Structural and Spectroscopic Properties of Antiferromagnetically Coupled FeUIMnU andFeI1Mn1I Complexes

The heterobimetallic complexes [FemMnnBPMP(02CCH2CH3)z1(BPh& (11, [FemMn11BPMP(0~CCH3)~l(BPh& (1'), and [FenMnnBPMP(02CCH2CH3)~](BPb) (2), in which BPMP is the anion of 2,6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol, were synthesized and characterized by proton NMR, EPR, and magnetization measurements. The high-spin Fe(II1) (S = V2) and high-spin Mn(I1) centers (S = V2) in 1 and 1' are antiferromagnetically coupled, resulting in an S = 0 ground state in these complexes. Variable temperature magnetic susceptibility measurements show that J = 23 cm-' for 1' (H = JSz*S2). The solid state structure of the FeIIMn" complex (2) was determined by X-ray crystallography. Complex 2 crystallizes in the triclinic space group Pi with the following unit cell parameters: a = 12.613(6) A, b = 15.037(13) A, c = 16.62(2) A, a = 82.070(5)", p = 81.180(5)", y = 67.940(5)", and 2 = 2. Its structure contains a @-phenoxo)bis@-carboxy1ato)dimetal cluster with an iron-manganese distance of 3.360(4) A. Complex 2 is remarkably rich in spectroscopic characteristics. The high-spin Fe(I1) (S = 2) and high-spin Mn(1I) (S = 5/2) ions are antiferromagnetically coupled affording an S = I12 ground state and an unusual EPR signal at 2.5 K. This resonance is broad with g, < 2 and exhibits hyperfine features due to the nuclear spin of the Mn(I1) ion ( I = V2). At temperatures above 10 K, a second resonance signal appears at g = 5.8 which is ascribed to a doublet within the first excited S = 3/2 manifold. Variable temperature studies of both the ground and excited state EPR signals have yielded values of J = 8 cm-l and = 5 cm-', which approximately agree with the multifield, variable temperature magnetic susceptibility data on the polycrystalline material.