Synthesis and reactivity of heteroleptic zinc(I) complexes toward heteroallenes

Abstract

Heteroleptic zinc(I) complexes L^1,2Zn–ZnCp* (L¹ = HC[C(CF₃)NC₆F₅]₂1; L² = HC[C(Me)NDipp]₂; Dipp = 2,6-i-Pr₂C₆H₃2) are synthesized by reactions of Cp*₂Zn₂ with L¹H and L²ZnH. 2 reacts with t-BuNCO to give unprecedented carbamate complex (4), while reactions with RN₃ gave bis-hexazene, triazenide, and trimeric azide complexes (5–7).

---

Since the discovery of Cp*₂Zn₂ (Cp* = C₅Me₅) by Carmona et al.,¹ Zn(I) complexes have received increasing interest.² In contrast to Cp*₂Zn₂, which is accessible by reactions of Cp*₂Zn with (C₂H₅)₂Zn and by reduction of Cp*₂Zn and ZnCl₂ with KH,³ the majority of Zn(I) complexes are synthesized by Wurtz-type coupling of organozinc halides RZnX.⁴ Such reactions also yield homotrinuclear Zn₃⁵ and heterotetranuclear Re₂Zn₂ complexes,⁶ but this pathway typically suffers from low yields. In contrast, ligand exchange reactions of Cp*₂Zn₂ with H-acidic ligands or potassium salts are more efficient routes for the synthesis of heteroleptic Zn(I) complexes.^7–9 Apart from their bonding nature, Zn(I) complexes are of interest due to their widespread reactivity, i.e. disproportionation,¹ acid–base,¹⁰ protonation,¹¹ redox,¹² and cluster formation reactions,¹³ respectively. Recently, we reported an isocyanate insertion reaction into one Zn–Cp* bond of Cp*₂Zn₂,¹⁴ revealing a new reaction pattern for Zn(I) complexes. Encouraged by this finding, we became interested in reactions of heteroleptic Zn(I) complexes Cp*ZnZnL with heteroallenes, and report herein on the synthesis of L¹ZnZnCp* 1 and L²ZnZnCp* 2, and reactions of 2 with t-BuNCO and three organoazides RN₃.

The reaction of L³H (L³ = HC[C(Me)NMes]₂, Mes = 2,4,6-Me₃C₆H₂) with Cp*₂Zn₂ in 1 : 1 and 1 : 2 molar ratio yielded the homoleptic Zn(I) complex L³₂Zn₂.⁷ In contrast, the reaction of Cp*₂Zn₂ with one or two equiv. of L¹H (L¹ = HC[C(CF₃)NC₆F₅]₂) in toluene at 6 °C gave the heteroleptic complex L¹ZnZnCp* 1 (Scheme 1), whereas L²H (L² = HC[C(Me)NDipp]₂, Dipp = 2,6-i-Pr₂C₆H₃) failed to react. Inspired by a Pd(II)-induced homocoupling reaction of RZnH,¹⁵ L²ZnH was reacted with Cp*₂Zn at 4 °C for 3 days, yielding L²Zn–ZnCp* (2) and other by-products (Fig. S37, ESI†). 2 also formed in the equimolar reaction of L²ZnH with Cp*₂Zn₂ (Scheme 1), whereas substitution of the second Cp* group by reacting 2 or Cp*₂Zn₂ with L²ZnH in 1 : 1 or 1 : 2 molar ratio failed, although the desired homoleptic Zn(I) complex L²₂Zn₂ is known.⁴ In contrast, Cp*₂Zn₂ reacted with the stronger reductant L²MgH to give zinc metal and L²MgCp* (3, ESI†).

Scheme 1 Synthesis of heteroleptic Zn(I) complexes 1 and 2.

The ¹H and ¹³C NMR spectra of 1 and 2 show characteristic resonances of the Cp* group and L¹ (1) and L² (2). The ¹⁹F NMR spectrum of 1 shows a resonance at −66.40 ppm (CF₃) and three resonances of the C₆F₅ groups in a 2 : 1 : 2 intensity ratio.

