Barium phosphidoboranes and related calcium complexes.

The attempted synthesis of [{Carb}BaPPh 2 ] (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 HPPh 2 ·BH 3 and HPPh 2 ·B(C 6 F 5 ) 3 instead of HPPh 2 . This allowed for the synthesis of [{Carb}Ae{PPh 2 ·BH 3 }] (Ae = Ba, 2; Ca, 3), [{Carb}Ca{(H 3 B) 2 PPh 2 }·(thf)] (4), [{Carb}Ba{PPh 2 ·B(C 6 F 5 ) 3 }] (5), [{Carb}Ba{O(B(C 6 F 5 ) 3 )CH 2 CH 2 CH 2 CH 2 PPh 2 }·thf] (6), [Ba{O(B(C 6 F 5 ) 3 )CH 2 CH 2 CH 2 CH 2 PPh 2 } 2 ·(thf) 1.5 ] (7) and [Ba{PPh 2 ·B(C 6 F 5 ) 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{PPh 2 ·B(C 6 F 5 ) 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·(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.