Organometallics 1995,14, 5099—5103
5099
Comparison of Ru-C Bond Characters Involved in Successive Reduction of RU-CO2 to RU-CH2OH Kiyotsuna Toyohara, Kiyoshi Tsuge, and Koji Tanaka* Institute for Molecular Science, Myodaiji, Okazaki 444, Japan Received June 22, 1995®
Comparison of Raman spectra of a series of cis-[Ru(bpy)2(CO)X]n+ complexes (X = CO, 0-2) and their 180- or deuteriumC(0)0H, C(0)0CH3, C02, CHO, and CH2OH; substituted analogs permit reasonable assignments of v(Ru—X) and v(Ru—CO) bands around 500 and 470 cm-1. The validity of the assignments of those bands led to identification of two configurational isomers of cts-[Ru(bpy)2(CO)(CH2OH)]+ with respect to the orientation of the CH2—OH bond. The v(Ru—X) bands shift to higher wavenumbers as the Ru—X bond distances (d(Ru—X)) become longer. Such unusual dependence of v(Ru—X) upon d(Ru—X) may be associated with multibond characters of the C=0, C=0, and C—O bonds in the Ru—X moieties.
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=
Introduction
namic sense, because the E° values (25 °C, pH 0) for HCOOH, CO, HCHO, CH3OH, and CH4 formation are -0.199, -0.103, -0.071, +0.030, and +0.169 V vs NHE, respectively. Recently, not only HCOOH and CO but also HCHO, CH3OH, HO(0)CCHO, and HO(0)CCH2OH have been obtained in electrochemical C02 reduction by [Ru(bpy)(trpy)(CO)]2+ in EtOH/H20 at -20 °C,6 in which Ru-C02, Ru-C(0)OH, Ru-CO, Ru-CHO, and Ru-CH2OH species have been suggested to participate in the catalytic cycle. Such multistep conversion of C02 on Ru is inevitably accomplished by variation in the carbon orbital of the Ru-C bond (sp2, sp, and sp3 which would also have a crucial influence on the formation energy of HCOOH, CO, HCHO, CH3OH, and CH4 in multielectron reduction of C02 by metal complexes. Vibrational spectroscopy may provide useful information about the Ru—C bond characters in the conversion from Ru—C02 to Ru—CH2OH. Vibrational studies on organometallic compounds, including metal—C02 adducts, have been well documented and elucidated the presence of various couplings of metal—carbon stretching modes with many vibrational modes of other ligands.7 We, therefore, have undertaken a Raman spectroscopic study of a series of cts-[Ru(bpy)2(CO)X]n+ (X = C02, C(0)OH, C(0)OCH3, CO, CHO, and CH2OH; = 0-2) complexes and their 180- or deuterium-substituted analogs in order to assign v(Ru-X) bands. Comparison' of the Raman spectra among those homologous complexes would permit reasonable assignments of the v(Ru-X) bands without serious variations in the extent of couplings with other
Conversion of C02 to highly reduced organic compounds has been attracting much attention from the
