Probing the Structural, Bonding, and Magnetic Properties of Cobalt

Nov 12, 2013 - The scatter plots of the reduced density gradient (RDG) versus ρ allow us to identify the different types of interactions, and the map...
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Probing the Structural, Bonding, and Magnetic Properties of Cobalt Coordination Complexes: Co−Benzene, Co−Pyridine, and Co− Pyrimidine Peng Shao, Xiao-Yu Kuang,* and Li-Ping Ding Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China S Supporting Information *

ABSTRACT: Neutral and anionic Co 1,2 (benzene) 1,2 , Co1,2(pyridine)1,2, and Co1,2(pyrimidine)1,2 complexes have been investigated within the framework of an all-electron gradient-corrected density functional theory. The ground-state structures for each size clusters were identified based on the geometry optimization. Meanwhile, their electron affinities and vertical detachment energies were predicted and compared with the experimental values. By analyzing the pattern of highest occupied molecular orbitals (HOMOs), we found that the bond formation of these Co−organic complexes mainly arises from the 3d/4s electrons of the cobalt atoms and the πcloud of the organic molecules. More importantly, we presented an approach to map and analyze the Co−organic interactions from another perspective. The scatter plots of the reduced density gradient (RDG) versus ρ allow us to identify the different types of interactions, and the maps of the gradient isosurfaces show a rich visualization of chemical bond and steric effects. Their magnetic properties were studied by determining the spin magnetic moments and visualizing the spin density distributions. Finally, the natural population analysis (NPA) charge was calculated to achieve a deep insight into the distribution of electron density and the reliable charge-transfer information. clusters.14,29−31 Especially, the cobalt−benzene clusters are one of the most interesting examples because of a significant exchange interaction and electron correlation of the 3d band of cobalt. Furthermore, cobalt is known to be an important catalytic metal in many industrial processes,32 including Fischer−Tropsch synthesis, in which it is combined with larger hydrocarbons. Bowen Jr. et al.18 have determined vertical detachment energies (VDEs) and adiabatic electron affinities (AEAs) of anionic ConBzm complexes by mass spectrometry and photoelectron spectroscopy. Very recently, our group33 has identified the ground-state (GS) geometries and studied the electronic and magnetic properties of ConBzm− (n ≤ 2, m ≤ 3) complexes using density functional theory (DFT). These previous studies all show that benzene molecules are unable to retain the magnetic moments of cobalt atoms. As is well-known, a substituent can modify the electronic structures and properties of a parent molecule. Azabenzene is obtained by replacing one or more CH groups in the benzene ring with a nitrogen atom. Pyridine is the simplest azabenzene with one N atom substituted, followed by diazine with the formula C4H4N2. Depending on the positions of nitrogen atoms, diazine exists in three isomers, pyrazine (1,4-diazine), pyrimidine (1,3-diazine), and pyridazine (1,2-diazine). These

1. INTRODUCTION Organometallic complexes, which consist of transition-metal (TM) clusters and various organic molecules such as benzene, cyclooctatetraene, pyrene, and coronene, have attracted considerable attention1−13 over the past few years. Lots of experimental14−20 and theoretical works10,16,17,21 have been devoted to these complexes due to their wide variety of structural and magnetic properties, as well as their broad potential applications in catalysis, polymers, and novel magnetic and optical materials. Such studies provide a convenient framework to study the interaction between the d electrons of TM atoms and π electrons of the organic molecules. On the other hand, investigations of identifying combinations of TM atoms/clusters and organic supports, which retain the high magnetic moments of these metal clusters,22 may provide significant ways toward discovering building blocks for novel magnetic materials. Among these organometallics, the TM−benzene complexes10−12 are the most studied systems as basic models for d−π bonding interactions as well as being potential precursors of extended metal−aromatic polymers. Various TM atoms supported on benzene molecules have been investigated extensively by both experiment18,19,23 and theory.11,23−30 For the early-TM atoms (Sc, Ti, V), the benzene molecules sandwich the metal atoms to form multiple-decker sandwich structures,14 while for the late-TM atoms (Fe, Co, Ni), the benzene molecules form the rice-ball structures with the TM © 2013 American Chemical Society

Received: September 7, 2013 Revised: November 6, 2013 Published: November 12, 2013 12998

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Table 1. Calculated Electron Affinities (eV) for Combenzenen, Compyridinen, and Compyrimidinen Clusters and Comparison with the Experimental Values Combenzenen EA (eV) (m, n) (0, (1, (1, (2, (2, a

1) 1) 2) 1) 2)

Compyridinen

Compyrimidinen

theo.

exp.

theo.

exp.

theo.

exp.

