Mixed Sandwich Compounds C - American Chemical Society

Mar 31, 2010 - †College of Physical Science and Technology, Southwest Jiaotong University ... School of Physics and Chemistry, Xihua University, Che...
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Organometallics 2010, 29, 1934–1941 DOI: 10.1021/om901113y

Mixed Sandwich Compounds C5H5MC8H8 of the First-Row Transition Metals: Variable Hapticity of the Eight-Membered Ring Hongyan Wang,*,† Xiaohong Chen,§ Yaoming Xie,‡ R. Bruce King,*,‡,§ and Henry F. Schaefer III‡ †

College of Physical Science and Technology, Southwest Jiaotong University Chengdu 610031, People’s Republic of China, ‡Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602, and §Research Center for Advanced Computation, School of Physics and Chemistry, Xihua University, Chengdu 610039, People’s Republic of China Received December 23, 2009

The C5H5MC8H8 complexes (M = Ti, V, Cr, Mn, Fe, Co, Ni) have been investigated by density functional theory. Only for titanium is the octahapto parallel ring sandwich (η5-C5H5)Ti(η8-C8H8) the lowest energy structure; this 17-electron complex has been synthesized and characterized structurally by X-ray crystallography. For vanadium, chromium, and manganese the (η5-C5H5)M(η6-C8H8) (M = V, Cr, Mn) structures with hexahapto C8H8 rings are the lowest energy structures. However, for the vanadium compound the octahapto parallel ring singlet spin state structure (η5C5H5)V(η8-C8H8) lies only ∼6 kcal/mol above the hexahapto triplet spin state (η5-C5H5)V(η6-C8H8) structure. The chromium and manganese (η5-C5H5)M(η6-C8H8) complexes have been synthesized. For iron the 17-electron tetrahapto structure (η5-C5H5)Fe(η4-C8H8) with four adjacent C8H8 carbons bonded to the iron is the lowest energy structure. For cobalt, two isomeric 18-electron tetrahapto structures, namely, (η5-C5H5)Co(η4-C8H8), with four adjacent C8H8 carbons bonded to the cobalt, and (η5-C5H5)Co(η2,2-C8H8), with two nonadjacent C8H8 double bonds bonded to the cobalt, lie within 5 kcal/mol in energy. Experimental work indicates the existence of both isomers in solution, but only (η5-C5H5)Co(η2,2-C8H8) has been isolated in the crystalline state. The lowest energy structure for the nickel complex is the 17-electron complex (η5-C5H5)Ni(η2-C8H8), with a dihapto C8H8 ring containing three uncomplexed conjugated CdC double bonds.

1. Introduction The chemistry of cyclooctatetraene metal complexes originates from the observation of Cope and Hochstein1 in 1950 that cyclooctatetraene forms a crystalline adduct with silver nitrate. This adduct was subsequently characterized structurally by Mathews and Lipscomb.2 Shortly thereafter three independent research groups3-6 synthesized the first cyclooctatetraene metal carbonyl derivative, namely, 1,2,3,4η4-C8H8Fe(CO)3, as a relatively high-yield product from the reaction of Fe(CO)5 with cyclooctatetraene (Figure 1). The chemistry of mixed sandwich complexes C5H5MC8H8 containing both five- and eight-membered rings can be viewed as dating back to the 1960 discovery of C5H5CoC8H8 by Nakamura and Hagihara as the product from the reaction of η5-C5H5Co(CO)2 with cyclooctatetraene.7 However, C5H5CoC8H8 is not a true sandwich compound, since the C8H8 ring is in the tub form, with only four of the eight *Corresponding authors. E-mail: [email protected]; rbking@ chem.uga.edu. (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. 1959, 957. (6) Nakamura, A.; Hagihara, N. Bull. Chem. Soc. Jpn. 1959, 32, 880. pubs.acs.org/Organometallics

Published on Web 03/31/2010

carbon atoms bonded to the cobalt atom, i.e., (η5-C5H5)Co(1,2,5,6-η2,2-C8H8), in which the cobalt atom attained the favored 18-electron configuration by receiving five electrons from the η5-C5H5 ligand and only four electrons from the 1,2,5,6-η2,2-C8H8 ligand (Figure 1).8 Furthermore, the 1,2, 5,6-η2,2 tetrahapto bonding of the C8H8 ring in C5H5CoC8H8 is different from the 1,2,3,4-η4 tetrahapto bonding of the C8H8 ring in C8H8Fe(CO)3 (Figure 1).9 Subsequent work showed that the 1,2,5,6-η2,2 and 1,2,3,4-η4 isomers of C5H5CoC8H8 have very similar energies and are in equilibrium in solution.10-12 The first true sandwich compound containing parallel fiveand eight-membered rings was the titanium derivative (η5C5H5)Ti(η8-C8H8), which is a 17-electron complex, discovered by Van Oven and de Liefde Meijer13 in 1969 as a product (7) Nakamura, A.; Hagihara, N. Bull. Chem. Soc. Jpn. 1960, 33, 425. (8) Wadepohl, H.; Merkel, R.; Pritzkow, H. Acta Crystallogr. C 1998, C54, 1095. (9) Dickens, B.; Lipscomb, W. N. J. Am. Chem. Soc. 1961, 83, 4862. (10) Moraczewski, J.; Geiger, W. E., Jr. J. Am. Chem. Soc. 1981, 103, 4779. (11) Albright, T. A.; Geiger, W. E., Jr.; Moraczewski, J.; Tulyathan, B. J. Am. Chem. Soc. 1981, 103, 4787. (12) Grzeszczuk, M.; Smith, D. E.; Geiger, W. E., Jr. J. Am. Chem. Soc. 1983, 105, 1772. (13) Van Oven, H. O.; de Liefde Meijer, H. J. J. Organomet. Chem. 1969, 19, 373. r 2010 American Chemical Society

