Cyclopentadienyl lanthanide borohydrides derived from the unsubstituted cyclopentadienyl ligand. Unprecedented structural diversity and ε-caprolactone polymerization

• The systematic study of the series of cyclopentadienyl borohydride Ln 3+ complexes. • Five different structural types of cyclopentadienyl Neodymium borohydride complexes. • Different complex cyclopentadienylborohydride lanthanide anions. • Neodymium cyclopentadienylborohydrides initiate ε-caprolactone polymerization. A series of mono-, bis- and tris(cyclopentadienyl) complexes of La, Nd and Tb with borohydride ligand were synthesized and structurally characterized. Various structural types of complex anions and cations containing cyclopentadienyl and borohydride ligands in the coordination sphere of Ln 3+ have been studied by single-crystal X-ray diffraction. Five different structural types of Cp-lanthanide borohydrides were found for neodymium: {CpNd(BH 4 ) 2 } in [(CpNd(BH 4 ) 2 (THF) 2 ] ( 2Nd) ; {Cp 2 Nd(BH 4 ) 2 } − in [Cp 2 Nd(BH 4 ) 2 ][Na(18-crown-6)(THF) 2 ] ( 3Nd) ; {Cp 2 Nd(BH 4 )} in [Cp 2 Nd(BH 4 )(THF)] (4Nd) ; {CpNd(18-crown-6)(BH 4 )} + in [Cp 2 Nd(BH 4 ) 2 ] 2 [CpNd(18-crown-6)(BH 4 )][Na(18-crown-6)(THF) 2 ] ( 6Nd) ; {Cp 3 Nd(BH 4 )} − in [Cp 3 Nd(BH 4 )][Na(18-crown-6)(THF) 2 ] ( 8Nd) . Complex anions {Cp 2 Nd(BH 4 ) 2 } − and {Cp 3 Nd(BH 4 )} − are able to replace each other in the crystal lattice. Mono- and bis(cyclopentadienyl)borohydride complexes of neodymium effectively initiate ε-caprolactone polymerization.