Synthesis, crystal structure and acidic properties in aqueous solution of phosphate ammine complexes of ruthenium nitrosyl

Inorganica Chimica Acta 413 (2014) 90–96

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Synthesis, crystal structure and acidic properties in aqueous solution of phosphate ammine complexes of ruthenium nitrosyl Maxim A. Il’in a,b,⇑, Alexander N. Makhinya a,b, Iraida A. Baidina a, Sergey V. Tkachev a a b

Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences, 3 Akad. Lavrentiev Ave., Novosibirsk 630090, Russia Department of Natural Sciences, Novosibirsk State University (National Research University), 2 Pirogova Str., Novosibirsk 630090, Russia

a r t i c l e

i n f o

Article history: Received 5 March 2013 Received in revised form 25 December 2013 Accepted 30 December 2013 Available online 14 January 2014 Keywords: Ruthenium nitrosyl Phosphate ammine complexes Synthesis Crystal structures Acidic properties NMR spectroscopy

a b s t r a c t First examples of phosphate ammine complexes of ruthenium nitrosyl trans-[Ru(NO)(NH3)4(HxPO4)] (NO3)x(2  x)H2O (x = 0, 1, 2) were obtained with good yields. Results of elemental analysis, X-ray, infrared, UV–Vis diffuse reflectance, and 31P NMR spectroscopies fit well with the formulas [Ru(NO)(NH3)4(PO4)]2H2O (1), [Ru(NO)(NH3)4(HPO4)]NO3H2O (2), and [Ru(NO)(NH3)4(H2PO4)](NO3)2 (3). In all the complexes, the phosphate group is in a trans-position to NO and is monodentate coordinated to ruthenium by one of the oxygen atoms. The pKa values 2.6 and 7.2 were calculated from potentiometric titration and 31P NMR for the reactions: trans-[Ru(NO)(NH3)4(H2PO4)]2+ + H2O ¡ trans-[Ru(NO)(NH3)4 (HPO4)]+ + H3O+ and trans-[Ru(NO)(NH3)4(HPO4)]+ + H2O ¡ trans-[Ru(NO)(NH3)4(PO4)] + H3O+, respectively. The rate constant for the reaction trans-[Ru(NO)(NH3)4(H2O)]3+ + H2PO4 ¡ trans-[Ru(NO)(NH3)4 (H2PO4)]2+ + H2O was estimated as kobs = 7  108 s1 at 70 °C in a dilute solution of H3PO4 (5 M). Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction The interest in transition-metal nitrosyls is primarily due to the presence in their structure of a coordinated group NO. It is wellknown that NO plays an important role in various physiological processes, including cardiovascular control, immunological responses, neuronal signaling and others [1–5]. A disfunction of the regulation in nitric oxide metabolism was implicated in several disease states. The control of local concentration of NO, which is essential for obtaining the therapeutic effect, can be achieved with exogenous sources of nitric oxide [6,7]. Due to high affinity of ruthenium to NO, ruthenium complexes are currently being investigated as potential therapeutic NO-scavengers and -deliverers. Generally this metal based drugs exhibit fewer toxicity problems than drugs based on other metals [8–10]. In addition to the prospects of using ruthenium nitrosyls as biologically active compounds, the ability of these complexes to form light-induced long-lived metastable linkage isomers is of interest as well [11–13]. In the isomers, the nitrosyl group is bound either through the oxygen atom (isonitrosyl, MS1) or through both oxygen and nitrogen in a bidentate side-on arrangement (g2, MS2). Due to the differences in the method of coordination of ⇑ Corresponding author at: Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences, 3 Akad. Lavrentiev Ave., Novosibirsk 630090, Russia. Tel.: +7 3833165633; fax: +7 3833309489. E-mail address: [email protected] (M.A. Il’in). http://dx.doi.org/10.1016/j.ica.2013.12.039 0020-1693/Ó 2014 Elsevier B.V. All rights reserved.

NO, ruthenium nitrosyls can be used as molecular constructive blocks for the synthesis of bifunctional compounds that combine photochromic properties with magnetic, conducting properties, etc. [14,15]. The successful development of these promising areas requires novel methods of synthesis and the actual data of structure and reactivity of the precursor complexes. To date, there is no information about the structure and synthesis methods of ruthenium nitrosyl complexes with coordinated ammonia molecules and phosphate ions. In this paper, we describe synthesis, spectroscopic and structural characterization of the complexes with coordinated four molecules of ammonia and PO43, HPO42 or H2PO4 ions, estimate their acidic properties and kinetics of formation in aqueous solutions.

