Fig 2 - uploaded by Lucy Saunders
Content may be subject to copyright.
(a) Crystal packing arrangement for the structure of 1 at 298 K, (b) hydrogen bonding interactions to the nitro – ( η 1 -NO2) ligand in molecule (B).
Source publication
Thermal and photochemical control of nitro–nitrito linkage isomerism in single-crystals of [Ni(medpt)(NO 2)(η 2 -ONO)] ac The known complex [Ni(medpt)(η 1 -NO 2)(η 2 -ONO)] 1 (medpt = 3,3′-diamino-N-methyldipropylamine) crystallises in the monoclinic space group P2 1 /m with 1.5 molecules in the asymmetric unit with two different η 1 -NO 2 ligand e...
Contexts in source publication
Context 1
... UV/visible spectrum of 1 exhibited a dominant absorption at 361 nm (see ESI † ). This is characteristic of a MLCT transition and irradiation in the tail of this absorption band would be appropriate for generating a metastable linkage isomer. 15 A crystal of 1 was first slow cooled to 100 K, in the dark, to ensure both molecules (A) and (B) adopt the 100% nitro – ( η 1 -NO 2 ) linkage isomer arrangement in the ground state (GS). The crystal was then irradiated with 400 nm LED light at 100 K for a period of 1 h, in situ on the diffractometer using a specifically designed LED ring array. This experimental set up has been described previously 16 and positions six LEDs in a ring ca. 1 cm from the crystal position. The crystal was also rotated during this period, helping to further ensure uniform illumination. After the irradiation period, a second X-ray dataset was then collected in the absence of further light. This dataset revealed that a change in both molecules is induced on excitation. Whilst 89% conversion to a nitrito – ( η 1 -ONO) isomer had been achieved for molecule (A), a minor 32% nitrito isomer was also identified for molecule (B). Linkage isomerism in molecule (B) necessarily breaks the mirror symmetry required at this site in P 2 1 / m. The N -methyl group, C(17), also breaks the original symmetry, preferring to sit just off the mirror plane in the metastable state (MS) structure ( Fig. 5). Given the asymmetric arrangement for the endo -nitrito excited state (ES), refinement in the original centrosymmetric space group P 2 1 / m does not sufficiently describe the MS structure of (B). It is not possible to adequately model the disorder across the mirror plane and, most importantly, to obtain a value for the MS conversion level in the molecule. As such, a more satisfactory model was obtained in P 2 1 . The reduction in symmetry raises the number of molecules in the asymmetric unit to Z ′ = 3 and, as only molecule (B) breaks the symmetry of the original GS cell, the other two molecules are essentially similar. This is confirmed by the fact that they are the same geometric isomer (molecule (A), Fig. 5) and their nitro : nitrito ratios refine to similar values. As such conversion levels reported for molecule A are an average over the two, now assigned as molecules (A) and (A ′ ). Apart from the photoexcited components of molecule (B), the majority of the ES structure still obeys the symmetry con- straints of the GS space group P 2 1 / m , approximately, it is unsurprising that refinement in the lower symmetry space group requires a TWIN instruction. This treatment returns a value for the Flack parameter of ca. 0.5 in all photoexcited datasets modelled in P 2 1 . No further change in the excitation level for any of the molecules could be induced on further irradiation, confirming that a photostationary state had been achieved. The MS structure remained constant on holding the crystal at 100 K in the dark, confirming it to be metastable under the experimental conditions. Variable temperature parametric studies were then conducted to determine the temperature range over which the photoactivated state is metastable. The nitrito isomers in both molecules remain constant on warming to 110 K, but begin to decrease on further heating. By 150 K molecule (B) has regained its GS arrangement and the structure is now best solved in the original P 2 1 / m unit cell. On further warming the nitrito isomer occupancy in molecule (A) continues to decrease, until at 135 K this molecule also returns to the GS nitro – ( η 1 -NO 2 ) arrangement. The nitro : nitrito ratios refined from all photocrystallographic data for molecules (A) and (B) are presented in Table 2. The crystal packing for 1 is largely unchanged on excitation and similar to that shown in Fig. 2, excepting the change in nitrite coordination for molecule (B) and the concomitant reduction in symmetry. The lack of strong intermolecular interactions to molecule (A) could suggest why a mix of linkage isomers is only found for this molecule. Despite the space group