ARTICLE pubs.acs.org/JPCC
Precise Chemical, Electronic, and Magnetic Structure of Binuclear Complexes Studied by Means of X-ray Spectroscopies and Theoretical Methods Karsten Kuepper,*,† David M. Benoit,‡,r Ulf Wiedwald,† Florian M€ogele,§ Andreas Meyering,|| Manfred Neumann,|| Jean-Paul Kappler,^ Loïc Joly,^ Stefan Weidle,# Bernhard Rieger,# and Paul Ziemann† Institute of Solid State Physics, ‡Nachwuchsgruppe Theorie-SFB 569, §Institute of Materials and Catalysis, Ulm University, Albert-Einstein-Allee 11, D-89069 Ulm, Germany Department of Physics, University of Osnabr€uck, Barbarastrasse 7, D-49069 Osnabr€uck, Germany ^ IPCMS UCMS 7504 CNRS—Universite de Strasbourg, 23 rue du Loess, BP 43, 67034 Strasbourg Cedex 2, France # WACKER—Lehrstuhl f€ur Makromolekulare Chemie, Technische Universit€at M€unchen, Lichtenbergstrasse 4, D-85747 Garching bei M€unchen, Germany
)
†
ABSTRACT: We investigate two planar complexes MnNi and CoNi (see Scheme 1) by X-ray photoelectron spectroscopy (XPS) and ultralow-temperature X-ray magnetic circular dichroism (XMCD). In this way the valence states as well as the presence of uncompensated magnetic moments are obtained. The magnetism has been probed at a temperature of 0.6 K in order to reveal the magnetic ground state properties. We find that divalent Ni ions are in a diamagnetic low spin ground state in both complexes; however, in MnNi a small fraction of divalent nickel high-spin ions leads to a residual XMCD signal, indicating parallel spin alignment with the Mn spins. Mn and Co are found to be in a divalent high-spin configuration in both compounds. Theoretically, we address the energetic ordering of the different possible spin states of the binuclear complexes using (zeroth-order) relativistic approximation density functional calculations and a triple-ζ quality basis set. These results show that intermediate-spin states are often favored over low-spin states for most both metal combinations, in qualitative agreement with our experimental observations.
’ INTRODUCTION Magnetic molecules have attracted huge interest since the discovery of single magnetic molecules like the Mn12Ac cluster1 exhibiting a magnetic bistability. Metallo-organic chemistry provides an enormous flexibility to synthesize transition metal based molecular complexes showing magnetic long-range interactions. Among the most promising magnetic molecules are polymetallic clusters which comprise transition metal ions multiply bridged via different ligands to mediate exchange interactions between the paramagnetic centers. As a result, ground states with relatively large total spin may be found.24 Besides the intensely studied dodecanuclear complexes [Mn12O12(O2CR)16 (H2O)x]n (n = 0, 1, 2; x = 3, 4), a number of other manganese containing clusters with nuclearity ranging from n = 2 to 84 have been investigated revealing a number of interesting properties such as a high spin ground state, huge magnetic anisotropy, blocking temperatures of a few Kelvin but also quantum tunneling of magnetization, and quantum interference,511 which make them potential candidates for practical applications in molecular spintronics.1214 However, for new memory applications like ultradense magnetic data storage arrays based on storing information on each single molecular magnet, small metallo-organic complexes which can be reliably deposited and maybe even controlled via a ferromagnetic surface are highly desirable. Recently, some remarkable progress in r 2011 American Chemical Society
this field has been reported, e.g., Scheybal et al. reported ferromagnetic coupling of Mn ions in manganese tetraphenylporphyrin on ferromagnetic substrates.15 Wende et al. were able to switch the magnetic ordering of iron porphyrin molecules via metallic Co and Ni substrates,16 Mannini et al. wired a star-shaped Fe4 complex to a gold surface which then showed single magnetic molecule behavior, i.e., an open magnetic hysteresis loop at low temperature,17 and also quantum tunneling of the magnetization via tailoring the orientation of the Fe4 complexes could be realized.18 Furthermore ferromagnetic coupling of Mn ions in manganese phthalocyanine (MnPc) with ferromagnetic surfaces via covalent bonding has been successfully demonstrated.19 Very recently, another interesting candidate for molecular spintronics in the form of azopyridine functionalized Ni-porphyrin has been reported by Venkataramani et al.20 In this paper a light-induced switching between a paramagnetic high spin state (5-fold bipyramidal coordinated Ni2+) and a diamagnetic low spin state (4-fold square planar coordinated Ni2+) has been demonstrated. In most square planar coordinated Ni2+ complexes the Ni ion is found in a diamagnetic low spin (singlet) ground state. A number of those complexes are subject to
Received: July 21, 2011 Revised: October 26, 2011 Published: October 28, 2011 25030
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Scheme 1. Synthesis of the Mono-/Binuclear Metal Complexes MnNi and CoNi from the Ligand 1
