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Articles • J. Braz. Chem. Soc. 21 (7) • 2010 • https://doi.org/10.1590/S0103-50532010000700022   copy

Chemical and photochemical properties of a ruthenium nitrosyl complex with the N-monosubstituted cyclam 1-(3-Propylammonium)-1,4,8,11-tetraazacyclotetradecane

Authorship SCIMAGO INSTITUTIONS RANKINGS

Abstracts

The amine-functionalized trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 complex (1-pramcyH = 1-(3-propylammonium)-1,4,8,11-tetraazacyclotetradecane) was synthesized from trans-[RuCl(tfms) (1-pramcyH)](tfms)2 (tfms = trifluoromethanesulfonate) in acidic aqueous solution in the presence of nitric oxide (NO). The complex was characterized by elemental, spectroscopic (UV-Vis, IR, ¹H and 13C NMR) and electrochemical analyses. Two pKa values (7.0 and 8.2) were estimated for trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 and were assigned to one of the cyclam nitrogen protons and to the protonated aminopropyl group. Reduction of trans-[Ru(NO)Cl(1-pramcyH)]3+ results in rapid loss of chloride followed by slower loss of NO, while irradiation of the complex in aqueous deaerated conditions suggests photochemical labilization of NO. The quantum yields for NO photoaquation decrease as the irradiation wavelength increases, being noticeable only at λirr < 370 nm, and increase as pH increases. The behavior of trans-[Ru(NO)Cl(1-pramcyH)]3+, which contains an aminopropyl substituted cyclam, parallels that reported for the analogous complex with the unsubstituted ligand, but differs from that described for the complex in which carboxypropyl is the substituent.

nitrosyl; ruthenium; nitric oxide donor; substituted cylam; mono-N-functionalized cyclam

O complexo aminofuncionalizado trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 (1-pramcyH = 1-(3-propilamônio)-1,4,8,11-tetraazaciclotetradecano) foi sintetizado através da reação do trans-[RuCl(tfms)(1-pramcyH)](tfms)2 (tfms = trifluorometanossulfonato) com óxido nítrico (NO) em solução aquosa ácida. O complexo foi caracterizado por análise elementar, espectroscópica (UV-vis, IV, RMN de ¹H e 13C) e eletroquímica. Dois valores de pKa (7,0 e 8,2) foram determinados para trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 e foram atribuídos a um dos prótons do grupo amina do cyclam e ao propilamônio. A redução do trans-[Ru(NO)Cl(1-pramcyH)]3+ leva à saída rápida de cloreto seguida de saída lenta de NO, enquanto a irradiação do complexo em solução aquosa desaerada resulta na labilização fotoquímica do NO. O rendimento quântico para a fotoaquação do NO diminui com o aumento do comprimento de onda de irradiação e com a diminuição do pH, e é observável apenas a λirr < 370 nm. O comportamento do trans-[Ru(NO)Cl(1-pramcyH)]3+ é semelhante ao do complexo análogo trans-[Ru(NO)Cl(cyclam)]2+, porém difere daquele do complexo com carboxipropil como substituinte.

ARTICLE

Chemical and photochemical properties of a ruthenium nitrosyl complex with the N-monosubstituted cyclam 1-(3-Propylammonium)-1,4,8,11-tetraazacyclotetradecane

Kleber Q. FerreiraI,II; Elia Tfouni* * e-mail: eltfouni@usp.br ,I

IDepartamento de Química, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo - USP, Av. Bandeirantes, 3900, 14040-901 Ribeirão Preto-SP, Brazil

IIDepartamento de Química Geral e Inorgânica, Instituto de Química, Universidade Federal da Bahia, Rua Barão de Jeremoabo, s/n, Campus Universitário de Ondina, 40170-115 Salvador-BA, Brazil

ABSTRACT

The amine-functionalized trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 complex (1-pramcyH = 1-(3-propylammonium)-1,4,8,11-tetraazacyclotetradecane) was synthesized from trans-[RuCl(tfms) (1-pramcyH)](tfms)2 (tfms = trifluoromethanesulfonate) in acidic aqueous solution in the presence of nitric oxide (NO). The complex was characterized by elemental, spectroscopic (UV-Vis, IR, 1H and 13C NMR) and electrochemical analyses. Two pKa values (7.0 and 8.2) were estimated for trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 and were assigned to one of the cyclam nitrogen protons and to the protonated aminopropyl group. Reduction of trans-[Ru(NO)Cl(1-pramcyH)]3+ results in rapid loss of chloride followed by slower loss of NO, while irradiation of the complex in aqueous deaerated conditions suggests photochemical labilization of NO. The quantum yields for NO photoaquation decrease as the irradiation wavelength increases, being noticeable only at λirr < 370 nm, and increase as pH increases. The behavior of trans-[Ru(NO)Cl(1-pramcyH)]3+, which contains an aminopropyl substituted cyclam, parallels that reported for the analogous complex with the unsubstituted ligand, but differs from that described for the complex in which carboxypropyl is the substituent.

