Calcium and heterometallic manganese–calcium complexes supported by tripodal pyridine-carboxylate ligands: structural, EPR and theoretical investigations

Abstract

Carboxylate-bridged Mn(II)–Ca(II) complexes are potentially relevant for mimicking the first stages of the Oxygen-Evolving Complex (OEC) assembly process. Here, we report on new homonuclear Ca(II) and heteronuclear Mn(II)–Ca(II) complexes with carboxylate-functionalized tripodal tris(2-pyridylmethyl)amine ligands, the heptadentate H₃tpaa, previously reported, and the new hexadentate H₂tpada, containing respectively three and two carboxylate units. The mononuclear [Ca(Htpaa)(OH₂)] (Ca₁) and dinuclear [Ca(tpada)(OH₂)₂]₂ (Ca₂) calcium complexes, as well as the tetranuclear [{Mn(tpaa)}₂{Ca(OH₂)₅(μ-OH₂)}₂][Mn(tpaa)]₂ (Mn₂Ca₂·2Mn) and dinuclear [Mn(tpada)ClCa(OH₂)_2.67(MeOH)_2.33]Cl (MnCa) heterometallic species have been structurally characterized; the syntheses of Ca₁ and Mn₂Ca₂·2Mn being previously reported by us (Inorg. Chem., 2015, 54, 1283). The Mn(II) and Ca(II) are linked by two μ_1,1-bridging carboxylates in MnCa, while only one μ_1,3-carboxylate bridge connects each Ca²⁺ ion to each Mn(II) in Mn₂Ca₂. A variable number of water molecules (n = 1 to 7) are coordinated to Ca in all complexes, most of them being involved in hydrogen-bond networks, in analogy to what occurs in the photosystem II. All donor atoms of the tpaa³⁻ and tpada²⁻ ligands are coordinated to the Mn²⁺ ions, despite the unusually long distance between the Mn²⁺ ion and the tertiary amine imposed by the constraining nature of the ligands, as supported by theoretical calculations. Solid state EPR spectroscopy, in combination with DFT calculations, has also shown that the Ca²⁺ ion has an effect on the electronic parameters (zero field splitting) of the linked Mn(II) in the case of MnCa (μ_1,1-carboxylate bridges). In Mn₂Ca₂ (μ_1,3-carboxylate bridge) the Ca²⁺ ion induces only slight structural changes in the Mn coordination sphere.