J. Phys. Chem. 1993,97, 500-502
500
Solvation Structure of Divalent Transition-Metal Ions in N,N-Dimethylformamide and N,N-Dimethylacetamide Kazabiko Ozlltsumi' Department of Chemistry, University of Tsukuba, Tsukuba 305, Japan
Mdoto KOide, HOnob S u d & pnd Shin-ichi Ishig~ro Department of Electronic Chemistry, Tokyo Institute of Technology at Nagatsuta, Midori- ku, Yokohama 227, Japan Received: July 29, 1992; In Final Form: October 20, 1992
The solvation structure of divalent transition-metal ions, manganese(II), ironCII), cobalt(II), nickel(II), ~ p p r (11), and zinc(II), in NJV-dimethylformamide (DMF) and N,N-dimethylacetamide (DMA) has been studied by the EXAFS (extended X-ray absorption fine structure) method. All the metal ions examined show the coordination number n = 6, suggtsting a six-coordinate octahedral structure in DMF. The manganese(II), cobalt(II), nickel(I1). and copper(I1) ions also shown = 6 in DMA as weJl,asin DMF. However, the coordination number of 4.6 is revealed for the zinc(I1) ion in DMA, which is significantly smaller, suggesting that a solvation equilibrium, [Zn(dma)4I2+ 2dma [Zn(dma)a12+, is established in the solution.
+
Iah.odrrction NJv-Dimethylfonnamide(DMF) and Nfl-dimethylacetamide (DMA) are aprotic donor solvents having similar solvent properties,1.2Le., relative dielectric constants t(DMF) 3: 36.71, c(DMA) = 37.78; donor numbers&(DMF) = 26.6, &(DMA) = 27.8;acceptor numbenA~(DMA)5 16.0,A~(DhlA)= 13.6. However, their interaction with metal ions is significantlydifferent. For example, [Co(dmf)6](Clod)* is obtained from DMF and also from DMF-CHICI~and DMF-CHjCl mixtures, while [Co(dma)s](C104)2 is isolated from DMA and [Co(dma)4](C104)2 from the DMA-CHzC12 and DMA-CH3Cl mi~tures.~ NMR and electronic spectra show that the cobalt(I1) and nickel(I1) ions exist as an equilibrium mixture of [M(dma)6l2' and [M(dma)4I2+(M = Co or Ni). The former is the main species at room temperature, and the latter predominates as temperature increases.r5 In DMF, [M(dmf)612+is the main species over a wide range of temperat~re.~.~ On the other hand, from conductometry, Kamiehka and Uruska indicated the [M(dma)612+ion to be a charge carrier in metal(I1) tetrafluoroborate-DMA solutions? while Grzybkowski and Pilarczyk claimed the [M(dma)4I2+ (M = Cu or Zn) ion as a charge carrier in metal(I1) perchlorate-DMA solution^.^ It is thus desirable to obtain direct structural information of the solvated metal ions in DMA. The solvation structure of metal ions in some nonaqueous solutions as well as in aqueous solution has been determined by diffraction, EXAFS (extended X-ray absorption fine structure), and computer-simulationmethods,10since the solvationstructure is essential to elucidate the thermodynamics and kinetics of metal ionsin solution. In DMF, the six-coordinate [Co(dmf)s12+,[Cu(dmf)612+, and [Cd(dmf)~]*+ structurar have been establishcd.11-13 However, a systematic structural determination of the first-row divalent transition-metal ions in DMF has never been carried out. Furthermore, no direct structural information on the solvation structure of these metal ions in DMA has been provided sofar. In thepresent study, weaimedat determiningthesolvation structure of the manganese(II), iron(II), cobalt(II), nickel(II), copper(II), and zinc(1I) ions in DMF and DMA by the EXAM method. The solvation structure of iron(I1) in DMA was not determinable because iron(I1) was easily oxidized.
