J . Phys. Chem. 1990, 9 4 , 8483-8493
8483
Pressure-Tuning Spectroscopy of Charge-Transfer Salts. X-ray Crystallography and Comparative Studies in Solution and in the Solid State T. M. Bockman,l H.-R. Chang? H. G.Drickamer,* and J. K. Kochi*-' D e p a r t m e n t of C h e m i s t r y , University of H o u s t o n , University Park, H o u s t o n , Texas 77204, and t h e School of Chemical Sciences, Department of Physics and M a t e r i a l s Research Laboratory, University of Illinois a t Urbana-Champaign, Urbana, Illinois 61801 (Received: January 16, 1990; In Final Form: May 7 , 1990)
The highly colored pyridinium (P') and cobaltocenium (C') iodides are charge-transfer salts by virtue of the new electronic absorption bands that follow Mulliken theory. X-ray crystallography establishes the relevant interionic separation and steric orientation of the cation/anion pairs P'I- and C'I- constrained for optimum charge-transfer interaction in the crystal lattice. Spectral comparisons of the charge-transfer (CT) transitions by absorption (solution) and by diffuse reflectance (solid-state) measurements reveals the commonality of contact ion pairs (CIP) in aprotic nonpolar solvents (dichloromethane) with those extant in crystalline charge-transfersalts. As such, the compression of the charge-transfer salts PI-dissolved in dichloromethane by the application of pressures up to 10 kbar leads to the expected blue shift of the CT bands. Such a compressional effect can be attributed to increased ground-state stabilization of CIP arising from the enhanced solute-solvent interactions by the pressure-induced change in the dielectric constant (of dichloromethane), since it quantitatively follows the solvatochromic trend (Figure 13). By comparison, the compression of the charge-transfer salts P+I- in the solid state by the application of pressures up to 140 kbar leads to unusual red shifts of the CT bands indicative of the dominance of destabilizing charge-transfer interactions. X-ray crystallographic analysis of the cation/anion packing in the unit cell suggests that compressional effects on cation/cation and anion/anion interactions in the crystal lattice are superimposed on the relevant CT interactions of cation/anion pairs. The effects of pressure on the cryptic CT bands of crystalline cobaltocenium iodide are also compared with those observed in solution.
Introduction Weak intermolecular interactions leading to molecular complexes of uncharged electron donors (D) and acceptors (A) are frequently manifested by the appearance of new charge-transfer absorption bands.3 The latter according to Mulliken4 effectively corresponds to the vertical electronic transition ( h v C T ) which transforms the neutral ground state of the molecular complex [D,A] to a polar excited states5 Accordingly, the charge-transfer (CT) excitation energy is equated to the vertical ionization potential of the electron donor less the electron affinity of the acceptor and the excited state interaction energy wp.6 In very weak complexes, the C T excited state is approximated by the ion pair [D'',A'] with the ionic interaction energy given by the Coulombic potential eZ/rD,A, where rD,A is the mean separation of the donor and acceptor.' In accord with this formulation, the application of pressure to electron donor-acceptor complexes, both in the solid state and in solution, uniformly results in a marked red shift of the C T band.* Thus, pressure-tuning spectroscopy as applied to charge-transfer complexes represents a viable probe for intermolecular interaction^.^ Weak intermolecular interactions represented by the diffuse London dispersion forces extant in neutral EDA complexes are replaced by dominant electrostatic forces in charge-transfer complexes with polar ground states, i.e., salts [D-,A+]. Indeed the critical importance of strong Coulombic interactionsI0 warrants the extension of pressure-tuning spectroscopy to solvation and ( I ) University of Houston, University Park.
(2) University of Illinois at Champaign-Urbana. (3) (a) Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949,7J,2703. (b) Woodward, R. B. J. Am. Chem. Soc. 1942,64,3058.(c) Brackman, W. Rec. Trav. Chim. 1949,68. 147. (d) Weiss, J. J. Chem. Soc. 1942,245. (4) Mulliken, R. S.J. Am. Chem. Soc. 1950,72, 600.
(5) Mulliken, R. S.; Person, W. B. Molecular Complexes; Wiley: New York, 1969. (6) See: Hanna, M. W.; Lippert, J. L. In Molecular Complexes; Foster, R.. Ed.; Elek Science: London, 1973; Vol. I , p 1 ff. (7) Tamres, M.; Strong, R. L. In Molecular Association; Foster, R., Ed.; Academic: New York, 1979; Vol. 2, p 332 ff. (8) For a summary, see: Offen, H. W. in ref 6, p 117 ff. However for an exception, see: Bentley, W. H.; Drickamer, H. G. J . Chem. Phys. 1965,42, 1573.
(9) (a) Drickamer, H. G. Acc. Chem. Res. 1986,19, 329. (b) Drickamer, H. G.; Stephens, D. R. J . Chem. Phys. 1959,30, 1518. (c) Aust, R. B.; Samara, G. A.; Drickamer, H. G. J. Chem. Phys. 1964,41.2003. (d) Kadhim, A. H.; Offen, H. W. J. Chem. Phys. 1968,48,749. (IO) See, e.g.: Berry, R. S.;Rice, S. A.; Ross, J. Physical Chemistry; Wiley: New York, 1980; p 972.
0022-3654/90/2094-8483$02.50/0
association in solution" and to solid-state organization12 of charge-transfer salts. Accordingly, in this study we selected three representative charge-transfer salts to examine under pressure, and we have carried out for the first time parallel studies in the solid state and in solution of CT structures characterized by X-ray crystallography.
Results The orange Kosower's salt N-methyl-p-carbomethoxypyridinium iodide (CMP'I-) was chosen as the prototypical charge-transfer salt owing to its extensive use in the evaluation of solvent polarity.13J4 For this class of *-acceptors, we found the deep red N-methylacridinium iodide (A&-) to be particularly intriguing, since it is among those exhibiting the smallest C T excitation energies. Contrastingly, the organometallic cation derived from cobaltocene represents a structurally distinctive class of electron acceptors, and it was examined as the deep yellow cobaltocenium iodide. The differences in the acceptor properties of these cationic moieties are given by the varying magnitudes of the reduction potentials, i.e.15
E",d(V vs SCE)
(CMP+I-)
(AC+I-)
-0.79
-0.43
(cp2co+I-) -0.95
(1 1) (a) Marcus, Y . Ion Solvation; Wiley: New York, 1985. (b) Burger, K. Solvation Ionic and Complex Formation Reactions in Non-Aqueous Solvenrs; Elsevier: New York, 1983. (1 2) (a) Silinsh, E. A. Organic Molecular Crystals; Springer Verlag: New York, 1980. (b) Wright, J. D. Molecular Crystals; Cambridge University Press: New York, 1987. (c) Herbstein, F. H. Crystalline r-Molecular
Compounds: Chemistry, Spectroscopy and Crystallography in Perspective in Structural Chemistry, Vol. 4; Dunitz, J. D., Ibers, J. A., Eds.; Wiley: New York, 1971. (d) Prout, C. K.; Kamenar, B. in ref 6, p 151 ff. (e) Sms, 2. G.; Klein, D. J. in ref 7, Vol. 1, p 2 ff. (1 3) For convenience, the pyridinium (P) and cobaltoccnium (C*) cations are hereafter specified as CMP+ for N-methyl-p-carbomethoxypyridinium, Ac+ for N-methylacridinium, and Cp,Co+ for cobaltocenium, respectively. (14) (a) Kosower, E. M.; Klinedinst, P. E. J . Am. Chem. Soc. 1956,78, 3493. (b) Kosower, E. M. J . Am. Chem. Soc. 1958,80,3253. (c) Davis, K. M. C. "Solvent Effects on Charge-Transfer Complexes" in Molecular Association; Foster, R., Ed.; Academic: London, 1975. (d) Reichardt, C. Solvent and Solvent Effects in Organic Chemistry, 2nd ed.; VCH Publishers: New York, 1988.
