Indirect and Direct Grafting of Transition Metals to Siliconoids.

Unsaturated charge-neutral silicon clusters (siliconoids) are important as gas-phase intermediates between molecules and the elemental bulk. With stable zirconocene- and hafnocene-substituted derivatives, we here report the first examples containing directly bonded transition-metal fragments that are readily accessible from the ligato-lithiated Si6 siliconoid (1Li) and Cp2 MCl2 (M=Zr, Hf). Charge-neutral siliconoid ligands with pending tetrylene functionality were prepared by the reaction of amidinato chloro tetrylenes [PhC(NtBu)2 ]ECl (E=Si, Ge, Sn) with 1Li, thus confirming the principal compatibility of such low-valent functionalities with the unsaturated Si6 cluster scaffold. The pronounced donor properties of the tetrylene/siliconoid hybrids allow for their coordination to the Fe(CO)4 fragment.

The synthesis of stable unsaturated silicon clusters (siliconoids)1 has attracted considerable interest because of the presumed intermediacy of the parent species in gas‐phase deposition processes2 as well as the fact that the unsubstituted vertices are reminiscent of the free valencies of bulk and nano silicon surfaces.3 Since the report of the first stable siliconoid,4 a variety of further examples have been prepared by the groups of Wiberg,5 Kyushin,6 Iwamoto,7 Breher,8 Fässler,9 Lips,10 and our group.11 The two recent syntheses of regioisomeric lithiated Si6 siliconoids (benzpolarenes)11d, 11f and their facile functionalization with suitable electrophiles considerably enlarged the scope of this emerging field towards the related Zintl anions (polyanionic, deltahedral clusters without any substituents). The presence of organic substituents in siliconoids confers higher solubility, while the electronic properties are retained as manifest in the wide dispersion of 29Si NMR shifts.4, 6, 8, 11, 12 While Zintl anions of Group 14 elements heavier than silicon have been frequently applied in the synthesis of partially substituted metalloid clusters,13 the grafting of substituents to silicon‐based Zintl anions has only been achieved very recently by the groups of Korber14 and Fässler,15 who independently reported the protonation of silicon Zintl anions to the partially H‐substituted anionic clusters [HSi9]3−, [H2Si9]2−, and [HSi4]3−.16 In addition, the Fässler group successfully transferred silyl substituents to Si9 4−.9

Zintl silicide anions have been employed as extraordinarily electron‐rich ligands towards transition‐metal centers.17 Conversely, the coordination of neutral siliconoids to metals has thus far remained elusive. Here, with zirconocene and hafnocene derivatives, we report the first examples of siliconoids bearing covalently attached transition‐metal functionalities. As attempts to coordinate charge‐neutral siliconoids to transition‐metal fragments in a dative manner remained inconclusive, we resorted to the grafting of amidinato tetrylene residues PhC(NtBu)2E (E=Si, Ge, Sn)18, 19, 20 to the Si6 scaffold. Given that tetrel(II) species are well established as ligands in catalysis,21 it was conceivable that tetrylene functionalization would facilitate the coordination of electron‐rich siliconoid moieties to transition‐metal centers. As will be elaborated further on, the targeted tetrylene–siliconoid hybrids are stable and indeed readily transformed into the corresponding Fe(CO)4 complexes.

Treatment of Cp2MCl2 (M=Zr, Hf) with the ligato‐lithiated siliconoid 1Li results in rapid and uniform conversion into the corresponding Group 4 metalated siliconoids 2 a and 2 b (Scheme 1). In contrast, reaction with one equivalent of titanocene dichloride led to a complicated mixture of products, presumably because of competing redox reactions. The diagnostically wide dispersion of 29Si signals confirmed the presence of uncompromised benzpolarene scaffolds.11d, 11f The signals of the privo‐silicon atoms appear at the usual low field at 162.6 (2 a) and 162.8 ppm (2 b), while the two unsubstituted nudo‐vertices give rise to two individual signals at characteristically high field at −233.5 and −240.6 ppm for 2 a and −232.1 and −240.3 ppm for 2 b (Table 1). The apparent symmetry reduction is typical for ligato‐substituted benzpolarenes and has been attributed to hindered rotation of the pending functionality.11d The other 29Si NMR chemical shifts are located within the usual range for saturated silicon atoms and vary only slightly with the introduced ligand (see the Supporting Information).

