Palladium(II), platinum(II), and platinum(IV) complexes containing trans-1,2-bis(diphenylphosphino)ethene or cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane: complete X-ray structural characterization of binuclear compounds

Abstract

Several novel binuclear Pd II, Pt II, and Pt IV complexes of trans-1,2-bis(diphenylphosphino)ethene ( trans-dppen) or cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane (dppcb) have been prepared and characterized by X-ray diffraction methods, 195Pt{ 1H} and 31P{ 1H} NMR spectroscopy, FAB mass spectrometry, IR spectroscopy, elemental analyses and melting points. The X-ray structure and NMR parameters of [Pt 2I 4( trans-dppen) 2] ( 1) confirm that homobimetallic complexes of the type [M 2L 4( trans-dppen) 2] (M=Ni, Pd, Pt; L=Cl −, I −, CN −) contain two square planar coordination units joined by two trans-dppen bridges in the solid as well as in the solution state. An analogous structure type is retained in the Pt IV compound [Pt 2Cl 8( trans-dppen) 2] ( 2). In contrast to the corresponding Pt II complex of the new tetradentate phosphine dppcb, the X-ray structure of [Pd 2Cl 4(dppcb)] ( 3) reveals only one conformation. This can be explained by the longer Pd–P bonds compared with the Pt–P bonds and the weaker square planar stabilization energy in 3. In 3 dppcb acts as a binuclear tetraligate single-bridging ligand combining two square planar coordination centres. The reaction of Pd(CN) 2 with dppcb leads to [Pd 2(CN) 4(dppcb)] ( 4). The removal of the coordinated chlorides in 3 by AgBF 4 followed by subsequent treatment with NaNO 2 produces [Pd 2(NO 2) 4(dppcb)] ( 5). In an analogous reaction with 1,10-phenanthroline (phen) or 2,2′-bipyridine (bipy), [Pd 2(dppcb)(phen) 2](BF 4) 4 ( 6) and [Pd 2(dppcb)(bipy) 2](BF 4) 4 ( 7) are formed. The complexes 4– 7 show structure types corresponding to the X-ray structure of 3. The same is true for the treatment of 3 with PMePh 2 or PMe 2Ph, where [Pd 2(dppcb)(PMePh 2) 4](BF 4) 2Cl 2 ( 8) and [Pd 2(dppcb)(PMe 2Ph) 4](BF 4) 4 ( 9) are obtained. However, the X-ray structure of 8 reveals that the chlorides are coordinated in the solid state, and the crystal structure consists of trans-[Pd 2Cl 2(dppcb)(PMePh 2) 2](BF 4) 2 ( 10). The flexibility and stereochemical demands of dppcb in the compounds 3– 10 and related species are discussed in view of the possible application of Pd II complexes containing bidentate tertiary phosphine ligands as catalysts for the alternating copolymerization of ethene and carbon monoxide. In this respect dppcb can be regarded as a combination of two bidentate phosphines, where the cyclobutane ring corresponds to a relatively rigid ligand backbone. This produces strain in the five-membered rings of 3– 10 which is released by ‘envelope’-folding in the X-ray structures of 3 and 10.