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Slipped versus zig-zag TCNQ stack upon steric control in conducting 1:4 diphosphonium-TCNQ salts
Synthetic Metals, 10 (1985) 415 - 425
415
SLIPPED V E R S U S ZIG-ZAG TCNQ STACK UPON STERIC CONTROL IN CONDUCTING 1:4 DIPHOSPHONIUM-TCNQ SALTS P. BATAIL, L. OUAHAB*, J.-F. HALET and J. PADIOU Laboratoire de Chimie du Solide et lnorganique Moldculaire, L.A. 254, Universitd de Rennes I, campus de Beaulieu, 35042 Rennes (France)
M. LEQUAN and R. M. LEQUAN Laboratoire de Recherche de Chimie Organique, Ecole Nationale Supgrieure de Chimie de Paris, 11, rue Pierre et Marie Curie, 75231 Paris Cddex 05 (France)
(Received April 16, 1984; in revised form December 25, 1984; accepted January 15, 1984) Abstract The structural and conducting properties of two 1:4 charge transfer diphosphonium-TCNQ salts, prepared by reaction of the corresponding iodides with TCNQ in hot acetonitrile, are compared. The diphosphoniumparaphenylene (DPPP 2+) salt resembles the classical analog 1:2 salt tetraphenylphosphonium-TCNQ, TPP(TCNQ)2, with slipped stacks and normal semiconducting behaviour. By contrast, the diphosphoniumcyclohexadiene (DPCH 2+) salt exhibits zig-zag TCNQ stacks and presents a phase transition at 205.5 K. This is described and discussed with special emphasis on a possible analogy between DPCH(TCNQ)4 and N-methyl-N-ethyl-morpholinium MEM(TCNQ)2, which has recently been thoroughly studied.
Introduction In the course of a general investigation of synthetic routes for the generation of organic iodides and subsequent reactions with electron acceptors, we recently prepared a series of diphosphonium 1:4 charge transfer salts [1] of TCNQ. This paper reports the structural and electrical properties of two materials, Dppp2+(TCNQ)42- and DPCH2+(TCNQ)42-, which achieve the same stoichiometry with comparable large diphosphonium dications.
DPPP
DPCH
*Permanent address: Institut de Chimie, Universit~ de Constantine, Algeria. 0379-6779/85/$3.30
Q Elsevier Sequoia/Printed in The Netherlands
416 Experimental Black needle-like crystals were grown from h o t solutions of TCNQ and the corresponding diphosphonium iodide in acetonitrile.
Electrical conductivity The d.c. electrical conductivities were measured by use of a standard four-probe technique. Gold wires 5 × 10 -2 mm in diameter were connect ed to the crystals with silver paint. The conductivity in both materials is highly anisotropic and much larger along the needle axis. Since the phase transition in DPCH(TCNQ)4 leads to cracking of the crystals, the experi m ent was re pr o d u ced for several crystals and the same consistent results were obtained.
Crystal structures X-ray r o tat i on photographs proved t hat the needle-shaped crystals are elongated along the ~ axis o f the centrosymmetrical triclinic unit cells of the DPPP 2+ and DPCH 2+ salts. The unit cell parameters (Table 1) were determined with a NONIUS CAD-4 a u t o m a t e d d i f f r a c t o m e t e r and the intensity data collection and re fin emen t results are summarized in Table 2. The structures were solved by a combination of direct m e t h o d s (MULTAN) and difference Fourier synthesis. The h y d rogen atoms were located on a Fourier map and introduced in the final cycle but n o t refined. The atomic parameters are given in Tables 3 and 4 [3].
Results and discussion In Figs. 1 and 2 we compare the single-crystal d.c. electrical conductivity along the needle axis and the pattern o f molecular overlap o f DPPP-
TABLE 1 Crystal data
a b c 7 v Z Dc
DPPP(TCNQ)4
DPCH(TCNQ)4
13.060(4) • 15.428(3) 8.060(3) 103.46(2)° 78.29(3) ° 92.40(3) ° 1650.5 h a 1 1.30
11.801(3) A 16.401(4) 8.753(3) 104.70(3)° 98.15(2)° 101.80(3)° 1570.2 A3 1 1.31
417 TABLE 2 Intensity data collection and refinement results Compound Formula Molecular weight Space group Crystal dimension (mm) Radiation Linear absorption coefficient (cm-1) Scan type Scan range (deg) 0 limits (deg) Data collected Unique data used
DPPP(TCNQ)a
P2N16CsoH46 1293.31 P1 0.45 × 0.18 × 0.04 Monochromatized MoK(~ 1.203 0/20 1.0 + 0.35 tan 0 1 - 25 6036 2209 0.036 0.044
R = ~,(I]Fol --IFelI)/Y, IFol R w = ( Y ~ w ( I F o l - IFcl)2/~,wFo2) 1/2
DPCH(TCNQ)4 P2N ~6C76H40 1239.22 P1 0.54 × 0.18 × 0.06 MoK(~ 0.73 0/20 1.0 + 0.45 tan 0 1 - 25 6235 2185 0.048 0.060
TABLE 3 Positional parameters with their estimated s.d. for DPPP(TCNQ)4 Atoms
x
y
z
