Pyridinium acceptors

Despite considerable efforts, the formulation in equation (42) remains incomplete owing to the high reactivity of organocuprates as well as their oligomeric nature. Accordingly, we select organoborates as stable electron donors to study alkyl additions to various pyridinium acceptors (by thermal and photoinduced electron transfer) via charge-transfer salts as follows. 

Alkylation of pyridinium acceptors with organoborates as electron donors... 

No. Pyridinium acceptor (Py+) Eli (V versus SCE) Charge-transfer band (nm) BMe4 BMePh3 BPh4 ... 

We have confirmed that this state is a powerful reductant by an investigation of the electron transfer quenching of [Ir(p-pz)(COD)]jf by a series of pyridinium acceptors with varying reduction potentials (Figure 2 Table I). For acceptors with reduction potentials of t -1.5 to -1.9 V (vs. SSCE, CH3CN), the quenching rate constants range from 8 x 10 to 1 x 10 M ls l. The important point is... 

Similarly, the back electron transfer reactions involving alkylated pyridinium acceptors are very rapid and no net photochemistry is observed. 

A significant amount of work has been carried out on complexes that are analogous to 8 in that they contain pyridinium acceptors directly coordinated to a photoactive metal center [82-85]. As noted above, in these complexes electronic coupling between the metal center and the pyridinium acceptor is comparatively large, and as a result the dynamics of photoinduced forward and back ET are best considered by using excited state decay theory [86]. In any event, these complexes have figured prominently in the study of ET in metal-organic dyads and some of the important discoveries made with them are briefly reviewed in this section. The Re(I) complex 10a (Scheme 5) has been featured in much of this work... 

Several systematic studies of the driving force dependence of the rate of forward and back ET in type 1 dyads (see Fig. 1) were carried out during the past decade. As might be expected, the type 1 dyads used in these investigations consist of covalently linked assemblies of metal complexes and organic quenchers used in early studies of bimolecular photoinduced ET reactions. Thus, the type 1 dyads consist of polypyridine Ru(II) complexes linked to pyridinium acceptors such as paraquat and diquat (quatemized 2,2 -bipyridine). 

Scheme 9 illustrates the sequence of events that occur when these Ru(II)-pyridinium type 1 dyads (16) are photoexcited. Visible light excitation produces the MLCT excited state, 17. Forward ET occurs via transfer of an electron from the bipyridine acceptor ligand to the covalently linked pyridinium acceptor to produce charge separated state 18, which features a d5 Ru(III) ion linked to the reduced pyridinium acceptor. Finally, back ET occurs by transfer of the odd electron from the pyridinium radical to the Ru(III) center. 

Gray and co-workers examined the driving force dependence of ET in the Ir2 dyads (34) by synthesizing a series of complexes in which the reduction potential of the pyridinium acceptor is varied [106,107]. Scheme 16 illustrates the... 

Similar to other d -d systems, the drnuclear iridium(I) complex [Ir(/x-pz)(COD)]2 (23) showed spin-allowed and spin-forbidden (da — pa) absorption bands at 498 and 585 nm, respectively. Under ambient conditions, the complex displayed fluorescence at 564 nm and phosphorescence at 687 nm, which were assigned to singlet and triplet excited states of (da — pa) character. The triplet excited state of the complex was a powerful reductant with an excited-state reduction potential E° (Ir2+ ) of-1.81 V vs. SSCE. Facile electron transfer reactions occurred between the excited complex and methyl viologen and other pyridinium acceptors. The absence of an inverted effect for the forward electron transfer reactions, and the presence of such inverted behavior for the back-electron-transfer reactions were observed and explained. ... 

Our initial interest in these systems was stimulated by observations of their photochemical electron-transfer reactivity (6.12). From spectroscopic and electrochemical studies, the 3(da pa) excited state is predicted to be a powerful reductant, with E (M2 /3M2 ) estimated to range from -0.8 to -2.0 V vs SSCE in CH3CN. That this state is a powerful reductant has been confirmed by investigation of the electron-transfer quenching of 3M2 by a series of pyridinium acceptors with varying reduction potentials ( X For several binudear complexes, the excited-state reduction potenfial cannot be calculated accurately due to the irreversibility of the ground-state electrochemistry but it can be estimated from bimolecular electron-transfer quenching experiments. 

Since ionization potentials of anionic donors and electron affinities of cationic acceptors are not readily available, Mulliken correlations for charge-transfer ion pairs are generally presented in a modified form using electrochemical oxidation or reduction potentials, respectively. A typical example of such a modified Mulliken plot with unit slope is shown in Figure 2 for the CT ion pairs of TpMo(CO)3 [Tp = hydrido-trM-(3,5-dimethylpyrazolyl)borate] as the donor and various pyr-idinium acceptors [127]. Similar (modified) Mulliken correlations with unit slopes have been found for numerous other ion pairs with pyridinium acceptors and Mn(CO)s [126], Co(CO)4 [118], or V(CO)e [118] as donors. It is important that the Coulombic work term (co) in Eq. 8 is explicitly included in all Mulliken evaluations of ion pairs with different structures since co reflects the electrostatic energy of the (ground-state) ion pair which strongly depends on the inter-ionic distance [125]. 

