Article pubs.acs.org/Organometallics
Bonding of Iron Tricarbonyl Units to Heptafulvene: Trimethylenemethane, Butadiene, and Allylic Coordination Modes Huidong Li,† Hao Feng,*,† Weiguo Sun,†,‡ Qunchao Fan,† Yaoming Xie,§ R. Bruce King,*,§ and Henry F. Schaefer, III§ †
School of Physics and Chemistry, Research Center for Advanced Computation, Xihua University, Chengdu 610039, People’s Republic of China ‡ Institute of Atomic and Molecular Physics, Sichuan University, Chengdu, Sichuan 610065, 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: Complexation of the very unstable free heptafulvene with iron carbonyls is known experimentally to lead to stable complexes, including two isomers of (η4-C7H6CH2)Fe(CO)3 as well as the binuclear (C7H6CH2)2Fe2(CO)6. Density functional theory shows that structures of the mononuclear (C7H6CH2)Fe(CO)3 with trimethylenemethane and butadiene subunits of the heptafulvene ligand bonded to the Fe(CO)3 moiety have nearly equal energies within ∼3 kcal/mol, consistent with the experimental observation of two isomers depending upon the synthetic method. For (C7 H6 CH2 )Fe2 (CO) 6 a trans-η 4 :η 4 structure with no iron−iron bond and a cis-η3:η3 allylic structure with an iron−iron bond are predicted to have energies within ∼3.0 kcal/mol. Comparison of the predicted ν(CO) frequencies for these two structures with the experimental ν(CO) frequencies for the structurally uncharacterized (C7H6CH2)Fe2(CO)6 suggests the cis-η3:η3 allylic structure for the latter. trimethylenemethaneiron tricarbonyl, [η4-(CH2)3C]Fe(CO)3, in which the four carbon atoms bonding to the Fe(CO)3 unit have a Y-shaped branched arrangement (Figure 1). The structure of trimethylenemethaneiron tricarbonyl was originally confirmed by electron diffraction in the gas phase7 and much later by X-ray crystallography in the solid state.8,9 The synthesis of trimethylenemethaneiron tricarbonyl was more difficult than that of butadieneiron tricarbonyl since trimethylenemethane is a diradical, is not stable in the free state, and thus is not available as a reagent to react with iron carbonyls. However, the dechlorination of CH2C(CH2Cl)2 with Fe2(CO)9 provides a good synthesis of trimethylenemethaneiron tricarbonyl. Heptafulvene is a ligand of interest in iron carbonyl chemistry, since it contains both butadiene and trimethylenemethane subunits (Figure 2). Free heptafulvene was first synthesized by Doering and Wiley in 1960.10 However, it was found to be stable only in dilute solutions at low temperatures. Attempts to isolate pure heptafulvene, even at −60 °C, gave only polymer. This instability limits severely the use of heptafulvene as a reagent for the preparation of metal complexes. However, heptafulvene metal complexes, including both of the heptafulveneiron tricarbonyl isomers in Figure
1. INTRODUCTION The discovery that two carbonyl groups in Fe(CO)5 can be replaced by a four-carbon hydrocarbon ligand dates back to the 1930 synthesis of butadieneiron tricarbonyl by Reihlen and coworkers1 by the reaction of Fe(CO)5 with butadiene in an autoclave. The currently accepted (η4-C4H6)Fe(CO)3 structure of butadieneiron tricarbonyl, anticipated from consideration of the 18-electron rule2,3 (Figure 1), was proposed by Hallam and Pauson4 in 1958 and subsequently confirmed by Mills and Robinson5 in 1963 using X-ray crystallography at −40 °C. The four carbon atoms of butadiene bonding to the Fe(CO)3 unit in butadieneiron tricarbonyl have a linear arrangement. Approximately 35 years later Emerson, Watt, and Pettit6 discovered an isomer of butadieneiron tricarbonyl, namely
Figure 1. Isomeric iron tricarbonyl complexes of butadiene and trimethylenemethane. © 2013 American Chemical Society
Received: July 1, 2013 Published: August 23, 2013 4912
