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

Barnett, B.R., Labios, L.A., Moore, C.E. , England, Jason, Rheingold, A.L., Wieghardt, K. and Figueroa, J.S. (2015) Solution Dynamics of Redox Noninnocent Nitrosoarene Ligands: Mapping the Electronic Criteria for the Formation of Persistent Metal-Coordinated Nitroxide Radicals. Inorganic Chemistry, 54 (14). pp. 7110-7121. ISSN 0020-1669

Full content URL: https://doi.org/10.1021/acs.inorgchem.5b01252

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Item Type:Article
Item Status:Live Archive

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(κ1-N-PhNO)2(CNArDipp2)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(η2-N,O-PhNO)(CNArDipp2)2 and thus is not a persistent species in solution. An equilibrium between trans-Pd(κ1-N-PhNO)2(CNArDipp2)2, Pd(η2-N,O-PhNO)(CNArDipp2)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(κ1-N-PhNO)2(CNArDipp2)2 is in competition with insertion-type chemistry of Pd(η2-N,O-PhNO)(CNArDipp2)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(κ1-N-ArNO)2(CNArDipp2)2 complex. While most substitution schemes failed to significantly perturb the kinetic lability of the nitrosoarene ligands in the corresponding trans-Pd(κ1-N-ArNO)2(CNArDipp2)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.

Keywords:transition metal complexes, redox active ligands, redox non-innocence, Electronic structure
Subjects:F Physical Sciences > F120 Inorganic Chemistry
Divisions:College of Science > School of Chemistry
ID Code:51905
Deposited On:11 Oct 2022 10:21

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