Tetra-, Tri-, and Mononuclear Manganese(II/III) Complexes of a Phenol-Based N2O2Capping Ligand: Use of Carboxylates as Ancillary Ligands in Tuning the Nuclearity of the Complexes

Abstract

Manganese(II/III) complexes of a phenol-based tetradentate ligand L(2-) [H(2)L = N,N'-dimethyl-N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)-ethylenediamine], namely, [Mn(4)(L)(2)(PhCOO)(6)] (1), [Mn(3)(L)(2)(CH(3)CH(2)COO)(2)(OMe)(2)].H(2)O (2), and [Mn(L){(CH(3))(3)CCOO}(CH(3)OH)].CH(3)OH (3), have been synthesized. The basicity and steric congestion provided by the carboxylate moiety used as an ancillary ligand have profound influence on tuning the nuclearity of these compounds. Results of X-ray crystallography, electronic spectroscopy, and variable-temperature (1.8-300 K) magnetic measurements have been used to characterize these compounds. Complex 1 has a very interesting centrosymmetric structure that involves two crystallographically equivalent binuclear [Mn(II)-Mn(III)] units, connected together by a pair of syn-anti bridging benzoates to generate a "dimer of dimers" structural motif. Compound 2 with propionate as the ancillary ligand, on the other hand, has a nearly linear Mn(III)-Mn(II)-Mn(III) core with antiferromagnetically coupled (J = -0.13 cm(-1)) metal centers. Compound 1 has an S(T) = 9 spin ground state with ferromagneticlly coupled metal centers (J(wb)= 2.8(1) and J(bb) = 0.09(2) cm(-1)) that failed to function as a single molecule magnet due to the presence of low-lying excited states with smaller spin values and a weak magnetic anisotropy. The electron paramagnetic resonance spectrum of 1 in the frozen solution (12 K) displays two signals in the g = 2 and g = 4 regions, each split into six lines due to (55)Mn (I = 5/2) superhyperfine couplings. The use of bulky pivalate as a replacement for benzoate provides enough steric bulk to generate a mononuclear species [Mn(L){(CH(3))(3)CCOO}(CH(3)OH)].CH(3)OH (3). The lone manganese(III) center in this compound has an octahedral geometry, completed by the tetradentate ligand L(2-) together with an axially coordinated methanol molecule and a monodentate pivalate. The latter two are connected by a hydrogen bond, thus stabilizing the monodentate carboxylate moiety. Redox behaviors (CV) of 1 and 3 are grossly similar, each undergoing a quasi-reversible reduction process at E(1/2) = -0.03 and -0.11 V, respectively, versus a Ag/AgCl reference.