Photochemical synthesis and structural characterization of (η 5-C 5Me 5) 3Mo 3(CO) 4(μ 2-H)(μ 3-O): an unprecedented 46-electron trimolybdenum cluster containing a localized monoprotonated MoMo double bond

Abstract

Irradiation of the 30-electron Mo 2(η 5-C 5Me 5) 2(CO) 4 and Re 2(CO) 10 in toluene solution (containing H 2O) afforded (in 1–2% yields) a novel triangular metal cluster, (η 5-C 5Me 5) 3Mo 3(CO) 4(η 2-H)(η 3-O) ( 1), which was characterized by a single-crystal X-ray diffraction study. Compound 1, of pseudo C s - m symmetry, has a triangulo-Mo 3(η 3-O) core with composite MoHMo and MoMo electron-pair bonds along one unusually short edge (2.660(1) Å) and Mo electron-pair bonds along the other two edges (2.916(1) and 2.917(1) Å). The edge-bridged hydride ligand, which displays a characteristic high-field proton NMR resonance at δ −17.79 ppm, was not found from the crystallographic determination but was located via a quantitative potential-energy-minimization method. This procedure unambiguously established that the optimized hydrogen position, which corresponds to a distinct coordination site with identical MoH distances of 1.85 Å, is the only one that can be sterically occupied by a metal-bound hydride ligand. This 46-electron species is the first electron-deficient trimolybdenum cluster containing a monoprotonated MoMo double bond; its existence is attributed to ligand overcrowding due to the bulky pentamethylcyclopentadienyl rings. Black (η 5- C 5Me 5) 3Mo 3(CO) 4(η 2-H)(η 3-O) · 1/2THF crystallizes with two formula species in a triclinic unit cell of P1 symmetry with a 8.603(4), b 11.115(4), c 19.412(11) Å, α 80.69(4)°, β 101.10(4)°, and γ 98.88(3)° at −40° C. Least-squares refinement (RAELS with 221 variables) of one independent Mo 3 molecule and a centrosymmetrically-disordered THF molecule converged at R 1( F) 5.62%, R 2( F 6.88% for 8460 independent diffractometry data ( I 0 ρ 3σ( I 0 collected at −40° C with Mo- K α radiation