2. Experimental 2.1. General All reagents and solvents were obtained from commercial sources and used as supplied. The complex trans-[Ru(NO)(NH3)4 (OH)]Cl2 was synthesized from (NH4)2[Ru(NO)Cl5] with a nearly quantitative yield (95%) [16]. (NH4)2[Ru(NO)Cl5] was obtained from commercial ruthenium(III) chloride hydrate according to the literature [17]. 31P NMR spectra were recorded on a Bruker Avance 500 spectrometer at 298 K, operating at 202.45 MHz.

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Chemical shifts (d(31P), ppm) were referenced to an 85% solution of H3PO4 as an external standard. FT-IR spectra were recorded on a Scimitar FTS 2000 Fourier spectrometer in the range 4000– 375 cm1. The samples for recording were prepared via the standard procedure by pressing the compounds in KBr pellets. UV–Vis diffuse reflectance spectra in the solid state were collected with a Shimatzu UV-3101PC spectrophotometer over the spectral range of 240–800 nm at room temperature. Reflectance spectra were converted to absorbance ones by using the Kubelka–Munk equation [18,19]. The electronic spectra in aqueous solutions (see Figs. S1 and S2) were recorded on a SF-102 model in the 200– 900 nm range at room temperature in a 1.0 cm quartz cell. Elemental analyses were performed on a EURO EA3000 (Euro Vector).

Samples for 31P NMR were prepared by dissolving exact weighed portions of 1 (4  103 M) and adding either concentrated hydrochloric acid or 5 M NaOH to adjust pH to an appropriate value (Table S1). The prepared solutions were placed into a tube and 31P NMR spectra were recorded at room-temperature. The acidic properties were also determined by potentiometric titration (Fig. S3) with 0.1 M HCl (l = 1.0 M KCl). All measurements were run in triplicate.

2.2. Synthesis of trans-[Ru(NO)(NH3)4(PO4)]2H2O (1)

2.6. Kinetics measurements

2.0 ml H3PO4 (85 %) was added under stirring to 0.5 g (1.7 mmol) solid trans-[Ru(NO)(NH3)4(OH)]Cl2. After 24 h at room temperature, the mixture was being heated for 5 h on a water bath at 95 °C. Then, 5 ml of concentrated HCl was added to the mixture at room temperature and a light yellow precipitate was formed a few minutes later. The precipitate was filtered, dissolved in a minimum volume of water (2 ml), and 0.5 ml of 5 M NaOH was added (until pH 9). The complex 1 was precipitated after 3 h. The precipitate of 1 was collected by filtration, washed with cold water (1 ml), acetone and dried in air. The yield was 90%. After a few days of standing at room temperature, the mother liquor yielded yellow crystals suitable for a single-crystal X-ray analysis. Anal. Calc. for H16N5O7PRu: H, 4.88; N, 21.21. Found: H, 4.91; N, 21.15%. FT-IR (cm–1): 3456 sh m(H2O); 3360–3120 bvs m(NH3); 1848 vs m(NO); 1621 m, 1591 m d(H2O), das(NH3); 1343 m, 1321 s, 1282 m ds(NH3); 1052 bvs, 971 vs mas(PO4); 922 bs, 876 sh, 853 m, 734 bm qr(NH3), ms(PO4); 663 bm, 644 m, 621 s, 589 m, 551 s, 501 m m(Ru–NO), das(PO4); 486 s m(Ru–NH3); 440 m ds(PO4); 415 w d(Ru–NO). UV–Vis: kmax = 450, 350 and 260 nm.

Trans-[Ru(NO)(NH3)4(H2PO4)]2+ was analysed in dilute solutions of o-phosphoric acid at 70 ± 1 °C by 31P NMR spectroscopy. The spectra were acquired using 300 scans at 24 ± 0.5 °C. The kinetic data were treated as a pseudo-first-order reaction, and thus, the rate constant (kobs) was estimated from the plots of ln(A0/A) vs. time (Fig. S4).

