The Energetics of Forming a Diiron Pentacarbonyl Complex of Indene

Nov 20, 2013 - ... Diiron Pentacarbonyl Complex of Indene: Stabilization of a High-Energy ... School of Physics and Chemistry, Research Center for Adv...
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The Energetics of Forming a Diiron Pentacarbonyl Complex of Indene: Stabilization of a High-Energy Tautomer Rong Jin,† Xiaohong Chen,† Quan Du,† Hao Feng,*,† Yaoming Xie,‡ and R. Bruce King*,‡ †

School of Physics and Chemistry, Research Center for Advanced Computation, Xihua University, Chengdu 610039, People’s Republic of China ‡ Department of Chemistry and Center for Computational Chemistry University of Georgia, Athens, Georgia 30602, United States S Supporting Information *

ABSTRACT: The reaction of indene with Fe2(CO)9 has been shown to result in stabilization of the unstable 7H tautomer of indene as the diiron pentacarbonyl complex (η5:η3-7H-C9H8)Fe2(CO)5. The diiron pentacarbonyl complexes of the four indene tautomers, including the stable and readily available 1H-indene as well as the unstable 2H-, 6H-, and 7H-indenes, have now been investigated by density functional theory. The lowest energy of all of the (indene)Fe2(CO)5 structures is the experimentally observed (η5:η3-7H-C9H8)Fe2(CO)5, derived from the highest energy indene tautomer, namely 7Hindene, which is not known in the free state. The lowest energy diiron pentacarbonyl complex of the stable 1H-indene tautomer, namely (η6:η2-1H-C9H8)Fe2(CO)5, lies ∼11 kcal/mol in energy above this experimentally known isomer. The lowest energy diiron pentacarbonyl complexes of the remaining two indene tautomers, namely (η4:η4-6H-C9H8)Fe2(CO)5 and (η4:η4-2H-C9H8)Fe2(CO)5, are still higher energy structures, lying ∼14 and ∼19 kcal/ mol, respectively, in energy above (η5:η3-7H-C9H8)Fe2(CO)5.

1. INTRODUCTION The special role of the cyclopentadienyl ligand in organometallic chemistry was first recognized in the early 1950s by the seminal discovery of ferrocene, (η5-C5H5)2Fe.1−4 The high stability of ferrocene reflects the stability of η5-C5H5M linkages and stimulated the development of extensive chemistry of a variety of cyclopentadienylmetal derivatives. In addition, deprotonation of the readily available indene was recognized as a source of the closely related η5-indenyl ligand (η5-C9H7). This soon led to the discovery of bis(indenyl)iron, (η5C9H7)2Fe, i.e., dibenzoferrocene, only two years after the original report of ferrocene.5,6 In addition to being a source of the five-electron-donor indenyl ligand, η5-C9H7, unprotonated free indene, C9H8, is a potential eight-electron-donor ligand through its benzene ring and the exocyclic CC double bond in its five-membered ring. Exploring this possibility, Cotton and Hanson synthesized (η5:η3-C9H8)Fe2(CO)5 by the reaction of indene with Fe2(CO)9 at room temperature.7 Formally, this product can be derived from Fe2(CO)9 by replacing four CO groups with a single eight-electron-donor indene ligand. X-ray crystallography of this product showed that the “extra” hydrogen of the CH2 group in the five-membered ring of free indene had migrated to the six-membered ring in its Fe2(CO)5 complex (Figure 1). As a result, all five carbons in the indene five-membered ring in (η5:η3-C9H8)Fe2(CO)5 are bonded to one of the iron atoms, leaving three sp2 carbon atoms in the indene six-membered ring for η3 allylic bonding to the other iron atom. Thus, the reaction of indene with Fe2(CO)9 results in isomerization to give a highenergy tautomer of indene, stabilized by complexation with iron carbonyl. This is related to the isomerization of diolefins by © 2013 American Chemical Society

Figure 1. The four possible isomers of indene and the structure of the iron carbonyl complex (η5:η3-C9H8)Fe2(CO)5 synthesized by Cotton and Hanson. The stable isomer of indene (1H-indene) is enclosed in a box. The iron complex is derived from 7H-indene.

reaction with Fe(CO)5, first reported more than 50 years ago by Arnet and Pettit.8 The isomerization of indene upon forming an Fe2(CO)5 complex raises the question of the relative stabilities of the four possible indene isomers (Figure 1) and their C9H8Fe2(CO)5 complexes. This paper reports the use of density functional theory to explore this possibility.

