Molecular Magnets Based on Nickel(II) Complexes with 3-Imidazoline Nitroxides and Alcohols

Crystallization of heterospin bischelate NiL2, where L is deprotonated nitroxide 4-(3‘,3‘,3‘-trifluoromethyl-2‘-oxopropylidene)-2,2,5,5-tetramethyl-3-imidazoline-1-oxyl, with alcohols allows a logical assembly of layered
[NiL2(CH3OH)2 (1) and NiL2(C2H5OH)2 (2)] or frame compounds [NiL2(HO(CH2)4OH) (3) and NiL2(HO(CH2)5OH) (4)]. The structures of complexes 1, 2, and 4 have been determined. Crystal data:  1, monoclinic, space
group P21/c, a = 8.929(2) Å, b = 15.773(3), c = 11.518(2), β = 118.84(1)°, Z = 2; 2, monoclinic, space group
P21/c, a = 9.923(5) Å, b = 15.992(5) Å, c = 11.486(2) Å, β = 120.63(3)°, Z = 2; 4, monoclinic, space group
C2/c, a = 21.028(4) Å, b = 10.943(2), c = 14.405(3), β = 110.61(3)°, Z = 4. The structure of 3 is very close
to the structure of 4 and has been reported earlier. Robust H-bonding between the OH groups of the coordinated
alcohols and the >N−•O groups of the neighboring fragments NiL2 leads to the formation of polymeric layers in
the solid. At the same time, hydrogen bonds form effective channels for magnetic interactions >N−•O···H−OR−Ni2+−RO−H···O•−N<. The magnetic phase transition to the weak ferromagnetic state below 7 K is inherent
in 1−4. For 2 and 4, this transition is induced by the external magnetic field that correlates with the elongation
of Ni−RO and RO···O•− distances in the exchange channels >N−•O···H−OR−Ni2+−RO−H···O•−N< in solids
2 and 4 compared to 1 and 3. It has been found that magnetic ordering occurs only within the polymeric layers
formed due to multiple hydrogen bonds. Investigation of the anisotropy of magnetic susceptibility performed on
large single crystals of 1−4 revealed an essentially different ordering of the magnetic moments of magnetic
sublattices in 1, 2 and in 3, 4.