Article pubs.acs.org/Organometallics
Alkyne Dichotomy: Splitting of Bis(dialkylamino)acetylenes, Dimethoxyacetylene, Bis(methylthio)acetylene, and Their Heavier Congeners To Give Carbyne Ligands in Iron Carbonyl Derivatives Huidong Li,†,‡ Hao Feng,*,†,‡ Weiguo Sun,†,‡ 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: The R2C2Fe2(CO)6 complexes obtained from reactions of the alkynes RCCR with iron carbonyls are known experimentally to exhibit the following two types of structures: (1) tetrahedrane structures in which the alkyne carbons are incorporated into a central Fe2C2 tetrahedron with a short FeFe distance, suggesting a formal double bond and (2) dichotomy structures in which the alkyne has split into two separate RC carbyne units bridging a singly bonded pair of iron atoms. The relative energies of these two structure types with different substituents R have now been examined using density functional theory. For the dialkylamino derivatives (R′2N)2C2Fe2(CO)6 (R′= CH3, C2H5) the dichotomy structures are preferred energetically over the tetrahedrane structures by 5−11 kcal/mol. For (CH3O)2C2Fe2(CO)6 the dichotomy and tetrahedrane structures have comparable energies. Dichotomy structures as well as tetrahedrane structures were found for (CH3E)2C2Fe2(CO)6 (E = S, Se) but not for (CH3Te)2C2Fe2(CO)6 or (CH3SO)2C2Fe2(CO)6. Only tetrahedrane structures were found for the alkyl derivatives R2C2Fe2(CO)6 (R = (CH3)3C, CH3, C6H5) and R′2C2Fe2(CO)6 (R′ = H, C5H4N).
1. INTRODUCTION The chemistry of acetylene metal carbonyl derivatives dates back to the 1956 synthesis1 of cobalt carbonyl derivatives of the type (alkyne)Co2(CO)6. Such compounds are readily obtained in good yield by reactions of Co2(CO)8 with a variety of alkynes under mild reaction conditions. Since then, the chemistry of (alkyne)Co2(CO)6 derivatives has developed extensively.2,3 Such compounds have proven to be useful in organic synthesis for reactions such as the Pauson−Khand synthesis of cyclopentenone derivatives.4 The structure of (alkyne)Co2(CO)6 derivatives can be described as a Co2C2 tetrahedrane with formal single bonds along each of the six edges of the tetrahedron (Figure 1). The cobalt atoms in such structure acquire the favored 18-electron configuration, and the carbon atoms achieve the stable octet. The experimental Co− Co distance of 2.462 Å is indicative of a formal single bond for this t-Bu2C2Co2(CO)6 structure.5,6 The subsequent major development in acetylene metal carbonyl chemistry was the extensive research by Hübel and coworkers7 on reactions of iron carbonyls with alkynes under diverse conditions to give a complicated variety of products including not only acetylene derivatives but also cyclobutadiene, cyclopentadienone, tropone, and ferrole iron carbonyl derivatives. Binuclear iron carbonyl derivatives with a simple alkyne ligand included dark green compounds claimed to have the general formula R2C2Fe2(CO)7 (R = Ph, tBu) and © 2012 American Chemical Society
inferred to have only terminal carbonyl groups on the basis of the infrared ν(CO) frequencies. However, this original formulation was found to be incorrect by an X-ray diffraction structure determination on the tert-butyl derivative. The latter was shown to contain a central Fe2C2 tetrahedrane unit with only six carbonyl groups, namely t-Bu2C2Fe2(CO)6, rather than the originally suggested seven carbonyl groups. The experimental FeFe distance of 2.316 Å,6,8 which is ∼0.15 Å shorter than the experimental Co−Co single-bond distance of 2.462 Å in the analogous (t-Bu2C2)Co2(CO)6, was suggested to be a formal double bond to give each iron atom the favored 18electron configuration. The structures of H2C2M2(CO)n (M = Co, Fe; n = 6−4) have been studied systematically by DFT methods.9,10 Another important development in acetylene metal carbonyl chemistry was the discovery of new types of compounds when dialkylamino-substituted acetylenes are used as reagents for reactions with metal carbonyls. Such products include a yellow solid of stoichiometry [(C2H5)2N]2C2Fe2(CO)6, first reported in 197611 as a product from the reaction of (C2H5)2NC CN(C2H5)2 with Fe(CO)5 or Fe2(CO)12. X-ray crystallography showed an unusual structure for this product, in which the carbon−carbon triple bond of the original (C2H5)2NC Received: August 30, 2012 Published: December 13, 2012 88