2.3. Synthesis of trans-[Ru(NO)(NH3)4(HPO4)]NO3H2O (2) 0.1 g (0.3 mmol) of complex 1 was dissolved in 0.25 ml of 1.5 M HNO3 at room temperature. The solution was evaporated to dryness in air. The dry residue was washed with acetone and dried in air. Yellow crystals suitable for a single-crystal X-ray analysis were obtained by the slow evaporation at room temperature of a part of the solvent from the reaction solution. Anal. Calc. for H15N6O9PRu: H, 4.03; N, 22.40. Found: H, 4.08; N, 22.39%. FT-IR (cm1): 3460 sh m(H2O); 3260 bvs, 3100 sh m(NH3); 1879 vs m(NO); 1655 sh, 1600 m d(H2O), das(NH3); 1384 vs, 1350 vs m(NO3); 1302 bs ds(NH3); 1125 bs dplane(POH); 1046 s, 980 bvs mas(PO4); 920 bs, 896 m, 864 bs, qr(NH3), ms(PO4); 775 sh, 715 w m(NO3); 640 m, 617 bm, 579 m, 520 bs m(Ru–NO), das(PO4), d(NO3); 479 s m(Ru– NH3); 460 sh ds(PO4); 400 w d(Ru–NO). UV–Vis: kmax = 435, 315 and 250 nm. 2.4. Synthesis of trans-[Ru(NO)(NH3)4(H2PO4)](NO3)2 (3) The compound 1 (0.1 g, 0.3 mmol) reacted with 1 ml of 1.5 M HNO3 via the same procedure described for 2 in order to give a yellow precipitate of 3. Yellow crystals of 3 suitable for a single-crystal X-ray analysis were formed similarly to crystals of 2 after a few days. Anal. Calc. for H14N7O11PRu: H, 3.36; N, 23.33. Found: H, 3.39; N, 23.38%. FT-IR (cm1): 3300 bvs, 3180 sh m(NH3); 1897 vs m(NO); 1605 bm d(H2O), das(NH3); 1360 bvs, 1350 bvs m(NO3); 1320 sh, 1250 bs ds(NH3); 1149 s dplane(POH); 1065 bvs, 973 vs mas(PO4); 897 s, 861 bs, 829 s qr(NH3), ms(PO4); 727 w m(NO3); 695 m, 638 m, 610 bw, 576 m, 535 s, 504 s m(Ru–NO), das(PO4), d(NO3);

474 m m(Ru–NH3); 411 m, ds(PO4); 400 sh d(Ru–NO). UV–Vis: kmax = 430, 308 and 244 nm. 2.5. Evaluation of acidity constants

2.7. Single-crystal structure analysis Diffraction data for 1–3 were collected on a Bruker X8 Apex CCD diffractometer with Mo Ka radiation (k = 0.71073 Å) at 296(2) K. The crystallographic characteristics and main indicators of the refinement are presented in Table 1. The structures were solved by direct methods and refined by a full-matrix least-squares treatment against |F|2 in an anisotropic-isotropic (for H) approximation with the SHELX-97 program set [20]. All non-H atoms of the main structural units were refined anisotropically. The H atoms were refined in their geometrically calculated positions; a riding model was used for this purpose. Further details may be obtained from the Cambridge Crystallographic Data Center upon quoting depository numbers CCDC 908557–908559. 3. Results and discussion 3.1. Synthetic aspects Three ruthenium nitrosyl complexes with PO43, HPO42 or H2PO4 ions have been prepared. Despite the apparent simplicity

Table 1 X-ray experimental details.

Empirical formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z l (mm1) Measured reflections Unique reflections (Rint) Parameters R1 [I P 2r(I)] wR2 [I P 2r(I)] Goodness-of-fit

1

2

3

H16N5O7PRu 330.22 orthorhombic Pbcm 9.3467(7) 9.9602(8) 10.7240(8) 90 90 90 998.35(13) 4 1.757 10251 1994 (0.0328) 106 0.0182 0.0421 1.016

H15N6O9PRu 375.22 monoclinic C2/c 28.0526(13) 6.6930(3) 13.8414(6) 90 115.810(1) 90 2339.56(18) 8 1.528 17028 4459 (0.0148) 183 0.0138 0.0374 1.058

