N-heterocyclic carbene adducts of molybdenum tetra-carboxylate complexes

Treatment of the quadruply bonded dimolybdenum complexes [Mo2(μ-O2CCF3)4] (1), [Mo2(μ-O2CCH3)4] (2) and [Mo2(μ-O2CBut)4] (3) with the N-heterocyclic carbenes (NHCs), L (La = IPr, Lb =IMes, Lc = IiPr2Me2 and Ld = IEt2Me2), respectively, results in the formation of a series of 1:1 and 1:2 adducts: In the case of the larger, and more sterically demanding, carbene ligands i.e. La and Lb, coordination occurs exclusively axially with respect to the [Mo2] complexes and only once in the case La, [Mo2(μ-O2CR)4(La)] (R = Me, tBu and CF3) (1a, 2a and 3a), and twice in the case of Lb, [Mo2(μ-O2CCF3)4(Lb)2] (2b). For the less sterically demanding carbene ligands (Lc and Ld) coordination to the [Mo2] core occurs twice, with a transoidal dis-tribution about the [Mo2] unit and is exclusively in equatorial positions, resulting in partial displacement of two of the bridg-ing {μ-O2CR} ligands in each complex. In the case of complexes 1a, 2a, 3a, 2b, [Mo2(μ-OTFA)2(OTFA)2(Lc)2] (1c), [Mo2(μ-OTFA)2(OTFA)2(Ld)2] (1d) [Mo2(μ-OAc)2(OAc)2(Ld)2] (2d) the solid state molecular structures have been unambiguously characterised by single crystal X-ray diffraction. As part of our study into the reactivity of NHC ligands with [Mo2(μ-O2CCH3)4], the heteroleptic NHC adduct [Mo2(μ-OAc)2Cl2(Ld)2] (5) formed from the reaction of complex 2d with trimethylsi-lylchloride has also been isolated and structurally characterized using single crystal x-ray diffraction. As part of our study the Mo-Mo stretching frequencies of these complexes have been analyzed by Raman spectroscopy