intense research due to their intriguing redox properties, opening a potential route to electrochemical or biocatalytic applications.2124 Recently, M€ogele et al.25 presented results on two-dimensional self-assembled monolayers of binuclear complexes comprising two different coordination sites,26,27 giving rise to more advanced applications in various fields such as sensors,2830 catalysis,31,32 or electrochemistry.3335 Moreover, these binuclear complexes are of particular interest for new building blocks for magnetic molecular electronics and storage devices due to the possibility of fine-tuning magnetic properties by usage of different transition metal ions within the same intramolecular ligand structure. First investigations of the magnetic properties of these complexes by means of superconducting quantum interference device (SQUID) magnetometry reveal magnetic moments between 0.27 and 1.73 μB per molecule, indicating a potential antiferromagnetic coupling of the two metal sites.25 However, in order to elucidate the internal electronic and magnetic properties in more detail, complementary experimental and theoretical approaches are required. Here we apply element-specific X-ray photoelectron spectroscopy (XPS) and X-ray magnetic circular dichroism (XMCD) to two particularly interesting binuclear complexes, namely, MnNi and CoNi (Scheme 1), in order to investigate the chemical, electronic, and magnetic properties. While the magnetization of MnNi could not be saturated within an external magnetic field of 5.5 T at a temperature of 2 K, CoNi exhibits a saturation moment of 1.73 μB per molecule.25 The synchrotron based technique of XMCD opens unique capabilities to probe magnetic properties in an element-specific way and, additionally, delivers information on orbital and spin moments,36,37 separately, thereby giving insight into the internal magnetic structure of MnNi and CoNi molecules. Complementary XPS measurements are sensitive to the charge transfer effects between transition metal ion and ligands. Our experiments are supported by relativistic firstprinciples electronic structure calculations, within the framework of density functional theory (DFT), and charge transfer multiplet simulations.
A detailed experimental and theoretical knowledge of the electronic structure is of general interest for any material whose properties are dominated by electron correlation effects; the experimental data can, e.g., be used to improve the theoretical approaches. In turn, with help of reliable localized and ab inito electronic structure approaches, one may predict the properties of a compound in question, opening a possibility to achieve desired properties for new materials, in our case potential new metallo-organic complexes interesting for molecular magnetism.
’ SYNTHESIS AND EXPERIMENTAL AND COMPUTATIONAL DETAILS The investigated binuclear metal complexes (MnNi and CoNi) were obtained as previously described by M€ogele et al.25 (Scheme 1). For the first complexation reaction the ligand 1 was treated with nickel acetate to afford the mononuclear complex 2. In the last step the metal(II) salts (MnCl2, MnNi; Co(OAc)2, CoNi), in the presence of lithium hydroxide (to deprotonate the hydroxy functions) and 2, were used to gain the binuclear complexes MnNi and CoNi, respectively. Magnetic susceptibilities were measured with a Quantum Design MPMS SQUID magnetometer. Dried powders of MnNi (17.58 mg) and CoNi (2.55 mg) were enclosed in gelatin capsules, and the total magnetic moment of the sample was determined as a function of temperature in external fields of μ0H = 5 mT and 1 T after cooling in zero field to 2 K. The magnetic signal of the container as obtained from a reference experiment on empty capsules was subtracted. The total signal (sample and container) was found in the range of 104 emu at an external field of 5 mT and 102 emu at 1 T being at least 2 orders of magnitude larger than the detection limit. Molar susceptibility χM was calculated using the sample mass, molar weight, and the external field. The error bar is on the order of 25%, mainly because of weighing of the powders and variations of capsules’ mass. The XMCD experiments were performed at the surface and interface microscopy (SIM) beamline of the Swiss Light Source 25031
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The Journal of Physical Chemistry C (SLS). We used the 7 T cryomagnetic TBT-XMCD endstation, working with a 3He4He dilution setup in order to reach base temperatures around 0.6 K.38 MnNi and CoNi powders were pasted on carbon tape before connecting the sample holder to the cryostat coldfinger. The spectra were recorded using the total electron yield (TEY). In order to control potential radiation damage effects, the intensity of the incoming photons was reduced to around 510% of the maximum photon flux, and data were taken at several spots on each sample (X-ray spot ∼150 μm). No significant changes could be observed in the Mn L2,3, Co L2,3, and Ni L2,3 spectra after several hours of X-ray exposure; thus degradation of the molecules by X-ray exposure or the generation of secondary electrons can be excluded. Complementary XPS measurements were performed at the Department of