5628
J . Phys. Chem. 1988, 92, 5628-5634
Vibrational Spectra and Normal-Coordinate Analysis of Tris( bipyridine)ruthenium(I I ) Prabal K. Mallick, Gerald D. Danzer, Dennis P. Strommen,* and James R. Kincaid* Chemistry Department, Marquette University, Milwaukee, Wisconsin 53233 (Received: November 9, 1987; In Final Form: April 5, 1988)
The infrared and resonance Raman spectra of the tris(bipyridine)ruthenium(II) complex and several of its deuteriated analogues are reported: specifically, those of the complexes of 3,3'-dideuterio-2,2'-bipyridine, 6,6'-dideuterio-2,2'-bipyridine, and perdeuterie2,2'-bipyridine. Multiple excitation lines are employed to obtain the resonance Raman spectra to facilitate observation of all A, fundamental vibrations associated with the ligand framework. Normal-coordinate calculations are carried out employing a systematic procedure based on a modified valence force field that is comparable to those used previously for aromatic molecules. The resultant (30-force constant) field reproduced observed frequencies with an average error of 1.5%.
-
Introduction Intense interest in polypyridine complexes of divalent ruthenium and other heavy-metal ions continues as a result of their inherently interesting and unusual excited-state properties and the potential utility of this class of compounds in solar energy conversion schemes.'-1° A better understanding of these properties will require the elucidation of the electronic and molecular structures of their ground and excited states. Resonance Raman (RR) spectroscopy has been shown to be a useful method to directly probe the excited-state as well as the ground-state species."-" In fact, the observation by Dallinger, Woodruff, and co-workers" of the time-resolved resonance Raman (TR3) spectrum of the exhibited bands excited-state species [ R ~ ( b p y ) ~*,~ +which ] characteristic of the bipyridine anion radial fragment, provided convincing evidence for the previously proposed "ligand-localized" excited ~ t a t e . This ~ ? ~ technique was later used by ourselves and others to investigate selective localization in mixed-ligand complexes." This demonstrated utility has prompted several research groups to undertake systematic R R studies of various members of this class of complexes in attempts to develop empirical correlations of spectra with structure.12-18 However, for molecules of this complexity, such empirical correlations are of limited utility, and a clear description of normal modes can be obtained only through carefully controlled normal-coordinate calculations. (1) Meyer, T. J. Prog. Inorg. Chem. 1983, 30, 389. (2) Kalyanasundaram, K. Coord. Chem. Rev. 1982, 46, 159. (3) DeArmond, M K.; Carlin, C. M. Chem. Rev. 1981, 36, 325. (4) Graetzel, M. Acc. Chem. Res. 1981, 14, 376. (5) Energy Resources through Photochemistry and Catalysis; Graetzel, M., Ed.; Academic: New York, 1983. (6) Conrad, D.; Allen, G. H.; Rillema, D. P.; Meyer, T. J. Inorg. Chem. 1983, 22, 1614. (7) Allen, G. H.; White, R. P.; Rillema, D. P.; Meyer, T. J. J . Am. Chem. Soc. 1984, 106, 2613. (8) Ferguson, J.; Kransz, E. R.; Maeder, M. J . Phys. Chem. 1985, 89,
1852. (9) Kitamura, N.; Kim, H.-B.; Kawanishi, Y.; Obata, R.; Tazuke, S. J . Phys. Chem. 1986, 90, 1488. (10) Chang, Y. J.; Orman, L. K.; Anderson, D. R.; Yabe, T.; Hopkins, J. B. J . Chem. Phys. 1987, 87, 3249. (11) (a) Dallinger, R. F.; Woodruff, W. H. J . Am. Chem. Soc. 1979, 101, 4391. (b) Bradley, P. G.; Kress, N.; Hornberger, B. A,; Dallinger, R. F.; Woodruff, W. H. J . Am. Chem. Soc. 1981, 103, 7441. (12) McClanahan, S.; Kincaid, J. J . Rnman Spectrosc. 1984, 15, 173. (13) Smothers, W. K.; Wrighton, M. S. J . Am. Chem. SOC.1983, 105, 1067. (14) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1982, 21, 3978. (15) (a) Chung, Y. L.; Leventis, N.; Wagner, P. J.; Leroi, G . E. J . Am. Chem. Soc. 1985, 107, 1416. (b) Kumar, C. V.; Barton, J. K.; Turro, N. J.; Gould, I. R. Inorg. Chem. 1987, 26, 1455. ( c ) Carroll, P. J.; Brus, L. E. J . Am. Chem. Soc. 1987, 109, 7613.
