PLATINUM(II) COMPLEXES WITH MONOCOORDINATED 2,9-DIMETHYL-1,10-PHENANTHROLINE AND PHOSPHINE-LIGANDS - EXCHANGE OF THE DONOR NITROGEN AND ROTATION ABOUT THE PT-P AND P-C BONDS STUDIED BY NMR-SPECTROSCOPY - ARENE STACKING AS AN INTRAMOLECULAR BRAKE

The structure and dynamic behavior of the platinum complexes cis- and trans-[PtX2(Me2-phen)(PPh3)] (X = Cl (1), Br (2),I (3); cis isomer, a; trans isomer, b), trans-[PtX(Me2-phen)(PPh3)2]X (X = Cl (4), Br (5),I (6)), and trans-[PtX(Me2-phen)(PBun3)2]X (X = Cl (7), Br (8), I (9)) containing monocoordinated 2,9-dimethyl-1,10-phenanthroline (Me2-Phen) are reported. Exchange of the donor atom (flipping) for the singly bonded Me2-phen and rotations about the Pt-P and P-C bonds of the phosphine have been investigated by NMR spectroscopy. For the first time the DELTAG* for flipping of a bidentate ligand with converging lone pairs of electrons has been measured and found to be strongly dependent upon the nature of the trans ligand (trans effect). The DELTAG* for rotation about the Pt-P bond is unusually high in the case of PPh3 ligands (DELTAG* > 10 kcal/mol) and does not change significantly among the different compounds examined, although the Pt-P bonds are 0.1 angstrom longer in complexes 4-6 (phosphine trans to another phosphine) than in complexes 1a-3a (phosphine trans to a halogen atom). The PBun3 complexes 7-9 exhibit much smaller DELTAG* values for phosphine rotation (<7 kcal/mol) but similar or slightly bigger DELTAG*'s for flipping of the Me2-Phen ligand. One phenyl ring of PPh3 (the most shielded one) exhibits restricted rotation about the P-C(ipso) bond with DELTAG* in the range 7.1-7.8 kcal/mol. The two remaining phenyls of PPh3 are not equivalent, and for one of them restricted rotation about the P-C(ipso) bond is also observed. The crystal structures of the complexes cis-[PtCl2(Me2-phen)(PPh3)] (1a) and trans-[PtCl(Me2-phen)(PPh3)2]Cl (4), as chloroform solvates, have been determined by X-ray diffraction methods. One phenyl of each phosphine ligand overlaps with the coordinated end of Me2-phen, the C(ipso) atom eclipsing the nitrogen atom of the phenanthroline; a second phenyl of the phosphine comes close to the second end of Me2-phen. A stacking interaction between a phenyl ring of PPh3 and the Me2-phen aromatic system appears though to be responsible for the high-energy barrier to phosphine rotation, thus simulating an electromagnetic brake. Crystal data of 1a.CHCl3: space group P2(1)/n, a = 15.996(6) angstrom, b = 17.996(6) angstrom, c = 12.314(4) angstrom, beta = 111.23(2)-degrees, Z = 4, 3726 reflections, R = 0.0322. Crystal data of 4.CHCl3: space group P2(1), a = 11.727(6) angstrom, b = 11.825(7) angstrom, c = 17.091(8) angstrom, beta = 94.96(2)-degrees, Z = 2, 4338 reflections, R = 0.0333.