Photocrystallographic identification of metastable nitrito linkage isomers in a series of nickel(ii) complexes

Single crystal photocrystallographic experiments and solid state Raman spectroscopy have been used to determine the low temperature, metastable structures of the nickel(ii) nitrito complexes Ni(aep)(2)(η(1)-ONO)(2) 1#O (aep = 1-(2-aminoethyl)piperidine), Ni(aem)(2)(η(1)-ONO)(2) 2#O (aem = 1-(2-aminoethyl)morpholine), and Ni(aepy)(2)(η(1)-ONO)(2) 3#O (aepy = 1-(2-aminoethyl)pyrrolidine and where the #O denotes the oxygen-bound nitrito metastable molecule). These linkage isomers of the equivalent nitro complexes Ni(aep)(2)(η(1)-NO(2))(2) 1, Ni(aem)(2)(η(1)-NO(2))(2) 2 and Ni(aepy)(2)(η(1)-NO(2))(2) 3 are formed by LED irradiation at temperatures below 120 K. The behavior of the three complexes upon irradiation is generally similar, but some subtle differences have been observed. From the crystallographic studies all three complexes 1-3 exhibit the endo-nitrito linkage isomer upon irradiation, however, for 3# (a crystal structure that contains components of both 3 and 3#O) an exo-nitrito isomer is also observed. Under conditions of 90-100 K, with blue light, the conversion percentages to the nitrito isomers, 1#O, 2#O and 3#O were 16%, 22% and 30%, respectively. At temperatures below 110 K all three nitrito isomers were stable for over four hours but while 2#O and 3#O could be detected at temperatures down to 30 K, at temperatures below 60 K the metastable structure 1#O appeared to be quenched and only the nitro isomer 1 was identified in the crystal. The solid state Raman spectra for 1#, 2# and 3# confirmed the photocrystallographic results with the nitrito isomers being identified from the O-N-O deformation vibrations.