Organometallics 1995, 14, 2366-2373
2366
Photochemistry of Monosubstituted Manganese Carbonyl Compounds, Mnz(CO)sL,in 3-Methylpentaneat Low Temperature. Flash Photolysis Studies in Hexane at Room Temperature Shulin Zhang, Xiaoqing Song, and Theodore L. Brown* School of Chemical Sciences and Beckman Institute for Advanced Science and Technology, 505 S . Mathews Avenue, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 Received December 27, 1994@ The photochemical behavior of monosubstituted manganese carbonyl dimers, Mnz(C0)gL (L = PMe3, P(n-Bu)a, PPh3, P(CH2Ph)3, and P(i-Pr)s), in 3-methylpentane at 93 K has been studied. Loss of CO leads to formation of the semibridging structure Mn2(C0),L@-q1,q2CO). Because loss of CO might occur from either of the two nonequivalent metals, and because the phosphine exhibits positional isomerism, several isomeric semibridging structures are possible, depending on the steric requirements of the phosphine. Each gives rise to a unique semibridging CO stretching frequency. Upon warming of the low-temperature glass, recombination with CO and intramolecular rearrangements of the semibridging forms occur in competition. Recombination with CO occurs most readily at the least sterically hindered sites. Thus, recombination to form the parent carbonyl, Mn2(CO)gL, occurs most readily with Mn2(CO)gPMe3 and most slowly with Mnz(CO)gP(i-Pr)3. Studies of the PMe3 and PPh3 complexes show t h a t recombination with CO leads to substantial quantities of the equatorial form, which isomerizes to the axial form as the solution is warmed. At room temperature the thermodynamic equilibrium between the equatorial and axial isomers of M ~ Z ( C O )lies ~ L heavily toward the axial form. Flash photolysis of hexane solutions of Mn2(C0)gL compounds in hexane solutions at room temperature reveal a complex decay of absorbances due to CO loss products. Analysis of the transient behavior leads to three pseudo-first-order rate constants: one is due to recombination of CO with Mnz(CO)g, formed via photochemical loss of L. The other two correspond to recombination of CO with Mn2(C0)gL intermediates in which the CO has been lost from either Mn center.
Introduction
bond in manganese carbonyl dimers and in analogous dimers involving r h e n i ~ m .The ~ kinetic stabilities of Dinuclear metal carbonyl compounds are subject to the semibridge structures formed from CO loss in Mnztwo primary photochemical processes, metal-metal (CO)lo,Mn2(CO)aL2,and Mn2(CO)s@-L-L)2(where L is bond homolysis and CO dissociation.lV2 This duality of a phosphorus donor) toward reaction with nucleophiles photochemicalprocesses was established early for Mn2or in oxidative addition reactions vary widely, largely (C0)lo in laser flash photolysis experiments, which because of variations in the steric requirements of the donor groups bound to the metals. revealed the presence of W-visible absorptions attributable to both Mn(C0)5 radicals and M n ~ ( C 0 ) g . ~ The study of CO dissociation from monosubstituted Both Mn(C015 and Mn2(CO)g were observed in hydromanganese carbonyl compounds, Mnz(CO)gL, at low carbon solution at room temperature via direct IR temperature in a glass affords an opportunity to test detection of the transient specie^.^ Shortly thereafter, various hypotheses regarding the properties of semibridge bonds formed following CO loss. If the semibridge bonds Hepp and Wrighton5 established that photolysis in are unsymmetrical, as assumed, there should be evi3-methylpentane (3MP) at 77 K leads to formation of a dence of isomers based on whether the metal from which CO-loss product with an IR absorption at 1760 cm-l, CO has photodissociated is that to which the phosphorus attributed to a linear semibridge bond analogous to that ligand is bound. The kinetic stability of a given established in Mn2(C0)5(d~pm12.~ More recently we semibridge isomer should depend on the position and have discussed the nature and stability of the semibridge steric bulk of the phosphine. The relative stabilities of the various possible isomers formed from CO loss can Abstract published in Advance ACS Abstracts, April 15, 1995. be assessed from observations of the IR spectra as the (1) GeofTroy, G. L.; Wrighton, M. S. Organometallic Photochemistry; Academic Press: New York, 1978. low-temperature glass is warmed. However, intercon(2) Meyer, T. J.; Caspar, J. V. Chem. Rev. 1985,85, 187. version among the isomers may compete with recom(3)(a) Rothberg, J.; Cooper, N. J.; Peters, K. S.; Vaida, V. J. Am. Chem. Sac. 1982,104,3536.(b) Yesaka, H.; Kobayashi, T.; Yasufuku, bination with CO, affording a complex reaction system. K.; Nagakura, S. J . Am. Chem. Sac. 1983,105,6249. Herein we report the photochemistry of Mnz(C0)gL (L (4)Church, S. P.; Hermann, H.; Grevels, F. W.; Schaffner, K. J . = PMe3, P(n-Bu13, PPh3, P(CH2Ph13, and P(i-Pr)s) in Chem. Soc., Chem. Commun. 1984,785. (5) Hepp, A. F.; Wrighton, M. S. J . Am. Chem. Sac. 1983,105,5934. 3MP at 93 K and above, following xenon flash lamp and @
(6)(a) Colton, R.;Commons, C. J.; Hoskins, B. F. J . Chem. SOC., Chem. Commun. 1975,363.(b) Commons, C. J.; Hoskins, B. F. Aust. J . Chem. 1975,28,1663.
