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

Abstract

Abstract f‐Element organometallic chemistry is dominated by cyclopentadienyl ligands. In contrast, isoelectronic metallole ligands with the general formula [EC 4 R 4 ] 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 [(η 5 ‐Cp Sn )M(η 5 ‐Cp ttt )] 2 ( 1 M  ; M = Y, Gd, Dy; Cp Sn = [SnC 4 ‐2,5‐(SiMe 3 ) 2 –3,4‐Me 2 ] 2− , Cp ttt = [1,2,4‐C 5 t Bu 3 H 2 ] − ), which form by virtue of Sn→M dative bonds. One‐electron reduction of 1 M with KC 8 /2.2.2‐cryptand produces the mono‐anionic complexes [{(η 5 ‐Cp Sn )M(η 5 ‐Cp ttt )} 2 ] − ( 2 M ), and two‐electron reduction gives di‐anionic [{(η 5 ‐Cp Sn )M(η 5 ‐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 {M 2 Sn 2 } 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 (−2 J formalism).