Tetra-, Tri-, and Mononuclear Manganese(II/III) Complexes of a Phenol-Based N2O2 Capping Ligand: Use of Carboxylates as Ancillary Ligands in Tuning the Nuclearity of the Complexe

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• 作者：Debdas Mandal^&#8224 ; ; Pabitra Baran Chatterjee^&#8224 ; ; Shubhajit Bhattacharya^&#8224 ; ; Ki-Young Choi^‡ ; ; Rodolphe Clrac^§ ; ; Mukti
• 刊名：Inorganic Chemistry
• 出版年：2009
• 出版时间：March 2, 2009
• 年：2009
• 卷：48
• 期：5
• 页码：1826-1835
• 全文大小：290K
• 年卷期：v.48,no.5(March 2, 2009)
• ISSN：1520-510X

文摘

Manganese(II/III) complexes of a phenol-based tetradentate ligand L²⁻ [H₂L = N,N′-dimethyl-N,N′-bis(2-hydroxy-3,5-dimethylbenzyl)-ethylenediamine], namely, [Mn₄(L)₂(PhCOO)₆] (1), [Mn₃(L)₂(CH₃CH₂COO)₂(OMe)₂]·H₂O (2), and [Mn(L){(CH₃)₃CCOO}(CH₃OH)]·CH₃OH (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 ⁵⁵Mn (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){(CH₃)₃CCOO}(CH₃OH)]·CH₃OH (3). The lone manganese(III) center in this compound has an octahedral geometry, completed by the tetradentate ligand L²⁻ 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.