(7) Brown, T. L.; Zhang, S. Znorg. Chem. 1995,34,1164.
0276-7333/95/2314-2366$09.OO/O 0 1995 American Chemical Society
Organometallics, Vol. 14,No. 5, 1995 2367
Monosubstituted Manganese Carbonyl Compounds
Table 1. IR Data for Mnz(CO)& in Hexane at 25 “C L P(n-Bu)s PPh3 PMe3 P(CHzPh)3 P(i-Pr)s
vrn
(cm-1)
2089,2006,1992,1971,1957,1933 2091,2011,1996,1974,1962,1939 2090,2009,1991,1972,1959,1937 2090,2010,1994,1973,1960,1934 2090,2030,1996,1974,1959,1930
PPh,
continuous s u n l a m p photolysis at 93 K, and flash photolysis in hexane under 1 atm of CO at room
temperature. Experimental Section All experiments were carried out under a n atmosphere of purified argon, employing Schlenk techniques. 3-Methylpentane (3MP, 99+%, Aldrich) was distilled over CaHz, passed over activated silica, and stored over freshly activated 4 molecular sieves. It was subjected to three freeze-pumpthaw cycles before use. Mnz(CO)lo was purchased from Strem and sublimed before use. Hexane was purified as described previ~usly.~CO (Matheson purity grade, minimum purity 99.99%)was obtained from Matheson Gas Products, Inc. Trace Fe(C015 was removed by passing the CO through a n activated charcoal trap in a sand bath at 180 “C, followed by passage through a second activated charcoal trap cooled in a dry iceacetone bath, a column of activated 3 A molecular sieves, and an oxygen trap purchased from American Scientific. Mnz(CO)sPPh3 was prepared from reaction of Mnz(CO)lo (0.56 mmol) and PPh3 (0.62 mmol) in the presence of Me3NO (0.65 mmol) in toluene (40 mL) overnight at room temperature.8 Mnz(CO)gP(CHzPh)3was prepared from the photoreaction (sunlamp) of Mnz(CO)lo (1.1mmol) with P(CHzPhI3 (1.0 mmol) in 80 mL of hexane/30 mL of toluene mixed solvent for 2 days. The other monosubstituted dimers Mnz(C0)gL (L = P(n-Bu)s, PMe3, and P(i-PrI3) were synthesized by “crosscoupling‘‘ photolysis of Mnz(C0)lo and the corresponding dimers MnZ(CO)& in hexane and/or toluene, using a sunlamp. The disubstituted dimers were prepared and fully characterized in these laboratories by S u l l i ~ a n . ~ All monosubstituted dimers were separated and purified by passing through columns packed with activated neutral alumina, using hexane as eluent. The compounds were characterized by their IR spectra, in particular noting the absence of bands due t o the reactant metal carbonyl compounds. The CO IR stretching mode absorptions in hexane solution are listed in Table 1, and the IR spectra are displayed in Figure 1. The IR spectrum of Mnz(CO)gPPh3 is consistent with that reported in the l i t e r a t ~ r e . ~The J ~ other monosubstituted compounds are new, t o our knowledge. IR spectra were recorded on a Perkin-Elmer Model 1710 FTIR spectrophotometer. A sealable cell with CaFz windows and a 1.0 mm pathlength, capable of variable-temperature use, was used for the low-temperature measurements. The solution of the dinuclear metal carbonyl (-0.5 to 1.0 mM) in 3MP was loaded into the IR cell under Ar. A SPECAC Model 21500 variable-temperature system was employed. The cell was cooled to 93 K using liquid nitrogen. After a solution in the cell has been flash-photolyzed while at low temperature, using a xenon flash lamp source (broadband UV-visible), the assembly is quickly moved to the IR spectrometer t o record spectra. To obtain the desired IR spectra at various temperatures, the solvent background spectra were subtracted. The reference spectra for this purpose were obtained by separately recording the solvent background at corresponding temperatures and saving them on a computer disk. Data analyses
a
(8)Koelle, U.J. Organomet. Chem. 1978,155, 53. (9)Sullivan, R.J.; Brown, T. L. J . A m . Chem. SOC.1991,113,9155. (10)(a) Fawcett, J. P.; Poe, A. J.;Twigg, M. V. J.Organomet. Chem. 1973,61,315. (b) Wawersik, H.; Basolo, F. h o g . Chim. Acta 1969, 3,113. (c) Jackson, R.A.; Poe, A.Inorg. Chem. 1978,17, 997.
