0 Copyright
Volume 5 , Number 3 , March 1986
1986
American Chemical Society
Photochemistry of Matrix-Isolated CH,Mn(CO),: Two Isomers of CH,Mn(CO),
Evidence for
Amanda Horton-Mastin, Martyn Poliakoff,” and James J. Turner* Department of Chemistry, University of Nottingham, Nottingham NG7 2RD, England Received July 2, 1985
UV photolysis of CH,Mn(CO), isolated in CHI or Ar matrices at 20 K provides IR and UV-vis evidence for two isomers of the coordinatively unsaturated fragment CH,Mn(CO),, with square-pyramidal structures 3b and 4b. Both isomers show a large shift between the position of their visible absorption maxima in Ar and CH4 matrices, consistent with an interaction of the matrix with the unsaturated metal center.
Introduction There is considerable interest, both experimental and theoretical, in the structure of coordinatively unsaturated transition-metal carbonyl species. Matrix isolation has been very successful in providing structural information about such carbonyl fragments and a large amount of work has centered around the geometry of five-coordinate d6 species.’ Early work by Perutz and Turner2r3involved Cr(C0)5, generated by UV photolysis of matrix-isolated Cr(CO)6. The structure was shown to be square-pyramidal, 1, by a combination of 13C0 enrichment and IR spectroscopy,2 a technique which has since been very widely a ~ p l i e d .This ~ square-pyramidal structure, 1, was quickly found to be in agreement with simple molecular orbital prediction^.^ Nevertheless, there have been arguments6 over the possibility that structure 1 was imposed by the matrix and “free” Cr(CO)Swould have a trigonal-bipyramidal structure. These suggestions have been finally disproved by very recent time-resolved infrared experiments7 which show that “naked” Cr(C0)5has the same ClVstructure in the gas phase as in condensed phases.2 The question then arose whether a square-pyramidal or pseudo-square-pyramidal structure would always be the most stable for a five-coordinate 16-electron species con(1)Burdett, J. K. Coord. Chem. Reu. 1978, 27, 1. Hitam, R. B.; Mahmoud, K. A.; Rest, A. J. Ibid. 1984,55,1. (2)Perutz, R. N.;Turner, J. J. Inorg. Chem. 1975, 14, 262. (3) Perutz, R. N.; Turner, J. J. J. Am. Chem. SOC.1975, 97, 4800. (4)Burdett, J. K.; Poliakoff, M.; Turner, J. J.; Dubost, H. Adu. I n frared R a m a n Spectrosc. 1976,2,1-52. (5)Burdett, J. K. J . Chem. SOC.,Faraday Trans. 2 1974, 70, 1599. Elian, M.; Hoffman, R. Inorg. Chem. 1975,14, 1058. (6) Kundig, E. P.; Ozin, G. A. J . Am. Chem. SOC.1974, 96, 3820. Burdett, J. K.;Graham, M. A.; Perutz, R. N.; Poliakoff, M.; Rest, A. J.; Turner, J. J.; Turner, R. F. Ibid. 1975,97, 4805. (7)Seder, T.A.; Church, S. P.; Ouderkirk, A. J.; Weitz, E. J . Am. Chem. SOC.1985,107, 1432.
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taining a d6 metal center. Would all M(CO),R species adopt structures 3 and 4 or would the pseudo-trigonal structure 5 be more stable in some cases?8 Molecular orbital calculationsg for Mn(C0)4X species made the interesting prediction that the trigonal structure 5c would be the most stable for Mn(CO),Br but for HMn(CO),, structures 3a, 4a, and 5a would all have similar energies. The difference between M I I ( C O ) ~ Band ~ HMn(CO), was attributedgto *-bonding effects of Br. Subsequent matrix isolation experiments,’O again involving 13C0 enrichment, showed that M I I ( C O ) ~ Bdid ~ indeed have the predicted pseudo-trigonal-bipyramidalstructure 5c. More recently HMn(CO),, generated by photolysis of HMn(CO),, was shown’l to have two isomers C, and C4”,3a and 4a, both clearly derived from the square-based-pyramidal M(C0)5 structure 1. Both isomers had UV-visible absorptions and could be interconverted by irradiation with visible light of the appropriate wavelength. In the matrix, the C, isomer 3a appeared to be the more stable. This work revealed a surprising difference between HMn(CO), and CH,Mn(CO), which had earlier been reported12 to have the trigonal structure 5b,similar to that of Mn(CO)4Br. This structure implies that CH3 is more like Br than H in its effect on molecular geometry, even though CH3 is unlikely to have significant K interactions. CH3Mn(CO), is one of the most easily generated carbonyl fragments containing an alkyl group and is poten(8) The other possible trigonal isomer with an axial R group and C,, symmetry would be expected to be Jahn-Teller unstable. (9)Lichtenberger, D. L.;Brown, T. L. J . Am. Chem. SOC.1978,100, 366. (10)McHugh, T. M.; Rest, A. J.; Taylor, D. J. J . Chem. SOC.,Dalton Trans. 1980,1803. (11)Church, S. P.; Poliakoff, M.; Timney, J. A,; Turner, J. J. Inorg. Chem. 1983,22,3259. (12)McHugh, T. M.; Rest, A. J. J . Chem. SOC.,Dalton Trans. 1980, 2823.
