Organometallics 1995, 14, 4409-4414
4409
Experimental and Theoretical Studies of the Gas-Phase Reactions of “Bare”Iron(1) with Tetralin Katrin Seemeyer, Roland H. Hertwig, Jan HruMk, Wolfram Koch,* and Helmut Schwarz” Institut f i r Organische Chemie der Technischen Universitat Berlin, Strasse des 17.Juni 135, 0-10623 Berlin, Germany Received March 29, 1995@ Extensive labeling studies have been employed to unravel the reaction mechanism of the
Fe+ mediated single and double dehydrogenation of tetralin (1). The reaction is both regioand stereospecific, leading first to the 1,2-dihydronaphthalene-Fe+complex (2) and finally to the naphthalene-Fe+ complex (3). In the course of the reaction the metal ion sticks to the same face of the hydrocarbon surface. The steric and electronic effects exerted by a CH3- or a CHsO-group attached at C(5) demonstrate that the metal ion migrates toward the initially saturated ring and is finally $-coordinated to this ring. The proposed reaction mechanism is supported by density functional calculations on the tetralin-Fe+ complex 1-Fe+ as well as on the reaction products, i.e., 1,2-dihydronaphthalene-Fe+ (2) and naphthalene-Fe+ (3).
Introduction Although metal ion-mediated dehydrogenationsin the gas phase have been studied frequently,l the elucidation of the reaction mechanisms is often difficult due to (i) WD equilibrations,2 (ii) the operation of competing A formechanisms, and/or (iii)“metabolic ~witching”.~ tunate case has been encountered for the metal complexes of tetralin (l),for which labeling experiments ,~ indicate that the M+-mediated (M = Fe,4 C O , ~Ag9 consecutive losses of HQand 2H2 (see Scheme 1)in the gas phase to generate the naphthalene-Me complexes 3 are highly specific. However, some questions regarding the bonding situation of 1-M+, 2, and 3 as well as the details of the reaction mechanism are still unresolved. For example: Does the saturated ring in 1-M+ affect the coordination of the metal ion to the benzene submoiety as compared to benzene ifself? Is there a bonding interaction with the additional double bond in 2? What is the hapticity of M+ in 3? If the mode of bonding in 1-Fe+, 2, and 3 deviates from that of M+(C6Hs),is this reflected in the bond dissociation energies (BDEs)? Does the metal ion influence the conformation of the nonaromatic ring? Which C-H position of the tetralin ring is first activated? Does the metal remain bonded to the aromatic ring throughout the whole reaction sequence or is it translocated to the initially saturated ring? Here, we report the experimental results of the Fe+mediated single and double dehydrogenation of tetralin @Abstractpublished in Advance ACS Abstracts, August 1, 1995. (1)(a) Eller, K.; Schwarz, H. Chem. Rev.1991,91,1121. (b) Eller, K. Coord. Chem. Rev. 1993,126,93. (2)For an extreme example of extensive WD exchange, see: Schwarz, J.; Schwarz, H. Chem. Ber. 1993,126,1257. (3)(a) Priisse, T.; Fiedler, A.; Schwarz, H. Helv. Chim. Acta 1991, 74,1127.(b) Seemeyer, K.; Priisse, T.; Schwartz, H. Helv. Chim. Acta 1993, 76, 1632. (c) Raabe, N.; Karrass, S.; Schwarz, H. Chem. Ber. 1994,127,261. (4) Seemeyer, K.; Priisse, T.; Schwarz, H. Helv. Chim. Acta 1993, 76, 113. (5)Huang, Y.;Profilet, R. D.; Ng, J. H.; Ranasinghe, Y. A.; Rothwell, I. P.; Freiser, B. S.Anal. Chem. 1994,66, 1050.
