Two in one: the bismuth bromido clusters in [Bi₆Cp*₆Br₉][Bi₇Br₂₄]

Introduction

Bismuth halides are well known to form diverse coordination polymers and anionic complexes as a result of their high Lewis acidity (Clegg et al., 1992, 1993; Wang et al., 1997; Erbe et al., 2010; Mansfeld et al., 2013). Thus, addition of sources for Br^- to bismuth bromide results in the formation of polybromido bismuthates of different stoichiometry (Robertson et al., 1967; McPherson and Meyers, 1968; Lazarini, 1977a,b). The structures are based on single or associated distorted octahedra, whereby association via corners, edges, and even faces was reported. For example, Ahmed et al. have prepared a variety of monomeric and oligomeric anions by the reaction between BiBr₃ with tetraphenylphosphonium bromide (Ph₄PBr) (Ahmed et al., 2001a,b), including [Bi₂Br₉]^3-, [Bi₂Br₈(CH₃COCH₃)₂]^2- and [Bi₆Br₂₂]^4-. Additional representative examples of polybromidobismuthate ions are [BiBr₅]^2-, [Bi₂Br₁₀]^4-, and [Bi₄Br₁₆]^4- (Norman, 1998). Diverse cations were used to stabilize such bromido bismuthates, e.g., [Ph₄P]⁺ (Ahmed et al., 2001a,b), [Me₃HP]⁺ (Clegg et al., 1991), [Me₂NH₂]⁺, [Et₂NH₂]⁺, and even [Fe(η⁵-C₅H₅)₂]⁺ (Norman, 1998).

We report on the synthesis and structure of [Bi₆Cp*₆Br₉][Bi₇Br₂₄] (1), containing heptanuclear [Bi₇Br₂₄]^3-, which is not reported for bismuth bromido complexes. The iodide analog was only recently described by Monakhov et al. as anionic part of the crystal structure of [Bi(OAc)₂ (thf)₄]₃[Bi₇I₂₄], which was obtained by the reaction of BiI₃ with [Pd(OAc)₂] (Monakhov et al., 2012). However, the first report on [Bi₇I₂₄]^3- dates back to 1996, but details on synthesis and structure were not given (Carmalt et al., 1996). In addition to the anionic cluster in 1, a cationic bismuth complex with Cp*Bi moieties is observed in the solid state. To date, there are only a few examples reported in the literature containing Cp*Bi moieties. For example, BiCp*Br₂ (A) was reported by Jutzi and Schwartzen in 1989, prepared from metallic bismuth and 5-bromopentamethyl-1,3-cyclopentadiene. Recently, the single crystal X-ray structure analysis of A was reported (Monakhov et al., 2011). Another example is [Al(η⁵-Cp*)₂{Bi(η⁵-Cp*)}(μ-AlI₄)(AlI₄)₂]*2C₇H₈ (B), which was obtained upon reaction of (AlCp*)₄ with BiI₃ in toluene (Üffing et al., 1999). The structure of B reveals in the solid state a chain of Cp*Bi units bridged by tetraiodoaluminate ions. Ionic bismuth halido clusters similar to 1 were reported recently (Monakhov et al., 2011). The reaction between BiX₃ (X=Cl, Br) and LiCp* gave compounds such as [Bi₆Cp*₅Cl₁₂][Bi₂Cl₇(thf)₂] and [{Bi₅Cp*₅Br₉}{BiBr₄}]₂. The title compound [Bi₆Cp*₆Br₉][Bi₇Br₂₄] (1) represents another example within the family of these novel organobismuth halido clusters.

Results and discussion

The pentamethylcyclopentadienyl-substituted bismuth bromido compound [Bi₆Cp*₆Br₉][Bi₇Br₂₄] (1) was obtained by the reaction of [FeCp*(CO)₂]₂ in CH₂Cl₂ with BiBr₃. Based on a recently reported synthesis protocol formation of [Bi{FeCp*(CO)₂}Br₂] (C) was expected (Wójcik et al., 2012). However, most likely as a result of a lower concentration used, compound C did not crystallize, and the title compound 1 was isolated instead. The mechanism of formation of 1 might be explained by the reaction sequence shown in Scheme 1. In the first step, C is formed, which converts into BiCp*Br₂ (A) by the formal loss of “Fe(CO)₂” (Eq. 1, 2). Reaction of A with unreacted BiBr₃ (Eq. 3) gave the title compound 1, in analogy to the reaction between BiX₃ (X=Cl, Br) and Cp*Li as described recently for similar Cp*–substituted bismuth halides (Monakhov et al., 2011). Monakhov et al. observed the formation of a mixture of BiCp*Br₂ (A) and [{Bi₅Cp*₅Br₉}{BiBr₄}]₂ upon reaction of LiCp* with BiBr₃. Our attempts to synthesize compound 1 either starting from this reaction mixture with appropriate stoichiometry for 1 or directly from A by addition of LiCp* were not successful so far. The ¹H NMR spectrum of compound 1 in CDCl₃ shows one singlet resonance at δ 1.77 assigned to the Cp* ligands, suggesting a η⁵-coordination mode.

