Electronic Structure and Reactivity of the Carbyne-Bridged Dimolybdenum Radical [Mo₂(η⁵-C₅H₅)₂(μ-CPh)(μ-PCy₂)(μ-CO)]⁺

Abstract

The unsaturated compound [Mo2Cp2(μ-CPh)(μ-PCy2)(μ-CO)] (1, Cp = η5-C5H5) reacts with trace amounts of water in the presence of [FeCp2]BF4 to give a mixture of the hydroxycarbyne complex [Mo2Cp2(μ-COH)(μ-CPh)(μ-PCy2)]BF4 (minor) and the hydroxo complex [Mo2Cp2(μ-CPh)(OH)(μ-PCy2)(CO)]BF4 (major product), with the latter rapidly rearranging to give the carbene isomer cis-[Mo2Cp2(μ-η1:η3-CHPh)(O)(μ-PCy2)(CO)]BF4 (Mo–Mo = 2.9435(3) Å). An analogous reaction takes place with phenol, to give selectively the related phenoxo complex [Mo2Cp2(μ-CPh)(OPh)(μ-PCy2)(CO)]BF4. In contrast, the reactions of 1 with H2SiPh2 or H3BNH2tBu in the presence of [FeCp2]BF4 result in the selective H transfer to the O atom of the carbonyl ligand, to give the mentioned hydroxycarbyne complex. All the above reactions can be rationalized by assuming the initial formation of the radical cation [Mo2Cp2(μ-CPh)(μ-PCy2)(μ-CO)]+ (2), a molecule displaying a somewhat weakened intermetallic bonding (Mo–Mo = 2.537 Å vs 2.493 Å in 1) and a linear semibridging carbonyl, with both the LUMO and most of the unpaired electron density being located at a single molybdenum atom, with a much smaller distribution over the oxygen atom of the carbonyl ligand, according to density functional theory calculations. As expected, the radical 2 adds rapidly a molecule of nitric oxide to give a diamagnetic product, but spontaneous decarbonylation also takes place to eventually give the 30-electron nitrosyl complex [Mo2Cp2(μ-CPh)(μ-PCy2)(μ-NO)]BF4. Deprotonation of cis-[Mo2Cp2(μ-η1:η3-CHPh)(O)(μ-PCy2)(CO)]BF4 gives the neutral carbyne complex cis-[Mo2Cp2(μ-CPh)(O)(μ-PCy2)(CO)] (Mo–Mo = 2.8024(5) Å), which upon protonation reverts to its carbene precursor, via the corresponding hydroxo complex. Related trans isomers can be prepared through protonation reactions of trans-[Mo2Cp2(μ-CPh)(O)(μ-PCy2)(CO)] (Mo–Mo = 2.8206(6) Å), a complex easily prepared by reacting the dicarbonyl [Mo2Cp2(μ-CPh)(μ-PCy2)(CO)2] with air.