Keywords: nitrosyl, ruthenium, nitric oxide donor, substituted cylam, mono-N-functionalized cyclam

RESUMO

O complexo aminofuncionalizado trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 (1-pramcyH = 1-(3-propilamônio)-1,4,8,11-tetraazaciclotetradecano) foi sintetizado através da reação do trans-[RuCl(tfms)(1-pramcyH)](tfms)2 (tfms = trifluorometanossulfonato) com óxido nítrico (NO) em solução aquosa ácida. O complexo foi caracterizado por análise elementar, espectroscópica (UV-vis, IV, RMN de 1H e 13C) e eletroquímica. Dois valores de pKa (7,0 e 8,2) foram determinados para trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 e foram atribuídos a um dos prótons do grupo amina do cyclam e ao propilamônio. A redução do trans-[Ru(NO)Cl(1-pramcyH)]3+ leva à saída rápida de cloreto seguida de saída lenta de NO, enquanto a irradiação do complexo em solução aquosa desaerada resulta na labilização fotoquímica do NO. O rendimento quântico para a fotoaquação do NO diminui com o aumento do comprimento de onda de irradiação e com a diminuição do pH, e é observável apenas a λirr < 370 nm. O comportamento do trans-[Ru(NO)Cl(1-pramcyH)]3+ é semelhante ao do complexo análogo trans-[Ru(NO)Cl(cyclam)]2+, porém difere daquele do complexo com carboxipropil como substituinte.

Introduction

The discovery of the participation of nitric oxide (NO) in a wide range of physiological processes and pathologies1 has launched investigations on NO donors, including metal nitrosyl complexes in solution or immobilized in matrices, aiming at the understanding of both fundamental aspects and biological activity for potential applications.2-43

Among the NO donors, ruthenium nitrosyl complexes are particularly attractive because they are stable, can be either water soluble or not, deliver NO at different rates upon activation by reduction at biologically accessible potentials and/or by light irradiation, in either solution or matrices.14-18,34,40-49 These properties can be tuned by the adequate choice of ligands.14, 18

It should be pointed out that interesting biological activities for several of these complexes have already been reported.14,17,21,33,34,37-39 Because NO can be either beneficial or harmful depending on its bioavailability, compounds capable of releasing NO in a specific biological target have potential biological applications and could be useful tools to study the physiological action of NO. Efforts from this laboratory have been directed toward this goal, using several strategies. One of such approaches involves the functionalization of ruthenium nitrosyl complexes, so they can be linked to important entities such as antibodies and to surfaces, in order to obtain selective NO donor drugs or devices.

We have been working with complexes such as trans-[Ru(NO)(NH3)4(py-X)]n+ (py-X are pyridine, substituted pyridines, pyrazine), trans-[Ru(NO)X(py-X)4]n+ (X = Cl-, OH-, H2O, NO2-, etc.), and [Ru(NO)X(mac)]n+ (X = Cl-, OH-, H2O, etc.; mac = tetraazamacrocycle).14,18,41,45 The tetraazamacrocyclic RuII and RuIII complexes exhibit some unusual features compared to alicyclic analogues.18 One of their fundamental properties is the size of the macrocyclic ring. A change in ring size has been shown to markedly affect the electronic spectra, redox potentials, and reactivities of the complexes.18 Substitution in the ligand may also affect some properties, such as reactivity.

We have reported the syntheses and reactivity of trans-[RuCl(L)(1-pramcyH)]n+ (1-pramcyH = 1-(3-propylammonium)cyclam), L = Cl-, H2O, OH- and tfms (trifluoromethanesulfonate), where cyclam has a pendant ammonium group.48 The loss of chloride from trans-[RuCl2(1-pramcyH)]2+ and trans-[RuCl2(tmc)]+ (tmc = tetramethylcyclam) is faster than that in the corresponding dichloro cyclam complex.18 However, in very few cases, the substitution in the cyclam ligand imparts a drastic shift from the expected properties of the metal complex.19,50,51

