Infrared Photoacoustic Spectra of the Solid Manganese(I) Carbonyl Halides, Mn(CO)sX and [Mn(CO)4X]2 (X = Cl, Br, I)

HONGQI LI and IAN S. BUTLER* Department of Chemistry, McGiU University, 801 Sherbrooke St. West, Montreal, Quebec H3A 2K6, Canada

Mid- and near-IR photoacoustic (PA) spectra have been measured at room temperature for two series of crystalline manganese(I) carbonyl halides: Mn(CO)sX and [Mn(CO)4X]2 (X = CI, Br, I). Vibrational as- signments are proposed for many of the observed bands. The PA spectra in the near-IR region (4200-3800 cm-l), Where the binvary v(CO) over- tones and combinations absorb, are useful spectral fingerprints for these organomanganese(I) complexes.

Index Headings: Infrared; Photoacoustic spectra.

INTRODUCTION

Over the past few years, there has been increasing worldwide interest in material sciences, particularly in the development and industrial applications of new ad- vanced materials, e.g., organometallic polymer deriva- tives as electrically conducting thin films and fibers, ~ organic oligomers and high polymers as liquid crystals, 2 and organometallic clusters as precursors to high-purity ceramics and coatings2 Although conventional trans- mission IR spectroscopy has played an important role in the structural characterization of many of these new ad- vanced materials, some of them are optically opaque and/ or insoluble in common organic solvents. Consequently, while these materials present a challenge for conven- tional IR spectroscopy, several new analytical IR tech- niques, including IR photoacoustic (PA) spectroscopy, 4~ can now be used to surmount this problem.

For the past six years, we have been exploring the application of IR PA spectroscopy in studying the mid- and near-IR spectra of solid organometallic complexes. The ultimate aim of the work is to use this spectroscopic technique to monitor the pyrolytic decomposition of or- ganometallic complexes in air, in the production of new metallized-oxide ceramic materials. Our IR studies have shown that PA data for the binary v(CO) overtone and combination region at ~4000 cm -~ provide useful struc- tural fingerprints for solid metal carbonyl ceramic pre- cursors.7-1L The technique is sensitive and nondestruc- tive, and neccessitates only milligram quantities of these expensive organometallic materials.

In this paper, we report room-temperature, mid- and near-IR PA spectra of two typical series of crystalline metal carbonyl halides: Mn(CO)sX and [Mn(CO)4X]2 (X = C1, Br, I). Vibrational assignments are proposed for both series of organomanganese(I) complexes.

Received 27 July 1992. * Author to whom correspondence should be sent.

EXPERIMENTAL

The crystalline Mn(CO)sX and [Mn(CO)IX]2 were prepared according to the literature methods? 2 The PA spectra were recorded on a Mattson Cygnus Model 25 FT-IR spectrometer with the aid of an EG&G Princeton Applied Research Model 6003 photoacoustic cell with a 5-mm-diameter, 1-mm-deep sample cup. The spectral conditions employed were: 100 scans, 8 cm -~ resolution, 0.084 cm s -~ mirror velocity, and air as conducting gas. The spectra are ratioed against a carbon-black back- ground. The solution and the KBr spectra of the com- plexes were recorded at 4 cm -~ resolution on a Bomem Michelson Model 100 FT-IR spectrometer fitted with a DTGS detector.

RESULTS AND DISCUSSION

The vibrational spectra of monosubstituted manga- nese pentacarbonyl complexes have been thoroughly ex- amined over the past 30 years. 13-1s Most of this work has been focused on the CO stretching fundamentals [~(CO)], although some assignments have been made for the 6(MnCO) and ~(MnC) modes on the basis of approximate force constant calculations and the method of local os- cillating dipoles. 1~,14 Some vibrational data have been re- ported for the halogen-bridged dimers? 9-22 The known binary ~(CO) overtones and combinations for manganese carbonyl complexes are restr icted to Mn2(CO)lo and its meta l -meta l bonded derivatives, 23-26 and M n ( C O ) 5 X . 16,27,2s No such near-IR data have been re- ported for any of the halogen-bridged dimers.

