Iodine(i) complexes incorporating sterically bulky 2-substituted pyridines
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This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY-NC 3.0 https://creativecommons.org/licenses/by-nc/3.0/ Iodine(i) complexes incorporating sterically bulky 2-substituted pyridines © 2022 The Author(s). Published by the Royal Society of Chemistry Published version Ward, Jas S.; Gomila, Rosa M.; Frontera, Antonio; Rissanen, Kari Ward, J. S., Gomila, R. M., Frontera, A., & Rissanen, K. (2022). Iodine(i) complexes incorporating sterically bulky 2-substituted pyridines. RSC Advances, 12(14), 8674-8682. https://doi.org/10.1039/d2ra01390h 2022
Iodine(I) complexes incorporating sterically bulky 2-substituted pyridines† Jas S. Ward, * a Rosa M. Gomila, b Antonio Frontera b and Kari Rissanen a The silver(I) and iodine(I) complexes of the 2-substituted pyridines 2-(diphenylmethyl)pyridine (1) and 2-(1,1diphenylethyl)pyridine (2), along with their potential protonated side products, were synthesised to investigate the steric limitations of iodine(I) complex formation. The complexes were characterised by 1 H and 1 H– 15 N HMBC NMR, X-ray crystallography, and DFT calculations. The solid-state structures for the silver(I) and iodine(I) complexes were extensively compared to the literature and analysed by DFT to examine the influence of the sterically bulky pyridines and their anions. Introduction Since their advent in the 1960s, 1,2 halogen(I) (also known as halonium) ions, X + (X ¼Br, I), stabilised by a pair of Lewis bases (L) in the form [L–X–L] + , have existed as examples of halogen group elements formally in the unusual +1 oxidation state, though it was not until the 1990s that they gained mainstream recognition due to the myriad of organic transformations they were dely demonstrated to effect. 3–5 In addition to this utility, halogen(I) ions possess other favourable properties arising from their s-hole interaction, 6 most notably the reliable high degree of linear directionality which has been fruitfully utilised in selfassembling supramolecular architectures, 7–9 and recently in coordination polymers such as halogen-bonded organic frameworks (XOFs). 10 Halogen(I) complexes can be straightforwardly synthesised in a one pot reaction by addition of an elemental halogen, X 2 (X ¼Br, I) to the analogous 2-coordinate silver(I) complex by Ag + to X + cation exchange, 11,12,14 or as was recently shown, also from 3coordinate silver(I) complexes via partial cation exchange. 15,16 The use of substituted pyridines as the stabilising Lewis bases dominates the literature of halogen(I) complexes, and of those examples, it is pyridines substituted in the 4-position which overwhelmingly comprise the largest subset aer pyridine itself. 17 The 4-position of coordinating pyridines is one that can be described as only electronically affecting halogen(I) ion formation, and has been previously utilised to explore that relationship in halogen(I) complexes. 11 With respect to the relationship of sterics toward halogen(I) formation, currently the iodine(I) complexes in the literature with the most steric bulk around the I + ion are those incorporating 2,6-dimethylpyridine (2,6-lutidine) and 2,4,6-trimethylpyridine, 18–22 as well as a single solid-state example of a bromine(I) complex with quinoline as the Lewis base, 23 though this is not including the molecular clamps reported by Erd´ elyi and co-workers as those ligands would also provide an additional stabilising contribution via the chelate effect. 9,11 Barluenga's reagent, [I(py) 2 ]BF 4 (py ¼pyridine), the ubiquitous iodination reagent for which iodine(I) chemistry owes its current renown, is commercially available and demonstrates a vast scope of utility, however, decomposition is observed over time. Therefore, an expansion of the pyridine scaffold would be an ideal starting point to explore the steric limitations of halogen(I) ions, and their potential applications toward a new generation of halogen(I) reagents. Results and discussion Synthesis and solution studies The silver(I) complexes [1–Ag–1]PF 6 (1a) and [2–Ag–2]PF 6 (2a) were synthesised quantitatively from the two sterically bulky pyridine-based ligands, 2-(diphenylmethyl)pyridine (1) and 2Scheme 1 The synthesis of silver(I)(1a,2a) and iodine(I)(1b,2b) complexes of 2-(diphenylmethyl)pyridine (1) and 2-(1,1-diphenylethyl) pyridine (2). a University of Jyvaskyla, Department of Chemistry, Jyv¨ askyl¨ a 40014, Finland. E-mail: james.s.ward@jyu. b Department of Chemistry, Universitat de les Illes Balears, Crts de Valldemossa km 7.6, 07122 Palma de Mallorca, Baleares, Spain †Electronic supplementary information (ESI) available: Synthesis, NMR, computational details, and X-ray. CCDC 2144042–2144045, 2150094–2150097. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d2ra01390h Cite this: RSC Adv., 2022, 12, 8674 Received 2nd March 2022 Accepted 14th March 2022 DOI: 10.1039/d2ra01390h rsc.li/rsc-advances 8674 |RSC Adv., 2022, 12,8674–8682 © 2022 The Author(s). Published by the Royal Society of Chemistry RSC Advances PAPER Open Access Article. Published on 21 March 2022. Downloaded on 3/22/2022 4:43:55 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online View Journal | View Issue