0 1990 American Chemical Society
8484 The Journal of Physical Chemistry, Vol. 94, No. 22, I990
A
Bockman et al. TABLE I: Bond Distances (Angstrom) and Bond Angles (Degrees) in Ac+I-
?
Bond Lengths
B
N-C(I) N-C(14) C(l)-C(6) C(3)-C(4) C(5)-C(6) C(7)-C(8) C(8)-C(13) C(I0)-C(I1) C(12)-C(13)
n
1.380(8) 1.492(8) 1.419 (8) 1.419(10) 1.420 ( I O ) 1.388 ( I O ) 1.412 (8) 1.415 (11) 1.400 (10)
N-C( 13) C( 1)-C(2) c~c(3) C(4)-C(5) C(6)-C(7) C(8)-C(9) C(9)-C(IO) C( I I)-C(12)
1.393 (9) 1.395 (11) 1.334 (11) 1.355 (12) 1.414 (11) 1.418( 1 1 ) 1.356 (12) 1.343 ( 1 3)
Bond Angles
C(ll-N-CII3) C(I3)-N-C(14) N-C( I)-C(6) C(I)-C(2)-C(3) C(3)-C(4)-C(5) C(I)-C(6)-C(5) C(5)-C(6)-C(7) C(7)-C(8)-C(9) C(9)-C(8)-C(13) C(9)-C(lO)-C(ll) C(ll)-C(l2)-C(l3) N-C(13)-C(12)
-0
O
e
"e
118.7 (6) 122.9 (5j 118.7 (6) 120.9(8) 119.9 (6) 119.8 (6) 120.2 (6) 120.2(7) 120.2 (7) 123.1 (8) 118.6 (6) 120.0(7)
A
Gs0
122.8 ( 5 ) C(lkN-C(I4) 118.3 i5j NlC(l)-C(2)' 118.3 (6) C(2)-C(I)-C(6) 121.2 (6) C(2) 1.8 V in their one-electron oxidation potentials The same spectral distinction was also apparent in the orange Kosower salt CMP'I- relative to the colorless CMP+03SCF3-,as differentiated by the well-resolved visible band in the former with ,A, = 445 nm in Figure 3B. The assignment of the new visible bands to interionic charge-transfer absorptions accords with Mulliken t h e ~ r y ,since ~ , ~ the red shift of the absorption band of A C T at -480 nm relative to that of C M P T a t 445 nm follows the difference of 0.36 V in the reduction potentials of the corresponding acceptor cations (vide supra.)15 Accordingly, A C T and C M P T are hereafter distinguished (from the triflates) as charge-transfer salts. It is noteworthy that the local excitation of the cationic moiety in these charge-transfer salts (e.g., &(A&) = 365 nm and A,,,(CMP+) = 270 nm) was the same as those found in the triflate salts. In this regard, the local bands of the cationic acceptor and the charge-transfer band of the salt are overlapping in cobaltocenium iodide. Thus Figure 3C shows that the prominent, resolved bands at A, 405 nm in the absorption spectrum of Cp,Co+I- and Cp2Co+PF6-at the same concentration in dichloromethane differ only in a slight band broadening [ A B (fwhm) = 1300 cm-I] and absorbance increase (AA = 0.1 M-' cm-I). Solvent and Salt Effects on the Absorption Spectra of Charge-Transfer Salts. The highly colored solutions of the charge-transfer salts in dichloromethane were completely bleached in water. This dramatic change is illustrated in Figure 4 by the disappearance of the charge-transfer band at A, = 445 nm for C M P T and Ash = 490 nm for A C T . The same color loss occurred in polar aprotic media such as acetonitrile, as shown by the disappearance of the charge-transfer band in Figure 4A. The marked variation in the charge-transfer absorbance in Figure 4 was readily ascribed to changes in the concentration of the C T contact ion pair (CIP) by ionic dissociation into solvent-separated ion pairs (SSIP) and free ions (FI).23 As such, the reversible equilibria between CIP and SSIP (FI) can be described by a dissociation constant Kclp applied to a particular solvent and temperature, the quantitative effects of which were evaluated spectrophotometrically by measuring the charge in the C T absorbance ACT at various concentrations C of the chargetransfer salt from the relationshipZS
-
ACT
KcIp = CCT
cCCT +- 2c
ACT
(1)
(22) For example, the irreversible anodic wave for iodide at E , = 0.47 V vs SCE at u = 500 mV s-I is shifted to E, > 2.3 V in MeCN containing 0.1 M tetrabutylammonium hexafluorophosphate. (23) Note that this measurement makes no distinction between SSIP and FI or of ion triplets, etc. For a discussion, see ref 24.
Bockman et al.
8486 The Journal of Physical Chemistry, Vol. 94, No. 22, 1990 0.50
au,
ado
azo
am
a io
a io
ao
0.0
w
B V A W W 0
IAVELEN[;TH (n3
VAVELEH;TH h)
Figure 4. Solvent effects on charge-transfer salts of 1 X IO-) M. (A) Ac+I-, (B) CMP'I-, and (C) Cp&o+I- in dichloromethane (-), water acetonitrile (-.-), and acetone (- - -).
B
A
L
w
(e-),
C
I
B
F
WAVUWGTH M
UAYEWCTH 0
MA-
W
Figure 5. Salt effects on the charge-transfer salts ( A ) A&-, (B)CMP+I-, and (C) Cp2Co+I-dissolved as 1 X IO-) M solutions in dichloromethane with (top-to-bottom) 0. I , 2, 4, and 8 equiv of added Bu,N+C104-.
The value of the dissociation constant KCrp= 3 X 10-6 M for A C T with ccIp = 920 M-I cm-' a t 530 nm compares with a value of Kelp = 2 X IO-' M for CMP+I- with tCIP= 1510 M-' cm-I at A,, of the charge-transfer band. The addition of small amounts of inert salts such as tetra-nbutylammonium perchlorate (TBAP) to solutions of chargetransfer salts also induced large changes in the intensity of the C T absorption bands, as illustrated in Figure 5 . The monotonic decrease in the CT absorbance with increasing amounts of TBAP was characteristic of the facile competition for the contact ion pair, i.e.24 A C T + Bu4N+CI0,-
KEX
Ac+C104- + Bu4N+I-
(2)
Such an ionic exchange effectively served to disassemble the intimate charge-transfer ion pair (CIP) without recourse to solvent variation as in SSlP and FI formation described above, and it is largely governed by electrostatic^.^^ Solvent and salt effects were observed on cobaltocenium iodide merely as absorbance changes of the unresolved principal band = 405 nm. Indeed the magnitude of the diminutions in at A,, Figures 4C and 5C is similar to that shown in Figure 3C in its principal reflection of the C T transition moment of the CIP since the values of KCrpand KEXare expected to be similar to those of (24) Bockman, T. M.; Kochi. J. K. J . Am. Chrm. Soc. 1989, 111,4669. ( 2 5 ) Drago, R. S.: Rose, N. J . J . Am. Chem. SOC.1959, 81, 6138.