These complexes present interesting configurations by exhibiting a κ3 denticity for the mono-N-substituted 1-(carboxypropyl)cyclam ligand in fac-[Ru(NO)Cl2 (κ3N4,N8,N11(1-carboxypropyl)cyclam)]Cl·H2O,19κ5 for the mono-N-substituted N-(2-methylpyridyl)cyclam ligand in cis-[Ru(Lpy)NO]3+ (Lpy = N-(2-methylpyridyl)-1,4,8,11-tetraazacyclotetradecane),50 and κ6 for the tetra-N-substituted 1,4,8,11-tetrakis(2-pyridylmethyl)cyclam in [Ru(HL)][ClO4]3·H2O (L = 1,4,8,11-tetrakis(2-pyridylmethyl)cyclam).51 However, when the substituent is ammoniumpropyl (1-(3-propylammonium)cyclam), the ruthenium complex adopts the expected trans configuration in trans-[RuCl(L)(1-pramcyH)]n+ (L = Cl-, H2O, OH-) (trans-[Ru(NO)Cl(1-pramcyH)]3+),48 instead of the fac configuration adopted in fac-[Ru(NO)Cl2(κ3N4,N8,N11(1-carboxypropyl)cyclam)]Cl•H2O when the substituent is carboxypropyl.

In order to verify the effect of the ammoniumpropyl substituent on the configuration of the nitrosyl complex, so as to obtain a wider variety of NO donors, in this paper we report the synthesis, characterization, electrochemical and photochemical properties of the trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 complex.

Experimental

Chemicals and reagents

Ruthenium trichloride (RuCl3•nH2O) (Strem; 40-45% Ru) was the starting material for the syntheses of the ruthenium complexes. Cyclam was purchased from Aldrich. Acetone, chloroform and ethanol were purified according to literature procedures.52 Doubly distilled water was used throughout this work. All other materials were reagent grade and were used without further purification.

Syntheses

The trans-[Ru(NO)Cl(cyclam)](PF6)2 complex25 was synthesized for comparison purposes. 1H NMR (acetonitrile-d3) δ 1.59 (m, 4 H, CH2), 1.94 (d, J 6.8 Hz, 2 H, CH2), 2.16 (d, 2 H, J 5.6 Hz, CH2), 2.61 (m, 4 H, CH2), 2.86 (q, J 5.4 Hz, 2 H, CH2), 3.09 (d, J 7.2 Hz, 2 H, CH2), 3.32 (m, 4 H, CH2), 5.12 (s, 2 H, NH), 5.66 (s, 2 H, NH). 13C NMR (acetonitrile-d3) d 26.66 (CH2), 28.83 (CH2), 49.76 (CH2), 52.42 (CH2), 52.93 (CH2), 55.07 (CH2)). Trans-[RuCl2(1-pramcyH)]Cl2,48trans-[RuCl(tfms)(1-pramcyH)](tfms)2,48trans-[RuCl(H2O)(1-pramcyH)](PF6)348 and trans-[RuCl(H2O)(cyclam)](PF6)248 complexes were prepared as described elsewhere.

Trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 was prepared by a slight modification of a procedure previously described for the synthesis of trans-[Ru(NO)Cl(cyclam)](PF6)2.25 An amount of 77 mg (0.10 mmol) of trans-[RuCl(tfms)(1-pramcyH)](tfms)2 was dissolved in 10 mL of 0.1 mol L-1 HPF6 solution in a three-necked flask under argon atmosphere and continuous stirring. Nitric oxide, which was generated by dropping 30% nitric acid onto Cu and passing it through a 6 mol L-1 solution of NaOH, was bubbled through the solution. After 4 h of bubbling, 1 mL of a saturated aqueous solution of NH4(PF6) was added, and the solution was concentrated to ca. 2 mL by rotary evaporation under reduced pressure. On cooling, the bright yellow solid, that slowly precipitated, was collected by filtration, washed with acetone, ether, and dried under vacuum. Yield: 45% (0.17 g; 0.046 mmol). Elemental analyses: Found: C, 17.98; H, 3.67; N, 9.65. Calc. for C13H32ClF18N6OP3Ru: C, 18.16; H, 3.75; N, 9.77%. UV-Vis λmax/nm (water) 272 (ε/(dm3 mol-1 cm-1) 3.1×103), 360 (4×102) and 455 (90). 1H NMR (acetonitrile-d3) δ 1.86 (m, 3 H, CH2), 2.15 (m, 3 H, CH2), 2.66 (m, 5 H, CH2), 2.78 (m, 4 H, CH2), 3.07 (t, J 4.5 Hz, 2 H, CH2), 3.29 (d, J 4.8 Hz, 3 H, CH2), 3.45 (d, J 5.2 Hz, 2 H, CH2), 3.55 (dd, J 5.3 Hz, 4 H, CH2), 5.36 (s, 3 H, NH), 6.06 (s, 1 H, NH), 6.26 (s, 2 H, NH). 13C NMR (acetonitrile-d3) δ 29.89 (CH2), 30.05 (CH2), 30.23 (CH2), 50.48 (CH2), 51.20 (CH2), 53.27 (CH2), 53.50 (CH2), 53.82 (CH2), 54.39 (CH2), 55.20 (CH2),55.80 (CH2), 56.10 (CH2), 56.54 (CH2).

