<i>s</i>-Block Metal-Lanthanide Bonding: Direct Comparison of Mg–Yb and Mg–Ca Complexes
Sandeep Kumar Thakur, Nil Roig, Johannes Maurer, Jens Langer, Mercedes Alonso, Sjoerd Harder
Abstract
Despite considerable progress in understanding the fundamental aspects of metal–metal bonding, lanthanide-metal bonding is limited to interactions between lanthanide (Ln) metals and electron-rich p - or d -block metals. Exemplified by a Mg–Yb complex, we present synthesis and extensive computational analyses of the first s -block metal-Ln bond. Key to its formation is the unique Mg 0 complex [(BDI*)MgNa] 2 ( 1 ) which can be viewed as a dimer consisting of two magnesyl anions, (BDI*)Mg –, bridged by two Na + cations (BDI* = HC[C( t Bu)N-DIPeP] 2, DIPeP = 2,6-CHEt 2 -phenyl). Reaction of dimeric 1 with (YbN’’ 2 ) 2 (N’’ = N(SiMe 3 ) 2 ) quantitatively gave the mixed complex [(BDI*)MgNa/YbN’’ 2 ] ( 2 -Yb) in which (BDI*)Mg – anions bridge between Yb 2+ and Na + cations (Mg–Yb: 3.466(1), Mg–Na: 3.415(1) Å). Atoms-In-Molecules shows Mg–Yb and Mg–Na bond paths corresponding to weak electrostatic interactions. Complex 2 -Yb is isostructural to previously reported [(BDI*)MgNa/CaN’’ 2 ] ( 2 -Ca). Weak Mg 0 -M 2+ (M = Ca, Yb) bonding is supported by facile exchange reactions with (SrN’’ 2 ) 2 leading to formation of [(BDI*)MgNa/SrN’’ 2 ] ( 2 -Sr). Energy decomposition analysis (EDA-NOCV) confirmed major electrostatic Mg 0 -M 2+ bonding (63–66%), while orbital interactions are less significant (19–20%). Both, 2 -Ca and 2 -Yb show considerable Mg 0 → M 2+ electron transfer leading to oxidation-state ambiguities (Mg 0 -M +II ↔ Mg +I -M +I ). Effective-Oxidation-State calculations assign the following metal oxidation states, Na +I -Mg 0 -M +II, with a reliability index of 69–72%. Despite structural and electronic similarities, there are differences in reactivities. Complex 2 -Ca reacts only as a 2-electron reducing agent whereas 2 -Yb can deliver up to three electrons. This study includes reactions with N 2 O, TEMPO, and Me 3 PX (X = O, S, Se). While only 2 -Ca could be converted in a complex with incorporated O 2–, complexes with S 2– or Se 2– have been isolated for both.