Duroquinone complexes

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)...

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). 

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. 

According to the MO treatment, all duroquinone complexes should have a singlet ground state and should be diamagnetic. This has been confirmed by experiment. However, the cycloocta-1,5-diene complexes of Ni with 2,5- or 2,6-dimethylquinone are paramagnetic in the solid state with moments of 1.5 and 2.75 B.M., respectively 64). If it is assumed that the total wave function for these complexes already contains ionic contributions, as indicated by (XXXI), it is probable that intramolecular oxidation takes place, causing the observed magnetic moments. 

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

Manganese carbonyl, Mn2(CO)io, when treated with tetracyclone at 140-150°, gives an air-sensitive product which on hydrolysis gives the T-cyclopentadienyl complex (LII) 215). Manganese carbonyl does not react with dimcthylacetylene in sunlight, but duroquinone was formed in a similar reaction of the alkyne with [Mn(CO)6] 1 155). Mn(—1) is iso-electronic with Fe(0), and it is suggested that the unstable quinone com-... 

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). 

Duroquinone reacts with Co(CO)2(C5H5) to give [00(0113)402]-0o(0sH5). The corresponding complexes of the other transition metals in this group also have been prepared (521). These complexes have well-defined electronic spectra and from the infrared spectra it has been concluded that the 7r-bonded duroquinone molecule must be nonplanar. 

Figure 17. (a) Duroquinone radical anion, (b) Asymmetric duroquinone radical anion-imidazole complex. 

CT complexes are formed and electron transfer occurs from excited molecules of anthracene derivatives to methylviologen in aqueous micellar media. Methylene blue quenches pyrene fluorescence by electron transfer in SDS micelles . E.lectron transfer between anthraquinone sulphonate radicals and duroquinone in SDS micellar solution occurs in the aqueous phase there is no evidence of intramicellar transfer. Photoionisation of... 

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... 

A further chemical which could be made is duroquinone. Uses which have been proposed for this include the use of the monoxime as a fungicide, particularly to protect seeds prior to planting(53). Duroquinone also forms a complex with Ni(0)(54) and with Ni(0) and cyclooctadiene(55), either of these complexes being claimed as catalysts for polymerizing unsaturated compounds such as acrylonitrile or phenylacetylene. Duroquinone may also have value in electrochromic devices(56,57) however, the requirements for this use are relatively stringent, namely it must have an oxidation/reduction potential within an acceptable range and it must be able to be cycled tens of thousands of times. The advantage of duroquinone is its lack of available reaction sites. Clearly the tetrachloro derivative would also be a possibility. Duroquinone may have a similar potential in electrical accumulators. 

Most of the listed complexes show strong absorption maxima, probably due to CT-transitions. The transitions in duroquinone-, cyclo-pentadienone-, and thiophene-dioxide-iron tricarbonyl have been assigned on the basis of qualitative MO considerations 408>. MO schemes have been established for Mn(CO)5X 193,195) and arene chromium tricarbonyls 92>. The band at 26670 cm-1 in C8HoCr(CO)3 has been attributed to a Cr- ring CT. We will return to this point in section E6. 

Electronic spectra and band assignments for simple olefin transition metal complexes are relatively scarce absorption spectra (with band assignments) of Zeise s salt (K[Pt(C2H4)Cls] H2O have been reported 335> as well as the spectra of other platinum olefin complexes 132>. Details about the spectra of Ni(duroquinone) 2 and olefin-Ni-duroquinone are available 409>, and the absorption data for the tetraphenylcyclobutadiene complexes of PdCl2 and NiCl2 have been given by Fritz 184>. 

The polarographic reduction of i/4-diene complexes was first noted for [M(i/4-l,4-quinone)Cp] (M = Rh or Ir, quinone = duroquinone or 2,6-di(t-butyl)quinone), which show one two-electron wave (M = Ir), or two one-electron waves (M = Rh) at potentials more negative than those of the free quinone (575). The cyclopentadienone compounds [M(i/4-C5R40)Cp] (M = CoorRh, R = Ph or C6F5) undergo reversible one-electron reduction to radical anions (e.g., M = Co, R = Ph, E° = — 1.46 V in thf). The cobalt complexes, more stable than those of rhodium, are generated by alkali metal reduction at 100 K and show ESR spectra characteristic of metal-based d9 species (374). 

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