(1,1-diphenylethyl)pyridine (2), respectively (Scheme 1). The synthesis of the iodine(I) analogues, [1–I–1]PF 6 (1b) and [2–I–2] PF 6 (2b), were performed by addition of an equivalent of elemental iodine. The reactions were all followed by 1 H and 1 H– 15 N correlated NMR spectroscopy. The 1 H NMR spectra of the free ligand 1,toAg + complex 1a, and nally to I + complex 1b revealed that all peaks demonstrate noticeable shis for each transformation, with the ranges of 0.04–0.56 ppm (1to 1a) and 0.01–0.84 ppm (1a to 1b), the most apparent being those for the downeld pyridyl and upeld methine resonances that are free from overlapping chemical shis with the pendant phenyl rings (Fig. 1). Whilst the 1 H NMR data did provide clear indications of clean reactions occurring, they could not themselves point toward the identity of the products. It should be noted that the pyridyl resonances in 1a (green) do not follow the trend of shiing toward downeld as observed for 1to 1b, which is likely due to the increased electron density on the pyridyl rings due to retro-donation with the Ag + metal centre. In contrast, from the 1 H– 15 N HMBC NMR experiments, the 15 N NMR chemical shisof65.4 ppm (1), 118.6 ppm (1a), and 165.7 ppm (1b) are characteristic of the desired conversions having been achieved, resembling results observed for other iodine(I) complexes of 4-substituted pyridine analogues. 12,13 Anticipating an increased likelihood of decomposition due to the steric hindrance of the substituents in the 2positions of the pyridyl rings, the protonated (1c) and hydronium (1d) complexes were deliberately synthesised for comparison, which gave 15 N NMR chemical shisof167.5 and 122.5 ppm, respectively. Given the reactivity of halogen(I) complexes, their propensity to decompose to protonated species, and the similarity of the 15 N NMR chemical shisof1a to 1d (Dd 15N ¼3.9 ppm) and 1b to 1c (Dd 15N ¼1.8 ppm), caution must always be taken in characterising halogen(I) species based solely on NMR spectroscopy data. Nevertheless, the identity of 1a and 1b were denitely conrmed by single crystal X-ray diffraction studies. The conversion of the free ligand 2to the Ag + complex 2a demonstrated similar changes in the 1 H NMR chemical shisas observed for 1to 1a, with a range of 0.07–0.90 ppm, and 15 N NMR chemical shisof65.1 ppm (2) and 111.4 ppm (2a). However, despite the precipitation of the AgI by-product of cation exchange upon addition of elemental iodine to effect the transformation of 2a to 2b, NMR studies suggested that the desired iodine(I) complex 2b had already begun to decompose within minutes of its inception. The 1 H NMR spectrum showed that the pyridyl protons were signicantly broadened, and the concomitant 1 H– 15 N HMBC experiment gave a 15 N NMR chemical shiof 121.5 ppm.‡This 15 N NMR chemical shi was far from the expected value of approximately 165 ppm for the desired iodine(I) complex 2b, though it did match well to the independently synthesised hydronium species [2–H–2]PF 6 (2d), which had a 15 N NMR chemical shiof 123.0 ppm. Solid-state studies The solid-state structure of 1a contained two half cations which self-completed by symmetry, and similarly 1b contained just one half, which in both instances ensured that all N–Ag–N and N–I–N angles were symmetry enforced, i.e., perfectly linear. The slightly elongated Ag–N bond lengths of 2.151(2) and 2.162(2) ˚ A in 1a, in combination with the I–N bond length of 2.273(3) ˚ Ain 1b, were as expected and unremarkable when compared to those observed for [Ag(pyridine) 2 ]PF 6 (2.129(6) ˚ A) and [I(pyridine) 2 ]PF 6 (2.268(2) ˚ A), 24,25 or even the more closely related 2-ethylpyridine (2-Etpy) derivatives [Ag(2-Etpy) 2 ]PF 6 (2.128(3)/2.130(3) ˚ A) and [I(2-Etpy) 2 ]PF 6 (2.270(2) ˚ A), 13 with only the silver(I) comparisons being slightly beyond a 3stolerance and therefore crystallographically distinguishable from one another. It should be noted that the previously reported [I(2Etpy) 2 ]PF 6 adopted a counterintuitive syn-conguration of the 2ethyl substituents, with concomitant loss of co-planarity (an angle of 32.4between the planes of the two pyridyl rings was found). However, in both 1a and 1b, the ligands were co-planar and, as expected, assumed anti-congurations (Fig. 2) due to steric considerations, with one of the two pendant phenyl rings pointing directly away from the Ag + or I + centres, respectively. Interestingly, the I + centre of 1b is noticeably exerting an increased repulsion on the 2-substituents, despite the ligands being further apart from one another and the I + when compared Fig. 1 The superimposed 1H NMR spectra of 1(red), 1a (green), and 1b (blue) of their non-overlapping pyridyl and methine resonances in CD 2 Cl 2 (500 MHz, 298 K). Fig. 2 The X-ray crystal structures of the cations of [1–Ag–1]PF 6 (1a; left) and [1–I–1]PF 6 (1b; right) showing the anti-configuration of the ligands due to steric considerations (PF 6 anions omitted for clarity; thermal ellipsoids at 50% probability). ‡The 1 H NMR spectrum was collected within 5 minutes of I 2 addition, but a satisfactory 1 H– 15 N HMBC experiment took several hours to complete to give the chemical shistated. © 2022 The Author(s). Published by the Royal Society of Chemistry RSC Adv., 2022, 12,8674–8682 | 8675 Paper RSC Advances Open Access Article. Published on 21 March 2022. Downloaded on 3/22/2022 4:43:55 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