Single crystals were grown from toluene solutions at −30 °C (1, Fig. S42, ESI†) and 4 °C (2, Fig. S43, ESI†). The complexes crystallize in the triclinic space group P1̄ (1) and the monoclinic space group P2₁ (2). The Zn1–Zn2 bond of 1 (2.2883(5) Å) is shorter than that of 2 (2.3008(2) Å) and other Zn(I) complexes, but comparable to those in [Zn]₈ clusters (2.27–2.29 Å).¹⁶ The Cp* groups are η⁵-bonded with Zn2–Cp*_(centr) distances of 1.8858(4) Å (1) and 1.9215(3) Å (2) and Cp*_(centr)–Zn1–Zn2 bond angles of 177.2° (1) and 177.5° (2), which are close to linearity as observed in Zn₂Cp*₂¹ and Cp*ZnZnL.^8b,14 The Zn1 atoms are three-coordinated, and the Zn1–N1/2 bonds of 1 (1.9917(13), 2.0140(13) Å) are longer than those of 2 (1.9580(10), 1.9598(10) Å), in accordance with the reduced electron donor capacity of the fluorinated L¹ ligand,¹⁷ but comparable to those of homoleptic complexes L²₂Zn₂ and L³₂Zn₂.^4,7

With heteroleptic complex 2 in hand, we explored its reactivity toward heteroallenes. Homoleptic Cp*₂Zn₂ reacted with RNCO (R = t-Bu, Dipp) at ambient temperature with insertion into one Zn–Cp* bond.¹⁴ In contrast, the reaction of heteroleptic Zn(I) complex 2 with t-BuNCO at 70 °C for 4 days gave complex 4 in 41% yield (Scheme 2), whereas no reaction occurred with DippNCO even at 100 °C. The formation of 4 results from insertion of t-BuNCO into the Zn–Zn and Zn–Cp* bonds and cleavage of a C=O bond accompanied by the formation of t-BuNC as was confirmed by in situ¹H NMR spectroscopy (Fig. S39, ESI†). Any attempts to isolate reaction intermediates by varying the temperature and the molar ratio of the reagents failed. However, an excess of t-BuNCO promotes the reaction, as 4 was not formed in a 1 : 1 molar ratio reaction at 70 °C. The analogous reaction of L³₂Zn₂⁷ with t-BuNCO also quantitatively gave a zinc carbamate complex 4S (Fig. S40 and S46, ESI†) and t-BuNC.

Scheme 2 Synthesis of complex 4.

Complex 4 is thermally stable and decomposes at 210 °C. The ¹H NMR spectrum shows two singlets of the t-Bu groups and five singlets of the Cp* group, indicating an asymmetric nature of the complex in solution. The ¹³C NMR spectrum also shows five singlets of the Me groups of the Cp* ligand as well as two singlets of the tertiary C atom of the t-Bu groups and resonances of the NCO units at 170.1 and 172.8 ppm, respectively.

Single crystals of 4, which crystallize in the monoclinic space group P2₁/c (Fig. 1), were grown from a saturated toluene solution at 4 °C. Both Zn atoms adopt distorted tetrahedral coordination spheres and are bridged by a carbamate unit.

Fig. 1 Molecular structure of 4. Thermal ellipsoids are drawn at 30% probability level. Parts of the ligands are drawn in wire/stick model, while H atoms are omitted for clarity.

The Zn1–O2/3 (2.0697(8), 2.0309(8) Å) bond lengths are comparable to those of the zinc carbamate complex L²ZnO₂CN(i-Pr)₂ (2.028(2), 2.041(1) Å),¹⁸ but longer compared to those of the carboxylate complexes [L²Zn(μ,η²-O₂CR)]₂ (R = H, Me, Ph, Oi-Pr), which range from 1.936 to 2.027 Å.¹⁹ The Zn2–N3/4 (1.9000(9), 1.9169(9) Å) bond lengths are virtually identical to that of Cp*Zn–Zn(N(t-Bu)C(Cp*)O) (1.9148(9) Å),¹⁴ while the Zn2–O3 distance (2.2780(8) Å) is rather long, indicating a rather weak coordinative interaction. The C–O (1.2819(12), 1.3273(12) Å) and C–N bond lengths (1.3270(13) Å) indicate a delocalized π-electron system within the carbamate unit. The Cp* ligand is η²-coordinated to Zn2, and the Zn–C bonds (2.4906(11), 2.5135(11) Å) are elongated compared to Zn-π complexes with η² interactions, i.e. alkyne-coordinated ZnBr₂ (2.217(5), 2.393(5) Å)²⁰ and [PhC(Nt-Bu)₂(Cp*)Si–Zn(Cp*)Cl] (2.2519(26), 2.1224(29) Å),²¹ but shorter than those in Zn(C₆F₅)₂(tol.) (2.784(2), 2.6847(15) Å)²² and arylacetylene-substituted calix[4]arene zinc complexes (2.7695(37), 3.0667(37) Å).²³

