Nickel isocyanide complexes

Virtually all work on nickel isocyanide complexes centers on nickel(O) species. Malatesta and Bonati 90) describe complexes of the formula NiL4 and Ni(CO) L4 jj. The former are formed in a variety of reactions, including reductions of nickel(II) in the presence of isocyanides, and by the replacement of other ligands by isocyanides. The latter are, of course, derivatives of Ni(CO)4. In addition, a few ill-defined nickel(II) complexes are reported, as is the formally nickel(I) species (C5H5NiCNC6Hj)2. 

A more complex reaction is involved in the cooligomerization of acetylenes and tert-butyl isocyanide using nickel acetate as the catalyst (Scheme 20)43 the nature of intermediate complexes leading to the formation of 2-cyano-5-terf-butylaminopyrroles has not been established. Cocyclization of tert-butyl isocyanide with coordinated hexafluoro-2-butyne gives rise to coordinated cyclopentadienone anils for molybdenum systems,44 hence the nature of acetylene substitutents and of the organometallic catalyst play crucial roles in these processes. The pyrrole products from the former reaction can be decomposed by sulfuric acid and the overall sequence provides a simple synthesis of 5-amino-2-cyanopyrroles (Scheme 20). 

Complex 61 is also accessible from the 2-nitrophenyl isocyanide complex 68 by reduction of the nitro group with Sn/HCl. Incomplete reduction of the nitro group in 68 with Raney-Nickel/hydrazine yields, after intramolecular cyclization, the complex 70 with the NH,NOH-stabilized benzimidazolin-2-ylidene ligand. Complex 69 with the 2-hydoxylamin-substituted phenyl isocyanide ligand presumably occurs as an intermediate in this reaction. The alkylation of both the NH,NH- and the NH, NOH-stabilized NHC ligands in 67 and 70, respectively, proceeds readily (Fig. 23) [184, 185]. 

Tetrakis(alkyl isocyanide) complexes of nickel(O), Ni(CNR)4, and the mixed isocyanide complexes with phosphines and unsaturated molecules are strictly analogous to the corresponding carbonyl complexes.23,24 They are generally more stable than [Ni(CN)4]4-. Mixed isocyanide complexes have been prepared by the reaction of Ni(cod)2 and CNBu followed by reaction with the appropriate phosphine or unsaturated molecules (alkenes, arylnitroso compounds, azo compounds, etc.) as outlined in equations (7) and (S).25... 

The t-butyl isocyanide complexes of nickel(O) are used as starting materials for the synthesis of a number of complexes with unsaturated groups n bonded to the nickel atom in reactions similar to those given by dioxygen or diazo complexes (Sections 50.2.7.2 and 50.2.6.3). 

The red-brown polynuclear complex Ni4[(CH3)3CNC]7 can be recrystallized from diethyl ether in a Dry Ice-acetone bath to give a microcrystalline material which displays terminal and bridging isocyanide stretching frequencies at 2020 and 1605 cm"1, respectively. This highly air-sensitive material may be used as an intermediate in the preparation of nickel isocyanide complexes of unsaturated molecules simply by the addition of the desired molecules to a hexane or ether suspension. 

J. S. Miller, and A. L. Balch, Preparation and Reactions of Tetrakis(methyl isocyanide) Complexes of Divalent Nickel, Palladium, and Platinum, Inorg. Chem. 11, 2069-2074 (1972). 

An interesting addition reaction has been found with the isocyanide complexes of the nickel triad and dialkylamines [Eq. (142)]. The metal atom is oxidized, with formation of diaminocarbenes (77). The ligand cis to the metal-bonded oxygen is attacked. This is shown in the X-ray structure of 177b (197). 

Most metals are electron-attracting with respect to isocyanide ligands and the coordinated ligand is sensitive to nucleophilic attack. This reversal in behaviour is called Umpolung , a term introduced by Seebach . Umpolung is an important feature in organometallic chemistry. An example of such a nucleophilic attack is the initiation step in the polymerization of isocyanides by nickel(II) complexes ... 

The intermolecular bonds of N=N jt-electrons to metal were, however, reported as a 7t-bonded azo-transition complex 3.7 (bis(tert-butyl isocyanide)(azobenzene)nickel) by Dickson and Ibers in 1972 as shown in Fig. 3.3 [15]. 

