Binuclear Cyclopentadienylmetal Cyclooctatetraene Derivatives of the

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Binuclear Cyclopentadienylmetal Cyclooctatetraene Derivatives of the First Row Transition Metals: Effects of Ring Conformation on the Bonding of an Eight-Membered Carbocyclic Ring to a Pair of Metal Atoms Xiuming Zhai,† Guoliang Li,† Qian-shu Li,*,†,‡ Yaoming Xie,§ R. Bruce King,*,†,§ and Henry F. Schaefer, III§ †

Center for Computational Quantum Chemistry, South China Normal University, Guangzhou 510631, People's Republic of China Institute of Chemical Physics, Beijing Institute of Technology, Beijing 100081, People's Republic of China § Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602, United States ‡

bS Supporting Information ABSTRACT: Binuclear Cp2M2(μ-C8H8) derivatives have been synthesized for M = V, Cr, Co, and Ni and have now been studied theoretically for the entire first row of transition metals from Ti to Ni. The early transition metal derivatives Cp2M2(μ-C8H8) (M = Ti, V, Cr. Mn) are predicted to form lowenergy cis-Cp2M2(μ-C8H8) structures with a folded C8H8 ring (dihedral angle ∼130) and short metalmetal distances suggesting multiple bonding. These predicted structures are close to the experimental structures for M = V, Cr with VtV and CrtCr bond lengths of ∼2.48 and ∼2.36 Å, respectively. The middle to late transition metals form trans-Cp2M2(μ-C8H8) structures (M = Mn, Fe, Co, Ni) with a twisted μ-C8H8 ring and no metalmetal bonding. The hapticity of the central μ-C8H8 ring in such structures ranges from five for Mn and Fe to four for Co and three for Ni and thus depend on the electronic requirements of the central metal atom. This leads to the favored 18-electron configuration for both metal atoms in the singlet Fe, Co, and Ni structures but only 17-electron metal configurations in the triplet Mn structure. In addition, the late transition metals form trans-Cp2M2(μC8H8) structures (M = Fe, Co, Ni), with the tub conformation of the μ-C8H8 ring functioning as a tetrahapto (M = Fe, Co) or trihapto (M = Ni) ligand to each CpM group. A μ-C8H8 ring in the tub conformation also bonds to two CpFe units as a bis(tetrahapto) ligand in both singlet and triplet cisCp2Fe2(μ-C8H8) structures.

1. INTRODUCTION The chemistry of cyclooctatetraene metal complexes originated from the observation of Cope and Hochstein1 in 1950 that cyclooctatetraene forms a crystalline adduct with silver nitrate, which was subsequently characterized structurally by Mathews and Lipscomb.2 Shortly thereafter, three independent research groups36 synthesized the first cyclooctatetraene metal carbonyl derivatives from the reaction of iron carbonyls with cyclooctatetraene (Figure 1). This reaction was found to give three products, namely, the mononuclear η4-C8H8Fe(CO)3 as the major product as well as the two binuclear derivatives trans-(η4,η4C8H8)Fe2(CO)6 and cis-(η5,η5-C8H8)Fe2(CO)5 as minor products. Within a few years of their original discovery, these three products were structurally characterized by X-ray diffraction.79 Further development of the chemistry of cyclooctatetraene metal complexes included the synthesis of compounds with cyclopentadienylmetal fragments rather than metal carbonyl fragments bonded to the cyclooctatetraene ring. Thus, shortly after the discovery of the iron carbonyl derivatives noted above, the analogous reaction of CpCo(CO)2 (Cp = η5-C5H5) with cyclooctatetraene was found to give both the mononuclear CpCo (η2,2-C8H8) and the binuclear trans-Cp2Co2(η2,2,η2,2-C8H8) r 2011 American Chemical Society

(Figure 2).10 Although the CpCo unit is isoelectronic and isolobal with the Fe(CO)3 unit, the bonding modes of the C8H8 ring to the CpCo unit(s) were found to be distinctly different from those to the Fe(CO)3 units. There are two types of stereochemistry for binuclear cyclooctatetraene metal complexes. In trans (or antifacial) complexes the metals are located on opposite sides of the ring so the two metal atoms are too far apart to form a direct metalmetal bond. Such complexes are most favorable for the later transition metals where the cyclooctatetraene ring can provide sufficient electrons for the metal to approach or attain the favored 18-electron configuration without any metalmetal bonding. Early transition metals requiring more electrons to approach the favored 18-electron configuration are expected to favor cis (or synfacial) complexes where both metals are on the same side of the cyclooctatetraene ring. In this case, direct metalmetal bonding is feasible. The earliest examples of binuclear cyclooctatetraene complexes exemplifying both stereochemistries are the original iron carbonyl complexes Received: December 20, 2010 Revised: February 17, 2011 Published: March 22, 2011 3133

dx.doi.org/10.1021/jp112077j | J. Phys. Chem. A 2011, 115, 3133–3143

The Journal of Physical Chemistry A

Figure 1. Three products originally obtained from iron carbonyls and cyclooctatetraene.

Figure 2. Products from the reaction of CpCo(CO)2 with cyclooctatetraene.

Figure 3. Structures of the known cis-Cp2M2(η5,η5-C8H8) derivatives (M = V, Cr).

trans-(η4,η4-C8H8)Fe2(CO)6 and cis-(η5,η5-C8H8)Fe2(CO)5 (Figure 1). For binuclear cyclopentadienylmetal cyclooctatetraene derivatives without carbonyl ligands, the trans stereochemistry is found in the original dicobalt complex trans-Cp2Co2(η2,2,η2,2C8H8) (Figure 2). A related nickel compound (η5-Me5C5)2Ni2(C8H8) was subsequently synthesized but has not been characterized structurally.11 More recently, the cis-Cp2M2(η5,η5-C8H8) derivatives (M = V,1214 Cr1518) have been synthesized (Figure 3). These latter compounds have short metalmetal distances, suggesting metalmetal multiple bonding, which has been discussed in detail in the literature.1923 This paper describes a systematic investigation of Cp2M2(C8H8) derivatives using density functional theory (DFT) methods. A major objective of this work is the understanding of the relative stabilities of cis and trans structures as well as changes in the hapticity of the metal-ring bonding and the apparent formal metalmetal bond order as the number of metal electrons is changed in the first row transition metals in going from titanium to nickel.

