2 research outputs found
Iron Complexes of Square Planar Tetradentate Polypyridyl-Type Ligands as Catalysts for Water Oxidation
The
tetradentate ligand, 2-(pyrid-2′-yl)-8-(1″,10″-phenanthrolin-2″-yl)-quinoline
(ppq) embodies a quaterpyridine backbone but with the quinoline C8
providing an additional sp<sup>2</sup> center separating the two bipyridine-like
subunits. Thus, the four pyridine rings of ppq present a neutral,
square planar host that is well suited to first-row transition metals.
When reacted with FeCl<sub>3</sub>, a μ-oxo-bridged dimer is
formed having a water bound to an axial metal site. A similar metal-binding
environment is presented by a bis-phenanthroline amine (dpa) which
forms a 1:1 complex with FeCl<sub>3</sub>. Both structures are verified
by X-ray analysis. While the Fe<sup>III</sup>(dpa) complex shows two
reversible one-electron oxidation waves, the Fe<sup>III</sup>(ppq)
complex shows a clear two-electron oxidation associated with the process
H<sub>2</sub>O–Fe<sup>III</sup>Fe<sup>III</sup> → H<sub>2</sub>O–Fe<sup>IV</sup>Fe<sup>IV</sup> → OFe<sup>V</sup>Fe<sup>III</sup>. Subsequent disproportionation to an FeO
species is suggested. When the Fe<sup>III</sup>(ppq) complex is exposed
to a large excess of the sacrificial electron-acceptor ceric ammonium
nitrate at pH 1, copious amounts of oxygen are evolved immediately
with a turnover frequency (TOF) = 7920 h<sup>–1</sup>. Under
the same conditions the mononuclear Fe<sup>III</sup>(dpa) complex
also evolves oxygen with TOF = 842 h<sup>−1</sup>
New Tridentate Ligand Affords a Long-Lived <sup>3</sup>MLCT Excited State in a Ru(II) Complex: DNA Photocleavage and <sup>1</sup>O<sub>2</sub> Production
Two new complexes, [RuÂ(tpy)Â(qdppz)]Â(PF6)2 (1; qdppz = 2-(quinolin-8-yl)ÂdipyridoÂ[3,2-a:2′,3′-c]Âphenazine, tpy = 2,2′:6′,2″-terpyridine)
and [RuÂ(qdppz)2]Â(PF6)2 (2), were investigated for their potential use as phototherapeutic
agents through their ability to photosensitize the production of singlet
oxygen, 1O2, upon irradiation with visible light.
The complexes exhibit strong RuÂ(dÏ€) → qdppzÂ(Ï€*)
metal-to-ligand charge transfer (MLCT) absorption with maxima at 485
and 495 nm for 1 and 2 in acetone, respectively,
red-shifted from the RuÂ(dÏ€) → tpyÂ(Ï€*) absorption
at 470 nm observed for [RuÂ(tpy)2]2+ (3) in the same solvent. Complexes 1 and 3 are not luminescent at room temperature, but 3MLCT emission
is observed for 2 with maximum at 690 nm (λexc = 480 nm) in acetone. The lifetimes of the 3MLCT states of 1 and 2 were measured using
transient absorption spectroscopy to be ∼9 and 310 ns in methanol,
respectively, at room temperature (λexc = 490 nm).
The bite angle of the qdppz ligand is closer to octahedral geometry
than that of tpy, resulting in the longer lifetime of 2 as compared to those of 1 and 3. Arrhenius
treatment of the temperature dependence of the luminescence results
in similar activation energies, Ea, from
the 3MLCT to the 3LF (ligand-field) state for
the two complexes, 2520 cm–1 in 1 and
2400 cm–1 in 2. However, the pre-exponential
factors differ by approximately two orders of magnitude, 2.3 ×
1013 s–1 for 1 and 1.4 ×
1011 s–1 for 2, which, together
with differences in the Huang–Rhys factors, lead to markedly
different 3MLCT lifetimes. Although both 1 and 2 intercalate between the DNA bases, only 2 is able to photocleave DNA owing to its 1O2 production upon irradiation with ΦΔ = 0.69. The present work highlights the profound effect of the ligand
bite angle on the electronic structure, providing guidelines for extending
the lifetime of 3MLCT RuÂ(II) complexes with tridentate
ligands, a desired property for a number of applications