ARTICLE pubs.acs.org/JPCA
Dithizone and Its Oxidation Products: A DFT, Spectroscopic, and X-ray Structural Study Karel G. von Eschwege,* Jeanet Conradie, and Annemarie Kuhn Department of Chemistry, PO Box 339, University of the Free State, Bloemfontein 9300, South Africa
bS Supporting Information ABSTRACT: Air oxidation of ortho-fluorodithizone resulted in the first X-ray resolved structure of a disulfide of dithizone, validating the last outstanding X-ray structure in the oxidation of dithizone, H2Dz, which proceeds via the disulfide, (HDz)2, to the deprotonated dehydrodithizone tetrazolium salt, Dz. Density functional theory calculations established the energetically favored tautomers along the entire pathway; in gas phase and in polar as well as nonpolar solvent environments. DFT calculations using the classic pure OLYP and PW91, or the newer B3LYP hybrid functional, as well as MP2 calculations, yielded the lowest energy structures in agreement with corresponding experimental X-ray crystallographic results. Atomic charge distribution patterns confirmed the cyclization reaction pathway and crystal packing of Dz. Time dependent DFT for the first time gave satisfactory explanation for the solvatochromic properties of dithizone, pointing to different tautomers that give rise to the observed orange color in methanol and green in dichloromethane. Concentratochromism of H2Dz was observed in methanol. Computed orbitals and oscillators are in close agreement with UVvisible spectroscopic experimental results.
1. INTRODUCTION Dithizone is one of the most well-known analytical reagents, used for the detection of a variety of metals.1 Every year during the last five years almost forty related publications appeared in the literature. However, since its introduction to analytical chemistry in 19252 only a limited amount of basic research had been done on this compound of which most was reviewed by Irving in 1977.1 Among these, a two-step oxidation path was proposed for the unsubstituted ligand, going from dithizone, H2Dz (1, green), via the disulfide, (HDz)2 (2, orange-red),3 to the fully oxidized dehydrodithizone, Dz (3, pale yellow).4 First step soft oxidation, to (HDz)2 only, is effected by the interaction with silver and/or iodine (Scheme 1), while full oxidation may be accomplished with a range of oxidizing agents like KMnO4, K3[Fe(CN)6],4 H2O2, or even air (in polar solvents).5 The disulfide undergoes spontaneous thermal scission, disproportionating into equal amounts of protonated H2Dz and deprotonated Dz products. Already more than a century ago similar disulfide formation-scission processes in other related sulfur species were observed.6 H2Dz, which also serves as a mild organic reducing agent, is itself the synthesis oxidation product of the metastable precursor, H4Dz (light brown).7 A recent cyclic voltammetry study in organic media describes the redox chemistry of dithizone in full.8 The two observed first oxidation waves were assigned to the electrochemical formation of the disulfide, 2, and the chemical precursor of Dz, HDz+. The reduction half cycle reveals essentially four waves during which HDz+ is systematically reduced to H3Dz, the anionic analogue of the thiocarbazide, H4Dz. The linear backbone of dithizone affords rotation to a variety of isomeric forms, as are depicted in Figure 1.9 Single crystal r 2011 American Chemical Society
X-ray crystallographic evidence supports dithizone isomer, 1a,10 and dehydrodithizone, 3a (Figures 6 and 7),11 in solid phase. A gas phase and semiempirical computational solvent study performed on selected dithizone isomers agrees with the former structure, 1a.12 To our knowledge, comprehensive ab initio density functional theory (DFT) invoking solvent media, and time dependent (TD) DFT had not been applied to these compounds before, especially also with regard to the disulfide, 2. Together with new X-ray crystallographic evidence related to the oxidation products, we resolved to treat the entire ensemble similarly, using high level DFT quantum computational techniques, in both gas phase and solvent environment. Apart from the importance of dithizone in numerous trace metal analyses, it also forms the basis of an ultrafast13 colorful photochromic reaction observed in at least nine different metal dithizonate complexes,14 a property that may find possible applications in the field of opto-electronics. The visible light photoinduced reaction results in isomerization around the CdN double bond, followed by a spontaneous thermal radiationless back-reaction to the original ground resting state.15
2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. General. Reagent chemicals and solvents were purchased from Sigma-Aldrich and used without further purification. 1 H NMR spectra were recorded on a 300 MHz Bruker Avance DPX 300 NMR spectrometer at 298 K. Chemical shifts are Received: August 25, 2011 Revised: November 18, 2011 Published: November 21, 2011 14637
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Scheme 1. Chemical Oxidation of Dithizone
Figure 2. DFT calculated relative energies of six H2Dz tautomers, 1af. B3LYP: ( gas phase, 0 DCM, and 2 MeOH. OLYP: gas phase. PW91: + gas phase. MP2: O gas phase.
