Ferromagnetic Interactions between Imino Nitroxides through Diamagnetic Metal Ions: Crystal Structures, Magnetism, and Electronic Properties of [MI(imino nitroxide)2](PF6) (M = CuIand AgI)

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Abstract
An orthogonal arrangement of imino nitroxide ligands in CuI and AgI complexes results in propagation of an intramolecular ferromagnetic interaction between the coordinated imino nitroxides through these diamagnetic ions. Reaction of [CuI(CH3CN)4](PF6) and AgI(PF6) with imino nitroxides in ethanol solution gave [CuI(immepy)2](PF6) (1) and [AgI(impy)2](PF6) (2), respectively (immepy = 2-(2-(6-methylpyridyl))-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy and impy = 2-(2-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy). Complex 1 crystallizes in the orthorhombic space group Pba2, which has a unit cell with a = 11.251(2) Å, b = 14.745(3) Å, c = 9.747(2) Å, V = 1617.0(5) Å3, and Z = 2. Refinement with 991 reflections observed [|Io| > 3σ(Io)] gave R = 0.044 (Rw = 0.053). In 1, imino nitroxides coordinate to the CuI ions in a tetrahedral fashion, the dihedral angle between coordinating imino nitroxides being 88.7°. Magnetic susceptibility measurement for 1 shows typical triplet−singlet behavior with strong ferromagnetic interactions (J = 55.1(6) cm-1 with g = 2.0; H = −JS1·S2). The UV−visible absorption spectrum of 1 shows strong bands at 464 and 766 nm assignable to charge-transfer bands from the Cu ion to the ligand NLUMO and SOMO, respectively. The π-back-donation to the SOMO induces the spin on the Cu ion and results in the strong ferromagnetic interaction. Complex 2 crystallizes in the orthorhombic space group P21cn which has a unit cell with a = 10.535(6) Å, b = 31.295(3) Å, c = 8.921(4) Å, V = 2941(1) Å3, and Z = 4. Refinement with 1882 reflections [|Io| > 3σ(Io)] gave R = 0.058 (Rw = 0.074). In 2, the four coordination sites of the AgI ion were completed with two imino nitroxides and the dihedral angle between the imino nitroxides is 79.2°. The EPR spectrum of a frozen ethanol solution of 2 shows a typical triplet signal with zero field splitting (|D| = 0.043 cm-1 and |E| = 0.014 cm-1). Curie plots for the Δm = 2 signal below 80 K gave a positive Weiss constant (θ = 4 K), suggesting an intramolecular ferromagnetic interaction in 2.

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