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Figure 1. Comparison of the metal-C8H8 bonding in (1,2,3,4η4-C8H8)Fe(CO)33-6 and (η5-C5H5)Co(1,2,5,6-η2,2-C8H8).7

Figure 2. Known C5H5MC8H8 derivatives with octahapto (M = Ti)13 and hexahapto (M = Cr, Mn) rings.15,16

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Mixed sandwich compounds (η5-C5H5)M(η6-C8H8) with a hexahapto C8H8 ring containing an uncomplexed CdC double bond are also known (Figure 2). The 17-electron chromium derivative19 (η5-C5H5)Cr(η6-C8H8) has been synthesized by the reaction of (η5-C5H5)CrCl2(THF) with C8H8 in the presence of iPrMgBr and has been structurally characterized by X-ray crystallography.15 The relatively unstable 18-electron complex (η5-C5H5)Mn(η6-C8H8) has been reported as a product from the photochemical reaction of (η5-C5H5)Mn(CO)3 with cyclooctatetraene.16 However, it has not been characterized structurally. In summary the mixed sandwich compounds C5H5MC8H8 are known for the first-row transition metals Ti, Cr, Mn, and Co but not for V, Fe, and Ni. These known complexes exhibit varying hapticities of the C8H8 rings ranging from eight for Ti to six for Cr and probably Mn and four for Co. Furthermore, an interesting feature of the chemistry of the mixed sandwich compounds is the apparent instability of the 18electron complexes (η5-C5H5)V(η8-C8H8) and (η5-C5H5)Mn(η6-C8H8) relative to the corresponding 17-electron complexes (η5-C5H5)Ti(η8-C8H8) and (η5-C5H5)Cr(η6-C8H8). In order to evaluate the possibilities in this area, we have performed comprehensive theoretical studies on the firstrow transition metal derivatives C5H5MC8H8 (M = Ti through Ni). Our results are reported in this paper.

2. Theoretical Methods

Figure 3. Structure of [(η5-C5H5)V]2[η7,η7-C8H8-C8H8], a dimer of C5H5VC8H8.

from the reaction of K2C8H8 with (η5-C5H5)TiCl3 or (η5C5H5)2TiCl2. The parallel sandwich structure (Figure 2) of this product was confirmed shortly thereafter by X-ray diffraction.14 The electron spin resonance spectrum of the paramagnetic (η5-C5H5)Ti(η8-C8H8) indicates that the unpaired electron resides essentially in the metal d(z2) orbital. A surprising feature of the chemistry of (η5-C5H5)M(η8-C8H8) derivatives is the difficulty in synthesizing (η5-C5H5)V(η8-C8H8) even though it has the favored 18-electron configuration, and the corresponding 17-electron complex (η5-C5H5)Ti(η8-C8H8) is rather stable, as noted above. Thus in 1959 one of the authors of this paper, after successfully synthesizing the 17-electron complex (η5-C5H5)V(η7-C7H7) from the thermal reaction of (η5-C5H5)V(CO)4 with cycloheptatriene,17 attempted the analogous synthesis of (η5-C5H5)V(η8-C8H8) by the thermal reaction of (η5-C5H5)V(CO)4 with cyclooctatetraene. However, no tractable product was obtained from this reaction. Furthermore, an attempt to synthesize (η5-C5H5)V(η8-C8H8) from the reaction of VCl3(THF)3 with a mixture of K2C8H8 and NaC5H5 led instead to the paramagnetic binuclear complex [(η5-C5H5)V]2[η7,η7-C8H8-C8H8] (Figure 3), in which two C8H8 rings have coupled through C-C bond formation, with the two vanadium atoms each bonded to a heptahapto C8H8 ring.18 This compound may be regarded as a dimer of (η5-C5H5)V(η8-C8H8). (14) Kroon, P. A.; Helmholdt, R. B. J. Organomet. Chem. 1970, 25, 461. (15) Angermund, K.; Betz, P.; Doehring, A.; Jolly, P. W.; Kr€ uger, C.; Schenfelder, K. U. Polyhedron 1993, 12, 2663. (16) Pauson, P. L.; Segal, J. A. J. Chem. Soc., Dalton Trans. 1975, 2387. (17) King, R. B.; Stone, F. G. A. J. Am. Chem. Soc. 1959, 81, 5263. (18) Bachmann, B.; Heck, J.; Meyer, G.; Pebler, J.; Schleid, T. Inorg. Chem. 1992, 31, 607.

