Structural and Spectroscopic Characterization of First-Row Transition Metal(II) Substituted Blue Copper Model Complexes with Hydrotris(pyrazolyl)borate

[CuL(SC6F5)] (1) (L = hydrotris(3,5-diisopropyl-1-pyrazolyl)borate anion) has been reported as a good model for
blue copper proteins [Kitajima, N.; Fujisawa, K.; Tanaka, M.; Moro-oka, Y. J. Am. Chem. Soc. 1992, 114, 9232−9233]. To obtain more structural and spectroscopic insight, the first-row transition metal(II) substituted complexes
of Cu(II) (1) to Mn(II) (2), Fe(II) (3), Co(II) (4), Ni(II) (5), and Zn(II) (6) were synthesized and their crystal structures
were determined. These model complexes have a distorted tetrahedral geometry arising from the tripodal ligand L.
The d value, which is defined by the distance from the N2S basal plane to the metal(II) ion, and the bond angles
such as N−M−N and S−M−N are good indicators of these structural distortions. The obtained complexes were
characterized by UV−vis absorption, EPR, NMR, far-IR, and FT-Raman spectroscopies and electrochemical and
magnetic properties. In UV−vis absorption spectra, the sulfur-to-metal(II) CT bands and the d−d transition bands
are observed for 1 and 3−5. For 1, the strong sulfur to Cu(II) CT band at 663 nm, which is one of the unique
properties of blue copper proteins, is observed. The CT energies of the Fe(II) (3), Co(II) (4), and Ni(II) (5) complexes
are shifted to higher energy (308 and 355 nm for 3, 311 and 340 nm for 4, 357 and 434 nm for 5) and are almost
the same as the corresponding Co(II)- and Ni(II)-substituted blue copper proteins. In the far-IR spectra, three far-IR
absorption bands for 2−6 at ca. 400, ca. 350, and ca. 310 cm-1 are also observed similar to those for 1. Other
properties are consistent with their distorted tetrahedral geometries.