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

Abstract

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 B(2)Cl(4) (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(O(2)C(6)H(4))(ttp)]. Chloride abstraction 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, [B(2)(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.

Three‐Coordinate Boron(III) and Diboron(II) Dications

“…As in III the metalloid centers in IV are four‐coordinate. An example for a dication with three‐coordinate boron(II) atoms was presented by Siebert, Brothers and co‐workers, however, its structural characterization was merely based on theoretical methods . Very recently, Braunschweig and co‐workers structurally characterized a related three‐coordinate diboron(II) dication which was isolated in low yield after chloride abstraction from an unsymmetrical diborane(4) adduct precursor …”

Facile Redox‐Induced Aromatic–Antiaromatic Interconversion of a β‐Tetracyano‐21,23‐Dithiaporphyrin under Ambient Conditions

“…The second type of isophlorins contain tetravalent elements, such as Si or Ge or their respective analogues . They complement isophlorins in terms of their charge valance because isophlorins serve as tetravalent ligands.…”

Pyrrole‐Bridged Porphyrin Nanorings

“…In recent years, the chemistry of isophlorins has been reactivated in light of its relevance to antiaromatic porphyrins. [12][13][14] Synthesis of pyrrole-bridged porphyrin rings 3 H-5 H is straightforward through Suzuki-Miyaura cross-coupling of 5,10-diaryl-15,20-dibromoporphyrin 1…”