Formation of sub-valent carbenoid ligands by metal-mediated dehydrogenation chemistry: coordination and activation of H2Ga{(NDippCMe)2CH}

Coordination and Activation of EH Bonds (E=B, Al, Ga) at Transition Metal Centers

“…Thus, a range of κ 2 -gallane complexes of the type M(CO) 4 (κ 2 -H 3 GaÁNHC) (where M ¼ Cr, Mo, and NHC ¼ 6-Dipp, 6-Mes) can be synthesized from M(CO) 4 (cod) in a manner analogous to the corresponding alanes and (like their aluminium counterparts) can be shown to be inert toward loss of H 2 (see Scheme 18). 49d By contrast, the reactions of the β-diketiminato-stabilized gallium dihydride {HC(MeCDippN) 2 }GaH 2 with a range of mono-and dinuclear metal carbonyl reagents demonstrate, perhaps more clearly than any other systems, the propensity for gallanes to lose dihydrogen and form 18 From a spectroscopic viewpoint, σ-complex 84 displays two CO stretching bands (1951/1886 cm À1 ), similar to those reported for 70 (1947/1879 cm À1 ), implying similar donor characteristics for the alane and gallane ligand (a finding also reflected in the very similar geometric structures of the two systems). 18,49a The corresponding bands for 85 are markedly red-shifted (1903/1837 cm À1 ), however, consistent with the stronger σ-donor capabilities of the gallylene ligand.…”

“…This mechanism is very similar to that proposed for the analogous reaction with Fe(CO) 5 , which generates the closely related (albeit mononuclear) iron gallylene complex Fe(CO) 4 Ga{(NDippCMe) 2 CH} (90). 18 In the case of Mn 2 (CO) 10 , however, the corresponding activation reaction at [Mn 2 (CO) 9 ] would generate a more sterically encumbered metal center in the initial step, and MndH reductive elimination is presumably therefore favored over H 2 loss, leading to the observed formation of HMn(CO) 5 and the monometallic manganese gallylene complex 88. Finally, it is of note that Himmel and coworkers have recently reported the catalytic dehydrogenation of an amine gallane with bicyclic guanidine ligands using an iridium-based catalyst {η 3 -1,3-(OP t Bu 2 ) 2 C 6 H 3 }IrH 2 .…”

“…Thus, Cr(CO) 4 [κ 2 -H 2 Ga{(NDippCMe) 2 CH}] (86) is the only simple κ 2 adduct of the Nacnac-stabilized gallane which can be trapped, albeit as a cocrystallite with the (dehydrogenated) gallylene system Cr(CO) 5 [Ga{(NDippCMe) 2 4 Ga {(NDippCMe) 2 CH} (89), obtained from the reactions with Co 2 (CO) 8 and Mn 2 (CO) 10 , two alternative dehydrogenation pathways are revealed, with either H 2 or MdH bonds acting as the ultimate hydrogen sink (Scheme 24). 18 In the case of the cobalt system 89, the reaction is thought (on the basis of computational studies) to proceed via extremely facile GadH oxidative addition to one metal center of the photolytically generated [Co 2 (CO) 7 ] fragment. This yields a (hydrido)cobalt gallyl complex with a single hydrogen atom attached to both cobalt and gallium atoms.…”

“…15 However, with the +1 oxidation state more readily accessible for gallium (and the GadH bond being inherently weaker than its lighter congeners), 3,17 the potential for novel and interesting chemistry initiated by the dehydrogenation of Ga(III) hydride complexes at transition metal centers is not only foreseeable but beginning to be realized in practice. 15,18 This chapter will primarily focus on reviewing the coordination and activation of AldH and GadH bonds at transition metal centers, making reference to key examples of related BdH σ-complexes in order to put fundamental issues of electronic structure and bonding into appropriate context. In the interests of space, and with a view to comparing the intrinsic electronic/geometric properties of the coordinated σ-bond, "tethered" systems in which the coordinated EdH bond forms part of an existing metal-bound ligand are not as a rule included.…”

“…2.682(3) Å for the related κ 1 -(H 3 BÁNMe 3 ) complex 9], 15a,25 although still longer, for example, than the direct (unsupported) MndGa bond found in Cp 0 Mn(CO) 2 [Ga {(NDippCMe) 2 CH}] [85, 2.311(2) Å , vide infra] 18. It does appear therefore that the gallane complex is further along the oxidative addition pathway than Scheme 21 Thermal and photolytic routes to (OC) 5 W(κ 1 -H 3 GaÁquin), 80.…”