H14N7O11PRu 420.22 triclinic  P1 7.4614(15) 7.6213(15) 13.346(3) 84.39(3) 75.33(3) 62.29(3) 649.8(2) 2 1.401 9341 6826 (0.012)2 388 0.0145 0.0390 1.055

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of synthesis of these compounds, some important details are to be considered more thoroughly. For the synthesis of the complex 1, concentrated H3PO4 was added to solid trans-[Ru(NO)(NH3)4 (OH)]Cl2 and the reaction mixture was being kept at room temperature for 24 h. This preconditioning is necessary for removing as much of resulting hydrogen chloride as possible before the heating. If the reaction mixture is heated immediately after the addition of H3PO4, a significant amount of insoluble trans-[Ru(NO)(NH3)4Cl]Cl2 [21] is formed, and the yield of complex 1 is reduced to 30%. We tried to synthesize the uncharged complex 1 by excluding the step of isolating a light yellow precipitate formed by adding concentrated HCl to the reaction mixture. For this purpose, to the reaction mixture obtained by trans-[Ru(NO)(NH3)4(OH)]Cl2 reacting with H3PO4 (24 h at room temperature), a solution of NaOH to adjust pH to 9 was added. In this case, the resulting precipitate, besides 1, contained a large amount of sodium salts of phosphoric acid. The attempts to isolate the complex 1 from the mixture in the pure form led to a decrease in its yield up to 20%. The complexes 2 and 3 were obtained by evaporation to dryness of solutions formed as the result of 1 being interacted with a solution of HNO3 taken in some excess to the calculated amount (1.25 for compound 2 and 2.5 for compound 3). 3.2. IR spectra The FT-IR spectra of all the compounds demonstrate very strong bands in the range 1850–1900 cm1 that belong to the stretching vibrations of nitrosyl and are typical of most ruthenium nitrosyls with a diamagnetic Ru(II) center and linearly coordinated NO+ [22–24]. Maximum of the absorption band m(NO) shifts to shorter wavelengths in the row of anions PO43, HPO42, H2PO4 coordinated in the trans–position to nitrosyls, cm1: 1848 (for 1), 1879 (for 2) and 1897 (for 3). Intense broad bands m(NH3) are observed for all the compounds at 3300–3100 cm1. Complexes that contain molecules of water of crystallization exhibit bands m(H2O) as a shoulder at 3500 cm1. Several weak bands in the range 2900– 2170 cm1 are not listed because they are undoubtedly overtone or combination bands. Due to the presence of phosphate ions the absorption bands are complicated. The bands mas(PO4) of asymmetric stretching vibrations are observed at 1250-970 cm1. Symmetric vibrations ms(PO4) overlap with the rocking mode qr(NH3) in the range of 920–715 cm1. The bands at 1240–1150 cm1 are observed for compounds containing protonated phosphate ions, and can be attributed to the deformation vibrations of the fragment P–O–H. The deformation vibrations das(PO4) and ds(PO4) are in the range 660-410 cm–1 and overlap with vibrations m(Ru–NO) and m(Ru–NH3). In addition, the spectra of complexes 2 and 3 exhibit vibrations d(NO3) in the same range. Stretching vibrations m(NO3) for these complexes are observed as very intense bands at 1380– 1350 cm1. The assignments of absorption bands were made according to the literature [22–26]. 3.3. UV–Vis spectroscopy The UV–Vis diffuse reflectance spectra for compounds 1–3 were transformed to absorbance using the Kubelka–Munk function [18,19]

FðRÞ ¼ ð1  RÞ2 =2R ¼ K=S; where R, K, and S represent reflectance, absorption, and scattering, respectively. The electronic spectra of 1–3 in the solid state are shown in Fig. 1. Similarly to spectra reported for other trans-tetraammine nitrosyl complexes [Ru(NO)(NH3)4(L)]n+ (L = NH3, OAc (acetate),

Fig. 1. Electronic absorption spectra of trans-[Ru(NO)(NH3)4(PO4)]2H2O (1), trans[Ru(NO)(NH3)4(HPO4)]NO3H2O (2), and trans-[Ru(NO)(NH3)4(H2PO4)](NO3)2 (3) in the solid state.

