Interactions of arylhydroxylamines and alkylaldoximes with a rhodium porphyrin

Abstract

Heme enzymes are involved in the binding and metabolism of hydroxylamine (RNHOH) and aldoxime (RCH=NOH) compounds (R = H, alkyl, aryl). We report the synthesis and X-ray crystal structure of a metalloporphyrin in complex with an arylhydroxylamine, namely that of (TPP)Rh(PhNHOH)(C₆H₄Cl) (TPP = tetraphenylpophryinato dianion). The crystal structure reveals, in addition to N-binding of PhNHOH to Rh, the presence of an intramolecular H-bond between the hydroxylamine -OH proton and a porphyrin N-atom. Results from density functional theory (DFT) calculations support the presence of this intramolecular H-bond in this global minimum structure, and a natural bond order (NBO) analysis reveals that this H-bond comprises a donor π N=C (porphyrin) to acceptor σ* O-H (hydroxylamine) interaction of 2.32 kcal/mol. While DFT calculations predict the presence of similar intramolecular H-bond interactions in the related aldoxime complexes (TPP)Rh(RCH=NOH)(C₆H₄Cl) in their global minima structures, the X-ray crystal structure obtained for the (TPP)Rh(CH₃(CH₂)₂CH=NOH)(C₆H₄Cl) complex is consistent with the local (non-global) minima conformation that does not have this intramolecular H-bond interaction.

Keywords: Aldoxime; DFT calculation; Hydroxylamine; Intramolecular H-bond; Porphyrin; Rhodium; X-ray structure.