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 paramagnetic, and data obtained by MS spectrometry revealed masses corresponding with both monomeric and dimeric nitrido/nitridyl complexes. NMR only reveals broad uncharacteristic signals and the complex is EPR silent at 8K or above. 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.