A new approach to predicting the carbonyl stretching frequencies of Co 2 (CO) 8 with D 3d symmetry

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1 Indian Journal of Chemistry Vol. 46A, September 2007, pp A new approach to predicting the carbonyl stretching frequencies of Co 2 (CO) 8 with D 3d symmetry Cemal Kaya*, Duran Karakaş & Elvan Üstün Department of Chemistry, Cumhuriyet University, 5840, Sivas, Turkey kaya@cumhuriyet.edu.tr Received 2 March 2007; revised 29 July 2007 This paper describes a method for determining C-O stretching frequencies of dicobalt octacarbonyl belonging to D 3d point group. The method is based on the variation of fundamental C-O stretching frequencies and C-O stretching force constants of axially substituted X 3 MCo(CO) 4 (M=Si, Ge, Sn and Pb; X=Cl, Ph and Et) molecules with the dipole moment of X 3 M-Co(CO) 4 bond. Frequencies of a () g, and e u modes have been evaluated from the graph of frequency versus dipole moment, and the frequencies of a g and e g modes determined from the equations derived by using the CO-factored force field. The calculated frequencies have been found to be consistent with the experimental values. With the use of the calculated frequencies, the C-O factored force constants of Co 2 (CO) 8 with D 3d symmetry have been calculated. In addition, frequencies of mono- 3 CO substituted species of Co 2 (CO) 8 have been estimated and compared with observed frequencies of the species. Dicobalt octacarbonyl was first synthesized by Mond et al.. Considerable attention has been focused on cobalt carbonyls and their derivatives, particularly in terms of their catalytic and selectivity in a wide variety of industrial important carbonylation and hydroformalytion (oxo-synthesis) reactions 2-8. In fact, dicobalt octacarbonyl is a catalyst precursor for these reactions, and may be involved in equilibria with active catalytic species 5. Dicobalt octacarbonyl exhibits interesting and unusually complex structural characteristics. In the solid state, the structure has C 2v point group symmetry (Fig. a), involving a pair of bridging CO groups 9,0. In solution, the nature of Co 2 (CO) 8 is far more difficult to establish. Although the carbonyl-bridged structure persists in solution, the molecule is highly fluxional and only one singlet 3 CO resonance is observed over a wide temperature range. A detailed examination of IR spectrum of Co 2 (CO) 8 in a range of different solvents and at different temperatures has shown the CO existence of at least three isomeric forms [Figs a, b and c] 2-6. Isomer (a) contains CO bridges, observed in solid state whereas isomers (b) and (c) correspond to nonbridged forms. Other evidences for the existence of the three isomeric forms for Co 2 (CO) 8 come from the Extended Hückel Theory (EHT) calculations 7 and application of the Ligand Polyhedral Model (LPM) 8. Isomer (b) has been assigned a D 3d symmetry and isomer (c) has been assigned a D 2d symmetry 9. In the IR spectrum of Co 2 (CO) 8, a large number of C-O stretching bands are observed in solution and at low temperature matrices. Some of them were unambiguously assignable to (a), (b) and (c) forms 5,6,9. For Co 2 (CO) 8 with D 3d symmetry, form (b), on the basis of group theory six carbonyl stretching modes are expected, 2a g, e g, 2 and e u. The a g and e g modes are Raman-active; the and e u modes are infrared-active. Three infrared-active carbonyl stretching bands were observed in the matrix isolation spectrum of Co 2 (CO) 9 8, and in hydrocarbon () solvents the frequencies of the and e u modes were reported by Noack 2. High-pressure Raman spectrum of Co 2 (CO) 8 was taken by Ernstbrunner and Kilner 20. Both IR and Raman data obtained in different phases and at different temperatures do not provide frequencies useful in force constants calculations because of the shifts due to phase change. Frequency shifts are not uniform for all bands 2,22. It is, therefore, important to develop methods for predicting frequencies of C-O stretching bands, especially infrared-inactive ones or of those not observed in IR spectrum. We aim herein to predict carbonyl Fig. Possible structures of Co 2 (CO) 8 in solution.

