Synthesis, Structure, and Reactivity of Disiloxa[3]tetrelocenophanes.

Tetramethyldisiloxa[3]metallocenophanes of the heavy group 14 elements germanium, 2a, tin, 2b, and lead, 2c, (tetrelocenophanes) have been synthesized by the reaction of dilithiated ligand, 1, with the corresponding element(II) chloride. The plumbocenophane, 2c, forms one-dimensional coordination polymers in the solid state, while the germanocenophane, 2a, and the stannocenophane, 2b, are monomeric. Furthermore, the reactivity of the stannocenophane, 2b, and the plumbocenophane, 2c, toward N-heterocyclic carbenes was explored. Although the coordination of carbene is reversible in solution at room temperature, the corresponding carbene complexes, 3a,b, could be structurally characterized, illustrating the Lewis acidity of the central atom in these metallocenophanes.

Introduction

Metallocenes with interlinked cyclopentadienyl moieties, so called ansa-metallocenes or metallocenophanes, have been known for over half a century, with examples for various transition metals.[1] They have attracted wide attention for their applications as catalysts in homogeneous catalysis or as monomers in ring-opening polymerization reactions.[2,3] In contrast, metallocenophanes of p-block elements are much less common, with only a handful of structurally characterized examples, mostly of group 14 elements (tetrelocenophanes).[4−10] This is particularly surprising, as the unbridged parent compounds have been known for a long time,[11−13] with stannocene and plumbocene being the first diorganotetrylene type compounds described in the literature.[14,15] Furthermore, these species could be of potential interest as precursors in the preparation of p-block metal-containing oligomers and polymers. With the great progress in low-valent group 14 chemistry in recent years,[16−19] and our group’s interest in the main group element-based metallocenophanes, our aim was to investigate the influence of different bridging motifs on the molecular geometry and reactivity of tetrelocenophanes. Whereas previous studies focused on disila[2]metallocenophanes,[9,10,20] we now report the first synthesis and structural characterization of three-atomic bridged metallocenophanes of p-block elements, in the form of disiloxa[3]tetrelocenophanes of group 14 elements germanium, tin, and lead, 2a–c. Although the tetramethyldisiloxa[3] bridging motif has been employed in transition metal-based metallocenophanes in the past,[21−26] no p-block metallocenophanes with three-atomic bridging motifs have been reported, so far.

Results and Discussion

The reaction of dilithiated ligand 1 with the corresponding element(II) chloride proceeds smoothly at 173 K to give the corresponding [3]tetrelocenophanes, 2a–c, in good yields (Scheme ). Stannocenophane, 2b, and plumbocenophane, 2c, exhibit very upfield-shifted NMR resonances in the 119Sn and 207Pb NMR spectra, respectively, (δ119Sn(2b) = −2124; δ207Pb(2c) = −4795), as is typical for tin(II)- and lead(II)-based metallocenes.[9,10]

Scheme 1

Synthesis of Tetramethyldisiloxa[3]tetrelocenophanes, 2a–c [a: E = Ge, D = 1,4-dioxane; b: E = Sn, D = thf; c: E = Pb, D = thf]

Crystals suitable for single crystal X-ray diffraction analysis were obtained from hexane solutions at 248 K, allowing for a structural characterization of tetrelocenophanes 2a–c (Figure ).

Figure 1

Molecular structures of (a) 2a, (b) 2b, and (c) 2c in the crystal (thermal ellipsoids at 50% probability level, except for the ansa-bridge of 2c in (c); H atoms omitted for clarity).

