Rhombic EPR Spectra Research Articles

Cycloruthenates(III) with CNO pincer-like ligands: Regioselective metallation of N-(4-R-benzoyl)-N′-(2-naphthylidene)hydrazines

Synthesis and characterization of a series of cyclometallated ruthenium(III) complexes with the Schiff bases N-(4-R-benzoyl)-N′-(2-naphthylidene)hydrazines (H2Ln, where n = 1–5 for R = H, Cl, Me, OMe and NO2, respectively) are described. All the complexes having the general molecular formula trans-[Ru(Ln)(PPh3)2Cl] (1–5) were characterized by elemental analysis, magnetic susceptibility, mass spectrometric, various spectroscopic (IR, UV-Vis and EPR) and electrochemical measurements. Molecular structures of trans-[Ru(L1)(PPh3)2Cl] (1) and trans-[Ru(L2)(PPh3)2Cl] (2) were confirmed by X-ray crystallography. In each of these two complexes, the metal center is in a distorted octahedral CNOClP2 coordination environment. The tridentate amidate-O, azomethine-N and the 2-naphthyl-C3 coordinating (L1/2)2– and the Cl– occupy the four equatorial positions and the P-atoms of the two triphenylphosphine molecules satisfy the two axial positions of the metal center. Electronic spectra of 1–5 in dichloromethane exhibit a ligand field transition within 1322–1160 nm and two ligand-to-metal and three intraligand transitions in the ranges 500–406 and 332–250 nm, respectively. The room temperature (300 K) magnetic moments of 1–5 are in the range of 1.86–1.99 µB. All five complexes display rhombic EPR spectra in frozen (77 K) dichloromethane-toluene (1:1) solution. A quasi-reversible reduction response in the range of –0.60–0.68 V and two irreversible oxidation responses around 0.9 and 1.4 V (vs. Ag/AgCl) were observed in the cyclic voltammograms of 1–5 in dichloromethane. Theoretical calculations were performed to get an insight of the regioselective ruthenation at the 3-position over the 1-position of the 2-naphthyl group of (Ln)2– in these complexes.

Creation of an unexpected plane of enhanced covalency in cerium(III) and berkelium(III) terpyridyl complexes

Controlling the properties of heavy element complexes, such as those containing berkelium, is challenging because relativistic effects, spin-orbit and ligand-field splitting, and complex metal-ligand bonding, all dictate the final electronic states of the molecules. While the first two of these are currently beyond experimental control, covalent M‒L interactions could theoretically be boosted through the employment of chelators with large polarizabilities that substantially shift the electron density in the molecules. This theory is tested by ligating BkIII with 4’-(4-nitrophenyl)-2,2’:6’,2”-terpyridine (terpy*), a ligand with a large dipole. The resultant complex, Bk(terpy*)(NO3)3(H2O)·THF, is benchmarked with its closest electrochemical analog, Ce(terpy*)(NO3)3(H2O)·THF. Here, we show that enhanced Bk‒N interactions with terpy* are observed as predicted. Unexpectedly, induced polarization by terpy* also creates a plane in the molecules wherein the M‒L bonds trans to terpy* are shorter than anticipated. Moreover, these molecules are highly anisotropic and rhombic EPR spectra for the CeIII complex are reported.

Fixation and sequestration of carbon dioxide by copper(II) complexes

The fixation of carbon dioxide ( $$\hbox {CO}_{2})$$ is an important global challenge. A significant increase of the atmospheric $$\hbox {CO}_{2}$$ due to the industrial emissions and a steady increase in combustion of fossil fuels is a widespread environmental concern. This article is a short literature review on the recent developments in the field of $$\hbox {CO}_{2}$$ activation and fixation by bioinspired copper(II) catalysts. In our laboratory, copper(II) complexes of bidentate ligands have been reported as catalysts for the fixation of $$\hbox {CO}_{2}$$ . The molecular structure of one of the complexes has shown unusual trigonal bipyramid geometry ( $$\tau $$ , 0.936) by the coordination of two ligand units and a water molecule. All the complexes exhibited a well-defined Cu(II)/Cu(I) redox potentials around 0.352 to 0.401 V in acetonitrile. The rhombic EPR spectra of the complexes indicate the existence of a geometrical equilibrium between trigonal bipyramidal and square pyramidal at 70 K. The d-d transitions around 750–800 and 930–955 nm further supports five coordination geometry in solution. These copper(II) complexes have successfully fixed atmospheric $$\hbox {CO}_{2}$$ as $$\hbox {CO}_{3}^{2-}$$ by using $$\hbox {Et}_{3}\hbox {N}$$ as sacrificial reducing agent and afforded [ $$\hbox {Cu(L)CO}_{3}$$ ( $$\hbox {H}_{2}\hbox {O}$$ )]. The $$\hbox {CO}_{3}^{2-}$$ bound complex has shown a distorted square pyramidal geometry ( $$\tau $$ , 0.369) around copper(II) center via the coordination of only one ligand unit, a carbonate, and water molecules. The catalysts are active enough to fix $$\hbox {CO}_{2}$$ for eight repeating cycles without any change in the efficiency. The fixation of $$\hbox {CO}_{2}$$ possibly proceeds via the formation of Cu(I)-species. This is supported by X-ray structure, which reveals distorted tetrahedral geometry by the coordination of two units of ligand. SYNOPSIS The fixation of carbon dioxide ( $$\hbox {CO}_{2})$$ is an important global challenge. This review summarizes the recent developments in $$\hbox {CO}_{2}$$ fixation by bioinspired Cu(II) catalysts.

