Deprotonation of N-heterocyclic carbenes to afford heterobimetallic organolanthanide complexes

Amido-tethered N-heterocyclic carbene (NHC) lanthanide complexes, [Ln(L)N"(2)] [L = t-BuNCH2-CH2{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"(2)Ln(L-)K(DME)](2) [Ln = Y (2), Sm (5); L- = the dianion t-BuNCH2-CH2{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"(mu-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(NCSiMe(3)CHNt-Bu)}]. The potassium-reduced NHC complex [K](+)[t-BuNHCH2{CH2C(NCHCHNt-Bu)}](center dot-) (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, H-1 and C-13 NMR spectroscopy, and CHN rnicroanalyses. Compound 3 has been characterized by H-1 and C-13 NMR spectroscopy, and compound 7 by EPR and H-1 NMR spectroscopy.