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Molecular and electronic structure of Tc2(O2CCH3)2Cl4 studied by multiconfigurational quantum chemical methods
Polyhedron 70 (2014) 144–147
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Polyhedron journal homepage: www.elsevier.com/locate/poly
Molecular and electronic structure of Tc2(O2CCH3)2Cl4 studied by multiconfigurational quantum chemical methods Tanya K. Todorova a, Frederic Poineau b, Paul M. Forster b, Laura Gagliardi c, Kenneth R. Czerwinski b, Alfred P. Sattelberger d,⇑ a
Department of Physical Chemistry, University of Geneva, CH-1211 Geneva, Switzerland Department of Chemistry, University of Nevada Las Vegas, Las Vegas, NV 89119, USA Department of Chemistry, Supercomputing Institute, and Chemical Theory Center, University of Minnesota, 207 Pleasant Street SE, Minneapolis, MN 55455, USA d Energy Engineering and Systems Analysis Directorate, Argonne National Laboratory, Lemont, IL 60439, USA b c
a r t i c l e
i n f o
Article history: Received 7 October 2013 Accepted 3 December 2013 Available online 7 December 2013 Keywords: Technetium DFT Metal–metal bonds Optical spectroscopy Electronic structure
a b s t r a c t The molecular and electronic structure, as well as the electronic absorption spectrum of Tc2(O2CCH3)2Cl4 were studied by multiconfigurational quantum chemical methods. The computed ground state geometry is in excellent agreement with the experimental structure determined by single crystal X-ray diffraction (SCXRD). The total bond order (i.e., 3.20) is consistent with the presence of a moderately strong quadruple Tc–Tc bond and is the largest bond order reported so far for a multiple Tc–Tc bonded complex. Effective bond order analysis indicates stronger p and d bonds for Tc2(O2CCH3)2Cl4 (i.e., 1.71 for p and 0.59 for d) than for Tc2Cl82 (i.e., 1.68 for p and 0.47 for d). The electronic absorption spectrum was recorded in dichloromethane and shows three distinct bands in the range 10 000–35 000 cm1. Assignment of the bands, as well as their excitation energies and intensities were performed at the CASSCF/CASPT2 level of theory. The lowest energy band corresponds to the d ? d⁄ transition; the next higher energy bands are attributed to d ? p⁄ and p ? d⁄ transitions, respectively. Ó 2014 Published by Elsevier Ltd.
1. Introduction Transition metal carboxylate complexes with multiple metal– metal bonds exhibit interesting catalytic and biological properties [1]. For example, dirhodium(II) tetraacetate is an efficient catalyst for metal carbene transformations [2], while dirhenium(III) dichlorotetraisobutyrate inhibits tumor growth [3]. The studies of the molecular and electronic structures of dinuclear carboxylate complexes are essential to understand their catalytic and biological properties; at a more fundamental level, the study of their electronic structure will permit a better understanding of the nature of metal–metal interactions in dinuclear complexes. For second row transition metals, metal–metal bonded dinuclear carboxylate complexes are encountered for elements of groups 6–9 [4a-4e]. One element whose metal–metal bond chemistry is poorly developed is technetium. Currently, only five Tc(III) quadruply metal–metal bonded dinuclear carboxylate complexes are structurally characterized [4c]. The nature of metal–metal bonding in these complexes, as well as the influence of ligands on the
bonding in the Tc26+ unit are not well understood. For example, the Tc–Tc separation within the Tc26+ unit depends of the nature and position of the ancillary ligands. Complexes with axial Cl ligands, e.g., Tc2(O2CCH3)4Cl2 (Tc–Tc = 2.1758(3) Å) [5] exhibit longer Tc–Tc distances than those with equatorial Cl ligands, e.g., Tc2Cl82 (Tc–Tc = 2.1560(3) Å) [6] and Tc2(O2CCH3)2Cl4 (Tc– Tc = 2.1500(6) Å) [7]. Recently, we revisited the structure and bonding in the Tc2X8n (X = Cl, Br; n = 2, 3) anions [6]. Using multiconfigurational calculations, we have shown that the p component in Tc2X82 is weaker than in Tc2X83 and likely the origin of a larger Tc–Tc separation in the Tc2X82 complexes. The focus of the present work is the study of the metal–metal bonding in Tc2(O2CCH3)2Cl4. Its calculated molecular structure is compared to the one determined experimentally by single crystal X-ray diffraction. The metal–metal bonding in the Tc26+ unit was quantified in terms of effective bond order and compared to that of Tc2Cl82. Finally, the experimental electronic absorption spectrum of Tc2(O2CCH3)2Cl4 was recorded and assignment of the bands was performed at the CASSCF/CASPT2 level of theory. 2. Experimental methods
Abbreviations: SCXRD, single crystal X-ray diffraction; DFT, density functional theory; CASPT2, Complete Active Space Second-order Perturbation Theory; EBO, effective bond order. ⇑ Corresponding author. E-mail address: [email protected] (A.P. Sattelberger). 0277-5387/$ - see front matter Ó 2014 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.poly.2013.12.001
The preparation of Tc2(O2CCH3)2Cl4 has been previously described in detail in Ref. [7]. The compound has been characterized in solid state by SCXRD, Infrared and X-ray absorption fine
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structure spectroscopy [7]. The compound has been characterized in dichloromethane by UV–Vis spectroscopy [7]. The UV–Vis spectrum obtained in Ref. [7] was used as a source for the electronic spectra presented in Section 3.3. 2.1. Theoretical calculations Quantum chemical calculations were performed using density functional theory (DFT) and multiconfigurational Complete Active Space SCF (CASSCF) methods [8], followed by second-order perturbation theory (CASPT2) [9]. Full geometry optimization of Tc2(O2CCH3)2Cl4 was performed at the DFT level using the TURBOMOLE 6.0 program package [10]. The TPSSh hybrid meta-GGA functional was chosen due to its established performance for transition metal compounds [11,12]. Triple-zeta valence plus polarization (def2TZVPP) basis sets were employed on all atoms, except for technetium, for which small-core, quasi-relativistic pseudopotentials were employed. The CASSCF/CASPT2 calculations were performed with the MOLCAS 7.4 package [13] at the DFT/TPSSh/def2-TZVPP optimized geometry. Relativistic all electron ANO-RCC basis sets of triple-zeta quality (VTZP) were used for Tc, Cl and O atoms, whereas ANO-RCC basis sets of double-zeta quality (VDZP) were used for C and H [14,15].The VTZP basis corresponds to the following contractions: 7s6p4d2f1g for Tc, 5s4p2d1f for Cl, and 4s3p2d1f for O. The contraction for C and H was 3s2p1d and 2s1p, respectively. Scalar relativistic effects were included using the Douglas-Kroll-Hess Hamiltonian [16]. The complete active space state interaction (CASSI) method [17,18] was applied to compute the transition probabilities. The computational costs arising from the two-electron integrals were drastically reduced by employing the Cholesky decomposition (CD) technique [18–21] combined with the Local Exchange (LK) screening [22]. This approach has proven to be very successful in the studies of metal–metal bonded compounds [23– 27]. 3. Results and discussion 3.1. Synthesis and X-ray structure determination The compound Tc2(O2CCH3)2Cl4 can be prepared in essentially quantitative yield from the reaction of Tc2(O2CCH3)4Cl2 with gaseous hydrochloric acid at 150 °C (Eq. 1).
