On the Equilibrium between Alkyne and Olefin Platinum(II) Complexes of Zeise’s Salt Type: Syntheses and Characterization of [K(18C6)][PtCl3(RCCR′)]

Zeise’s type alkyne complexes [K­(18C6)]­[PtCl3(RCCR′)] (R/R′ = Me/Me, 3; Et/Et, 4; Me/t-Bu, 5; t-Bu/t-Bu, 6; Me/Ph, 7; Me/CO2Me, 8; RCCR′ = COC, 9; COC = cyclooctyne; 18C6 = 18-crown-6) were obtained from the cis-but-2-ene complex [K­(18C6]­[PtCl3(cis-but-2-ene)] (2) and the requisite alkyne via a ligand substitution reaction. 1H NMR spectroscopically determined equilibrium constants showed that the formation of all these alkyne complexes is endergonic, except that of the cyclooctyne complex 9, which is strongly exergonic. The identities of the complexes were proved by microanalysis and NMR spectroscopy (1H, 13C). X-ray diffraction analyses of complexes 5–7 exhibited slightly elongated CC triple bonds (1.23(1)–1.24(1) Å) and a back bending of the substituents on the alkyne between 16(1)° and 21(1)°. In contrast, a longer CC bond (1.27(1) Å) was found for the cyclooctyne complex 9, whereas the C–CC angles in this complex (26.8(7)°/26.0(7)°) are the same as in the noncoordinated cyclooctyne (26(2)°). Quantum chemical calculations on the DFT level of theory of the complex anions [PtCl3(RCCR′)]− (3a′–9a′) showed analogous structural features for the coordinated alkynes. Furthermore, energy decomposition analysis exhibited that the extraordinarily high stability of the cyclooctyne complex 9a′ can be understood in terms of a very low preparation energy of the cyclooctyne (due to the “prebended” structure of the noncoordinated COC) and a relatively high (instantaneous) interaction energy. NBO analyses made clear that the π back-donation in complexes bearing alkynes with electron-withdrawing substituents (Ph, CO2Me) and in the COC complex is significantly greater than that in complexes bearing alkynes with alkyl substituents (Me, Et, t-Bu).