Figure 2. Variation of the charge-transfer transition energy ( cr) with the reduction potential ( °red) of pyridinium acceptors in CT ion pairs with TpMo(CO)3 as the donor [127].

Tetraalkylborates are mild and selective alkylation reagents [186, 187], and they are commonly considered as sources of nucleophilic alkyl groups (R ) just as borohy-drides are depicted as hydride (H ) sources. However, since organoborates represent excellent electron donors (see Table 5, Section 2.2.1), the question arises as to what role electron donor-acceptor interactions play in the nucleophilic alkyl transfer. Phenyl- and alkyl-substituted borate ions form highly colored charge-transfer salts with a variety of cationic pyridinium acceptors [65], which represent ideal substrates to probe the methyl-transfer mechanisms. Most pyridinium borate salts are quite stable in crystalline form (see for example Figure 5C), but decompose rapidly when dissolved in tetrahydrofuran to yield methylated hydropyridines (Eq. 65). 

Charge-transfer activation of the charge-transfer salts effects a spontaneous electron transfer [18] from the borate donor to the pyridinium acceptor which results in the formation of a radical pair (Eq. 67). 

The greatest increases in (3zzz values are associated with tlie displacement (R ) of the anion from the pyridinium acceptor. To understand the origin of this increment. 

The structure of donor-acceptor porphyrin complexes such as 68 has very recently been modified to make a new family of amphiphilic porphyrin dyes with polar pyridinium acceptor head groups and hydrophobic dialkyl-aniline donors (Figure 1.34). The free porphyrins and... 

The pyridinium acceptor is covalently attached to the dithiolate donor which provides a rr-electron for the ILCT transition. 

Molecular dyads of ruthenium(ii)- or osmium(ii)-bis(terpyridine) chromophores and expanded pyridinium acceptors have been used to demonstrate the effect of the bridge and the metal ions to the photophysical properties of linear systems. In particular, via ultrafast transient absorption spectroscopy, an equilibration between MLCT and photo-induced charge-separated excited states has been observed demonstrating that intramolecular photoinduced electron transfers can occur within multicomponent systems in spite of driving forces virtually approaching zero. ... 

Table 8 Bimolecular Quenching Rate Constants for the Oxidadve Quenching Reactions of 25b, 30, and 31 with Pyridinium Acceptors...

Similar low-energy absorption bands have also been observed for other polynuclear copper(I) acetylide complexes such as 24b [102], 24e [111], 25b [103], 26a [102], 27c [108], and 31 [106] with different pyridinium acceptors. 

Besides, the photoexcited complex has also been found to react with a series of pyridinium acceptors such as MV [129]. The electron transfer nature of the photoreaction mechanism has been established by the appearance of the characteristic MV cation radical absorption in the transient absorption difference spectrum. The reaction has been shown to be reversible with a back-electron transfer rate constant of 1.5 x 10 dm mol" s" . From the oxidative quenching experiments with a series of structurally related pyridinium acceptors, an excited state reduction potential of [Au2 /AU ] of -1.6(1) V vs. SSCE [/fT In KV = 0.58(10) V vs. SSCE, = 0.9(K10) eV] has been estimated by three-parameter, nonlinear, least-squares fits to the equation ... 

The phosphorescent states of the polynuclear gold(I) complexes 50a, b and 51a, b have also been found to react with MV [132,133]. The quenching mechanism has been found to be electron transfer in nature. The excited state reduction potentials of these polynuclear gold(I) phosphine complexes have also been estimated by oxidative quenching experiments with a series of structurally related pyridinium acceptors. For example, the bimolecular quenching rate constants for the photoreactions between 50b and the pyridinium acceptors are listed in Table 14. A plot of iRT/F) n fc, vs. Eu2 values of the pyridinium ions is displayed in Fig. 10. The excited state reduction potentials, RT In Xkv values. 

Table 15 Excited State Reduction Potentials, RT In Kks/ Values, and Reoiganization Energies for the Electron Transfer Reactions of 50a-50b, 51a-51b with Pyridinium Acceptors...

The photoredox properties of 54b have also been studied by oxidative quenching experiments wiA a series of structurally related pyridinium acceptors [138]. The reorganization energy for the electron transfer reaction has been determined to be 1.66(7) eV. The outer-sphere contribution is estimated to be 0.75 eV. The remaining 0.91 eV suggests that there are substantial inner-sphere changes associated with the formation of Au(I)Au(Il) species. This is in line with the observation of a large Stokes shift. 

The triplet state also is a strong photoreductant in its reactions with alkylated pyridinium acceptors in acetonitrile solution. As predicted from outer-sphere electron transfer theory, a plot of / rin (Aq) versus (A /A) is linear for quenchers with values of (AVA) that are less than, or approximately equal to, the value of for the IrJ/Irz couple. The plotted data for this fit range from... 

Zhu, D. and Kochi, J.K., Alkylation of pyridinium acceptors via thermal and photoinduced electron transfer in charge-transfer salts with organoborates, Organometallics, 18,161, 1999. 

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