Striking Differences between the Solution and Solid-State Reactivity of Iron PNP Pincer Complexes with Carbon Monoxide

Several new iron(II) complexes of the types [Fe(PNP)X2] (X = Cl, Br) containing tridentate PNP pincer-type ligands based on 2,6-diaminopyridine and 2,6-diaminopyrimidine have been prepared. They all exhibit intermolecular Fe−X···H−N hydrogen bonds, forming supramolecular networks in the solid state. In the case of X = Cl these compounds react readily with gaseous CO both in the solid state and in solution to give selectively the octahedral complexes cis- and trans-[Fe(PNP)(CO)(Cl)2], respectively, whereas with X = Br mixtures of cis and trans isomers are obtained. These transformations are accompanied by color and spin-state changes. CO binding is fully reversible in all cases, and heating solid samples of either cis- or trans-[Fe(PNP)(CO)(X)2] leads to complete regeneration of analytically pure [Fe(PNP)(X)2]. Mössbauer spectroscopy confirmed the high-spin nature of the parent five-coordinate Fe(II) complex (δ = 0.80(1) mm s−1) and the shift to two different low-spin octahedral species after reaction with CO in the solid (δ = 0.13(1) mm s−1) or in solution (δ = 0.15(1) mm s−1). Magnetization studies led to a magnetic moment close to 4.9 μB, reflecting the expected four unpaired d-electrons in [Fe(PNP)Cl2], which were confirmed by DFT calculations. The DFT study of the reaction pathway for CO capture led to low energy barriers (≤3.4 kcal mol−1). The cis−trans isomerization reaction was found to take place with a low energy barrier (10.8 kcal mol−1), after initial loss of chloride, and involves also spin-state changes