ARTICLE pubs.acs.org/crystal
Argentophilic Interactions in Mono-, Di-, and Polymeric Ag(I) Complexes with N,N0-Dimethyl-piperazine-2,3-dithione and Iodide Angela Serpe,*,† Flavia Artizzu,† Luciano Marchio,*,‡ Maria Laura Mercuri,† Luca Pilia,† and Paola Deplano*,† † ‡
Dipartimento di Chimica Inorganica ed Analitica, Universita di Cagliari, Cittadella di Monserrato, I-09042, Monserrato, Cagliari, Italy Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Universita di Parma, Parco Area delle Scienze 17A, I-43100 Parma, Italy
bS Supporting Information ABSTRACT: A variety of silver(I) neutral complexes differing in stoichiometries and/or crystallographic arrangements have been obtained by reacting [HMe2pipdt]I3 (1) (Me2pipdt = N,N0 dimethyl-piperazine-2,3-dithione) with silver as a metal powder and as 1þ metal ion. Once dissolved in THF, 1 reacts with silver powder, affording complexes of different ligand/metal ratios: [Ag(Me2pipdt)2]I3 (2a) and [Ag(Me2pipdt)I]2(3) (in the two different crystallographic arrangements 3a and 3b). The polymer [Ag2(Me2pipdt)I2]n (4) and the monomer [Ag(Me2pipdt)2]I3 (2b) are instead obtained when 1 reacts with AgSbF6 in CH3CN. The short intermetallic distances found in the dimeric and polymeric structures [dAg-Ag(Å) = 3.3267(9) (3a), 3.0873(5) (3b), 2.8139(9)/3.1460(9)/3.2420(8) (4)] suggested possible argentophilic interactions in the complexes. These interactions have been investigated applying density functional theory (DFT) calculations and the atoms-in-molecules (AIM) analysis on 3a and 3b, on a fragment of 4, and on a Ag2I2 model. This model mimics the rhomboidal fragment which occurs in most of the described structures and provides the tools to ascertain the occurrence and the strength of the argentophilicity as a function of the intermetallic distance (2.7-3.5 Å) for the isolated Ag2I2 moiety. The obtained results highlight argentophilicity in 3b and 4.
’ INTRODUCTION Currently, silver chemistry has gained increasing interest addressed to achieve a satisfactory structure/properties relationship, useful to provide suitable tools to prepare compounds with desired predictable properties for conventional or new applications. In particular, silver(I) compounds showing a variety of structures1 - spanning from monomers,2 to oligomers,2d,3 polymers2b,3c,4 and clusters4d,5 - have been prepared and characterized. Some of these species exhibit peculiar properties that make them attractive for new applications in the field of opto-electronic or magnetic materials,6 sensors,6 or antiviral and antimicrobial drugs.5c,7 It has been shown that argentophilicity, relatable to d10-d10 closed-shell weak interactions in silver(I) compounds,8 plays a crucial role in promoting both the aggregation of silver centers producing a wide array of extended supramolecular networks,2d,9 and leading properties such as luminescence,4b,4d,4e,5a,10 conductivity,11 or colossal thermal expansion.12 The role of the ligand in favoring and enforcing argentophilicity is important as shown by the large number of complexes containing “ligand-supported” interactions10e,13 in comparison with the rare cases of “ligandunsupported” ones.10d,10e,14 Thione/thiolate ligands have shown to be very effective in adjusting the coordination framework in silver(I) complexes and in promoting metal-metal interactions r 2011 American Chemical Society
thanks to their capability to work as bridging ligands.1,5b,5c,15 The affinity of the “soft” iodide ligand toward silver is also wellknown. Reagents prepared in our laboratory by reacting polyfunctional thione-donors16 and diiodine,17 formed by neutral adducts or triiodide salts of the monoprotonated ligand, have been shown to be powerful oxidation reagents toward noblemetal (NMs). In particular, these reagents, which are noncytotoxic, have been shown to be capable of dissolving gold,18 palladium,19 and in some cases platinum,20 in very mild conditions, thanks to the presence of combined coordination-oxidation properties in the same molecule. For these features, the cited reagents have been satisfactorily employed for applicative purposes in the field of NMs dissolution and recovery from different kinds of Hi-Tech scraps,17a such as exhausted three-way catalysts (TWC)21 or electric and electronic equipment (WEEE).22 Therefore, these reagents seem to us promising to oxidize silver metal as well to form silver(I) compounds and to provide suitable ligands to favor argentophilic interactions among the metals.
