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This article is downloaded from

http://researchoutput.csu.edu.au It is the paper published as:

Author: B. Kovac, K. Kowalski and I. Novak

Title: Electronic Structure of 2,5-dimethylazaferrocene

Journal: Journal of Organometallic Chemistry ISSN: 0022-328X

Year: 2011

Volume: 696

Issue: 8

Pages: 1664-1667

Abstract: The electronic structures of 2,5-dimethylazaferrocene has been studied by UV photoelectron spectroscopy (UPS) and high-level DFT methods. UPS data have been used to assign the previously reported spectrum of its radical-cation. We show that aza substitution enhances mixing/interaction between Fe 3d and ligand pi-orbitals. DOI/URL: http://dx.doi.org/10.1016/j.jorganchem.2011.01.034 http://researchoutput.csu.edu.au/R/-?func=dbin-jump-full&object_id=23948&local_base=GEN01-CSU01 Author Address: inovak@csu.edu.au/ CRO Number: 23948

http://researchoutput.csu.edu.au/http://dx.doi.org/10.1016/j.jorganchem.2011.01.034http://researchoutput.csu.edu.au/R/-?func=dbin-jump-full&object_id=23948&local_base=GEN01-CSU01http://researchoutput.csu.edu.au/R/-?func=dbin-jump-full&object_id=23948&local_base=GEN01-CSU01

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Electronic Structure of 2,5-dimethylazaferrocene

Branka Kovač

Ruđer Bošković Institute, Bijenička cesta 54, HR-10000 Zagreb, Croatia

bkovac@irb.hr

Konrad Kowalski

Department of Organic Chemistry, Faculty of Chemistry, University of Łódź, Tamka 12, 91-403

Łódź, Poland

kondor15@wp.pl

Igor Novak*

Charles Sturt University, POB 883, Orange NSW 2800, Australia

inovak@csu.edu.au

Abstract: The electronic structures of 2,5-dimethylazaferrocene has been studied by UV

photoelectron spectroscopy (UPS) and high-level DFT methods. UPS data have been used to

assign the previously reported spectrum of its radical-cation. We show that aza substitution

enhances mixing/interaction between Fe 3d and ligand π-orbitals.

Keywords: ferrocenes, photoelectron spectroscopy

mailto:bkovac@irb.hrmailto:inovak@csu.edu.au

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Introduction

Ferrocene and its derivatives have been investigated at great length due to their interesting

chemical bonding (electronic structure) and their applications in synthetic chemistry and material

science [1-3]. Heteroferrocenes and in particular azaferrocenes have been investigated to a much

lesser extent [4]. The electronic structure of 1,1-diazaferrocene has been reported previously [5].

However, the electronic structure of monoazaferrocene, which can be considered as a “missing

link” in the sequence between ferrocene and diazaferrocene, has not been studied yet. The

valence electronic structure of ferrocenes has been studied by UV photoelectron spectroscopy [6]

shown to be a very good method for this purpose.

Experimental and Theoretical Methods

A sample of 2,5-dimethylazaferrocene studied in this work was prepared and purified as

described previously [7]. Photoelectron spectra (Fig. 1) were recorded on 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 FWHM of the 3p-1

2P3/2 Ar

+ ← Ar (

1S0) line. The sample was

recorded at 40 oC. The measured spectra were reproducible and showed no signs of

decomposition.

DFT calculations were performed with GAUSSIAN 09 software [8] using density functional

hcth [9] and Stuttgard-Dresden pseudopotential basis set [10]. The optimized structure

corresponded to the minimum on the potential energy surface as was inferred from the absence

of imaginary harmonic vibrational frequencies. The first vertical ionization energy (E1) was

calculated by subtracting the total electron energy obtained for the optimized structure of neutral

molecule from the energy resulted by a single point calculation on the molecular radical-cation

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with the same geometry. Higher vertical ionization energies were then calculated by 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.

The calculated geometries (bond lengths, angles) were in good agreement with the molecular

structures of 2,5-dimethylazaferrocene determined by X-ray diffraction [11]. The agreement

between the calculated and measured bond lengths and angles was within 0.01 Å and 10,

respectively [see Supplementary Data for calculated geometry].

Results and Discussion

Photoelectron spectra

The assignment of the UPS spectra of (II) (Fig. 1) is summarized in Table 1 and based on the

comparison with previously reported spectra and assignments reported for 1,1’-

dimethylferrocene (I) and 2,2’,5,5’-tetra-tert-butyl-diazaferrocene (III) [12,5].

