Lauren E.
Hatcher
*a,
Lucy K.
Saunders
b and
Ben A.
Coulson
a
aSchool of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, CF10 AT, UK. E-mail: HatcherL1@cardiff.ac.uk
bDiamond Light Source, Harwell Science and Innovation Campus, Fermi Ave, Didcot, OX11 0DE, UK
First published on 23rd December 2022
We present a charge density study of two linkage isomer photoswitches, [Pd(Bu4dien)(NO2)]BPh4·THF (1) and [Ni(Et4dien)(NO2)2] (2) using Hirshfeld Atom Refinement (HAR) methods implemented via the NoSpherA2 interface in Olex2. HAR is used to explore the electron density distribution in the photoswitchable molecules of 1 and 2, to gain an in-depth understanding of key bonding features and their influence on the single-crystal-to-single-crystal reaction. HAR analysis is also combined with ab initio calculations to explore the non-covalent interactions that influence physical properties of the photoswitches, such as the stability of the excited state nitrito-(η1-ONO) isomer. This insight can be fed back into the crystal engineering process to develop new and improved photoswitches that can be optimised towards specific applications.
While single-crystal X-ray diffraction (SCXRD) refinements using the traditional Independent Atom Model (IAM) provide atomic-scale information before, after, and even during photo-switching, often information on NCIs is, at best, only inferred from the refined parameters (e.g. bond lengths and angles). Experimental charge density refinements can directly refine the electron density based on more accurate, non-spherical models e.g. multipolar refinements, providing unique understanding of the fine electron density.6–8 However, such experiments ideally require very high-resolution data (<0.5 Å). As many issues typical for photocrystallographic studies (e.g. radiation damage from light and/or X-rays, stimuli-induced phase transitions, or significant disorder resulting from partial conversion to the excited-state) can significantly limit the diffraction data quality obtained, these studies present a significant challenge for accurate experimental charge density analysis.
More recently, semi-empirical approaches have been proposed that sit between experimental charge density refinements and ab initio calculations.9 These include multipole-based databank approaches, e.g. the Invarioms10 or ELMAM11 methods, although these methods are typically limited to organic and bioorganic compounds where fewer atom types are required. The NoSpherA2 (Non-Spherical Atoms in Olex2) approach, implemented in the crystal structure refinement software Olex2,12 has already been applied to several interesting crystallographic problems, including those involving metal atoms.13–15 The software utilises Hirshfeld atom refinement (HAR) to calculate non-spherical atomic form factors, then refines these non-spherical atom shapes against the experimental electron density obtained by SCXRD.9 Through this combination of quantum mechanical calculations and experimental electron density refinement, a greatly improved crystallographic model is obtained that can provide new insight into the intra- and intermolecular bonding. The NoSpherA2 approach has some advantages over fully-experimental charge density analysis as it can be applied to materials that do not diffract to such high resolution as needed for multipolar refinements, and more readily to materials that contain disorder.
We herein present an application of NoSpherA2 to photo-switchable linkage isomer crystals. Using HAR, we investigate the electron density distribution (EDD) and NCIs present in the ground-state (GS) and excited-state (ES) isomers of two known systems: [Pd(Bu4dien)(NO2)]BPh4·THF (1),16 and [Ni(Et4dien)(NO2)2] (2),17,18 which can both be fully-converted between their nitro-(η1-O2) GS and photoinduced endo-nitrito-(η1-NO) ES at 100 K. As well as dealing with the limited resolution imposed by in situ irradiation of the crystals, 1 and 2 present different challenges for HAR analysis: while 1 has a large asymmetric unit containing 130 atoms, including a heavy PdII metal centre, 2 contains fewer atoms but its d8 octahedral NiII centre requires an open-shell wavefunction calculation. By understanding the EDD and NCIs present in these systems we can explore the nature of the bonding between the isomerisable ligand and the metal. This insight is used to rationalise properties, e.g. the stability of the photoexcited state, knowledge that can be applied to rationally-design new materials for particular applications. The results show the applicability of HAR for photocrystallographic refinements and recommend its future application to other photoswitchable and photocatalytic materials.
