Nickel II Complexes

It has been known that square-planar Ni(II) complexes are most stable with the 14-membered macrocyclic ligands. The t3q)ical 14-membered macrocycle is [14]aneN4, which is called cyclam. Therefore, many Ni(II) complexes synthesized are those with 14-membered macro-cyclic ligands that form 6-5-6-5-membered chelate rings with the metal 

Free ligands containing methylenediamine linkages have not been isolated because such ligands decompose when the metal ion is taken out of the complex. 

The functional groups can be attached to the macrocyclic ligands by using methods similar to Eqs. (1) and (2). The Ni(II) complexes of pentaaza macrocyclic ligands, 3b-3g, in which functional groups are appended at the bridgehead nitrogen atom, are prepared by the template reaction of [Ni(2,3,2-tet)] and formaldehyde with carboxamide or sulfonamide in the presence of base. The complexes 3b and 3c are 

Nitroethane also has been utilized in place of primary amine or amide as a locking fragment in the template condensation reaction of amines and formaldehyde for the synthesis of macrocyclic complexes. For example, the square-planar Ni(II) complex of Li was prepared by the reaction of NiCen) , formaldehyde, and nitroethane 17). 

Structure of [Ni (10)](PF6)2. [Reprinted with permission from (18). Copyright 1994, The Royal Society of Chemistry.] 

A similar complication pertains in the reactions of tris(hydroxy-methyl)aminomethane ( tris ) with two sterically protected versions of [PdCl(dien)], namely [PdCl(l,4,7-Me3dien)] and [PdCl(l,l,7,7-Me4dien)], in water between pH values 7 and Reverse chloride anation of [Pd(OH2)-(Me dien)] results in equilibration, and anation by tris itself is slow. The reactions were encountered when the tris was used to buffer solutions for other reactions. 

Easy expansion of coordination number to six is also a common feature of nickel(II) chemistry, and several examples of coordinated perchlorate leading to this condition, at least in the solid state, have been revealed. Perchlorate bridges 

Solutions of [Ni(cyclam)] in water or D2O are known to be paramagnetic, the result of a fast equilibrium between the square-planar species and trans octahedral [Ni(cyclam)(OH2)2], present to about 30% at 25 C. Recently a minor isomer of [Ni(cyclam)] has been observed by NMR spectroscopy in the solutions and assigned as the R,5,R,5 isomer/ That this does not participate in the rapid (NMR time-scale) equilibrium with water illustrates another fine energy balance in such compounds. 

Kinetic studies of nucleophilic substitution reactions of traw5-[NiBr(QBr5)L2] (L is PPh2Et or PPh2Pr) by SCN , NJ, NO2, and I reveal the operation of the usual two-term rate law at these sterically hindered complexes/  

The conversion of ds-[PtCl2(PBu3)(PhCN)] to its trans-isomer proceeds smoothly in a variety of solvents after an induction period, caused by generation of PhCN and [Pt2Cl4(PBu3)2]. The pseudofirst-order process is autocatalytic. It 

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In the presence of 6-iodo-l-phenyl-l-hexyne, the current increases in the cathodic (negative potential going) direction because the hexyne catalyticaHy regenerates the nickel(II) complex. The absence of the nickel(I) complex precludes an anodic wave upon reversal of the sweep direction there is nothing to reduce. If the catalytic process were slow enough it would be possible to recover the anodic wave by increasing the sweep rate to a value so fast that the reduced species (the nickel(I) complex) would be reoxidized before it could react with the hexyne. A quantitative treatment of the data, collected at several sweep rates, could then be used to calculate the rate constant for the catalytic reaction at the electrode surface. Such rate constants may be substantially different from those measured in the bulk of the solution. The chemical and electrochemical reactions involved are... 

The first polyphosphino maeroeyeles designed speeifieally for use as transition metal binders were reported in 1977 in back-to-baek eommunications by Rosen and Kyba and their eoworkers. The maeroeyeles reported in these papers were quite similar in some respeets, but the synthetic approaches were markedly different. DelDonno and Rosen began with bis-phosphinate 18. Treatment of the latter with Vitride reducing agent and phosphinate 19, led to the tris-phosphine,20. Formation of the nickel (II) complex of 20 followed by double alkylation (cyclization) and then removal of Ni by treatment of the complex with cyanide, led to 21 as illustrated in Eq. (6.15). The overall yield for this sequence is about 10%. 