change the data in Table 3 confirm there is no dramatic shift in cell parameters, although a small +16(3) Å 3 (0.9%) increase in cell volume is observed on excitation that is just significant in terms of the esds. As noted in the earlier variable temperature studies, the hydrogen bond network observed in 1 could provide an explanation for the considerably lower photoconversion level achieved for molecule (B). Hydrogen bond data for the GS and MS are given in the ESI. † Several intermolecular hydrogen bonds are rearranged following photoconversion in 1 . No hydrogen bonds to the nitrite ligand exist in the GS arrangement for molecule (A), suggesting it is better placed to undergo isomerisation as there are fewer strong contacts pro- viding a barrier to conversion. However, excitation to the nitrito isomer creates new intermolecular hydrogen bonds involving these groups in molecules (A) and (A ′ ) (Fig. 6). For- mation of these new, weak interactions will be energetically favourable and so should favour the formation of the metastable nitrito – ( η 1 -ONO) isomer in both cases. Additionally, the nitrite ligand in molecule (B) is constrained in the GS by two symmetry-related N(5) – H(5D) ⋯ O(11) contacts, becoming bonds N(23) – H(23C) ⋯ (O11) and N(5) – H(5D) ⋯ O(12) when the mirror symmetry is lost upon excitation (Fig. 7, left). These must be broken for isomerisation to occur and in their place a new intermolecular N(23) – H(23A) ⋯ (O12A) hydrogen bond to the nitrito isomer must additionally be formed (Fig. 7, right). The breaking and re-forming of these bonds requires extra energy, making conversion in molecule (B) less favourable. Hirshfeld surfaces were constructed separately around molecules (A) and (B) in turn, such that fingerprint plots could be generated for each of these geometric isomers individually (ESI † ). Firstly, the overall plots are significantly different for molecules (A) and (B), confirming that they occupy quite different environments within the crystal. Considering each molecule individually, the overall plot shapes are similar for GS and MS, confirming there is little change in the overall structure on excitation. For both molecules (A) and (B), only three key types of interaction contribute to the fingerprint plot: while N ⋯ H and O ⋯ H contacts must include changes involving the nitrite groups, H ⋯ H close contacts will solely represent the rest of the structure. Both the N ⋯ H and O ⋯ H plots display sharp, spike features that represent hydrogen bonding. 17 For molecule (A), a subtle increase in these features is observed on moving to the MS reflecting the formation of new hydrogen bonds to the metastable nitrito ligands while none exist to the GS nitro isomer, as discussed previously. For molecule (B), a difference between GS and MS contacts is also evident and in particular the formation of a single, shorter O ⋯ H hydrogen bond is observed, comparing well with the observations described by Fig. 7. By contrast, the H ⋯ H plots for both molecules show little change between GS and metastable state, again confirming that the key differences are only the result of the photochemical linkage isomerisation. Finally, it has been shown that analysis of the reaction cavity, defined as the region encompassing the photoactive part of the molecule, can be used to rationalise the progress of photoreactions in the solid-state. 18 In the context of 1 , the reaction cavity is defined as the region encapsulating a photoactive nitrite group. The cavity volume can be determined by removing the nitrite ligand in question from the crystal structure and conducting a void space calculation on the remaining structure, using the CCDC software package Mercury. 19 No void space was found to be present in any of the structures of 1 before the removal of the photoactive frag- ment. Using a contact surface calculation with a probe radius of 1.2 Å and a grid spacing of 0.1 Å, reaction cavity volumes were determine for molecules (A) and (B) in both the GS and MS structures (Table 4). To ensure fair comparison between all molecules, cavity volumes are given as a percentage of the overall cell volume and are additionally normalised by the number of that molecule in the unit cell ( i.e. divided by 3 for molecule (A) and by 2 for molecule (B), given the 2 : 1 ratio of (A) to (B)). The reaction cavity volume is calculated to be considerably larger for molecule (A) than for molecule (B). Additionally, while Δ V c , the change in reaction cavity volume on photoactivation, is found to increase for molecule (A), a decrease is observed for molecule (B). This indicates there is less space for nitro – nitrito rearrangement to occur in molecule (B) ...