SCXRD data collection and processing were completed as described above. For both 1 and 2, diffraction data could be obtained to a resolution of d = 0.6 Å in both the GS and ES.
Comparison of the GS and ES maps for both the deformation density and Laplacian show differences in charge distribution in the Pd–N bonds to Bu4dien following excitation, with the valence shell charge concentrations (VSCCs) more localised in the GS compared to the ES (see Fig. 2(c) vs (d)), which is indicative of a change in the electronegativity of the ligand. These differences indicate a stronger Pd-to-Bu4dien interaction in the ES isomer and likely reflect the change in electronic structure at Pd(1) following conversion between nitro-(η1-O2) to nitrito-(η1-NO) isomers. This is supported by a comparison of the experimental bond lengths from the crystal structure data (Tables S1.2/S1.3†). The Pd–N bonds to Bu4dien all shorten on excitation, with small but significant changes of ΔPd(1)–N(2) = −0.0119(12) Å and ΔPd(1)–N(4) = −0.0051(11) Å for the cis-coordinated donors and ΔPd(1)–N(3) = −0.0105(13) Å for the donor trans- to the nitrite group. These changes also remain consistent in the geometry-optimised structures.
Fig. 1(c)/(d) and 2(c)/2(d) provide a good visual analysis of the difference in electron density distribution within the nitro-(η1-O2) and nitrito-(η1-NO) groups. The Laplacian maps are particularly informative, with clear delocalisation evident across the N–O bonds of the GS isomer that is markedly reduced in the ES. Similarly, the GS deformation density (Fig. 1(c)) clearly shows both CC in the O(1) and O(2) lone pairs and a fairly high level of CC in the N–O bonds (strong blue features), while the ES map (Fig. 1(d)) shows much lower CC in the N–O bonds of nitrito-(η1-NO). Instead, there appears to be a very strong localisation of charge in the N(1A) lone pair, a feature also backed up by the theoretical results. From topological analysis, there is a shift in the relative positions of the (3,−1) bond critical points (BCPs) between the GS and ES isomers, with BCPs equally positioned at 47% along both N → O directions in nitro-(η1-O2), but moving 2% closer to the central N(1A) atom in the nitrito-(η1-ONO) ES (see Fig. S1.2 and Tables S1.4/1.5†). Theoretical Natural Bond Orbital (NBO) analysis also agrees with this bonding picture. In the GS nitro-(η1-O2), the σ(N–O) bonds are almost completely delocalised with 51.4% localisation on N(1) in N(1) → O(1) and 50.3% in N(1) → O(2) (Table S1.6†) and bond orders of 1.44 and 1.45 respectively (Table S1.7†). Conversely, for ES nitrito-(η1-ONO) the σ(N–O) bonds are more polar and the electron density more localised, with 60.4% localisation on N(1A) in N(1A) → O(1A) and 55.8% in N(1A) → O(2A), and unequal bond orders of 1.18 and 1.78. These results confirm that, while the electron density in nitro-(η1-O2) is considerably delocalised across both N–O bonds, suggesting a partial double bond character in each, in the ES this delocalisation is much reduced. This is in line with the experimental bond lengths (Tables S1.2/S1.3†), and with other theoretical studies in the literature investigating the electron density distribution in related metal-nitrite complexes.30
The change in EDD within the Bu4dien and nitrite ligands between GS and ES structures clearly reflects the NO2 → ONO switching. Comparing the GS and ES geometries about Pd(1) in the experimental crystal structures, the bond between Pd(1) and the nitrite ligand elongates by +0.0248(13) Å on excitation (Tables S1.2/1.3†). This indicates poorer overlap between O(1A) and Pd(1) in the ES, compared to that of N(1A) and Pd(1) in the GS, and is reflected in the deformation density (Fig. 1(c)/(d)). In the GS, the nitro-(η1-O2) ligand is well-placed to provide good σ-donation to Pd(1) via its N(1) lone pair, which is clearly aligned along the N(1)–Pd(1) bonding direction to match with the acceptor lobe of the PdII4dx2−y2 orbital. Conversely, the ES map shows that nitrito-(η1-NO) is less well-matched for donor–acceptor bonding, as the lone pairs on O(1A) do not align as well with the 4dx2−y2 orbital. It is clear there