Synthesis and reductive chemistry of bimetallic and trimetallic rare-earth metallocene hydrides with (C5H4SiMe3)1− ligands

The reductive chemistry of [Cp'2Ln(μ–H)(THF)x]y [Ln = Y, Dy, Tb; Cp' = (C5H4SiMe3)1−; x = 2, 0 and y = 2, 3] was examined to determine if these hydrides would be viable precursors for 4fn5d1 Ln2+ ions that could form 5d1-5d1 metal–metal bonded complexes. The hydrides were prepared by reaction of the chlorides, [Cp'2Ln(μ–Cl)]2, 1-Ln, with allylmagnesium chloride to form the allyl complexes, [Cp'2Y(η3–C3H5)(THF)], 2-Ln, which were hydrogenolyzed. The solvent-free reaction of solid 2-Ln with 60 psi of H2 gas in a Fischer-Porter apparatus produced, in the Y case, the trimetallic species, [Cp'2Y(μ–H)]3, 3-Y, and in the Dy and Tb cases, the bimetallic complexes [Cp'2Ln(μ–H)(THF)]2, 4-Ln (Ln = Dy, Tb). The latter complexes could be converted to 3-Dy and 3-Tb by heating under vacuum. Isopiestic data indicate that 3-Y solvates to 4-Y in THF. Reductions of 4-Y, 4-Dy, and 4-Tb with KC8 in the presence of a chelate such as 2.2.2-cryptand or 18-crown-6 all gave reaction products with intense dark colors characteristic of Ln2+ ions. In the yttrium case, with either chelating agent, the dark green product gives a rhombic EPR spectrum (g1 = 2.01, g2 = 1.99, g3 = 1.98, A = 24.1 G) at 77 K. However, the only crystallographically-characterizable products obtainable from these solutions were Ln3+ polyhydride anion complexes of composition, [K(chelate)]{[Cp'2Ln(μ–H)]3(μ–H)}. Reduction of 1-Y with KC8 in the presence of 2.2.2-cryptand also yields an intensely colored product with an axial EPR spectrum (gx = gy = 2.05, Ax = Ay = 35.5 G; gz = 2.07, Az = 34.5) similar to that of (Cp'3Y)1− ion, but crystals were not obtained from this system.

Catalytic fixation of atmospheric carbon dioxide by copper(ii) complexes of bidentate ligands.

