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Size-Dependent Ligand Quenching of Ferromagnetism in Co3(benzene)n+ Clusters Studied with XMCD Spectroscopy Scott T. Akin, Vicente Zamudio-Bayer, Kaining Duanmu, Georg Leistner, Konstantin Hirsch, Christine Buelow, Arkadiusz Lawicki, Akira Terasaki, Bernd von Issendorff, Donald G. Truhlar, J. Tobias Lau, and Michael A. Duncan J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b01839 • Publication Date (Web): 25 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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The Journal of Physical Chemistry Letters

J. Phys. Chem. Lett.

Size-Dependent Ligand Quenching of Ferromagnetism in Co3(benzene)n+ Clusters Studied with XMCD Spectroscopy Scott T. Akin,1 Vicente Zamudio-Bayer,2,3 Kaining Duanmu,4 Georg Leistner,2,5 Konstantin Hirsch,2,5 Christine Bülow,2,5 Arkadiusz Ławicki,2 Akira Terasaki,6,7 Bernd von Issendorff,3 Donald G. Truhlar,4* J. Tobias Lau,2* and Michael A. Duncan1* 1

Department of Chemistry, University of Georgia, Athens, Georgia 30602, U.S.A. Institut für Methoden und Instrumentierung der Forschung mit Synchrotronstrahlung, Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Straße 15, 12489 Berlin, Germany 3 Physikalisches Institut, Universität Freiburg, Stefan-Meier-Straße 21, 79104 Freiburg, Germany 4 Department of Chemistry, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, Minneapolis, Minnesota 55455, U.S.A. 5 Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany 6 East Tokyo Laboratory, Genesis Research Institute, Inc., 717-86 Futamata, Ichikawa, Chiba 272-0001 Japan 7 Department of Chemistry, Faculty of Science, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan 2

Abstract Cobalt-benzene cluster ions of the form Co3(bz)n+ (n = 0−3) were produced in the gasphase, mass-selected, and cooled in a cryogenic ion trap held at 3−4 K. To explore ligand effects on cluster magnetic moments, these species were investigated with X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) spectroscopy. XMCD spectra yield both the spin and orbital angular momenta of these clusters. Co3+ has a spin magnetic moment of µS = 6 µB and an orbital magnetic moment of µL = 3 µB. Co3(bz)+ and Co3(bz)2+ complexes were found to have spin and orbital magnetic moments identical to the values for ligand-free Co3+. However, coordination of the third benzene to form Co3(bz)3+ 1 ACS Paragon Plus Environment


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completely quenches the high spin state of the system. Density functional theory calculations elucidate the spin states of the Co3(bz)n+ species as a function of the number of attached benzene ligands, explaining the transition from septet to singlet for n = 0→3.

Keywords: metal clusters, magnetism, metal-ligand interactions, X-ray magnetic circular dichroism, density functional theory, electronic structure

Table-of-contents graphic:

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The magnetism of transition metal clusters and nanoparticles has significant potential applications in areas such as magnetic storage or spintronics.1-2 Measurements of cluster magnetic properties have been conducted on isolated clusters in the gas phase3-13 and for clusters supported on surfaces.14-18 The seminal work on magnetic moments of neutral metal clusters employed Stern-Gerlach deflection experiments in molecular beams.3-13 For the smallest cluster sizes, these experiments suffer from difficulties cooling the small metal clusters into their lowestenergy states. However, recent work has applied X-ray magnetic circular dichroism spectroscopy to trapped and cooled gas phase metal cluster ions.19-26 Unlike Stern-Gerlach experiments on neutrals, this approach yields magnetic moments that can be separated into spin and orbital components.27,28 In the present study, we extend these new measurements to study the effects of benzene ligand coating on cobalt cluster ions. The magnetic moments of small clusters or nanoparticles may be enhanced significantly compared to those observed for the same species in the bulk.1 Per-atom spin moments for clusters may be 50% higher than bulk moments, while their orbital moments may be an order of magnitude greater.1,19,20,22,29-31 Electron delocalization and crystal-field effects are responsible for the lower magnetic moments in the bulk. Small transition metal atom clusters may have both spin and orbital contributions to their magnetic moment3-13,26 and are superparamagnetic in the gas phase, i.e., they are paramagnetic with a large total moment. Significant orbital angular momentum is possible because the atoms in such clusters are mostly on the surface, with low coordination. The trends in magnetism for isolated metal clusters of various sizes have been well studied, as well as the temperature dependence of the magnetization.3-26,28-33 Cobalt, iron, and nickel clusters have been studied because of their expected large magnetic moments. However, applications require materials distributed on surfaces or suspended in inert solids or films, and this raises questions about metal-support interactions. Consequently, atoms and small clusters of 3 ACS Paragon Plus Environment