Context 2
... development of photocrystallography, the study of the structures of molecules in photoactivated metastable or excited states using single-crystal X-ray crystallography, has led to the study of a range of chemical processes in the solid- state including cycloaddition reactions, 2 LIESST spin cross- over transformations 3 and the determination of the structures of molecular complexes that have transient lifetimes. 4 One of the most fascinating areas of research has been that of solid-state photoactivated linkage isomerism 5 in which ambidentate ligands alter their coordination mode to give a metastable or short-lived species. One of the features common to the solid-state process is that incomplete conversion from one linkage isomer to another occurs and, in the majority of the early cases reported, conversions of less than 50% are observed before a stationary photo-state is reached or crystal degradation occurs. 6 Only in a few cases where the “ reaction cavity ” 7 around the photoactive ligand has been “ engineered ” to provide sufficient flexibility have high levels of conversion or, indeed, 100% conversion been observed. 8 If the generation and manipulation of these metastable states is to have any “ real-world ” applications it is essential to be able to control both the rate and level of conversion. It is already known that the generation of metastable states causes changes in colour, refractive index or non-linear optical properties of solid-state complexes, 9 and we, 10 and others, 5 d , g are exploring both steric and kinetic effects that favour the formation of metastable linkage isomers. The majority of photocrystallographic studies that have been carried out on linkage isomers have been on complexes that contain only one ambidentate ligand or where the ambidentate ligands are related by crystallographic symmetry. A notable excep- tion is the microsecond time-resolved study on [Cu(2,9-dimethyl- 1,10-phenanthroline)(1,2-bis(diphenylphosphino)etane)][PF ] which shows that the two independent copper( I ) centres distort to different extents upon photoactivation. 11 The differences were attributed to differences in the crystal environment of the two independent molecules. Thus, it is apparent that the molecular environment within the crystal is impor- tant in determining the level of conversion to the short-lived or metastable state. We decided to probe this aspect further by including in our programme of studies on photoactivated nitro – nitrito interconversions 5 a ,12 by carrying out a photocrystallographic study on [Ni(medpt)( η 1 -NO 2 )( η 2 -ONO)] 13 (medpt = 3,3 ′ -diamino- N -methyldipropylamine). The system crystallises in the monoclinic space group P 2 1 / m with one-and-a-half molecules in the asymmetric unit and has η 1 -NO 2 ligand environments that have different symmetry restraints. We now report the results of this study. The complex [Ni(medpt)( η -NO 2 )( η -ONO)] 1 was prepared by literature methods 13 and obtained as violet crystals by evaporation from methanol. The crystal structure was re-determined initially at 298 K. The complex crystallises in the monoclinic space group P 2 1 / m with 1.5 molecules in the asymmetric unit. The molecular structures of the two unique molecules are illustrated in Fig. 1. While molecule (A) occupies a general position, molecule (B) is located on the mirror plane such that the mirror bisects Ni(2), N(11), N(12), N(14), O(13), O(14) and C(17). As such there is a 2 : 1 ratio of (A) to (B) molecules in the crystal. Both nickel centres are six-coordinate, bound to one monodentate nitrite, one nitrito – ( η 2 -O,ON) group and one tridentate amine ligand. Although identical in their constituent components, the two independent molecules are in fact geometric isomers of one another. In molecule (A) the amine binds meridionally such that the N -methyl substituent points downwards away from the monodentate nitrite. However in molecule (B) the amine, though also meridionally coordinated, is inverted such that its N -methyl group points upwards towards the nitrite. This geometric difference was not identified in the original study, 13 but it provides an explanation as to why the two molecules must adopt crystallographically distinct environments. The monodentate nitrite ligand adopts nitro – ( η 1 -NO 2 ) geometry as the major form in both species and, while this is the sole isomer present for molecule (B), in molecule (A) a minor endo -nitrito – ( η 1 -ONO) component is also identified at 22%. The crystal packing for 1 is illustrated in Fig. 2. Molecules of the same isomer align in rows along the a -axis (into the plane of the paper in Fig. 2) and there are no significant interactions between the molecules within each row. The symmetry-related nitro oxygen atoms O(21) of molecule (B) are involved in a N(5) – H(5D) ⋯ O(11) contact to a neigh- bouring molecule (A). This creates hydrogen bonded chains throughout the structure, involving both species and running parallel to the b -axis. By contrast, there are no hydrogen bonds involving the nitrite in molecule (A) and these are oriented away from the bulky amine groups. Given that a thermal equilibrium between the nitro and nitrito isomers had been observed previously in the complex [Ni(Et 4 dien)(NO 2 ) 2 ], 14 the structure of 1 was determined at intervals over the temperature range 100 – 298 K to assess the effect of temperature on the nitro : nitrito ratio in molecule (A). In this procedure the crystal was slowly cooled from 298 to 100 K, in situ on the diffractometer and in the absence of light. Cooling was paused at regular intervals to allow collec- tion of a full single-crystal X-ray dataset, from which the nitro : nitrito isomer ratio for molecule (A) was then refined at each temperature. The crystal was held at each stage for 5 min to allow the temperature to equilibrate, before a new experiment was conducted. The results of the study are presented in Table 1. Molecule (B) showed no change in coordination of the nitro group with temperature reflecting the differing intermolecular interactions of the two molecules. The nitro : nitrito ratio was observed to change on cooling, with conversion to the nitro – ( η 1 -NO 2 ) isomer increasing as the temperature was lowered. By 150 K no evidence of the endo -nitrito – ( η 1 -ONO) isomer could be found in difference maps, confirming that a 100% nitro isomer had been achieved on cooling. As for [Ni(Et 4 dien)(NO 2 ) 2 ], 14 this result indicates the two linkage isomers exist in a thermodynamic equilibrium at ambient temperature and the position of this equilibrium can be shifted by varying the temperature. The nitro – ( η 1 -NO 2 ) isomer appears to be the more thermo- dynamically stable arrangement, as it is preferred at low temperature. Although only limited data were obtained in this variable temperature experiment, it was possible to perform a kinetic analysis of the equilibrium. A Van ' t Hoft plot was constructed for the data between 298 – 200 K and used to approximate thermodynamic data using eqn ...
