Tetramethyleneethane as a Bis(allylic) Ligand: 16-Electron Cobalt

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Tetramethyleneethane as a Bis(allylic) Ligand: 16-Electron Cobalt Configurations in Preference to Cobalt−Cobalt Bonding 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: Recently (2010) O’Hare and collaborators have synthesized and structurally characterized a tetramethyleneethane (TME) cobalt carbonyl complex, namely, trans-(η3,η3TME)Co2(CO)6, by reaction of NaCo(CO)4 with 2,3-bis(bromomethyl)butadiene (TMEBr2). Motivated by this experimental discovery, the structures and energetics of the series of TMECo2(CO)n derivatives (n = 6, 5, 4, 3, 2) have now been investigated by density functional theory. The lowest energy TMECo2(CO)6 structure is this experimentally known trans-(η3,η3TME)Co2(CO)6 structure consisting of two (η3-allyl)Co(CO)3 units linked by a C−C bond joining the central carbons of the allylic ligands in each half. However, the corresponding cisTMECo2(CO)6 structure lies only ∼3 kcal/mol above the global minimum. The lowest energy TMECo2(CO)5 and TMECo2(CO)4 structures consist of (η3-allyl)Co(CO)3 and/or (η3allyl)Co(CO)2 building blocks linked through the central allyl carbon atoms to give the TME ligand. The cobalt atoms in the (η3-allyl)Co(CO)2 portions of these structures have only a 16-electron configuration rather than the normally favored 18-electron configuration. Such TMECo2(CO)n structures (n = 6, 5, 4) are found with both cis and trans stereochemistries, corresponding to locations of the cobalt atoms on the same or opposite sides, respectively, of the plane of the TME ligand. In the only low-energy TMECo2(CO)3 structure a Co→Co dative bond from the Co(CO)2 cobalt to the Co(CO) cobalt gives both cobalt atoms a 16-electron configuration. For TMECo2(CO)2 both of the low-energy structures have bis(tetrahapto) η4,η4-TME ligands in which the two central carbon atoms of the TME ligand bridge both cobalt atoms. bonds (Figure 2).1 The structure of the known carbonyl derivative (η3,η3-TME)[Co(CO)3]2 bears a similar relation to the long-known2,3 (η3-C3H5)Co(CO)3 as biphenyl bears to benzene. Thus coupling two (η3-C3H5)Co(CO)3 units through the central carbon atoms of the η3-allyl ligands gives the experimentally observed1 (η3,η3-TME)[Co(CO)3]2 structure. Note that both cobalt atoms in (η3,η3-TME)[Co(CO)3]2 have the favored 18-electron configuration without a cobalt− cobalt bond. Tetramethyleneethane is also formally a dimer of allene arising from coupling two allene units through the central carbon atoms of each allene. Indeed the syntheses of the first tetramethyleneethane metal carbonyl complexes from allene and reactive metal carbonyl systems predate the recent synthesis 1 of (η3,η3-TME)[Co(CO)3]2 by approximately 45 years.4 Thus, Nakamura and Hagihara4 first reported in 1965 the synthesis of (η3,η3-TME)[Fe(CO)3]2 from Fe2(CO)9 and allene. In that case, a metal−metal bond between the iron atoms in (η3,η3-TME)[Fe(CO)3]2 is required to give each iron atom the favored 18-electron configuration. Therefore it is not surprising that X-ray crystallography shows (η3,η3-TME)[Fe(CO)3]2 to have both Fe(CO)3 groups on the same side of the TME unit (i.e., a cis structure)

1. INTRODUCTION The tetramethyleneethane (TME) ligand is of interest in providing two η3-allylic units in sufficiently close proximity to facilitate coordination to a pair of bonded metal atoms (Figure 1). However,

Figure 1. Free tetramethyleneethane (TME) diradical and its bonding to a dimetal unit (M2) as a bis(trihapto) ligand. efficient syntheses of TME−metal complexes have proven to be difficult since tetramethyleneethylene is a transient diradical (Figure 1). Recently (2010), however, O’Hare and co-workers1 developed much more efficient syntheses of binuclear unsubstituted tetramethyleneethane dimetal complexes using 2,3-bis(bromomethyl)butadiene (TMEBr2) or dipotassium tetramethyleneethanediide (K2TME) as sources of the TME ligand. Using such reagents as TME sources, the binuclear cobalt carbonyl derivative TME[Co(CO)3]2 was prepared from TMEBr2 and NaCo(CO)4 and the binuclear cyclopentadienylcobalt derivative TME(CoCp*)2 (Cp* = η5-Me5C5) from K2TME and Cp*Co(acac). Both of these binuclear cobalt complexes were shown by X-ray crystallography to have trans structures without cobalt−cobalt © 2012 American Chemical Society

Special Issue: F. Gordon A. Stone Commemorative Issue Received: November 18, 2011 Published: January 31, 2012 2887

