Supramolecular Architecture in Gold (I) and Gold (III) 2

Article pubs.acs.org/crystal

Supramolecular Architecture in Gold(I) and Gold(III) 2-Pyridineformamide Thiosemicarbazone Complexes by Secondary Interactions: Synthesis, Structures, and Luminescent Properties Alfonso Castiñeiras,* Nuria Fernández-Hermida, Rebeca Fernández-Rodríguez, and Isabel García-Santos Departamento de Química Inorgánica, Facultad de Farmacia, Universidad de Santiago de Compostela, E-15782 Santiago de Compostela, Spain S Supporting Information *

ABSTRACT: Reaction of the thiosemicarbazone ligands 2-pyridineformamide thiosemicarbazone (HAm4DH) or 2-pyridineformamide 3-hexamethyleneiminylthiosemicarbazone (HAm4hexim) with the gold(I) compound [AuCl(tdg)] (tdg = thiodiglycol = 2,2′-thiodiethanol) gives the complexes [Au(H2Am4DH)2]Cl3·H2O (1) and [Au(HAm4hexim)2]Cl (2). The room temperature oxidation of solutions of 2 in water/methanol leads to the crystallization of a gold(III) complex of formula [Au(Am4hexim)Cl]Cl·2H2O (3). Single-crystal X-ray diffraction analysis revealed that in compounds 1 and 2 gold(I) is linearly coordinated to sulfur atoms from two monodentate thiosemicarbazone molecules. Packing architectures and hydrogen bonding networks obtained using graph set notations are discussed for all complexes. Weak agostic interactions exist in both 1 [(N−H)···Au; H···Au = 2.70 and 2.82 Å] and 2 [(C−H)···Au; H···Au = 2.79 Å], and gold−gold contacts are present in the solid state in 1 (Au···Au = 3.83 Å) but not in 2 (Au···Au = 5.46 Å). The fluorescence of the ligands and complexes of gold(I) was studied. In addition, elemental analysis data and spectroscopic properties in the solid state and in solution are also described for complexes 1 and 2.

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gold(III) complexes include Au···halide,19 Au···π,20 and various kinds of hydrogen bonds21 [including E−H···X, E−H···π, C− H···E (E = O, N, X), C−H···n (n = lone pair electrons) interactions], Au···(H−E) agostic interactions22,23 (threecenter two-electron interactions between an electron-deficient metal atom and a C−H or N−H σ bond),24,25 and π−π stacking and anion interactions, although the closed d10 configuration of Au(I) appears to cancel any intermetallic bonding. When the ligand is a nitrogenated heterocyclic thione,26 such as a heterocyclic thiosemicarbazone, it may coordinate to other transition metals through an N atom to form supramolecules. The presence of multiple donor atoms within the same ligand can give rise to multiple coordination modes and allow the formation of complexes with different stabilities and reactivities.27 Nevertheless, the coordination chemistry of thiosemicarbazones with gold(I) remains relatively unexplored28 [as does that of gold(III) thiosemicarbazones].29 In this paper we describe the synthesis and reactivity of gold(I) complexes with 2-pyridineformamide thiosemicarbazone (HAm4DH) and 2-pyridineformamide 3-hexamethyleneiminylthiosemicarbazone (HAm4hexim) (Scheme 1); the crystal and molecular structures of [Au(H2Am4DH)2]Cl3·H2O (1), [Au(HAm4hexim)2]Cl (2), and [Au(Am4hexim)Cl]Cl·2H2O

INTRODUCTION The chemistry of gold is undergoing a significant expansion because both the metal and its complexes in different oxidation states present special features that make them appropriate for a growing number of medical and industrial applications. Numerous complexes of gold exhibit biological properties,1 mainly of an antitumoral nature2 but also, to a lesser extent, of an antibacterial nature.3 Work in the past decade has concerned complexes with linear Cl−Au−P,4 Cl−Au−S,5 P−Au−N,6 P−Au−S,7 and S−Au− S8 cores; the P−Au−S compound auranofin is used against rheumatoid arthritis and has also been screened for antitumoral activity.9 Gold complexes with a gold−sulfur bond are also of interest for applications in the fields of thin films, glasses, and ceramics, and for their luminescence and optical properties.10 Gold(I) is of particular interest,11,12 but gold(III) has also attracted attention because its complexes are isoelectronic and isosteric with complexes of platinum(II) and could be excellent candidates for evaluation as antitumor agents. In the luminescence of gold(I) complexes, a wide variety of electronic transitions have been implicated, including intraligand and metal-centered transitions, ligand-to-metal and metal-to-ligand charge transfer (LMCT and MLCT) and, of particular note, ligand-to-metal−metal bond charge transfer (LMMCT).10a,13,14 The latter process involves aurophilic (Au···Au) interactions,15 which are due to correlation and relativistic effects16,17 and are analogous to hydrogen bonds in their stabilization of crystal structures.18 Other secondary interactions that influence the structures and chemical and physical properties of gold(I) and © 2012 American Chemical Society

Received: November 22, 2011 Revised: December 29, 2011 Published: January 3, 2012 1432

