Metallocene versus Metallabenzene Isomers of Nickel, Palladium, and

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Metallocene versus Metallabenzene Isomers of Nickel, Palladium, and Platinum Yi Zeng,† Hao Feng,*,† R. Bruce King,*,‡ and Henry F. Schaefer, III‡ †

School of Physics and Chemistry, Research Center for Advanced Computation, Xihua University, Chengdu, China 610039 Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602, United States

‡

S Supporting Information *

ABSTRACT: The relative energies of singlet and triplet metallocene, metallabenzene, and metallacyclopentadiene C10H10M isomers (M = Ni, Pd, Pt) have been examined using density functional theory. For the C10H10Ni system, the experimentally known triplet nickelocene (η5Cp)2Ni is the lowest energy isomer by ∼17 kcal/mol with respect to singlet nickelocene. For the C10H10Pd system, the triplet and singlet palladocene structures have similar energies within ∼2 kcal/mol. However, the singlet palladocene has a “slipped ring” (η3-Cp)2Pd structure with two trihapto Cp rings. The C10H10Pt system is different since the platinabenzene CpPtC5H5 isomer is the lowest energy structure. This is in accord with the synthesis of stable substituted CpPtC5H3R2 platinabenzenes by Haley and co-workers [Haley, M. M.; et al. Organometallics 2004, 23, 1174]. However, the slipped ring singlet platinocene (η3-Cp)2Pt to the isomeric platinocene lies only ∼2 kcal/mol above the platinabenzene global minimum, so the energy barrier for conversion of the platinabenzene must be substantial. The following general observations can be made regarding the relative stabilities of isomeric C10H10M (M = Ni, Pd, Pt) structures: (1) Triplet structures become less favorable energetically than isomeric singlet structures in the sequence Ni < Pd < Pt. (2) Slipped metallocene structures with trihapto η3-Cp rather than pentahapto η5-Cp rings leading ultimately to 16- rather than 18-electron metal configurations become increasingly favorable energetically in the sequence Ni < Pd < Pt. (3) Metallabenzene (η5-Cp)MC5H5 structures with pentahapto Cp rings are always more favorable energetically than isomeric metallacyclopentadiene (η6-C6H6)MC4H4 structures with hexahapto benzene rings.

1. INTRODUCTION The seminal discovery of the sandwich compound ferrocene, Cp2Fe (I in Figure 1: M = Fe; Cp = η5-C5H5) in 19511,2 was

nickelocene is the least reactive toward air oxidation and can be handled in air in the solid state.8 The range of pure metallocenes Cp2M of the second and third row transition metals with no additional ligands is much more limited since such stable metallocenes follow the 18electron rule more strictly. Thus, ruthenocene and osmocene with 18-electron metal configurations are very stable molecules (I in Figure 1: M = Ru, Os). However, attempts to synthesize rhenocene, Cp2Re, with a 17-electron rhenium configuration for the rhenium atom, led instead to the hydride Cp2ReH with the favored 18-electron rhenium configuration.9 Cationic metallocenes of the second and third row transition metals with 18-electron metal configurations such as Cp2M+ (M = Rh, Ir) and Cp2M2+ (M = Pd, Pt)10−12 are also very stable species. In addition palladium forms the binuclear perpendicular complex Cp2Pd2L2 [L = P(iPr)3] in which the cyclopentadienyl ligands bridge the pair of palladium atoms.13 However, attempts to synthesize palladocene or platinocene by analogy with nickelocene starting from Pd(II) or Pt(II) salts and metal cyclopentadienides have been unsuccessful.19 Thus, until now, there are no reliable reports of the corresponding metallocenes of either Pd(II) or Pt(II). Gusev et al. have attributed the failure to synthesize such metallocenes to the 20-electron configuration of the central metal atom.10−12

Figure 1. Three types of C10H10M isomers.

