected cyclohexadiene moiety, which results from hydride transfer to the ortho-position of the mesityl substituent. Two consecutive boron-to-ligand hydride transfers eventually result in reductive N-N bond cleavage to give triazaboroles as the final product. [ABSTRACT FROM AUTHOR]

Despite the current interest in structure and reactivity of sub-valent main group compounds, neutral boron analogues of N-heterocyclic carbenes have been elusive due to their high reactivity. Here we provide evidence that 2-electron reduction of a (formazanate)BF2 precursor leads to NaF elimination and formation of an N-heterocyclic boron carbenoid, and describe the formation of a series of unusual BN heterocycles that result from trapping of this fragment. Subsequent chemical oxidation by XeF2 demonstrates that the trapped (formazanate)B fragment retains carbenoid character and regenerates the boron difluoride starting material in good yield. These results indicate that the formazanate ligand framework provides a unique entry into sub-valent boron chemistry. [ABSTRACT FROM AUTHOR]

Ligand cooperativity provides (transition) metal complexes with new reactivities in substrate activation and catalytic reactions, but usually the ligand acts as an internal (Brønsted) base, while the metal acts as a (Lewis) acid. We describe the synthesis and stepwise activation of a new phosphane-pyridine-amide ligand PNNH2 in combination with RhI. The ligand is susceptible to stepwise proton and hydride loss from the nitrogen arm (imine formation) and deprotonation at the pyridylphosphine arm (dearomatization), giving rise to amine complex 1, amido species 2, imine complex 3 and dearomatized compound 4. Complex 4 bears a dual-mode cooperative PNN′ ligand containing both a (nucleophilic) basic methine fragment and a reactive (electrophilic) imine moiety. The basic ligand arm enables substrate deprotonation while the imine ligand arm enables reversible 'storage' of the activated (nucleophilic) form of a sulfonamide substrate at the ligand. In combination with metal-based reactivity, this allows for the mono-alkylation of o-toluenesulfonamide with iodomethane. Compounds 1, 3 and 4 are structurally characterized. We also report the first structurally characterized example of an aminal in the coordination sphere of rhodium, complex 5, [Rh(CO)( PNN′′)], formed by sequential NH activation of sulfonamide by the dearomatized ligand PNN′ and follow-up nucleophilic attack of anionic sulfonamide onto the imine fragment. [ABSTRACT FROM AUTHOR]

The cationic tantalum complex {[η6-Ar-CMe2-η5-C5H4]TaPr}[B(C6F5)4] (1; Ar = 3,5-Me2C6H3) serves as a starting materialfor a series of neutral, monocationic, and dicationic derivatives.The cationic hydride {[η6-Ar-CMe2-η5-C5H4]TaH}[B(C6F5)4] (2) that results from hydrogenolysisof 1inserts the di- and trisubstituted olefins cyclopenteneand 2-methyl-2-pentene; it reacts with styrene to give the 2,1-insertionproduct, for which the Ta–CH(Me)Ph group is bound in a σ3-allylic fashion. A neutral complex is obtained from 1by reaction with Br–, and a dicationicderivative is available by hydride abstraction from 2using the Lewis acidic trityl cation. All compounds described hereretain the unusual ansa-(η5-cyclopentadienyl,η6-arene) coordination mode of the ligand that stabilizes theformally Ta(III) center. X-ray structures and DFT calculations showthat the metal–arene interaction contains a significant πback-donation component (arene←Ta(III)) that differs only slightlyin the series, despite the variation of charge at the metal center. [ABSTRACT FROM AUTHOR]