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Figure 1. Products from alkynes and metal carbonyls. of d functions, contracted following Hood et al., and designated (14s11p6d/10s8p3d).17 For Se, one set of pure spherical harmonic d functions with orbital exponent αd(Se) = 0.338 is added to the Ahlrichs contracted DZ basis sets, designated (14s11p6d/8s6p3d).18 For Te, the Hay and Wadt effective core potential (ECP) with 46 core electrons was used. The corresponding basis set is designated as (3s3p1d/2s2p1d) augmented with the polarization d orbital αd(Te) = 0.237.19 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.20−26 The reliability of such DFT methods is influenced by the quality of the approximate exchange and correlation functionals. Three differently constructed DFT functionals, namely the B3LYP* method, the BP86 method, and the M06-L method, were used in the present study. The original B3LYP method is a hybrid HF/DFT method, combining the three-parameter Becke functional (B3) with the Lee−Yang−Parr (LYP) generalized gradient correlation functional.27,28 This method includes exact exchange and is calibrated by fitting three parameters to a set of experimental results. However, decreasing the amount of the Hartree−Fock exchange from the 20% incorporated in the original B3LYP to the 15% in the modified B3LYP* has been shown to describe more accurate singlet− triplet splitting for the first-row transition metals. Therefore, the B3LYP* method rather than the original B3LYP was used.29−31 The BP86 method combines Becke’s 1988 exchange functional (B) with Perdew’s 1986 gradient-corrected correlation functional (P86).32,33 This method does not include exact exchange and is mainly deduced by forcing the functional to satisfy certain exact constraints based on first principles. The third functional used in this work is the hybrid meta-GGA DFT method M06-L developed by Truhlar’s group.34 This functional reflects some progress that is essential for expanding the applicability of Kohn−Sham DFT. The M06-L functional was constructed using three strategies, namely constraint satisfaction, modeling the exchange-correlation hole, and empirical testing. The M06-L functional, suggested by Truhlar et al., appears to be one of the best functionals for the study of organometallic and inorganic thermochemistry. In this connection our results show that the Fe− Fe distances predicted by the M06-L method are much closer to the experimental results than those obtained by the other two methods. Therefore, only the M06-L results are reported in the present paper. The B3LYP* and BP86 results are available in the Supporting Information. The geometries of all structures were fully optimized using the BP86, B3LYP*, and M06-L methods. The ultrafine grid (99, 590) is adopted for evaluating integrals numerically in the Gaussian09 program.35 Harmonic vibrational frequencies and infrared intensities were determined at the same levels. In the search for minima, low-magnitude vibrational frequencies may be suspect, because the numerical integration procedures used in existing DFT methods have significant limitations.36 For structures with small imaginary vibrational frequencies, the even finer integration grid (120, 974) was used for further evaluation. All of the final optimized structures reported in this paper have only real vibrational frequencies unless otherwise indicated. For the structures with imaginary frequencies, the corresponding normal modes are given in the Supporting Information.
CN(C2H5)2 underwent complete cleavage to give two discrete (C2H5)2NC ((diethylamino)carbyne) ligands (Figure 2).12 The
Figure 2. Canonical structures for the alkyne dichotomy products (R2N)2Fe2(CO)6.
