Reactions of Nickel Complexes

A comparative study of the displacement of L (pyridine, 4-chloro-pyridine, and 4-aminopyridine) from [PdL(dien)] in the presence of Cl, and at varying pH, has been made. The presence of intermediate species [PdCIL(dien)] were detected (except for L = 4-aminopyridine) which appear to be five-coordinate. The roles of acid and base in these reactions were complicated. 

The square-planar cations [PdHL3] (L = PEt3, PMePh2, or PPh3) are fluxional on the NMR time scale. Retention of coupling at the high temperature limits indicated that intramolecular processes commute the phosphines. The nickel analogs behave similarly. 

A number of studies have contributed toward elucidating the factors which control geometry changes between square-planar and tetrahedral structures. A series of complexes of structure (4) has been investigated by 

The equilibria between square-planar nickel(II) complexes and their octahedral adducts have been examined in CH2CI2 and CHCI3 solutions using the lipophilic ligand JV-cetyl cyclam, (5). With apical ligands Cl and 

NCS complexes of Co and Ni have been followed spectroscopically and calorimetrically in dmf. For the cobalt complexes, geometry change from octahedral to tetrahedral occurs at the third step, Eq. (8). For nickel(II), however, octahedral geometry persists, even for [Ni(NCS)3(dmf)3] and [Ni(NCS)4(dmf)2] -. 

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Platinum, ruthenium, and mixed platinum-ruthenium species supported on silica Various alumina-supported nickel complexes Dispersion of metallic species during treatments in 02 and H2 at temperatures up to 473 K Reaction of nickel complexes and interaction with support at temperatures... 

Some related carbonylation reactions of nickel complexes have been postulated to proceed via Ni—H species (128), and reactions of acetylenes with iron carbonyls in alkaline solution (233, 234) maY proceed via Fe—H intermediates (249). 

On the contrary, reactions of nickel complexes often provide five-coordinate species [76], ft has been clarified that five-coordinate insertion pathway for carbonylation of cafionic monomethylnickel derivatives is energetically favorable under reaction conditions of copolymerizing CO and ethylene (Scheme 7.11) [77]. 

Few examples of this mechanism have been clearly demonstrated because of tfie difficulty in establishing that this path occurs from experimental data. The most well-established examples are reactions of nickel complexes with aryl halides Studied by Tsou and Kochi. The rate of the reaction of Ni(PEt3)jWith aryl halides was shown to be first order in nickel and in ArX and retarded by added PEtj, Ortho-methyl substituents had little effect on the rate. Because of the lack of steric effect, electron transfer was proposed to occur after formation of a TT-complex between Ni(PEt3)j and ArX, rather than by direct insertion of the metal into the carbon-halogen bond by a three-centered mechanism. Moreover, the products of the reaction included the Ni(I) species L3NiX and arene. Tliese products are likely to result from the pathway in Scheme 7.4, involving electron transfer from Ni(0) to the aryl halide and escape of the aryl radical from the solvent cage. Other studies of oxidative additions of aryl halides and sulfonates to Ni(0) complexes have been reported. " ... 

Tetramethylcyclobutadiene dichloride [76404-16-5] can be prepared by reaction of nickel carbonyl and 3,4-dichlorotetramethylcyclobutene (CBD) in polar solvents (103). The complex is black-violet, mp 185°C (dec). 

Stable enolates such as diethyl malonate anions react with allyl sulfones (or acetates) in the presence of nickel complexes to give a mixture of the a- and /-product83. The regioselectivity is generally poor in the nickel-catalyzed reaction, but the molybdenum-catalyzed reaction is selective for alkylation at the more substituted allylic site, thereby creating a quaternary carbon center84. 

The reaction of alkenylcarbene complexes and alkynes in the presence of Ni(0) leads to cycloheptatriene derivatives in a process which can be considered as a [3C+2S+2S] cycloaddition reaction [125]. As shown in Scheme 77, two molecules of the alkyne and one molecule of the carbene complex are involved in the formation of the cycloheptatriene. This reaction is supposed to proceed through the initial formation of a nickel alkenylcarbene complex. A subsequent double regioselective alkyne insertion produces a new nickel carbene complex, which evolves by an intramolecular cycloprop anation reaction to form a nor-caradiene intermediate. These species easily isomerise to the observed cycloheptatriene derivatives (Scheme 77). 

Stone et al. 43, 60, 61) have studied reactions of nickel and palladium isocyanides and a number of fluorocarbons which give a remarkable variety of products. With Ni(CNBu )4 and fluoroolefins [C2F4, (CF3)2C=C(CN)2 and CF2 = CFCF = CF2] themetallocyclic complexes (XXXIV), (XXXV), and (XXXVI) are observed 60,61) the complex Ni(CNBu )2(CjF5), analo-... 

G. P. Chiusoli Catalysis of some insertion reactions by nickel complexes, pp, 169-199 (31). 

With P-donors a variety of different structures are formed. Two complexes (359a,b) with a five-coordinate square pyramidal geometry and one S-donor in the apical position are reported.934,933 In a related complex (360) the nickel center reveals a distorted square planar geometry, because one xanthate ligand switched to a monodentate coordination mode. 6 By reaction of [NiL2] complexes with Ph2P(CH2)2P(Ph)(CH2)2PPh2, compound (362) is formed, which consists of a five-coordinate cation and a hexacoordinate anion. 7... 

