Hydride Derivatives Research Articles

Boron Chemistry – A Rapidly Expanding Research Field

Guest Editors Vladimir Bregadze and Zuowei Xie give a short introduction to the field and history of boron chemistry. This thematic issue is devoted to boron chemistry. This field of chemistry quickly started to develop when scientists tried to investigate boron hydrides in the middle of the 20th century after Alfred Stock's discovery. At that time, the leading world countries started to invest a lot of money in research into this field, in view of the good prospects for the application of boron hydrides as rocket fuel. However the toxicity and flammability of these compounds prevented the investigations in this field. So, attempts were directed to synthesize less explosive and less toxic boron hydride derivatives. It was found that polyhedral boron hydrides are less toxic and not flammable. The first polyhedral boron hydrides were described in 1959 and 1960 by Hawthorne and Pitochelli1 with the examples of decahydro-closo-decaborate [B10H10]2– and dodecahydro-closo-dodecaborate [B12H12]2–. In these clusters, the whole polyhedron consist of boron atoms. At the end of 1963 the first communications describing the synthesis of icosahedral carboranes, a new unusual class of polyhedral compounds whose cage consisted of ten boron atoms and two carbon atoms,2, 3 were simultaneously published in USSR and USA. Although papers on derivatives known as lower and medium carboranes had already been published,4 the publications on higher icosahedral carboranes made a great impression on the scientific community of that time. It became clear that a new, important page in the development of chemistry was thus opened. The subsequent vigorous development of this field demonstrated that the discovery of carboranes is as significant as the discovery of metallocenes and fullerenes for the development of modern chemistry or even science as a whole. The boron hydrides and polyhedral boron compounds proved to be interesting from both practical and theoretical points of view. The studies of their structure and reactivity formed the basis for important chemical concepts such as three-center two-electron bonds, multicenter bonds, and three-dimensional aromaticity. The unusual structures and properties of polyhedral borates and carboranes, and metallaboranes and metallacarboranes derived from them, caught the attention of scientists to this field and led to the preparation of a great number of new compounds. A comprehensive monograph on carboranes was recently presented by Grimes.5 In addition to boranes and carboranes, other types of boron compounds also received an enormous amount of attention. As shown in this thematic issue, a total of 41 articles highlight the different aspects of current boron chemistry and encompass a broad spectrum of compounds bearing one to ten boron atoms. This field continues to remain important and popular as can be seen by the contributions from all over the world. We hope that readers will find this timely cluster issue interesting and stimulating. We wish to thank all contributors, reviewers, and the EurJIC editorial team for their high-quality and professional work. Vladimir Bregadze graduated from the Chemistry Department of M.V. Lomonosov Moscow State University in 1960. He received his Ph.D. and D.Sc. degree, Professor of Chemistry in USSR Academy of Sciences. He is now Head of Laboratory of Organoaluminum and Boron Compounds of the A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences. In 1976 V.I.Bregadze received the USSR State Prize for the Development and Application of New Materials in Industry, and in 1996 he received the Russian Federation State Prize in Science and Technology for the “Chemistry of Carboranes and Polyhedral Boranes”. His fields of interest are organic and inorganic derivatives of boron and main group metals, the chemistry of polyhedral boranes and carboranes, the study of their reactivity and application in medicine (antitumour activity, boron neutron capture therapy) and for the design of materials. He is Adjunct Professor of the Beijing University of Chemical Technology. He has published over 400 research papers, reviews and chapters in books. Zuowei Xie obtained a B.Sc. from Hangzhou University in 1983, a M.Sc. from Shanghai Institute of Organic Chemistry (SIOC), the Chinese Academy of Sciences in 1986, and his Ph.D. in 1990, working in a joint program between SIOC and Technische Universität Berlin. After serving as a Research Associate in SIOC and a Postdoctoral Fellow at University of Southern California, he joined the chemistry faculty of The Chinese University of Hong Kong in 1995, where he is now a Choh-Ming Li Professor of Chemistry. He has co-authored over 270 papers and received several prestigious awards including the State Natural Science Prize in 2008 and the Chinese Chemical Society Yao-Zeng Huang Award in Organometallic Chemistry in 2010.

