Benzannulated tris(2-mercapto-1-imidazolyl)- hydroborato ligands: tetradentate κ~4-S_3H binding and access to monomeric monovalent thallium in an S_3 coordination environment

摘要

The benzannulated tris(mercaptoimidazolyl)borohydride sodium complex, Tm~(Bu~tBenz)Na, has been synthesized via the reaction of NaBH_4 with 1-tert-butyl-1,3-dihydro-2H-benzimidazole-2-thione, while Tm~(MeBenz)K has been synthesized via the reaction of KBH_4 with 1-methyl-1,3-dihydro-2H-benzimidazole- 2-thione. The molecular structures of the solvated adducts, {Tm~(Bu~tBenz)Na(THF)}_2(μ-THF)_2 and Tm~(MeBenz)- K(OCMe_2)_3, have been determined by X-ray diffraction, which demonstrates that the TmR ligands in these complexes adopt different coordination modes to that in {Tm~(MeBenz)Na}_2(μ-THF)_3. Specifically, while the Tm~(MeBenz) ligand of the sodium complex {Tm~(MeBenz)Na}_2(μ-THF)_3 adopts a κ~3-S_3 coordination mode, the potassium complex Tm~(MeBenz)K(OCMe_2)_3 adopts a most uncommon inverted κ4-S_3H coordination mode in which the potassium binds to all three sulfur donors and the hydrogen of the B-H group in a linear K?H-B manner. Furthermore, the Tm~(Bu~tBenz) ligand of {Tm~(Bu~tBenz)Na(THF)}_2(μ-THF)_2 adopts a κ~3-S_2H coordination mode, thereby demonstrating the flexibility of this ligand system. The monovalent thallium compounds, Tm~(MeBenz)Tl and Tm~(Bu~tBenz)Tl, have been obtained via the corresponding reactions of Tm~(MeBenz)Na and Tm~(Bu~tBenz)Na with TlOAc. X-ray diffraction demonstrates that the three sulfur donors of the TmRBenz ligands of both Tm~(MeBenz)Tl and Tm~(Bu~tBenz)Tl chelate to thallium. This coordination mode is in marked contrast to that in other TmRTl compounds, which exist as dinuclear molecules wherein two of the sulfur donors coordinate to different thallium centers. As such, this observation provides further evidence that benzannulation promotes κ~3-S_3 coordination in this system.