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2. THEORETICAL METHODS Density functional theory (DFT) methods, which approximate electron correlation effects, have evolved as a practical and effective computational tool, especially for organometallic compounds.2438 In this paper, two DFT methods (B3LYP and BP86) were used. The B3LYP method is an HF/DFT hybrid method using Becke’s three-parameter exchange functional (B3)39 and the LeeYangParr generalized gradient correlation functional (LYP).40 The BP86 method is a pure DFT method combining Becke’s 1988 exchange functional (B)41 with Perdew’s 1986 gradient correlation functional (P86).42 For carbon the double-ζ plus polarization (DZP) basis set used here adds one set of pure spherical harmonic d functions with orbital exponent Rd(C) = 0.75 to the standard Huzinaga Dunning contracted DZ sets, and is designated (9s5p1d/ 4s2p1d).43,44 For H, a set of p polarization functions Rp(H) = 0.75 is added to the HuzinagaDunning DZ set. For the first row transition metals, in our loosely contracted DZP basis set, the Wachters’ primitive sets are used, but augmented by two sets of p functions and one set of d functions, and contracted following Hood et al. and designated (14s11p6d/10s8p3d).45,46 In the present paper, the geometries of all structures were fully optimized using both the B3LYP and BP86 methods. Vibrational frequencies were determined by analytically evaluating the second derivatives of the energy with respect to the nuclear coordinates. The geometries of all of the computations were carried out with the Gaussian 03 program,47 exercising the fine grid option (75 radial shells, 302 angular points) for evaluating integrals numerically,48 while the tight (108 hartree) designation is the default for the self-consistent field (SCF) convergence. In the search for minima, low magnitude imaginary vibrational frequencies are suspect, because the numerical integration procedures used in existing DFT methods have significant limitations.48 Thus, an imaginary vibrational frequency of magnitude less than 50i cm1 should be considered questionable. In some cases, a finer (120, 974) integration grid was used for the optimization to investigate small imaginary vibrational frequencies. All of the final optimized structures reported in this paper have only real vibrational frequencies, unless otherwise indicated. The wave function stability was routinely tested for all of the structures obtained. All reported structures have stable wave functions; otherwise, the optimization was continued until a stable wave function was found. The structures in this paper are designated as M-aX, where M is the name of the transition metal, the symbol a is the order of relative energy, and X stands for the spin state (singlets and triplets are designated as S and T, respectively). Thus, the global minimum of singlet Cp2Ti2(μ-C8H8) is labeled as Ti-1S. 3. RESULTS 3.1. Titanium Complexes Cp2Ti2(μ-C8H8). Triplet and singlet structures, namely, Ti-1T and Ti-2S, respectively, are predicted for Cp2Ti2(μ-C8H8) with almost identical geometries (Figure 4 and Table 1). The C8H8 rings in these structures are bent with two planar sets of five carbon atoms sharing two carbon atoms at the “hinge” of the bend. The dihedral angles between these two sets of carbon atoms are 129 (Table 1). The TiTi bond distances are ∼2.6 Å, suggesting high order multiple bonds. No relevant examples of compounds with titaniumtitanium bonds are known experimentally for comparison. However, in a previous 3134

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theoretical study,49 the presumed formal TitTi triple bond in Cp2Ti2(CO)6 and the formal Ti:Ti quadruple bond are predicted to have lengths of ∼2.8 and ∼2.6 Å, respectively. This would suggest a formal quadruple bond in Ti-1T, thereby giving each titanium atom the 17-electron configuration for a binuclear triplet. 3.2. Vanadium Complexes Cp2V2(μ-C8H8). The vanadium complex Cp2V2(μ-C8H8) has been characterized experimentally1214 and studied theoretically at the SCF1921 and CASSCF22,23 levels. A singlet Cp2V2(μ-C8H8) structure is observed experimentally with a suggested singlettriplet energy separation (ΔH) based on NMR spectroscopy.14 Our DFT methods predict the triplet structure V-1T to be the global minimum with the singlet V-2S lying above V-1T by 16.4 (B3LYP) or 1.7 kcal/mol (BP86) (Figure 5 and Table 2). Usually the B3LYP method predicts a lower relative energy for the lower triplet state than the BP86 method.50 The true value for the triplet-singlet splitting should lie between that predicted by each method. Thus, our theoretical results essentially agree with the experimental results. The previous theoretical study predicted a C2v minimum. However, our C2v structure has a small imaginary frequency (∼60i cm1). Following the corresponding normal mode leads to the minimum with C2 symmetry. Both Cp2V2(μ-C8H8) structures V-1T and V-2S are predicted to have bent μ-C8H8 rings with dihedral angles of ∼130 similar to the Cp2Ti2(μ-C8H8) structures discussed above (Figure 5 and Table 2). The VtV distance in V-1T is predicted to be 2.473 Å (B3LYP) or 2.499 Å (BP86), indicating a formal triple bond to give each V atom a 17-electron configuration as expected for a binuclear triplet. This predicted VtV distance is close to the experimental VtV distance of 2.439 Å, as determined by X-ray crystallography.12 The VtV triple bond distance in V-1T is also close to the experimental VtV triple bond distance of 2.459 Å in Cp2V2(CO)5.51,52 The V 3 V quad bond distance in V-2S is predicted to be significantly shorter than that in V-1T, namely, 2.255 (B3LYP) or 2.333 Å (BP86). This suggests the formal quadruple bond to give each vanadium atom in V-2S the favored 18-electron configuration. 3.3. Chromium Complexes Cp2Cr2(μ-C8H8). The optimized Cp2Cr2(μ-C8H8) structures (Figure 6 and Table 3) are analogous to those of the titanium and vanadium complexes discussed

above. The singlet Cp2Cr2(μ-C8H8) structure Cr-1S is known experimentally.1518 In the present theoretical study, the singlet structure Cr-1S and the triplet structure Cr-2T are predicted to be nearly degenerate in energy. The BP86 method predicts Cr-1S to be lower by 4.8 kcal/mol, whereas the B3LYP method predicts Cr-2T to be lower by 8.1 kcal/mol. The tendency of the B3LYP method to favor triplet spin state structures relative to the BP86 method has been established previously.50 In Table 3, our theoretical CrtCr distance of 2.362 (B3LYP) or 2.356 Å (BP86) for the singlet Cr-1S is close to the experimental value of 2.390 Å, determined by X-ray crystallography, and can correspond to the formal triple bond required to give both chromium atoms in Cr-1S the favored 18-electron configuration. It should be noted that the formal CrtCr triple bonds in the related (η5-R5C5)2 Cr2(CO)4 (R = H,53 Me54,55) are found by X-ray crystallography to be significantly shorter at 2.24 Å. This difference could be a consequence of the geometry of the metal bonding to the μ-C8H8 ring making it unfavorable for the two chromium atoms to come closer together than ∼2.3 Å. The CrdCr distance in the triplet Cr-2T is significantly longer at 2.583 (B3LYP) or 2.506 Å (BP86) and thus can correspond to the formal double bond required to give both chromium atoms 17-electron configurations for a binuclear triplet. 3.4. Manganese Complexes Cp2Mn2(μ-C8H8). Three low energy structures for Cp2Mn2(μ-C8H8) were found (Figure 7 and Table 4). Two of these structures have cis conformations, whereas the third structure is a trans structure. In the singlet cisCp2Mn2(μ-C8H8) structure, Mn-1S the MndMn distance of 2.705 (B3LYP) or 2.696 Å (BP86) is significantly longer than the metalmetal distances in the earlier transition metal derivatives Cp2M2(μ-C8H8) (M = Ti, V, Cr). In addition, the MndMn distance in Mn-1S is significantly shorter than the experimental MnMn single bond distance of 2.9256 or 2.895 Å57 in Mn2(CO)10 determined by X-ray crystallography. This suggests a formal double bond, thereby giving both manganese atoms in Mn-1S the favored 18-electron configuration. The triplet Cp2Mn2(μ-C8H8) structure Mn-2T (Figure 7 and Table 4) lies in energy 1.4 kcal/mol (B3LYP) above Mn-1S or 1.7 kcal/mol (BP86) below Mn-1S, so that these two structures are essentially degenerate in energy. The MndMn distance in

Figure 4. Optimized structures of Cp2Ti2(μ-C8H8).