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reasonably high CO dissociation energy, consistent with the ability to isolate this tricarbonyl as a stable compound. However, the CO dissociation energy of the dicarbonyl (η3-C3H5)Co(CO)2 was found to be ∼30 kcal/mol higher than that of the tricarbonyl, suggesting reasonable stability of the 16-electron dicarbonyl. This dicarbonyl appears to be an intermediate in the substitution reactions of the tricarbonyl (η3-C3H5)Co(CO)3 with various ligands (e.g., phosphines) to give (η3-C3H5)Co(CO)2L derivatives. In addition, the dicarbonyl (η3-C3H5)Co(CO)2 has been detected spectroscopically as a product from the photolysis of the tricarbonyl (η3-C3H5)Co(CO)3 in lowtemperature matrices at 12 K.7

2. THEORETICAL METHODS Electron correlation effects have been included via density functional theory methods, which have been shown to be a practical and effective computation tool, especially for organometallic compounds.8−14 Two DFT methods, B3LYP and BP86, were used in the present study. The reliability of such DFT methods is related to the chosen approximate exchange−correlation energy functional. The B3LYP and the BP86 methods are constructed in very different ways. The B3LYP method is a hybrid HF/DFT method using a combination of the three-parameter Becke functional (B3) with the Lee−Yang−Parr (LYP) generalized gradient correlation functional.15,16 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).17,18 This method does not include exact exchange and is mainly deduced by forcing the functional to satisfy certain exact constraints based on first principles. When these two very different DFT methods agree, reasonable predictions can be made. For most of the compounds investigated in this work, the two methods agree quite well. In this work the double-ζ plus polarization (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 Huzinaga−Dunning standard contracted DZ sets and are designated (9s5p1d/4s2p1d).19,20 For H, a set of p polarization functions αp(H) = 0.75 is added to the Huzinaga−Dunning DZ sets. For cobalt, our loosely contracted DZP basis set uses the Wachters’ primitive set augmented by two sets of p functions and one set of d functions. These are contracted following Hood et al. and designated (14s11p6d/10s8p3d).21,22 The geometries of all structures were fully optimized using both the DZP B3LYP and DZP BP86 methods. The 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,23 in which the fine grid (75, 302) is the default for evaluating integrals numerically, and the tight designation is the default for the energy convergence.

Figure 2. Structures of the tetramethyleneethane dicobalt complexes synthesized by O’Hare and collaborators.1 with an Fe−Fe distance of 2.927(3) Å, corresponding to a formal single bond (Figure 3).

Figure 3. Early examples of tetramethyleneethane metal carbonyls obtained in low yield from reactions of allenes with metal carbonyls. Another tetramethyleneethane metal carbonyl complex obtained from allene is the manganese carbonyl complex (η3,η3-TME)[Mn(CO)4]2 (Figure 3), isolated in low yield from the photochemical reaction of Mn2(CO)10 with allene.5 In this case a Mn−Mn bond is not needed to give both manganese atoms the favored 18-electron configuration. Therefore (η3,η3-TME)[Mn(CO)4]2 adopts a trans structure, similar to that of (η3,η3-TME)[Co(CO)3]2 discussed above (Figure 2). The structure of (η3,η3-TME)[Mn(CO)4]2 is related to the structure of the long-known6 (η3-C3H5)Mn(CO)4 by coupling through the central carbon atom of the η3-allylic unit. The 18-electron rule suggests that decarbonylation of (η3,η3TME)Co2(CO)6 to give (TME)Co2(CO)n (n = 5, 4, 3) should lead to derivatives having formal cobalt−cobalt bonds of order 6 − n. Thus (TME)Co2(CO)5 would be expected to have a formal Co−Co single bond, (TME)Co2(CO)4 to have a formal CoCo double bond, etc. This paper uses density functional theory (DFT) to evaluate such possibilities for (TME)Co2(CO)n (n = 5, 4, 3) derivatives. Somewhat surprisingly, no low-energy structures with short cobalt−cobalt distances (suggesting formal double or triple bonds) were found. Instead, structures with 16-electron (η3-allylic)Co(CO)2 subunits were found. Only for the highly unsaturated dicarbonyl (TME)Co2(CO)2 were low-energy structures found with short cobalt−cobalt distances suggesting multiple bonding. The stability of (η3-allylic)Co(CO)2 units having cobalt atoms with 16-electron configurations raises questions concerning the thermodynamics of the simple (η3-C3H5)Co(CO)n (n = 3, 2, 1) system. Therefore, such structures were also optimized using the same DFT methods and the thermodynamics of their CO dissociation reactions investigated. The known2,3 (η3-C3H5)Co(CO)3 was found to have a