2 KAYA et al.: CARBONYL STRETCHING FREQUENCIES OF Co 2 (CO) 8 WITH D 3d SYMMETRY 389 stretching frequencies of Co 2 (CO) 8 belonging to D 3d symmetry (form b, in Fig. ) by using the correlation curves between C-O stretching frequencies and force constants of axially substituted X 3 MCo(CO) 4 and dipole moment of X 3 M-Co(CO) 4 bond. Theoretical It is well-known that the CO-factored force field, generally known as the approximate Cotton- Kraihanzel force field, is widely used to evaluate C-O stretching force constants and CO-CO interaction constants in metal carbonyls and their derivatives However, the application of this simplified force field to complexes having more than one fundamental vibration belonging to the same non-degenerate irreducible representation leads to undetermined algebraic system. On the basis of the CO-factored force field, C-O stretching force constants and CO- CO interaction constants for Co 2 (CO) 8 with D 3d symmetry are defined in Fig. 2. The secular equations for the mentioned molecule derived by employing energy factored force field are given in Table. As can be seen from Table, the number of unknowns (force contants) is more than the number of observable C-O stretching modes, and the secular equations for Co 2 (CO) 8 with D 3d symmetry are undetermined. In order to solve this undetermined algebraic system, we have used our earlier results 3. The Co 2 (CO) 8 with D 3d symmetry is closely related to axially substituted X 3 MCo(CO) 4 molecules with C 3v symmetry because in the Co 2 (CO) 8 molecule the Co(CO) 4 group on one cobalt is cited in an axial position as the X 3 M group in the X 3 MCo(CO) 4 molecule. Therefore, the approach to solving the COfactored force fields of X 3 MCo(CO) 4 molecules could be valid for Co 2 (CO) 8 molecule belonging to D 3d symmetry. Eq. (5) reported by us earlier 32 has been modified as follows for the form Co 2 (CO) 8 considered in this work: + km = ( 2 ) () 4µ km = ( 2 ) 4µ () where and 2 are the parameters of a g and a g ; and 2 are the parameters of a () 2u and a 2u modes, respectively. When Eqs () and are inserted in the equations given in Table, the following relations are obtained: k = [3( + ) + ( ) + 8( )] (3) Fig. 2 Definition of CO-factored force constants in Co 2 (CO) 8 with D 3d symmetry (k and k 2 are CO-stretching force constants; k c, k c, k m, k t, p and q are CO-CO interaction constants). Table Secular equations for Co 2 (CO) 8 molecule with D 3d symmetry Secular equations a ( k + + p + q) - ( + km) µ 2 2 3µ 3µ ( k + k ) µ ( k + k )- ( k + - p - q) - ( - km) µ 2 2 3µ 3µ ( k - k ) µ ( k - k )- Symmetry species a g () a g () µ ( k k + q p) e g c µ ( k k q + p) e u c a The stretching and interaction constants are defined in Fig. ; µ represents the reciprocal of the reduced mass of the CO group; =4π 2 c 2 υ 2 where υ is the frequency in cm -. k2 = [( + ) + 3( 2 + (4) k c = [3( + ) + ( ) 4( )] (5) = [( + ) ( 2 + (6) kt = [( ) + 3( 2 (7) km = [( ) ( 2 (8) p = [3( ) + ( 2 2 ) 4( 3 3 )] (9) q = [3( ) + ( 2 2 ) + 8( 3 3 )] (0) where 3 and 3 are parameters of e g and e u modes, respectively.

3 390 INDIAN J CHEM, SEC A, SEPTEMBER 2007 It is obvious from Eqs (3)-(0) that the CO-factored force constants can be calculated from fundamental C-O stretching frequencies of the molecule under study or that fundamental frequencies can be estimated from force constants if they are known. Many years ago, Kahn and Bigorgne 33 plotted frequencies of a mode of axially substituted X 3 MCo(CO) 4 (M=Si, Ge, Sn and Pb; X=Cl, Ph, and Et) against dipole moments of X 3 M-Co(CO) 4 bond; and obtained a linear correlation. They also emphasized that the frequency at µ=0 corresponds to that of Co 2 (CO) 8. In this work, using frequencies of Kahn and Bigorgne 33, we have drawn graphs of all frequencies of X 3 MCo(CO) 4 against dipole moments of X 3 M-Co(CO) 4 bond. The curves obtained are given in Fig. 3, where a linear correlation is obtained for all frequencies. Axially substituted X 3 MCo(CO) 4 molecules belong to C 3v point group. In order to find the correlations between D 3d and C 3v point groups, we have employed the method of descending symmetries 34 and the results obtained are presented follows: C 3v D 3d Our value is also very close to this. Although Bor and Noack 5 assigned tentatively the band observed at 207 cm - in hydrocarbon solvent to Low temperature form (Fig. a), in their first papers 4,35 they independently proposed that the same band belongs to high temperature form (Fig. b). Our results confirm the latter. In order to determine frequencies of the remaining bands [a g and e g modes, the band of a () 2u mode was reported by Noack and Bor 5, 2069 cm - ] of Co 2 (CO) 8 molecule under study, we have made use of Eqs (3) and (4), and graphs of C-O stretching force constants in X 3 MCo(CO) 4 molecules against dipole moments of X 3 M-Co(CO) 4 bond. The CO-factored force constants of axially substituted X 3 MCo(CO) 4 molecules have been taken from our previous studies 3. As can be seen from Fig. 4, the variation of both k and k 2 with the dipole moment of X 3 M-Co(CO) 4 bond is linear as in the case of frequencies. This enables us to estimate C-O stretching force constants (k and k 2 ) of Co 2 (CO) 8 with D 3d symmetry, assuming that the force constants at µ=0 belong to the mentioned complex. k and k 2 were found to be and A E A g, A 2u E g, E u () From this correlation, one can conclude that a () and a in C 3v point group correspond to a g and a 2u in D 3d point group, respectively, and e in the former group corresponds to e u in the latter group. These results enable us to predict particularly the () frequency of a g mode of Co 2 (CO) 8 with D 3d symmetry from the graph of frequency versus dipole moment, which is infrared-inactive. Results and Discussion From Fig. 3, the frequencies corresponding to µ=0 were found to be 206.2, and cm -. As suggested by Kahn and Bigorgne 33, the three frequencies may be assigned to Co 2 (CO) 8. The correlation between C 3v and D 3d point groups leads to a () g =206.2, a 2u = and e u = cm -. The frequency of e u mode determined here is very close to that reported by Noack et al. 5,6, 2022 cm -, certainly assigning to Co 2 (CO) 8 with D 3d symmetry. The band of a 2u mode was observed only in argon matrix 9 at cm -. Since there is a shift of approximately 4 cm - to higher frequencies in the matrix isolation spectra, our value for mode appears to be acceptable. In addition, as reported by Bor 4, the band at 2044 was assigned to Co 2 (CO) 8 with D 3d symmetry. Fig. 3 Graphs of frequencies versus dipole moment. Fig. 4 Variation of k and k 2 of X 3 MCo(CO) 4 molecules with dipole moment of X 3 M-Co(CO) 4 bond.

4 KAYA et al.: CARBONYL STRETCHING FREQUENCIES OF Co 2 (CO) 8 WITH D 3d SYMMETRY mdyn/å, respectively. With the use of these force constants and the frequencies, ( ), 2069 ( ), ( 2 ) and ( 3 ) cm -, 2 and 3 (frequencies of a g and e g modes) were calculated from Eqs (3) and (4) as and cm -. It should be noted that the frequency of cm - is in excellent agreement with that of Ernstbrunner and Kilner 20, 2035 cm -. The C-O stretching frequencies of Co 2 (CO) 8 belonging to D 3d point group determined here and the CO-factored force constants calculated from Eqs (3)-(0) are given in Table 2. To check the validity of the procedures applied here, C-O stretching frequencies of isotopically enriched species of Co 2 (CO) 8 were estimated by using the force constants given in Table 4 and the results obtained were compared with observed frequencies of the species. The spectra of 3 CO-enriched Co 2 (CO) 8 were reported by Noack and Bor 5. They proposed that the bands at 20, 99 and 975 cm - belong to Co 2 (CO) 8 with D 3d symmetry. The secular equations for the equatorially, and axially substituted mono 3 CO species, and fully substituted 3 CO species of Table 2 Calculated C-O stretching frequencies and force constants for Co 2 (CO) 8 with D 3d symmetry Frequency (cm - ) Force constants (mdyn/ Å) a Taken from ref. 33. () a g a g e g () e u a k k 2 k c k c k m k t p q Table 3 Secular equations a for isotopically enriched Co 2 (CO) 8 with D 3d symmetry Equatorially substituted species Assignments µ * k - 2µ k µ q 2µ p µ k µ k c c m ( + )- ( + ) 2µ * k µ k k 2µ p µ q p 2µ k 2µ k c c c m µ * q 2µ p µ k - 2µ µ km µ 6a 2µ * p µ ( q + p) 2µ µ ( k + ) - 2µ km 2µ µ * 2µ µ km 2µ km µ k2 - µ kt µ * km 2µ km µ 2µ µ kt µ k2 - µ ( k - ) - µ ( q - p) 2a µ q - p µ k - k - ( ) ( c) ( + )- ( + ) ( + ) ( + )- Axially substituted species µ k 2k µ q 2 p 3µ * k 3µ k c c m µ q 2 p µ k 2 3µ * km 3µ 4a 3µ 3µ km µ * k2 - µ kt 3µ km 3µ µ * kt µ k2 - µ ( k - ) - µ ( q - p) 2e µ q - p µ k - k - ( ) ( c) Fully substituted species ( k + + p + q) - ( + km) µ * 2 2 3µ * 3µ *( k + k ) µ *( k + k )- ( k + - p - q) - ( - km) µ * 2 2 3µ * 3µ *( k - k ) µ *( k - k )- ( k q p) ( k q p) µ * e g µ * e u a µ and µ* denote the reciprocal of the reduced mass of 2 CO and 3 CO, respectively. 2a g 2