Figure 5. Optimized structures of Cp2V2(μ-C8H8).

Table 1. Bond Distances (in Å), Total Energies (E in Hartree), Relative Energies (ΔE and ΔEZPE in kcal/mol)a, Spin Expectation Values (ÆS2æ), Electronic States, and C8H8 Ring Dihedral Angles (D, in degrees) for the (CpTi)2(μ-C8H8) Structures Ti1Ti2 Ti-1T Ti-2S a

Ti1C1

Ti1C2

Ti1C3

Ti1C4

Ti1C5

E

ΔE

ΔEZPE

ÆS2æ

state

D ()

B3LYP

2.588

2.381

2.215

2.294

2.215

2.319

2395.82839

0.0

0.0

2.02

3

A

129.3

BP86

2.563

2.343

2.211

2.293

2.211

2.343

2396.07304

0.0

0.0

2.01

3

A1

128.9

B3LYP BP86

2.600 2.646

2.323 2.313

2.195 2.183

2.270 2.255

2.192 2.179

2.322 2.311

2395.82755 2396.07079

0.5 1.4

0.1 0.7

0.00 0.00

1

A A

129.3 128.1

1

ΔEZPE are relative energies after zero-point energy (ZPE) corrections. Because the ZPE corrections are very small, we use the ΔE values in discussion. 3135



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Mn-2T of 2.699 (B3LYP) or 2.679 Å (BP86) is essentially the same as that in Mn-1S, so that it could conceivably be considered as a formal double bond giving both manganese atoms the favored 18-electron configuration. The different spin multiplicities in Mn1S and Mn-2T can arise from the different natures of the formal double bonds in the two structures. In Mn-1S the formal double bond can be considered to be a σ þ π bond with the usual σ bond supplemented by a two-electron π-bond using one of the orthogonal pair of π-orbitals of the MndMn bond. The other π component of the manganesemanganese bond in the Mn-1S structure is empty and thus does not participate in the metal metal bonding. However, in Mn-2T, the formal double bond can be considered as a σ þ 2/2π double bond with a single electron “half-bond” in each of the π components of the MndMn bond, leading to the triplet spin state in a structure where both metal atoms have the favored 18-electron configuration. Such a σ þ 2/2π double bond is similar to that in dioxygen, although the two unpaired electrons in Mn-2T are in bonding orbitals. The third Cp2Mn2(μ-C8H8) structure Mn-3T is a trans structure (Figure 7 and Table 4). In fact, manganese is the earliest transition metal for which a trans Cp2M2(μ-C8H8) structure is observed at competitive energies. The key feature of Mn-3T is that the metal atoms are slipped to the side of the C8H8 ring, which twists to two nearly planar C4 fragments. The trans conformation of Mn-3T and the twist conformation of the C8H8 ring forces the manganese atoms beyond bonding distance, that is, the Mn 3 3 3 Mn distance is 3.655 (B3LYP) or 3.598 Å (BP86). However, each manganese atom is within bonding distance of five of the eight carbon atoms of the C8H8 ring, so that two of the carbon atoms are shared by both manganese atoms. The twist conformation of the C8H8 ring allows three-center two-electron (3c-2e) bonding of two of the ring carbon atoms to both manganese atoms so that the C8H8 ring (considered as a neutral ligand) can function as an effective five-electron donor to each metal atom. Thus, the manganese atoms each have the 17-electron configuration for a binuclear triplet. 3.5. Iron Complexes Cp2Fe2(μ-C8H8). Five structures for Cp2Fe2(μ-C8H8) were obtained. The global minimum Fe-1S has cis stereochemistry with the cyclooctatetraene ring in the tub form (Figure 8 and Table 5). In Fe-1S, four adjacent carbons of the C8H8 ring are bonded to one iron atom and the remaining four carbons are bonded to the other iron atom, so that no C8H8 carbons are shared by both iron atoms. This structure is isoelectronic with the known58 cation [CpRh(μ-C8H8)Rh(η4-C7H8)]þ and the known59 dication [(η5-Me5C5)2Co2(μ-C8H8)]2þ. The FeFe distance of ∼3.0 Å in Fe-1S can be interpreted as a formal single bond, which is lengthened because of the configuration of the C8H8 ring. The Cp2Fe2(μ-C8H8) structure Fe-1S is related to the structure of the known compound9 (η5,η5-C8H8)Fe2(CO)4

(μ-CO), in which, however, five rather than four of the C8H8 carbons are within bonding distance of each iron atom. The FeFe single bond of 2.742 Å in (η5,η5-C8H8)Fe2(CO)4(μ-CO) is significantly shorter than that in Fe-1S, probably because of differences in the conformation of the C8H8 ring. A second singlet Cp2Fe2(μ-C8H8) structure Fe-2S has a different conformation of the C8H8 ring with “twist” rather than “tub” geometry (Figure 8 and Table 5). Structure Fe-2S lies 4.6 kcal/mol (B3LYP) or 3.8 kcal/mol (BP86) above Fe-1S. As in the trans-Cp2Mn2(μ-C8H8) structure Mn-3T, five carbon atoms of the twisted C8H8 ring are within bonding distance of each iron atom in Fe-2S, so that the C8H8 ring functions as a fiveelectron donor to each iron atom. The trans stereochemistry of Fe-2S places the iron atoms 3.648 (B3LYP) or 3.593 Å (BP86) from each other so there is clearly no direct FeFe bond. However, five electrons from the Cp ligand and five electrons from the C8H8 ring gives each iron atom the favored 18-electron configuration in Fe-2S. The triplet Cp2Fe2(μ-C8H8) structure Fe-3T, geometrically similar to Fe-1S, lies 2.0 (B3LYP) or 17.1 kcal/mol (BP86) above the global minimum Fe-1S. However, the long Fe 3 3 3 Fe distance of 3.487 (B3LYP) or 3.453 Å (BP86) in Fe-3T indicates no direct bonding between the two Fe atoms, thereby giving each Fe atom the 17 electron configuration expected for a binuclear triplet. The “bent” triplet60 Cp2Fe2(μ-C8H8) structure Fe-4T lies 7.5 (B3LYP) or 23.0 kcal/mol (BP86) above the global minimum Fe-1S. The structure Fe-4T has an unsymmetrical geometry with the chair η5,η3-C8H8 ring in which three carbon atoms are bonded to one iron atom and the remaining five carbon atoms are bonded to the other iron atom. The iron atoms in Fe-4T are too far apart for direct bonding. Therefore, the iron atom bonded to five carbon atoms of the C8H8 ligand has the favored 18electron configuration but the iron atom bonded to only three carbon atoms of the C8H8 ligand has a 16-electron configuration. This latter iron atom accounts for the triplet spin state of Fe-4T. The final Cp2Fe2(μ-C8H8) structure Fe-5T lies 9.1 (B3LYP) or 23.7 kcal/mol (BP86) above the global minimum Fe-1S. The C8H8 ring in Fe-5T is bonded to each iron atom as an η2,2 ligand using the carbon atoms of two nonconjugated bonds. The Fe 3 3 3 Fe distance at 3.859 (B3LYP) or 3.829 Å (BP86) is clearly

Figure 6. Optimized structures of Cp2Cr2(μ-C8H8).

