The University of Manchester Research Post Hartree-Fock calculations of pnictogen-uranium bonding in EUF3 (E = N-Bi) DOI: 10.1039/C8CC05581E Document Version Accepted author manuscript Link to publication record in Manchester Research Explorer Citation for published version (APA): Atkinson, B., Hu, H-S., & Kaltsoyannis, N. (2018). Post Hartree-Fock calculations of pnictogen-uranium bonding in EUF (E = N-Bi). Chemical Communications, 79, 11100–11103. https://doi.org/10.1039/C8CC05581E 3 Published in: Chemical Communications Citing this paper Please note that where the full-text provided on Manchester Research Explorer is the Author Accepted Manuscript or Proof version this may differ from the final Published version. If citing, it is advised that you check and use the publisher's definitive version. General rights Copyright and moral rights for the publications made accessible in the Research Explorer are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Takedown policy If you believe that this document breaches copyright please refer to the University of Manchester’s Takedown Procedures [http://man.ac.uk/04Y6Bo] or contact [email protected] providing relevant details, so we can investigate your claim. Download date:26. Sep. 2021 Journal Name Post Hartree-Fock calculations of pnictogen-uranium bonding in EUF3 (E = N-Bi) Benjamin E. Atkinson, Han-Shi Hu,‡ and Nikolas Kaltsoyannis∗ NUF is identified as having a N ≡ U triple bond, as has 3 i 5 TIPS been previously found (Andrews et al., Angew. Chem. and (N(CH2CH2NSi Pr3)3). Tren has subsequently been suc- Int. Ed., 2008, 47, 5366). By contrast, while previously cessfully employed in isolating a range of pnictogen-actinide TIPS reported calculations on PUF and AsUF (Andrews et al., complexes, including U(Tren )(PH) featuring a double U−P 3 3 6 TIPS Inorg. Chem., 2009, 48, 6594) gave a E ≡ U triple bond, our bond, and the tetrameric [U(Tren )(AsK2)]4 featuring a − 7 calculations suggest a single bond for both molecules, with U−As triple bonding interaction. antibonding p* and non-bonding 5fU orbitals significantly Andrews et al. also reported data for PUF3 and AsUF3. They occupied, and highly multiconfigurational wavefunctions. obtained argon matrix isolated IR spectra of the three molecules, We propose this difference to be due to the smaller [6,6] with U-F stretches being observed in the 520-620 cm – 1 region. ∗ ∗ active space used (s, p, p and s ) in the previous studies. The only pnictogen-uranium stretch observed was for NUF3 (at In our calculations, a [6,16] active space was employed in 938 cm – 1); the P-U stretch and As-U stretch were not observed order to include uranium f-orbitals and pnictogen d-orbitals. but were calculated to lie outside the window of the experimen- tal spectra. Density functional theory (DFT) calculations were Uranium’s central role in nuclear power necessitates a detailed performed alongside complete active space-SCF (CASSCF) cal- knowledge of its chemistry, including at the most fundamental culations with second-order perturbation theory (CASPT2). A 6 level. Pnictogen-actinide chemistry is an understudied area, and electrons in 6 orbitals ([6,6]) active space was employed, which ∗ ∗ in addition to being of fundamental interest, an understanding included the s, p, p and s orbitals of the pnictogen-uranium of uranium-nitrogen chemistry is important for developing future bond. Effective U-E bond orders were 2.78, 2.39 and 2.21 for applications of the actinides, such as the potential for uranium ni- NUF3, PUF3 and AsUF3 respectively, indicating significant multi- tride to be used as a replacement for uranium oxide fuels, given ple bonding, with a weakening of p bonding down group 15. its higher melting point, thermal conductivity and increased den- In this contribution, we report superior post Hartree-Fock cal- 1 sity. culations on EUF3 (E = N-Bi). Calculations were performed at The terminal uranium nitride molecules which have been stud- the CASSCF 8 and CASPT2 levels of theory, 9,10 but with a sub- ied spectroscopically were summarised by King and Liddle in stantially larger active space than previously used; 6 electrons in 2 2013; the molecules UN, UN2, NUNH (featuring both a dou- 16 orbitals. This was chosen to include the uranium 5f, 6d and 3 4 ble and triple U-N bond) and NUF3 were all isolated in in- 7s valence orbitals, along with the pnictogen np valence orbitals. −14 ert gas matrices, and all feature U− N stretching frequencies For NUF3, the principal conclusion from the present larger active between 938-1051 cm – 1. 