Nickel n Complexes

A quantitative consideration on the origin of the EFG should be based on reliable results from molecular orbital or DPT calculations, as pointed out in detail in Chap. 5. For a qualitative discussion, however, it will suffice to use the easy-to-handle one-electron approximation of the crystal field model. In this framework, it is easy to realize that in nickel(II) complexes of Oh and symmetry and in tetragonally distorted octahedral nickel(II) complexes, no valence electron contribution to the EFG should be expected (cf. Fig. 7.7 and Table 4.2). A temperature-dependent valence electron contribution is to be expected in distorted tetrahedral nickel(n) complexes for tetragonal distortion, e.g., Fzz = (4/7)e(r )3 for com-... 

A nickel-catalyzed conversion of primary alkyl iodides in neat diethylzinc, Scheme 29, was reported by Vettel et al33 Although many nickel(n) complexes are suitable catalysts for this reaction, the best results were obtained with... 

It is proposed that the reaction proceeds through (i) oxidative addition of a silylstannane to Ni(0) generating (silyl)(stannyl)nickel(n) complex 25, (ii) insertion of 1,3-diene into the nickel-tin bond of 25 giving 7r-allylnickel intermediate 26, (iii) inter- or intramolecular allylation of aldehydic carbonyl group forming alkoxy(silyl)nickel intermediate 27, and (iv) reductive elimination releasing the coupling product (Scheme 69). 

Early attempts at an asymmetric hydroalumination utilized a chiral -butylsalicylidenime complexed to a nickel(n) complex 117.128 When racemic 3,7-dimethyl-1-octene 116 was treated with 0.2mol% of the nickel complex 117 and 0.3 equiv. of TIBA at 0°C, followed by hydrolysis, the alkene 118 with 1.2% ee was obtained. The unreacted olefin 119 was recovered and found to have an ee of 1.8% (Scheme 14). 

Figure 10.3 shows the CV of a nickel(n) complex as a function of scan rate (continuous lines), together with data simulated by the DigiSim package (solid and open circles). The agreement between experiment and theory is seen to be very close. 

Nickel(n).—Complexes. A detailed examination of the cr stal field description of tetragonal nickel(ii) complexes revealed the possible existence of and... 

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 metal ion does, however, introduce a new subtlety into these reductions. The reduction of the two imine groups in the nickel(n) complex 4.10 is readily achieved with Na[BH4], The free tetraamine ligand would be expected to exhibit a facile pyramidal inversion at each nitrogen atom, whereas in the nickel(n) complex this inversion is not possible without significant weakening (or breaking) of the Ni-N bonds. In macrocyclic complexes it is very often found that the complex obtained by the reduction of a co-ordinated imine does not possess the same stereochemistry as that obtained by the direct reaction of the free amine with metal ion. 

The analogy between imines and carbonyls was introduced earlier, and just as 1,3-dike-tonate complexes undergo electrophilic substitution reactions at the 2-position, so do their nitrogen analogues. Reactions of this type are commonly observed in macrocyclic ligands, and many examples are known. Electrophilic reactions ranging from nitration and Friedel-Crafts acylation to Michael addition have been described. Reactions of 1,3-diimi-nes and of 3-iminoketones are well known. The reactions are useful for the synthesis of derivatised macrocyclic complexes, as in the preparation of the nickel(n) complex of a nitro-substituted ligand depicted in Fig. 5-12. 

In addition to the charge control over the reaction discussed above, there is also a marked element of conformational control over alkylation reactions. This is seen clearly in the methylation of the nickel(n) complex of the tetraaza macrocyclic ligand, cyclam (Fig. 5-32). Reaction of the nickel complex with methylating agents allows the formation of a A, A V",A "-tetramethylcyclam complex. In this product, each of the four nitrogen atoms is four-co-ordinate and tetrahedral, and specific configurations are associated with each. Of the four methyl groups in the product, two are oriented above the square plane about the nickel, and two below it. 

Transamination reactions of this type have found some synthetic application. The synthesis of the nickel(n) complex of a macrocycle indicated in Fig. 5-58 clearly involves... 

As mentioned above, reactions of this type have been widely used in the synthesis of macrocyclic ligands. Indeed, some of the earliest examples of templated ligand synthesis involve thiolate alkylations. Many of the most important uses of metal thiolate complexes in these syntheses utilise the reduced nucleophilicity of a co-ordinated thiolate ligand. The lower reactivity results in increased selectivity and more controllable reactions. This is exemplified in the formation of an A -donor ligand by the condensation of biacetyl with the nickel(n) complex of 2-aminoethanethiol (Fig. 5-78). The electrophilic carbonyl reacts specifically with the co-ordinated amine, to give a complex of a new diimine ligand. The beauty of this reaction is that the free ligand cannot be prepared in a metal-free reac-... 

Figure 5-78. The nickel(n) complex of 2-aminoethanethiol reacts smoothly with biacetyl at nitrogen to give a diimine ligand.

Many exotic electrophiles have been shown to react with co-ordinated thiolate for example new disulfide bonds may be formed by reaction with S2C12. The nickel(n) complex of a very unusual tetrasulfide macrocyclic ligand may be prepared by this method (Fig. 5-83). Notice that this reaction utilises the nickel complex of the N2S2 ligand prepared by a metal-directed reaction in Fig. 5-78. 

Unfortunately, this macrocycle cannot be prepared as a free ligand by this method. The starting diimine 6.10 could apparently be prepared from 2-aminoethanethiol and biacetyl. However, we saw in Fig. 5-79 that the direct reaction of 2-aminoethanethiol with 1,2-dicarbonyls leads to a range of cyclic and acyclic products, rather than to products such as 6.10. However, we also saw in Fig. 5-78 that the nickel(n) complex (6.12) of the 6.12 could be obtained if the reaction was conducted in the presence of an appropriate salt. 

