Ultrashort MnâMn Bonds in Organometallic Complexes - The Journal

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Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX

Ultrashort Mn−Mn Bonds in Organometallic Complexes Tomás Alonso-Lanza,*,† Jhon W. González,† Faustino Aguilera-Granja,‡,† and Andrés Ayuela† †

Departamento de Física de Materiales, Facultad de Químicas, UPV/EHU, Centro de Física de Materiales CFM-MPC CSIC−UPV/EHU, Donostia International Physics Center (DIPC), 20018 San Sebastián, Spain ‡ Instituto de Física, Universidad Autónoma de San Luis de Potosí, 78000 San Luis Potosí S.L.P., México S Supporting Information *

ABSTRACT: Manganese metallocenes larger than the experimentally produced sandwiched MnBz2 compound are studied using several density functional theory methods. First, we show that the lowest energy structures have Mn clusters surrounded by benzene molecules, in socalled rice-ball structures. We then find a strikingly short bond length of 1.8 Å between pairs of Mn atoms, accompanied by magnetism depletion. The ultrashort bond lengths are related to Bz molecules caging a pair of Mn atoms, leading to a Mn−Mn triple bond. This effect is also found when replacing benzenes by other molecules such as borazine or cyclopentadiene. The stability of the Mn−Mn bond for Mn2Bz2 is further investigated using dissociation energy curves. For each spin configuration, the energy versus distance plot shows different spin minima with barriers, which must be overcome to synthesize larger Mn−Bz complexes.

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which has been used to increase the octane level of gasoline.29 Experiments on TM−Bz compounds have been performed, leading to their synthesis in the gas phase and their characterization using spectroscopic methods18,30−32 and in magnetic experiments.33,34 TM−Bz compounds are present in many types of molecular clusters. Small Mn−Bz compounds, such as MnBz2, have been synthesized.18,35 As for Mn adatoms in graphene, Mn sandwiched between Bz molecules prefers to be in a hollow position.36,37 Theoretical studies are required to understand larger complexes of inner Mn clusters surrounded by Bz molecules to be produced experimentally. Dimetallocenes and multimetallocenes have been largely investigated using density functional theory (DFT) calculations. Different external molecules have been considered, such as Bz, cyclopentadiene, and even fullerenes.38 Similarly, many different central metal atoms have been studied, leading to alkalineearth dimetallocenes,39−42 transition metal dimetallocenes,43−45 heterodinuclear compounds,46,47 and zinc isoelectronic elements, such as Cd and Hg.48 Although multinuclear metallocenes of Co, Cu, Ni, and other transition metals have been described,43−45,49−54 the study of larger manganese benzene compounds is still to be done. We herein investigate organometallic compounds in which Bz molecules form cage clusters with a few Mn atoms, MnnBzm. The benzene molecules strongly affect the bonding between manganese atoms, differently from other transition metals. Manganese benzene compounds are also interesting because they are in the middle between two trends, sandwich-like

INTRODUCTION A number of organometallic compounds have been synthesized and used in a wide range of applications.1−5 The synthesis of ferrocene6−9 stimulated a search for related metallocenes, such as cobaltocene,10 nickelocene,11 rhodocene,12 and manganocene,13 in which a transition metal atom is located between two cyclopentadiene molecules. Similar compounds have been proposed combining transition metal atoms with benzene molecules (TM−Bz compounds).14 The basic unit of such a compound is a transition metal atom over a single Bz ring, known as a half-sandwich.15−17 Such basic units can be combined to form larger compounds with either sandwich-like structures for early transition metals (Sc, Ti, and V) or rice-ball structures for late transition metals (Fe, Co, and Ni).18 TM−Bz compounds look promising for future applications. For instance, sandwich-like molecules have been proposed as conductors involving spin transport,19 either isolated bridging the tips of Cu nanocontacts19,20 or pile up building infinite wires having Sc, Ti, V, Cr, and Mn.21 Sandwiched molecules and infinite wires containing V, Nb, and Ta have been found to provide strong magnetic anisotropy.22 We are interested in characterizing larger clusters with rice-ball structures, specifically in the study of Mn−Bz compounds, to gain knowledge on their still unfamiliar structures and properties. The field of organometallics was recently boosted following a breakthrough in the synthesis of binuclear metallocenes, socalled dimetallocenes, such as decamethyldizincocene23,24 Zn2(η5C5Me5)2 and Zn2(η5C5Me4Et)2. Compounds containing Mn with many nuclei have plenty of uses in organometallics.25 Some previously described molecules containing Mn−C bonds are dimanganese decacarbonyl26−28 Mn2(CO)10 and methylcyclopentadienyl manganese tricarbonyl (CH3C5H4)Mn(CO)3, © XXXX American Chemical Society