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these metals have been investigated on several surfaces using a variety of techniques.29-47 Small neutral cobalt clusters have been studied with Stern-Gerlach experiments and found to have magnetic moments in the range of 2 µB per atom.4-9,11 The spin and orbital magnetic moments of the smallest cation clusters of cobalt (8−22 atoms) have recently been reported, with µS and µL values of 2.0−2.7 and 0.4−1.0 µB per atom, respectively.19,22 The cobalt dimer cation has also been measured, with µS and µL values of 2.5 and 0.5 µB per atom.26 Ligand interactions and their effects on the magnetism of neutral clusters have also been investigated.48-53 Such studies are interesting because naturally occurring metal clusters are coordinated, and chemistry involving metal clusters often occurs in the solution phase. In general, the greater coordination of ligated clusters is expected to reduce their orbital magnetic moment, but ligand effects on spin moments are also possible. Thus, ligand addition often reduces magnetic moments, but in some cases it has been found to enhance the magnetism. Knickelbein has investigated ligand effects on neutral nickel clusters with added carbon monoxide, oxygen, and hydrogen, as well as iron clusters coated with hydrogen.51-53 All three ligands reduced the magnetic moments of nickel clusters,51,52 but hydrogen enhanced the magnetic moments of iron clusters.52 In other experiments, Stern-Gerlach measurements were conducted on a variety of transition metal-benzene clusters, including those believed to have multiple-decker sandwich structures or those with multi-metal cores and adsorbed benzene.54-56 Whereas magnetic moments in the 1.9–2.5 µB per atom range were observed for naked cobalt clusters, those with adsorbed benzene had significantly reduced moments. However, these measurements were made on neutrals, and it was not possible to separate the orbital versus spin components of the magnetic moments or to correlate the ligand effect with the specific electronic structure of the system. Various computational studies have also been performed on metalbenzene systems, predicting both enhanced or quenched magnetic moments in different 4 ACS Paragon Plus Environment

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systems.57-65 Here we use X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) spectroscopy in conjunction with computational work to study the cobaltbenzene system, focusing on Co3+ with different numbers of attached benzene ligands. Com(bz)n+ clusters are produced and studied in the Nanocluster Trap endstation at the BESSY II beamline UE52-PGM shown in Figure 1.66 Com+ clusters produced with a magnetron source are interacted at room temperature with ≈10-6 mbar benzene vapor in the hexapole ion guide region of the instrument to form Com(bz)n+ species (see mass spectrum in Figure S1 of the Supporting Information). These ions are mass selected and held in a cryogenic ion trap cooled to 3−4 K. X-ray absorption causes dissociation, producing Co+ and benzene+ ions along with additional smaller hydrocarbon ions resulting from the fragmentation of benzene (see fragmentation mass spectrum in Figure S2 of the Supporting Information). The fragmentation pattern varied for different clusters, but did not vary with the X-ray energy. The combined intensities of all fragment ions were recorded as a function of photon energy to produce XAS spectra.66 Figure 2 shows the resulting spectra for the Co3(bz)n+ (n = 0−3) complexes. The red dashed lines mark the onsets of the L3 (2p3/2 → 3d) edge (777.5 ± 0.3 eV) and the L2 (2p1/2 → 3d) edge (792.8 ± 0.3 eV) for the ligand-free Co3+ cluster. Clusters with an increasing number of benzenes exhibit a shift to higher energy in the L3 onset (777.9 ± 0.2, 777.8 ± 0.1, 779.0 ± 0.2 eV for n = 1, 2, 3) and additional intensity in the second, higher energy L3 peak. The n = 1 and 2 spectra are quite similar to that of Co3+, but the n = 3 spectrum changes significantly. The L3 edge for Co3(bz)3+ has an even greater intensity in the second L3 peak and a new third feature. Such an appearance of a new line together with the large shift in energy is usually associated with a substantial change in the electronic structure, such as an oxidation state change on the metal.67 These multiplet features likely result from the shifts in the 3d-based valence states 5 ACS Paragon Plus Environment


