Alkyne complexes of platinum. Part 3. The synthesis and crystal structure of bis(diphenylacetylene)platinum, and studies on related compounds

Abstract

The bis(alkyne)platinum complexes [Pt(R¹C₂R²)₂][R¹= R²= Ph, CH₂SiMe₃, C₆H₄OMe-4, or C(OH)Me₂; R¹= Bu^t, R²= SiMe₃; R¹= Ph, R²= SiMe₃] have been prepared by treating [Pt(cod)₂](cod = cyclo-octa-1,5-diene) with excess of the alkyne. This method was unsuccessful for the synthesis of the related compounds [Pt(R¹C₂R²)₂](R¹= R²= Me, Et, Bu^t, or C₆H₄Me-4; R¹= Bu^t, R²= Me; R¹= Ph, R²= Me) but these species are readily prepared from [Pt(C₂H₄)₃] and the respective alkynes. A single-crystal X-ray diffraction study of [Pt(PhC₂Ph)₂] has shown that the crystals are monoclinic, space group P2/n, with Z= 2 in a unit cell of dimensions a= 13.163(5), b= 6.062, c= 14.354(7)Å, and β= 115.04(3)°. The structure has been solved by heavy-atom methods from automated diffractometer data, and refined to R 0.038 (R′ 0.046) for 3 433 reflections. The molecule adopts an essentially tetrahedral configuration, the angle between the two [graphic omitted] planes being 82°. The platinum atom lies on a crystallographic two-fold axis of rotation, at a mean distance of 2.022(5)Å from the acetylenic carbon atoms which are 1.291(5)Å apart. Co-ordination of the diphenylacetylene to the metal shows the expected bending of the phenyl groups away from the platinum atom [Ph–C–C 153.2(5)°]. The acetylenic stretches in the i.r. spectra of the complexes [Pt(R¹C₂R²)₂] occur in the range 1 840–1 924 cm^–1, ca. 150 cm^–1 above the corresponding bands in the spectra of complexes [Pt(R¹C₂R²)(PR₃)₂], reflecting less metal–acetylene back bonding in the former. For all the complexes [Pt(R¹C₂R²)₂] there is a downfield shift of the acetylenic-carbon resonances in the ¹³C n.m.r. spectra of 30–40 p.p.m. upon metal co-ordination.

Equimolar amounts of [Pt(cod)₂] and PhCCPh afford [Pt(PhC₂Ph)(cod)], while a second mol of the acetylene yields [Pt(PhC₂Ph)₂]. In contrast, in the presence of excess of the alkynes 4-MeC₆H₄CCC₆H₄Me-4′, C₆F₅CCC₆F₅, and 4-MeOC₆F₄CCC₆F₄OMe-4′, [Pt(cod)₂] yields monoalkyne complexes [Pt(alkyne)(cod)]. Treatment of the fluoroarylalkyne-platinum compounds with CNBu^t gives [Pt(R¹C₂R²)(CNBu^t)₂](R¹= R²= C₆F₅ or C₆F₄OMe-4). Addition of [Pt(PhC₂Ph)₂] to light petroleum solutions containing excess of L = CNBu^t, PMe₃, PEt₃, or PPh₃ yields the complexes [Pt(PhC₂Ph)L₂]. By using samples containing enriched Ph¹³CCPh, ¹³C n.m.r. chemical shifts and ¹⁹⁵Pt–¹³C coupling constants have been measured for these compounds, and for those species containing phosphine ligands the ¹³P n.m.r. spectra also recorded.