5251
Organometallics 1995, 14, 5251-5257
Matrix Photochemistry of trans-[Cp*Fe(CO)]aOr-CO)~-CH2):Generation of the Cis Isomer and of a Double-CO-Loss Photoproduct Yvonne H. Spooner, Ellen M. Mitchell, and Bruce E. Bursten” Department of Chemistry, The Ohio State University, Columbus, Ohio 43210 Received June 28, 1995@ The photochemistry of [Cp*Fe(CO)]z@-CO)@-CH2)(7*; Cp* = r5-C5Me5) has been investigated in a matrix of 3-methylpentane at low temperatures. Compound 7* undergoes CO loss to form the triply bridged product [Cp*Fe]&-C0)2@-CHd (8*),a species similar to the CO-loss product from [CpFe(C0)]2@-C0)2(1). Compound 8* undergoes a subsequent photochemical CO loss to generate the double-CO-loss product Cp*2Fe2@-r1:r2-CO)(-CH2) (9*), which contains a semibridging CO ligand. Because of the preference for CH2 to serve as a bridging ligand, 9* is very different from the double-CO-loss product from 1,i.e. [CpFe(C0)lz (3),which has terminal CO ligands. Compound 9* thermally reacts with CO to reform 8*. Thermal ramping experiments demonstrate t h a t 8* thermally reacts with CO to re-form 7*, with a preference for the formation of the thermodynamically unfavored cis isomer. The cis isomer of 7* converts to the trans isomer via a first-order process t h a t is considerably slower than cidtrans isomerization for 1 or [Cp*Fe(CO)]z@-CO)z(1*). A scheme is proposed to explain the formation of cis-7* in preference to truns-7*. The photochemistry of the Cp-containing, alkylidene-bridged compounds [CpFe(C0)]2@-CO)@-CH2)( 7 )and [CpFe(C0)12@-CO)@-CHCHd(7’) is not as straightforward as that of 7*, owing to multiple isomers and low solubility.
Introduction
Fe-Fe double bond (eq 3)., We have recently demon-
The photochemistry of dinuclear organometallic complexes has been and continues to be an area of intense interest.l Most of the dinuclear complexes that have been studied photochemicallyare symmetric complexes . these complexes can be considered dimers [MGL’bL”,..12; of two identical mononuclear fragments, either with an unsupported metal-metal bond, such as the M2(CO)lo (M = Mn, Re) systems, or with an even number of bridging ligands, such as in [CpFe(C0)12(p-C0)2(1;Cp = q5-C5H5)and [Cp*Fe(C0)12(p-C0)2(l*;Cp* = v5-C5Me5). A major reason for the interest in these symmetric systems is that they exhibit two very different photochemical reactions, namely photochemical homolysis into two identical mononuclear radical fragments (eq 1) and photochemical ligand dissociation (eq 2). The homolysis: [MIA&’, L”,... 12
+ hv - 2’ML,L’b L”,...(1)
-
ligand dissociation: [MLaL’bL’’e...124- hv M & ~ - 1 ~ 2 b ~ ~L. .(2) .
+
factors that govern which of the above processes occurs, and to what extent, are still poorly understood. Frozen-matrix photochemistry has been an extremely useful technique for detecting ligand dissociation in symmetric complexes (eq 21, especially those with easily probed ligands such as CO. For example, symmetric complex 1 undergoes CO loss to yield the dinuclear complex CpzFea(p-CO)s (2),2a paramagnetic compound that contains three bridging CO ligands and a formal Abstract published in Advance ACS Abstracts, September 15,1995. (1) (a) Geoffroy, G. L.; Wrighton, M. S. Organometallic Photochemistry; Academic Press: New York, 1979. (b) Meyer, T. J.; Caspar, J. V. Chem. Reu. 1985,85,187-218. @
[CpFe(CO)I,@-CO),
1
+ hv - Cp2Fe2@-CO),+ CO 2
(3)
strated that complex 2 undergoes a second CO loss t o yield the highly reactive double-CO-loss product [CpFe(CO)]2 (31,a remarkable product inasmuch as both CO ligands are terminal (eq 4h4 Complex 3 thus apparently
+
Cp2Fe2@-CO), hv 2
-
Cp(OC)FeEFe(CO)Cp
3
+ CO (4)
possesses an unsupported Fe-Fe triple bond. We have also recently reported double-CO loss from the symmetric dimer Mnz(C0)lo (4h5 Photolysis of 4 leads initially t o the formation of Mnz(C0)g (51, which has a semibridging CO ligand.6 Continued photolysis leads to additional CO loss and the formation O f Mn2(C0)8 (6), a species that, like 3,contains only terminal CO ligands. The above studies on the symmetric complexes 1 and 4 demonstrate the remarkable ease with which CO can change its bonding mode, from terminal to semibridging (2) (a) Caspar, J. V.; Meyer, T. J. J. Am. Chem. SOC.1980, 102, 7794-7795. (b) Hooker, R. H.; Mahmoud, K. A,; Rest, A. J. J. Chem. SOC.,Chem. Conzmun. 1983, 1022-1024. (c) Hepp, A. F.; Blaha, J. P.; Lewis, C.; Wrighton, M. S. Organometallics 1984, 3, 174-177. (3) Blaha, J. P.; Bursten, B. E.; Dewan, J. C.; Frankel, R. B.; Randolph, C. L.; Wilson, B. A.; Wrighton, M. S. J. Am. Chem. SOC. 1985,107,4561-4562. (4)Kvietok, F. A.; Bursten, B. E. J. Am. Chem. SOC.1994, 116, 9807-9808. (5) Kvietok, F. A.; Bursten, B. E. Organometallics 1996,14,23952399. (6) (a) Hepp, A. F.; Wrighton, M. S. J.Am. Chem. Soc. 1983, 105, 5934-5935. (b) Dunkin, I. R.; Hlrter, P.; Shields, C. J. J.Am. Chem. Soc. 1984,106,7248-7249. (c) Zhang, H.-T.; Brown, T. L. J.Am. Chem. SOC.1993, 115, 107-117.
