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We controlled the hydrophilicity of metal-organic frameworks (MOFs) to achieve high proton conductivity and high adsorption of water under low humidity conditions, by employing novel class of MOFs, {NR(3)(CH(2)COOH)}[MCr(ox)(3)]·nH(2)O (abbreviated as R-MCr, where R = Me (methyl), Et (ethyl), or Bu (n-butyl), and M = Mn or Fe): Me-FeCr, Et-MnCr, Bu-MnCr, and Bu-FeCr. The cationic components have a carboxyl group that functions as the proton carrier. The hydrophilicity of the cationic ions was tuned by the NR(3) residue to decrease with increasing bulkiness of the residue: {NMe(3)(CH(2)COOH)}(+) > {NEt(3)(CH(2)COOH)}(+) > {NBu(3)(CH(2)COOH)}(+). The proton conduction of the MOFs increased with increasing hydrophilicity of the cationic ions. The most hydrophilic sample, Me-FeCr, adsorbed a large number of water molecules and showed a high proton conductivity of ~10(-4) S cm(-1), even at a low humidity of 65% relative humidity (RH), at ambient temperature. Notably, this is the highest conductivity among the previously reported proton-conducting MOFs that operate under low RH conditions.

The reaction between the [Mn(BS)(H2O)]+ monomeric and [Mn2(μ-BS)2(H2O)2]2+ dimeric cations and [Fe(CN)6]3- gave rise to cation−anion interaction via the formation of [FeC⋮NMn] bridges. Depending on the nature of the Schiff base and regardless of the stoichiometry used, either the trimeric anion [{Mn(BS)}2{Fe(CN)6}]- (BS = 3-MeOsalen, 6; 5-Clsalen, 7; 5-Brsalen, 8; salcy, 10) or the pentameric cation [{Mn(BS)}4{Fe(CN)6}]+ (BS = saltmen, 9) is formed, which has been assembled by the K+ cation or the ClO4 - anion, respectively. The X-ray analysis of 6 revealed a two-dimensional network layer structure. The magnetic measurements showed its metamagnetic behavior, where the ferromagnetic interaction operates within each layer and the antiferromagnetic interaction operates between the layers. The Neel temperature, T N, is 9.2 K, and the critical field at 2 K is 300 Oe. The temperature dependent magnetic susceptibilities of 7 and 8 are in agreement with a discrete, symmetrical, trinuclear structure Mn(III)Fe(III)Mn(III) (S Mn = 2, S Fe = 1/2, S Mn = 2) with a ferromagnetic spin coupling between the Mn(III) and Fe(III) ions, a small antiferromagnetic intertrimer interaction, and a large zero-field splitting of the Mn(III) ion. The structure of 9 consists of a two-dimensional layer containing as the repeating unit a cyclic dodecamer. The layers stack along the c axis, and ClO4 - anions are positioned between the layers. The magnetic measurements showed this compound's ferromagnetic behavior. There are, in fact, two kinds of intralayer magnetic interactions, the interaction between the Fe(III) and Mn(III) ions bridged by CN groups and the interaction between two Mn(III) ions in the dimer [Mn2(saltmen)2], both being ferromagnetic. The interlayer magnetic interaction is ferromagnetic. All of the interactions render to 9 an overall ferromagnetic behavior.