Ligand exchange in zirconocene dichloride–diethylzinc bimetallic systems

The reaction between zirconocene dichloride and excess diethylzinc in d-6-benzene solution was studied. It was found that the exchange reaction between Cp2ZrCl2 and Et2Zn is accompanied by the formation of such complexes as bis-(cyclopentadienyl)ethylzirconium chloride (EtZrCp2Cl), a zirconium-organozinc complex, and bis-(cyclopentadienyl)diethylzirconium (Et2ZrCp2). It was also found that as a result of ligand exchange in zirconocene dichloride–diethylzinc bimetallic systems, the zirconium-organozinc complex is formed in minor amounts. An assessment of the thermodynamic stability of the obtained products is given based on the results of DFT analysis. The description of the NMR spectral data of the obtained organozirconium complexes is carried out.


Introduction
Chain transfer reactions between compounds of transition metals and alkyls, haloalkanes of Group II -III metals play an important role in the catalysis of homogeneous polymerization of olefins [1,2]. A special place among chain transfer reactions is occupied by the exchange reactions of zirconocene complexes and organoaluminum compounds of various structures. Despite the low activity of these systems in polymerization reactions, they are important for organic and organometallic chemistry. σ-Ligand exchange between metallocene dichloride-and organoaluminum compounds -based systems underlies the generation of hydride and alkyl organometallic complexes initiating the reactions of hydro-, carboand cyclometalation of alkynes and alkenes. Functionally substituted products of hydrometalation and carbometalation obtained in situ are important synthons for the organometallic synthesis of various classes of organic molecules [3]. The nature of the interaction of metallocene complexes of zirconium and organoaluminum compounds has been studied in detail and continues to be studied by L.V. Parfenova with co-workers [4,5]. However, there are no literature data on the study of the exchange reactions of zirconocene complexes with organocin reagents.

Results and discussion
We found that the reaction of Cp 2 ZrCl 2 with 4 equiv. Et 2 Zn in d-6benzene solution at room temperature is accompanied by the alkylation of zirconocene dichloride and the formation after 5 min bis-(cyclopentadienyl)ethylzirconium chloride (EtZrCp 2 Cl) 1 and bimetallic species 2 in a ratio of ~ 1 : 1 (Scheme 1).
The structural identification of the obtained compounds was carried out by 1D and 2D methods of NMR spectroscopy under standard conditions. The ratio of intensities of three singlet signals of protons of cyclopentadiene rings (δ = 5.81, 5.95, and 6.01 ppm) is equal to 1: 4: 3, that confirms the presence of a residue of unreacted zirconocene dichloride (δ = 6.01 ppm) and, respectively, the formation of three products as a result of the reaction ligand exchange. The presence of compound 1 in the resulting reaction mixture is indicated by the nuclear Overhauser effect, that was observed in the NOESY spectrum between the protons of the ethyl group (δ = 0.96 -1.01 ppm and δ = 1.29 -1.33 ppm, 1 H NMR) and protons of cyclopentadiene rings (δ = 5.81 ppm) (Scheme 2, Figures  a) and b)). Analysis of the 1 H NMR spectral data shows that the signal with a chemical shift δ = 5.95 ppm refers to two products with different concentrations. Signals of the protons of the methylene groups in the range δ = 0.24 -0.29 ppm, as well as of the methyl group in the range of δ = 1.22 -1.25 ppm. at the 1 H NMR spectrum, according to the conclusions of [10], may indicate the formation of the second component of the reaction mixturebimetallic complex 2. We believe that the formation of the product 2 is result of the reaction of Cp 2 ZrCl 2 with an excess of Et 2 Zn via bimetallic β-C-H activation. The experimentally observed intense gas evolution after addition of Et 2 Zn to a solution of Cp 2 ZrCl 2 and benzene serves as additional evidence of the formation of the complex under consideration. Spectral data for compound with a similar structure, formed as a result of the reaction of Cp 2 ZrCl 2 with Et 3 Al, are presented in the work of Negishi [10]. The authors of [11], when they examined the interaction of Cp 2 ZrCl 2 with AlMe 3 , note a low rate of formation of the corresponding zirconium-aluminum organic complex relative to the target product Cp 2 ZrMe. The ratio of signal intensities at the 1 H NMR spectrum obtained by us also indicates the formation of bimetallic complex 2 in minor amounts. The structure of the product of complete ethylation of zirconocene dichloride 3 is identified in the 1  The optimization of the geometric parameters of the compounds under study and the solution of the vibrational problem were carried out in the GAUSSIAN 09 program [12] within the framework of the density functional theory (DFT) using the B3LYP functional and the LANL2DZ basis set. The absence of imaginary frequencies of normal vibrations in each case indicates the thermodynamic stability of molecules 1, 2 and 3. The values of the Gibbs free energy presented in Table 1 cause the shift of thermodynamic equilibrium in the reaction system towards the reaction products.

Conclusions
Thus, as a result of our study, we have discovered for the first time ligand exchange between zirconocene dichloride and diethylzinc. It was found that the interaction of Cp 2 ZrCl 2 with an excess of Et 2 Zn in a d-6-benzene solution leads to the formation of organometallic complexes such as bis-(cyclopentadienyl)ethylzirconium chloride (EtZrCp 2 Cl), zirconium-organozinc complex and bis-(cyclopentadienyl)diethylzirconpium (Et 2 ZrCp 2 ). The ligand exchange process is accompanied by the formation of a zirconiumorganozinc complex in minor amounts. For all the compounds received, the thermodynamic stability was assessed based on the analysis of the results of the DFT study.
NMR Examination of the Reaction of Cp 2 ZrCl 2 with 4 Equiv of Et 2 Zn.
An argon-filled dry round-bottomed flask was charged with Cp 2 ZrCl 2 (0.5 mmol, 0,146 g), benzene-d6 (0.7 ml), and ZnEt 2 (2 mmol, 0.2 ml). The mixture was stirred at room temperature for 5 min and transfer via cannula to NMR tube. NMR spectra were recorded at 298 K. NMR examination of the reaction mixture indicated that Cp 2 ZrEtCl 1 was formed in 83% yield:

Conflicts of interest
The authors declare no competing financial interest.