to the Ag + in 1a. This distortion is not readily apparent in the previously discussed bond lengths or angles, but can be quantied by comparison of the I + /pyridyl(C2)/I + /pyridyl(C6) through-space distances in 1b of 3.074(5)/3.252(4) ˚ A, which are reminiscent of those observed for [I(2-Etpy) 2 ]PF 6 of 3.086(2)/ 3.244(2) ˚ A, and are clearly deviating from the more alike pair of distances of Ag + /pyridyl(C2)/Ag + /pyridyl(C6) in 1a of 3.065(2)/3.070(2) ˚ A and 3.050(3)/3.085(2) ˚ A. Whilst a few examples of discrete silver(I) complexes have been reported with similarly sterically bulky 2-substituted pyridines, 17,26 though all less sterically encumbered than 1a and 2a, no examples of such sterically endowed iodine(I) complexes currently exist in the literature. The solid-state structure of 2a (Fig. 3) revealed signicantly lengthened Ag–N bond lengths of 2.210(2) and 2.210(2) ˚ A, which are, as best as can be determined, 17 some of the longest known to date for a discrete, linear silver(I) complex incorporating 2substituted pyridines, only rivalled by those of [Ag(4-(phenylethynyl)pyridine) 2 ] + (2.214(5) and 2.217(5) ˚ A). 27 Whilst longer Ag–N bond lengths are present in the literature, in those instances the elongation can be associated to the complexes exhibiting signicantly distorted N–Ag–N angles due to partial coordination from another donor to the silver(I) centre, such as coordination from an anion, 28,29 or from strain caused by the ligand system due to it being dimeric/polymeric in nature, 30 or both. 31 Unlike in 1a and 1b, the pyridyl ligands in 2a are not coplanar, nor do they assume an anti-conguration, with a N–Ag– N angle of 173.04(7)and an angle of 78.4between the planes of the two pyridyl rings. If the deviation from linearity of the N–I–N angle in 2b was similar to that observed for 2a, then this could contribute to the high reactivity of 2b, given that the largest observed deviation from linearity for an iodine(I) complex is for [I(2-Etpy) 2 ]PF 6 with a N–I–N angle of 173.62(10). 13 Whilst an increase in reactivity for 2b was anticipated relative to 1b due to the slightly increased steric bulk of its substituent, the rapid decomposition of 2b was particularly striking in comparison to the only minimal decomposition that was observed aer 8 days for a sample of 1b kept in solution for the duration. The persistence of 1b is drastically longer than for many other known iodine(I) complexes incorporating Lewis bases with sterically negligible substituents in the 2-positions, such as 2-ethylpyridine and 1-ethylpiperidine, 14 both of which demonstrated behaviour resembling that of 2b.§. Computational studies DFT calculations (M06-2X/def2-TZVP level of theory, see ESI† and theoretical methods below for details) were performed to investigate the counterintuitive syn-conguration of 2a and also to characterise computationally the elusive 2b structure that could not be structurally characterised by X-ray diffraction methods. Fig. 4 shows the optimised structure of 2a and the hypothetical anti-conguration (denoted as 2a0), that exhibits a perfectly linear N–Ag–N angle and co-planar pyridyl rings. This conguration is 2.3 kcal mol 1 less stable than the syn-conguration, in line with the experimental observation. The DFT optimised structure exhibits a N–Ag–N angle of 172 and angle between the pyridyl rings of 81, in good agreement with the experimental values (cf. 173and 78, respectively). Similar agreement, including distances, was observed for the rest of complexes (see ESI, Table S2†), thus giving reliability to this level of theory. The larger stability of the syn-conguration in 2a is most likely due to the contribution of van der Waals interactions between the methyl groups and the aromatic rings, as revealed by the noncovalent interaction plot analysis (NCIPlot index, see Fig. S43 in the ESI†). The geometries of the synand anti-congurations of compound 2b were also calculated (Fig. 5). The calculations reveal that both isomers are practically isoenergetic (the synconguration is only 0.2 kcal mol 1 more stable). A likely explanation is that the stabilisation due to the co-planarity of the rings in the anti-conguration is more important in the iodine(I) complex 2b than in the silver(I) complex 2a. In fact, the Fig. 3 The X-ray crystal structure of the cation of [2–Ag–2]PF 6 (2a) (PF 6 anion omitted for clarity; thermal ellipsoids at 50% probability). Fig. 4 M06-2X/def2-TZVP optimised geometries of the syn (a) and anti (b) configurations of the cation of 2a, with indication of the relative energy. The methyl groups are represented in red. §Despite decomposition of [I(2-Etpy) 2 ]PF 6 being observed within minutes of its synthesis by NMR studies, it was robust enough for its solid-state structure to be obtained. 13 8676 |RSC Adv., 2022, 12,8674–8682 © 2022 The Author(s). Published by the Royal Society of Chemistry RSC Advances Paper Open Access Article. Published on 21 March 2022. Downloaded on 3/22/2022 4:43:55 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
