Journal of Structural Chemistry, Vol. 38, No. 4, 1997

STUDIES OF THE ELECTRONIC AND SPATIAL STRUCTURE OF CADMIUM(II) DIALKYLDITHIOCARBAMATE MOLECULES IN NONAQUEOUS MEDIA UDC 539.26;539.196.3;541.49

L. N. Mazalov, S. B. l~renburg, N. V. Bausk, S. V. Tkachev, S. M. Zemskova, and S. V. Larionov

CdK EXAFS and 113Cd NMR spectra were measured for solutions of cadmium dialkyldithiocarbamates in organic solvents with different donor numbers (tn'butylphosphin~ methylene chloride, benzene, dibutylsulfide, pyridine, dimethylsulfoxide) and for some model compounds. The parameters of the local surroundings of the Cd atoms for complex forms in solutions were determined from EXAFS data using the EXCURV-92 package. Spatial structure models of the complex forms in a metal chelate-nonaqueous solvent system are suggested. It is established that coordination of nitrogen atoms by cadmium atoms in pyridine solutions is realized for both Cd[(C2Hs)2NCS2]2 and Cd[(n-C4Hg)2NCS2] 2. For the tn'butylphosphine solution of Cd[(n-C4Hg)2NCS2]2, additional coordination of the phosphorus atoms of tn'butylphosphine molecules by cadmium is established.

INTRODUCTION

Solvation is a complex phenomenon that involves energy and structural changes in a solute-solvent system. In monographs [1, 2] solvafion is treated as a donor-acceptor reaction accompanied by a thermal effect, which may be expressed via the donor number of the solvent. The result of this process is coordination of solvent molecules to metal ions and withdrawal of the ligand anion into the outer sphere. However, these ideas were developed on the basis of the data obtained for solutions of metal salts in nonaqueous media and are not applicable to chelate compounds in which the anion is a chelate ligand. Traditional models of physicochemical properties of such metal complexes in solutions use interactions of two types: 1) specific chemical interaction of the metal atom with ligands of the first coordination sphere and 2) nonspecific interaction of the inner sphere of the metal complex with medium (solvent) as dielectric continuum. For electroneutral complexes, the interaction inside the complex (in the inner sphere) is much greater than that with the medium (solvent). Another model used for these systems is based on the idea of outer-spheric coordination of solvent molecules by complex molecules [3]. There are cases when organic solvent molecules are directly coordinated by the metal atoms of a molecular compound (alkylbenzenes -- antimony trichloride) in solutions with characteristic fast exchange of free (in solution) and bonded (in complexes) sblvent molecules [4, 5]. On the other hand, there are examples of solid metal complexes with organic solvent molecules: [Hg((i-C3H7)OCS2)(P(c-C6HIOa)2(CH2C12)](CI04)and Ni(C2HsOCS2)B• (B = Phen, 2,2'-Bipy, en; A = (C2H5)20); the nature of coordination of solvent molecules is not discussed [6, 7]. It seems important to investigate formation of metal chelate complexes having hydrophobic substituents of different lengths in series of nonaqueous solvents with varying donating abilities. Examples of such chelates are metal dithiocarbamates, whose specific feature is the ability of some part of strained four-membered metallocyeles to be rearranged into bridged chain cycles, which leads to additional coordination sites in the chelate molecule. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated from Zhumal Strulaumoi Khimff, Vol. 38, No. 4, pp. 723-731, July-August, 1997. Original article submitted February 20, 1997. 0022-4766/97/3804-0601518.00 01997 Plenum Publishing Corporation

601

Organic solutions of zinc and cadmium dithiocarbamates are used in spray pyrolyses to prepare ftlms of zinc and cadmium sulfides. Thus thin film~ of CdS of satisfactory quality are formed when a pyridine solution of cadmium diethyldithiocarbamate Cd[(C2Hs)2NCS2]2 (below CdL2) is sprayed at T = 300-400"C [8]. It was found experimentally that not all organic solvents are suitable for these purposes, and empirical rules for solvent selection were formulated. In the NMR studies of the state of metal chelates in nonaqueous media it was shown that the nudear chemical shift of the central atom changes considerably depending on the nature of the solvent used (6 varies by more than 100 ppm) [9, 10]; this is attributed to the energy and structural changes in the first coordination sphere of the central atom. Prospects for using EXAFS spectroscopy for investigating the structure of aqueous solutions of metal ions are outlined in [11]. In [12] it is reported that the rate constant of ligand exchange for metal complex hydrates in aqueous solutions correlates with the Debye-Waller factors obtained from the EXAFS data. In [13] EXAFS measurements provided information about the structure of the inner and outer hydrate shells for some metal ions in aqueous solutions. The works investigating the state of different types of complexes in nonaqueous solutions generally use IR and optical spectroscopy. Few works use the NMR of heavy nuclei and still fewer studies employ EXAFS, XANES, and X-ray fluorescent spectroscopy [14-16]. EXPERIMENTAL