Electron correlation effects were included by employing density functional theory (DFT) methods, which have evolved as a practical and effective computational tool, especially for organometallic compounds.20-24 The reliability of such density functional theory (DFT) methods is governed by the quality of the approximate exchange-correlation (XC) energy functional. We initially chose two DFT methods, namely, the B3LYP and the BP86 methods, which are constructed in very different ways. The B3LYP method is a hybrid HF/DFT method using a combination of the threeparameter Becke functional (B3) with the Lee-Yang-Parr (LYP) generalized gradient correlation functional.25,26 This method includes exact exchanges and is calibrated by fitting three parameters to a set of experimental results. The BP86 method combines Becke’s 1988 exchange functional (B) with Perdew’s 1986 gradient-corrected correlation functional method (P86).27,28 This method does not include exact exchange and is mainly deduced by forcing the functional to satisfy certain exact constraints based on first principles. When these two very different DFT methods agree, confident predictions can be made. However, Reiher and collaborators have found that B3LYP always favors the high-spin state and BP86 favors the low-spin state for a series of the Fe(II)-S complexes.29 In this work a similar tendency for the B3LYP method to favor higher spin states relative to the BP86 method was observed for the C5H5MC8H8 derivatives (M = Cr, Mn, Fe, and Co). However, for all of the C5H5MC8H8 derivatives investigated in this work, both methods agree on the lowest energy structures. (19) M€ uller, J.; Menig, H. J. Organomet. Chem. 1975, 96, 83. (20) Ehlers, A. W.; Frenking, G. J. Am. Chem. Soc. 1994, 116, 1514. (21) Li, J.; Schreckenbach, G.; Ziegler, T. J. Am. Chem. Soc. 1995, 117, 486. (22) Jonas, V.; Thiel, W. J. Chem. Phys. 1995, 102, 8474. (23) Brynda, M.; Gagliardi, L.; Wimark, P. O.; Power, P. P.; Roos, B. O. Angew. Chem., Int. Ed. 2006, 45, 3804. (24) Zhao, Y.; Truhlar, D. G. J. Chem. Phys. 2006, 124, 224105. (25) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (26) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (27) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (28) Perdew, J. P. Phys. Rev. B 1986, 33, 8822. (29) Reiher, M.; Salomon, O.; Hess, B. A. Theor. Chem. Acc. 2001, 107, 48.

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Figure 4. Optimized geometries for the C5H5TiC8H8 structures (bond distances are in A˚). In Figures 4 to 10 the upper distances were determined by the B3LYP method and the lower distances by the BP86 method. The geometries of all structures were fully optimized using both the DZP B3LYP and DZP BP86 methods. The harmonic vibrational frequencies were determined at the same levels by evaluating analytically the second derivatives of the energy with respect to the nuclear coordinates. The corresponding infrared intensities were evaluated analytically as well. All of the computations were carried out with the Gaussian 03 program, in which the fine grid (75,302) is the default for evaluating integrals numerically, while the tight (10-8 hartree) designation is the default for the energy convergence.30 For carbon the double-ζ plus polarization (DZP) basis set used here adds one set of pure spherical harmonic d functions with an orbital exponent Rd(C) = 0.75 to the standard Huzinaga-Dunning contracted DZ sets and is designated (9s5p1d/ 4s2p1d).31,32 For H, a set of p polarization functions Rp(H) = 0.75 is added to the Huzinaga-Dunning DZ sets. For the firstrow 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, contracted following Hood et al., and designated (14s11p6d/10s8p3d).33,34 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.35 Thus, an imaginary vibrational frequency of magnitude less than 50i cm-1 should imply that there is a minimum with energy very similar to that of the stationary point in question. In some cases a finer (99,590) integration grid was used for the optimization to check the small imaginary vibrational frequencies. All of the final optimized structures reported in this paper have only real vibrational frequencies unless otherwise indicated.

3. Results and Discussion 3.1. Molecular Structures. The geometries of the complexes C5H5MC8H8 (M=Ti, V, Cr, Mn, Fe, Co, and Ni) were optimized in the electronic doublet and quartet states for Ti, Cr, Fe, and Ni and singlet and triplet states for V, Mn, and Co without any symmetry constraints. The optimizations started with sandwich structures having parallel C5H5 and C8H8 rings. The equilibrium geometries of the energetically low-lying species of C5H5MC8H8 are shown in Figures 4 to 10, with all C-C (30) Frisch, M. J.; et al. Gaussian 03, Revision A.1; Gaussian, Inc.: Pittsburgh, PA, 2003. (31) Dunning, T. H. J. Chem. Phys. 1970, 53, 2823. (32) Huzinaga, S. J. Chem. Phys. 1965, 42, 1293. (33) Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033. (34) Hood, D. M.; Pitzer, R. M.; Schaefer, H. F. J. Chem. Phys. 1979, 71, 705. (35) Papas, B. N.; Schaefer, H. F. J. Mol. Struct. 2006, 768, 275.

Wang et al.

Figure 5. Optimized geometries for the C5H5VC8H8 structures.

Figure 6. Optimized geometries for the C5H5CrC8H8 structures.

