J. Phys. Chem. 1988,92, 6892-6899
6892
ARTICLES Salts of the 1,I,2,3,3-Pentacyanopropentde Anion: Crystallographic and Spectroscopic Studies T. J. Johnson, K. W.Hipps, and R. D. Willett* Department of Chemistry and Chemical Physics Program, Washington State University, Pullman, Washington 99164-4630 (Received: October 21, 1987; In Final Form: January 26, 1988)
The rubidium, potassium, and pyridinium salts of the title monoanion are prepared and their crystal structures determined and compared to other reported structures having the PCP ion as a component. These three structures show that the C8N5anion is easily distorted from its self-consistent field (SCF)-calculated C, geometry. The potassium and rubidium structures are isomor hic, crystallizing in the monoclinic space group P2'/c (Z= 4) with a = 9.571 (2) A, b = 12.726 (3) A, c = 7.757 (1) and fi = 99.89 (2)' for the rubidium structure at room temperature and a = 9.591 ( 5 ) A, b = 12.433 (3)A, c = 7.371 (4)A, and ,3 = 100.61 (4)O for the potassium structure determined at 143 (f3) K. 'dinium pentacyanopropenide crystallizes in the orthorhombic space group P2'2,2 with a = 7.657 (1) A, ,3 = 6.546 ( 1 ) y a n d c = 12.642 (2) A (Z= 2). Low-temperature electronic emission spectra are recorded for all three salts and are qualitatively similar to the lowtemperature fluorescence reported for the cesium salt of pentacyanopropenide (CsPCP). Diffuse reflectance spectra are reported in the UV-vis region and evaluated by using extended Hiickel theory. FTIR and Raman spectra of these salts are used in conjunction with SCF calculations to label additional normal modes of PCP. Previously unobserved C=N stretching modes are used to further the assignment of the in-plane fundamentals.
1,
Introduction The cyanocarbons and cyanocarbanions were first studied extensively in the 1950s and 1960s by a Du Pont group headed by Midd1eton.'s2 Tetracyanoethylene (TCNE) is the most widely studied of the cyanocarbons, and its rich (redox) chemistry has a vast l i t e r a t ~ r e . ~Of . ~ the cyanocarbanions, a few, such as the tetracyanoethylene monoanion (TCNE'-) are radicals. Most, however, are relatively stable closed-shell species, such as the 1,1,2,3,3-pentacyanopropenide (PCP) monoanion (see Figure 1). Led by the pioneering work of Devlin? both free radical species such as TCNE"%'J6 and closed shell species such as the very small tricyanomethanide anion, TCM,* have been the subject of an (1) Middleton, W. J.; Little, E. L.; Coffman, D. D.; Engelhardt, V. A. J . Am. Chem. Soc. 1958,80,2795. See other papers in this issue; the work of the Du Pont group comprises the entire issue. (2) Middleton, W. J. US. Patents 2 766 135, 2 766 243, 2 766 246, 2166247, 1956. (3) (a) Dhar, D. N. Chem. Rev. 1967, 67, 611. (b) Michaelian, K. H.; Rieckhoff, K. E.; Voigt, E. M. Can. J . Spectrosc. 1980, 25, 148. (4) (a) Fatiadi, A. J. Synthesis 1986,4,249. (b) Poradowska, H.; Nowak, K. Wiad. Chem. 1982,36,649. (5) (a) Hinkel, J. J.; Devlin, J. P. J . Chem. Phys. 1973, 58, 4750. (b) Moore, J. C.; Smith, D.; Youhne, Y.; Devlin, J. P. J . Phys. Chem. 1971, 75, 325. (6) Pons, S.;Khoo, S. B.; Bewick, A,; Datta, M.; Smith, J. J.; Hinman, A. S.;Zachmann, G. J . Phys. Chem. 1984,88, 3575. (7) (a) Matsuzaki, S.;Mitauishi, T.; Toyoda, K. Chem. Phys. Lett. 1982, 91,296. (b) Erley, W.; Ibach, H. J. Phys. Chem. 1987,91,2947. (c) Datta, M.; Jansson, R. E. W.; Freeman, J. J. Spectrosco. Leu. 1986, 19, 129. (8) Hipps, K. W.; Aplin, A. T. J. Phys. Chem. 1985,89, 5459. See earlier references therein. (9) Mazur, U.; Hipps, K. W. J. Phys. Chem. 1983,87,4641. (10) H i m , K. W.; Keder, J. W. J . Phys. Chem. 1983.87, 3186. (11) Jensen, W. P.; Jacobson, R. A. Inorg. Chim. Acta 1981, 50, 189. (12) Sim, G. A,; Woodhouse, D. I.; Knox, G. R. J. Chem. SOC.,Dalton Trans. 1979, 629. ( 1 3 ) Bruce, M. I.; Wallis, R. C.; Skelton, B. W.; White, A. H. J. Chem. SOC.,Dalton Tram. 1981, 2205. (14) Bertolasi, V.; Gilli, G. Acta Crystollogr. Sect. C: Struct. Commun. 1983, C39, 1242. (15) Miller, J. S.;Calabrese, J. C.; Rommelmann, H.; Chittipeddi, S.R.; Zhang, J. H.; Reiff, W. M.; Epstein, A. J. J. Am. Chem. Soc. 1987, 109, 769. (16) Dixon, D. A.: Miller, J. S. J. Am. Chem. SOC.1987. 109.3656. and references therein.
