Insight into the Electronic Structure of Formal Lanthanide(II

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Insight into the Electronic Structure of Formal Lanthanide(II) Complexes using Magnetic Circular Dichroism Spectroscopy Valerie E. Fleischauer,†,∥ Gaurab Ganguly,‡,∥ David H. Woen,§ Nikki J. Wolford,† William J. Evans,*,§ Jochen Autschbach,*,‡ and Michael L. Neidig*,† †

Department of Chemistry, University of Rochester, Rochester, New York 14627, United States Department of Chemistry, University at Buffalo, State University of New York, Buffalo, New York 14206-3000, United States § Department of Chemistry, University of California, Irvine, California 92697-2025, United States Downloaded via NOTTINGHAM TRENT UNIV on August 16, 2019 at 18:29:47 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Magnetic circular dichroism (MCD) spectroscopy has been utilized to evaluate the electronic structure of the tris(cyclopentadienyl) rare-earth complexes [K(2.2.2cryptand)][LnCp′3] (Ln = Y, La, Pr, Eu, Gd; Cp′ = C5H4SiMe3), which contain ions in the formal +2 oxidation state. These complexes were chosen to evaluate the 4fn5d1 electron configuration assignments of the recently discovered La(II), Pr(II), and Gd(II) ions versus the traditional 4fn+1 configuration of the long-known Eu(II) ion. The 4d1 Y(II) complex provided another benchmark in the MCD study. Transitions with f-orbital character were observed in the NIR MCD spectra of the 4f25d1 complex [PrCp′3]−. This study provides the first direct observation of f−f transitions in such Ln(II) species. The broadening of these transition for Pr(II) provides further confirmation of the 4fn5d1 versus 4fn+1 electronic configurations previously proposed and supported by restricted active-space (RAS) calculations. For further insight into the electronic structure of these [LnCp′3]− complexes, experimental UV−vis MCD spectroscopy was coupled with spectral calculations, which allowed for the assignment of transitions. The sensitivity of UV−vis MCD to spin−orbit coupling (SOC) and the increased spectral resolution in comparison to electronic absorption spectroscopy enabled identification of low-energy nd to (n + 1)p transitions in this class of complexes. Combined, these studies provide further insight into the electronic transitions and overall electronic structure of low-valent lanthanide(II) organometallic complexes.

1. INTRODUCTION While there have been significant advancements in the synthesis of organometallic complexes featuring f-block elements, analysis of their electronic structure and bonding remains underdeveloped in comparison to their d-block analogues. One recent development in synthetic lanthanide chemistry that presents challenges in spectroscopic and computational analysis involves the discovery that the formal +2 oxidation state is available not only for the traditional ions Eu(II), Yb(II), Sm(II), Tm(II), Dy(II), and Nd(II) but also for all of the lanthanides except radioactive Pm.1−8 It was found that tris(silylcyclopentadienyl) complexes such as [K(2.2.2-cryptand)][LnCp″3]1,8 and [K(2.2.2-cryptand)][LnCp′3]2−5 (Cp′′ = C5H3(SiMe3)2, Cp′ = C5H4SiMe3) were accessible by potassium reduction of 4fn LnCp″3 and LnCp′3 precursors. This was surprising as some of the calculated Ln(III)/Ln(II) redox potentials were more negative than the −2.9 V vs SHE reduction potential of potassium.6,7,9 However, the calculated potentials were for 4fn → 4fn+1 reductions, whereas structural, spectroscopic, and magnetic data along with density functional theory (DFT) analysis suggested the © XXXX American Chemical Society

reduction involves addition of an electron to a 5d orbital, i.e. 4fn → 4fn5d1.2−8,10 The unusual 4fn5d1 electronic configurations were probed by X-ray absorption near-edge spectroscopy (XANES) of the L2/ L3 edge of [LnCp′3]− for Ln = Pr, Nd, Sm, Gd, Tb, Dy, Y, Ho, Er, Lu.11 Overall, this study found that the shift in the L2/L3 peak position for compounds predicted to be of 4fn+1 configurations (Sm, Tm, Yb) was large (>6.6 eV), and the remaining compounds predicted to have 4fn5d1 configurations had much smaller shifts in the L2/L3 edge (≤1.9 eV). This subtle energy change was said to be consistent with changes observed for transition-metal L-edge experiments where dorbital occupancies have a smaller effect on the edge energy. Furthermore, DFT calculations focusing on the neardegenerate 4f and 5d orbitals of holmium and samarium agreed with the XANES experimental data. While techniques such as absorption spectroscopy and XAS can provide insights into electronic structure and oxidation Received: May 13, 2019