Scheme 1
Y+
M+ \
1-M’
M+
2
3
(1)in the gas phase. Preliminary results were reported previ~usly.~ The reaction mechanism has been elucidated by employing deuterium-labeled isotopomers and studying 5-substituted derivatives of tetralin. BDEs were determined by Cooks’ kinetic method6 and are augmented by quantum chemical calculations employing density functional theory (DFTh7 This level of theory incorporates in an approximate fashion exchange interactions and electron correlation, which are both crucial for a proper description of open shell transition metal compounds. Density functional theory in the form of the local spin density approximation (LSDA) with additional nonlocal corrections has already successfully been applied to open shell transition metal system^.^,^ While the geometries resulting from DFT calculations are generally in remarkable accord with experimental data or accurate calculations,1°bond dissociation energies are often significantly too large.7bJ1 Fortunately, it has been shown that this behavior is systematic; hence, meaningful corrections can be made, and relative stabilities are reproduced correctly.12 Another appeal(6)(a) Cooks, R.G.; Kruger, T. L. J.Am. Chem. Sot. 1977,99,1279. (b) McLuckey, S.A.; Cameron, D.; Cooks, R. G. J.Am. Chem. SOC.1981, 103,1313.(c) Wright, L. G.; McLuckey, S. A.; Cooks, R. G. Int. J.Mass Spectrom. Ion Phys. 1982,42, 115. (d) Chen, L.-2.; Miller, J. M. Org. Mass Spectrom. 1992,27,883. (7)(a) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and Molecules; Oxford University Press: New Yory, 1989. (b) Ziegler, T.Chem. Reu. 1991,91,651. (8)Ziegler, T.;Lee, J. Can. J.Chem. 1994,72,783. (9)Russo, T.V.;Martin, R. L.; Hay, P. J. J.Chem. Phys. 1994,101, 7729. (10)See, e.g.: Density Functional Methods in Chemistry; Labanowski, J. K., Andzelm, J. W., Eds.; Springer-Verlag: Berlin, 1991. (11)(a) Hertwig, R. H.; Koch, W. J. Comput. Chem. 1996,in press. (b) Becke, A. D. J.Chem. Phys. 1992,96,2155.(c) Ibid. J. Chem. Phys. 1992,97,9173. (d) Ibid. J. Chem. Phys. 1993,98,5648.
0276-733319512314-4409$09.00/00 1995 American Chemical Society
4410 Organometallics, Vol. 14, No. 9, 1995
Seemeyer et al.
Experimental Section As the experimental setup has been described repeatedly,13J4a brief description may suffice: A 1:l mixture of Fe(COh and tetralin or its derivatives is bombarded with 100eV electrons in the chemical ionization source (repeller voltage close to 0 V) of a modified four-sector tandem mass spectrometer of BEBE configuration (B stands for magnetic sector and E for electric sector), in which MS-I is the original VG ZABHF-2F part and MS-I1 is a n AMD 604 mass ~pectr0meter.I~ Although the actual mechanism by which the complexes are formed is yet unknown, the pressure in the ion source is high enough t o permit collisional cooling, thus increasing the lifetime such that time-delayed decompositions after ca. 1pus take place (metastable ion (MI) dissociation). To this end, the organometallic complexes of Fe+ and tetralin (and its derivatives) having 8-keV translational energy are mass-selected by means of B( l)E(l). Unimolecular reactions occurring in the field-free region between E(1) and B(2) were monitored by scanning B(2). Spectra were recorded on-line and averaged by using signal-averaging techniques employing the AMD Intectra data system. In typical experiments, 15-20 spectra were recorded. All compounds were synthesized by standard laboratory procedures, purified by chromatographic means, and fully characterized by spectroscopic methods.
1-Fe*
Computational Details
2' n
n
We applied the local spin density functional appr~ximationl~ augmented by nonlocal corrections for exchange and correlation due t o BeckelGand Perdew17(BPI. The calculations were carried out utilizing the program DGAUSS18 employing a DZVP all electron basis set for carbon and hydrogen, and a DZVP basis combined with a norm conserving pseudopotential (NCPP)lS describing the [Ar] core for iron. The geometries presented in Figure 1 were fully optimized in Cartesian coordinates without imposing any symmetry constraints. In a recent MCPF investigation, Bauschlicher et aL20 identified the ground state of the FeC(C6H6) complex as 4Az. We, therefore, also expect 1-Fe+, 2, and 3 to be quartet states.2I
Results and Discussion
U
2"
3 Figure 1. Calculated structures of 1-Fe+, 2, and 3. Bond lengths in A, bond angles in degrees.
ing aspect of DFT is its rather modest demand with respect to computing resources, which makes the method an ideal tool to computationally approach this problem.
