Xiaofei
Sun
*a,
Grégory
Nocton
b and
Peter W.
Roesky
*ac
aInstitute of Inorganic Chemistry (AOC), Karlsruhe Institute of Technology (KIT), Kaiserstr. 12, 76131, Karlsruhe, Germany. E-mail: xiaofei.sun@kit.edu; roesky@kit.edu
bLCM, CNRS, Ecole Polytechnique, Institut Polytechnique de Paris, 91120 Palaiseau, France
cInstitute of Nanotechnology (INT), Karlsruhe Institute of Technology (KIT), Kaiserstr. 12, 76131, Karlsruhe, Germany
First published on 23rd December 2024
Cyclononatetraenyl (Cnt) is a nine-membered monoanionic aromatic ligand. Despite its early discovery in 1963, it has been rarely utilised in coordination chemistry, which is mainly due to its large diameter and easy skeletal rearrangement. Only in 2017, the first lanthanide Cnt complex was synthesised, marking the beginning of a new era in organolanthanide chemistry. Since then, the chemistry displayed by Cnt has expanded rapidly in lanthanide chemistry. Due to the ligand flexibility, both classical planar η9-Cnt ligands and heavily bent ones with lower hapticity were found in Ln coordination complexes. These novel structure motifs exhibit reactivity which differs significantly from traditional cyclopentadienyl (Cp) and cyclooctatetraenediide (Cot) complexes. Some of the obtained compounds also show interesting photoluminescence and magnetic properties. This review presents an overview of the synthesis, structure and reactivity of the growing family of lanthanide Cnt compounds.
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Fig. 1 Aromatic four- to nine-membered carbocycles used as ligands in lanthanide chemistry. For the four-membered ring only a substituted derivate was used as ligand. |
Following the widespread use of Cp ligands, the 10π-aromatic cyclooctatetraenediide (Cot) dianion has also played a crucial role in the chemistry of f-elements for more than five decades.7,8 Lanthanide complexes featuring Cp or Cot ligands have become important not only due to their unique structures but also their potential applications in catalysis, electrochemistry, and material science. Beyond the ubiquitous Cp and Cot rings in organolanthanide chemistry, other aromatic carbocycles have also been employed as ligands. A significant step was made by Cloke et al. in 1987 when they synthesized several lanthanide bis(arene) sandwich compounds with bulky benzene derivatives using electron-beam vaporization techniques.9 Very recently, the first examples of lanthanide complexes comprising the four-membered substituted cyclobutadienyl dianion have also been disclosed.10,11
It is well-established that the dianionic Cot ligand can serve as a bridging unit to form lanthanide multidecker species.12–14 In addition, the purely carbon-based seven-membered cycloheptatrienyl trianion has also been employed in organolanthanide chemistry, acting as bridging ligand for inverse sandwich and multidecker compounds.15–17
All the carbocycles with ring sizes from four to eight have been utilised in organolanthanide chemistry for over 20 years. The next larger aromatic carbocycle is the nine-membered 10π-electron cyclononatetraenyl (Cnt) monoanion, which was only introduced in organolanthanide chemistry in 2017.18
The Cnt anion has been known since the 1960s with the corresponding alkali metal salts (Li(Cnt) or K(Cnt)) being synthesized from Cot in a two-step procedure.19,20 On the basis of their NMR data and the strong UV absorption, their aromatic character has been confirmed. However, synthesising stable complexes with Cnt ligands were found to be extremely challenging due to the large ring size and tendency of skeletal rearragement.21,22 Still, Cnt remains attractive as a planar π-ligand in coordination chemistry, especially for generating novel sandwich complexes with ligands beyond the classical smaller ring systems. Early studies on the coordination chemistry of Cnt ligands with transition metal precursors were reported already in the 1970s. The two examples are the synthesis of the heteroleptic compounds, [(Cp)TiII(Cnt)]23 and [(Cp)2NbIII(Cnt)],24 where the spectroscopic observations revealed η3- and η7-coordination modes for the Ti and Nb compounds, respectively. More recently, in 2009, a tetrapalladium Cnt-based sheet sandwich-type complex was reported and the molecular structure was disclosed.25
The Cnt anion can exist as two different isomers, Cnt-trans and Cnt-cis (Fig. 2). Only recently, the molecular structure of the potassium salt of the Cnt ligand (K(Cnt)) was reported by Nocton26 and Roesky27 independently. Depending on the reaction and crystallisation conditions, the outcome of the formation of K(Cnt) slightly differs.28 Nocton et al. reported that recrystallisation of K(Cnt) from diethyl ether, led to single crystals with disorder for the C1 atom (Fig. 3, right), which is linked to the formation of the cis–cis–cis–cis isomer (Cnt-cis) and the cis–cis–cis–trans isomer (Cnt-trans) forming a structure resembling a pac-man. Both isomers are present in solution, as evidenced by NMR spectroscopy. For the cis-isomer, the monoanionic charge is delocalized over the entire ring, as shown in the fully planar structure and the nearly equivalent C–C bond distances, resulting a D9h symmetry. In contrast, for the trans analogue, one of the carbon atoms is inside the ring, and the negative charge is mainly localised over there. Therefore, C2v symmetry is assumed in first approximation. Roesky et al. obtained single crystals of K(Cnt) from dimethoxyethane and the structure showed only the Cnt-cis form, with the potassium ion exhibiting complete η9-coordination towards the ring (Fig. 3, left).
