Aminomethyl-bipyridine Bearing Two Flexible Nitronyl-nitroxide Arms: A New Podand for Complexation of Transition Metals in a Facial or Meridional Conformation

Transition metal complexes of 6-aminomethyl-bis[methyl-2-(4,5-dihydro-4,4,5,5- tetramethylimidazolinyl-3-oxide-l-oxy)]-2,2‘-bipyridine, bpyN(NIT)2, 1, have been synthesized and characterized by FAB-MS, UV−vis, FT-IR, and
EPR spectroscopies, elemental analysis, and susceptibility measurements. Single-crystal X-ray diffraction studies
have been performed on all compounds giving the following crystal data:  bpyN(NIT)2, 1, triclinic, P1̄, Z = 2, a =
10.7224(4) Å, b = 11.0995(4) Å, c = 13.1134(3) Å, α = 114.101(9)°, β = 97.476(9)°, γ = 99.667(9)°; ZnbpyN(NIT)2, 2, hexagonal, P32, Z = 3, a = 15.4545(3) Å, b = 15.4545(3) Å, c = 13.5594(3) Å; NibpyN(NIT)2, 3,
hexagonal, P32, Z = 3, a = 15.2867(1) Å, b = 15.2867(1) Å, c = 13.7160(1) Å; CubpyN(NIT)2, 4, triclinic, Pl̄, Z
= 2, a = 11.8640(4) Å, b = 13.2023(4) Å, c = 13.2661(5) Å, α = 90.539(9)°, β = 104.983(9)°, γ = 113.252(9)°. The two radicals of the free ligand 1 are almost perpendicular to one another in the solid state, favoring a
weak ferromagnetic interaction (J/kB = 8.8 K). The complexes obtained by wrapping the ligand around a single
metal center gave rise to two different coordination schemes where the two radicals of 1 adopt a ON(3)O meridional
(with Ni and Zn) or a ON(3)O facial conformation (with Cu), which strongly affects the magnetic and electronic
properties (O accounts for the coordinated oxygen atoms of the nitroxide radicals and N(3) accounts for the tertiary
amine). For 2, a model of a dimer has been used giving rise to a weak antiferromagnetic interaction between the
radicals (J/kB = −5.3 K). For 3, a very strong intramolecular antiferromagnetic coupling has been found and estimated
at J/kB = −230 K and J‘/kB = −110 K between the nickel and each radical using an asymmetric model of a trimer.
For 4, an unusual magnetic behavior is observed, dominated by antiferromagnetic interactions with a residual
plateau at χT = 0.63 emu·K·mol-1. Molecular modeling at the CASSCF level is in keeping with an antiferromagnetic
coupling of the radical bound with the Cu(II) in the equatorial position. The combined structural, electronic, and
magnetic characteristics suggest that the use of a flexible molecule provide an additional approach for fine-tuning
magnetic interactions.