−1.18 0.19 0.60 0.98 0.83

−1.15a ∼0 ± 0.10b 0.50 ± 0.10b 1.31 ± 0.10b 1.00 ± 0.10b

−0.72 0.60 1.02 0.96 1.35

−0.62a 0.71c 1.06c 0.86c 1.70c

−0.28 0.95 1.44 1.19 1.34

∼0a

Reference 45. bReference 18. cReference 4.

of the clusters were relaxed fully without any symmetry constraints. Due to the spin polarization, each initial structure was optimized at various possible spin multiplicities to determine the preferred spin state. In order to confirm that the optimized geometry corresponds to a local minimum in potential energy, each of them was followed by an analysis of harmonic vibrational frequencies. In addition, AEAs were calculated as the difference between the total energy of the anion and the neutral clusters at their respective GS geometries. The VDEs were calculated as the energy difference between the anion and its neutral counterpart both at the GS geometry of the anion. They are given by the following definitions:

azabenzene rings are important constituents of both natural and synthetic compounds.34,35 For example, pyridine, pyridazine, and pyrazine are starting materials in the production of insecticides, herbicides, and several pharmaceutical drugs. Pyrimidine is the precursor of cytosine, thymine, and uracil in DNA or RNA. Because the azabenzene molecules contain nitrogen bases and an aromatic ring, they can function as a σ donor, π acceptor, or π donor.35,36 Then, some interesting questions emerge. Do the geometries and nature of the cobalt− benzene complexes change as the benzene is replaced by the azabenzene molecule? Can the Co atoms retain their high magnetic moments in Co−azabenzene complexes? To our knowledge, among Co−azabenzene systems, only Con(pyridine)m− have been studied by Bowen’s group4 using the experimental photoelectron spectroscopy. Meanwhile, they also performed preliminary theoretical studies on anionic complexes Co1, 2(pyridine)−. Motivated by both this imbalance and the absence of any studies on Co−benzene or Co−azabenzene complexes, we performed a systematic study on neutral and negatively charged Co1,2(benzene)1,2, Co1,2(pyridine)1,2, and Co1,2(pyrimidine)1,2 in the framework of DFT. Here, we choose pyrimidine to investigate due to it having the highest aromaticity36 and lowest relative energies37 among the three types of diazines.

VDE = Eneutral at optimized anion geometry − Eoptimized anion

AEA = Eoptimized neutral − Eoptimized anion

The calculated results and some experimental values are listed in Tables 1 and 2. Good agreement between them further supports the suitability of our computational method and helps determine the obtained GS structures. Table 2. Calculated VDEs (eV) for Combenzenen, and Compyridinen Clusters and Compared with the Experiment Values Combenzenen

2. COMPUTATIONAL METHODS Geometry optimizations and frequency analyses of neutral and anionic Co 1 , 2 (benzene) 1 , 2 , Co 1 , 2 (pyridine) 1 , 2 , and Co1,2(pyrimidine)1,2 clusters were carried out in the framework of a DFT-based method using the GAUSSIAN 03 program.38 The gradient-corrected Becke’s exchange39 combined with Perdew−Wang’s correlation40 and BPW91 functionals was employed for these systems. It has been demonstrated that this level of theory is very reliable in predicting the structural, electronic, and magnetic properties for Co−organic complexes based on recent reports.13,33,41 The all-electron basis set 6311+G*,42 which contains both diffuse function and polarization, was used for all of the atoms. To search for various structures of our complexes, we used the fully optimized geometries of the pure neutral and anionic benzene, pyridine, and pyrimidine as the starting point. Then, we wrote a shell script that randomly places cobalt atoms around the bare organic complexes fixed at the center of a sphere, with a restriction that all of the Co atoms should reside inside of the sphere and not have too close contact (≤1 Å) with any atoms in the organic complexes. A great number of possible initial structures, containing sandwich, nonsandwich, and riceball types, were considered for geometry optimization. Furthermore, the structures of Co−organic complexes in previous studies13,33,41,43,44 were also employed as a guide. All

VDE (eV) (m, n) (1, 1) (1, 2) (2, 1) (2, 2) a

theo. 0.22, 0.77 0.71 0.83, 1.43 0.91, 1.34

exp.a

0.70, 1.45, 2.20 1.43, 1.79, 2.25 3.05 1.11, 1.50, 1.79

Compyridinen exp.b

theo. 0.74, 1.35 1.23 1.02, 1.59 1.14, 1.66

0.74, 0.99, 1.25, 1.49, 1.95, 2.32 1.21 0.95, 1.50 1.89

Reference 18. bReference 4.

3. RESULTS AND DISCUSSION On the basis of the method described above, a number of optimized isomers for neutral and negatively charged Co1,2(benzene)1,2, Co1,2(pyridine)1,2, and Co1,2(pyrimidine)1,2 clusters are obtained. We only list the GS structures together with their symmetries, multiplicities, and magnetic moments in Figures 1−3. For convenience, the Co1,2(benzene)1,20/−, Co1,2(pyridine)1,20/−, and Co1,2(pyrimidine)1,20/− complexes are designated as Co 1,2 Bz 1,2 0/− , Co 1,2 Pyrid 1,2 0/− , and Co1,2Pyrim1,20/−, respectively. Furthermore, the Co−C bond lengths for all of the complexes are listed in the Supporting Information (Tables S1−S3), and the atom numerations to identify Co and C atoms are shown in Figures 1−3, 12999

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Figure 1. The GS structures of neutral and anionic Co1,2(benzene)1,2 complexes. Their symmetries, multiplicities, magnetic moments, and calculated vertical transition energies are also given.

Figure 3. The GS structures of neutral and anionic Co1,2(pyrimidine)1,2 complexes. Their symmetries, multiplicities, magnetic moments, and calculated vertical transition energies are also given.