Fe−Fe distance of 2.482 Å in this alkyne dichotomy product suggests an iron−iron single bond. This gives each iron atom in [(C2H5)2N]2C2Fe2(CO)6 the favored 18-electron configuration if each of the two bridging Et2NC units donates three electrons to the central Fe2 unit. Formally, the bridging (dialkylamino)carbyne ligands can donate three electrons to the central Fe2 unit by forming a double bond with one iron atom and a single bond with the other iron atom (Figures 1 and 2). Subsequently, both [(CH3)2N]2C2Fe2(CO)6 and [(C2H5)2N]2C2Fe2(CO)6 were obtained as one of several types of products from R2NNO (R = CH3, C2H5) and Fe2(CO)9.13 The cited experimental studies show that the following two very different types of R2C2Fe2(CO)6 products can be obtained from alkynes and iron carbonyls: (1) tetrahedrane derivatives with a central Fe2C2 tetrahedron and an FeFe distance indicative of a formal double bond and (2) alkyne dichotomy products in which the alkyne has split into two separate RC units, forming the wingtips of an Fe2C2 butterfly with an Fe−Fe distance indicative of a formal single bond. So far, the alkyne dichotomy products have been obtained with bis(dialkylamino)acetylenes and the tetrahedrane derivative with di-tert-butylacetylene. This paper uses density functional theory to predict the relative stabilities of these two very different structural types for R2C2Fe2(CO)6 derivatives for a wide variety of alkynes RCCR (R = N(C2H5)2, N(CH3)2, CH3X (X = O, S, Se, Te, SO), C(CH3)3, CH3, H, C6H5, C5H4N).
2. THEORETICAL METHODS Double-ζ plus polarization (DZP) basis sets were used in this research. For H, a set of p polarization functions αp(H) = 0.75 is added to the Huzinaga−Dunning DZ sets.14,15 For carbon, nitrogen, oxygen, or sulfur one set of pure spherical harmonic d functions with orbital exponent αd(C) = 0.75, αd(N) = 0.80, αd(O) = 0.85, and αd(S) = 0.70 is added to the standard Huzinaga−Dunning contracted DZ sets. These basis sets are designated (4s1p/2s1p) for H, (9s5p1d/4s2p1d) for C, N, and O, and (12s8p1d/6s4p1d) for S. For the transition metal iron, in our loosely contracted DZP basis set, the Wachters primitive sets are used,16 but augmented by two sets of p functions and one set 89
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3. RESULTS 3.1. (Dialkylamino)acetylenes. The iron carbonyl derivatives from bis(dialkylamino)acetylenes are discussed first, since alkyne dichotomy has been observed experimentally in the diethylamino derivative [(C2H5)2N]2C2Fe2(CO)6.12 Two low-lying structures were obtained for the methyl analogue [(CH3)2N]2C2Fe2(CO)6 (Figure 3). The global minimum is
Figure 3. Optimized [(CH3)2N]2C2Fe2(CO)6 structures. The number under each structure in Figures 3 to 6 is its relative energy (in kcal/ mol).
Figure 4. Optimized [(C2H5)2N]2C2Fe2(CO)6 structures.
the C2v dichotomy structure Me2N-d (the letter d stands for dichotomy). The predicted Fe−Fe distance of 2.528 Å (M06L) can be interpreted as a formal single bond. The N C(bridging) distance of 1.312 Å (M06-L) is consistent with a double bond. In Me2N-d, each N atom has a formal positive charge, while each Fe atom has a formal negative charge, thereby giving each iron atom to have the favored 18-electron configuration. The second [(CH3)2N]2C2Fe2(CO)6 structure Me2N-t (the letter t stands for tetrahedrane) with C2 symmetry has a central Fe2C2 tetrahedron (Figure 2). Me2N-t is predicted to lie in energy 5.0 kcal/mol (M06-L) above Me2N-d. The FeFe distance of 2.425 Å (M06-L) in Me2N-t is ∼0.1 Å shorter than the Fe−Fe single-bond distance in Me2N-d. This suggests a formal FeFe double bond in Me2N-t, thereby leading to the favored 18-electron configurations. When the four methyl groups in [(CH3)2N]2C2Fe2(CO)6 are replaced by ethyl groups, more conformers for [(C2H5)2N]2C2Fe2(CO)6 are found, arising from different orientations of the ethyl groups. Four low-lying structures were obtained for [(C2H5)2N]2C2Fe2(CO)6 (Figure 4), all having C2 symmetry. The global minimum is the dichotomy structure Et2N-d-1, which was first synthesized by King and Harmon11 and structurally characterized by Cash et al. using X-ray crystallography.12 The Fe−Fe distance of 2.505 Å (M06-L) is somewhat longer than the experimental value of 2.482 Å.12 In Et2N-d-1, the two ethyl groups in each (C2H5)2NC unit are in anti positions. The second dichotomy [(C2H5)2N]2C2Fe2(CO)6 structure Et2N-d-2 has a geometry similar to that of Et2N-d-1, but the two ethyl groups in each CN(C2H5)2 are in syn positions. Et2N-d-2 lies only 1.8 kcal/ mol (M06-L) above Et2N-d-1. The Fe−Fe distance of 2.495 Å (M06-L) in Et2N-d-2 is similar to that in Et2N-d-1. The two tetrahedrane [(C2H5)2N]2C2Fe2(CO)6 structures Et2N-t-1 and Et2N-t-2 were also found with FeFe distances of 2.429 Å (M06-L) for Et2N-t-1 and 2.418 Å (M06-L) for Et2N-t-2. These FeFe distances are ∼0.06 Å shorter than the corresponding dichotomy structures, consistent with the formal double bonds required to give all the iron atoms the favored 18electron configurations. The predicted energy for structure Et2N-t-1 is 5.5 kcal/mol (M06-L) above Et2N-d-1, whereas that for Et2N-t-2 is 5.0 kcal/mol (M06-L) above Et2N-d-1.