(16) Caspar, J. V.; Westmoreland, T. D.; Allen, G. H.; Bradley, P. G.; Meyer, T. J.; Woodruff, W. H. J . Am. Chem. SOC.1984, 106, 3492. (17) (a) McClanahan, S. F.; Dallinger, R. F.; Holler, F. J.; Kincaid, J. R. J . Am. Chem. Soc. 1985,107,4853. (b) Mabrouk, P. A.; Wrighton, M. S. Inorg. Chem. 1986, 25, 526. (c) Kumar, C. V.; Barton, J. K.; Gould, I. R.; Turro, N. J.; Van Houten, J. Inorg. Chem. 1988, 27, 648. (18) Poizat, 0.;Sourisseau, C. J . Phys. Chem. 1984, 88, 3007.
0022-3654/88/2092-5628$01.50/0
While the theory of normal-coordinate calculations is well developed,lg the problem is usually undertermined for large polyatomic molecules (such as that of interest here), and the experimental frequencies can be reproduced by a large number of derived force fields, most of which are not physically meaningful. To minimize this problem, it is necessary to obtain experimental frequencies for as many isotopically labeled species as is practical and to proceed in a cautious manner in deriving a physically reasonable force field that satisfactorily reproduces isotopic frequency shifts. The few previously reported normal-coordinate calculations on systems of this type were hampered by the lack of I R and Raman data for specifically deuteriated In the present work, the vibrational spectra (both IR and RR) are reported for R ~ ( b p y ) and ~ ~ +three selectively deuteriated analogues. In addition, multiple excitation lines are employed in acquiring the R R spectra so as to ensure that fundamental modes that are enhanced under either MLCT or ~ - 7 r excitation * are observed. These efforts provide a relatively large number of reliable experimental frequencies for a reasonable number of isotopic analogues. The derivation of the final force field was also carefully conducted. Thus, a systematic procedure23 was employed that restricted perturbations in diagonal and off-diagonal elements by maintaining certain fixed relationships among these during the minimization process.
Experimental Section The preparation of the isotopically labeled ligands and complexes has been previously described.12 Raman spectra were obtained from aqueous solutions that M in metal complex and 0.50 M in typically were 0.5 X N a 2 S 0 4 ,the 983-cm-' sulfate band being used as an internal standard. Spectra were obtained by circulating the solution through a 1-mm glass or quartz capillary tube by using a peristaltic pump. The laser beam was focused onto the sample by using a glass achromat or quartz focusing lens. The scattered radiation was collected (90' geometry) by a conventional two-lens collection system using either glass or quartz lenses. Dispersion (typical spectral band pass = 3 cm-') was achieved by using a Spex Model 1403 double monochromator equipped with a Spex Model DMlB controller and a Hamamatsu R928 photomultiplier tube. A Spectra Physics Model 2025-05 argon ion laser was used for excitation. The I R spectra (as KBr pellets) were obtained by using either a Perkin-Elmer Model 610 or a Nicolet 5DXB FT-IR. A Raman spectrum of a KBr pellet of R ~ ( b p y ) ~ showed C l ~ no substantial differences from the solution spectrum, thus confirming the validity (19) Wilson, E. B., Jr.; Decius, J. C.; Cross, P. C. Molecular Vibrations; McGraw-Hill: New York, 1955. (20) Strukl, J. S.; Walter, J. L. Spectrochim. Acta, Part A 1971, 27, 209. (21) Strukl, J. S.; Walter, J. L. Spectrochim. Acta, Part A 1971, 27, 223. (22) Neto, N.; Muniz-Miranda, M.; Angeloni, L.; Castellucci, E. Spectrochim. Acta, Part A 1983, 39, 97. (23) Kincaid, J. R.; Strommen, D. P., work in progress.
