6.9 CRECIMIENTO ECONOMICO Y CONVERGENCIA

CoIII _{and Co}II _{encapsulation complexes in aqueous solution, k}

ex, has been determined

for a range of substituted sarcophagine-type cage complexes (Table 1.2). The substitutional inertness of the cage complexes in the cobalt(III) and cobalt(II) forms enables these rate constants to be measured by the racemisation method invented by Dwyer and Gyarfas55_{. Thus, the rate of electron transfer for the [Co((X,Y)-sar)]}3+/2+

self-exchange reaction was followed by monitoring the change in optical activity with time after an optically-active form of the CoIII-cage, ∆ -[Co((X,Y)-sar)]3+ (or Λ- [Co((X,Y)-sar)]3+_{), was mixed with the reduced form of the enantiomeric cage}

complex, Λ-[Co((X,Y)-sar)]2+ (or ∆-[Co((X,Y)-sar)]2+_{) (equation 1.3).}18,56 _Electron

transfer clearly leads to racemisation (equation 1.3).18,55–57

∆-[CoII_((X,Y)-sar)]2+ + Λ-[CoIII_((X,Y)-sar)]3+ k₁₂

∆-[CoIII_((X,Y)-sar)]3+ + Λ-[CoII_((X,Y)-sar)]2+ _(1.3)

The reactant ions in the electron self-exchange reaction between [Co((X,Y)- sar)]3+ and [Co((X,Y)-sar)]2+ complexes are coordinatively saturated MN₆ complexes in both oxidation states.18,31 _{Structual studies of the cage complexes in the solid state}

showed that the saturated cage ligand forms a tight sheath about the metal-ion in both oxidation states.18,31 _{Furthermore, the ligand exchange kinetics of the cobalt(II)-cage}

Table 1.2. Electron self-exchange rate constants (kex) for the MIII/II couple (where M =

Fe, Ni, Mn, Co, Ru) of sarcophagine-type cage complexes in aqueous solution at 25 °C.

Complex kex

(M-1 _s-1₎

Medium

[Co((di-AZA)-sar)]3+/2+ 5.1 ± 0.3a,b 0.2 M NaCl

[Co((Cl)₂-sar)]3+/2+ _3.0a,c _{0.2 M NaCl}

[Co((Cl, CH₃)-sar)]3+/2+ _2.4a,c _{0.2 M NaCl}

[Co((ClCH₂, CH₃)-absar)]3+/2+ _7.3a,c _{0.2 M NaCl}

[Co((NH₃)₂-sar)]5+/4+ _{(2.4 ± 0.3) × 10}-2)a,d _{0.2 M Li/HClO4}

[Co((N(CH₃)₃)₂-sar)]5+/4+ _{(1.1 ± 0.1) × 10}-2)a,e _{0.2 M NaCl} lel₃-[Co((NH₃)₂-char)]5+/4+ _{(3.8 ± 0.2) × 10}-2)a,f _{0.1 M Na/HClO4}

mer-lel₃-[Co((NH₃)₂-pnsar)]5+/4+ _{(3.3 ± 0.1) × 10}-2)a,g _{0.2 M CF3SO3H/Na}

mer-ob₃-[Co((NH₃)₂-pnsar)]5+/4+ _{1.00 ± 0.01}a,g _{0.2 M CF3SO3H/Na}

[Co((AZA, CH₃)-N₃S₃-sar)]3+/2+ _{2.2 × 10}4)h,i _{0.2 M NaCl}

[Co((CH₃)₂-S₆-sar)]3+/2+ _{2.8 × 10}4)j,k _{0.1 M}

[Co(sar)]3+/2+ 2.1 ± 0.2a,d 0.2 M NaCl

[Mn(sar)]3+/2+ 30 ± 2a,l 0.1 M CF₃SO₃H

[Ni(sar)]3+/2+ (5.3 ± 0.3) × 103)h,m 0.25 M CF₃SO₃H/Na

[Ru(sar)]3+/2+ 1.2 × 105)n 0.1 M CF₃SO₃H/Na

[Fe(sar)]3+/2+ 8 × 105)j,o,p 0.1 M DCl

a _{Determined polarimetrically.} b _{ref. 18.} c _{ref. 8.} d _{ref. 56.} e _{ref. 17.} f _{ref. 29.} g _ref.

32. h _{Determined by a stopped-flow circular dichroism technique.} i _{ref. 58.}

j _{Determined by an NMR line-broadening method.} k _{ref. 27.} l _{ref. 11f.} m _{ref. 11g.} n

