Synthesis and properties of 1,3-Bis(2,6-diisopropylphenyl)-2-(trimethylstannyl)- 2,3-dihydro-1H-1,3,2-diazaborole

3.1 Synthesis of 1,3-bis(2,6-diisopropylphenyl)-2-(trimethylstannyl)- 2,3-dihydro-1H-1,3,2-diazaborole (1)

A solution of Li[B{N(Dipp)CH}₂] (2.50 mmol) in thf (25 mL) was added dropwise to a cooled (–30°C) solution of Me₃SnCl (0.50 g, 2.50 mmol) in thf (20 mL). The resulting mixture was stirred at –30°C for 1 h and slowly warmed up to room temperature. After removal of all volatile compounds in vacuo, the yellow residue was extracted into hexane. Filtering from LiCl and evaporation of the solvents gave the diazaborole 1 as a yellow solid, which was purified by sublimation (90°C/10⁻³ mbar). Single crystals of 1 were obtained by recrystallization from hexane (yield: 0.56 g, 41%). – ¹H NMR (300 MHz, C₆D₆): δ=–0.17 (s, 9H, SnMe₃), 1.17 (d, 12H, ³J_H,H=6.9 Hz, CHMe₂), 1.28 (d, 12H, ³J_H,H=6.9 Hz, CHMe₂), 3.19 (sept, 4H, ³J_H,H=6.9 Hz, CHMe₂), 6.37 (s, 2H, N–CH=CH–N); 7.12–7.26 (m, 6H, 7.73–7.71, ph-H). – ¹³C{H} NMR (126 MHz, C₆D₆): δ=–10.8 (Sn(CH₃)₃), 23.9 (CH(CH₃)₂), 25.9 (CH(CH₃)₂), 29.1 (CH(CH₃)₂), 122.5 (N–CH=CH–N), 123.9 (ph-CH), 127.8 (ph-CH), 140.4 (ph-CC), 146.5 (ph-CN). – ¹¹B{¹H} NMR (96 MHz, C₆D₆): δ=29.6 (h_1/2=350 Hz). – ¹¹⁹Sn{¹H} NMR (112 MHz, C₆D₆): δ=–148.3 (q, ¹J_119Sn,11B=984 Hz; cf. Fig. 3). – MS (ESI⁺): m/z (%)=552 [M]⁺. – Anal. for C₂₉H₄₅BN₂Sn (551.2): calcd. C 63.19, H 8.23, B, N 5.08; found C 63.34, H 8.18, N 4.47.

3.2 Reaction of 1 with Li[Me]

A solution of Li[Me] (0.36 mmol) in 0.2 mL Et₂O and 1 mL thf was added to a solution of the diazaborole 1 (0.36 mmol) in 5 mL thf at –78°C. The resulting mixture was slowly warmed up to room temperature. The ¹H/¹³C/¹¹B/¹¹⁹Sn NMR spectra of the reaction solution revealed exclusively the signals of 2 and methane (δ_H,[D8]thf=0.16). After addition of 0.1 mL H₂O to the reaction mixture, signals appeared in the ¹H/¹³C/¹¹B/¹¹⁹Sn NMR spectra of the solution which could be assigned to the diazaborole 1.

2: ¹H NMR (500 MHz, [D₈]thf): δ=–0.59 (s, 9H, SnMe₃), 1.15 (d, 12H, ³J_H,H=6.9 Hz, CHMe₂), 1.16 (d, 12H, ³J_H,H=6.9 Hz, CHMe₂), 3.08 (sept, 4H, ³J_H,H=6.9 Hz, CHMe₂), 5.82 (s, 1H, N–CH=CLi–N); 7.05–7.30 (m, 6H, ph-H). – ¹³C{¹H} NMR (126 MHz, [D₈]thf): δ=–11.4 (Sn(CH₃)₃), 22.7/23.0 (CH(CH₃)₂), 24.4/24.6 (CH(CH₃)₂), 27.6/27.7 (CH(CH₃)₂), 121.5/121.9 (N–CH=CLi–N), 123.9/124.1 (ph-CH), 127.3/128.8 (ph-CH), 142.6 (ph-CN), 145.8/146.0 (ph-CC), 149.1 (ph-CN). – ¹¹B{¹H} NMR (160 MHz, [D₈]thf): δ=26.9. – ¹¹⁹Sn{¹H} NMR (187 MHz, [D₈]thf): δ=–143.8 (q, ¹J_119Sn,11B=1031 Hz).

