No 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 H3BNH2 t Bu 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.

The title complex was formed instantaneously by reacting [Mo2Cp2(μ-CPh)(μ-PCy2)(μ-CO)] with [FeCp2]BF4 in the presence of diphenyl disulfide, but it could not be isolated as a pure material. It most likely displays bridging phosphide and thiolate ligands and essentially terminal carbonyl (Mo–C = 1.987 Å) and carbyne (Mo–C = 1.820 Å) ligands, according to density functional theory calculations. In solution this complex released CO spontaneously to yield the 30-electron complex [Mo2Cp2(μ-CPh)(μ-PCy2)(μ-SPh)]BF4 (Mo–Mo = 2.4772(6) Å), and attempts to crystallize it at low temperature yielded instead the electron-precise aquo derivative [Mo2Cp2(μ-CPh)(μ-PCy2)(μ-SPh)(CO)(OH2)]BF4 (Mo–Mo = 2.7820(4) Å). The addition of CO induced the coupling between the carbyne and phosphide ligands, to yield the tricarbonyl phosphinocarbene complex [Mo2Cp2(μ-η1:η1,κ1-CPhPCy2)(μ-SPh)(CO)3]BF4 (Mo–Mo = 2.928(1) Å). This coupling process was reversible, since the latter complex could be decarbonylated stepwise by thermal methods, to give first the 32-electron phosphinocarbene derivative [Mo2Cp2(μ-η1:η1,κ1-CPhPCy2)(μ-SPh)(CO)2]BF4 (Mo–Mo = 2.7863(4) Å) and eventually yielding [Mo2Cp2(μ-CPh)(μ-PCy2)(μ-SPh)]BF4 via the title complex. In contrast to this behavior, the reaction of the title complex with CN t Bu did not induce a P–C coupling process, but eventually led to the displacement of the thiolate ligand to give the dication trans-[Mo2Cp2(μ-CPh)(μ-PCy2)(CN t Bu)4]2+ via the bis(isocyanide) complex cis-[Mo2Cp2(μ-CPh)(μ-PCy2)(μ-SPh)(CN t Bu)2]BF4, with these differences being probably steric in origin.

The title compound reacted with HBF4·OEt2 at room temperature to give a mixture of the agostic-like, phosphine-bridged complex [Mo2Cp2(μ-CPh)(μ-κ1:η2-PHCy2)(CO)2]BF4 (major) and the carbene-bridged complex [Mo2Cp2(μ-η1:η2-CHPh)(μ-PCy2)(CO)2]BF4 (minor). It readily added a molecule of HCCCO2Me or a single Se atom at its Mo2C(carbyne) center to give with high yield the corresponding propenylylidene- or selenoacyl-bridged derivatives [Mo2Cp2{μ-η2:η3-CPhCHC(CO2Me)}(μ-PCy2)(CO)2] and [Mo2Cp2{μ-η,κ:η,κ-C(Ph)Se}(μ-PCy2)(CO)2], respectively. In contrast, the addition of a neat donor at the metal site can induce a reversible carbyne–carbonyl coupling, as observed in the reaction with N2CPh2 to give the ketenyl derivative [Mo2Cp2{μ-η1:η2-C(Ph)CO}(μ-PCy2)(CO)(κ1-N2CPh2)].

The unsaturated methoxycarbyne complex [Mo 2 Cp 2 (μ-COMe)(μ-CPh)(μ-PCy 2 )](CF 3 SO 3 ) (Cp = η 5 -C 5 H 5 ; Mo−Mo = 2.4707(3) Å) reacted with CO (293 K, 40 bar) or CNR (233 K, R = t Bu, Xyl) to give the corresponding methoxyalkynebridged derivatives [Mo 2 Cp 2 {μ-η 2 :η 2 -C(OMe)CPh}(μ-PCy 2 )L 2 ]-(CF 3 SO 3 ) following from a reductive C−C coupling between methoxycarbyne and benzylidyne ligands (L = CO, CNR). This coupling could be fully reversed for the dicarbonyl product upon photolysis in tetrahydrofuran solution. The related hydroxycarbyne complex [Mo 2 Cp 2 (μ-COH)(μ-CPh)(μ-PCy 2 )]BF 4 reacted analogously with CO (293 K, 4 bar) to give the hydroxyalkyne-bridged derivative [Mo 2 Cp 2 {μ-η 2 :η 2 -C(OH)CPh}(μ-PCy 2 )(CO) 2 ]BF 4 (Mo−Mo = 2.6572(5) Å) as a result of C−C coupling between hydroxycarbyne and benzylidyne ligands, but this process could not be reversed photochemically. The latter complex could be prepared more efficiently via protonation of the ketenyl precursor [Mo 2 Cp 2 {μ-C(Ph)CO}(μ-PCy 2 )(CO) 2 ] with HBF 4 •OEt 2 in dichloromethane solution. The hydroxycarbyne complex also reacted with CN t Bu and CNXyl to give C−C coupled products, but different than anticipated: in both cases this reaction yielded selectively the corresponding aminoalkyne-bridged derivatives [Mo 2 Cp 2 {μ-η 2 :η 2 -C(NHR)CPh}(μ-PCy 2 )-(CNR) 2 ]BF 4 (Mo−Mo = 2.6525(5) Å when R = t Bu), as a result of H-transfer from hydroxycarbyne to isocyanide ligands and subsequent C−C coupling between aminocarbyne and benzylidyne ligands.