Methylene Blue Interactions: Complexation and Metallate Formation

Inorganica Chimica Acta 360 (2007) 1799–1808 www.elsevier.com/locate/ica

Mercury(II)–methylene blue interactions: Complexation and metallate formation

Mani Mohan Raj a, Allimuthu Dharmaraja a, Savaridasson Jose Kavitha a, Krishnaswamy Panchanatheswaran a,*, Daniel E. Lynch b

a School of Chemistry, Bharathidasan University, Tiruchirappalli 620024, Tamil Nadu, India b Faculty of Health and Life Sciences, Coventry University, Coventry CV1 5FB, UK

Received 28 March 2006; received in revised form 4 September 2006; accepted 12 September 2006 Available online 19 October 2006

Abstract

Reactions of HgX2 (X = Cl, Br, I) with methylene blue (MB) chloride dihydrate and the reaction of HgCl2 with MB nitrate dihydrate have been undertaken in an attempt to prepare metal derivatives of MB. The products were characterized by elemental analysis, UV–Vis, 1 IR, H NMR spectroscopy and X-ray crystallography. The reaction of HgCl2 with MB chloride dihydrate and subsequent crystallization in a DMF/H2O mixture yielded the products (1 and 2) with different crystal morphologies. The structure of complex 1 represents the first structural report of a complex of MB with any metal ion. The efficacy of MB to act as a ligand in spite of its cationic nature is thus demonstrated. The structure of 1 comprises a distorted tetrahedral geometry around Hg(II); the coordination valencies being provided 2 by three chloride ions and a MB cation. The structure of 2 is a salt consisting of an HgCl4 anion and two MB cations. The reaction of 2 HgI2 and HgBr2 with MB chloride dihydrate yielded salts 3 and 4 with Hg2X6 (X = Br or I) anions and MB cations. A mixed salt 5, whose anions comprise mercury(II), chloride and nitrate species resulted from the reaction between HgCl2 with MB nitrate dihydrate. The reaction of Hg2F2 with MB chloride dihydrate and the crystallization of the resulting product 6 in aqueous DMF yielded crystals of (MB)2HgCl4 Æ H2O. 2006 Elsevier B.V. All rights reserved.

Keywords: Methylene blue; Metal–dye interaction; Metallate; Mercury(II)

1. Introduction interacting with Hg(II), Cd(II), Ag(I) has been postulated for the removal of these ions from waste water [9], while Methylene blue (MB), which is a ‘‘dyestuﬀ that made the preparation and properties of the gallium chloride– medical history’’ [1], is used as a stain and an agent for MB complex has been reported [10]. However, no struc- blood product decontamination [2,3]. It had been used to tural report of the metal complex of MB is at present treat urinary tract infections, to distinguish between can- known. Therefore, the study of interactions of MB with cerous and normal tissues [4], an antidote for cyanide poi- metal ions will be helpful to understand its applications. soning in humans, and an antiseptic in veterinary medicine The Cambridge Structural Database (version 5.25) [11] [5]. It is also used in photodynamic therapy [6], which is an includes four MB crystal structures containing the pheno- antineoplastic therapy, aided by the absorption of light. It thiazinium moiety, which are the chloride pentahydrate is also one of the most frequently used counter ions for ion- [12], triiodide [13], thiocyanate [14], bis(malenonitriledi- pair formation [7] with salicylic acid [8]. The idea of MB thiolato)cuprate(II) [15]. Additionally the structures of urate hexahydrate [16] and nitrate dihydrate [17] salts are reported. This investigation has been undertaken to under- * Corresponding author. Tel.: +91 431 2351352. E-mail address: [email protected] (K. Panchanatheswaran). stand the interactions between MB salts and diﬀerent mercury halides.

2. Experimental 1483, 1432, 1388, 1351, 1329, 1240, 1214, 1170, 1133, 1037, 882, 820, 661, 617, 532. UV–Vis (CH3OH) kmax/ 1 2.1. Preparation of 3,7-bis(dimethylamino)phenothiazin-5- nm: 653, 610 (sh), 291. H NMR (200 MHz, DMSO-d6): ium nitrate dihydrate (MB nitrate dihydrate) d 3.14 (s, 12H), 7.19 (s, 4H), 7.62 (s, 2H).

