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Bridged diquaternary salts of di-2-pyridyl ketone and their radical cations: crystal structure, raman and resonance raman studies
Journal
of Molecular Structure,
Elsevier Science
Publishers
B.V.,
159 (1987)
265-278
Amsterdam
-
Printed
in The Netherlands
BRIDGED DIQUATERNARY SALTS OF DI-2-PYRIDYL KETONE AND THEIR RADICAL CATIONS: CRYSTAL STRUCTURE, RAMAN AND RESONANCE RAMAN STUDIES
D. J. BARKER Department
and L. A. SUMMERS
of Chemistry,
R. P. COONEY*, Department (Received
University
G. R. CLARK
of Chemistry, 18 December
of Newcastle,
New South
Wales 2308
(Australia)
and C. E. F. RICKARD
University
of Auckland,
Auckland
(New Zealand)
1986)
ABSTRACT Raman spectra are reported for two bridged diquaternary salts of di-2-pyridyl ketone. The crystal structure of one parent dication is also reported. Crystal data: C,,H,,Br,N,O, M, = 372.05, a = 9.369(l), i, = 11.036(4), c = 7.115(L) A, u = 97.60(2), p = 90.12(l), y = 67.79(l)“, V = 674.20 a’, triclinic, space group Pl; Z = 2, D, = 1.83, D, = 1.832 g cme3 , MoKa, h = 0.71069 A, p = 63.54 cm-‘, F(OOO) = 364, T = 292 f 1 K, R = 0.033, Resonance Raman spectra of the coloured SW = 0.032 for 1330 (I > 2.5~) reflections. radical cations obtained from the diquaternary salts by one-electron reduction are also reported. Changes in the frequency of the carbonyl stretching vibration of the parent dications are correlated with changes in the conformation of the conjugated system. Changes in the Raman spectrum of a parent dication between the solid state and aqueous solution appear to arise from a modification of molecular geometry in solution. The decrease in frequency of the carbonyl stretching mode on radical formation is attributed to a decrease in the carbonyl bond order, due to delocalisation of the odd electron. INTRODUCTION
The extensive interest in the chemistry of diquaternary salts of 2,2’- and 4,4’-bipyridine is prompted by their wide range of applications, including use in solar energy conversion systems, and as electrochromic displays and herbicides [l] . Their ability to be reduced to stable radical cations at potentials around -0.4V (NHE) is central to their utility in the above areas. The intense colour of these radicals has enabled their study by resonance Raman spectroscopy [2, 31. Our resonance Raman study of the radical cations derived from cyclic diquaternary salts of 2,2’-bipyridine (I) demonstrated the ability of the technique to identify subtle structural differences in the radicals [3]. As part of a continuing study into the vibrational spectra of a range of bipyridinium compounds, we report a crystal structure determination and Raman and resonance Raman spectra for the cyclic diquater*Author
to whom
0022-2860/87/$03.50
correspondence o 1987
should
be addressed.
Elsevier
Science
Publishers
B.V.
266
nary salts of di-2pyridyl radical cations.
ketone
(IIa:
it = 2, IIb: n = 3) and their respective
cfpu&3 *n
III
*n
IV
EXPERIMENTAL
Preparations The diquaternary salts* IIa (DPK2’+) and IIb (DPK3’+) were prepared in the dibromide form by the method of Black and Summers [4]. As reported, DPK3 dibromide is difficult to obtain in a crystalline form, and was obtained as a finely divided powder after numerous recrystallisations. Water was distilled and fractionated under nitrogen. Zinc dust (99%) from May and Baker Ltd. was used to reduce chemically the parent dications DPK2*+ and DPK3*: Solutions were purged with high purity nitrogen (99.9%). Spectroscopy The resonance Raman spectra of DPK2t and DPK3’ were reproducible, but exhibited a severe fluorescent background, thought to arise from the presence of trace decomposition products of the radicals. This may have been due in part to some sample decomposition under laser illumination, but *The systematic names of the compounds employed (DPK2”) = 7,13-dihydro-13-oxo-6FZ-dipyrido[l,2~:2’,1’g] IIb (DPK3”) = 6,7,8,14-tetrahydro-14-oxo-dipyrido[l,2~:2’,1’-d]
in this study are as follows: IIa [1,4]-diazepinediium; and [1,5]diazocinediium.
267
it is also known that the radicals are not particularly stable under ambient light conditions. Black and Summers [4] have reported that the one-electron reduction of the parent dications is not completely reversible. The decomposition products, however, are not strongly coloured and do not yield a spectrum under the conditions used to excite the resonance Raman spectra. Because of the high fluorescent background arising from radical instability and the need, as a consequence, to correct for this large background, recorded spectral intensities were not considered sufficiently precise for the construction fo detailed excitation profiles. The equipment used to record the Raman spectra has been described previously [5]. A circulating solution cell used for the recording of resonance Raman spectra has also been described previously [3]. A Nicolet MX-1E Fourier-transform infrared spectrometer was used to record infrared spectra. Crystal structure
determination
Crystals of DPK2’+ form as yellow needles by vapour diffusion of acetone into an aqueous solution of the compound. A crystal suitable for intensity data collection was mounted on a fine glass fibre and positioned on a Nonius CAD-4 diffractometer. Unit cell dimensions were derived from least-squares fits to the observed setting angles of twenty-five reflections, using %-filtered MO&Y radiation. The absence of symmetry other than the centre in the diffraction pattern showed the crystal class to be triclinic, and the structure has been solved and satisfactorily refined in space group Pi. Intensity data collection employed the 20/w scan technique with a total background/peak count time ratio of l/2. The omega scan angle was 1.0 + 0.35 tan0. Reflections were counted for either 80 s or until a(l)/1 was 0.020.* Crystal alignment and possible decomposition were monitored throughout the data collection by measuring three selected standard reflections after every hour. However, no non-statistical variations were recorded. The data were corrected for Lorentz and polarisation effects, and for absorption using empirical absorption curves derived from azimuthal scans. The maximum and minimum absorption correction factors were 1.000 and 0.839, respectively.** The structure was solved from heavy-atom Patterson and electron density maps. Refinement was by full-matrix least-squares procedures, minimising the function Z w( I F. I - IFc I)’ . Atomic scattering factors and dispersion corrections were for neutral atoms. The two bromines per asymmetric unit comprise one bromine on a general position, and two half-bromines con*u(Z) = (20.1166/iWZ)(C + 3B)“*, where the constant term is the maximum possible scan rate, NPZ is the ratio of the maximum possible scan rate to the scan rate for the measurement, C = total counts, B = total background. **Computing was carried out using the SDP suite of programs on a PDP-11 for initial data processing, and SHELX-76 on an IBM 4341 computer for structure solution and refinement.
268
strained to lie on crystallographic centres of symmetry. After initial isotropic refinement, anisotropic thermal parameters were refined for all nonhydrogen atoms. A difference electron density map revealed discrete positions for all hydrogen atoms, and these were subsequently included in the least-squares refinement with individual isotropic temperature factors. The refinement converged with R and R, ([Cw(lF, I - IFc 1)*/C, IF, I’] l”) being 0.033 and 0.032 respectively. Reflection weights were w = 0.989/ (o*(F) + gF2), with final g being 4.77 X 10e4. Final atomic coordinates for non-hydrogen atoms are listed in Table 1. Anisotropic thermal parameters, hydrogen atom coordinates, dihedral torsion angles, and tables of observed and calculated structure factor amplitudes have been deposited with B.L.L.D. as Supplementary Publication number S.U.P. 264330 (10 pages). DESCRIPTION
OF THE CRYSTAL
STRUCTURE
The crystals consist of discrete diquatemary cations of di-2-pyridyl ketone, and bromide anions. The two bromide ions in the crystallographic asymmetric unit comprise one bromide in a general position, and two half-bromide ions occupying crystallographic centres of symmetry. The atomic numbering scheme and geometry for the cation are outlined TABLE Fractional
1 coordinates
and equivalent
isotropic
Atom
X
Y
Wl)
0.4636( 1) 0.0 0.0 0.2407(6) 0.2169(6) 0.5292(6) 0.2969(B) 0.2132(7) 0.1218(B) 0.0403( 10) 0.0457( 13) 0.1346(11) 0.3142(B) 0.4804(B) 0.4448( 8) 0.5022(10) 0.6416(10) 0.7267( 10) 0.6666(g)
0.3201( 1) 0.0 ::05003(5)
Br(2) Br(3) O(1) N(1) N(2) C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(l0) C(l1) C(12) C(13)
aBeq = %71’ z‘ iZ jUijU~Cl,‘Cli.aj.
