Synthesis and Structural Characterization of Two New Main Group Element Carboranylamidinates

: Two new main group element carboranylamidinates were synthesized using a bottom-up approach starting from o -carborane, ortho -C 2 B 10 H 12 ( 1 , = 1,2-dicarba- closo -dodecaborane). The ﬁrst divalent germanium carboranylamidinate, GeCl[HL Cy ] ( 3 , [HL Cy ] − = [ o -C 2 B 10 H 10 C(NCy)(NHCy)] − , Cy = cyclohexyl), was synthesized by treatment of GeCl 2 (dioxane) with 1 equiv. of in situ-prepared Li[HL Cy ] ( 2a ) in THF and isolated in 47% yield. In a similar manner, the ﬁrst antimony(III) carboranylamidinate, SbCl 2 [HL i Pr ] ( 4 , [HL i Pr ] − = [ o -C 2 B 10 H 10 C(N i Pr)(NH i Pr)] − ), was obtained from a reaction of SbCl 3 with 1 equiv. of Li[HL i Pr ] in THF (56% yield). The title compounds were fully characterized by analytical and spectroscopic methods as well as single-crystal X-ray diffraction. Both compounds 3 and 4 are monomeric species in the solid state, and the molecular geometries are governed by a stereo-active lone pair at the metal centers.

Inorganics 2019, 7, x FOR PEER REVIEW 2 of 8 amidinate anions [23,24]. In this contribution, we report the synthesis and full characterization of the first germanium(II) carboranylamidinate as well as the first antimony compound of this type.

Synthesis and Characterization of GeCl[HL Cy ] (3) and SbCl2[HL iPr ] (4)
The synthetic protocol leading to the title compounds is outlined in Scheme 2. In the first step, the lithium carboranylamidinates 2a and 2b were prepared in a one-pot reaction from o-carborane (1) and the corresponding carbodiimide. Subsequent reaction of 2a with 1 equiv. of the readily accessible germanium(II) precursor GeCl2(dioxane) [25] led to formation of GeCl[HL Cy ] (3) as the first carbonylamidinate of divalent germanium. Compound 3 was isolated in 47% yield as colorless, blocklike crystals after recrystallization from toluene. In a similar manner, the first antimony(III) carboranylamidinate, SbCl2[HL iPr ] (4) was prepared from SbCl3 and 1 equiv. of Li[HL iPr ] (2b) in THF. After crystallization from toluene, compound 4 could be isolated in 56% yield as colorless, needlelike crystals which, like 3, are significantly moisture-sensitive. In both cases, the complex having a [HL] − -type ligand is the only identified product, and no evidence for the formation of products with [L] 2− ligands has been observed. Consequently, the divalent germanium precursor turned out to react with Li[HL] in a similar manner as GeCl4 [22], while the reaction of SbCl3 took a different course than that of PhPCl2 [20].
Both title compounds 3 and 4 were fully characterized through the usual set of elemental analyses and spectroscopic methods. The 1 H-and 13 C-NMR data of 3 were in good agreement with the expected composition. In the 1 H-NMR spectrum, a singlet at δ 8.06 ppm could be assigned to the uncoordinated NH functionality of the amidine unit. High molecular mass peaks in the mass spectrum of 3 were detected at m/z 457 (87% rel. int.) [M − H] + and 422 (13% rel. int.) [M − Cl] + . The absence of peaks at higher molecular masses confirmed the monomeric nature of 3. In the IR spectrum of 3, typical bands of the amidine moiety were observed at 3403 cm −1 (νN-H), 1577 cm −1 (νC=N), and 1260 cm −1 (νC-N). A medium strong band at 2584 cm −1 can be assigned to the carborane cage (νB-H) [22]. The antimony derivative 4 was fully characterized in the same manner. The 1 H-NMR spectrum of 4 displayed a characteristic signal pattern of the two chemically inequivalent isopropyl groups (two doublets and two septets). In this case, the NH resonance could not be observed. However, the presence of a [HL iPr ] − ligand in 4 was confirmed by a sharp νN-H band at 3396 cm −1 in the IR spectrum. Additional characteristic bands of the amidine group were observed at 1605 cm −1 (νC=N) and 1251 cm −1 (νC-N), and the carborane backbone gave rise to a series of strong bands around 2590 cm −1 (νB-H) [22]. In the mass spectrum of 4, the highest molecular mass peak at m/z 426 (60% rel. int.) could be assigned to the ion [M − Cl] + .

