A convenient route to mixed cationic group 13/14/15 compounds

Abstract

The formation of novel cationic mixed main group compounds is reported revealing a chain composed of different elements of group 13, 14, and 15. Reactions of different pnictogenylboranes R₂EBH₂·NMe₃ (E = P, R = Ph, H; E = As, R = Ph, H) with the NHC-stabilized compound IDipp·GeH₂BH₂OTf (1) (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene) were carried out, yielding the novel cationic, mixed group 13/14/15 compounds [IDipp·GeH₂BH₂ER₂BH₂·NMe₃]⁺ (2a E = P; R = Ph; 2b E = As; R = Ph; 3a E = P; R = H; 3b E = As; R = H) by the nucleophilic substitution of the triflate (OTf) group. The products were analysed by NMR spectroscopy and mass spectrometry and for 2a and 2b also by X-ray structure analysis. Further reactions of 1 with H₂EBH₂·IDipp (E = P, As) resulted in the unprecedented parent complexes [IDipp·GeH₂BH₂EH₂BH₂·IDipp][OTf] (5a E = P; 5b E = As), which were studied by X-ray structure analysis, NMR spectroscopy and mass spectrometry. Accompanying DFT computations give insight into the stability of the formed products with respect to their decomposition.

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Introduction

Carbon- and main group element-containing catena compounds based on C–El (e.g., El = B, Si, P) building blocks have been known for a long time and are used for various applications.^1a–f For instance, 1,3-disilabutane, H₃SiCH₂SiH₂CH₃, serves as a precursor for SiC thin film deposition in several CVD (Chemical Vapor Deposition) processes.² Similarly, inorganic chain compounds that consist of two different main group elements are well known in the literature, especially in the case of group 13/15 ³ and 14/15 ⁴ compounds, respectively. However, comparable compounds that are assembled by three different elements of group 13, 14 and 15 are very limited, in particular, if heavier homologues are considered.⁵ For example, in 2004 Tokitoh and coworkers reported the neutral silylboranephosphine I, which was synthesised by reaction of a sterically crowded silylene with BH₃·PPh₃ (Chart 1).⁶ Alternatively, the anionic phosphineboranestannate II was synthesized by Wright and coworkers through the reaction of LiSn(NMe₂)₃ with the phosphine-borane ^tBu₂PH·BH₃.⁷ Examples for chain-like group 13/14/15 compounds which are stabilized by β-diketiminate ligands are the very recently reported compound III by Schulz⁸ and compound IV by von Hänisch and coworkers.⁹ Furthermore, the von Hänisch group reported unexpected reactivity of a four-membered Ga/P heterocycle towards bulky N-heterocyclic carbene (NHC) ligands, leading to the silylphosphinogallane V.¹⁰ In addition, Inoue and coworkers prepared the structurally related silylaminoborane IMe₄·BH₂NHSiH₂Si^tBu₃ (IMe₄ = 1,3,4,5-tetra(methyl)imidazole-2-ylidene)¹¹ and the NHC-stabilized compound VI from a salt-metathesis reaction.¹²

Chart 1 Selected examples for mixed group 13/14/15 chain compounds.

The few existing examples of linear group 13/14/15 compounds are usually short (3-membered) chains and possess organic groups to stabilize them. Thus, the only examples of parent group 13/14/15 chains containing only hydrogen substituents, including the triple mixed hydrides H₃TetPH₂BH₃ (Tet = Si, Ge) and our recently reported compound [IDipp·GeH₂BH₂PH₃][OTf] (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene), could not be isolated or show very limited stability, which prevented their full characterization.¹³ Thus, it is still very desirable to develop efficient synthetic routes for longer parent chain compounds, as they might represent suitable single-source precursors to bulk and nanodimensional ternary solids of tuneable composition and function, which can serve as small bandgap semiconductors for optoelectronics.¹⁴

Our group has a special interest in hydride-substituted (parent) group 13/15 compounds, and donor/acceptor stabilization is an elegant method for the development of several group 13/15 parent compounds of the type LA·H₂EE‘H₂·LB (E = group 15 element, E’ = group 13 element, LA = Lewis acid, LB = Lewis base).¹⁵ This concept was successfully refined to the LB-only stabilization of pnictogenyltriels H₂EE‘H₂·LB.¹⁶ Besides neutral compounds, the NMe₃-stabilized pnictogenylboranes H₂EBH₂·NMe₃ (E = P, As) could also be applied to build up cationic chain complexes.^3c,d Hence, we wondered if it is possible to obtain long mixed group 13/14/15 chain compounds (group 14 element ≠ C) by the reaction of LB-stabilized pnictogenylboranes with the mixed group 13/14 starting material IDipp·GeH₂BH₂OTf (1) and if so, this approach could enable access to the longest cationic group 13/14/15 chains bearing only hydrogen substituents.