Cationic titanium(IV) complexes with ansa-(η5-cyclopentadienyl,η6-arene) ligands were synthesized and characterized by X-ray crystallography. The strength of the metal-arene interaction in these systems was studied by variable-temperature NMR spectroscopy. Complexes with a C1 bridge between the cyclopentadienyl and arene moieties feature hemilabile coordination behavior of the ligand and consequently are active ethylene trimerization catalysts. Reaction of the titanium(IV) dimethyl cations with CO results in conversion to the analogous cationic titanium(ll) dicarbonyl species. Metal-to-ligand backdonation in these formally low-valent complexes gives rise to a strongly bonded, partially reduced arene moiety. In contrast to the η6-arene coordination mode observed for titanium, the more electron-rich vanadium(V) cations [cyclopentadienyl-arene]V(NiPr2)(NC6H4-4-Me)+ feature η¹-arene binding, as determined by a crystallographic study. The three different metal-arene coordination modes that we experimentally observed model intermediates in the cycle for titanium-catalyzed ethylene trimerization. The nature of the metal-arene interaction in these systems was studied by DFT calculations. [ABSTRACT FROM AUTHOR]

Here we present a novel dynamic organocatalyst, based on a firstgeneration molecular motor core, able to control the stereochemical outcome of the Henry reaction using both light and heat as external stimuli. [ABSTRACT FROM AUTHOR]

The selective α‐deuteration of α,β‐unsaturated nitriles using the strong base tBuOK or a metal‐ligand cooperative Ru pincer catalyst is described. With D2O as deuterium source and glyme as solvent at 70 °C, tBuOK is an efficient catalyst for deuteration at the α‐C(sp2) position of cinnamonitriles, providing access to a broad range of deuterated derivatives in good to excellent yields and with very high levels of deuterium incorporation. While the tBuOK‐catalysed protocol does not tolerate base‐sensitive functional groups, cinnamonitrile derivatives containing a benzylic bromide or ester moiety were deuterated in excellent yields using Milstein's ruthenium PNN pincer catalyst. Moreover, the activity for H/D exchange of the metal‐ligand cooperative Ru catalyst is found to be significantly higher than that of tBuOK, allowing reactions to proceed well even at room temperature. A mechanistic proposal is put forward that involves deprotonation of the cinnamonitrile α‐CH position when using tBuOK as catalyst, whereas H/D exchange catalysis with the Ru PNN pincer likely proceeds via (reversible) oxa‐Michael addition of D2O. [ABSTRACT FROM AUTHOR]

Abstract: Water addition to α,β‐unsaturated nitriles would give facile access to the β‐hydroxy‐nitriles, which in turn can be hydrogenated to the γ‐amino alcohols. We have previously shown that alcohols readily add in 1,4‐fashion to these substrates using Milstein's Ru(PNN) pincer complex as catalyst. However, attempted water addition to α,β‐unsaturated nitriles gave the 3‐hydroxynitriles in mediocre yields. On the other hand, addition of benzyl alcohol proceeded in excellent yields for a variety of β‐substituted unsaturated nitriles. Subsequent treatment of the benzyl alcohol addition products with TMSCl/FeCl3 resulted in the formation of 3‐hydroxy‐alkylnitriles. The 3‐benzyloxy‐alkylnitriles obtained from oxa‐Michael addition also could be hydrogenated directly in the presence of acid to give the amino alcohols as their HCl salts in excellent yields. Hydrogenation under neutral conditions gave a mixture of the secondary and tertiary amines. Hydrogenation in the presence of base and Boc‐anhydride gave the orthogonally bis‐protected amino alcohols, in which the benzyl ether can subsequently be cleaved to yield Boc‐protected amino alcohols. Thus, a variety of molecular scaffolds with a 1,3‐relationship between O‐ and N‐functional group is accessible starting from oxa‐Michael addition of benzyl alcohol to α,β‐unsaturated nitriles. [ABSTRACT FROM AUTHOR]

The article presents a research on the stabilization of zirconium (Zr) and titanium (Ti) cations through interactions with ferrocenyl fragment. It discusses how early metal cations react with ferrocene wherein the carbon-hydrogen (CH)-activation of ferrocene has resulted in stable but active polymerization catalysts. It shows that electron-rich metal fragments have the ability to stabilize electrophillic ZR cations through combining Iron (Fe) and Zr interactions with agnostic CH bonds.