is less electron density available for n → 4d* donation, with a much lighter blue region aligned with the red sigma hole at the metal in Fig. 1(d). The majority of electron density on O(1A) is instead localised in the lone pair on the other side of the atom, which is not involved in metal–ligand bonding. Indeed, in the 3D representation (ESI Movie 2†) only this non-bonding lone pair is readily observed, which compares well to theoretical analysis of similar complexes in the literature.30 The fact that nitrito-(η1-NO) is a less delocalised system, as argued above, likely explains the lack of an obvious donor lone pair for the Pd(1)–O(1A) interaction. The localisation of charge within the nitrito ligand itself provides less density for dative bonding to PdII, making nitrito-(η1-NO) the poorer σ-donor. This could also explain why nitro-(η1-NO2) is the thermodynamically-favoured isomer at ambient conditions, while the weaker bound nitrito-(η1-NO) is a metastable state. However, despite these visual observations, topological analysis does confirm that a bond path exists between Pd(1) and O(1A), with a (3,−1) BCP located along this path (Fig. S1.2 and Table S1.5†).
(1) |
In regions of both covalent bonding and NCIs, s will have very small, near-zero values. Thus, it is a useful indicator to identify intra- and intermolecular bonding features. NCI analysis extends this by using density derivatives (specifically the second eigenvalue of the Laplacian, λ2) to distinguish between different types of NCIs. Specifically, the value of the function sign(λ2ρ) determines whether an NCI is non-bonding (sign(λ2ρ) > 0, for e.g. a close-contact steric interaction, or bonding (sign(λ2ρ) < 0), for e.g. a hydrogen bond.31
Fig. 3(a) and (b) show scatterplots of s vs sign(λ2ρ), computed for the GS nitro-(η1-O2) and ES nitrito-(η1-NO) photoactive cations respectively. In these plots, sharp features at low values of s correspond to NCIs, with the red-green-blue colour-coding highlighting the value of sign(λ2ρ), and thus the type of NCI represented (red = non-bonding, green = van der Waals and blue = bonding NCIs). The scatterplots essentially provide a fingerprint of the unique combination of NCIs for the GS and ES, respectively, and a quick visual comparison between them immediately highlights the similarities and differences between isomers. Fig. 3(c) and (d) show 3D representations of the same information, superimposed onto the molecules as isosurfaces of s = 0.5 that are colour-coded according to the same red-green-blue scale.
Fig. 3 Non-covalent interaction (NCI) analysis for GS and ES isomers of 1, using the reduced density gradient (RDG, s). Theoretical analysis: (a) plot of s vs sign(λ2ρ) for the GS nitro-(η1-O2) cation and (b) plot of s vs sign(λ2ρ) for the ES nitrito-(η1-NO) cation, providing a fingerprint of the NCIs in each isomer [generated in Multiwfn25 and visualised in Gnuplot33]. (c) 3D plot of s isosurfaces (s = 0.5 a.u.) for the GS nitro-(η1-O2) cation and (d) the ES nitrito-(η1-NO) cation, highlighting the key NCIs in each isomer: colour-coding links to (a)/(b) plots with red = regions of sign(λ2ρ) > 0 i.e. steric interactions, green = regions of sign(λ2ρ) ≈ 0 and blue = regions of sign(λ2ρ) < 0 i.e. bonding interactions e.g. hydrogen bonds [generated in Multiwfn and visualised in VMD26]. HAR analysis: (e) 2D s (RDG) map for the GS structure of 1 in the Pd(1), N(1), O(1), O(2) plane and (f) 2D s (RDG) map for the ES structure of 1 in the Pd(1), O(1A), N(1A), O(2A) plane, showing key NCIs in the asymmetric unit [generated by HAR in NoSpherA29]. |
Common features of GS and ES plots are steric repulsions (red regions) at the positions of ring critical points (RCPs) in the chelating Bu4dien ligands and other steric repulsions between the butyl moieties and Pd(1). The scatterplots show that these steric interactions change only marginally on excitation. There are also two bonding-type intermolecular interactions from butyl hydrogens H(6B) and H(14A) to Pd(1) (light blue surfaces) in both the GS and ES molecules, which match with (3,−1) BCPs identified in the topological analysis (Tables S1.4/1.5†).