Results and discussion

Reactions of IDipp·GeH₂BH₂OTf (1) with the LB-stabilized diphenyl substituted pnictogenylboranes Ph₂EBH₂·NMe₃ (E = P, As) led to the formation of the unprecedented mixed cationic group 13/14/15 chains as OTf⁻ salts of [IDipp·GeH₂BH₂EPh₂BH₂·NMe₃]⁺ (2a E = P; 2b E = As) (Scheme 1) and represent, to our knowledge, the longest cationic, mixed element 13/14/15 (group 14 element ≠ C) chain compounds so far. After stirring the reaction mixture overnight, the Ge–B–E–B–N chains 2a and 2b could be isolated in high yields of 94 and 93%, respectively. The reactions were performed in Et₂O solutions, as the cationic complexes precipitate out of the reaction mixture upon formation and can then be isolated as pure white powders. Compounds 2a and 2b are well soluble in more polar solvents like CH₂Cl₂ or THF and are stable in solution and as solids, at ambient temperatures under an inert atmosphere. After the successful formation of the organo-substituted cationic group 13/14/15 chains in 2a and 2b, we wondered if the all hydrogen-substituted Ge-B-E-B chains [IDipp·GeH₂BH₂EH₂BH₂·NMe₃][OTf] (3a E = P; 3b E = As) could be synthesized analogously. Reactions were performed under the same conditions as for 2a and 2b by using IDipp·GeH₂BH₂OTf (1) and the parent pnictogenylboranes H₂EBH₂·NMe₃ (E = P, As) as starting materials. Similar to the diphenyl-substituted compounds, the formed ionic products 3a and 3b precipitate out of the reaction mixture and could be isolated as white powders, however, in slightly lower yields of 68 and 74%, respectively (Scheme 1). While both products are stable as solids at ambient temperatures in an inert atmosphere, they slowly decompose within several days in solutions of CH₂Cl₂ or THF.

Scheme 1 Synthesis of the group 13/14/15 chain compounds 2a, 2b, 3a and 3b. Yields are given in parentheses.

While single crystals of the diphenyl-substituted compounds 2a and 2b were successfully obtained (Fig. 1) by layering a THF solution of the corresponding compounds with n-hexane, numerous attempts to crystalize the parent compounds 3a and 3b failed due to their rather fast decomposition in solution, leading only to the crystallization of [IDippH][OTf].

Fig. 1 Molecular structures of the cations in 2a (top) and 2b (bottom) in the solid state with thermal ellipsoids at a 50% probability level. Hydrogen atoms bound to carbon atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: 2a: C1–Ge1 2.009(2), Ge1–B2 2.051(3), B1–P1 1.937(3), P1–B2 1.969(3), B2–N1 1.609(3); C1–Ge1–B1 111.86(10), Ge1–B1–P1 112.17(14), P1–B2–N1 116.50(18). 2b: C1–Ge1 2.0075(18), Ge1–B1 2.054(2), B1–As1 2.047(2), As1–B2 2.072(2), B2–N1 1.599(3); C1–Ge1–B1 112.01(8), Ge1–B1–As1 111.19(10), As1–B2–N1 114.43(13).

