Electrochemical properties of dinuclear [Ru([n]aneS4)] complexes of 2,3-bis(2-pyridyl)pyrazine

The syntheses of three new dinuclear [Ru([n]aneS(4))] complexes, where n = 12, 14, 16, bridged by the ligand 2,3-bis(2-pyridyl)pyrazine, (dpp) are reported. The absorption spectra of the complexes show changes in the energy of the MLCT bands within the series, indicating that the thiacrown ligands stabilise the Ru(II) oxidation state to different degrees. Electrochemical studies are also consistent with these observations, and reveal that the pi-acceptor properties of [n]aneS(4) ligands lead to metal based oxidation couples occurring at potentials that are more anodic than those observed in the analogous dinuclear [Ru(bpy)(2)](2+) complex. Despite the back-bonding properties of the thiacrown ligands leading to a reduction in ligand-bridge mediated metal-metal coupling, electrochemical interactions between the metals are still considerable.     

Redox non-innocence of thioether crowns: elucidation of the electronic structure of the mononuclear Pd(III) complexes [Pd([9]aneS3)2]3+ and [Pd([18]aneS6)]3+

The Pd(II) complexes [Pd([9]aneS(3))(2)](PF(6))(2)·2MeCN (1) ([9]aneS(3) = 1,4,7-trithiacyclononane) and [Pd([18]aneS(6))](PF(6))(2) (2) ([18]aneS(6) = 1,4,7,10,13,16-hexathiacyclooctadecane) can be oxidized electrochemically or chemically oxidized with 70% HClO(4) to [Pd([9]aneS(3))(2)](3+) and [Pd([18]aneS(6))](3+), respectively. These centers have been characterized by single crystal X-ray diffraction, and by UV/vis and multifrequency electron paramagnetic resonance (EPR) spectroscopies. The single crystal X-ray structures of [Pd(III)([9]aneS(3))(2)](ClO(4))(6)·(H(3)O)(3)·(H(2)O)(4) (3) at 150 K and [Pd([18]aneS(6))](ClO(4))(6)·(H(5)O(2))(3) (4) at 90 K reveal distorted octahedral geometries with Pd-S distances of 2.3695(8), 2.3692(8), 2.5356(9) and 2.3490(6), 2.3454(5), 2.5474(6) ?, respectively, consistent with Jahn-Teller distortion at a low-spin d(7) Pd(III) center. The Pd(II) compound [Pd([9]aneS(3))(2)](PF(6))(2) shows a one-electron oxidation process in MeCN (0.2 M NBu(4)PF(6), 293 K) at E(1/2) = +0.57 V vs. Fc(+)/Fc assigned to a formal Pd(III)/Pd(II) couple. Multifrequency (Q-, X-, S-, and L-band) EPR spectroscopic analysis of [Pd([9]aneS(3))(2)](3+) and [Pd([18]aneS(6))](3+) gives g(iso) = 2.024, |A(iso(Pd))| = 18.9 × 10(-4) cm(-1); g(xx) = 2.046, g(yy) = 2.041, g(zz) = 2.004;?|A(xx(Pd))| = 24 × 10(-4) cm(-1), |A(yy(Pd))| = 22 × 10(-4) cm(-1), |A(zz(Pd))| = 14 × 10(-4) cm(-1), |a(xx(H))| = 4 × 10(-4) cm(-1), |a(yy(H))| = 5 × 10(-4) cm(-1), |a(zz(H))| = 5.5 × 10(-4) cm(-1) for [Pd([9]aneS(3))(2)](3+), and g(iso) = 2.015, |A(iso(Pd))| = 18.8× 10(-4) cm(-1); g(xx) = 2.048 g(yy) = 2.036, g(zz) = 1.998; |a(xx(H))| = 5, |a(yy(H))| = 5, |a(zz(H))| = 6 × 10(-4) cm(-1); |A(xx(Pd))| = 23× 10(-4) cm(-1), |A(yy(Pd))| = 22 × 10(-4) cm(-1), |A(zz(Pd))| = 4 × 10(-4) cm(-1) for [Pd([18]aneS(6))](3+). Both [Pd([9]aneS(3))(2)](3+) and [Pd([18]aneS(6))](3+) exhibit five-line superhyperfine splitting in the g(zz) region in their frozen solution EPR spectra. Double resonance spectroscopic measurements, supported by density functional theory (DFT) calculations, permit assignment of this superhyperfine to through-bond coupling involving four (1)H centers of the macrocyclic ring. Analysis of the spin Hamiltonian parameters for the singly occupied molecular orbital (SOMO) in these complexes gives about 20.4% and 25% Pd character in [Pd([9]aneS(3))(2)](3+) and [Pd([18]aneS(6))](3+), respectively, consistent with the compositions calculated from scalar relativistic DFT calculations.     

Synthesis, structure, and electronic properties of RuN6 dinuclear Ru-Hbpp complexes

A series of RuN(6) dinuclear Ru-Hbpp complexes (Hbpp is the dinucleating tetraaza ligand 3,5-bis(pyridyl)pyrazole) of general formula {[Ru(II)(R(2)-trpy)(MeCN)](2)(μ-R(1)-bpp)}(3+), 10(3+)-14(3+), (R(1) = H, Me, or NO(2). and R(2) = H, Me, MeO; see Scheme 1) has been prepared from their Cl(-) or AcO(-) bridged precursors. The complexes have been characterized by UV-vis, NMR, CV, and some by X-ray. Complexes 10(3+)-14(3+), Ru(2)(II,II), were oxidized by 1 equiv in solution, leading to the mixed valence Ru(2)(II,III) complexes 10(4+)-14(4+) containing one unpaired electron and were characterized by EPR and UV-vis-near-IR, which showed metal-centered spin and the presence of low-energy IVCT bands. The H(ab) parameter indicates a relatively strong electronic coupling between the two ruthenium centers (class II). Further two electron oxidation in solution of the 10(3+)-14(3+) led to the formation of EPR silent Ru(2)(III,III) complexes 10(5+)-14(5+), that were further characterized by UV-vis-NIR. TD-DFT calculations are employed to assign the nature of the UV-vis transitions for the complexes in the various oxidation states, which are of metal to ligand charge transfer (MLCT) type for Ru(2)(II,II) and ligand to metal charge transfer (LMCT) type for Ru(2)(III,II) and Ru(2)(III,III).     