Physics, University of Osnabr€uck, Osnabr€uck, Germany, using a PHI 5600CI multitechnique spectrometer with monochromatic Al Kα, i.e., 1486.6 eV radiation with 0.3 eV full width at half-maximum. The overall resolution of the spectrometer is 1.5% of the pass energy of the analyzer, 0.35 eV in the present case. XPS measurements were performed at room temperature with spectra calibration based on C 1s states evolving from adsorbed hydrocarbons (285.0 eV). XAS and XMCD spectra were simulated within the charge transfer multiplet-model using the TT-multiplet program.3941 After calculating the atomic energy levels of the initial (2pn3dm) and final (2pn13dm+1) states, they were reduced to 80% of their HartreeFock values and a D4h square planar crystal field was considered, which has been approximated by the crystal field parameters 10Dq, Ds, and Dt. The relations Ds = 19/126*10Dq and Dt = 2/35*10Dq were applied as suggested by Miedema et al.42 for square planar geometries. The hopping integrals for the x2y2, z2, xy, and xy/yz orbitals have been set to 2.0, 2.0, 1.0, and 1.0 eV, which corresponds to the default values of the TTmultiplet program.3941 Finally, we considered an external magnetic field energy of 0.01 eV and charge transfer by introducing 3dm+1L states and broadened the simulated spectra considering lifetime broadening and spectrometer resolution. The energetic hierarchy of the different possible spin states of the binuclear complexes is examined using zeroth-order scalar relativistic approximation (ZORA)43 density functional calculations and a triple-ζ quality basis set, as implemented in the ORCA package. The geometry of each binuclear complex is first optimized using unrestricted BeckePerdew44,45 exchange correlation functional (ZORA-UBP86) for the different possible spin states. We use the def2-TZVPP-ZORA basis set from Pantazis et al.46 (def2-TZVP-ZORA for UBP86) and the resolution of the identity (RI) approximation, along with the appropriate auxiliary basis set, in order to speed up the calculations. The resulting geometries are then reoptimized using the unrestricted OPBE functional which is a combination of OPTX exchange47 with the PBE correlation functional.45 This particular functional has been shown48 to improve energetic ordering of spin states of transition metal complexes compared to UBP86. Moreover, OPBE leads to better geometries than the hybrid B3LYP functional49 where the presence of a portion of Hartree Fock exchange tends to overstabilize high-spin states.50
’ RESULTS AND DISCUSSION In Figure 1 we present the Ni 2p XPS from the CoNi complex and various reference compounds. The Ni 2p spectrum of CoNi
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Figure 1. Ni 2p XPS of the CoNi complex with some spectra of reference compounds for comparison, namely, NiO,62 NiII(tbu-salcn) (extracted from Shimazaki et al.22), NiII(pc) (extracted from Ottaviano et al.53), and NiII(AcAc)2 (extracted from Perera et al.54). The spectra have been brought to a common binding energy scale.
Figure 2. Co 2p XPS of the CoNi complex with a CoO spectrum for comparison, namely, CoO.
comprises two peaks located around 853.9 eV (2p3/2) and 873.9 eV (2p1/2). In contrast to the octahedrally coordinated Ni2+ ion in NiO (top spectrum in Figure 1) no satellites due to charge transfer are visible. Note that the double peak structure of this reference spectrum on the low binding energy side (labeled “I”) can be attributed to an intrinsic feature of NiO.51,52 Furthermore the Ni 2p spectra of the square planar complexes nickel phthalocyanine53 and of the vapor phase of nickel acetylacetonate54 are shown in Figure 1. The spectrum of Ni phthalocyanine shows a similar structure compared to that of the CoNi complex, with somewhat sharper 2p3/2 and 2p1/2 peaks, and no visible charge transfer satellites. Finally, we compare the Ni 2p XPS of CoNi with that of NiII(tbu-salcn) (salcn = N,N0 -(disalicylidene)-1,2cyclohexanediamine)),22 since this complex exhibits an identical ligand structure around the Ni2+ ion. These two Ni 2p XPS are almost identical. The absence of any satellite structure is consistent with a diamagnetic Ni2+ ion (singlet state).23 In contrast, the Co XPS of the CoNi complex exhibits a satellite structure due to charge transfer excitations (Figure 2). We compare the Co 2p XPS with a reference spectrum of CoO. The spectra are similar to each other with somewhat different charge transfer satellite structures and intensities. Specifically, the charge transfer satellites are closer to the Co 2p3/2 and 2p1/2 main peaks in CoNi. The overall similarity, however, between these spectra clearly indicates that Co is divalent and high spin 25032
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Figure 3. Top panel: Left and right polarized Co L2,3 XAS and the resulting XMCD performed in an external magnetic field of 6.5 T strength at a temperature of 0.6 K. Bottom panel: The corresponding isotropic XAS and XMCD signals in comparison with charge transfer multiplet simulations. For clarity, simulated spectra are slightly shifted with respect to the experimental data.