dx.doi.org/10.1021/cg2015435 | Cryst. Growth Des. 2012, 12, 1432−1442

Crystal Growth & Design

Article

[Au(H 2 Am4DH) 2 ]Cl 3·H 2 O (1). Reagents and solvents: H[AuCl4]·3H2O (39 mg, 0.1 mmol), water (3 mL), thiodiglycol (24 mg, 0.2 mmol), methanol (20 mL), HAm4DH (40 mg, 0.2 mmol). Yield: 33 mg (54%). Elemental analysis: found, C 23.56, H 2.88, N 19.38, S 8.85%; calc. for C14H22AuCl3N10OS2 (713.25), C 23.56, H 3.11, N 19.62, S 8.98%. ESI MS, m/z: 392.02, [Au(HAm4DH)]+; 587.08, [Au(HAm4DH)2]+; 978.09, [Au2(HAm4DH)2(Am4DH)]+. IR (νmax/cm−1): 3410 m ν(OH), 3284 m, 3226 m, 3128s, 3095s ν(NH), 2824 m ν(CH), 1685 m, 1662 m, 1623s, 1596s, 1550s, 1530s ν(CN + CC), 1457 m, 1448 m, 1408s, 1372w, 1287s, 1237 m, 1176 m, 1160 m, 1091 m ν(NN), 1048 m, 995vw, 938vw, 916vw, 855w, 788 m ν(CS), 779 m, 728 m, 659w, 623s Δ(CCC), 528w, 438vw, 426vw, 405vw φ(CC), 366s ν(Au−S). 1H NMR (DMSOd6, ppm): 11.07 (2H, bs, N3H), 9.82 (2H, bs, N1H), 9.10 (4H, bs, N5H), 8.70 (2H, m, H1), 8.05 (8H, m, N4H + H3 + H4), 7.60 (2H, m, H2). 1H NMR (MeOH-d4, ppm): 8.70 (2H, d, H1), 8.49 (2H, d, H4), 8.06 (2H, d, H3), 7.66 (2H, m, H2). 13C NMR (DMSO-d6, ppm): 152.6 (C1), 149.0 (C5), 143.8 (C6), 138.9 (C3), 126.1 (C2), 121.3 (C4). ΛM (10−3 M, MeOH): 301 S cm2 mol−1. Single crystals were grown by evaporation of a methanol/acetonitrile solution over 7 days. [Au(HAm4hexim)2]Cl·3/2H2O (2 + 3/2H2O). Reagents and solvents: H[AuCl4]·3H2O (39 mg, 0.1 mmol), water (3 mL), thiodiglycol (24 mg, 0.2 mmol), methanol (20 mL), HAm4hexim (56 mg, 0.2 mmol). Yield: 41 mg (22%). Elemental analysis: found, C 38.03, H 5.05, N 17.18, S 7.30%; calc. for C26H41AuClN10O1.5S2 (824.22), C 38.35, H 5.08, N 17.20, S 7.88%. ESI MS, m/z: 474.11, [Au(HAm4hexim)]; 1224.35, [Au2(Am4hexim)2]+; 978.09, [Au2(C7H9N5S)2(C7H9N5S)]+. IR (νmax/ cm−1): 3407s ν(OH), 3286 m, 3160 m ν(NH), 2918s, 2853s ν(CH), 1672s, 1606 m, 1582 m, 1570w, 1518s ν(CN + CC), 1495s, 1456 m, 1431s, 1408 m, 1368 m, 1312 m, 1291w, 1265 m, 1247 m, 1195 m, 1176 m, 1148 m, 1092s ν(NN), 995s, 973 m, 901 m, 852vw, 795 m ν(CS), 744vw, 721vw, 668vw, 630vw Δ(CCC), 616vw, 587vw, 526w, 507 m, 473 m, 423vw, 406vw φ(CC), 355s ν(Au−S). 1H NMR (DMSO-d6, ppm): 11.75 (2H, bs, N2H), 9.10 (4H, bs, N5H), 8.70 (2H, d, H1), 8.14 (2H, d, H4), 8.07 (2H, td, H3), 7.66 (2H, dd, H2); 3.74 (8H, s, Ha), 1.65 (8H, s, Hc), 1.37 (8H, s, Hb). 13C NMR (DMSO-d6, ppm): 150.3 (C1), 144.3 (C5), 143.7 (C6), 139.0 (C3), 128.2 (C2), 122.2 (C4), 50.3(Ca), 27.9 (Cb), 26.8 (Cc). ΛM (10−3 M, MeOH): 70 S cm2 mol−1. Single crystals of 2 were grown by evaporation of solvent from the mother liquor over four days. When the mother liquor was left for a longer period, a change in color from yellow to brown occurred and [Au(Am4hexim)Cl]Cl·2H2O (3) crystallized. Crystal Structure Determinations. Diffraction data were obtained at 100.0(1) K on a Bruker X8 KappaAPEXII diffractometer from crystals of 1, 2, and 3 mounted on glass fibers. The data were processed with SAINT32 and were corrected for absorption using SADABS.33 The structures were solved by direct methods,34 which revealed the positions of all non-hydrogen atoms. These were refined on F2 by a full-matrix least-squares procedure using anisotropic displacement parameters.34 All hydrogen atoms were located in difference maps and included as riders with isotropic displacement parameters constrained to 1.2Ueq of their carrier atoms. In 2 the contribution of the density of disordered solvent molecules was subtracted from the measured structure factors using the SQUEEZE option.35 Subsequent refinement then converged with R factors and parameter errors significantly better than for all attempts to model the disordered water. Molecular graphics were generated with DIAMOND.36 The crystal data, experimental details, and refinement results are summarized in Table 1. CCDC reference numbers 730127, 730128, and 730129 contain the supplementary crystallographic data for 1, 2, and 3, respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Spectrophotometric and Spectrofluorimetric Measurements. UV−vis absorption spectra were recorded on a UV−vis Kontron Instruments Uvikon 810P spectrophotometer with solute concentrations of about 10−5 M in spectroscopic grade methanol at room temperature. Fluorescence spectra were recorded on a Jovin Yvon-Spex Fluoromax-2 spectrofluorimeter, and the samples for fluorescence measurements

Scheme 1

(3) are described, and the photophysical properties of the ligands and complexes 1 and 2 are discussed.

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EXPERIMENTAL SECTION

Materials. All reagents and solvents were commercially available except for 2-pyridineformamide thiosemicarbazone (HAm4DH) and 2-pyridineformamide 3-hexamethyleneiminylthiosemicarbazone (HAm4hexim), which were prepared as reported previously.30,31 Elemental analyses were performed with a Fisons 1108 microanalyzer. Mass spectra were obtained on a BIOTOF II API 4000 spectrometer for ESI. Fourier transform infrared (FT-IR) spectra were recorded from KBr disks (4000−400 cm−1) or polyethylenesandwiched Nujol mulls (500−100 cm−1) with a Bruker IFS-66 V spectrophotometer and are reported in the synthesis section using the following abbreviations: vs = very strong, s = strong, m = medium, w = weak, vw = very weak, sh = shoulder, br = broad. 1H and 13C NMR spectra were obtained from DMSO-d6 or MeOH-d4 solutions at room temperature on a Bruker AMX 300 spectrometer. 1H NMR spectra of MeOH-d4 solutions of HAm4hexim and 2 from +20 to −80 °C were recorded on a Varian Inova500 NMR spectrometer. Chemical shifts, in ppm, are reported relative to Me4Si using the solvent signal as reference, with the atoms numbered as in Scheme 4. Heteronuclear multiple quantum correlation (HMCQ) and heteronuclear multiple bond correlation (HMBC) spectra were run in the same solvents. The molar conductivities of 10−3 M solutions in methanol were measured with a CRISON MicroCM 2202 conductivimeter. Preparation of the Complexes. Synthesis of gold complexes was achieved as shown in Scheme 2, as follows.

