Published: November 21, 2011

r 2011 American Chemical Society 14637 dx.doi.org/10.1021/jp208212e | J. Phys. Chem. A 2011, 115, 14637–14646

ARTICLE

pubs.acs.org/JPCA

Dithizone and Its Oxidation Products: A DFT, Spectroscopic, andX-ray Structural StudyKarel 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

1. INTRODUCTION

Dithizone is one of the most well-known analytical reagents,used for the detection of a variety of metals.1 Every year duringthe last five years almost forty related publications appeared inthe literature. However, since its introduction to analyticalchemistry in 19252 only a limited amount of basic research hadbeen done on this compound of which most was reviewed byIrving in 1977.1 Among these, a two-step oxidation path wasproposed for the unsubstituted ligand, going from dithizone,H2Dz (1, green), via the disulfide, (HDz)2 (2, orange-red),

3 tothe fully oxidized dehydrodithizone, Dz (3, pale yellow).4 Firststep soft oxidation, to (HDz)2 only, is effected by the interactionwith silver and/or iodine (Scheme 1), while full oxidationmay beaccomplished with a range of oxidizing agents like KMnO4,K3[Fe(CN)6],

4 H2O2, or even air (in polar solvents).5 Thedisulfide undergoes spontaneous thermal scission, disproportio-nating into equal amounts of protonated H2Dz and deproto-nated Dz products. Already more than a century ago similardisulfide formation-scission processes in other related sulfurspecies were observed.6 H2Dz, which also serves as amild organicreducing agent, is itself the synthesis oxidation product of themetastable precursor, H4Dz (light brown).7 A recent cyclicvoltammetry study in organic media describes the redox chem-istry of dithizone in full.8 The two observed first oxidation waveswere assigned to the electrochemical formation of the disulfide, 2,and the chemical precursor of Dz, HDz+. The reduction half cyclereveals essentially four waves during which HDz+ is system-atically reduced to H3Dz

�, the anionic analogue of the thio-carbazide, H4Dz.

The linear backbone of dithizone affords rotation to a varietyof isomeric forms, as are depicted in Figure 1.9 Single crystal

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 studyperformed on selected dithizone isomers agrees with the formerstructure, 1a.12 To our knowledge, comprehensive ab initiodensity functional theory (DFT) invoking solvent media, andtime dependent (TD) DFT had not been applied to thesecompounds before, especially also with regard to the disulfide,2. Together with new X-ray crystallographic evidence related tothe oxidation products, we resolved to treat the entire ensemblesimilarly, using high level DFT quantum computational techni-ques, in both gas phase and solvent environment.

Apart from the importance of dithizone in numerous tracemetal analyses, it also forms the basis of an ultrafast13 colorfulphotochromic reaction observed in at least nine different metaldithizonate complexes,14 a property that may find possibleapplications in the field of opto-electronics. The visible lightphotoinduced reaction results in isomerization around the CdNdouble bond, followed by a spontaneous thermal radiationlessback-reaction to the original ground resting state.15

2. EXPERIMENTAL AND COMPUTATIONAL METHODS

2.1. General. Reagent chemicals and solvents were purchasedfrom Sigma-Aldrich and used without further purification.1H NMR spectra were recorded on a 300 MHz Bruker AvanceDPX 300 NMR spectrometer at 298 K. Chemical shifts are

Received: August 25, 2011Revised: November 18, 2011

ABSTRACT: Air oxidation of ortho-fluorodithizone resulted in thefirst X-ray resolved structure of a disulfide of dithizone, validating thelast outstanding X-ray structure in the oxidation of dithizone, H2Dz,which proceeds via the disulfide, (HDz)2, to the deprotonateddehydrodithizone tetrazolium salt, Dz. Density functional theorycalculations established the energetically favored tautomers alongthe entire pathway; in gas phase and in polar as well as nonpolarsolvent environments. DFT calculations using the classic pure OLYPand PW91, or the newer B3LYP hybrid functional, as well as MP2 calculations, yielded the lowest energy structures in agreementwith corresponding experimental X-ray crystallographic results. Atomic charge distribution patterns confirmed the cyclizationreaction pathway and crystal packing of Dz. Time dependent DFT for the first time gave satisfactory explanation for thesolvatochromic properties of dithizone, pointing to different tautomers that give rise to the observed orange color in methanol andgreen in dichloromethane. Concentratochromism ofH2Dz was observed inmethanol. Computed orbitals and oscillators are in closeagreement with UV�visible spectroscopic experimental results.

