Ligand Determination of a Copper Complex by Cu 2p X-Ray Photoelectron Spectroscopy

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1 ANALYTICAL SCIENCES DECEMBER 1994, VOL Ligand Determination of a Copper Complex by Cu 2p X-Ray Photoelectron Spectroscopy Jun KAwAI*', Sei TSUBOYAMA*2, Kazuhiko ISHIZU*3, Kazuo MIYAMURA*4 and Masahiko SABURI*4 *'Department of Materials Science and Engineering, Kyoto University, Sakyo, Kyoto , Japan *2RIKEN (The Institute of Physical and Chemical Research),.Wako, Saitama , Japan *3Department of Chemistry, Ehime University, Matsuyama 790, Japan *4Department of Applied Chemistry, University of Tokyo, Hongo, Tokyo 113, Japan The copper 2p X-ray photoelectron spectra (XPS) of cupric complexes, the structures of which have been well characterized, were measured. The intensity of the so-called "shake-up" satellite relative to the main line was also measured. It was found that the satellite intensity decreased as the cupric complex became covalent. It was also found using this satellite intensity that a cupric complex of (S)-3-aminohexahydroazepine (abbreviated as S-ahaz), [Cu(Sahaz)2](BF4)2, is directly coordinated by an F atom. Keywords X-Ray photoelectron spectroscopy, copper(li) complex, satellite peak The copper 2p3;2 X-ray photoelectron spectra (XPS) of copper(ii) compounds have a high-energy satellite at ev in binding energy, which has been called a "shake -up" satellite, since it was believed to originate from an electron shake-up at the time of 23 ~2 2 electron photoionization.' However, according to studies by Kotani and Toyozawa2, Asada and Sugano3, Larsson4, van der Laan et al.5, and Ghijsen et al.6, it has been clarified that the so-called "shake-up" satellite (at around 943 ev in binding energy) reflects the valence-electron configuration of the ground state (3d9), and that the socalled "main line" (at around 934 ev) reflects a chargetransfer state (3d10L-'). Here, L denotes a Ligand. For the one-electron energy level of divalent copper compounds, the ligand (L) p level is usually deeper than the copper 3d level in the ground state, since the valenceelectron configuration of cupric compounds is nominally 3d9. The valence hole usually exists in the 3d orbital. It is a big molecular orbital if the local structure of copper is CuL4 and has D4d symmetry. Although the 3d orbital becomes deeper by 9-10 ev (=Q) after 2p electron photoionization, the L p orbital is not affected by the copper core hole potential. Consequently, the energylevel ordering is inverted; the Cu 3d level becomes deeper than the L p orbital. If a photoelectron leaves from the copper compound slowly enough, the molecular orbitals of the same symmetry cannot cross each other (avoided crossing'). Thus, the Cu 3d hole is transferred to the shallower Lp orbital after the creation of a 2p core hole (adiabatic limit). This situation can be realized by a 2p threshold excitation using tunable synchrotron radiation. The final state of this process corresponds to the 2p main line. On the other hand, if the photoionization of the 2p electron is sufficiently fast the Cu 3d hole remains in the Cu 3d atomic orbital with a finite probability (ca. 30% depending on the covalency, described below) after photoionization of the 2p electron (sudden limit). The final state of this process corresponds to the so-called "shake-up" satellite. Although the real photoionization process is intermediate between these two limits, the sudden approximation well describes the experimental result when we measure the Cu 2p XPS spectra excited by Mg or Al Ka radiation.8'9 In the sudden approximation, the so-called "shake-up" satellite intensity relative to the main line is expressed as (Ref. 6) Is/ Im = 1 / tan2(8'-e), (1) where tan(20)=2t/a and tan(20')=2t/(d -Q). Here, T is the orbital hybridization between the Cu 3d and L p orbitals, A the one-electron orbital energy difference between the Cu 3d and L p orbitals, and Q (described above) the Coulomb attraction energy between the Cu 2p-' core hole and the 3d electron. The ground state total wave function of a cupric compound may be expressed as 1G ~(Cu2+L2-)cos 0 - Ji(Cu+L-)sin 0, (2)