P N(1) N(2) N(3) N(4) N(5) N(6) S(7) N(8) C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10) C(ll) C(12) C(13) C(14) C(15) C(16) C(17) C(18) C(19)
0.66435 (7) 0.4554 (3) 0.3579 (3) 0.3897 (3) 0.2802 (3) 0.1254 (3) 0.0107 (3) 0.2013 (3) 0.1114 (3) 0.3845 (2) 0.4079 (2) 0.3954 (3) 0.3603 (2) 0.3380 (2) 0.3498 (2) 0.3961 (3) 0.4288 (3) 0.3741 (3) 0.3472 (3) 0.3711 (3) 0.3107 (3) 0.1015 (3) 0.1370 (3) 0.1509 (3) 0.1289 (3) 0.0942 (3) 0.0807 (3) 0.0839 (3)
0.91565 (6) 0.2240 (2) 0.3897 (2) 0.6563 (2) 0.8224 (2) 0.6889 (2) 0.8585 (2) 0.2710 (2) 0.4419 (2) 0.4508 (2) 0.4421 (2) 0.5117 (2) 0.5963 (2) 0.6058 (2) 0.5359 (2) 0.3793 (2) 0.2934 (2) 0.3862 (2) 0.6679 (2) 0.6606 (2) 0.7533 (2) 0.6377 (2) 0.5523 (2) 0.4849 (2) 0.4962 (2) 0.5822 (2) 0.6499 (2) 0.7064 (2)
0.6500 --0.1103 --0.3945 0.7030 0.4341 1.0048 0.7395 0.1831 --0.1001 0.0170 0.1690 0.2996 0.2927 0.1414 0.0103 --0.1177 --0.1126 --0.2705 0.4288 0.5801 0.4293 0.5954 0.5985 0.4657 0.3146 0.3112 0.4442 0.7346
Beq (A 2) (1) (4) (4) (4) (4) (4) (5) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4)
3.16 (2) 6.0 (1) 5.23 (9) 5.57 (9) 6.0 (1) 6.2 (1) 7.2 (1) 6.1 (1) 5.29 (9) 2.67 (8) 2.90 (8) 3.09 (8) 2.84 (8) 2.89 (8) 2.90 (8) 3.05 (8) 3.89 (9) 3.69 (9) 3.27 (9) 3.78 (9) 3.81 (9) 3.33 (9) 3.79 (9) 3.50 (9) 3.36 (9) 3.54 (8) 3.76 (9) 3.77 (9)
(continued overleaf)
418 TABLE 3 (continued) Atoms
x
C(20) C(21) C(22) C(23) C(24) C(25) C(26) C(27) C(28) C(29) C(30) C(31) C(32) C(33) C(34) C(35) C(36) C(37) C(38) C(39) C(40)
0.1068 0.0433 0.1417 0.1751 0.1240 0.7823 0.8085 0.9000 0.9633 0.9371 0.8472 0.6922 0.7598 0.7849 0.7428 0.6778 0.6523 0.5727 0.5972 0.5237 0.6054
y (3) (3) (2) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (2) (3) (3) (3)
0.6964 0.7911 0.4265 0.3408 0.4359 0.8891 0.8016 0.7854 0.8538 0.9400 0.9583 0.9924 0.9632 1.0204 1.1050 1.1339 1.0781 0.9644 0.9735 1.0093 0.8192
z (2) (3) (2) (2) (2) (2) (2) (3) (3) (2) (2) (2) (2) (3) (3) (2) (2) (2) (3) (3) (2)
Beq (~2)
0.8839 0.7386 0.1779 0.1799 0.0245 0.5016 0.4222 0.3054 0.2692 0.3469 0.4640 0.8258 0.9106 1.0450 1.0981 1.0143 0.8780 0.5647 0.4055 0.3429 0.7063
(5) (5) (4) (4) (4) (4) (5) (5) (5) (5) (4) (4) (4) (4) (4) (5) (4) (4) (4) (4) (5)
4.6 (1) 4.8 (1) 3.53 (9) 3.99 (9) 3.96 (9) 3.15 (8) 4.4 (1) 5.7 (1) 5.4 (1) 4.4 (1) 3.77 (9) 3.05 (8) 4.6 (1) 5.4 (1) 5.0 (1) 4.6 (1) 3.72 (9) 3.03 (8) 3.98 (9) 4.17 (9) 4.5 (1)
TABLE 4 Positional parameters with their estimated s.d. for DPCH(TCNQ)a Atoms
x
P N(1) S(2) N(3) N(4) N(5) N(6) N(7) N(8) C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10) C(11)
0.6400 0.7858 0.5888 0.3867 0.5778 1.0559 0.8667 0.8607 0.6549 0.6284 0.6687 0.6351 0.5594 0.5215 0.5533 0.6608 0.7313 0.6206 0.5223 0.4464
(1) (3) (4) (4) (4) (4) (4) (5) (4) (3) (4) (4) (4) (3) (4) (3) (4) (4) (4) (4)
Beq (A 2)
y
z
--0.00120 (8) 0.4284 (3) 0.2855 (2) 0.7405 (3) 0.8788 (2) 0.3619 (3) 0.2296 (2) 0.8245 (3) 0.6674 (3) 0.5101 (3) 0.5906 (3) 0.6622 (3) 0.6614 (3) 0.5817 (3) 0.5090 (2) 0.4351 (3) 0.4328 (3) 0.3536 (3) 0.7355 (3) 0.7373 (3)
--0.0257 --0.1674 0.0963 0.5475 0.2674 --0.9771 --0.6916 --0.5053 --0.2629 0.1258 0.0896 0.1603 0.2752 0.3120 0.2416 0.0505 --0.0697 0.0792 0.3422 0.4555
(2) (5) (5) (5) (5) (5) (5) (6) (5) (5) (5) (5) (5) (5) (5) (5) (5) (5) (5) (5)
3.19 (3) 5.1 (1) 5.1 (1) 5.3 (1) 5.1 (1) 6.1 (1) 4.8 (1) 6.7 (1) 5.8 (1) 2.7 (1) 2.9 (1) 3.2 (1) 2.8 (1) 3.0 (1) 2.8 (1) 2.8 (1) 3.5 (1) 3.4 (1) 3.1 (1) 3.7 (1)
(continued on facing page)
419 TABLE 4 (continued) Atoms
x
C(12) C(13) C(14) C(15) C(16) C(17) C(18) C(19) C(20) C(21) C(22) C(23) C(24) C(25) C(26) C(27) C(28) C(29) C(30) C(31) C(32) C(33) C(34) C(35) C(36) C(37) C(38)
0.5552 0.8984 0.9391 0.9075 0.8342 0.7949 0.8254 0.9308 1.0014 0.8948 0.7975 0.8342 0.7190 0.6885 0.7156 0.7595 0.7756 0.7504 0.7059 0.7566 0.7524 0.8459 0.9466 0.9489 0.8562 0.6013 0.4912
y (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (5) (5) (6) (5) (4) (4) (5) (5) (5) (5) (4) (4)
z
0.8146 0.4487 0.5297 0.6023 0.6014 0.5217 0.4490 0.3735 0.3693 0.2942 0.6753 0.7569 0.6712 0.0534 0.1432 0.1830 0.1339 0.0446 0.0049 --0.0440 --0.1314 --0.1620 --0.1063 --0.0202 0.0119 0.0716 0.0884
(3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (4) (4) (3) (3) (3) (3) (3) (4) (3) (3) (3)
0.2999 --0.6854 --0.7191 --0.6481 --0.5319 --0.4958 --0.5690 --0.7597 --0.8790 --0.7211 --0.4589 --0.4863 --0.3500 --0.1652 --0.1315 --0.2373 --0.3812 --0.4189 --0.3107 0.0475 --0.0079 0.0422 0.1481 0.2055 0.1530 0.1364 0.1214
/1 E
"~-2 -I-+4+ 4-
-3
+ + ÷
-~. 3
I
I
I
~.
