Reduction of Rare-Earth Stannole Sandwich Complexes to Tin-Based Radical Ligands and Tin-Tin Bonds

f-Element organometallic chemistry is dominated by cyclopentadienyl ligands. In contrast, isoelectronic metallole ligands with the general formula [EC₄R₄]^2-, where E is a heavier group 14 element, are rare in the f-block, particularly stannole ligands. Here, we describe the synthesis of the dimetallic stannole complexes [(η⁵-Cp^Sn)M(η⁵-Cp^ttt)]₂ (1_M ; M = Y, Gd, Dy; Cp^Sn = [SnC₄-2,5-(SiMe₃)₂-3,4-Me₂]^2-, Cp^ttt = [1,2,4-C₅ ^tBu₃H₂]^-), which form by virtue of Sn→M dative bonds. One-electron reduction of 1_M with KC₈/2.2.2-cryptand produces the mono-anionic complexes [{(η⁵-Cp^Sn)M(η⁵-Cp^ttt)}₂]^- (2_M), and two-electron reduction gives di-anionic [{(η⁵-Cp^Sn)M(η⁵-Cp^ttt)}₂]^2- (3_M) as [K(2.2.2-crypt)]⁺ salts. Studies of the stannole complexes using crystallography, UV/vis and EPR spectroscopy, magnetometry and computational methods reveal that the reduction steps generate tin-tin bonds through population of a delocalized molecular orbital that spans the {M₂Sn₂} rings, with attendant dearomatization of the stannole rings. Complexes 2_M are the first tin-radical ligands bound to rare earth elements. Spin density calculations of 2_Y and 2_Gd reveal significant build-up of unpaired spin on the tin atoms, with magnetic measurements on 2_Gd yielding an unprecedentedly large tin-gadolinium exchange coupling constant of -112 cm^-1 (-2J formalism).