A Survey of the Angular Distortion Landscape in the Coordination Geometries of High-Spin Iron(II) 2,6-Bis(pyrazolyl)pyridine Complexes

Reaction of 2,4,6-trifluoropyridine with sodium 3,4-dimethoxybenzenethiolate and 2 equiv of sodium pyrazolate in tetrahydrofuran at room temperature affords 4-(3,4-dimethoxyphenylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine (L), in 30% yield. The iron(II) complexes [FeL2][BF4]2 (1a) and [FeL2][ClO4]2 (1b) are high-spin with a highly distorted six-coordinate geometry. This structural deviation from ideal D2d symmetry is common in high-spin [Fe(bpp)2]2+ (bpp = di{pyrazol-1-yl}pyridine) derivatives, which are important in spin-crossover materials research. The magnitude of the distortion in 1a and 1b is the largest yet discovered for a mononuclear complex. Gas-phase DFT calculations at the ω-B97X-D/6-311G** level of theory identified four minimum or local minimum structural pathways across the distortion landscape, all of which are observed experimentally in different complexes. Small distortions from D2d symmetry are energetically favorable in complexes with electron-donating ligand substituents, including sulfanyl groups, which also have smaller energy penalties associated with the lowest energy distortion pathway. Natural population analysis showed that these differences reflect greater changes to the Fe-N{pyridyl} σ-bonding as the distortion proceeds, in the presence of more electron-rich pyridyl donors. The results imply that [Fe(bpp)2]2+ derivatives with electron-donating pyridyl substituents are more likely to undergo cooperative spin transitions in the solid state. The high-spin salt [Fe(bpp)2][CF3SO3]2, which also has a strong angular distortion, is also briefly described and included in the analysis.

Spin State Behavior of A Spin-Crossover Iron(II) Complex with N,N′-Disubstituted 2,6-bis(pyrazol-3-yl)pyridine: A Combined Study by X-ray Diffraction and NMR Spectroscopy

A series of three different solvatomorphs of a new iron(II) complex with N,N′-disubstituted 2,6-bis(pyrazol-3-yl)pyridine, including those with the same lattice solvent, has been identified by X-ray diffraction under the same crystallization conditions with the metal ion trapped in the different spin states. A thermally induced switching between them, however, occurs in a solution, as unambiguously confirmed by the Evans technique and an analysis of paramagnetic chemical shifts, both based on variable-temperature NMR spectroscopy. The observed stabilization of the high-spin state by an electron-donating substituent contributes to the controversial results for the iron(II) complexes of 2,6-bis(pyrazol-3-yl)pyridines, preventing ‘molecular’ design of their spin-crossover activity; the synthesized complex being only the fourth of the spin-crossover (SCO)-active kind with an N,N′-disubstituted ligand.

In Situ NMR Search for Spin-Crossover in Heteroleptic Cobalt(II) Complexes

Here we report the first successful attempt to identify spin-crossover compounds in solutions of metal complexes produced by mixing different ligands and an appropriate metal salt by variable-temperature nuclear magnetic resonance (NMR) spectroscopy. Screening the spin state of a cobalt(II) ion in a series of thus obtained homoleptic and heteroleptic compounds of terpyridines (terpy) and 2,6-bis(pyrazol-3-yl)pyridines (3-bpp) by using this NMR-based approach, which only relies on the temperature behavior of chemical shifts, revealed the first cobalt(II) complexes with a 3-bpp ligand to undergo a thermally induced spin-crossover. A simple analysis of NMR spectra collected from mixtures of different compounds without their isolation or purification required by the current method of choice, the Evans technique, thus emerges as a powerful tool in a search for new spin-crossover compounds and their molecular design boosted by wide possibilities for chemical modifications in heteroleptic complexes.

Spectral properties of supramolecular systems based on cobalt(ii)/manganese(iii) phthalocyanine and fullero[60]pyrrolidines with PET

Photoinduced electron transfer in the metallophthalocyanine–fullerene dyads was confirmed and the main “chemical structure – spectral properties” dependences were revealed.

Stereochemistry of Hexacoordinated Zn(II), Cu(II), Ni(II), and Co(II) Complexes with Iminodiacetamide Ligands

Metal complexes of iminodiacetamide (imda) ligands and metal ions Zn(II), Cu(II), Ni(II), and Co(II) were prepared using eight imda ligands (L1-L8) substituted with groups of different steric and electronic properties on the central amine N atom (H atom, methyl, isopropyl, and benzyl) and the para position of the phenyl rings (nitro and dimethylamino). The effect of these substituents on the stoichiometry (ML and ML₂), geometry, and stereochemistry (mer, trans-fac, cis-fac) of the complexes was studied in the solid state, in solution, and by density functional theory calculations. Single-crystal and powder X-ray diffraction, thermogravimetry, and IR spectroscopy showed that in the solid state imda ligands preferentially form trans-fac ML₂ complexes, with the exception of the cis-fac complex 7_Zn. NMR spectroscopy of diamagnetic Zn(II) and paramagnetic Co(II) complexes revealed the formation of both ML and ML₂ complexes in solution, which was also confirmed by UV-vis titrations. Variable-temperature NMR was used to study the effect of the substituent on the central amine N atom on the Zn-N bond strength and nitrogen inversion. The relative stabilities of the isomers were rationalized by computations and the optimized structures used for geometry analysis.