Dinuclear Olefin and Alkyne Complexes of Platinum(II)

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Abstract

Bis(olefin) complexes of Zeise's dimer type – [{PtCl2(cis-MeHC=CHMe)}2] (2) or [{PtCl2(c-Hex)}2] (3, c-Hex = cyclohexene) – were prepared by treatment of K2[PtCl4] in the presence of SnCl2 or K[PtCl3(cis-MeHC=CHMe)] with the corresponding olefin. Zeise's dimer, [{PtCl2(H2C=CH2)}2] (1), was found to react in chloroform or dichloromethane with the internal alkynes RC≡CtBu (R = Me, tBu), provided that the cleaved-off ethene was removed, to yield bis(alkyne) complexes of Zeise's dimer type [{PtCl2(RC≡CtBu)}2] (R = Me, 11; tBu, 12). Without removal of the ethene, mononuclear mixed ethene/alkyne complexes cis-[PtCl2(H2C=CH2)(RC≡CtBu)] (R = Me, 8; tBu, 9) and cis-[PtCl2(H2C=CH2)2] (10) were formed. Analogous reactions with alkynes RC≡CR′ bearing sterically less demanding substituents (R, R′ = Me, Et, nPr, Ph) led to (cyclobutadiene)platinum(II) complexes [PtCl2(C4R2R′2)] (R/R′ = Me/Me 4; Et/Et 5; Me/nPr 6) and [PtCl2(C4Me2Ph2)] (7). Furthermore, use of acetone (instead of CHCl3/CH2Cl2) as solvent in the reactions between 1 and sterically undemanding substituted alkynes resulted in bridge cleavage of Zeise's dimer (1) and in aldol addition and condensation reactions, thereby confirming why Chatt et al. in his analogous classical experiments (1961) observed only decomposition. Complexes 27 and 11/12 were isolated in pure states and fully characterized by elemental analysis, NMR spectroscopy, and X-ray diffraction measurements (2, 11, 12). For the dinuclear bis(olefin) (13) and bis(alkyne) complexes (11, 12) solvent-dependent equilibria between the transoid and cisoid isomers were observed, and these could even be ascertained crystallographically for complex [{PtCl2(tBuC≡CtBu)}2] (12), which crystallized both as the transoid isomer 12a and as the cisoid isomer 12b·CHCl3. Furthermore, consistent with DFT calculations, NMR measurements provided evidence of fast rotation and of hindered rotation of olefin and alkyne ligands, respectively, resulting in conformational isomers in 8 and 11.

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