Steric Effects in Redox Reactions and Electron Transfer Rates

8 Steric Effects in Redox Reactions and Downloaded by UNIV OF ARIZONA on December 16, 2012 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/ba-1997-0253.ch008

Electron Transfer Rates Rodney J. Geue, John V. Hanna, Arthur Höhn, C. Jin Qin, Stephen F. Ralph, Alan M. Sargeson, and Anthony C. Willis Research School of Chemistry, The Australian National University, Canberra, Australian Capital Territory 0200, Australia, and Commonwealth Scientific and Industrial Research Organisation, Division of Coal and Energy Technology, North Ryde, New South Wales 2113, Australia

Several key cobalt(II/III) hexaamine systems are discussed in which steric factors enhance electron transfer rates by a factor of more than 10 and effect redox potential changes in excess of 1.2 V. In some instances conformational isomers alone show dramatic differences. Elec-tron transfer rates between ob and lel conformations of metal ion cages arising from Λ[Co(R-pn) ] and Λ[Co(S-pn) ] ions (pn = 1, 2-propanediamine) vary by more than 10 fold. The larger cavity [Co(Me -tricosaneN )] ions (Me -tricosaneN = 1,5,5,9,13,13,20,20octamethyl-3,7,11,15,18,22-hexaazabicyclo[7.7.7]tricosane) have orange and blue conformations that differ in redox potential by >0.5 V. X-ray crystal structure views of the orange form and of the unusually air-sta -ble cobalt(II)complex as their nitrate salts are presented. Also, solid -state Co NMR and reflectance spectra imply a very weak ligand field at the cobalt site of the blue form. 6

3

3

3

2+/3+

3

2+/3+

3

8

6

3+

8

6

59

STERIC EFFECTS HAVE ALWAYS BEEN INTERESTING

especially in their ability to influence isomer distributions and stereochemistry (I). In more recent time, however, they have been impressive in their ability to influence other proper ties. It is that aspect that this chapter will address and especially in relation to the metal ion cage redox potentials and electron transfer reactions. To begin with, the self-exchange electron transfer of the [Co(sepulchrate)] (Figure 1) couple is a good example. Structural studies and molecular mechanics calculations of the two complex ions indicate that both 3+/2+

© 1997 American Chemical Society

In Electron Transfer Reactions; Isied, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1997.

137

138

E L E C T R O N TRANSFER REACTIONS Steric Effects on Electron Transfer 3+/2+ [Co(sep)] .[Co(sep)]3+

i \

2+

[CoCNH ) l2t[Co(NH3) ]3+ 3

6

6

Downloaded by UNIV OF ARIZONA on December 16, 2012 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/ba-1997-0253.ch008

R.C Figure 1. The [Co (sepulchrate)] structure and proposed energetics for the electron transfer reactions of the cage and [Co0JH^J . 3+/2+

oxidation states are strained relative to the [ C o ( N H ) ] or [ C o ( e n ) ] (en = 1,2-ethanediamine) couples (2, 3). In both instances the metal ion and the organic cage compromise their needs to give a minimum energy structure, but the strain is evident in both parts. Crudely, it can be argued that the cavity of the cage is a little too large for Co(III) and too small for Co(II). This condi tion means that the strain in both ground states helps the reactant pair toward the activated complex, and the free energy barrier for electron transfer is thereby effectively lowered (Figure 1). The process largely contributes to a 10 fold enhancement in rate relative to the parent couple [ C o ( N H 3 ) ] . It is a very good example of an "entatic state" (4). Also, it is an important demonstra tion of how influential these steric forces can be in affecting electron transfer rates and the reversibility of such redox couples. 3