0 1988 American Chemical Society
Tris( bipyridine)ruthenium(II)
The Journal of Physical Chemistry, Vol. 92, No. 20, 1988 5629
RR
i
180.00
950.00
1800.08
FREQUENCY( c M - ~ ) Figure 1. Infrared and resonance Raman spectra of Ru(bpy)& 983-cm-' band of the sulfate internal standard.
Resonance Raman spectra obtained with 457.9-nm excitation; the asterisk denotes
of using directly measured IR (KBr) and Raman (solution) spectra in the fitting procedure.
Results and Discussion Spectral Data. The infrared and resonance Raman (RR) spectra of Ru(bpy)32+are shown in Figure 1. As is well-known, the R R spectra are dominated by the totally symmetric highfrequency modes associated with the ligand framework."-'* The general spectral pattern is consistent with an effective C , symmetry; Le., it is appropriate to interpret the spectra in terms of a single coordinated bipyridine. Careful inspection of the IR and R R spectra indicates that while A , modes are formally allowed under the C , point group, their intensity in the I R spectrum is extremely weak. Finally, the quality of the R R data, together with the selective enhancement of A I modes, renders the identification of most of the A l fundamentals rather straightforward. The lower quality spectra in the low-frequency region of the I R and the appearance of out-of-plane modes below -900 cm-' (ref 18) complicates the Selection of B2 fundamentals. While the majority of the A l modes are effectively enhanced under MLCT (457.9 nm) excitation, some are not. However, excitation at shorter wavelengths (363.8 or 350.7 nm) facilitates observation of these, presumably via enhancement associated with the ?M* transition located at -280 nm. This behavior is evident upon comparison of the spectra of Ru(bpy)?+ obtained with 363.8 nm and that obtained with 457.9-nm excitation (Figure 2). Thus, the weak 1450-cm-' band observed with 363.8-nm excitation is not enhanced with the 457.9-nm line. In addition, the 1265-cm-' feature is more evident in the 363.8-nm spectrum, while the 1276-cm-' mode is stronger with 457.9-nm excitation.
Also shown in Figure 2 are the R R spectra of Ru(3,3'-bpyd2)2+. It is interesting to note that the enhancement of a-a* active modes, relative to MLCT-active modes, may change as a function of deuteriation. Thus, in addition to expected frequency shifts upon deuteriation, the forms of the normal modes evidently change to the extent that the enhancement efficiency under T-T* excitation also is altered. The frequencies of all observed features in the spectrum of each isotopic analogue are listed in Table I. In this table, all of the Al and B2 fundamental modes are assigned to observable I R and RR features. In addition, several proposed combinations and overtones, as well as observed out-of-plane (IR) bands,18 are indicated. Calculations. The atom numbering scheme and descriptions of internal coordinates are illustrated in Figure 3. The system is considered as a single coordinated bipyridine, having C , symmetry. The 57 fundamental modes of vibration of this 21-atom framework transform as 20Al + 9A2 19B2 9B1. We have confined our calculation to the in-plane A l and B2 symmetry classes. The standard Wilson G F matrix approach19 was employed, by using the familiar normal-coordinate programs developed by S c h a c t ~ c h n e i d e r . ~The ~ general valence force field (GVFF) employed in this calculation is more appropriate than a UreyBradley type field for large polyatomic aromatic molecules such as that of interest here. The rationale for this choice has been clearly explained by others.2s As is typical for calculations
+
+
(24) Schachtschneider, J. H. Technical Report No. 231-64 and 57-65; Shell Development Co., Emoryville, CA. (25) Neto, N.; Scrocco, M.;Califano, S.Spectrochim.Acta, Parr A 1966, 22, 1981.
5630 The Journal of Physical Chemistry, Vol. 92, No. 20, 1988
Mallick et al.