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induced by the benzene bonding. The onset of the Co3(bz)3+ L3 edge occurs at 779.0 ± 0.2 eV, a large shift of 1.5 eV from that for Co3+. Delocalization of 3d orbitals weakens the 2p-3d Coulomb repulsion. This increases 2p electron binding energies and leads to a blue-shift in the absorption onset.68,69 The trend in the L2 edge mirrors that of the L3 edge, with blue shifts in the onsets (792.7 ± 0.2, 792.8 ± 0.1, 793.7 ± 0.2 eV for n = 1, 2, 3) and a second feature becoming more prominent with the addition of one and two benzenes. Again, the third benzene produces even greater intensity in the second peak at the L2 edge and the appearance of a third feature here. Figure 3 shows the XMCD spectra, taken from the difference of the positive and negative circularly polarized spectra (the individual spectra are shown as Figure S3 in the Supporting Information). The XMCD spectra for n = 0−2 are similar, exhibiting a strong negative dichroism at the L3 edge followed by a weaker positive dichroism at the L2 edge. In stark contrast to this, Co3(bz)3+ exhibits virtually no dichroism, indicating that it has no net magnetic moment. The determination of spin and orbital moments from the XMCD spectra depends on the detected total magnetization (see raw data in Supporting Information, p. S8), the number of unoccupied d orbitals (holes), and the cluster temperature (see below). The ratios of the orbital to spin angular momenta in these clusters are determined from the integrated intensities of the dichroism at the L3 and L2 edges and the application of the XMCD sum rules.27,28 These ratios are: 0.47 ± 0.05, 0.47 ± 0.10, and 0.49 ± 0.29 for the Co3+(bz)n, n = 0−2 clusters respectively. The value for Co3+(bz)3 was not determined as there is no dichroism for the n = 3 cluster. While the orbital/spin ratio is the most reliable quantity in XMCD, this ratio is derived from the difference of two spectra recorded with opposite polarizations. Consequently, the noise observed in the XMCD process is compounded by the noise recorded in the individual spectra. The noise is worse for larger clusters which have signal distributed over more fragmentation channels. The 6 ACS Paragon Plus Environment