2 100
2050
2000
1950
1900
Wavenumber (cm-l) Figure 1. FTIR spectra of MnZ(C0)gL compounds in hexane at 298 K. were performed using a combination of ASYST-based programs and Slidewrite Plus, version 5.0. In variable-temperature experiments, after the photoproducts have been generated by photolysis at 93 K in the cell, the liquid nitrogen is removed and the cell temperature allowed to increase. The temperature increase was determined to reproducibly obey the “natural” warmup function T = a - ,b exp(-yt), where a = 312 K, ,b = 220 K, y = 2.7 x lo-* K s-l, and t is time in seconds. The cell temperature was measured by a thermocouple located at the lower end of the cell. The flash photolysis apparatus has been described previously.ll The 10-4-10-5 M solutions in hexane were prepared under a n argon atmosphere and degassed by three freezepump-thaw cycles. After the degassed solution warmed to room temperature, purified CO was then added to the solution to a total of 1 atm with constant shaking of the flask for 10 min to ensure saturation by CO. Mnz(CO)TLz* is known to have a broad absorption around 510 nm with maximum of 500 nm for L = PMe3 and 550 nm for L = P(i-Pr)3.11b In the present study, transient absorbances were monitored at both 510 and 530 nm. The time-dependent transient absorbances are analyzed by fitting into three firstorder processes. Except for slight differences in the relative amplitudes of the three processes, the kinetic behaviors are the same for both monitoring wavelengths. Since photolysis of Mnz(C0)gL in solution at room temperature results in formation of Mnz(C0)lo and Mnz(CO)& in each experiment, only the trace from the first flash is used for kinetics analysis. The experiments are repeated with freshly prepared solutions. Reported rate constants are averages of three or more flash experiments.
Results and Discussion
Not a great deal of structural information is available for monosubstituted manganese carbonyl compounds. (11)(a) Sullivan, R.J.; Brown, T. L. J . Am. Chem. SOC.1991,113, 9162. (b) Herrick, R. S.; Brown, T. L. Inorg. Chem. 1984,23,4550. (c)
Herrick, R. S.; Herrinton, T. R.; Walker, H. W.; Brown, T. L. Organometallics 1985, 4 , 42.
Zhang et al.
2368 Organometallics, Vol. 14,No. 5, 1995
0.50
/
1738
0.30 0.10
6
a 1 flash
-0.10
8 flashes
+ 10 min sunlamp
-0.30
+ 40 min sunlamp + 80 min sunlamp
-0.50 2150
2100
2050
2000
1950
1900
1760
1710
1660
Wavenumber (cm-') Figure 2. DifferenceFTIR spectra of a 1mM solution of Mnz(CO)gP(n-Bu)3in 3MP at 93 K, before and following successive degrees of photoirradiation as listed. The crystal structure of Mnz(C0)gPPhMez reveals that the phosphine occupies an axial position on the manganese.12 The IR spectrum in benzene,13with bands at 2094 (w), 2016 (s), 1993 (vs), 1969 (sh), and 1938 (m), is characteristic of axial m o n o s u b ~ t i t u t i o n . ~ ~In J~J~ Mnz(CO)gP(OCH)3and related phosphite derivatives, the phosphite is assigned to an axial location on the basis of the IR ~ p e c t r a . ~ Equatorial substitution was proposed for Mnz(C0)gPH3,17on the basis of the small size of the PH3 ligand and the concordance of the IR pattern (2094 (w), 2023 (s), 2012 (s), 1996-1990 (vs), 1970 (m), 1955 (m)) with that of Mnz(CO)g(nitrile)compounds, which are presumed to be equatorially s u b ~ t i t u t e d . ' ~ The crystal structure of Mn2(CO)gPPhzH reveals the ligand t o be in an equatorial location. The P-H bond lies over the Mn-Mn bond, and the two metals are in a staggered configuration with respect to one another.18 The infrared spectra of five monosubstituted phosphine derivatives of Mnz(C0)g are shown in Figure 1. The frequencies of the major observed IR bands are listed in Table 1. The strongest pattern of absorptions is that characteristic of axially-substituted derivative^.'^ On the other hand, nearly all the compounds exhibit weak absorptions associated with equatorial substitution. For example, the shoulder at about 2013 cm-l, and the weak absorption at 1959 cm-l, in the spectrum of Mn2(CO)gPMe3 are attributable to the eq-isomer. A weak absorbance in the vicinity of 1960 cm-l, characteristic of the eq-isomer, is seen in all the spectra. We conclude that the dominant isomer in solution is axMnz(C0)gL but that in most instances some eq-Mnn(C0)gLis also present. (It should be noted that at room temperature the equilibrium between equatorial and (12)Liang, M.; Singleton, E.; Reimann, R. J. Orgunomet. Chem. 1973,56,C21. (13)Reimann, R.; Singleton, E. J. Chem. Soc., Dalton Trans. 1976, 2109. (14)Ziegler, M. L.; Haas, H.; Sheline, R. K. Chem. Ber. 1966,98, 2454. (15)Harris, G.W.;Coville, N. J. J. Crystullogr. Spectroscopic Res. 1989,19,451. (16)Chan, H. S.0.; Hor, T. S. A.; Tan, K.-L.; Leong, Y.-P. Znorg. Chim. Acta 1991,184,23. (17)Fischer, E.0.; Herrmann, W. Chem. Ber. 1972,105,286. (18)Giordano, R.; Sappa, E.; Tiripicchio,A.; Camellini, M. T.; Mays, M. J.; Brown, M. P. Polyhedron 1989,8,1855.