Received: November 11, 2010 Revised: January 14, 2011 Published: February 21, 2011 1278
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Crystal Growth & Design Chart 1
Here we describe the results obtained by reacting the triiodide salt [HMe2pipdt]I3 (1), prepared by mixing N,N0 -dimethylpiperazine-2,3-dithione (Me2pipdt, see Chart 1) with iodine in a 1:1.5 molar ratio, toward silver as a metal powder and as a monocation. Theoretical studies were performed by using the Bader’s theory of atoms-in-molecules (AIM)23-25 to investigate possible argentophilic interactions in the obtained products.
’ EXPERIMENTAL SECTION Materials and Instrumentation. Reagents and solvents were purchased from Aldrich and used without further purification. Microanalyses were performed on a Carlo Erba CHNS elemental analyzer model EA1108. IR spectra (4000-200 cm-1) were recorded on KBr pellets with a FTIR Bruker Equinox 55 spectrometer. FT-Raman spectra (resolution 4 cm-1) were recorded on a Bruker RFS100 FT-spectrometer, fitted with an indium-gallium-arsenide detector (room temperature) and operating with an excitation frequency of 1064 nm (Nd:YAG laser). The power level of the laser source varied between 20 and 40 mW. The solid samples were introduced in a capillary tube and then fitted into the compartment designed for a 180° scattering geometry. Electronic spectra (800-200 nm) were performed at room temperature (25 °C) on CH3CN solutions (∼10-4 mol dm-3) by using a 0.5 mm thick quartz cell and recorded on a Cary 5 spectrophotometer. Data Collection and Structure Determination. Table 1 summarizes the most relevant crystal data for the samples 2a, 2b, 3a, 3b, and 4. Single crystal data were collected with a Bruker AXS Smart 1000 (3a) and with a Bruker AXS Smart 1000 (2a, 2b, 3b, and 4) area detector diffractometers, Mo KR: λ = 0.71073 Å. The unit cell parameters were obtained using 60 ω-frames of 0.5° width and scanned from three different zones of reciprocal lattice. The intensity data were integrated from several series of exposures frames (0.3° width) covering the sphere of reciprocal space.26 An absorption correction was applied using the program SADABS27 with min and max transmission factors of 0.5821.000 (2a), 0.625-1.000 (2b), 0.425-1.000 (3a), 0.731-1.000 (3b), and 0.587-1.000 (4). The structures were solved by direct methods (SIR9728,29 and SIR200429) and refined on F2 with full-matrix leastsquares (SHELXL-9730), using the Wingx software package.31 Nonhydrogen atoms were refined anisotropically for all compounds, and the hydrogen atoms were placed at their calculated positions. The maximum and minimum peaks on the final difference Fourier maps corresponded to 1.678/-1.521 e Å-3 (2a), 1.145/-0.885 e Å-3 (2b), 0.690/-0.707 e Å-3 (3a), 0.859/-0.748 e Å-3 (3b), and 1.318/-1.745 e Å-3 (4). Graphical material was prepared with the ORTEP3 for Windows32 and Mercury 2.033 programs. CCDC Nos. 792904-792908 contain the supplementary crystallographic data for this paper. Computational Details. Density functional theory calculations were performed on the dinuclear complexes and on a fragment of the polymeric complex containing the three putative Ag-Ag interactions. For both entities, X-ray structures were used without geometry optimization. Two different density functionals were used in the study: (1) the Becke three-parameter exchange functional with Lee-Yang-Parr correlation functional (B3LYP),34,35 and (2) the Slater local spin density exchange functional with Vosko-Wilk-Nusair correlation functional
ARTICLE
(SVWN).36,37 The Stuttgard-Dresden effective core potential along with the SDD valence basis set was used for silver and iodine atoms,38-40 whereas all other atoms were treated with the 6-31G basis set.41,42 In order to better comprehend the interaction occurring between the two silver atoms in the polymeric complex, the Ag2I2 fragment was optimized with the B3LYP and SVWN density functionals and with the MøllerPlesset MP2 method,43,44 using the SDD basis set. For all systems, that is, dinuclear complexes, polynuclear fragment of 4 and Ag2I2 model, the topological analysis of the electron density was performed with the AIM2000 Release 1 package.45 In addition, the properties of the electron density between the two metal atoms in the Ag2I2 model were investigated by calculating the properties of the electron density as a function of the silver-silver distance (from 2.71 to 3.51 Å, step 0.1 Å) at the MP2/SDD theory level. All the calculations have been performed with the Gaussian 03 program suite.46 In Supporting Information details on the application of AIM theory are provided. Synthesis of [HMe2pipdt]I3 (1). The salt was prepared as described in refs 17a and 20. Anal. found (Calcd.) for C6H11N4S4I3 (555.999): C%, 13.2 (12.96), H% 2.0 (1.99), N% 5.0 (5.04), S% 12.1 (11.53).