Reductive Dehydrogenation of a Stannane via Multiple Sn−H Activation by Frustrated Lewis Pairs

A bulky substituted stannane Ar*SnH3 (Ar*=2,6-(2',4',6'-triisopropylphenyl)phenyl) was treated with the well-known frustrated Lewis pair (FLP) PtBu3 /B(C6 F5 )3 in varying stoichiometries. To some degree, hydride abstraction and adduct formation is observed, leading to [Ar*SnH2 (PtBu3 )]+ which is rather unreactive toward further dehydrogenation. In a competing process, the FLP proved to be capable of completely striping-off hydrogen and hydrides to generate the first cationic phosphonio-stannylene [Ar*Sn(PtBu3 )]+ . This behavior provides insight into the activation/abstraction mechanism processes involved in these Group 14 hydride derivatives.

Mechanism of H2 Production by Models for the [NiFe]-Hydrogenases: Role of Reduced Hydrides.

The intermediacy of a reduced nickel-iron hydride in hydrogen evolution catalyzed by Ni-Fe complexes was verified experimentally and computationally. In addition to catalyzing hydrogen evolution, the highly basic and bulky (dppv)Ni(μ-pdt)Fe(CO)(dppv) ([1](0); dppv = cis-C2H2(PPh2)2) and its hydride derivatives have yielded to detailed characterization in terms of spectroscopy, bonding, and reactivity. The protonation of [1](0) initially produces unsym-[H1](+), which converts by a first-order pathway to sym-[H1](+). These species have C1 (unsym) and Cs (sym) symmetries, respectively, depending on the stereochemistry of the octahedral Fe site. Both experimental and computational studies show that [H1](+) protonates at sulfur. The S = 1/2 hydride [H1](0) was generated by reduction of [H1](+) with Cp*2Co. Density functional theory (DFT) calculations indicate that [H1](0) is best described as a Ni(I)-Fe(II) derivative with significant spin density on Ni and some delocalization on S and Fe. EPR spectroscopy reveals both kinetic and thermodynamic isomers of [H1](0). Whereas [H1](+) does not evolve H2 upon protonation, treatment of [H1](0) with acids gives H2. The redox state of the "remote" metal (Ni) modulates the hydridic character of the Fe(II)-H center. As supported by DFT calculations, H2 evolution proceeds either directly from [H1](0) and external acid or from protonation of the Fe-H bond in [H1](0) to give a labile dihydrogen complex. Stoichiometric tests indicate that protonation-induced hydrogen evolution from [H1](0) initially produces [1](+), which is reduced by [H1](0). Our results reconcile the required reductive activation of a metal hydride and the resistance of metal hydrides toward reduction. This dichotomy is resolved by reduction of the remote (non-hydride) metal of the bimetallic unit.

ChemInform Abstract: Alkaline Earths as Main Group Reagents in Molecular Catalysis

AbstractReview: [147 refs.

Alkaline earths as main group reagents in molecular catalysis.

The past decade has witnessed some remarkable advances in our appreciation of the structural and reaction chemistry of the heavier alkaline earth (Ae = Mg, Ca, Sr, Ba) elements. Derived from complexes of these metals in their immutable +2 oxidation state, a broad and widely applicable catalytic chemistry has also emerged, driven by considerations of cost and inherent low toxicity. The considerable adjustments incurred to ionic radius and resultant cation charge density also provide reactivity with significant mechanistic and kinetic variability as group 2 is descended. In an attempt to place these advances in the broader context of contemporary main group element chemistry, this review focusses on the developing state of the art in both multiple bond heterofunctionalisation and cross coupling catalysis. We review specific advances in alkene and alkyne hydroamination and hydrophosphination catalysis and related extensions of this reactivity that allow the synthesis of a wide variety of acyclic and heterocyclic small molecules. The use of heavier alkaline earth hydride derivatives as pre-catalysts and intermediates in multiple bond hydrogenation, hydrosilylation and hydroboration is also described along with the emergence of these and related reagents in a variety of dehydrocoupling processes that allow that facile catalytic construction of Si-C, Si-N and B-N bonds.