Scheme 2. Synthesis of Gold Thiosemicarbazone Complexes (tdg = thiodiglycol)

Thiodiglycol was added slowly to a yellow solution of H[AuCl4]·3H2O in water. The resulting colorless solution was added dropwise, in 1:2 molar ratio, to a solution of the appropriate thiosemicarbazone in methanol. The mixture was stirred for 2 h, and the resulting yellow microcrystalline precipitate was filtered off, washed with ether, and dried over anhydrous CaCl2. 1433



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Table 1. Crystal and Structure Refinement Data for Compounds 1−3 compound empirical formula formula weight color/habit wavelength/Å temperature/K crystal size/mm crystal system space group unit cell dimensions a/Å b/Å c/Å α/° β/° γ/° volume /Å3 Z calc density/Mg/m3 absorp coeff/mm−1 F(000) θ range/° limiting indices/h,k,l refln collect/unique (Rint) completeness θ/° absorp correct. max/min transm data/parameters final R indices [I > 2σ(I)] R indices (all data) goodness-of-fit on F2 largest diff peak/hole, e Å−3

1

2 C26H38AuClN10S2 787.20 yellow/needle 0.71973 100(2) 0.36 × 0.03 × 0.01 triclinic P1̅ (No. 2)

C13H22AuCl2N5O2S 580.20 black/prism 0.71973 100(2) 0.23 × 0.18 × 0.13 monoclinic P21/c (No. 14)

10.5139(9) 10.9917(10) 11.1514(10) 64.305(4) 84.672(4) 81.941(4) 1149.07(18) 2 2.063 6.962 692 1.96−26.41 −12/13, −11/13, 0/13 21719/4671 (0.0556) 26.41 (99.0%) SADABS 0.7222/0.3096 4671/280 R1 = 0.0453 wR2 = 0.1073 R1 = 0.0578 wR2 = 0.1126 1.017 3.162/−2.224

5.4560(3) 10.6411(7) 15.0027(9) 101.207(4) 93.868(4) 96.916(4) 844.50(9) 1 1.548 4.590 392 1.39−26.47 −6/6, −13/13, 0/18 21759/3479 (0.0737) 26.47 (99.8%) SADABS 0.9555/0.2888 3479/187 R1 = 0.0486 wR2 = 0.1239 R1 = 0.0504 wR2 = 0.1255 1.076 3.667/−3.052

18.0161(6) 11.4234(3) 9.2310(3) 90 98.8540(10) 90 1877.15(10) 4 2.053 8.250 1120 1.14−26.41 −22/22, 0/14, 0/11 42365/3863 (0.0470) 26.41 (99.9%) SADABS 0.4135/0.2527 3863/217 R1 = 0.0181 wR2 = 0.0377 R1 = 0.0216 wR2 = 0.0385 1.065 0.655/−0.978

were diluted until the maximum absorption was below 0.1, typically, 0.05. Blank solutions were measured in the same way and the background signal was subtracted. Fluorescence quantum yields were determined relative to quinine sulfate in 0.2 M H2SO4 (ΦS = 0.546 at 298 K),37 using the equation:38

ΦX =

3

C14H22AuCl3N10OS2 713.85 yellow/needle 0.71973 100(2) 0.22 × 0.06 × 0.05 triclinic P1̅ (No. 2)

oxygen, H2O2) and solvents (CH3CN, DMF) without any positive results. All three complexes are air-, light-, and moisturestable solids at room temperature. The compositions and molecular formulas of the two gold(I) complexes are consistent with their elemental analysis data and mass spectra. In the mass spectra, the formation of the complexes is supported by the detection of peaks that correspond to [Au(HAm4DH)]+ and [Au(HAm4DH)2]+ for 1 and [Au(HAm4hexim)] for 2. The molar conductivities of the complexes in methanol (301 S cm2 mol−1 for 1 and 70 S cm2 mol−1 for 2) are consistent with their formulation as 3:1 and 1:1 electrolytes, respectively.39 Crystal Structure Analysis of (1) and (2). Molecular Structure. The crystal structures of compounds 1 and 2 were determined by single-crystal X-ray crystallography. Selected bond lengths and angles are listed in Table 2. The molecular structure of complex 1 (Figure 1) does not have any crystallographically imposed symmetry but has a linear S−Au−S geometry with an S(1)−Au(1)−S(2) angle of 177.24(7)°, which is similar to values of 175.11(5)−178.8(2)° found in previously reported bis(thiolate)gold(I) complexes.40 The two Au−S bonds are almost identical in length [2.2946(19) and 2.2938(19) Å] and are within the range reported for many other sulfur donor ligands.41 In particular, they are very similar to those found in complex 2 [2.2952(16) Å] and, within experimental error, are equal to the distances observed in [Au2(3-NO2-Hbtsc)4]Cl2·2CH3CN

AS × FX × nX2

ΦS AX × FS × nS2

where Ai denotes the absorption at the excitation wavelength, Fi is the integrated fluorescence intensity, ni is the refractive index of the solvent, and ΦS is the quantum yield of the fluorescence standard. Subindexes i = X or S refer to sample (ligands HAm4DH and HAm4hexim, and complexes 1 and 2) or standard, respectively.

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RESULTS AND DISCUSSION The thiosemicarbazones HAm4DH and HAm4hexim reacted with [AuCl(tdg)] (tdg = thiodiglycol = 2,2′-thiodiethanol) to give the mononuclear gold(I) complexes [Au(H2Am4DH)2]Cl3·H2O (1) and [Au(HAm4hexim)2]Cl (2), respectively. The disproportionation of Au+ in 2 on standing for prolonged periods in water/methanol in air at room temperature led to crystallization of the AuIII complex [Au(Am4hexim)Cl]Cl·2H2O (3) (Scheme 2); this complex was previously obtained by us in the form [Au(Am4hexim)Cl]Cl·CH3OH.29c This reaction was repeated using other oxidants (strong flow of 1434



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Table 2. Selected Bond Lengths and Angles in the Thiosemicarbazones and Compounds 1 and 2a compound

HAm4DH31

1

2

HAm4hexim32

Distances (Å) Au(1)−S(1) Au(1)−S(2) S(1)−C(17) S(2)−C(27) C(17)−N(14) C(27)−N(24) C(17)−N(13) C(27)−N(23) N(12)−N(13) N(22)−N(23) C(16)−N(12) C(26)−N(22) Au(1)···Au(1)a Au(1)···Cl(1)b Au(1)···Cl(2)a

2.2948(19) 2.2938(19) 1.735(7) 1.733(7) 1.309(9) 1.321(9) 1.336(9) 1.327(9) 1.396(8) 1.383(8) 1.299(9) 1.306(10) 3.8309(6) 3.896(2) 3.489(2)

S(1)−Au(1)−S(2) C(17)−S(1)−Au(1) C(27)−S(2)−Au(1) S(1)−C(17)−N(13) S(2)−C(27)−N(23) S(1)−C(17)−N(14) S(2)−C(27)−N(24) S(1)−Au(1)···Cl(1)b S(2)−Au(1)···Cl(1)b S(1)−Au(1)···Cl(2)a S(2)−Au(1)···Cl(2)a