Synthesis of [Ag(Me2pipdt)2]I3 (2a) and [AgI(Me2pipdt)]2 (3a and 3b). A THF solution of 1 (∼100 mg in 100 mL) was allowed to
react with Ag powder (2:1 molar ratio, 9.72 mg; 5-8 μm, 99.9%) under stirring and under Ar atmosphere. The solution turned rapidly from redbrown to brown, and a quantitative dissolution of the Ag powder and the precipitation of 3 in form of brown microcrystals was observed in approximately 24 h. The solution, recovered by filtration, was concentrated under reduced pressure and submitted to slow-diffusion of Et2O. After about 24 h, well-shaped crystals of 2a (dark brown long needles with high solubility in solvents such as THF, CH3CN, acetone, and very low solubility in Et2O), suitable for X-ray studies, and crystals of 3 (mainly red-brown plates (3a) and few dark brown prisms (3b)), insoluble in most common organic solvents, coprecipitated from the solution. The solid was collected and treated with THF where 2a is selectively dissolved. 3a and 3b were separated from the solution and washed with Et2O. 2a was crystallized from the THF solution by slow-diffusion of Et2O. Characterization of 2a: yield: 22%; Anal. found (Calcd.) for C12H20N4S4AgI3 (837.152): C%, 17.5 (17.22), H% 2.2 (2.41), N% 6.7 (6.69), S% 15.5 (15.32); Raman (cm-1): 2970(mw); 2904(w), 1516(w), 1435(w), 1398(w), 1356(w), 1242(mw), 1129(m), 1107(mw), 546(mw), 293(m), 131(s), 114(vs), 85(sh), UV-vis (λ, nm) (ε 10-3, mol-1 dm3 cm-1): 223(26.0), 293(36.0), 361(23.0). Characterization of 3: yield: 78%; Anal. found (calcd.) for C6H10N2S2AgI (409.061): C%, 18.4 (17.62), H% 2.8 (2.46), N% 6.9 (6.84), S% 15.8 (15.68).
Synthesis of [Ag(Me2pipdt)2]I3 (2b) and [Ag2I2(Me2pipdt)]n (4). By slow addition of a CH3CN solution of AgSbF6 (62 mg in 50 mL) to 1 in the same solvent with a 1:1 molar ratio (100 mg in 50 mL), color solution changed from orange to brown and precipitation of AgI (71% yield) immediately occurred. After filtration, well-shaped crystals of 4 suitable for X-ray studies (shiny black blocks, 21% yield) were obtained by slow evaporation of the solvent. The crystals were recovered by decantation, washed with petroleum ether (40-60°), dried, and then fully characterized. Anal. found (Calcd.) for Ag2C6H10N2S2I2 (643.832): C% 11.8 (11.19); H% 1.5 (1.56); N% 4.6 (4.35); S% 10.0 (9.96); MIR (on KBr pellets, cm-1): 2935(vw), 2870(vw), 1532(w), 1510(vs), 1503(sh), 1437(vw), 1427(w), 1417(sh), 1392(m), 1382(m), 1352(s), 1338(sh), 1279(m), 1258(m), 1245(vw), 1192(w), 1118(m), 1108(m), 890(w), 545(m). Raman (cm-1): 2997(w), 2951(m), 2908(ms), 1508(m), 1434(m), 1392(m), 1351(s), 1277(w), 1236(s), 1151(vw), 1105(s), 883(vw), 860(vw), 681(w), 545(ms), 480(w), 424(w), 385(w), 367(w), 295(vw), 228(w), 160(m), 136(m), 109(sh), 99(vs), 75(s). Well-shaped crystals of 2b (8% yield, dark brown needles) were also collected by submitting the CH3CN mother solution to slow-diffusion of Et2O. The crystals were separated by decantation, washed with Et2O, and then characterized by single-crystal X-ray crystallograpy. 1279
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Table 1. Summary of Crystallographic Data for [Ag(Me2pipdt)2]I3 (2a and 2b), [AgI(Me2pipdt)]2 (3a and 3b) and [Ag2I2(Me2pipdt)]n (4) [Ag(Me2pipdt)2]I3 (2) 2a
a
[AgI(Me2pipdt)]2 (3)
2b
3a
[Ag2I2(Me2pipdt)]n (4)
3b
empirical formula
C12H20AgI3N4S4
C12H20AgI3N4S4
C12H20Ag2I2N4S4
C12H20Ag2I2N4S4
formula weight
837.13
837.13
818.10
818.10
C6H10Ag2I2N2S2 643.82
color, habit crystal size, mm
black, needle 0.55 0.15 0.15
black, needle 0.27 0.23 0.11
red-brown, plate 0.27 0.22 0.11
dark brown, prism 0.28 0.18 0.05
black, block 0.18 0.13 0.08
crystal system
triclinic
orthorhombic
monoclinic
monoclinic
triclinic
space group
P1
Pbca
P21/n
P21/n
P1
a, Å
8.214(1)
9.328(1)
7.597(1)
11.382(3)
8.338(1)
b, Å
8.763(1)
15.087(1)
16.811(2)
9.242(3)
9.352(1)
c, Å
18.791(3)
33.966(3)
8.875(1)
11.923(4)
9.884(1)
R, deg
80.311(1)
90
90
90
104.497(1)
β, deg γ deg
81.940(2) 65.782(1)
90 90
96.872(5) 90
118.18(1) 90
107.909(2) 102.916(2)
V, Å3
1212.1(3)
4780.1(7)
1125.3(2)
1105.5(6)
671.2(1)
Z
2
8
2
2
2
T, K
293(2)
293(2)
293(2)
293(2)
293(2)
λ Å (Mo KR)
0.71073
0.71073
0.71073
0.71073
0.71073 3.186
F(calc), Mg/m3