axial isomers is expected to be fa~i1e.l~) The fact that Mn2(CO)gPMe3 exists to a large extent as the axial isomer is rather surprising, given that Mnz(CO)s(PMes)z exists largely in the eq,eq-form.llbJ9 Photolysis of Mnz(C0)gL in 3MP a t 93 K results in loss of CO, with formation of a semibridge, yielding Mnz(CO),L@-7;11,7;12-CO).Figure 2 shows the difference FTIR spectra obtained during photolysis of Mnz(CO)gP(n-Bu)s, initially using a xenon flash lamp and then under continuous irradiation with a sunlamp. Loss of starting material is accompanied by the appearance of new bands, including one at 2133 cm-' due to free CO. Of most interest is the band structure in the region of the semibridging CO stretch. After several xenon lamp flashes there already is evident in the 1660-1760 cm-l region a multiplet of absorptions. Since the multiplet structure appears early in the photolysis process, when only a low level of conversion of the starting dimer has occurred, the bands cannot be ascribed to a secondary photolysis step, such as loss of a second CO. Use of the sunlamp permits a higher degree of conversion to semibridge forms than can conveniently be obtained by repeated xenon lamp flashes, but it does not appear to give rise to significant secondary photoproduct formation. The possibility exists that a t least one of the observed IR bands might result from loss of a phosphine rather than CO in the photolysis process. However, aside from the much lower likelihood of the phosphine escaping from the solvent cage in the low temperature 3MP glass, this possibility can readily be ruled out because the resulting species, Mn2(CO)&-v1,v2-CO),would give rise to an absorption at 1758 cm-1,5920where no absorption is seen (Figure 2). It should also be noted that upon warming the 3MP glass following photolysis, there is eventually a recombination with CO to re-form the starting material with at least 95% efficiency. Scheme 1 illustrates the various isomeric CO-loss products that might result from the photolysis process. CO might be lost from either of the two distinct metal centers, giving rise to two sets of isomers that are (19)Zhang, H.-T.; Brown, T. L. J. Am. Chem. SOC.1993,115,107. (20)Zhang, S.;Zhang, H.-T.; Brown, T. L. Organometallics 1992, 11, 3929.
Organometallics, Vol. 14,No.5, 1995 2369
Monosubstituted Manganese Carbonyl Compounds Scheme 1
1
e
n
L P(i-Pr),
1800
1750
1700
1650
1600
Wavenumber (cm-l) Figure 3. Difference FTIR spectra in the region of the semibridging CO stretch mode for Mnz(CO)gLcompounds in 3MP solution, before and following sunlamp irradiation at 93 K and upon subsequent warming. The spectra in each case were taken at 93 K (-1, 143 K (..*), and 183 K (- - -).
distinguished within each set by the location of the phosphine substituent relative t o the semibridge. Because the photochemical event results in the deposition of excess energy in the system beyond that needed for CO dissociation, there is energy available t o cause rearrangements of the dinuclear CO-loss products. Six distinct isomers, each with a characteristic semibridge stretching frequency, are possible. From photolysis studies of disubstituted compounds in 3MP at low temperature, it can be expected that the different locations of the phosphine relative to the
semibridge will give rise to frequency shifts on the order of 15-20 cmP1.l9 Thus, it is reasonable to associate the pattern of semibridge stretching frequencies shown in Figure 2 with the existence of the various isomers shown in Scheme 1. Figure 3 shows the pattern of bands at 93 K for all five compounds studied and, upon warming, at 143 and 183 K. As seen, phosphines other than P(nBu)~ also give rise to multiple bands, but in no case are the bands as well-distinguished as in Figure 2. To assign the bands in Figure 2 to the possible isomeric forms, it is useful to begin with the question of whether the set of isomers based on loss of CO from the phosphine-bearing Mn (2, 4, 6) should differ as a class from those based on loss from the other Mn (1,3, 5). We have recently shown that, in symmetricallysubstituted manganese dimers, there is a remarkable constancy in the quantity A, defined as A = YATCO Ysb, where VATCO is the average of the terminal CO stretching frequencies in the semibridging species Mn2(CO)8-,L,(p-q1,q2-CO) (n = 2 , 4 ) and Ysb is the frequency of the semibridging CO ~ t r e t c h .This ~ constancy, which gives rise to a value for A in the range of 245 cm-l, can be interpreted as resulting from a more or less constant degree of electronic stabilization of the semibridge bond with increasing degree of substitution at the metal centers. When substitution is unsymmetrical, as in Mn2(C0)6(a-diimine)(p~-y~,11~-Co),~~ a smaller value for A is seen. In this case, CO photodissociates from the Mn bearing the a-diimine ligand. The smaller value for A arises because the center to which the semibridge CO is bound in q2 fashion is a poorer Lewis acid by virtue of replacement of two CO groups by the diimine ligand. (Unsymmetricalsubstitution in the opposite sense, i.e., in which the substituting ligand is on the metal to which the semibridgingCO is q 1 coordinated, might conversely result in a larger value for A. However, this instance has not yet been realized experimentally.) The semibridging CO in isomer 1 in Scheme 1 will exhibit a value of V& which, to first approximation, will be close to that seen in ax,ax-Mnz(CO)6L2(p-q1,q2-CO), about 1700 cm-1.7J9 It could have a somewhat lower value because, by being coordinated to an Mn(C014 rather than t o an Mn(C0)sL center, it is bonded t o a stronger Lewis acid. On the other hand, the terminal (21) an der Graaf, T.; Stufiens, D. J.; Oskam, A.; Goubitz, K. Znorg. Chem. 1991,30,599.