Diphosphorus-bridged heterometallic anions and hydrides derived from reactions of complex [Mo2Cp2(μ-PCy2)(μ-κ2:κ2-P2)(CO)2]− with precursors of 16-electron metal carbonyl fragments

Reaction of the Li+ salt of the title anion with [Fe2(CO)9] or tetrahydrofuran adducts of formula [MLn(THF)] gave the trinuclear species Li[Mo2MCp2(μ-PCy2)(μ-κ2:κ2:κ1-P2)(CO)2Ln], (MLn = Fe(CO)4, MnCp′(CO)2, Mo(CO)5, W(CO)5; Cp′ = η5-C5H4Me), some of which could be isolated as the corresponding PPN+ salts (PPN = N(PPh3)2). These products followed from further coordination of the P2 ligand to the incoming 16-electron MLn fragment via the lone electron pair at its basal P atom, and displayed characteristically high P–P couplings (1JPP ca. 500Hz) indicative of retention of a strong P–P bond. In addition, these products underwent a fluxional process derived from the swing of the P2 unit around the Mo–Mo axis with concomitant exchange of the MLn fragment between P atoms. Reaction of the title anion with excess of the above carbonyl complexes involved further addition of a second MLn fragment to the P atom still bearing a lone electron pair, thus leading to the tetranuclear anionic derivatives Li[Mo2M2Cp2(μ-PCy2)(μ-κ2:κ2:κ1:κ1-P2)(CO)2L2n], most of which could be isolated as the corresponding PPN+ salts. For the Mo and W derivatives, this reaction also involved a trans to cis isomerisation at the Mo2(CO)2 moiety of the parent anion. All tetranuclear anions displayed fluxional behaviour in solution, comparable to that of the parent anion in the case of the Fe and Mn derivatives, but likely involving rotation of the P2 ligand in the case of the cisoid Mo and W derivatives. Protonation of the above trinuclear anions with [NH4]PF6 led to decomposition except for the Mn complex, this yielding a poorly characterized diphosphenyl derivative [MnMo2Cp2Cp′(μ-κ1,η2:κ2:κ1-HP2)(μ-PCy2)(CO)4] displaying an agostic-like P–H–Mo interaction, eventually decomposing to the known phosphide-bridged complex [MnMo2Cp2Cp′(μ3-P)(μ-PCy2)(CO)4]. In contrast, protonation of the tetranuclear anions yielded the unsaturated hydride derivatives [Mo2M2Cp2(H)(μ-PCy2)(μ-κ2:κ2:κ1:κ1-P2)(CO)L2n], (M = Mn, Mo, W), with a formal intermetallic double bond (Mo–Mo = 2.7412(6) Å for the tungsten complex) as a result of loss of a CO ligand from the Mo2(CO)2 moiety after protonation, probably favoured by the presence of excess of adducts [MLn(THF)] in these reaction mixtures.

Four-coordinate cobalt pincer complexes: electronic structure studies and ligand modification by homolytic and heterolytic pathways.

A family of cobalt chloride, methyl, acetylide and hydride complexes bearing both intact and modified tert-butyl substituted bis(phosphino)pyridine pincer ligands has been synthesized and structurally characterized and their electronic structures evaluated. Treatment of the unmodified compounds with the stable nitroxyl radical, TEMPO (2,2,6,6-tetramethylpiperidin-1-yloxidanyl) resulted in immediate H- atom abstraction from the benzylic position of the chelate yielding the corresponding modified pincer complexes, ((tBu)mPNP)CoX (X = H, CH3, Cl, CCPh). Thermolysis of the methyl and hydride derivatives, ((tBu)PNP)CoCH3 and ((tBu)PNP)CoH, at 110 °C also resulted in pincer modification by H atom loss while the chloride and acetylide derivatives proved inert. The relative ordering of benzylic C-H bond strengths was corroborated by H atom exchange experiments between appropriate intact and modified pincer complexes. The electronic structures of the modified compounds, ((tBu)mPNP)CoX were established by EPR spectroscopy and DFT computations and are best described as low spin Co(II) complexes with no evidence for ligand centered radicals. The electronic structures of the intact complexes, ((tBu)PNP)CoX were studied computationally and bond dissociation free energies of the benzylic C-H bonds were correlated to the identity of the X-type ligand on cobalt where pure σ donors such as hydride and methyl produce the weakest C-H bonds. Comparison to a rhodium congener highlights the impact of the energetically accessible one-electron redox couple of the first row metal ion in generating weak C-H bonds in remote positions of the supporting pincer ligand.

Oxidative addition of σ bonds to an Al(I) center.

