Stanislav Záliš

Ultrafast excited-state dynamics of rhenium(I) photosensitizers [Re(Cl)(CO)3(N,N)] and [Re(imidazole)(CO)3(N,N)]+: diimine effects.

Femto- to picosecond excited-state dynamics of the complexes [Re(L)(CO)(3)(N,N)](n) (N,N = bpy, phen, 4,7-dimethyl-phen (dmp); L = Cl, n = 0; L = imidazole, n = 1+) were investigated using fluorescence up-conversion, transient absorption in the 650-285 nm range (using broad-band UV probe pulses around 300 nm) and picosecond time-resolved IR (TRIR) spectroscopy in the region of CO stretching vibrations. Optically populated singlet charge-transfer (CT) state(s) undergo femtosecond intersystem crossing to at least two hot triplet states with a rate that is faster in Cl (∼100 fs)(-1) than in imidazole (∼150 fs)(-1) complexes but essentially independent of the N,N ligand. TRIR spectra indicate the presence of two long-lived triplet states that are populated simultaneously and equilibrate in a few picoseconds. The minor state accounts for less than 20% of the relaxed excited population. UV-vis transient spectra were assigned using open-shell time-dependent density functional theory calculations on the lowest triplet CT state. Visible excited-state absorption originates mostly from mixed L;N,N(•-) → Re(II) ligand-to-metal CT transitions. Excited bpy complexes show the characteristic sharp near-UV band (Cl, 373 nm; imH, 365 nm) due to two predominantly ππ*(bpy(•-)) transitions. For phen and dmp, the UV excited-state absorption occurs at ∼305 nm, originating from a series of mixed ππ* and Re → CO;N,N(•-) MLCT transitions. UV-vis transient absorption features exhibit small intensity- and band-shape changes occurring with several lifetimes in the 1-5 ps range, while TRIR bands show small intensity changes (≤5 ps) and shifts (∼1 and 6-10 ps) to higher wavenumbers. These spectral changes are attributable to convoluted electronic and vibrational relaxation steps and equilibration between the two lowest triplets. Still slower changes (≥15 ps), manifested mostly by the excited-state UV band, probably involve local-solvent restructuring. Implications of the observed excited-state behavior for the development and use of Re-based sensitizers and probes are discussed.

Boron Atoms as Spin Carriers in Two‐ and Three‐Dimensional Systems

Paramagnetic compounds with at least partially boron-centered electron spin can be constructed using either the prototypically electron-accepting boron atoms bridged by planar pi-conjugated organic systems, or by taking advantage of the three-dimensional delocalized bonding in oligonuclear borane, haloborane, or carborane clusters. The concept of mixed valency can thus be transferred from organic and transition-metal chemistry to main-group-element molecules, and density functional theory is capable of reproducing the variable spin distribution.

Oxygen Reduction Catalyzed by a Fluorinated Tetraphenylporphyrin Free Base at Liquid/Liquid Interfaces

The diprotonated form of a fluorinated free base porphyrin, namely 5-(p-aminophenyl)-10,15,20-tris(pentafluorophenyl)porphyrin (H(2)FAP), can catalyze the reduction of oxygen by a weak electron donor, namely ferrocene (Fc). At a water/1,2-dichloroethane interface, the interfacial formation of H(4)FAP(2+) is observed by UV-vis spectroscopy and ion-transfer voltammetry, due to the double protonation of H(2)FAP at the imino nitrogen atoms in the tetrapyrrole ring. H(4)FAP(2+) is shown to bind oxygen, and the complex in the organic phase can easily be reduced by Fc to produce hydrogen peroxide as studied by two-phase reactions with the Galvani potential difference between the two phases being controlled by the partition of a common ion. Spectrophotometric measurements performed in 1,2-dichloroethane solutions clearly evidence that reduction of oxygen by Fc catalyzed by H(4)FAP(2+) only occurs in the presence of the tetrakis(pentafluorophenyl)borate (TB(-)) counteranion in the organic phase. Finally, ab initio computations support the catalytic activation of H(4)FAP(2+) on oxygen.

