Further attempts to rationalise the co-ordination chemistry of manganese with Schiff base ligands and supplementary carboxylate donors

Abstract

Some manganese(III) complexes of Schiff base ligands with ancillary carboxylate donors have been found to exhibit structural diversity, although some patterns emerged. Thus, when the ligands 3CH₃O-salen and 3CH₃O-salpn [3CH₃O-salen = dianion of N,N′-bis(3-methoxysalicylidene)ethane-1,2-diamine, 3CH₃O-salpn = dianion of N,N′-bis(3-methoxysalicylidene)propane-1,3-diamine] are used in conjunction with carboxylates RCO₂^– (R = Me, Et, Prⁿ or CH₂Ph) unidentate carboxylate bonding occurs as in the crystallographically observed [Mn(3CH₃O-salen)(O₂CMe)(H₂O)]·2H₂O 1, [Mn(3CH₃O-salen)(O₂CCH₂Ph)(H₂O)]·H₂O 4, and [Mn(3CH₃O-salpn)(O₂CCH₂Ph)(H₂O)] 5. On the other hand, employing the more sterically encumbered carboxylates Bu^tCO₂^– and PrⁱCO₂^–, bidentate chelating binding of the carboxylate occurs, as in [Mn(3CH₃O-salpn)(O₂CBu^t)] 2 and [Mn(3CH₃O-salpn)(O₂CPrⁱ)] 3. The reactivity of [Mn(salpn)(acac)] (acac = acetylacetonate) with Me₃SiCl and aliphatic carboxylic acids, RCO₂H (R = Me, Et, Prⁿ, Buⁿ, Prⁱ or Bu^t), has also been investigated. A dimer species [{Mn(salpn)Cl}₂]·CH₃CN 6 was isolated from the reaction of Me₃SiCl with [Mn(salpn)(acac)] while the carboxylic acids seem to lead to the isolation of monomers, such as [Mn(salpn)(O₂CPrⁱ)] 7. This synthetic route has also been applied to the preparation of related complexes, in which the manganese(III) centre is not attainable with reliability via the aerobic oxidation of a manganese(II) precursor, although some rare examples have been obtained by the latter method, such as [Mn(5NO₂-salen)(O₂CMe)(H₂O)], 8, [5NO₂-salen = dianion of N,N′-bis(5-nitrosalicylidene)ethane-1,2-diamine]). Single crystals were grown from a dimethylformamide solution of the material of stoichiometry Mn(3Br,5NO₂-salpn)(O₂CMe)·H₂O [3Br,5NO₂-salpn = dianion of N,N′-bis(3-bromo-5-nitrosalicylidene)propane-1,3-diamine], isolated from this route and found to consist of the unexpected [{Mn(µ-3Br,5NO₂-salpn)(µ-O)}₂]·3DMF 9, apparently containing a manganese(IV) species, in spite of the electron withdrawing nature of the substituents on the aromatic rings of the ligand.