Context 3
... development of photocrystallography, the study of the structures of molecules in photoactivated metastable or excited states using single-crystal X-ray crystallography, has led to the study of a range of chemical processes in the solid- state including cycloaddition reactions, 2 LIESST spin cross- over transformations 3 and the determination of the structures of molecular complexes that have transient lifetimes. 4 One of the most fascinating areas of research has been that of solid-state photoactivated linkage isomerism 5 in which ambidentate ligands alter their coordination mode to give a metastable or short-lived species. One of the features common to the solid-state process is that incomplete conversion from one linkage isomer to another occurs and, in the majority of the early cases reported, conversions of less than 50% are observed before a stationary photo-state is reached or crystal degradation occurs. 6 Only in a few cases where the “ reaction cavity ” 7 around the photoactive ligand has been “ engineered ” to provide sufficient flexibility have high levels of conversion or, indeed, 100% conversion been observed. 8 If the generation and manipulation of these metastable states is to have any “ real-world ” applications it is essential to be able to control both the rate and level of conversion. It is already known that the generation of metastable states causes changes in colour, refractive index or non-linear optical properties of solid-state complexes, 9 and we, 10 and others, 5 d , g are exploring both steric and kinetic effects that favour the formation of metastable linkage isomers. The majority of photocrystallographic studies that have been carried out on linkage isomers have been on complexes that contain only one ambidentate ligand or where the ambidentate ligands are related by crystallographic symmetry. A notable excep- tion is the microsecond time-resolved study on [Cu(2,9-dimethyl- 1,10-phenanthroline)(1,2-bis(diphenylphosphino)etane)][PF ] which shows that the two independent copper( I ) centres distort to different extents upon photoactivation. 11 The differences were attributed to differences in the crystal environment of the two independent molecules. Thus, it is apparent that the molecular environment within the crystal is impor- tant in determining the level of conversion to the short-lived or metastable state. We decided to probe this aspect further by including in our programme of studies on photoactivated nitro – nitrito interconversions 5 a ,12 by carrying out a photocrystallographic study on [Ni(medpt)( η 1 -NO 2 )( η 2 -ONO)] 13 (medpt = 3,3 ′ -diamino- N -methyldipropylamine). The system crystallises in the monoclinic space group P 2 1 / m with one-and-a-half molecules in the asymmetric unit and has η 1 -NO 2 ligand environments that have different symmetry restraints. We now report the results of this study. The complex [Ni(medpt)( η -NO 2 )( η -ONO)] 1 was prepared by literature methods 13 and obtained as violet crystals by evaporation from methanol. The crystal structure was re-determined initially at 298 K. The complex crystallises in the monoclinic space group P 2 1 / m with 1.5 molecules in the asymmetric unit. The molecular structures of the two unique molecules are illustrated in Fig. 1. While molecule (A) occupies a general position, molecule (B) is located on the mirror plane such that the mirror bisects Ni(2), N(11), N(12), N(14), O(13), O(14) and C(17). As such there is a 2 : 1 ratio of (A) to (B) molecules in the crystal. Both nickel centres are six-coordinate, bound to one monodentate nitrite, one nitrito – ( η 2 -O,ON) group and one tridentate amine ligand. Although identical in their constituent components, the two independent molecules are in fact geometric isomers of one another. In molecule (A) the amine binds meridionally such that the N -methyl substituent points downwards away from the monodentate nitrite. However in molecule (B) the amine, though also meridionally coordinated, is inverted such that its N -methyl group points upwards towards the nitrite. This geometric difference was not identified in the original study, 13 but it provides an explanation as to why the two molecules must adopt crystallographically distinct environments. The monodentate nitrite ligand adopts nitro – ( η 1 -NO 2 ) geometry as the major form in both species and, while this is the sole isomer present for molecule (B), in molecule (A) a minor endo -nitrito – ( η 1 -ONO) component is also identified at 22%. The crystal packing for 1 is illustrated in Fig. 2. Molecules of the same isomer align in rows along the a -axis (into the plane of the paper in Fig. 2) and there are no significant interactions between the molecules within each row. The symmetry-related nitro oxygen atoms O(21) of molecule (B) are involved in a N(5) – H(5D) ⋯ O(11) contact to a neigh- bouring molecule (A). This creates hydrogen bonded chains throughout the structure, involving both species and running parallel to the b -axis. By contrast, there are no hydrogen bonds involving the nitrite in molecule (A) and these are oriented away from the bulky amine groups. Given that a thermal equilibrium between the nitro and nitrito isomers had been observed previously in the complex [Ni(Et 4 dien)(NO 2 ) 2 ], 14 the structure of 1 was determined at intervals over the temperature range 100 – 298 K to assess the effect of temperature on the nitro : nitrito ratio in molecule (A). In this procedure the crystal was slowly cooled from 298 to 100 K, in situ on the diffractometer and in the absence of light. Cooling was paused at regular intervals to allow collec- tion of a full single-crystal X-ray dataset, from which the nitro : nitrito isomer ratio for molecule (A) was then refined at each temperature. The crystal was held at each stage for 5 min to allow the temperature to equilibrate, before a new experiment was conducted. The results of the study are presented in Table 1. Molecule (B) showed no change in coordination of the nitro group with temperature reflecting the differing intermolecular interactions of the two molecules. The nitro : nitrito ratio was observed to change on cooling, with conversion to the nitro – ( η 1 -NO 2 ) isomer increasing as the temperature was lowered. By 150 K no evidence of the endo -nitrito – ( η 1 -ONO) isomer could be found in difference maps, confirming that a 100% nitro isomer had been achieved on cooling. As for [Ni(Et 4 dien)(NO 2 ) 2 ], 14 this result indicates the two linkage isomers exist in a thermodynamic equilibrium at ambient temperature and the position of this equilibrium can be shifted by varying the temperature. The nitro – ( η 1 -NO 2 ) isomer appears to be the more thermo- dynamically stable arrangement, as it is preferred at low temperature. Although only limited data were obtained in this variable temperature experiment, it was possible to perform a kinetic analysis of the equilibrium. A Van ' t Hoft plot was constructed for the data between 298 – 200 K and used to approximate thermodynamic data using eqn ...