Cl, OH [27]; L = SO32 or NO2 [28]; L = P(OEt)3 (triethyl phosphite) [29]), the UV–Vis spectra of 1–3 present three bands. The intense absorption bands at about 250 nm are composed of a series of electronic transitions in which the major contribution is the metal-ligand charge transition, and a minor contribution is a ligand-ligand charge transfer. The second absorption band in the range of 350–300 nm can be assigned to a spin-allowed d–d transition. The third absorption band (weak and broad) at about 430 nm in the electronic spectra of the complexes 1–3 was assigned to a mixture of two transitions: a ligand-field spin-forbidden transition and a charge transfer process d–p⁄(NO). All these assignments are in agreement with the published data for octahedral Ru(II) trans-tetraammine nitrosyl complexes [27–30]. The similar electronic spectra of aqueous solutions of 1 at pH 0.5, 5.5 and 10, and the corresponding extinction coefficients are shown in the section ‘‘Supplementary material’’ (Fig. S1, S2). 3.4. Acid properties and kinetics of formation in aqueous solutions The acid dissociation constants of H2PO4 and HPO42 ions coordinated in the trans-position to nitrosyls were estimated by potentiometric titration and 31P NMR. At different pH values in the aqueous solutions of 1, 2 or 3 the following equilibria are established:

ƒ! trans-½RuðNOÞðNH3 Þ4 ðH2 PO4 Þ2þ þ H2 O ƒƒƒ ƒƒƒ ƒ trans-½RuðNOÞðNH3 Þ4 ðHPO4 Þþ þ H3 Oþ

ð1Þ

ƒ! trans-½RuðNOÞðNH3 Þ4 ðHPO4 Þþ þ H2 O ƒƒƒ ƒƒƒ ƒ trans-½RuðNOÞðNH3 Þ4 ðPO4 Þ þ H3 Oþ :

ð2Þ

The pKa’s for the reactions (1) and (2) were calculated from potentiometric titrations as 2.6 ± 0.2 (pKa(H2PO4coord.)) and 7.2 ± 0.2 (pKa(HPO42coord.)). Due to the fast proton exchange between the protonated (HPO42coord. or H2PO4coord.) and deprotonated (PO43coord.) forms, the separate lines, typical of these forms, were absent from the 31P NMR spectrum in the intermediate acidity region. The effect of pH on the chemical shift of an average signal was successfully used to determine the acidity constant (e.g., [Pt(dien)(H2PO4)]+ and [Rh(NH3)5(H2PO4)] by 31P NMR [31,32]; trans-[Pt(NH3)2(H2O)2]2+ by 15N NMR [33]). We estimated the pKa(H2PO4coord.) and pKa (HPO42coord.) at room temperature by using the effect of acidification on the chemical shift of the coordinated phosphate groups in the 31P NMR spectrum of a solution of 1.

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The 31P NMR spectrum of an aqueous solution of the complex 1 displays one peak at 9.0 ppm. Upon the addition of alkali up to pH 10, the observed peak slightly shifts to the lower fields (d = 9.2 ppm) and stops changing at higher pH (up to pH 12). The sequential acidification of the solution of the complex 1 with hydrochloric acid leads to an increasing shift of the line at d = 9.1 ppm to the higher fields (Fig. 2): first to d = 5.2 ppm at pH 5.5, and to d = 2.8 ppm with a further acidification (pH 0.5). The values of pKa(H2PO4coord.) and pKa(HPO42–coord.) were estimated as recommended [34], considering that pKa = pH at d1/2, which is equal to a half the difference between the chemical shifts of the two neighboring chemical forms. The 31P NMR chemical shifts as the function of pH of the solution are presented in Fig. 3. The curves are similar to the titration curves of polybasic acids, in which horizontal sections correspond to the prevalence of the specific chemical forms. The pKa’s for the reactions (1) and (2) were obtained from 31P NMR as 2.5 ± 0.2 and 7.4 ± 0.5, respectively. Thus, very close values of pKa(H2PO4coord.) and pKa(HPO42coord.) which were obtained by two independent methods confirmed that these results are correct. These data show that the acidic properties of coordinated anions H2PO4coord. and HPO42coord. increase by about 4 orders of magnitude compared with the free anions of o-phosphoric acid (see Table 2). Such difference in acidic properties after coordinating for other ligands has been observed [16,30,32]. Moreover, as seen in Table 2, the acidity of the ligands in the complexes of ruthenium is influenced by the nature of the trans-ligands. For example, the pKa of the ligand in the trans-position to the nitrosyl is smaller in trans-[Ru(NO)(NH3)4(H2O)]3+ (pKa = 1.4 [16] or 3.1 [40]) than in trans-[Ru(NH3)5(H2O)]3+ (pKa = 4.1 [36]). This same behavior is also observed in the complexes with coordinated phosphorous acid (although with a little less noticeable): the pKa(trans-[Ru(NO)(NH3)4(P(OH)3)]3+) = 0.74 [30] against the pKa (trans-[Ru(NH3)4(H2O)(P(OH)3)]3+) = 1.0 [37]. This order is consistent with the increasing p-acceptor and decreasing r-donor strengths of the NO ligand, thus the replacement of other ligands by the nitrosyl gives the metal center a Ru(III) character [35]. In addition to the above evaluations of acid dissociation constants, the efficient rate constant for the reaction