Reduction of both nickel porphyrins and thiaporphyrins to Ni1 species has been studied by EPR and 2H NMR spectroscopy.179, 2 58 The Ni1 complex of 5,10,15,20-tetraphenyl-21-thiaporphyrin has been isolated and characterized. Reaction of this complex with sulfur dioxide produced a paramagnetic five-coordinated Ni1 S02 adduct, while reaction with nitrogenous base ligands (amines, pyridines, imidazoles) yielded five- and six-coordinate complexes. In addition, the crystal structure of Ni1 diphenyldi-p-tolyl-21-thiaporphyrin has been determined. The coordination geometry about the nickel center is essentially square planar with extremely short Ni—N and Ni—S bonds (Ni—N = 2.015(2) A, 2.014(12) A, and 1.910(14) A and Ni—S = 2.143(6) A).2359... 

In a series of studies of the spectroscopy and photochemistry of nickel(O) -a-diimine complexes, the structural differences among the complexes NiL2 and Ni(CO)2L (L Q-diimine) have been examined by means of molecular orbital calculations and electronic absorption Raman resonance studies.2471, 472 Summing up earlier work, the noninnocence of a-diimine ligands with a flat — N=C—C=N— skeleton in low-valent Ni chemistry and the course of substitution reactions of Ni° complexes with 1,4-diaza-1,3-dienes or a,a -bipyridine have been reviewed.2473... 

In the study of effects of ultrasound on the aqueous reactions of nickel, we found some interesting results, for example, the colloidal formation of Ni-DMG complex and degassing of NH3 during different experiments. When 25 ml of 0.001 M NiSC>4 solution was complexed with 5 ml of 1% dimethyl glyoxime (DMG) in faintly alkaline ammonia medium and sonicated for 30 minutes and compared with another set of 25 ml of complexed solution which was stirred mechanically, a colloidal solution of Ni-DMG complex was formed in sonicated condition. Particles of Ni-DMG complex did not settle even after keeping 3 1 h because of their smaller size, in sonicated solution, whereas in the unsonicated condition large particles of Ni-DMG complex settled down immediately. 

An important contribution to an understanding of the mechanism of propene dimerization has been obtained by studying the reaction of nickel-hydride model complexes (55). The formation of the propyl- and isopropyl-nickel complexes 48 and 49 has been observed in the reactions of HNi(PR3)Cl complexes (50) with propene at -78°C [Eq. (15)] ... 

Similar studies on the reactions of butadiene and bis(ir-allyl)palladium were carried out by Wilke and co-workers (4). Unlike the reactions with nickel complexes, no cyclization took place, and 1,6,10-dodecatriene... 

This review is an attempt to rationalize the main reaction patterns observed so far in organonickel chemistry. Synthetic work in organic chemistry has found an exceedingly valuable tool in the use of nickel complexes. The reason for this lies in the fact that nickel possesses a very favorable combination of properties to meet the requirement for an organic reaction to take place via coordination. Let us consider, for example, which basic steps occur when organic ligands react on a transition metal to form C—C bonds. 

Hydride transfer from organic substrates to olefins (219) or halides (220), catalyzed by halogeno(triphenylphosphine)nickel complexes, and halide replacement reactions (example 13, Table VIII) by hydrolytic cleavage of nickel complexes have also been described. 

Replacement reactions of aromatic halides or other halides with SCN, NCO, or N02 can be easily carried out by oxidation of nickel complexes with copper salts (examples 8-11, Table XI). 

Beyond palladium, it has recently been shown that isoelectronic metal complexes based on nickel and platinum are active catalysts for diyne reductive cyclization. While the stoichiometric reaction of nickel(O) complexes with non-conjugated diynes represents a robust area of research,8 only one example of nickel-catalyzed diyne reductive cyclization, which involves the hydrosilylative cyclization of 1,7-diynes to afford 1,2-dialkylidenecyclohexanes appears in the literature.7 The reductive cyclization of unsubstituted 1,7-diyne 53a illustrates the ability of this catalyst system to deliver cyclic Z-vinylsilanes in good yield with excellent control of alkene geometry. Cationic platinum catalysts, generated in situ from (phen)Pt(Me)2 and B(C6F5)3, are also excellent catalysts for highly Z-selective reductive cyclization of 1,6-diynes, as demonstrated by the cyclization of 1,6-diyne 54a.72 The related platinum bis(imine) complex [PhN=C(Me)C(Me)N=Ph]2Pt(Me)2 also catalyzes diyne hydrosilylation-cyclization (Scheme 35).72a... 

Bis(oxazoline)-type complexes, which have been found useful for asymmetric aldol reactions, Diels-Alder, and hetero Diels-Alder reactions can also be used for inducing 1,3-dipolar reactions. Chiral nickel complex 180, which can be prepared by reacting equimolar amounts of Ni(C10)4 6H20 and the corresponding (J ,J )-4,6-dibenzofurandiyl-2,2 -bis(4-phenyloxazoline) (DBFOX/Ph) in dichloromethane, can be used for highly endo-selective and enantioselective asymmetric nitrone cycloaddition. The presence of 4 A molecular sieves is essential to attain high selectivities.88 In the absence of molecular sieves, both the diastereoselectivity and enantioselectivity will be lower. Representative results are shown in Scheme 5-55. 

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