6

3 + / 2 +

3

3+/2+

6

6

3+/2+

Another example of some published results (5) involving cages (Figure 2) arising from A[Co(S-pn) ] and A[Co(R-pn) ] is displayed i n Table I (pn = 1,2-propanediamine). These two molecules stabilize two conformations of the cage ligand, namely the lel and ob forms. Their structures have been estab lished by X-ray crystallographic analysis (Gainsford, G. J., Department of Sci entific and Industrial Research, New Zealand; Robinson, W. T., University of Canterbury, New Zealand, unpublished data), and the respective conforma tions are stabilized because of the demand that the methyl groups adopt an equatorial orientation for the five-membered Co-pn-type ring systems. Thus, the two diastereoisomers shown are not interconvertible and are readily sepa rated by chromatography. Several important features arose from this study. The ob and lel isomers had vastly different redox potentials for the Co(III)/(II) couple, ΔΕ ~ 0.3 V (~10 in equilibrium constant), and the self exchange electron transfer rate constants differed by —30 fold. The two isomers shown i n Figure 2 have a facial arrangement of the methyl groups (fac), and there are equivalent isomers where the methyl groups are arranged meridionally (mer). The regiospecificity 3

3+

3

3

3+

3

3

z

5



8.

G E U E ET AL.

139

Steric Effects in Redox Reactions and ET Rates

o^ /flC-A-[Co{(NH ) -(i?)-Me3sar}]

te/ /ac-A-[Co{(NH3) -(5)-Me sar}] r

2

3

5+

r

3

Figure 2. Stable conformers of the [Co((NH^ Me -sar)] with permission from reference 5. Copyright 1989.)

5+

cages. (Reproduced

5+

3

2

Table I. Rate Constants and Redox Potentials for Diastereoisomers of [Co{(NH ) ,Me -sar}] ions at 25 °C 3

2

5+/4+

3

Rate Constants (M sr )

Isomers

-2

Enantiomeric pairs fac-M mer-lel fac-ob mer-ob [Co{(NH3) sar}] /

hi 0.03F 0.033* 0.97 LOO 0.025

3

3

2

3

5+

0.015 0.015 -0.295 -0.325 0.02

a

3

2

E'(Vvs. SHE)

1

fl

4+

Diastereoisomeric pairs

^1,2

mer-A-lel + fac-A-ob mer-A-lel + fac-A-ob fac-A-lel + fac-A-ob fac-A-lel +fac-A-ob mer-A-lel + mer-A-ob mer-A-lel + mer-A-ob fac-A-lel + mer-A-ob fac-A-lel + mer-A-ob 3

3

3

3

3

3

3

3

3

Λ

3

3

3

0.34

fl

3

3

0.34

fl

3

3

54 , 89& 45 40° 32 17 14 13 10°, 53*

0.31

a

fl

fl

0.31

N O T E : S H E means standard hydrogen electrode. observed rate constant at conditions of 25 ± 0.1 °C in 0.1 M CF SO H/0.1 M C F S 0 N a . 3

3

^Calculated rate constant from the Marcus cross-reaction relationship:

3

3

k = {kn-k^.K^.f^ 12

SOURCE: Reproduced with permission from reference 5. Copyright 1989.


140

E L E C T R O N TRANSFER REACTIONS

of these substituents did not affect the redox properties much at all, but this stereochemical factor, the combinations of chirality about the metal and the chiral C center, generate twelve pairs of redox couples to give electron transfer rate constants (Table I). Their values show that ΔΔ and ΔΛ combinations differ little. The variations arising from the mer and fac orientations of the methyl group vary by as much as a factor of 5, but the lel and ob combinations vary by —2000-fold. What is also evident is a substantial change in for the Co(III) lel and ob forms from —480 to —450 nm, respectively, for the absorp tion band of A - » T parentage. Clearly, the ob conformer has a stronger ligand field than the lel form, and these effects are carried through to the redox properties. However, they all emanate from the demand of the methyl group to remain in an equatorial position in all the isomers. The other infer ence one can gather from these data is that ΔΔ and ΛΔ combinations and mer-mer,fac-fac, and mer-fac combinations do not influence the docking of the ions in the activated complex in any profound way. So for very similar mol ecules, the steric effects influenced the conformations and thereby the redox and electron transfer rates substantially. Then it became interesting to see what happened when the cage was expanded in size. This cage expansion clearly would enhance its ability to sta bilize larger metal ions, and the result (6) is displayed in Figure 3b. Compared with a comparable sarcophagine-type cage, Figure 3a, the redox potential is now much more positive, clearly indicating the C o ion is relatively stabilized in the larger cavity. For the Co(III) complex, the λ (band origin A -> T ) occurs at a longer wavelength, and the complex is crimson instead or the yei3