Our IR PA results and proposed vibrational assign- ments for Mn(CO)sX and [Mn(CO)4X]2 (X = C1, Br, I) are given in Tables I and II, respectively. Several rep- resentative PA spectra, as well as some comparisons with conventional transmission IR measurements, are shown in Figs. 1-5.

Mid-IR Spectra of the Mn(CO)sX Complexes. The Mn(CO)sX (X = C1, Br, I) complexes are expected to have C4, symmetry, and the 30 predicted normal modes and their spectral activities are: 7a~(IR/Raman) + a2(inactive) + 4bl(Raman) + 2b2(Raman) + 8e(IR/Ra- man). From Table I, the observed IR PA spectra of the Mn(CO)sX molecules in the mid-IR region closely par- allel the spectra obtained by the traditional KBr disk method. In each case, the weak a l (eq) /.,(CO) mode is un- split, but the actual band position is higher in energy in the solid than it is in CC14 solution. The a~ (~) u(CO) modes have almost identical energies in the PA and KBr spec-

Volume 46, Number 12, 1992 0003-7028/92/4612-178552.00/0 APPLIED SPECTROSCOPY 1785 © 1992 Society for Applied Spectroscopy

TABLE I. Infrared PA and transmission spectra of Mn(CO)sX (cm ')."

M n ( C O ) s C 1 M n ( C O ) ~ B r M n ( C O ) s I

S o l u t i o n K B r P A S S o l u t i o n K B r P A S S o l u t i o n K B r P A S A s s i g n m e n t s

4516 w 4 5 1 0 w 4531 w 4466 w 4448 w 4 4 4 5 w 4284 w 4 2 6 6 w ¢ 4272 w 4 2 5 0 w b 4257 w 2a~ (2) 4 2 1 8 s h 4195 w

4188 s b 4 1 8 0 m 4178 s ~ 4 1 7 5 m 4165 s b 4 1 6 1 w a~ (2~ + e 4 1 6 1 m b 2b , 4132 w b 4 1 2 7 m s b 4137 w a~ (') + a , (z)

4 1 0 0 v w b 4 1 3 2 m 4 0 9 3 v w ¢ 4 1 2 8 m 4 0 8 0 s h b 4 1 0 6 m 2e 3 9 7 8 m s b 3 9 9 8 sh 3 9 8 2 m b c 4 0 0 0 s h 3 9 8 9 m b 4 0 2 0 m 2a~ (~)

3 9 6 6 m 3 9 7 0 m

2 1 3 8 w 2 1 4 3 w 2142 s 2 1 3 4 w 2 1 3 8 w 2 1 3 6 m 2 1 3 3 w 2 1 3 1 w 2 1 3 0 w : ( 2 ) - ~ 2054 s 2 0 6 9 m ~ 2064 s 2051 s 2 0 6 5 m ~ 2061 s 2050 s 2035 s 2055 s

2047 s ~ 2043 s 2 0 2 2 w 2 0 1 7 w 2025 s 2 0 2 0 w 2022 sh 1 9 9 9 m 1996 s 2000 s 2 0 0 0 m 2000 s 2004 s 2 0 0 1 m 2008 s 2012 S a l ( ' ) J

1988 s 1990 s V13co

1 9 5 2 w 1956 s 1 9 5 4 w 1 9 5 8 m 1 9 6 9 w 1972 s

~(CO)

6 4 0 m 6 4 7 m t 6 3 3 m 6 2 4 s 6 3 2 m 6 2 8 s 631 s 617 w M - C O b e n d i n g a n d 584 w 5 8 7 m 582 w 585 w 585 w 579 w M - C s t r e t c h i n g 547 w 5 5 0 m 544 w 542 w 541 w 5 3 7 m 4 0 9 w 416 w 427 w

" N o t e : s = s t r o n g ; w = w e a k ; s h b D a t a f r o m Ref . 27. c D a t a f r o m Ref . 16.