2 854 ANALYTICAL SCIENCES DECEMBER 1994, VOL. 10 where Cue+L2- is the ionic state and Cu+L- the covalent state. That is to say, the electronic state of a divalent copper compound is expressed by a resonance of ionic and covalent states. After creating the 2p-' hole by X- ray photoionization, the wave function becomes 02, ~ 0(Cu2+L2-)cos 0' - c(cu+l-)sin 0'. (3) The covalency is related to the orbital hybridization (T) of the compound. If d is small, then the compound is covalent, and if I T I is large the compound is also covalent, since I T I is roughly proportional to the overlap integral, S=<Cu 3d I L p>. The dynamic charge transfer from the ligand to the copper is larger for covalent cupric compounds than for ionic compounds, since the level inversion between Cu 3d and L p is more drastic in an ionic compound than that in covalent compounds. Consequently, I,IIm is a measure of the covalency, or a measure of the mixing of a covalent state with an ionic state. The satellite intensity (I,/Im) is small for covalent cupric compounds and large for ionic cupric compounds. These have been well established for simple solids such as CuF2, CuC12, CuBr2 and CuO. Kawai et al.10 have measured four kinds of cupric complexes in order to determine the relative satellite intensity, from which a correlation between the covalency of the compound and the satellite intensity was found. Recently, Fujiwara et al." again measured and showed the validity of this for cupric complexes. On this basis we can determine the ligand element of an unknown complex by measuring the satellite intensity of the 2p3/2 XPS. A copper(i1) complex of (S)-3-aminohexahydroazepine (abbreviated as S-ahaz), [Cu(S-ahaz)2](BF4)2, was selected for ligand element analysis because the detailed crystal structure had not been determined until now.12 This is mainly because a good crystalline sample for X-ray structural Fig. I Perspective view of complexes I-IV. Cu (green), 0 (red), N (blue), C (black), Cl (yellow) and Br (brown).

3 ANALYTICAL SCIENCES DECEMBER 1994, VOL analysis has not yet been obtained. n Experimental Four cupric complexes (See Fig. 1), the crystal structures of which had been well characterized and were similar to [Cu(S-ahaz)2](BF4)2, were selected for standard XPS spectral measurements. Complex I was the same as that shown in Fig. 4 of Ref. 13; complex II was as shown in Fig. 7 of Ref. 14; complex III was a monomer of complex I and the same as Fig. 1 of Ref. 13; and complex IV was as shown in Fig. 4 of Ref. 14. The local coordination states of the copper ion are shown in Fig. 2. The copper 2p3/2 XPS spectra of the four cupric complexes (I, II, III and IV), [Cu(S-ahaz)2](BF4)2, CuO, CuF2.2H2O, CuCl2, CuBr2 and copper(ii) phthalocyanine [abbreviated as Cu(Pc)] were measured by a VG ESCA LAB MK-II spectrometer equipped with an Al or Mg anode X-ray source (the applied power was typically 14 kv and ma). The samples were of the powder form and put on 3M adhesive tape attached to the sample holder. The amount of the required sample was ca. 10 mg. The pass energy of the spherical electron energy analyzer was 50 ev. The pressure of the vacuum chamber was ca Torr during the measurements. The step size was 0.35 ev with a dwelling time of 100 ms from 920 to 980 ev by the binding energy. It took less than 30 s to obtain one spectrum. Such a short measuring time was required in order to avoid sample decomposition, since the samples could be easily damaged by thermal radiation from the X-ray tube.10 To compensate for the weak signal due to the short measuring time, numerical smoothing was required. The measured spectra were thus smoothed by the 5- point-savitzky-golay method15 with 2-20 iterations (depending on S/N). This smoothing was essential to Fig. 3 Measured Cu 2p3/2 X-ray photoelectron spectra of cupric complexes after being smoothed and backgroundsubtracted. The spectra are shifted in order to compare the profile. Fig. 4 Measured Cu 2p3/2 X-ray photoelectron spectra of [Cu(S-ahaz)2](BF4)2, CuF2.2H2O, Cu0 and Cu(Pc) after being smoothed and background-subtracted. The spectra are shifted in order to compare the profile. Fig. 2 Local coordination structure of complexes I -IV. minimize the reduced chi-square (x2), which was a measure of the variance of fit, in the peak separation procedure. After being smoothed, an inelastic background was subtracted from the spectra using Shirley's method.15 The measured spectra after being numerically processed are shown in Figs. 3 and 4. These spectra were then separated into several Gaussian- Lorentzian mixing functions in order to evaluate I5/Im (Table 1). All of the spectral processing was performed using the VGS 5000/5250 Enhanced Data Processing System on a DEC micro PDP 11/73 computer.

