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We report the preparation of enantiomerically pure constrained geometry complexes (cgc) of the rare‐earth metals bearing a pentadienyl moiety (pdl) derived from the natural product (1R)‐(−)‐myrtenal. The potassium salt 1, [Kpdl*], was treated with ClSiMe2NHtBu, and the resulting pentadiene 2 was deprotonated with the Schlosser‐type base KOtPen/nBuLi (tPen=CMe2(CH2Me)) to yield the dipotassium salt [K2(pdl*SiMe2NtBu)] (3). However, 3 rearranges in THF solution to its isomer 3’ by a 1,3‐H shift, which elongates the bridge between the pdl and SiMe2NtBu moieties by one CH2 unit. This is crucial for the successful formation of various monomeric C 1‐ or dimeric C 2‐symmetric rare‐earth cgc complexes with additional halide, tetraborohydride, amido and alkyl functionalities. All compounds have been extensively characterised by solid‐state X‐ray diffraction analysis, solution NMR spectroscopy and elemental analyses., Proton migration as key to success. A 1,3‐H shift enables the synthesis of the first enantiomerically pure pentadienyl‐based constrained geometry complexes, which can be further functionalised to silyamido and alkyl compounds.

Bis(alkyl)nickel(II) complexes [(tmeda)NiR2] (R = CH2SiMe3 (1a), CH2CMe3 (1b), CH2CMe2Ph (1c)) can be prepared from [(tmeda)Ni(acac)2] and MgR2. This approach is equivalent to and, in some cases, e...

The reaction of 1,2‐dipiperidinoacetylene (1) with 0.5 equivalents of SnCl2 or GeCl2⋅dioxane afforded the 1,2,3,4‐tetrapiperidino‐1,3‐cyclobutadiene tin and germanium dichloride complexes 2 a and 2 b, respectively. A competing redox reaction was observed with excess amounts of SnCl2, which produced a tetrapiperidinocyclobutadiene dication with two trichlorostannate(II) counterions. Heating neat 1 to 110 °C for 16 h cleanly produced the dimer 1,3,4,4‐tetrapiperidino‐3‐buten‐1‐yne (3); its reaction with stoichiometric amounts of SnCl2 or GeCl2⋅dioxane furnished the 1,3,4,4‐tetrapiperidino‐1,2‐cyclobutadiene tin and germanium dichloride complexes 4 a and 4 b, respectively. Transition‐metal complexes containing this novel four‐membered cyclic bent allene (CBA) ligand were prepared by reaction of 3 with [(tht)AuCl], [RhCl(CO)2]2, and [(Me3N)W(CO)5] to form [(CBA)AuCl] (5), [(CBA)RhCl(CO)2] (6), and [(CBA)W(CO)5] (7). The molecular structures of all compounds 2–7 were determined by X‐ray diffraction analyses, and density functional theory (DFT) calculations were carried out to rationalise the formation of 3 and 4 a., Cyclic bent allenes as ligands: The thermal dimerisation of 1,2‐dipiperidinoacetylene affords 1,3,4,4‐tetrapiperidino‐3‐buten‐1‐yne, which provides access to main‐group and transition‐metal complexes of a strongly electron‐donating four‐membered cyclic bent allene ligand. The formation of these complexes as well as the dimerisation of 1,2‐dipiperidinoacetylene were also studied by computational methods.

A series of Co(I), Co(II), and Ni(II) compounds were prepared by employing the open pentadienyl 2,4-(Me3C)2C5H5 (Pdl′) and some edge-bridged variants (ePdl) such as 6,6′-dimethylcyclohexadienyl (dm...

International audience; Living systems carry out the reduction of N2 to ammonia (NH3) through a series of protonation and electron transfer steps under ambient conditions using the enzyme nitrogenase. In the chemical industry, the Haber-Bosch process hydrogenates N2 but requires high temperatures and pressures. Both processes rely on iron-based catalysts, but molecular iron complexes that promote the formation of NH3 on addition of H2 to N2 have remained difficult to devise. Here, we isolate the tri(iron)bis(nitrido) complex [(Cp'Fe)3(μ3-N)2] (in which Cp' = η5-1,2,4-(Me3C)3C5H2), which is prepared by reduction of [Cp'Fe(μ-I)]2 under an N2 atmosphere and comprises three iron centres bridged by two μ3-nitrido ligands. In solution, this complex reacts with H2 at ambient temperature (22 °C) and low pressure (1 or 4 bar) to form NH3. In the solid state, it is converted into the tri(iron)bis(imido) species, [(Cp'Fe)3(μ3-NH)2], by addition of H2 (10 bar) through an unusual solid-gas, single-crystal-to-single-crystal transformation. In solution, [(Cp'Fe)3(μ3-NH)2] further reacts with H2 or H+ to form NH3.

Four- and five-coordinate high-spin (S = 2) Fe(II) complexes bearing the pyrrolyl-derived tBuPNP pincer ligand [tBuPNP = 2,5-(tBu2PCH2)2(C4H2N)] have been prepared. All three complexes were characterized by X-ray diffraction, zero-field 57Fe Mossbauer spectroscopy and solid-state magnetic susceptibility studies. Furthermore, the experimentally determined Mossbauer parameters of compounds 4–6 can be modeled satisfactorily by density functional theory (DFT) computations at the B97D level of theory. Introduction of the sterically demanding bis(trimethylsilyl)amido moiety induces significant distortions from ideal tetrahedral geometry in [(tBuPNP)FeN(SiMe3)2] (4), featuring an Fe(II) high-spin state with large negative axial zero-field splitting and slow paramagnetic relaxation at low temperature, as evidenced by measurements of the solid-state magnetic susceptibility and zero-field 57Fe Mossbauer spectroscopy. Replacing the chlorido ligand in [(tBuPNP)FeCl] (3) by the bidentate acetylacetonato (acac) ligand enforces a trigonal–bipyramidal structure in [(tBuPNP)Fe(acac)] (5), in which the Fe(II) atom also adopts a high-spin configuration. Reaction of 3 with adamantyl azide allowed us to confirm the inherent lability of the phosphine coordination, since the azide inserts into the Fe P bond to form [(tBuPNPN3Adamantyl)FeCl] (6), in which the tBuPNP ligand is transformed to a phosphino-pyrrolyl-phosphazide ligand framework (tBuPNPN3Adamantyl). This motif can be considered as a trapped Staudinger intermediate along the reaction pathway of free phosphines with organic azides to form phosphinimines. Bidentate binding of the phosphazide sidearm in κN:κN-fashion to the Fe(II) atom enforces an s-anti coordination of the P1–N2–N3–N4 moiety, which raises the barrier for the thermodynamically favorable N2 elimination and allows this intermediate to be trapped. Complex 6 contains a high-spin Fe(II) atom with a square-pyramidal structure.

