Hossain Ferdaus, Rigsby Matthew A, Duncan Cole T, Milligan Paul L, Lord Richard L, Baik Mu-Hyun, Schultz Franklin A
Department of Chemistry and Chemical Biology, Indiana University-Purdue University Indianapolis, 402 North Blackford Street, Indianapolis, Indiana 46202, USA.
Inorg Chem. 2007 Apr 2;46(7):2596-603. doi: 10.1021/ic062224+. Epub 2007 Mar 2.
The manganese(III)-bis[poly(pyrazolyl)borate] complexes, Mn(pzb)2SbF6, where pzb- = tetrakis(pyrazolyl)borate (pzTp) (1), hydrotris(pyrazolyl)borate (Tp) (2), or hydrotris(3,5-dimethylpyrazolyl)borate (Tp*) (3), have been synthesized by oxidation of the corresponding Mn(pzb)2 compounds with NOSbF6. The Mn(III) complexes are low-spin in solution and the solid state (microeff = 2.9-3.8 microB). X-ray crystallography confirms their uncommon low-spin character. The close conformity of mean Mn-N distances of 1.974(4), 1.984(5), and 1.996(4) A in 1, 2, and 3, respectively, indicates absence of the characteristic Jahn-Teller distortion of a high-spin d4 center. N-Mn-N bite angles of slightly less than 90 degrees within the facially coordinated pzb- ligands produce a small trigonal distortion and effective D3d symmetry in 1 and 2. These angles increase to 90.0(4)degrees in 3, yielding an almost perfectly octahedral disposition of N donors in Mn(Tp*)2+. Examination of structural data from 23 metal-bis(pzb) complexes reveals systematic changes within the metal-(pyrazolyl)borate framework as the ligand is changed from pzTp to Tp to Tp*. These deformations consist of significant increases in M-N-N, N-B-N, and N-N-B angles and a minimal increase in Mn-N distance as a consequence of the steric demands of the 3-methyl groups. Less effective overlap of pyrazole lone pairs with metal atom orbitals resulting from the M-N-N angular displacement is suggested to contribute to the lower ligand field strength of Tp* complexes. Mn(pzb)2+ complexes undergo electrochemical reduction and oxidation in CH3CN. The electrochemical rate constant (ks,h) for reduction of t2g4 Mn(pzb)2+ to t2g3eg2 Mn(pzb)2 (a coupled electron-transfer and spin-crossover reaction) is 1-2 orders of magnitude smaller than that for oxidation of t2g4 Mn(pzb)2+ to t2g3 Mn(pzb)22+. ks,h values decrease as Tp* > pzTp > Tp for the Mn(pzb)2+/0 electrode reactions, which contrasts with the behavior of the comparable Fe(pzb)2+/0 and Co(pzb)2+/0 couples.
通过用五氟化锑亚硝酰（NOSbF6）氧化相应的二[聚（吡唑基）硼酸根]锰（II）化合物，合成了锰（III）-双[聚（吡唑基）硼酸根]配合物Mn(pzb)2SbF6，其中pzb- = 四（吡唑基）硼酸根（pzTp）（1）、三（吡唑基）硼酸根（Tp）（2）或三（3,5-二甲基吡唑基）硼酸根（Tp*）（3）。锰（III）配合物在溶液和固态中均为低自旋态（磁矩eff = 2.9 - 3.8 玻尔磁子）。X射线晶体学证实了它们罕见的低自旋特性。在1、2和3中，平均Mn-N距离分别为1.974(4)、1.984(5)和1.996(4) Å，非常接近，这表明不存在高自旋d4中心特有的 Jahn-Teller 畸变。在面式配位的pzb-配体中，N-Mn-N咬角略小于90度，在1和2中产生了小的三角畸变和有效的D3d对称性。在3中，这些角度增加到90.0(4)度，在Mn(Tp*)2+中N供体几乎呈完美的八面体排列。对23种金属-双（pzb）配合物的结构数据进行研究发现，随着配体从pzTp变为Tp再变为Tp*，金属-（吡唑基）硼酸根骨架内发生了系统性变化。这些变形包括M-N-N、N-B-N和N-N-B角度的显著增加，以及由于3-甲基的空间需求导致Mn-N距离的最小增加。由于M-N-N角位移导致吡唑孤对与金属原子轨道的重叠效果较差，这被认为是Tp配合物配体场强度较低的原因。Mn(pzb)2+配合物在乙腈中发生电化学还原和氧化。将t2g4 Mn(pzb)2+还原为t2g3eg2 Mn(pzb)2（一个耦合电子转移和自旋交叉反应）的电化学速率常数（ks,h）比将t2g4 Mn(pzb)2+氧化为t2g3 Mn(pzb)22+的速率常数小1 - 2个数量级。对于Mn(pzb)2+/0电极反应，ks,h值随着Tp > pzTp > Tp而降低，这与可比的Fe(pzb)2+/0和Co(pzb)2+/0电对的行为形成对比。
