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Synthesis and characterization of alkyne complexes analogous to Zeise's salt — [K(18-cr-6)][PtCl3(nPrCCH)], the first complex of platinum(II) with a terminal alkyne

1995; Elsevier BV; Volume: 234; Issue: 1-2 Linguagem: Inglês

10.1016/0020-1693(95)04467-n

1873-3255

Dirk Steinborn, Matthias Tschoerner, André von Zweidorf, Joachim Sieler, Horst Bögel,

Catalytic Alkyne Reactions

The reaction of K2[PtCl4] with crown ether 18-cr-6 yields in methylene dichloride [K(18-cr-6)]2[PtCl4] (2) and in water [K(18-cr-6)]2[Pt2Cl6] (3). 3 reacts with alkynes with internal triple bonds to give alkyne complexes of Zeise's salt type [K(18-cr-6)][PtCl3(RCCR′)] [R, R′ = Me, Me (4a); Et, Et (4b); Me, Et (4c); Me, nPr (4d); Ph, Ph (4e); C(OH)Me2, C(OH)Me2 (4f)]. Complexes 4a-f (Tdec 130–186 °C) were characterized by elemental analysis as well as by NMR, IR and Raman spectroscopy. Due to coordination of the alkyne to PtCl3− vibration frequencies ν(CC) are lowered to 1937–2059 cm−1 corresponding to a relative lowering Δνν of about 10%. The molecular structures of 4a (monoclinic, P21m, a = 8.795(2), b = 11.447(3), c = 12.544(2) Å, β = 109.66(2)°, Z = 2, R1 = 0.0292) and 4b (triclinic, P1, a = 9.172(2), b = 10.918(3), c = 14.645(4) Å, α = 98.32(2), β = 99.43(2), γ = 112.67(2)°, Z = 2, R1 = 0.0401) were elucidated by X-ray analysis. The angle between the planes PtCl3 and Pt(CC) in 4a is exactly 90° and in 4b it is 81°. The lengthening of the CC bond is small in accordance with ab initio MO calculations reflecting the dominance of σ donation over π back-donation. The KCltrans distance in 4a is very short (3.077(4) Å) and the angle between the mean plane of the crown ether and the PtCl3 group is only 20.9°. In 4b there is a contact of K to Cltrans (3.126(5) Å) and Clcis (3.356(6) Å) whereas the interplanar angle PtCl3/18-cr-6 amounts to 105.5°. Complexes 4 can be interconverted by ligand substitution reactions and the equilibrium constant for 4a and 4b is K = 3.02. Thus [K(18-cr-6)][PtCl3(nPrCCH)] (4g) was prepared by treatment of 4a with 1-pentyne at −20 °C. 4g (Tdec 95 °C), the first platinum(II) complex containing an alkyne with a terminal triple bond, was characterized by elemental analysis and spectroscopically (ν(CC) 1937 cm−1, 2J(Pt,H) 70.6 Hz). Complexes 4 react with ethylene to give [K(18-cr-6)][PtCl3(H2CCH2)] (5) representing Zeise's salt in which the potassium cation is coordinated by a crown ether.