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.