Racemization and Isotopic Exchange of Tris(acetylacetonato)silicon(IV) Perchlorate in Organic Solvents

Racemization of trisacetylacetonatosilicon(IV) ions and their isotopic exchange reaction with acetylacetone-14C (Hacac) have been studied in tetrachloroethane (TCE) and acetonitrile (AN). No solvolysis takes place at least below 60°C, and the racemization rate R is expressed as follows: R=[complex] (k1+k2[acid]) (in TCE) and R=[complex] (k3+k4[base]) (in AN). The rate constants and activation parameters at 50°C are as follows: rate constants, 4.79×106 s−1, 1.35×10−4 M−1 s−1, 3.87×10−5 s−1 and 1.19×10−1 M−1 s−1, activation enthalpy (ΔH/kcal mol−1) 31.6±0.1, 29.3±1.9, 25.7±0.2 and ca. 18, and activation entropy (ΔS/cal mol−1K−1) 14.6, 14.2, 0.53, −5.75, respectively, for k1, k2 (for trichloroacetic acid), k3 and k4 (for 2,6-lutidine). No isotopic exchange is observed within 6 hr at 60°C in the solvents. The racemization should proceed intra-molecularly and presumably via an intermediate of coordination 5 with one of the ligands as unidentate.