viewpoint of utilization of C02 as a Cl resource. Taking into account that C02 reduction of Cu electrodes generates a variety of organic molecules such as CH4, C2H2, C2HsOH, and C3H7OH,* electrochemical C02 reduction by metal complexes may provide suitable models for the understanding of the multistep conversion of C02 to organic compounds on a metal surface. Although most C02 reductions using metal complexes give only CO and/ or HCOOH,2-5 multielectron reduction of C02 is more favorable than two-electron reduction in a thermody1
4*),
Abstract published in Advance ACS Abstracts, October 1, 1995. (1) Hori, Y.; Murata, A.; Takahashi, R.; Suzuki, S. J. Chem. Soc., Chem. Commun. 1988, 17. (2) Electrochemical CO2 reduction and mechanisms: (a) Arana, C.; Keshavarz, M.; Potts, K. T.; Abruna, H. D. Inorg. Chim. Acta 1994, 225, 285. (b) Steffey, B. D.; Miedaner, A.; Maciejewski-Farmer, M. L; Bematis, P. R.; Herring, A. M.; Allured, V. S.; Carperos, V.; DuBois, D. L. Organometallics 1994, 13, 4844. (c) Szymaszek, A.; Pruchnik, F. Rhodium Express 1994,5,18. (d) Fujita, E.; Haff, J.; Sanzenbacher, R.; Elias, H. Inorg. Chem. 1994, 33, 4627. (e) Ogura, K.; Sugihara, H.; Yano, J.; Higasa, M. J. Electrochem. Soc. 1994, 141, 419. (f) ®
Chardon-Noblat, S.; Collomb-Dunand-Sauthier, M.-N.; Deronzier, A.; Ziessel, R.; Zsoldos, D. Inorg. Chem. 1994, 33, 4410. (g) Christensen, P.; Hamnett, A.; Muir, A. V. G.; Timney, J. A.; Higgins, S. J. Chem. Soc., Faraday Trans. 1994, 90, 459. (h) Kimura, E.; Wada, S.; Shionoya, M.; Okazaki, Y. Inorg. Chem. 1994, 33, 770. (i) Kolodsick, K. J.; Schrier, P. W.; Walton, R. A. Polyhedron 1994, 13, 457. (j) Collomb-Dunand-Sauthier, . N.; Deronzier, A.; Ziessel, R. J. Chem. Soc., Chem. Commun. 1994, 189. (k) Haines, R. J.; Wittrig, R. E.; Kubiak, C. P. Inorg. Chem. 1994, 33, 4723. (I) Collomb-DunandSauthier, M.-N.; Deronzier, A.; Ziessel, R. Inorg. Chem. 1994,33, 2961. (m) Halmann, . M., Ed. Chemical Fixation of Carbon Dioxides·, CRC Press: London, 1993; p 67. (n) Bhugun, I.; Lexa, D.; Saveant, J.-M. J. Am. Chem. Soc. 1994, 116, 5015. (o) Sullivan, B. P., Ed. Electrochemical and Electrocatalytic Reduction of Carbon Dioxide·, Elsevier: Amsterdam, 1993, and references therein. (3) Photochemical reduction of CO2: (a) Ishitani, O.; George, M. W.; Ibusuki, T.; Johnson, F. P. A.; Koike, K; Nozaki, K; Pac, C.; Turner, J. J.; Westwell, J. R. Inorg. Chem. 1994, 33, 4712. (b) Matsuoka, S.; Yamamoto, K.; Ogata, T.; Kusaba, M.; Nakashima, N.; Fujita, E.; Yanagida, S. J. Am. Chem; Soc. 1993, 115, 601. (c) Calzaferri, G.; Haedener, K; Li, J. J. Photochem. Photobiol., A 1992, 64, 259. (d) Kimura, E.; Bu, X.; Shinomiya, M.; Wada, S.; Maruyama, S. Inorg. Chem. 1992, 31, 4542 and references therein. (4) Insertion of CO2 into —H. (a) Ratliff, K S.; Lentz, R. E.; Kubiak, C. P. Organometallics 1992,11,1986. (b) Jeget, C.; Fouassier, M.; Mascett, J. Inorg. Chem. 1991, 30, 1521. (c) Jeget, C.; Fouassier, M.; Tranquille, M.; Mascett, J. Inorg. Chem. 1991, 30,1529. (d) Pugh, J. R.; Bruce, M. R. M.; Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1991, 30, 86. (e) Sullivan, B. P.; Meyer, T. J. J. Chem. Soc., Chem. Commun.
vibrational modes.