soon followed by the discovery of analogous metallocenes of the other first row transition metals from vanadium to nickel. Ferrocene, with the favored 18-electron configuration3−7 for the central iron atom, was the most robust of these metallocenes and the only one stable toward air oxidation under normal conditions. However, the stability in an inert atmosphere of all of these first row transition metallocenes with metal electron configurations ranging from 15 for Cp2V to 20 for Cp2Ni clearly indicated that stable metallocenes violating the 18-electron rule could be synthesized for the first row transition metals. Among the metallocenes of the first row transition metals violating the 18-electron rule, the 20-electron © 2014 American Chemical Society

Received: September 30, 2014 Published: December 8, 2014 7193

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consistent cc-pVTZ-PP basis sets were used.60,61 With these ECPs, 28 core electrons (1s22s22p63s23p63d10) are included in the effective cores for Pd, and 60 core electrons (1s22s22p63s23p63d104s24p64d104f14) for Pt. Structural optimizations were performed using the Gaussian 09 package.62 Vibrational frequencies were determined by evaluating analytically the second derivatives of the energy with respect to the nuclear coordinates. The ultrafine grid, i.e., the pruned (99, 590) grid, was used for the computation of twoelectron integrals.63 Natural bond orbital (NBO) analyses64−66 were carried out to discuss the nature of the electronic structures for these systems. The C10H10M structures are designated as N-M-m, where N = I, II, and III for metallocene, metallabenzene, and metallacyclopentadiene isomers, respectively (Figure 1), M designates the metal (Ni, Pd, and Pt), and m (multiplicity) represents the spin state, i.e., s for singlets, and t for triplets.

Metallabenzenes, in which a CH group of benzene has been formally replaced by an isolobal transition metal and its associated ligands, have experienced a flurry of recent activity because of their interest as novel aromatic systems.14−16 Metallabenzenes in which the metal unit is a CpM unit (II in Figure 1) are isomers of metallocenes. Thorn and Hoffmann first predicted theoretically metallabenzenes in 1979.17 In that seminal work, they proposed that electronic delocalization stabilizes three hypothetical classes of metallabenzene compounds. In 1982, the first metallabenzene, an osmabenzene, was synthesized by Roper et al.18 Despite the subsequent tremendous progress made in metallabenzene chemistry during the subsequent 30 years, currently known metallabenzene derivatives are limited to late transition metals almost entirely of the second and third rows, such as Os,19,20 Ir,21 Pt,22 and Ru,23 and to recently reported examples with the middle transition metal Re.24 A number of experimental and theoretical studies show that certain metallabenzenes have low thermal stability and tend to rearrange to more thermodynamically stable cyclopentadienyl complexes.25−29 For the metallabenzenes incorporating the group 10 metals, only a single nickelabenzene was reported,30 but with the nickelabenzene ring π-bonded to ruthenium. No palladabenzene has been reported as yet. In 2004, Haley and co-workers31 first synthesized platinabenzenes and characterized them structurally using X-ray crystallography. Such platinabenzenes were found by Haley and co-workers to be thermally robust. An alternative type of metallocene isomer has a fivemembered metallacyclopentadiene (metallole) ring with a hexahapto benzene ring bonded to the ring metal atom (III in Figure 1). Such species are unknown experimentally. However, related metallacyclopentadiene rings are found in Fe,32,33 Co,34−36 Ni,37−42 Ru,43−48 Rh,49,50 and Ir51 derivatives. Such metallacyclopentadiene derivatives include the metallametallocenes CpM(C4H4M′)Cp (M, M′ = Fe, Co, Ni)52 and dinickelametallocenes (η5-CpNiC4H4)2M (M = Fe, Co, Ni)53 that have been the subjects of recent theoretical studies. In the past few years, a number of theoretical studies on metallocenes, metallabenzenes, and metallacyclopentadienes have been reported in the literature.29,54,55 These studies mainly focused on their structure and bonding, electronic structures, reactivity, and mechanisms of formation. Despite this considerable amount of research, theoretical studies comparing the relative energies of metallocene, metallabenzene, and metallacyclopentadiene isomers for the group 10 metals (Ni, Pd, Pt) are rather limited. This work reports the relative energetics of the metallocene, metallabenzene, and metallacyclopentadiene isomers of the C10H10M systems. Our objective is threefold: (1) Determination of the most favored isomer for each system; (2) elucidation of aromaticity in these compounds; and (3) analysis of the chemical bonding to unravel the reason for the different isomer energetics.