5 10lIT(K-I)
6
7
Fig. 1. Comparison of d.c. conductivity of the two salts.
Beq (~2) (6) (5) (5) (5) (5) (5) (5) (5) (6) (5) (5) (6) (6) (6) (7) (7) (6) (7) (6) (5) (6) (7) (8) (8) (7) (6) (6)
3.6 2.9 3.1 3.2 3.0 3.2 3.0 3.4 3.9 3.5 3.4 4.4 3.9 3.6 4.7 5.9 6.3 7.4 5.9 3.2 4.9 6.0 6.8 7.1 5.5 4,6 4.4
(1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (2) (2) (2) (2) (1) (1) (2) (2) (2) (2) (1) (1)
~
,r~
[:]ppp2"[TCNQ) ~-
~
'
'
/
/
l
3.33
3.25
DPCH2"(TCNQ}~-
i
Fig. 2. Comparison of the slipped (left) and zig-zag (right) patterns of molecular overlap. (a) The two possible arrangements; (b) projection of the crystal structure perpendicular to the stacking direction and (c) the molecular overlap viewed along the normal to the TCNQ planes.
(C)
(b)
3 20
(a)
\
\__
,TQ=
0
421
(TCNQ)4 and DPCH(TCNQ)4. Both are good single-chain semiconductors with identical low activation energies and comparable room-temperature conductivities of 0.2 and 0.4 ( ~ cm) -1 respectively. The DPCH 2+ salt is remarkable because it shows (i) a fairly unusual zig-zag TCNQ stack [2] and (ii) a phase transition at 202.5 K, while DPPP(TCNQ)4 exhibits the c o m m o n slipped stack with no phase transition. Molecular dimensions and interactions between ions Drawings of the DPPP and DPCH cations and interatomic distances and angles of the two TCNQ anions in the asymmetric unit are presented in Figs. 3 and 4 respectively. Each of the cations is crystallographically required to have 1 molecular symmetry.
C(2B)
~ \
~
cc261a
/
c(2g)
/
c,3,1
,
(a)
C(3~U3~) C(32)I/C(35) m
ci3al.
IO'L~ (b)
'~
/~
~
_
c(27)
Fig. 3. Perspective views o f t h e DPPP 2+ (a) and DPCH 2+ (b) ions. E s t i m a t e d s t a n d a r d deviat i o n s for n o n - h y d r o g e n a t o m s are a p p r o x i m a t e l y 0.005 A ; b o n d angles are w i t h i n 0.3 0.4 ° .
422
N(2)
N(~.}
(~
c16)
ClS)
=Z+
%=
~/
..-7
L) N(I}
C)N(3)
N(8)
N(6)
%_ ~_
c(~7)
)
.~2 -~/
c(18)
1.340
C(2O)
L)NIH
(J N(5)
(a) N(~
NIl)
c(2)
\~
L,.)N(2) N(5)
I~
c(31
?k
~Z
~/
~)N(31 N(7)
(b) Fig. 4. Bond distances in TCNQ molecules. Estimated standard deviations are approximately 0.005 A. (a) and (b) are related to DPPP and DPCH cations respectively.