4 856 ANALYTICAL SCIENCES DECEMBER 1994, VOL. 10 Table 1 Measured satellite intensity of copper complexes (standard deviations are indicated in the parentheses) Measurements were made at least three times in order to check the reproducibility. Discussion The covalency of the four complexes and CuO increases as one goes from CuO, I, III, II to IV: 04(CuO) < N202C1(I) < N40(III) = N4Br(II) < N4(IV). (4) This is because the ordering of the electronegativity16 is 0(3.44) > Cl(3.16) > N(3.04) > Br(2.96) (the electronegativity is written in parentheses). Though it seems that Cu-N is more ionic than Cu-Br from the electronegativity difference, the square-planar CuN4 provides p electrons which are very delocalized; thus, complex IV is more covalent than complex II. The agreement between the ordering expressed in (4) and that shown in Table 1 is satisfactory. Therefore the satellite intensity is a measure of the covalency or the delocalization of the Cu 3d electron in cupric complexes. We plotted (Fig. 5) the satellite intensity of CuO, CuF2 2H2O, CuCl2, CuBr2 and Cu(Pc) against the electronegativity of the neighboring atoms, finding that the satellite intensity is also correlated with the covalency. A few exceptions are rationalized as follows: (i) Although the electronegativity of Cl is less than that of 0, the satellite intensity of CuC12 in Fig. 5 is greater than that of CuO. This is rationalized by saying that CuCl2 is, in fact, more ionic than CuO, which is a more reasonable conclusion chemically. Therefore, the electronegativity of CuCl2 should be shifted to the right of CuO in Fig. 5. (ii) The electronegativity of CuF2.2H2O plotted in Fig. 5 is that of F. The Cu atom interacts with 0 as well as F in the solid. Therefore, the satellite intensity of CuF2.2H2O is considered to be smaller than that of anhydrate CuF2. Thus, the position of CuF2 without H2O should be shifted to the right in Fig. 5. The measured [Cu(S-ahaz)2](BF4)2 satellite intensity was 0.62, as shown in Fig. 5. The least-square line in Fig. 5 indicates that IS/Im being 0.62 means that the electronegativity of the neighboring atom of [Cu(Sahaz)2](BF4)2 is as large as 4.2. This indicates that [Cu(S-ahaz)2](BF4)2 is a very ionic compound. Such an ionic bond in [Cu(S-ahaz)2](BF4)2 is only formed between Cu and F. It is already known that the local Fig. 5 Measured satellite intensity (ISIIm) plotted against the electronegativity of the neighboring atoms. crystal structure of [Cu(S-ahaz)2](BF4)2 is similar to that of IV in Fig It is thus concluded that the local structure of [Cu(S-ahaz)2](BF4)2 is similar to II in Fig. 1, in which Cl is replaced by F. In conclusion, we have shown that the relative satellite intensity (ISIIm) of the X-ray photoelectron spectra is a measure of the covalency of cupric compounds. Alternatively, UV or visible spectrochemical measurements of complexes are usually performed in solution. Thus, the local structure of a solid is not always conserved in a solution. However, XPS measurements are possible for solid specimens as received. This method therefore has potential to become a powerful tool for determining the local structure of a center metal ion in a complex. Though one of the shortcomings of the present method is that it cannot provide detailed crystal structure, such as does X-ray crystallography, it does not require large-size crystals, which are required in X-ray crystallography. The present method has been applied to determine the local structure of [Cu(S-ahaz)2](BF4)2, which has not well crystallized and the amount synthesized was very small. Though it has not yet been proved in other methods, the Cu is found to be coordinated not only by N, but also by F. Thanks are due to Ms. Aiko Nakao (RIKEN) for her help with the XPS measurement. References 1. A. Rosencwaig, G. K. Wertheim and H. J. Guggenheim, Phys. Rev. Lett., 27, 479 (1971). 2. A. Kotani and Y. Toyozawa, J. Phys. Soc. Jpn., 37, 563 (1974). 3. S. Asada and S. Sugano, J Phys. Soc. Jpn., 41, 1291

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