New copper(ii) complexes, [Cu(L1)2(H2O)](ClO4)2, 1 [L1 = 2-pyridin-2-yl-quinoline], [Cu(L2)2(H2O)](ClO4)2, 2 [L2 = 2-pyridin-2-yl-quinoxaline], [Cu(L3)2(H2O)](ClO4)2, 3 [L3 = 6,7-dimethyl-2-pyridin-2-yl-quinoxaline], [Cu(L4)2(H2O)](ClO4)2, 4 [L4 = 4-phenyl-2-pyridin-2-yl-quinoline] and [Cu(L5)2(H2O)](ClO4)2, 5 [L5 = 4-phenyl-2-pyridin-2-yl-quinazoline], were synthesized and characterized as catalysts for selective fixation of atmospheric CO2. The molecular structure of 2 was determined by single-crystal X-ray studies and shown to have an unusual trigonal bipyramid geometry (τ, 0.936) around the copper(ii) center, with the coordination of two ligand units and a water molecule. The Cu-Nquin (2.040, 2.048 Å) bonds are slightly longer than the Cu-Npyr (1.987 Å) bonds but shorter than the Cu-Owater bond (2.117 Å). Well-defined Cu(ii)/Cu(i) redox potentials of around 0.352 to 0.401 V were observed for 1-5 in acetonitrile. The electronic absorption spectra of 1-5 showed ligand-based transitions at around 208-286 nm with a visible shoulder at around 342-370 nm. The d-d transitions appeared at around 750-800 and 930-955 nm in acetonitrile. The rhombic EPR spectra of 1-5 exhibited three different g values gx, 2.27-2.34; gy, 2.06-2.09; and gz, 1.95-1.98 at 70 K. Atmospheric CO2 was successfully fixed by 1-5 using Et3N as a sacrificial reducing agent, resulting in CO32--bound complexes of type [Cu(L)CO3(H2O)] that display an absorption band at around 614-673 nm and a νst at 1647 cm-1. This CO32--bound complex of 1 was crystallized from the reaction mixture and it displayed a distorted square pyramidal geometry (τ, 0.369) around the copper(ii) center via the coordination of only one ligand unit, a carbonate group, and water molecules. Furthermore, treatment of the carbonate-bound Cu(ii) complexes with one equivalent of H+ under N2 atmosphere resulted in the liberation of bicarbonate (HCO3-) and regenerated the parent complexes. These regenerated catalysts were active enough to fix CO2 in eight repeating cycles without any change in efficiency. The fixation of CO2 possibly occurs via the formation of Cu(i)-species, which is accompanied by the formation of an MLCT band at around 450-500 nm. The rates of Cu(i)-species formation, kobs, were determined and found to be 5.41-10.31 × 10-3 s-1 in the presence of Et3N in acetonitrile at 25 °C. Interestingly, the copper(i)-species of 3 has been successfully crystallized and displayed a distorted tetrahedral geometry through the coordination of two units of ligand L3.

A copper(II) complex of benzimidazole-based ligand: synthesis, structure, redox aspects and fluorescence properties

Employing 1-(2-methoxybenzyl)-2-(2-methoxyphenyl)-1H-benzimidazole (bpb) as a monodentate ligand, a new greenish-blue copper(II) complex, [Cu(bpb)2(NO3)2] (1a), has been synthesized. 1a has been characterized analytically and spectroscopically. The X-ray crystal structure of 1a reveals that it adopts a cis disposition with respect to the ligands. The solid state structure of 1a is stabilized by intramolecular offset face-to-face π–π stacking. Non-covalent supramolecular edge-to-face C–H⋯π interactions with neighboring molecules give 1-D supramolecular chains that further lead to the formation of an assembled 3-D supramolecular metal-organic framework via hydrogen bonding interactions. 1a shows blue fluorescence most likely due to intramolecular offset face-to-face π–π stacking. At room temperature, 1a is one-electron paramagnetic. It shows a rhombic EPR spectrum with g1 = 2.12, g2 = 2.42, and g3 = 2.52 in the solid state at liquid nitrogen temperature. In cyclic voltammetry, 1a displays a one-electron oxidative Cu(II)/Cu(III) couple. Our DFT calculations, corroborate the observed experimental results of 1a.

Chiral and achiral vanadyl lactates with vibrational circular dichroism: Toward the chiral metal cluster in nitrogenase

A series of chiral and achiral neutral vanadyl complexes with N-heterocycle chelated ligands [V2O2(S-lact)2(bpy)2] (1), [V2O2(S-lact)2(phen)2] (2), [V2O2(R-lact)(S-lact)(bpy)2] (3), [V2O2(R-lact)(S-lact)(phen)2] (4) (H2lact=lactic acid, phen=1,10-phenanthroline, and bpy=2,2′-bipyridine) have been obtained under hydrothermal condition. The complexes feature bidentate lactate that chelates to vanadium atom through their α-alkoxido and carboxylato groups, while the other coordination sites are occupied by N-heterocycle ligand and terminal oxygen without coordinated or crystallized water molecule. Unusual short bond distances are observed for the coordinated lactates with V(IV)–Oα-alkoxy and V(IV)–Ocarboxy 1.986(7)Å in average, which imply the influence of full deprotonation in the chelated ring compared with those of 2.15Å in iron vanadium cofactor of V-nitrogenase. Enhanced activity and the red-shifts of vibrational circular dichroism (VCD) have been observed for the chiral neutral vanadyl complexes 1 and 2 and supported by B3LYP calculation. The carboxylato vibrations in VCD are observed in 1660, 1649, 1600, 1474, 1443cm−1 and 1664, 1653, 1636, 1483, 1431cm−1 for 1 and 2 respectively. Their absolute configurations of chiral metal center are assigned as ΛS. The EPR spectra of 1 and 3 with bpy ligands appear to be isotropic, while 2 and 4 with phen ligands display textbook rhombic EPR spectra, and the g values obtained are gxx 2.4727, gyy 1.9975, gzz 1.6456 for 2; and gxx 2.4585, gyy 1.9976, gzz 1.6412 for 4 respectively.