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DFT method, combining the three-parameter Becke functional (B3) with the Lee−Yang−Parr (LYP) generalized gradient correlation functional.21,22 This method includes exact exchange and is calibrated by fitting three parameters to a set of experimental results. The BP86 method combines Becke’s 1988 exchange functional (B) with Perdew’s 1986 gradient-corrected correlation functional method (P86).23,24 This method does not include exact exchange and is mainly deduced by forcing the functional to satisfy certain exact constraints on the basis of first principles. When these two conceptually different DFT methods agree, confident predictions can be made. The double-ζ plus polarization (DZP) basis sets were used for the optimizations with the B3LYP and BP86 methods. For carbon and oxygen one set of pure spherical harmonic d functions with orbital exponents αd(C) = 0.75 and αd(O) = 0.85 is added to the standard Huzinaga−Dunning contracted DZ sets. This basis set is designated (9s5p1d/4s2p1d).25,26 For H, a set of p polarization functions αp(H) = 0.75 is added to the Huzinaga−Dunning DZ sets. For the first-row transition metals, in our loosely contracted DZP basis sets, the Wachters primitive sets are used, but augmented by two sets of p functions and one set of d functions, contracted following Hood et al. and designated (14s11p6d/10s8p3d).27,28 Dispersion and basis set effects were studied using the Grimme functional B97D29 including dispersion and the much larger def2-TZVPP basis sets30 for all elements. Since previous studies show that the BP86 method usually gives ν(CO) vibrational frequencies closest to the experimental results without using scaling factors,31,32 the def2-TZVPP BP86 method was also used for the lower energy (C7H6CH2)Fe2(CO)6 structures. The geometries of all structures were fully optimized using the DZP B3LYP, DZP BP86, and def2-TZVPP B97D methods as well as the def2-TZVPP BP86 method for a few key structures. Harmonic vibrational frequencies were determined at the same levels by evaluating analytically the second derivatives of the energy with respect to the nuclear coordinates. The corresponding infrared intensities were evaluated analytically as well. All of the computations were carried out with the Gaussian 09 program,33 in which the fine grid (75, 302) is the default for evaluating integrals numerically, while the tight designation is the default for the energy convergence. For structures with small imaginary vibrational frequencies, the finer integration grid (120, 974) was used for further evaluation. In the search for minima, low-magnitude imaginary vibrational frequencies may be suspect, because the numerical integration procedures used in existing DFT methods have significant limitations.34 All of the final optimized structures reported in this paper have only real vibrational frequencies unless otherwise indicated. For (C7H6CH2)Fe2(CO)6 species both singlet and triplet spin state structures were optimized. Only the lowest energy structures are reported in this paper. Each (C7H6CH2)Fe2(CO)6 structure is
Figure 2. Two isomeric (η4-C7H6CH2)Fe(CO)3 complexes derived from heptafulvene.
2,11,12 have been prepared by indirect methods in which the heptafulvene unit is generated from an eight-carbon skeleton after attachment of the Fe(CO)3 moiety. The structure of the trimethylenemethane-bonded isomer has been determined by X-ray crystallography.13 The binuclear heptafulveneiron carbonyl complex (C7H6CH2)Fe2(CO)6 was synthesized by reaction of the trimethylenemethane-bonded mononuclear (C7H6CH2)Fe(CO)3 with excess Fe2(CO)9 in hexane at room temperature.12 This binuclear derivative was found to have limited solubility in organic solvents. It has not been characterized structurally, owing to the difficulty in growing suitable single crystals. This paper describes a density functional theory study on heptafulveneiron carbonyls of the types (C7H6CH2)Fe(CO)3 and (C7H6CH2)Fe2(CO)6 in which one or two Fe(CO)3 moieties are bonded to the heptafulvene ligand. Of particular interest was the investigation of possible structures for the experimentally known12 but structurally uncharacterized (C7H6CH2)Fe2(CO)6. In addition, this study compares the energies of isomeric structures with butadiene-type bonding and trimethylenemethane bonding of the hydrocarbon ligand to the metal atom.