Table 1. DFT B3LYP (1st Line), OLYP (2nd Line), PW91 (3rd Line), and MP2 (4th Line) Calculated Relative Energies of Different Tautomers of Dithizone, 1, the Disulfide of Dithizone, 2, and Dehydrodithizone, 3a relative energy (kJ mol1) structure
H2Dz, 1
a
c
b
c
d
0 0
0 2.2
0
0
0
0
0
0
27.5 (39.2) [40.5]
11.3
2.1
33.5
17.5
0
32.9
68.1
15.6
20.8
11.3
76.6
62.2 (46.2) [42.6] 60.3
9.4 14.8
19.6 33.3
60.9
75.6
43.0
62.8
17.3
90.0
64.6 (74.0) [74.3]
40.4
78.8 55.9 e
Dz, 3
0 (0) [0] 0
71.9
Figure 1. Tautomers of dithizone, H2Dz, 1. Compound 1a corresponds to the crystal structure of H2Dz.28
reported relative to SiMe4 at 0 ppm. Ultraviolet and visible spectra were recorded from dilute solutions in quartz cuvettes, utilizing a Varian Cary 50 Probe UV/visible spectrophotometer. 2.2. Synthesis. The following adapted method was used for the preparation of ortho-fluorodithizone7 and its disulfide4 derivative. 2.2.1. (HDz)2. Synthesis from unsubstituted dithizone as described elsewhere.4 2.2.2. (o-FHDz)2. In a 100 mL beaker, ortho-fluoroaniline (5 g, 39 mmol) was added to a mixture of concentrated hydrochloric acid (20 mL) and water (35 mL) at 0 C. Diazotization was done by the slow addition of sodium nitrite (3.5 g, 50 mmol), while stirring (ca. 1 h), until all the aniline was dissolved. In a 500 mL beaker, the diazo solution was added, with stirring, to a cold mixture of sodium acetate (80 g, ca. 1 mol), glacial acetic acid (45 mL, 0.75 mol), and water (25 mL). Nitromethane (6.8 g, 111 mmol) was added after 10 min. After stirring for 2 h at room temperature, the volume was increased with water to 400 mL. After stirring for another 1 h, the red formazyl derivative was filtered in a large B€uchner funnel,
(HDz)2, 2
35.3 b 53.0
48.1 (61.8) [63.5] 62.1 61.5 36.5
f
43.1 (44.1) [43.4] 60.6 64.2 40.4
a
All energies were calculated in gas phase (no brackets), and for 1 also in dichloromethane (round brackets) and methanol [square brackets] solvents. b PW91 2d geometry did not converge. c Computed gas-solvent energy differences are normalized to zero. Relative to the gas phase, solvent energies of 1a are (16.8) and [18.8].
washed with copious amounts of water and then with a small amount of ethanol and ether. Drying of the product at 70 C yielded crude nitroformazan (1.03 g, 7.4%). Nitroformazan (0.9 g) was suspended in absolute ethanol (150 mL) in a 200 mL Erlenmeyer flask. The mixture was 14638
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Figure 3. Highest occupied molecular orbitals of (a) 1a and1b, H2Dz, (b) 2a, (HDz)2, and (c) 3a, Dz. Phenyl hydrogens are omitted. (C, dark gray; H, light gray; N, purple; S, yellow).
