4344
J . Phys. Chem. 1987,91, 4344-4360
Characterization of the Charge-Transfer Reaction between Decamethylferrocene and 7,7,8,8-Tetracyano-p-quinodimethane (1:l). The 57FeMossbauer Spectra and Structures of the Paramagnetic Dimeric and the Metamagnetic One-Dimensional Salts and the Molecular and Electronic Structures of [TCNQ]" ( n = 0, -1, -2) Joel S. Miller,*la Jian H. Zhang,Ib William M. Reiff,*lb David A. Dixon,*la L. D. Preston,lc,d Arthur H. Reis, Jr.,lc,eElizabeth Gebert,je Michael Extine," Jan Troup,lf Arthur J. Epstein,'g and Michael D. Wardla Central Research and Development Department,? E. I. du Pont de Nemours & Co., Inc., Wilmington, Delaware 19898; Molecular Structure Corporation, College Station, Texas 77840; Department of Chemistry, Northeastern University, Boston, Massachusetts 021 15; Chemistry Dioision, Argonne National Laboratory, Argonne, Illinois 60439; and the Xerox Webster Research Laboratories, Webster, New York 14580 (Received: September 23, 1986; In Final Form: March 19, 1987) The charge-transfer reaction of decamethylferrocene, Fe(C5Me5),, Fc*, and 7,7,8,8-tetracyano-p-quinodimethane, TCNQ, has been characterized. Three major reaction products of varying stoichiometry, conductivity, and magnetism are formed: a I-D metamagnetic 1:1 salt, [Fc*]'+[TCNQ]'-; a paramagnetic [Fc*],'+[TCNQ],*-; 1:l dimeric salt, [Fc*],'+[TCNQ],~-; and a conducting 1:2 salt of [Fc*]'+[TCNQ],- composition. The crystal and molecular structures of the 1:l paramagnetic dimeric and metamagnetic one-dimensional salts were solved. The ions in the dimeric phase crystallize in the centrosymmetric monoclinic space group P2]/c with a = 9.708 (1) A, b = 12.211 (2) A, c = 23.585 (4) A, fl = 95.01 ( 1 ) O , 2 = 4, and R = 0.058 for 3665 independent reflections. The structure consists of discrete stacks of DAAD dimeric (D = Fc*; A = TCNQ) units. One-dimensional stacking of ions is not observed. The decamethylferrocenium cations have average Fe-C distances of 2.096 (7) A, longer than the 2.050 A reported for neutral decamethylferrocene. The C5Me5rings appear eclipsed; however, as a result of the disorder only one ring was partially resolvable. The C5Me5rings are essentially parallel to the TCNQ moieties and separated by 3.554 A. The TCNQ anions form a tight aBdimer (separated by a short distance of 3.147 A) and are slipped along the short TCNQ axis. Bonding arises through filling of the bonding aBdimer orbital. The magnetic susceptibility of powder samples of [FC*]~'+[TCNQ~J~~obeys the Curie expression above 1.5 K suggesting very little magnetic coupling between the intradimer S = Fe(II1)'s which are separated by 14 A. The [TCNQIz2-dimer is strongly antiferromagnetically coupled. The zero-field Mossbauer spectra of the dimeric phase above 1.4 K are typical of S = [Fc*]'+ and exhibit rapidly relaxing paramagnetic behavior. However, relatively small external fields ( < 5 kG) are all that is necessary to induce slow paramagnetic relaxation and a substantial internal hyperfine field (-350 kG) at the decamethylferrocenium ions. Ions in the linear chain metamagnetic phase crystallize in the centrosymmetric triclinic space group PI. The unit cell at -106 "C is a = 8.635 (4) A, b = 9.384 (6) A, c = 10.635 (9) A; (Y = 116.76 ( 5 ) O , fl = 112.58 (5)'; y = 72.49 (4)O, V = 701.8 A3, Z = 1 , and R, = 0.060 for 1156 reflections. The poorer quality of this structure is due to the inability to grow suitable crystals of this kinetically favored phase with respect to the thermodynamically favored dimeric phase. The structure consists of discrete one-dimensional chains comprised of alternating radical cation donors, D, and anion acceptors, A, Le., -DADA.