The reaction between an Fe(III) complex and O(2) to afford a stable catalytically active diiron(IV)-mu-oxo compound is described. Phosphonium salts of orange five-coordinated Fe(III)-TAML complexes with an axial aqua ligand ([PPh(4)]1-H(2)O, tetraamidato macrocyclic Fe(III) species derived from 3,3,6,6,9,9-hexamethyl-3,4,8,9-tetrahydro-1H-1,4,8,11-benzotetraazacyclotridecine-2,5,7,10(6H,11H)-tetraone) react rapidly with O(2) in CH(2)Cl(2) or other weakly coordinating solvents to produce black mu-oxo-bridged diiron(IV) complexes, 2, in high yields. Complexes 2 have been characterized by X-ray crystallography (2 cases), microanalytical data, mass spectrometry, UV/Vis, Mossbauer, and (1)H NMR spectroscopies. Mossbauer data show that the diamagnetic Fe-O-Fe unit contains antiferromagnetically coupled S = 1 Fe(IV) sites; diamagnetic (1)H NMR spectra are observed. The oxidation of PPh(3) to OPPh(3) by 2 was confirmed by UV/Vis and GC-MS. Labeling experiments with (18)O(2) and H(2)(18)O established that the bridging oxygen atom of 2 derives from O(2). Complexes 2 catalyze the selective oxidation of benzylic alcohols into the corresponding aldehydes and bleach rapidly organic dyes, such as Orange II in MeCN-H(2)O mixtures; reactivity evidence suggests that free radical autoxidation is not involved. This work highlights a promising development for the advancement of green oxidation technology, as O(2) is an abundant, clean, and inexpensive oxidizing agent.
Novel chelators, i.e., 4-(2-pyridyl)-1,2,3-triazole derivatives, were synthesized by means of Cu(I)-catalyzed 1,3-dipolar cycloaddition and used to prepare luminescent Re(I) complexes [ReCl(CO)(3)(Bn-pyta)], [ReCl(CO)(3)(AcGlc-pyta)] and [ReCl(CO)(3)(Glc-pyta)] (Bn-pyta = 1-benzyl-4-(2-pyridyl)-1,2,3-triazole, AcGlc-pyta = 2-(4-(2-pyridyl)-1,2,3-triazol-1-yl)ethyl 2,3,4,6-tetra-O-acetyl-beta-d-glucopyranoside, Glc-pyta = 2-(4-(2-pyridyl)-1,2,3-triazol-1-yl)ethyl beta-d-glucopyranoside). X-Ray crystallography of Bn-pyta and Glc-pyta indicated an azocompound-like structure while the 1,2,4-triazole isomer has an azine character. [ReCl(CO)(3)(Bn-pyta)] crystallized in the monoclinic system with space group P2(1)/n. Bn-pyta ligand coordinates with the nitrogen atoms of the 2-pyridyl group and the 3-position of 1,2,3-triazole ring, which is a very similar coordinating fashion to that of the 2,2'-bipyridine derivative. The glucoconjugated Re(I) complexes [ReCl(CO)(3)(AcGlc-pyta)] and [ReCl(CO)(3)(Glc-pyta)] hardly crystallized, and were analyzed by applying extended X-ray absorption fine structure (EXAFS) analysis. The EXAFS analyses suggested that the glucoconjugation at the 1-position of the 1,2,3-triazole makes no influence to the coordinating fashion of 4-(2-pyridyl)-1,2,3-triazole. [ReCl(CO)(3)(Bn-pyta)] showed a blue-shifted maximum absorption (333 nm, 3.97 x 10(3) M(-1) cm(-1)) compared with [ReCl(CO)(3)(bpy)] (371 nm, 3.35 x 10(3) M(-1) cm(-1)). These absorptions were clearly assigned to be the mixed metal-ligand-to-ligand charge transfer (MLLCT) on the basis of time-dependent density functional theory calculation. The luminescence spectrum of [ReCl(CO)(3)(Bn-pyta)] also showed this blue-shifted feature when compared with that of [ReCl(CO)(3)(bpy)]. The luminescence lifetime of [ReCl(CO)(3)(Bn-pyta)] was determined to be 8.90 mus in 2-methyltetrahydrofuran at 77 K, which is longer than that of [ReCl(CO)(3)(bpy)] (3.17 micros). The blue-shifted electronic absorption and elongated luminescence lifetime of [ReCl(CO)(3)(Bn-pyta)] suggested that 4-(2-pyridyl)-1,2,3-triazole functions as an electron-rich bidentate chelator.