The article presents a study on the nitrous oxide (N2O) complexation by frustrated lewis pairs (FLP). It states that N2O is considered as a minor constituent of the atmosphere but it posed a strong environmental oxidant that is ∼300 times more potent as a greenhouse gas compare to carbon dioxide. The study suggests that frustrated lewis pairs bind nitrous oxide to give intact linkages.

The Yin – Yang symbol. on the cover symbolizes the complementary nucleophilic and electrophilic ligand arms of the new cooperative PNN pincer ligand, strengthened by the two Kung ‐ Fu fighters making balancing, harmonizing moves. For more details see the Full Paper by J. I. van der Vlugt, B. de Bruin et al. on page 12683 ff. [ABSTRACT FROM AUTHOR]

Invited for the cover of this issue is the group of Bas de Bruin and Jarl Ivar van der Vlugt at the University of Amsterdam. The image depicts the complementary nucleophilic and electrophilic ligand arms of a new cooperative PNN pincer ligand. Read the full text of the article at . [ABSTRACT FROM AUTHOR]

The article describes the cationic tantalum complexes bearing an arene-substituted cyclopentadienyl ligand involving ansa-(n5-C5H4>,n6-arene) coordination. According to the authors, the coordination of the pendant arene allows resonance stabilization of low-valent species, resulting in ambivalent character. They add the cationic ansa-Cp-arene tantalum complexes can display both migratory insertion and oxidative addition reactions.

The enantiomeric homogeneity of light-driven molecular motors based on overcrowded alkenes is crucial in their application as either unidirectional rotors or as chiral multistate switches. It was challenging to obtain these compounds as single enantiomers via the established synthetic procedures due to loss of optical purity in the key step, i.e., the Barton-Kellogg olefination reaction. Searching for strategies to avoid racemization, a new class of light-driven molecular motors was designed, synthesized, and studied. The stereochemical integrity was fully preserved throughout the synthesis, and on the basis of photochemical and kinetic studies using UV/vis, CD, and ¹H NMR spectroscopy, it was established that they still function properly as unidirectional molecular motors. [ABSTRACT FROM AUTHOR]

The first catalytic enantioselective synthesis of ambiphilic phosphine boronate esters is presented. The asymmetric boration of α,β-unsaturated phosphine oxides catalyzed by a copper bisphosphine complex affords optically active organoboronate esters that bear a vicinal phosphine oxide group in good yields and high enantiomeric excess. The synthetic utility of the products is demonstrated through stereospecific transformations into multifunctional optically active compounds. [ABSTRACT FROM AUTHOR]

The first catalytic enantioselective synthesis of ambiphilic phosphine boronate esters is presented. The asymmetric boration of α,β-unsaturated phosphine oxides catalyzed by a copper bisphosphine complex affords optically active organoboronate esters that bear a vicinal phosphine oxide group in good yields and high enantiomeric excess. The synthetic utility of the products is demonstrated through stereospecific transformations into multifunctional optically active compounds. [ABSTRACT FROM AUTHOR]

Alkali metal salts of redox-active formazanate ligands were prepared, and their structures in the solidstate and in solution are determined. The nitrogen-rich [NNCNN] backbone of formazanates results in a varied coordination chemistry, with both the internal and terminal nitrogen atoms available for bonding with the alkali metal. The potassium salt K[PhNNC(p-tol)NNPh]·2THF (1-K) is dimeric in the solid state and even in THF solution, as a result of the K atom bridging via interaction with a terminal N atom and the aromatic ring of a second unit. Conversely, for the compounds Na[MesNNC(CN)NNMes]·2THF (2-Na) and Na[PhNNC(tBu)NNPh] (3-Na) polymeric and hexameric structures are found in the solid state respectively. The preference for binding the alkali metal through internal N atoms (1-K and 2-Na) to give a 4-membered chelate, or via internal/external N atoms (5-membered chelate in 3-Na), contrasts with the 6-membered chelate mode observed in our recently reported formazanate zinc complexes. [ABSTRACT FROM AUTHOR]

