Isostructural Pd^II and Pt^II pyrophosphato complexes: Polymorphism and unusual bond character in d⁸-d⁸ systems

Isostructural, "clamshell"-like, neutral dimeric pyrophosphato complexes of general formula {[M(bipy)]₂(μ-P₂O ₇)} [M = Pd^II (1) or Pt^II (2)] were synthesized and studied through single-crystal X-ray diffraction, IR, ³¹P NMR spectroscopy, and MALDI-TOF mass spectrometry. Compound 1 was synthesized through the reaction of palladium(II) acetate, 2,2′-bipyridine (bipy), and sodium pyrophosphate (Na₄P₂O₇) in water. Compound 2 was prepared through two different routes. The first involved the reaction of the Pt^IV precursor Na₂PtCl₆, bipy, and Na₄P₂O₇ in water, followed by reduction in DMF. The second involved the reaction of the Pt^II precursor K ₂PtCl₄, bipy, and Na₄P₂O₇ in water. Both complexes crystallize in the monoclinic chiral space group Cc as hexahydrates, 1 · 6H₂O (1a, yellow crystals) and 2 · 6H₂O (2a, orange crystals), and exhibit a zigzag chain-like supramolecular packing arrangement with short and long intra/intermolecular metal-metal distances [3.0366(3)/4.5401(3) Å in 1a; 3.0522(3)/4.5609(3) Å in 2a]. A second crystalline phase of the Pt species was also isolated, with formula 2 · 3.5H₂O (2b, deep green crystals), characterized by a dimer-of-dimers (pseudo-tetramer) structural submotif. Green crystals of 2b could be irreversibly converted to the orange form 2a by exposure to air or water, without retention of crystallinity, while a partial, reversible crystal-to-crystal transformation occurred when 2a was dried in vacuo. ³¹P NMR spectra recorded for both 1 and 2 at various pHs revealed the occurrence of a fluxional protonated/deprotonated system in solution, which was interpreted as being composed, in the protonated form, of [HO=PO₃]⁺ (P_α) and O=PO₃ (P_β) pyrophosphate subunits. Compounds 1 and 2 exhibited two successive one-electron oxidations, mostly irreversible in nature; however, a dependence upon pH was observed for 1, with oxidation only occurring in strongly basic conditions. Density functional theory and atoms in molecules analyses showed that a d⁸-d⁸ interaction was present in 1 and 2.