Figure 4. Isotropic XAS (left plus right polarized spectra) and the (within in the signal-to-noise ratio nonexisting) XMCD signal recoded at the Ni L2,3 edges in comparison with charge transfer multiplet simulations. For clarity, the simulated XAS is slightly shifted with respect to the experimental data.
(s = 3/2), and the Ni is divalent with a low spin singlet state in CoNi at room temperature. Next, we discuss the low temperature magnetic properties probed by XMCD. The top panel of Figure 3 displays the Co L2,3 XAS of CoNi excited with left and right circularly polarized light in an external magnetic field of 6.5 T. The spectra were recorded at a temperature about 600 mK, thus the sample is expected being close to its magnetic ground state. At the Co L2,3 edges of CoNi a strong magnetic dichroism is observed, in particular at the L3
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Figure 5. Top panel: Left and right polarized Mn L2,3 XAS and the resulting XMCD performed in an external magnetic field of 6.5 T strength at a temperature of 0.6 K. Bottom panel: The corresponding XAS (isotropic) and XMCD signals in comparison with charge transfer multiplet simulations. Note the constant offset of experimental and simulated data introduced for clarity.
edge a small shoulder at low photon energy is followed by an intense main peak around 781 eV comprising a double structure in form a kink followed by the absolute maximum in intensity at 781.5 eV. Another feature with positive sign can be seen at 785 eV. At the L2 edge, a rather weak and broad dichroic behavior can be observed, spanning the range from around 795 to about 799 eV. At the Ni L2,3 edges no XMCD was found (Figure 4), confirming a low spin state in an external field of 6.5 T at 0.6 K. From these experiments we can conclude that the Co ions are responsible for the overall magnetic moment in this complex, hence ruling out an antiferromagnetic coupling between the divalent Co and Ni ions in CoNi as suggested by SQUID magnetometry.25 We also compare our experimental results with corresponding charge transfer simulations. Scanning through the parameter space of the crystal field parameters 10Dq, Ds, and Dt, and the charge transfer potential Δ we find almost perfect agreement between the Ni L2,3 XAS and a charge transfer simulation with 10Dq = 2.75 eV, Ds = 0.16 eV, Dt = 0.415 eV, and Δ = 7 eV (cf. Figure 4), resulting in a simulated Ni2+ singlet state with a 93.2% 3d8 and 6.8% 3d9L charge transfer configuration. In the case of the Co L2,3 XAS and XMCD we find the best agreement using the parameters 10Dq = 0.5 eV, Ds = 0.03 eV, Dt = 0.075 eV, and Δ = 6 eV. Whereas no perfect agreement is found, it still appears to be the best fit compared to the simulated spectra with lower or higher crystal field strength. The overall agreement between XAS and the corresponding simulation is rather good, but we find 25033
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Figure 7. χT plots taken at external fields of 5 mT (upper panel) and 1000 mT (lower panel) as a function of temperature of CoNi and MnNi molecules after subtraction of the signal of the gelantine capsule. The insets show the inverse of the susceptibility as function of temperature. Figure 6. Top panel: Left and right polarized Ni L2,3 XAS and the resulting XMCD performed in an external magnetic field of 6.5 T strength at a temperature of 0.6 K. Bottom panel: The corresponding XAS (isotropic) and XMCD signals in comparison with a charge transfer multiplet simulation of the XAS. Note the constant offset of experimental and simulated data introduced for clarity.
