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Infrared spectrum of matrix-isolated Fe(CO)5 in the FeC stretching region
Spectrochimica
Acta, Vol. 3~4, pp. 1753 to 1774.
Pergamon
Press, 1975.
Printed
in Northern
Ireland
RESEARCH NOTE INFRARED
SPECTRUM Fe--C
OF MATRIX-ISOLATED STRETCHING
(Received
16 Jammry,
The vibrational spectrum of Fe(CO), has been studied extensively by many workers. EDGELL et al. [l] assigned two Fe-C stretching bands (gaseous i.r.) at 474(AsX) and 431 cm-‘(E’) in agreement with its D,, symmetry. BIGOR~NE [2] studied the i.r. and Raman spectra of Fe(CO), and its derivatives, and assigned the former to the E’ and the latter to the A,” species. This reversal of band assignments was followed by CATALIOTTI el al. [3] who assigned the i.r. bands of crystalline Fe(CO), at 480 and 433 cm-’ to the E’ and A,” species, respectively. Later, JONES el al. [4] have carried out an extensive vibrational study on Fe(CO), and its ‘*C and IsO derivatives, and “for lack of better evidence,” accepted Bigorgne’s assignments of these two modes. SWANSON et al. [6] were the first to measure the i.r. spectrum of matrix-isolated Fe(CO),. They report five CO stretching bands in Ar and Xe matrices at co. 20 K, three of which become weaker while the other two become stronger by annealing below the migration temperature of the matrix atoms. They assigned the former three bands to t,he molecules of C,, or lower symmetry, and the latter two bands to symmetry. POLIAROFP and the molecules of I),, TURNER [6] also measured the i.r. spectrum of Fe(CO), in seven different matrices. Depending upon the nature of the matrix, they observed three to seven CO stretching bands. They postulated that there are at least two different sites in these matrices and that relative concentrations of these sites are determined by the “spray-on” condition and the annealing process. As stated above, the band assignments in the Fe-C stretching region has not yet been conclusive. Furthermore, the matrix-isolation spectrum of Fe(CO), has not been measured in the Fe-C stretching region. We have, therefore, carried out an i.r. study of matrixisolated Fe (CO) s in the 600400 cm-r region.
Fe(CO),-matrix samples were prepared by depositing the pre-mixed gases onto a CsI window at ca. 20 K which was cooled by a Cryogenic Technology Model 20 closed-cycle refrigerator. Fe(CO), was purified by a one-step distillation in a glass vacuum line. Matrix gases, argon, nitrogen and methane, were purified by slow passage through a liquid nitrogen bath prior to sample preparation. In most experiments, the matrix to Fe(CO), mole ratio was 1000/l Spectra were recorded on a Beckman or greater. IR-12 i.r. spectrophotometer using standard calibration techniques.
Fe(CO),
IN THE
REGION 1976)
RESULTS AKD DISCUSSIOH Since the i.r. spectrum of matrix-isolated Fe(CO), in the CO stretching region has been reported by previous workers [6, 61, no discussion will be given here except to mention that our spectrum in an Ar matrix is in complete agreement with that of POLIAKOFF and TURNER [S]. Figure 1 shows the i.r. spectrum of Fe(CO), in an Ar matrix in the 600-400 cm-r region. As stated before, the gas-phase i.r. spectrum exhibits two broad bands at 474 and 431 cm-’ [I]. However, the Ar matrix spectrum at 20 K exhibits at least seven bands (487, 479, 477, 463, 464, 434 and 409 cm-i) in the same region. As is seen in Fig. 2, the N, matrix gives six bands (484, 477, 447, 436, 426 and 414 cm-i) and the CH, matrix exhibits five bands (484, 478, 443, 426 and 416 cm-‘) at 20 K. These results suggest that each matrix gives a set of sites which is different in number and symmetry.