(5) CO formation in protic media: (a) Ishida, H.; Katsuyuki, F.; Ohba, T.; Ohkubo, K; Tanaka, T.; Terada, T.; Tanaka, T. J. Chem. Soc., Dalton Trans. 1990, 2155. (b) Ishida, H.; Tanaka, K.; Morimoto, M.; Tanaka, T. Organometallics 1986, 5, 724. (c) Tanaka, K.; Morimoto, M.; Tanaka, T. Chem. Lett. 1983, 901. (d) Choudhury, D.; ColeHamilton, D. J. J. Chem. Soc., Dalton Trans. 1982, 1885. (6) (a) Nagao, H.; Mizukawa, T.; Tanaka, K. Inorg. Chem. 1994, 33, 3415. (b) Nagao, H.; Mizukawa, T.; Tanaka, K. Chem. Lett. 1993, 955. (7) (a) Caballol, R.; Sanchez, . E.; Barthelat, J. C. J. Phys. Chem. 1987, 91,1328. (b) Jegat, C.; Fouassier, M.; Tranquille, M.; Mascetti, J.; Tommasi, I.; Aresta, M.; Ingold, F.; Dedieu, A. Inorg. Chem. 1993, 32, 1279. (c) Jegat, C.; Fouassier, M.; Mascetti, J. Inorg. Chem. 1991, 30, 1521. (d) Jegat, C.; Fouassier, M.; Tranquille, M.; Mascetti, J. Inorg. Chem. 1991, 30, 1529.
1984, 1244.
0276-7333/95/2314-5099$09.00/0
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1995 American Chemical Society
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Organometallics, Vol. 14, No. 11, 1995
Experimental Section Materials. Commercially available H2180 and NaBD4 were used without further purification. Preparation of Cl” and PFe” salts of czs-[Ru(bpy)2(CO)2]2+ was described elsewhere.8 The remaining complexes czs-[Ru(bpy)2(CO)(COOH)](PF6), czs-[Ru(bpy)2(C0)(C02)], and czs-[Ru(bpy)2(CO)(CH2OH)](PF6) were prepared by reactions of cts-[Ru(bpy)2(CO)2](PF6)2 with BiuNOH and NaBH4, as reported previously.513'6®'9 Deuteriumsubstituted czs-[Ru(bpy)2(CO)(CD2OH)]+ was prepared by the reaction of czs-[Ru(bpy)2(CO)2]2+ with NaBD4 in CH3CN/H2O.
Synthesis of [Ru(bpy)2(CO)(CHO)](PFe). An addition of molar excess of NaBH4 to a colorless MeOH/H20 (2:1 v/v) solution of czs-[Ru(bpy)2(CO)2](PF6)2 at -5 °C resulted in gradual precipitation of yellow czs-[Ru(bpy)2(CO)(CHO)](PF6) (1), which was collected by filtration and washed with cold a 1.5
water; yield 75%. Anal. Caled: C, 42.93; H, 2.76; N, 9.11. Found: C, 42.64; H, 2.85; N, 9.04. IR spectrum (KBr): 1608 and 13C NMR: 13.9 (r(C=0)) and 1950 cm”1 (v(C=0)). and 265 (-CHO). Similarly, deuterium-substituted czs-[Ru(bpy)2(CO)(CDO)]+ was prepared by a reaction of czs-[Ru(bpy)2(CO)2]2+ with NaBD4 in CH30H/H20. Raman Spectroscopy. Raman spectra were measured on a Perkin-Elmer FT-Raman 2000 equipped with a Nd:YAG laser (laser power 500 mW at Rayleigh scattering 1064 nm, resolution 4 cm”1) and an InGaAs detector. All measurements were carried out in KBr disks or solutions (ca. 50 mmol/dm3) in 10 mm diameter glass tubes at room temperature. X-ray Analysis of [Ru(bpy)2(CO)(CH2OH)](PF6). Although we have reported the structure of the hydroxymethyl complex,6® detailed investigations of the IR and Raman spectra of czs-[Ru(bpy)2(CO)(CH2OH)]+ suggested the existence of two isomers in the solid state. We carefully examined the final difference Fourier map again and found that the oxygen atom of the hydroxymethyl group was disordered over two sites, giving two isomers. The least-squares calculation including the disordered oxygen atom and the structure was successfully refined. All calculations were carried out on a Silicon Graphics IRIS Indigo computer system using TEXSAN.10 The R and Rw values converged to 0.048 and 0.053 with the disordered oxygen atom, while they were 0.059 and 0.070 without the disordered atom. The bond lengths and angles are almost the same values as those in the previously determined structure, except for the hydroxymethyl group. The crystallographic data, final atomic parameters, and bond lengths and angles have been deposited as supporting information.
