Synthesis, Structure, and Magnetism of Mono- and Binuclear Manganese(II) Compounds of Nitronyl Nitroxide Substituted Phosphine Oxides

Complexes of manganese(II)-containing aminoxyl radical substituted phosphine oxide ligands are reported. The
compounds [(o-nitronyl nitroxide-phenyl)diphenylphosphine oxide]bis(hexafluoroacetylacetonato)manganese(II),
3, and bis{[(p-nitronyl nitroxide-phenyl) diphenylphosphine oxide]bis(hexafluoroacetylacetonato)manganese(II)},
4, prepared by addition of the free radical phosphine oxides to Mn(hfac)2, were structurally characterized. Complex
3 is mononuclear, containing an O,O-chelating ortho-substituted radical phosphine oxide ligand, while in 4 the
para-substituted ligands bridge two Mn(hfac)2 units to yield a binuclear molecular rectangle. The magnetic behavior
of both systems is dominated by a strong antiferromagnetic Mn(II)−aminoxyl interaction (J = −213 (3), −218
(4) cm-1 with ℋ = −JSMn·Srad) to give effective S = 2 ground state units. The S = 3 excited state is populated
at high temperatures. At low temperatures a decrease in χMT in both complexes is attributable primarily to inter-
or intramolecular antiferromagnetic interactions rather than zero-field splitting (ZFS) of the S = 2 ground state.
For the bimetallic compound, the magnetic data indicate that ligand-mediated interactions between the Mn(II)
spin carriers are weak. The powder EPR spectra of both systems have been recorded and successfully simulated,
giving a ZFS parameter D = 0.112 cm-1. Crystals of 3 are triclinic, space group P1̄ with a = 10.6672(19) Å, b
= 13.270(6) Å, c = 15.363(3) Å, α = 93.84(2)°, β = 108.054(16)°, γ = 105.69(3)°, and Z = 2. Crystals of 4
are monoclinic, space group P21/a with a = 12.463(6) Å, b = 19.315(3) Å, c = 17.084(9) Å, α = 90°, β =
98.49(2)°, γ = 90°, and Z = 2.