Octa-coordination in complexes of lanthanides with N2 confirmed by matrix-isolation IR spectroscopy and DFT calculations

Abstract

Although the pure existence of binary dinitrogen-lanthanide metal complexes could be established some twenty years ago as unstable species in cryogenic matrices, the controversy about the highest possible coordination number (hexa- or octa-coordinated) prevails up to today. In this study lanthanide (Ln) dinitrogen complexes, synthesized in neat dinitrogen cryogenic matrices at less than 10 K, were investigated by infrared and UV-visible spectroscopy. Surprisingly, only some of the thermally evaporated metals react directly with N2 to yield compound, while in most cases isolated metal atoms are observed in the neat dinitrogen matrix – which react upon photoexcitation. The formed complexes were identified on the basis of the characteristic shifts of the N2 stretching frequency by assistance from DFT calculations. The latter results facilitated also a revision of related literature data. Our joint experimental-theoretical analysis confirmed the formation of octa-coordinated end-on Ln(NN)8 complexes in cryogenic matrices.