anti-conguration facilitates the back-donation from the iodine atom (free lone pair) to the p-system, which is important in iodine(I) complexes. 14 This effect compensates for the van der Waals interactions that are established in the syn-conguration, which are also less important in 2b, as revealed by the NCIPlot index (Fig. S43†). The geometries of the Ag + and I + compounds (2a and 2b) in the anti-conguration are very similar. In contrast, those in the syn-conguration (2a0and 2b0) are more different, especially regarding the pyridyl ring plane angles that differ by 21(cf. 60in 2b0and 81in 2a0). Finally, the dissociation energies (measured as [L–I–L] + to I + + 2L; E dis ) and I–N distances of the iodine(I) complexes 1b and 2b were compared with previously reported examples instead incorporating the quinuclidine (quin) and dimethylaminopyridine (DMAP) ligands (Table 1). 14 The results show the greatest stability for the [I(quin) 2 ] + and [I(DMAP) 2 ] + complexes followed by 1b and 2b in line with the I–N distances that are signicantly longer in the latter complexes. The lowest dissociation energy corresponds to compound 2b, which agrees with its higher reactivity as observed in the NMR experiments. Anion and packing effects In halogen(I) chemistry, the BF 4 and PF 6 anions are traditionally used owing to their weakly coordinating nature, which therefore do not complicate the well-established Ag + to X + (X ¼Br, I) cation exchange process used to synthesise halogen(I) complexes. In the solid state, both the silver(I) complexes 1a and 2a showed meaningful interactions with their respective PF 6 anions, though with 2a as a discrete ion pair and 1a as a continuous 1D array of Ag + /F–PF 4 –F/Ag + /F–PF 4 –F contacts (Fig. 6). The closest Ag + /F distances of 2.918(2)/ 2.990(2) ˚ A for the two independent molecules in 1a were below the combined van der Waals radii of for these atoms (Ag + F¼3.19 ˚ A), though are comparable to other previously reported linear silver(I) complexes, such as [Ag(2-Etpy) 2 ]PF 6 and [Ag(DMAP) 2 ]PF 6 . 12,13 Similarly, 1a was able to successfully synthesise 1b via cation exchange, just as has been reported for the aforementioned literature complexes. However, the closest Ag + /F distance of 2.810(1) ˚ Ain2a is signicantly shorter than those observed in 1a, and is reminiscent of silver(I) complexes with more strongly coordinating anions, such as those bearing potential oxygen donors like the nitrate anion. The more strongly bound PF 6 anion in 2a is likely an outcome of the sterically bulky ligands preventing optimal electronic stabilisation for the Ag + centre, which was similarly indicated by the long Ag–N bond lengths observed in 2a (vide supra). With respect to 1b, similar to 2a, it also existed as a discrete ion pair. Previous studies have conrmed that iodine(I) ions intrinsically impose a linear 2-coordination sphere, and unlike their silver(I) counterparts, are insensitive to the identity of the anion present. 25 This is apparent in 1b with a pair of shortest I + /F distances of 3.618(5), which were reminiscent of those observed in [I(2-Etpy) 2 ]PF 6 (shortest I + /F distances ¼3.693(2) ˚ A), 13 both of which were well over the combined van der Waals radii of these atoms (cf. van der Waals radii of I + F ¼3.45 ˚ A), indicating that neither were meaningful interactions. Intrigued by the inuence of the anions on the silver(I) precursors, the BF 4 ,[1–Ag–1]BF 4 (1e) and [2–Ag–2]BF 4 (2e), and OTf, [1–Ag–1]OTf (1f; OTf ¼triate) and [2–Ag–2]OTf (2f), anion analogues were also prepared and crystallised so comparisons could be made (Fig. 7). These anions were selected as they are commonly used in the preparation of halogen(I) complexes, so were more relevant than anions such as nitrate, and safer than others such as the perchlorate anion. Both 1e and 2e exist as discrete ion pairs with the BF 4 anions, with the shortest Ag + /F Fig. 5 M06-2X/def2-TZVP optimised geometries of the syn (a) and anti (b) configurations of the cation of 2b, with indication of the relative energy. The methyl groups are represented in red. Table 1 Dissociation energies (E dis , kcal mol 1 ), I–N distances (d,˚ A) and N–I–N angles (a,) at the M062X/def2-TZVP level of theory 1b 2b(syn)2b (anti) [I(quin) 2 ] + [I(DMAP) 2 ] + E dis 126.8 119.8 119.6 179.7 181.8 d2.273 2.305 2.301 2.288 2.245 a180 173.8 180 180 180 Fig. 6 The packing of three molecules of 1a showing the 1D network of short Ag + –F intermolecular interactions (all distances in ˚ A; ligands simplified for clarity). © 2022 The Author(s). Published by the Royal Society of Chemistry RSC Adv., 2022, 12,8674–8682 | 8677 Paper RSC Advances Open Access Article. Published on 21 March 2022. Downloaded on 3/22/2022 4:43:55 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