In order to perform X-ray structure analysis on salts of the parent chains 3a/b, attempts to exchange the [OTf]⁻ anions with the larger weakly coordinating anions (WCAs) [BArF₂₀]⁻ (BArF₂₀ = [B(C₆F₅)₄]) and [TEF]⁻ (TEF = [Al{OC(CF₃)₃}₄]) were carried out, as the bigger anions might promote a more favorable packing of the ion pairs within the solid state. However, those attempts failed to yield crystals. Thus, experiments to exchange the rather small LB NMe₃ with the sterically more demanding NHCs were performed. Out of the NHCs used IMe₄, IMes (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene) and IDipp, only the latter was suitable for the crystallization of the desired products (vide infra). Whereas only decomposition was observed for the reaction of 1 and H₂PBH₂·IMe₄, the parent compound [IDipp·GeH₂BH₂PH₂BH₂·IMes][OTf] (4) could be isolated as white powder in 68% yield from the reaction of 1 and H₂PBH₂·IMes. While 4 was verified by NMR spectroscopy, all crystallization attempts failed and only decomposition was observed. Similarly to H₂PBH₂·IMes, reactions of 1 with H₂EBH₂·IDipp (E = P, As) in Et₂O result in the formation of the desired parent complexes [IDipp·GeH₂BH₂PH₂BH₂·IDipp][OTf] (5a E = P; 5b E = As) in high yields for 5a (91%) and good yields for 5b (69%) (Scheme 2).

Scheme 2 Synthesis of the parent compounds 5a and 5b. Yields are given in parentheses.

The ¹H NMR spectra in CD₂Cl₂ of all isolated group 13/14/15 chain compounds show signals for the GeH₂-moieties in the expected range of δ = 3.20–3.57 ppm. Furthermore, the ³¹P NMR spectrum of 2a contains a broad singlet at δ = −24.9 ppm due to the coupling with the ¹¹B nuclei and shows a downfield shift compared to the starting material Ph₂PBH₂·NMe₃ (δ = −39.5 ppm).¹⁷ Analogously, the hydrogen-substituted compounds 3a, 4 and 5a each display a broad ³¹P NMR triplet resonance with almost identical chemical shifts for the PH₂ moieties (δ = −112.1 to −116.6 ppm; ¹J_P,H = 334–340 Hz). The ¹¹B NMR spectra of 2a – 3b reveal two broad signals for the BH₂ moieties in the range of δ = −6.4 to −10.8 ppm and δ = −37.2 to −43.5 ppm. In contrast to the NMe₃-stabilized compounds 2a – 3b, the ¹¹B NMR spectra of 5a and 5b show a significantly upfield-shifted signal for the terminal boron atom within the Ge–B–E–B chain (5aδ = −34.8 ppm; 5bδ = −33.3 ppm) due to the exchange of NMe₃ with IDipp. This is also observed for the substitution with IMes in compound 4 (δ = −35.3 ppm). Similar to the NMe₃-stabilized compounds, a broad signal for the second boron atom is found in the range of δ = −39.6 to −42.8 ppm. Moreover, all molecular ion peaks of the products 2a–5b (except 4) are detected in the ESI-MS spectra (see ESI† for details).

The solid state structures of 2a and 2b reveal similar C–Ge bond lengths [2a: 2.009(2) Å; 2b: 2.0075(18) Å] (Fig. 1). In addition, the Ge−B distances are almost identical [2a: 2.051(3) Å; 2b: 2.054(2) Å]. The P–B bond lengths of 2a [1.937(3) and 1.969(3) Å] and the As–B bond lengths of 2b [2.047(2) and 2.072(2) Å] are in the expected range for single bonds. The B–P–B angle measures 119.27(13)°, while the corresponding B–As–B angle is slightly wider [122.45(9)°]. Both B–N distances [2a: 1.609(3) Å; 2b: 1.599(3) Å] are within the expected range of single bonds.¹⁸ In the Ge–B–E–B chain (2a: E = P, 2b: E = As) all substituents adopt a staggered conformation about the Ge−B and both E–B bond axes. Specifically, an antiperiplanar conformation is present about the Ge1–B1 axis, whilst a synperiplanar arrangement is found about the B1–E1-axis, and a synclinal arrangement exists about the E1–B2-axis. Similar to 2a and 2b, single crystals of 5a and 5b could be obtained by layering of a CH₂Cl₂ solution of the products with n-hexane at room temperature. Both compounds 5a and 5b crystallize with two independent molecules in the asymmetric unit and show a very disordered Ge–B–E–B (E = P, As) chain (Fig. 2; see the ESI† for details). In the solid state both compounds reveal Ge−B single bond distances. The P–B bond lengths of compound 5a and the As–B bond lengths of 5b are in the expected range for single bonds.¹⁸