Table 2. Bond Distances (in Å), Total Energies (E in Hartree), Relative Energies (ΔE and ΔEZPE in kcal/mol)a, Spin Expectation Values (ÆS2æ), Electronic States, and C8H8 Ring Dihedral Angles (D, in degrees) for the Cp2V2(μ-C8H8) Structures V1V2 V-1T

a

V1C2

V1C3

V1C4

V1C5

E

ΔE

ΔEZPE

ÆS2æ

state

D ()

B3LYP

2.473

2.273

2.150

2.227

2.149

2.275

2584.89589

0.0

0.0

2.03

3

BP86

2.499

2.268

2.139

2.212

2.138

2.272

2585.19789

0.0

0.0

2.01

3

130.5

2.439

2.246

2.128

2.199

2.128

2.246

2.255 2.333

2.265 2.247

2.142 2.133

2.247 2.235

2.145 2.134

2.262 2.244

2584.86983 2585.19525

16.4 1.7

16.9 2.2

0.00 0.00

1

131.3 129.1

expt12 V-2S

V1C1

B3LYP BP86

B B A A

1

130.7

ΔEZPE are relative energies after zero-point energy (ZPE) corrections. 3136



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Table 3. Bond Distances (in Å), Total Energies (E in Hartree), Relative Energies (ΔE and ΔEZPE in kcal/mol)a, Spin Expectation Values (ÆS2æ), Electronic States, and Dihedral Angles (D, in Degrees) for the Cp2Cr2(μ-C8H8) Structures Cr1Cr2 Cr-1S

a

Cr1C2

Cr1C3

Cr1C4

Cr1C5

E

ΔE

ΔEZPE

ÆS2æ

state

D ()

B3LYP

2.362

2.225

2.084

2.170

2.084

2.224

2785.77267

0.0

0.0

0.00

1

BP86

2.356

2.242

2.076

2.147

2.076

2.243

2786.14684

0.0

0.0

0.00

1

133.9

2.390

2.261

2.061

2.101

2.061

2.261

B3LYP

2.583

2.257

2.102

2.155

2.100

2.250

2785.78553

8.1

9.4

2.12

3

130.5

BP86

2.506

2.247

2.087

2.171

2.087

2.253

2786.13923

4.8

4.0

2.04

3

130.1

Expt17 Cr-2T

Cr1C1

A A A A

132.2

ΔEZPE are relative energies after zero-point energy (ZPE) corrections.

Figure 7. Optimized structures of Cp2Mn2(μ-C8H8).

a nonbonding distance. Therefore each Fe atom in Fe-5T has the 17-electron configuration for a binuclear triplet. 3.6. Cobalt Complexes Cp2Co2(μ-C8H8). Four low energy structures were found for Cp2Co2(μ-C8H8) (Figure 9 and Table 6). Because a cobalt atom needs only nine electrons from the external ligands, a Cp2Co2(μ-C8H8) structure in which each cobalt atom receives four electrons from the μ-C8H8 ring does not require a cobaltcobalt bond for each metal to achieve the favored 18-electron configuration. Therefore, Cp2Co2(μ-C8H8) structures with trans stereochemistry and the two cobalt atoms too far apart for direct bonding should be favorable. Thus, it is not surprising that the singlet trans-Cp2Co2(μ-C8H8) structure Co-1S is predicted to be the global minimum in agreement with experimental results.61,62 Structure Co-1S has been categorized as a “pseudo-triple-decker”58 or a “near miss” triple decker.63 The μ-C8H8 ring is bonded to each cobalt atom as an η2,2 ligand using the four carbon atoms from two nonadjacent double bonds. Thus, the cobalt-ring bonding in the singlet structure Co-1S is very similar to that in the triplet trans-Cp2Fe2(μ-C8H8) structure Fe-5T (Figure 8 and Table 5). The cobalt atoms in Co-1S have the favored 18-electron configuration despite the fact that they are too far apart for any direct cobaltcobalt bonding. The cis-Cp2Co2(μ-C8H8) structure Co-2S lying 1.6 (B3LYP) or 4.2 kcal/mol (BP86) above the global minimum Co-1S is the only Cp2Co2(μ-C8H8) structure with cis stereochemistry found in this work (Figure 9 and Table 6). As in Co-1S, both cobalt atoms in Co-2S are bonded to distinct sets of four carbon atoms in the μ-C8H8 ring. However, unlike Co-1S the sets of four C8H8 carbons bonded to a given cobalt atom are adjacent carbon atoms corresponding to two adjacent CdC double bonds of the ring. Despite the cis stereochemistry of Co-2S, the Co 3 3 3 Co distance of 3.761 (B3LYP) or 3.836 Å (BP86) is far too long for a direct cobaltcobalt bond. However, as in Co-1S, both cobalt atoms in Co-2S have the favored 18-electron configuration, even without a cobaltcobalt bond. A third singlet Cp2Co2(μ-C8H8) structure Co-3S was found in this work, at 15.5 (B3LYP) or 15.1 kcal/mol (BP86) above the

