The molecular and electronic structure of N,N′-ethylenebis(acetylacetonylideiminato)oxovanadium(IV) and the electronic structure of its thio analogue

Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 1–5

The molecular and electronic structure of N,N
-ethylenebis(acetylacetonylideiminato)oxovanadium(IV) and the electronic structure of its thio analogue Barry L. Westcott a,∗ , Guy Crundwell a , Thomas R. Burkholder a , Laura A. Michelsen a , Caroline B. Gardner a , Nadine E. Gruhn b , Allen D. Hunter c , Penny Miner c , Mathias Zeller c b

a Department of Chemistry, Central Connecticut State University, 1615 Stanley Street, New Britain, CT 06050, USA The Center for Gas Phase Electron Spectroscopy, Department of Chemistry, The University of Arizona, Tucson, AZ 85721, USA c Department of Chemistry, Youngstown State University, Youngstown, OH 44512, USA

Received 30 August 2003; received in revised form 21 November 2003; accepted 24 November 2003

Abstract The electronic structures of the title complexes—VO(acen) and VS(acen)—and the free H2 (acen) ligand are probed using gas-phase UV-photoelectron spectroscopy [acen = N,N
-ethylenebis(acetylacetonylideiminato)]. The effect of the different axial donors on the metal center is examined, as is the effect that the oxo and thio ligands have on the acen orbitals. We find that the oxo and thio donors primarily affect the metal center and that the ligand periphery remains mostly unchanged. © 2003 Elsevier B.V. All rights reserved. Keywords: Photoelectron spectroscopy; Vanadium; Vanadyl; Acen; Petroleum

1. Introduction Complexes of vanadium can account for over 75% of the transition metal impurities found in crude petroleum [1–3]. Approximately half of the vanadium complexes have been identified as derivatives of vanadyl porphyrins [4], and complexes of this type have been the primary focus of previous studies. The structural nature—both molecular and electronic—of the non-porphyrin complexes, however, remains a mystery; since these impurities have deleterious effects on catalysts in the petroleum refining process, a better understanding of these impurities is necessary. We have been studying the electronic structure of complexes of vanadium in order to correlate the effect that ligand connectivity has on the electronic structure and reactivity of the metal center. Our primary method of studying electronic structure is gas-phase ultraviolet photoelectron spectroscopy. This ∗ Corresponding author. Tel.: +1-860-832-2677; fax: +1-860-832-2704. E-mail address: [email protected] (B.L. Westcott).

0368-2048/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2003.11.007

technique gives a picture of the molecular orbital energies and, consequently, insight into the bonding and reactivity of complexes. Previously we reported the electronic structure of VO(oep) and VO(pc) [oep: octaethyl porphyrin, and pc: pthalocyanin] [5], and complexes of the type VO(ac) [ac = 2,4-pentanedione and its fluorinated and alkylated derivatives] [6]. In the porphyrin complexes, the singly-occupied vanadium d orbital did not lie highest in energy as would be expected from the Aufbau principle described by Bohr [7]. Instead, the HOMO in these complexes was a porphyrin-based ␲-orbital, which contributes to the unusual stability of these complexes. The singly-occupied orbital in the acac complexes (O4 coordination sphere) was the HOMO, following the expectations of the Aufbau principle. Here we present the photoelectron spectra of two complexes—VO(acen) and VS(acen)—that are accepted models for non-porphyrin complexes found in crude oil deposits, according to Crans et al. [8]. Both complexes possess an N2 O2 coordination sphere about the metal center allowing comparison between the electronic structures of these two complexes with the acac complexes that have been previously reported.

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B.L. Westcott et al. / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 1–5

band profile and the number of peaks necessary for a statistically good fit. For the HeII fits the peak positions and half-widths were fixed with respect to those of the HeI fit. Only the peak amplitudes were allowed to vary to account for changes in photoionization cross-section. Confidence limits for the relative integrated peak areas are about 5% with the primary source of uncertainty being the determination of the baseline. The baseline arises from electron scattering and is taken to be linear over the small energy range of these spectra. We have described the fitting procedures in more detail elsewhere [13]. 2.3. Computational studies Fig. 1. ORTEP representation of VO(acen). Ellipsoids at 50% probablility.

