INTRODUCTION
Diimine ligated late-transition-metal complexes are capable of catalyzing not only Heck, Suzuki, and Hiyama coupling reactions,1 but also (co)polymerization of olefins.2,3 In this context, the catalytic copolymerization of ethylene and polar monomers is a straightforward and atom-economic method to synthesize functionalized polyolefin materials of industrial importance.3-15 However, there are many challenges in these copolymerization systems, such as low reaction activity and low polar monomer incorporation. It is obvious that further study of such copolymerization mechanisms at molecular and electronic levels is highly required to develop more efficient copolymerization systems. In this instance, the issues of large size of such catalytic systems and computational resources could rule the highly accurateab-initio wavefunction based methods out. Density functional theory (DFT) offered an ideal solution to study the mechanism of such copolymerization systems.16-22 However, many density functionals have no broad application range, and the application of individual functionals to specific systems needs performance test according to either experimental or ab-initio results. It is therefore necessary to choose a suitable and accurate DFT method to deal with the system of interest. Given the importance of developing the ethylene/polar monomer copolymerization system, it is worth of performing a benchmark study of DFT functionals for the mechanistic study of such copolymerizations catalyzed by late transition metal complexes.
In 2006, Martin et al.23 performed a benchmark study on C-C and C-H activations by bare palladium atom, and found thatmeta -GGA functionals have good performances. Recently, Checink et al. conducted thermochemical benchmark set of closed-shell metal organic system including carbene-ligated Pd complex and presented a series of applicable functionals. It is noteworthy that two commonly used functionals, BP86(D3) and B3LYP(D3) had poor performance.24 Schreckenbach et al. carried out a benchmark study on thermodynamics of hydrocarbon isomerization and olefin monomer insertion, but without consideration of metal catalysis. They found that the dispersion augmented BPBE, PBEPBE, and B3LYP functionals showed better performance compared with other functional investigated.25 Maron et al. studied the solvation and dispersion effects on group 3 and 4 metallocene-catalyzed propylene coordination and insertion reactions. It was revealed that dispersion effects generally overestimate the stability of olefin complexes and B3PW91 functional performed well in a comprehensive consideration of computational cost and precision.26 Although significant progress has been achieved in this field, as far as we are aware, the benchmark study of theoretical methods for olefin copolymerization systems catalyzed by late transition metal has not been reported to date. In the present work, a benchmark study on the insertion of three monomers, viz. ethylene, methyl acrylate (MA) and vinyl bromide (VB) into (α-diimine)Pd-Me bond has been conducted, respectively, where there is available experimental data (Table 1).4, 27 By screening 67 functional methods, a series of functionals have been found to perform well in the prediction of insertion activation energies of these monomers, and their intersection could be suitable for dealing with such copolymerization systems. It is also found that double-zeta basis sets are sufficient for solvation single-point calculations with SMD or CPCM models.
Table 1. Kinetic Data for Monomer Insertion into (α-diimine)Pd-Me Bond in Dichloromethane.