Decoding catalytic activity of platinum carbene hydrosilylation catalysts

A series of complexes of the formula [Pt(dvtms)(ImPy-R)] (dvtms = 1,1,3,3-tetramethyl-1,3-divinyldisiloxane, ImPy-R = 2-R-imidazo[1,5-a]pyridine-3-ylidene; R = 4-cyanophenyl (4a), 4-trifluoromethylphenyl (4b), phenyl (4c), 4-methoxyphenyl (4d), mesityl (4e), pentafluorophenyl (4f), tert-butyl (4g)) are presented. These compounds are synthesized from Karstedt's catalyst [Pt₂(dvtms)₃] and the respective imidazo[1,5-a]pyridinium salts using ^tBuOK as base. 4a-g were characterized (¹H, ¹³C, ¹⁹F, ¹⁹⁵Pt NMR, EA, IR, UV-Vis) and investigated by cyclic voltammetry. The compounds are efficient and selective catalysts in the model hydrosilylation reaction of oct-1-ene with HSi₃O₂Me₇. The combination of spectroscopic and theoretical studies on the B3LYP/6-31G∗∗ level of theory reveals a distinct correlation of HOMO energy levels E_HOMO, oxidation potential E_Ox and catalytic activity (TOF). The impact of the substitution pattern introduced at the ImPy-R ligand can be interpreted based on electronics (Hammet correlation) and sterics of the residues R.