A study of unsaturated pyrazolate-bridged diruthenium carbonyl complexes

The reaction of Ru-3(CO)(12) with approximately 3 equiv of 3,5-di-tert-butylpyrazole, (dbpz)H, in hexane at 170 degreesC in a stainless steel autoclave afforded the unsaturated, pyrazolate-bridged diruthenium. complex [Ru-2(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 [Ru-2(CO)(6)(dbpz)(2)] (1b); its structure is analogous to that of the previously determined complexes [Ru-2(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 [Ru-2(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.