Key differences include changes in van der Waals interactions (green regions, sign(λ2ρ) ≈ 0), i.e. weak intramolecular C–H⋯O contacts between the butyl hydrogens and the nitrite group, which are clear in the 3D surface plots and the scatterplots. The sharper and longer green “spikes” in the ES plot indicate a decrease in s and so shorter, stronger contacts on excitation, which is supported by a comparison of the H(6A)⋯O(2)/O(2A) and H(14A)⋯O(2)/O(2A) short contact distances between the GS and ES structures. However, the biggest change in NCIs between the GS and ES isomers involves a new interaction between Pd(1) and the terminal O(2A) atom in the ES nitrito-(η1-NO) ligand. This is highlighted by the dark blue region along the O(2A) → Pd(1) direction in Fig. 3(d), and new blue features at ca. −0.043 sign(λ2ρ) in Fig. 3(b) that are absent in Fig. 3(a). These results indicate that there is a stabilising contact between O(2A) and the metal centre, which can be classified as a bonding NCI. This conclusion is backed up by the bond path identified between Pd(1) and O(2A) in the topological analysis, with a (3,−1) BCP 37% along the O(2A) → Pd(1) direction (Fig. S1.2(c) and Table S1.5†).
Fig. 3(e), (f) and S1.3† show 2D plots of s, that have been generated by HAR in NoSpherA2 and so additionally take into account intermolecular interactions within the asymmetric unit between cation, anion and THF solvent molecules. The HAR analysis confirms similar features to those in the theoretical NCI plots, with the key Pd(1)⋯O(2A) interaction in the ES clearly evident in Fig. 3(f). The ES deformation density also provides additional evidence of an interaction (Fig. 1(d)). The depletion of density at PdII (red lobe) has some extension towards O(2A), while the orientation of the O(2A) lone pair also aligns with this depletion, suggesting a degree of orbital overlap that supports a bonding interaction.
For complex 2, the EDD is seen to change between the GS and ES structures, although the changes are generally more subtle than those observed for 1. As for the Pd-complex, the deformation density maps clearly show dative covalent bonding from all ligands to NiII. For the equatorially-coordinated Et4dien ligand, n → 3d* donation from the N(3), N(4) and N(5) lone pairs is clearly observed, with strong alignment between these blue (+ve) density accumulation regions and the red (−ve) density depletion for the 3dx2−y2 antibonding acceptor orbital on Ni(1). It is evident in comparing GS and ES Laplacian maps in Fig. 5 that the VSCCs are more diffuse, albeit slightly, and have more extension along the N → Ni bonding direction for the GS than for the ES, indicating a stronger Ni-to-Et4dien interaction prior to excitation. Theoretical topology analysis neither supports nor contradicts these visual observations, showing no significant change in the positions of the BCPs in the Ni(1)–N(3), Ni(1)–N(4) or Ni(1)–N(5) bonds, as a percentage of the overall bond length, between the GS and ES structures (Fig. S2.3†). However, a comparison of the experimental bond lengths confirms an expansion of the Et4dien coordination sphere on excitation, with all 3 Ni–N bond distances undergoing a small but significant increase (ΔNi(1)–N(3) = +0.0049(15) Å, ΔNi(1)–N(4) = +0.0061(14) Å and ΔNi(1)–N(5) = +0.0098(14) Å, Tables S2.2/S2.3†). These observations are in direct contrast to analysis of 1, although the changes again likely reflect the change in the EDD that occurs on excitation.