Elemental analyses

Analyses were performed at the Departamento de Química, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, University of São Paulo, using an Elemental Analyzer CE Instruments, model EA 1110.

Spectra

Electronic absorption spectra were recorded on a Hewlett-Packard model 8452A spectrophotometer using quartz cells. Infrared absorption spectra were obtained in Nujol mulls, in water or in acetonitrile, on a Bomen MB-102 spectrophotometer. 1H and 13C NMR spectra were obtained in 5 mm NMR tubes on a Bruker WH400 spectrometer in acetonitrile-d3 or D2O. The EPR experiments were conducted in frozen acetonitrile (77 K) in an ESP-300E Bruker instrument operated with an X-band microwave bridge.

Electrochemical measurements

Cyclic voltammetry and controlled potential electrolysis were performed with a PARC 273 potentiostat/galvanostat. All tests were carried out using a conventional three-electrode cell. Glassy carbon and platinum gauze were used as working electrodes for cyclic voltammetry and coulometry, respectively. Ag/AgCl and platinum wire were used as the reference and auxiliary electrodes respectively. Electrochemical data were obtained in different media, as follows: pH 1 (CF3SO3H/CF3SO3Na, µ = 0.1 mol L-1); pH 4.32 (CF3COOH/CF3COONa, µ = 0.1 mol L-1); pH 1 (HCl/KCl, µ = 0.1 mol L-1); pH 7.9 (NaH2PO4•H2O/Na2HPO4, µ = 0.1 mol L-1); pH 6 (LiCl, µ = 0.2 mol L-1); and acetonitrile, µ = 0.1 mol L-1, containing tetrabutylammonium hexafluorophosphate [tba(PF6)]. All solutions were deaerated by bubbling high purity argon, and thermostated using a Haake FK ultracryostat. The reported E1/2’ values are the arithmetic means of the Epa and Epc values.

The spectroelectrochemical measurements in the UV-Vis region were carried out in a quartz cell with 0.030 cm optical path, using gold mini-grid, Ag/AgCl and platinum wire as working, reference and auxiliary electrodes respectively. Analogous measurements in the infrared region were carried out using gold mini-grid, Ag wire and gold wire as working, reference and auxiliary electrodes respectively, mounted in a CaF2 window with 0.020 cm optical path. Successive UV-Vis or IR spectra for trans-[Ru(NO)Cl(cyclam)](PF6)2 and trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 were recorded during the reduction process of the complexes at 25 ºC, with applied potentials of –500 mV vs. Ag/AgCl in acetonitrile solutions. The pH measurements were performed using a 430 Analion or a Corning pH meters.

Photolyses

Monochromatic irradiations at 313, 334 and 370 nm were carried out using a 150 W Xenon lamp in an Oriel Universal Arc Lamp Source (model 6253). For photolysis at the appropriate wavelengths, the irradiation wavelength was selected with Oriel interference filters, with an average band path of 10 nm.

The progress of the photoreactions was monitored spectrophotometrically on a MB Bomem 102 FTIR spectrophotometer, using a ZnSe ATR crystal, or on an HP8452A diode array spectrophotometer for in-situ vibrational and electronic spectroscopy respectively. For most runs, the initial concentration of the complex was ca. 10-2 (infrared experiments) to 10-3 mol L-1 (UV-Vis experiments) in the following buffers: CF3SO3H/CF3SO3Na (µ = 0.1 mol L-1; pH 1), CH3COOH/CH3COONa (µ = 0.1 mol L-1; pH 4.75); CH3COOH/CH3COONa (µ = 0.1 mol L-1; pH 4.90), or Na2HPO4/NaH2PO4 (µ = 0.1 mol L-1; pH 7.4). The collimated beam intensities ranged from 1×10-9 to 4×10-8 einstein-1 cm-2 as determined by ferrioxalate actinometry. The chemical actinometer potassium tris(oxalato)ferrate(III) was prepared according to Calvert and Pitts.53 After the equilibration of the cell holder temperature, photolysis begun by irradiating the sample for a period of time ranging from 0 to 7200 s, with increments of 1200 s.