-. The decamethylferrocenium cation has a staggered conformation with an average Fe-C distance of 2.090 A. This is equivalent to that observed for [Fc*]'+ in the dimeric phase. The C5Me5rings are staggered and are canted with respect to the [TCNQ]'- moieties by 3.9'. The distance between the TCNQ and the C5Me5rings is 3.43 A. The [TCNQJ'- anion represents the first structural characterization of an isolated [TCNQJ" in the solid state. The one-dimensional phase exhibits metamagnetic properties with a Nee1 temperature of 2.55 K. The intrachain Fe"'-Fe"' separation is 10.549 A. Below -3.5 K the Mossbauer spectra of this phase undergo progressive magnetic hyperfine splitting for the decamethylferrocenium singlet (bFG+ (293 K) = 0.34 mm/s) leading to a highly broadened six transition Zeeman pattern at 2.7 K consistent with slow paramagnetic relaxation. Between 2.7 and 2.5 K the latter spectrum undergoes a further abrupt transition to a narrow line width ten transition pattern corresponding to the overlap of two Zeeman patterns with corresponding to the patterns are 390 and 432 kG. Ab a 2:1 intensity ratio. At 1.60 K the internal hyperfine fields, HINT, initio molecular orbital calculations with the STO-3G basis set were carried out for [TCNQ]", n = 0, -1, -2. The calculated structure and scaled vibrational frequencies are in excellent agreement with the observed values. The charge distributions are given together with the spin distribution for [TCNQJ'-. To confirm the results of the MO calculations, [TCNQI2- (as the [Co(C5Me5),]+(2: 1) salt) was structurally characterized for the first time. The ions crystallize in the centrosymmetric orthorhombic space group Pbca with a = 15.465 (6) A, b = 23.557 (9) A, c = 26.794 (1 1) A, Z = 8, R, = 0.085. The structures of C5and DSddecamethylcobaltocenium cations have been reported for the first time. The better determined D5, cation possesses average C d , C-C, and C-Me distances of 2.048, 1.42 and I SO6 A, respectively. The average C5 ring-Co( 1) centroid distance is 1.653 A. These values are comparable to that reported for isoelectronic Fe(C,Me,),. The structure of [TCNQ12- has also been determined for the first time. The dianion is planar within the experimental error and possesses D2,, local symmetry. The C6 ring is benzene-like with an average ring C-C distance of 1.41 A. The average C=C(CN),. C-CN, and C=N distances are I .44, 1.42, and 1.15 A, respectively.
-
Introduction There has been an increasing interest in the chemical and physical properties of the reaction products between inorganic complexes and electron-withdrawing organic acceptors such as 7,7,8,8-tetracyano-p-quinodimethane,TCNQ.2-4 In some cases, such materials exhibit higher than expected electrical conductivity, unique structures with unusual oxidation states, and unexpected 'Contribution No. 4099 0022-3654/S7/2091-4344$0l.50/0
magnetic properties. With our discovery5 of metamagnetic properties in [Fc*]'+[TCNQ]'- (Fc* = Fe(C,(CH,),),) we have ( 1 ) (a) E. I . du Pont de Nemours & Co., Experimental Station, E328 Wilmington, DE 19898. (b) Northeastern University, Boston, MA 021 15. (c) Chemistry Division, Argonne National Laboratory,Argonne, IL 60439. (d) Undergraduateresearch participant from Knox College, Galesburg, IL. (e) Current address: Dean of the Faculty Office, Brandeis University, Waltham, MA 02254. (f) Molecular Structure Corporation, College Station, TX 77840. ( 9 ) Xerox Webster Research Laboratory, Webster, NY 14580; current address: Departments of Physics and Chemistry, The Ohio State University, Columbus, OH 43210-1106.
0 1987 American Chemical Society
Reaction of Fe(C,Me,), and TCNQ
The Journal of Physical Chemistry, Vol. 91, No. 16, 1987 4345
focused our attention on understanding the chemical and physical features so that we can design, prepare, and ultimately study a ferromagnetic organometallic ~ y s t e m . ~Thus, , ~ we have undertaken the systematic study of the charge-transfer reactions between various substituted ferrocenes, e.g., Fe(CS(CH3)s)2,4-9"2-15 Fe(C,H,CH,),,lo Fe(CsH4)2(CH2)3,11 and organic acceptors such as TCNQ,,-" perfluoro-TCNQ,I5 tetracyanoethylene (TCNE),6,7,13 [C(CN),]-,I4 and 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ),', to understand the effects of the steric and electronic constraints of the structure on the properties of this class of compounds. Recently, the synthesis and characterization of a ferromagnetic complex have been r e a l i ~ e d , ~and J ~ contrasting the structural