2. THEORETICAL METHODS Electron correlation effects have been included to some degree using density functional theory (DFT) methods, which have evolved as a practical and effective computational tool, especially for organometallic compounds.14−20 The reliability of such density functional theory (DFT) methods is governed by the quality of the approximate exchange-correlation (XC) energy functional. Two differently constructed DFT methods, namely the B3LYP and the BP86 methods, were used in the present study. The B3LYP method is a hybrid HF/
Figure 3. Isomeric iron tricarbonyl complexes of (a) butadiene and (b) trimethylenemethane. The numbers in parentheses are the relative energies with zero-point correction (ΔE in kcal/mol) predicted by the B3LYP/DZP, BP86/DZP, and B97D/def2-TZVPP methods. 4913
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Table 1. Comparison of the Predicted Bond Lengths (Å) in Trimethylenemethaneiron Tricarbonyl, [η4-(CH2)3C]Fe(CO)3, with the Experimental Values Fe1−C1 Fe1−C2 Fe1−C3 Fe1−C4 Fe1−C5 Fe1−C6 Fe1−C7 C1−C2 C1−C3 C1−C4 av error
exptl
B3LYP
B3LYP − exptl
BP86
BP86 − exptl
B97D
B97D − exptl
1.945 2.124 2.133 2.134 1.798 1.798 1.798 1.429 1.429 1.430
1.954 2.139 2.139 2.139 1.799 1.799 1.799 1.434 1.434 1.434
+0.009 +0.015 +0.006 +0.005 +0.001 +0.001 +0.001 +0.005 +0.005 +0.004 +0.005
1.959 2.145 2.145 2.145 1.784 1.784 1.784 1.443 1.443 1.443
+0.014 +0.021 +0.012 +0.011 −0.014 −0.014 −0.014 +0.014 +0.014 +0.013 +0.006
1.956 2.154 2.154 2.154 1.793 1.793 1.793 1.428 1.428 1.428
+0.011 +0.030 +0.021 +0.020 −0.005 −0.005 −0.005 −0.001 −0.001 −0.002 +0.006
designated as aX-b, where a is the number of CO groups, X relates to the spin state (S = singlet or T = triplet), and b orders the structures according to their BP86 relative energies. The mononuclear structures (C7H6CH2)Fe(CO)3 are designated as MaX-b where a, X, and b have the same significance as for the binuclear structures.
ligand. The energy difference between M3S-1 and M3S-2 is only 1.6 kcal/mol (B3LYP), 2.4 kcal/mol (BP86), or 3.8 kcal/ mol (B97D). In addition, the transition state between M3S-1 and M3S-2 predicts an energy barrier of 17.3 kcal/mol (B3LYP), 12.7 kcal/mol (BP86), or 12.1 kcal/mol (B97D) for their interconversion. This energy barrier is high enough to suggest the coexistence of M3S-1 and M3S-2. The transition state between M3S-1 and M3S-2 is predicted to have an imaginary frequency of 49i cm−1 (B3LYP), 36i cm−1 (BP86), or 42i cm−1 (B97D). The corresponding normal mode involves the translation and rotation of the Fe(CO)3 group from the Yshaped trimethylenemethane coordination in M3S-1 to the trans-butadiene coordination mode in the transition structure and then to the cis-butadiene coordination mode in M3S-2. 3.3. Binuclear (C7H6CH2)Fe2(CO)6 Structures. Five energetic low-lying (C7H6CH2)Fe2(CO)6 structures, namely four singlet structures (6S-1, 6S-2, 6S-3, and 6S-4) and one triplet structure (6T-5), were optimized (Figure 5 and Table S1 (Supporting Information)). The lowest energy structure is the Cs trans-(η4:η4-C7H6CH2)Fe2(CO)6 structure 6S-1, in which a trimethylenemethane subunit of the heptafulvene ligand is