Ortho-ruthenation of 1-naphthalenyl in 1-naphthaldehyde 4-R-benzoylhydrazones: Ruthenium(III) CNO pincer complexes

In methanol, reactions of [Ru(PPh3)3Cl2], 1-naphthaldehyde 4-R-benzoylhydrazones (H2nabhR, where R = H, Me, OMe, Cl and NO2) and NaOAc in 1:1:2 mol ratio provide ortho-metallated ruthenium(III) complexes of general formula trans-[Ru(nabhR)(PPh3)2Cl] (1–5) in 50–58% yields. Elemental analysis, magnetic susceptibility, spectroscopic (IR, UV–Vis and EPR) and cyclic voltammetric measurements were used to characterize the complexes. Single crystal X-ray structures of 1 (R = H), 2 (R = Me) and 5 (R = NO2) show pincer like coordination mode of nabhR2−. The 1-naphthalenyl ortho-C, the azomethine-N and the amidate-O donor nabhR2−, two mutually trans PPh3 and the chloride assemble a distorted octahedral trans-CNOClP2 coordination sphere around the trivalent metal centre. In the electronic spectra, dichloromethane solutions of 1–5 display multiple strong bands within 506–272 nm due to ligand to metal charge transfer and intraligand transitions. The room temperature (298 K) magnetic moments (μeff) of 1–5 are within 1.92–1.99 μB and they display rhombic EPR spectra in frozen (130 K) dichloromethane–toluene (1:1). Cyclic voltammetry with dimethylformamide solutions of the complexes reveals ligand substituent sensitive Ru(III) → Ru(II) reduction and Ru(III) → Ru(IV) oxidation in the potential ranges −0.27 to −0.36 V and 0.94–1.13 V (vs. Ag/AgCl), respectively.

Structural Characterization ofNitrosomonas europaeaCytochromec-552 Variants with Marked Differences in Electronic Structure

Nitrosomonas europaea cytochrome c-552 (Ne c-552) variants with the same His/Met axial ligand set but with different EPR spectra have been characterized structurally, to aid understanding of how molecular structure determines heme electronic structure. Visible light absorption, Raman, and resonance Raman spectroscopy of the protein crystals was performed along with structure determination. The structures solved are those of Ne c-552, which displays a "HALS" (or highly anisotropic low-spin) EPR spectrum, and of the deletion mutant Ne N64Δ, which has a rhombic EPR spectrum. Two X-ray crystal structures of wild-type Ne c-552 are reported; one is of the protein isolated from N. europaea cells (Ne c-552n, 2.35 Å resolution), and the other is of recombinant protein expressed in Escherichia coli (Ne c-552r, 1.63 Å resolution). Ne N64Δ crystallized in two different space groups, and two structures are reported [monoclinic (2.1 Å resolution) and hexagonal (2.3 Å resolution)]. Comparison of the structures of the wild-type and mutant proteins reveals that heme ruffling is increased in the mutant; increased ruffling is predicted to yield a more rhombic EPR spectrum. The 2.35 Å Ne c-552n structure shows 18 molecules in the asymmetric unit; analysis of the structure is consistent with population of more than one axial Met configuration, as seen previously by NMR. Finally, the mutation was shown to yield a more hydrophobic heme pocket and to expel water molecules from near the axial Met. These structures reveal that heme pocket residue 64 plays multiple roles in regulating the axial ligand orientation and the interaction of water with the heme. These results support the hypothesis that more ruffled hemes lead to more rhombic EPR signals in cytochromes c with His/Met axial ligation.