2.294
2.326
2.414
2.458
μ, mm-1
5.001
5.072
4.859
4.945
7.795
θ range, deg no. of rfln/obsv F > 4σ(F)
2.21-30.99 7634/4718
2.40-27.28 5352/4264
2.42-31.14 3589/2908
2.04-27.52 2540/2085
2.31-30.00 3900/3432
R1a
0.0429
0.0295
0.0251
0.0250
0.0310
wR2b
0.1069
0.0668
0.0637
0.0634
0.0749
R1 = Σ||Fo| - |Fc||/Σ|Fo|. b wR2 = [Σ[w(Fo2 - Fc2)2]/Σ[w(Fo2)2]]1/2, w = 1/[σ2(Fo2) þ (aP)2 þ bP], where P = [max(Fo2,0) þ 2Fc2]/3.
’ RESULTS AND DISCUSSION Synthesis and Molecular Structures. The reactions of 1 with silver metal powder and the silver(I) salt AgSbF6 are summarized in Scheme 1 and described in detail in the Experimental Section. As shown in Scheme 1a, silver powder is dissolved quantitatively under the reported experimental conditions in approximately 1 day. A small amount of 3 in the form of microcrystals is recovered from the solution on standing, while 2a and an additional amount of 3 coprecipitated by Et2O diffusion into the THF solution. Crystals of 2a can be separated manually or by recrystallization from acetone/Et2O. On the other hand, when the reagent 1 was reacted with the silver(I) salt AgSbF6, a precipitation of AgI immediately occurred. 2b, 4, and other unidentified products were precipitated from the recovered solution as described in the Experimental Section (Scheme 1b). The molecular structure of 2a is reported in Figure 1. The metal is in a distorted tetrahedral environment bound by two S,S chelate ligands. The reason for the deviation from the ideal geometry derives by the ligands bite angles of ∼84°, since the Ag-S bond lengths vary in the relatively narrow range of 2.490(1)-2.556(1) Å, Table 2. The counterion is represented by the almost linear and slightly asymmetrical I3- anion. The molecular structure of the mononuclear complex 2b is reported in Figure 1 and the metal environment is analogous to that of 2a, defined by two S,S chelate ligands. Although 2a and 2b present the same stoichiometry, the two compounds differ by the relative orientation of the two ligands, and in 2b the metal geometry is less distorted than in 2a.
As for 2a, in 2b the counterion is represented by an almost linear I3- anion. Spectroscopic characterization of 2a agrees with X-ray diffractometric results. In fact, in the solid state, the presence of a slightly asymmetrical triiodide unit is supported by Raman peaks at 114(vs), 131(s), and 85(sh) cm-1 which can be assigned to the symmetrical and antisymmetrical stretching and to the bending of the triiodide unit, respectively.47 The triiodide anion is preserved in CH3CN solution, as shown by the presence of typical bands at 293 nm and 361 nm in the UV-vis region. Crystals of 3 were collected, washed by Et2O, and characterized by X-ray crystallography, showing two different molecular arrangements (a and b) corresponding to the same dimeric formula. The molecular structures of 3a and 3b are reported in Figure 2, whereas selected geometric parameters for both structures are reported in Table 3. The two complexes mainly differ by the nature of the donor atom that bridges the two metal centers, the S(1) sulfur atoms in 3a, and the iodine atom in 3b. In 3a the ligand chelates a metal with both sulfur atoms and bridges a second silver atom with the S(2) atom (μ2-κ2S:S0 mode). The Ag-S(1) and Ag-S(2) bond distances are of comparable lengths even though Ag-S(1) is ∼0.05 Å longer than Ag-S(2) due to the involvement of the S(1) atoms in the Ag-S-Ag bridge. In addition, the Ag-S(1)0 distance is ∼0.3 Å longer than the other Ag-S ones, and ∼0.2 Å longer than the Ag-I bond distance. Accordingly, the geometry exhibited by Ag is intermediate between the tetrahedral and the trigonal pyramidal with the apical position occupied by the bridging S(1)0 atom. With this respect, the sum of the base angles defined by S(1), I, S(2), and Ag, is ∼346° (cfr. 360° for a trigonal pyramidal geometry and 328.5° for a tetrahedral one). 1280
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Scheme 1. Reaction Scheme of 1 with Silver Powder (a) and Silver 1þ Ion (b); i = THF, r.t., 2:1, 24 h; ii = Crystallization from the Solution with Et2O; iii = CH3CN, r.t., 1:1a,b
a
Very small amount. b 71% yield.