Similar publications
![Figure 1. Chemical structure of pyrrolo[1,2-a][1,10] phenanthroline...](profile/Marius-Prelipceanu/publication/276436975/figure/fig1/AS:345997881430020@1459503731364/Chemical-structure-of-pyrrolo1-2-a1-10-phenanthroline-derivatives-investigated_Q320.jpg)
Thermally stimulated processes have been studied in thin films of phenanthroline derives to describe the states that had been localized. Ultraviolet Photoelectron Spectroscopy had checked out before thin films of new pyrrolo[l,2-a][l,10] phenanthroline derivatives [1], in order to further applications in optoelectronic devices. The investigated com...
Citations
... c) In the case of the ambidentate ligand NO2 -, the possible binding modes for the nitrite ligand in a transition metal complex are explained in [61,62]. Further work on the NO2was carried out mainly by the Raithby group [63][64][65][66][67][68][69]. ...
The evolving focal point of interest to tune structure-property relationship in solid state materials exhibiting two or more accessible energy states, have centralized emphasis on
transition metal complexes with ambidentate ligands owing to their switching ability as linkage isomers by external stimulus of light. Complete, stable at room temperature, optically controlled, and reversible interconversion of these photoinduced states is ideally desirable to find real-time potential applications ranging from data storage devices to smart windows. Principally, the knowledge of all 'structures' during the whole photocycle, i.e., starting from the ground state (GS) and during all the steps involving transient or metastable states before its recovery back to the initial state GS, is a prerequisite to interpret correctly the structure property relationships. The thesis is based on the study of photoswitchable nitrosyl complexes of three peculiarly different types {M(NO)m}n (m = 1,2; n = 6,8,10) and their analysis with coupled photocrystallographic and spectroscopic methods to provide complete information about the geometrical structural changes and energetics after photoexcitation. In terms of photocrystallographic analysis, a detailed structural investigation in steady-state, comparative steady-state study of continuous wave vs pulsed laser irradiation sources, followed by squeezing all the photons to single-shot one-pulse photocrystallography for preparation of inhouse ms-time resolved, and finally, an attempt to reach to ps-time resolved photocrystallographic analysis are presented. This step-wise methodology contributes to a better understanding of the mechanism of structural dynamics and to comprehend the structure-property relationships of these photoswitchable molecular complexes.
... [4] Molecular photoswitches are typically organic molecules with extended aromatic systems responsive to light. [5] Nevertheless, transition-metal complexes possessing ambidentate ligands, such as NO, NO 2 , or SO 2 [2,6,7,[8][9][10][11][12][13][14][15] constitute a promising and readily modifiable group of this kind of compounds. [9] The ambidentate ligands are the key fragments of these molecules because they can bind to a metal center in multiple ways which can be controlled optically. ...
... [6,8,11,15,[18][19][20]21] There are, though, rather limited examples of high-conversion photoswitches of this kind, and/or complexes which work at higher temperatures. [10,11,[13][14][15][22][23][24] Furthermore, an aspect which is still under debate is the mechanism of light-and thermally-induced transformations between linkage isomers. There are few papers which undertake this issue supporting experimental presumptions with theoretical modelling. ...
... 43.7 Å 3 ) when compared to other literature-reported photoswitchable compounds of this kind ( Figure 1d). [10,12,14,19] Table 1. Selected crystal structure parameters of the studied complex 'dark' crystal structure at 100 K (2 nd column), crystal structures after irradiation at 100 K (3 rd column), at 150 K (4 th column), and at 200 K (5 th column) (for more details see the Supporting Information, Table S1). ...
