Dinickelametallocenes: Sandwich Compounds of the First-Row

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Dinickelametallocenes: Sandwich Compounds of the First-Row Transition Metals (M = Fe, Co, Ni) with Two Pentahapto Planar Nickelacycle Ligands 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 610039, China Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602, United States

‡

S Supporting Information *

ABSTRACT: Buchalski and co-workers showed in 2008 that reactions of the nickelafluorenyl anion CpNiC12H8− with MX2 (M = Co, Ni) give the dinickelametallocene sandwich compounds (CpNiC12H8)2M. We now report theoretical studies on the related bis(nickelacyclopentadienyl)metal derivatives (CpNiC4H4)2M and bis(nickelaindenyl)metal derivatives (CpNiC8H6)2M as well as the bis(nickelafluorenyl)metal derivatives. The structures of the lowest energy bis(nickelacyclopentadienyl) sandwich compounds (CpNiC4H4)2M may be derived from those of the corresponding metallocenes Cp2M by replacing a CH group in each Cp ring with an isolobal CpNi unit. The Ni−M distances of ∼2.5 Å indicate formal single bonds and thus a pentahapto η5-CpNiC4H4 ligand. The spin states of the lowest energy (CpNiC4H4)2M derivatives are similar to those of the corresponding metallocenes Cp2M, namely, singlet, doublet, and triplet for M = Fe, Co, and Ni, respectively. Fusion of benzene rings to the nickelacyclopentadienyl rings to give first the bis(nickelaindene) sandwich compounds (CpNiC8H6)2M and then the experimentally known bis(nickelafluorene) sandwich compounds (CpNiC12H8)2M lowers the energy of the higher spin state. As a result, the lowest energy (CpNiC12H8)2Co structure is not the doublet spin state of Cp2Co and (CpNiC4H4)2Co but instead a quartet spin state. This is in accord with experimental work showing (CpNiC12H8)2Co to have a magnetic moment of ∼3.7 μB, indicating three unpaired electrons and thus the predicted quartet spin state. central atoms (Figure 2).20 Their Ni−Ni distances of ∼2.4 Å and the Ni−Co distances of ∼2.37 Å determined by X-ray

1. INTRODUCTION Metallametallocenes are compounds in which one or both cyclopentadienyl groups of a metallocene are replaced by a metallacyclopentadiene ring (Figure 1). The first example of

Figure 2. Three types of sandwich dinickelametallocenes (M = Fe, Co, Ni). Some cyclopentadienyl (Cp) and fluorenyl (Fl) derivatives were synthesized by Buchalski and co-workers and characterized structurally by X-ray crystallography.20

Figure 1. Metallacyclopentadienes as ligands in sandwich compounds.

such a metallametallocene was the cobalt derivative (CpCoC4H4)CoCp (Cp = η5-C5H5), first reported in 1972 by Rosenblum and co-workers as one of several products from the photolysis of CpCo(CO)2 with α-pyrone.1 Subsequently a great variety of metallametallocenes with just one metallacycle have been synthesized and structurally characterized. Metals found in such metallacycle derivatives include ruthenium,2−7cobalt,8−10 rhodium,9,11,12 iridium,13 and nickel.14−19 In addition, sandwich compounds containing two nickelafluorenyl rings around a central metal atom have been synthesized by Buchalski and co-workers.20 Most metallametallocenes contain only one metallacycle. The few dimetallametallocenes with two metallacycles that have been reported include the dinickelametallocenes (η 5 CpNiC12H8)2M (M = Co, Ni) with cobalt and nickel as © XXXX American Chemical Society

crystallography suggested formal metal−metal single bonds. A key to the synthesis of these dinickelametallocenes is the availability of the corresponding nickelacyclopentadienide or nickelafluorenide anion from nickelocene and the corresponding cyclopentadienide or fluorenide anions. Reactions of such nickelacyclic anions with transition metal halides give the corresponding dinickelametallocenes. Such syntheses parallel the original syntheses of metallocenes from reactions of sodium or lithium cyclopentadienide with metal halides. Received: May 28, 2014

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dx.doi.org/10.1021/om500574m | Organometallics XXXX, XXX, XXX−XXX

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Table 1. M−Ni Distances, NPA Natural Atomic Charges, Metal Electron Configurations, Traditional Formal M−Ni Bond Orders (M Is the Exocyclic Metal), Wiberg Bond Indices (WBIs), HOMO and LUMO Energies (in eV), and HOMO−LUMO Gaps (in eV) for All 18 Structures Obtained by the M06-L Methoda metal electron configuration

