The new palladium complexes with NHC and phosphine ligands, cis-PdCl 2 (L 2 )(PPh 3 ) (2), cis-PdBr 2 (L 2 )(PPh 3 ) (3), and cis-PdCl 2 (L 3 )(PPh 3 ) (4) were prepared following a general protocol of a one pot reaction between PdCl 2 (COD), PPh 3 , and the ligand precursors LH 3 Y (Y=Cl, Br, BF 4 ) (L 2 = 1,3-dibenzylimidazolin-2-ylidene; L 3 =1,3-dibenzylimidazol-2-ylidene). The cis-PdCl 2 (L 3 )(PCy 3 ) complex (5) was prepared by the ligand substitution reaction between 4 and PCy 3 . The palladium complexes with NHC and pyridine complexes, trans-PdCl 2 (L)(py) (6: L = L 2 ; 7: L = L 3 ) were obtained by heating a mixture of PdCl 2 (COD) and LH 3 BF 4 in pyridine. A similar reaction condition using CH 3 CN as solvent with KO t Bu as base afforded cis-PdCl 2 (L 3 ) 2 (8). Complexes 2-8 were successfully characterized by X-ray crystallographic studies, among which, intriguingly, two polymorphs of 8 were obtained. Thermogravimetric analysis showed that the cis-PdX 2 (NHC)(PR 3 ) complexes are more thermally stable than the trans-PdCl 2 (NHC)(py) complexes. Together with the known cis-PdCl 2 (L 1 )(PCy 3 ) (1) (L 1 =1-benzyl-3-(N-phenylacetamido)imidazol-2-ylidene), they were screened for the direct arylation reaction between aryl halides and alkynes. The result indicate that the carbene/phosphine complexes 1-5 are superior precatalysts than 6-8 with higher activities than the commonly-used system of Pd(OAc) 2 /2PPh 3 .

A computational approach has been designed for accurately determining enthalpies of formation (ΔH(f)) for the carotene species. This approach, named correlation corrected atomization (CCAZ), is based on the concept of bond and group additivity, and is applied along with density functional theory (DFT). Corrections to the deficiencies in DFT were divided into 1,2-, 1,3-, and 1,4- atomic interactions, which were determined by comparisons with the G3 data of the training set. When comparing predictions from CCAZ combined with two different DFT methods (B3LYP and MPWB1K), fairly accurate prediction is expected. In contrast, DFT using the atomization and isodesmic schemes resulted in poor predictions of ΔH(f). The equivalent methods, atomic equivalent (AEQ) and group equivalent (GEQ) provide improved predictions; however, the accuracies are lower than that of CCAZ.

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This study investigates the applications of computational approaches in the prediction of enthalpies of formation (ΔH(f)) for C-, H-, and O-containing compounds. Molecular mechanics (MM4) molecular mechanics method, density functional theory (DFT) combined with the atomic equivalent (AE) and group equivalent (GE) schemes, and DFT-based correlation corrected atomization (CCAZ) were used. We emphasized on the application to xanthophylls, C-, H-, and O-containing carotenoids which consist of ∼ 100 atoms and extended π-delocaization systems. Within the training set, MM4 predictions are more accurate than those obtained using AE and GE; however a systematic underestimation was observed in the extended systems. ΔH(f) for the training set molecules predicted by CCAZ combined with DFT are in very good agreement with the G3 results. The average absolute deviations (AADs) of CCAZ combined with B3LYP and MPWB1K are 0.38 and 0.53 kcal/mol compared with the G3 data, and are 0.74 and 0.69 kcal/mol compared with the available experimental data, respectively. Consistency of the CCAZ approach for the selected xanthophylls is revealed by the AAD of 2.68 kcal/mol between B3LYP-CCAZ and MPWB1K-CCAZ.

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