Deduced from electron transfer cross-reactions by application of the Marcus correlation (ref. 50). o_{ref. 59.} p _{Corrected for the [Fe}II_(sar)]2+ _{high-spin–low-spin equilibrium (K}

complexes are slow on the timescale of electron transfer in the [Co((X, Y)-sar)]3+/2+

exchange reaction (see above). Thus, it is unlikely that electron transfer between two encapsulated metal-ions involves the formation of an inner-sphere bridge between the two metal-ion centres. The cobalt(II)-cages also reduces O₂ quantitatively to H₂O₂ in aqueous solution and regenerates the intact cobalt(III)-cage complex.61 _Labile

cobalt(II)-amine complexes generally react rapidly to give peroxy-bridged CoIII _dimers

and clearly this pathway is denied by the cage.18 _{A study of medium effects on the}

kinetics of the [Co(sar)]3+/2+ _{electron exchange reaction in aqueous solution showed}

that small monatomic anions such as the Cl- _{ion do not have a catalytic effect on the rate}

of electron transfer.56 _{The tight fit of the encapsulating ligand about the metal in both}

oxidation states, the relatively slow ligand exchange compared with the rates of electron transfer, the absence of any significant anion dependence for the rate of electron

exchange and their general inability to increase the coordination number are only consistent with an outer-sphere electron transfer mechanism in these encapsulated metal-ion systems.18,56

The CoIII/II _{electron exchange reaction in the hexa-aza cage complexes is}

accompanied by a change from a low spin (t2g 6

) CoIII ground state to a high spin (t2g 5

eg

2

) CoII _{ground state.}18 _{This involves a substantial change in the inner-sphere structure of}

the cage complex including large changes in Co—N bond lengths (∆d (Co—N) = 0.19

Å) and a large difference in the twist angle of the two trigonal N₃ faces of the encapsulated ion ( (CoIII) = 56°, (CoII) = 28°).8,18,11a A large degree of inner- sphere reorganization is also required for unencapsulated cobalt(III)ammine and amine complexes (e.g., [Co(NH₃)₆]3+/2+)62, but the electron-exchange rate constant for the cobalt cage complex is 105_–106 _{times greater than the rate constant for the}

unencapsulated cobalt ion complexes18_{. This large enhancement in the rate of outer-}

sphere electron transfer has been rationalized on the basis of ligand strain induced in the ground state of the cobalt(II) and cobalt(III)-cage complexes.8,30, _{In this model, the}

cage ligand may be viewed as a "spring" that is strained in both oxidation states.63 _This

is evident in the structural differences and distortions18,11a _{plus strain calculations for}

both the CoIII _{and Co}II _{cage complexes as each ion moves towards the the nuclear}

configuration of the activated complex thus lowering the activation barrier for electron transfer.30,64

A further large rate enhancement in the CoIII/II _{electron self-exchange rate of cage}

complexes was noted for cage ligands with thioether donor atoms. The electron self- exchange rate constant for the cobalt-ion cage complex with an N₃S₃ donor-atom set, [Co((AZA, CH₃)-N₃S₃-sar)]3+/2+,58 is ca. 104 times greater than that for the N₆ donor- atom analogue, [Co((AZA, CH₃)-sar)]3+/2+ (Table 1.2)18. The CoIII/II electron self exchange rate constant for the homoleptic thioether cage comple, [Co((CH₃)₂-S₆- sar)]3+/2+ is comparable to that of the N₃S₃ analogue (Table 1.2).27 The CoII complex of the thioether-cage complexes have a predominantly low-spin ( t2g

6

eg

1

) electronic configuration in the ground state.27,65 _{Electron-exchange in the thioether-cage}

complexes therefore involves significantly less inner-sphere reorganization than in the high-spin CoII _{hexa-aza cage complexes.}

The electron self-exchange rate constants for the MIII/II _{couple of the}

sarcophagine complexes, [M(sar)]3+/2+ _{(M = Mn, Ni, Fe, Ru), have been determined}

(Table 1.2). The MnIII/II)11f _{and Ni}III/II)11g _{self-exchange rate constants were}

determined by the racemisation method outlined above. The electron self-exchange rate constant for the FeIII/II _{cage complex was determined by following the broadening in}

the 1_{H NMR signal of the cap proton in the [Fe(sar)]}2+ _{complex due to electron}

exchange with the paramagnetic FeIII complex, [Fe(sar)]3+, in D₂O.59 The electron self-exchange rate constant for the RuIII/II _{cage complex was deduced from the}

measured kinetics of outer-sphere cross-reactions with a series of well characterised outer-sphere co-reactants using the Marcus correlation50 _{(see also Chapter 2, Section}

2.1.2). The electron-transfer rate-constants span a factor of 105 _{from the Co system to}

the Fe, Ru system. The large range in the electron transfer rate constants is largely determined by differences in the inner-sphere reorganization energy required for electron transfer in each [M(sar)]3+/2+ _couple.11h

The transition metal-ion cage complexes provide a well-defined series of outer- sphere electron-transfer reagents with tuneable metal-centred reduction potentials and

accurately determined electron self-exchange rate constants. These outer-sphere redox agents have been used as probes of the fundamental aspects of outer-sphere electron transfer in solution including aspects of the Marcus-Hush formalism. This aspect of the redox chemistry of the sarcophagine-type cage complexes together with some of the anomalies in the application of the Marcus-Hush formalism to these systems are outlined in Chapter 2.

Chapter 1

Part B: Advances in the Chemistry of Sarcophagine-Type Cage

Complexes

1.4 Metal-Ion Template Syntheses of Poly-Aza Cage Ligand Complexes in Non-