Letter pubs.acs.org/JPCL
Combined Ab Initio Computational and Infrared Spectroscopic Study of the cis- and trans-Bis(glycinato)copper(II) Complexes in Aqueous Environment Oliver M. D. Lutz,‡,§ Christoph B. Messner,‡,§ Thomas S. Hofer,*,† Matthias Glaẗ zle,⊥ Christian W. Huck,‡ Günther K. Bonn,‡ and Bernd M. Rode† †
Theoretical Chemistry Division, Institute of General, Inorganic and Theoretical Chemistry, ‡Institute of Analytical Chemistry and Radiochemistry, and ⊥General and Inorganic Chemistry Divison, Institute of General, Inorganic and Theoretical Chemistry, Leopold-Franzens University, Innsbruck, Innrain 80-82, 6020 Innsbruck, Austria S Supporting Information *
ABSTRACT: The cis- and trans-bis(glycinato)copper(II) complexes in aqueous solution have been investigated by means of a combined theoretical and experimental approach. The conducted quantum mechanical charge field molecular dynamics (QMCF-MD) studies, being the first quantum mechanical simulations of organometallic complexes by this method, yielded accurate structural details of the investigated isomers as well as novel dynamic data, which has successfully been confirmed and extended by subsequent mid-infrared measurements. The spectroscopic results, critically assessed by adjacent multivariate data analysis, indicate an isomeric stability at ambient conditions, vanishing at elevated temperatures.
SECTION: Molecular Structure, Quantum Chemistry, and General Theory
S
the literature of whether the isomers are stable in an aqueous environment.8,12,16,17 The QMCF-MD simulations of the cis- and trans-bis(glycinato)copper(II) complexes delivered reliable structural as well as dynamical data. Both isomers proved stable within the 30 ps of sampling, and no interconversions of the two isomers were observed. Figure 1 depicts the different conformers of the cis- and trans-bis(glycinato)copper(II) complexes observed in the simulations. The cis complex formed a six-fold-coordinated tetragonal bipyramid over 50% of the sampling time with the glycinato molecules equatorially and the two water molecules axially bonded (Figure 1a). For the other 50% of the simulation period, a five-fold-coordinated tetragonal pyramid with one water molecule axially bonded was observed (Figure 1b). The trans complex formed three different geometries, tetragonal bipyramidal with two water molecules axially bonded (∼50%, Figure 1c), tetragonal pyramidal with one water molecule axially bonded (∼30%, Figure 1d), and tetragonal bipyramidal with a water molecule in the plane (∼20%, Figure 1e). Table 1 lists peak maxima obtained from radial distribution functions (RDFs) in comparison with an extended X-ray absorption fine structure (EXAFS) spectro-
ophisticated studies of copper-based bis(amino-acidato) complexes are of particular relevance considering their crucial role in biochemistry1 as well as in astrobiology.2,3Copper amino acid complexes have been extensively studied in the past by various experimental approaches4−11 as well as by theoretical investigations.12−16 As most of today’s experimental platforms still lack the ability to capture ultrafast processes such as structural interconversions or hydrogen bonding phenomena, it seemed promising to approach the investigation of this complex by means of a molecular dynamics simulation on the femtosecond time scale. Due to the fact that charge transfer as well as many-body effects play a significant role in this system, a quantum mechanical treatment of the complex and its close proximity is mandatory for an accurate description of intra- and intermolecular interactions. The QMCF-MD approach combines an accurate quantum mechanical description of the complex with a well-established solvent potential treatment in a large simulation box, yielding reliable structural as well as dynamical data of the bis(glycinato)copper complexes. Additionally, a mid-infrared spectroscopic study of the solvated complexes is undertaken, and the observed frequencies are compared to the theoretically predicted frequencies. Furthermore, multivariate data analysis of the mid-infrared spectra indicated isomeric stability of the complexes, being discussed in conjunction with the results of the QMCF-MD simulations, possibly solving the ambiguities in © 2013 American Chemical Society
Received: February 8, 2013 Accepted: April 10, 2013 Published: April 10, 2013 1502
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Figure 1. Different conformers of cis- and trans-bis(glycinato)copper(II) observed in the QMCF-MD simulations (VMD, visual molecular dynamics50).
Table 1. Average Water Coordination Numbers (CNav), Bond Distances (peak maxima of RDFs), and Mean Residence Times of the cis and the trans Complex Obtained from QMCF-MD Simulationsa experimental datab
QMCF-MD CNav Cu−Ogly (Å) Cu−Ngly (Å) Cu−Ow (Å) Cu−Cα (Å) MRT0.5 (ps)
cis
trans
1.5 1.97 2.11 2.30 2.94 3.1
1.7 1.97 2.15 2.20 2.96 4.1
Table 2. Predicted trans Content Values in Percent for the Heated cis Samples
cis
trans
heating time
trans cont.a
trans cont.b
trans cont.c
trans cont.d
5 min 30 min 60 min
5% 53% 102%
8% 50% 99%
3% 51% 94%
8% 51% 96%
a
c
d
1.95 /1.95 1.99c/1.95d 2.40c 2.84c/2.80d
c
Based on a recalculation of the PLSR model with the heated cis samples (cross validation). bBased on a recalculation of the PLSR model with the heated cis samples (test set validation). cBased on a predicition via the original PLSR model (cross validation). dBased on a prediction via the original PLSR model (test set validation).
d
1.95 /1.95 1.99c/1.95d 2.40c 2.84c/2.80d
Table 3. Experimentally Observed IR Absorption Bands Compared to the Theoretically Derived Data
a
The bond distances are compared to experimentally derived data. The EXAFS technique is unable to distinguish the cis from the trans complex.8 cReference 8. dReference 48.
b
QMCF-MDa
scopic study.8 All observed distances are in reasonable agreement with the experimental data, considering that EXAFS studies rely on mean distances, whereas theoretically obtained bond lengths are computed from RDF peak maxima. While the glycinato ligands are coordinated bidentate throughout the simulation period, the water molecules undergo 12 (trans) and 15 (cis) exchange processes during the 30 ps of sampling. The mean residence times (MRTs)18 of the coordinating water molecules are 3.1 (cis) and 4.1 ps (trans), which are very short compared to those of copper in pure water, where a MRT value of ∼230 ps was estimated by 17O NMR spectroscopy.19 This higher water exchange rate, indicating weaker Cu−O bonds, can be attributed to two effects, (1) electron donating ligands are known to increase the mobility of first-shell water molecules20,21 and (2) the Cu−O distance is elongated compared to that of copper in pure water.22 Table 3 lists the simulation-derived IR absorption bands obtained via velocity autocorrelation functions (VACFs)23 in comparison to the experimentally observed bands. The band locations are in good agreement with the experiments, readily indicating differences between cis- and trans-bis(glycinato)copper(II) especially in the lower spectral region. Exemplarily, Figure 2 shows experimentally derived spectra of aqueous cis- and trans-bis(glycinato)copper(II).
experiment
assigned
cis
trans
cis
trans
mode
1626
1634
1596
1596
νCO2,as
1414
1427
1435
1436
δCH2,as
1404
1400
1393
1388
νCO2,s
1329
1328
1330
1330
ωCH2
1148
1159
1125
1139
τNH2
1016
1047
1053
1058
ωNH2
940
929
921
913
ρNH2
a
The frequencies have been scaled in accordance with the recommendations of Scott and Radom.49
The isomeric stability observed in the two simulations led to the experimental part of the study, the syntheses of the solid isomers, the preparation of solution samples, and the subsequent recording of mid-infrared spectra as well as the establishment of a quantitative partial least-squares regression (PLSR) model24 for the characterization of the hypothesized thermal isomerization in the liquid state. The spectra of pure cis- and trans-bis(glycinato)copper(II) solutions show significant differences (Figure 2), pointing toward the hypothesis that the two isomers do not interconvert. In order to improve the plausibility of this hypothesis, the spectra of mixtures containing 20, 40, 60, and 80% of the 1503
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cis samples from the trans isomer and the mixtures, further underlining the previously outlined theory. The thermal isomerization of cis-bis(glycinato)copper(II) presented by Delf et al.,11 describing the solid-state conversion from the cis to the trans isomer, does not provide evidence that the isomerization is also observable in the liquid state. Thus, and in order to corroborate the previous hypothesis, it seemed promising to heat samples of dissolved cis-bis(glycinato)copper(II) in a Teflon-lined Parr bomb. After a number of tests (temperature range of 90−180 °C in 20 °C increments), a temperature of 150 °C was found to represent the lowest temperature required for the liquid-state isomerization to be observed. For the corresponding solid-state isomerization, Delf et al.11 reported a temperature of 180−210 °C. The cis solutions were kept at 150 °C for a period of 5, 30, and 60 min in order to investigate the rate of isomerization over time. Subsequent mid-infrared measurements and an inclusion of the spectra in the previous PLSR models (Figure 3) indicate a timecorrelated formation of trans-bis(glycinato)copper(II) at elevated temperatures. In order to further improve the plausibility of the model, the previously established PLSR model was utilized for a prediction of the heated cis samples, yielding identical prediction values. In Table 2, the independently predicted values are compared to the outcomes based on the recalculated PLSR model; however, for the sake of visibility, mean values were utilized for each sample group. Figure 4 shows the 3D score plot of a principal component
Figure 2. ATR FT-IR spectra of (a) the cis- and (b) transbis(glycinato)copper(II) complex in an aqueous environment.
dissolved trans isomer were utilized, together with the isomerically pure solution spectra, to establish a PLSR model. As data pretreatment, the baseline of the employed spectra was normalized via linear baseline correction,24 and the obtained PLSR model was verified, besides conventional cross validation (five samples per segment), via test set validation (32 calibration samples, 18 validation samples). The linear plots in Figure 3, representing the PLSR data, clearly separate pure
Figure 4. 3D PCA scores indicating the heat-induced isomeric conversion from cis- to trans-bis(glycinato)copper(II).
analysis (PCA) model,24 differentiating between pure cis, trans, and heated cis solutions, thus underlining the findings from the previous PLSR model. With the first principal component influencing the model by 82% (Figure 4), the plausibility of the hypothesis is strengthened further. A prolonged monitoring of the pure cis samples over a period of 10 days, investigating whether the conversion from cis- to trans-bis(glycinato)copper(II) also occurs at ambient conditions, confirmed the cis complex’s stability, thus further underlining the hypothesis that aqueous cis-bis(glycinato)copper(II) is isomerically transformable only at elevated temperatures. The Unscrambler software package 24 offers a direct evaluation of the wavenumber regions influencing the components in the PCA and PLSR plots. Regions of 1598 (±36), 1387 (±27), and 1090 cm−1 (±90 cm−1) have been
Figure 3. PLSR models for the cis/trans-bis(glycinato)copper(II) isomerization based on cross validation (top) and test set validation (bottom). The models indicate that neither the cis nor the trans isomers interconvert at ambient conditions. Adding the heated cis samples to the calibrations indicates a cis to trans conversion at an elevated temperature of ≥150 °C. 1504
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identified to influence the model significantly. These findings are in agreement with the simulation data, concluding that especially the peak areas around νCO2,s, τNH2, and ωNH2 are shifted, comparing the cis and the trans isomers. In conclusion, this work comprises the first ab initio QMCFMD simulation study of an organometallic complex in an aqueous environment. The significant sampling period (>30 ps) as well as the substantially large QM region both contributed toward the quality of the results. As the theoretically derived IR absorption bands, being directly connected to the bond force constants, depend on the second derivative of the energy with respect to the nuclear coordinates, they can be seen as a sensitive probe of the simulation’s accuracy. Moreover, the presented results are of significant biochemical relevance, considering the observed alterations in water exchange rates and, consequently, also the reaction kinetics, when the ionic center coordinates to amino acids or proteins. The infrared spectroscopic investigation of the solvated complexes confirmed the theoretically observed isomeric stability at ambient conditions; however, the thermal isomerization, to date having only been reported for the solid complex, was indicated to also occur in aqueous solutions of bis(glycinato)copper(II).
tional effort. Appropriate starting structures were obtained from 200 ps long classical simulations with the previously mentioned Amber parm99 force field parameters. Prior to the 30 ps of sampling, the simulation box was re-equilibrated for 4 ps with an intermittent heating period to 400 K to adjust the system to the QM/MM conditions.
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EXPERIMENTAL METHODS
Chemicals and Reagents. Copper(II) acetate monohydrate (98%, Sigma Aldrich Chemical Co., Vienna, Austria), glycine (99%, Sigma Aldrich Chemical Co., Vienna, Austria), water (LC-MS grade, Sigma Aldrich Chemical Co., Vienna, Austria), and ethanol (99.9%, Gatt-Koller GmbH, Absam, Austria) were not purified further prior to the syntheses. Syntheses. Because isomerically pure bis(glycinato)copper(II) is unavailable via principal suppliers, the cis isomer was synthesized according to the preparation reported by O’Brien.17 The crude product, after having been filtered off from the icecooled liquid remainder, was recrystallized once from water/ ethanol 50:50 v/v, yielding bright blue acicular cis-bis(glycinato)copper(II). The corresponding trans isomer was synthesized twice utilizing the two procedures reported by O’Brien,17 yielding the lustrous platy product. Saturated solutions of the isomers in water were prepared prior to the measurements. After thorough mixing, the solutions were centrifuged for 2 min at 13 000 rpm (eppendorf centrifuge 5415 R) in order to ensure absence of the solid product in the liquid phase. Because of the fact that the mid-infrared spectra of solid cis- and trans-bis(glycinato)copper(II) differ significantly in the lower wavenumber region,10 the spectra were recorded between 1800 and 800 cm−1 at a resolution of 4 cm−1. Pure water was utilized for the background correction measurements during the IR spectroscopy, enabling sensivitve detection of the solute. Product Characterization. Mid-infrared attenuated total reflectance (ATR) spectroscopy (Spectrum 100, PerkinElmer, Seer Green, United Kingdom) of the solid cis and trans samples yielded spectra (see the Supporting Information) in excellent agreement with the data published by Condrate and Nakamoto10 as well as the results by Delf et al.11 and the works by Herlinger et al.46 However, because this work discusses the hydration of bis(glycinato)copper(II), the reader is referred to the respective works10,11,46 providing detailed band assignments of the solid complexes. The synthesized cis-bis(glycinato)copper(II) crystals have been characterized by X-ray powder diffraction, carried out in transmission geometry on a flat sample, utilizing a Stoe Stadi P powder diffractometer with MoKα1 radiation (Ge(111) monochromator, λ = 70.93 pm). The obtained pattern is in excellent agreement with the theoretical pattern based on single-crystal diffraction data47 (see the Supporting Information). Vis absorption measurements of the cis and trans complex solutions have been conducted, both yielding maxima at 630 nm, in excellent agreement with a previous study.4 Because it is known that solid bis(glycinato)copper(II) exists as a monohydrate, losing its single-crystal water molecule at ≥160 °C,11 the product may be further characterized by heating significant sample amounts (50 mg), resulting in the lighter anhydrous complex, as was previously shown by Delf et al.11 The weight loss observed during heating of the monohydrate until weight consistency reflected the stochiometric quantity of
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THEORETICAL METHODS The QMCF-MD approach,22,25−27 being of similar nature as the conventional QM/MM-MD methodology,28,29 utilizes a substantially larger quantum mechanically treated region that is further separated into two subregions, the core and the layer zone. The main advantage of this extension is the possible renunciation of non-Coulombic interaction potentials between core zone particles and particles located in the MM region. The versatility of this framework has been proven in the past for cations,30−32 anions,33,34 as well as small organic molecules.35,36 The simulations of the two isomers were carried out in cubic boxes with a side length of 39.2 Å, each containing 2000 water molecules. As Cu(II) was placed in the center of the core region, its radius was chosen to be 1.0 Å. The radius of the layer zone was set to 6.5 Å, and smoothing22 was applied between 6.3 and 6.5 Å. Due to the location of the glycinato molecules in the layer zone, non-Coulombic glycinato−water potentials were constructed using Amber parm99 force field parameters.37 The simulation environment was established with a density of 0.997 g·cm−3, and the temperature was set to 298 K using the Nosé− Hoover chain dynamics38 with a chain length of 5. The integration of the equations of motion was realized with the velocity Verlet algorithm, and the integrator time step was chosen to be 0.2 fs in order to provide a proper description of hydrogen movement. To account for the non-negligible longrange Coulombic interactions, the reaction field method39 was employed (ϵ = 78.36), and the Coulombic cutoff was established at 15 Å. The quality as well as the feasibility of the employed Hartree−Fock (HF) formalism40 was proven via cluster calculations (Gaussian09 software package41). Minor electron correlation effects were observed as the binding energies obtained at HF and MP/2 levels of theory differ only slightly (see the Supporting Information). HF was preferred over the hybrid functional B3LYP as the latter gains no computational speed and overestimates the effect of electron correlation (see the Supporting Information). For the copper ion, the well-established42 LANL2DZ ECP basis set,43,44 was utilized and for all other atoms, the 6-31G(d,p) basis set45 provided a good compromise between accuracy and computa1505
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(27) Hofer, T. S.; Rode, B. M.; Pribil, A. B.; Randolf, B. R. In Theoretical and Computational Inorganic Chemistry; van Eldik, R., Harvey, J., Eds.; Advances in Inorganic Chemistry; Academic Press: New York, 2010; Vol. 62, pp 143−175. (28) Field, M. J.; Bash, P. A.; Karplus, M. J. Comput. Chem. 1990, 11, 700−733. (29) Bakowies, D.; Thiel, W. J. Phys. Chem. 1996, 100, 10580−10594. (30) Schwenk, C.; Rode, B. ChemPhysChem 2004, 5, 342−348. (31) Messner, C. B.; Hofer, T. S.; Randolf, B. R.; Rode, B. M. Phys. Chem. Chem. Phys. 2011, 13, 224−229. (32) Lutz, O. M. D.; Hofer, T. S.; Randolf, B. R.; Rode, B. M. Chem. Phys. Lett. 2012, 536, 50−54. (33) Pribil, A. B.; Hofer, T. S.; Randolf, B. R.; Rode, B. M. J. Comput. Chem. 2008, 29, 2330−2334. (34) Pribil, A. B.; Hofer, T. S.; Vchirawongkwin, V.; Randolf, B. R.; Rode, B. M. Chem. Phys. 2008, 346, 182−185. (35) Weiss, A. K. H.; Hofer, T. S.; Randolf, B. R.; Bhattacharjee, A.; Rode, B. M. Phys. Chem. Chem. Phys. 2011, 13, 12173−12185. (36) Weiss, A. K. H.; Hofer, T. S.; Randolf, B. R.; Rode, B. M. Phys. Chem. Chem. Phys. 2012, 14, 7012−7027. (37) Cornell, W. D.; Cieplak, P.; Bayly, C. I.; Gould, I. R.; Merz, K. M.; Ferguson, D. M.; Spellmeyer, D. C.; Fox, T.; Caldwell, J. W.; Kollman, P. A. J. Am. Chem. Soc. 1995, 117, 5179−5197. (38) Martyna, G. J.; Klein, M. L.; Tuckerman, M. J. Chem. Phys. 1992, 97, 2635−2643. (39) Adams, D. J.; Adams, E. M.; Hills, G. J. Mol. Phys. 1979, 38, 387−400. (40) TURBOMOLE, V5.10, a development of University of Karlsruhe and Forschungszentrum Karlsruhe GmbH; 2009. (41) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision A.02; Gaussian Inc.: Wallingford, CT, 2009. (42) Schwenk, C. F.; Rode, B. M. J. Chem. Phys. 2003, 119, 9523− 9531. (43) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270−283. (44) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 284−298. (45) Hariharan, P. C.; Pople, J. A. Theor. Chem. Acc. 1973, 28, 213− 222. (46) Herlinger, A. W.; Wenhold, S. L.; Long, T. V. J. Am. Chem. Soc. 1970, 92, 6474−6481. (47) Lamshoft, M.; Ivanova, B. J. Coord. Chem. 2011, 64, 2419−2442. (48) Carrera, F.; Marcos, E.; Merkling, P.; Chaboy, J.; Munoz-Paez, A. Inorg. Chem. 2004, 43, 6674−6683. (49) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502−16513. (50) Humphrey, W.; Dalke, A.; Schulten, K. J. Mol. Graphics 1996, 14, 33−38.
bis(glycinato)copper(II) losing a single water molecule, thus further underlining the identity of the product.
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ASSOCIATED CONTENT
* Supporting Information S
The computed binding energies for cis- and trans-bis(glycinato)copper(II) are listed in a tabular manner. The infrared spectra of the solid products are shown with the peak maxima indicated. The X-ray powder diffraction pattern of cisbis(glycinato)copper(II) is compared to the theoretical pattern based on single-crystal diffraction data.47 This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Author Contributions §
The two first authors declare equal contribution to this work.
Notes
The authors declare no competing financial interest.
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REFERENCES
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