The semiquinone-ruthenium combination as a remarkably invariant feature in the redox and substitution series [Ru(Q)(n)(acac)(3-n)](m), n = 1-3; m = (-2), -1, 0, +1, (+2); Q = 4,6-Di-tert-butyl-N-phenyl-o-iminobenzoquinone.

Three new compounds, [Ru(Q(*-))(acac)(2)] = 1, [Ru(Q(*-))(2)(acac)] = 2, and [Ru(Q(*-))(3)] = 3, were obtained and characterized as Ru(III) complexes with 4,6-di-tert-butyl-N-phenyl-o-iminobenzosemiquinone (Q(*-)) ligands. All three systems show multiple electron transfer behavior, which was analyzed using electron paramagnetic resonance (EPR) and UV-vis-near-infrared (NIR) spectroelectrochemistry. (1)H NMR spectroscopy and a crystal structure analysis suggest antiferromagnetically spin-spin coupled Ru(III) and Q(*-) in 1, similar to that in the related compound 4 with unsubstituted o-iminobenzosemiquinone. However, in contrast to 4(n) (Remenyi, C.; Kaupp, M. J. Am. Chem. Soc. 2005, 127, 11399), the system 1(m) exhibits unambiguously metal-centered electron transfer, producing ions [Ru(IV)(Q(*-))(acac)(2)](+) = 1(+) and [Ru(II)(Q(*-))(acac)(2)](-) = 1(-), both with EPR-evidenced ligand-based spin, as also supported by DFT calculations. Compared with the related redox system [Ru(Q)(bpy)(2)](k) (5(k)) (k = 0-3), the spectroelectrochemical similarity suggests corresponding electronic structures except for the 1(+)/5(3+) pair (Ru(IV)(Q(*-))(acac)(2)](+) (1(+)) versus [Ru(III)(Q(0))(bpy)(2)](3+) (5(3+))). Compound 2, a three-spin system [Ru(III)(Q(*-))(2)(acac)] obtained in the all-cis configuration, possesses a complicated magnetic behavior including strong intramolecular antiferromagnetic coupling (J(Ru-Q), on the order of -10(3) cm(-1) and J(Q-Q), -10(2) cm(-1)) and weak intermolecular antiferromagnetic and ferromagnetic interactions. Strong intramolecular coupling leads to one unpaired electron at low temperatures, as also supported by the radical-type EPR signal of the solid and of solutions, which diminishes at higher temperatures. The up-down-up spin arrangement for the ground state of {(Q(*-))-Ru(III)-(Q(*-))} (S = 1/2) is confirmed by DFT calculations for 2. Oxidation to 2(+) leaves the UV-vis-NIR spectrum almost unchanged, whereas reduction to 2(-) and 2(2-) produces low-energy absorptions. The ligand-centered spin for 2(2-) = [Ru(II)(Q(*-))(Q(2-))(acac)](2-) suggests the [Ru(II)(Q(*-))(2)(acac)](-) formulation for 2(-). Compound 3, obtained as a structurally characterized mer isomer, has a predominantly ligand-centered highest occupied molecular orbital (HOMO), as evident from the EPR signal of the intermediate 3(+) and as supported by DFT calculations. In contrast, electron addition proceeds to yield a metal/ligand mixed spin intermediate 3(-) according to EPR, in agreement with ca. 25% calculated metal character of the lowest unoccupied molecular orbital (LUMO). The near-infrared absorption of 3 at 1280 nm corresponds to the HOMO-LUMO transition (ligand-to-metal/ligand-to-ligand charge transfer). Oxidation to 3(+) produces a weak broad band at about 2500 nm, while the reduction to 3(-) gives rise to an intense absorption feature at 816 nm. The valence state alternatives are being discussed for all spectroelectrochemically accessible species, and the individual results are compared across this unique substitution and redox series involving a highly noninnocent ligand/metal combination. All established oxidation state formulations involve the iminosemiquinone-ruthenium entity, illustrating the remarkable stability of that arrangement, which corroborates the use of this combination in water oxidation catalysis.

