Ligand Ni complexes

Fig. 7.21 [Gl] Carbosilan dendrimer with pincer ligands (Ni"-complex).

The chemistry of zerovalent nickel complexes will be discussed in Section 4. One of the most exciting developments in this area is the recent introduction of N-heterocyclic carbene ligands. Ni complexes of these hgands have been shown to be important catalysts and reagents for a number of important organic transformations. This section wiU describe briefly the practical methods for preparing Ni-carbene complexes and will outline some of the catalyhc reactions promoted by these complexes. This section wih also describe the preparation and reactivihes of other types of Ni(0) complexes. 

Polymerization of alkynes by Ni" complexes produces a variety of products which depend on conditions and especially on the particular nickel complex used. If, for instance, O-donor ligands such as acetylacetone or salicaldehyde are employed in a solvent such as tetrahydrofuran or dioxan, 4 coordination sites are available and cyclotetramerization occurs to give mainly cyclo-octatetraene (cot). If a less-labile ligand such as PPhj is incorporated, the coordination sites required for tetramerization are not available and cyclic trimerization to benzene predominates (Fig. A). These syntheses are amenable to extensive variation and adaptation. Substituted ring systems can be obtained from the appropriately substituted alkynes while linear polymers can also be produced. 

Metal-ligand vibrations have been identified using metal isotopes, e.g. the Ni—P stretching band which appears at 273.4 cm for NiCl2(PEt3)2 is shifted to 267.5 cm" for the Ni complex. 

The hydrosilylation of alkynes has also been studied using as catalysts Pt, Rh, Ir and Ni complexes. The improvement of the regioselectivity of the catalyst and the understanding of stereoelectronic factors that control it have been major incentives for the ongoing research. From numerous studies involving non-NHC catalysts, it has been established that there is a complex dependence of the product ratio on the type of metal, the aUcyne, the metal coordination sphere, the charge (cationic versus neutral) of the catalytic complex and the reaction conditions. In the Speier s and Karstedt s systems, mixtures of the thermodynamically more stable a- and -E-isomers are observed. Bulky phosphine ligands have been used on many occasions in order to obtain selectively P-f -isomers. 

Enynes 79 can also undergo cycloisomerisation reactions in presence of NHC/ transition metal complexes (Scheme 5.32). The cycloadduct 124 can be prepared either in presence of complex 110 [36] or in presence of a NHC-Ni complex (prepared in situ from a mixture of [ (COD) ] and IDTB 123 [37]). In the latter case, the active catalytic species is believed to be a Ni-H species that is generated via C-H activation of the carbene ligand. 

Cyanide binds to a series of Ni complexes with N302 chelate ligands, and EPR spectra of the adducts indicate quasi-axial symmetry with a g pattern typical of low-spin, six-coordinate complexes with axial elongation and with a 2Ay ground state. No direct spin interaction of the unpaired electron with the carbon atom of the cyanide takes place.70... 

With respect to the ring size, it has been stated that neither the redox potentials nor the half-lives of the Ni species are directly correlated to the cavity of the macrocyclic ligand, but the redox potentials are dependent on solvation effects.139 The effect of fused benzene rings and ring conformation has been monitored.140 In Ni complexes of fluorine-containing cyclams (25) the higher oxidation state becomes successively destabilized with respect to Ni, while the lower oxidation state (i.e., Ni1) becomes successively stabilized.141... 

The Ni complex of corrole (octaalkyl derivative (71)) is a S= 1/2 low-spin Ni11 species with a ligand-centered 7r-cation radical, as evidenced by the isoptropic EPR signal at g 2.01.291,292... 

Ni11 complexes of various amine/thiophenolate ligands have been investigated (for Ni complexes of ortho-aminothiophenol see Section 6.3.4.9.2(iv)). Thiophenolate is expected to exhibit decreased bridging tendency compared to aliphatic thiolates, but complexes (466) and (467) still consist of two pseudo-octahedral NiL fragments bridged via thiolate donors. In contrast to the parent complex (466), the thiolate and thioether donors in (467) coordinate cis to the Ni center... 

Upon recrystallization, [Ni(tpzlmtacn)]2+ affords [Ni(L)(MeCN)]2+ (L = l,4-bis(pyrazol-l-ylmethyl)-l,4,7-triazacyclononane) via a N-dealkylation reaction and loss of a pendent arm.1420 More rational routes to Ni complexes of tacn ligands with only one or two pendent arms have been developed.1431,1432 In [Ni(L)(X) ]x (e.g., L= l-(3-aminopropyl)-l,4,7-triazacyclononane (n = 2) or l-(l-methylimidazol-2-ylmethyl)-l,4,7-triazacyclononane (n = 2) or l,4-bis(l-methyl-imidazol-2-ylmethyl)-l,4,7-triazacyclononane (n = 1), the coordination sphere is completed by additional ligands that bind either terminal (X = C1 , H20) or bridge two metal ions (X = N3 , OH, oxalate). The Ni11 complex of l,4-bis(2-pyridylmethyl)-l,4,7-triazacyclononane has been shown to be extremely inert to ligand dissociation in aqueous solution.1433 In (562), the tacn ligand provides a single bidentate arm.1434... 

Aliphatic amines, which are hard bases, are unfavorable for coordination to soft Ni°. If the principles of ligand preorganization and donor prepositioning are applied, however, (diamine)Ni° complexes are accessible, which has been demonstrated for complex (1020) with a rigid N,N -dimethyl-3,7-diazabicyclo[3.3.1 ]nonane ligand.2468... 

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... 

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