Nickel complexes ethylene

The synthesis of pure bis(phenylsodium)nickel-ethylene (13) is achieved by reaction of a mixture of NaC6H5/LiC6H5 (Na/Li = 2-4 1) with CDTNi(O) (1) in the molar ratio (NaC6H5 + LiC Hs)/Ni = 2 1 in the presence of ethylene (79). The phenyllithium-containing nickel-ethylene complex 9 remains dissolved while the bis(phenylsodium)nickel-ethylene (13) precipitates out as an orange-red powder. 

The N2 in 14 can be displaced by carbon monoxide or ethylene. Ethylene and 14 react to give the calculated amount of N2 and the nickel-ethylene complex (9), as well as free LiC6H5 and Et20 (14, 22). 

In addition to the neutral nickel/phosphine complexes used in the Shell Higher Olefins Process (SHOP), cationic Ni-complexes such as [(mall)Ni(dppmo)][SbF6] (see Figure 5.2-7) have attracted some attention as highly selective and highly active catalysts for ethylene oligomerization to HAOs [106]. 

Nickel atoms have also been allowed to react with C2H4 under cryogenic conditions (101,123). Depending on the metal-concentration conditions and the deposition temperature, either mononuclear species, Ni(C2H4) , n = 1-3(123), or multinuclear species, Ni2(C2H4) ,m = 1-2, and Ni3(CjH4)i, may be isolated. Unlike the copper complexes, these species are all colorless the mononuclear ethylene complexes each dis-... 

Nickel(II) complexes of ligands 38 (R=H,Me R =H,Me,Et,Tr,CH30 R =H, CH3O R =H, F, CH3O) are highly active catalysts for ethylene polymerization [86,159], whereas palladium(II) complexes possess catalytic properties in the copolymerization of CO and alkenes [160] (Scheme 36). 

Table II also lists several isomerizations and skeletal rearrangements (examples 4-7) which are related to butadiene-ethylene dimerization. Protonation of phosphorus-containing nickel(O) complexes is sufficient to achieve skeletal rearrangement of 1,4-dienes in a few seconds at room temperature, probably via cyclopropane intermediates (example 6, Table II). For small ring rearrangements see Bishop (69).

This is the case for secondary and tertiary alkyl bromides. If the stability is high, however, as, for example, with primary alkyl bromides, the organo nickel(III) complex is further reduced to an alkyl nickel(II) complex which loses the alkyl group in form of the alkyl anion. An electroinactive Ni(II) species remains. The number of regenerative cycles is consequently low. The structure of the ligand also influences the lifetime of the alkyl nickel(ni) complex thus, a less stable complex is formed in the case of [A,A -ethylene-bis(salicylidene-irainato)]nickel(II) ([Ni(salen)]) as compared with (5,5,7,12,12,14-hexamethyl-l,4,8,ll-tetraazacyclo-tetradecane)nickel(II) ([Ni(teta)] ), and hence the former complex favors the radical pathway even with primary alkyl halides. 

Tridentate ligands of the linear type having donor sets P3, AS3 or P As3. t, where the donor atoms are connected by o-phenylene, ethylene or trimethylene chains, were found to form stable complexes with nickel(II) salts.1350 Listings of the most common ligands and nickel(II) complexes are given in Tables 67 and 68, respectively. 

The stability of the olefin complexes seems to be determined by the steric and electronic characters of both the phosphorus ligand and the olefin (22). For example, ethylene complexes have only been isolated for the cases with sterically large ligands such as P(0-o-tolyl)3 and PPh3 however, maleic anhydride forms a stable isolable complex with the smaller P(0-p-tolyl)3 ligand. The nickel-ethylene bond strength is estimated to be 39 kcal/mol based on values of 36 kcal/mol for 1-hexene and 42 kcal/mol for acrylonitrile [when L = P(0-o-tolyl)3] (22). 

Somewhat greater success has been reported in respect of homo-oligo-merization of olefins, though once again reports are few, and focused predominantly on nickel. The complexes TpxNiCl (Tpx = TpMs 13, TpMs 14, TpMs 15 Ms = mesityl, Tpw = HB(pzN4s)2(pz5Ms), TpMs = HB(pzMs) (pz5 ) were screened in the presence of methylaluminoxane (MAO) cocatalyst at 0 °C under 30 bar ethylene. Under these conditions, both 13 and 14 showed appreciable activity, with very high selectivity for 1-butene (95-96%) within the C4 fraction, equivalent to 81% selectivity overall.13 In contrast, 15 was found to be completely inactive under these conditions. 

