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.

杂自旋双螯合配合物NiL₂（其中L为去质子化氮氧自由基4-(3',3',3'-三氟甲基-2'-氧代亚丙基)-2,2,5,5-四甲基-3-咪唑啉-1-氧基）与醇类共结晶，可理性组装得到层状配合物[NiL₂(CH₃OH)₂（1）与NiL₂(C₂H₅OH)₂（2）]或骨架型配合物[NiL₂(HO(CH₂)₄OH)（3）与NiL₂(HO(CH₂)₅OH)（4）]。配合物1、2和4的结构已得到解析。晶体学数据如下：1属单斜晶系，空间群P2₁/c，晶胞参数a=8.929(2) Å，b=15.773(3) Å，c=11.518(2) Å，β=118.84(1)°，Z=2；2属单斜晶系，空间群P2₁/c，晶胞参数a=9.923(5) Å，b=15.992(5) Å，c=11.486(2) Å，β=120.63(3)°，Z=2；4属单斜晶系，空间群C2/c，晶胞参数a=21.028(4) Å，b=10.943(2) Å，c=14.405(3) Å，β=110.61(3)°，Z=4。配合物3的结构与4极为相近，且已在早前研究中被报道。配位醇的羟基与相邻NiL₂片段的>N−•O基团间存在强氢键作用，使得固态下形成聚合物层。与此同时，氢键构筑了有效的磁相互作用通道：>N−•O···H−OR−Ni²⁺−RO−H···O•−N<。1~4均存在7 K以下转变为弱铁磁态的本征磁相变。对于2和4，该相变可由外磁场诱导，这一现象与相较于1和3，2、4中交换通道>N−•O···H−OR−Ni²⁺−RO−H···O•−N<内的Ni−RO及RO···O•−键长伸长存在关联。研究表明，磁有序仅存在于由多重氢键构筑的聚合物层内。对1~4的大尺寸单晶开展磁化率各向异性研究后发现，1、2与3、4的磁亚晶格磁矩排布存在显著差异。