Ruthenium nitrosyl complexes with 1,4,7-trithiacyclononane and 2,2'-bipyridine (bpy) or 2-phenylazopyridine (pap) coligands. Electronic structure and reactivity aspects

The present article describes ruthenium nitrosyl complexes with the {RuNO}(6) and {RuNO}(7) notations in the selective molecular frameworks of [Ru(II)([9]aneS(3))(bpy)(NO(+))](3+) (4(3+)), [Ru(II)([9]aneS(3))(pap) (NO(+))](3+) (8(3+)) and [Ru(II)([9]aneS(3))(bpy)(NO˙)](2+) (4(2+)), [Ru(II)([9]aneS(3))(pap)(NO˙)](2+) (8(2+)) ([9]aneS(3) = 1,4,7-trithiacyclononane, bpy = 2,2'-bipyridine, pap = 2-phenylazopyridine), respectively. The nitrosyl complexes have been synthesized by following a stepwise synthetic procedure: {Ru(II)-Cl} → {Ru(II)-CH(3)CN} → {Ru(II)-NO(2)} → {Ru(II)-NO(+)} → {Ru(II)-NO˙}. The single-crystal X-ray structure of 4(3+) and DFT optimised structures of 4(3+), 8(3+) and 4(2+), 8(2+) establish the localised linear and bent geometries for {Ru-NO(+)} and {Ru-NO˙} complexes, respectively. The crystal structures and (1)H/(13)C NMR suggest the [333] conformation of the coordinated macrocyclic ligand ([9]aneS(3)) in the complexes. The difference in π-accepting strength of the co-ligands, bpy in 4(3+) and pap in 8(3+) (bpy < pap) has been reflected in the ν(NO) frequencies of 1945 cm(-1) (DFT: 1943 cm(-1)) and 1964 cm(-1) (DFT: 1966 cm(-1)) and E°({Ru(II)-NO(+)}/{Ru(II)-NO˙}) of 0.49 and 0.67 V versus SCE, respectively. The ν(NO) frequency of the reduced {Ru-NO˙} state in 4(2+) or 8(2+) however decreases to 1632 cm(-1) (DFT: 1637 cm(-1)) or 1634 cm(-1) (DFT: 1632 cm(-1)), respectively, with the change of the linear {Ru(II)-NO(+)} geometry in 4(3+), 8(3+) to bent {Ru(II)-NO˙} geometry in 4(2+), 8(2+). The preferential stabilisation of the eclipsed conformation of the bent NO in 4(2+) and 8(2+) has been supported by the DFT calculations. The reduced {Ru(II)-NO˙} exhibits free-radical EPR with partial metal contribution revealing the resonance formulation of {Ru(II)-NO˙}(major)?{Ru(I)-NO(+)}(minor). The electronic transitions of the complexes have been assigned based on the TD-DFT calculations on their DFT optimised structures. The estimated second-order rate constant (k, M(-1) s(-1)) of the reaction of the nucleophile, OH(-) with the electrophilic {Ru(II)-NO(+)} for the bpy derivative (4(3+)) of 1.39 × 10(-1) is half of that determined for the pap derivative (8(3+)), 2.84 × 10(-1) in CH(3)CN at 298 K. The Ru-NO bond in 4(3+) or 8(3+) undergoes facile photolytic cleavage to form the corresponding solvent species {Ru(II)-CH(3)CN}, 2(2+) or 6(2+) with widely varying rate constant values, (k(NO), s(-1)) of 1.12 × 10(-1) (t(1/2) = 6.2 s) and 7.67 × 10(-3) (t(1/2) = 90.3 s), respectively. The photo-released NO can bind to the reduced myoglobin to yield the Mb-NO adduct.     

Self-assembly of electroactive thiacrown ruthenium(II) complexes into hydrogen-bonded chain and tape networks

A family of coordination complexes has been synthesized, each comprising a ruthenium(II) center ligated by a thiacrown macrocycle, [9]aneS(3), [12]aneS(4), or [14]aneS(4), and a pair of cis-coordinated ligands, niotinamide (nic), isonicotinamide (isonic), or p-cyanobenzamide (cbza), that provide the complexes with peripherally situated amide groups capable of hydrogen bond formation. The complexes [Ru([9]aneS(3))(nic)(2)Cl]PF(6), 1(PF(6)); [Ru([9]aneS(3)) (isonic)(2)Cl]PF(6), 2(PF(6)); [Ru([12]aneS(4))(nic)(2)](PF(6))(2), 3(PF(6))(2); [Ru([12]aneS(4))(isonic)(2)](PF(6))(2), 4(PF(6))(2); [Ru([12]aneS(4)) (cbza)(2)](PF(6))(2), 5(PF(6))(2); [Ru([14]aneS(4))(nic)(2)](PF(6))(2), 6(PF(6))(2); [Ru([14]aneS(4))(isonic)(2)](PF(6))(2), 7(PF(6))(2); and [Ru([14]aneS(4))(cbza)(2)](PF(6))(2), 8(PF(6))(2) have been characterized by NMR spectroscopy, mass spectrometry, and elemental analysis. UV/visible spectroscopy shows that each complex exhibits an intense high-energy band (230-255 nm) assigned to a pi-pi* transition and a lower energy band (297-355 nm) assigned to metal-to-ligand charge-transfer transitions. Electrochemical studies indicate good reversibility for the oxidations of complexes with nic and isonic ligands (|I(a)/I(c)| = 1; DeltaEp < 100 mV), In contrast, complexes 5 and 8, which incorporate cbza ligands, display oxidations that are not fully electrochemically reversible (|I(a)/I(c)| = 1, DeltaEp > or = 100 mV). Metal-based oxidation couples between 1.32 and 1.93 V versus Ag/AgCl can be rationalized in term of the acceptor capabilities of the thiacrown ligands and the amide-bearing ligands, as well as the pi-donor capacity of the chloride ligands in compounds 1 and 2. The potential to use these electroactive metal complexes as building blocks for hydrogen-bonded crystalline materials has been explored. Crystal structures of compounds 1(PF(6)).H(2)O, 1(BF(4)).2H(2)O, 2(PF(6)), 3(PF(6))(2), 6(PF(6))(2)CH(3)NO(2), and 8(PF(6))(2) are reported. Four of the six form amide-amide N-H...O hydrogen bonds leading to networks constructed from amide C(4) chains or tapes containing R(2)(2) (8) hydrogen-bonded rings. The other two, 2(PF(6)) and 8(PF(6)), form networks linked through amide-anion N-H...F hydrogen bonds. The role of counterions and solvent in interrupting or augmenting direct amide-amide network propagation is explored, and the systematic relationship between the hydrogen-bonded networks formed across the series of structures is presented, showing the relationship between chain and tape arrangements and the progression from 1D to 2D networks. The scope for future systematic development of electroactive tectons into network materials is discussed.     