423 Bond distances and angles are as expected for DF'PP 2+ when compared with those of the related tetraphenyl-phosphonium monocation in TPP(TCNQ)2 [4]. Consistent with previous structural work [5], bond distances and angles in DPCH show no evidence of delocalization of the positive charge over the central ring. Rather, the cyclohexadiene moiety is non-planar and adopts a pronounced chair conformation with a dihedral angle of 31.2 °. The conformation of the cyclohexadiene has already been discussed in these systems and the particular planar [5a] and boat [5b and c] geometries reported so far result from steric factors. Although this is the first example of a chair conformation, it is likely that in this case too the presence of bulky phenyl substituents at the phosphorus atom induces the observed conformation, which met the requirement for centrosymmetry. In any case the difference in energy in comparison with the boat conformation is expected to be very small. Like analogous 1:2 salts TPP(TCNQ)2 and TMPD(TCNQ)2 [6], stoichiometry imposes an effective charge of --0.5 e on each TCNQ in DPPP(TCNQ)4 and DPCH(TCNQ) 4. Indeed, the expected shift from a quinonoid to a benzenoid structure with increasing charge is observed in each of the individual planar TCNQ units which satisfy m m m symmetry with respect to their bond lengths (Fig. 4). In addition, the degrees of charge transfer are estimated to be 0.54 and 0.46 respectively for DPPP(TCNQ)4 and DPCH(TCNQ)4. These values are deduced from the recently proposed empirical scheme [7 ] based on the concommittant respective lengthening and shortening of the formal double and single bonds of the TCNQ structure as it becomes more benzenoid. In DPPP(TCNQ}4 the shortest intermolecular distances involve weak van der Waals interactions between methyl groups of the cation and nitrogen atoms of the anions (C40 ... N7, 3.33 A). The cation-anion interactions are even weaker in DPCH(TCNQ)4 since the closest distance is a long P ... N of 3.67 A, much larger than the expected van der Waals distance (3.4/~). Crystal structures and intrastack interactions The crystal packings of DPPP(TCNQ)4 and DPCH(TCNQ)4 are dominated by stacks of nearly parallel TCNQ molecules (Fig. 5). The stacking axis is not colinear with the elongation axis of the crystals. Rather, they develop along [101], i.e., at angles of 65 ° and 59 ° with the high-conductivity axis of the DPPP 2+ and DPCH 2+ salts respectively. The intrastack molecular overlap patterns are displayed in Fig. 2. A c o m m o n feature in both materials is the classical ring-external bond type of overlap observed between alternating TCNQ units along the column (Fig. 5(b)). In principle, this mode of overlapping would generate either a slipped or a zig-zag stack {Fig. 5(a)). The vast majority of segregated charge transfer TCNQ salts indeed exhibit slipped stacks and rather few examples are known to present the alternative zig-zag configuration. This is the case for the tetramethyl-p-phenylenediamine complex TMPD(TCNQ)2 [6] and for the
424
(a)
C
Fig. 5. Stereoscopic views of the crystal packing of (a) DPPP(TCNQ)4 and (b) DPCH(TCNQ)4. high-temperature structure found above the semiconductor intermediate conductor transition at 335 K for MEM(TCNQ)2 [8]. The most significant difference between the structures of DPPP(TCNQ)4 and DPCH(TCNQ)4 is in the mode of stacking (Fig. 2(b)). Both slipped and zig-zag stacks are illustrated here, for the DPPP and DPCH salts respectively. In both materials the stack is only slightly dimerized and dimers are related by a centre of s y m m e t r y along the stack. The intra- and inter-dimer distances (3.20 and 3.27 A) in DPPP(TCNQ)4 are identical with those of the related complex TPP(TCNQ)2 [4]. The corresponding distances (3.25 and 3.33 A) in DPCH(TCNQ)4 are close to the mean interplanar spacing of 3.24 A observed in the regular stack of TMPD(TCNQ)2. They are also close and in fact average to the value of 3.29 A found in the regular zig-zag stack above 335 K in MEM(TCNQ) 2 [8]. Those observations all substantiate the similarity between the zig-zag TCNQ stack conformation in DPCH(TCNQ)4 and that of TMPD(TCNQ)2 and high-temperature MEM(TCNQ)2. It is then n o t e w o r t h y that the regular zig-zag stacking found in the latter structure results from considerable structural changes since a strong dimerization associated with hardly any interdimer overlap has been re-
425 p o r t e d b e l o w 335 K [ 8 a ] . This r e o r g a n i z a t i o n o f the T C N Q stack and an increase o f the disorder o f the MEM m o l e c u l e are responsible for the threefold decrease in c o n d u c t i v i t y at the transition. In D P C H ( T C N Q ) 4 a phase transition is also observed (Fig. 1) with a sharp decrease in c o n d u c t i v i t y . In this material, however, as in the p a r e n t D P P P ( T C N Q ) 4 , n o evidence for disorder o f the c a t i o n is f o u n d in the r o o m t e m p e r a t u r e structure. We suggest t h a t in D P C H ( T C N Q ) 4 at t h e phase transition, a p r o f o u n d structural change also o c c u r s in the a l m o s t regular a n i o n stacks. T h e y m i g h t b e c o m e strongly dimerized with possibly s o m e significant misoverlapping o f the dimers, w h i c h w o u l d explain the d r o p in c o n d u c t i v i t y . This, as well as a n y i n h e r e n t instability o f the zig-zag m o d e o f stacking o f the T C N Q units, requires f u r t h e r testing b y careful physical m e a s u r e m e n t s above and b e l o w the phase transition.
References 1 M. Lequan, R. M. Lequan, P. Batail, J. F. Halet and L. Ouahab, Tetrahedron Lett., 24 (1983) 3107. DPPP = diphosphoniumparaphenylene, DPCH = diphosphoniumcyclohexadiene, TCNQ = tetracyano-p-quino-dimethane. TPP = tetraphenylphosphonium; TMPD = tetramethyl-p-phenylenediamine; MEM = N-methyl-N-ethylmorpholonium. 2 For a general discussion of the mode of stacking in TCNQ compounds see: F. H. Herbstein, Perspectives in Structural Chemistry, Vol. IV, John Wiley, New York, 1971, pp. 166 - 395. 3 B. A. Frenz, The Enraf-Nonius CAD-4 SDP-A real-time system for concurrent X-ray data collection and crystal structure solution, in H. Schenk, R. Olthof-Hazekamp, H. van Koningsveld and G. C. Bassi (eds.), Computing in Crystallography, Delft Univ. Press, 1978, pp. 64 - 71. A complete listing of bond distances and angles and of the observed and calculated structure factors is available on request. 4 P. Goldstein, K. Seff and K. N. Trueblood, Acta Cryst., B24 (1968) 778. 5 a) N. G. Bokii and Yu. T. Struchkov, Zh. Struckt. Khim. SSSR, 6 (1965) 571. b) J. N. Brown and L. M. Trefonas, J. Heterocyclic Chem., 9 (1972) 187. c) L. D. Cheung and L. M. Trefonas, J. Heterocyclic Chem., 9 (1972) 991. 6 A. W. Hanson, Acta Cryst., B24 (1968) 768. 7 T. J. Kistenmacher, T. J. Emge, A. N. Bloch and D. O. Cowan, Acta Cryst., B38 (1982) 1193. 8 (a) B. van Bodegom and A. Bosch, Acta Cryst., B37 (1981) 863; (b) M. Morrow, W. N. Hardy, J. F. Carolan, A. J. Bertinsky, L. Weiler, V. K. Gujral, A. Janossy, K. Holczer, G. Mihlay, G. Grunner, S. Huizinga, A. Werwey and G. A. Sawatsky, Can. J. Phys., 58 (1980) 334.