A novel synthesis for dialkyl cobalt compounds [(tmeda)CoR2] is presented. In these complexes tmeda is readily replaced by an NHC or a bidentate phosphine ligand to form 3- and 4-coordinate compounds, respectively. [(ItBu)Co(CH2SiMe3)2] (ItBu = 1,3-di-tert-butylimidazolin-2-ylidene) serves as an efficient, homogeneous olefin hydrogenation pre-catalyst and allows the preparation of the novel cobalt bis(alkyne) complex [(ItBu)Co(η2-PhCCPh)2].

The half-sandwich complex [Cp′Fe(μ-I)]2 (1; Cp′ = η5-1,2,4-(Me3C)3C5H2) is cleaved when heated in toluene to form a cation–anion pair [{Cp′Fe(η6-toluene)}+{Cp′FeI2}−] (2), in which the two Fe(II) atoms adopt different spin states, i.e., a low-spin (S = 0) and a high-spin (S = 2) configuration. Upon oxidation of 1 with C2H4I2, the thermally stable 15VE species [Cp′FeI2] (3) can be isolated, in which the Fe(III) atom adopts an intermediate spin (S = 3/2) configuration. Complex 3 is an excellent starting material for further functionalizations and it reacts with Mg(CH2SiMe3)2 to form the unprecedented Fe(III) (S = 3/2) bis(alkyl) complex [Cp′Fe(CH2SiMe3)2] (4). The respective spin states of complexes 2–4 are confirmed by single-crystal X-ray crystallography, zero-field 57Fe Mossbauer spectroscopy, and solid-state magnetic susceptibility measurements. In contrast to the related 14VE high-spin (S = 2) Fe(II) alkyl species [Cp′FeCH(SiMe3)2], which resists the reaction with H2 as a consequence of a spin-induced reaction barrier, complex 4 reacts cleanly with H2 (8 bar) in cyclohexane to yield iron hydrides [{Cp′Fe}2(μ-H)3] (5) and [Cp′Fe(μ-H)2]2 (6) in a 1 : 4 ratio. However, when the hydrogenation of 4 is carried out in benzene, a green 19VE [Cp′Fe(η6-C6H6)] (A) intermediate is formed, which dimerizes to the bis(cyclohexadienyl)-bridged product [(Cp′Fe)2(μ2–η5:η5-C12H12)] (7). Further evidence for the intermediacy of [Cp′Fe(η6-C6H6)] (A) was gathered by X-band EPR and UV/vis spectroscopy. Interestingly, attempts to oxidize 7 with AgSbF6 proceeded via C–C bond cleavage instead of metal oxidation to form [Cp′Fe(C6H6)][SbF6] (8).

The reaction of the cycloheptatrienyl-cyclopentadienyl (Cht-Cp) titanium sandwich complexes [(η7-C7H7)Ti(η5-C5R5)] (R = H, Me) with n-BuLi followed by treatment with the chlorophosphanes ClPR'2 (R' = Cy, tBu) afforded the Cht-substituted phosphanes [(η7-C7H6PR'2)Ti(η5-C5R5)] (R = H, Me; R' = Cy, tBu). Together with their previously reported Cp-substituted analogues [(η7-C7H7)Ti(η5-C5H4PR2)] (R = Cy, tBu), these complexes were employed as ancillary phosphane ligands in palladium-catalyzed Suzuki-Miyaura coupling between sterically encumbered aryl bromides and aryl boronic acids. The unusually short reaction times indicated rapid generation of catalytically active Pd(0) species, and the reaction of palladium(II) acetate, Pd(OAc)2, with an excess of of the phosphorus ligands afforded linear diphosphane palladium(0) complexes. These reactions gave no indication of phosphane oxide formation, and it is supposed that the Cht-Cp titanium sandwich moiety serves as an integrated reducing agent. Accordingly, the reaction of [(η7-C7H7)Ti(η5-C5M5)] with Pd(OAc)2 or Ag(OAc) resulted in a three-electron redox reaction with formation of the half-sandwich complex [(η5-C5Me5)Ti(OAc)3] together with 7,7'-bi-1,3,5-cycloheptatriene (ditropyl) and elemental Pd or Ag metal. Upon reaction with silver(I) trifluoromethanesulfonate, Ag(OTf), a four-electron redox process was observed which furnished the tropylium titanate complex [C7H7][(η5-C5Me5)Ti(OTf)4].

Half-sandwich complexes of the N-heterocyclic carbene-phosphinidene adduct [(IPr)PH] (1, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) were prepared by its reaction with dimeric complexes of the type [LMCl2]2, which afforded the three-legged piano-stool complexes [LMCl2{HP(IPr)}] (9a/9b: M = Ru/Os, L = η6-p-cymene; 10a/10b: M = Rh/Ir, L = η5-C5Me5). Their conversion into the corresponding carbene-phosphinidenide complexes [LMCl{P(IPr)}] (11a/11b: M = Ru/Os; 12a/12b: M = Rh/Ir) with a two-legged piano stool geometry was studied by NMR spectroscopy in the presence of the strong base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). Alternatively, the complexes 11 and 12 were isolated in high yields from the reactions of the carbene-phosphinidene adduct [(IPr)PTMS] (2) with [LMCl2]2, whereby formation of the metal-phosphorus bonds was accompanied by elimination of trimethylsilyl chloride (Me3SiCl). Theoretical calculations reveal a strong polarization of the phosphorus ligands upon metal complexation, wh...

