Energetics of Variable Hapticity of Carbocyclic Rings in

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Energetics of Variable Hapticity of Carbocyclic Rings in Cyclopentadienylmetal Carbonyl Systems of the Second Row Transition Metals C5H5M(CO)nCmHm (M = Ru, Tc, Mo, Nb) Including Mechanistic Studies of Carbonyl Dissociation Zhihui Zhang,† Xuejun Feng,*,† Qun Chen,† Mingyang He,† Yaoming Xie,‡ and R. Bruce King*,‡ †

School of Petrochemical Engineering, Changzhou University, Changzhou 213164, PR China Center for Computational Chemistry and Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States

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‡

S Supporting Information *

ABSTRACT: Decarbonylation of the experimentally known CpRu(CO)2(η1C5H5), CpMo(CO)2(η3-C7H7), and CpNb(CO)2(η4-C8H8) (Cp = η5-C5H5), each with uncomplexed 1,3-butadiene units in the CnHn ring, as well as the related CpTc(CO)2(η2-C6H6), to give the corresponding carbonyl-free derivatives CpM(ηn-CnHn) derivatives has been studied by density functional theory. For ruthenium, technetium, and molybdenum the coordinated CnHn ring of the intermediate monocarbonyl CpM(CO)(ηn−2-CnHn) contains an uncomplexed CC double bond and each decarbonylation step proceeds with a significant energy barrier represented by a higher energy transition state. However, decarbonylation of CpNb(CO)2(η4-C8H8) to the monocarbonyl proceeds without an energy barrier, preserving the tetrahapto coordination of the C8H8 ring to give CpNb(CO)(η4-C8H8) in which the niobium atom has only a 16-electron configuration. All of the monocarbonyl derivatives CpM(CO)(CnHn) are predicted to be strongly energetically disfavored with respect to disproportionation to give CpM(CO)2(CnHn) + CpM(CnHn). This allows us to understand the failure to date to synthesize any of the monocarbonyl derivatives.

1. INTRODUCTION A seminal development in organometallic chemistry was the discovery that planar carbocyclic rings could bind to transition metals through all of their carbon atoms. This was first manifested in the serendipitous synthesis of the sandwich compound ferrocene (Cp2Fe; Cp = η5-C5H5; Figure 1) by two

18-electron configuration. Sandwich compounds of the types (η7-C7H7)2M and (η8-C8H8)2M with larger fully bonded carbocyclic rings could not be obtained for d-block metals. However, sandwich compounds containing two seven-membered rings, e.g., the anion4 [(η-C7H7)2U]−, and two eightmembered rings, e.g., “uranocene”,5 (η8-C8H8)2U, were synthesized for f-block metals including uranium. Shortly after the discovery of ferrocene in the 1950s, a variety of transition metal organometallic complexes with single pentahapto Cp rings were discovered. Most of the time, the favored structures and stoichiometries were found to have the favored 18-electron configuration6,7 for the central transition metals. Therefore, it was initially surprising to be able to isolate a stable Cp2Fe(CO)2 species from the reaction of CpFe(CO)2I with sodium cyclopentadienide.8 Simple counting of electrons in Cp2Fe(CO)2 led to an excessive 22electron configuration for the central iron atom, assuming both Cp rings to be the only type of Cp ligand known at that time, namely, a pentahapto Cp ring. However, it was soon realized that the favored 18-electron configuration was achieved for the iron atom in Cp2Fe(CO)2 by only partially bonding one of the Cp rings to the central iron atom using only one of the five carbon atoms in what is now known as a monohapto η1-Cp ligand (Figure 2).9 The iron atom in a (η5-Cp)Fe(η1Cp)(CO)2 complex indeed has the favored 18-electron

Figure 1. Stable sandwich compounds containing five- to eightmembered planar carbocyclic rings.

independent research groups in 1951, namely, that of Kealy and Pauson1 and that of Miller, Tebboth, and Tremaine.2 A key feature of ferrocene is the full pentahapto bonding of each Cp ring to the central iron atom using all five of its carbon atoms. Later, even the notably stable and normally relatively unreactive benzene ring was also shown to bind to transition metals in an analogous manner using all six carbon atoms, as indicated by the discovery of dibenzenechromium by Fischer and Hafner.3 Ferrocene and dibenzenechromium are particularly stable sandwich compounds because of the 18-electron rule favoring structures in which the central metal atom has an © XXXX American Chemical Society

Received: June 6, 2018

A

DOI: 10.1021/acs.organomet.8b00387 Organometallics XXXX, XXX, XXX−XXX

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sandwich compound (η5-Cp)Mo(η7-C7H7). Instead, this gave the dicarbonyl CpMo(CO)2(η3-C7H7) with a trihapto η3-C7H7 ring (Figure 2).23,24 This color change in this reaction was particularly dramatic since the green (η7-C7H7)Mo(CO)2I converted to the orange CpMo(CO)2(η3-C7H7) during this reaction. Subsequently, the red (η3-C7H7)Co(CO)3, also with a trihapto η3-C7H7 ring,24 was also synthesized with a structure closely related to that of (η4-C8H8)Fe(CO)3. The carbonylfree sandwich compound CpMo(η7-C7H7) with a heptahapto η7-C7H7 ring was initially prepared in low yield by the reaction of MoCl5 with a mixture of CpMgBr, iPrMgBr, and cycloheptatriene25 and later in much better yield by the reaction of (η7-C7H7)Mo(NCMe)I2 with sodium cyclopentadienide.26 The corresponding methylcyclopentadienyl derivative (η5-C5H4Me)Mo(η7-C7H7) has been structurally characterized by X-ray diffraction.26 Another early observation involving seven- and eightmembered ring cyclopolyolefins is relevant to the results discussed in this paper. The simple thermal reaction of CpV(CO)4 with boiling cycloheptatriene was reported in 1959 to result in the loss of all four carbonyl groups to give the sandwich compound CpV(η7-C7H7).27 This is a stable purple sublimable solid that could be handled for brief periods in air despite the 17-electron configuration of the central vanadium atom. However, an analogous reaction of CpV(CO)4 with cyclooctatetraene failed to give an analogous sandwich compound CpV(η8-C8H8), which might be expected to be more stable than CpV(η7-C7H7) because of the 18-electron configuration of the vanadium atom in CpV(η8-C8H8). A fairly recent theoretical study28 was consistent with this experimental observation in showing that a hexahapto CpV(η6-C8H8) with an uncomplexed CC double bond in the eight-membered ring was the lowest energy structure rather than an isomeric structure with an octahapto η8-C8H8 ring. The niobium compound CpNb(CO)2(η4-C8H8) with a tetrahapto η4-C8H8 ring rather than a carbonyl free CpNbC8H8 isomer was found to be the product from the photolysis of CpNb(CO)4 with cyclooctatetraene.29 The corresponding carbonyl-free sandwich compound CpNb(η8-C8H8) with an octahapto η8-C8H8 ring has not been reported. This body of experimental results, some within 15 years of the original 1951 discovery of ferrocene, suggest some stability of metal carbonyl derivatives of cyclopolyolefins partially bonded to transition metals leaving two uncomplexed double bonds. In order to understand better the formation of such species and their possible decarbonylation to carbonyl-free sandwich compounds containing fully bonded cyclopolyolefins, we have used density functional theoretical methods to study the successive decarbonylation of second row transition metal derivatives of the type CpM(CO)2(ηn−4-CnHn) (n = 5, M = Ru; n = 6; M = Tc; n = 7, M = Mo; n = 8, M = Nb) to form the corresponding carbonyl-free sandwich complexes CpM(ηnCnHn). We chose these systems of the second row transition metals since the species CpRu(CO)2(η1-Cp),12,13 CpMo(CO)2(η3-C7H7)23,24 and CpNb(CO)2(η4-C8H8)29 are all known experimentally.

