Alexei B. Burdukov

Molecular Ferromagnets Based on Metal Complexes with 3-Imidazoline Nitroxides

Abstract The approach to the design of molecular magnets based on metal complexes with 3-imidazoline nitroxides is discussed. Both polymeric bischelate complexes demonstrating bulk magnetization and molecular model compounds are considered. Some aspects of quantum-chemical treatment of exchange interactions in metal-nitroxide exchange clusters are presented.

Metal complexes of the nitrile-substituted nitroxides

Abstract The chelates of the nitrile-substituted 3-imidazoline nitroxides with copper(II), cobalt(II), and nickel(II) have been synthesized and studied with X-ray diffraction and magnetic susceptibility measurements. The influence of the nitrile substituent on structural and magnetic features of the complexes are discussed.

The first example of the cubic NbO-type metal–organic framework derived from a metal bis-chelate

Abstract The series of compounds M(dbmCN)2 (dbmCN=dibenzoylacetonitrile, M=Cu, Co, Ni, Mn, Fe) has been synthesized and structurally characterized. The compounds exhibit robust metal-organic framework of cubic symmetry, and are capable of clathration of small molecules.

Two Cu(hfac)2 complexes with the imidazoline nitroxide 4-amido-2,2-dimethyl-5,5-dimethoxy-3-imidazoline-1-oxyl. Conversion of a discrete molecular structure to a chain structure as a result of stoichiometric changes

Abstract The reaction of copper hexafluoroacetylacetonate with the stable nitroxide 4-amido-2,2-dimethyl-5,5-dimethoxy-3-imidazoline-l-oxyl (L) in a ratio of 2:1 in hexane yielded the Cu(hfac) 2 -saturated coordination chain polymer of composition [Cu(hfac) 2 ] 2 L. In [Cu(hfac) 2 ] 2 L, half of the Cu(hfac) 2 molecules coordinate L in bidentate fashion by the amide-group oxygen and the imine nitrogen of the heterocycle. The other Cu(hfac) 2 molecules arrange the above {Cu(hfac) 2 L} fragments in polymer chains by alternate coordination of either the N-O group, or the amide-group oxygens of two {Cu(hfac) 2 L} fragments to axial positions. As a result [Cu(hfac) 2 ] 2 LL involves three copper ions with coordination number 6 and with different coordination environments. For metal complexes with nitroxides, this mode of chain formation was observed for the first time. The magnetic properties of the complex are described in terms of an exchange cluster model with ferromagnetic intracluster exchange interactions.

Metal chelates of the amide-substituted nitroxides: the influence of hydrogen bonding on the structure of metal-nitroxide complexes

Abstract The amide function was introduced into metal chelates derived from 3-imidazoline-1-oxyl enaminoketones (MCo, Ni) with the aim of investigation of the influence of hydrogen bonding upon the structure and properties of metal-nitroxide complexes. The complexes obtained manifest strong tendency to include different solvents due to clathration, hydrogen bonding, or hydrogen bonding combined with coordinative bonding. The hydrogen bond patterns vary from linear chains arising from amide–amide interactions to complex 3D motifs formed with participation of the solvent molecules (water, methanol). The hydrogen bonds can act as magnetically innocent intermolecular junctions as well as exchange pathways between the paramagnetic centers. The magnetic properties of the complexes are presented and discussed in connection with their structure.

The competition of basicity and steric factors in the nitroxide donor functions in metal complexes: The study of M(hfac)2(M = Co, Ni) adducts with 3-imidazoline nitroxides

Abstract The similar nitroxides 2,2,5,5-tetramethyl-4-phenyl-3-imidazoline-1-oxyl (L1) and 2,2-dimethyl-5,5-dimethoxy-4-phenyl-3-imidazoline-1-oxyl (L2) from complexes of different composition on reaction with cobalt and nickel hexafluoroacetylacetonates: M(hfac)2L21 and M(hfac)2 (H2O)2L22. In the M(hfac)2L21 complexe the nitroxides are bonded to the metal ion through the nitroxide oxygen but the imine nitrogen does not participate in coordination for steric reasons. In M(hfac)2(H2O)2L22 the nitroxide moieties are H-bonded to the coordinated water molecules through the imine nitrogen atoms and methoxy oxygen atoms. The nitroxide L2 is unstable in the presence of Co(hfac)2 and on removal of water decomposes to yield the adduct with nitrone Co(hfac)2L23, L3 = 2,2-dimethyl-5-methoxy-4-phenyl-2H-imidazol 1-oxide. The complexes M(hfac)2L21 demonstrate strong antiferromagnetic coupling between the metal ion and nitroxides.

