Synthesis, Structure, and Properties of Low-Spin Manganese(III)−Poly(pyrazolyl)borate Complexes

• Published in: Inorganic chemistry 2007-04, Vol.46 (7), p.2596-2603
• Main Authors: Hossain, Ferdaus, Rigsby, Matthew A, Duncan, Cole T, Milligan, Paul L, Lord, Richard L, Baik, Mu-Hyun, Schultz, Franklin A
• Format: Article
• Language: English
• Subjects: Borates - chemistry; Crystallography, X-Ray; Electrochemistry; Manganese Compounds - chemical synthesis; Manganese Compounds - chemistry; Models, Molecular; Molecular Conformation; Pyrazoles - chemistry
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic062224+

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 (μeff = 2.9−3.8 μB). 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) Å 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 90o within the facially coordinated pzb- ligands produce a small trigonal distortion and effective D 3 d symmetry in 1 and 2. These angles increase to 90.0(4)o 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 (k s,h) for reduction of t2g 4 Mn(pzb)2 + to t2g 3eg 2 Mn(pzb)2 (a coupled electron-transfer and spin-crossover reaction) is 1−2 orders of magnitude smaller than that for oxidation of t2g 4 Mn(pzb)2 + to t2g 3 Mn(pzb)2 2+. k s,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.