Oxamidate-Bridged Dinuclear Five-Coordinate Nickel(II) Complexes: A Magneto−Structural Study

The preparation, spectroscopic characterization, and magnetic study of three new oxamidate-bridged nickel(II) dinuclear
complexes of formulas {[Ni(Me3[12]aneN3)]2(μ-oa)}(PF6)2 (1), {[Ni(Me3[12]aneN3)]2(μ-dmoa)}(PF6)2 (2), and {[Ni(Me3[12]aneN3)]2(μ-dpoa)}(PF6)2 (3) (Me3[12]aneN3 = 2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene, oa = oxamidate,
dmoa = N,N‘-dimethyloxamidate, and dpoa = N,N‘-diphenyloxamidate) are reported. The crystal structures of two
of them (1 and 3) have been determined. 1 and 3 crystallize in the monoclinic system, space group P21/c, with Z
= 2 and a = 7.901(4) Å, b = 13.597(6) Å, c = 17.565(10) Å, and β = 96.46(4)° for 1 and a = 13.854(3) Å, b
= 17.469(4) Å, c = 12.543(3) Å, and β = 116.22(3)° for 3. The structures of 1 and 3 consist of dinuclear
{[Ni(Me3[12]aneN3)]2(μ-oa)}2+ and {[Ni(Me3[12]aneN3)]2(μ-dpoa)}2+ cations and hexafluorophosphate anions. Each
nickel in 1−3 is five-coordinate, and the substitution of the hydrogen atom of the amidate nitrogen of 1 by a methyl
(2) or a phenyl (3) group causes a significant modification of the stereochemistry of the nickel(II) ions from square
pyramidal toward trigonal bipyramidal (τ values of 0.12 and 0.48 for 1 and 3, respectively). The NOESY spectrum
of 3 has allowed us to achieve the assignment of the phenyl protons of the N,N‘-diphenyloxamidate. The value of
magnetic coupling between the two nickel(II) ions across the oxamidate bridge [J = −57.0 (oa, 1), −38.0 (dmoa,
2) and −30.5 cm-1 (dpoa, 3)] is very sensitive to this stereochemical change, and its variation is explained on the
basis of orbital considerations. DFT type calculations have been performed to analyze and substantiate the trend
of the magnetic coupling in 1−3.