then distilled under reduced pressure. DMA was dried for 3 days over BaO and then distilled under reduced pressure. DMF and DMA solvatesof metal(I1) tetraflwroboratc were prepared from hydrates of metal(I1) tetrafluoroborate and dried in vacuo over P2O5. Sample solutions were prepared by dissolving metal(11) tetrafluoroborate-DMF and -DMA solvates in DMF and DMA, respectively. The concentration of the metal ions was set around 0.5 mol dm-j. All test solutions were treated in a drybox over P205. EXAFS Meuwewnts. EXAFS spectra were measured around the K edge of interest using the BL6B or BL7C station at the Photon Factoryof theNational Laboratoryfor High Energy Physics.14 The white synchrotron radiations were monochromatized by using a Si( 111) double crystal. In order to reduce higher-order harmonics, the two crystals were detuned by 80%, 50%,40%, 3096,2096, and 20% to the fundamentals above the K edge of manganese, iron, cobalt, nickel, copper, and zinc atoms, respectively. A glass fiber filter, which had been dried at 100 OC in an air bath, was immersed in a sample solution and then sealed in a Mylar bag in order to prevent evaporation of solvents and to avoid moisture. The apparent absorbance is obtained as In &/I), where I and IOare X-ray intensities with and without a sample, respectively. The intensitiesIOand Iwere simultaneously measured by ionization chambers filled with N2 and N2(85%) Ar( 15%) gas, respectively. Details of the data reduction of raw EXAFS spectra have been described e l s e ~ h e r e . ~The ~ - ~ threshold ~ energy of a K-shell electron EOwas selected as the position of the half-height of the edge jump in each sample. A curve-fitting procedure in the k space for the refinements of structure parameters was applied to the Fourier filtered k 3 x ( k ) avalues to minimize the error-square sum U = ZIP(x(k),u- x(k)alcd)2. The model function X(k)-ld is obtained according to the single-electron and singlescattering theory.18-21The values of the backscattering amplitude F(r,k) and the total scattering phase shift u ( k ) were taken from tables by Teo and Lee.22 In the fitting procedures, the EOvalue and the mean free path of the photoelectron A were determined in advance from the standard sample (an aqueousmetal(I1) tetrafluoroborate solution containing [M(H20)6I2+), and then they were used as constants in the course of the structural analysis of unknown samples, while the distance between central and scattering atoms r, the Debye-Waller factors u, and the number of scattering atoms n were optimized as variables.
+
Experimental Section h p k Solutionr. All chemicals used were of reagent grade.
DMF was dried for several weeks over molecular sieve 4A and 0022-3654/58/2097-0500304.00/0
(6
1993 American Chemical Society
Solvation Structure in DMF and DMA
The Journal of Physical Chemistry, Vol. 97, No. 2, 1993 501
TABLE I: Strpetpre puunctcrr for the Solvated Dinknt Mdrl 1- in DMF,DMA,and W a M 0 I I
n
metal
interaction
parameter
DMF
Mn2+
Mn-0
r/pm
Fd+
F
r/pm u/pm
w+
c0-0
216(1) 6.9 (1) 5.8 (1) 210(1) 7.0 (1) 5.8 (1) 208(1) 6.5 (1) 5.9(1) 204(1) 6.1 (1) 5.9(1) 196(1) 6.2 (1) 4.1 (1) 229(2) lO(1) 2.0(2) 208(1) 7.9 (1) 5.8(1)
u/pm
CD
n
I 0 c
4
\
3 x
n
r/pm a/pm
ri
&
n
Ni2+ 2
6
IO
2
6 1 0
2
Ni-O
r/pm u/pm
6 1 0 1 4
w10-2 pn-1 Figure 1. EXAFS spectra in the form of k3x(k)for sample solutions.
n
Cu2+
Cu-0,
r/pm u/pm
Cu-O,
r/pm
n
u/pm n
Zn2+
Zn-0
r/pm u/pm n
2
0
2
0
2
.
. . .DMA . .
,
217(1) 7.2(1) 6b 211 (1) 6.9 (1) 6b
208 (1)
207(1) 7.1 (1) 5.5(1) 205 (1) 6.8 (1) 5.9(1) 196(1) 6.0(1) 4.1 (1) 227(2) l6(2) 1.8 (4) 199(1) 7.2 (1) 4.6(1)
a Standarddeviationsof curve fits are given in p n t h - . were kept constant duruing the calculations.
DMF,.
water
DMA 216(1) 7.3 (1) 5.8 (1)
,
Zb 207(1) 8.2(1) 6b
Thevalua
Water. .
,
7.0(1) Sb 205(1) 6.4 (1) 6b 197(1) 5.8 (1) 4b 229(2) 12(1)
,
,
,
4
I
.
Pn
Figure 2 Fourier transforms F(r) of the k3x(k)curves shown in Figure 1, uncorrected for the phase shift.