= s h o u l d e r ; m = m e d i u m .

tra. The strong e and al (eq) p(CO) modes of the chloro- and bromo-derivatives are split into doublets in the KBr spectra, while these modes appear as singlets for Mn(CO)sI. There is some PA band truncation of the very strong e and a l (eq) p ( C O ) modes due to signal saturation. 29 The bands are somewhat broadened by this effect, but their positions are maintained. The weak band occurring

in all three KBr spectra at ~1956 cm -1 is most likely attributable to a g(13CO) satellite mode. For Mn(CO)~C1 and Mn(CO)sBr, this band is much more intense in the PA spectra. Finally, the absence of any bands due to the IR-inactive bl v(CO) modes indicates that the structures of the Mn(CO)sX complexes do indeed conform to the expected C4v point group symmetry.

TABLE II. Infrared PA and transmission spectra of [Mn(CO)sXh (cm-'). a

[Mn(CO)4C1]2 [ M n ( C O ) 4 B r ] 2 [Mn(CO)4I ]2

S o l u t i o n K B r P A S S o l u t i o n K B r P A S S o l u t i o n K B r P A S A s s i g n m e n t s

4 4 9 5 w 4496 w 4 4 2 5 w 4425 w 4419 w

4207 w 4 1 8 0 w 2a~ (2~ 4 1 8 7 m 4172 w 4 1 2 9 m 4 1 0 9 m 4 0 8 4 w a~ (2) + a~ (~ 4 0 7 6 m 4 0 7 0 m 4061 w 2b~ 4 0 1 8 m 4 0 0 0 m 3 9 9 7 m 2a~ (2)

3 9 7 9 m 3 9 6 0 m 3 9 4 6 m 3 9 4 3 m 2b2 3 9 0 1 m 3901 s 3 9 0 3 m 1950 × 2

2 1 0 4 w 2 1 0 9 w 2 1 0 9 m 2 0 9 9 w 2 1 0 2 m 2 0 9 9 m 2 0 8 7 m 2 0 8 7 m 2081 s a~(2) '~ 2 0 4 5 s 2 0 5 4 m ] 2 0 4 5 s h 2 0 4 2 w 2 0 5 4 m ~ 2 0 4 2 s h 2 0 3 3 s 2 0 4 2 m 2 0 3 5 s h b,

2039 m~ 2036mj 2027m 2013s 2021s } 2025s} 2012m 2018m~ 2010sh~ 2008m 2015m~ 2010sh } al (')

2010 s 2010 s 2009 s J 2006 s ] 2003 s J 1997 s 1 9 7 8 m 1982 s 1977 s 1 9 7 7 m 1 9 8 2 m 1972 s 1 9 7 5 m 1982 s h 1970 s b= )

1 9 7 7 m 1 9 7 6 m 1951 s 1949 s 1952 s 1950 s 1955 s 1953 s

~(CO)

663 m 666 m 6 5 8 m 662 m 652 m "~ 631 w 630 m 6 3 0 m 628 m 6 2 8 m 617 s L 607 m 607 s h 609 s h M - C O b e n d i n g a n d 601 m 6 0 0 m 600 m 601 s 603 m 591 s 545 w 545 w 545 w 546 w 541 w 542 w [ M - C s t r e t c h i n g

441 w J 422 w 421 w 424 w

a N o t e : w = w e a k ; m = m e d i u m ; s = s t r o n g ; s h = s h o u l d e r .

1786 Volume 46, Number 12, 1992

O

<

PA

goo 21'oo 2obo 19bo ! 1800 cm-1

Wavenumbers

FIG. 1. Infrared spectra of Mn(CO)~I in the 2250-1800 cm -~ region; in KBr disk (top); PA spectrum (bottom).

Four IR-active carbonyl bending modes [5(MnCO); a~ + 3e] and three manganese-carbon stretching modes [g(MnC); 2al + e] are expected in the 700-300 cm -~ region for the C4o-symmetry Mn(CO)sX molecules. Usu- ally, the vibrational modes in this region absorb much more weakly than do the g(CO) modes, and the bands are often overlapped. From Table I and Fig. 1, it is ev- ident that the PA spectra in this region are closely similar to the corresponding KBr spectra.

