Designer ligands. Part 9. 1 Catalytic activity of biomimetic cobalt(II) and copper(II) complexes of multidentate ligands

A series of Co(II) and Cu(II) complexes have been prepared, and their structures investigated using a combination of microanalytical, spectroscopic, electrochemical and X-ray crystallographic techniques. The capacity of the complexes to oxidise phenol and catechol substrates has been examined, and while some of the Cu(II) complexes catalyse oxidative coupling of 2,4-di- t-butylphenol (DTBP) to the corresponding biphenyl (9–13%), the Co(II) complexes catalyse efficient conversion (up to 92%) of 3,5-di-t -butylcatechol (DTBC) to the o - quinone within 24 h.


Introduction
][4][5][6] Some time ago, Reglier et al. 7 described the synthesis and phenolase activity of a dinuclear Cu(I) complex, designed to model the enzyme tyrosinase, while very recently, Comba and co-workers 8 reported the X-ray crystal structures of three Cu(II) complexes which exhibit catecholase activity.Progress towards the elucidation of tyrosinase structures has been reviewed by van Gelder et al., 9 and it is apparent that the active sites contain a pair of suitably spaced copper atoms capable of accommodating a bridged dioxygen molecule.In our own investigations in this area, we have explored the preparation of ligands which are capable of chelating two copper atoms and in which the co-ordinating moieties are separated by biphenyl, 10 1,10-phenanthroline 11 and acyclic spacers. 1 Simple Co(II) complexes have also been shown to form metal complexoxygen adducts, 2 and Co(II) complexes have been used to model biological processes. 12In this communication, we report the preparation and catalytic activity of Co(II) and Cu(II) complexes of selected multidentate ligands.

Scheme 3
The mid-IR frequency shifts (relative to the free ligands) of the amide NH and carbonyl (amide I) bands exhibited by the diamide complexes were used to establish whether coordination occurs through the amide nitrogen or oxygen atoms, 13 and whether coordination involving nitrogen donors is accompanied by deprotonation.Vibrational spectra were run in the far-IR region to determine the disposition (tetrahedral or octahedral) of the chloride ligands in the Co(II) complexes.Although the two Co-Cl bands characteristic of tetrahedral geometry about the metal centre (at ca.301 and 324 cm -1 ) 14 were not present in the IR spectra of the Co(II) complexes, a strong, broad band was observed, in each case, at ca. 300 cm -1 .The presence of a single band is attributed to accidental degeneracy of the symmetric and antisymmetric Co-Cl stretches, and is considered to indicate a distorted tetrahedral cobalt geometry.This assumption is supported by the fact the amine complex 13 [shown by single crystal X-ray analysis to contain a tetrahedral cobalt ion (Figure 1)] exhibits a single far-IR band at 298 cm -1 .The coordination geometry of cobalt complexes may also be inferred from characteristic absorption bands in the visible region, and absorption data for the complexes are summarised in Table 2.In all cases, the observed absorptions correspond to the 4 A 2 → 4 T 1 (P) transition, which is characteristic of tetrahedral cobalt coordination; the fine structure evident in the spectra is attributed to spin-orbit coupling effects and to transitions involving doublet states.

Cyclic voltammetry
Cyclic voltammetry was used to confirm the oxidation level(s) of the metal centres in the dinuclear cobalt complexes 2, 3, 7a-c and 10.The observation (Table 3) of a single, strong peak at ca.1 V (which is not exhibited by the corresponding ligand) indicates oxidation of Co(II) to Co(III) on the cyclic voltammetric time scale and suggests that, in each complex, the metal centres are symmetrically located.Comparison of the oxidation potentials of the biphenyl diamide complexes (2 and 3) with those of the 1,10-phenanthroline diamide complexes (7a-c) reveals that the latter have slightly higher oxidation potentials -an observation attributed to the greater electron-withdrawing capacity of the 1,10-phenanthroline nucleus.The fact that the 1,10phenanthroline diamine complex 10 exhibits a lower oxidation potential than the diamide complexes 7a-c is attributed to electron-withdrawal by the carbonyl oxygen of the amide group, which renders the diamide complexes more resistant to oxidation.The complexity of the data has precluded unambiguous assignment of peaks in the cyclic voltammograms of the copper complexes.