0 1986 American Chemical Society
406
Organometallics, Vol. 5 , No. 3, 1986 I
A
Horton-Mastin et al. e
I
I
(a) ArICH,
1
2
A 3a 9:H
? b ?=Me
La 2 : i i L b R:Me
0
?.
5a R = H 5b R z M e 5 c R = 3r
tially an excellent model system for studying M-C interactions. This has prompted us to reinvestigate the structure of CH,Mn(CO),. The original workersI2assigned structure 5b to CH,Mn(CO), on the basis of I3CO enrichment. However, our own more recent experiments” with HMn(CO), led to the realization that 13CO-enrichment techniques cannot always distinguish between a C, square-pyramidal structure, such as 3, and a CZutrigonal-bipyramidal structure, 5. In this case, it can be shown that the original I3CO datal2 for CH,Mn(CO), are equally consistent with the square-pyramidal structure13 3b. In this paper, we report IR results which show that matrix-isolated CH,Mn(CO), exists in two interconvertible isomeric forms, one of which must be the C4”isomer 4b. Both isomers have UV-vis absorption bands, which are substantially blue-shifted when the matrix material is changed from Ar to CH,. In 16-electron, five-coordinate systems, such shifts have always been associated with square-pyramidal or pseudo-square-pyramidal fragm e n t ~ . ~ , ~ We , ” , therefore ~~ assign the square-pyramidal structure 3b to the second isomer of CH,Mn(CO),.
Experimental Section The matrix isolation apparatus at Nottingham has been described previously.15 IR spectra were obtained with use of a Nicolet 7199 FT-IR interferometer (32K data points, 262K Transform points, 0.5 cm-’ resolution) and UV-vis spectra with Perkin-Elmer Lambda 5 spectrophotometer with a Model 3600 Data Station. All UV spectra were recorded with 2-nm slit width. CH,Mn(CO) was a gift from Dr. J. Spencer, and matrix gases (Messer Griesheim) were used without further purification. The photolysis source was a Philips HPK 125W Hg arc, with Balzers interference filters where appropriate.
Results Figure 1 shows IR spectra produced by UV photolysis of CH3Mn(CO)5in a CHI matrix and subsequent irradiation with visible light. Before photolysis, the IR spectrum of CH3Mn(CO)5shows the overall pattern (2al + e) expected for a square-pyramidal Mn(CO),X species with local C,, symmetry but each mode is somewhat split by (13) The I3CO data can be fitted with a C, force fieldI5 with kl = 1600.2, k2 = 1665.7, k3 = 1593.1, k1.2 = 40.0, k1,z 56.56, k22 = 53.75, and k Z 3= 25.94 Nm-l, where “2” refers to the two equivalent C 6 groups. The original workers12were unable to find a solution baaed on a C, force field but in the absence of other evidence might have been expected to favor the higher symmetry CZcsolution which has two fewer force constants. (14)Poliakoff, M. Inorg. Chem. 1976, 15, 2022; 1976, 15, 2892. (15) Church, S. P. Ph.D. Thesis, University of Nottingham, 1982.