Scheme 1

Synthesis of ligato‐metallocene‐substituted Si6 siliconoids 2 a and 2 b. 2 a: M=Zr; 2 b: M=Hf. Tip=triisopropylphenyl. The “naked” positions 1 and 3 are referred to as nudo, the NMR‐deshielded position 2 as privo, the mono‐substituted positions 4 and 6 as ligato, and the remote position 5 as remoto.11f

Table 1

Selected analytical data of metallocene‐substituted siliconoids 2 a and 2 b.

	δ(29Si2) [ppm]	δ(29Si1/3) [ppm]	Si1–Si3 [Å]	Si4–M [Å], exp	Si4–M [Å], calc	λ max [nm]
2 a (M=Zr)	162.6	−233.5 −240.6	2.588(2)	2.782(1)	2.741	521
2 b (M=Hf)	162.8	−232.1 −240.3	2.588(1)	2.770(1)	2.738	497

The longest‐wavelength absorptions in the UV/Vis spectra are at λ max=521 nm (2 a) and 497 nm (2 b), and therefore slightly red‐shifted in comparison with previously reported ligato‐substituted siliconoids (λ max=364–477 nm).11d, 11f Single crystals were obtained by crystallization from hexane/toluene in 53 % (2 a) and 66 % (2 b) yield, and their structures were confirmed by X‐ray diffraction in the solid state (Figure 1).

Figure 1

Representative molecular structure of siliconoid 2 b in the solid state.30 Hydrogen atoms omitted for clarity. Thermal ellipsoids set at 50 % probability. For the structure of 2 a, see the Supporting Information.

The distances between the unsubstituted bridgehead silicon atoms Si1 and Si3 (2 a: 2.588(2) Å; 2 b: 2.588(1) Å) are significantly shorter than those of the peraryl‐substituted global‐minimum isomer11b and in line with previously reported persilabenzpolarenes with electropositive groups in the ligato‐position.11d, 11f The electronic nature of the substituent apparently affects the electron density available for cluster bonding.

The Zr−Si bond length of 2.782(1) Å in 2 a is in between that reported for a disilenyl‐substituted zirconocene chloride on the one hand (2.7611 Å)22c and silyl‐substituted zirconocene complexes on the other hand (2.813–2.8214 Å).22 Similarly, the Hf−Si bond of 2.7702(9) Å in 2 b is shorter than those reported for tetracoordinated hafnium compounds (2.835–2.888 Å).23 The experimental bond lengths were satisfyingly reproduced by DFT calculations for 2 a and 2 b at the BP86+D3(BJ)/def2‐SVP level of theory (Table 1).

In solution, even smallest traces of water lead to the progressive hydrolysis of ligato‐zirconocene‐functionalized siliconoid 2 a as indicated by the gradual appearance of a second set of signals, including a characteristic Si−H resonance at 4.103 ppm, in the 1H NMR spectrum.22c The Si−Hf bond of ligato‐hafnocene‐functionalized siliconoid 2 b exhibits a considerably higher stability towards hydrolysis (see the Supporting Information for details). In view of the instability of the covalent silicon–metal bond in 2 a and 2 b, it is unsurprising that attempts to directly graft later transition metals to siliconoids have failed thus far. In the same vein, the dative coordination of charge‐neutral siliconoids to transition metals is also unknown. We therefore considered the functionalization of the Si6 scaffold with an auxiliary tetrylene ligand in order to facilitate coordination.

Tetrylenes are known for their excellent σ‐donating properties. As the reaction of ligato‐lithiated benzpolarene 1Li with Jutzi's silicocene affords the cluster‐expanded Si7 siliconoid11e instead of the simple substitution product, we chose the N‐heterocyclic chloro tetrylenes of the Roesky type in the expectation that the nitrogen donors adjacent to the SiII center would tame its electron deficiency sufficiently to allow for the isolation of a silylene‐functionalized Si6 siliconoid. In fact, the tetrylenes [PhC(NtBu)2]ECl are known to readily undergo nucleophilic substitution of the chlorine substituent.19 Treatment of the ligato‐lithiated benzpolarene 1Li with 1.1 equivalents of [PhC(NtBu)2]ECl19, 20 indeed leads to rapid conversion into uniform products (E=Si, Ge, Sn; Scheme 2) accompanied by precipitation of LiCl. 29Si NMR analysis showed the diagnostic wide dispersion of chemical shifts (as discussed above for 2 a and 2 b), and thus confirmed the anticipated integrity of the benzpolarene scaffolds suggesting the formation of siliconoids 3 a–c. An additional signal in the 29Si NMR spectrum of 3 a was assigned to the pending silylene center.20a, 20c

Scheme 2

Synthesis of the tetrylene‐functionalized Si6 siliconoids 3 a–c. 3 a: E=Si; 3 b: E=Ge; 3 c: E=Sn.