The above compound was prepared by adding a solu- 2.4. Preparation of 3,7-bis(dimethylamino)phenothiazin-5- tion of MB chloride trihydrate (0.373g, 1mmol) in water/ ium hexaiododimercurate(II) (4) MeOH to a solution of silver nitrate (0.34g, 2mmol) in water. The solution was filtered. Crystals of MB nitrate The above compound was obtained by the reaction of dihydrate separated as violet crystals when the filtrate HgI2 (0.45 g, 1 mmol) with MB chloride dihydrate was allowed to evaporate. Yield: 0.25g (65%); m.p. (0.35 g, 1 mmol) adopting the same procedure as described >320 C. Anal. Calc. for C16H22N4O5S: C, 50.25; H, 5.80; in Section 2.3. Yield: 0.53 g (61%); m.p. >320 C. Anal. N, 14.65; S, 8.38. Found: C, 49.98; H, 5.79; N, 14.41; S, Calc. for C16H18I3HgN3S Æ [(CH3)2NCOH]: C, 24.31; H, 7.98%. IR (cm1): 3418, 1599, 1486, 1443, 1384, 1356, 2.68; N, 5.97; S, 3.42. Found: C, 24.79; H, 2.34; N, 5.45; 1 1252, 1176, 1147, 1082, 961, 884, 612. UV–Vis (CH3OH) S, 3.89%. IR (cm ): 2818, 1593, 1479, 1436, 1384, 1346, 1 kmax/nm: 653, 601 (sh), 292. H NMR (200 MHz, D2O): 1327, 1244, 1168, 1133, 1035, 944, 879, 825, 776, 663. 3 1 d 2.97 (s, 12H), 6.60 (s, 2H), 6.84 (d, J = 10.0, 2H), 7.06 UV–Vis (CH3OH) kmax/nm: 653, 601 (sh), 291. H NMR 3 (d, J = 8.01, 2H). (200 MHz, DMSO-d6): d 3.36 (s, 12H), 7.48 (s, 4H), 7.87 (d, 3J = 9.58, 2H). 2.2. Preparation of 3,7-bis(dimethylamino)phenothiazin-5- ium-jN10-trichloromercury(II) (1) and 3,7- 2.5. Synthesis of 3,7-bis(dimethylamino)phenothiazin-5-ium bis(dimethylamino)phenothiazin-5-ium octachlorodinitratotetramercurate(II) (5) tetrachloromercurate(II) (2) To a solution of HgCl2 (0.07 g, 0.27 mmol) in methanol, To a solution of HgCl2 (0.271 g, 1 mmol) in methanol, MB nitrate dihydrate (0.1 g, 0.26 mmol) also in methanol MB chloride dihydrate (0.355 g, 1 mmol) in the same sol- was added slowly. The blue precipitate was formed slowly vent was added slowly. The blue precipitate was formed and the reaction mixture was stirred for few minutes for immediately and stirred for a few minutes for completion completion of the reaction. The precipitate was filtered of the reaction. The precipitate was filtered and recrystal- and recrystallized in an ethanol/water mixture. Yield: lized in DMF/H2O resulting in two types of crystals viz., 0.15 g (65%); m.p 320 C. Anal. Calc. for green needles (1) and red brown cubic crystals (2), which C16H18Cl4Hg2N4O3S: C, 21.61; H, 2.04; N, 6.30; S, 3.61. were mechanically separated. Combined yield: 0.42 g Found: C, 21.63; H, 2.23; N, 6.19; S, 3.81%. IR (cm1): (72%). For 1, m.p. chars at 227 C. Anal. Calc. for 2915, 1738, 1599, 1483, 1435, 1385, 1351, 1336, 1244, C16H18Cl3HgN3S: C, 32.49; H, 3.07; N, 7.11; S, 5.42. 1222, 1174, 1137, 1037, 945, 886, 835, 805, 772, 532. UV– 1 1 Found: C, 32.24; H, 3.21; N, 7.15; S, 5.42%. IR (cm ): Vis (CH3OH) kmax/nm: 652, 608 (sh), 291. H NMR 1597, 1491, 1385, 1353, 1251, 1177, 1142, 1037, 945, 883, (200 MHz, DMSO-d6): d 3.35 (s, 12H merged with residual 3 844, 801, 665, 532, 451. UV–Vis (CH3OH) kmax/nm: 651, H2O), 7.51 (s, 4H), 7.89 (d, J = 10.2, 2H). 1 610 (sh), 291, 244. H NMR (200MHz, DMSO-d6): d 3.36 (s, 12H), 7.48 (s, 4H), 7.88 (s, 2H). For 2, m.p. chars 2.6. Preparation of 3,7-bis(dimethylamino)phenothiazin-5- at 240 C. Anal. Calc. for C32H36Cl4HgN6S2: C, 42.18; H, ium tetrachloromercurate(II) monohydrate (6) 3.98; N, 9.22; S, 7.04. Found: C, 42.00; H, 4.32; N, 9.51; 1 S, 6.78%. IR (cm ): 1593, 1489, 1436, 1384, 1349, 1317, Reaction of Hg2F2 (0.25 g, 0.57 mmol) with MB chlo- 1241, 1168, 1134, 1028, 941, 876, 789, 665, 536. UV–Vis ride dihydrate (0.2 g, 0.56 mmol) adopting the procedure (CH3OH) kmax/nm: 651, 610 (sh), 291. described in Section 2.3 yielded the above compound. Yield: 0.19 g (70%); m.p. >320 C. Anal. Calc. for 2.3. Preparation of 3,7-bis(dimethylamino)phenothiazin-5- C32H38Cl4HgN6OS2: C, 41.36; H, 4.09; N, 9.04; S, 6.90. ium hexabromodimercurate(II) (3) Found: C, 41.36; H, 3.98; N, 9.19; S, 6.79%. IR (cm1): 1593, 1488, 1439, 1386, 1324, 1253, 1222, 1170, 1140, To a solution of HgBr2 (0.20 g, 0.56 mmol) in hot meth- 1037, 949, 878, 849, 820, 787, 665 532, 447. UV–Vis anol, MB chloride dihydrate (0.2 g, 0.56 mmol) also in (CH3OH) kmax/nm: 651, 609 (sh), 291. methanol was slowly added. The blue precipitate was formed immediately and stirred for a few minutes for com- 2.7. Instrumentation pletion of the reaction. The precipitate was filtered and recrystallized in a DMF/water mixture. Yield: 0.27 g Elemental analysis was carried out using a model Ele- (68%); m.p. >320 C. Anal. Calc. for C16H18Br3HgN3S: menter varioEL III at the Sophisticated Test and Instru- C, 26.52; H, 2.50; N, 5.80; S, 4.42. Found: C, 26.56; H, mentation Centre (STIC), Cochin, India. The IR spectra 2.38; N, 5.73; S, 4.44%. IR (cm1): 2922, 1738, 1594, were obtained using KBr pellets on a Perkin–Elmer Table 1 The crystal data and structure refinement parameters of the structures of compounds 1–6