The chelates CdL2 and Cd[(n-C4H9)2NCS2]2 (below CdL2) were prepared according to the standard procedure by the exchange reaction of Cd(CH3COO)2- 2SH20 and NaL or NaL' [17]. Synthesis of mixed-ligand complexes CdL2B (B = Phen, 2,2'-Bipy) is described in [18]. Organic solvents of reagent grade [chloroform, benzene, acetonitrile, acetone, ethylacetate, dlmethylformamlde (DMF), dimethylsulfoxide (DMSO), pyridine (Py), dibutylsulfide (DBS), and tributylphosphine (TBP)] were used without further purification. NMR spectra were recorded on a CXP-300 pulse spectrometer with an operating frequency of 66.57 MHz for 113Cd. The chemical shifts t~ (positive downfield) were determi-ed relative to the external standard - aqueous 1.5 M solution of Cd(C104) 2. The CdK EXAFS spectra were recorded using synchrotron radiation of the VEPP-3 ring (G. Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences). Ring settings during the measurements: energy 2 GeV, current 50 mA. The CdK absorption spectra (energy 26,711 eV) were measured by chinking the sample using At/He and Xe ionization chambers as monitoring and final detectors, respectively. A channel-cut Si(111) single crystal was used as a single-unit double-crystal monochromator. During the measurements, the station was monitored using microcomputers via CAMAC modules. The data obtained were processed with the EXCURV-92 package using a SUN (Sparc Classic) computer. The phase and amplitude characteristics were calculated in the X,z approximation using the package procedures. For analyzing the local surroundings of Cd atoms, Fourier-faltered data were fitted with k2 weighing in the range of photoelectron wave vectors from 3 to 10 /~-1. In the fitting procedure, the error of determination of interatomic distances was not higher than 0.02/I. in all cases. The factor of amplitude damping So caused by many-electron effects was determined from the results of the fitting procedure for solid model complexes with known structure. Solid complexes CalL2, CdL2(2,2'-Bipy), and CdL2Phen were used as model compounds. Different models of local surroundings of Cd atoms were checked during the fitting procedure to examine the possibility of further coordination of solvent molecules by cadmium. RESULTS AND DISCUSSION To obtain primary information about the structure of solvate complexes formed by dissolving metal chelates in nonaqueons solvents with different donor numbers, we recorded and analyzed the 113Cd NMR spectra of cadmium dialkyldithiocarbamates in solvents with low (methylene chloride, benzene), medium (ethylacetate), and high (pyridine, TBP) donor numbers. NMR spectra of cadmium complexes in solvent mixtures were also studied (methylene chloride-benzene, methylene chloride,-ethylacetate, methylene chloride-pyridine, methylene chloride-TBP, chloroform-TBP). 602