Results Raman spectra have been widely employed to assess metal—carbon bond characters of organometallic complexes, and metal-carbon stretching modes usually emerge in the range 1000—200 cm”1.11 To assign v(Ru— CO) and v(Ru-X) bands of czs-[Ru(bpy)2(CO)X]n+ (X = C02, C(0)OH, C(0)OCH3, CO, CHO, and CH2OH; = 0-2) in the Raman spectra, oxygen or hydrogen atoms in the substituent X were replaced by their isotopes because of the difficulty in synthesis of czs-[Ru(bpy)2(13CO)2]2+ as the starting compound for the preparation of the series of complexes. Substitutions of the oxygen (8) Kelly, J. M.; O’Connell, C. M. J. Chem. Soc., Dalton Trans. 1986, 253. (9) (a) Tanaka, H.; Tzeng, B. C.; Nagao, H.; Peng, S. M.; Tanaka, K. Organometallics 1992, 11, 3171. (b) Tanaka, K.; Tzeng, B.-C.; Nagao, H.; Peng, S.-M.; Tanaka, K. Inorg. Chem. 1993, 32, 1508. (10) TEXSAN: Single Crystal Structure Analysis Software, Version 1.6; Molecular Structure Corp., The Woodlands, TX 77381, Í993. (11) (a) Hartley, F. R.; Patai, S., Eds. The Chemistry of the MetalCarbon Bond', Wiley: Chichester, U.K., 1982; Vol. 1, references therein, (b) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds·, Wiley-Interscience: New York, 1986, and references therein.
1. Raman spectra of (a) czs-[Ru(bpy)2(C160)2](Cl)2 in H2160 and (b) czs-[Ru(bpy)2(C180)2](Cl)2 in H2180/CH3OH (4/1 (v/v)).
Figure
atoms of czs-[Ru(bpy)2(C160)2]2+ and czs-[Ru(bpy)2(CO)(C1602)] by 180 were conducted by taking advantage of a facile equilibration reaction (eq 1) in H2180.12 The
[Ru(bpy)2(C0)2]2+=§^ colorless
[Ru(hpy)2(C0)(C(0)0H)]+ yellow [Ru(bpy)2(C0)(C02)] (1) red assignments of the Ru-C stretching bands in the Raman spectra of czs-[Ru(bpy)2(CO)2]2+ and czs-[Ru(bpy)2(C0)(C02)] were, therefore, based on the isomer shifts caused by the replacement of 160 by 180. The conversion from cis-[Ru(bpy)2(C160)2]Cl2 to czs-[Ru(bpy)2(C180)2]C12 was easily monitored by the disappearance of the two v(C160) bands at 2098.7 and 2052.3 (m) cm”1 of the former and appearance of the two v(C180) bands at 2049.7 and 2002.7 (m) cm”1 of the latter in the Raman spectra in H2180/CH30H (4:1 v/v) (Figure 1). Besides those v(CO) bands, only one band at 443.6 (m) cm”1, observed in H2160/CH30H, shows an isotope shift to 430.0 (m) cm”1 in H2180/CH30H. Not only the band positions but also the patterns of all other peaks detected in H2160 and H2180 are identical within ±1.0 cm”1. The 443.6 cm”1 band, therefore, is reasonably assigned to vsym(Ru—CO) of cts-[Ru(bpy)2(CO)2]2+. The va9ym(Ru—CO) band in the Raman spectrum of czs-[Ru(bpy)2(CO)2]Cl2 may be weakened by the local C2y symmetry of czs-[Ru(bpy)2(CO)2]2+.13 Similarly, the v(CO) band at 1947.9 (m) cm”1 in cis[Ru(bpy)2(C160)(C1602)] in CH30H/H2160 (4:1 v/v) moves to 1908.9 (m) cm”1 in CH30H/H2180 (4:1 v/v) (Figure 2). Furthermore, the shift of a band at 1242.3 cm”1 in CH30H/H2160 to 1224.8 cm”1 in CH30H/H2180 is associated with the vsym(C02) band. On the basis of an assignment of the vasym(C02) band at 1442.5 cm”1 in the IR spectrum, the band may be obscured by the change (12) Ishida, H.; Tanaka, K.; Tanaka, T. Organometallics 1987, 6,
181.
