Ruthenium-cobalt mixed-metal nitrido and nitrene carbonyl clusters: Structure, reactivity, and 15N NMR spectroscopy

Abstract

The reaction of [Ru3(μ-H)2(CO)9(μ3-NOMe)] (1) with [Cp* Co(CO)2] (Cp* = η5-C5Me5) in refluxing THF afforded the new clusters [Ru3Co(μ-H)(CO)9(η5-C5 Me5)(μ4-N)] (2), [Ru2Co(CO)6(μ3-CO)(η5-C 5Me5)(μ3-NH) (3), [Ru5(μ-H)(CO)9(μ-CO)3(η 5-C5Me5)(μ4-NH)] (4), and [Ru6Co(μ3-H)(CO)8(μ-CO)3 (μ4-η2-CO)(η5-C5Me 5)3(μ4-N)] (5). Clusters 2 and 3 have typical butterfly nitrido and triangular nitrene structures, whilst compound 4 is a novel square-pyramidal pentaruthenium μ4-NH carbonyl cluster with a Cp* ligand transferred from the external mononuclear Co complex. Cluster 4 can also be obtained by the reaction of 1 with pentamethylcyclopentadiene in the presence of 1,3-cyclohexadiene, albeit in low yield. Compound 5 consists of a rare Ru6Co metal framework where a basal Ru-Ru edge of the central Ru4 tetrahedron is capped by a Cp*Co fragment and another edge is bridged by a diruthenium unit. Complex 2 was found to be converted into 3 upon deprotonation with 1,8-diazabicyclo[5.4.0]undec-7-ene, on refluxing with [Cp*Co(CO)2], or on exhaustive electrolysis. Upon thermolysis of 2 with PhC2Ph in n-octane, [Ru3Co(CO)6(μ-CO)2(η5-C 5Me5)(μ4-NH)(μ4-η 2-PhC2Ph)] (6) and [Ru3Co(CO)5(μ-CO)2(η5-C 5Me5){μ4-η3-NC(O)C 2(C4H4)CHCH(Ph)}(μ4-η 2-PhC2Ph)] (7) were obtained in moderate yields. The structures of 6 and 7 both consist of a slightly twisted Ru3Co square base with a quadruply bridging PhC2Ph ligand. Cluster 6 is capped by a μ-NH moiety on the opposite side, while in 7 a CO ligand and an ortho-carbon atom of a Ph group from another PhC2Ph ligand interact with the μ4-N nitrido atom. Molecular orbital calculations have been carried out in order to gain further insight into the electronic structures of these interesting square-planar Ru3Co and Ru4 clusters. The butterfly Ru2Co2 nitrido complex [Ru2Co2(CO)9(η5-C 5Me5)(μ4-N)] (8) was isolated from the reaction of [Ru3Co(CO)12(μ4-N)] with [Cp*Co(CO)2] in refluxing THF as a result of metal exchange of an Ru(CO)3 fragment by a Cp*Co moiety. The redox properties of the compounds described herein have been investigated by means of cyclic voltammetry and controlled potential coulometry. Cluster 6 was shown to be able to tolerate both the addition of two electrons and the removal of one electron without significant structural change. The chemical environments of the nitrogen atoms of these clusters have been investigated by analysis of their 15N NMR chemical shifts.