Catalytic activity
The ability of the complexes to oxidise phenol and/or catechol substrates using molecular oxygen was examined, using, as substrates, 2,4-di-t-butylphenol (DTBP) and 3,5-di-t-butylcatechol (DTBC).In addition to its oxidation to DTBC [which may be oxidised, in turn, to 3,5di-t-butyl-o-quinone (DTBQ)], DTBP can also be oxidised to a coupled biphenyl product. 16The catalytic activity of the various ligands was examined in DMF (chosen for its ability to dissolve the complexes), while 1 H NMR spectroscopy was used to detect the products of the oxidation reactions catalysed by the complexes.Given the excellent conversions observed for the Co(II) complexes (Table 1), the catalytic recyclability of these systems was also investigated.Of all the complexes examined, only the Cu(II) complexes 4, 8a and 8b catalysed the oxidative coupling of DTBC to afford 3,3',5,5'-tetra-t-butyl-2,2'-dihydroxybiphenyl in low yield (9-13% conversion).A mechanism for such oxidative coupling of phenols has been proposed by Kushioka, 16 while the formation of other biphenyl products from the reactions of phenols with copper complexes have been reported. 17In contrast, all of the Co(II) complexes tested and all, but one, of the Cu(II) complexes catalysed conversion of DTBC to the corresponding o-quinone (Table 1).The Co(II) complexes proved to be significantly more efficient, generally effecting efficient conversion to the o-quinone within 24 h and with excellent recyclability.
Although all of the Cu(II) complexes, with the exception of complex 11, appear to be polymeric, the catalytic activity of complexes 4, 8a and 8b suggests that, in these cases at least, the copper ions are sufficiently close to permit dioxygen bridging and subsequent substrate binding; a Cu-Cu separation of ca 3.6 Å in the dioxygen bridged complexes is considered necessary for biomimetic activity. 6Alternatively, polymer-monomer equilibria in solution could account for the presence of monomeric systems.In computer models of the Co(II) complexes 3 and 7b, the octahedral Co ions are separated by 3.1 Å and 3.4 Å, respectively.Interestingly, Yamami et al. 12 have reported Co-Co separations of 3.4-3.5Å in the crystal structures of Co(II) complexes which catalyse the disproportionation of hydrogen peroxide.

Experimental Section
General Procedures.Infrared spectra were recorded on a Perkin Elmer 2000 spectrophotometer using potassium bromide discs (4000-400 cm -1 ), hexachlorobutadiene mulls (HCBD; 4000-2000 and 1500-1250 cm -1 ), polyethylene discs (500-30 cm -1 ) or nujol mulls (500-30 cm -1 ).NMR spectra were obtained from CDCΡ 3 solutions on a Bruker AMX400 NMR spectrometer and are referenced using the solvent signals (δ H 7.25 and δ C 77.0 ppm).Electrochemical data were obtained using a Bio-Analytical Systems (BAS) CV-50 voltammograph.The measurements were carried out under nitrogen using freshly distilled, dried, degassed DMF.A glass carbon electrode (diameter = 3.0 mm) was used as the working electrode and a platinum wire as the counter electrode.A non-aqueous reference electrode was employed.The electrode solution for the nonaqueous reference electrode was prepared by dissolving 0.01 M AgNO 3 in 0.1 M TEAP (tetraethylammonium perchlorate) in DMF, resulting in an Ag * Ag + (TEAP/DMF) electrode; TEAP was employed as an electrolyte.Exploratory modelling of selected ligands and complexes was undertaken using the MSI Cerius2 platform on a Silicon Graphics O 2 computer. 13The synthesis and characterisation of the ligands have been reported previously. 10,11balt complex 2. To a stirred solution of CoCl   1), to give substrate:complex molar ratios of 100:1.The resulting mixtures were aerated by stirring vigorously for 1-5 d.At the conclusion of each reaction period, the mixture was concentrated to dryness in vacuo and the residue analysed by 1 H NMR spectroscopy to determine the substrate:product ratio.Recyclability was established by adding fresh substrate, DMF and Et 3 N to the residue from the initial reaction, and stirring for 24 h.The solvent was evaporated in vacuo and the residual material analysed as before.

Figure 1 .
Figure 1.X-Ray crystal structure of complex 13, showing the crystallographic numbering.

Table 1 .
Catalytic data for the Co(II) and Cu(II) complexes a Using DTBC as substrate in the presence of Et 3 N to afford the o-quinone.b After 3h.c Not determined.d In the absence of Et 3 N.

Table 2 .
Electronic absorption bands for the cobalt complexes