177.26(7) 104.9(2) 106.0(3) 117.0(5) 116.9(5) 123.7(6) 122.6(6) 75.82(5) 103.30(5) 103.94(5) 76.35(6)

C(17)−S(1)···S(2)−C(27) Au(1)−S(1)−C(17)−N(14) Au(1)−S(2)−C(27)−N(24)

−172(1) −15.3(7) 11.7(7)

2.2952(16) 1.695(2) 1.701(2) 1.316(3) 1.315(3) 1.343(2) 1.338(2) 1.386(2) 1.391(2) 1.292(2) 1.295(2)

1.777(6)

1.732(4)

1.337(8)

1.364(5)

1.324(8)

1.330(5)

1.378(7)

1.363(4)

1.296(8)

1.306(5)

5.4560(4)

Angles (°) 180.0c 102.9(2) 119.99(15) 120.53(15) 123.06(15) 122.86(14)

122.7(5)

124.5(3)

120.9(5)

120.4(3)

Torsion Angles (°)

a

−180.0 76.2(5)

Symmetry transformations: a, (−x, 1 − y, −z) (1); (−1 + x, y, z) (2); b, (1 − x, 1 − y, −z); c, (1 − x, 2 − y, 1 − z) (S1−Au1−S1*, 2).

Figure 1. A view of the bis(H2Am4D)gold(I) cation in 1.

The C−S distances in the new compounds, 1.734(7) and 1.733(7) Å in 1 and 1.777(6) Å in 2, are longer than those in crystals of the respective free ligands [1.695(2) and 1.706(2) Å in HAm4DH30 and 1.732(4) Å in HAm4hexim31] and lie at or above the upper end of the usual range for gold(I) complexes of thioureas and heterocyclic thiones, 1.67−1.73 Å.43 Indeed, the 1.777(6) Å observed in 2 is close to the lengths of 1.78−1.80 Å reported for homoleptic two-coordinate thiolate complexes of gold(I).44 Careful analysis of the bond lengths and angles (Table 2) lends further support to the notion that in the thiosemicarbazone moieties of 1 and 2 considerable charge delocalization is inherited from the free thiosemicarbazone (Scheme 3). Apparently in keeping with this, the Au−S−C angles in both 1 and 2 are narrower than average values: in 1

(3-NO 2 -Hbtsc = 3-nitrobenzaldehyde thiosemicarbazone)28c [2.279(5) and 2.278(4) Å] and in [Au(H2Plhexim)]2Cl2·S8·6H2O [HPlhexim = bis(2-pyridyl)ethanedione bis{3-hexamethyleniminylthiosemicarbazone}] [2.282(2) Å].28d In all of these compounds, the Au−S bond lengths are considerably shorter than the sum of the covalent radii of the elements (2.36 Å),42 indicating a degree of dπ−dπ bonding between gold and sulfur. Compound 2 is the first example of a bis(thiosemicarbazone)gold(I) complex with a perfectly centrosymmetric crystal structure (Figure 2). The gold atom lies at an inversion center and is coordinated to two S atoms at equal distances [2.2952(16) Å] with an S(1)−Au(1)−S(1)a angle of 180.0° (a: −x + 1, −y + 2, −z + 1). 1435



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chain with base vector [111] (see Table 3 and Figure 3; molecular packing for complex 2 is included in the Supporting Table 3. Hydrogen Bonds in 1 and 2 d(D−H) (Å)

D−H···A N(14)−H(14B)···Au(1) N(24)−H(24A)···Au(1) N(11)−H(11A)···Cl(2) N(13)−H(13A)···Cl(1) N(14)−H(14A)···Cl(2) N(14)−H(14B)···Cl(2)a N(15)−H(15A)···Cl(1) N(15)−H(15B)···Cl(3)b N(21)−H(21A)···O(1)c N(23)−H(23A)···Cl(3)d N(24)−H(24A)···Cl(1)e N(24)−H(24B)···O(1)c N(25)−H(25A)···Cl(1)f N(25)−H(25B)···Cl(3)d O(1)−H(1A)···Cl(3) O(1)−H(1B)···Cl(2)

Figure 2. Structure of the bis(HAm4hexim)gold(I) cation in 2 showing two C−H···Au agostic interactions. Symmetry code: a, (−x, 1 − y, −z).

Scheme 3. Delocalization System in the Thiosemicarbazone Moiety

C(19)−H(19A)···Au(1) N(12)−H(12C)···S(1) N(15)−H(15A)···Cl(1)a N(15)−H(15A)···Cl(1)a N(15)−H(15B)···Cl(1)3

d(H···A) (Å)

Compound 0.88 0.88 0.88 0.88 0.88 0.88 0.88 0.88 0.88 0.88 0.88 0.88 0.88 0.88 0.89 0.89 Compound 0.99 0.88 0.88 0.88 0.88

1a 2.82 2.70 2.24 2.42 2.39 2.37 2.29 2.50 1.88 2.40 2.83 2.06 2.37 2.37 2.25 2.30 2b 2.79 2.43 2.30 2.30 2.44

d(D···A) (Å)

∠(DHA) (°)

3.279(6) 3.279(6) 3.016(6) 3.303(6) 3.203(6) 3.170(6) 3.163(7) 3.253(7) 2.692(8) 3.281(6) 3.541(7) 2.912(8) 3.312(7) 3.249(6) 3.112(6) 3.109(6)

122.7 114.4 146.5 175.9 153.2 150.9 170.9 144.7 153.3 179.7 139.1 162.5 136.6 173.0 161.0 151.1

3.621(8) 2.886(6) 3.156(7) 3.156(7) 3.206(8)

141.9 112.4 164.2 164.2 146.3

a Symmetry transformations: a, (−x, −y + 1, −z + 1); b, (−x + 1, −y + 1, −z + 1); c, (x, y, z − 1); d, (−x, −y, −z + 1); e, (−x + 1, −y + 1, −z); f, (x − 1, y − 1, z). bSymmetry transformations: a, (x − 1, y, z); b, (−x + 1, −y + 1, −z).