2. THEORETICAL METHODS All computations were performed using double-ζ plus polarization (DZP) basis sets. The DZP basis sets used for carbon and oxygen add one set of pure spherical harmonic d functions with orbital exponents αd(C) = 0.75 and αd(O) = 0.85 to the standard Huzinaga−Dunning contracted DZ sets9,10 and are designated as (9s5p1d/4s2p1d). For hydrogen, a set of p polarization functions αp(H) = 0.75 is added to Received: September 16, 2013 Published: November 20, 2013 7418

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the Huzinaga−Dunning DZ set, which is H(4s1p/2s1p). The loosely contracted DZP basis set for iron is the Wachters primitive set11 augmented by two sets of p functions and a set of d functions, contracted following Hood, Pitzer, and Schaefer,12 designated as (14s11p6d/10s8p3d). Electron correlation effects were considered by employing density functional theory (DFT) methods, which have evolved as a practical and effective computational tool, especially for organometallic compounds.13−19 Three DFT methods were used in this study. The first DFT method is based on the B3LYP functional, which is a hybrid HF/DFT functional combining the three-parameter Becke functional (B3)20 with the Lee−Yang−Parr (LYP) generalized gradient correlation functional.21 The second DFT method used in the present paper is BP86, which combines Becke’s 1988 exchange functional (B)22 with Perdew’s 1986 gradient-corrected correlation functional (P86).23 These two DFT methods usually predict similar molecular geometries. However, Reiher et al. have found that B3LYP always favors the high-spin state, whereas BP86 favors the low-spin state.24 For this reason, they proposed a new parametrization for the B3LYP functional, named B3LYP*, which provides electronic state orderings in better agreement with experiment.25 Since we are interested in reliable values for singlet−triplet splittings, we use exclusively the optimized B3LYP* geometries and energy orderings for the discussions in the text. However, we have also studied all of the systems by the B3LYP and BP86 methods and present the results in the Supporting Information, All three DFT methods are found to give virtually identical differences for the tautomer energies with the same spin multiplicity. The geometries of all structures were fully optimized. The vibrational frequencies and infrared intensities were determined analytically. Since the BP86 method has been shown to predict vibrational frequencies closer to experimental values than the B3LYP method without using any scaling factors,26,27 we discuss only the BP86 ν(CO) frequencies in the text. All computations were carried out using the Gaussian 09 program.28 The fine grid (75, 302) was the default for the numerical evaluation of the integrals, while the finer grid (120, 974) was used only to evaluate the small imaginary vibrational frequencies.29 In the present paper, the symbols mH-0 are used to denote the free indene tautomers, where m stands for the position of the CH2 carbon atom in the indene skeleton (see Figure 1). For the (C9H8)Fe2(CO)5 complexes, the symbols mH-Fe-Sn or mH-Fe-Tn are used, where n orders the isomers according to their energies, and S and T stand for singlet and triplet spin states, respectively. Thus, the global minimum of the (7H-C9H8)Fe2(CO)5 structure is 7H-Fe-S1. All of the (C9H8)Fe2(CO)5 structures studied in the present paper have C1 symmetry.

Figure 2. Optimized geometries of the four indene tautomers.

Table 1. Total Energies (E, in hartree) and Relative Energies (ΔE, in kcal/mol) Predicted by the B3LYP* Method for the Four Tautomers of the Free Indene −E ΔE

1H-0 (Cs, 1A′)

2H-0 (C2v, 1A1)

6H-0 (Cs, 1A′)

7H-0 (Cs, 1A′)