Copper complexes with non-innocent ligands: probing CuII/catecholato-CuI/o-semiquinonato redox isomer equilibria with EPR spectroscopy

Abstract The temperature-dependent intramolecular metal/ligand electron transfer equilibrium (L)Cu II (Q 2− )⇌(L)Cu I (Q − ) has been quantitatively analyzed by EPR as a function of the non-innocent o -quinonoid ligand Q and of the co-ligand L. Suitable co-ligands were 1-methyl-(2-methylthiomethyl)-1 H -benzimidazole and 1-diphenylphosphino-2-dimethylaminoethane (dde) which can tolerate both copper oxidation states. EPR hyperfine data were extracted for a variety of alkyl- and methoxy-substituted o -semiquinones and their copper(I) complexes in solution. A close match of Q and Cu redox orbitals is essential for observing the redox isomer equilibrium which has been similarly reported for aminoxidase enzymes. Efforts to isolate the mixed-ligand compounds from solution failed due to coordination disproportionation. The new [Cu(dde) 2 ](ClO 4 ) was structurally characterized with comparable CuN and CuP distances of 2.24–2.27 A and a PCuP angle of 142.41(5) o , the ‘2+2’ coordination pattern could be reproduced by DFT calculations.

A Five-Center Redox System: Molecular Coupling of Two Noninnocent Imino-o-benzoquinonato-Ruthenium Functions through a π Acceptor Bridge

Combining the concepts of noninnocent behavior of metal/ligand entities and the coupling of redox-active moieties via an electronically mediating bridge led to the synthesis and the structural, electrochemical, and spectroscopic characterization of [Cl(Q)Ru(mu-tppz)Ru(Q)Cl](n) where Q(o) is 4,6-di-tert-butyl-N-phenyl-o-iminobenzoquinone and tppz(o) is 2,3,5,6-tetrakis(2-pyridyl)pyrazine, the available oxidation states being Ru(II,III,IV), Q(o,*-,2-), and tppz(o,*-,2-). One-electron transfer steps between the n = (2-) and (4+) states were studied by cyclic voltammetry and by EPR, UV-vis-NIR spectroelectrochemistry for the structurally characterized anti isomer of [Cl(Q)Ru(mu-tppz)Ru(Q)Cl](PF(6))(2), 2(PF(6))(2), the only configuration obtained. The combined investigations reveal that 2(2+) is best described as [Cl(Q(*-))Ru(III)(mu-tppz(o))Ru(III)(Q(*-))Cl](2+) with antiferromagnetic coupling between the ruthenium(III) and the iminosemiquinone components at each end. A metal-based spin as evident from large g factor anisotropy (EPR) and an intense intervalence absorption band at 1850 nm in the near-infrared (NIR) suggest that oxidation occurs at both iminosemiquinones to yield two Ru(II,III)-bonded quinones, implying redox-induced electron transfer. Reduction takes place stepwise at the metal centers yielding iminosemiquinone complexes of Ru(III,II) as evident from radical complex EPR spectra with small (99,101)Ru hyperfine contributions. After complete metal reduction to ruthenium(II) the bridging ligand tppz is being reduced stepwise as apparent from typical NIR absorption bands around 1000 nm and from small g anisotropy of the monoanion [Cl(Q(*-))Ru(II)(mu-tppz(*-))Ru(II)(Q(*-))Cl](-). A structure-based DFT calculation confirms the Ru-Cl character of the HOMO and the iminoquinone-dominated LUMO and illustrates the orbital interaction pattern of the five electron transfer active components in this new system.

Relaxation dynamics of Pseudomonas aeruginosa Re(I)(CO)3(alpha-diimine)(HisX)+ (X = 83, 107, 109, 124, 126)Cu(II) azurins.