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uncertainties in the dichroism are therefore larger, producing greater uncertainties in the orbital/spin ratios determined. This and the possible range of cluster temperatures can result in different assignments for the orbital and spin angular momenta. With the orbital/spin ratio measured, the number of holes and the cluster temperature must be adjusted using chemical insight and experience from other similar experiments on this instrument to determine µS and µL. We initially assign the number of 3d holes as 2 per atom because of the atomic (3d74s2) and ionic (3d8) configurations. This means there are two unpaired electrons on each cobalt atom, six unpaired electrons total for the cluster, and a value of µS = 2S = 6µB. Since the ratio of orbital/spin moments is essentially 0.5 for Co3+ through Co3(bz)2+, the orbital moment is then µL = 3µB, or 1µB per atom, which is significantly higher than the value of ≈ 0.14 µB in bulk cobalt.70 Other assignments for the number of holes, and therefore the resulting µS and µB values, are also possible, if they produce moments consistent with those measured and temperatures within the expected range of the experiment. On the basis of previous work, the temperature of the ions should be about 15−30 K. This is higher than the temperature of the trap walls (3−4 K), consistent with some RF heating of the ions during their trapping time.20-26 Because S is quantized (±1/2 per electron) and because the total number of electrons in the system is even, the possible values for S corresponding to reasonable temperatures are 2S = 4, 6, and 8. We exclude 2S = 4 for Co3(bz)+ because as the temperature required for this assignment (T = 4 K) is exactly that of the trap, thus not allowing for any rf heating. All other states lead to more reasonable temperatures of 12 K < T < 31 K for all cluster sizes. Since we observe very similar spectra for Co3+, Co3(bz)+, and Co3(bz)2+, it is likely that their spin states are the same, and we also rule out 2S = 4 for Co3+ and Co3(bz)2+. We prefer 2S = 6 over 2S = 8 because the lower value is 7 ACS Paragon Plus Environment


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consistent with the typical rf heating in our ion trap. Figure S4 in the Supporting Information shows different values of µS and their correspondence to different temperature values. Therefore, the values of µS = 6µB and µL = 3µB fit best with all the known experimental parameters. As shown below, this assignment also fits best with the theory. A more detailed description of the analysis is presented in the Supporting Information. Figure 4 shows the cluster size dependence of the orbital and spin angular momenta for n = 0−3. For the n = 0−2 clusters, the values of µS and µL are 6 and 3 µB, respectively. As the error bars for Co3(bz)2+ are relatively large, it is not possible to determine its ground state unambiguously. However, as noted above, µS = 6 is the most likely choice. Finally, Co3(bz)3+ shows an abrupt change to no significant dichroism at the L3 or L2 edge, indicating that µS = 0 and µL = 0. These assignments are consistent with theoretical predictions, which are discussed in more detail below. The addition of the first two benzene molecules to the Co3+ cluster have virtually no effect on the magnetic moment, but the third benzene quenches the ferromagnetic spin coupling completely. To understand the change in spin states of Co3(bz)n+, we first calculated the electronic structure of Co3+ using density functional theory (DFT)71 with the def2-TZVP basis set72 and six exchange-correlation functionals that have been shown to be accurate for 3d transition metals.73 In particular, we selected the best local functional, M06-L,74 the four hybrid functionals M08SO,75 SOGGA11-X,76 B97-3,77 and MPW1B95,78 and one range-separated hybrid functional with post-SCF molecular-mechanics damped-dispersion energy, ωB97X-D.79 The geometry of the lowest-energy structure calculated by M06-L is given on page S32 of the Supporting Information. We found severe spin contamination in calculations on the singlet and triplet states, and the solution to the self-consistent-field equations is not a spin eigenfunction; based on previous experience, we chose to use the variational method,73,80 which directly uses the broken8 ACS Paragon Plus Environment

