RESUMO
The first examples of diboron complexes of the tetrapyrroles octaethylporphyrazine (OEPz) and 2,9,16,23-tetra-t-butyl-phthalocyanine (Pc) are reported, counterpoints to the better known monoboron tripyrroles, subporphyrazine and subphthalocyanine. Two stereochemical possibilities are observed, with cisoid-B2 OF2 (OEPz), both cisoid-B2 OPh2 (OEPz) and transoid-B2 OPh2 (OEPz), transoid-B2 OF2 (Pc) and cisoid-B2 OPh2 (Pc) having been isolated and characterised, including structure determinations for the OEPz complexes. This variation in stereochemistry, which can be extended to include the previously reported transoid-B2 OF2 (porphyrin), cisoid-[B2 OF2 (corrole)]- , and both transoid- and cisoid-B2 OF2 (calixphyrin), prompted a wider DFT study to elucidate the factors influencing the stereochemical preferences. This shows that the cisoid/transoid preference is correlated to the ease with which the macrocycle accommodates a rectangularly distorted N4 cavity.
RESUMO
Boron complexes of calix[4]phyrins (1.1.1.1) were prepared by reacting the free-base ligands with BF3·Et2O. The reaction conditions can be efficiently tailored to produce mono- or di-boron calixphyrins. Mono-BF2 calixphyrins with boron coordinating to either the dipyrrin, BF2[H(Calix)], or dipyrromethane, BF2[H(Calix)] and BF2[H2(Calix)]+, bonding sites were isolated. The dipyrromethane isomer, BF2[H(Calix)], isomerises into BF2[H(Calix)] which kinetic studies and DFT calculations indicate is an intramolecular process. Two isomers of B2OF2(Calix) were isolated, one isomer bonding via the dipyrrin sites with the FBOBF moiety in cisoid geometry, and the second isomer bonding via the dipyrromethane sites with the FBOBF moiety in transoid geometry. Although the cisoid/dipyrrin isomer was calculated to be most energetically favourable for B2OF2(Calix), the isolation of the transoid/dipyrromethane isomer is postulated to occur via the presumed intermediate (BF2)2(Calix), for which DFT indicated a preference for transoid/dipyrromethane geometry.
