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Computational Investigations of Ligand Fluxionality in Fe3(C8H8)3 Ryan J. Martinie, Brandon J. Burkhart, and Roger L. DeKock* Department of Chemistry and Biochemistry, Calvin College, 1726 Knollcrest Circle, Grand Rapids, Michigan 49546, United States
bS Supporting Information ABSTRACT: The recently synthesized tris(μ-1,3,5,7-cyclooctatetraene)triiron, Fe3(C8H8)3, was investigated by DFT computational studies. In the gas phase, the cluster adopts C3h symmetry and pseudo-η3:η5 hapticity to each of the cyclooctatetraene (COT) ligands, which exist in a π-delocalized V conformation with each ligand bridging two metal atoms. A higher symmetry structure, D3h, lies only slightly above the C3h structure and represents a transition state between two energetically identical C3h structures. The computed charge on the iron atoms is near 0 by both the Hirshfeld and Voronoi methods and comports with the compound having a formal oxidation state of 0. In addition, the cluster can adopt a second conformation with the three COT ligands in a π-delocalized tub configuration with a decrease in stability of only ∼3 kcal/mol. The ligands can also rotate about the metal metal framework with a transition barrier of only ∼1 kcal/mol. Dissociation of a single COT ligand requires ∼57 kcal/mol. The low-energy ligand conformational changes, sliding, and rotation likely make significant contribution to the experimentally observed fluxionality of the ligands. A triplet structure was also examined and found to be nearly isoenergetic with the singlet. The triplet potential energy surface resembles that of the singlet in that a variety of low-energy transformations can contribute to ligand fluxionality.
’ INTRODUCTION The molecule tris(μ-1,3,5,7-cyclooctatetraene)triiron, Fe3(C8H8)3, was recently synthesized in the Grubbs laboratory.1 This system contains a central trinuclear transition-metal triangle (Figure 1). Systems of this sort have been of interest since the synthesis of Fe3(CO)12 by Dewar and Jones in 1907.2 The question of substituting the carbonyl ligands for hydrocarbon ligands was longstanding but ultimately solved by the synthesis of Lavallo and Grubbs.1 This Lavallo and Grubbs molecule is a “48 electron” cluster3,4 and is therefore expected to be stable and closed shell. The 3 iron atoms each contribute 8 valence electrons, for a total of 24. Each of the 3 cyclooctatetraene (COT) ligands donates 8π electrons, for a total of 48. With a 48-electron count, each metal atom would achieve an “effective atomic number”, EAN, of 16 electrons, except that the cluster forms metal metal single bonds, increasing the EAN to 18. The molecule Fe3(COT)3 has a 1:1 stoichiometry of metal atoms to ligands. Recently, COT coordinated to the group 3 metals Sc, Y, and La has been characterized in matrix isolation experiments.5 These molecules are predicted (DFT) to have C8v symmetry, with the COT ligand in a planar configuration. Hence, these molecules can be thought of formally as M2+ and COT2 . Whereas the free ligand COT has alternating π bonds and exists in a tub shape,6 the COT2 ligand has 4n + 2 π electrons and is a completely delocalized aromatic molecule.7 These M(COT) complexes with the ligand formally existing as COT2 remind us of the sandwich compound U(COT)2, one of the early known “ring whizzers”.8 r 2011 American Chemical Society
Two molecules closely related to Fe3(COT)3, Fe(COT)2 and Ru(COT)2, exhibit a different bonding motif.9,10 In these molecules the ligand is decidedly not a planar COT2 ring. Rather, these structures are best described as M((1 6-η)-C8H8)((1 4-η)C8H8). In this configuration the metals achieve an 18-electron count. The Fe3(COT)3 molecule exhibits yet another bonding motif to the metal framework, namely that of a delocalized “V” conformation, as shown in Figure 1. The coordination of the ligands has been described as η3 and η5, and the Fe Fe bonds have been described as single bonds.1 The iron atoms were alluded to as Fe(0), although Lavallo and Grubbs deferred to computational modeling to help ascertain the formal oxidation state of the metal atoms. Lavallo and Grubbs report a number of interesting experimental properties of Fe3(COT)3, most notably a single 1H NMR resonance indicative of ligand fluxionality and paramagnetism. In this work, we investigate the fluxionality, the paramagnetism, and the structure and bonding in Fe3(COT)3 through computational DFT experiments. Large organic molecules are known to adopt numerous conformations on metal surfaces,11,12 and conformational changes are important in the growth of molecular structures on solid surfaces.13,14 We believe that our computational work investigating the fluxionality of COT in Fe3(COT)3 is pertinent to such studies. Received: June 28, 2011 Published: September 07, 2011 5196
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Figure 1. Fe3(COT)3 as reported by Lavallo and Grubbs.
