XPS-Characterization of Heterometallic Coordination Compounds with Optically Active Ligands

The heterometallic optical complexes [Cu 2 Co(S,S(+)cpse) 3 (H 2 O) 3 ]⋅4H 2 O (1) and [Cu 2 Ni(S,S(+)cpse) 3 (H 2 O) 3 ]⋅10H 2 O (2) were obtained from the mononuclear copper(II) compound by the addition of nickel(II) or cobalt(II) chlorides, where (H 2 cpse) is the acetyl amino alcohol derivative N-[2-hydroxy-1(R)-methyl-2(R)-phenylethyl]-N-methylglycine. In comparison with the homotrinuclear copper(II) compound [Cu 3 (S,S(+)cpse) 3 (H 2 O) 3 ]⋅8H 2 O reported previously, the substitution of a copper(II) atom by one cobalt(II) ion gave place to a heterotrinuclear compound 1, which presents ferromagnetic-antiferromagnetic behaviour. When substituting a copper(II) by a nickel(II) ion, the trinuclear compound 2 showed an antiferromagnetic coupling.Themagnetic behaviour of the heterotrinuclear compounds is driven by the nature of the metal ion which was introduced in the copper(II) triangular array.The ligand and its coordination compounds were characterized by IR, UV-Vis-NIR.Their chemical was confirmed by photoelectron spectroscopy (XPS).


Introduction
Investigation of novel inorganic-organic compounds mixed represents one of the most active areas of materials science and chemical research.The structural diversity of compounds bearing with more than one metal ion may give place to potential applications in research fields such as electrical conductivity, magnetism, photomechanism, host-guest chemistry, ion exchange, shape specificity, and catalysis [1,2].
Mixed metal states can allow access to several electronic states and cooperative effects between them [3].There is a particular emphasis on heterobimetallic complexes, and much of this research has been directed towards transition metal clusters.High-spin multiplicity in a polymetallic entity could be stabilized without imposing ferromagnetic interactions between the nearest magnetic centers.To achieve this, two high local spins are aligned in the same direction due to antiferromagnetic interactions with a small spin located between them [4][5][6].
Chiral ligands have been used in the formation of an extended homochiral two-dimensional (2D) sheet, which, in turn, leads to a conglomerate structure, constructed through the homochiral stacking of the 2D sheets [7].Also, such photochemical switches controlled by photoisomerization can be used in the formation of cholesteric phases on nematic hosts and helical pitches [8].In the technological field, chiral magnet may participate in the process for storage information and gas storage [9,10].
The d-block metal ions are borderline acids, having a strong tendency to coordinate with N-donors as well as O-donors [11].The acetyl pseudoephedrine derivative S,S(+)⋅ H 2 cpse is a flexible enantiomerically pure ligand, with three potential coordination sites: the nitrogen which is an atom, and the carboxylic and the alcohol groups and (Figure 1).
In this work, we present the synthesis of the heterotrinuclear compounds [Cu 2 Ni(S,S(+)cpse) 3  the OH group in the mononuclear copper(II) compound [Cu(S,S(+)Hcpse) 2 ]⋅2H 2 O, which gives place to an oxobridge between the metal ions [12].They were spectroscopically characterized by IR, electronic absorption, atomic absorption, EPR, and magnetic susceptibility at variable temperature.Photoemission spectroscopy provided direct electronic information about core-level states at higher binding energies.The heterometallic compounds were spectroscopically compared with trinuclear copper(II) compound [Cu 3 (S,S(+)cpse) 3 (H 2 O) 3 ]⋅8.5 reported previously [13].This characterization allowed us to understand structural similarities between homometallic and heterometallic compounds.Finally, it is considered that these results contributed significantly to the structural elucidation and the electronic structure of coordination compounds [14][15][16].

Results and Discussion
3.1.Heterotrinuclear Compounds 1 and 2. When reacting the copper(II) mononuclear [Cu(S,S(+)Hcpse) 2 ]⋅2H 2 O (obtained as previously described) [17] in basic conditions, with cobalt(II) or nickel(II) chlorides, the corresponding heterotrinuclear compounds [Cu 2 Co(S,S(+)cpse In these compounds the ligand is coordinated by the nitrogen atom, the carboxylic group and the alcohol group, as observed from their IR spectra [19], where the corresponding stretching bands are shifted to lower energies upon coordination to the metal ions (Table 1).The V (C-OH) st. is not Table 1: Selected IR bands for the ligand and its coordination compounds.

