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

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{NCHCHNiPr})]), LnCp(L)2 (Ln = Y) 2, and the homoleptic tetrakis(NHC) complex Th(L)44 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[triple bond, length as m-dash]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

Protonolysis and thermolysis reactions of functionalised NHC-carbene boranes and borates

A set of β-ketoimidazolium and β-ketoimidazolinium salts of the general formula [R1C(O)CH2{CH[NCR3CR3N(R2)]}]X (R1 = tBu, naphth; R2 = iPr, Mes, tBu; R3 = H, Me, (H)2; X = Cl, Br) show contrasting reactivity with superhydride bases MHBEt3; two are reduced to chiral β-alcohol carbene–boranes R1CH(OH)CH2{C(BEt3)[NCR3CR3N(R2)]} 2 (R1 = tBu; R2 = iPr, Mes; R3 = H), two with bulky R2 substituents are reduced to chiral β-borate imidazolium salts [R1CH(OBEt3)CH2{CH[NCR3CR3N(R2)]}]X 3 (R1 = tBu, naphth; R2 = Mes, tBu; R3 = H, Me; X = Cl, Br), and the two saturated heterocycle derivatives remain unreduced but form carbene–borane adducts R1C(O)CH2{C(BEt3)[NCR3CR3N(R2)]} 4 (R1 = tBu, naphth; R2 = Mes; R3 = (H)2). Heating solutions of the imidazolium borates 3 results in the elimination of ethane, in the first example of organic borates functioning as Brønsted bases and forming carbene boranes R1CH(OBEt2)CH2{C[NCR3CR3N(R2)]} 5 (R1 = naphth; R2 = Mes; R3 = Me). The ‘abnormal’ carbene borane of the form 2 R1CH(OH)CH2{CH[NC(BEt3)CR3N(R2)]} (R1 = tBu; R2 = tBu; R3 = H), is also accessible by thermolysis of 3, suggesting that the carbene–borane alcohol is a more thermodynamically stable combination than the zwitterionic imidazolium borate. High-temperature thermolysis also can result in complete cleavage of the alcohol arm, eliminating tert-butyloxirane and forming the B–N bound imidazolium borate 7. The strong dependence of reaction products on the steric and electronic properties of each imidazole precursor molecule is discussed