increasing number of spectroscopic studies. One of the these anions, PCP, has attracted a great deal of attention due to both its richly structured low-temperature fluorescenceg and the fact that a monolayer of the adsorbed anion can be seen on a surface in the absence of any surface enhancement mechanism.I0 In inorganic chemistry, PCP has been seen to act as both a simple counterion"~12*'sand as an N-coordinated ligand to a ruthenium metal center.I3 And very recently, two crystal structures have shown that the r-system of the PCP anion may play a role in intermolecular charge transfer in systems having both organic14 and inorganic cation^.'^ The percyano systems are also being investigated by Miller and co-workers as possible candidates for organic ferromagnetic materia1s.l6 However, much of the spectroscopic data reported on the PCP anion have been obtained by using solid samples of the cesium salt, C S P C P . ' ~ * 'As ~ was demonstrated by a crystal structure,'* the cesium counterions provide a highly symmetric environment for the PCP ion. It turns out that the lattices obtained by using pyridinium (PyH'), rubidium, and potassium counterions provide a much less symmetric environment for the anion (vide infra). In these lattices, as in the 2-(dimethylamino)-S-methyl1,3-thiazolium pentacyanopropenide (DMATPCP) s t r u c t ~ r e ,the ' ~ PCP anion is distorted from a C, geometry, and more spectroscopic information about PCP is made available. With this in mind, the mpe of the present paper is 3-fold. First, we report the crystal structures of PyHPCP, RbPCP, and KPCP and show that the anion is very distortable along the r-bonding framework. These results are compared to other known structures containing the PCP anion. Second, diffuse reflectance spectra of the four salts are reported showing relative shifts in the absorption band edge; these spectra are modeled by extended Huckel theory (EHT) and compared to the low-temperature emission spectra. Third, vibrational (Raman and FTIR) data will be used along with GAUSSIAN 82 SCF calculations to further the vibrational assignment of the in-plane fundamentals of the PCP anion. (17) Mazur, U.; Hipps, K. W. J. Phys. Chem. 1982, 86, 2854. (18) Hipps, K. W.; Geiser, U.; Mazur, U.; Willett, R. D. J . Phys. Chem. 1984,88, 2498.
0022-3654/88/2092-6892.$01.50/00 1988 American Chemical Society
1,1,2,3,3-PentacyanopropenideAnion
The Journal of Physical Chemistry, Vol. 92, No. 24, 1988 6893
QKf
"""\d.,. Kb
Figure I. ORTIP drawing of the first conformation of the PCP anion and the first coordination sphere of cations from the 143 K structure of potassium 1,1,2,3,3-pentacyanopropenide. Experimental Section Materials. The pyridinium salt of PCP was prepared by literature method^.'^ Although the cesium salt may be easily obtained by metathesis with CsCl in aqueous solution, for the rubidium and potassium salts, excesses of the alkali halides are needed to complete metathesis. For RbPCP and KPCP approximately 5- and 15-fold molar excesses of RbCl and KCl, respectively, were mixed with PyHpCP in water. These procedures were repeated three or four times each until bands characteristic of the PyH+ ion disappeared from the infrared spectrum. Thereafter, all samples were treated with activated charcoal in acetone solution to remove organic impurities, and after about six or eight such treatments were precipitated with ether." All samples were treated at least 20 times in this manner, and some samples were given in excess of 50 acetone/charcoal treatments. Elemental analysis (Galbraith) found (calculated): PyHPCP: C, 63.38 (63.41); N, 34.31 (34.13); H, 2.53 (2.46). R b P C P C, 38.09 (38.19); N, 27.73 (27.84). KPCP: C, 46.14 (46.82); N, 34.14 (34.13). Crystals for X-ray use were grown from either water or wateracetone solutions. Since these water-recrystallized samples had trace amounts of fluorescent impurities, they were not used for emission or Raman experiments. X-ray powder patterns were recorded to verify that the specimens used for X-ray work had the same crystal structures as the acetone-ether-precipitated microcrystals used for spectroscopy. It was determined that the structures of the two materials were the same for each of the K+, Rb+, and PyH+ salts. In addition, high-resolution FTIR spectra of the water-grown crystals were also recorded to verify that no peaks were shifted from the acetone-ether spectroscopy samples. Diffuse Reflectance and Emission. Diffuse reflectance spectra were recorded on a Perkin-Elmer Model 330 UV-visible absorption spectrometer. An integrating sphere accessory with a barium sulfate lining was used. Spectra were stored by an LSI 11/23 computer and plotted with a Tektronix plotter. Emission spectra were recorded on a previously described instrumentg except that a C o S 0 4 / N i S 0 4 solution filter with a Corning 7-54 glass filter was used and the emitted light was dispersed by a SPEX 1702 0.75-m single monochromator. Raman and FTIR Spectra. Raman spectra were recorded by using the 