Q276-7333l95/2314-5251$Q9.QQlQ 0 1995 American Chemical Society
Spooner et al.
5252 Organometallics, Vol. 14, No. 11, 1995
to bridging. In these cases, the double-CO-loss products 3 and 6 apparently contain no bridging or semibridging CO ligands, results that are surprising and nonintuitive. We were therefore curious as to how this double-COloss chemistry would be altered in the case of a dinuclear complex that contained a ligand which strongly preferred a bridging rather than terminal bonding mode. A simple way of achieving this goal is to replace a bridging CO ligand in one of the symmetric complexes with a bridging CH2 ligand, to yield one of the wellknown methylene-bridged dinuclear complexes [M&L'&"c...l@-CH2), where, in this instance, a,b, and c are not necessarily even integers.' Alkylidene ligands in low-valent complexes show a strong preference for bridging coordination; we therefore thought that any single- or multiple-CO-loss products would retain a methylene bridge and that the remaining CO ligands could be forced into a different coordination environment than in the symmetric complexes. The present study will address in detail the photochemistry of one of the series of methylene-bridged analogs of the Fp2 systems (l),namely the [($-C5R5)Fe(C0)12@-CO)+-CHR) systems (7,R = R = H;7*,R = CH3, R' = H; 7',R = H, R = CH3).* These particular systems are natural extensions of our studies on the symmetric complexes, for they are easily synthesized (especially given the methodology developed by Wright and Nelsong) and analyzed, and iron hydrocarbyl systems are becoming increasingly important in organic synthesis.1° Further, we have previously investigated the photochemistry of 7'with respect to the generation of CO-loss and radical photoproducts and to the kinetics of photosubstitution.ll Two conclusions of this earlier study are particularly relevant to the present one: (1) the CHCH3 bridge deactivates the homolysis process for 7' and (2) compound 7' undergoes CO loss to generate (8'1, an analog of 2, albeit with CpzFez@-CO)z@-CHCH3) lower symmetry (eq 5).
+
[CpFe(C0)12@-CO)(p-CHCH3) hv
-
Cp,Fe,@-CO),@-CHCH,)
+ CO ( 5 )
8' For reasons that will be discussed below, we shall focus on the photochemistry of the Cp* derivative 7*, which is more amenable to study than 7 and 7'. We shall demonstrate that complex 7* definitively undergoes double-CO loss, like that exhibited by the symmetric dimers. Further, we shall see that the methylene bridge, because of its unwillingness to adopt a terminal configuration, confers a very different structure on the double-CO-loss product, namely one in which the remaining CO ligand occupies a semibridging position. These studies will thus demonstrate another marked difference in the photochemistry of the methylene(7)See, for example: (a) Herrmann, W. A. Adv. Organomet. Chem. 1982,20,159-263. (b) Herrmann, W. A. J . Organomet. Chem. 1983, 250,319-343. (c) Holton, J.;Lappert, M. F.; Pearce, R.; Yarrow, P. I. W. Chem. Reu. 1983,83,135-201. (8) Kao, S. C.; Lu, P. P. Y.; Pettit, R. Organometallics 1982,1,911918. (9)Wright, M.E.;Nelson, G. 0. J . Organomet. Chem. 1984,263, 371-373. (10)See, for example: Fatiadi, A. J. J . Res. Natl. Inst. Stand. Technol. 1991,96, 1-113. (11)McKee, S. D.; Bursten, B. E. J . Am. Chem. Soc. 1991, 113, 1210-1217.
bridged systems relative to their symmetric, carbonylcontaining congeners.