(13) Neither vasym(Ru—CO) nor vBym(Ru—CO) was detected in the IR spectrum due to strong bands of bpy ligands.
Reduction
ofRu-COi
Organometallics, Vol. 14, No. 11, 1995
to RU-CH2OH
Figure 2. Raman spectra of (a) cis-[Ru(bpy )2(C 160)(C16C>2)]
in H2160/CH30H and (b) czs-[Ru(bpy)2(C180)(C1802)](Cl)2 in H2180/CH30H (4/1 (v/v)).
in the optical intensity around 1450 cm-1 in the Raman spectra. Besides the isotope shifts of the v(CO) and v(C02) bands, two other bands undergo isotope shifts in the region 1000-200 cm-1; the bands at 520.2 (m) and 474.7 (w) cm-1 in CH30H7H2160 shift to 509.1 (m) and 472.7 (s) cm"1 in CH30H/H2160 (Figure 2). In accordance with this, an IR spectrum of a ?;1-C02 titanium complex showed isotopic shifts of the bands of 1187 and 722 cm-1 on 180 substitution and a strong coupling of the Ti—C stretching mode with the O—C—O bending mode is suggested by normal coordination calculations.78 The replacement of oxygen atoms between cis-[Ru(bpy)2(C160)(C1602)] and czs-[Ru(bpy)2(C180)(C1802)] would have more serious effect on the v(Ru—C02) mode than on the v(Ru—CO) mode. From 11.1 the extent of the isotope shifts of two bands ( and 2.0 cm-1) the main contributions to the 520.1 and 474.7 cm-1 bands of czs-[Ru(bpy)2(C160)(C1602)], therefore, result from v(Ru—C02) and v(Ru—CO) modes, respectively. In the equilibrium reaction of eq 1, czs-[Ru(bpy)2(CO)(C(0)OH)]+ always exists as an equilibrium mixture with czs-[Ru(bpy)2(CO)2]2+ and czs-[Ru(bpy)2(C0)(C02)] or either of them, which hampered the measurement of the Raman spectra of czs-[Ru(bpy)2(C0)(C(0)0H)]+ in aqueous solution. Accordingly, the assignment of v(Ru—C(O)OH) and v(Ru—CO) bands of czs-[Ru(bpy)2(CO)(C(0)OH)](PF6) was conducted by comparison of the Raman spectra of czs-[Ru(bpy)2(C0)(C(0)0CH3)](PF6) in KBr disks.14 The v(CO) bands of czs-[Ru(bpy)2(CO)(C(0)OH)](PF6) and cis-[Ru(bpy)2(C0)(C(0)0CH3)](PF6) are observed at 1973.7 (m) and 1951.3 (m) cm-1, respectively. On the other hand, both Raman spectra are quite similar to each other in the region 1000-200 cm-1 within ±1.0 cm-1, except for two bands at 511.3 (m) and 473.1 (s) cm""1 in czs-[Ru(bpy)2(CO)(C(0)OH)[(PF3) and at 518.1 (m) and 471.7 cm-1 (s) in czs-[Ru(bpy)2(C0)(C(0)0CH3)](PF6). The relative shifts of both bands between the two complexes ( 6.8 and 1.4 cm-1) indicate that the 511.3 and 473.1 cm""1 bands of czs-[Ru(bpy)2(C0)(C(0)0H)[(PF6) mainly reflect the v(Ru—C(O)OH) and v(Ru—CO) modes, respectively,
5101
Figure 3. Raman spectra of (a) czs-[Ru(bpy)2(CO)(CHO)](PF6) and (b) czs-[Ru(bpy)2(CO)(CDO)](PF6) in KBr disks.