The pyrrolyl-based iron pincer compounds [(tBuPNP)FeCl] (1), [(tBuPNP)FeN2] (2), and [(tBuPNP)Fe(CO)2] (3) were prepared and structurally characterized. In addition, their electronic ground states were probed by various techniques including solid-state magnetic susceptibility and zero-field 57Fe Mossbauer and X-band electron paramagnetic resonance spectroscopy. While the iron(II) starting material 1 adopts an intermediate-spin (S = 1) state, the iron(I) reduction products 2 and 3 exhibit a low-spin (S = 1/2) ground state. Consistent with an intermediate-spin configuration for 1, the zero-field 57Fe Mossbauer spectrum shows a characteristically large quadrupole splitting (ΔEQ ≈ 3.7 mm s–1), and the solid-state magnetic susceptibility data show pronounced zero-field splitting (|D| ≈ 37 cm–1). The effective magnetic moments observed for the iron(I) species 2 and 3 are larger than expected from the spin-only value and indicate an incompletely quenched orbital angular momentum and the presence of spin–orbit co...

The enantiomerically pure pentadienyl (Pdl*) ligand derived from the natural product (1R)-(−)-myrtenal forms with MCl3 (M = La, Ce, Pr, and Nd) the corresponding homoleptic [(η5-U-Pdl*)3M compounds (1-M). These complexes were fully characterised by 1H NMR spectroscopy, elemental analyses and X-ray diffraction. They exhibit in solution and solid state idealized C3 symmetry, and their molecular structures also reveal that the Pdl* ligand adopts a U-conformation and coordinates exclusively with its less sterically encumbered face to the rare-earth metal atom. For the paramagnetic representatives an assignment of the 1H NMR resonances based on a simple dipolar model gave satisfactory results.

Phenylene-bridged hybrid phosphine-imidazolin-2-imine RP,NR′ ligands (R=Ph, Cy, i-Pr, t-Bu; R′=Me, i-Pr) were prepared from 1,2-dibromobenzene by palladium-catalyzed C–N coupling with 1,3,4,5-tetramethylimidazolin-2-imine or 1,3-diisopropyl-4,5-dimethylimidazolin-2-imine, followed by lithiation with tert-butyllithium and reaction with the chlorophosphines (R2PCl). Their reaction with the dimeric iridium complex [Ir(cod)Cl]2 (cod=1,5-cyclooctadiene) and subsequent anion exchange with sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBArF24) or potassium hexafluorophosphate (KPF6) afforded iridium complexes of the type [(RP,NR′)Ir(cod)]BArF24 or [(RP,NR′)Ir(cod)]PF6, respectively. The latter PF6 salts were structurally characterized, revealing short Ir—N bonds as an indication of electron-rich nitrogen donor atoms. The former complexes were tested for their applicability in catalytic H/D exchange. In particular, the complex with the ligand t-BuP,NMe showed remarkable performance in H/D exchange with a broad range of aromatic substrates, including ketones, amides, esters, heterocycles and nitro compounds, and also promoted H/D exchange at aromatic Boc-protected amines, benzylamines and methoxy derivatives.

Since the first seminal report of boron-centred nucleophiles, the area of boryl anions has developed only sporadically and requires further systematisation. The boryl anions of type NHC–B(CN)2− (NHC = N-heterocyclic carbene) described herein complete a consistent series with the known anions cAAC–B(CN)2− [cAAC = cyclic(alkyl)(amino)carbene] and B(CN)32−. A novel approach towards NHC-stabilised cyanoboranes based on alkylthio-cyano exchange at boron is presented, and in contrast to other methods affords the products in better purity and yield. Reduction of suitable NHC–dicyanoboranes gave two unprecedented examples of NHC–B(CN)2− boryl anions. The latter were shown to react as boron-centred nucleophiles with facile formation of B–E bonds, where E = C, Si, Sn, P, Au. Bonding analysis by DFT calculations suggests a systematic variation of the energy of the boron-centred HOMO depending on the carbene, which in turn can control the nucleophilic character.

The reactions of the molybdenum alkylidyne complex [MesCMo{OCMe(CF3)2}3] (1) with the diaminoacetylenes R2NCCNR2 (2, NR2 = 4-methylpiperidinyl; 3, NR2 = NEt2; Mes = 2,4,6-trimethylphenyl) afforded the metallacyclobutadiene (MCBD) complexes 4 and 5. In contrast to all other MCBD complexes, 4 and 5 are paramagnetic and best described as Mo(IV) species containing an anionic diaminodicarbene of the type [(R2N)CC(Mes)C(NR2)]−.

The reaction of the ferrocene-bridged bis(imidazolin-2-imine) ligand N,N′-bis(1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene)-1,1′-ferrocenediamine [fc(NIm)2] with MCl2 salts furnished the complexes [{fc(NIm)2}MCl2] (M = Mn, Fe, Co, Ni, Cu, Zn), which were structurally and spectroscopically characterized. The iron complex was oxidized by reaction with ferrocenium hexafluoroantimonate, and both compounds, [{fc(NIm)2}FeCl2] and [{fc(NIm)2}FeCl2]SbF6, were further studied by means of Mossbauer spectroscopy, SQUID magnetometry, and single-crystal X-ray diffraction, revealing that oxidation occurred at the tetrahedral FeN2Cl2 moiety. The CuX (X = Cl, Br, I) complexes of fc(NIm)2 were synthesized, affording dimeric (X = Cl, Br) or polymeric (X = I) structures. In addition, the ligand bis(N,N,N′,N′-tetraisopropyl-2,3-diaminocyclopropen-1-ylidene)-1,1′-ferrocenediamine [fc(NCyp)2], was prepared and the trimetallic complex [{fc(NCyp)2}Pd2Cl4] was synthesized and structurally characterized. The attempted synthesis of the corresponding nickel complex furnished single crystals of the composition [{fc(NHCyp)2}Cl(NiCl3)]; X-ray diffraction analysis afforded the first crystal structure of the [Ni2Cl6]2– anion.