514.5-nm line of a Spectra Physics argon-ion laser as the source. After passing through a Pellin-Broca prism monochromator and a narrow-pass interference filter, the beam i m p inged on the microcrystalline sample held in a capillary tube. For the low-temperature spectra, the sealed capillary was immersed directly in an optical Dewar filled with liquid nitrogen. (Typical laser powers for the room-temperature spectra were -5 mW or less.) Scattered light was collected and dispersed by an ISA (Jobin-Yvon) 1.O-m double monochromator. Spectra were recorded in constant band-pass mode; for the 514.5-nm line and the slit widths used, resolution was typically 2.5 cm-'. The detector (19) Diekinson, C. L.; Wiley, D. W.; McKusick, B. C. J. Am. Chem. SOC. 1960,82, 6132.
was a Hamamatsu R666 thermoelectrically cooled phototube equipped with a SPEX DPC2 photon counter. An IBM PC/XT computer with locally constructed interface and software served to control the experiment and record the spectra.20 All reported spectra are the sums of multiple scans. FTIR spectra were recorded both as the appropriate alkali halide pellet (KBr, RbCl, CsCl) and as nujol mulls to verify the absence of cation exchange. With the exception of the 1 5 0 0 - ~ m - ~ band in CsPCP, no significant shifts were observed between the alkali halide and the nujol spectra. The instrument was an IBM IR98 FTIR spectrometer with a liquid-nitrogen-cooled M C T detector. The spectra were recorded at 2-cm-' resolution. CorJtputational Methods. Extended Huckel calculations were performed by using the program modified for the IBM P C by Bartmess and Thomas.21 Input geometries were generated from the crystal structures by the crystallographic software.22 For the calculations reported here the method 3 option was employed. This option scales the Coulomb integrals (VOIEs) according to the Mulliken populations and accounts for the overall charge of the anion by a pseudo-Madelung correction. SCF-HF calculations were performed by using the GAUSSIAN 82 program on a VAX 11-785 machine. Geometries were optimized by using the Berny analytical technique, and force constants and fundamental frequencies were determined in a subsequent calculation. Due to hard-disk storage limitations and the size of the anion, a minimal basis set (STO-3G) was used throughout . Crystallography. Powder patterns were recorded by using standard film techniques with a Cu K a X-ray source. Structures of all crystals were first determined at room temperature (293 f 3 K) with a Nicolet R3m/E diffractometer with Mo K a (A = 0.7 10 69 A) radiation. For reasons described below, the structure of the potassium salt was redetermined while being cooled by a stream of gas from a liquid nitrogen reservoir; the measured temperature was T = 143 f 3 K. Data were collected using either 8-28 (PyHPCP, RbPCP) or o (KPCP) scans. Both the orientation matrix and the lattice parameters were automatically optimized by using a linear least-squares fit to a large (218) number of reflections with high 28 values. Absorption corrections were made for the rubidium salt; since the KPCP and PyHPCP samples were composed of relatively light atoms, these data were not corrected for absorption. The stabilities of the crystals were monitored with three check reflections; crystal decomposition was minimal. Data collection parameters are summarized in Table I. The final agreement factors, R and R,, are also shown in Table I, where R = CllFol - IFcll/CIFol and R , = (CW(lF0l- I~c1)2/CWI~olz11'z. Data reduction was executed in the context of the SHELXTL package from Nicolet with scattering factors from standard sources.22 Structures were determined by using direct methods in all cases. For the pyridinium salt the C-H and N-H bond distances were held fixed at 0.96 A,and the hydrogens given a fixed isotropic thermal parameter of 0.06 A2. An ORTEP drawing of the anion (taken from the low-temperature KPCP structure) is presented in Figure 1 along with its first coordination sphere; the same numbering scheme for the anion will be used to describe all of the alkali (Cs, Rb, K) salt structure^.^^ Solution and Description of Crystal Structures The structure solution and refinement involved treatment of disorder problems in all three structures. For the pyridinium salt, the cations nominally lie with their principal axes along the 2-fold (20) Johnson, T. J. Ph.D. Dissertation, Chemical Physics Program, Washington State University, Pullman, WA 99164, May 1987. (21) Bartmess, J. E.;Thomas, D. Quuntum Chemistry Program Exchange; OCMP _ - - - - 01 1. (22) (a) Campana, C. F.; Shepard, D. F.; Litchman, W. M. Inorg. Chem. 1981, 20, 4039. (b) Structure Determination Operation Manual; Nicolet Instrument: Madison, WI, 1980; Section E.2. (c) International Tables for X-ray Crystallography, 4th ed.; Hahn, T., Ed.; Reidel: Dordrecht, Holland, Boston, MA, 1983. (23) The atomic numbering used for CSPCP and other PCP structures from the literature has been changed to be consistent with the present scheme.