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combined C-term MCD experimental and computational methodology to systems of lower symmetry.

state, MCD provides improved spectral resolution, especially in cases where there are overlapping absorption bands. A- and Bterm transitions in MCD are temperature independent and arise from Zeeman splitting of the excited state and fieldinduced mixing of zero-field states, respectively. In the case of C-term MCD, transitions are temperature dependent and arise from the removal of a ground-state degeneracy in the presence of a magnetic field and are particularly sensitive to changes in electronic structure of paramagnetic centers with a large spin− orbit coupling constant, such as the lanthanides. Specifically, the observation of f−f transitions in MCD allows for more detailed insight into orbital information. Finally, MCD is extremely sensitive to the orbital location of unpaired electrons, showing drastic differences in spectra when spinstate changes occur.12 While initial spectroscopic studies have provided critical insight into the ground-state electronic structures of this lanthanide series, the unique ground-state characteristics motivated us to further probe the electronic structure of these complexes. In particular, our ability to probe the nearinfrared (NIR) region (where f−f transitions would be expected) using magnetic circular dichroism (MCD) spectroscopy as well as more highly resolved studies of higher energy electronic transitions allows for the development of our understanding of excited states to complement the existing ground-state insights. While MCD has been utilized to study both A- and B-term spectra of LnCp3 type compounds at room temperature13−17 as well as a series of LnX3·3H2O compounds (X = Cl, trifluoroacetylacetonate),18 the use of C-term MCD spectroscopy to study lanthanide compounds is limited.19 Early C-term MCD studies of f-elements by Denning and co-workers provided detailed insight into f-orbital energies, spin−orbit interactions, excited states, and ligand field energies in neptunium complexes.20,21 More recently, we have used MCD spectroscopy combined with DFT calculations of Cterm MCD spectra to predict the observed C-term transitions in the octahedral [UCl6]− ion.12 Herein, we exploit the high resolution of MCD spectroscopy for the study of the electronic structure in selected Ln(II) complexes of formula [K(2.2.2-cryptand)][LnCp′3] (Figure 1). The sensitivity of MCD to changes in electronic structure

