Structure−Reactivity Correlations for Complex Formation Reactions between Square-Planar Metal Centers and Thioethers

Kinetics for complex formation between Pd(H 2 O) 4 2+ and thioethers of largely varying electronic and steric properties, viz . MeSCH 2 COOH, ( n -Pr) 2 S, EtSCH 2 CH 2 OH, S(CH 2 CH 2 CH 2 OH) 2, S(CH 2 CH 2 OH) 2, EtSCH 2 COOH, S(CH 2 COOH) 2, S(CH 2 CH 2 COOH) 2, ( i -Pr) 2 S, ( s -Bu) 2 S, ( t -Bu) 2 S, and protonated thiomorpholine, S(C 2 H 4 ) 2 NH 2 +, has been studied by use of stopped-flow spectrophotometry in an acidic aqueous medium. Second-order rate constants k 1 298 are 1.61 × 10 4, 8.0 × 10 4, 3.79 × 10 4, 3.69 × 10 4, 2.21 × 10 4, 1.84 × 10 4, 1.91 × 10 3, 1.34 × 10 4, 1.52 × 10 4, 7.75 × 10 3, 900, and 5.2 × 10 3 M -1 s -1, respectively. The reactivity toward Pd(H 2 O) 4 2+ of all thioethers studied so far can be described as a function of their σ-donor properties as expressed by the sum of the Taft constants, ∑σ*, and their steric requirements as defined by cone angles, θ, by use of the equation: log k 1 = (9.9 ± 0.3) − (0.67 ± 0.05)∑σ* − (0.059 ± 0.003)θ. Similarly, second-order rate constants k 298 reported previously for reactions between thioethers and Pd(dien)H 2 O 2+ and Pt(dien)Br + are described by log k 298 = (10.5 ± 0.6) − 0.67∑σ* − (0.081 ± 0.006)θ and log k 298 = (4.6 ± 0.6) − 0.72∑σ* − (0.080 ± 0.006)θ, respectively. Hence, the reactivity trends of thioethers toward square-planar complexes can be given a general interpretation in terms of intrinsic, electronic, and steric parameters, by use of log k = γ + α∑σ* + βθ. Large variations in both electronic and steric properties of the entering ligands indicate that there is no “duality behavior” in the reactions of thioethers with square-planar metal centers, as claimed in previous literature. No steric threshold is observed for these sterically unhindered systems. There is a rough compensation effect between Δ H 1 ⧧ and Δ S 1 ⧧, i . e . a smaller Δ H 1 ⧧ is usually accompanied by a larger negative Δ S 1 ⧧, indicating that all thioethers react via the same mechanism. It appears that the much lower reactivity observed for the highly branched ( t -Bu) 2 S is primarily caused by a high activation enthalpy. Volumes of activation have been determined for a series of thioethers with a constant cone angle, viz . EtSCH 2 COOH, S(CH 2 COOH) 2, and S(CH 2 CH 2 COOH) 2 through high-pressure stopped-flow measurements. Values of Δ V 1 ⧧ are −7.9 ± 0.5, −8.1 ± 0.4, and −7.6 ± 0.3 cm 3 mol -1, respectively. These values together with that for Et 2 S (−8.7 ± 0.1 cm 3 mol -1 ) determined previously shows that variation of electronic properties, steric factors being kept constant, change the reactivity markedly, but have no observable influence on the activation volumes. Stability constants β 1 for five palladium thioether complexes derived as the ratio between rate constants for forward and reverse reactions vary between (1.2 ± 0.3) × 10 4 and (3.2 ± 0.7) × 10 4 M -1 .