347.55557 0.0

347.52122 21.6

347.51574 25.0

347.51069 28.2

Between these two tautomers, the Cs tautomer 6H-indene (6H0) lies 25.0 kcal/mol above 1H-0. The highest energy tautomer 7H-indene (7H-0) lies at an even higher energy of 28.2 kcal/ mol above 1H-0. 3.2. (C9H8)Fe2(CO)5 Structures Derived from the Various Indene Tautomers. 3.2.1. Pentacarbonyl(7Hindene)diiron, (7H-C9H8)Fe2(CO)5. Four low-lying (7H-C9H8)Fe2(CO)5 structures were found, namely one singlet and three triplets (Figure 3). The predicted global minimum is the singlet 7H-Fe-S1. All three triplet (7H-C9H8)Fe2(CO)5 structures are

3. RESULTS AND DISCUSSION 3.1. The Tautomers of Indene. There are four indene tautomers, depending on the location of the CH2 group. The stable tautomer is 1H-indene, a common inexpensive hydrocarbon available commercially. The three remaining tautomers are 2H-indene, 6H-indene, and 7H-indene (Figures 1 and 2). We have investigated the relative stabilities of the four indene tautomers. Their geometries and energies predicted by the B3LYP* method are shown in Figure 2 and Table 1. As expected, the known stable 1H-indene (1H-0) is the global minimum, lying more than 20 kcal/mol in energy below each of the other tautomers (Table 1). This tautomer is the only indene tautomer with a benzenoid six-membered ring. The C2v tautomer 2H-indene (2H-0) lies 21.6 kcal/mol (B3LYP*) in energy above 1H-0. This tautomer has an o-quinonoid arrangement of the four CC double bonds, consistent with its higher energy relative to 1H-0. The remaining two indene tautomers have the CH2 group in the six-membered ring and a fulvene arrangement of three of the four CC double bonds.

Figure 3. The four (7H-C9H8)Fe2(CO)5 structures. 7419

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predicted to have strong terminal ν(CO) frequencies at 2017, 1989, and 1967 cm−1, which agree well with the experimental results of 2038, 1983, and 1971 cm−1.7 There is one low ν(CO) frequency for 7H-Fe-S1 (1848 cm−1) and for 7H-Fe-T1 (1812 cm−1), consistent with the prediction of a single semibridging CO group by the BP86 method. The other ν(CO) frequencies ranging from 1920 to 2017 cm−1 are in the typical range for the terminal CO groups. 3.2.2. Pentacarbonyl(1H-indene)diiron, (1H-C9H8)Fe2(CO)5. Three low-lying singlet structures were found for pentacarbonyl-1H-indenediiron (1H-C9H8)Fe2(CO)5 (Figure 4 and Table 4). All of the pentacarbonyl(1H-indene)diiron isomers have a nearly planar six-membered ring and a nonplanar fivemembered ring, owing to the CH2 sp3 carbon atom in the latter. The lowest-lying (1H-C9H8)Fe2(CO)5 structure 1H-Fe-S1 lies 10.7 kcal/mol above the global minimum 7H-Fe-S1 (Figure 4 and Table 4). Structure 1H-Fe-S1 has a bridging (η6:η2-μ-C9H8) ligand bonded to the Fe(CO)2 group as a hexahapto ligand and to the Fe(CO)3 group as a dihapto ligand. The Fe−Fe distance of 2.966 Å can be considered to be a long formal single bond. However, this can be regarded as a dative bond from the Fe(CO)2 iron atom to the Fe(CO)3 iron atom thereby giving each iron atom the favored 18-electron configuration. The (1H-C9H8)Fe2(CO)5 structure 1H-Fe-S2 lies 29.5 kcal/ mol in energy above 7H-Fe-S1 (Figure 4 and Table 4). It has a bridging CO group and a bridging η2:η2-μ-C9H8 ligand, leaving two uncomplexed CC double bonds in the indene system of lengths 1.384 and 1.391 Å. The short predicted FeFe distance of 2.424 Å can correspond to a formal triple bond, thereby giving each Fe atom the favored 18-electron configuration. The (1H-C9H8)Fe2(CO)5 structure 1H-Fe-S3, lying 32.7 kcal/mol in energy above 7H-Fe-S1, has a bridging η6:η2-μC9H8 ligand and a bridging CO group to connect the two fragments (Figure 4 and Table 4). The bridging CO group in 1H-Fe-S3 has an unusually short Fe−O distance of 2.056 Å and exhibits an unusually low ν(CO) frequency at 1603 cm−1 indicating it to be a four-electron-donor bridging η2-μ-CO group. The long Fe···Fe distance of 3.501 Å indicates the lack of an iron−iron bond. However, each Fe atom in 1H-Fe-S3 has the favored 18-electron configuration. The harmonic ν(CO) frequencies predicted by the BP86 method for these three (1H-C9H8)Fe2(CO)5 structures are given in Table 5. The ν(CO) frequency for the usual type of two-electron-donor bridging CO group in 1H-Fe-S2 is 1832 cm−1. However, the ν(CO) frequency for the four-electrondonor bridging η2-μ-CO group in 1H-Fe-S3 is 1603 cm−1, which is ∼230 cm−1 below that of the normal bridging CO group in 1H-Fe-S2. The terminal ν(CO) frequencies for the terminal CO groups in these (1H-C9H8)Fe2(CO)5 structures are significantly higher, ranging from 1918 to 2020 cm−1. 3.2.3. Pentacarbonyl(2H-indene)diiron, (2H-C9H8)Fe2(CO)5. Four low-lying (2H-C9H8)Fe2(CO)5 structures (two triplets and two singlets) are found (Figure 5 and Table 6). In the two singlet (2H-C9H8)Fe2(CO)5 structures, 2H-Fe-S1 and 2H-FeS2, both the five-membered ring and the six-membered ring are distorted away from planarity. In the triplet structure 2H-FeT1, the five-membered ring is nonplanar, while the sixmembered ring is roughly planar. In 2H-Fe-T2, the sixmembered ring is nonplanar, while the five-membered ring is roughly planar.

