Steric Protection of Rhodium-Nitridyl Radical Species

In an attempt to synthesize a mononuclear rhodium nitridyl complex with a reduced tendency to undergo nitridyl radical N-N coupling we synthesized a bulky analog of Milstein’s bipyridine-based PNNH ligand, bearing a tert-butyl group at the 6’ position of the bipyridine moiety. A three-step synthetic route toward this new bulky tBu₃PNNH ligand was developed, involving a selective nucleophilic substitution step, followed by a Stille coupling and a final hydrophosphination step to afford the desired 6-(tert-butyl)-6'-((di-tert-butylphosphino)methyl)-2,2'-bipyridine (tBu₃PNNH) ligand. This newly developed tBu₃PNNH ligand was incorporated in the synthesis of the sterically protected azide complex [Rh(N₃)(tBu₃PNNH)]. We explored N₂ elimination form this species using photolysis and thermolysis, hoping to synthesize a mononuclear rhodium complex with a terminal nitrido moiety. Characterization of the reaction product(s) using NMR, coldspray HR-ESI-MS and EPR spectroscopy shows that the material is both EPR and NMR silent, and data obtained by MS spectrometry revealed masses corresponding with both monomeric and dimeric nitrido/nitridyl complexes. The combined data point to formation of a paramagnetic [(tBu₃PNN)Rh(µ-N)Rh(tBu₃PNN)] species. It thus seems that despite its three tBu groups the new ligand is not bulky enough to prevent formation of Rh-N-Rh bridged species. However, the increased steric environment does prevent further reaction with carbon monoxide, which is unable to coordinate to rhodium.