Département de chimie moléculaire

Carboxylate-bridged Mn(II)-Ca(II) complexes are potentially relevant for mimicking the first stages of the Oxygen-Evolving Complex (OEC) assembly process. Here, the authors 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, contg. resp. 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 were structurally characterized; the syntheses of Ca1 and Mn2Ca2·2Mn being previously reported by the authors (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$1,3-carboxylate bridge connects each Ca2+ ion to each Mn(II) in Mn2Ca2. A variable no. of water mols. (n = 1 to 7) are coordinated to Ca in all complexes, most of them being involved in hydrogen-bond networks, in analogy to what occurs in the photosystem II. All donor atoms of the tpaa3- and tpada2- ligands are coordinated to the Mn2+ ions, despite the unusually long distance between the Mn2+ ion and the tertiary amine imposed by the constraining nature of the ligands, as supported by theor. calcns. Solid state EPR spectroscopy, in combination with DFT calcns., also showed that the Ca2+ ion has an effect on the electronic parameters (zero field splitting) of the linked Mn(II) in the case of MnCa ($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. [on SciFinder(R)]