The thiolate-bridged diiron carbonyl complex [{Fe(μ-PyBPT-κ 3 N,C,S)(CO) 2 }Fe(CO) 3 ] (1) consists of two units, Fe(PyBPT)(CO) 2 and Fe(CO) 3 , where the N,C,S-pincer ligand PyBPT is a doubly deprotonated form of 3′-(2″-pyridyl)-1,1′-biphenyl-2-thiol. The two Fe complex units are connected through a thiolate S atom, π coordination, and an Fe−Fe bond. Diiron complex 1 reacted with 1 equiv of dimethylphenylphosphine to form the CO substitution product [{Fe(μ-PyBPT-(2), which has a polarized Fe−Fe bond. A further reaction of 3 with PMe 2 Ph produced the N,C,S-pincer iron(II) complex trans-[Fe(PyBPT-κ 3 N,C,S)(CO)(PMe 2 Ph) 2 ] (4) and the iron(0) complex trans-[Fe(CO) 3 (PMe 2 Ph) 2 ]. 1,2-Bis(diphenylphosphino)benzene (dppbz) abstracted the Fe(CO) 3 unit from 1 to give the dimeric diiron(II,II) complex [{Fe(μ-PyBPT-κ 3 N,C,S)(CO) 2 } 2 ] (7) and the iron(0) complex [Fe(CO) 3 (dppbz)]. The asymmetric bridging ligand PyBPT and coordination of the phosphines induce polarization of the Fe−Fe bond, which leads to the formation of the iron(II) and iron(0) complexes via heterolytic Fe−Fe cleavage. Electrochemical properties of 3 and 4 were investigated by cyclic voltammetry. Complex 3 showed two one-electron-reduction processes, the potentials of which are ca. 0.4 V more negative than those of 1. Electrocatalytic proton reduction by 3 was investigated, and the efficiency was comparable to that of 1.
Treatment of 4-(2′-pyridyl)dibenzothiophene (PyDBT) with the ruthenium carbonyl cluster [Ru 3 (CO) 12 ] gave the diruthenium(II) complex [Ru(µ-PyBPT-κ 3 N,C,S)(CO) 2 ] 2 (1), where PyBPT denotes a dianion of 3′-(2′′-pyridyl)-1,1′-biphenyl-2-thiol. The tridentate-N,C,S PyBPT ligand provides a pincer structure consisting of a six-membered thiaruthenacycle and a five-membered azaruthenacycle. The thiolatecontaining NCS pincer ligand in 1 is produced by cleavage of a carbon-sulfur bond adjacent to a pyridyl group in PyDBT. The corresponding reactions using 4-(4′-methyl-2′-pyridyl)dibenzothiophene (4-MePyDBT) and 4-(6′-methyl-2′-pyridyl)dibenzothiophene (6-MePyDBT) afforded the diruthenium(II) complexes with the same pincer framework [Ru(µ-4-MePyBPT-κ 3 N,C,S)(CO) 2 ] 2 (2) and [Ru(µ-6-MePyBPT-κ 3 N,C,S)(CO) 2 ] 2 (3), respectively. The much slower formation of 3 certifies the reaction path through the initial coordination of the pyridyl group to Ru or the formation of an N,S-chelate structure. Indeed, PyDBT showed the chelating ability in the ruthenium(II) complex [Ru(η 6 -C 6 H 6 )(PyDBTκ 2 N,S)Cl]CF 3 SO 3 (4). Complex 1 contains C i and C 2 symmetrical isomers, 1a and 1b, respectively, which were separated. The latter isomerized to 1a in DMSO-d 6 at 80 °C. The stepwise formation of the same NCS pincer ligand was established in the reaction of [Rh(µ-Cl)(CO) 2 ] 2 with PyDBT. The facile reaction at room temperature produced the mononuclear rhodium(I) complex cis-[RhCl(CO) 2 (η 1 -N-PyDBT)] (5). The isolated complex 5 was converted to the tetranuclear rhodium(I/III/III/I) complex [{Rh(µ-PyBPTκ 3 N,C,S)}(µ-Cl) 2 {Rh(CO) 2 }] 2 (6) at 100 °C for 3 days.