saturated (20 min) with ammonia gas. Then, for a period of 3 h, hydrogen sulfide gas, with the use of a Gibbs apparatus, was passed through the solution, resulting in the formation of thiocarbazide. After reduction was complete, as indicated by a change of color from red to orange-yellow, while stirring, the solution was slowly added to a water/ice (500 mL) mixture. The dirty-white carbazide was filtered off by suction, and washed with water. The unstable thiocarbazide was oxidized to red thiocarbazone by adding cold 2% methanolic potassium hydroxide solution (50 mL). Stirring was continued until complete dissolution. Dark green dithizone was precipitated by the addition of the former thiocarbazone solution to dilute hydrochloric acid (1%, 100 mL). The product was filtered and again precipitated from an alcoholic alkali solution to which diluted hydrochloric acid was added. After filtration, the product was washed with water, followed by a little ethanol and ether. Repeating the aforementioned precipitation procedure three times yielded spectroscopically (1H NMR) pure ortho-fluorodithizone (0.22 g, 26%). Mp: 114 C. UV/vis (dichloromethane): λmax = 450 and 621 nm. 1H NMR (300 MHz, CDCl3): δ 7.417.29 and 8.028.10 (8 H, 2 m, C6H4F). 2.2.2.1. Auto-Oxidation. o-Fluorodithizone was dissolved in toluene with the addition of a few drops of n-hexane. Autooxidation in the presence of atmospheric oxygen yielded a deep red solution 24 h later, containing crystals suitable for single crystal X-ray data collection. Mp: 128129 C. UV/vis (diethyl ether): λmax = 254, 299, and 405 nm. 2.2.2.2. Chemical Oxidation. o-Fluorodithizone (0.05 g, 0.2 mmol) was dissolved in dichloromethane (20 mL) and oxidized under sonication (12 min), together with a dichloromethane solution (5 mL) of iodine (0.25 g, 2 mmol) and water (15 mL). The solvent was removed under reduced pressure at room temperature. The residue was dissolved in ether and passed through a short silica column. The first red fraction was collected and the solvent evaporated at 40 C on a rotary evaporator. The glassy solid was left to stand overnight. UV/vis (diethyl ether): λmax = 254, 299, and 405 nm. During workup, the product protonated, yielding the parent compound, thus excluding the possibility of further characterization. 2.3. Quantum Computational Methods. DFT calculations employed the PW91 (PerdewWang, 1991) exchange and correlation functional16 and the OLYP17 (OPTX exchange
functional combined with the LeeYangParr correlation functional18), GGA (generalized gradient approximation), and a TZP (triple ζ polarized) basis set as implemented in ADF 2009 (Amsterdam density functional).19 TDDFT,20 time-dependent density functional theory, implemented in the ADF program, was used for the calculation of excitation energies. Calculations in solution, as contrasted to the gas phase, were done using the conductor like screening model (COSMO)21 of solvation as implemented in ADF.22 Calculations were also done using the B3LYP23 (B3 Becke 3-parameter exchange and LeeYangParr correlation) functional for both exchange and correlation as implemented in the Gaussian09 program package,24 using the 6-311G(d,p)25 (valence double-ζ) basis set. Calculations in methanol solvent (dielectric constant = 32.6) and CH2Cl2 (dielectric constant = 8.9) were performed with the solvent model IEFPCM. Geometries obtained from B3LYP calculations were used to obtain single point MP2 energies, and NBO analysis was performed by using the NBO 3.1 module26 in Gaussian09. Whether artificially generated atomic coordinates or coordinates obtained from X-ray crystal data (Cambridge Crystallographic Database) were used in the input files, optimizations for corresponding compounds resulted in similar optimized geometries. Accuracy of different computational methods was evaluated by comparing the root-mean-square deviations (rmsd’s) between the optimized molecular structure and the crystal structure, using only nonhydrogen atoms in the backbones of molecules. Rmsd values were calculated using the 00 rms Compare Structures00 utility in ChemCraft, version 1.6.27 No symmetry limitations were imposed in the calculations.