Fig. 4(c)/(d) and 5(c)/5(d) again support visual analysis of the density changes within the nitrite ligands and, in the case of 2, the 2D contour plots are useful to study the EDD in both the isomerising η1-NO2 and photoinert η2-O,ON groups. The deformation density plots in Fig. 4(c)/(d) again clearly show the dative donor–acceptor bonding between the spectator nitrito-(η2-,N) ligand and NiII, with evidence of good orbital overlap of the O(3) lone pair with 3dx2−y2, and the O(4) lone pair with the (also antibonding) 3dz2 orbital in the GS and ES. As for Et4dien, the Laplacian plots (Fig. 5(c)/(d)) indicate slightly more extension of the O(3) and O(4) VSCCs towards Ni(1) in the GS, which is supported by a slight increase in the Ni–O bond distances after excitation (ΔNi(1)–O(3) = +0.0080(14) Å and ΔNi(1)–O(4) = +0.0109(14) Å) indicating a weakened interaction. The Laplacian plots (Fig. 5(c)/(d)) clearly show the delocalisation of charge across N(2), O(3) and O(4), which is supported by very similar theoretical bond orders for N(2)–O(3) and N(2)–O(4) (Table S2.7†). Finally, comparison of the isomerising η1-nitrite ligands completes the picture of how the EDD changes as a result of photoswitching. Within both the GS and ES ligands there is again clear delocalisation across the GS N(1), O(1) and O(2) and the ES N(1A), O(1A), O(2A) atoms, respectively, although it is evident that the VSCCs are more localised for the ES nitrito-(η1-NO) ligand than for GS nitro-(η1-O2), in line with, though less pronounced than, the differences seen for 1. This increased localisation in the ES is also broadly supported by the results from theoretical NBO analysis. The σ(N–O) bond in the GS is 55.5% localised on O in N(1)–O(1) and 51.3% in N(1)–O(2), as an average over the α and β spin orbitals, with bond orders of 1.30 and 1.21 respectively (Table S2.7†). This transforms to an average of 54.4% localisation on O in N(1A)–O(1A) and 56.3% in N(1A)–O(2A) in the ES, with less equal bond orders of 1.16 and 1.34, indicating that, overall, the σ(N–O) bonds are slightly more polar in the ES. For the ES nitrito-(η1-NO) ligand, as in 1, charge is primarily concentrated into the N(1A) lone pair, which is evident in the deformation density (Fig. 4(d)) and in the positions of the calculated N–O BCPs, which both move symmetrically 1% closer to the central nitrogen atom in the ES (Fig. S2.3†).
Despite the similarities in the EDD within the isomerising ligands, experimental bond lengths show that the NiII–nitrite bond distance actually decreases by −0.0221(14) Å on excitation of 2, which is again the opposite change to that seen in 1. This decrease in the bond length is not particularly well evidenced in the deformation density (Fig. 4(c) vs (d)) where there appears to be stronger matching of the GS N(1) lone pair with the 3dz2 acceptor orbital compared to the corresponding O(1A) → Ni(1) donation in the ES. Similarly, comparison of the Laplacian plots (Fig. 5(c) vs (d)) shows a larger, more diffuse region of −∇2ρ(r) at N(1) that has greater extension towards the metal than the corresponding ES feature, which would typically indicate better n → 3d* donation in the GS.