they lie at the lower end of the range found in most gold(I) complexes with sulfur donor ligands, and in 2 they are halfway between this range and the 100.4(4)° or 101.4(4)° observed in bis(thiourea)gold(I) bromide [another bis(thione)gold(I) complex with an exceptionally long C−S bond, 1.77(1) Å]45 and in the cationic mesoionic46 thiolate complex bis(1,4,5trimethyl-1,2,4-triazolium-3-thiolate)gold(I) [in which the C−S distances are 1.73(1) and 1.76(1) Å].43 The above results suggest that compound 2 is more thiolate in nature than compound 1. This is also supported by the Au−S−C−N torsion angles [in 1, −15.3(7)° for Au(1)− S(1)−C(17)−N(14) and 11.6(7)° for Au(1)−S(2)−C(27)− N(24); in 2, 76.2(5)° for Au(1)−S(1)−C(17)−N(14)], since the greater the double bond character of C−S, the more orthogonal one would expect the Au−S to be with respect to both the σ and π bonds, and hence closer to the plane of the ligand. Hydrogen Bond Networks and Packing Analysis. Both compounds form hydrogen bonds in the crystal lattices, and the number of such bonds per molecule depends on the molecular topology and the presence of hydrogen bonding centers. Therefore, an analysis of the hydrogen bond network topology was carried out using the graph set notation terminology introduced by Etter47a and revised by Bernstein.47b In compound 2, hydrogen bonding is limited to the intramolecular N(12)−H(12)−S(1) bond that is characteristic of the E’ configuration, and to the formation of the ring motif R42(8) that links the N(15) and N(15)n atoms (n: −x, 1 − y, −z) of neighboring molecules through Cl(1)a (a: −1 + x, y, z) and Cl(1)b (b: 2 − x, 1 − y, −z), thus creating an infinite one-dimensional

Information, SM7). Compound 1, with its three chloride counterions and its unsubstituted N(4)H2 groups, has a much richer hydrogen bond network, and this involves all five nitrogen atoms (Figure 4). In both ligands of 1, N(1), N(3), N(4), and N(5) act as donors through all their hydrogen atoms, while N2 acts as an acceptor of one prong of each of the forked bonds of N(1) and N(4). Cl(1) accepts from N(13) and N(15), and Cl(3)d from N(23) and N(25) (d: −x, −y, 1 − z), to form the ring motif R21(7), while Cl(2) accepts one prong of each of the forked bonds of N(11) and N(14)−H(14A) to form the ring motif R21 (10). Additionally, R42(8) rings that bridge between two different molecules are formed by N(15) and N(25)g together with Cl(1) and Cl(3)b (b: 1 − x, 1 − y, 1 − z; g: 1 + x, 1 + y, z), and by N(14) and N(14)a together with Cl(2) and Cl(2)a (a: −x, 1 − y, 1 − z). Finally, short C42(8) chains parallel to the b axis between N(14) and N(23)d are mediated by Cl(2), H−O(1)−H, and Cl(3); the oxygen atom also acts as an acceptor in forming the ring motif R41(10) with N(21)c and N(24)c (c: x, y, −1 + z), thus contributing to the construction of an extended three-dimensional hydrogen bond network that, to the best of our knowledge, is unprecedented in hydrated gold(I) thiosemicarbazones. The intermolecular interactions in compound 1 differ from those in 2 due to the existence of a gold−gold interaction (Figure 5), albeit a weak one. The Au−Au length is 3.8309(6) Å, and this value lies considerably outside the usual range (2.76−3.40 Å).15 In terms of intramolecular interactions, both 1 and 2 feature short Au···H distances that suggest the presence of weak agostic 1436



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Figure 3. Perspective view of the chain formed by N−H···Cl hydrogen bonds in 2. Symmetry code: a, (1 − x, 2 − y, 1 − z).

contacts and the Au···Au distance is 4.9623(2) Å (Figure 9). Analysis of the relationships between neighboring pyridine rings did not show any strong π−π interactions, but there are moderately strong interactions between the chelate rings of neighboring cations, a weak interaction between the metal and a neighboring chelate ring, and two weak X−H···π interactions. An analysis of short ring interactions did not show any strong π−π interactions in the packing, but there are moderately strong chelate ring−chelate ring interactions (Table 6) between gold chelates of cationic moieties. In addition, a weak metal− metal chelate ring interaction and two weak X−H···π interactions are also present to stabilize the packing (molecular packing for complex 3 is included in the Supporting Information, SM8). Spectral Studies. The IR spectra of the complexes exhibit bands due to ν(OH) and ν(NH) vibrations in the range 3158−3407 cm−1. The bands associated with ν(CN) and ν(CC) appear in the range 1519−1623 cm−1 at lower and higher frequencies, respectively, than in the spectra of the ligands.30,31 The fact that the ν(CS) band appears at lower frequencies than in the spectra of the uncoordinated thiosemicarbazones  at 788 and 795 cm−1 for 1 and 2, respectively  shows coordination of the sulfur to the gold center. Furthermore, coordination by the sulfur is also evidenced by the presence of bands at 366 cm−1 (1) and 355 cm−1 (2), which are attributed to Au−S stretching modes. In the 1H NMR spectra, the pyridine proton signals appear at 7.60−8.70 ppm (within their normal range), the N(4)H signals of 1 appear at 8.05 ppm, and those of the N(5)H protons of both complexes appear at 9.10 ppm, that is, downfield from their counterparts in the spectra of the ligands and other complexes of these thiosemicarbazones.30,31,48,49 Furthermore, in the case of 1 these signals are downfield from the N(4)H signals, which is quite unusual for this kind of thiosemicarbazone. The hydrazinic proton signal of 1 appears downfield from its position in HAm4DH (10.05 ppm), at 11.07 ppm; this is not sufficiently low field to suggest intramolecular hydrogen bonding50 (so compound 1 seems to retain the E configuration of the uncoordinated ligand, with the hydrazinic proton on N(3); see Scheme 4) but is low enough field to suggest hydrogen bonding with the solvent. In the spectrum of 2 the hydrazinic proton signal lies at 11.75 ppm, which is closer to its position at 12.71 ppm in HAm4hexim (which has an E′ configuration, with the proton on N2; see Scheme 4) than to its position at around 10 ppm in other neutral metal complexes of this ligand, in which the ligand adopts E configuration.50a−c Furthermore, as the crystallographic results (vide supra) show the solid-state configuration of 2 to be E′ (which makes it the first complex of HAm4hexim to retain this configuration on coordination), it seems likely that E′ is also the configuration adopted in solution.