Mid-IR Spectra of the [Mn(CO)4X]2 Complexes. Some solution and solid-state IR data have been reported for the [Mn(CO)4X]2 (X = C1, Br, I) complexes. 14,19,2° An x-ray diffraction study of [Mn(CO)4Br]2 has provided strong evidence for a halogen-bridged dimeric structure with D2h symmetry) ° According to the work of E1-Sayed and Kaesz ~9 and Orgel, 21 the IR spectra of the compounds in the v(CO) region exhibit three strong bands [b3J 2), b~,, b2,] and a relatively weak band at higher energy [B3J ~)] in CC14 and other solvents. Cotton 14 has treated the vi- brational spectra of the dimers in terms of the C2v local symmetry of the cis-Mn(CO)4X2 fragments, for which four ~(CO) modes (2a~ + b~ + b2) are again predicted. The observed IR spectra are fully in agreement with both spectroscopic treatments, but we have preferred to use Cotton's approach in the analysis of our solid-state IR PA data. Some typical conventional and PA spectra are shown in Figs. 2 and 3.

In the KBr spectra, the a~ ~), bl, and b2 v(CO) modes are all split into doublets due to solid-state effects, while the higher-energy a~ ~2> modes remain unsplit (Fig. 2). Although the b~ ~(CO) modes of all three compounds are not readily apparent in the PA spectra, the binary over- tones (2b~) do show up quite clearly at ~4070 cm -~ (Ta- ble II). This difference arises because the ~(CO) funda- mentals are extremely intense and the PA absorptions

CCI4

o3 o <

KBr

2250 2160 2070 1980 1890 1800 cm -1

Wavenumbers

FIG. 2. Infrared spectra of [Mn(CO)4C1]2 in the 2250-1800 cm 1 re- gion; in CC14 solution (top); in KBr disk (bottom).

become saturated (large absorption coefficients ~).29 This saturation effect leads to considerable overlap between the bl and al (1) ,(CO) fundamentals.

In the low-energy region, where the 5(MnCO) and v(MnC) modes absorb, the KBr and PA spectra are al- most identical (Table II). There is, however, some de- tectable solid-state splitting of one 5(MnCO) mode in the KBr spectra. The vibrational assignments for the dimers are not as well documented as those for the monomers, but we have made some attempts based on the C2v local symmetry of the cis-Mn(CO)4X2 moieties.

Near-IR PA Spectra of Mn(CO)sX and [Mn(CO)4Xh Complexes. The binary v(CO) overtones and combina-

8

o

3000

Fro. 3. region.

~ o ~'00 ls~ lo~o 500 Cm-1

Wavenumbers

Infrared PA spectrum of [Mn(CO)4C1]2 in the 3000-500 cm -1

APPLIED SPECTROSCOPY 1787

FIG. 5. region.

\ 4600 4500 44100 43100 4200 4100 4(~X) 3900

CRI-1 Wavenumbers

Inf ra red P A s p e c t r u m of Mn(CO)sBr in the 4600-3900 cm Fro. 4. region.

tions occur in the 4100-3800 cm -1 region with much lower intensities than those of the parent fundamentals. Despite their weakness, however, these overtone and combination bands can still be useful in locating the positions of formally IR-inactive ~(CO) modes. More- over, from a theoretical standpoint, these and higher- order overtone and combination bands are the chief source of anharmonicity data2 ~ One of the major advantages of the photoacoustic effect is that it extends the linear de- tection range of bands with small absorption coefficients ~.29 A decrease in the PA modulation frequency leads to an increase in the signal-to-noise ratios, and, consequent- ly, the quality of the near-IR spectra is greatly improved. In addition, conventional near-IR spectroscopy is often quite tedious because the solvents and KBr must be completely dry so that the sample signals can be detected free from any interference due to water absorptions. Spe- cial long-pathlength liquid cells have sometimes been constructed in an effort to increase the intensities of the sample signals, but even then suitable nonhydrocarbon

cm'l Wavenumbers

Inf ra red PA s p e c t r u m of [Mn(CO)4Cl]2 in the 4600-3800 cm -~

T A B L E III. Infrared PA binary v(CO) overtone and combination data for solid Mn(CO)sBr (cm 1).

