Time-Resolved Energy Dispersive Extended X-Ray Absorption of Fine Structure (EXAFS) Measurements of Oligomerisation Reaction Catalysed by Nickel Homogeneous Catalyst and Alkyl-Aluminium Co-Catalyst

Energy Dispersive EXAFS (EDE) data has been analysed for the nickel (3-diketonate catalysts, Ni(dpm)2, using curved wave theory with ab initio phaseshifts. The derived parameters for nickel intermediate compounds were in good agreement, within experimental error of 0.02-0.05 A for the bond distances. A stopped flow system was used for in situ time-resolved EDE studies on the nickel systems at room temperature. The growth of a pre-edge feature and change at the top of the edge can be clearly seen to show the reaction reached completion in less than 6 s indicates averaged distances of 3 Ni-O/C 2.00 A.


INTRODUCTION
The oligomerisation of alkenes such as       (Statistical errors are given in parentheses)

Figure -4 : Reaction consistent with the dispersive EXAFS results for the Ni(dpm) 2 catalyst reaction with AIEt 2 (OEt) in timescales of 16 s (1000 scans x 16 ms).
sphere. The edge structure of the Ni K-edge spectrum shows a marked growing peak at the shoulder before the edge 11 . Figure 2 showed the normalised spectrum of the reagent solution is compared with 1 sl , 2 nd , 9 th , 20 th , 30 th and after 20 minutes indicates a changing process during the reaction. As observed above, 1.000 scans provide a better signal to noise ratio, although significant time resolution is lost. A good fit to all these spectra and their Fourier transform were obtained with a good approximation to the Ni-0 (1.85 to 1.95 A) and N-C bond distance (1.90 to 2.00 A) 12 . Refining the coordination numbers and bond distances for Ni-0 and Ni-C produces the data as tabulated in Table 1. The decreased in oxygen coordination number in the first shell was noted 30 s after the addition of AIEt 2 (OEt) indicating a change in the square planar geometry of Ni(dpm) 2 . This was followed by the appearance of carbon atom coordination number in the second shell with 2.8 (4) carbon atoms and maintained to 3.1 (8) carbon atoms after 20 minutes as shown in the Figure 3. Time-resolved EDE and curve fitting analysis proposed 13 a reaction as displayed in Figure  4.
Comparison of each spectrum showed that species 1 is observed in the first spectrum.
The resulting background subtracted, k 3 -weighted, Ni K-edge dispersive EXAFS and Fourier transform are presented in Figure 5 (a-d), together with the best experimental fits. It is assumed that the precatalyst started to react with the co-catalyst within 16s (Figure 5a). After 32 s, a reduced number of oxygen atoms in the first coordination sphere correspond to species 2 ( Figure 5b). The coordination number shows one ligand dpm is attached to the nickel centre (average of 1.6 to 2.2 oxygen atoms) within experimental error 14 . The second shell is fitted with a carbon atom from the ethyl group and hexene, and shows some inconsistency that either 2 or 3 carbon atoms are bonded. At this point, it js crucial to confirm whether intermediate forms first or the product is being produced as initial rapid changes occurred from 1 st to the 10 th spectrum 15 .
Analysis of the 5 th spectrum (Figure 5c) revealed the apparent of 2.7 (9) carbon atoms at 1.932 (27) A, meanwhile, on 6 th spectrum, 2.6 (4) carbon atoms is fitted at 1.927 (22) A on the second shell. Analysis of the final (30 th ) spectrum shows more consistent parameters and corresponds to species 2. 3.0 (5) carbon atoms are fitted at 1.970 (25) A in the second shell. The spectrum was also recorded after 20 minutes of reaction. Analysis of the spectrum ( Figure  5d) indicated that organometallic 2 is present in the solution, with 3.1 (8) carbon atoms well defined at 1.972 (31) A. The solution remained homogeneously dark brown. However, detection on the dimp'isation of hexene and oligomeric α-olefin was not investigated in this research.
The experiment with the higher ratio was performed on the Ni(dpm) 2 catalyst using AIEt 2 (OEt) of 1.6 M solution as 1:13.3 ratio of Ni(dpm) 2 :AIEt 2 (OEt). This run using 100 accumulations with 20 ms integration time showed changes after the 6 th spectra but the data quality is not very good.

CONCLUSIONS
The characterization of the organometallic complex at different ratios of alkyl-aluminum co-catalyst, Ni(dpm) 2   Ni-C2.00A. The Ni(dpm)2:AIEt2(OEt):1-hexene of ratio of 1:6:20 were acquired within 16 s for each spectrum, EDE measurements using a new rectangular Si(111) monochromator and Hamamatsu S4874 photodiode array at Station 9.3, Dares bury Laboratory have yielded consecutive spectra data taken at 1-1000 accumulations with 2-30 ms integration time to follow the changes during initiation of the catalytic reaction. Using the stronger reagent of AIEt3, EDE was able to follow the reaction within 31 ms (15 ms dead-time).
The success of these time-resolved EXAFS experiments has shown that this method is reliable to use for further investigations on reactions with a millisecond dead time.