The Al(I) compound NacNacAl (1, NacNac = [ArNC(Me)CHC(Me)NAr](-) and Ar = 2,6-Pr(i)2C6H3) reacts with H-X (X = H, Si, B, Al, C, N, P, O) σ bonds of H2, silanes, borane (HBpin, pin = pinacolate), allane (NacNacAlH2), phosphine (HPPh2), amines, alcohol (Pr(i)OH), and Cp*H (Cp* = pentamethylcyclopentadiene) to give a series of hydride derivatives of the four-coordinate aluminum NacNacAlH(X), which are characterized herein by spectroscopic methods (NMR and IR) and X-ray diffraction. This method allows for the syntheses of the first boryl hydride of aluminum and novel silyl hydride and phosphido hydride derivatives. In the case of the addition of NacNacAlH2, the reaction is reversible, proving the possibility of reductive elimination from the species NacNacAlH(X).

Reactivity of Coordinatively Unsaturated Bis(N-heterocyclic carbene) Pt(II) Complexes toward H2. Crystal Structure of a 14-Electron Pt(II) Hydride Complex

The reactivity toward H2 of coordinatively unsaturated Pt(II) complexes, stabilized by N-heterocyclic carbene (NHC) ligands, is herein analyzed. The cationic platinum complexes [Pt(NHC')(NHC)](+) (where NHC' stands for a cyclometalated NHC ligand) react very fast with H2 at room temperature, leading to hydrogenolysis of the Pt-CH2 bond and concomitant formation of hydride derivatives [PtH(NHC)2](+) or hydrido-dihydrogen complexes [PtH(H2)(NHC)2](+). The latter species release H2 when these compounds are subjected to vacuum. The X-ray structure of complex [PtH(IPr)2][SbF6] revealed its unsaturated nature, exhibiting a true T-shaped structure without stabilization by agostic interactions. Density functional theory calculations indicate that the binding and reaction of H2 in complexes [PtH(H2)(NHC)2](+) is more favored for derivatives bearing aryl-substituted NHCs (IPr, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene and IMes = 1,3-dimesityl-1,3-dihydro-2H-imidazol-2-ylidene) than for those containing tert-butyl groups (I(t)Bu). This outcome is related to the higher close-range steric effects of the I(t)Bu ligands. Accordingly, H/D exchange reactions between hydrides [PtH(NHC)2](+) and D2 take place considerably faster for IPr and IMes* derivatives than for I(t)Bu ones. The reaction mechanisms for both H2 addition and H/D exchange processes depend on the nature of the NHC ligand, operating through oxidative addition transition states in the case of IPr and IMes* or by a σ-complex assisted-metathesis mechanism in the case of I(t)Bu.

Intramolecular C–H Oxidative Addition to Iridium(I) in Complexes Containing aN,N′-Diphosphanosilanediamine Ligand

The iridium(I) complexes of formula Ir(cod)(SiNP)(+) (1(+)) and IrCl(cod)(SiNP) (2) are easily obtained from the reaction of SiMe2{N(4-C6H4CH3)PPh2}2 (SiNP) with [Ir(cod)(CH3CN)2](+) or [IrCl(cod)]2, respectively. The carbonylation of [1][PF6] affords the cationic pentacoordinated complex [Ir(CO)(cod)(SiNP)](+) (3(+)), while the treatment 2 with CO gives the cation 3(+) as an intermediate, finally affording an equilibrium mixture of IrCl(CO)(SiNP) (4) and the hydride derivative of formula IrHCl(CO)(SiNP-H) (5) resulting from the intramolecular oxidative addition of the C-H bond of the SiCH3 moiety to the iridium(I) center. Furthermore, the prolonged exposure of [3]Cl or 2 to CO resulted in the formation of the iridium(I) pentacoordinated complex Ir(SiNP-H)(CO)2 (6). The unprecedented κ(3)C,P,P' coordination mode of the [SiNP-H] ligand observed in 5 and 6 has been fully characterized in solution by NMR spectroscopy. In addition, the single-crystal X-ray structure of 6 is reported.