6894 The Journal of Physical Chemistry, Vol. 92, No. 24, 1988
Johnson et al.
TABLE I: PCP Salts Structure Determiaatioa Data PyHPCP RbPCP emp formula C13HSN6 RbCsNS mol wt, g/mol 246.23 251.59 crystal class orthorhombic monoclinic space group P21212 P2IlC systematic absences hOO, h = odd hOl, I = odd OkO, k = odd OkO, k = odd lattice constants 7.657 (1) 9.571 (2) a, A 6.546 (1) 12.726 (3) b, 8, 12.642 (2) 7.757 (1) c, A 90 90 a,deg 90 99.89 (2) I% deg 90 90 Y. deg vol., A3 633.7 (2) 930.7 (3) 24: 25" < 20 < 40' centering rflctns 18: 23" < 20 < 35" collctn temp RT RT crystal size 0.41 mm X 0.31 mm X 0.16 mm X 0.18 mm X 0.38 mm 0.05 mm calcd density, g/cm3 1.29 1.80 Z 2 4 g, cm-l 0.80 55.66 diffractometer syst Nicolet R3m/E Nicolet R3m/E Mo Ka radiation with graphite monochromator Zr filter = 0.71069 A) data collctn method 8-28 scans 6-26 scans scan range, deg 2.0 1.8 scan speed, deg/min 3.O-29.3 4.0-29.3 411, 321, 131 check rflctn 211, 141, iZi abs corrctn none empirical transmission 0.176 max, 0.055 min 252 480 F(O00) total rflctns 729 1910 20 (max), deg 50 50 unique rflctns 676, 549 obsd 1637, 1164 obsd R 0.0575; F > 3a(F) 0.0368; F > 3a(F) 0.0833 0.0342 Rw 0.001 A/u (mean) 0.23 1 A/u (max) -0.006 for U2,, C(7) -2.552 for Ull, C(14) total parameters refined 103 142 anisotropic on all thermal parameters all anisotropic for first nonhydrogen conformation; isotropic for second conformation
(x
KPCP, low temp KCENS
KPCP, RT" KCENS 205.22 monoclinic
205.22 monoclinic P2dc h01, I = odd O M ) , k = odd
p2l/C
9.591 (5) 12.433 (4) 7.371 (4) 90 100.61 (4) 90 863.9 (7) 25: 22" < 20 < 38O 143 (h5) K 9 observable faces, largest face: 0.40 X 0.40 mm 1.58 4 5.57 Nicolet R3m/E graphite monochromator
9.643 (4) 12.479 (2) 7.465 (3) 90 101.23 (3) 90 881.1 (5) 25: 2". 20 < 38O RT 9 observable faces, largest face: 0.40 mm X 0.40 mm 1.55 4 5.46 Nicolet R3m/E graphite monochromator
scans 1.o 4.0-29.0 020,33i, i30 none
w
408 2329 55 1989, 1663 obsd 0.0263; F > 64F) 0.0354 0.182 -1.083 for U12,N(3) 181 all anisotropic for first conformation; isotropic for second conformation
408 2607 52 1726, 1376 obsd 0.0298; F > 6u(F) 0.0396 0.132 -1.259 for extcntn 181 all anisotropic for first conformation; isotropic for second conformation
w
hOl, I = odd
OM),
k = odd
scans
1.o
4.0-29.0 020, 33i, i30 none
