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

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.