No abstract

A series of bisNHC chelate ligands with alkyl bridges of different chain lengths and their palladium complexes have been prepared. The influence of the different bridges on the solid-state structure and reactivity of the complexes has been studied. The catalytic activity of the palladium complexes was successfully tested in the Mizoroki−Heck reaction and the catalytic CH activation of methane. The ethylene-bridged palladium complex showed the highest catalytic activity in the CH activation of methane and the Mizoroki−Heck coupling of bromoarenes, while for unreactive chloroarenes bisNHC complexes with longer bridges give better results.

A series of novel pyrimidine functionalized palladium(II)(NHC) complexes with aryl and alkyl substituents [1-(2-Pyrimidyl)-3-(aryl or alkyl)imidazoline-2-ylidene palladium(II) chlorides] was synthesized by transmetalation via the corresponding silver complexes. All compounds have been fully characterized by 1H and 13C NMR spectroscopy, elemental analysis, and in two cases by X-ray single crystal structures. Different solid state structures were observed for aryl and alkyl substituted ligands: for the sterically less demanding methyl substituent, a complex is formed, where two ligands are coordinated to one metal center [Pd(L)2Cl]+, while in the mesityl case a [Pd(L)Cl2] structure was observed. They show good catalytic activity in the CH activation of methane as well as in the Mizoroki-Heck reaction, where especially the methyl substituted complex shows a remarkably high activity combined with a very high selectivity.

Photoluminescence measurements show that platinum(II) tetracarbene complexes, which could be obtained via different synthetic routes, are interesting lead structures for the development of blue emitters for PhOLEDs with good quantum yields and high photostability.

Addition of BF(3).OEt(2) to ethereal solutions of the Ni(II) beta-diketiminates [Me(2)NN]Ni(R)(2,4-lutidine) (R = Et (1), Pr (2)) allows the isolation of the neutral beta-H agostic monoalkyls [Me(2)NN]Ni(R) (R = Et (3), Pr (4)). X-ray studies of primary alkyls 3 and 4a reveal acute Ni-C(alpha)-C(beta) angles with short Ni-C(beta) distances, indicating structures along the beta-H elimination pathway. Positional disorder of the alkyl group in the X-ray structure of 4 corresponds to partial (22%) occupancy by the secondary alkyl [Me(2)NN]Ni(CHMe(2)) (4b). Variable-temperature NMR spectra of 3 and 4 reveal fluxional behavior that result from beta-H elimination, in-plane rotation of the beta-CH(3) group, and a tetrahedral triplet structure for 3 that were investigated by density functional theory calculations at the Becke3LYP/6-31G level of theory without simplifications on the beta-diketiminate ancillary ligand. Calculations support low temperature NMR studies that identify the linear beta-H agostic propyl isomer 4a as the ground state with the branched beta-H agostic isomer 4b slightly higher in energy. NMR studies and calculations show that the beta-agostic 3 reluctantly coordinates ethene and that 3 is the ground state for this ethylene oligomerization catalyst. The thermodynamic isotope effect K(H)/K(D) = 1.3(2) measured for the loss of 2,4-lutidine from 1 to form beta-agostic 3 was also examined by DFT calculations.