Supramolecular catalysis may also involve the combination of a host cavity and a metal active site as in the bis(diphenylphosphino)calix[4]arene nickel(II) complex 12.40 which acts as an efficient catalyst for ethylene and propylene polymerisation, and in tandem with zirconocene dichloride, for the formation of linear low-density polyethylene. In the latter case the complex gives very little branching - a significant advantage. The key to the effectiveness of the catalyst involves calixarene-induced changes in the bite angle at the Ni(II) centre, which is square planar in the active form of the catalyst.29... 

DFT/MM calculations on ethylene polymerization by nickel diimine complexes have been applied within Car-Parrinello molecular dynamics simulations [40, 41]. A first set of calculations was used to refine the computed energy barrier for the termination step. The enthalpy barrier computed in the calculations described above was 18.6 kcal/mol, a value which decreased to 14.8 kcal/mol at 25 °C in the molecular dynamics calculation, in better agreement with experiment [40]. A second study analyzed the capture of the olefin by the catalyst [41], and found that this process, which has no en-thalpic barrier, has an entropic barrier. 

An X-ray study of [Ni(monothioacac)2J showed it to have an essentially planar cis structure, with the bond lengths suggesting that C- O has more double-bond character than C - S.588 The structure of the nickel(n) complex of cis-l-mercapto-2-(p-bromobenzoyl)ethylene also reveals a cis configuration.590... 

Under the reaction conditions the precursor complex probably generates a nickel-hydride species, which then initiates the oligomerization reaction. Evidence for this comes from the studies on the reactions of 7.17. As shown by 7.6, on reaction with ethylene 7.17 eliminates styrene and produces a nickel-hydride complex. A model catalytic intermediate 7.18 has been characterized by single-crystal X-ray studies. Complex 7.18 reacts with ethylene to give a nickel-ethyl species in a reversible manner. This is shown by reaction 7.7. Reactions 7.6 and 7.7 are strong evidence for the involvement of a nickel-hydride catalytic intermediate. 

Consequently, new dilithium-nickel-olefin complexes with tetra- or pentacoordinated nickel atoms are formed, e.g., the Li2Ni complexes Li2Ni[(CH3)2NCH2CHCHCH2N(CH3)2]3 (18), (LiTMEDA)2Ni(C2H4)3 (19), (LiTMEDA NifCVHw (20), and (LiTHF)2Ni(C4H6)3 (21), by reaction with N,N,N, TV -tetramethy lbutene-2-diamine, ethylene, norbor-nene, or butadiene (14, 31-33). 

Nickel allyl complexes in the presence of chiral bidentate ligands catalyze the enantioselective codimerization of ethylene with norbornene and with styrene 129... 

SYNTHESIS OF NICKEL-ORGANIC COMPLEXES GRAFTED ON THE SURFACE OF SiO AS CATALYSTS FOR ETHYLENE OLIGOMERIZATION REACTION... 

One of the first mechanistic proposals for the hydrocarboxylation of alkenes catalyzed by nickel-carbonyl complexes came from Heck in 1963 and is shown in Scheme 24. An alternate possibility suggested by Heck was that HX could add to the alkene, producing an alkyl halide that would then undergo an oxidative addition to the metal center, analogous to the acetic acid mechanism (Scheme 19). Studies of Rh- and Ir-catalyzed hydrocarboxylation reactions have demonstrated that for these metals, the HX addition mechanism, shown in Scheme 24, dominates with ethylene or other short-chain alkene substrates. Once again, HI is the best promoter for this catalytic reaction as long as there are not any other ligands present that are susceptible to acid attack (e g. phosphines). 

Ethylene (tert-phosphine) complexes of zero-valent nickeP and platinum have been known for years. Analogous palladium complexes can be synthesized along the same lines as those reported for the nickel compounds, using ethoxy-diethylaluminum(III) as the reducing agent in the presence of ethylene. These palladium-ethylene complexes may serve as starting materials for oxidative addition reactions, since the ethylene ligand is loosely bonded. ... 

Athene carbonates. Ethylene carbonate (I) is formed when ethylene oxide and carbon dioxide are heated at 100° in benzene solution containing this nickel(0) complex. Under similar conditions, 2-methyl-1,2-epoxypropane is converted into 1,1-dimethyl-ethylene carbonate and 2,3-epoxybutanc into 1,2-dimethylethylene carbonate. Several other catalysts of the type L2Ni(0) are effective. 

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