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

Abstract

[CuL(SC(6)F(5))] (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 N(2)S 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.

Cationic tris(pyrazolyl)borate bipyrimidine complexes

“…The 1 H NMR spectra of both complexes reveal appropriately shifted resonances due to the paramagnetic nickel centre with the borohydride resonances found at -10.82 and -9.77 ppm for 1 and 2, respectively [25][26][27]. The presence of a single pyrazolyl hydrogen peak at 71.14 and 70.83 ppm for 1 and 2, respectively, and a single methyl resonance for 2 indicate fast rotation of the Tp R ligand.…”

Hydrogen-bonded mononuclear nickel(II) benzoate complexes: synthesis and structural studies

“…1. The nickel center in this complex is five-coordinate with nickelnitrogen bond distances in the range of 2.045(2)-2.147 (19) Å which is usual for nickel bonded Tp complexes [14,20]. The metal-nitrogen bond distance of coordinated Pz Ph,Me H is almost the same as the Ni-N bond (2,X=H,3,4,5,6,7,8,9,10, Scheme 1 Synthesis of complexes 1-10 Table 2).…”

Distorted tetrahedral nickel-nitrosyl complexes: spectroscopic characterization and electronic structure

“…In general, the protons closest to the paramagnetic metal ion suffer the largest chemical shifts and the greatest line broadening. However, the 4-H proton of the pyrazolyl ring is observed to have the largest downfield shift to 81.09 ppm (integration of 3 protons) in [Ni(SC 6 F 5 )(L1)] [70]. This is a general tendency for complexes containing the Tp ligand [69,70,83].…”

“…However, the 4-H proton of the pyrazolyl ring is observed to have the largest downfield shift to 81.09 ppm (integration of 3 protons) in [Ni(SC 6 F 5 )(L1)] [70]. This is a general tendency for complexes containing the Tp ligand [69,70,83]. The large shift comes from the deshielding effects of the aromatic pyrazolyl ring current and the paramagnetic contact shift [83,84].…”

“…S12). The previous assignments of the [Ni(SC 6 F 5 )(L1)] complex are used here to further analyze the data [70]. In general, the protons closest to the paramagnetic metal ion suffer the largest chemical shifts and the greatest line broadening.…”