Alkyne complexes of platinum. Part 4. Stepwise formation of di- and tri-platinum complexes with bridging alkyne ligands; crystal structure of [Pt 3 {µ-(η 2 -PhC 2 Ph)} 2 (PEt 3 ) 4 ]

Reaction of the compound [Pt(PhC2Ph)2] with [Pt(C2H4)(PPh3)2] or [Pt2(µ-cod)(PEt3)4](cod = cyclo-octa-1,5-diene) affords diplatinum complexes [Pt2(µ-PhC2Ph)(PhC2Ph)(PR3)2](R = Ph or Et). The triphenylphosphine compound has also been prepared by treating PhCCPh with [Pt(C2H4)2(PPh3)]. The terminal alkyne ligand in [Pt2(µ-PhC2Ph)(PhC2Ph)(PPh3)2] bonds a third platinum atom on treatment with [Pt(C2H4)(PPh3)2] to give the triplatinum complex [Pt3(µ-PhC2Ph)2(PPh3)4]. The latter can also be prepared by addition of 2 mol equivalents of [Pt(C2H4)(PPh3)2 to [Pt(PhC2Ph)2], and the triethylphosphine-triplatinum analogue is similarly obtained using [Pt2(µ-cod)(PEt3)4]. In view of the novelty of these compounds, a single-crystal X-ray diffraction study has been carried out on [Pt3(µ-PhC2Ph)2(PEt3)4], crystals of which are monoclinic, space group C2/c(no. 15), with Z= 4 in a unit cell of dimensions a= 17.047(2), b= 13.677(2), c= 25.073(3)Å, and β= 105.79(1)°. The structure has been solved by heavy-atom methods from 3 038 intensity data [I 3.0σ(I)] measured on a four-circle diffractometer at 300 K, and refined to R 0.047 (R′ 0.046). The three platinum atoms adopt an open V-shaped configuration with an internuclear distance of 2.904Å and an interbond angle of 144°, while the acetylenic units form transverse bridges across the two Pt–Pt vectors on the convex side of the V. The phenyl groups bend away from the metal atoms to give a C–C–Ph angle of 139°, and the whole molecule is constrained crystallographically to C2 symmetry. The ethyl groups of the phosphine ligands are ill defined and possibly disordered.The diplatinum complexes [Pt2(µ-RC2R)(PMe3)4](R = Ph or C6F5) have also been prepared, and [Pt2(µ-PhC2Ph)(PEt3)4] characterised spectroscopically. Reaction of [Pt(PhC2Ph)(CNBut)2] with [Pt(C2H4)(PPh3)2] yields trans-[Pt2(µ-PhC2Ph)(CNBut)2(PPh3)2], scrambling of the CNBut and PPh3 ligands having occurred. The stereochemistry of this complex, and its PhCCC6H4OMe-4 analogue, has been established by 31P and 13C-labelling n.m.r. studies. The modes of formation of the various compounds are discussed.