Figure 2. The GS structures of neutral and anionic Co1,2(pyridine)1,2 complexes. Their symmetries, multiplicities, magnetic moments, and calculated vertical transition energies are also given.

respectively. For negatively charged complexes, the photodetachment transition energies from a spin multiplicity (M = 2S + 1, where S is the total spin) change of ±1 are calculated and presented on the right side of the GS structures of the anions in Figures 1−3. 3.1. Co−Benzene. We first discuss our results of neutral and anionic Co−benzene complexes to elucidate their geometries and electrophilic properties. To our knowledge, the geometrical structures of neutral Co−benzene complexes have been extensively explored by Zhang et al.43 and Kurikawa et al.32 Besides, the GS geometries of negatively charged complexes have been identified by our group.33 It worth pointing out that the GS structures of each type of complex obtained in the present studies are in good agreement with the previous results.32,33,43 As seen in Figure 1, in general, the structures remain unchanged following an electron attachment. In the GS structures of both CoBz and CoBz− complexes, the cobalt atom occupies the η6 site on top of the benzene ring and binds with six carbon atoms. The neutral cluster is a doublet state with C2v symmetry, while the triplet anion has C2 symmetry with a slightly distorted benzene ring. The relative energies of the corresponding quartet and quintet states are 0.425 and 0.966 eV, respectively. The electron affinity (EA) of CoBz is calculated to be near zero (0.19 eV). This may help explain why its photoelectron spectra cannot be observed in potodetachment experiments.18 Furthermore, the calculated electron detachment energies are 0.22 and 0.77 eV, which correspond to transitions from the anion triplet to neutral doublet and quartet, respectively. In Co2Bz0/− complexes, the two Co atoms prefer to form dimers, with the dimer bond axis occupying the η6 site and 13000

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Information (Table S4). From Table S4 (Supporting Information), it can be seen that the binding energies gradually increase with the increasing cluster size. The D0 values for the negative species are larger than those for the neutrals. 3.2. Co−Pyridine. In optimizing the structures of Co− pyridine complexes, both σ- and π-bonding modes are considered by us. The σ-type complex is formed by Co bonding to the nitrogen atom, and the π-type complex is formed by Co binding to the six-membered π ring. As can be seen from Figure 2, the π-type structures tend to be the GSs of Co−pyridine complexes. By the comparison between Co− benzene (Figure 1) and Co−pyridine (Figure 2), it is interesting to find that the GS structures of Co−pyridine are similar to those of Co−benzene complexes. This indicates that replacing one CH group in the benzene ring with a nitrogen atom has little effect on the geometries of Co−organic complexes. In the GS structure of neutral CoPyrid, the cobalt atom occupies the η6 site on top of the pyridine ring and binds with six-membered atoms, whereas the Co atom in the anion occupies the η4 site and connects with four carbon atoms. The preferred spin states of them are the neutral doublet and anion triplet states, respectively. Furthermore, their EA is calculated to be 0.60 eV, which agrees well with the experimental value (0.71 eV).4 This value is larger than that of CoBz (0.19 eV), indicating that the N substituent enhances its ability to attract electrons. Just like the Co2Bz0/− complexes, the GS structures of Co2Pyrid0/− also have the Co2 dimer bond axis occupying the η6 site of the pyridine ring. What is even more interesting is that the Co−Co bond lengths of the neutral (2.119 Å) and anionic (2.182 Å) Co2Pyrid clusters are almost identical to those of Co2Bz (2.120 Å) and Co2Bz− (2.183 Å), respectively. The vertical transition energies from the GS of the anion to the neutral having the same geometry as the anion but with spin multiplicities that differ by ±1 are calculated to be 1.02 and 1.59 eV. These match the experimental transitions (around 0.95 and 1.50 eV)4 rather well, as shown in Table 2. The GS structures of neutral and anionic CoPyrid2 have been obtained by us for the first time. As shown in Figure 2, they are also the tilted sandwich structures, which are similar to those of CoBz20/−. The Co atom has η6 coordination with the bottom benzene, whereas it binds to three and two carbon atoms of the upper benzene in the neutral and anion, respectively. Our calculated electron affinity is 1.03 eV, which is in excellent agreement with the experimental value (1.06 eV).4 This EA is larger than that of the isolated Co2 dimer (0.91 eV). The singlet anion has one transition to the neutral doublet state, and the transition energy is 1.23 eV. With regard to Co2Pyrid20/− complexes, their GSs are the sandwich configurations with the following features. The molecular axes of Co2 dimers, with bond lengths of 2.322 (in the neutral) and 2.392 Å (in the anion), show parallel orientation to the planes of the pyridine rings. One Co atom is bonded with the C sites of pyridine, while the other Co atom acts as a bridge linking the two nitrogen atoms. This type of bridge coordination yields a highly compact geometry for neutral and anionic Co2Pyrid2. The preferred spin states of them are the neutral triplet and anion doublet states, respectively. For the anion doublet state, the VDEs are calculated to be 1.14 and 1.66 eV, which correspond to the transitions from it to the neutral triplet and singlet states, respectively.