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Fig. 3 The molecular structures of KCnt·(dme)2 (left) and KCnt·Et2O (right) in the solid state with thermal ellipsoids at 40% probability.† |
Already in 1973, more than half a century ago, Steitwieser et al. attempted to incorporate the Cnt ligand in organolanthanide complexes.29 They aimed to synthesise the heteroleptic sandwich complex [(Cnt)LnIII(Cot)] (Ln = Ce, Pr, Nd and Sm). However, instead of the expected product, the one-pot reaction between LnCl3, K2(Cot), and K(Cnt) in THF led to the formation of a new class of mono(cyclooctatetraenediide) compounds, the dimeric chlorides [LnIII(Cot)(thf)2Cl]2 (1-Ln, Ln = Ce, Pr and Sm) (Scheme 1).
The large diameter (approximately 4.1 Å) of the Cnt ligand appears to be particularly well-suited for metal centres with large ionic radii such as heavy main group metals like Ba, Pb, Bi, and the f-elements. In 2005, Sitzmann et al. reported the reaction between K(Cnt) and BaI2 in THF, which resulted in the formation of the homoleptic complex [Ba(η9-Cnt)2] (2) (Scheme 2).30 Although no single crystals could be obtained for structure determination, DFT calculations proposed the sandwich-type structure of 2. Further spectroscopic methods (NMR spectroscopy and EI MS) indicated the successful isolation of 2.
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Fig. 4 Molecular structures of [LnII(η9-Cnt)2] (3-Ln, Ln = Sm, Eu, Tm, Yb) in the solid state with thermal ellipsoids at 40% probability.† |
3-Sm | 3-Eu | 3-Tm | 3-Yb | |
---|---|---|---|---|
Distances are given in Å and angles are given in deg (°). | ||||
C–C | 1.33(1)–1.47(2) | 1.353(15)–1.448(13) | 1.32(1)–1.47(2) | 1.33(3)–1.43(3) |
C–C (av.) | 1.39(4) | 1.39(2) | 1.39(4) | 1.39(4) |
M–C | 2.833(8)–2.958(6) | 2.796(13)–2.957 | 2.736(8)–2.760(8) | 2.74(2)–2.86(2) |
M–C (av.) | 2.88(3) | 2.87(4) | 2.75(1) | 2.79(3) |
M–CntCt | 2.061 | 2.065 | 1.91 | 1.932 |
CntCt–CntCt | 4.122 | 4.131 | 3.82 | 3.865 |
Ring size (max C⋯C–C) | 4.05 | 4.02 | 4.02 | 4.06 |
CntCt–Ln–CntCt | 180 | 180 | 180 | 180 |
In addition to the experimental findings on 3-Eu, the electronic structure and photoluminescence properties of 3-Eu were further analysed via quantum computational methods.34 Ligand-field density functional theory (LFDFT), a DFT-based model incorporating the configuration interaction algorithm through a model Hamiltonian, was employed for the calculations. In the optimized structure of 3-Eu, both the eclipsed D9h and the staggered D9d conformations, were found to be nearly degenerate, suggesting that the Cnt ligand can freely rotate in the gas phase. The calculated molecular orbital diagram reveals that the Eu 4f orbitals contribute only weakly to the predominantly ionic chemical bonding. Time-dependent DFT (TD-DFT) and LFDFT calculations explained the photoluminescence emission as electric dipole-allowed Eu 4f-5d transitions between the lowest excited 4f65d1 state of the EuII ion and the 4f75d0 ground state. The low quantum yield in toluene solution at room temperature may be indicative of non-radiative decay caused by strong interactions with solvent molecules.