Ruthenium complexes containing 2-(2-nitrosoaryl)pyridine: structural, spectroscopic, and theoretical studies

Ruthenium complexes containing 2-(2-nitrosoaryl)pyridine (ON(^)N) and tetradentate thioether 1,4,8,11-tetrathiacyclotetradecane ([14]aneS4), [Ru(ON(^)N)([14]aneS4)](2+) [ON(^)N = 2-(2-nitrosophenyl)pyridine (2a), 10-nitrosobenzo[h]quinoline (2b), 2-(2-nitroso-4-methylphenyl)pyridine, (2c), 2-(2-nitrosophenyl)-5-(trifluoromethyl)pyridine (2d)] and analogues with the 1,4,7-trithiacyclononane ([9]aneS3)/tert-butylisocyanide ligand set, [Ru(ON(^)N)([9]aneS3)(C≡N(t)Bu)](2+) (4a and 4b), have been prepared by insertion of a nitrosonium ion (NO(+)) into the Ru-aryl bond of cyclometalated ruthenium(II) complexes. The molecular structures of the ON(^)N-ligated complexes 2a and 2b reveal that (i) the ON(^)N ligands behave as bidentate chelates via the two N atoms and the bite angles are 86.84(18)-87.83(16)° and (ii) the Ru-N(NO) and N-O distances are 1.942(5)-1.948(4) and 1.235(6)-1.244(5) ?, respectively. The Ru-N(NO) and N-O distances, together with ν(N═O), suggest that the coordinated ON(^)N ligands in this work are neutral moiety (ArNO)(0) rather than monoanionic radical (ArNO)(?-) or dianion (ArNO)(2-) species. The nitrosated complexes 2a-2d show moderately intense absorptions centered at 463-484 nm [ε(max) = (5-6) × 10(3) dm(3) mol(-1) cm(-1)] and a clearly discriminable absorption shoulder around 620 nm (ε(max) = (6-9) × 10(2) dm(3) mol(-1) cm(-1)), which tails up to 800 nm. These visible absorptions are assigned as a mixing of d(Ru) → ON(^)N metal-to-ligand charge-transfer and ON(^)N intraligand transitions on the basis of time-dependent density functional theory (TD-DFT) calculations. The first reduction couples of the nitrosated complexes range from -0.53 to -0.62 V vs Cp(2)Fe(+/0), which are 1.1-1.2 V less negative than that for [Ru(bpy)([14]aneS4)](2+) (bpy = 2,2'-bipyridine). Both electrochemical data and DFT calculations suggest that the lowest unoccupied molecular orbitals of the nitrosated complexes are ON(^)N-centered. Natural population analysis shows that the amount of positive charge on the Ru centers and the [Ru([14]aneS4)] moieties in 2a and 2b is larger than that in [Ru(bpy)([14]aneS4)](2+). According to the results of the structural, spectroscopic, electrochemical, and theoretical investigations, the ON(^)N ligands in this work have considerable π-acidic character and behave as better electron acceptors than bpy.     

Redox non-innocence of thioether macrocycles: elucidation of the electronic structures of mononuclear complexes of gold(II) and silver(II)

The mononuclear +2 oxidation state metal complexes [Au([9]aneS(3))(2)](2+) and [Ag([18]aneS(6))](2+) have been synthesized and characterized crystallographically. The crystal structure of the Au(II) species [Au([9]aneS(3))(2)](BF(4))(2) shows a Jahn-Teller tetragonally distorted geometry with Au-S(1) = 2.839(5), Au-S(2) = 2.462(5), and Au-S(3) = 2.452(5) A. The related Ag(II) complex [Ag([18]aneS(6))](ClO(4))(2) has been structurally characterized at both 150 and 30 K and is the first structurally characterized complex of Ag(II) with homoleptic thioether S-coordination. The single-crystal X-ray structure of [Ag([18]aneS(6))](ClO(4))(2) confirms octahedral homoleptic S(6)-thioether coordination. At 150 K, the structure contains two independent Ag(II)-S distances of 2.569(7) and 2.720(6) A. At 30 K, the structure retains two independent Ag(II)-S distances of 2.615(6) and 2.620(6) A, with the complex cation retaining 3-fold symmetry. The electronic structures of [Au([9]aneS(3))(2)](2+) and [Ag([18]aneS(6))](2+) have been probed in depth using multifrequency EPR spectroscopy coupled with DFT calculations. For [Au([9]aneS(3))(2)](2+), the spectra are complex due to large quadrupole coupling to (197)Au. Simulation of the multifrequency spectra gives the principal g values, hyperfine (A) and quadrupole (P) couplings, and furthermore reveals non-co-incidence of the principal axes of the P tensor with respect to the A and g matrices. These results are rationalized in terms of the electronic and geometric structure and reveal that the SOMO has ca. 30% Au 5d(xy)() character, consistent with DFT calculations (27% Au character). For [Ag([18]aneS(6))](2+), detailed EPR spectroscopic analysis confirms that the SOMO has ca. 26% Ag 4d(xy)() character and DFT calculations are consistent with this result (22% Ag character).     

