Kinetics

Photophysics stopped-flow spectrometer, Model SF. 17 MV. Kinetic data acquired with the stopped-flow spectrometer were automatically logged and stored on an

Archimedes 410/1 computer. A 100 W tungsten-halide lamp and a 150 W xenon lamp were used as the light sources for reactions monitored in the visible or near UV (λ > 350 nm) and in the far ultra-violet regions of the spectrum, respectively. All reactions were carried out under anaerobic conditions to prevent scavenging of the oxygen sensitive cobalt(II)-cage reductants. For this reason, the stopped-flow drive and mixer assemblies were enclosed within a plastic glove bag inflated with a positive pressure of N₂. This arrangement was sufficient to maintain the reservoir, mixing and stop syringes and the observation chamber under an inert atmosphere. A small grain of Zn(Hg) was added to the exterior reservoir syringe containing the reductant solution as an extra precaution against scavenging of CoII complexes by trace dioxygen. The temperature of the mixing syringes and reaction chamber was maintained at 25.0 ± 0.2 °C by water circulated through a Techne C-85A thermostat. The water contained in the thermostat circuit was continually purged with a steady stream of N₂.

Reactant solutions (normally 25 mL or less total volume) were purged with catalytically purified Ar saturated with H₂O for at least 30 min prior to the addition of Zn(Hg) to the reductant solutions. The solutions were purged with Ar until the

appropriate color change in the reductant solution was observed (at least 1 h).

Immediately prior to monitoring a reaction, the drive syringes and mixing chamber were thoroughly flushed with N₂-purged H₂O, then twice with freshly purged reactant

solutions.

All reactions were monitored by following the optical-absorbance change with time after equal volumes (typically 50 µL) of each reactant solution were mixed. A

non-linear regression programme based on the Marquardt algorithm was used to fit a single exponential function with a floating endpoint (equation 2.20) to the absorbance traces (380 to 400 data points).

A_t =

(

)

obs

t₊

A_e (2.20)

where A_t is the absorbance at time t (s), A₀ is the initial absorbance, A_e is the absorbance at equilibrium and kobs is the observed pseudo first-order rate constant (s-1). The fitting routine was terminated when the improvement in the normalised variance after an iteration was < 0.1%. This analysis yielded absorbance changes (A₀ - A_e) and the pseudo first-order rate constant for each run. The kinetics of electron transfer for a reaction were monitored at two wavelengths wherever a suitable absorbance change occurred. At least seven to ten runs were carried out at each wavelength. The reported rate constants represent the mean and standard deviation of these multiple runs at both wavelengths.

2.6 References

(1) Sutin, N. In Progress in Inorganic Chemistry, Lippard, S. J. Ed; John Wiley and Sons: USA, 1983, vol. 30, pp 441–498.

(2) Meyer, T. J.; Taube, H. In Comprehensive Coordination Chemistry, Wilkinson, G; Gillard, R. D.; McCleverty, J. A. Eds.; Pergamon Press: 1987; vol. 1, pp 331–383.

(3) Newton, M. D. Int. J. Quantum Chem. 1980, 14, 363. (4) Marcus, R. A. Ann. Rev. Phys. Chem. 1964, 15, 155

(5) Marcus, R. A. Angew. Chem. Int. Ed. Engl. 1993, 32, 1111. (6) Hush, N. S. Trans. Faraday Soc. 1961, 57, 557.

(7) Reynolds, W. L.; Lumry, R. W. In Mechanisms of Electron Transfer, The Ronald Press Company: New York, USA, 1966; Chapter 6.

(8) Marcus, R. A. J. Phys. Chem. 1963, 67, 853.

(9) Chou, M.; Creutz, C.; Sutin, N. J. Am. Chem. Soc. 1977, 99, 5615.

(10) Creaser, I. I.; Sargeson, A. M.; Zanella, A. W. Inorg. Chem. 1983, 22, 4022. (11) Anderson, A.; Bonner, H. A. J. Am. Chem. Soc. 1954, 76, 3826.

(12) Bernhard, P.; Sargeson, A. M. Inorg. Chem. 1987, 26, 4122. (13) Silverman, J.; Dodson, R. W. J. Phys. Chem. 1952, 56, 846. (14) Ford-Smith, M. H; Sutin, N. J. Am. Chem. Soc. 1961, 83, 1830. (15) Stynes, H. C.; Ibers, J. A. Inorg. Chem. 1971, 10, 2304.

(16) Rillema, D. P.; Endicott, J. F.; Patel, R. C. J. Am Chem. Soc. 1972, 94, 394. (17) Buhks, E.; Bixon, M.; Jortner, J.; Navon, G. Inorg. Chem. 1979, 18, 2014. (18) Hendry, A. J. PhD Thesis, The Australian National University, 1986. (19) Geue, R. J.; Hendry, A. J.; Sargeson, A. M. J. Chem. Soc., Chem Commun.

1989, 1646.

(20) Bull, D. J. PhD Thesis, The Australian National University, 1991. (21) Bogsanyi, D.; Creaser, I. I. Unpublished Data.