3. RESULTS 3.1. Molecular Structures. Both singlet and triplet structures have been optimized for the TMECo2 (CO)n systems. However, the triplet structures for TMECo2(CO)n (n = 6, 5, and 4) have energies more than 30 kcal/mol above the lowest energy singlet structure. Even for the highly unsaturated TMECo2(CO)n (n = 3 and 2) systems, the triplet structures lie higher in energy than the singlet structures using the BP86 method. Thus, only low-lying singlet structures are discussed in the present paper. 3.1.1. TMECo2(CO)6. Two low-lying structures, namely, the trans 6S-1 and cis 6S-2 structures, were obtained for TMECo2(CO)6 (Figure 4). The global minimum 6S-1 is predicted to be the experimentally known1 C2h singlet structure trans-(η3,η3-TME)Co2(CO)6, which has been synthesized by 2888

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central C−C bond connecting the two allylic units of the TME ligand. The barrier for the conversion of the cis structure 6S-2 to the trans structure 6S-1 is very low (∼1 kcal/mol), suggesting that 6S-2 will collapse to the global minimum 6S-1 at room temperature. This is probably the reason that the cis structure 6S-2 has not been observed experimentally. 3.1.2. TMECo2(CO)5. The global minimum for TMECo2(CO)5 depends on the DFT method. The BP86 method predicts the global minimum to be the singlet Cs structure 5S-1 with a bridging carbonyl group, i.e., cis-(η 3 ,η 3 -TME)Co2(CO)4(μ-CO) (Figure 6). This bridging carbonyl group in 5S-1 exhibits a ν(CO) frequency of 1857 cm−1 (BP86), which is more than 100 cm−1 below the terminal ν(CO) frequencies, in accord with expectation. The Co−Co distance of 2.741 Å (B3LYP) or 2.707 Å (BP86) can be interpreted as a formal single bond, giving each cobalt atom the favored 18-electron configuration. This Co−Co distance in 5S-1 is closer to the experimental 2.700(2) Å Co−Co distance in the unbridged Co2(CO)8 structure in a C60 mixed crystal than the experimental 2.5301(8) Å Co−Co distance in the doubly bridged Co2(CO)6(μ-CO)2 structure.24 The second and third structures 5S-2 and 5S-3 have singlet (η3,η3-TME)Co2(CO)5 geometries with all terminal carbonyl groups (Figure 6). These two structures are similar except 5S-2 has trans stereochemistry while 5S-3 has cis stereochemistry. Structure 5S-2 lies 6.3 kcal/mol above 5S-1 by the BP86 method but 3.6 kcal/mol below 5S-1 by the B3LYP method. Similarly structure 5S-3 lies 7.7 kcal/mol above 5S-1 by the BP86 method but 2.6 kcal/mol below 5S-1 by the B3LYP method. Thus, the three TMECo2(CO)5 structures are nearly degenerate in energy, suggesting that the TMECo2(CO)5 may be fluxional. The theoretical Co−C(TME) distances of 2.03 to 2.09 Å suggest that both Co atoms are coordinated by separate η3-allyl ligands. The long Co···Co distance of 3.904 Å (B3LYP) or 3.874 Å (BP86) indicates no direct interaction between the two cobalt atoms, despite the cis stereochemistry. In both of these TMECo2(CO)5 structures the Co(CO)3 cobalt atom acquires the favored 18-electron configuration, but the Co(CO)2 cobalt atom favors only a 16-electron configuration. 3.1.3. TMECo2(CO)4. Two energetically low-lying structures, namely, 4S-1 and 4S-2, are predicted for TMECo2(CO)4 (Figure 7). These two structures are (η3,η3-TME)Co2(CO)4 geometric isomers, i.e., the cis isomer 4S-1 and the trans isomer 4S-2. Structures 4S-1 and 4S-2 are essentially degenerate in energy, with their predicted energies lying within 1 kcal/mol by both B3LYP and BP86 methods. In both TMECo2(CO)4 structures the Co−C(TME) distances (2.03 and 2.06 Å) indicate that each Co atom is coordinated by an η3-allyl ligand. The Co···Co distance in 4S-1 of 3.814 Å (B3LYP) or 3.275 Å (BP86) suggests no direct Co···Co interaction despite the cis stereochemistry. 3.1.4. TMECo2(CO)3. The lowest energy TMECo2(CO)3 structure by the BP86 method is the Cs structure 3S-1 with cis stereochemistry and each Co atom coordinated to an η3-allyl ligand, i.e., cis-(η3,η3-TME)Co2(CO)3 (Figure 8). The Co−Co distance of 2.884 Å (B3LYP) or 2.712 Å (BP86) can be interpreted as a dative single bond donating an electron pair from the Co(CO)2 cobalt atom to the Co(CO) cobalt atom. Attempted optimization of the corresponding trans-TMECo2(CO)3 structure led to a much higher energy structure since the trans stereochemistry does not provide for the possibility of a direct Co−Co bond like the cis structure 3S-1.