0
0 " . -1
O
D
.
.
!
.
L :::: .*
a
.
.
,ursyi):::
12
14
Ib
O
D
D
D 0
18
22
20
E, n e r g y
,
21
kK
2b
28
':::e*>
Figure 6. Typical experimental shift of the charge-transfer band of C M P T in dichloromethane by compression from atmosphericpressure ( 0 )to IO kbar (0) at 25 OC. The ordinate is the normalized absorbance 1%
uo/o
Ac+I- and C M P V described above. Pressure Effects on the Absorption Bands of Charge-Transfer Salts in Solution. The compression of charge-transfer salts in solution was examined in dichloromethane owing to their extensive association as contact ion pairs in this medium. The typical shift of the charge-transfer band of CMP+I- with the application of pressure is illustrated in Figure 6,and the well-defined CT absorption band could be computer fitted to a single Gaussian with A,, = 444 nm at atmospheric pressure and A,, = 415 nm at
The Journal of Physical Chemistry, Vol. 94, No. 22, 1990 8487
Spectroscopy of Charge-Transfer Salts
A
B 2
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L
23 22 -
I
I
$
,
,
,
1 I
a
-
7
1
0
5
.!
x
cn a W
10
Pressure (kbor)
Figure 7. Pressure dependence of the charge-transfer band of (A) CMP'I- and (B) Ac+I- in dichloromethane evaluated at ,A, The data (A)of Ewald and Scudder (ref 26) in acetone are also included for comparison.
IO kbar (which was close to the upper limit of the solvent freezing). Accompanying this spectral shift was a corresponding decrease in the charge-transfer absorbance that typically diminished by roughly 50% upon increasing the pressure to 6 kbar. The monotonic blue shift of the absorption maximum of the CT band at various pressures is plotted in Figure 7A, together with that previously measured by Ewald and Scudder in acetone for comparative purposes.26 Figure 7A also shows that the same blue shift a plied to the compression data evaluated at Am = 0.5 (Le., Indeed the latter was useful in analyzing the effect of pressure on A C T since the hypsochromic shift of the CT absorbance maximum at 480 nm (Figure 7B) was progressively obscured by overlap with the local absorption of the acridinium acceptor. Compressional shifts similar to those observed in Figure 7 were also observed with C M P T in methyl acetate and A C T in methyl formate. Although the quantitative measurements were complicated by the low solubility of the charge-transfer salts in these nonpolar media, the blue shifts of the absorption bands similar to those in Figure 7 were unmistakable. The hypsochromic shift observed with C M P T in dichloromethane was reversible, and the CT absorption recovered fully upon pressure release from 6 kbar. However the slight hysteresis observed upon depressurization from 10 kbar may reflect the occurrence of an irreversible chemical process at higher pressures. Owing to the presence of unresolved CT bands in Figure 3C, CbCo+I- was examined over an extended range of pressures using three polymeric media. For example Figure 8 presents the spectral change accompanying the pressure increase from 6 to 142 kbar of CbCo+l- dissolved in poly(methy1 methacrylate), together with the computer-fitted Gaussian deconvolutions of the experimental CT envelopesm The pressure dependence of each of the Gaussian components always exhibited a red shift both in polystyrene and poly(methy1 methacrylate) (see Figures 9A and 9B). Such a consistent bathochromic change contrasted with the pressure behavior of Cp2Co+I- dissolved in poly(styrenesu1fonic acid), as shown in Figure 9C. It should be emphasized that poly(methy1 methacrylate) (PMMA) and polystyrene (PS) are both weakly (26) Ewald, A. H.; Scudder, J. A. J . Phys. Chem. 1972, 76, 249.
(27) When the maximum of the CT band cannot be clearly resolved, the value of A,.$ [Le., the wavelength at which the CT absorbance log ([,,/I) is equal to 0.51 has been used as a suitable alternative. For example, see: Tsubomura, H.; Mulliken, R. S. J . Am. Chem. Soc. 1960,82, 5966. (28) Hammack, W. S.; Drickamer, H. G.; Hendrickson, D. N. Chem. Phys. Lett. 1980. I S I , 469.
(0)and at
&.5( 0 ) .
-E -b 0
0
Energy
,
kK
Figure 8. Typical shift in the absorption band of Cp2Co+I- in poly(methyl methacrylate) upon compression from (A) 6 to (e) 142 kbar showing the gaussian components from the computer-fitted deconvolutions.
polar polymeric media as opposed to the highly ionizing poly(styrenesulfonic acid) (PSSA). As a result, Cp2Co+I- (dissolved as ion pairs in PSSA) are likely to exist as extensive aggregates in PMMA and PS to account for the contrasting red shift in the charge-transfer band. Pressure Effects on Crystalline Charge-Transfer Salts. The crystalline charge-transfer salts C M P T and A C T were initially examined in an alumina matrix by intimate grinding for the measurement of the diffuse reflectance spectra.29 Spectral (29) Cf. Takahashi,Y.; Sankarawman,S.; Kochi, J. K. J. Am. Chem. Soc. 1909, I 11, 2954.
8488 The Journal of Physical Chemistry, Vol. 94, No. 22, I990
Bockman et a].
A
C
i 30
i
" X
P u O
0
o o o o o 0 0
1
I
I
i
/Illlllllllljl/j SO
100
150
0
15
0
" " 1 " 1 ' 1 " ' ' 50 I00
150
50
100
Pressure ( kbor 1 Figure 9. Pressure dependence of the individual Gaussian components of the absorption band of Cp&o+I- in (A) polystyrene,
isn
( e )poly(methy1
m&hacrylate), and (C) poly(styrenesulfonic acid).