Synthesis and structural characterization of the first metal complex with an indole nitroxide

Abstract Copper hexafluoroacetylacetonate reacts with an indole nitroxide (1,2-dihydro-3-oxo-2,2-diphenyl-3H-indole-1-oxyl) in hexane to yield a trinuclear complex with two bridging nitroxides, bis-[μ-(1,2-dihydro-3-oxo-2,2-diphenyl3-H-indole1-1-oxyl)-O; O′]hexakis(hexafluoroacetylacetonato)tricopper(II), which has been structurally characterized by X-ray crystallography. The nitroxide functions of the paramagnetic ligands are coordinated to the terminal copper atoms. Magnetic measurements revealed strong antiferro-magnetic coupling between the spins of the nitroxides and of the terminal copper atoms.

Crystal Structure of Copper(II) and Cobalt(II) Hexafluoroacetylacetonate Complexes with 2,2,5,5-Tetramethyl-4-phenyl-3-imidazoline-1-oxyl-3-oxide

The crystal structures of the copper(II) and cobalt(II) hexafluoroacetylacetonate complexes with the nitroxide 2,2,5,5-tetramethyl-4-phenyl-3-imidazoline-1-oxyl-3-oxide (PhImOx) Cu(hfac)2(PhImOx), Co(hfac)2(PhImOx), and Co(hfac)2(PhImOx)2 have been determined. The former two are chain type complexes, whereas the latter has a molecular structure. The coordination polyhedra of the copper and cobalt atoms are tetragonally distorted octahedra. In the two chain type compounds, the octahedra are formed from the hfac anion, nitroxyl, and N-oxide O atoms; in the molecular complex, the O atoms of the N∸O groups are not coordinated to the metal atoms. In the structure of Cu(hfac)2(PhImOx), F...F contacts equal to 2.805 Å have been found.

Complexes with Nitr' oxides Containing Different Substituents in the α-Position to N·-O Group Competition of Basicity and Sterical Availability of the Donor Functions of Nitroxides in Metal Complexes with Sterically Hindered Imidazolines

Abstract In the nitroxide molecule, change of the substituent in the α-position to the N·-O group affects both the donor properties of this group and its steric availability. Metal complexes with three nitroxide pairs are used to show that the above change of the α-substituent in the nitroxide molecule can induce structural transformation of the molecular complex itself (and, as a consequence, change its magnetic properties), change the magnetic properties of the solid complex, or change the chemical behavior of the nitroxide.

Synthesis, structure, and magnetic properties of mixed-ligand complexes of manganese and gadolinium hexafluoroacetylacetonates with 4-phenyl-2,2,5,5-tetramethyl-3-imidazoline-3-oxide-1-oxyl nitroxide

Complexes of manganese(II) and gadolinium(III) hexafluoroacetylacetonates with stable nitroxide 4-phenyl-2,2,5,5-tetramethyl-3-imidazoline-3-oxide-1-oxyl (L), Mn(hfac)2L2 and Gd(hfac)3L2, have been synthesized. The results of an X-ray study of these complexes (Mn(hfac)2L2: a=21.762(4), b=9.685(1), c=22.073(3) Å, β=114.94(1)°, space group Cc, Z=4; Gd(hfac)3L2: a=22.079(4), b=11.562(2), c=21.302(4) Å, β=112.25(1)°, space group Cc, Z=4) have shown that both compounds have a molecular structure with the paramagnetic ligand coordinated via the oxygen atom of the N-oxide group. According to the results of static magnetic susceptibility measurements, the energies of exchange interactions between unpaired electrons of L and of metal ions are less than 1 cm−1 in magnitude.