Figure 4. Two optimized TMECo2(CO)6 structures. The numbers in parentheses are the relative energies (ΔE in kcal/mol) predicted by the B3LYP and BP86 methods, respectively, while the bond distances in Å are predicted by the BP86 method.

reaction of TMEBr2 with two equivalents of Na[Co(CO)4] and structurally characterized by single-crystal X-ray diffraction. The Co−C(TME) distances of 2.04 and 2.09 Å predicted by BP86 for 6S-1 are close to the experimental values of 2.02 and 2.09 Å (Table S7 in the Supporting Information). The Co−C(CO) distances obtained by both methods (1.778 and 1.814 Å by BP86, Figure 4) are also close to the experimental values of 1.780 and 1.818 Å. The theoretical C−C bond distance of 1.490 Å between the two middle carbon atoms in the TME ligand is close to the experimental value of 1.487 Å. The theoretical distances for the other four C−C bonds in TME of 1.440 Å (BP86) are also close to the experimental values of 1.414 Å or 1.420 Å. The infrared-active ν(CO) frequencies of 2029, 1987, and 1986 cm−1 predicted by the BP86 method agree with the two strongest experimental ν(CO) frequencies of 2053 and 1985 cm−1 observed in a KBr disk.1 The experimentally observed structure was also studied theoretically by the same authors using the BP86 method.1 The second structure 6S-2 is the C2 singlet cis-(η3,η3TME)Co2(CO)6 (Figure 4), lying in energy only 3.2 kcal/mol (B3LYP) or 3.6 kcal/mol (BP86) above 6S-1. The Co−Co distance of 4.203 Å (B3LYP) or 4.211 Å (BP86) in 6S-2 indicates no direct cobalt−cobalt interaction. The Co−C−C−Co dihedral angle is ∼50° in this cis structure. In both 6S-1 and 6S-2 both cobalt atoms have the favored 18-electron configurations. The kinetic stability of 6S-2 was investigated by a search for the transition state between structures 6S-1 and 6S-2. This transition state (TS in Figure 5) was found to have a single imaginary vibrational frequency of 24i cm−1 (B3LYP) or 22i cm−1 (BP86), corresponding to internal rotation around the

Figure 5. Energies after zero-point energy correction (kcal/mol, in parentheses) predicted by the B3LYP and BP86 methods for 6S-1, 6S-2, and the transition state between them. 2889

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Figure 6. Three optimized TMECo2(CO)5 structures. In Figures 6 to 10, the numbers in parentheses are the relative energies (ΔE in kcal/mol) predicted by the B3LYP and BP86 methods, respectively, while the bond distances in Å are predicted by the BP86 method.

20 kcal/mol above these cis structures. The two structures 2S-1 and 2S-2, however, display quite different geometries from the structures with larger numbers of carbonyl groups, since the middle C−C bond in TME is in a perpendicular position to bridge the two Co atoms so that the TME moiety coordinates to each cobalt atom as a tetrahapto ligand. The BP86 method predicts the C2v structure 2S-1, i.e., cis-(η4,η4-TME)Co2(CO)2, to be the global minimum lying in energy 2.2 kcal/mol below 2S-2, while the B3LYP method predicts structure 2S-2 to lie 4.7 kcal/mol below 2S-1. The closeness of the energies of 2S-1 and 2S-2 suggests TMECo2(CO)2 to be a fluxional system. In the TMECo2(CO)2 structure 2S-1, the short CoCo distance of 2.244 Å (B3LYP) or 2.264 Å (BP86) can be interpreted as a formal double bond, so that each Co atom has a 16-electron configuration if the six π-electrons of TME are divided equally by the two cobalt atoms (Figure 9). This distance is relatively short for a double bond but comparable to the experimental25 FeFe double bond distance of 2.265 Å in (η5-C5H5)2Fe2(μ-CO)3. The shortness of the CoCo double bond in 2S-1 can relate to the geometrical constraints of the tetrahapto bonding of the η4,η4-TME ligand to each cobalt atom. However, an alternative formulation of the Co−Co bond as a relatively long quadruple bond, which would give each cobalt atom the normally favored 18-electron configuration, cannot be ruled out. Structure 2S-2 has a four-electron-donor carbonyl, i.e., cis-(η4,η4-TME)Co2(CO)4(η2-μ-CO), confirmed by a short Co−O distance of 2.553 Å (B3LYP) or 2.636 Å (BP86) and an unusually low ν(CO) frequency of 1799 cm−1 (BP86). The Co−Co distance of 2.486 Å (B3LYP) or 2.430 Å (BP86) predicted for 2S-2 is interpreted as a relatively long dative triple bond from the cobalt atom bonded to both carbonyl groups to the cobalt atom bonded only to the fourelectron-donor bridging η2-μ-CO group. This gives each cobalt atom in 2S-2 the normally favored 18-electron configuration. Four of the six electrons for such a dative triple bond in 2S-2 arise from the cobalt atom bonded to both CO groups (the “left” cobalt atom in Figure 9), and the remaining two electrons arise from the cobalt atom bonded to only one CO group, namely, the η2-μ-CO group (the “right” cobalt atom in Figure 9). 3.1.6. Mononuclear (C3H5)Co(CO)n Derivatives. The structures of the mononuclear (C3H5)Co(CO)n (n = 3, 2, 1) were optimized by the same DFT methods (Figure 10). The only low-lying structure predicted for C3H5Co(CO)3 is the singlet Cs structure (η3-C3H5)Co(CO)3 known experimentally.2,26−28 The predicted Co−C distances and the ν(CO) frequencies are comparable to the experimental values. The only structure obtained for C3H5Co(CO)2 is also a Cs singlet (η3-C3H5)Co(CO)2 structure.