It is known that solid CdL2 is a dlmer with CdS5 coordination node (trigonal bipyramid) [19]. According to the results of molecular weight determination, the dimer structure is preserved in low-polar organic solvents (benzene); the monomer forms predominate in very dilute solutions (-<10 -3 M) [20]. The enthalpy of this transformation was estimated in [21]: - 4 0 kJ/mole Cd for CdL2 and - 1 5 kJ/mole Cd for CdlJ2 . It was also shown in [21] that the CdL2 and Cdl.~ molecules easily add two Py molecules or a 2,2'-bipyridine (below 2,2'-Bipy) molecule; thermal effects of these reactions were measured ( - 3 1 and - 4 3 kJ/mole for CdL~Py and CdL~(2,2'-Bipy), respectively). Earlier, we investigated reactions of CdL 2 with different nitrogen-containlng heterocycles and solved the crystal structures of CdL2(2,2'-Bipy), CdL2Phen (Phen = 1,10-phenanthroline), CdL2Im (Ira = imidazole) complexes [22, 23]. In [24] it was shown that CdL 2 molecules are acceptors with respect to phosphines PR 3 (R = CH3, C2H5). Thus CdL2 molecules are undoubtedly acceptors. The ll3cd NMR spectra of some cadmium diethyldithiocarbamate complexes with nitrogen-containlng heterocycles in chloroform were investigated in [25]. The characteristic shift of cadmium in the coordination node containing two sterieally strained four-membered cycles SCSCd is -330-340 ppm [25]. Addition of nitrogen-contalning donors leads to increased screening of the central nucleus and to an upfield shift of the caclmhlnl signal by approximately 100 ppm. Here we study the ll3cd NMR spectra of solutions of some cadmium dibutyldithiocarbamate complexes with nitrogen-containlng heterocycles (Phen, 2,2'-Bipy) in methylene chloride. In agreement with the results of [25], these systems show an upfield shift of the Cd signal by 100 ppm relative to the NMR signal for CdI.~ due to increased screening of the central nucleus caused by the addition of nitrogen-containing donors to CN = 6. Table 1 gives the chemical shifts 6113Cd for dialkyldithiocarbamates in different solvents. The chemical shift of cadmium changes by different values up to 160 ppm. In solvents with pronounced donating ability (DMF, Py, DMSO), the ll3Cd NMR signal is shifted upfield by -100 ppm, which may be due to coordination of solvent molecules to the cadmium chelate. At the same time, as we can see from the analysis of NMR spectra, DBS, which possesses donating ability due to the presence of an unshared pair on the sulfilr atom, shows less tendency toward screening than the solvent molecules having unshared pairs on the nitrogen (Py) or oxygen (DMF) atoms. Comparing the cadmium chemical shifts for CdL2 and CdI~, we can note that the shift changes by 8 ppm on passing from methylene chloride to benzene for CdL2 but does not change for Cdl.~. Probably, the reason lies in the differences in solvation of ligands with substituents differing greatly in size. The CdL2 molecule has two coordination sites that are capable of interacting with the solvent. Assuming that the solvent is coordinated to the metal and blocking these free coordination sites on the metal, we obtain a decrease in At3 of cadmium due to variation of the solvent. Indeed, on passing from chloroform to a (4:3 v/v) chloroform-pyridine mixture, A6 is - 5 8 ppm for CdL2 and + 16 ppm for CdL2Phen. These data indicate formation of outer- and possibly inner-spheric complexes of cadmium dialkyldithiocarbamates with solvent molecules.

TABLE 1. ll3Cd Chemical Shifts of the CdL2 and Cdl_~ Complexes in Different Solvents Solvent

Donor number (ace. to Gutman) Concentration, M

TBP CHzCI2 Acetone Ethylacetate DBS DMF CsHsN CsHsN CsHsN DMSO

0.0 17.0 17.1 26.6 33.1 33.1 33.1 29.8

cat;2 6113Cd

Concentration, M

6113Cd

0.001 0.002 0.018

375.9 343.3 335.7

0.004 0.01 0.001 0.011 0.05 0.014

316 238 233.7 232.9 233.7 221.5

0.051 0.027 0.042 0.023 0.006 0.049 0.045

381.7 334_5 334.3 326.9 326.6 320.7 244.3

0.054 0.043

236_5 223

Note. TBP - tributylphosphine, DBS - dibutylsulfide, DMF - dimethylformamide, DMSO -- dlmethylsulfoxide. 603

