Carbonyl−Metal Clusters with Mixed O,N-Donor Ligands:  Reactivity of the Ureato Cluster [Ru₃(μ-H)(μ₃-HNCONMe₂)(CO)₉] with Phosphines. Structural Characterization of a Triphenylphosphine Derivative and of a Bis(diphenylphosphido) Derivative Which Also Contains a Bridging η¹-Phenyl Ligand

The ureato-bridged carbonyl cluster [Ru₃(μ-H)(μ₃-HNCONMe₂)(CO)₉] (1) (H₂NCONMe₂ = N,N-dimethylurea) reacts with 1 equiv of triphenylphosphine or diphenylphosphine to give the isostructural derivatives [Ru₃(μ-H)(μ₃-HNCONMe₂)(L)(CO)₈] (L = PPh₃, PPh₂H). The X-ray structure of [Ru₃(μ-H)(μ₃-HNCONMe₂)(PPh₃)(CO)₈] indicates that the substitution has taken place on an equatorial position cis to the bridging NH fragment. Treatment of 1 with bis(diphenylphosphino)methane (dppm) affords a mixture of the symmetric (C_s) and asymmetric (C₁) isomers of [Ru₃(μ-H)(μ₃-HNCONMe₂)(μ-dppm)(CO)₇]. All these reactions take place at room temperature. The phenyl-bridged bis(diphenylphosphido) derivative [Ru₃(μ-η¹-Ph)(μ₃-HNCONMe₂)(μ-PPh₂)₂(CO)₆] has been prepared by reaction of 1 with 2 equiv of PPh₃ in the presence of alumina and has been characterized by X-ray diffraction methods. The thermolysis of [Ru₃(μ-H)(μ₃-HNCONMe₂)(PPh₂H)(CO)₈] in refluxing THF leads to the phosphido-bridged derivatives [Ru₃(μ₃-HNCONMe₂)(μ-PPh₂)(μ-CO)₂(CO)₆] and [Ru₃(μ-H)(μ₃-HNCONMe₂)(μ-PPh₂)₂(CO)₆]. It has been observed that carbonyl substitution reactions on compound 1 are easier than those previously reported on clusters containing N- and/or P-donor bridging ligands. This seems to be related to the hard−soft character of the bridging ligand:  the harder the ligand the more reactive is the cluster in carbonyl substitution reactions.