Table 2. Relevant Geometric Parameters (Å and [°]) of the X-ray Structures of 2a and 2b 2a Ag-S(11)
2.505(1)
C(11)-S(11)
1.660(4)
Ag-S(12)
2.490(1)
C(21)-S(21)
1.673(4)
Ag-S(21)
2.535(1)
C(12)-S(12)
1.683(4)
Ag-S(22)
2.556(1)
C(22)-S(22)
1.675(4)
I(1)-I(2)
2.953(6)
I(2)-I(3)
2.8850(6)
S(11)-Ag-S(12)
121.51(5)
C(11)-S(11)-Ag
103.9(1)
S(11)-Ag-S(21) S(11)-Ag-S(22)
83.98(4) 133.07(5)
C(21)-S(21)-Ag C(12)-S(12)-Ag
102.8(1) 104.8(2)
S(21)-Ag-S(12)
136.57(5)
C(22)-S(22)-Ag
102.9(2)
S(21)-Ag-S(22)
104.42(5)
I(1)-I(2)-I(3)
176.49(2)
S(12)-Ag-S(22)
83.78(4) 2b
Figure 1. Molecular structures of 2a (above) and 2b (below). Solid thermal ellipsoids are depicted at the 30% probability level. 0
The Ag-Ag distance is 3.3267(9) Å, and it is slightly shorter than the sum of the van der Waals radii (3.44 Å) suggesting a weak metal-metal interaction mediated by the bridging ligand.48 The dinuclear complex 3b presents the ligand S,S chelate as in 2a, but with the Ag-S bond distances that are, on average, ∼0.06 Å longer than those of the latter complex, Tables 2 and 3. The Ag-I distances are ∼0.2 Å longer than the Ag-S ones, but overall the bond distances are more symmetric, so that the metal geometry can be described as distorted tetrahedral. Another interesting feature exhibited by this complex derives from the Ag-Ag distance, which is considerably shorter than in 3a (3.0873(5) Å, indicating a certain degree of metal-metal interaction.
Ag-S(11)
2.525(1)
C(11)-S(11)
1.682(4)
Ag-S(12)
2.499(1)
C(21)-S(21)
1.678(4)
Ag-S(21) Ag-S(22)
2.565(1) 2.578(1)
C(12)-S(12) C(22)-S(22)
1.673(4) 1.668(4)
I(1)-I(2)
2.893(5)
I(2)-I(3)
2.958(5)
S(11)-Ag-S(12)
125.26(5)
C(11)-S(11)-Ag
101.7(1)
S(11)-Ag-S(21)
82.03(3)
C(21)-S(21)-Ag
97.3(1)
S(11)-Ag-S(22)
124.66(4)
C(12)-S(12)-Ag
103.5(1)
S(21)-Ag-S(12)
126.47(5)
C(22)-S(22)-Ag
102.8(1)
S(21)-Ag-S(22)
120.54(4)
I(1)-I(2)-I(3)
179.07(1)
S(12)-Ag-S(22)
83.52(4)
The molecular structure of the polymeric complex 4 is reported in Figure 3, and it provides an additional example of variability to the structures noted so far. In fact, the ligand shows the same type of μ2-κ2S:S0 coordination mode with respect to two silver atoms, Ag(1) and Ag(2) as found for 3a. However, while in 3a the coordination environment 1281
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Figure 3. Molecular structure of the polymeric complex 4. Symmetry codes: 0 = -x; -y; -z, 00 = -1 - x; -y; -z, 000 = 1 þ x; y; z. Solid thermal ellipsoids are depicted at the 30% probability level.
Table 4. Relevant Geometric Parameters (Å and [°]) of the X-ray Structures of 4a Figure 2. Above, molecular structures of 3a (symmetry code 0 = -x; -y; -z), and below molecular structure of 3b (symmetry code = -x; -y; -z). Solid thermal ellipsoids are depicted at the 30% probability level.