Factors that influence the antiproliferative activity of half sandwich Ru(II)-[9]aneS3 coordination compounds: activation kinetics and interaction with guanine derivatives

Half sandwich Ru(ii)-[9]aneS3 complexes ([9]aneS3 = 1,4,7-trithiacyclononane) are being studied for their antiproliferative activity. We investigated here the activation kinetics of three such complexes, namely [Ru([9]aneS3)(en)Cl](PF(6)) (1), [Ru([9]aneS3)(bpy)Cl](PF(6)) (2) and [Ru([9]aneS3)(pic)Cl] (3) (en = 1,2-diaminoethane, pic = picolinate), and their interaction with DNA model bases. The aim of the study was to assess how they are affected by the nature and charge of the chelating ligand. The model reactions of 1-3 with the guanine derivatives 9-methylguanine (9MeG), guanosine (Guo), and guanosine 5'-monophosphate (5'-GMP) were studied by NMR spectroscopy. All reactions lead, although with different rates and to different extents, to the formation of monofunctional adducts with the guanine derivatives N7-bonded to the Ru center. Two products, the complexes [Ru([9]aneS3)(en)(9MeG-N7)](PF(6))(2) (4) and [Ru([9]aneS3)(pic)(9MeG-N7)](PF(6)) (10), were structurally characterized also by X-ray crystallography. The structure of 4 is stabilized by strong intramolecular H-bonding between an NH of en and the carbonyl O6 of 9MeG. The kinetics of aquation and anation of complexes 2 and 3, as well as the kinetics and the mechanism of the reaction of complexes 1-3 with the biologically more relevant 5'-GMP ligand were studied by UV-Vis spectroscopy. The rate of the reaction of 1-3 with 5'-GMP depends on the nature of the chelating ligand rather than on the charge of the complex, decreasing in the order 3≈2 > 1. The measured enthalpies and entropies of activation (ΔH(≠) > 0, ΔS(≠) < 0) support an associative mechanism for the substitution process.     

Synthesis and study of Ru,Rh,Ru triads: modulation of orbital energies in a supramolecular architecture

Supramolecular trimetallic complexes [((tpy)RuCl(BL))(2)RhCl(2)](3+) where tpy = 2,2':6',2' '-terpyridine and BL = dpp or bpm [dpp = 2,3-bis(2-pyridyl)pyrazine and bpm = 2,2'-bipyrimidine] have been synthesized and characterized. The mixed-metal complexes couple a reactive rhodium(III) center to two ruthenium(II) light absorbers to form a light absorber-electron collector-light absorber triad. The variation of the bridging (dpp and bpm) and terminal (tpy in lieu of bpy) ligands has some profound effects on the properties of these complexes, and they are remarkably different from the previously reported [((bpy)(2)Ru(bpm))(2)RhCl(2)](5+) system. The electrochemical data for both title trimetallics consist of overlapping Ru(III/II) couples for both terminal metals at 1.12 V versus the Ag/AgCl reference electrode. Cathodically an irreversible Rh(III/I) reduction followed by bridging ligand reductions is seen. This is indicative of highest occupied molecular orbitals (HOMO) localized on the terminal ruthenium metal centers and a lowest unoccupied molecular orbital (LUMO) residing on the rhodium. This rhodium-based LUMO is in contrast to the bpy analogue [((bpy)(2)Ru(bpm))(2)RhCl(2)](5+), which has a bpm(pi) localized LUMO. This orbital inversion by terminal ligand variation illustrates the similar energy of these Rh(dsigma) and bpm(pi) orbitals within this structural motif. Both title trimetallics possess broad, low-energy Ru --> BL charge transfer absorbances at 540 nm (dpp) and 656 nm (bpm). A comparison of the spectroscopic, electrochemical, and spectroelectrochemical properties of these trimetallic complexes is presented.     

Half-sandwich RuII-[9]aneS3 complexes with dicarboxylate ligands: synthesis, characterization and chemical behavior

With the aim of further developing the structure-activity relationship in biologically active half-sandwich Ru(ii)-[9]aneS(3) complexes ([9]aneS(3)=1,4,7-trithiacyclononane), a series of new mono- and dinuclear complexes bearing the chelating dicarboxylate ligands oxalate (ox), malonate (mal) and methylmalonate (mmal), have been synthesized and studied. Treatment of the precursor [Ru([9]aneS(3))(dmso)(3)][CF(3)SO(3)](2) (7) with equivalent amounts of K(2)(dicarb) afforded the corresponding neutral complexes with the general formula [Ru([9]aneS(3))(dmso-S)(eta(2)-dicarb)] (where dicarb=ox (1), mal (2) and mmal (3)), while using half an equivalent of K(2)(ox), the symmetric dimer [{Ru([9]aneS(3))(dmso-S)}(2)(mu-eta(4)-ox)][CF(3)SO(3)](2) (4) was isolated. The reaction of with the oxalato complex fac-[Ru(dmso-S)(3)(dmso-O)(eta(2)-ox)] (9) yielded two asymmetric dimers, namely [{Ru([9]aneS(3))(dmso-S)}(mu-eta(4)-ox){fac-Ru(dmso-S)(3)(CF(3)SO(3))}][CF(3)SO(3)] (5) and [{Ru([9]aneS(3))(dmso-S)}(mu-eta(4)-ox){fac-Ru(dmso-S)(3)(dmso-O)}][CF(3)SO(3)](2) (6), depending on the reaction conditions. All new complexes were structurally characterized, both in solution (by NMR spectroscopy) and in the solid state (by X-ray crystallography). The chemical behavior of the complexes in aqueous solution was studied by UV-Vis and NMR spectroscopy in view of their potential antitumor activity: the monomers partially release a dmso ligand to yield the monofunctional aqua adduct [Ru([9]aneS(3))(eta(2)-dicarb)(H(2)O)], while the dimers rapidly open up the oxalato bridge to give two mononuclear fragments. Splitting of the asymmetric dimers 5 and 6 occurs selectively and the ox moiety remains bonded to the fac-Ru(dmso-S)(3) fragment. A detailed comparison of the structural and chemical features of 1-6 with those of similar dicarboxylate complexes possessing the fac-Ru(dmso-S)(3) fragment in place of Ru([9]aneS(3)) allows us to draw a number of general conclusions on the binding preferences of dicarb ligands on the octahedral Ru(II) center.     