Complex 2 was then reacted with organoazides RN₃ (R = Ph, Ad, SiMe₃). In contrast to homoleptic L³₂Zn₂, which was found to react with RN₃ with formation of zinc hexazene [(L³Zn)₂(μ-η²:η²-N₆R₂)] (R = Ph, 2,6-i-Pr₂C₆H₃) or dimeric zinc azide complexes [(L³Zn)(μ-N₃)]₂ (R = Me₃Si, Me₃Sn),^12c the reaction of heteroleptic complex 2 with 2 equiv. of AdN₃ at 70 °C for 2 days yielded the first bis-hexazene complex 5 (Scheme 3), which is thermally stable in solution up to 100 °C and in the solid state (decomposition temperature > 300 °C), respectively. Complex 5 is likely formed via the Cp*Zn(μ-η²:η²-N₆Ad₂)ZnL² intermediate, followed by intramolecular elimination of Cp*₂Zn.

Scheme 3 Synthesis of complexes 5–7.

In contrast, the reaction of 2 with PhN₃ gave the zinc triazenide complex 6 in 48% yield (Scheme 3). Alkaline or alkaline earth metal triazenides are typically formed in reactions of aryl azides and organolithium and magnesium complexes,²⁴ hence the formation of 6 likely results from a nucleophilic attack of the Cp* ligand. Since no reaction was observed in a control experiment of L²ZnCp* with PhN₃, we assume that the first reaction step is an insertion reaction of PhN₃ into the Zn–Zn bond of 2. Low-valent metal complexes are known to react with RN₃ with formation of metal triazenides as was shown for homo- (Al, Cr) and heterobimetallic (In–Zn) complexes,²⁵ while a dinuclear iron complex was formed by the reaction of an Fe–N₂ complex with AdN₃.²⁶ The reaction of 2 with Me₃SiN₃ occurred with reductive elimination of Si₂Me₆ as was reported for the analogous reaction of L³₂Zn₂^12c and formation of complex 7 featuring a pseudo triangular Zn₃N₉ moiety (Scheme 3). 7 also formed in 78% yield in the reaction of L²ZnH and Me₃SiN₃.

¹H and ¹³C NMR spectra of 5 and 6 show resonances of L² and Ad (5) and Cp* and Ph (6), while 7 shows two sets of resonances of L² due to two conformers in solution, which form a temperature-dependent equilibrium as confirmed by VT-¹H NMR (Fig. S41, ESI†). IR spectra show absorption bands of the hexazene (1265, 1218 cm⁻¹, 5), triazenide (1313, 1255 cm⁻¹, 6) and azide groups (2157, 2124 cm⁻¹, 7).

Single crystals of 5–7 were grown from toluene solutions. Complexes 5 and 6 crystallize in the monoclinic space groups P2/n and P2₁/m and complex 7 in the triclinic space group P1̄ (Fig. 2). Complex 5 contains two bridging hexazene ligands. The Zn1–N1/2 bonds within the C₃N₂Zn ring are slightly shorter than the Zn1–N3/6 bonds (2.0024(19), 2.0378(18) Å) in the neighbouring N₄Zn ring, but comparable to Zn–N3/6 bonds (2.0079(18), 1.9817(19) Å) in the nonadjacent N₄Zn ring. The N5–N6 distance (1.400(3) Å) is typical for a single bond, while the other N–N bond lengths (1.297(3)–1.301(3) Å) of the hexazene unit indicate an allyl-like nature as was previously reported for metal hexazene complexes.^12c,27 The Zn atom in triazene complex 6 is tetrahedrally coordinated by four N atoms of the L² ligand and the triazene group. The N–N bond lengths within the ZnN₃ metallacycle (1.297(6), 1.307(5) Å) indicate a delocalized π-electron system within the N₃ moiety. The Zn–N2 bonds (1.9592(10) Å) are shorter than the Zn–N1/3 bonds (2.0524(13), 2.0869(14) Å). The only structurally characterized zinc triazenide complex [Dipp₂N₃]₂Zn, which was prepared by an ethane elimination reaction of ZnEt₂ with Dipp₂N₃H,²⁸ shows comparable structure parameters.

