Structural and Spectroscopic Trends in a Series of Half-Sandwich Scorpionate Complexes

Hydrotris(pyrazol-1-yl)borates (i.e., HB{pz}₃^–, Tp) and tris(pyrazol-1-yl)methanes (i.e., HC(pz)₃, Tpm) are tridentate face-capping ligands, formally isolobal to the cyclopentadienyl anion (C₅H₅^–, Cp), that have been extensively utilized in bioinorganic and organometallic chemistry. A wide variety of half-sandwich complexes can be supported, which have been exploited as enzyme active site models and as functional catalysts. For example, Tp-supported copper complexes (i.e., [Tp^Br3Cu(NCMe)]) have been used as nitrene transfer catalysts, promoting olefin aziridination and amination of aliphatic and aromatic C–H bonds. Analogous catalysis was also reported using [Tpm^RCu(NCMe)]BF₄ in ionic liquids. Fifteen half-sandwich scorpionate complexes [(L)M(NCMe)₃](BF₄)_n (L = tris(3,5-dimethylpyrazol-1-yl)methane, Tpm^Me,Me, n = 2, 1^M, M = Mn, Fe, Co, Ni; L = tris(3-phenylpyrazol-1-yl)methane, Tpm^Ph, n = 2, 2^M, M = Mn, Fe, Co, Ni; L = hydrotris(3,5-dimethylpyrazol-1-yl)borate, [Tp^Me,Me]⁻, n = 1, 3^M, M = Fe, Co, Ni; L = hydrotris(3-phenyl-5-methylpyrazol-1-yl)borate, [Tp^Ph,Me]⁻, n = 1, 4^M, M = Mn, Fe, Co, Ni) were prepared by addition of the tripodal ligands to solvated [M(NCMe)_x]²⁺ (M = Mn, x = 4; M = Fe, Co, Ni, x = 6) precursor complexes. The product complexes were characterized by ¹H NMR (except M = Mn), UV–vis–NIR, and FTIR spectroscopy. The structures of 2^Mn, 2^Ni, 3^Fe, 3^Co, and 4^Fe were determined by X-ray crystallography. The data were consistent with complexes of high-spin divalent metal ions in idealized piano-stool geometries in all cases. Consequent lability of the acetonitrile ligands will enable use of these complexes as synthetic precursors and as catalysts. Comparison to previously reported structures of 1^Fe, 1^Co, 2^Fe, and 2^Co, the triflate salt analogues of 4^Co and 4^Ni, as well as related sandwich complexes (e.g., [(Tp^Me,Me)₂M]) and solvated metal dications [M(NCMe)₆]²⁺ reveals numerous trends in M–N bond lengths. Primary among these are the Irving–Williams series, with significant structural effects also arising from ligand charge and sterics. Systematic trends in spectroscopic data were also observed which further elucidate these issues,

Structural and Spectroscopic Trends in a Series of Half-Sandwich Scorpionate ComplexesShengwen Liang, Haoshuang Wang, Tapash Deb, Jeffrey L. Petersen, Gordon T. Yee, and Michael P. Jensen. Inorganic Chemistry Article ASAP