Uranium versus Thorium: A Case Study on a Base-Free Terminal Uranium Imido Metallocene

Abstract

The structure of and bonding in two base-free t e r m i n a l a c t i n i d e i m i d o m e t a l l o c e n e s , [ η 5 -1 , 2 , 4 -(Me 3 C) 3 C 5 H 2 ] 2 An�N(p-tolyl) (An = U (1), Th (1′)) are compared and connected to their individual reactivity. While structurally rather similar, the U(IV) derivative 1 is slightly more sterically crowded. Furthermore, density functional theory (DFT) studies imply that the 5f orbital contribution to the bonding within the individual actinide imido An�N(p-tolyl) moieties is significantly larger for 1 than for 1′, which makes the bonds between the [η 5 -1,2,4-(Me 3 C) 3 C 5 H 2 ] 2 U 2+ and [(p-tolyl)N] 2− fragments more covalent. Therefore, steric and electronic factors impact the reactivity of these imido complexes. For example, complex 1 is inert toward internal alkynes, but it readily forms Lewis base adducts [η 5 -1,2,4-(Me 3 C) 3 C 5 H 2 ] 2 U�N(p-tolyl)(L) (L = OPMe 3 (6), dmap (9), PhCN (14), and 2,6-Me 2 PhNC (17)) with Me 3 PO, 4-dimethylaminopyridine (dmap), nitrile, PhCN, or isonitrile 2,6-Me 2 PhNC. It may also react as a nucleophile or undergo a [2 + 2] cycloaddition with CS 2 , isothiocyanates, thioketones, ketones, lactides, and acyl nitriles, forming the four-or five-membered metallaheteroacycles, terminal sulfido, or oxido complexes, and cyanide amidate complexes, respectively. In contrast, after the addition of aldehyde p-tolylCHO, the tetranuclear complex [η 5 -1,2,4-(Me 3 C) 3 C 5 H 2 ] 4 [OCH(p-tolyl)CH(p-tolyl)O] 2 U 4 O 4 (10) is isolated. However, while 1 is unreactive toward dicyclohexylcarbodiimide (DCC), an equilibrium exists in benzene solution between N,N′-diisopropylcarbodiimide (DIC), 1, and the four-membered metallaheterocycle [η 5 -1,2,4-(Me 3 C) 3 C 5 H 2 ] 2 U[N(p-tolyl)C(�N i Pr)N( i Pr)] (12). Furthermore, 1 may also engage in single-and two-electron transfer processes. It is singly oxidized by Ph 3 CN 3 , CuI, Ph 2 S 2 , and Ph 2 Se 2 , yielding the uranium(V) imido complexes [η 5 -1,2,4-(Me 3 C) 3 C 5 H 2 ] 2 U�N(p-tolyl)(X) (X = N 3 (20), I (22), PhS (23), and PhSe (24)), or is doubly oxidized by organic azides (RN 3 ) and 9-diazofluorene, forming the uranium(VI) bis-imido metallocenes [η 5 -1,2,4-(Me 3 C) 3 C 5 H 2 ] 2 U�N(p-tolyl)(=NR) (R = p-tolyl (18), mesityl (19)) and [η 5 -1,2,4-(Me 3 C) 3 C 5 H 2 ] 2 U=N(p-tolyl)[=NN=(9-C 13 H 8 )] (21), respectively.

Synthesis and Structure of [η5-1,2,4-(Me3Si)3C5H2]2Th(bipy) and Its Reactivity toward Small Molecules

“…The Th–N(1) distance is 2.631(4) Å, consistent with a datively coordinated nitrogen atom, whereas the Th–N(2) distance is 2.351(4) Å. In contrast to the uranium cyanide complexes (Cp 3 t Bu ) 2 U­(OSiMe 3 )­(CN), (Cp 3 t Bu ) 2 U­[OC­(Ph)­N­( p -tolyl)]­(CN), and (Cp 2 t Bu ) 2 U­[OC­(Ph)­N­( p -tolyl)]­(CN), but analogous to the uranium isocyanide complex (Cp 3tms ) 2 U­(OSiMe 3 )­(NC), and thorium isocyanide complexes [(3-Mes-C 3 H 2 N 2 ) 2 BH 2 ] 2 Th­[4-(Me 3 C)­bipy]­(NC) and (Cp 3 t Bu ) 2 Th­(OSiMe 3 )­(NC), the CN – ligand in 19 coordinates to the Th 4+ ion by its nitrogen atom instead of the carbon atom, due to the higher Lewis acidity of the Th 4+ ion, and the electron deficient nature of the ligand 1,2,4-(Me 3 Si) 3 C 5 H 2 , which also increases the Lewis acidity of the metal ion . The experimentally observed preference for N- over C-coordination is also reflected in DFT computations, predicting the isocyanide linkage isomer to be energetically more favorable than its cyanide counterpart (Cp 3tms ) 2 Th­[4-(Me 3 C)­bipy]­(CN) by Δ G (298 K) = −2.4 kcal/mol (see Supporting Information for details).…”

