ABSTRACT
A series of hemilabile ligands of alpha-aminoaldimines and their methylpalladium complexes have been prepared and characterized. Neutral square-planar methylpalladium complexes in the form of [R(1)R(2)NCMe(2)CH horizontal lineNR]Pd(Me)Cl (R = Me, R(1) = R(2) = Me (3a); R = Me, R(1) = R(2) = Et (3b); R = Et, R(1) = R(2) = Me (4a); R = (n)Pr, R(1) = R(2) = Me (5a); R = (i)Pr, R(1) = R(2) = Me (6a); R = (i)Pr, R(1) = R(2) = Et (6b); R = (i)Pr, (R(1), R(2)) = c-C(4)H(8) (6c); R = (i)Pr, R(1) = (i)Pr, R(2) = H (6d); R = (i)Pr, R(1) = (t)Bu, R(2) = H (6e); R = (t)Bu, R(1) = R(2) = Me (7a); R = (t)Bu, R(1) = R(2) = Et (7b); R = (t)Bu, (R(1), R(2)) = c-C(4)H(8) (7c); R = (t)Bu, R(1) = (i)Pr, R(2) = H (7d); R = (t)Bu, R(1) = (t)Bu, R(2) = H (7e); R = Ph, R(1) = R(2) = Me (8a); R = Ph, R(1) = R(2) = Et (8b)) show geometrical isomerism. The relative ratios of trans/cis isomers appear to be predominated by the steric hindrance between the Pd-bound methyl group and imino or amino substituents (R and R(1) and R(2)). The NMR studies for the substitution reaction of (COD)Pd(Me)Cl with Et(2)NCMe(2)CH horizontal lineN(i)Pr at -20 degrees C indicate that cis-6b is the major kinetic product, which isomerizes to the thermodynamic product in trans form quantitatively above -5 degrees C. Kinetic results show that the ligand substitution reaction likely undergoes an associative pathway, and the isomerization reaction proceeds via an intramolecular process that comprises imine dissociation and recoordination.
ABSTRACT
Ethylene polymerization catalyzed by Ni(ii) complexes that bear new bidentate ligands with a functional hybrid of amine and imine has been studied. A class of new alpha-aminoaldimines and their nickel complexes [R(1)R(2)NCMe(2)CH[double bond, length as m-dash]N(2,6-R(3)(2)C(6)H(3))]NiBr(2) (R(1) = R(2) = Me, R(3) = Me (Ni-); R(3) = (i)Pr (Ni-); R(1) = R(2) = Et, R(3) = H (Ni-); Me (Ni-); (i)Pr (Ni-); R(1) = R(2) = (n)Pr, R(3) = (i)Pr (Ni-); (R(1)R(2)) = c-C(3)H(6) R(3) = (i)Pr (Ni-); (R(1)R(2)) = c-C(4)H(8), R(3) = (i)Pr (Ni-)) were synthesized. The molecular structures of six nickel complexes were determined by X-ray crystallography, showing distorted tetrahedral configurations. The SQUID data of Ni- confirms its ground state of triplet spin. Using methylaluminoxanes (MAO) as the activator, the nickel complexes are found to catalyze ethylene polymerization under moderate pressure and ambient temperature. The activity reaches to 10(6) g PE mol Ni(-1) h(-1), and increases with the ethylene pressure in the range of 14-28 bar. The highly branching PE products have M(n) approximately 10(5) with PDI < 2. The amine and imine functionalities demonstrate independent control to the polymerization reactions, wherein the activity appears to be facilitated by using the catalysts installed with bulky imino substituents as well as with less sterically hindered amino substituents. This is ascribed to the C(2) unsymmetric coordination in the square planar resting state in which the bulky polymer chain prefers cis to the imine and the small ethylene monomer is cis to the amine.
ABSTRACT
New alpha-aminoaldimines with the formula of Et(2)NCMe(2)CH[double bond, length as m-dash]NR (R = (i)Pr, (t)Bu, Ph) and their dichloro or diacetato complexes of Ni, Pd, Pt are prepared and structurally characterized. A nickel complex is in a distorted tetrahedral configuration, and the Pd and Pt complexes () are of square planar form. The alpha-aminoaldimines can chelate to the metal in a C(2)-unsymmetric bidentate motif through the hetero functionalities of amine and imine, which show comparable trans influence. Square planar organometallic palladium derivatives bearing alpha-aminoaldimines, including Pd-methyl, Pd-acetyl, and Pd-(eta(2)-acetylnorboryl) (), are also synthesized. The latter two species are a result of CO-insertion into Pd-methyl complexes and ensuing norbornene-insertion, respectively. The geometrical isomerism is found in the trans configuration in most studied cases. Such a stereoselectivity results from the thermodynamic stability governed predominantly by steric control. The stereoselectivity is also supported by DFT calculations.
