Photoelectron spectroscopy of some substituted ferrocenes

Journal of Organometallic Chemistry 727 (2013) 64e67

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Photoelectron spectroscopy of some substituted ferrocenes Branka Kova
c a, Igor Novak b, *, Obadah S. Abdel-Rahman c, Rainer F. Winter c RuCer Bo
skovic Institute, Bijeni
cka cesta 54, HR-10000 Zagreb, Croatia Charles Sturt University, POB 883, Orange NSW 2800, Australia c Fachbereich Chemie, Universität Konstanz, Universitätsstraße 10, D-78453 Konstanz, Germany a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 October 2012 Received in revised form 22 November 2012 Accepted 23 November 2012

The electronic structures of six substituted ferrocenes containing bromo, carbonyl, and phosphanyl substituents have been studied by UV photoelectron spectroscopy (UPS) and DSCF/TDDFT calculations. Our UPS data show that splitting of Fe3d ionizations strongly decreases vs. ferrocene upon introducing benzoyl, chlorophosphine or dialkylphosphanyl groups. Ó 2012 Elsevier B.V. All rights reserved.

Keywords: Substituted ferrocenes Photoelectron spectroscopy Quantum chemistry (Spectro)electrochemistry

1. Introduction Ferrocenes are a large class of organometallics with many practical applications. The important applications pertinent to the substituted ferrocenes studied in this work include the use of ferrocene derivatives which are bidentate phosphines as ligands in the palladium-catalysed intramolecular cross-coupling reactions or the use of benzoyl ferrocenes as anionic photoinitiators [1]. Sterically bulky [1,10 -bis(di-tert-butylphosphino)ferrocene]palladium(II) dichloride (Scheme 1) is a Pd(II) complex of an electronrich bidentate ferrocene phosphine ligand. The large bite angle imparts high activity to the catalyst. In the SuzukieMiyaura reaction, for example, even unactivated aryl chlorides or sterically very hindered substrates undergo reaction to give the corresponding coupling products in high yields. In addition, its high stability in air is one of the features of this catalyst, which increases its utility in a number of fields. These catalysts enable green chemistry operations using water as solvent. Ferrocene IV is the precursor for making compound VI (Scheme 2) and both compounds have been studied in this work. As the substituents modify properties of ferrocenes, various spectroscopic and electrochemical methods have been used to investigate substituted ferrocenes [2]. NMR has shown that the extent of transmission of electronic effects between individual * Corresponding author. E-mail addresses: [email protected] (B. Kova
c), [email protected] (I. Novak), [email protected] (R.F. Winter). 0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2012.11.037

rings via the metal centre is approximately 22% and that proton chemical shifts in substituted ferrocenes correlate with Hammett sP constants [2a]. Another study has shown that there is correlation between proton chemical shifts and electron withdrawing/electron donating properties of the substituent [2b]. Mössbauer spectroscopy and the measurement of redox potentials showed that the presence of a carbonyl group in the a-position significantly influences the electron density in the e2g and a1g Fe3d orbitals but to different extents [2c]. The measured redox potentials of carbonyl substituted ferrocenes are in keeping with the electron withdrawing property of the carbonyl group [2c]. The application of benzoyl ferrocenes as anionic photoinitiators is closely related to the presence of low lying electronic excited states which have considerable metal-to-ligand charge transfer character (MLCT) [2d,e]. The resonance structure associated with MLCT is shown in Scheme 1. NEXAFS spectroscopy with synchrotron radiation source [3] has revealed that the electron withdrawing/electron donating characteristics of the substituents as well as their p-conjugation with the Cp ring strongly affect Fe2p and C1s core transition energies. We present in this work the HeI/HeII UV photoelectron study of the compounds shown in Scheme 2. The UPS studies of substituted ferrocenes are numerous [4] and include variable photon energy studies of ferrocene itself [4b,c]. Our present work focuses on ferrocene derivatives which have significant practical applications mentioned above. We wish to relate the electronic structure to properties of substituted ferrocenes which are responsible for these applications.

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Scheme 1. A catalytically active Pd complex and a resonance structure related to MLCT.

obtained by single point calculations on the molecular radical cations with the same geometries. Higher vertical ionization energies were then calculated by the time dependent density functional (TD-DFT) method which gives electronic excitation energies for the molecular cation. The excitation energies added to E1 then gave approximate values for higher vertical ionization energies. This method called DSCF/TD has already been used successfully for the assignment of photoelectron spectra [6]. The basis set used for iron was of the effective core potential type [5b].