Fig. 2 Molecular structure of 5–7. Thermal ellipsoids drawn at 30% probability level, parts of the ligands are in a wire/stick model, and hydrogen atoms are omitted for clarity. 7 contains two molecules in the asymmetric unit (only the Zn(1)-containing molecule is discussed).

In contrast to dimeric [(L³Zn)(μ-N₃)]₂,²⁹ complex 7 forms a pseudo-triangular Zn₃N₉ moiety with bridging N₃ units, resulting in an almost planar Zn₃N₉ metallacycle (r.m.s. deviation from the least-squares plane 0.0655 Å), and each Zn atom is further coordinated by one L² ligand. The Zn–N bonds (1.952(5)–2.027(5) Å) within the Zn₃N₉ moiety are slightly longer than those in the C₃N₂Zn rings (1.945(4)–1.959(5) Å). The N–N–N angles of 178.0(7)°, 179.1(7)° and 178.9(7)° are almost linear, and the N–N bond lengths range from 1.147(7) to 1.192(7) Å.

To summarize, heteroleptic Zn(I) complexes L^1/2ZnZnCp* (1, 2) were synthesized and reactions of L²ZnZnCp* 2 with heteroallenes are reported. The reaction with t-BuNCO proceeded with insertion into both the Zn–Zn and the Zn–Cp* bonds and formation of carbamate complex 4, whereas reactions with RN₃ yielded unprecedented bis-hexazenide, triazenide, and trimeric azide complexes 5–7, respectively.

S. S. acknowledges support by the University of Duisburg-Essen.

Conflicts of interest

There are no conflicts to declare.

