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

The reaction of anhydrous EuCl3 with one equiv. of lithium salt of a three-carbon bridged bis(guanidinate) Li2L1 (L-1 = [Pr-i(Me3Si)NC((NPr)-Pr-i)N(CH2)(3)NC((NPr)-Pr-i)N(SiMe3)Pr-i]) in THF afforded chloride (EuLCl)-L-III-Cl-1(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 L-3, [(EuL3)-L-II](2) (L-3 = [Pr-i(Me3Si)NC((NPr)-Pr-i)N(CH2)(3)N(SiMe3)C((NPr)-Pr-i)(2)]) (2), through the redistribution of one guanidinate in L-1 during the reduction. Complex 2 was structurally characterized to be a binuclear complex in which two Eu metals are connected together by two L-3 ligands that adopted a mu-eta(1):eta(2):eta(2) coordination mode for one L-3 ligand and mode for the other. Treatment of the in situ formed (EuLCl)-L-III-Cl-2(THF)(2) (L-2 = 1,8-C10H6(NC((NPr)-Pr-i)((NHPr)-Pr-i)}(2)) by the reaction of EuCl3 with 0.5 equiv. of [(Li2L2Li2)-Li-2] in THF with Na/K alloy yielded a novel Eult complex [(EuL2)-L-II(THF)(2) (3) in good yield. Complex 3 was characterized by an X-ray crystal structure analysis. Complex 3 features an unusual mu-eta(1):eta(2):eta(2) 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.