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reported relative to SiMe4 at 0 ppm. Ultraviolet and visible spectrawere recorded from dilute solutions in quartz cuvettes, utilizing aVarian 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 hydro-chloric acid (20 mL) and water (35 mL) at 0 �C. Diazotizationwas done by the slow addition of sodium nitrite (3.5 g, 50mmol),while stirring (ca. 1 h), until all the aniline was dissolved. In a 500mLbeaker, the diazo solution was added, with stirring, to a coldmixture of sodiumacetate (80 g, ca. 1mol), glacial acetic acid (45mL,0.75mol), and water (25mL). Nitromethane (6.8 g, 111mmol) wasadded after 10 min. After stirring for 2 h at room temperature, thevolumewas increasedwith water to 400mL. After stirring for another1 h, the red formazyl derivative was filtered in a large B€uchner funnel,

washed with copious amounts of water and then with a small amountof ethanol and ether. Drying of the product at 70 �C yielded crudenitroformazan (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

Scheme 1. Chemical Oxidation of Dithizone

Figure 1. Tautomers of dithizone, H2Dz, 1. Compound 1a correspondsto the crystal structure of H2Dz.

28

Figure 2. DFT calculated relative energies of sixH2Dz tautomers, 1a�f.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 Energiesof Different Tautomers of Dithizone, 1, the Disulfide ofDithizone, 2, and Dehydrodithizone, 3a

relative energy (kJ mol�1)

structure H2Dz, 1 (HDz)2, 2 Dz, 3

a 0 (0) [0]c 0 0

0 0 2.2

0 0 0

0 0 0

b 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

c 62.2 (46.2) [42.6] 9.4 19.6

60.3 14.8 33.3

60.9 75.6 43.0

62.8 17.3 90.0

d 64.6 (74.0) [74.3] 40.4

71.9 35.3

78.8 b

55.9 53.0

e 48.1 (61.8) [63.5]

62.1

61.5

36.5

f 43.1 (44.1) [43.4]

60.6

64.2

40.4aAll energies were calculated in gas phase (no brackets), and for 1 also indichloromethane (round brackets) and methanol [square brackets]solvents. b PW91 2d geometry did not converge. cComputed gas-solventenergy differences are normalized to zero. Relative to the gas phase,solvent energies of 1a are (16.8) and [18.8].

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saturated (20 min) with ammonia gas. Then, for a period of 3 h,hydrogen sulfide gas, with the use of a Gibbs apparatus, waspassed through the solution, resulting in the formation ofthiocarbazide. After reduction was complete, as indicated by achange of color from red to orange-yellow, while stirring, thesolution was slowly added to a water/ice (500 mL) mixture.The dirty-white carbazide was filtered off by suction, andwashed with water. The unstable thiocarbazide was oxidizedto red thiocarbazone by adding cold 2% methanolic potassiumhydroxide solution (50 mL). Stirring was continued untilcomplete dissolution.Dark green dithizone was precipitated by the addition of

the former thiocarbazone solution to dilute hydrochloric acid(1%, 100mL). The product was filtered and again precipitated froman alcoholic alkali solution to which diluted hydrochloric acidwas added. After filtration, the product was washed with water,followed by a little ethanol and ether. Repeating the aforemen-tioned precipitation procedure three times yielded spectrosco-pically (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 7.41�7.29 and8.02�8.10 (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. Auto-oxidation in the presence of atmospheric oxygen yielded a deepred solution 24 h later, containing crystals suitable for singlecrystal X-ray data collection.Mp: 128�129 �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) andoxidized under sonication (12 min), together with a dichlor-omethane solution (5 mL) of iodine (0.25 g, 2 mmol) andwater (15 mL). The solvent was removed under reducedpressure at room temperature. The residue was dissolved inether and passed through a short silica column. The first redfraction was collected and the solvent evaporated at 40 �Con a rotary evaporator. The glassy solid was left to standovernight. UV/vis (diethyl ether): λmax = 254, 299, and405 nm. During workup, the product protonated, yieldingthe parent compound, thus excluding the possibility of furthercharacterization.2.3. Quantum Computational Methods. DFT calcula-

tions employed the PW91 (Perdew�Wang, 1991) exchange andcorrelation functional16 and the OLYP17 (OPTX exchange

functional combined with the Lee�Yang�Parr correlationfunctional18), GGA (generalized gradient approximation), anda TZP (triple ζ polarized) basis set as implemented in ADF 2009(Amsterdam density functional).19 TDDFT,20 time-dependentdensity functional theory, implemented in the ADF program,was used for the calculation of excitation energies. Calculationsin solution, as contrasted to the gas phase, were done using theconductor like screening model (COSMO)21 of solvation asimplemented in ADF.22

Calculations were also done using the B3LYP23 (B3 Becke3-parameter exchange and Lee�Yang�Parr correlation) func-tional for both exchange and correlation as implemented in theGaussian09 program package,24 using the 6-311G(d,p)25