Received: August 7, 2017 Revised: October 30, 2017 Published: October 30, 2017 A

DOI: 10.1021/acs.jpcc.7b07828 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Geometries of the MnmBzn molecules studied here. Mn, C, and H atoms are shown as red, green, and white spheres, respectively. Note that the total spin magnetic moment is null or small for all cases.

tials with nonlinear core corrections factorized in the Kleinman−Bylander form. The basis set size was double-ζ plus polarization orbitals with an energy shift of 0.05 eV. Using the SIESTA package, the same minima were obtained for the Mn2Bz2 and Mn3Bz3 molecules.

structures for early transition metals and rice-ball structures for late transition metals.18 We find that the clusters contain Mn− Mn multiple (triple) bonds with ultrashort interatomic distances and depleted magnetism. We then focus on the electronic structure of the smaller Mn2Bz2 molecule to further improve our understanding of such molecules. The two Bz molecules covering the Mn2 molecule appear to stabilize a nonmagnetic local minimum in the inner Mn dimer. We verify that these results also appear when replacing the caging benzenes with other molecules. Furthermore, ultrashort bonds are also found in larger Mn−Bz compounds, indicating that the effect is robust and that strong dimerization is induced in larger molecules. Because most experiments work with charged clusters, we have checked that positively charged Mn−Bz clusters also present ultrashort Mn−Mn bonds. Lastly we study the synthesis of the Mn2Bz2 molecule, as a first step toward larger Mn−Bz compounds, and we show that different high local spin magnetic moment states create barriers to such syntheses.

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RESULTS AND DISCUSSION Metallization Induced Superdimerization. The ground state geometries of the Mn2Bz2, Mn3Bz3, Mn4Bz3, and Mn4Bz4 molecules are shown in Figure 1. We also consider the roles of isomers for each cluster, and study other geometries and magnetic solutions. We first determine whether a sandwich or rice-ball structure is preferred. The rice-ball geometry has lower energy for each of the four Mn−Bz clusters studied here. The results for the sandwich structures are shown in the Supporting Information. We perform calculations for several isomers with different magnetic moments. For instance, a symmetric Mn3Bz3 isomer with 5 μB has an energy difference of 1 eV from the ground state, and a distorted Mn3Bz3 isomer with 7 μB lies 0.5 eV higher in energy. All the isomers are lying higher in energies than the structures presented in Figure 1. All the ground state molecules have strikingly short bond lengths between the Mn couples. The Mn2Bz2 molecule is the simplest system to show this effect with a Mn−Mn distance of 1.8 Å. The ultrashort bond lengths are unexpected because larger distances between Mn atoms have been reported, such as 3.4 Å for the Mn dimer,59−61 2.93 Å for dimanganese decacarbonyl,26,28 2.4−2.6 Å for manganese carbides,62,63 and the broad distance range of 2.25−2.95 Å for several Mn bulk allotropes.61 In the presence of ligands, the Mn−Mn distance decreases in the range from above 3 to 2.8 Å, as reported by adding Cl atoms.64 On the other hand, analogous short multiple bonds have been shown theoretically and experimentally for other transition metals, such as in ultrashort Cr dimers (1.73 Å) within organic molecules.65

