Deprotonation of N-Heterocyclic Carbenes to Afford Heterobimetallic Organolanthanide Complexes

Amido-tethered N-heterocyclic carbene (NHC) lanthanide complexes, [Ln(L)N‘ ‘2] [L = t-BuNCH2CH2{C(NCHCHNt-Bu)}; N‘ ‘ = N(SiMe3)2; Ln = Y (1), Sm (4)], when treated with potassium
naphthalenide, undergo regiospecific deprotonation at the NHC backbone to generate the Ln(III)−K(I)
heterobimetallic complexes [N‘ ‘2Ln(L-)K(DME)]2 [Ln = Y (2), Sm (5); L- = the dianion t-BuNCH2CH2{C(NCCHNt-Bu)}], which are the first examples of NHC ligands that C,C‘-bridge two metals. For
yttrium(III), which has a high reduction potential, it appears that reduction injects an electron into the
heterocycle ring, thus priming the NHC for selective H atom abstraction. However, when complex 4,
which has a much lower and accessible reduction potential, is treated with potassium naphthalenide, a
higher proportion of metal-centered reduction chemistry is observed; only a small percentage of 4 is
reduced at the NHC, giving correspondingly much lower yields of 5. Attempts to isolate the Sm(II)
species observed in the reduction of 4 resulted in isolation of KN‘ ‘ and 4; the putative Sm(II) complex
is apparently unstable with respect to disproportionation and ligand redistribution. Treatment of 1 with
KC8 affords no reaction; treatment of 4 with KC8 affords [Sm(L)N‘ ‘(μ-OCH3)]2 (6), a product of metal-mediated radical C−O activation and cleavage of the DME solvent. Compound 2 reacts quantitatively
with Me3SiCl to eliminate KCl and generate the complex [Ln(L‘)N‘ ‘2] (3) [L‘ = t-BuNCH2CH2{C(NCSiMe3CHNt-Bu)}]. The potassium-reduced NHC complex [K]+[t-BuNHCH2CH2{C(NCHCHNt-Bu)}]•- (7) is also reported; 7 is the first chemically generated, stable NHC radical anion. Compounds
2, 5, and 6 have been characterized by X-ray crystallography, 1H and 13C NMR spectroscopy, and CHN
microanalyses. Compound 3 has been characterized by 1H and 13C NMR spectroscopy, and compound
7 by EPR and 1H NMR spectroscopy.