A lack of synergy? An unusual actinide-ligand bonding mode

Nik Kaltsoyannis Department of Chemistry University College London

What should I talk about?

“Anything you like, as long as you are enthusiastic”

Outline of presentation • • Part 1 A very brief introduction to actinide chemistry

The f elements

by N Kaltsoyannis and P Scott, Oxford University Press (1999)

The Chemistry of the Actinide and Transactinide Elements

, 3 rd Morss, N. Edelstein, and J. Fuger (eds), Springer (2006) Edition, L. R. Part 2 Unusual metal-ligand bonding modes in molecular uranium complexes

H Li Be Na Mg K Rb Cs Fr Ca Sr Ba Ra Sc Y La Ac Just checking…..

Ti Zr Hf Rf V Nb Ta Db Cr Mo W Sg Mn Tc Re Bh Fe Ru Os Hs Co Rh Ir Mt Ni Pd Pt Cu Ag Au Zn Cd Hg B Al C Si Ga In Tl Ge Sn Pb N P As Sb Bi O S Se Te Po F Cl Br I At He Ne Ar Kr Xe Rn Element 89 Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Element 90 Element 103

The ground electronic configurations of the actinides

Element

Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium

Electronic configuration

[Rn]6d 2 7s 2 [Rn]5f 2 6d 1 7s 2 [Rn]5f 3 6d 1 7s 2 [Rn]5f 4 6d 1 7s 2 [Rn]5f 6 7s 2 [Rn]5f 7 7s 2 [Rn]5f 7 6d 1 7s 2 [Rn]5f 9 7s 2 [Rn]5f 10 7s 2 [Rn]5f 11 7s 2 [Rn]5f 12 7s 2 [Rn]5f 13 7s 2 [Rn]5f 14 7s 2 [Rn]5f 14 6d 1 7s 2

The shapes of the seven 5

f

orbitals (cubic set). 5

f y

3 , 5

f x

3 , 5

f z

3 5

f x

(

z

2-

y

2) , 5

f y

(

z

2-

x

2) , 5

f z

(

x

2-

y

2) 5

f xyz

The oxidation states adopted by the actinide elements in their compounds +7 +6 +5 +4 +3 +2 Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr The most stable oxidation state in aqueous solution is represented by the black circles. Open circles indicate other oxidation states adopted and squares indicate that the oxidation state is found only in solids.

Radial distribution functions of selected atomic orbitals of U 6+ (Enrique Batista, B3LYP, all-electron, 2 nd order DK)

The particular challenges posed to quantum chemistry by the actinides 1 Lots of electrons.

2 Heavy elements  relativistic effects are important (scalar - modification of atomic orbital energies – and spin-orbit).

3 Large number of valence atomic orbitals of similar radial distribution and energy (5f, 6p, 6d, 7s, 7p)  actinide complexes are frequently open-shell, with many closely-spaced electronic states. The correct description of electron correlation effects is extremely important (and difficult) in these cases.

Part 2 Unusual metal-ligand bonding modes in molecular uranium complexes The classic Dewar-Chatt-Duncanson view of synergic bonding Qualitative MO scheme for CO s donation from filled CO 3 s orbital p acceptance “backbonding” into vacant CO 2 p orbital Schematic view of synergic bonding between CO and a transition metal

Qualitative MO scheme for octahedral ML 6 with p acceptor ligands (e.g. CO)

Are there CO complexes of the actinides? Two views of (C 5 Me 5 ) 3 U(CO) Evans et al. JACS 125 (2003) 13831 [{(L)U} 2 (

µ:

h

1 ,

h

1

-CO)] Meyer et al. JACS 127 (2005) 11242 “The hard, oxophilic f elements typically have a low binding affinity for the soft p CO ligand, and carbonyl complexes do not readily form” bonding

f orbital to carbonyl 2 p backbonding: the electronic structures of (C 5 H 5 ) 3 U(CO) and (C 5 H 5 ) 3 U(OC) B.E. Bursten and R.J. Strittmatter, JACS 109 (1987) 6606.

“Two major interactions of (C 5 H 5 ) 3 U(CO) are discussed. The CO 3 s lone pair interacts primarily with the empty U 6d orbitals to form the U-CO s bond, and extensive U 5f → CO 2 p backbonding is observed”

R N N N R U N N N R R R N N U N N R R=SiMe 2 Bu t P. Roussel and P. Scott, JACS 120 (1998) 1070.

R N N N R U N N N R R N U R N N N R

H 2 N H 3 N NH 2 U N N H 2 N H 2 N NH U 2 NH 3 NH 2  Back bonding without s bonding N.Kaltsoyannis and P. Scott,

Chem. Commun.

(1998) 1665.

What is the oxidation state of the uranium atoms in [(C 5 Me 5 ) 2 U] 2 ( h

µ 6 :µ 6

-C 6 H 6 )?

Realistic possibilities include (a) U(II) and neutral benzene (b) U(III) and (benzene) 2 (most likely from experiment) and (c) U(IV) and (benzene) 4-

How well does calculation reproduce the experimental geometry?

Interatomic distance/ Å U-U U1-Cp* (av) U2-Cp* (av) C-C (benzene, complex) C-C (benzene, free) U1-C (benzene, av) U1-C (benzene, max) U1-C (benzene, min) U2-C (benzene, av) U2-C (benzene, max) U2-C (benzene, min) Exp.

4.396

2.840

2.830

1.440

1.390

2.621

2.733

2.547

2.628

2.730

2.538

Calc.

4.406

2.860

2.840

1.440

1.394

2.634

2.719

2.591

2.627

2.674

2.532

So why is the benzene ring so non-planar?

Hückel energies of the carbocyclic ring p orbitals

Calculation suggests (a) each uranium gives up two electrons to the cp* ligands (b) each uranium has two 5f-based electrons (c) four electrons (two per uranium) are used to form a uranium/arene d bond

•The localisation properties of the four uranium/arene δ bonding electrons determine the formal oxidation state of the metal centres. •Population analysis indicates that these electrons have an approximately equal contribution from both metal and arene, and hence the oxidation state of the uranium atoms is best described as +3.

•The benzene ring is not neutral. Rather, it carries a charge close to -2, as there is transfer of uranium 5f electron density into the benzene e 2u C-C π* molecular orbitals. The benzene ring is thus no longer H ückel aromatic, and is significantly non-planar as a result.

W.J. Evans, S.A. Kozimor, J. W. Ziller and N. Kaltsoyannis, JACS 126 (2004) 14533.

Arene-bridged diuranium complexes: inverted sandwiches supported by d backbonding P.L. Diaconescu, P.L. Arnold, T.A. Baker, D.J. Mindiola and C.C. Cummins, JACS 122 (2000) 6108.

( m -C 7 H 8 )[U(N[Ad]Ar) 2 ] 2 The two near degenerate d backbonding orbitals of ( m -C 6 H 6 )[U(NH 2 ) 2 ] 2