123456 1799–1808 (2007) 360 Acta Chimica Inorganica / al. et Raj M.M.

Molecular formula C16 H18Cl3HgN3SC32H36Cl4HgN6S2 C16 H18Br3HgN3SC16H18I3HgN3SC32H36Cl8Hg4N8O6S2 C32H40Cl4HgN6O2S2 Formula weight 591.33 911.20 724.71 865.68 1778.79 947.23 Temperature (K) 120(2) 120(2) 120(2) 120(2) 120(2) 120(2) Radiation, wavelength (A˚ )MoKa, 0.71073 Mo Ka, 0.71073 Mo Ka, 0.71073 Mo Ka, 0.071073 Mo Ka, 0.71073 Mo Ka, 0.71073 Space group P1 P21/c P1 P21/cP21/nP21/n Crystal system triclinic monoclinic triclinic monoclinic monoclinic monoclinic a (A˚ ) 7.8629(9) 9.5614(2) 7.6135(14) 10.7291(16) 7.8622(3) 11.6935(4) b (A˚ ) 9.9734(12) 14.8963(4) 8.1798(12) 19.789(3) 16.9512(6) 11.5109(6) c (A˚ ) 12.1936(15) 23.7802(7) 16.308(3) 10.6334(8) 34.3394(13) 13.4190(6) a () 94.303(4) 90 88.186(12) 90 90 90 b () 100.697(5) 91.094(2) 78.310(7) 113.778(6) 90.8700(10) 101.364(3) c () 97.840(5) 90 88.422(11) 90 90 90 V (A˚ 3) 925.86(19) 3386.38(15) 993.8(3) 2066.0(4) 4576.0(3) 1770.82(14) Z 242442 3 Dcalc (g/cm ) 2.121 1.787 2.422 2.783 2.582 1.776 l (mm1 ) 8.861 5.018 13.883 12.038 13.989 4.806 Crystal size (mm) 0.60 · 0.06 · 0.04 0.32 · 0.22 · 0.18 0.02 · 0.07 · 0.42 0.16 · 0.10 · 0.07 0.36 · .02 · 0.01 0.08 · 0.18 · 0.23 Reﬂections collected/ 9768/9768 32048/6645 19100/3833 30822/4726 38424/8826 20189/3479 unique Rint 0.0000 0.0449 0.1630 0.0446 0.0606 0.0511 Tmin and Tmax 0.076 and 0.718 0.297 and 0.465 0.068 and 0.769 0.249 and 0.486 0.081 and 0.873 0.404 and 0.700 Goodness-of-ﬁt on F2 0.784 1.007 1.476 1.066 1.023 1.073 Final R indices [I >2r(I)] R1 = 0.0957, R1 = 0.0277, R1 = 0.1622, R1 = 0.0786, R1 = 0.0353, R1 = 0.0294, wR2 = 0.2320 wR2 = 0.0603 wR2 = 0.4122 wR2 = 0.2447 wR2 = 0.0671 wR2 = 0.0660 1801 1802 M.M. Raj et al. / Inorganica Chimica Acta 360 (2007) 1799–1808