Mixed-metal supramolecular complexes coupling phosphine-containing Ru(II) light absorbers to a reactive Pt(II) through polyazine bridging ligands

Supramolecular bimetallic Ru(II)/Pt(II) complexes [(tpy)Ru(PEt(2)Ph)(BL)PtCl(2)](2+) and their synthons [(tpy)Ru(L)(BL)](n)()(+) (where L = Cl(-), CH(3)CN, or PEt(2)Ph; tpy = 2,2':6',2'-terpyridine; and BL = 2,2'-bipyrimidine (bpm) or 2,3-bis(2-pyridyl)pyrazine (dpp)) have been synthesized and studied by cyclic voltammetry, electronic absorption spectroscopy, mass spectral analysis, and (31)P NMR. The mixed-metal bimetallic complexes couple phosphine-containing Ru chromophores to a reactive Pt site. These complexes show how substitution of the monodentate ligand on the [(tpy)RuCl(BL)](+) synthons can tune the properties of these light absorbers (LA) and incorporate a (31)P NMR tag by addition of the PEt(2)Ph ligand. The redox potentials for the Ru(III/II) couples occur at values greater than 1.00 V versus the Ag/AgCl reference electrode and can be tuned to more positive potentials on going from Cl(-) to CH(3)CN or PEt(2)Ph (E(1/2) = 1.01, 1.55, and 1.56 V, respectively, for BL = bpm). The BL(0/-) couple at -1.03 (bpm) and -1.05 V (dpp) for [(tpy)Ru(PEt(2)Ph)(BL)](2+) shifts dramatically to more positive potentials upon the addition of the PtCl(2) moiety to -0.34 (bpm) and -0.50 V (dpp) for the [(tpy)Ru(PEt(2)Ph)(BL)PtCl(2)](2+) bridged complex. The lowest energy electronic absorption for these complexes is assigned as the Ru(d pi) --> BL(pi*) metal-to-ligand charge transfer (MLCT) transition. These MLCT transitions are tuned to higher energy in the monometallic synthons when Cl(-) is replaced by CH(3)CN or PEt(2)Ph (516, 452, and 450 nm, for BL = bpm, respectively) and to lower energy when Pt(II)Cl(2) is coordinated to the bridging ligand (560 and 506 nm for BL = bpm or dpp). This MLCT state displays a broad emission at room temperature for all the dpp systems with the [(tpy)Ru(PEt(2)Ph)(dpp)PtCl(2)](2+) system exhibiting an emission centered at 750 nm with a lifetime of 56 ns. These supramolecular complexes [(tpy)Ru(PEt(2)Ph)(BL)PtCl(2)](2+) represent the covalent linkage of TAG-LA-BL-RM assembly (TAG = NMR active tag, RM = Pt(II) reactive metal).     

Enhancement of metal-metal coupling at a considerable distance by using 4-pyridinealdazine as a bridging ligand in polynuclear complexes of rhenium and ruthenium

Novel polynuclear complexes of rhenium and ruthenium containing PCA (PCA = 4-pyridinecarboxaldehyde azine or 4-pyridinealdazine or 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene) as a bridging ligand have been synthesized as PF(6-) salts and characterized by spectroscopic, electrochemical, and photophysical techniques. The precursor mononuclear complex, of formula [Re(Me(2)bpy)(CO)(3)(PCA)](+) (Me(2)bpy = 4,4'-dimethyl-2,2'-bipyridine), does not emit at room temperature in CH(3)CN, and the transient spectrum found by flash photolysis at lambda(exc) = 355 nm can be assigned to a MLCT (metal-to-ligand charge transfer) excited state [(Me(2)bpy)(CO)(3)Re(II)(PCA(-))](+), with lambda(max) = 460 nm and tau < 10 ns. The spectral properties of the related complexes [[Re(Me(2)bpy)(CO)(3)}(2)(PCA)](2+), [Re(CO)(3)(PCA)(2)Cl], and [Re(CO)(3)Cl](3)(PCA)(4) confirm the existence of this low-energy MLCT state. The dinuclear complex, of formula [(Me(2)bpy)(CO)(3)Re(I)(PCA)Ru(II)(NH(3))(5)](3+), presents an intense absorption in the visible spectrum that can be assigned to a MLCT d(pi)(Ru) --> pi(PCA); in CH(3)CN, the value of lambda (max) = 560 nm is intermediate between those determined for [Ru(NH(3))(5)(PCA)](2+) (lambda(max) = 536 nm) and [(NH(3))(5)Ru(PCA)Ru(NH(3))(5)](4+) (lambda(max) = 574 nm), indicating a significant decrease in the energy of the pi-orbital of PCA. The mixed-valent species, of formula [(Me(2)bpy)(CO)(3)Re(I)(PCA)Ru(III)(NH(3))(5)](4+), was obtained in CH(3)CN solution, by bromine oxidation or by controlled-potential electrolysis at 0.8 V in a OTTLE cell of the [Re(I),Ru(II)] precursor; the band at lambda(max) = 560 nm disappears completely, and a new band appears at lambda(max) = 483 nm, assignable to a MMCT band (metal-to-metal charge transfer) Re(I) --> Ru(III). By using the Marcus-Hush formalism, both the electronic coupling (H(AB)) and the reorganization energy (lambda) for the metal-to-metal intramolecular electron transfer have been calculated. Despite the considerable distance between both metal centers (approximately 15.0 Angstroms), there is a moderate coupling that, together with the comproportionation constant of the mixed-valent species [(NH(3))(5)Ru(PCA)Ru(NH(3))(5)](5+) (K(c) approximately 10(2), in CH(3)CN), puts into evidence an unusual enhancement of the metal-metal coupling in the bridged PCA complexes. This effect can be accounted for by the large extent of "metal-ligand interface", as shown by DFT calculations on free PCA. Moreover, lambda is lower than the driving force -DeltaG degrees for the recombination charge reaction [Re(II),Ru(II)] --> [Re(I),Ru(III)] that follows light excitation of the mixed-valent species. It is then predicted that this reverse reaction falls in the Marcus inverted region, making the heterodinuclear [Re(I),Ru(III)] complex a promising model for controlling the efficiency of charge-separation processes.     