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symmetry lowest-energy solutions with MS equal to the target S value. Table 1 shows that all the functionals predict that the triplet and septet states are 11−26 kcal/mol lower in energy than the singlet and quintet states, and the septet state is 0.1−1.6 kcal/mol lower in energy than the triplet state. The latter result, that the septet is the ground state, agrees with the assigned spin from the XMCD spectra. These results can be explained as follows. The Mulliken spin densities and NBO81-84 analysis indicate a local electron configuration of 4s2/33d8 on each cobalt atom for singlet though septet, while only the d electrons contribute to the spin magnetic moments. Figure 5 shows that the singlet and quintet must have one cobalt atom with a low spin configuration, while the triplet and septet have all high-spin cobalt atoms, which are more favorable. This explains why triplet and septet have much lower energies. The only difference between the triplet and the septet is magnetic coupling; all three cobalt atoms are ferromagnetically coupled in the septet, but one of them is anti-ferromagnetically coupled to the other two atoms in the triplet. We then calculated the electronic structures of Co3(bz)n+, n = 1−3, using DFT with M06L and M08-SO. We found that Co3+ is bent and for n = 1 and 2 that structures with side-on benzene are higher in energy than structures with Co sitting above the center of the benzene ring, so we only consider the latter. We also used the variational method to determine the energies for spin-contaminated singlets and triplets. The M06-L results in Table 2 show that the energy gap between the singlet or quintet and the triplet or septet decreases as more benzenes are coordinated to Co3+. Mulliken spin densities and NBO analysis show that the electronic structures of the Co3+ core do not change for any of the spin states of Co3(bz)+. This explains why the lowest-energy spin state of Co3(bz)+ remains a septet. However, the electronic structures of the Co3+ core do change for some cases when n > 1. In particular, the spin densities on the three cobalt atoms change to -2.6, 1.3, 1.3 for singlet Co3(bz)2+, to 1.5, 1.3, 1.3 for quintet 9 ACS Paragon Plus Environment


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Co3(bz)3+, and to -2.2, 1.1, 1.1 for singlet Co3(bz)3+. These changes do not affect the energy ordering of the former two species, but they significantly lower the energy of singlet Co3(bz)3+, thus making it the ground spin state. Therefore, according to the M06-L results, Co3(bz)n+ complexes have septet ground states for n = 0, 1, 2 and a singlet ground state for n = 3, which agrees with the XMCD spectra. Next we further analyze the electronic structure change for Co3(bz)3+, in which each cobalt atom is approximately sitting above the center of a benzene ring (the line through cobalt and the benzene center is approximately normal to the benzene plane). The Co-benzene-center distances are 1.55, 1.55, and 1.62 Å respectively; the average is 1.57 Å. We created a dissociation path by retaining the geometry of the Co3+ core and simultaneously pulling each benzene away along the cobalt-benzene center line by the same percentage of the equilibrium distance. The energy changes are calculated with M06-L/def2-TZVP and are shown in Figure 6, where the abscissa is the average Co-benzene-center distance (D). From the discussion above, we know that the electronic structures of the singlet and quintet Co3+ cores in Co3(bz)3+ are different from those of bare Co3+, so when the benzenes are pulled away, at some point their electronic structures will change. We found such changes in electronic structure near 2.5 Å for the quintet and near 2.8 Å for the singlet, which means the spin densities on the three cobalt atoms for the quintet are 1.5, 1.3, 1.3 at D < 2.5 Å and 0, 2, 2 at D ≥ 2.5 Å, and those for the singlet are -2.2, 1.1, 1.1 at D < 2.8 Å and 0, 2, -2 at D ≥ 2.8 Å. The figure shows that the triplet and septet rapidly become the lower-energy spin states as the benzenes are pulled away, which illustrates that the interaction with benzenes changes the electronic structures of singlet and quintet Co3+ at long distance, but this interaction significantly lowers their energies only at short distance.


