Reactions of Iron Atoms with Benzene - The Journal of Physical

J. Phys. Chem. A 2010, 114, 1657–1664

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Reactions of Iron Atoms with Benzene Stewart F. Parker* ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, United Kingdom ReceiVed: June 25, 2009; ReVised Manuscript ReceiVed: December 7, 2009

The reaction of iron atoms with benzene has been studied for nearly 40 years. Despite this, there is no agreement as to the nature of the products. With the aid of density functional theory calculations of the energetics and the infrared spectra of the various species, the present work provides a rationalization of the conflicting reports regarding the nature of the products of the reaction of iron atoms with benzene in low-temperature matrices. At low temperature in dilute benzene matrices, Fe(η6-C6H6) and Fe(η6-C6H6)2 are the major products. At high iron concentrations, Fe2(η2-C6H6) is also formed. In pure benzene at low temperature, Fe(η6-C6H6)2 and Fe(η6-C6H6)(η4-C6H6) are formed. None of the species undergo photoexcitation to give insertion products HFe(C6H5). In pure benzene at 77 K, Fe(η6-C6H6)(η4-C6H6) is the major product, together with small amounts of Fe(η6-C6H6)2 and iron clusters. The infrared spectra of pure benzene are complicated by the activation of infrared forbidden modes by the presence of the metal atom. I. Introduction The reaction of iron atoms with benzene in cryogenic matrices has been studied for nearly 40 years.1-8 Despite this, there is no agreement as to the nature of the products. At low iron and benzene concentrations, the product is believed2,4,6,8 to be Fe(η6C6H6) (1). (See Figure 1 for the structures.) In concentrated matrices or in pure benzene, the situation is more complicated. There is evidence from UV-vis5 and Mo¨ssbauer6 spectroscopies of the presence of at least two species being present. Furthermore, the ratio of the complexes depends on the matrix deposition temperature: at 7 K, it is ∼1:1, and at 77 K, it is ∼1:3. There is no evidence of the two species interconverting.6 However, an infrared spectroscopic study detected only a single species.7 Proposed assignments of the species have included the 20 electron Fe(η6-C6H6)2 (4) complex and an 18 electron Fe(η6C6H6)(η4-C6H6) (5) complex. In this article, with the aid of density functional theory (DFT) calculations of the energetics and simulation of the infrared spectra of the various species, I show how the various scenarios can be rationalized into a unified picture. II. Experimental Section DFT calculations were carried out using GAUSSIAN039 and DMOL3.10 For the GAUSSIAN03 calculations, the MPW1PW91 functional was used with the 6-31G+(d,p) basis set. For the DMOL3 calculations, the BLYP functional with the DNP basis set was used. This is a double numerical precision grid that includes polarization functions on all atoms including hydrogen. In all cases, a geometry optimization was followed by a frequency calculation. The output from both programs contains the vibrational modes in the harmonic approximation, the atomic displacements for each mode, the calculated infrared intensity of each mode, and the zero-point energy (needed for the total energy calculation). The atomic displacements enable visualization of the modes that aid assignments; they are also all that is required to generate the inelastic neutron scattering11 (INS) spectrum using the program ACLIMAX.12 The INS spectrum * Corresponding author. Tel: +44 (0)1235 446182. Fax: +44 (0)1235 445720. E-mail: [email protected].

of Cr(η6-C6H6)6 (Aldrich, 97%, used as received) and benzene was recorded using TOSCA13 and MAPS,11,14 respectively, at ISIS14 (Chilton, U.K.) and the infrared spectrum with a Bruker Vertex70 Fourier transform infrared spectrometer (256 scans at 4 cm-1 resolution) with the sample loaded onto a Pike single reflection diamond attenuated total internal reflection accessory. III. Results A. Validation of Methods. To check that the methods used gave reliable results, benzene and bisbenzene chromium, Cr(η6C6H6)6, were calculated with the same procedures. For Cr(η6C6H6)6 with D6h symmetry, the C-H, C-C, and Cr-C distances are 1.085, 1.415, and 2.138 Å (GAUSSIAN03) and 1.088, 1.425, and 2.201 Å (DMOL3) respectively. These are in very good agreement with the experimental15 gas-phase values of 1.090 ( 0.005, 1.423 ( 0.002, and 2.150 ( 0.002. They are also in good agreement with recent ab initio calculations: 1.096, 1.418, and 2.146 Å 16 and 1.085, 1.415, and 2.138 Å.17 Comparison of the calculated and experimental inelastic neutron scattering (INS) spectra11 is a stringent test of the calculation because this requires that both the transition energies and amplitudes of motion of the atoms in each mode are calculated correctly. In addition, there are no selection rules, so all modes are observable. Figures 2 and 3 show the comparison of the experimental spectra of Cr(η6-C6H6)6 and as calculated by GAUSSIAN03 and DMOL3 for the INS and infrared spectra, respectively. As expected, the agreement is excellent for the internal modes in terms of both wavenumber and intensity accuracy, and they are very similar to those of the most recent ab initio study.16 Because the results of both methods are for the isolated, that is, gasphase molecule, the external modes below 200 cm-1 are not reproduced by the calculations. It can be seen that scaling the GAUSSIAN03 results by 0.972 is required for good agreement, wheras no scaling is required for the DMOL3 results. On the basis of these observations, all calculated spectra in this article have been scaled by 0.972 for GAUSSIAN03 results and are not scaled for DMOL3 results. B. Fe(C6H6). In agreement with Wang et al.,8 with GAUSSIAN03, the triplet (S ) 3) Fe(η6-C6H6) C6V structure (1) is found to be the lowest energy structure with the quintet (S ) 5) and singlet (S ) 1) structures 0.400 and 1.852 eV higher

