Porphyrin complexes containing coordinated BOB groups: synthesis, chemical reactivity and the structure of [BOB(tpClpp)]2+.

Abstract

The reactions of boron halides with free base porphyrins under conditions where partial hydrolysis of the boron halides can occur give diboron porphyrin complexes containing BOB moieties in which each boron is bonded to two porphyrin nitrogen atoms. BF3 ·OEt2 with H2 (por) gives B2OF2 (por) (por = tpp, ttp, tpClpp, oep) which has an asymmetric structure in which one boron lies in the porphyrin plane (Bip) while the other lies above it (Boop). BCl3 ·MeCN with H2 (por) gives B2O2 (BCl3 )2 (por) which contains a four-membered B2O2 ring and is stable only in the presence of excess BCl3 . BBr3 with Li2 (tpClpp) gives the dicationic complex [B2O(tpClpp)] 2+ as its [BBr4 ]− salt, and is the first example of a boron porphyrin containing three-coordinate boron to be structurally characterised. B2O2 (BCl3 )2 (por) can be chromatographed on basic alumina to give the hydroxyboron complex B2O(OH)2 (por), which is deduced from its NMR spectra and DFT calculations to have a structure analogous to B2OF2 (por). The OH protons are shifted upfield to near d −4 (Boop-OH) and −10 (Bip-OH) by the diamagnetic porphyrin ring current. The reaction of either B2O2 (BCl3 )2 (por) or B2O(OH)2 (por) (por = ttp, tpClpp) with alcohols (ROH, R = Et, 4-C6H4CH3 ) gives B2O(OR)2 (por), which can in turn be converted to B2O(OR)(OH)(por) by repeated chromatography. The reaction of PhBCl2 with H2 (por) (por = ttp, tpClpp) gives B2O(Ph)(OH)(por) which has been characterised by spectroscopy in concert with DFT calculations. It is a further example of the B2OF2 (por) structural type, in which the phenyl group is coordinated to the out-of-plane boron and the OH group to the in-plane boron, as are its derivatives B2O(Ph)(X)(tpClpp) (X = F, OEt). Steric drivers for the facile hydrolysis of haloboron porphyrins relative to their dipyrromethene and expanded porphyrin counterparts are discussed.