Synthesis and Structural Characterization of Two New Main Group Element Carboranylamidinates

Abstract

Two new main group element carboranylamidinates were synthesized using a bottom-up approach starting from o-carborane, ortho-C₂B₁₀H₁₂ (1, = 1,2-dicarba-closo-dodecaborane). The first divalent germanium carboranylamidinate, GeCl[HL^Cy] (3, [HL^Cy]⁻ = [o-C₂B₁₀H₁₀C(NCy)(NHCy)]⁻, Cy = cyclohexyl), was synthesized by treatment of GeCl₂(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, SbCl₂[HL^iPr] (4, [HL^iPr]⁻ = [o-C₂B₁₀H₁₀C(NⁱPr)(NHⁱPr)]⁻), was obtained from a reaction of SbCl₃ with 1 equiv. of Li[HL^iPr] 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.

1. Introduction

Dodecahedral carborane cage compounds of the composition C₂B₁₀H₁₂ [1] are of tremendous scientific and technological interest due to a variety of practical applications, including the synthesis of polymers and ceramics [2], catalysts [3,4,5], radiopharmaceuticals [6], and non-linear optical materials [7]. The novel chelating ligand type of ortho-carboranylamidinates was first synthesized in our laboratory in 2010 by in-situ metalation of o-carborane, ortho-C₂B₁₀H₁₂ (1, = 1,2-dicarba-closo-dodecaborane) with n-butyllithium, followed by treatment with 1 equiv. of a 1,3-diorganocarbodiimide [8]. They combine the carborane cage with the versatile chelating amidinate anions, [RC(NR′)₂]⁻ [9,10,11,12] in one ligand system. In the resulting lithium ortho-carboranylamidinates Li[(o-C₂B₁₀H₁₀)C(NR)(NHR)] (= Li[HL]; 2a: R = ⁱPr, 2b: R = Cy (cyclohexyl)), a proton is formally shifted from a carboranyl carbon atom to the amidinate unit, resulting in an amidine moiety acting as a monodentate N-donor functionality (Scheme 1a). The lithium derivatives were further treated with various metal and non-metal chloride precursors to yield carboranylamidinates of e.g., Sn(II) and Cr(II) [8], Rh(I) and Ir(I) [13,14,15,16], Fe(II) and Fe(III) [17,18], Mo(II), Mn(II), Co(II), Ni(II), Cu(II) [18,19], Ti(IV), Zr(IV), Si(IV), Ge(IV), Sn(IV), Pb(IV), and P [20,21,22]. In the case of reactions with Cp₂TiCl₂, Cp₂ZrCl₂, PhPCl₂, and various dichlorosilanes R₂SiCl₂, formal dehydrochlorination led to complexes with dianionic [(o-C₂B₁₀H₁₀)C(NR)₂]²⁻ (= [L]²⁻) ligands having a deprotonated amidine group [20,22]. In a recent study, we have shown that the formation of this product class is preferred for highly Lewis-acidic centers, while “soft” metal centers form stable complexes with the original [(o-C₂B₁₀H₁₀)C(NR)(NHR)]⁻ (= [HL]⁻) ligand [22]. In all cases (i.e., for both [HL]⁻- and [L]²⁻-type ligands, and independent from the choice of the central atom), the ligand adopts a characteristic κC,κN-chelating coordination mode instead of the “normal” κN,κN′-chelating mode of coordinated 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.

2. Results and Discussion

2.1. 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, block-like 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, 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 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 ¹H- and ¹³C-NMR data of 3 were in good agreement with the expected composition. In the ¹H-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 ¹H-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]⁺.

2.2. 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 = ⁱPr) 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 germanium(II) derivative 3, the stereo-active lone pair leads to a trigonal-pyramidal coordination environment of the Ge center (Figure 1). At 204.0(5) and 229.4(2) pm, respectively, the Ge–C and Ge–Cl bond lengths are expectedly longer than in the previously reported germanium(IV) derivative GeCl₃[HL^iPr] (Ge–C 195.6(2) pm, Ge–Cl 226.4(1) pm) [22]. However, the Ge–N distances are very similar in both compounds (3: 205.3(5) pm, GeCl₃[HL^iPr]: 204.8(2) pm). Rather untypical for carboranylamidinates, the molecules in 3 are assembled through weak N–H⋯Cl hydrogen bonds to infinite supramolecular chains (Figure 2). In the previously reported complexes with [HL]^–-type ligands, no hydrogen bonding with participation of the amidine NH moiety has been observed [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 ligands being formally replaced by a lone pair. Different from 3, the amidine NH moiety in 4 is not involved in hydrogen bonding.

3. Experimental Section

3.1. 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. ¹H-NMR (400 MHz) and ¹³C-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).