bond distances given in angstroms. In these figures the upper distances were obtained by the B3LYP method and the lower distances were obtained by the BP86 method. The structures are designated as M-aX, where M is the symbol of the central metal atom, a orders the structures according to relative energies, and X designates the spin states, using S, D, T, and Q for singlets, doublets, triplets, and quartets, respectively. 3.1.1. C5H5TiC8H8. Two structures, namely, a doublet Ti-1D and a quartet Ti-2Q, are found for C5H5TiC8H8 (Figure 4 and Table 1). The global minimum Ti-1D has a true sandwich structure with the titanium atom lying between parallel η5-C5H5 and η8-C8H8 rings. The average Ti-C bond distances to the C8H8 ligand in Ti-1D, namely, 2.371 A˚ (B3LYP) or 2.367 A˚ (BP86), are a bit shorter than the Ti-C bond distances to the C5H5 ligand, namely, 2.392 A˚ (B3LYP) or 2.373 A˚ (BP86). This is in agreement with the structure determination on (η5-C5H5)Ti(η8-C8H8) by X-ray crystallography, which indicates Ti-C distances of 2.326 to 2.333 A˚ to the η8-C8H8 ligand and longer distances of 2.344 to 2.363 A˚ to the η5-C5H5 ligand.14 The other optimized C5H5TiC8H8 structure is the quartet Ti-2Q (Figure 4 and Table 1). This is a very high energy structure, lying 49.2 kcal/mol (B3LYP) or 52.6 kcal/mol (BP86) above the global minimum Ti-1D, and thus is of questionable chemical significance. In Ti-2Q the η5-C5H5 ring is planar. However, the C8H8 ring is a nonplanar hexahapto ligand with two carbon atoms beyond bonding distance to the titanium atom. The uncomplexed CdC double bond formed by these two carbon atoms of length 1.387 A˚ (B3LYP) or 1.357 A˚ (BP86) is appreciably shorter than the other CdC distances in the η6-C8H8 ring. 3.1.2. C5H5VC8H8. The triplet electronic state (η5C5H5)V(η6-C8H8) structure (V-1T) (Figure 5 and Table 2) was found to be the global minimum, unlike C5H5TiC8H8

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Figure 7. Optimized geometries for the C5H5MnC8H8 structures.

Figure 8. Optimized geometries for the C5H5FeC8H8 structures.

Figure 9. Optimized geometries for the C5H5CoC8H8 structures.

discussed above (Figure 4 and Table 1). The singlet (η5-C5H5)V(η8-C8H8) structure (V-2S) lies 7.7 kcal/mol (B3LYP) or 5.4 kcal/mol (BP86) above the triplet structure V-1T (Figure 5 and

Table 2). This may account for the difficulty in preparing this 18-electron complex by the simple thermal reaction of C5H5V(CO)4 with cyclooctatetraene.

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Wang et al. Table 2. Bond Distances (in A˚), Total Energies (E in hartree), Relative Energies (ΔE in kcal/mol), and HOMO-LUMO Gaps for the C5H5VC8H8 Structures V-1T (Cs)

V-C8H8 V-C5H5 -HOMO(R) -LUMO(R) gap/eV -E ΔE ÆS2æ

Figure 10. Optimized structures of C5H5NiC8H8. Table 1. Bond Distances (in A˚), Total Energies (E in hartree), Relative Energies (ΔE in kcal/mol), and HOMO-LUMO Gaps for the C5H5TiC8H8 Structures Ti-1D (Cs)

M-C8H8a M-C5H5b -HOMO -LUMO gap/eV -E ΔE ÆS2æ

BP86

B3LYP

BP86

2.371 2.392 0.16067 0.02429 3.71 1352.77006 0.0 0.76

2.367 2.373 0.10282 0.05449 1.32 1352.87704 0.0 0.76

2.500 2.395 0.14843 0.03619 3.05 1352.69167 49.2 3.76

2.419 2.374 0.10054 0.06635 0.93 1352.79332 52.6 3.76

a Average M-C8H8 ring bonded distance. bAverage M-C5H5 ring distance.

The hexahapto η6-C8H8 ring of the triplet C8H8 global minimum V-1T, like that in the quartet titanium derivative Ti-2Q (Figure 4), has two carbon atoms of the eight-membered ring beyond bonding distance to the vanadium atom and corresponding to an uncomplexed double bond (Figure 5). The corresponding CdC distance in V-1T of 1.373 A˚ (B3LYP) or 1.374 A˚ (BP86) is shorter than the other C-C distances in the C8H8 ring. In the surprisingly higher energy singlet structure V-2S, the η5-C5H5 and η8-C8H8 rings are parallel, corresponding to the favored 18-electron configuration. The average V-C bond distances in both C5H5VC8H8 structures are shorter for the C5H5 ligand than for the C8H8 ligand. 3.1.3. C5H5CrC8H8. The (η5-C5H5)Cr(η6-C8H8) (Cr-1D) doublet electronic state structure with a hexahapto η6-C8H8 ring is found as a true minimum with no imaginary vibrational frequencies (Figure 6 and Table 3). In the hexahapto η6-C8H8 ring of Cr-1D the CdC distance of the two uncomplexed carbon atoms is relatively short at 1.337 A˚ (B3LYP) or 1.347 A˚ (BP86). The chromium atom in Cr-1D has a 17-electron configuration. This predicted structure Cr-1D is consistent with the experimentally reported structure15 of (η5C5H5)Cr(η6-C8H8), synthesized19 by reaction of (η5-C5H5)CrCl2(THF) with C8H8 in the presence of iPrMgBr. A quartet (η5-C5H5)Cr(1,2,5,6-η2,2-C8H8) structure (Cr-2Q) was also found for C5H5CrC8H8, lying 4.9 kcal/mol (B3LYP) or 21.9 kcal/mol (BP86) above Cr-1D. In Cr-2Q the C8H8 ligand is a tetrahapto 1,2,5,6-η2,2-C8H8 ligand, which is bonded to the chromium atom through two isolated CdC double bonds. This gives the chromium atom in Cr-2Q the expected 15-electron configuration for a quartet. The uncomplexed CdC distances in the 1,2,5,6-η2,2-C8H8 ligand are 1.374 A˚