Figure 2. Some of the original metal complexes containing partially bonded cyclic polyolefins leaving two uncomplexed conjugated CC double bonds in a butadiene subnetwork.

configuration in which the monohapto η1-Cp ligand has two uncomplexed CC double bonds. The analogous ruthenium derivatives (η5-Cp)2Ru (ruthenocene)10,11 and (η5-Cp)(η1Cp)Ru(CO)2 were subsequently synthesized.12,13 A related problem arose shortly thereafter in 1959 a study of iron carbonyl complexes of cyclooctatetraene. The simple thermal reaction of cyclooctatetraene with iron pentacarbonyl was found by several research groups14−17 to give readily the stable volatile dark red complex (C8H8)Fe(CO)3. This complex raised another dilemma since a planar fully complexed C8H8 ring using all eight carbon atoms would provide another example of a metal complex with an unfavorable 22-electron configuration for the central metal atom.18 Since X-ray structural determinations were not generally available at the time (C8H8)Fe(CO)3 was first synthesized, an attempt was made to elucidate the structure of C8H8Fe(CO)3 by the then new method of proton NMR spectroscopy. However, this further confused the issue since all of the protons in C8H8Fe(CO)3 were found to be equivalent in an NMR spectrum taken under ambient conditions. This was initially thought to imply an octahapto η8-C8H8 ring confirming the unfavorable 22-electron configuration of the iron atom in C8H8Fe(CO)3. Only when an X-ray structural determination became available on C8H8Fe(CO)3 did an explanation for this dilemma become clear.19 Thus, the C8H8 ring in C8H8Fe(CO)3 was shown to be an tetrahapto η4-C8H8 ring with two uncomplexed conjugated CC double bonds in a butadienelike subunit similar to the η1-Cp ring in (η5-Cp)(η1Cp)Fe(CO)2 discussed above (Figure 2). The single proton NMR resonance in C8H8Fe(CO)3 under ambient conditions was later shown to separate upon sufficient cooling into the pattern of multiple resonances expected for an (η4-C8H8)Fe(CO)3 structure with a bent tetrahapto η4-C8H8 ring.20,21 This observation led to the development of the concept of fluxional organometallic molecules of which (η4-C8H8)Fe(CO)3 and (η5-Cp)Fe(CO)2(η1-Cp) were among the first examples.22 In such fluxional molecules the metal atom moves around the cyclic ligand at a rate faster than the NMR time scale. A final early example of partial bonding of a carbocyclic ring to a metal atom was found in cycloheptatrienylmetal chemistry. The reaction of the readily available C7H7Mo(CO)2I with sodium cyclopentadienide was investigated by analogy with the reaction of CpFe(CO)2I with sodium cyclopentadienide discussed above9 with the hope that this would lead to the

2. THEORETICAL METHODS Density functional theory (DFT) has been found to be a practical and effective computational method, especially for organometallic compounds.30−36 In this study, we use two DFT methods. The first is a newer generation functional, MPW1PW91, which is a combination of the modified Perdew−Wang exchange functional B


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Figure 3. Optimized CpRu(CO)nC5H5 (n = 2, 1, 0) structures with distances in Å. with Perdew−Wang’s 91 gradient correlation functional.37 This functional has been shown to be good for some heavy transition metal compounds.38 The second DFT method is the BP86 method, which uses Becke’s 1988 exchange functional with Perdew’s 1986 gradient corrected correlation functional.39,40 The Stuttgart double-ζ basis sets with an effective core potential (ECP)41,42 were used for the second-row transition metals ruthenium, technetium, molybdenum, and niobium. In these basis sets, 28 core electrons in the transition metal atoms are replaced by an effective core potential (ECP). This effective core approximation includes scalar relativistic contributions, which may become significant for the heavy transition metal atoms. The valence basis sets are contracted from (8s7p6d) primitive sets to (6s5p3d). For the carbon, oxygen, and hydrogen atoms, the all-electron double-ζ plus polarization (DZP) basis sets were used. These are derived from Huzinaga and Dunning’s contracted double-ζ contraction set43 by adding spherical harmonic polarization functions with the orbital exponents αd(C) = 0.75, αd(O) = 0.85, and αp(H) = 0.75. For the C5H5Ru(CO)2C5H5, C5H5Tc(CO)2C6H6, C5H5Mo(CO)2C7H7, and C5H5Nb(CO)2C8H8 systems, there are 296, 316, 336, and 356 contracted Gaussian basis functions, respectively. All of the computations were performed using the Gaussian 09 program,44 in which the fine grid (75,302) is the default for evaluating integrals numerically.45 Because of significant limitations in the numerical integration procedures, some low imaginary vibrational frequencies arise from numerical errors and are given less weight in the analysis. Therefore, we do not always follow such very low imaginary vibrational frequencies. The geometries of all structures considered were fully optimized using both the MPW1PW91 and BP86 methods. The vibrational frequencies and the corresponding infrared intensities were determined analytically at the same level of theory. All vibrational frequency results are given in the Supporting Information. Only the structures with singlet states are reported in the present paper, since the corresponding triplet state structures have significantly higher energies. In this paper the CpM(CO)n(CmHm) molecules are designated as M-n where n is the number of carbonyl groups. For example, the C5H5Ru(CO)2C5H5 structure is denoted as Ru-2.