3

3

3

l

lg

X

3

X


3

2 +

X

ιη3Χ

λη^αΚΙΠ) 470 nm

^ C o d l l ) 516 nm

E° -0.45 V ( S H E )

E° +0.08 V ( S H E )

ket 2 M - V

k,! 0.5M-V 25°C

1

25°C

X

x

lg

?

1

(a)

(b)

(c)

Figure 3. Spectra maxima, redox potentials and electron transfer self-exchange rate constants for sar and tricosane cages; a is [Co(Me -sar)] , b is [Co(facMeg-O^tricosaneN^] ^ ^, and c is [Co(Me -D tncosaneN^] . 2

3

2

8

3h

3+/2+

3+/2+


8.

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141


low-orange characteristic of Co(III) hexaamine compounds in general. These data imply a weaker ligand field and an expanded cavity, and that shows up as longer Co(III)-N bonds (>2.02 Â). The electron transfer self-exchange rate, however, does not alter very much. Now the cage is a slightly better fit for Co(II) and a much poorer fit for Co(III), and crudely it can be viewed as a trade-off in terms of steric strain for the two ions; that is, relative to [Co-(Me sar)] the total steric strain component for the reorganization energy of [Co (fac-Me -tricosaneNg)] is not very different, but is just distributed in a dif ferent manner. The structure of the Co(III) ion (Figure 4) shows three rather 2

3+/2+

3+/2+


5

Figure 4. The structure of A-[Co(foc-Mec-D tncosaneN$ in its (PF^} H 0 salt. Selected bond lengths (A) and angles (°) (averaged assuming C symmetry over η = 1-3) are as follows. Bond length Co-N(n2) 2.021 (±0.004), Co-N(n6) 2.024 (± 0.013); bond angles: N(n2)-Co-N(n6) 95.2 (±0.% Co-N(n2)-C(n3) 120.1 (±0.5), Co-N(n6)-C(n5) 120.7 (±0.7); torsion angles: N(n6)-Co-N(n2)-C(n3) -19.9 (± 1.8), N(n2)-Co-N(n6)-C(n5) 15.9 (±2.2). 3h

3+

3

3


2


142


Figure 5. Visible spectra and cyclic voltammetry for [Co(fac-Me$-D tricosaneN^] ion. 3h

3+

distorted chairlike six-membered rings with the methyl groups all equatorial and C symmetry overall. The cyclic voltammetry also shows essentially a reversible couple (Figure 5), and the synthesis of this ion (Scheme I) is remark ably stereospecific with the overall yields at the moment being 25-30%. In summary then, we have this picture of a more positive redox potential for the larger cage but not much change in electron transfer rate. What happens, then, if three more methyl groups are added to the annular six-membered rings as shown in Figure 3c? The added methyl group will clearly confuse the chelate i n relation to which conformation it should adopt, and the steric forces overall should clearly increase. This synthesis can be readily accom plished by using 2-methyl propanal instead of propanal i n the previous synthe sis. The reaction works quite well, but in the final step we get a Co(II) complex whose structure is given i n Figure 6, instead of the Co(III) ion that normally arises from the B H ~ reduction of the imine groups in the initial triimine tricosane product. The structure has quite a different conformation to that of the [Co (Me -tricosaneN ] ion. Now the annular rings are skew-boat so the two methyl groups are equivalent, and a plane through the three skeletal C atoms of the ring is parallel to the C axis of the ion. Moreover, the complex is now quite difficult to oxidize to the Co(III) state, which is now destabilized considerably. 3

4

ni

5

6

3+

3




8. G E U E ET AL.

Figure 6. Structure of the [Co (^e -D firicosaneN^ ll

8

3}


143


144


Ε (vs SCE, mv)

Figure 7. Cyclic voltammetry of [Co (Me -D 1ricosaneN^] n

8

km in water at 22 °C.

