Photosubstitution in Ru

TABLE 17.21 Wavelength dependence of the quantum yield of photosubstitution in [Ru(NH3)s 1] + as a function of the electron-withdrawing abilities of L. 

However, Reveco et al.181 observed a pronounced temperature dependence for the emission quantum yield in [Ru(bpy)2(NPP)]+ and concluded there to be an energy separation between emitting and deactivating states. The complete absence of photosubstitution in this complex led them to propose that the deactivating state was MLCT with significant singlet character, and not ligand field. Allen et al.182 had previously considered the possibility of this interpretation. 

The tris(diimine)Ru(II) complexes are inert to photosubstitution in RT aqueous solutions however at higher T, [RufbipyljP undergoes measurable photoaquation in strongly acidic solutions . Quantum yields for this are small (2 X 10 in 1 M aq HCl... 

The tris(diimine)Ru(II) complexes are inert to photosubstitution in RT aqueous... 

Arakawa and coworkers [45] developed the on-line photoreaction cell depicted in Figure 5.3 and performed a series of studies on the detection of reaction intermediates in photosubstitution and photooxidation of Ru(II) complexes. The photosubstitution of Ru(bpy)2B [bpy = 2,2 -bipyridine B = 3,3 -dimethyl-2,2 -bipyridine (dmbpy) or 2-(aminomethyl)pyridine (ampy)] was studied in acetonitrile and pyridine. Irradiation of Ru(bpy)2B and related complexes yields a charge-transfer excited species with an oxidized Ru center and an electron localization on the bpy moiety. The excited-state complex underwent ligand substitution via a stepwise mechanism that includes ant] bidentate ligand (Scheme 5.6). Photoproducts such as Ru(bpy)2S2 (S = solvent molecule) and intermediates with a monodentate (mono-hapfo-coordination) B ligand, Ru(bpy)2BS, and Ru(bpy)2BSX+ (X=C10/, PF ) were detected. Other studies also identified photo-oxidized products of several mixed-valence Ru(II) complexes upon irradiation (7i> 420 nm) [31b, 46]. 

A photophysical study of cis-[Ru(bipy)2L2] , where L = pyridine, pyridazine (35), 7V-methylimidazole (28), and related ligands is of relevance to photosubstitution at ruthenium(II) as well as to photoredox processes. Irradiation of an aqueous solution of [Ru(bipy)2Cl(NO)] at pH 5 leads to addition of OH" to the nitrosyl ligand, albeit with a low quantum yield. The mechanism of ligand substitution in [Ru(pc)(py)2] depends on the solvent (whether this is a potential ligand or not) substitution here is thought to be photoredox induced, via an Ru(I)(pc ) species. Photochemical studies on substitution in related diimine complexes include those at [Ru(LL)3] , where LL = biquinolyl (36), " or the pyrazole derivative (37). " Both complexes are considerably more photolabile than [Ru(bipy)3] the photodissociation excited state of the biquinolyl complex is easily populated at room temperature. On the other hand, the dinuclear complex [(H3N)4Ru(bipym)Ru(NH3)4) , bipym = bipyrimidine (38),... 

Styrylpyridine photochemistry has been important in one other system. The photoprocesses in the complexes W(CO)sX (X = pyridine, 2-styrylpyridine, and 4-styrylpyridine) have been investigated.129 Unlike the XRe(CO)3(taz s-styryl-pyridine)2 and Ru(II)-styrylpyridine complexes having lowest IL and CT states, respectively, the W(CO)5X complexes have lowest LF excited states with only a small contribution from W - pyridyl CT. The one-electron diagram for low-spin d6, Cnv complexes shown in Scheme 7 is appropriate here. Both photosubstitution and photoisomerization reactions are found for the W(CO)5X complexes and some quantum yield data are found in Table 23.129 The data show that substitution efficiencies for... 

Quantum yields have been measured for the photoaquation of a large range of substituted pyridine complexes of the type [Ru(NH3)5(pyX)]2+.53 The marked dependence of quantum yields on the nature of X indicates that metal-to-ligand-charge-transfer (MLCT) excited states are not involved in the photosubstitution. Presumably, a ligand-field excited state is responsible. Evidence has been reported for a simple outer-sphere reduction of cytochrome c by [Ru(NH3)6]2+.54 Such a... 

A particularly promising feature of the Ru(terpy)(phen)(L)2+ series, in relation to future molecular machine and motors, is related to the pronounced effect of steric factors on the photochemical reactivity of the complexes [84]. When the bulkiness of the spectator phenanthroline moiety was increased, the steric congestion of the coordination sphere of the ruthenium complex also increased. This increased congestion was qualitatively correlated to the enhanced photoreactivities of these complexes (Fig. 14). More specifically, changing phen for dmp increased by one to two orders of magnitude the quantum yield of the photosubstitution reaction of L by pyridine with L = dimethylsulfide or 2,6-dimethoxybenzonitrile. 

In the most congested case, (Ru(terpy )(dmp)(CH3SCH3)2+), the photosubstitution quantum yield was shown to be

room temperature in pyridine, which is an extremely high value in ruthenium(II) photochemistry. The control of the bulkiness of the spectator chelates, leading to the control of the congestion of the complex and, hence, to the efficiency of ligand photoexpulsion, is a specific feature of the Ru(terpy)(phen)(L)2+ core. This... 

Weber W, Ford PC (1986) Photosubstitution reactions of the ruthenium(II) arene complexes Ru(r 6-arene)L32+ (L = ammonia or water) in aqueous solution. Inorg Chem 25 1088-1092... 

The Ru(II) complexes of saturated amines, e.g., RuAjL, where Ru is in the 2+ or 3-1- oxidation state, A is NHj or similar amine such as ethylenediamine/2 and L is a charged or neutral ligand, or of diimines and Ru(AA)jL2, where Ru is in the 2+ oxidation state and AA is a diimine, such as 2,2 -bipyridine (bipy) or 1,10-phenanthroline (1,10-phen), are photosubstitution active. For example, irradiation of aq Ru(NH3) p in the ligand-field (LF) region leads principally to simple photoaquation ... 

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