and electronic properties of these similar compounds is crucial to the ultimate understanding of cooperative magnetic interactions in charge-transfer complexes. The first system studied consisted of a pair of unusual materials isolated from the reaction of decamethylferrocene, Fc*, and TCNQ. The resultant 1:l charge-transfer compounds include the thermodynamically favored paramagnetic dimeric [ Fe(C,(CH,),)2]2'+[TCNQ]2Z-, and the kinetically favored 1-D metamagnetic ~ a l t , [Fe(C5(CH3),),]"[TCNQ]*-. ~-~ Herein, we report the molecular and crystal structure of these two 1:l salts and their low-temperature Mossbauer spectra. To better understand the microscopic spin interactions in the solid which lead to the observed cooperative magnetic properties, a better understanding of the electronic structure is imperative. Consequently we have devoted considerable computational effort toward ab initio calculations on TCNQ, [TCNQ]'-, and [TCNQI2- with the STO-3G basis setI6 in order to obtain optimized geometries and to obtain information about the electronic structure. Although larger basis set c a l c ~ l a t i o n shave ~ ~ been previously performed on these species, they were done at a fixed (2) For example: (a) Goldberg, S. 2.;Eisenberg, R.; Miller, J. S.;Epstein, A. J. J. Am. Chem. SOC.1976,98, 5173. (b) Goldberg, S. 2.;Spivack, B.; Stanley, G.; Eisenberg, R.; Braitsch, D. M.; Miller, J. S.; Abkowitz, M. J. Am. Chem. SOC.1977,99,110. (c) Endres, H.; Keller, H. J.; Moroni, W.; Nothe, D. Z . Naturforsch. B. 1976,31,1322. (d) Siedel, A. R. J. Am. Chem. SOC. 1975,97,5931. (e) Miles, M. G.; Wilson, J. D. Inorg. Chem. 1975,14, 2357. (f) Shibaeva, R. B.; Atovmyan, L. 0.;Orfanova, M. N. Chem. Commun. 1969 1494. (9) Williams, R. M.; Wallwork, S. C. Acta Crystallogr. 1968,24B, 168. (h) Lequan, R.-M.; Lequan, M.; Jaoven, G.; Ovahab, L.; Batail, P.; Padiou, J.; Sutherland, R. G. J . Chem. Soc., Chem. Commun. 1985, 116. (3) Melby, L. R.; Harder, R. S.; Hertler, W. R.; Mahler, W.; Benson, R. E.; Mochel, W. E. J. Am. Chem. SOC.1962,84, 3374. (4) Endres, H. In Extended Linear Chain Compounds; Miller, J. S., Ed.; Plenum: New York, 1983; Vol. 3, pp 263-312. (5) Reis, A. H., Jr.; Preston, L. D.; Williams, J. M.; Peterson, S. W.; Candela, G. A.; Swartzendruber, L. J.; Miller, J. S. J. Am. Chem. SOC.1979, 101, 2756. (6) Miller, J. S.; Reiff, W. M.; Epstein, A. J. Isr. J. Chem., in press. (7) Miller, J. S.;Krusic, P. J.; Epstein, A. J.; Reiff, W. M.; Zhang, J. H. Mol. Cryst. Liq. Cryst. 1985,120,21. (8) (a) Candela, G. A,; Swartzendruber, L.; Miller, J. S.; Rice, M. J. J. Am. Chem. SOC.1979.101. 2755. (b) Miller. J. S.: Reis. A. H.. Jr.: Gebert. E.; Ritsko, J. J.; Salaneck,'W. R.; Kovnat, L:; Cape, T. W.; Van Duyne,R: P. J. Am. Chem. SOC.1979,101, 71 11. (9) Miller, J. S.; Reis, A. H., Jr.; Candela, G. A. Lect. Notes Phys. 1979, 96,313. (IO) Wilson, S. R.; Corvan, P. J.; Seiders, R. P.; Hodgson, D. S.; Brookhart, M.; Hatfield, W. E.; Miller, J. s.;Reis, A. H., Jr.; Rogan, P. K.; Gebert, E.; Epstein, A. J. Molecular Metals; Hatfield, W. E., Ed.; Plenum: New York, 1979; p 407. (1 1) Willi, C.; Reis, A. H., Jr.; Gebert, E.; Miller, J. S. Inorg. Chem. 1981, 20, 313, 318. (12) (a) Gebert, E.; Reis, A. H., Jr.; Miller, J. S.; Rommelmann, H.; Epstein, A. J. J. Am. Chem. SOC.1982,104, 4403. (b) Miller, J. S.; Krusic, P. J.; Dixon, D. A.; Zhang, J. H.; Reiff, W. M.; Anderson, E.; Epstein, A. J. J. Am. Chem. SOC.1986,108,4459. (13) (a) Miller, J. S.; Calabrese, J. C.; Epstein, A. J.; Zhang, J. H.; Reiff, W. M. J . Chem. SOC.,Chem. Commun. 1986,1026. (b) Miller, J. S.; Calabrese, J. C.; Dixon, D. A.; Epstein, A. J.; Chittipeddi, S. R.; Zhang, J. H.; Reiff, W. M. J. Am. Chem. SOC.1987,109, 769. (14) Dixon, D. A.; Calabrese, J. C.; Miller, J. S. J. Am. Chem. SOC.1986, 108, 2582. (15) Miller, J. S.; Zhang, J. H.; Reiff, W. M. J. Am Chem. SOC.,in press. (16) Hehre, W. J.; Stewart, R. F.; Pople, J. A. J. Chem. Phys. 1969,51, 2657. (17) (a) Johansen, H. Int. J. Quantum Chem. 1975,9,459.(b) Jonkman, H. T.; van der Velde, G. A.; Nieuwport, W. C. Chem. Phys. Lett. 1974,25, 62.
