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Theoretical Study of Divalent Bis(Pentaisopropylcyclopentadienyl) Actinocenes

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Theoretical Study of Divalent Bis(Pentaisopropylcyclopentadienyl) Actinocenes doi.org/10.26434/chemrxiv.9640931.v1 Theoretical Study of Divalent Bis(Pentaisopropylcyclopentadienyl) Actinocenes Jason Yu, Filipp Furche Submitted date: 17/08/2019 • Posted date: 19/08/2019 Licence: CC BY-NC-ND 4.0 Citation information: Yu, Jason; Furche, Filipp (2019): Theoretical Study of Divalent Bis(Pentaisopropylcyclopentadienyl) Actinocenes. ChemRxiv. Preprint. The existence of divalent bis(pentaisopropylcyclopentadienyl) actinocene compounds, An(CpiPr5)2 for An = (Th, U, Pu, Am, Bk, No, Lr), is assessed by density functional theory (DFT) calculations with scalar-relativistic small core pseudopotentials. The calculations predict ground states with significant 6d occupation for Th, U, and Lr, whereas Am, Bk, and No exhibit 5f ground states. A mixed ground state with predominant 5f character is found for Pu. The complexes exhibit a linear coordination geometry and high S10 symmetry except for Pu(CpiPr5)2 and Am(CpiPr5)2, which are found to be bent by 11 and 12 degrees , respectively. Absorption spectra are simulated with time-dependent density functional theory (TD-DFT) and compared to experimental spectra of known tris(C4H4SiMe3) and tris(C5H3(SiMe3)2) compounds [M. E. Fieser et al., J. Am. Chem. Soc. 2015, 137, 369-382] as well as recently synthesized divalent lanthanide analogs Dy(CpiPr5)2 and Tb(CpiPr5)2 [C. Gould et al., J. Am. Chem. Soc. 2019, accepted]. Thermodynamic stability is assessed by calculation of adiabatic reduction potentials of the trivalent precursors [An(CpiPr5 )2] +, and the feasibility of further reduction to obtain as yet unknown monovalent molecular actinide complexes is discussed. File list (2) diAc_paper.pdf (854.40 KiB) view on ChemRxiv download file diAc_SI.pdf (5.46 MiB) view on ChemRxiv download file Theoretical Study of Divalent Bis(Pentaisopropylcyclopentadienyl) Actinocenes Jason M. Yu and Filipp Furche∗ Department of Chemistry, University of California Irvine, 1102 Natural Sciences II, Irvine, CA 92697-2025, USA E-mail: fi[email protected] Abstract The existence of divalent bis(pentaisopropylcyclopentadienyl) actinocene compounds, iPr5 An(Cp )2 for An = (Th, U, Pu, Am, Bk, No, Lr), is assessed by density functional theory (DFT) calculations with scalar-relativistic small core pseudopotentials. The calculations predict ground states with significant 6d occupation for Th, U, and Lr, whereas Am, Bk, and No exhibit 5f ground states. A mixed ground state with pre- dominant 5f character is found for Pu. The complexes exhibit a linear coordination iPr5 iPr5 geometry and high S10 symmetry except for Pu(Cp )2 and Am(Cp )2, which are found to be bent by 11◦ and 12◦, respectively. Absorption spectra are simulated with time-dependent density functional theory (TD-DFT) and compared to experimental spectra of known tris(C4H4SiMe3) and tris(C5H3(SiMe3)2) compounds [M. E. Fieser et al., J. Am. Chem. Soc. 2015, 137, 369-382] as well as recently synthesized diva- iPr5 iPr5 lent lanthanide analogs Dy(Cp )2 and Tb(Cp )2 [C. Gould et al., J. Am. Chem. Soc. 2019, accepted]. Thermodynamic stability is assessed by calculation of adiabatic iPr5 + reduction potentials of the trivalent precursors [An(Cp )2] , and the feasibility of further reduction to obtain as yet unknown monovalent molecular actinide complexes is discussed. 1 Introduction Bis(cyclopentadienyl) metallocenes of the transition-metal elements are prototypical organo- metallic compounds.1{4 The interaction of metal-centered valence d orbitals with π orbitals of the ligands in these species gives rise to rich coordination chemistry, enabling applica- tions in numerous synthetic and catalytic purposes including stereoselective addition,5 olefin 6{8 9 polymerization, and N2 activation. A central goal of organoactinide chemistry is the dis- covery, isolation, and characterization of new compounds which exhibit chemistry similar to the transition-metals. However, π-bonded coordination compounds in low valence states are rare for the actinide elements, and no bis(cyclopentadienyl) actinocene analog exists.10 From an electronic structure perspective, this discrepancy may be rationalized by the properties of the actinides' 5f valence shell, which is less available for coordinative bonding due to its contracted nature and high nodality.11 Rather than forming bis(cyclopentadienyl) metal- locenes known from transition metal elements, it was predicted by early calculations12 and experimentally verified13 that the actinides instead preferred tetravalent bis(COT) (COT = 8 14{17 η − C8H8) structures with primarily electrostatic bonding. Synthetic attempts towards bis(cyclopentadienyl) structures have produced linear or nearly linear species that include