bonds. In 1 the gold atom lies 2.82 and 2.70 Å from H(14B) and H(24A), respectively, and in 2 it lies 2.79 Å from H(19A). All of these distances are within the reported range for similar Au···H interactions, 1.95−3.20 Å.22 The existence of an agostic bond in 1 is further supported by the IR spectrum, in which the stretching vibration of the most flexible NH bond lies at a wavenumber about 100 cm−1 lower than that in the free ligand (3095 vs 3208 cm−1) (see Supporting Information SM1−SM4), an observation that is characteristic of an M···H−X (X = C, N, O) agostic bond.24,25 Further evidence for this structure is provided by the flatness of the AuS2H2 core, the atoms of which have an r.m.s. deviation of only 0.10 Å from the least-squares plane [which lies at an angle of 20.1(3)° from the thiosemicarbazone plane]. Crystal Structure Analysis of (3). Molecular Structure. The molecular structure of [Au(Am4hexim)Cl]Cl·2H2O (3) is shown in Figure 6 and selected bond lengths and angles are listed in Table 4. A compound of formula [Au(Am4hexim)Cl]Cl·CH3OH with a similar molecular structure has been reported previously.29c The Am4hexim anion acts as a tridentate ligand and is coordinated to the gold(III) center through its pyridyl nitrogen, N(11), azomethine nitrogen, N(12), and thiolate sulfur. A chloride ligand [Au−Cl = 2.2796(7) Å] completes the square planar coordination, in which the atom with the greatest deviation from the least-squares plane [0.019(1) Å] is N(12) and the deviation of the Au atom is 0.0054(8) Å. The tridentate ligand has Z, E, and Z configurations with respect to the bonds C(15)− C(16), C(16)−N(12), and N(13)−C(17), respectively. Although the N(12) bond is trans to Cl(1), the Au(1)−N(12) bond is shorter than Au(1)−N(11) because of the negative charge on the azomethine fragment. The Au−Cl bond is shorter and the Au(1)−N12 and Au−S bonds longer in 3 than in [Au(Am4hexim)Cl]Cl·CH3OH,29c but the bond lengths in the thiosemicarbazone moiety are virtually the same in these two complexes, and in both cases reflect the influence of deprotonation when compared with those of HAm4hexim31 or 2. Hydrogen Bond Networks and Packing Analysis. The molecular packing in 3 is governed by hydrogen bonding involving the [Au(Am4hexim)Cl]+ cation as a donor [through the formamide nitrogen N(15)], the chloride anion as an acceptor, and two water molecules as donors and acceptors (Table 5). The packing exhibits a layered structure (Figure 7) in which the N(15) atom of each [Au(Am4hexim)Cl]+ cation forms part of a hydrogen bonded ring of graph set R43(8) involving two water molecules and a chloride anion; the rings associated with two neighboring, centrosymmetrically related cations are linked by O−H···Cl bonds to create a cuboid hydrogen bonding structure in which the O and Cl atoms form a ring of graph set R64(12), and successive cuboids are themselves linked by O−H···Cl bonds (Figure 8). There are no metal−metal 1437



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Figure 4. View of compound 1 showing secondary interactions (hydrogen bonds, Au···Cl and N−H···Au agostic interactions) as dashed lines. Symmetry codes: a, (x, 1 − y, 1 − z); b, (1 − x, 1 − y, 1 − z); c, (x, y, −1+z); d, (−x, −y, 1 − z); e, (1 − x, 1 − y, −z); f, (−1 + x, −1 + y, z); g, (1 + x, 1 + y, z); h, (−x, −y, −z).

Table 4. Selected Bond Lengths and Angles in 3 and [Au(Am4hexim)Cl]Cl·CH3OH29c compound Au(1)−N(12) Au(1)−N(11) Au(1)−S(1) Au(1)−Cl(1) N(12)−Au(1)−N(11) N(12)−Au(1)−S(1) N(11)−Au(1)−S(1) N(12)−Au(1)−Cl(1) N(11)−Au(1)−Cl(1) S(1)−Au(1)−Cl(1)

Figure 5. Perspective view of the bis(H2Am4hexim)gold(I) cations of 1, showing the gold−gold distance. Symmetry code: a, (−x, 1 − y, −z).

[Au(Am4hexim)Cl]Cl·CH3OH

3 Distances (Å) 1.965(2) 2.070(2) 2.2642(7) 2.2795(7) Angles (°) 80.94(9) 84.80(7) 165.69(6) 179.32(7) 98.69(8) 95.59(3)

1.958(7) 2.073(6) 2.259(2) 2.290(2) 80.3(2) 85.4(2) 165.7(2) 177.2(2) 99.3(2) 94.93(7)

Table 5. Hydrogen Bonding in 3a D−H···A

d(D−H) (Å)

d(H···A) (Å)

d(D···A) (Å)

∠(DHA) (°)

N(15)−H(15A)···O(2)b N(15)−H(15B)···Cl(2)b O(1)−H(1A)···Cl(2)c O(1)−H(1B)···Cl(2)a O(2)−H(2B)···O(1)d O(2)−H(2A)···Cl(2)c

0.88 0.88 0.91 0.93 0.88 0.91

1.96 2.41 2.21 2.22 1.85 2.26

2.823(3) 3.200(2) 3.112(2) 3.149(2) 2.731(3) 3.166(2)

164.6 149.6 172.3 173.8 175.3 178.0

Symmetry transformations: a, (−x + 1, y − 1/2, −z + 1/2); b, (x, −y + 1/2, z − 1/2); c, (−x + 1, −y + 1, −z + 1); d, −x + 1, y + 1/2, −z + 1/2). a

that act as counterions,51 this is attributable to the pyridine nitrogen being protonated, as in the solid state (vide supra). An attempt to clarify this point by recording the NMR spectrum in MeOH-d4 failed because the spectrum did not show NH signals, with H-D exchange being greater in this solvent than in DMSOd6. To the best of our knowledge, compound 1 is the first example of a pyridine thiosemicarbazone complex to contain a protonated pyridine N.

Figure 6. Molecular structure of [Au(Am4hexim)Cl]Cl·2H2O (3).

The 1H NMR spectrum of 1 shows a signal at 9.82 ppm and, in light of data for protonated uncoordinated thiosemicarbazones 1438