2. Experimental and theoretical methods

3. Results and discussion

Samples of the compounds IeVI were obtained from Sigmae Aldrich. The selection of compounds was governed not only by their chemical importance but also by their thermal stability and volatility. The last two properties are essential when using the UPS method since the sample needs to be vaporized without decomposition. The photoelectron spectra (Figs. 16e21 in Supplementary materials) were recorded on a Vacuum Generators UV-G3 spectrometer and calibrated with small amounts of Xe gas which was added to the sample flow. The spectral resolution in HeI and HeII spectra was 25 meV and 70 meV, respectively, when measured as the FWHM of the 3p1 (2P3/2) Arþ ) Ar (1S0) line. The vertical ionization energy values have been determined from intensity maxima. The samples of compounds IeVI were recorded at 130, 140, 40, 130, 100 and 170  C, respectively. The measured spectra were reproducible and showed no signs of decomposition e.g. no sharp peaks corresponding to small molecules as decomposition products were observed. The exception is the low intensity peak in the spectrum of IV which can be attributed to HCl decomposition product. The spectra were reproducible over long time intervals. After the measurements the sample residues were inspected and showed no discolouration or charring. DFT calculations were performed with GAUSSIAN 03 software [5] at the B3LYP/6-31G(d,p) level and included full geometry optimization for each molecule. The optimized structures corresponded to the minima on the potential energy surface as was inferred from the absence of imaginary harmonic vibrational frequencies. The vertical ionization energies were calculated for the optimized structures using the DSCF/TD method [6] at the 631 þ G(d,p) level. The first vertical ionization energies (E1) were calculated by subtracting the total electron energy obtained for the optimized structures of the neutral molecules from the energies

3.1. Photoelectron spectra The photoelectron spectra of IeVI are shown in Figs. 16e21 (Supplementary materials) and their assignments are summarized in Table 1. The assignments are based on the results of DSCF/TD calculations, comparison with the spectra of related compounds and on the relative band intensity changes on going from HeI to HeII radiation. We note that HeII/HeI atomic photoionization cross-section ratios for C2p, O2p, P3p, Cl3p, Br4p and Fe3d orbitals are 0.31, 0.64, 0.41, 0.05, 0.06 and 1.81, respectively [7]. These values indicate that (relative to molecular orbitals with predominantly C2p character) bands corresponding to ionizations from orbitals with Fe3d or O2p character will increase in intensity on going from HeI to HeII. The orbitals with halogen nP character will show a prominent decrease in relative intensity under the same conditions. HeII/HeI intensities can thus be used as an aid in the assignments. We shall first briefly summarize important results for the spectra of individual compounds. 3.2. Photoelectron spectra of I and II (carbonyl derivatives) Comparison of spectra of I and II with the spectra of ferrocene and acetophenone [8] shows that the lowest ionization energy bands at 7.13 and 7.35 eV (in I) and 7.0 and 7.25 eV (in II) correspond to ionizations from orbitals with Fe3d character (e2g and a1g in the parent ferrocene). This assignment is also consistent with the pronounced increase in relative intensity of these bands on going from HeI to HeII (Figs. 16 and 17 in supplementary materials). The increase in relative intensity of the band at 8.95 eV in I and II suggests that it corresponds to ionization from the oxygen lone pair of the carbonyl group. Bands at 10.1 and 10.3 eV

Scheme 2. The compounds studied in this work.

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66

Table 1 Experimental band maxima (Ei), calculated vertical ionization energies (DSCF/TDF in eV), band assignments and relative HeII/HeI band intensities.a,b Ferrocene derivative

Band

Ei  0.05/eV

DSCF/TDF

MO

HeII/HeI

I

XeA BeI

7.13, 7.35 8.95e9.30

6.88, 7.23 9.08, 9.22 9.46, 9.63 9.68, 9.72 9.84, 9.94 6.78, 7.51 8.20,8.21,9.02 9.70 9.86,9.88, 9.92

Fe3d nCO, pCO

1.0 0.53

Fe3d nCO, pCO

1.0 0.5 0.5 0.05

II

XeA BeD E FeJ

7.0, 7.25 8.95 9.6 10.1e10.3

pCp, pCp pB, pB pCp, pCp pCp pCp

nBr nBr pCp

pCp III

IV

V

VI

a b

XeA BeC DeE FeG XeA BeG HeO

6.95, 7.3 8.6, 9.05 9.45 10.38, 11.0 7.58e7.7 9.25e9.95 11.55

XeA B CeF

6.75, 7.05 7.55 8.7e9.65

XeA BeC DeG

6.55, 6.85 7.6 8.5e9.65

6.47, 7.18 9.05, 9.12, 9.61, 9.78 10.25, 10.85 7.19, 7.82 9.46, 9.54 10.18, 10.26 10.29, 10.30 10.91, 10.94 6.16, 6.90 7.68 9.06, 9.36 9.37, 9.41 5.90, 6.68 7.58, 7.65 8.96, 9.01 9.18, 9.22

