Durene-capped porphyrin complexes of iron(II) and their interaction with imidazoles, isonitriles, CO, and O₂
Creators: name: David, Shantha K.
Publisher: name: University of British Columbia
Year: 2010

description: Based on well developed procedures, durene-4/4 (la) and durene-7/7 (3a) capped porphyrins containing a totally hydrophobic cavity have been prepared: cyclization of the chain-linked dipyrromethane dimer (4) allows for porphyrin formation despite distortion of the porphyrin skeleton from planarity caused by the imposition of a tight cap. The crystal structure of the Fe[sup III](lb)Cl complex shows considerable distortion of the porphyrin from planarity. The uv-visible spectral trends for the free base porphyrins indicate that some porphyrin distortion exists also in the durene-5/5 free base (2a), while the 7/7-derivative (3a) is essentially flat. Proton nmr data suggest that the durene moiety in the 7/7-base is suspended closer to the porphyrin plane than in the tighter capped-5/5 and -4/4 analogs. The heme derivatives, Fe[sup II](durene-por), have been studied with respect to their interaction with imidazoles, isocyanides, CO, and O₂. The binding constants of 1,5-dicyclohexyl-(Dclm) and 1,2-dimethyl-(1,2-Me₂lm)-imidazoles to the unhindered side of the four-coordinate hemes are similar within the durene series despite differences in the porphyrin plane distortion; for steric reasons, these imidazoles do not coordinate on the capped (or distal) side of the heme. The size of the distal pocket in the five-coordinate durene-7/7 and -5/5 systems has been probed using the bulky isocyanides, tosylmethylisocyanide (TMIC) and t-butylisocyanide (t-BuNC), which differ in their spacial requirements on binding to the iron. The extremely restrictive distal environment of the durene-4/4 system, however, inhibits the coordination of either [See thesis for Diagram] isocyanide. The Fe[sup II](durene-7/7)(Dclm) complex exhibits a reduced overall affinity for CO relative to simple flat, open hemes; this is manifested in a depressed association rate for CO, and is interpreted as a distal steric effect as a result of the durene-cap. The durene-5/5 and -A/A systems also show reduced CO affinities compared to open hemes, but this results predominantly from increased dissociation rates for CO from the six-coordinate Fe[sup II ](Por)(Dclm)(CO) complexes because of proximal steric strain induced by the porphyrin plane distortion. The carbonyl stretching frequencies for the Fe[sup II](Por)(B)(CO) species (B = methylimidazole or 1,2-Me₂lm) of the durene series are discussed with respect to relative destabilization of the Fe-C-O moiety. The five-coordinate Fe[sup II] (durene-por) (B) systems (B - Dcl-m or 1,2-Me₂im) bind O₂ reversibly to similar extents, implying a negligible effect of porphyrin skeletal distortion. A 10-fold reduced affinity for CO by the Fe[sup II] (durene-4/4)(B) complex relative to durene-5/5 (or other less distorted hemes) is therefore interpreted in terms of proximal steric discrimination of CO relative to O₂ within this severely distorted system. Thermodynamic data for CO and O₂ binding to the hindered Fe[sup II](durene-4/4)(B) systems are discussed in comparison to those for planar heme systems. Significantly the five-coordinate durene heme complexes all show considerably higher K[sup CO]/K [sup O₂] ratios (M values) relative to encumbered hemes that incorporate polar amide functions in their distal environments. This is entirely consistent with the concept of electronic interactions within the distal binding pocket stabilizing the Fe-O₂ moiety and increasing the affinity of the heme toward O₂, relative to CO. The implications of such model studies for binding to the reversible O₂ and CO carrying hemoproteins are discussed.