The synthesis, structure, and reactivity of an actinide metallacyclopropene were comprehensively studied. The reduction of [η(5)-1,2,4-(Me3C)3C5H2]2ThCl2 (1) with potassium graphite (KC8) in the presence of diphenylacetylene (PhC≡CPh) yields the first stable actinide metallacyclopropene [η(5)-1,2,4-(Me3C)3C5H2]2Th(η(2)-C2Ph2) (2). The magnetic susceptibility data show that 2 is indeed a diamagnetic Th(IV) complex, and density functional theory (DFT) studies suggest that the 5f orbitals contribute to the bonding of the metallacyclopropene Th-(η(2)-C═C) moiety. Complex 2 shows no reactivity toward alkynes, but it reacts with a variety of heterounsaturated molecules such as aldehyde, ketone, carbodiimide, nitrile, organic azide, and diazoalkane derivatives. DFT studies complement the experimental observations and provide additional insights. Furthermore, a comparison between Th and group 4 metals reveals that Th(4+) shows unique reactivity patterns.

Reduction of (η 5 -C 5 Me 5 ) 2 ThCl 2 (1) with potassium graphite (KC 8 ) in the presence of 2,2′-bipyridine forms the thorium bipy metallocene (η 5 -C 5 Me 5 ) 2 Th-(bipy) (2) in good yield. Complex 2 was fully characterized and reacts with various small molecules. For example, 2 serves as a source for the (η 5 -C 5 Me 5 ) 2 Th(II) fragment when exposed to conjugated alkynes, elemental sulfur and their organic derivatives, diazabutadiene, carbodiimide, CS 2 , isothiocyanate, and organic azides. Furthermore, treatment of 2 with ketone Ph 2 CO, thio-ketone Ph 2 CS, imine PhCHNPh, and nitrile PhCN results in C−C bond coupling products (η 5 -C 5 Me 5 ) 2 Th[(bipy)(Ph 2 CO)] (10), (η 5 -C 5 Me 5 ) 2 Th[(bipy)(Ph 2 CS)] (11), (η 5 -C 5 Me 5 ) 2 Th[(bipy)(PhCHNPh)] (12), and (η 5 -C 5 Me 5 ) 2 Th[(bipy)(PhCN)] (13), respectively, in quantitative conversion.

The reaction of the base-free terminal thorium imido complex [{η(5)-1,2,4-(Me3C)3C5H2}2Th=N(p-tolyl)] (1) with p-azidotoluene yielded irreversibly the tetraazametallacyclopentene [{η(5)-1,2,4-(Me3C)3C5H2}2Th{N(p-tolyl)N=N-N(p-tolyl)}] (2), whereas the bridging imido complex [{[η(5)-1,2,4-(Me3C)3C5H2]Th(N3)2}2{μ-N(p-tolyl)}2][(n-C4H9)4N]2 (3) was isolated from the reaction of 1 with [(n-C4H9)4N]N3. Unexpectedly, upon the treatment of 1 with 9-diazofluorene, the NN bond was cleaved, an N atom was transferred, and the η(2)-diazenido iminato complex [{η(5)-1,2,4-(Me3C)3C5H2}2Th{η(2)-[N=N(p-tolyl)]}{N=(9-C13H8)}] (4) was formed. In contrast, the reaction of 1 with Me3SiCHN2 gave the nitrilimido complex [{η(5)-1,2,4-(Me3C)3C5H2}2Th{NH(p-tolyl)}{N2CSiMe3}] (5), which slowly converted into [{η(5)-1,2,4-(Me3C)3C5H2}{η(5):κ-N-1,2-(Me3C)2-4-CMe2(CH2NN=CHSiMe3)C5H2}Th{NH(p-tolyl)}] (6) by intramolecular C-H bond activation. The experimental results are complemented by density functional theory (DFT) studies.

Reduction of (η 5 -C 5 Me 5 ) 2 ThCl 2 (1) with potassium graphite (KC 8 ) in the presence of 1,4-diphenylbutadiyne (PhCCCCPh) yields the first actinide metallacyclocumulene, the thorium metallacyclopentatriene (η 5 -C 5 Me 5 ) 2 Th(η 4 -C 4 Ph 2 ) (2). The structure and reactivity of 2 were investigated in detail; structural parameters and density functional theory (DFT) studies confirm the presence of a metallacyclopentatriene unit in 2. Furthermore, DFT computations also indicate a notable contribution of the 5f orbitals to the bonding of the metallacyclopentatriene Th−(η 4 -CCCC) moiety. While complex 2 shows no reactivity toward alkynes, it reacts with a variety of heterounsaturated molecules such as isothiocyanates, carbodiimides, aldehydes, ketones, nitriles, pyridines, and diazoalkane derivatives. DFT studies complement the experimental observations and provide additional insights. Furthermore, in comparison to group 4 metals, the thorium metallacyclopentatriene 2 exhibits distinctively different reactivity patterns.

The synthesis, electronic structure, and reactivity of a uranium metallacyclocumulene were studied. Reduction of [(η 5 -C 5 Me 5 ) 2 UCl 2 ] (1) with potassium graphite (KC 8 ) in the presence of 1,4-bis(trimethylsilyl)butadiyne (Me 3 SiCC−C CSiMe 3 ) forms the uranium metallacyclocumulene [(η 5 -C 5 Me 5 ) 2 U{η 4 -C 4 (SiMe 3 ) 2 }] (2) in good yield. Magnetic susceptibility data confirm that 2 behaves as a U(IV) complex, and density functional theory (DFT) studies indicate a substantial 5f orbital contribution to the bonding of the metallacyclopentatriene U(η 4 -CCCC) moiety, leading to more covalent bonds between the [(η 5 -C 5 Me 5 ) 2 U] 2+ and [η 4 -C 4 (SiMe 3 ) 2 ] 2− fragments than those found in the related Th(IV) compound. Consequently, very different reactivity patterns emerge; e.g., 2 can act as a synthetic equivalent for the (η 5 -C 5 Me 5 ) 2 U(II) fragment when reacted with conjugated species such as butadiyne, bipy, and diazabutadiene derivatives. Alternatively, the [(η 4 -Me 3 SiCCCCSiMe 3 )] 2− moiety in 2 may react as a nucleophile when exposed to a variety of simple heterounsaturated molecules such as aldehydes, ketones, nitriles, isothiocyanates, carbodiimides, pyridines, and organic azides. DFT studies are included to complement the experimental observations.