Experimental Section Materials. Standard solvent and reagent purification techniques were used.12 Tetrahydrofuran was dried and distilled over potassiumhenzophenone. Hexanes (Mallinckrodt Analytical Reagent) were washed with concentrated sulfuric acid and distilled water, predried with MgS04, and distilled over potassium metal. Methylcyclohexane and 3-methylpentane were purchased from Aldrich, dried over Na/K alloy, and vacuum-transferred t o Kontes-sealed flasks for storage in an inert-atmospheredrybox. Chloromethyl pivalate and K(secCdH&BH (as K-Selectride)were purchased from Aldrich and used as received. Pentamethylcyclopentadienyliron dicarbonyl dimer (12) was purchased from Strem and used without further purification. Cyclopentadienyliron dicarbonyl dimer (1) was synthesized according to the standard literature pre~arati0n.I~ The methylene-bridged compounds (75-c5R5)z(R = H, CH3) were prepared via F~~(CO)&L-CO)QL-CH~) literature methods9 and purified on a refluxing column with hot hexanes,14 and the ethylidene-bridged compound [q5CsH5]2Fe2(CO)2QL-CO)~-CHCH3) was prepared by literature methods.* All synthetic manipulations were performed under an atmosphere of dry argon using standard Schlenk techniques. Argon was purified by passage through consecutive columns of activated Rydox catalyst and Drierite.I2 Alumina used for column separations was heated under vacuum until fully activated and converted to grade I11 via the addition of 6%, by weight, deionized water.12 Instrumentation. The following instrumentation was utilized: Bruker WM FT lH (250 MHz) NMR spectrometer; Nicolet Magna 550 FT-IR spectrometer; Cary-17 UV/vis spectrometer, upgraded by Olis. The matrix isolation studies employed a SPECAC Model 21500 variable-temperaturesystem equipped with CaF2jacket windows. The present studies used a cell of CaFz windows with lead spacers and a path length of 1 mm. The irradiation source was a 200 W broadspectrum, medium-pressure Hg-vapor UV/vis lamp (Oriel) equipped with an IR filter. Matrix Isolation Experiments. Matrices were prepared by cooling solutionsof the samples below the glass-transition temperature. The concentration of the photoactive species was between 0.5 and 2.0 mM. Fresh solutions were prepared for each matrix run. The matrix cell was placed 29-34 cm from the exit filter of the irradiation source. Most of the matrix experiments were performed at 93 K using liquid-nitrogen cooling and a thermocouple temperature controller monitored at the cell jacket. During portions of this study, the matrix was warmed to a specified temperature by removing liquid nitrogen from the Dewar, warming the matrix to the desired temperature, and then immediately recooling the matrix to 93 K. This procedure took between 5 and 35 min for warming (depending on final temperature) and approximately 10 min for the full recooling process, including allowances for reequilibrating at the lower temperature. All spectra were recorded at 93 K, except as noted. Because of differences in heat capacity,the measuredjacket temperature is an upper bound t o the actual temperature of the matrix, and we estimate the potential uncertainty in the actual matrix temperatureduring these warmup procedures to be as large as 10 K, dependent (12)(a) Gordon, A.J.; Ford, R. A. The Chemist's Companion; Wiley: New York, 1972.(b) Shriver, D.F. The Manipulation ofAir-Sensitive Compounds; Krieger: Malbar, FL, 1969.(c) Perrin, D.D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory Chemicals; Pergamon: Oxford, U.K., 1988. (13)King, R. B. Organometallic Syntheses; Academic Press: New York, 1965;Vol. 1. (14)Casey, C.P.;Fagan, P. J.; Miles, W. H. J.Am. Chem. Soc. 1982, 104,1134-1136 (supplementary materials).
Photochemistry of trans-[Cp*Fe(CO)Idp-CO)(p-CH2)
Organometallics, Vol. 14, No. 11, 1995 5253
on the final temperature. Because of these temperature uncertainties, multiple runs were performed to demonstrate the reproducibility of the results presented here. During the matrix experiments,the times required to obtain IR and UV/vis spectra were approximately 3-5 min for IR (32 scans, 2 cm-’ resolution) and 14-28 min for U V l v i s (16 and 32 scans per data point, respectively). The IR window of interest was 2400 to 1600cm-l, but only the region from 22001700 cm-l is reported because this is the smallest window containing all the peaks of interest. The UVlvis observation window was from 800 to 200 nm.
Results and Discussion
As was the case in our earlier studies of double-COloss photoproducts, our main probe of the photochemical processes that occur in the frozen matrices is infrared spectroscopy of the CO-stretching region. The use of IR spectroscopy as our principal probe is one of the reasons that we have focused on the Cp* complex [Cp*Fe(CO)12@-CO)@-CH2)(7*). As was the case in complex 1*, the steric bulk of the Cp* ligands forces 7* to adopt an essentially trans-only equilibrium conformation; in contrast, complexes l , 7 , and 7’ exist as cidtrans mixtures in solution.14J5J6Thus, the IR spectra of 7* and its photoproducts are much easier to interpret than ~ ?have ~ those of 7 and 7’. As with our other ~ t u d i e s ,we carried out the matrix experiments in neat 3-methylpentane (3-MP),which provides a softer matrix than