global minimum Co-1S (Figure 9 and Table 6). Like Co-1S structure Co-3S has trans stereochemistry and disjoint sets of four carbon atoms of the μ-C8H8 ring bonded to each cobalt atom. However, the four carbon atoms of each disjoint set consist of three adjacent carbons as a η3-allylic unit plus a fourth “isolated” carbon atom. Each cobalt atom in Co-3S, like those in Co-1S and Co-2S, has the favored 18-electron configuration. A triplet Cp2Co2(μ-C8H8) structure Co-4T was found at 6.5 (B3LYP) or 25.7 kcal/mol (BP86) above the Co-1S global minimum. The B3LYP method predicts two very small imaginary vibrational frequencies (27i and 7i cm1), which were removed by a finer integration grid (120, 974), indicating that they arise from numerical integration error. 3.7. Nickel Complexes Cp2Ni2(μ-C8H8). The cyclooctatetraene ring in Cp2M2(μ-C8H8) derivatives of the electron-rich late transition metals, namely, Fe, Co, and Ni, separates into two diene units.58 These units are symmetrically bonded to the metal atoms as tetrahapto ligands in the Fe and Co derivatives but as trihapto ligands in the Ni derivatives. The local environment of the nickel atoms in the lowest energy Cp2Ni2(μ-C8H8) structures is very similar to that in the known64,65 18-electron trihaptoallyl derivative CpNi(η3-C3H5). Six energetically low-lying structures are found for Cp2Ni2(μC8H8) (Figure 10 and Table 7). The global minimum Ni-1S has cis stereochemistry with each nickel atom bonding to three adjacent carbon atoms of the μ-C8H8 ring leaving one uncomplexed CdC double bond in the ring. As expected, the predicted CdC distance for this double bond of 1.366 (B3LYP) or 1.386 Å (BP86) is shorter than the other ring carboncarbon distances, in accord with expectation. Each nickel atom in Ni-1S attains the favorable 18-electron configuration by receiving three electrons from the μ-C8H8 ring and five electrons from the Cp ring. The trans-Cp2Ni2(μ-C8H8) structure Ni-2S is predicted to lie 5.1 (B3LYP) or 2.7 kcal/mol (BP86) above the global minimum Ni-1S (Figure 10 and Table 7). Like Ni-1S, each Ni atom in Ni-2S is connected to three carbon atoms in the cyclooctatetraene ring to satisfy the 18-electron rule. Again the lone noncoordinated double bond in the μ-C8H8 is substantially shorter than the other ring carboncarbon bonds at 1.37 Å. However, unlike Ni-1S, the three carbons of the μ-C8H8 ring bonded to a given nickel atom are not three adjacent carbon atoms like an η3-allylic unit but instead correspond to two adjacent carbon atoms of a CdC double bond and a third isolated carbon atom. The lowest energy triplet Cp2Ni2(μ-C8H8) structure Ni-3T lies 5.1 kcal/mol below Ni-1S by B3LYP but 5.2 kcal/mol above Ni-1S by BP86 (Figure 10 and Table 7). This is consistent with the observations of Reiher, Solomon, and Hess,50 that the B3LYP method predicts lower energies for triplet spin state structures than the BP86 method relative to corresponding singlet state 3137



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Table 4. Bond Distances (in Å), Total Energies (E in Hartree), Relative Energies (ΔE and ΔEZPE in kcal/mol)a, Spin Expectation Values (ÆS2æ), Electronic States, and Dihedral Angles (D in degrees) for the Cp2Mn2(μ-C8H8) Structures Mn1Mn2

Mn1C2

Mn1C3

Mn1C4

Mn1C5

E

ΔE

ΔEZPE

ÆS2æ

state

Mn-1S

B3LYP

2.705

2.264

2.044

2.107

2.043

2.276

2998.82228

0.0

0.0

0.00

1

2.696 2.699

2.267 2.259

2.039 2.060

2.096 2.112

2.037 2.061

2.282 2.270

2999.21716 2998.82011

0.0 1.4

0.0 0.4

0.00 2.18

1

Mn-2T

BP86 B3LYP BP86

2.679

2.264

2.046

2.095

2.047

2.302

2999.21979

1.7

2.1

B3LYP

3.655

2.137

2.083

2.098

2.216

2.135

2998.81593

4.0

BP86

3.598

2.102

2.068

2.074

2.200

2.079

2999.19868

11.6

Mn-3T a

Mn1C1

D ()

A

130.3

A 3 A

131.7 132.7

2.06

3

B

134.0

2.8

2.05

3

B

10.5

2.03

3

B


Figure 8. Optimized structures of Cp2Fe2(μ-C8H8).

Table 5. Bond Distances (in Å), Total Energies (E in Hartree), Relative Energies (ΔE and ΔEZPE in kcal/mol)a, Spin Expectation Values (ÆS2æ), and Electronic States for the Cp2Fe2(μ-C8H8) Structures Fe1Fe2

Fe1C1

Fe1C2

Fe1C3

Fe1C4

E

ΔE

ΔEZPE

ÆS2æ

B3LYP

2.990

2.168

2.047

2.050

2.123

3224.27533

0.0

0.0

0.000

1

BP86

2.981

2.119

2.026

2.035

2.085

3224.71930

0.0

0.0

0.000

1

Fe-2S

B3LYP

3.648

2.121

2.039

2.031

2.148

3224.26794

4.6

4.0

0.000

1

3.593 3.487

2.099 2.081

2.027 2.029

2.015 2.030

2.135 2.076

3224.71319 3224.27214

3.8 2.0

3.3 0.5

0.000 2.085

1

Fe-3T

BP86 B3LYP BP86

3.453

2.076

2.023

2.020

2.069

3224.69208

17.1

15.7

2.030

3

b

Fe-1S

Fe-4T Fe-5T

state A A A A B

3

B

3 0

B3LYP

4.212

2.175

1.993

2.175

3224.26332

7.5

7.3

2.141

BP86

3.982

2.065

2.006

2.065

3224.68270

23.0

21.5

2.037

A

B3LYP

3.859

2.062

2.061

2.063

2.064

3224.26077

9.1

6.3

2.075

3

BP86

3.829

2.066

2.035

2.036

2.072

3224.68152

23.7

21.2

2.032

3

3 00

A B B

ΔEZPE are relative energies after zero-point energy (ZPE) corrections. b Fe-4T has Cs geometry and a small imaginary vibrational frequency (14i cm1) predicted by B3LYP. a

structures. We thus conclude that the Cp2Ni2(μ-C8H8) structures Ni-3T and Ni-1S are essentially degenerate. The triplet Cp2Ni2(μ-C8H8) structure Ni-3T is very similar to the singlet Cp2Co2(μ-C8H8) structure Co-1S (Figure 9 and Table 6) and

the triplet Cp2Fe2(μ-C8H8) structure Fe-5T (Figure 8 and Table 5), in having trans stereochemistry and η2,2 coordination of the μ-C8H8 ring to each metal atom. For Ni-3T this gives each nickel atom a 19-electron configuration corresponding to a 3138



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binuclear triplet. Such 19-electron metal configurations are rather rare in stable transition metal organometallic compounds but are found in cobaltocene66 (Cp2Co) and CpFe(η6-Me6C6).67 The singlet Cp2Ni2(μ-C8H8) structure Ni-4S lies 8.2 (B3LYP) or 6.5 kcal/mol (BP86) above the lowest energy singlet structure Ni-1S (Figure 10 and Table 7). Structure Ni-4S has a geometry similar to Ni-2S, except for the locations of the three carbon atoms in the μ-C8H8 ring that are bonded to each nickel atom. There is also one uncomplexed CdC double bond (∼1.38 Å) in the eightmembered ring between C7 and C8 in Ni-4S (Figure 10). A triplet Cp2Ni2(μ-C8H8) structure Ni-5T with cis stereochemistry is also found at 0.1 (B3LYP) or 13.2 kcal/mol (BP86) above the Ni-1S structure (Figure 10 and Table 7). Each nickel atom is bonded to only two carbons of the μ-C8H8 ring. This leaves two uncomplexed nonadjacent CdC double bonds in the μ-C8H8 ring. Even though Ni-5T has cis stereochemistry, its Ni 3 3 3 Ni distance is predicted to be relatively long at 4.640 (B3LYP) or 3.819 Å (BP86), indicating the absence of a direct nickelnickel interaction. An analogous trans-Cp2Ni2(μ-C8H8) structure Ni-6T is found at 0.8 (B3LYP) or 15.1 kcal/mol

(BP86) above Ni-1S. However, in Ni-6T the four carbon atoms bonded to the nickel atom correspond to adjacent double bonds so that the uncomplexed double bonds are conjugated. In both Ni-5T and Ni-6T, each nickel atom receives five electrons from the Cp ring and two electrons from two carbons of the μ-C8H8 ring, thereby giving them the 17-electron configurations anticipated for binuclear triplets.