NPA NiCpFe-S NiCpFe-T NiInFe-S NiInFe-T NiFlFe-S NiFlFe-T NiCpCo-D NiCpCo-Q NiInCo-D NiInCo-Q NiFlCo-D NiFlCo-Q NiCpNi-S NiCpNi-T NiInNi-S NiInNi-T NiFlNi-S NiFlNi-T a

C2 C2 Ci Ci Ci Ci C2 C2 Ci Ci Ci Ci C2 C2 Ci Ci Ci Ci

M−Ni bond length

M

Ni

M

Ni

WBI for M−Ni

formal bond order

HOMO

LUMO

gap

2.393 2.454 2.412 2.419 2.425 2.412 2.473 2.424 2.438 2.429 2.416 2.394 2.567 2.467 2.396 2.447 2.383 2.420

0.09 0.49 0.19 0.56 0.27 0.65 0.47 0.91 0.55 0.95 0.66 0.99 0.75 0.84 0.72 0.90 0.75 0.93

0.75 0.71 0.75 0.72 0.76 0.74 0.72 0.69 0.73 0.71 0.76 0.71 0.70 0.74 0.72 0.72 0.76 0.73

18 18 18 18 18 18 19 19 19 19 19 19 16 20 20 20 20 20

18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18

0.20 0.10 0.23 0.15 0.23 0.17 0.10 0.11 0.13 0.11 0.19 0.12 0.06 0.09 0.15 0.10 0.20 0.10

1 1 1 1 1 1 1 1 1 1 1 1 0 1 1 1 1 1

−4.29 −3.39 −4.06 −3.78 −3.96 −3.89 −3.51 −3.56 −3.65 −3.85 −3.85 −4.05 −3.45 −3.62 −3.69 −3.88 −3.69 −4.01

−2.39 −2.25 −2.51 −2.77 −2.65 −3.29 −2.21 −1.87 −2.87 −1.97 −3.38 −1.96 −2.52 −1.91 −3.11 −1.96 −3.43 −1.86

1.90 1.14 1.54 1.01 1.31 0.59 1.31 1.70 0.78 1.88 0.47 2.09 0.93 1.71 0.57 1.92 0.26 2.15

Global minima structures are in italics. closed-shell structures and the unrestricted approaches for open-shell structures. Standard double-ζ plus polarization (DZP) basis sets were adopted. For hydrogen, a set of p polarization functions αp(H) = 0.75 is added to the Huzinaga−Dunning DZ sets. For carbon, one set of pure spherical harmonic d functions with orbital exponent αd(C) = 0.75 is added to the Huzinaga−Dunning standard contracted DZ sets.29,30 For Fe, Co, and Ni, our loosely contracted DZP basis set, derived from the Wachters’ primitive set,31 is used after being augmented by two sets of p functions and one set of d functions and then contracted using the method of Hood, Pitzer, and Schaefer.32 These DZP basis sets are designated as (4s1p/2s1p) for hydrogen, (9s5p1d/4s2p1d) for carbon, and (14s11p6d/10s8p3d) for Fe, Co, and Ni. Unless otherwise specified, the results discussed in this paper were obtained using these basis sets with the M06-L method. The combination of the M06-L DFT method and these double-ζ plus polarization basis sets has been shown to predict structures in transition metal chemistry that are generally consistent with experiment.33 However, in a few cases where the energy separations between different spin states were very small, the optimizations were repeated with the larger basis sets, def2-svp and def2-tzvp, using the M06-L method. The structural optimizations were performed using Gaussian09.34 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 two-electron integrals.35 In some cases, the finer grid (120, 974) was used for checking small imaginary vibrational frequencies. Natural bond orbital (NBO) analyses36−38 were carried out to provide information on the chemical bonding in these systems.

Previous work on metallametallocenes with one metallacyclopentadiene ring, CpM(C4H4M′)Cp (M and M′ = Fe, Co, Ni), showed that relatively electron-rich transition metals were preferred for the endocyclic positions in the metallacyclopentadiene ring of the heterometallic CpM(C4H4M′)Cp (M ≠ M′) structures.21 We now explore the structures and energetics of the sandwich dinickelametallocenes containing two nickelacyclic rings. These nickelacyclic rings include nickelacyclopentadienyl rings in (η5-CpNiC4H4)2M, 1-nickelaindenyl rings in (η5-CpNiC8H6)2M, and 9-nickelafluorenyl rings in (η5-CpNiC12H8)2M (M = Fe, Co, Ni) related to the species studied by Buchalski and co-workers.20 In these sandwich compounds, the central metal atom is coordinated simultaneously to both nickelacycles, and the endocyclic nickel atoms in the nickelacycles are bonded to external cyclopentadienyl ligands (Figure 2). The optimized structures are designated NiRM-m, where NiR represents the nickelacyclic rings, namely, NiCp for nickelacyclopentadienyl, NiIn for 1nickelaindenyl, and NiFl for 9-nickelafluorenyl; M represents the exocyclic metal (M = Fe, Co, Ni); and m (multiplicity) represents the spin state, i.e., S for singlets, D for doublets, T for triplets, and Q for quartets.