Reaction of l,3-bis(phenylmethyl)imidazol-2-ylidene with nickel tetra carbonyl gives [(t (C)-1,3-bis(phenylmethyl)imidazol-2-ylidene)Ni(CO)3] (970M2472). Complexes of composition [Ni(CO)2L2] with imidazol-2-ylidenes are also known (93JOM(459)177). Another species to be mentioned in this respect is bis(l,3-dimesitylimidazol-2-ylidene)nickel(0) (94JA4391). 1,3-Dicyclohexylimidazol-2-yUdene substitutes triphenylphosphine or THF from [NiX LJ (X = Cl, Br L PPhj, THF) to yield the stable nickel(II) complexes 69 (X = C1, Br R = Cy) (97OM2209). Another preparation of nickel(II) derivatives is the interaction of... 

Although the aqua nickel(II) complex A was confirmed to be the active catalyst in the Diels-Alder reaction, no information was available about the structure of complex catalyst in solution because of the paramagnetic character of the nickel(II) ion. Either isolation or characterization of the substrate complex, formed by the further complexation of 3-acryloyl-2-oxazolidinone on to the l ,J -DBFOX/ Ph-Ni(C104)2 complex catalyst, was unsuccessful. One possible solution to this problem could be the NMR study by use of the J ,J -DBFOX/Ph-zinc(II) complex (G and H, Scheme 7.9) [57]. 

Among the J ,J -DBFOX/Ph-transition(II) metal complex catalysts examined in nitrone cydoadditions, the anhydrous J ,J -DBFOX/Ph complex catalyst prepared from Ni(C104)2 or Fe(C104)2 provided equally excellent results. For example, in the presence of 10 mol% of the anhydrous nickel(II) complex catalyst R,R-DBFOX/Ph-Ni(C104)2, which was prepared in-situ from J ,J -DBFOX/Ph ligand, NiBr2, and 2 equimolar amounts of AgC104 in dichloromethane, the reaction of 3-crotonoyl-2-oxazolidinone with N-benzylidenemethylamine N-oxide at room temperature produced the 3,4-trans-isoxazolidine (63% yield) in near perfect endo selectivity (endo/exo=99 l) and enantioselectivity in favor for the 3S,4J ,5S enantiomer (>99% ee for the endo isomer. Scheme 7.21). The copper(II) perchlorate complex showed no catalytic activity, however, whereas the ytterbium(III) triflate complex led to the formation of racemic cycloadducts. 

In the nitrone cycloaddition reactions catalyzed by the l ,J -DBFOX/Ph transition metal complexes also, the diastereo- and enantioselectivities were found to depend upon the presence of MS 4 A [71]. Thus, both the selectivities were much lowered in the iron(II) or nickel(II) complex-catalyzed reactions without MS 4 A,... 

Enantioselectivities were found to change sharply depending upon the reaction conditions including catalyst structure, reaction temperature, solvent, and additives. Some representative examples of such selectivity dependence are listed in Scheme 7.42. The thiol adduct was formed with 79% ee (81% yield) when the reaction was catalyzed by the J ,J -DBFOX/Ph aqua nickel(II) complex at room temperature in dichloromethane. Reactions using either the anhydrous complex or the aqua complex with MS 4 A gave a racemic adduct, however, indicating that the aqua complex should be more favored than the anhydrous complex in thiol conjugate additions. Slow addition of thiophenol to the dichloromethane solution of 3-crotonoyl-2-oxazolidinone was ineffective for enantioselectivity. Enantioselectivity was dramatically lowered and reversed to -17% ee in the reaction at -78 °C. A similar tendency was observed in the reactions in diethyl ether and THF. For example, a satisfactory enantioselectivity (80% ee) was observed in the reaction in THF at room temperature, while the selectivity almost disappeared (7% ee) at 0°C. 

A similar catalytic dimerization system has been investigated [40] in a continuous flow loop reactor in order to study the stability of the ionic liquid solution. The catalyst used is the organometallic nickel(II) complex (Hcod)Ni(hfacac) (Hcod = cyclooct-4-ene-l-yl and hfacac = l,l,l,5,5,5-hexafluoro-2,4-pentanedionato-0,0 ), and the ionic liquid is an acidic chloroaluminate based on the acidic mixture of 1-butyl-4-methylpyridinium chloride and aluminium chloride. No alkylaluminium is added, but an organic Lewis base is added to buffer the acidity of the medium. The ionic catalyst solution is introduced into the reactor loop at the beginning of the reaction and the loop is filled with the reactants (total volume 160 mL). The feed enters continuously into the loop and the products are continuously separated in a settler. The overall activity is 18,000 (TON). The selectivity to dimers is in the 98 % range and the selectivity to linear octenes is 52 %. 

