Barium phosphidoboranes and related calcium complexes

The attempted synthesis of [{Carb}BaPPh₂] (1) showed this barium-phosphide and its thf adducts, 1·thf and 1·(thf)₂, to be unstable in solution. Our strategy to circumvent the fragility of these compounds involved the use of phosphinoboranes HPPh₂·BH₃ and HPPh₂·B(C₆F₅)₃ instead of HPPh₂. This allowed for the synthesis of [{Carb}Ae{PPh₂·BH₃}] (Ae = Ba, 2; Ca, 3), [{Carb}Ca{(H₃B)₂PPh₂}·(thf)] (4), [{Carb}Ba{PPh₂·B(C₆F₅)₃}] (5), [{Carb}Ba{O(B(C₆F₅)₃)CH₂CH₂CH₂CH₂PPh₂}·thf] (6), [Ba{O(B(C₆F₅)₃)CH₂CH₂CH₂CH₂PPh₂}₂·(thf)_1.5] (7) and [Ba{PPh₂·B(C₆F₅)₃}₂·(thp)₂] (8) that were characterised by multinuclear NMR spectroscopy (thp = tetrahydropyran). The molecular structures of 4, 6 and 8 were validated by X-ray diffraction crystallography, which revealed the presence of Ba⋯F stabilizing interactions (ca. 9 kcal mol^-1) in the fluorine-containing compounds. Compounds 6 and 7 were obtained upon ring-opening of thf by their respective precursors, 5 and the in situ prepared [Ba{PPh₂·B(C₆F₅)₃}₂]. By contrast, thp does not undergo ring-opening under the same conditions but affords clean formation of 8. DFT analysis did not highlight any specific weakness of the Ba-P bond in 1·(thf)₂. The instability of this compound is instead thought to stem from the high energy of its HOMO, which contains the non-conjugated P lone pair and features significant nucleophilic reactivity.