title: 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.
authors.affiliation: †Department of Chemistry and Biochemistry, University of California San Diego, 9500 Gilman Drive, MC 0358, La Jolla, California 92093-0358, United States.
authors.forename: Brandon R
authors.initials: BR
authors.lastname: Barnett
authors.affiliation: †Department of Chemistry and Biochemistry, University of California San Diego, 9500 Gilman Drive, MC 0358, La Jolla, California 92093-0358, United States.
authors.forename: Liezel A
authors.initials: LA
authors.lastname: Labios
authors.affiliation: †Department of Chemistry and Biochemistry, University of California San Diego, 9500 Gilman Drive, MC 0358, La Jolla, California 92093-0358, United States.
authors.forename: Curtis E
authors.initials: CE
authors.lastname: Moore
authors.affiliation: ‡Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany.
authors.forename: Jason
authors.initials: J
authors.lastname: England
authors.affiliation: †Department of Chemistry and Biochemistry, University of California San Diego, 9500 Gilman Drive, MC 0358, La Jolla, California 92093-0358, United States.
authors.forename: Arnold L
authors.initials: AL
authors.lastname: Rheingold
authors.affiliation: ‡Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany.
authors.forename: Karl
authors.initials: K
authors.lastname: Wieghardt
authors.affiliation: †Department of Chemistry and Biochemistry, University of California San Diego, 9500 Gilman Drive, MC 0358, La Jolla, California 92093-0358, United States.
authors.forename: Joshua S
authors.initials: JS
authors.lastname: Figueroa