Synthesis and Characterization of GeCl[HL Cy ] (3) and SbCl 2 [HL iPr ] (4)
The synthetic protocol leading to the title compounds is outlined in Scheme 2. In the first step, the lithium carboranylamidinates 2a and 2b were prepared in a one-pot reaction from o-carborane (1) and the corresponding carbodiimide. Subsequent reaction of 2a with 1 equiv. of the readily accessible germanium(II) precursor GeCl 2 (dioxane) [25] led to formation of GeCl[HL Cy ] (3) as the first carbonylamidinate of divalent germanium. Compound 3 was isolated in 47% yield as colorless, block-like crystals after recrystallization from toluene. In a similar manner, the first antimony(III) carboranylamidinate, SbCl 2 [HL iPr ] (4) was prepared from SbCl 3 and 1 equiv. of Li[HL iPr ] (2b) in THF. After crystallization from toluene, compound 4 could be isolated in 56% yield as colorless, needle-like crystals which, like 3, are significantly moisture-sensitive. In both cases, the complex having a [HL] − -type ligand is the only identified product, and no evidence for the formation of products with [L] 2− ligands has been observed. Consequently, the divalent germanium precursor turned out to react with Li[HL] in a similar manner as GeCl 4 [22], while the reaction of SbCl 3 took a different course than that of PhPCl 2 [20].
Both title compounds 3 and 4 were fully characterized through the usual set of elemental analyses and spectroscopic methods. The 1 H-and 13 C-NMR data of 3 were in good agreement with the expected composition. In the 1 H-NMR spectrum, a singlet at δ 8.06 ppm could be assigned to the uncoordinated NH functionality of the amidine unit. High molecular mass peaks in the mass spectrum of 3 were detected at m/z 457 (87% rel. int.) [M − H] + and 422 (13% rel. int.) [M − Cl] + . The absence of peaks at higher molecular masses confirmed the monomeric nature of 3. In the IR spectrum of 3, typical bands of the amidine moiety were observed at 3403 cm −1 (ν N-H ), 1577 cm −1 (ν C=N ), and 1260 cm −1 (ν C-N ). A medium strong band at 2584 cm −1 can be assigned to the carborane cage (ν B-H ) [22]. The antimony derivative 4 was fully characterized in the same manner. The 1 H-NMR spectrum of 4 displayed a characteristic signal pattern of the two chemically inequivalent isopropyl groups (two doublets and two septets). In this case, the NH resonance could not be observed. However, the presence of a [HL iPr ] − ligand in 4 was confirmed by a sharp ν N-H band at 3396 cm −1 in the IR spectrum. Additional characteristic bands of the amidine group were observed at 1605 cm −1 (ν C=N ) and 1251 cm −1 (ν C-N ), and the carborane backbone gave rise to a series of strong bands around 2590 cm −1 (ν B-H ) [22]. In the mass spectrum of 4, the highest molecular mass peak at m/z 426 (60% rel. int.) could be assigned to the ion [M − Cl] + .

