Pseudohalides. Part IV. Infrared spectra and thermochemistry of the complexes PhBX2,L (X = Cl, Br, NCO, or NCS; L = C5H5N or EtOAc), and relative acceptor strengths of halogeno- and pseudohalogenoboranes; with notes on the chemistry of four-co-ordinate isocyanato- and isothiocyanato-boranes

Abstract

Eight complexes PhBX₂,L (X = Cl, Br, NCO, or NCS; L = C₅H₅N or EtOAc) have been prepared and characterised. Their i.r. spectra have been examined and assignments are proposed. The most significant feature is probably the progressive decrease in ν(CO) in the series PhBX₂,EtOAc from 1639 (X = NCO), to 1597 (X = NCS), 1582 (X = Cl), and 1563 cm.^–1(X = Br) compared with 1714 cm.^–1 in free EtOAC. These changes reflect the inverse order of acceptor strengths (X = Br > Cl > NCS > NCO) in PhBX₂. The same result is found calorimetrically, using pyridine as the ligand; but for the EtOAc–PhBX₂ system there are complicating side reactions (the 1 : 1 complexes dissolve exothermically in excess ligand). The condensed-phase heats of the reaction (where l = liquid and c = crystal): PhBX₂(l)+ py(l)= PhBX₂,py(c) are –39·1, –33·2, ∼–28·7, and –26·2 kcal. mole^–1 for X = Br, Cl, NCS, and NCO, respectively.

The compounds B(NCO)₃,py, B(NCS)₃,py, o-C₆H₄O₂BNCO,py, Ph₂BNCO,py, and PhB(Cl)NCO,py are reported. B(NCX)₃,py (X = O or S), was obtained from BY₃,py (Y = halogen) and AgNCX (probably by a predissociation mechanism); the other complexes were prepared directly from their components. Reactions with alcohols and amines are described. Four-co-ordinate pseudohalogenoboranes generally behave like their three-co-ordinate progenitors. An exception is found in the B(NCO)₃,py–RR¹NH system which leads to B(NH·CO·NRR¹)₃.