-
400
Frequency cm-1
Fig. 1. Infrared spectrum of Fe(CO), in an Ar matrix at 20, 34,38 and 40 K. 1773
Research note
1774
As the temperature of an Ar matrix is raised from 20 to 38 K, the relative intensity of the 463 versus 464 cm-r band changes as is shown in Fig. 1. At 40 K, only four bands are observed at ca. 487 (weak), 479 (medium), 464 (weak) and 434 cm-r (strong). At 100 K, the spectrum is similar to that of microcrystalline Fe(CO), reported by CA~ALIOTTI et al. [31; only two bands remain at ca. 480 and 434 cm-i. This result suggests that two bands at 479 and 434 cm-i are probably due to molecules situated at D,, sites. However, the observed change in relative intensity of the 463 versus 464 cm-i band mentioned above seems to imply that there are two more sites of symmetry lower than D,, in an Ar matrix. If the Ar matrix at 20 K is warmed quickly to 60 K, the spectrum shown in Fig. 2 is obtained; three bands of almost equal intensity are observed at 480, 440 and 428 cm-i, the latter two being a doublet. As the temperature is raised to 100 K, the 480 cm-’ band remains unchanged while the doublet coalesces into a single band at 434 cm-i. Since the latter frequency is close to the average of two frequencies of the doublet, it is most reasonable to assign these doublet peaks as the E’ mode split slightly by a distortion of D,, symmetry. This distortion may be small so that the A,’ -I-
P
/
2oK
CH.3
/
-;./
/
4co
REFEFUZHCES
;;YP 100 K
I
500
Acknowledgement-The authors are grateful to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the support of this research. J. DAVID BROWN DAVID E. TEVAULT ALAN D. CORMIER KAZUO NAEAMOTO Department of Chemistry Marquette University Milwaukee, WI 63233 U.S.A.
70K
50K
Y
modes are not observed in the i.r. spectrum. As is seen in Fig. 2, similar doublets are observed after the N, and CH, matrix molecules are boiled off. These results suggest that, regardless of the matrix used, these small distortions oocur immediately after the Furthermore, boiling-off of the matrix molecules. our observations cast doubt upon the previously accepted assignment that the higher frequency band near 480 cm-i is due to the E’ species 12-41. In view of our new findings, we propose to assign the 480 and 434 cm-i bands to the A,” and E’ species, respectively, thus reviving the original assignment made by EDQELL et al. [l]. In D,, symmetry, the E’ mode is both i.r. and Raman active whereas A,# mode is only i.r. active. A previous investigator [2] assigned the i.r. band at 476 cm-’ of the liquid (480 cm-l of the solid) to the E’ stretching mode since the liquid Reman spectrum exhibits a Although another Raman band band at 482 cm-‘. was reported at 491 cm-’ and assigned to an E” (Raman active) bending mode, the presence of these two bands in this region is not obvious as pointed out by JONES et al. [4]. It is probable that only one band exists in this region which may be assigned to the E” bending mode. The Raman band at 448 -r was previously assigned to another E” bending mode. This band oan be assigned to the E’ stretching mode although it may overlap with the E” bending mode.
I
I
I
I
500
looi+
400
I
500
I
I
I
[I] [2] [3] [4]
400
Frequencycm-’ Fig. 2. Infrared spectrum of Fe(CO), in Ar, N, and CH, matrices at 20, 60-70 and 100 K.
[S] [6]
W. F. EDOELL, W. E. WILSON and R. SUMMITT, Spectrochim. Acta 19,863 (1963). M. BIQORONE, J. Organometal. Chem., 24, 211 (1970). R. CATALIOTTI, A. FOFFANI and L. MARCHETTI, Irwrg. Chem. 10, 1694 (1971). L. H. JONES, R. S. MCDOWELL, M. GOLDBLATT and R. I. SWANSON, J. Chem. Phys., 57,206O (1972). B. I. SWANSON, L. H. JONES and R. R. RYAN, J. Mol. Spectry., 45, 324 (1973). M. POLIAKOFF and J. J. TURNER, J. Chem. SOL, Dalton, 1361 (1973).
