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

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
D3d 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 (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.