(valence double-ζ) basis set. Calculations in methanol solvent(dielectric constant = 32.6) andCH2Cl2 (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 per-formed by using the NBO 3.1 module26 in Gaussian09.Whether artificially generated atomic coordinates or coordi-

nates obtained from X-ray crystal data (Cambridge Crystal-lographic Database) were used in the input files, optimizationsfor corresponding compounds resulted in similar optimized geo-metries. Accuracy of different computational methods was evalu-ated by comparing the root-mean-square deviations (rmsd’s)between the optimized molecular structure and the crystal struc-ture, using only nonhydrogen atoms in the backbones ofmolecules.Rmsd values were calculated using the 00rms Compare Structures00utility 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 computationaltechniques make it an ideal tool for the investigation of dithizonechemistry, known for its solvatochromic and photochromicproperties, and a variety of possible tautomers, redox species,and complexes. Establishing energetically favored geometriesfrom the outset lays the foundation made for explaining orpredicting reaction pathways, charge distributions, and energytransitions in relevant species. For the study of the oxidationchemistry of dithizone, the most probable isomers of H2Dz (1),(HDz)2 (2), and Dz (3), which are considered here, are presentedin Figures 1, 4, and 6. Geometries 1a and 3a agree with reportedstructures,10,11 while X-ray crystallographic evidence in support of

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).

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2a is for the first time reported, see Figure 11. The influence of anonpolar solvent, dichloromethane, and a polar solvent, methanol,is also investigated.3.1.1. Dithizone. DFT computed gas phase geometry opti-

mized energies of six dithizone tautomers, 1a�f, involving threedifferent functionals, B3LYP, OLYP and PW91, and two solvents(using B3LYP), are graphically correlated in Figure 2. Singlepoint MP2 energies for DFT/B3LYP optimized geometries arealso included.

The two geometries, 1a and 1b, represent the lowest energyconformations of dithizone (Table 1), an observation that is,among others, ascribed to the linear structures of these twotautomers. This is in agreement with �NdN� (azobenzene)and �CdC� (stilbene) conjugated systems where the linear(trans) configurations are of slightly lower energy than cisconfigurations.29 Energy differences of 37 to 79 kJ mol�1

between linear 1a and bent 1c, d, and e were calculated. Apartfrom 1a, 1c is the only other conjugated structure; nevertheless,

Table 2. Selected X-ray Crystallographic10,11 and Quantum Computational Bond Lengths, Bond Angles, and Torsion Angles ofH2Dz, Dz, and (o-FHDz)2

a

DFT program ADF ADF Gaussian

functional PW91 OLYP B3LYP

basis set TZP TZP 6-311G(d,p)

X-ray

compound H2Dz Dz H2Dz Dz H2Dz Dz H2Dz Dz

rmsdb 0.137 0.037 0.123 0.094 0.091 0.047

Bond Lengths (Å)

S�C 1.712 1.697 1.723 1.667 1.709 1.667 1.728 1.676

C�N2 1.334 1.358 1.364 1.399 1.368 1.396 1.356 1.388

N2�N1 1.299 1.313 1.300 1.316 1.300 1.311 1.290 1.310

N1�C(Ph) 1.380 1.443 1.393 1.430 1.390 1.436 1.400 1.435

Bond Angles (deg)

S�C�N2 124.23 124.90 125.27 125.87 126.07 125.95 125.72 125.94

C�N2�N1 116.70 105.11 112.63 105.84 112.81 105.91 114.40 105.87

N2�N1�C(Ph) 122.22 123.60 123.68 122.74 123.04 122.68 123.17 122.83

(o-FHDz)2 monomer I II I II I II I II

Bond Lengths (Å)

S�S 2.084 2.119 2.117 2.143

S�C 1.799 1.787 1.810 1.809 1.813 1.815 1.802 1.802

C�N2 1.379 1.391 1.377 1.380 1.388 1.383 1.389 1.388

N2�N1 1.256 1.275 1.277 1.277 1.273 1.272 1.260 1.259

N1�C(Ph) 1.439 1.419 1.403 1.406 1.406 1.404 1.411 1.407

C�N3 1.310 1.309 1.314 1.318 1.314 1.315 1.304 1.302

N3�N4 1.307 1.315 1.317 1.317 1.315 1.317 1.313 1.313

N4�C(Ph) 1.421 1.400 1.391 1.392 1.393 1.395 1.496 1.394

F�C(Ph)c 1.356 1.364 1.357 1.370 1.359 1.374 1.344 1.358

Bond Angles (deg)