do other common hydrocarbon matrix materials.17 Matrix Photochemistry of 7*. The methylenebridged derivative 7* is an analog of the “Fp*2” system 1*, in which a bridging carbonyl ligand has been replaced by a bridging methylene ligand. Because trans-l* has centrosymmetric Ca point symmetry, it exhibits a very simple two-band IR spectrum, with one band each in the terminal and bridging CO-stretching regions. The replacement of a bridging CO by a bridging CH2 ligand to form trans-7* reduces the molecular symmetry t o a maximum of CZ,and the IR spectrum of trans-7* could, in principle, show three CO-stretching bands. However, the transoid arrangement of the terminal CO ligands is expected to render the symmetric stretch of these ligands to be very weak. Figure la shows the IR spectrum of trans-l* as a frozen solution (1 mM) in neat 3-MP a t 93 K. As expected, two prominent bands are observed, a t 1915 and 1765 cm-l. These absorptions correspond roughly to the antisymmetric stretch of the terminal CO ligands and to the stretch of the bridging CO, respectively. The simplicity of this starting spectrum greatly assists in the elucidation of subsequent photochemical changes. Irradiation of the frozen matrix of trans-7* produces significant spectral changes. After 6 min of broad(15) (a) Cotton, F. A.; Yagupsky, G. Inorg. Chem. 1967, 6 , 15-20. (b) Fischer, R. D.; Vogler, A.; Noack, K. J . Organomet. Chem. 1967, 7, 135. (c) Noack, K. J. Organomet. Chem. 1967, 7, 151. (d) Manning, A. R. J. Chem. SOC.A 1968, 1319-1324. (e) Bullitt, J. G.; Cotton, F. A.; Marks, T. J. J . Am. Chem. SOC.1970,92, 2155-2156. (0McArdle, P.; Manning, A. R. J. Chem. SOC.A 1970, 2133-2136. (g) Bullitt, J. G.; Cotton, F. A.; Marks, T. J. Inorg. Chem. 1972, 11, 671-676. (h) Gansow, 0. A.; Burke, A. R.; Vernon, W. D. J . Am. Chem. Soc. 1972, 94,2550-2552. (i) Adams, R. D.; Cotton, F. A. Inorg. Chim. Acta 1973, 7, 153-156. (i) Kirchner, R. M.; Marks, T. J.; Kristoff, J. S.; Ibers, J. A. J . Am. Chem. Soc. 1973,95,6602-6613. (16)The cis isomer of 7’ has additional isomers owing to the possibility of exo and endo orientations of the ethylidene methyl group relative to the Cp ligands. These additional isomers can make NMR analysis of reactions of 7’ more complex that that of either 7 or 7*. (17) Ling, A. C.; Willard, J. E. J. Phys. Chem. 1968, 72,1918-1923.
Figure 1. Progressive IR spectra during the irradiation of [Cp*Fe(C0)32(u-CO)(u-CH2)(7*)in 3-methylpentane at 93 K (a) spectrum prior to irradiation; (b) spectrum after 6 min of irradiation; (c) spectrum after 40 min of irradiation; (d)spectrum after 80 min of irradiation; (e) spectrum after 180 min of irradiation. The peak labels in spectra b and c refer to the compound numbers in Scheme 1. spectrum W irradiation, the peak a t 1915 cm-l shows a decrease in intensity, while the one at 1765 cm-l increases in intensity. In addition, new well-defined bands appear at 1802 and 2132 cm-l (Figure lb). These changes are consistent with the photochemical formation of a CO-loss product that has CO-stretching modes a t 1802 and 1765 cm-l (coincident with trans-7*). This species is readily identified as the C2” triply bridged complex Cp*zFe2@-C0)2@-CHd(8*;Scheme 11, a complex analogous to CpzFez@-CO)z@-CHMe)( 8 ) ,which we have observed in the solution and matrix photochemistry of 7’.11J8 The presence of two bridging-CO stretching modes in 8* suggests that its structure is analogous to the structurally characterized molecule The Cp*2Fe2@-C0)3(2*), the CO-loss product from l*.3 1802 and 1765 cm-l bands are assigned to the symmetric (AI) and antisymmetric (B1 or B2) CO-stretching modes of 8*, respectively; the symmetric stretch is expected to be a t higher energy than the antisymmetric stretch,19 and, because the angle between the CO ligands is expected to be -120”, the antisymmetric stretch should have the greater intensity. A comparison of the UV/vis spectra of 7* and 8* shows a marked shift in the position of the prominent visible band in each spectrum (Figure 21, and the matrix noticeably changes color from purple to orange. Complex 8* exhibits a welldefined electronic absorption at 482 nm, consistent with (18)McKee, S. D. Ph.D. Dissertation, The Ohio State University, 1990. (19) Cotton, F. A.; and Kraihanzel, C. S. J . Am. Chem. Soc. 1962, 84,4432.
5254 Organometallics, Vol. 14,No. 11, 1995
Spooner et al.
(a) 1915cm.l band of 7'
-
\
-
300
400
500 600 wavelength (nm)
700
a
band of E'
=
0.2 4,0\m. 2.0
b
1
200
0.3
-0 . 0 0 1
100
200
100
200
03,
I
Figure 2. UV/vis spectra in 3-methylpentane at 93 K (a) spectrum of 7* prior to irradiation; (b) spectrum obtained after irradiation and back-reaction of 9* to 8*. After this procedure, the predominant species present in the matrix (by IR spectroscopy) is 8*. Scheme 1. Photochemical CO-LossProcesses and Thermal Back-ReactionsExhibited by tmns-7* upon Irradiation in a Matrix of Frozen 3-Methylpentane H H
." 0.0 0
200
100
0
Figure 3. Temporal behavior of relevant IR bands during the irradiation of 7* in 3-MP at 93 K: (a) the 1915 cm-l band of 7*;(b) the 1802 cm-' band of 8*; (c) the 1729 cm-' band of 9*; (d) the 2132 cm-' band of free CO. Abscissas are in minutes; ordinates are integrated intensities in units of absorbance units per centimeter.