distances of 2.828(5)/2.965(8) ˚ A (the BF 4 anion was found to equally occupy two positions in the solid state) and 2.743(2) ˚ A, respectively. As expected, the OTf anion complexes 1f and 2f were observed as neutral T-shaped species, with the OTf anion bound to the Ag + centres with Ag + –O bond lengths of 2.685(2) and 2.578(2) ˚ A, respectively. These matched reported examples of OTf silver(I) complexes incorporating 2-substituted pyridines in the literature, [Ag(2-methoxypyridine) 2 ]OTf and [Ag(2methylsulfanylpyridine) 2 ]OTf, 32,33 which demonstrated the same T-shaped geometry and similar Ag + –O bond lengths of 2.679(3) and 2.673(3) ˚ A (minor position (19%) of the disordered OTf anion ignored), respectively. These solid-state observations were reected in the solutionstate studies of the 1 H– 15 N HMBC determined 15 N NMR chemical shis (Table 2). The values show that the comparisons of the BF 4 (1e and 2e) and PF 6 (1a and 2a) complexes only exhibit negligible differences for the same ligand (1a vs. 1e and 2a vs. 2e), whilst the OTf complexes showed small, but signi- cant, differences of 4.1 (1a vs. 1f) and 1.4 (2a vs. 2f) ppm when compared to their PF 6 analogues, possibly indicating that the less sterically encumbered complex 1f continues to interact with the OTf anion in solution, at least more so than 2f. A common feature of all silver complexes is that the anion is close to the Ag(I) atom, establishing semi-coordination bonds, or coinage bonds (CiB) according to the nomenclature proposed by some authors. 34,35 These contacts likely inuence the Ag–N distance along with the bulkiness of the ligands. The CiBs in complexes 1a, e, f and 2a, e, f were analysed using the quantum theory of atoms-in-molecules (QTAIM). 36 Fig. 8 shows the QTAIM representation of the six compounds showing in all cases a bond critical point (CP, represented as red sphere) and bond path (orange line) connecting one atom of the anion to the silver atom, thus conrming the existence of the interaction. In all cases both the Laplacian (V 2 r) of the electron density and the total energy density (H r ) at the bond CP are positive, thus revealing that the Ag/F(O) contacts are noncovalent in nature (CiBs). This is corroborated by the small values or r(see Table 3), conrming the weak and closed shell nature of the CiBs. The strength of these CiBs was estimated by using the total energy density at the bond CP and the equation proposed by Espinosa et al. 37 This method is convenient to evaluate the noncovalent interaction without the contribution of the pure coulombic attraction between the counterions. These values are indicated in Fig. 8 (annotated in red close to the CPs). The energies range from 2.3 kcal mol 1 in 1a to 9.5 kcal mol 1 in 2f, in line with the Ag/anion distances (see Table 3). It is interesting to highlight that for the complexes with the shortest Ag/ anion distance of each series (complexes 1f and 2f) the density (r, see Table 3) at the bond CP is greater than that in the rest of Fig. 7 The X-ray crystal structures of [1–Ag–1]OTf (1f; left) and [2– Ag–2]OTf (2f; right) showing the bound OTf anions (pyridyl substituents simplified for clarity; thermal ellipsoids at 50% probability). Table 2 The 1 H– 15 N HMBC determined 15 N NMR chemical shifts (ppm) for the silver(I) complexes [1–Ag–1] + and [2–Ag–2] + with different anions (BF 4 ,PF 6 , and OTf) Complex d N Complex d N [1–Ag–1]PF 6 (1a)–118.6 [2–Ag–2]PF 6 (2a)–111.4 [1–Ag–1]BF 4 (1e)–117.6 [2–Ag–2]BF 4 (2e)–111.9 [1–Ag–1]OTf (1f)–114.5 [2–Ag–2]OTf (2f)–110.0 Fig. 8 QTAIM analysis (only the Ag/anion contact is represented for clarity) of compounds 1a (a), 2a (b), 1e (c), 2e (d), 1f (e) and 2f (f). The dissociation energies are indicated in red adjacent to the bond CPs (red spheres). Table 3 The X-ray Ag–N and Ag/X(X¼F, O) distances, density (r, a.u.) at the bond CPs represented in Fig. 8 and Wiberg bond indexes (WBI) for all the silver(I) complexes synthesised in this work Complex Ag–NAg–Xr10 2 WBI (Ag–N) 1a 2.151 2.990 0.97 0.164 & 0.159 1e 2.150 & 2.155 2.828 1.46 0.144 & 0.150 1f 2.170 & 2.175 2.685 2.23 0.138 & 0.133 2a 2.210 2.818 1.45 0.092 & 0.095 2e 2.215 & 2.219 2.743 1.75 0.081 & 0.091 2f 2.224 & 2.223 2.578 2.67 0.080 & 0.082 8678 |RSC Adv., 2022, 12,8674–8682 © 2022 The Author(s). Published by the Royal Society of Chemistry RSC Advances Paper Open Access Article. Published on 21 March 2022. Downloaded on 3/22/2022 4:43:55 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
complexes, thus suggesting a greater charge transfer from the anion to the Ag(I), thus causing the elongation and weakening of the N–Ag–N bonds, as evidenced by the longer Ag–N distances and smaller Wiberg bond indexes (see Table 3) 38 in complexes 1f and 2f. Conclusions In conclusion, silver(I) and iodine(I) complexes of sterically bulky 2-substituted pyridines were synthesised and spectroscopically characterised to investigate the steric limitations of iodine(I) ion formation. Three of these complexes, including two silver(I) and one iodine(I) complex, were also denitively conrmed by X-ray diffraction studies, thus demonstrating the possibility to form iodine(I) complexes with sterically bulky ligands in close proximity to the I + centre. The effect of the anions on the sterically bulky silver(I) complexes was examined and further explored with a range of different anions, commonly used in halogen(I) chemistry, through extensive DFT studies. DFT calculations were utilised to explain the formation of the syn-isomer of 2a, as well as to study the decreased stability of 2b with respect to 1b and other iodine(I) complexes reported in the literature, highlighting the potential of steric control in future halogen(I) chemistry. Experimental General considerations All reagents and solvents were obtained from commercial suppliers and used without further purication, except for 2- (diphenylmethyl)pyridine (1) and 2-(1,1-diphenylethyl)pyridine (2) which were synthesised according to literature procedures. 