which would also have the favored 18-electron configuration for the ruthenium atom, led instead to Ru-2. In Ru-2, the uncoordinated CC−CC moiety in the η1-C5H5 ring is trans to the CO groups. Besides this trans structure, we have found two other conformers arising from the internal rotation around the Ru−C(η1-C5H5) bond. One is Ru-2-cis (Table S1) with the uncoordinated CC−CC moiety cis to the CO groups and lying 4.1 kcal/mol (MPW1PW91) or 4.0 kcal/mol (BP86) above Ru-2 with a small imaginary vibrational frequency of 36i cm−1 (MPW1PW91) or 33i cm−1 (BP86). Following the corresponding normal mode leads to gauche conformer Ru-2-gauche, which is a genuine minimum, lying 1.8 kcal/mol (MPW1PW91) or 2.0 kcal/mol (BP86) above Ru-2 (Table S1). Lowest-energy CpRu(CO)C5H5 structure Ru-1 has Cs symmetry. With one less CO ligand than Ru-2, one of the cyclopentadienyl ligands increases its hapticity from 1 to 3 to give a CpRu(CO)(η3-C5H5) structure in which the central ruthenium atom retains the favored 18-electron configuration. The eight Ru−C(ring) bonding distances are predicted to be ∼2.4 Å or less. In the η3-C5H5 ring, a CC distance of 1.371 Å (MPW1PW91) or 1.387 Å (BP86) is predicted for the uncoordinated double bond. Conformer Ru-1-cis with the C C double bond cis to the CO group has been found lying 0.9 kcal/mol (MPW1PW91) or 2.2 kcal/mol (BP86) in energy above Ru-1. However, structure Ru-1-cis is not a genuine minimum, but has a small imaginary vibrational frequency of 21i cm−1 (MPW1PW91) or 29i cm−1 (BP86), which is related to the internal rotation. Following the corresponding normal mode leads to gauche conformer Ru-1-gauche, lying above Ru1 by 0.7 kcal/mol (MPW1PW91) or 2.2 kcal/mol (BP86). Thus, this gauche conformer is very close to the cis conformer in geometry and in energy. Lowest-energy Cp2Ru structure Ru-0 with D5h symmetry is the well-known sandwich compound ruthenocene.10,11,46 The Ru−C distances are predicted to be 2.180 Å (MPW1PW91) or 2.196 Å (BP86), and the C−C distances are ∼1.440 Å. These calculated distances are close to the experimental Ru−C and C−C distances of 2.21 and 1.43 Å, respectively, found for Cp2Ru in an early X-ray crystallography study.11 A D5d conformer of Cp2Ru is predicted to lie slightly higher in energy at ∼0.7 kcal/mol by both methods. This D5d structure has one imaginary vibrational frequency of 54i cm−1 by MPW1PW91 or 53i cm−1 by BP86. Following the corresponding normal mode leads to the D5h minimum. The ruthenium atom in both Cp2Ru structures has the favored 18-electron configuration.

3. RESULTS AND DISCUSSION 3.1. CpRu(CO)nC5H5 (n = 2, 1, 0) Structures. The lowestenergy CpRu(CO)2C5H5 structure Ru-2 (Cs symmetry) has two CO ligands with Ru−CO distances of 1.859 Å (MPW1PW91) or 1.861 Å (BP86) (Figure 3). Structure Ru2 has one pentahapto η5-Cp ring and one monohapto η1-C5H5 ring, indicated by short Ru−C(ring) bonding distances of 2.2− 2.3 Å, while the nonbonding Ru−C(ring) distances are longer than 3 Å. Thus, the ruthenium atom in Ru-2 has the favored 18-electron configuration. Attempted optimization of the (η3C5H5)2Ru(CO)2 isomer with two trihapto η3-C5H5 rings, C


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thereby giving the central technetium atom the favored 18electron configuration. The four uncoordinated carbon atoms in the benzene ring are an uncomplexed butadiene unit, i.e., CC−CC, with distances of 1.37 Å for the two CC double bonds and 1.42 Å for the C−C single bond. In Tc-2, the uncoordinated CC−CC moiety lies cis to the CO groups. Another conformer, Tc-2-trans (Table S2), with the uncoordinated CC−CC subunit trans to the CO groups has essentially the same energy, lying above Tc-2 by only 0.02 kcal/mol (MPW1PW91) or below Tc-2 by 0.19 kcal/mol (BP86). Lowest-energy monocarbonyl CpTc(CO)C6H6 structure Tc-1 (Cs symmetry) has a Tc−CO distance of 1.890 Å (MPW1PW91) or 1.903 Å (BP86) (Figure 5). It has one η5Cp ring and one tetrahapto benzene ring with an uncomplexed CC double bond cis to the CO group thereby giving the technetium atom the favored 18-electron configuration. Conformer Tc-1-trans with the CC double bond trans to the CO group has been found of similar energy, lying only 2.2 kcal/mol (MPW1PW91) or 0.5 kcal/mol (BP86) above Tc-1. The trans conformer, Tc-1-trans, is predicted to be a genuine minimum with no imaginary vibrational frequencies by both DFT methods. Carbonyl-free CpTcC6H6 Tc-0 has a tiny imaginary vibrational frequency of 28i cm−1 (MPW1PW91) or 29i cm−1 (BP86). However, this is due to a numerical error, since this imaginary frequency is removed by using the finer (99, 590) integration grid. The Tc−C distances are ∼2.24 Å for the C5H5 ring and ∼2.20 Å for the C6H6 ring. Structure CpTcC6H6 is a sandwich structure with one η5-C5H5 ring and one η6-C6H6 ring to give the technetium atom the favored 18-electron configuration. Although CpTcC6H6 has not been synthesized, the valence isoelectronic rhenium compound CpRe(η6-C6H6) has been synthesized by ultraviolet irradiation of ReCl5 with a CpMgBr/iPrMgBr/cycloheptatriene mixture48 or by co-condensation of rhenium vapor with a mixture of benzene and cyclopentadiene.49 The potential energy curve for the stepwise dissociation of carbonyl groups from Tc-2 to give Tc-0 via Tc-1 is shown in Figure 6. The predicted dissociation energy of losing one CO group from Tc-2 to give Tc-1 is substantial at 48.2 kcal/mol via a high barrier of 49.9 kcal/mol (MPW1PW91). This indicates that Tc-2 is stable both thermodynamically and

The potential energy curve for the stepwise dissociation of carbonyl groups from Ru-2 and Ru-1 is shown in Figure 4. The