-
the visible transitions we have observed in 8' (485 nm)ll and 2* (515 nm).20 Further irradiation of the matrix leads to neardisappearance of the IR bands due to 7' and 8* and the appearance of an absorption at 1729 cm-l, which is attributed to the new photoproduct 9* (Figure lc-e). The formation of 9* occurs only after 8* has been made, and 8* can be nearly completely converted to 9* a t 93 K. During this subsequent irradiation, the matrix changes color from orange to pale green. Figure 3 shows plots of IR band intensity (measured as integrated peak areas) vs time for isolated bands of 7*, 8*, 9*, and free CO. These plots demonstrate the facile formation of 8* from 7* and the equally facile conversion of 8* to 9*. The intensity of the 2132 cm-l band for free CO appears to be increasing throughout these conversions, evidence
that the process 8* 9* involves additional CO evolution. However, the small intensity of the 2132 cm-' absorption makes it difficult to arrive at definite conclusions concerning the 8* 9* conversion. In order to explore this conversion further, and to arrive a t a more convincing conclusion as to the identity of 9*, we have used the observation that 9* thermally converts back to 8* a t 93 K. Thermal Back-Reaction of 9* to 8*. The thermal back-reaction of 9* to 8* is readily apparent in the difference spectrum shown in Figure 4. The matrix was irradiated for 180 min at 93 K, which leads to the spectrum in Figure le. The matrix was then allowed to react in the dark at 93 K for an additional 90 min. The spectrum in Figure 4 was obtained by subtracting the spectrum after 180 min irradiation from the spectrum obtained after 180 min irradiation plus 90 min thermal reaction. The negative peak at 1729 indicates that 9* is consumed during the dark reaction period, and the positive peaks at 1765 cm-' and 1802 cm-l show the conversion back to 8*. No conversion to 7* is evident, probably because there is insufficient thermal 7* conversion. The energy at 93 K to effect the 8* negative peak at 2132 cm-I shows conclusively that CO is consumed in the conversion of 9* to 8*. We therefore conclude that 9* is a double-CO-loss product from 7* that has the formula Cp*2Fez(CO)(CHz). The 1729 cm-l absorption of the CO in 9* seems most consistent with a linear semibridging p-qJ:q2-C0.21 Thus, the likely structure for 9* is two Cp*Fe fragments linked by a semibridging CO and, we assume, an intact methylene bridge (Scheme 1). The comparison of this structure to the double-CO-loss product from 1 (or 1*) is quite striking. As noted above, we have proposed that double-CO loss from 1 leads to the species 3 with only terminal CO's and a formal Fe-Fe triple bond.4 In contrast, the apparent instability of CH2 in a terminal conformation forces 9* to adopt multiple bridging ligands
(20) Bursten, B. E.; McKee, S. D.; Platz, M. S. J.Am. Chem. Soc. 1989,111, 3428-3429.
(21) (a) Crabtree, R. H.; Lavin, M. Inorg. Chem. 1986,25,805-812. (b)Zhang, S.; Brown, T. L. J.Am. Chem. SOC.1993,115, 1779-1789.
-
trans 7.
8'
1915 cm" 1765 cm"
1801 cm" 1766 c"'
H U
-
CIS 7'
1965 cm" 1764 cm.'
9.
1729 cm"
-
-
Photochemistry of trans-CCpYFe(C0)1~~-CO)(~-CH2)
Organometallics, Vol. 14, No. 11, 1995 5255
0.241
022j
1
1
0.20
1
I’
0 181
I 0 16-
0 140 12-
A b I
010.
