Reversible Exchange of (η5-Cyclopentadienyl)(dinitrosoalkane)cobalt Complexes with Alkenes. Kinetic and Spectroscopic Evidence for C5H5Co(NO)2 as a Reactive Intermediate

Article abstract of DOI:10.1021/ja00348a011

Heating (η⁵-cycopentadienyl)(2,3-dimethyl-2,3-dinitrosobutane)cobalt (1), prepared from 2, NO, and 2,3-dimethyl-2-butene, with cis-endo-5,6-dicarbomethoxynorbornene (2b) in aromatic solvents converted it quantitatively into 2,3-dimethyl-2-butene (4) and 3b (the adduct formed from 2b and 2/NO).The reaction was first order in <1> with k_obsd = (4.3 +/- 0.3) x 10^-4 s^-1 at 75 deg C, E_a = 29.4 +/- 0.4 kcal/mol, and A = 1,3x10¹⁵ s^-1.Addition of alkene 4 inhibited the reaction in a linear fashion, consistent with a mechanism involving reversible dissociation of 1 into 2,3-dimethyl-2-butene and CpCo(NO)2, followed by reaction of this dinitrosyl with 2b, as shown in Scheme I.This olefin exchange occurs stereospecifically since complexes 5 and 7 exclusively regenerate (E)- and (Z)-3-methyl-2-pentene, respectively.Complex 1 also undergoes olefin exchange upon photolysis in the presence of 2b with Φ₃₀₀ = 1.12x10^-3.Treatment of 2 with nitric oxide generates a new species which is stable in solution at room temperature.This material is too reactive to allow isolation, but its solutions can be studied spectroscopically.It exhibits two bands in the IR (1609 and 1690 cm^-1) and a single resonance in the proton NMR (δ 4.22); in the UV it absorbs at λ_max 255 nm.Adding alkene to a solution of this material rapidly produces cobalt dinitrosoalkanes (Scheme III), identical with those formed on reaction of alkenes with 2/NO.On the basis of these observations this reactive species is proposed to be CpCo(NO)2 (10).The kinetics of the reaction between 10 and alkenes have been studied by using rapid-scan UV-visible spectrophotometry.These experiments have shown that the rate is second order overall, first order in both dinitrosyl and alkene, and the bimolecular rate constants for a series of olefins are reported.An electronically modified derivative of 10, (CO2Me)C5H4Co(NO)2 (16), was prepared from the corresponding dimer 15 and nitric oxide.This complex was noticeably more stable than its parent counterpart but was still too reactive to isolate.