Figure 4. Potential energy curve for the dissociation energies (kcal/ mol) and Gibbs free energies (kcal/mol, in parentheses) for loss of a single carbonyl from CpRu(CO)nC5H5 (n = 2, 1).

predicted dissociation energy of losing one CO group from Ru-2 to give Ru-1 is 35.6 kcal/mol through a rather high energy barrier of 42.4 kcal/mol (MPW1PW91). This indicates that Ru-2 is stable both thermodynamically and kinetically. For Ru-1, the CO dissociation reaction is exothermic, liberating 7.7 kcal/mol as predicted by MPW1PW91. This suggests that Ru1 may not be viable species. However, a significant energy barrier of 20.8 (= 56.4 − 35.6) kcal/mol (MPW1PW91) is predicted for CO dissociation from Ru-1 suggesting the possibility of kinetic stability. This potential energy curve is consistent with that reported by Yates et al.47 In their paper, the two minima 1M and 2M for CpRu(CO)2(η1-C5H5) correspond to our Ru-2 and Ru-2-gauche, respectively. 3.2. CpTc(CO)nC6H6 (n = 2, 1, 0) Structures. The lowestenergy CpTc(CO)2C6H6 structure Tc-2 (Cs symmetry) has two CO ligands with Tc−CO distances of 1.884 Å (MPW1PW91) or 1.889 Å (BP86) (Figure 5). The cyclopentadienyl ring in Tc-2 is pentahapto (η5-Cp), whereas the benzene ring is dihapto (η2-C6H6) to the technetium atom

Figure 5. Optimized structures of CpTc(CO)nC6H6 (n = 2, 1, 0) with distances in Å. D


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ring is cis to the CO groups. We have found a trans conformer, Mo-2-trans (Table S3), of energy similar to that of Mo-2, lying only 0.4 kcal/mol (MPW1PW91) or 0.1 kcal/mol (BP86) in energy above Mo-2. Lowest-energy monocarbonyl CpMo(CO)C7H7 structure Mo-1 (Cs symmetry) has an Mo−CO distance of 1.972 Å (MPW1PW91) or 1.978 Å (BP86) (Figure 7). It has one pentahapto η5-Cp ring and one pentahapto η5-C7H7 ring to give the molybdenum atom the 18-electron configuration. In Mo-1, the uncoordinated CC moiety in the η5-C7H7 ring is cis to the CO group. A trans conformer, Mo-1-trans (Table S3), is found to lie above Mo-1 by 13.6 kcal/mol (MPW1PW91) or below Mo-1 by 11.8 kcal/mol (BP86). CpMoC7H7 structure Mo-0 (Cs symmetry) has a sandwich structure with one pentahapto η5-Cp ring and one heptahapto η7-C7H7 ring, thereby giving the molybdenum atom the favored 18-electron configuration (Figure 7). This species has been synthesized by two methods,25,26 and its methyl derivative (η5-C5H4Me)Mo(η7-C7H7) is structurally characterized by X-ray crystallography.26 The experimental average Mo−C distances to the η5-C5H5 and η7-C7H7 rings of 2.31 and 2.25 Å in (η5-C5H4Me)Mo(η7-C7H7) are close to those of 2.33 and 2.28 Å, respectively, predicted for Mo-0. The potential energy curve for the stepwise dissociation of carbonyl groups from Mo-2 to give Mo-0 via Mo-1 is shown in Figure 8. The predicted dissociation energy of losing one CO

Figure 6. Potential energy curve for the carbonyl dissociation energies (kcal/mol) and Gibbs free energies (kcal/mol, in parentheses) from CpTc(CO)nC6H6 (n = 2, 1).

kinetically. For monocarbonyl Tc-1, carbonyl dissociation is essentially thermoneutral (0.0 kcal/mol by MPW1PW91) suggesting that Tc-1 is not viable. However, the MPW1PW91 method predicts a high energy barrier of 30.4 (= 78.6 − 48.2) kcal/mol for CO dissociation from Tc-1. We have also considered an isomer CpTcH(CO)2C6H5, in which a hydrogen atom in the C6H6 ring has migrated to the technetium atom from the benzene ring. The remaining phenyl radical is bonded to the technetium atom through a single carbon atom, i.e., as a monohapto ligand giving the technetium atom the favored 18-electron configuration (Figure S1). However, this isomer has a significantly higher energy than that of Tc-2 by 16.3 kcal/mol (MPW1PW91) or 13.5 kcal/mol (BP86). 3.3. CpMo(CO)nC7H7 (n = 2, 1, 0) Structures. The CpMo(CO)nC7H7 (n = 2, 1, 0) structures are all of Cs symmetry with real vibrational frequencies, except for Mo-2 with a tiny imaginary vibrational frequency (11i cm−1) using the BP86 method. Lowest-energy CpMo(CO)2C7H7 structure Mo-2 has two CO ligands with Mo−CO distances of 1.951 Å (MPW1PW91) or 1.958 Å (BP86). Structure CpMo(CO)2C7H7 has one pentahapto η5-Cp ring and one trihapto η3-C7H7 ring bonded to the molybdenum atom to give the molybdenum atom the favored 18-electron configuration. In Mo-2 the uncoordinated CC−CC moiety in the η3-C7H7

Figure 8. Potential energy curve for the dissociation energies (kcal/ mol) and Gibbs free energies (kcal/mol, in parentheses) for loss of a single carbonyl from CpMo(CO)nC7H7 (n = 2, 1).

Figure 7. Optimized CpMo(CO)nC7H7 (n = 2, 1, 0) structures with distances in Å. E


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Figure 9. Optimized structures of CpNb(CO)nC8H8 (n = 2, 1, 0) with bond distances in Å.