10.1021/jp905958b  2010 American Chemical Society Published on Web 01/07/2010

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Figure 1. Structures of the iron complexes considered. Fe(η6-C6H6) (1), Fe(η4-C6H6) (2), Fe(η2-C6H6) (3), Fe(η6-C6H6)2 (4), Fe(η6-C6H6)(η4-C6H6) (5), Fe(η6-C6H6)(η2-C6H6) (6), Fe(η4-C6H6)2 (6a), Fe(η2-C6H6)2 (7), Fe2(η6-C6H6) (8), Fe2(η2-C6H6) (9), Fe2(// C6H6) (10), Fe(η6-C6H6)Fe (11), Fe2(// C6H6)2 (12), (η6-C6H6)Fe2(η6-C6H6) (13), µ2-(η3,η3-C6H6)[(η6-C6H6)Fe]2 (14), HFe(C6H5) (S ) 1) (15), HFe(C6H5) (S ) 3) (16), HFe(C6H5) (S ) 5) (17), and Fe(η6-C6H6)(η4-C6H8) (C6H8 ) 1,4-cyclohexadiene) (18).

in energy. The structure of (1) is in good agreement with that found by Pandey et al.;17 a comparison is given in Table S1 of the Supporting Information. Initial structures of the type Fe(η4-C6H6) (S ) 1,3,5) (2) and Fe(η2-C6H6) (S ) 1,3) (3) either did not converge or converged to Fe(η6-C6H6). However, Fe(η2-C6H6) (S ) 5) converged to the same structure, (3), with a similar energy to that of (1) S ) 5. With DMOL3, a different picture emerges. In this case, Fe(η2C6H6) (S ) 3) (3) is found to be the lowest energy structure with all other starting structures tested either dissociating or converging to (1). (See Table 1.) Figure 4 shows a comparison of the experimental8 infrared spectrum of iron codeposited with a low concentration of benzene in solid Ar at 12 K (Figure 4a) with the GAUSSIAN03 calculated infrared spectrum of (1), S ) 3 (Figure 4b) and the

DMOL3 calculated infrared spectra of (1), S ) 3 (Figure 4c) (the calculated infrared spectrum of (1), S ) 1 is almost identical to that of (1), S ) 3) and (3) S ) 3 (Figure 4d). For (1) S ) 3, the GAUSSIAN03 and DMOL3 calculated infrared spectra are very similar, although the absolute intensities of the GAUSSIAN03 spectrum are about twice as large as those of the DMOL3 spectrum. The experimental conditions favor the formation of the monoiron monobenzene product, and the Figure clearly shows that this has the Fe(η6-C6H6) C6V structure (1), as found by Wang et al.8 Although DMOL3 predicts (3) S ) 3 to be a lower energy state, the calculated spectrum is incompatible with the observed spectrum. A complete list of the transition energies is given in Table S2 of the Supporting Information. C. Fe(C6H6)2. As shown in Table 2, for bisbenzeneiron, a remarkable result is obtained with GAUSSIAN03. Irrespective

Reactions of Iron Atoms with Benzene

Figure 2. Comparison of (a) experimental and ab initio calculated INS spectra of Cr(η6-C6H6)2, (b) GAUSSIAN03 scaled by 0.972, and (c) DMOL3 (no scaling).

Figure 3. Comparison of (a) experimental and ab initio calculated infrared spectra of Cr(η6-C6H6)2, (b) GAUSSIAN03 scaled by 0.972, and (c) DMOL3 (no scaling).

of the starting structure, for S ) 1, Fe(η6-C6H6)(η4-C6H6) (5) is the stable structure, whereas for S ) 3, again irrespective of the starting structure, Fe(η6-C6H6)2 (4) is the stable structure. With DMOL3, a somewhat different picture is found: for S ) 1, 3, 5, (4) is a transition state with multiple imaginary modes. The lowest energy state found is (5) S ) 1, although Fe(η6C6H6)(η2-C6H6) (6) S ) 3 is very close in energy. A third structure, Fe(η2-C6H6)2 (7) S ) 5, is found at higher energy. When this was used as an initial structure for GAUSSIAN03, the same structure was obtained for S ) 5, albeit 2.397 eV above the lowest energy state. (S ) 1 or 3 did not converge.) Figure 5 shows the experimental8 infrared spectrum of iron codeposited with a low concentration of benzene in solid Ar at 12 K after UV-visible photolysis and annealing at 35 K (Figure 5a) together with the GAUSSIAN03 calculated infrared spectra of (4), S ) 3 (Figure 5b) and (5), S ) 1 (Figure 5c). Figure 5d is the experimental2 infrared spectrum of iron codeposited with pure benzene at 77 K. (The bands resulting from the presence of iron are marked with 1, †, or *, and the remainder are observed in solid benzene.) In agreement with the assignments of Ball et al.7 and Wang et al.,8 Figure 5a can be assigned to a mixture of (1), S ) 3 and (4), S ) 3.