-0.2933(5) -0.2445(5) -0.1058(6) -0.1975(6) -0.1775(7) -0.2532(B) -0.3496(10) -0.3664(B) -0.3174( 7) -0.3570(6) -0.1307(6) -0.0312(B) -0.0465(g) --+X1636(9) -0.2579(9)
temperature
factors z
0.2057( 1) 0.5 0.5 0.0666( 7) 0.1772(7) 0.2923(7) 0.1224(9) 0.0753(B) -0.0820( 9) -0.1316(11) -0.0221( 14) 0.1344(12) 0.3438(B) 0.2867( 9) 0.2215(B) 0.2328(10) 0.3088( 11) 0.3730( 10) 0.3662( 10)
Be, 4.14 3.36 4.67 4.16 2.84 2.57 2.92 2.60 3.44 4.71 6.12 4.90 2.87 2.82 2.46 3.26 4.04 4.22 3.64
(a’;”
in Fig. 1. Bond lengths and angles are listed in Table 2. Where possible, corresponding values in the two halves of the cation are listed side by side to facilitate comparisons. There is consistent agreement (values differ by no more than 30) between the two halves of the cation for all bonds and angles except for those involving the two nitrogen atoms, where differences of up to 50 are found. The view of the cation shown in Fig. 2 is that looking down the carbonyl 0(1)-C(l) bond, and it can be seen that the cation is twisted asymmetrically with respect to the dipyridyl rings. Planes of best fit, and atom displacements therefrom, are given in Table 3. The pyridyl ring N(2)C(13) is nearly parallel to the carbonyl group, but N(l)-C(6) is twisted by 27.1”. A full listing of torsion angles about each bond is deposited with the supplementary data. Any conjugative effects which might be present between the carbonyl group and the pyridyl rings would be expected to operate only through the N( 2) pyridyl ring as judged from their coplanarity. We note, however, that the C(l)-C(2) and C(l)-C(9) bond lengths are indistinguishable at 1.504(9) and 1.501(g) 8. Intermolecular approach distances have been calculated. The following list gives all those shorter than 3.4 A : Br( 1) ***C( 1) 3.26; Br( 1) - - - C(2) 3.35; Br(1) ***C(9) 3.36; C(3).**0(1) 3.21; C(4)*..0(1) 3.01, C(lO)***O(l) 3.36; C(11) ***O(1) 3.07 a. Some of these contacts are particularly short, and these interactions could contribute to the asymmetry observed in the cation. The stereopair diagram (Fig. 3) shows the unit cell packing arrangement. RESULTS
AND DISCUSSION
The Raman spectra of the dibromide salts of DPK22+ in the solid state and in aqueous solution, and DPK32+ in the solid state, are presented in Fig. 4. The resonance Raman spectra of DPK2t and DPK3? are presented in Fig. 5.
Fig. 1. Cation
geometry
and atomic
numbering.
270 TABLE
2
Bond lengths
(a ) and angles (degrees)
Atoms
Bond
Atoms
Bond
0(1k-w) Nl)--C(2) Nl)--C(6) N(l)+(7) C(l)-C(2) C(2)--c(3) C(3)-C(4) C(4)--c(5) C(5)--c(6)
1.209( 7) 1.348(7) 1.321(9) 1.484(8) 1.504(9) 1.398( 9) 1.345( 10) 1.386( 12) 1.378(11)
C(7)-C(8) N(2)-C(9) N(2)--C(l3) N(2)--‘J8) C(l)-C(9) C(9)-C(lO) C(lO)-C(ll) C(ll)-C(12) C(12)-C(13)
1.491(9) 1.368(8) 1.353(9) 1.472(8) 1.501(9) 1.387(10) 1.372(11) 1.375(12) 1.353(11)
Atoms
Angle
Atoms
Angle
0(1v-31)--c(2) C(2)-w)--c(9) C(2)_-N(lW(6)
115.6(6) 128.6(6) 121.9(6) 119.8(5) 118.4(6) 124.6(5) 118.7(6) 116.6(6) 120.5(7) 119.0(7) 119.6(8) 120.3(8) 111.1(5)
C(2)-N(l)*(7) C(6)-N(lH(7) N(l)--C(2)-C(l) N(l)--C(2)--C(3) C(l)--c(2k-C(3) C(2)-c(3)-C(4) C(3)-C(4)+(5) C(4)--c(5)-C(6) C(5)-C(6)-N(1) N(lH(7)--C(8)
Fig. 2. Edge-on
115.7(6) C(9)-N(2)-C(13) C(9)-N(2)--c(8) C(13)-N(2)%(8) N(2)-C(9)-C(l) N(2)-C(9)--C(lO) C(~)Hx9)-C(lO) C(9)-C(lO)~(ll) c(1o)-C(ll)--c(12) C(ll)-C(12)--c(13) C(12)-C(13)-N(2) N(2)--C(8)+X7)
119.3(6) 123.1(6) 117.5(6) 125.4(6) 118.4(7) 116.2(6) 121.4(8) 119.1(8) 118.6(8) 123.1(8) 113.5(5)
view of the cation.
Raman spectra of the parent
dications
The crystal structure of DPK2’+ as the dibromide salt indicates that the molecule is not symmetrically oriented about the carbonyl group. The angles between the carbonyl group and the individual pyridine rings are -2” and The spectroscopic implications of this asymmetry in +24”, respectively. DPK2’+ are discussed below. Because of the recrystallisation difficulties (see Experimental section), a good quality single-crystal of DPK3’+ dibromide could not be prepared. However, on the basis of analogy, certain tentative
271 TABLE 3 Planes of best fit Plane I. N(l), C(2), C(3), C(4), C(5), C(6) Equation 0.567X - 0.586Y - 0.5792 - 1.585 = 0 Displacements (d ) O.Oll(5) C(1) N(1) 0.000(S) O(1) C(2) -0.008( 7) C(3) C(9) 0.007(9) C(4) C(7) 0.003(11) C(5) -0.012(9) C(6) Plane 2. N(2), C(9), C(lO), C(ll), C(12), C(13) Equation 0.336X - 0.264Y - 0.9042 - 0.203 = 0 Displacements (A) -0.007(5) N(2) C(1) 0.013(6) C(9) O(1) -0.004(8) C(l0) C(2) -0.010(8) C(11) C(8) 0.017(8) C(12) -0.009( 8) C(l3) Plane 3. O(l), C(l), C(2), C(9). Equation 0.422X - 0.251Y - 0.8712 - 0.553 = 0 Displacements (a) 0.006(5) O(1) N(1) -0.016(7) C(l) N(2) 0.005( 6) C(2) C(7) 0.005(S) C(9) C(8) Angles between normals to planes Plane 1 Plane 2 Plane 1 Plane 3 Plane 2 Plane 3
A
-0.053(7) -0.542(5) 0.476(7) 0.032(7)
0.121(7) 0.147(5) 0.264(6) 0.020(7)
-0.503(5) 0.013(5) -1.081(6) -0.056(7)
29.7” 27.1” 5.3”
b 0
c
Fig. 3. Stereopair diagrams showing the unit cell contents.
conclusions can be drawn as to its structure, relative to that determined for DPK2’+. The crystal structure of the carbocyclic analogue of DPK2*+ (IIIa: n = 2) has been determined [ 61, in which the torsional angles are reported to be +14” and +30”. Polarographic [7] and acidity [8] measure-
272
I 900
I
1200 Raman
1500 Shift/cm-’
3
Fig. 4. Raman spectra of DPK2 dibromide as (a) solid and (b) 0.3 M aqueous solution, and (c) DPK3 dibromide as solid. (Exciting line for (a) and (b): 514.5 nm Ar’; for (c), 476.5 nm Ar’. Band-pass for (a): 5 cm-’ ; for (b) and (c), 10 cm-’ .)
