J. Phys. Chem. 1984,88, 3269-3214
3269
Solvation Studies in Matrices: Fourier Transform Infrared Data for the Stepwise Solvation of Li'NOC Ion Pairs by Unidentate and Bldentate Aromatic Amines J. Paul Devlin* and Keith Consanit Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 (Received: August 31, 1983; In Final Form: January 27, 1984) The severe distortion of the anion by the contacting cation in the isolated ion pair Li+NO< causes a splitting of the degenerate nitrate v3 mode of 262 cm-'. This large Av3 value is reduced to less than 100 cm-I for the completely solvated ion pair isolated in glassy Lewis-base solvent matrices. This collapse of Av3 provides a spectroscopic probe which has been used to observe the stepwise solvation of Li+N03- by the aromatic amines pyridine (py, monodentate) and phenathroline (phen, bidentate) in an infrared study of argon matrices containing Li+N0< and varying amounts of these Lewis-base solvents. py was observed to solvate the cation in five steps with each step revealed by an -35-cm-' reduction in the value of Av3. Solvation by phen was accompanied by three reductions in Av3 each of notably different magnitude (63, 39, and 100 cm-'). Surprisingly, complete solvation, which is believed to mirror the case of the liquid solution solvate, is apparently characterized by Li+ five-coordinate to py (py,.Li+N03-) and six-coordinate to phen (phen3-Li+NO3-).Since 6Av3, the reduction in Av3 for a solvation step, is clearly not correlated with the decrease in ligand-cation bond strength that is anticipated for increasing coordination number, the value of 6Av3 is presumed to be comparably sensitive to the steric forces between the solvent and nitrate ligands that become large for high coordination numbers. Recognition of this composit nature of 6Av3 (caused partly by cation charge neutralization, partly by steric forces) allows rationalization of the irregularity in 6Av3 for the three phen solvation steps. For a given coordination number the steric forces are expected to be greater for the monodentate than for the bidentate ligands, a difference that may be basic to the observed differences in the py and phen solvation sequences. This composit view of 6Av3 makes it clear that 6Av3values can be used as a measure of the solvent-cation interaction strength only for the solvation steps for which steric effects are minimal, Le., the 0-1 and, perhaps, the 1-2 steps.
Introduction The significance of ion solvation to the stability and reactivity of ionic solutions cannot be overstated.' In the absence of solvation energies ionic solutions would not exist while the reactivity of a solvated ion is typically solvent dependent and unlike that of the gas phase. Further, the inner solvation shell may dictate the kinetics of ion electrode processes and the mobilities of ions in biological systems are influenced by the nature of the solvent sheaths experienced under varying conditions. For such reasons a detailed description of ion solvation has been an important objective and, though elusive, is beginning to evolve from the application of the modern methods of physical chemistry. The vibrational spectroscopy of the solvates of polyatomic ion pairs, S,.M+XO,,,-, isolated in low-temperature matrices has been established as one promising approach to the understanding of ion pair solvation.2-6 By varying the solvent content in argon matrices and noting the response of the cation distortion of the anion, as gauged from the magnitude of the degeneracy splitting of the strongly absorbing vj antisymmetric stretching mode (i.e,, Av3), one can identify the individual solvate molecules, S,,.M+XO,, for various metal-oxyanion pairs. Here, n typically varies from 1 to nmax,where nmx resembles the cation solvation number for the corresponding liquid s ~ l u t i o n . ~ * ~ Initially it was presumed that most of the large decrease (>150 cm-') in Av3 that accompanies a switch from an argon to a pure glassy solvent matrix, and which was attributed to a neutralization of the cation charge by charge transfer from solvent ligand molecules, would occur in the early stages of solvation ( n = 1 or 2) with later stages producing only minor variations in the vga and V3b band positions.6 This presumption has now been discredited by a body of matrix data which strongly indicates that the later solvation stages are as effective as the first in reducing the cation distortion of the anion.2 The faulty assumption was based on the knowledge that AHf for gas-phase cation solvates is large for the 0-1 step but decreases monotonically for subsequent solvation stages.' The matrix ion-pair solvation data do not parallel this trend, a fact that clearly points to the influence of a second important factor, other than the strength of the cationsolvent interaction, in the reduction of Present address: Department of Natural Sciences, Eastern Montana College, Billings, MT.