39,40 For structural NMR assignments, 1 H NMR and 1 H– 15 N NMR correlation spectra were recorded on a Bruker Avance III 500 MHz spectrometer at 25 CinCD 2 Cl 2 . Chemical shis are reported on the dscale in ppm using the residual solvent signal as internal standard (CH 2 Cl 2 in CD 2 Cl 2 :d H 5.32), or for 1 H– 15 N NMR spectroscopy, to an external d 3 -MeNO 2 standard. For the 1 H NMR spectroscopy, each resonance was assigned according to the following conventions: chemical shi (d) measured in ppm, observed multiplicity, observed coupling constant (JHz), and number of hydrogen atoms. Multiplicities are denoted as: s (singlet), d (doublet), t (triplet), m (multiplet), and br (broad). For the 1 H– 15 N HMBC spectroscopy, spectral windows of 4 ppm ( 1 H) and 300 ppm ( 15 N) were used, with 1024 points in the direct dimension and 512 increments used in the indirect dimension, with subsequent peak shape analysis being performed to give the reported 15 N NMR resonances. The single crystal X-ray data for 1c was collected at 120 K using an Agilent SuperNova dual wavelength diffractometer with an Atlas detector using mirror-monochromated Cu-Ka(l¼ 1.54184 ˚ A) radiation. The single crystal X-ray data for 1a,1b and 2a was collected at 120 K using an Agilent SuperNova diffractometer with an Eos detector using mirror-monochromated MoKa(l¼0.71073 ˚ A) radiation The program CrysAlisPro 41 was used for the data collection and reduction on the SuperNova diffractometer, and the intensities were absorption corrected using a Gaussian face index absorption correction method. All structures were solved by intrinsic phasing (SHELXT) 42 and rened by full-matrix least squares on F 2 using the OLEX2, 43 utilizing the SHELXL-2015 module. 44 Anisotropic displacement parameters were assigned to non-H atoms and isotropic displacement parameters for all H atoms were constrained to multiples of the equivalent displacement parameters of their parent atoms with U iso (H) ¼1.2 U eq (aromatic) or U iso (H) ¼1.5 U eq (alkyl) of their respective parent atoms. The X-ray single crystal data and CCDC numbers of all new structures are included below. Synthesis and characterisation All silver(I) and iodine(I) complexes were prepared using the same quantitative general methods, which are given below using [1–Ag–1]PF 6 (1a) and [1–I–1]PF 6 (1b) as examples. Free ligand 2-(diphenylmethyl)pyridine (1). 1 H NMR (500 MHz, CD 2 Cl 2 )d8.56 (d, J¼4.0 Hz, 1H), 7.62 (td, J¼7.7, 1.7 Hz, 1H), 7.30 (t, J¼7.4 Hz, 4H), 7.23 (d, J¼7.3 Hz, 2H), 7.19 (d, J¼ 7.2 Hz, 4H), 7.17–7.11 (m, 2H), 5.65 (s, 1H); 15 N NMR (500 MHz, CD 3 CN) d65.4. The solid-state structure is known. 45 Synthesis of [1–Ag–1]PF 6 (1a). A DCM (3 mL) solution of 1 (24.5 mg, 0.1 mmol) was added to AgPF 6 (12.6 mg, 0.05 mmol) and the resulting colourless solution stirred for 1.5 hours. All volatiles removed under reduced pressure to leave a white solid. 1 H NMR (500 MHz, CD 2 Cl 2 )d8.00 (d, J¼4.5 Hz, 1H), 7.87 (td, J ¼7.9, 1.6 Hz, 1H), 7.39–7.33 (m, 7H), 7.17 (d, J¼8.0 Hz, 1H), 7.05 (d, J¼6.7 Hz, 4H), 5.47 (s, 1H); 15 N NMR (500 MHz, CD 3 CN) d118.6. Crystals suitable for single crystal X-ray diffraction were obtained from a DCM solution of 1a vapour diffused with DIPE. Crystal data for 1a: CCDC-2144042, [C 36 H 30 AgN 2 ]PF 6 ,M¼ 743.46, colourless plate, 0.08 0.19 0.29 mm 3 , triclinic, space group P 1 (No. 2), a¼8.5415(3) ˚ A, b¼11.2632(6) ˚ A, c¼ 16.8492(7) ˚ A, a¼91.301(4),b¼97.022(3),g¼101.834(4),V¼ 1572.71(12) ˚ A 3 ,Z¼2, D calc ¼1.570 g cm 3 ,F(000) ¼752, m¼ 0.76 mm 1 ,T¼120.0(1) K, q max ¼29.2, 7314 total reections, 5852 with I o >2s(I o ), R int ¼0.028, 7314 data, 461 parameters, 186 restraints, GooF ¼1.06, 0.57 < dDr<0.55 e˚ A 3 ,R[F 2 > 2s(F 2 )] ¼0.036, wR(F 2 )¼0.079. Synthesis of [1–I–1]PF 6 (1b). ACD 2 Cl 2 (0.5 mL) solution of 1 (9.8 mg, 0.04 mmol) was added to AgPF 6 (5.1 mg, 0.02 mmol) and the resulting colourless solution stirred for 1.5 hours. I 2 (5.1 mg, 0.02 mmol) was added as a solid and the mixture sonicated for 1 minute to give a red solution and a yellow precipitate, which was used as is for NMR spectroscopic studies. 1 H NMR (500 MHz, CD 2 Cl 2 )d8.84 (d, J¼4.8 Hz, 1H), 8.06 (td, J ¼7.8, 1.3 Hz, 1H), 7.42–7.31 (m, 7H), 7.23 (d, J¼7.8 Hz, 1H), 6.99 (d, J¼6.6 Hz, 4H), 5.82 (s, 1H); 15 N NMR (500 MHz, CD 3 CN) d165.7. Crystals suitable for single crystal X-ray diffraction were obtained by evaporation of a DCM : pentane (1 : 3) solution of 1b. Crystal data for 1b: CCDC-2144043, [C 36 H 30 IN 2 ]PF 6 , M¼762.49, colourless plate, 0.04 0.10 0.23 mm 3 , monoclinic, space group C2/c,a¼20.8912(5) ˚ A, b¼8.1839(2) ˚ A, c¼ 18.9412(7) ˚ A, b¼94.605(3),V¼3227.95(16) ˚ A 3 ,Z¼4, D calc ¼ 1.569 g cm 3 ,F(000) ¼1528, m¼1.11 mm 1 ,T¼120.0(1) K, q max ¼28.0, 3844 total reections, 2994 with I o >2s(I o ), R int ¼ © 2022 The Author(s). Published by the Royal Society of Chemistry RSC Adv., 2022, 12,8674–8682 | 8679 Paper RSC Advances Open Access Article. Published on 21 March 2022. Downloaded on 3/22/2022 4:43:55 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