Absence of a magnetic interaction in a dinuclear copper complex? The case of a crossed axial–equatorial oxalate coordination mode

A simple and easy preparation of the hydrogen oxalate ligand (HOOCCOO)−, with only one deprotonated carboxylic function, is described. 13C NMR spectroscopy furnishes a straightforward characterization of this interesting ligand. This ligand coordinates in its monodeprotonated form to a CuII complex with a free coordination position, yielding a complex that presents a very weak antiferromagnetic interaction. In a subsequent step, the second deprotonation allows the isolation of an oxalato-bridged dinuclear CuII complex, which is characterized by structural determination, the copper ions being pentacoordinate in an equivalent centrosymmetry-related environment. DFT calculations show that the magnetic interaction through the oxalato bridge is strictly equal to 0cm−1, demonstrating that the singlet and triplet states of this oxalato-bridged dinuclear CuII complex have the same energy. This dinuclear complex presents a rhombic EPR spectrum, with three well-defined g values, at room temperature as well as at 100K.

Ruthenium(III) cyclometallates: Regioselective metallation of 1-pyrenyl in 1-pyrenaldehyde 4-R-benzoylhydrazones

Reactions of [Ru(PPh3)3Cl2] with 1-pyrenaldehyde 4-R-benzoylhydrazones (H2pnbhR, where R = H, Me, OMe, Cl and NO2) in presence of NaOAc afford ortho-metallated ruthenium(III) complexes of formula trans-[Ru(pnbhR)(PPh3)2Cl] (1–5). The complexes have been characterized by elemental analysis, magnetic susceptibility, spectroscopic (IR, UV–vis and EPR) and cyclic voltammetric measurements. Molecular structure of 2 (R = Me) determined by single crystal X-ray crystallography shows a distorted octahedral CNOClP2 coordination sphere around the trivalent metal centre assembled by the 1-pyrenyl ortho-C, azomethine-N and amidate-O donor pnbhMe2−, two mutually trans PPh3 and the chloride. Electronic spectra of 1–5 in dichloromethane display several strong bands within 555–276 nm due to ligand to metal charge transfer and ligand centred transitions. The complexes are one-electron paramagnetic (μeff = 1.90–1.99 μB) and display rhombic EPR spectra in frozen (130 K) dichloromethane–toluene (1:1). Cyclic voltammograms of the complexes in dimethylformamide display a ligand substituent sensitive metal centred one electron reduction couple in the E1/2 range −0.24 to −0.31 V (vs. Ag/AgCl).

Switching Orientation of Two Axial Imidazole Ligands between Parallel and Perpendicular in Low-Spin Fe(III) and Fe(II) Nonplanar Porphyrinates

We have reported here the synthesis, structure, and properties of low-spin bis-imidazole-coordinated Fe(III) and Fe(II) complexes of 5,10,15,20-tetrakis(pentafluorophenyl)-2,3,7,8,12,13,17,18-octachloroporphyrin, [Fe(III)(TFPPCl(8))(L)(2)]ClO(4) and Fe(II)(TFPPCl(8))(L)(2) (L = 1-methylimidazole, 4-methylimidazole, imidazole). The X-ray structure of Fe(II)(TFPPCl(8))(1-MeIm)(2) is reported here, which demonstrated the near-perpendicular axial ligand orientation (dihedral angle between two 1-methylimidazoles is 80.7°) for Fe(II) porphyrins in a highly saddle-distorted macrocyclic environment. Oxidation of Fe(II)(TFPPCl(8))(L)(2) using thianthrenium perchlorate produces [Fe(III)(TFPPCl(8))(L)(2)]ClO(4), which was also isolated in the solid state and characterized spectroscopically. The complex gives rhombic EPR spectra in both solid and solution phases at 77 K and thus represents a rare example of nearly parallel axial ligand orientations for the unhindered imidazoles in a saddle-distorted porphyrin macrocycle. Geometry optimization using DFT also converged to the parallel axial alignment when 1-methylimidazole was used as the axial ligand (the dihedral angle between two axial ligands is 8.6°). The potential energy surface (PES) scan results also show that the relatively parallel axial orientations are energetically preferred for Fe(III), while perpendicular orientations are preferred for the Fe(II) complexes reported here. Bulk oxidation of Fe(II)(TFPPCl(8))(L)(2) in dichloromethane at a constant potential under nitrogen converts it to [Fe(III)(TFPPCl(8))(L)(2)]ClO(4), which gives identical EPR spectra at 77 K and which upon reduction regenerates Fe(II)(TFPPCl(8))(L)(2) again. Thus, we have demonstrated here very rare examples of Fe porphyrins in which the relative axial imidazole orientations switch between parallel and perpendicular just upon changing the oxidation states of iron from +3 to +2, respectively, in a nonplanar porphyrinic environment. These observations could be immensely important for understanding the possible effects of axial histidine orientations on similar macrocyclic deformations observed in various heme proteins.