RHdt9
The extracted EXAFS spectra x ( k ) weighted by k3 and their Fourier transforms p(r)I (uncorrected for the phase shift) of the sample solutions are shown in Figures 1 and 2, respectively. The first intense peaks observed in the p(r)l curves are due to the M-O bonds in the first solvation sphere of the respective metal ions, since DMF, DMA, and water are all oxygen-donorsolvents. The manganese(I1). iron(II), cobalt(II), nickel(II), copper(II), and zinc(I1) ions in water aresurrounded with six water molecules and have an octahedral [M(H~O)S]~+ ~ t r u c t u r e . ~The ~ J position ~ and height of the first peaks in the p(r)lcurves are practically the same for these metal(I1) ions in DMF and water. Thus, the metal(I1) ions are expected to have an octahedral [M(dmf)6I2+ structure in DMF as well as in water. The peak shapes for the manganese(II),cobalt(II), nickel(II), andcoppcr(I1) ionsinDMA are very similar to those in DMF and water, and the metal(I1) ions are also expected to have an octahedral [M(dma)612+ structure. In the case of the zinc(I1) ion in DMA, a remarkable difference in the p(r)I curve is seen, Le., the peak of the DMA solution appears at an appreciably shorter r position (155 pm) compared with that of DMF and water (163 pm for both DMF and water). The structure of the zinc(I1) ion in DMA is thus expected to be different from that in DMF and water. The structure parameters of the metal(I1) ions in solution were finally determined by a least-squares calculation applied to the Fourier filtered k3x(k) values in the region 3.5 < k / l W pm-1 < 11.5. The inverse Fourier transformation of the F(r) values was carried out over the r range to include the main peak in Figure 2 for each sample. The EOand X values were evaluated in advance from an aqueous metal(I1) tetrafluoroborate solution containing the [M(&O)S]~+ion. The M 4 bond lengths within [M(Hz0)6l2+ were also allowed to vary in order to check the reproducibility of the values. The best fit values are given in Table I, and the solid lines calculated using the parameter values thus obtained reproduce well the experimental points, as shown
4
6
8
10
4
6
8
IO
4
6
8
1012
&/lo-* pn-' Figwe 3. Fourier filtered k3x(k) curvca for sample s o l ~ t i ~ nThe ~. observed values arc shown by dots and calculated on- using parameter values in Table I by solid lines.
in Figure 3. The M-O bond lengths in the hydrated metal(I1) ions are in good agreement with the literature and thus the EOand A values are well approximated in the present study. The interatomic distance, the Debyc-Waller factor, and the solvation number for the metal(I1) ions in DMF and DMA were then optimized by adopting the EO and A values thus evaluated. The parameter values obtained are also given in Table I. Disclmsim
Since the coordination number converged to the the values close to six as seen in Table I, the manganese(II), iron(II), cobalt(11), nickel(II), coppcr(II), and zinc(I1) ions in DMF have a six-coordinate octahedral structure like in water. The M-O bond lengths in DMF are practically the same as those in water. Also, the Co-0 and C u 4 bond lengths in DMF agree well with those previously determined within experimental uncertainties.' 1,12 In DMA, the solvation number of the manganese(II), cobalt(11). nickel(I1). and copper(I1) ions was determined to be six, indicating that these ions have a six-coordinate octahedral structure as well as in DMF. However, the coordination number of 4.6 was obtained for the zinc@) ion in DMA. Considering that the zinc(I1) ion favorably forms either a six-coordinate structure as in DMF and water or a four-coordinate structure as in hexamethylphosphoric triamide (HMPA)2s and 1,1,3,3-
502 The Journal of Physical Chemistry, Vol. 97, No. 2, 1993
tetramethylurea (TMU),26 the zinc(I1) ion may exist as an equilibrium mixture of the six- and four-coordinatecomplexes in DMA. It is then deduced that 70% of zinc(I1) ions exist as [Zn(dma)4I2+. The measured Zn-O bond length in DMA is 199 pm, which is shorter than that of six coordination, e&, 208 pm for [Zn(dmf)J2+ in DMF and 207 pm for [Zn(H20)sl2+in water, while it is longer than that of four coordination, e.g. 193 pm for [Zn(hmpa),12+inHMPA,Zs195 pmfor [Zn(tmu)4I2+in TMU,26 and 197 pm for a ZnO If we use the Zn-0 bond lengths of 208 and 195 pm for six and four coordination, respectively, the average Zn-O bond length of the zinc(I1) ion in DMA is calculated to be 198 pm, which is in good agreement with the observed one (199 pm). Thus, we propose a solvation equilibrium, [Zn(dma)4I2++ 2dma = [Zn(dma)6I2+,in DMA. It has been known that the limiting molar conductance of the copper(I1) and zinc(I1) ions is slightly larger than that of the manganese(II), cobalt(II), and nickel(I1) ions in DMAe8v9As d i s c d above, the zinc(I1) ion mainly exists as [Zn(dma)4I2+ in DMA. Similarly, the copper(I1) ion exists as [Cu(dma)4I2+ to be a charge carrier in DMA, because the axial Cu-0 bond is weaker than the equatorial Cu-0 one. This is especially so for [Cu(dma)6I2+ascomparedwith [cu(dmf)Sl2+and [Cu(H20)a12+, because of the larger steric interaction of solvating DMA molecules. Obviously, the fourcoordinate [M(dma)4]2+is smaller than the six-coordinate [M(dma)612+in size to lead to a larger conductivityin solution. Hence, the copper(I1) and zinc(I1) ions are expected to have larger conductivitythan the manganese(II), cobalt(II), and nickel(I1) ions. The M-O bond distance in DMA, which is thought to be a measure of the M-0 bond strength, is elongated in the order, Cu(1I) < Zn(I1) < Ni(I1) < Co(I1) < Mn(I1). The order should be in parallel with the frequency shifts of the C = O bond of DMA molecules bound to the metal ions. Kamiefika and Uruska have reported the carbonyl frequenciesof DMA molecules bound to the metal(I1) ions in a DMA-chlorobenzene mixture of 0.72 mole fraction chlorobenzene.8 In fact, the C = O bond frequencies shift to the higher side in the order, Cu(I1) < Zn(I1) < Ni(I1) < Co(I1) < Mn(II), suggesting that the M - O bond strength decreases in thesame order. This is in agreement with our result. To conclude, the coordination of DMA molecules to a metal(11) ion is sterically hindered to some extent, which may result
Ozutsumi et al. in specificaspects with respect to the thermodynamics, dynamics, and structure of metal(I1) ions in DMA.