3. RESULTS AND DISCUSSION 3.1. Isomeric Iron Tricarbonyl Complexes of Butadiene and Trimethylenemethane. The isomeric iron tricarbonyl complexes of butadiene and trimethylenemethane were optimized (Figure 3). All of the methods used in the present research predict the butadieneiron tricarbonyl structure to lie only ∼1.0 kcal/mol in energy below the trimethylenemethaneiron tricarbonyl structure. This is consistent with the fact that both structures have been synthesized as stable compounds. Using the C3v trimethylenemethaneiron tricarbonyl structure as an example, the predicted interatomic distances are seen to be very close to the experimental distances (Table 1). 3.2. Isomeric Mononuclear Heptafulvene Complexes (C7H6CH2)Fe(CO)3. The two low-lying, nearly energetically degenerate (C7H6CH2)Fe(CO)3 structures M3S-1 and M3S-2, each with a η4-heptafulvene ligand, are found, consistent with the fact that both of these isomers have been synthesized (Figure 4).11,12 In both M3S-1 and M3S-2 the iron atoms attain the favored 18-electron configuration. In M3S-1 four carbon atoms of the heptafulvene ligand function as a Y-shaped trimethylenemethane ligand. However, in M3S-2 four carbon atoms of the heptafulvene ring function as a linear butadiene
Figure 5. Five optimized (C7H6CH2)Fe2(CO)6 structures. The upper distances were obtained by the B3LYP method and the lower distances by the BP86 method. The numbers in parentheses are the relative energies with zero-point corrections (ΔE in kcal/mol) predicted by the B3LYP/DZP, BP86/DZP, and B97D/def2-TZVPP methods, respectively. The last numbers in the parentheses under 6S-1, 6S-2, and 6S-3 are the relative energies predicted at the BP86/def2-TZVPP level.
Figure 4. The optimized lowest energy mononuclear (C7H6CH2)Fe(CO)3 structures. The relative energies with zero-point correction (ΔE in kcal/mol) of B3LYP/DZP, BP86/DZP, and B97D/def2-TZVPP are given in parentheses. 4914
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Table 2. Harmonic Vibrational Frequencies (in cm−1) and Infrared Intensities (in Parentheses, in km/mol) for the (C7H6CH2)Fe2(CO)6 Structures Predicted by the BP86 Methoda exptl 6S-1 6S-2 6S-3 6S-4 6T-5 a
DZP def2-TZVPP DZP def2-TZVPP DZP def2-TZVPP
2040 2039 (7) 2037 (5) 2036 (777) 2042 (787) 2037 (785) 2043 (788) 1954 (27) 1954 (511)
2017 (1995) 2023 (2073) 1997 (1558) 2003 (1578) 1998 (1584) 2003 (1590) 1958 (182) 1959 (814)
1980 1977 (865) 1981 (880) 1983 (1108) 1988 (1156) 1990 (1141) 1994 (1156) 1975 (234) 1960 (517)
1972 (1008) 1976 (1028) 1971 (150) 1974 (159) 1968 (113) 1971 (125) 1984 (1192) 1964 (1039)
1967 1971 1957 1960 1957 1960 2002 1997
(380) (551) (164) (168) (56) (67) (1611) (2439)
1967 1970 1951 1954 1953 1955 2038 2017
(538) (396) (211) (233) (247) (268) (725) (52)
The frequencies in boldface are those with high IR intensities close to experiment.