Unusual Structure, Fluxionality, and Reaction Mechanism of Carbonyl Hydrosilylation by Silyl Hydride Complex [(ArN)Mo(H)(SiH2Ph)(PMe3)3

The reactions of bis(borohydride) complexes [(RN=)Mo(BH4)2(PMe3)2] (4: R = 2,6-Me2C6H3; 5: R = 2,6-iPr2C6H3) with hydrosilanes afford new silyl hydride derivatives [(RN=)Mo(H)(SiR'3)(PMe3)3] (3: R = Ar, R'3 = H2Ph; 8: R = Ar', R'3 = H2Ph; 9: R = Ar, R'3 = (OEt)3; 10: R = Ar, R'3 = HMePh). These compounds can also be conveniently prepared by reacting [(RN=)Mo(H)(Cl)(PMe3)3] with one equivalent of LiBH4 in the presence of a silane. Complex 3 undergoes intramolecular and intermolecular phosphine exchange, as well as exchange between the silyl ligand and the free silane. Kinetic and DFT studies show that the intermolecular phosphine exchange occurs through the predissociation of a PMe3 group, which, surprisingly, is facilitated by the silane. The intramolecular exchange proceeds through a new non-Bailar-twist pathway. The silyl/silane exchange proceeds through an unusual Mo(VI) intermediate, [(ArN=)Mo(H)2(SiH2Ph)2(PMe3)2] (19). Complex 3 was found to be the catalyst of a variety of hydrosilylation reactions of carbonyl compounds (aldehydes and ketones) and nitriles, as well as of silane alcoholysis. Stoichiometric mechanistic studies of the hydrosilylation of acetone, supported by DFT calculations, suggest the operation of an unexpected mechanism, in that the silyl ligand of compound 3 plays an unusual role as a spectator ligand. The addition of acetone to compound 3 leads to the formation of [trans-(ArN)Mo(OiPr)(SiH2Ph)(PMe3)2] (18). This latter species does not undergo the elimination of a Si-O group (which corresponds to the conventional Ojima's mechanism of hydrosilylation). Rather, complex 18 undergoes unusual reversible β-CH activation of the isopropoxy ligand. In the hydrosilylation of benzaldehyde, the reaction proceeds through the formation of a new intermediate bis(benzaldehyde) adduct, [(ArN=)Mo(η(2)-PhC(O)H)2(PMe3)], which reacts further with hydrosilane through a η(1)-silane complex, as studied by DFT calculations.

Group 13 β-Ketoiminate Compounds: Gallium Hydride Derivatives As Molecular Precursors to Thin Films of Ga2O3

Bis(β-ketoimine) ligands, [R{N(H)C(Me)-CHC(Me)═O}(2)] (L(1)H(2), R = (CH(2))(2); L(2)H(2), R = (CH(2))(3)), linked by ethylene (L(1)) and propylene (L(2)) bridges have been used to form aluminum, gallium, and indium chloride complexes [Al(L(1))Cl] (3), [Ga(L(n))Cl] (4, n = 1; 6, n = 2) and [In(L(n))Cl] (5, n = 1; 7, n = 2). Ligand L(1) has also been used to form a gallium hydride derivative [Ga(L(1))H] (8), but indium analogues could not be made. β-ketoimine ligands, [Me(2)N(CH(2))(3)N(H)C(R')-CHC(R')═O] (L(3)H, R' = Me; L(4)H, R' = Ph), with a donor-functionalized Lewis base have also been synthesized and used to form gallium and indium alkyl complexes, [Ga(L(3))Me(2)] (9) and [In(L(3))Me(2)] (10), which were isolated as oils. The related gallium hydride complexes, [Ga(L(n))H(2)] (11, n = 3; 12, n = 4), were also prepared, but again no indium hydride species could be made. The complexes were characterized mainly by NMR spectroscopy, mass spectrometry, and single crystal X-ray diffraction. The β-ketoiminate gallium hydride compounds (8 and 11) have been used as single-source precursors for the deposition of Ga(2)O(3) by aerosol-assisted (AA)CVD with toluene as the solvent. The quality of the films varied according to the precursor used, with the complex [Ga(L(1))H] (8) giving by far the best quality films. Although the films were amorphous as deposited, they could be annealed at 1000 °C to form crystalline Ga(2)O(3). The films were analyzed by powder XRD, SEM, and EDX.