2370 Organometallics, Vol. 14, No. 5, 1995
Zhang et al.
0 I1 CO groups on the Mn to which the semibridging CO is 03 m bonded in r1fashion should have an average stretching frequency slightly higher than those in Mn2(CO)&@q1,q2-C0), because the other manganese center is less electron-donating than in the reference compound, by virtue of lacking a phosphine substituent. (The value of YATCO for the CO groups on the Mn(C014L center is hypothetical; there is no way of distinguishing the CO stretching modes on the two metal centers, because of extensive coupling.) The first effect noted should be larger than the second. Thus, we expect that Ysb for isomer 1 will be in the vicinity of 1690 cm-l. Isomers 3 and 5 should exhibit frequencies on the order of 15-20 cm-l on either side of this, as was observed for such Wavenumber (cm-I) isomerism in M ~ ~ ( C O ) ~ [ P ( ~ - B U ) ~ ~ ~ ~ - ~ ~ , ~ ~ - C O ) . ~ ~ Figure 4. Difference FTIR spectra in the region of the The semibridging CO in isomer 2 in Scheme 1 will semibridging CO stretch mode for Mnz(CO)gP(n-Bu)sfolexhibit a value of Ysb which, to first approximation, will lowing xenon flash lamp and sunlamp irradiation (see be close to that seen in Mnz(CO)s(~-)71,r2-CO), about caption for Figure 1)at 93 K. The spectra were taken at 1760 cm-l. It could have a somewhat higher value than 93 K (-), 143 K and 183 K (- - -1 as the glass was warmed. this because it is v2 coordinated to a metal center that is a weaker Lewis acid than that in Mnz(C0)8@-ql,~,~~stages of warming, as represented by the 143 K specCO) by virtue of the presence of the phosphine on the trum, there is a clear loss in total IR intensity, resulting metal center. On the other hand, the (hypothetical) from recombination with CO. As expected, the lower value for VATCO should be slightly lower for the CO frequency bands disappear first, because these are groups on the metal bearing the five CO groups, because associated with isomers 1,3,and 5. There is a question of the presence of the electron-releasing phosphine on of the assignment of the 1709 and 1722 cm-l bands. The the second metal center. Assuming that the first effect 1722 cm-l band disappears at a quite low temperature, is more important than the second, we would predict a even though it might be expected to belong to the 2,4, value of a'&, for isomer 2 of about 1770 cm-l. Isomers 4 6 isomer set. However, there is in this temperature and 6 should exhibit bands about 15 cm-l on either side interval some conversion of isomers, as evidenced by a of this. slight increase in intensity of the 1738 cm-l band, in Comparison of these predictions with the observed spite of the fact that there should be some, although a bands for Mn2(C0)7P(n-Bu)3@-r1,r2-CO),Figure 2, resmaller, loss in all the higher frequency bands due to veals that, while the lower frequency bands are about reaction with CO. Thus, the 1722 cm-l band may where expected, the higher frequency bands are lower disappear because of interconversion to the 1738 cm-l than predicted. Nevertheless, the general separation (and possibly the 1748 cm-l) band. Alternatively, the into bands spaced at about 15-20 cm-l intervals is 1722 cm-l band may belong to the 1, 3,5 isomer set, observed. and the 1709 cm-l band to the 2, 4, 6 isomer set. Further insight into the origins of the bands comes Although all the details of the changes in relative from consideration of their behavior as the glass is band intensities with temperature cannot be assigned warmed. Previous work has established that steric with certainty, the general behavior of the system is in factors play a major role in determining the relative good accord with expectations. The changes occurring kinetic stabilities of the semibridge structures toward in the spectra of the other monosubstituted compounds, recombination with CO. The presence of a phosphine shown in Figure 3, reveal behavior that supports the on the metal center to which the CO group is bound in interpretations given above. In Mn2(C0)7P(i-Pr)3@r2fashion results in a lower rate of recombination with v ~ , ~ ~ ~ - only C O )two , bands are seen. These can be CO, in proportion to the steric requirement of the ascribed to isomers 2 (1741 cm-l) and 1 (1717 cm-l), phosphine. Thus, we expect that isomers 1, 3, and 5 on the grounds that the large steric requirement of P(iwill undergo recombination with CO at lower temperPr)3 precludes its location in an equatorial position. ature than isomers 2, 4, and 6. Indeed, upon recombination with CO with warming, A second aspect is the interconversion of the isomers. only the axial isomer is seen, whereas the equatorial Isomers 2, 4, and 6 can interconvert during a momenisomer is formed in substantial quantity when less tary opening up of the semibridge, which