Michael P. Bubnov

First structurally characterized mixed-halogen nickel(III) NCN-pincer complex

A square-pyramidal mixed-halogen nickel(III) NCN-pincer complex (PipeNCN)NiClBr (where PipeNCN=2,6-bis(piperidinomethyl)phenyl) was structurally characterized. Bromine occupies apical position; pincer ligand and chlorine atom are in the basal plane. EPR detects that complex in solution exists as a mixture of two structural isomers with bromine or chlorine atoms in the top of pyramid.

Ferromagnetic Coupling in the Heterospin Bis-Catecholato–Manganese(IV) Complex with Pyridine Substituted by Nitronyl-nitroxide

A new bis(3,6-di-tert-butyl-catecholato)manganese complex with two 4-NIT-Py ligands was synthesized and characterized [4-NIT-Py = pyridine substituted at position 4 with nitronyl-nitroxide radical, 2-(pyridin-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl]. X-ray diffraction indicated an octahedral environment of the manganese atom with a trans arrangement of ligands. Bonds lengths in the inner coordination core of the metal and in the chelate cycles that are representative of the charge distribution between the metal and ligands displayed a Mn(IV)(Cat2-)2 charge distribution. Variable-temperature magnetic susceptibility measurements detected intramolecular ferromagnetic coupling between the Mn(IV) S = 3/2 spins and spins of nitronyl-nitroxyls and intermolecular ferromagnetic interactions of spins of adjacent nitronyl-nitroxide fragments in a chain of molecules at low temperatures. The last phenomenon is revealed by short contacts between nitronyl-nitroxide radicals of adjacent complex molecules.

EPR study of intramolecular dynamics in o-semiquinonic nickel complexes with PCP-pincer ligand in solution

A number of square pyramidal o-semiquinonic nickel complexes with 2,6-bis(diphenylphosphinomethyl)phenyl were investigated by EPR spectroscopy in solution. The complexes have flexible coordination spheres and can exist in solution as two isomers rapidly (on the EPR timescale) interconverting one to another at room temperature. The equilibrium is entropy driven and does not significantly depend on the substituents in the semiquinone ring. The spin density distribution depends on mesomeric properties and bulkiness of substituents in ortho-position to oxygen atoms.

Valence–Tautomeric Interconversion in a Bis(dioxolene)cobalt Complex with Iminopyridine Functionalized by a TEMPO Moiety. Phase Transition Coupled with Monocrystal Destruction

Iminopyridine modified by TEMPO nitroxide was utilized for the synthesis of an octahedral bis(o-semiquinonato)cobalt complex. Variable-temperature magnetic susceptibility measurements detect a valence tautomeric transformation in the temperature range 200-300 K. A reproducible hysteresis loop of about 40 K width is observed on the magnetic moment temperature dependence in the transition region. Differential scanning calorimetry measurements confirm different temperatures of phase transitions accompanying a valence-tautomeric transformation upon heating and cooling. Attempts to study the structural changes associated with the valence-tautomeric transformation by single-crystal X-ray diffraction failed because of the crystal destruction taking place upon cooling from 220 K. The powder X-ray diffraction pattern indicated an essential change of the unit cell upon cooling from 240 K.

An EPR study of the intramolecular dynamics in o-semiquinonic nickel complexes with a diphosphorous pincer ligand.

Square pyramidal o-semiquinonic nickel complexes with 2,6-bis(diphenylphosphinomethyl)phenyl have a flexible coordination sphere and exist in solution as the mixture of tautomers rapidly, according to EPR time scale, interconverting at room temperature.