Tetra-, Tri-, and Mononuclear Manganese(11/111) Complexes of a Phenol-Based N_2O_2 Capping Ligand: Use of Carboxylates as Ancillary Ligands in Tuning the Nuclearity of the Complexes

摘要

Manganese(1l/III) complexes of a phenol-based tetradentate ligand L~2- [H_2L = N,N'-dimethyl-N,W-bis(2-hydroxy_3,5-dimethylblzyl)-ethylenediamine], namely, [Mn_4(L)_2(PhCOO)6] (1),[Mn_3(L)_2(CH_3CH_2COO)_2(OMe)_2] _ H_2O (2), and [Mn(L){(CH_3)_3CCOO}(CH_3OH)] _ CH_3OH (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 crystalIographically equivalent binuclear [Mn~(__)-Mn~(___)] 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~(___)-Mn~(__)-Mn~(___) 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 ~55Mn (_= 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)_3CCOO}(CH_3OH)] _ CH_3OH(3). The lone manganese(lll) 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/AgCI reference.