some differences in the XMCD fine structure. In particular, the simulated XMCD shows a second resolved peak of negative sign at 782.5 eV, visible in the experiment only as a small shoulder. Moreover, the L2-XMCD signal is overestimated in the simulation. Nevertheless, overall satisfactory agreement between experiment and simulation has been obtained confirming the Co ions to be in a divalent high spin sate with a 90.1% 3d7 and 9.9% 3d8L configuration. The differences might be explained by an asymmetric square planar crystal field resulting from different CoO bond length and/or angles, leading to a partly “threedimensional” crystal field. Indeed, our density functional theory calculations indicate the possibility of such crystal field distortions within the CoO4 cluster (see Table 3 and discussion below). As to extract the spin and orbital magnetic moments from the Co XMCD and XAS, we applied the sum rules experimentally confirmed by Chen et al.36 From the experiment we find a spin moment mspin = 0.85 μB/Co atom and an orbital moment of morb = 0.45 μB/Co atom. Since the spin sum rule leads to underestimated moments for spectra of ionic compounds due to corehole Coulombic interactions,55,56 we have to correct the result of the spin sum rule. The spin sum correction factor depends on several parameters such as crystal field symmetry, Slater integral reduction, charge transfer, and 3d spin orbit coupling. Teramura et al.55 derived a correction factor of 1/ 1.086 = 0.921 for a Co2+ ion in an octahedral crystal field with 10Dq = 1.5 eV. A more recent work concludes an error of 520% in dependence of the above-mentioned parameters.56 Thus the
spin moment is in the range of mspin = 0.93 μB/Co atom. and mspin = 1.02 μB/Co atom. The overall magnetic moment per Co ion and molecule is between 1.38 and 1.47 μB per atom at T = 0.6 K, in comparison to 1.73 μB per molecule at T = 2 K found by SQUID magnetometry.25 Qualitatively, we find a similar situation for the MnNi complex. Whereas the XMCD shows a strong dichroism at Mn L2,3, almost no XMCD signal is found arising at the Ni L2,3 edges (Figures 5 and 6). The Mn L2,3 XAS consists of a shoulder around 641.5 eV, the L3 main peak is located at 642.5 eV followed by two relative maxima at 643.9 and 646.2 eV. The L2 XAS consists of two maxima around 653.1 and 655.0 eV. The overall shape of the XAS clearly indicates a Mn2+ valence state; also the XMCD is pretty similar compared to XMCD spectra of other Mn2+ spectra of star-shaped molecules.57,58 The multiplet simulation (Figure 5, bottom panel) matches the experimental data almost perfectly, only the shoulder located at 641.5 eV in the XAS is not visible in the simulation. We used 10Dq = 0.5 eV, Ds = 0.03 eV, and Dt = 0.075 eV as crystal field parameters, and Δ = 9 eV as charge transfer potential, resulting in a 93.5% 3d5 and 6.5% 3d6L ground state configuration. Thus, the divalent Mn ions are rather ionic, as are the divalent Co ions and Ni ions in these binuclear complexes. Sum rule analysis yields a spin moment of mspin = 1.46 μB/Mn atom and an orbital moment of morb = 0.06 μB/Mn atom. The small orbital moment can be expected due to the almost pure half filled 3d5 configuration of the Mn2+ ions, where all d electrons are balanced within the (mL contributions. As to the spin sum rule correction Teramura et al.55 derive a correction factor of 1/0.68 = 1.47 for Mn2+ (10Dq = 1.5 eV), Piamonteze et al.56 report a similar result, i.e., 30%. Gambardella et al.59 obtain a correction factor of 1/0.556 = 1.8 for diluted Mn2+ ions in GaAs and Ge. In comparison to the results obtained by SQUID magnetometry,25 Hence, we find a significantly higher magnetic moment, in the range of 2.21 and 2.69 μB/Mn atom 25034
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(XMCD) compared to 1.25 μB/molecule probed with SQUID magnetometry.25 This is likely due to the unsaturated moment at T = 2 K and 5.5 T, whereas the XMCD measurements were performed at a temperature of 0.6 K in a 6.5 T field. According to the XAS results, the divalent Ni ions are predominantly in the diamagnetic low spin state also in the MnNi complex; however, at the L3 shoulder located around 854 eV (Figure 6, upper panel) a residual XMCD appears, as well as even weaker at the L3 maximum (854.8 eV), which indicates that a small fraction of the Ni2+ might be present in the high spin form. Note that the sign of this quite small dichroism is nevertheless unambiguously negative, indicating a potential ferromagnetic coupling of the small Ni high spin fraction with spins of the Mn ions in an external magnetic field. Furthermore, an origin of a monocunclear Ni based complex 2 (Scheme 1) can be excluded since the mononuclear complex has been found to be diamagnetic be means of SQUID magnetometry. Furthermore