Ian M. Bartlett

Structure and bonding in redox-active d4, d5 and d6 alkyne complexes: metal–alkyne moieties as electron sinks

X-Ray structural studies of the two redox-related pairs [Cr(CO)2(η-PhCCPh)(η6-C6HMe5)]0/1+ and [Mo(CO)2(η-PhCPh)(Tp′)]0/1+[Tp′= hydrotris(3,5-dimethyl-pyrazolyl)borate] are consistent with the HOMO of the d6, Cr0 alkyne complex being an antibonding M–alkyne π⊥ orbital.

Redox routes to arenechromium complexes of two-, three- and four-electron alkynes; structure and bonding in paramagnetic [Cr(CO)L(η-RCCR)(η-arene)]+

X-Ray structural studies on the redox pair [Cr(CO)2(η-PhCCPh)(η-C6Me5H)]z (z = 0 and 1) show that one-electron oxidation of the neutral complex results in a shortening of the Cr–Calkyne bonds and a lengthening of the Cr–C(O) bonds, consistent with depopulation of a HOMO antibonding with respect to the metal–alkyne interaction. Oxidation leads to an increase in the substitutional lability of the Cr–CO bonds so that [Cr(CO)2(η-RCCR)(η-C6Me6)]+ (R = Ph or C6H4OMe-p) reacts with Lewis bases to give [Cr(CO)L(η-RCCR)(η-C6Me6)]+ {L = CNXyl, P(OMe)3 and P(OCH2)3CEt}, X-ray studies on which show a rotation of the alkyne to align with the remaining Cr–CO bond. ESR spectroscopic studies on [Cr(CO)L(η-RCCR)(η-C6Me6)]+ show delocalisation of the unpaired electron onto the alkyne ligand, consistent with its description as a three-electron donor. The cations [Cr(CO)L(η-RCCR)(η-C6Me6)]+ undergo both one-electron reduction and oxidation, and chemical oxidation of [Cr(CO){P(OCH2)3CEt}(η-p-MeOC6H4CCC6H4OMe-p)(η-C6Me6)]+ with AgPF6 gives the dication [Cr(CO){P(OCH2)3CEt}(η-p-MeOC6H4CCC6H4OMe-p)(η-C6Me6)]2+. Thus the two-electron alkyne of [Cr(CO)2(η-RCCR)(η-C6Me6)] is converted into the four-electron alkyne of [Cr(CO)L(η-RCCR)(η-C6Me6)]2+ by an ECE (E = electrochemical, C = chemical) process in which all of the intermediates have been fully characterised.

Molybdenum based alkyne-isocyanide coupling reactions; synthesis of a reactive diiminometallacyclopentene complex

The complex [Mo(CO)(PhCCPh)2(η-C5Me5)][BF4] reacted with three equivalents of 2,6-dimethylphenyl isocyanide, CNxyl, to give the tris(isocyanide) complex [Mo(PhCCPh)(CNxyl)3(η-C5Me5)][BF4] 1. With four equivalents of CNxyl, alkyne–isocyanide coupling leads to the formation of the diiminometallacyclopentene [Mo{C(Nxyl)C(Ph)C(Ph)CNxyl}(CNxyl)2(η-C5Me5)][BF4] 2 which decomposes in thf at room temperature to give the tetrakis(isocyanide) complex [Mo(CNxyl)4(η-C5Me5)][BF4] 3. In dichloromethane, 2 gives the diiminocyclobutene xylNCC(Ph)C(Ph)CNxyl 4, the η2-iminoacyl complex [MoCl(CNxyl)2(η2-xylNCCH2Cl)(η-C5Me5)][BF4] 5, formed by an addition reaction with dichloromethane, and the metallacyclobutene complex [Mo{C(Ph)C(Ph)CN(H)xyl}(CNxyl)3(η-C5Me5)][BF4]26. Complex 5 is more efficiently synthesized by photolysis of 3 in dichloromethane, but the same reaction in chloroform produces the dichloromethyl complex [MoCl(CHCl2)(CNxyl)3(η-C5Me5)][BF4] 7 which thermally decomposes to [MoCl2(CNxyl)3(η-C5Me5)][BF4] 8. Reaction of 2 with HCl in diethyl ether results in protonation of the metallacyclic ring and formation of the iminium metallacyclopentene [MoCl{C(Nxyl)C(Ph)C(Ph)CN(H)xyl}(CNxyl)2(η-C5Me5)][BF4] 9. Compound 3 is oxidised by AgBF4 to give the molybdenum(IV) complex [MoF(CNxyl)4(η-C5Me5)][BF4]210. The molecular structures of 3–6, 7/8 and 10 have been determined by X-ray crystallography.

Oxidatively induced carbon chain growth at a ditungsten centre

X-Ray structural and EHMO studies show that two electrons are sequentially removed from a W–W δ* orbital on oxidation of the bis(metallacyclopentadiene) complex [W2(µ-C4Me4)2(η-C5H5)2] to [W2(µ-C4Me4)2(η-C5H5)2]z+(z= 1,2); the dication undergoes C–C coupling and proton loss in boiling MeCN to give [W2(µ-C8Me7CH2)(η-C5H5)2]+ which adds hydride ion to form the metallacyclononatetraene complex [W2(µ-C8Me8)(η-C5H5)2].

Structure and bonding in the d4/d3 alkyne redox pairs [WX(CO)(MeCCMe)Tp′]z (X = F, Cl, Br and I; z = 0 and 1): halide stabilisation of electron deficient metal alkyne complexes

X-Ray structural and EPR spectroscopic studies of the redox-related pairs [WX(CO)(MeCCMe)Tp′]z (X = F, Cl, Br and I; z = 0 and 1) [Tp′ = hydrotris(3,5-dimethylpyrazolyl)borate] are consistent with the HOMO of the d4 (z = 0) species being π-bonding with respect to the W–CO bond, π-antibonding with respect to the W–X bond, and δ-bonding with respect to the W–alkyne bond.