almost perpendicular to the carboatomic ring. The difference between them is in the Co−Co bond length; it is 2.120 Å in the neutral, while it is elongated to be 2.183 Å in the anion. Compared with the isolated Co2 dimers (Table S5, Supporting Information), both of the Co−Co bond lengths are elongated in complexes. Moreover, the EA of Co2Bz is calculated to be 0.98 eV, which is larger than that of the isolated Co2 dimer (0.91 eV). The electron detachment transition energy from the anion quartet to neutral quintet is calculated to be 0.83 eV, which is much smaller than that of the transition from the quartet to triplet (1.43 eV). This could be understood by the fact that the quintet is the preferred spin state for neutral clusters; thus, it is much easier to transform from the quartet to quintet. In the case of CoBz2, both the neutral and anionic clusters are tilted sandwich structures with the Co atom sandwiched between the two benzene molecules. The upper benzene molecule is tilted with respect to the bottom benzene plane. The Co atom has η6 coordination with the bottom benzene and η2 coordination with the upper benzene molecule. The bond distances of the η6−η2 coordination for CoBz2 and CoBz2− are listed in Table S1 (Supporting Information). For the neutral complex, a quartet ideal sandwich structure is obtained, but it is 1.571 eV higher in energy than the most stable configuration. Our GS asymmetric structure is consistent with the results of earlier electric deflection,23 infrared spectroscopy, and electron paramagnetic resonance experiments,46 which all suggest that CoBz2 assumes an asymmetric structure. Our obtained EA value of CoBz2 is 0.60 eV, which agrees well with the measured value (0.50 ± 0.10 eV).18 The small EA value of CoBz2 may reflect the repulsion effects of the two benzene molecules as well as the repulsion of the added electron. There is only one transition for anionic CoBz2¯ corresponding to transformation from the anion singlet to neutral doublet state, and the transition energy is 0.71 eV. This value corresponds to the first prominent transitions (0.70 eV) in the experimental photoelectron spectrum18 of CoBz2−, as shown in Table 2. During our geometry optimization of Co2Bz20/− complexes, three types of initial structures are considered, (a) the parallel sandwich structure, that is, the Co−Co bond is sandwiched between two benzene rings and lies parallel to them; (b) the perpendicular structure, in which the Co2 axis lies perpendicular to the plane of the benzene molecule; and (c) the one-end open sandwich structure, that is, Co and benzene are alternatively piled up. Finally, the lowest-energy configurations of Co2Bz2 and Co2Bz2− are found to be the triplet and doublet parallel sandwich structures, respectively. In these two structures, the benzene molecules are all bent and linked by Co atoms. The Co−Co bond length in the neutral cluster is 2.210 Å, which is shorter than that of the Co2 dimer (2.328 Å) in the anion. The VDEs corresponding to transitions from the anion doublet to neutral triplet and singlet are calculated to be 0.91 and 1.34 eV, respectively. These two values correspond to the first two obvious peaks centered at 1.11 and 1.50 eV in the photoelectron spectrum of Co2Bz2−, as revealed in Table 2. Furthermore, our calculated EA (0.83 eV) is also in agreement with the measured value (1.00 eV).18 The good agreements between the experimental and theoretical results lends considerable credence to our obtained structure and spin multiplicities for Co2Bz20/−. In addition, we have calculated the binding energies (D0) of Co and Co2 with the benzene molecules for Co1,2Bz1,2 and Co1,2Bz1,2−. All of the values are listed in the Supporting 13001

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Figure 4. The HOMOs of the GS structures of Co1,2(benzene)1,20/−, Co1,2(pyridine)1,20/−, and Co1,2(pyrimidine)1,20/− complexes.

3.3. Co−Pyrimidine. In the case of pyrimidine, we also considered the initial geometries with Co chelating to both nitrogen atoms or binding to the six-membered π ring. Both σand π-type configurations are found for Co−pyrimidine complexes. The GS structure of neutral CoPyrim is tested to be the σ configuration with a Co−N bond length of 1.821 Å. For anionic species, the existence of the most stable π configuration is attributed to the aromaticity of the sixmembered ring of pyrimidine. Previous studies36 have shown that the pyrimidine has the highest aromaticity among the three types of diazines molecules (pyrimidine, pyrazine, and pyridazine). Both the neutral and anionic clusters have the low point symmetry (Cs) because of the meta-arrangement of the two nitrogen atoms. The EA is calculated to be 0.95 eV. Compared with the EA of CoPyridine (0.60) and CoBz (0.19 eV), we find that the values continue to grow along with the increasing number of nitrogen atoms. This further confirms that the N substituent may enhance their abilities to attract electrons. Furthermore, the calculated VDEs are 1.12 and 1.98 eV, which correspond to transitions from the anion triplet to neutral doublet and neutral quartet states, respectively. Unfortunately, there is no experimental value reported on the EA and VDE of Co−pyrimidine complexes. We hope that our

studies might offer more detailed information for further investigations and facilitate the experiment in the future. The GS of Co2Pyrim is a quintet with Co 2 lying perpendicular to the pyrimidine ring, yielding an η6 sandwich structure. A quartet GS is found for the anionic Co2Pyrim− complex, in which the molecular Co2 axis deviates from a perpendicular direction to the ring. This phenomenon is also found in the other two anionic Co2-type complexes (Co2Bz− and Co2Pyrid−). The Co−Co bond lengths in neutral and anionic Co2Pyrim clusters are calculated to be 2.117 and 2.179 Å, respectively, which are both slightly shorter than those in corresponding species of Co2Bz and Co2Pyrid. Usually, the EAs of identical-sized Co−organic complexes will increase with the increasing of substituted nitrogen atoms. Indeed, the EA for Co2Pyrim (1.19 eV) is larger than the calculated EA for Co2Bz and Co2Pyrid (0.60 and 1.02 eV). As for CoPyrim20/− complexes, their GS structures are very different from those of the CoBz2 and CoPyrid2 clusters. The lowest-energy configuration of neutral CoPyrim2 is a doublet quasi-linear structure, in which the Co atom is bonded with two nitrogen sites and acts as a bridge linking the two pyrimidine molecules. The planes of the two pyrimidine molecules lie perpendicular to each other. For Co2Pyrim−, a rice-ball structure with a singlet state is found to be the most stable 13002