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METHODS We performed DFT calculations using the Vienna Ab initio Simulation Package (VASP), based on the projected augmented wave method (PAW).55,56 For the exchange and correlation functionals we used the Perdew−Burke−Ernzerhof (PBE) form of the generalized gradient approximation (GGA).57 We chose a cubic unit cell with sides of 30 Å in order to avoid interaction between images. We used an electronic temperature of 25 meV and a cutoff energy of 500 eV. The Mn−Bz molecules were relaxed until the forces were less than 0.006 eV/Å. The results obtained using VASP were reproduced using the SIESTA package within a GGA and applying the PBE functional for selected cases. We used a mesh cutoff of 250 Ry and sampled at the Γ-point. The atomic cores were described using nonlocal norm-conserving relativistic Troullier−Martins58 pseudopotenB


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occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) gaps are in the range 1.17−0.72 eV. Taking into account the well-known issue of gap underestimation for the generalized gradient approximation, the gap values suggest that the molecules are stable. The gap narrows as the size of the molecule increases. We could have followed the series by looking for larger molecules with ultrashort Mn−Mn bonds, such as Mn5Bz5, Mn6Bz6, and so on. Nevertheless, it appears that placing five or more Bz rings around a nucleus of larger Mn clusters is a real challenge due to the lack of space. Molecular beam experiments use charged systems; therefore, we determine how our neutral Mn−Bz clusters are affected when positively charged. An electron was removed from the structures, which were fully relaxed again. The results show there are two different trends depending on the size. Smaller clusters, Mn2Bz2+ and Mn3Bz3+, preserve the ultrashort distance of the Mn−Mn bonds. Note that the Mn2Bz2+ cluster now has a magnetic moment of 1 μB and that the Mn3Bz3+ increases from 1 μB for the neutral case to 2 μB. Larger complexes, Mn4Bz3+ and Mn4Bz4+, lose the ultrashort Mn−Mn bond distance and display bulk-like behavior. The Mn−Mn distances increase to values within the interval 2.3−2.6 Å. The local magnetic moments of Mn increase to values within the range 2.0−3.5 μB, and the Mn atoms are antiferromagnetically coupled. We have also looked at the values of the local magnetic moments of the Mn2Bz2+ cluster. We found that there is approximately 0.5 μB on each manganese atom, summing up to the total magnetic moment of 1 μB. Similar results are obtained for Mn3Bz3+. The localization of the magnetic moment on manganese atoms shows that the electron was removed from molecular orbitals localized on manganese atoms. We believe that the Mn−Mn bond in the Mn2Bz2 and Mn3Bz3 clusters is notably stronger than that in the Mn4Bz3 and Mn4Bz4 clusters. The enlargement of Mn−Mn distances from 1.8 Å in Mn2Bz2 to 1.94 Å for Mn4Bz4 supports this idea. Removing one electron is more likely to modify the weaker bonding structure of the largest Mn−Bz clusters. We have tested the robustness of the ultrashort Mn−Mn bond by carrying out DFT calculations including the van der Waals interaction (vdW-DF)66 within the VASP method.67,68 We have relaxed the Mn2Bz2 cluster from the ground state distance and the ultrashort Mn−Mn bond is preserved, with zero total and local magnetic moments. To get information around the minimum, we have also carried out a single point calculation of Mn2Bz2 for a Mn−Mn distance of 2.4 Å, lying 1.72 eV above the ground state, a value that is similar to 2 eV as given in Figure 2a. We have also tested the role of the strong correlation in manganese atoms by computing the Mn2Bz2 cluster including the Hubbard U term in the VASP code, following the Ueff = U − J approach.69 We have relaxed the Mn2Bz2 cluster from the ground state distance using values of Ueff = 1, 2, and 3 eV. We find that the ultrashort Mn−Mn bond is preserved and that there is no emergence of local or total magnetism. We further test when ultrashort Mn−Mn bonds would appear by replacing the caging benzenes with other organic molecules, such as cyclopentadiene, C5H6, and even inorganic molecules, such as borazine, B3H6N3.70−72 After careful atomic relaxations, both types of caging compounds for Mn2 present ultrashort Mn−Mn bonds accompanied by magnetism depletion. [For the cyclopentadiene-caged Mn2, the ultrashort Mn−Mn distance is 1.85 Å. The Mn local magnetic moments

