Diimine complexes Ru

Transition metal-carbonyl-diimine complexes [Ru(E)(E ) (CO)2(a-diimine)] (E, E = halide, alkyl, benzyl, metal fragment a-diimine = 1, 4-diazabutadiene or 2,2 -bipyridine) are widely studied for their unconventional photochemical, photophysical, and electrochemical properties. These molecules have a great potential as luminophores, photosensitizers, and photoinitiators of radical reactions and represent a challenge to the understanding of excited-state dynamics. The near-UV/visible electronic spectroscopy of [Rn(X)(Me)(CO)2(/Pr-DAB)] (X = Cl or I iPr-DAB = A,A -di-isopropyl-l,4-diaza-l,3-butadiene) has been investigated throngh CASSCF/C ASPT2 and TD-DFT calculations on the model complexes [Ru(X)(Me)(CO)2(Me-DAB)] (X = Cl or I) (Table 2). 

There are more examples of a second type in which the chirality of the metal center is the result of the coordination of polydentate ligands. The easiest case is that of octahedral complexes with at least two achiral bidentate ligands coordinated to the metal ion. The prototype complex with chirality exclusively at the metal site is the octahedral tris-diimine ruthenium complex [Ru(diimine)3 with diimine = bipyridine or phenanthroline. As shown in Fig. 2 such a complex can exist in two enantiomeric forms named A and A [6,7]. The bidentate ligands are achiral and the stereoisomery results from the hehcal chirality of the coordination and the propeller shape of the complex. The absolute configuration is related to the handness of the hehx formed by the hgands when rotated... 

Ru(CN)jNO reactions with OH , SH and SOj" resemble those of the nitroprusside ion, with attack at the coordinated nitrosyl to give analogous transients and similar second-order rate constants. Ruthenium(II) complexes of the general type Ru(N2), Nj = biden-tate hgands, are important reactants. The relative inertness of Ru(NH3) + and Ru(diimine)f+ towards substitution makes these complexes definite, although weak, outer-sphere reductants (Tables 5.4, 5.5, 5.6 and 5.1). Ruthenium(ll) complexes of the general type Ru(diimine)f +, and particularly the complex Ru(bpy)j+, have unique excited state properties. They can be used as photosensitizers in the photochemical conversion of solar energy. Scheme 8.1 ... 

Leland and Powell also studied ECL obtained from reaction of [(bpy)3Ru]3+ with trialkylamines [47], Since the mechanism involves an electron transfer from the amine to Ru3+, there exists an inverse relationship between the first ionization potential of the amine and ECL intensity. The relative intensity of [(bpy)3Ru]2+ ECL was found to be ordered tertiary > secondary > primary. Quaternary ammonium ions and aromatic amines do not produce ECL with Ru(II) diimine complexes. Brune and Bobbitt subsequently reported the detection of amino acids by [(bpy)3Ru]2+ ECL [28,29], Employing capillary electrophoresis for separation, the presence of various amino acids can be detected directly by reaction with [(bpy)3Ru]3+ generated in situ with up to femtomo-lar sensitivity and with a selectivity for proline and leucine over other amino acids. The formation of an amine radical cation intermediate is characteristic of proposed mechanisms of both aliphatic amines and amino acids. 

From a band centered at 465 nm, typical of a Ru(diimine)2+ complex [96], a new spectrum is obtained by irradiation, which corresponds to a RuC12 (diimine)2 complex (/.max = 562 nm). As expected, the MLCT band for the photochemical product is strongly bathochromically shifted from that of the tris-diimine complex. The presence of a clean isosbestic point at 485 nm indicates that the photochemical reaction is selective and quantitative. This has been confirmed by JH NMR spectroscopy and by thin-layer chromatography. Only one photochemical product was detected and the starting complex 342+ has completely disappeared. Similar experiments have been performed with the rotaxane 352+. The reaction is represented in a schematic fashion in Fig. 20. 

Although ferryl intermediates of horseradish peroxidase and microperoxidase-8 have been produced in reactions with photogenerated [Ru(bpy)3]3+ [5], analogous experiments with P450s were unsuccessful, presumably due to the inefficiency of electron transfer from the buried heme active site through the protein backbone [6]. Photoactive molecular wires (sometimes referred to as metal-diimine wires, sensitizer-tethered substrates, or electron tunneling wires) were developed to circumvent this problem by providing a direct ET pathway between [Ru(bpy)3]3+ and the heme. These molecular wires, which combine the excellent photophysical properties of metal-diimine complexes... 

Fig. 2. The isosbestic points at 446 and 556 nm in the absorption spectra are matched by an isoemissive point at 685 nm indicating only two species present in solution, both of which are emissive. The shift in emission maximum from 606 nm in neutral solutions to 728 nm upon addition of acid may have interesting sensor applications. The results for 4 stand in contrast with results from dppz-containing Ru(II) tris diimine complexes, where dppz = dipyrido-ipyridophenazine, in which reversible protonation of quinoxaline N atoms leads to quenching of emission. Luminescence in frozen solvent glasses for 4 at 77 K is much stronger ( = 0.044 for the qdt complex), but still broad and without resolved structure.

A series of ruthenium (II) diimine complexes containing oxa-thiacrown derived from 1,10-phenanthroline have been synthesized and characterized <2007IC720>. The crystal stmctures of [Ru(bpy)2200](PF6)2, [Ru(bpy)2201](ClC>4)2, [Ru(bpy)2202](C104)2 have been determined. The luminescence properties of [Ru(bpy)2200](C104)2 were found to be sensitive and selective toward the presence of Hgz+ ions in an acetonitrile solution. 

As noted above, MLCT excited states of diimine metal complexes are both better reductants and oxidants than the ground-state species. This property has been exploited by Gray, Barton, and others for the study of proteins, DNA, and other biological molecules. Flash/quench experiments were first developed to provide a high driving force method to measure rates of electron transfer in proteins. In these experiments an excited diimine complex, typically a member of the Ru(bipy)2(diimine) + family of complexes, is oxidatively quenched with Ru(NH3)6 +, Co(NH3)5CP+, or methyl viologen, or reductively quenched with / -methoxy-A,A-dimethylaniline to yield a highly active redox species. 

Concerning the relaxation processes of Ru(II) tris(diimine) complexes whose lowest excited state is the MLCT state, a good linear relationship between the logarithm of the nonradiative decay rate ( a) and the energy gap ( )oo( MLCT)), the energy difference from the ground state to the MLCT state, is observed. This is termed as the Energy Gap Law (5,27). In a series of rhe-nium(I) bipyridine tricarbonyl complexes, such a linear correlation was also observed (Fig. 4) (26,27), where the net... 

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