All of the compounds crystallize solvent free from hexanes. Full X-ray crystallographic analysis was performed on the previously unreported complex, 3.2-I,
and compared to the solid state structures of 3.2-F (Figure 3.2.1), 3.2-Cl,1,2 and 3.2-Br.2
The structure of 3.2-I is depicted in Figure 3.2.12, and the crystallographically
determined structural parameters for compounds 3.2-F, 3.2-Cl, 3.2-Br, and 3.2-I are
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Figure 3.2.12. 30% probability thermal ellipsoid plot of compound 3.2-I. Hydrogen atoms have been removed for clarity. Selected bond lengths: Ce(1)–N(1): 2.2153(9) Å,
Ce(1)–I(1): 2.9980(2) Å. Reprinted from work submitted to Inorganic Chemistry, 2014.
Table 3.2.3. Comparison of bonding metrics from the crystallographically determined structures of 3.2-F, 3.2-Cl, 3.2-Br, and 3.2-I. (a) data reported in reference 1. (b) N* = N(SiMe3)2. Adapted from work submitted to Inorganic Chemistry, 2014.
Complex M–N* (Å)b M–X (Å) M–N*3 (Å) 3.2-F 2.201(10)– 2.254(9) 2.065(6), 2.065(7) 0.166(5), 0.197(5) 3.2-Cla 2.217(3) 2.597(2) 0.365(3) 3.2-Bra 2.219(7) 2.7662(17) 0.373(9) 3.2-I 2.2153(9) 2.9980(2) 0.3539(10)
Compounds 3.2-Cl, 3.2-Br, and 3.2-I crystallized in the rhombohedral R3c space
group with crystallographically imposed C3 symmetry axes along the Ce–X bond
vectors.1,2 Compound 3.2-F crystallized in the monoclinic P21/c space group with two
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X bond vectors. The Ce–N* (N* = N(SiMe3)2) bond distances present in compounds 3.2-
Cl, 3.2-Br, and 3.2-I were equivalent within experimental error and intermediate to the
range of Ce–N* bond distances present in compound 3.2-F. As expected, the Ce–X bonds
lengthened with coordination of larger heteroatoms (Ce–F in 3.2-F < Ce–Cl in 3.2-Cl <
Ce–Br in 3.2-Br < Ce–I in 3.2-I). As a result of the interactions between the coordinated
halide ligand and the bulky bis(trimethylsilyl)amide ligands, the compounds showed increasing pyramidilization with increasing size of the halide ligand (3.2-F < 3.2-Cl <
3.2-Br) as evidenced by the increased displacement of the cerium ion from the plane
formed by the amide nitrogen atoms. Compound 3.2-I showed an exception to this trend,
and was slightly less pyramidilized than 3.2-Cl and 3.2-Br despite the large size of the
iodide ligand since the long Ce–I bond distance (2.9980(2) Å) reduced steric bulk in the complex. This Ce–I bond distance in 3.2-I was shorter than the Ce–I bond distances in
reported molecular cerium(IV) iodide complexes of 3.1284(6) and 3.1414(11) Å.6,7
3.2.6 Spectroscopic characterization of cerium(IV) halide complexes
NMR analysis of compounds 3.2-F, 3.2-Cl, 3.2-Br, and 3.2-I in benzene-d6
showed sharp spectral features at chemical shifts consistent with the formally diamagnetic nature of the complexes. The 1H NMR shifts ranged from 0.39–0.49 ppm and trended according to the electron-rich nature of the halide, with the more electron donating halides leading to less deshielding of the bis(trimethylsilyl)amide protons (δ
3.2-F < δ 3.2-Cl < δ 3.2-Br < δ 3.2-I). The 13C NMR shifts ranged from 4.75–6.44 ppm
and displayed a similar trend.
In the infrared region of the absorption spectra, compounds 3.2-F, 3.2-Cl, 3.2-Br,
109 vibrations of the silylamide ligand environment. The Ce–X stretch was observed experimentally only in the case of 3.2-F (at 493 cm-1) (Figure 3.2.7). The remaining Ce–
X stretches were beyond the low energy limit of the spectrometer.
