Synthesis, platinum(II) complexes and structural aspects of the new tetradentate phosphine cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane

Abstract

Several novel dimers of the composition [M 2Cl 4( trans-dppen) 2] (M=Ni ( 1), Pd ( 2), Pt ( 3)) containing trans-1,2-bis(diphenylphosphino)ethene ( trans-dppen) have been prepared and characterized by X-ray diffraction methods, NMR spectroscopy ( 195Pt{ 1H}, 31P{ 1H}), elemental analyses, and melting points. The intramolecular [2+2] photocycloaddition of the two diphosphine-bridges in 3 produces [Pt 2Cl 4(dppcb)] ( 4), where dppcb is the new tetradentate phosphine cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane. Neither 1 nor the free diphosphine trans-dppen shows this reaction. In the case of 2 the photocycloaddition is slower than in 3. This difference can be explained by the shorter distance between the two aliphatic double bonds in 3 than in 2, but also different transition probabilities within ground and excited states of the used metals could be involved. Furthermore, variable-temperature 31P{ 1H} NMR spectroscopy of 2 or 3 reveals a negative activation entropy of 2 for the [2+2] photocycloaddition, but a positive of 3. The removal of chloride from 4 by precipitating AgCl with AgBF 4, and subsequent treatment with 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen) leads to [Pt 2(dppcb)(bipy) 2](BF 4) 4 ( 5) and [Pt 2(dppcb)(phen) 2](BF 4) 4 ( 6), respectively. In an analogous reaction of 4 with PMe 2Ph or PMePh 2, [Pt 2(dppcb)(PMe 2Ph) 4](BF 4) 4 ( 7) and [Pt 2(dppcb)(PMePh 2) 4](BF 4) 4 ( 8) are formed. Complexes 1– 8 show square–planar coordinations, where the compounds 4– 8 have also been characterized by the above mentioned methods together with fast atom bombardment mass spectrometry ( 7, 8). The crystal structure of 4 reveals two conformations, which arise from an energetic competition between the sterical demands of dppcb and an ideal square–planar environment of Pt(II). The free tetraphosphine dppcb can be obtained easily from 4 by treatment with NaCN. It has been characterized fully by the above methods including 13C{ 1H} and 1H NMR spectroscopy. The X-ray structure analysis shows the pure MMMP-enantiomer in the solid crystal, which is therefore optically active. This chirality is induced by a conformation of dppcb, where all four PPh 2 groups are non-equivalent. Variable-temperature 31P{ 1H} NMR spectroscopy of dppcb confirms this explanation, since the single signal at room temperature is split into two doublets at 183 K. The goal of this article is to demonstrate the facile production of a new tetradentate phosphine from a diphosphine precursor via Pt(II) used as a template.