J. Phys. Chem. A, Vol. 114, No. 4, 2010 1659 Comparison of Figure 5a and 5d indicates a considerable difference between the species obtained by deposition at low temperature and that from deposition of pure benzene at 77 K. Both spectra are representative of those that have been measured by several different groups: therefore, Figure 5a (reproduced from Wang et al.8) is similar to that obtained by Ball et al.7 and Efner et al.,2 whereas spectra similar to Figure 5d (reproduced from Efner et al.2) have been obtained by Aleksanyan and Kurtikyan3 and Skell et al.4 The band at 730 cm-1 (marked with 1) may be due to (4) S ) 3, as the wavenumber is up shifted in pure benzene;7 however, there are a number of other bands (marked with † or *) that have no correspondence with the calculated spectrum of (4) S ) 1. Approximately half of the new bands in pure benzene (those marked with †) are in reasonable agreement with the calculated spectrum of (5) S ) 1 and are so assigned. The two bands observed at 485 and 540 cm-1 are calculated for (5) at 435 and 507 cm-1 by GAUSSIAN03 and 376 and 486 cm-1 by DMOL3, both of which are in only modest agreement with the experimental values. However, both of these modes involve a partial folding of the deformed η4-C6H6 ring and thus are very sensitive to the details of the interaction. The dihedral angle between the two halves of the ring is 41.3° for the GAUSSIAN03 structure and 38.6° for the DMOL3 structure supporting this suggestion, whereas, for the undeformed η6-C6H6 ring, the torsion (oscillation about the C6 axis of benzene) is calculated at 401 and 420 cm-1 by GAUSSIAN03 and DMOL3, respectively. In contrast with the GAUSSIAN03 results, DMOL3 found that (5) S ) 1 was the lowest energy state found with (6) S ) 3 close by and at higher energy (7) S ) 5 with (4) S ) 1, 3, 5 as transition states. Figure 6 compares the experimental2 spectrum of iron codeposited with pure benzene at 77 K with the DMOL3 calculated spectra of the three stable species. Only (5) S ) 1 shows reasonable agreement in terms of wavenumbers and relative intensities with the bands marked with *. Therefore, both the GAUSSIAN03 and DMOL3 results are consistent with the presence of (5) S ) 1 in pure benzene at 77 K. Structural data (Table S1) and vibrational energies (Table S2) of (4) and (5) are given in the Supporting Information. D. Fe2(C6H6). Ball et al.7 have provided convincing evidence of the presence of Fe2(C6H6) in Ar/C6H6 matrices deposited at 12 K. As shown in Figure 1 and Table 3, a wide range of structures were investigated with GAUSSIAN03 and DMOL3. The problem is complicated by the large number of possible spin states for a di-iron complex. In no case was it possible to obtain a stable structure with GAUSSIAN03; the structure either failed to converge or gave structures with imaginary modes. The “best” result was for Fe2(η2-C6H6) (9) S ) 7 with one imaginary mode with an energy of -30 cm-1. With DMOL3, obtaining stable structures was straightforward. Consistent with the GAUSSIAN03 result, Fe2(η2-C6H6) (9) S ) 7 was the lowest energy structure obtained, although Fe2(// C6H6) (10) S ) 5, 7 also resulted in relatively low-energy structures. What might have been expected to be the stable structure, Fe2(η6-C6H6) (8), the di-iron analogue of (1), was significantly higher in energy. Structures starting from a Cs geometry, that is, where the Fe-Fe bond is not perpendicular to the centroid of the benzene ring, converged to the C6V structure. Figure 7 compares the DMOL3 calculated infrared spectra of (9) S ) 7, (10) S ) 5, and (8) S ) 3 (those of (8) S ) 5, 7 are almost identical to that of (8) S ) 3) with a stick diagram of the measured modes of Fe2(C6H6) in Ar/C6H6 (the complete spectrum was not published). It can be seen that there is excellent agreement with the calculated spectrum of Fe2(η6-

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TABLE 1: Comparison of Structures and Energetics in the Fe - Benzene System initial structure

spin state (S)

converged structure GAUSSIAN03

energy above lowest energy state (eV)

converged structure DMOL3


Fe(η6-C6H6) (1)

1 3 5 1 3 5 1 3 5

Fe(η6-C6H6) (1) Fe(η6-C6H6) (1) Fe(η6-C6H6) (1) Fe(η6-C6H6) (1) Fe(η6-C6H6) (1) unable to converge Fe(η6-C6H6) (1) Fe(η6-C6H6) (1) Fe(η2-C6H6) (3)

1.852 GSa 0.400 1.852 GS

Fe(η6-C6H6) (1) Fe(η6-C6H6) (1) dissociates Fe(η6-C6H6) (1) Fe(η6-C6H6) (1) dissociates dissociates Fe(η2-C6H6) (3) dissociates

0.607 0.615

Fe(η4-C6H6) (2) Fe(η2-C6H6) (3)

a

1.852 GS 0.419

0.607 0.615 GS

GS ) lowest energy (ground) state, TS ) transition state; the number of imaginary modes is given in parentheses.