FT-IR spectrophotometer. Electronic spectra were atom. The high Rint value for compound 3 was the result recorded in CH3OH on a CARY 300 Bio UV–Vis Varian of poor crystal quality resulting in weak high-angle data. spectrophotometer. The 1H NMR spectra were recorded using a Bruker 200 MHz instrument with TMS as an inter- nal reference. 3. Results and discussion

2.8. Crystallography 3.1. Synthesis of MB complex/metallates

Data were collected on a Bruker-Nonius KappaCCD The formation of compounds 1–6 is illustrated in area detector diffractometer, with / and x scans chosen Scheme 1. These products are isolated in 60–70% yield of to give a complete asymmetric unit. Cell refinement [18] the crystalline materials, the yields being calculated using gave cell constants whose dimensions are listed, along with methylene blue salt as the limiting reagent. The colours the other relevant crystallographic details for each com- of the resultant crystals range from violet through green pound in Table 1. Absorption corrections were applied to red, which are most probably due to surface phenome- [19] to each data set. The structures were solved by direct non effects rather than the actual solid-state colours of methods and refined using the SHELX-97 package [20]. All the bulk crystals. Solubility of the products differs widely. hydrogen atoms, except the water hydrogen atoms in 6, MB nitrate dihydrate and its salt with HgCl2 (5) are soluble were included in the refinement at calculated positions, in in water and methanol, unlike the other compounds. The the riding-model approximation, with and C–H distances reaction of Hg2F2 on MB chloride dihydrate produced a ˚ 2 of 0.95 (aromatic H atoms) and 0.98 A (CH3 H atoms). salt containing the HgCl4 ion, indicating that MB can 2þ The water hydrogen atoms in 6 were input in generated neither coordinate to the Hg2 ion, nor induce the forma- positions and refined using an O–H distance restraint of tion of any salt derived from Hg(I). The formation of 6 is 0.88 A˚ . The isotropic displacement parameters for all best explained by the anion metathesis and subsequent oxi- hydrogen atoms were set equal to 1.25Ueq of the carrier dation induced by the MB chloride dihydrate.

Cl Cl Cl Hg _ _ _ _ _ 2- 2- Cl Cl N + + Hg Hg and 2MB + 2MB + H O Cl Cl Cl Cl . 2 H3C + CH3 Cl Cl N S N _ _ __

CH3 CH3 1 2 6 Hg2F2 HgCl2

AgNO3 - H3C + CH3 + N SN MB + NO3 - CH3 Cl CH3 HgCl2

HgX 2 X = Br or I _ _ 2- Cl Cl Hg Cl Hg _ _ O O 2- Cl X X X O N Cl N O + O O + Hg Hg + 2MB + 2MB Hg Cl Hg X X X _ _ O O Cl Cl N 3 and 4 N O O O O _ _ n