I II III IV

Scheme 1. Substituted ferrocenes

The changes in the HeII/HeI band intensity related to the photoionization cross-sections below

were also used as an aid in the assignment, especially for the bands corresponding to the

ionizations of nitrogen lone pair and iron 3d orbitals. The HeII/HeI photoionization cross-section

ratios for C, N and Fe 3d atomic orbitals are 0.31, 0.44 and 1.81, respectively [13]. We expect

that the bands corresponding to ionizations from nitrogen lone pair and metal 3d orbitals should

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increase in relative intensity (vs. C2p based MO) on going from HeI to HeII radiation as

compared to those arising from carbon based orbitals. Such increase is observed for the bands at

6.9-7.2 and 9.4 eV. The TD-DFT results agree well with the experimental vertical ionization

energies obtained for II (Table 1). Taking into account the changes in HeII/HeI intensity,

comparison with the established assignments of the spectra of related ferrocenes and relative

band intensities we conclude that the electronic bands in the range 6.9 -7.5 eV correspond to

ionization from Fe 3d orbitals. The bands at 8.5, 9.05 and 9.4 eV can be assigned to

cyclopentadienyl (Cp) and pyrrolyl (Pyr) localized π-orbitals and to the nitrogen lone pair,

respectively. The assignment of the nitrogen lone pair is consistent with the increase in relative

intensity of the band at 9.4 eV on going from HeI to HeII radiation observed in our spectra.

Assignment of the UPS spectra of the title molecule is thus unambiguous and consistent with the

results obtained by other methods.

Our UPS data enabled us to assign and confirm the identity of the UV-Vis spectrum of the

radical-cation (ionized 2,5-dimethylferrocene) in solution reported previously [11] (Fig. 2). The

bands in gas phase UPS spectra correspond to different electronic states of the ionized molecule

(radical-cation) and the difference between vertical ionization energies (ΔE) thus approximately

equals the UV-Vis transition energies in the corresponding radical-cation assuming that solution

effects are negligible. Such a comparison is shown in Table 2 where a good agreement is

observable between experimental UV-Vis and UPS values confirming that the UV-Vis spectrum

reported previously [11] indeed belongs to radical-cation of the title molecule. In addition, our

UPS data allowed us to assign transitions in the UV-Vis spectrum of the radical-cation as ligand-

metal transitions.

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Intramolecular interactions

The most interesting part of the electronic structure analysis of azaferrocenes concerns the

extent of the intramolecular interactions between nitrogen lone pairs. Gleiter and co-workers

have suggested that such interaction is negligible as indicated by the absence of splitting of the

nitrogen lone pair bands in the spectrum of III (Scheme 1) due to spatial compactness of

nitrogen lone pair orbitals [5]. Another pertinent question is how much nitrogen 2p character is

acquired by Fe 3d and Cp π-orbitals upon aza substitution of ferrocene. We note that Fe 3d

orbital (ionization) energies increase from 6.76 and 7.10 eV measured for I to 6.90 and 7.20 eV,

respectively, measured for II (Table 1). Concomitantly, ligand π-orbitals are destabilized with

ionization energies decreasing from 8.59 and 9.12 eV to 8.5 and 9.05 eV, respectively. Noting

that Fe 3d orbitals have lower ionization energy compared to N2p we can rationalize this

observation by suggesting that Fe 3d orbitals acquire some N2p character while ligand π-orbitals

acquire some Fe 3d character. To put it in another way, diaza substitution enhances the mixing

between Fe 3d and ligand π-orbitals. N.B. the N2p orbital in question is not the in-plane lone

pair, but rather the out-of-plane N2p orbital of π-symmetry. The observed effect has both

resonance and inductive nature. The existence of inductive contribution can be gauged from the

observation that the energy separation between Fe 3d orbitals changes little on going from I to II

(0.34 eV → 0.30 eV). A similarly small change in energy separation of ligand π-orbitals is

observed between I and II (0.53 eV → 0.55 eV). However, if the effect was solely inductive the

two ligand π-orbitals should be stabilized upon aza substitution in equal measure rather than

destabilized. What intramolecular interactions occur upon aza substitution of both rings? In order

to answer this question we compare the energies of iron and ligand π-orbitals in II with those of

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analogous ones in III [5]. Inspection of UPS data presented in Table 1 reveals that one of the

ligand π-orbitals (e1g) becomes stabilized by 0.15 eV while one of the Fe 3d orbitals (a1g)

becomes strongly destabilized by 0.5 eV on going from II to III. In fact the ordering of the first

two Fe 3d ionizations in diaza derivative becomes inverted; HOMO ionization switches from e2g

orbital as in ferrocene and in 2,5-dimethylazaferrocene to becoming a1g orbital in 2,2’,5,5’-tetra-

tert-butyl-diazaferrocene. This change in order in III can best be explained by suggesting that the

observed effect is of predominantly resonance type since inductive effect would lower both Fe