The [Ru(CO)H(PNN)] pincer complex based on a dearomatised PNN ligand (PNN: 2-di- tert-butylphosphinomethyl-6-diethylaminomethylpyridine) was examined for its ability to isomerise alkenes. The isomerisation reaction proceeded under mild conditions after activation of the complex with alcohols. Variable-temperature (VT) NMR experiments to investigate the role of the alcohol in the mechanism lend credence to the hypothesis that the first step involves the formation of a rearomatised alkoxide complex. In this complex, the hemilabile diethylamino side-arm can dissociate, allowing alkene binding cis to the hydride, enabling insertion of the alkene into the metal-hydride bond, whereas in the parent complex only trans binding is possible. During this study, a new uncommon Ru0 coordination complex was also characterised. The scope of the alkene isomerisation reaction was examined. [ABSTRACT FROM AUTHOR]

Reactions of N,N-dimethylaniline, N-isopropylaniline, 1,4-C6H4(CH2NHtBu)2,and benzyldimethylamine with the Lewis acid B(C6F5)3have been studied. In the case of N,N-dimethylaniline the combination of the Lewisacid and base forms an almost completely noninteracting frustratedLewis pair, while the corresponding reactions of N-isopropylaniline and benzyldimethylamine with B(C6F5)3afford the adducts (PhNHiPr)B(C6F5)3(1) and PhCH2NMe2B(C6F5)3(2), respectively. 1,4-C6H4(CH2NHtBu)2reacts with 2 equiv of B(C6F5)3to give rise to an inseparablemixture of the mono- and bis-amine–borane adducts as well asan iminium salt derived from hydride abstraction from a benzylic carbonof the diamine. Subsequent selected reactions of the Lewis acid/basecombinations with H2afforded [PhNMe2H][HB(C6F5)3] (3), [PhCH2NMe2H][HB(C6F5)3] (4), and [1,4-C6H4(CH2NH2tBu)2][HB(C6F5)3]2(5). Reactions with CO2gave PhCH2NMe2CO2B(C6F5)3(6), [PhNiPrH2][PhNiPrCO2B(C6F5)3] (8), [1,4-C6H4(CH2NH2tBu)(CH2NtBuCO2B(C6F5)3] (9), and [C5H6Me4NMeH]2[1,4-C6H4(CH2NtBuCO2B(C6F5)3)2] (10). Interestingly, the species 4also reacts with CO2to give [PhCH2NMe2H][HCO2B(C6F5)3] (7). Species 8reacts with HSiEt3to yield [PhNiPrC(OSiEt3)OB(C6F5)3] (11). The adduct 2also reacts with ethene, 2,4-hexadiyne, and the alkynesHCCR (R = n-Bu, Ph, tBu,SiMe3) to give the 1,2- addition products PhCH2NMe2CH2CH2B(C6F5)3(12), PhCH2NMe2C(CCMe)=CMeB(C6F5)3(13), and the ammoniumalkynylborate salts [PhCH2NMe2H][RCCB(C6F5)3] (R = n-Bu (14a), Ph (14b), tBu (14c), SiMe3(14d)). In contrast, the reactionof 1-hexyne with the Lewis pair PhNMe2and B(C6F5)3affords the trans-1,2-addition productPhNMe2C(C4H9)=CH(B(C6F5)3) (15). This chemistry demonstratesthe generality of such FLP chemistry, extending it to N-based Lewisbases and thereby expanding the scope for applications of FLP chemistry. [ABSTRACT FROM AUTHOR]