2. EXPERIMENTAL METHODS 2.1. General Synthetic and Spectroscopic Procedures. All lanthanide complexes were synthesized according to previously reported literature procedures, where full characterization of all compounds including elemental analysis, X-ray crystal structures, and UV−vis spectra are available.2−5 All samples for MCD were prepared in an inert-atmosphere glovebox equipped with a liquid nitrogen fill port to allow for sample freezing at 77 K in the glovebox. Lanthanide complexes were stored at −30 °C in the glovebox and quickly weighed with a cold spatula before they were dissolved in 2-methyltetrahydrofuran at −30 °C over several minutes. The cold solutions were then loaded into precooled copper MCD cells containing high-purity quartz disks and a 2 mm thick Teflon gasket using a precooled syringe. Sample cells were directly frozen and stored under liquid nitrogen. Sample cells were loaded into the MCD sample compartment under liquid nitrogen. A modified sample compartment incorporating focusing optics and an Oxford Instruments SM4000-7T superconducting magnet/cryostat was used which permitted measurements from 1.6 to 290 K with magnetic fields up to 7 T. UV−vis MCD spectra were collected using a Jasco J-715 spectropolarimeter and a shielded S-20 photomultiplier tube. NIR spectra were collected with a Jasco J-730 spectropolarimeter and a liquid nitrogen cooled InSb detector. All MCD spectra were baseline-corrected against zero-field scans. 2.2. Computational Details. The molecular structures of the [LnCp′3]− (Ln = Y, Gd, Pr) complexes were optimized with Kohn− Sham density functional theory (DFT) using the hybrid meta-GGA TPSSh22 functional. The Def2-SVP basis set was used for all nonmetal atoms. A small-core pseudopotential23 and the corresponding quasirelativistic basis set were used for yttrium, while f-in-core pseudopotentials24 and the corresponding quasi-relativistic basis sets were used for praseodymium and gadolinium. The f-in-core pseudopotentials are convenient for treating partially filled f shell occupancies of lanthanides within a single reference framework such as DFT, although any details that are related to the open f shells are lost. Electronic absorption spectra for [YCp′3]−, [PrCp′3]−, and [GdCp′3]− were first computed with time-dependent Kohn−Sham theory (TD-DFT) linear response theory (Figures S5−S7). Solvent effects on the spectra were probed with the help of the conductor-like polarizable continuum solvent model (C-PCM),25,26 using the dielectric constant of THF. The optimizations and TD-DFT calculations were conducted using the Gaussian09 program package.27 Multireference self consistent field (SCF) and configuration interaction (CI) wave function calculations at the restricted activespace (RAS) level were used to calculate the electronic states and absorption spectra. The RAS calculations were performed with the second-order Douglas−Kroll−Hess (DKH2) all-electron relativistic Hamiltonian28,29 and all-electron ANO-RCC-VTZP basis sets,30,31 using a current development version of Molcas/OpenMolcas.32 Since the RAS calculations are computationally demanding, the SiMe3 substituents on the Cp′ rings were replaced with hydrogen (Cp). After truncation, the complexes afforded a plane of symmetry and the Cs point group was used for the calculations. A comparison of absorption spectra for [YCp′3]− vs [YCp3]− and for [GdCp′3]− vs [GdCp3]− at the TD-DFT level revealed that, while the substituents affect the intensities for the higher bands considerably due to the diffuse nature of the excitations (vide infra), the effect on the absorption onset is relatively minor (see the Supporting Information). The model systems were therefore used for subsequent wave function calculations and focused on the low-energy bands. Additional computational details can be found in the Supporting Information. The calculated spectral data reported herein were obtained with the inclusion of spin−orbit coupling (SOC) via state interaction of the RAS spin-states (RASSI). Moreover, corrections to the state energies from the dynamic electron correlation turned out to be important and

Figure 1. Representative structure of [Ln(C5H4SiMe3)3]− complexes (left) and X-ray crystal structure of [Pr(C5H4SiMe3)3]− (right) with thermal ellipsoids drawn at the 30% probability level and hydrogen atoms omitted for clarity. Reproduced from ref 4.

and orbital population makes this a valuable technique for the study of the formally Ln(II) molecules due to the presence of 4fn5d1 or 4fn+1 electronic ground states. The NIR spectral region is particularly exciting, as it can identify f−f transitions for this class of molecules for the first time. Detailed ab initio calculations complement this experimental approach and help to explain the nature of the observed electronic transitions in the UV−vis region. This work extends the utility of our B

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Organometallics were obtained from post SCF multiconfigurational pair-density functional theory (MC-pDFT) (denoted as MC-pDFT//CI) with the on-top tLSDA density functional recently developed by Gagliardi et al.33 In the discussion below, “RAS level” refers to the calculations corrected by MC-pDFT and includes SOC unless a specific spin-state is discussed. The absorption spectra over the experimentally observed range were generated by broadening individual vertical transition peaks with normalized Gaussian function. The sigma parameter used to broaden the Gaussian functions was 2800 cm−1 for each absorption spectrum provided in this paper unless otherwise noted. The band peak energies and calculated state energies relative to the ground state are given in the figures as photon wavenumbers in units of 103 cm−1. Magnetic circular dichroism (MCD) spectra were calculated according to the following equation for each electronic transition: ÄÅ ÉÑ ÅÅ i ∂f (E) y i ÑÑ Δε C zyz jj ÅÅ jj Ñ z z zf (E)ÑÑÑ ∝ ÅÅAjj− + jjB + z z z z ÅÅ k ∂E { j ÑÑ E k T B { (1) ÅÇ ÑÖ k

