Low coordinate metal complexes can exhibit superlative physicochemical properties, but achieving low coordination numbers for the lanthanides (Ln) is problematic due to the tendency to maximize electrostatic contacts in their predominantly ionic bonding regimes. Despite these challenges, formally two-coordinate bent, [Ln{C(SiMe3)3}2] (Ln = Sm, Eu, Yb), and near-linear, [Ln{N(SiiPr3)2}2] (1-Ln; Ln = Sm, Eu, Tm, Yb), Ln2+ complexes have been isolated. However, examples of formally two-coordinate Ln complexes in the most common +3 oxidation state have remained elusive due to the greater charge density and electrostatic forces of Ln3+ centers. Here we report the isolation and characterization of the bent, formally two-coordinate Ln3+ complexes [Ln{N(SiiPr3)2}2][B(C6F5)4] (2-Ln; Ln = Sm, Tm, Yb), which are synthesized by halide abstraction of the heteroleptic three-coordinate precursors [Ln{N(SiiPr3)2}2(X)], (3-Ln; X = Cl, Ln = Sm, Tm; X = F, Ln = Yb) using [H(SiEt3)2][B(C6F5)4]. Complexes 3-Ln were accessed by the oxidation of 1-Ln with either tBuCl or [FeCp2][PF6]; a Eu3+ analogue could not be accessed by adapting these methods. Interrogation of the electronic structures of 2-Ln and 3-Ln by EPR spectroscopy, magnetic studies and ab initio calculations revealed that the bent complex 2-Yb expresses a pseudo-planar rather than a pseudo-axial crystal field in the solid state. This unanticipated result is in opposition to the predicted electronic structure for two-coordinate systems, providing crucial information for the construction of Ln complexes with enhanced magnetic and optical properties through molecular design.

Electronic structures of bent, formally two-coordinate lanthanide(III) cations

2019 Fall National Meeting

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