3. RESULTS AND DISCUSSION 3.1. Structures. Recent advances in quantum computational techniques make it an ideal tool for the investigation of dithizone chemistry, known for its solvatochromic and photochromic properties, and a variety of possible tautomers, redox species, and complexes. Establishing energetically favored geometries from the outset lays the foundation made for explaining or predicting reaction pathways, charge distributions, and energy transitions in relevant species. For the study of the oxidation chemistry of dithizone, the most probable isomers of H2Dz (1), (HDz)2 (2), and Dz (3), which are considered here, are presented in Figures 1, 4, and 6. Geometries 1a and 3a agree with reported structures,10,11 while X-ray crystallographic evidence in support of 14639
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Table 2. Selected X-ray Crystallographic10,11 and Quantum Computational Bond Lengths, Bond Angles, and Torsion Angles of H2Dz, Dz, and (o-FHDz)2a DFT program
ADF
ADF
Gaussian
functional
PW91
OLYP
B3LYP
basis set
TZP
TZP
6-311G(d,p)
X-ray compound
H2Dz
Dz
rmsdb
H2Dz
Dz
H2Dz
Dz
H2Dz
Dz
0.137
0.037
0.123
0.094
0.091
0.047
Bond Lengths (Å) SC
1.712
1.697
1.723
1.667
1.709
1.667
1.728
1.676
CN2 N2N1
1.334 1.299
1.358 1.313
1.364 1.300
1.399 1.316
1.368 1.300
1.396 1.311
1.356 1.290
1.388 1.310
N1C(Ph)
1.380
1.443
1.393
1.430
1.390
1.436
1.400
1.435
SCN2
124.23
124.90
125.27
125.87
126.07
125.95
125.72
125.94
CN2N1
116.70
105.11
112.63
105.84
112.81
105.91
114.40
105.87
N2N1C(Ph)
122.22
123.60
123.68
122.74
123.04
122.68
123.17
122.83
I
II
I
II
Bond Angles (deg)
(o-FHDz)2 monomer
I
II
I
II
1.809
2.117 1.813
1.815
2.143 1.802
1.802
Bond Lengths (Å) SS SC
2.084 1.799
1.787
2.119 1.810
CN2
1.379
1.391
1.377
1.380
1.388
1.383
1.389
1.388
N2N1
1.256
1.275
1.277
1.277
1.273
1.272
1.260
1.259
N1C(Ph)
1.439
1.419
1.403
1.406
1.406
1.404
1.411
1.407
CN3
1.310
1.309
1.314
1.318
1.314
1.315
1.304
1.302
N3N4
1.307
1.315
1.317
1.317
1.315
1.317
1.313
1.313
N4C(Ph)
1.421
1.400
1.391
1.392
1.393
1.395
1.496
1.394
FC(Ph)c
1.356
1.364
1.357
1.370
1.359
1.374
1.344
1.358
SSC
101.99
100.22
103.92
102.84
105.18
105.82
103.05
103.92
SCN2
122.64
123.93
122.58
124.25
120.68
124.43
123.21
122.60
Bond Angles (deg)
SCN3
123.86
124.27
122.35
121.82
126.11
121.77
122.71
122.77
CN2N1
114.81
114.70
114.91
115.36
115.51
115.21
115.74
115.60
CN3N4
117.52
119.47
119.13
119.06
119.26
120.45
120.91
120.93
N2N1C(Ph)
113.44
113.19
114.04
113.60
114.61
114.47
114.94
114.88
N3N4C(Ph)
119.61
118.00
121.54
121.60
122.02
122.13
122.05
122.11
CSSC
104.0
Torsion Angles (deg) 108.1
62.3
112.9
a
Monomers I and II of the disulfide are indicated. Root mean square deviations (rmsd) are given for H2Dz and Dz. b Rmsd values, in angstroms, are rootmean-square atom positional deviations, calculated for nonhydrogen atoms for the best three-dimensional superposition of the calculated structures on the experimental structures. c FC(Ph) distances on different phenyl rings within each monomer differ but are similar in the two monomers.
2a is for the first time reported, see Figure 11. The influence of a nonpolar solvent, dichloromethane, and a polar solvent, methanol, is also investigated. 3.1.1. Dithizone. DFT computed gas phase geometry optimized energies of six dithizone tautomers, 1af, involving three different functionals, B3LYP, OLYP and PW91, and two solvents (using B3LYP), are graphically correlated in Figure 2. Single point MP2 energies for DFT/B3LYP optimized geometries are also included.
The two geometries, 1a and 1b, represent the lowest energy conformations of dithizone (Table 1), an observation that is, among others, ascribed to the linear structures of these two tautomers. This is in agreement with NdN (azobenzene) and CdC (stilbene) conjugated systems where the linear (trans) configurations are of slightly lower energy than cis configurations.29 Energy differences of 37 to 79 kJ mol1 between linear 1a and bent 1c, d, and e were calculated. Apart from 1a, 1c is the only other conjugated structure; nevertheless, 14640
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Figure 6. Isomers of dehydrodithizone, Dz, 3. Tautomer 3a corresponds to a crystal structure.11
Figure 4. Isomers of the dithizone disulfide, (HDz)2, 2. Isomer 2a corresponds to the crystal structure reported here.