4
Table 3. Relevant Geometric Parameters (Å and [°]) of the X-ray Structures of 3a and 3ba 3a Ag-I
2.6780(7)
C(1)-S(1)
1.683(3)
Ag-S(1)
2.587(1)
C(2)-S(2)
1.681(3)
Ag-S(2)
2.544(1)
C(1)-C(2)
1.527(4)
Ag-S(1)0 S(1)-Ag-I
2.859(1) 121.78(3)
Ag-Ag0 S(1)-Ag-S(2)
3.3267(9) 82.34(3)
S(1)-Ag-S(1)0
104.87(3)
S(2)-Ag-I
141.43(3)
I-Ag-S(1)0
107.00(3)
S(2)-Ag-S(1)’
92.93(3)
Ag-S(1)-Ag0
75.13(3)
S(2)-Ag-Ag0
86.58(3)
C(1)-S(1)-Ag
104.3(1)
C(2)-S(2)-Ag
104.8(3)
3b Ag-I
2.8085(4)
C(1)-S(1)
1.672(2)
Ag-I0 Ag-S(1)
2.8119(4) 2.5959(7)
C(2)-S(2) C(1)-C(2)
1.673(2) 1.524(3)
Ag-S(2)
2.5845(8)
Ag-Ag0
3.0873(5)
S(1)-Ag-I
127.46(2)
I-Ag-I0
113.36(1)
S(1)-Ag-S(2)
81.09(2)
C(1)-S(1)-Ag
100.40(8)
S(2)-Ag-I
102.88(2)
C(2)-S(2)-Ag
103.84(8)
a0
= -x; -y; -z.
of the metal is achieved by means of three sulfur atoms and one iodine anion, in 4, Ag(1) is surrounded by two sulfur atoms and two bridging iodines, and Ag(2) presents one sulfur atom and three iodines in its coordination sphere. The Ag(1)-sulfur distances (Table 4) are significantly longer than in the other complexes, but they reflect the same trend: the bridging sulfur S(1) exhibits the longer Ag-S bond distance. Also in this case, both metals present a distorted tetrahedral geometry, but it is more difficult to rationalize the type of
00
Ag(1)-Ag(1)
2.8139(9)
Ag(1)-I(2)00
2.9131(6)
0
Ag(2)-Ag(2)
3.1460(9)
Ag(2)-S(2)
2.657(1)
Ag(1)-Ag(2)0 Ag(1)-S(2)000
3.2420(8) 2.690(1)
Ag(2)-S(1) Ag(2)-I(1)
2.609(2) 2.8597(6)
Ag(1)-I(1)
2.755(5)
Ag(2)-I(1)0
2.9076(6)
Ag(1)-I(2)
2.865(6) 105.71(2)
I(2)-Ag(1)-I(2)00
121.725(16)
69.799(17)
I(1)-Ag(2)-I(1)0
113.889(17)
Ag(1)-I(1)-Ag(2) 0
Ag(1)-I(1)-Ag(2)
Ag(1)-I(2)-Ag(1)00 58.275(16)
Ag(1)00 -Ag(1)-Ag(2)0 84.02(2)
Ag(2)-I(1)-Ag(2)0 66.111(17)
Ag(2)0 -Ag(2)-Ag(1)0 88.94(2)
I(1)-Ag(1)-I(2) a0
00
110.234(18)
= -x; -y; -z, 00 = -1 - x; -y, -z, 000 = x - 1; y, z.
distortion due to the different type of bridges: I(1) adopts the μ3 mode, I(2) the μ3 mode, and the ligand the μ2-κ2S:S0 one. The polymeric chains of 4 run parallel to the a crystallographic axis, Figure 4. Again, the metals give rise to metal-metal interactions and, according to the Ag-Ag distances, these are likely to be of different strengths: d[Ag(1)-Ag(1)00 ] = 2.8139(9) Å, d[Ag(2)Ag(2)0 ] = 3.1460(9) Å, and d[Ag(1)-Ag(2)0 ] = 3.24208(8) Å. Many factors (molar ratio of the reagents, solvent, experimental conditions, etc...) coupled with the versatile coordination requirements of silver(I) and the wide donor capability of the ligands could heavily affect the obtained products.15d,49 Topology of the Electron Density. The atoms-in-molecules (AIM) analysis of electron density23-25 has proved to be a powerful tool for the study of various chemical interactions.50 These include covalent and ionic bonds,51-53 hydrogen bonds,54,55 and agostic interactions.56 Within the capabilities of AIM are also weaker types of interactions that are usually more difficult to be elucidated such as van der Waals57 or metal-metal ones.58-60 A rule of thumb criterion for the individuation of a weak interaction is usually based on the analysis of the interatomic distance, and when this is lower than the sum of the van der Waals radii, then an interaction is likely to be present.61 However, this only provides a qualitative description and more accurate and quantitative picture 1282
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Crystal Growth & Design
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Table 5. Relevant Geometric Parameters (Å and [°]) of the Optimized Ag2I2 Model System B3LYP
S-VWN
MP2
Ag-I
2.826
2.730
2.807
Ag-Ag I-I
2.856 4.880
2.664 4.766
2.818 4.855
Ag-I-Ag
60.68
58.40
60.26
I-Ag-I
119.32
121.60
119.74
Table 6. Analysis of the BCPs in the Optimized Ag2I2 Modela contact B3LYP SVWN MP2
F (e/Å3)
r2(F) (e/Å5)b
εc
Ag-Ag
0.2046
1.8446
0.2949
Ag-I
0.2877
1.9556
0.0367
Ag-Ag Ag-I
0.3132 0.3484
2.8270 2.0590
0.1166 0.0474
Ag-Ag
0.2155
2.0496
0.3376
Ag-I
0.2979
2.0434
0.0480
a
Figure 4. Portion of the crystal packing of the polymeric complex 4. The hydrogen atoms were omitted for clarity.