The synthesis of the frustrated carbene-borane 4-(9-borabicyclo[3.3.1]nona-9-yl)-1,3-di-tert-butylimidazolin-2-ylidene (6) was accomplished by deprotonation of the N-heterocyclic carbene (NHC) 1,3-di-tert-butylimidazolin-2-ylidene (1) at the 4-position with n-butyllithium, followed by addition of 9-chloro-9-borabicyclo[3.3.1]nonane (BBN-Cl). The abnormal carbene-borane adduct 5, with the BBN-Cl moiety at the 4-position, was obtained from the reaction of 1 with BBN-Cl; however, its conversion into 6 by treatment with K[N(SiMe3)2] (potassium hexamethyldisilazide, KHMDS) was not achieved. The reactions of 6 with the rhodium(I) complexes [Rh(COD)Cl]2 (COD = 1,4-cyclooctadiene) and [Rh(CO)2Cl]2 afforded the complexes cis-[(6)Rh(COD)Cl] (7) and cis-[(6)Rh(CO)2Cl] (8), with the latter allowing assessment of the σ-donor/π-acceptor properties of 6 by IR spectroscopy. The reactivity of 6 towards carbon dioxide, phenylacetylene and acetonitrile was studied, affording the labile adduct 6·CO2 (9), whereas irreversible deprotonation and formation of the imidazolium phenylalkynylborate 10 and the imidazolium cyanomethylborate 11 was observed for PhCCH and CH3CN. The molecular structures of 5, 7, 8, 10, and 11 were established by X-ray diffraction analyses.

Sterically encumbered ferrocenium salts with four, five, or six tert-butyl substituents and a variety of counteranions ([BF4]–, [SbF6]–, and [PF6]–) were prepared. The electronic structures of these materials were investigated by various physical techniques including electron paramagnetic resonance (EPR) and Mossbauer spectroscopy and solid-state magnetic susceptibility measurements. Spin–orbit coupling and low-symmetry distortions split the electronic ground states (2E2g) in these compounds into two Kramers doublet states (Ψa and Ψb); the energy difference between these states was evaluated by EPR spectroscopy and magnetic susceptibility measurements and decreases with increasing steric demand at the ferrocenium moiety. Further information on the first excited state (2A1g) was obtained by solid-state magnetic susceptibility measurements, which also indicated that an increasing steric bulk resulted in a decreasing energy gap between the 2E2g(Ψa) and 2A1g states. The Mossbauer spectra revealed temperature-dependent line shapes attributed to unusually slow paramagnetic relaxation on the 57Fe Mossbauer time scale. The pertinent spin–spin and spin–lattice (Orbach and Raman) relaxation parameters were determined for these materials and further substantiate the trends observed by our EPR spectroscopic and solid-state magnetic susceptibility studies.

Addition of N-heterocyclic carbenes [L = 1, 3-di-tert-butylimidazolin-2-ylidene (ItBu), 1, 3-diisopropyl-4, 5-dimethylimidazolin-2-yildene (IiPrMe2), 1, 3-di-mesitylimidazolin-2-yildene (IMes), and 1, 3-bis(2, 6-diisopropylphenyl)imidazolin-2-yildene (IPr)] to manganocene [(C5H5)2Mn] forms the neutral Lewis base adducts [(C5H5)2Mn(L)] (2–5) in moderate yields. Complexes 2–4 were structurally characterized and their solid-state magnetism was investigated between T = 2.6–300 K. In contrast to literature reports, these materials exclusively adopt a high-spin (S = 5/2) ground state.

This work presents the synthesis, characterization, and application of several new metal(I) complexes with trifluoromethylpyridine-containing N-heterocyclic carbene (NHC) ligands. The metal of choice was gold(I) for compounds 7 – 10, rhodium(I) for 11 – 12, and iridium(I) for 13 – 14, respectively. The trifluoromethylpyridine moiety was incorporated, along with other biologically active moieties, with the intention of modifying the lipophilicity of the complexes, so that the transport of the active units (M–NHC) through the cell wall barrier is facilitated. The biological activity of the complexes was investigated. In vitro assessment of antitumor activity in a panel of 12 human tumor cell lines by a monolayer assay revealed good potency (mean IC50 12.6 μm) and tumor selectivity for one compound. The solid-state structures of two solvates of compound 7, one with MeOH and one with THF, were determined by X-ray diffraction analysis.

The first preparations of half-open and open manganocenes were accomplished. Treatment of KPdl′ (Pdl′ = 2,4-(Me3C)2C5H5)) with [{(η5-Cp”)Mn(thf)(μ-I)}2] (Cp″ = 1,2,4-(Me3C)3C5H2) and MnI2(thf)2 resulted in the formation of [(η5-Cp″)Mn(Pdl′)] (2) and [(Pdl′)2Mn] (4), respectively. Both compounds adopt a high-spin (S = 5/2) ground state. Maximum spin states are rather unusual for pentadienyl complexes, since these ligands generally stabilize transition metal complexes in their low-spin state. In addition, the electronic structure of 2 was compared to its closed analogue [(η5-Cp″)Mn(η5-Cp′)] (Cp′ = 1,3-(Me3C)2C5H3), which also adopts the high-spin configuration because of steric hindrance destabilizing the electronically more favorable low-spin state. Reaction of KPdl′ with [(C5H5)2Mn] yields the trimetallic compound [{(η5-Pdl′)Mn(η5-C5H5)]}2Mn] (5) concomitant with the formation of 2,4,7,9-tetra-tert-butyl-1,3,7,9-decatetraene (Pdl′2), indicating reduction of two Mn atoms. Solid-state magnetic susceptibilit...

A series of (allyl)palladium(II) complexes of anionic N-heterocyclic carbenes that contain a weakly coordinating borate moiety (WCA-NHC) were prepared by deprotonation of the carbene IPr with n-butyllithium in the 4-position, which was followed by addition of B(C6F5)3 and reaction of the resulting lithium carbene–borate with [(η3-allyl)Pd(μ-Cl)]2 (allyl = 2-propenyl, 2-butenyl, 2-methyl-2-propenyl, 1-phenyl-2-propenyl). In polar solvents such as tetrahydrofuran, chloropalladate complexes of the type [Li(THF)3][(WCA-NHC)PdCl(η3-allyl)] were formed, whereas the reactions in toluene afforded neutral complexes of the type [(WCA-NHC)Pd(η3-allyl)], which exhibit an intramolecular interaction with the N-aryl groups of the carbene ligands. The allyl-palladium complexes were employed as catalysts for the Buchwald–Hartwig amination of various aryl halides with morpholine.