Figure 5. Numbering system of H2Dz (1a), (o-FHDz)2, and Dz (3a) relates to Table 2. The monomers that make up the disulfide are distinguished by Roman numerals I and II.
this attribute does not limit the large energy difference between these two geometries. The relative energy of tautomer 1a is consistently significantly lower than that of 1b (>20 kJ mol1), the structure differing from 1b, d, and e by not having hydrogens bonded to sulfur but instead symmetrically to the nitrogens bordering phenyl rings. Through conjugation along the symmetrical 1a structure, the formation of a diradical is prevented, which alleviates violation of the octet rule on the inner two nitrogens. π-Electron delocalization along the entire backbone, also including the phenyl rings, is thus contributing toward stabilization of the dithizone molecule, see Figure 3, 1a. As one of the frontier orbitals, only the HOMO is shown. Similar extended orbitals are observed in especially HOMO-2 and HOMO-3 (See Supporting Information, Figure SI-1). Compounds 1b, d, e, and f, on the contrary, adhere to the octet bonding order around all atoms, which in turn limits π delocalization along the backbone. HOMO’s in the latter tautomers reveal the expected more localized bonding order through out the molecule, i.e., better defined alternating single and double bonds along the backbone, as may be seen in Figure 3, 1b. A second atom bonded to sulfur, as also in the disulfide, 2, is responsible for limiting π conjugation.
The general energy trend obtained by using the three functionals, gas and solvent environment, was roughly confirmed by MP2 single point energy calculations performed on B3LYP optimized geometries. When also considering solvent environment calculations, the over all general order in stability of all six tautomers is not altered. Regardless of the difference in dielectric constants between dichloromethane (ε = 8.9) and methanol (ε = 32.6), very similar energies were obtained for these two solvents (see Figure 2). This result indicates that hydrogen bonding in protic solvents like methanol, as opposed to nonprotic DCM, is not contributing significantly toward stabilization of any of the tautomers. Therefore, based on relative energies alone, tautomer 1a, agreeing with the solid state structure, is the computationally favored geometry, also in solution. Table 2 compares computed bond lengths, bond angles, and torsion angles of H2Dz (1a), (HDz)2 (2a), and Dz (3a) to the corresponding single crystal X-ray crystallography data. In general, the best agreement with experimental data as reflected by the smallest root-mean-square atom positional deviations (rmsd) were obtained for B3LYP calculations. Small deviations in the CSSC torsion angle in the disulfide, 2a, resulted in excessive decreases in the rmsd’s calculated for corresponding computed structures and are therefore omitted. This torsion angle (104.0 in the crystal structure, see section 3.3) is closely simulated by PW91 (108.1) and B3LYP (112.9). OLYP optimized 2a to a minimum energy structure with a torsion angle of 62.3. Comparison between bond lengths and bond angles for all three compounds, 1a3a, are otherwise in very close agreement with crystal data. Computed bond lengths in general deviate not more than 0.03 Å, as mostly seen in the CN2 bonds, while bond angles may deviate by up to 4 at most. 3.1.2. Dithizone Disulfide. The two monomers of the disulfide, 2a, each maintain the linear configuration of the parent compound, 1a, however, now corresponding almost exactly to the slightly higher energy dithizone structure, 1b. Instead of H being bonded to S as in dithizone 1b, one proton is lost during the oxidation process, resulting in adjacent sulfur radicals combining to form the SS bonded disulfide. In the optimized structure of 2a, the remaining imine proton, (N4)H, (see Figure 5 for atom numbering) forms an intramolecular hydrogen bond with the nitrogen N1 of the adjacent monomer, exactly as observed in the X-ray crystal structure (Figure 11). The relative energy of isomer 2a is only about 10 kJ mol1 (B3LYP) less than that of isomers 2b and 2c. The latter structures allow for NH---N hydrogen bonding within each monomer of the disulfide. Compound 2d is the least stable 14641
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Table 3. Selected Mulliken Atomic Charges in Dz, 3a, and Its Precursor, HDz+ a HDz+
Dz, 3a PW91 OLYP S
B3LYP
PW91 OLYP B3LYP
0.32 0.35 0.29 (0.49) [0.47] 0.05 0.04 0.43 0.23 0.25 0.28
C
Scheme 2. OLYP and PW91 Computed Mulliken Relative Atomic Charge Distribution during the Oxidation of Dithizone
0.01 0.01
0.01 0.10
0.09 0.12
(0.14)
[ 0.13]
0.01
0.07
0.03
0.05 0.10
0.07 0.12
0.04 0.13
N1 0.12
0.14
0.14 (0.10) [0.11] 0.04 0.08 0.27
0.04
0.04
0.04
0.05 0.13 0.07
N2 0.19 0.19 0.17 (0.18) [0.18] 0.04 0.10 0.11 0.25 0.24 0.25
0.13 0.25 0.19
N3 0.19 0.19 0.17 (0.18) [0.18] 0.11 0.08 0.11 0.25 0.24 0.25 N4 0.12 0.04
0.14 0.04
0.14 0.11 0.12
0.14 (0.10) [0.11] 0.23 0.27 0.27 0.04 0.17 0.15 0.15
a
Calculations done in gas phase are indicated without brackets, dichloromethane in round brackets, methanol in square brackets, and NBO natural atomic charges in the following lines.