5 392 INDIAN J CHEM, SEC A, SEPTEMBER 2007 Table 4 Calculated and observed frequencies for the species with one equatorial 3 CO, one axial 3 CO and fully 3 CO Frequencies (cm - ) Species Calc. Obs. Assignments Equatorially substituted a a a a a a a b a a a Axially substituted a a a a a b e a e Fully substituted a g a g a e g a e u a Taken from refs 35 and 5; b Taken from ref. 20. Co 2 (CO) 8 belonging to D 3d point group were derived by reported procedures 36 and are presented in Table 3. The calculated frequencies are given in Table 4. Inspection of Table 4 reveals that there exists a rather good fit between calculated and observed frequencies, with a maximum error of 2 cm -. Although Bor and Noack 5 suggested that the band at 200 cm - belongs to the solid structure, our calculations show that it belongs to the structure understudy. Conclusions There exists a linear relation between fundamental C-O stretching frequencies of X 3 MCo(CO) 4 complexes and dipole moment of X 3 M-Co(CO) 4, (Fig. 3). The correlation between C 3v and D 3d point groups enables us to estimate the frequencies of a () g, a 2u and e u modes of Co 2 (CO) 8 with D 3d symmetry. () Such a calculation is especially important for a g mode because its band is not observed in IR spectra. The linear correlation between C-O stretching force constants of X 3 MCo(CO) 4 complexes and dipole moment of X 3 M-Co(CO) 4 bond (Fig. 4) allows the calculation of C-O stretching force constants (k and k 2 ) and frequencies of a g and e g modes of Co 2 (CO) 8. The present method may be used to determine the C-O stretching bands belonging to Co 2 (CO) 8 with D 3d symmetry in the solution IR spectra. References Mond L, Hirtz H & Cowap M D, J Chem Soc, 97 (90) Harrod J F & Chalk A J, J Am Chem Soc, 87 (965) Hanlan L A, Huber H, Kunding E P, Mc Garvey B R & Ozin G A, J Chem Soc, 97 (975) Whyman R, J Organometal Chem, 8 (974) Heck R F, Organotransition Metal Chemistry (Academic Press, New York) Pino P, Piacenti F & Bianchi M, Organic Synthesis via Metal Carbonyls, Vol II (Wiley, New York) Orchin M & Rupilius W, Catal Rev, 6 (972) 85 8 Seelig F F, Z Naturforsch, B, 3 (976) Mills O S & Robinson G, Proc Chem Soc, (959) Summer G G, Klug H P & Alexander L E, Acta Crystallogr, 7 (964) 732. Evans J, Johnson B F G, Lewis J, Matheson T W & Norton J R, J Chem Soc Dalton Trans, (978) Noack K, Helv Chim Acta, 47 (964) Noack K, Helv Chim Acta, 47 (964) Bor G, Spectrochim Acta, 9 (963) Bor G & Noack K, J Organometal Chem, 64 (974) Bor G, Dieter U K & Noack K, J Chem Soc Commun, (976) Bellagamba V, Ercoli R, Gamba A & Suffritti G B, J Organometal Chem, 90 (980) 38 8 Johnson B F G & Parisini E, Inorg Chim Acta, 98 (992) Sweany R L & Brown T L, Inorg Chem, 6 (977) Ernstbrunner E E & Kilner M, JCS Dalton, (975) Haines L M & Stiddard M H B, Inorg Chem Rdiochem, 2 (969) Cotton F A, Musca A & Yagupsky G, Inorg Chem, 6 (967) Cotton F A & Kraihanzel C S, J Am Chem Soc, 84 (962) Üstün E Ö & Kaya C, J Molec Struct Theochem, 70 (2004) Kaya C & Karakaş D, Indian J Chem, 46A (2007) Braterman P S, Metal Carbonyl Spectra (Academic Press, London) Kaya C, Spectrochim Acta Part A, 50 (994) Kaya C, Spectrochim Acta Part A, 52 (996) Karakaş D & Kaya C, J Organometal Chem, 640 (200) Zengin N & Kaya C, J Organometal Chem, 658 (2002) Kaya C, J Organometal Chem, 586 (999) Kaya C, J Organometal Chem, 575 (999) Kahn O & Bigorgne M, J Organometal Chem, 0 (967) Orchin M & Jaffe H H, Symmetry, Orbitals and Spectra (Wiley-Interscience, John Wiley & Sons, Inc. New York) Noack K, Spectrochim Acta, 9 (963) Wilson Jr E B, Decius J F & Cross P C, Molecular Vibrations (Mc Graw-Hill, New York) 955.