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

Reactions of the perhalogenated polyhedral pnictogenaboranes closo -1,2-Pn 2 B 4 Hal 4 (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-Pn 2 B 4 Br 4 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-Pn 2 B 4 Br 3 ( 1-PB and 1-AsB ) and 4,6-(4-bromobut-1-oxy) 2 - closo -1,2-Pn 2 B 4 Br 2 ( 2-PB and 2-AsB ). 11 B and 31 P 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-P 2 B 4 Cl 4 , closo -1,2-As 2 B 4 Cl 4 , and the mixed closo- 1,2-AsPB 4 Cl 4 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 31 P NMR data of 1-PB has allowed the experimental determination of the coupling constant 1 J ( 31 P(1), 31 P(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 31 P NMR spectra in the area of boron cluster chemistry was decisive for the structural characterization of 1-PB and 2-PB .