3.2. Dimethoxyacetylene and Its Heavier Congeners. The dimethoxyacetylene system (CH3O)2C2Fe2(CO)6 and its heavier congeners (CH3X)2C2Fe2(CO)6 (X = S, Se, Te, SO) were also studied in order to provide other examples of R2C2Fe2(CO)6 with lone pairs on the R substituents (Figure 5). Three low-lying structures were found for (CH3O)2C2Fe2(CO)6, including one dichotomy structure (MeO-d) and two tetrahedrane structures (MeO-t-up and MeO-t-down). MeO-t-up has the two methyl groups pointing upward, whereas MeO-t-down has the two methyl groups pointing downward (in Figure 5). The lowest energy structure predicted is the tetrahedrane structure MeO-t-down. The dichotomy structure MeO-d lies 3.8 kcal/mol (M06-L) above MeO-t-down. Another tetrahedrane structure, MeO-t-up, lies 4.9 kcal/mol (M06-L) higher in energy than MeO-t-down. The Fe−Fe distance of 2.581 Å (M06-L) predicted for MeO-d is ∼0.2 Å longer than the FeFe distance of 2.405 Å (M06-L) for MeO-t-down and ∼0.1 Å longer than that of 2.474 Å (M06L) for MeO-t-up. This is consistent with an Fe−Fe single bond in MeO-d and FeFe double bonds in MeO-t-down and MeO-t-up. Each iron atom in each (CH3O)2C2Fe2(CO)6 structure in this way has the favored 18-electron configuration. Three structures were found for the (CH3S)2C2Fe2(CO)6 system, similar to the analogous (CH3O)2C2Fe2(CO)6 system (Figure 6). The two tetrahedrane structures MeS-t-down and MeS-t-up are predicted to be very close in energy by all three DFT methods. These two structures differ only in the orientation of the methyl groups, namely upward for MeS-tup and downward for MeS-t-down, as depicted in Figure 6. The dichotomy structure MeS-d lies higher in energy than MeS-t-down by 12.1 kcal/mol (M06-L). The Fe−Fe distance of 2.544 Å (M06-L) in MeS-d is consistent with a formal single Fe−Fe bond. In contrast, the iron−iron distance in MeS-tdown of 2.380 Å (M06-L) and that in MeS-t-up of 2.445 Å (M06-L) can be interpreted as FeFe double bonds. As discussed for the (CH3O)2C2Fe2(CO)6 system, each iron atom in the three (CH3S)2C2Fe2(CO)6 structures has the favored 18electron configuration. Similar studies were done on the (CH3Se)2C2Fe2(CO)6, (CH3Te)2C2Fe2(CO)6, and (CH3SO)2C2Fe2(CO)6 systems (see the Supporting Information). The two tetrahedrane 90
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Figure 5. Optimized (CH3O)2C2Fe2(CO)6 structures.
Figure 6. Optimized (CH3S)2C2Fe2(CO)6 structures.
