Solid‐State Interconversions: Unique 100 % Reversible Transformations between the Ground and Metastable States in Single‐Crystals of a Series of Nickel(II) Nitro Complexes

Mark R Warren,Chick C Wilson,Stefanie Schiffers,Michael W George,John E Warren,Robert J Deeth,Christopher H Woodall,Simon J Teat,Andrew L Johnson,Simon K Brayshaw,Anna J Warren,Timothy L Easun,Paul R Raithby

doi:10.1002/chem.201302053

Abstract

The solid-state, low-temperature linkage isomerism in a series of five square planar group 10 phosphino nitro complexes have been investigated by a combination of photocrystallographic experiments, Raman spectroscopy and computer modelling. The factors influencing the reversible solid-state interconversion between the nitro and nitrito structural isomers have also been investigated, providing insight into the dynamics of this process. The cis-[Ni(dcpe)(NO2)2] (1) and cis-[Ni(dppe)(NO2)2] (2) complexes show reversible 100 % interconversion between the η1-NO2 nitro isomer and the η1-ONO nitrito form when single-crystals are irradiated with 400 nm light at 100 K. Variable temperature photocrystallographic studies for these complexes established that the metastable nitrito isomer reverted to the ground-state nitro isomer at temperatures above 180 K. By comparison, the related trans complex [Ni(PCy3)2(NO2)2] (3) showed 82 % conversion under the same experimental conditions at 100 K. The level of conversion to the metastable nitrito isomers is further reduced when the nickel centre is replaced by palladium or platinum. Prolonged irradiation of the trans-[Pd(PCy3)2(NO2)2] (4) and trans-[Pt(PCy3)2(NO2)2] (5) with 400 nm light gives reversible conversions of 44 and 27 %, respectively, consistent with the slower kinetics associated with the heavier members of group 10. The mechanism of the interconversion has been investigated by theoretical calculations based on the model complex [Ni(dmpe)Cl(NO2)].

Highlights

The pioneering work on the structural determination of metastable species was performed by Woike et al in the 1990s by investigating the structure of sodium nitroprusside,[6] and has continued with Coppens and Woike, and co-workers, separately, demonstrating that photogenerated, low-temperature linkage isomerism occurs in a wide range of transition metal nitro, nitrosyl and sulfur dioxide complexes in the solid state.[7]
Single-crystal X-ray photocrystallographic and solid-state Raman spectroscopy studies of these materials, containing two nitro groups in both cis or trans arrangements, supported by bulky phosphine ligands, show that they undergo high levels of interconversion between the nitro and nitrito linkage isomers in single crystals when irradiated with UV LEDs
An initial computational study of the model complex [Ni(dmpe)Cl(NO)2] demonstrated that B3LYP appears to provide a viable method for modelling these systems and we will report a fuller analysis of the role of HF exchange in these systems plus extend the calculations to the dinitro complexes of Ni, Pd and Pt in due course

Summary

Introduction

As a result of this approach, we recently reported the first example of a solid state 100 % conversion, reversible linkage isomerisation between a [Ni(dppe)Cl(h1-NO2)] nitro complex and the metastable [Ni(dppe)Cl(h1-ONO)] nitrito isomer upon irradiation with UV light at temperatures between 100–160 K (Scheme 1).[12] Warming the sample to above 160 K caused immediate reversion to the ground-state structure and the process could be repeated without apparent crystal decomposition. For cis-[Ni(dppe)(NO2)2] (2), complete conversion from the ground-state nitro-isomer to metastable nitrito-isomer was observed in the Raman experiment after 80 min irradiation at 100 K.

Results

Conclusion