2. THEORETICAL METHODS Three DFT methods were used in this work. The popular B3LYP22 method combines the three-parameter Becke functional (B3)23 with the Lee−Yang−Parr generalized gradient correlation functional (LYP).24 The BP86 method combines Becke’s 1988 exchange functional (B) with Perdew’s 1986 gradient-corrected correlation functional (P86).25,26 The B3LYP and BP86 methods predict geometries in generally good agreement with each other. However, these two DFT methods predict rather different singlet−triplet separations. Usually B3LYP prefers high-spin states, whereas BP86 prefers low-spin states.27 Therefore, in addition, the meta-GGA density functional theory (DFT) method, M06-L, developed by Truhlar and Zhao28 was used. In general, the restricted approaches are used for

3. RESULTS AND DISCUSSION 3.1. Wiberg Bond Indices as Indicators of Metal− Nickel Bonding. The Wiberg bond indices (WBIs) of the metal−nickel bonds as determined by NBO analyses are shown to correlate roughly with the formal bond orders suggested by the M−Ni distances and electron counting (Table 1). Thus, WBI values of 0.09 to 0.20 are taken as indicators of formal single bonds corresponding to M−Ni distances ranging from B


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Figure 3. Equilibrium geometries and relative energies (kcal/mol) for the Ni−Fe−Ni systems (hydrogens omitted for clarity).

3.2. Iron Sandwiches with Nickelacyclic Rings. The three singlet structures NiCpFe-S, NiInFe-S, and NiFlFe-S are the global minima for the NiCpFe, NiInFe, and NiFlFe systems (Figure 3). The singlet NiCpFe-S is predicted to lie 16.5 kcal/ mol in energy below the corresponding triplet NiCpFe-T, consistent with our previous report21 that the singlet CpFe(C4H4Ni)Cp (FeNi-S) lies 15.0 kcal/mol in energy below the corresponding triplet FeNi-T. This singlet−triplet splitting decreases upon fusion of benzene rings to the nickelacycle. Thus, NiInFe-S and NiFlFe-S lie only 5.0 and 0.5 kcal/mol below their corresponding triplet structures, respectively, suggesting interesting magnetic properties. The small energy separation of 0.5 kcal/mol between NiFlFe-S and NiFlFe-T becomes 1.2 or 1.3 kcal/mol using the def2-svp or def2-tzvp basis sets, respectively. The HOMO−LUMO gaps for the singlets NiCpFe-S, NiInFe-S, and NiFlFe-S (1.90 to 1.31 eV) and the triplets NiCpFe-T, NiInFe-T, and NiFlFe-T (1.14 to 0.59 eV) both decrease monotonically upon fusion of benzene rings to the nickelacyclopentadienyl rings. The predicted Fe−Ni bond distances of 2.393, 2.412, and 2.419 Å in NiCpFe-S, NiInFe-S, and NiFlFe-S, respectively,

2.39 to 2.47 Å. A lower WBI of 0.06 with a correspondingly longer M−Ni distance of ∼2.57 Å in NiCpNi-S is taken as an indicator of no significant direct metal−metal bonding. In previous theoretical studies of the metallametallocenes with one metallacyclopentadienyl ring, namely, CpM′(C4H4M)Cp (M, M′ = Fe, Co, Ni), WBIs of 0.22 to 0.27 were found for systems with formal metal−metal single bonds.21 Lower WBIs of 0.08 to 0.12 were taken to imply a lack of significant metal−metal bonding. Previous studies of WBIs in metal−metal bonded derivatives of the first-row transition metals such as iron39 indicate typical values of 0.2 to 0.3 for unbridged formal metal− metal single bonds. The WBIs of bridged metal−metal single bonds can be even lower such as an Fe−Fe WBI of 0.11 in Fe2(CO)9 owing to the role played by multicenter bonding in many of these highly bridged systems. 40,41 A similar phenomenon can occur in the dinickelametallocene sandwich compounds discussed in this paper since the M−Ni bonds in these structures are effectively bridged by hydrocarbon units. Therefore, the use of WBI values to ascertain formal metal− metal bond orders must be used with caution in these systems. C


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Figure 4. Equilibrium geometries and relative energies (kcal/mol) for Ni−Co−Ni systems.