Palladium(ii) complexes with a bidentate, anionic formazanate ligand are described. Attempts to prepare mono(formazanate) palladium alkyl complexes often leads to the homoleptic bis(formazanate) complex, which shows rich electrochemistry due to the redox-active nature of the ligands. Performing salt metathesis between the precursor [Pd(COD)(CH3)Cl] and the potassium salt of the ligand in the presence of tetrabutylammonium chloride yields a square planar mono(formazanate) palladate complex through coordination of chloride anion. Ligand exchange allows binding of unsaturated molecules and evaluation of the reactivity of the Pd–CH3 fragment. Using this approach, insertion reactions of CO, isocyanide and methyl acrylate into the Pd–CH3 bond are demonstrated. [ABSTRACT FROM AUTHOR]

The synthesis of mono(formazanate) iron complexes is described. In the presence of tetrabutylammonium halides, salt metathesis reactions afford the ferrate(ii) complexes [Bu4N][LFeX2] (L = PhNNC(p-tol)NNPh; X = Cl, Br) in good yield, and the products are characterized in detail. The high-spin ferrate(ii) complexes show cyclic voltammograms that are consistent with reversible, ligand-based one-electron reduction. The halides in these ferrate(ii) compounds are labile, and are displaced by 4-methoxyphenyl isocyanide (4 equiv.) as evidenced by formation of the low-spin, cationic octahedral complex [LFe(CNC6H4(p-OMe))4][Br]. Thus, a straightforward route to mono(formazanate) iron(ii) complexes is established. [ABSTRACT FROM AUTHOR]

The highly regio- and enantioselective (up to >99:1 dr, up to 99:1 er) desymmetrization of meso-1,4-dibromocycloalk-2-enes using asymmetric allylic substitution with organolithium reagents to afford enantioenriched bromocycloalkenes (ring size of 5 to 7) has been achieved. The cycloheptene products undergo an unusual ring contraction. The synthetic versatility of this Cu(I)-catalyzed reaction is demonstrated by the concise stereocontrolled preparation of cyclic amino alcohols, which are privileged chiral structures in natural products and pharmaceuticals and widely used in synthesis and catalysis. [ABSTRACT FROM AUTHOR]

Symmetric molecular motors based on two overcrowded alkenes with a notable absence of a stereogenic center show potential to function as novel mechanical systems in the development of more advanced nanomachines offering controlled motion over surfaces. Elucidation of the key parameters and limitations of these third-generation motors is essential for the design of optimized molecular machines based on light-driven rotary motion. Herein we demonstrate the thermal and photochemical rotational behavior of a series of third-generation light-driven molecular motors. The steric hindrance of the core unit exerted upon the rotors proved pivotal in controlling the speed of rotation, where a smaller size results in lower barriers. The presence of a pseudo-asymmetric carbon center provides the motor with unidirectionality. Tuning of the steric effects of the substituents at the bridgehead allows for the precise control of the direction of disrotary motion, illustrated by the design of two motors which show opposite rotation with respect to a methyl substituent. A third-generation molecular motor with the potential to be the fastest based on overcrowded alkenes to date was used to visualize the equal rate of rotation of both its rotor units. The autonomous rotational behavior perfectly followed the predicted model, setting the stage for more advanced motors for functional dynamic systems. [ABSTRACT FROM AUTHOR]

Biological molecular motors translate their local directional motion into ordered movement of other parts of the system to empower controlled mechanical functions. The design of analogous geared systems that couple motion in a directional manner, which is pivotal for molecular machinery operating at the nanoscale, remains highly challenging. Here, we report a molecular rotary motor that translates light-driven unidirectional rotary motion to controlled movement of a connected biaryl rotor. Achieving coupled motion of the distinct parts of this multicomponent mechanical system required precise control of multiple kinetic barriers for isomerization and synchronous motion, resulting in sliding and rotation during a full rotary cycle, with the motor always facing the same face of the rotor. [ABSTRACT FROM AUTHOR]