Tc2 ðO2 CCH3 Þ4 Cl2 þ 2HCl ! Tc2 ðO2 CCH3 Þ2 Cl4 þ 2HO2 CCH3
ð1Þ
In previous studies, technetium trichloride (a-TcCl3) was obtained from the reaction of Tc2(O2CCH3)4Cl2 with HCl(g) at 300 °C [7]. The mechanism of formation of a-TcCl3 mimics the one described for rhenium; the M2(O2CCH3)2Cl4 (M = Tc, Re) complexes are formed as intermediates in the early stage of these reactions [28]. We believe that the rhenium reaction could be further optimized with respect to the formation of Re2(O2CCH3)2Cl4 by lowering the reaction temperature. The crystallographic structure of Tc2(O2CCH3)2Cl4 has been reported as a footnote in Ref. [7], but not fully analyzed. Therefore, prior to initiating the theoretical study, it was necessary to analyze in more detail its experimental structure. The compound Tc2(O2CCH3)2Cl4 crystallizes in the triclinic space group P-1 (a = 6.0303(12) Å, b = 6.5098(13) Å, c = 8.3072(16) Å; a = 112.082(2)°, b = 96.667(3)°, and c = 108.792(3)°) and is isostructural to Re2(O2CCH3)2Cl4 [28]. The compound exhibits an eclipsed conformation and consists of two transacetate ligands bridging the Tc26+ unit and four terminal chlorides (Fig. 1). The four chlorine atoms are located in a plane that is perpendicular to the plane formed by the O atoms of the acetate
Fig. 1. Ball and stick representation of Tc2(O2CCH3)2Cl4. View perpendicular to the Tc–Tc bond (top left) and view along the Tc–Tc bond (top right). View of the Tc2(O2CCH3)2Cl4 chain along the a-axis (bottom). Distances are in Å. Carbon atoms are in black, oxygen in red, chlorine in green and technetium in grey.
ligands. The Cl atoms are not chemically equivalent, and two sets of Tc–Cl distances are observed: Tc–Cl1 = 2.3339(7) Å and Tc–Cl2 = 2.2903(8) Å. The interatomic distances and angles in Tc2(O2CCH3)2Cl4 are presented in Table 1. The conversion of Tc2(O2CCH3)4Cl2 to Tc2(O2CCH3)2Cl4 is accompanied by a slight decrease in the Tc–Tc distance (i.e., 0.02 Å) and a rearrangement of the chloride ligands. The Tc–Tc distance (2.1500(6) Å) in Tc2(O2CCH3)2Cl4 is essentially identical to the one found in (n-Bu4N)2[Tc2Cl8] (i.e., 2.1560(3) Å) and consistent with the presence of a quadruple Tc–Tc bond [6]. The nature of the Tc–Tc bonding in Tc26+ unit will be discussed in Section 3.2. We note that the Re-Re distance in Re2(O2CCH3)2Cl4 is 2.2084(3) [28]. A difference in the range of 0.04–0.06 Å is typical for the Tc–Re pairs of compounds with identical ligands [4b-4c]. In the crystal, the Tc2(O2CCH3)2Cl4 units form infinite chains that run along the a-axis (Fig. 1, bottom). Within a chain, the dinuclear complexes are in van der Waals contact via the Cl1 and Tc atoms (Tc0 Cl1 = Tc Cl10 = 2.900 Å) and the Cl1 and Cl2 atoms (Cl1 Cl20 = Cl1 Cl2 = 3.301 Å). Van der Waals interactions between dimers of adjacent chains also occur between the hydrogen atoms of the methyl groups; such interactions result in a distortion
Table 1 Selected bond lengths [Å] and angles [°] in Tc2(O2CCH3)2Cl4. Tc–O2 Tc–O1 Tc–Tc1 Tc–Cl2 Tc–Cl1 O2–Tc–O1 O2–Tc–Tc1 O1–Tc–Tc1 O2–Tc–Cl2 O1–Tc–Cl2 Tc1–Tc–Cl2 O2–Tc–Cl1 O1–Tc–Cl1
2.021(1) 2.022(2) 2.1500(6) 2.2903(8) 2.3339(7) 177.57(8) 91.32(6) 90.56(6) 89.27(6) 91.80(6) 103.12(2) 88.69(6) 89.40(6)
O1–C1 O2–C10 C1–O2’ C1–C2
1.269(3) 1.285(3) 1.285(3) 1.488(4)
Tc1–Tc–Cl1 Cl2–Tc–Cl1 C1–O1–Tc C10 –O2–Tc O1–C1–O20 O1–C1–C2 O20 –C1–C2
102.89(2) 153.94(3) 119.32(18) 118.22(18) 120.5(3) 120.5(2) 119.0(3)
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of the acetate ligand and the C2 and C20 atoms are respectively positioned outside the planes formed by the C1 and O1 and C10 and O2 atoms. Finally, the shortest separation between the Tc atoms of adjacent complexes (Tc Tc0 = 4.133 Å) precludes metal–metal interaction between the complexes. The DFT geometry optimization of Tc2(O2CCH3)2Cl4 revealed that the calculated Tc–Tc, Tc–Cl and Tc–O bond distances are in good agreement with the experimental values (Table 2), with the largest deviation found for the Tc–Tc distance (within 3%). It is important to mention that the calculations performed on a finite size Tc2(O2CCH3)2Cl4 cluster result in four equivalent Tc–Cl distances. This is a clear indication that the packing effects along the a-axis are responsible for the two nonequivalent sets of Tc–Cl distances observed in the Tc2(O2CCH3)2Cl4 crystal structure. Comparison with the structure of Tc2Cl82 optimized at the same level of theory (TPSSh/def2-TZVPP) (Table 3) confirms the experimental observation that the Tc–Tc bond distances are shorter in Tc2(O2CCH3)2Cl4 than in Tc2Cl82. 3.2. Electronic structure The CASSCF/CASPT2 calculations were performed in order to investigate the nature of the Tc-Tc bonding in Tc2(O2CCH3)2Cl4. The CASSCF wave function was analyzed in terms of its natural orbitals and their occupation numbers. In the CASSCF calculations, the complete active space contains twelve electrons in twelve active orbitals (12/12). This space comprises one 4dr, two 4dp and one 4dd Tc–Tc bonding orbitals and the corresponding antibonding orbitals, two Tc–carboxylate p bonding and the corresponding p antibonding orbitals. In the subsequent CASPT2 calculations, orbitals up to and including the 3d for Tc, 2p for Cl, 1s for C and O were kept frozen. The 12 molecular orbitals forming the active space along with their occupation numbers are presented in Fig. 2. The dominant electronic configuration r2p4d2 in the 1A1g ground state has a weight in the range