The interpretation of the labeling results for tetralin (Table 1)is both unambiguous and straightforward and only compatible with a sequence of regiospecific 1,2eliminations involving hydrogen atoms from C(l)/C(2) and/or C(3)/C(4). For example, the data of la,c, and in particular the absence of HD loss for the first dehydrogenation, rule out a 2,3-elimination to generate the 1,4(12)(a) Hertwig, R. H.; HruSak, J.; Schroder, D.;Koch, W.; Schwarz, H. Chem. Phys. Lett. 1996,236,194. (b) Holthausen, M. C.; Heinemann, C.; Cornehl, H. H.; Koch, W.; Schwarz, H., J . Chem. Phys. 1994, 102,4931. (13)(a) Prusse, T. Ph.D. Thesis, Technische Universitat Berlin, Berlin, Germany, 1991;D83. (b) Priisse, T.;Schwarz, H. Organometallics 1989,8,2856. (14)(a) Srinivas, R.; Siilzle, D.; Koch, W.; DePuy, C. H.; Schwarz, H. J . Am. Chem. SOC.1991,113, 5970. (b) Srinivas, R.;Siilzle, D.; Weiske, T.; Schwarz, H. Int. J . Mass Spectrom. Ion Proc. 1991,107, 369. (15)(a) Slater, J.C. Phys. Reu., 1951,81,285.(b)Vosko, S.J.;Wilk, L.; Nusair, M. Can. J . Phys. 1980,58,1200. For a more recent and simpler formulation, see also: Perdew, J. P.; Wang, Y. Phys. Rev. B 1992,45,13244. (c) Gunnarson, 0.; Lundqvist, I. Phys. Rev. B 1977, 4274. (16)Becke, A. D. Int. J . Quantum Chem. 1983,23,1915. J.Chem. Phys. 1986,84,4524;Phys. Rev. A 1988,38,3098. (17)Perdew, J. P. Phys. Rev. B 1986,33,8822. (18)DGAUSS 2.3,Cray Research, Inc., 1994. (19)For a review on norm-conserving pseudopotentials, see: (a) Pickett, W. E. Comput. Phys. Rep. 1989,9. (b) Chelikowski, J. R.; Cohen, M. L. University of Minnesota Institute Research Report UMSI 911339;University of Minnesota: Minneapolis, MN, 1991. (20)Bauschlicher, C. W., Jr.; Partridge, H.; Langhoff, S. R. J . Am. Chem. SOC.1992,96,3273.
Gas-Phase Reactions of “Bare”Fe(I) with Tetralin
Scheme 2 M’
a
b
b’
dihydronaphthalene-Fe+ complex. Therefore, the product of the first reaction must correspond to the 1,2dihydronaphthalene-Fe+ complex 2 as shown in Scheme 1, and the mechanism is that of a combination of oxidative addition, 8-hydrogen transfer, and reductive elimination of molecular hydrogen. The data of the racemic pairs of lc,d convincingly demonstrate that the H(D) transfer t o the metal ion center follows a syn-process. The complex of cis-labeled tetralin IC does not undergo HD loss; in contrast, the MI mass spectrum of the trans-complex Id contains signals for both H2 and HD elimination, while D2 is not eliminated. Furthermore, as demonstrated earlier,22in the overall reaction the metal ion does not switch from one surface of the x-system to the other. This is evidenced by the MI spectrum of le-Fe+. If the Fe+ cation would change the surface, one must observe the combined losses of H2 HD, irrespective from which side of the complex the first molecule of H2 originates. This is not the case. Rather, the isotope distribution demands that throughout the whole reaction sequence the metal ion sticks t o the same plane of the hydrocarbon surface to which it was bound initially. In addition, earlier work22suggests that during these reactions the x-coordinated metal ion migrates toward the initially nonaromatic ring. For bicyclic [ML,(polyene)] complexes, where ML, = Cr(C0)3 and CrCp, a theoretical analysis23of haptotropic rearrangements2* has been conducted in great detail. Although this analysis does not pertain-in a strict sense-to the present study, there are some relations worth considering. In fact, three pathways deserve t o be distinguished (Scheme 2). The metal ion migrates in a symmetrical fashion toward the unsaturated ring across the central C(9)/ C(l0) bond (Scheme 2, path a). This path is not expected to be significantly affected by substituents attached to C(5);consequently, in the elimination of the first molecule of H2, the positions C(l)/C(2) and C(3)/
+
(21)In order to confirm this assumption we have investigated the sextet state of the complexes 1-Fe*, 2, and 3. The total energies for 1-Fe’, 2, and 3 are nearly degenerate for both the quartet and sextet states. We rationalize this as follows: The calculated energy needed to excite the sextet ground state of the Fe’ atom to its quartet state is overestimated by about 12 kcaUmo1, which is not unexpected for DFT methods. On the other hand the order in the total energies between quartet and sextet is presumably reversed if we look at the complexes since we expect the quartet to be the ground state here in analogy to Fe’-benzene. Consequently, the quartet should be significantly lower in energy. But, because this error in the atomic splitting is being propagated into the molecular total energies, the splitting between quartet and sextet structures in the case of 1-Fe+, 2, and 3 is reduced by about the same amount (12kcaUmo1) in our calculations. Hence, we believe that all three complexes are in fact quartet states. (22) Seemeyer, K.; Schwarz, H. Helv. Chim. Acta 1993, 76, 2384. (23)Albright, T. A.; Hofmann, P.; Hoffmann, R.; Lillya, C. P.; Dobosh, P. A. J . Am. Chem. SOC.1983, 105, 3396. (24)Ark, N.T.; Elian, M.; Hoffmann, R. J . Am. Chem. SOC.1978, 100, 110.