B3LYP

BP86

B3LYP

BP86

2.317 2.312 0.16187 0.05516 2.90 1447.26497 0.0 2.08

2.278 2.285 0.12831 0.07549 1.44 1447.39814 0.0 2.04

2.333 2.310 0.13818 0.03487 2.81 1447.25277 7.7 0.00

2.329 2.280 0.08389 0.06702 0.46 1447.38947 5.4 0.00

Table 3. Bond Distances (in A˚), Total Energies (E in hartree), Relative Energies (ΔE in kcal/mol), and HOMO-LUMO Gaps for the Lowest Lying C5H5CrC8H8 Structures Cr-1D (C1)

Ti-2Q (Cs)

B3LYP

V-2S (Cs)

Cr-C8H8 Cr-C5H5 -HOMO(R) -LUMO(R) gap/eV -E ΔE ÆS2æ

Cr-2Q (Cs)

B3LYP

BP86

B3LYP

BP86

2.146 2.224 0.19676 0.02304 4.73 1547.70648 0.0 0.83

2.134 2.202 0.16088 0.05525 2.87 1547.88075 0.0 0.78

2.314 2.343 0.17248 0.07284 2.71 1547.69872 4.9 4.22

2.192 2.261 0.13588 0.07458 1.67 1547.84580 21.9 3.85

(B3LYP) or 1.372 A˚ (BP86), which is shorter than the other C-C distances in Cr-2Q and similar to the uncomplexed CdC bonds in the hexahapto η6-C8H8 rings of Ti-2Q (Figure 4), V-1T (Figure 5), and Cr-1D (Figure 6). 3.1.4. C5H5MnC8H8. Three structures are found for C5H5MnC8H8 (Figure 7 and Table 4). The global minimum is the singlet Mn-1S, (η5-C5H5)Mn(η6-C8H8), with a planar η5-C5H5 ring but a nonplanar hexahapto η6-C8H8 ring with a relatively short CdC distance of 1.334 A˚ (B3LYP) or 1.344 A˚ (BP86) between the two uncomplexed carbon atoms. Structure Mn-1S has the favored 18-electron configuration. For the triplet state two structures are found, namely, (η5C5H5)Mn(η4-C8H8) (Mn-2T) and (η5-C5H5)Mn(η2,2-C8H8) (Mn-3T). Structure Mn-2T lies 1.4 kcal/mol (B3LYP) or 10.8 kcal/mol (BP86) above the global minimum Mn-1S (Figure 7 and Table 4). In Mn-2T the C8H8 ligand is a 1,2,3,4-η4tetrahapto ligand, bonded to the manganese atom through two adjacent double bonds. The CdC uncomplexed doublebond distances in this η4-C8H8 ligand are relatively short at 1.371 A˚ (B3LYP) or 1.383 A˚ (BP86). In the other triplet structure (η5-C5H5)Mn(η2,2-C8H8) (Mn-3T), the η2,2-C8H8 ring is bonded to the Mn atom through two nonadjacent double bonds. Structure Mn-3T lies 5.5 kcal/mol (B3LYP) or 16.5 kcal/mol (BP86) above the Mn-1S global minimum. The manganese atoms in Mn-2T and Mn-3T have the 16-electron configurations expected for a triplet. 3.1.5. C5H5FeC8H8. Two stable doublet spin state structures, namely, (η5-C5H5)Fe(η4-C8H8) (Fe-1D) and (η5-C5H5)Fe(η2,2-C8H8) (Fe-2D), are predicted for C5H5FeC8H8 (Figure 8 and Table 5). The global minimum structure is Fe-1D, in which the C8H8 ring is a 1,2,3,4-tetrahapto ligand bonded to the iron atom through four adjacent carbon atoms, similar to the cyclooctatetraene ligand in the known3-7,9 (η4-C8H8)Fe(CO)3. The higher energy (η5-C5H5)Fe(η2,2-C8H8) structure Fe-2D, at 7.5 kcal/mol (B3LYP) or 7.9 kcal/mol (BP86) above Fe-1D, has no imaginary vibrational frequency by BP86 but a small

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Table 4. Bond Distances (in A˚), Total Energies (E in hartree), Relative Energies (ΔE in kcal/mol), and HOMO-LUMO Gaps for the C5H5MnC8H8 Structures Mn-1S (C1)

Mn-C8H8 Mn-C5H5 -HOMO(R) -LUMO(R) gap/eV -E ΔE imaginary freq ÆS2æ

Mn-2T (C1)

Mn-3T (C1)

B3LYP

BP86

B3LYP

BP86

B3LYP

BP86

2.135 2.158 0.19233 0.02264 4.62 1654.22187 0.0 none 0.00

2.121 2.137 0.13386 0.05470 2.15 1654.42066 0.0 none 0.00

2.117 2.222 0.17063 0.07073 2.72 1654.22404 1.4 none 2.21

2.099 2.188 0.16154 0.09047 1.93 1654.40340 10.8 14i 2.08

2.070 2.272 0.19945 0.05174 4.02 1654.21316 5.5 none 2.18

2.049 2.243 0.16955 0.08141 2.40 1654.39434 16.5 none 2.06

Table 5. Bond Distances (in A˚), Total Energies (E in hartree), Relative Energies (ΔE in kcal/mol), and HOMO-LUMO Gaps for the C5H5FeC8H8 Structures Fe-1D (C1)

Fe-C8H8 Fe-C5H5 -HOMO(R) -LUMO(R) gap/eV -E ΔE imaginary freq ÆS2æ

Fe-2D (Cs)

Fe-3Q (C1)