ƒ! trans-½RuðNOÞðNH3 Þ4 ðH2 OÞ3þ þ H2 PO4 ƒƒƒ ƒƒƒ ƒ trans-½RuðNOÞðNH3 Þ4 ðH2 PO4 Þ2þ þ H2 O

ð3Þ

Fig. 2.

31

Fig. 3. Chemical shifts in the

31

P NMR spectra of 1 as a function of pH.

Table 2 pKa in free anions of o-phosphoric acida and some ruthenium complexes.b Protonated form

pKa

Refs.

H2PO4 HPO42 trans-[Ru(NO)(NH3)4(H2PO4)]2+ trans-[Ru(NO)(NH3)4(HPO4)]+ [Ru(NH3)5(H2O)]3+ trans-[Ru(NO)(NH3)4(H2O)]3+

6.82 11.25 2.6 7.2 4.1 1.4 3.1 1.0 0.74

[38] [38] this work this work [36] [16] [40] [37] [30]

trans-[Ru(NH3)4(H2O)(P(OH)3)]3+ trans-[Ru(NO)(NH3)4(P(OH)3)]3+ a b

The pKa values for H3PO4 (2 and 3 steps) at 25 °C and ionic strength l = 1.0. The ligand that correspond to pKa is in bold.

was determined from 31P NMR in 5.03 M solution of H3PO4 at 70 °C. The starting complex trans-[Ru(NO)(NH3)4(OH)]2+ is fast protonated and trans-[Ru(NO)(NH3)4(H2O)]3+ is formed in acidic solutions at room temperature [16,40]. The 31P NMR spectra for aqueous solutions of trans-[Ru(NO)(NH3)4(H2O)]3+ (4.0  102 M) with H3PO4 after heating (70 °C) were recorded. In the spectra peaks at 0.47 and 2.54 ppm were observed. The peak at 0.47 ppm corresponds to free o-phosphoric acid [32]. The peak at 2.54 ppm

P NMR spectra of 1 (4  103 M) at pH 10.0, 5.3, and 0.5.

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Due to the rapid exchange between H3PO4 and H2PO4 in solution and over 100-fold excess of H3PO4 compared with ruthenium we could consider [H2PO4] as constant value (0.25 M). Therefore kinetic equation should correspond to pseudo-first order reaction: d[A]/dt = kobs[A]. Really the change of the peak (2.54 ppm) area from time (Fig. S4) is well described by linear dependence for the first-order reaction and kobs = 7  108 s1. This reaction is slow, because the presence of the nitrosyl has the delabilizing effect on the trans-ligand. Such influence of NO for other ligands has been observed. For example, the substitution of the coordinated water molecule in trans-[Ru(NO)(NH3)4(H2O)]3+ by chloride ion proceeds about 30-fold times slower than in [Ru(NH3)5(H2O)]3+ (kCl = 3.7  106 M1 s1 and 8.7  105 M1 s1, respectively; 40 °C, l = 2 M NaCl, [H+] = 1.0  102 M) [40]. 3.5. X-ray crystal structures

Fig. 4. X-ray diffraction structure of 1 (a), and 2 (b); thermal ellipsoids are drawn on the 50% probability level.

corresponds to anion H2PO4coord., the intensity of which increased with time. The rate of the process (3) depends on the concentration of dihydrophosphate ion and complex ion:

d½A=dt ¼ k  ½A  ½H2 PO4 ; where A ¼ ½RuðNOÞðNH3 Þ4 ðH2 OÞ3þ :

