Nickel rapid complexation

As with other first-row transition metals, copper complexes are not expected to be satisfactory singlet oxygen photogenerators, because of the rapid deactivation of excited states in the presence of partially filled d-orbitals. The exceptional case of the copper(II) benzochlorin iminium salt ((18), M = Cu) has already been referred to (Section 9.22.5.6) this showed bioactivity, although the nickel(II) complex ((18), M = Nin) was inactive.195... 

The monomers dealt with can be polymerized by various mechanisms, not only by ROMP. For example, a rapid polymerization of norbornadiene occurs using a homogeneous catalytic system consisting of nickel acetylacetonate or a nickel-phosphine complex, such as nickel bis-(tri-n-butylphosphine) dichloride (NiCl2(TBP)2) or nickel bis-(tricyclohexylphosphine) dichloride (NiCl2(TBP)2). Nickel acetylacetonate as catalyst is known to initiate rather a classical vinyl polymerization (7). The classical vinyl polymerization... 

The most common nickel(O) complexes are those containing phosphorus, arsenic and antimony as donor atoms. Besides the Malatesta and Cenini book/1 which specifically deals with metal(O) complexes, nickel(O) complexes have been summarized in books and review articles which report complexes with phosphine, arsine and stibine ligands.31-37 Actually the nickel(O) complexes with these ligands amount to hundreds and the number of new complexes which are synthesized is increasing very rapidly, making nickel(0) phosphine chemistry a very extensive topic. 

The coordination chemistry of macrocyclic ligands has been extensively studied and aspects of isomerism have been considered in numerous systems.241 Methods whereby two diastereomers of complexes of tetra- N-methylcyclam may be isolated have been discussed previously.184 This, however, is a relatively simple system and it is usually necessary to consider isomerism due to the presence of asymmetric atoms in the chelate arms, as well as that due to asymmetric donor atoms that may be rendered stable to inversion by coordination. An example of a system exhibiting this level of complexity is afforded by the nickel(II) complexes of the macrocyclic ligands generated by reduction of the readily prepared macrocycle (46). These ligands contain two asymmetric carbon atoms and four asymmetric nitrogen atoms but, because AT-inversion is rapid, it is conventional to consider that only three separable stereoisomers exist. There is an enantiomeric pair, (47a) and (47b), which constitutes the racemic isomer (R, R ), and an achiral (R, S ) diastereomer (47c), the meso isomer. 

It is possible to perturb a spin equilibrium by photoexciting one of the isomers. Among the possible radiative and nonradiative fates of the excited state is intersystem crossing to the manifold of the other spin state. Internal conversion within this manifold ultimately results in the nonequilibrium population of the ground state. If these processes are rapid compared with the relaxation time of the spin equilibrium, then the dynamics of the ground state spin equilibrium can be observed. This experiment was first performed for spin equilibria with a coordination-spin equilibrium of a nickel(II) complex (85). More recently a similar phenomenon has been observed in the solid state at low temperatures (41). The nonequilibrium distribution can be trapped for long periods at... 

Planar-tetrahedral equilibria of nickel(II) complexes were the first spin-equilibria for which dynamics were measured in solution. It had been known that such complexes were in relatively rapid equilibrium in solution at room temperature, for their proton NMR spectra were exchange averaged, rather than a superposition of the spectra of the diamagnetic and paramagnetic species. At low temperatures, however, for certain dihalodiphosphine complexes, it is possible to slow the exchange and observe separate resonances for the two species. On warming the lines broaden and coalesce and kinetics parameters can be obtained. Two research groups reported such results almost simultaneously in 1970 (99,129). Their results and others reported subsequently are summarized in Table V. 

When a solution of 19 and excess HCN was warmed to 25°C, rapid formation of propionitrile was observed along with a small amount of ethane. Ethane formation is accompanied by irreversible oxidation of 19 (vCN = 2152 cm-1) to nickel dicyanide complexes, which precipitate from solution and give a new broad IR band at 2170 cm . Product propionitrile (in CH2C12) appears at 2252 cm-1 with a shoulder at 2240 cm 1 due to (C2H5CN)NiL3. Most likely, this oxidation arises by attack of HCN on the 16-electron intermediate 19. 

A major breakthrough in the use of Nobin as an asymmetric phase-transfer catalyst came when Belokon and coworkers applied it to the alkylation of glycine-derived nickel(II) complex 11a under the conditions shown in Scheme 8.13 [25], Representative results are given in Table 8.1, which illustrate that benzylic and allylic halides react very rapidly and highly enantioselectively to produce a-amino acids. Intrigu-ingly, in this case (R)-Nobin catalyzes the formation of (R)-amino acids, which is the opposite enantioselectivity to that observed for the alkylation of alanine derivative 16b [21,24],... 