3. RESULTS AND DISCUSSION 3.1. Molecular Structures. 3.1.1. The C10H10Ni System. The nickelocenes are 20-electron open-shell systems with two pentahapto η5-Cp ligands. Such systems have two obvious very low-lying conformations, namely, eclipsed (D5h) and staggered (D5d). The total energy of the D5h structure is slightly lower by 0.12 kcal/mol than its corresponding D5d structure (Figure S1 and Table S1, Supporting Information). However, because the energy difference between D5h and D5d is so small, the D5d conformation may be observed under certain conditions. The well-known ferrocene is found to be a D5h closed-shell molecule in the gas phase, but ferrocene apparently favors the D5d conformation in the condensed phase. The energy difference including zero-point vibrational energies between D5h and D5d for ferrocene at the M06-L/cc-PVTZ was predicted to be 0.8 kcal/mol, in good agreement with the experimental value of 0.9 ± 0.3 kcal/mol.67−69 The closed-shell Cs singlet nickelocene I-Ni-s lies 17.4 kcal/mol higher in energy than the triplet spin state isomer I-Ni-t (D5h). The Ni−C distances in I-Ni-s (Figure 2) clearly indicate one pentahapto η5-Cp and one trihapto η3Cp ligand for the Ni atom, thereby giving the Ni atom the favored 18-electron configuration. The uncomplexed CC double bond length of 1.357 Å in the trihapto η3-Cp ligand in I-Ni-s is significantly shorter than any other of the C−C bonds in the Cp rings. The singlet nickelabenzene II-Ni-s is a high energy C10H10Ni isomer, lying 37.4 kcal/mol above the triplet nickelocene I-Ni-t, suggesting facile rearrangement to the nickelocene (Figure 2). The isomeric singlet nickelacyclopentadiene structure III-Ni-s is an even higher energy structure at 47.6 kcal/mol above I-Ni-t. The central C−Ni−C angle of the five-membered NiC4 ring in III-Ni-s (81.9°) is smaller than that of the six-membered NiC5 ring in II-Ni-s (90.8°). The Ni atoms in II-Ni-s and III-Ni-s both have the favored 18-electron configuration. The triplet spin state nickelabenzene and nickelacyclopentadiene isomers are even higher energy structures, lying 62.4 and 74.4 kcal/mol above I-Ni-t, respectively (Figure S1 and Table S1). 3.1.2. The C10H10Pd System. Similar to the M = Ni system, the palladocene structures have lower energies than the isomeric palladabenzene and palladacyclopentadiene structures (Figure 3). The two conformers of the triplet palladocenes, namely, the eclipsed (D5h) and staggered (D5d) structures, are predicted to be genuine minima with all real vibrational frequencies and to have the same energies within 0.02 kcal/mol

2. THEORETICAL METHODS The meta-GGAs density functional theory (DFT) method, M06-L, developed by Truhlar et al.56 was used, since it is sometimes the best functional for transition metal energetics57 and was used in previous work52,53 with metallocycles containing transition metals. For the H, C, and Ni atoms, the correlation-consistent cc-pVTZ basis sets were used.58,59 For the Pd and Pt atoms, the Stuttgart−Cologne MCDHF (multiconfiguration Dirac−Hartree−Fock adjusted) effective core potentials (ECP) and the corresponding correlation7194