geometry and the geometry has not previously been optimized at the ab initio level. The force fields for the closed shell species TCNQ and [TCNQIz- were also calculated. Following our recent discovery'* of a means to structurally and spectroscopically characterize dianions we have prepared [TCNQIz- as the 2:l [Co(C,Me,),]+ salt and structurally characterized it for the first time as an aid in calibrating the theoretical results. Experimental Section
Synthesis. Decamethylferrocene was obtained from either Organometallics, Inc. (E. Hampstead, N H ) or Strem Chemicals (Newburyport, MA). Decamethylferrocenium fluoroborate was prepared by a literature route13bwhereas decamethylcobaltocenium hexafluorophosphae was obtained from Strem Chemicals and converted to Co(C,Me,), by a literature procedure.19 TCNQ was obtained from Aldrich Chemical Co. (Milwaukee, WI) and recrystallized twice from suitably pure acetonitrile., The salt [NMe,] [P-(NC),CC,H,(C(O)CN] was prepared by the literature route.20 The charge-transfer salts of TCNQ were prepared in a H E 501 Vacuum Atmosphere drybox (2 weeks. Crystals collected were shiny purple platelets (yield -35%). Alternatively this complex can be prepared electrochemically. A divided H-cell (medium porosity fritted glass separator) was assembled containing a saturated dichloromethane solution previously prepared from microcrystalline [Fe(C,Me,)2]2[TCNQ]2,0.04 M Fe(C5Me5), (0.34 mmol), and 0.05 M Bu4N[TCNQI3(0.43 mmol) in the working compartment, and 0.05 M NBu4[TCNQ] (0.43 mmol) in the counter electrode compartment. A platinum wire electrode (1.0 X 0.1 cm) was placed in each compartment and a constant current of +300 pA applied across the cell at 25 OC. After 24 h (2.7 X lo4 Faradays) the mixture was filtered to yield 95 mg of purple, X-ray quality crystals of [Fe(CSMe5)2],[TCNQ]2(79% Faradaic yield). When electrolysis was performed at +30 pA flat, octagonal shaped crystals were formed in similar yields which were greater than 1OX larger (0.5 X 0.2 cm) than those grown at the higher current density. [ Fe( CSMeS)z]'+[p-(NC),CC6H4C( 0)CiV-aO. 25 MeCN was prepared from [Fe(C,Me,),]+.[BF,]- (100 mg; 0.242 mmol) dissolved in 20 mL of hot acetonitrile. To this solution was added [NMe4]'[p-(NC),CC,H4C(O)CN]- (65 mg; 0.242 mmol) in 20 mL of hot acetonitrile. Upon cooling, dark red crystals of [Fe(C,MeS)2]*+ [p-(NC) &C6H4C(0)C N ] - precipitated. Recrystallization was carried out in acetonitrile (107 mg; 0.203 mmol; 84%). Anal. Calcd (%) for C,, sH,47SN3z,0Fe:C = 71.29; H (18) Miller, J. S.; Dixon, D. A. Science 1987,235,871. (19) Robbins, J. L.; Edelstein, M.; Spenser, B.; Smart, J. C., J. Am. Chem. SOC.1982,104, 1882. (20) Hertler, W. R.; Hartzler, H. D.; Acker, D. S.;Benson, R. E. J . A m . Chem. SOC.1962,84, 3387.
4346 The Journal of Physical Chemistry, Vol. 91, No. 16, 1987
mol formula
mol wt, daltons space group
z
temp, O C extinctions cell constants a, A b, A c,
A
cy,
deg
P> deg Y3
deg
v,A3
calcd density radiation wavelength, 8, absorpn coeff, cm-I max 28; deg scan width (variable), deg variable scan speed, deg/min crystal volume, cm3 no. of reflections used for refinement total no. of reflections counted R for all reflections R, for reflections above u
Miller et al.