Fig. 2 Major parts of the disordered molecular structures of the cations in 5a (top) and 5b (bottom) in the solid state with thermal ellipsoids at a 50% probability level. Only one cation of the asymmetric unit is depicted, respectively. Hydrogen atoms bound to carbon atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]:5a: C1–Ge1 1.955(3), Ge1–B1 2.014(7), B1–P1 1.934(6), P1–B2 1.922(13), B2–C2 1.665(11); C1–Ge1–B1 111.3(2), Ge1–B1–P1 105.7(3), B1–P1–B2 114.8(4), P1–B2–C2 116.1(8). 5b: C1–Ge1 1.994(3), Ge1–B1 2.055(8), B1–As1 2.076(6), As1–B2 2.034(10), B2–C2 1.595(9); C1–Ge1–B1 112.1(2), Ge1–B1–As1 103.4(3), B1–As1–B2 119.0(4), As1–B2–C2 112.2(7).

Computational studies of the gas phase reactions of IDipp·GeH₂BH₂OTf with R₂EBH₂·LB leading to the ion pairs reveal that those processes are exothermic for all cases (Table 1). The favorability increases in the order 3b < 2b < 3a < 2a < 5b < 5a and thus, confirms the experimental findings well. In detail, the formation of the phosphorus derivatives is more exothermic (by 18–35 kJ mol⁻¹) than for the corresponding arsenic compounds. Furthermore, the formal substitution of NMe₃ by IDipp is exothermic by 75–80 kJ mol⁻¹ and therefore significantly increases the stability of the respective hydride-only-substituted products.

Table 1 Thermodynamic characteristics for gas phase processes. Reaction energies ΔE₀°, standard enthalpies ΔH₂₉₈° and Gibbs energies ΔG₂₉₈° in kJ mol⁻¹, standard reactions entropies ΔS₂₉₈° in J mol⁻¹ K⁻¹. B3LYP/def2-TZVP level of theory

Process	ΔE0°	ΔH298°	ΔS298°	ΔG298°
1 + Ph2PBH2·NMe3 = 2a	−77.9	−69.0	−183.6	−14.3
1 + Ph2AsBH2·NMe3 = 2b	−51.3	−43.8	−168.9	6.6
1 + H2PBH2·NMe3 = 3a	−67.5	−58.1	−173.9	−6.2
1 + H2AsBH2·NMe3 = 3b	−39.9	−31.8	−168.5	18.4
1 + H2PBH2·IDipp = 5a	−122.8	−113.8	−151.0	−68.7
1 + H2AsBH2·IDipp = 5b	−88.1	−80.0	−146.7	−36.3
3a + IDipp = 5a + NMe3	−79.4	−82.0	−33.7	−72.0
3b + IDipp = 5b + NMe3	−75.8	−78.2	−35.4	−67.6

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Conclusions

The results show that the synthetic concept to generate cationic linear chain compounds by using IDipp·GeH₂BH₂OTf (1) in the reactions with different pnictogen sources could be successfully applied to obtain the longer, parent, mixed group 13/14/15 chain compounds containing a Ge–B–E–B (E = P, As) sequence. As a result, the diphenyl-substituted compounds 2a and 2b were synthesized in very good yields and were fully characterized. In addition, the cationic chain compounds of only hydrogen-substituted parent group 13/14/15 elements [IDipp·GeH₂BH₂EH₂BH₂·NMe₃][OTf] (3a E = P; 3b E = As) could be isolated successfully. Subsequent substitution of NMe₃ with IDipp for the group 13/14/15 chains containing only hydrogen substituents allowed X-ray structure analysis on the unprecedented parent compounds 5a and 5b. Further investigations are directed to generate anionic or neutral chain compounds of mixed group 13/14/15 elements.

Data availability

All experimental procedures, spectroscopic data, information on the theoretical calculations and crystallographic data can be found in the ESI.†

Author contributions

M. T. A. and R. C. performed the experimental work. M. T. A wrote the original draft. A. Y. T. performed the DFT calculations. M. Seidl, Y. Z. and M. J. F. performed single crystal X-ray experiments and interpreted the structural data. E. R. and M. Scheer supervised and acquired funding for the project.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by joint DFG-RSF grant (RSF project 21-43-04404 and DFG project Sche 384/41-1). The use of computational resources of the research center «Computing Center» of the research park of St. Petersburg State University is acknowledged. M. T. A. thanks the Deutscher Akademischer Austauschdienst (DAAD) for travel support. E. R. thanks NSERC of Canada for funding.