2. Experimental 2.1. Synthesis All starting materials were used as received from Fisher Chemicals. All three compounds—acen, VO(acen), VS(acen)—were synthesized according to the procedures of McCarthy et al. [9], Boucher et al. [10], and Callahan and Durand [11]. Green–black plates of VO(acen) (Fig. 1) suitable for X-ray diffraction were obtained via sublimation at 80 ◦ C, and X-ray data1 was collected according to published procedures [12]. 2.2. Photoelectron spectroscopy The HeI (21.2 eV) and HeII (40.8 eV) photoelectron spectra were recorded using instrumentation and procedures described previously [13]. Samples of H2 (acen) and VO(acen) showed no signs of impurity during sublimation in the instrument at 85–100 ◦ C for H2 (acen) and 140–180 ◦ C for VO(acen). The spectrum of VS(acen) showed signs of VO(acen) impurity below temperatures of 150 ◦ C; however, no further impurities were observed in the range from 155 to 180 ◦ C. A small amount of non-volatile solid remained in the sample cell at the end of each experiment. During data collection the instrument resolution (measured using FWHM of the argon 2 P3/2 peak) was 0.020–0.025 eV HeI and 0.022–0.028 eV for HeII. The spectra were fit analytically with asymmetric Gaussian peaks with a confidence limit of peak positions and width deviations generally considered as ±0.02 eV (≈3σ level) by Lichtenberger and Copenhaver [14]. For the different complexes, the HeI spectrum was fit first. The number of peaks used in each fit was based on the features of the 1 Crystallographic data: VO(C H N O ), M = 289.22 g/mol, mon12 18 2 2 oclinic P21 /n, a = 15.4274(12), b = 11.77868(8), c = 8.0647(6) Å, b = 118.0610(10)◦ , V = 1293.19(16) Å3 , Z = 4, ρcalc = 1.486 g/cm3 , λ(MoKα) = 0.71073 Å, T = 90(2) K, 12,580 reflections measured, 3,177 unique (Rint = 0.0239), R1 on F (wR2 on F 2 ) = 0.0338 (0.0909) on 3072 observed reflections.

Two types of ab initio quantum mechanical calculations were performed using the GAMESS-US suite of programs described by Shmidt et al. [15]. In the first set, effective core potentials (ECP’s) using valence basis sets (VBS) at the -31G level were used for heavy atoms using the SKBJC basis set [16–18]. With this method, the 10 core electrons of V are combined to form an effective core charge and the contraction scheme (8s,8p,6d) → [4s,4p,3d] is employed for the remaining 13 electrons. For the main group elements, two core electrons of C, O and N are combined to form an effective core charge and the contraction scheme (4s,4p) → [2s,2p] is employed for the remaining valence electrons. The H basis set is an unscaled -31 basis. In the second set, the Ahlrichs VTZ basis set was used [19].2 This method uses a (14s,9p,5d) → [8s,5p,3d] contraction scheme for V and (10s,6p) → [6s,3p] for the second row elements. The hydrogen basis uses a (5s) → [3s] contraction. The VO(acen) and VS(acen) energies were calculated based on the experimental crystal structures (this work, [20]), while the H2 (acen) energies were obtained from the optimized geometries using SBKJC ECP and Ahlrichs VTZ basis sets.

3. Results and discussion 3.1. Molecular structure The crystal structure of VO(acen) was previously reported by Bruins and Weaver [21]; however, in that case the VO(acen) samples were obtained by recrystallization 2 Ahlrichs’ and Roos’ basis sets were obtained from the Extensible Computational Chemistry Environment Basis Set Database, Version 12/18/02, as developed and distributed by the Molecular Science Computing Facility, Environmental and Molecular Sciences Laboratory which is part of the Pacific Northwest Laboratory, P.O. Box 999, Richland, Washington 99352, USA, and funded by the US Department of Energy. The Pacific Northwest Laboratory is a multi-program laboratory operated by Battelle Memorial Institue for the US Department of Energy under contract DE-AC06-76RLO 1830. Contact David Feller or Karen Schuchardt for further information.