A straightforward synthetic route to arylazoindazoles via nucleophilic aromatic substitution is presented. Upon deprotonation of the NH group, a C6F5-substituted formazan undergoes facile cyclization as a result of intermolecular nucleophilic substitution (SNAr). This new class of azo photoswitches containing an indazole five-membered heterocycle shows photochemical isomerization with high fatigue resistance. In addition, the Z-isomers have long thermal half-lives in the dark of up to several days at room temperature. The fluorinated indazole group offers a handle for further functionalization and tuning of its properties, as it is shown to be susceptible to a subsequent, highly selective nucleophilic displacement reaction. [ABSTRACT FROM AUTHOR]

Inexpensive and readily available organomagnesium reagents were used for the catalytic enantioselective alkylation of enolizable N-sulfonyl ketimines. The low reactivity and competing enolization of the ketimines was overcome by the use of a copper-phosphine chiral catalyst, which also rendered the transformation highly chemoselective and enantioselective for a broad range of ketimine substrates. [ABSTRACT FROM AUTHOR]

Inexpensive and readily available organomagnesium reagents were used for the catalytic enantioselective alkylation of enolizable N-sulfonyl ketimines. The low reactivity and competing enolization of the ketimines was overcome by the use of a copper-phosphine chiral catalyst, which also rendered the transformation highly chemoselective and enantioselective for a broad range of ketimine substrates. [ABSTRACT FROM AUTHOR]

Redox-active formazanate ligands are emerging as tunable electron-reservoirs in coordination chemistry. Here we show that boron diphenyl complexes with formazanate ligands, despite their (formal) negative charge, can be further reduced by up to two electrons. A combined crystallographic, spectroscopic and computational study establishes that formazanate ligands are stable in mono-, di- and trianionic form. [ABSTRACT FROM AUTHOR]

The pincer complex (PNN)RuH(CO), with a de-aromatized pyridine in the ligand backbone, is shown to react with nitriles in a metal–ligand cooperative manner. This leads to the formation of a series of complexes with new Ru–N(nitrile) and C(ligand)–C(nitrile) bonds. The initial nitrile cycloaddition products, the ketimido complexes 3, have a Brønsted basic (nitrile-derived) Ru–N fragment. This is able to deprotonate a CH2 side-arm of the pincer ligand to give ketimine complexes (4) with a de-aromatized pyridine backbone. Alternatively, the presence of a CH2 group adjacent to the nitrile functionality can lead to tautomerization to an enamido complex (5). Variable-temperature NMR studies and DFT calculations provide insight in the relative stability of these compounds and highlight the importance of their facile interconversion in the context of subsequent nitrile transformations. [ABSTRACT FROM AUTHOR]

Despite its status as one of the world's most prevalent and deadly bacterial pathogens, Mycobacterium tuberculosis (Mtb) infection is not routinely diagnosed by rapid and highly reliable tests. A program to discover Mtb-specific biomarkers recently identified two natural compounds, 1-tuberculosinyl adenosine (1-TbAd) and N6-tuberculosinyl adenosine (N6-TbAd). Based on their association with virulence, the lack of similar compounds in nature, the presence of multiple stereocenters, and the need for abundant products to develop diagnostic tests, synthesis of these compounds was considered to be of high value but challenging. Here, a multigram-scale stereoselective synthesis of 1-TbAd and N6-TbAd is described. As a key-step, a chiral auxiliary-mediated Diels-Alder cycloaddition was developed, introducing the three stereocenters with a high exo endo ratio (10:1) and excellent enantioselectivity (>98% ee). This constitutes the first entry into the stereoselective synthesis of diterpenes with the halimane skeleton. Computational studies explain the observed stereochemical outcome. [ABSTRACT FROM AUTHOR]

The catalytic enantioselective synthesis of ambiphilic phosphine boronate esters is presented. [ABSTRACT FROM AUTHOR]

