Nickel complexes benzonitrile

The exact nature of the complex from which the final reductive elimination occurs remains subject to speculation. The involvement of a five-coordinate nickel complex RNiLsCN-A appears likely, based on the observation that reductive elimination of benzonitrile from PhNi(Et3P)2(CN) is promoted by adding triethyl phosphite, and that reductive elimination of propionitrile from EtNi[P(0-o-tolyl)3](C2H4)(CN) is first order in triaryl phosphite concentration. ... 

Aryne complexes of late transition metals are very reactive towards both nucleophiles (amines, alcohols, water) and electrophiles (iodine). They also undergo insertion reactions with CO, alkenes and alkynes,but while the behaviour of ruthenium complexes is somewhat similar to that of titanium or zirconium complexes, the reactivity of nickel complexes is rather different [6,8]. Examples of these reactions that are particularly interesting for the purposes of this chapter are shown in Schemes 8 and 9. Ruthenium complex 33 undergoes insertion of a molecule of benzonitrile,benzaldehyde or di(p-tolyl)acetylene to yield met-allacycles 40,41 and 42, respectively (Scheme 8). Further insertion of a second unsaturated molecule into these metallacycles has not been observed [25,27]. 

The positions of the three principal bands (A, B, and C) are listed in Table IX for several complexes. B and C appeared when Ni[P-(C6H5)3]3 was added to a solution of the nitrile (band B). Therefore it is reasonable to assign band B and C> respectively, to the free nitrile and to the nitrile coordinated on the nickel(0). A shift of v (CN) to lower frequency on complexation of benzonitrile to Ni[P(0-o-C6H4CH3)3]3 has been reported recently (20). A further proof that nickel(0) complex is responsible for band C is its disappearance on addition of an aromatic halide. 

In this context it is interesting to note that benzonitrile, Ph—C=N, trimerizes to a triazine on a Raney nickel surface. It was assumed that Jt-bonded nitriles were involved in the reaction mechanism.10 This reaction resembles the well-known template synthesis of phthalocyanine complexes from phthalodinitrile. Formation of linear polymers [—C(R)—N—] occurs on heating aryl or alkyl cyanides with metal halides.11... 

The first two of these compounds occur at sufficiently high concentrations in some crudes to enable production of them from petroleum. Benzonitrile has also been detected [6]. Much of the trace metal content of petroleums, in particular vanadium and nickel, is present in association with petroporphy-rins, which are polycyclic pyrroles closely related in structure to the hemes and chlorophylls. These materials are examples of the more complex nitrogen heterocycles to be found in petroleum. These particular heterocycles with their complexed metal atoms contribute much to our present knowledge of the original biogenesis of the petroleum hydrocarbons [6]. 

An example of reversible intramolecular electron transfer has now been reported though not for a metal-metal system. It had previously been shown that the complexes [Ni (TPP)] (TPP =tetraphenylporphinate) when electrochemically oxidized in benzonitrile solution yield first the brown-coloured nickel(m) complex, which then decays at a measurable rate by a process of ligand-to-metal electron transfer [reaction (7), forward step (TPP )"=radical-ion] ... 

One final example of C-CN cleavage has appeared that involves rhodium instead of nickel [58]. Reduction of [Rh(dippe)Cl]2 with potassium graphite led to a species assigned as [Rh(dippe)]2K2(THF)2. Here the rhodium is formally in the — 1 oxidation state, so the complex is isoelectronic with [Ni(dippe)]. The complex reacts with benzonitrile to give the C-CN cleavage product, just as seen with nickel. Use of labeled substrate Ph CN gives a product that can be readily characterized as K [Rh (dippe)(Ph)( CN)] using H, C, and P NMR spectroscopy. Further study of this reaction was not possible, as sources of protons readily lead to the known complex [(dippe)Rh]2(p-H)(p-N=CHPh) reported by Fryzuk [59]. 

Nickel catalyzed O-H additions to imsaturated systems are limited to the hydration reactions of nitriles for the synthesis of amides. These reactions have been widely studied by Prof. J. J. Garcia and his research group and their results recently reviewed [60]. They reported the isolation and characterization of Ni(0) complexes of type [(dippe)Ni(ri -NCR], R = aryl, heteroaryl or alkyl, derived from the reductive interaction of [(dippe)NiH]2 with organic cyanides. The catalytic and synthetic utility of these complexes was demonstrated first in the catalytic hydration of benzonitrile and acetonitrile [61] and then extended to dicyanobenzenes [62], to mono- and dicyanoalkanes [63] and finally to cyanopyridines [64]. 

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