While germanocenophane, 2a, and stannocenophane, 2b, are monomeric in the solid state, plumbocenophane, 2c, exhibits a one dimensional polymeric structure in the crystal in which each lead atom is coordinated by three cyclopentadienyl ligands, similar to what is found for plumbocene (Cp2Pb) in the solid state,[27−29] and unlike what was reported earlier for a permethylated Si[2]plumbocenophane.[9] The intermolecular Pb–CCp distances are longer (337.9–395.1 pm) than the intramolecular ones (243.0–256.0 pm), suggesting rather weak intermolecular interactions, which is in line with the assumption that plumbocenophane, 2c, is monomeric in solution. The metal–Cp bonds in 2a–c are within normal ranges for group 14 metallocenes.[10] When comparing the dihedral angles of the Cp planes, α, in [3]tetrelocenophanes 2a–c to the angles α found in the previously reported permethylated [2]tetrelocenophanes,[9] one conspicuous difference is that [3]tetrelocenophanes 2a–c exhibit a more bent geometry, much like the unbridged parent compounds germanocene (Cp2Ge),[30] stannocene (Cp2Sn),[31] and plumbocene (Cp2Pb).[27−29] This is because of the stereochemically active lone pair at the central atom. The influence of the Si–O–Si ansa-bridge in tetrelocenophanes 2a–c on the dihedral angle of the Cp-planes, α, is comparably small (Table ). This is because of the fact that the Si–O–Si moiety is very flexible and that the ansa-bridge is not bound to be positioned on the far side relative to the lone pair of the central atom (Chart , 2-I), but can be located laterally (Chart , 2-II). This is more pronounced in germanocenophane, 2a, with shorter Ge–Cp bonds and a larger dihedral angle α between the Cp planes, and less pronounced in stannocenophane, 2b (2a: ϕ[32] = 56.1°; 2b: ϕ[32] = 51.1°), and indicates that the lone pair has a stronger effect on the dihedral angle α than the ansa-bridge in tetrelocenophanes 2a–c. In all cases, however, packing effects in the crystal have to be taken into consideration as well.

Table 1

Selected Bond Lengths and Angles in 2a–c, 3a,b and Cp2(Ge/Sn/Pb)a

	E-Cp [pm]	αb [deg]	βc [deg]	γd [deg]
Cp2Ge	223.4	50.2	156.9–157.4
Cp2Sn	237.0–243.7	45.9–46.7	146.9–147.4
Cp2Pb	244.1–252.7	53.6	131.0–134.6
2a	222.1–225.3	44.9	157.1	143.9
2b	239.2–241.6	49.8	147.7	144.5
2c	243.0–256.0	36.3–54.2	137.1–151.5	143.7–149.8
3a	257.0	37.6	140.9	148.0
3b	260.5	37.0	140.5	150.8

Range values given because multiple molecules are found in the asymmetric unit and/or the bond is found more than once in the molecule.

Dihedral angle of Cp planes

Cpcentroid–E–Cpcentroid angle

Si–O–Si angle.

Chart 1

Illustration of Different Orientations of the ansa-Bridge Relative to the Lone Pair at the Central Atom in Disiloxa[3]tetrelocenophanes 2

As tetrelocenophanes 2a–c could be of interest as monomers in the preparation of the corresponding polymetallocenylsiloxanes, we explored the possibility to prepare an analogous disiloxanyl-bridged distannocene, which would be a two-repeat-unit model compound of such a polymer. This is in analogy to a report of Zuckerman and co-workers on the synthesis of a xylyl-bridged distannocene.[6] However, when dilithiated ligand, 1, was reacted with two equivalents of cyclopentadienyltin(II) chloride, the 1H, 13C, and 119Sn NMR spectra showed exclusive formation of stannocenophane, 2b, and stannocene, 4 [δ119Sn = −2124 (2b), −2199 (4)] (Scheme ). It remains unclear, whether a bridged distannocene is initially formed and undergoes a subsequent redistribution reaction to give 2b and 4, or if cyclopentadienyllithium is eliminated from a monosubstituted intermediate, giving 2b and reacting with cyclopentadienyltin(II) chloride to give 4. In either case, entropic effect might be the driving force for the formation of 2b and 4 as the sole products of the reaction.