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recorded in MeOH-d4 (see Supporting Information SM5 and SM6). On lowering the temperature to −80 °C, however, magnetic inequivalence is observed and the signals of the protons on carbons a and b become broader (Scheme 4). However, there is no significant upfield shift relative to the corresponding signals in free HAm4hexim, which suggests that in compound 2 there are no agostic interactions in methanol, in contrast to the solid state (vide supra). The 13C NMR spectra of 1 and 2 in DMSO-d6 did not contain a thiocarbonyl signal, which appears at 176.7 ppm in the spectrum of HAm4DH and at 178.0 ppm in the spectrum of HAm4hexim. The other signals show only very small shifts relative to those for the uncoordinated ligands. Photophysical Study. The photophysical study and the fluorescent behavior, used to discuss the electronic origin of the optical transitions, were established by quantitative comparison of absorption and fluorescence excitation spectra and by determination of the fluorescence quantum yields. Absorption and luminiscence spectroscopic data for ligands and complexes are listed in Table 7. The electronic spectra of the thiosemicarbazones and complexes 1 and 2 are shown in Figure 10. The spectra of HAm4DH and HAm4hexim show π → π* transitions at 244 and 254 nm, respectively, azomethine n → π* transitions at 324 and 331 nm, respectively, and thiocarbonyl n → π* transitions at 378 and 392 nm, respectively. Coordination to gold(I) increased the absorbance of both compounds at almost all wavelengths, the main exception concerning HAm4hexim at the wavelength of its azomethine n → π* peak.52 In compound 1, coordination appears to increase the energy of the π → π* transition and split the azomethine n → π* peak. In 2, coordination increased the energy of the thiocarbonyl n → π* and eliminated the azomethine n → π* peak from the gross spectrum, presumably by shifting it to a wavelength at which it was masked by neighboring peaks. HAm4DH is nonfluorescent but undergoes a CHEF effect upon coordination to Au in 1, which, in methanol at room temperature, shows a quantum yield ΦF = 0.02. The UV−vis absorption and fluorescence spectra of 1 are shown in Figure 11. Excitation of complex 1 at 317 nm gave rise to a fluorescence emission spectrum with a single band with a maximum at 384 nm. The excitation spectrum measured around the emission maximum shows a strong band at 287 nm with a small feature at 231 nm. The absorption spectra of the ligand and 1 are quite similar, which suggests that the metal does not have a significant effect on the nuclear and electronic structure and does not perturb the aggregation equilibria of the ligand in the ground electronic state. However, the fact that the excitation spectrum does not resemble the absorption spectrum suggests a molecular aggregation; similarly, the large energy gap between the emission and excitation maxima in 1 is typical of Au-centered emissions in Au(I) complexes.53 It is known from the literature that gold(I) complexes with Au···Au contacts can split the Au(6pz) and S(pπ) orbitals and create an intramolecular dσ*-pσ excitedstate that generates emission.54 Considering that the X-ray structure of 1 shows the existence of a very weak gold−gold contact in the solid state, we suggest that the luminescence is based on a gold-centered emission due to the formation of dimers or other aggregates.55 Furthermore, such molecular association could be modified upon excitation; in particular, it could be enhanced in the excited state by metal-metal bonding.55a On the basis of crystallographic data, 1 is a good candidate to exhibit

Figure 7. Portion of the packing diagram of 3 viewed parallel to the y axis. Hydrogen bonds are indicated by dashed lines, as are long Au···Au contacts. Symmetry codes: a, (x, 1/2 − y, 1/2 − z); b, (x, 1/2 − y, −1/2 + z).

Figure 8. Perspective view of compound 3 showing classical hydrogen bonds (indicated by dashed lines). Symmetry codes: a, (1 − x, −1/2 + y, 1/2 − z); b, (x, −1/2 + y, z); c, (1 − x, −y, 1 − z); d, (1 − x, −3/2 + y, 1/2 − z).

Figure 9. Structure of two symmetrically related cations of 3 showing the Au···Au distance.

In the temperature range 0−20 °C, the hexamethylene protons of 2 appear as sharp singlets in the 1H NMR spectra 1439



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Table 6. Short π−π Stacking Interactions in Complex 3a ring

d[Cg(I)−Cg(J)] (Å)

d⊥[Cg(I)−P(J)] (Å)

d⊥[Cg(J)−P(I)] (Å)

α (°)

β (°)

Cg(1)···Cg(2)a Cg(1)···Cg(3)a Cg(2)···Cg(1)b Cg(3)···Cg(1)b

3.6488 3.5394 3.6488 3.5394

3.393 3.378 3.451 3.295

3.07 5.07 3.07 5.07

18.96 21.42 21.58 17.36

21.58 17.36 18.96 21.42

Symmetry transformations: a, (x, 1/2 − y, 1/2 + z); b, (x, 1/2 − y, −1/2 + z). Centroids: Cg(1), Au(1)/S(1)/C(17)/N(13)/N(12); Cg(2), Au(1)/N(11)/C(15)/C(16)/N(12); Cg(3), N(11)/C(11)/C(12)/C(13)/C(14)/C(15); Cg(4), N(14)/C(18)/C(19)/C(20)/C(21)/C(22)/ C(23). a

Scheme 4. Configurations of the Free Ligands

Table 7. Absorption and Luminescence Data for the Thiosemicarbazones and Their Gold(I) Complexes 1 and 2 compound HAm4DH 1 HAm4hexim 2

absorption λmax/nm (ε, dm3 mol−1 cm−1) 203(18038), 244(6889), 324(12186), 378(2727) 201(20074), 212(18309), 317(22571), 326(22268), 379(2951) 219(18953), 254(11233), 331(6464), 392(15600) 203(38690), 254(19774), 373(24692)

emission λmax/nm

excitation λmax/nm

ΦF

Figure 11. UV−vis absorption (gray shaded area), emission (red), and excitation (blue) spectra of 1.

no luminescence 384

287

0.02

407

331

0.15

407

331

0.01

spectra show single bands with maxima at 407 nm. The excitation spectra recorded at the emission peak show an intense band at 331 nm together with a small feature at about 241 nm in HAm4hexim and 238 nm in 2. The profiles of the fluorescence spectra of HAm4hexim and 2, aside from a difference in fluorescence quantum yields, indicate that the origin of the luminescence is the same and that intraligand transitions are involved. This is also supported by the similarity in their absorption spectra. Remarkably, in HAm4hexim and 2 the maxima are nearly coincident, but the excitation and absorption spectra for the two compounds are clearly different (Figures 12 and 13) and a significant Stokes’ shift of 5641 cm−1

Figure 10. Electronic absorption spectra of HAm4DH, HAm4hexim, 1 and 2 in methanol at 298 K.

such excited states due to the existence of very weak gold−gold contacts in the solid state and other secondary interactions, such as agostic interactions, hydrogen bonds, Au···Cl, etc., that could also be involved in the fluorescence. The fluorescence emission spectra of HAmhexim and 2 with excitation at 331 nm are shown in Figures 12 and 13. Both

Figure 12. UV−vis absorption (gray shaded area), emission (red), and excitation (blue) spectra of HAm4hexim. 1440



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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data in CIF and pdf formats, packing diagram of unit cells of compounds 1−3, spectroscopic characterization figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +0034-881-815090.

