Several new iron(II) complexes of the types [Fe(PNP)X 2 ] (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 3 3 3 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 ]. M€ ossbauer 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 μ B , reflecting the expected four unpaired d-electrons in [Fe(PNP)Cl 2 ], which were confirmed by DFT calculations. The DFT study of the reaction pathway for CO capture led to low energy barriers (e3.4 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.

Anhydrous CoCl2 or [NiCl2(DME)] reacts with the ligand PCPMe-iPr (1) in the presence of nBuLi to afford the 15e and 16e square planar complexes [Co(PCPMe-iPr)Cl] (2) and [Ni(PCPMe-iPr)Cl] (3), respectively. Complex 2 is a paramagnetic d7 low-spin complex, which is a useful precursor for a series of Co(I), Co(II), and Co(III) PCP complexes. Complex 2 reacts readily with CO and pyridine to afford the five-coordinate square-pyramidal 17e complexes [Co(PCPMe-iPr)(CO)Cl] (4) and [Co(PCPMe-iPr)(py)Cl] (5), respectively, while in the presence of Ag+ and CO the cationic complex [Co(PCPMe-iPr)(CO)2]+ (6) is afforded. The effective magnetic moments μeff of all Co(II) complexes were derived from the temperature dependence of the inverse molar magnetic susceptibility by SQUID measurements and are in the range 1.9 to 2.4 μB. This is consistent with a d7 low-spin configuration with some degree of spin–orbit coupling. Oxidation of 2 with CuCl2 affords the paramagnetic Co(III) PCP complex [Co(PCPMe-iPr)Cl2] (7), while the synthesis of the diamagnetic Co(I) complex [Co(PCPMe-iPr)(CO)2] (8) was achieved by stirring 2 in toluene with KC8 in the presence of CO. Finally, the cationic 16e Ni(II) PCP complex [Ni(PCPMe-iPr)(CO)]+ (10) was obtained by reacting complex 3 with 1 equiv of AgSbF6 in the presence of CO. The reactivity of CO addition to Co(I), Co(II), and Ni(II) PCP square planar complexes of the type [M(PCPMe-iPr)(CO)]n (n = +1, 0) was investigated by DFT calculations, showing that formation of the Co species, 6 and 8, is thermodynamically favorable, while Ni(II) maintains the 16e configuration since CO addition is unfavorable in this case. X-ray structures of most complexes are provided and discussed. A structural feature of interest is that the apical CO ligand in 4 deviates significantly from linearity, with a Co–C–O angle of 170.0(1)°. The DFT-calculated value is 172°, clearly showing that this is not a packing but an electronic effect.

Dynamic spin interchange where crystals explode with preservation of magnetic memory is observed for a mononuclear hysteretic Fe(iii) Schiff-base compound.