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ligands corresponding to a 16-electron configuration for the Pd atom. Thus, the bonding Pd−C distances to the η3-Cp rings range from 2.12 to 2.26 Å, whereas the nonbonding Pd···C distances range from 2.56 to 2.75 Å. The uncomplexed CC double bonds in the η3-Cp rings are relatively short at 1.35− 1.37 Å. The palladium environment in the (η3-Cp)2Pd structure I-Pd-s is thus similar to the known70 bis(allyl)palladium (η3-C3H5)2Pd. As for nickel, the singlet palladabenzene structure II-Pd-s is a significantly higher energy C 10 H 10 Pd isomer, lying 17.1 kcal/mol above the palladocene I-Pd-t (Figure 3). This suggests facile rearrangement of the palladabenzene II-Pd-s to the isomeric palladocene. The isomeric singlet palladacyclopentadiene III-Pd-s is a somewhat higher energy structure, lying 19.8 kcal/mol above the palladocene I-Pd-t. The central C−Pd−C angle of the five-membered PdC4 ring in III-Pd-s (78.5°) is smaller than that of the six-membered PdC5 ring in II-Pd-s (87.5°). The Pd atoms in II-Pd-s and III-Pd-s both have the favored 18-electron configuration. The triplet palladabenzene and palladacyclopentadiene isomers are much higher energy structures, lying 49.1 and 55.3 kcal/mol in energy above I-Pd-t, respectively (Figure S2 and Table S2). 3.1.3. The C10H10Pt System. The platinum C10H10Pt system differs from its palladium and nickel analogues C10H10M (M = Ni, Pd), since the singlet platinabenzene II-Pt-s is the lowest energy isomer, lying 2.1 and 13.3 kcal/mol in energy below the isomeric singlet platinocene I-Pt-s and platinacyclopentadiene III-Pt-s, structures, respectively (Figure 4). This is in good agreement with the synthesis and structural characterization of the substituted platinabenzenes (η5-Me5C5)PtC5H3PhR (R =

Figure 2. Equilibrium geometries and relative energies (kcal/mol in parentheses) for four C10H10Ni isomers.

Figure 3. Equilibrium geometries and relative energies (kcal/mol) for the C10H10Pd isomers.

confirming a fluxional system (Figure S2 and Table S2, Supporting Information). The two pentahapto η5-Cp ligands with Pd−C distances of 2.379 Å for the two triplet palladocenes give the Pd atoms an electron-rich 20-electron configuration, analogous to the experimentally known nickelocene (structure I-Ni-t in Figure 2). The singlet Cs I-Pd-s lies only ∼2 kcal/mol higher in energy above the triplet structures, suggesting a system with interesting magnetic properties. The Pd−C distances in I-Pd-s (Figure 2) indicate two trihapto η3-Cp

Figure 4. Equilibrium geometries and relative energies (kcal/mol) for the C10H10Pt isomers. 7195