’ METHODOLOGY All computational studies were performed using the Amsterdam Density Functional 2009.01 and 2010.02 suite of programs15 17 using the BLYP18 20 density functional and an ADF basis set TZP (triple-ζ basis set with one polarization function) for transition metals and DZP (double-ζ basis set with one polarization function) for carbon and hydrogen. “Medium” frozen core approximations were used (transition metal, 3p frozen; carbon, 1s frozen). Representative structures were also optimized using the BP86-D19,21,22 density functional; this change in functional had no effect on the conclusions of our study. Energies are reported without zero point energy correction. Given the very low energy barriers we obtain for transition states in this work, the zero point energy corrections will be miniscule for energy differences between electronic states. Analytical frequencies were obtained for all structures, and the results confirmed stable structures (no imaginary frequencies) as well as transition states (one imaginary frequency). We have devoted the majority of our effort to examination of the singlet cluster molecule. A separate section in the Results and Discussion is dedicated to consideration of the triplet. Bond order analyses were performed using the Nalewajski Mrozek (N-M) bond index.23 The particular index that we employed is the trace (ΔP)2 and was referred to as a 3-index set. Note that these bond order results are dependent upon the chosen atomic reference states. For example, the Re Re bond order in the classic quadruple-bonded Re2Cl82 is about 2.9 when the reference states are the spherically averaged (normal) atomic reference states in ADF.23 However, if we first compute the electronic structure of ReCl4 ions, and these are used as the constituent fragments for the Re2Cl82 ion, the fragment fragment bond order is near 4.0.24 Of course, a chemist would interpret the fragment fragment bond order as a Re Re bond. That is, we obtain a quadruple bond, as expected from rudimentary bonding theory. In the case of Fe3(COT)3 we are unable to examine the Fe Fe bond order in the same way that we could for the Re2Cl82 ion. That is, the bridging ligands make it impossible to partition the molecule such that the fragment fragment interaction is primarily comprised of a metal metal interaction. For example, starting from three Fe(COT) fragments, each fragment fragment interaction is composed of both metal metal and metal ligand interactions. Hence, we report computed bond orders only from atomic fragments as the reference. Use of atomic fragments as reference in computed bond orders has its own nuances. In the case of simple main-group compounds, such as the COT ligand encountered here, computed bond orders agree well with chemical intuition, as we shall see below. However, in the case of electron donor acceptor bonds, things are not so simple. In brief, computed bond orders for donor acceptor “single” bonds are often significantly less than 1. For example, bond orders for Ni2+ NH3 bonds are computed to be in the range 0.3 0.5.23 In fact, this result is not surprising in that the bonding pair of electrons is more localized on the ammonia ligand in comparison to a pure covalent bond. That is, what a chemist might glibly call a “single” bond in fact is less than a single bond in this more nuanced approach. We will need to be aware of this nuance when we examine computed M C bond orders, because of their donor acceptor character in the Dewar Chatt Duncanson model of metal ligand bonding.
Figure 2. Fe3(COT)3 in C3h symmetry. Bond lengths not displayed can be inferred from symmetry. Labels indicate bond lengths in angstroms.
