The impact of hydrogen bonding on 100% photo-switching in solid-state nitro-nitrito linkage isomers

Two crystal systems: [Pd(Et(4)dien)(NO2)]OTf [1] and [Pt(Et(4)dien)(NO2)]OTf [2] (Et(4)dien = N,N,N',N'-tetraethyldiethylene-triamine, OTf = trifluoromethanesulfonate) are investigated by steady-state photo-crystallographic methods. Both structures contain intermolecular hydrogen bonds to the ground state nitro-(eta(1)-NO2) isomer, which are previously shown to limit the achievable level of nitro -> nitrito photo-conversion. Irradiation at 100 K induces a mixture of endo-ONO and exo-ONO isomers in 1 and 2, with overall incomplete photo-activation. In contrast, irradiation at higher temperatures leads to much higher conversion, with 100% excitation in 1 at 150 K. The results show that the detrimental effects of hydrogen bonding on the photo-reaction are overcome at higher temperature, adding a new dimension of control to the isomerisation process.