S�S�C 101.99 100.22 103.92 102.84 105.18 105.82 103.05 103.92

S�C�N2 122.64 123.93 122.58 124.25 120.68 124.43 123.21 122.60

S�C�N3 123.86 124.27 122.35 121.82 126.11 121.77 122.71 122.77

C�N2�N1 114.81 114.70 114.91 115.36 115.51 115.21 115.74 115.60

C�N3�N4 117.52 119.47 119.13 119.06 119.26 120.45 120.91 120.93

N2�N1�C(Ph) 113.44 113.19 114.04 113.60 114.61 114.47 114.94 114.88

N3�N4�C(Ph) 119.61 118.00 121.54 121.60 122.02 122.13 122.05 122.11

Torsion Angles (deg)

C�S�S�C 104.0 108.1 62.3 112.9aMonomers I and II of the disulfide are indicated. Rootmean square deviations (rmsd) are given for H2Dz andDz.

bRmsd values, in angstroms, are root-mean-square atom positional deviations, calculated for nonhydrogen atoms for the best three-dimensional superposition of the calculated structures onthe experimental structures. c F�C(Ph) distances on different phenyl rings within each monomer differ but are similar in the two monomers.

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this attribute does not limit the large energy difference betweenthese two geometries.The relative energy of tautomer 1a is consistently significantly

lower than that of 1b (>20 kJmol�1), the structure differing from1b, d, and e by not having hydrogens bonded to sulfur but insteadsymmetrically to the nitrogens bordering phenyl rings. Throughconjugation along the symmetrical 1a structure, the formation ofa diradical is prevented, which alleviates violation of the octet ruleon the inner two nitrogens. π-Electron delocalization along theentire backbone, also including the phenyl rings, is thus con-tributing toward stabilization of the dithizone molecule, seeFigure 3, 1a. As one of the frontier orbitals, only the HOMO isshown. Similar extended orbitals are observed in especiallyHOMO-2 and HOMO-3 (See Supporting Information, FigureSI-1). Compounds 1b, d, e, and f, on the contrary, adhere to theoctet bonding order around all atoms, which in turn limits πdelocalization along the backbone. HOMO’s in the latter tauto-mers reveal the expected more localized bonding order throughout the molecule, i.e., better defined alternating single anddouble bonds along the backbone, as may be seen in Figure 3, 1b.A second atom bonded to sulfur, as also in the disulfide, 2, isresponsible for limiting π conjugation.

The general energy trend obtained by using the three func-tionals, gas and solvent environment, was roughly confirmed byMP2 single point energy calculations performed on B3LYPoptimized 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 betweendichloromethane (ε = 8.9) and methanol (ε = 32.6), very similarenergies were obtained for these two solvents (see Figure 2).This result indicates that hydrogen bonding in protic solventslike methanol, as opposed to nonprotic DCM, is not contributingsignificantly toward stabilization of any of the tautomers. There-fore, based on relative energies alone, tautomer 1a, agreeing withthe 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 thecorresponding single crystal X-ray crystallography data. In gen-eral, the best agreement with experimental data as reflected bythe smallest root-mean-square atom positional deviations (rmsd)were obtained for B3LYP calculations. Small deviations in theC�S�S�C torsion angle in the disulfide, 2a, resulted inexcessive decreases in the rmsd’s calculated for correspondingcomputed structures and are therefore omitted. This torsionangle (104.0� in the crystal structure, see section 3.3) is closelysimulated by PW91 (108.1�) and B3LYP (112.9�). OLYPoptimized 2a to a minimum energy structure with a torsionangle of 62.3�. Comparison between bond lengths and bondangles for all three compounds, 1a�3a, are otherwise in veryclose agreement with crystal data. Computed bond lengths ingeneral deviate not more than 0.03 Å, as mostly seen in theC�N2 bonds, while bond angles may deviate by up to 4� at most.3.1.2. Dithizone Disulfide. The twomonomers of the disulfide,

2a, each maintain the linear configuration of the parent com-pound, 1a, however, now corresponding almost exactly to theslightly higher energy dithizone structure, 1b. Instead of H beingbonded to S as in dithizone 1b, one proton is lost during theoxidation process, resulting in adjacent sulfur radicals combiningto form the S�S bonded disulfide. In the optimized structure of2a, the remaining imine proton, (N4)H, (see Figure 5 for atomnumbering) forms an intramolecular hydrogen bond with thenitrogen N1 of the adjacent monomer, exactly as observed in theX-ray crystal structure (Figure 11).The relative energy of isomer 2a is only about 10 kJ mol�1

(B3LYP) less than that of isomers 2b and 2c. The latterstructures allow for NH---N hydrogen bonding within eachmonomer of the disulfide. Compound 2d is the least stable

Figure 4. Isomers of the dithizone disulfide, (HDz)2, 2. Isomer 2acorresponds 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 aredistinguished by Roman numerals I and II.