B.L. Westcott et al. / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 1–5

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Table 1 GAMESS calculated energies and orbital compositions for the valence orbitals of H2 (acen) Calculated energy (eV)

Orbital composition

−7.82 −8.23 −9.98 −10.53

48% 40% 70% 57%

C10, 19% O3, 16% N2, 4% C3, 4% C9 C3, 24% N1, 13% C2, 9% O2, 6% C10 O3, 11% C12, 7% C10 N1, 10% C5, 7% C4, 7% C6, 5% O2

Atom numbering scheme is consistent with that of the crystal structure of VO(acen) from Fig. 1.

Fig. 2. HeI photoelectron spectra of acen, VO(acen), and VS(acen).

from benzene, and the room temperature crystal structure was based on 954 reflections that were uncorrected for absorption effects. Although the refinement model contained all 36 atoms, only the vanadium and vanadyl oxygen were refined anisotropically. Furthermore, all hydrogen atoms were located by difference maps and were not constrained to ideal distances or geometries e.g. C7 formed two bonds with hydrogens at distances of 1.122 and 0.874 Å with a ∠H–C–H bond angle of 73◦ . Therefore, the crystal structure of VO(acen) has been redetermined, and this new model was used as the basis geometry for our DFT calculations. There are several differences between our structure and that reported previously. First, the bond lengths between atoms in VO(acen) have been determined to a greater degree of accuracy; many differ significantly from the values published previously. For example, the V=O bond was initially reported to be 1.585 ± 0.007; however, this study finds the V=O bond to be 1.6094 (±0.0011). As for the V–acen interactions, in our determination, the V–O bonds to the ligand are longer on average, and the V–N bonds to the ligand are shorter on average. 3.2. Electronic structure The HeI photoelectron spectra of the acen ligand and the two vanadium complexes are shown in Fig. 2. The free ligand shows three distinct features in the region from 7.5 to 11.5 eV. These features are assigned as ionizations arising from orbitals that are an admixture of oxygen and nitro-

gen lone pairs, in agreement with the computational data (Table 1). A more specific assignment is difficult because of the covalency of this system. Attempts to collect data with different ionizing sources—i.e., HeII or NeI—would not lead to a more specific assignment, as the photoionization cross-sections of N and O—from the data of Yeh and Lindau [22]—are not significantly different. Since our focus is on the effect that the ligand has on the metal center, a more unambiguous assignment of the ligand spectrum is not necessary, as it would not provide further insight into the electronic structure at the metal center. The spectra of the metallated complexes are quite similar. The first spectral feature—6.80 eV for VO(acen), 6.52 eV for VS(acen)—corresponds to ionization from the primarily metal-based dx2 −y2 -orbital, in agreement with the results of the EPR studies of Callahan and Durand [11]. The 0.28 eV destabilization of the metal-based SOMO is attributed to the lower electronegativity of the S atom versus O, and is in agreement with electrochemical studies of Seangprasertkij and Riechel [23]. HeII spectra of VO(acen) and VS(acen) exhibit growth in the band labeled 1 for both complexes, consistent with the photoionization cross-section changes for V [22] (Fig. 3, Table 2). Computational data supports the assignment of band 1 with the results from the SBKJC basis set showing 98% V dx2 −y2 character for the HOMO in both complexes (Fig. 4, Table 3). At higher ionization potentials, the features from the ligand are clearly observed and can be correlated to the free ligand spectrum, with an expected destabilization of these orbitals due to the interaction with the metal center. Due to the low symmetry of the complex (Cs idealized, C1 in the solid state), an unambiguous assignment of the ligand-based Table 2 Relative areas of peaks upon changing photon sourcea Peak

1 2 3 4 5 5a 6 a

VO(acen)

VS(acen)

HeI

HeII

HeI

HeII

1.00 2.67 2.92 1.94 1.11 – 9.49

1.00 1.73 1.64 1.37 0.71 – 6.13

1.00 2.50 2.36 2.82 2.50 7.42 19.35

1.00 2.03 1.40 2.09 1.20 1.65 9.39

Peak 1 is taken as the reference.

