Synthesis and molecular structures of divalent bridged bis(guanidinate) europium complexes and their application in intermolecular hydrophosphination of alkenes and alkynes

Abstract

The reaction of anhydrous EuCl₃ with one equiv. of lithium salt of a three-carbon bridged bis(guanidinate) Li₂L¹ (L¹ = [ⁱPr(Me₃Si)NC(NⁱPr)N(CH₂)₃NC(NⁱPr)N(SiMe₃)ⁱPr]) in THF afforded chloride Eu^IIIL¹Cl(THF)₂ (1). The reduction reaction of complex 1 with Na/K alloy in a molar ratio of 1 : 1.2 in THF gave a novel Eu^II complex supported by an unexpected new bridged bis(guanidinate) ligand L³, [Eu^IIL³]₂ (L³ = [ⁱPr(Me₃Si)NC(NⁱPr)N(CH₂)₃N(SiMe₃)C(NⁱPr)₂]) (2), through the redistribution of one guanidinate in L¹ during the reduction. Complex 2 was structurally characterized to be a binuclear complex in which two Eu metals are connected together by two L³ ligands that adopted a μ-η¹:η²:η² coordination mode for one L³ ligand and a μ-η²:η²:η² mode for the other. Treatment of the in situ formed Eu^IIIL²Cl(THF)₂ (L² = 1,8-C₁₀H₆{NC(NⁱPr)(NHⁱPr)}₂) by the reaction of EuCl₃ with 0.5 equiv. of [Li₂L²₂Li₂] in THF with Na/K alloy yielded a novel Eu^II complex [Eu^IIL²(THF)]₂ (3) in good yield. Complex 3 was characterized by an X-ray crystal structure analysis. Complex 3 features an unusual μ-η¹:η²:η² coordination mode of the bridged bis(guanidinate) ligand onto Eu^II. Complexes 2 and 3 are efficient pre-catalysts for the intermolecular hydrophosphination of alkenes and alkynes to give exclusively anti-Markovnikov products and mainly anti-addition products for the alkyne reactions. For these transformations, the best performances were observed with complex 2.