Electronic Structures and Reactivity Patterns of Dipalladium(11,11) and Diplatinum(11,11) Complexes

Abstract

This work explores the electronic structures and reactivity patterns of diplatinum(II, II) and dipalladium(II, II) complexes. Complexes containing metal-metal bonds play important roles in both inorganic and organometallic chemistry. Among the many examples of these complexes, dimers of square planar RhI, IrI, and PtII centers comprise a special class that feature attractive d8–d8 interactions. The unique electronic structure characteristic of these complexes gives rise to chemical, photochemical, and photophysical properties that have engaged researchers for the past 35 years. One of the best-known examples of these compounds is tetrakis(µ-pyrophosphito)platinum(II), Pt(pop). Herein, the photophysical properties of Pt(pop) are compared with its fluoroborated analogue, Pt(pop-BF2). This complex possesses eight BF2 groups that replace the hydrogen atoms located between each “pop” ligand. When compared with Pt(pop), Pt(pop-BF2) has a much greater singlet lifetime (1.56 ns) and singlet quantum yield (0.27). The enhancement is the result of a drastically slower 1A2u → 3A2u intersystem crossing rate. In particular, the thermal barrier to intersystem crossing is significantly higher in Pt(pop-BF2) (2230 cm-1 vs. 1190 cm-1). We believe this is primarily the result of the increased rigidity of the complex afforded by the BF2 groups. The rigidity increases the energy of symmetry-lowering vibrational modes, which are necessary to promote spin-orbit mixing of the 1dσ*pσ and 3dσ*pσ states. Despite the many examples of M-M bonded d8–d8 complexes of RhI, IrI, and PtII in the literature, until recently there were no PdII complexes fitting this description. Our investigations of clamshell-shaped PdII dimers [(2-phenylpyridine)Pd(µ-X)]2 and [(2-p-tolylpyridine)Pd(µ-X)]2 (X = OAc or TFA) revealed short Pd–Pd distances (~ 2.85 A). The molecules adopt this unusual geometry in part because of a d8–d8 bonding interaction between the two Pd centers. Density functional theory (DFT) and ab initio (AI) analyses confirm the presence of a Pd–Pd bonding interaction in [(2-phenylpyridine)Pd(µ-X)]2 and show that the HOMO is a dz2 σ*Pd–Pd antibonding orbital, while the LUMO and proximal unoccupied orbitals are mainly located on the 2-phenylpyridine rings. Computational analyses of other PdII–PdII dimers that have short Pd–Pd distances yield an orbital ordering similar to that of [(2-phenylpyridine)Pd(µ-X)]2, but quite different from that found for d8–d8 dimers of Rh, Ir, and Pt. This difference in orbital ordering arises because of the unusually large energy gap between the 4d and 5p orbitals in Pd, and may explain why Pd d8–d8 dimers do not exhibit the distinctive photophysical properties of related Rh, Ir, and Pt species. Our work on PdII–PdII electronic structures led us to employ these complexes as electrocatalysts in the regioselective chlorination of C–H bonds. Previous work on d8–d8 complexes has established that when treated with halogens (Cl2, Br2, or I2), the complex undergoes two-center oxidative addition to form an axially coordinated X–d7–d7–X species. Similar products are observed following electrochemical oxidation in the presence of a halide (Cl–, Br–, or I–). Recently, related PdII–PdII complexes were found to selectively chlorinate benzo[h]quinoline through reductive elimination from a Cl–PdIII–PdIII–Cl species. This led us to probe the viability of the analogous electrochemical route. Cyclic voltammetry, spectroelectrochemistry, and bulk electrolysis measurements confirm that electrochemical oxidation of PdII–PdII yields the identical Cl–PdIII–PdIII–Cl intermediate, which is capable of reductive chlorination of C–H bonds. Additional evidence for formation of axially coordinated bromide and acetate species is also presented. Over 10 turnovers of 10-chlorobenzo[h]quinoline were achieved at 80% isolated yield. Further research into the area may lead to a potentially versatile, useful, and green route for C–H bond functionalization reactions.