Figure 6. Isomers of dehydrodithizone, Dz, 3. Tautomer 3a corre-sponds to a crystal structure.11

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isomer, with a relative energy of more than 35 kJ mol�1 higherthan that of the most stable isomer, 2a. Compound 2a, again withlinear geometry (in each monomer), corresponds to the crystalstructure reported in section 3.3.When comparing bond distances between the two monomers,

all three functionals are noted to give bond lengths and anglesthat are more similar than what is observed in the crystalstructure. As a consequence of the HDzmoieties being no longersymmetrical like in the mother compound, H2Dz, the largedegree of delocalization is also alleviated, with better definedalternating 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 unstabledisulfide, cyclization follows, with loss of the remaining imineproton. Optimization of this molecule gives the lowest energystructure, 3a, resembling corresponding crystal data,11 as seen inrmsd values <0.1 (Table 2). When either the deprotonated linear3c or folded 3b structures (Figure 6) were used as computationalinput geometries, optimization resulted in different twistedgeometries, consequently the optimization of 3c was forced by

symmetry (C2v) to stay linear. However, using input where themolecule is more tightly folded, i.e., the phenyl-bonded nitrogensbeing closer together, the geometry immediately converged tothe ring-closed, 3a. The open structures have energies relatedto the minimum 3a (i.e., 3b, B3LYP and OLYP), or more than15 kJ mol�1 higher when twisted into a slightly different con-formation (i.e., 3b, PW91).In 1961 already Ogilvie and Corwin suggested dehydrodithi-

zone to be meso-ionic by nature, i.e., with a negative charge onS and the positive charge delocalized over the entire tetrazole ring,including all four nitrogens.30 Jian et al. more recently computedMulliken atomic charge distributions (MD) as well as naturalpopulation analyses (NPA) on Dz, using B3LYP, HF, and MP2methods.31 Under the NPA approach, different basis sets con-sistently assigned negative charges to both S and the tetrazolering, while the two phenyls are positive. However, depending onthe basis set that was used, the MD method gave results thatagreed either with Ogilvie, or with the NPA. On the basis of theinconsistency of MD and due to protonation expected to occuronly at negatively charged nitrogens, NPA was proposed by Jianas giving a better description of reality, i.e., with negative chargeson both S and all four nitrogen atoms.In the present investigation, B3LYP gave MD consistent with

the NPA results of Jian et al. using the HF and MP2 methods, ingas phase as well as in a solvent environment, i.e., both S andN1�N4 are negative, see Table 3. A difference is howeverobserved in the PW91 and OLYP optimized MD values, wherethe Dz nitrogens, N1 and N4, which are bonded to phenyl rings,each have positive charges, while nitrogens N2 and N3 arenegatively charged. These latter results partially disagree withboth Ogilvie and Jian but, nevertheless, reconcile the notion of ameso-ionic molecule based on crystallographic packing patterns(Figure 7) and protonation of Dz at negative nitrogen positions.In support of the OLYP and PW91, MD results illustrates crystalpacking where the apex S of every Dzmolecule is electrostaticallyheld in place by the positively charged N1 and N4 atoms ofneighboring molecules. The sulfur atom is at a distance of 3.550Å from the plane of neighboring tetrazole rings. This ionic typepacking can not be supported by a negative charge distributed

Table 3. Selected Mulliken Atomic Charges in Dz, 3a, and ItsPrecursor, HDz+a

Dz, 3a HDz+

PW91 OLYP B3LYP PW91 OLYP B3LYP

S �0.32 �0.35 �0.29 (�0.49) [�0.47] �0.05 �0.04 �0.43

�0.23 �0.25 �0.28 0.01 0.07 0.03

C 0.01 0.01 0.09 (0.14) [ 0.13] 0.05 0.07 0.04

0.01 0.10 0.12 0.10 0.12 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 �0.14 �0.11 �0.12

N4 0.12 0.14 �0.14 (�0.10) [�0.11] 0.23 0.27 �0.27

0.04 0.04 0.04 �0.17 �0.15 �0.15aCalculations done in gas phase are indicated without brackets, dichloro-methane in round brackets, methanol in square brackets, and NBOnatural atomic charges in the following lines.

Figure 7. Ionic-type crystal packing of Dz, 3a,11 corresponds to thepacking of related structures (o-OCH3)Dz and (m-F)Dz.

11.