renders the bulky ligands are involved (vide infra). Consistent with metal center coordinatively unsaturated and thus liable the relative assignment of the two bands, the 1717 cm-l to intramolecular rearrangement. This process might band disappears at the lower temperature. If this be expected to occur at temperatures which are even assignment is correct, the close correspondence in lower than those at which isomers 1, 3 and 5 undergo frequencies argues for assignment of the 1722 cm-l intermolecular reaction with CO. The same mode of band in Mn2(C0)7P(n-Bu)&-r1,v2-CO) to the isomer set interconversion is not available to isomers 1, 3,and 5 ; (1,3,51. a momentary opening of the semibridge does not change In each of the compounds studied there is an IR band the coordination environment a t the metal which bears in the 2077-2080 cm-l interval due to a terminal CO the phosphine. stretch of the semibridging form, which is effectively The changes in the IR bands due to semibridging CO invariant to the particular isomer. The intensity of this groups in Mn2(C0)7P!n-Bu)3@-r1,q2-CO)upon warming absorption thus can be used as a measure of the total are shown in greater detail in Figure 4. In the initial amount of semibridging form present. A second mea(.-a),
Organometallics, Vol. 14, No. 5, 1995 2371
Monosubstituted Manganese Carbonyl Compounds P(n-Bu),
I . . ’ .
’
I
”
’
”
”
”
”
-0-
2079
-0-
I.A.
-
’
PPh, -I-2090
-0-
2013
-0-
2080
-0-
1.A.
PMe, -m-
2091
-0-
2013
-0-
2078
-0-
I.A.
P(CH,Ph),
80
120
160
200
products. We see initial formation of a relatively large quantity of the equatorial isomer upon formation of Mn2(CO)gPMe3or Mnz(CO)gPPh3. The eq-form constitutes about 70% of the Mnz(CO)gPMe3formed initially upon reaction with CO, in the temperature range 120160 K. This isomer is kinetically stable upon warming to about 210 K, at which point a substantial fraction of the eq-isomer converts to the ax-form. Analogous behavior was observed for Mn2(CO)gPPh3, except that the eq- ax-conversion occurs at a lower temperature. The fact that the axiaUequatoria1 isomer distribution in the initial products of recombination with CO is different from the thermodynamic one supports the arguments for the presence of semibridging isomers in which the phosphine is located in an equatorial as well as axial position. In summary, the present study has provided convincing evidence for formation of semibridging isomeric forms upon photodissociation of CO from Mnz(C0)gL compounds in a low-temperature glass. The number and variety of these isomers for Mnz(CO)gP(n-Bu)sis consistent with loss of CO from either metal center, with formation of linear semibridging structures. The smaller phosphines can apparently occupy equatorial sites in the photoproduct, Mn2(C0)8LCu-r1,r2-CO).Upon warming of the glass, recombination with CO is evident from the loss in absorbances in the semibridging CO stretch region of the IR. The reactivity of any given isomer toward recombination, for which the evidence suggests an associative p r o ~ e s s ,is~ clearly reduced by steric crowding at the metal center which the incoming CO must approach. Thus, recombination is observed t o occur most readily for isomers 1,3,and 5. In general, reactivity is inversely related to the steric requirement of the phosphine. Recombinations with CO of isomers in which the phosphine occupies an equatorial site, 3, 5, 4,and 6, appear to lead in substantial measure, if not exclusively, to the eq-Mn2(CO)gL product, which isomerizes upon further warming largely to ax-Mnz(CO)sL. The Mn2(CO)sL@-y1,y2-CO) compounds, particularly for L = P(n-Bu)s, represent interesting examples, in a low molar mass system, of “rugged energy l a n d s ~ a p e s ” . ~ ~ A given Mn2(C0)8L(p-r1,y2-CO)system is in principle capable of existing in a variety of energy states. These states can be categorized in a hierarchy with at least three distinct levels, as illustrated schematically in Figure 6: (a) The phosphine may be on either of the two nonequivalent metals, giving rise to the (1, 3, 5) and (2, 4,6) isomer sets. (b) It may occupy any of three geometrically distinct positions at each metal. (c) The ligand may exist in a variety of conformational states in each location. This last level of complexity is potentially of interest for ligands such as P(n-BuI3, P(CHzPhI3, or PPh3; the various conformational states could vary considerably in the degree of steric access they allow for an incoming reagent. Recombinations with CO as well as possibly other reactions, such as oxidative addition of HX,8 are associative processes characterized by smaller free energy barriers than those separating many of the possible states of the system. The reaction barriers are much lower than those for interconversion of the { 1,3,5}and
-0-
2079
-0-
I.A.