at 300-400 cm-1. Structural data (Table S3) and vibrational energies (Table S4) of (8) and (14) are given in the Supporting Information. F. Insertion Reactions of Fe(C6H6). One of the features of matrix-isolated metal atoms is the readiness with which they will insert into X-H (X ) C, N, O, F) bonds on photoexcitation, for example.19-22 In contrast, iron does not7,8 photoinsert into a C-H bond of benzene, even though it readily inserts into the less-reactive C-H bond of methane.19,20 Consideration of possible insertion products shows that stable products are formed with S ) 1, 3, 5 with the structures (15), (16), and (17), respectively. The structures are 2.848 (S ) 1), 1.303 (S ) 3), and 0.232 eV (S ) 5) higher in energy than (1) S ) 3. IV. Discussion Figure 4. Comparison of (a) experimental2,8 infrared spectrum of iron codeposited with a low concentration of benzene in solid Ar at 12 K and ab initio calculated infrared spectra of Fe monobenzene complexes; (b) Fe(η6-C6H6) (1), S ) 3, GAUSSIAN03, scaled by 0.972; (c) Fe(η6C6H6) (1), S ) 3, DMOL3 (no scaling); and (d) Fe(η2-C6H6) (3), S ) 3, DMOL3 (no scaling). Experimental spectra reproduced from refs 2 and 8 with the permission of Elsevier.

C6H6) S ) 3 (8), suggesting that this is the experimental structure, even though it is not the lowest energy structure found. E. Fe2(C6H6)x. In pure benzene, Mo¨ssbauer6 spectroscopy indicates that ∼10% of the iron is present as clusters on condensation at 7 K and ∼20% on condensation at 77 K. UV-vis5 spectroscopy also suggests the presence of Fe clusters interacting with benzene. However, because the only infrared spectra of pure benzene matrices are for low iron concentrations, only di-iron-containing structures were considered: Fe2(// C6H6)2 (12),18 (η6-C6H6)Fe2(η6-C6H6) (13), and µ2-(η3,η3-C6H6)[(η6C6H6)Fe]2 (14). With both GAUSSIAN03 and DMOL3, for all multiplicities tested (S ) 5, 7, 9), (12) resulted in structures with large imaginary modes. A variant of (12) where the rings were not constrained to be parallel behaved similarly, indicating that this is not a viable structure of iron. For (13), the structure calculated with GAUSSIAN03 for (S ) 7) had one imaginary mode, the magnitude of which was only 4 cm-1, suggesting that this species may exist. The imaginary mode is the antiphase rotation of the rings about the axis containing the Fe-Fe bond, which suggested that a staggered structure might be more stable; however, the derived C6V structure had the same imaginary mode but with a magnitude of 34 cm-1. For (S ) 1), (14) is an 18 electron complex and both GAUSSIAN03 and DMOL3 find that it is stable with no imaginary modes. Figure 8 compares the calculated infrared spectra with that of iron in a pure benzene matrix. There is no evidence of the presence of (13); however, it is possible that (14) may be present, as indicated by the bands

Infrared spectra of the products of the reaction of iron atoms and dimers with benzene in cryogenic matrices have been recorded by several groups2-4,7,8 over a period of almost 40 years. The spectra show a high degree of consistency with each other, showing that the same products were obtained in each case. In the earliest work, Efner et al.2 assigned the product of the reaction of iron and benzene solely to Fe(η6-C6H6) (1). Subsequent work5-8 and the analysis presented here shows that this is not correct and other products are present. The basis of their assignment was the behavior of the band at 366 cm-1; in C6D6, this shifted to 349 cm-1; in a mixed Ar/C6H6/C6D6 matrix, only the same two bands were observed. The expectation was that if two or more coordinated benzene molecules were present, then additional bands due to the mixture of isotopomers should be present. However, inspection of Figures 4 and 5 shows that only (1) has a mode with significant intensity in this region. Therefore, whereas the assignment to a 1:1 Fe/C6H6 complex is correct, it is the assumption that this was the only product that is incorrect. As an aside, the technique of using a mixture of C6H6 and C6D6 would not yield the expected result of a triplet of bands for Fe(η6-C6H6)2 (4). This is because the intense feature at 250 cm-1 is the asymmetric Fe-ring stretch; in essence, the iron atom oscillates between the two rings, and thus there is very little involvement of the benzene rings in the motion. The isotopic shift is only 5 cm-1 for (4)-D12 and 3 cm-1 for (4)-D6, which would be difficult to observe. The most significant result in this article is the demonstration that S ) 1 Fe(η6-C6H6)(η4-C6H6) (5) is present in pure benzene matrices. GAUSSIAN03 indicates that it is almost 1 eV higher in energy than S ) 3 Fe(η6-C6H6)2 (4), so its formation is surprising. Note that the relative energies are for the isolated gas-phase species; presumably, the matrix stabilizes (5) relative to (4). (5) has been detected only in pure benzene and is preferentially formed at higher temperatures. At 10 K, the ratio


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TABLE 2: Comparison of Structures and Energetics in the Fe-(benzene)2 System initial structure

spin state (S)





1 3 5 1 3 5 1 3 5

Fe(η6-C6H6)(η4-C6H6) (5) Fe(η6-C6H6)2 (4) Fe(η6-C6H6)2 (4) Fe(η6-C6H6)(η4-C6H6) (5) Fe(η6-C6H6)2 (4) ∼Fe(η6-C6H6)2 (4) unable to converge unable to converge Fe(η2-C6H6)2 (7)