Scheme 2

Reaction of Dilithiated Ligand, 1, with Cyclopentadienyltin(II) Chloride, To Give Stannocenophane, 2b, and Stannocene, 4

In light of these results, it remains unclear if Zuckerman and co-workers had actually obtained a xylyl-bridged distannocene, as no 119Sn NMR chemical shifts or structural characterization were reported.[6]

Polymetallocenes are most commonly prepared by ring-opening polymerizations starting from the corresponding metallocenophanes, by cleavage of either the metal–Cp bond or the ansa-bridge-Cp bond.[2] Because previous studies of stannocene carbene complexes revealed that the coordination of a strong σ donor can weaken the metal–Cp bond,[33] this was further explored as a potential mechanism for metal-Cp bond activation in tetrelocenophanes, by reacting stannocenophane, 2b, with 1,3,4,5-tetramethyl-imidazole-2-ylidene. Upon mixing stannocenophane 2b and NHC in benzene at room temperature and examining the mixture by multinuclear NMR spectroscopy, a downfield shift of the 119Sn resonance was observed (δ119Sn = −1761; Δδ119Sn = 363 vs 2b), along with a resonance of δ13C = 200.3 for the carbene carbon atom. These findings clearly indicate a coordination of NHC to the tin atom in stannocenophane 2b. Likewise, when plumbocenophane 2c was reacted with 1,3,4,5-tetramethyl-imidazole-2-ylidene in toluene at room temperature, a similar coordination was observed (Scheme ).[34]

Scheme 3

Reaction of Disiloxa[3]tetrelocenophanes, 2b,c, with NHC To Give NHC Complexes 3a,b (2b/3a: E = Sn; 2c/3b: E = Pb; NHC = 1,3,4,5-Tetramethyl-imidazole-2-ylidene)

Crystals of complexes 3a,b suitable for single crystal X-ray diffraction analysis, could be obtained from thf (3a) and toluene (3b) solutions at 248 K, allowing for structural characterization (Figure ). Noteworthy, plumbocenophane NHC complex 3b represents the first example of a carbene adduct of a bis(cyclopentadienyl)lead compound. The Sn–CNHC bond in complex 3a is 245.7 pm, the Pb–CNHC bond in complex 3b is 258.2 pm. This is comparably long (e.g.,: {(Me3Si)3Si}2Sn–NHC: 228.7 pm;[35] {(Me2Si)2((Me3Si)2Si)2}Sn–NHC: 229.5 pm;[35] Ter(H)Pb–NHC: 233.2 pm[36]), but similar to what has been observed previously for stannocene carbene complexes.[33] The E–Cp bonds in complexes 3a,b are significantly elongated compared to stannocenophane 2b and plumbocenophane 2c, respectively (Table ), which indicates a weakening of the metal Cp bonds upon coordination of NHC. This is in line with the previous observations in stannocene carbene complexes[33] and can be rationalized by the donation of the carbene into the antibonding lowest unoccupied molecular orbital (LUMO) of the metallocene (Figure ).

Figure 2

Molecular structure of (a) stannocenophane NHC complex 3a and (b) plumbocenophane NHC complex 3b in the crystal (thermal ellipsoids at 50% probability level, H atoms omitted for clarity).

Figure 3

Isosurface plot of the LUMO (Kohn–Sham orbital) of stannocenophane 2b (B3LYP-D3/def2-TZVP, isovalue = 0.035).

The LUMO of stannocenophane 2b and plumbocenophane 2c, as is typical for tetrelocenes in general, corresponds to an antibonding interaction of a p-orbital at the central atom with the cyclopentadienyl ligand orbitals.[12,13,33] Therefore, the NHC coordinates to the metal atom in 3a,b. In a “side-on” fashion and donation of electron density into the LUMO weakens the E–Cp bond. This is further illustrated by the fact that complex 3a is extremely reactive and decomposes in solution at room temperature within hours. The fact that only a weakening and no cleavage of the E–Cp bonds is observed, might be due to the fact that the carbene coordination is relatively weak. This is in line with calculated complexation energies of ΔE = −65.9 kJ mol–1 (ΔG298 = −5.3 kJ mol–1) for 3a and ΔE = −72.2 kJ mol–1 (ΔG298 = −17.9 kJ mol–1) for 3b (complexation energies refer to the reactions 3a,b → 2b,c + NHC and are calculated at B3LYP-D3/def2-TZVP[37]). Therefore, 3a,b might exist in a dissociative equilibrium in solution and an averaged set of signals is observed in the NMR spectra at room temperature.

