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The mixed-valence complexes [(bpy)(2)(Cl)Os(III)(BL)Os(II)(Cl)(bpy)(2)](3+) and [(tpy)(bpy)Os(III)(BL)Os(II)(bpy)(tpy)](5+) (bpy is bipyridine; tpy is 2,2':6',2' '-terpyridine; BL is a bridging ligand, either 4,4'-bipyridine (4,4'-bpy) or pyrazine (pz)) have been prepared and studied by infrared and near-infrared measurements in different solvents. For BL = 4,4'-bpy, there is clear evidence for localized Os(II) and Os(III) oxidation states in the appearance of the expected two interconfigurational dpi --> dpi bands at Os(III) and additional, broad absorption features in the near-infrared arising from intervalence transfer (IT) transitions. For [(bpy)(2)(Cl)Os(pz)Os(Cl)(bpy)(2)](3+) and [(tpy)(bpy)Os(pz)Os(bpy)(tpy)](5+), unusually intense nu(pz) bands appear in the infrared at 1599 cm(-)(1) (epsilon = 2600 M(-)(1) cm(-)(1)) for the former and at 1594 cm(-)(1) (epsilon = 2020 M(-)(1) cm(-)(1)) for the latter. They provide an oxidation state marker and evidence for localized oxidation states. A series of bands appear in the near-infrared from 2500 to 8500 cm(-)(1) that can be assigned to a combination of interconfigurational dpi --> dpi and IT transitions. In CD(3)CN, in the mid-infrared, bands arising from nu(bpy) ring stretching modes from 1400 to 1500 cm(-)(1) are averaged for [(bpy)(2)(Cl)Os(pz)Os(Cl)(bpy)(2)](3+) or significantly perturbed for [(tpy)(bpy)Os(pz)Os(bpy)(tpy)](5+) compared to electronically isolated Os(II) and Os(III) complexes. The pyrazine-bridged complexes have properties that place them in a new class of mixed-valence molecules, Class II-III having properties associated with both Class II and Class III in the Robin and Day classification scheme. The characteristic features of this class are that oxidation states are localized because of vibrational coupling but that solvent orientational motions are uncoupled because of rapid intramolecular electron transfer.

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Spectra of the mixed-valence complexes [(bpy)2ClOs(L)Ru(NH3)5]4+ (L = 4,4‘-bipyridine, pyrazine; bpy = 2,2‘-bipyridine) are highly solvent dependent. They provide oxidation state specific spectral markers which show that oxidation states for L = 4,4‘-bpy are OsIII−RuII in solvents of donor number (DN) < 14. In solvents of high donor number (DN > 15) they are OsII−RuIII. The isomers coexist at donor numbers 14−15 and the distribution between them tuned in mixtures of acetonitrile and propylene carbonate demonstrating that intramolecular electron transfer can be induced by varying the solvent. Both OsIII−RuII and OsII−RuIII display broad, solvent-dependent intervalence transfer (IT) bands in the near infrared (NIR). There are two isomers of [(bpy)2ClOs(pz)Ru(NH3)5]4+ as well. In one, which is dominant in solvents of DN < 22, the oxidation states are [(bpy)2ClOsIII(pz)RuII(NH3)5]4+. This is shown by the appearance of dπ → dπ marker bands for OsIII (at 4200 and 5800 cm-1 in CD3CN), ν(bpy) OsIII bands in the mid-IR, and the pattern of UV−visible bands. For this isomer a narrow, structured IT band is observed at ∼8100 cm-1 which is nearly solvent independent in solvents of DN ≥ 11.9. The second isomer, [(bpy)2ClOsII(pz)RuIII(NH3)5]4+, is dominant at DN > 24. It has IT bands in the NIR which are broad and solvent dependent. It is concluded that there is significant through-pyrazine electronic coupling between OsIII and RuII in [(bpy)2ClOsIII(pz)RuII(NH3)5]4+, but localized oxidation states, and a residual solvent−vibrational barrier to intramolecular electron transfer continue to exist. The exchanging electron is in a dπ(RuII) orbital largely orthogonal to the bridge. In [(bpy)2ClOsII(pz)RuIII(NH3)5]4+ electronic coupling is much less. Its properties are consistent with those of Class II in the Robin and Day mixed-valence classification scheme. The two isomers coexist in trimethyl phosphate (DN = 23.0) and formamide (DN = 24.0). From the temperature dependence of the mixed-valence equilibrium in trimethyl phosphate, [(bpy)2ClOsIII(pz)RuII(NH3)5]4+ ⇌ [(bpy)2ClOsII(pz)RuIII(NH3)5]4+, K(297K) = 2.8, ΔH° = −8.2 ± 2 kcal/mol, and ΔS° = −27 ± 6 cal mol-1 deg-1. Because of the difference in the extent of electronic delocalization between the two isomers, intramolecular electronic transfer is a complex event involving coupled electronic and nuclear motions. There is no simple relationship between optical and thermal electron transfer.

