Steric and Electronic Influences of Internal Alkynes on the Formation of Thorium Metallacycles: A Combined Experimental and Computational Study

Abstract

The formation of thorium metallacyclopentadiene and metallacyclopropene complexes is significantly influenced by the steric and electronic properties of the internal alkyne employed during their syntheses. The reduction of (η 5 -C 5 Me 5 ) 2 ThCl 2 (2) with potassium graphite (KC 8 ) or lithium in the presence of internal phenyl(alkyl)acetylenes ( PhCCR) selectively yields the corresponding C ssymmetric thorium metallacyclopentadienes (η 5 -C 5 Me 5 ) 2 Th[η 2 -C(Ph)C(R)C-(Ph)C(R)] (R = Me (4), i Pr (5), C 6 H 11 (6)), while the phenyl(silyl)acetylene PhCCSiHMe 2 gives the C 2v -symmetric metallacyclopentadiene (η 5 -C 5 Me 5 ) 2 Th[η 2 -C(SiHMe 2 )C(Ph)C(Ph)C(SiHMe 2 )] (7). However, the sterically more encumbered acetylene derivative PhCCSiMe 3 affords the chloro metallacyclopropene complex [(η 5 -C 5 Me 5 ) 2 Th(η 2 -C 2 Ph(SiMe 3 ))(Cl)][Li(EDM) 2 ] (8), whereas Me 3 SiC CSiMe 3 forms the chloro alkenyl complex [(η 5 -C 5 Me 5 ) 2 Th[C(SiMe 3 )CHSiMe 3 ](Cl) (9), in which the chloro metallacyclopropene intermolecularly activates a C−H bond of the solvent (C 7 H 8 ). Density functional theory (DFT) studies complement the experimental findings and rationalize the selectivity observed in the C−C bond formation.

Influence of the 5f Orbitals on the Bonding and Reactivity in Organoactinides: Experimental and Computational Studies on a Uranium Metallacyclopropene

“…However, in the hypothetical thorium complex (η 5 -C 5 Me 5 ) 2 Th­[η 2 -C 2 (SiMe 3 ) 2 ] ( 2 ′), the metal contribution to the bonding of the Th­[η 2 -C 2 (SiMe 3 ) 2 ] moiety decreases notably (18.2% Th for Th–C σ bond and 7.8% Th for Th–(CC) π bond) (Table ), suggesting a more polarized and therefore more ionic bond between (η 5 -C 5 Me 5 ) 2 Th and alkyne fragment [η 2 -C 2 (SiMe 3 ) 2 ], and the π-donation from the π-MO of the coordinated alkyne to the metal atom is significantly less pronounced (resonance structure B in Figure ). A direct comparison of the bonding in 2 to that of the hypothetical thorium complex (η 5 -C 5 Me 5 ) 2 Th­[η 2 -C 2 (SiMe 3 ) 2 ] ( 2 ′) shows that the 5f orbital contribution to the U–C σ (36.0%) and U–(CC) π (51.5%) bonds in 2 is substantially larger than that of the 5f orbitals in 2 ′ (12.4% for Th–C σ bond and 27.7% for Th–(CC) π bond), which is consistent with the previously investigated systems. ,, This difference reflects itself in the reactivity of the uranium complex 2 when compared to the thorium metallacyclopropenes. , …”

“…This is consistent with a strong coordination of the bis­(trimethylsilyl)­acetylene moiety to the uranium atom, and therefore the metallacyclopropene fragment stays intact in toluene solution even at high temperatures. However, in contrast to the thorium metallacyclopropenes, , the coordinated bis­(trimethylsilyl)­acetylene in 2 can be exchanged with internal alkynes. For example, addition of diphenylacetylene (PhCCPh) at room temperature gives the metallacyclopentadiene complex (η 5 -C 5 Me 5 ) 2 U­(η 2 -C 4 Ph 4 ) ( 3 ) − in quantitative conversion (Scheme ).…”

“…Density functional theory (DFT) shows that 5f orbitals contribute substantially to the σ- and π-bonds of the U-(η 2 -CC) moiety and that the bonds between the (η 5 -C 5 Me 5 ) 2 U 2+ and [η 2 -C 2 (SiMe 3 ) 2 ] 2– fragments are more covalent than those of the related thorium metallacyclopropene complex. Although the coordinated alkyne in the thorium metallacyclopropenes is inert to alkyne exchange, it reacts as a nucleophile toward heterounsaturated molecules or as a strong base inducing the inter- or intramolecular C–H bond activations. , In contrast, uranium complex 2 serves as a synthetically useful Cp* 2 U­(II) synthon, as illustrated by its reaction with unsaturated molecules such as alkynes, imines, bipy, carbodiimide, organic azides, hydrazine, and azo derivatives in which the coordinated alkyne was readily replaced during the course of the reaction. This relates complex 2 to group 4 metallacyclopropenes. , Further investigations on the intrinsic reactivity of actinide metallacyclopropenes and uranium metallacyclocumulene complex 6 are ongoing and will be reported in due course.…”

Uranium versus Thorium: Synthesis and Reactivity of [η5‐1,2,4‐(Me3C)3C5H2]2U[η2‐C2Ph2]

“…The molecular structure of 2 is shown in Figure 1, and selected bond lengths and angles are listed in Table 1. The relevant C(41)−C(42) distance of 1.33(2) Å agrees with the value found for a typical double bond (1.331 Å) [6a] and is essentially identical to those found in the uranium metallacyclopropene ( η 5 ‐C 5 Me 5 ) 2 U[ η 2 ‐C 2 (SiMe 3 ) 2 ] (1.338(11) Å) [6f] and the thorium metallacyclopropenes [ η 5 ‐1,2,4‐(Me 3 C) 3 C 5 H 2 ] 2 Th( η 2 ‐C 2 Ph 2 ) (1.343(4) Å) [6a] and [( η 5 ‐C 5 Me 5 ) 2 Th( η 2 ‐C 2 Ph(SiMe 3 ))(Cl)][Li{MeO(CH 2 CH 2 O) 2 Me} 2 ] (1.360(7) Å), [6e] indicating a doubly reduced alkyne ligand, [ η 2 ‐C 2 Ph 2 ] 2− . The angle (33.2(6)°) of C(41)‐U(1)‐C(42) also parallels that in the uranium metallacyclopropene ( η 5 ‐C 5 Me 5 ) 2 U[ η 2 ‐C 2 (SiMe 3 ) 2 ] (33.3(3)°) [6f] and the C‐Th‐C angle (32.6(1)°) in the related thorium metallacyclopropene [ η 5 ‐1,2,4‐(Me 3 C) 3 C 5 H 2 ] 2 Th( η 2 ‐C 2 Ph 2 ) [6a] .…”

Insertion Chemistry of Lutetacyclopropene toward Unsaturated C−O/C−N Bonds

“…Studies on the synthesis and reactivity of metallacyclopropenes, which act as key intermediates in organic synthesis and catalytic reactions, are of great interest in organometallic and coordination chemistry. [1] As a consequence, the metallacyclopropenes of early transition metals, [2,3] main group elements, [4] and actinides [5] have been extensively studied in the last decades. Reacitivity investigation indicates these metallacyclopropenes can often insert one unsaturated CÀ N or CÀ O bond to give the corresponding five-membered metallacycles (Scheme 1a), while the coupling of one metallacyclepropene and two molecules of unsaturated substrates is rarely observed.…”