and thus likewise corresponds to a formal single bond. This gives each iron atom in 6S-3 the favored 18-electron configuration. A tiny imaginary vibrational frequency of 14i cm−1(BP86) or 13i cm−1 (B97D) is predicted in 6S-3, which becomes almost insignificant at 3i cm−1 when using the (120, 974) integration grid. The B3LYP method predicts only real vibrational frequencies for 6S-3. The fourth hexacarbonyl structure is a much higher energy Cs singlet cis-(η3:η2,1-C7H6CH2)Fe2(CO)6 structure 6S-4, lying 17.9 kcal/mol (B3LYP), 15.8 kcal/mol (BP86), or 15.8 kcal/ mol (B97D) above 6S-1 (Figure 5 and Table S17 (Supporting Information)). The Fe−C distances in 6S-4 suggests that the heptafulvene ligand coordinates to one Fe(CO)3 moiety as a η3allylic ligand using three adjacent carbon atoms. The heptafulvene ligand is also coordinated to the other Fe(CO)3 moiety as a η3 ligand but not involving three adjacent carbon atoms in contrast to the η3 coordination of the heptafulvene ligand to both iron atoms in both 6S-2 and 6S-3. This uncomplexed CC double bond in 6S-4 is the exocyclic double bond, as in 6S-3. The Fe−Fe distance of 2.824 Å (B3LYP), 2.810 Å (BP86), or 2.883 Å (B97D) in 6S-4 is consistent with a single bond, thereby giving each iron atom in 6S-4 the favored 18-electron configuration. The lowest energy triplet hexacarbonyl structure is the trans(η3:η3-C7H6CH2)Fe2(CO)6 structure 6T-5, lying 5.7 kcal/mol (B3LYP), 17.3 kcal/mol (BP86), or 12.3 kcal/mol (B97D) in energy above the lowest energy structure 6S-1 (Figure 5 and Table S17 (Supporting Information)). The discrepancies between the B3LYP and BP86 results for the singlet−triplet splitting in (C7H6CH2)Fe2(CO)6 are consistent with the observation by Reiher and collaborators35,36 that B3LYP favors higher spin states whereas the BP86 method favors lower spin states. The true values are expected to lie between the results from the two methods. The Fe−C distances in 6T-5, like those in 6S-2, 6S-3, and 6S-4, suggest coordination of two η3-allylic subunits from the heptafulvene ligand to each Fe(CO)3 moiety. However, the long Fe···Fe distance of 4.974 Å (B3LYP), 4.867 Å (BP86), or 4.831 Å (B97D) clearly indicates the absence of direct iron−iron bonding. This gives each iron atom a 17electron configuration, consistent with a binuclear triplet spin state structure. 3.4. QTAIM Analysis and Molecular Orbital Analysis. In order to compare the trimethylenemethane and heptafulvene ligands, the trimethylenemethaneiron tricarbonyl, [η 4 (CH2)3C]Fe(CO)3, structure was optimized as well using the same DFT methods (Figure 3 and Table S4 (Supporting Information)). The optimized structure agrees well with the experimental structure. Farrugia et al.9 have studied the interaction between the Fe atom and the trimethylenemethane
coordinated to one Fe(CO)3 moiety and the remaining free butadiene subunit is coordinated to the other Fe(CO)3 moiety. The long Fe...Fe distance of ∼4.6 Å clearly indicates the absence of a direct iron−iron bond, thereby giving each iron atom in 6S-1 the favored 18-electron configuration. The second predicted hexacarbonyl structure is the C1 singlet cis-(η3:η3-C7H6CH2)Fe2(CO)6 structure 6S-2, lying only 2.8 kcal/mol (B3LYP) or 2.9 kcal/mol (BP86) above 6S-1 (Figure 5 and Table S17 (Supporting Information)). Using the B97D functional with the larger def2-TZVPP basis set, reverses the energy order so that structure 6S-2 actually lies 1.4 kcal/mol below structure 6S-1. In 6S-2 the Fe−C distances indicate coordination of a η3-allylic subunit of the heptafulvene ligand to each Fe(CO)3 moiety, leaving an uncomplexed C6C7 double bond of length 1.360 Å (B3LYP), 1.375 Å (BP86), or 1.362 Å (B97D). This CC distance is significantly shorter than the other C−C distances in the heptafulvene ligand. The predicted Fe−Fe distance of 2.964 Å (B3LYP), 2.913 Å (BP86), or 2.976 Å (B97D) in 6S-2 can be considered to be a single bond, thereby giving each