Organometallics, Vol. 14,No. 9, 1995 4411 C(4) should participate equally. In contrast, if the metal ion follows the unsymmetrical paths b orb’ (Scheme 2), one expects a preference for C(l)/C(2) or C(3)/C(4)depending on the nature of the substituent. For non or weakly complexing substituents, e.g., R = CH3, M+ is likely to be deflected away from R to the region of C(8)/ C(9) (Scheme 2, path b). Consequently, the loss of the first H2 molecule is predicted to preferentially involve C(l)/C(2). In contrast, substituents which are capable t o compensate for the loss of nelectron complexation, e.g., R = OCH3, are expected to direct the metal ion toward the region C(5)/C(lO),thus favoring the activation of C(3)/C(4)C-H-bonds (Scheme 2, path b’). The data for the Fe+ complexes of 4 and 5 (see Table 1)show that there is indeed a preferred directionality operative for the elimination of the first molecule of molecular hydrogen. For the 5-CHs-substituted tetralin 4, comparison of the isotopomers 4a,b clearly demonstrates that the CH3-group directs the metal ion away from its neighbors C(3)/C(4). This is evidenced by the increased ratio for the loss of HfiD = 5.6 from 4a; in contrast, from the isotopomer 4b this ratio drops to 1.3. If one assumes that the kinetic isotope effects associated with metal ion-mediated dehydrogenation of 5-substituted tetralins are approximately the same as for the tetralin isotopomer of lb, which amounts to 1.9 f.0.2,25from a simple algebraic analysis it follows from the data of 4a,b that the steric hindrance imposed by the CH3-group amounts t o a factor of ca. 2. Thus, the oxidative insertion of the metal ion in the sterically more easily accessible C(l)/C(2)region (path b) is preferred. We consider the effect of the methyl group to be steric rather than electronic in nature since the a-constants of the linear free energy relationship,26 which describe the electronic effect of a substituent at an aromatic ring, do not differ very much for an H-atom or a methyl group. Furthermore, if the methyl group would exert an electronic effect, this would be expected to have the same direction as that of the methoxy substituent (see below); this, however, is not the case. In contrast, the CH30 substituent a t C(5), i.e., 5, clearly directs the metal ion toward the sterically more congested C(3)/C(4)region. This follows immediately from the large H f i D ratio of 14.5 for the complex 5a-Fe+. Obviously the loss of complexation energy associated with the migration of the Fe+ is partially compensated for by coordination of the metal ion to the lone pair electrons of the methoxy group. The experiments with the 5-substituted derivatives of tetralin show that the metal ion, as expected, has to migrate toward the initially nonaromatic ring in order to induce the dehydrogenation. However, the experiments do not reveal the details of this migration: for example, does the oxidative addition commence with an insertion in the C(1) or the C(2) C-H bond? Therefore, DFT calculations were conducted in the hope to provide this missing link. The optimized structures of the quartet states of 1-Fe+, 2, and 3 are shown in Figure 1. Comparsion between the complexed hydrocarbons and those of the free ligands is made in Table 2. (25) For a comprehensive, detailed analysis of the isotope effects associated with the metal ion-mediated dehydrogenation of tetralin, see ref 4. (26) (a) Taft, R. W. Steric Efects in Organic Chemistry; Newman, M. S., Ed.; Wiley: New York, 1956. (b) Taft, R. W.; Prize, E.; Fox, I. L.; Lewis, I. C.; Andersen, K. K.; Davis, G. T. J . Am. Chem. 1963,85, 709,3146.(c)Exner, 0. Correlation Analysis in Chemistry; Chapman, N. B., Shorter, J., Eds.: Plenum Press: New York, 1978.
