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

Several novel dimers of the composition [M2Cl4(trans-dppen)(2)] (M=Ni (1), Pd (2), Pt (3)) containing trans-1,2-bis-(diphenylphosphino)ethene (trans-d ppen) have been prepared and characterized by X-ray diffraction methods, NM R spectroscopy (Pt-195{H-1}, P-31{H-1}), elemental analyses, and melting po ints. The intramolecular [2 + 2] photocycloaddition of the two diphosphine- bridges in 3 produces [Pt2Cl4(dppcb)] (4), where dppcb is the new tetradent ate phosphine cis,trans,cis-1,2,3,4-tetrakis-(diphenylphosphino)cyclobutane . Neither 1 nor the free diphosphine trans-dppen shows this reaction. In th e case of 2 the photocycloaddition is slower than in 3. This difference can be explained by the shorter distance between the two aliphatic double bond s in 3 than in 2, but also different transition probabilities within ground and excited states of the used metals could be involved. Furthermore, vari able-temperature P-31{H-1} NMR spectroscopy of 2 or 3 reveals a negative ac tivation entropy of 2 for the [2 + 2] photocycloaddition, but a positive of 3. The removal of chloride from 4 by precipitating AgCl with AgBF4, and su bsequent treatment with 2,2'-bipyridine (bipy) or 1,10-phenanthroline (phen ) leads to [Pt-2(dppcb)(bipy)(2)](BF4)(4) (5) and [Pt-2(dppcb)(phen)(2)](BF 4)(4) (6), respectively. In an analogous reaction of 4 with PMe2Ph or PMePh 2, [Pt-2(dppcb)(PMe2Ph)(4)](BF4)(4) (7) and [Pt-2(dppcb)(PMePh2)(4)](BF4)(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 s tructure of 4 reveals two conformations, which arise from an energetic comp etition between the sterical demands of dppcb and an ideal square-planar en vironment of Pt(II). The free tetraphosphine dppcb can be obtained easily f rom 4 by treatment with NaCN. It has been characterized fully by the above methods including C-13{H-1} and H-1 NMR spectroscopy. The X-ray structure a nalysis shows the pure MMMP-enantiomer in the solid crystal, which is there fore optically active. This chirality is induced by a conformation of dppcb , where all four PPh2 groups are non-equivalent. Variable-temperature P-31{ H-1} 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. (C) 1999 Elsevier Science S.A. All rights reserved.