To date, no structurally characterized nitrosamine complexes of ruthenium with coordinated phosphate ions have been known. The crystal structures of 1, 2 and 3 have been found by a single-crystal X-ray analysis. The structure of the complex particles with atom numbering and the relevant thermal vibration ellipsoids are shown in Figs. 4 and 5. The crystal data and structure refinements are given in Table 1. The main interatomic distances and bond angles of the compounds 1 and 2 are given in Table 3; those of the compound 3 are given in Table 4. Yellow crystals of 1, 2 and 3 crystallize in the orthorhombic, monoclinic and triclinic system, respectively. The structure of 1 is formed from neutral complexes trans-[Ru(NO)(NH3)4(PO4)] and crystallization water molecules. The structures of 2 and 3 are ionic and contain complex cations trans-[Ru(NO)(NH3)4(HPO4)]+ (2) or trans-[Ru(NO)(NH3)4(H2PO4)]2+ (3) and nitrate anions. The crystals of 2 also contain a crystallization water molecule. In the structure of 3, there are two crystallographically independent ruthenium complexes with different orientations in the unit cell. In all three complexes, the phosphate group is trans to the nitrosyl and monodentately coordinated to ruthenium by one of the oxygen atoms. The coordination polyhedron of ruthenium in all the complexes is a slightly distorted octahedron (RuN5O). Deviations of the valence cis-angles from the ideal 90° at Ru atoms achieve 8.4°. In the equatorial plane of the complex cations, there are four molecules of ammonia, while nitroso and phosphate groups occupy axial positions. Bond lengths Ru–N(NH3) are in the range 2.07–2.12 Å, with the average of 2.10 Å. In the linear fragment Ru–NO, the angle deviates from 180° by 4.3, 2.8, 5.0° in the compounds 1-3, respectively. The average bond lengths Ru–N and N–O are equal to 1.73 and 1.15 Å. These values are typical of the known ruthenium ammine nitrosyl complexes [39–41]. The average length of bonds

Fig. 5. X-ray diffraction structure of 3; thermal ellipsoids are drawn on the 50% probability level.

M.A. Il’in et al. / Inorganica Chimica Acta 413 (2014) 90–96 Table 3 Selected geometric parameters for 1 and 2.

4. Conclusions

1

2

Bond lengths (Å) N–O Ru(1)–N Ru(1)–N(1) Ru(1)–N(2) Ru(1)–O(1) P(1)–O(1) P(1)–O(2)

1.151(2) 1.7369(16) 2.1034(12) 2.0923(11) 1.9846(12) 1.5872(13) 1.5237(10)

1.1506(10) 1.7361(8) 2.1032(7) 2.1111(8) 1.9935(6) 1.5578(7) 1.5281(7)

Bond angles (°) Ru(1)–N–O N–Ru(1)–N(1) N–Ru(1)–N(2) N(1)–Ru(1)–N(2) Ru(1)–O(1)–P(1) O(1)–P(1)–O(3)

175.70(15) 91.62(5) 94.85(5) 90.48(5) 130.72(7) 105.09(7)

177.23(7) 94.92(3) 92.63(3) 92.72(3) 131.58(4) 105.43(4)

Table 4 Selected geometric parameters for 3. Bond lengths (Å) N(1)–O(1) N(2)–O(2) Ru(1)–N(1) Ru(2)–N(2) Ru(1)–N(11) Ru(2)–N(21) Ru(1)–N(12) Ru(2)–N(22) Ru(1)–O(11) Ru(2)–O(21) P(1)–O(11) P(2)–O(21) P(1)–O(12) P(2)–O(22)

Bond angles (°) 1.151(8) 1.148(7) 1.754(6) 1.710(5) 2.074(5) 2.113(4) 2.086(5) 2.122(4) 1.970(5) 2.015(4) 1.563(5) 1.496(5) 1.476(4) 1.517(4)

95

Ru(1)–N(1)–O(1) Ru(2)–N(2)–O(2) N(1)–Ru(1)–N(11) N(2)–Ru(2)–N(21) N(1)–Ru(1)–N(12) N(2)–Ru(2)–N(22) N(11)–Ru(1)–N(12) N(21)–Ru(2)–N(22) Ru(1)–O(11)–P(1) Ru(2)–O(21)–P(2) O(11)–P(1)–O(13) O(21)–P(2)–O(23)