of the strength of a specific interaction can be obtain from the AIM analysis. According to the X-ray structures, the dinuclear and the polymeric complexes may exhibit a metal-metal bond. In addition, since the Ag2I2 moiety is present in 3a, 3b and in the polymeric complex 4, this fragment has been optimized by means of DFT (B3LYP and SVWN) and the MP2 methods. Because of the limited number of atoms, it was then possible to compare the electronic properties of this fragment when using DFT methods and the computationally demanding but usually more accurate MP2 method.62 The optimized models with the B3LYP functional and the MP2 method show comparable geometric parameters, whereas when using the SVWN functional a significant shortening (∼0.1 Å) of all the bond distances is observed, Table 5. The properties of selected bond critical points (BCPs) are summarized in Table 6, and a picture of the charge density in the Ag2I2 plane is reported in Figure 5. From a qualitative point of view, the topology of F is similar when using DFT (B3LYP and SVWN) or MP2 methods. There can be evidenced four BCPs linking the silver and iodine atoms, and a BCP interposed between the two metals that attests the occurrence of a silver-silver interaction. For all BCPs, the MP2 method and the B3LYP functional give very similar results as confirmed by the values of the F and r2(F), whereas when using the SVWN functional, a greater charge accumulation for all BCPs is found. In particular, the BCP between the two metal centers is characterized by a considerable increase of F and r2(F) implying a greater interaction between the two silver atoms. Since these results may be relatable to the shorter bond distances of the optimized model with the SVWN functional, the topology of F was calculated for the Ag2I2 model with the MP2 method by varying the Ag-Ag separation in the 2.7-3.5 Å range (0.1 Å step), which approximately defines the lower and upper limits of putative Ag-Ag interactions. As can be seen in Table 7, the BCP
Because of fragment symmetry the properties of only two representative bonds are described. b Laplacian of F (r2(F)), that represents the sum of the Hessian eigenvalues (λ1 þ λ2 þ λ3). The value of r2(F) at the BCP allows to define the interaction occurring between two atoms as closed-shell (ionic, r2(F) > 0) or shared (covalent, r2(F) < 0). c ε = ellipticity, (λ1/λ2 - 1).
Figure 5. Contour lines of the charge density of the optimized Ag2I2 model (MP2/SDD). (3; -1) bond critical points = 2, (3; þ1) ring critical points = þ.
between the two silver atoms is present in the 2.7-3.2 Å interval (the sum of van der Waals radii for two silver atoms amounts to 3.44 Å), whereas in the 3.3-3.5 Å interval, there is no longer an electron density accumulation between the two metal centers, as confirmed by the appearance of a ring critical point (RCP)63 in place of the BCP. Moreover, at 3.2 Å, ε has the very large value of 6.57, indicative of an incipient bond rupture. This suggests that the maximum distance at which an Ag-Ag interaction commences to be observed for the Ag2I2 moiety is 0) or shared (covalent, r2(F) < 0). b ε = ellipticity, (λ1/λ2 - 1). c Bond critical point. d Ring critical point. a
Figure 6. Above, contour lines of the charge density of 3a in the plane that defines the Ag-S interaction responsible of the dimerization (left), and in the plane containing the I-Ag-S atoms (right). Below, contour lines of the charge density of the Ag2I2 rhomboid of 3b. B3LYP/SDD-631G. (3; -1) bond critical points = 2, (3; þ1) ring critical points = þ.