Using the def2-svp or def2-tzvp basis sets for NiInCo-Q/ NiInCo-D, the energy difference is predicted to be 0.05 or 0.3 kcal/mol, and the Gibbs free energy difference to be 0.8 or 1.5 kcal/mol, respectively. This prediction of a quartet ground state for NiFlCo-Q is consistent with the experimental observation of a magnetic moment of 3.704 μB, corresponding to a quartet ground state with three unpaired electrons.20 The predicted Co−Ni distances for the cobalt sandwiches with nickelacyclic rings range from 2.394 to 2.473 Å, with corresponding WBIs ranging from 0.19 to 0.10 (Table 1 and Figure 4). These can be interpreted as Co−Ni single bonds, thereby giving each cobalt atom a 19-electron configuration and each nickel atom the favored 18-electron configuration. The spin densities (Figure S2) of the six structures including the three doublet structures and the three quartet structures are localized ∼70% on the central cobalt atoms. This is consistent with their 19-electron configurations and similar to the cobalt atom in the very stable cobaltocene Cp2Co.43 The predicted Co−Ni distance of 2.394 Å in NiFlCo-Q is close to the experimental values of 2.3711(8) and 2.379(1) Å for the analogous structure determined by X-ray crystallography.20 The HOMO−LUMO gaps for the doublets decrease monotonically from 1.31 to 0.47 eV as benzene rings are fused to the nickelacyclopentadiene ring on going from

are close to the experimental value of 2.419 Å in the perphenylated CpFe(C4Ph4Ni)Cp, as determined by X-ray crystallography.42 The three Fe−Ni distances coupled with the WBI values of 0.20 to 0.23 correspond to formal single Fe−Ni bonds, thereby giving both the iron and nickel atoms the favored 18-electron configurations. The triplet structure NiCpFe-T has an Fe−Ni distance of 2.454 Å, which is longer than the corresponding singlet NiCpFe-S but still can correspond to a Fe−Ni single bond between the iron and the nickel atoms. The other two Fe−Ni distances in NiInFe-T (2.419 Å) and NiFlFe-T (2.412 Å) can also be interpreted as the formal single bonds in view of the WBIs of 0.15 and 0.17. In NiCpFe-T, NiInFe-T, and NiFlFe-T (Figure S1) the central iron atoms bear ∼90% of the spin density. 3.3. Cobalt Sandwiches with Nickelacyclic Rings. The doublet NiCpCo-D is the global minimum on the potential energy surface of the NiCpCo system, lying 25.4 kcal/mol below the quartet NiCpCo-Q (Figure 4). The doublet NiInCoD is still the global minimum for the NiInCo system, but the quartet NiInCo-Q lies only 9.8 kcal/mol above NiInCo-D. In the NiFlCo system, the global minimum shifts to the quartet NiFlCo-Q, lying 0.7 kcal/mol below the doublet NiFlCo-D. The inclusion of the entropic contribution lowers the energy of NiFlCo-Q by 2.3 kcal/mol on the potential energy surface. D


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Figure 5. Equilibrium geometries and relative energies (kcal/mol) for Ni−Ni−Ni systems.

value of 0.09, suggesting a weak Ni−Ni single bond. This gives both endocyclic (external) nickel atoms an 18-electron configuration but the central nickel atom a 20-electron configuration similar to that for the stable nickelocene Cp2Ni.44 In this connection 41% of the spin density (Figure S3) is located on the central nickel atom, in accord with its 20electron configuration. The singlet NiCpNi-S lies only 0.6, 0.5, or 1.1 kcal/mol in energy above its triplet isomer NiCpNi-T using the DZP, def2-svp, and def2-tzvp basis sets with the M06L method, respectively, thus predicting interesting magnetic properties. The Ni−Ni distance in the singlet NiCpNi-S is significantly lengthened to 2.567 Å. This longer Ni−Ni distance with the smallest WBI of 0.06 indicates the lack of a formal Ni− Ni bond. The NiInNi subgroup is also a fluxional system, since the singlet NiInNi-S lies only 2.9 kcal/mol in energy above the global minimum NiInNi-T. The bonding Ni−Ni distances in NiInNi-T (2.447 Å) and NiInNi-S (2.396 Å) correspond to WBIs of 0.10 and 0.15, respectively, thereby providing the