Crystal and Molecular Structures
Both title compounds 3 and 4 crystallize from toluene in solvent-free form with one monomeric molecule in the asymmetric unit. Crystal structure determinations confirmed the presence of one monoanionic carboranylamidinate ligand attached to the metal center in a typical κC,κN-chelating mode. The protonated NHR residue (3: R = Cy; 4: R = i Pr) is directed away from the metal center and does not contribute to coordinative saturation thereof. Both 3 and 4 exist as the antirotamer in the crystal (relating to the orientation of the NHR group relative to the carboranyl group). In both compounds, the C-N bond to the metal-attached nitrogen (N1) is shorter than the C-N bond to the protonated nitrogen (N2), which is in agreement with the presence of a formal double bond between C1 and N1. The observed C-N distances resemble those observed in previously described complexes with [HL] − ligands [21,22].
In the antimony(III) derivative 4, the central Sb atom displays a pseudo-trigonal-bipyramidal coordination by the κCκN-chelating [HL iPr ] − ligand, two chlorido ligands, and a stereo-active lone pair (Figure 3). The axial positions are occupied by the nitrogen donor (N1) and one of the chlorine atoms (Cl2), with the N1-Sb1-Cl2 angle being 163.63(5)°. This assignment is in agreement with the Sb1-Cl2 bond lengths of 249.7(1) pm, which is considerably longer than the equatorial Sb1-Cl1 bond (234.8(1) pm). The Sb1-C3 bond is 218.6(2) pm and therefore slightly longer than the mean value for tetra-coordinated Sb(III) compounds in the Cambridge Structural Database (214 pm for 664 entries with R1 ≤ 0.075) [26]. The same is true for the Sb1-N1 bond, which is 237.0(2) pm (mean value for 167 CSD entries with R1 ≤ 0.075: 230 pm) [26]. The molecular structure of 4 is closely related to those of the previously reported ECl3[HL] compounds (E = Ge, Sn) [22], with one of the equatorial chlorido

Crystal and Molecular Structures
Both title compounds 3 and 4 crystallize from toluene in solvent-free form with one monomeric molecule in the asymmetric unit. Crystal structure determinations confirmed the presence of one monoanionic carboranylamidinate ligand attached to the metal center in a typical κC,κN-chelating mode. The protonated NHR residue (3: R = Cy; 4: R = i Pr) is directed away from the metal center and does not contribute to coordinative saturation thereof. Both 3 and 4 exist as the antirotamer in the crystal (relating to the orientation of the NHR group relative to the carboranyl group). In both compounds, the C-N bond to the metal-attached nitrogen (N1) is shorter than the C-N bond to the protonated nitrogen (N2), which is in agreement with the presence of a formal double bond between C1 and N1. The observed C-N distances resemble those observed in previously described complexes with [HL] − ligands [21,22].
In the antimony(III) derivative 4, the central Sb atom displays a pseudo-trigonal-bipyramidal coordination by the κCκN-chelating [HL iPr ] − ligand, two chlorido ligands, and a stereo-active lone pair (Figure 3). The axial positions are occupied by the nitrogen donor (N1) and one of the chlorine atoms (Cl2), with the N1-Sb1-Cl2 angle being 163.63 (5) • . This assignment is in agreement with the Sb1-Cl2 bond lengths of 249.7(1) pm, which is considerably longer than the equatorial Sb1-Cl1 bond (234.8(1) pm). The Sb1-C3 bond is 218.6(2) pm and therefore slightly longer than the mean value for tetra-coordinated Sb(III) compounds in the Cambridge Structural Database (214 pm for 664 entries with R 1 ≤ 0.075) [26]. The same is true for the Sb1-N1 bond, which is 237.0(2) pm (mean value for 167 CSD entries with R 1 ≤ 0.075: 230 pm) [26]. The molecular structure of 4 is closely related to those of the previously reported ECl 3 [HL] compounds (E = Ge, Sn) [22], with one of the equatorial chlorido ligands being formally replaced by a lone pair. Different from 3, the amidine NH moiety in 4 is not involved in hydrogen bonding.

General Procedures and Instrumentation
All reactions were carried out in oven-dried or flame-dried glassware under an inert atmosphere of dry argon employing standard Schlenk and glovebox techniques. The solvent THF was distilled from sodium/benzophenone under nitrogen atmosphere prior to use. GeCl2(dioxane) was prepared according to a published procedure [25]. All other starting materials were purchased from commercial sources and used without further purification. 1 H-NMR (400 MHz) and 13 C-NMR (100.6 MHz) spectra were recorded in THF-d8 solution on a Bruker DPX 400 spectrometer (Bruker BioSpin, Rheinstetten, Germany). IR spectra were measured with a Bruker Vertex 70V spectrometer (Bruker Optics, Rheinstetten, Germany) equipped with a diamond ATR unit between 4000 cm −1 and 50 cm −1 . Microanalyses (C, H, N) were performed using a VARIO EL cube apparatus (Elementar Analysensysteme, Langenselbold, Germany).