Figure 7. The two optimized TMECo2(CO)4 structures.

Figure 8. Optimized TMECo2(CO)3 structure and a schematic representation of the Co→Co dative bond in this structure.

3.1.5. TMECo2(CO)2. The two low-lying singlet TMECo2(CO)2 structures 2S-1 and 2S-2 by the BP86 method have cis stereochemistry with direct cobalt−cobalt bonds (Figure 9). The corresponding trans structures, lacking the possibility for direct cobalt−cobalt bonds, lie more than

Figure 9. The two optimized TMECo2(CO)2 structures. 2890

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Ni(CO)4, Fe(CO)5, and Cr(CO)6 of 27, 41, and 37 kcal/mol, respectively.31 Table 1 also shows the energies for the disproportionation reactions 2 TMECo2(CO)n → TMECo2(CO)n+1 + TMECo2(CO)n−1. Such disproportionation reactions for the tetracarbonyl cis-TMECo2(CO) 4 indicate that cis-TMECo2(CO)4 (4S-1) is favored energetically with respect to cisTMECo2(CO)5 and cis-TMECo2(CO)3, and thus is a possible synthetic target. Table 2 reports the dissociation energies of the mononuclear derivatives (C3H5)Co(CO)n for loss of a single carbonyl by the Table 2. Energies (kcal/mol) for Carbonyl Dissociation of C3H5Co(CO)n Derivativesa (C3H5)Co(CO)3 → (C3H5)Co(CO)2 + CO (C3H5)Co(CO)2 → (C3H5)Co(CO) + CO a

Three low-lying structures within ∼5 kcal/mol (BP86) are obtained for the monocarbonyl (C3H5)Co(CO). The global minimum predicted by the BP86 method is a C1 singlet (η3C3H5)Co(CO). A closely related Cs singlet (η3-C3H5)Co(CO) structure lies only 1.6 kcal/mol (B3LYP) or 0.3 kcal/mol (BP86) above the C1 structure. A C1 triplet (η3-C3H5)Co(CO) structure lies 2.4 kcal/mol above the singlet C1 structure predicted by the BP86 method. However, the B3LYP method predicts it to lie 12.1 kcal/mol below the singlet. This is, again, an example of the different splitting between the singlet and triplet structures predicted by the two DFT methods, as discussed by Reiher and collaborators.29,30 Briefly, the inclusion of exact exchange in B3LYP favors high spin states compared to BP86. 3.2. Thermochemistry. Table 1 reports the energies of the single carbonyl dissociation reactions for the lowest lying structures for TMECo2(CO)n (n = 6, 5, 4, 3, 2),

TMECo2(CO)n → TMECo2(CO)n − 1 + CO All such dissociation energies are substantial, namely, 18 to 48 kcal/mol by B3LYP or 25 to 50 kcal/mol by BP86, and similar to the experimental carbonyl dissociation energies for Table 1. Energies (kcal/mol) for Carbonyl Dissociation and Disproportionation of TMECo2(CO)n Derivatives B3LYP

BP86

25.8

25.3

18.3

37.3

48.2

50.9

45.3

41.7

−7.5

12.2

29.9

13.5

−2.9

−9.3

BP86

22.0 51.6

31.6 60.0

The data are based on the lowest singlet structures.

reactions (C3H5)Co(CO)n → (C3H5)Co(CO)n−1 + CO (n = 3, 2). The carbonyl dissociation energy from (C3H5)Co(CO)3 to give (C3H5)Co(CO)2 is significant at 22.0 kcal/mol (B3LYP) or 31.6 kcal/mol (BP86) and comparable to the experimental values for the homoleptic carbonyls Cr(CO)6, Fe(CO)5, and Ni(CO)4 discussed above.31 Further dissociation of a CO group from (C3H5)Co(CO)2 to give (C3H5)Co(CO) requires a much higher energy of 51.6 kcal/mol (BL3YP) or 60.0 kcal/ mol (BP86), suggesting that the dicarbonyl might exist under certain conditions. This is supported experimentally by the spectroscopic detection of the dicarbonyl (C3H5)Co(CO)2 as a product from the photolysis of (C3H5)Co(CO)3 in lowtemperature matrices at 12 K.7 3.3. Atomic Population, Natural Bonding Orbital Analyses, and Cobalt−Cobalt Bonding. Previous studies of the Wiberg bond indices (WBIs) in metal−metal bonded derivatives suggest typical values of 0.2 to 0.3 for unbridged formal metal−metal single bonds.32 This is consistent with the calculated WBI values for the formal single Co−Co bonds in the TMECo2(CO)n derivatives ranging from 0.13 to 0.20 (Table 3). As expected, the WBI values for the nonbonding Co···Co distances are close to zero, namely, in the range 0.01 to 0.02. Generally, the WBI values confirm the formal Co−Co bond order assigned by the Co−Co distances and the electron counting discussed above. However, for the TMECo2(CO)2 structures 2S-1 and 2S-2, assigned to have rather short cobalt− cobalt single and double bonds but only 16-electron cobalt configurations, respectively, the WBIs are abnormally high but perhaps not high enough to suggest formal long cobalt−cobalt triple and quadruple bonds for 2S-1 and 2S-2, respectively, leading to normally favorable 18-electron configurations for the metal atoms. Table 3 also shows the natural atomic charges on the Co atoms in the cis-TMECo2(CO)n singlet structures (n = 6, 5, 4, 3, 2). The natural charge on a given cobalt atom appears to be related to the number of carbonyl groups to which it is bonded, with an increasing number of Co−CO bonds leading to an increased natural negative charge. This suggests that the negative charge on the cobalt atom from the OC→Co forward bonding is not completely counterbalanced by the concurrent Co→CO π→π* back-bonding. Thus for the structures with all terminal carbonyls, the Co atoms bonded to three CO groups have natural charges of −1.3 (Table 3), the Co atoms bonded