4. DISCUSSION The preferred structures for the Cp2M2(μ-C8H8) derivatives depend on the position of the transition metal (M) in the Periodic Table. This relates to the number of electrons required to approach the favored 18-electron configuration after allowing for a maximum of five effective electrons from the μ-C8H8 ring to each metal atom in the binuclear complex. 4.1. Early Transition Metal Derivatives (M = Ti, V, Cr). The early transition metals (Ti, V, Cr) require metalmetal bonding in addition to the electrons from the Cp and μ-C8H8 rings to approach the favored 18-electron configuration in their Cp2M2(μ-C8H8) complexes. For this reason, the cis (synfacial) stereochemistry is required to make such metalmetal bonding feasible. The μ-C8H8 ring in such complexes have a “diptych” conformation consisting of two sets of five coplanar carbon atoms with a fold dihedral angle of ∼130. The two carbon atoms common to both sets of five coplanar carbon atoms may be regarded as “hinges.” Each metal atom is within bonding distance of all five carbon atoms of one of the coplanar sets. A μ-C8H8 ring bonded to a pair of metal atoms in this manner effectively donates four electrons to each of the metal atoms. The chromium and vanadium Cp2M2(μ-C8H8) derivatives (M = V,1214 Cr1518) have been synthesized and structurally characterized by X-ray crystallography. Both theory and experiment suggest that singlet and triplet states have similar energies. A singlet Cp2Cr2(μ-C8H8) structure requires a formal CrtCr triple bond for each chromium atom to attain the favored 18electron configuration. The predicted CrtCr distance of ∼2.36 Å for singlet Cp2Cr2(μ-C8H8) (Cr-1S in Figure 6) is close to the experimental value of 2.39 Å for Cp2Cr2(μ-C8H8) determined by X-ray crystallography.12 However, it is somewhat longer than the experimental CrtCr distance68 of 2.24 Å for the formal triple bond in Cp2Cr2(CO)4. For triplet Cp2Cr2(μ-C8H8), the significantly longer predicted CrdCr distance of ∼2.54 Å is consistent with the formal double bond required to give the chromium atom the 17-electron configurations for a binuclear triplet.

Figure 9. Optimized structures of Cp2Co2(μ-C8H8).

Table 6. Bond Distances (in Å), Total Energies (E in Hartree), Relative Energies (ΔE and ΔEZPE in kcal/mol)a, Spin Expectation Values (ÆS2æ), and Electronic States for the (CpCo)2(μ-COT) Structures Co1Co2 Co-1S Co-2S Co-3S Co-4T

Co1C1

Co1C2

Co1C3

Co1C4

E

ΔE

ΔEZPE

ÆS2æ

state

B3LYP

3.812

2.042

2.043

2.043

2.042

3462.40627

0.0

0.0

0.00

1

BP86

3.801

2.031

2.031

2.031

2.031

3462.88248

0.0

0.0

0.00

1

B3LYP

3.761

2.050

1.987

1.986

2.051

3462.40366

1.6

2.3

0.00

1

BP86

3.836

2.060

1.979

1.979

2.051

3462.87576

4.2b

4.7

0.00

1

B3LYP

3.840

2.054

2.019

2.154

2.593

3462.38151

15.5

15.7

0.00

1

BP86

3.854

2.064

1.995

2.121

2.572

3462.85847

15.1

15.1

0.00

1

B3LYP

3.914

2.183

2.245

3462.39596

6.5

4.4

2.05

BP86

3.837

2.110

2.215

3462.84160

25.7

23.6

2.03

A A A A A A

3 00

A

3 00

A

ΔEZPE are relative energies after zero-point energy (ZPE) corrections. b Co-2S has a C2 geometry and a small imaginary vibrational frequency (22i cm1) predicted by BP86. a

3139



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Figure 10. Optimized structures of Cp2Ni2(μ-C8H8).

Table 7. Bond Distances (in Å), Total Energies (E in Hartree), Relative Energies (ΔE and ΔEZPE in kcal/mol)a, Spin Expectation Values (ÆS2æ), and Electronic States for the Cp2Ni2(μ-C8H8) Structures Ni1Ni2

Ni1C1

Ni1C2

Ni1C3

Ni1C4

E

B3LYP

3.856

2.044

1.944

2.061

2.855

3713.50145

BP86

3.901

2.055

1.932

2.065

2.844

Ni-2S

B3LYP

3.936

2.027

2.041

1.986

Ni-3T

BP86 B3LYP

3.946 3.960

2.013 2.174

2.011 2.173

BP86

3.869

2.119

B3LYP

4.003

BP86

Ni-1S

Ni-4S Ni-5T Ni-6T a

ΔE

ΔEZPE

ÆS2æ

0.0

0.0

0.00

1

A

3713.97807

0.0

0.0

0.00

1

A

2.810

3713.49336

5.1

4.3

0.00

1

A

2.019 2.171

2.836 2.172

3713.97381 3713.50950

2.7 5.1

2.0 7.3

0.00 2.04

1

A B

2.122

2.122

2.119

3713.96979

5.2

3.2

2.02

3

B

2.139

2.050

2.477

1.975

3713.48837

8.2

7.7

0.00

1

A

4.016

2.099

2.027

2.437

2.016

3713.96777

6.5

6.0

0.00

1

A

B3LYP

4.640

2.801

2.037

2.037

2.801

3713.50129

0.1

0.8

2.03

3

B2

BP86

3.819

2.275

2.020

2.023

2.308

3713.95710

13.2

12.0

2.03

3

B

B3LYP

4.751

2.762

2.029

2.047

2.879

3713.50018

0.8

0.1

2.03

3

B

BP86

4.611

2.866

2.033

2.019

2.780

3713.95404

15.1

14.1

2.02

3

B

state

3


The lowest energy structure of the vanadium derivative Cp2V2(μ-C8H8) is a triplet (V-1T in Figure 5) with a predicted VtV distance of ∼2.48 Å a bit longer than the experimental value12 of ∼2.44 Å. This is interpreted as the formal triple bond needed to give both vanadium atoms the 17-electron configurations for a binuclear triplet. Furthermore, this VtV distance is close to the experimental VtV triple bond distance of 2.459 Å in Cp2V2(CO)5.51,52 For singlet Cp2V2(μ-C8H8), a formal V 3 V quadruple bond is required to give both vanadium atoms the favored 18-electron configuration. This is consistent with the predicted V 3 V quadruple bond distance of only ∼2.3 Å in singlet Cp2V2(μ-C8H8) (V-S2 in Figure 5). The titanium derivative Cp2Ti2(μ-C8H8), unlike the chromium and vanadium derivatives, has never been synthesized. However, the simple sandwich compound CpTi(η8-C8H8) has been synthesized69 and structurally characterized by X-ray crystallography.70 The singlet and triplet Cp2Ti2(μ-C8H8)