415
SLIPPED V E R S U S ZIG-ZAG TCNQ STACK UPON STERIC CONTROL IN CONDUCTING 1:4 DIPHOSPHONIUM-TCNQ SALTS P. BATAIL, L. OUAHAB*, J.-F. HALET and J. PADIOU Laboratoire de Chimie du Solide et lnorganique Moldculaire, L.A. 254, Universitd de Rennes I, campus de Beaulieu, 35042 Rennes (France)
M. LEQUAN and R. M. LEQUAN Laboratoire de Recherche de Chimie Organique, Ecole Nationale Supgrieure de Chimie de Paris, 11, rue Pierre et Marie Curie, 75231 Paris Cddex 05 (France)
(Received April 16, 1984; in revised form December 25, 1984; accepted January 15, 1984) Abstract The structural and conducting properties of two 1:4 charge transfer diphosphonium-TCNQ salts, prepared by reaction of the corresponding iodides with TCNQ in hot acetonitrile, are compared. The diphosphoniumparaphenylene (DPPP 2+) salt resembles the classical analog 1:2 salt tetraphenylphosphonium-TCNQ, TPP(TCNQ)2, with slipped stacks and normal semiconducting behaviour. By contrast, the diphosphoniumcyclohexadiene (DPCH 2+) salt exhibits zig-zag TCNQ stacks and presents a phase transition at 205.5 K. This is described and discussed with special emphasis on a possible analogy between DPCH(TCNQ)4 and N-methyl-N-ethyl-morpholinium MEM(TCNQ)2, which has recently been thoroughly studied.
Introduction In the course of a general investigation of synthetic routes for the generation of organic iodides and subsequent reactions with electron acceptors, we recently prepared a series of diphosphonium 1:4 charge transfer salts [1] of TCNQ. This paper reports the structural and electrical properties of two materials, Dppp2+(TCNQ)42- and DPCH2+(TCNQ)42-, which achieve the same stoichiometry with comparable large diphosphonium dications.
DPPP
DPCH
*Permanent address: Institut de Chimie, Universit~ de Constantine, Algeria. 0379-6779/85/$3.30
Q Elsevier Sequoia/Printed in The Netherlands
416 Experimental Black needle-like crystals were grown from h o t solutions of TCNQ and the corresponding diphosphonium iodide in acetonitrile.
Electrical conductivity The d.c. electrical conductivities were measured by use of a standard four-probe technique. Gold wires 5 × 10 -2 mm in diameter were connect ed to the crystals with silver paint. The conductivity in both materials is highly anisotropic and much larger along the needle axis. Since the phase transition in DPCH(TCNQ)4 leads to cracking of the crystals, the experi m ent was re pr o d u ced for several crystals and the same consistent results were obtained.
Crystal structures X-ray r o tat i on photographs proved t hat the needle-shaped crystals are elongated along the ~ axis o f the centrosymmetrical triclinic unit cells of the DPPP 2+ and DPCH 2+ salts. The unit cell parameters (Table 1) were determined with a NONIUS CAD-4 a u t o m a t e d d i f f r a c t o m e t e r and the intensity data collection and re fin emen t results are summarized in Table 2. The structures were solved by a combination of direct m e t h o d s (MULTAN) and difference Fourier synthesis. The h y d rogen atoms were located on a Fourier map and introduced in the final cycle but n o t refined. The atomic parameters are given in Tables 3 and 4 [3].
Results and discussion In Figs. 1 and 2 we compare the single-crystal d.c. electrical conductivity along the needle axis and the pattern o f molecular overlap o f DPPP-
TABLE 1 Crystal data
a b c 7 v Z Dc
DPPP(TCNQ)4
DPCH(TCNQ)4
13.060(4) • 15.428(3) 8.060(3) 103.46(2)° 78.29(3) ° 92.40(3) ° 1650.5 h a 1 1.30
11.801(3) A 16.401(4) 8.753(3) 104.70(3)° 98.15(2)° 101.80(3)° 1570.2 A3 1 1.31
417 TABLE 2 Intensity data collection and refinement results Compound Formula Molecular weight Space group Crystal dimension (mm) Radiation Linear absorption coefficient (cm-1) Scan type Scan range (deg) 0 limits (deg) Data collected Unique data used
DPPP(TCNQ)a
P2N16CsoH46 1293.31 P1 0.45 × 0.18 × 0.04 Monochromatized MoK(~ 1.203 0/20 1.0 + 0.35 tan 0 1 - 25 6036 2209 0.036 0.044
R = ~,(I]Fol --IFelI)/Y, IFol R w = ( Y ~ w ( I F o l - IFcl)2/~,wFo2) 1/2
DPCH(TCNQ)4 P2N ~6C76H40 1239.22 P1 0.54 × 0.18 × 0.06 MoK(~ 0.73 0/20 1.0 + 0.45 tan 0 1 - 25 6235 2185 0.048 0.060
TABLE 3 Positional parameters with their estimated s.d. for DPPP(TCNQ)4 Atoms
x
y
z
P N(1) N(2) N(3) N(4) N(5) N(6) S(7) N(8) C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10) C(ll) C(12) C(13) C(14) C(15) C(16) C(17) C(18) C(19)
