Ligands centered reduction

Larger dendrimers based on a Ru(bpy)2+ core and containing up to 54 peripheral methylester units (12) have recently been obtained [29a]. Both the metal-centered oxidation and ligand-centered reduction processes become less reversible on increasing dendrimer size [29b]. 

Spectral modulation in the NIR region has been reported for the complex [Ru(L8)3]2+ which undergoes six ligand-centered reductions, two per ligand.17 It initially shows no absorption... 

Combination and rearrangement of Eqs. (16) and (23) lead to expression (24) that correlates linearly the ligand-centered reduction potential with the metal centered oxidation potential (d is the denticity of the reducible LL ligand). The slope of this line is Yl/Ym, and the intercept is a constant for the particular reducible ligand [66]. 

Relationships between redox potentials and the energy hv) of a metal-to-ligand charge transfer (M LCT) band have been well documented and expressed, for complexes [M(LL)WXYZ], in a simplified way, by Eq. (25) in which C is a constant and A (Redox) is given by Eq. (26), that is, the difference between the metal centered oxidation potential and the ligand centered reduction potential [67]. 

Ru(bpy)3]2 +, in which bpy is 2,2 -bipyridine ligand. This compound is thermodynamically stable, kinetically inert, and shows outstanding electrochemical52 and photochemical510 properties. It exhibits a metal-centered reversible oxidation (below 2 V versus SCE) process in MeCN at room temperature and six distinct reversible ligand-centered reduction processes in dimethylformamide at 219 K.53... 

These carboxylate ions are weaker counterions than CP and Pp, so that the corresponding dendrimers tend to ionize more easily and to give clearer signals in MALDI-TOF mass spectra. As to the electrochemical properties, their cyclic voltammetries show a reversible metal centered oxidation and two reversible ligand-centered reduction processes at potential values very similar to those of the corresponding dendrimers with CP counterions. Therefore, the [Ru(tpy)2]2+ complexes are electrochemically equivalent and can efficiently store charges. 

The tris-bidentate complexes, [Os(phen)2(das)]2+, [Os(bpy)2(das)]2+, [Os(phen)2(dpae)]2+, [Os(phen)(das)2]2+, and [Os(bpy)(das)2]2+, are prepared by refluxing cis-[Os(N-N)2Cl2] or [Os(N-N)C14] with the appropriate diarsine in ethylene glycol. These complexes exhibit reversible Os(III/II) oxidations and a reversible reduction that is centred at the phen or bpy ligand. In the case of [Os(bpy)2(das)]2+, a second ligand-centered reduction is observed in acetonitrile. The electronic absorption and emission spectra of these complexes have also been reported (97). [Os(bpy)(das)2]2+ has been used as an effective sensitizer in a photoelec-trochemical half-cell (312). 

No ligand centered reduction has been observed in a cathodic region up to — 2.0 V (vs SCE) the HOMO-LUMO energy separation in corrole resulted then to be larger than 3.3 V, i.e. much greater than that present in porphyrins. This is consistent with the lower skeletal symmetry of the corrole structure with respect to porphyrin which is expected to increase the separation between HOMO and LUMO [46]. 

The [Fe(terpy)2] cation is low-spin, as demonstrated by Mossbauer, electronic, H NMR, and resonance Raman spectroscopy and magnetic measurements (20,184,187, 228, 266). Similarly, spectroscopic studies of the iron(III) cation have indicated a low-spin ( B) ground term (382). There have been numerous electrochemical studies of the bis complexes (177, 200, 256, 298, 332, 344, 373, 378, 379, 397, 398). Ligand-centered reductions to formal oxidation states of iron(I), iron(O), and iron( — 1) and oxidations to iron(III) are observed. The complex [Fe(terpy)L][C104]2 [L = tris(2 -pyridyl)l,3,5-triazine (Fig. 15)] has been prepared (399, 442). 

Although the uncharged tris(3,5-di-t-butylcatecholate) complex of iron [Fe(DTBC)3] has been extensively studied, " the proposed bonding in these reports is unclear. The most common formulation is as an ionic salt between iron(lll) and three semiquinone anion radicals, Fe +(DTBSQ 03 However, the magnetic moment is 2.9 BM (consistent with an 5 = 2/2 spin state) and the electrochemistry indicates a ligand-centered reduction. Both of these characteristics are analogous to ferrate dianion. 

A cobalt complex 8 containing a redox active tetradentate bis-iminopyridine framework has been reported to support a water reduction catalyst, with activities of observed rate constant, of 10 M s derived from voltammetry measurements (Scheme 3) [16]. Ligand-centered reduction of the coordinated imine function has been proposed as the first electrocatalytic step followed by protonation. Notably, this compound was shown to operate even under basic conditions at pH 8 (buffer) to give 10 liter of H (mol catalyst" h" ) albeit with a modest Faradaic efficiency of only 60%. 

Scheme 4 Ligand-centered reduction in electrocatalytic proton reduction mediated by a nickel(II) bisaryliminopyridine electrocatalyst...

The various model compounds also exhibited characteristic optical absorption changes upon reduction of Ni(II) to Ni(I) [62, 64, 65, 68, 73]. A detailed discussion is beyond the scope of this review, but it is worth noting that optical absorption spectra (as can EPR) can easily distinguish between metal- and ligand-centered reductions [64]. 

Transition metal bipyridineand terpyridine complexes were used as redox reporter groups for anions. The bimetallic receptor 27 was found to respond to chloride anions in CH3CN, as evidenced by a cathodic shift of — I lOmV in the ligand-centered reduction couple of the two -substituted bipyridine groups. 

The Lever model has typically been applied to octahedral-type six-coordinate complexes, with metal-centered redox processes, but extensions to other types of complexes have also been proposed, namely, to square-planar four-coordinate and five-coordinate Rh /Rh complexes [39, 76, 77], to sandwich complexes [23, 24, 31, 32], to Ru clusters [78-80], and also to complexes with ligand-centered reduction processes [24]. Relevant or representative cases are discussed in the following sections. 

Similar Ru(II) polypyridine units were connected to dirhodium(ll) tetracarboxylate platforms to form the supramolecular assemblies 80-82 (Figures 9.37 and 9.38) [166], exhibiting severed metal-centered oxidation and ligand-centered reduction processes, and intense LC and MLCT bands centered on the Ru(II) units. Efficient energy transfer from MLCT (Ru-based) to the lowest-energy... 

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