Room temperature. TABLE 11: Experimental and Theoretical Bond Lengths (Aogstroms) in Various Salts of the 1,1,2,3,3-Pentacynsopropetlide Anionc STO-3G KPCP KPCP Rul'TcpPCPb DMATPCPC DMFePCPd bond (Ck) CSPCP'~ PyHPCP RbPCP (-130 "C) (RT)' (ref 13) (ref 14) (ref 15; -100 "C) 1.411 (7) 1.405 (6) 1.449 (8) 1.437 (3) 1.436 (4) C(1)-C(2) 1.439 1.424 (10) 1.421 (6) 1.422 (4) 1.440 (8) 1.432 (3) 1.437 (5) 1.440 1.426 (7) 1.430 (6) C(2)-C(3) 1.423 (9) 1.437 (7) 1.425 (3) 1.395 (7) 1.382 (4) 1.402 (6) 1.399 (2) 1.399 (3) C(2)-C(4) 1.395 1.366 (9) 1.405 (5) 1.396 (2) 1.468 (6) 1.454 (2) 1.452 (3) 1.479 1.454 (7) 1.465 (8) C(4)-C(5) 1.511 (13) 1.454 (5) 1.451 (4) 1.395. 1.398 (7) 1.382 (4)* 1.372 (7) 1.388 (2) 1.386 (3) C(4)-C(6) 1.331 (10) 1.370 (6) 1.396 (2)* 1.426 (2) 1.425 (4) 1.440* 1.434 (7) 1.430 (6)* 1.431 (7) C(6)C(7) 1.477 (12) 1.429 (5) 1.425 (3)* 1.385 (12) C(6)-C(8) 1.439* 1.418 (7) 1.405 (6)* 1.406 (7) 1.440 (4) 1.431 (5) 1.421 (5) 1.422 (4)* 1.142 (7) 1.141 (7) 1.138 (8) 1.153 (3) 1.135 (5) C(1)-N(1) 1.159 1.142 (9) 1.145 (6) 1.143 (3) 1.152 (7) 1.141 (6) 1.122 (8) 1.139 (3) 1.127 (5) 1.112 (10) C(3)-N(2) 1.159 1.141 (7) 1.155 (3) 1.128 (7) 1.129 (8) 1.124 (7) 1.151 (4) 1.146 (5) 1.152 (14) C(5)-N(3) 1.157 1.145 (5) 1.142 (4) C(7)-N(4) 1.159* 1.134 (7) 1.141 (6)* 1.140 (7) 1.143 (2) 1.142 (6) 1.106 (11) 1.155 (5) 1.155 (3). 1.159* 1.134 (6) 1.141 (7)* 1.141 (7) 1.135 (4) 1.136 (6) C(8)-N(5) 1.111 (11) 1.139 (6) 1.143 (3)* "The asterisk denotes a redundant coordinate. Figure 1 displays the numbering scheme used in this table. 'RuTTcpPCP is ruthenium(I1) (triphenylphosphine)(trimethoxyphosphine)(cyclopentadienyl)ptacyanopropenide: RU[P(OM~)~](PP~~)(~)-C~H~)PCP. cDMATPCP is 2-(dimethylamino)-5-methy1-1,3-thiazoliurnpentacyanopropenide: [C6HllN2S]+[C3(CN)S]-.dDMFePCP is decamethylferrocenium pentacyanoe Room temperature. propenide: [Fe(CSMe5)2]'+[C3(CN)S]-. c axis. However, upon solution of the structure it became apparent
that there was a great deal of disorder in the pyridinium ion as evidenced by large thermal parameters in the plane of the cation (Le., approximately in the b direction, &). Since the ellipsoids indicated some sort of torsional disorder, the structure was refined with the cation held as a rigid hexagon in two symmetry related orientations, each with half occupancy. The first orientation (shown in Figure 2) has contact distances from the p ridinium proton to N(1a') of 2.674 A and to N(1b) of 2.133 In the second orientation (generated by the 2-fold c axis in the horizontal
H.
direction in Figure 2) these contact distances are reversed. The angle between the principal axes of the two orientations is approximately 24O. The single diffraction experiment cannot distinguish between a static statistical distribution of cations in different orientations or a librational mode of all the cation^.^^^*^ (24) It is psible to determine whether a disorder follows a behavior proportional to kT by doing crystal structures at a series of different temperatures. See,for example: Brock, C. p.; Morelan, G. L.J . Phys. Chem. 1986, 90, 563 1.