Acta, Vol. 3~4, pp. 1753 to 1774.
Pergamon
Press, 1975.
Printed
in Northern
Ireland
RESEARCH NOTE INFRARED
SPECTRUM Fe--C
OF MATRIX-ISOLATED STRETCHING
(Received
16 Jammry,
The vibrational spectrum of Fe(CO), has been studied extensively by many workers. EDGELL et al. [l] assigned two Fe-C stretching bands (gaseous i.r.) at 474(AsX) and 431 cm-‘(E’) in agreement with its D,, symmetry. BIGOR~NE [2] studied the i.r. and Raman spectra of Fe(CO), and its derivatives, and assigned the former to the E’ and the latter to the A,” species. This reversal of band assignments was followed by CATALIOTTI el al. [3] who assigned the i.r. bands of crystalline Fe(CO), at 480 and 433 cm-’ to the E’ and A,” species, respectively. Later, JONES el al. [4] have carried out an extensive vibrational study on Fe(CO), and its ‘*C and IsO derivatives, and “for lack of better evidence,” accepted Bigorgne’s assignments of these two modes. SWANSON et al. [6] were the first to measure the i.r. spectrum of matrix-isolated Fe(CO),. They report five CO stretching bands in Ar and Xe matrices at co. 20 K, three of which become weaker while the other two become stronger by annealing below the migration temperature of the matrix atoms. They assigned the former three bands to t,he molecules of C,, or lower symmetry, and the latter two bands to symmetry. POLIAROFP and the molecules of I),, TURNER [6] also measured the i.r. spectrum of Fe(CO), in seven different matrices. Depending upon the nature of the matrix, they observed three to seven CO stretching bands. They postulated that there are at least two different sites in these matrices and that relative concentrations of these sites are determined by the “spray-on” condition and the annealing process. As stated above, the band assignments in the Fe-C stretching region has not yet been conclusive. Furthermore, the matrix-isolation spectrum of Fe(CO), has not been measured in the Fe-C stretching region. We have, therefore, carried out an i.r. study of matrixisolated Fe (CO) s in the 600400 cm-r region.
Fe(CO),-matrix samples were prepared by depositing the pre-mixed gases onto a CsI window at ca. 20 K which was cooled by a Cryogenic Technology Model 20 closed-cycle refrigerator. Fe(CO), was purified by a one-step distillation in a glass vacuum line. Matrix gases, argon, nitrogen and methane, were purified by slow passage through a liquid nitrogen bath prior to sample preparation. In most experiments, the matrix to Fe(CO), mole ratio was 1000/l Spectra were recorded on a Beckman or greater. IR-12 i.r. spectrophotometer using standard calibration techniques.
Fe(CO),
IN THE
REGION 1976)
RESULTS AKD DISCUSSIOH Since the i.r. spectrum of matrix-isolated Fe(CO), in the CO stretching region has been reported by previous workers [6, 61, no discussion will be given here except to mention that our spectrum in an Ar matrix is in complete agreement with that of POLIAKOFF and TURNER [S]. Figure 1 shows the i.r. spectrum of Fe(CO), in an Ar matrix in the 600-400 cm-r region. As stated before, the gas-phase i.r. spectrum exhibits two broad bands at 474 and 431 cm-’ [I]. However, the Ar matrix spectrum at 20 K exhibits at least seven bands (487, 479, 477, 463, 464, 434 and 409 cm-i) in the same region. As is seen in Fig. 2, the N, matrix gives six bands (484, 477, 447, 436, 426 and 414 cm-i) and the CH, matrix exhibits five bands (484, 478, 443, 426 and 416 cm-‘) at 20 K. These results suggest that each matrix gives a set of sites which is different in number and symmetry.
