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) . 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)– , dihydrobis(thioxotriazolyl)borate (bt)– [17, 18], and dihydrobis(1-methyl-5-thiotetrazolyl)borate (bttMe)–  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 (btMe)–. 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(btMe)3], for instance, is emissive in the solid state at λmax = 674 nm .
2 Results and discussion
The title complex [Bi(btMe)(phen)Cl2] is a mixed-ligand complex based on (btMe)– 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(btMe)], which we recently reported . 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(btMe)(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(btMe)(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(btMe)(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].
The packing of the [Bi(btMe)(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.
3 Experimental section
All manipulations were carried out under dry nitrogen with standard Schlenk and high-vacuum techniques. Solvents were purified and dried by standard methods immediately prior to use. Chemicals were obtained from commercial sources and used without further purification. NMR spectra were recorded in CDCl3 with a Bruker DRX 500 spectrometer.
The title complex was prepared by mixing BiCl3 (0.157 g, 0.5 mmol) with phenanthroline (0.09 g, 0.5 mmol) followed by the slow addition of [Na(btMe)] (0.147 g, 0.5 mmol) prepared as described recently . The resulting mixture was stirred at room temperature for 3 h and then filtered. The solvent was removed in a rotary evaporator, and the resulting solid was washed with small amounts of methanol and n-pentane. Crystals suitable for X-ray diffraction were grown by slow evaporation of a chloroform solution of the complex. Yield 0.230 g, 62 %, color: dark red, m.p. 137 °C. – 1H NMR (CDCl3, note that atom numbering differs from that in the solid state): δ = 9.21 (4H, dd, H(2,9)), 8.26 (4H, dd, H(4,7)), 7.81 (4H, s, H(5,6)), 7.64 (4H, dd, H(3,8)), 6.17 (s, 2H, CH), 2.20 ppm (s, 6H, H3C). – 13C NMR (CDCl3): δ = 187.1 (C = S), 150.4 (C2, C9), 146.2 (C1a, C10a), 139.8 (CH-btMe), 136.0 (C4, C7), 128.7 (C4a, C6a), 127.0 (C5, C6), 123.1 (C3, C8), 108.9 (C–CH3), 17.4 ppm (H3C).
3.2 Crystal structure determination
Single crystals suitable for X-ray diffraction measurement were suspended in a paratone-N/paraffin oil mixture, mounted on a glass fiber and transferred onto the goniometer of the diffractometer. The structures were solved by direct methods and refined by full-matrix least-squares cycles (programs Shelxs-97 and Shelxl-97 ). The structures are represented using the program Ortep-III . X-Ray diffraction data were recorded at 100(2) K using a Nonius Kappa-CCD diffractometer. Empirical formula C22H20BBiCl8N4S4; Mr = 972.05, crystal size: 0.24 × 0.10 × 0.04 mm. Crystal system triclinic, space group a = 10.1151(2) Å, b = 10.3285(2) Å, c = 17.4505(3) Å, α = 99.166(2)°, β = 102.910(2)°, γ = 110.150(2)°, Z = 2, V = 1611.70(5) Å3. Radiation: MoKα, λ = 0.71073 Å, μ = 6.416 mm–1, θmax = 27.49° (completeness 99.6 %), F(000) = 936 e, reflections measured = 38133, unique reflections (Rint) = 7371 (0.070), R1 (I > 2 σ(I)) = 0.0290, wR2 (all data) = 0.0646, ρmax/min = 1.44/–1.37 e Å–3. Programs used: Shelxs, Shelxl, Collect, Ortep-III [27, 28–30].
CCDC 102124517 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
We thank Peter Mester for NMR measurements. M. I. acknowledges with special thanks the Deutscher Akademischer Austausch Dienst (DAAD) for providing a PhD fellowship.
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Published Online: 2015-02-10
Published in Print: 2015-03-01