Spin-crossover in a pseudo-tetrahedral bis- (formazanate) iron(ll) complex ( l) is described. Structural, magnetic, and spectroscopic analyses indicate that this compound undergoes thermal switching between an S=0 and an S=2 state, which is very rare in four-coordinate complexes. The transition to the high-spin state is accompanied by an increase in Fe-N bond lengths and a concomitant contraction of intraligand N-N bonds. The latter suggests that stabilization of the low-spin state is due to the ^-acceptor properties of the ligand. One-electron reduction of 1 leads to the formation of the corresponding anion, which contains a low-spin (S= 1/2) Fe(l) center. The findings are rationalized by electronic structure calculations using density functional theory. [ABSTRACT FROM AUTHOR]

A palladium-catalyzed direct synthesis of symmetric biaryl compounds from aryl halides in the presence of tBuLi is described. In situ lithium-halogen exchange generates the corresponding aryl lithium reagent, which undergoes a homocoupling reaction with a second molecule of the aryl halide in the presence of the palladium catalyst (1 mol %). The reaction takes place at room temperature, is fast (1 h), and affords the corresponding biaryl compounds in good to excellent yields. The application of the method is demonstrated in an efficient asymmetric total synthesis of mastigophorene A. The chiral biaryl axis is constructed with an atropselectivity of 9:1 owing to catalyst-induced remote point-to-axial chirality transfer. [ABSTRACT FROM AUTHOR]

A palladium-catalyzed direct synthesis of symmetric biaryl compounds from aryl halides in the presence of tBuLi is described. In situ lithium-halogen exchange generates the corresponding aryl lithium reagent, which undergoes a homocoupling reaction with a second molecule of the aryl halide in the presence of the palladium catalyst (1 mol %). The reaction takes place at room temperature, is fast (1 h), and affords the corresponding biaryl compounds in good to excellent yields. The application of the method is demonstrated in an efficient asymmetric total synthesis of mastigophorene A. The chiral biaryl axis is constructed with an atropselectivity of 9:1 owing to catalyst-induced remote point-to-axial chirality transfer. [ABSTRACT FROM AUTHOR]

The article highlights the chemical equations involved in consecutive dynamic resolutions of phosphine oxides.

An unprecedented catalytic pathway for oxa-Michael addition reactions of alcohols to unsaturated nitriles has been revealed using a PNN pincer ruthenium catalyst with a dearomatized pyridine backbone. The isolation of a catalytically competent Ru-dieneamido complex from the reaction between the Ru catalyst and pentenenitrile in combination with DFT calculations supports a mechanism in which activation of the nitrile through metal-ligand cooperativity is a key step. The nitrile-derived Ru-N moiety is sufficiently Brønsted basic to activate the alcohol and initiate conjugate addition of the alkoxide to the α,β-unsaturated fragment. This reaction proceeds in a concerted manner and involves a six-membered transition state. These features allow the reaction to proceed at ambient temperature in the absence of external base. [ABSTRACT FROM AUTHOR]

An unprecedented catalytic pathway for oxa-Michael addition reactions of alcohols to unsaturated nitriles has been revealed using a PNN pincer ruthenium catalyst with a dearomatized pyridine backbone. The isolation of a catalytically competent Ru-dieneamido complex from the reaction between the Ru catalyst and pentenenitrile in combination with DFT calculations supports a mechanism in which activation of the nitrile through metal-ligand cooperativity is a key step. The nitrile-derived Ru-N moiety is sufficiently Brønsted basic to activate the alcohol and initiate conjugate addition of the alkoxide to the α,β-unsaturated fragment. This reaction proceeds in a concerted manner and involves a six-membered transition state. These features allow the reaction to proceed at ambient temperature in the absence of external base. [ABSTRACT FROM AUTHOR]