Scheme 2. OLYP and PW91 Computed Mulliken RelativeAtomic Charge Distribution during the Oxidation ofDithizone

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over all four nitrogen atoms, which would otherwise haverepelled the negatively charged S atom.NBO natural atomic charge patterns in the Dz molecule

computed from the optimized geometries of all three functionalsare without exception in agreement with the better PW91 andOLYP MD values.As a consequence of both OLYP and PW91 MD and NBO

methods giving the most realistic atomic charge distributionpattern in dehydrodithizone up to date, in an attempt to explaincyclization, the same methods were extended to all the speciesinvolved during the final oxidation step of H2Dz to Dz. Scheme 2indicates the signs of the partial MD charges next to every atomor group.The positive charge in the intermediary cationic species (bet-

ween 2a and 3a) is largely hosted by the phenyl rings, which is

practically canceled in the neutral product, 3a. The electro-static repulsion due to similar charges on nitrogens N1 and N4,as obtained by methods mentioned in the previous paragraph(see Table 3), would certainly prevent ring-closure. Here, onlyOLYP and PW91 MD again provides a realistic representation,whereby N1 and N4 have opposite charges in the intermediaryspecies, thus allowing for electrostatic attraction and ring-closure to take place, with hydrogen loss only at this point.Losing the imine proton at an earlier point in time wouldgenerate a geometry related to 3b and 3c, causing chargedistribution to be symmetrical in the two halves of themolecule,thus also preventing cyclization to form the tetrazole ring. Thisfinding, i.e., of the imine proton staying intact during thelifetime of the cationic intermediary species, is also consistentwith the cyclic voltammetry (CV) study of dithizone that wasrecently reported.8 Here, it was proposed that, on the relativelyfast CV time scale, HDz+ does not lose the proton to form Dz,but during the ensuing reduction half cycle, it is immediatelyreduced to (HDz)2 and H2Dz.3.2. Electronic Spectra. The intense color of dihtizone,

together with its potential to isomerize, to be solvatochromic,concentratochromic, and ultimately also photochromic in metalcomplexes,15 lend it ideally to TDDFT studies toward increasedunderstanding of the fundamental properties of this versatilemolecule. Present and previous15 experience established theB3LYP functional as implemented in Gaussian as most accurate

Figure 8. B3LYP TDDFT (methanol as solvent) computed oscillators(bars) overlaid with Doppler broadening curves (---) and experimentalUV�visible spectra (—) of H2Dz (1a), (HDz)2 (2a), and Dz (3a) inmethanol.

Figure 9. Experimental spectra of H2Dz (lines) and B3LYP computedoscillators (bars) for 1b (see corresponding oscillators of 1a in Figure 8).Top: 1 is solvatochromic. Dilute methanol solutions are orange (---),with one absorption band (tautomer 1b), and dichloromethane solu-tions are green (—), with two absorption bands in the visible spectrum(tautomer 1a). Bottom: 1 is concentratochromic in methanol. Dilutesolutions are orange (b, ε = 13 800 L mol�1 cm�1 at λmax = 475 nm),while increasing the concentration changes the solution color to green(a, ε = 12 500 L mol�1 cm�1 at λmax = 455 nm, and ε = 14 600 Lmol�1 cm�1 at λmax = 592 nm).

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The Journal of Physical Chemistry A ARTICLE

in its simulation of experimental UV�visible spectra of dithizoneand its derivatives and complexes.The oscillators of compounds 1a�f, 2a, and 3a in both gas

phase and solvent (CH2Cl2 and CH3OH) were obtained bymeans of B3LYP TDDFT calculations. As for dithizone, onlygeometry 1a and in the solvent environment, gives a theore-tical spectrum that resembles the experimentally obtainedspectrum in methanol as solvent; see Figure 8 (top). Oscilla-tors are overlaid with Doppler broadening curves to betterillustrate correlations with experimental spectra. A significantdifference was noted between gas phase and solvent calcula-tions; the lowest energy oscillator appears at 705 nm in gasphase, while it occurs at 626 nm in DCM and 611 nm inmethanol. The experimental peak in methanol is at 593 nm.The second calculated oscillator in methanol lies at 428 nm,corresponding to the 455 nm experimental peak maximum,while several weaker oscillators below the local minimum at ca.370 nm also agrees well with the UV part of the spectrum of 1a inmethanol.The solvent computed spectrum of the disulfide of unsub-

stituted dithizone, 2a, corresponds almost exactly with the exper-imental spectrum; see Figure 8 (middle). Several strong oscilla-tors appear from ca. 370 to 450 nm, which, when overlaid with abroadening curve, give a peak maximum at 421 nm, lying veryclose to 416 nm, which was observed experimentally. The twooscillators that appear at the longest wavelengths are evidentlythe cause for the unsymmetrical peak shape as may also clearly beseen in the experimental spectrum. The difference in the gasphase computed spectrum is much less than for 1a, with peaks ingeneral at only a few nanometers higher than in the solventspectra.Comparison between computed and experimental spectra of