0
r
008-
b
:
006-
A
If
**
.Io4/ -0 06 21W
2WO
1900
1800
W a v n u m b m (cml) ..o 10
0
2000
°
8
1900
8
1800
2100
WavNlUmbeK (cml)
Figure 4. Difference IR spectrum from the irradiation and thermal back-reaction of 7* in 3-methylpentane at 93 K. The spectrum shown is the difference A - B, where A and B are defined as follows: (A) spectrum obtained after irradiation of the matrix for 180 min followed by 90 min of thermal (dark) reaction at 93 K; (B)spectrum obtained immediately after irradiation of the matrix for 180 min. As shown, the prodominant species aRer the initial irradiation is 9*. During the thermal reaction, 9* and CO are consumed (negative peaks) while 8* is produced (positive peaks). and a formal Fe-Fe double bond. We will be comparing the bonding and electronic structures of these doubleCO-loss diiron products in a later publication. The progressive conversions 7* 8* 9* under photolytic conditions are reproducible and nearly complete a t 93 K. The ease of the conversions does not depend upon the starting concentration of 7* in the matrix, which provides evidence for the reasonable expectation that all the CO-loss processes are unimolecular. It is notable that 3-MP is a relatively “soft” (low-viscosity)matrix material for these studies; if we use a more viscous matrix material, such as methylcyclohexane (MCH),the same photochemical conversion of 7* to 9* occurs, albeit not nearly as readily. Variable-Temperature Studies of the Thermal Back-Reactions: Formation of cis-7*. As noted earlier, at 93 K there is sufficient thermal energy for the back-reaction 9* CO 8* to occur quite readily. However, the reaction of 8* with CO to produce starting material 7* does not occur at 93 K to any significant extent over a period of 2 h. These observations are consistent with the notion that 9* is more coordinatively unsaturated than 8* and that, therefore, 9* should react with CO more easily than does 8*. Interestingly, when the matrix was warmed in the dark by only 5-10 K, the back-reaction 8* CO 7*
- -
+
-
+
-
Figure 6. Progressive IR spectra following the irradiation of 7* for 6 min in 3-MP at 93 K: (a) spectrum at 93 K, prior to thermal ramping; (b) spectrum after ramping the matrix to 123 K; (c) spectnim after ramping the matrix to 138 K; (d) spectrum after ramping the matrix t o 148 K. The peak labels in spectra a and d refer to the compound numbers in Scheme 1.The temperature of the matrix was ramped to a final temperature; the matrix was then cooled to 93 K and reequilibrated prior to collecting the spectra. could be detected readily after only 30 min. This observation suggested to us that controlled warming of the matrix could be revealing. Figure 5 shows a series of IR spectra obtained by ramping the temperature of the matrix cell in the dark t o successively higher temperatures, a t a nominal warming speed of 2-3 Wmin, followed by thermal quenching to 93 K to record each spectrum. Several observations are notable. First, as indicated by the disappearance of the peak at 1729 cm-l, the back-reaction 9* CO 8* is essentially complete by the time the matrix is warmed to 123 K. Second, as expected, the peak for free CO disappears as the back-reactions occur. Third, the appearance of the terminal-CO peak for trans-7* (1915 cm-l) is accompanied by the concurrent appearance of another terminal-CO band of comparable intensity at 1967 cm-l, which is due to a new species. At the same time, the peak in the bridging40 region at 1765 cm-’ broadens significantly, suggesting that the new species may also have a peak in that region. When the matrix is melted and allowed to equilibrate at room temperature, the new species converts entirely to truns-7*, with no consumption or evolution of CO. These observations are consistent with the identification of the new species as cis7*, which is thermodynamically unstable with respect to the trans isomer. The peak at 1967 cm-l is likely the symmetric stretching mode of cis-7*, which is expected to be a t higher energy than the observed antisymmetric stretch of trans-7*. The antisymmetric terminal-CO-stretchingmode of cis-7* is expected to be allowed but to be very weak. We cannot provide a
+
-
5256 Organometallics, Vol.14,No. 11, 1995
Spooner et al.
definitive assignment of its position, although it may be nearly coincident with the strong antisymmetric stretching mode of trans-7*, as is the case for the corresponding bands of cis- and t r ~ n s - 1 Likewise, .~~ the bridging-CO modes in the two isomers are expected to be a t nearly the same energy, as are the antisymmetric bridging-CO-stretch modes in cis- and trans-1.15 Several aspects of the formation of cis-7* and its isomerization t o trans-7* are of interest. First, the conversion cis-7" trans-7* follows first-order kinetics, which is consistent with an intramolecular process. The process is slow: at room temperature, the first-order rate constant is on the order of s-1.22 This rate constant is similar to that obtained by Casey et al. for the cidtrans equilibration of trans-enriched 7 , a process for which they report a half-life of 10 min at 36 O C , 1 4 and by Altbach et al. for the cis/trans isomerization of 7 in various solvents.23 The cis/trans isomerization of 7 is much slower than that for 1,24and we see the same phenomenon in the Cp*-containing systems. Moore, Poliakoff, and Turner have reported the first-order rate constant for the conversion of photochemically generated cis-l* to trans-l* in cy~lohexane.~~ They find a rate constant of ca. 50 s-l at 24 "C, roughly 5 orders of magnitude greater than that for the isomerization of 7*. Several mechanisms have been proposed for the cis/ trans isomerization of methylene-bridged (and related) complexes, most of which involve an unbridged isomer with a terminal CH2 ligand.14,23y26It is generally accepted that the cidtrans isomerization is expected to be slower for 7 and 7* than for 1 and 1* because the latter can readily utilize an unbridged isomer t o facilitate the isomerization; if 7 and 7* isomerize via the same mechanism, the unbridged intemediate that contains a terminal alkylidene is presumed to be significantly higher in energy than the unbridged isomers of 1 and l*.27 Regardless of the true nature of the intermediate in the cis/trans isomerization of 7*, the present studies reinforce the notion that it is a higher energy process than that in the parent carbonyl systems.