1 The first uranium nitride molecule space calculations is very similar to that calculated by Andrews et isolable at ambient conditions was reported by King et al. in 2013, al. with a [6,6] active space, i.e. there is a U−N triple bond. A featuring the highly sterically hindered polydentate TrenTIPS lig- U-N bond length of 1.753 Å was calculated, vs. 1.759 Å from L. Andrews et al.. Geometric parameters are summarised in Fig. 1, and key vibrational frequencies in Table 1. The wavefunction is 2 4 School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL. largely monoconfigurational with the s p configuration making United Kingdom; E-mail: [email protected] 1 up 86.6% of the wavefunction. The A1 state was calculated to y Electronic Supplementary Information (ESI) available: computational methodol- be the ground state; the first excited state, the 3A00 state, predom- ogy, energies and natural orbital occupancies of excited states, full vibrational fre- inantly of s 1p45 f 1 character, lies 1.60 eV higher in energy. The quencies and optimised geometric parameters. See DOI: 10.1039/b000000x/ U z Present address: Department of Chemistry, Tsinghua University, Haidian District, vast majority of the active electrons are inside the equivalent of Beijing 100084, China. Andrew’s [6,6] active space (i.e. the s and p bonding and anti- Journal Name, [year], [vol.], 1–4 | 1 bonding orbitals) with only 0.042 electrons in the remaining 10 orbitals of the active space. This suggests that, for NUF3, the [6,6] active space is perfectly adequate. 2.743 Å 2.871 Å 1.753 Å 122.6° 105.3° 1 σ 2 π 3 π 4 π* 5 π* 105.1° 105.4° (1.93) (1.08) (1.06) (0.67) (0.66) 105.0° 2.050 Å 2.049 Å 93.7° 106.1° 106.2° 126.6° 93.7° 2.045 Å 127.6° 2.052 Å 2.051 Å NUF3 PUF3 AsUF3 1 6 5fU 7 5fU 8 3dP /σ 9 3dP 10 3dP 11 3dP Fig. 1 The ground state geometries of NUF3 (left, C3v, A1), PUF3 (mid- (0.26) (0.25) (0.02) (0.02) (0.02) (0.01) 1 0 1 0 dle, Cs, A ) and AsUF3 (right, Cs, A ). Bond lengths given in ångstrom, bond angles in degrees. Blue: nitrogen, black: phosphorus, pink: ar- senic, red: uranium, yellow: fluorine. The assignment of a triple bond for NUF3 is clear; the effective bond order BOeff = 2:73. This is firmly supported by the U-N bond critical point (BCP) ellipticity e = 0:00 (Table 2). BCP elliptici- ties measure the extent to which the electron density is preferen- 12 3dP 13 3dP/σ* 14 4pP/3dP 15 4pP/3dP 16 σ* tially accumulated in a given plane containing the bond path con- (0.01) (0.01) (0.00) (0.00) (0.00) necting two chemically bonded atoms. 11 Cylindrically symmetric Fig. 2 The natural orbitals of the active space of PUF (1A0, CASPT2), bonds (e.g. single or triple bonds) have ellipticities (close to) 3 shown at the 0.05 isosurface value. Occupancy number given in brack- zero, while double bonds feature significantly non-zero ellipticity ets. values. The triple bond is reflected in both the calculated and ob- served stretching frequencies. In argon matrix IR absorption spec- tra, Andrews et al. observed the N-U stretching frequency at 938 derstood by the significant extent to which orbitals outside of this cm – 1, compared to a calculated 921 cm – 1 (at the [6,6] CASPT2 [6,6] active space are occupied. There are 0.52 electrons occupy- level of theory). In this work, the N-U stretching frequency is cal- ing nonbonding 5 fU orbitals, and 0.07 electrons in nonbonding – 1 culated to be 928 cm (Table 1), though given that argon matrix 3dP. In the [6,6] active space, these electrons must occupy ei- effects are not included in the gas phase calculations, this slight ther the p bonding or antibonding orbitals, or the s antibonding improvement in the agreement may be fortuitous. Two very small orbital, favouring the triple bonded electronic structure.z The – 1 – 1 imaginary frequencies are present (at 9:52i cm and 0:73i cm ), effective bond order BOeff = 1:35 suggests either a single bond though these are likely artefacts of the numerical frequency calcu- or a very weak double bond. The BCP ellipticity value (Table lation, as is the 1 cm – 1 difference between the two components 2) strongly suggests a single bond assignment, and this is also of the E symmetry U-F stretch. supported by the delocalisation index DI(UjN) = 0:79. The latter By contrast to NUF3, increasing the size of the active space has metric is the Quantum Theory of Atoms-in-Molecules (QTAIM) 11 a significant effect on the geometry of PUF3. A [6,6] active space measure of bond order.