The occurrence of two sets of signals each in the 29Si and 119Sn NMR spectra of 3 b and 3 c suggested the presence of rotational isomers in solution. Indeed, 29Si and 119Sn solid‐state NMR spectra of 3 a–c show just a single set of signals (Table 2 and the Supporting Information). A VT‐NMR study in toluene solution revealed the onset of coalescence at 343 K for germylene‐substituted 3 b although the barrier proved to be too high to allow for accurate determination of the coalescence temperature (>70 °C). The 29Si and 1H NMR spectra of stannylene‐substituted 3 c show broad signals with poor signal‐to‐noise ratios at room temperature, suggesting that the coalescence temperature may be within reach. Accordingly, VT‐NMR analysis of 3 c at low temperature (226 K) revealed sharpened signals in the 1H NMR spectrum as well as a second set of less intense signals in the 29Si NMR spectrum (Table 2).

Table 2

Selected NMR shifts of the tetrylene‐functionalized siliconoids 3 a–c.

	δ(29Si2) [ppm]	δ(29Si1/3) [ppm]	δ(29Si2) solid [ppm]	δ(29Si1/3) solid [ppm]	δ(119Sn) [ppm]	δ(119Sn) solid [ppm]
3 a	166.7	−244.6 −260.7	160.0	−250.5 −262.6	–	–
3 b major	167.3	−245.4 −261.1	163.4	−248.9 −261.2	–	–
3 b minor	165.6	−233.9 −238.1	–	–	–	–
3 c major	162.3	−232.6 −236.9	–	–	267.8	–
3 c minor	168.8	−242.3 −259.3	162.9	−244.3 −259.9	336.5	332.0

–

–

–

–

–

–

–

–

–

–

–

The longest‐wavelength absorption bands in the UV/Vis spectra are at λ max=472 nm (3 a), 436 nm (3 b), and 436 nm (3 c). Single crystals were obtained in 72 % (3 a), 78 % (3 b), and 74 % (3 c) yield, and the structures were confirmed by X‐ray diffraction studies (Figure 2). The structure of the Si6 cluster core is hardly influenced by the nature of the tetrylene ligand. The distances between the unsubstituted bridgehead silicon atoms Si1 and Si3 (3 a: 2.6039(9) Å; 3 b: 2.612(2) Å; 3 c: 2.6149(8) Å) are slightly shorter than in the global‐minimum isomer Si6Tip6 11b and in line with previously reported ligato‐functionalized siliconoids with electropositive groups.11d, 11f The bonds between Si4 and the pending tetrylene (3 a: 2.4294(9) Å; 3 b: 2.493(2) Å; 3 c: 2.6753(6) Å) are longer than typical single bonds.24, 25, 26, 27

Figure 2

Representative molecular structure of silylene‐functionalized siliconoid 3 a in the solid state.30 Hydrogen atoms omitted for clarity. Thermal ellipsoids set at 50 % probability. For the structures of 3 b and 3 c, see the Supporting Information.

In order to probe the suitability of 3 a–c as neutral ligands towards transition metals, we attempted the coordination to Fe(CO)4 in a proof‐of‐principle study (Scheme 3). Complexes 4 a–c are obtained in a straightforward manner by stirring a benzene solution of 3 a–c with 4 (4 a), 5 (4 b), or 1.5 (4 c) equivalents of Fe2(CO)9 18, 19a, 28 at room temperature. Complexes 4 a–c were fully characterized by X‐ray diffraction on single crystals, elemental analysis, as well as multinuclear NMR, UV/Vis, and IR spectroscopy.

Scheme 3

Reaction of the tetrylene‐functionalized Si6 siliconoids 3 a–c with Fe2(CO)9 to afford the corresponding Fe(CO)4 complexes 4 a–c.