References

1. (a) H. I. Schlesinger, N. W. Flodin and A. B. Burg, J. Am. Chem. Soc., 1939, 61, 1078–1083; (b) F. Gol, G. Hasselkuss, P. C. Knüppel and O. Stelzer, Z. Naturforsch., B: J. Chem. Sci., 1988, 43, 31–44; (c) I. Kovacs and G. Fritz, Z. Anorg. Allg. Chem., 1993, 619, 1491–1493; (d) A. Wörsching and G. Fritz, Z. Anorg. Allg. Chem., 1984, 512, 131–163; (e) G. Fritz, Angew. Chem., Int. Ed. Engl., 1987, 26, 1111–1132; (f) D. P. Gates, in New Aspects in Phosphorus Chemistry V, Springer, Berlin, Heidelberg, 2005, vol. 250, pp. 107–126.
2. (a) J.-H. Boo, K.-S. Yu, Y. Kim, S. H. Yeon and I. N. Jung, Chem. Mater., 1995, 7, 694–698; (b) K.-W. Lee, K.-S. Yu and Y. Kim, J. Cryst. Growth, 1997, 179, 153–160; (c) C. R. Stoldt, C. Carraro, W. R. Ashurst, D. Gao, R. T. Howe and R. Maboudian, Sens. Actuators, A, 2002, 97–98, 410–415.
3. (a) C. Marquardt, C. Thoms, A. Stauber, G. Balázs, M. Bodensteiner and M. Scheer, Angew. Chem., Int. Ed., 2014, 53, 3727–3730; (b) C. Marquardt, T. Kahoun, A. Stauber, G. Balázs, M. Bodensteiner, A. Y. Timoshkin and M. Scheer, Angew. Chem., Int. Ed., 2016, 55, 14828–14832; (c) C. Marquardt, G. Balázs, J. Baumann, A. V. Virovets and M. Scheer, Chem. - Eur. J., 2017, 23, 11423–11429; (d) O. Hegen, C. Marquardt, A. Y. Timoshkin and M. Scheer, Angew. Chem., Int. Ed., 2017, 56, 12783–12787; (e) L. Tuscher, C. Ganesamoorthy, D. Bläser, C. Wölper and S. Schulz, Angew. Chem., Int. Ed., 2015, 54, 10657–10661; (f) M. Köster, A. Kreher and C. von Hänisch, Dalton Trans., 2018, 47, 7875–7878.
4. (a) K. Hassler and U. Katzenbeisser, J. Organomet. Chem., 1990, 399, C18–C20; (b) K. Hassler and S. Seidl, Monatsh. Chem., 1988, 119, 1241–1244; (c) M. S. Balakrishna, P. Chandrasekaran and P. P. George, Coord. Chem. Rev., 2003, 241, 87–117; (d) M. Driess, S. Block, M. Brym and M. T. Gamer, Angew. Chem., Int. Ed., 2006, 45, 2293–2296; (e) M. Balmer and C. von Hänisch, Z. Anorg. Allg. Chem., 2018, 644, 1143–1148; (f) A. R. Dahl, A. D. Norman, H. Shenav and R. Schaeffer, J. Am. Chem. Soc., 1975, 97, 6364–6370.
5. (a) M. Fan, R. T. Paine, E. N. Duesler and H. Nöth, Z. Anorg. Allg. Chem., 2006, 632, 2443–2446; (b) S. Inoue and K. Leszczyńska, Angew. Chem., Int. Ed., 2012, 51, 8589–8593; (c) R. J. Wilson, J. R. Jones and M. V. Bennett, Chem. Commun., 2013, 49, 5049–5051; (d) M. Kapitein, M. Balmer and C. von Hänisch, Phosphorus, Sulfur, Silicon Relat. Elem., 2016, 191, 641–644; (e) D. Wendel, A. Porzelt, F. A. D. Herz, D. Sarkar, C. Jandl, S. Inoue and B. Rieger, J. Am. Chem. Soc., 2017, 139, 8134–8137.
6. T. Kajiwara, N. Takeda, T. Sasamori and N. Tokitoh, Organometallics, 2004, 23, 4723–4734.
7. M. Fernández-Millán, L. K. Allen, R. García-Rodríguez, A. D. Bond, M. E. G. Mosquera and D. S. Wright, Chem. Commun., 2016, 52, 5993–5996.