lI1,2,3,3-PentacyanopropenideAnion
The Journal of Physical Chemistry, Vol. 92, No. 24, 1988 6895
TABLE III: Experimental and Theoretical Bond Angles (degrees) in Various Salts of the 1,1,2,3,1Peatacy~aopropenideAniona STO-3G KPCP KPCP DMATbond (C,) CsPCP" F'yHPCP RbPCP (-130 "C) (RTIb RuTTcuPCP'~ PCP" 123.8 124.0 (4) 124.1 (4) 123.4 (4) 122.5 (2) 123.1 (2) 123.3 (6) 123.0 (4) 120.6 119.3 (5) 118.8 (4) 119.2 (4) 120.3 (2) 119.4 (3) 120.0 (3) 120.6 (7) 115.6 116.7 (4) 117.0 (4) 117.3 (4) 117.1 (2) 117.5 (3) 116.9 (3) 116.1 (5) 115.0 115.2 (4) 114.6 (3) 114.5 (4) 114.9 (1) 113.7 (4) 115.3 (6) 115.6 (2) 115.0* 115.1 (4) 114.6 (3)* 116.6 (4) 116.2 (1) 116.1 (2) 116.2 (3) 113.5 (7) 129.7 (4) 131.2 (8) 130.0 130.7 (5) 128.9 (4) 128.9 (1) 128.3 (2) 130.0 (3) 123.8* 124.1 (4)* 123.0 (4) 121.6 (4) 120.4 (2) 123.9 (3) 120.1 (3) 121.9 (7) 120.6* 120.5 (5) 118.8 (4)* 120.0 (4) 122.0 (1) 121.8 (2) 121.3 (3) 120.6 (8) 115.6' 117.0 (4). 118.4 (4) 114.7 (3) 116.5 (4) 117.6 (2) 118.1 (3) 117.5 (7) 176.6 178.5 (6) 174.3 (5) 177.7 (6) 178.3 (2) 177.2 (6) 176.8 (4) 178.5 (5) 179.0 179.7 (6) 179.1 (8) 179.5 (4) 177.7 (7) 178.2 (3) 179.1 (4) 178.2 (4) 179.6 (6) 180.0 180.0 (1) 177.2 (5) 178.1 (3) 176.9 (4) 177.2 (9) 177.5 (4) 179.0' 179.6 (7) 179.5 (4)* 174.9 (6) 176.1 (2) 179.2 (4) 175.6 (4) 169.3 (11) 176.6* 176.7 ( 5 ) 174.3 (5)* 176.8 (6) 175.4 (4) 173.9 (6) 179.1 (4) 174.3 (8)
DMFePCPL5 (-100 "C) 122.7 (2) 120.7 (2) 116.6 (2) 115.2 (1) 115.2 (1)* 129.7 (3) 122.7 (2)* 120.7 (2)* 116.6 (2)* 178.3 (3) 179.4 (5) 180 (4) 179.4 (5)* 178.3 (3)*
"The asterisk denotes a redundant coordinates. Figure 1 displays the numbering scheme used in this table. bRoom temperature. Nllll
A
Figure 2. ORTEP drawing of the relative packing in the structure of pyridinium 1,1,2,3,3-pentacyanopropenidewith hydrogen bonding indicated by dotted lines. The view is approximately down the a axis with the c axis horizontal. All PyH' cations are shown only in the orientation where they are H-bonding to the uppermost PCP anions.
Detailed structural data are presented inTables I1 and 111. The room-temperature structure of KPCP was solved, yielding K and PCP positions. However, difference electron density maps yielded additional peaks (1.1-1.2 e-/A3) attributed to a second orientation for the PCP anion. Therefore, the data were recollected at low temperature in which the structure of a portion of the second conformation became more distinct. As can be seen in Figure 3, the two conformations occupy essentially the same amount of space. This second conformation was thus included in the model and allowed to refine with isotropic temperature factors and with the occupancy of the second conformation optimized as a free variable. In addition, since some of the cyano bond lengths became artificially long, an approximate value of 1.11 f 0.03 A was chosen and allowed to refine for the second conformation C=N bond lengths.22 Full optimization of the second conformation was not attempted. The value of the occupancy free variable after refinement showed that the second orientation occupied 10.4% of the sites. From application of Hamilton's test this construct represents a significant improvement since the R value dropped from 4.12% before the second orientation was added to 2.63% after it was added while increasing from 128 to 181 the number of least-squares parameters.26 For the rubidium salt, the room-temperature difference peaks on the Fourier map indicated the presence of the second conformation also, but the original difference peaks were only about half as large. Since the two structures are isomorphic, the atomic coordinates for the second conformation were grafted from the KPCP low-temperature results. However, the solution was more difficult because the least-squares refinement diverged upon letting the positional and isotropic thermal parameters vary. The pro(25) A table of atomic positions is available for CsPCP as supplementary material, as are tables of atomic positions and structure factors for the other three salts. (26) Hamilton, C.K. Acta Crystallogr. 1965, 18, 502.
