Structural differences in nickel(II) and palladium(II) complexes containing cis-1,2-bis(diphenylphosphino)ethene or 1,2-bis(diphenylphosphino)ethane

Abstract

Monometallic Ni II and Pd II complexes of cis-1,2-bis(diphenylphosphino)ethene ( cis-dppen) or 1,2-bis(diphenylphosphino)ethane (dppe) have been prepared and characterized by X-ray diffraction methods, 31P 1H NMR spectroscopy, FAB mass spectrometry, elemental analyses and melting points. The X-ray structure of [NiCl 2( cis-dppen)] ( 1) is given for the first time. In 1 the mean deviation of the nickel, the phosphorus and the chlorine atoms from a least-squares plane through these atoms is only 0.025 Å. The ethene bridge is nearly coplanar with this plane, which can be explained by a π-bonding interaction. It is shown that the small deviation from coplanarity are caused by a crystal packing effect. The reaction of Ni(CN) 2 with cis-dppen leads to the novel compound [Ni(CN) 2( cis-dppen)] ( 2). The removal of chloride in 1 by AgBF 4 followed by the reaction with 1,10-phenanthroline (phen) leads to the novel octahedral Ni II complex [Ni( cis-dppen)(phen) 2](BF 4) 2 ( 3). In an analogous manner 2,2′-bipyridine (bipy) produces [Ni( cis-dppen)(bipy) 2](BF 4) 2 ( 4). The reaction of trans-1,2-bis(diphenylphosphino)ethene ( trans-dppen) with PdCl 2 and subsequent treatment with LiI leads to the unexpected product [PdI 2(dppe)] ( 5). Compound 5 is also fully characterized by an X-ray structure analysis for the first time. The palladium, phosphorus and iodine atoms show a mean deviation of only 0.017 Å from a least-squares plane through these atoms. However, the PdP 2C 2 chelate ring has a twist conformation, where one carbon atom of the aliphatic chain of dppe lies −0.420 Å to one side, and the second carbon atom 0.318 Å to the other side of the coordination plane. In 5 the PPdP chelate angle of 85.59(5)° is significantly smaller than the corresponding PNiP angle of 87.79(4)° in 1. Furthermore, the X-ray structure of the recently reported complex [Pd( cis-dppen) 2](BPh 4) 2 ( 6) is given. The palladium atom is located on a centre of symmetry leading to the complete planarity of the coordination plane due to crystallographic constraints. However, the two carbon atoms of the ethene bridge of cis-dppen lie 0.547 and 0.532 Å, respectively, to one side of the coordination plane. Compound 6 significantly shows the smallest PMP chelate angle of 84.78(3)° compared with the corresponding values in 1 and 5. A comparison of the X-ray structures of 1,5 and 6 with several known X-ray structures of Ni II and Pd II complexes containing cis-dppen or dppe is given. Differences in the chelate angles and coplanar parts of these structures are discussed with respect to the observed reactivities of compounds 1–6.