The role of 5f-orbital participation in unexpected inversion of the σ-bond metathesis reactivity trend of triamidoamine thorium(iv) and uranium(iv) alkyls

Abstract

We report on the role of 5f-orbital participation in the unexpected inversion of the s-bond metathesis reactivity trend of triamidoamine thorium(IV) and uranium(IV) alkyls. Reaction of KCH 2 Ph with [U(Tren TIPS)(I)] [2a, Tren TIPS ¼ N(CH 2 CH 2 NSiPr i 3) 3 3À ] gave the cyclometallate [U {N(CH 2 CH 2 NSiPr i 3) 2 (CH 2 CH 2 NSiPr i 2 C[H]MeCH 2)}] (3a) with the intermediate benzyl complex not observable. In contrast, when [Th(Tren TIPS)(I)] (2b) was treated with KCH 2 Ph, [Th(Tren TIPS)(CH 2 Ph)] (4) was isolated; which is notable as Tren N-silylalkyl metal alkyls tend to spontaneously cyclometallate. Thermolysis of 4 results in the extrusion of toluene and formation of the cyclometallate [Th {N(CH 2 CH 2 NSiPr i 3) 2 (CH 2 CH 2 NSiPr i 2 C[H]MeCH 2)}] (3b). This reactivity is the reverse of what would be predicted. Since the bonding of thorium is mainly electrostatic it would be predicted to undergo facile cyclometallation, whereas the more covalent uranium system might be expected to form an isolable benzyl intermediate. The thermolysis of 4 follows well-defined first order kinetics with an activation energy of 22.3 AE 0.1 kcal mol À1 , and Eyring analyses yields DH ‡ ¼ 21.7 AE 3.6 kcal mol À1 and DS ‡ ¼ À10.5 AE 3.1 cal K À1 mol À1 , which is consistent with a s-bond metathesis reaction. Computational examination of the reaction profile shows that the inversion of the reactivity trend can be attributed to the greater f-orbital participation of the bonding for uranium facilitating the s-bond metathesis transition state whereas for thorium the transition state is more ionic resulting in an isolable benzyl complex. The activation barriers are computed to be 19.0 and 22.2 kcal mol À1 for the uranium and thorium cases, respectively, and the latter agrees excellently with the experimental value. Reductive decomposition of "[U(Tren TIPS)(CH 2 Ph)]" to [U(Tren TIPS)] and bibenzyl followed by cyclometallation to give 3a with elimination of dihydrogen was found to be endergonic by 4 kcal mol À1 which rules out a redox-based cyclometallation route for uranium.

Triamidoamine uranium(IV)–arsenic complexes containing one-, two- and threefold U–As bonding interactions

“…In an attempt to combine uranium oxidation and potassium abstraction into one step, 4 was treated with AgBPh 4 , but this resulted in the formation of intractable mixtures. When oxidation was attempted with PbI 2 , only the uranium(IV) cyclometallate [U{N(CH 2 CH 2 NSi i Pr 3 ) 2 (CH 2 CH 2 NSi i Pr 2 CHMeCH 2 )}] (6) could be isolated 67 . We note that if the synthesis of 3c is not limited to 2 h and is stirred for two days, the diuranium(IV) triarsenide complex [{U(Tren TIPS )} 2 (μ:η 3 -η 3 -As 3 )][K(B15C5) 2 ] 7 is the only isolable product.…”

Thorium–phosphorus triamidoamine complexes containing Th–P single- and multiple-bond interactions

“…These complexes can be prepared by more than one route using a new thorium(IV) separated ion pair [Th(Tren TIPS )(DME)][BPh 4 ] ( 1 ) or the thorium(IV)-cyclometallate complex [Th{N(CH 2 CH 2 NSiPr i 3 ) 2 (CH 2 CH 2 NSiPr i 2 C(H)MeCH 2 )}] ( 4 ) (ref. 66). Quantum chemical calculations reveal that the Th–P bonds in this study are more ionic than the U–P analogues, where comparisons are available, and in some instances show surprising levels of 7s- as well as 6d- and 5f-orbital participation in these Th–P bonds.…”

Small Molecule Activation Mediated by a Thorium Terminal Imido Metallocene

“…Nevertheless, more recent studies indicate that 5f orbitals play a significant part in the bonding of organothorium compounds, thus leading to a distinctively different reactivity from that of group 4 complexes. 7 In the course of our investigations in this area, we have recently prepared a base-free terminal thorium imido metallocene [η 5 -1,2,4-(Me 3 C) 3 C 5 H 2 ] 2 ThN(p-tolyl) (1). 8−10 Gratifyingly, complex 1 shows a rich reaction chemistry including the activation of element sulfur, 9 Si−H bonds, 11 and NN bonds in diazoalkanes.…”