Structural studies on iminophosphine ligands and their palladium complexes

The crystal and molecular structures of the iminophosphine o-(Ph₂P)C₆H₄CH=NC₆H₄OMe-4 (1) and its palladium complexes [Pd(³-C₃H₅){o-(Ph₂P)C₆H₄CH=NC₆H₄OMe-p}]BF₄ (2) and [Pd(²-fn){o-(Ph₂P)C₆H₄CH=NC₆H₄OMe-4}] [fn=fumaronitrile, (3)] have been determined by X-ray analysis. In the free ligand (1), the planar imino group of E configuration is oriented, relative to the PPh₂ unit, so that the CH=N hydrogen atom points towards phosphorus, with the nitrogen atom on the opposite side. In (2) and (3) the iminophosphine behaves as a P,N-chelate ligand, this coordination mode being achieved by the imino group rotation of 169.3° and 145.3°, respectively, around its bond with the ortho disubstituted phenyl ring. Complex (2) shows a structural disorder with two different orientations of the allyl ligand. The trigonal planar coordination around the central metal in complex (3) involves the P- and N-donor atoms of (1) and the ²-bound olefin, with a marked lengthening of the olefinic carbon-carbon bond. In both the complexes, the chelate six-membered ring of the iminophosphine with palladium is not coplanar with the N-Pd-P coordination plane, the imino carbon atom and the ortho disubstituted phenyl group lying on the same side out of the N-Pd-P plane, whereas the N-substituent and one of the PPh₂ groups are on the opposite side. The ¹H-n.m.r. spectra at low temperatures of (2) and (3), and of [Pd(²-tmetc){o-(Ph₂P)C₆H₄CH=NCMe₃}] [tmetc=tetramethyl ethylenetetracarboxylate, (4)] are interpreted on the basis of a non-rigid conformation of the chelate iminophosphine, which undergoes a fast dynamic process whereby the N- and P-substituents move above and below the coordination plane.