1
1
L
900.00
1300.00
Raman
1700.0D
shift i c m - ’ 1
Figure 2. Resonance Raman spectra of Ru(bpy),CI2 and Ru(3,3’-bpy-d2),Cl2. H
H /
Figure 3. Atom numbering and internal coordinate labeling scheme.
involving these large polyatomics, the problem is grossly underdetermined in that there are many more adjustable parameters (force constants) than available experimental frequencies. In general, this complication is partially remedied by the acquisition of the spectra of isotopically labeled analogues and by judicious
elimination of off-diagonal (interaction) force constants. In the present case, we have obtained the I R and R R spectra of three isotopically labeled analogues in addition to those of the natural-abundance species. Our approach for minimizing the number of force constants is described below. Force Field Treatment. In some works, the number of force constants are reduced by using an average value for ring stretching force constants (i.e., all v(C-C) constants are restricted to the same value). However, we have opted to maintain the integrity of these constants inasmuch as there exist useful relationships26between bond length or bond order and stretching force constants. We have utilized these relationships, along with a reliable crystal structure determination for RU(~PY)~(PF&,*’ to arrive at initial estimates of the stretching force constants. Initial values for angle bending, stretch-bend interaction, and Ru-N stretching force constants were estimated from consideration of the derived force field for pyridine2* and other Several points (26) Burgi, H.-B.; Dunitz, J. D. J. Am. Chem. SOC.1987, 109, 2924. (27) Rillema, D. P.; Jones, D. S.; Levy, H. A. J. Chem. SOC.,Chem. Commun. 1979, 849.
Tris(bipyridine)ruthenium( 11)
The Journal of Physical Chemistry, Vol. 92, No. 20, 1988 5631
TABLE I: Observed Frequencies for Ru(bpy),*+ and Deuteriated Analogues' hn 3,3'-d7 6,6'-d2 dn assgt hn Resonance Raman Bands 1608 1563 1491 1450 1320 1276 1264 1176
1590 1553' 1547 1468 1430 1407 1298 1260 1250 1224 1213 1130
1602 1557 1485' 1474 1388 1404 1378 1319 1259 1196 1170 1116
1575 1536* 1529 1449* 1427 1305 1202
1067
3,3'-d7
6,6'-d2
dn
1078 1028
1030
85 1 1018 1002
1016 1043 1028 767 668 370 340 283 255 230 143
1255 1210 1020 1007 864
908 765 658 365 335 277
1042 915 757 666 368 339 278
assgt
728 640 363 332
228 155
Infrared Bands 1732 1667 1628 1628 1632 1600 1555
1584 1540
1602
1487 1462 1441
1458 1422
1441
1394 1261 1383 1293
1425 1400 1260 1380 1289
1573 1565 1530
1487
1420 1385 1310 1268
1106 985 1067
1010 965 899
883 838 808
1109 1000
867 845
1025 939 900
834
830 790
1333
781 774
1229 1209 1139
1260 1236 1158 1138
775
1295
760
733
660 645 1142 1000 978
737
736 700
1227
1276 1244 1227 1160
1121 1100 1066 1042 1025
639 610
668 653 640
661 632 622 592
421
#Asterisksdenote an isotopic impurity. regarding the diagonal constants are worth mentioning. W e have included a separate force constant for the in-plane hydrogen wagging coordinate at the 3,3'-position while restricting the other hydrogen wagging force constants to a single initial value. Such a distinction seems justified in view of the greater degree of steric interaction at the 3,3'-positions and the unique chemical properties of these hydrogens as demonstrated by their facile exchange with deuterium under certain solution conditions.12 In addition, while the initial values of the v(C,-Cj) stretching constants were estimated from relationships based on the measured bond lengths, the initial estimate of the v(C-N) stretching constant (Le,, one constant) was based on previously determined force fields.22.28 One of the most commonly employed strategies to eliminate off-diagonal elements is to neglect stretch-stretch interaction constants that do not include a common atom (Le., only the 1-2 stretch-stretch interactions are retained in this type of approximation). However, as was pointed out by Scherer and Overend?O in the case of highly 'conjugated (e.g., aromatic) systems, both 1-3 and 1-4 stretch-stretch interactions should be considered to be important. This concept has been convincingly supported and employed by Califano and co-workers in their studies of the vibrational spectra of polycyclic aromatic hydrocarbon^.^^ Therefore, in addition to the 1-2 stretchstretch interactions, the 1-3 and 1 - 4 interaction constants have been included in the present (28) Cummings, D. L.; Wood,J. L.J . Mol. Struc. 1974, 20, 1. (29) Bigotto, A.; Costa, G.; Galasso, V.; DeAlti, G. Spectrochim. Acto, Part A 1970, 26, 1939. (30) Scherer, J. R.; Overend, J. Spectrochim. Acta 1961, 17, 719.
