Diboryl and Diboranyl Porphyrin Complexes: Synthesis, Structural Motifs, and Redox Chemistry: Diborenyl Porphyrin or Diboranyl Isophlorin?

• Published in: Chemistry : a European journal Vol. 13; no. 21; pp. 5982 - 5993
• Main Authors: Weiss, Andre, Hodgson, Michael C., Boyd, Peter D. W., Siebert, Walter, Brothers, Penelope J.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 01.01.2007; WILEY‐VCH Verlag; Wiley
• Subjects: boron; Boron - chemistry; Catechols - chemistry; Chemistry; Chemistry, Multidisciplinary; Chlorides - chemistry; density functional calculations; Fluorides - chemistry; Hydrogen - chemistry; Iodides - chemistry; isophlorin; Lithium - chemistry; Magnetic Resonance Spectroscopy; Models, Molecular; Molecular Structure; Oxidation-Reduction; Physical Sciences; porphyrinoids; Porphyrins - chemical synthesis; Porphyrins - chemistry; redox chemistry; Science & Technology; Temperature; isophlorin; density functional calculations; porphyrinoids; LI-7; CRYSTAL-STRUCTURE; LITHIUM; boron; redox chemistry
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200700046

The syntheses of diboryl porphyrin complexes [(BX2)2(ttp)] (ttp: dianion of tetra‐p‐tolylporphyrin) and the BB single‐bond diboranyl complexes [(BX)2(ttp)] (X=F, Cl, Br, I) are given. The former are prepared from the reactions of BX3 (X=F, Cl) with [Li2(ttp)] and the latter from B2Cl4 (X=Cl), the reaction of SbF3 with [(BCl)2(ttp)] (for X=F), and, in the cases of X=Br or I, in a remarkable reductive coupling reaction resulting directly from the reaction of BBr3 or BI3 with [Li2(ttp)]. Density functional theory (DFT) calculations on the thermochemical parameters for the reductive coupling reactions (and those calculated for related dipyrromethene complexes) indicate that a combination of the reducing ability of bromide and iodide ions combined with the constrained environment of the porphyrin ligand contribute to the driving force. The reductive coupling is also observed in the reaction of [(BCl2)2(ttp)] with nBuLi to give [(BnBu)2(ttp)], which was characterised crystallographically. The reaction of [(BCl)2(ttp)] with catechol gives a boron catecholato porphyrin complex, [B2(O2C6H4)(ttp)]. Chloride ion from [(BCl)2(ttp)] gives the planar dication [B2(ttp)]2+, whereas chemical reduction of [(BCl)2(ttp)] by using magnesium anthracenide gives a neutral complex, [B2(ttp)], in which the TTP ligand has been reduced by two electrons to give an unusual example of an isophlorin complex. The cationic and neutral complexes [B2(ttp)]2+ and [B2(ttp)] were characterised through a combination of spectroscopic data that is supported by DFT calculations on the porphine analogues. Spontaneous reduction! Spontaneous reductive coupling occurs in the reaction of BBr3 and BI3 with [Li2(por)] (por: unspecified porphyrin dianion) to give the BB‐bonded products [(BX)2(por)] (X=halide). Subsequent halide removal gives the dication [B2(por)]2+, and further chemical reduction results in the neutral species, [B2(por)], which contains the unusual 20‐electron antiaromatic isophlorin macrocycle (see graphic).