A range of tetrahedral bis(formazanate)zinc complexes with different steric and electronic properties of the formazanate ligands were synthesized. The solid-state structures for several of these were determined by X-ray crystallography, which showed that complexes with symmetrical, unhindered ligands prefer coordination to the zinc center via the terminal N atoms of the NNCNN ligand backbone. Steric or electronic modifications can override this preference and give rise to solid-state structures in which the formazanate ligand forms a 5-membered chelate by binding to the metal center via an internal N atom. In solution, these compounds show dynamic equilibria that involve both 5- and 6-membered chelates. All compounds are intensely colored, and the effect of the ligand substitution pattern on the UV-vis absorption spectra was evaluated. In addition, their cyclic voltammetry is reported, which shows that all compounds may be electrochemically reduced to radical anionic (L2Zn-) and dianionic (L2Zn2-) forms. While unhindered NAr substituents lie in the plane of the ligand backbone (Ar = Ph), the introduction of sterically demanding substituents (Ar = Mes) favors a perpendicular orientation in which the NMes group is no longer in conjugation with the backbone, resulting in hypsochromic shifts in the absorption spectra. The redox potentials in the series of L2Zn compounds may be altered in a straightforward manner over a relatively wide range (~700 mV) via the introduction of electron-donating or -withdrawing substituents on the formazanate framework. [ABSTRACT FROM AUTHOR]

The detection of nuclear radiation necessitates the availability of new generations of tunable blue emitting fluorophores with high emission quantum yields. Here we show that pyridyl-1,2,4-triazole based diphenyl boron complexes can provide highly tuneable emission through facile modification of the C5 position of the 1,2,4-triazolato ring. The series of complexes prepared show a wide range of emission from near-UV to green, enabling fine control over the spectral overlap with detectors used in scintillator technology. [ABSTRACT FROM AUTHOR]

New methodology for making novel materials is highly desirable. Here, an 'ingredients' approach to functional self-assembled hydrogels was developed. By designing a building block to contain the right ingredients, a multi-responsive, self-assembled hydrogel was obtained through a process of template-induced self-synthesis in a dynamic combinatorial library. The system can be switched between gel and solution by light, redox reactions, pH, temperature, mechanical energy and sequestration or addition of MgII salt. [ABSTRACT FROM AUTHOR]

The synthesis of bis(formazanate) zinc complexes is described. These complexes have well-behaved redox-chemistry, with the ligands functioning as a reversible electron reservoir. This allows the synthesis of bis(formazanate) zinc compounds in three redox states in which the formazanate ligands are reduced to 'metallaverdazyl' radicals. The stability of these ligand-based radicals is a result of the delocalization of the unpaired electron over four nitrogen atoms in the ligand backbone. The neutral, anionic, and dianionic compounds (L2Zn0/−1/−2) were fully characterized by single-crystal X-ray crystallography, spectroscopic methods, and DFT calculations. In these complexes, the structural features of the formazanate ligands are very similar to well-known β-diketiminates, but the nitrogen-rich (NNCNN) backbone of formazanates opens the door to redox-chemistry that is otherwise not easily accessible. [ABSTRACT FROM AUTHOR]

The synthesis of bis(formazanate) zinc complexes is described. These complexes have well-behaved redox-chemistry, with the ligands functioning as a reversible electron reservoir. This allows the synthesis of bis(formazanate) zinc compounds in three redox states in which the formazanate ligands are reduced to 'metallaverdazyl' radicals. The stability of these ligand-based radicals is a result of the delocalization of the unpaired electron over four nitrogen atoms in the ligand backbone. The neutral, anionic, and dianionic compounds (L2Zn0/−1/−2) were fully characterized by single-crystal X-ray crystallography, spectroscopic methods, and DFT calculations. In these complexes, the structural features of the formazanate ligands are very similar to well-known β-diketiminates, but the nitrogen-rich (NNCNN) backbone of formazanates opens the door to redox-chemistry that is otherwise not easily accessible. [ABSTRACT FROM AUTHOR]