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Figure 5. Plots of the RDG versus the electron density for the selected complexes.

from σ-type (in Co2Bz) to dz2-type (in Co2Bz−) in the higherenergy HOMO(β). As for the tilted sandwich CoBz2 complex, HOMO(α) and HOMO(β) clearly indicate the bond formation between π electrons of the benzene molecule and 3d orbitals of the Co atom. CoBz2− is a closed-shell system; therefore, its spin-up and spin-down orbitals are degenerate. The formations of 3d−π bonds are also accomplished in its HOMO. In Co2Bz20/− complexes, the orbital density is mostly localized on cobalt atoms, and the π cloud of benzene becomes very weak. Their HOMOs are dominated by those with antibonding or nonbonding character and show 3d−2p bond signatures between benzene and the Co2 dimer. We now describe the bond formation between the metal cobalt atom and pyridine molecule. As we discussed above, replacing one CH group in the benzene ring with a nitrogen atom has little effect on the geometries of Co−organic complexes. From Figure 4, we can clearly see that the bonding natures of Co−pyridine complexes are also similar to those of the corresponding size Co−benzene complexes, with only few exceptions. Their HOMOs also mainly arise from the 3d/4s electrons of the cobalt atoms and the π cloud of the pyridine molecules. However, the π cloud of pyridine is slightly weaker than that of benzene, corresponding to the aromaticity of pyridine being less than that of benzene. This is because the substituted nitrogen atom disturbs the π system. When two CH groups of benzene are substituted by nitrogen atoms, their geometry structures have some changes. Accordingly, their bond formations also change obviously. For CoPyrim, the spin-down HOMO(β) has higher energy than the spin-up HOMO(α) and presents a σ-type bond between the Co atom and the nitrogen atom of pyrimidine. This further confirms that it is a σ-type configuration, as mentioned in section 3.3. The patterns of HOMOs for CoPyrim−, Co2Pyrim, and Co2Pyrim− are very similar to those of corresponding Co−pyridine complexes, which are

configuration. The Co atom has η2 carbon coordination with the two pyrimidine molecules. The vertical transition energy from the singlet anion to the doublet neutral is calculated to be 2.12 eV. In addition, our calculated EA of CoPyrim2 is 1.44 eV. Just like Co2Bz20/− and Co2Pyrid20/− complexes, the distorted sandwich structures in which the Co2 dimer lies parallel to the two pyrimidine rings are found to be the GS geometries of neutral and anionic Co2Pyrim2. The pyrimidine molecules in these two structures are significantly distorted and vary from the planar structure to the three-dimensional structure. It worth noting that the two pyrimidine rings in the neutral sandwich structure are staggered, with only threemembered atoms of the ring overlapped. The preferred spin states of neutral and anionic clusters are triplet and doublet states, respectively. For anionic Co2Pyrim2−, the energies of the vertical transitions to the neutral triplet and singlet are calculated to be 1.73 and 2.15 eV, respectively. 3.4. Bonding Properties Analysis. In order to understand the bonding nature of the neutral and anionic Co1,2(benzene)1,2, Co1,2(pyridine)1,2, and Co1,2(pyrimidine)1,2 complexes, we have analyzed the pattern of their highest occupied molecular orbitals (HOMOs). Both the spin-up HOMO(α) and spin-down HOMO(β) of them are displayed in Figure 4. As can be seen from this figure, the Co−C and Co−N covalent bond formation mainly arises from the 3d/4s electrons of the cobalt atoms and the π cloud of the three types of organic molecules. As for the CoBz complex, both the HOMO(α) and HOMO(β) have a π-type bond between the Co atom and benzene molecule but with mixed Co dxz characters. The spinup α orbital is the HOMO of CoBz−, which consists mainly of the cobalt 4s orbital and the π cloud of benzene. In the case of the Co2Bz cluster, both HOMO(α) and HOMO(β) are mainly formed by the cobalt dimer σ orbital. With an extra electron attaching, it is obvious that the orbital of the Co atom changes 13003

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Figure 6. Gradient isosurfaces (s = 0.3 au) for the selected complexes.

mainly formed by cobalt 3d/4s electrons or the Co2 dimer σ orbital. In the case of CoPyrim2, only a σ-type bond is observed on the N−Co−N axis, which acts as a bridge and links the two pyrimidine molecules. In Co2Pyrim20/− complexes, the two Co atoms in identical clusters show different signatures. One atom shows dx2-type orbital, while the other one shows dxy-type orbital. 3.5. Interaction in Co−Organic Complexes. Molecular structure is governed by various interactions, such as chemical bonds and weak interactions. Weak interactions, including hydrogen bonds, electrostatic interactions, steric repulsion, and so on, are the driving force in most biochemical processes. In this section, we present an approach to map and analyze the interactions within Co−benzene, Co−pyridine, and Co− pyrimidine complexes from a new perspective. This approach depends on the electron density and reduced density gradient (RDG). The quantum mechanical electron density (ρ) is the key quantity in DFT. The RDG, which is the first derivative of electron density, is defined as s = 1/(2(3π2)1/3)|∇ρ|/ρ3/4, where 1/(2(3π2)1/3 can be denoted by Cs, and the value of it is about 0.162. RDG is a fundamental dimensionless quantity in DFT used to describe the deviation from a homogeneous electron distribution.47,48 To explore the feature of interactions in Co− organic complexes, we perform the plots of the RDG versus electron density for some selected clusters that represent different types of structures of our systems. The scatter diagrams for these selected complexes are shown in Figure 5. From these maps, we can get the basic pattern of interactions. Now, we take the plot of CoBz as an example to illustrate the features. The top left side points (small density and large reduced gradient) correspond to the edge of this complex. The

