New insights in Mn–Ca chemistry from the use of oximate-based ligands: {MnII/III22Ca2} and {MnIV2Ca2} complexes with relevance to both low- and high-valent states of the oxygen-evolving complex

The initial use of quinoline-2-aldoxime (qaoH) and 2,6-diacetylpyridine dioxime (dapdoH2) in heterometallic Mn–Ca chemistry has afforded a mixed-valence {MnII/III22Ca2} cluster containing several {Mn4CaOx} subunits and a ‘butterfly’-like {MnIV2Ca2} complex, respectively. The one-pot reaction of Mn(O2CPh)2·2H2O and Ca(O2CPh)2·H2O with qaoH and NEt3 in a 1:1:1:1 molar ratio in a solvent mixture comprising MeCN/MeOH gave complex [Mn22Ca2O14(OH)4(OMe)6(O2CPh)22(qao)2(MeCN)2(H2O)4](OH)2 (1; 6MnII/16MnIII) in 20% yield. A similar reaction with that of 1, albeit with Mn(O2CPh)2·2H2O and Ca(NO3)2·4H2O in the presence of dapdoH2 and NEt3, in a molar ratio of 1:1:2:2 in MeCN/MeOH, led to the smaller nuclearity complex [Mn2Ca2(OMe)2(NO3)2(dapdo)4] (2; 2MnIV) in 40% yield. The reported compounds demonstrate structural and magnetic relevance to both low- and high-valent states of the OEC.