B3LYP

BP86

B3LYP

BP86

B3LYP

BP86

2.084 2.165 0.18063 0.06723 3.09 1766.94960 0.0 none 0.835

2.071 2.116 0.15518 0.12058 0.938 1767.15488 0.0 none 0.768

2.060 2.216 0.19670 0.03525 4.39 1766.93766 7.5 21i 0.791

2.047 2.165 0.16401 0.07440 2.44 1767.14226 7.9 none 0.765

2.215 2.277 0.17142 0.09304 2.13 1766.94085 5.5 none 4.18

2.085 2.201 0.13712 0.11406 0.627 1767.12713 17.4 none 3.85

Table 6. Bond Distances (in A˚), Total Energies (E in hartree), Relative Energies (ΔE in kcal/mol), and HOMO-LUMO Gaps for the C5H5CoC8H8 Structures Co-1S (C1)

Co-C8H8 Co-C5H5 -HOMO -LUMO gap/eV -E ΔE ÆS2æ

Co-2S (Cs)

Co-3S (C1)

Co-4T (C1)

Co-5T (C1)

B3LYP

BP86

B3LYP

BP86

B3LYP

BP86

B3LYP

BP86

B3LYP

BP86

2.057 2.102 0.18286 0.06172 3.30 1886.01599 0.0 0.00

2.060 2.076 0.15907 0.09224 1.82 1886.24980 0.0 0.00

2.037 2.144 0.18959 0.03762 4.14 1886.01006 3.7 0.00

2.022 2.115 0.16421 0.08232 2.23 1886.24207 4.9 0.00

2.044 2.139 0.19290 0.03887 4.19 1885.99338 14.2 0.00

2.036 2.106 0.15985 0.06908 2.47 1886.22544 15.2 0.00

2.094 2.213 0.16567 0.08859 2.19 1886.00220 8.7 2.36

2.051 2.142 0.13235 0.09605 0.99 1886.21350 22.8 2.05

2.048 2.253 0.18940 0.06259 3.45 1886.00526 6.7 2.08

2.137 2.221 0.13135 0.10370 0.75 1886.20834 25.9 2.03

imaginary vibrational frequency of 21i cm-1 by B3LYP. The η2,2-C8H8 ring in Fe-2D is bonded to the iron atom through two nonadjacent double bonds. The iron atoms in Fe-1D and Fe-2D have 17-electron configurations consistent with the doublet spin states. A quartet (η5-C5H5)Fe(η3-C8H8) structure (Fe-3Q) lies 5.5 kcal/mol (B3LYP) or 17.4 kcal/mol (BP86) above Fe-1D (Figure 8 and Table 5). Structure Fe-3Q has a trihapto η3C8H8 ring bonded to the iron atom through three carbon atoms, leaving five of the C8H8 carbon atoms outside the bonding distance of the iron atom. Two of the three unpaired electrons in Fe-3Q arise from the 16-electron iron configuration, whereas the third unpaired electron is delocalized within the five carbon atoms of the η3-C8H8 ligand not bonded to the iron atom. 3.1.6. C5H5CoC8H8. Three distinct singlet C5H5CoC8H8 structures are found (Figure 9 and Table 6). The global minimum Co-1S is a (η5-C5H5)Co(η4-C8H8) structure containing a 1,2,3,4-tetrahapto η4-C8H8 ring bonded to the cobalt atom through four adjacent carbon atoms similar to the η4-C8H8 ligands in the known3-6,9 (η4-C8H8)Fe(CO)3 (Figure 1) and the predicted structure (η5-C5H5)Fe(η8-C8H8) (Fe-1D in Figure 8). The small imaginary vibrational

frequency of 6i cm-1 found in the initial B3LYP optimization of Co-1S disappears when the larger integration grid (99,590) is used. The (η5-C5H5)Co(η2,2-C8H8) structure (Co-2S), in which the η2,2-C8H8 ring is bonded to the cobalt atom through two nonadjacent double bonds, lies only 3.7 kcal/mol (B3LYP) or 4.9 kcal/mol (BP86) above the Co-1S global minimum. The third singlet structure (η5-C5H5)Co(η3,1-C8H8) (Co-3S) has a still different type of tetrahapto C8H8 ligand, in which three adjacent carbon atoms of the C8H8 ligand and an isolated carbon atom are within bonding distance of the central cobalt atom, leaving two uncomplexed CdC bonds. Structure Co-3S lies at the relatively high energy of 14.2 kcal/mol (B3LYP) or 15.2 kcal/mol (BP86) above the global minimum Co-1S. The (η5-C5H5)Co(η2,2C8H8) structure (Co-2S) is the structure found experimentally and characterized by X-ray diffraction,8 even though it is predicted to lie ∼4 kcal/mol above the global minimum. However, the two C5H5CoC8H8 structures Co-1S and Co-2S appear to be in equilibrium in solution,10-12 consistent with their small energy difference. Two triplet spin state structures are found for C5H5CoC8H8. The lowest lying triplet structure Co-4T (Figure 9 and Table 6) has a trihapto η3-C8H8 ring and lies 8.7 kcal/mol

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Table 7. Bond Distances (in A˚), Total Energies (E in Hartree), Relative Energies (ΔE in kcal/mol), and HOMO-LUMO Gaps for the C5H5NiC8H8 Structures Ni-1D (C1)

Ni-C8H8 Ni-C5H5 -HOMO(R) -LUMO(R) gap/eV -E ΔE ÆS2æ

Ni-2Q (C1)