relatively high energy structures, lying at least 15 kcal/mol above 7H-Fe-S1 (Table 2). In 7H-Fe-S1 there is a η5:η3-μTable 2. Total Energies (E, in hartree), Relative Energies (ΔE, in kcal mol−1), Fe−Fe Distances (in Å), and Spin Expectation Values (⟨S2⟩) for the Four (7H-C9H8)Fe2(CO)5 Structures, as Predicted by the B3LYP* Method (Figure 3)a

ΔE Fe−Fe ⟨S2⟩ a

7H-Fe-S1 (1A)

7H-Fe-T1 (3A)

7H-Fe-T2 (3A)

7H-Fe-T3 (3A)

0.0 2.838 0.0

15.8 3.746 2.05

16.2 3.628 2.05

27.9 2.774 2.06

None of the structures have any imaginary vibrational frequencies.

C9H8 bridging ligand bonded to the Fe(CO)2 moiety as a η5 ligand and to the Fe(CO)3 moiety as a η3 ligand. This is in good agreement with the observed experimental structure, determined by X-ray crystallography.7 The predicted Fe−Fe distance in 7H-Fe-S1 of 2.838 Å (B3LYP*) is comparable to the experimental value of 2.782 Å, and suggests the formal Fe− Fe single bond required to give the each Fe atom the favored 18-electron configuration. In the structure 7H-Fe-S1, the sixmembered ring of 7H-indene is not planar, since the methylene carbon atom is distorted out of the plane of the other five carbon atoms, also in agreement with experiment.7 The two triplet (7H-C9H8)Fe2(CO)5 structures 7H-Fe-T1 and 7H-Fe-T2, lying ∼16 kcal/mol above 7H-Fe-S1 (B3LYP*), also have η5:η3-μ-C9H8 bridging ligands bonded to Fe(CO)2 and Fe(CO)3 moieties similar to the singlet structure 7H-Fe-S1 (Figure 3 and Table 2). The predicted long Fe···Fe distances of >3.6 Å in 7H-Fe-T1 and 7H-Fe-T2 indicate the lack of a direct iron−iron interaction. This gives each iron atom a 17-electron configuration consistent with the triplet spin state. The other triplet structure 7H-Fe-T3 has an η2:η4-μ-C9H8 bridging ligand and lies 27.9 kcal/mol above 7HFe-S1. The structure 7H-Fe-T3 also has five terminal CO groups. However, the Fe(CO)2 fragment is bonded to the η4 six-membered ring as a η4 ligand and the Fe(CO)3 fragment is bonded to the five-membered ring as a η2 ligand. The Fe−Fe distance of 2.774 Å in 7H-Fe-T3 suggests a formal Fe−Fe single bond, thereby giving the each Fe atom the 17-electron configuration for a binuclear triplet. The six-membered rings in the three triplet (7H-C9H8)Fe2(CO)5 structures, like those in 7H-Fe-S1, are also distorted from planarity. The theoretical ν(CO) vibrational frequencies predicted by the BP86 method for these four (7H-C 9 H 8 )Fe 2 (CO) 5 structures are given in Table 3. The structure 7H-Fe-S1 is Table 3. Harmonic ν(CO) Vibrational Frequencies (in cm−1) Predicted by the BP86 Method for the Four (7HC9H8)Fe2(CO)5 Structuresa BP86 7H-FeS1 7H-FeT1 7H-FeT2 7H-FeT3