iron atom the favored 18electron configuration. The strong ν(CO) frequencies of 2036, 1997, and 1983 cm−1 for 6S-2 predicted by the BP86/DZP method or 2042, 2003, and 1988 cm−1 predicted by the BP86/ def2-TZVPP method are closer to the strong experimental ν(CO) frequencies of 2040 and 1980 cm −1 for the experimentally synthesized but structurally uncharacterized12 (C7H6CH2)Fe2(CO)6 than are the predicted strong ν(CO) frequencies of 2017, 1977, and 1972 cm−1 (BP86/DZP) or 2023, 1981, and 1976 cm−1 (BP86/def2-TZVPP) for 6S-1 (Table 2). This suggests that the experimentally observed (C7H6CH2)Fe2(CO)6 product has structure 6S-2 with the heptafulvene coordinated as a η3:η3 allylic ligand and a formal Fe−Fe single bond rather than structure 6S-1 with the heptafulvene coordinated as a η4:η4 ligand and no Fe−Fe bond. The third hexacarbonyl structure is the singlet Cs cis-(η3:η3C7H6CH2)Fe2(CO)6 structure 6S-3, lying 6.9 kcal/mol (B3LYP), 5.1 kcal/mol (BP86), or only 2.1 kcal/mol (B97D) in energy above 6S-1 (Figure 5 and Table S17 (Supporting Information)). The Fe−C distances in 6S-3, like those in 6S-2, suggest that two η3-allylic subunits within the heptafulvene ring are coordinated separately to the two Fe(CO)3 moieties. The uncomplexed C1C2 double-bond distance of 1.356 Å (B3LYP), 1.369 Å (BP86), or 1.356 Å (B97D) is shorter than any of the other C−C bonds in the heptafulvene ligand. In 6S-3 the uncomplexed CC double bond is the exocyclic double bond of the heptafulvene ligand, whereas in 6S-2 the uncomplexed CC double bond is in the seven-membered ring. The Fe−Fe distance of 2.953 Å (B3LYP), 2.934 Å (BP86), or 3.005 Å (B97D) in 6S-3 is similar to that in 6S-2 4915
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carbon atoms in detail. The bond characteristics of the compounds were analyzed using the AIM37,38 theory of Bader and carried out using Multiwfn software.39 Consistent with their result, only one bond path is observed between the TMM ligand and the Fe atom by the QTAIM topological analysis (Figure 6). The (η4-C4H6)Fe(CO)3 structure and the
Figure 7. Frontier molecular orbitals of (a) trimethylenemethaneiron tricarbonyl, [η4-(CH2)3C]Fe(CO)3, and (b) the heptafulveneiron tricarbonyl, (C7H6CH2)Fe(CO)3, structure with a TMM subunit coordinated to the Fe(CO)3 group. These frontier molecular orbitals are primarily responsible for the Fe−TMM unit bonding.
Figure 6. Bond paths in the (a) [η4-(CH2)3C]Fe(CO)3, (b) (η4C4H6)Fe(CO)3, and (c, d) (C7H6CH2)Fe(CO)3 structures.
Turning to the binuclear species, the DFT results on the hexacarbonyl (C7H6CH2)Fe2(CO)6 are particularly interesting, since the hexacarbonyl has been synthesized but not characterized structurally.12 The lowest energy (C7H6CH2)Fe2(CO)6 structure 6S-1 uses all eight carbon atoms to bond to two separate Fe(CO)3 groups. For such bonding the eight carbon atoms of the heptafulvene ligand are partitioned into two four-carbon subunits, namely a trimethylenemethane subunit and a cis-butadiene subunit. Each subunit is bonded to an Fe(CO)3 moiety as a η4 ligand, leading to local iron environments similar to those of the known stable Fe(CO)3 complexes of trimethylenemethane6−9 and butadiene.4,5 The trans stereochemistry of the two Fe(CO)3 moieties keeps the iron atoms too far apart for a metal−metal bond. The preference of (C7H6CH2)Fe2(CO)6 for the η4:η4 structure 6S-1 is not clear since the alternative η3:η3 allylic structure 6S-2 lies only ∼3 kcal/mol above 6S-1 (B3LYP and BP86) or slightly below 6S-1 (B97D). In this η3:η3 allylic structure 6S-2 only six of the heptafulvene carbon atoms are bonded to the iron atoms, leaving an uncomplexed CC double bond. Since only six of the eight π electrons of the heptafulvene ligand are involved in its bonding to the two iron atoms, an Fe−Fe bond is required in 6S-2 to give each iron atom the favored 18-electron configuration. This Fe−Fe bond appears in the frontier molecular orbital HOMO-1 as well as in the bond path analysis (Figure 8). η3:η3 allylic ligand−metal bonding to an Fe2(CO)6 moiety similar to that in 6S-2 is found
