Phthalocyanines and their metal complexes were discovered in 1920 by accident, when 1,2-dicyanobenze — Physical Chemistry — Thermodynamics Chemistry Question
Metal Phthalocyanines: Mechanism of Reduction
Phthalocyanines and their metal complexes were discovered in 1920 by accident, when 1,2-dicyanobenzene (phthalonitrile) was heated in a copper jar. An amazingly thermally stable blue powder was collected. Besides thermal stability, metal phthalocyanines also have a property of being excellent catalysts of a number of oxidation reactions. This feature of phthalocyanines is due to the ability of the dianionic phthalocyanine (Pc) ligand to stabilize metals in various oxidation states; this is illustrated by the following problem.
Given the atom connectivity in a metal-free phthalocyanine molecule provided below, [VISUAL] draw the structure of iron(III) phthalocyanine chloride, with a correct pattern of double bonds.
Model Answer
Structure showing iron(III) phthalocyanine chloride. The central iron is coordinated to four pyrrole-like nitrogens in the core of the phthalocyanine ring, and a chlorine ligand is attached to the iron (as Fe-Cl), with a symmetric alternating double bond pattern throughout the phthalocyanine ring system.
Dithionite anion occurs in aqueous solution at equilibrium with its monomer, SO2 –, a free radical species. Draw the Lewis structure of dithionite anion and write a reaction its dissociation into SO2 –.
Model Answer
The Lewis structure of dithionite anion consists of an S-S single bond, with each sulfur atom bonded to two oxygen atoms (possessing appropriate double bonds and formal negative charges distributed over the oxygens). Dissociation reaction: S2O4^2- <=> 2 SO2^-
Another reduced sulfur species, sodium hydrosulfoxylate, NaHSO2, is also known. Show which common sulfur species can be used to synthesize sequentially both a dithionite anion and a hydrosulfoxylate anion using suitable reducing agents.
Model Answer
Sulfur dioxide, SO2, can be used to synthesize both species. Electrochemical reduction or appropriate reducing agents can be used:
SO2 + e- → SO2^-
SO2^- + H+ + e- → HSO2^-
Or dimerization to dithionite: 2 SO2 + 2 e- → S2O4^2-
This question concerns the dithionite reduction of phthalocyanine complexes.
i) The following kinetic equation was obtained for the iron(III) phthalocyanine (PcFeIII) reduction to iron(II) phthalocyanine by dithionite:
S2O4 2– + PcFeIII → PcFeII + sulfur containing products; the reaction is relatively fast.
rate1 = k [PcFeIII][S2O4 2–]
ii) By contrast, for the iron(II) phthalocyanine reduction to iron(I) phthalocyanine the following kinetic equation was obtained:
S2O4 2– + PcFeII → PcFeI + sulfur containing products; the reaction is very slow.
rate2 = k [PcFeII][S2O4 2–] 0.5.
iii) For cobalt(II) phthalocyanine reduction with dithionite to Co(I) phthalocyanine, yet another kinetic equation could be obtained:
S2O4 2– + PcCoII → PcCoI + sulfur containing products; the reaction is slow.
rate3 = k3 [S2O4 2–]
Propose mechanisms for the reactions above that would allow you to account for the difference in the observed kinetic orders.
Model Answer
i) PcFeIII is a strong enough oxidant to react with S2O4^2- directly in a fast, bimolecular one-step reaction.
ii) S2O4^2- is in rapid equilibrium with its monomer radical: S2O4^2- <=> 2 SO2^-. PcFeII is a poor oxidizing agent, making its reaction with the monomeric SO2^- radical the slow, rate-limiting step: PcFeII + SO2^- → products, which explains the 0.5 order in dithionite.
iii) The rate-limiting step is the slow dissociation of dithionite into its monomer radicals: S2O4^2- → 2 SO2^- (slow, rate-limiting). This is followed by a fast reaction of PcCoII with the radical: PcCoII + SO2^- → products (fast), giving a rate law that is independent of PcCoII concentration and first-order in dithionite.