subtraction of the diffuse reflectance spectrum of the corresponding triflate salts resulted in a charge-transfer spectrum of A C T showing a resolved band at ,A, = 510 nm and a pair of partially resolved bands at ,A, = 340 and 480 nm for CMP+I-. Although these charge-transfer spectra correspond to those obtained in solution (compare Figure 3), a closer spectral inspection was allowed by the solid-state absorption spectra obtained with finely ground samples of crystalline charge-transfer salts uniformly suspended in a mineral oil matrix for transmission measurements through a diamond cell.3o The typical red shift in the absorption band of CMP+I- with increasing pressure in Figure 10 differs dramatically from the blue shift observed in solution (see Figure 7). Moreover the solid-state absorption band, unlike that observed in solution (Figure 6 ) , could only be deconvoluted by employing the three Gaussians shown in Figure IO-with the major components I and I1 in both CMP+I- and A C T showing the same pressure dependence of the red shift (Figure 11). The minor spectral component 111 grew in importance with pressure, but was relatively pressure insensitive. The absorption spectrum of a crystalline sample of Cp2Co+Idispersed in a cesium chloride matrix was successfully deconvoluted into a pair of major and 3 minor bands (Figure 12A) similar to that previously reported for C ~ , C O + P F ~ - .Unlike ~' the latter, however, all the bands in the absorption spectrum of Cp,Co+I(30) Jurgensen, C. W.; Drickamer, H. G. Phys. Reu. 1984, 830, 7202. Note the range of applied pressures differs in the solution and crystal studies owing to the different compressibilities of liquids and solids.28 Thus the change in density is the critical factor in assessing the change in intermolecular separations and not the pressure range. The latter in solution studies was limited by the solvent freezing. Experiments in frozen solutions, as suggested by one referee, were avoided in view of the uncertain fate (e&, crystalline, amorphous, solid solution, clathrate, etc.) of the ionic salt in pressure-frozen dichloromethane. (31) (a) Roginski, R. T.; Moroz, A.; Hendrickson, D. N.; Drickamer, H. G. J . Phys. Chem. 1988, 92,4319. For the theoretical interpretation of the five bands in the visible absorption spectrum of cobaltccenium ion in solution, see ref 45. (b) Alternatively, a referee suggests that the anomalous red-shift of Cp,Co+l- may arise from a solvent polarizability/solute dipole interaction. However we disfavor this explanation for two reasons. Firstly, the overall shift of the bands throughout the pressure range were found to be roughly comparable for PS and PMMA despite the considerable differences in their polarizabilities [Michel, P.;Dugas. J.; Cariou. J. M.;Martin, L. J. Mucromol Sci. Phys. 1986,825. 3791. Secondly, the interaction is expected to have the same dependence on Abcl [it., the difference in dipole moments between ground and excited states] and the same sign as the solvent dipole/solute dipole term responsible for the blue shifts of CT salts in fluid solutions. [See discussion by Nicol, M.; Wild, S. M. Yancey, J. J. Chem. Phys. 1973,58, 4350. Nicol, M. F. Appl. Specrrosc. Reu. 1974, 8, 1831. Polarizability changes between ground and excited states, although pertinent to the interpretation of piezochromism of nonpolar solutes [see: Nicol, M. J . Am. Opt. Soc. 1965,55. 1176. Robertson, W. W.; Weigang, 0.E.,Jr.; Matsen. F. A. J . Mol. Specrrosc. 1957, I, I ] is expected to be of minor importance in accounting for the shifts in the CT bands of salts.
A I
a 4/\ /
Energy , kK Figure 10. Typical shift in the solid-state CT band of C M P V (mineral oil) upon the pressure increase from 53 to 120 kbar showing the change in the Gaussian components obtained from the computer-fitted deconvolution.
red-shifted under compression (Figure 12B), although the magnitudes of the shifts were considerably smaller than those observed with the pyridinium iodides (Figure 11). A modest increase in the relative intensities of the low energy bands was also observed.
Discussion The well-defined 1:l crystalline salts from the pyridinium and cobaltocenium iodides CMP+I-, A&-, and Cp2Co+I- exhibit the desired properties for comparative studies of compression on charge-transfer salts in solution and in the solid state. I. Structures of Contact Ion Pairs in Solution from X-ray Crystallography. The X-ray crystallographic structures shown in Figures 1 and 2 delineate the interionic separations and the spatial cation/anion orientations that are pertinent to these crystalline charge-transfer salts. Indeed the close relationship between the diffuse reflectance spectra of crystalline salts with the absorption spectra of the salts dissolved in dichloromethane
I
r
( 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0
100
50
Prassuro
50
151:
100
Prsssuro
(
kbnr
)
Figure 11. Pressure dependence of the Gaussian components of the absorption spectra of (A) A C T and (B) C M P T suspended in mineral oil.
( 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0
sa
Pressure,
100
1SO
kbar
Figure 12. (A) Solid-state absorption spectrum of Cp,CotI- suspended in mineral oil. (B) Pressure dependence of the Gaussian components shown on the left.
underscores the essential unity of the charge-transfer transitions (hv,) in the solid state and in solution. We therefore conclude that the critical interionic separations rA+D' in Figures 1 and 2 are also pertinent to hvCTof these salts in solution. As such, the brightly colored solutions of charge-transfer salts derive directly from contact ion pairs (CIP)-closely related in kind to those defined by X-ray crystallography. Furthermore the chargetransfer spectrophotometry in eq 1 establishes such contact ion pairs to be the dominant species in dichloromethane; and the quantitative treatment of the primary salt effects demonstrates that the charge-transfer salts of the type examined in this study are purely ionic salts indistinguishable from the more commonly used electrolytes such as tetrabutylammonium perchlorate (TBAP) in eq 2.24 Since conductivity studies have shown that free ions not particularly important in di~hloromethane,~~ the pronounced hypochromic effect of added inert salts on the charge-transfer spectra serves as a useful and diagnostic probe for the relevant CIPs in aprotic solvents. 11. Compressional Effects on Contact Ion Pairs in Solutions. The application of pressure to condensed phases leads to decreased intermolecular and interionic separations with the resultant in(32) Gordon, J . E. Organic Chemistry of Electrolyte Solutions; Wiley: New York, 1975: p 375 ff.
TABLE 111: Pressure Dependence of the Charge-Transfer Spectra of C M P T and A C T in Dichloromethane charge-transfer EO' pressure shiftb salt (IO' cm-I) (cm-' kbar-I) u/aoc CMPtIAmaxd 22.5 +I50 0.73 A0.5d 17.1 +170 ha,F 23.3 +180 0.36 Amaxd
A0.5d
Ac'I19.1 17.1
+220
-
'Energy of band maximum (Amax) or of absorption at absorbance = 0.5 (&) at atmospheric pressure. bLinearized pressure shift of A,, or &,5 between 0 and 10 kbar. 'Ratio of absorbance at 6 kbar to that measured at 1 bar. CHIC], solution. 'In acetone solution from ref 26.
crease in charge-transfer force^.'^ Such compressional effects are expected to lead to the ground-state stabilization of ionic charge-transfer salts.34 Indeed this formulation is consistent with (33) As predicted by Mulliken, R.S. J . Am. Chem. Sof. 1952, 74, 81 1 . See also: Drickamer, H. G.; Frank, C. W. Electronic Transitions and rhe High Pressure Chemistry and Physics of Solids: Chapman and Hall: London, 1973: p 98 ff.
Bockman et al.
8490 The Journal of Physical Chemistry, Vol. 94, No. 22, 199r0
24
TABLE I V Pressure-DependentSpectra of Crystalline Charge-Transfer Salts Em,," band pressure shiftb fractionalC CT salt (IO' cm-') assignment (cm-I kbar-I) area change Ac'l22.9 So S l d -9.9 0.34 -2 1 0.39 19.0 CT So TIC -0 0.96 13.4 CMP'I23.0 CT' -20 0.99 19.1 CT -24 0.79 TIC -0 2.04 13.4
L
-C
m
0
$ 4
Peak position of Gaussian-reduced absorption maximum derived from the transmission spectrum at 5.5 kbar pressure. bLinearized pressure dependence of energy of absorption maximum as a function of pressure. Negative sign denotes red-shift. CAreaof each resolved peak relative to that at the highest pressure attained ( A c V = 85 kbar, CMP'I- = 140 kbar). dSee ref 40a. CVideinfra. /See ref 40b.