merits for the carbocyclic analogue of DPK3’+ (IIIb: n = 3) have been interpreted in terms of a more twisted structure (i.e. greater torsional angles) compared to IIIa. Furthermore, crystal structures determined for bridged diquaternary salts of 2,2’-bipyridine (I: n = 2,4) [9, lo] have shown that an increase in the length of the bridging chain forces the aromatic system further from planarity. It is therefore expected that DPK32+ will exhibit increased torsional angles, relative to those measured for DPK22+. The Raman spectra of the parent dications (IIa, b) are complex, due to the species’ size and low molecular symmetry. However, our study of the bridged diquaternary salts of 2,2’-bipyridine [ 31 has shown that subtle differences in molecular geometry can be identified from complex spectra. It was shown that trends in the frequencies of certain conjugation-sensitive
273
9bo Raman
Go0 Shift /cm-’
1 00
Fig. 5. Resonance Raman spectra of (a) DPK2t and (b) DPK3t (0.01 M in water, 514.5 nm Ar+, Band-pass: 10 cm-’ slit).
bands common to each spectrum, could be correlated with known differences in structure. Pyridine ring stretching modes in the region of 1490-1630 cm-l, for example, were utilised in this manner. It is anticipated that the corresponding modes in the spectra of DPK2*+ and DPK3*+ should likewise be sensitive to changes in conjugation across the Ar-CO-Ar system (where Ar refers to an aromatic ring). The vibrational frequency and intensity of the symmetric stretching vibration of the carbonyl group has been shown to be particularly sensitive to its steric and electronic environment [ 11-151. It would be expected that a difference in conjugation between DPK2*+ and DPK3*+ should manifest itself in different stretching vibrations for the respective carbonyl groups. The respective carbonyl stretching frequencies observed for DPK2*+ and DPK3*+ (in the solid state) are 1682 cm-’ and 1708 cm-‘. The dependence of the carbonyl stretching frequency on conjugation has been illustrated by de Roos [13] for a series of ortho-substituted benzophenones. It was shown that increasing the size of the substituent caused an increase in the C----O stretching frequency, presumably as a result of a decrease in conjugation between the benzene rings and the carbonyl group. In view of this, it seems reasonable to attribute the 26 cm-’ shift in the C=G frequency between DPK2*+ and DPK3*’ to a decrease in the conjugation, brought about by the more twisted structure expected for the latter (see above). A striking difference in the solid state spectrum of DPK2*+ compared to
274
that of DPK3’+ is in the relative intensities of the respective carbonyl bands. Shigorin [ 151 has shown that the Raman intensity of the carbonyl stretching band increases dramatically as the extent of conjugation associated with the group increases. Therefore the significant increase in relative intensity of the C=O band between DPK22+ and DPK3’+ is as predicted from the crystal structure of the former (this study) and the more twisted structure anticipated for the latter. As discussed above, certain pyridine ring stretching modes were found to be sensitive to changes in conjugation in a series of bridged diquaternary salts of 2,2’-bipyridine [3]. It is uncertain whether bands in the corresponding region (1500-1620 cm-‘) of the spectra of DPK22+ and DPK32+ (see Fig. 4) can be correlated with conjugation differences between the two species. The complex nature of the spectrum of DPK22+ in this region (three sets of double peaks) contrasts with the simpler pattern observed for DPK3’+ (one dominant and two weak bands). There is, however, an apparent shift to higher frequencies of the bands in this region between DPK22+ and DPK32+, consistent with a localising of electron density within the two pyridine rings of the latter. The crystal structure of DPK22+ as the dibromide salt (see above) indicates that the two pyridine rings are not equivalent in their orientation, relative to the carbonyl group (torsional angles of -2” and +24”). As a consequence, the extent of Ar+0 conjugation should be different for each pyridine ring. This would account for the peak-doubling (see above) in the ring stretching mode region (1500-1620 cm-‘) of the spectrum of DPK22+. The intense group of bands in the 1050-1200 cm-’ region of the spectra of DPK22+ and DPK32+ could not be unambiguously assigned, since a number of corresponding vibrations of analogous molecules occurs in this region. Blazevic and Colombo [16] have assigned the symmetric Ar-C-Ar stretching vibration in benzophenone to a band at 1149 cm-‘, and bands in this region of the spectra of various quatemised pyridines have been assigned to in-plane C-H deformations, mixed with N’-C stretching modes [ 171. Figure 4 shows the considerable difference in the Raman spectrum of DPK22+ between the solid state and aqueous solution. The most noticeable changes between solid and solution spectrum involve the carbonyl band and the pyridine ring stretching modes (see Table 1). The C=O band undergoes a +13 cm-’ shift, together with a ca. five-fold decrease in relative intensity on dissolution. There is a simplification of the ring stretching mode region (three single peaks), as well as a shift in each band towards higher frequencies on dissolution. In view of the above observations regarding the conjugation-sensitivity of the frequencies and intensities of these bands, we ascribe the change in the Raman spectrum of DPK22’ between solid and solution to a modification of the molecular geometry on dissolution. The evidence suggests a change from C, point group symmetry for the DPK22’ species in the crystal to C2 symmetry for the corresponding solution species. This evidence consists, firstly, of increases in the frequencies of the ring stretching
275
modes (1500-1700 cm-‘) which favour higher torsional angles and increased electronic localization between the aromatic rings. Secondly, the simplification of the spectrum on going from crystal to solution favours a symmetrical orientation of the C=O bond (along a CZ axis) relative to the aromatic rings. Other factors influencing the change in spectrum of DPK2’+ on dissolution, including H-bonding in aqueous solution and the loss of interactive forces present in the solid state, must be considered. It is not possible to directly elucidate the effect that these factors have on the Raman spectrum of DPK2*+, since it is essentially insoluble in organic solvents. Consequently, examination of the behaviour of the carbonyl band in the spectra of related compounds is necessary. The carbonyl stretching frequency observed for a range of ketones, both conjugated and isolated, has been shown using both Raman and infrared spectroscopy to be extremely dependent upon the physical state of the compound [ll, 14, 161. In ketones as different as benzophenone [ll, 161 and acetone [ll, 181, the carbonyl frequency is observed to increase between pure compound and dissolution in non-interacting media. A decrease in frequency is observed for these molecules, however, as the H-bonding strength of the solvent increases [ 111 (see Table 4). The much-studied case of benzophenone provides an example of the frequency sensitivity (solid: 1650 cm-l [16]; solution: 1673 cm-’ (hexane) [ll]; 1657 cm-’ (ethanol) [ll]). Similar behaviour is observed for di-2pyridyl ketone (DPK), from which DPK2’+ is prepared (solid: 1677 cm-‘; solution: 1693 cm-’ (hexane), 1676 cm-’ (water)). In view of this sensitivity of the carbonyl frequency to changes in physical state, the conformational significance of the +13 cm-’ shift between solid and solution spectra for DPK*+ depends on the interpretation of changes in other regions of the spectrum. Specifically, data for benzophenone [16] and di-2-pyridyl ketone (in the present study) indicate that the aromatic ring stretching mode frequencies are not sensitive to changes in physical state of the molecules (see Table 1). Therefore it is suggested that the positive shift in frequencies observed for these bands between the solid and solution spectra of DPK2*+ (see Table 1) is associated with a change in the ArCO torsional angles on dissolution (see preliminary discussion above). The five-fold decrease in relative intensity of the carbonyl band between the solid and solution spectra of DPK2*+ cannot be confidently interpreted in terms of the proposed conformational changes, since similar intensity decreases are noted between solid and solution spectra for benzophenone [ 161 and di-2-pyridyl ketone. Resonance
Raman spectra of the radical cations
Dilute coloured aqueous solutions of the radical monocations derived from DPK2*+ and DPK321 exhibit resonance Raman spectra (see Fig. 5). No evidence for dimer formation was observed for DPKB? and DPK3$, as the
276 TABLE 4 Vibrational frequencies of selected modes of bridged diquaternary salts of di-2-pyridyl ketone ( DPK22’, DPK3’+) and related molecules Species
State
Ring stretches (cm-‘)
v(C=o) (cm-‘)
Detecteda
DPK2” DPK2*+ DPKP .* DPK3” DPK3 + DPK
solid H,O H,C solid H,C solid hexane CHCl, H,C solid hexane benzene ethanol
1502,1519,1586,1610,1621 1514,1588,1617 1584 1528,1617 1579 1568,1583 1571,1585 1571,1585 1569,1584 1597 1600 -
1682 1695 1637 1708 1634 1677 1693 1684 1676 1650 1673 1662 1657
R R RR R RR R IR R, IR R Rb IRC Rb IRC
BP
aDetected by R, Raman; RR, resonance Raman or IR, infrared. bRef. 16, ‘Ref. 11.