In all cases under consideration, the 113Cd NMR spectra contain one line, whose position changes when the composition of the solvent changes, suggesting fast (in the NM_R time scale) exchange between the free solvent molecules (B) in the volume and the solvent molecules associated with CdL2, e.g., in the CdL2B solvate complex. The character of this exchange depends on the temperature and composition of the solvent. It is of interest to investigate the 113Cd NMR spectra at different temperatures and with different compositions of the solvent. As solvent here we use methylene chloride and mixtures of the latter with solvents with different donor numbers, including benzene (low donor number, -0.1), ethylacetate (medium donor number, -17), and pyridine (high donor number, -33). We tried to retard the exchange by lowering the temperature. Benzene and pyridine have comparatively high melting points, which hinder profound cooling. We thought that methylene chloride, having melting point -95.1~ and a low donor number, would be able to perform the role of a noncomplexing inert medium. The 113Cd chemical shifts of CdL2 in CH2C12 at different temperatures, with different additions, and with different molar ratios (CdL 2 : addition) are listed in Table 2. From the analysis of the chemical shifts in Table 2 one can see that with benzene and ethyl acetate additions (donor numbers 0.1 and 17, respectively), methylene chloride may not be used as an inert medium. The ratio of solvent to addition concentrations is certainly important, but the interaction with the addition is not manifested for benzene or ethylacetate. The presence of two signals at low temperatures indicates that the solution contain~ at least two cadmium compounds differing in structure or composition. This may be due to different numbers of solvent molecules in the outer or inner sphere. However, we cannot exclude that one of the signals is due to the presence of the starting chelate dimers. These unidentified forms must also exist at room temperature in a ratio depending on the corresponding equilibrium constant and its temperature dependence. When pyridine is added to the solution (Table 2), the interaction of dissolved CdL2 with pyridine manifests itself quite distinctly. The rate of exchange increases with concentration of pyridine in solution, and methylene chloride, evidently, also participates in solvate formation. When the concentration of pyridine in solution decreases, so does the rate of exchange between the solvate shell and the free pyridine molecules. Thus the exchange may be "frozen out," at least partially, so that the signal with 6 = 224 ppm is likely"to be due to association of CdL2 with pyridine. When tributylphosphine is added to the solution (Table 2), the results are interpreted more unambiguously.

TABLE 2. Chemical Shifts 6113Cd of the CdL2 Complex in CH2CI2 at Different Temperatures, with Different Additions, and at Different Molar Ratios of Concentrations (CdL2 : addition) T, K

No addition

298

336

Addition CdL2: CdL2: CdL2: CdL2: CtH 6 = 1:3 CH3COOC2H5 = 1:3 CsH5N = 1:50 CsHsN = 1:3 336 337 301 304

233

CdL2:TBP = 1:25 534(w.b.) 385(w.b.) 244(w.b.) 501(w.b.) 4OO

297

213 203

340(br.b.)

295(55%) 224(br.b.)

200

506 (triplet) 406 (singlet) 506 (triplet, 78%) 406 (slnglet,

CdL2:TBP = 1:3 393

403 505 (triplet) 403 (doublet)

22%) 506 (triplet,

193

2O%) 405(doublet,

80%) 183

351(67 %)

270(33 %)

351(64%) 274(36%)

351(70%) 270 (30%)

270(br.b.)

Note: br.b. - broad band, w.b. - weak band; relative intensities of bands given in percent. 604

As in the case of pyridine, the rate of exchange increases with the concentration of free TBP in solution. The presence of Cd-P spin-spin coupling allows one to establish formation of inner-spheric complexes conta;nl,g one and two TBP molecules, the former complex being more labile. The 31p-113Cd spin-spin coupling constant is 1629 Hz for the complex with two TBP molecules and 1523 Hz for the complex with one TBP molecule; this corresponds to the value given in [24]. TBP is probably capable of withdrawing phenanthroline from CdL-2Phen. In the 4:3 (v/v) chloroform-TBP mixture, the 113Cd chemical shifts for Cdl.~Phen and CdL2 at room temperature coincide; when the mixture is cooled to 200 K, the spectra of CdL,2Phen show splitting of the exchange signal into a singlet and two triplets. In this case, further research is required. CdK EXAFS spectra were measured for some complexes in the solid state and for solutions in organic solvents. We checked whether the asymmetric surroundings formed by the sulfur atoms around the Cd atoms, characteristic for the solid state, are preserved in a pyridine solution of Cdl.~. For this purpose, the initial model included the sulfur atoms lying in pairs at equal distances. As a result of the fitting procedure, it was established that the four sulfur atoms in the solution of the complex lie at equal distances from the cadmium atoms. The results of the fitting procedure are considerably improved when one or two nitrogen atoms, which may be coordinated to cadmium atoms in solution, are introduced in sequence into the model: the fitting index F characterizing the consistency of the model with experimental spectra ([16]) decreased by factors of 1.7 and 2.0, respectively (Fig. 1). According to the results of the fitting procedure, the Cd-S and Cd-N interatomic distances are 2.61 and 2.36 A, respectively, for the model with one nitrogen atom and 2.63 and 2.40 ~ respectively, for the model with two nitrogen atoms. Analogous models, taking into account the asymmetric arrangement of the sulfur atoms and coordination of one or two nitrogen atoms by the cadmium atoms, were checked for pyridine solutions of CdL2. For this chelate, it was established that the sulfur atoms lie symmetrically and that the introduction of one and two nitrogen atoms into the model decrease the fitting index by factors of 1.7 and 1.9, respectively. According to the results of the fitting procedure, the Cd--S and Cd-N interatomic distances are 2.60 and 2.34 A, respectively, for the model with one nit_rogen atom and 2.62 and 2.40 A, respectively, for the model with two nitrogen atoms; these results are in agreement with the distances calculated for CdL2(2,2'-Bipy ) and CdL2Phen by XRD analysis in [22]. The data obtained indicate that coordination of nitrogen atoms by cadmium atoms in a pyridine solution is realized for both Cdl_~ and CdI-2 and that replacement of L by L' does not produce a significant effect on the Cd-N distances. It was interesting to verify that benzene solutions contain [Cdl_~2 dlmers, serving as a basis for packing the complex in the solid state, as indicated by the NMR data. For this purpose, during the fitting of EXAFS data, we analyzed a model of surroundings of cadmium atoms including five sulfur atoms: two pairs of sulfur atoms lying at close distances and one sulfur atom of the bridging bidentate diethyldithiocarbamate, which is coordinated to the neighboring Cd atom in the dlmer, lying at a distance of 2.77 A.. This model gave a fitting index that is 1.8 times smaller than that for the monomer model including four sulfur atoms at equal distances. The fitting procedure for this model showed that two sulfur atoms are at a distance of 2.60 ,t, two at a distance of 2.55 A, and one at a distance of 2.75 A.. These values are close to analogous distances in CdLz: 2.60, 2.63 A; 2x2.52/~; 2.77 A. (XRD data [19]). Variation of k2z(k) 0.40-

