Photochemistry and Photophysics of Coordination Compounds

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60 N.A.P. Kane-Maguire Fig. 19 Representative trans-[Cr(macrocycle)(ONO)2] + complexes The only product of 436 nm continuous photolysis of I in deaerated pH 7 aqueous solution was trans-[Cr(cyclam)(H2O)(ONO)] 2+ ,formedinasubstitution step in only low quantum yield (φaq = 0.009). However, when the same photolysis was performed in air-saturated solution, a very different product was formed in much higher yield (φO2 = 0.27). On the basis of mass spectral and EPR evidence, this Cr final product was formulated as the oxo-Cr(V) species, trans-[Cr(cyclam)(O)(ONO)] 2+ . In addition, NO gas release was confirmed employing an NO-specific electrode sensor. Transient absorption spectral studies indicated that NO release occurred in a rapidly reversible earlier step (see Scheme 2) involving homolytic cleavage of coordinated nitrite ion in I to yield the transitory oxo-Cr(IV) product, trans- [Cr(cyclam)(O)(ONO)] + . Scheme 2 Photoinitiated reaction scheme for trans-[Cr(cyclam)(ONO)2] + In deaerated solution, the transient rapidly reforms the parent complex, resulting in simple photoaquation being the only observed net reaction. Under air-saturated conditions, however, the transient species is very rapidly scavenged by dissolved O2 to generate the oxo-Cr(V) final product, leading to the net release of NO gas. The overall mechanism is summarized in Scheme 2. In an effort to utilize this reaction scheme in a more practical NO delivery system, the Ford group subsequently synthesized a series of complexes where
Photochemistry and Photophysics of Coordination Compounds: Chromium 61 Fig. 20 Mechanism of NO release following light absorption by a pendant aromatic antennae strongly absorbing pendant aromatic chromophores were attached to the macrocyclic ring [128–130]. Two of these second-generation Cr(III)nitrito systems are shown in Fig. 19, where molecules II and III have anthracenyl and pyrenyl pendant arms, respectively. The normally strong fluorescence of these tethered aromatics was largely quenched upon Cr(III) coordination, consistent with fast intramolecular energy transfer to the lower lying Cr(III) ligand field excited states followed by NO gas release according to Scheme 2 [128]. The overall NO-generating process for these promising lightgathering antennae systems is depicted in Fig. 20 for the pyrenyl-pendant complex (molecule III). 8.2 Photogeneration of Nitrido Complexes from Cr(III) Coordinated Azide The complex [Cr(NH3)5N3] 2+ containing the azido ligand, N3 – ,wasthesubject of several photochemical studies during the 1970s [131–134]. The results from irradiations in the LMCT region (λ ≤ 330 nm)identifiedthepresenceof two competing processes, involving the formation of azide radical, N3 · and nitrene, N – , intermediates (Eqs. 3 and 4, respectively) [132–134]: [CrIII (NH3)5N3] 2+ + hν → [CrII (NH3)5(N3·)] 2+ → 1.5N2 [CrIII (NH3)5N3] 2+ + hν → [CrIII (NH3)5N] 2+ +N2 Product analysis was complicated by the thermal reactions of the radical species generated. However, based in part on the differences expected in the N2 gas yields for the two processes, Katz and Gafney [132, 134] concluded that initial nitrene formation was the dominant reaction pathway. Although the fi- (3) (4)
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280 Topics in Current Chemistry Edi
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Photochemistry and Photophysics of
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Volume Editors Prof. Vincenzo Balza
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Topics in Current Chemistry Also Av
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X Preface nation Compounds. The ven
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Contents Photochemistry and Photoph
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258 Author Index Volumes 251-280 Be
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Subject Index Alkyne bridges 148 An
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Subject Index 273 Rh(III) cyclometa
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Photochemistry and Photophysics of Coordination Compounds

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