New cyanide-bridged MnIII–MIII heterometallic dinuclear complexes constructed from [MIII(AA)(CN)4]− building blocks (M = Cr and Fe): synthesis, crystal structures and magnetic properties

Abstract

Three Mn^III–M^III (M = Cr and Fe) dinuclear complexes have been obtained by assembling [Mn^III(SB)(H₂O)]⁺ and [M^III(AA)(CN)₄]⁻ ions, where SB is the dianion of the Schiff-base resulting from the condensation of 3-methoxysalicylaldehyde with ethylenediamine (3-MeOsalen²⁻) or 1,2-cyclohexanediamine (3-MeOsalcyen²⁻): [Mn(3-MeOsalen)(H₂O)(µ-NC)Cr(bipy)(CN)₃]·2H₂O (1), [Mn(3-MeOsalen)(H₂O)(µ-NC)Cr(ampy)(CN)₃][Mn(3-MeOsalen)(H₂O)₂]ClO₄·2H₂O (2) and [Mn(3-MeOsalcyen)(H₂O)(µ-NC)Fe(bpym)(CN)₃]·3H₂O (3) (bipy = 2,2′-bipyridine, ampy = 2-aminomethylpyridine and bpym = 2,2′-bipyrimidine). The [M(AA)(CN)₄]⁻ unit in 1–3 acts as a monodentate ligand towards the manganese(III) ion through one of its four cyanide groups. The manganese(III) ion in 1–3 exhibits an elongated octahedral stereochemistry with the tetradentate SB building the equatorial plane and a water molecule and a cyanide-nitrogen atom filling the axial positions. Remarkably, the neutral mononuclear complex [Mn(3-MeOsalen)(H₂O)₂]ClO₄ co-crystallizes with the heterobimetallic unit in 2. The values of the Mn^III–M^III distance across the bridging cyanide are 5.228 (1), 5.505 (2) and 5.265 Å (3). The packing of the neutral heterobimetallic units in the crystal is governed by the self-complementarity of the [Mn(SB)(H₂O)]⁺ moieties, which interact each other through hydrogen bonds established between the aqua ligand from one fragment with the set of phenolate- and methoxy-oxygens from the adjacent one. The magnetic properties of the three complexes have been investigated in the temperature range 1.9–300 K. Weak antiferromagnetic interactions between the Mn^III and M^III ions across the cyanido bridge were found: J_MnM = −5.6 (1), −0.63 (2) and −2.4 cm⁻¹ (3) the Hamiltonian being defined as H = −JS_Mn·S_M. Theoretical calculations based on density functional theory (DFT) have been used to substantiate both the nature and magnitude of the exchange interactions observed and also to analyze the dependence of the magnetic coupling on the structural parameters within the Mn^III–N–C–M^III motif in 1–3.