=
=
(14) No appreciable change was observed in the Raman spectra of [Ru(bpy)2(C0)(C(0)0H)](PF6) in CH3CN and in KBr disks.
4. Raman spectra of (a) czs-[Ru(bpy)2(CO)(CH2OH)](PF6) and (b) czs-[Ru(bpy)2(CO)(CD2OH)](PF6) in KBr disks.
Figure
because the v(Ru—C(O)OH) mode would undergo more pronounced perturbation than the v(Ru—CO) mode by
esterification of the C(0)OH moiety. A yellow powder of czs-[Ru(bpy)2(CO)(CHO)](PFe) was quite stable in the solid state, but the formyl complex begins to decompose at -15 °C in CH3CN solution. Therefore, the Raman spectrum of czs-[Ru(bpy)2(CO)(CHOXKPFe) was measured in a KBr disk. The substitution of the CHO moiety of czs-[Ru(bpy)2(CO)(CHO)](PFe) by CDO causes a small shift of the v(CO) band from 1950.0 to 1946.9 cm-1. Besides the shift of the v(CO) band, a comparison of the spectra of czs-[Ru(bpy)2-
(COXCHO)](PF6) and czs-[Ru(bpy)2(CO)(CDO)](PF6) demonstrates the isotope shift of only two bands from 518.9 (m) and 472.3 (w) cm-1 of the former to 515.6 (m) and 471.3 cm-1 (w) of the latter (Figure 3). Such shifts also permit assignment of the 518.9 (m) and 472.3 cm-1 (w) bands of czs-[Ru(bpy)2(CO)(CHO)]+ to the v(Ru-CHO) and v(Ru—CO) modes, respectively. The v(CO) band ofczs-[Ru(bpy)2(CO)(CH2OH)](PF6) is
dependent on the medium. In the solid state, a small splitting of the v(CO) band is observed at 1925 and 1934 cm-1, while two v(CO) bands clearly appear at 1944.5 and 1986.9 cm-1 in CH2C12. Similarly, czs-[Ru(bpy)2-
Toyohara et al.
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b
a
Figure 5. Molecular structures of two conformers of cts-[Ru(bpy)2(CO)(CH2OH)](PF6). The thermal ellipsoids are drawn at the 30% level.
Table
1.
Relevant Raman Bands and Bond Parameters of [Ru(bpy)2(CO)X]n+ (X CO, CHO, CH2OH; n = 0-2)“ v(Ru—X), cm-1
X(color) CO26, (red)
C(0)OH (yellow) C(0)0CHs (yellow) CO6 (colorless)
CHO (yellow) CH2OHc (orange) 0
v(Ru—CO),
unlabeled
labeled
unlabeled
520.2 511.3 518.1 443.6
509.1
474.4 473.1 471.7
430.0
518.9 558.9 523.5
515.6 534.6 511.1
The methods of isotope labeling
are
472.3 477.7 471.6
detailed in the text.
cm
1
labeled 472.7
471.3 475.7
~470 6
v(CO),
1
cm
=
C02, C(0)OH, C(0)0CH3,
unlabeled
d(Ru-CO), Á
d(Ru-X),
labeled
ZX-Ru-CO,
1947.9 1973.7 1951.2 2098.7 2052.3 1950.0 1934.3 1923.1
1908.9
1.81
2.06
88.5
1.80 1.87 1.91
2.04 1.87 1.91
88.5 88.8
1.85
2.19
90.9
2049.7 2002.7 1946.9 1934.9 1923.7
Á
deg
Labeled with 180.c Labeled with D.
