Synthesis and characterization of alkyne complexes analogous to Zeise's salt — [K(18-cr-6)][PtCl 3( nPrCCH)], the first complex of platinum(II) with a terminal alkyne

Abstract

The reaction of K 2[PtCl 4] with crown ether 18-cr-6 yields in methylene dichloride [K(18-cr-6)] 2[PtCl 4] ( 2) and in water [K(18-cr-6)] 2[Pt 2Cl 6] ( 3). 3 reacts with alkynes with internal triple bonds to give alkyne complexes of Zeise's salt type [K(18-cr-6)][PtCl 3(RCCR′)] [R, R′ = Me, Me ( 4a); Et, Et ( 4b); Me, Et ( 4c); Me, nPr ( 4d); Ph, Ph ( 4e); C(OH)Me 2, C(OH)Me 2 ( 4f)]. Complexes 4a-f ( T dec 130–186 °C) were characterized by elemental analysis as well as by NMR, IR and Raman spectroscopy. Due to coordination of the alkyne to PtCl 3 − vibration frequencies ν(CC) are lowered to 1937–2059 cm −1 corresponding to a relative lowering Δν ν of about 10%. The molecular structures of 4a (monoclinic, P2 1 m , a = 8.795(2), b = 11.447(3), c = 12.544(2) A ̊ , β = 109.66(2)°, Z = 2, R 1 = 0.0292) and 4b (triclinic, P 1 , a = 9.172(2), b = 10.918(3), c = 14.645(4) A ̊ , α = 98.32(2), β = 99.43(2), γ = 112.67(2)°, Z = 2, R 1 = 0.0401) were elucidated by X-ray analysis. The angle between the planes PtCl 3 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Cl trans distance in 4a is very short (3.077(4) Å) and the angle between the mean plane of the crown ether and the PtCl 3 group is only 20.9°. In 4b there is a contact of K to Cl trans (3.126(5) A ̊ ) and Cl cis (3.356(6) A ̊ ) whereas the interplanar angle PtCl 3/18-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)][PtCl 3( nPrCCH)] ( 4g) was prepared by treatment of 4a with 1-pentyne at −20 °C. 4g ( T dec 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, 2 J(Pt,H) 70.6 Hz). Complexes 4 react with ethylene to give [K(18-cr-6)][PtCl 3(H 2CCH 2)] ( 5) representing Zeise's salt in which the potassium cation is coordinated by a crown ether.