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Themechanisms of CO2insertion into the U–Nbond of a silylamido NHC (N-heterocyclic carbene) U(III) complex wereinvestigated theoretically. An earlier reported mechanism involvinga reversible NHC carboxylation was found to be energetically too demanding,and a novel four-step mechanism featuring a direct CO2insertioninto the U–N bond is proposed. [ABSTRACT FROM AUTHOR]

The thorium bipyridyl metallocene (Cp 3 t Bu ) 2 Th(bipy) ( 1 ; Cp 3 t Bu = η 5 -1,2,4-(Me 3 C) 3 C 5 H 2 ) shows a rich reactivity toward a series of small molecules. For example, complex 1 may act as a synthon for the (Cp 3 t Bu ) 2 Th(II) fragment as illustrated by its reactivity toward to CuI, hydrazine derivative (PhNH) 2 , Ph 2 E 2 ( E = S, Se), elemental sulfur (S 8 ) and selenium (Se), organic azides, CS 2 , and isothiocyanates. Moreover, in the presence of polar multiple bonds, such as those in ketones Ph 2 CO and (CH 2 ) 5 CO, aldehydes p -MePhCHO and p -ClPhCHO, seleno-ketone ( p -MeOPh) 2 CSe, nitriles PhCN, Ph 2 CHCN, C 6 H 11 CN, and p -(NC) 2 Ph, and benzoyl cyanide PhCOCN, C-C coupling occurs to furnish (Cp 3 t Bu ) 2 Th[(bipy)(Ph 2 CO)] ( 10 ), (Cp 3 t Bu ) 2 Th[(bipy)((CH 2 ) 5 CO)] ( 11 ), (Cp 3 t Bu ) 2 Th[(bipy)( p -MePhCHO)] ( 12 ), (Cp 3 t Bu ) 2 Th[(bipy)( p -ClPhCHO)] ( 13 ), (Cp 3 t Bu ) 2 Th[(bipy){( p -MeOPh) 2 CSe}] ( 14 ), (Cp 3 t Bu ) 2 Th[(bipy)(PhCN)] ( 16 ), (Cp 3 t Bu ) 2 Th[(bipy)(Ph 2 CHCN)] ( 17 ), (Cp 3 t Bu ) 2 Th[(bipy)(C 6 H 11 CN)] ( 18 ), [(Cp 3 t Bu ) 2 Th] 2 {μ-(bipy)[ p -Ph(CN) 2 ](bipy)} ( 20 ), and (Cp 3 t Bu ) 2 Th{(bipy)[PhC(CN)O]} ( 21 ), respectively. Nevertheless, ketazine (PhCH═N) 2 or benzyl nitrile PhCH 2 CN forms the dimeric complexes [(Cp 3 t Bu )Th] 2 [μ-NC(Ph)(bipy)] 2 ( 15 ) and (Cp 3 t Bu ) 2 Th[(bipy){C(═CHPh)NH}] ( 19 ), respectively. In contrast, C-N bond cleavage and C-C coupling processes occur upon addition of isonitriles Me 3 CNC and C 6 H 11 NC to 1 to yield the thorium isocyanido amido complexes (Cp 3 t Bu ) 2 Th[4-(Me 3 C)bipy](NC) ( 22 ) and (Cp 3 t Bu ) 2 Th[4-(C 6 H 11 )bipy](NC) ( 23 ), respectively. Furthermore, a single-electron transfer (SET) process ensues when 1 equiv of CuI is added to 1 to yield the Th(VI) bipyridyl iodide complex (Cp 3 t Bu ) 2 Th(I)(bipy) ( 3 ).