Table 1. [K(2.2.2-cryptand)][Ln(C5H4SiMe3)3] Compounds and Their Previously Predicted Electronic Configurations compound −

predicted electron configuration 1

[YCp′3] [LaCp′3]− [PrCp′3]−

4d 5d1 4f2 5d1

[EuCp′3]− [GdCp′3]−

4f7 4f7 5d1

NIR MCD transitions (cm−1) none none 7103, 8600, 10950, 12640, 13600 none none

the f−f transitions, no d−d transitions are observed in the NIR region of either of these nd1 complexes. In contrast to these nd1 ground state complexes, weak NIR transitions were observed in [PrCp′3]− (Figure S1) and can be fit to at least five transitions (Figure 2). This is consistent with the presence of low-energy, Laporte-forbidden f−f transitions. The appearance of these f−f transitions in [PrCp′3]− further supports the ground-state electronic configuration, which was previously assigned as 4f 2 5d 1 . Neither [GdCp′3 ] − nor [EuCp′3 ] − exhibited any NIR f−f transitions, consistent with their previously assigned 4f75d1 and 4f7 electronic configurations, respectively, for which all f−f transitions are spin-forbidden. The absence of any electronic transitions in the NIR region for the gadolinium and europium complexes also indicates the absence of low-energy 4f to 5d electronic transitions in the NIR region. While these NIR MCD studies provide the first direct observation of weak NIR transitions in one of these complexes, further corroborating their previously assigned ground-state electronic configurations, it is noteworthy that the assigned f−f transitions in [PrCp′3]− (4f25d1) are significantly broader than would be expected. Typically, lanthanide f−f transitions in the NIR region are very narrow (fwhm ∼50 cm−1),38 but the observed transitions in [PrCp′3]− exhibit broader line widths (fwhm ∼600 cm−1), intermediate between those typically observed for f−f transitions and d−d transitions in transition metals (fwhm ∼1000 cm−1).39−42 This broadening suggests that the ground-state molecular orbitals containing the “felectrons” are unlikely to be purely f-orbital in nature but, instead, contain additional d-orbital character. This 4f/5d mixing has previously been proposed for similar systems with the 4fn5d1 configuration and is believed to result from the reduced symmetry of the complexes in the tris-Cp′ ligand environment combined with the large number of neardegenerate metal-centered orbitals.5 Indeed, a model RAS calculation with three electrons in seven f- and five d-orbitals for [PrCp3]− revealed a range of electronic states below 10000 cm−1 that are strongly multiconfigurational, mixing f−f with d−f and d−d excited configurations. Currently, simulated Cterm MCD spectra of Laporte-forbidden f−f transitions observed in the NIR region are not available with the theoretical methods used for this study. Computations of Laporte-allowed C-term spectra in the UV−vis region have been successfully applied, as shown later in this report (vide infra) and previously with the [UCl6]− ion.12 3.2. UV−Vis MCD of [LnCp′3][K(2.2.2-cryptand)] Complexes: Spectra and Computations. While NIR MCD provided direct insight into low-energy f-centered electronic transitions in [K(2.2.2-cryptand)][LnCp′3] complexes, MCD in the UV−vis region yields high-resolution information on higher energy electronic transitions. In the

The MCD intensity depends on three Faraday terms denoted as A, B, and C, and the expressions for the isotropic MCD have been previously reported.34 The A-term arises from an excitation to an orbitally degenerate state. The B-term arises from the magnetic field induced mixing of the zero-field states. For an open-shell ground state electronic configuration (i.e., for paramagnetic molecules), there is an additional temperature-dependent C-term that usually dominates the MCD at low temperatures due to a 1/(kBT) prefactor. MCD spectral envelopes were generated from the calculated state energies, and the values for A, B, and C for each transition were based on eq 1. An MCD code developed in house12 was used for these calculations. Band C-term bands were simulated by Gaussian functions f(E) while the A-term bands have a Gaussian derivative shape. To calculate the B-terms we have used a sum over states (SOS) approach.35 MCD spectra were calculated for 5 K, the temperature at which the experimental measurements were taken. While a more elaborate treatment of the C-term calculation has been previously demonstrated,36,37 this method is beyond the scope of this study.