’ RESULTS AND DISCUSSION Structure and Bonding in Fe3(COT)3. Our study began with an examination of the structure and bonding of Fe3(C8H8)3. On the basis of X-ray crystallography, Lavallo and Grubbs report a nearly equilateral iron triangle with each bridging COT ligand coordinating in a manner approaching η3 and η5 (Figure 1), although specific structural data was not included.1 In our studies, the system adopts C3h symmetry25 and agrees with a nearly equilateral metal triangle and bridging COT ligands. Calculated metal metal bond lengths of 2.94 Å (Figure 2) are in substantial agreement with the experimentally observed values of 2.815 2.830 Å (Figure 1). These bond lengths are indicative of an M M single bond.26 Our results indicate a bond order of ∼0.6 (Figure 4). For reasons elaborated in the Methodology, we interpret this result as a single bond. The coordination of the ligands has been described as η3 and η5. In our theoretical model, there is little difference between the η3 and η5 cases. Specifically, the Fe C bond lengths (to the five nearest carbons) are 2.09, 2.11, 2.11, 2.63, and 2.63 Å for the “η3 case” and 2.10, 2.10, 2.10, 2.46, and 2.46 Å for the “η5 case” (Figure 2). As we see, the η3 and η5 cases are very similar, and both lie somewhere between η3 and η5. Furthermore, it would take only a slight “slippage” of the ligand to make the η3 and η5 cases identical. The symmetry of the overall molecule would then be D3h rather than C3h. The perturbation required to move from C3h to D3h is so small that the C3h structure is only 0.1 kcal/mol lower in energy than the D3h structure. A single imaginary frequency revealed that the D3h structure was in fact a very low energy transition state. Free COT adopts a tub configuration (Figure 3A).6 However, in Fe3(COT)3, the COT ligands adopt an unusual V configuration (Figure 3B). The angle of this V is ∼135°. Unlike the alternating single and double bonds in free COT (calculated bond orders of 1.10 and 1.90, Figure 3), COT ligands in Fe3(COT)3 exhibit nearly complete delocalization around the ring, with computed bond orders ranging from 1.26 to 1.33. Bond orders differ by only 0.07, compared to a difference of 0.80 in free COT. The bond lengths are consistent with the bond orders; in free COT, the bond lengths differ by 0.13 Å, while those in bound COT differ by only 0.015 Å. Binding with the iron atoms in this cluster causes a remarkable change in the stability of the V conformation. When COT is not 5197
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Figure 5. Fe(COT) geometry with η4 coordination. The COT ligand geometry is approximately a “half tub”. Figure 3. Free COT (A) and COT in Fe3(COT)3 (B). Labels indicate bond length/bond order. Bond orders reported for B are from cluster calculations.
Figure 4. Fe3(COT)3 and calculated bond orders. M COT labels at the bottom right indicate the sum of bonding to the nearest five carbons on a single ligand. Figure 6. Conformational changes (V to tub) in Fe3(COT)3.
bound in the cluster, our theoretical studies indicate that the V conformation is less stable than the tub by 35.7 kcal/mol. However, when the COT ligand binds to the iron atoms, the V conformation is more stable by ∼1 kcal/mol (see below). One way to interpret the metal ligand bonding in the Fe3(COT)3 complex is to take advantage of the fact that each iron atom is in nearly η5 coordination to two adjacent COT ligands. This η5 coordination brings to mind the structure of ferrocene. To that end, we have also completed a theoretical study of ferrocene in order to draw comparisons between ferrocene and the Fe3(COT)3 complex. The computed Fe C bond length in ferrocene is 2.12 Å. The average bond length to the nearest six carbon atoms in Fe3(COT)3 is 2.10 Å (Figure 2). (The average Fe C distance for the other four carbon atoms is 2.54 Å. The latter bond length is larger because these carbon atoms occupy a bridging position.) As we might expect from the similarity in the Fe C bond distances between the two complexes, the Fe C bond orders are also very similar (Figure 4). To wit, the Fe C bond order in ferrocene is 0.48; the six nearest-neighbor Fe C bond orders in Fe3(COT)3 are 0.40, 0.44, and 0.44 to the first ligand and 0.40. 0.50, and 0.50 to the second ligand. (The bridging carbon atoms have lower Fe C bond orders, 0.21 and 0.32, as expected due to the longer bond lengths.) In brief, the Fe C bond lengths and bond orders are very similar in Fe3(COT)3 and FeCp2. The C C bond distances within the COT ligand of Fe3(COT)3 are also very similar to those within the Cp ligand of ferrocene. The longest C C distance is 1.44 Å, and the shortest is 1.42 Å (Figure 2), whereas the five computed C C bond lengths in FeCp2 are 1.43 Å. Correspondingly, the average C C bond order in Fe3(COT)3 is 1.31 (Figures 3 and 4), and that in FeCp2 is 1.32. Oxidation State of Fe Atoms. Our results indicate a Hirshfeld charge and Voronoi deformation density27 on each iron atom of