8. A. Bücker, C. Wölper, G. Haberhauer and S. Schulz, Chem. Commun., 2022, 58, 9758–9761.
9. S. Schneider and C. von Hänisch, Eur. J. Inorg. Chem., 2021, 4655–4660.
10. M. Balmer, M. Kapitein and C. von Hänisch, Dalton Trans., 2017, 46, 7074–7081.
11. G. Dübek, D. Franz, C. Eisenhut, P. J. Altmann and S. Inoue, Dalton Trans., 2019, 48, 5756–5765.
12. D. Franz, T. Szilvási, A. Pöthig and S. Inoue, Chem. - Eur. J., 2019, 25, 11036–11041.
13. (a) A. B. Burg and E. S. Kuljian, J. Am. Chem. Soc., 1950, 72, 3103–3107; (b) J. E. Drake and J. Simpson, Inorg. Chem., 1967, 6, 1984–1986; (c) J. E. Drake and J. Simpson, Chem. Commun., 1967, 249–250; (d) J. E. Drake and C. Riddle, J. Chem. Soc. A, 1968, 7, 1675–1678; (e) N. Goddard and J. E. Drake, J. Chem. Soc. A, 1969, 662–665; (f) M. T. Ackermann, M. Seidl, F. Wen, M. J. Ferguson, A. Y. Timoshkin, E. Rivard and M. Scheer, Chem. – Eur. J., 2022, 28, e202103780.
14. (a) A. G. Norman, J. M. Olson, J. F. Geisz, H. R. Moutinho, A. Mason, M. M. Al-Jassim and S. M. Vernon, Appl. Phys. Lett., 1999, 74, 1382–1384; (b) T. Watkins, A. V. G. Chizmeshya, L. Jiang, D. J. Smith, R. T. Beeler, G. Grzybowski, C. D. Poweleit, J. Menéndez and J. Kouvetakis, J. Am. Chem. Soc., 2011, 133, 16212–16218; (c) R. Jia, T. Zhu, V. Bulović and E. A. Fitzgerald, J. Appl. Phys., 2018, 123, 175101; (d) T. K. Purkait, A. K. Swarnakar, G. B. de Los Reyes, F. A. Hegmann, E. Rivard and J. G. C. Veinot, Nanoscale, 2015, 7, 2241–2244; (e) A. A. Omaña, R. K. Green, R. Kobayashi, Y. He, E. R. Antoniuk, M. J. Ferguson, Y. Zhou, J. G. C. Veinot, T. Iwamoto, A. Brown and E. Rivard, Angew. Chem., Int. Ed., 2021, 60, 228–231; (f) M. M. D. Roy, A. A. Omaña, A. S. S. Wilson, M. S. Hill, S. Aldridge and E. Rivard, Chem. Rev., 2021, 121, 12784–12965.
15. (a) U. Vogel, A. Y. Timoshkin and M. Scheer, Angew. Chem., Int. Ed., 2001, 40, 4409–4412; (b) U. Vogel, P. Hoemensch, K.-C. Schwan, A. Y. Timoshkin and M. Scheer, Chem. – Eur. J., 2003, 9, 515–519.
16. (a) K.-C. Schwan, A. Y. Timoskin, M. Zabel and M. Scheer, Chem. - Eur. J., 2006, 12, 4900–4908; (b) C. Marquardt, A. Adolf, A. Stauber, M. Bodensteiner, A. V. Virovets, A. Y. Timoshkin and M. Scheer, Chem. – Eur. J., 2013, 19, 11887–11891.
17. C. Marquardt, T. Jurca, K.-C. Schwan, A. Stauber, A. V. Virovets, G. R. Whittell, I. Manners and M. Scheer, Angew. Chem., Int. Ed., 2015, 54, 13782–13786.
18. P. Pyykkö and M. Atsumi, Chem. – Eur. J., 2009, 15, 12770–12779.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2219857, 2219858, 2219859 and 2219860. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2sc06594k

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