A mixed-ligand dihydrobis(2-mercapto-4-methylthiazolyl)borate) bismuth complex: [Bi(bt^Me)(phen)Cl₂]

1 Introduction

Boron-centered anionic ligands with tripodal and dipodal nature are the basis of a wide range of complexes with enormous variations in structure and reactivity [1–7]. Numerous modifications including analogues with soft binding sites have been reported, e.g., the tripodal (hydrotris-(methimazolyl)borate (tm)^– (methimazolyl = 1-methyl-1H-imidazole-2(3H)-thione-3-yl) [6]. Such soft ligands are of interest for the stabilization of low-valent electron-rich metal species [8–13]. Difunctional variations such as dihydrobis(methimazolyl)borate (bm)^– [14, 15], dihydrobis(2-thiopyridone)borate (bmp)^– [16], dihydrobis(thioxotriazolyl)borate (bt)^– [17, 18], and dihydrobis(1-methyl-5-thiotetrazolyl)borate (btt^Me)^– [19] have been used to complex a variety of metal ions in the search for new substances for applications in radiopharmaceutical and bio-inorganic chemistry [19–22]. Some of them show (B)H···M binding motifs [19–22]. We have recently reported on bismuth(III) complexes of dihydrobis(2-mercapto-benzimidazolyl)borate (bb) and dihydrobis(2-mercapto-4-methylthiazolyl)borate (bt^Me)^–. These studies have been performed in the search for new photophysically interesting systems. Complexes of the mentioned ligands were intended to combine π systems that function as a chromophoric unit with the heavy-atom effects of bismuth (spin-orbit coupling) in a sense related to the efficient triplet emitter and organic light-emitting diode (OLED) material tris(pyridylphenyl)iridium(III) [23–25] but in contrast to that based on a much cheaper metal source. Our recently reported complex [Bi(bt^Me)₃], for instance, is emissive in the solid state at λ_max = 674 nm [26].

2 Results and discussion

The title complex [Bi(bt^Me)(phen)Cl₂] is a mixed-ligand complex based on (bt^Me)^– as primary ligand with 1,10-phenanthroline and chloro co-ligands. It was prepared from the consecutive reaction of bismuth(III) chloride BiCl₃ with phenanthroline and the ligand source [Na(bt^Me)], which we recently reported [26]. The dark red compound can be crystallized from chloroform for purification (62 % yield) and to afford a specimen for X-ray diffraction. The compound was further characterized by ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy of solutions in CDCl₃. As molecules of [Bi(bt^Me)(phen)Cl₂] are chiral, the virtual equivalence of NMR signals (listed in the Experimental section) of quasi symmetry-related atoms must be due to fast molecular fluxionality or other exchange processes in solution.

Figure 1 shows the molecular structure of [Bi(bt^Me)(phen)Cl₂] in the solid state. The bismuth atom is six-coordinate with deviation from a regular octahedron due to a distortion resulting from the stereochemically active lone pair of electrons. The presence of this lone pair, assumed to occupy the space directed opposite to the 1,10-phenanthroline ligand, becomes particularly obvious from the angles Cl(2)–Bi–S(3) (119.9(1)°), which is much larger than 90°, and Cl(1)–Bi–S(1) (161.0(1)°), which is much smaller than 180°. The interpretation of this binding motif is supported by the restrictions of the bite angle of the 1,10-phenanthroline ligand N(3)–Bi–N(4), which is quite acute at 67.4(1)°. There are no further attractive contacts to other molecules of [Bi(bt^Me)(phen)Cl₂] at this site of the lone pair, because the next atoms in this direction belong to two chloroform solvent molecules present in the crystal. The intermolecular Bi···H(1B) distance of 3.15(3) Å in this case is too long to be indicative of agostic interactions, as has been observed in other cases [19–22].

Fig. 1

Lewis formula and molecular structure and of [Bi(bt^Me)(phen)Cl₂]; displacement ellipsoids are drawn at 50 % level. Selected important structural parameter values (Å, deg): Bi–S(1) 2.740(1), Bi–S(3) 2.993(1), Bi–N(3) 2.483(3), Bi–N(4) 2.480(3), Bi–Cl(1) 2.708(1), Bi–Cl(2) 2.667(1); S(1)–Bi–S(3) 89.9(1), N(3)–Bi–N(4) 67.4(1), Cl(1)–Bi–Cl(2) 93.7(1), Cl(2)–Bi–S(1) 92.0(2), Cl(2)–Bi–S(3) 119.9(1), Cl(1)–Bi–S(1) 161.0(1), Cl(1)–Bi–N(3) 91.3(1), Cl(1)–Bi–N(4) 80.9(1), N(3)–Bi–S(1) 75.3(1), N(3)–Bi–S(3) 86.3(1), N(4)–Bi–S(1) 81.5(1), N(4)–Bi–S(3) 153.6(1).

The packing of the [Bi(bt^Me)(phen)Cl₂] molecules (see Fig. 2) is dominated by π-π dispersive type interactions between the 1,10-phenanthroline ligands to form piles in a stair-like fashion. Interactions are restricted to the two planes #1 defined as (C13 C17 C16 C15 C14 C12) and #3 defined as (N4 C20 C19 C18 C16 C17). Plane #1 makes a contact to plane #3 of a neighboring molecule [symmetry code 2_676 (1–x, 2–y, 1–z)] at an angle: 2.1°, a centroid-centroid distance of 3.783 Å and a centroid shift distance of 1.685 Å. Plane #3 makes a contact to the same plane of a neighboring molecule [symmetry code 2_676 (1–x, 2–y, 1–z)] at an angle of 0°, a centroid-centroid distance of 3.785 Å, and a centroid shift distance 1.752 Å.

Fig. 2

Packing of [Bi(bt^Me)(phen)Cl₂] molecules in the crystal showing the π-π dispersive type interactions between the 1,10-phenanthroline ligands.

3 Experimental section