frequencies and relative intensities of all key bands remained unchanged over a range of concentrations. This is as reported for the radical cations derived from the diquaternary salts of 2,2’-bipyridine (I: 12 = 2, 3) [3], though it is in contrast with data reported for diquaternary salts of 4,4’-bipyridine [2]. All resonance Raman lines in the spectra of DPK2’ and DPK3? were polarised and so were attributed to totally symmetric vibrations. Black and Summers [4] have proposed on the basis of analogy with benzophenone radical anion (BP;) [19] that the odd electron in DPKB? and DPK3 should reside partially on the carbonyl oxygen (IV). This would cause a lowering in the carbonyl bond order, relative to that in the dication, manifesting itself in the vibrational spectrum as a decrease in frequency. Such a shift in the carbonyl frequency between parent and reduced species has been observed for benzophenone and its derivatives, though there is dispute as to the actual frequency in the case of BP;. Eargle [20] has used electron density data to assign a band at ca. 1550 cm-’ in the infrared spectrum of BP; to the carbonyl vibration. Juchnovski and co-workers [21] have employed isotopic substitution to make their assignment to a band ca. 1400 cm-‘. No attempt was made by Aleksandrov et al. [22] to assign any peaks in the resonance Raman spectrum of BP;. An intense band occurring in the resonance spectra of DPK2’ and DPK3t is assigned to the carbonyl at 1637 cm-’ and 1634 cm-‘, respectively, stretching mode, since the frequencies are too high to be attributed to the ring stretching modes. The latter were observed at 1500-1620 cm-’ in the
277
parent spectra, and are expected to decrease in frequency on radical formation, in accord with observations for the related modes in the 2,2’-bipyridine study [3]. The frequency shift of the assigned carbonyl band between parent and radical for DPK2*+ and DPK32+ is -45 cm-’ and -74 cm-‘, respectively. The difference is attributed to a greater conformational adjustment required for the latter to attain a near-planar Ar-CO-Ar configuration. This frequency difference can also ‘be related to differences in the half-wave potentials reported for the two species (E (SCE): DPK22+ -0.11 V; DPK32+ -0.17 V) [4]. It is interesting to note that the corresponding frequency shift reported for benzophenone (-110 cm-’ or -272 cm-’ [20, 211) is considerably greater than those observed for DPK22+ and DPK32+. This presumably reflects the considerable destabilisation caused by loss of aromaticity of the rings in BP:, and the consequent localising of the odd electron on the carbonyl group. Only one ring stretching mode is found to be strongly enhanced in the resonance spectrum of each species (DPKB’: 1584 cm-‘: DPK3+: 1579 cm-‘), at a lower frequency than that observed for the dominant ring mode in the parent spectra. This is consistent with the expectation of lowered ring force constants on radical formation, due to increased conjugation. In the 2,2’-bipyridine study [ 31, interpretations based on the frequency-dihedral angle dependence of certain bands suggested a more planar structure for the radical cation of Ib (n = 3) than for Ia (n = 2). However, the differences in frequencies of these key bands between the resonance Raman spectra of the radical forms of Ia and Ib were far greater than those observed for the carbony1 and ring stretching bands in the spectra of DPKB? and DPK3+. It is therefore concluded that the conjugated structures of the aromatic systems of DPK2t and DPK3’ are similar. As noted above for the dication spectra, the intense peaks in the lOOO1400 cm-’ region of each spectrum (see Fig. 5) could not be assigned with any confidence, due to the large number of possible vibrations. However, differences between the spectra of DPK2+ and DPK3’ in this region are marked (see Fig. 5). The medium-intensity peaks at 1199 cm-’ and 1231 cm-’ in the spectrum of DPK2t are seen to shift to 1149 cm-l and 1282 cm-‘, respectively, and to decrease in intensity in the spectrum of DPK3+. It is tentatively proposed that these shifts are due to vibrational coupling, which will be more pronounced as the aromatic system tends towards coplanarity. If this is the case, then it suggests that the the Ar-CO-Ar system of DPK3+ is nearer to co-planarity than that of DPK2+. A similar trend was observed for the analogous radical cations derived from Ib and Ia [3]. While no intense resonance Raman lines were observed in the region below 900 cm-‘, less intense features may have been obscured by the fluorescent background. In our earlier study of the 2,2’-bipyridine-based radicals [3], only very weak bands were observed in this region of the resonance Raman spectra, and these played no part in the interpretation.
218 ACKNOWLEDGEMENT
One of the authors (D.J.B.) is grateful to the Commonwealth Department of Education for a Commonwealth Postgraduate Research Award. REFERENCES 1 L. A. Summers, in A. R. Katritzky (Ed.), Advances in Heterocyclic Chemistry, Vol. 35, Academic Press, New York, 1984, pp. 281-374. 2 M. Forster, R. B. Girling and R. E. Hester, J. Raman Spectrosc., 12 (1982) 36. 3 D. J. Barker, R. P. Cooney and L. A. Summers, J. Raman Spectrosc., 16 (1985) 265. 4 A. L. Black and L. A. Summers, J. Chem. Sot. C, (1970) 2394. 5 D. J. Barker, R. P. Cooney and L. A. Summers, J. Raman Spectrosc., in press. 6 J. P. Reboul, J. C. Soyfer, B. Cristau, N. Darbon, Y. Oddon and G. Pepe, Acta Crystallogr., Sect. C, 39 (1983) 600. 7 V. M. Kazakova, I. G. Makarov, A. I. Samokhvalova and D. V. Ioffe, J. Struct. Chem., 15 (1974) 209. 8 R. Stewart, M. R. Granger, R. B. Moodie and L. J. Muenster, Can. J. Chem., 41 (163) 1065. 9 J. E. Derry and T. A. Hamor, Nature (London), 221 (1969) 464. 10 J. E. Derry and T. A. Hamor, J. Chem. Sot. Chem. Commun., (1970) 1284. 11 M. Ito, K. Inuzuka and S. Imanishi, J. Chem. Phys., 31(1959) 1694. 12 L. J. Bellamy and R. L. Williams, Trans. Faraday Sot., 55 (1959) 14. 13 A. M. de Roos, Rec. Trav. Chim. Pays-Bas, 87 (1968) 1359. 14 R. S. Becker, J. Mol. Spectrosc., 3 (1959) 1. 15 D. Shigorin, Dokl. Akad. Nauk. S.S.S.R., 96 (1954) 769. 16 J. Blazevic and L. Colombo, J. Raman Spectrosc., 11 (1981) 143. 17 E. Spinner, J. Chem. Sot., (1963) 3660. 18 G. Fini, P. Mirone and P. Patella, J. Mol. Spectrosc., 28 (1968) 144. 19 P. H. Rieger and G. K. Fraenkel, J. Chem. Phys., 37 (1962) 2811. 20 D. H. Eargle, J. Chem. Sot. (B), (1970) 1556. 21 I. Juchnovski, T. Kolev and I. Rashkov, Spectrosc. Lett., 18 (1985) 171. 22 I. V. Aleksandrov, Y. S. Bobovich, V. G. Maslov and A. N. Sidorov, JETP Lett., 17 (1973) 219.