of 67% and is mixed with a fraction (12%) with the electronic configuration r2p4d⁄2. The Tc–Tc bonding was quantified in terms of effective bond order (EBO), defined as (gb–ga)/(gb+ga), where gb is the occupation number for the bonding natural orbital, ga is the occupation number for the antibonding natural orbital. The occupancy of d and d⁄ orbitals are respectively 1.59 and 0.41 which gives an EBO of 0.59 for the d bond (Table 4). The EBO value for the r bond is 0.90 and the corresponding value for the p bond is 1.71. The r, p and d bonds are stronger in Tc2(O2CCH3)2Cl4 than in Tc2Cl82 (Table 4) which is consistent with the decrease of the Tc–Tc distance observed. The total bond order in Tc2(O2CCH3)2Cl4 is 3.20 which is the largest value reported so far for a multiple Tc-Tc bonded dimer [6,27,29]. It is similar to the one calculated in Re2Cl82 (i.e., 3.25) and consistent with the presence of a moderately strong quadruple bond [22].
Fig. 2. Active orbitals and their occupation numbers for Tc2(O2CCH3)2Cl4.
Table 4 Effective bond order (EBO) values for Tc2Cl82 and Tc2(O2CCH3)2Cl4. Tc2Cl82
Tc2(O2CCH3)2Cl4
0.88 1.68 0.47 3.03
0.90 1.71 0.59 3.20
EBO
r p d Total bond order
3.3. Spectroscopy In order to gain a better understanding of the electronic structure of Tc2(O2CCH3)2Cl4, spectroscopic measurements and CASSCF/CASPT2 calculations were performed. The UV–Vis spectrum of Tc2(O2CCH3)2Cl4 (Fig. 3) was recorded between 35 000 and 10 000 cm1 [7]. The electronic absorption spectrum exhibits three bands at 15 455, 28 450 and 32 100 cm1 and is similar to the one reported for Tc2Cl82 with the lowest energy band shifted by 742 cm1 to higher energy in Tc2(O2CCH3)2Cl4 [30]. Computed excitation energies along with their intensities are reported in Table 5. In previous theoretical studies on Tc2Cl82, the lowest energy band (exp. 14 713 cm1) was attributed to the d ? d⁄ transition [29]. Our calculations confirm that the lowest energy band in the
Table 2 Calculated and experimental Tc–Cl, Tc–O and Tc–Tc bond distances [Å] in Tc2(O2CCH3)2Cl4.
Calculated TPSSh/def2-TZVPP Experimental
Tc–Tc (Å)
Tc–Cl (Å)
Tc–O (Å)
2.080 2.150
2.278 2.312
2.021 2.022
Table 3 Calculated and experimental Tc–Cl and Tc–Tc bond distances (Å) in Tc2Cl82.
Calculated TPSSh/def2-TZVPP Experimental
Tc–Tc (Å)
Tc–Cl (Å)
2.124 2.156
2.392 2.322
Fig. 3. UV–Vis spectra of (A) Tc2(O2CCH3)2Cl4 and (B) Tc2Cl82 recorded in CH2Cl2.
T.K. Todorova et al. / Polyhedron 70 (2014) 144–147 Table 5 Experimental band maxima (cm1), absorption coefficients e (M1 cm1), assignments, calculated CASPT2 excitation energies (cm1) and their intensities for Tc2(O2CCH3)2Cl4. Experimental
e, M1 cm1
Assignment
Excitation energies
Intensity
15 455 28 450 32 100
2170 8420 11 980
d ? d⁄ d ? p⁄ p ? d⁄
17 733 27 553 31 721
0.41E-2 0.36E-3 0.31E-3
spectrum of Tc2(O2CCH3)2Cl4 corresponds to the d ? d⁄ transition and its excitation energy is predicted at 17 733 cm1. The difference between the calculated and experimental position (2278 cm1) is slightly larger than the one reported for Tc2Cl82 (1834 cm1). The d ? d⁄ transition in Tc2Cl82 was predicted at 16 547 cm1 using CASPT2 calculation with the VTZP basis set [29]. However, the calculations do reproduce the blue shift of the d ? d⁄ transition observed experimentally for Tc2(O2CCH3)2Cl4 relative to Tc2Cl82 (Fig. 3). The next band is attributed to the d ? p⁄ transition and its calculated position (27 553 cm1) matches well with the experimental value (28 450 cm1). Finally, the transition at 32 100 cm1 is assigned to p ? d⁄ transition and is predicted at 31 721 cm1. In the spectrum of Tc2Cl82, the highest energy band (33 016 cm1) was proposed by analogy with Re2Cl82 to be the p ? p⁄ transition. The corresponding p ? p⁄ transition for Tc2(O2CCH3)2Cl4 is computed at 37 746 cm1, and the d ? p⁄ (CO2) at 39 500 cm1. 4. Conclusion The structure, bonding and spectroscopic properties of Tc2(O2CCH3)2Cl4 have been investigated by combined theoretical and experimental methods. The calculated molecular structure of the ground state is in very good agreement with the structure determined experimentally. The computed structural parameters are within 3% of the experimental values. Calculations confirm that the Tc–Tc separations is slightly shorter in Tc2(O2CCH3)2Cl4 than in the Tc2Cl82- anion. The Tc–Tc bonding was quantified in terms of effective bond order. The calculated total effective bond order in Tc2(O2CCH3)2Cl4 (i.e., 3.20) is consistent with the presence of a moderately strong quadruple Tc–Tc bond and is the largest bond order calculated so far for a multiple Tc–Tc bonded dimer. Moreover, the p and d components in the Tc26+ core are stronger in Tc2(O2CCH3)2Cl4 than in Tc2Cl82. A strengthening of the p and d bonds is likely the origin of the shorter Tc–Tc separation in Tc2(O2CCH3)2Cl4. The electronic spectrum of Tc2(O2CCH3)2Cl4 in dichloromethane is similar to that of Tc2Cl82. Assignment of the transitions was performed at the CASSCF/CASPT2 level of theory and the lowest energy band assigned to the d ? d⁄ transition. In agreement with experimental data, the calculation predicts this transition to be shifted to higher energy relative to the one in Tc2Cl82. In order to better understand the nature of metal–metal bonding in dinuclear technetium carboxylates complexes, current theoretical work on [Tc2(O2CCH3)4X2]n (X = Cl, Br, n = 0, 1-) is in progress and the results will be reported in due course.
