σ-Donor versus η6-π-Arene Interactions in Monomeric Europium(II) and Ytterbium(II) Thiolates − An Experimental and Computational Study

The base-free, hydrocarbon-soluble complexes M(SAr*)2 (4a: M = Eu, orange; 4b: M = Yb, purple; Ar* = 2,6-Trip2C6H3; Trip = 2,4,6-iPr3C6H2) have been prepared by a protolysis reaction of the Grignard-analogous compound RLnI (R = 2-F3CC6H4) with HSAr*. Compound 4b has been characterized by melting point, elemental analysis, 1H, 13C, and 171Yb NMR, IR spectroscopy, UV/Vis spectrophotometry, mass spectrometry, and X-ray crystallography. Additionally, the purple YbIII complex YbI2(SAr*)(THF)3 (3) and the yellow DME adduct Yb(SAr*)2(dme)2 (5) have been synthesized and characterized by X-ray crystallography. The solid-state structures of 4a and 4b (as THF or benzene hemisolvates) show monomeric units with the metal atom bonded to two terminal thiolate ligands [av. M−S: 281.7 pm (4a)/269.1 pm (4b)]. Additional η6-π-arene interactions are observed to two C6H2 rings of the Trip substituents [av. M···C: 306.5 pm (4a)/297.3 pm (4b)]. The binding energy for these interactions has been determined by variable-temperature 1H NMR spectroscopy to be 54.3 kJ mol−1 (in methylcyclohexane) and 57.1 kJ mol−1 (in toluene). The nature and strength of the η6-π-arene bonding has been studied by ab initio calculations on the model systems [M(arene)]2+ (arene = C6H6, C6F6, 1,3,5-Me3C6H3, 1,3,5-iPr3C6H3) and Yb(SH)2(C6H6)n (n = 1, 2). The results are compared with the binding energies in the σ-donor adducts [M(THF)]2+, [M(dme)]2+, and [M(imidazol-2-ylidene)]2+.