Bryan A. Albani

Unusually Efficient Pyridine Photodissociation from Ru(II) Complexes with Sterically Bulky Bidentate Ancillary Ligands

The introduction of steric bulk to the bidentate ligand in [Ru(tpy)(bpy)(py)]2+ (1; tpy = 2,2′:2′,6″-terpyridine; bpy = 2,2′-bipyridine; py = pyridine) to provide [Ru(tpy)(Me2bpy)(py)]2+ (2; Me2bpy = 6,6′-dimethyl-2,2′-bipyridine) and [Ru(tpy)(biq)(py)]2+ (3; biq = 2,2′-biquinoline) facilitates photoinduced dissociation of pyridine with visible light. Upon irradiation of 2 and 3 in CH3CN (λirr = 500 nm), ligand exchange occurs to produce the corresponding [Ru(tpy)(NN)(NCCH3)]2+ (NN = Me2bpy, biq) complex with quantum yields, Φ500, of 0.16(1) and 0.033(1) for 2 and 3, respectively. These values represent an increase in efficiency of the reaction by 2–3 orders of magnitude as compared to that of 1, Φ500 < 0.0001, under similar experimental conditions. The photolysis of 2 and 3 in H2O with low energy light to produce [Ru(tpy)(NN)(OH2)]2+ (NN = Me2bpy, biq) also proceeds rapidly (λirr > 590 nm). Complexes 1–3 are stable in the dark in both CH3CN and H2O under similar experimental conditions. X-ray crystal structures and theoretical calculations highlight significant distortion of the planes of the bidentate ligands in 2 and 3 relative to that of 1. The crystallographic dihedral angles defined by the bidentate ligand, Me2bpy in 2 and biq in 3, and the tpy ligand were determined to be 67.87° and 61.89°, respectively, whereas only a small distortion from the octahedral geometry is observed between bpy and tpy in 1, 83.34°. The steric bulk afforded by Me2bpy and biq also result in major distortions of the pyridine ligand in 2 and 3, respectively, relative to 1, which are believed to weaken its σ-bonding and π-back-bonding to the metal and play a crucial role in the efficiency of the photoinduced ligand exchange. The ability of 2 and 3 to undergo ligand exchange with λirr > 590 nm makes them potential candidates to build photochemotherapeutic agents for the delivery of drugs with pyridine binding groups.

Optimizing the Electronic Properties of Photoactive Anticancer Oxypyridine-Bridged Dirhodium(II,II) Complexes

A series of partial paddlewheel dirhodium compounds of general formula cis-[Rh2(xhp)2(CH3CN)n][BF4]2 (n = 5 or 6) were synthesized {xhp = 6-R-2-oxypyridine ligands, R = -CH3 (mhp), -F (fhp), -Cl (chp)}. X-ray crystallographic studies indicate the aforementioned compounds contain two cis-oriented bridging xhp ligands, with the remaining sites being coordinated by CH3CN ligands. The lability of the equatorial (eq) CH3CN groups in these complexes in solution is in the order -CH3 > -Cl > -F, in accord with the more electron rich bridging ligands exerting a stronger trans effect. In the case of cis-[Rh2(chp)2(CH3CN)6][BF4]2 (5), light irradiation enhances the production of the aqua adducts in which eq CH3CN is replaced by H2O molecules, whereas the formation of the aqua species for cis-[Rh2(fhp)2(CH3CN)6][BF4]2 (7) is only slightly increased by irradiation. The potential of both compounds to act as photochemotherapy agents was evaluated. A 16.4-fold increase in cytotoxicity against the HeLa cell line was observed for 5 upon 30 min irradiation (λ > 400 nm), in contrast to the nontoxic compound 7, which is in accord with the results from the photochemistry. Furthermore, the cell death mechanism induced by 5 was determined to be apoptosis. These results clearly demonstrate the importance of tuning the ligand field around the dimetal center to maximize the photoreactivity and achieve the best photodynamic action.

Steric and Electronic Factors Associated with the Photoinduced Ligand Exchange of Bidentate Ligands Coordinated to Ru(II)

In an effort to create a molecule that can absorb low energy visible or near‐infrared light for photochemotherapy (PCT), the new complexes [Ru(biq)2(dpb)](PF6)2 (1, biq = 2,2′‐biquinoline, dpb = 2,3‐bis(2‐pyridyl)benzoquinoxaline) and [(biq)2Ru(dpb)Re(CO)3Cl](PF6)2 (2) were synthesized and characterized. Complexes 1 and 2 were compared to [Ru(bpy)2(dpb)](PF6)2 (3, bpy = 2,2′‐bipyridine) and [Ru(biq)2(phen)](PF6)2 (4, phen = 1,10‐phenanthroline). Distortions around the metal and biq ligands were used to explain the exchange of one biq ligand in 4 upon irradiation. Complex 1, however, undergoes photoinduced dissociation of the dpb ligand rather than biq under analogous experimental conditions. Complex 3 is not photoactive, providing evidence that the biq ligands are crucial for ligand photodissociation in 1. The crystal structures of 1 and 4 are compared to explain the difference in photochemistry between the complexes. Complex 2 absorbs lower energy light than 1, but is photochemically inert although its crystal structure displays significant distortions. These results indicate that both the excited state electronic structure and steric bulk play key roles in bidentate photoinduced ligand dissociation. The present work also shows that it is possible to stabilize sterically hindered Ru(II) complexes by the addition of another metal, a property that may be useful for other applications.