0
E W
.7
.e
.Q
1.0
€ -I / € + 2 Figure 13. Pressure-dependent changes in the charge-transfer band of CMP+I- plotted as change in the solvent polarity function of dichloroThe solvent-dependent changes in the excitation energy methane (0). of CMP'I- in the neat solvents pyridine ( b ) ,acetone ( q ) ,and nbutyronitrile (+) are included for comparison.
the spectral blue shift shown by CMP+I- and Ac'I- in Figure 7.26 The magnitudes of the pressure dependence of the charge-transfer bands in Table I11 are large and more or less comparable in C M P V and A c V . The charge-transfer bands of pyridinium iodides are also known to be quite solvent dependent-the transition energy (hvCT) of C M P V being the empirical indicator of solvent polarity on the Z scale.35 The blue shift of the charge-transfer band observed with increasing solvent polarity was attributed by Kosower to the preferential solvation of the ionic ground state of the iodide salt relative to the uncharged CT excited Solvent polarity can also be expressed as the functionflc) = (c - I)/(€ + 2 ) where c is the static dielectric constant of the ~ o l v e n t . ~For ~ , molecules ~~ such as dichloromethane with permanent dipole moments, the compressional change in c can be calculated from the modified form of the Clausius-Mossotti equation: (c - I ) / p ( t + 2) = constant, owing to the direct relationship of applied pressure with solvent compressibility ( p = density).38 The smooth blue shift of the charge-transfer band of C M P I - with the pressure-induced change of the solvent polarity function is illustrated in Figure 13. Most revealingly, the rigorous fit of the CT data obtained in three separate (neat) solvents (pyridine, acetone, and n-butyronitrile) to this relationship indicates that the compressional effect can be attributed solely to solventsolute interactions. As such, a separate pressure-induced change in the interionic separation of contact ion pairs in solution must be secondary to their ground-state (34) Since the intermolecular interactions in such CT salts are dominated by electrostatic forces, the effects of pressure can be considered primarily as a perturbation of the ionic ground state relative to the uncharged excited state, as previously elucidated by studies of solvatochromi~m.~~~ For the description of piezochromic band shifts as a subclass of solvatochromism, see ref 39c. (35) Kwower, E. M. J . Am. Chem. Soc. 1958.80, 3261. (36) Rettig. W. J . Mol. Srrucr. 1982,84, 303. See also: Kirkwood, J . G.; Westheimer, F. H. J. Chem. Phys. 1938, 6, 506. (37) Hammack, W. S.;Hendrickson, D. N.; Drickamer, H. G. J . Phys. Chem. 1989, 93, 3483. (38) (a) Debye, P. Polor Molecules,; Dover: New York, 1945. (b) Danforth, W. E.; Phys. Reu. 1931, 38, 1224. (e) Bridgman, P. W. h o c . Am. Acad. Scf. 1931, 66, 185. Note that the prominent role of solvent polarity in accounting for the pressure shifts in Figure 13 does not preclude other contributions (e.&. volume changes on excitation, density-indcpendent changs in the dielectric constant, etc.). Although the Clausius-Mossotti equation has not been directly verified for dichloromethane,Vedam [Vedam, K.; Limsuwan, P. J . Chem. Phys. 1978,69,4772] established its accuracy in the prediction of the dielectric constant (within 10%) of a wide range of solvents (from pentane to water). An inaccuracy of 10% in e could result in an error in Af) of at most 2%. which is clearly insufficient to affect the fit in Figure 13.
stabilization by increased solvent polarity. This conclusion also coincides with the substantial diminution in the C T absorbance observed at higher pressures (see Table III), as a result of CIP d i s ~ o c i a t i o nunder ~ ~ more polar circumstances. The importance of the direct interaction of the solute dipole with the solvent dipole for the C T transition of ion pairs is underscored by the large value of B in the McRae relationship398
2772
+1
where q is the zero-frequency refractive index. For C T salts of the type examined in this study, the values of B are unusually large and diagnostic of a dominance of dipolar contributions (signifying large dipole changes upon excitation).39b The McRae equation can be rewritten to emphasize the pressure dependence (Le., density) on the band shift as39c Av = K[(** - 1 ) / ( 2 v 2
+ I ) ] + Cp + Fp2
(4)
where C includes the dipole-dipole and dipole-polarizability contributions and F the polarizability-polarizability terms. Thus the linear relationship in Figure 13 is the natural consequence of Kosower's formulation14bof the spectral transition in chargetransfer salts. I l l . Compressional Effects on Crystalline Charge- Transfer Salts. In the solid state, the charge-transfer bands of polycrystalline C M P T and Ac'I- both shift to lower energies upon pressurization (Table IV). Although this trend accords with the usual behavior of CT molecular crystals from uncharged donor/acceptor pairs, it is singularly different from the blue shifts observed in solution. The distinctive behavior of charge-transfer salts in the solid state and in solution clearly arises from the different environments of the cation/anion pairs. Thus contact ion pairs are primarily relevant to charge-transfer salts in solution, and pressurization increases the individual dipolar coupling to the solvent dielectric and results in the ground-state stabilization of CIP. On the other hand in the solid state, the individual cationlanion interactions leading to C T stabilization (blue shift) are difficult to consider separately from the neighboring cation/cation and anion/anion interactions resulting in CT destabilization (red shift). Indeed, the crystal structure of Ac'I- in Figure 1B shows the juxtapositions of A C T , Ac+Ac+, and 1-1- pairs in the unit cell. Thus the packing diagrams indicate that crystals of Ac'Iconsist not only of discrete C T ion pairs but also of stacks of acridinium cations and iodide anions that are partially overlapping.4i The application of pressure (isotropically) apparently (39) (a) McRae, E. G. J . P h p Chem. 1957,61,562. (b) For example, B = 7900,14000 and 16000 cm- for CMPI-, Q+Co(CO), and Q+V(CO),-, respectively (where Q+ = N-methylquinolinium), M. S.Paley, unpublished results. (c) Robertson, W. W.; King, A. D., Jr. J . Chem. Phys. 1961.34, 151 I . (40) (a) Bendig, J.; Kreysig, D.; Kawski, A. Z . Naturforsch. 1981, 36A, 30. (b) The pair of bands at E, = 23.0 and 19.1 cm-' show parallel behavior upon compression and may arise from solid-state effects on the CT transition. Note CMP'OTf shows no absorption in this region.
The Journal of Physical Chemistry, Vol. 94, No. 22, 1990 8491
Spectroscopy of Charge-Transfer Salts Pressure-Dependent Spectrum of Crystalline C ~ , C O ~ I - ~ pressure shift' fractionald ( 10' cm-') assignment (cm-' kbar-lI area change
TABLE V
Ems?
~
2 7 4 0 (26.5) 24.20 (24.2) 21.06 (19.5) 17.90 (16.4) 14.5 (13)
'AI, 'A,, IA,, 'A,, ,A,,
----
'E, a !E,, b 3E18
)E2, a JE,,
-7.8 -8.5 -6.9 -4.8 -2.3
(+4.2) (+6.2) (+7.5) (+4.3) (0)
1.06 0.86 1.41 1 .84 2.10
Dispersed in CsCI. bGaussian-resolved absorption maximum derived from transmission spectrum at 5 kbar. Absorption maxima and pressure shifts of the corresponding bands of Cp,CotPFC from ref 31 in parentheses. Linearized pressure dependence of the absorption maximum. dRatio of the area of the resolved band at 120 kbar to that at 5 kbar.
