Synthesis, Molecular Structure, and Physical Properties of the Complexes [{PhB(pz)2(CH2SMe)}2M] (M = MnII, FeII; pz = pyrazol‐1‐yl) Containing a Novel [N,N,S]‐Heteroscorpionate Ligand

Abstract

AbstractTwo transition metal complexes [L2M] {M = FeII (2), MnII (3)} of the novel [N,N,S] scorpionate ligand L = [PhB(pz)2(CH2SMe)]– have been synthesized and characterized by X‐ray crystallography, magnetic measurements, cyclic voltammetry, and various spectroscopic techniques. In the solid state, both complexes 2 and 3 possess a distorted octahedral ligand sphere, but adopt different configurations (2: cis; 3: trans). According to X‐ray diffraction data, Mössbauer spectroscopy, and SQUID measurements, compound 2 exists as low‐spin complex in the solid state at temperatures T ≤ 20 °C (μeff ≈ 0.5 μB). Upon heating, the magnetic moment μeff increases continuously to a value of 1.8 μB at 90 °C, which indicates a temperature dependent high‐spin ↔ low‐spin transition above room temp. Solutions of 2 in CDCl3 showed paramagnetic behavior at 25 °C (μeff = 3.1 μB; Evans NMR method). Cyclic voltammetry (CH2Cl2, [Bu4N][PF6]; vs. FcH/FcH+) on 2 reveals a reversible FeII/FeIII redox transition at E1/2 = –0.16 V. Density functional theory (B3LYP‐D/def2‐TZVPP//B3LYP‐D/SVP level) has been used to evaluate relative energies of the cis and trans isomers of 2 in their high‐spin and low‐spin states. Theoretical and experimental investigations have been extended to the related literature‐known homoleptic FeII complexes [{PhB(pz)3}2Fe], [{PhB(pz)(CH2SMe)2}2Fe], and [{PhB(CH2SMe)3}2Fe]. The following order of ligand‐field strengths has been established: [PhB(CH2SMe)3]– < [PhB(pz)(CH2SMe)2]– < [PhB(pz)2(CH2SMe)]– < [PhB(pz)3]–. The only significant deviation from this systematic behavior became evident for the unexpectedly low solution magnetic moment of [{PhB(CH2SMe)3}2Fe], which finds no parallel in the solid state. Computed spin‐state splittings for the entire series of complexes, however, confirm the trend in solution magnetic moments and we relate the disaccording experimental observations to the presence of a second stereoisomer in crystals of [{PhB(CH2SMe)3}2Fe] which increases the paramagnetism of solid‐state samples of this complex.