the clearly defined resonances in the NMR spectrum24 of the mononuclear complex 2 confirm that there is no paramagnetic substance/residue in the substance. Thus, if present, such a fraction cannot contribute to the XMCD signal. Nevertheless, the presence of another Ni containing impurity phase, which then might be responsible for the observed XMCD signal, cannot be entirely excluded. However, this might be an interesting detail, indicating potential ferromagnetic superexchange in binuclear MnNi complexes. Ferromagnetic coupling in binuclear planar complexes is not very common, e.g., investigated in a mixed valence iron(II/ III) complex,60 and very recently in a series of vanadium(II/III) complexes.61 In order to verify this investigation and shed some more light on the question as to whether it is possible to synthesize a binuclear complex with ferromagnetically coupled transition metal ions of the same valence, it is desirable to tailor
the carbon-based ligand chemistry and/or to change the positions of the transition metal ions in these complexes. Such experiments are currently under way. Variable temperature direct measurements of the magnetic susceptibility were performed on CoNi and MnNi in applied fields of 5 mT and 1 T over a temperature range of 260 K. The data are plotted as χMT over T (Figure 7). The two molecules show quite different behavior as function of temperature and external field. In a small field of 5 mT, the magnetic susceptibility of CoNi reaches an absolute maximum in intensity of around 72 cm3 mol1 K at around 4 K before a decrease sets in, indicating overall ferromagnetic interactions. Since the Ni ions have been found to be diamagnetic by the XMCD experiments, these interactions must be of intermolecular nature. In higher external fields (1 T) the maximum of the susceptibility occurs already at 10 K, followed by a quite sharp decrease, indicating antiferromagnetic intermolecular interactions at low temperatures in higher external magnetic fields. This may be an explanation for the lower magnetic moment obtained by XMCD (1.38 μB/Co at., 0.6 K, 6.5 T) compared to SQUID magnetometry (1.73 μB/molecule, 2 K, 5.5 T). The magnetic susceptibility of MnNi in a small applied field of 5 mT shows a sharp increase from around 26 to almost 75 cm3 mol1 K between 44 and 34 K. Then the susceptibility decreases constantly to 20 cm3 mol1 K at 2 K. In contrast to CoNi the maximum of the susceptibility does not shift toward higher temperatures in higher external fields (Figure 7, lower panel), possibly indicating that a phase transition takes place in MnNi around 34 K. Furthermore the Table 2. Selected (average) Bond Distances and Out-ofPlane Angles for CoNi Complexes Obtained at the RIUBP86/def2-TZVP and RI-UOPBE/def2-TZVPP Level of Theorya
Table 1. Relative Energies of the Possible Spin States of CoNi and MnNi Complexes Obtained at the RI-UBP86/def2-TZVP and RI-UOPBE/def2-TZVPP Level of Theorya
doublet CoNi
CoNi spin state doublet
a
UBP86 0
MnNi UOPBE 0
UBP86 0
BP86 OPBE BP86
d(CoO1) d(CoO2)
relative energy (kJ/mol)
UOPBE 0
quartet
+28
2
89
91
sextet octet
+121
+88
110 7
154 57
Both sets of calculations use the zeroth-order relativistic approximation (ZORA) approach. All values are relative to the total energy of the doublet state and given in kJ/mol.
quartet
1.81 1.86
1.80 1.85
1.87 2.01
OPBE 1.87 2.05
sextet BP86 1.87 2.00
OPBE 1.87 2.00
d(NiO2)
1.89
1.88
1.88
1.86
1.96
1.94
d(NiN)
1.83
1.81
1.83
1.82
1.89
1.88
d(CoNi)
2.85
2.84
2.99
3.04
2.97
2.98
— (O1O2O2O1) 0.9
0.1
21.4
32.1
16.7
28.6
— (NO2O2N)
2.2
4.8
5.9
5.0
5.1
2.4
a
The two sets of oxygen atoms are labeled differently: O1 corresponds to the oxygen atoms closest to the aromatic rings and O2 stands for the oxygen atoms between Co and Ni. Both sets of calculations use the zeroth-order relativistic approximation (ZORA) approach. Distances are in angstroms and angles in degrees.
Figure 8. Graphical representation of the spin density for CoNi complex for different spin states (A, doublet; B, quartet; C, sextet). The Ni atom is shown in pink, oxygen in red, nitrogen in blue, hydrogen in white, and the spin density is shown in green (note that the cobalt atom is covered by the spin density). The isosurface is obtained from ZORA-UOPBE/def2-TZVPP calculations and is drawn at 0.005 e/Å. 25035
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Figure 9. Graphical representation of the spin density for MnNi complex for different spin states (A, doublet; B, quartet; C, sextet; D, octet). The Ni atom is shown in pink, oxygen in red, nitrogen in blue, hydrogen in white, and the spin density is shown in green (note that the manganese atom is covered by the spin density). The isosurface is obtained from ZORA-UOPBE/def2-TZVPP calculations and is drawn at 0.005 e/Å.