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Acknowledgments Funding for this research was provided by a SISGR Grant from the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. 47824B. The computational part of this study was supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, Heavy Elements Chemistry Program, US Department of Energy, under Grant DE-SC002183. The authors thank Trevor Low and Julie Bertoia for outstanding health physics support. References [1] M.A.S. Aquino, Coord. Chem. Rev. 170 (1998) 141. [2] F.A. Cotton, C.A. Murillo, R.A. Walton (Eds.), Multiple Bonds between Metal Atoms, third ed., Springer, NY, 2005 (Chapter 13). [3] N. Shtemenko, P. Collery, A. Shtemenko, Anticancer Res. 27 (2007) 2487. [4] (a) Reference 2, chapter 4.; (b) Reference 2, chapter 8.; (c) Reference 2, chapter 7.; (d) Reference 2, chapter 9.; (e) Reference 2, chapter 12. [5] W.M. Kerlin, F. Poineau, K.R. Czerwinski, P.M. Forster, A.P. Sattelberger, Polyhedron 48 (2013) 115. [6] F. Poineau, P.M. Forster, T.K. Todorova, L. Gagliardi, A.P. Sattelberger, K.R. Czerwinski, Dalton Trans. 41 (2012) 2869. [7] F. Poineau, E.V. Johnstone, P.F. Weck, E. Kim, P.M. Forster, B.L. Scott, A.P. Sattelberger, K.R. Czerwinski, J. Am. Chem. Soc. 132 (2010) 15864. [8] B.O. Roos, P.R. Taylor, P.E.M. Siegbahn, Chem. Phys. 48 (1980) 157. [9] K. Andersson, P.A. Malmqvist, B.O. Roos, J. Chem. Phys. 96 (1992) 1218. [10] R. Ahlrichs, M. Baer, M. Haeser, H. Horn, C. Koelmel, Chem. Phys. Lett. 162 (1989) 165. [11] F. Furche, J.P. Perdew, J. Chem. Phys. 124 (2006) 044103. [12] M.P. Waller, H. Braun, N. Hojdis, M. Bühl, J. Chem. Theory Comput. 3 (2007) 2234. [13] G. Karlström, R. Lindh, P.A. Malmqvist, B.O. Roos, U. Ryde, V. Veryazov, P.O. Widmark, M. Cossi, B. Schimmelpfennig, P. Neogrady, L. Seijo, Comput. Mater. Sci. 287 (2003) 222. [14] B.O. Roos, R. Lindh, P.A. Malmqvist, V. Veryazov, P.O. Widmark, J. Phys. Chem. A 109 (2005) 6575. [15] B.O. Roos, R. Lindh, P.A. Malmqvist, V. Veryazov, P.O. Widmark, J. Phys. Chem. A 108 (2005) 2851. [16] B.A. Hess, Phys. Rev. A 33 (1986) 3742. [17] P.A. Malmqvist, Int. J. Quantum Chem. 30 (1986) 479. [18] P.A. Malmqvist, B.O. Roos, Chem. Phys. Lett. 155 (1989) 189. [19] F. Aquilante, P.A. Malmqvist, T.B. Pedersen, A. Ghosh, B.O. Roos, J. Chem. Theory Comput. 4 (2008) 694. [20] F. Aquilante, T.B. Pedersen, R. Lindh, B.O. Roos, A.S. De Meras, H. Koch, J. Chem. Phys. 129 (2008) 024113. [21] F. Aquilante, T.B. Pedersen, L. Gagliardi, R. Lindh, J. Chem. Phys. 130 (2009) 154107. [22] F. Aquilante, T.B. Pedersen, R. Lindh, J. Chem. Phys. 126 (2007) 194106. [23] F. Ferrante, L. Gagliardi, B.E. Bursten, A.P. Sattelberger, Inorg. Chem. 44 (2005) 8476. [24] L. Gagliardi, B.O. Roos, Inorg. Chem. 42 (2003) 1599. [25] B.O. Roos, A. Borin, L. Gagliardi, Angew. Chem., Int. Ed. 46 (2007) 1469. [26] M. Brynda, L. Gagliardi, B.O. Roos, Chem. Phys. Lett. 471 (2009) 1. [27] F. Poineau, P.M. Forster, T.K. Todorova, L. Gagliardi, A.P. Sattelberger, K.R. Czerwinski, Inorg. Chem. 49 (2010) 6646. [28] S.N. Esjornson, P.E. Fanwick, R.A. Walton, Inorg. Chim. Acta 162 (1989) 165. [29] F. Poineau, L. Gagliardi, P.M. Forster, A.P. Sattelberger, K.R. Czerwinski, Dalton Trans. 30 (2009) 5954. [30] F. Poineau, A.P. Sattelberger, S.D. Conradson, K.R. Czerwinski, Inorg. Chem. 47 (2008) 1991.