Fig. 6 Non-covalent interaction (NCI) analysis for the GS and ES isomers of 2, using the reduced density gradient (RDG, s). Theoretical analysis: (a) plot of s vs sign(λ2ρ) for GS nitro-(η1-O2) and (b) plot of s vs sign(λ2ρ) for ES nitro-(η1-NO), providing a fingerprint of the NCIs in each isomer [generated in Multiwfn25 and visualised in Gnuplot33]. (c) 3D plot of s isosurfaces (s = 0.5 a.u.) for GS nitro-(η1-O2) and (d) 3D plot of s isosurfaces (s = 0.5 a.u.) for ES nitrito-(η1-NO), highlighting the key NCIs in each isomer: colour-coding links to (a)/(b) plots with red = regions of sign(λ2ρ) > 0 i.e. steric interactions, green = regions of sign(λ2ρ) ≈ 0 and blue = regions of sign(λ2ρ) < 0 i.e. bonding interactions e.g. hydrogen bonds [generated in Multiwfn and visualised in VMD26]. HAR analysis: (e) 2D s (RDG) map for the GS structure of 2 in the Ni(1), N(1), O(1), O(2) plane and (f) 2D s (RDG) map for the ES structure of 2 in the Ni(1), O(1A), N(1A), O(2A) plane, showing key NCIs in the asymmetric unit [generated by HAR in NoSpherA29. |
It is less surprising that in 2 the nitrite ligand forms a stabilising hydrogen bonding NCI with the available N–H donor on Et4dien. This intramolecular N–H⋯O bond can be classified as moderately-strong34 and it is clear that it is the most important NCI to the nitrite ligand for complex 2. A comparison of the NCI analyses (for 1 and 2) indicate that the N(4)–H(4)⋯O(2A) in 2 is a stronger and more stabilising contact than the Pd(1)⋯O(2A) interaction, which is the only key stabilising NCI to the nitrite in 1. This is evident in a comparison of the EDDs, where for 2 there is clear matching of regions of electron accumulation on O(2A) and depletion at H(4) for the formation of the bonding NCI (Fig. 4(d)), compared with poorer overlap in 1 between the available O(2A) lone pair with the 4dx2−y2 acceptor orbital in Fig. 1(d). The NCI analysis also supports this comparison. Contrasting the ES scatterplots for 1 (Fig. 3(b)) and 2 (Fig. 6(b)), we can see that the hydrogen bond interaction in 2 is associated with a smaller reduced density gradient of s ≈ 0.05, indicating a more strongly bonding NCI, compared with a value of s ≈ 0.10 for the Pd(1)⋯O(2A) interaction. Theoretical studies on related metal-nitrite systems in the literature predict similar stabilising interactions between endo-nitrito-(η1-NO) and suitable donor groups within the molecule, where available,30 however a broader investigation of other linkage isomer switches, capable of achieving different final ES population levels, is required to make a thorough assessment of how necessary such NCIs are to facilitate good nitro → nitrito photoswitching. Another key comparison is that the formal metal–nitrite bonding interaction Pd(1)–N(1)/O(1A) in 1 is lengthened and weakened on excitation, while conversely the analogous Ni(1)–N(1)/O(1A) bond in 2 appears to strengthen with irradiation. It is possible that this difference reflects HSAB theory, as it might be expected that the “hard” O-donor in the ES nitrito-(η1-NO) should have better affinity for NiII than for PdII, as the 3d8 metal is also expected to be the Lewis acid.
All of the above results indicate that the nitrito-(η1-NO) isomer should be a more stable ES for complex 2 relative to complex 1, which should have some manifestation in the physical properties of each system. For 2, we note that the nitrito-(η1-NO) isomer can be thermally-occupied and is present at room temperature,17 while conversely, in 1 the ES nitrito-(η1-NO) isomer has only ever been observed as a light-induced metastable state. This fits with the conclusion from charge density analysis that nitrito-(η1-NO) is better stabilised in 2 than in 1, and potentially explains the thermal accessibility of endo-nitrito in complex 2 under ambient conditions.
It should also be noted that the nitrite ligands in 1 and 2 are involved in van der Waals NCIs (green in the NCO analyses) with alkyl moieties on the ethylenetriamine co-ligands, which are found to shorten quite significantly in the ES of 1, but do not change significantly for 2. However, as the NCI analysis clearly shows that these C–H⋯O contacts have a less bonding character than the Pd(1)⋯O(2A) and N(4)–H(4)⋯O(2A) interactions, this indicates that they are less likely to be as influential.