Synthesis and Decarbonylation Reactions of Diiron Cyclopentadienyl Complexes with Bent-Phosphinidene Bridges

The phosphinidene complexes [Fe2Cp2(μ-PR)(μ-CO)(CO)2] (Cp = η5-C5H5; R=Cy, Ph, Mes, Mes*; Mes = 2,4,6-C6H2Me3, Mes* = 2,4,6-C6H2tBu3) were readily prepared in high yields through a two-step procedure starting from the corresponding phosphine complexes [Fe2Cp2(μ-CO)2(CO)(PH2R)]. When R = Cy, Ph, Mes, the first step required the oxidation of the phosphine complex with 1 equiv of [FeCp2]BF4, this being followed by spontaneous dehydrogenation to yield the cationic phosphide complexes [Fe2Cp2(μ-PHR)(μ-CO)(CO)2]BF4, while the formation of the related PMes*H-bridged complex required a double oxidation of the corresponding neutral precursor in the presence of a deprotonating agent. In all cases the final step involved the deprotonation of the above cations with strong bases such as M(OH) (M = Na, K). Attempts to decarbonylate the above phosphinidene complexes by either thermolysis or photolysis in solution gave results strongly dependent on the nature of R. Thus, the photolysis of the cyclohexylphosphinidene complex gave a mixture of the clusters [Fe4Cp4(μ3-PCy)2(μ3-CO)(μ-CO)(CO)] (Fe−Fe distances 2.6115(7) and 2.5332(8) Å) and [Fe3Cp3(μ3-PCy)(μ-PCyH)(μ-CO)2], while the thermal decarbonylation of the mesitylphosphinidene complex in refluxing toluene gave the trinuclear derivative [Fe3Cp3(μ-PMes)(μ-PMesH)(μ-CO)2(CO)], having a trigonal phosphinidene ligand bridging two iron atoms (Fe−P = 2.161(8), 2.165(8) Å). The formation of all these compounds presumably involves the participation of the corresponding intermediate complex [Fe2Cp2(μ-PR)(μ-CO)2], a species likely to contain a trigonal (four-electron-donor) phosphinidene ligand, not detected. In contrast, the supermesitylphosphinidene complex undergoes a C(tBu)−H oxidative addition to the P atom under mild thermal conditions to give the phosphine derivative [Fe2Cp2(μ-CO)2(CO){PH(CH2CMe2)C6H2tBu2}], while its photolysis gave a mixture of the above phosphine complex and the phosphide−hydride derivatives cis- and trans-[Fe2Cp2(μ-H){μ-P(CH2CMe2)C6H2tBu2}(CO)2], the latter resulting from the oxidative addition of a P−H bond to the dimetal center.

A nickel–manganese catalyst as a biomimic of the active site of NiFe hydrogenases: a combined electrocatalytical and DFT mechanistic study

The dinuclear nickel–manganese complex [Ni(xbsms)Mn(CO)3(H2O)]+ (H2xbsms = 1,2-bis(4-mercapto-3,3-dimethyl-2-thiabutyl)benzene) is reported as a bio-inspired mimic of the active site of NiFe hydrogenases catalyzing hydrogen evolution from trifluoroacetic acid in DMF with an overpotential requirement of 860 mV. This is higher than that displayed by Ni–Ru systems [Canaguier et al., Chem.–Eur. J., 2009, 15, 9350–9364] but similar to that found for related noble metal free Ni–Fe mimics [Canaguier et al., Chem. Commun. 2010, 46, 5876–5878]. A combined electrochemical and theoretical (DFT) study suggests a heterolytic mechanism for hydrogen evolution from a hydride derivative. The structure of the active intermediate, with a bridging hydride ligand between Ni and Mn, resembles that of the Ni–C active state of NiFe hydrogenases.

Synthesis and Redox Behavior of Bis(3‐methylthio‐1‐azulenyl)methyl Cations and Dications Connected by 2‐Thienyl and 2,5‐Thiophenediyl Spacers

AbstractNovel bis(3‐methylthio‐1‐azulenyl)methyl cations 3a+ and 3b+ and dications 4a2+ and 4b2+ were synthesized by thehydride abstraction reaction of the corresponding hydride derivatives 7a, 7b, 8a, and 8b, which were readily prepared by the acid‐catalyzed condensation reaction of 6a and 6b with the corresponding aldehyde and dialdehyde with DDQ. The pKR+ values of novel cations 3a+ and 3b+ and dications 4a2+ and 4b2+ were determined spectrophotometrically to examine the thermodynamic stability of these carbocations. The redox behavior of the mono‐ and dications was also examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV), which revealed their reversible electrochemical reduction properties, except for the case of dication 4b2+. Moreover, a significant color change was observed by visible spectroscopy under electrochemical reduction conditions. Dications 4a2+ and 4b2+ were also chemically reduced by Zn powder to afford thienoquinoid compounds 9a and 9b, respectively.