0.802 GS unable to converge 0.802 GS TS (2)

Fe(η6-C6H6)2 (4) Fe(η6-C6H6)2 (4) Fe(η6-C6H6)2 (4) Fe(η6-C6H6)(η4-C6H6) (5) Fe(η6-C6H6)(η2-C6H6) (6) Fe(η2-C6H6)2 (7) Fe(η4-C6H6)2 (6a) Fe(η4-C6H6)2 (6a) Fe(η2-C6H6)2 (7)

TS (3)a TS (4) TS (3) GS 0.087 1.324 TS (1) TS (1) 1.324

Fe(η6-C6H6)2 (4) Fe(η6-C6H6)(η4-C6H6) (5) Fe(η2-C6H6)2 (7)

a

2.397


Figure 5. Comparison of (a) experimental8 infrared spectrum of iron codeposited with a low concentration of benzene in solid Ar at 12 K after UV-visible photolysis and annealing at 35 K and ab initio calculated infrared spectra (GAUSSIAN03, scaled by 0.972) of (b) Fe(η6-C6H6)2 (4), S ) 3 and (c) Fe(η6-C6H6)(η4-C6H6) (5), S ) 1. (d) Experimental2 infrared spectrum of iron codeposited with pure benzene at 77 K (the bands resulting from the presence of iron are marked with 1 ) (4), † ) (5), or * ) perturbed benzene (see text); the remainder are observed in pure solid benzene). Experimental spectra reproduced from refs 2 and 8 with the permission of Elsevier.

of (4):(5) is ∼1:1, however, the strongest band of (5) at 716 cm-1 is less than half as intense as that of (4) at 707 cm-1, 51 versus 127 km mol-1, making (5) difficult to detect in the presence of (4). At 77 K, the ratio of (4):(5) is ∼1:3, simplifying detection. Argon freezes at 74 K, whereas benzene freezes at 278 K, and hence the heat of sublimation of benzene is considerable

Figure 6. Comparison of (a) experimental2 infrared spectrum of iron codeposited with pure benzene at 77 K (the bands resulting from the presence of iron are marked with 1 ) (4), † ) (5), or * ) perturbed benzene, (see text); the remainder are observed in pure solid benzene) and ab initio calculated infrared spectra (DMOL3, no scaling) of: (b) Fe(η6-C6H6)(η4-C6H6) (5), S ) 1; (c) Fe(η6-C6H6)(η2-C6H6) (6) S ) 3; and (d) Fe(η2-C6H6)2 (7), S ) 5. All calculated spectra are on the same ordinate scale. The experimental spectrum was reproduced from ref 2 with the permission of Elsevier.

∼50 kJ mol-1, and this may provide the necessary activation energy for the formation of (5). There must also be a very large barrier between (4) and (5) because there is no evidence6-8 of the two species interconverting either thermally or photochemically. Iron is known to form a large number of complexes of the type Fe(η6-arene)(η4-diene)23-25 (of which (5) is an unusual example although the corresponding Ru complex,26 S ) 1 Ru(η6C6H6)(η4-C6H6), is known and stable at room temperature). One such complex is Fe(η6-C6H6)(η4-C6H8)27 (17) (C6H8 ) 1,4-

TABLE 3: Comparison of Structures and Energetics in the Fe2-(benzene)x (x ) 1 or 2) System initial structure Fe2(η6-C6H6) (8)

>Fe2(η2-C6H6) (9) Fe2(// C6H6) (10) Fe(η6-C6H6)Fe (11)

a

spin state (S) 1 3 5 7 9 5 7 9 5 7 9 1 5 7



Fe2(η6-C6H6) (8)

TS (2)

Fe2(η6-C6H6) (8)

TS (2)

Fe2(η2-C6H6) (9)

TS (1)

Fe2(// C6H6) (10)

TS (1)

Fe(η6-C6H6)Fe (11) Fe(η6-C6H6)Fe (11) Fe(η6-C6H6)Fe (11)

TS (4) TS (2) TS (2)



Fe2(η6-C6H6) (8) Fe2(η6-C6H6) (8) Fe2(η6-C6H6) (8) Fe2(η6-C6H6) (8) Fe2(η6-C6H6) (8) Fe2(η6-C6H6) (8) Fe2(η2-C6H6) (9) Fe2(η2-C6H6) (9) Fe2(// C6H6) (10) Fe2(// C6H6) (10) Fe2(// C6H6) (10) Fe(η6-C6H6)Fe (11) Fe(η6-C6H6)Fe (11) Fe(η6-C6H6)Fe (11)

TS (1)a 1.208 1.402 1.468 TS (2) 1.402 GS TS (2) 0.299 0.334 TS (1) TS (2) 3.317 TS (2)


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Figure 7. DMOL3 calculated infrared spectra of (a) (9) S ) 7, (b) (10) S ) 5, (c) (8) S ) 3, and (d) a stick diagram of the measured7 modes of Fe2(C6H6) in Ar/C6H6; the relative intensities of the modes are indicated, s ) strong, w ) weak.

Figure 9. Comparison of (a) the experimental2 infrared spectrum of iron codeposited with pure benzene at 77 K (the bands resulting from the presence of iron are marked with 1 ) (4), † ) (5), or * ) perturbed benzene (see text); the remainder are observed in pure solid benzene), (b) the INS spectrum of solid benzene, (c) the mode energies30 for the gas-phase molecule (the doubly degenerate modes are indicated by a double length line), (d) GAUSSIAN03 calculated infrared spectra of Fe(η6-C6H6)(η4-C6H8) (17) (C6H8 ) 1,4-cyclohexadiene), and (e) the experimental line positions27 of (17). The experimental spectrum was reproduced from ref 2 with permission of Elsevier.