To probe this further, solid state 13C and 119Sn CP/MAS NMR measurements of complex 3a were conducted (Figure ). Stannocenophane NHC complex 3a exhibits a 119Sn NMR chemical shift in the solid state of δ119Sn = −1360, which is downfield shifted by Δδ119Sn = 401 compared to what is observed in solution at room temperature. The carbene-carbon atom exhibits a 13C NMR chemical shift of δ13C = 196.5 in the solid state, with a C–Sn coupling constant of 1JCSn = 587 Hz. This clearly indicates that an averaged set of signals is observed in solution at room temperature and that stannocenophane NHC complex 3a in fact exists in a dissociative equilibrium with stannocenophane 2b + NHC.

Figure 4

119Sn{1H} CP/MAS NMR spectra of stannocenophane NHC complex 3a. Upper trace: 13 kHz MAS, lower trace: 10 kHz MAS [δiso = −1360; δ11 = −907(±7); δ22 = −1210(±6); δ33 = −1962(±1); Ω(δ11–δ33) = 1055].

To further verify this, the stoichiometry of the reaction was varied. When two equivalents of stannocenophane, 2b, are mixed with one equivalent of NHC, only one set of signals is observed in the 1H and 13C NMR spectra, along with one signal in the 119Sn NMR spectra at δ119Sn = −1976. This is upfield shifted by Δδ119Sn = 215, compared to what was observed for solutions derived from 3a (δ119Sn = −1761), indicating a fast equilibrium between carbene complex, 3a, and uncoordinated 2b + NHC.[38] In addition, when a solution of 3a or 3b is treated with aluminum chloride, almost instantaneous formation of NHC·AlCl3 and metallocenophanes 2b,c is observed by multinuclear NMR spectroscopy (Scheme ).[39]

Scheme 4

Reaction of NHC Complexes 3a,b with AlCl3 (2b/3a: E = Sn; 2c/3b: E = Pb; NHC = 1,3,4,5-Tetramethyl-imidazole-2-ylidene)

This further supports the assumption that the NHC coordination is reversible and that 3a,b exist in a dissociative equilibrium with 2b,c + NHC.

Conclusions

In summary, the synthesis and structural characterization of tetramethyldisiloxa[3]germanocenophane, 2a, tetramethyl-disiloxa[3]stannocenophane, 2b, and tetramethyldisiloxa[3]plumbocenophane, 2c is reported. In case of stannocenophane, 2b, it was shown that it can also be obtained by treatment of dilithiated ligand 1, with two equivalents of cyclopentadienyltin(II) chloride, which might reveal Zuckerman’s report of a distannocene in a different light. Furthermore, stannocenophane 2b, and plumbocenophane 2c react with 1,3,4,5-tetramethyl-imidazole-2-ylidene (NHC) to give the corresponding carbene complexes 3a,b. These complexes are believed to exist in a dissociative equilibrium in solution at room temperature, as indicated by comparison of solution and solid state CP/MAS 119Sn NMR spectroscopy in case of 3a, as well as abstraction of NHC by treatment of 3a,b with aluminum chloride and calculated complexation energies (ΔE) of just −65.9 kJ mol–1 (3a) and −72.2 kJ mol–1 (3b).