NiCpCo-D to NiFlCo-D. The HOMO−LUMO gap of 2.09 eV for the quartet NiFlCo-Q is the largest with the most kinetic stability, consistent with the homologous compound synthesized and isolated by Buchalski and co-workers.20 3.4. Nickel Sandwiches with Nickelacyclic Rings. The triplet structures NiCpNi-T, NiInNi-T, and NiFlNi-T are the global minima for the NiCpNi, NiInNi, and NiFlNi systems, respectively. Structure NiInNi-T has a miniscule imaginary frequency of 7i cm−1 by the M06-L method even using the finer integration grid (120, 974). However, it is a genuine minimum with all real frequencies by the B3LYP and BP86 methods. The significantly larger HOMO−LUMO gaps of the triplet structures (1.71 to 2.15 eV) relative to the singlet structures (0.93 to 0.26 eV) arise from the raising of the LUMO energies and may relate to the energetic preference of the triplet structures over isomeric singlet structures. The Ni−Ni distance of 2.467 Å for the global minimum NiCpNi-T in the NiCpNi system corresponds to a small WBI E


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central and outer nickel atoms with 20- and 18-electron configurations, respectively. The spin density (Figure S3) of NiInNi-T is mainly located 46% on the exocyclic nickel atom in accord with its 20-electron configuration. The experimentally known20 triplet NiFlNi-T structure is clearly the lowest energy structure for the NiFlNi system, lying 9.5 kcal/mol below the corresponding singlet NiFlNi-S (Figure 5). The experimental magnetic moment of 2.34 μB for NiFlNiT is close to the spin-only value of √8 = 2.83 μB for a triplet spin state. The predicted Ni−Ni distances of 2.420 Å in NiFlNi-T are reasonably close to the experimental values of 2.3981(5) and 2.3995(5) Å determined by X-ray crystallography.20 The predicted HOMO−LUMO gap of 2.15 eV for NiFlNi-T is the largest of any of the sandwich nickelametallocenes found in this work, consistent with its observed stability. The Ni−Ni distances, including the 2.383 Å distance in the singlet NiFlNi-S, correspond to the WBIs of 0.10 and 0.20 for NiFlNi-T and NiFlNi-S, respectively, indicating the formal single bonds required to give the central nickel atom a 20-electron configuration and the outer nickel atom the favored 18-electron configuration. The central nickel atom also possesses ∼49% of the spin density of NiFlNi-T, consistent with its 20-electron configuration (Figure S3).

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was supported by the Program for New Century Excellent Talents in University (Grant No. NCET-10-0949), the fund of the Key Laboratory of Advanced Scientific Computation, Xihua University, China, and the U.S. National Science Foundation (Grants CHE-1057466, CHE-1054286, and CHE-1361178).

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REFERENCES

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4. SUMMARY The structures of the lowest energy bis(nickelacyclopentadienyl) sandwich compounds (CpNiC4H4)2M may be derived from those of the corresponding metallocenes Cp2M by replacing a CH group in each Cp ring with an isolobal CpNi unit. The Ni−M distances of ∼2.5 Å indicate formal single bonds and thus a pentahapto η5CpNiC4H4 ligand. The spin states of the lowest energy (CpNiC4H4)2M derivatives are similar to those of the corresponding metallocenes Cp2M, namely, singlet, doublet, and triplet for M = Fe, Co, and Ni, respectively. Fusion of benzene rings to the nickelacyclopentadienyl rings to give first the bis(nickelaindenyl) sandwich compounds (CpNiC8H6)2M and then the experimentally known bis(nickelafluorenyl) sandwich compounds (CpNiC12H8)2M lowers the energy of the higher spin state. As a result, the lowest energy (CpNiC12H8)2Co structure is not the doublet spin state of Cp2Co and (CpNiC4H4)2Co but instead a quartet spin state. This agrees with experimental work by Buchalski and coworkers, who find (CpNiC12H8)2Co to have a magnetic moment of ∼3.7 μB, indicating three unpaired electrons and thus a quartet spin state.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1 to S3: spin density for the triplet, doublet, and quartet structures; Tables S1 to S3: metal−metal distances, total energies, relative energies, zero-point energies, enthalpies, free energies, numbers of imaginary frequencies, and spin expectation values for the 18 structures with the M06-L, B3LYP, and BP86 methods; Tables S4 to S21: optimized coordinates of the 18 structures; Tables S22 to S39: harmonic vibrational frequencies and infrared intensities for the 18 structures; complete Gaussian reference (ref 34). The .xyz file contains the computed Cartesian coordinates of all of the molecules reported in this study. This file may be opened as a text file to read the coordinates or opened directly by a molecular modeling program such as Mercury (version 3.3 or later, http://www.ccdc.cam.ac.uk/pages/Home.aspx) for visualF


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Dinickelametallocenes: Sandwich Compounds of the First-Row

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