Figure 8. Comparison of (a) the experimental2 infrared spectrum of iron codeposited with pure benzene at 77 K (the bands resulting from the presence of iron are marked with 1 ) (4), † ) (5), or * ) perturbed benzene (see text); the remainder are observed in pure solid benzene) and the GAUSSIAN03 calculated infrared spectra of (b) µ2-(η3,η3C6H6)[(η6-C6H6)Fe]2 (14) and (c) (η6-C6H6)Fe2(η6-C6H6) (13). Part b is ×2 ordinate expanded relative to part c. Experimental spectrum reproduced from ref 2 with the permission of Elsevier.

cyclohexadiene). Figure 9 compares the GAUSSIAN03 calculated infrared spectrum (9d) and the experimental line positions,27 (9e), of (17) with the spectrum of iron in pure benzene, (9a). As might be expected from the similarity of the structures of (5) and (17), the wavenumber positions are similar (see Figure 5c for the spectrum of (5)); however, the intensity distribution is very different and does not match that observed for iron in pure benzene. It is concluded that (17) is not present, consistent with UV-vis5 and Mo¨ssbauer6 studies. Structural data (Table S1) and vibrational energies (Table S2) of (17) are given in the Supporting Information. For S ) 3, Fe(η6-C6H6)2 (4), the structure is very similar to those found by Wang et al.8 and Pandey et al.17 (See Table S1 in the Supporting Information). In (4) the benzene ligands are largely undistorted from the free molecule, as is the η6coordinated ligand in S ) 1 Fe(η6-C6H6)(η4-C6H6) (5). In contrast, the η4-coordinated ligand in (5) is strongly distorted.

The uncomplexed C-C bond has shortened to 1.342 Å (GAUSSIAN03)/1.353 Å (DMOL), comparable to a conventional CdC, whereas the complexed C-C bonds have lengthened somewhat. (See Table S1 in the Supporting Information.) The dihedral angle in the η4-coordinated ligand, 40.9° (GAUSSIAN03)/38.8° Å (DMOL), is remarkably similar to that calculated for the cyclohexadiene ligand in Fe(η6-C6H6)(η4-C6H8) (C6H8 ) 1,4-cyclohexadiene) (18). Both of these calculated values are very close to that observed by X-ray crystallography in the isoelectronic Ru(η6-HMB)(η4-HMB)28 (HMB ) hexamethylbenzene), where the dihedral angle in the η4-HMB is 42.8°. The presence of (5) is able to account for about half of the new bands observed when iron is codeposited with pure benzene at 77 K (See Figure 5.) One possible assignment for the remaining features, those marked with *, is that they are due to clusters, Fex(C6H6)y, with x ) 2 and y g 2 as the most likely candidates. Zhang and Wang18 were able to generate a large variety of stable structures for dicobalt and tricobalt interacting with benzene using DMOL3. In the cobalt study, the lowest energy species had structure (12), so the iron analogue was investigated to see if this was also true for iron. Zhang and Wang18 also found that the symmetric structure (13) was stable but was 1.041 eV higher in energy than the lowest energy structure. However, iron appears to have a marked preference for symmetrical bonding to benzene, as shown by (8) rather than the lower energy (9) being the observed Fe2(C6H6) species, so (13) was also investigated. (14) would appear to be an unlikely choice; however, the complex with toluene29 in place of benzene is known and is stable. (14) also has the advantage that each iron atom in the complex is an 18 electron species. Figure 8 compares the experimental spectrum with those calculated for (13) and (14). The presence of (13) is very difficult to judge because the most intense mode at 251 cm-1 (in which the Fe2 oscillates between the rings) is at the edge of the data


J. Phys. Chem. A, Vol. 114, No. 4, 2010 1663

TABLE 4: Assignment of the Infrared Spectra of Solid Benzene Co-Condensed with Iron Vapour GAUSS

DMOL -1

Efner et al.2

Aleksanyan et al.3

Shobert et al.4

assignment

609 w 725 m

370 395 485 540 ∼600 715

Fe(η6-C6H6)(η4-C6H6) Fe(η6-C6H6)(η4-C6H6) Fe(η6-C6H6)(η4-C6H6) Fe(η6-C6H6)(η4-C6H6) C6H6 ν6 E2g (IR forbidden) Fe(η6-C6H6)(η4-C6H6) and Fe(η6-C6H6)2

785 855

780 m 859 w

775 855

968 1175

930 w 968 m 1175 m

1435

1438 m

370 485

position/cm (intensity/km mol-1)

position/cm-1 (intensity/km mol-1)

353 387 434 508

(9.50) (4.49) (41.52) (37.62)

371 402 377 486

(4.54) (2.55) (22.04) (26.83)

717, 708

(51.08) (127.30)

683

(38.50)

Fe(η6-C6H6)(η4-C6H6) C6H6 ν10 E1g (IR forbidden)

785

(25.30)

763

(44.26)

965 1175

Fe(η6-C6H6)(η4-C6H6) Fe(η6-C6H6)(η4-C6H6) C6H6 ν9 E2g (IR forbidden)

958 986

(17.58) (8.36)