Experimental Section

All manipulations were carried out under an inert gas atmosphere (argon 5.0), using either Schlenk line techniques of a glovebox. Dilithiated ligand 1,[21−24,26] 1,3,4,5-tetramethyl-imidazole-2-ylidene,[40] and cyclopentadienyltin(II) chloride[41−43] were synthesized according to literature-known procedures. NMR-spectra were recorded on Bruker AVANCE III 300 (solution NMR), Bruker AVANCE III 400 (solution NMR), and Bruker Ascend 400WB (solid state NMR) spectrometers. 1H and 13C NMR spectra were referenced using the solvent signals [δ1H (C6HD5) = 7.16; δ13C (C6D6) = 128.06]. 29Si, 119Sn, and 207Pb NMR spectra were referenced using external standards [δ29Si(SiMe4) = 0; δ119Sn(SnMe4) = 0; δ207Pb(PbMe4) = 0]. Elemental analysis were performed on an Elementar vario MICRO cube (in some cases low carbon values were repeatedly reproducibly observed, presumably because of the formation of silicon carbide). Single crystal X-ray diffraction analysis was carried out at low temperatures on Bruker AXS X8 Apex CCD and Bruker AXS D8 Venture diffractometers operating with graphite monochromated Mo Kα radiation. Structure solution and refinement were performed using SHELX.[44] Crystal structures have been deposited with the Cambridge Crystallographic Data Centre (CCDC) and are available free of charge from the Cambridge Structural Database (reference numbers: 1903471, 1903472, 1903473, 1903474, 1941170).

Synthesis of 2a–c

Dilithiated ligand 1 (500 mg, 1.82 mmol) and the respective element dichloride (GeCl2 1,4-dioxane: 421 mg, 1.82 mmol; SnCl2: 346 mg, 1.82 mmol; PbCl2: 507 mg, 1.82 mmol) were added to a Schlenk flask and cooled to 173 K. Precooled thf was added and the solution was stirred at 173 K for 90 min. Subsequently, the mixture was allowed to warm to room temperature and stirred for an additional 150 min. All volatiles were removed in vacuo and the residue was suspended in hexane. After filtration, the solution was concentrated and stored at 248 K to obtain 2a–c as crystalline solids.

Yields: 2a: 500 mg/82%; 2b: 490 mg/70%; 2c: 460 mg/54%.

2a: 1H NMR (400.13 MHz, 296 K, C6D6): δ 0.35 (s, 12H, Si–CH3), 6.02 (t, 3JHH = 2 Hz, 4H, Cp), 6.28 (t, 3JHH = 2 Hz, 4H, Cp); 13C{1H} NMR (100.62 MHz, 296 K, C6D6): δ 1.6 (Si–CH3), 113.7 (Cp), 118.3 (Cp), 121.7 (Cp); 29Si{1H} INEPT NMR (79.49 MHz, 296 K, C6D6): δ −4.5. Elemental analysis for C14H20GeOSi2: Calcd: 50.48%, C; 6.05%, H. Found: 49.41%, C; 6.11%, H.

2b: 1H NMR (400.13 MHz, 297 K, C6D6): δ 0.37 (s, 12H, Si–CH3), 6.06 (t, 3JHH = 2 Hz, 2JHSn = 20 Hz, 4H, Cp), 6.25 (t, 3JHH = 2 Hz, 4H, Cp); 13C{1H} NMR (100.62 MHz, 297 K, C6D6): δ 1.8 (Si–CH3), 115.2 (Cp), 116.9 (Cp), 118.2 (Cp, 1JCSn = 49.5 Hz, 51.7 Hz); 29Si{1H} INEPT NMR (79.49 MHz, 297 K, C6D6): δ −5.6; 119Sn{1H} NMR (148.89 MHz, 297 K, C6D6): δ −2124. Elemental analysis for C14H20OSi2Sn: Calcd: 44.35%, C; 5.32%, H. Found: 43.75%, C; 5.34%, H.

