Synthesis, characterization, and molecular structures of nitrosyl nitrito complexes of osmium porphyrins: Disproportionation of nitric oxide in its reaction with Os(P)(CO) (P = porphyrinato dianion)

Abstract

The Os(P)(NO)(ONO) compounds (P = TTP, TMP, OEP, TmTP; TTP = 5,10,15,20-tetra-p-tolylporphyrinato dianion, TMP = 5,10,15,20-tetramesitylporphyrinato dianion, OEP = octaethylporphyrinato dianion, TmTP = tetra(m-tolyl)porphyrinato dianion) have been prepared from the reaction of the precursor carbonyl complexes Os(P)(CO) with excess nitric oxide. Nitrous oxide was detected as a by-product of the reaction. The IR spectra of the Os(P)(NO)(ONO) compounds (as KBr pellets) reveal bands in the 1790–1804 cm–1 range that are assigned to υNO. The IR spectra also reveal two new bands for each complex in the 1495–1531 and 913–962 cm–1 ranges indicative of O-bound nitrito ligands. The linearity of the bound NO groups and the O-binding of the trans nitrito ligands in the Os(P)(NO)(ONO) complexes are evident in the single-crystal X-ray crystal structures of the TTP and TMP derivatives. The kinetics of the reaction were studied by stopped-flow mixing techniques. Spectroscopic analysis of rapidly mixed solutions of Os(P)CO and NO in toluene showed a biphasic approach to the Os(P)(NO)(ONO) and N2O products, owing to the starting material Os(P)CO scavenging CO formed during the reaction to give Os(P)(CO)2 (KCO = 106 M–1). The dicarbonyl was the only transient species observed. It is proposed that the rate-determining step of the reaction leading to Os(P)(NO)(ONO) is NO displacement of CO from Os(P)(CO) via initial formation of an unstable 19 electron Os(P)(NO)(CO) intermediate.Key words: osmium, nitric oxide, X-ray, nitrosyl, porphyrin, kinetics.