We focus on the locally unpolarized Mn2Bz2 molecule with a Mn−Mn distance of around 1.8 Å. The loss of magnetism is a hallmark of the ultrashort bonds between Mn atoms capped with Bz molecules. The magnetic moments of irregular systems, such as Mn3Bz3 and Mn4Bz3, are almost depleted. In the unit consisting of a Mn atom over a benzene molecule, the magnetic moment decreases from 5 to 3 μB.16 However, the total elimination of the magnetism for the Mn2Bz2 molecule is still surprising. When combining two Mn2Bz2 units to form a Mn4Bz4 molecule, the larger compound also has ultrashort bonds, and the total and local magnetic moments remain zero. The Bz molecules around the Mn atoms form a distorted tetrahedron, tilted because of neighboring Bz rings. There are two short bond lengths of 1.94 Å and four larger bond lengths of 2.56 Å. The short bond lengths are between the opposite atom pairs, labeled 1−2 and 3−4 in Figure 1. The two cases Mn2Bz2 and Mn4Bz4 have ultrashort distances and total magnetism depletion. The ultrashort Mn−Mn bonds also occur for other molecules, such as Mn3Bz3 and Mn4Bz3. The Mn3Bz3 ground state geometry depletes most of the magnetism, and gives an ultrashort distance of 1.88 Å between Mn(1) and Mn(3). The third Mn atom, Mn(2), moves away and remains bonded mainly to Mn(1), with a bond length of 2.22 Å. The side position of Mn(2) bends the Bz molecule associated with Mn(1). The total spin magnetic moment is 1 μB, localized mainly on Mn(2) with 1.53 μB. The Mn(1) atom has antiferromagnetic coupling with a magnetic moment of −0.57 μB, and the Mn(3) local moment decreases to a very small value of 0.28 μB. [Small spin polarization is induced in the C atoms (0.06 μB being the highest value for one C atom) with opposite sign to the closest Mn atom.] The added Mn−Bz unit distorts the Mn2Bz2 geometry, giving nonzero local magnetic moments on the Mn atoms. The peculiar distorted structure is due to the large energy gain because of the ultrashort Mn−Mn bond. The ultrashort bond phenomenon is therefore found partially for Mn3Bz3. The dimerized Mn4Bz4 and Mn3Bz3 structures indicate that the ultrashort bond mechanism is caused by two Mn atoms caged by Bz molecules. We next consider when the short Mn−Mn bonds accompanied by magnetism quenching occur by removing a Bz molecule from an already dimerized structure. We consider the Mn4Bz3 molecule shown in Figure 1. The structure is obtained by removing a Bz molecule from the final Mn4Bz4 geometry, and fully relaxing the geometry again. The Mn4Bz3 molecule becomes reorganized because the Mn(4) atom has lost its Bz molecule. The Mn(1), Mn(2), and Mn(3) atoms form an equilateral triangle with sides of 2.62 Å. The Mn(4) atom is bonded to the other three Mn atoms with short bond lengths of 2.05 Å, which are larger than those found in Mn4Bz4. In this case, the local magnetic moments are antiferromagnetically coupled, with small values of 1.07 μB for Mn(1), Mn(2), and Mn(3) and −0.83 μB for Mn(4). Most carbon atoms have a negative induced magnetic moment of 0.02−0.03 μB. Although the local manganese magnetism is reduced and the bonds are short, it seems that the presence of two Bz molecules caging two Mn atoms is key to the ultrashort Mn−Mn bond scheme. We now consider the stability of these compounds to discuss their possible synthesis. We compute the formation energy, per manganese atom, for MnnBzm clusters toward separated Mn atoms and Bz molecules, using the expression Ef = (nEMn + mEBz − Etotal)/n. We find that the formation energies are always favorable, in the range 1.76−2.18 eV. Furthermore, the highest C