932 960

(21.63) (12.44)

1373 1435

Fe(η6-C6H6)(η4-C6H6) Fe(η6-C6H6)(η4-C6H6) and Fe(η6-C6H6)2

1363 1436 1437 1443 1443

(7.80) (3.48) (2.68) (7.39) (7.34)

1355 1431 1448

(6.67) (2.09) (2.12)

available, so a small error in the mode energy would make it unobservable. The next strongest mode at 958 cm-1 unfortunately occurs in a spectral region that is completely obscured by the intense in-plane bending modes of the uncomplexed benzene. The 958 cm-1 mode is unusual in that it is the antiphase ring breathing mode of the two rings, which is infrared forbidden in the parent molecule. On the basis of the structured envelope at 300-400 cm-1, Figure 8c suggests that (14) may be present, although (5) also makes some contribution at this energy. Therefore, whereas iron-benzene clusters are undoubtedly present, those investigated do not match the observed bands very well. If all of the new bands were assigned to clusters, then this would require a large proportion of the iron to be present as clusters, and this does not fit with the other spectroscopic evidence. Complete lists of the vibrational energies of (4), (5), (14), and (18) are given in Tables S2 and S4. An alternative assignment was suggested by Aleksanyan and Kurtikyan.3 They noted that the bands fall at positions corresponding to infrared forbidden modes of benzene and concluded that they were activated by defects caused by the presence of iron. If this were the case, then the bands should appear at the

same positions irrespective of the metal. They note that this is observed for codeposition of benzene with Cr, Mn, Fe, Co, Ni, or Pd (although no spectra are shown). Inspection of Figure 1 of ref 2 confirms that this is the case for codeposition of benzene with Fe, Co, or Ni. This idea finds further support in Figure 9 where the infrared spectrum of an iron containing benzene matrix (Figure 9a) is compared with the INS spectrum of pure solid benzene (Figure 9b) (the gas-phase energies30 are indicated by the stick diagram (Figure 9c).). The absence11 of any selection rules in INS spectroscopy means that all modes are allowed. It can be seen that the unassigned modes (those marked with *) have counterparts in the INS spectrum, whereas those assigned to (4) and (5) (1 and †) do not. Table 4 gives a complete assignment of all new bands in the infrared spectrum of solid benzene introduced by the presence of iron atoms. Kafafi et al.31 have shown that the precursor for the photoinsertion of iron atoms into a C-H bond of methane to give HFeCH3 is an Fe · · · H-CH3 complex. GAUSSIAN03 calculations of the energies of Fe · · · H-CH3 and HFeCH3 show that the latter is a lower energy state for S ) 1, 3, 5 (with S ) 5 the lowest energy state). Calculations for an analogous

Figure 10. Schematic of the products of the codeposition of iron atoms with benzene in low-temperature matrices. The products in the red and magenta areas are found at∼12 K in dilute (C6H6/Ar 1:100 or less) matrices, and the products in the magenta and the green areas are found in pure benzene matrices.

1664


Fe · · · H-C6H5 complex result in dissociation. It is possible that even though the photoinsertion product is stable, the absence of a suitable precursor prevents its formation on irradiation. Whereas the use of ab initio methods to assign matrix infrared spectra is almost de rigueur now, one of the novel features of this work is the use of two different computational methods to determine relative energies and assign the infrared spectra. This was originally prompted by the inability of GAUSSIAN03 to locate a stable structure for Fe2(C6H6), despite the convincing evidence7 of its existence. As a check of the methods used, Fe2CO was calculated with GAUSSIAN03, and the results are in good agreement with those obtained by Tremblay et al.32 In contrast, DMOL3 was readily able to locate a variety of structures; however, it does not seem to predict the relative energies accurately. The structure that gives the best agreement with the experimental spectrum of Fe2(C6H6) is 1.2 eV above the lowest energy structure found. A similar situation was found for Fe(C6H6). It is encouraging to note that when the same structure is obtained by both methods, the predicted infrared spectra are in good agreement in terms of wavenumber positions and relative intensities. The absolute intensities of the DMOL3 spectra are usually about half those of the GAUSSIAN03 spectra. The most pleasing result with GAUSSIAN03 was the demonstration that for Fe(C6H6)2, (5) was the stable state for S ) 1 and (4) was the stable state for S ) 3. Unfortunately, it was not possible to reproduce this result using DMOL3 using a range of basis sets and functionals because structures of the type of (4) invariably had imaginary modes. This suggests that the relative energies of the two species are very sensitive to the method used. Therefore, whereas GAUSSIAN03 indicates that (5), S ) 1 is nearly 1 eV higher in energy than (4), S ) 3, DMOL3 indicates that (5), S ) 1 is the lower energy structure. This sensitivity indicates that the energies of the two structures are not dissimilar rationalizing the existence of both forms. V. Conclusions The present work provides a rationalization of the conflicting reports regarding the nature of the products of the reaction of iron atoms with benzene in low-temperature matrices. The results are summarized in Figure 10. At low temperature in dilute benzene matrices, (1) and (4) are the major products. Either thermally or photochemically, (1) reacts with benzene to give (4). At high iron concentrations, (8) is also formed. In pure benzene at low temperature, (4) and (5) are formed. The two species do not interconvert, and there is no evidence of (1) reacting to give (5). None of the species undergo photoexcitation to give (14), (15), or (16) or their products coordinated with benzene. In pure benzene at 77 K, (5) is the major product, together with small amounts of (4) and iron clusters. The infrared spectra of pure benzene are complicated by the activation of forbidden infrared modes induced by defects and reduction of the translational symmetry caused by the presence of the iron atom. It is clear from this work that the use of more than one theoretical method is beneficial and is highly recommended for future work. The two methods employed here, GAUSSIAN03 and DMOL3, seem able to explore different, although partially overlapping regions of the parameter space. Both methods produce reliable infrared spectra, with DMOL3 having the advantage that no scaling is necessary. GAUSSIAN03 has the advantage that the relative energies are more reliable in that the lowest energy states found are those that are observed experimentally.