0.043, 3844 data, 230 parameters, 51 restraints, GooF ¼1.09, 0.74 < dDr<0.75 e˚ A 3 ,R[F 2 >2s(F 2 )] ¼0.046, wR(F 2 )¼0.100. Synthesis of [H(1)]PF 6 (1c). A MeOH (5 mL) solution of 1 (159.4 mg, 0.65 mmol) was diluted with H 2 O (1 mL), then conc. HCl (0.1 mL, excess) was added. Aer 5 minutes of stirring, [NH 4 ]PF 6 (158.9 mg, 0.975 mmol) was added and stirred for a further 5 minutes. The total volume was reduced under reduced pressure to approximately half, at which time a white precipitate was observed. Addition H 2 O (7 mL) was added to induce further precipitation, and the white solid collected by ltration (N. B. the dried precipitate was very static prone). Recovered yield ¼0.150 mg (0.38 mmol, 59%). 1 H NMR (500 MHz, CD 2 Cl 2 )d12.27 (s.br, 1H), 8.37 (d, J¼5.5 Hz, 1H), 8.34 (td, J¼8.0, 1.3 Hz, 1H), 7.77 (t, J¼6.4 Hz, 1H), 7.56 (d, J¼8.1 Hz, 1H), 7.43–7.31 (m, 6H), 7.12 (d, J¼7.0 Hz, 4H), 5.91 (s, 1H); 15 N NMR (500 MHz, CD 3 CN) d167.5. Crystals suitable for single crystal X-ray diffraction were obtained from a DCM solution of 1c vapour diffused with pentane. Crystal data for 1c: CCDC2144044, [C 18 H 16 N]PF 6 ,M¼391.29, colourless plate, 0.05 0.16 0.25 mm 3 , monoclinic, space group P2 1 /c,a¼11.5946(3) ˚ A, b¼19.6879(4) ˚ A, c¼15.0819(4) ˚ A, b¼100.548(2),V¼ 3384.62(14) ˚ A 3 ,Z¼8, D calc ¼1.536 g cm 3 ,F(000) ¼1600, m¼ 2.05 mm 1 ,T¼120.0(1) K, q max ¼76.6, 6629 total reections, 5486 with I o >2s(I o ), R int ¼0.032, 6629 data, 475 parameters, no restraints, GooF ¼1.08, 0.66 < dDr<0.25 e˚ A 3 ,R[F 2 >2s(F 2 )] ¼0.052, wR(F 2 )¼0.145. Synthesis of [1–H–1]PF 6 (1d). ACD 2 Cl 2 (0.5 mL) solution of 1 (4.9 mg, 0.02 mmol) was added to 1c (7.8 mg, 0.02 mmol), and the resulting colourless solution stirred for 15 minutes before being used for NMR spectroscopic studies. 1 H NMR (500 MHz, CD 2 Cl 2 ) d8.21 (d, J¼4.4 Hz, 1H), 8.00 (td, J¼7.8, 1.6 Hz, 1H), 7.46 (t, J¼ 5.7 Hz, 1H), 7.39–7.26 (m, 7H), 7.10 (d, J¼7.1 Hz, 4H), 5.96 (s.br, 0.5H), 5.67 (s, 1H); 15 N NMR (500 MHz, CD 3 CN) d122.5. Synthesis of [1–Ag–1]BF 4 (1e). Prepared analogously to 1a using AgBF 4 (9.7 mg, 0.05 mmol). 1 H NMR (500 MHz, CD 2 Cl 2 ) d8.09 (d, J¼4.5 Hz, 2H), 7.85 (td, J¼7.9, 1.5 Hz, 2H), 7.39–7.31 (m, 14H), 7.14 (d, J¼8.0 Hz, 2H), 7.06 (d, J¼7.1 Hz, 8H), 5.51 (s, 2H); 15 N NMR (500 MHz, CD 3 CN) d117.6. Crystals suitable for single crystal X-ray diffraction were obtained from a DCM solution of 1e vapour diffused with pentane. Crystal data for 1e: CCDC-2150094, [C 36 H 30 AgN 2 ]BF 4 $2(CH 2 Cl 2 ), M¼855.15, colourless plate, 0.15 0.38 0.52 mm 3 , monoclinic, space group P2 1 ,a¼9.5033(4) ˚ A, b¼15.3402(6) ˚ A, c¼13.3461(5) ˚ A, b¼ 108.197(4),V¼1848.32(13) ˚ A 3 ,Z¼2, D calc ¼1.537 g cm 3 , F(000) ¼864, m¼0.89 mm 1 ,T¼120.0(1) K, q max ¼27.8, 7174 total reections, 6559 with I o >2s(I o ), R int ¼0.032, 7174 data, 516 parameters, 95 restraints, GooF ¼1.08, 0.59 < dDr<0.92 e˚ A 3 ,R[F 2 >2s(F 2 )] ¼0.037, wR(F 2 )¼0.100. Synthesis of [1–Ag–1]OTf (1f). Prepared analogously to 1a using AgOTf (12.8 mg, 0.05 mmol). 1 H NMR (500 MHz, CD 2 Cl 2 ) d8.17 (d, J¼4.4 Hz, 2H), 7.81 (td, J¼7.8, 1.6 Hz, 2H), 7.38–7.28 (m, 14H), 7.10–7.04 (m, 10H), 5.56 (s, 2H); 15 N NMR (500 MHz, CD 3 CN) d114.5. Crystals suitable for single crystal X-ray diffraction were obtained from a DCM solution of 1f vapour diffused with Et 2 O. Crystal data for 1f: CCDC-2150095, [C 36 H 30 AgN 2 ][CF 3 O 3 S], M¼747.56, colourless block, 0.21 0.40 0.44 mm 3 , triclinic, space group P 1 (No. 2), a¼ 11.3073(5) ˚ A, b¼11.6452(4) ˚ A, c¼15.0627(7) ˚ A, a¼68.117(4), b¼85.174(4),g¼71.906(4),V¼1748.34(14) ˚ A 3 ,Z¼2, D calc ¼ 1.420 g cm 3 ,F(000) ¼760, m¼0.69 mm 1 ,T¼120.0(1) K, q max ¼29.2, 8052 total reections, 6904 with I o >2s(I o ), R int ¼0.029, 8052 data, 424 parameters, no restraints, GooF ¼1.05, 0.44 < dDr<0.54 e˚ A 3 ,R[F 2 >2s(F 2 )] ¼0.033, wR(F 2 )¼0.077. Free ligand 2-(1,1-diphenylethyl)pyridine (2). 1 H NMR (500 MHz, CD 2 Cl 2 )d8.58 (dt, J¼3.8, 0.8 Hz, 1H), 7.57 (td, J¼7.9, 1.9 Hz, 1H), 7.27 (t, J¼7.4 Hz, 4H), 7.21 (t, J¼7.2 Hz, 2H), 7.15– 7.08 (m, 5H), 7.03 (d, J¼8.0 Hz, 1H), 2.20 (s, 3H); 15 N NMR (500 MHz, CD 3 CN) d65.1. Synthesis of [2–Ag–2]PF 6 (2a). Prepared analogously to 1a using 2(25.9 mg, 0.1 mmol). 1 H NMR (500 MHz, CD 2 Cl 2 )d7.89 (td, J¼8.1, 1.7 Hz, 1H), 7.68 (d, J¼4.9 Hz, 1H), 7.50 (d, J¼ 8.1 Hz, 1H), 7.36–7.28 (m, 7H), 7.02 (d, J¼7.0 Hz, 4H), 2.12 (s, 3H); 15 N NMR (500 MHz, CD 3 CN) d111.4. Crystals suitable for single crystal X-ray diffraction were obtained from a DCM solution of 2a vapour diffused with DIPE. Crystal data for 2a: CCDC-2144045, [C 38 H 34 AgN 2 ]PF 6 ,M¼771.51, colourless block, 0.13 0.20 0.37 mm 3 , monoclinic, space group I2/a,a¼ 19.5839(3) ˚ A, b¼9.2056(1) ˚ A, c¼36.7734(5) ˚ A, b¼97.273(1),V ¼6576.23(15) ˚ A 3 ,Z¼8, D calc ¼1.558 g cm 3 ,F(000) ¼3136, m¼ 0.73 mm 1 ,T¼120.0(1) K, q max ¼28.8, 7810 total reections, 6708 with I o >2s(I o ), R int ¼0.030, 7810 data, 435 parameters, no restraints, GooF ¼1.04, 0.38 < dDr<0.39 e˚ A 3 ,R[F 2 >2s(F 2 )] ¼0.033, wR(F 2 )¼0.070. Attempted synthesis of [2–I–2]PF 6 (2b). Prepared analogously to 1b using 2(10.4 mg, 0.04 mmol). 