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

Fifteen half-sandwich scorpionate
complexes [(L)­M­(NCMe)3]­(BF4)n (L = tris­(3,5-dimethylpyrazol-1-yl)­methane,
TpmMe,Me, n = 2, 1M, M = Mn, Fe, Co, Ni; L = tris­(3-phenylpyrazol-1-yl)­methane, TpmPh, n = 2, 2M, M =
Mn, Fe, Co, Ni; L = hydrotris­(3,5-dimethylpyrazol-1-yl)­borate, [TpMe,Me]−, n = 1, 3M, M = Fe, Co, Ni; L = hydrotris­(3-phenyl-5-methylpyrazol-1-yl)­borate,
[TpPh,Me]−, n = 1, 4M, 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 2Mn, 2Ni, 3Fe, 3Co, and 4Fe 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 1Fe, 1Co, 2Fe, and 2Co, the triflate salt analogues of 4Co and 4Ni, as well as
related sandwich complexes (e.g., [(TpMe,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.