Parker Acknowledgment. Computing resources (time on the SCARF computer used to perform the GAUSSIAN03 calculations) were provided by STFC’s e-Science facility. We thank The Rutherford Appleton Laboratory for access to neutron beam facilities. Supporting Information Available: Structural data and a complete list of vibrational energies of (1), (4), (5), (8), (14), and (18). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Timms, P. L. J. Chem. Soc., Chem Commun. 1969, 1033. (2) Efner, H. F.; Tevault, D. E.; Fox, W.;B.; Smardzewski, R. R. J. Organomet. Chem. 1978, 146, 45. (3) Aleksanyan, V. T.; Kurtikyan, T. S. Koord. Khim. 1977, 3, 1548. (English translation in: Russ. J. Coord. Chem. 1977, 3, 1209. (4) Shobert, A. L.; Hisatsume, L. C.; Skell, P. S. Spectrochim. Acta A 1984, 40, 609. (5) Morand, P. D.; Francis, C. G. Organometallics 1985, 4, 1653. (6) Parker, S. F.; Peden, C. H. F. J. Organomet. Chem. 1984, 272, 411. (7) Ball, D. W.; Kafafi, Z. H.; Hauge, R. H.; Margrave, J. L. J. Am. Chem. Soc. 1986, 108, 6621. (8) Wang, Y.; Szczepanski, J.; Vala, M. Chem. Phys. 2007, 342, 107. (9) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.05; Gaussian, Inc.: Wallingford, CT, 2004. (10) Delley, B. J. Chem. Phys. 2000, 113, 7756. (11) Mitchell, P. C. H.; Parker, S. F.; Ramirez-Cuesta A. J.; Tomkinson, J. Vibrational Spectroscopy with Neutrons: With Applications in Chemistry, Biology, Materials Science, and Catalysis; World Scientific: Singapore, 2005. (12) Ramirez-Cuesta, A. J. Comput. Phys. Commun. 2004, 157, 226. (13) Colognesi, D.; Celli, M.; Cilloco, F.; Newport, R. J.; Parker, S. F.; Rossi-Albertini, V.; Sacchetti, F.; Tomkinson, J.; Zoppi, M. Appl. Phys. A: Mater. Sci. Process. 2002, 74, S64. (14) ISIS Home Page. www.isis.rl.ac.uk. (15) Haaland, A. Acta Chem. Scand. 1965, 19, 41. (16) Be´rces, A.; Zeigler, T. J. Phys. Chem. 1994, 98, 13233. (17) Pandey, R.; Bijan, K.; Rao, B. K.; Jena, P.; Blanco, M. A. J. Am. Chem. Soc. 2001, 123, 3799. (18) Zhang, X.; Wang, J. J. Phys. Chem. A 2008, 112, 296. (19) Ozin, G. A.; McCaffrey, J. G. J. Am. Chem. Soc. 1982, 104, 7351. (20) Billups, W. E.; Margrave, J. L.; Konarski, M. M.; Hague, R. H. J. Am. Chem. Soc. 1982, 104, 7393. (21) Parnis, J. M.; Ozin, G. A. J. Phys. Chem. 1989, 93, 1204. (22) Cho, H. G.; Andrews, L. J. Phys. Chem. A 2008, 112, 12293. (23) Beard Jr., L. K.; Silvon, M. P.; Skell, P. S. J. Organomet. Chem. 1981, 209, 245. (24) Ittel, S. D.; Tolman, C. A. Organometallics 1982, 1, 1432. (25) Brodt, C.; Niu, S.; Pritzkow, H.; Stephan, M.; Zenneck, U. J. Organomet. Chem. 1993, 459, 283. (26) Fischer, E. O.; Elschenbroich, C. Chem. Ber. 1970, 103, 162. (27) Fischer, E. O.; Miller, J. Z. Naturforsch. 1962, 176, 776. (28) Huttner, G.; Lange, S. Acta Crystallogr. 1972, B28, 2049. (29) Schneider, J. J.; Specht, U.; Goddard, R.; Kru¨ger, C.; Ensling, J.; Gu¨tlich, P. Chem. Ber. 1995, 128, 941. (30) Shimanouchi, T. Tables of Molecular Vibrational Frequencies Consolidated; National Bureau of Standards: Washington, D.C., 1972; Vol. 1, p 152. (31) Kafafi, Z. H.; Hauge, R. H.; Margrave, J. L. J. Am. Chem. Soc. 1985, 107, 6134. (32) Tremblay, B.; Gutsev, G.; Manceron, L.; Andrews, L. J. Phys. Chem. A 2002, 106, 10525.

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Reactions of Iron Atoms with Benzene - The Journal of Physical

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