Dehydrogenation, Methyl Elimination and Insertion Reactions of the Agostic Methyl‐Bridged Complex [Mo₂Cp₂(μ‐κ¹:η²‐CH₃)(μ‐PtBu₂)(μ‐CO)]

Abstract

The high unsaturation of the title complex enabled it to react with a wide variety of molecules under mild conditions, whereby the agostic methyl ligand underwent unusual or unprecedented processes. Methane elimination occurred in the reactions with PPh H and SiPh H , this being followed in the latter case by Si-H bond oxidative addition to give the hydride silylene derivative [Mo Cp H(μ-PtBu )(μ-SiPh )(CO)]. Dehydrogenation, however, was the dominant process in the room temperature reaction with [Fe (CO) ], to give the unsaturated methylidyne cluster [Mo FeCp (μ -CH)(μ-PtBu )(CO) ] (Mo-Mo=2.6770(8) Å). In contrast, PMe elimination took place in the reaction with P , to give the unsaturated triphosphorus complex [Mo Cp (μ-η :η -P )(μ-PtBu )] (Mo-Mo=2.6221(3) Å). Yet a most remarkable reaction occurred with BH ⋅THF, involving insertion of two BH units and dehydrogenation to yield [Mo Cp (μ-B H Me)(μ-PtBu )(CO)], with the novel methyldiboranyl ligand acting as a 5-electron donor due to the presence of two 3-centre, 2-electron B-H-Mo interactions, according to spectroscopic data and DFT calculations (Mo-Mo ca. 2.65 Å).

Electronic Structure and Donor Ability of an Unsaturated Triphosphorus-Bridged Dimolybdenum Complex

“…The structure of 2 in the crystal (Figure ) was determined during our preliminary study of the reactivity of the methyl complex 1 . The acyclic P 3 ligand bridges symmetrically the dimetal center, with the external P atoms being tightly bound to the Mo atoms, as judged from the short Mo–P distances of ca.…”

“…The formation of 2 formally results from elimination of methylphosphinidene (“PMe”) between 1 and P 4 , a rare process itself for which we cannot quote a precedent, and decarbonylation; unfortunately, NMR analysis of the crude reaction mixture did not enable us to determine the fate of this unstable phosphinidene. As for the formation of 3 , first unnoticed in our preliminary study of this reaction, one might be tempted to think it as derived from reaction of white phosphorus with the dicarbonyl complex [Mo 2 Cp 2 (μ-κ 1 :η 2 -CH 3 )­(μ-P t Bu 2 )­(CO) 2 ], which is the actual precursor of compound 1 , therefore a potential contaminant of the starting material in this reaction. However, separated experiments revealed that the above dicarbonyl complex does not react with P 4 at 333 K. It did it, however, in toluene solution at ca.…”

“…ORTEP diagram (30% probability) of compound 2 , with t Bu (except their C 1 atoms) and H atoms omitted for clarity . Selected bond lengths (Å) and angles (deg): Mo1–Mo2 = 2.6221(3); Mo1–P1 = 2.3952(8); Mo2–P1 = 2.3952(8); Mo1–P2 = 2.6308(8); Mo2–P2 = 2.6255(8); Mo1–P3 = 2.3988(8); Mo2–P3 = 2.3909(8); Mo1–P4 = 2.4224(7); Mo2–P4 = 2.4131(7); P1–P2 = 2.1556(12); P2–P3 = 2.1442(12).…”

“… a NMR data recorded in C 6 D 6 solution at 121.48 MHz and 293 K, with chemical shifts (δ) in ppm relative to external 85% aqueous H 3 PO 4 , and P–P couplings ( J PP ) in hertz. Labels according to the figure shown above (E = electrophile). b Data taken from ref . c In tetrahydrofuran solution; averaged resonance for P 1 and P 2 atoms; see text. d Resonance for the PH 2 ligand. e In CD 2 Cl 2 solution. f Assignment of the P 1 and P 3 resonances might be exchanged; see text. g In toluene solution. …”

“…Recently, we found that the unsaturated methyl-bridged complex [Mo 2 Cp 2 (μ-κ 1 :η 2 -CH 3 )­(μ-P t Bu 2 )­(μ-CO)] ( 1 ) reacted selectively with P 4 under relatively mild conditions (333 K) to give the triphosphorus-bridged derivative [Mo 2 Cp 2 (μ-η 3 :η 3 -P 3 )­(μ-P t Bu 2 )] ( 2 ) in good yield, in a process formally involving the elimination of methylphosphinidene (PMe) . There are two main points of interest concerning this compound: in the first place, we note that 2 is a rare example of a complex bearing a noncyclic P 3 ligand bridging a dimetal center in a symmetrical η 3 :η 3 mode.…”