scattered points in the middle and right sides correspond to the chemical bonds. Regions around the nuclei have the larger density values and appear beyond the right edge of the plot. There is a spike in the low-density region ranging from 0.035 to 0.105 au. These points are the signature of the interaction between the cobalt atom and benzene molecule. It worth pointing out that the electron density values can be an indicator of the interaction strength to some degree. The plot of CoPyrid is similar to that of CoBz, with only a slight change in the spike region. This indicates the N substituent has little effect on the interaction between Co and the organic molecule. As for the CoPyrid− complex, there is another spike that appeared at about 0.026 au. This corresponds to the steric effect in the center of the ring. The CoPyrim has a σ configuration and Co− N bond; therefore, its scatter diagram is very different from those of CoBz and CoPyrid, which have a π-bonding mode. In the plot of the sandwich CoBz2 structure, two spikes represent the Co atom interacting with two benzene molecules. To show a rich visualization of the interactions in these studied complexes, we provide the gradient isosurfaces in Figure 6. Isosurfaces are generated for s = 0.3 au. As can be seen in Figure 6, the different types of interactions have been clearly shown in these maps. The π-type interaction between Co and organic complexes assumes a basin shape. The elliptical slab between Co and the N atom of pyrimidine is the σ-type interaction. In CoPyrid−, CoPyrim, CoPyrim2, and Co2Pyrim2 complexes, the regions in the center of the rings show a strong steric effect (also called nonbond overlap). It originates from the fact that each atom in a molecule occupies a certain amount of space. When atoms are brought together, hindrance will be induced at the expense of shape, energy, reactivity, and so forth. 13004

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Figure 7. Charge and spin density distributions in the GS structures of neutral Co1,2(benzene)1,2, Co1,2(pyridine)1,2, and Co1,2(pyrimidine)1,2 complexes.

Figure 8. Charge and spin density distributions in the GS structures of anionic Co1,2(benzene)1,2−, Co1,2(pyridine)1,2−, and Co1,2(pyrimidine)1,2− complexes.

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maps can show a rich visualization of the spin density distribution. From Figures 7 and 8, we can clearly see that all of the spin densities reside on the cobalt atoms, which appear d orbital sharp in most of the complexes. The unpaired d electrons of the cobalt atom can place some spin density at its own atomic orbitals. In Co−organic0/− and Co2−organic20/− (organic = benzene, pyridine, and pyrimidine) complexes, a four-lobe orbital shape is centered at the metal atom. The spin densities residing on the Co atom of Co2−organic2 are obviously more than those of Co2−organic2− complexes. This is in accordance with the results of their calculated magnetic moment. As for Co2−organic0/−, a remarkable feature of them is the coexistence of a different type of d orbital sharp that appears on the two Co atoms. For Co−organic2−, there is no electron density that appears in this complex, which further confirms that their magnetic moment is zero. A recently reported result50 clearly confirms that the four-lobed shape is independent of the type of basis functions used. Such a spin distribution stems therefore from the principles of quantum mechanics and should be amenable to experimental determination. In order to achieve deep insight into the charge distribution and the reliable charge-transfer information, we have calculated the natural population analysis (NPA) for the lowest-energy species of Co1,2(benzene)1,20/−, Co1,2(pyridine)1,20/−, and Co1,2(pyrimidine)1,20/− complexes. The atomic charges of neutral and negatively charged clusters are shown in Figures 7 and 8, respectively. Furthermore, the colors of various atoms are fixed from red to cyan, corresponding to their atomic charge changes from positive 0.6 to negative −0.6 e. From these two figures, we can see that all of the hydrogen atoms in both neutral and anionic clusters possess positive charges in the range of 0.144−0.248 e. The six-membered carbon or nitrogen atoms of the organic ring always have very negative charges. The atomic charges of cobalt atoms have positive values in neutral complexes with the exception of Co2−organic. This indicates that the electron transfer from the Co atom to organics, namely, cobalt atoms, acts as an electron donor in these neutral clusters. By comparison between neutral and anionic complexes, we find that the extra electron is mainly localized on the organic molecules, apart from that in CoBz and CoPyrid. As for these two exceptive clusters, 0.833 and 0.685 e extra electrons go to compensate for the positive charges of Co atoms.