An efficient nitrite nickel(II) photoswitch, with the 1‐phenyl‐2‐hydroxyimino‐3‐[(2’‐dimethylamino)ethyl]imino‐1‐propanone moiety used as the ancillary ligand, is reported. In the ground‐state (‘dark’) crystal structure, the studied compound exists predominantly as the nitro‐(η¹‐N(O)2) isomer, however, traces of the exo‐ and endo‐nitrito‐(η¹‐ONO) forms are detected both at 100 K (4–5 % each) and under ambient conditions (~9 % each). When excited with the 405–530 nm LED light, the nitro‐to‐nitrito isomerization takes place. The total conversion exceeds 90 %. The exo‐nitrito linkage isomer constitutes the dominant photo‐generated form, whereas the relative population of both nitrito species depends on temperature. The reaction is fully reversible and reproducible. The photo‐products are stable up to 200 K. The system constitutes a good model case for the reaction mechanism studies. Thus, experimental and theoretical investigations on the photo‐isomerism were conducted and are presented in detail. Eventually, the nitro→exo‐nitrito→endo‐nitrito reaction pathway is proposed.
... The feasibility of the isomerization process solely via thermal energy in the absence of light was investigated using controlled heating experiments. [49][50][51] Two NO 2 -PDI solutions of the same concentration were prepared in ACN and TOL, and were completely covered with aluminium foil to avoid any unwanted photoexcitation. Both the solutions were heated at 50 C for 30 min, followed by heating at 70 C for 30 min and nally heating at 150 C for 10 min. ...
The discovery of vibrant excited-state dynamics and distinctive photochemistry has established nitrated polycyclic aromatic hydrocarbons as an exhilarating class of organic compounds. Herein, we report the atypical photorearrangement of nitro-perylenediimide (NO2-PDI) to nitrito-perylenediimide (ONO-PDI), triggered by visible-light excitation and giving rise to linkage isomers in the polar aprotic solvent acetonitrile. ONO-PDI has been isolated and unambiguously characterized using standard spectroscopic, spectrometric, and elemental composition techniques. Although nitritoaromatic compounds are conventionally considered to be crucial intermediates in the photodissociation of nitroaromatics, experimental evidence for this has not been observed heretofore. Ultrafast transient absorption spectroscopy combined with computational investigations revealed the prominence of a conformationally relaxed singlet excited-state (SCR 1) of NO2-PDI in the photoisomerization pathway. Theoretical transition state (TS) analysis indicated the presence of a six-membered cyclic TS, which is pivotal in connecting the SCR 1 state to the photoproduct state. This article addresses prevailing knowledge gaps in the field of organic linkage isomers and provides a comprehensive understanding of the unprecedented photoisomerization mechanism operating in the case of NO2-PDI.
... 6−11 Transition-metal complexes in which the metal center is coordinated by molecular fragments that can exist in multiple isomeric forms are among the potential functional materials of this kind. 12 Examples of ambidentate ligands known to display solid-state linkage isomerism include NO, 13,14 NO 2 ,8,12,[15][16][17][18][19][20][21][22][23][24][25] SO 2 ,[26][27][28][29] SCN,30 and N 2. 31 Transition-metal complexes containing nickel, 12,22,24,25 cobalt, 32−34 iron, 35 ruthenium, 13,36 osmium, 37 palladium, or platinum [19][20][21]23 centers are the most representative examples of such systems. Light-induced structural changes in crystals can be investigated using photocrystallographic methods. ...
... 6−11 Transition-metal complexes in which the metal center is coordinated by molecular fragments that can exist in multiple isomeric forms are among the potential functional materials of this kind. 12 Examples of ambidentate ligands known to display solid-state linkage isomerism include NO, 13,14 NO 2 ,8,12,[15][16][17][18][19][20][21][22][23][24][25] SO 2 ,[26][27][28][29] SCN,30 and N 2. 31 Transition-metal complexes containing nickel, 12,22,24,25 cobalt, 32−34 iron, 35 ruthenium, 13,36 osmium, 37 palladium, or platinum [19][20][21]23 centers are the most representative examples of such systems. Light-induced structural changes in crystals can be investigated using photocrystallographic methods. ...
... Indeed, it is essential to ensure the space necessary for the NO 2 group to undergo light-induced linkage isomerization. 8,[15][16][17][18]22 Naturally, not only the size of the cavity but also its shape are relevant so as to ensure the smallest possible changes in the unit cell upon the molecular transformation. 19 Hence, the reaction cavity volumes were calculated for all examined crystal Inorganic Chemistry pubs.acs.org/IC ...