240
Temperature (K) Figure 5. Variation with temperature of (a) the 20772080 cm-l band, common to all states of the semibridge form, Mn2(CO)sLOL-r1,r2-C0),- 0 -, (b) the integrated intensity of all semibridging CO stretch bands, - 0 -, and (c) normalized intensities of the bands due to cwcMnz(CO)gL, - -, and eq-Mnz(CO)gL,- 17 - (L = CH3, PPhs). sure, less precise because of uncertainties in the baseline as the temperature is varied, is the integrated intensity of all the semibridging CO absorptions. These two measures are shown as a function of temperature in Figure 5 for four of the compounds studied. In each case the 3MP glass was warmed at the same controlled rate, starting from 93 K. It is evident that steric factors are important in the reactivity toward CO. Mn2(CO)gPMe3 is formed at the lowest temperature of any of the four compounds; Mnz(CO)gP(n-Buh is formed at a lower rate, followed by Mn2(CO)gPPh3and Mnz(CO)gP(CH2Ph)3. The order of reactivity is roughly inversely related to the steric requirements of the phosphine ligand.22 Also shown are the normalized intensities of bands for the PMe3 and PPh3 derivatives that are characteristic of ax-Mna(CO)gLand eq-Mnz(CO)gL,the immediate products of recombination with CO. In these two cases, by careful use of difference spectra, it was possible to estimate the relative amounts of eq- and ax-isomeric (22) (a) Brown, T. L. Inorg. Chem. 1992,31,1286. (b)Brown, T. L.; Lee, K. J. Coord. Chem. Rev. 1993,128,89. (23) Zhang, H.-T.; Zhang, S.; Brown, T. L. Unpublished kinetics studies, to be submitted for publication.
(24) Frauenfelder, H.;Sligar, S. G.; Wolynes, P.G.Science 1991, 254,1598.
2372 Organometallics, Vol. 14, No. 5, 1995
Zhang et al.
n
transient concentration of the CO-loss product, denoted as Mni(CO)sL* or Mnz(CO)g*,followingflash photolysis. A A representative decay curve, for Mn2(CO)sP(i-Pr)3*,is shown in Figure 7. Analysis of this and other decay curves as a sum of exponential terms reveals that there are three components. The solid line in Figure 7 shows the fit of the calculated decay to the observed data. Similar results were obtained for L = PMe3, P(n-Bu)s, and PPh3. Table 2 lists the calculated second-order rate constants corresponding to each component of the decay, assuming that each process is pseudo-first-order, first order each in metal carbonyl intermediate and CO, with Ligand the latter present in excess concentration of 13.5 mM.25 Conformational Also listed are second-order rate constants taken from States the literature for related processesllb and corrected as necessary to reflect the newer assumed value of CO concentration in hexane under 1 atm pressure. Figure 6. Schematic illustration of the energy surfaces connecting the various substates of Mn2(CO)~L@-~1,$-CO). The largest of the rate constants has essentially the The illustration is meant to show the multilevel character same value for all four compounds studied. I t matches of the energy surface. It further illustrates the lower well the value previously determined for recombination energy barriers to interconversions of isomers 1, 3,and 5 of CO with Mn2(CO)g*.llb This process, which correas compared with isomers 2 , 4 , and 6, the large barrier to sponds to only a small fraction of the total change in interconversionof the two sets of isomers, and the existence absorbance, is apparently due to reaction 1. The Mnsof ligand conformational states for each isomer.
-
(2, 4,
S} isomer sets and possibly also than those for
Mn,(CO),*
k, + CO Mn,(CO),,
interconversion among isomers 1,3,and 5. I t appears that interconversions among isomers 2,4, and 6 occur (CO)g* arises from photochemical loss of phosphine a t about the rates of recombination of those isomers rather than CO from the parent compound. Our results with CO or slower. It is likely that interconversion of contrast with those of Banister et a1.,26who observed conformational states of the ligand occur more rapidly relatively more extensive phosphine ligand loss on roomthan intermolecular reaction processes, although there temperature photolysis of some CpMn(C0)2PR3 comis no experimental evidence on this point. Thus there pounds. may exist a Boltzmann population distribution of ligand The other two decays we ascribe to recombination of conformational states for each of the six,isomers that CO with Mnz(CO)sL*, according to eqs 2 and 3. The may exist. Whether this distribution is of significance for the reactivities of the isomers will depend on the k2 CO Mn,(CO),L (2) particular ligand and attacking reagent. Mn*(CO),-Mn(CO),L The various states of M ~ ~ ( C O ) ~ L ( U - V ~ ,are ~;~~-CO) k3 formed in a nonequilibrium distribution under irradiaMn(CO),-Mn*(CO),L CO Mn,(CO),L (3) tion at low temperature. Thus the overall reactivity at low temperature or upon warming will be a complex faster process is associated with recombination of CO function of the distribution of substates and the barriers with the isomer in which L is on the Mn center to which to interconversion