Scheme 1. Interaction between MB and mercury salts. M.M. Raj et al. / Inorganica Chimica Acta 360 (2007) 1799–1808 1803

3.2. Spectroscopy 3.2.1. Molecular and crystal structure of 1 The MB cation can be represented by the resonance The IR spectra of 1–6 have peaks due to the presence structures I, II and III, with the atom numbering used of the MB units. The C¤C/C¤N stretching frequencies throughout this paper indicated over structure II. observed at 1599 cm1 in MB chloride dihydrate and

MB nitrate dihydrate are essentially unshifted, with the 1 10 9 11 N 14 maximum shift being 6 cm1 observed in 4. Similarly, N 2 8 1 the frequency 1395 cm , observed for the stretching of 3 3 + (CH3)2N SN(CH3)2 (CH3)2N 12 SN(CH13 7 3)2 the same bond in MB chloride dihydrate, is shifted + 4 5 6 7 (I) (II) downward by 10 cm1 in the IR spectra of 1, 2, 4, 5 and 6, while the downward shift registered for 3 is only 5cm1. Further, the C–N stretching frequency observed 1 N at 1147 cm in MB nitrate for N(CH3)2 is shifted to + 1136, 1134, 1133, 1133, 1140 and 1137 in 1, 2, 3, 4, 5 (CH ) N(CHS ) 1 3 2 N 3 2 and 6, respectively. The strong peak at 961 cm , due (III) to the free nitrate ion in MB nitrate dihydrate, is shifted to 945 cm1 in 5 and is relatively weak. This indicates that the nitrate ion is involved in coordination with The predominant resonance forms of the MB cation in Hg(II) in the above product. MB nitrate dihydrate [17] are either II or III, as evident There is no significant shift in the UV–Vis spectrum of from the shortening of the C1–C2, C4–C12, C8–C9 and the products 1–6 when compared to MB chloride dihydrate C6–C13 bonds (average length 1.367(4) A˚ ) compared with itself. The anion seems to have no effect on the solution other C–C bonds in the ring (average length 1.423(4) A˚ ). absorption spectrum of the products. The 1H NMR of The equality of the C13–S5 (1.728(3) A˚ ) and C12–S5 MB chloride dihydrate and MB nitrate dihydrate in D2O (1.728(3) A˚ ) bonds, suggest that S is not involved in conju- gave rise to two doublets and one singlet in the region of gation, precluding the contribution of I. The C7–N7 and 6.60–7.10 ppm and all the methyl protons gave rise to C3–N3 bonds are similar, being 1.340(4) A˚ and another singlet at 2.99 ppm as expected. The 1H NMR of 1.335(4) A˚ , respectively in length. The bond lengths and compounds 3, 4 and 5 in DMSO-d6 gave two signals in the bond angles are comparable to other similar MB deriv- the aromatic region in the ratio of 1:2. These signals are atives, discussed in this investigation. The schematic dia- broad and shifted downfield compared to the signals grams for the formation all the products are given in obtained in D2O for MB chloride dihydrate and MB Scheme 1. nitrate dihydrate indicating significant interaction with The X-ray crystal structure of 1 (Fig. 1) shows that in DMSO. The 1H NMR for the products 2 and 6 could the product, MB coordinates to the mercury through the not be recorded, since they were insoluble in common hard site, viz nitrogen (N10) atom and not through S(5). organic solvents including DMSO. In order to ascertain This is partly expected since in the resonance hybrid of the structure of the solids, X-ray crystallographic investiga- MB, (N10) carries no formal positive charge. Due to the tions were undertaken. symbiotic effect [21] of the three chlorine atoms attached