Notes and references

1. I. Resa, E. Carmona, E. Gutierrez-Puebla and A. Monge, Science, 2004, 305, 1136.
2. (a) C.-S. Cao, Y. Shi, H. Xu and B. Zhao, Coord. Chem. Rev., 2018, 365, 122; (b) R. H. Duncan Lyngdoh, H. F. Schaefer and R. B. King, Chem. Rev., 2018, 118, 11626; (c) T. Li, S. Schulz and P. W. Roesky, Chem. Soc. Rev., 2012, 41, 3759; (d) E. Carmona and A. Galindo, Angew. Chem., Int. Ed., 2008, 47, 6526; (e) A. Grirrane, I. Resa, A. Rodríguez and E. Carmona, Coord. Chem. Rev., 2008, 252, 1532; (f) R. Ayala and A. Galindo, J. Organomet. Chem., 2019, 898, 120878.
3. A. Grirrane, I. Resa, A. Rodríguez, E. Carmona, E. Alvarez, E. Gutierrez-Puebla, A. Monge, A. Galindo, D. del Río and R. A. Andersen, J. Am. Chem. Soc., 2007, 129, 693.
4. (a) M. Ma, L. Shen, H. Wang, Y. Zhao, B. Wu and X.-J. Yang, Organometallics, 2020, 39, 1440; (b) M. Juckel, D. Dange, C. de Bruin-Dickason and C. Jones, Z. Anorg. Allg. Chem., 2020, 646, 603; (c) A. Stasch, Chem. – Eur. J., 2012, 18, 15105; (d) Y. Liu, S. Li, X.-J. Yang, P. Yang, J. Gao, Y. Xia and B. Wu, Organometallics, 2009, 28, 5270; (e) Z. Zhu, M. Brynda, R. J. Wright, R. C. Fischer, W. A. Merrill, E. Rivard, R. R. Wolf, J. C. Fettinger, M. M. Olmstead and P. P. Power, J. Am. Chem. Soc., 2007, 129, 10847; (f) I. L. Fedushkin, A. A. Skatova, S. Y. Ketkov, O. V. Eremenko, A. V. Piskunov and G. K. Fukin, Angew. Chem., Int. Ed., 2007, 46, 4302; (g) Y.-C. Tsai, D.-Y. Lu, Y.-M. Lin, J.-K. Hwang and J.-S. K. Yu, Chem. Commun., 2007, 4125; (h) X.-J. Yang, J. Yu, Y. Liu, Y. Xie, H. F. Schaefer, Y. Liang and B. Wu, Chem. Commun., 2007, 2363; (i) Z. Zhu, R. J. Wright, M. M. Olmstead, E. Rivard, M. Brynda and P. P. Power, Angew. Chem., Int. Ed., 2006, 45, 5807; (j) Y. Z. Wang, B. Quillian, P. R. Wei, H. Y. Wang, X. J. Yang, Y. M. Xie, R. B. King, P. V. Schleyer, H. F. Schaefer and G. H. Robinson, J. Am. Chem. Soc., 2005, 127, 11944.
5. J. Hicks, E. J. Underhill, C. E. Kefalidis, L. Maron and C. Jones, Angew. Chem., Int. Ed., 2015, 54, 10000.
6. T. D. Lohrey, L. Maron, R. G. Bergman and J. Arnold, J. Am. Chem. Soc., 2019, 141, 800.
7. S. Schulz, D. Schuchmann, U. Westphal and M. Bolte, Organometallics, 2009, 28, 1590.
8. (a) H. P. Nayek, A. Lühl, S. Schulz, R. Köppe and P. W. Roesky, Chem. – Eur. J., 1773, 2011, 17; (b) S. Schulz, S. Gondzik, D. Schuchmann, U. Westphal, L. Dobrzycki, R. Boese and S. Harder, Chem. Commun., 2010, 46, 7757.
9. (a) K. Mayer, L. Jantke, S. Schulz and T. F. Fässler, Angew. Chem., Int. Ed., 2017, 56, 2350; (b) S. Gondzik, D. Bläser, C. Wölper and S. Schulz, Chem. – Eur. J., 2010, 16, 13599.
10. (a) D. Schuchmann, U. Westphal, S. Schulz, U. Flörke, D. Bläser and R. Boese, Angew. Chem., Int. Ed., 2009, 48, 807; (b) M. Carrasco, R. Peloso, I. Resa, A. Rodríguez, L. Sánchez, E. Álvarez, C. Maya, R. Andreu, J. J. Calvente, A. Galindo and E. Carmona, Inorg. Chem., 2011, 50, 6361; (c) P. Jochmann and D. W. Stephan, Chem. – Eur. J., 2014, 20, 8370.
11. (a) S. Schulz, D. Schuchmann, I. Krossing, D. Himmel, D. Bläser and R. Boese, Angew. Chem., Int. Ed., 2009, 48, 5748; (b) M. Carrasco, R. Peloso, A. Rodríguez, E. Álvarez, C. Maya and E. Carmona, Chem. – Eur. J., 2010, 16, 9754; (c) K. Freitag, H. Banh, C. Ganesamoorthy, C. Gemel, R. W. Seidel and R. A. Fischer, Dalton Trans., 2013, 42, 10540; (d) H. Banh, C. Gemel, R. W. Seidel and R. A. Fischer, Chem. Commun., 2015, 51, 2170.
12. (a) S. Gondzik, D. Bläser, C. Wölper and S. Schulz, J. Organomet. Chem., 2015, 783, 92; (b) S. Gondzik, S. Schulz, D. Bläser and C. Wölper, Chem. Commun., 2014, 50, 1189; (c) S. Gondzik, S. Schulz, D. Bläser, C. Wölper, R. Haack and G. Jansen, Chem. Commun., 2014, 50, 927; (d) J. Gao, S. Li, Y. Zhao, B. Wu and X.-J. Yang, Organometallics, 2012, 31, 2978.