4412 Organometallics, Vol. 14, No. 9, 1995
Seemeyer et al.
Table 1. Unimolecular Single and Double Dehydrogenation of Fe+ Complexes of Tetralin and Its Derivatives" neutral eliminated
no.
ligand
m
Hz
HD
2HzorDz
HatHD
35
65
la
37
8
lb
29
IC
41
Id
31
12
le
29
8
4
26
4kl
28
5
67
18
14
68
1
4b
5
22
Sa
29
5b
20
HZ+ D1 or 2HD
55
59
12
43
44
74
78
2
1'b
68
79
1
z (1%)from Sa-Fe+, which is not expected to a Data a r e normalized to 2 fragment intensities = 100%. "he quite minor loss of two H take place if only the positions C(l)/C(2)and/or (C3)/C(4) serve as Hz sources, originates from a new channel in which the CH3O-substituent participates. This follows from the combined loss of Hz HD from the CDaO-labeled isotopomer Sb. It has been conjectured by a reviewer t h a t this minor channel may result from Fe+ in a n excited state.
+
The salient feature of the three calculated structures 1-Fe+, 2, and 3 is the $-coordination of the metal ion. The corresponding iron-carbon bond distances are within the range of 2.23-2.34 A. Due to the absence of any symmetry constraints in 1-Fe+ and 2 and a low Cz-symmetry in 3,the Fe-C bond lengths are not equal. From a comparison of the data collected in Table 2, it becomes evident that the structure of the ligand is only marginally altered by the presence of the iron. We note, however, that in each case the C-C bond lengths of the
aromatic moiety are elongated by 0.01-0.02 A as a result of the complexation with the iron. The preference for the central position of the iron in all three complexes can be explained in terms of a MO picture by an overlap of the HOMO of the ligand with an empty sd-hybridized orbital of Fe+, similar to the bonding of Fe+ t o benzene, which has recently been studied by Bauschlicher et a1.20 The total atomic charge on iron in 1-Fe+ is reduced to 0.72e, according to the Mulliken population analysis. This charge transfer
Organometallics, Vol. 14, No. 9, 1995 4413
Gas-Phase Reactions of “Bare”Fe(I) with Tetralin
Table 2. Selected Bond Lengths, Bond Angles, and Torsion Angles of 1-Fe+, 2 , and 3 and the Respective Free Ligands Tetralin, Dihydronaphthalene, and Naphthalene”
c1-c2 c2-c3 c3-c4 C4-clO C9-C10 c10-C6 c5-cS cS-c7 c7-CS
Cs-C9 c9-c1 c1-c2-c3 C3-C4-C10
C1o-C9-C1 c1o-c4-c3-c2
cg-c1-cz-c3 a
1-Fe+
tetralin
1.54 1.54 1.54 1.52 1.43 1.45 1.42 1.42 1.43 1.44 1.52 109.6 114.1 121.4 51.9 39.0
1.54 1.54 1.54 1.53 1.42 1.41 1.40 1.41 1.40 1.41 1.52 110.7 113.2 113.3 46.7 46.7
All angles in degrees and bond lengths in
2’ 1.36 1.50 1.54 1.52 1.45 1.43 1.42 1.43 1.43 1.43 1.47 122.1 112.6 119.2 47.2 1.3
dihydronaphthalene
3
1.36 1.52 1.56 1.52 1.42 1.40 1.41 1.41 1.41 1.41 1.47 122.0 114.5 119.4 36.0 0.3
1.42 1.43 1.42 1.45 1.46 1.43 1.39 1.43 1.39 1.43 1.45 120.7 120.2 120.2 -1.0 0.8
naphthalene 1.39 1.43 1.39 1.43 1.44 1.43 1.39 1.43 1.39 1.43
1.43 120.3 120.8 118.9 0.0
0.0
A.