The 29Si NMR spectra of 4 a–c show a similarly wide distribution in chemical shifts as those of 3 a–c, albeit with distinctly different numerical values. In the 29Si NMR spectrum, the additional signal of the silylene moiety of 4 a at 110.0 ppm is drastically downfield‐shifted compared to that of 3 a (48.0 ppm). Similarly, the stannylene side arm of 4 c shows a 119Sn NMR signal at 456.3 ppm (Table 3). A related LSnCl–Fe(CO)4 complex resonates at much higher field at 255 ppm.19a 1H and 13C NMR analyses of 4 a–c in C6D6 confirmed the presence of two singlet resonances each assignable to the tert‐butyl groups.

Table 3

Selected NMR shifts of Fe(CO)4 complexes of tetrylene‐substituted siliconoids 4 a–c.

	δ(29Si2) [ppm]	δ(29Si1/3) [ppm]	δ(29Si2) solid [ppm]	δ(29Si1/3) solid [ppm]	δ(119Sn) [ppm]	δ(119SnCP/MAS) [ppm]
4 a	165.1	−198.2 −230.4	158.2	−195.9 −231.2	–	–
4 b	163.7	−203.0 −231.4	156.7	−202.2 −232.8	–	–
4 c	160.2	−201.7 −230.7	154.1	−200.3 −232.8	456.3	469.2

–

–

–

–

According to the UV/Vis spectra of 4 a–c, the longest‐ wavelength absorption bands are observed at λ max=470 nm (4 a), 469 nm (4 b), 466 nm (4 c) and thus slightly blue‐shifted compared to those of 3 a–c. The Fe(CO)4 complexes 4 a–c exhibit IR characteristics of tetrylene–Fe(CO)4 complexes,28a, 29 with CO stretching modes at υ=1899, 1913, 1948, 2022 cm−1 (4 a), 1908, 1918, 1949, 2025 cm−1 (3 b), and 1902, 1914, 1942, 2015 cm−1 (4 c). The donor strength of 4 a–c can be classified by the asymmetric carbonyl absorptions at υ=2022 cm−1 (4 a), 2025 cm−1 (4 b), and 2015 cm−1 (4 c), which indicate a slightly lower ligand‐to‐metal σ‐donation compared to other tetrylene Fe(CO)4 complexes such as {[PhC(NtBu)2]SiOtBu}Fe(CO)4 (υ=2026 cm−1)28a and {[PhC(NtBu)2]GeCl}Fe(CO)4 (υ=2042 cm−1).28c Single crystals of 4 a–c were obtained in 75 % (4 a), 60 % (4 b), and 70 % (4 c) yield, and the structures were confirmed to be isosteric by X‐ray diffraction studies (Figure 3). The distances between the bridgehead silicon atoms Si1–Si3 (4 a: 2.560(1) Å; 4 b: 2.566(1) Å; 4 c: 2.5756(7) Å) are slightly shorter than in 3 a–c. This is indicative of increased electron density within the cluster scaffold upon formation of the transition‐metal complex. The bonds Si−E in 4 a–c (4 a: 2.384(1) Å; 4 b: 2.4422(8) Å; 4 c: 2.5896(5) Å) are equally shortened compared to 3 a–c and now in the typical range of Si−E single bonds (E=Si, Ge, Sn).24, 25, 26, 27 The Fe(CO)4 complexes 4 a–c exhibit typical Fe−E distances (4 a: 2.279(1) Å; 4 b: 2.3496(5) Å; 4 c: 2.4957(3) Å).18, 19a, 28a, 28b, 28c

Figure 3

Representative molecular structure of the Fe(CO)4 complex of 4 a in the solid state.30 Hydrogen atoms omitted for clarity. Thermal ellipsoids set at 50 % probability. For the structures of 4 b and 4 c, see the Supporting Information.

In conclusion, with 2 a and 2 b, we have reported the first transition‐metal‐substituted neutral siliconoids. The reaction of an anionic Si6 siliconoid with PhC(NtBu)2ECl (E=Si, Ge, Sn) gave rise to siliconoids with pending Roesky‐type silylene, germylene, and stannylene moieties, an unprecedented feature in silicon cluster chemistry. Unlike in the case of the previously attempted grafting of a Cp*‐substituted silylene fragment,11e the electrophilicity of the tetrylenes of the Roesky type is sufficiently low, thereby avoiding the otherwise observed expansion of the cluster core. As proof of concept for the suitability of these novel ligands in the coordination to transition metals, we synthesized and characterized the corresponding Fe(CO)4 complexes 4 a–c.

Conflict of interest

The authors declare no conflict of interest.

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supplementary

Click here for additional data file.