In terms of the photostability of the ES nitrito-(η1-NO) arrangements, the HAR and NCI analyses do not provide any significant new understanding. For both 1 and 2, nitrito-(η1-NO) is the photoinduced metastable state, which suggests that it should be less stable than nitro-(η1-O2). Additionally, comparison of the photoreaction rates and metastable limits indicates that the ES isomer is more favourable in 1, as it can be accessed more quickly (>15 min irradiation for 100% population in 1, vs ∼1 h for 2) and remains metastable to a higher critical temperature (240 K in 1vs 140 K in 2).16,18 It is therefore evident that the photoexcited state stability must be influenced by other factors than the EDD. These likely include the absorption properties and photophysics of the material, kinetic factors e.g. the relative kinetic lability of the differing metal centres, and steric influences from the surrounding crystal lattice. Many of these factors have been discussed by us5,28,35,36 and others37–43 previously, and this conclusion highlights the complexity of rationally-designing solid-state photoswitchable crystals and the importance of considering the many, and often competing, influential factors that govern the photoreaction.
The analysis of the EDD in 1 and 2 agrees that there is a more pronounced localisation of charge in the ES nitrito-(η1-NO) ligands than in GS nitro-(η1-O2). The results for complex 1 highlight that this indicates a reduction in the σ-donor ability of nitrito-(η1-NO), as the greater degree of localisation provides less density for dative bonding to the metal centre. It follows from this observed change in σ-donor ability that nitro-(η1-O2) is a stronger-field ligand than nitrito-(η1-NO), reflecting the fact that nitro-(η1-O2) is typically reported to be higher in the spectrochemical series.44 Given this, it might be expected that some evidence of a change in trans-influence can be found in the bond lengths, CCs and BCP positions between Pd(1)/Ni(1) and the auxiliary ligands in 1 and 2, respectively.
As discussed earlier, for complex 1 while the Pd–nitrite bond distance increases on excitation, the experimental and theoretical Pd–Bu4dien bond lengths all shorten in the ES, including the Pd(1)–N(3) distance directly trans- to the isomerising group. Thus, at first glance there does appear to be a shift in trans-influence on excitation of 1, based on bond length changes alone, as the switch to the weaker-field nitrito-(η1-NO) donor is expected to correlate with a shortening, and thus strengthening, of the metal–ligand bond length trans- to itself (i.e. the Pd(1)–N(3) distance). Conversely, comparison of the calculated Pd–N bond orders for the GS and ES structures tends not to support this observation. Table S1.7† shows that the Pd(1)–N(3) bond order actually decreases from 0.63 in the GS to 0.59 in the ES, despite the observed (and calculated) bond shortening, although the cis-coordinated Pd(1)–N(2) and Pd(1)–N(4) bonds do show the expected bond order increase. The topological analysis does not provide strong evidence for either interpretation, with no significant shift in the positions of (3,−1) BCPs, as a percentage along their bond paths (Fig. S1.3†). However, a visual comparison of Fig. 1(a) and (b) reveals that the N(3) lone pair is more diffuse along the Pd → N direction in the ES than in the GS, which would indicate a strengthening of the Pd(1)–N(3) bond and therefore support the interpretation of some trans-influence evident in complex 1. A similar analysis can be completed for complex 2 to try and assess the validity of significant trans-influence. In contrast to 1, the Ni–nitrite bond distance actually decreases on excitation, which is at odds with the visual assessment of the EDD in the GS and ES and does not support the idea that nitrito-(η1-NO) is the weaker-field ligand in this case. All metal–ligand distances to the auxiliary Et4dien and nitrito-(η2-,N) ligands are found to increase in the ES isomer, with the largest change in the Ni(1)–O(4) bond (trans- to the isomerising group and so competing for the 3dz2 acceptor orbital). The theoretical bond orders agree with the experimental bond length changes (Table S2.7†), indicating a strengthening of the Ni–nitrite interaction on excitation and a corresponding weakening in the bonding interactions to the auxiliary ligands. Topological analysis again provides only limited information, with very little change observed on excitation excepting that while the Ni(1)–O(4) BCP moves 1% closer to NiII in the ES isomer on excitation, the Ni(1)–N(1)/O(1A) BCP mirrors this change, moving 1% closer to the nitrite ligand (Fig. S2.3†). In summary, though the changes for complex 2 are the reverse of those seen in complex 1, in both systems there appears to be some synergistic changes in the EDD of the isomerising nitrite group and the ligands trans- to them, which must compete for the same d-orbitals on the metal. As such, there appears to be some evidence of trans-influence in the EDD for both 1 and 2, although the question of whether nitro-(η1-O2) or nitrito-(η1-NO) is the weaker field ligand in both ligand fields is not clear.
