A Study of Unsaturated Pyrazolate-Bridged Diruthenium Carbonyl Complexes

The reaction of Ru₃(CO)₁₂ with approximately 3 equiv of 3,5-di-tert-butylpyrazole, (dbpz)H, in hexane at 170 °C in a stainless steel autoclave afforded the unsaturated, pyrazolate-bridged diruthenium complex [Ru₂(CO)₅(dbpz)₂] (2). The spectroscopic and structural analysis indicated that this compound contains a vacant coordination site at one axial position trans to the Ru−Ru linkage. This compound reacted with CO at room temperature to give the saturated complex [Ru₂(CO)₆(dbpz)₂] (1b); its structure is analogous to that of the previously determined complexes [Ru₂(CO)₆(pz)₂], pz = pyrazolate and 3,5-dimethyl pyrazolate, except that the equatorial CO ligands show a skew arrangement, an indication of steric constraints within the coordination sphere. Treatment of 2 with benzyl isocyanide or with pyridine in toluene leads to the formation of substitution products [Ru₂(CO)₄(L)(dbpz)₂], L = CNCH₂Ph and pyridine, in which the incoming donor ligand occupies an equatorial position at the saturated Ru center. The structural significance of this pyrazolate-bridged diruthenium complex 2 is discussed.