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The M08-SO results are shown in the Supporting Information, and they basically agree with the M06-L results except for Co3(bz)3+, which is predicted to be a septet by M08-SO. Considering that M08-SO has 57% Hartree-Fock exchange and that functionals with high Hartree-Fock exchange are well known to favor high spin states, this difference is not surprising. We also did a test by adjusting the percentage of Hartree-Fock exchange in MPW1B95. This functional has 31% Hartree-Fock exchange, and the singlet of Co3(bz)3+ is calculated to be 27.8 kcal/mol higher in energy than the septet, but the energy gap drops to 15.6 kcal/mol if the Hartree-Fock exchange is adjusted down to 20%. In light of this sensitivity of the predicted spin state to the character of the functional, it is encouraging that all six functionals studied here, which – as mentioned above – were selected based on previous tests for spin-state energetics, agree with the experimentally determined ground spin states for Co3+, and M06-L agrees for all Co3(bz)n+ species. In this study we show that a sudden and large (9 µB) quenching of magnetic moments occurs for the Co3+ cluster through the addition of benzene ligands. Using XMCD as a probe, we determine that Co3(bz)+ and Co3(bz)2+ both preserve the spin and orbital magnetic moments of Co3+, while Co3(bz)3+ has a µS = 0 and µL = 0 ground state, with quenched spin and orbital magnetic moments. DFT calculations agree with this conclusion, finding a spin state change to a singlet for the n = 3 complex. Computational studies on multi-atom transition metal clusters are notoriously problematic, as these systems possess many electronic states lying close in energy. Experimental measurements of magnetism are likewise challenging. In this particular case, DFT with M06-L calculations predict at most two relatively close electronic states, with the lowest predicted state matching the assignment from the low temperature XMCD measurements for all clusters sizes. The combined approach of XMCD measurements and DFT calculations makes it possible to see the electronic state energetics and spatial aspects of electron distributions that first 11 ACS Paragon Plus Environment


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cause and then quench the magnetism in this system. The XMCD method described here provides the potential to more clearly elucidate the magnetic properties of ligand-coated metal nanoclusters and to reveal the effects of their local molecular environments.

Experimental A liquid nitrogen cooled magnetron gas aggregation source is used to produce cobalt cluster ions.66 Benzene vapor is added into the hexapole ion guide section of the apparatus, where collisions with cobalt cluster ions produce the desired cobalt-benzene cluster ions. These ions are size selected in a quadrupole mass filter, turned 90° into a radiofrequency quadrupole ion trap, and interrogated with X-ray absorption spectroscopy and X-ray magnetic circular dichroism spectroscopy at the L2,3 edge of cobalt and the K-edge of carbon. X-ray absorption is detected via the yield of all ions produced by photodissociation. For XMCD measurements, the total magnetic moments of size-selected clusters are aligned by a homogenous magnetic field with µ0H = 5 T of a superconducting solenoid that is placed around the ion trap.20-26 The ion trap housing and electrodes are cooled to a temperature of 3−4 K by a flow of liquid helium, and the cluster ions are thermalized by a constant flow of precooled helium buffer gas to a temperature of 10−30 K. The monochromatic and elliptically polarized soft X-ray synchrotron radiation beam from an undulator beamline (UE52-PGM) at the Berlin synchrotron radiation facility BESSY II is coupled in along the trap axis for photoexcitation of the clusters at the cobalt L2,3-edge and carbon K-edge. Photoexcitation at the cobalt L2,3-edge leads to dipole-allowed transitions from 2p core levels into unoccupied d and s valence states as well as to direct valence and core-level photoionization. In this process, 3d states are predominantly probed because of the large transition matrix element, which leads to Xray absorption cross sections that are larger by one order of magnitude than transitions into the 12 ACS Paragon Plus Environment

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higher ns and nd (n > 3) states. This excitation scheme allows us to probe the magnetic moments of cobalt benzene cluster cations, which are carried by the 3d electrons. The 2p core hole that is created in the X-ray absorption process relaxes via Auger decay, leading to highly charged clusters which dissociate into a number of fragment cations that are also stored in the ion trap. Bunches of parent ions and product ions are extracted from the ion trap by a pulsed exit aperture potential, and are detected by a reflectron time-of-flight mass spectrometer. To record ion yield spectra as a measure of X-ray absorption, time-of-flight mass spectra were measured for parallel (σ+) and antiparallel (σ-) orientations of the magnetic field H and photon helicity σ with a total data acquisition time of 8−24 s per photon energy step. These XAS and XMCD spectra were recorded with a typical photon flux of 1−5 x 1012 photons per second, and an energy resolution of 250 meV. Onsets and uncertainties were determined as described previously.69 All spectra were normalized to the incident photon flux, monitored with a GaAsP photodiode mounted on-axis behind the ion trap.

