Photo- and Thermoswitchable Half-Sandwich Nickel(II) Complex: [Ni(η⁵-C₅H₅)(IMes)(η¹-NO₂)]

Abstract

A new photoswitchable half-sandwich nitro Ni II complex with the N-heterocyclic carbene ligand was synthesized and successfully crystallized. The compound constitutes an analogue of a known nitrate nickel(II) compound, [Ni(η 5 -Cp)(IMes)(NO 3 )] (Cp = cyclopentadienyl, C 5 H 5 ; IMes = 1,3-bis(2,4,6trimethylphenyl)imidazol-2-ylidene), and crystallizes with two symmetry-independent molecules in the asymmetric unit. When irradiated with 470 nm lightemitting diode light, the complex molecules undergo a photoisomerization reaction in the solid state. Formation of the respective NO 2 group linkage isomers was studied photocrystallographically at different temperatures. After irradiation of the single-crystal sample for ca. 3 h at 100 K approximately 20% conversion of the η 1nitro (Ni−NO 2 ) ligand to its exo-nitrito (Ni−ONO) form was observed. At ∼130 K the exo-nitrito binding mode transforms further to the more stable endo-nitrito conformation, whereas at temperatures higher than 175 K the crystal converts back to the ground state. It appears that the photoisomerization reaction can be to some extent triggered, or additionally stimulated, thermally. Consequently, the highest conversion level (ca. 90%) was achieved after sample irradiation at 145 K. Despite similar molecular energies and intermolecular interactions, the two symmetry-independent molecules behave somewhat differently upon irradiation and temperature changes. The photocrystallographic findings and molecular aspects were supplemented by theoretical computations, including the quantum-mechanics/molecular-mechanics approach.

Photocrystallographic and spectroscopic studies of a model (N,N,O)-donor square-planar nickel(II) nitro complex: in search of high-conversion and stable photoswitchable materials

“…In the case of the QM/MM results, the energetic differences between the isomers are more emphasized (+18.0 and +31.2 kJ mol À1 , for the endoand exo-nitrito forms, respectively) than for the isolated molecule. A similar conclusion was drawn in our previous study (Kutniewska et al, 2019), but there the crystal confining effect was less pronounced. In addition, the above analysis confirms the validity of our refined structural model of all existing ground and metastable states.…”

“…However, note the above conclusions must be treated rather qualitatively. The estimated reaction cavity volume standard deviation amounted to about 0.5 Å 3 (Kutniewska et al, 2019), and the reported changes are within this range.…”

“…The presence of two linkage isomers in the QTNiNO 2 crystal structure at 100 K suggests that the nitro-to-nitrito transformation triggered by temperature rise and/or photoexcitation could be expected. There is already a number of examples in the literature of nickel nitro coordination compounds exhibiting such behaviour in the solid-state upon either photo or thermal activation (Chattopadhyay et al, 2005;Nakamoto et al, 1958;Finney et al, 1981;Kutniewska et al, 2019). Consequently, to verify these anticipations, multitemperature X-ray diffraction studies were conducted, starting at 100 K. On slow heating the occupation of the minor endo-nitrito component indeed started to increase, as shown in Fig.…”

“…In the latter approach the central molecule is modelled with QM, whereas the crystal-field effects are modelled with MM. It has already been proven that the socalled QM/MM method is capable of describing the crystal environment reasonably well, and comprises a reliable theoretical support for photocrystallographic findings (Kutniewska et al, 2019;Jarzembska et al, 2014;Makal et al, 2011;Benedict et al, 2011). The isolated-molecule and QM/MM optimization results are compared in Fig.…”

“…Hence, in the current contribution, which details a part of our wider project dedicated to the search for novel functional materials (Kutniewska et al, 2019;Jarzembska et al, 2017), a new photo-active nitro-group nickel(II) complex is introduced. The compound designed and synthesized by us, QTNiNO 2 (shown in the scheme below), aside from the key nitro group, contains a [2-methyl-8-aminoquinoline]-1-tetralone (QT) ligand chelating the metal centre.…”