The electronic absorption spectra of 3.2-F, 3.2-Cl, 3.2-Br, and 3.2-I collected in
hexanes revealed broad transitions in the UV/Visible region (Figure 3.2.13). These features were assigned as ligand-to-metal charge transfer (LMCT) transitions and are characteristic of cerium(IV) coordination compounds.7,27,28,30,39,43 The related homoleptic
cerium(IV) silylamide complex Ce[N(SiHMe2)2]4 underwent LMCT transitions at 19,084
and 22,727 cm-1.44 Based on this comparison, the LMCT features centered at ~20,000 cm-
1
and ~27,000 cm-1 were assigned from donation of electron density from the silylamide
ligands to the cerium cation. In the spectra of 3.2-F, 3.2-Cl, 3.2-Br, and 3.2-I, the high
energy feature varied by 2200 cm-1 and the low energy feature varied by 2602 cm-1 (Table 3.2.4). Considering that these LMCT transitions described a transfer of electron density from filled orbitals with primarily silylamide ligand character to vacant 4f metal orbitals, the variability of these features indicated that the identity of the halide ligand impacted the donating ability of the silylamide ligands. The coordination of smaller, more electron- rich halides to the cerium center stabilized the ground electronic state of the complex, resulting in higher energy LMCT transitions as a result of an increased energy difference between the ground and excited states.
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Figure 3.2.13. The electronic absorption spectra of 3.2-F, 3.2-Cl, 3.2-Br, and 3.2-I,
collected in hexanes. Adapted from work submitted to Inorganic Chemistry, 2014.
Table 3.2.4. LMCT transition energies and molar absorptivities from the electronic absorption spectra of 3.2-F, 3.2-Cl, 3.2-Br, and 3.2-I. Adapted from work submitted to
Inorganic Chemistry, 2014. Complex E (cm-1) / ε (M-1cm-1) 3.2-F 21100 / 5932 28102 / 2891 3.2-Cl 19880 / 3826 27460 / 2524 3.2-Br 19147 / 438 25499 / 3089 3.2-I 18896 / 4698 26191 / 3291
3.2.7 Electrochemical characterization of cerium(IV) halide complexes
Electrochemical analysis was undertaken in order to describe the effect of the ligand environment on the thermodynamic stability of the cerium(IV) cation. Electrochemical data was collected in THF with 0.1 M [nPr4N][BArF4]. Electrochemical
111 Ce[N(SiMe3)2]3 upon dissolution in THF and was therefore unstable under the
experimental conditions. Comparing the electrochemical behavior of 3.1 (Figure 3.2.5) to
those of the halide functionalized products demonstrated that the electron rich, electronegative halide ligands stabilized the cerium(IV) center, reflected in the shift of the cerium(III/IV) couple to more reducing potentials (Figure 3.2.14 and Table 3.2.5). Compound 3.2-F was stabilized by 0.25 V compared to 3.2-Cl and 3.2-Br, which were
observed at comparable potentials of –0.30 and –0.31 V, respectively. The similarity in the reducing ability of 3.2-Cl and 3.2-Br was consistent with a reported family of
cerium(IV) aryloxide compounds.7
As seen in Figure 3, the more sterically restricted coordination environments in compounds containing the larger halide ligands resulted in more reversible electrochemical features, indicated by the small values of ΔE. The steric demand of the complexes containing large halide ligands (2-Br > 2-Cl > 2-F) reduced the
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Figure 3.2.14. Isolated Ce(III/IV) redox couple in the cyclic voltammograms of 3.2-F,
3.2-Cl, and 3.2-Br collected in THF with 0.1 M [nPr4N][BArF4]; [analyte] = ca. 1 mM; ν
= 0.05 V/sec. Adapted from work submitted to Inorganic Chemistry, 2014.
Table 3.2.5. Electrochemical data for compounds 3.1, 3.2-F, 3.2-Cl, and 3.2-Br collected in THF with 0.1 M [nPr4N][BArF4]; [analyte] = ca. 1 mM; ν = 0.05 V/sec. (a) data
reported in reference 27. Adapted with permission from work submitted to Inorganic Chemistry, 2014. Complex E1/2 (V vs. Fc/Fc+) ΔE (V) 3.1 +0.35 0.64 3.2-F –0.56 0.26 3.2-Cla –0.30 0.15 3.2-Br –0.31 0.08
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3.2.8 Computational analysis of CeX[N(SiMe3)2]3 (X = F–, Cl–, Br–, I–)
Electronic structure calculations of compounds 3.2-F, 3.2-Cl, 3.2-Br, and 3.2-I
were performed using Gaussian 09 at the B3LYP level of theory in order to describe the nature of metal ligand bonding in the complexes. The geometry optimized gas phase structures were found to be in excellent agreement with the crystallographically determined bond lengths and bond angles. In each complex, the LUMO–LUMO+6 molecular orbitals were primarily non-bonding 4f orbitals localized on the cerium atom. Figure 3.2.15 shows the calculated electronic manifold for each complex and depictions of molecular orbitals that include metal ligand orbital overlap.