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Ph and tBu) derived from the parent C10H10Pt by replacing all of the hydrogen atoms on the Cp ring with methyl groups and two of the hydrogen atoms in the PtC5 ring by organic groups. In the platinabenzene ring of II-Pt-s, the two predicted Pt−C distances of ∼1.95 Å and the central C−Pt−C angle of 87.7° are close to those in the palladium analogue II-Pd-s. In addition, they are close to the experimental values of ∼1.95 Å and 89.5°, respectively, found in the substituted platinabenzenes (η5-Me5C5)PtC5H3PhR (R = Ph and tBu) using X-ray crystallography.31 Similarly, in the platinacyclopentadiene ring of III-Pt-s, the Pt−C distances of 1.997 Å and the central C−Pt−C angle of 78.6° are close to those in the palladium analogue III-Pd-s. The singlet platinocene structure I-Pt-s lies ∼13 kcal/mol in energy below the isomeric triplet platinocene structure 1-Pt-t (Figure 4). The Pt−C distances in I-Pt-s indicate two trihapto η3-Cp rings to provide the Pt atom with a 16-electron configuration similar to the known70−72 bis(allyl)platinum (η3-C3H5)2Pt. The triplet platinocene structures I-Pt-t have dihapto η2-Cp and pentahapto η5-Cp rings to give the Pt atom a 17-electron configuration to account for one of the unpaired electrons in the triplet spin state. The other unpaired electron in the triplet I-Pt-t can be located at least formally on the dihapto η2-Cp ring. The D5h and D5d metallocene structures with two pentahapto η5-Cp rings are predicted to be transition states with large imaginary frequencies, lying ∼17 kcal/mol above I-Pt-s (Figure S3 and Table S3, Supporting Information). 3.2. Aromaticity Analysis. The NICS (nucleus-independent chemical shifts) proposed by Schleyer et al.73 was adopted as an effective aromaticity criterion. NICS is determined as the negative value of the absolute shielding value computed at a ring center or at some other out-of-plane points of the ring. The more negative the NICS values, the more aromatic the ring is. In this work, the NICS values are computed at the center of the five-membered ring (5-MR) of the cyclopentadienyl ligand for the metallocene (I-type) and the metallacyclopentadiene (III-type) structures, and the six-membered ring (6-MR) of the metallabenzene (II-type) structures. For the isomeric metallocenes, metallabenzenes and metallacyclopentadienes, the NICS values follow the sequence: −24.3 ppm (I-Ni-t) < −4.8 ppm (II-Ni-s) < 3.9 ppm (III-Ni-s), −18.2 ppm (I-Pd-t) < −3.7 ppm (II-Pd-s) < 4.8 ppm (III-Pd-s), and −4.3 ppm (IIPt-s) < −2.0 ppm (I-Pt-s) < 2.6 ppm (III-Pt-s), in agreement with increasing energies. In all cases, the metallacyclopentadiene (III-type) structures are slightly antiaromatic, as indicated by the slightly positive NICS values. 3.3. Frontier Molecular Orbitals. In order to provide some insight into the bonding of the metallocenes, metallabenzenes, and metalloles, their frontier molecular orbitals (MOs) were investigated (Figure 5). Two electrons occupy the degenerate SOMO of the D5h triplet spin state metallocene structures I-M-t. This SOMO involves antibonding π interactions of the dyz orbitals of the transition metal with the π orbitals to the two Cp rings. These two metal electrons are in excess of the favored 18-electron metal configuration and weaken the metal−ring interactions relative to the 18-electron metallocenes Cp2M (M = Fe, Ru, Os). The degenerate LUMO for I-M-t incorporates antibonding interactions of the dxy/dx2−y2 metal orbital set with the δ orbitals of the Cp rings. The singlet metallocene structures I-M-s have lower Cs symmetry than the corresponding triplet structures. In the singlet structures, one of the Cp rings is effectively bonded as a

Figure 5. Frontier molecular orbitals of the C10H10M isomers.

trihapto η3-Cp ligand rather than the usual pentahapto η5-Cp ligand (Figure 5). The HOMOs of the I-M-s structures are pure metal dz2 orbitals having antibonding interactions with the Cp rings. The LUMOs of the singlet metallocene structures I-M-s correspond to similar antibonding interactions of metal dxz or dyz orbitals with Cp ring orbitals. The frontier MOs of the metallabenzenes II-M-s involve interactions of the metal dxz or dyz orbitals with π orbitals of the carbon atoms of the metallabenzene ring (Figure 5). The HOMOs represent bonding interactions contributing to the π-orbital systems of the metallabenzene rings. The LUMOs represent similar antibonding interactions. The frontier MOs of the metallacyclopentadienes III-M-s are very different than those of the metallabenzenes (Figure 5). Thus, the HOMOs correspond to antibonding interactions of metal d orbitals with the π-systems of the metallacyclopentadiene rings, there being no involvement of the η6-benzene ring. Conversely, the LUMOs of III-M-s involve purely δ* antibonding interactions with the δ orbitals of the η6-benzene π-system with no involvement of the metallacyclopentadiene ring.

4. CONCLUSIONS The triplet nickelocene (η5-Cp)2Ni (I-Ni-t) is the lowest energy C10H10Ni structure, in accord with the synthesis of triplet nickelocene as a stable compound (Figure 2). The corresponding singlet Cp2Ni structure I-Ni-s, lying ∼17 kcal/ mol in energy above the triplet (η5-Cp)2Ni structure, has a “slipped” Cp ring corresponding to trihapto rather than pentahapto bonding to the nickel atom. The resulting singlet (η3-Cp)(η5-Cp)Ni structure has the favored 18-electron configuration. The nickelabenzene (η5-Cp)NiC5H5 (II-Ni-s) and nickelacyclopentadiene (η6-C6H6)NiC4H4 (III-Ni-s) isomers have much higher energies of ∼37 and ∼47 kcal/mol, respectively, above triplet (η5-Cp)2Ni, and thus, are not predicted to be involved in the chemistry of C10H10Ni. The situation is rather different with the C10H10Pd isomers (Figure 3). The triplet and singlet Cp2Pd metallocenes (I-Pd-t and I-Pd-s) have similar energies within ∼2 kcal/mol (Figure 3). The triplet C10H10Pd structure (I-Pd-t) is a true η5-Cp2Pd metallocene with D5h symmetry. However, in the singlet Cp2Pd metallocene (I-Pd-s), both Cp rings have “slipped” to form 7196

