Structural and Chemical Effects of the P^tBu₂Bridge at Unsaturated Dimolybdenum Complexes Having Hydride and Hydrocarbyl Ligands

Abstract

A high-yield synthetic route for the preparation of the unsaturated anion [MoCp(μ-PBu)(μ-CO)] (2) was implemented, via two-electron reduction of the chloride complex [MoCp(μ-Cl)(μ-PBu)(CO)] (1). Reaction of 2 with [NH][PF] led to the formation of the 30-electron complex [MoCp(H)(μ-PBu)(CO)] (3), in which the hydride ligand adopts an uncommon terminal disposition. DFT analysis of the electronic structure of 3 gave support to the presence of a M≡M triple bond in this complex following from a σδδ configuration, a view also supported by the high electron density accumulated at the corresponding Mo-Mo bond critical point. In contrast, reactions of 2 with IMe or ClCHPh gave the alkyl-bridged complexes [MoCp(μ-κ:η-CHR)(μ-PBu)(CO)] (R = H (4a), Ph (4b)), which in solution display agostic Mo-H-C interactions. Decarbonylation of 4a took place rapidly under photochemical conditions to give the 30-electron complex [MoCp(μ-κ:η-CH)(μ-PBu)(μ-CO)] (7), with a stronger agostic coordination of its methyl ligand. In contrast, irradiation of 4b led to the formation of the benzylidyne derivative [MoCp(μ-CPh)(μ-PBu)(μ-CO)] (9), following from fast decarbonylation and dehydrogenation of the bridging benzyl ligand. Low-temperature photochemistry allowed for the NMR characterization of an intermediate preceding the hydrogen elimination, identified as the carbene hydride [MoCp(H)(μ-CHPh)(μ-PBu)(CO)] (10), a product which evolves slowly by H elimination to the benzylidyne derivative. Analogous dehydrogenation of the methyl ligand in 7 could be accomplished upon moderate heating, to yield the corresponding methylidyne derivative [MoCp(μ-CH)(μ-PBu)(μ-CO)] (9). A complete reaction mechanism accounting for these photochemical reactions was elaborated, based on the reaction intermediates identified experimentally and on extensive DFT calculations. Surprisingly, for both systems the C-H bond activation steps are relatively easy thermal processes occurring with modest activation energies after photochemical ejection of CO, with a rate-determining step involving the formation of agostic carbenes requiring also a strong structural reorganization of the central MoPC rings of these molecules.

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

“…It was then of interest to examine the reactivity of related agostic complexes at even more unsaturated centres, as in the case of the monocarbonyl derivative [Mo 2 Cp 2 (μ‐κ 1 :η 2 ‐CH 3 )(μ‐PCy 2 )(μ‐CO)], which features an agostic methyl ligand bridging over an intermetallic triple bond, but these studies were hampered by the high air‐sensitivity of this molecule. Recently, however, we found that the stability of this type of complex could be significantly improved by using the bulkier P t Bu 2 ligand as support of the dimetal centre . Indeed we found that the targeted methyl complex [Mo 2 Cp 2 (μ‐κ 1 :η 2 ‐CH 3 )(μ‐P t Bu 2 )(μ‐CO)] ( 1 ) could be conveniently prepared in situ upon photolysis of the corresponding dicarbonyl precursor [Mo 2 Cp 2 (μ‐κ 1 :η 2 ‐CH 3 )(μ‐P t Bu 2 )(CO) 2 ] (Scheme ), and this gave us the opportunity to explore in detail its chemical behaviour.…”

“…Methane elimination again takes place in this case, but also the oxidative addition of the second Si−H bond, to give the hydride silylene derivative [Mo 2 Cp 2 H(μ‐P t Bu 2 )(μ‐SiPh 2 )(CO)] ( 2 ) in high yield . Spectroscopic data for 2 denote the presence of a terminal hydride ( δ H =−5.20 ppm), a semi‐bridging carbonyl ( (CO)=1776 cm −1 , δ C =282 ppm) and a bridging silylene ligand ( δ Si =214 ppm), and suggest that the structure of this complex is related to that of the hydride benzylidene complex [Mo 2 Cp 2 H(μ‐P t Bu 2 )(μ‐CHPh)(CO)], an intermediate species detected in the photochemical dehydrogenation of the agostic benzyl complex [Mo 2 Cp 2 (μ‐κ 1 :η 2 ‐CH 2 Ph)(μ‐P t Bu 2 )(CO) 2 ] and studied by DFT methods . Interestingly, compound 2 undergoes a fluxional process, fast on the NMR timescale, that renders equivalent metal centres down to 173 K, likely involving the reversible formation and cleavage of a Si−H bond, which would be analogous to the process computed for the mentioned hydride benzylidene complex.…”

“…[11] Spectroscopic data for 2 denote the presence of at erminal hydride( d H = À5.20 ppm), as emi-bridging carbonyl (ñ(CO) = 1776 cm À1 , d C = 282 ppm) and abridging silylene ligand (d Si = 214 ppm),a nd suggest that the structure of this complex is related to that of the hydride benzylidene complex [Mo 2 Cp 2 H(m-PtBu 2 )(m-CHPh)(CO)],a ni ntermediate species detected in the photochemical dehydrogenation of the agostic benzylc omplex [Mo 2 Cp 2 (m-k 1 :h 2 -CH 2 Ph)(m-PtBu 2 )(CO) 2 ]a nd studied by DFT methods. [9] Interestingly,c ompound 2 undergoes af luxional process, fast on the NMR timescale, that renders equivalent metal centres down to 173 K, likelyi nvolving the reversible formation and cleavage of aS i ÀHb ond, which would be analogous to the process computed for the mentioned hydride benzylidene complex. We note that the Ru 2 complexes [Ru 2 Cp* 2 (m-H) 2 (m-SiRR')] (d Si 265-310 ppm), [12] seem to be the only other complexes identified previously as having silylene ligands bridging over (formally) triple intermetallic bonds.…”

“…Recently,h owever,w ef ound that the stability of this type of complex could be significantly improved by using the bulkier PtBu 2 ligand as support of the dimetal centre. [9] Indeed we found that the targeted methyl complex [Mo 2 Cp 2 (m-k 1 :h 2 -CH 3 )(m-PtBu 2 )(m-CO)] (1)c ould be conveniently prepared in situ upon photolysis of the corresponding dicarbonyl precursor [Mo 2 Cp 2 (m-k 1 :h 2 -CH 3 )(m-PtBu 2 )(CO) 2 ]( Scheme 1), and this gave us the opportunity to explore in detail its chemical behaviour. At the time, it was clear that compound 1 was significantly activatedw ith respect to CÀHb ond cleavage, sincei tu ndergoes clean intramolecular dehydrogenation upon gentle heating at 353 Kt oy ield the corresponding methylidyne derivative, in contrastt ot he behaviour of dicarbonyl complex[ Mo 2 Cp 2 (mk 1 :h 2 -CH 3 )(m-PCy 2 )(CO) 2 ], which only yielded methylidyned erivatives under photochemical activation.…”

Reply to the ‘Comment on “Hydride, gold(i) and related derivatives of the unsaturated ditungsten anion [W2Cp2(μ-PCy2)(μ-CO)2]−”’ by M. Green, Dalton Transactions, 2018, 47, DOI: 10.1039/C8DT00044A

“…Isomer 1T coexists in solution with the hydride-bridged isomer when M = W and R = Cy, 3,4 while it is fully dominant when M = Mo and R = t Bu. 5 The representation of the intermetallic interaction for this second isomer is a triple bond under all schemes, including the CBC method.…”