1848 (295), 1953 (154), 1967 (949), 1989 (1132), 2017 (854) 1812 (383), 1932 (296), 1962 (851), 1972 (1273), 2008 (837) 1938 (270), 1949 (619), 1963 (1066), 1977 (869), 2011 (831) 1920 (264),1937(89), 1956 (1011), 1976 (1249), 2007 (915)

exptl7 1971, 1983, 2038

a

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Figure 4. The three (1H-C9H8)Fe2(CO)5 structures.

Table 4. Energies Relative to the Global Minimum 7H-Fe-S1 (ΔE, in kcal/mol) and the Fe−Fe Distances (in Å) for the Three (1H-C9H8)Fe2(CO)5 Structures, as Predicted by the B3LYP* Methoda ΔE Fe−Fe a

1H-Fe-S1 (1A)

1H-Fe-S2 (1A)

1H-Fe-S3 (1A)

10.7 2.966

29.5 2.424

32.7 3.501

Table 6. Energies Relative to the Global Minimum 7H-Fe-S1 (ΔE, in kcal/mol) and Fe−Fe Distances (in Å) for the Four (2H-C9H8)Fe2(CO)5 Structures, as Predicted by the B3LYP* Methoda

ΔE Fe−Fe ⟨S2⟩

None of these structures have any imaginary vibrational frequencies. a

Table 5. Harmonic ν(CO) Vibrational Frequencies (in cm−1) Predicted by the BP86 Method for the (1HC9H8)Fe2(CO)5 Structuresa 1H-Fe-S1 1H-Fe-S2 1H-Fe-S3

2H-Fe-S1 (1A)

2H-Fe-S2 (1A)

2H-Fe-T1 (3A)

2H-Fe-T2 (3A)

19.4 2.849 0.0

20.4 2.742 0.0

31.0 3.104 2.086

32.3 2.663 2.082

None of these structures have any imaginary vibrational frequencies.

η4:η4-μ-C9H8 ligand. The structure 2H-Fe-S2 differs from 2HFe-S1 in having a bridging CO group. This shortens the Fe−Fe distance from 2.849 Å in 2H-Fe-S1 to 2.742 Å in 2H-Fe-S2. In both 2H-Fe-S1 and 2H-Fe-S2 the Fe−Fe bonds can be interpreted as formal single bonds, thereby giving each iron atom the favored 18-electron configuration. The triplet (2H-C9H8)Fe2(CO)5 structure 2H-Fe-T1, lying 31.0 kcal/mol in energy above 7H-Fe-S1, has all five terminal CO groups and a η3:η4-μ-C9H8 bridging indene ligand (Figure 5 and Table 6). The indene ligand is bonded to the Fe(CO)2 moiety through the six-membered ring as a η3 ligand and to the Fe(CO)3 moiety through the five-membered ring as a η4 ligand. The Fe···Fe distance of 3.104 Å in 2H-Fe-T1 is too long for an iron−iron bond. Another triplet (2H-C9H8)Fe2(CO)5 structure, 2H-Fe-T2, lies only 1.3 kcal/mol in energy above 2H-FeT1 so that the two triplet (2H-C9H8)Fe2(CO)5 structures 2HFe-T1 and 2H-Fe-T2 can be considered to be degenerate. The structure 2H-Fe-T2 has one bridging CO group and a η4:η2-μC9H8 bridging indene ligand with an uncomplexed CC double bond of length 1.359 Å. The predicted Fe−Fe distance in 2H-Fe-T2 of 2.663 Å (B3LYP*) is consistent with a formal single bond. This gives each iron atom in 2H-Fe-T2 the 17electron configuration for a binuclear triplet, assuming that the iron atom bonded to the six-membered indene ring has a formal positive charge and the iron atom bonded to the fivemembered indene ring has a formal negative charge. The ν(CO) frequencies predicted by the BP86 method for these four (2H-C9H8)Fe2(CO)5 structures are given in Table 7.

