New complexes of divalent thulium with substituted phospholyl and cyclopentadienyl ligands

In order to try to stabilise the Tm^II ion with phospholyl ligands lacking methyl groups at the 3 and 4 positions, the following ligand precursors were prepared: sodium 2,5-di-tert-butylphospholyl [Na(Htp)] (3) and sodium 2,5-bis(trimethylsilyl)phospholyl [Na(Hsp)] (8), Because of its similar steric bulk, we also decided to use sodium di-tert-butylcyclopentadienyl [NaCp^tt] (9) as a ligand precursor. Compound 3 was obtained by sodium cleavage of the P-P bond of 2,5,2′,5′-tetra-tert-butyl-1,1′- diphosphole (2). Halogen-lithium exchange in 1,4-diiodo-1,4-bis(trimethylsilyl) buta-1,3-diene (4) followed by reaction with PhPCl₂ afforded 1-phenyl-2,5-bis(trimethylsilyl)phosphole (5). Treatment of 5 with lithium metal followed by oxidative treatment with iodine gave 2,5,2′,5′- tetrakis(trimethylsilyl)-1,1′-diphosphole (7) which could be transformed into 8 by treatment with sodium. Treatment of 3, 8 and 9 with [TmI ₂(THF)₃] in diethylether yielded three new organothulium(II) complexes [Tm(Htp)₂(THF)] (10), [Tm(Hsp) ₂(THF)] (11) and [Tm(Cp^tt)₂(THF)] (12), respectively, which could be isolated pure as crystalline solids in low to moderate yields. Complexes 10, 11 and 12 were studied by X-ray crystallography and their structures compared with those of two previously described complexes, [Tm(Cp″)₂(THF)] 1 [Cp″ = 1,3-bis(trimethylsilyl) cyclopentadienyl] and [Tm(Dtp)₂(THF)] (13) (Dtp = 3,4-dimethyl-2,5-di-tert-butylphospholyl). Although the phospholyl ligands may be slightly more bulky than their cyclopentadienyl counterparts, the structures of 1, 10, 11 and 12 are remarkably similar and are substantially different from that of the more bulky 13.