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B.L. Westcott et al. / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 1–5

Fig. 4. MOLEKEL representation of the SOMO of VO(acen) [30].

(7.36 eV) [6], and the acen complex is the most easily ionized (6.80 eV) [20]. Stability of porphyrin complexes has been established by Gruhn et al. [26], and can be attributed to the exceptional stability of the ligand itself. The relative Fig. 3. HeI/HeII spectra of VS(acen).

ionizations is difficult. In the spectrum of VS(acen), a spectral feature is seen at 8.44 eV (band 5a) that is not observed in the spectrum of VO(acen). This feature is assigned to ionization from an orbital predominately localized on the thio moiety because upon changing ionizing source from HeI to HeII, band 5a shrinks significantly, suggesting a large amount of sulfur character in this band [22].Computational data for orbitals other than the HOMO are inconclusive, showing substantial mixed character with contributions from the axial ligand—either O or S—and the atoms involved in the delocalized ␲-system of the acen ligand (Fig. 5, Table 4). According to Bancroft and Hu [24]and Green [25], the relative stability of metal complexes can be inferred by the energy required to remove the d electrons. When comparing the series VO(oep), VO(acac)2 , and VO(acen) we find the d1 electron in the porphyrin complex is the most difficult to remove (7.50 eV) [5], the acac complex is intermediate Table 3 GAMESS calculated energies and orbital compositions for the valence orbitals of VO(acen) Calculated energy (eV) −3.54 −8.16 −8.69 −11.13 −11.36 −11.55

Orbital composition 98% V1 26% C3, 23% C10, 12% N1, 11% N2, 7% O2, 7% O3 28% C10, 25% C3, 10% N2, 9% N1, 6% O3, 5% O2, 5% C11 55% O1, 13% O2, 10% V1 35% O1, 22% O2, 9% V1, 6% O2, 5% N2 24% O2, 11% O1, 9% O3, 8% N2, 7%N1, 5% C1, 5% V1, 5% C3

Atom numbering scheme is consistent with that of the crystal structure of VO(acen) from Fig. 1.

Fig. 5. MOLEKEL representations of the HOMO-1 (top) and HOMO-2 (bottom) for VO(acen) [30].

B.L. Westcott et al. / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 1–5 Table 4 GAMESS calculated energies and orbital compositions for the valence orbitals of VS(acen) Calculated energy (eV)

Orbital composition

−4.63 −7.65 −7.83 −8.97

98% V1 74% S1, 5% V1, 4% C10 85% S1, 5% V1 25% C10, 15% S1, 13% C3, 12% N2, 7% N1, 5% O1, 5% V1 33% S1, 19% C3, 10% V1, 9% C10, 8% N1 32% S1, 29% V1, 12% C3, 10% C10

−9.12 −9.70

Atom numbering scheme is consistent with that of the crystal structure of VO(acen) from Fig. 1.

stability of the acac complex with respect to the acen complex can be explained through simple electronegativity arguments. Since the O atoms are more electronegative than the N atoms, the N atoms will necessarily be better donors to the metal center, leading to a destabilization in the orbital energy of the metal-based orbital. Similar behavior has been observed previously by Lichtenberger et al. [27] in the photoelectron spectra of dimetal complexes with O and N donors, as well as in Re complexes where a halogen ligand is varied amongst Cl, Br, and I [28,29].

4. Supplementary material Listings of results and input parameters for the computation studies are available from the authors. The full details of the crystal structure of VO(acen) have been deposited with the Cambridge Structural Data Centre, CCDC No. 218164. Copies of this information may be obtained free of charge from: The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44-1233-336033; e-mail: [email protected] or http://www.ccdc.cam. ac.uk).

Acknowledgements We wish to thank Dr. T. Cundari (University of North Texas) for helpful discussions regarding the application of GAMESS to vanadyl complexes. G.C., C.G., L.M., and B.W. were supported by CSU-AAUP and CCSU Student-Faculty Research grants. M.Z. was supported by NSF grant 0111511, and the diffractometer was funded by NSF grant 0087210, by Ohio Board of Regents grant CAP-491, and by Y.S.U.

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