Computational All density functional theory (DFT) calculations as well as the Mulliken spin densities and NBO analysis were carried out using a locally modified Gaussian09, MNGFM-6.7.85,86 Lowest energy structures Co3(bz)n+ (n = 0−3) for singlet through nonet spin states were optimized with no symmetry restrictions. Magnetic anisotropy energies and density of states for the most stable spin states of Co3(bz)n+ (n = 0−3) were calculated using VASP 87,88 and are shown in the SI.



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ASSOCIATED

CONTENT

Supporting Information Mass spectra, additional XAS spectra and more complete details of the computational work. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR

INFORMATION

Corresponding Authors *Email: [email protected] (DGT), [email protected] (JTL), [email protected] (MAD). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

We gratefully acknowledge the generous support for this work from the Air Force Office of Scientific Research through grant no. FA9550-15-1-0088 (MAD). Travel support for STA was provided by the Deutsche Forschungsgemeinschaft project FOR1282, while that for MAD was provided by the Alexander von Humboldt Foundation through a Senior Fellowship. Beam time was provided by HZB on the UE52-PGM Nanocluster Trap endstation at BESSY II. The apparatus was supported by the German Federal Ministry of Education and Research grant BMBF-05K13V. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 and those of the University of Minnesota Supercomputing Institute. The density functional portion of the work 14 ACS Paragon Plus Environment

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was supported as part of the Inorganometallic Catalysis Design Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award DE-SC0012702.



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Table 1. Relative energies (kcal/mol) of singlet through nonet spin states of Co3+.

functional

singlet

triplet

quintet

septet

nonet

M06-L

24.1

1.6

25.6

0.0

15.0

M08-SO

12.2

1.1

11.7

0.0

26.9

SOGGA11-X

17.5

0.4

17.0

0.0

20.8

B97-3

16.8

0.1

16.7

0.0

19.8

ωB97X-D

16.6

0.8

16.3

0.0

25.1

MPW1B95

16.7

0.6

16.6

0.0

20.3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 2. Relative energies of Co3(bz)n+ in kcal/mol for singlet through nonet spin-states at the M06-L/def2-TZVP level. Co3(bz)n+

singlet

triplet

quintet

septet

nonet

0

24.1

1.6

25.6

0.0

15.0

1

12.8

3.2

9.4

0.0

22.8

2

11.8

1.0

8.6

0.0

30.2

3

0.0

0.2

7.8

4.8

65.9


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Figure Captions

Figure 1. Schematic view of the instrument. The cluster ion beam travels from bottom right to top left. The X-ray beam intersects cooled ions in the quadrupole trap.

Figure 2. X-ray absorption spectra of Co3(bz)n+ (n = 0−3). The dashed red lines show the onset of the L3 and L2 absorption for Co3+.

Figure 3. XMCD spectra for Co3(bz)n+ (n = 0−3). The intensities are directly comparable between all XMCD traces. The transition from a large XMCD (n = 0−2) to vanishing XMCD (n = 3) indicates the quenching of magnetic moments.

Figure 4. Spin (black) and orbital (red) angular momenta from XMCD spectra for Co3(bz)n+ (n = 0−3). The data shown are the "raw" magnetization values per atom. These values are multiplied by two for the partial magnetization observed and by three to get the moments for the entire cluster. Theoretical structures shown as insets have septet (n = 0−2) and singlet (n = 3) spin states. Structures were optimized at the M06-L/Def2-TZVP level.

Figure 5. Electronic structures of the singlet through septet spin states of Co3+.

Figure 6. Energy change with increasing average Co-benzene-center distance for the Co3(bz)3+ cluster. If the electronic structure of the Co3+ core in these clusters were exactly the same as that in isolated Co3+, the lowest energy state at large separation would be the septet.



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Figure 1.


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Figure 2.



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Figure 3.


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Figure 4.



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Figure 5.


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Figure 6.


Size-Dependent Ligand Quenching of ... - ACS Publications

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