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.