structures are predicted to lie within 2 kcal/mol of energy and thus are essentially degenerate. Simple electron counting suggests a formal TiTi quintuple bond for singlet Cp2Ti2 (μ-C8H8) and a formal TiTi quadruple bond for triplet Cp2Ti2(μ-C8H8) to give the titanium atoms the favored 18electron configuration. No relevant experimental examples of compounds with titaniumtitanium multiple bonds are known to provide a basis for comparing our predicted TiTi bond lengths in the Cp2Ti2(μ-C8H8) structures with experimental values. If the titanium atoms in singlet Cp2Ti2(μ-C8H8) are assumed to have only a 16-electron configuration, like the titanium atoms in a number of other very stable compounds such as CpTi(η7-C7H7),71 Cp2TiCl2,72,73 and so on, then only a TitTi triple bond is required. The predicted TitTi distance of 2.63 Å in singlet Cp2Ti2(μ-C8H8) is actually shorter than the 2.80 Å predicted in a recent study49 for the formal TitTi triple bond in the experimentally unknown molecule Cp2Ti2(CO)6. 3140


The Journal of Physical Chemistry A The TiTi distance of 2.57 Å predicted for triplet Cp2Ti2 (μ-C8H8) is very similar to that predicted for the corresponding singlet. The comparison of this 2.57 Å TiTi distance in triplet Cp2Ti2(μ-C8H8) with the much longer 2.80 Å distance for the TitTi triple bond predicted49 for Cp2Ti2(CO)6 suggests the possibility of a formal Ti 3 Ti quadruple bond in triplet Cp2Ti2(μC8H8), thereby giving both titanium atoms the 17-electron configurations for a binuclear triplet. 4.2. Central Transition Metal Derivatives (M = Mn, Fe). The Cp2M2(μ-C8H8) derivatives for the central transition metals Mn and Fe have not been reported in the literature. Our theoretical studies predict both cis- and trans-Cp2M2(μ-C8H8) structures to lie within 10 kcal/mol of energy of each other. Both the singlet and triplet cis-Cp2Mn2(μ-C8H8) structures, namely, Mn-1S and Mn-2T, respectively, have “diptych” folded μ-C8H8 rings similar to those found in the early transition metal derivatives discussed above. The singlet and triplet structures have similar MndMn distances of ∼2.7 Å, which can be allocated (with discretion) to the formal double bonds needed to give each manganese atom the favored 18-electron configuration. These MndMn double bond distances fall between the experimental MnMn single bond distance of 2.895 Å in the most recent X-ray diffraction study57 of Mn2(CO)10 and the experimental MntMn triple bond distance of 2.17 Å found by X-ray crystallography74 in Cp2Mn2(μ-CO)3. A variety of conformations of the μ-C8H8 ring are found in the five Cp2Fe2(μ-C8H8) structures (Figure 8). However, the “diptych” conformation of the C8H8 found in the Ti, V, Cr, and Mn rings is no longer found in either the singlet structure Fe-1S or the triplet structure Fe-3T of cis-Cp2Fe2(μ-C8H8) (Figure 8). Instead, the μ-C8H8 ring has the tub conformation, with each iron bonded to four carbon atoms from the bottom of the tub. The FeFe distance of 2.98 Å in the singlet Fe-1S is reasonable, although rather long, for a formal single bond to give each iron atom the favored 18-electron configuration. The Fe 3 3 3 Fe distance of ∼3.47 Å in the triplet Fe-3T is ∼0.5 Å longer than that in the singlet Fe-1S and clearly too long for direct ironiron bonding. Thus, without an ironiron bond, each iron atom in Fe-3T has the 17-electron configuration for a binuclear triplet. The twisted conformation of the μ-C8H8 ring found in the triplet manganese structure Mn-3T is also found in the iron complex trans-Cp2Fe2(μ-C8H8) Fe-2S (Figure 8). Each iron atom has the favored 18-electron configuration in Fe-2S with a local environment similar to ferrocene. Although, trans-Cp2Fe2(μ-C8H8) has not been synthesized, the isoelectronic dication trans-[(η5-Me5C5)2Co2(μ-C8H8)]2þ has been synthesized as the PF6 salt.59 A structure with the twist configuration of the μ-C8H8 ring has been suggested for this dication.59,62 However, this structure has not been confirmed by X-ray crystallography. 4.3. Late Transition Metal Derivatives (M = Co, Ni). All of the Cp2Co2(μ-C8H8) structures have the central μ-C8H8 ring functioning as a four-electron donor ligand to each cobalt atom, so that each cobalt atom has the favored 18-electron configuration without any cobaltcobalt bond. The lowest energy singlet structure Co-1S (Figure 9) is the structure found experimentally.10 The μ-C8H8 ring has the same tub conformation and same bonding to the metal atoms as in the trans-Cp2Fe2(μC8H8) structure Fe-5T (Figure 8) discussed above. The μ-C8H8 ring in the related cis structure Co-2S (Figure 9) also has a tub conformation but with the CpCo groups coordinated to sets of four adjacent cobalt atoms at the bottom of the tub. Variations of the twist conformation of the μ-C8H8 ring are found in the

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remaining two Cp2Co2(μ-C8H8) structures, both of which have trans stereochemistry. The nickel atoms in the Cp2Ni2(μ-C8H8) structures need only three electrons each from the μ-C8H8 ring to attain the favored 18-electron configuration. This leaves an uncomplexed CdC double bond in the μ-C8H8 moiety in the singlet structures Ni1S, Ni-2S, and Ni-4S with a relatively short CdC distance of ∼1.37 Å. The rather unstable compound (η5-Me5C5)2Ni2(μC8H8) has been synthesized from NiCl2, LiC5Me5, and Li2C8H8 but has not yet been structurally characterized.11 A structure similar to Ni-1S has been suggested for (η5-Me5C5)2Ni2(μC8H8). The triplet trans-Cp2Ni2(μ-C8H8) structure Ni-3T (Figure 10) has a similar tub arrangement of the μ-C8H8 ring with tetrahapto bonding to each nickel atom as the trans-Cp2Co2(μ-C8H8) structure Co-1S (Figure 9) and trans-Cp2Fe2(μ-C8H8) structure Fe-5T. For Ni-3T, this leads to a 19-electron configuration for each nickel atom. Although the 19-electron configuration is rare in stable transition metal complexes, it is known in cobaltocene (Cp2Co)66 as well as the mixed sandwich compound CpFe(η6C6Me6).67 Triplet Cp2Ni2(μ-C8H8) structures are also found in which only two adjacent carbon atoms of the central μ-C8H8 ring are bonded to each nickel atom, thereby giving the nickel atoms 17electron configurations for a binuclear triplet (Figure 10). With such a bonding scheme, the μ-C8H8 is functioning as a monoolefin ligand toward each nickel atom. In the cis-Cp2Ni2 (μ-C8H8) structure Ni-5T, the two pairs of carbon atoms bonded to the nickel atoms are on opposite sides of the μ-C8H8 ring, so that the two uncomplexed CdC double bonds are not adjacent. However, in the trans-Cp2Ni2(μ-C8H8) structure Ni-6T, the pairs of carbon atoms bonded to each nickel atom are adjacent, so that the uncomplexed CdC double bonds of the μ-C8H8 form a conjugated 1,3-diene unit.

