Electronic Structures and Photoredox Chemistry of Tungsten(0) Arylisocyanides

Conspectus The high energy barriers associatedwith the reaction chemistryof inert substrates can be overcome by employing redox-active photocatalysts.Research in this area has grown exponentially over the past decade,as transition metal photosensitizers have been shown to mediate challengingorganic transformations. Critical for the advancement of photoredoxcatalysis is the discovery, development, and study of complexes basedon earth-abundant metals that can replace and/or complement establishednoble-metal-based photosensitizers. Recent work has focusedon redox-active complexes of 3d metals,as photosensitizers containing these metals most likely would be scalable.Although low lying spin doublet (spin flip) excitedstates of chromium(III) and metal-to-ligand charge transfer (MLCT)excited states of copper(I) have relatively long lifetimes, the electronicexcited states of many other 3d metal complexes fall on dissociativepotential energy surfaces, owing to the population of highly energetic & sigma;-antibonding orbitals. Indeed, we and other investigators haveshown that low lying spin singlet and triplet excited states of robustclosed-shell metal complexes are too short-lived at room temperatureto engage in bimolecular reactions in solutions. In principle, thisproblem could be overcome by designing and constructing 3d metal complexescontaining strong field & pi;-acceptor ligands, where thermallyequilibrated MLCT or intraligand charge transfer excited states mightfall well below the upper surfaces of dissociative 3d-3d states. Notably,such design elements have been exploited by investigators in veryrecent work on redox-active iron(II) systems. Another approach, onewe have actively pursued, is to design and construct closed-shellcomplexes of earth-abundant 5d metals containing very strong & pi;-acceptorligands, where vertical excitation of 5d-5d excited states at theground state geometry would require energies far above minima in thepotential surfaces of MLCT excited states. As this requirement ismet by tungsten(0) arylisocyanides, these complexes have been thefocus of our work aimed at the development of robust redox-activephotosensitizers. In the following Account, we review recentwork on homoleptic tungsten(0)arylisocyanides. Originally reported by our group 45 years ago, W(CNAr)(6) complexes have exceptionally large one- and two-photon absorptioncross-sections. One- or two-photon excitation produces relativelylong-lived (hundreds of nanoseconds to microsecond) MLCT excited statesin high yields. These MLCT excited states, which are very strong reductantswith E & DEG;(W+/*W-0) = -2.2to -3.0 V vs Fc([+/0]), mediate photocatalysis oforganic reactions with both visible and near-infrared (NIR) light.Here, we highlight design principles that led to the development ofthree generations of W(CNAr)(6) photosensitizers; and wediscuss likely steps in the mechanism of a prototypal W(CNAr)(6)-catalyzed base-promoted homolytic aromatic substitution reaction.Among the many potential applications of these very bright luminophores,two-photon imaging and two-photon-initiated polymerization are oneswe plan to pursue.

Automated summary

The use of redox-active photocatalysts can overcome the high energy barriers in the reaction chemistry of inert substrates. Over the past decade, transition metal photosensitizers have been extensively studied for mediating challenging organic transformations. The focus of recent research is on redox-active complexes of 3d metals, as they are more scalable.