[Fe(C5Me5)212(Cl2H,N4)2 C64H68Fe2N8
1061.14 P2,/c (C:,, No. 14) 4 room temp hOl, 1 = 2n + 1 OkO. k = 2n + 1 9.708 (1) 12.211 (2) 23.585 (4) 90.00 95.01 (1) 90.00 2785.1 (7) 1.265" Mo K a X = 0.71073 5.84 45 (2.0 + A(a2 0.49-29.30 0.157 X lod 3042 4253 0.058
530.57 pi
(c,,NO. 2)
1
-106 f 1 none
8.635 (4) 9.384 (6) 10.635 (9) 116.76 (5) 112.58 (5) 72.49 (4) 701.8 1.26 Mo Kcy X = 0.71073 5.8 50
0.7 + 0.350 tan B 2-20 ( w ) 1.32 X loa 1156 1929 0.071 0.060
904.04 Pbca (&:, No. 61) 8 25 f 1 Okl, k = 2n + 1 h01, I = 2n + 1 hkO, h = 2n + 1 15.465 (6) 23.557 (9) 26.749 (1 1) 90 90 90 9761 (11) 1.23 M OK a X = 0.71073 7.2 44 0.8 + 0.14 tan 0 2-5 ( w ) 0.275 X 2643 6613 0.073 0.085
"Observed density (C6Hi2/BrCH2CH2Br) = 1.270 g/cm3. bVariable.
(5%) for C 5 3 H 6 5 5 N 4 5 C ~C2 = : 72.05, H = 7.47, N = 7.13, and = 6.60; N = 8.58; Found: C = 71.19; H = 6.50; N = 8.41. Co = 13.34; observed: C = 71.85, H = 7.48, and N = 7.49. Infrared spectra (Nujol): vCN = 2216 (w), 2183 (s), and 2154 Infrared spectra (Nujol) vCN = 2105 (s) and 2150 (s) cm-I. (m), vc0 = 1625 (m), and vo-.c(cn) = 1567 (s) cm-'. These values Physical Properties. 57Fe Mossbauer Spectra. Zero-field compare with the respective values of 2205, 2180, 2150, 1640, Mossbauer spectra were determined by using a conventional and 1590 cm-I reported for [NMe4]+[p-(NC)2C6H4C(0)CN]-.20 [Fe(C5Me5)2]'+[TCNQ],- was prepared from the neutral constant acceleration spectrometer with a source of 50 MCi 57C0 molecules. A solution containing 100 mg (0.306 mmol) of Feelectroplated onto the surface and annealed into the body of a (C5Me5), dissolved in 30 mL of warm acetonitrile was added to 6-pm-thick foil of high-purity rhodium in a hydrogen atmosphere. The details of cryogenics, temperature control, etc. have been a solution containing 125 mg (0.612 mmol) of TCNQ dissolved in 30 mL of warm MeCN. After the volume was reduced to 25 described previously.21a X-ray Data Collection. Dimeric Phase. A flat, shiny dark mL and slowly cooled to room temperature, fine needle crystals purple crystal of [Fc*]~'+[TCNQ],~was selected for data colformed and were collected via vacuum filtration [yield 134 mg lection at room temperature on a Syntex P2, diffractometer.22a (60%)]. A small amount of the 1:1 phase simultaneously formed; A detailed description of the crystal alignment procedure has been recrystallization from MeCN lead to the isolation of our anapreviously reported.,Ib Final cell parameters and other experilytically pure sample. Elemental analysis: calculated (%) for mental information are presented in Table I. C4H3*FeN8: C = 71.93, H = 5.21; N = 15.25, and Fe = 7.61; observed (Galbraith), Fe = 71.83; H = 5.09, and N = 15.03. IR: Intensity data (4253 reflections) were measured in the range of 4' C 26' C 45' with three check reflections automatically vCN (Nujol) = 2202 (m) cm-'. Crystals suitable for single-crystal inserted after every 47 measurements; these remained unchanged. X-ray diffraction analysis could not be prepared. The two-probe Linear Chain Phase. A green platelet crystal of [Fepressed pellet room temperature conductivity was measured to (C,Me,),] [TCNQ]23having approximate dimensions of 0.22 X be 0.1 ohm-' cm-l. 0.20 X 0.03 mm was mounted on a glass fiber with its long axis This 1.2 complex can also be prepared electrochemically. A roughly parallel to the 6'-axis of the goniometer. Preliminary divided H-cell (medium porosity fritted glass separator) was examination and data collection were performed on an Enrafassembled with [Fe(C5Me5),][BF4] (25 mg, 0.06 mmol) and Nonis CAD4 computer-controlled K axis diffractometer equipped TCNQ (50 mg; 0.25 mmol) in 8 mL of 0.1 M NBu4[BF4]/ with a graphite crystal, incident beam monochromator. acetonitrile in the working compartment, and 8 mL of 0.1 M NBu4[BF4]/acetonitrile in the counter electrode compartment. Of the 1929 reflections collected, 1850 were unique. As a check A platinum wire electrode ( 1 .O X 0.1 cm) was placed in each on crystal and electronic stability, three representative reflections compartment and an Ag/AgCl electrode in the working comwere measured every 41 min. The intensities of these standards partment. The working electrode was biased at +0.30 V (vs. remained constant within experimental error throughout data collection. No decay or absorption corrections were applied and Ag/AgCl), and after 16 h (1.7 X Faradays) black fibrous crystals of [Fe(C5Me,),] [TCNQ], were obtained on the working (21) (a) Cheng, C.; Reiff, W. M. Inorg. Chem. 1977, 16, 2097. (b) Reis, electrode