0.66435 (7) 0.4554 (3) 0.3579 (3) 0.3897 (3) 0.2802 (3) 0.1254 (3) 0.0107 (3) 0.2013 (3) 0.1114 (3) 0.3845 (2) 0.4079 (2) 0.3954 (3) 0.3603 (2) 0.3380 (2) 0.3498 (2) 0.3961 (3) 0.4288 (3) 0.3741 (3) 0.3472 (3) 0.3711 (3) 0.3107 (3) 0.1015 (3) 0.1370 (3) 0.1509 (3) 0.1289 (3) 0.0942 (3) 0.0807 (3) 0.0839 (3)
0.91565 (6) 0.2240 (2) 0.3897 (2) 0.6563 (2) 0.8224 (2) 0.6889 (2) 0.8585 (2) 0.2710 (2) 0.4419 (2) 0.4508 (2) 0.4421 (2) 0.5117 (2) 0.5963 (2) 0.6058 (2) 0.5359 (2) 0.3793 (2) 0.2934 (2) 0.3862 (2) 0.6679 (2) 0.6606 (2) 0.7533 (2) 0.6377 (2) 0.5523 (2) 0.4849 (2) 0.4962 (2) 0.5822 (2) 0.6499 (2) 0.7064 (2)
0.6500 --0.1103 --0.3945 0.7030 0.4341 1.0048 0.7395 0.1831 --0.1001 0.0170 0.1690 0.2996 0.2927 0.1414 0.0103 --0.1177 --0.1126 --0.2705 0.4288 0.5801 0.4293 0.5954 0.5985 0.4657 0.3146 0.3112 0.4442 0.7346
Beq (A 2) (1) (4) (4) (4) (4) (4) (5) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4)
3.16 (2) 6.0 (1) 5.23 (9) 5.57 (9) 6.0 (1) 6.2 (1) 7.2 (1) 6.1 (1) 5.29 (9) 2.67 (8) 2.90 (8) 3.09 (8) 2.84 (8) 2.89 (8) 2.90 (8) 3.05 (8) 3.89 (9) 3.69 (9) 3.27 (9) 3.78 (9) 3.81 (9) 3.33 (9) 3.79 (9) 3.50 (9) 3.36 (9) 3.54 (8) 3.76 (9) 3.77 (9)
(continued overleaf)
418 TABLE 3 (continued) Atoms
x
C(20) C(21) C(22) C(23) C(24) C(25) C(26) C(27) C(28) C(29) C(30) C(31) C(32) C(33) C(34) C(35) C(36) C(37) C(38) C(39) C(40)
0.1068 0.0433 0.1417 0.1751 0.1240 0.7823 0.8085 0.9000 0.9633 0.9371 0.8472 0.6922 0.7598 0.7849 0.7428 0.6778 0.6523 0.5727 0.5972 0.5237 0.6054
y (3) (3) (2) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (2) (3) (3) (3)
0.6964 0.7911 0.4265 0.3408 0.4359 0.8891 0.8016 0.7854 0.8538 0.9400 0.9583 0.9924 0.9632 1.0204 1.1050 1.1339 1.0781 0.9644 0.9735 1.0093 0.8192
z (2) (3) (2) (2) (2) (2) (2) (3) (3) (2) (2) (2) (2) (3) (3) (2) (2) (2) (3) (3) (2)
Beq (~2)
0.8839 0.7386 0.1779 0.1799 0.0245 0.5016 0.4222 0.3054 0.2692 0.3469 0.4640 0.8258 0.9106 1.0450 1.0981 1.0143 0.8780 0.5647 0.4055 0.3429 0.7063
(5) (5) (4) (4) (4) (4) (5) (5) (5) (5) (4) (4) (4) (4) (4) (5) (4) (4) (4) (4) (5)
4.6 (1) 4.8 (1) 3.53 (9) 3.99 (9) 3.96 (9) 3.15 (8) 4.4 (1) 5.7 (1) 5.4 (1) 4.4 (1) 3.77 (9) 3.05 (8) 4.6 (1) 5.4 (1) 5.0 (1) 4.6 (1) 3.72 (9) 3.03 (8) 3.98 (9) 4.17 (9) 4.5 (1)
TABLE 4 Positional parameters with their estimated s.d. for DPCH(TCNQ)a Atoms
x
P N(1) S(2) N(3) N(4) N(5) N(6) N(7) N(8) C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10) C(11)
0.6400 0.7858 0.5888 0.3867 0.5778 1.0559 0.8667 0.8607 0.6549 0.6284 0.6687 0.6351 0.5594 0.5215 0.5533 0.6608 0.7313 0.6206 0.5223 0.4464
(1) (3) (4) (4) (4) (4) (4) (5) (4) (3) (4) (4) (4) (3) (4) (3) (4) (4) (4) (4)
Beq (A 2)
y
z
--0.00120 (8) 0.4284 (3) 0.2855 (2) 0.7405 (3) 0.8788 (2) 0.3619 (3) 0.2296 (2) 0.8245 (3) 0.6674 (3) 0.5101 (3) 0.5906 (3) 0.6622 (3) 0.6614 (3) 0.5817 (3) 0.5090 (2) 0.4351 (3) 0.4328 (3) 0.3536 (3) 0.7355 (3) 0.7373 (3)
--0.0257 --0.1674 0.0963 0.5475 0.2674 --0.9771 --0.6916 --0.5053 --0.2629 0.1258 0.0896 0.1603 0.2752 0.3120 0.2416 0.0505 --0.0697 0.0792 0.3422 0.4555
(2) (5) (5) (5) (5) (5) (5) (6) (5) (5) (5) (5) (5) (5) (5) (5) (5) (5) (5) (5)
3.19 (3) 5.1 (1) 5.1 (1) 5.3 (1) 5.1 (1) 6.1 (1) 4.8 (1) 6.7 (1) 5.8 (1) 2.7 (1) 2.9 (1) 3.2 (1) 2.8 (1) 3.0 (1) 2.8 (1) 2.8 (1) 3.5 (1) 3.4 (1) 3.1 (1) 3.7 (1)
(continued on facing page)
419 TABLE 4 (continued) Atoms
x
C(12) C(13) C(14) C(15) C(16) C(17) C(18) C(19) C(20) C(21) C(22) C(23) C(24) C(25) C(26) C(27) C(28) C(29) C(30) C(31) C(32) C(33) C(34) C(35) C(36) C(37) C(38)
0.5552 0.8984 0.9391 0.9075 0.8342 0.7949 0.8254 0.9308 1.0014 0.8948 0.7975 0.8342 0.7190 0.6885 0.7156 0.7595 0.7756 0.7504 0.7059 0.7566 0.7524 0.8459 0.9466 0.9489 0.8562 0.6013 0.4912
y (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (4) (5) (5) (6) (5) (4) (4) (5) (5) (5) (5) (4) (4)
z
0.8146 0.4487 0.5297 0.6023 0.6014 0.5217 0.4490 0.3735 0.3693 0.2942 0.6753 0.7569 0.6712 0.0534 0.1432 0.1830 0.1339 0.0446 0.0049 --0.0440 --0.1314 --0.1620 --0.1063 --0.0202 0.0119 0.0716 0.0884
(3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (4) (4) (3) (3) (3) (3) (3) (4) (3) (3) (3)
0.2999 --0.6854 --0.7191 --0.6481 --0.5319 --0.4958 --0.5690 --0.7597 --0.8790 --0.7211 --0.4589 --0.4863 --0.3500 --0.1652 --0.1315 --0.2373 --0.3812 --0.4189 --0.3107 0.0475 --0.0079 0.0422 0.1481 0.2055 0.1530 0.1364 0.1214
/1 E
"~-2 -I-+4+ 4-
-3
+ + ÷
-~. 3
I
I
I
~.