Synthesis, characterization and photophysical properties of quinolin-8-olato chelated osmium(II) organometallics bearing a pendant imine-phenol motif and electrogeneration of trivalent analogue

The reaction of Os(RL1)(PPh3)2(CO)Br, 1b, with qui-nolin-8-ol (HQ), 2, has furnished complexes of the type [Os(RL2)(PPh3)2(CO)(Q)], 3, in excellent yield (RL1 is C6H2O-2-CHNHC6H4R(p)-3-Me-5, RL2 is C6H2OH-2-CHNC6H4R(p)-3-Me-5 and R is Me, OMe, Cl). In this process, quinolin-8-olato (Q) undergoes five-membered chelation, the iminium-phenolato function tautomerizing to the imine-phenol function. The trans geometry of the Os(PPh3)2 fragment is consistent with the occurrence of a single 31P resonance near –6.0 ppm in 3. In dichloromethane solution, 3 displays a quasireversible 3+/3 couple near 0.40 V vs. SCE (3+ is the osmium (III) analogue of 3). Coulometrically generated solutions of 3+ displays a strong absorption near 340 nm, 415 nm and 500 nm and are one-electron paramagnetic (low-spin d5, S = 1/2) and show rhombic EPR spectra in 1:1 dichloromethanetoluene solution at 77 K with g values near 2.44, 2.20, 1.83. Distortion parameters using the observed g values have been computed. Solutions of 3 absorb near 420 nm and emit near 510 nm at 298 K and 580 nm at 77 K. The fluorescence is believed to originate from the 3MLCT state.

Paramagnetic ruthenium-biimidazole derivatives [(acac)2RuIII(LHn)]m, n/m = 2/+, 1/0, 0/−. Synthesis, structures, solution properties and anion receptor features in solution state

The paramagnetic ruthenium-biimidazole complexes [(acac)(2)Ru(III)(LH(-))] (1 = red-brown), [(acac)(2)Ru(III)(LH(2))](ClO(4)) (2 = pink) and Bu(4)N[(acac)(2)Ru(III)(L(2-))] (3 = greenish yellow) comprising of monodeprotonated, neutral and bideprotonated states of the coordinated biimidazole ligand (LH(n), n = 1, 2, 0), respectively, have been isolated (acac(-) = acetylacetonate). Single-crystal X-ray diffraction of 1 reveals that the asymmetric unit consists of three independent molecules: A-C, where molecule A corresponds to complex 1 and the other two molecules B and C co-exist as a hydrogen bonded dimeric unit perhaps between the cationic 2(+) and anionic 3(-). The packing diagram further reveals that the molecule A in the crystal of 1 also forms a hydrogen bonded dimer with the neighbouring another unit of molecule A. The formation of [(acac)(2)Ru(III)(LH(2))](ClO(4)) (2) has also been authenticated independently by its single-crystal X-ray structure. The packing diagram of 2 shows multiple hydrogen bonds between the N-H protons of coordinated LH(2) and the counter ClO(4)(-). Paramagnetic complexes show (1)H NMR spectra over a wide range of chemical shift, delta (ppm), +10 to -35 in CDCl(3). One-electron paramagnetic 1-3 (mu/B.M. approximately 1.9) exhibit distinct rhombic-EPR spectra with relatively large g anisotropic factors: <g> 2.136-2.156 and Deltag 0.65-0.77, typical for distorted octahedral ruthenium(III) complexes. The complexes 1-3 are inter-convertible as a function of pH. The pK(a1) and pK(a2) of 6.8 and 11, respectively, for 2 are estimated by monitoring the pH dependent spectral changes. The Ru(III)-Ru(IV) couple near 1.25 V vs. SCE remains almost invariant in 1-3 whereas the corresponding Ru(III)-Ru(II) couple varies appreciably in the range of -0.52 to -0.85 V vs. SCE based on the protonated-deprotonated states of the coordinated biimidazole ligand. Compounds 1-3 exhibit one weak ligand to metal charge transfer (LMCT) transition near 500 nm and intense intraligand transitions in the higher energy UV region. The spectrophotometric titrations of 2 with the TBA (TBA = tetrabutylammonium) salts of a wide variety of anions, F(-), Cl(-), Br(-), I(-), HSO(4)(-), OAc(-), H(2)PO(4)(-) in CH(3)CN reveal that the possible hydrogen bonds between the N-H protons of LH(2) in 2 and Cl(-) or Br(-) or I(-) or HSO(4)(-) or H(2)PO(4)(-) anion are rather weak or negligible. However, in presence of excess H(2)PO(4)(-) anion, the molar ratio of 2 to H(2)PO(4)(-) being 1 : 4, simple liberation of one N-H proton of the coordinated LH(2) in 2 has been taken place which in effect yields 1 and H(3)PO(4). On the contrary, the spectrophotometric titrations of 1 : 1 molar solution of 2 and OAc(-) or F(-) anion suggest the initial formation of hydrogen bonds between the N-H protons of LH(2) in 2 and the anion with the calculated log K value of 5.92 or 4.7, respectively, which eventually leads to the transfer of one of the N-H protons of LH(2) in 2 to the anion, resulting in 1 and HOAc or HF. On addition of excess OAc(-) to the above solution of 1 (molar ratio of OAc(-) to 1, 4 : 1), further hydrogen bonding between the N-H proton of LH(-) in 1 and OAc(-) occurs but without the abstraction of the N-H proton of LH(-). However, excess F(-) anion concentration (molar ratio of anion to 1, 5 : 1) facilitates the removal of the remaining N-H proton of LH(-) in 1 which in turn yields 3 incorporating the bideprotonated form L(2-).

