Trinucleation of Arylazo Oxime Ensembles. 2. < sup > 1 < /sup > Linear Fe < sup > II < /sup > Ni < sup > II < /sup > Fe < sup > II < /sup > and Related Systems

Abstract

The tris(arylazo oximato)iron(II) anion, Fe(RL)3-, reacts smoothly with 3d metal(II) ions, affording heterometallic trinuclear species of type MIIFeII2(RL)6(M = Mn, Co, Ni, Zn; HRL is RC(=NOH)N=NPh with R = Me, Ph). The structure of NiFe2(MeL)6·CH2Cl2has been determined X-ray crystallographically. The complex belongs to the space group P21/n, with Z = 4. Unit cell dimensions are a = 29.607 (18) Å, b = 19.732 (11) Å, c = 10.040 (4) Å, β = 90.53 (4)°, and V= 5865 (5) Å3. Refinement was achieved to R = 0.0734 and Rw= 0.0933. The trinuclear complex consists of two Fe(MeL)3-segments coordinating the centrally placed nickel(II) ion with the help of six pendant oximato oxygen atoms. The NiO6octahedron thus produced is nearly regular. Within each Fe(MeL)3-unit the three ligands (coordination at oxime and azo nitrogen atoms) are facially disposed. The FeN6 coordination sphere is a trigonally compressed pseudooctahedron. The FeNiFe angle is 178.2 (1)°. The two Fe(MeL)3-units within the same molecule have the same chirality (ΔΔ or ΛΛ)—the crystal as a whole being racemic. The dichloromethane molecule is disordered in the crystal. All the trinuclear complexes display intense MLCT bands localized within the Fe(RL)3-units at ~600 and ~400 nm. The M = Ni and Co species show ligand field transitions at ~1300 nm (Dq ≈ 760 cm-1). The powder EPR spectrum of MnFe2(MeL)6(S = 5/2) in pure form is closely similar to those of the same complex doped (1%) into MFe2(MeL)6 (M = Ni, Co, Zn). In all cases the dominant signal occurs at g ≈ 2, showing that the zero-field splitting (and hence distortion of the MO6octahedron) is small as required by the X-ray structure of the M = Ni complex. The chromium(III) complex [CrFe2(MeL)6]ClO4·2H2O also reported here has a relatively large zero-field splitting (D≈ 0.5 cm-1). In dichloromethane solution the quasi-reversible MIIIFe2(MeL)6+/MIIFe2(MeL)6couples have the following E°298 values: M = Cr, -0.9 V (estimated); M = Mn, +0.51 V; M = Fe, +0.05 V; M = Co, +0.69 V; M = Ni, +1.3 V (estimated). In the trinuclear complexes the two bound Fe(MeL)3-units undergo successive quasi-reversible 1e reductions, affording a pair of overlapping but clearly observable cyclic voltammetric couples in all cases. The pair, which expectedly shifts to higher potentials in going from a bivalent (e.g., E°298for the M = Ni complex being -0.79 and -0.93 V) to a trivalent (E°298for the M = Cr complex being -0.43 and -0.59 V) central metal ion, is diagnostic of trinucleation. © 1986, American Chemical Society. All rights reserved.