1931 (217), 1941 (183), 1969 (1053), 1984 (933), 2019 (926) 1832 (386), 1918 (197), 1951 (865), 1968 (1649), 2002 (755) 1603 (143), 1958 (421), 1963 (439), 1969 (1432) 2020 (888)

a

The infrared intensities (in km/mol) are given in parentheses. The bridging ν(CO) frequencies are give in boldface.

Figure 5. The four (2H-C9H8)Fe2(CO)5 structures.

Table 7. Harmonic ν(CO) Vibrational Frequencies (in cm−1) Predicted by the BP86 Method for the Four (2HC9H8)Fe2(CO)5 Structuresa

The singlet structure 2H-Fe-S1 is the lowest-lying (2HC9H8)Fe2(CO)5 structure (Figure 5 and Table 6). However, 2H-Fe-S1 lies 19.4 kcal/mol above the (7H-C9H8)Fe2(CO)5 global minimum 7H-Fe-S1. The structure 2H-Fe-S1 has a η4:η4-μ-C9H8 ligand bridging the central Fe2 unit. The Fe(CO)2 unit is bonded to the indene six-membered ring, and the Fe(CO)3 unit is bonded to the indene five-membered ring. The structure 2H-Fe-S2, lying only 1.1 kcal/mol above 2H-Fe-S1, has a geometry similar to that of 2H-Fe-S1, also with a bridging

2H-Fe-S1 2H-Fe-S2 2H-Fe-T1 2H-Fe-T2

1935 (155), 1963 (12), 1973 (972), 1981 (1335), 2026 (840) 1831 (321), 1953 (75,) 1963 (1065), 1987 (1180), 2012 (820) 1949 (229), 1925 (717), 1959 (1000), 1973 (989) 2012 (799) 1823 (370), 1933 (529), 1963 (641), 1969 (1370) 2009 (906)

a

The infrared intensities (in km/mol) are given in parentheses. The bridging ν(CO) frequencies are given in boldface.

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Figure 6. The three (6H-C9H8)Fe2(CO)5 structures.

The ν(CO) frequencies for the bridging CO groups range from 1823 to 1831 cm−1. However, the ν(CO) frequencies for the terminal CO groups are significantly higher, ranging from 1933 to 2026 cm−1, consistent with expectation. 3.2.4. Pentacarbonyl(6H-indene)diiron, (6H-C9H8)Fe2(CO)5. Three low-lying (6H-C9H8)Fe2(CO)5 structures (one singlet and two triplets) are obtained (Figure 6 and Table 8). In the

Table 9. Harmonic ν(CO) Vibrational Frequencies (in cm−1) Predicted by the BP86 Method for the Pentacarbonyl(6H-indene)diiron (C9H8)Fe2(CO)5 Isomersa 6H-Fe-S1 6H-Fe-T1 6H-Fe-T2 a

The infrared intensities (in km/mol) are given in parentheses. The bridging ν(CO) frequencies are given in boldface.

Table 8. Energies Relative to the Global Minimum 7H-Fe-S1 (ΔE, in kcal/mol) and the Fe−Fe Distances (in Å) for the Three (6H-C9H8)Fe2(CO)5 Structures, Predicted by the B3LYP* Methoda ΔE Fe−Fe ⟨S2⟩ a

6H-Fe-S1 (1A)

6H-Fe-T1 (3A)

6H-Fe-T2 (3A)

13.5 2.736 0

29.3 2.764 2.069

35.7 2.674 2.090

1814 (337), 1954 (45), 1968 (1062), 1991 (1173), 2016 (803) 1827 (280), 1940 (571), 1961 (834), 1973 (130), 2004 (882) 1934 (86), 1940 (265), 1954 (1057), 1972 (1586), 2009 (739)