A series of Au(I) complexes (12, 13, 14, 15, 16) and Ag(I) complexes (17, 18, 19, 20) derived from imidazo[1,5-a]pyridin-3-ylidenes were synthesized from AuCl(SMe2) or by reacting silver(I) acetate with 2,5-dimethylimidazo[1,5-a]pyridin-2-ium iodide or imidazo[1,5-a]pyridin-2-ium salts, and were characterized using NMR spectroscopy, mass spectrometry and elemental analyses. In addition, the Au(I) complex 13 and the Ag(I) complex 19 were characterized using single-crystal X-ray diffraction. Using paclitaxel as a standard, all Au(I) and Ag(I) N-heterocyclic carbene complexes were evaluated for their in vitro anti-tumour activity against 12 cell lines using a monolayer cell survival and proliferation assay. The highest anticancer activity was found for complexes 15, 13 and 14 with mean IC50 values of 10.09, 10.42 and 12.28 μM, respectively. Copyright © 2016 John Wiley & Sons, Ltd.

Deprotonation of 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene (IDipp) with n-butyllithium at the 4-position and addition of B(C6F5)3 afforded the lithium-carbene borate complex [(WCA-NHC)Li(toluene)] with a weakly coordinating anionic (WCA) moiety. The reaction of the lithiocarbene with the dimeric allyl complex [(η3-C3H5)Ni(μ-Cl)]2 in tetrahydrofuran (THF) afforded the chloronickelate complex [Li(THF)n][(WCA-IDipp)NiCl(η3-C3H5)] (1 ≤ n ≤ 3), which was used to generate the complex [(WCA-IDipp)Ni(η3-C3H5)], which is expected to achieve stabilization through an intramolecular arene-metal interaction (“π-face donation”) involving the N-aryl groups of the carbene ligand, by elimination of LiCl in toluene solution. The labile complex [(WCA-IDipp)Ni(η3-C3H5)], which undergoes decomposition by C–H activation and/or reductive elimination, can be stabilized by addition of carbon monoxide or phosphines to form novel neutral complexes of the type [(WCA-IDipp)NiL(η3-C3H5)] (L = CO, PMe3, PMe2Ph, PMePh2).

International audience; The molybdenum and tungsten 2,4,6-trimethylbenzylidynes [MesC M{OC(CF3)(2)Ph}(3)] (12a, M = Mo; 12b, M = W) were prepared and structurally characterized as related complexes to already known [MesC M{OC(CF3)(2)Me}(3)] (MoF6, M = Mo; WF6, M = W). While treatment of 12a with 3-hexyne yielded the propylidyne complex [EtC Mo{OC(CF3)(2)Ph}(3)] (13), the tungsten congener 12b formed isolable metallacyclobutadiene (MCBD) species 14-16 by reaction with 3-hexyne, 1-phenyl-1-propyne, and 2,4-hexadiyne, which can be correlated with the higher electrophilicity of the tungsten complex. Furthermore, the labile MCBD [(C3Et3 )Mo{OC(CF3)(2)Me}(3)] (17) was isolated at low temperature from the reaction of the highly active MoF6 catalyst with 3-hexyne and could be characterized by X-ray diffraction analysis. At room temperature, the same reaction afforded [EtC Mo{OC(CF3)(2)Me}(3)] (18), and the equilibrium reaction with 3-hexyne to form 17 was additionally studied by variable temperature NMR spectroscopy, which allowed determining Delta H degrees and Delta S degrees for the formation of MCBD 17. The experimental thermodynamic data were used to set the benchmark for DFT calculations. Moreover, the deprotiometallacyclobutadiene complex (DPMCBD) [C-3(Mes)(Ph)}Mo{OC(CF3)(3)}(2)] (19), prepared from [MesC Mo{OC(CF3)(3)}(3)] (MoF9) and phenylacetylene, was isolated and structurally characterized as a decomposition product of terminal alkyne metathesis and employed in the polymerization of phenylacetylenes.

A novel synthesis for dialkyl cobalt compounds [(tmeda)CoR2] is presented. In these complexes tmeda is readily replaced by an NHC or a bidentate phosphine ligand to form 3- and 4-coordinate compounds, respectively. [(ItBu)Co(CH2SiMe3)2] (ItBu = 1,3-di-tert-butylimidazolin-2-ylidene) serves as an efficient, homogeneous olefin hydrogenation pre-catalyst and allows the preparation of the novel cobalt bis(alkyne) complex [(ItBu)Co(η2-PhCCPh)2].

The coordination chemistry of the sterically encumbered pentadienyl ligand pdl' (pdl' = 2,4-(Me3C)2C5H5) towards a series of rare-earth metals was systematically explored and the resulting metal complexes were fully characterized by several techniques including X-ray diffraction, elemental analysis, NMR spectroscopy and solid-state magnetic susceptibility studies. Three different reaction products were isolated depending on the redox-potentials and ionic radii of the metal atoms. They can be classified as (a) salt-metathesis, (b) metal reduction-ligand oxidation and (c) ligand deprotonation products. While for the larger and difficult to reduce metal ions, M = La, Ce, Pr and Nd, trivalent compounds [(η5-pdl')3M] (1-M) were isolated, for the more readily reduced metal ions the corresponding divalent compounds [[(η5-pdl')2M(thf)n] (2-M; with M = Sm (n = 2); Eu, Yb (n = 1)) were formed. A more complex structural motif was observed for the smaller and also difficult to reduce metal ions, M = Sc, Y, Gd, Tb, Dy, Ho, Er, Tm and Lu, which yielded the bimetallic complexes of the type [(pdl')(pdl'-1H)(pdl'-2H)M2(thf)2] (3-M). In these dimeric complexes the pdl' ligand acts as a result of deprotonation reactions not only as a monoanionic [pdl']-, but also as a dianionic [pdl'-1H]2- and a trianionic [pdl'-2H]3- ligand scaffold, which form unusual structural motifs including a six-membered metallacycle. Solid-state magnetic susceptibility studies revealed the expected free-ion magnetic moments at T = 300 K for all investigated compounds, whereas at lower temperatures crystal-field effects dominate. Furthermore, for 3-Gd, 3-Tb, 3-Dy and 3-Er weak ferromagnetic exchange interactions were observed at low temperature.

