An Unsaturated Four‐Coordinate Dimethyl Dimolybdenum Complex with a Molybdenum–Molybdenum Quadruple Bond

Abstract

We describe the synthesis and the molecular and electronic structures of the complex [Mo Me {μ-HC(NDipp) } ] (2; Dipp=2,6-iPr C H ), which contains a dimetallic core with an Mo-Mo quadruple bond and features uncommon four-coordinate geometry and has a fourteen-electron count for each molybdenum atom. The coordination polyhedron approaches a square pyramid, with one of the molybdenum atoms nearly co-planar with the basal square plane, in which the trans coordination position with respect to the Mo-Me bond is vacant. The other three sites are occupied by two trans nitrogen atoms of different amidinate ligands and the methyl group. The second Mo atom occupies the apex of the pyramid and forms an Mo-Mo bond of length 2.080(1) Å, consistent with a quadruple bond. Compound 2 reacts with tetrahydrofuran (THF) and trimethylphosphine to yield the mono-adducts [Mo Me(μ-Me){μ-HC(NDipp) } (L)] (3⋅THF and 3⋅PMe , respectively) with one terminal and one bridging methyl group. In contrast, 4-dimethylaminopyridine (dmap) forms the bis-adduct [Mo Me {μ-HC(NDipp) } (dmap) ] (4), with terminally coordinated methyl groups. Hydrogenolysis of complex 2 leads to the bis(hydride) [Mo H {μ-HC(NDipp) } (thf) ] (5⋅THF) with elimination of CH . Computational, kinetic, and mechanistic studies, which included the use of D and of complex 2 labelled with C (99 %) at the Mo-CH sites, supported the intermediacy of a methyl-hydride reactive species. A computational DFT analysis of the terminal and bridging coordination of the methyl groups to the Mo≣Mo core is also reported.

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

“…As mentioned above, complex 1⋅thf was previously prepared by our group for specific synthetic purposes [28] . It was obtained from [Mo 2 (μ‐OAc) 2 (μ‐Ad Dipp2 ) 2 ] and an excess of LiMe, followed by elimination of LiMe from the intermediate complex [Mo 2 (μ‐Me){(μ‐Me) 2 Li(thf)}(μ‐Ad Dipp2 ) 2 ] and hydrogenolysis of the resulting neutral dimethyl [Mo 2 (Me) 2 (μ‐Ad Dipp2 ) 2 (thf)] [27] . An overall 45 % yield was quoted.…”

“…Alike its dimethyl precursor [Mo 2 (Me) 2 (μ‐Ad Dipp2 ) 2 (thf)], [27] complex 1⋅thf underwent facile removal of the weakly bound molecules of tetrahydrofuran to generate the unsolvated dihydride [Mo 2 (H) 2 (μ‐Ad Dipp2 ) 2 ] ( 1 ), with terminal Mo−H bonds. Complex 1 was isolated as an extremely air‐sensitive, bright‐red crystalline solid.…”

“…To avoid molecular association and formation of oligomers, our ligands of choice were bulky aminopyridinates and amidinates, [26, 27] particularly the amidinate anion ArNC(H)NAr − , with the specific Ar group 2,6‐ i Pr 2 C 6 H 3 (Ar substituent and amidinate ligand will be, respectively, abbreviated as Dipp and Ad Dipp2 ). Thus, a few years ago we described some lithium trimethyl dimolybdenum(II) complexes [26] and related neutral dimethyl dimolybdenum(II) species [27] containing Ad Dipp2 ligands. These included the four‐coordinate, fourteen‐electron compound [Mo 2 (CH 3 ) 2 (μ‐Ad Dipp2 ) 2 ] [27] .…”

“…Thus, a few years ago we described some lithium trimethyl dimolybdenum(II) complexes [26] and related neutral dimethyl dimolybdenum(II) species [27] containing Ad Dipp2 ligands. These included the four‐coordinate, fourteen‐electron compound [Mo 2 (CH 3 ) 2 (μ‐Ad Dipp2 ) 2 ] [27] . Next, we considered enlarging these results to embrace analogous hydride complexes.…”

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

“…16 The structure of 2 is therefore similar to that found previously for the bis(methyl) complex analogue. 15 Complex 2 possesses poor thermal stability in solution, but in the presence of ethylene decomposition is retarded, allowing its crystallization from concentrated toluene solutions at 0°C. Figure 1 shows an ORTEP view of the molecules of 2 that contain a trans- [Et-Mo≣Mo-Et] We next centred attention on the nature of the green intermediate A.…”