Diolefin complexes of transition metals as 'venus fly-trap' templates

English: The aim of this study was to gain a further insight into the bonding modes of 1,5-cyclooctadiene to various middle to late transition metal centres. The intension was to design a theoretical model of the influences of the variations of the metal centres and ligands on the “Venus fly-trap” system. Two principle geometric features were classified in the 1,5- cyclo-octadine when co-ordinated, i.e. the (bite) angle of the olefinic moieties to the metal centre, and the (jaw) angle, i.e. the dihedral angle between the two planes formed between the alkane ethylene groups. The latter mimicking the “jaws” of the Venus flytrap. A range of 1,5-cyclo-octadiene metal complexes were successfully synthesized and characterized via NMR and IR spectroscopy, the metal centers chosen were that of rhodium(I), platinum(II) and palladium(II). The ligands selected were divided into three categories namely · β-diketonato · β-enaminonato · tropolonato From each of these categories ligands were selected to vary the electron withdrawing or donating ability. Both symmetrical and non-symmetrical ligands were made use of, to see the influence of the nitrogen trans effect on the 1,5-cyclo-octadiene moiety. Finally, the tropolonato category added the influence of the smaller five membered metal chelate ring. A single crystal X-ray crystallographic study of the complexes was undertaken. The reported X-ray crystallographic structure determinations include the following complexes: [Pd(cod)(acac)]PF6 (1, Monoclinic P21/n, R = 3.59 %), [Pd(cod)(acac)]BF4 (2, Orthorhombic Pca21, R = 2.24 %), [Pd(cod)(thtfac)]PF6 (3, Monoclinic P21/n, R = 2.43 %), [Pd(cod)(thtfac)]BF4 (4, Monoclinic P21/n, R = 2.83 %), [Pd(cod)(tfacac)]PF6 (5, Triclinic P1 , R = 13.41 %), [Pd(cod)(hfacac)]PF6 (6, Monoclinic P21/c, R = 2.60 %), [Pt(cod)(acac)] (7, Orthorhombic Pca21, R = 1.75 %), [Pt(cod)(acac)]PF6 (8, Monoclinic P21/n, R = 3.02 %), [Pt(cod)(dbm)]BF4 (9, Triclinic P1), [Pt(cod)(thtfac)]BF4 (10, Monoclinic P21/n, R =2.80 %), [Pd(cod)(3Br-trop)]PF6 (11, Triclinic P1, R = 5.90 %), [Pd(cod)(3Br-trop)]BF4 (12, Triclinic P1, R = 3.40 %), [Pd(cod)(trop)]PF6 (13, Tetragonal P42bc, R = 4.08 %), [Pt(cod)(trop)]PF6 (14, Tetragonal P42/mbc, R = 3.45 %), [Pt(cod)(trop)]BF4 (14, Tetragonal P42bc, R = 2.91 %), [Pt(cod)(3Br-trop)]PF6 (16, Monoclinic C2/c, R = 4.38 %), [Pt(cod)(3Br-trop)]BF4 (17, Triclinic P1, R = 2.66 %), [Pd(cod)(NH-acac)]BF4 (18, Monoclinic P21/c, R = 2.24 %), [Pt(cod)(NH-acac)]BF4 (19, Monoclinic P21/c, R = 2.09 %), [Pt(cod)(NH-acac)]PF6 (20, Monoclinic C2/m, R = 3.88 %), [Pt(cod)(NMe-acac)]BF4 (21, Orthorhombic P212121, R = 1.50 %), [Pt(cod)(NMeacac)] PF6 (22, Monoclinic C2, R = 2.44 %), [Pt(cod)(NPh-acac)]BF4 (23, Monoclinic P21/n, R = 3.25 %), [Pt(cod)(NPh-acac)]PF6 (24, Triclinic P1, R = 3.29 %), [Rh(cod)(acac)] (25, Monoclinic Cc, R = 2.21 %), [Rh(cod)(dbm)] (26, Monoclinic Cc, R = 2.80 %), [Rh(cod)(thtfac)] (27, Monoclinic P21/n, R = 4.75 %) and [Rh(cod)(trop)] (28, Orthorhombic P212121, R = 1.39 %). The co-ordination geometry of the crystal structures was square planar, for palladium(II) and platinum(II) crystal structures resulting in a cationic charged species which was balanced with either BF4 - or PF6 - counter ions. Extensive hydrogen bonding was observed for the solid state structures with some interesting metal ring chelate interactions. The twist angle, the distortion from the square planar co-ordination geometry, for the crystal structures was found to be of a similar order. A theoretical DFT study was carried out on the group 7-11 transition metals with acetylacetone (Hacac), trifluoroacetylacetone (Htfacac), hexafluoroacetylacetone (Hhfacac), 4-aminopent-3-en-2-one (HNH-acac), 4-(methylamino)pent-3-en-2-one (HNMe-acac), 4-anilinopent-3-en-2-one (HNPh-acac), tropolone (Htrop) and tribromotropolone (H3Br-trop). The comparison of the calculated structures with the Xray crystal structures was found to be in good agreement with R-values of greater than 98 %. The molecular orbitals showed the influences of the π orbital delocalization and the bonding orbitals of the cyclo-octadiene with the various transition metal centres and provided an easy graphical method to compare both the variations of ligands and metal centres. Both the molecular orbital energies and band gap energies were presented and reflected the changes in the cyclo-octadiene complexes. A push-pull effect was observed though the use of the “core” co-ordination geometry, where the metal displays a lateral movement within the co-ordination area (Atot). The bite and jaw angles were determined for all the theoretical structures as well as the solid state crystal structures obtained. A maximum opening of c.a. 15° for the β-diketonato complexes while both the β-enaminonato and tropolonato complexes have a c.a. 14° variation in the jaw angle affected by both the metal and trans ligand manipulations.