The half-sandwich complex [Cp'Fe(μ-I)]

Two new Pt(II) complexes with imidazolin-2-imines as ancillary ligands, [Pt(DMEAImiPr)Cl2] and [Pt(DPENImiPr)Cl2], were synthesized and characterized. Substitution reactions of these complexes with nucleophiles – thiourea (TU), L-methionine (L-Met), L-histidine (L-His) and guanosine-5′-monophosphate (5′-GMP) – were carried out in 25 mM Hepes buffer in the presence of 30 mM NaCl. The reactions were monitored using variable-temperature UV-Vis spectrophotometry and were followed under pseudo-first-order conditions with a large excess of nucleophiles. A slightly higher reactivity was found for [Pt(DMEAImiPr)Cl2], while the reactivity of the nucleophiles decreased in the order TU > L-Met > L-His > 5′-GMP. The negative values reported for the entropy of activation confirmed an associative substitution mode. Spectrophotometric acid–base titrations were performed to determine the pKa values of the coordinated water molecules in the diaqua complexes [Pt(DMEAImiPr)(H2O)2]2+ and [Pt(DPENImiPr)(H2O)2]2+. Solubility measurements revealed good solubility of the studied imidazolin-2-imine complexes in water. The crystal structure of [Pt(DMEAImiPr)Cl2] was determined by X-ray diffraction analysis. The coordination geometries around the platinum atoms are distorted square-planar; the [Pt(DMEAImiPr)Cl2] complex displays Pt–N distances of 2.0162(19) and 2.0663(19) A. Attempts to coordinate Au(III) ions to different imidazolin-2-imine ligands did not result in the formation of coordination complexes, but rather in the reduction of the Au(III) precursor. This was evidenced by the X-ray crystal structure of [(DACH(ImiPrH)2)(AuCl2)2], which formed during the reaction of KAuCl4 with the ligand DACH(ImiPr)2.

Addition of N-heterocyclic carbenes (L = 1,3-di-tert-butylimidazol-2-ylidene (ItBu), 1,3-di-iso-propyl-4,5-dimethylimidazol-2-yildene (IiPr2Me2), 1,3-mesitylimidazol-2-yildene (IMes) and 1,3-di-(2,6-di-isopropylphenyl)imidazol-2-yildene (IPr)) to the iron half-sandwich complex [Cp′FeI]2 (Cp′ = η5-1,2,4-(Me3C)3C5H2, 1) forms the neutral, 16VE adducts [Cp′FeI(L)] (2–5) in moderate to excellent yields. These complexes were structurally characterised. The NHC ligand binds strongly to the Fe(II) atom, so that no exchange is observed on the NMR and chemical time scale. Fe(II) atoms in the starting material 1 adopt a high-spin configuration (S = 2) and are weakly antiferromagnetically coupled at low temperatures. Furthermore, in contrast to previous reports on related [(η5-C5Me5)FeCl(NHC)] systems, in which the Fe(II) atoms assume an intermediate spin (S = 1), no spin state change occurs upon coordination of the NHC ligand; the Fe(II) atoms in complexes 2–5 retain their high-spin state (S = 2) as shown by solid state magnetic susceptibility and zero-field 57Fe Mossbauer spectroscopy investigations. Density functional theory (DFT) studies at the B3LYP level of theory also agree with a well separated S = 2 ground state for compounds 2–5. Surprisingly for Fe(II) high-spin systems, compounds 1–5 exhibit slow paramagnetic relaxation in their Mossbauer spectra; this can be traced to spin–spin and spin–lattice relaxation processes with unusually large spin–lattice relaxation barriers. A structural model is proposed that associates these processes with crystal packing effects.

The bis(imidazolin-2-imine) ligand N,N'-bis(1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene)-1,1'-ferrocenediamine, fc(NIm)2 (1) was prepared. Its reaction with [NiCl2(dme)] (dme = 1,2-dimethoxyethane) or [PdCl2(MeCN)2] afforded the tetrahedral, paramagnetic complex [(1-κ(2)N,N')NiCl2] (6a) or the diamagnetic, square-planar complex [(1-κ(2)N,N')PdCl2] (6b), respectively. For the latter, slow rearrangement to the ionic complex [(1-κFe,κ(2)N,N')PdCl]Cl, [7]Cl, was observed, which was followed by (1)H NMR and UV/vis spectroscopy. Treatment of [7]Cl with NaBF4 afforded [7]BF4; the palladium atoms in both cations adopt square-planar environments with short Fe-Pd bonds (ca. 2.65 Å). In addition, a series of dicationic complexes of the type [(1-κFe,κ(2)N,N')ML](BF4)2 (8a: M = Ni, L = MeCN; 8b: M = Pd, L = MeCN; 9a: M = Ni, L = PMe3; 9b: M = Pd, L = PMe3) was prepared from 6a (M = Ni) or [7]BF4 by chloride abstraction with NaBF4 or AgBF4 in the presence of acetonitrile or trimethylphosphine, respectively. In the presence of triphenylphosphine, the palladium(II) complex [(1-κFe,κ(2)N,N')Pd(PPh3)](BF4)2 (10) was isolated. Iron-nickel and iron-palladium bonding in these complexes was studied experimentally by NMR, UV/vis, and Mössbauer spectroscopy and by cyclic voltammetry. Detailed DFT calculations were carried out for the cations [(1-κFe,κ(2)N,N')M(MeCN)](2+) in the 8a/8b couple, with Bader's atoms in molecules theory revealing the presence of noncovalent, closed-shell metal-metal interactions. Potential energy surface scans with successive elongation of the Fe-M bonds allow an estimation of the iron-metal bond dissociation energies (BDE) as BDE(Fe-Ni) = 11.3 kcal mol(-1) and BDE(Fe-Pd) = 24.3 kcal mol(-1).