Diastereoselective preparation of a-alkyl-a-amino acids is also possible using chiral Schiff base nickel(II) complexes of a-amino acids as Michael donors. The synthetic route to glutamic acid derivatives consists of the addition of the nickel(II) complex of the imine derived from (.S )-,V-[2-(phenylcarbonyl)phenyl]-l-benzyl-2-pyrrolidinecarboxamide and glycine to various activated olefins, i.e., 2-propenal, 3-phenyl-2-propenal and a,(f-unsaturated esters93- A... 

Stcric and electronic factors influencing the structure of nickel(II) complexes. E. Uhlig, Coord. Chem. Rev., 1973,10, 227-264 (152). 

Mechanisms of ligand replacement in octahedral nickel(II) complexes— an update. R. G. Wilkins, Comments Inorg. Chem., 1983, 2, 187-201 (54). 

Salmonella typhimurium, 6,582 esters, nickel(II) complexes hydrolysis, 6, 424... 

Here we comment on the shape of certain spin-forbidden bands. Though not strictly part of the intensity story being discussed in this chapter, an understanding of so-called spin-flip transitions depends upon a perusal of correlation diagrams as did our discussion of two-electron jumps. A typical example of a spin-flip transition is shown inFig. 4-7. Unless totally obscured by a spin-allowed band, the spectra of octahedral nickel (ii) complexes display a relatively sharp spike around 13,000 cmThe spike corresponds to a spin-forbidden transition and, on comparing band areas, is not of unusual intensity for such a transition. It is so noticeable because it is so narrow - say 100 cm wide. It is broad compared with the 1-2 cm of free-ion line spectra but very narrow compared with the 2000-3000 cm of spin-allowed crystal-field bands. 

Figure 4-7. Spectrum of a typical, octahedral nickel(ii) complex.

In octahedral symmetry, the F term splits into Aig + T2g + Tig crystal-field terms. Suppose we take the case for an octahedral nickel(ii) complex. The ground term is 2g. The total degeneracy of this term is 3 from the spin-multiplicity. Since an A term is orbitally (spatially) non-degenerate, we can assign a fictitious Leff value for this of 0 because 2Leff+l = 1. We might employ Van Vleck s formula now in the form... 

First, consider an octahedral nickel(ii) complex. The strong-field ground configuration is 2g g- The repulsive interaction between the filled 2g subshell and the six octahedrally disposed bonds is cubically isotropic. That is to say, interactions between the t2g electrons and the bonding electrons are the same with respect to x, y and z directions. The same is true of the interactions between the six ligands and the exactly half-full gg subset. So, while the d electrons in octahedrally coordinated nickel(ii) complexes will repel all bonding electrons, no differentiation between bonds is to be expected. Octahedral d coordination, per se, is stable in this regard. 

The change from high- to low-spin configurations is necessarily discontinuous. A given complex is either on one side of the divide or the other. We conclude this section with a look at how the steric role of the d shell can affect angular geometries within a series of just high-spin, nominally tetrahedral nickel(ii) complexes. 

The Chelate Effect and Polydentate Ligands 147 Table 8-1. Stability constants for some nickel(ii) complexes of ammonia and 1,2-diaminoethane. 

However, consideration in terms of the ionic radius or the LFSE shows that both factors predict that the maximum stabilities will be associated with nickel(ii) complexes, as opposed to the observed maxima at copper(ii). Can we give a satisfactory explanation for this The data presented above involve Ki values and if we consider the case of 1,2-diaminoethane, these refer to the process in Eq. (8.13). 

The readily accessible oxalamidine derivative PhN = C(NHBu )C(NHBu ) = NPh provides another useful entry into the coordination chemistry of oxalamidinato ligands. Scheme 195 summarizes the results of an initial study. Mono- and dinuclear complexes of Ti, Zr, and Ta have been isolated and fully characterized. Silylation of both N-H functions was achieved by subsequent treatment with 2 equivalents of n-butyllithium and MesSiCl. The preparation of a nickel(II) complex failed and gave a hydrobromide salt instead. ... 

The electronic spectra of a range of dithio- and perthiocarboxylato-nickel(II) complexes and their pyridine adducts show the presence of a variety of structures in solution, but complete interpretation of the spectra was prevented by lack of a complete MO treatment of these complexes (378). 

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