-
400
Frequency cm-1
Fig. 1. Infrared spectrum of Fe(CO), in an Ar matrix at 20, 34,38 and 40 K. 1773
Research note
1774
As the temperature of an Ar matrix is raised from 20 to 38 K, the relative intensity of the 463 versus 464 cm-r band changes as is shown in Fig. 1. At 40 K, only four bands are observed at ca. 487 (weak), 479 (medium), 464 (weak) and 434 cm-r (strong). At 100 K, the spectrum is similar to that of microcrystalline Fe(CO), reported by CA~ALIOTTI et al. [31; only two bands remain at ca. 480 and 434 cm-i. This result suggests that two bands at 479 and 434 cm-i are probably due to molecules situated at D,, sites. However, the observed change in relative intensity of the 463 versus 464 cm-i band mentioned above seems to imply that there are two more sites of symmetry lower than D,, in an Ar matrix. If the Ar matrix at 20 K is warmed quickly to 60 K, the spectrum shown in Fig. 2 is obtained; three bands of almost equal intensity are observed at 480, 440 and 428 cm-i, the latter two being a doublet. As the temperature is raised to 100 K, the 480 cm-’ band remains unchanged while the doublet coalesces into a single band at 434 cm-i. Since the latter frequency is close to the average of two frequencies of the doublet, it is most reasonable to assign these doublet peaks as the E’ mode split slightly by a distortion of D,, symmetry. This distortion may be small so that the A,’ -I-
P
/
2oK
CH.3
/
-;./
/
4co
REFEFUZHCES
;;YP 100 K
I
500
Acknowledgement-The authors are grateful to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the support of this research. J. DAVID BROWN DAVID E. TEVAULT ALAN D. CORMIER KAZUO NAEAMOTO Department of Chemistry Marquette University Milwaukee, WI 63233 U.S.A.
70K
50K
Y
modes are not observed in the i.r. spectrum. As is seen in Fig. 2, similar doublets are observed after the N, and CH, matrix molecules are boiled off. These results suggest that, regardless of the matrix used, these small distortions oocur immediately after the Furthermore, boiling-off of the matrix molecules. our observations cast doubt upon the previously accepted assignment that the higher frequency band near 480 cm-i is due to the E’ species 12-41. In view of our new findings, we propose to assign the 480 and 434 cm-i bands to the A,” and E’ species, respectively, thus reviving the original assignment made by EDQELL et al. [l]. In D,, symmetry, the E’ mode is both i.r. and Raman active whereas A,# mode is only i.r. active. A previous investigator [2] assigned the i.r. band at 476 cm-’ of the liquid (480 cm-l of the solid) to the E’ stretching mode since the liquid Reman spectrum exhibits a Although another Raman band band at 482 cm-‘. was reported at 491 cm-’ and assigned to an E” (Raman active) bending mode, the presence of these two bands in this region is not obvious as pointed out by JONES et al. [4]. It is probable that only one band exists in this region which may be assigned to the E” bending mode. The Raman band at 448 -r was previously assigned to another E” bending mode. This band oan be assigned to the E’ stretching mode although it may overlap with the E” bending mode.
I
I
I
I
500
looi+
400
I
500
I
I
I
[I] [2] [3] [4]
400
Frequencycm-’ Fig. 2. Infrared spectrum of Fe(CO), in Ar, N, and CH, matrices at 20, 60-70 and 100 K.
[S] [6]
W. F. EDOELL, W. E. WILSON and R. SUMMITT, Spectrochim. Acta 19,863 (1963). M. BIQORONE, J. Organometal. Chem., 24, 211 (1970). R. CATALIOTTI, A. FOFFANI and L. MARCHETTI, Irwrg. Chem. 10, 1694 (1971). L. H. JONES, R. S. MCDOWELL, M. GOLDBLATT and R. I. SWANSON, J. Chem. Phys., 57,206O (1972). B. I. SWANSON, L. H. JONES and R. R. RYAN, J. Mol. Spectry., 45, 324 (1973). M. POLIAKOFF and J. J. TURNER, J. Chem. SOL, Dalton, 1361 (1973).