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Mn−Mn molecular levels with opposite spins. Note that highspin solutions for Mn2Bz2 require larger distances because of the larger electronic repulsion between Mn levels. To improve our understanding of the ultrashort Mn−Mn bonds, we carry out spin-polarized single point calculations for the Mn2Bz2 molecule. The Mn atoms are separated by 0.1 Å in each step, and the Bz rings are maintained in the same relative positions with respect to the Mn atoms. We plot the energy against Mn−Mn distance in Figure 2a. The local magnetic moments of Mn are zero up to a distance of 2.4 Å, and there is a deep minimum at a distance of 1.8 Å. The energy increases sharply by more than 2 eV as the Mn atoms separate. Two different DFT codes give the minimum at 1.8 Å, even when the Mn2Bz2 molecule is distorted and allowed to relax again. The large energy gain is associated with the multiple (triple) bond between Mn atoms, as shown by considering the molecular orbitals and electron localization function, as described in the Supporting Information. We further explore the nature of Mn2Bz2 bonding to determine when both inner Mn atoms are locally spincompensated. We study the isolated Mn2 molecule, varying the Mn−Mn distance from 1 to 4 Å, as shown in Figure 2b. When the magnetization is released, the total magnetic moment varies from 2 μB for distances shorter than 1.7 Å to 10 μB for distances larger than 2.4 Å. We have also calculated the energy versus distance curves for fixed total spin values of 0, 2, 6, and 10 μB. We find that each spin curve has a different minimum. The global minimum for Mn2 is around 2.5 Å, as previously reported at this level of calculation (LDA/GGA).73−77 The antiferromagnetic curve is almost degenerated to that of 10 μB for long distances, in agreement with previous calculations.78 There is a delicate balance between exchange and splitting, and small changes in distance can modify the magnetic characteristics of the Mn dimer.79 Although DFT has been shown to be successful for many systems, the choice of exchange− correlation functionals is key for some systems, such as the Mn dimer. Note, for example, that obtaining the minimum caused by van der Waals interactions60,80,81 at distances larger than 3 Å requires using hybrid functionals82 or even the CASSCF (complete active space self-consistent field) method,59 which reproduces the antiferromagnetic coupling and the bond length larger than 3 Å obtained in experiments.61,83−86 The strong Mn−Mn bonding in the Mn2 molecule at short distances can therefore be computed accurately enough using generalized gradient approximation (GGA) functionals. The low-spin solutions (with 0 and 2 μB) shrink to local minima at bond lengths of about 1.8 Å, as reported previously.87,88 The minimum at short distances is also favored in the bonding in Mn2Bz2 because the magnetic moment in the 3d Mn levels is decreased by the Bz molecules, so the Mn−Mn distances are dramatically reduced. Implications for the Synthesis of Mn2Bz2. The MnBz2 molecule has been synthesized and identified using spectroscopic techniques in the same way as for other TM−Bz compounds.18 Larger Mn−Bz compounds are, however, difficult to synthesize.89 It seems that electron spin restrictions may prevent their synthesis. Here, we investigate the path for the synthesis of Mn2Bz2 by estimating energy barriers. The possible barriers are extracted from the energy versus Bz−Bz distance curves for different total magnetic moments, as shown in Figure 3. Further computational details are given in the Supporting Information. The total magnetic moment decreases from long to short distances, and a barrier appears as the

Figure 2. (a) Total energy versus Mn−Mn bond length for the Mn2Bz2 cluster. (b) Total energy versus distance for the Mn2 dimer. Each curve has the total magnetic moment fixed at a different value.

