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Coordination‐Induced Spin Crossover (CISCO) through Axial Bonding of Substituted Pyridines to Nickel–Porphyrins: σ‐Donor versus π‐Acceptor Effects
Author(s) -
Thies Steffen,
Bornholdt Claudia,
Köhler Felix,
Sönnichsen Frank D.,
Näther Christian,
Tuczek Felix,
Herges Rainer
Publication year - 2010
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201000603
Subject(s) - pyridine , chemistry , porphyrin , nickel , crystallography , enthalpy , spin crossover , spectrochemical series , entropy of activation , stereochemistry , medicinal chemistry , ligand field theory , photochemistry , reaction rate constant , thermodynamics , kinetics , organic chemistry , physics , ion , quantum mechanics
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 K 1S and K 2 , and for the first time also the corresponding thermodynamic parameters Δ H 1S , Δ H 2 , Δ S 1S , and Δ S 2 , are determined for the reaction of a nickel‐porphyrin (Ni‐tetrakis(pentafluorophenyl)porphyrin) with different 4‐substituted pyridines by temperature‐dependent NMR spectroscopy. The association constants K 1S and K 2 are correlated with the basicity of the 4‐substituted pyridines (R: OMe>H>CO 2 Et>NO 2 ) whereas the Δ H 1S values exhibit a completely different order (OMeCO 2 Et>NO 2 ). 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 d xz and d yz 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 K 1S relative to K 2 . For nitropyridine K 2 is 17 times larger than K 1S and in the case of methoxypyridine K 2 and K 1S are almost equal.