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

Introduction Uranyl UO 2 2+ is the most common functional unit in the chemistry of U(VI). Imido ligand NR 2- is isoelectronic with the oxo O 2- 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.

Possible analogues - 1 “Synthesis and structure of the first U(VI) organometallic complex” D.S.J. Arney, C.J. Burns and D.C. Smith, JACS 114 (1992) 10068

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

On the valence electronic structure of UO 2 2+ In 1999….what was generally agreed upon: UO 2 2+ 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  g,  u,  g and  u symmetry U 6+ UO 2 2+ O 2 4-

In 1999….what was generally not agreed upon: The ordering of these four molecular orbitals ReferenceComputational methodMolecular orbital ordering Cornehl et al., Angew. Chem. Int. Ed. Engl. 35 (1996) 891. CCSD/quasi-relativistic pseudopotentials  g <  g <  u <  u K.G. Dyall, Mol. Phys. 96 (1999) 511. DHF/all electron  g <  u <  g <  u Zhang et al., J. Phys. Chem. A 103 (1999) CISD/relativistic ECPs  g <  u <  u <  g De Jong et al., J. Mol. Struct. (THEOCHEM) 458 (1999) 41. DHF+CCSD(T)/all electron  g <  u ~  g <  u

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

Possible reasons for the destabilisation of the  u MO of UO Overlap between the U f  and O 2p orbitals is small because the position of the angular nodes of the f  orbital gives rise to extensive overlap cancellation in regions of different phase. Figure 5.10 from The f elements by N. Kaltsoyannis and P. Scott 2The “pushing from below” mechanism. The  u orbital is destabilised by a filled-filled interaction with the U 6p semi-core orbitals. C.K. Jørgensen and R. Reisfeld, Struct. Bonding 50 (1982) 121

Molecular orbital energy level diagrams for UO 2 2+ N. Kaltsoyannis, Inorg. Chem. 39 (2000) R.G. Denning, JPCA 111 (2007) 4125.

Don’t believe everything you read in textbooks…. Overlap between the uranium valence orbitals and the oxygen p levels decreases in the order f  > f  > d  > d  

A better qualitative molecular orbital energy level diagram for UO 2 2+ Always include the actinide 6p orbitals in your calculations!

Molecular orbital energy level diagrams for UN 2, UON + and UO 2 2+

Molecular orbital energy level diagrams for OUNPH 3 3+ and U(NPH 3 ) 2 4+

Why is  below  in OUNPH 3 3+ and U(NPH 3 ) 2 4+ ? 6a 1 O-U-N-P-H  4e U-N  Walsh diagram for elongation of the U-N bond in OUNPH 3 3+

Molecular orbital energy level diagrams for OUNPH 3 3+ and U(NPH 3 ) 2 4+ with U 6p in core and valence

Density functional theory calculations on UO 2 2+ confirm the valence MO ordering proposed by Denning (on the basis of experimental data) and indicate that the  u 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 UO 2 2+, UON + and UN 2 indicates that the uranium– element  bonding MOs are in all cases more stable than the uranium–element  bonding levels, and that U–N bonding is significantly more covalent than U–O. The U–N bonds in OUNPH 3 + and U(NPH 3 ) 2 4+ are longer and less covalent than their equivalents in UON + and UN 2, and the U–N  bonding MOs are more stable than the U– N  levels. This reversal of the U–N  /  MO ordering with respect to UON + and UN 2 is due to a combination of two factors: (a)stabilising N–P(–H)  contribution to the U–N  MO(s) (b)increased U–N distance which destabilises the U–N  bonding levels. As with UN 2 (and UO 2 2+ ) the d  MO of U(NPH 3 ) 2 4+ is much more stable than the f . This is partly due to the destabilising influence of the pushing from below mechanism on the f  MO, which is found to operate in the iminato systems to a similar extent as in UO Summary

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

Never say never…. T.W. Hayton, J.M. Boncella, B.L. Scott, P.D. Palmer, E.R. Batista and P.J. Hay, Science 310 (2005) 1941 T.W. Hayton, J.M. Boncella, B.L. Scott, P.D. Palmer, E.R. Batista and P.J. Hay, JACS 128 (2006) 10549

Comparison of UO 2 2+, UN 2 and U(NMe) 2 I 2 (THF) 2

“Overall, we can say that the U-N bonding orbitals in U(NMe) 2 I 2 (THF) 2 are of the same type as those of the UO 2 2+ 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, JACS 128 (2006) 10549