2+

3h

The cyclic voltammetry for the [ C o ( M e - t r i c o s a n e N ) ] ion in water (Figure 7) is no longer so reversible. It has an oxidation wave at 0.84 V (vs. standard hydrogen electrode (SHE)), and there is considerable asymmetry i n the whole cyclic voltammogram. The system is chemically reversible, however, and it is possible to quantitatively oxidize the Co(II) ion to Co(III) coulometrically. Also, it can be seen that a distortion at ~250 mV on the reduction wave [vs. saturated calomel electrode (SCE)] indicates the presence of another Co(III) species. The oxidized solution was therefore examined in considerable detail, and two Co(III) salts have been isolated from the solution under different condi tions, one orange and the other blue. These two salts have the same constitu tion except for water content. The orange form is isolated from water as the nitrate salt, whereas the blue form is isolated with ethanol from the concen trated mother liquor. If the solid orange form is dropped into ethanol it turns blue immediately, at least on the surface. Are the two forms isomers, or is one a hexaamine and the other a partly dissociated cage complex? Reflectance spectra (Figure 8) show that they are clearly different. The orange form has an absorption maximum at 470 nm, whereas the blue form has a maximum at about 600 nm. The nitrate salt of the orange form gives crystals suitable for X-ray crystallographic analysis except that at —20 °C X-irradiation generates the blue form and crystallinity is lost. However, at low temperature (-60 °C) a good structure was obtained (Figure 9), quite different from that of the Co(II) complex. A skew-boat configuration of the annular six-membered rings is retained, but the complex now has an ob n

8

6

2+

3


8. G E U E ET AL.

hoo

urange

•

•·

Blue

80

/

τ

40 "

ι

\

7 "

.

m

1

V

/

...... I.....

• 60 ppm, as well as some rearrangement of the 20^40-ppm resonances being observed. Although only six resonances are clearly observed for the blue salt, consistent with D symmetry, the intrinsic broadness of the solid-state N M R resonances (in comparison to their solution N M R counterparts) may preclude the observation of further peak structure linked to a reduction from D symmetry. 1 3

3

5 9

1 3

1 3

3

1 3

3

3


8.

G E U E ET AL.

147



Solid

*-»«-..

ISO

I

Ιιι.

100

SO

0 ppm

• • • •-

180

100

80

0

—

ppm

Figure 11. C solid-state NMR spectra of [Co(Me -D tncosaneN^](N0^ : blue (4H 0) and orange (5H 0) isomers. 13

8

2

3h

3

2

The C o M A S N M R spectra (Figure 12) more clearly highlight large elec tronic and structural differences between the orange and blue forms. The very sharp C o resonance obtained for the orange salt indicates a very symmetrical cobalt environment, which eliminates any substantial quadrupolar interaction at the metal site. The C o spectrum for the blue form, however, reveals an enormous downfield shift of —4000 ppm for the cobalt resonance relative to that of the orange form. This shift is coupled with a spectacular increase i n linewidth attributed to an increased nuclear quadrupolar interaction experi enced by the Co nucleus. These spectral changes are consistent with a reduced ligand field strength at the cobalt site(s) in the blue complex implied by the downfield shift to higher deshielding, and an increased electric field gradient reflecting a significant electronic distortion from D symmetry about the Co nucleus. In total, the spectroscopic data for the Co(III) complex imply that there are two conformational isomers that interconvert in solution and the blue form 5 9