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Polyhedron journal homepage: www.elsevier.com/locate/poly
Molecular and electronic structure of Tc2(O2CCH3)2Cl4 studied by multiconfigurational quantum chemical methods Tanya K. Todorova a, Frederic Poineau b, Paul M. Forster b, Laura Gagliardi c, Kenneth R. Czerwinski b, Alfred P. Sattelberger d,⇑ a
Department of Physical Chemistry, University of Geneva, CH-1211 Geneva, Switzerland Department of Chemistry, University of Nevada Las Vegas, Las Vegas, NV 89119, USA Department of Chemistry, Supercomputing Institute, and Chemical Theory Center, University of Minnesota, 207 Pleasant Street SE, Minneapolis, MN 55455, USA d Energy Engineering and Systems Analysis Directorate, Argonne National Laboratory, Lemont, IL 60439, USA b c
a r t i c l e
i n f o
Article history: Received 7 October 2013 Accepted 3 December 2013 Available online 7 December 2013 Keywords: Technetium DFT Metal–metal bonds Optical spectroscopy Electronic structure
a b s t r a c t The molecular and electronic structure, as well as the electronic absorption spectrum of Tc2(O2CCH3)2Cl4 were studied by multiconfigurational quantum chemical methods. The computed ground state geometry is in excellent agreement with the experimental structure determined by single crystal X-ray diffraction (SCXRD). The total bond order (i.e., 3.20) is consistent with the presence of a moderately strong quadruple Tc–Tc bond and is the largest bond order reported so far for a multiple Tc–Tc bonded complex. Effective bond order analysis indicates stronger p and d bonds for Tc2(O2CCH3)2Cl4 (i.e., 1.71 for p and 0.59 for d) than for Tc2Cl82 (i.e., 1.68 for p and 0.47 for d). The electronic absorption spectrum was recorded in dichloromethane and shows three distinct bands in the range 10 000–35 000 cm1. Assignment of the bands, as well as their excitation energies and intensities were performed at the CASSCF/CASPT2 level of theory. The lowest energy band corresponds to the d ? d⁄ transition; the next higher energy bands are attributed to d ? p⁄ and p ? d⁄ transitions, respectively. Ó 2014 Published by Elsevier Ltd.
1. Introduction Transition metal carboxylate complexes with multiple metal– metal bonds exhibit interesting catalytic and biological properties [1]. For example, dirhodium(II) tetraacetate is an efficient catalyst for metal carbene transformations [2], while dirhenium(III) dichlorotetraisobutyrate inhibits tumor growth [3]. The studies of the molecular and electronic structures of dinuclear carboxylate complexes are essential to understand their catalytic and biological properties; at a more fundamental level, the study of their electronic structure will permit a better understanding of the nature of metal–metal interactions in dinuclear complexes. For second row transition metals, metal–metal bonded dinuclear carboxylate complexes are encountered for elements of groups 6–9 [4a-4e]. One element whose metal–metal bond chemistry is poorly developed is technetium. Currently, only five Tc(III) quadruply metal–metal bonded dinuclear carboxylate complexes are structurally characterized [4c]. The nature of metal–metal bonding in these complexes, as well as the influence of ligands on the
bonding in the Tc26+ unit are not well understood. For example, the Tc–Tc separation within the Tc26+ unit depends of the nature and position of the ancillary ligands. Complexes with axial Cl ligands, e.g., Tc2(O2CCH3)4Cl2 (Tc–Tc = 2.1758(3) Å) [5] exhibit longer Tc–Tc distances than those with equatorial Cl ligands, e.g., Tc2Cl82 (Tc–Tc = 2.1560(3) Å) [6] and Tc2(O2CCH3)2Cl4 (Tc– Tc = 2.1500(6) Å) [7]. Recently, we revisited the structure and bonding in the Tc2X8n (X = Cl, Br; n = 2, 3) anions [6]. Using multiconfigurational calculations, we have shown that the p component in Tc2X82 is weaker than in Tc2X83 and likely the origin of a larger Tc–Tc separation in the Tc2X82 complexes. The focus of the present work is the study of the metal–metal bonding in Tc2(O2CCH3)2Cl4. Its calculated molecular structure is compared to the one determined experimentally by single crystal X-ray diffraction. The metal–metal bonding in the Tc26+ unit was quantified in terms of effective bond order and compared to that of Tc2Cl82. Finally, the experimental electronic absorption spectrum of Tc2(O2CCH3)2Cl4 was recorded and assignment of the bands was performed at the CASSCF/CASPT2 level of theory. 2. Experimental methods
Abbreviations: SCXRD, single crystal X-ray diffraction; DFT, density functional theory; CASPT2, Complete Active Space Second-order Perturbation Theory; EBO, effective bond order. ⇑ Corresponding author. E-mail address: [email protected] (A.P. Sattelberger). 0277-5387/$ - see front matter Ó 2014 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.poly.2013.12.001
The preparation of Tc2(O2CCH3)2Cl4 has been previously described in detail in Ref. [7]. The compound has been characterized in solid state by SCXRD, Infrared and X-ray absorption fine
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structure spectroscopy [7]. The compound has been characterized in dichloromethane by UV–Vis spectroscopy [7]. The UV–Vis spectrum obtained in Ref. [7] was used as a source for the electronic spectra presented in Section 3.3. 2.1. Theoretical calculations Quantum chemical calculations were performed using density functional theory (DFT) and multiconfigurational Complete Active Space SCF (CASSCF) methods [8], followed by second-order perturbation theory (CASPT2) [9]. Full geometry optimization of Tc2(O2CCH3)2Cl4 was performed at the DFT level using the TURBOMOLE 6.0 program package [10]. The TPSSh hybrid meta-GGA functional was chosen due to its established performance for transition metal compounds [11,12]. Triple-zeta valence plus polarization (def2TZVPP) basis sets were employed on all atoms, except for technetium, for which small-core, quasi-relativistic pseudopotentials were employed. The CASSCF/CASPT2 calculations were performed with the MOLCAS 7.4 package [13] at the DFT/TPSSh/def2-TZVPP optimized geometry. Relativistic all electron ANO-RCC basis sets of triple-zeta quality (VTZP) were used for Tc, Cl and O atoms, whereas ANO-RCC basis sets of double-zeta quality (VDZP) were used for C and H [14,15].The VTZP basis corresponds to the following contractions: 7s6p4d2f1g for Tc, 5s4p2d1f for Cl, and 4s3p2d1f for O. The contraction for C and H was 3s2p1d and 2s1p, respectively. Scalar relativistic effects were included using the Douglas-Kroll-Hess Hamiltonian [16]. The complete active space state interaction (CASSI) method [17,18] was applied to compute the transition probabilities. The computational costs arising from the two-electron integrals were drastically reduced by employing the Cholesky decomposition (CD) technique [18–21] combined with the Local Exchange (LK) screening [22]. This approach has proven to be very successful in the studies of metal–metal bonded compounds [23– 27]. 3. Results and discussion 3.1. Synthesis and X-ray structure determination The compound Tc2(O2CCH3)2Cl4 can be prepared in essentially quantitative yield from the reaction of Tc2(O2CCH3)4Cl2 with gaseous hydrochloric acid at 150 °C (Eq. 1).
Tc2 ðO2 CCH3 Þ4 Cl2 þ 2HCl ! Tc2 ðO2 CCH3 Þ2 Cl4 þ 2HO2 CCH3
ð1Þ
In previous studies, technetium trichloride (a-TcCl3) was obtained from the reaction of Tc2(O2CCH3)4Cl2 with HCl(g) at 300 °C [7]. The mechanism of formation of a-TcCl3 mimics the one described for rhenium; the M2(O2CCH3)2Cl4 (M = Tc, Re) complexes are formed as intermediates in the early stage of these reactions [28]. We believe that the rhenium reaction could be further optimized with respect to the formation of Re2(O2CCH3)2Cl4 by lowering the reaction temperature. The crystallographic structure of Tc2(O2CCH3)2Cl4 has been reported as a footnote in Ref. [7], but not fully analyzed. Therefore, prior to initiating the theoretical study, it was necessary to analyze in more detail its experimental structure. The compound Tc2(O2CCH3)2Cl4 crystallizes in the triclinic space group P-1 (a = 6.0303(12) Å, b = 6.5098(13) Å, c = 8.3072(16) Å; a = 112.082(2)°, b = 96.667(3)°, and c = 108.792(3)°) and is isostructural to Re2(O2CCH3)2Cl4 [28]. The compound exhibits an eclipsed conformation and consists of two transacetate ligands bridging the Tc26+ unit and four terminal chlorides (Fig. 1). The four chlorine atoms are located in a plane that is perpendicular to the plane formed by the O atoms of the acetate
Fig. 1. Ball and stick representation of Tc2(O2CCH3)2Cl4. View perpendicular to the Tc–Tc bond (top left) and view along the Tc–Tc bond (top right). View of the Tc2(O2CCH3)2Cl4 chain along the a-axis (bottom). Distances are in Å. Carbon atoms are in black, oxygen in red, chlorine in green and technetium in grey.
ligands. The Cl atoms are not chemically equivalent, and two sets of Tc–Cl distances are observed: Tc–Cl1 = 2.3339(7) Å and Tc–Cl2 = 2.2903(8) Å. The interatomic distances and angles in Tc2(O2CCH3)2Cl4 are presented in Table 1. The conversion of Tc2(O2CCH3)4Cl2 to Tc2(O2CCH3)2Cl4 is accompanied by a slight decrease in the Tc–Tc distance (i.e., 0.02 Å) and a rearrangement of the chloride ligands. The Tc–Tc distance (2.1500(6) Å) in Tc2(O2CCH3)2Cl4 is essentially identical to the one found in (n-Bu4N)2[Tc2Cl8] (i.e., 2.1560(3) Å) and consistent with the presence of a quadruple Tc–Tc bond [6]. The nature of the Tc–Tc bonding in Tc26+ unit will be discussed in Section 3.2. We note that the Re-Re distance in Re2(O2CCH3)2Cl4 is 2.2084(3) [28]. A difference in the range of 0.04–0.06 Å is typical for the Tc–Re pairs of compounds with identical ligands [4b-4c]. In the crystal, the Tc2(O2CCH3)2Cl4 units form infinite chains that run along the a-axis (Fig. 1, bottom). Within a chain, the dinuclear complexes are in van der Waals contact via the Cl1 and Tc atoms (Tc0 Cl1 = Tc Cl10 = 2.900 Å) and the Cl1 and Cl2 atoms (Cl1 Cl20 = Cl1 Cl2 = 3.301 Å). Van der Waals interactions between dimers of adjacent chains also occur between the hydrogen atoms of the methyl groups; such interactions result in a distortion
Table 1 Selected bond lengths [Å] and angles [°] in Tc2(O2CCH3)2Cl4. Tc–O2 Tc–O1 Tc–Tc1 Tc–Cl2 Tc–Cl1 O2–Tc–O1 O2–Tc–Tc1 O1–Tc–Tc1 O2–Tc–Cl2 O1–Tc–Cl2 Tc1–Tc–Cl2 O2–Tc–Cl1 O1–Tc–Cl1
2.021(1) 2.022(2) 2.1500(6) 2.2903(8) 2.3339(7) 177.57(8) 91.32(6) 90.56(6) 89.27(6) 91.80(6) 103.12(2) 88.69(6) 89.40(6)
O1–C1 O2–C10 C1–O2’ C1–C2
1.269(3) 1.285(3) 1.285(3) 1.488(4)
Tc1–Tc–Cl1 Cl2–Tc–Cl1 C1–O1–Tc C10 –O2–Tc O1–C1–O20 O1–C1–C2 O20 –C1–C2
102.89(2) 153.94(3) 119.32(18) 118.22(18) 120.5(3) 120.5(2) 119.0(3)
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of the acetate ligand and the C2 and C20 atoms are respectively positioned outside the planes formed by the C1 and O1 and C10 and O2 atoms. Finally, the shortest separation between the Tc atoms of adjacent complexes (Tc Tc0 = 4.133 Å) precludes metal–metal interaction between the complexes. The DFT geometry optimization of Tc2(O2CCH3)2Cl4 revealed that the calculated Tc–Tc, Tc–Cl and Tc–O bond distances are in good agreement with the experimental values (Table 2), with the largest deviation found for the Tc–Tc distance (within 3%). It is important to mention that the calculations performed on a finite size Tc2(O2CCH3)2Cl4 cluster result in four equivalent Tc–Cl distances. This is a clear indication that the packing effects along the a-axis are responsible for the two nonequivalent sets of Tc–Cl distances observed in the Tc2(O2CCH3)2Cl4 crystal structure. Comparison with the structure of Tc2Cl82 optimized at the same level of theory (TPSSh/def2-TZVPP) (Table 3) confirms the experimental observation that the Tc–Tc bond distances are shorter in Tc2(O2CCH3)2Cl4 than in Tc2Cl82. 3.2. Electronic structure The CASSCF/CASPT2 calculations were performed in order to investigate the nature of the Tc-Tc bonding in Tc2(O2CCH3)2Cl4. The CASSCF wave function was analyzed in terms of its natural orbitals and their occupation numbers. In the CASSCF calculations, the complete active space contains twelve electrons in twelve active orbitals (12/12). This space comprises one 4dr, two 4dp and one 4dd Tc–Tc bonding orbitals and the corresponding antibonding orbitals, two Tc–carboxylate p bonding and the corresponding p antibonding orbitals. In the subsequent CASPT2 calculations, orbitals up to and including the 3d for Tc, 2p for Cl, 1s for C and O were kept frozen. The 12 molecular orbitals forming the active space along with their occupation numbers are presented in Fig. 2. The dominant electronic configuration r2p4d2 in the 1A1g ground state has a weight in the range