Considering that the coordinated water of trans-[RuCl(1-pramcyH)(OH2)]3+ has a pKa of 3.1,48 the calculated NO quantum yield was based on the concentrations of the photoproduct trans-[RuCl(1-pramcyH)(L)]n+ (L = H2O, OH–), obtained by spectroscopic determination with absorbance readings at λ = 356 nm for the photolysis at pH = 1. At this pH, trans-[RuCl(1-pramcyH)(OH2)]3+ (λmax/nm = 356, ε/(dm3 mol-1 cm-1) = 2900) is formed. At pHs 4.75 and 4.90, a mixture of trans-[RuCl(1-pramcyH)(OH2)]3+ and trans-[RuCl(1-pramcyH)(OH)]2+ (λmax = 315, ε = 830) is obtained. At pH 7.4, only trans-[RuCl(1-pramcyH)(OH)]2+ (λmax = 310, e = 1.0 x 103) is produced.

The calculated øNO values were plotted versus % of the reaction (ø x % R). The extrapolated spectroscopic quantum yield at R = 0% was taken as øNO for the photoaquation of NO from trans-[Ru(NO)Cl(1-pramcyH)]3+. Evaluation of øNO at R = 0 % eliminates possible complications resulting from secondary photolysis of primary reaction products and inner filter effects.

Results and Discussion

Syntheses

The synthesis of the 1-pramcyH complex uses trans-[Ru(tfms)Cl(1-pramcyH)](tfms)2 as precursor, which, in acidic aqueous solution, aquates with a specific rate constant, k1, of 6.5×10-2 s-1 at pH 1, forming trans-[RuCl(OH2)(1-pramcyH)]3+.48 As for the cyclam complex, trans-[Ru(NO)Cl(cyclam)](PF6)2, trans-[RuCl(H2O)(1-pramcyH)]3+ generates trans-[Ru(NO)Cl(1-pramcyH)]3+ in the presence of NO. The nitrosyl complex was obtained as its hexafluorophosphate salt, trans-[Ru(NO)Cl(1-pramcyH)](PF6)3, which is soluble in water, acetonitrile, methanol and dimethylsulfoxide, and in a lesser extent in acetone.

Because the complex was isolated in acidic medium (0.1 mol L-1 HPF6), the aminopropyl group is protonated. Potentiometric titration allowed estimation of two pKa values (7.0 and 8.2) for trans-[Ru(NO)Cl(1-pramcyH)](PF6)3, which should be ascribed to one of the cyclam nitrogen protons and to the protonated propylammonium. Considering that the cyclam nitrogen of the related complexes trans-[RuCl(cyclam)(NHC(O)4-py)]+ (py = pyridine), trans-[RuCl2(1-pramcyH)]2+, and trans-[RuCl(OH)(1-pramcyH)]2+ have pKa values of 7.9, 8.0, and 7.8, respectively,48,54 and free propylammonium has a pKa of 9.8,55,56 it is difficult to undoubtedly assign the pKa values. However, it is more likely that the pKa of 7.0 refers to the cyclam nitrogen and that of 8.2 to propylammonium, value that is lower than 9.8 due to the charge effect of the Ru metal center, as expected.

IR, EPR, NMR and electronic spectra

In nitrosyl complexes, an IR absorption band assigned to NO stretching in the 1950-1800 cm-1 region is associated with a linear structure for Ru-N-O and a nitrosonium character (NO+).57 Ruthenium complexes of this type are often represented by the resonance form RuII(NO+). As pointed out earlier, this formulation is one of several resonance forms (others being RuIII(NO) and RuIV(NO–)), and, following Enemark and Feltham’s58 notation, the {Ru-NO}6 complexes are highly delocalized. The IR spectrum of nujol mulls of trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 displays three peaks at 1880 cm-1, 1865 cm-1 and 1842 cm-1 (Figure 1). In the IR spectra of ruthenium nitrosyl complexes recorded from Nujol mulls or KBr pellets, the NO stretching band sometimes appears as two or more peaks or as one peak with one or two shoulders. This feature has been assigned to solid state effects.18 However, only one peak at 1875 cm-1 appears in the spectrum of the aqueous solution of trans-[Ru(NO)Cl(1-pramcyH)](PF6)3; it is shifted to 1864 cm−1 in acetonitrile (Figure 2), indicating a solvent dependence for the n(NO). These peaks support the nitrosonium character of NO in this complex. An attempt to obtain an EPR spectrum for trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 showed no signal, giving further support to the IR assignment.



The 1H and 13C NMR spectra of trans-[Ru(NO)Cl (1-pramcyH)](PF6)3 are consistent with the presence of the 3-propylammonium group (-(CH2)3NH3+) bound to a cyclam nitrogen. The 1H NMR chemical shifts in the 5-7.5 ppm range can be assigned to the hydrogen atoms linked to the nitrogens of cyclam and those of the 3-propylammonium group (Figure S1 in Supplementary Information).