5. CONCLUSION The preferred structures for the binuclear cyclooctatetraene derivatives Cp2M2(μ-C8H8) of the first row transition metals depend on the electronic requirements of the transition metals relating to the position of the metal in the Periodic Table. Thus, the early transition metal derivatives Cp2M2(μ-C8H8) (M = Ti, V, Cr, Mn) are predicted to form low-energy cis-Cp2M2(μC8H8) structures with a folded “diptych” C8H8 ring having a dihedral angle of ∼130. The predicted metalmetal distances in these early transition metal derivatives are short, suggesting the multiple bonding required for the metal atoms to approach the favored 18-electron configurations for singlet structures and 17-electron configurations for binuclear triplet structures. These predicted structures are close to the experimental structures for Cp2M2(μ-C8H8) (M = V, Cr) with VtV and CrtCr bond lengths of ∼2.48 and ∼2.36 Å, respectively. The middle to late transition metals form rather different trans-Cp2M2(μ-C8H8) structures (M = Mn, Fe, Co, Ni) with a twisted μ-C8H8 ring and no metalmetal bonding. The hapticities of the bonds from the central μ-C8H8 ring to each metal atom in such structures range from five for Mn and Fe to four for Co and three for Ni and thus depend on the electronic requirements of the central metal atom. This leads to the favored 18-electron configurations for both metal atoms in the singlet Fe, Co, and Ni structures but only 17-electron metal configurations in the triplet Mn structure. In addition, the late transition 3141


The Journal of Physical Chemistry A metals form trans-Cp2M2(μ-C8H8) structures (M = Fe, Co, Ni), with the tub conformation of the μ-C8H8 ring functioning as a tetrahapto (M = Fe, Co) or trihapto (M = Ni) ligand to each CpM group. A trans-Cp2Co2(η2,2,η2,2-C8H8) structure of this type is known experimentally.10 A μ-C8H8 ring in the tub conformation also bonds to two CpFe units as a bis(tetrahapto) ligand in both singlet and triplet cis-Cp2Fe2(μ-C8H8) structures.

’ ASSOCIATED CONTENT

bS

Supporting Information. Table 1: Vibrational frequencies (in cm1) and infrared intensities (in km/mol, given in parentheses) of the Cp2M2(μ-C8H8) (M = Ti, V, Cr, Mn, Fe, Co, Ni) complexes at the B3LYP/DZP and BP86/DZP levels; Table 2: Cartesian coordinates of the optimized Cp2M2(μ-C8H8) (M = Ti, V, Cr, Mn, Fe, Co, Ni) structures at the B3LYP/DZP and BP86/DZP levels; complete Gaussian 03 reference (ref 47). This material is available free of charge via the Internet at http:// pubs.acs.org.

’ ACKNOWLEDGMENT We are indebted to the 111 Project (B07012) and the National Natural Science Foundation (20873045 and 20973066) of China as well as the U.S. National Science Foundation (Grants CHE0749868 and CHE-0716718) for support of this research. ’ REFERENCES (1) Cope, A. C.; Hochstein, F. A. J. Am. Chem. Soc. 1950, 72, 2515. (2) Mathews, F. S.; Lipscomb, W. N. J. Am. Chem. Soc. 1958, 80, 4745. (3) Manuel, T. A.; Stone, F. G. A. Proc. Chem. Soc., London 1959, 90. (4) Manuel, T. A.; Stone, F. G. A. J. Am. Chem. Soc. 1960, 82, 366. (5) Rausch, M. D.; Schrauzer, G. N. Chem. Ind. (London) 1959, 957. (6) Nakamura, A.; Hagihara, N. Bull. Chem. Soc. Jpn. 1959, 32, 880. (7) Dickens, B.; Lipscomb, W. N. J. Am. Chem. Soc. 1961, 83, 4862. (8) Dickens, B.; Lipscomb, W. N. J. Am. Chem. Soc. 1961, 83, 489. (9) Fleischer, E. B.; Stone, A. L.; Dewar, R. B. K.; Wright, J. D.; Keller, C. E.; Pettit, R. J. Am. Chem. Soc. 1966, 88, 3158. (10) Fritz, H. P.; Keller, H. Z. Naturforsch. 1961, 16b, 348. (11) Wilke, G.; Fuss, B.; Khouzamik, F.; Gersdorf, J. J. Organomet. Chem. 1985, 290, 77. (12) Elschenbroich, C.; Heck, J.; Massa, W.; Nun, E.; Schmidt, R. J. Am. Chem. Soc. 1983, 105, 2905. (13) Weber, J.; Chermette, H.; Heck, J. Organometallics 1989, 8, 2544. (14) Bachmann, B.; Friedemann, H.; Heck, J.; W€unsch, M. Organometallics 1989, 8, 2523. (15) Breil, H.; Wilke, G. Angew Chem., Int. Ed. 1966, 5, 898. (16) M€uller, J.; Holzinger, W.; K€ohler, F. H. Chem. Ber. 1976, 109, 1222. (17) Elschenbroich, C.; Heck, J.; Massa, W.; Schmidt, R. Angew. Chem., Int. Ed. 1983, 22, 330. (18) Heck, J.; Rist, G. J. Organomet. Chem. 1988, 342, 45. (19) Poumbga, C.; Daniel, C.; Benard, M. J. Am. Chem. Soc. 1991, 113, 1090. (20) Luthi, H. P.; Bauschlicher, C. W. J. Am. Chem. Soc. 1987, 109, 2046. (21) Mougenot, P.; Demuynck, J.; Benard, M.; Bauschlicher, C. W. J. Am. Chem. Soc. 1988, 110, 4503. (22) Richter, U.; Heck, J.; Reinhold, J. Inorg. Chem. 1999, 38, 77. (23) Richter, U.; Heck, J.; Reinhold, J. Inorg. Chem. 2000, 39, 658. (24) Ehlers, A. W.; Frenking, G. J. Am. Chem. Soc. 1994, 116, 1514. (25) Delly, B.; Wrinn, M.; L€uthi, H. P. J. Chem. Phys. 1994, 100, 5785.

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Binuclear Cyclopentadienylmetal Cyclooctatetraene Derivatives of the

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