Barium phosphidoboranes and related calcium complexes

The attempted synthesis of [{Carb}BaPPh2] (1) showed this barium-phosphide and its thf adducts, 1 center dot thf and 1 center dot(thf)2, to be unstable in solution. Our strategy to circumvent the fragility of these compounds involved the use of phosphinoboranes HPPh2 center dot BH3 and HPPh2 center dot B(C6F5)3 instead of HPPh2. This allowed for the synthesis of [{Carb}Ae{PPh2 center dot BH3}] (Ae = Ba, 2; Ca, 3), [{Carb}Ca{(H3B)2PPh2}center dot(thf)] (4), [{Carb}Ba{PPh2 center dot B(C6F5)3}] (5), [{Carb}Ba{O(B(C6F5)3)CH2CH2CH2CH2PPh2}center dot thf] (6), [Ba{O(B(C6F5)3)CH2CH2CH2CH2PPh2}2 center dot(thf)1.5] (7) and [Ba{PPh2 center dot B(C6F5)3}2 center dot(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 BaMIDLINE HORIZONTAL ELLIPSISF 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 center dot B(C6F5)3}2]n. 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 center dot(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. Although a stable {N<^>N<^>N}BaPH2 phosphide supported by a bis(imino)carbazole is elusive, barium and calcium phosphidoborane complexes incorporating BH3 or B(C6F5)3 moities have been obtained. NMR, crystallographic and computational data are provided.