In the already described 1H NMR spectrum of trans-[Ru(NO)Cl(cyclam)](PF6)2 obtained in D2O,25 the NH’s hydrogen signals are absent because of fast H-D exchange.18,60 Hence, the 1H NMR spectrum of trans-[Ru(NO)Cl(cyclam)](PF6)2 was obtained in acetonitrile-d3 for comparison, and it has a relatively simple resonance pattern regarding the NH-cyclam hydrogens, as a result of the higher symmetry of the cyclam moiety in this geometry.25 There are two signals in the 5-7.5 ppm range (δ = 5.12 and 5.66 ppm) (see Experimental section), with 2:2 intensities, assigned to four NH hydrogens of cyclam. In its turn, the 1H NMR spectrum of trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 in acetonitrile-d3 shows three signals of NH hydrogens with 3:1:2 intensities (Figure S1). The signal with δ 5.36 ppm was assigned to the hydrogen atoms linked to the nitrogen of 3-propylammonium group, while the signals with δ 6.06 and δ 6.26 ppm were assigned to hydrogen atoms linked to the cyclam nitrogens. These signals were absent in the 1H NMR in D2O because of fast H-D exchange. The integral of the signals assigned to the carbon chain hydrogens of the 1-(3-propylammonium)cyclam ligand is consistent with 26 hydrogens in this molecule (see Experimental section).

The 13C NMR spectrum of trans-[Ru(NO)Cl(cyclam)](PF6)2 in acetonitrile-d3 displays six signals in the 26-56 ppm range assigned to the CH2 aliphatic carbons present in the macrocyclic ligand, and is consistent with that of trans-[RuCl(cyclam)(4-acpy)](BF4)2.59 In its turn, the 13C NMR spectrum of trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 in acetonitrile-d3 shows thirteen peaks (Figure S2), assigned to the CH2 aliphatic carbons present in the 1-(3-propylammonium)cyclam ligand. The structure of this complex in solution is shown in Figure 3, and is similar to those of trans-[Ru(NO)Cl(cyclam)](PF6)2,25trans-[RuCl(cyclam)(4-acpy)](BF4)2,59 and trans-[RuCl2(cyclam)]Br.61 This trans configuration is the thermodinamically expected one, unlike that of fac-[Ru(NO)Cl2(κ3N4,N8,N11(1-carboxypropyl)cyclam)]+, a very closely related cyclam monosubstituted complex, where the cyclam ring N (with the carboxypropyl pendant arm instead of an aminopropyl) is not coordinated to the ruthenium, resulting in κ3 denticity.19


The electronic spectrum of trans-[Ru(NO)Cl(1-pramcyH)]3+ (Figures S3 and S4) is similar to those of other ruthenium nitrosyl complexes,17,18,25 especially that of the related cyclam complex, trans-[Ru(NO)Cl(cyclam)]3+, as expected. This is due to the similarity of the ligands and, therefore, the bands were assigned by analogy. The spectrum of a pH 1 aqueous solution of trans-[Ru(NO)Cl(1-pramcyH)]3+ displays one band at 272 nm (ε/(dm3 mol-1 cm-1) = 3.1×103), ascribed to a ligand to metal charge transfer (LMCT) [pπ(Cl) → eg(Ru)], and another band at 360 nm (ε = 4×102) assigned to at least a spin-allowed d-d transition plus an additional contribution from a metal to ligand charge transfer (MLCT) due to a dπ(Ru)→ π*(NO) transition. The band at 455 nm (ε = 90) was assigned to a ligand field transition with a possible contribution from another dπ(Ru)→ π*(NO) MLCT transition.

Redox potentials

The cyclic voltammetry study of trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 in acetonitrile [25º C; 100 mV s-1; µ = 0.1 mol L-1 tba(PF6)] reveals three electrochemical processes in the -1.6 V to + 1 V range (vs. Ag/AgCl): Ep2c = -1350 mV, Ep1c = -320 mV and Ep1a = -220 mV (Figure S5). The ratio of the Ep1c and Ep1a current peak heights approaches unity as the scan rate increases, consistent with a coupled chemical reaction, assigned to the slow loss of NO following reduction to trans-[Ru(NO)Cl(1-pramcyH)]2+, similarly to trans-[Ru(NO)Cl(cyclam)]+.25 If the [RuII-NO+] formalism is considered, the Ep1c and Ep1a processes can be attributed to the NO+/NO0 (or {RuNO}6/7) couple by analogy with trans-[Ru(NO)Cl(cyclam)]2+, for which Ep1c = -210 mV and Ep1a = -150 mV (vs. Ag/AgCl) were obtained (equation 1). The previously reported values of Ep1c = -262 mV and Ep1a = -82 mV (vs. Ag/AgCl)25 were probably due to an ohmic drop. The Ep2c process at -1.35 V can be assigned to the {RuNO}7/8 process, similarly to trans-[Ru(NO)Cl(cyclam)]n+ (equation 2).25