leads to a small elongation of the aromatic C-C bonds (0.02 A) and an out-of-planedistortion of the C-H bonds by 4”. The conformation of the saturated ring in 1-Fe+ is not significantly altered as compared to uncomplexed tetralin in that, in both systems, the nonaromatic ring adopts a half-chair conformation. With respect to possible sites for the first C-H bond activation in the course of the dehydrogenation reaction, the distances of the two relevant axial hydrogen atoms in 1-Fe+ (as shown in Figure 1)are of importance. According to the stereoelectronic principle27 these H atoms should be preferred, as their C-H bond orbitals are parallel to the n-orbitals of the aromatic ring so that stabilization of the newly formed n-orbital is achieved at an early stage. The distance of the iron to the axial hydrogen in position C(1) amounts t o 3.56 A, whereas the hydrogen in position C(2) is further away (3.79 &. If the saturated ring of 1-Fe+ would adopt a boat-like conformation, the axial hydrogen atom bound to C(2) would be the nearest. These simple geometrical arguments in conjunction with the experimental results of the tetralin isotopologues point to the conclusion that the first C-H bond activation commences at C(1). This is also in accord with investigations for the deprotonatiod protonation of the Cr(C0)3-2-methyltetralin complex in the condensed phase.28 The theoretical investigation of intermediate 2 requires some care. Whereas for 1-Fe+ the conformation of the nonaromatic ring is by and large unaffected by the metal-ion complexation, there exists a difference for 2. In fact, two different structures were located. In one case (2’) the C(3)-methylene group is slightly bent toward the iron center with a hydrogen atom in an axial position a t a distance of 3.9 A t o the iron. In a second conformation 2” the C(3)-methylene group is turned away from the iron. In the latter case the only axial hydrogen is located at C(4) a t a distance of only 3.4 A to the iron. Hence, the second H2 elimination is likely to occur at C(4) starting from structure 2’. Once the second dehydrogenation step is completed both rings have achieved aromaticity. Yet, the theoretical analysis (27) (a) Deslongchamps, P. Stereoelectronic Effects in Organic Chemistry; Pergamon Press: Oxford, 1983. (b) Winterfeldt, E. Prinzipien und Methoden der stereoselektiven synthese; Vieweg & Sohn: Braunschweig, Germany, 1987. (28) Schmalz, H.-G.; Volk, T.; Batz, J. W. In preparation.
Table 3. Comparison of Bond Dissociation Energies of Fe+L (in kcal/mol) as Determined by Cooks’ Kinetic Method and Calculated by Density Functional (BPI for the Quartet States of Fe+L” exptl benzene-Fe+ tetralin-Fe+ (l-Fe+) dihydronaphthalene-Fe+( 2 ) dihydronaphthalene-Fe+ (2”) naphthalene-Fe+ (3)
A
48.630(55)31 0 53.1 (59.5) 4.5 52.2 (58.6) 3.6 51.2(57.6)
2.6
computed 57.1 63.2 63.7 62.5 60.3
A
0 6.1 6.6 5.4 3.2
The data in brackets refer to the anchor point of BDE(Fe+CsHs) = 55 kcavmol; A denotes the bond dissociation energy relative to BDE(Fe+-C&Is). a
clearly points to a $-coordination of the metal to one ring only. As the mechanism of dehydrogenation implies an oxidative insertion of the metal ion in a C-H bond of the saturated ring, it is very likely that in the course of the reaction the metal undergoes a translocation from the aromatic part to the originally saturated region of tetralin. The structure of the final product of the double dehydrogenation reaction, i.e., the naphthalene-Fe+ complex 3 is quite similar to those of 1-Fe+ and 2. As already mentioned, the metal ion is $coordinated and the geometric features of the coordinated aromatic ring are only marginally affected by the presence of the uncomplexed ring. We note that the hydrogens are bent away from the plane by ca. 2”. The charge transfer component reduces the Mulliken charge on iron to 0.77e. Next, we turn to the thermochemistry of the single and double dehydrogenation of tetralin. Based on the AHf values29 for tetralin (6.0 kcaVmol), dihydronaphthalene (28.0 kcaVmol), and naphthalene (35.9 kcall mol), in the absence of Fe+ these reactions are endothermic. In order t o probe the effect of Fe’ on the reaction enthalpy, we have both measured and calculated bond dissociation energies of “bare”Fe+ to tetralin, dihydronaphthalene, and naphthalene. Considering a simple thermodynamic cycle it becomes clear that this endothermicity does not change dramatically as the BDEs differ by only a few kcaVmol (Table 3). In addition, the heats of dehydrogenation can be assessed theoretically by subtraction of the calculated total (29) (a)Lias, S. G.; Liebman, J. F.; Levin, R. D. J.Phys. Chem. Rec Data 1984,13,695. (b)Lias, S. G.; Liebman, J. F.; Levin, R. D.; Kafafi, S. A. NIST Standard Reference Database, Positive Ion Energetics, Version 2.01; Gaithersburg, MD, January 1994.