-
(22)Because of the limitations of our matrix cell upon warming to room temperature, it was not possible to maintain a constant temperature while monitoring the decay of cis-7* to trans-7*. The kinetic data were obtained over a temperature range of 18-26 "C.Six data points were obtained by measuringthe integrated intensity I of the 1967 cm-l band of cis-7* over a 55 min interval, after which the band was too small to be integrated. A plot of -ln(Z) vs time was linear ( R = 0.99). The first-order rate constant obtained from this plot is 7 x s-l, which corresponds to a half-life of ca. 16 min. (23)Altbach, M. I.; Muedas, C. A.; Korswagen, R. P.; Ziegler, M.L. J . Organomet. Chem. 1986,306, 375-383. (24)Adams, R. D.; Cotton, F. A. J. Am. Chem. SOC.1973,95,65896594. (25)Moore, B. D.; Poliakoff, M.; Turner, J. J. J. Am. Chem. Soc. 1986,108, 1819-1822. (26)(a) Adams, R. D.; Brice, M. D.; Cotton, F. A. Inorg. Chem. 1974, 13,1080-1085. (b)Dyke, A. F.; Knox, S. A. R.; Mead, K A.;Woodward, P. J. Chem. SOC.,Chem. Commun. 1981, 861-862. (c) Dyke, A. F.; Knox, S. A. R.; Morris, M. J.;Naish, P. J. J.Chem. SOC.,Dalton Trans. 1983,1417-1426. (d) Colborn, R. E.;Dyke, A. F.; Knox, S. A. R.; Mead, K. A,; Woodward, P. J . Chem. Soc., Dalton Trans. 1983, 2099-2108. (e) Theopold, K. H.;Bergman, R. G. J.Am. Chem. Soc. 1983,105,464475. (0Casey, C. P.; Gable, K P.;Roddick, D. M. Organometallics 1990, 9,221-226. (g) Ueno, K.; Hamashima, N.; Ogino, H. Organometallics 1992,11,1435-1437. (h)Fong, R. H.; Lin, C.-H.;Idmoumaz, H.; Hersh, W. H. Organometallics 1993, 12, 503-516. (27)Moore et aLZ6derived activation parameters for the conversion of cis-l* to trans-l*. If we assume the same preexponential factor that they obtained (which presupposes a nearly identical mechanism, similar electronic rearrangement during isomerization, and negligible solvent effects),we find that the activation energy for cis-7* trans7* is about 100 kJ mol-', ca. 30 kJ mol-' greater than the activation energy for cis-l* trans-l*.
-
-
A second interesting aspect of the formation of cis-7* concerns the relative amounts of cis- and trans-7* produced during the reaction of 8* with CO. It is apparent in Figure 5 that the 1967 cm-' band for cis7* is larger than the band at 1915 cm-l, which is exclusively or predominantly due to trans-7*. Previous results on 1 would lead us to expect that the 1967 cm-l band is intrinsically more intense than the 1915 cm-l band. Cotton et al. have estimated the relative intensities of the corresponding bands in 1, namely the symmetric (AI) mode of cis-1 at 1998 cm-l and the antisymmetric (B,) mode of trans-1 at 1954 cm-l.15g They found a n intensity ratio A1:BU= 3.80:3.04= 1.25. If we assume this intensity ratio for the bands of cisand trans-7*, the data presented in Figure 5 indicate that the reaction of 8* with CO leads t o initial preferential formation of the cis isomer over the trans isomer (eq 6). For example, the ratio of the integrated intensi-
8*
+ CO - cis-7* (major product) + trans-7*
(minor product) ( 6 )
ties of the 1967 and 1915 cm-l bands in Figure 5b is 1.6, which, using the assumed ratio of absolute intensities, implies that ca. 55-60% of the product is the cis isomer.28 At the warmer temperatures for Figure 5c,d, at which cidtrans isomerization may begin to occur at a significant rate, the percentage of cis isomer decreases t o ca. 50% and 45%, respectively. The detailed mechanism for the addition of CO to 8* is not known. Nevertheless, we can propose a simple scheme that rationalizes the apparent preferential formation of cis-7* (Scheme 2 ) . We shall assume the following. (i) CO adds directly to an Fe atom of 8* along the bisectors of the dihedral angles formed by the three bridging ligands in 8*. Under this assumption, there are two exo (one for each Fe atom) and four endo sites of attack with respect to the pCH2 ligand. (ii) The incoming CO (denoted C in Scheme 2) ultimately becomes one of the terminal CO ligands of 7*. One of the bridging CO ligands of 8* (denoted A or B) becomes the other terminal CO ligand of 7*, while the other one remains bridging. (iii) The "unbridging" of CO A or B in 8* occurs via a "least-motion" rearrangement in which the unbridged CO ultimately ends up on the side of the plane defined by the other two bridging ligands that requires the lesser atomic motion. Under these assumptions, exo attack by carbonyl C leads exclusively to the formation of cis-7*; this direction of attack preserves the mirror symmetry of the system, and thus it makes no difference whether carbonyl A or B undergoes bridging-to-terminal conversion. If carbonyl C attacks along an endo direction, the mirror symmetry of the system is broken, and we predict that different isomers will be formed, depending on which of the bridging carbonyls moves to a terminal position. If the bridging CO that is nearer t o the incoming ligand C is the one that unbridges, cis-7* will be formed, whereas unbridging of the other bridging CO will lead to formation of trans-7*. If the site of attack (ex0 or endo) and (28)Because cis-7* isomerizes to trans-7*, in principle it is possible that the trans-7* observed in Figure 5b is due to isomerization rather than the direct reaction of 8* with CO. However, because the isomerization process is slow even at room temperature (vide supra), it is more likely that both cis- and trans-7* are formed directly according to eq 6 .
