Coordination‐Induced Spin Crossover (CISCO) through Axial Bonding of Substituted Pyridines to Nickel–Porphyrins: σ‐Donor versus π‐Acceptor Effects

Abstract

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 DeltaH(1S), DeltaH(2), DeltaS(1S), and DeltaS(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 DeltaH(1S) values exhibit a completely different order (OMe<H>CO(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 pi 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.

Rational design of a room temperature molecular spin switch. The light-driven coordination induced spin state switch (LD-CISSS) approach

“…22,38 Our calculations confirm the increase in binding strength of pyridine as a function of electron withdrawing substituents in meso-position of the porphyrin. The calculated association energy of pyridine to Ni-TPPF 20 (−6.96 kcal mol −1 ) is 3.6 times higher than to the Ni-TPP (−1.96 kcal mol −1 ).…”

“…[22][23][24] Binding enthalpy is smaller compared to pyridines which is compensated by a smaller entropy. As expected the binding enthalpy for the meso phenyl substituted record player 1 is much lower (−1.19 kcal mol −1 ) whereas entropy is almost equal (−9.52 cal mol −1 K −1 ).…”

“…are included as well22 Relative energy includes two uncoordinated pyridine molecules. b Relative energy includes one uncoordinated pyridine.…”

Tuning the electrical conductance of metalloporphyrin supramolecular wires

“…Numerous studies have examined the effect of the axial ligand on the redox2829 and photovoltaic properties of metalloporphyrins30. Nickel porphyrin with coordinating axial ligands are paramaganetic (S = 1) in contrast to four-coordinate species (S = 0)3132. Therefore, to accurately calculate the transport properties of these molecules spin polarized transport calculations must be carried out.…”

Singly versus Doubly Reduced Nickel Porphyrins for Proton Reduction: Experimental and Theoretical Evidence for a Homolytic Hydrogen‐Evolution Reaction

“…X‐ray diffraction studies revealed that Ni‐P crystallized in the tetragonal space group $I\bar{4}2d$ with the Ni atom located at the crystallographically required S 4 axis. The macrocycle of Ni‐P is saddled rather than planar, and those pentafluorophenyl groups at trans positions are staggered, a conformation commonly seen in metal porphyrins 39. The four identical N−Ni−N bond angles at 90° and Ni−N bond distances at 1.9242(14) Å suggest that the Ni atom is located perfectly at the center of the planar square defined by the four N atoms.…”