Photochemistry and Photophysics of Coordination Compounds

62 N.A.P. Kane-Maguire
nal fate of the nitrene intermediate was not established, Sriram and Endicott
noted that quasi-thermodynamic calculations suggested the lowest energy
product ground state for this system would be a Cr(V) species [133].
More recent photochemical investigations on azido complexes of Cr(III)
have focused on systems containing tetradentate ligands such as N2O4 Schiffbases
[135] and N3 or N4 macrocyclic ligands [136, 137] as stable non-leaving
groups. For many of these systems, air-stable, solid products have been isolated,
and have been fully characterized by a variety of spectroscopic probes
(including X-ray crystallography). These studies provide convincing evidence
for the formation of stable Cr(V) complexes containing the nitrido ligand,
N3– , formed via the generic photoreaction shown in Eq. 5:
[CrIII – N3] 2+ + hν → [CrV ≡ N] 2+ +N2
(5)
Evidence for a Cr(V) product is based on the diagnostic EPR signature displayed
by this d1 metal ion. The presence of a Cr ≡ Ntriplebondinthe
product is also in accord with the short Cr – N bond distances observed in Xray
crystallographic studies, and the presence of a strong infrared absorption
in the 1020–1150 cm –1 region [135–138].
During the present review period, the charge transfer photochemistry of
several azido-Cr(III) complexes containing new Schiff-base ligands as the
non-leaving groups were examined [139, 140]. In the first of these papers,
no crystallographic evidence was presented for Cr(V)-nitrido product formation,
but this product assignment was strongly supported by EPR and infrared
spectral results [139]. In the second contribution, a potentially valuable
biochemical application of azido-Cr(III) photochemistry is reported by Shrivastava
and Nair [140]. An azido-Cr(III) Schiff-base complex was irradiated
in the presence of bovine serum albumin (BSA), and the photolyte examined
by sodium dodecyl sulfate-polyacrylamide disc electrophoresis (SDS-PAGE).
The SDS-PAGE results revealed that the BSA protein was cleaved at multiple
sites, non-specifically into smaller peptide fragments. The protein cleavage
was attributed to the azido-Cr(III) complex binding at multiple sites, and being
subsequently converted to the reactive nitrido-Cr(V) species upon light
activation. This light-promoted protease activity bears some analogies with
the photonuclease activity discussed earlier for [Cr(diimine)3] 3+ interactions
with DNA (Sect. 7.1). A non-selective photonuclease could be utilized in
a variety of applications, including protein sequencing.
9
Final Comments
In this chapter, the author has attempted to provide an overview of recent
progress in the field of Cr(III) photochemistry and photophysics, with a more
detailed focus on certain topics of interest. For cohesiveness, it was not pos-