Fig. 1. Molecular structure of 3,7-bis(dimethylamino)phenothiazin-5-ium-jN10-trichloromercury(II) (1). 1804 M.M. Raj et al. / Inorganica Chimica Acta 360 (2007) 1799–1808 to the mercury center, Hg(II) is rendered a hard Lewis acid involved in C–H...Cl hydrogen bonds with C...Cl and thus preferentially coordinates to the hard Lewis base distances, shorter than 3.54(27) A˚ , reported for similar dis- ˚ site (N10). The Hg–N bond distance of 2.779(19) A tance in HgCl2(SCHN(CH3)2)2 [23,24]. In the three-dimen- observed in 1, falls in the range of 2.257(5)–2.957(6) A˚ , sional array the chlorine atoms interlink phenothiazinium reported for the complex of tris[(1-methylimidazol-2- cation layers via C–H...Cl hydrogen bonds. yl)methyl]amine with mercury(II) [22]. Thus it is longer than common Hg–N bonds, and indicates moderate inter- 3.2.2. Molecular and crystal structure of 2 action between the mercury and nitrogen atoms. The aver- The asymmetric unit of 2 consists of two MB cations and a ˚ 2 age Hg–Cl distances (2.417(4) A) are considerably shorter HgCl4 anion. Its molecular structure (Fig. 2) shows signif- 2 than the analogous distances observed in the HgCl4 ion icant diﬀerences in bonding parameters with 1. The Hg–Cl in 2 and 6 (2.492(1) and 2.4955(10) respectively). This rep- bond distances (2.515(1), 2.489(1), 2.469(1) and 2.496(1) A˚ ) resents the ﬁrst structural report of any metal complex with are considerably longer than those observed in 1. This may MB. In the crystal structure all three chlorine atoms are signify weaker interactions between the 4Cl and the Hg2+

Fig. 2. Molecular structure of 3,7-bis(dimethylamino)phenothiazin-5-ium tetrachloromercurate(II) (2).

Fig. 3. Molecular structure of 3,7-bis(dimethylamino)phenothiazin-5-ium hexabromodimercurate(II) (3) (symmetry code: 0 = x, y, z). M.M. Raj et al. / Inorganica Chimica Acta 360 (2007) 1799–1808 1805 metal center in contrast to the weak and strong bonds the final refinement. No further discussion on the structure around Hg(II) in 1. It is to be noted that the Hg–N bond is is given here save its gross molecular structure. The struc- 2 relatively weak, resulting in strong interactions between ture of 3 consists of two MB cations and a Hg2Br6 anion. Hg(II) and the chlorine in 1. The X-ray crystal structure The Hg–Br bond distances are comparable to the corre- shows the supramolecular chain constructed by C–H...Cl sponding distances of 2.4693(5) A˚ for Hg–Br bonds (termi- and C–H...N hydrogen bonding interactions. In the crystal nal) and 2.9997(5) A˚ for Hg–Br bonds (bridged), as structure of 2, pairwise hydrogen-bonding motifs are formed reported in [HgBr2(ABPPY)]2 [26]. (ABPPY = a-acetyl-a- between two MB units, via CH...N distances shorter than benzoylmethylenetriphenylphosphorane). 2.75 A˚ [25], the upper limit of such interactions. 3.2.4. Molecular and crystal structure of 4 3.2.3. Molecular and crystal structure of 3 The structure of 4 (Fig. 4) consists of two MB cations 2 The molecular structure of 3 is shown in Fig. 3. High andaHg2I6 anion. It is significant that in the structures residual electron density surrounds the mercury atom in of 3 and 4 the anions are aggregated, unlike in 2.We

Fig. 4. Molecular structure of 3,7-bis(dimethylamino)phenothiazin-5-ium hexaiododimercurate(II) (4) (symmetry code: 0 = x, y, z; 00 = x, y + 1/2, z + 1/2).