Figure 10. Lowest energy C3H5Co(CO)n (n = 3, 2, 1) structures.

trans-TMECo2(CO)6 (6S-1) → cis-TMECo2(CO)5 (5S-1) + CO cis-TMECo2(CO)5 (5S-1) → cis-TMECo2(CO)4 (4S-1) + CO cis-TMECo2(CO)4 (4S-1) → cis-TMECo2(CO)3 (3S-1) + CO cis-TMECo2(CO)3 (3S-1) → cis-TMECo2(CO)2 (2S-1) + CO 2 cis-TMECo2(CO)5 (5S-1) → trans-TMECo2(CO)6 (6S-1) + cis-TMECo2(CO)4 (4S-1) 2 cis-TMECo2(CO)4 (4S-1) → cis-TMECo2(CO)5 (5S-1) + cis-TMECo2(CO)3 (3S-1) 2 cis-TMECo2(CO)3 (3S-1) → cis-TMECo2(CO)4 (4S-1) + cis-TMECo2(CO)2 (2S-1)

B3LYP

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Table 3. Atomic Charges and Wiberg Bond Indices for the cis-TMECo2(CO)n Singlet Structures

cis-TMECo2(CO)6 (6S-2) cis- TMECo2(CO)5 (5S-1) cis- TMECo2 (CO)5 (5S-3) cis- TMECo2(CO)4 (4S-1) cis- TMECo2(CO)3 (3S-1) cis- TMECo2(CO)2 (2S-1) cis- TMECo2(CO)2 (2S-2)

natural charges on Co1/Co2

Wiberg bond index

Co−Co distance (Å)

B3LYP

BP86

B3LYP

BP86

B3LYP

BP86

formal Co−Co bond order

−1.29/−1.29 −0.99/−0.99 −0.57/−1.28 −0.58/−0.58 +0.01/−0.65 −0.14/−0.14 +0.34/−0.64

−1.27/−1.27 −0.98/−0.98 −0.55/−1.26 −0.54/−0.54 +0.08/−0.62 −0.14/−0.12 +0.43/−0.65

0.01 0.23 0.02 0.10 0.20 0.54 0.48

0.01 0.20 0.02 0.02 0.13 0.55 0.42

4.211 2.707 3.874 3.274 2.712 2.246 2.430

4.203 2.740 3.904 3.814 2.884 2.244 2.486

0 1 0 0 1 2,4 1,3

to two CO groups have negative charges of −0.5 to −0.6, while those bonded to a single CO group have charges of +0.1 to −0.1. However, with the Co−O interaction in a four-electrondonor bridging η2-μ-CO group (e.g., in 2S-2), the natural charge for the related Co atom increases to about +0.4.