The silylamide ligands in compounds 3.2-F, 3.2-Cl, 3.2-Br, and 3.2-I interacted
with the cerium ion through both σ and π donation. The molecular orbitals involved in electron donation via Ce–N σ and π interactions are shown in the top-most orbitals depicted for each complex in Figure 3.2.15. The trends in the energy differences between the molecular orbitals with significant Ce–N interactions and the LUMO orbital in each complex (3.2-F > 3.2-Cl > 3.2-Br > 3.2-I) were largely consistent with the trends
observed in the lower and higher energy LMCT transitions in the electronic absorption spectra (Figure 3.2.13).
The bonding interactions between the cerium ion and the halide ligands in compounds 3.2-F, 3.2-Cl, 3.2-Br, and 3.2-I were largely ionic in all cases, but molecular
orbitals with overlap of atomic orbitals in both the σ and π orientations were observed. These molecular orbitals are depicted in the bottom-most orbitals represented for each compound in Figure 3.2.15. The metal halide bonding interactions in the σ orientation showed increased overlap in compounds containing the heavier halides due to
114 contributions from more diffuse valence atomic orbitals. In contrast, the metal halide interactions in the π orientation observed in 2-F and 2-Cl were minimized in the
compounds containing the heavier halide ligands.
Figure 3.2.15. Normalized computationally determined valence orbital diagrams of 3.2- F, 3.2-Cl, 3.2-Br, and 3.2-I. The diagrams have been normalized to the molecular orbitals containing Ce–N σ interactions. Occupied molecular orbitals are shown in blue and unoccupied molecular orbitals are shown in red. The molecular orbitals that show Ce–X and Ce–N interactions in the σ and π orientations are shown. From top to bottom, Ce–N π interactions, Ce–N σ interactions, Ce–X π interactions, and Ce–X σ interactions.
Reprinted from work submitted to Inorganic Chemistry, 2014.
Using natural population analysis, natural charges of the cerium and halide ions and the Ce–X Mayer bond orders were calculated in order to describe the ionicity of the metal-halide bonds in compound 3.2-F, 3.2-Cl, 3.2-Br, and 3.2-I (Table 3.2.6). These
115 metrics indicated a decrease in the ionicity of the metal-halide bonds when the cerium ion interacted with the heavier halide ions. The observed decrease in the natural charges of the amide nitrogens in complexes containing heavier halides indicated that the identity of the halide impacted the interaction between the cerium and the amide ligands, as was observed spectroscopically. Previous comparison of reduction potentials and bonding metrics for cerium(IV) molecular complexes has indicated that no correlation exists between the degree of bond ionicity and the thermodynamic stability of the cerium(IV) ion.44 Likewise, comparison of the bonding metrics shown in Table 3.2.6 with the electrochemical data shown in Figure 3.2.14 and Table 3.2.5 did not indicate a correlation between the degree of ionicity of the Ce–X bond and the thermodynamic stability of the cerium(IV) ion.
Table 3.2.6. Natural charges and Mayer bond orders calculated for 3.2-F, 3.2-Cl, 3.2-Br, and 3.2-I. Adapted from work submitted to Inorganic Chemistry, 2014.
Complex qCe qX qN (avg.) MBOCe–X
3.2-F 2.081 –0.540 –1.649 0.853
3.2-Cl 1.902 –0.460 –1.628 1.028
3.2-Br 1.859 –0.410 –1.626 0.951
3.2-I 1.840 –0.421 –1.622 1.064
3.2.9 Synthesis and characterization of cerium(IV) pseudohalide complexes
In an attempt to extend this series of compounds to include the azide congener Ce(N3)[N(SiMe3)2]3, 3.2-F was reacted with Me3Si–N3 in THF at R.T., followed by
crystallization from a cold hexanes solution. As seen in Scheme 3.2.4, this route did lead to anion exchange as was the case in the syntheses of 3.2-Cl, 3.2-Br, and 3.2-I, but the
116
reaction in this case formed the dimeric compound {Ce[N(SiMe3)2]3(μ-
N3)Ce[N(SiMe3)2]3(N3)} (3.2-N3) (Scheme 3.2.4).