0.08 and 0.00, respectively, supporting the oxidation state of the iron atoms as Fe(0), as suggested in the original synthesis paper.1 Computational support for the Fe(0) label when a COT ligand is bound to Fe is provided by a single study performed on the monomer Fe(COT). If the formal oxidation state were Fe(II), we would expect a planar COT ligand and η8 coordination, as found in the group 3 (Sc, Y, La) M(COT) complexes.5 Instead, the structure obtained is Fe((1 4-η)-C8H8) (Figure 5). Properties of Fe3(COT)3: Fluxionality and Paramagnetism. The Fe3(COT)3 cluster exhibits a number of interesting experimental properties, among them a single, broad, paramagnetically shifted 1H NMR resonance.1 The presence of a single 1H NMR peak suggests ligand fluxionality. For the C3h conformation, we would expect five proton NMR peaks. In contrast, the presence of a single broad peak indicates that all protons are in an identical spin environment, and therefore the conformation of the molecule must be changing to the extent that, on the NMR time scale, every proton occupies each position. We examine the following contributors to ligand fluxionality: conformational changes, sliding, rotation, and single ligand dissociation. Ligand Fluxionality: Ligand Conformational Changes. The most energetically stable conformation of the Fe3(COT)3 molecule contains three COT ligands in the V conformation described above. However, our theoretical studies indicate that each ligand may independently move to a tub conformation, similar to the case for free COT, destabilizing the cluster by only ∼1 kcal/mol (Figure 6).28 Two ligands can also move to the tub conformation, or all three can move to the tub conformation, destabilizing by ∼2.1 and ∼2.6 kcal/mol, respectively (Figure 6). The low-energy transition between tub- and V-shaped ligands in Fe3(COT)3 may make an important contribution to the experimentally observed ligand fluxionality.1 The transition barrier is quite 5198
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Figure 7. Rocking transformation through a D3h transition state.
Figure 8. Rotation of a single ligand. Two carbon atoms are labeled in order to demonstrate the rotation.
Figure 9. Dissociation of a single COT ligand, with M M bond lengths.
low and is certainly attainable at ambient temperature.29 Therefore, the easy interconversion of the ligands between V and tub conformations could make a significant contribution to ligand fluxionality and the presence of a single NMR resonance. Ligand Fluxionality: Ligand Sliding. As noted above, the lowenergy C3h geometry is very similar in energy to the D3h conformation. As a result, the COT ligands can rock back and forth between the two energetically identical C3h structures, through the D3h transition state (Figure 7).30 The transition barrier for this transformation is extremely low, only 0.1 kcal/mol. This rocking movement might also contribute to observed ligand fluxionality. Ligand Fluxionality: Ligand Rotation. Whereas ligand conformational changes and rocking would create some fluxional character and peak broadening in the COT ligands, they likely would not cause all protons to exist in an identical spin environment, as indicated by a single resonance; ligand rotation, on the other hand, would. Therefore, the feasibility of ligand rotation was also investigated. As a ligand rotates, we would expect each carbon in the COT ring to move one position over, at which point the conformation would be identical with the starting conformation
(Figure 8). Furthermore, the midway point between these two states is the tub form of the COT ligand (Figure 8). As stated above, the tub state is only ∼1 kcal/mol less stable that the V conformation; therefore, it is expected that this rotation will be low energy. Our computational studies confirm this intuition, with a transition barrier of ∼1 kcal/mol. This rotation is notable in a nonplanar ligand. Overall, computational investigation of ligand fluxionality reveals a bonding picture in which the potential energy surface is very flat, and ligands can freely alter their conformation with minimal energy penalty. This bonding picture strongly supports the experimental NMR evidence of fluxionality. Ligand Fluxionality: Ligand Dissociation. According to Lavallo and Grubbs, the fluxional NMR resonance of Fe3(COT)3 in solution “suggests the formation of a new species that contains iron and at least one coordinated and fluxional COT ligand”.1 This might suggest the dissociation of one or more COT ligands. We computed the ligand dissociation energy of a single ligand to be 57.2 kcal/mol (Figure 9).33 This dissociation energy is not large but should not be attainable at ambient temperatures. The much lower barriers to ligand conformational change, sliding, and 5199
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barrier of ∼0.1 kcal/mol, rotate with a barrier of ∼1 kcal/mol, and dissociate with a penalty of 57 kcal/mol. These first three low-energy transitions likely make significant contributions to the observed fluxionality of the ligands. Finally, the triplet and singlet states of the molecule are essentially isoenergetic and the triplet state potential energy surface also allows for low-energy ligand fluxionality.