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Organometallics trihapto ligands. The palladium atom in the resulting (η3-Cp)2Pd structure has a 16-electron configuration similar to the palladium atom in the known diallylpalladium, (η3-C3H5)2Pd.70 The isomeric palladabenzene (II-Pd-s) and palladacyclopentadiene (III-Pd-s) structures are relatively high energy structures lying ∼17 and ∼20 kcal/mol, respectively, above triplet (η5-Cp)2Pd. The platinum C10H10Pt system is markedly different from the nickel and palladium analogues since the platinabenzene (η5-Cp)PtC5H5 (II-Pt-s) is the lowest energy structure, albeit by only ∼2 kcal/mol (Figure 4). This is consistent with the experimental isolation of stable substituted (η5-Me5C5)PtC5H3R2 platinabenzene derivatives by Haley and co-workers and suggests significant activation energies for the isomerization of platinabenzenes. 31 The predicted geometry of the platinabenzene ring in II-Pt-s is very similar to that found by X-ray crystallography for the substituted platinabenzenes. The singlet platinocene (η3-Cp)2Pt (I-Pt-s), lying only ∼2 kcal/mol above the isomeric platinabenzene (η5-Cp)PtC5H5 (II-Pt-s), has a “double slipped ring” structure with two trihapto η3-Cp rings similar to its palladium analogue. The isomeric platinacyclopentadiene (η6-C6H6)PtC4H4 (III-Pt-s) and triplet metallocene Cp2Pt (I-Pt-t) structures lie at the significantly higher energies of ∼13 and ∼15 kcal/mol, respectively. The relative stabilities of the C10H10M (M = Ni, Pd, Pt) isomers can be summarized by the following observations: (1) Triplet structures become less favorable energetically than isomeric singlet structures in the sequence Ni < Pd < Pt. (2) Slipped metallocene structures with trihapto η3-Cp rather than pentahapto η5-Cp rings leading ultimately to 16rather than 18-electron metal configurations become increasingly favorable energetically in the sequence Ni < Pd < Pt. (3) Metallabenzene (η5-Cp)MC5H5 structures with pentahapto Cp rings are always more favorable energetically than isomeric metallacyclopentadiene (η6-C6H6)MC4H4 structures with hexahapto benzene rings.

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REFERENCES

This research was supported by the National Natural Science Foundation of China (21403170), Chunhui Plan of Ministry of Education of China (Z2014063), the Open Research Subject of Key Laboratory of Research Center for Advanced Computation (szjj2014-082), New Century Excellent Talents in University of China (NCET-10-0949), and the U.S. National Science Foundation (Grants 389CHE-1057466, CHE-1054286, and CHE-1361178).

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S3: Singlet and triplet geometries for the C10H10M (M = Ni, Pd, Pt) systems. Tables S1−S3: Total energies (E, in hartree), relative energies (ΔE), zero-point energies (ZPE), enthalpies (ΔH), free energies (ΔG, in kcal/mol), and spin expectation values ⟨S2⟩ for the C10H10M systems (M = Ni, Pd, Pt) predicted by the M06-L method. Tables S4−S25: Optimized coordinates of the C10H10M (M = Ni, Pd, Pt) structures. Tables S26−S47: Harmonic vibrational frequencies (in cm−1) and infrared intensities (in parentheses, in km/mol) for the C10H10M (M = Ni, Pd, Pt) structures. Complete Gaussian 09 reference. This material is available free of charge via the Internet at http://pubs.acs.org.

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Corresponding Authors

*E-mail: [email protected] (R.B.K.). *E-mail: [email protected] (H.F.). Notes

The authors declare no competing financial interest. 7197

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