Isocyanide complexes have found numerous applications in organic synthesis and catalysis. Isocyanides undergo polymerization in the presence of many transition metal complexes, for instance, metal carbonyls, metallocenes, cyclopentadienyl carbonyls, nickel(II), palladium(II), and cobalt(II) complexes. Exceptionally high activity is exhibited by nickel and cobalt carbonyls. The resulting polymers are Schiff bases ... 

Protonation of Ni(CNXyl)(triphos) to afford the carbene complex results in oxidation of Ni(0) to Ni(ii). This is evident in the change of coordination geometry from tetrahedral to square planar. The G(5)-N(l) bond distance in the carbene complex (1.268(5) A) is shorter than a typical G-N single bond (1.472(5) A), but longer than the G-N triple bond of isocyanides (1.157(5) A). The three substituents on the carbene carbon are coplanar, consistent with the sp character of the carbon atom. The nickel-carbon bond distance of the carbene complex /(Ni(l)-G(5)), 1.860(4) A, is 0.07 A longer than the Ni-G distance of the Ni(0) isocyanide complex. Overall, the metrical parameters fall into a range that reflects essentially no 7r-carbene character and significant iminium formyl character. 

In contrast to the above results, the nickel isocyanide complexes react with tolane to afford diiminocyclobutenes in variable yield (Suzuki and Takizawa, 1972). These reactions presumably proceed via mixed isocyanide-acetylene complexes since these can be prepared independently and gave the same product on thermolysis. The imino compounds are readily hydrolyzed to cyclobutenediones. 

Yamamoto and Yamazaki also suggest that the higher steric requirements of tert-h xty and 2,6-dimethylphenyl isocyanides determine the lack of an acylimino-nickel complex in their reactions with C5H5Ni(PPhj)CH3 (166). 

Substantially more work has been done on reactions of square-planar nickel, palladium, and platinum alkyl and aryl complexes with isocyanides. A communication by Otsuka et al. (108) described the initial work in this area. These workers carried out oxidative addition reactions with Ni(CNBu )4 and with [Pd(CNBu )2] (. In a reaction of the latter compound with methyl iodide the complex, Pd(CNBu )2(CH3)I, stable as a solid but unstable in solution, was obtained. This complex when dissolved in toluene proceeds through an intermediate believed to be dimeric, which then reacts with an additional ligand L (CNBu or PPh3) to give PdL(CNBu )- C(CH3)=NBu I [Eq. (7)]. 

In the reaction of Ni(CNBu )4 and methyl iodide oligomerization of the isocyanide was observed the only isolable nickel complex was (I), shown below. This product is believed to arise through sequential insertions of three isocyanides into a nickel-carbon bond. Upon further treatment with additional isocyanide at a temperature greater than 60° C one obtains a polymer (RNC) presumably through multiple isocyanide insertion reactions. The addition of benzoyl chloride to Ni(CNBu )4 gave two isolable compounds Ni(CNBu )3(COPh)Cl (74%) and (II) (8.2%). This latter reaction, and the isolation of (II) in particular, suggests that the proposed mechanism for polymerization of isocyanides is reasonable. 

One other reaction deserves mention. From bis(cyclooctadiene)nickel and butadiene (31), and in the presence of an isocyanide (RNC, R = cyclohexyl, phenyl, tcrt-butyl) two organic oligomeric products are obtained, 1 -acylimino-11 -vinyl-3,7-cycloundecadiene and 1 -acylimino-3,7,11 -cyclo-dodecatriene. In each, one isocyanide has been incorporated. An analogous reaction with carbon monoxide had been reported earlier. The proposed mechanism of these reactions, via a bis-7r-allyl complex of nickel, is probably related to the mechanism described for allylpalladium complexes above. 

Some of the most interesting work on nickel(O) complexes has been carried out by Otsuka et al. (107, 110). These workers have succeeded in obtaining a complex, [Ni(CNBu )2],. This complex is prepared from bis(l,5-cyclooctadiene)nickel and the isocyanide, carefully restricting the amount of the latter to 2 moles per mole of nickel [Eq. (28)]. 

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