De novo design of a non-natural fold for an iron–sulfur protein: Alpha-helical coiled-coil with a four-iron four-sulfur cluster binding site in its central core

Using a ‘metal-first’ approach, we computationally designed, prepared, and characterized a four-iron four-sulfur (Fe 4S 4) cluster protein with a non-natural α-helical coiled-coil fold. The novelty of this fold lies in the placement of a Fe 4S 4 cluster within the hydrophobic core of a four-helix bundle, making it unique among previous iron–sulfur (FeS) protein designs, and different from known natural FeS proteins. The apoprotein, recombinantly expressed and purified from E. coli, readily self-assembles with Fe 4S 4 clusters in vitro. UV–Vis absorption and CD spectroscopy, elemental analysis, gel filtration, and analytical ultracentrifugation confirm that the protein is folded and assembled as designed, namely, α-helical coiled-coil binding a single Fe 4S 4 cluster. Dithionite-reduced holoprotein samples have characteristic rhombic EPR spectra, typical of low-potential, [Fe 4S 4] + (S = 1/2), with g values of g z, y = (1.970, 1.975), and g x = 2.053. The temperature, and power dependence of the signal intensity were also characteristic of [Fe 4S 4] + clusters with very efficient spin relaxation, but almost without any interaction between adjacent clusters. The new design is very promising although optimization is required, particularly for preventing aggregation, and adding second shell interactions to stabilize the reduced state. Its main advantage is its extendibility into a multi-FeS cluster protein by simply duplicating and translating the binding site along the coiled-coil axis. This opens new possibilities for designing protein-embedded redox chains that may be used as “wires” for coupling any given set of redox enzymes.

Mononuclear ruthenium(III) complexes containing chelating thiosemicarbazones: Synthesis, characterization and catalytic property

Mononuclear ruthenium(III) complexes of the type [RuX(EPh(3))(2)(L)] (E=P or As; X=Cl or Br; L=dibasic terdentate dehydroacetic acid thiosemicarbazones) have been synthesized from the reaction of thiosemicarbazone ligands with ruthenium(III) precursors, [RuX(3)(EPh(3))(3)] (where E=P, X=Cl; E=As, X=Cl or Br) and [RuBr(3)(PPh(3))(2)(CH(3)OH)] in benzene. The compositions of the complexes have been established by elemental analysis, magnetic susceptibility measurement, FT-IR, UV-vis and EPR spectral data. These complexes are paramagnetic and show intense d-d and charge transfer transitions in dichloromethane. The complexes show rhombic EPR spectra at LNT which are typical of low-spin distorted octahedral ruthenium(III) species. All the complexes are redox active and display an irreversible metal centered redox processes. Complex [RuCl(PPh(3))(2)(DHA-PTSC)] (5) was used as catalyst for transfer hydrogenation of ketones in the presence of isopropanol/KOH and was found to be the active species.

A deliberate synthesis of a mononuclear trans-dimethoxide complex of ruthenium(III). X-ray crystal structure, electrochemistry and electron paramagnetic resonance

Using bis(3,5-dimethylpyrazol-1-yl)methane as the bidentate N donor ligand L, the yellow compound trans-[Ru IIIL 2(OMe) 2]ClO 4 · CH 2Cl 2 is synthesized. It is a rare example of a mononuclear dialkoxo complex of Ru(III). It shows a quasireversible Ru(II/III) couple at −0.65 V versus NHE in acetonitrile at a Pt electrode. Its magnetic moment at room temperature corresponds to one unpaired electron. It displays a rhombic EPR spectrum in acetone at 77 K with g = 2.219, 2.062 and 1.855.