2c: 1H NMR (300.13 MHz, 299 K, C6D6): δ 0.41 (s, 12H, Si–CH3), 6.04 (m, 4H, Cp), 6.24 (m, 4H, Cp); 13C{1H} NMR (75.48 MHz, 299 K, C6D6): δ 1.9 (Si–CH3), 116.8 (Cp), 117.7 (Cp), 118.9 (Cp); 29Si{1H} NMR (59.63 MHz, 299 K, C6D6): δ −7.1; 207Pb{1H} NMR (62.51 MHz, 299 K, C6D6): δ −4795. Elemental analysis for C14H20OPbSi2: Calcd: 35.95%, C; 4.31%, H. Found: 36.4%, C; 4.93%, H.

Synthesis of 3a

Stannocenophane 2b (50.0 mg, 132 μmol) and 1,3,4,5-tetramethyl-imidazole-2-ylidene (16.0 mg, 129 μmol) were mixed, 2 mL of thf, precooled to 193 K, were added, and the resulting mixture was stirred for 10 min at 193 K. Storage of the solution at 248 K yielded yellow crystals of stannocenophane carbene complex 3a.

Yield: 10 mg/17%. 1H NMR (400.13 MHz, 296 K, C6D6): δ 0.54 (s, 12H, Si–CH3), 1.34 (s, 6H, C–CH3), 3.13 (s, 6H, N–CH3), 6.30–6.36 (m, 8H, Cp); 13C{1H} NMR (100.62 MHz, 296 K, C6D6): δ 2.3 (Si–CH3), 8.1 (C–CH3), 33.6 (N–CH3), 115.9 (Cp), 116.7 (Cp), 123.3 (C–CH3), 200.3 [carbene-C (detected by 1H–13C-HMBC)]; 13C{1H} CP/MAS (13 kHz) NMR (100.67 MHz, 295 K): δ 2.6 (Si–CH3), 3.3 (Si–CH3), 7.6 (CH3), 10.2 (CH3), 34.2 (CH3), 111.3, 112.9, 117.8, 120.1, 123.2, 125.3, 196.5 (carbene-C, 1JCSn = 587 Hz); 29Si{1H} NMR (79.49 MHz, 296 K, C6D6): δ −5.7; 119Sn{1H} NMR (111.68 MHz, 299 K, C6D6): δ −1761; 119Sn{1H} CP/MAS (13 kHz) NMR (149.05 MHz, 295 K): δiso −1360. Elemental analysis for C21H32N2OSi2Sn: Calcd: C, 50.11%; H, 6.41%; N, 5.57%. Found: C, 50.11%; H, 6.40%; N, 5.72%.

Synthesis of 3b

A solution of Plumbocenophane 2c (80.0 mg, 171 μmol) in 3 mL of toluene was added to a solution of 1,3,4,5-tetramethyl-imidazole-2-ylidene (21.0 mg, 169 μmol) in 3 mL of toluene and the mixture was stirred for 5 min. Storage of the solution at 248 K yielded large yellow crystals of plumbocenophane carbene complex 3b.

Yield: 38 mg/38%. 1H NMR (400.13 MHz, 293 K, C6D6): δ 0.64 (s, 12H, Si–CH3), 1.32 (s, 6H, C–CH3), 3.04 (s, 6H, N–CH3), 6.34 (m, 4H, Cp), 6.39 (m, 4H, Cp); 13C{1H} NMR (100.63 MHz, 293 K, C6D6): δ 2.4 (Si–CH3), 8.2 (C–CH3), 34.3 (N–CH3), 114.4 (Cp), 117.0 (Cp), 119.2 (Cp), 123.7 (C–CH3); 29Si{1H} INEPT NMR (79.49 MHz, 293 K, C6D6): δ −7.0; 207Pb NMR (62.51 MHz, 295 K, C6D6): no signal could be detected in the 207Pb NMR spectrum in a range of −2000 to −6800 ppm.

Elemental analysis for C21H32N2OPbSi2: Calcd: C, 42.62%; H, 5.45%; N, 4.73%. Found: C, 43.06%; H, 5.59%; N, 4.40%.