Duroquinone, nickel complexes

The observed spectra of some duroquinone-nickel complexes with olefins have been correlated by means of semiquantitative molecular-orbital theory by Schrauzer and Thy ret (48). In the case of n complexes of polynuclear hydrocarbons, such as naphthalene and anthracene, although their spectra are recorded, no conclusions have been drawn with regard to structure nor has any theoretical work been reported. Similar remarks apply to complexes of nonalternant hydrocarbons such as azulene. Although innumerable complexes of olefins with various transition metals are known and admirably reviewed (84), no theoretical discussion of even a qualitative nature has been provided of their electronic spectra. A recent qualitative account of the electronic spectra of a series of cyclopentadienone, quinone, and thiophene dioxide complexes has been given by Schrauzer and Kratel (85). 

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. 

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)...

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). 

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... 

Table I. Only olefins containing at least two roughly parallel double bonds are suitable as ligands and it follows from the NMR spectra of the complexes that they must have highly symmetrical structures, e.g., (XXIII, XXIV, and XXV) (55, 56). An X-ray structural determination of cycloocta-1,5-diene-duroquinone-nickel has been performed by Click and Dahl (56a). The space group is i (2/n) with four molecules per unit cell, a = 14.26,...

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.

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. 

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... 

Nickel carbonyl reacts with tetracyclone to give the complex [Ni-(tetracyclone) 2] (215) and with duroquinone to give the complex [Ni-(duroquinone)2] (189). 

Schrauzer and Thyret have described (528, 529, 531) the synthesis of olefin-Ni(O) complexes containing a quinone, in particular, duro-quinone, as a ligand. The red, diamagnetic duroquinone complexes are obtained by reaction of nickel carbonyl with the quinone in excess olefin. They are stable in air and soluble in polar organic solvents and water. Those olefins which form the coiiqilex contain essentially parallel double bonds, e.g., norbornadiene, dicyclopentadiene, 1,5-cycloocta-diene, 1,3,5-cyclooctatriene, or cyclooctatetraene. 

Crystallographic analysis (245) of the (COD)Ni(duroquinone) complex has shown discrete monomeric molecules with the nickel atom located between the boat form of the C OD ring and the duroquinone ring (Fig. 5). The respective orientation of the ligands is indicative of a... 

With other quinones, the only olefin yielding stable complexes is 1,5-cyclooctadiene. The quinones employed have been trimethyl-p-benzoquinone, 2,5- and 2,6-dimethyl-p-benzoquinone (531), and vitamin E quinone (530). In general, these complexes show higher water solubility, higher dipole moments, and more marked paramagnetism than do the duroquinone complexes. The paramagnetism suggests that there is some electron transfer from nickel to quinone and that the nickel may indeed have an oxidation state midway between Ni(0) and Ni(II). 

In a first discussion of the possible mechanism (Schrauzer, e< al., 32, 33) it was assumed that the products are formed within tt complexes in a concerted fashion ( it complex multicenter reaction ). Norbom-adiene forms many complexes with transition metals in which it is symmetrically coordinated. Some pertinent examples are (XII) (1, 2), (XIII) (35), and (XIV) (36). Ni(CO)4 reacts with acrylonitrile to produce bisacrylonitrile nickel, Ni(CH2=CHCN)2 which adds phosphines forming 1 1 and 1 2 adducts (37, 38). Acrylonitrile is a relatively strong TT-bonding ligand and may back-accept charge from the metal via d -p bonding. In this regard it is related to duroquinone (tetra-methylquinone), which is known to form Ni(0) it complexes of the type Ni(Dqu)2 or (XIV). The most likely intermediate in the reaction... 

Difluoro-2,3,5,6-tetramethyl-1,4-diboracyclohexa-2,5-diene displaces CO from a number of metal carbonyls to form 7r-complexes. The crystal structure of [(C4Me4B2F2)2Ni] (37) shows that all six ring atoms are attached to the nickel, and that the geometry is very like that of the isoelectronic duroquinone complex. 

The fact that quinones may form tt complexes with transition metals was first recognized by Sternberg et al. 53), who found that butyne reacts with iron pentacarbonyl in sunlight to afford duroquinone-iron tricarbonyl (XIX). These authors also reported that manganese pentacarbonyl hydride yields durohydroquinone under similar conditions whereas nickel carbonyl did not react 53a). However, more recent work has established that duro-quinone and some other substituted quinones are capable of forming Ni(0) complexes, most of which are surprisingly stable. 

C. Complexes of Nickel with Duroquinone and Cyclic Olefins... 

When p-benzoquinone or toluquinone are treated with nickel carbonyl in the presence of cycloocta-l,5-diene only Ni(II)-hydroquinone or quin-hydrone salts, respectively, are formed. When the somewhat less strongly oxidizing 2,5- or 2,6-dimethyl quinones are treated with nickel carbonyl under the same conditions cycloocta-l,5-diene-dimethylquinone-Ni(0) complexes are obtained. These substances resemble their duroquinone... 

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