Well-Defined, Molecular Bismuth Compounds: Catalysts in Photochemically Induced Radical Dehydrocoupling Reactions

Abstract

A series of diorgano(bismuth)chalcogenides, [Bi(di-aryl)EPh], has been synthesised and fully characterised (E=S, Se, Te). These molecular bismuth complexes have been exploited in homogeneous photochemically-induced radical catalysis, using the coupling of silanes with TEMPO as a model reaction (TEMPO=(tetramethyl-piperidin-1-yl)-oxyl). Their catalytic properties are complementary or superior to those of known catalysts for these coupling reactions. Catalytically competent intermediates of the reaction have been identified. Applied analytical techniques include NMR, UV/Vis, and EPR spectroscopy, mass spectrometry, single-crystal X-ray diffraction analysis, and (TD)-DFT calculations.

Keywords: bismuth; chalcogens; dehydrocoupling; photocatalysis; radical reactions.

Scheme 1

Well‐defined, molecular bismuth compounds for catalysed radical reactions: species applied in thermally‐ vs. photochemically‐induced transformations.

Scheme 2

Synthesis of compounds 2‐EPh via routes A and B and molecular structures of 2‐SePh and 2‐TePh in the solid state. Cp=C₅H₅. Displacement parameters are drawn at the 50 % probability level. Selected bond lengths (Å) and angles (°): 2‐SePh: Bi1−C1, 2.284(11); Bi1−C14, 2.303(10); Bi1−Se1, 2.7285(11); Bi1−S1, 2.966(3); C1‐Bi1‐C14, 97.9(3); C1‐Bi1‐Se1, 90.0(3); S1‐Bi1‐Se1, 157.23(6). 2‐TePh: Bi1−C1, 2.288(3); Bi1−C14, 2.289(3); Bi1−Te1, 2.9296(3); Bi1−S1, 3.0007(7); C1‐Bi1‐C14, 96.98(9); C1‐Bi1‐Te1, 93.92(7); S1‐Bi1‐Te1, 161.818(14).

Figure 1

Experimental UV/Vis spectrum of 2‐SPh in THF (solid black line), five lowest energy calculated transitions (blue bars), and molecular orbitals involved in the two calculated absorptions with highest intensity (A1 and A4). Isovalue=0.03.

Scheme 3

a) Synthesis of compounds 4 and 5; i: Na(OTEMP), r.t. THF, 2 h; ii: PhSiH₃, 60 °C, benzene, 4 d. b, c) Molecular structures of 4 and 5 in the solid state. Displacement parameters are drawn at the 50 % probability level. Selected bond lengths (Å) and angles (°): 4: Bi1−C1, 2.274(8); Bi1−C14, 2.271(9); Bi1−O1, 2.184(5); Bi1−S1, 2.936(2); N1−O1, 1.459(8); C1‐Bi1‐C14, 91.5(3); C1‐Bi1‐O1, 92.3(3); Bi1‐O1‐N1, 106.1(4); S1‐Bi1‐O1, 159.76(15); ∑C/O‐N1‐C/O, 333.9(6). 5: Bi1−C1, 2.262(7); Bi1−C14, 2.289(7); Bi1−Bi1‘, 3.0211(5); C1‐Bi1‐C14, 104.8(3); C1‐Bi1‐Bi1‘, 91.59(18); C1‐Bi1‐Bi1‘‐C1‘, 180.

Scheme 4

Tentatively suggested catalytic cycle for dehydrocoupling of S1 with TEMPO to give P1, catalysed by 2‐SPh (for further details see Supporting Information). [Bi]=[Bi(C₆H₄CH₂)₂S].