single bond from the cobalt atom in the (η3-allyl)Co(CO)2 unit to the cobalt atom in the (η3-allyl)Co(CO) unit. This effectively gives both cobalt atoms in 3S-1 16-electron configurations. The dative Co→Co bond is parallel to the C−C single bond joining the center carbon atoms of the two η3-allyl units, i.e., the “central” carbon−carbon bond in the TME ligand. All of the low-energy binuclear TMECo2(CO)n (n = 6, 5, 4, 3) structures have the TME moiety functioning as a bis(trihapto) ligand, thus bonding to each cobalt atom in a similar manner to the allyl−cobalt bond in the well-known2,3,26−28 (η3-C3H5)Co(CO)3. For the two low-energy structures 2S-1 and 2S-2 of the more highly unsaturated TMECo2(CO)2, the TME moiety functions as a bis(tetrahapto)ligand with the two “central” TME carbons bonded to both cobalt atoms (Figure 9). Structures 2S-1 and 2S-2 have the “central” carbon−carbon bond of the TME ligand perpendicular to the cobalt−cobalt bond. A similar mode of bonding a TME ligand to a pair of metal atoms is found in the recently synthesized1 complex TME(CoCp*)2 (Figure 2). Assignment of the formal cobalt−cobalt bond orders in these two low-energy TMECo2(CO)2 structures is rather difficult. In 2S-1 both carbonyl groups are terminal carbonyl groups. The Co−Co distance of ∼2.25 Å in 2S-1 is short for the formal double bond to give each cobalt atom a 16-electron configuration but long for the formal quadruple bond to give each cobalt atom an 18-electron configuration. The Wiberg bond index of 0.55 for the cobalt−cobalt bond in 2S-1 is very high for a formal double bond but low for a formal quadruple bond using the 0.13 to 0.20 range for the WBIs of formal Co−Co bonds as a general guide. An analogous problem occurs for the TMECo2(CO)2 structure 2S-2, in which one of the carbonyl groups is a four-electron-donor bridging η2-μ-CO group. Thus the Co−Co distance of ∼2.45 Å in 2S-2 is short for a formal single bond to give each cobalt a 16-electron configuration but long for the formal triple bond to give each cobalt atom an 18electron configuration. Similarly the WBI of ∼0.45 for the cobalt−cobalt in 2S-2 is extraordinarily high for a formal single bond but somewhat low for a formal triple bond. These characteristics of the cobalt−cobalt bonds in both of the lowenergy TMECo2(CO)2 structures suggest that they are best regarded as resonance hybrids between a canonical form with a lower formal Co−Co bond order and 16-electron configurations for the cobalt atoms and a canonical form with a higher formal Co−Co bond order and 18-electron configurations for the cobalt atoms. The two low-energy TMECo2(CO)6 structures are a cis/trans pair of (η3,η3-TME)Co2(CO)6 structures 6S-1 and 6S-2 (Figure 4). These structures may simply be regarded as two (η3-allyl)Co(CO)3 units related to the well-known2,3,26−28 (η3-C3H5)Co(CO)3 joined together by a C−C bond between

4. DISCUSSION Tetramethyleneethane is viable as the dianion TME2−, which consists of two allyl anions linked by a C−C bond between the central carbon atoms. The dipotassium salt K2TME of this dianion has recently1 been shown to be a useful reagent for the synthesis of the cyclopentadienylcobalt derivative TME(CoCp*)2. The stability of the TME2− dianion suggests that the cobalt formal oxidation state in the TMECo2(CO)n derivatives is Co(I) with a d8 electronic configuration. Such d8 metal atoms (e.g., Co(I), Rh(I), Ir(I), Ni(II), Pd(II), Pt(II)) typically form square-planar complexes with 16-electron metal configurations. If the reasonable assumption is made that the four-electrondonor allyl anion is a bidentate monoanion, then an (η3allyl)Co(CO)2 unit can formally be considered as such a 16electron square-planar Co(I) complex. Evidence for the existence of 16-electron (η3-allyl)Co(CO)2 units in TMECo2(CO)n derivatives include the following: (1) For TMECo2(CO)5 the structures 5S-2 and 5S-3 containing an (η3-allyl)Co(CO)2 unit with a 16-electron configuration for the cobalt atom and no cobalt−cobalt bond have similar energies to 5S-1, which has a Co−Co single bond and 18-electron configurations for both cobalt atoms (Figure 6). (2) The only low-energy TMECo2(CO)4 structures 4S-1 and 4S-2 found in this work are a cis/trans isomer pair of structures consisting of two (η3-allyl)Co(CO)2 units with 16-electron cobalt configurations linked through the central carbon atoms without a cobalt−cobalt bond (Figure 7). No low-energy TMECo2(CO)4 structures with cobalt−cobalt bonds and 18-electron cobalt configurations were found. Structures with favorable 18- and 16-electron cobalt configurations without cobalt−cobalt bonds are found for TMECo2(CO)n (n = 6, 5, 4), leading to the possibility of lowenergy cis−trans isomer pairs. However, for the more highly unsaturated TMECo2(CO)n (n = 3, 2), structures with cobalt− cobalt bonds are required to give the cobalt atoms 16-electron configurations. Therefore all of the low-energy TMECo2(CO)n (n = 3, 2) structures have cis stereochemistry. For the tricarbonyl TMECo2(CO)3 only a single low-energy structure, 3S-1 (Figure 8), is found. This structure consists of an (η3allyl)Co(CO)2 dicarbonyl unit linked to an (η3-allyl)Co(CO) monocarbonyl unit. This linkage occurs not only through the center carbons of the allyl unit but also through a dative Co→Co 2892