Figure 1. IR spectra in the C-0 stretching region illustrating reversible formation of isomer B of CH,Mn(CO),. Spectra were recorded after photolysis of CH,Mn(CO), isolated in a CHI matrix (CH,M~I(CO)~:CH,,1:4000) at 14 K (a) after deposition; (b) after 12-min UV irradiation, following by 7-min irradiation with X = 403 nm; (c) 7 min further irradiation with X = 489 nm; (d) 20 min more photolysis with X = 403 nm. The bands are labeled as follows: B, “CIU” isomer of CH,Mn(CO),; black, C, isomer of CH,M~I(CO)~; unlabeled, unreacted CH3Mn(CO)j. (Note the expanded absorbance scale in the high wavenumber region and that a is plotted on a reduced absorbance scale compared to b-d).
“matrix” effect@ (Figure la). On photolysis, new IR bands are observed. Apart from a band at 2138 cm-l due to free CO (not illustrated), five IR bands can be assigned to v(C-0) vibrations of primary photoproducts (see Figure l b where four of the bands are colored black and the fifth band is marked B). On subsequent irradiation, the four black bands remain with constant relative intensities and therefore belong to one molecular species, while the band B changes substantially in intensity suggesting that it may be due to a second species (compare parts c and d of Figure 1). Three of the four black bands correspond almost exactly to those originally reportedI2 for CH,Mn(CO), (see Table I) while the fourth band (arrowed in Figure 1) overlaps with an absorption of the parent CH3Mn(CO), and might have easily been missed in the earlier w0rk.l‘ Thus, the black bands are reasonably assigned to CH,Mn(CO),. Is the fifth band (B in Figure 1) due to a distinct chemical species or merely a “matrix splitting”? This question can be answered by examining the UV-vis spectra of the matrix (Figure 2). These spectra show two a b (16) See, e.g.: Horton-Mastin, A,; Poliakoff, M. Chem. Phys. L e t t . 1984, 109, 587 and references therein.
(17) There is some evidence for the presence of this band, unresolved from that of CH3Mn(CO), in Figure 1 of ref 12.
Organometallics, Vol. 5, No. 3, 1986 407
Photochemistry of Matrix-Isolated CH,Mn(CO), Table I. Wavenumbers (cm-') of CH,Mn(CO), and HMn(C0)4 Isolated in CH4 Matrices at 20 K CH3Mn(C0)4 HMn(CO), a b C assignt C, Isomer 3 2086.6 1997.3 1989.1 1952.Id
2086.9 1997.4
2089.9 1996.6d
e
e
1952.0
1964.9
1960.0d
e
"Clun Isomer 4 1966.3
a' a" a' a' e
"This work. *Reference 12, note that the bands have been reassigned. Reference 11. d Mean value of matrix splitting. e Not reported. A
I
I
LOO
500
600
700
nrn
F i g u r e 2. UV-visible spectra recorded in the same experiment as the IR spectra in Figure 1: (-) corresponding to l b ; - - -, corresponding to IC; corresponding to Id. (Note that these spectra have been corrected for background scatter by computer subtraction of the spectrum recorded before any irradiation of the matrix. The two absorption maxima are assigned to isomers A and B of CH,Mn(CO),.) e-,
sorption maxima (marked A and B) which change substantially in relative intensities during the experiment. Matrix-isolated metal carbonyls have broad unresolved electronic absorptions which do not display detectable matrix splittings. Thus the visible absorptions A (414 nm) and B (505 nm) must arise from two different and distinct species. Absorption A is clearly associated with the IR bands of CH3Mn(C0),, while absorption B shows the same growth and decay pattern as the IR band B (Figure 1). What is the nature of this second species B? B is not Mn(CO)5,the IR bands of which are known.l8 A careful analysis of the IR spectra showed that B was formed from CH,Mn(CO), by loss of CO and that CH3Mn(CO), and B could be interconverted by irradiation with the appropriate wavelength visible light, which also causes some regeneration of the parent CH3Mn(CO),. For example, in CH, matrix