The cyclic voltammetric behavior of trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 in 0.1 mol L-1 CF3SO3H/CF3SO3Na at pH 1 and 25 ºC is very similar to that of trans-[Ru(NO)Cl(cyclam)]3+.25 At a 100 mV s-1 scan rate, there are three electrochemical processes, namely Ep1c = –390 mV, Ep2c = –530 mV, and Ep2a = –400 mV vs. Ag/AgCl (Figure 4). At these conditions, peak 1a (see ahead) is enveloped by peak 2a; peaks 2c and 2a are pH dependent in the 1-9 pH range, while the others are pH independent. When the cyclic voltammogram is run in 0.2 mol L-1 LiCl at pH 6 or in 0.1 mol L-1 HCl/KCl at pH 1, only two peaks are detected, Ep1c at -370 mV and Ep1a at -290 mV vs. Ag/AgCl; peaks 2a and 2c are no longer observed (Figure S6). These results are consistent with suppression of the chloride dissociation, and the observed redox process is assigned to the {RuNO}6/7 couple for trans-[Ru(NO)Cl(1-pramcyH)]3+ (equation 1).


Reduction of trans-[Ru(NO)Cl(1-pramcyH)]3+ at -400 mV should be followed by a fast chloride release, as verified by a test with silver nitrate, to form trans-[Ru(NO)(OH2)(1-pramcyH)]3+ (equation 3), which then loses NO at a smaller rate, to form trans-[Ru(OH2)2(1-pramcyH)]3+ (equation 4). The pH dependent 2c/2a pair of peaks was attributed to the {RuNO}6/7 couple in trans-[Ru(NO)(OH2)(1-pramcyH)]4+ (equation 5), after loss of chloride.

Only when the scan range is extended further to –1.5 V (vs. Ag/AgCl) in 0.1 mol L-1 CF3SO3H/CF3SO3Na at pH 1, and the coordinated NO0 undergoes a further reduction process {RuNO}7/8 (equation 6), or at smaller scan rates, is it possible to observe the presence of the trans-[Ru(H2O)2(1-pramcyH)]3+ species (equation 7) in the repetitive scan mode. The trans-[Ru(H2O)2(1-pramcyH)]3+ species exhibits a reversible, pH dependent, one electron electrochemical process at E1/2 = -50 mV close to the reported value, at pH 1, of -100 mV48 and of -155 mV for trans-[Ru(H2O)2(cyclam)]2+.61 The possibility that the Ep3c process may involve five electrons and result in reduction of NO to amine, as reported for some cases,62-64 is under investigation in our laboratories.

From plots of Ep2cversus pH it was possible to estimate the equilibrium constants for the reactions represented in equations 8 and 9.

The estimated pKa value of 2 for the trans-[Ru(NO)(OH2)(1-pramcyH)]4+ species is close to those of trans-[Ru(NO)(OH2)(cyclam)]3+ and trans-[Ru(NO)(OH2)(NH3)4]3+ species, pKa = 3.1.25,42 As in the case of the latter two complexes and others, such as trans-[Ru(salen)(NO)(OH2)]+ with a pKa of 4,40 it indicates a strong RuIII character.

Spectroelectrochemistry

The infrared and UV-Vis spectral changes observed for trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 when it is submitted to a controlled potential electrolysis at –500 mV vs. Ag/AgCl in acetonitrile (µ = 0.1 mol L-1 tba(PF6)) are shown in Figures 5 and 6. We were unable to avoid some solvent evaporation during the experiments, which may explain the absence of clean isosbestic points. The intensity of the n(NO) band at 1864 cm−1 decreases, while a new peak appears at 1810 cm-1, the intensity of which increases. These changes are consistent with the reduction of the nitrosyl ligand, forming trans-[Ru(NO)Cl(1-pramcyH)]2+, and are similar to results previously described in the literature.18,25 However, it has been reported that the spectra of reduced froms of nitrosyl complexes show the n(NO) peak at much lower wavenumbers.65,66



The electronic absorption spectral changes observed for trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 consist of an absorbance decrease at 263 nm with a concomitant increase at 291 nm. The final electronic absorption spectrum is assigned to the trans-[Ru(NO)Cl(1-pramcyH)]2+ complex. Likewise, the UV-Vis monitoring of the non-exhaustively controlled potential electrolysis of trans-[RuCl(cyclam)(NO)](PF6)2, carried out in the same conditions to promote reduction of NO+ to NO, shows absorbance decrease at 254 nm with concomitant increase at 298 nm, which in acetonitrile and under a reductive potential should denote coordinated NO0.

