Thorium alkyls

The catalytic hydrogenation of inserted carbon monoxide is by no means limited to nonconjugated thorium alkyl precursors. 

The Cp3ThR complexes have been used to measure metal alkyl bond disruption enthalpies for a series of thorium alkyl complexes for both solution and gas phases. The thorium carbon bonds appear to be relatively strong when compared to other metal-carbon linkages, and solution bond disruption enthalpies range from ca. 75 to 90kcalmol . ... 

There is a class of cationic bis(pentamethylcyclopen-tadienyl)thorium alkyls that deserve special mention. Marks and coworkers have discovered that treatment of Cp 2ThMe2 with protolytic reagents, in the form of sterically encumbered ammonium salts, provides an efi cient route to Cp 2ThMe+X complexes (equation 60). These complexes exhibit characteristic low-field thorium-methyl NMR signals that... 

The photochemistry of the methyl complex ThCpaCHa differed markedly from that of thorium alkyls containing /3 hydrogens [Eq. (3)]. The reaction was appreciably slower and was only 50% complete after... 

Marks and co-workers (10, 10a) also studied the photochemistry of uranium alkyls in toluene solution at ambient (10°C) and low (-196°C) temperatures. As with the thorium alkyls, mixtures of the corresponding alkane and alkene were produced in near quantitative yield [e.g., Eq. (6)]. 

The crystal structure supports the ri -haptidty of the butadiene ligand. The average Th-C distance to the terminal carbon atoms of the butadiene ligand (2.57(3) A) is only slightly smaller than that to the internal carbon atoms (2.74(2) A), and are comparable to those found in other thorium alkyl complexes. The C(l)-C(2) and C(3)-C(4) average distances (average of four independent molecules in the unit cell) is 1.46(5) A, which is compared to the average C(2)-C(3) distance of 1.44(3) A. 

Figure 6. First and second bond disruption enthalpies for bls(penta-methylcyclopentadlenyl) thorium alkyl, metallacycle, and hydride complexes ...

Cyclopentadienyls are again widely used as spectator ligands. Equation 15.37 shows how a thorium alkyl is hydrogenolyzed by H2... 

In thermochemical investigations, Sonnenberger, Morss, and Marks [23] have determined CpaThR thorium-alkyl ligand bond disruption enthalpies... 

Fig. 23.18 Free energy vs reaction coordinate diagrams for the insertion of CO into thorium alkyl and hydride bonds at about equal concentrations at — S0°C and 1 atm CO pressure [85]. The positions of the reactants have been arbitrarily placed at the same energy level. Because it was not possible to determine the position of the presumed aUcyl or hydride carbonyl intermediate (Th(CO)(R)), these pre-equilibria have been omitted from the energy profiles.

Scheme 14.1 Reaction of Cp2AnMo2 with 2-picoline. The thorium alkyl complex Cp 2 Th(CH3)2 and 2-picoline react to give preferential sp C-H bond activation in the presence of a reactive sp C-H bond, while the analogous uranium Cp 2 U( CH3 )2 readily reacts with only the ortho 2-picoline sp C-H bond. Reprinted with permission from Yang et al. [51 Copyright 2008, American Chemical Society...

Thorium compounds of anionic nitrogen-donating species such as [Th(NR2)4], where R = alkyl or sdyl, are weU-known. The nuclearity is highly dependent on the steric requirements of R. Amides are extremely reactive, readily undergoing protonation to form amines or insertion reactions with CO2, COS, CS2, and CSe2 to form carbamates. Tetravalent thorium thiocyanates have been isolated as hydrated species, eg, Th(NCS)4(H20)4 [17837-16-0] or as complex salts, eg, M4 Th(NCS)g] vvH20, where M = NH, Rb, or Cs. 