0022-3654/84/2088-3269$01.50/0
the anion distortion during solvation. This second factor, namely, the steric forces that arise between the solvent ligands and anion ligand of the cation and which operate to progressively increase the cation-anion bond length as n increases, will be examined in a later section of this paper. However, it is noteworthy here that the presumption that was based on the gas-phase cation-solvation data, combined with a failure to fully recognize the influence of contaminant water on the ion-pair matrix-solvation spectra, led to the misassignment of certain solvate spectra in earlier reports.8 It has been firmly established that the nitrate anion in the M t N 0 3 - ion pairs acts as a bidentate ligandgJO(a penchant of the oxyanions that was originally established from matrix isolation data for the perchlorate ion in M+C104- species"). Further, the ab initio calculations also suggest that the NO3- bidentate character is retained for Li+N03- solvated by one, two, or three HzO molecules,98 while infrared data for M+C104- in glassy NH3 and glassy HzO matrices strongly indicate that the completely solvated Mt ion also favors a bidentate association with the oxy a n i ~ n .It~ has been repeatedly noted that cation distortion of the anion is significant, even for M+N03- completely solvated in the pure glassy solvent mat rice^.^ Perhaps the clearest indication that direct cationanion contact is maintained in the glassy solvent matrices is the Av3 value of 59 cm-' observed for Li+N03- in glassy (1) See, for example, Conway, B. D. "Ionic Hydration in Chemistry and Biophysics";Conway, B. E., Ed.; Elsevier: New York, 1981. (2) Toth, J. P.; Ritzhaupt, G . ; Devlin, J. P. J . Phys. Chem. 1981, 85, 1387-91. (3) Smyrl, N.; Devlin; J. P. J . Phys. Chem. 1973, 77, 3067-70. (4) Toth, J. P.; Thornton, C.; Devlin, J. P. J . Solution Chem. 1978, 7, 783-94. (5) Draeger, J.; Ritzhaupt, G.; Devlin, J. P. Inorg. Chem. 1979, 18, 1808-1 1 . (6) Ritzhaupt, G.; Devlin, J. P. J . Phys. Chem. 1975, 79, 2265-69. (7) See, for example, (a) Castleman, A. W.; Holland, P. M.; Linsay, D. M.; Peterson, K. I. J . Am. Chem. SOC.1978,100, 6039-45. (b) Kebarle, P. "Modern Aspects of Electrochemistry",Conway, B. E.; Bockris, J. O M . , Eds., Plenum: New York, 1974, Vol. 11, Chapter 1. (8) In particular the first solvation step was overlooked for both ACN and THF in ref 2. Thus, the correct nmaxvalue for THF is 4, not 3, and, as will be described elsewhere (ref 20), the u3,-ujb solvate frequencies are 151C-1278, 1487-1291, 1465-1315, and 1430-1328 for n = 1-4, respectively. (9) (a) Moore, J. C.; Devlin, J. P. J . Chem. Phys. 1978,68, 826-31. (b) Amlof, J.; Ischenko, A. A. Chem. Phys. Lett. 1979, 61, 79. (10) Beattie, I. R.; Ogden, J. S.; Price, D. D. J . Chem. Sor., Dalton Trans. 1979, 1460. (11) Ritzhaupt, G . ;Devlin, J. P. J . Chem. Phys. 1975, 62, 1982.