favors the destabilizing interactions. In this regard, it is relevant to note the strong red shift of the charge-transfer band of K+TCNQ-, which also has a stacked structure of ions4, In addition to the marked piezochromic shift of the chargetransfer bands to lower energies upon compression, the absorption tails of C M P T and A&- increase in importance relative to the other transitions (see Table IV). These low energy absorptions, resolved into bands at 13 400 cm-' in both A C T and CMP'I-, are tentatively assigned to spin-forbidden S TI transitions43 since they are expected to be more important in iodide than triflate salts owing to the increased spin-orbit coupling in the heavier atom-especially upon the compression of these charge-transfer salts.44 I V. Cryptic Charge-Transfer Bands: Compressional Behavior of Cobaltocenium Iodide. The five resolved bands in Figure 12 of solid-state spectrum of Cp2Co+l- bear a strong resemblance to those recently reported31 for the spectrum of Cp2Co+PF6--with the exception that they are all shifted to higher energies. The partial spectral assignments of the d-d transitions of C P , C O + ~ ~ in Table V were based on a simple extrapolation to the chargetransfer salts, since the solution studies in Figures 3 and 5 showed no clear spectral resolution of the charge-transfer bands of Cp2Co+I- from ligand field bands of the cobaltocenium cation. Most importantly, the consistent red shift of all of these bands in Cp,Co+l-, as illustrated in Figure 12, is in marked contrast to the consistent blue shift of the bands previously documented for Cp2Co+PF6-.31 The spectral shifts to lower energies upon the compression of Cp,Co+I- undoubtedly arises from the same structural considerations on the charge-transfer bands that apply to C M P I - and Ac+I- (vide supra)-the effect of pressure leading to an increase in the ground-state energies of the cationic cobaltocenium stacks and resultant lowering of the ionic-to-neutral CT transitions. Indeed this qualitative description receives support from the X-ray crystal structure of Cp,Co+I- in Figure 2B showing the presence of contiguous stacks of Cp,Co+ in a tetragonal unit cell. The dominance of the cation-cation interactions relative to the stabilizing cation-anion interactions is underscored by the sizeable separation of 4.75 A between Co and I centers relative to the interannular separation of 3.52 A between Cp ligands. Be that as it may, the charge-transfer forces in Cp,Co+I- are not
-
(41) According to this qualitative formulation, the difference between pressure effects on CT salts in solution and in the solid state result from the dominence of ion-pair (dipolar) interactions with solvent in contrast to ionpair/ion (multipolar) interactions within the crystal lattice. Although the X-ray structures in Figures 1B and 2B help to identify the latter, the quantitative assessment of each interaction sufficient to allow the net (destabilizing) effect to be predicted is a difficult computational problem. For a discussion, see: Silinsh. E. A,, in ref 12a, p 96 f. (42) Tkacz, M.; Jurgensen, C. W.; Drickamer, H. G. J. Chem. Phys. 1986, 84, 649. Tanaka, J.; Tanaka, M.: Kawai, T.; Takabe, T.; Maki, 0. Bull. Chem. Soc. Jpn. 1976,49, 2358. (43) (a) Scott, D. R.; Becker, R. S.J . Chem. Phys. 1961,35, 516. Note the use of ethyl iodide as a solvent for the enhancement of singlet to triplet transitions in ferrocene. (b) Birks, J. B. Phorophysics of Aromatic Molecules; Wiley: London, 1970: p 211 f. (44) See: Stephens, D. R.; Drickamer, H. G. J . Chem. Phys. 1961, 35, 429. (45) Sohn, Y. S.;Hendrickson, D. N.; Gray, H. B. J . Am. Chem. Soc. 1970, 92, 3233. See also: Rukhlyada, N. N.; Leonova, E. V.: Kochetkova, N. S.;Bychkov, N. V. Isv. Akad. Nauk SSSR, Ser Khim. 1983, 1325.
TABLE VI: Pressure-Dependent Spectra of CpzCotI- in Polymeric Media polymeric E,,/ pressure shiftb fractionalc solvent (10' cm-I) (cm-l kbar-I) area change PMMA~ 29.16 -1 5 1.92 26.48 -22 1.57 24.33 -28 1.55 20.53 -20 2.26 PS' 29.43 -2 1 1.47 26.53 -23 1.04 24.17 -27 2.00 -1 1 2.89 20.06 PSSAf 27.47 +9.2 1.39 24.64 +6.3 1.58 19.68 +2.8 a Gaussian-resolved absorption maximum at low (PMMA, 5 kbar; PS, 8 kbar; PSSA, 7 kbar) pressure. Linearized pressure dependence of the shift of the absorption maximum. cRatio of the area of the resolved Gaussian absorption band in the normalized spectrum at the highest pressures attained (PMMA, 142 kbar; PS, 134 kbar; PSSA, 133 kbar) to that at low pressure. dPoly(methyl methacrylate). Polystyrene. fPoly(styrenesu1fonic acid).
sufficient to generate new, resolved absorption bands. Nonetheless they are sufficiently affected by interionic compression to markedly alter the trend of the d-d transitions of ~ o b a l t o c e n i u m . ~ ~ - ~ ~ The spectral studies of Cp,Co+I- in polymeric media were designed to probe the behavior of individual ion pairs in solution, but the consistent red shift observed upon compression in Figures 9A,B differs from the expected shift to higher energies, as found for CMP+I- and Ac+I- (Figure 7). This anomalous behavior may arise from ionic aggregates showing crystal-like behavior, especially in such nonpolar media as polystyrene and poly(methy1 methacrylate). In contrast the polar polymer, poly(styrenesu1fonic acid), can act as a true solvent (owing to the presence of highly ionizable ArS03H groups) and the absorption bands of Cp2Co+Iundergo shifts to higher energies in this medium (Table VI), and it is reminiscent of that previously observed in crystalline C~,CO+PF~-.~'
Experimental Section Materials. Cobaltocene was prepared according to King.47 Acridine (Eastman) and methyl isonicotinate (Aldrich) were converted to their methiodides with methyl iodide according to the method of VogeL4* Ac+I- and CMP'I- were both recrystallized from a mixture of acetonitrile and ethyl acetate prior to use. Cp2Co+I-. To a suspension of cobaltocene (2.98 g, 16 mmol) in benzene (30 mL) under an argon atmosphere was added dropwise a solution of iodine (2.0 g, 7.8 mmol) in benzene (100 mL). The suspension was stirred for 2 h and filtered in air. This precipitate was recrystallized twice from a mixture of acetonitrile and ethyl acetate to yield 2.63 g (53%) of yellow crystals. Samples were prepared for X-ray crystallography by dissolving 10 mg into acetonitrile contained in a small vial. The vial was placed inside a 1 0 0 " round-bottom flash containing some diethyl ether and the capped flask was allowed to stand. Vapor diffusion at room temperature produced golden-yellow plates that were mounted for X-ray crystallography. IR (KBr): 3058 (s), 1824 (m), 1414 (s), 1113 (s), 1065 (br), 859 (s), 500 (s), 462 (vs) ~m-I.4~ The polymeric media consisting of polystyrene (PS,Aldrich, MW
-
(46) (a) For the mixing of CT and local bands, see, for example: Vanquickenborne, L. G.; Haspeslagh, L.; Hendrickx, M. in Excited States and Reactive Intermediates; Lever, A. B. P.,Ed.; American Chemical Society, Washington, DC, 1986; p 12. See also Wright, J. D. in ref 12b. (b) Note that irradiation of the local bands of CT complexes often gives rise to the same photochemistry as the direct irradiation of the CT band. See: Hilinski, E. F.; Rentzepis, P. M. J . Am. Chem. Soc. 1985,107,5907. Sankararaman, S.; Perrier, S.;Kochi, J. K. J . Am. Chem. SOC.1989, I I I , 6448. (47) King, R. B. Organometallic Syntheses; Academic: New York, 1965; Vol. I , p 70. (48) Vogel, A. I. Textbook ofPractical Organic Chemistry; Wiley: New York, 1956. (49) Van den Akker, M.;Jellinek, F. Rec. Trav. Chim. 1971, 90, 1101.