Figure 3. ORTEP drawing of the first conformation and circle drawing of the second conformation of the PCP anion from the structure of potassium 1,1,2,3,3-pentacyanopropenide.For the first conformation,the carbons are numbered 1-8, and the nitrogens 1-5. For the second conformation 10 was added to each of these values.
cedure used, therefore, was to hold the occupancy of the second conformation fixed at approximately 6%, to fix the atoms in the second set, and to let the positions of the first conformation refine. In a similar fashion, the positions and occupancies of the first conformation were then fixed, and the second conformation carbons allowed to refine. In the final cycles the atoms of the second conformation were held fixed, and the first conformation positions again allowed to vary. The refined occupancy of the second conformation was 5.7%, approximately half that of the KPCP structure. Tables of atomic coordinates and equivalent isotropic thermal parameters for the PyH structure and for the first conformation of the R b and K (low temperature) structures are presented in Table IV.z5 In contrast to the structure of CsPCP, the RbPCP and KPCP structures have a rather unsymmetrical arrangement of the alkali cations about the PCP anion as displayed in Figure 1. Each anion has seven nearest-neighbor alkali cations, while each cation has eight nearest-neighbor nitrogens. The data presented in Tables I1 and I11 reveal certain trends. It is seen that the KPCP and RbPCP structures have significant out-of-plane bending.27 Several of the cyano groups, especially N(4)-C(7)-C(6) and N(5)-C(8)-C(6), are bent from a linear geometry due to the out-of-plane interactions with the neighboring cations. The amount of C-C-N bending is much greater in the rubidium salt than, for example, in the cesium structure. In RbPCP the average C-C-N angle (first conformation only) is 176.8O compared to 178.8O for CsPCP. The distortion from C, symmetry lifts the equivalency of the cyano groups, removing, as we shall see, an accidental degeneracy in the frequencies of some of the C=N stretches. Interestingly, the distortion from (27) A table of calculated least-squares planes for all of the salts is available as supplementary material.
Johnson et al.
6896 The Journal of Physical Chemistry, Vol. 92, No. 24, 1988
Nanometers
TABLE I V Fractioaril Caordhatea u w l Eqdvdat htro@c Therrrml Palrmeters in Pyrieum Peatecyewpropenidcand tlte Majority Positions in Rubidium Pentecyanopropesideend Potassium Pentecyrnopropeoide
atom N(l) C(1) c(2j C(3) N(2) C(4) C(5) N(3) N(4) C(6) C(7) C(8) C(9) C(10)
xla
vlb
-,
ZIC
400
500
700
U (eguiv) . _ -
Pyridinium 0.4032 (8) 0.4132 ( 8 ) 0.4192 (5j 0.3421 (6) 0.2797 (6)
1,1,2,3,3-Pentacyanopropenide 0.2499 (8) 0.9401 (4) 0.126 (2) 0.2041 (7) 0.0269 (4) 0.084 (2) 0.1671 i 5 j 0.1364 (3) 0.064 (1) 0.2037 (4) 0.073 (1) 0.3178 (7) 0.2576 (3) 0.099 (2) 0.4370 (6) 0.059 (2) 0.0000 0.1819 (4) 0.5000 0.068 (2) o.oo00 0.2978 (4) 0.5000 0.3871 (4) 0.101 (2) o.Ooo0 0.5000 0.7416 (4) 0.060 (3) 0.4981 (26) 0.5304 (14) 0.050 (3) 0.3446 0.7061 0.4308 0.598 1 0.074 (4) 0.3010 0.4274 0.5256 0.087 ( 5 ) 0.4432 0.4914 0.081 ( 5 ) 0.6290 0.561 1 0.5588 0.065 (4) 0.6726 0.6691 0.5621
I
I 25
30
Rb N(l) C(l) C(2) C(3) N(2) C(4) C(5) N(3) C(6) C(7) N(4) C(8) N(5)
Rubidium 0.8043 (1) 0.4199 (6) 0.5349 (7) 0.6800 ( 5 ) 0.7599 (7) 0.8263 ( 5 ) 0.7426 ( 5 ) 0.8801 (6) 0.9845 (6) 0.6919 ( 5 ) 0.7686 (6) 0.8203 ( 5 ) 0.5669 ( 5 ) 0.4684 ( 5 )