niobium atom the favored 18-elecron configuration (Figure 9). The Nb−C distances for both rings are very similar at ∼2.42 Å. The niobium atom in CpNbC8H8 has the favored 18-electron configuration. The potential energy curve for the stepwise loss of carbonyl groups from Nb-2 to give Nb-0 via Nb-1 is shown in Figure 10. The carbonyl dissociation energy from Nb-2 to Nb-1 is

group from Mo-2 to Mo-1 is 35.9 kcal/mol (MPW1PW91) via a high energy barrier of 49.1 kcal/mol (MPW1PW91). This indicates that Mo-2 is stable both thermodynamically and kinetically as indicated by its synthesis from C7H7Mo(CO)2I and NaC5H5. Carbonyl dissociation from Mo-1 to give Mo-0 is only slightly endothermic at 5.0 kcal/mol (MPW1PW91). However, there is a high barrier of 35.9 (= 71.8 − 35.9) kcal/ mol for such carbonyl dissociation. We have also considered an isomer CpMoH(CO)2C7H6 of CpMo(CO)2C7H7, in which a hydrogen atom in the C7H7 ring has migrated to the molybdenum atom and the remaining C7H6 unit is connected to the molybdenum atom as a carbene ligand containing three uncomplexed CC double bonds (Figure S2). This gives the molybdenum the favored 18electron configuration. However, CpMoH(CO)2C7H6 is a much higher energy structure, lying 33.7 kcal/mol (MPW1PW91) or 27.6 kcal/mol (BP86) above Mo-2 and thus not likely to be of chemical significance. 3.4. CpNb(CO)nC8H8 (n = 2, 1, 0) Structures. The three CpNb(CO)nC8H8 (n = 2, 1, 0) structures all have Cs symmetry (Figure 9). Lowest-energy CpNb(CO)2C8H8 structure Nb-2 has one pentahapto η5-Cp ring and one tetrahapto η4-C8H8 ring bonded to the niobium atom. Structure Nb-2 has two carbonyl ligands with Nb−CO distances of 2.065 Å (MPW1PW91) or 2.062 Å (BP86). Thus, the niobium atom has the favored 18-electron configuration. The uncoordinated butadienoid CC−CC moiety in the η4-C8H8 ring is trans to the CO groups. A cis conformer, Nb-2-cis (Table S4), is found to lie only slightly above Nb-2 in energy by 1.7 kcal/mol (MPW1PW91) or 2.4 kcal/mol (BP86). Lowest-energy CpNb(CO)C8H8 structure Nb-1 (Cs symmetry) has one CO ligand with an Nb−CO distance of 2.028 Å (MPW1PW91) or 2.031 Å (BP86) (Figure 9). Structure Nb-1 has one fully bonded pentahapto η5-Cp ring. However, the cyclooctatetraene ligand in Nb-1 is only a tetrahapto η4C8H8 ring rather than a hexahapto η6-C8H8 ring based on bonding Nb−C(ring) distances less than 2.45 Å. Thus, the niobium atom in Nb-1 has only a 16-electron configuration. The uncoordinated butadienoid CC−CC moiety in the η4-C8H8 ring is trans to the CO group. A cis conformer, Nb-1cis (Table S4), is found to lie above Nb-1 by 5.6 kcal/mol (MPW1PW91) or below Nb-1 by 5.2 kcal/mol (BP86). Lowest-energy carbonyl-free CpNbC8H8 structure Nb-0 (Cs symmetry) has a sandwich structure with one pentahapto η5Cp ring and one octahapto η8-C8H8 ring thereby giving the

Figure 10. Potential energy curve for the dissociation energies (kcal/ mol) and Gibbs free energies (kcal/mol, in parentheses) for loss of a single carbonyl from CpNb(CO)nC8H8 (n = 2, 1).

34.5 kcal/mol (MPW1PW91) without a transition state. Our scanning work shows that the energy of Nb-2 monotonically increases when a CO group gradually leaves to reach finally a value equal to the energy sum of Nb-1 plus CO. Figure 10 shows that there is no energy barrier for the reaction of Nb-1 to combine with CO. This may be a consequence of the bonding of the C8H8 ring to the niobium atom remaining tetrahapto when Nb-2 is converted to Nb-1. However, for Nb1, carbonyl dissociation is endothermic at 18.5 kcal/mol via transition state Nb-TS1. However, the energy of transition state Nb-TS1 is slightly lower than that of the dissociation products. The IRC (intrinsic reaction coordinate) analysis shows that there is a complex CpNbC8H8···CO with a long Nb···C distance of 4.246 Å, which is 1.5 kcal/mol lower in energy than the dissociation products (Nb-0 plus CO). Figure 10 shows that Nb-2 is stable both thermodynamically and kinetically consistent with the synthesis of C 5 H 5 Nb(CO)2C8H8 by photolysis of CpNb(CO)4 with cyclooctatetraene. However, Nb-1 and Nb-0 are of higher energies than the transition states leading to their formation. F


Article

Organometallics

Table 1. Energies and Free Energies (kcal/mol) for Carbonyl Dissociation from C5H5M(CO)nCmHm (M = Ru, Tc, Mo, Nb) Structures after ZPVE Corrections MPW1PW91

BP86

dissociation reaction

ΔEZPVE

ΔG298

ΔEZPVE

ΔG298

CpRu(CO)2C5H5 → CpRu(CO)C5H5 + CO CpRu(CO)C5H5 → Cp2Ru + CO CpTc(CO)2C6H6 → CpTc(CO)C6H6 + CO CpTc(CO)C6H6 → CpTcC6H6 + CO CpMo(CO)2C7H7 → CpMo(CO)C7H7 + CO CpMo(CO)C7H7 → CpMoC7H7 + CO CpNb(CO)2C8H8 → CpNb(CO)C8H8 + CO CpNb(CO)C8H8 → CpNbC8H8 + CO

33.5 −9.4 45.4 −1.5 33.2 3.7 31.8 17.7

24.2 −17.8 37.6 −11.3 24.3 −5.2 21.3 7.6

36.3 0.3 49.8 4.7 39.6 9.9 37.0 26.2

27.8 −8.0 41.7 −4.8 28.9 1.4 28.0 16.2

Table 2. Energies and Free Energies (kcal/mol) for Disproportionation Reactions of 2CpM(CO)CmHm → CpMCmHm + CpM(CO)2CmHm (M = Ru, Tc, Mo, Nb) after ZPVE Corrections MPW1PW91

BP86

disproportionationreaction

ΔEZPVE

ΔG298

ΔEZPVE

ΔG298

2CpRu(CO)Cp → Cp2Ru + CpRu(CO)2C5H5 2CpTc(CO)C6H6 → CpTcC6H6 + CpTc(CO)2C6H6 2CpMo(CO)C7H7 → CpMoC7H7 + CpMo(CO)2C7H7 2CpNb(CO)C8H8 → CpNbC8H8 + CpNb(CO)2C8H8