The reaction of the nickel(n) complex 6.12 with l,2-bis(dibromomethyl)benzene occurs smoothly to give the nickel(n) complex of the macrocycle (6.13) in respectable yield (Fig. 6-8). If co-ordinating anions are present, these occupy the axial sites to give... 

Sometimes this deactivation is so great that co-ordinated amines are non-nucleophilic. This is particularly likely when the ligand is co-ordinated to a non-labile metal centre. However, even in these cases, all is not lost. We may also use the enhanced acidity of ligands co-ordinated to a metal centre to generate reactive nucleophiles which would not otherwise be readily accessible. For example, nickel(n) complexes of deprotonated diamines may be prepared, and react with dialkylating agents to yield macrocyclic complexes (Fig. 6-10). To clarify this, consider the reaction in Fig. 6-10 in a little more detail. The amine 6.14 is reactive and unselective, and does not give the desired macrocycle upon reaction with the ditosylate. Deprotonation of the amine under mild conditions is not pos-... 

Figure 6-10. The nickel(n) complex of the deprotonated ligand (H2L = 6.14) reacts smoothly with the ditosylate to give a nickel(n) macrocyclic complex.

Figure 6-11. The reaction of 6.16 with BF3 gives the nickel(n) complex of a new macrocyclic ligand. The ligand is dianionic, with the charge formally localised upon the tetrahedral boron atoms.

Figure 6-14. The reduction of co-ordinated macrocyclic imines provides a method for the preparation of macrocyclic amines. The reaction above illustrates one of the standard methods for the preparation of cyclam. The metal ion may be removed from the nickel(n) complex by prolonged reaction with cyanide.

Figure 6-18. The condensation of the [Ni(en)3]2+ salts with acetone yields nickel(n) complexes of a new tetraaza macrocyclic ligand.

The role of the metal ion may be purely conformational, acting to place the reactants in the correct spatial arrangement for cyclisation to occur, or it may play a more active role in stabilising the enol, enolate, imine or enamine intermediates. The prototypical example of such a reaction is shown in Fig. 6-18. The nickel(n) complex of a tetradentate macrocyclic ligand is the unexpected product of the reaction of [Ni(en)3]2+ with acetone. There are numerous possible mechanisms for the formation of the tetradentate macro-cyclic ligand and the exact mechanism is not known with any certainty. 

Figure 6-24. The nickel(n) complex of cyclam is extremely stable. To remove the metal it is necessary to react the complex with the strong field ligand, cyanide. In this case, the thermodynamic driving force for demetallation comes from the very high stability of the [Ni(CN)4]2 ion.

Figure 6-28. The template formation of the nickel(n) complex of 6.28. The nickel(n) ion is the correct size for the cavity of the macrocycle. The macrocycle is doubly deprotonated to give a neutral complex.

In a similar manner, nickel(li) has the correct ionic radius for the bonding cavity of the fourteen-membered ring, tetraazamacrocycle 6.28. The reaction of 6.29 with nickel(n) acetate in the presence of base gives the nickel(n) complex of 6.28 (Fig. 6-28). This is an example of a template reaction that involves a nucleophilic displacement as the ring-formation process. 

In many cases it is possible to utilise the hole size effects for the synthesis of specific types of macrocycle. Thus, a tetradentate macrocycle (6.33) is expected to be obtained from a template condensation of 2,6-diacetylpyridine with 1,5,9-triazanonane in the presence of small, first-row transition metal dications. The hole size of 6.33 closely matches the size of these metal ions. This is indeed what happens when Ni2+ (r = 0.8 A) is used as a template for the condensation and the nickel(n) complex of 6.33 is obtained in good yield (Fig. 6-32). However, when Ag+ (r = 1.0 A) is used as a template, the metal ion... 

The nature of the additional nucleophile may be varied. For example, the reaction of the nickel(n) complex 6.56 with formaldehyde and methylamine gives the macrocyclic complex 6.57 (Fig. 6-46). Again, it is not clear whether the first steps of the reaction involve reaction with formaldehyde, followed by attack of amine upon the imine, or initial formation of an electrophile such as H2C=NMe, which attacks 6.56. 

What should we do to observe a three-dimensional template effect First, we should choose a reaction type that we know to be effective for the formation of macrocyclic ligands and extend the methodology to a kinetically inert cP or d6 metal centre. Let us reconsider the reaction, that we first encountered in Fig. 6-11. In this reaction, a dioximato complex reacted with BF3 to give the nickel(n) complex of a dianionic macrocycle (Fig. 7-1). 

Although we approached this section in terms of kinetically inert metal centres, it is also possible to build such encapsulating ligands about relatively labile metal ions. For example, although the square-planar nickel(n) complex 7.2 may be formed, this can react with an excess of dimethylglyoximedihydrazone and formaldehyde to give the nickel(n) analogue of 7.3. 

In some cases a whole series of dehydrogenation reactions may proceed sequentially to yield aromatic or highly conjugated products. An example of this is seen in the aerial oxidation of the nickel(n) complex of the macrocycle formed by the template condensation of biacetyl bishydrazone with formaldehyde. The product of the oxidation is the fully aromatic dianionic macrocyclic complex (Fig. 9-24). 

Methylcyclam and 1,5-dimethylcyclam form planar nickel(n) complexes, but... 

The ligands H2[HPhHen2], H2[HPhHentn], H2[HPhHtn2] (see Scheme 1, p. 230) form diamagnetic planar nickel(n) complexes.101 The 1H n.m.r. spectra of the nickel(n) complexes of two macrocycles derived from (88) have been reported. The ligands co-ordinate in a planar manner via the four N atoms.480... 

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