3. RESULTS AND ANALYSIS 3.1. NIR MCD Spectra of [LnCp′3][K(2.2.2-cryptand)] Complexes. The NIR energy region of lanthanide complexes provides valuable insight into f−f transitions and the underlying f-electron configurations of the corresponding ground state. Much like d−d transitions, f−f transitions are Laporte-forbidden, resulting in weak signal intensity, which can be challenging to observe by electronic absorption spectroscopy. Due to the SOC contributions, as well as the large orbital angular momentum associated with both d- and f-orbitals, Cterm MCD spectroscopy has the advantage of facilitating the measurement of Laporte-forbidden transitions. Traditionally, this technique has been used to observe low-energy d−d transitions in transition-metal complexes and metalloproteins, but it is equally useful for measuring f−f transitions. Here, Cterm MCD was employed to evaluate NIR transitions in Ln(II) complexes of this structural class that covered the range of electronic structures previously assigned: La (5d1), Pr (4f25d1), Gd (4f75d1), and Eu (4f7), as well as Y (4d1) as a pure d-electron analogue. Initial NIR MCD measurements focused on [YCp′3]−, which has a 4d1 ground-state configuration. The 5 K, 7 T MCD spectrum indicated that no electronic transitions were present in the NIR region, consistent with the lack of f-orbital occupation (and f−f transitions) expected for the 4d1 groundstate configuration. [LaCp′3 ]− also exhibited no NIR transitions by MCD, consistent with its previously assigned 5d1 ground-state electronic configuration (Table 1). Much like C

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Figure 3. MCD spectra at 5 K and 7 T (blue) and individual transition component fits (red) of [K(2.2.2-cryptand)][Ln(C5H4SiMe3)3] complexes in the UV−visible region.

further evaluate their electronic structures and establish the broader utility of calculated C-term MCD spectra in lower symmetry complexes. Due to the significant computational cost of these calculations, three complexes were selected for analysis. [YCp′3]− was chosen as a transition-metal comparison with no f-orbitals present to be involved in bonding. [PrCp′3]− and [GdCp′3]− were chosen due to their unusual ground-state electronic configurations as well as the variation in partially filled vs half-filled f-orbital manifold configurations (4f25d1 vs 4f75d1) on the resulting excited-state spectra. As mentioned in section 2.2, RAS calculations were performed on [LnCp3]− systems where the SiMe3 substituents on the Cp′ rings were replaced with hydrogen (Cp). The RASSCF calculated ground-state spin multiplicities of [YCp3]−, [PrCp3]−, and [GdCp3]− are doublet, quartet, and nonet, respectively, due to one (4d1), three (4f25d1), and eight (4f75d1) unpaired electrons. For the lanthanide complexes, the spin states where the unpaired electrons in 4f and 5d are antiferromagnetically coupled are close (0.25 eV (RASSCF) and 0.09 eV (MC-pDFT//CI) higher) in energy. In agreement with previously reported DFT calculations,4 the excess electron relative to a +3 oxidation state of the metal occupies a nd orbital (n = 4, 5), instead of an (n − 1)f-orbital in the case of the lanthanides. The three Cp rings create a ligand field with an approximate 3-fold rotational axis of symmetry around the metal, which lifts the degeneracy of the nd shell. If the metal and the Cp centroids are in the x,y plane, and z coincides with the 3-fold axis, the calculated splitting of valence nd and diffuse (n + 1)d-orbitals turned out to be in order of energy: dz2 < (dxz, dyz) < (dx2−y2, dxy). The diffuse (n + 1)p-orbitals also split accordingly, with energies pz < (px, py). The experimental UV−vis MCD spectrum of [YCp′3]− consists of two negative and three positive features which

Figure 2. MCD spectra at 5 K and 7 T (blue) and individual transition component fits (red) of [K(2.2.2-cryptand)][Ln(C5H4SiMe3)3] complexes in the NIR region.

UV−vis spectral region, all of the complexes show more intense and unique field-dependent transitions regardless of electronic configuration (Figure 3 and Figure S2). The electronic transitions in this lanthanide series are clearly more complex than those previously observed in electronic absorption, which commonly produce broad and nondistinct features (vide infra), demonstrating the value of the increased spectral resolution using MCD. Due to the complexity of transitions in this energy region, the most significant insight into the nature of these higher energy transitions derives from coupling these experiments to theoretical calculations, which were pursued for a subset of the molecules. This study extends this methodology to the [K(2.2.2-cryptand)][LnCp′3] complexes studied herein to D