0 1984 American Chemical Society
3270 The Journal of Physical Chemistry, Vol. 88, No. 15, 1984
Devlin and Consani
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Me2S0,4 a dissociating solvent which shows no v3 splitting for glass Knudsen ovens were used for the production of L i N 0 3 ion NO3- in the liquid solution.lZ If ion pairs do not dissociate in pairs ( T 295 “C) and phen vapor ( T 70 “C). py and, to such a dissociating matrix, then surely they will remain in contact a lesser extent, bpy, were sufficiently volatile for generation of for the more weakly dissociating solvents, a view that has been the solvent vapor stream from room temperature reservoirs. The confirmed by the similarity of the solution and matrix spectra for argon matrix-isolated LiNO, samples, as well as the matrices THF, MeCN, DMF,4 and, now, pyridine. Thus, the favored containing LiNO,/solvent/argon mixtures, were prepared at 12 interpretation of the matrix-solvation data has been in terms of K in a cryostat cooled by an Air Products Displex CS-202 solvation of the M+ ions engaged in a direct (bidentate) coorclosed-cycle refrigerator. The lithium nitrate, Fisher certified grade, was recrystallized dination with the oxyanion. Such an approach ignores the possibility that the spectrum of twice and dried by repeated melting under vacuum in the Knudsen an ion pair at the nth stage of cation solvation can be influenced oven. The phen (Baker Analyzed Reagent) and bpy (99*% Aldrich) were purified (and dried in the former case) by subby the state of solvation of the anion. In fact, the unambigious limination under an active vacuum at least twice prior to transferal interpretation of the full range of matrix solvation spectra has to storage containers. The py (Aldrich, Spectrophotometric grade) not been possible for systems such as the water solvates because and deuterated py (Stohler Isotopes 99.5%, lot 1325) were dried HzO--NO< H bonding is of sufficient strength to seriously distort with calcium hydride and vacuum distilled to storage containers. the anion.13 For such systems useful solvation data may be limited These containers were the source of the py and py-d, used for to the first and second M+-solvation stages that occur, preferpremixing with argon in vapor reservoirs to give the desired matrix entially to anion solvation, in matrixes which have a solvent mole concentrations. The source of the argon varied from experiment percentage in the 0.5-1.0 range. to experiment (MG Scientific, or Union Carbide), but the purity The evidence is quite good that solvation of ion pairs by uniwas never less than 99.98%. dentate aprotic solvents such as THF, ACN, and MezSO occurs without serious spectroscopic effects from anion s o l v a t i ~ n . ~It- ~ - ~ ~ All infrared spectra were measured with a Digilab FTS-20C vacuum spectrometer with resolution nominally 2 cm-’, but reis less certain that the nitrate ion does not, at a particular solvation duced to -3 cm-’ by triangular apodization of the interferograms. stage, switch from a bidentate to a monodentate relationship with Spectra were composed of from 100 to 500 interferograms the cation. Such a switch should produce an oversize decrease (coadded every 10 scans) and processed with a fast Fourier in Av, (Le., 6Av3)for that stage since, in the monodentate structure, transform in the “double precision” mode. Spectra for mixed v~~ (the symmetric v 3 component) lies closer to the nitrate v1 solvent-argon samples were measured for fresh deposits at 12 frequency than does Vjb and is expected to be pushed to higher K for matrices with solvent mole percentages of 1, 5, 10, 42, and frequency through a V ~ - V , resonant ~ i n t e r a ~ t i o n . ’ ~This effect 50 for py, and with comparable samples for phen. Spectra for is clear from a comparison of the monodentate and bidentate these samples were also measured after annealing in the 15-30 normal coordinate calculations of Hester et al.15 One such K range. oversize solvation step (2-3) for T H F could possibly result from a bi- to monocoordination switch accompanying the addition of Spectroscopic Results the third solvent molecule.