As well as the possible influence of HSAB rules, discussed above, another explanation for the apparently conflicting bond length changes between 1 and 2 is that there are competing steric and electronic effects that have a combined influence on the geometric parameters seen. For example, in complex 1 it is evident that in the ES the PdII 4dx2−y2 orbital is forced to tilt slightly to accommodate bonding to O(1A) (Fig. 1(c) vs (d)). This results in better overlap between 4dx2−y2 and donor lone pairs on Bu4dien, particularly for N(3), which can also account for the shortened Pd–N bond lengths. This is supported by more diffuse VSCCs for the ES isomer, indicating better donor–acceptor overlap (Fig. 2(d)) and by a reduction of the RMS deviation from the ideal square plane in the ES (GS RMSD for Pd(1), N(1), (N2), N(3) and (N4) = 0.0976, compared to ES RMSD for Pd(1), O(1A), N(2), N(3), N(4) = 0.0652). It is possible that, to accommodate the required geometry changes for best Pd–Bu4dien overlap, whilst also maintaining the Bu4dien’s chelating “bite” around PdII, the Pd(1)–N(3) bond is also forced to contract, regardless of any underlying trans-influence. For 2, though any reorientation of 3dz2 is less obvious in Fig. 4(c)/(d), any tilting would be less well accommodated by the bidentate nitrito-(η1-O,ON) ligand, which necessarily has a more restricted “bite” angle, leading to an overall lengthening of the Ni–η2-nitrito interaction. Steric crowding around each metal centre may also have an effect on the achievable metal–ligand overlap, which necessarily varies for the two different coordination environments. If steric and electronic influences are in competition, this clearly complicates the interpretation of simple geometric parameters, e.g. bond lengths and angles, which makes a stronger argument for the use of more involved analyses, such as HAR and charge density studies, to further investigate the complex variations in metal–ligand bonding between the GS and ES.
Finally, it should be noted that, while the HAR analysis presented allows some analysis of intermolecular interactions within the asymmetric unit, it does not account for interactions between adjacent asymmetric units and so provides no insight into the influence of the wider crystal structure. This is the key disadvantage for semi-empirical methods over experimental charge density studies, e.g. multipolar refinements, as some level of approximation must still be made in the wavefunction calculation. Previous studies have shown it is possible to take into account some of these wider interactions, for example by running HAR on dimers of the target unit across symmetry positions and assessing how the EDD changes in reference to the isolated unit.13 Unfortunately, this is beyond the scope of the current study as, due to the size and complexity of 1 and 2 this approach is too computationally intensive to be viable. For 1 and 2, intermolecular NCIs to the nitrite ligand are exclusively C–H⋯O and C–H⋯N short contacts that, while expected to be weaker in nature, may still have a combined effect in stabilising the nitro-(η1-O2) and/or nitrito-(η1-NO) isomers.16,18 As such, future work will look to improve on these limitations, aiming to incorporate nearest neighbours into the HAR analysis.
Footnote |
† Electronic supplementary information (ESI) available: Crystal structure data are not submitted to the CCDC as these structures are determined previously, CSD ref codes: WABGUE, WABHAL, WUZWIZ (1) and IYIPUC, IYIQIR, IYIRAK (2). See DOI: https://doi.org/10.1039/d2fd00158f |
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