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Figure 4. RAS-level calculated (of [LnCp3]−) vs experimental (of [K(2.2.2-cryptand)][LnCp′3]) absorption (top) and MCD (bottom) spectra at 5 K. The calculated MCD spectrum for [YCp3]− has been shifted to higher energy by 2000 cm−1 for better comparison with the experimental spectrum.

can be fit to at least nine transitions between 14500 and 36000 cm−1 (Table S1, Figure 3, and Figure S3). Calculations of this complex determine that the ground state of [YCp′3]− is singleconfigurational. The accepting orbital corresponding to the first transition shows strong d−p mixing due to lack of an inversion center in the system. The experimental and calculated MCD spectra for [YCp′ 3 ] − and [YCp 3 ] − , respectively are compared in Figure 4A. The calculated MCD spectrum qualitatively reproduces the pattern of the lowest-energy MCD bands. Due to necessary compromises in the computational approach, the active space used to generate the MCD spectrum does not include orbitals that would be needed to reproduce the spectrum beyond about 25000 cm−1. The doublet (4d1) ground-state configuration is spatially nondegenerate. In the linear perturbation theory formalism employed here for the magnetic field, the C-terms vanish unless the ground state has some degree of spatial degeneracy: for instance, from an unquenched orbital angular momentum or via SOC. (We also consider the MCD spectra to be far off the saturation limit.) Accordingly, with the inclusion of SOC, the ground spin state acquires some spatial degeneracy through mixing with excited spin states that are spatially degenerate. Therefore, according to the calculations, both B- and C-terms have significant contributions to the MCD spectrum at 5 K. At room temperature, the B-terms would completely dominate the low-energy MCD spectrum according to the analysis per eq 1. Experimentally, [PrCp′3]−, with a 4f25d1 ground state configuration, has a UV−vis MCD spectrum with two negative and two positive features (Figure S3). These can be fit to at least nine transitions in the energy region of 15000−33000 cm−1 (Figure 3). Among the three complexes selected for calculations, [PrCp′3]− can be considered as the most challenging due to the f2 configuration (not empty, filled, or half filled) which gives rise to a multiconfigurational groundstate. We found that the calculated absorption spectrum is the same irrespective of whether SOC is included in the

calculation or not. The calculation reveals a very broad band stretching from approximately 10000 to 25000 cm−1, with large number of contributing states that arise from excitations out of 5dz2 combined with different f2 microstates. As previously mentioned due to computational limitations, we were unable to generate the bands visible in the experimental spectrum at higher energy (Figure 4B). The calculations lead to an assignment of the lowest intense excitations primarily to 5dz2 → diffuse 6p transitions with contributions from 5dz2 and 5dyz. At higher energies, 5dz2 → 4f transitions also contribute to the absorption intensity. The calculated MCD spectrum for [PrCp3]− below 25000 cm−1 reproduces the experiment rather nicely. Unlike [YCp′3]−, the ground state of [PrCp′3]− is orbitally degenerate due to the f2 occupations. Therefore, C-terms dominate the MCD spectrum even in the absence of SOC. In the calculations, at 5 K the Aand B-terms are 1 order of magnitude smaller than the C-terms and can be neglected. Note that a simple MO diagram to describe the different transitions (5d to 4f, or 5d to diffuse 6p/ π*) is not straightforward, as the 4f and 5d orbital energies are dependent on the number of electrons in each shell.43,44 For the studied systems, the 4fn5d1 configuration is below 4fn+15d0 in total energy if n is not 0 or 6 because of the added electron repulsion within the compact 4f shell. This repulsion also causes the 4f orbital energy to be different in the 4fn5d1 and the 4fn+15d0 configurations, respectively. The experimental UV−vis MCD spectrum of [GdCp′3]− consists of three negative and two positive features in the energy region between 20000 and 36000 cm−1. These features can be fit to at least ten transitions (Figure 3). Similar to the case for [YCp′3]−, the ground spin state is spatially nondegenerate but mixes due to SOC with spatially degenerate excited spin states. The calculated MCD spectrum for [GdCp′3]− was found to be completely dominated by Cterms at 5 K. However, at room temperature the spectrum is predicted to be a complicated interplay of A-, B-, and C-terms (Figure S9). In contrast, the calculated absorption spectrum is E

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character of the transitions follows an increase in energy.4 The future application of C-term MCD to other ligand types in Ln(II) chemistry will enable further insight into electronic structure and bonding in low-valent Ln chemistry.