* Typically, the simultaneous occurrence of a range of solvates L i N 0 3 / A r Codepositions. As a prelude to the work on the v3b band in the matrices, (e.g., n = 2, 3, and 4) each having a solvation of L i N 0 3 isolated in argon matrices, the isolation of pair of moderate bandwidth, causes some crowding in the v3 L i N 0 3 in pure solid argon was reinvestigated.I6 Although no spectral range. This crowding can limit the accuracy of the deviation from the assignment of the major bands was necessary, positioning of the solvate bands and has discouraged quantiative there were some interesting developments. In particular, sharp applications of the data. This crowding effect can be minimized bands at 1520 and 1279 cm-’ (in previously published spectra) in two ways: (1) by placing emphasis on the lower solvates ( n have been nearly eliminated through a determined effort to reduce = 1, 2, and possibly 3) which can be formed without anion solwater contamination in the matrices. These two bands have, vation so that the v3 bands are sharp and easily positioned or, (2) therefore, been assigned as the v~~ and Vjb absorptions of HzO. reducing the potential number of solvates by using bi- (or mulLiNO,, the ion-pair monohydrate, giving an excellent fit to the tidentate ligands, such as glyme, bipyridine, or phenanthroline, .~~ these bands theoretical 6Av3value (21 vs. 25 ~ m - l ) Previously, for which the number of solvate bands might be expected to be were tacitly attributed to argon matrix splittings. Their correct nearly halved. Both of these approaches have been incorporated assignment has importance since subsequent studies have shown into the present study of Li+NO< solvation by the aromatic amines that SI.LiN03 has v~~and Vjb absorptions at very nearly these same pyridine (py), bipyridine (bpy), and phenanthroline (phen). two frequencies for several ligands (THF,* ACN, NH3, and, to a lesser degree, py). Further, the present study revealed that, if the LiN03/Ar ratio is deliberately increased, bands at 1308, 1326, 1345, 1479, and -1515 cm-’ appear and steadily increase in intensity, clearly indicating the production of ion-pair multimers. This is in qualitative agreement with early studies using COz as bpy (trans form) phen a matrix.” The spectrum of the u3 region for LiN0, isolated in argon, Experimental Section containing a minimum of multimer and hydrate absorption, is Experimentation was divided into three phases: the meapresented in Figure 3, curve a. The presence of a trace of water surement of the infrared spectra of (1) L i N 0 3 isolated in pure in this, and other samples presented in this paper, is indicated by argon matrices, (2) LiNO, isolated in glassy matrices of 100% bands near 1600 cm-I, though the aromatic amines also absorb py, bpy, or phen at liquid nitrogen temperatures, and (3) L i N 0 3 in this region. isolated in argon matrices containing solvent molecules (py or LiN03/Solvent Codepositions. The infrared spectra of the phen). region of interest are presented in Figures 1 and 2, for L i N 0 , The pure “glassy” solvent matrix samples were prepared in a codeposited with py, with bpy, and with phen, at liquid-nitrogen standard liquid-nitrogen-cooled infrared cell. Resistance-heated coolant temperatures. The bands for the solvent molecules may be identified by comparison with the pure “glassy” solvent spectra (12) Findlay, T. J. V.; Symons, M. C. R. J . Chem. SOC.,Faraday Trans. that are included in each case. The positions of v~~and Vjb, 141 1 I 1976, 72, 820.
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(13) See, for example, Irish, D. E.; Brooker, M. H. Ado. Infrared Raman Spectrosc. 1978, 2, 275-77. (14) The ion-pair-solvation MO calculations (ref 9) also predict a smaller ug splitting for mono- vs. bidentate anion coordination with the cation. (15) (a) Britzinger, H.; Hester, R. E. Inorg. Chem. 1966, 5, 980-5. (b) Grossman, W. E. L.; Hester, R. E. Ibid. 1966, 5 , 1308-12.
(16) The original study of LiNOJ in an argon matrix is described in Smith, D.; James, D. W.; Devlin, J. P. J . Chem. Phys. 1971, 54, 4437-42. (17) Pollard, G.; Smyrl, N.; Devlin, J. P. J . Phys. Chem. 1972, 76, 1826-31.
The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 3271
Solvation of Li+N03- by Aromatic Amines I
n
Figure 1. Infrared bands for v,, and
v3b ( X ) of Li+N03- completely solvated in glassy pyridine matrices. Curves a and c are for pure glassy py and py-d5,respectively, while curves b and d are of Li+N03-in the corresponding matrices.
)O
1600
1500
1400
1300
1200
cm.1 Figure 3. Infrared bands (curves c and e) for u3a and v3b of the first and
second py and phen solvates of Li+NO< in argon matrices with 01.5% concentration of the solvent. Curve a is for LiCNO
