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

• Photoexcitation completes reaction of thermally produced lanthanide atoms with N 2 • Several lanthanide-N 2 complexes have been identified in the cryogenic N 2 matrix • The maximum coordination number of N 2 to lanthanides proved to be 8 (end-on form) 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 N 2 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 N 2 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.