Photochemical studies

Irradiation of trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 in a deaerated 0.1 mol L-1 HPF6 solution with light of λ = 334 nm results in the decrease in intensity of the ν(NO) band at 1875 cm−1 (Figure 7). This decrease undoubtedly suggests the photochemical labilization of NO (equation 10). Similar changes in the infrared spectrum had already been observed with 10-2 mol L-1trans-[RuCl(NO)(cyclam)]2+ and trans-[RuCl([15]aneN4)NO]2+ solutions at pH 7,17 and with trans-[RuCl(NO)(cyclam)]2+ in a xerogel matrix. 16 Addition of silver nitrate solution to the photolyzed solution did not evidence formation of AgCl, indicating that chloride is not labilized as far as this assay is considered.


The UV-Vis spectral changes during photolysis of trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 with 334 nm light at pH 1 are shown in Figure 8. There is a broad absorption increase in the 300-390 nm region, which is consistent with the formation of the RuIII aquo complex photoproduct, trans-[Ru(OH2)Cl(1-pramcyH)]3+ (equation 10), as observed for trans-[RuCl(NO)(cyclam)](PF6)227 and other ruthenium ammine nitrosyl complexes.16,17 At this pH, trans-[Ru(OH2)Cl(1-pramcyH)]3+ has an absorption band at λmax/nm = 356 (ε/(dm3 mol-1 cm-1) = 2900). At pH 4.75 or 4.90, the photoproduct is mainly trans-[Ru(OH)Cl(1-pramcyH)]2+, which absorbs at λmax = 356 (ε = 830). At pH 7.4, trans-[RuIII(OH)Cl(1-pramcy)]+ absorbs at λmax = 356 (ε = 2900).


The photoproducts quantum yields for the photolysis of trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 are shown in Table 1.

The quantum yields pattern for the photolysis of trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 is similar to those of other nitrosyl ruthenium(am(m)ine) complexes.14,17,18,46,67 The quantum yields decrease as pH decreases and as the irradiation wavelength increases, being noticeable only at λirr < 370 nm. The larger quantum yields achieved at larger pH values can be explained, as in the case of the analogous trans-[Ru(NO)(NH3)4(py-X)]3+ complexes,46 as follows. The trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 complex has 3 pKa values, and as the pH increases, in addition to other species a larger fraction of the product is in the hydroxo form. Different species are not necessarily expected to have the same quantum yields. Also, the hydroxo complex is likely to be much less reactive toward the back reaction with NO than the corresponding aquo product, and, thus, has larger quantum yields. As a matter of fact, the increase in quantum yield with larger pH is also consistent with the synthesis of trans-[Ru(NO)Cl(1-pramcyH)]3+ (see Experimental).

Conclusions

The procedures described here result in the successful synthesis of a ruthenium nitrosyl complex with the substituted 1-(3-propylammonium)cyclam, trans-[Ru(NO)Cl(1-pramcyH)](PF6)3. Unlike 1-carboxypropyl, which results in a fac configuration for the nitrosyl complex, the aminopropyl results in the thermodynamically expected trans arrangement. This amine functionalized complex can be linked to a peptide, or an antibody, for selective biological activity. It can also be bound to a support in a device, such as a stent, to form a drug eluting stent. The behavior of trans-[Ru(NO)Cl(1-pramcyH)]3+, which contains a substituted cyclam, parallels that of the cyclam analog, suggesting that these complexes could maintain their properties when linked to a biomolecule. Electrochemical and photochemical experiments suggest the labilization of NO. Like other ruthenium am(m)mine nitrosyl complexes, such as trans-[Ru(NO)(NH3)4(py-X)]n+ and trans-[RuCl(NO)(cyclam)](PF6)2, trans-[Ru(NO)Cl(1-pramcyH)](PF6)3 is attractive for potential biological applications because it is stable, water-soluble, and can deliver NO upon activation by reduction at a biologically accessible potential and/or by irradiation with light. Current research in this lab is directed toward these goals for potential applications of the complexes as NO donors.

Supplementary Information

1H NMR, 13C NMR and electronic absorption spectra, as well as cyclic voltammograms recorded for trans-[Ru(NO)Cl(1-pramcyH)](PF6)3, are available free of charge at http://jbcs.sbq.org.br, as a pdf file.

Acknowledgments

The authors thank the Brazilian agencies FAPESP, CNPq and CAPES for financial support.

Received: October 27, 2009

Web Release Date: April 29, 2010

FAPESP helped in meeting the publication costs of this article.

Supplementary Information






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    • Publication Dates

    • Publication in this collection
      30 July 2010
    • Date of issue
      2010

    History

    • Received
      27 Oct 2009
    • Accepted
      29 Apr 2010
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