Reactivity of a trans-[H–MoMo–H] unit towards alkenes and alkynes: bimetallic migratory insertion, H-elimination and other reactions

Abstract

Complex [Mo2(H)2{μ-HC(NDipp)2}2(THF)2], (1·THF), reacts with C2H4 and PhCH[double bond, length as m-dash]CH2 to afford hydrido-hydrocarbyl and bis(hydrocarbyl) derivatives of the Mo[quadruple bond, length as m-dash]Mo bond. Reversible migratory insertion and β-hydrogen elimination, as well as reductive elimination and other reactions, have been uncovered. PhC[triple bond, length as m-dash]CH behaves instead as a Brönsted-Lowry acid towards the strongly basic Mo-H bonds of 1·THF.

Experimental and Computational Studies on Quadruply Bonded Dimolybdenum Complexes with Terminal and Bridging Hydride Ligands

“…Indeed, the H atoms of the terminal methyl group of the hexane molecules establish short contacts (2.7–3.0 Å) with the methyl groups of the i Pr groups of the aryl rings. When the ligands in the trans complexes become bulkier, for example, X=Me [27] or Et, [37] the aryl rings become progressively rotated (8 and 18°, respectively). In a case with two different ligands (X=vinyl, Et), [37] the two aryl groups close to the vinyl ligand are slightly less rotated than those on the ethyl side, 10(4) and 14(1)°, respectively.…”

“…Similarly, the structure of [Mo 2 (μ‐Ad Dipp2 ) 2 (H) 2 (thf) 2 ] (both calculated and experimental [35] ) and the X‐ray structure of [Mo 2 (μ‐Ad Dipp2 ) 2 (H)(μ‐H)(PMe 3 )] present rotation angles of 20 and 24°, associated with thf and PMe 3 , respectively. Another example has X=C≡CPh, L=thf [37] . In this case, the sense of the rotation of the aryls (by around 16°) is such that they get away from the thf while approaching the acetylide in trans , clearly telling us that C≡CPh has much less steric hindrance than thf.…”

Supported σ‐Complexes of Li−C Bonds from Coordination of Monomeric Molecules of LiCH3, LiCH2CH3 and LiC6H5 to Mo≣Mo Bonds

“…[37][38][39] Multiply bonded dimolybdenum complexes are no exception, as exemplified by Chisholm's triply bonded [Mo 2 (CH 2 CH 3 ) 2 (NMe 2 ) 4 ], [40] and also by the closely related Mo� � Mo compound [Mo 2 (CH 2 CH 3 ) 2 (μ-Ad Dipp2 ) 2 ], that at room temperature undergoes β-H elimination within minutes. [41] We suggest that the reluctance of the above complexes to experience β-H elimination is due to the difficulty encountered by the β hydrogen atoms to approach the Mo atom, as a result of the rigidity of the five-membered HÀ MoÀ LiÀ CH 2 (CH 3 )À Mo metallacycle. Though as solids the new compounds can be manipulated at room temperature, storage at À 20 °C is advisable.…”

“…. [41] With reference to the Mo 2 Li skeleton of the Mo 2 Li(C)(H) rings, the orientation of the alkyl groups in complexes 4•LiCH 2 R and 5•LiCH 2 R (R = H, CH 3 ) relative to the pertinent MoÀ Li edge of the triangle is such that the sp 3 hybrid orbital available for bonding to the MoÀ Li pair (represented by arrows in Figure S6a in the SI) is directed to that bond but much closer to the molybdenum than to the lithium atom. This fact can be attributed to the higher covalent character of the MoÀ C bond compared to the LiÀ C bond.…”

Supported σ‐Complexes of Li−C Bonds from Coordination of Monomeric Molecules of LiCH3, LiCH2CH3 and LiC6H5 to Mo≣Mo Bonds

“…The stability of the LiCH 2 CH 3 complexes 4⋅LiCH 2 CH 3 ⋅O 2 CH and 5⋅LiCH 2 CH 3 is additionally surprising considering the well‐known tendency of M−CH 2 CH 3 complexes to undergo β‐H elimination [37–39] . Multiply bonded dimolybdenum complexes are no exception, as exemplified by Chisholm's triply bonded [Mo 2 (CH 2 CH 3 ) 2 (NMe 2 ) 4 ], [40] and also by the closely related Mo≣Mo compound [Mo 2 (CH 2 CH 3 ) 2 (μ‐Ad Dipp2 ) 2 ], that at room temperature undergoes β‐H elimination within minutes [41] . We suggest that the reluctance of the above complexes to experience β‐H elimination is due to the difficulty encountered by the β hydrogen atoms to approach the Mo atom, as a result of the rigidity of the five‐membered H−Mo−Li−CH 2 (CH 3 )−Mo metallacycle.…”