of Molecular Structure,
Elsevier Science
Publishers
B.V.,
159 (1987)
265-278
Amsterdam
-
Printed
in The Netherlands
BRIDGED DIQUATERNARY SALTS OF DI-2-PYRIDYL KETONE AND THEIR RADICAL CATIONS: CRYSTAL STRUCTURE, RAMAN AND RESONANCE RAMAN STUDIES
D. J. BARKER Department
and L. A. SUMMERS
of Chemistry,
R. P. COONEY*, Department (Received
University
G. R. CLARK
of Chemistry, 18 December
of Newcastle,
New South
Wales 2308
(Australia)
and C. E. F. RICKARD
University
of Auckland,
Auckland
(New Zealand)
1986)
ABSTRACT Raman spectra are reported for two bridged diquaternary salts of di-2-pyridyl ketone. The crystal structure of one parent dication is also reported. Crystal data: C,,H,,Br,N,O, M, = 372.05, a = 9.369(l), i, = 11.036(4), c = 7.115(L) A, u = 97.60(2), p = 90.12(l), y = 67.79(l)“, V = 674.20 a’, triclinic, space group Pl; Z = 2, D, = 1.83, D, = 1.832 g cme3 , MoKa, h = 0.71069 A, p = 63.54 cm-‘, F(OOO) = 364, T = 292 f 1 K, R = 0.033, Resonance Raman spectra of the coloured SW = 0.032 for 1330 (I > 2.5~) reflections. radical cations obtained from the diquaternary salts by one-electron reduction are also reported. Changes in the frequency of the carbonyl stretching vibration of the parent dications are correlated with changes in the conformation of the conjugated system. Changes in the Raman spectrum of a parent dication between the solid state and aqueous solution appear to arise from a modification of molecular geometry in solution. The decrease in frequency of the carbonyl stretching mode on radical formation is attributed to a decrease in the carbonyl bond order, due to delocalisation of the odd electron. INTRODUCTION
The extensive interest in the chemistry of diquaternary salts of 2,2’- and 4,4’-bipyridine is prompted by their wide range of applications, including use in solar energy conversion systems, and as electrochromic displays and herbicides [l] . Their ability to be reduced to stable radical cations at potentials around -0.4V (NHE) is central to their utility in the above areas. The intense colour of these radicals has enabled their study by resonance Raman spectroscopy [2, 31. Our resonance Raman study of the radical cations derived from cyclic diquaternary salts of 2,2’-bipyridine (I) demonstrated the ability of the technique to identify subtle structural differences in the radicals [3]. As part of a continuing study into the vibrational spectra of a range of bipyridinium compounds, we report a crystal structure determination and Raman and resonance Raman spectra for the cyclic diquater*Author
to whom
0022-2860/87/$03.50
correspondence o 1987
should
be addressed.
Elsevier
Science
Publishers
B.V.
266
nary salts of di-2pyridyl radical cations.
ketone
(IIa:
it = 2, IIb: n = 3) and their respective
cfpu&3 *n
III
*n
IV
EXPERIMENTAL
Preparations The diquaternary salts* IIa (DPK2’+) and IIb (DPK3’+) were prepared in the dibromide form by the method of Black and Summers [4]. As reported, DPK3 dibromide is difficult to obtain in a crystalline form, and was obtained as a finely divided powder after numerous recrystallisations. Water was distilled and fractionated under nitrogen. Zinc dust (99%) from May and Baker Ltd. was used to reduce chemically the parent dications DPK2*+ and DPK3*: Solutions were purged with high purity nitrogen (99.9%). Spectroscopy The resonance Raman spectra of DPK2t and DPK3’ were reproducible, but exhibited a severe fluorescent background, thought to arise from the presence of trace decomposition products of the radicals. This may have been due in part to some sample decomposition under laser illumination, but *The systematic names of the compounds employed (DPK2”) = 7,13-dihydro-13-oxo-6FZ-dipyrido[l,2~:2’,1’g] IIb (DPK3”) = 6,7,8,14-tetrahydro-14-oxo-dipyrido[l,2~:2’,1’-d]
in this study are as follows: IIa [1,4]-diazepinediium; and [1,5]diazocinediium.
267
it is also known that the radicals are not particularly stable under ambient light conditions. Black and Summers [4] have reported that the one-electron reduction of the parent dications is not completely reversible. The decomposition products, however, are not strongly coloured and do not yield a spectrum under the conditions used to excite the resonance Raman spectra. Because of the high fluorescent background arising from radical instability and the need, as a consequence, to correct for this large background, recorded spectral intensities were not considered sufficiently precise for the construction fo detailed excitation profiles. The equipment used to record the Raman spectra has been described previously [5]. A circulating solution cell used for the recording of resonance Raman spectra has also been described previously [3]. A Nicolet MX-1E Fourier-transform infrared spectrometer was used to record infrared spectra. Crystal structure
determination
Crystals of DPK2’+ form as yellow needles by vapour diffusion of acetone into an aqueous solution of the compound. A crystal suitable for intensity data collection was mounted on a fine glass fibre and positioned on a Nonius CAD-4 diffractometer. Unit cell dimensions were derived from least-squares fits to the observed setting angles of twenty-five reflections, using %-filtered MO&Y radiation. The absence of symmetry other than the centre in the diffraction pattern showed the crystal class to be triclinic, and the structure has been solved and satisfactorily refined in space group Pi. Intensity data collection employed the 20/w scan technique with a total background/peak count time ratio of l/2. The omega scan angle was 1.0 + 0.35 tan0. Reflections were counted for either 80 s or until a(l)/1 was 0.020.* Crystal alignment and possible decomposition were monitored throughout the data collection by measuring three selected standard reflections after every hour. However, no non-statistical variations were recorded. The data were corrected for Lorentz and polarisation effects, and for absorption using empirical absorption curves derived from azimuthal scans. The maximum and minimum absorption correction factors were 1.000 and 0.839, respectively.** The structure was solved from heavy-atom Patterson and electron density maps. Refinement was by full-matrix least-squares procedures, minimising the function Z w( I F. I - IFc I)’ . Atomic scattering factors and dispersion corrections were for neutral atoms. The two bromines per asymmetric unit comprise one bromine on a general position, and two half-bromines con*u(Z) = (20.1166/iWZ)(C + 3B)“*, where the constant term is the maximum possible scan rate, NPZ is the ratio of the maximum possible scan rate to the scan rate for the measurement, C = total counts, B = total background. **Computing was carried out using the SDP suite of programs on a PDP-11 for initial data processing, and SHELX-76 on an IBM 4341 computer for structure solution and refinement.
268
strained to lie on crystallographic centres of symmetry. After initial isotropic refinement, anisotropic thermal parameters were refined for all nonhydrogen atoms. A difference electron density map revealed discrete positions for all hydrogen atoms, and these were subsequently included in the least-squares refinement with individual isotropic temperature factors. The refinement converged with R and R, ([Cw(lF, I - IFc 1)*/C, IF, I’] l”) being 0.033 and 0.032 respectively. Reflection weights were w = 0.989/ (o*(F) + gF2), with final g being 4.77 X 10e4. Final atomic coordinates for non-hydrogen atoms are listed in Table 1. Anisotropic thermal parameters, hydrogen atom coordinates, dihedral torsion angles, and tables of observed and calculated structure factor amplitudes have been deposited with B.L.L.D. as Supplementary Publication number S.U.P. 264330 (10 pages). DESCRIPTION
OF THE CRYSTAL
STRUCTURE
The crystals consist of discrete diquatemary cations of di-2-pyridyl ketone, and bromide anions. The two bromide ions in the crystallographic asymmetric unit comprise one bromide in a general position, and two half-bromide ions occupying crystallographic centres of symmetry. The atomic numbering scheme and geometry for the cation are outlined TABLE Fractional
1 coordinates
and equivalent
isotropic
Atom
X
Y
Wl)
0.4636( 1) 0.0 0.0 0.2407(6) 0.2169(6) 0.5292(6) 0.2969(B) 0.2132(7) 0.1218(B) 0.0403( 10) 0.0457( 13) 0.1346(11) 0.3142(B) 0.4804(B) 0.4448( 8) 0.5022(10) 0.6416(10) 0.7267( 10) 0.6666(g)
0.3201( 1) 0.0 ::05003(5)
Br(2) Br(3) O(1) N(1) N(2) C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(l0) C(l1) C(12) C(13)
aBeq = %71’ z‘ iZ jUijU~Cl,‘Cli.aj.