Photoinduced relaxation processes of five structurally characterized Pseudomonas aeruginosa Re(I)(CO)(3)(alpha-diimine)(HisX) (X = 83, 107, 109, 124, 126)Cu(II) azurins have been investigated by time-resolved (ps-ns) IR spectroscopy and emission spectroscopy. Crystal structures reveal the presence of Re-azurin dimers and trimers that in two cases (X = 107, 124) involve van der Waals interactions between interdigitated diimine aromatic rings. Time-dependent emission anisotropy measurements confirm that the proteins aggregate in mM solutions (D(2)O, KP(i) buffer, pD = 7.1). Excited-state DFT calculations show that extensive charge redistribution in the Re(I)(CO)(3) --> diimine (3)MLCT state occurs: excitation of this (3)MLCT state triggers several relaxation processes in Re-azurins whose kinetics strongly depend on the location of the metallolabel on the protein surface. Relaxation is manifested by dynamic blue shifts of excited-state nu(CO) IR bands that occur with triexponential kinetics: intramolecular vibrational redistribution together with vibrational and solvent relaxation give rise to subps, approximately 2, and 8-20 ps components, while the approximately 10(2) ps kinetics are attributed to displacement (reorientation) of the Re(I)(CO)(3)(phen)(im) unit relative to the peptide chain, which optimizes Coulombic interactions of the Re(I) excited-state electron density with solvated peptide groups. Evidence also suggests that additional segmental movements of Re-bearing beta-strands occur without perturbing the reaction field or interactions with the peptide. Our work demonstrates that time-resolved IR spectroscopy and emission anisotropy of Re(I) carbonyl-diimine complexes are powerful probes of molecular dynamics at or around the surfaces of proteins and protein-protein interfacial regions.

Ligand-to-ligand charge transfer states and photochemical bond homolysis in metalcarbon bonded platinum complexes

Abstract This paper reviews the photochemical PtC bond homolysis reactions of metalcarbon bonded platinum compounds. Three types of complexes are considered: [Pt(R) 2 (COD)], [Pt(R) 2 (α-diimine)] and [Pt(Me) 4 (α-diimine)]. This photoreactivity originates in the involvement of the σ-bonded co-ligands in low-lying excited states. This involvement was proved by time-dependent density-functional theory (TD DFT) calculations and resonance Raman (rR) spectroscopy. In the case of the platinum(II) complexes, the contribution of the co-ligand to the high-lying filled orbitals increases with increased σ-donor strength of this ligand. Concurrently, the photoreactivity increases dramatically in the case of the [Pt(R) 2 (COD)] complexes. The corresponding α-diimine analogues are much less reactive due to the smaller co-ligand participation in high-lying filled orbitals. The platinum(IV) complexes [Pt(Me) 4 (α-diimine)] are very photoreactive, due to the fact that the HOMO has almost exclusively σ(C ax PtC ax ) character.

An Odd‐Electron Complex [Ruk(NOm)(Qn)(terpy)]2+ with Two Prototypical Non‐Innocent Ligands

Six combinations of oxidation states are conceivable for the paramagnetic title complex. Single-crystal X-ray diffraction, spectroscopic analysis (IR, EPR at conventional and high frequency), and DFT calculations establish that it is the iminosemiquinone radical structure that is formed: [Ru(k)(NO(m))(Q(n))(terpy)](2+) (k = 2+, m = 1+, n = 1-).

Singlet diradical complexes of ruthenium and osmium: geometrical and electronic structures and their unexpected changes on oxidation.

Reaction of HL, HLa (2-[(2-N-phenylamino)phenylazo]pyridine), HLb (2-[{2-N-(4-methylphenyl)amino}phenylazo]pyridine), or HLc (2-[{2-N-(4-chlorophenyl)amino}phenylazo]pyridine), with KRuO4 or OsO4 and PPh3 under exhaustive deoxygenation (PPh3 --> OPPh3) yields diamagnetic compounds ML2. Crystal structure determination for M(La)2 indicates the radical dianion state, L2(.-), for the ligands as evident from the typical N-N bond length of about 1.33 A for a one-electron reduced azo function. The resulting spin-coupled complexes, MIV(L(2.-))2, can be oxidized in two reversible one-electron steps, as probed by cyclic voltammetry and UV-vis-NIR spectroelectrochemistry. The paramagnetic intermediates, [M(La)2]+, are distinguished by intense NIR absorption, largely metal-centered spin as revealed by EPR, and, in the case of [Os(La)2]I3, by crystallographically determined shortening of the NN bond to about 1.30 A, corresponding to a coordinated unreduced azo function. Thus, oxidation of the complex MIV(L(2.-))2 involves partial reduction of the metal in [MIII(L-)2]+ because intramolecular double electron transfer is offsetting the external charge removal. Density-functional theory calculations were employed to confirm the structural features and to support the spectroscopic assignments.