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numbers of imaginary vibrational frequencies (Nimg), and spin expectation values ⟨S2⟩ for the C3H5Co(CO)n (n = 3, 2, 1) structures; Tables S7 to S11: Cobalt−carbon(TME) distances (in Å) for the TMECo2(CO)n structures (n = 6 to 2); Tables S12 to S21: Optimized coordinates of the TMECo2(CO)n structures (n = 2 to 6); Tables S22 to S26: Optimized coordinates of the C3H5Co(CO)n structures (n = 3, 2, 1); Tables S27 to S36: Harmonic vibrational frequencies (in cm−1) and infrared intensities (in parentheses, in km/mol) for TMECo2(CO)n (n = 6 to 2); Tables S37 to S41: Harmonic vibrational frequencies (in cm−1) and infrared intensities (in parentheses, in km/mol) of C3H5Co(CO)n (n = 3, 2, 1); complete Gaussian09 reference (ref 23).This material is available free of charge via the Internet at http://pubs.acs.org.

the two central carbon atoms of each trihapto(allyl) unit. The trans isomer 6S-1 is the lower energy of these two isomers by ∼3 kcal/mol and has been synthesized and characterized structurally by X-ray crystallography.1 Since most of the low-energy TMECo2(CO)n (n = 6, 5, 4, 3) structures can be constructed from (allylic)Co(CO)3 and (allylic)Co(CO)2 building blocks, the structures were optimized for the mononuclear species (η3-C3H5)Co(CO)n (n = 3, 2, 1). The expected experimental (η3-C3H5)Co(CO)3 structure was found. The predicted energy for the dissociation of a carbonyl group from the tricarbonyl (η3-C3H5)Co(CO)3 to give the dicarbonyl (C3H5)Co(CO)2 is 22.0 kcal/mol (B3LYP) or 31.6 kcal/mol (BP86), comparable to the experimental 27 kcal/mol carbonyl dissociation energy for the stable metal carbonyl Ni(CO)4.31 However, further dissociation of a carbonyl group from (C3H5)Co(CO)2 to give (C3H5)Co(CO) requires a much higher energy of 51.6 kcal/mol (BL3YP) or 60.0 kcal/mol (BP86), suggesting that the dicarbonyl might exist under certain conditions. Thus the dicarbonyl (C3H5)Co(CO)2 appears to be an intermediate in the substitution reactions of the tricarbonyl (η3-C3H5)Co(CO)3 with various ligands (e.g., phosphines) to give (η3-C3H5)Co(CO)2L derivatives. In addition, the dicarbonyl (C3H5)Co(CO)2 has been detected spectroscopically as a product from the photolysis of (C3H5)Co(CO)3 in low-temperature matrices at 12 K.7

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Corresponding Author

*E-mail: [email protected]; [email protected].

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 (Grand No. NCET-10-0949), China, and the U.S. National Science Foundation (Grants CHE-1057466 and CHE1054286).

5. SUMMARY The lowest energy TMECo2(CO)6 structure is the experimentally known1 trans-(η3,η3-TME)Co2(CO)6 structure consisting of two (η3-allyl)Co(CO)3 units linked by a C−C bond joining the central carbons of the allylic ligands in each half. However, the corresponding cis-TMECo2(CO)6 structure lies only ∼3 kcal/mol above this global minimum. The lowest energy TMECo2(CO)5 and TMECo2(CO)4 structures consist of (η3-allyl)Co(CO)3 and/or (η3-allyl)Co(CO)2 building blocks linked through the central allyl carbon atoms. The cobalt atoms in the (η3-allyl)Co(CO)2 portions of these structures have only 16-electron configurations rather than the normally favored 18-electron configuration. Such TMECo2(CO)n structures (n = 6, 5, 4) are found with both cis and trans stereochemistries corresponding to locations of the cobalt atoms on the same or opposite sides, respectively, of the plane of the tetramethyleneethane ligand. For the more highly unsaturated TMECo2(CO)3 and TMECo2(CO)2 cobalt−cobalt bonds are required even to give each cobalt atom a 16-electron configuration. In the only low-energy TMECo2(CO)3 structure, 3S-1, a Co→Co dative bond from the Co(CO)2 cobalt to the Co(CO) cobalt gives each cobalt atom a 16-electron configuration. Both low-energy TMECo2(CO)2 structures have bis(tetrahapto) η4,η4-TME ligands in which the two central carbon atoms of the TME ligand bridge both cobalt atoms.

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AUTHOR INFORMATION

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DEDICATION This paper is dedicated to the memory of R.B.K.’s major professor, Gordon Stone, in recognition of his vast pioneering contributions to organometallic chemistry as well as the role that he played as a mentor for R.B.K.’s career in organometallic chemistry.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

Figures S1 to S5: Optimized structures of TMECo2(CO)n (n = 6 to 2) predicted by the B3LYP and BP86 methods; Figure S6: The lowest lying structures of C3H5Co(CO)n (n = 3, 2, 1); Tables S1−S5: Total energies (E, hartree), relative energies (ΔE, kcal/mol), Co−Co bond lengths (Å), numbers of imaginary vibrational frequencies (Nimg), and spin expectation values ⟨S2⟩ for the TMECo2(CO)n structures (n = 6 to 2); Table S6: Total energies (E, hartree), relative energies (ΔE, kcal/mol), 2893

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