Systematics of BX3 and BX2+ Complexes (X = F, Cl, Br, I) with Neutral Diphosphine and Diarsine Ligands

The coordination chemistry of the neutral diphosphines, R2P­(CH2)2PR2 (R = Me or Et) and o-C6H4(PR′2)2 (R′ = Me or Ph), and the diarsine, o-C6H4(AsMe2)2, toward the Lewis acidic BX3 (X = F, Cl, Br, and I) fragments is reported, including several rare complexes incorporating BF3 and BF2+. The studies have revealed that the flexible dimethylene linked diphosphines form [(BX3)2{μ-R2P­(CH2)2­PR2}] exclusively, confirmed by multinuclear NMR (1H, 11B, 19F­{1H}, and 31P­{1H}) and IR spectroscopy and microanalytical data. Crystallographic determinations of the four BX3 complexes with Et2P­(CH2)2PEt2 confirm the 2:1 stoichiometry and, taken together with the spectroscopic data, reveal that the Lewis acid behavior of the BX3 fragment toward phosphine ligands increases in the order F ≪ Cl ∼ Br < I. The first diphosphine- and diarsine-coordinated dihaloboronium cations, [BX2{o-C6H4(EMe2)2}]+ (E = P, As), are obtained using the rigid, preorganized o-phenylene linkages. These complexes are characterized similarly, and the data indicate that the complexes with o-C6H4(AsMe2)2 are much more labile and readily decomposed than the phosphine analogues. X-ray crystallographic studies on [BX2{o-C6H4­(PMe2)2}]­[BX4] (X = Cl, Br), [BI2{o-C6H4­(PMe2)2}]­[I3], and [BCl2{o-C6H4­(AsMe2)2}]­[BCl4] confirm the presence of distorted tetrahedral coordination at boron through a chelating diphosphine or diarsine and two X ligands, with d(B–P) revealing a similar increase in Lewis acidity down group 17. Comparison of d(B–P) and d(B–As) reveals an increase of ca. 0.08 Å from P to As. Reaction of BCl3 with the diphosphine dioxide Ph2P­(O)­CH2P­(O)­Ph2 gives the ligand-bridged dimer [(BCl3)2{Ph2P­(O)­CH2­P­(O)­Ph2}], while using either BF3 gas or [BF3(SMe2)] gives a mixture containing both [(BF3)2{μ-Ph2P­(O)­CH2P­(O)­Ph2}] and the unexpected difluoroboronium salt, [BF2{Ph2P­(O)­CH2P­(O)­Ph2}]­[B2F7] containing a chelating phosphine oxide. The structure of the latter was confirmed crystallographically.