Cyclometallated ruthenium(III) complexes: Synthesis, structure and properties

Abstract Reactions of Schiff bases (H 2 apahR) derived from acetophenone and acid hydrazides, triethylamine and [Ru(PPh 3 ) 3 Cl 2 ] (1:2:1 mole ratio) in methanol provide cyclometallated ruthenium(III) complexes of formula trans -[Ru(apahR)(PPh 3 ) 2 Cl] in 74–81% yields. The complexes have been characterized by elemental analysis, magnetic susceptibility, spectroscopic (infrared, electronic and EPR) and electrochemical measurements. X-ray crystal structures of two representative complexes have been determined. In each complex, the metal centre is in distorted octahedral CNOClP 2 coordination sphere assembled by the C,N,O-donor meridionally spanning apahR 2− , the chloride and the two mutually trans -oriented PPh 3 molecules. All the complexes are one-electron paramagnetic ( μ eff. = 1.85–1.98 μ B ) and display rhombic EPR spectra in frozen (120 K) dichloromethane-toluene (1:1) solution. Electronic spectra of the complexes display several absorptions within 470–270 nm due to ligand-to-metal charge transfer and ligand centred transitions. The complexes are redox active and display a Ru(III) → Ru(II) reduction and a Ru(III) → Ru(IV) oxidation in the potential ranges −0.66 to −0.70 V and 0.75 to 0.80 V (vs. Ag/AgCl), respectively.

Ru(PPh3)3Cl2]‐Mediated Transformation of –CH3 into =CHPHPh3 in Biacetylmonoxime Acetyl‐/Aroylhydrazones: Ruthenium(III) Complexes with the Transformed Ligands

AbstractRuthenium(III) complexes of general formula trans‐[Ru(tL)(PPh3)Cl2] have been isolated in 58–73 % yields from reactions between [Ru(PPh3)3Cl2] and tridentate Schiff bases (H2L) derived from biacetylmonoxime and acid hydrazides in methanol. The X‐ray structures of the complexes reveal unprecedented conversions of methyl groups in the biacetyl fragments of the Schiff bases into triphenyl‐vinyl‐λ5‐phosphane (=CHPHPh3) moieties in the transformed ligands (tL–). The metal centre in each complex is in a distorted octahedral N2OPCl2 coordination sphere made up of the meridionally spanning N,N,O‐donor tL–, PPh3 and two mutually trans‐oriented chlorides. In their electronic spectra, the complexes each display four absorptions due to ligand‐to‐metal and ligand‐centred transitions in the 463–260 nm range. All the complexes are one‐electron paramagnetic (μeff. at 298 K = 1.81–2.06 μB) and display rhombic EPR spectra in frozen (120 K) dichloromethane/toluene (1:1) solutions. Cyclic voltammograms of the complexes in dimethylformamide solutions show substituent‐sensitive, quasi‐reversible RuIV/RuIII and RuIII/RuII couples in the potential ranges of 0.46 to 0.54 V and –0.53 to –0.42 V (vs. Ag/AgCl), respectively.(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2009)

Synthesis of dinitrosyl iron complexes (DNICs) with intramolecular hydrogen bonding

HSCH 2CONHCH 3 and HSCH 2CON(CH 3) 2 containing a peptide bond are prepared for the synthesis of DNICs with/without intra-molecular hydrogen bonding, respectively. The IR ν(NO) bands of [(NO) 2Fe(SCH 2CONHCH 3) 2] − ( 2) appears at 1751, 1700 cm −1. In complex 2, the presence of intramolecular [NH⋯S] hydrogen bonding was verified by the observation of IR spectroscopy with N−H stretching frequency 3334 cm −1 (CDCl 3) and subsequently confirmed by single-crystal X-ray diffraction showing N−S distance of 2.94 Å. Complex 2 displays the rhombic EPR spectrum with g 1 = 2.039, g 2 = 2.031 and g 3 = 2.013 at in frozen H 2O. Complexes 2 and 3 rapidly release NO when exposed to light. The time needed for photolysis reactions of 2 is two times faster than that of 3 in less polar solvent. Representative time courses for the photolability of 2 and 3 in THF display the NO-off ability: 2 > 3.