Recent studies in phthalocyanine chemistry

Abstract Our recent synthetic studies In polynuclear phthalocyanine chemistry are reviewed. The electronic coupling which occurs between the various phthalocyanine units are probed by electronic absorption and emission spectroscopy. Many of the complexes are effective as oxygen reduction electrocatalysts and these studies, carried out with the catalysts as monolayers on ordinary pyrolytic graphite surfaces, are discussed. Under certain circumstances, the cobalt(II) phthalocyanine species will disproportionate into Coil) and Co(III) species thereby providing the potential for multi-electron catalysis. The general utility of polynuclear phthalocyanines as multi-electron catalysts is explored. A. INTRODUCTION Since their accidental synthesis in Scotland, by Imperial Chemical Industries, in 1928, phthalocyanines (Pc) have enjoyed considerable industrial importance for use in dyestuffs, paints, colours for metal surfaces, fabrics and plastics. In recent years there has been considerable interest in developing their use in electrochromic devices, organic catalysis, electrocatalysis, photocatalysis, photovoltaic devices, lithium batteries, fuel cells, pollution control (especially desulfurisation) etc. Many of the uses cited in the preceding sentence involve a redox process in which two or more electrons are exchanged per reaction sequence, e.g. water or hydrogen sulfide oxidation and oxygen or thionyl chloride reduction. Of especial interest to us has been the design of multi-electron redox catalysts capable of a concerted catalysis of multi-electron processes. Such multi-electron reagents might be facilitated by one or more of several strategies. These include the use of mononuclear species capable of a simultaneous 2 (or more) electron redox process, the use of aggregation to produce a cooperative result between mononuclear molecules, and the design of polynuclear reagents. All three of these approaches have been considered of late . In this paper, we shall review our progress towards the multi-electron catalysts. B. SPECTRO-ELECTROCHEMISTRY OF IRON AND COBALT TETRASULFONATED PHTHALOCYANINES (TsPc) These water soluble species were studied by laying down 1 or more nonolayers on an ordinary pyrolytic graphite (OPG) surface Trends in the pH dependencies of the various redox species are due, in part, to differences in axial coordination of the central metal atoms. Aggregation o All of these processes are one-electron, diffusion controlled and quasi-reversible or reversible (ix. v'2 (v = scan rate). In DMF/TBAP the Co(III)/Co(II) and Pc(-1)/Pc(-2) couples shift negatively and positively to values of -0.02V and +0.38V vs Fc/Fc respectively, so that the first oxidation now occurs on the cobalt atom (pathway la). When scanned over a limited potential range ( The bulk solution is 5-coordinate (DMF)Co(II)Pc(-2) The potential of couple (6-coord) is more negative than couple (5-coord) because of the strong predilection of Co(III) for 6-coordination. Thus (DHF)2Co(II)Pc(-2) (2a, right-hand side) is oxidised first (anodic wave). During the cathodic scan, in the limited scan in Disproportionation will occur in a range of solvents provided they do not bind too strongly to Co and inhibit attack of 0H. This chemistry is based upon the equilibria:- where When contacted with various reagents the resulting chemistry was monitored by electronic spectroscopy. A film of Co(I)TNPc will reduce protons (dilute acid), and alkaline carbonate rapidly and alkaline sulfite very slowly. The oxidation product in the film, however, is Co(II)Pc not Co(III)Pc. Firstly, two 1,3-diiminoisoindoline units are linked together via a variety of bridges including direct linkage (zero bridge) and using a common benzene ring (so-called -1 link). These are then condensed with 5-neopentoxyl,3-diiminoisoindoline to generate mononuclear tetraneopemtoxyphthalocyanine (by self-condensation of the latter molecules) and the new binuclear system (scheme I) The binuclear species Include, using cobalt derivatives as example, (the number in parenthesis is the number of bridging atoms connecting the two phthalocyanine units):- The use of pentaerythritol as a template for connecting four 1,3-diiminoisoindoline groups, led to the successful synthesis of the remarkable tetranuclear species (II), being tetra[tri(neopentoxy)phthalocyanato]cobalt attached via a C(CH2O)4 core In studying the metal-free and cobalt complexes of these various species, certain features were evident. i) In condensed phases (frozen solution glass, crystal, Nujol mull etc), thse complexes may exhibit intermolecular aggregation. In dilute solution, < 5 x 10 M, intermolecular aggregation is generally absent (but not in the tetranuclear species). ii) Some binuclear phthalocyanines may close upon themselves ('clamshell' behaviour) via cofacial intramolecular aggregation. Specifically, only Cat(4), t-BuCat(4) and EtMeO(5) exhibit intramolecular cofacial aggregation. Such species exist in a dynamic equilibrium with 'open' conformations. iii) The electronic spectra of the metal-free species show evidence of the coupling of the transition moments on each phthalocyanine ring, expected to be more important here than in the porphyrins, because of the larger visible region transition moments of the former Cat(4) Some variation in this sequence can be expected to occur with the isomer mix isolated during column chromatography of these species. As indicated above, cofacial metal-free binuclear species additionally emit at 750nm, at liquid nitrogen temperature, with a lifetime of about 5Ons compared with about 5ns for the 700nm emission. Coupling of the transition moments on each phthalocyanine ring results in the Q state coupling to yield an upper and lower level. Each level further splits in the lower symmetry of the metal-free derivative Interestinly, many of the polynuclear species also exhibit emission (very short-lived ca 5-10ns), from th Soret region, near 425mm F. ELECTRONIC COUPLING IN POLYNUCLEAR COBALT DERIVATIVES The cobalt complexes do not emit but electronic coupling may be detected through consideration of their absorption spectra. Extensive coupling occurs in many of the cobalt(II) polynuclear phthalocyanine derivatives. This is exemplified by blue shifting of the Q band absorption I. OXYGEN REDUCTION WITH POLYNUCLEAR COBALT DERIVATIVES Cyclic voltammetry studies on the binuclear cobalt species show redox waves corresponding almost exactly with those observed for the mononuclear control molecule. Evidently the degree of electronic coupling is insufficient to cause a marked departure in redox behaviour, i.e. reduction or oxidation occurs at both rings as though they were unconnected. This is not quite true of the tetranuclear cobalt species which shows significant aggregation at the concentrations used for voltammetry. In addition to waves due to the unaggregated and mononuclear-like component, additional waves due to the aggregated species occur. Spectroelectrochemistry in the region of these additional waves provides no evidence for any mixed valence species being formed. Studies have been undertaken by laying down a monolayer or more of a cobalt polynuclear phthalocyanine on an OPG electrode and studying the oxygen reduction chemistry. By using a rotating electrode, it is possible to obtain values for the kinetic current (Koutecky-Levich analysis), being a relative measure of the efficiency of oxygen reduction [161. Hydrogen peroxide is the product with the cobalt catalysts in alkaline solution, while, under similar conditions, FeTNPc yields the four electron reduction to water [301. The kinetic currents for the polynuclear cobalt(II) species are greater than that for the mononuclear species (pH 13, NaOH, OPG electrode) according to the sequence (relative numbers in parentheses):- Thus the binuclear cobalt species are undoubtedly more efficient for oxygen reduction than is the mononuclear control molecule, and the tetranuclear cobalt species is the most efficient. However the improvement over the control molecule, while definite, is not very dramati c. Of more potential interest, is the observation that the increase in kinetic currents from one species to another parallels, reasonably well, the degree of electronic coupling as indicated by half-bandwidths (5). Thus the premise that the efficiency depends upon electronic coupling seems vindicated. It is evidently true to say that these species exhibit multi-electron redox characteristics and more highly coupled species are now being investigated