Coordinate compound and ligand
] as (COO − )- observed due to deprotonation of the OH group and the bridging oxygen coordination to the metal centres.
In the electronic spectrum of [Cu 2 Co(S,S(+)cpse for the cobalt(II) ion in a square-based pyramid geometry; the latter is a broad band which is superimposed with the expected transition for the copper(II) ion at ca. 14000 cm −1 [20].Additionally, there is a CT M-L band at 31336 cm −1 (Figure 2).The spectrum was compared with the corresponding mononuclear compounds.The contribution of the copper(II) and cobalt(II) metal ions is observed in the electronic transitions.
The reflectance spectrum of the [Cu 2 Ni(S,S(+)cpse) 3 ⋅ (H 2 O) 3 ]⋅10H 2 O compound 2 shows transitions in the near infrared that were assigned for a nickel(II) in a square-based pyramidal geometry, 3 E  ← 3 E at 5218 cm −1 and 3 A 2 ← 3 E at 7047 cm −1 ; the rest of the expected transitions are overlapped with the copper(II) transition at 14285 cm −1 , in the same region as for the trinuclear copper(II 3).In accordance with the atomic absorption, elemental analysis, ultraviolet-infrared visible diffuse reflectance, and infrared spectroscopy, it is proposed that the heterotrinuclear compounds present a similar structure to that of In XPS interpretation, the general rule is that the BE of the central atom increases as the electronegativity of the attached atoms or groups increases [21].The difference on  electronegativities when substituting copper(II) by cobalt(II) or nickel(II) is not significant, but the charge transfer between metal ions is observed.
In the XPS valence band spectra for these compounds mainly two peaks are observed.The first one with a single fine structure at a binding energy of 2-8 eV originates from photoemission out of the d 9 ground state configuration of copper(II), for which the photo-hole is screened by a charge transfer from the ligand O2p electrons towards the Cu (labelled by d 9 → d 9 L with L meaning the hole in the O2p ligand states L).For the three compounds there exists a contribution of copper(II).However, heterometallic compounds 1 and 2 present an additional contribution from the cobalt(II), Co 3d (1 eV), and nickel(II) Ni 3d (2 eV) (Figure 6).In the crystalline structure for [Cu 3 (S,S(+)cpse) 3 (H 2 O) 3 ]⋅8H 2 O there exists a stable six-membered ring (Cu1-O-Cu2-O-Cu3-O-Cu1) in a chair conformation (Figure 7).For this arrangement there is a higher electronic delocalization between metal ions and the oxygen atoms from of the ligand.For the heterometallic compounds this electronic delocalization is also expected.In consequence, the effect on their charge transfer is observed which is a shift to positive energy at Fermi level.The second one appears at about 14 eV and presents contributions of the ligand.
In Figure 8, the binding energies for copper 2p 3/2 , 2p 1/2 , nickel 2p 3/2 , 2p 1/2 , and cobalt 2p  peak at 942.80) and 2p 1/2 in 952.48 eV (charge transfer satellite peak at 962.71 eV).The compounds 1 and 2 present similar peaks to those of the homotrinuclear copper(II) compound in the expected region for copper(II); therefore this behaviour suggests electronic similarities between the trinuclear compounds.The compounds 1 and 2 spectra for cobalt (781.53 eV, charge transfer satellite peak at 786.87 and 2p 1/2 in 797.42 eV, charge transfer satellite peak at 802.96 eV) and nickel (855.20 eV, charge transfer satellite peak at 861.22 and 2p 1/2 in 872.87 eV, charge transfer satellite peak at 879.47 eV) were compared with CoO [22] and NiO.The satellites were used as a finger print for the recognition of high-spin cobalt(II) and nickel(II).This behaviour is consistent with a 3d 8 nickel(II) ion and 3d 7 cobalt(II) ion.No evidence of other nickel(II) and cobalt(II) transitions was observed from their spectra.Hence this suggests that in these compounds the metal ions do not contain different oxidation states other than 2 + [23].
The observed binding energy of the N 1s XPS transition, at 399.93 eV, is in agreement with the reported values for organic amines coordinated with metal atoms in solid state [15].The nitrogen atom shows a similar electronic behaviour in all studied compounds (Figure 8).

