Metastable Linkage Isomerism in [Ni(Et₄dien)(NO₂)₂]: A Combined Thermal and Photocrystallographic Structural Investigation of a Nitro/Nitrito Interconversion

Abstract

Photocrystallography is a rapidly developing technique that enables the full three-dimensional structure of a molecule in a metastable or excited state to be elucidated when a single crystal of the complex is photoactivated by visible or UV light.[1] Pioneering work in the areas of both molecular and macromolecular photocrystallography has allowed the crystallographic characterization of species with microsecond and sub-microsecond lifetimes. [2, 3] Of equal importance are the detailed studies on the identification and stabilization of metastable species [4,5] because of potential photorefractive applications, including data storage and optical switching. [6] This area has been pioneered by Coppens et al., and they, and others, have now successfully determined the structures of a number of metastable species including several transitionmetal-nitrosyl, -nitro, and -SO 2 complexes. [7][8][9] A major limitation in the study of these systems has been the relatively low level of conversion from the ground state to the metastable state that can be achieved in the single crystal. Controlled, complete reversible interconversion is desirable if these materials are to find real applications as optical switches or in data-storage devices. Examples of high conversions into metastable species are rare; however, notable studies include the first example of a reversible 100 % conversion of the nickel(II)-nitro complex [Ni(dppe)(h 1 -NO 2 )Cl] (dppe = 1,2-bis(diphenylphosphino)ethane) into the nitrito isomer [Ni-(dppe)(h 1 -ONO)Cl] when irradiated with UV light-emitting diodes (LEDs) [10] at temperatures below 160 K. A 92 % lightinduced conversion from the h 1 -NO isomer to the h 1 -ON

Thermal and photochemical control of nitro–nitrito linkage isomerism in single-crystals of [Ni(medpt)(NO2)(η2-ONO)]

“…2 and the data, returning an estimation of EA = -1.6 (± 0.05) kJ mol -1 . This value is smaller than that obtained for [Ni(Et4dien)(NO2)2], 14 but is still comparable. The values for ΔH, ΔS and EA are indicative of a low energy barrier between the isomers, suggesting they are close in energy.…”

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

“…24 This is in contrast to the majority of photo-switchable nitrite linkage isomers reported, which more typically show an expansion of the crystal on activation. [6][7][8] The unit cell reduction is anisotropic: while the b and c axes contract by −0.1256IJ5) Å and −0.230IJ1) Å respectively, the a-axis lengthens by 0.0867(5) Å. Given that the 1-D chains of metal cations extend parallel to the a-axis, it might be argued that the expansion in this direction is a direct result of the isomerisation.…”

“…1 Photo-switching between linkage isomers can be achieved in both solution and the solid-state, with irradiation of the solid leading to the production of a metastable state (MS) isomer below some critical temperature. 2 In situ photocrystallographic studies have to-date revealed the structures of various MS linkage isomers, including nitrosyl, 3,4 nitrite, [5][6][7][8] sulfur dioxide, [9][10][11] and di-nitrogen 12 systems.…”

Photocrystallographic studies of novel metal–nitrite compounds

“…[4] More recently, we have also reported interesting thermal and photo-isomerisation behaviour in the complex [Ni(Et4dien)(NO2)2], (Et4dien = N,N,N',N'-tetraethyldiethylenetriamine). [5] Steady-state photocrystallographic studies with this system will be discussed. Excitation with 500 nm LED light produces a metastable endonitrito isomer at low temperature (100 K), with 100 % photo-conversion in the single crystal.…”