’ ASSOCIATED CONTENT Figure 10. Optimized geometry of triplet Fe3(COT)3.
rotation suggest that these are likely the primary factors influencing the observed single fluxional resonance. One notable aspect of ligand dissociation was the structural change to the molecule following the removal of a single COT ligand. Upon dissociation of the ligand, all three M M bonds shortened significantly, from 2.90 Å to 2.29 Å, and the M M bond adjacent to the removed COT ligand shortened the most, from 2.90 Å to 2.17 Å (Figure 9). Bond orders increase correspondingly; bond orders on the longer M M bonds increase from 0.58 to 1.03, and the shorter bond increases from 0.58 to 2.28. These bond orders could be interpreted as Fe Fe double bonds and triple bonds, respectively. Paramagnetism. Upon NMR analysis of Fe3(COT)3, the observed shift is indicative of paramagnetism.1 The difference in energetic stabilities of the triplet and singlet cases is extremely small. The most stable singlet structure and the most stable triplet structure differ in energy by less than 0.1 kcal/mol.31 It is likely, therefore, that the triplet is in equilibrium with the singlet, giving rise to a paramagnetic NMR resonance. In contrast with the singlet structure, the optimized triplet cluster adopts C2v symmetry, with one ligand in the tub conformation (Figure 10). X-ray diffraction data indicate a uniform V conformation of the ligands in the solid state.1 However, on the basis of the highly flexible character of ligands, it seems likely that solvation effects and solid-state packing forces may alter the structure. The fluxional character of the ligands described above for the singlet state is also evident in the triplet.32 The transition from the C3h cluster, with all ligands in the V conformation, to the C2v cluster results in an increase in stability of 2.9 kcal/mol (compared to a 1.4 kcal/mol decrease in the singlet); this value also corresponds well to the transition barrier to ligand rotation, as discussed for the singlet. Moving all three ligands to the tub conformation requires 3.0 kcal/mol (compared to 2.6 kcal/mol in the singlet). Single ligand dissociation requires 63.7 kcal/mol (compared to 57.2 kcal/mol in the singlet). Overall, these investigations suggest that while there are subtle differences in the triplet and singlet energy surfaces, both are sufficiently flat as to allow significant low-energy conformational changes in the ligands, resulting in ligand fluxionality.