(C0)(CD20H)](PFe) shows the same splitting of v(CO) in both the solid and solutions. Furthermore, four bands at 558.9 (m), 523.5 (m), 477.7 (w), and 471.6 (sh) cm-1 of czs-[Ru(bpy)2(CO)(CH2OH)(PF6) undergo isotope shifts to 534.6 (m), 511.1 (m), 475.7 (w), and ~470 (sh) cm-1, respectively, in the Raman spectrum of czs-[Ru(bpy)2(C0)(CD20H)](PF6) (Figure 4). The isotope shifts of the four bands in solid czs-[Ru(bpy)2(C0)(CH20H)]+ and czs-[Ru(bpy)2(C0)(CD20H)]+ were also detected in CH3CN at almost identical wavenumbers. This is in contrast to the pattern of the Raman spectra of czs-[Ru(bpy)2(CO)X]n+ (X = C02, C(0)0H, C(0)0CH3, CHO), showing one medium v(Ru—X) band and a weak v(Ru—CO) band around 530 and 470 cm-1, respectively. The appearance of two medium and two weak bands in the Raman spectra of czs-[Ru(bpy)2(C0)(CH20H)]+ in this region is not explained by the disorder between CO and CH2OH of czs-[Ru(bpy)2(C0)(CH20H)](PF6), suggested in a previous X-ray crystal analysis.6® The discrepancy, therefore, motivated us to reexamine the crystal structure of czs-[Ru(bpy)2(C0)(CH20H)](PF6) to confirm configurational isomers. Reinvestigation of the X-ray analysis of the crystal structure of [RutbpyMCO)(CH2OHKPF6) clearly showed the presence of two isomers with respect to the orientation of the RUCH2—OH bond directed below and above the equatorial plane. The former is the same structure as the previously reported one with an Ru-C(22)-0(2) bond angle of 105° (Figure 5a). The latter has a Ru—C(23)—0(3) angle of 108°, and a weak interaction between 0(3) and the carbonyl carbon, estimated from the relatively short distance (3.1 Á), would give two v(Ru—X) and v(Ru—CO) bands in the
Raman spectra. Thus, the four bands (558.9 (m), 523.5 (m), 477.7 (w), and 471.6 (w) cm""1) of czs-[Ru(bpy)2(CO)(CH2OH)](PF6) are concluded to result from the two isomers. From the relative intensities of these four bands, the 558.9 (m) and 477.7 (w) cm-1 pair and the 523.5 (m) and 471.6 (w) cm-1 pair are reasonably assigned to v(Ru—CH2OH) and v(Ru—CO) modes, respectively, of the two isomers. The finding of two isomers of czs-[Ru(bpy)2(C0)(CH20H)]+ also results from the reasonable assignment of the v(Ru—CH2OH) bands and v(Ru—CO) bands.
Discussion From the large isotope shift in the Raman spectra between czs-[Ru(bpy)2(C160)2]2+ and czs-[Ru(bpy)2(C180)2]2+, vsym(Ru—Cu160) is straightforwardly assigned at 443 cm-1. A series of czs-[Ru(bpy)2(CO)X]n+ complexes (X = CO2, C(0)0H, C(0)0CH3, CHO, and 0—2) shows one characteristic mediumCH2OH; intensity band in the range 558.9—511.3 cm-1 and a weak band around 470 cm-1. The medium intensity band undergoes obvious isotope shifts compared to the weak one, and all the remaining bands in these complexes are almost invariant with the substituent X in the region from 1000 to 200 cm-1. The bands around 530 (m) and 470 (w) cm'1, therefore, are reasonably associated with the Ru—X and Ru—CO stretching modes, respectively, and the coupling of v(Ru—CO) and r(Ru—X) modes would be very small due to a nearly perpendicular X-Ru-CO bond angle (Table 1). In addition, the deviation of the Raman spectrum of cis—
Reduction ofRu-C02 to Ru-CH2OH
Figure 6. Plot of v(C—O) bands vs v(Ru—X) bands of cis0-2; X CO, C(0)0H, C(0)0CH3, [Ru(bpy)2(CO)X]n+ ( =
=
C02, CHO, CH2OH).