“…In analogy to its uranium counterpart (Cp 3tms ) 2 U­(bipy) ( 3′ ) (Figure ), we propose that 3 initially reacts with mesitylN 3 to give the imido complex (Cp 3tms ) 2 ThN­(mesityl). However, unlike (Cp 3tms ) 2 UN­(mesityl), the thorium imido complex (Cp 3tms ) 2 ThN­(mesityl) is unstable and activates the C–H bond of a methyl group on a trimethylsilyl substituent to yield an amido complex, which can be traced to the more ionic bonding between the [(Cp 3tms ) 2 Th] 2+ and [mesitylN] 2– moieties . Finally, insertion of a second molecule of mesitylN 3 into the Th–CH 2 group of the amido intermediate yields product 5 (Scheme ).…”

Intrinsic Reactivity of [η5-1,3-(Me3C)2C5H3]2Th(bipy) toward Small Molecules

“…Moreover, the reactivity differences between the uranium bipy complex (Cp 2tms ) 2 U­(bipy) forming a terminal imido bipy adduct (Cp 2tms ) 2 U­(NSiMe 3 )­(bipy) (Table ) and 1 yielding an amido pyridyl complex 10 can be rationalized by the bonding within the [Cp 2 An] 2+ and [RN] 2– fragments in the imido intermediates. The ThNR bonds are more ionic than those of UNR, and therefore, deprotonation is more favorable for the (Cp 2 t Bu ) 2 Th­(NSiMe 3 )­(bipy) intermediate, whereas this does not occur in the case of the uranium imido derivative (Cp 2tms ) 2 U­(NSiMe 3 )­(bipy) . The molecular structure of 10 is presented in Figure , and selected bond distances and angles are listed in Table .…”

Ligand–Metal Complementarity in Rare-Earth and Actinide Chemistry

“…They find appreciable covalent character in the U–C aryl bond and were able to excellently reproduce the experimental data, allowing them to describe trends in the 5f contribution to the bonding in these complexes and to compare this to other U VI organometallic complexes . Continuing the theme of actinide-ligand covalency, the groups of Zi, Ding, and Walter reported a comparative tour de force case study between thorium­(IV) and uranium­(IV) imido complexes, [An­(Cp ttt ) 2 (=NC 6 H 4 -4-Me)] (An = Th, U) . In this work, small structural differences, such as the uranium complex being slightly more sterically congested due to increased covalency, and hence tighter ligand binding, and the smaller size of U IV versus Th IV , are used to rationalize a substantial body of divergent reactivity.…”

“…For example, the thorium complex reacts with internal alkynes to give [Th­(Cp ttt ) 2 {N­(C 6 H 4 -4-Me)­C 2 R 2 -κ N , C }] (R = Me, Ph), whereas the uranium analogue is unreactive. Conversely, because the redox chemistry of thorium is limited, the reaction with Ph 2 Se 2 results in the Cp ttt anion acting as a sacrificial reductant, producing a half-sandwich complex product, [Th­(Cp ttt )­(SePh)­{μ-N­(C 6 H 4 -4-Me)}] 2 , whereas the uranium analogue undergoes one-electron oxidation to give [U­(Cp ttt ) 2 (NC 6 H 4 -4-Me)­(SePh)] . Eisen and co-workers continued the theme of small-molecule reactivity with Th IV bent-metallocenes by exploring the reactivity of [Th­(Cp*) 2 (Cl) 2 ] with substituted cyclopropenyl imines, finding an unexpected temperature dependence .…”