In 2018, the group of Nocton extended the bis(Cnt) chemistry to other divalent lanthanides, Sm, Tm and Yb.26 They discovered that both the starting material and the solvent are crucial for the high yield synthesis of [LnII(η9-Cnt)2]. The trans/cis isomer mixture of K(Cnt) was necessary for the successful high yield synthesis and when the reactions between K(Cnt) and LnI2 were performed in coordinating solvents (THF, Et2O), the yields remained very low. In contrast, reacting K(Cnt) with LnI2 (Ln = Sm, Eu, Tm, Yb) in toluene with a few drops of THF, crystals of 3-Ln were obtained in good yields (33–62%) from the toluene filtrate (Scheme 4). 1H NMR spectroscopy of the diamagnetic 3-Yb and paramagnetic 3-Sm both indicated the existence of three isomers [LnII(Cnt-cis)2], [LnII(Cnt-trans)2] and [LnII(Cnt-cis)(Cnt-trans)] in solution. Complete isomerisation to the [LnII(Cnt-cis)2] isomers took a few days and the yields were extremely low, which suggested that a better isomerisation solvent is required.
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Scheme 4 Synthesis of the homoleptic Ln bis(Cnt) complexes as isomer mixture and the further isomerisation to the [LnII(Cnt-cis)2] complexes 3-Ln. |
Dissolving the crystals of 3-Yb in THF led to the formation of the ion pair [YbII(Cnt)(thf)4][Cnt] (4-Yb) with one coordinated and one uncoordinated Cnt ligand (Scheme 4). SCXRD analysis revealed that the coordinated one exhibits a 1:
1 ratio disorder between the trans and cis forms, while the uncoordinated one consists entirely of the cis isomer. Treating the crystals of 3-Yb with acetonitrile resulted in the full decoordination of the two Cnt ligands from the YbII centre and the formation of the ion pair [YbII(CH3CN)7][Cnt]2 (5-Yb). The YbII ion exhibits a distorted pentagonal bipyramidal geometry and the two Cnt anions are in cis configuration. A pivotal step to get access to pure [YbII(Cnt-cis)2] is to dry the crystals of 5-Yb under 10−3 mbar vacuum for 8 h (Scheme 4). Applying similar procedures for the other divalent Ln complexes also led to the clean formation of [LnII(Cnt-cis)2] (3-Ln, Ln = Sm, Eu and Tm).
3-Sm, 3-Tm and 3-Yb are isomorphous to the previously reported 3-Eu. The high symmetry of the sandwich structures can be reflected from their crystallographic features as the LnII atoms are positioned at the centre of inversion, leading to rigorously linear sandwich motifs. Additionally, the Cnt rings exhibit disorder with altering stacked D9h and eclipsed D9d symmetry, which could potentially result in interesting spectroscopic properties. For example, very recently, Vitova et al. investigated the electronic structure of [SmII(η9-Cnt)2] (3-Sm) by valence band high-energy resolution resonant inelastic X-ray scattering (VB-RIXS), high-resolution X-ray absorption near-edge structure (HR-XANES) and quantum chemical computations. The results demonstrated that the ionic bond of 3-Sm can be photo-modulated and the Sm–C bond covalency increases by transfer of the SmII 4f electrons to the 5d orbitals.35 This change leads to a reconfiguration of 3-Sm with a detectable contraction in Sm–C bond lengths and increased disorder in the local SmII coordination environment.
The synthesis began with the preparation of the half-sandwich compounds [(η8-Cot)LnIIII(thf)n)] (Ln = Nd, Sm, Dy and Er; n = 2 for Sm, Dy, Er, n = 3 for Nd) from cyclooctatetraene, Ln metal and iodine, following a slightly modified procedure by Mashima et al.37 Refluxing K(Cnt) with [(η8-Cot)LnIIII(thf)n)] in toluene yielded the super sandwich complexes 6-Ln (Ln = Nd, Sm, Dy and Er) in moderate yields of 31–36% after crystallisation from toluene (Scheme 5). Despite some disorder in the crystal structures, the 6-Nd and 6-Sm complexes exhibit perfect sandwich-type structure motifs with η8-coordinated Cot and η9-coordinated Cnt rings in nearly ideal coplanar arrangement (Fig. 5). Notably, the smaller Cot ligand is much closer to the LnIII centre compared to the larger Cnt ligand, which is mostly attributed to the dianionic charge of Cot which results in a stronger electrostatic attraction. Greater disorder was found in the crystal structures of the smaller lanthanides, 6-Dy and 6-Er, and detailed investigation of the structures were published later.38
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Scheme 5 Synthesis of the heteroleptic sandwich complexes [(Cnt)LnIII(η8-Cot)] (6-Ln, Ln = Nd, Sm, Dy and Er) from [(η8-Cot)LnIIII(thf)n)] and K(Cnt). |
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Fig. 5 Molecular structures of 6-Nd (left) and 6-Sm in the solid state with thermal ellipsoids at 40% probability.† |
It is known that equatorial ligands are beneficial for enhancing the magnetic anisotropy of ErIII due to the prolate nature of its highest mJ state. 6-Er exhibited single molecule magnet (SMM) behaviour, characterised by an open butterfly-like magnetic hysteresis loop up to 10 K, which was well-reproduced by CASSCF calculations. The magnetic properties of 6-Er indicate that the combination of the Cot/Cnt ligands induces a strong equatorial ligand field. Conversely, 6-Dy showed less promising magnetic properties since the highest mJ state with oblate shape is best to be stabilised by axial ligands, instead of the equatorial Cot/Cnt ligands.