Nitrosyl derivatives of the unsaturated dihydrides [Mn 2(μ-H) 2(CO) 6(μ-L 2)] (L 2 = Ph 2PCH 2PPh 2, (EtO) 2POP(OEt) 2)

The title dimanganese complexes react with NO (5% in N 2) at room temperature to give as major products the corresponding hexanitrosyl derivatives [Mn 2(NO) 6(μ-L 2)] in moderate yields, and they react rapidly with NO 2 to give the corresponding hydride derivatives [Mn 2(μ-H)(μ-NO 2)(CO) 6(μ-L 2)], these having a nitrite ligand bridging the dimetal centre through the N and O atoms. The dppm-bridged dihydride also reacts selectively at 273 K with (PPN)NO 2 to give first the nitro derivative (PPN)[Mn 2(μ-H)(H)(NO 2)(CO) 6(μ-dppm)], which then transforms into the nitrosyl complex (PPN)[Mn 2(μ-CO)(CO) 5(NO)(μ-dppm)] at room temperature or above (dppm = Ph 2PCH 2PPh 2; PPN + = [N(PPh 3) 2] +). The latter anion reacts with (NH 4)PF 6 to give the hydride-bridged nitrosyl complex [Mn 2(μ-H)(μ-NO)(CO) 6(μ-dppm)] and with [AuCl(PPh 3)] to give the trinuclear cluster [AuMn 2(μ-NO)(CO) 6(μ-dppm)(PPh 3)] (Mn–Au = ca. 2.68 Å; Mn–Mn = 2.879(2) Å). Both products are derived from the addition of the added electrophile at the intermetallic bond and rearrangement of the nitrosyl ligand into a bridging position. In contrast, methylation of the anion with CF 3SO 3Me takes place at the nitrosyl ligand to yield the unstable methoxylimide derivative [Mn 2(μ-NOMe)(CO) 6(μ-dppm)]. Analogous reactions at the nitrosyl ligand take place upon the addition of HBF 4·OEt 2 to the nitrosyl-bridged hydrides [Mn 2(μ-H)(μ-NO)(CO) n(μ-dppm) m] (n = 6, m = 1; n = 4, m = 2) to give the corresponding hydroxylimide derivatives [Mn 2(μ-H)(μ-NOH)(CO) n(μ-dppm) m]BF 4, which were also thermally unstable and could not be isolated nor fully characterized.

Siloxane and silsesquioxane molecules—Precursors for silicate materials

Preparation of ceramics by sol–gel method has been known for many years, but recently it has been developed as a method for the synthesis of nanostructural ceramic materials. Hydrolytic polycondensation of simple molecules [e.g. most widely used tetraethoxysilane (TEOS)] leads to xerogel materials that can contain macromolecules of distinct random, ladder and cage or partial cage structure. In order to obtain well-defined silsesquioxanes it is preferred to start the process with more complex molecules, bringing in a specific framework that can govern the structure of the product. In the presented work alkoxy derivatives of cyclosiloxanes and polysiloxanes as well as a hydride derivative of octahedral silsequioxane (T 8 H) were applied as precursors in the process of hydrolytic polycondensation. Depending on the reaction conditions, silsesquioxane macromolecules or silica material of ordered structure were obtained. We have prepared mesoporous organiosilica materials without using any template or surfactant whatsoever. The meso-pores are created due to the unique structure of initial oligosiloxane or silsequioxane molecules and the specific interactions in the used catalyst/solvent system. In the case of octasilsesquioxane precursor, the condensation process gives directly mesoporous silica material. Dried polysilsesquioxanes were heated at the temperature of 600 °C in argon or air atmosphere (pyrolysis or ceramization). In the atmosphere of argon SiC x O y glass materials were obtained.

Chemistry of Unsaturated Group 6 Metal Complexes with Bridging Hydroxy and Methoxycarbyne Ligands. 6. C−E Bond Formation and C−O Bond Cleavage Processes in the Reactions of [Mo2(η5-C5H5)2(μ-COMe)(μ-PCy2)(μ-CO)] with Several p-Block Elements (E) and Their Hydride Derivatives

