Rhodium and Iridium Mediated C-H and O-H Bond Activation of Two Schiff Base Ligands: Synthesis, Characterization and Catalytic Properties of the Organometallic Complexes

Reaction of [Rh(PPh3)3Cl] with two Schiff base ligands, viz. N-(2′-hydroxyphenyl)furan-2-aldimine (H2L1) and N-(2′-hydroxyphenyl)thiophene-2-aldimine (H2L2), in refluxing toluene affords organorhodium complexes of type [Rh(PPh3)2(L)Cl] (L = L1 and L2). Similar reaction with [Ir(PPh3)3Cl] yields organoiridium complexes of type [Ir(PPh3)2(L) (H)] (L = L1 and L2). Crystal structures of [Rh(PPh3)2(L1)Cl] and [Ir(PPh3)2(L2) (H)] have been determined, where the imine ligands are found to bind to the metal centers as CNO-donors. Structures of [Rh(PPh3)2(L2)Cl] and [Ir(PPh3)2(L1) (H)] have been optimized by density functional theory method. Formation of the organometallic complexes is believed to proceed via C-H and O-H bond activation of the imine ligands. All four complexes show intense absorptions in the visible and ultraviolet regions. Cyclic voltammetry on the complexes shows an oxidation on the positive side of SCE and a reduction on the negative side. The organoiridium complexes are found to efficiently catalyze Suzuki-type C-C cross coupling reactions.

Side-Arm Assisted Anilido-Imine Based Rare-Earth Metal Complexes for Isoprene Stereoselective Polymerization

Anilido-imine ligands o-C6H4(NHAr1)(CH=NAr2), in which Ar1 is 2,6-diisopropylbenzyl group and Ar2 contains fluorine (HL1) or methoxyl (HL2) group on ortho-position of phenyl substituent, were synthesized for constructing rare-earth metals based complexes of 1a–1c (HL1 based Sc, Lu, Y) and 2a–2c (HL2 based Sc, Lu, Y). Based on their NMR spectra and X-ray single-crystal structures, the side-arm group of -F and -OMe is identified to chelate to the corresponding central metal. The twisted angles between two planes formed by chelated heteroatoms (N, N, F for HL1 and N, N, O for HL2) are observed, in which the largest dihedral angle (53.3°) for HL1-Y and the smallest dihedral angle (44.32°) for HL2-Sc are detected. After being activated by AliBu3 and [Ph3C][B(C6F5)4], these catalysts showed great activity for isoprene polymerization. Bearing the same ligand HL1, smaller scandium based complex 1a and middle size of lutetium based 1b provided lower cis-1,4-selectivity (57.3% and 64.2%), larger yttrium complex 1c displayed high cis-1,4-selectivity (84%). Chelating by crowded HL2, small size of scandium complex 2a provided impressive trans-1,4-selectivity (93.0%), middle lutetium based 2b displayed non-selectivity and larger yttrium complex 2c showed clear cis-1,4-selectivity (83.3%). Moreover, 2a/AliBu3 system showed the quasi-living chain transfer capability.

Ag(I) Complexes of Imine Derivatives of Unexpected 2-Thiophenemethylamine Homo-Coupling and Bis-(E)-N-(furan-2-ylmethyl)-1-(quinolin-2-yl)methanimine

Imines are fundamental organic compounds used as synthetic intermediates and as ligands in coordination chemistry. They are also found to be important pharmacophores in various bioactive compounds. In this report, two Schiff bases were prepared using the traditional condensation of 4-pyridinecarboxaldehyde with 2-thiophenemethylamine and 2-quinolinecarboxaldehyde with furfurylamine to form (E)-1-(pyridin-4-yl)-N-(thiophen-2-ylmethyl)methanimine (L1) and (E)-N-(furan-2-ylmethyl)-1-(quinolin-2-yl)methanimine (L2) respectively. L1 and L2 were complexed with silver perchlorate in 2:1 [M:L] stoichiometry to obtain complexes 1 and 2, respectively. The crystal structures of 1 and 2 were unequivocally determined by single-crystal X-ray diffraction analysis. The resulting structures revealed 2 to be a four-coordinate as expected. In contrast, an unexpected chemoselective hydrolytic cleavage of one mole of the (CH=N) imine ligands occurred in complex 2 and, further, the amines (thiophenemethylamine) homo-coupled to form a new imine ligand derivative in situ (L1a) before coordinating to the Ag(I) center along with L1. This observation described an alternative synthetic route to be explored to synthesize a diverse range of imine derivatives, which involves the Ag(I)-promoted homo-coupling of amines. Herein, the crystal structures of Ag(I) complexes of pyridinyl [Ag(L1)(L1a)]ClO4 (1) and quinolinyl [Ag(L2)2]ClO4 (2) Schiff bases are presented.

Dendrimers Functionalized with Palladium Complexes of N-, N,N-, and N,N,N-Ligands

Pyridine, pyridine imine, and bipyridine imine ligands functionalized by a phenol have been synthesized and characterized, in many cases by X-ray diffraction. Several of these N-, N,N-, and N,N,N,-ligands have been grafted onto the surface of phosphorhydrazone dendrimers, from generation 1 to generation 3. The complexation ability of these monomers and dendrimers towards palladium(II) has been assayed. The corresponding complexes have been either isolated or prepared in situ. In both cases, the monomeric and dendritic complexes have been tested as catalysts in Heck couplings and in Sonogashira couplings. In some cases, a positive dendritic effect has been observed, that is, an increase of the catalytic efficiency proportional to the dendrimer generation.

Pre-organisation of ligand donor sets modulates the supramolecular structure of bis(pyridyl-imine) silver(I) chelates

Three bis(pyridyl-imine) ligands 1,2-bis(2-pyridylaminomethyl)benzene (L1), 1,3-bis(2- pyridylaminomethyl)benzene (L2) and 1,4-bis(2-pyridylaminomethyl)benzene (L3) were synthesised and complexed to silver(I) salts forming four novel complexes; 1,2-bis(2-pyridylaminomethyl)benzene-silver(I) hexafluoroantimonate(V) [Ag(L1)](SbF6), bis(1,3-bis(2-pyridylaminomethyl)benzene-di-silver(I)) bis(hexafluoroantimonate(V)) [Ag2-µ-(L2)2](SbF6)2, catena-1,4-bis(2-pyridylaminomethyl)benzene-silver(I)) hexafluoroantimonate(V)...