4. DISCUSSION The lowest energy (1H-C9H8)Fe2(CO)5 structure 1H-Fe-S1 (Figure 4) derived from the stable tautomer of indene, namely 1H-indene (1H-0 in Figure 2), has the six-membered ring bonded to an Fe(CO)2 unit, thereby giving that iron atom the favored 18-electron configuration. In addition, the CC double bond in the five-membered 1H-indene ring is bonded to the Fe(CO)3 unit, which would give that iron atom only a 16-electron configuration. However, the Fe−Fe distance in 1HFe-S1 of ∼2.97 Å is consistent with a dative bond from the Fe(CO)2 iron to the Fe(CO)3 iron, thereby giving each iron atom the favored 18-electron configuration. The lowest energy (C9H8)Fe2(CO)5 structure is not this (1H-C9H8)Fe2(CO)5 structure 1H-Fe-S1 but instead the (7HC9H8)Fe2(CO)5 structure 7H-Fe-S1 (Figure 3) derived from the unstable 7H tautomer of indene (7H-0 in Figure 2). Thus, 7H-Fe-S1 lies 10.7 kcal/mol below the lowest energy structure 1H-Fe-S1 derived from the stable tautomer of indene 1H-0. The structure 7H-Fe-S1 is known experimentally, having been synthesized from Fe2(CO)9 and indene and structurally characterized by X-ray crystallography.7 In 7H-Fe-S1 the “extra” indene hydrogen has migrated to the six-membered indene ring so that the five-membered ring can bond to the Fe(CO)2 unit as a η5 ligand similar to that in the thousands of known stable η5-C5H5 metal complexes. This leaves three adjacent sp2 carbons uniquely in the indene six-membered ring to bond to the Fe(CO)3 moiety as a η3 ligand. An Fe−Fe single bond of ∼2.84 Å length in 7H-Fe-S1 gives each iron atom the favored 18-electron configuration. The bonding of the bicyclic indene ligand to the Fe2(CO)5 moiety in 7H-Fe-S1 is very similar to the bonding of the bicyclic azulene ligand to the Fe2(CO)5 moiety in the known azulene diiron pentacarbonyl complex (η5:η3-C10H8)Fe2(CO)5, which has also been synthesized and structurally characterized by X-ray crystallography.30,31 In this azulene complex, as in 7HFe-S1, the five-membered ring of the bicyclic hydrocarbon is bonded to the Fe(CO)2 unit as a η5 ligand. Furthermore, in the azulene complex three adjacent carbon atoms in the azulene seven-membered ring are bonded to the Fe(CO)3 unit. The experimental Fe−Fe single-bond distance of 2.782 Å in the

None of these structures have any imaginary vibrational frequencies.

singlet structure 6H-Fe-S1, the five-membered ring is only slightly distorted from planarity but the six-membered ring is severely nonplanar. In 6H-Fe-T1, both rings are only slightly distorted away from planarity. In 6H-Fe-T2, the five-membered ring is nearly coplanar and the six-membered ring is severely distorted away from planarity. The singlet (6H-C9H8)Fe2(CO)5 structure 6H-Fe-S1, lying 13.5 kcal/mol above the global minimum 7H-Fe-S1, has four terminal CO groups, one bridging CO group, and a bridging η4:η4-μ-C9H8 indene ligand (Figure 6 and Table 8). In 6H-FeS1, the predicted Fe−Fe distance of 2.736 Å suggests the formal single bond required to give each Fe atom the favored 18-electron configuration. The triplet (6H-C9H8)Fe2(CO)5 structure 6H-Fe-T1, lying 29.3 kcal/mol above 7H-Fe-S1, has a semibridging CO group and a bridging η5:η2-μ-C9H8 indene ligand (Figure 6 and Table 8). The predicted Fe−Fe distance of 2.764 Å in 6H-Fe-T1 is consistent with a formal single bond. The other triplet (6HC9H8)Fe2(CO)5 structure, 6H-Fe-T2, is a significantly higher energy structure, lying 35.7 kcal/mol above 7H-Fe-S1. The structure 6H-Fe-T2 has five terminal CO groups and a bridging η1:η4-μ-C9H8 indene ligand. The Fe−Fe distance in 6H-Fe-T2 is predicted to be 2.674 Å, suggesting a formal single bond similar to that in 6H-Fe-T1. The ν(CO) frequencies predicted by the BP86 method for these three (6H-C9H8)Fe2(CO)5 structures are given in Table 9. The ν(CO) frequencies for the bridging and semibridging CO groups are 1814 and 1827 cm−1. The ν(CO) frequencies for terminal CO groups are significantly higher, ranging from 1934 to 2016 cm−1. 7422