Roesky and Nocton further expanded the chemistry of the super sandwich complexes to additional late lanthanides, Tb, Ho, Tm and Lu.38 In an alternative synthesis to the one described earlier (Scheme 5), the reactions between the half-sandwich starting material [(η8-Cot)LnIIII(thf)n)] and K(Cnt) were performed in acetonitrile/toluene mixture at room temperature (Scheme 6). For the smallest element Lu, the synthesis was optimised by refluxing the borohydride precursor [(η8-Cot)LuIII(BH4)(thf)2)] and K(Cnt) in toluene (Scheme 6). The analogues YbIII sandwich complex was not formed due to reduction by the Cot ligand. Sufficient drying before recrystallisation from toluene is crucial in order to remove the coordinating solvent molecules (THF or acetonitrile), as incomplete removal results in crystal structures with partially coordinated solvent molecules and lower hapticities than η9 for the Cnt ligands.
The properties of 6-Ln were analysed in toluene-d8 solution using 1H NMR spectroscopy. Depending on the nature of the LnIII, the spectra showed either two broad or well-defined sharp singlet signals for the Cnt and Cot protons at room temperature. Complex 6-Lu exhibited no fluxional behaviour from −80 °C to 80 °C. Interestingly, for the 6-Tb complex, the Cnt ligand consisted initially of purely cis-configuration, but it partially isomerised to 6-Tb′ over the course of several days (Scheme 7). This isomerisation process was followed by NMR spectroscopy and the amount of 6-Tb′ increased over the time. 6-Tb′ consists of an η8-coordinating Cot ring and the Cnt-trans ligand, in which one of the carbon atoms is inside of the ring. This configuration has been known since the 1960s and was recently discussed for the divalent lanthanidocene complexes 3-Ln.26,28,39,40 Complex 6-Tb′ with Cnt-trans was also observed in the solid-state structure. Depending on the reaction temperature and crystallisation method, single crystals of the TbIII sandwich complex were found to contain either only the Cnt-cis isomer or a disorder of Cnt-cis/Cnt-trans isomers (Fig. 6). When the reaction was performed in toluene/acetonitrile mixture at room temperature, the isolated crystals from toluene consist of solely the Cnt-cis isomer.
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Fig. 6 Molecular structures of [(Cnt)TbIII(η8-Cot)] with purely Cnt-cis (left) and Cnt-cis/Cnt-trans disorder (right) in the solid state with thermal ellipsoids at 40% probability.† |
Synthesising the TbIII species in hot toluene resulted in a mixture of sandwich compounds with Cnt-cis and Cnt-trans isomers. Among all 6-Ln complexes, this isomerisation has only been detected for the TbIII species.
Molecular structures of all 6-Ln complexes were studied by SCXRD analyses. Among the late lanthanides (Ln = Tb, Dy, Ho, Er, Tm and Lu) (Fig. 6), only the TbIII complex 6-Tb displayed two almost coplanar rings with the Cnt ligand in the η9-coordination mode (Fig. 6 left). As for the smaller lanthanides, DyIII to LuIII, the solid-state structures exhibit significant disorder, with the structure of 6-Lu being the most disordered one (Fig. 7). The disorder in the structures posed challenges in the refinement. The Ln is positioned close to the centre of inversion with 0.5 occupancy, and a 0.5/0.5 disorder of both the Cot/Cnt rings were found. The optimal approach to model this disorder problem is to maintain the Cot ring relatively planar, which is consistent with its typical planarity reported in the literature. The CtCot–LnIII distances nicely correlate to the lanthanide contraction, gradually decreasing from CtCot–TbIII (1.804 Å) to CtCot–LuIII (1.653 Å). In contrast, the Cnt exhibited some degree of deformation from planarity, which is indicative for a lower hapticity. A careful evaluation of the structural metrics revealed that from TbIII to LuIII, the lanthanide contraction results in a transition of the hapticity of the Cnt ligand from η9 (Tb) to η6 (Lu) and meanwhile the Cnt ring shows greater deviation of planarity for the smaller LnIII ions (Fig. 6 and 7). Furthermore, different measurement temperatures also have slight effect on the coordination mode.