1,1,2,3,3-Pentacyanopropenide 0.3390 (1) -0.0492 (1) 0.7069 (4) -0.1961 (7) 0.7184 (4) -0.1317 (7) 0.7315 (3) -0.0431 (6) 0.0098 (7) 0.6376 ( 5 ) 0.5665 (4) 0.0519 (7) 0.8298 (4) -0.0007 ( 5 ) 0.8256 (4) 0.1171 (6) 0.8181 (4) 0.2081 (7) 0.9278 (4) -0.0518 (6) 1.0195 (4) 0.0166 (7) 1.0965 (3) 0.0688 (6) 0.9412 (4) -0.1747 (6) 0.9561 (4) -0.2774 (10)
K N(l) C(l) C(2) C(3) N(2) C(4) C(5) N(3) C(6) C(7) N(4) C(8) N(5)
Potassium 0.7957 (1) 0.4162 (3) 0.5347 (2) 0.6813 (2) 0.7591 (3) 0.8241 (2) 0.7461 (2) 0.8858 (2) 0.9951 (4) 0.6945 (2) 0.7690 (2) 0.8223 ( 2 ) 0.5621 (4) 0.4625 (2)
1,1,2,3,3-Penta~yanopropenide~ 0.3297 (1) -0.0529 (1) 0.025 (1) 0.7023 (2) -0.1867 (3) 0.029 (1) 0.7096 (1) -0.1217 (3) 0.019 (1) 0.7201 (2) -0.0360 (2) 0.019 (1) 0.6228 (2) 0.0130 (3) 0.023 (1) 0.5467 (2) 0.0514 (3) 0.033 (1) 0.8200 (1) 0.0072 (2) 0.019 (1) 0.8152 (1) 0.1239 (2) 0.021 (1) 0.8090 (3) 0.2188 (5) 0.026 (1) 0.9220 (1) -0.0437 (2) 0.021 (1) 1.0170 (1) 0.0256 (2) 0.023 (1) 1.0964 (1) 0.0766 (2) 0.031 (1) 0.9351 (4) -0.1704 (7) 0.027 (1) 0.9498 (1) -0.2770 (2) 0.031 (1)
0.048 (1) 0.061 (2) 0.051 (3) 0.039 (2) 0.045 (2) 0.068 (2) 0.039 (2) 0.040 (2) 0.062 (2) 0.042 (2) 0.046 (2) 0.061 (2) 0.044 (2) 0.066 (2)
a The pyridinium ring was refined as a rigid hexagon riding off of the ring nitrogen N(4). The equivalent isotropic U is defined as one-third of the trace of the orthogonalized U,,tensor (A). bFor the 143 K data set.
planarity occurs almost entirely by cyano group bending and dihedral twisting; the sums of the angles at the propene carbons C(2), C(4), and C(6) for both the RbPCP and KFCP structures are within 0.1 O of 360.0°, In fact, it is true for all of the reported structures that they are very nearly planar about the central carbons. Also to be noted is the similarity of the bond angles (and lengths) around N ( l ) , C(1), and C(2) for the PCP that is N coordinated to a ruthenium center13 compared to the alkali salts and inorganic complexes where it acts as a counterion. Apparently the metal-to-ligand back-bonding has little manifestation in these geometric parameters. However, there is a lengthening of the C - C propene bond on the side closest to the cation (C(2)-C(4)) relative to the other formally equivalent C - C bond C(4)-C(6). With regard to the bond lengths of Table 11, the apparent shortening of the C=N bonds in the KPCP structure a t higher temperatures is an artifact of the torsional motion of the anions.% (28) Libration corrections have not been made to these bond lengths to facilitate comparison with other values obtained from the literature. See: Busing, W. R.;Levy, H. A. Acta Crystallogr. 1964, 17, 142.
20
15
cm-' x 1 0 - ~ Figure 4. Room-temperature diffuse reflectance spectra of PyHPCP, KPCP, RbPCP,and CsPCP. The rewlution was 4 nm. For comparison, the first HOMO-LUMO transition energies calculated with extended Hiickel theory are as follows: PyHPCP, 514.7 nm; KPCP, 537.5 nm; RbPCP, 519.3 nm; CsPCP, 533.4 nm.
Beyond this, there is very little trend seen in the cyano bond lengths. For the carbons, the most conspicuous trend is observed for the C - C propene bonds. Except for those cases where PCP is crystallographically restricted to a 2-fold axis (PyHPCP, DMFePCP) or has a nearly 2-fold symmetric environment (CsPCP), the anion shows the greatest distortability along the propene bonds. Comparison of the C(2)-C(4) and C(4)