Table 3. Selected (average) Bond Distances and Out-of-Plane Angles for MnNi Complexes Obtained at the RI-UBP86/def2TZVP and RI-UOPBE/def2-TZVPP Level of Theorya doublet MnNi d(CoO1) d(CoO2)
quartet
sextet
BP86
OPBE
BP86
OPBE
1.83 1.92
1.84 1.90
1.84 1.92
1.84 1.91
BP86 1.92 2.13
octet OPBE 1.93 2.16
BP86 1.94 2.18
OPBE 1.94 2.18
d(NiO2)
1.91
1.98
1.91
1.94
1.89
1.88
1.96
1.95
d(NiN)
1.84
1.87
1.85
1.85
1.84
1.82
1.89
1.88
d(MnNi)
2.87
2.94
2.88
2.86
3.14
3.15
3.16
3.17
— (O1O2O2O1)
2.6
1.2
1.9
1.6
13.1
12.6
14.7
13.3
— (NO2O2N)
3.0
4.5
1.3
2.4
4.5
4.4
7.0
7.1
a
The two sets of oxygen atoms are labeled differently: O1 corresponds to the oxygen atoms closest to the aromatic rings and O2 stands for the oxygen atoms between Mn and Ni. Both sets of calculations use the zeroth-order relativistic approximation (ZORA) approach. Distances are in angstroms and angles in degrees.
decrease of the susceptibility between 34 and 2 K of MnNi occurs on a minor slope compared to the result of CoNi, indicating weaker antiferromagnetic intermolecular interactions for MnNi. Combining this result with the fact that the MnNi molecule could not be magnetically saturated in an external field of 5.5 T and at a temperature of 2 K, one can qualitatively understand the higher magnetic moment probed by XMCD (2.21 μB/Mn atom, 0.6 K, 6.5 T) compared to SQUID (1.25 μB/molecule, 2 K, 5.5 T). In order to gain more insight into the electronic properties of CoNi and MnNi, we use density functional theory to examine the relative energy, the distribution of spin density, and the geometry of each different spin state. The computed relative
energies are shown in Table 1. We see that, for CoNi, the lowspin doublet is favored by the UBP86 functional, with the higher spin states being about 30 and 120 kJ/mol higher in energy for the quartet and sextet, respectively. The situation changes with UOPBE, as this functional predicts the lowest energy spin state to be the quartet by 2 kJ/mol followed very closely by the doublet and then the sextet spin state, about 90 kJ/mol higher. We note that such lowering of energy for the intermediate spin state is consistent with the observations of other groups, e.g., ref 48 who also pointed out the deficiencies of UBP86. Nevertheless, both functionals lead to a similar qualitative ordering that favors low/ intermediate-spin states over the high-spin state. This ranking is 25036
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The Journal of Physical Chemistry C also consistent with our experimental observation. Figure 8 shows a graphical representation of the spin density for CoNi. We see that both doublet and quartet have a large spin density on the Co ion while showing a negligible population on the Ni center. As expected, the high-spin state shows a significant participation of the Ni ion and, interestingly, we also observe a delocalization of the spin population on the organic framework. In order to examine possible distortions of the ligand field around the Co atom for all three spin states, we report in Table 2 a selection of interatomic distances and two out-of-plane angles, one for the Co center (O1O2O2O1) and one for the Ni center (NO2O2N). We note that, despite the quantitative differences in the energetic ordering between UBP86 and UOPBE, both functionals give very similar geometries in all cases. We see that the low-spin complex is planar and that the oxygen atoms are arranged in a square around the Co atom. As the total spin of the complex increases, we observe a lengthening of the metalmetal distance and a distortion of the square planar environment around the Co atom. The distortion happens already for the quartet state and remains for the sextet. It is also worth noting that, in all spin states, the environment around the Ni atom remains nearly planar but that the NiO distances increase with total spin values. The energetic situation for MnNi (see Table 1) is similar to that of CoNi, with both functionals yielding the same spin state ordering. Here, the sextet state is predicted to be the lowest spin state, followed by the quartet, the octet, and finally the doublet. Once again, we see that the intermediate spin states are favored and, crucially, this time the high-spin octet is energetically favored over the low-spin doublet. This could explain our experimental observation of a tendency for MnNi to be more likely to accommodate a high-spin Ni atom, while this was not the case for CoNi. The spin density for MnNi is shown in Figure 9. We observe that the spin density around the Mn atom is more spherical than that for Co in CoNi, which is not surprising given the larger number of unpaired electrons for Mn. As was the case for Co in CoNi, we note that the doublet (low-spin), quartet, and sextet spin (both intermediate spin) densities are mainly localized on the Mn atom, with only minimal involvement of the Ni atom. The high-spin state shows an extensive delocalization of the spin density on both metal atoms and on the organic framework. The computed distances and out-of-plane angles for all four possible spin states are shown in Table 3. As noted previously, there is little difference between the predicted geometries obtained with UBP86 and those obtained with UOPBE. The doublet and quartet states retain a square-planar environment for the Mn atom and have similar geometries. The sextet and octet states show marked increase of the metalmetal distance combined with a distortion of the planar environment around the Mn center. The planarity around the Ni atom is roughly maintained for all spin states.