Reaction of NaPyrtBu2 [PyrtBu2 = 2, 5-(Me3C)2C4H2N] and KPyrtBu2Me2 [PyrtBu2Me2 = 2, 5-(Me3C)2-3, 4-Me2C4N] with MnI2(thf)3 resulted in the isolation of the first bis(pyrrolyl)manganese(II) complexes. However, different coordination modes are realized depending on the identity of the Pyr-ligand. For example, the MnII atom is κN coordinate in [(κN-PyrtBu2)2Mn] (1-Mn), whereas several different species were isolated with the more bulky PyrtBu2Me2 ligand. Depending on the reaction conditions the κN-coordinate complex [(κN-PyrtBu2Me2)2Mn] (2-Mn′) is initially formed, but converts on sublimation to the η3 coordinate isomer [(η3-PyrtBu2Me2)2Mn] (2-Mn′). In some cases we could also identify a THF adduct of 2-Mn to give [(κN-PyrtBu2Me2)(η2-PyrtBu2Me2)Mn(thf)] [2-Mn(thf)], in which one PyrtBu2Me2 ligand binds in a κN coordination mode, while the other is best described as η2-PyrtBu2Me2. These complexes were characterized by elemental analyses and X-ray diffraction studies. Density functional theory (DFT) computations confirm that the potential energy surface between the different hapticities is rather flat for these MnII high-spin species (S = 5/2).

This work presents the synthesis, characterization and application of eleven new gold (I) complexes 13–23 with 1,2,4-oxadiazole-containing N-heterocyclic carbene (NHC) ligands and of the NHC silver(I) complex 24. The 1,2,4-oxadiazole unit, which can be found in a variety of biologically active natural products such as phidianidines or quisqualic acid, was incorporated, along with a variety of other biologically active moieties (anthracene, indole, 2-pyridine, 2,3,4,5-tetra-O-acetyl-D-glucopyranose, quincorine and quincoridine), in order to change the lipophilicity of the complexes, so that the transport of the active units (M-NHC) though the cell wall barrier is facilitated. The biological activity of the complexes was investigated. In vitro assessment of anti-tumor activity in a panel of 12 human tumor cell lines by a monolayer assay revealed impressive potency (mean IC50

The phosphorus-functionalized cycloheptatrienylcyclopentadienyl (ChtCp) titanium sandwich complex [(η7-C7H7)Ti(η5-C5H4PtBu2)] (7) forms a frustrated Lewis pair (FLP) in combination with the borane B(C6F5)3, and this FLP undergoes heterolytic cleavage of dihydrogen and fixation of carbon dioxide and nitrous oxide with a noticeable colorimetric response, which is visible to the naked eye. In their UV/Vis spectra, the H2, CO2 and N2O fixation products exhibit a bathochromic shift of the absorption band that can be assigned to a HOMOLUMO excitation of the ChtCp titanium (troticene) moiety.

The reaction of the N-heterocyclic carbene 1,3-di-tert-butyl-4,5-dimethylimidazolin-2-ylidene (1b) with trimethylsilyl iodide, triflate and triflimidate [Me3SiX, X = I, CF3SO3 (OTf), (CF3SO2)2N (NTf2)] by mixing the neat, liquid starting materials afforded the corresponding 2-(trimethylsilyl)imidazolium salts [(1b)SiMe3]X as highly reactive, white crystalline solids. Only the triflimidate (X = NTf2) proved to be stable in solution and could be characterized by means of NMR spectroscopy (in C6D5Br) and X-ray diffraction analysis, whereas dissociation into free 1b and Me3SiOTf was observed for the triflate system, in agreement with the trend derived by DFT calculations; the iodide was too insoluble for characterization. The compounds [(1b)SiMe3]X showed the reactivity expected for frustrated carbene–silylium pairs, and treatment with carbon dioxide, tert-butyl isocyanate and diphenylbutadiyne gave the 1,2-addition products [(1b)CO2SiMe3]X (X = I, OTf, NTf2), [(1b)C(NtBu)OSiMe3]OTf and [(1b)C(Ph)C(SiMe3)CCPh]OTf, respectively. Upon reaction with [AuCl(PPh3)], metal–chloride bond activation was observed, with formation of the cationic gold(I) complexes [(1b)Au(PPh3)]X (X = OTf, NTf2).

The synthesis of C2 symmetric enantiomerically pure open Ca and Sr metallocenes, [(η(5)-pdl*)2Ca(thf)] (1) and [(η(5)-pdl*)2Sr(thf)2] (2) (pdl* = dimethylnopadienyl) is described and these complexes were fully characterized. The solid state structures confirm that the pdl* ligands coordinate exclusively with the less sterically demanding site to the Ca and Sr atoms. These complexes are active catalysts for the controlled ring opening polymerization (ROP) of rac-lactide to give heterotactically enriched polylactides (PL) with narrow polydispersities (PDI = 1.29-1.31) and without adding further activators.

Deprotonation of the N-heterocyclic carbene 1,3-di-tert-butylimidazolin-2-ylidene (1) with n-butyl lithium in the 4-position, followed by addition of bis(2,4,6-trimethylphenyl)boronfluoride, Mes2BF, afforded 4-[bis(2,4,6-trimethylphenyl)boryl]-1,3-di-tert-butylimidazolin-2-ylidene (6), which contains non-interacting carbene and borane moieties and can thus be regarded as a potential intramolecular frustrated Lewis pair (FLP). Alternatively, 6 can be conveniently prepared from 1 by reaction with Mes2BF via the abnormal carbene–borane adduct 7, which could be converted into 6 by treatment with potassium hexamethyldisilazide, K(HMDS). The reactions of 6 with the rhodium(I) complexes [Rh(COD)Cl]2 and [Rh(CO)2Cl]2 furnished [(6)Rh(COD)Cl] (8) and [(6)Rh(CO)2Cl] (9), with the latter allowing an assessment of the donor properties of 6 by IR spectroscopy. 6 reacted with carbon dioxide and tert-butyl isocyanate to form the adducts 6·CO2 (11) and 6·tBuNCO (12), which proved unstable in solution, with partial dissociation into the starting materials. The X-ray crystal structures of 6, 7, 8·2CH2Cl2 and 12 are reported.