Fe3d

pCp pCp pCp pCp nBr nBr Fe3d nP nP

pCp pCp pCp pCp

nCl (8) Fe3d nP

pCp pCp pCp pCp

Fe3d nP nP

pCp pCp pCp pCp

1.0 0.5 0.4 0.2 1.0 0.44 0.44 0.44 0.09 1.0 0.3 0.6 0.6 1.0 0.3 0.4 0.4

Subscripts Cp ¼ cyclopentadienyl; b ¼ benzene. For bands unresolved in HeII the same HeII/HeI ratio is shown.

correspond to ionization of bromine lone pairs as is evident from their very substantial decrease in intensity on going from HeI to HeII. It is important to note that the energy splitting of Fe3d ionizations in I and II (0.22 and 0.25 eV, respectively) is noticeably smaller than in the parent ferrocene (0.35 eV). This reduced splitting is consistent with the earlier report [2c] that the carbonyl group of benzoyl substituted ferrocenes interacts preferentially with one rather than both types of Fe3d orbitals [2c]. The strong interaction between the carbonyl and the ferrocenyl group as demonstrated by UPS is consistent with the use of carbonyl substituted ferrocenes as photoinitiators [1a]. Both Fe3d ionization energies in I and II are higher than in parent ferrocene which can at least in part be ascribed to the inductive effect of the carbonyl group. In I the shift of Fe3d ionizations is larger than in II. This can be rationalized by observing that phenyl and carbonyl groups in I are both electron withdrawing, while the electron withdrawing effect of the carbonyl group of II is mitigated by the attached electron donating alkyl substituent.

3.4. Photoelectron spectra of IVeVI (phosphine derivatives) The three phosphine substituted derivatives contain the bands at lowest energies with their Fe3d ionizations ranging from 6.55 to 7.70 eV. The sequence of Fe3d ionization energies is IV > ferrocene > V > VI. This order follows the inductive effect and is attributed to the presence of strongly electron withdrawing e PCl2 groups in IV and electron donating ePtBu2 groups in V and VI. Fe3d band splitting in V and VI is similar to ferrocene, but decreases to less than half of the ferrocene value in IV. The next band manifold contains ionizations from four Cp p-orbitals and two phosphorus P3p lone pairs (Table 1). The large band at 11.55 eV in the spectrum of IV contains eight chlorine lone pair ionizations Cl3p as can be readily deduced from its intensity decrease with HeII radiation (Table 1). The ionization energies of the phosphorus lone pairs (nP) are also of interest. The phosphorus lone pair ionizations can be assigned with the aid of DSCF/TD calculations and on comparison with UPS spectra of PCl3 and tBu2PCH2PtBu2 [10,11]. In PCl3 the nP ionization energy is 10.7 eV [10] and thus much higher than the nP energies in IV which appear within the 9.25e9.95 eV manifold. The decrease of nP ionization energy on going from PCl3 to IV can be attributed to the replacement of one electron withdrawing Cl substituent by the electron donating ferrocenyl substituent. In IV the nP ionization energies are approximately 1.7 eV larger than in V or VI. This stabilizing inductive effect is again related to the presence of chlorine substituents in IV. The quasidegenerate nP ionization energies in V and VI are 7.55 and 7.6 eV, respectively which is very close to the 7.8 eV value measured in the UPS of tBu2PCH2PtBu2 for two quasi-degenerate nP ionizations [11]. This closeness of energy values indicates that there is little interaction between nP orbitals via either ferrocene or eCH2-group relays in the ground state. As already commented there is little electronic interaction (electron density transmission) between the two alkylphosphine substituents in VI via the ferrocene moiety, presumably because ferrocenyl and alkylphosphine moieties are both electron donating. On the other hand significantly reduced splitting of Fe3d ionizations observed in IV suggests the presence of Table 2 UVevis spectroscopic data for radical cations of IeVI compared with electronic transitions calculated from UPS data.a,b Compound I

II

III

3.3. Photoelectron spectra of III (bromoferrocene) The two lowest ionization energy bands in the photoelectron spectra can again be readily assigned as arising from Fe3d orbitals. Their ionization energies are almost identical to those in ferrocene itself. The bands in the 8.6e9.45 eV region correspond to four ionizations from Cp p-orbitals. The bromine lone pair ionization energies in III may be compared to those of bromobenzene (10.63, 11.21 eV) [9]. The separation of lone pair ionization energies in the two molecules differs by only 0.04 eV, however, the bromine lone pairs in III are shifted towards lower ionization energies vs. bromobenzene by approximately 0.25 eV. This could be due to the well known electron donating ability of the ferrocenyl group [2].