derive from the slight depletion of the electron density at the metal centers exerted by the more electronegative iodides. In 3a, the Ag-Ag separation (3.3267(9) Å) may be indicative of a metal-metal contact. This value is nevertheless greater than the range in which such an interaction occurs, as determined for the Ag2I2 model. In fact, by the analysis of F and r2(F), it emerges that a RCP is present between the two metals, ruling out an Ag-Ag interaction, Figure 6 and Table S1, Supporting Information. The interaction holding together the two [Ag(Me2pipdt)I] units is that occurring between the silver and sulfur atoms of two symmetry related moieties, Figure 6. A different scenario is found when inspecting F and r2(F) of 3b. In fact, the topology of F shows that the two metals are actually interacting, as predicted by the relatively short distance present between them (3.0873(5) Å). The properties of the
Figure 7. Above, polynuclear fragment of the complex 4 used in the calculations. Below, contour lines of the charge density of the polynuclear fragment. The diagram on the right refers to the plane defined by the I(6), Ag(2), Ag(3), and I(8) atoms, B3LYP/SDD-6-31G. (3; -1) bond critical points = 2, (3; þ1) ring critical points = þ.
electron density at the Ag-Ag BCP are in line with the values reported for the Ag2I2 model, Table S2, Supporting Information. A more complex situation is observed for the polymeric complex 4. Three types of Ag-Ag interactions are predictable according to the intermetallic distance, Table S3, Supporting Information. Nevertheless the topology of F, Figure 7, indicates that the metals interacts only in the two Ag2I2 rhomboids, where the AgAg separation is 2.8139(9) and 3.1460(9) Å, respectively. The values of F and r2(F) are greater for the shorter Ag-Ag distance in agreement with the calculation performed on the Ag2I2 model. Furthermore, in the rhomboid characterized by the longer Ag-Ag distance, the RCP are very close to the Ag-Ag BCP suggesting an incipient bond rupture. This is reflected also in the ε value, which is greater for the longer Ag-Ag separation. The two rhomboids are connected through the bridging sulfur atom of one Me2pipdt ligand and a iodide ion, so that two silver 1284
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Crystal Growth & Design atoms of two different rhomboids are located at 3.2420(8) Å from each other. Neverthless, no BCP is found between them ruling out any metal-metal interaction. According to this analysis, the polymeric chain is composed of couples of alternating rhomboidal Ag2I2 fragments connected by the bridging ligand and the iodide anion. As found for the dinuclear complexes, the calculations performed with the SVWN functional yield values of F and r2(F) that are slightly greater than those obtained with the B3LYP functional.
’ CONCLUDING REMARKS Reagent 1 has proved to be capable of oxidizing silver metal and to provide suitable ligands to form silver(I) compounds and to favor the argentophilic interactions among the metals. A variety of silver(I) neutral complexes, [Ag(Me2pipdt)2]I3 (2a and 2b), [Ag(Me2pipdt)I]2 (3a and 3b), and [Ag2(Me2pipdt)I2]n (4), differing in stoichiometry, nuclearity, and/or crystallographic arrangements, have been obtained and structurally characterized. Silver-silver distances in di- and polynuclear complexes shorter than the sum of van der Waals radii were analyzed by AIM method with the view to highlight the presence of “ligand supported” argentophilic interactions and then predict the properties of the complexes. The theoretical approach has allowed us to point out the presence of argentophilicity in the dinuclear complex 3b, and for two of the three short Ag-Ag distances of the polymeric complex 4. These results suggest that 3b and 4 complexes are promising candidates to be investigated for optical and/or conducting properties. The high-dimensionality of the crystal structure of the polymeric complex is a further peculiarity which adds interest for possible applications of this complex as an optoelectronic material. Finally, the presented results highlight the peculiar capability of the selected reagent in the silver metal activation, which makes it promising for applications in the field of silver recovery from different kind of materials. ’ ASSOCIATED CONTENT
bS
Supporting Information. Details on AIM theory and Tables S1, S2, and S3 containing the analysis of the RCPs and/ or BCPs in 3a, 3b, and 4, respectively, are provided. Supplementary structural data are available at CCDC reference numbers 792904 (2a), 792905 (2b), 792906 (3a), 792907 (3b), 792908 (4), in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*(A.S.) Phone: þ39(0)706754489; fax: þ39(0)706754456; e-mail:
[email protected]. (L.M.) Phone: þ39(0)521905424; fax: þ39(0)521905557; e-mail:
[email protected]. (P.D.) Phone: þ39(0)706754680; fax: þ39(0)706754456; e-mail:
[email protected].
’ ACKNOWLEDGMENT This research was supported by Universita di Cagliari and Universita di Parma. ’ REFERENCES (1) (a) Gimeno, M. C.; Laguna, A. Silver and Gold. In Comprehensive Coordination Chemistry II; McCleverty, J. A., Meyer, T. J., Fenton, D. E.,
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