A series of novel Pd(ii) complexes with chelating mono(imidazolin-2-imine) and bis(imidazolin-2-imine) ligands were synthesized. The crystal structures of [Pd(DMEAIm(iPr))Cl2] and [Pd(DPENIm(iPr))Cl2] were determined by X-ray diffraction analysis. The reactivity of the six Pd(ii) complexes, namely, [Pd(en)Cl2], [Pd(EAIm(iPr))Cl2], [Pd(DMEAIm(iPr))Cl2], [Pd(DPENIm(iPr))Cl2], [Pd(BL(iPr))Cl2] and [Pd(DACH(Im(iPr))2)Cl2], were investigated. Spectrophotometric acid-base titrations were performed to determine the pKa values of the coordinated water molecules in [Pd(en)(H2O)2](2+), [Pd(EAIm(iPr))(H2O)2](2+), [Pd(DMEAIm(iPr))(H2O)2](2+), [Pd(DPENIm(iPr))(H2O)2](2+), [Pd(BL(iPr))(H2O)2](2+) and [Pd(DACH(Im(iPr))2)(H2O)2](2+). The substitution of the chloride ligands in these complexes by TU, l-Met, l-His and Gly was studied under pseudo-first-order conditions as a function of the nucleophile concentration and temperature using stopped-flow techniques; the sulfur-donor nucleophiles have shown better reactivity than nitrogen-donor nucleophiles. The obtained results indicate that there is a clear correlation between the nature of the imidazolin-2-imine ligands and the acid-base characteristics and reactivity of the resulting Pd(ii) complexes; the order of reactivity of the investigated Pd(ii) complexes is: [Pd(en)Cl2] > [Pd(EAIm(iPr))Cl2] > [Pd(DMEAIm(iPr))Cl2] > [Pd(DPENIm(iPr))Cl2] > [Pd(BL(iPr))Cl2] > [Pd(DACH(Im(iPr))2)Cl2]. The solubility measurements revealed good solubility of the studied imidazolin-2-imine complexes in water, despite the fact that these Pd(ii) complexes are neutral complexes. Based on the performed studies, three unusual features of the novel imidazolin-2-imine Pd(ii) complexes are observed, that is, good solubility in water, very low reactivity and high pKa values. The coordination geometries around the palladium atoms are distorted square-planar; the [Pd(DMEAIm(iPr))Cl2] complex displays Pd-N distances of 2.013(2) and 2.076(2) A, while the [Pd(DPENIm(iPr))Cl2] complex displays similar Pd-N distances of 2.034(4) and 2.038(3) A. The studied systems are of interest because little is known about the substitution behavior of imidazolin-2-imine Pd(ii) complexes with bio-molecules under physiological conditions.

The novel boryl anions NHC–B(CN)2 – complete a series of cyanoborates with continuously increasing boron-centred nucleophilicity., Since the first seminal report of boron-centred nucleophiles, the area of boryl anions has developed only sporadically and requires further systematisation. The boryl anions of type NHC–B(CN)2 – (NHC = N-heterocyclic carbene) described herein complete a consistent series with the known anions cAAC–B(CN)2 – [cAAC = cyclic(alkyl)(amino)carbene] and B(CN)3 2–. A novel approach towards NHC-stabilised cyanoboranes based on alkylthio-cyano exchange at boron is presented, and in contrast to other methods affords the products in better purity and yield. Reduction of suitable NHC–dicyanoboranes gave two unprecedented examples of NHC–B(CN)2 – boryl anions. The latter were shown to react as boron-centred nucleophiles with facile formation of B–E bonds, where E = C, Si, Sn, P, Au. Bonding analysis by DFT calculations suggests a systematic variation of the energy of the boron-centred HOMO depending on the carbene, which in turn can control the nucleophilic character.

The 15-valence electron (VE), high-spin (S = 3/2) half-sandwich complex [Cp'Fe(IiPr

Facile pseudohalide activation occurs in the reaction of SCN–, SeCN– and N3– with the iron half-sandwich [Cp′Fe(μ-I)]2., The iron half-sandwich [Cp′Fe(μ-I)]2 (Cp′ = 1,2,4-(Me3C)3C5H2, 1) reacts with the pseudohalides NCO–, SCN–, SeCN– and N3– to give [Cp′Fe(μ-NCO)]2 (2), [Cp′Fe(μ-S)]2 (3), [Cp′Fe(μ-Se2)]2 (4) and [Cp′Fe(μ-N)]2 (5), respectively. Various spectroscopic techniques including X-ray diffraction, solid-state magnetic susceptibility studies and 57Fe Mössbauer spectroscopy were employed in the characterization of these species. Mössbauer spectroscopy shows a decreasing isomer shift with increasing formal oxidation state, ranging from Fe(ii) to Fe(iv), in complexes 1 to 5. The sulfido-bridged dimer 3 exhibits strong antiferromagnetic coupling between the Fe(iii) centers. This leads to temperature-independent paramagnetism (TIP) at low temperature, from which the energy gap between the ground and the excited state can be estimated to be 2J = ca. 700 cm–1. The iron(iv) nitrido complex [Cp′Fe(μ-N)]2 (5) shows no reactivity towards H2 (10 atm), but undergoes clean reactions with CO (5 bar) and XylNC (Xyl = 2,6-Me2C6H3) to form the diamagnetic isocyanate and carbodiimide complexes [Cp′Fe(CO)2(NCO)] (7) and [Cp′Fe(CNXyl)2(NCNXyl)] (8), respectively. All compounds were fully characterized, and density functional theory (DFT) computations provide useful insights into their formation and the electronic structures of complexes 3 and 5.

A series of enantiomerically pure −SiMe2NR2 (R = Me, Et) substituted pentadienyl ligands were prepared starting from the natural product (1R)-(−)-myrtenal. Deprotonation with a Schlosser superbase yields the corresponding potassium salts, which were characterized by various spectroscopic techniques. In solution these neutral N-donor-substituted pentadienyl systems predominantly adopt a U conformation, but in two cases the rare S conformation was also observed as a minor component in solution. Addition of 18-crown-6 allowed the molecular structures of two of these potassium pentadienyls to be determined by X-ray diffraction. Interestingly, η5 and κN coordination of the pentadienyl system to the [K(18-crown-6)]+ cation was observed. Furthermore, these ligand systems also coordinate to transition metals and form an open titanocene, open vanadocenes, open chromocenes, and half-open trozircenes with [TiCl3(thf)3], [VCl3(thf)3], CrCl2, and [(η7-C7H7)ZrCl(tmeda)], respectively. These complexes were characterized...