A Fractional Bond Order of 1/2 in Pd₂⁵⁺−Formamidinate Species; The Value of Very High-Field EPR Spectra

Reaction of Pd2(DAniF)4, 1, (DAniF = di-p-anisylformamidinate) with 1 equiv of AgPF6 in CH2Cl2 at or below −10 °C produces the paramagnetic species [Pd2(DAniF)4]PF6, 1-PF6, that has been studied by X-ray crystallography, UV−vis spectroscopy, electrochemistry, and multifrequency (9.5, 34.5, 110, and 220 GHz) EPR spectroscopy. Upon oxidation of the precursor, the Pd−Pd distance decreases by 0.052 Å from 2.6486(8) to 2.597(1) Å. The EPR spectra show broad signals with line widths of about 1000 G. The spectra collected at high field show a large spread of g tensor components (∼0.03), but these are masked at lower frequencies (9.5 and 34.5 GHz). A reinvestigation using high-field EPR of the p-tolyl analogue, which is the only other structurally characterized Pd25+ species (Cotton, F. A.; Matusz, M.; Poli, R.; Feng, X. J. Am. Chem. Soc. 1988, 110, 1144), shows that this species, which had been reported to give an isotropic 9.5 GHz EPR spectrum, also gives anisotropic 110 and 220 GHz EPR spectra with a similarly large spread of g tensor components consistent with the unpaired electron residing in a metal-based MO. The results of these studies and calculations using density functional theory are consistent with the oxidation being metal-based, resulting in an uncommon Pd25+ species with a Pd−Pd bond order of 1/2.