dehydrodithizone, 3a, shows close resemblance in general shape,but the former is red-shifted; see Figure 8 (bottom). The smallpeak in the visible part of the spectrum that gives 3a its paleyellow color is shifted by about 90 nm, from the experimental411 nm to about 500 nm, while the prominent peak maximumat 252 nm is red-shifted to 278 nm. Compound 3a is a drama-tic example of solvent effect. The longest wavelength oscillatoroccurs at 505 nm in methanol, 540 nm in DCM, and at 946 nmin the gas phase. In the present study, the effect of solventappears to bemore pronounced in delocalizedmolecules, as both1a and 3a are. This result is consistent with previous discussionsabout the stabilization effect of solvents compared to gas phasecalculations (Figure 2 and Table 1). The gas phase�solventcomputed differences seen for 1a and 3a are conclusive evidenceof how indispensable the solvent environment is in TDDFTcomputations, especially in comparisons with experimentalspectra.Confidence in the relative accuracy of the above method

brought about an attempt to explain the solvatochromic propertyof dithizone (see Figure 9, top) by the application of TDDFT.Dilute solutions of dithizone, 1, are orange, while more con-centrated solutions (not very soluble in methanol) are green.Decreasing the concentration results in the fading of the 600 nmpeak, while the molar absorptivity of the 450�480 nm peakstays relatively unchanged, at about 12 500 L mol�1 cm�1; seeFigure 9. Anionic dithizonate, HDz�, is also known for beingorange. Therefore, both the oscillators of the six tautomericforms of 1 and that of HDz� were compared with the observedorange spectrum of 1 in methanol (Figure 9, spectrum b).An undisputed correlation between the experimental (orange)

spectrum was found with the computed spectrum of tautomer1b, which is also the tautomer of the second lowest energy, nextto 1a. The close agreement of the computed oscillator at 446 nm,overlapping with the experimental peak at ca. 450 nm, isillustrated in Figure 9. The same is true for the shorter

Table 4. Crystal Data and Structure Refinement

empirical formula C26H18F4N8S2 3 0.5(C6H14)

formula weight 625.69

temperature 100 (2) K

wavelength 0.71073 Å

crystal system triclinic

space group P1

unit cell dimentions a = 7.6537 (3) Å, b = 12.4352 (6) Å,

c = 16.3279 (7) Å

α = 73.674 (2)�β = 85.270 (2)�γ = 76.176 (2)�

volume 1447.96 (11) Å3

Z 2

density Dx = 1.435 Mg m�3

absorbtion coefficient 0.25 mm�1

F(000) crystal

description

646 red plate

crystal size 0.05 � 0.30 � 0.35 mm

theta range for data collection 3.4�24.3�index ranges �10 e h e 10, �16 e k e 15,

�21 e l e 21

reflections collected 26 154

independent reflections 7129 [R(int) = 0.054, R(σ) = 0.0668]

absorbtion correction multiscan SADABS (Bruker, 2009)

R[F2 > 2σ(F2)] 0.059

data/restraints/parameters 7129/0/388

largest diff. peak and hole 0.64 e �3 and �0.44 e �3

data collection

Bruker APEX CCD area-

detector diffractometer

7129 independent reflections

radiation source: fine-focus

sealed tube

4629 reflections with I > 2σ(I)

graphite monochromator Rint = 0.054

ω scans θmax = 28.3�, θmin = 3.0�absorption correction: multiscan

SADABS (Bruker, 2009)

h = �10 f 10

Tmin = 0.920, Tmax = 0.987 k = �16 f 15

26154 measured reflections l = �21 f 21

refinement

refinement on F2 primary atom site location: structure-

invariant direct methods

least-squares matrix: full secondary atom site location:

difference Fourier map

R[F2 > 2σ(F2)] = 0.059 hydrogen site location: inferred

from neighboring sites

wR(F2) = 0.168 H-atom parameters constrained

S = 1.05 w = 1/[σ2(Fo2) + (0.0819P)2 + 0.3004P],

where P = (Fo2 + 2Fc

2)/3

7129 reflections (Δ/σ)max < 0.001

388 parameters Δæmax = 0.64 e Å�3

0 restraints Δæmin = �0.44 e Å�3

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The Journal of Physical Chemistry A ARTICLE

wavelength oscillators, i.e., between 250 and 350 nm. All othertautomers, as well as anionic HDz�, gave significantly differentoscillator peak patterns, and may it therefore confidently beproposed that structure 1b gives rise to the orange color ofdithizone in dilute methanolic solutions. This structure does notrequire isomerization but simply the intramolecular transfer ofthe 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 nonproticDCM environment. This finding, however, is not contradictory tothe above evidence in favor of methanol assisting in an intramo-lecular proton transfer reaction, i.e., proton (N4)H in 1a beingtransferred to S in 1b, which has little to do with the relativeenergies of the two tautomers in different solvents.Figure 9 (bottom) also shows concentratochromism observed