hardly affected by SOC, as observed for the other complexes. The first intense excitations in the calculation (ca. 17000 and 21000 cm−1) involve diffuse pz and px/py acceptor orbitals, respectively (Figure 4C). The MCD intensity for these sets of transitions is opposite, with a negative MCD band around 21000 cm−1. In comparison to the experiment, it appears that in the calculation the first excitation is also overestimated in its dipole strength (absorption) as far as the MCD C-term is concerned. This leads to the appearance of a positive first MCD band in the calculation that is not resolved in the experiment.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00315. Cartesian coordinates for the calculated structures (XYZ) Additional experimental MCD spectra and data regarding the TD-KST absorption spectra (PDF)

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CONCLUSIONS MCD spectroscopy of the [K(2.2.2-cryptand)][LnCp′3] complexes has provided additional insight into the electronic structures of these reduced Ln(II) complexes. NIR MCD enabled the first direct evaluation of low-energy f−f transitions in this class of complexes, and these were found to exist only within [PrCp′3]−, further corroborating the previously assigned 4f25d1 electronic ground-state configuration. In particular, the unusual broadening of the observed f−f transitions for [PrCp′3]− was attributed to some 5d character in the “felectron” orbitals contributing to the f−f transitions. Finally, the lack of NIR transitions in [GdCp′3]− and [EuCp′3]− further supported their previous assignments of half-filled fshell configurations (4f75d1 and 4f7, respectively). Due to the sensitivity of UV−vis MCD to SOC and the increased spectral resolution in comparison to electronic absorption spectroscopy, higher resolution insight into the electronic transitions of these complexes was possible, broadening our understanding of these complexes. Focusing on [YCp′3]−, [PrCp′3]−, and [GdCp′3]−, it is found that the lowest energy transitions in the UV−vis region are electric dipole allowed ndz2 to diffuse (n + 1)pz transitions, followed by dz2 to quasi-degenerate (n + 1)px/py transitions. Due to the noncentrosymmetric structures, the transitions afford some d− d character. The diffuse nature of the low-energy states is likely to render them sensitive to the presence and dynamics of the solvent, which would require efforts going beyond a continuum model to describe reliably in the calculations. However, it is clear that the strongly reducing ligand environment does not easily accept an additional electron and, consequently, ligandcentered excited states are hardly present in our calculations. Previous electronic absorption studies assigned the transitions in the UV−vis region to d−d transitions with significant ligand character. We do find that such transitions exist as well, but at higher energy. Overall, the combination of C-term MCD spectroscopy and theoretical calculations provides a robust method for evaluation of electronic structure in [K(2.2.2-cryptand)][LnCp′3] complexes. These studies provide the first direct observation of f−f transitions in such species, providing further confirmation of the 4fn5d1/4fn+1 electronic configurations previously proposed.3−5,11 The broadening observed in the f−f transitions further supports the mixed 4f/5d ground state predicted for the Pr complex, mixing which was also previously supported by natural population analysis, where the small changes in the 4f and 5d population suggest this same mixed ground-state configuration.5 In addition, studies focused on the higher energy UV−vis region of this series of Ln(II) complexes provided higher resolution spectra of the previously reported d−d and d−π* transitions, where the increase in ligand

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for W.J.E.: [email protected]. *E-mail for J.A.: jochena@buffalo.edu. *E-mail for M.L.N.: [email protected]. ORCID

David H. Woen: 0000-0002-5764-1453 William J. Evans: 0000-0002-0651-418X Jochen Autschbach: 0000-0001-9392-877X Michael L. Neidig: 0000-0002-2300-3867 Author Contributions ∥

V.E.F. and G.G. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.L.N. gratefully acknowledges support for this work from the U.S. Department of Energy, Office of Science, Early Career Research Program, under Award DE-SC0016002. J.A. and G.G. acknowledge support from the U.S. Department of Energy, Office of Basic Energy Sciences, Heavy Element Chemistry program, under grant DE-SC0001136. W.J.E. and D.H.W. thank the U.S. National Science Foundation for support of the synthetic work under CHE-1855328. We thank the Center for Computational Research (CCR) at the University at Buffalo for providing computational resources.

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