Halide exchange of (Cp 3tms ) 2 ThCl 2 ( 1 ; Cp 3tms = η 5 -1,2,4-(Me 3 Si) 3 C 5 H 2 ) with Me 3 SiI furnishes (Cp 3tms ) 2 ThI 2 ( 2 ), which is then reduced with potassium graphite (KC 8 ) in the presence of 2,2'-bipyridine to give the thorium bipyridyl metallocene (Cp 3tms ) 2 Th(bipy) ( 3 ) in good yield. Complex 3 was fully characterized and readily reacted with various small molecules. For example, 3 may serve as a synthetic equivalent for the (Cp 3tms ) 2 Th(II) fragment when exposed to CuI, Ph 2 S 2 , organic azides, and CS 2 . Moreover, upon the addition of thiobenzophenone Ph 2 CS, p -methylbenzaldehyde ( p -MeC 6 H 4 )CHO, benzophenone Ph 2 CO, amidate PhCONH( p -tolyl), seleno-ketone ( p , p '-dimethoxy), selenobenzophenone ( p -MeOPh) 2 CSe, di( p -tolyl)methanimine ( p -tolyl) 2 C═NH, 1,2-di(benzylidene)hydrazine (PhCH═N) 2 , and nitriles PhCN, PhCH 2 CN, and Ph 2 CHCN C-C coupling results to give (Cp 3tms ) 2 Th[(bipy)(Ph 2 CS)] ( 8 ), (Cp 3tms ) 2 Th[(bipy)( p -MePhCHO)] ( 9 ), (Cp 3tms ) 2 Th[(bipy)(Ph 2 CO)] ( 10 ), (Cp 3tms ) 2 Th[(bipy){( p -tolylNH)(Ph)CO}] ( 11 ), (Cp 3tms ) 2 Th[(bipy){( p -MeOPh) 2 CSe}] ( 12 ), (Cp 3tms ) 2 Th[(bipy){( p -tolyl) 2 CNH}] ( 13 ), (Cp 3tms ) 2 Th[(bipy)(PhCHNN═CHPh)] ( 14 ), (Cp 3tms ) 2 Th[(bipy)(PhCN)] ( 16 ), (Cp 3tms ) 2 Th[(bipy)(PhCH 2 CN)] ( 17 ), and (Cp 3tms ) 2 Th[(bipy)(Ph 2 CHCN)] ( 18 ), respectively. However, when thiazole is added to 3 , the dimeric sulfido complex [(Cp 3tms ) 2 Th] 2 [μ-(bipy)CH 2 NCHCHS] 2 ( 15 ) can be isolated. Moreover, the addition of isonitriles such as Me 3 CNC and PhCH 2 NC to 3 results in C-N bond cleavage and C-C coupling processes to form the thorium isocyanido amido complexes (Cp 3tms ) 2 Th[4-(Me 3 C)bipy](NC) ( 19 ) and (Cp 3tms ) 2 Th[4-(PhCH 2 )bipy](NC) ( 20 ), respectively. Nevertheless, upon exposure of 3 to (trimethylsilyl)diazomethane Me 3 SiCHN 2 , the bis-amido complex (Cp 3tms ) 2 Th[5,6-(Me 3 SiCH)bipy] ( 21 ), concomitant with N 2 release, is isolated.

A simple and efficient two-step method for the construction of novel 2,4,9a-trisubstituted-4a,9a-dihydroindeno[2,1- d ][1,3]oxazin-9-ones has been developed. The NHC-catalyzed aza-benzoin reaction of o -alkenyl benzaldehydes with N -acylarylimines afforded 1-( o -alkenylaryl)-2-amido-2-aryl-1-ethanones, which underwent regioselective 5-exo-trig radical cyclization to furnish the three-ring-fused heterocyclic products, generally in good yields. The synthetic method displayed good tolerance toward the nature of substituents, substitution pattern, and steric hindrance of o -alkenyl benzaldehydes. Based on this method, the synthesis of unprecedented dihydrobenzo[6,7]indeno[2,1- d ][1,3]oxazin-7-ones and dihydropyrido[2',3':3,4]cyclopenta[1,2- d ][1,3]oxazin-9-ones has been achieved by employing o -alkenylnaphthaldehyde and o -alkenylnicotinaldehyde as substrates. The regioselectivity between 5- exo-trig and 6- endo-trig radical cyclization reactions of different 1-( o -alkenylaryl)-2-amido-2-aryl-1-ethanones were elucidated with DFT calculations.