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Fig. 7 Top: Molecular structures of [(Cnt)LnIII(η8-Cot)] (6-Ln, Ln = Dy, Ho, Er, Tm and Lu) in the solid state with thermal ellipsoids at 40% probability.† Bottom: Representation of size-dependent coordination modes of the Cnt ligand in the [(Cnt)LnIII(η8-Cot)] (6-Ln) sandwich complexes. |
Magnetic measurements of 6-Tb, 6-Dy, 6-Ho and 6-Tm showed that the large aromatic Cnt ligand is well suited for the prolate ErIII and TmIII ions. However, none of the four compounds behave as SMMs, which is different compared to the erbium congener 6-Er. Ab initio computational studies indicated that the deviation from planarity and the reduced hapticity of the Cnt do not significantly impact the overall anisotropy in the case for the prolate ions but influence the composition and ratio of mixed mJ states in non-adapted oblate ions.
Utilising these reversible properties, the CeIII and TbIII complexes 6-Ln can act as switchable luminophores. Significant differences were observed in the photoluminescence properties of the solvent-free neutral sandwich compounds [(Cnt)LnIII(η8-Cot)] (6-Ln) compared to the solvated ionic analogues [LnIII(thf)4(η8-Cot)][Cnt] (8-Ln). While 6-Ln did not show significant light emission, the ionic versions 8-Ln exhibited intense photoluminescence upon UV-light irradiation. The two forms were easily switched by adding THF to 6-Ln and applying vacuum to 8-Ln. While the emission properties changed, the colours of the respective crystalline materials also changed, which clearly indicated the successful solvation and desolvation.
Besides acting as switchable luminophores, the magnetic properties of the ErIII and NdIII complexes 6-Er and 6-Nd showed significant different SMM behaviours after THF addition. For instance, the neutral Er complex [(Cnt)ErIII(η8-Cot)] (6-Er) exhibits good SMM behaviour with high energy barriers, whereas the THF-solvated analogue 9-Er showed faster relaxation and lower barriers. In contrast to 6-Er and 9-Er, the Nd system showed more distinct behaviour. While [(Cnt)NdIII(η8-Cot)] (6-Nd) behaved as an SMM, the SMM character was entirely shut off upon solvation and formation of 8-Nd. Theoretical calculations further provided explanation of the different magnetic behaviour of these compounds. The almost pure character mJ ground states of 6-Nd accounted for its SMM character. Computational analysis also explained the lack of SMM character of 8-Nd, which is attributed to the pseudo-planar anisotropy of its ground doublet state with highly mixed mJ values.
Spontaneous isomerisation of Cnt-trans to Cnt-cis occurs at room temperature, but by maintaining the temperature at 233 K, a trans:cis ratio of 80:
20 can be achieved. By performing the synthesis under light exclusion, this ratio can be increased to 95
:
5. Instead of the mixture of different isomers (Scheme 4), by having the Cnt-trans ligand (K(Cnt-trans)) in high purity in hand, the reaction between SmI2 and K(Cnt-trans) was performed under light protection in toluene, resulting in the formation of pure [Sm(Cnt-trans)2] (3-Sm-trans, Scheme 9). Although pure 3-Sm-trans was obtained according to NMR spectroscopy, crystallisation always led to a mixture of different isomers, likely due to the unavoidable slow isomerisation during the crystallisation process and the solubility differences between the trans and cis isomers.
Furthermore, to deepen the understanding of how light and solvent affect the outcome of the synthesis of [(Cnt)LnIII(η8-Cot)] (6-Ln) and to obtain a series of heteroleptic sandwich complexes analogues to 6-Ln with the Cnt either in purely cis or trans form, the synthesis was carefully performed under different conditions. Analogous to the synthesis described earlier (Scheme 6), when reacting K(Cnt) and [(η8-Cot)LnIIII(thf)n)] (Ln = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho; n = 2, 3) in a toluene/acetonitrile mixture under ambient light, 6-Ln complexes with pure Cnt-cis were obtained. Alternatively, performing the reaction in non-coordinating solvent (toluene) under light protection using K(Cnt-trans) and [(η8-Cot)LnIIII(thf)n)] (Ln = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho; n = 2, 3), crystalline products with a high ratio of [(Cnt-trans)LnIII(8-Cot)] (6-Ln-trans) were obtained from concentrated toluene solutions in all cases (Fig. 8). The trans:
cis ratios were analysed using 1H NMR spectroscopy and SCXRD analysis. For the obtained crystals of 6-Ln-trans, the trans
:
cis ratio varied depending on the rare-earth metal ion. Crystals of the Tb, Dy and Ho complexes consisted of 100% of the trans isomer, while the ratios for the other rare-earth metals were 71% (Gd), 69% (Sm), 58% (Nd), 55% (Pr), 50% (Ce), 25% (La) and 73% (Y), respectively. The Ln–C distances for both the Cot and Cnt rings, as well as the Ln–C1 distance decrease along the lanthanide series, with a notable exception in 6-Ho, due to changes in the coordination mode (Table 2).38 Solid structures only represent the portion of the product mixture that crystallised. Since Cnt-trans is more soluble than Cnt-cis, the observed ratios according to XRD analyses are not accurate. Therefore, further solution analysis was performed. Complex 3-Sm-trans showed five signals in 2
:
2
:
2
:
2
:
1 molar ratio in the 1H NMR spectrum, consistent with the Cnt-trans having C2v symmetry in solution. For the series of 6-Ln-trans complexes, both isomers were identified according to the 1H NMR spectra. Using that method allowed the correct identification of both isomers in solution.