−42.9 −46.9 −29.5 −14.1

−42.0 −48.9 −29.5 −13.7

−36.0 −45.1 −29.7 −10.8

−35.8 −46.5 −27.5 −11.8

within 5 kcal/mol of being thermoneutral. However, similar to CpRu(CO)C5H5, there is a transition state (Figure 4) with a considerable barrier of 30.4 kcal/mol. This indicates that Tc-1 is thermodynamically disfavored but stable kinetically. The carbonyl dissociation thermochemistry of the molybdenum systems CpMo(CO)nC7H7 (n = 2, 1, 0) is similar to that of the ruthenium and technetium systems discussed above. Thus, the dicarbonyl CpMo(CO)2C7H7 is viable toward CO dissociation. However, CpMo(CO)C7H7 is disfavored thermodynamically but stable kinetically. For the niobium derivatives, both the dicarbonyl CpNb(CO)2C8H8 and the monocarbonyl CpNb(CO)C8H8 systems have significant energies (and free energies) for carbonyl dissociation, suggesting thermodynamic viability and kinetic stability toward CO loss. This may relate to the fact that the carbonyl-free CpNb(η8-C8H8) has apparently never been synthesized (or at least reported in the literature). Another criterion for the viability of the monocarbonyls is the energy for the disproportionation reactions of the following type:

We have also considered a special isomer CpNbH(CO)2C8H7, in which a hydrogen atom in the C8H8 ring has migrated to the niobium atom and the remaining C8H7 radical is bonded to the niobium atom through a ring carbon and an agostic hydrogen from an adjacent CH group (Figure S3). With the pentahapto η5-C5H5 ring and the agostic hydrogen atom, the niobium atom has the 18-electron configuration. However, CpNbH(CO)2C8H7 is a very high energy structure lying 58.6 kcal/mol (MPW1PW91) or 57.4 kcal/mol (BP86) above Nb-2 and thus does not appear to be chemically relevant. 3.5. Thermochemistry. Table 1 reports the dissociation energies with the ZPVE corrections (ΔEZPVE) and the dissociation free energies at 298 K (ΔG298) for loss of a single carbonyl group from the C5H5M(CO)nCmHm (M = Ru, Tc, Mo, Nb; n = 2, 1) structures, namely CpM(CO)n CmH m → CpM(CO)n − 1CmH m + CO

The predicted dissociation energy (ΔE) for loss of the first CO group from the dicarbonyl CpRu(CO)2(η1-C5H5) to give CpRu(CO)C5H5 is ∼35 kcal/mol, and the corresponding free energy is lower by ∼9 kcal/mol (Table 1). These values are similar to the experimental dissociation energies for Ni(CO)4, Fe(CO)5, and Cr(CO)6 of 27, 41, and 37 kcal/mol, respectively.50 Thus, the CpRu(CO)2C5H5 structure is viable toward CO loss. However, dissociation of the second CO group from CpRu(CO)C5H5 to give Cp2Ru appears to be essentially thermoneutral with a negative free energy (ΔG), indicating that the monocarbonyl CpRu(CO)C5H5 species is not thermodynamically viable. However, a transition state (RuTS1 in Figure 2) with a significant energy barrier of 20.8 kcal/ mol makes CpRu(CO)C5H5 kinetically stable. The carbonyl dissociation energy (ΔE) from CpTc(CO)2C6H6 to give CpTc(CO)C6H6 is substantial at ∼45− 50 kcal/mol depending on the method with the corresponding free energy (ΔG) being ∼8 kcal/mol lower (Table 1). Thus, the CpTc(CO)2C6H6 structure is thermodynamically viable toward CO loss. The energy for the further dissociation of the final CO group from CpTc(CO)C6H6 to give CpTcC6H6 is

2CpM(CO)CmH m → CpMCmH m + CpM(CO)2 CmH m (M = Ru, Tc, Mo, Nb)

Such processes for all four monocarbonyls were found to be strongly exothermic with similar values for both ΔE and ΔG ranging from −12 kcal/mol for the niobium derivative C5H5Nb(CO)C8H8 to ∼47 kcal/mol for the technetium derivative CpTc(CO)C6H6 (Table 2). This suggests that none of the monocarbonyls are viable species.

4. CONCLUSION Our thermochemical data quite clearly account for the failure to observe experimentally any of the monocarbonyls CpM(CO)(CnHn) (M = Ru, n = 5; M = Tc, n = 6; M = Mo, n = 7; M = Nb, n = 8). The clearest demonstration is in the observation that all of the disproportionation reactions 2CpM(CO)(CnHn) → CpM(CO)2(CnHn) + CpM(CnHn) are strongly exothermic (Table 2). In addition, CO G


Organometallics

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dissociation processes from the monocarbonyls CpM(CO)(CnHn) to give CpM(CnHn) except for the anomalous CpNb(CO)(η4-C8H8) have negative free energies (ΔG) indicating that the monocarbonyls are not viable species. This may relate to extra stabilization of the corresponding dicarbonyls CpM(CO)2(CnHn) by the conjugation of the two uncomplexed CC double bonds in a butadiene-like unit. The most stable sandwich compounds generally appear to be those containing pentahapto η5-Cp rings, notably the metallocenes (η5-Cp)2M of which the group 8 metallocenes (M = Fe, Ru, Os) are generally observed to be the most stable and least reactive because of their favorable 18-electron configuration. Thus, it is not surprising that the increase in energy of ∼28 kcal/mol in losing two CO groups in going from CpRu(CO)2(η1-Cp) to Cp2Ru + 2CO is the lowest among the four systems studied. Furthermore, the activation energy for CO loss from CpRu(CO)(η3-Cp) to Cp2Ru of ∼21 kcal/mol is significantly lower than that for CO loss from CpTc(CO)(η6-C6H6) or CpMo(CO)(η7-C7H7). The cyclooctatetraene system CpNb(CO)n(C8H8) (n = 2, 1, 0) has some distinctive features relative to the other three systems. Although sandwich compounds containing octahapto η8-C8H8 rings have been synthesized for f-block and group 4 metals, notably the so-called “actinocenes” (η8-C8H8)2An (An = Th, Pa, U, Np, Pu),5 the group 5 metal derivatives CpM(η8C8H8) (M = V, Nb, Ta) remain unknown despite their favorable 18-electron configurations. Thus, the thermal reaction of CpV(CO)4 with cyclooctatetraene failed to give CpV(C8H8) under similar conditions that the reaction of CpV(CO)4 with cycloheptatriene gave the 17-electron sandwich compound CpV(η7-C7H7).27 Furthermore, the photochemical reaction of CpNb(CO)4 with cyclooctatetraene appears to go only as far as the dicarbonyl CpNb(CO)2(C8H8).29 One of the authors in 197651 related the difficulty in preparing sandwich compounds containing a fully bonded η8-C8H8 ring to the area of the planar octagon covered by the four CC double bonds bonding to the transition metal by introducing the concept of the “elplacarnet tree.” The assumption was implied that the extent of the orbitals of a dblock metal was not sufficient for efficient overlap with all of the relevant orbitals of a planar cyclooctatetraene ring. In accord with the difficulty in fully bonding an octahapto η8C8H8 ring to a group 5 metal, the energy increase in going from CpNb(CO)2(η4-C8H8) to CpNb(η8-C8H8) + 2CO of ∼51 kcal/mol is the highest in the four CpM(CO)n(CmHm) systems studied in this work. In addition, the lowest energy structure of the monocarbonyl CpNb(CO)(η4-C8H8) is unusual since the cyclooctatetraene ligand remains tetrahapto in losing a CO group from CpNb(CO)2(C8H8) to CpNb(CO)(C8H8) even though it gives the niobium atom only a 16electron configuration. However, 16-electron configurations of stable complexes of the early transition metals are not unusual as indicated by 16-electron titanium complexes such as CpTi(η7-C7H7), Cp2TiCl2, and Cp2TiR2 (R = alkyl or aryl). Also of interest is the conversion of CpNb(CO)2(η4-C8H8) (Nb-2) to CpNb(CO)(η4-C8H8) (Nb-1) without a transition state (Figure 10). This is possible because conversion of Nb-2 to Nb-1 does not require reorganization of the C8H8 ligand since it is the same tetrahapto ligand in both complexes.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00387. Total energies (E, hartree), relative energies (ΔE, kcal/ mol), and the number of imaginary vibrational frequencies (Nimg) for the C5H5M(CO)nC5H5 (M = Ru, Tc, Mo, Nb; n = 2, 1, 0) isomers; total energies and the imaginary vibration frequencies for the transition states of loss of a CO group for M-2 and M-1 by the MPW1PW91 method; harmonic vibrational frequencies (cm−1) and their infrared intensities (km/mol) for the C5H5M(CO)nC5H5 (M = Ru, Tc, Mo, Nb; n = 2, 1, 0) isomers (PDF) Coordinates of the optimized structures (XYZ)

