Nitrogen-based analogues of the uranyl ion

1. Nik Kaltsoyannis Department of Chemistry University College London Nitrogen-based analogues of the uranyl ion

2. Introduction Uranyl UO22+ is the most common functional unit in the chemistry of U(VI). Imido ligand NR2- is isoelectronic with the oxo O2- ligand, and the two groups can often be interchanged in transition metal complexes. The alkyl or aryl R group provides a variable unavailable in oxo chemistry  synthesis of the isoelectronic imido analogues of uranyl is highly desirable. However, synthesis of imido uranyl analogues has proved very difficult. In 1996 Denning speculated that the isolation of U(NR)2 is not possible because U(VI) is too oxidising.

3. Possible analogues - 1 “Synthesis and structure of the first U(VI) organometallic complex” D.S.J. Arney, C.J. Burns and D.C. Smith, JACS114 (1992) 10068

4. Possible analogues - 2 “Stable Analogues of the Uranyl Ion Containing the -N=U=N- Group” D.R. Brown and R.G. Denning, Inorg. Chem.35 (1996) 6158 i.e. How good is this analogy?

5. On the valence electronic structure of UO22+ In 1999….what was generally agreed upon: UO22+ has 12 valence electrons (coming from the oxygen 2p and uranium 5f, 6d and 7s orbitals) These electrons are accommodated in four molecular orbitals, of pg, pu, sg and su symmetry U6+ UO22+ O24-

6. In 1999….what was generally not agreed upon: The ordering of these four molecular orbitals

7.  two questions: Is this the correct orbital ordering? If so, why is the su orbital so much less stable than the others?

8. Possible reasons for the destabilisation of the su MO of UO22+ • Overlap between the U fs and O 2p orbitals is small because the position of the angular nodes of the fs orbital gives rise to extensive overlap cancellation in regions of different phase. • The “pushing from below” mechanism. The su orbital is destabilised by a filled-filled interaction with the U 6p semi-core orbitals. C.K. Jørgensen and R. Reisfeld, Struct. Bonding50 (1982) 121 Figure 5.10 from The f elements by N. Kaltsoyannis and P. Scott

9. Molecular orbital energy level diagrams for UO22+ N. Kaltsoyannis, Inorg. Chem.39 (2000) 6009. R.G. Denning, JPCA111 (2007) 4125.

10. Don’t believe everything you read in textbooks…. Overlap between the uranium valence orbitals and the oxygen p levels decreases in the order fs > fp > dp > ds 

11. A better qualitative molecular orbital energy level diagram for UO22+ Always include the actinide 6p orbitals in your calculations!

12. Molecular orbital energy level diagrams for UN2, UON+ and UO22+

13. Molecular orbital energy level diagrams for OUNPH33+ and U(NPH3)24+

14. Why is s below p in OUNPH33+ and U(NPH3)24+? 4e U-N p 6a1 O-U-N-P-H s Walsh diagram for elongation of the U-N bond in OUNPH33+

15. Molecular orbital energy level diagrams for OUNPH33+ and U(NPH3)24+ with U 6p in core and valence

16. Summary Density functional theory calculations on UO22+ confirm the valence MO ordering proposed by Denning (on the basis of experimental data) and indicate that the su HOMO is destabilised with respect to the other valence MOs on account of a filled‑filled interaction with the uranium 6p semi‑core orbitals (the pushing from below mechanism). Comparison of the isoelectronic series UO22+, UON+ and UN2 indicates that the uranium–element p bonding MOs are in all cases more stable than the uranium–element s bonding levels, and that U–N bonding is significantly more covalent than U–O. The U–N bonds in OUNPH3+ and U(NPH3)24+ are longer and less covalent than their equivalents in UON+ and UN2, and the U–N s bonding MOs are more stable than the U–N p levels. This reversal of the U–N s/p MO ordering with respect to UON+ and UN2 is due to a combination of two factors: • (a) stabilising N–P(–H) s contribution to the U–N s MO(s) • (b) increased U–N distance which destabilises the U–N p bonding levels. As with UN2 (and UO22+) the ds MO of U(NPH3)24+ is much more stable than the fs. This is partly due to the destabilising influence of the pushing from below mechanism on the fs MO, which is found to operate in the iminato systems to a similar extent as in UO22+.

17. So how good is this analogy? “It is certainly correct of Denning to describe uranium bis iminato complexes as structural analogs of the uranyl ion, it is not clear that the analogy can be fully extended to the electronic structure” N. Kaltsoyannis, Inorg. Chem.39 (2000) 6009

18. Never say never…. T.W. Hayton, J.M. Boncella, B.L. Scott, P.D. Palmer, E.R. Batista and P.J. Hay, Science310 (2005) 1941 T.W. Hayton, J.M. Boncella, B.L. Scott, P.D. Palmer, E.R. Batista and P.J. Hay, JACS128 (2006) 10549

19. Comparison of UO22+, UN2 and U(NMe)2I2(THF)2

20. “Overall, we can say that the U-N bonding orbitals in U(NMe)2I2(THF)2 are of the same type as those of the UO22+ fragment…although the ordering is different. This difference in order is an indication that the “pushing from below” mechanism proposed for uranyl does not exert a strong influence in the present case due perhaps to a smaller involvement of the uranium 6p orbital in the binding MOs” T.W. Hayton, J.M. Boncella, B.L. Scott, P.D. Palmer, E.R. Batista and P.J. Hay, JACS128 (2006) 10549