Binuclear palladium(II), platinum(II) and platinum(IV) complexes containing 1,2-bis(diphenylphosphino)acetylene: Different orientations of the diphosphine-bridges due to metal-phosphorus dπ-dπ back bonding

Article abstract of DOI:10.1016/S0020-1693(96)05449-7

Several novel Pd(II), Pt(II) and Pt(IV) dimers containing 1,2-bis(diphenylphosphino)acetylene (dppa) as bridging ligand have been prepared and characterized by X-ray diffraction methods, NMR spectroscopy (¹⁹⁵Pt{¹H), ³¹P{¹H)), IR spectroscopy, elemental analyses and melting points. The X-ray structures of the former reported complexes [Pd₂Cl₄(dppa)₂] (1) and [Pt₂Cl₄(dppa)₂] (2) are given for the first time: 1: monoclinic, P2₁/n, a = 14.401(3), b = 12.100(2), c = 15.880(3) A, β = 91.36(3)°, Z = 2, R = 0.039 for 6432 observed reflections (I> 3σ(I)); 2: orthorhombic, Pbcn, a = 13.995(3), b = 17.669(4), c = 21.960(4) A, Z = 4, R = 0.052 for 2332 observed reflections (I> 3σ(I)). These two X-ray structures are discussed with respect to the former given ambiguous interpretation of the X-ray structure of [PtPdCl₄(dppa)₂]. Replacement of the chlorides in 1 or 2 by iodides leads to [Pd₂I₄(dppa)₂] (3) or [Pt₂I₄(dppa)₂] (4). 4 is also fully characterized by an X-ray structure analysis for the first time: 4: monoclinic, P2₁/c, a = 10.586(2), b = 30.526(6), c = 18.383(4) A, β = 100.09(3)°, Z = 4, R = 0.061 for 4698 observed reflections (I> 3σ(I)). The X-ray structures of 1,2 and 4 reveal different orientations of the dppa bridges due to metal-phosphorus dπ-dπ back bonding leading to different angles between the square-planar coordination units, respectively. Replacement of the chlorides in 1 by NO₂^- leads to [Pd₂(NO₂)₄(dppa)₂] (5), whereas in 2 only one chloride can be substituted by NO₂^- and [Pt₂Cl₃(NO₂) (dppa)₂] (6) is formed. A similar reaction of 2 with 2,2′-bipyridine (bipy) produces [Pt₂Cl₂(dppa)₂(bipy)](BF₄) ₂ (7). Oxidation of 2 by chlorine gives the Pt(IV) dimer [Pt₂Cl₈(dppa)₂] (8). An equimolar amount of PdCl₂ reacts with one triple bond of 2 leading to [Pt₂PdCl₆(dppa)₂] (9). In an analogous manner [Pd₄Cl₈(dppa)₂] (10) is formed from 1 and two equimolar amounts of PdCl₂. The stability and reactivity of complexes 1-10 are discussed with respect to the X-ray structures of 1,2,4 and [ PtPdCl₄(dppa)₂].