CyclooctatetraenylCompounds. Sandwich-type complexes of cyclooctatetraene (COT), CgH g, are well known. The chemistry of thorium—COT complexes is similar to that of its Cp analogues in steric number and electronic configurations. Thorocene [12702-09-9], COT2Th, (16), the simplest of the COT derivatives, has been prepared by the interaction of ThCl [10026-08-1] and two equivalents of K CgHg. Thorocene derivatives with alkyl-, sdyl-, and aryl-substituted COT ligands have also been described. These compounds are thermally stable, air-sensitive, and appear to have substantial ionic character. 

Hydrocarbyl Complexes. Stable homoleptic and heteroleptic uranium hydrocarbyl complexes have been synthesized. Unlike the thorium analogues, uranium alkyl complexes are generally thermally unstable due to P-hydride elimination or reductive elimination processes. A rare example of a homoleptic uranium complex is U(CH(Si(CH2)3)2)3, the first stable U(I11) homoleptic complex to have been isolated. A stmctural study indicated a triganol... 

The reaction of thorium and uranium triscyclopentadienide chloride with Grignard and lithium reagents has also led to a variety of thermally stable alkyls, aryls and allyls. These complexes will be discussed in a later part of this review. 

Table 12.5 Thorium and uranium alkyl/hydride complexes on various supports, including respective C CP MAS NMR data and assignments (5 in ppm) .

The polymerization of ethylene was also qualitahvely inveshgated by pulse injec-hons of ethylene into helium flowing over thorium (67) and uranium (86) metallocene hydrocarbyl complexes supported on 7-AI2O3.950 at 25 °C, both revealing similar achvihes [171, 173]. Supported thorium half-sandwich complexes 65 exhibited higher achvity than surface species, resulhng from coordinatively more saturated tris(cyclopentadienyl) and metallocene U/Th-alkyl/hydride complexes, that is, 77, 79, 82, 90 and 91 [171]. C CP MAS NMR spectra revealed no clear evidence of ethylene insertion into [Th-CHs] or [AL5-CH3] moiehes of material... 

Phenol and alkyl-substituted phenols yield dibenzofuran and alkyldiben-zofurans by catalytic dehydration-dehydrogenation over thorium or cerium oxides. This reaction can also be achieved by thermal cracking of... 

Carboxylic acids can be converted to symmetrical ketones by pyrolysis in the presence of thorium oxide. In a mixed reaction, formic acid and another acid heated over thorium oxide give aldehydes. Mixed alkyl aryl ketones have been prepared by heating mixtures of ferrous salts.1717 When the R group is large, the methyl ester rather than the acid can be decarb-methoxylated over thorium oxide to give the symmetrical ketone. 

The conversion of CO + H2 (syn-gas) to hydrocarbons and oxygenates (Fischer-Tropsch chemistry)119 is of considerable industrial importance and recently the activation and fixation of carbon monoxide in homogeneous systems has been an active area for research.120,121 The early transition elements and the early actinide elements, in particular zirconium124 and thorium,125 126 supported by two pentamethylcyclopentadienyl ligands have provided a rich chemistry in the non-catalytic activation of CO. Reactions of alkyl and hydride ligands attached to the Cp2M centers with CO lead to formation of reactive tf2-acyl or -formyl compounds.125,126 These may be viewed in terms of the resonance forms (1) and (2) shown below. 

Amine reagents also extract thorium from carbonate solutions and the use of a primary amine, RNH3C1, where R = C10 to Cl3 alkyl, as a 20% solution in kerosene allowed the concentrations of impurities in the extracted thorium to be reduced by factors of 33.8 for UVI, 111.4 for MoVI, 18.9 for Zrlv and 6167 for Mg11.177 The extracted thorium species was shown to be of the composition (RNH3)4Th(C03)4(H20)x. Di(tridecyl)amine has been used to extract thorium from barren uranium process liquors in the Blind River plant in Canada147 and flowsheets for the recovery of lanthanides, U03 and high-purity Th(S04)2 from the Elliot Lake area in Ontario using Primene/isodecanol have been described.178... 

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