among them in comparison with the the semibridging CO is +bonded, i.e., the {1,3,6}set, barriers to reaction. Recombination with CO, for ex2. The slower rate process corresponds to recombieq ample, is not characterized by a single exponential decay nation at the other, more crowded Mn center, reprebut rather by a sum of such decays, each characteristic sented by the (2, 4, S} isomer set, eq 3. The fact that of a distinct species that does not readily interconvert the fit to a three-component decay is essentially exact with other substates under reaction conditions. indicates that each of the three components is precisely When Mn2(CO)gL is flash-photolyzed at room tema single exponential decay. Thus, reactions 2 and 3 perature, many of the interconversions between subrepresent effectively single processes, suggesting that states will occur rapidly with respect to bimolecular the equilibration among the equatorial and axial isoreaction with a subtrate, e.g. CO. In this case, the mers in each of the two sets is rapid at room temperanumber of characteristic reaction rates will be smaller ture with respect to the rate of recombination with CO. but still not reduced to a single characteristic rate Although at equilibrium there may be small amounts constant. The complexity of the reaction system should of the equatorial isomer present in the Mnz(CO)sL* be discernible from analysis of the absorbance decay intermediates, as suggested by the spectra of Figure 1 following flash photolysis, in which all the substates of for the products, the axial isomer probably predominates the system contribute more or less equally to a single in each case, resulting in the appearance of only a single UV-visible absorption which is monitored in following decay process. the decay. We have studied the transient behavior of four Mn2(25) Carbon Monoxide; Cargill, R. W., Ed.; Solubility Data Series; Pergamon Press: New York, 1990; pp 51-52. (C0)gL compounds in hexane a t room temperature, (26)Banister, J. A.; George, N. W.; Grubert, S.; Howdle, S. M.; under 1atm CO pressure. The absorbance at either 510 Jobling, M.; Johnson, F. P. A.; Morrison, S.L.; Poliakoff, M.; Schubert, or 530 nm was monitored to provide information on the U.; Westwell, J. R. J. Organomet. Chem. 1994, 484, 129.
+ +
-
Organometallics, Vol. 14, No. 5, 1995 2373
Monosubstituted Manganese Carbonyl Compounds 0.25
020,
I
0.20-
0.15-0.02
0.02
0.07
0.11
0.16
0.20 @-I)
0.10-
0.00 -0.10
I
I
0.00 0.10
1
I
0.20
I
0.30 0.40
I
1
I
I
0.50
0.60
0.70
0.80
Seconds
0.90
(E-1)
Figure 7. Transient decay of the absorbance at 510 nm following xenon flash lamp photolysis of a 2.0 x M hexane at 296 K. The continuous solid line represents a computed fit to the experimental results. solution of Mn~(C0)sP(i-Pr)~ The insert shows the residuals for two- or three-parameter least-squares fits. The two-parameterfit is noticeably poorer at short times. Table 2. Second-Order Rate Constants under 1 atm CO ([COI = 13.6 mMF L PMe3 P(n-Bu)a
PPh3 P(i-Pr13
co PMe3 P(n-Bu)a
P(i-Pr13
kib
1.9 x 2.0 x 2.1 1.8 2.1 3.5 6.7
105 105
105 105 105df 104ef
10-~kf
10-3k3C
6.1 3.1
7.3 5.0 4.1 1.6
4.2 2.5
103ef 1.0 x 102ef
a M-1 s-1. b f15%. 3~10%. For Mnz(C0)9*.llbe For Mnz(C0)7Lz*.llb f Corrected to [CO] = 13.5 mM.
The k2 values are larger than the k3 in each case, as expected. The variation among the four compounds studied represents only a 3-fold variation. The k3 values are approximately 1 order of magnitude smaller than k2 in each case. One might expect that an increasing steric requirement of the phosphine would be reflected more strongly in the isomer in which the phosphine is bound to the metal center a t which CO is recombining. While this is true to a limited degree, the effect is not large; the k3 values span slightly less than a 5-fold range. When the k3 values are compared with the corresponding rate constants for recombination of CO with
Mnz(C0)7Lz*, listed in Table 1, a curious pattern emerges. While k3 for L = P(i-Pr)s is larger than for the disubstituted compound, as expected, k3 for L = PMe3 is smaller than for the disubstituted analog, and k3 for L = P(n-Bu)s is essentially the same. The PMe3 comparison may be related to the fact that in Mn2(CO)s(PMe& the PMe3 ligands are predominantly in the equatorial positions, whereas PMe3 is predominantly in the axial position in Mnz(CO)gPMe3. Thus, the PMe3 substituents may be located in equatorial positions in Mn2(C0)7(PMe3)2* and in the axial position in Mn2(CO)sPMe3*. The dinuclear compounds which are the subject of this study are not the only examples in which complexity can arise. In reactions proceeding via thermal or photodissociation of a CO or other ligand from heterodinuclear compounds, or from cluster compounds in which internal rearrangement reactions compete with intermolecular processes, and in various ligand-substituted systems similar considerations might apply.
Acknowledgment. This research was supported by the National Science Foundation through Grant NSFCHE92-13531. 0M940984Z
