Noel A.P. Kane-Maguire

Photoredox behavior and chiral discrimination of DNA bound M(diimine)3n+ complexes (M=Ru2+, Cr3+)

Abstract An overview is given of the rapidly developing field of bioinorganic chemistry involving the interaction of Ru(diimine)32+ complexes with double-stranded DNA. This contribution focusses on the photoredox behavior of these DNA bound systems and the presence of chiral discrimination in the binding process. Cr(diimine)33+ complexes are attractive new candidates for such studies in view of their long-lived room temperature luminescence and strong excited state oxidizing power. For these Cr(III) systems we observe strong quenching of the emission signal in the presence of B-DNA, and provide evidence that this quenching is associated with a photoredox process involving guanine base oxidation. We also demonstrate the value of capillary electrophoresis (CE) as a technique for assessing the relative binding constants and the stereoselectivities of transition metal interactions with DNA. In addition, we explore the use of CE as an alternative approach to help resolve literature disagreements between different Ru(phen)32+/DNA binding models.

Photochemistry and Photophysics of Coordination Compounds: Chromium

The study of the photochemistry and photophysics of octahedral and pseudo-octahedral Cr(III) complexes has a rich history. An initial discussion is devoted to a general appraisal of the state of these two subjects up to December 1998, after providing a framework of state energy levels and radiative and non-radiative relaxation processes relevant to Cr(III) systems. The remaining sections cover some of the more active areas in the Cr(III) field, such as ultrafast dynamics, photosubstitution, thermally activated excited state relaxation, energy transfer, and photoactivated redox processes (both intermolecular and intramolecular). Each of these sections begins with an overview of the subject area, and then one or more representative papers from the recent literature are selected for more detailed discussion.

Chiral separations of transition metal complexes using capillary zone electrophoresis

Several buffer additives that may facilitate chiral separation for optically active transition metal (TM) systems are investigated using capillary zone electrophoresis. The TM complexes evaluated exhibit considerable heterogeneity with respect to total complex charge (0 to 4+), ligand type, and identity of the central metal including Ru2+, Ni2+, Cr3+, and Co3+, threo-D[+]-Isocitrate, potassium antimonyl-d-tartrate and dibenzoyl-L-tartrate are identified as the most efficient chiral selectors. Interestingly, TM complexes exhibiting a (3+) total complex charge exhibit a reversal of enantiomer elution order versus all other complexes when separated using the tartrate additives. Operating parameters including pH, temperature, and capillary length are discussed, and chiral separations of complex mixtures are demonstrated.

The resolution of cis-[Ru(phen)2(CH3CN)2]2+ (phen = 1,10-phenanthroline), and its use in the synthesis of chiral cis-[Ru(phen)2X2]n+ species (n = 0, 2; X = CN−, Cl−, py)

Abstract A convenient general method is described for the isolation of optically active ruthenium(II) complexes, cis-[Ru(phen)2X2]n+. The resolution strategy involves the initial preparation of the Δ and Λ isomers of cis-[Ru(phen)2(CH3CN)2](PF6)2. These precursor compounds were characterized by UV-Vis, 1H NMR and CD spectral analysis, while enantiomeric purity was verified by conversion to optically active [Ru(phen)3]2+. Subsequent nucleophilic displacement of both coordinated CH3CN by X− (where X− = CN−, Cl−, py) proceeds with near complete retention of configuration. The isolation of the optically active neutral species cis-[Ru(phen)2(CN)2] and cis-[Ru(phen)2Cl2] is significant, since the traditional resolution method via diastereoisomer formation is not directly applicable. Furthermore, cis-[Ru(phen)2(CN)2] may serve as a valuable chiral building block for an extensive series of polymetallic complexes where CN− functions as a bridging ligand. The availability of authentic samples of resolved cis-[Ru(phen)2Cl2] is important for the quantitative assessment of stereoselectivity in the covalent binding of such racemic complexes with DNA.

Synthesis of cis-[Cr(diimine)2(1-methylimidazole)2]3+ Complexes and an Investigation of Their Interaction with Mononucleotides and Polynucleotides

A protocol is presented for the synthesis of chromium(III) complexes of the type cis-[Cr(diimine)2(1-methylimidazole)2](3+). These compounds exhibit large excited-state oxidizing powers and strong luminescence in solution. Emission is quenched by added guanine, yielding rate constants that track the driving force for guanine oxidation. The cis-[Cr(TMP)(DPPZ)(1-MeImid)2](3+) species binds strongly to duplex DNA with a preference for AT base sites in the minor groove and may serve as a precursor for photoactivated DNA covalent adduct formation.

Reversible intermolecular energy transfer between chromium(III) complexes in fluid solution

Abstract : Excitation energy transfer from Cr(en)3(3+), Cr(NH3)5(NCS)(2+), trans-CR(en)2(NCS)2(+), and cis-Cr(en)2(NCS)2(+) to Cr(CN)6(3-) is observed in room temperature solutions in water, dimethylsulfoxide, or dimethylformamide. The processes were observed by emission intensity measurements and, especially, by lifetime measurements on both the donor and the acceptor emission. The rate of energy transfer is essentially diffusion controlled in all cases and thus not very sensitive to the charge or ligation of the donor. In addition, evidence for reverse excitation energy transfer is presented; the lifetime of Cr(CN)6(3-) emission is dependent on the concentration of the Cr(III) ammine. It is shown that in this type of coupled system a full kinetic analysis is required to obtain correct bimolecular energy transfer rate constants as those obtained from conventional Stern-Volmer plots can be seriously in error. (Author)

Cyclic voltammetry of silyl- and stannyl-tricobalt cluster compounds

The electrochemical behavior of the clusters PhM′Co3(CO)n (M′ = Si, n = 11 and M′ = Sn, n = 12) has been examined via cyclic voltammetry in CH2Cl2 solution. In both cases the radical anions PhM′Co3(CO)n⨪ are unstable, rearranging to Co(CO)4 and other products. However, for the silicon compound, where there is one formal cobalt—cobalt bond, the radical anion is sufficiently stable to allow for detection of anion reoxidation at medium scan speeds. The half-life of PhSiCo3(CO)11⨪ has been calculated as 2.3 s at 25°C.