Figure 7. Ionic-type crystal packing of Dz, 3a,11 corresponds to the packing of related structures (o-OCH3)Dz and (m-F)Dz.11.
isomer, with a relative energy of more than 35 kJ mol1 higher than that of the most stable isomer, 2a. Compound 2a, again with linear geometry (in each monomer), corresponds to the crystal structure reported in section 3.3. When comparing bond distances between the two monomers, all three functionals are noted to give bond lengths and angles that are more similar than what is observed in the crystal structure. As a consequence of the HDz moieties being no longer symmetrical like in the mother compound, H2Dz, the large degree of delocalization is also alleviated, with better defined alternating double and single bonds in the (HDz)2 backbone. 3.1.3. Dehydrodithizone. Dehydrodithizone, Dz, is the stable oxidation product of dithizone. After scission of the unstable disulfide, cyclization follows, with loss of the remaining imine proton. Optimization of this molecule gives the lowest energy structure, 3a, resembling corresponding crystal data,11 as seen in rmsd values 2σ(F2)]
0.059
data/restraints/parameters
7129/0/388
largest diff. peak and hole
0.64 e Å3 and 0.44 e Å3
data collection Bruker APEX CCD area-
7129 independent reflections
detector diffractometer radiation source: fine-focus
4629 reflections with I > 2σ(I)
sealed tube graphite monochromator
Rint = 0.054
ω scans
θmax = 28.3, θmin = 3.0
absorption correction: multiscan h = 10 f 10 SADABS (Bruker, 2009) Tmin = 0.920, Tmax = 0.987
k = 16 f 15
26154 measured reflections refinement
l = 21 f 21
refinement on F2
primary atom site location: structure-
least-squares matrix: full
secondary atom site location:
R[F2 > 2σ(F2)] = 0.059
hydrogen site location: inferred
wR(F2) = 0.168 S = 1.05
H-atom parameters constrained w = 1/[σ2(Fo2) + (0.0819P)2 + 0.3004P],
invariant direct methods difference Fourier map from neighboring sites
where P = (Fo2 + 2Fc2)/3 7129 reflections
(Δ/σ)max < 0.001
388 parameters
Δæmax = 0.64 e Å3
0 restraints
Δæmin = 0.44 e Å3
spectrum was found with the computed spectrum of tautomer 1b, which is also the tautomer of the second lowest energy, next to 1a. The close agreement of the computed oscillator at 446 nm, overlapping with the experimental peak at ca. 450 nm, is illustrated in Figure 9. The same is true for the shorter 14644
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Figure 10. Structural packing in the (o-FHDz)2 crystal, indicating two disulfides and four hexane solvent molecules per unit cell. Hydrogens are omitted.