calculation. All other stretch-stretch interactions have been neglected. A survey of previous normal-coordinate calculations for large polyatomic molecules reveals the fact that there is little agreement regarding the importance and magnitude of various types of stretch-bend and bend-bend interaction constants. In general, those stretch-bend interactions that involve a common bond are always included. In addition, the interaction of the hydrogen wagging motion with the stretching coordinate that shares only a common carbon atom are often included (i.e., the interaction of v(C3-C4) with 6(CzC3H); see Figure 3 and Table IJI). In agreement with ~ t h e r s , " ~ . this ~ constant is set equal in magnitude and opposite in sign to force constant number 28. Similar considerations have also been applied to the interactions between v(C-N) and 6(C3C2Czt)as well as between v(C2-C3) and 6(NCZC2,). In the absence of well-defined relationships for estimating the values of these interaction constants, their initial values were comparable to those derived for bipyridine.22 The only bend-bend interaction constants included were those involving the angles associated with the 2,2'-positions (i.e., force constant numbers 10 and 11). Furthermore, these bend-bend interaction constants were incorporated into the Z-matrix as functions of force constant numbers 10 and 11 and are therefore not independent. On the basis of the points discussed above, we arrived at a 30-constant force field whose initial values (i.e., before entering the fitting procedure) are in accord with the results of previous calculations and chemical intuition. To include the 1-3 and 1 - 4 stretch-stretch interactions without increasing the number of independent force constants, we have employed a strategy that links these interactions to the 1-2 stretch-stretch interaction
5632 The Journal of Physical Chemistry, Vol. 92, No. 20, 1988
Mallick et al.
TABLE [I: Calculated Frequencies and Potential Energy Distribution
ha no.
obs.
A, Modes 1 2
3 4 5 6 7 8 9 10 11
12 13 14 15
16 17
18 19 20 B2 Modes 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
1608 1563 1491 1450 1320 1276 1264 1176 1067 1028 766 668 370 283
1600 1555 1462 1420 1310 1244 1227 1160 1121 1100 1010 660 645
calc.
3,3’-d2 obs. calc.
6,6’-dz obs. calc.
3071 307 1 3070 2288 1602 1547 1455 1424 1298 1278 1197
3072 307 1 3070 2284 1601 1550 1459 1421 1321 1296 1196 1133 1102 1058 858 762 675 368 290 206
3072 307 1 3070 3069 1605 1557 1471 1435 1333 1296 1224 1182 1115 1096 1036 772 684 371 294 207
1590 1547 1468 1430 1298 1260 1224 1130 1151 1083 1028 1047 908 889 765 770 658 674 365 368 277 290 205
3072 307 1 3070 3069 1616 1565 1482 1426 1322 1259 1185 1145 1115 1076 989 697 653 468 283
307 1 307 1 3070 2289 1610 1556 1456 1400 1317 1224 1159 1120 1091 987 886 691 644 457 283
1584 1540 1422 1394 1293 1229 1209 1139 1106 985 883 639 610
1602 1557 1474 1388 1319 1259 1196 1116 1030 915 757 666 368 278
1602 1441 1400 1289 1236 1209 1138 1109 1000 900 653 640
3072 307 1 3070 2284 1611 1560 1469 1420 1305 1221 1157 1137 1104 979 856 696 647 459 280
da obs.
1575 1529 1427 1305 1255 1210 1020 1007 864 851 728 640 363
1565 1530 1333 1295 1227 1142 1000 978 867 845 834 632 622
calc.
Ru(bpy-ha) PED,%
2288 2286 2285 2283 1591 1522 1409 1339 1297 1203 1042 986 848 833 828 736 659 359 284 196
u(C-H), 99 u(C-H), 99 u(C-H), 99 u(C-H), 99 u(C~-C~), 43; u(CZ-C2/), 14; v(C-N), 14; ~(C3
