Reactivity of Perhalogenated Octahedral Phospha- and Arsaboranes toward THF: A Joint Experimental/Computational Study

Reactions of the perhalogenated polyhedral pnictogenaboranes closo-1,2-Pn₂B₄Hal₄ (Pn = P, As; Hal = Cl, Br) with Lewis bases are presently being studied with a focus on rationalizing the sites of nucleophilic attacks on clusters bearing σ-holes. These σ-holes are localized both on pnictogens and, for Hal = Br, on bromine atoms, as revealed by electrostatic potential (ESP) and intrinsic bond orbital (IBO) analyses. Surprisingly, the attack of the cyclic ether THF on closo-1,2-Pn₂B₄Br₄ does not occur on the site with the largest positive partial charge, centered in the middle of the pnictogen-pnictogen vector. Instead, presumably promoted by the positivated bromine substituents, THF inserts into the boron-bromine bonds of the negatively charged boron atoms opposite to the pnictogen atoms to form 4-(4-bromobut-1-oxy)-closo-1,2-Pn₂B₄Br₃ (1-PB and 1-AsB) and 4,6-(4-bromobut-1-oxy)₂-closo-1,2-Pn₂B₄Br₂ (2-PB and 2-AsB). ¹¹B and ³¹P chemical shift computations at various levels support the assignments of the signals, which reflect the correctness of the molecular geometries in solutions. The Lewis-acidic perchlorinated analogues closo-1,2-P₂B₄Cl₄, closo-1,2-As₂B₄Cl₄, and the mixed closo-1,2-AsPB₄Cl₄ bear negative charges. These negative charges are revealed by the V_s,max values when computing the electrostatic potentials both on the boron and the chlorine atoms. Due to this negative charge, the analogues do not react with THF unless they are heated above 66 °C, where they slowly decompose to borate esters B(OR)₃ without the formation of concrete intermediates. The evaluation of ³¹P NMR data of 1-PB has allowed the experimental determination of the coupling constant ¹J(³¹P(1), ³¹P(2)) = |143| Hz in a closo-diphosphaborane for the first time, which agrees well with the computed value of -178 Hz. The pioneering joint experimental vs computational interpretation of ³¹P NMR spectra in the area of boron cluster chemistry was decisive for the structural characterization of 1-PB and 2-PB.