5 9

5 9

3


148

E L E C T R O N TRANSFER REACTIONS 7486

orange isomer Co single pulse SW = 1 χ 10 Hz


6

i

blue ii Co single pulse SW = l x l 0 H z 6

orange isomer

Figure 12. Co solid-state NMR spectra of [Co(Me -D fncosaneN^(N0^ : (4H 0) and orange (5H 0) isomers. 59

&

2

3}

3

blue

2

has a very weak ligand field, bordering on the high-spin state. Despite the lack of a structure for the blue form, it is clear that the average C o - N bond length should be >2.1 Â, much longer than has been observed hitherto. It is also likely that its structure is close to that of the Co(II) dinitrate salt. Simply, the two fea sible structures are related to the RR or SS chirality of the two caps (Figure 13). (RR or SS describe the right- or left-handed helical relationship between the N - C bonds of the caps and the C axis of the complex ion.) In the ASS (ARR) form, the ob conformation is evident, whereas i n the ARR (or ASS) form the lel conformer obtains. The ARR (lel ) conformer is essentially more extended along the C axis and is thereby able to destabilize a smaller ion. The interconversion between the ARR and ASS forms therefore does not require a configuration change about Co or Ν and is simply a conformational change for the three six-membered annular chelates and the cap configurations. This m

3

3

3

3

3



8.

G E U E ET AL.


A-lcl

A-ob

3

149

3

Figure 13. Projected structures of the blue (lelj) and orange (ob^ isomers of [Co (Me -D tncosaneN^)] . 8

3h

3+

change appears to occur on a time scale faster than a second but slower than a millisecond, and it may be possible to slow the rate of this process and observe both isomers independently. It is also evident that these two forms must be nearly equienergetic even in water although favoring the ob form somewhat. It is also obvious that the blue form must be the good oxidant (—0.8 V vs. SHE). Overall, the major factor in these studies is that the redox potential can be changed by at least 1.2 V with the same metal ion couple and the same set of ligating atoms merely by influencing the steric factors in a modest way in the cage. It is obvious also that the Co(III)-N bond lengths have been extended a considerable way toward the Co(II)-N bond length (2.22 Â), which itself is longer than normal for a high-spin Co(II) saturated amine complex. It is even possible that the Co(III)-N bond length in the blue isomer is close to the highspin Co(III)-N value. It follows that the intramolecular reorganization energy component of the electron transfer barrier may be small and the rate of elec tron self-exchange fast between the blue Co(III) isomer and the Co(II) form. It remains to be seen what that self-exchange rate constant is for the cobalt(II)/(III) octamethyl tricosaneN couple. 3

6

Acknowledgments This chapter is based on a paper presented in honor of Henry Taube, a stimu lating researcher, teacher, and friend, on the occasion of his 80th birthday Sym posium.


150


References 1 2.


3. 4. 5. 6.

Buckingham, D. Α.; Sargeson, A. M . Topics in Stereochemistry; Wiley: New York, 1971; pp 219-277. (a) Creaser, I. I.; Geue, R. J.; Harrowfield, J. M . ; Herlt, A. J.; Sargeson, A. M . ; Snow, M . R.; Springborg, J. J. Am. Chem. Soc. 1982, 104, 6016, and references therein; (b) Geue, R. J.; Pizer, R.; Sargeson, A. M. Abstracts of Papers, National Meeting of the American Chemical Society, Las Vegas, NV; American Chemical Society: Washington, DC, 1982; INOR 62. Geselowitz, D. Inorg. Chem. 1981, 20, 4457. Vallee, B. L . Williams,R.P.Proc.Natl.Acad.Sci.U.S.A. 1968, 59, 498. Geue, R. J.; Hendry, A. J.; Sargeson, A. M. J. Chem. Soc. Chem. Commun. 1989, 1646. Geue, R. J.; Höhn, Α.; Ralph, S. F.; Sargeson, A. M.; Willis, A. C. J. Chem. Soc. Chem. Commun. 1994, 1513. ;


Steric Effects in Redox Reactions and Electron Transfer Rates

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