Rate data for the iodination of pyrazole in aqueous solution showed the reaction to be first-order in both iodine and heterocycle and an inverse first-order [H+] dependence was found over the pH range 5.96-6.74 (64JA2857). A kinetic study of the aqueous iodination of pyrazole coordinated to Ni2+ showed the coordinated ligand to react more rapidly, and a [H+] dependence that differed from that of the free ligand (82JA2460). However, the results of this study should be viewed with caution, as the presence of several nickel-pyrazole complexes in solution necessarily leads to uncertainties about the exact nature of the reactive species. 

The effect of the charge may also be seen in some nickel(n) complexes. The neutral complex 4.7, containing two anionic ligands obtained by the deprotonation of the salicylaldehyde derivative, is completely stabilised towards hydrolysis. In contrast, the monocationic complex 4.8, containing two neutral 1,10-phenanthroline ligands, is rapidly hydrolysed to the corresponding salicylaldehyde complex (Fig. 4-25). Presumably, the overall positive charge of the complex promotes the attack of the nucleophile upon the electrophilic carbon centre. 

The three-coordinate intermediate in Equation 19 would then allow for greater mobility of the groups from which reductive elimination could proceed.(19) Rearrangement followed by a very rapid reductive elimination the cis isomer is also a possibility. However, our unsuccessful attempts to synthesize cis-aryl-methyl-nickel(ll) complexes still leave open the question of reductive elimination from such stereoisomers. The results we obtained in studies with the bidentate diphosphine ligand (dppe), though qualitatively in this direction, unfortunately lack definitiveness as yet. 

The Mechanism of the cross coupling reaction can be accommodated by an oxidative addition of 1-bromopropene to iron(l) followed by exchange with ethylmagnesium bromide and reductive elimination. Scheme 3 is intended to form a basis for discussion and further study of the catalytic mechanism. In order to maintain the stereospecificity, the oxidative addition of bromo-propene in step a should occur with retention. Similar stereochemistry has been observed in oxidative additions of platinum(O) and nickel(O) complexes.(32)(33) The metathesis of the iron(lll) intermediate in step b is ixp icted to be rapid in analogy with other alkylations.(34) The formation of a new carbon-carbon bond by the redilcTive elimination of a pair of carbon-centered ligands in step c has been demonstrated to occur... 

This paper reviews the measurement of this parameter, both in cubic and in non-cubic complexes, and discusses its significance. A novel method for the rapid calculation of Dq and B from the spectra of cubic molecules of high-spin d, dP, d , and d ions is introduced, and the spectra of some tetragonally distorted nickel (II) complexes are interpreted in terms of unusually low, crystal field strengths for the axial ligands. 

The dimeric allylic nickel halide complexes are simpler in that no problems of reaction with organo-aluminium compounds are involved. They are of low activity and give polymer of low molecular weight owing to a rapid monomer transfer reaction [61]. The active centre is formed by dissociation of the complex in the presence of monomer and since the rate is proportional to [Ni] and [M] the extent of dissociation is small and the complex with monomer of low stability. 

The quenching efficiencies of the free ligands fall off rapidly as the excitation energy of the donor (Ex) decreases from 70 to 60 kcal/mol. The quenching rate constants of the nickel(II) complexes show a slight decrease In this region but are then fairly constant (kq = 3 X 10 M s ) down to Ex =... 

Nickel tetracarbonyl undergoes a rapid oxidative addition of the Si-Si bond of 1, highly strained fluorinated disilane, at room temperature to give ffve-membered cyclic bis(organosilyl)nickel(II) complex 2, which then reacts with terf-buty-lacetylene to give six-membered disilacyclohexadiene derivatives 3 as a mixture of the regioisomers (Eq. 1) [10]. A similar bis-silylation reaction of alkynes with bis(organosilyl)nickel(II) complex has been reported in the reaction of bis(trichlorosilyl)(bipy)nickel(II) (bipy 2,2 -bipyridyl), which is prepared by dialkyl(bipy)nickel(II) with trichlorosilane [11]. 

Nickel(O) complexes rapidly oligomerize aliene selectivity depends on the detailed ligand environment . Allylnickel complexes are isolable from these reactions and are involved in the catalytic cycle. Treatment of bis(cyclooctadiene)Ni at — 30°C with, sequentially, aliene and triphenylphosphine affords ... 

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