6-Y-trans | 6-La-trans | 6-Ce-trans | 6-Pr-trans | 6-Nd-trans | 6-Sm-trans | 6-Gd-trans | 6-Tb-trans | 6-Dy-trans | 6-Ho-trans | |
---|---|---|---|---|---|---|---|---|---|---|
Distances are given in Å. | ||||||||||
Ln–CCot (av.) | 2.542(8) | 2.694(4) | 2.54(4) | 2.636(6) | 2.625(9) | 2.595(6) | 2.570(7) | 2.552(10) | 2.541(5) | 2.478(7) |
Ln–CCnt-C8 (av.) | 2.806(9) | 2.905(8) | 2.881(15) | 2.86(2) | 2.87(2) | 2.833(8) | 2.826(9) | 2.818(11) | 2.802(8) | 2.792(9) |
Ln–C1A | 2.657(10) | 2.851(16) | 2.750(12) | 2.749(15) | 2.79(2) | 2.687(13) | 2.703(17) | 2.664(16) | 2.660(9) | 2.673(11) |
Ln–CCnt (av.) | 2.776(9) | 2.894(10) | 2.85(1) | 2.84(2) | 2.85(2) | 2.804(9) | 2.801(11) | 2.787(13) | 2.774(8) | 2.768(9) |
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Fig. 8 Molecular structures of [(Cnt-trans)LnIII(η8-Cot)] (6-Ln-trans, Ln = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho) in the solid state with thermal ellipsoids at 40% probability.† |
The photo-isomerisation of the Cnt ligand, as well as complex 3-Sm and 6-Ln, were studied in detail. The solutions were directly irradiated in NMR tubes at different wavelengths, and the resulting isomer ratios were quantified using 1H NMR spectroscopy. The isomerisation of the Cnt ligand from Cnt-trans to Cnt-cis was monitored using 427 nm irradiation, showing a pseudo-first-order reaction rate, with complete conversion achieved after 5 min. Under 427 nm irradiation, complex 3-Sm-trans isomerised via the intermediate 3-Sm-trans–cis to 3-Sm within 30 min, with both steps showing pseudo-first-order reaction rates. However, complete photochemical analysis was not conducted for the isomerisation of the Cnt ligand and the 3-Sm complex.
The photo-isomerisation of the heteroleptic complexes 6-Ln and further analysis are more challenging due to strong paramagnetic nature of some lanthanides and overlapping signals of the isomers in the 1H NMR spectra. For the compounds where integration was possible, irradiation at different wavelengths produced different trans:
cis ratios (Table 3). Kinetic studies on complex 6-Sm showed that the photo-stationary state (PSS) was reached after 30 min of irradiation, meaning the trans:cis ratio no longer changed. This behaviour was studied for all the 6-Ln (Ln = Y, La, Ce, Sm, Tb, Dy and Lu) complexes, and it varied depending on the irradiation wavelengths and metal ion. For the early lanthanides La and Ce, as well as Y, different wavelengths showed minor changes in the PSS ratios, consistently favouring the trans isomers (Table 3). In contrast, for the later lanthanides, Sm, Tb, Dy and Lu, the trans ratios showed significant differences depending on the irradiation wavelengths. Under high-energy irradiation, the trans isomers were favoured while under low energy irradiation, the cis isomers were preferentially formed. These variations might be explained by differences in the energy barriers and absorption cross-sections of the isomers. The photo-isomerisation process of these systems is complex, DFT and TD-DFT were performed to offer a preliminary orbital explanation.