1 H NMR (500 MHz, CD 2 Cl 2 )d8.23 (s.br, 1H), 7.96 (td, J¼7.7, 0.9 Hz, 1H), 7.43 (d, J ¼7.6 Hz, 2H), 7.36–7.28 (m, 7.0 Hz, 6H), 7.05 (d, J¼7.2 Hz, 4H), 2.22 (s, 3H); 15 N NMR (500 MHz, CD 3 CN) d121.5 (decomposition product). Synthesis of [H(2)]PF 6 (2c). A MeOH (5 mL) solution of 2 (168.6 mg, 0.65 mmol) was diluted with H 2 O (1 mL), then conc. HCl (0.1 mL, excess) was added. Aer 5 minutes of stirring, [NH 4 ]PF 6 (158.9 mg, 0.975 mmol) was added and stirred for a further 5 minutes. The total volume was reduced under reduced pressure to approximately half, at which time a white precipitate was observed. Addition H 2 O (7 mL) was added to induce further precipitation, and the white solid collected by ltration. Product was initially observed as a colourless oil, which solidied aer several hours. Recovered yield ¼91.6 mg (0.23 mmol, 35%). 1 H NMR (500 MHz, CD 2 Cl 2 )d9.12 (s. very br, 1H), 8.74 (dd, J¼5.8, 0.9 Hz, 1H), 8.42 (td, J¼8.1, 1.5 Hz, 1H), 7.86 (dd, J¼6.3, 1.0 Hz, 1H), 7.70 (d, J¼8.2 Hz, 1H), 7.43–7.36 (m, 6H), 7.08 (dd, J¼8.0, 1.3 Hz, 4H), 2.39 (s, 3H); 15 N NMR (500 MHz, CD 3 CN) d180.4. Synthesis of [2–H–2]PF 6 (2d). ACD 2 Cl 2 (0.5 mL) solution of 2 (5.2 mg, 0.02 mmol) was added to 2c (8.1 mg, 0.02 mmol), and the resulting colourless solution stirred for 15 minutes before being used for NMR spectroscopic studies. 1 H NMR (500 MHz, CD 2 Cl 2 )d9.86 (s.br, 0.5H), 8.59 (d, J¼4.4 Hz, 1H), 7.98 (td, J¼ 8.0, 1.6 Hz, 1H), 7.44 (dd, J¼5.6, 0.8 Hz, 1H), 7.39–7.24 (m, 7H), 7.08 (d, J¼7.2 Hz, 4H), 2.28 (s, 3H); 15 N NMR (500 MHz, CD 3 CN) d123.0. Synthesis of [2–Ag–2]BF 4 (2e). Prepared analogously to 1a using 2(25.9 mg, 0.1 mmol) and AgBF 4 (9.7 mg, 0.05 mmol). 1 H 8680 |RSC Adv., 2022, 12,8674–8682 © 2022 The Author(s). Published by the Royal Society of Chemistry RSC Advances Paper Open Access Article. Published on 21 March 2022. Downloaded on 3/22/2022 4:43:55 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
NMR (500 MHz, CD 2 Cl 2 )d7.90 (td, J¼8.1, 1.7 Hz, 2H), 7.69 (d, J ¼5.0 Hz, 2H), 7.49 (d, J¼8.1 Hz, 2H), 7.37–7.28 (m, 14H), 7.02 (d, J¼7.0 Hz, 8H), 2.12 (s, 6H); 15 N NMR (500 MHz, CD 3 CN) d111.9. Crystals suitable for single crystal X-ray diffraction were obtained from a DCM solution of 2e vapour diffused with Et 2 O. Crystal data for 1a: CCDC-2150096, [C 38 H 34 AgN 2 ]BF 4 ,M¼ 713.35, colourless block, 0.21 0.38 0.50 mm 3 , monoclinic, space group I2/a,a¼18.1676(3) ˚ A, b¼9.4569(2) ˚ A, c¼ 37.4652(6) ˚ A, b¼100.690(2),V¼6325.2(2) ˚ A 3 ,Z¼8, D calc ¼ 1.498 g cm 3 ,F(000) ¼2912, m¼0.69 mm 1 ,T¼120.0(1) K, q max ¼28.6, 7397 total reections, 6250 with I o >2s(I o ), R int ¼ 0.043, 7397 data, 417 parameters, no restraints, GooF ¼1.04, 0.38 < dDr<0.64 e˚ A 3 ,R[F 2 >2s(F 2 )] ¼0.036, wR(F 2 )¼0.079. Synthesis of [2–Ag–2]OTf (2f). Prepared analogously to 1a using 2(25.9 mg, 0.1 mmol) and AgOTf (12.8 mg, 0.05 mmol). 1 H NMR (500 MHz, CD 2 Cl 2 )d7.86 (td, J¼8.0, 1.7 Hz, 2H), 7.79 (d, J¼4.2 Hz, 2H), 7.42 (d, J¼8.1 Hz, 2H), 7.36–7.26 (m, 14H), 7.04 (d, J¼7.2 Hz, 8H), 2.12 (s, 6H); 15 N NMR (500 MHz, CD 3 CN) d110.0. Crystals suitable for single crystal X-ray diffraction were obtained from a CHCl 3 solution of 2f vapour diffused with pentane. Crystal data for 1a: CCDC-2150097, [C 39 H 34 AgN 2 ] [CF 3 O 3 S], M¼775.61, colourless plate, 0.06 0.07 0.32 mm 3 , monoclinic, space group I2/a,a¼19.7243(4) ˚ A, b¼9.1083(2) ˚ A, c¼37.8599(7) ˚ A, b¼95.788(2),V¼6767.0(2) ˚ A 3 ,Z¼8, D calc ¼ 1.523 g cm 3 ,F(000) ¼3168, m¼0.72 mm 1 ,T¼120.0(1) K, q max ¼26.9, 8076 total reections, 6092 with I o >2s(I o ), R int ¼ 0.063, 8076 data, 451 parameters, 101 restraints, GooF ¼1.07, 0.57 < dDr<0.55 e˚ A 3 ,R[F 2 >2s(F 2 )] ¼0.046, wR(F 2 )¼0.090. Theoretical methods For the optimisations and single point calculations the M06-2X 46 / def2-TZVP 47 level of theory and the Turbomole 7.2 program 48 was used. This level of theory was previously used to study similar complexes. 14–16 The def2-TZVP implementation used in this work employs for Ag the ECP-28 set and scalar relativistic effects. 47 Frequency calculations were used to verify that the geometries correspond to true minima on the potential surface (no imaginary frequencies). No symmetry constraints were imposed for the calculations. Solvent calculations were considered using the conductor-like screening model (COSMO). 49 QTAIM and NCIplot index, that is adequate to reveal noncovalent interactions in real space, 50 were computed at the same level of theory by means of the MULTIWFN program 51 and represented using the VMD soware. 52 The Wiberg bond index was computed using the NBO 7.0 program 53 at the same level of theory. Conflicts of interest There are no conicts to declare. Acknowledgements The authors gratefully acknowledge the Magnus Ehrnrooth Foundation (J. S. W.), the MICIU/AEI of Spain (A. F. project PID2020-115637GB-I00, FEDER), the Academy of Finland (K. R. grant no. 317259), and the University of Jyvaskyla, Finland for nancial support. Notes and references 1 J. A. Creighton, I. Haque and J. L. Wood, Chem. Commun., 1966, 229. 2 I. Haque and J. L. Wood, J. Mol. Struct., 1968, 2, 217–238. 3 J. Barluenga, J. M. Gonz´ alez, M. A. Garcia-Martin, P. J. Campos and G. Asensio, J. Chem. Soc., Chem. Commun., 1992, 1016–1017. 4 J. Ezquerra, C. Pedregal, C. Lamas, J. Barluenga, M. P´ erez, M. A. Garc´ ıa-Mart´ ın and J. M. Gonz´ alez, J. Org. Chem., 1996, 61, 5804–5812. 5 G. Espu˜ na, G. Arsequell, G. Valencia, J. Barluenga, M. P´ erez and J. M. Gonz´ alez, Chem. Commun., 2000, 1307–1308. 6 J. Pancholi and P. D. Beer, Coord. Chem. Rev., 2020, 416, 213281. 7 L. Turunen, U. Warzok, R. Puttreddy, N. K. 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