1253288-45-7Relevant academic research and scientific papers
Coordination-induced spin crossover (CISCO) through axial bonding of substituted pyridines to nickel-porphyrins: σ-Donor versus π-acceptor effects
Thies, Steffen,Bornholdt, Claudia,Koehler, Felix,Soennichsen, Frank D.,Naether, Christian,Tuczek, Felix,Herges, Rainer
, p. 10074 - 10083 (2010)
Nickel-porphyrins, with their rigid quadratic planar coordination framework, provide an excellent model to study the coordination-induced spin crossover (CISCO) effect because bonding of one or two axial ligands to the metal center leads to a spin transition from S=0 to S=1. Herein, both equilibrium constants K1S and K2, and for the first time also the corresponding thermodynamic parameters ΔH1S, ΔH2, ΔS1S, and ΔS2, are determined for the reaction of a nickel-porphyrin (Nitetrakis(pentafluorophenyl) porphyrin) with different 4-substituted pyridines by temperature-dependent NMR spectroscopy. The association constants K1S and K2 are correlated with the basicity of the 4-substituted pyridines (R: OMe>H>CO2Et-NO2) whereas the ΔH1S values exhibit a completely different order (OMeCO2Et> NO2). 4-Nitropyridine exhibits the largest binding enthalpy, which, however, is overcompensated by a large negative binding entropy. We attribute the large association enthalpy of nitropyridine with porphyrin to the back donation of electrons from the Ni dxz and dyz orbitals into the π orbitals of pyridine, and the negative association entropy to a decrease in vibrational and internal rotation entropy of the more rigid porphyrin-pyridine complex. Back donation for the nitro-and cyanopyridine complexes is also confirmed by IR spectroscopy, and shows a shift of the N-O and C-N vibrations, respectively, to lower wave numbers. X-ray structures of 2:1 complexes with nitro-, cyano-, and dimethylaminopyridine provide further indication of a back donation. A further trend has been observed: the more basic the pyridine the larger is K1S relative to K2. For nitropyridine K2 is 17 times larger than K1S and in the case of methoxypyridine K2 and K1S are almost equal.
