Steric Protection of Rhodium-Nitridyl Radical Species

28 May 2019, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

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.

Keywords

bulky ligand
rhodium
azide complexes
photolysis
nitridyl

Supplementary materials

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