2-Pyridinethione (2-mercaptopyridine, H-2mp) undergoes rapid oxidative addition with 2 mol of the 17-electron organometallic radical .Cr(CO)3Cp* (where Cp* = C5Me 5), yielding hydride H-Cr(CO)3Cp* and thiolate (η1-2mp)Cr(CO)3CP*. In a slower secondary reaction, (η1-2mp)Cr(CO)3CP* loses CO generating the N,S-chelate complex (η2-2mp)Cr(CO) 2Cp* for which the crystal structure is reported. The rate of 2-pyridine thione oxidative addition with .Cr(CO) 3Cp* (abbreviated .Cr) in toluene best fits rate = kobs[H-2mp][.Cr]; kobs(288 K) = 22 ± 4 M-1 s-1; ΔH‡ = 4 ± 1 kcal/mol; ΔS‡= - 40 ± 5 cal/mol K. The rate of reaction is the same under CO or Ar, and the reaction of deuterated 2-pyridine thione (D-2mp) shows a negligible (inverse) kinetic isotope effect (kD/kH = 1.06 ± 0.10). The rate of decarbonylation of (η 1-2mp)Cr(CO)3Cp* forming (η 2)-2mp)Cr(CO)2Cp* obeys simple first-order kinetics with kobs (288 K) = 3.1 × 10-4 s-1, ΔH‡ = 23 ± 1 kcal/mol, and ΔS‡ = + 5.0 ± 2 cal/mol K. Reaction of 4-pyridine thione (4-mercaptopyridine, H-4mp) with .Cr(CO)3Cp* in THF and CH2Cl 2 also follows second-order kinetics and is approximately 2-5 times faster than H-2mp in the same solvents. The relatively rapid nature of the thione versus thiol reactions is attributed to differences in the proposed 19-electron intermediate complexes, [.(S=C5H 4N-H)Cr(CO)3Cp*] versus [.(H-S-C 6H5)Cr(CO)3Cp*]. In comparison, reactions of pyridyl disulfides occur by a mechanism similar to that followed by aryl disulfides involving direct attack of the sulfur-sulfur bond by the metal radical. Calorimetric data indicate Cr-SR bond strengths for aryl and pyridyl derivatives are similar. The experimental conclusions are supported by B3LYP/6-311+G(3df,2p) calculations, which also provide additional insight into the reaction pathways open to the thione/thiol tautomers. For example, the reaction between H. radical and the 2-pyridine thione S atom yielding a thionyl radical is exothermic by ≈30 kcal/mol. In contrast, the thiuranyl radical formed from the addition of H. to the 2-pyridine thiol S atom is predicted to be unstable, eliminating either H. or HS. without barrier.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry