Symmetry-Dependent Vibrational Circular Dichroism Enhancement in

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Spectroscopy and Photochemistry; General Theory

Symmetry-Dependent Vibrational Circular Dichroism Enhancement in Co(II) Salicylaldiminato Complexes Gennaro Pescitelli, Steffen Lüdeke, Marcin Górecki, and Lorenzo Di Bari J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03764 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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The Journal of Physical Chemistry Letters

Symmetry-Dependent Vibrational Circular Dichroism Enhancement in Co(II) Salicylaldiminato Complexes Gennaro Pescitelli,†* Steffen Lüdeke,‡ Marcin Górecki,†,§ Lorenzo Di Bari,† †

Department of Chemistry and Industrial Chemistry, University of Pisa, Pisa, Italy

‡

Institute of Pharmaceutical Sciences, University of Freiburg, Freiburg, Germany

§

Institute of Organic Chemistry, Polish Academy of Sciences, Warsaw, Poland

AUTHOR INFORMATION Corresponding Author * Department of Chemistry and Industrial Chemistry, University of Pisa, Via Moruzzi 13, 56126 Pisa, Italy; E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT. Chiral coordination compounds of Co(II) and other open-shell metal complexes display enhanced vibrational circular dichroism (VCD) spectra associated with the existence of low-lying excited states (LLES). In addition to the enhancement, a series of Co(II) salicylaldiminato complexes exhibit an almost monosignate pattern of VCD bands, a unique feature if compared with the usual alternation of positive and negative signals. Frequency and excited-state calculations reveal that VCD enhancement and sign reversal selectively affect normal modes of B symmetry of the C2-symmetric pseudo-tetrahedral species, thanks to their combination with one or more LLES having the same B symmetry. This proves the strict relation between VCD enhancement and monosignate appearance and demonstrates an unprecedented symmetry dependence of the two phenomena.

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Vibration circular dichroism (VCD) is a powerful chiroptical technique able to provide detailed structural information on a large variety of chemical species.1-2 The most important limitation of VCD are the generally very low recorded signal intensities, usually 5 orders of magnitude smaller than the corresponding IR absorbance signals. Some biological samples in aggregated form (fibrils, spray-dried films, etc.) have intrinsically strong VCD signals, often referred to as enhanced VCD.1, 3-4 In a different and more general approach, one may conceive a moiety capable of generating enhanced VCD signals as local structural probe when linked to a molecular or biomolecular species.5-7 The smallest imaginable probe is a metal ion bound to a ligand, whose – usually weak – VCD signals may be enhanced upon binding. In fact, Co(II),6, 8-12 Co(III),13-16 Ni(II),8-9, 17 Cu(II),17 other M(III) (M= metal),18-19 and several Ln(III) ions (Ln= lanthanide) 20-21 have been demonstrated to be endowed with VCD enhancing properties. The common feature of all mentioned metal complexes is the presence of unpaired d- or f-electrons responsible for low-lying electronic states (LLES). The current theory proposed by Nafie in 200422 relates the VCD enhancement to the communication between ligand-centered vibrational transitions of the ground state and metal-centered LLES,1, 22 which requires going beyond the Born-Oppenheimer approximation.5, 22 LLES theory has replaced previous interpretations based on ring current theory13-15, 23 and on Fano-type interference mechanism.8, 12, 24 Recently, we reported a series of bis[(R or S)-N-(1-(Ar)ethyl)salicylaldiminato] complexes of Co(II) with different aryl groups (1-4, Scheme 1). The compounds were prepared by reacting the Schiff base ligands with Co(II) acetate or sulfate in the presence of a base, and were characterized by various methods including X-ray crystallography.25 Complexes 1-4 have metalcentered Δ/Λ-chirality and exhibit a continuum of absorption and circular dichroism bands spanning from the UV to the mid-IR region, which we dubbed (chiro)optical super-spectrum. In


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particular, they show various electronic d-d transitions in the NIR (6000-12000 cm–1) and IR regions (1800-3200 cm–1).25 These latter characteristically overlap with C-H stretching vibrations, a phenomenon previously observed only for Co(II) and Ni(II) sparteine and isosparteine,8-9, 12 and to a lesser extent for Co(III) bis(biuretato) complexes.16 The proximity or even the direct superposition between ligand-centered vibrational states and metal-centered LLES is then particularly evident for compounds 1-4. VCD signals of 1-4 are enhanced with respect to the free ligands or the homoleptic Zn(II) and Cu(II) complexes26-27 by at least one order of magnitude according to the respective g-factors (Δε/ε). Figure 1 shows the IR/VCD spectrum of compound (R)-1 (the spectra of the other species in Scheme 1 are reported in the Supporting Information, Figure S1). The strongest VCD signals appear in the region of the C=N stretching vibration around 1600 cm−1 with Δε  1 M–1 cm–1 and g  5·10−4, and in the region between 1300-1450 cm−1 with Δε  0.3 M–1 cm–1 and g  2.5·10−4. X

X

(S) *

N

(R) 

O

 Co

N

N

*

O

Co O

*

N *

(S) X -(S)-1, X=H 2, X=OCH3 3, X=Cl 4, X=Br

*

O

*

(R) X -(R)-1, X=H 2, X=OCH3 3, X=Cl 4, X=Br

Scheme 1. Structure of bis[(R/S)-N-(1-(Ar)ethyl)salicylaldiminato] Λ/Δ-cobalt(II) complexes 1-4. A given ligand configuration (R or S) induces a dominant chirality-at-metal (Λ and Δ, respectively) in the C2-symmetrical pseudo-tetrahedral geometry.


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Figure 1. Experimental and calculated IR and VCD spectra for (R)-1. Spectra calculated at the B3LYP/def2-TZVP level in vacuo as the Boltzmann-weighted average over 2 conformers, with relative internal energies estimated at the D3BJ-B3LYP/def2-TZVP/PCM level. Plotting parameters: Lorentzian bandwidth 4 cm–1; frequency scaling 0.975. The framed region is expanded in Figure 2; vertical arrows indicate the most relevant calculated positive VCD bands with no experimental correspondence. In addition to the enhancement, the VCD spectra of 1-4 are strikingly almost monosignate, namely, nearly all bands are negative for (R) enantiomers and positive for (S) enantiomers. For the (R) series the ratio between the integrals of negative and positive peaks in the range 950-1750 cm−1 is 20:1, while for the (S) series the proportion is inverted. Normally, one would expect 1:1 proportion for VCD peaks in the mid-IR range. The fact that the monosignate appearance of VCD spectra and their enhancement are related phenomena is suggested by several observations. The single sign appearance has not been observed in the VCD spectra of the homoleptic


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diamagnetic Zn(II) and Cu(II) complexes of 1-4.26-27 On the contrary, VCD spectra somewhat biased toward one sign have been reported previously for some tris-chelated Co(III) complexes1415

– also featuring LLES – though they were interpreted at that time through the ring-current

theory.28 We must however mention that the effect observed in those studies was comparably mild while the extent of the bias observed for our compounds 1-4 is unprecedented. Interestingly enough, this bias cannot be reproduced by density functional theory (DFT) calculations of VCD spectra which, in their current implementation, are limited to molecules with well-separated ground and electronic excited states.1, 5, 9, 22 This is certainly not the case for compounds 1-4 and other Co(II) or Ni(II) complexes,8-9, 12 where the fundamental C-H stretching vibrations, and possibly the overtones of several other vibrations, overlap with the lowest electronic excited state. The calculated IR and VCD spectra for (R)-1 at the B3LYP/def2-TZVP level are shown in Figure 1. These spectra are the Boltzmann-weighted average of 2 conformers, both with Λ-(R) configuration (Figure S2, Supporting Information), which is the dominant diastereomeric species in solution;25 their populations were estimated at the D3BJ-B3LYP/def2-TZVP level of theory with PCM for chloroform. In the mid-IR region, the IR spectrum is well reproduced by DFT calculations, demonstrating that the set of input structures is correct, and the level of calculation is appropriate. On the contrary, the simulated VCD spectrum shows a poor agreement with the experiment, because the observed bias for a negative sign for (R)-1 is not reproduced by the DFT calculations. Moreover, the intensity of several VCD bands is underestimated, especially between 1410 and 1650 cm–1 (framed region in Figure 1). As mentioned above, this failure is evidently associated with the approximations underlying the current theory for VCD calculations implemented in Gaussian,29 which does not include the effects of LLES.


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Figure 2. (a) Calculated VCD spectrum in the 1350-1650 cm–1 range for the C2-symmetrized structure of (R)-1 with band assignment, corresponding to the framed region in Figure 1, compared with the experimental spectrum; plotting parameters same as in Figure 1. (b) Atom displacements (blue arrows) and the overall transition dipole (dark yellow arrow) for normal modes no. 134 (left) and 135 (right), with their respective symmetries. To better understand and interpret VCD calculation results, and to provide further insight into the VCD enhancement phenomenon, we benefit from the fact that, with 92% Boltzmann population at 300K (estimated at D3BJ-B3LYP/def2-TZVP/PCM level of calculation), the lowest-energy conformer of (R)-1 was largely dominant over the other conformers (Figure S2, Supporting Information). The strong preference is dictated by the constrained coordination geometry and various non-bonding interactions among the two ligand moieties, which render the remaining conformers much less favored and well separated from the absolute minimum. Under these conditions, Boltzmann averaging procedure is expected to be reasonably accurate. More importantly, the dominant conformer was almost C2-symmetric with only a very small deviation between the actual lowest-energy structure and the closest C2-symmetric structure (Figure S3, Supporting Information). The IR/VCD spectrum calculated on this latter structure with enforced C2 symmetry was almost identical with the Boltzmann-averaged IR/VCD spectrum (FigureS4,


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Supporting Information), which justifies the use of such a structure for further analysis. In fact, the use of a symmetrized structure has the great advantage that its normal modes could be classified according to their symmetry (Table S1, Supporting Information), either A (i.e. oriented along the C2 axis) or B (i.e. oriented perpendicular to the C2 axis, Figure 2). Very interestingly, we found that all calculated normal modes with A symmetry and negative rotational strengths corresponded to negative experimental VCD bands, meaning that these normal modes were correctly reproduced by the calculations. On the contrary, all normal modes with B symmetry and positive rotational strengths did not have an experimental correspondence in the sense that the corresponding experimental VCD bands were either negative or negligible (see vertical arrows in Figure 1). In other words, the influence of the paramagnetic Co(II) core seems to selectively affect normal modes with B symmetry. The phenomenon is illustrated by a series of bands calculated between 1425 and 1475 cm–1 (scaled frequencies, Figure 2). This region is dominated by four normal modes (134, 135, 140, and 141), which can be described as aromatic C-H in-plane bending vibrations. Modes 135 and 141 have B symmetry and strong positive rotational strengths; modes 134 and 140 have A symmetry and moderate-to-strong negative rotational strengths. In the experimental spectrum, two weak-to-moderate negative VCD signals are found for (R)-1 in this region (Figure 2). There is no direct counterpart for these negative bands in the calculated spectrum, which can be explained by cancellation or reversal of bands from B-symmetric normal modes. Due to the limitations of VCD calculations, it is not possible to assess the extent of enhancement for each populated conformer. Still, the analysis based on the C2-symmetrical lowest-energy structure clearly demonstrates that the enhancement is symmetry-dependent, which explains the sign bias.


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The currently accepted interpretation of the VCD enhancement is based on the vibronic coupling between ground-state vibrations and low-lying excited states (LLES), which contribute effectively to the magnetic dipole term for the vibrational transitions. It was therefore worth examining the low-lying electronic transitions of Co(II) compounds 1-4 and other pertinent molecular models. According to time-dependent DFT (TDDFT) calculations run at the B3LYP/def2-TZVP level, all the first three electronic transitions of the pseudo-tetrahedral complex 1 (calculated for the C2-symmetric structure in its most stable quartet state) are magnetic-dipole allowed transitions with B symmetry (rightmost column in Figure 3). In fact, they have negligible electric transition dipole moments (oscillator strengths f < 0.001) and sizable magnetic transition moments (> 1.3 au for the first two transitions),30 oriented perpendicular to the C2 axis. Obviously, VCD enhancement selectively occurs with those vibrations sharing the same B symmetry of the electronic transitions most likely involved in the vibronic coupling, meaning that the symmetry of the Co(II) LLES states is crucial for the symmetry-dependent enhancement.


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Figure 3. Transition energies and symmetries for the first excited states computed for models 5-7 and compound 1 at the B3LYP/def2-SVP level arranged in a Walsh-type diagram. In parentheses are reported the molecular symmetry and the value (90°) imposed to the dihedral angle defined by the two N–Co–O planes in 6 and 7a; in 7b and 1, the value is 76°. For details on how the models were constructed see the Supporting Information. To further explore the dependence of LLES on the nature and geometry of ligands around a Co2+ core, we used fragment models 5-7 (Figure 3): Co(NH3)42+ with Td (5a) and C2v (5b) symmetry; Co(H2O)2(NH3)22+ (6) with C2v symmetry; Co(N⁀O)2 where N⁀O is a minimal fragment representing the first-sphere coordination of the salicylaldiminato ligand of 1-4, both in the same geometry as in 1 (7b) and with 90° dihedral between the two N–Co–O moieties (7a). The geometries of the various models were optimized at the B3LYP/def2-SVP level imposing the desired symmetry whenever necessary, then TDDFT calculations were run on each structure in its quartet state at the same level of theory. The results are summarized in Figure 3. The


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calculations lack necessary elements to make them quantitatively accurate (e.g. spin-orbit coupling, doubly-excited states), however, the qualitative picture is already very informative. The Walsh-type diagram allows one to follow the evolution of the first two triply degenerate excited states expected for a tetrahedral d7 Co(II) system, namely the lowest-lying 4A24T2 transition and the higher-lying 4A24T1(F) transition,31-33 upon progressive symmetry lowering and ligand replacement.34 It appears that the sequence of excited states, including their symmetry, depends crucially on both the nature of first-sphere coordinating groups and on the coordination geometry. In particular, the favorable situation encountered for compounds 1-4, namely the fact that the first three excited states all have B symmetry, is both related to the type of ligand and to the pseudo-tetrahedral geometry. This finding may explain why the symmetrydependent VCD enhancement observed for 1-4 is unprecedented in the literature and suggests that a tailored design of Co(II) complexes would make it possible to reproduce single-sign VCD for other Co(II) compounds. In conclusion, the occurrence of VCD enhancement in Co(II) complexes – and conceivably in other metal complexes – is due to the coupling between vibrational and electronic states which must fulfil two requirements: (1) a favorable resonance energy term, hence requiring the presence of LLES close to fundamental or overtone vibrations; (2) a symmetry match. The latter can be appreciated only for compounds endowed with some symmetry elements, which is probably the reason why it has so far been overlooked in the literature. For quasi-tetrahedral Co(II) complexes with C2 symmetry, our analysis demonstrates that the VCD enhancement involves both vibrational and electronic states of B symmetry. Although this symmetry requirement is embedded in the framework of current theories of VCD enhancement,5, 22 it has never been openly stated within the LLES context. On the other hand, the interference


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mechanism for VCD enhancement explicitly contains the combination of vibrational electric dipoles with electronic magnetic dipoles.12, 35 The first-order mixing between two states with respectively non-zero electric and magnetic transition dipoles is the source of rotational strengths in the so-called dynamic or μ·m coupling mechanism for optical activity.32, 36 This mixing is naturally subjected to symmetry rules for a constructive interaction between transition densities,32 37 the same rules which seem to hold for symmetry-dependent VCD enhancement. To the best of our knowledge, the correlation of symmetry, sign bias, and enhancement depicted in this study is the first demonstration of symmetry dependence of VCD enhancement. We think that our finding strongly supports the validity of the LLES theory and calls for extra efforts from the theoretical chemistry community for its implementation in VCD calculations.


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge: computational section, additional experimental data and additional computational data (PDF).

AUTHOR INFORMATION The authors declare no competing financial interests.

ACKNOWLEDGMENT Mohammed Enamullah and Christoph Janiak are gratefully acknowledged for their collaboration in this project. G.P. acknowledges the CINECA award under the ISCRA initiative for the availability of high performance computing resources and support.


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REFERENCES (1) Nafie, L. A. Vibrational Optical Activity. Principles and Applications. John Wiley & Sons: Chichester, U.K.; 2011. (2) Polavarapu, P. L. Chiroptical Spectroscopy: Fundamentals and Applications. CRC Press: Boca Raton, FL; 2016. (3) Ma, S.; Cao, X.; Mak, M.; Sadik, A.; Walkner, C.; Freedman, T. B.; Lednev, I. K.; Dukor, R. K.; Nafie, L. A. Vibrational Circular Dichroism Shows Unusual Sensitivity to Protein Fibril Formation and Development in Solution. J. Am. Chem. Soc. 2007, 129, 12364-12365. (4) Kurouski, D.; Lombardi, R. A.; Dukor, R. K.; Lednev, I. K.; Nafie, L. A. Direct Observation and Ph Control of Reversed Supramolecular Chirality in Insulin Fibrils by Vibrational Circular Dichroism. Chem. Commun. 2010, 46, 7154-7156. (5) Domingos, S. R.; Hartl, F.; Buma, W. J.; Woutersen, S. Elucidating the Structure of Chiral Molecules by Using Amplified Vibrational Circular Dichroism: From Theory to Experimental Realization. ChemPhysChem 2015, 16, 3363-3373. (6) Domingos, S. R.; Huerta-Viga, A.; Baij, L.; Amirjalayer, S.; Dunnebier, D. A. E.; Walters, A. J. C.; Finger, M.; Nafie, L. A.; de Bruin, B.; Buma, W. J.; Woutersen, S. Amplified Vibrational Circular Dichroism as a Probe of Local Biomolecular Structure. J. Am. Chem. Soc. 2014, 136, 3530-3535. (7) Domingos, S. R.; Sanders, H. J.; Hartl, F.; Buma, W. J.; Woutersen, S. Switchable Amplification of Vibrational Circular Dichroism as a Probe of Local Chiral Structure. Angew. Chem. Int. Ed. 2014, 53, 14042-14045. (8) Barnett, C. J.; Drake, A. F.; Kuroda, R.; Mason, S. F.; Savage, S. Vibrational Electronic Interaction in the Infrared Circular-Dichroism Spectra of Transition-Metal Complexes. Chem. Phys. Lett. 1980, 70, 8-10. (9) He, Y. N.; Cao, X. L.; Nafie, L. A.; Freedman, T. B. Ab Initio VCD Calculation of a Transition-Metal Containing Molecule and a New Intensity Enhancement Mechanism for VCD. J. Am. Chem. Soc. 2001, 123, 11320-11321. (10) Berardozzi, R.; Badetti, E.; Carmo dos Santos, N. A.; Wurst, K.; Licini, G.; Pescitelli, G.; Zonta, C.; Di Bari, L. Co(II)-Induced Giant Vibrational CD Provides a New Design of Methods for Rapid and Sensitive Chirality Recognition. Chem. Commun. 2016, 52, 8428-8431. (11) Arrico, L.; Angelici, G.; Di Bari, L. Taking Advantage of Co(II) Induced Enhanced VCD for the Fast and Sensitive Determination of Enantiomeric Excess. Org. Biomol. Chem. 2017, 15, 9800-9803. (12) Drake, A. F.; Hirst, S. J.; Kuroda, R.; Mason, S. F. Optical Activity of Tetrahedral Dihalo[(-)--Isosparteine] Cobalt(II) Complexes. Inorg. Chem. 1982, 21, 533-538.


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(13) Yung, D. A.; Lipp, E. D.; Nafie, L. A. Vibrational Circular Dichroism in Bis(Acetylacetonato)(L-Alaninato)Cobalt(III). Isolated Occurrences of the Coupled Oscillator and Ring Current Intensity Mechanisms. J. Am. Chem. Soc. 1985, 107, 6205-6213. (14) Young, D. A.; Freedman, T. B.; Lipp, E. D.; Nafie, L. A. Vibrational Circular-Dichroism in Transition-Metal Complexes. 2. Ion Association, Ring Conformation, and Ring Currents of Ethylenediamine Ligands. J. Am. Chem. Soc. 1986, 108, 7255-7263. (15) Freedman, T. B.; Young, D. A.; Oboodi, M. R.; Nafie, L. A. Vibrational CircularDichroism in Transition-Metal Complexes .3. Ring Currents and Ring Conformations of AminoAcid Ligands. J. Am. Chem. Soc. 1987, 109, 1551-1559. (16) Johannessen, C.; Thulstrup, P. W. Vibrational Circular Dichroism Spectroscopy of a Spin-Triplet Bis-(Biuretato) Cobaltate(III) Coordination Compound with Low-Lying Electronic Transitions. Dalton Trans. 2007, 1028-1033. (17) Merten, C.; Hiller, K.; Xu, Y. J. Effects of Electron Configuration and Coordination Number on the Vibrational Circular Dichroism Spectra of Metal Complexes of Trans-1,2Diaminocyclohexane. Phys. Chem. Chem. Phys. 2012, 14, 12884-12891. (18) Sato, H.; Mori, Y.; Fukuda, Y.; Yamagishi, A. Syntheses and Vibrational Circular Dichroism Spectra of the Complete Series of [Ru((-)- or (+)-tfac)n(acac)3-n] (n=0-3, tfac=3Trifluoroacetylcamphorato and acac = Acetylacetonato). Inorg. Chem. 2009, 48, 4354-4361. (19) Sato, H.; Taniguchi, T.; Nakahashi, A.; Monde, K.; Yamagishi, A. Effects of Central Metal Ions on Vibrational Circular Dichroism Spectra of Tris-(-Diketonato)Metal(III) Complexes. Inorg. Chem. 2007, 46, 6755-6766. (20) Lo Piano, S.; Di Pietro, S.; Di Bari, L. Shape-Conserving Enhancement of Vibrational Circular Dichroism in Lanthanide Complexes. Chem. Commun. 2012, 48, 11996-11998. (21) Górecki, M.; Carpita, L.; Arrico, L.; Zinna, F.; Di Bari, L. Chiroptical Methods in a Wide Wavelength Range for Obtaining Ln3+ Complexes with Circularly Polarized Luminescence of Practical Interest. Dalton Trans. 2018, 47, 7166-7177. (22) Nafie, L. A. Theory of Vibrational Circular Dichroism and Infrared Absorption: Extension to Molecules with Low-Lying Excited Electronic States. J. Phys. Chem. A 2004, 108, 7222-7231. (23) Nafie, L. A.; Freedman, T. B. Ring Current Mechanism of Vibrational CircularDichroism. J. Phys. Chem. 1986, 90, 763-767. (24) Hansen, A. E. Interference Effects in Natural Circular Dichroism Spectra. Chem. Phys. Lett. 1978, 57, 588-591. (25) Pescitelli, G.; Lüdeke, S.; Chamayou, A.-C.; Marolt, M.; Justus, V.; Górecki, M.; Arrico, L.; Di Bari, L.; Islam, M. A.; Gruber, I.; Enamullah, M.; Janiak, C. Broad-Range Spectral


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Analysis for Chiral Metal Coordination Compounds: (Chiro)Optical Superspectrum of Cobalt(II) Complexes. Inorg. Chem. 2018, 57, 13397-13408. (26) Chamayou, A. C.; Lüdeke, S.; Brecht, V.; Freedman, T. B.; Nafie, L. A.; Janiak, C. Chirality and Diastereoselection of /-Configured Tetrahedral Zinc Complexes through Enantiopure Schiff Base Complexes: Combined Vibrational Circular Dichroism, Density Functional Theory, 1H-NMR, and X-Ray Structural Studies. Inorg. Chem. 2011, 50, 1136311374. (27) Chamayou, A. C.; Makhloufi, G.; Nafie, L. A.; Janiak, C.; Lüdeke, S. Solvation-Induced Helicity Inversion of Pseudotetrahedral Chiral Copper(II) Complexes. Inorg. Chem. 2015, 54, 2193-2203. (28) This theory apply to species containing a cyclic moiety formed by metal ligation or intramolecular hydrogen bonds. Hence, a ring current is generated upon vibrational excitation, whose magnetic moment overrides the effects from the contributions of the vibrational magnetic dipole moments to the sign of a VCD band. (29) Stephens, P. J. Theory of Vibrational Circular Dichroism. J. Phys. Chem. 1985, 89, 748752. (30) For comparison, the value calculated at same level of theory for the magnetic-dipole allowed n–π* transition of acetone or formaldehyde is 1.2 au. (31) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry. 6th ed.; John Wiley & Sons: New York; 1999. (32) Mason, S. F. Molecular Optical Activity and the Chiral Discriminations. Cambridge University Press: Cambridge; 1982. (33) Mason, S. F.; Peacock, R. D. Complexes of Some First-Row Transition Elements with (– )-Spartein. J. Chem. Soc. Dalton Trans. 1973, 226-228. (34) Thulstrup, P. W.; Broge, L.; Larsen, E.; Springborg, J. On the Electronic Structure and Spectroscopic Properties of a Pseudo-Tetrahedral Cationic Cobalt(II) Tetraamine Complex ([35]Adamanzane)Cobalt(II). Dalton Trans. 2003, 3199-3204. (35) Drake, A. F.; Kuroda, R.; Mason, S. F.; Peacock, R. D.; Stewart, B. The d-Electron Optical Activity of Tetrahedral Dichloro[(–)-Spartein]-Cobalt(II). J. Chem. Soc. Dalton Trans. 1981, 976-980. (36) Höhn, E. G.; Jr., O. E. W. Electron Correlation Models for Optical Activity. J. Chem. Phys. 1968, 48, 1127-1137. (37) Schellman, J. A. Symmetry Rules for Optical Rotation. Acc. Chem. Res. 1968, 1, 144151.


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Figure 1. Experimental and calculated IR and VCD spectra for (R)-1. Spectra calculated at the B3LYP/def2TZVP level in vacuo as the Boltzmann-weighted average over 2 conformers, with relative internal energies estimated at the D3BJ-B3LYP/def2-TZVP/PCM level. Plotting parameters: Lorentzian bandwidth 4 cm–1; frequency scaling 0.975. The framed region is expanded in Figure 2; vertical arrows indicate the most relevant calculated positive VCD bands with no experimental correspondence. 86x84mm (300 x 300 DPI)



Figure 2. (a) Calculated VCD spectrum in the 1350-1650 cm–1 range for the C2-symmetrized structure of (R)-1 with band assignment, corresponding to the framed region in Figure 1, compared with the experimental spectrum; plotting parameters same as in Figure 1. (b) Atom displacements (blue arrows) and the overall transition dipole (dark yellow arrow) for normal modes no. 134 (left) and 135 (right), with their respective symmetries. 170x50mm (300 x 300 DPI)


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Figure 3. Transition energies and symmetries for the first excited states computed for models 5-7 and compound 1 at the B3LYP/def2-SVP level arranged in a Walsh-type diagram. In parentheses are reported the molecular symmetry and the value (90°) imposed to the dihedral angle defined by the two N–Co–O planes in 6 and 7a; in 7b and 1, the value is 76°. For details on how the models were constructed see the Supporting Information. 85x89mm (300 x 300 DPI)



TOC graphics 49x49mm (300 x 300 DPI)


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Symmetry-Dependent Vibrational Circular Dichroism Enhancement in

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