ACKNOWLEDGMENTS We gratefully acknowledge the financial support by the Xunta de Galicia (INCITE08PXIB203128PR). (1) (a) Kostova, I. Anticancer Agents Med. Chem. 2006, 6, 19. (b) Barnard, P. J.; Berners-Price, S. J. Coord. Chem. Rev. 2007, 251, 1889. (c) Bruijnincx, P. C. A.; Sadler, P. J. Curr. Opin. Chem. Biol. 2008, 12, 197. (2) (a) Milanic, V.; Dou, Q. P. Coord. Chem. Rev. 2009, 253, 1649. (b) Ott, I. Coord. Chem. Rev. 2009, 253, 1670. (c) Bindoli, A.; Rigobello, M. P.; Scutari, G.; Gabbiani, C.; Casini, A.; Messori, L. Coord. Chem. Rev. 2009, 253, 1692. (3) Navarro, M. Coord. Chem. Rev. 2009, 253, 1619. (4) McKeage, M. J.; Maharaj, L.; Berners-Price, S. J. Coord. Chem. Rev. 2002, 232, 127. (5) Abdou, H. E.; Mohamed, A. A. Jr.; Fackler, J. P.; Burini, A.; Galassi, R.; López de Luzuriaga, J. M.; Olmos, M. E. Coord. Chem. Rev. 2009, 253, 1661. (6) Kunnz, P. C.; Kassack, M. U.; Aamacher, A. Dalton Trans. 2009, 7741. (7) (a) Nomiya, K.; Yamamoto, S.; Noguchi, R.; Yokoyama, H.; Kasuga, N. Ch.; Ohyama, K.; Kato, Ch. J. Inorg. Biochem. 2003, 95, 298. (b) Bermejo, E.; Casas, J. S.; Couce, M. D.; Sánchez, A.; SánchezGonzález, A.; Sordo, J.; Varela, J. M.; Vázquez-López, E. M. J. Inorg. Biochem. 2008, 102, 184. (8) (a) Howard-Lock, H. E. Metal-Base Drugs 1999, 6, 201. (b) Shaw, C. F. III Chem. Rev. 1999, 99, 2589. (c) Tiekink, E. R. T. Gold Bull. 2003, 365, 117. (9) (a) Sutton, B. M.; McGusty, E.; Walz, D. T.; DiMartino, M. J. J. Med. Chem. 1972, 15, 1095. (b) Mirabelli, C. K.; Johnson, R. K.; Hill, D. T.; Faucette, L. F.; Girard, G. R.; Kuo, G. Y.; Sung, Ch. M.; Crooke, S. T. J. Med. Chem. 1986, 29, 218. (c) Hill, D. T.; Sutton, B. M. Cryst. Struct. Commun. 1980, 9, 679. (10) (a) Schmidbaur, H. In Gold. Progress in Chemistry, Biochemistry and Technology; Schmidbaur, H., Ed.; John Wiley and Sons Ltd.: Chichester, 1999. (b) Hashmi, A. S. K.; Schwarz, L.; Chio, J.-H.; Frost, T. M. Angew Chem., Int. Ed. 2000, 39, 2285. (c) Gorin, D. J.; Davis, N. R.; Toste, F. D. J. Am. Chem. Soc. 2005, 127, 11260. (d) Asao, N.; Sato, K.; Manggenbateer, Y.; Yamamoto, Y. J. Org. Chem. 2005, 70, 3682. (e) Corti, M.; Laguna, A.; Thompson, D. Gold Bull. 2006, 39, 226. (f) Fernández, E. J.; Laguna, A.; López de Luzuriaga, J. M. Dalton Trans. 2007, 1969. (g) Tiekink, E. R. Inflammopharmacology 2008, 16, 138. (11) Tiekink, E. R. T.; Kang, J.-G. Coord. Chem. Rev. 2009, 253, 1627. (12) Yam, V. W. W.; Cheng, E. Ch.-Ch. Chem. Soc. Rev. 2008, 37, 1806. (13) (a) Gimeno, M. C.; Laguna, A. Silver and Gold. In Comprehensive Coordination Chemistry II; Elsevier: New York, 2003; Vol. 6. (b) Gimeno, M. C.; Laguna, A. Gold Bull. 2003, 36, 83. (14) (a) Jones, W. B.; Yuan, J.; Narayanaswamy, R.; Young, M. A.; Elder, R. C.; Bruce, A. E.; Bruce, M. R. M. Inorg. Chem. 1995, 34, 1996. (b) Yam, V. W. W.; Chan, C. L.; Li, C. K.; Wong, K. M. C. Coord. Chem. Rev. 2001, 216−217, 173. (15) Schmidbaur, H.; Schier, A. Chem. Soc. Rev. 2008, 37, 1931. (16) Schmidbaur, H.; Cronje, S.; Djordjevic, B.; Schuster, O. Chem. Phys. 2005, 311, 151.

Figure 13. UV−vis absorption (gray shaded area), emission (red), and excitation (blue) spectra of 2.

implies significant reorganization of low-frequency modes in the excited state. Therefore, the considerable decrease in the fluorescence, as indicated by quantum yield values (ΦF = 0.15 for the free ligand and ΦF = 0.01 in the complex), shows that the gold atom partially enhances the radiationless deactivation channels of the ligand. In conclusion, these results suggest that the chemical modification of the side chain bound to the N4 position of the thiosemicarbazone affects the fluorescence; HAm4DH is nonfluorescent whereas HAm4hexim has a quantum yield ΦF = 0.15. While HAm4DH is not emissive, the coordination of this ligand to the gold atom in 1 produces a CHEF effect with ΦF = 0.02. The fluorescence of this complex is attributed to an aggregation equilibrium, where the aurophilic bonding in the excited state between adjacent complexes could be involved. In contrast to the above, the fluorescence is quenched when the HAm4hexim coordinates in 2, but both compounds have similar emissive behavior  apart from a difference in the fluorescence quantum yield. Therefore, the emission in 2 is due to the localized excited state of the ligand and the presence of the metal partially quenches the fluorescence. The crystal data for 2 indicate the absence of aurophilic interactions in 2 in the solid state, and this study reveals that these interactions are also absent in solution.

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CONCLUSIONS

HAm4DH and HAm4hexim form the stable gold(I) complexes [Au(H2Am4DH)2]Cl3·H2O (1) and [Au(HAm4hexim)2]Cl (2). In 1 the thiosemicarbazone ligand is protonated and in 2 its hydrazinic hydrogen is located on N2. In 1 there is a very weak gold−gold interaction with a distance of 3.83 Å [slightly longer than the van der Waal’s diameter of gold(I)], but the formation of a similar bond in 2 appears to be prevented by the bulky azepane ring. The oxidation of Au+ in 2 to the gold(III) complex [Au(Am4hexim)Cl]Cl·2H2O (3) on standing in water/ methanol solution in air at room temperature is attributable to a dismutation reaction. Coordination to gold(I) in 1 endows the nonluminescent ligand HAm4DH with moderate luminescence (ΦF = 0.02), but the luminescence of HAm4hexim is quenched (ΦF = 0.15 for the ligand, 0.01 for 2); it seems likely that all the luminescence bands are due to intraligand transitions. 1441



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Supramolecular Architecture in Gold (I) and Gold (III) 2

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