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Organometallics

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azulene complex (η5:η3-C10H8)Fe2(CO)5 is essentially identical with the experimental Fe−Fe distance of 2.782 Å in the indene complex (η5:η3-7H-C9H8)Fe2(CO)5. In the azulene complex (η5:η3-C10H8)Fe2(CO)5 the uncomplexed portion of the azulene ligand is one of the CC double bonds in the seven-membered azulene ring. However, in the indene complex (η5:η3-7H-C9H8)Fe2(CO)5 the uncomplexed portion of the indene ligand is the CH2 group in the six-membered indene ring. The lower energy of the (η 5 :η 3 -7H-C 9 H 8 )Fe 2 (CO) 5 derivative 7H-Fe-S1 of the unstable indene tautomer 7H-0 relative to the (η6:η2-1H-C9H8)Fe2(CO)5 derivative 1H-Fe-S1 derived from the stable indene tautomer 1H-0 by ∼11 kcal/mol is a consequence of the favorability of η5 bonding of a cyclopentadienyl ring to a transition metal relative to other types of hydrocarbon bonding to transition metals. However, the higher energy (η6:η2-1H-C9H8)Fe2(CO)5 structure 1H-FeS1 is a probable intermediate in the reported synthesis7 of 7HFe-S1 from indene and Fe2(CO)9. Thus, the activation energy of ∼11 kcal/mol for the 1H-0 → 1H-Fe-S1 → 7H-Fe-S1 pathway is much lower than the activation energy of ∼28 kcal/ mol for the 1H-0 → 7H-0 → 7H-Fe-S1 pathway in the conversion of indene to the known (η5:η3-7H-C9H8)Fe2(CO)5 (i.e., 7H-Fe-S1). In order to gain a more complete picture of the (indene)Fe2(CO)5 system, the diiron pentacarbonyl complexes of the remaining two indene tautomers (Figure 2) were also optimized using the same density functional methods. The lowest energy Fe2(CO)5 derivatives of these unstable indene tautomers were found to lie at significantly higher energies above the lowest energy (indene)Fe2(CO)6 structure 7H-Fe1S. Thus, the lowest energy Fe2(CO)5 complex of 6H-indene is the bis(tetrahapto) (6H-η4:η4-C9H8)Fe2(CO)5 structure 6HFe-1S, lying ∼14 kcal/mol above the global minimum 7H-Fe1S (Figure 6). In 6H-Fe-1S a cis-1,3-diene unit in each 6Hindene ring is bonded to an iron atom with a Fe−Fe single bond of predicted length 2.736 Å, giving each iron atom the favored 18-electron configuration. The lowest energy Fe2(CO)6 complex of 2H-indene, namely 2H-Fe-1S (Figure 5), is an even higher energy structure, lying ∼19 kcal/mol above the global minimum. In structure 2H-Fe-1S, as in structure 6H-Fe-1S, a cis 1,3-diene unit in each indene ring is bonded to an iron atom with an Fe−Fe single bond of predicted length ∼2.85 Å to give each iron atom the favored 18-electron configuration.

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Article

ACKNOWLEDGMENTS We are indebted to the New Century Excellent Talents in University (Grant No. NCET-10-0949), Chinese National Natural Science Foundation (Grant No. 11174236), and the U.S. National Science Foundation (Grant CHE-1057466) for the support of this research.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Tables giving the total energies, relative energies, Fe−Fe distances, harmonic vibrational frequencies, and infrared intensities for the tautomers of free indene and pentacarbonyl(indene)diiron, (C9H8)Fe2(CO)5. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for H.F.: [email protected]. *E-mail for R.B.K.: [email protected]. Notes

The authors declare no competing financial interest. 7423

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