M−P versus MM Bonds as Protonation Sites in the Organophosphide-Bridged Complexes [M₂Cp₂(μ-PR₂)(μ-PR‘₂)(CO)₂], (M = Mo, W; R, R‘ = Ph, Et, Cy)

Abstract

The unsaturated complexes [W2Cp2(mu-PR2)(mu-PR'2)(CO)2] (Cp = eta5-C5H5; R = R' = Ph, Et; R = Et, R' = Ph) react with HBF4.OEt2 at 243 K in dichloromethane solution to give the corresponding complexes [W2Cp2(H)(mu-PR2)(mu-PR'2)(CO)2]BF4, which contain a terminal hydride ligand. The latter rearrange at room temperature to give [W2Cp2(mu-H)(mu-PR2)(mu-PR'2)(CO)2]BF4, which display a bridging hydride and carbonyl ligands arranged parallel to each other (W-W = 2.7589(8) A when R = R' = Ph). This explains why the removal of a proton from the latter gives first the unstable isomer cis-[W2Cp2(mu-PPh2)2(CO)2]. The molybdenum complex [Mo2Cp2(mu-PPh2)2(CO)2] behaves similarly, and thus the thermally unstable new complexes [Mo2Cp2(H)(mu-PPh2)2(CO)2]BF4 and cis-[Mo2Cp2(mu-PPh2)2(CO)2] could be characterized. In contrast, related dimolybdenum complexes having electron-rich phosphide ligands behave differently. Thus, the complexes [Mo2Cp2(mu-PR2)2(CO)2] (R = Cy, Et) react with HBF4.OEt2 to give first the agostic type phosphine-bridged complexes [Mo2Cp2(mu-PR2)(mu-kappa2-HPR2)(CO)2]BF4 (Mo-Mo = 2.748(4) A for R = Cy). These complexes experience intramolecular exchange of the agostic H atom between the two inequivalent P positions and at room-temperature reach a proton-catalyzed equilibrium with their hydride-bridged tautomers [ratio agostic/hydride = 10 (R = Cy), 30 (R = Et)]. The mixed-phosphide complex [Mo2Cp2(mu-PCy2)(mu-PPh2)(CO)2] behaves similarly, except that protonation now occurs specifically at the dicyclohexylphosphide ligand [ratio agostic/hydride = 0.5]. The reaction of the agostic complex [Mo2Cp2(mu-PCy2)(mu-kappa2-HPCy2)(CO)2]BF4 with CN(t)Bu gave mono- or disubstituted hydride derivatives [Mo2Cp2(mu-H)(mu-PCy2)2(CO)2-x(CNtBu)x]BF4 (Mo-Mo = 2.7901(7) A for x = 1). The photochemical removal of a CO ligand from the agostic complex also gives a hydride derivative, the triply bonded complex [Mo2Cp2(H)(mu-PCy2)2(CO)]BF4 (Mo-Mo = 2.537(2) A). Protonation of [Mo2Cp2(mu-PCy2)2(mu-CO)] gives the hydroxycarbyne derivative [Mo2Cp2(mu-COH)(mu-PCy2)2]BF4, which does not transform into its hydride isomer.

Mild P–P Bond Cleavage in the Methyldiphosphenyl Complex [Mo2Cp2(μ-PCy2)(μ-κ2:κ2-P2Me)(CO)2] To Give Novel Phosphide-Bridged Trinuclear Derivatives

“…Compound 1 reacted readily at room temperature with stoichiometric amounts of [Fe 2 (CO) 9 ] to give the trinuclear phosphide methylphosphinidene complex [Mo 2 FeCp 2 ( 3 -P)(-PCy 2 )( 3 -PMe)(CO) 5 ] (2a), formally derived from addition of the 14-electron fragment Fe(CO) 3 to a MoP 2 face of the Mo 2 P 2 tetrahedral core of 1 with full scission of the PP bond (Scheme 2).…”

“…These reactions, however, proved to be rather complex and strongly dependent on stoichiometry and reaction conditions, and we here report on those reactions incorporating a single metal fragment, while those leading to tetranuclear derivatives of 1 will be reported separately. 9 As it will be shown below, all reactions incorporating a single metal-fragment eventually involve the cleavage of the PP bond in the diphosphenyl ligand of 1 under mild conditions, to yield derivatives containing phosphide and methylphosphinidene ligands, with the latter being sometimes de-coordinated along the process.…”

Bis(η7-cycloheptatrienyl)tri-μ-hydrido-dimolybdenum(0,I)

“…The Mo-Mo bond length of 2.5732 (4) Å is, as expected, shorter than the Mo-Mo bonds in monohydrido (Mo-H-Mo) complexes, where this distance lies in the range 3.4056 (5)-3.540 (1) Å (Petersen et al, 1981;Darensbourg et al, 1980;Lin et al, 1993). The monohydrido complex [Mo 2 (η 5 -C 5 H 5 ) 2 (µ-H)(SnPh 3 )(CO) 2 (PCy 2 H)] reported by Alvarez et al (2006) contains a formal Mo≡Mo bond with a length (2.5730 (6) Å) almost identical to that in the title complex. There is only one report of a dihydrido complex with a quadruply bonded (3) Å) unit (Jones et al, 1980).…”

“…For related literature, see: Alvarez et al (2006); Darensbourg et al (1980); Jones et al (1980); Lin et al (1993); Sü ss- Fink & Therrien (2007); Petersen et al (1981); Shima & Suzuki (2005); Tamm et al (2004Tamm et al ( , 2006. Table 1 Selected geometric parameters (Å , ).…”

The coordination chemistry of the proton

“…Protonation of the complex occurs at phosphorus and the corresponding P-H hydrogen atom is described as engaging in an agostic hydrogen interaction. 21 As we have protonated the 31 P-NMR probe directly, we can now monitor the electronic situation concerning the former Mo-P bond. The expectation is that protonation of phosphorus would result in a significant downfield shift of the phosphorus resonance.…”