B3LYP

BP86

B3LYP

BP86

3.189 2.189 0.18415 0.07871 2.87 2011.56004 0.0 0.79

2.911 2.143 0.13288 0.11217 0.564 2011.79013 0.0 0.76

3.192 2.199 0.16766 0.10881 1.60 2011.53585 15.2 3.78

3.172 2.165 0.13683 0.00705 3.53 2011.75139 24.3 3.76

(B3LYP) or 22.8 kcal/mol (BP86) above the global minimum Co-1S. The trihapto η3-C8H8 ring gives the cobalt atom in Co-4T a 17-electron configuration, thereby accounting for one of the two unpaired electrons in the triplet spin state. The second unpaired electron in Co-4T is apparently delocalized in the five carbon atoms (of the η3-C8H8 ring) not bonded to the cobalt atom. A second triplet C5H5CoC8H8 structure (Co-5T) is also found. However, the B3LYP and BP86 methods predict different Co-C2 and Co-C9 distances in the Co-5T structures starting from the same initial geometry (Figure 9 and Table 6). The predicted Co-C2 and Co-C9 distances are 2.208 and 2.069 A˚ by BP86. Starting from the BP86 Co-5T structure and optimizing it with B3LYP lengthens the Co-C2 and Co-C9 distances to 2.968 A˚. Thus the B3LYP method predicts a (η5-C5H5)Co(η2-C8H8) structure for Co5T with a dihapto η2-C8H8 ring and a 16-electron cobalt configuration lying 6.7 kcal/mol above the global minimum Co-1S. However, the BP86 method predicts a (η5-C5H5)Co(η2,2-C8H8) structure for Co-5T with a tetrahapto η2,2-C8H8 ring lying 25.9 kcal/mol above Co-1S. The chair conformation of the 1,2,5,6-η2,2-C8H8 ring in the triplet BP86 structure Co-5T is very different from the boat conformation of the 1,2,5,6-η2,2-C8H8 ring in the singlet structure Co-2S even though the η2,2-C8H8 rings are bonded to the cobalt atoms in Co-2S and Co-5T in the same manner. 3.1.7. C5H5NiC8H8. Doublet and quartet (η5-C5H5)Ni2 (η -C8H8) structures are found (Figure 10 and Table 7). Both structures (Ni-1D and Ni-2Q) are true minima without any imaginary vibrational frequencies. The global minimum is the doublet Ni-1D, (η5-C5H5)Ni(η2-C8H8), in which the η5-C5H5 ring is planar but the C8H8 ring is nonplanar with three conjugated uncomplexed CdC double bonds. The nickel atom in Ni-1D has a 17-electron configuration. The quartet (η5-C5H5)Ni(η2-C8H8) (Ni-2Q) lies 15.2 kcal/mol (B3LYP) or 24.3 kcal/mol (BP86) above the doublet Ni1D. The quartet Ni-2Q also has a dihapto η2-C8H8 ring, but this ring is planar, unlike that in Ni-1S. 3.2. Orbital Analysis. Figure 11 show the frontier molecular orbitals of the (η5-C5H5)M(η8-C8H8) complexes with parallel rings, namely, Ti-1D (Figure 4), and V-2S (Figure 5). These are seen to involve primarily the metal d orbitals, with the MO incorporating the z2 orbital being of the highest energy. This MO is occupied by a single electron (i.e., the SOMO) in the 17-electron complex (η5-C5H5)Ti(η8-C8H8) and filled by a lone pair (i.e., the HOMO) in the 18-electron complex (η5-C5H5)V(η8-C8H8). The occupancy of the z2 orbital by a single electron in (η5-C5H5)Ti(η8-C8H8) (Ti-1D in Figure 4) is consistent with the experimentally observed electron spin resonance spectrum.14

Figure 11. Frontier molecular orbitals of (η5-C5H5)Ti(η8C8H8) and (η5-C5H5)V(η8-C8H8).

The two (η5-C5H5)M(η8-C8H8) MOs energetically below the z2 MO, namely, the HOMO-1 and HOMO-2 orbitals, involve the x2-y2 and xy orbitals, respectively, which are of suitable symmetry for δ bonding to the two rings. Figure 11 shows that this δ bonding from the metal atom occurs almost entirely to the eight-membered ring, with essentially no δ bonding to the five-membered ring. This is consistent with the availability of two electron pairs in the essentially degenerate e2 orbitals of δ symmetry in the C8H82- ring for such δ bonding to the metal atom. The two MOs below those involving the x2-y2 and xy orbitals, namely, the HOMO-3 and HOMO-4 orbitals, involve the xz and yz orbitals, respectively, which are of suitable symmetry for π bonding to the two rings. Figure 11 shows that this π bonding occurs almost entirely to the five-membered ring with essentially no π bonding from these orbitals to the eight-membered ring.

