McMaster Chemistry: Emslie

Ultra-Rigid Non-Carbocyclic Ligands in Actinide Chemistry

Introduction to Actinide Chemistry

Our work on actinide chemistry is focused on thorium and uranium since these elements are only slightly radioactive, not particularly toxic (similar to Pb), naturally abundant and commercially available. Thorium and Uranium are in fact, the heaviest elements that can be studied in macroscopic quantities a typical synthetic chemistry laboratory.

People often assume that thorium and uranium are rare and highly radioactive elements. However, this is not the case. For example, the soil removed to build a typical family home (~100 tonnes) contains ~1kg of uranium and ~3kg of thorium. There are also many thorium and uranium-containing minerals, and the half lives for Th-232, U-238 and U-235 are between 0.7 and 14 billion years, so the amount of radioactivity from these isotopes is fairly small (see the table below for more information).

Click on this link for more information on the properties of thorium and uranium.

Isotope	Natural Abundance	Decay mode / energy	Half Life (billions of years)	Specific Activity	Activity per g of Th_nat or U_nat
²³²Th	100.00 %	a / 4.0 MeV	14.05	4.07 kBq/g	4.07 kBq/g
²³⁸U	99.27 %	a / 4.2 MeV	4.468	12.445 kBq/g	12.356 kBq/g
²³⁵U	0.72 %	a / 4.4 MeV g / 0.21 MeV	0.7038	80.011 kBq/g	0.568 kBq/g
²³⁴U	0.0055 %	a / 4.8 MeV	0.0002455	231.3 MBq/g	12.356 kBq/g
U_nat	100 %	a	n/a	n/a	25.280 kBq/g

Why Study the Actinides ?

1. Early actinide elements are endowed with a unique set of features which suggest the potential to access modes of reactivity which differ from those accessible elsewhere in the periodic table. These features are:

--- Potential for significant covalency and f-orbital involvement in bonding (unlike the lanthanides).

--- Formation of complexes of high Lewis acidity

--- Large ionic radii - similar or larger than those of the lanthanides, and significantly larger than those for transition metals in the same oxidation state [revised effective ionic radii (C.N. 6) of Shannon and Prewitt: U³⁺ 1.03, La³⁺ 1.03, Lu³⁺ 0.86, Th⁴⁺ 0.94, U⁴⁺ 0.89, Ce⁴⁺ 0.87, Hf⁴⁺ 0.71, Ti⁴⁺ 0.61, U⁵⁺ 0.76, Ta⁵⁺ 0.64, V⁵⁺ 0.54, U⁶⁺ 0.73, W⁶⁺ 0.60, Cr⁶⁺ 0.44 Å].

--- A distinctly non-lanthanide-like variety of oxidation states is accessible for several of the early actinides [e.g. U(III)-U(VI)].

--- An aversion to the formation of actinide-actinide bonds.

2. Ligand design and understanding of actinide-ligand bonding is of great importance in the development of new methods for selective actinide complexation and actinide/element separation. Potential applications of these methods include analytical actinide separation prior to detection (for biological or environmental monitoring), nuclear fuel reprocessing (for re-use or transmutation), environmental remediation, and in vivo decomplexation.

Organometallic Actinide Complexes in the Literature

Compared with the Organometallic chemistry of most transition metals, actinide organometallic chemistry is far less developed. In addition, organoactinide chemistry has been dominated by the use of carbocyclic ancillaries such as:

- C₅R₅ ligands (most organoactinide chemistry was developed using C₅R₅ complexes by Marks, Eisen and Evans)
- C₅R₅-based ligands [e.g. Me₂Si(C₅Me₄)₂ or (tBuN)SiMe₂(C₅Me₄)]
- the tetramethylphospholyl anion_x
- dianionic C₈R₈ or pentalene ligands
- carboranes_x
- arenes_x
- the cycloheptatrienyl trianion

The chemistry of non-carbocycle supported organoactinide complexes is much less developed, but is of great interest due to the enormous structural and electronic versatility afforded by such ligands. Non-cyclopentadienyl actinide(IV) bis-hydrocarbyl (alkyl, aryl or allyl) complexes are particularly rare, and the majority of this work has focused on the use of monoanionic ancillaries:

- alkoxy/aryloxy (Th/U) ligands_x
- amidinate (U) ligands_x
- tris(pyrazolyl)borate (U) ligands

Goals in Organothorium and Uranium Chemistry

Our focus is on the preparation of non-carbocyclic organoactinide complexes due to their largely untapped potential in both traditional organometallic catalysis and in the development of novel reactivity. These studies are directed towards sterically hindered and unusually rigid ligands, since they are expected to:

(a) Allow access to coordination environments which are dictated by design rather than the preferences of the central metal and/or co-ligands._x
(b) Ensure that well intentioned steric bulk is positioned in such a way as to maximize its effectiveness.

As a result, various modes of decomposition are expected to become less favourable, especially those involving sterically hindered transition states or the formation of dinuclear or bis-ligand complexes. Rigid ligands are also expected to be more amenable to steric tuning since the effects of steric bulk are not easily mitigated by alterations in the ligand geometry or hapticity, and are therefore more predictable.

Extremely rigid, bulky and planar 2,6-bis(anilidomethyl)pyridine and 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene ligands recently allowed the synthesis of a range of thorium(IV) complexes, including:

--- the first crystallographically characterized thorium dialkyls supported by a multidentate non-carbocyclic ancillary._x--- a thorium complex, [LTh(CH₂SiMe₃)₂], with thermal stability comparable or greater than the Cp* analogue.
--- the first non-cyclopentadienyl actinide alkyl cations (all are rare examples of arene-coordinated metal alkyl cations)._x
--- the first example of double alkyl abstraction to form a dicationic actinide complex._x
--- a unique 'Grignard adduct' in which a thorium dihalide is bound (via Th-X-Mg linkages) to an MeMgX unit (X = Cl or Br).