3.2. 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 ⁿBuLi 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. 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 C₁₅H₃₃B₁₀ClGeN₂ (457.59 g·mol⁻¹): C, 39.37; H, 7.27; N, 6.12; found C, 38.88; H, 7.20; N, 5.99. ¹H NMR (400.1 MHz, THF-d₈, 23 °C): δ 8.06 (s, NH), 3.30–3.22 (m, CH), 3.15–3.03 (m, CH), 1.85–0.67 (m, Cy/BH) ppm. ¹³C NMR (100.6 MHz, THF-d₈, 23 °C): δ 157.5 (CN(NH)), 56.0 (CH), 53.8 (CH), 34.3 (Cy), 26.2 (Cy) ppm. IR (ATR): ν 3403 w (ν_N–H), 3305 w, 3066 w, 2929 m, 2854 m (ν_B–H), 2634 w, 2582 s, 2113 w, 1661 w, 1577 s (ν_C=N), 1531 s, 1464 w, 1449 m, 1366 w, 1348 w, 1332 m, 1300 w, 1260 w (ν_C–N), 1243 w, 1229 w, 1192 w, 1146 w, 1078 m, 1059 m, 1042 m, 1022 m, 973 w, 940 w, 921 w, 907 w, 890 m, 868 w, 843 m, 820 m, 799 w, 790 w, 767 w, 729 m, 718 m, 693 m, 656 m, 593 w, 558 w, 541 w, 507 w, 480 w, 446 w, 410 w, 380 w, 361 w, 300 s, 266 s, 227 m, 197 m, 176 m, 158 m, 121 m, 113 m, 98 m, 75 m, 66 m cm⁻¹. MS (EI): m/z (%) 457 (87) [M − H]⁺, 422 (13) [M − Cl]⁺, 367 (47) [M − Cy + H]⁺, 351 (14) [M − GeCl]⁺, 339 (17) [M − Cy − Cl]⁺, 295 (60) [M − 2Cy]⁺, 269 (69) [M − GeCl − Cy]⁺, 255 (100) [C₄H₇]⁺, 83 (83) [Cy]⁺, 187 (60) [M − GeCl − 2 Cy + 2H]⁺, 98 (26) [NCy + H]⁺, 58 (16) [M − Cl − 2Cy + H]⁺.

3.3. Synthesis of Compound 4

In a similar manner as for 3, a solution of Li[HL^iPr] was prepared from 1 (0.95 g, 6.56 mmol) in THF (50 mL), a 2.5 M solution of ⁿBuLi in hexanes (2.7 mL, 6.56 mmol) and 1,3-diisopropylcarbodiimide (0.83 g, 1 mL, 6.56 mmol) [8]. The addition of solid SbCl₃ (1.50 g, 6.56 mmol) produced a yellow solution and precipitation of a small amount of black solid (presumably Sb). Work-up as described for 3 afforded compound 4 as colorless, needle-like, moisture-sensitive crystals in 56% isolated yield (1.70 g). M.p. 141 °C. Elemental analysis calculated for C₉H₂₅B₁₀Cl₂N₂Sb (462.07 g·mol⁻¹): C, 23.39; H, 5.45; N, 6.06; found C, 23.50; H, 5.47; N, 6.10. ¹H NMR (400.1 MHz, THF-d₈, 23 °C): δ 3.26 (sept, 2 H, CH, J = 6.4 Hz), 3.15 (sept, 2 H, CH, J = 6.4 Hz), 1.48–1.16 (br m, BH), 0.86 (d, 6 H, CH₃, J = 6.4 Hz), 0.55 (d, 6 H, CH₃, J = 6.4 Hz) ppm. ¹³C NMR (100.6 MHz, THF-d₈, 23 °C): δ 153.2 (CN(NH)), 50.3 (CH), 47.8 (CH), 23.1 (CH₃), 23.0 (CH₃) ppm. IR (ATR): ν 3396 w (ν_N–H), 3375 w, 2970 w, 2930 w, 2873 w, 2599 m, 2590 m (ν_B–H), 2568 w, 2113 w, 1999 w, 1738 w, 1605 m (ν_C=N), 1530 m, 1459 w, 1390 w, 1370 w, 1333 w, 1289 w, 1251 w (ν_C–N), 1159 w, 1122 m, 1067 m, 1038 w, 969 w, 947 w, 930 w, 899 w, 872 w, 856 w, 838 w, 815 w, 760 w, 735 w, 681 w, 665 w, 634 w, 621 w, 597 w, 575 w, 555 w, 539 w, 517 w, 480 w, 455 w, 412 w, 380 w, 341 m, 303 w, 249 s, 213 m, 193 s, 160 s, 141 s, 113 s, 78 s cm⁻¹. MS (EI): m/z (%) 426 (60) [M − Cl]⁺, 368 (31) [M − Cl − ⁱPr − CH₃]⁺, 326 (24) [Sb(C₂H₁₀B₁₀)CNH + H]⁺, 270 (10) [M − SbCl₂]⁺, 256 (20) [M − SbCl₂ − CH₃ + H]⁺, 227 (97) [M − SbCl₂ − ⁱPr]⁺, 213 (18) [M − SbCl₂ − ⁱPr − CH₃ + H]⁺, 192 (54) [SbCl₂]⁺, 170 (25) [(C₂H₁₀B₁₀)CNH + H]⁺, 120 (9) [Sb]⁺, 462 (3) [M]⁺, 69 (35) [CNⁱPr]⁺, 58 (100) [HNⁱPr]⁺.

3.4. X-ray Crystallography

Single crystal X-ray intensity data of 3 and 4 were collected on a STOE IPDS 2T diffractometer [27] equipped with a 34 cm image plate detector, using graphite-monochromated Mo Kα radiation, at T = 100(2) K. The structure was solved by dual-space methods (SHELXT-2014/5) [28] and refined by full matrix least-squares methods on F² using SHELXL-2017/1 [29]. Crystallographic data for the compounds (see Supplementary Materials) have been deposited at the CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies of the data can be obtained free of charge on quoting the depository numbers 1899321 (3) and 1899321 (4) (Fax: +44-1223-336-033; E-Mail: [email protected], http://www.ccdc.cam.ac.uk).

4. 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.