Dismutation of Tricyanoboryllead Compounds: The Homoleptic Tetrakis(tricyanoboryl)plumbate Tetraanion

Abstract

A series of unprecedently air-stable (tricyanoboryl)plumbate anions was obtained by the reaction of the boron-centered nucleophile B(CN)₃ ^2- with triorganyllead halides. Salts of the anions [R₃ PbB(CN)₃ ]^- (R=Ph, Et) were isolated and found to be stable in air at room temperature. In the case of Me₃ PbHal (Hal=Cl, Br), a mixture of the anions [Me_4-n Pb{B(CN)₃ }_n ]^n- (n=1, 2) was obtained. The [Et₃ PbB(CN)₃ ]^- ion undergoes stepwise dismutation in aqueous solution to yield the plumbate anions [Et_4-n Pb{B(CN)₃ }_n ]^n- (n=1-4) and PbEt₄ as by-product. The reaction rate increases with decreasing pH value of the aqueous solution or by bubbling O₂ through the reaction mixture. Adjustment of the conditions allowed the selective formation and isolation of salts of all anions of the series [Et_4-n Pb{B(CN)₃ }_n ]^n- (n=2-4) including the homoleptic tetraanion [Pb{B(CN)₃ }₄ ]^4- .

Keywords: Anions; Boron; Cyanoborates; Dismutations; Lead.

Figure 1

Synthesis of compounds with electron‐precise Pb−B bonds.

Figure 2

Syntheses of [Ph₄P]1^Ph and K1^Et , crystal structures of 1^Ph ([Ph₄P]⁺ salt) and ² _∞[C(OPbEt₃)₃ 1^Et ] (top; ellipsoids are drawn at the 25 % probability level except for the H atoms that are either depicted with arbitrary radii or omitted for clarity, C atoms of the phenyl groups of 1^Ph and the Et groups of the C(OPbEt₃)₃ ⁺ ion are shown as wire‐ and‐stick models), formation of K1^Me and K₂ 2^Me , and ¹¹B and ²⁰⁷Pb NMR spectra of 1^Me and 2^Me (bottom).

Figure 3

²⁰⁷Pb{¹H} NMR spectra of 1^Et , 2^Et (CD₃CN), and 3^Et ([D₇]DMF) (top); ²⁰⁷Pb{¹¹B,¹H}, ²⁰⁷Pb{¹¹B}, and ²⁰⁷Pb{¹H} NMR spectra of 3^Et (H₂O*) and ²⁰⁷Pb{¹¹B} and ²⁰⁷Pb NMR spectra of 4 (H₂O*) (bottom, left), and ²⁰⁷Pb CP/MAS NMR spectra of [Me₂DABCO]₃(3^Et )₂⋅4 KI and [Me₂DABCO]₂ 4⋅2 KI (bottom, right).

Figure 4

Synthesis of [NEt₄]₂ 2^Et , [Fe(phen)₃]₃(3^Et )₂⋅1.5 (K₂SO₄), and [Me₂DABCO]₃(3^Et )₂⋅4 KI (top) and crystal structures of 2^Et ([Fe(bpy)₃]2^Et ) and 3^Et [Me₂DABCO]₃(3^Et )₂⋅n H₂O, ellipsoids are drawn at the 35 % probability level, bottom).

Figure 5

Synthesis of [Me₂DABCO]₂ 4⋅2 KI (top), ¹¹B{¹H} NMR spectra of a reaction mixture of K1^Et to result in 3^Et (16 h in HCl(aq), top spectrum) followed by removal of all volatiles under vacuum and successive addition of HCl(aq) and removal of all volatiles under reduced pressure) (middle), crystal structure of 4 ([Me₂DABCO₂]4⋅5 DMSO, ellipsoids are drawn at the 35 % probability level), and one of the triply degenerate HOMOs of 4 (B3LYP/6–311++G(d,p)/SDD/COSMO(H₂O), bottom).

Figure 6

Cyclic voltammograms of K1^Ph , K1^Et , [NEt₄]₂ 2^Et , and [Me₂DABCO]₃(3^Et )₂.

Figure 7

¹¹B NMR spectra of the reaction products of 1^Et , 2^Et , 3^Et , and 4 after irradiation of solutions in CH₃CN, water, and DMSO at 375 nm for 24 hours. Water was added to the reaction mixtures of 1^Et in CH₃CN and 4 in DMSO to ensure dissolution of all boron species. The dark‐grey side product, most likely metallic lead, was separated before the NMR spectroscopic studies.