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

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 (B_ip) while the other lies above it (B_oop). 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 δ −4 (B_oop-OH) and −10 (B_ip-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.