become about 0.1 μB, which are not fully depleted in comparison to the Bz−Mn molecule because a H atom from cyclopentadiene is bound to a Mn atom. For the borazinecaged Mn2, the Mn−Mn distance is 1.82 Å and the local and total magnetic moments are fully depleted.] Then, the ultrashort Mn−Mn bond effect, accompanied by a significant decrease in the magnetic moment of Mn, seems to be robust, and it is expected to be found for other molecules caging manganese atoms. Simplest Case: Mn2Bz2. We focus on the smaller Mn2Bz2 molecule to investigate the ultrashort Mn−Mn bond, also found in larger molecules. The fully relaxed Mn2Bz2 molecule has an unexpected Mn−Mn bond length of 1.8 Å, as shown in Figure 1. However, the Mn dimer has been found to be stabilized by van der Waals interactions59 and to have a bond length of more than 3 Å. Bulk metallic Mn is predicted to have interatomic distances of between 2.25 and 2.95 Å, depending on the allotropic structure. The Mn−Mn distance decreases when shifting from van der Waals-like bonds to metallic-like bonds. However, our results are surprising because the Mn− Mn bonds can be ultrashort. This means that the Mn−Mn bonding mechanism in hybrid Mn−Bz molecules is different from the one seen in Mn clusters and bulk Mn.60 We find a nonmagnetic solution for the Mn2Bz2 molecule that contrasts with the high-spin solutions obtained from most DFT calculations for the Mn dimer.60,61,73−76 Benzene molecules reduce spin-splitting in the Mn atoms, and electrons fill the minority-spin bonding molecular orbitals instead of the majority-spin antibonding molecular orbitals. The ultrashort Mn−Mn distance is therefore related to the loss of magnetism. For spin-compensated solutions, the Mn dimer in Mn2Bz2 can be further compressed because electrons between the Mn atoms suffer less electronic repulsion when they fill the same D



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ACKNOWLEDGMENTS This work was partially supported through project FIS201676617-P funded by the Spanish Ministry of Economy and Competitiveness MINECO, by the Basque Government under the ELKARTEK project (SUPER), and by the University of the Basque Country (Grant IT-756-13). T.A.-L. acknowledges a grant provided by the MPC Material Physics CenterSan Sebastián. F.A.-G. acknowledges the DIPC for their generous hospitality. The authors also acknowledge the support of the DIPC computer center.

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Figure 3. Mn2Bz2 total energies for different distances between the benzene molecules. The curves were obtained by fixing the total magnetic moment.

Mn2Bz2 molecule forms at high spin. We can overcome this barrier by, for instance, bringing the Mn−Mn core into a lowspin state sandwiched between distant Bz molecules. We propose that using thermal or optical pumping techniques on the Mn cluster and Bz molecule precursors could help to find routes for synthesizing the Mn2Bz2 molecule and larger Mn−Bz clusters.

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CONCLUSIONS We studied Mn−Bz molecules with rice-ball configurations. We found a multiple (triple) bond between the inner manganese atoms in the core metal cluster. Two hallmarks of this effect are a strikingly short Mn−Mn bond length of about 1.8 Å and a lack of magnetism. These can be explained because every two Bz molecules stabilize the two neighboring Mn atoms with no spin polarization. Furthermore, other molecules such as borazine and cyclopentadiene have a similar effect on manganese atoms. We also found that the ground states of larger molecules, such as Mn3Bz3 and Mn4Bz4, stabilize the irregular structures of the molecules with ultrashort Mn−Mn bonds. Lastly, we investigated the roles of different spins, looking at the Mn2Bz2 energy as the Bz−Bz distance varied. There are energy barriers that could be overcome by canceling local Mn spins using optical or thermal techniques. These results suggest that further experimental progress can be made in synthesizing larger Mn−Bz clusters that are of interest in the field of organometallic compounds.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b07828. Molecular orbitals and electron localization function of Mn2Bz2, information about the energy barrier calculations, Mn−Bz sandwiches compounds, and charge state of Mn−Bz compounds (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tomás Alonso-Lanza: 0000-0002-3005-8663 Jhon W. González: 0000-0003-2238-5522 Notes

The authors declare no competing financial interest. E


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