We hope that the characteristics of different types of regions in scatter graphs and gradient isosurfaces will be helpful to understand the internal mechanism in the future. 3.6. Magnetic and Electronic Properties. It is wellknown that the spin magnetic moment of a free cobalt atom is 3.0 μB. In the standard bulk phases, it is ferromagnetic and carries 1.7 μB/atom. Thus, it is interesting to elucidate how the magnetic moments of Co atoms can be further affected when they interact with benzene, pyridine, and pyrimidine. Because the orbital magnetic moments are usually very small compared with the spin magnetic moments in metals, the spin magnetic moment is thought to be a reasonable estimate of the total magnetic moment for a given metal−organic system. In other words, the spin magnetic moment is equal to the spin multiplicity minus one. This has been confirmed by our earlier work.33,49 The spin multiplicity and magnetic moment of each complex are listed below the corresponding structures in Figures 1−3. By the comparison between them, it is interesting to find that the preferred spin multiplicities of identical-sized Co−benzene, Co−pyridine, and Co−pyrimidine complexes show surprising consensus. That is to say, the identical-sized Co−organic complexes have the same magnetic moments. This may be understood by the fact that the CH group has seven total electrons, which is isoelectronic with the nitrogen atom. It is well-known that the magnetic moment is relevant to the unpaired electrons. In this case, replacing one CH group with a nitrogen atom has no effect on the total number of unpaired electrons of the whole complex, as well as its magnetic moment. Due to the magnetic moments of the three types of complexes having the same evolution against the cluster size, we take the Co−benzene as an example to elucidate their magnetic properties. In present study, the GS, of neutral CoBz, CoPyrid, and CoPyrim are tested to have a 1.0 μB spin magnetic moment. The previous studies have shown that the magnetic moments of Co(pyrene)41 and Co(coronene)13 are also calculated to be 1.0 μB. Compared with the magnetic moment of a free cobalt atom (3.0 μB), we can conclude that the magnetic moment of the Co atom becomes significantly reduced when it interacts with an organic molecule. As for the neutral Co2Bz, its spin magnetic moment is 4 μB, which corresponds to 2 μB per cobalt atom. It is tempting to compare the spin magnetic moment per Co atom in Co2Bz with free Co atom’s spin moment (3 μB) and conclude that the spin magnetic moment of Co in Co2Bz is reduced from its atomic value. However, we think that we should consider a more appropriate scenario, in which the Co2 dimer is taken as one unit interacting with a benzene molecule. The bare Co2 dimer has a spin magnetic moment only of 4 μB; thus, it can be seen that the benzene molecule can maintain the spin magnetic moment of the Co2 dimer. In the neutral doublet CoBz2 and triplet Co2Bz2 complexes, the cobalt has a spin magnetic moment of 1 μB/Co atom. Thus, following the trend first seen in CoBz and then again in CoBz2 and Co2Bz2, the spin magnetic moment of the Co atoms gets significantly reduced from its atomic state. This quenching is brought about by the interaction of Co 3d electrons with the π electrons of benzene. Another interesting aspect is that the magnetic moments of all of the anion clusters are consistently 1 μB less than those of their corresponding neutrals, with the exception of CoBz−. In addition, the electron density maps from the spin selfconsistent field (SCF) for our studied complexes (isovalue = 0.03 au) are also given in Figures 7 and 8. These spin density

4. CONCLUSIONS We have performed a systematic investigation on both neutral and negatively charged Co1,2(benzene)1,2, Co1,2(pyridine)1,2, and Co1,2(pyrimidine)1,2 complexes by means of all-electron calculations. Their GS geometries, bonding natures, interactions, and electronic and magnetic properties are studied. All of the results are summarized as follows. (1) On the basis of geometry optimization, we find that the identical-sized Co−benzene and Co−pyridine have similar GS structures but are different from those of Co−pyrimidine complexes. In the former two types of complexes, the cobalt atoms prefer to occupy the η6 site on top of the ring and form sandwich structures when they interact with one or two organic molecules. In Co2−organic systems, two cobalt atoms are likely to form dimers, with the dimer bond axis almost perpendicular to the organic ring. However, for Co−pyrimidine complexes, both σ- and π-type configurations are identified. Furthermore, 13006