6-Y-trans (%) | 6-La-trans (%) | 6-Ce-trans (%) | 6-Sm-trans (%) | 6-Tb-trans (%) | 6-Dy-trans (%) | 6-Lu-trans (%) | |
---|---|---|---|---|---|---|---|
NMR | 61 | 55 | 79 | 89 | 87 | 75 | 0 |
XRD | 73 | 25 | 50 | 69 | 100 | 100 | 0 |
PSS 370 nm | 96 | 93 | 97 | 64 | 78 | 76 | 69 |
PSS 370 nm | 99 | 98 | 100 | 52 | 79 | 75 | 71 |
PSS 427 nm | 97 | 99 | 94 | 28 | 50 | 48 | 29 |
PSS 440 nm | 95 | 99 | 86 | 23 | 39 | — | 20 |
PSS 450 nm | — | — | — | — | — | 33 | — |
PSS 467 nm | 92 | 99 | 64 | 13 | 28 | — | 11 |
PSS 525 nm | 90 | 98 | 88 | 12 | 1 | 6 | 9 |
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Scheme 10 Synthesis of the half sandwich complexes 10-Ln and reaction between [SmIII(BH4)3(thf)3] and K(Cnt). |
Single crystals of 10-Ln were obtained from the concentrated toluene solutions at −10 °C in 66% and 72% yields for 10-La and 10-Ce. The two isostructural complexes with the general formula of [(η9-Cnt)LnIII(BH4)3(thf)] exhibit three-legged piano-stool-type structure motifs (Fig. 9). In these structures, the LnIII-ions are being η9-coordinated by the planar Cnt ring, two terminal BH4 ligands and one THF molecule. The molecular structures resemble similarities to that of the LnIII half sandwich borohydride compounds with the sterically-demanding silyl-functionalized Cot ligand.44 The binding mode of the Cnt ring was further confirmed by Raman spectroscopy, which show clearly assignable characteristic ring vibrations (νasym(η9-Cnt) = 1520 cm−1 (10-La), 1522 cm−1 (10-Ce); νsym(η9-Cnt) = 678 cm−1 (10-La), 679 cm−1 (10-Ce)) and (ν(Ln-Cnt) = 145 cm−1 (10-La), 140 cm−1 (10-Ce)). In the 1H NMR spectra, a singlet signal was detected for the Cnt protons at 7.01 ppm for the diamagnetic 10-La and at 5.41 ppm for the paramagnetic 10-Ce. The BH4 proton signals were found at 1.26–0.61 ppm for 10-La and at 35.2 ppm for 10-Ce.
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Fig. 9 Molecular structures of [(η9-Cnt)LnIII(BH4)3(thf)] (10-Ln, Ln = La, Ce) in the solid state with thermal ellipsoids at 40% probability.† |
Solvation experiments revealed that dissolving 10-La and 10-Ce in warm THF led to the formation of the ionic species [LnIII(BH4)2(thf)5][Cnt] (11-Ln) (Ln = La, Ce, Scheme 11), in which the Cnt ligands decoordinate from the LnIII ions. Instead, five THF molecules are coordinated to the metal centres in their equatorial positions. The two terminal BH4 anions coordinate to the LnIII centres in their axial positions, resulting in cationic parts that exhibit pentagonal bipyramidal geometries. A similar displacement of the Cnt ligand by THF was also found for the super sandwich complexes [(Cnt)LnIII(η8-Cot)] (6-Ln).41 These results clearly demonstrate the labile coordination between the LnIII ions and the Cnt ligand, in comparison to the strong coordination with the hard THF donor. Additionally, the Cnt is also weaker bound to the LnIII ions compared to the smaller Cp ligand, as these compounds do not dissociate in THF. Interestingly, the displacement of Cnt by THF from 10-Ln to 11-Ln is fully reversible as the half-sandwich compounds 10-Ln were formed again after drying the crystals of 11-Ln under vacuum. This was proven by Raman spectroscopy of the dried crystals, indicating their potential use for switchable materials.
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Scheme 11 Top left: Equilibrium between complexes 10-Ln and 11-Ln upon reaction with THF or drying under vacuum. Right: Polymeric complex 12-La formed after drying 10-La at 60 °C under vacuum and crystallisation from toluene. Bottom: Molecular structures of 11-La and 12-La in the solid state with thermal ellipsoids at 40% probability.† |
Furthermore, the THF in complex 10-La was fully removed after drying it under high vacuum at 60 °C for several days, which produced the poorly soluble polymeric species [LaIII(μ-η2:η2-BH4)2(η3-BH4)(η9-Cnt)] (12-La) in extremely low yield, as only a few single crystals were obtained from toluene (Scheme 10). The polymeric structure is best described as a trigonal-prismatic coordination polymer, in which each LaIII ion is coordinated by one Cnt ring (η9), one terminal BH4 ligand (η3), and two bridging BH4 (μ-η2:η2) ligands. NMR spectroscopy showed a single peak for the Cnt both in the 1H (δ 7.44 ppm) and 13C NMR (δ 113.4 ppm) spectra and confirmed the presence of the BH4 groups.