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AUTHOR INFORMATION

Corresponding Authors

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

Zhihui Zhang: 0000-0002-8744-7897 R. Bruce King: 0000-0001-9177-5220 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, China, and the Priority Academic Program Development of Jiangsu Higher Education Institutions in China.

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REFERENCES

(1) Kealy, T. J.; Pauson, P. L. A new type of organo-iron compound. Nature 1951, 168, 1039−1040. (2) Miller, S. A.; Tebboth, J. A.; Tremaine, J. F. Dicyclopentadienyliron. J. Chem. Soc. 1952, 632−635. (3) Fischer, E. O.; Hafner, W. Di-benzol-chrom − Uber Aromatenkomplexe von Metallen I. Z. Naturforsch., B: J. Chem. Sci. 1955, 10, 665−668. (4) Arliguie, T.; Lance, M.; Nierlich, M.; Vigner, J.; Ephritikhine, M. Synthesis and crystal structure of [K(C12H24O6)][U(η-C7H7)2], the first cycloheptatrienyl sandwich compound. J. Chem. Soc., Chem. Commun. 1995, 183−184. (5) Streitwieser, A., Jr.; Mü ller-Westerhoff, U. Bis(cyclooctatetraenyl)uranium (uranocene). A new class of sandwich complexes that utilize atomic f orbitals. J. Am. Chem. Soc. 1968, 90, 7364−7364. (6) Pyykkö , P. Understanding the eighteen-electron rule. J. Organomet. Chem. 2006, 691, 4336−4340. (7) Landis, C. R.; Weinhold, F. 18-electron Rule and the 3c/4e Hyperbonding Saturation Limit. J. Comput. Chem. 2016, 37, 237− 241. (8) Hallam, B. F.; Pauson, P. L. Dicyclopentadienyliron dicarbonyl. Chem. Ind. 1955, 653−653. (9) Piper, T. S.; Wilkinson, G. Alkyl and aryl derivatives of cyclopentadienyl compounds of chromium, molybdenum, tungsten, and iron. J. Inorg. Nucl. Chem. 1956, 3, 104−124. (10) Wilkinson, G. The preparation and some properties of ruthenocene and ruthenicinium salts. J. Am. Chem. Soc. 1952, 74, 6146. H