Two photoswitchable nickel(II) nitro coordination compounds and their copper(II) analogues are reported. In all these systems, the metal center is chelated by (N,N,O)-donor ligands containing either 2-picolylamine or 8-aminoquinoline fragments. The studied compounds were thoroughly investigated using crystallographic and spectroscopic techniques supplemented by computational analysis. They are easy to synthesize and stable, and all compounds undergo the nitro group isomerization reaction. Nevertheless, there are significant differences between the copper and nickel systems regarding their structural and switchable properties. According to the solid-state IR spectroscopy results, 400-660 nm light irradiation of the ground-state (η2-O,O')-κ-nitrito copper(II) complexes at 10 K induces a rather moderate conversion to a metastable linkage isomer, which is visible only up to approximately 60-80 K. In turn, upon visible light irradiation (ca. 530 nm excitation wavelength), the ground-state nitro isomers of the examined nickel(II) complexes transform into the endo-nitrito forms. It was possible to achieve about 35% conversion for both nickel(II) systems and to determine the resulting crystal structures at 160 K in the case of single crystals after 30-45 min of exposure to LED light (crystals decayed with longer irradiation), and roughly 95% conversion was achieved for thin-film samples as indicated by the IR spectroscopy results. Traces of the endo-nitrito linkage isomers remained up to 200-220 K, and the isomerization reaction was proven to be fully reversible.
... Particularly, NO 2 -containing Ni II and Pd II chelate complexes with polyamines and polyphosphines have been reported. [4][5][6][7][8][9][10][11][12][13][14][15] The reported systems are constructed from large, sterically-demanding ancillary ligands. Such components can dictate the crystal packing and are responsible for a suitable cavity around the potentially isomerisable nitrite group. ...
... Thus, intermolecular interactions, which can potentially assist or hinder linkage isomerism, become of particular importance. 9,14 We have recently drawn our attention to helical metallocomplexes fabricated from polydentate Schiff base ligands, comprising two symmetrically related pyridyl coordination sites derived from benzyldihydrazone, which are monohelical due to the constraint rotation around the C-C bond. [18][19][20] Upon coordination, the rotating freedom of the two N-N bonds is also reduced, and the ligand can be locked in a twist conformation (Scheme 1). ...
We report the design, structural, spectroscopic and computational characterizations of the two new quasi-aromatic Möbius chelate coordination compounds fabricated from Cd(NO3)2·4H2O and a bulky helical organic ligand derived from benzildihydrazone...
... Although noncovalent interactions including hydrogen-bonding, π-π stacking, electrostatic interactions, and van der Waals interactions are weak compared to covalent bonding, they play a vital role in the formation, physical properties and chemical processes of functional materials. [1][2][3][4][5][6][7][8][9][10] Noncovalent interactions have been utilized in the development of functional materials, [11][12][13] for tailoring crystallization, [14][15][16] or the study of host-guest interactions. [17][18][19] Reliably identifying noncovalent interactions is crucial in the understanding and development of novel functional materials. ...
Noncovalent interactions are essential in the formation and properties of a diverse range of materials. However, reliably identifying noncovalent interactions remains challenging using conventional methods such as X-ray diffraction, especially in nanocrystalline, poorly crystalline or amorphous materials which lack long-range lattice periodicity. Here, we demonstrate the accurate determination of deviations in the local structure and tilting of aromatic rings during the temperature-induced first order structural transition in the 1 : 1 adduct of 4,4'-bipyridinium squarate (BIPY:SQA) from the low temperature form HAZFAP01 to high temperature HAZFAP07 by X-ray pair distribution function. This work demonstrates how pair distribution function analyses can improve our understanding of local structural deviations resulting from noncovalent bonds and guide the development of novel functional materials.
... The first system of this kind, [Ni(dppe)(NO 2 )Cl] [dppe = 1,2-bis(diphenylphosphino)ethane], which was confirmed to undergo 100% conversion between its ground state and the nitrito isomer, was described in the work by Warren et al. (2009). Raithby and co-workers later published a few more articles on transition-metal nitrocoordination compounds capable of achieving high-conversion levels upon light irradiation (Skelton et al., 2015;Warren et al., 2014;Hatcher, Bigos et al., 2014;Hatcher et al., 2011Hatcher et al., , 2019. Nevertheless, up to now the metastable species have usually been present in the crystal structures only at relatively low temperatures. ...
... The reaction cavity volume computed for the 100 K crystal structure [using MERCURY (Macrae et al., 2020); the rolling-probe method: probe radius of 1.2 Å , grid spacing of 0.1 Å ] is equal to 33.2 Å 3 per one complex molecule. This volume is comparable to the respective literature-reported values derived for other NO 2 metal complexes undergoing photo-induced transformations (Hatcher, Bigos et al., 2014;Hatcher & Raithby, 2017). ...
A new, cheap, easy-to-synthesize and air-stable photoswitchable nickel(II) complex, QTNiNO 2 , is reported. The metal centre in QTNiNO 2 is coordinated by a nitro group and a [2-methyl-8-aminoquinoline]-1-tetralone ligand. The compound crystallizes in the tetragonal space group I 4 1 / a with one complex molecule comprising the asymmetric unit, and the crystals are stable under ambient conditions. Irradiation of the solid-state form of QTNiNO 2 with 530–660 nm LED light at 160 K converts the ambidentate nitro moiety fully to the nitrito linkage isomer which is stable up to around 230 K, as indicated by IR spectroscopy measurements. The structures of all species present in the examined crystals and their thermal stability were confirmed via X-ray multi-temperature and photocrystallographic experiments. The impact of temperature on the (photo)isomerization reaction taking place in a single crystal was additionally investigated. The experimental results are supported by computational analyses of crystal packing and intermolecular interactions that influence the isomerization process studied.