-0.2933(5) -0.2445(5) -0.1058(6) -0.1975(6) -0.1775(7) -0.2532(B) -0.3496(10) -0.3664(B) -0.3174( 7) -0.3570(6) -0.1307(6) -0.0312(B) -0.0465(g) --+X1636(9) -0.2579(9)
temperature
factors z
0.2057( 1) 0.5 0.5 0.0666( 7) 0.1772(7) 0.2923(7) 0.1224(9) 0.0753(B) -0.0820( 9) -0.1316(11) -0.0221( 14) 0.1344(12) 0.3438(B) 0.2867( 9) 0.2215(B) 0.2328(10) 0.3088( 11) 0.3730( 10) 0.3662( 10)
Be, 4.14 3.36 4.67 4.16 2.84 2.57 2.92 2.60 3.44 4.71 6.12 4.90 2.87 2.82 2.46 3.26 4.04 4.22 3.64
(a’;”
in Fig. 1. Bond lengths and angles are listed in Table 2. Where possible, corresponding values in the two halves of the cation are listed side by side to facilitate comparisons. There is consistent agreement (values differ by no more than 30) between the two halves of the cation for all bonds and angles except for those involving the two nitrogen atoms, where differences of up to 50 are found. The view of the cation shown in Fig. 2 is that looking down the carbonyl 0(1)-C(l) bond, and it can be seen that the cation is twisted asymmetrically with respect to the dipyridyl rings. Planes of best fit, and atom displacements therefrom, are given in Table 3. The pyridyl ring N(2)C(13) is nearly parallel to the carbonyl group, but N(l)-C(6) is twisted by 27.1”. A full listing of torsion angles about each bond is deposited with the supplementary data. Any conjugative effects which might be present between the carbonyl group and the pyridyl rings would be expected to operate only through the N( 2) pyridyl ring as judged from their coplanarity. We note, however, that the C(l)-C(2) and C(l)-C(9) bond lengths are indistinguishable at 1.504(9) and 1.501(g) 8. Intermolecular approach distances have been calculated. The following list gives all those shorter than 3.4 A : Br( 1) ***C( 1) 3.26; Br( 1) - - - C(2) 3.35; Br(1) ***C(9) 3.36; C(3).**0(1) 3.21; C(4)*..0(1) 3.01, C(lO)***O(l) 3.36; C(11) ***O(1) 3.07 a. Some of these contacts are particularly short, and these interactions could contribute to the asymmetry observed in the cation. The stereopair diagram (Fig. 3) shows the unit cell packing arrangement. RESULTS
AND DISCUSSION
The Raman spectra of the dibromide salts of DPK22+ in the solid state and in aqueous solution, and DPK32+ in the solid state, are presented in Fig. 4. The resonance Raman spectra of DPK2t and DPK3? are presented in Fig. 5.
Fig. 1. Cation
geometry
and atomic
numbering.
270 TABLE
2
Bond lengths
(a ) and angles (degrees)
Atoms
Bond
Atoms
Bond
0(1k-w) Nl)--C(2) Nl)--C(6) N(l)+(7) C(l)-C(2) C(2)--c(3) C(3)-C(4) C(4)--c(5) C(5)--c(6)
1.209( 7) 1.348(7) 1.321(9) 1.484(8) 1.504(9) 1.398( 9) 1.345( 10) 1.386( 12) 1.378(11)
C(7)-C(8) N(2)-C(9) N(2)--C(l3) N(2)--‘J8) C(l)-C(9) C(9)-C(lO) C(lO)-C(ll) C(ll)-C(12) C(12)-C(13)
1.491(9) 1.368(8) 1.353(9) 1.472(8) 1.501(9) 1.387(10) 1.372(11) 1.375(12) 1.353(11)
Atoms
Angle
Atoms
Angle
0(1v-31)--c(2) C(2)-w)--c(9) C(2)_-N(lW(6)
115.6(6) 128.6(6) 121.9(6) 119.8(5) 118.4(6) 124.6(5) 118.7(6) 116.6(6) 120.5(7) 119.0(7) 119.6(8) 120.3(8) 111.1(5)
C(2)-N(l)*(7) C(6)-N(lH(7) N(l)--C(2)-C(l) N(l)--C(2)--C(3) C(l)--c(2k-C(3) C(2)-c(3)-C(4) C(3)-C(4)+(5) C(4)--c(5)-C(6) C(5)-C(6)-N(1) N(lH(7)--C(8)
Fig. 2. Edge-on
115.7(6) C(9)-N(2)-C(13) C(9)-N(2)--c(8) C(13)-N(2)%(8) N(2)-C(9)-C(l) N(2)-C(9)--C(lO) C(~)Hx9)-C(lO) C(9)-C(lO)~(ll) c(1o)-C(ll)--c(12) C(ll)-C(12)--c(13) C(12)-C(13)-N(2) N(2)--C(8)+X7)
119.3(6) 123.1(6) 117.5(6) 125.4(6) 118.4(7) 116.2(6) 121.4(8) 119.1(8) 118.6(8) 123.1(8) 113.5(5)
view of the cation.
Raman spectra of the parent
dications
The crystal structure of DPK2’+ as the dibromide salt indicates that the molecule is not symmetrically oriented about the carbonyl group. The angles between the carbonyl group and the individual pyridine rings are -2” and The spectroscopic implications of this asymmetry in +24”, respectively. DPK2’+ are discussed below. Because of the recrystallisation difficulties (see Experimental section), a good quality single-crystal of DPK3’+ dibromide could not be prepared. However, on the basis of analogy, certain tentative
271 TABLE 3 Planes of best fit Plane I. N(l), C(2), C(3), C(4), C(5), C(6) Equation 0.567X - 0.586Y - 0.5792 - 1.585 = 0 Displacements (d ) O.Oll(5) C(1) N(1) 0.000(S) O(1) C(2) -0.008( 7) C(3) C(9) 0.007(9) C(4) C(7) 0.003(11) C(5) -0.012(9) C(6) Plane 2. N(2), C(9), C(lO), C(ll), C(12), C(13) Equation 0.336X - 0.264Y - 0.9042 - 0.203 = 0 Displacements (A) -0.007(5) N(2) C(1) 0.013(6) C(9) O(1) -0.004(8) C(l0) C(2) -0.010(8) C(11) C(8) 0.017(8) C(12) -0.009( 8) C(l3) Plane 3. O(l), C(l), C(2), C(9). Equation 0.422X - 0.251Y - 0.8712 - 0.553 = 0 Displacements (a) 0.006(5) O(1) N(1) -0.016(7) C(l) N(2) 0.005( 6) C(2) C(7) 0.005(S) C(9) C(8) Angles between normals to planes Plane 1 Plane 2 Plane 1 Plane 3 Plane 2 Plane 3
A
-0.053(7) -0.542(5) 0.476(7) 0.032(7)
0.121(7) 0.147(5) 0.264(6) 0.020(7)
-0.503(5) 0.013(5) -1.081(6) -0.056(7)
29.7” 27.1” 5.3”
b 0
c
Fig. 3. Stereopair diagrams showing the unit cell contents.
conclusions can be drawn as to its structure, relative to that determined for DPK2’+. The crystal structure of the carbocyclic analogue of DPK2*+ (IIIa: n = 2) has been determined [ 61, in which the torsional angles are reported to be +14” and +30”. Polarographic [7] and acidity [8] measure-
272
I 900
I
1200 Raman
1500 Shift/cm-’
3
Fig. 4. Raman spectra of DPK2 dibromide as (a) solid and (b) 0.3 M aqueous solution, and (c) DPK3 dibromide as solid. (Exciting line for (a) and (b): 514.5 nm Ar’; for (c), 476.5 nm Ar’. Band-pass for (a): 5 cm-’ ; for (b) and (c), 10 cm-’ .)