The development of new DNA cleavage agents is of interest for the designing of synthetic restriction enzymes,"] for an understanding of the structure of DNA, ['] and for pharmaceutical application^.^^] We have recently directed our ef-forts at investigating functional metal complexes that initiate DNA cleavage by unusual mechanisms,141 and have reported efficient cleavage of DNA by the dicationic complex lL5I (bpy = 2,2'-bipyridine, tpy = terpyridine). 1 was generated via oxidation of 2 [Eqs. (a) and (b)J.["J [Ru"OHz(bpy)(tpy)12+ [Ru"'OH(bpy)(tpy)12+ + e-+ H + (a) 2 [Ru"'OH(bpy)(tpy)12+ [ R~'~O ( b p y ) ( t p y ) ]~+ + e -+ H + (b) 1The DNA cleavage with 1 can be effected either stoichiometrically by addition of 1 to the DNA or electrocatalytically by application of a potential of 0.8 V to a solution of DNA and 2.['I In attempts to increase the binding affinity of these agents, we have now prepared the dicationic complex 3 (dppz = dipyridophenazine, see Scheme 1).The planar dppz ligand imparts high DNA affinity to the complex 4."] Studies with topoisomerase showed that 4 unwinds DNA by 30°, strongly implicating an intercalative binding mode. Complex 4 has very attractive photophysical properties, but does not exhibit DNA cleavage reactivity. Complex 3 also exhibits a high DNA affinity while retaining the DNA cleavage reactivity of the Ru"OH,/RU'"O functionality. Furthermore, the X-ray crystal structure of this complex shows extensive x-stacking of the planar dppz ligand (see Fig. 3), which may provide insight into the high DNA affinity of dppz complexes.Reaction of Ru (tpy)Cl, with dppz in the presence of triethylamine affords the complex [RuCl(dppz)(tpy)]CI. Treatment of an aqueous solution of this complex with AgCIO, leads to the precipitation of 3-(C104), . H,O, in the form of black crystals suitable for X-ray structure analysis. The complex exhibits a typical Ru" --t py(x*) metal-ligand charge transfer (MLCT) absorption band with A,,,,, = 482 nm (E = 12000 M-lcm-'). In aqueous solution, the complex is oxidized sequentially to the dicationic Ru" complex [Eqs. (c) and (d)]. 3 [Ru'"OH(dppz)(tpy)]*+ + e -+ H' (C) [Ru"'(dppz)(tpy)12+ [ R~'~O ( d p p z ) ( t p y ) ]~+ + e -+ H + ( 4 5 At pH 7, E,,,(Ru~"/Ru") = 0.56 V and E,,,(Ru'"/Ru"') = 0.62 V (SCE), compared to 0.49 V and 0.62 V in the case of the complex 2.[61 The pK, value of the aqua ligand in 3 is 8.6 compared to a pK, of 9.7 in 2.16] The structure of 3 is shown in Figure 1 .[81 The coordination about the Ru center is very similar to that in both [R~"OH,(tmen)(tpy)]~+ (tmen = N,N,N',N',-tetramethylethylenediamine)lgl and 2." Thus, incorporation of the strongly intercalating dppz ligand has not altered the electronic properties of the Rul'OH, functionality that are vital to the DNA cleavage reactivity.Complex 3 binds very strongly to DNA. We have determined the binding constants for a series of complexes related to 2 using an electrochemical method based on the cyclic voltammetric currents for the Ru"'/Ru" redox pairs of the complexes with and without DNA.1...