Photochemistry of trans-[Cp*Fe(CO)ldp-CO)(p-CH2)
Scheme 2. Simplified Scheme for the Addition of CO to 8* To Form a Mixture of cis- and tmns-7* (See Text)= H
H
'C'
n
endo
-6 A
endo
+ ex0
oxo attack: A unbridger
Organometallics, Vol. 14, No. 11, 1995 5257 chose to try the same experiments with the Cp analogs 7 and 7'. Two factors contributed to making these additional studies less successful than we had hoped. First, as noted earlier, the presence of multiple stereoisomers in 7 and 7' greatly complicates the IR spectra of the starting materials and the photoproducts. Second, the Cp derivatives are markedly less soluble in the matrix solvents than is the Cp* derivative. Because of this lower solubility, we were not able to achieve the same concentrations of starting materials in the frozen matrix without significant spectral broadening, possibly due to aggregation or precipitation at low temperatures. Irradiation of the matrix of 7 for 10 min leads to the evolution of free CO (detected in the IR spectrum at 2132 cm-l) and new IR peaks at 1837 and 1800 cm-l. These latter bands are consistent with the formation of the CO-loss photoproduct Cp2Fe2(p-CO)2(p-CH2)(8);the approximately 35 cm-l blue shift in the bridging-CO IR bands of 8 relative to those of 8* is reasonable upon the replacement of electron-rich Cp* ligands with Cp ligand^.^^^^^ Additional bands at 1990 and 1734 cm-l are also observed, and these appear simultaneously a t rates similar t o those due t o 8. These bands, which are not necessarily from the same product, do not correlate with any of those observed in the photochemistry of 7*. Continued irradiation of the matrix of 7 leads to the appearance of multiple complex peaks, and we have been unable to assign any of these definitively t o the Cp-containing analog of 9. The matrix photochemistry of the ethylidene-bridged complex 7' in frozen 3-MP is similar t o that of 7 and is similar to previous matrix studies of 7' in a 90% MCH, 10%3-MP matrix.18 The spectrum of the frozen matrix shows severe broadening and shifting relative to the room-temperature spectrum. Upon irradiation, free CO is produced, as is the triply bridged CO-loss product Cp2Fe2@-CO)&-CHMe) ( 8 ) ,which exhibits CO-stretching modes at 1833 and 1795 cm-l. As was the case for 7 , we were unable to find definitive evidence for the formation of a double-CO-loss product analogous t o 9.
-IIXYP* cis * 7'
B unbridger
cp
k"
-
cis 7'
endo attack: A unbridger
-11
-
trans 7'
B unbridges cis .r
a The labels A-C denote carbonyl ligands. It is assumed that attack of carbonyl C causes either A or B to undergo bridgeto-terminal conversion and that this conversion is a leastmotion process. According t o the scheme, attack exo to the pCH2 ligand produces only cis-7*, whereas attack in one of the endo positions will lead to an equal mixture of cis- and truns-7*. Because there are twice as many endo sites as exo sites, the scheme predicts a 2:l ratio of cis-7*:trans-7* produced.
the choice of which CO undergoes bridging-to-terminal conversion (A or B) are both statistical, the scheme predicts that the product ratio will be cis-7*:truns-7*= 2:l. Obviously, this simplified scheme neglects any energetic differences between different sites of attack, nor does it account for the relative rates of addition of CO to 8* and cis/trans isomerization of 7*, but the statistical treatment does lead t o preferential formation of cis-7*, as observed. Our proposal of the kinetic formation of a thermodynamically unfavorable cis isomer is similar to the situation observed in the fast solution photochemistry of trans-l* by Moore, Poliakoff, and Turner.25 These authors found that photogenerated 'FeCp*(CO)z radicals could recombine t o form cis-l* as well as trans-1*, with the cis isomer rapidly reverting t o the trans isomer in room-temperature solution. It is interesting to note that, if the scheme outlined in Scheme 2 is valid, the addition of CO to 2* t o form 1* would lead to 2:l preferential formation of cis-l* over trans-l*. Thus, it strikes us that some of the cis-l* observed by these authors might result from CO addition to 2*, in addition to that formed by radical recombination. Matrix Photochemistry of 7 and 7'. Given the photochemistry exhibited by the Cp* derivative 7*, we
Conclusion Each of the unsymmetric methylene-bridged diiron complexes 7 , 7',and 7* exhibits photochemical CO loss in frozen 3-MP t o form a triply bridged intermediate ( € 4 8 ,or 8*) that is analogous to the CO-loss photoproducts from the Fp2 and Fp*2 systems 1 and l*. The Cp* derivative 7* exhibits loss of a second CO to generate a double-CO-loss product that we propose has the identity Cp*2Fe2(p-q1:q2-CO)(p-CH2) The strong bridging preference of the methylene bridge in 7* has therefore led to a double-CO-loss photoproduct that is decidedly different from that obtained from 1 or l*. These studies of photochemical response to structural modification are ongoing.
I
Acknowledgment. We gratefully acknowledge the National Science Foundation (Grant CHE-9208703)for support of this research. We also thank Dr. Frank Kvietok for assistance and helpful discussions. OM9505064 (29) Vitale, M.; Lee, K. IC;Hemann, C. F.; Hille, R.; Gustafson, T. L.; Bursten, B. E. J.Am. Chem. SOC.1996,117, 2286-2296.