(22) Gahan, L. R.; Hambley, T. W.; Sargeson, A. M.; Snow, M. R. Inorg. Chem.

(23) Dubs, R. V.; Gahan, L. R.; Sargeson, A. M. Inorg. Chem. 1983, 22, 2523. (24) Miles, E. Unpublished Results.

(25) Bernhard, P.; Sargeson, A. M. J. Am. Chem. Soc. 1989, 111, 597.

(26) Bernhard, P.; Bürgi, H, -B; Raselli, A.; Sargeson, A. M. Inorg. Chem. 1989, 28, 3234.

(27) Comba, P.; Engelhardt, L. M.; Harrowfield, J. MacB.; Horn, E.; Sargeson, A. M.; Snow, M. R.; White, A. H. Inorg. Chem. 1985, 24, 2327.

(28) Hagen, K. S.; Hauser, A.; Martin, L. L.; Martin, R. L.; Sargeson, A. M. J.

Chem. Soc., Chem Commun. 1988, 1313.

(29) Hagen, K.; Martin, L. L. Unpublished Data.

(30) Brunschwig, B. S.; Creutz, C.; Macartney, D. H.; Sham, T-K.; Sutin, N.

Faraday Discuss. Chem Soc. 1982, 74, 113.

(31) Hupp, J. T.; Weaver, M. J. Inorg. Chem. 1983, 22, 2557.

(32) Geue, R. J.; McCarthy, M. G.; Sargeson, A. M. J. Am. Chem. Soc. 1984, 106, 8282.

(33) Lay, P. A. PhD Thesis, The Australian National University, 1981.

(34) Geue, R. J.; Hambley, T. W.; Harrowfield, J. M.; Sargeson, A. M.; Snow, M. R.

J. Am. Chem. Soc. 1984, 106, 5478.

(35) Creaser, I. I.; Geue, R. J.; Harrowfield, J. MacB.; Herlt, A. J.; Sargeson, A. M.; Snow, M. R.; Springborg, J. J. Am. Chem. Soc. 1982, 104, 6016.

(36) Creaser, I. I. Unpublished Results.

(37) Kokholm Petersen, G. In Redox Measurements; their theory and technique: Radiometer A/S Technical Bulletin: 1966; ST40.

Chapter 3

Reactions of -Methylene Aliphatic Aldehydes with Cobalt(III)

Tris(1,2-diamine) Complexes:

New Routes to Symmetrically-

Substituted Hexa-Aza Cage Ligands

3.1 Introduction

The series of cobalt(III) tris(1,2-diamine) complexes that include [Co(en)₃]3+ ₍₁₎

(en = 1,2-ethanediamine) (Figure 3.1) and lel₃-[Co(chxn)₃]3+ _{(2) (chxn = trans-1,2-}

cyclohexanediamine) is an important class of precursor complexes for metal-ion template encapsulation reactions carried out under conventional aqueous-phase

conditions (see Chapter 1). In an aqueous reaction medium, both sets of primary amine nitrogens arranged arranged around the two trigonal faces of these cobalt(III) tris(1,2- diamine) cations can be capped by condensation with formaldehyde and the carbon acid nucleophile -_{CH2NO2 to give the corresponding symmetrically di-nitro substituted}

cobalt(III) hexa-aza cage complexes. The tripodal cobalt(III) hexaamine complex, [Co(sen)]3+ (3), has a similar set of coordinated primary nitrogen centres arranged around the open trigonal face of the cation (see Figure 3.1). In aqueous solution, the [Co(sen)]3+ _{ion can be capped with} -_{CH2NO2 and formaldehyde under conditions}

similar to those used to cap the [Co(en)₃]3+ _{and lel3-[Co(chxn)3]}3+ _cations.1,2 _As

indicated in Chapter 1, this kind of chemistry can also be used to cap the open trigonal face of the [Co(sen)]3+ ion with formaldehyde and a range of nucleophiles derived from weak carbon acids in acetonitrile solution. The similar reactivities of the tripodal

complex and the tris(1,2-diamine) complexes towards encapsulation reactions in aqueous solution therefore implied that it might be possible to use non-aqueous

synthetic methods to cap the trigonal faces of [Co(en)₃]3+ _{and lel3-[Co(chxn)3]}3+ _with

A range of aldehydes and ketones with weakly acidic methylene units alpha to the carbonyl function (I) have been used to cap the trigonal face of the [Co(sen)]3+ complex under non-aqueous conditions (see Chapter 1).

O R C H₂ 'R (I)

The established reactivity of aliphatic aldehydes as capping reagents for the tripodal complex under non-aqueous conditions indicated that this type of carbon acid might prove suitable for encapsulation reactions based on the structurally related cobalt(III) tris(1,2-diamine) complexes. Thus, in the present work, the co-condensation of α-methylene aliphatic aldehydes and formaldehye with [Co(en)3]3+ and lel3-

[Co(chxn)₃]3+ has been investigated as a possible new, more versatile, route to

symmetrically substituted sarcophagine-type cage ligands, both with flexible and fixed chelate ring conformations.

3.2 Results