Binary over tones Expec ted F o u n d and combina t ions (cm ~) (cm -~) A

2a~ (2) 4272 4272 0 2e 4122 4128 - 6 2al (') 4008 4000 8 al (2) + e 4197 4195 2 . 2bl 4160 4175 - 1 5

solvents have to be found which minimize the possibility of any signal overlapping.

Figures 4 and 5 illustrate the near-IR PA spectra of Mn(CO)sBr and [Mn(CO)4C1]2, respectively. For Mn(CO)sBr, the binary overtones of the e and al (2) ~(CO) modes and the a l (1) + e combination are quite strong, while the weaker a i (]) binary overtone is still readily dis- tinguished. The peaks appearing at ~4180 cm -1 are at- tributed to the A~-symmetry binary overtone of the IR- forbidden bl v(CO) fundamental. The complete selection rules for binary overtones and combinations under CIo symmetry have been given previously in the litera- ture.22. 32

Our analysis of the PA binary v(CO) overtones and combinations of the Mn(CO)sX and [Mn(CO)4X]2 (X = C1, Br, I) molecules are shown in Tables III and IV, respectively. A comparison of our monomer data with those from earlier work 2v,2s shows that the assignments are quite self-consistent. In the PA spectra, even the solid-state split components of the ai and e v(CO) fun- damentals exhibit overtones at ~3966 cm-L The very weak peaks detected at about 4520 and 4450 cm -1 are probably due to ternary combinations. The discrepancies in Table IV between the observed and calculated posi- tions of the binary ~(CO) overtones and combinations are the direct result of anharmonicity effects, and these are comparable to the values obtained by using conven- tional IR spectroscopy. 24,2s It is evident that PA spec- troscopy affords reliable and accurate data in the near- IR region.

No assignments have been made previously for the binary v(CO) overtones and the combinations of the C2v- local symmetry halogen-bridged dimers. The IR PA spec- tra measured in the 4600-3600 cm -1 region in the present work did yield some useful results. The proposed band assignments are given in Tables II and IV; the necessary selection rules are available in Ref. 32. The discrepancies in band positions, in this case due to the v(CO) anhar- monicities, are similar to those for the monomers and other related complexes2 ~ It should be mentioned that the discrepancies may be partly due to coupling between harmonics of the same symmetry, e.g., the A~-symmetry overtones, 2b~ and 2a~ (1).

TABLE IV. Infrared PA binary v(CO) overtone and combination data for solid [Mn(CO)4Brh (cm-l).

Binary over tones Expec ted F o u n d and combina t ions (cm -1) (cm - ' ) A

2al (2) 4198 4207 - 9 2b~ 4084 4070 14 2al (~) 4012 4000 12 2b2 3944 3946 - 2 a~ (') + a~ (2) 4105 4109 --4

1788 Volume 46, Number 12, 1992

CONCLUSIONS

Organometallic compounds are often darkly colored and air and/or solvent sensitive. They are normally syn- thesized in small quantities because the starting mate- rials can be quite expensive. Infrared PAS is an ideal method for the spectroscopic characterization of solid organometallic complexes because it is nondestructive and particularly good for small amounts of opaque ma- terials. Furthermore, near-IR PAS has a distinct advan- tage over conventional near-IR methods for organome- tallic complexes because the signal-to-noise ratios can be increased in the long-wavelength region by changing the modulation frequency. As a result, high-quality near- IR spectra can be obtained for the binary v(CO) overtone and combinations without the technical difficulties nor- mally encountered with conventional near-IR tech- niques. Therefore, IR PA spectroscopy offers a useful supplement to conventional IR spectroscopy for the or- ganometallic chemist. As more developments take place in IR PA instrumention and computer software, the dif- ficulties encountered with band truncation for some of the more intense PA signals will eventually be overcome.

ACKNOWLEDGMENTS

I.S.B. thanks the following agencies for the award of research grants: NSERC (Canada), EMR (Canada), and FCAR (Quebec). H.L. ac- knowledges McGill University for the award of a graduate teaching assistantship.

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APPLIED SPECTROSCOPY 1789