5 10lIT(K-I)
6
7
Fig. 1. Comparison of d.c. conductivity of the two salts.
Beq (~2) (6) (5) (5) (5) (5) (5) (5) (5) (6) (5) (5) (6) (6) (6) (7) (7) (6) (7) (6) (5) (6) (7) (8) (8) (7) (6) (6)
3.6 2.9 3.1 3.2 3.0 3.2 3.0 3.4 3.9 3.5 3.4 4.4 3.9 3.6 4.7 5.9 6.3 7.4 5.9 3.2 4.9 6.0 6.8 7.1 5.5 4,6 4.4
(1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (1) (2) (2) (2) (2) (1) (1) (2) (2) (2) (2) (1) (1)
~
,r~
[:]ppp2"[TCNQ) ~-
~
'
'
/
/
l
3.33
3.25
DPCH2"(TCNQ}~-
i
Fig. 2. Comparison of the slipped (left) and zig-zag (right) patterns of molecular overlap. (a) The two possible arrangements; (b) projection of the crystal structure perpendicular to the stacking direction and (c) the molecular overlap viewed along the normal to the TCNQ planes.
(C)
(b)
3 20
(a)
\
\__
,TQ=
0
421
(TCNQ)4 and DPCH(TCNQ)4. Both are good single-chain semiconductors with identical low activation energies and comparable room-temperature conductivities of 0.2 and 0.4 ( ~ cm) -1 respectively. The DPCH 2+ salt is remarkable because it shows (i) a fairly unusual zig-zag TCNQ stack [2] and (ii) a phase transition at 202.5 K, while DPPP(TCNQ)4 exhibits the c o m m o n slipped stack with no phase transition. Molecular dimensions and interactions between ions Drawings of the DPPP and DPCH cations and interatomic distances and angles of the two TCNQ anions in the asymmetric unit are presented in Figs. 3 and 4 respectively. Each of the cations is crystallographically required to have 1 molecular symmetry.
C(2B)
~ \
~
cc261a
/
c(2g)
/
c,3,1
,
(a)
C(3~U3~) C(32)I/C(35) m
ci3al.
IO'L~ (b)
'~
/~
~
_
c(27)
Fig. 3. Perspective views o f t h e DPPP 2+ (a) and DPCH 2+ (b) ions. E s t i m a t e d s t a n d a r d deviat i o n s for n o n - h y d r o g e n a t o m s are a p p r o x i m a t e l y 0.005 A ; b o n d angles are w i t h i n 0.3 0.4 ° .
422
N(2)
N(~.}
(~
c16)
ClS)
=Z+
%=
~/
..-7
L) N(I}
C)N(3)
N(8)
N(6)
%_ ~_
c(~7)
)
.~2 -~/
c(18)
1.340
C(2O)
L)NIH
(J N(5)
(a) N(~
NIl)
c(2)
\~
L,.)N(2) N(5)
I~
c(31
?k
~Z
~/
~)N(31 N(7)
(b) Fig. 4. Bond distances in TCNQ molecules. Estimated standard deviations are approximately 0.005 A. (a) and (b) are related to DPPP and DPCH cations respectively.