and the behavior of B is quite consistent with a second isomer of CH3Mn(CO),. B has only one strong v(C-0) IR band, which can only be assigned to a Cb square-pyramidal structure, 4b. Group theory predicts two v(C-0) bands for this structure (a, e) with the a, band expected to be very weak. All other plausible structures are predicted to have at least two strong v(C-0) IR bands. Further support for our assignment is the frequency of the IR band, only 6 cm-' from the band of the C4"isomer of HMn(CO), (see Table I). What is the structure of isomer A of CH3Mn(C0)4? We have observed four IR-active v(C-0) bands for this isomer. The band pattern is similar to that of HMn(CO), which has structure 3a. However, four IR bands would be equally consistent with the pseudo-square-pyramid 3b (3a' + a") or the pseudo-trigonal-bipyramid 5b (2al + bl + bz). Similarly, the 13C0 data12J3 could be fitted to either structure. We can, however, distinguish between the two structures on the basis of UV-vis spectra. Perutz and Turner discovered3 that matrix-isolated Cr(CO), was not "naked" but interacted with the matrix material via the empty coordination site to give a pseudo-six-coordinate species, 2. The strength of this interaction depended on the matrix material. The interaction, which was less with Ar than with CH, or Xe, manifested itself in a substantial shift in the wavelength of the visible absorption of Cr(CO), between one matrix and another, e.g., A,, = 533 (Ar) and 494 nm (CH,). Similar interactions of Cr(CO), have since been observed in cryogeniclg and room-temperature solutions20,21and even in the gas phase.22 The interaction, which has been discussed in detail p r e v i o u ~ l yinvolves ,~~ a 0 interaction involving the empty al LUMO of Cr(CO),, which is largely dZzin character. This orbital is raised in energy both by direct interaction with the matrix and also by the change in axial/equatorial bond angle 8 shown somewhat exaggeratedly in 9, a change which simultaneously raises the energy of the LUMO and lowers the energy of the HOMO.23
+
$-!2&
9
X
A similar interaction would be expected for any 16electron square-pyramidal or pseudo-square-pyramidal fragment, e.g., 3 or 4, and has been observed for several systems, e.g., Cr(C0),CS4 and HMn(CO),.'l By contrast, a trigonal or pseudotrigonal fragment, e.g., 5 , does not have a suitable LUMO for interaction with the matrix. Thus we can distinguish between structures 3b and 5b for CH3Mn(CO),. Structure 3b should show a substantial blue-shift in A,, from Ar to CH, matrices, while structure 5b should not. The spectra are shown in Figure 3. Between CH, and Ar matrices there were substantial shifts in the UV/visible absorption maxima of both photoproducts. These matrix shifts [CH,Mn(CO), (A), 414 (CH,) to 457 nm (Ar), 2250 cm-l; CH3Mn(CO), (B), 505 (CH,) to 565 nm (Ar), 2100 cm-l] are similar to those of Cr(C0)53( - 1600 cm-I) and HMn(CO),ll (3a; 2500 cm-l), both of which are known to interact with the matrix. The shift in the band of B, which
Similar evidence for two photoproducts was obtained in Ar matrices but the IR spectra are somewhat more split by "matrix effects". This scheme is very similar to that observed" for the two isomers of HMn(CO),, 3a and 4a,
(19) Simpson, M. B.; Poliakoff, M.; Turner, J. J.; Maier, W. B. 11; McLaughlin, J. G. J . Chem. SOC.,Chern. Cornmun. 1983, 1355. (20) Kelly, J. M.; Long, C.; Bonneau, R. J. Phys. Chern. 1983,87,3344. (21) Welch, J. A,; Peters, K. S.; Vaida, V. J . Phys. Chem. 1982, 86,
(18) Church, S. P.; Poliakoff, M.; Timney, J. A.; Turner, J. J. J . Am. Chern. SOC.1981, 103, 7517.
1941. (22) Breckenridge, W. H.; Sinai, N. J . Phys. Chern. 1981, 85, 1351. (23) Burdett, J. K.; Grzybowski, J. M.; Perutz, R. N.; Poliakoff, M.; Turner, J. J.; Turner, R. F. Inorg. Chem. 1978, 17, 147.