IV

V

VI

a

lexp/nm (UVevis)

Ei/eV (UVevis)

DE/eV

303 457 634 310 458 636 309 385 458 566 639 678 302 440 595 875 303 438 580 878 309 439 868

4.09 2.71 1.96 4.00 2.70 1.95 4.01 3.22 2.71 2.19 1.94 1.82 4.10 2.82 2.08 1.42 4.09 2.83 2.14 1.41 4.01 2.82 1.43

4.20 2.60 1.95 3.95 2.60 1.95 4.05 3.43 2.50 2.10 1.95 1.75 3.97 2.37 2.05 1.55 4.10 2.90 2.10 1.65 4.15 2.80 1.65

Ionic states & transitions

(UPS) J / A(11.55e7.35) Fe3d ) s J / B(11.55e8.95) nCO ) s I / A(9.30e7.35) Fe3d ) pCp J / A(11.2e7.25) Fe3d ) s G / X (9.6e7.0) Fe3d ) pCp B / X(8.95e7.00) Fe3d ) pCp G / X(11.0e6.95) Fe3d ) nBr F / X(10.38e6.95) Fe3d ) nBr E / X (9.45e6.95) Fe3d ) pCp C / X (9.05e6.95) Fe3d ) pCp G / C (11.0e9.05) pCp ) nBr C / A(9.05e7.3) Fe3d ) pCp H / X(11.55e7.58) Fe3d ) nCl F / X(9.95e7.58) Fe3d ) pCp D / A(9.75e7.7) Fe3d ) pCp B / A(9.25e7.7) Fe3d ) nP G / X(10.85e6.75) Fe3d ) s F / X(9.65e6.75) Fe3d ) pCp G / B(10.85e7.55) nP ) s C / A(8.7e7.05) Fe3d ) nP I / A(11.0e6.85) Fe3d ) s G / A(9.65e6.85) Fe3d ) pCp D / A(8.5e6.85) Fe3d ) nP

The UVevis values are from Supplementary materials. UVevis data include solvent effects, UPS do not hence some discrepancies between UPS and UVevis values for electronic transitions. b

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interaction/mixing between P3p lone pairs and the ferrocene moiety. In that case the dichlorophosphine units are electron poor while the ferrocenyl moiety is electron donating [2]. Finally, the replacement of one tBu group by the ferrocenyl group on going from tBu3P to V or VI reduces nP ionization energy only marginally, from 7.7 eV in tBu3P [10] to 7.55 or 7.6 eV in V and VI, respectively. The UPS study is important since it explains why compounds like e.g. VI can be used in Pd catalysts. The replacement of one tBu group in tBu3P by ferrocenyl does not alter the electron donating ability of the phosphorus atom (as indicated by their similar ionization energies) yet allows the phosphine ligand to accommodate a very wide bite angle in its catalytically active palladium complexes. 3.5. Spectroelectrochemical measurements We have measured the redox potentials of the studied ferrocenes and the electronic spectra of the associated ferrocenium ions (see Supplementary materials). The half-wave potentials correlate in a linear fashion with lowest (HOMO) ionization energies of Fe3d orbitals. However, the correlation coefficient of 0.954 suggests that there are possible effects of differing solvation energies embedded in E½ values which are not accounted for by measurements under vacuum conditions. Our proposed assignments of electronic transitions in the radical cations of IeVI in Table 2 are based on TD-DFT calculations and most of them correspond to ligand-to-metal type transitions. 4. Conclusion We have shown on the basis of UPS data that stabilizing resonance interaction between iron and benzoyl moieties in benzoyl ferrocene is of the order of 0.1 eV. This conclusion is based on the comparison of Fe3d ionization energies in the spectra of I and II. This information is useful for an understanding of the photoinitiator properties of benzoyl ferrocenes. We have also shown that ferrocenyl moiety is a poor electron relay (based on comparing phosphorus lone pair ionization energies in V and VI) which suggests the possibility that not only bidentate but also polydentate ferrocene phosphine ligands (binding two Pd centres) can be devised with potential catalytic activity. Acknowledgements I.N. thanks the Faculty of Science, Charles Sturt University for the financial support of this work through the CSU Competitive Grant OPA 4838. B.K. thanks the Ministry of Science, Education and Sports

67

of the Republic of Croatia for the financial support through Project 098-0982915-2945. O. S. A.-R. thanks the DAAD for financial support through a PhD grant.