Fig. 5. Molecular structure of 3,7-bis(dimethylamino)phenothiazin-5-ium octachlorodinitratotetramercurate(II) (5) (symmetry positions: a =1+x, y, z; b = 1+x, y, z). 1806 M.M. Raj et al. / Inorganica Chimica Acta 360 (2007) 1799–1808 attribute the formation of the anion aggregates in 3 and 4 oxygens not involved in any bonding to mercury. Two oxy- 2 to the initial formation of HgX4 (X=Br, I) and subse- gens (O2 and O1) of the other nitrate ion exhibit l1 and l2 quent conglomerations. The stability of the above anions bonding modes leaving only one oxygen non-bonded to a must be traced to the soft–soft metal–halide interactions mercury atom. The overall structure is an anionic coordi- in both 3 and 4. The crystal structure of 4 is stabilized by nation polymer consisting of chloride, nitrate and Hg(II) weak C–H...I(C...I distances are 3.79 A˚ ) hydrogen-bond- species in three-, four- and five-coordinate environments. ing interactions. This compares well with the correspond- All the mercury atoms display coordination modes consisting C...I distance of 3.78(2) A˚ reported for ing of two strong bonds with chlorine atoms. The rest of HgI2(SCHN(CH3)2)2 [23,24]. the bonds surrounding the mercury atoms are longer than typical covalent bonds yet are shorter than the sum of the 3.2.5. Molecular and crystal structure of 5 van der Waals’ radii of atoms (Hg 1.55; Cl 1.75; O 1.52 A˚ ) The molecular structure of 5 (Fig. 5) indicates that it is a [27]. These bonds are considered to be weak bonds. coordination polymer whose asymmetric unit consists of The novel anionic polymeric layer consists of two 2 MB cations and a Hg4Cl8ðNO3Þ2 anion. The lowest chains; one comprising four-coordinated mercury atoms Hg–Hg distance in the asymmetric unit is 3.918 A˚ indicat- alternatively bridged by chlorine and nitrate oxygen atoms. ing no formal Hg–Hg associations. The average bridging The other chain consists of tri- and penta-coordinated mer- Hg–Cl distance (3.046(2) A˚ ) is long when compared to that cury, again alternatively bridged by chlorine and nitrate of normal Hg–Cl bonds (2.290(2) A˚ ) [26], although they do oxygen atoms. Fig. 6 shows that the crystal structure is sta- lie within the sum of the van der Waals’ radii 3.30 A˚ [27]. bilized by C–H...Cl and C–H...O hydrogen-bonding The four Hg atoms differ in their coordination modes. interactions. The C...O distance of 3.272(9) A˚ is within The environment around Hg1 is a distorted square pyra- the range 3.12–3.48 A˚ , reported for similar inter-atomic mid and the mercury atoms Hg2 and Hg4 exhibit four distances in {[Cd2(L)(NO3)4] Æ 2CH3CN}1(2.2CH3CN) coordination. Hg3 can be considered as three coordinate (L=2,5-bis(2-pyridyl-methylsulfanylmethyl)pyrazine) [28]. with a T-shaped geometry. The two nitrate ions of the The cationic columns are interposed between the anionic asymmetric unit each display different bonding modes. layers, leading to the formation of a supramolecular Both of them act as bridging ligands. In one, the nitrate three-dimensional array. oxygen (O4) exhibits a l2-bonding mode with other two

Table 2 Signiﬁcant intermolecular interactions for compounds 1–6 D–H ...A D–H H...AD...A D–H...A Hydrogen bonds 1 C6–H6...Cl1a 0.95 2.74 3.614(15) 154 2 C2A–H2A...Cl1 0.95 2.72 3.644(4) 165 C9B–H9B...N10Bb 0.95 2.61 3.554(5) 177 C71B–H72B...N10Ac 0.98 2.55 3.504(5) 165 4 C31–H33...I1d 0.98 2.85 3.79(2) 162 5 C6B–H6B...O6e 0.95 2.41 3.272(9) 151 C32A–H34A...Cl8f 0.98 2.77 3.751(7) 175 6 O1W–H1W...N10 0.88 2.12 3.001(5) 177 O1W–H2W...Cl2g 0.88 2.58 3.392(4) 153 C4–H4...Cl1 0.95 2.70 3.569(4) 153 C71–H72...O1Wh 0.98 2.40 3.338(5) 161 p–p interactions Cg(I)–Cg(J) Distance b (slip angle) a (dihedral angle) 1 Cg1–Cg3i 3.540(8) 18.95 0.29 2 Cg5–Cg2j 3.4267(18) 8.25 1.20 4 Cg4–Cg3k 3.563(11) 19.85 2.88 5 Cg2–Cg3l 3.673(4) 15.61 1.92 6 Cg2–Cg2m 3.417(2) 11.62 5.20 Note. Cg1 and Cg2 are the centers of the thiazinium rings in 1–5 and Cg3, Cg4 and Cg5 are the centers of the aromatic rings in 1–5; Cg2 is the center of the aromatic ring in 6. b is the angle between ring normal and the vector connecting the ring centroids. Symmetry transformations used to generate equivalent atoms: a = x, 1 y, z;b=1 x,2 y, z;c=1 x, 1/2 + y, 1/2 z;d=1 x, y, 2 z; e = 1/2 x, 1/2 + y, 1/2 z; f = 5/2 x, 1/2 + y, 1/2 z; g =1 x, y,1 z;h=1+x, y, z;i=1 x,1 y, z;j=1 x, Fig. 6. Packing diagram in 3,7-bis(dimethylamino)phenothiazin-5-ium 1 y, z;k=x, 1/2 y, 1/2 + z;l=1+x, y, z; m = 3/2 x, y, octachlorodinitratotetramercurate(II) (5), showing the supramolecular 3/2 z. sheet, assisted by C–H...Cl and C–H...O hydrogen bonding interactions. M.M. Raj et al. / Inorganica Chimica Acta 360 (2007) 1799–1808 1807