1-(2-Picolyl)-substituted 1,2,3-triazole as novel chelating ligand for the preparation of ruthenium complexes with potential anticancer activity

The 1,4-disubstituted 1,2,3-triazole ligand prepared by click chemistry 1-(2-picolyl)-4-phenyl-1H-1,2,3-triazole (ppt) was investigated as novel chelating ligand for Ru(II) complexes with potential antitumor activity. The preparation and structural characterization, mainly by NMR spectroscopy in solution and by X-ray crystallography in the solid state, of four new Ru(II) complexes is reported: two isomeric Ru-dmso compounds, trans,cis-[RuCl(2)(dmso-S)(2)(ppt)] (1) and cis,cis-[RuCl(2)(dmso-S)(2)(ppt)] (2), and two half-sandwich Ru-[9]aneS(3) coordination compounds, [Ru([9]aneS(3))(dmso-S)(ppt)][CF(3)SO(3)](2) (3) and [Ru([9]aneS(3))Cl(ppt)][CF(3)SO(3)] (4). In all compounds ppt firmly binds to ruthenium in a bidentate fashion through the pyridyl nitrogen atom and the triazole N2, thus forming a puckered six-membered ring. The chemical behavior in aqueous solution of the water-soluble complexes 3 and 4 was studied by UV-Vis and NMR spectroscopy and compared to that of the previously described organometallic analogue [Ru(η(6)-p-cymene)Cl(ppt)][Cl] (5) in view of their potential antitumor activity. Compounds 3-5 were tested also in vitro for cytotoxic activity against two human cancer cell lines, one sensitive and one resistant to cisplatin, in comparison with cisplatin. Compound 4, the one that aquates faster, was found to be more cytotoxic than cisplatin against human lung squamose carcinoma cell line (A-549).     

Linear-type S-bridged triruthenium complexes with aliphatic aminothiolate ligands: synthesis, characterization, and properties

Treatment of [RuCl(2)(DMSO)(4)] with 2-aminoethanethiol (Haet) in ethanol gave a dicationic triruthenium complex, [Ru[Ru(aet)(3)](2)]Cl(2) ([1]Cl(2)). Complex [1]Cl(2) was also obtained by treatment of RuCl(3).nH(2)O with excess Haet in water. When [1](2+) was chromatographed on a cation-exchange column of SP-Sephadex C-25, meso (DeltaLambda) and racemic (DeltaDelta/LambdaLambda) isomers of the corresponding tricationic complex, [Ru[Ru(aet)(3)](2)](3+) ([2](3+)), were eluted with aqueous NaNO(3). The racemic isomer of [2](3+) was optically resolved into DeltaDelta and LambdaLambda isomers by using [Sb(2)(R,R-tartrato)(2)](2-) as a resolving agent. The molecular structures of DeltaLambda- and DeltaDelta/LambdaLambda-[2](NO(3))(3) were determined by X-ray crystallography. In these complexes, the central Ru atom is coordinated by six thiolato groups from two terminal fac-(S)-[Ru(aet)(3)] units in an octahedral geometry, forming a linear-type S-bridged triruthenium structure. The spectroelectrochemical studies on the electronic absorption and CD spectra, together with the electrochemical studies, demonstrated that [1](2+) and [2](3+) are interconvertible with each other through a one-electron redox process, retaining the chirality of the triruthenium structure. Their electronic structures were investigated on the basis of EPR and magnetic susceptibility measurements, which indicated that [1](2+) and [2](3+) have spin ground states of S(t) = 0 and S(t) = 1/2, respectively. The corresponding L-cysteinato complex, [Ru[Ru(L-cys-N,S)(3)](2)](3-), which was formed from RuCl(3).nH(2)O and excess L-cysteine (L-H(2)cys) in water followed by air oxidation, is also presented.     