equatorial donor ligands bound to the metal.18{20 Nevertheless, the higher metal oxidation states of these compounds preclude much of the organometallic activity exhibited by their transition-metal congeners. Substituted cyclopentadienyl ligands21{25 have led to the isolation of the first divalent 0 0 −1 26{28 lanthanide compounds with three equatorial Cp (Cp = (C5H4SiMe3) ) ligands. Iso- II 00 −1 II 0 −1 II 00 −1 00 lation of the actinide compounds [Th (Cp )3] , [U (Cp )3] , and [Pu (Cp )3] (Cp = −1 (C5H3(SiMe3)2 − 1; 3) ) followed soon after, with those for Th and U analogously exhibit- ing (5f)0(6d)2 and (5f)3(6d)1 configurations akin to transition-metal species.29{32 The unex- pected discovery of divalent actinide compounds with substituted cyclopentadienyl ligands suggested that other organoactinide compounds with unconventional electronic structure could be accessed with suitable ligands. 2 iPr5 Recently, divalent Dy and Tb complexes with bis(Cp ) ligands (iPr = −CH(CH3)2) iPr5 were synthesized by reduction of trivalent Ln(Cp )2I intermediates and characterized by X-ray crystallography, magnetic measurements, and density functional theory (DFT) calcu- lations, representing the first examples of bis(cyclopentadienyl) lanthanocene compounds.33 These structures maintain a (4f)n(5d)1 ground state configuration and exhibit covalent σ- bonding between the cyclopentadienyl ligands and metal-based orbitals, resembling the bond- ing in transition-metal metallocenes. Here we assess the viability of bis(pentaisopropylcyclopentadienyl) actinocene compounds iPr5 (Figure 1) by characterizing the electronic structure of An(Cp )2 for An = (Th, U, Pu, Am, Bk, No, Lr). Our computational methodology has been extensively validated in prior iPr5 33 work on crystallographically characterized lanthanide complexes such as Dy/Tb(Cp )2, −1 34 −1 27 0 −1 00 −1 26,28 [LnfN(SiMe3)2g3] , [Ln(C5Me4H)3] , [Ln(Cp )3] , [Ln(Cp )3] , as well as the 00 −1 31 0 −1 29 00 −1 30 35 organoactinide complexes [Th(Cp )3] , [U(Cp )3] , [Pu(Cp )3] , and U(C5Me4H)3NO. The current study focuses on key qualitative features of bis(CpiPr5 ) actinide compounds in- cluding trends in dominant electronic configurations of the metal, symmetries, and properties relevant to guide synthetic efforts. More detailed calculations including spin-orbit interac- tions, a quantitative treatment of near-degeneracy effects, and a more complete description of low-lying excited states with different spin multiplicities are left to future work. iPr5 Figure 1: Side-on and end-on views of An(Cp )2 molecular structure. Hydrogen atoms are omitted for clarity. 3 Computational Details Structure optimizations using the TPSS meta-generalized gradient approximation (meta- 36 iPr5 GGA) density functional were performed for An(Cp )2 starting from the X-ray structure iPr5 33 of Dy(Cp )2. Stuttgart-Cologne scalar-relativistic small core effective core potentials (ECPs) were used for An = (Th, U, Pu, Am, Bk, No, Lr).37 The corresponding valence basis set38,39 was used for the actinide atoms, and a double-ζ quality basis set (def2-SV(P))40 was used for the ligands. The exchange-correlation terms were evaluated on a DFT quadrature grid of size m4 and total energies were converged in the SCF procedure to a tolerance of 10−7 hartrees. The resolution of the identity approximation (RI-J)41 and D3 dispersion corrections by Grimme and co-workers42 were employed. Optimized structures were obtained to a threshold of 10−4 a.u. Each converged structure was verified to be a minimum by harmonic vibrational analysis.43 Electronic configurations were assigned based on inspection of molecular orbitals and their natural atomic orbital populations.44 Molecular coordinate files, orbital occupations, and visualizations of frontier orbital contours are available as supporting information (SI). TD-DFT calculations to study electronic excitations were performed at optimized ge- ometries with the gauge-invariant TPSSh hybrid functional.36,45 Absorption spectra were simulated from the results using normalized Gaussian functions scaled by predicted oscilla- tor strength. A root mean square width of 0.15 and empirical blue shift of 0.15 eV were used, consistent with parameters chosen to fit experimental spectra for the divalent lanthanocenes iPr5 33 Dy/Tb(Cp )2. The lowest 10 excitations for each dipole allowed transition were com- puted for An = (Th, U, Bk, No, Lr) and the lowest 30 for An = (Pu, Am). To study the synthetic accessibility of the divalent and monovalent bis(CpiPr5 ) actinide compounds, im- plicit solvent corrections were included in DFT through the Conductor-like Screening Model (COSMO).46 These calculations were performed using the dielectric constant for tetrahy- drofuran ( = 7:520).47 The harmonic oscillator-rigid rotor approximation at 298.15 K was used for free-energy calculations. 4 Molecular orbital plots
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