Fig. 7. Molecular structure of 3,7-bis(dimethylamino)phenothiazin-5-ium tetrachloromercurate(II) monohydrate (6) (symmetry codes: 0 = x + 1/2, y, z + 1/2; 00 = x + 1/2, y, z + 1/2; 000 = x, y, z). 3.2.6. Molecular and crystal structure of 6 be formed. The reaction of MB chloride dihydrate with In contrast to the structure of 2 the asymmetric unit of 6 Hg2F2 results in the oxidation of Hg(I) to Hg(II) with the 2 2 (Fig. 7) consists of a MB cation, half of the HgCl4 anion formation of the HgCl4 anion. The results presented in and a water molecule. Hg–Cl distances resemble those in 2 this paper have demonstrated that the reaction of MB salts and the structure is stabilized by C–H...Cl hydrogen- with other metal containing species is worth pursuing. bonding interactions. Additionally C–H...Ow and Ow– H...N interactions are also observed with H...Ow and 5. Supplementary material ˚ Hw...N distances of less than 2.8 A, the permissible limit for such interactions [29]. CCDC 601772, 601773, 601769, 601774, 601771 and 601770 contain the supplementary crystallographic data 3.2.7. p–p stacking interactions in compounds 1–6 for 1, 2, 3, 4, 5 and 6. These data can be obtained free of One of the important structural features of methylene charge via http://www.ccdc.cam.ac.uk/conts/retriev- blue derivatives is p–p stacking interactions. Two modes ing.html, or from the Cambridge Crystallographic Data of stacking were observed in the above products via paral- Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: lel and anti-parallel stacking in which like atoms either (+44) 1223-336-033; or e-mail: [email protected]. eclipse or remain staggered. The rings are stacked anti-parallel with the dihedral angles (a) ranging from 0.29 to Acknowledgement 5.20 in all the compounds except in 5, in which parallel stacking is observed. The b angle, which represents the The authors thank the EPSRC National Crystallogra- extent of non-eclipsing, ranges from 8.2 to 19.9. The phy Service (Southampton, England). shortest distance between the adjacent rings vary from 3.417(2) A˚ , observed in 6, to 3.673(4) A˚ in 5 (Table 2). References These distances are comparable to 3.43 A˚ and 3.50 A˚ , observed in MB dye included uric acid crystals and MB [1] H.W. Roesky, K. Mockel, Chemical Curiosities, translated by T.N. chloride pentahydrate, respectively [16,12]. Mitchel, W.E. Russey, New York, VCH, 1996, p. 76. [2] M. Wainwright, Int. J. Antimicrob. Agents 16 (2000) 381. [3] M. Wainwright, R.M. Giddens, Dyes & Pigments 57 (2003) 245. 4. Conclusions [4] E. Gurr, Staining-Practical and Theoretical, 2nd ed., Williams and Warkins, Baltimore, MD, 1962 (p. 303). It is established by the present work that MB can act as [5] L. Michaelis, S. Granick, J. Am. Chem. Soc. 67 (1945) 1212. a ligand to Hg(II). The stability of HgCl 2 ion is best [6] D. Gabrielli, E. Belisle, D. Severino, A.J. Kowaltowski, M.S. 4 Baptista, Photochem. Photobiol. 79 (3) (2004) 227. explained by the presence of multipoint C–H...Cl interac- [7] L. Michalis, S. Granick, J. Am. Chem. Soc. 67 (1945) 1212. tions. The tendency for metallate formation is so strong [8] S. Basu, S.K. Ghosh, S. Kundu, S. Nath. S. Panigrahi, S. Praharaj, 2 that mixed anions of the type Hg4Cl8ðNO3Þ2 can also T. Pal, Chem. Phys. Lett. 407 (2005) 493. 1808 M.M. Raj et al. / Inorganica Chimica Acta 360 (2007) 1799–1808

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