Figure 7. Optimized R2C2Fe2(CO)6 (R = tBu, Me, Ph, C5H4N, H) structures.
of these species, only tetrahedrane structures could be optimized. Attempted optimization of the corresponding dichotomy structures always led to tetrahedrane structures. The [(CH3)3C]2C2Fe2(CO)6 tetrahedrane structure tBu-t has been synthesized experimentally and characterized by X-ray crystallography.6,8 Our theoretical results for this tetrahedrane structure agree well with experiment. Thus, the predicted Fe Fe distance of 2.337 Å (M06-L) is close to the experimental value of 2.316 Å and suggests the formal double bond required to give each iron atom the favored 18-electron configuration (Table 1). The other four tetrahedrane R2 C 2Fe 2 (CO)6 structures C5H4N-t, Ph-t, Me-t, and H-t have similarly short FeFe distances ranging from 2.350 to 2.453 Å.
structures are nearly degenerate in energy for the (CH 3 Se) 2 C 2 Fe 2 (CO) 6 , (CH 3 Te) 2 C 2 Fe 2 (CO) 6 , and (CH3SO)2C2Fe2(CO)6 systems (Tables S5−S7 in the Supporting Information). Dichotomy structures do not appear to exist for the (CH3Te)2C2Fe2(CO)6 and (CH3SO)2C2Fe2(CO)6 systems, since attempted optimization of these dichotomy structures led to the corresponding tetrahedrane structures. For (CH3Se)2C2Fe2(CO)6, the BP86 and B3LYP* methods predict the dichotomy structures to lie more than 9 kcal/mol above the lowest energy tetrahedrane structure. However, the attempted M06-L optimization of the (CH3Se)2C2Fe2(CO)6 dichotomy structure led to the tetrahedrane structure. 3.3. Other Acetylenes. R2C2Fe2(CO)6 (R = (CH3)3C, CH 3 , C 6 H 5 , C 5 H 4 N, H) derivatives from alkyl- and arylacetylenes were included in this study (Figure 7). For all 91
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Table 1. Predicted Fe−Fe Distances (in Å) [(CH3)3C]2C2Fe2(CO)6 (tBu-t) (C5H4N)2C2Fe2(CO)6 (C5H4N-t) (C6H5)2C2Fe2(CO)6 (Ph-t) (CH3)2C2Fe2(CO)6 (Me-t) H2C2Fe2(CO)6 (H-t) (CH3O)2C2Fe2(CO)6 (MeO-d) (CH3O)2C2Fe2(CO)6 (MeO-t-down) (CH3O)2C2Fe2(CO)6 (MeO-t-up) (CH3S)2C2Fe2(CO)6 (MeS-d) (CH3S)2C2Fe2(CO)6 (MeS-t-down) (CH3S)2C2Fe2(CO)6 (MeS-t-up) (CH3Se)2C2Fe2(CO)6 (MeSe-t-down) (CH3Se)2C2Fe2(CO)6 (MeSe-t-up) (CH3Te)2C2Fe2(CO)6 (MeTe-t-down) (CH3Te)2C2Fe2(CO)6 (MeTe-t-up) (CH3SO)2C2Fe2(CO)6 (MeSO-t-down) (CH3SO)2C2Fe2(CO)6 (MeSO-t-up) [(C2H5)2N]2C2Fe2(CO)6 (Et2N-d-1) [(C2H5)2N]2C2Fe2(CO)6 (Et2N-d-2) [(C2H5)2N]2C2Fe2(CO)6 (Et2N-t-1) [(C2H5)2N]2C2Fe2(CO)6 (Et2N-t-2) [(CH3)2N]2C2Fe2(CO)6 (Me2N-d) [(CH3)2N]2C2Fe2(CO)6 (Me2N-t)
M06-L
exptl
2.337 2.417 2.369 2.350 2.373 2.581 2.405 2.474 2.544 2.380 2.445 2.377 2.441 2.396 2.431 2.409 2.412 2.505 2.495 2.429 2.418 2.528 2.425
2.316
Table 2. Comparison of the C(bridging)−X (X = N, O, S, Se) Distances (in Å, M06-L) in the Dichotomy Structures and the Corresponding Tetrahedrane Structures C(bridging)−X [(CH3)2N]2C2Fe2(CO)6 (Me2N-d) [(CH3)2N]2C2Fe2(CO)6 (Me2N-t)
1.312 1.350
1.907/1.907 1.889/2.306
[(C2H5)2N]2C2Fe2(CO)6 (Et2N-d-1)
1.308
1.907/1.913
exptl
2.482
Fe(1): 1.902/1.903 Fe(2): 1.909/1.914