merits for the carbocyclic analogue of DPK3’+ (IIIb: n = 3) have been interpreted in terms of a more twisted structure (i.e. greater torsional angles) compared to IIIa. Furthermore, crystal structures determined for bridged diquaternary salts of 2,2’-bipyridine (I: n = 2,4) [9, lo] have shown that an increase in the length of the bridging chain forces the aromatic system further from planarity. It is therefore expected that DPK32+ will exhibit increased torsional angles, relative to those measured for DPK22+. The Raman spectra of the parent dications (IIa, b) are complex, due to the species’ size and low molecular symmetry. However, our study of the bridged diquaternary salts of 2,2’-bipyridine [ 31 has shown that subtle differences in molecular geometry can be identified from complex spectra. It was shown that trends in the frequencies of certain conjugation-sensitive
273
9bo Raman
Go0 Shift /cm-’
1 00
Fig. 5. Resonance Raman spectra of (a) DPK2t and (b) DPK3t (0.01 M in water, 514.5 nm Ar+, Band-pass: 10 cm-’ slit).
bands common to each spectrum, could be correlated with known differences in structure. Pyridine ring stretching modes in the region of 1490-1630 cm-l, for example, were utilised in this manner. It is anticipated that the corresponding modes in the spectra of DPK2*+ and DPK3*+ should likewise be sensitive to changes in conjugation across the Ar-CO-Ar system (where Ar refers to an aromatic ring). The vibrational frequency and intensity of the symmetric stretching vibration of the carbonyl group has been shown to be particularly sensitive to its steric and electronic environment [ 11-151. It would be expected that a difference in conjugation between DPK2*+ and DPK3*+ should manifest itself in different stretching vibrations for the respective carbonyl groups. The respective carbonyl stretching frequencies observed for DPK2*+ and DPK3*+ (in the solid state) are 1682 cm-’ and 1708 cm-‘. The dependence of the carbonyl stretching frequency on conjugation has been illustrated by de Roos [13] for a series of ortho-substituted benzophenones. It was shown that increasing the size of the substituent caused an increase in the C----O stretching frequency, presumably as a result of a decrease in conjugation between the benzene rings and the carbonyl group. In view of this, it seems reasonable to attribute the 26 cm-’ shift in the C=G frequency between DPK2*+ and DPK3*’ to a decrease in the conjugation, brought about by the more twisted structure expected for the latter (see above). A striking difference in the solid state spectrum of DPK2*+ compared to
274
that of DPK3’+ is in the relative intensities of the respective carbonyl bands. Shigorin [ 151 has shown that the Raman intensity of the carbonyl stretching band increases dramatically as the extent of conjugation associated with the group increases. Therefore the significant increase in relative intensity of the C=O band between DPK22+ and DPK3’+ is as predicted from the crystal structure of the former (this study) and the more twisted structure anticipated for the latter. As discussed above, certain pyridine ring stretching modes were found to be sensitive to changes in conjugation in a series of bridged diquaternary salts of 2,2’-bipyridine [3]. It is uncertain whether bands in the corresponding region (1500-1620 cm-‘) of the spectra of DPK22+ and DPK32+ (see Fig. 4) can be correlated with conjugation differences between the two species. The complex nature of the spectrum of DPK22+ in this region (three sets of double peaks) contrasts with the simpler pattern observed for DPK3’+ (one dominant and two weak bands). There is, however, an apparent shift to higher frequencies of the bands in this region between DPK22+ and DPK32+, consistent with a localising of electron density within the two pyridine rings of the latter. The crystal structure of DPK22+ as the dibromide salt (see above) indicates that the two pyridine rings are not equivalent in their orientation, relative to the carbonyl group (torsional angles of -2” and +24”). As a consequence, the extent of Ar+0 conjugation should be different for each pyridine ring. This would account for the peak-doubling (see above) in the ring stretching mode region (1500-1620 cm-‘) of the spectrum of DPK22+. The intense group of bands in the 1050-1200 cm-’ region of the spectra of DPK22+ and DPK32+ could not be unambiguously assigned, since a number of corresponding vibrations of analogous molecules occurs in this region. Blazevic and Colombo [16] have assigned the symmetric Ar-C-Ar stretching vibration in benzophenone to a band at 1149 cm-‘, and bands in this region of the spectra of various quatemised pyridines have been assigned to in-plane C-H deformations, mixed with N’-C stretching modes [ 171. Figure 4 shows the considerable difference in the Raman spectrum of DPK22+ between the solid state and aqueous solution. The most noticeable changes between solid and solution spectrum involve the carbonyl band and the pyridine ring stretching modes (see Table 1). The C=O band undergoes a +13 cm-’ shift, together with a ca. five-fold decrease in relative intensity on dissolution. There is a simplification of the ring stretching mode region (three single peaks), as well as a shift in each band towards higher frequencies on dissolution. In view of the above observations regarding the conjugation-sensitivity of the frequencies and intensities of these bands, we ascribe the change in the Raman spectrum of DPK22’ between solid and solution to a modification of the molecular geometry on dissolution. The evidence suggests a change from C, point group symmetry for the DPK22’ species in the crystal to C2 symmetry for the corresponding solution species. This evidence consists, firstly, of increases in the frequencies of the ring stretching
275
modes (1500-1700 cm-‘) which favour higher torsional angles and increased electronic localization between the aromatic rings. Secondly, the simplification of the spectrum on going from crystal to solution favours a symmetrical orientation of the C=O bond (along a CZ axis) relative to the aromatic rings. Other factors influencing the change in spectrum of DPK2’+ on dissolution, including H-bonding in aqueous solution and the loss of interactive forces present in the solid state, must be considered. It is not possible to directly elucidate the effect that these factors have on the Raman spectrum of DPK2*+, since it is essentially insoluble in organic solvents. Consequently, examination of the behaviour of the carbonyl band in the spectra of related compounds is necessary. The carbonyl stretching frequency observed for a range of ketones, both conjugated and isolated, has been shown using both Raman and infrared spectroscopy to be extremely dependent upon the physical state of the compound [ll, 14, 161. In ketones as different as benzophenone [ll, 161 and acetone [ll, 181, the carbonyl frequency is observed to increase between pure compound and dissolution in non-interacting media. A decrease in frequency is observed for these molecules, however, as the H-bonding strength of the solvent increases [ 111 (see Table 4). The much-studied case of benzophenone provides an example of the frequency sensitivity (solid: 1650 cm-l [16]; solution: 1673 cm-’ (hexane) [ll]; 1657 cm-’ (ethanol) [ll]). Similar behaviour is observed for di-2pyridyl ketone (DPK), from which DPK2’+ is prepared (solid: 1677 cm-‘; solution: 1693 cm-’ (hexane), 1676 cm-’ (water)). In view of this sensitivity of the carbonyl frequency to changes in physical state, the conformational significance of the +13 cm-’ shift between solid and solution spectra for DPK*+ depends on the interpretation of changes in other regions of the spectrum. Specifically, data for benzophenone [16] and di-2-pyridyl ketone (in the present study) indicate that the aromatic ring stretching mode frequencies are not sensitive to changes in physical state of the molecules (see Table 1). Therefore it is suggested that the positive shift in frequencies observed for these bands between the solid and solution spectra of DPK2*+ (see Table 1) is associated with a change in the ArCO torsional angles on dissolution (see preliminary discussion above). The five-fold decrease in relative intensity of the carbonyl band between the solid and solution spectra of DPK2*+ cannot be confidently interpreted in terms of the proposed conformational changes, since similar intensity decreases are noted between solid and solution spectra for benzophenone [ 161 and di-2-pyridyl ketone. Resonance
Raman spectra of the radical cations
Dilute coloured aqueous solutions of the radical monocations derived from DPK2*+ and DPK321 exhibit resonance Raman spectra (see Fig. 5). No evidence for dimer formation was observed for DPKB? and DPK3$, as the
276 TABLE 4 Vibrational frequencies of selected modes of bridged diquaternary salts of di-2-pyridyl ketone ( DPK22’, DPK3’+) and related molecules Species
State
Ring stretches (cm-‘)
v(C=o) (cm-‘)
Detecteda
DPK2” DPK2*+ DPKP .* DPK3” DPK3 + DPK
solid H,O H,C solid H,C solid hexane CHCl, H,C solid hexane benzene ethanol
1502,1519,1586,1610,1621 1514,1588,1617 1584 1528,1617 1579 1568,1583 1571,1585 1571,1585 1569,1584 1597 1600 -
1682 1695 1637 1708 1634 1677 1693 1684 1676 1650 1673 1662 1657
R R RR R RR R IR R, IR R Rb IRC Rb IRC
BP
aDetected by R, Raman; RR, resonance Raman or IR, infrared. bRef. 16, ‘Ref. 11.