0.00

-0.40

1

'

!

"

|

4.0 6.0 8.0 ~;. k-,' Fig. 1. Experimental function of CdK EXAFS for a pyridine solution of the CdJ.~ complex (solid line) and theoretical curve for the model with two additionally coordinated nitrogen atoms (dashed line).

~z(k)

605

0.16-

9 0.12-

0.08

go o, 0.00

'

o.o 1:o s

,

9

.i

31o 4.o, ,A

Fig. 2. Moduli of the Fourier transform k2x(k) of CdK EXAFS for solutions of the Cdl~ complex in tributylphosphine (solid line) and in pyfidine (dashed line).

the number of sulfur atoms in the surroundings of the cadmium atoms gives 2.0, 2.13, and 1.18 for Cd-S distances of 2.60, 2.55, and 2.75 ~, respectively. The resulting interatomic distances and coordination numbers support the existence of dimers in benzene solutions. If the Cd-Cd atomic pair were identified at a distance of 3.30 L~, as in the solid complex, this would be an unambiguous support of the existence of dimers in solution. However, since the dlmer is relatively free to make vibrations with all degrees of freedom in solution, and due to high dynamic components of the Debye-Waller factors, this fact may not be unambiguously established from EXAFS data with the existing level of noise. For this solution, a model including the CdL2 monomer in which the benzene molecule is coordinated by the cadmium atom was also tested. This model gave only slightly improved fitting index (by a factor of 1.3). Analysis of EXAFS data with the assumption that the solution contzln.~ simultaneously monomers and dimers coordinating solvent molecules is not possible because of many parameters in the fitting procedure. The tn'butylphosphine solution of CdI~ differs from solutions in other solvents in that the amplitude of the fundamental maximum on the radial distribution function obtained by processing EXAFS data increases by 25%, indicating that the atoms are coordinated by TBP (F~g. 2). However, the introduction of one or two phosphorus atoms in addition to the four sulfur atoms decreases the fitting index by only 20%. The fitting gave Cd-S and Cd-P distances of 2.56 and 2.73/~ respectively. The discrepancy between the model and the experimental data may be caused by the fact that the available version of the EXCURV-92 package uses the harmonic approximation to describe the relative arrangement of atoms. In solution, coordination of phosphorus atoms by Cd atoms occurs at adistance that is slightly longer (2.73/~) than the characteristic distance of chemical bonding in a solid complex (2.57/~ according to the data of [24]). The form of the potential well in which the phosphorus atoms lie in this case must differ from harmonic. A correction for znharmonicitywill be made after a new version of the EXCURV program package has been obtained from the Synchrotron Center in Daresbury. This work was supported by RFFR grant No. 96-03-33166. REFERENCES 1.

2. .

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