in small yields (10 mg; 11% Faradaic yield). A. H., Jr.; Willi, C.; Siege], S.; Tani, B. Inorg. Chem. 1979, 18, 1859. [Co(C5Me5),],+[TCNQ12-.0.5 MeCN was prepared from the (22) (a) Fortran Operations Manual: Syntex P2,; Syntex Analytical neutral molecules. To a solution containing 120 mg (0.364 mmol) Instruments: Cupertino, CA 1975. (b) CAD4 Operations Manual; Enrafof Co(C,Me,), dissolved in 25 mL of warm acetonitrile was added Nonius: Delft, The Netherlands, 1977. (23) Previously we reported that this compound had a monoclinic unit a solution containing 37 mg (0.182 mmol) of TCNQ in 5 mL of cell.8b The R, was -25% (R = 11.8%)due to small crystals and partial warm MeCN. After concentration of the solution to one-third decomposition arising from partial reaction with oxygen to form [pthe original volume, the dark brown solution was cooled overnight (NC)2C6H4C(0)CN]-,vide infra. Although the gross 1-Dstructural features at -30 OC. The crystals that formed were collected via vacuum was correct, the redetermined triclinic structure supplies details previously not reported. filtration [yield 140 mg (89%)]. Elemental analysis, calculated
The Journal of Physical Chemistry, Vol. 91, No. 16, 1987 4341
Reaction of Fe(C5Me5), and TCNQ an extinction correction was not considered necessary. Cell constants and an orientation matrix for data collection were obtained from least-squares refinement, using the setting angles of 25 reflections in the range 2 < 8 < 12’. There were no systematic absences; the space group was assumed to be Pi (No. 2). Final cell parameters as well as the experimental information are presented in Table I. The data were collected at a temperature of -106 f 1 “ C by using the w-I9 scan technique. For intense reflections an attenuator was automatically inserted in front of the detector and the attenuator factor was 20.7. [ C O ( C ~ M ~ ~ ) ~ ] ~ [ T C NMeCN. Q].O.~ A red-brown rectangular crystal of [Co(C,Me,),] ,[TCNQ] composition having approximate dimensions of 0.27 X 0.30 X 0.34 mm was mounted in a glass capillary with its long axis roughly parallel to the 4 axis of the goniometer. Preliminary examination and data collection were performed on the Enraf-Nonius CAD4 system described above. Cell constants and an orientation matrix for data collection were obtained from least-squares refinement, using the setting angles of 17 reflections in the range 9 < I9 < 13’. From the systematic absences of Okl k = 2n + 1; h011= 2n 1; hkO h = 2n 1 and from subsequent least-squares refinement, the space group was determined to be Pbca (No. 61). The data were collected as above; see Table I. (Due to fracturing of the crystal, attempts to take data at -70 ‘C failed.) A total of 6613 reflections were collected, of which 5736 were unique and not systematically absent. As a check on crystal and electronic stability, three representative reflections were measured every 30 min. The slope of the least-squares line through a plot of intensity vs. time was -9 (1) counts/hour which corresponded to a total loss in intensity of 7.2%. The decay correction factors on I ranged from 0.965 to 1.133 with an average value of 1.026. No absorption correction was made. Solution and Structure Refinement. Dimer Phase. Absorption, Lorentz, and polarization corrections were applied.z4 The structure was solved by direct methods using the program MULTAN-^^.'^ The 26 atoms found from the MULTAN solution were used in the initial structure factor calculation and Fourier mapz6 from which the remaining non-hydrogen atoms were located. Full-matrix least-squares isotropic refinement of the 37 non-hydrogen atoms and a scale factor resulted in an R of 0.12. A difference Fourier map based on the refined coordinates of the non-hydrogen atoms revealed the positions of those hydrogen atoms attached to the [TCNQ]’- and to the methyl groups of only one of the cyclopentadienyl rings. The hydrogen atoms of the methyl groups of the second C5ring were not identifiable, probably due to disorder in the ring, which was evident from the larger than normal thermal parameters. Full-matrix least-squares anisotropic refinement of all non-hydrogen atoms with the hydrogen atom parameters remaining fixed with isotropic thermal parameters equal to the refined parameter of the carbon atom of which they are attached resulted in the final R, of 0.058 for all 3042 reflections where p > up. A final difference Fourier was featureless. The quantity minimized during refinement is CwlFOZ - F?l2 where F, and F, are the observed and calculated structure ~, factor amplitudes. Weights are assigned as w = l / ( ~ F 2 ) a(l) = [SC + T ~ ( &+ &) + p2p]”z where SC is the scan count, T is the scan to background time ratio, PI and p2 are the background counts on each side of a peak, I is the net intensity, and p is an ignorance factor, set to 0.02. The agreement indices are defined as follows:
+
+
R = CllFol - I~cll/CIFol
(4) The standard deviation of an observation of unit weight was 1.39. The highest peak in the final difference Fourier had a height af 0.65 e-/A3 with an estimated error based on AF3I of 0.09. Plots of Cw(lFo- Fc1)2vs. F,, reflection order in data collection, sin ( e l k ) , and various classes of indices showed no unusual trends. All calculations for the 1-D phase were performed on a PDP-11/60 based TEXRAY~’ system. A listing of F, and F, can be found as supplementary material. [ C O ( C ~ M ~TCNQI-0.5 ~ ) ~ ] ~ [ MeCN. The structure was solved by direct methods. Using 420 reflections (minimum E of 1.85) and 17 606 relationships, a total of 32 phase sets were produced. A total of two atoms was located from an E map prepared from the phase set with probability statistics: absolute figure of merit = 1.21, residual = 11.40, and IC/ zero = 3.154. The remaining atoms were located in succeeding difference Fourier syntheses. The acetonitrile solvent molecule was refined at 0.5 occupancy to account for small positional disorder at that site in the lattice. Hydrogen atoms were added to the structure factor calculations at their calculated positions, but their positions were not refined. The structure was refined in full-matrix least squares in the same manner as reported for the linear chain phase; vide supra. Only the 2643 reflections with Iobd > 3a were used. The final cycle of refinement included 535 variable parameters and converged (largest shift 0.40 esd) with the R factors given in Table I. The standard deviation of an observation of unit weight was 1.97. There were 103 correlation coefficients greater than 0.50. The highest correlation coefficient was 0.8 1 between parameters 405 and 408. The highest peak in the final difference Fourier was 0.56 e-/A with an estimated error based on A F 3 0 of 0.09. Plots
(1)
wR$ = [ c w l F O z- F , 2 1 z / c ~ ( F ~ ) 2 ] ’ / 2
(2)
G O F = [ C W ~ F -; F2l2/(N0
(3)
- NR)]’”
where No is the number of independent reflections and NR is the number of parameters varied. The value for the goodness of fit, GOF, is 2.16. Scattering factors for non-hydrogen atoms were taken from Cromer and Waber27and for the metal atoms were modified for the real and imaginary components of anamolous dispersion.28 Hydrogen scattering factors were taken from the literat~re.,~A listing of F, and F, can be found as supplementary material. See supplementary material available paragraph at the end of the text. Linear Chain Phase. The structure of the linear chain phase was solved by using the Patterson heavy-atom method which revealed the position of the Fe atom. The remaining atoms were located in succeeding difference Fourier syntheses. Hydrogen atoms were located and their positions and isotropic thermal parameters were refined. The structure was refined in full-matrix - lFCl2l least-squares where the function minimized was and the weight w is defined as 4F2/aZ(F,2).The standard deviation on intensities, a(F?), is defined as u 2 ( F , )= [Sz(C+ R’B) + (pF,2)2]/L2where S is the scan rate, C i s the total integrated peak count, R is the ratio of scan time to background counting time, B is the total background count, L is the Lorentz-polarization factor, and the parameter p is a factor introduced to downweight intense reflections. Here p was set to 0.060. Anomalous dispersion effects were included in Fc;30the values f o r f ’ a n d y w e r e those of C r ~ m e r Only . ~ ~ the 1156 reflections with Zobd > 3u were used. The final cycle of refinement included 220 variable parameters and converged (largest parameter shift was 0.1 1 times its esd) with unweighted, R, eq 1, and weighted, R,, eq 4, agreement factors of 0.060 and 0.071, respectively.
(24) DATALIB and DATASORT were written by H. A. Levy and locally adapted (Argonne) for the IBM 370/195. (25) MULTAN-74 was written by P. Main, M. M. Wolfson, and G. Germain and locally adapted (Argonne) for the IBM 370/195. (26) SSOUR, SSXFLS, and SSFFE are Sigma 5 versions of the programs FOURIER by R. J. Dellaca and W. T. Robinson, ORXFLS~ by W. R. Busing and H. A. Levy, and ORFFE by W. R. Busing and H. A. Levy.