Acknowl6dgmeat. EXAFS measurements were performed under the approval of the Photon Factory Program Advisory Committee(Proposa1No.91-005). Theworkhasbeen financially supported by the Grant-in-Aid for Scientific Research (No. 04640572) from the Ministry of Education, Science, and Culture of Japan. Rdereocea md Notes (1) Gutmann, V. The Donor-Acceplor Approach to Molecular Interacrfons;Plenum: New York, 1971. (2) Riddick, J. A,; Bunger, W. B.; Sakano, T. K. Organic Soluents, 4th 4.;Wiley-Interscience: New York, 1986. (3) Wayland, B. B.; Fitzgerald, R. J.; Drago, R.S. J. Am. Chem. Soc.
1966,88,4600. (4) Gutmann,V.; Beran, R.;Kerber, W. Monatsh. Chem. 1972,103, 164. (5) Lincoln,S. F.; Hounslow, A. M.; Boffa, A. N. Inorg. Chem. 1986, 25, 1038. (6) Matwiyoff, N. A. Inorg. Chem. 1966,5,788. (7) Drago, R. S.;Meek, D. W.; Joesten, M. D.; LaRoche, L. Inorg. Chem. 1%3,2, 124. (8) Kamiellska, E.; Unreka, I. Electrochim. Acra 1977,22, 181. (9) Grzybkowski,W.G.;PilarnyL,M.Electrochfm.Acta 1987,32,1601. (10) Marcus, Y. Chem. R N . 1988,88,1475 and references therein. (1 1) Ozutsumi, K.; Tohji, K.; Udagawa, Y.; Ishiguro, S. Bull. Chem.Soc. Jpn. 1991,64, 1528. (12) Ozutsumi, K.; Ishiguro, S.; Ohtaki, H. Bull. Chem. Soc. Jpn. 1988, 61,945. (13) Ozutsumi, K.; Takamuku, T.; Ishiguro, S.; Ohtaki,H. Bull. Chem. Soc. Jpn. 1989,62, 1875. (14) Nomura, M.;Koyama, A,; Sahrai, M. KEK Report 91-1,National Laboratory for High Energy Physics, Tsukuba, Japan, 1991. (15) Ozutsumi, K.; Kawashima, T. Inorg. Chfm.Acta 1991, 180, 231. (16) Ozustumi, K.; Miyata, Y.; Kawashima, T. J. Inorg. Blochem. 1991, 44,97. (17) Ozutsumi, K.; Kawashima, T. Polyhedron 1992,11, 169. (18) Sayers, D. E.; Stem, E. A.; Lytle,F. W. Phys. Reu. Lctr. 1971,27, 1204. (19) Stem, E. A.Phys. Rev. B 1974,10, 3027. (20) Stem,E. A.;Sayers,D.E.;Lytlc,F. W.Phys.Rm. B1975,11,4836. (21) Lengeler, B.; Eisenberger, P. Phys. RN. B 1980,21,4507. (22) TCO, B.-K.; Lee, P. A. J. Am. Chem. Sa.1979,101,2815. (23) Ohtaki,H.; Yamaguchi, T.; Ma&, M.Bull. Chem. Soc. Jpn. 1976, 49,701. (24) Nomura, M.;Yamaguchi, T. J. Phys. Chem. 1988,92,6157. (25) Ozutsumi, K.; Abe, Y.; Ishiguro, S. To be published. (26) Inada, Y.; Ozutsumi, K.; Funahashi, S. To be publihed. (27) Abrahams, S. C.;Bemstein, J. L. Acra Crysrallogr., Sect. B 1969, 25, 1233.