8492
The Journal of Physical Chemistry, Vol. 94, No. 22, I990
9000), poly(methy1 methacrylate) (PMMA, Aldrich, MW 12000), and poly(styrenesu1fonicacid) (PSSA, Polysciences, MW 70000) were used as obtained. Instrumentation. Infrared spectra were measured on a Nicolet IODX FT-IR spectrometer. All measurements of liquid samples were made with 1 .O mm NaCl cells. Solvent absorptions were compensated for by digital subtraction. Solid samples were measured in KBr disks. Solution UV-vis spectra were recorded on a Hewlett Packard 8450-A diode-array spectrometer. Crystalline CT salts were recorded as 10% dispersions in silica gel. The spectra were recorded on a Perkin-Elmer 330-A spectrophotometer equipped with a Hitachi H210-2101 integrating sphere accessory using an alumina disk as the reference. ' H N M R spectra were recorded on a JEOL FX90Q FT NMR spectrometer operating at 89.55 MHz. UV-Vis Spectroscopy of Charge-TransferSalts. Solvents for spectrophotometry were purified and stored under an argon atmosphere. Acetonitrile (Fisher) was allowed to stand with 0.1% KMn04 overnight, and the mixture was refluxed for 1 h. The brown Mn02 residue was removed by filtration. The acetonitrile was fractionally distilled through activated (Woelm Super-]) alumina and degassed by three freeze-pump-thaw cycles. Dichloromethane (Baker) and chloroform (Mallinckrodt) were each distilled from P20! and redistilled from CaH,. Acetone (Fisher) was fractionally distilled from P205. Tetra-n-butylammonium perchlorate (TBAP, Aldrich) was recrystallized from ethyl acetate and hexane and dried overnight at room temperature in vacuo. All spectroscopic measurements were made under an argon atmosphere in a IO-mm UV cell equipped with a Teflon valve. Salt Effects on the Solution Spectra of Charge-Transfer Salts. A solution of the C T salt in CH2CI2(2 X M) was prepared in a I-cm quartz cuvette. The UV-vis spectrum was acquired, an aliquot of a stock solution of TBAP in CH2CI2(0.2 M) added, and the spectrum was remeasured. Since the total volume change arising from the multiple additions of the stock solution was less than IO%, no correction was applied. Results were plotted as the relative decrease in the absorbance at the C T maximum (AcT)/(AcT)o versus the mole fraction of C T salt. Dissociation Constants of Charge-Transfer Salts. The method of Drago and RoseZSas modified by Schramm and ZinksOwas used for the determination of Kclp in eq 1. Concentrations of the C T salts that ranged from 5 X IO4 to 1 X M in CH2C12were prepared and the spectra recorded. Curves were plotted for the apparent dissociation constant as a function of trial values of the extinction coefficient according to the modified equation, where ACT represents C T absorbance at A,, C represents the concentration of the C T salt, and tClprepresents the trial values of the extinction coefficient. The intersections of the curves were averaged and reported as tCT and KcIp. Spectroscopic Measurements under Pressure. Pressure-dependent spectra utilized a spectrometer described earlier.30 Light from a tungsten-halogen lamp (Oriel 6333; 100 W) passed through a monochromator (Kratos GM 252 equipped with a 500-nm blaze grating) and it was focussed onto the cell with a quartz light guide. Transmitted light was passed from the back face of the cell and was guided to a photomultiplier tube (EM1 9558QA) cooled to -60 "C. An EGG Ortec 995 dual photon counter interfaced with an IBM PC-XT microcomputer recorded the signal from the photomultiplier tube. Solid-state (crystal and polymer) pressure-dependent spectra were obtained using a gasketed diamond-anvil Pressures were determined by the ruby-fluorescence method. Monochromatic light from an Omnichrome OM 439-38 He-Cd laser (441.7 nm) was focussed onto a ruby incorporated into the sample. Luminescence from the ruby was passed through a monochromator (Spex 1672) and then focussed onto a photomultiplier (EM1 9593) equipped for photon counting. Counts were recorded every 0.5 A and the pressure was determined according to the known linear pressure dependence of the emission maximum.s2 Pressure-dependent solution spectra (50) Schramm, C.; Zink, J . I. J . Am. Chem. Soc. 1979, 101, 4554. ( 5 1 ) See Jayaraman. N . Reu. Sci. Insrumen. 1986, 57, 1013.
Bockman et al. were recorded by placing the dichloromethane solution in a stainless-steel inner cell with pistons incorporating sapphire windows.s3 The cell was placed inside a larger bomb, which was filled with the pressure-transmitting fluid (isobutyl alcohol). The bomb was pressurized by using a hydraulic pump and intensifier previously calibrated using a manganin gauge. The change in the dielectric constant of CHzC12with pressure was obtained from the Clausius-Mossotti (vide supra) where c is the static dielectric constant and p is the density. The constant, evaluated at atmospheric pressure, was 0.5 12, consistent with the value for other nonpolar solvents.s4 The density of CH2C12 at various pressures was obtained from the compressibility data of Bridgman.ss Infrared spectra of crystalline Cp2Co+I- in mineral oil were acquired by using a pressure cell equipped with infraredtransmitting type IIa diamonds. The FT-IR spectrometer (Nicolet 7199) was equipped with an 800 cm-l HgCdTe detector and a Perkin-Elmer 4: 1 beam condenser. Preparation of Samples for Spectroscopy. Crystalline CT salts were ground with mineral oil ( A C Tand CMP+I-) or with CsCl (CpZCo+I-)prior to loading into the gasket of the diamond cell. Polymer films containing C T salts were prepared by dissolving the appropriate weight of the salt in methanol (for PSSA film) or dichloromethane (for PMMA or PS) and adding the polymer (for a 5% W:W concentration of salt to polymer). The solvent was evaporated on a glass slide. The transparent film was broken into pieces and loaded into the gasket of the diamond cell. Infrared Spectrum of Cp2Co+Tat High Pressure. The infrared spectrum of Cp2Co+I- was quite similar to that of ferrocene and the pressure dependence of the IR absorptions was practically identical. Since the enhancement of the symmetrical stretch v 5 with pressure was qualitatively the same as that observed for Cp2Fe,s6it was plausibly ascribed to coupled vibrations of paired CpZCo+ moieties in the unit cell. Such an arrangement was confirmed by X-ray crystallography (Figure 2). The detailed similarity of the ferrocene and cobaltocenium iodide spectra both at atmospheric pressure and at enhanced pressures indicated that the perturbation of the energy levels of Cp2Co+ by I- was not accompanied by specific counterion effects on the vibrational energy levels. X-ray Crystallography of Charge-Transfer Salts. N Methylacridinium Iodide (Ac+I-). A large, strawberry colored prismatic block having approximate dimensions 0.28 X 0.44 X 0.74 mm was mounted in a random orientation on a Nicolet R3m/V automatic diffractometer. The radiation used was Mo Kcu monochromatized by a highly ordered graphite crystal. Final cell constants, as well as other information pertinent to data collection and refinement, were space group PI, triclinic; cell constants a = 7.442 (2), b = 8.368 (2), and c = 10.316 (3) A; a = 105.22 (2)', p = 91.18 ( 2 ) O , y = 99.09 (2)'; V = 61 1 A3; molecular formula C,,H,,N+I-; formula weight 321.17; formula units per cell 2 = 2; density p = 1.75 g absorption coefficient p = 26.6 cm-I; radiation (Mo Ka) X = 0.71073 A; collection range '4 < 28 < 50'; scan width A0 = 1.20 + (Ka2 - Ka,)'; scan speed range 2.0 to 15.0' min-I; total data collected 2148; independent data f > 3u(I), 2038; total variables 149; R = ZllFol - ~ F c ~ ~ / Z ~ F o ~ , 0.035; R, = [Zw(lFJ - lFc1)2/ZwlFolZ]i/2, 0.043; weights w = C ( F ) - ~ The . Laue symmetry was determined to be 1,and the space group was shown to be either PI or Pi. Intensities were measured by using the D scan technique, with the scan rate depending on the count obtained in rapid pre-scans of each reflection. Two standard reflections were monitored after every 2 h or every 100 data collected, and these showed no significant variation. In reducing the data, Lorentz and polarization corrections were applied, as well as an empirical absorption correction based on (52) Piermarini, G. J.; Block, S.; Barnett, J . D. J . Appl. Phys. 1975, 46, 2774. Note that pressure measurements in diamond anvil cells commence at 5-10 kbar owing to the initial compression inherent to the loading. (53) Okamoto, B. Y . Ph.D. Thesis, University of Illinois. 1974. (54) Salman, 0. A.; Drickamer, H. G.J . Chem. Phys. 1982, 77, 3329. (55) Bridgman, P. W . Proc. Am. Acad. Arts Sci. 1948, 76, 72. (56) Roginski, R. T.; Shapley, J . R.; Drickamer, H. G . J . Phys. Chem. 1988, 92, 4316.