Article

Organometallics (11) Hardgrove, G. L.; Templeton, D. H. The crystal structure of ruthenocene. Acta Crystallogr. 1959, 12, 28−32. (12) Cotton, F. A.; Marks, T. J. New evidence for the 1,2-shift pathway in fluxional monohaptocyclopentadienylmetal compounds. J. Am. Chem. Soc. 1969, 91, 7523−7524. (13) Campbell, C. H.; Green, M. L. H. Evidence for a 1,2-shift mechanism for the rearrangement of σ-cyclopentadienyl compounds of iron, ruthenium, and copper. J. Chem. Soc. A 1970, 0, 1318−1323. (14) Manuel, T. A.; Stone, F. G. A. Cyclooctatetraene-iron complexes. Proc. Chem. Soc. London 1959, 90−90. (15) Manuel, T. A.; Stone, F. G. A. Cyclooctatetraene iron tricarbonyl and related compounds. J. Am. Chem. Soc. 1960, 82, 366−372. (16) Rausch, M. D.; Schrauzer, G. N. Cyclooctatetraene-iron tricarbonyl and cyclooctatetraene-diiron hexacarbonyl. Chem. Ind. 1959, 957−958. (17) Nakamura, A.; Hagihara, N. Cycloöctatetraene iron tricarbonyl. Bull. Chem. Soc. Jpn. 1959, 32, 880−881. (18) Cotton, F. A. Bonding of cycloöctatetraene to metal atoms: simple theoretical considerations. J. Chem. Soc. 1960, 400−406. (19) Dickens, B.; Lipscomb, W. N. Molecular structure of C8H8Fe(CO)3. J. Am. Chem. Soc. 1961, 83, 4862−4863. (20) Kreiter, C. G.; Maasbol, A.; Anet, F. A. L.; Kaesz, H. D.; Winstein, S. Valency tautomerism in metal-olefin complexes: cyclooctatetraenemolybdenum, -chromium and − iron tricarbonyls. J. Am. Chem. Soc. 1966, 88, 3444−3445. (21) Cotton, F. A.; Davison, A.; Faller, J. W. Concerning the structure of cyclooctatetraeneiron tricarbonyl in solution. J. Am. Chem. Soc. 1966, 88, 4507−4509. (22) Bratton, W. K.; Cotton, F. A.; Davison, A.; Musco, A.; Faller, J. W. Fluxional behavior of cyclooctatetraeneruthenium tricarbonyl. Proc. Natl. Acad. Sci. U. S. A. 1967, 58, 1324−1328. (23) King, R. B.; Bisnette, M. B. π-Cyclopentadienyl-π-cycloheptatrienyl derivatives of chromium and molybdenum. Tetrahedron Lett. 1963, 4, 1137−1141. (24) King, R. B.; Bisnette, M. B. Some cycloheptatrienyl derivatives of chromium, molybdenum and cobalt. Inorg. Chem. 1964, 3, 785− 790. (25) Fischer, E. O.; Wehner, H. W. Cyclopentadienylmolybdenum(0)-cycloheptatrienyl. J. Organomet. Chem. 1968, 11, P29−P30. (26) Green, M. L. H.; Ng, D. K. P.; Tovey, R. C.; Chernega, A. N. Synthesis and reactions of η-cycloheptatrienyl derivatives of molybdenum. J. Chem. Soc., Dalton Trans. 1993, 3203−3212. (27) King, R. B.; Stone, F. G A. Pi-Cyclopentadienyl-pi-cycloheptatrienyl vanadium,. J. Am. Chem. Soc. 1959, 81, 5263−5264. (28) Wang, H.; Chen, X.; Xie, Y.; King, R. B.; Schaefer, H. F., III The mixed sandwich compounds C5H5MC8H8 of the first row transition metals: Variable hapticity of the eight-membered ring,. Organometallics 2010, 29, 1934−1941. (29) King, R. B.; Hoff, C. D. Some new reactions of cyclopentadienyltetracarbonylniobium. J. Organomet. Chem. 1982, 225, 245−251. (30) Ziegler, T.; Autschbach, J. Theoretical methods of potential use for studies of inorganic reaction mechanisms. Chem. Rev. 2005, 105, 2695−2722. (31) Bü hl, M.; Kabrede, H. Geometries of transition-metal complexes from density-functional theory. J. Chem. Theory Comput. 2006, 2, 1282−1290. (32) Brynda, M.; Gagliardi, L.; Widmark, P. O.; Power, P. P.; Roos, B. O. A quantum chemical study of the quintuple bond between two chromium centers in [PhCrCrPh]: trans-bent versus linear geometry. Angew. Chem., Int. Ed. 2006, 45, 3804−3807. (33) Sieffert, N.; Bühl, M. Hydrogen generation from alcohols catalyzed by ruthenium− triphenylphosphine complexes: multiple reaction pathways. J. Am. Chem. Soc. 2010, 132, 8056−8070. (34) Schyman, P.; Lai, W.; Chen, H.; Wang, Y.; Shaik, S. The directive of the protein: how does cytochrome P450 select the mechanism of dopamine formation? J. Am. Chem. Soc. 2011, 133, 7977−7984.

(35) Adams, R. D.; Pearl, W. C.; Wong, Y. O.; Zhang, Q.; Hall, M. B.; Walensky, J. R. Tetrarhena-heterocycle from the palladiumcatalyzed dimerization of Re2(CO)8(μ-SbPh2)(μ-H) exhibits an unusual host−guest behavior. J. Am. Chem. Soc. 2011, 133, 12994− 12997. (36) Lonsdale, R.; Olah, J.; Mulholland, A. J.; Harvey, J. N. Does compound I vary significantly between isoforms of cytochrome P450? J. Am. Chem. Soc. 2011, 133, 15464−15474. (37) Adamo, C.; Barone, V. Exchange functionals with improved long-range behavior and adiabatic connection methods without adjustable parameters: The mPW and mPW1PW models. J. Chem. Phys. 1998, 108, 664−675. (38) Zhao, S.; Wang, W.; Li, Z.; Liu, Z. P.; Fan, K.; Xie, Y.; Schaefer, H. F. Is the uniform electron gas limit important for small Ag clusters? Assessment of different density functionals for Agn (n ≤ 4). J. Chem. Phys. 2006, 124, 184102. (39) Becke, A. D. Density-functional exchange-energy approximation with correct asymptotic behavior. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098−3100. (40) Perdew, J. P. Density-functional approximation for the correlation energy of the inhomogeneous electron gas. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 33, 8822−8824. (41) Dolg, M.; Stoll, H.; Preuss, H. A combination of quasirelativistic pseudopotential and ligand-field calculations for lanthanoid compounds. Theor. Chim. Acta 1993, 85, 441−445. (42) Bergner, A.; Dolg, M.; Kuechle, W.; Stoll, H.; Preuss, H. Abinitio energy-adjusted pseydopotentials for elements of groups 13−17. Mol. Phys. 1993, 80, 1431−1441. (43) Dunning, T. H. Gaussian basis functions for use in molecular calculations. I. Contraction of (9s5p) atomic basis sets for the firstrow atoms. J. Chem. Phys. 1970, 53, 2823−2833. (44) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision E.01; Gaussian, Inc.: Wallingford, CT, 2009. (45) Papas, B. N.; Schaefer, H. F. Concerning the precision of standard density functional programs: Gaussian, Molpro, NWChem, Q-Chem, and Gamess. J. Mol. Struct.: THEOCHEM 2006, 768, 175− 181. (46) Xu, Z.-F.; Xie, Y.; Feng, W.-L.; Schaefer, H. F. Systematic investigation of electronic and molecular structures for the first transition metal series metallocenes MCp2 (M = V, Cr, Mn, Fe, Co, and Ni). J. Phys. Chem. A 2003, 107, 2716−2729. (47) Ariafard, A.; Tabatabaie, E. S.; Yates, B. F. Mechanistic studies of ligand fluxionality in [M(η5-Cp)(η1-Cp)(L)2]n. J. Phys. Chem. A 2009, 113, 2982−2989. (48) Fischer, E. O.; Wehner, H. W. Cyclopentadienyl-rhenium(I)benzene. a new metallorganic base and its behavior in acetylation according to Friedel-Crafts. Chem. Ber. 1968, 101, 454. (49) Green, M. L. H.; O’Hare, D. Synthesis of η-arene-rhenium compounds using rhenium atoms. Crystal structures of [Re2(ηC6H6)(η-C6H8)2(μ-H)2] and [Re(η-C6H6)(η-C6H8)H]. J. Chem. Soc., Dalton Trans. 1987, 403−410. (50) Sunderlin, L. S.; Wang, D.; Squires, R. R. Bond strengths in 1strow-metal carbonyl anions. J. Am. Chem. Soc. 1993, 115, 12060− 12070. I


Article

Organometallics (51) King, R. B. The elplacarnet tree: A complement to the periodic table for the organometallic chemist. Isr. J. Chem. 1976, 15, 181−188.

J


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