wavelength oscillators, i.e., between 250 and 350 nm. All other tautomers, as well as anionic HDz, gave significantly different oscillator peak patterns, and may it therefore confidently be proposed that structure 1b gives rise to the orange color of dithizone in dilute methanolic solutions. This structure does not require isomerization but simply the intramolecular transfer of the N4 proton to the neighboring sulfur; see Figure 1. Earlier (section 3.1) it was pointed out that methanol does not alter the calculated energy of dithizone with respect to a nonprotic DCM environment. This finding, however, is not contradictory to the above evidence in favor of methanol assisting in an intramolecular proton transfer reaction, i.e., proton (N4)H in 1a being transferred to S in 1b, which has little to do with the relative energies of the two tautomers in different solvents. Figure 9 (bottom) also shows concentratochromism observed in methanolic solutions of 1, i.e., a change in color due to a change in concentration. The occurrence of this phenomenon is rather unique. Poly(thienylacetylenes)32 and fullerenes33 are reported to exhibit concentratochromism, manifesting as a gradual shift in wavelength due to changes in concentration. Dithizone behaves differently in that an entire new absorption band appears at 592 nm in methanol at higher concentrations (spectrum a), while the 475 nm absorption band of the dilute orange solution (spectrum b) is slightly blue-shifted to 455 nm. As this new observation is not part of the objectives of this article, an investigation into possible reasons for this effect is reserved for another study. 3.3. X-ray Crystallography. Because of the relative instability of the disulfide of dithizone, 2a, as opposed to the very stable meso-ionic final oxidation product, dehydrodithizone, 3a, the former would not be a natural choice in any crystallography attempt. The growth of crystals of 2a that are suitable for single crystal data collection happened coincidentally, during an attempt to recrystallize the ortho-fluoro derivative of dithizone from toluene/hexane. Exposure to air proved to be sufficient to partially oxidize (o-F)H2Dz to the disulfide, which crystallized out of solution within a day. Results from the
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Figure 11. X-ray crystal structure of the disulfide of ortho-fluorodithizone, with intramolecular ππ interaction between the phenyl rings at the top, and H-bond between the imine proton (N4)H and nitrogen N1.
consequent X-ray data collection are listed in Tables 2 (bond lengths and angles) and 4 (crystal data and structure refinement). Additional data may be found under Supporting Information. (HDz)2, 2a, crystallized in a triclinic crystal system in the P1 space group (Table 4). There are two disulfide and four hexane solvent molecules per unit cell; see Figure 10. The SS single bond between the two monomers allows free rotation around this bond. The CSSC torsion angle of 104.0 is an indication of the relative rotation between the two linear and relatively flat HDz monomers. This torsion angle is ascribed to the short intramolecular distance of ca. 3.46 Å between two phenyl rings suggesting ππ interaction, as well as an intramolecular hydrogen bond of 2.927 Å between the (N4)H of monomer I and N1 of monomer II; see Figure 7. On the basis of the H-bonding guidelines provided by Jeffrey,34 this bond is classified as weak, being relatively long (>2.13 Å), and having a small bonding angle of 91.94. A typical SS single bond in a disulfide may be 2.04 Å long, as seen in the ((NH2)2CS)2 dithio compound.35 The SS bond length in 2a, of 2.084 Å, is slightly longer, being indicative of a weaker bond. Bond distances through-out the monomeric back bones are most closely resembled by that observed in the dithizone derivative that is methylated on the sulfur position, S-methyldithizone.36 This compound, which is an analogue of 1b (Figure 1), does not allow the large degree of delocalization observed in dithizone, 1a. Double bonds N1dN2 and CdN3 in 2a are, respectively, 1.256 and 1.275 Å and 1.379 and 1.391 Å in the two disulfide monomers. The SC single bonds are 1.799 and 1.787 Å, which is more than 0.08 Å longer than the corresponding bond in 1a. Bond angles along the backbone are, as expected, rather similar to related bonds in H2Dz and (SMe)HDz, e.g., the CN1N2 bond angles in 2a are 114.81 and 114.70, compared to that of 112.37 in H2Dz. The crystallography (CIF) file may be viewed under Supporting Information. 14645
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The Journal of Physical Chemistry A
4. CONCLUSIONS Both ADF implemented functionals, PW91 and OLYP, and GO9-B3LYP gave good structural representations of dithizone and its oxidation products, with Gaussian (B3LYP) being the method of choice for TDDFT computations of UVvisible electronic transitions, and ADF (PW91 and OLYP) for the purpose of realistically assigning Mulliken atomic charge distributions. Single point MP2 for DFT optimized geometries gave validation for DFT computed energy trends. These computational techniques allowed convincing evidence in support of favored geometries in dithizone chemistry, both in the gas phase and in the solvent environment. Resolving the crystal structure of a disulfide of dithizone, (HDz)2, gave evidence in favor of the last outstanding structure in a series of structurally resolved derivatives that include H4Dz, H2Dz, HDz, (S-CH3)HDz, MHDz, and Dz. ’ ASSOCIATED CONTENT
bS
Supporting Information. GO9-B3LYP TDDFT calculations of all H2Dz tautomers, 1af, and the favored (HDz)2 and Dz tautomers, 2a and 3a, in gas, DCM, and MeOH media; CIF file information. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: 27-51-4012194. Fax: 27-51-4446384. E-mail: veschwkg@ ufs.ac.za.
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