3d orbital energies by approximately the same extent . Why is resonance (mixing) effect due to

aza substitution stronger in diaza compared to monoaza azaferrocene? We suggest that one

possible cause is the gradual reduction of molecular symmetry occurring on aza- substitution

which lifts restrictions on ligand-metal orbital interactions in the parent ferrocene. The

symmetry of the parent ferrocene (D5d ) is progressively reduced by Cp ring substitution to C2v

(1,1’-dimethylferrocene), Cs (2,5-dimethylazaferrocene) and C1 symmetry (2,2’,5,5’-tetra-tert-

butyl-diazaferrocene). In III the nitrogen atoms on the two pyrrolyl rings are rotated away from

each other to reduce steric crowdance by bulky tert-butyl groups as shown in Scheme 1. This

proposed structure is modelled after the structure of nonaza analogue, 2,2’,5,5’-tetra-tert-

butylferrocene (IV) whose structure has been determined by X-ray diffraction [14]. This

“twisting” of the Pyr rings is also consistent with the description of III provided by Janiak et al.

[5].

Conclusion

We have investigated the electronic structure of 2,5-dimethylazaferrocene and compared it

with 2,2’,5,5’-tetra-tert-butyl-diazaferrocene and 1,1’-dimethylferrocene. We suggest that upon

electrochemical oxidation of III the first electron is most likely removed from Fe 3d orbital. Our

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results further suggest that increasing the degree of aza substitution of ferrocene leads to

increased metal-ligand -orbital mixing/interactions. The nature of these interactions also

changes in diazaferrocene where η5-bonding mode (pentahapto) is replaced by η

1 (monohapto)

and η2 (dihapto) interactions between iron and ligand π-orbitals: η

1 (monohapto) interactions in

particular involve nitrogen localized orbitals and the metal atom. Such changed interactions have

been postulated previously on the basis of electron localization function analysis [15].

Acknowledgements

I.N. thanks the Faculty of Science, Charles Sturt University for the financial support of this work

through the Seed Grant A105-954-639-3495. B.K. thanks the Ministry of Science, Education and

Sports of the Republic of Croatia for the financial support through Project 098-0982915-2945.

K.K. is grateful to the University of Łódź for generous support of his research.

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Table 1. Vertical ionization energies and band assignments for some ferrocenesa,b

Ferrocene derivative Band Ei 0.2 /eV DFT/eV MO (symmetry) HeII/HeI

Ferrocene X 6.91 Fe 3d (e2g)

A 7.26 Fe 3d (a1g)

B 8.81 π (Cp) (e1u)

C 9.35 π (Cp) (e1g)

I X 6.76 Fe 3d (e2g)

A 7.10 Fe 3d (a1g)

B 8.59 π (Cp) (e1u)

C 9.12 π (Cp) (e1g)

II (this work)

X 6.90 7.0 Fe 3d (e2g) 1.0/1.0

A 7.20 7.3, 7.4 Fe 3d (a1g) 1.0/1.0

B 8.5 8.5, 8.7 π (Cp) (e1u) 0.4/0.5

C 9.05 9.1 ,9.2 π (Pyr) (e1g) 1.4/1.2

D 9.40 9.4 n 1.4/1.2

E 10.05 9.6 σ 0.3/0.5

III X 6.7 Fe 3d (a1g)

A 6.9 Fe 3d (e2g)

B 8.40 10.12 π (Pyr) (e1u)

C 9.20 10.76 π (Pyr) (e1g)

D 9.60 11.24 n awe retain D5d orbital symmetry labels for all ferrocenes to emphasize the relationship

between their MO characters b

Pyr designates substituted pyrrolyl ligands

Table 2. UV-Vis spectroscopic data for 2,5-dimethylazaferrocene radical-cation

(experimental: [11]) compared with transitions calculated from UPS dataa,b

exp

/ nm

(UV-Vis)

E eV

(UV-Vis)

ΔE eV

(UPS)

Radical-cation: ionic states and transitions

735 1.68 1.6 B X (8.5-6.9) π→Fe 3d

697 1.78 1.85 C A (9.05-7.2) π→Fe 3d

570 2.18 2.15 C X (9.05-6.9) π→Fe 3d athe numbers in brackets refer to the difference between ionization energies

btransition ΔE = 7.2-6.9 = 0.3 eV would appear at 4133 nm which is out of the range

of UV-Vis spectrum

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Fig. 1 HeI/HeII spectra of II

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Fig. 2 UV-Vis spectrum of neutral 2,5-dimethylazaferrocene and its radical-cation

from ref. [11]. The absorption maxima for the cation are indicated by vertical arrows.