Synthesis of Compound 3
A solution of Li[HL Cy ] was prepared as described previously [8] by treatment of 1 (0.95 g, 6.56 mmol) in THF (50 mL) with a 2.5 M solution of n BuLi in hexanes (2.7 mL, 6.56 mmol) followed by addition of 1,3-dicyclohexylcarbodiimide (1.35 g, 6.56 mmol). After stirring for 2 h at r.t., GeCl2(dioxane) (1.52 g, 6.56 mmol) was added as a solid and stirring was continued for 24 h. The reaction mixture was evaporated to dryness, and the solid residue was extracted with toluene (2 × 20 mL). The combined extracts were filtered and the clear, yellow filtrate was concentrated to a total volume of ca.

General Procedures and Instrumentation
All reactions were carried out in oven-dried or flame-dried glassware under an inert atmosphere of dry argon employing standard Schlenk and glovebox techniques. The solvent THF was distilled from sodium/benzophenone under nitrogen atmosphere prior to use. GeCl 2 (dioxane) was prepared according to a published procedure [25]. All other starting materials were purchased from commercial sources and used without further purification. 1 H-NMR (400 MHz) and 13 C-NMR (100.6 MHz) spectra were recorded in THF-d 8 solution on a Bruker DPX 400 spectrometer (Bruker BioSpin, Rheinstetten, Germany). IR spectra were measured with a Bruker Vertex 70V spectrometer (Bruker Optics, Rheinstetten, Germany) equipped with a diamond ATR unit between 4000 cm −1 and 50 cm −1 . Microanalyses (C, H, N) were performed using a VARIO EL cube apparatus (Elementar Analysensysteme, Langenselbold, Germany).

Synthesis of Compound 3
A solution of Li[HL Cy ] was prepared as described previously [8] by treatment of 1 (0.95 g, 6.56 mmol) in THF (50 mL) with a 2.5 M solution of n BuLi in hexanes (2.7 mL, 6.56 mmol) followed by addition of 1,3-dicyclohexylcarbodiimide (1.35 g, 6.56 mmol). After stirring for 2 h at r.t., GeCl 2 (dioxane) (1.52 g, 6.56 mmol) was added as a solid and stirring was continued for 24 h. The reaction mixture was evaporated to dryness, and the solid residue was extracted with toluene (2 × 20 mL). The combined extracts were filtered and the clear, yellow filtrate was concentrated to a total volume of ca. 10 mL. Crystallization at r.t. for a few days afforded 3 (1.39 g, 47%) as colorless, block-like, moisture-sensitive crystals. M.p. 177 • C (dec. ca. 220 • C). Elemental analysis calculated for C 15

Conclusions
To summarize the results reported here, two new carboranylamidinates of main group elements in low oxidation states were prepared and structurally characterized. Compound 3 represents the first carboranylamidinate species containing divalent germanium, while 4 is the first antimony carboranylamidinate. Both compounds were formed in a straightforward manner from the corresponding Li[HL] derivative, and no products containing dianionic [L] 2− ligands were obtained. This finding meets the expectation in view of the previously discussed influence of the "hardness" of the central atom on the resulting product [22], as Ge(II) and Sb(II) are rather soft. In both products, the molecular geometries are governed by a stereo-active lone pair at the metal centers. Due to their chloro functions, both compounds should be promising starting materials for further derivative chemistry.
Author Contributions: N.H. and F.Z. performed the experimental work. P.L. and F.E. carried out the crystal structure determinations. L.H. measured the IR and NMR spectra, and S.B. measured the mass spectra and carried out the elemental analyses. F.T.E. conceived and supervised the experiments. F.T.E. and P.L. wrote the paper.