inmethanolic solutions of 1, i.e., a change in color due to a changein concentration. The occurrence of this phenomenon is ratherunique. Poly(thienylacetylenes)32 and fullerenes33 are reportedto exhibit concentratochromism, manifesting as a gradual shift inwavelength due to changes in concentration. Dithizone behavesdifferently in that an entire new absorption band appears at592 nm inmethanol at higher concentrations (spectrum a), whilethe 475 nm absorption band of the dilute orange solution(spectrum b) is slightly blue-shifted to 455 nm. As this newobservation is not part of the objectives of this article, aninvestigation into possible reasons for this effect is reserved foranother study.3.3. X-ray Crystallography. Because of the relative instabil-

ity of the disulfide of dithizone, 2a, as opposed to the verystable meso-ionic final oxidation product, dehydrodithizone,3a, the former would not be a natural choice in any crystal-lography attempt. The growth of crystals of 2a that are suitablefor single crystal data collection happened coincidentally,during an attempt to recrystallize the ortho-fluoro derivativeof dithizone from toluene/hexane. Exposure to air proved to besufficient to partially oxidize (o-F)H2Dz to the disulfide, whichcrystallized out of solution within a day. Results from the

consequent X-ray data collection are listed in Tables 2 (bondlengths and angles) and 4 (crystal data and structure re-finement). Additional data may be found under SupportingInformation.(HDz)2, 2a, crystallized in a triclinic crystal system in the P1

space group (Table 4). There are two disulfide and four hexanesolvent molecules per unit cell; see Figure 10. The S�S singlebond between the two monomers allows free rotation aroundthis bond. The C�S�S�C torsion angle of 104.0� is anindication of the relative rotation between the two linear andrelatively flat HDz monomers. This torsion angle is ascribed tothe short intramolecular distance of ca. 3.46 Å between twophenyl rings suggesting π�π interaction, as well as an intra-molecular hydrogen bond of 2.927 Å between the (N4)H ofmonomer I and N1 of monomer II; see Figure 7. On the basis ofthe H-bonding guidelines provided by Jeffrey,34 this bond isclassified as weak, being relatively long (>2.13 Å), and having asmall bonding angle of 91.94�.A typical S�S single bond in a disulfide may be 2.04 Å long, as

seen in the ((NH2)2CS)2 dithio compound.35 The S�S bondlength in 2a, of 2.084 Å, is slightly longer, being indicative of aweaker bond. Bond distances through-out the monomeric backbones are most closely resembled by that observed in thedithizone 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 delocalizationobserved in dithizone, 1a. Double bonds N1dN2 and CdN3 in2a are, respectively, 1.256 and 1.275 Å and 1.379 and 1.391 Å inthe two disulfide monomers. The S�C single bonds are 1.799and 1.787 Å, which is more than 0.08 Å longer than thecorresponding bond in 1a. Bond angles along the backboneare, as expected, rather similar to related bonds in H2Dz and(SMe)HDz, e.g., the C�N1�N2 bond angles in 2a are 114.81and 114.70�, compared to that of 112.37� in H2Dz.The crystallography (CIF) file may be viewed under Support-

ing Information.

Figure 10. Structural packing in the (o-FHDz)2 crystal, indicating twodisulfides and four hexane solvent molecules per unit cell. Hydrogensare omitted.

Figure 11. X-ray crystal structure of the disulfide of ortho-fluorodithi-zone, with intramolecular π�π interaction between the phenyl ringsat the top, and H-bond between the imine proton (N4)H andnitrogen N1.

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The Journal of Physical Chemistry A ARTICLE

4. CONCLUSIONS

Both ADF implemented functionals, PW91 and OLYP, andGO9-B3LYP gave good structural representations of dithizoneand its oxidation products, with Gaussian (B3LYP) being themethod of choice for TDDFT computations of UV�visibleelectronic transitions, and ADF (PW91 and OLYP) for the pur-pose of realistically assigning Mulliken atomic charge distribu-tions. Single point MP2 for DFT optimized geometries gavevalidation for DFT computed energy trends. These computa-tional techniques allowed convincing evidence in support offavored geometries in dithizone chemistry, both in the gasphase and in the solvent environment. Resolving the crystalstructure of a disulfide of dithizone, (HDz)2, gave evidence infavor of the last outstanding structure in a series of structurallyresolved derivatives that includeH4Dz,H2Dz,HDz

�, (S-CH3)HDz,MHDz, and Dz.

’ASSOCIATED CONTENT

bS Supporting Information. GO9-B3LYP TDDFT calcula-tions of all H2Dz tautomers, 1a�f, and the favored (HDz)2 andDz tautomers, 2a and 3a, in gas, DCM, and MeOH media; CIFfile information. This material is available free of charge via theInternet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*Tel: 27-51-4012194. Fax: 27-51-4446384. E-mail: veschwkg@ufs.ac.za.

’ACKNOWLEDGMENT

TheCentral Research Fund of the University of the Free State,Bloemfontein, and the National Research Foundation of SouthAfrica are gratefully acknowledged.

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