The Journal of Physical Chemistry, Vol. 94, No. 22, 1990 8493
Spectroscopy of Charge-Transfer Salts TABLE VII: Final Atomic Coordinates ( X l o ' ) and Equivalent Isotropic Displacement Parameters (A2 X Id)for the Non-Hydrogen Atoms' A&-
I N C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(I0) C(11) C(12) C(13) C(14)
I co C(1) C(2) C(3) C(4) C(5)
-1535 ( I ) 3197 (7) 3147 (8) 3616 (IO) 3480 (IO) 2954 (10) 2552 (IO) 2624 (9) 2151 (9) 2159 (8) 1616 (IO) 1620 (10) 2156 (11) 2687 (IO) 2690 (8) 3692 (12)
5864 (1) 8268 (6) 8184 (8) 6838 (8) 6769 (IO) 8112 (IO) 9476 (9) 9544 (8) 10926 (8) 10940 (8) 12289 (9) 12287 (10) 10913 (11) 9612 (IO) 9596 (8) 6822 (9)
0 0 601 (33) -1354 -1190 807 1918
cp2c0+10 0 1437 (79) 600 -1481 -1 963 -148
7291 ( I ) 7251 (5) 8568 (6) 8984 (7) 10257 (7) 11248 (7) 10903 (7) 9544 (6) 9148 (7) 7806 (6) 7392 (8) 6077 (8) 5133 (8) 5479 (7) 6833 (6) 6211 (7)
0 5000 3485 (33) 347 1 3594 3684 3630
45 (1) 35 (2) 36 (2) 45 (3) 53 (3) 53 (3) 48 (3) 38 (2) 41 (2) 38 (2) 50 (3) 58 (3) 59 (3) 50 (3) 37 (2) 56 (3) 79 51 34 34 34 34 34
(1) (1)
(5) (5) (5) (5) (5)
"Equivalent isotropic U defined as one-third of the trace of the orthogonalized U, tensor.
9 scans of nine reflections having X values between 70 and 90°. The structure was solved by interpretation of the Patterson map, which revealed the position of the iodine atom. The remaining non-hydrogen atoms were located in subsequent difference Fourier syntheses. The usual sequence of isotropic and anisotropic refinement was followed, after which all hydrogens were entered in ideal calculated positions and constrained to riding motion, with a single variable isotropic temperature factor. The methyl group was treated as a separate rigid body and allowed to rotate freely. After all non-hydrogen shiftlesd ratios were less than 0.2, convergence was reached at the agreement factors listed above. No unusually high correlations were noted between any of the variables in the last cycle of full-matrix least squares refinement, and the final difference density map showed a maximum peak of about 0.6 e A-3 located near iodine. All calculations were made by using Nicolet's SHELXTL PLUS (1987) series of crystallographic programs. The final atomic coordinates are listed in Table VII. Cp2Co+l-. A large orange-yellow square plate (0.45 X 0.45 X 0.12 mm) was mounted as described above. The final cell constants and other crystallographic data were space group 1422 (tetragonal), with cell constants a = 6.719 (2) and c = 11.668
(4) A; V = 527 AS,molecular formula CloHloCoI;fw = 316.03;
Z = 2; p = 1.99 g ~ m - p~ =; 44.7 cm-I; I > 3 4 0 , 2 1 8 ; R = 0.069; R, = 0.069. The Laue symmetry was determined to be 4/mmm, and from the systematic absences noted the space group was shown to be I422,14mm, Iam2, or I4lmmm. Since the cell constants indicated only two molecules per unit cell, it was immediately evident that in any of the above space groups the cyclopentadienes would have to be heavily disordered. Therefore after data collection a second, much thinner plate was mounted in order to verify the cell constants and Laue group. The axial length was verified by doubling the length of each axis and remeasuring the intensities of the low-angle reflections at slow scan rates. Since none of the "odd" reflections were observed (considered at the level of 3u), the presence of a superlattice was deemed unlikely. Intensities were measured by using the 820 scan technique. In reducing the data, Lorentz and polarization corrections were applied, as well as an empirical absorption correction based on 9 scans of seven reflections having X values between 70 and 90°. Since all five possible space groups presuppose heavy disorder, 1422 was arbitrarily chosen for initial refinement. The structure was solved by use of the SHELXTL direct methods program, which revealed the positions of the Co and I atoms. The unitary structure factors displayed acentric statistics, although little weight was placed on this due to the suspected disorder. The Cp carbon atoms were located in subsequent Fourier syntheses and were found to be close to the c axis, which means that both rings have to be disordered over at least four different positions about a common centroid. Essentially, the rings appear as toroids of electron density, and thus for the sake of convenience a rigid body model was used, with each carbon having a population factor of 25%. Refinements in each of the other four space groups resulted in almost identical R values; however the planes of the two Cp rings about Co showed somewhat larger deviations from parallelism than in the I422 refinement. Therefore the final refinement was made in 1422 although none of the other four space groups can be categorically excluded. After all shiftlesd ratios were less than 0.1, convergence was reached at the agreement factors listed above. The only unusually high correlation noted between any of the variables in the last cycle of full-matrix least-squares refinement involved the z coordinate of the rigid body Cp ring and one of its rotational variables. The final difference density map showed a maximum peak of about 1.0 e A-3 located quite close to the iodine atom (Table VII). Acknowledgment. We thank J. D. Korp for crystallographic assistance and the National Science Foundation, Texas Advanced Research Program, and the Robert A. Welch Foundation (J.K.K.) and the Materials Science Division of the Department of Energy (under contract no. DE-AC02-76ERO-1198 to HGD) for financial support. Registry No. 11087-17-5.
CMP'I-,
7630-02-6; A&-,
948-43-6; Cp2Co+I-,