Note 1: An NMR tube was charged with [D₈]thf (0.5 mL) and the diazaborole 1 (0.05 mmol). To this solution 0.1 mL of a 1.8 m solution of Li[Me] (0.18 mmol) in Et₂O was added at room temperature. The NMR spectra of the reaction solution revealed the signals of 2, methane, and the diazaborole 1. Additionally, signals of the products of a side reaction between Li[Me] and Et₂O appeared in the ¹H and ¹³C NMR spectra which can be assigned to ethene (δ_H,[D8]thf=5.34) and Li[OEt] (δ_H,[D8]thf=1.23(t), 3.78(q)).

Note 2: A solution of 1 (0.05 mmol) and Li[tBu] (0.19 mmol) in 0.5 mL [D₈]thf was kept for 3 day at room temperature. The ¹H/¹³C/¹¹B/¹¹⁹Sn NMR spectra revealed exclusively the signals of 1 and Li[tBu].

Note 3: A solution of 1 (0.05 mmol) and Na[SitBu₃] [15] (0.1 mmol) in 0.5 mL [D₈]thf was kept for 3 day at room temperature. The ¹H/¹³C/¹¹B/²⁹Si/¹¹⁹Sn NMR spectra revealed exclusively the signals of 1 and Na[SitBu₃] [15].

3.3 Reaction of Li[B{N(Dipp)CH}₂] with P₄

A solution of Li[B{N(Dipp)CH}₂] (0.03 g, 0.08 mmol) in thf (0.8 mL) was added to a cooled solution (–78°C) of P₄ (0.04 mmol) in thf (0.3 mL). After 45 min at –78°C, the solution was allowed to reach room temperature. All volatiles were removed in vacuo and [D₈]thf (0.5 mL) was added. According to the ³¹P NMR spectrum (Fig. 5) the tetraphosphenediide Li₂[{HC(Dipp)N}₂B–PPPP–B{N(Dipp)CH}₂] (3) was formed as the main product (60% P) and the triphosphide Li[{HC(Dipp)N}₂B–PPP–B{N(Dipp)CH}₂] (δ_P=665.1, 175.4 ppm; ¹J_PP=500 Hz) as a side product (10% P) of this reaction (cf. the analogous reaction behavior of M[SitBu₃] (M=Li, Na, K) [7] and Na[SiPhtBu₂] [8] towards P₄; see Table 1 and Refs. [7], [9], [10], [11], [12], [13]).

Fig. 5:

AA′ part of the ³¹P{¹H} NMR spectrum of Li₂[{HC(Dipp)N}₂B–P(1)P(2)P(3)P(4)–B{N(Dipp)CH}₂] (3) in [D₈]thf. Chemical shifts and coupling constants of 3: δ=364.5, –29.4 ppm; ¹J_P(2),P(3)=–509.8, ¹J_P(1),P(2)=–434.3, ²J_P(1),P(3)=–3.7, ³J_P(1),P(4)=178.9 Hz [16].

Selected data for 3: ¹H NMR (250 MHz, [D₈]thf): δ=1.12 (d, 12H, ³J_H,H=7 Hz, CH(CH₃)₂), 1.17 (d, 12H, ³J_H,H=7 Hz, CH(CH₃)₂), 3.02 (sept, 4H, ³J_H,H=7 Hz, CH(CH₃)₂), 6.27 (s, 2H, (N)HC=CH(N), 7.13–7.26 (6H, m, ph-H). – ³¹P{¹H} NMR (101.25 MHz, [D₈]thf) see Fig. 5 and Table 1. – ¹¹B{¹H} NMR (80.25 MHz, [D₈]thf): δ=22.0.

Note: A solution of the diazaborole 1 (0.10 mmol) in 0.5 mL [D₈]thf was added to a solution of P₄ (0.04 mmol) in thf (0.3 mL). After 1 d at room temperature the ¹H/¹³C/¹¹B/³¹P/¹¹⁹Sn NMR spectra of the solution revealed the signals of 1 and P₄.