[(C2H5)2N]2C2Fe2(CO)6 (Et2N-d-2) [(C2H5)2N]2C2Fe2(CO)6 (Et2N-t-1) [(C2H5)2N]2C2Fe2(CO)6 (Et2N-t-2)
1.308 1.344 1.343
1.911/1.916 1.895/2.290 1.900/2.289
(CH3O)2C2Fe2(CO)6 (MeO-d) (CH3O)2C2Fe2(CO)6 (MeO-t-down) (CH3O)2C2Fe2(CO)6 (MeO-t-up)
1.291 1.331
1.874/1.884 1.878/2.237
1.340
1.836/2.211
(CH3S)2C2Fe2(CO)6 (MeS-d) (CH3S)2C2Fe2(CO)6 (MeS-t-down) (CH3S)2C2Fe2(CO)6 (MeS-t-up)
1.661 1.708 1.712
1.879/1.895 1.916/2.206 1.874/2.192
1.859
1.925/2.197
1.863
1.883/2.191
[(CH3)3C]2C2Fe2(CO)6 (tBu-t)
1.508
1.963/2.148
exptl
1.513/1.519
Fe(1): 2.060/2.130 Fe(2): 2.044/2.094
(CH3Se)2C2Fe2(CO)6 (MeSe-d) (CH3Se)2C2Fe2(CO)6 (MeSe-t-down) (CH3Se)2C2Fe2(CO)6 (MeSe-t-up)
4. DISCUSSION The R2C2Fe2(CO)6 complexes obtained from reactions of the alkynes RCCR with iron carbonyls are known experimentally to exhibit the following two types of structures (Figure 2): (1) tetrahedrane structures in which the alkyne carbons are incorporated into a central Fe2C2 tetrahedron with a short FeFe distance suggesting a formal double bond and (2) dichotomy structures in which the alkyne has split into two separate RC carbyne units bridging the two iron atoms. The effects of substituents on the preferred R 2 C 2 Fe 2 (CO) 6 structures (R = (C2H5)2N, (CH3)2N, CH3X (X = O, S, Se, Te, SO), (CH3)3C, CH3, H, C6H5, C5H4N) have now been examined using density functional theory. Two types of structures have been observed experimentally for two such complexes. For the amino substituents (C2H5)2N and (CH3)2N, the dichotomy structures are favored in energy over the corresponding tetrahedrane structure, in accord with experiment.12 For the methoxy (CH3O) substituent the dichotomy and tetrahedrane structures are nearly energetically degenerate. However, the CH3S, CH3Se, CH3Te, and CH3SO substituents favor the tetrahedrane structure over the dichotomy structure. Only the tetrahedrane structures are found for the alkyl and aryl R2C2Fe2(CO)6 derivatives (R = (CH3)3C, CH3, C6H5, C5H4N) as well as for H2C2Fe2(CO)6. In these cases attempts to optimize the dichotomy structures led to the corresponding tetrahedrane structures. These observations suggest that a lone electron pair on the substituent R is required to give a stable dichotomy R2C2Fe2(CO)6 structure. Such structures are stabilized by zwitterionic resonance structures with a formal double bond from the carbyne carbon to the substituent atom bearing the lone pair (Figure 2). For the (R2NC)2Fe2(CO)6 structures the C−N distances in the dichotomy structures are ∼0.03 Å shorter than those in the corresponding tetrahedrane structures, consistent with the contribution of the zwitterionic structure with a CN formal double bond (Table 2). Similarly for
Fe−C(bridging)