[Ru(bpy)2(CO)(CH2OH)]+ from those of the other complexes has led to the confirmation of the two isomers. These facts suggest small or negligible contributions of other moieties to the v(Ru—X) and v(Ru—C) bands in the cis-[Ru(bpy)2(CO)X]n+ series. The v(Ru—X) and v(Ru—CO) bands tentatively assigned are also collected in Table 1. The v(CO) and v(Ru—X) bands in cis-[Ru(bpy)2(CO)X]n+ are in the range 2098.9 (X = CO) to 1934.3 cm"1 (X = CH2OH), and 558.9 (X = CH2OH) to 443.5 cm-1 (X = CO), respectively, and the shift of v(Ru—CO) is quite small compared with that of v(Ru—X). Although no clear correlation is observed between v(Ru—X) and the mass of X (from CO to C(0)0CH3) or the charge of the complexes ( = 0-2), v(CO) bands move to lower wavenumbers as v(Ru—X) bands shift to higher ones (Figure 6). The order of the shift of the v(CO) bands to lower wavenumbers (X = CO < C(0)0H < CHO < C02 < CH2OH) correlates with the electron-donating ability of the substituent X. The correlation between v(Ru—X) and v(CO) in Figure 6, therefore, is explained by an enhancement of o- and -bonding characters of the Ru—X and Ru—CO bonds, respectively, with increasing
electron-donating ability of X. The crystal structures of most of these complexes, including the two isomers of cis-[Ru(bpy)2(CO)(CH2OH)]+, have been determined by X-ray analysis.611 The Ru—CO and Ru—X bond distances are also summarized in Table 1. There is no clear correlation between the Ru—CO bond distance and v(Ru—X). On the other
Organometallics, Vol. 14, No. 11, 1995
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7. Relationship between Ru—X bond lengths (d(Ru—X)) and v(Ru—X) bands of cis-[Ru(bpy)2(CO)X]n+ (n = 0—2; X = CO, C(0)OH, C(0)OCH3, C02, CHO, CH2OH).
Figure
hand, a plot of the Ru—X bond distance (d(Ru—X)) against the v(Ru—X) band gives a linear correlation (Figure 7). The gradual shortening of the Ru—X bond distances from Ru—CH2OH to Ru—C02 to Ru—C(O)OCH3 to Ru—CO is reasonably ascribed to the contraction of the radius of the carbon atom with the change of the hybridization from sp3 to sp2 to sp. The order of the shortening of Ru-X distances also reflects an increase in the (1 — interactions between Ru and carbon. The order of the -electron-acceptor ability is, therefore, inverse to that of -electron-donor ability of the substituents estimated from the shift of v(CO) bands of cis-[Ru(bpy)2(CO)X]n+ (CH2OH > C02 > CHO > C(O)OH > CO). The unusual relationship of v(Ru—X) bands shifting to higher wavenumbers as the Ru—X distances are lengthened (Figure 7) may be explained by an assumption that the multiple-bond characters between carbon and oxygen ( , C=0, and C—O) in the substituent X (CO, C(0)OH, C(0)OCH3, C02, CHO, CH2OH) have more influence on the observed v(Ru—X) band than do the Ru—C bond characters in these cis[Ru(bpy)2(CO)X]'l+ complexes.
Supporting Information Available: Lists of crystallographic data, calculated positional parameters, anisotropic thermal parameters, and bond distances and angles for [Ru(bpy)2(CO)(CH2OH)](PF6) (11 pages). Ordering information is given on any current masthead page. OM950483T