frequencies and relative intensities of all key bands remained unchanged over a range of concentrations. This is as reported for the radical cations derived from the diquaternary salts of 2,2’-bipyridine (I: 12 = 2, 3) [3], though it is in contrast with data reported for diquaternary salts of 4,4’-bipyridine [2]. All resonance Raman lines in the spectra of DPK2’ and DPK3? were polarised and so were attributed to totally symmetric vibrations. Black and Summers [4] have proposed on the basis of analogy with benzophenone radical anion (BP;) [19] that the odd electron in DPKB? and DPK3 should reside partially on the carbonyl oxygen (IV). This would cause a lowering in the carbonyl bond order, relative to that in the dication, manifesting itself in the vibrational spectrum as a decrease in frequency. Such a shift in the carbonyl frequency between parent and reduced species has been observed for benzophenone and its derivatives, though there is dispute as to the actual frequency in the case of BP;. Eargle [20] has used electron density data to assign a band at ca. 1550 cm-’ in the infrared spectrum of BP; to the carbonyl vibration. Juchnovski and co-workers [21] have employed isotopic substitution to make their assignment to a band ca. 1400 cm-‘. No attempt was made by Aleksandrov et al. [22] to assign any peaks in the resonance Raman spectrum of BP;. An intense band occurring in the resonance spectra of DPK2’ and DPK3t is assigned to the carbonyl at 1637 cm-’ and 1634 cm-‘, respectively, stretching mode, since the frequencies are too high to be attributed to the ring stretching modes. The latter were observed at 1500-1620 cm-’ in the
277
parent spectra, and are expected to decrease in frequency on radical formation, in accord with observations for the related modes in the 2,2’-bipyridine study [3]. The frequency shift of the assigned carbonyl band between parent and radical for DPK2*+ and DPK32+ is -45 cm-’ and -74 cm-‘, respectively. The difference is attributed to a greater conformational adjustment required for the latter to attain a near-planar Ar-CO-Ar configuration. This frequency difference can also ‘be related to differences in the half-wave potentials reported for the two species (E (SCE): DPK22+ -0.11 V; DPK32+ -0.17 V) [4]. It is interesting to note that the corresponding frequency shift reported for benzophenone (-110 cm-’ or -272 cm-’ [20, 211) is considerably greater than those observed for DPK22+ and DPK32+. This presumably reflects the considerable destabilisation caused by loss of aromaticity of the rings in BP:, and the consequent localising of the odd electron on the carbonyl group. Only one ring stretching mode is found to be strongly enhanced in the resonance spectrum of each species (DPKB’: 1584 cm-‘: DPK3+: 1579 cm-‘), at a lower frequency than that observed for the dominant ring mode in the parent spectra. This is consistent with the expectation of lowered ring force constants on radical formation, due to increased conjugation. In the 2,2’-bipyridine study [ 31, interpretations based on the frequency-dihedral angle dependence of certain bands suggested a more planar structure for the radical cation of Ib (n = 3) than for Ia (n = 2). However, the differences in frequencies of these key bands between the resonance Raman spectra of the radical forms of Ia and Ib were far greater than those observed for the carbony1 and ring stretching bands in the spectra of DPKB? and DPK3+. It is therefore concluded that the conjugated structures of the aromatic systems of DPK2t and DPK3’ are similar. As noted above for the dication spectra, the intense peaks in the lOOO1400 cm-’ region of each spectrum (see Fig. 5) could not be assigned with any confidence, due to the large number of possible vibrations. However, differences between the spectra of DPK2+ and DPK3’ in this region are marked (see Fig. 5). The medium-intensity peaks at 1199 cm-’ and 1231 cm-’ in the spectrum of DPK2t are seen to shift to 1149 cm-l and 1282 cm-‘, respectively, and to decrease in intensity in the spectrum of DPK3+. It is tentatively proposed that these shifts are due to vibrational coupling, which will be more pronounced as the aromatic system tends towards coplanarity. If this is the case, then it suggests that the the Ar-CO-Ar system of DPK3+ is nearer to co-planarity than that of DPK2+. A similar trend was observed for the analogous radical cations derived from Ib and Ia [3]. While no intense resonance Raman lines were observed in the region below 900 cm-‘, less intense features may have been obscured by the fluorescent background. In our earlier study of the 2,2’-bipyridine-based radicals [3], only very weak bands were observed in this region of the resonance Raman spectra, and these played no part in the interpretation.
218 ACKNOWLEDGEMENT
One of the authors (D.J.B.) is grateful to the Commonwealth Department of Education for a Commonwealth Postgraduate Research Award. REFERENCES 1 L. A. Summers, in A. R. Katritzky (Ed.), Advances in Heterocyclic Chemistry, Vol. 35, Academic Press, New York, 1984, pp. 281-374. 2 M. Forster, R. B. Girling and R. E. Hester, J. Raman Spectrosc., 12 (1982) 36. 3 D. J. Barker, R. P. Cooney and L. A. Summers, J. Raman Spectrosc., 16 (1985) 265. 4 A. L. Black and L. A. Summers, J. Chem. Sot. C, (1970) 2394. 5 D. J. Barker, R. P. Cooney and L. A. Summers, J. Raman Spectrosc., in press. 6 J. P. Reboul, J. C. Soyfer, B. Cristau, N. Darbon, Y. Oddon and G. Pepe, Acta Crystallogr., Sect. C, 39 (1983) 600. 7 V. M. Kazakova, I. G. Makarov, A. I. Samokhvalova and D. V. Ioffe, J. Struct. Chem., 15 (1974) 209. 8 R. Stewart, M. R. Granger, R. B. Moodie and L. J. Muenster, Can. J. Chem., 41 (163) 1065. 9 J. E. Derry and T. A. Hamor, Nature (London), 221 (1969) 464. 10 J. E. Derry and T. A. Hamor, J. Chem. Sot. Chem. Commun., (1970) 1284. 11 M. Ito, K. Inuzuka and S. Imanishi, J. Chem. Phys., 31(1959) 1694. 12 L. J. Bellamy and R. L. Williams, Trans. Faraday Sot., 55 (1959) 14. 13 A. M. de Roos, Rec. Trav. Chim. Pays-Bas, 87 (1968) 1359. 14 R. S. Becker, J. Mol. Spectrosc., 3 (1959) 1. 15 D. Shigorin, Dokl. Akad. Nauk. S.S.S.R., 96 (1954) 769. 16 J. Blazevic and L. Colombo, J. Raman Spectrosc., 11 (1981) 143. 17 E. Spinner, J. Chem. Sot., (1963) 3660. 18 G. Fini, P. Mirone and P. Patella, J. Mol. Spectrosc., 28 (1968) 144. 19 P. H. Rieger and G. K. Fraenkel, J. Chem. Phys., 37 (1962) 2811. 20 D. H. Eargle, J. Chem. Sot. (B), (1970) 1556. 21 I. Juchnovski, T. Kolev and I. Rashkov, Spectrosc. Lett., 18 (1985) 171. 22 I. V. Aleksandrov, Y. S. Bobovich, V. G. Maslov and A. N. Sidorov, JETP Lett., 17 (1973) 219.