Ruthenium complexes of 2-[(4-(arylamino)phenyl)azo]pyridine formed via regioselective phenyl ring amination of coordinated 2-(phenylazo)pyridine: isolation of products,X-ray structure,and redox and optical properties

Aromatic ring amination reactions in the ruthenium complex of 2-(phenylazo)pyridine is described. The substitutionally inert cationic brown complex [Ru(pap)(3)](ClO(4))(2) (1) (pap = 2-(phenylazo)pyridine) reacts smoothly with aromatic amines neat and in the presence of air to produce cationic and intense blue complexes [Ru(HL(2))(3)](ClO(4))(2) (2) (HL(2) = 2-[(4-(arylamino)phenyl)azo]pyridine). These were purified on a preparative TLC plate. The X-ray structure of the new and representative complex 2c has been solved to characterize them. The results are compared with those of the starting complex, [Ru(pap)(3)](ClO(4))(2) (1). The transformation 1 --> 2 involves aromatic ring amination at the para carbon (with respect to the diazo function) of the pendant phenyl rings of all three coordinated pap ligands in 1. The transformation is stereoretentive, and the amination reaction is regioselective. The extended ligand HL(2) coordinates as a bidentate ligand and chelates to ruthenium(II) through the pyridine and one of the azo nitrogens. The amine nitrogen of this bears a hydrogen atom and remains uncoordinated. Similarly, the amination reaction on the mixed-ligand complex [Ru(pap)(bpy)(2)](ClO(4))(2) produces the blue complex [Ru(HL(2))(bpy)(2)](ClO(4))(2) (3) as anticipated. The reactions of [RuCl(2)(dmso)(4)] and [Ru(S)(2)(L)(2)](2+) (dmso = dimethyl sulfoxide, S = labile coordinated solvent, L = 2,2'-bipyridine (bpy) and pap) with the preformed HL(2) ligand have been explored. The structure of the representative complex [RuCl(2)(HL(2a))(2)] (5a) is reported. It has the chlorides in trans configuration while the pyridine as well as azo nitrogens are in cis geometry. Optical spectra and redox properties of the newly synthesized complexes are reported. All the ruthenium complexes of HL(2) are characterized by their intense blue solution colors. The lowest energy transitions in these complexes appear near 600 nm, which have been attributed to intraligand charge-transfer transitions. For example, the lowest energy visible range transition in [Ru(HL(2b))(3)](2+) appears at 602 nm and its intensity is 65 510 M(-1) cm(-1). All the tris chelates show multiple-step electron-transfer processes. In [Ru(HL(2))(3)](2+), six reductions waves constitute the complete electron-transfer series. The electrons are believed to be added successively to the three azo functions. In the mixed-ligand chelates [Ru(HL(2))(pap)(2)](2+) and [Ru(HL(2))(bpy)(2)](2+) the reductions due to HL(2), pap, and bpy are observed.     

Cyclometallated Pt(II) and Pd(II) complexes with a trithiacrown ligand

We report the synthesis and full characterization for a series of cyclometallated complexes of Pt(II) and Pd(II) incorporating the fluxional trithiacrown ligand 1,4,7-trithiacyclononane ([9]aneS3). Reaction of [M(C insertion mark N)(micro-Cl)]2 (M = Pt(II), Pd(II); C insertion mark N = 2-phenylpyridinate (ppy) or 7,8-benzoquinolinate (bzq)) with [9]aneS3 followed by metathesis with NH4PF6 yields [M(C insertion mark N)([9]aneS3)](PF6). The complexes [M(C insertion mark P)([9]aneS3)](PF6) (M = Pt(II), Pd(II); Cinsertion markP = [CH2C6H4P(o-tolyl)2-C,P]-) were synthesized from their respective [Pt(C insertion mark P)(micro-Cl)]2 or [Pd(C insertion mark P)(micro-O2CCH3)]2 (C insertion mark P) starting materials. All five new complexes have been fully characterized by multinuclear NMR, IR and UV-Vis spectroscopies in addition to elemental analysis, cyclic voltammetry, and single-crystal structural determinations. As expected, the coordinated [9]aneS3 ligand shows fluxional behavior in its NMR spectra, resulting in a single 13C NMR resonance despite the asymmetric coordination environment of the cyclometallating ligand. Electrochemical studies reveal irreversible one-electron metal-centered oxidations for all Pt(II) complexes, but unusual two-electron reversible oxidations for the Pd(II) complexes of ppy and bzq. The X-ray crystal structures of each complex indicate an axial M-S interaction formed by the endodentate conformation of the [9]aneS3 ligand. The structure of [Pd(bzq)([9]aneS3)](PF6) exhibits disorder in the [9]aneS3 conformation indicating a rare exodentate conformation as the major contributor in the solid-state structure. DFT calculations on [Pt([9]aneS3)(ppy)](PF6) and [Pd([9]aneS3)(ppy)](PF6) indicate the HOMO for both complexes is primarily dz2 in character with a significant contribution from the phenyl ring of the ppy ligand and p orbital of the axial sulfur donor. In contrast, the calculated LUMO is primarily ppy pi* in character for [Pt([9]aneS3)(ppy)](PF6), but dx2-y2 in character for [Pd([9]aneS3)(ppy)](PF6).     

Oxidative homolysis of a nitrosylchromium complex

Metal(III)-polypyridine complexes [M(NN)(3)](3+) (M = Ru or Fe; NN = bipyridine (bpy), phenanthroline (phen), or 4,7-dimethylphenanthroline (Me(2)-phen)) oxidize the nitrosylpentaaquachromium(III) ion, [Cr(aq)NO](2+), with an overall 4:1 stoichiometry, 4 [Ru(bpy)(3)](3+) + [Cr(aq)NO](2+) + 2 H(2)O --> 4 [Ru(bpy)(3)](2+) + [Cr(aq)](3+) + NO(3)(-) + 4 H(+). The kinetics follow a mixed second-order rate law, -d[[M(NN)(3)](3+)]/dt = nk[[M(NN)(3)](3+)][[Cr(aq)NO](2+)], in which k represents the rate constant for the initial one-electron transfer step, and n = 2-4 depending on reaction conditions and relative rates of the first and subsequent steps. With [Cr(aq)NO](2+) in excess, the values of nk are 283 M(-1) s(-1) ([Ru(bpy)(3)](3+)), 7.4 ([Ru(Me(2)-phen)(3)](3+)), and 5.8 ([Fe(phen)(3)](3+)). In the proposed mechanism, the one-electron oxidation of [Cr(aq)NO](2+) releases NO, which is further oxidized to nitrite, k = 1.04x10(6) M(-1) s(-1), 6.17x10(4), and 1.12x10(4) with the three respective oxidants. Further oxidation yields the observed nitrate. The kinetics of the first step show a strong correlation with thermodynamic driving force. Parallels were drawn with oxidative homolysis of a superoxochromium(III) ion, [Cr(aq)OO](2+), to gain insight into relative oxidizability of coordinated NO and O(2), and to address the question of the "oxidation state" of coordinated NO in [Cr(aq)NO](2+).     