Magnetic Properties of the Trinuclear Compounds for
1 and 2. To investigate the magnetic interactions in the heterotrinuclear compounds, EPR spectra and the magnetic susceptibilities of powdered samples were measured from 4 to 300 K.
The EPR spectrum at 115 K for [Cu 2 Co(S,S(+)cpse) 3 ⋅ (H 2 O) 3 ]⋅4H 2 O (1) corresponds to a high-spin d 7 configuration either in an orbitally nondegenerate ground state ( 4 A 2 ) or in an orbitally degenerate ground state ( 4 T 1 ) in which the orbital levels are separated by spin-orbit coupling.The orbital angular moment is quenched, and the combined effects of crystal field symmetry and mixture of excited terms through second-order spin-orbit may lead to a splitting the two doublets.The resonance absorption features appear at  value ∼5.6, 3.4 and splitting ∼2.09.Hyperfine structure resolved in both ∼5.6 and ∼2.09 regions corresponds to a coupling for the cobalt(II) I = 7/2.However, the EPR spectra depicted in Figure 11 show the overlap signals for copper(II) and cobalt(II) for pentacoordinate complexes of high spin.
The plot of    versus  (300-4 K), show a change in the slope.It involves an increase in    as temperature is lowered, followed by a rapid drop at temperatures <13.75 K.The drop in    below 17 K suggests the presence of a weak intermolecular antiferromagnetic interaction, but after 17 K the ferromagnetic behaviour is shown (Figure 12).This compound presents a magnetic moment of 8.15 B.M. at 298 K, per all metallic atoms, which is indicating the contribution of the copper(II) and cobalt(II) atoms.

Conclusions
Cobalt-copper and nickel-copper trinuclear heterometallic compounds, with the chiral ligand (S,S(+)cpse), were studied by photoelectron spectroscopy (XPS).The found core levels for Cu 2p, Ni 2p, Co 2p, C 1s, O 1s, and N 1s led to the the identification the chemical structure in correspondence with the crystalline structure for homotrinuclear copper(II) compound.In the valence band, the effects due to charge transfer between the metal ions and the ligand were found.These results were in agreement with the spectroscopic and analytical characterization (IR, UV-V is reflectance diffuse, elemental analysis, and atomic absorption).The substitution of a copper(II) atom by one the cobalt(II) ion gave place to a heterometallic compound 1, which is ferromagneticantiferromagnetic magnetic transition.The substitution of a copper(II) atom by one the nickel(II) atom in compound 1, showed an antiferromagnetic coupling.The magnetic behaviour of the heterotrinuclear compounds is driven by  the nature of the metal ion which was introduced in the copper(II) triangular array.
of the C 1s region shows the following order: (C-CH 2 ) 285.00, (C-CH 3 ) 285.67, (CH 3 -N) 285.96 eV, (CH 2 -N) 286.46, (C-O-M; M = metal ion) 287.20 eV, and (C=O) 289.09 eV.The binding energy depends on the specific environment where the functional groups are located.In this context, it is found that C-C is after of C-N and C-O.The increase in bond polarity leads shifted to higher energy.These contributions for C1s level show coordination of the ligand towards metal by nitrogen and oxygen atoms (Figure 9).For O 1s core level the binding energy corresponding to (O-Cu-O) 530.79 eV, (C=O) 531.67 eV, (C-O) 532.38 eV, (H 2 O-Cu) 533.36, and satellite at 532.50 eV are shown.The allocations were realized for trinuclear copper(II) compound.For compound 1 contributions for O-Co-O and compound 2 O-Ni-O are additionally observed (Figure 10).Contributions due to H 2 O molecule are correlated with the amount of molecules outside the coordination sphere: compound 2 > trinuclear copper(II) compound > compound 1.