Solution Dynamics of Redox Noninnocent Nitrosoarene Ligands: Mapping the Electronic Criteria for the Formation of Persistent Metal-Coordinated Nitroxide Radicals.

Abstract

The redox-noninnocence of metal-coordinated C-organo nitrosoarenes has been established on the basis of solid-state characterization techniques, but the solution-phase properties of this class of metal-coordinated radicals have been relatively underexplored. In this report, the solution-phase properties and dynamics of the bis-nitrosobenzene diradical complex trans-Pd(κ(1)-N-PhNO)2(CNAr(Dipp2))2 are presented. This complex, which is best described as containing singly reduced phenylnitroxide radical ligands, is shown to undergo facile nitrosobenzene dissociation in solution to form the metalloxaziridine Pd(η(2)-N,O-PhNO)(CNAr(Dipp2))2 and thus is not a persistent species in solution. An equilibrium between trans-Pd(κ(1)-N-PhNO)2(CNAr(Dipp2))2, Pd(η(2)-N,O-PhNO)(CNAr(Dipp2))2, and free nitrosobenzene is established in solution, with the metalloxaziridine being predominantly favored. Efforts to perturb this equilibrium by the addition of excess nitrosobenzene reveal that the formation of trans-Pd(κ(1)-N-PhNO)2(CNAr(Dipp2))2 is in competition with insertion-type chemistry of Pd(η(2)-N,O-PhNO)(CNAr(Dipp2))2 and is therefore not a viable strategy for the production of a kinetically persistent bis-nitroxide radical complex. Electronic modification of the nitrosoarene framework was explored as a means to generate a persistent trans-Pd(κ(1)-N-ArNO)2(CNAr(Dipp2))2 complex. While most substitution schemes failed to significantly perturb the kinetic lability of the nitrosoarene ligands in the corresponding trans-Pd(κ(1)-N-ArNO)2(CNAr(Dipp2))2 complexes, utilization of para-formyl or para-cyano nitrosobenzene produced bis-nitroxide diradical complexes that display kinetic persistence in solution. The origin of this persistence is rationalized by the ability of para-formyl- and para-cyano-aryl groups to both attenuate the trans effect of the corresponding nitrosoarene and, more importantly, delocalize spin density away from the aryl-nitroxide NO unit. The results presented here highlight the inherent instability of metal-coordinated nitroxide radicals and suggest a general synthetic strategy for kinetically stabilizing these species in solution.