dimethoxyacetylene and its heavier congeners, the C−XR (X = O, S, Se) distances in the dichotomy R2C2Fe2(CO)6 structures are ∼0.04 Å (for O) or 0.05 Å (for S and Se) shorter than those in the corresponding tetrahedrane structures (Table 2). This is again consistent with the contribution of CX doubly bonded canonical forms to the dichotomy structures. The contribution of the zwitterionic canonical forms (Figure 2) to the R2C2Fe2(CO)6 dichotomy structures is also supported by the natural atomic charges determined using NBO analysis (Table 3).37 Thus, the natural charges on the Fe atoms in the dichotomy R2C2Fe2(CO)6 structures are 0.35−0.45 more negative than those in the isomeric tetrahedrane structures, consistent with the formal negative charge on the iron atoms in the zwitterionic canonical form. Correspondingly, the R2NC groups in the (R2NC)2Fe2(CO)6 dichotomy structures and the REC groups in the (REC)2Fe2(CO)6 dichotomy structures (E = O, S, Se) are ∼0.3 more positive than those in the isomeric tetrahedrane structures. This is consistent with the formal positive charges on these heteroatoms in the zwitterionic canonical forms of the dichotomy structures. The CN and CE (E = O, S, Se) double bonds in the zwitterionic canonical forms of the (R2NC)2Fe2(CO)6 and (REC)2Fe2(CO)6 structures imply approximate trigonal sp2 hybridization for the nitrogen and chalcogens. Such a hybridization scheme is supported by predicted angles of ∼120° around the N or O atom in the R2NC and ROC units, respectively. For example, in the R2NC units the lone pairs originating from the N atom lie in the π orbitals perpendicular 92
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Table 3. Natural Atomic Charges and Wiberg Bond Indices (WBI) for the Fe−Fe Bond in the Predicted Structures using the M06-L Methoda natural charge [(CH3)3C]2C2Fe2(CO)6 (tBu-t) (CH3)2C2Fe2(CO)6 (Me-t) H2C2Fe2(CO)6 (H-t) (C5H4N)2C2Fe2(CO)6 (C5H4N-t) (C6H5)2C2Fe2(CO)6 (Ph-t) (CH3Te)2C2Fe2(CO)6 (MeTe-t-down) (CH3Te)2C2Fe2(CO)6 (MeTe-t-up) (CH3SO)2C2Fe2(CO)6 (MeSO-t-down) (CH3SO)2C2Fe2(CO)6 (MeSO-t-up) (CH3O)2C2Fe2(CO)6 (MeO-d) (CH3O)2C2Fe2(CO)6 (MeO-t-down) (CH3O)2C2Fe2(CO)6 (MeO-t-up) (CH3S)2C2Fe2(CO)6 (MeS-d) (CH3S)2C2Fe2(CO)6 (MeS-t-down) (CH3S)2C2Fe2(CO)6 (MeS-t-up) (CH3Se)2C2Fe2(CO)6 (MeSe-t-down) (CH3Se)2C2Fe2(CO)6 (MeSe-t-up) [(C2H5)2N]2C2Fe2(CO)6 (Et2N-d-1) [(C2H5)2N]2C2Fe2(CO)6 (Et2N-d-2) [(C2H5)2N]2C2Fe2(CO)6 (Et2N-t-1) [(C2H5)2N]2C2Fe2(CO)6 (Et2N-t-2) [(CH3)2N]2C2Fe2(CO)6 (Me2N-d) [(CH3)2N]2C2Fe2(CO)6 (Me2N-t) a
WBI
bond order
Fe
C
X
0.65 0.69 0.65 0.54 0.59 0.56 0.52 0.57 0.54 0.30 0.58 0.47 0.33 0.56 0.50 0.56 0.51 0.33 0.33 0.58 0.59 0.32 0.59
2 2 2 2 2 2 2 2 2 1 2 2 1 2 2 2 2 1 1 2 2 1 2
−1.314 −1.326 −1.350 −1.302 −1.389/−1.248 −1.310 −1.303 −1.318 −1.315 −1.810 −1.444 −1.382 −1.687 −1.353 −1.313 −1.345 −1.310 −1.758 −1.763 −1.388 −1.394 −1.767 −1.391
+0.048 +0.095 −0.104 +0.027 +0.048/+0.014 −0.433 −0.412 −0.216 −0.215 +0.713 +0.395 +0.358 −0.010 −0.185 −0.208 −0.274 −0.291 +0.504 +0.511 +0.220 +0.223 +0.516 +0.224
−0.012 −0.633 +0.242 +0.215 −0.072/−0.067 +0.792 +0.800 +1.259 +1.226 −0.510 −0.516 −0.513 +0.435 +0.337 +0.340 +0.501 +0.510 −0.473 −0.471 −0.462 −0.464 −0.445 −0.444
X is the atom connected to the bridging carbon atom.
Figure 8. Highest occupied canonical molecular orbitals (HOMOs) in the dichotomy structures [(CH3)2N]2C2Fe2(CO)6 and (CH3E)2C2Fe2(CO)6 (E = O, S).