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(6) Knickelbein, M. B. Magnetic Moments of Bare and BenzeneCapped Cobalt Clusters. J. Chem. Phys. 2006, 125, 044308−044314. (7) Simon, A.; Joblin, C. Thermochemistry and Infrared Spectroscopy of Neutral and Cationic Iron−Polycyclic Aromatic Hydrocarbon Complexes of Astrophysical Interest: Fundamental Density Functional Theory Studies. J. Phys. Chem. A 2007, 111, 9745−9755. (8) Takegami, R.; Hosoya, A.; Suzumura, J.-I.; Yada, K.; Nakajima, A.; Yabushita, S. Ionization Energies and Electron Distributions of OneEnd Open Sandwich Clusters: Eun(C8H8)n (n = 1−4). Chem. Phys. Lett. 2005, 403, 169−174. (9) Wang, Y.; Szczepanski, J.; Vala, M. Vibrational Spectroscopy of Neutral Complexes of Fe and Polycyclic Aromatic Hydrocarbons. Chem. Phys. 2007, 342, 107−118. (10) Kandalam, A. K.; Rao, B. K.; Jena, P.; Pandey, R. Geometry and Electronic Structure of Vn(Bz)m Complexes. J. Chem. Phys. 2004, 120, 10414−10423. (11) Pandey, R.; Rao, B. K.; Jena, P.; Blanco, M. A. Electronic Structure and Properties of Transition Metal−Benzene Complexes. J. Am. Chem. Soc. 2001, 123, 3799−3808. (12) Pandey, R. B.; Rao, K.; Jena, P.; Newsam, J. M. Unique Magnetic Signature of Transition Metal Atoms Supported on Benzene. Chem. Phys. Lett. 2000, 321, 142−150. (13) Kandalam, A. K.; Kiran, B.; Jena, P.; Li, X.; Grubisic, A.; Bowen, K. H. Ground State Structures and Photoelectron Spectroscopy of [Com(coronene)]− Complexes. J. Chem. Phys. 2007, 126, 084306− 084314. (14) Kurikawa, T.; Takeda, H.; Hirano, M.; Judai, K.; Arita, T.; Nagao, S.; Nakajima, A.; Kaya, K. Electronic Properties of Organometallic Metal−Benzene Complexes [Mn(benzene)m (M = Sc−Cu)]. Organometallics 1999, 18, 1430−1438. (15) Hoshino, K.; Kurikawa, T.; Takeda, H.; Nakajima, A.; Kaya, K. Structures and Ionization Energies of Sandwich Clusters (Vn(benzene)m). J. Phys. Chem. 1995, 99, 3053−3055. (16) Weis, P.; Kemper, P. R.; Bowers, M. T. Structures and Energetics of Vn(C6H6)m+ Clusters: Evidence for a Quintuple-Decker Sandwich. J. Phys. Chem. A 1997, 101, 8207−8213. (17) Yasuike, T.; Nakajima, A.; Yabushita, S.; Kaya, K. Why Do Vanadium Atoms Form Multiple-Decker Sandwich Clusters with Benzene Molecules Efficiently? J. Phys. Chem. A 1997, 101, 5360− 5367. (18) Gerhards, M.; Thomas, O. C.; Nilles, J. M.; Zheng, W.-J.; Bowen, K. H. Cobalt−Benzene Cluster Anions: Mass Spectrometry and Negative Ion Photoelectron Spectroscopy. J. Chem. Phys. 2002, 116, 10247−10252. (19) Zheng, W.-J.; Nilles, J. M.; Thomas, O. C.; Bowen, K. H. Photoelectron Spectroscopy of Titanium−Benzene Cluster Anions. Chem. Phys. Lett. 2005, 401, 266−270. (20) Zheng, W.-J.; Nilles, J. M.; Thomas, O. C.; Bowen, K. H. Photoelectron Spectroscopy of Nickel−Benzene Cluster Anions. J. Chem. Phys. 2005, 122, 044306−044310. (21) Valencia, I.; Castro, M. Theoretical Study of The Structural and Electronic Properties of the Fen(C6H6)m, n ≤ 2; m ≤ 2 Complexes. Phys. Chem. Chem. Phys. 2010, 12, 7545−7554. (22) Billas, I. M. L.; Chătelain, A.; de Heer, W. A. Magnetism from the Atom to the Bulk in Iron, Cobalt, and Nickel Clusters. Science 1994, 265, 1682−1684. (23) Rayane, D.; Allouche, A.; Antoine, R. R.; Broyer, M.; Compagnon, I.; Dugourd, P. Electric Dipole of Metal−Benzene Sandwiches. Chem. Phys. Lett. 2003, 375, 506−510. (24) Bauschlicher, C. W.; Partridge, H.; Langhoff, S. R. Theoretical Study of Transition-Metal Ions Bound to Benzene. J. Phys. Chem. 1992, 96, 3273−3278. (25) Dargel, T. K.; Hertwig, R. H.; Koch, W. How Do Coinage Metal Ions Bind to Benzene? Mol. Phys. 1999, 96, 583−591. (26) Chaquin, P.; Costa, D.; Lepetit, C.; Che, M. Structure and Bonding in a Series of Neutral and Cationic Transition Metal− Benzene η6 Complexes [M(η6-C6H6)]n+ (M = Ti, V, Cr, Fe, Co, Ni, and Cu). Correlation of Charge Transfer with the Bathochromic Shift of the E1 Ring Vibration. J. Phys. Chem. A 2001, 105, 4541−4545.

our calculated EAs and VDEs of these GS structures show good agreement with the experimental values. (2) The pattern of HOMOs reveals that the bond formation of Co−organic complexes mainly arises from the 3d/4s electrons of the cobalt atoms and the π cloud of the three types of organic molecules. The bonding natures of Co− pyridine complexes are similar to those of the corresponding size Co−benzene complexes, while those of Co−pyrimidine change obviously. Due to the substituted nitrogen atom disturbing the π system, the π cloud of pyridine and pyrimidine is slightly weaker than that of benzene. (3) On the basis of the electron density (ρ) and RDG, we present an approach to map and analyze the interactions within Co−benzene, Co−pyridine, and Co−pyrimidine complexes. The scatter plots of the RDG versus ρ allow us to identify the different types of interactions in our studied complexes. Meanwhile, the maps of the gradient isosurfaces show a rich visualization of various chemical bonds and steric effects. (4) By determining their spin magnetic moments, we find that the magnetic moments of Co atoms become significantly reduced when they interact with the organic molecules, and the N substituent appears to have no effect on the total magnetic moment of the complex. All of the spin densities reside on the cobalt atoms, which appear d orbital sharp in most of the complexes. In addition, the NPA shows that the extra electron is mainly localized on the organic molecules, apart from that in CoBz and CoPyrid.

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ASSOCIATED CONTENT

S Supporting Information *

The low-lying structures, structural parameters, and calculated properties for Co−organic complexes and the isolated cobalt dimer. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +86 28 85405515. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 11274235 and No. 11104190) and the Doctoral Education Fund of the Education Ministry of China (No. 20100181110086 and No. 20110181120112).

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REFERENCES

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