Long-Range Spin Coupling: A Tetraphosphine-Bridged Palladium Dimer

The dipalladium compound [{(adt)Pd}2(μ-tpbz)] (1) (adt = bis(p-anisyl)-1,2-ethylenedithiolate, tpbz = 1,2,4,5-tetrakis(diphenylphosphino)benzene) has been synthesized from [{Cl2Pd}2(μ-tpbz)] by transmetalation employing (adt)SnMe2. The cyclic voltammogram (CV) of 1 reveals reversible oxidation waves at 0.00 V and +0.50 V (vs [Fc]+/Fc) with current amplitude twice that for identical processes in the monopalladium compound [(adt)Pd(dppb)] (2) (dppb = 1,2-bis(diphenylphosphino)benzene), an observation indicating each wave involves simultaneous one-electron oxidations at each metallodithiolene fragment. This assignment is affirmed by density functional theory (DFT) calculations that show the redox-active molecular orbital (MO) is principally composed of the dithiolene S2C2 π-system, and by spectroelectrochemical UV−vis of [1]2+, which displays hallmark low energy charge transfer (CT) bands. Dication [1]2+ is a diradical with a near degenerate singlet−triplet ground state; fluid solution electron paramagnetic resonance (EPR) spectra validate the DFT-derived isotropic exchange coupling, J′ = −6.3 cm−1. The frozen solution X-band EPR spectrum of [1]2+ is consistent with a spin-triplet bearing a very faint half-field (“ΔMS = 2”) signal. It is successfully simulated with an amazingly small zero field splitting, D = −15 × 10−4 cm−1 and negligible rhombicity (E/D = 0.008). These zero-field splitting parameters, which stem from the long-range dipolar spin coupling, are very accurately reproduced using a multipoint dipole model with an optimized interspin distance of 12.434 Å. With the framework reported herein for understanding how the weak interaction of two spins is mediated by tpbz, this bridging ligand can now be incorporated into extended systems with tailored chemical and physical properties for use in a variety of molecular-based electronic and magnetic devices.