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Scheme 12 Synthesis of the tris(Cnt) complexes 13-Ln and reaction between K(Cnt) and LnI3 (Ln = Sm, Yb), leading to [LnII(Cnt)2] (3-Sm and 3-Yb). |
SCXRD analysis revealed that all the complexes 13-Ln (Y-Er) are isostructural (Fig. 10). Instead of the typical η9-coordination modes in the bis(Cnt) sandwich type complexes 3-Ln, all the Cnt ligands are consistently η4-coordinating to the LnIII centres. Despite significant bending of the Cnt ligands around the LnIII ions, their aromaticity remains intact, without significant changes in C–C bond distances or C(sp2) hybridisation. There are interesting trends which nicely correlate with the ionic radii along the lanthanide series (Table 4): The shortest Ln–C bond was found to be 2.63(2) Å for the gadolinium complex 13-Gd and 2.54(1) Å for the thulium complex 13-Tm. A similar trend was observed for the average Ln–C4 distance, reflecting the lanthanide contraction along the series. Overall, the average Ln–C9 distance does not show any obvious trend and is rather similar for all seven compounds, suggesting a dynamic coordination behaviour of the Cnt ligand. An increase in LnIII size pushes the closest C atoms away but brings the farthest ones closer.
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Fig. 10 Molecular structures of [LnIII(Cnt)3] (13-Ln, Ln = Y, Gd, Tb, Dy, Er, Ho, Tm) in the solid state with thermal ellipsoids at 40% probability.† |
13-Y | 13-Gd | 13-Tb | 13-Dy | 13-Ho | 13-Er | 13-Tm | |
---|---|---|---|---|---|---|---|
Distances are given in Å.a Average of the nine C atoms in the Cnt ring.b Average of the four closest C atoms in the Cnt ring. | |||||||
Ln–C (closest) | 2.58(1) | 2.63(2) | 2.61(2) | 2.58(3) | 2.59(2) | 2.56(2) | 2.54(1) |
Ln–C (farthest) | 4.40(18) | 4.41(18) | 4.40(19) | 4.41(18) | 4.42(18) | 4.43(18) | 4.48(3) |
Ln–C9 (av.)a | 3.35(7) | 3.38(9) | 3.37(10) | 3.37(10) | 3.37(11) | 3.37(10) | 3.40(3) |
Ln–C4 (av.)b | 2.69(14) | 2.73(12) | 2.71(13) | 2.70(14) | 2.69(14) | 2.69(15) | 2.57(16) |
Due to the lability of the Cnt ligand in THF, 1H NMR spectra were recorded for all 13-Ln complexes in toluene-d8 to ensure their stability. For the Gd complex 13-Gd, the 1H NMR spectrum remained silent. For the diamagnetic Y complex 13-Y, only one singlet signal for the Cnt protons was found at δ 6.27 ppm. For all the other paramagnetic Ln species, this Cnt proton signal appeared as broad singlet at δ 295.5 ppm (13-Tb), δ 287.5 ppm (13-Dy), δ 273.0 ppm (13-Ho), δ 226.5 ppm (13-Er) and δ 264.3 ppm (13-Tm), showing a clear trend along the Ln series. The coordination environment is not temperature dependent, as the VT 1H NMR of 13-Y showed little change from −80 °C to 80 °C. According to UV-vis spectroscopy, typical ligand-based π to π* transitions were detected for all complexes in the visible region, with 13-Er and 13-Tm exhibiting hypersensitive f–f transitions. An additional charge-transfer band was observed for 13-Tm. Magnetic susceptibility measurements for all paramagnetic complexes 13-Ln matched well with the nature of the trivalent LnIII ions. None of the tris(Cnt) complexes displayed single molecule magnet behaviour. Computational studies further interpreted the electronic structures and the magnetic behaviour, suggesting that the Cnt ligands in 13-Ln provide a spherical crystal field.
Footnote |
† Reproduced from the CIF file having CCDC numbers of 1861445 (KCnt·Et2O), 1894445 (KCnt·(dme)2), 1861447 (3-Sm), 1544973 (3-Eu), 1861450 (3-Tm), 1861446 (3-Tm), 1894447 (6-Nd), 1894448 (6-Sm), 2073514, 2073526 (6-Tb), 2073516 (6-Dy), 2073518 (6-Ho), 2073520 (6-Er), 2073522 (6-Tm), 2073525 (6-Lu), 2125024 (7-La), 2125026 (8-Ce), 2125029 (9-Er), 2267628 (10-La), 2267629 (10-Ce), 2267630 (11-La), 2267631 (11-Ce), 2267632 (12-La), 2113686 (13-Y), 2113687 (13-Gd), 2113688 (13-Tb), 2113689 (13-Dy), 2113691 (13-Er), 2113690 (13-Ho), 2113692 (13-Tm), 2370453 (6-Y-trans), 2370455 (6-La-trans), 2370457 (6-Ce-trans), 2370459 (6-Pr-trans), 2370461 (6-Nd-trans), 2370463 (6-Sm-trans), 2370465 (6-Gd-trans), 2370466 (6-Tb-trans), 2370467 (6-Dy-trans), 2370468 (6-Ho-trans). |
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