3.4 Synthesis of 1,3-bis(2,6-diisopropylphenyl)-2-(chlorodimethylstannyl)- 2,3-dihydro-1H-1,3,2-diazaborole (4)

GaCl₃ (0.10 g, 0.60 mmol) was added to a solution of 1 (0.31 g, 0.57 mmol) in benzene (10 mL). The resulting mixture was stirred for 1.5 h at room temperature. After removal of all volatile compounds in vacuo, 4 was obtained by sublimation (100°C/10⁻³ mbar) as a colorless solid (yield: 0.19 g, 60%). – ¹H NMR (300 MHz, C₆D₆): δ=0.10 (s, 6H, ²J_119Sn,H=49 Hz, SnClMe₂), 1.17 (d, 12H, ³J_H,H=6.8 Hz, CHMe₂), 1.29 (d, 12H, ³J_H,H=6.8 Hz, CHMe₂), 3.18 (sept, 4H, ³J_H,H=6.8 Hz, CHMe₂), 6.35 (s, 2H, ⁴J_119Sn,H=13.5 Hz, N–CH=CH–N), 7.11–7.26 (m, 6H, ph-H). – ¹¹B{¹H} NMR (96 MHz, C₆D₆): δ=28.9 (¹J_SnB=1095 Hz, h_1/2=220 Hz). – ¹¹⁹Sn{¹H} NMR (112 MHz, C₆D₆): δ=84.7 (q, ¹J_119Sn,11B=1095 Hz). – Anal. for C₂₈H₄₂BClN₂Sn (571.6): calcd. C 58.83, H 7.41, N 4.90; found C 58.64, H 7.58, N 4.72.

Note: AsCl₃ (0.22 g, 1.19 mmol) was added to a solution of 1 (0.01 g, 0.01 mmol) in C₆D₆ (0.5 mL). After 1 d at room temperature the ¹H/¹³C/¹¹B/¹¹⁹Sn NMR spectra of the solution revealed the signals of 4.

3.5 Synthesis of 1,3-bis(2,6-diisopropylphenyl)-2-(dichloromethylstannyl)- 2,3-dihydro-1H-1,3,2-diazaborole (5)

GaCl₃ (0.12 g, 0.68 mmol) was added to a solution of 1 (0.18 g, 0.32 mmol) in benzene (10 mL). The resulting mixture was stirred for 1.5 h at room temperature. After removal of all volatile compounds in vacuo, 5 was obtained by sublimation (100°C/10⁻³ mbar) as a colorless solid (yield: 0.09 g, 45%). – ¹H NMR (300 MHz, C₆D₆): δ=0.41 (s, 3H, ²J_119Sn,H=53 Hz, SnCl₂Me), 1.10 (d, 12H, ³J_H,H=6.9 Hz, CHMe₂), 1.31 (d, 12H, ³J_H,H=6.9 Hz, CHMe₂), 3.06 (sept, 4H, ³J_H,H=6.9 Hz, CHMe₂), 6.27 (s, 2H, ⁴J_119Sn,H=20 Hz, N–CH=CH–N), 7.07–7.20 (m, 6H, ph-H). – ¹¹B{¹H} NMR (96 MHz, C₆D₆): δ=27.2 (¹J_119Sn,11B=1380 Hz, h_1/2=160 Hz). – ¹¹⁹Sn{¹H} NMR (112 MHz, C₆D₆): δ=106.3 (q, ¹J_119Sn,11B=1380 Hz). – Anal. for C₂₇H₃₉BCl₂N₂Sn (592.0): calcd. C 54.77, H 6.64, N 4.73; found C 53.94, H 6.38, N 4.67.

3.6 Crystal structure determinations

Data for 1 (Table 2) were collected on a STOE IPDS II two-circle diffractometer with graphite-monochromatized MoK_α radiation (λ=0.71073 Å) [17] and corrected for absorption using the program Platon [18]. The structure was solved by Direct Methods using the program shelxs [17] and refined against F² with full-matrix least-squares techniques using the program shelxl [19].

Three isopropyl groups are disordered over two positions with site occupation factors of 0.61(1), 0.54(3) and 0.58(3) for the major occupied site. Bond lengths and angles in the disordered groups were restrained to have the same values as in a non-disordered group. All disordered atoms were isotropically refined.

CCDC 1530547 (1) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.