of 67% and is mixed with a fraction (12%) with the electronic configuration r2p4d⁄2. The Tc–Tc bonding was quantified in terms of effective bond order (EBO), defined as (gb–ga)/(gb+ga), where gb is the occupation number for the bonding natural orbital, ga is the occupation number for the antibonding natural orbital. The occupancy of d and d⁄ orbitals are respectively 1.59 and 0.41 which gives an EBO of 0.59 for the d bond (Table 4). The EBO value for the r bond is 0.90 and the corresponding value for the p bond is 1.71. The r, p and d bonds are stronger in Tc2(O2CCH3)2Cl4 than in Tc2Cl82 (Table 4) which is consistent with the decrease of the Tc–Tc distance observed. The total bond order in Tc2(O2CCH3)2Cl4 is 3.20 which is the largest value reported so far for a multiple Tc-Tc bonded dimer [6,27,29]. It is similar to the one calculated in Re2Cl82 (i.e., 3.25) and consistent with the presence of a moderately strong quadruple bond [22].
Fig. 2. Active orbitals and their occupation numbers for Tc2(O2CCH3)2Cl4.
Table 4 Effective bond order (EBO) values for Tc2Cl82 and Tc2(O2CCH3)2Cl4. Tc2Cl82
Tc2(O2CCH3)2Cl4
0.88 1.68 0.47 3.03
0.90 1.71 0.59 3.20
EBO
r p d Total bond order
3.3. Spectroscopy In order to gain a better understanding of the electronic structure of Tc2(O2CCH3)2Cl4, spectroscopic measurements and CASSCF/CASPT2 calculations were performed. The UV–Vis spectrum of Tc2(O2CCH3)2Cl4 (Fig. 3) was recorded between 35 000 and 10 000 cm1 [7]. The electronic absorption spectrum exhibits three bands at 15 455, 28 450 and 32 100 cm1 and is similar to the one reported for Tc2Cl82 with the lowest energy band shifted by 742 cm1 to higher energy in Tc2(O2CCH3)2Cl4 [30]. Computed excitation energies along with their intensities are reported in Table 5. In previous theoretical studies on Tc2Cl82, the lowest energy band (exp. 14 713 cm1) was attributed to the d ? d⁄ transition [29]. Our calculations confirm that the lowest energy band in the
Table 2 Calculated and experimental Tc–Cl, Tc–O and Tc–Tc bond distances [Å] in Tc2(O2CCH3)2Cl4.
Calculated TPSSh/def2-TZVPP Experimental
Tc–Tc (Å)
Tc–Cl (Å)
Tc–O (Å)
2.080 2.150
2.278 2.312
2.021 2.022
Table 3 Calculated and experimental Tc–Cl and Tc–Tc bond distances (Å) in Tc2Cl82.
Calculated TPSSh/def2-TZVPP Experimental
Tc–Tc (Å)
Tc–Cl (Å)
2.124 2.156
2.392 2.322
Fig. 3. UV–Vis spectra of (A) Tc2(O2CCH3)2Cl4 and (B) Tc2Cl82 recorded in CH2Cl2.
T.K. Todorova et al. / Polyhedron 70 (2014) 144–147 Table 5 Experimental band maxima (cm1), absorption coefficients e (M1 cm1), assignments, calculated CASPT2 excitation energies (cm1) and their intensities for Tc2(O2CCH3)2Cl4. Experimental
e, M1 cm1
Assignment
Excitation energies
Intensity
15 455 28 450 32 100
2170 8420 11 980
d ? d⁄ d ? p⁄ p ? d⁄
17 733 27 553 31 721
0.41E-2 0.36E-3 0.31E-3
spectrum of Tc2(O2CCH3)2Cl4 corresponds to the d ? d⁄ transition and its excitation energy is predicted at 17 733 cm1. The difference between the calculated and experimental position (2278 cm1) is slightly larger than the one reported for Tc2Cl82 (1834 cm1). The d ? d⁄ transition in Tc2Cl82 was predicted at 16 547 cm1 using CASPT2 calculation with the VTZP basis set [29]. However, the calculations do reproduce the blue shift of the d ? d⁄ transition observed experimentally for Tc2(O2CCH3)2Cl4 relative to Tc2Cl82 (Fig. 3). The next band is attributed to the d ? p⁄ transition and its calculated position (27 553 cm1) matches well with the experimental value (28 450 cm1). Finally, the transition at 32 100 cm1 is assigned to p ? d⁄ transition and is predicted at 31 721 cm1. In the spectrum of Tc2Cl82, the highest energy band (33 016 cm1) was proposed by analogy with Re2Cl82 to be the p ? p⁄ transition. The corresponding p ? p⁄ transition for Tc2(O2CCH3)2Cl4 is computed at 37 746 cm1, and the d ? p⁄ (CO2) at 39 500 cm1. 4. Conclusion The structure, bonding and spectroscopic properties of Tc2(O2CCH3)2Cl4 have been investigated by combined theoretical and experimental methods. The calculated molecular structure of the ground state is in very good agreement with the structure determined experimentally. The computed structural parameters are within 3% of the experimental values. Calculations confirm that the Tc–Tc separations is slightly shorter in Tc2(O2CCH3)2Cl4 than in the Tc2Cl82- anion. The Tc–Tc bonding was quantified in terms of effective bond order. The calculated total effective bond order in Tc2(O2CCH3)2Cl4 (i.e., 3.20) is consistent with the presence of a moderately strong quadruple Tc–Tc bond and is the largest bond order calculated so far for a multiple Tc–Tc bonded dimer. Moreover, the p and d components in the Tc26+ core are stronger in Tc2(O2CCH3)2Cl4 than in Tc2Cl82. A strengthening of the p and d bonds is likely the origin of the shorter Tc–Tc separation in Tc2(O2CCH3)2Cl4. The electronic spectrum of Tc2(O2CCH3)2Cl4 in dichloromethane is similar to that of Tc2Cl82. Assignment of the transitions was performed at the CASSCF/CASPT2 level of theory and the lowest energy band assigned to the d ? d⁄ transition. In agreement with experimental data, the calculation predicts this transition to be shifted to higher energy relative to the one in Tc2Cl82. In order to better understand the nature of metal–metal bonding in dinuclear technetium carboxylates complexes, current theoretical work on [Tc2(O2CCH3)4X2]n (X = Cl, Br, n = 0, 1-) is in progress and the results will be reported in due course.
