Photochemistry and Photophysics of Coordination Compounds

44 N.A.P. Kane-Maguire
the rate of direct racemization is much larger than that of acid hydrolysis
to a cis-[Cr(phen)2(H2O)2] 3+ product. In contrast, at pH ≥ 11, the rate
of optical activity loss matches closely that for base hydrolysis to a cis-
[Cr(phen)2(OH)2] + product,anditwassuggestedthatthelossofoptical
activity under basic conditions occurs primarily via the hydrolysis path. However,
these data did not preclude the possibility that a substantial fraction of
rotation loss might occur via direct racemization, provided there is significant
retention of optical configuration in the base hydrolysis reaction.
A recent paper demonstrates that chiral capillary electrophoresis (CE)
provides a very effective direct probe of the extent of racemization of
parent Λ-[Cr(phen)3] 3+ , while simultaneously determining the optical purity
of the hydrolysis product [49]. The electropherogram shown in Fig. 5
(electropherogram A) is for a mixture of rac-[Cr(phen)3] 3+ and cis-rac-
[Cr(phen)2(H2O)2] 3+ (where 50 mM dibenzoyl-l-tartrate was employed as
the capillary chiral additive), while the electropherogram for parent Λ-
[Cr(phen)3] 3+ prior to photolysis is provided in Fig. 5 (electropherogram B).
Fig. 5 Electropherograms of A an aqueous mixture of 1 mM rac-[Cr(phen)3] 3+ and 1 mM
cis-rac-[Cr(phen)2(H2O)2] 3+ ,andB a 1 mM aqueous solution of Λ-[Cr(phen)3] 3+ ,using
50 mM sodium dibenzoyl-l-tartrate as chiral additive in 25 mM borate buffer, pH 9.2
(20 ◦ C). Detection wavelength = 320 nm
The corresponding electropherograms for Λ-[Cr(phen)3] 3+ solutions irradiated
for 24 min at 350 nm at pH 2.2 and pH 11.6, respectively, are presented
in Fig. 6.
The pH 2.2 data (Fig. 6) show significant formation of ∆-[Cr(phen)3] 3+ ,
but no bands (in agreement with expectation) for cis-[Cr(phen)2(H2O)2] 3+
product. The corresponding pH 11.6 results (Fig. 6) are strikingly different.
ThereisnoevidencefordirectΛ-[Cr(phen)3] 3+ racemization, while extensive
hydrolysis is observed with no apparent retention of absolute configuration.
In a control experiment, no loss of optical activity was observed on irradiating
a ∆-cis-[Cr(phen)2(OH)2] + solution for 60 min at pH 11.6. These results
provide direct verification that optical rotation loss for Λ-[Cr(phen)3] 3+