4. Conclusions Theoretical studies on the C5H5MC8H8 complexes predict the titanium derivative to be the 17-electron parallel ring sandwich (η5-C5H5)Ti(η8-C8H8), in accord with experimental observations.13 However, the lowest energy structure of the vanadium derivative is not the expected 18-electron analogous parallel ring sandwich (η5-C5H5)V(η8-C8H8), but instead a 16electron structure (η5-C5H5)V(η6-C8H8) containing a hexahapto nonplanar C8H8 ring with an uncomplexed CdC double bond. The observation of the octahapto sandwich structure (η5-C5H5)Ti(η8-C8H8) as the global minimum for the titanium derivative but the corresponding vanadium derivative (η5C5H5)V(η8-C8H8) as only a higher energy structure may relate to the larger covalent radius36 of titanium (1.40 A˚) relative to vanadium (1.35 A˚). This smaller vanadium atom cannot be at an optimum bonding distance to all eight carbon atoms of the C8H8 ring, so a hexahapto structure (η5-C5H5)V(η6-C8H8) becomes more favorable than the octahapto structure (η5C5H5)V(η8-C8H8) even though the octahapto structure is the one with the favorable 18-electron configuration. These observations suggest that octahapto planar ring η8C8H8 metal complexes are favorable only for the earliest transition metals with the largest atomic radii. Furthermore, the instability of the 18-electron octahapto (η5-C5H5)V(η8-C8H8) complex relative to the 16-electron hexahapto (36) Slater, J. C. J. Chem. Phys. 1964, 41, 3199.

Article

(η5-C5H5)V(η6-C8H8) complex may account for the difficulty in synthesizing C5H5VC8H8 from (η5-C5H5)V(CO)4 and cyclooctatetraene, even though the 17-electron parallel ring sandwich (η5-C5H5)V(η7-C7H7) can be synthesized readily from a thermal reaction of (η5-C5H5)V(CO)4 with cycloheptatriene.17 Furthermore, a [C5H5VC8H8]2 dimer (Figure 3)18 is known in which the eight-membered rings bond to the vanadium atom through only seven rather than eight carbon atoms. Chromium and manganese resemble vanadium in that analogous (η5-C5H5)M(η6-C8H8) (M = Cr, Mn) structures with hexahapto nonplanar C8H8 rings are the lowest energy structures for these metals. The chromium complex (η5C5H5)Cr(η6-C8H8), with a 17-electron configuration similar to the known17 vanadium cycloheptatrienyl sandwich complex (η5-C5H5)V(η7-C7H7), has been synthesized and characterized structurally by X-ray diffraction.15 The manganese complex (η5-C5H5)Mn(η6-C8H8) with the favorable 18-electron configuration has also been synthesized but not yet characterized structurally.16 The later transition metals, namely, iron, cobalt, and nickel, require fewer electrons from the C8H8 ring in their C5H5MC8H8 complexes to approach the favored 18-electron configuration. Thus, structures with tetrahapto C8H8 rings, having two uncomplexed CdC double bonds, are the lowest energy structures for the iron and cobalt derivatives. The lowest energy structure for the iron derivative is the 17-electron complex (η5-C5H5)Fe(η4-C8H8), in which four adjacent carbon atoms of the C8H8 ring are bonded to the iron atom, leaving two conjugated uncomplexed CdC double bonds. For the cobalt derivative, two isomeric 18-electron structures with tetrahapto C8H8 rings are found within ∼5 kcal/mol of each other. This is consistent with the experimental observation of two C5H5CoC8H8 isomers in equilibrium in solution.10-12 The lowest energy structure is (η5-C5H5)Co(η4-C8H8) in which four adjacent carbon atoms of the C8H8 ring are bonded to the (37) Ondracek, J.; Schehlmann, V.; Maixner, J.; Kratochvil, B. Collect. Czech. Chem. Commun. 1990, 55, 2447.

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cobalt atom. However, the experimentally observed structure in the crystalline state8 is (η5-C5H5)Co(η2,2-C8H8) in which two nonadjacent CdC double bonds are bonded to the cobalt atom, leaving two uncomplexed nonadjacent CdC double bonds. In (η5-C5H5)Co(η2,2-C8H8) the tetrahapto bonding of the eight-membered ring to the cobalt atom is similar to the tetrahapto bonding of the eight-membered 1,5-cyclooctadiene ring to the cobalt atom in the very stable37 (η5-C5H5)Co(η2,2C8H12). The third possible tetrahapto C5H5CoC8H8 structure, namely, (η5-C5H5)Co(η3,1-C8H8), in which the C8H8 bonds to the cobalt atom through a three-carbon η3-allylic unit and a nonadjacent single carbon atom with two uncomplexed nonadjacent CdC double bonds, is also found, but it lies at a considerably higher energy. Finally, for the nickel complex a 17-electron structure (η5-C5H5)Ni(η2-C8H8), with an uncomplexed 1,3,5-triene system in the dihapto C8H8 ring, is the lowest energy structure. Among the later transition metal C5H5MC8H8 structures, neither the iron nor the nickel complex has been synthesized.

Acknowledgment. We are grateful to the China National Science Foundation (Grants 10774104 and 10974161), Sichuan Province Youth Science and Technology Foundation (2008-20-360), and the U.S. National Science Foundation (Grants CHE-0716718 and CHE074986) for support of this work. H.W. thanks the Faculty of Chemistry and Chemical Engineering of Babes- -Bolyai University in Cluj-Napoca, Romania, for providing CPU time. Supporting Information Available: Bond distances M-Cp, M-cot (in A˚) for the (C5H5)M(C8H8) (M = Ti, V, Cr, Mn, Fe, Co, Ni) structures (Tables S1 to S7); complete tables of harmonic vibrational frequencies for (C5H5)M(C8H8) (M = Ti, V, Cr, Mn, Fe, Co, Ni) structures (Tables S8 to S27); optimized coordinates for the C5H5MC8H8 (M = Ti, V, Cr, Mn, Fe, Co, Ni) structures; complete Gaussian reference (ref 30). This material is available free of charge via the Internet at http:// pubs.acs.org.