423 Bond distances and angles are as expected for DF'PP 2+ when compared with those of the related tetraphenyl-phosphonium monocation in TPP(TCNQ)2 [4]. Consistent with previous structural work [5], bond distances and angles in DPCH show no evidence of delocalization of the positive charge over the central ring. Rather, the cyclohexadiene moiety is non-planar and adopts a pronounced chair conformation with a dihedral angle of 31.2 °. The conformation of the cyclohexadiene has already been discussed in these systems and the particular planar [5a] and boat [5b and c] geometries reported so far result from steric factors. Although this is the first example of a chair conformation, it is likely that in this case too the presence of bulky phenyl substituents at the phosphorus atom induces the observed conformation, which met the requirement for centrosymmetry. In any case the difference in energy in comparison with the boat conformation is expected to be very small. Like analogous 1:2 salts TPP(TCNQ)2 and TMPD(TCNQ)2 [6], stoichiometry imposes an effective charge of --0.5 e on each TCNQ in DPPP(TCNQ)4 and DPCH(TCNQ) 4. Indeed, the expected shift from a quinonoid to a benzenoid structure with increasing charge is observed in each of the individual planar TCNQ units which satisfy m m m symmetry with respect to their bond lengths (Fig. 4). In addition, the degrees of charge transfer are estimated to be 0.54 and 0.46 respectively for DPPP(TCNQ)4 and DPCH(TCNQ)4. These values are deduced from the recently proposed empirical scheme [7 ] based on the concommittant respective lengthening and shortening of the formal double and single bonds of the TCNQ structure as it becomes more benzenoid. In DPPP(TCNQ}4 the shortest intermolecular distances involve weak van der Waals interactions between methyl groups of the cation and nitrogen atoms of the anions (C40 ... N7, 3.33 A). The cation-anion interactions are even weaker in DPCH(TCNQ)4 since the closest distance is a long P ... N of 3.67 A, much larger than the expected van der Waals distance (3.4/~). Crystal structures and intrastack interactions The crystal packings of DPPP(TCNQ)4 and DPCH(TCNQ)4 are dominated by stacks of nearly parallel TCNQ molecules (Fig. 5). The stacking axis is not colinear with the elongation axis of the crystals. Rather, they develop along [101], i.e., at angles of 65 ° and 59 ° with the high-conductivity axis of the DPPP 2+ and DPCH 2+ salts respectively. The intrastack molecular overlap patterns are displayed in Fig. 2. A c o m m o n feature in both materials is the classical ring-external bond type of overlap observed between alternating TCNQ units along the column (Fig. 5(b)). In principle, this mode of overlapping would generate either a slipped or a zig-zag stack {Fig. 5(a)). The vast majority of segregated charge transfer TCNQ salts indeed exhibit slipped stacks and rather few examples are known to present the alternative zig-zag configuration. This is the case for the tetramethyl-p-phenylenediamine complex TMPD(TCNQ)2 [6] and for the
424
(a)
C
Fig. 5. Stereoscopic views of the crystal packing of (a) DPPP(TCNQ)4 and (b) DPCH(TCNQ)4. high-temperature structure found above the semiconductor intermediate conductor transition at 335 K for MEM(TCNQ)2 [8]. The most significant difference between the structures of DPPP(TCNQ)4 and DPCH(TCNQ)4 is in the mode of stacking (Fig. 2(b)). Both slipped and zig-zag stacks are illustrated here, for the DPPP and DPCH salts respectively. In both materials the stack is only slightly dimerized and dimers are related by a centre of s y m m e t r y along the stack. The intra- and inter-dimer distances (3.20 and 3.27 A) in DPPP(TCNQ)4 are identical with those of the related complex TPP(TCNQ)2 [4]. The corresponding distances (3.25 and 3.33 A) in DPCH(TCNQ)4 are close to the mean interplanar spacing of 3.24 A observed in the regular stack of TMPD(TCNQ)2. They are also close and in fact average to the value of 3.29 A found in the regular zig-zag stack above 335 K in MEM(TCNQ) 2 [8]. Those observations all substantiate the similarity between the zig-zag TCNQ stack conformation in DPCH(TCNQ)4 and that of TMPD(TCNQ)2 and high-temperature MEM(TCNQ)2. It is then n o t e w o r t h y that the regular zig-zag stacking found in the latter structure results from considerable structural changes since a strong dimerization associated with hardly any interdimer overlap has been re-
425 p o r t e d b e l o w 335 K [ 8 a ] . This r e o r g a n i z a t i o n o f the T C N Q stack and an increase o f the disorder o f the MEM m o l e c u l e are responsible for the threefold decrease in c o n d u c t i v i t y at the transition. In D P C H ( T C N Q ) 4 a phase transition is also observed (Fig. 1) with a sharp decrease in c o n d u c t i v i t y . In this material, however, as in the p a r e n t D P P P ( T C N Q ) 4 , n o evidence for disorder o f the c a t i o n is f o u n d in the r o o m t e m p e r a t u r e structure. We suggest t h a t in D P C H ( T C N Q ) 4 at t h e phase transition, a p r o f o u n d structural change also o c c u r s in the a l m o s t regular a n i o n stacks. T h e y m i g h t b e c o m e strongly dimerized with possibly s o m e significant misoverlapping o f the dimers, w h i c h w o u l d explain the d r o p in c o n d u c t i v i t y . This, as well as a n y i n h e r e n t instability o f the zig-zag m o d e o f stacking o f the T C N Q units, requires f u r t h e r testing b y careful physical m e a s u r e m e n t s above and b e l o w the phase transition.
References 1 M. Lequan, R. M. Lequan, P. Batail, J. F. Halet and L. Ouahab, Tetrahedron Lett., 24 (1983) 3107. DPPP = diphosphoniumparaphenylene, DPCH = diphosphoniumcyclohexadiene, TCNQ = tetracyano-p-quino-dimethane. TPP = tetraphenylphosphonium; TMPD = tetramethyl-p-phenylenediamine; MEM = N-methyl-N-ethylmorpholonium. 2 For a general discussion of the mode of stacking in TCNQ compounds see: F. H. Herbstein, Perspectives in Structural Chemistry, Vol. IV, John Wiley, New York, 1971, pp. 166 - 395. 3 B. A. Frenz, The Enraf-Nonius CAD-4 SDP-A real-time system for concurrent X-ray data collection and crystal structure solution, in H. Schenk, R. Olthof-Hazekamp, H. van Koningsveld and G. C. Bassi (eds.), Computing in Crystallography, Delft Univ. Press, 1978, pp. 64 - 71. A complete listing of bond distances and angles and of the observed and calculated structure factors is available on request. 4 P. Goldstein, K. Seff and K. N. Trueblood, Acta Cryst., B24 (1968) 778. 5 a) N. G. Bokii and Yu. T. Struchkov, Zh. Struckt. Khim. SSSR, 6 (1965) 571. b) J. N. Brown and L. M. Trefonas, J. Heterocyclic Chem., 9 (1972) 187. c) L. D. Cheung and L. M. Trefonas, J. Heterocyclic Chem., 9 (1972) 991. 6 A. W. Hanson, Acta Cryst., B24 (1968) 768. 7 T. J. Kistenmacher, T. J. Emge, A. N. Bloch and D. O. Cowan, Acta Cryst., B38 (1982) 1193. 8 (a) B. van Bodegom and A. Bosch, Acta Cryst., B37 (1981) 863; (b) M. Morrow, W. N. Hardy, J. F. Carolan, A. J. Bertinsky, L. Weiler, V. K. Gujral, A. Janossy, K. Holczer, G. Mihlay, G. Grunner, S. Huizinga, A. Werwey and G. A. Sawatsky, Can. J. Phys., 58 (1980) 334.