2,2'-Dipyridylketone (dpk) as ancillary acceptor and reporter ligand in complexes [(dpk)(Cl)Ru(mu-tppz)Ru(Cl)(dpk)]n+ where tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine

The tppz-bridged diruthenium(II) complex [(dpk)(Cl)Ru(II)(mu-tppz)Ru(II)(Cl)(dpk)](ClO4)2, [2](ClO4)2, and mononuclear [(dpk)(Cl)Ru(II)(tppz)](ClO4), [1](ClO4) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine, dpk = 2,2'-dipyridylketone], have been synthesized. The 260 mV separation between successive one-electron oxidation couples in [2]2+ translates to a relatively small comproportionation constant, Kc, of 2.5 x 10(4) for the intermediate. It is shown how electrochemical data (E(ox), E(red), Kc) reflect the donor/acceptor effects of ancillary ligands L in a series of systems [(L)ClRu(mu-tppz)RuCl(L)]n, particularly the competition between L and tppz for electron density from the metal. According to EPR (g1 = 2.470, g2 = 2.195, and g3 = 1.873 at 4 K) the intermediate [2]3+ is a mixed-valent Ru(II)Ru(III) species which shows a rather narrow intervalence charge transfer (IVCT) band at 1800 nm (epsilon = 1500 M(-1) cm(-1)). The width at half-height (Deltanu(1/2)) of 700 cm(-1) of the IVCT band is much smaller than the calculated value of 3584 cm(-1), obtained by using the Hush formula Deltanu(1/2) = (2310E(op))(1/2) (E(op) = 5556 cm(-1), energy of the IVCT transition) which would be applicable to localized (Class II) mixed-valent Ru(II)Ru(III) systems. Valence delocalization in [2]3+ is supported by the uniform shift of the nu(C=O) band of the N,N'-coordinated dpk ligands from 1676 cm(-1) in the Ru(II)Ru(II) precursor to 1690 cm(-1) in the Ru(2.5)Ru(2.5) form, illustrating the use of the dpk acceptor to act as reporter ligand via the free but pi-conjugated organic carbonyl group. The apparent contradiction between the moderate value of Kc and the narrow IVCT band is being discussed considering "borderline" or "hybrid" "Class II-III" concepts of mixed-valency, as well as coordination aspects, i.e., the bis-tridentate nature of the pi-acceptor bridging ligand. Altogether, the complex ions [1]+ and [2]2+ display four and five successive reduction processes, respectively, involving both tppz- and dpk-based unoccupied pi orbitals. The one-electron reduced form [2]+ has been assigned as a tppz*- radical-anion-containing species which exhibits a free-radical-type EPR signal at 4K (g(parallel) = 2.002, g(perpendicular) = 1.994) and one moderately intense ligand-based low-energy band at 965 nm (epsilon = 1100 M(-1) cm(-1)).     

New half sandwich Ru(II) coordination compounds for anticancer activity

With the aim of expanding the structure-activity relationship investigation, the series of Ru(II) half sandwich coordination compounds of the type [Ru([9]aneS3)(chel)(L)](n+) previously described by us (where [9]aneS3 is the neutral face-capping ligand 1,4,7-trithiacyclononane, chel is a neutral or anonic chelating ligand, L = Cl(-) or dmso-S, n = 0-2) was extended to 1,4,7-triazacyclononane ([9]aneN3). In addition, new neutral N-N, and anionic N-O and O-O chelating ligands, i.e. dach (trans-1,2-diaminocyclohexane), pic(-) (picolinate), and acac(-) (acetylacetonate), were investigated in combination with both [9]aneS3 and [9]aneN3. Overall, ten new half-sandwich complexes were prepared and fully characterized and their chemical behaviour in aqueous solution was established. The single-crystal X-ray structures of eight of them, including the versatile precursor [Ru([9]aneN3)(dmso-S)(2)Cl]Cl (9), were also determined. The results of in vitro antiproliferative tests performed on selected compounds against MDA-MB-231 human mammary carcinoma cells confirmed that, in this series, only compounds that hydrolyse the monodentate ligand at a reasonable rate show moderate activity, provided that the chelate ligand is a hydrogen bond donor.     

What a difference ancillary thienyl makes: unexpected additional stabilization of the diruthenium(III,II) but not the diosmium(III,II) mixed-valent state in tetrazine ligand-bridged complexes

The Ru(2)(III,II) mixed-valent state is strongly stabilized in [(bpy)(2)Ru(mu-bttz)Ru(bpy)(2)](5+) (3(5+), bttz = 3,6-bis(2-thienyl)-1,2,4,5-tetrazine, as evident from lowered oxidation potentials and isolability, a strongly increased comproportionation constant K(c) = 10(16.6), and a high-energy intervalence charge transfer band at 10100 cm(-1). Curiously, no such effects were observed for the diosmium(III,II) analogue, whereas the related systems [(bpy)(2)M(mu-bmptz)M(bpy)(2)](5+), bmptz = 3,6-bis(4-methyl-2-pyridyl)-1,2,4,5-tetrazine, exhibit conventional behavior, i.e., a slightly higher K(c) value of the Os(2)(III,II) analogue. EPR signals were observed at 4 K for 3(5+) but not for the other mixed-valent species, and high-frequency (285 GHz) EPR was employed to study the diruthenium(II) radical complexes 2(3+) and 3(3+).