(27) International Tables for X-ray Crystallography;Kynock Birmingham, England, 1974; p 71. (28) International Tables for X-ray Crystallography;Kynock: Birmingham, England, 1974; p 148. (29) International Tables f o r X-ray Crystallography;Kynock: Birmingham, England, 1974; p 102. (30) Ibers, J. A.; Hamilton, W. C. Acta Crystallogr. 1964, 17, 781. (31) Cruickshank, D. W. J. Acta Crystallogr. 1949, 2, 154. (32) TEXRAY is a trademark of the Molecular Structure Corporation. (33) Frenz, B. A. In Computing in Crystallography;Schenk, H., OlthofHazelkamp, R., van Konigsveld, H., Bassi, G. C., Eds.; Delft University Press: Delft, Holland, 1978; pp 64-7 1.
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The Journal of Physical Chemistry, Vol. 91, No. 16, 1987
of CwlF,I - IFC/)vs. IF& reflection order in data collection, sin (Blh),and various classes of indices showed no unusual trends. All calculations were performed on a VAX-11/750 computer using SDP-PLUS.34 A listing of F, and F, can be found as supplementary material. Molecular Orbital Calculations. All calculations were done with the STO-3G basis setI6 in DZhsymmetry. The calculations on the closed-shell species, TCNQ and [TCNQ12-, were done in the R H F formalism with the program GRADSCF~' on a CRAY-1A computer. The geometries were gradient ~ p t i m i z e d . ~ The ~ force fields were determined at the optimum geometry using analytic technique^.^^ Infrared intensities were also calculated analytically and are presented only for qualitative insights. The calculations on the [TCNQ]'- radical anion were done in the U H F formalism starting from the T C N Q wave functions. These calculations were done with the program HOND03* on an IBM-3081 computer. The U H F formalism is employed so that the extra electron remains bound and to provide information about spin populations and densities. The geometry was gradient optimized.
Results and Discussion Chemistry. The reaction of TCNQ and metallocenes can result in a variety of product^.^ Specifically with Fe(C,Me,), three phases form, namely a 1:1 1-D (kinetic), 1:l dimeric (thermodynamic), and 1:2 conducting phase. These phases are the result of in situ electron transfer when the charge-transfer complexes are crystallized from solutions of Fe(C5Me,), and TCNQ. The conducting 1.2 phase is formed by stoichiometric control of Fe(C5MeS), and TCNQ during crystallization. The dimeric phase is formed under conditions which allow slow crystallization, while the 1-D phase is formed only upon rapid crystallization. Since the dimer phase is thermodynamically preferred, growth of large single crystals of the I-D phase is difficult. Additionally, the presence of an isolated [TCNQJ'- in the 1-D phase destabilizes the crystal as it reacts in the solid state with oxygen to form S = 0 [p-(NC)&H&(0)CN]-.8 Since conventional techniques for the preparation of large single crystals of the 1-D phase were not successful, we investigated electrochemical growth as an alternative method. Electrocrystallization is analogous to metathesis methods which rely on controlled introduction of the components by slow diffusion. However, more precise control of the crystallization process is possible since the introduction of a redox active component can be regulated by the applied potential or current. Although synthesis of the 1-D phase was not accomplished with this technique, large crystals of the dimeric phases as well as crystals of the 1:2 phase were grown successfully by electrochemical methods.39 Electrocrystallization of the dimer phase was accomplished by oxidation of Fe(C,MeS), in the presence of [TCNQJ'- anion (eq 5) or in the reverse manner by reduction of TCNQ in the presence Fe(CsMes),
- e-
ITcNQ1'
dimer phase
of [Fe(CSMe5),]'+, eq 6. Both methods are feasible since Eo-
(34) Oneida Research Services, Inc., Whitesboro, NY. 13492. (35) GRADSCF is an ab initio gradient program system designed and written by A. Komornicki at Polyatomic Research and supported on grants through NASA-Ames Research Center. (36) (a) Komornicki, A.; Ishida, K.; Morokuma, K.; Ditchfield, R.; Conrad, M. Chem. Phys. Left. 1977, 45, 595. (b) McIver, J. A.; Komornicki, A., Jr. Chem. Phys. Lett. 1971, 10, 303. (c) Pulay, P. In Applications of Electronic Structure Theory; Schaefer, H. F., 111, Ed.; Plenum: New York, 1977; Chapter 4. (d) Komornicki, A,; Pauzat, F.; Ellinger, Y. J . Phys. Chem. 1983, 87, 3847. (37) King, H. F.; Komornicki, A. J . Chem. Phys. 1986, 84, 5645. (38) (a) Dupuis, M.; Rys, J.; King, H. F. J . Chem. Phys. 1976, 65, 1 1 I . (b) King, H. F.; Dupuis, M.; Rys, J. National Resource for Computer Chemistry Software Catalog, vol. I , Program QH02 (HONDO), 1980. (39) Ward, M. D. Inorg. Chem. 1986, 25, 4444.
Miller et al. TABLE 111: Interatomic Distances" in Dimer Phase: [Fe(C&fe&MTCNQI, bond distance,
bond distance.
A atoms A (a) Distances within the IFe(C