’ CONCLUSIONS The recently synthesized tris(cyclooctatetraene)triiron, Fe3(COT)3, was investigated through DFT computational studies. In the gas phase, the molecule adopts C3h symmetry and pseudoη3:η5 hapticity to each of the COT ligands, which exist in a unique π-delocalized V conformation. The cluster bonding agrees with an oxidation state of Fe(0), with computed metal charges near 0. In addition, the COT ligands freely adopt a tub configuration, destabilizing by only ∼1 kcal/mol, rock with a
bS
Supporting Information. Text, a figure, and tables giving additional methodological details, an additional explanation of orbital occupancy in D3h triplet Fe3(COT)3, and Cartesian coordinates of reported structures. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT R.J.M. acknowledges the support of a grant from the Calvin College Integrated Science Research Institute, funded by the Howard Hughes Medical Institute. We gratefully acknowledge the donors of the Petroleum Research Fund, administed by the American Chemical Society, for partial support of this research. Computer hardware was provided by a Major Research Instrumentation Grant from the National Science Foundation, Award No. OCI-0722819. We further acknowledge conversations with R. B. King and H. F. Schaefer, University of Georgia, regarding the structure and bonding of Fe3(C8H8)3. ’ REFERENCES (1) Lavallo, V.; Grubbs, R. H. Science 2009, 326, 559–562. (2) Dewar, J.; Jones, H. O. Proc. R. Soc. London, Ser. A 1907, 79, 66–80. (3) Mingos, D. M. P.; Wales, D. J. Introduction to Cluster Chemistry; Prentice Hall: Englewood Cliffs, NJ, 1990. (4) Dyson, P. J.; McIndoe, J. S. Transition Metal Carbonyl Cluster Chemistry; Taylor & Francis: Oxford, U.K., 2000. (5) Lee, J. S.; Lei, Y.; Kumari, S.; Yang, D.-S. J. Chem. Phys. 2009, 131, 104304. (6) Thomas, P. M.; Weber, A. J. Raman Spectrosc. 1978, 7, 353–357. (7) Katz, T. J. J. Am. Chem. Soc. 1960, 82, 3784–3785. (8) Streitwieser, A.; Mueller-Westerhoff, U. J. Am. Chem. Soc. 1968, 90, 7364. (9) Allegra, G.; Colombo, A.; Immirzi, A.; Bassi, I. W. J. Am. Chem. Soc. 1968, 90, 4455–4456. (10) Bennett, M. A.; Neumann, H.; Willis, A. C.; Ballantini, V.; Pertici, P.; Mann, B. E. Organometallics 1997, 16, 2868–2878. (11) Barlow, S. M.; Raval, R. Surf. Sci. Rep. 2003, 50, 201–341. (12) Rosei, F.; Schunack, M.; Naitoh, Y.; Jiang, P.; Gourdon, A.; Laegsgaard, E.; Stensgaard, I.; Joachim, C.; Besenbacher, F. Prog. Surf. Sci. 2003, 71, 95–146. (13) Weigelt, S.; Busse, C.; Petersen, L.; Rauls, E.; Hammer, B.; Gothelf, K. V.; Besenbacher, F.; Linderoth, T. R. Nat. Mater. 2006, 5, 112–117. (14) Richardson, N. Nat. Mater. 2006, 5, 91–92. (15) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Guerra, C. F.; van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001, 22, 931–967. (16) Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Theor. Chem. Acc. 1998, 99, 391–403. 5200
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(17) ADF2009.01; SCM Theoretical Chemistry, Vrije Universiteit, Amsterdam, The Netherlands; http://www.scm.com. (18) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (19) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (20) Johnson, B. G.; Gill, P. M. W.; Pople, J. A. J. Chem. Phys. 1993, 98, 5612. (21) Perdew, J. P. Phys. Rev. B 1986, 33, 8822. (22) Grimme, S. J. Comput. Chem. 2006, 27, 1787–1799. (23) Michalak, A.; DeKock, R. L.; Ziegler, T. J. Phys. Chem. A 2008, 112, 7256–7263. (24) Work done in our laboratory. (25) C3h symmetry is not fully supported by ADF. Therefore, the calculation was run in Cs symmetry, though C3h symmetry was effectively retained. This applies throughout. (26) Hess, C. R.; Weyherm€uller, T.; Bill, E.; Wieghardt, K. Angew. Chem., Int. Ed. 2009, 48, 3703–3706. (27) Fonseca Guerra, C.; Handgraaf, J.-W.; Baerends, E. J.; Bickelhaupt, F. M. J. Comput. Chem. 2004, 25, 189–210. (28) These theoretical studies were performed using the D3h conformation of the molecule, since the energy difference between the C3h and D3h structures is miniscule. (29) The NMR reported by Lavallo and Grubbs was measured at ambient temperature. (30) The D3h conformation shows one imaginary frequency, supporting its role as a transition state. (31) For the BP86-D functional, this energy difference is 2.3 kcal/mol. (32) We do not report a D3h structure for the triplet state of Fe3(COT)3. We were unable to achieve nonfractional orbital occupations in this state; see the Supporting Information for additional details. (33) The single ligand dissociation energies are dependent on the chosen functional: BLYP (57 kcal/mol), BP86 (113 kcal/mol).
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