175.4(6) 174.6(6) 95.2(2) 95.5(2) 96.2(3) 94.2(2) 89.1(2) 88.9(2) 137.1(3) 137.3(3) 107.0(2) 109.5(3)

The interaction of trans-[Ru(NO)(NH3)4(OH)]Cl2 with the concentrated o-phosphoric acid results in the formation of complexes with a monodentately coordinated phosphate ion. In this work, previously unknown complexes trans-[Ru(NO)(NH3)4(PO4)]2H2O, trans-[Ru(NO)(NH3)4(HPO4)](NO3)H2O and trans-[Ru(NO)(NH3)4 (H2PO4)](NO3)2H2O were isolated with high yields and characterized. According to the XRD, all these complexes contain almost a linear fragment Ru–N–O. Spectroscopic data indicate the presence of the ligand NO+ bound to the metal center Ru(II). In the IR spectra of these complexes, m(NO) is shifted to shorter wavelengths with an increase in the degree of protonation of the phosphate ions coordinated in the trans-position to nitrosyl: from 1848 (for PO43) to 1897 (for H2PO4) cm1. The presence of nitrosyl in these complexes causes an about 104-fold increase in the acidity of the ophosphoric acid compared with that of free H3PO4. Thus, aqueous solution of trans-[Ru(NO)(NH3)4(PO4)]2H2O at pH > 9.5 mainly contains the form [Ru(NO)(NH3)4(PO4)]; at 4.5 < pH < 5.5, the prevailing form is [Ru(NO)(NH3)4(HPO4)]+; and at pH < 1, the predominant form is [Ru(NO)(NH3)4(H2PO4)]2+. Estimation of kobs = 7  108 s1 (5.03 M H3PO4, 70 °C) shows that substitution of H2O to H2PO4 in trans-[Ru(NO)(NH3)4(H2O)]3+ is quite slow, yet this process can be realized in concentrated solutions of H3PO4 with almost qualitative yield during several hours heating at 90–100 °C. Acknowledgments The authors are very grateful to Dr. Gennadiy Kostin for the critical reading of the manuscript. We thank to Dr. Anna Zubareva for the elemental analysis. The work was partially supported by Russian Foundation for Basic Research (project 14-03-31314). Appendix A. Supplementary material

Ru–O(HxPO4) is 1.99 Å. In coordination octahedra, ruthenium atom is displaced from the equatorial plane N4 towards the NO group by 0.13 Å, and the plane N4 itself is inclined to the axis of ON–Ru–O by 3–4°, which can be explained by the presence of the bulky ligand HxPO4. In the phosphate groups, bond lengths P–O with the oxygen coordinated to Ru are equal to 1.59, 1.56, 1.53 Å for the compounds 1–3, respectively. Terminal bonds P– O with the free oxygen have the length 1.48–1.53 Å, bond lengths P–OH are equal to 1.58 (for 2) and 1.56 Å (for 3). Angles Ru–O–P increase in the series 1–3 as follows: 130.7°, 131.6°, 137.2°. Variation in the valence angles at the phosphorus atoms fits into the range 104.2–114.6°. Geometric characteristics of the secondsphere nitrate anions are usual for the complexes of ruthenium nitrosyl [41,42]. Bonds N–O are in the range 1.197–1.321 Å, with the mean of 1.26 Å. Bond angles O–N–O vary from 114.8° to 125.6°, with the average of 120°. In general, all the structures are layered. In the crystals of all the compounds, structural fragments are connected via hydrogen bonds. In the crystals of 1 and 2, structural fragments are connected by hydrogen bonds O–H  O, involving oxygen atoms of the phosphate groups and crystallization water molecules (distances O  OW are 2.74 and 2.67, 2.77 Å, respectively). In the structure of 3, hydrogen bonds O–H  O are realized between coordinated H2PO4 ions with the O  O distances estimated as 2.53 and 2.56 Å. In addition, all coordinated ammonia molecules in 1–3 are also involved in the formation of hydrogen bonds N–H  O (with the lengths N  O of 2.88–3.31 Å). The estimations of the shortest distances between the central atoms of Ru for the structures of 1–3 are equal 6.28, 6.69 and 7.02 Å, respectively.

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