Synthesis and structural characterization of bis and tris(2-mercapto-1-methylbenzimidazolyl)hydroborato complexes: benzannulation promotes κ3-coordination

Abstract

The benzannulated bis and tris(mercaptoimidazolyl)borohydride compounds, [BmMeBenz]Na and [TmMeBenz]Na, have been synthesized via the reactions of NaBH4 with two and three equivalents of 1-methyl-1,3-dihydro-2H-benzimidazole-2-thione, respectively. X-ray diffraction studies on the THF adducts, {μ-[BmMeBenz]Na(THF)₂}₂ and {[TmMeBenz]Na}₂(μ-THF)₃, indicate that both compounds are dinuclear but differ according to the nature of the bridging ligand. Specifically, {μ-[BmMeBenz]Na(THF)₂}₂ possesses bridging [BmMeBenz] ligands and terminal THF ligands, while {[TmMeBenz]Na}₂(μ-THF)₃ possesses terminal [TmMeBenz] ligands and bridging THF ligands. The tris(mercaptoimidazolyl)borohydride ligand of {[TmMeBenz]Na}₂(μ-THF)₃ coordinates in a κ³-manner, which is in marked contrast to the κ²-, κ¹- and κ⁰-modes that have been reported for various [TmMe]Na derivatives. Density functional theory (DFT) geometry optimization calculations of the anions [TmMeBenz]⁻ and [TmMe]⁻ in the gas phase indicate that the conformation required for κ³-S₃ coordination, i.e. one in which the three sulfur donors point away from the B-H group, is relatively more stable for [TmMeBenz]⁻ than for [TmMe]⁻, and thus provides a rationalization for the observation that benzannulation enables κ³-coordination of tris(mercaptoimidazolyl)borohydride ligand in {[TmMeBenz]Na}₂(μ-THF)₃. Furthermore, comparison of the molecular structure and IR spectroscopic properties of [TmMeBenz]Re(CO)₃ with those of [TmMe]Re(CO)₃ indicates that benzannulation reduces the electron donating properties of the ligand, but has little effect on its steric properties. {μ-[BmMeBenz]Na(THF)₂}₂ and {[TmMeBenz]Na}₂(μ-THF)₃ react with [Me₃PCuCl]₄ to give [BmMeBenz]CuPMe₃ and [TmMeBenz]CuPMe₃, the first pair of structurally related bis and tris(mercaptoimidazolyl)hydroborato copper(I) compounds.

Tricarbonylrhenium(I) Complexes with N‐Heterocyclic Thiones and Selenols

“…[6][7][8] Recently, they have also been considered for potential radiopharmaceutical applications as tricarbonyl rhenium and technetium compounds. They have been included as coordination sites of pincer-type chelators [9,10] or used as components for so-called [2+1] complexes. [11] The structural chemistry of rhenium(I) tricarbonyl complexes with simple N-heterocyclic thiols mainly focuses to mercaptopyridine, mercaptoimidazol, mercaptopyrimidine or related ligands.…”

Benzannulated tris(2-mercapto-1-imidazolyl)hydroborato ligands: tetradentate κ4-S3H binding and access to monomeric monovalent thallium in an [S3] coordination environment

“…8 In order to obtain a more sterically demanding variant of this ligand, we have now extended this method to the synthesis of [Tm Bu t Benz ]Na (Scheme 1), which features bulky t-butyl substituents. The molecular structure of the THF adduct {[Tm Bu t Benz ]Na(THF)} 2 (μ-THF) 2 has been determined by X-ray diffraction, as illustrated in Figure 2.…”

“…While the Na–S bond lengths are comparable to those in {[Tm MeBenz ]Na} 2 (μ-THF) 3 and its bis (mercaptoimidazolyl) counterpart, {μ-[Bm MeBenz ]Na(THF) 2 } 2 , as summarized in Table 1, the Na•••H–B distance [2.12(5) Å] is short by comparison to those in {μ-[Bm MeBenz ]Na(THF) 2 } 2 [2.45(2) Å and 2.48(2) Å]. 8 Furthermore, the mean Na•••H distance in structurally characterized compounds listed in the Cambridge Structural Database is 2.45 Å. 11 Thus, it is evident that the Na•••H–B interaction in {[Tm Bu t Benz ]Na(THF)} 2 (μ-THF) 2 must be considered significant.…”

“…Although there are relatively few structurally characterized [Tm R ]M alkali metal derivatives, 1b,8,13,14 it is evident that there exists a variety of structural types, which are influenced by the composition of the crystal. For example, (i) the aqua complex [Tm Me ]Na•4.5H 2 O is composed of discrete [Tm Me ] − anions ( i.e.…”

“…6,7 Moreover, with the synthesis of [Tm MeBenz ]Na, we recently reported that benzannulation of this ligand system is also possible (Figure 1) and that such derivitization has an impact on the coordination mode adopted by the ligand. 8 Here, we report the synthesis of a more sterically demanding benzannulated ligand, namely [Tm Bu t Benz ], and demonstrate how it allows the isolation of the first monomeric monovalent [Tm R ] thallium compound that features κ 3 - S 3 coordination.…”