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Organometallics 1986,5, 408-411
- -
inverse Berry pseudorotation, C,, CZu C,, already invoked in the isomerization of Cr(C0)4CS14and HMn(CO)4.12
Figure 3. UV-visible spectra illustrating the substantial shift in absorption maxima of the two isomers of CH3Mn(C0)4between CH4 and Ar matrices: (a) C, isomer A and (b) Clu isomer B. Spectra were obtained by computer subtraction of spectra similar to those in Figure 2. Note that the spectra for Ar and CH, matrices were obtained in different experiments.
we have already identified as the ClUisomer of CH,Mn(CO),, confirms our assignment. The shift in the absorption of the other isomer A is exactly as predicted for structure 3b and allows us to eliminate the trigonal structure 5b. Thus, the isomers of CH,Mn(CO), have structures 3b and 4b, both based on a square pyramid. Their photochemical interconversion can readily be explained by the
Conclusions Thus, contrary to earlier reports, CH,Mn(CO), has two isomers, 3b and 4b, similar in structure to those of HMn(C0)411and unlike the reported structure10 of Mn(C0)4Br. Thus, CH, is finally shown to be more akin to H than Br in its effects on molecular geometry. An interesting theoretical question still remains to be answered, namely, why the C, isomers 3a and 3b should be the predominant products in our photolysis experiments? Again, the interaction between the unsaturated metal center and the matrix underlines the enormous reactivity of these fragments. In hydrocarbon solution a t room temperature, “CH,Mn(CO),” will almost certainly be a pseudooctahedral species with a weakly coordinated solvent molecule occupying the vacant coordination site. Now that the isomers of CH,Mn(CO), have been characterized, they form promising model systems for studying M-C interactions. For example, following the pioneering work of M ~ K e a n , it ~ ,should now be possible to use IR bands in the v(C-H) region to examine differences in Mn-CH, bonding between the two isomers and CH,Mn(C0)j. Acknowledgment. We thank the SERC and B P Research Ltd for support. We are grateful to Dr. S. P. Church, Dr. M. A. Healy, Mr. J. G. Gamble, Dr. G. E. Morris, and Dr. A. J. Rest for their help and comments. Registry No. 3b, 71518-84-8;4b, 99684-96-5;CH,Mn(CO),, 13601-24-6. (24) Long, C.; Morrison, A. R.; McKean, D. C.; McQuillan, G. P. J . Am. Chem. SOC.1984, 106, 7418 and references therein.
Synthesis of Os,(CO),,(CH,)(p-I) with an $-Methyl Ligand and Its Insertion of CO To Give Acetyl Derivatives Eric D. Morrison, Sherri L. Bassner, and Gregory L. Geoffroy’ Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 Received May 24, 1985
Protonation of the anionic cluster [ O S ~ ( C O ) ~ ~ ( ~ - C Hwith ~ ) ( HBF4.Et20 ~ - I ) ] ~ yields O S ~ ( C O ) ~ ~ ( C H ~ ) ( ~ - I ) (5), one of the few examples of an alkyl-substituted cluster. Methyl cluster 5 reacts with CO to give predominantly the ?‘-acetyl cluster Os3(CO)ll(~1-C(OJCH3)(~-I) (7) along with small amounts of the bridging (6). The bridging acetyl cluster 6 can also be synthesized in high acetyl cluster OS~(CO)~~(~-O=CCH~)(~-I) yield from the reaction of O S ~ ( C O ) ~ , ( ~with C H ~HI. ) When the +acetyl cluster 7 is heated under vacuum, it loses CO and gives a mixture of the pacetyl cluster 6 and the original methyl cluster 5 . Insertion of CO into metal-alkyl bonds in mononuclear complexes is a textbook reaction with numerous examples and many important synthetic application^.'-^ In contrast, insertion of CO into metal-alkyl bonds in well-charac(1) Wojcicki, A. Adu. Organomet. Chem. 1973, 11, 87.
(2) Calderazzo, F. Angew. Chem., Int. Ed. Engl. 1977, 16, 299. (3) Collman, J. P.; Hegedus, L. S. ‘Principles and Applications of
Organotransition Metal Chemistry”; University Science Books: Oxford,
terized alkyl-substituted clusters has not been documented. However, CO insertion into transient metal-alkyl bonds has been proposed for several cluster reactions. For example, Kaesz et al., observed formation of the p p r o (2; X = C1, pionyl complexes Ru3(CO)ln(~-O=CEt)(~-X) Br, I) fromthe reaction of ( , ~ H ) R U ~ ( C O ) ~(X ~ (= ~ -C1, X) (4) Kampe, C. E.; Boag, N. M.; Kaesz, H. D. J . Am. Chem. SOC.1983, 105, 2896.
1980
0276-7333/86/2305-0408$01.50/0
0 1986 American Chemical Society
