Tetranuclear manganese carboxylate clusters with imidazole-carboxylate chelating ligands. X-ray crystal structure of the 4-imidazoleacetate complex

Procedures are described for the synthesis of tetranuclear manganese carboxylate complexes with ligands derived from 4-imidazoleacetic acid (imidacH) and 4-imidazolecarboxylic acid (imidcarbH). Reaction mixtures containing NBu(4)(n)imidac or NHex(4)(n)imidcarb generated in situ and either [Mn3O(O2CMe)(6)(py)(3)](ClO4) or Mn(O2CR)(2), RCO2H (R = Me, Ph) and NR'4MnO4 (R' = Bu-n, Hex(n)) give red-brown solutions from which the products (NBu4n)[Mn4O2(O2CR)(7)(imidac)(2)] (R = Me (1); R = Ph (2)) and (NHex(4)(n))[Mn4O2(O2CR)(7)(imidcarb)(2)] (R = Me (3); R = Ph (4)) can be obtained. The X-ray crystal structure of 2 reveals that the anion consists of an [Mn-4(mu (3)-O)(2)](8+) core with the four Mn atoms disposed in a 'butterfly' arrangement and the O atoms triply bridging each 'wing'. Peripheral ligation is provided by seven bridging O2CPh- ligands and two chelating imidac(-) groups. All available characterization data are consistent with the same core for complexes 1, 3 and 4, with the structures varying only in the peripheral ligation. H-1 NMR spectroscopy of all four complexes indicates that the structure is maintained in solution and the majority of the resonances have been assigned on the basis of T-1 measurements and substitution and exchange experiments. Cyclic voltammetry reveals the presence of a single quasi-reversible oxidation processes for complexes 1 and 2. (C) 2000 Elsevier Science Ltd. All rights reserved.