Appendix A. Supplementary materials Supplementary materials related to this article can be found at http://dx.doi.org/10.1016/j.jorganchem.2012.11.037.

References [1] a) Y. Yamaguchi, C. Kutal, Macromolecules 33 (2000) 1152; b) S. Pascual, P. de Mendoza, A.A.C. Braga, F. Maseras, A.M. Echavarren, Tetrahedron 64 (2008) 6021; c) B. Milde, M. Lohan, C. Schreiner, T. Rüffer, H. Lang, Eur. J. Inorg. Chem. (2011) 5437; d) M.A. Zuideveld, B.H.G. Swennenhuis, M.D.K. Boele, Y. Guari, G.P.F. van Strijdonck, J.N.H. Reek, P.C.J. Kamer, K. Goubitz, J. Fraanje, M. Lutz, A.L. Spek, P.W.N.M. van Leeuwen, J. Chem. Soc. Dalton Trans. (2002) 2308; e) B.H. Lipshutz, B.R. Taft, A.R. Abela, Platin. Met. Rev. 56 (2012) 62; f) C. Moser, A. Orthaber, M. Nieger, F. Belaj, R. Pietschnig, Dalton Trans. (2006) 3879. [2] a) S.A. Butter, H.C. Beachell, Inorg. Chem. 10 (1966) 1820; b) D.W. Slocum, T.R. Engelmann, R. Lewis, R.J. Kurland, J. Chem. Eng. Data 13 (1968) 378; c) A.G. Nagy, Acta Chim. Hung. 119 (1985) 33; d) W. Ding, C.T. Sanderson, R.C. Conover, M.K. Johnson, I.J. Amster, C. Kutal, Inorg. Chem. 42 (2003) 1532; e) Y. Yamaguchi, C. Kutal, Inorg. Chem. 39 (1999) 4861. [3] E. Otero, R.G. Wilks, T. Regier, R.I.R. Blyth, A. Moewes, S.G. Urquhart, J. Phys. Chem. A 112 (2008) 624. [4] a) I. Novak, B. Kova
c, J. Organomet. Chem. 691 (2006) 53 (and references quoted therein); b) C. Cauletti, J.C. Green, M.R. Kelly, P. Powell, J. Van Tilborg, J. Robbins, J. Smart, J. Electron Spectrosc. Relat. Phenom. 19 (1980) 327; c) G. Cooper, J.C. Green, M.P. Payne, Mol. Phys. 63 (1988) 1031. [5] a) M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery Jr., R.E. Stratmann, J.C. Burant, et al., Gaussian 03, Revision E01, Gaussian, Inc., Pittsburgh, PA, 2003; b) M. Dolg, U. Wedig, H. Stoll, H. Preuss, J. Chem. Phys. 86 (1987) 866. [6] a) U. Salzner, J. Phys. Chem. A 114 (2010) 10997; b) R.G. Egdell, J.C. Green, Inorg. Chim. Acta 361 (2008) 462. [7] J.J. Yeh, Atomic Calculation of Photoionization Cross-sections and Asymmetry Parameters, Gordon and Breach, Langhorne, 1993. [8] Lj. Pa
sa-Toli c, B. Kova
c, L. Klasinc, S.M. Shevchenko, S.P. McGlynn, Int. J. Quantum Chem. QCS 24 (1990) 799. [9] K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki, S. Iwata, Handbook of HeI Photoelectron Spectra of Fundamental Organic Molecules, Japan Scientific Societies Press, Tokyo, 1981. [10] M.F. Lappert, J.B. Pedley, B.T. Wilkins, O. Steltzer, E. Unger, J. Chem. Soc. Dalton Trans. (1975) 1207. [11] F. Eisenträger, A. Göthlich, I. Gruber, H. Heiss, C.A. Kiener, C. Krüger, J.U. Notheis, F. Rominger, G. Scherhag, M. Schultz, B.F. Straub, M.A.O. Vollanda, P. Hofmann, New J. Chem. 27 (2003) 540.