Spin-state equilibria in non-aqueous solution and quantum-mechanical investigations of iron(II) and nickel(II) complexes with 4-substituted 2,6-bis(benzimidazol-2-yl)pyridines

authored by: Wolfgang Linert, Michael Konecny, Franz Renz
Abstract: Cationic complexes with a series of tridentate ligands, L = 4X-substituted 2,6-bis(benzimidazol-2-yl)pyridines, [ML₂][ClO₄]₂ (M = Fe or Ni; X = H, OH or Cl), were isolated and characterized, together with the free pyridines, by elemental analysis, Fourier-transform IR, ¹H NMR and UV/VIS spectroscopy. The syntheses were performed via condensation of o-phenylenediamine with 4-substituted pyridine-2,6-dicarboxylic acids. Ligand-field parameters were estimated for the nickel complexes. The [FeL₂]²⁺ species show thermally induced spin-crossover behaviour (¹A₁→⁵T_2g) which has been investigated in methanol, nitromethane and 20% (v/v) dimethylformamide in MeOH. The behaviour is complicated by two complex dissociation equilibria, for which equilibrium constants have been evaluated. Ligand substitution is reflected in a change of the spin state in solution (μ_exptl = 2.50, X = H; 4.19, OH; and 4.49 μ_B, Cl at 295 K, in MeOH) and in the metal-to-ligand charge-transfer band (500-557 nm); when M = Fe and X = H there is a pronounced spin-crossover equilibrium in methanolic solution (μ_exptl = 1.31-3.45 μ_B for 213-328 K). A small variation of the magnetic moments when M = Fe and X = OH (μ_exptl = 3.77-4.73 μ_B at 220-332 K) might indicate a temperature-variable population of the ⁵E_g sublevel or variation in hydrogen bonding. The results are compared with quasi-relativistic quantum-mechanical calculations, and the spin-crossover behaviour of the new ligands, L, with substituents X = CHO, NH₂, CN, Me, NO₂, OH, CONH₂, COCl, SH, F, Cl, Br or I has been estimated. The differences in the calculated heats of formation between the high-and low-spin forms of [FeL₂]²⁺ when plotted against Δδ (=¹H NMR para increment for substituents X in benzene) show a turning point in the region around X = H and in this region spin-crossover behaviour is observed. Outside this region there is very little or no such behaviour and it is therefore possible to predict the spin-crossover behaviour for other substituents X from the Δδ value.
Organisation(s): Institute of Inorganic Chemistry
External Organisation(s): TU Wien (TUW)
Type: Article
Journal: Journal of the Chemical Society, Dalton Transactions
Pages: 1523-1531
No. of pages: 9
ISSN: 1472-7773
Publication date: 1994
Publication status: Published
Peer reviewed: Yes
ASJC Scopus subject areas: General Chemistry
Electronic version(s): https://doi.org/10.1039/DT9940001523 (Access: Unknown)

BibTeX

@article{0d1c6eba99ea4078b352c7b05f1593e2,
title = "Spin-state equilibria in non-aqueous solution and quantum-mechanical investigations of iron(II) and nickel(II) complexes with 4-substituted 2,6-bis(benzimidazol-2-yl)pyridines",
abstract = "Cationic complexes with a series of tridentate ligands, L = 4X-substituted 2,6-bis(benzimidazol-2-yl)pyridines, [ML2][ClO4]2 (M = Fe or Ni; X = H, OH or Cl), were isolated and characterized, together with the free pyridines, by elemental analysis, Fourier-transform IR, 1H NMR and UV/VIS spectroscopy. The syntheses were performed via condensation of o-phenylenediamine with 4-substituted pyridine-2,6-dicarboxylic acids. Ligand-field parameters were estimated for the nickel complexes. The [FeL2]2+ species show thermally induced spin-crossover behaviour (1A1→5T2g) which has been investigated in methanol, nitromethane and 20% (v/v) dimethylformamide in MeOH. The behaviour is complicated by two complex dissociation equilibria, for which equilibrium constants have been evaluated. Ligand substitution is reflected in a change of the spin state in solution (μexptl = 2.50, X = H; 4.19, OH; and 4.49 μB, Cl at 295 K, in MeOH) and in the metal-to-ligand charge-transfer band (500-557 nm); when M = Fe and X = H there is a pronounced spin-crossover equilibrium in methanolic solution (μexptl = 1.31-3.45 μB for 213-328 K). A small variation of the magnetic moments when M = Fe and X = OH (μexptl = 3.77-4.73 μB at 220-332 K) might indicate a temperature-variable population of the 5Eg sublevel or variation in hydrogen bonding. The results are compared with quasi-relativistic quantum-mechanical calculations, and the spin-crossover behaviour of the new ligands, L, with substituents X = CHO, NH2, CN, Me, NO2, OH, CONH2, COCl, SH, F, Cl, Br or I has been estimated. The differences in the calculated heats of formation between the high-and low-spin forms of [FeL2]2+ when plotted against Δδ (=1H NMR para increment for substituents X in benzene) show a turning point in the region around X = H and in this region spin-crossover behaviour is observed. Outside this region there is very little or no such behaviour and it is therefore possible to predict the spin-crossover behaviour for other substituents X from the Δδ value.",
author = "Wolfgang Linert and Michael Konecny and Franz Renz",
year = "1994",
doi = "10.1039/DT9940001523",
language = "English",
pages = "1523--1531",
journal = "Journal of the Chemical Society, Dalton Transactions",
issn = "1472-7773",
publisher = "Royal Society of Chemistry",
number = "10",
}