... 20 Building on seminal work by Coppens on metal-nitrosyl compounds, 21 a variety of linkage isomer systems have been studied including nitrosyl, [22][23][24][25] sulfur dioxide, [26][27][28][29] dinitrogen 30 and nitrite species. 20,[31][32][33][34][35][36][37][38] In particular, nitro nitrito isomerism is well studied by photocrystallographic methods, with the first 100 % conversion in a single crystal reported in 2009 for [Ni(dppe)(Cl)(η 1 -NO2)]. 35 Several other crystals capable of 100 % conversion have since been designed, and there is also interest in systems capable of room temperature photoswitching for real world applications. ...
Single-crystal-to-single-crystal nitro → nitrito isomerism is reported for the novel rhenium(I)-bipyridine complex [Re(OMe2-bpy)(CO)3(η¹-NO2)], achieving a maximum conversion of 66 % to a photoinduced nitrito-(η¹-text-decoration:underline"ONO) isomer under continuous illumination. The 3D X-ray structure of the photoinduced isomer is determined by steady-state and psuedo-steady-state photocrystallographic methods, providing insight into the structural changes required to accommodate photoswitching. Photocrystallographic kinetic studies follow the progress of photoswitching with time, determining a reaction rate constant of k = 0.38(2) min⁻¹ at 150 K. Linkage isomerism is fully-reversible on warming, and pseudo-steady-state experiments confirm that the photoexcited state is retained, at measurable occupancy, up to a temperature of 240 K. These results confirm the validity of combining photoactive linkage isomer and Re(I) photocatalyst chemistries, and the detailed determination of the photoexcited state structure will facilitate the future design of new photoactive Re(I) crystals for a range of applications.
... An example is [Ni(MeDPT)(NO2)2], which contains two, crystallographically distinct nitro-(η 1 -NO2) environments in its GS structure. 16 One NO2 group participates in no significant hydrogen bonding and is found to reach 89% photo-activation in the excited state (ES). By contrast, a conversion level of just 32 % is achieved at the second NO2 position, which is bound by moderately strong hydrogen bonds through both of its oxygen atoms. ...
... By contrast, a conversion level of just 32 % is achieved at the second NO2 position, which is bound by moderately strong hydrogen bonds through both of its oxygen atoms. 16 This result suggests that, when designing new linkage isomer switches, fragments containing hydrogen bond donor groups should be avoided. This would be an unfortunate design limitation, given that hydrogen bonds are traditionally one of crystal engineering's most important and useful tools. ...
... This hydrogen bond motif is significant, as it directly involves the potentially photo-active nitro-(η 1 -NO2) groups. The hydrogen bonds must be disrupted for photo-isomerisation to proceed and, given the literature precedent, 16 could be likely to hinder high photo-conversion in 1. The OTf anions sit in discrete pockets, with each OTf participating in short contacts with 6 nearest neighbour cations. ...
Two crystal systems: [Pd(Et4dien)(NO2)]OTf [1] and [Pt(Et4dien)(NO2)]OTf [2] (Et4dien = N,N,N’,N’-tetraethyldiethylenetriamine, OTf = trifluoromethanesulfonate) are investigated by steady-state photocrystallographic methods. Both structures contain intermolecular hydrogen bonds to the ground state nitro-(η1-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.
... For example, the Cu−O1B bond length is 2.010(18) Å, while the Cu−O2B distance is considerably longer at 2.66(2) Å. Similar quasi-bidentate interactions of nitrite with a Cu(II) centre in a tris(2-pyridyl) complex have previously been reported by Woollard-Shore et al. [20] who found a long Cu−O interaction of 2.5620 Å. [51][52][53][54]). ...
The copper-containing nitrite reductases (CuNIRs) are a class of enzymes that mediate the reduction of nitrite to nitric oxide in biological systems. Metal–ligand complexes that reproduce the salient features of the active site of CuNIRs are therefore of fundamental interest, both for elucidating the possible mode of action of the enzymes and for developing biomimetic catalysts for nitrite reduction. Herein, we describe the synthesis and characterization of a new tris(2-pyridyl) copper complex ([Cu1(NO2)2]) that binds two molecules of nitrite, and displays all three of the common binding modes for NO2⁻, with one nitrite bound in an asymmetric quasi-bidentate κ²-ONO manner and the other bound in a monodentate fashion with a linkage isomerism between the κ¹-ONO and κ¹-NO2 binding modes. We use density functional theory to help rationalize the presence of all three of these linkage isomers in one compound, before assessing the redox activity of [Cu1(NO2)2]. These latter studies show that the complex is not a competent nitrite reduction electrocatalyst in non-aqueous solvent, even in the presence of additional proton donors, a finding which may have implications for the design of biomimetic catalysts for nitrite reduction.