’ CONCLUSION We investigated the internal chemical, electronic, and magnetic properties of the planar binuclear complexes MnNi and CoNi in detail by applying the experimental techniques XPS, XMCD, and SQUID-magnetometry and addressing the results theoretically by means of charge transfer multiplet simulations and density functional calculations. XPS on CoNi indicates a Co2+ high spin state and a low spin (diamagnetic) Ni2+ at room temperature. Also at low temperatures around 0.6 K, very close to
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the magnetic ground state, we find the Ni2+ ions to be completely (CoNi) or predominantly (MnNi) in the low-spin state in both compounds. The Co ions in CoNi are in a high-spin state, carrying the overall magnetic moment in this complex. XMCD on the MnNi complex reveals the Mn ions to be in a divalent high-spin state. We can explain different magnetic moments found by SQUID magnetometry and XMCD in terms of intermolecular interactions. This result might indicate a potential avenue to the synthesis of a binuclear complex with ferromagnetically coupled transition metal ions. Such complexes are now being prepared applying different carbon-based ligand chemistry and/or to change the positions of the transition metal ions in these complexes. The combination of the experimental and theoretical approaches here will be used to tailor more advanced magnetic binuclear complexes.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses r
Department of Chemistry, The University of Hull, Cottingham Rd., Kingston upon Hull, HU6 7RX, U.K.
’ ACKNOWLEDGMENT D.M.B. gratefully acknowledges a generous grant (“SFB-569/ TP-N1”) from the Deutsche Forschungsgemeinschaft (DFG), the Ministry for Education and Research (Bundesministerium f€ur Bildung und Forschung), and the Ministry for Science, Research and Arts Baden-W€urttemberg (Ministerium f€ur Wissenschaft, Forschung und Kunst Baden-W€urttemberg) for computational time on the bwGRID cluster (http://www. bw-grid.de). The authors from Ulm University are thankful for the general support by DFG within SFB 569. Part of this work has been performed at the Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerland. Financial and travel support within the framework of the EU’s Seventh Framework Programme are gratefully acknowledged. We thank the beamline scientists of the SIM beamline for excellent technical support. ’ REFERENCES (1) Sessoli, R.; Gatteschi, D.; Caneschi, A.; Novak, M. A. Magnetic bistability in a metal-ion cluster. Nature 1993, 365, 141-XI. (2) Kahn, O. Molecular Magnetism; VCH: New York, 1993. (3) Gatteschi, D. Molecular Nanomagnets; Oxford University Press: Oxford and New York, 2006. (4) Blundell, S. J. Molecular magnets. Contemp. Phys. 2007, 48, 275–290. (5) Friedman, J. R.; Sarachik, M. P.; Tejada, J.; Ziolo, R. Macroscopic measurement of resonant magnetization tunneling in high-spin molecules. Phys. Rev. Lett. 1996, 76, 3830–3833. (6) Thomas, L.; Lionti, F.; Ballou, R.; Gatteschi, D.; Sessoli, R.; Barbara, B. Macroscopic quantum tunnelling of magnetization in a single crystal of nanomagnets. Nature 1996, 383, 145–147. (7) Milios, C. J.; Raptopoulou, C. P.; Terzis, A.; Lloret, F.; Vicente, R.; Perlepes, S. P.; Escuer, A. Hexanuclear Manganese(III) SingleMolecule Magnets. Angew. Chem., Int. Ed. 2003, 43, 210–212. (8) Tasiopoulos, A. J.; Vinslava, A.; Wernsdorfer, W.; Abboud, K. A.; Christou, G. Giant single-molecule magnets: A {Mn84} torus and its supramolecular nanotubes. Angew. Chem., Int. Ed. 2004, 43, 2117–2121. (9) Miyasaka, H.; Clerac, R.; Wernsdorfer, W.; Lecren, L.; Bonhomme, C.; Sugiura, K. -.; Yamashita, M. A dimeric manganese(III) tetradentate 25037
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