4414 Organometallics, Vol. 14, No. 9, 1995 energies of the dehydrogenated species and Ha from the one of the hydrogenated parent compounds. By doing so we obtain the following results: Comparing the complexed and the free hydrocarbon system, we find that the heats of dehydrogenation differ by less than 1 kcaVmol. This difference lies within the error of the method, especially as zero-point vibrational energies have not been considered. Hence, complexation with iron does not change the reaction enthalpy of the dehydrogenation reaction of these hydrocarbons. A complete investigation of the reaction pathway, including transition states, which would also allow for an estimate on the thermodynamic barriers of these reactions would be highly demanding for this system and has therefore not been performed. The experimental relative bond dissociation energies have been estimated by using Cooks’ kinetic method6 of determining the intensities of unimolecular losses of ligands L and L‘ from mixed bis-ligated complexes Fe+(L)(L’). Using the known BDE(Fe+-ben~ene)~O,~l as a reference (Table 3) these relative values can be converted to absolute BDEs. The experimental and calculated BDEs for Fe+(CsHs) and 1-Fe+, 2, and 3 are collected in Table 3. The previously calculated20BDE (47.6 kcaVmol) for the C6u symmetrical structure of Fe+(C&) is in good agreement with the experimental value of 48.6 kcaV mo130 obtained recently in our laboratory; the agreement with a value reported by Freiser and co-workers (55 kcaVm01)~l is less pleasing. For the purpose of calibration of our computational method we recalculated the BDE for Fe+(C&) using the same method and basis set as employed for 1-Fe+, 2, and 3. Apparently, our result of 57.1 kcaVmol for BDE(Fef-C6H6) agrees well with Freiser’s31 value. We note, however, that the density functional approach used in the present calculations tends to significantly overestimate binding energies. Although we are not able to unambiguously decide (30) Schroder, D.; Schwarz, H. J . Orgunomet. Chem., in press. (31)(a) Hettich, R. L.; Jackson, T. C.; Stanko, E. M.; Freiser, B. S. J . A m . Chem. SOC.1986, 108, 5086. (b) Bruckner, S. W.; Freiser, B. S . Polyhedron 1988, 16/17, 1583 and references cited therein.
Seemeyer et al. which of the two experimental values is more accurate, we suppose the value of >55 kcdmol as too high. In fact, a more recent experimental determination of the BDE of Fe+(C&) that uses an entirely different approach leads to a BDE of 51 kcaVm01.~~ As t o the relative BDEs, the disagreement is much less pronounced; in fact, we find satisfying agreement between theory and experiment. In any case, the data in Table 3, from both theory and experiment, strongly suggest that the driving force in the single and double dehydrogenation of tetralin-which, in the metal-free system and when complexed with iron, is endothermic-is provided by the energy gained in the initial attachment of “bare” Fe+ to the z-system of tetralin 1.
Conclusions We have shown that the bonding of Fe+ to tetralin 1 is similar to that of Fe+-benzene; this $-coordination is retained also in the dihydronaphthalene-Fef complex 2 and the naphthalene-Fe+ complex 3. Both our labeling experiments and the geometrical analysis of the structures resulting from our calculations strongly suggest that the first C-H bond activation occurs at C(1). Furthermore, the results obtained for 5-substituted derivatives of tetralin indicate that iron migrates across the hydrocarbon system with a directionality determined by the nature of the substituent. The BDEs of tetralin, dihydronaphthalene and naphthalene are slightly higher than the one of Fe+-benzene and remain almost unchanged for all three complexes involved in the dehydrogenation reaction of tetralin. Therefore the endothermicity of the single and double dehydrogenation of tetralin is retained, if complexed with iron.
Acknowledgment. We are grateful to the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie for the financial support of our work, t o Dr. D. Schroder for stimulating comments. R.H.H. thanks Cray Research, Inc. for generously providing CPU time, and especially to Dr. G. Fitzgerald. OM950232K (32) Armentrout, P. B. Private communication, January 1995.