The functionalization of CO 2 mediated by a series of U(IV) mixed-sandwich compounds, (COT TIPS2 )Cp*UR (R = -CH 3 , -CH 2 Ph, -CH 2 TMS, -CH(TMS) 2 , -NHPh, -OPh, -SPh, -SePh; COT TIPS2 = C 8 H 6 (Si i Pr 3 -1,4) 2 ; Cp* = C 5 Me 5 ; TMS = SiMe 3 ), was investigated by the density functional theory method. A two-step mechanism was revealed, in which the insertion of CO 2 into the U-C bond was identified as the rate-determining step via a transition state featured by a four-membered ring with a free-energy barrier of 18.8 kcal/mol to the reaction of the (COT TIPS2 )Cp*UCH 3 system. The whole reaction was strongly exothermic by 45.0 kcal/mol. Substitution effect was discussed, including the bulkiness of the R group and the nature of the ligating atom, and steric hindrance and electrostatic interactions were found to be responsible for the observed variation in reactivity. The reactivity of U(III) and U(IV) complexes in CO 2 functionalization was also compared and discussed. The results were consistent with experimental studies and complemented with molecular level of understanding on the mechanisms of CO 2 functionalization promoted by tetravalent U complexes.

The synthesis, structure, and reactivity of a uranium metallacyclopropene were comprehensively studied. Reduction of (η(5)-C5Me5)2UCl2 (1) with potassium graphite (KC8) in the presence of bis(trimethylsilyl)acetylene (Me3SiC≡CSiMe3) allows the first stable uranium metallacyclopropene (η(5)-C5Me5)2U[η(2)-C2(SiMe3)2] (2) to be isolated. Magnetic susceptibility data confirm that 2 is a U(IV) complex, and density functional theory (DFT) studies indicate substantial 5f orbital contributions to the bonding of the metallacyclopropene U-(η(2)-C═C) moiety, leading to more covalent bonds between the (η(5)-C5Me5)2U(2+) and [η(2)-C2(SiMe3)2](2-) fragments than those in the related Th(IV) compound. Consequently, very different reactivity patterns emerge, e.g., 2 can act as a source for the (η(5)-C5Me5)2U(II) fragment when reacted with alkynes and a variety of heterounsaturated molecules such as imines, bipy, carbodiimide, organic azides, hydrazine, and azo derivatives.

We report our recent DFT mechanistic study on the functionalization of CO2 and CS2 promoted by a trivalent uranium complex (Tp*)2UCH2Ph. In the calculations, the uranium atom is described by a quasi-relativistic 5f-in-core ECP basis set (LPP) developed for the trivalent uranium cation, which was qualified by the calculations with a quasi-relativistic small core ECP basis set (SPP) for uranium. According to our calculations, the functionalization proceeds in a stepwise manner, and the CO2 or CS2 does not interact with the central uranium atom to form a stable complex prior to the reaction due to the steric hindrance from the bulky ligands but directly cleaves the U-C (benzyl) bond by forming a C-C covalent bond. The released coordination site of uranium is concomitantly occupied by one chalcogen atom of the incoming molecule and gives an intermediate with the uranium atom interacting with the functionalized CO2 or CS2 in an η(1) fasion. This step is followed by a reorientation of the (dithio)carboxylate side chain of the newly formed PhCH2CE2(-) (E = O, S) ligand to give the corresponding product. Energetically, the first step is characterized as the rate-determining step with a barrier of 9.5 (CO2) or 25.0 (CS2) kcal/mol, and during the reaction, the chalcogen atoms are reduced, while the methylene of the benzyl group is oxidized. Comparison of the results from SPP and LPP calculations indicates that our calculations qualify the use of an LPP treatment of the uranium atom toward a reasonable description of the model systems in the present study.