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Acknowledgments Funding for this research was provided by a SISGR Grant from the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. 47824B. The computational part of this study was supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, Heavy Elements Chemistry Program, US Department of Energy, under Grant DE-SC002183. The authors thank Trevor Low and Julie Bertoia for outstanding health physics support. References [1] M.A.S. Aquino, Coord. Chem. Rev. 170 (1998) 141. [2] F.A. Cotton, C.A. Murillo, R.A. Walton (Eds.), Multiple Bonds between Metal Atoms, third ed., Springer, NY, 2005 (Chapter 13). [3] N. Shtemenko, P. Collery, A. Shtemenko, Anticancer Res. 27 (2007) 2487. [4] (a) Reference 2, chapter 4.; (b) Reference 2, chapter 8.; (c) Reference 2, chapter 7.; (d) Reference 2, chapter 9.; (e) Reference 2, chapter 12. [5] W.M. Kerlin, F. Poineau, K.R. Czerwinski, P.M. Forster, A.P. Sattelberger, Polyhedron 48 (2013) 115. [6] F. Poineau, P.M. Forster, T.K. Todorova, L. Gagliardi, A.P. Sattelberger, K.R. Czerwinski, Dalton Trans. 41 (2012) 2869. [7] F. Poineau, E.V. Johnstone, P.F. Weck, E. Kim, P.M. Forster, B.L. Scott, A.P. Sattelberger, K.R. Czerwinski, J. Am. Chem. Soc. 132 (2010) 15864. [8] B.O. Roos, P.R. Taylor, P.E.M. Siegbahn, Chem. Phys. 48 (1980) 157. [9] K. Andersson, P.A. Malmqvist, B.O. Roos, J. Chem. Phys. 96 (1992) 1218. [10] R. Ahlrichs, M. Baer, M. Haeser, H. Horn, C. Koelmel, Chem. Phys. Lett. 162 (1989) 165. [11] F. Furche, J.P. Perdew, J. Chem. Phys. 124 (2006) 044103. [12] M.P. Waller, H. Braun, N. Hojdis, M. Bühl, J. Chem. Theory Comput. 3 (2007) 2234. [13] G. Karlström, R. Lindh, P.A. Malmqvist, B.O. Roos, U. Ryde, V. Veryazov, P.O. Widmark, M. Cossi, B. Schimmelpfennig, P. Neogrady, L. Seijo, Comput. Mater. Sci. 287 (2003) 222. [14] B.O. Roos, R. Lindh, P.A. Malmqvist, V. Veryazov, P.O. Widmark, J. Phys. Chem. A 109 (2005) 6575. [15] B.O. Roos, R. Lindh, P.A. Malmqvist, V. Veryazov, P.O. Widmark, J. Phys. Chem. A 108 (2005) 2851. [16] B.A. Hess, Phys. Rev. A 33 (1986) 3742. [17] P.A. Malmqvist, Int. J. Quantum Chem. 30 (1986) 479. [18] P.A. Malmqvist, B.O. Roos, Chem. Phys. Lett. 155 (1989) 189. [19] F. Aquilante, P.A. Malmqvist, T.B. Pedersen, A. Ghosh, B.O. Roos, J. Chem. Theory Comput. 4 (2008) 694. [20] F. Aquilante, T.B. Pedersen, R. Lindh, B.O. Roos, A.S. De Meras, H. Koch, J. Chem. Phys. 129 (2008) 024113. [21] F. Aquilante, T.B. Pedersen, L. Gagliardi, R. Lindh, J. Chem. Phys. 130 (2009) 154107. [22] F. Aquilante, T.B. Pedersen, R. Lindh, J. Chem. Phys. 126 (2007) 194106. [23] F. Ferrante, L. Gagliardi, B.E. Bursten, A.P. Sattelberger, Inorg. Chem. 44 (2005) 8476. [24] L. Gagliardi, B.O. Roos, Inorg. Chem. 42 (2003) 1599. [25] B.O. Roos, A. Borin, L. Gagliardi, Angew. Chem., Int. Ed. 46 (2007) 1469. [26] M. Brynda, L. Gagliardi, B.O. Roos, Chem. Phys. Lett. 471 (2009) 1. [27] F. Poineau, P.M. Forster, T.K. Todorova, L. Gagliardi, A.P. Sattelberger, K.R. Czerwinski, Inorg. Chem. 49 (2010) 6646. [28] S.N. Esjornson, P.E. Fanwick, R.A. Walton, Inorg. Chim. Acta 162 (1989) 165. [29] F. Poineau, L. Gagliardi, P.M. Forster, A.P. Sattelberger, K.R. Czerwinski, Dalton Trans. 30 (2009) 5954. [30] F. Poineau, A.P. Sattelberger, S.D. Conradson, K.R. Czerwinski, Inorg. Chem. 47 (2008) 1991.