two (C7H6CH2)Fe(CO)3 structures M3S-1 and M3S-2 were also tested by the QTAIM topological analysis. It can be seen that all expected bond paths are observed in the (η4C4H6)Fe(CO)3 structure, which is different from the situation in the [η4-(CH2)3C]Fe(CO)3 structure. For the (C7H6CH2)Fe(CO)3 structures M3S-1 with a TMM unit coordinated to the Fe(CO)3 moiety, only one bond path is observed, similar to that in [η4-(CH2)3C]Fe(CO)3, which means that the Cβ atoms in the TMM unit interact with the Fe atom through the delocalized electron density. This phenomenon of the absence of the expected bond paths has been also reported for some sandwich compounds and transition-metal carbonyl compounds containing the TMM ligand.40,41 Comparing the (C7H6CH2)Fe(CO)3 structure M3S-2 having the butadiene unit coordinated to the Fe(CO)3 group with the (η4C4H6)Fe(CO)3 structure, we can see that only two bond paths can be found between the butadiene unit and the Fe atom in M3S-2, whereas all four expected four bond paths are observed in (η4-C4H6)Fe(CO)3. This may indicate that in the heptafulvene ligand of M3S-2 the butadiene subunit is more delocalized than in the butadiene ligand of (η4-C4H6)Fe(CO)3. Although only one bond path is observed between the TMM unit and the Fe atom by QTAIM topological analysis (Figure 6), we believe that strong interactions exist between the Fe atom and the Cβ atoms of the TMM unit, as indicated by the “umbrella conformation” of the TMM units in both structures, leading to the nonplanar TMM units in both structures. Additional physicochemical evidence also suggests significant interaction between the Fe atom and the Cβ atoms.9 Thus, the frontier molecular orbitals (Figure 7) indicate significant overlap between the Fe(CO)3 fragment and the TMM unit. In addition, the frontier molecular orbitals of trimethylenemethaneiron tricarbonyl, [η4-(CH2)3C]Fe(CO)3, as well as the (C7H6CH2)Fe(CO)3 structure with a TMM subunit coordinated to the Fe(CO)3 group responsible for the interaction between Fe atom and TMM unit are similar.
Figure 8. Bond path of the binuclear (C7H6CH2)Fe2(CO)6 structure 6S-2 and the frontier molecular orbital. 4916
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in the tetramethyleneethane complex [η3:η3-(CH2)4C2]Fe2(CO)6, originally obtained from Fe3(CO)12 and allene.42,43 The predicted Fe−Fe single-bond distance of ∼2.93 Å for 6S-2 is very close to the experimental Fe−Fe single-bond distance of 2.927 Å in [η3:η3-(CH2)4C2]Fe2(CO)6, determined by X-ray crystallography.43
complete Gaussian09 reference (ref 33). This material is available free of charge via the Internet at http://pubs.acs.org.
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Corresponding Author
*E-mail:
[email protected] (R.B.K.);
[email protected]. edu.cn (H.F.).
4. SUMMARY The experimental results on the mononuclear (C7H6CH2)Fe(CO)3 indicate the existence of two stable isomers having structures M3S-1 and M3S-2, which were prepared by different synthetic methods using different indirect methods to synthesize the heptafulvene ligand after bonding the Fe(CO)3 moiety to a suitable eight-carbon precursor.11−13 In each of these structures four carbon atoms of the heptafulvene ligand are bonded to the Fe(CO)3 moiety as a η4 ligand, leaving two uncomplexed CC double bonds. In the slightly lower energy structure M3S-1 the four bonded carbon atoms of the heptafulvene ligand come from the trimethylenemethane subunit, leaving an uncomplexed cis-butadiene subunit. In M3S-2 the four bonded carbon atoms of the heptafulvene ligand come from a cis-butadiene subunit within the sevenmembered ring, leaving an uncomplexed trans-butadiene subunit including the exocyclic CC double bond. The closeness of energy of the (C7H6CH2)Fe(CO)3 isomers M3S-1 and M3S-2 (energy difference