Introduction of a heavy atom into photosensitizers generally facilitates intersystem crossing and improves the quantum yield (Phi(Delta)) of singlet oxygen ((1)O(2)), which is a key species in photodynamic therapy (PDT). However, little information is available about the physiological importance of this heavy-atom effect. The aim of this study is to examine the heavy-atom effect in simple metallochlorins in vitro at the cellular level. 1,3-Dipolar cycloaddition of azomethine ylide to 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato palladium(II) and platinum(II) afforded metallochlorins 4b and 4c in yields of 17.1 and 12.9%, respectively. The Phi(Delta) values increased in the order of 4a (0.28) < 4b (0.89) < 4c (0.92) in C(6)D(6). The photocytotoxicity of 4a, 4b, and 4c was evaluated in HeLa cells at a light dose of 16 J x cm(-2) with lambda > 500 nm and increased in the order of 4a < 4b < 4c at the concentration of 0.5 microM. The photocytotoxicity of 4b and 4c was significantly inhibited by addition of sodium azide, but not D-mannitol, suggesting that (1)O(2) is the major species causing cell death. Our results clearly indicate that 4b and 4c act as efficient (1)O(2) generators due to the heavy-atom effect in a cellular microenvironment as well as in nonphysiological media.
Photochemical reactions of [Fe(CO)5] with dibenzothiophene (DBT) derivatives bearing a N-donor group produced a series of C,S-bridged diiron carbonyl complexes [{Fe(μ-L′-κ3 N,C,S)(CO)2}Fe(CO)3], as previously reported for 4-(2′-pyridyl)dibenzothiophene (L1), where L′ represents the N,C,S-tridentate ligands L1′–L5′, formed by C–S bond cleavage of L1–L5, respectively. The DBT derivatives used in this study have Schiff base or oxazoline moieties at the 4-position: L2 = PhCH2NCH-DBT, L3 = 2-MeOC6H4CH2NCH-DBT, L4 = (S)-PhC(Me)HNCH-DBT, L5 = (R)-4-isopropyl-2-oxazolinyl-DBT. The diiron complexes were characterized by NMR, absorption, and circular dichroism spectroscopy, and the dinuclear structures bridged by thiolate S and aryl C atoms were established by X-ray crystallography. The diiron complex [{Fe(μ-L′-κ3 N,C,S)(CO)2}Fe(CO)3] consists of two units, Fe(L′)(CO)2 and Fe(CO)3: the latter unit is located on a thiolate-containing metallacycle in the former one. The chiral Schiff base ligand precursor L4 gave a 55:45 mixture of two diastereomers for [{Fe(μ-L4′-κ3 N,C,S)(CO)2}Fe(CO)3], while chiral L5 with an (R)-4-isopropyl-2-oxazolinyl group afforded [{Fe(μ-L5′-κ3 N,C,S)(CO)2}Fe(CO)3] in a 9:1 diastereomeric ratio. The diiron carbonyl complexes of the N,C,S-tridentate ligands (L1′L5′) showed two reversible redox couples for [Fe2(μ-L′)(CO)5]0/– and [Fe2(μ-L′)(CO)5]−/2–. The two-electron-reduced forms undergo protonation and act as electrocatalysts for proton reduction of acetic acid in acetonitrile.
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