The unsaturated complex [Mo2Cp2(μ-COMe)(μ-PCy2)(μ-CO)] (1) reacts with several HERn molecules (HERn = HSPh, HPHPh, HSnPh3) in toluene at 263−288 K to give the corresponding derivatives [Mo2Cp2(μ-ERn)(μ-PCy2)(CO)2]. A related reaction takes place in toluene at 263 K with the diphosphite (EtO)2POP(OEt)2 to give a mixture of the diethoxyphosphide derivatives [Mo2Cp2(μ-PCy2){μ-P(OEt)2}(CO)2] and [Mo2Cp(η5-C5H4CH2Ph)(μ-PCy2){μ-P(OEt)2}(CO)2], the latter incorporating a benzyl group from the solvent, and with iodine (in dichloromethane at room temperature) to give first the iodide complex [Mo2Cp2(μ-I)(μ-PCy2)(CO)2] and then the triiodide derivative [Mo2Cp2(μ-I)I2(μ-PCy2)(CO)2]. In all of the above reactions, neat demethylation of the methoxycarbyne ligand takes place. In contrast, the silanes HSiPh3 and H2SiPh2 (unreactive toward 1 in refluxing toluene) react slowly under visible−UV irradiation at room temperature to give in good yields the novel complexes [Mo2Cp2{μ-C(2-C6H4SiPh2OMe)}(μ-PCy2)(μ-CO)] and [Mo2Cp2(μ-CSiPh2OMe)(μ-PCy2)(μ-CO)], having arylcarbyne and silylcarbyne ligands, respectively, the latter resulting from the overall elimination of H2 and insertion of Ph2SiC6H4 and SiPh2 fragments, respectively, into the strong C−OMe bond of the carbyne ligand. The reactions of 1 with elemental selenium and sulfur involve the incorporation of three to four chalcogen atoms to the dimetal center under mild conditions, to give respectively the carbene-seleniolate derivative [Mo2Cp2{μ-C(OMe)C(O)Se}(μ-PCy2)(μ-Se2)] and the O-methyldithiocarbonate [Mo2Cp2(μ-PCy2){μ-S2C(OMe)}(S)2], the latter derived from an unexpected coupling of two sulfur atoms to the carbyne ligand.

Icosahedral Ga‐Centred Nickel Carbonyl Clusters: Synthesis and Characterization of [H 3– n Ni 12 (μ 12 ‐Ga)(CO) 22 ] n– ( n = 2, 3) and [Ni 14.3 (μ 12 ‐Ga)(CO) 24.3 ] 3– Anions

The reaction of [Ni 5 (CO) 12 ] 2- or [Ni 6 (CO) 12 ] 2- with GaCl 3 in dichloromethane under a nitrogen atmosphere affords a mixture of [Ni 12+x (μ 12 -Ga)(CO) 22+x ] 3- (x = 0-3) clusters. Short exposure of the above mixture to a carbon monoxide atmosphere leads to the green icosahedral [Ni 12 (μ 12 -Ga)(CO) 22 ] 3- trianion, which was isolated and characterized as its [NnBu 4 ] + salt. In contrast, crystallization of the above mixture in the presence of Ni(CO) 4 enabled isolation of a cocrystallized mixture of [Ni 14 (μ 12 -Ga)(CO) 24 ] 3- (70%) and [Ni 15 (μ 12 -Ga)(CO) 25 ] 3- (30 %). As inferable from its structure, the additional three Ni(CO) moieties condense onto interlayer faces of the icosahedron. Protonation of [Ni 12 (μ 12 -Ga)-(CO) 22 ] 3- affords the corresponding [HNi 12 (μ 12 -Ga)(CO) 22 ] 2- hydride derivative, which was isolated in a pure state and fully characterized. All of the above compounds conform to the cluster-borane analogy, by the inclusion principle, and none exhibits relevant redox behaviour.

Mechanism of hydrogen evolution catalyzed by NiFe hydrogenases: insights from a Ni–Ru model compound

DFT modeling has been used to investigate a previously proposed mechanism of proton reduction catalyzed by [Ni(xbsms)Ru(CO)(2)Cl(2)] (H(2)xbsms = 1,2-bis(4-mercapto-3,3-dimethyl-2-thiabutyl)benzene), a bio-inspired mimic of NiFe hydrogenases based on a Ni-Ru framework. Protonation of the 2e(-)-reduced compound, from which a chloride anion has been eliminated, results in the formation of a semi-bridging hydride derivative with structural features comparable to those of the Ni-C state catalytic intermediate of native hydrogenases. The present study thus provides structural and functional insights into the enzymatic mechanism including the possible involvement of a bridging hydride derivative and heterolytic formation of a dihydrogen molecule on a {Ni(mu-S)(2)M} framework.