Synthesis, Structures, and Magnetic Properties of Heterodimetal Bis(μ-hydroxo)chromium(III)nickel(II) Complexes with Tpa Derivatives Having Sterically Bulky Substituents

A series of heterodinuclear bis(μ-hydroxo)chromium(III)nickel(II) complexes was newly prepared:  [(phen)2Cr(μ-OH)2Ni(tpa)](ClO4)3·0.5H2O (1), [(phen)2Cr(μ-OH)2Ni(Me-tpa)](ClO4)3·2H2O (2), [(phen)2Cr(μ-OH)2Ni(Me2-tpa)](ClO4)3·2H2O (3), and [(phen)2Cr(μ-OH)2Ni(Me3-tpa)](ClO4)3·3H2O (4), where phen is 1,10-phenanthroline and tpa, Me-tpa,
Me2-tpa, and Me3-tpa are tris(2-pyridylmethyl)amine, [(6-methyl-2-pyridyl)methyl]bis(2-pyridylmethyl)amine, bis[(6-methyl-2-pyridyl)methyl](2-pyridylmethyl)amine, and tris[(6-methyl-2-pyridyl)methyl]amine, respectively. X-ray crystallography revealed that the structures of 1−4 resemble one another having an edge-shared bioctahedral structure
with a Cr(μ-OH)2Ni unit (crystal data:  1·C2H5OH, triclinic, P1̄, a = 13.179(4) Å, b = 13.685(4) Å, c = 14.260(4)
Å, α = 84.95(2)°, β = 77.65(1)°, γ = 90.21(2)°, V = 2502(1) Å3, Z = 2, R = 0.103, Rw = 0.097; 2·C2H5OH,
triclinic, P1̄, a = 13.214(2) Å, b = 13.657(2) Å, c = 14.417(3) Å, α = 95.205(5)°, β = 102.583(4)°, γ =
90.720(3)°, V = 2527.3(8) Å3, Z = 2, R = 0.090, Rw = 0.122; 3·C2H5OH, triclinic, P1̄, a = 13.276(2) Å, b =
13.696(2) Å, c = 14.454(2) Å, α = 95.640(3)°, β = 102.821(4)°, γ = 90.174(3)°, V = 2549.5(6) Å3, Z = 2, R
= 0.087, Rw= 0.119; 4, triclinic, P1̄, a = 10.8916(9) Å, b = 14.268(2) Å, c = 17.522(2) Å, α = 84.498(9)°, β
= 74.313(7)°, γ = 72.402(7)°, V = 2498.6(5) Å3, Z = 2, R = 0.060, Rw= 0.088). Chromium and nickel ions are
coordinated by two phen's and Men-tpa, respectively, to complete a distorted octahedral coordination sphere.
Introduction of the 6-methyl group(s) onto the pyridyl group(s) results in the elongation of the Ni−N bond distances
due to an unfavorable steric interaction between the methyl group and the bridging hydroxide group:  systematic
elongation of the Ni−N bond distances and the Cr···Ni separations accompanied by an increase in the Cr−O−Ni
angles was observed as the number of the methyl groups increases. Variable-temperature magnetic susceptibility
measurements of 1−4 (4.2−300 K) indicated that magnetic interactions between Cr(III) and Ni(II) ions are
systematically modulated from a very weak antiferromagnetic interaction to a ferromagnetic interaction as the number
of the methyl groups increases; the exchange integrals J's for 1−4 are estimated to be −1.4, +0.0, +4.1, and +7.4
cm-1, respectively. The magneto−structural relationship is discussed in terms of the change in the magnetic orbital
energies of nickel(II) centers arising from the change in the Ni−N bond distances.