Trinuclear nickel cluster chemistry: The development of new capping ligands with emphasis on cooperativity between ligand and cluster and the variation of bridging bisphosphines

In previous studies, trinuclear nickel clusters of the type [Ni 3(dppm)3(μ3-I)(μ3-X)] n+ (X = I–, n = 0; X = CNR, n = 1; dppm = bis(diphenylphosphino)methane) were shown to be electrocatalysts for the reduction of CO2 to CO and CO32- where the CO2 binds through the C atom to a Ni. In order to design a cluster in which the oxygen atom of CO2 is engaged to suppress the disproportionation reaction, a cluster with two active sites was developed, and tin was chosen due to its high electrophilicty. This work presents the first examples of a tin capped trinuclear nickel cluster. The reaction of [Ni3(dppm)3(μ 3-I)2] with SnCl2 and NaCl gave [Ni3(dppm) 3(μ3-I)(μ3-SnCl3)] (1) in moderate yields. This 48e– cluster is capped with a stannane fragment, similar to the R3C– fragment. The structure of the reduced stannane cluster was shown by cyclic voltammetry (CV) to be [Ni3(dppm)3(μ3-I)(μ 3-SnCl2)]. The dichlorotin, or stannylene, capped trimer [Ni3(dppm)3(μ3-I)(μ3-SnCl 2)](PF6) was synthesized by the reaction of 1 with Tl(PF6). 1 was reduced with 2 equivalents of Na metal yielding [Ni3(dppm)3(μ3-I)(μ 3-SnCl)], and when reduced in the presence of CO2, a white precipitate was observed. It was determined to be C2O4 2–. The disproportionation reaction has been suppressed, and the Ni and Sn appear to work in cooperation with each other. To determine the effects different bisphosphines would have on the properties the trinuclear nickel clusters, dppm was substituted with depm [bis(diethylphosphino)methane]. Unlike the dppm analog, [Ni3(depm)3(μ3-I) 2]+ (4) was stable only as the 1 + cluster, and the cationic cluster will undergo substitution of one iodides with a π-acid, like CO (5) and CNMe (5), and with SnCl3– to form [Ni3(depm) 3(μ3-I)(μ3-SnCl3)] (7). The depm clusters showed an upfield shift in the 31P NMR of at least 3 ppm over the dppm-bridged trimers, and the reduction potential of the trimers increased 300 mV to a more negative potential. When 4 is reduced in the presence of CO2 5 was observed both in the CV and in the spectroelectrochemistry (SEC) experiment