0.69. This is consistent with the formal FeFe double bonds in the tetrahedrane structures required to give each iron the favored 18-electron configuration. The nature of the iron−iron interactions in both the dichotomy and tetrahedrane R2C2Fe2(CO)6 structures has also been investigated by examining the frontier molecular orbitals (MOs). For the dichotomy structures Me2N-d, MeO-d, and Me2S-d, the highest occupied molecular orbital (HOMO) corresponds to the bonding orbital of the Fe−Fe single bond, which is formed by overlap of the dz2 orbitals from both iron atoms (Figure 8). For the tetrahedrane structure Me2N-t the bonding orbitals corresponding to the two components of the formal FeFe double bond are HOMO-1 and HOMO-7 (Figure 9). The other high-lying bonding MOs, such as those from HOMO-2 to HOMO-6, are related to the ligands rather than the iron−iron bonding. Each of these two Fe−Fe bonding orbitals (HOMO-1 and HOMO-7) is tilted in the opposite direction from the Fe−Fe axis. The MOs for the other tetrahedrane structures show similar features. The MOs in Figures 8 and 9 thus support our interpretation of the iron− iron bonds in the R2C2Fe2(CO)6 structures as single bonds in
to the R2N plane and through overlap of such orbitals with suitable iron orbitals can transfer electron density to the iron atoms. This lowers drastically the energies of the dichotomy (R2NC)2Fe2(CO)6 structures relative to the isomeric tetrahedrane structures. A similar effect can occur in the (ROC)2Fe2(CO)6 systems but for such derivatives is weaker, owing to the lower basicity of ether oxygen atoms relative to amino nitrogen atoms. This can rationalize why for [(C2H5)2N]2C2Fe2(CO)6 the dichotomy structure lies 5−10 kcal/mol in energy below the isomeric tetrahedrane structure (depending on the method) whereas for (CH3O)2C2Fe2(CO)6 the energies of the isomeric dichotomy and tetrahedrane structures are essentially the same. Previous studies of Wiberg bond indices (WBIs) in metal− metal-bonded derivatives suggest typical values of 0.2−0.3 for unbridged formal metal−metal single bonds involving d-block transition metals such as iron.38 Similar WBI values ranging from 0.30 to 0.33 were found for the Fe−Fe bonds in the dichotomy R2C2Fe2(CO)6 structures, consistent with formal single bonds (Table 3). For the tetrahedrane R2C2Fe2(CO)6 structures the WBIs are approximately twice those of the dichotomy R2C2Fe2(CO)6 structures, ranging from 0.47 to 93
dx.doi.org/10.1021/om3008426 | Organometallics 2013, 32, 88−94
Organometallics
Article
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Figure 9. Two molecular orbitals related to the two components of the formal FeFe double bond in the tetrahedrane structure Me2N-t.
the dichotomy isomers but as double bonds in the tetrahedrane isomers.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Figures S1−S3, giving optimized structures of (CH3X)2C2Fe2(CO)6 (X = Se, Te, SO) predicted by the BP86, M06-L, and B3LYP* methods, Tables S1−S8, giving total energies (E, hartree), relative energies (ΔE, kcal/mol), Fe−Fe bond lengths (Å), and numbers of imaginary vibrational frequencies (Nimg) for the different structures, Tables S9−S32, giving harmonic vibrational frequencies (in cm−1) and infrared intensities (in parentheses, in km/mol) for the different structures, Tables S33−S56, giving optimized coordinates of the different structures, Table S57, giving Fe−Fe distances (in Å) predicted by three different functionals, Tables S58−S67, giving optimized coordinates of the structures with small imaginary frequency and the normal mode coordinates of the corresponding imaginary frequency, and text giving the complete Gaussian09 reference (ref 35). This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*E-mail:
[email protected] (H.F.);
[email protected]. edu (R.B.K.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The research was supported by the Scientific Research Fund of the Key Laboratory of the Education Department of Sichuan Province (Grant No. 10ZX012), the fund of the Key Laboratory of Advanced Scientific Computation, Xihua University, the Program for New Century Excellent Talents in University (Grant No. NCET-10-0949), China, and the U.S. National Science Foundation (CHE-1057466 and CHE1054286).
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REFERENCES
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