Bis duroquinone -nickel

Most quinones, instead of forming rr complexes, react with nickel carbonyl yielding salt like materials. With p-benzoquinone, for instance, a nearly black, insoluble, hydroscopic and paramagnetic (ja f = 3.3 B.M.) material of composition Ni(quinone)2 was obtained which is best formulated as Ni (quinone)2 and considered as a metallic quinhydrone type of compound 54). However, duroquinone behaves exceptionally. In a smooth reaction all four molecules of carbon monoxide are evolved and red, crystalline, diamagnetic bis(duroquinone)-nickel (XX) (54) is obtained. This complex is stable in air and begins to decompose without melting at 205° C. It is nearly insoluble in all solvents except dichloromethane, in which it is monomeric. In the infrared spectrum, the quinone C=0 stretching frequency appears at 1577 cm (in free duroquinone it is 1629 

Since the donor strength of duroquinone is rather small, the capacity of the nickel atom in stabilizing two duroquinone molecules by dative n bonding is probably limited. The situation is similar to that in the nickel complexes of the 1,1-dicyano- and 1,1,2-tricyanoethylenes which also form only 1 1 complexes (Section IV, B, 3). During thermal decomposition of (XX) monomeric, coordinately unsaturated Ni(duroquinone) is formed. This species is evidently very reactive and rapidly associates to (XXI). It may be trapped in the monomeric state if generated in the presence of electron donors such as cyclic dienes and some phosphines. Under these conditions new complexes of composition olefin-Ni(0)-duroquinone and (phosphine)2-Ni(0)-duroquinone are formed (55, 56, 57). 

---

Scheme 7.2 Preparation of bis(duroquinone) nickel(O) (10) and the related 1,5-cyclooctadiene(duroquinone) complex 11 (the methyl groups of the quinone ligands are omitted for clarity)...

Bis-duroquinone-nickel(0) 731 24 is one of the few examples of five-mem-bered ring chelates coordinated by oxygen. Coordination by sulphur leads to... 

It was found that if bis(duroquinone)-nickel is thermally decomposed in the presence of cyclooctatetraene, cyclooctatetraene-duroquinone-nickel, (XXII), is obtained (56). A more convenient synthesis was found shortly thereafter in a one-step reaction of duroquinone with nickel carbonyl in the presence of the polyolefin. Using the same procedure, analogous complexes with various other olefins were made, the properties of which are listed in... 

All complexes of this kind are quite polar and water soluble. The permanent dipole moments are caused by the fact that the olefinic ligands are predominantly donors and only weak acceptors so that the duroquinone molecule interacts with filled 3d orbitals of nickel even more strongly than in the case of bis(duroquinone)-nickel. Consequently, the quinone C=0 groups are more polarized than in the parent compound bis(duroquinone)-nickel (see Table I). The particularly high stability and polarity of the... 

Molecular Orbital Theory has been applied (60) to complexes of the type duroquinone-Fe(CO)3 and has also been used for bis(duroquinone)-nickel (61). The calculation was performed for the two possible symmetries, D21,... 

Fig. 2. Electronic spectra of some duroquinone-Ni(0) complexes, recorded in methanol solution (tfJ). x-x-x Bis(duroquinone)-nickel A-A-A-cyclooctadiene-duroquinone-nickel -0-0-0- norbornadiene-duroquinone-nickel.

Fig. 3. Experimental term-level schemes for duroquinone, bis(duroquinone)-nickel, and cyclooctatetraene-duroquinone-nickel (61).

Quinone complexes may be prepared either from acetylenes or from the quinone itself (see Figure 74). With nickel tetracarbonyl, most quinones react forming salt-like complexes which are best formulated as the oxidation products Ni2+ (quinone)2 [43,44]. These complexes are paramagnetic, insoluble and hydroscopic and do not behave like those others where the quinone is ni-bonded to the nickel. Tetramethylquinone (duro-quinone), which is a less oxidizing quinone, reacts readily with nickel tetracarbonyl forming the diamagnetic, red bis-duroquinone nickel, 8.12, m.p. 205° [48]. 

Bis(duroquinone)-nickel readily loses a duroquinone ligand, either thermally or by displacement with olefins such as cyclo-octa-1,5-diene, cyclo-octatetraene and norbornadiene (see Figure 75) [44,48]. These olefin complexes are remarkably stable, decomposing between 120 and 200°C. Analogous complexes of the other less oxidizing quinones, such as 2,5- or 2,6-dimethylquinones, are formed by treatment of nickel... 

Fig. 75. Some reactions of bis-duroquinone nickel the methyl groups are omitted for...

All the duroquinone-nickel complexes are diamagnetic. Attempts to prepare similar compounds with o-quinones have failed. It is also impossible to prepare bis(trimethylquinone)-nickel. Instead, the Ni(II) salt of tri-methylquinone is isolated (55). Evidently a slight increase in the electron affinity of the quinone -n system is sufficient to induce oxidation of nickel. 

The reaction of nickel carbonyl with duroquinone in the presence of an excess of triphenylphosphine produces the insoluble Ni(duroquinone). The same reaction in the presence of tri- -butylphosphine, however, yields red, crystalline air-sensitive bis(trihutylphosphine)-duroquinone-nickel (XXVIII), Eq. (20), (57). 

Since cyclooctadiene has no suitable low-lying unoccupied orbitals some of the 3d electrons of nickel are expected to have a relatively high antibonding character. It is therefore not surprising that the nickel complex is extremely reactive, air-sensitive, and very unstable in solutions even in the absence of oxygen. Carbon monoxide at room temperature completely displaces the cyclooctadiene molecules and yields nickel carbonyl (99). Acrylonitrile reacts with (LII) under similarly mild conditions, forming bis(acrylo-nitrile)-nickel 101), while duroquinone, well below room temperature, affords cyclooctadiene-duroquinone-nickel 101). These reactions uniquely demonstrate the close interrelationship between all complexes of zero-valent nickel. 

---