Investigations on Low-Valent Group 8 and 9 Metalloradicals

Tetradentate, monoanionic, tris(phosphino)silyl ligands were chelated to group 8 and 9 transition metals to stabilize complexes with unusual oxidation states and/or geometries. Initial studies with the [SiPPh3]− ligand on ruthenium established the flexibility of this ancillary ligand in stabilizing complexes with strongly trans influencing ligands in trans dispositions. A related ligand scaffold, [SiPiPr3]−, was subsequently used to stabilize mononuclear complexes of Ru(I) and Os(I), the first examples to be isolated and thoroughly chracterized. EPR spectroscopy and DFT calculations supported their metalloradical character, and further studies highlighted their reactivity in both one- and two-electron redox processes. The ability of the [SiPiPr3]− scaffold to stabilize d7 metalloradicals of group 8 metals was extended to group 9 metals, and a series of d7 complexes of cobalt, rhodium, and iridium were synthesized in which their ancillary ligands, oxidation states, spin states, and geometry are conserved. Similar to the previously reported [SiPiPr3]Fe(N2) complex, the related [SiPiPr3]Ru(N2) complex was shown to exhibit N−N coupling of organic azides to yield azoarenes catalytically. Detailed mechanistic studies conclusively showed that the Ru(III) imide species, whose iron analog is the key intermediate in the [SiPiPr3]Fe system, is not involved in the mechanism for the [SiPiPr3]Ru system. Instead, a mechanism in which free nitrene is released during the catalytic cyle is favored. Finally, hybrid ligands with multiple thioether donors in place of phosphine donors on the [SiPR3]− scaffold were synthesized to stabilize a number of dinitrogen complex of iron. These complexes featured rare examples of S−Fe−N2 linkages.