13. (a) H. Banh, J. Hornung, T. Kratz, C. Gemel, A. Pöthig, F. Gam, S. Kahlal, J.-Y. Saillard and R. A. Fischer, Chem. Sci., 2018, 9, 8906; (b) K. Freitag, C. Gemel, P. Jerabek, I. M. Oppel, R. W. Seidel, G. Frenking, H. Banh, K. Dilchert and R. A. Fischer, Angew. Chem., Int. Ed., 2015, 54, 4370; (c) T. Bollermann, K. Freitag, C. Gemel, R. W. Seidel, M. von Hopffgarten, G. Frenking and R. A. Fischer, Angew. Chem., Int. Ed., 2011, 50, 772; (d) H. Banh, K. Dilchert, C. Schulz, C. Gemel, R. W. Seidel, R. Gautier, S. Kahlal, J.-Y. Saillard and R. A. Fischer, Angew. Chem., Int. Ed., 2016, 55, 3285.
14. B. Li, C. Wölper, K. Huse and S. Schulz, Chem. Commun., 2020, 56, 8643.
15. (a) M. Chen, S. Jiang, L. Maron and X. Xu, Dalton Trans., 2019, 48, 1931; (b) S. Jiang, M. Chen and X. Xu, Inorg. Chem., 2019, 58, 13213.
16. P. Cui, H.-S. Hu, B. Zhao, J. T. Miller, P. Cheng and J. Li, Nat. Commun., 2015, 6, 6331.
17. (a) K. Huse, C. Wölper and S. Schulz, Eur. J. Inorg. Chem., 2018, 3472; (b) K. Huse, H. Weinert, C. Wölper and S. Schulz, Dalton Trans., 2020, 49, 9773.
18. M. H. Chisholm, J. Gallucci and K. Phomphrai, Inorg. Chem., 2002, 41, 2785.
19. (a) G. Feng, C. Du, L. Xiang, I. del Rosal, G. Li, X. Leng, E. Y.-X. Chen, L. Maron and Y. Chen, ACS Catal., 2018, 8, 4710; (b) C. Scheiper, C. Wölper, D. Bläser, J. Roll and S. Schulz, Z. Naturforsch., B: J. Chem. Sci., 2014, 69, 1365; (c) D. R. Moore, M. Cheng, E. B. Lobkovsky and G. W. Coates, J. Am. Chem. Soc., 2003, 125, 11911; (d) M. Cheng, E. B. Lobkovsky and G. W. Coates, J. Am. Chem. Soc., 1998, 120, 11018.
20. H. Lang, N. Mansilla and G. Rheinwald, Organometallics, 2001, 20, 1592.
21. S. Schäfer, R. Köppe, M. T. Gamer and P. W. Roesky, Chem. Commun., 2014, 50, 11401.
22. A. Guerrero, E. Martin, D. L. Hughes, N. Kaltsoyannis and M. Bochmann, Organometallics, 2006, 25, 3311.
23. E. Bukhaltsev, I. Goldberg, R. Cohen and A. Vigalok, Organometallics, 2007, 26, 4015.
24. (a) S. G. Alexander, M. L. Cole, C. M. Forsyth, S. K. Furfari and K. Konstas, Dalton Trans., 2009, 2326; (b) S.-O. Hauber, F. Lissner, G. B. Deacon and M. Niemeyer, Angew. Chem., Int. Ed., 2005, 44, 5871.
25. (a) W. Uhl, R. Gerding, S. Pohl and W. Saak, Chem. Ber., 1995, 128, 81; (b) C. Ni, B. D. Ellis, G. J. Long and P. P. Power, Chem. Commun., 2009, 2332; (c) M. D. Anker, Y. Altaf, M. Lein and M. P. Coles, Dalton Trans., 2019, 48, 16588.
26. R. E. Cowley, N. J. DeYonker, N. A. Eckert, T. R. Cundari, S. DeBeer, E. Bill, X. Ottenwaelder, C. Flaschenriem and P. L. Holland, Inorg. Chem., 2010, 49, 6172.
27. (a) R. E. Cowley, J. Elhaïk, N. A. Eckert, W. W. Brennessel, E. Bill and P. L. Holland, J. Am. Chem. Soc., 2008, 130, 6074; (b) S. J. Bonyhady, S. P. Green, C. Jones, S. Nembenna and A. Stasch, Angew. Chem., Int. Ed., 2009, 48, 2973; (c) S. J. Bonyhady, C. Jones, S. Nembenna, A. Stasch, A. J. Edwards and G. J. McIntyre, Chem. – Eur. J., 2010, 16, 938; (d) J. A. Bellow, P. D. Martin, R. L. Lord and S. Groysman, Inorg. Chem., 2013, 52, 12335; (e) L. Fohlmeister and C. Jones, Aust. J. Chem., 2014, 67, 1011; (f) S. Gondzik, C. Wölper, R. Haack, G. Jansen and S. Schulz, Dalton Trans., 2015, 44, 15703; (g) C. Stienen, S. Gondzik, A. Gehlhaar, R. Haack, C. Wölper, G. Jansen and S. Schulz, Organometallics, 2016, 35, 1022.
28. N. Nimitsiriwat, V. C. Gibson, E. L. Marshall, P. Takolpuckdee, A. K. Tomov, A. J. P. White, D. J. Williams, M. R. J. Elsegood and S. H. Dale, Inorg. Chem., 2007, 46, 9988.
29. G. Bendt, S. Schulz, J. Spielmann, S. Schmidt, D. Bläser and C. Wölper, Eur. J. Inorg. Chem., 2012, 3725.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental, analytical (NMR, IR spectra, elemental analysis) and crystallographic data of 1–7. CCDC 2111218 (1), 2111219 (2), 2111220 (3), 2111221 (4), 2111225 (4S), 2111222 (5), 2111223 (6) and 2111224 (7). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1cc05617d

---