1,2-eq,eq-[Re2(CO)8(THF)2]: A Reactive Re2(CO)8Fragment That Easily Activates H−H and C−H Bonds

The reaction of [Re 2 (μ-H) 2 (CO) 8 ] ( 1 ) with diazomethane at 193 K in THF- d 8 gives the unstable [Re 2 (μ-H)(μ-CH 3 )(CO) 8 ] derivative ( 2 ) containing a methyl group arising from the insertion of CH 2 into a Re−H−Re bond. Isotopic perturbation of the equilibria by partial deuteration demonstrated that the methyl bridges the Re−Re bond in an unsymmetrical way, with a fast exchange between one agostic and two terminal C−H bonds. At temperatures higher than 253 K, 2 decomposes, in THF solution, with CH 4 elimination to give the novel red complex 1,2- eq,eq -[Re 2 (CO) 8 (THF) 2 ] ( 3 ), which was characterized by NMR spectroscopy and X-ray analysis. In the solid form a staggered conformer of C 2 symmetry was found. 13 C NMR analysis revealed the presence, in wet THF, of the aquo complexes [Re 2 (CO) 8 (THF)(H 2 O)] ( 4 ) and [Re 2 (CO) 8 (H 2 O) 2 ] ( 5 ), whose formation is favored at low temperature (Δ H ° for the formation of 5 from 4: −14.4(2) kJ mol - 1 ). In solution, due to the lability of the THF ligands, 3 behaves as a “lightly stabilized” Re 2 (CO) 8 fragment, capable of activating different E−H bonds. Reaction with HCl in THF leads to [Re 2 (μ-H)(μ-Cl)(CO) 8 ], while with H 2 the unsaturated starting material [Re 2 (μ-H) 2 (CO) 8 ] is obtained. In THF solution, at room temperature, reactions with phenylacetylene, styrene, and acetaldehyde give the derivatives of C−H activation [Re 2 (μ-H)(μ-C⋮CPh)(CO) 8 ], [Re 2 (μ-H)(μ-CH C(H)Ph)(CO) 8 ], and [Re 2 (μ-H)(μ-η 2 -C(Me)O)(CO) 8 ]. Moreover, the activation of an sp 3 C−H bond in ethyl acetate occurs slowly when 3 is dissolved in the reactant itself, the resultant product being [Re 2 (μ-H)(μ-η 2 -CH 2 C(O)OEt)(CO) 8 ].