Homo- and heteroleptic alkoxycarbene f-element complexes and their reactivity towards acidic N–H and C–H bonds

Abstract

The reactivity of a series of organometallic rare earth and actinide complexes with hemilabile NHC-ligands towards substrates with acidic C-H and N-H bonds is described. The synthesis, characterisation and X-ray structures of the new heteroleptic mono- and bis(NHC) cyclopentadienyl complexes LnCp2(L) 1 (Ln = Sc, Y, Ce; L = alkoxy-tethered carbene [OCMe2CH2(1-C{NCHCHN(i)Pr})]), LnCp(L)2 (Ln = Y) , and the homoleptic tetrakis(NHC) complex Th(L)4 4 are described. The reactivity of these complexes, and of the homoleptic complexes Ln(L)3 (Ln = Sc 3, Ce), with E-H substrates is described, where EH = pyrrole C4H4NH, indole C8H6NH, diphenylacetone Ph2CC(O)Me, terminal alkynes RC≡CH (R = Me3Si, Ph), and cyclopentadiene C5H6. Complex 1-Y heterolytically cleaves and adds pyrrole and indole N-H across the metal carbene bond, whereas 1-Ce does not, although 3 and 4 form H-bonded adducts. Complexes 1-Y and 1-Sc form adducts with CpH without cleaving the acidic C-H bond, 1-Ce cleaves the Cp-H bond, but 2 reacts to form the very rare H(+)-[C5H5](-)-H(+) motif. Complex 1-Ce cleaves alkyne C-H bonds but the products rearrange upon formation, while complex 1-Y cleaves the C-H bond in diphenylacetone forming a product which rearranges to the Y-O bonded enolate product.

Early/Late Heterobimetallic Tantalum/Rhodium Species Assembled Through a Novel Bifunctional NHC‐OH Ligand

“…The solid‐state structures, shown in Figure , reveal that the alkoxy‐NHC ligand is bound to tantalum in a chelate (O,C) fashion in both species. The ligand C NHC ‐Ta‐O bite angles (75.69(16)° in 3 and 76.74(15)° in 4 ) are relatively tight for six‐membered metallacycles, but compare well with those reported in the literature for related early‐TM complexes . Both complexes adopt a pseudo‐square‐pyramidal geometry ( τ 5 =0.31 for 3 , τ 5 =0.09 for 4 ) with the tert ‐butylimido in 3 and the neopentylidene in 4 in the axial positions, and the alkoxy‐carbene and the two neopentyls in the square‐planar base.…”

The key role of R–NHC coupling (R = C, H, heteroatom) and M–NHC bond cleavage in the evolution of M/NHC complexes and formation of catalytically active species

“…Protonolysis reactions proceed very easily for Li/NHC 105 , 108 and Mg/NHC 112 complexes. Other complexes, including Ag I /NHC, 191 , 193 , 197 – 199 , 201 Al III /NHC, 119 In III /NHC, 119 Y III /NHC 125 and Ce III /NHC, 125 Mn I /NHC, Zn II /NHC, 213 Mo VI /NHC 130 and Ni II /NHC, 214 are also susceptible to protonolysis; the efficiency depends on the structure of NHC ligands, co-ligands and reaction conditions ( Table 1 column 3). For example, (NHC) 2 NiX 2 complexes (X = Cl, Br, I) suffer facile hydrolysis in aqueous MeCN or THF at 70 °C to give azolium salts and Ni(OH) 2 .…”

CH Bond Activation by f‐Block Complexes

“…We recently showed that C À H activation of a range of substrates with relatively acidic CÀH bonds was possible with cerium and yttrium complexes of N-heterocyclic carbenes (NHCs). [107] (88) in which one of the newly formed cyclopentadienyl ions coordinates to the two imidazolium CÀH groups rather than the Y cation (Scheme 32 c).…”