Scheme 1

Proposed reaction scheme for the formation of compound 1.

The title compound [Bi₆Cp*₆Br₉][Bi₇Br₂₄] (1) crystallizes in the trigonal space group R-3 with the lattice parameters a=22.731(5) Å, b=22.731(5) Å, c=19.515(5) Å, V=8732(4) ų and Z=3 (crystallographic data are provided inTable 1). Selected bond lengths and angles are listed in Table 2, the structure of the cation is shown in Figure 1 and the structure of the anion in Figure 2.

Figure 1

Thermal ellipsoid plot of the structure of the cationic part of 1, [Bi₆Cp*₆Br₉]³⁺ (1a).

Thermal ellipsoids are set at 30% probability level. Hydrogen atoms are omitted for clarity. Symmetry equivalent atoms are given by a: -x+y, 1-x, z; b=-1/3+y, 1/3-x+y, 1/3-z; c=2/3-x, 4/3-y, 1/3-z.

Figure 2

Thermal ellipsoid plot of the structure of the anionic part of 1, [Bi₇Br₂₄]^3- (1b).

Thermal ellipsoids are set at 30% probability level. Hydrogen atoms are omitted for clarity. Symmetry equivalent atoms are given by d=y, 1-x+y, -z; e=1+x-y, x, -z; f=1-x+y, 2-x, z.

The cation [Bi₆Cp*₆Br₉]³⁺ (1a) is comprised of six bismuth atoms located at the corners of an octahedron, eight μ₃-bromides cap the trigonal faces of the octahedral core, and one bromide located in the center of the Bi₆ octahedron showing μ₆-coordination. Six Cp* ligands are η5-bonded to the bismuth atoms and comprise the periphery of the metal bromido cluster. The hexanuclear {Bi₆} octahedron is a well known structural unit reported for bismuth oxido compounds (Sundvall, 1983; James et al., 1996; Whitmire et al., 2000; Mehring et al., 2006) and even {Bi₆O₉} moieties with interstitial oxygen atoms are reported as part of larger structures (Miersch et al., 2013). However, to the best of our knowledge, {Bi₆X₉} moieties (X=halogen) as reported here are rare. Similar arrangements are restricted to recently reported examples such as [Bi₆Cp*₅Cl₁₂]⁺ (Monakhov et al., 2011).

In 1a, the bromine atoms Br1 and Br3 bridge three bismuth atoms while the interstitial atom Br2 is located in the center of the Bi₆ octahedron with symmetric coordination to the six bismuth atoms with a Bi1-Br2 distance of 3.1977(7) Å. The bond distances assigned to Bi1 are Bi1-Br1, 3.1276(12) Å, Bi1-Br3 3.1377(14) Å, and Bi1-Br3b 3.1606(13) Å. The distances between the carbon atoms in the Cp* ring and the Bi atom are similar and range from 2.521(10) to 2.565(12) Å, which indicates that the Cp* ligand is symmetrically bonded to the bismuth atom with η⁵-coordination. The Bi-Cp*_centroid bond distance of 2.234(6) Å is similar to Bi-Cp* bond distances as reported for B (2.243 Å) (Üffing et al., 1999) and [Bi₆Cp*₅Cl₁₂]⁺ (2.250 Å) (Monakhov et al., 2011). The [Bi₆Cp*₆Br₉]³⁺ cluster might be described to be built from six Cp*Bi-based units as presented in Figure 3C.

Figure 3

View of structural arrangements of the anion and the cation of [Bi₆Cp*₆Br₉][Bi₇Br₂₄] (1).

(A) Bi octahedron containing an interstitial Br atom inside the {Bi₆} cage; (B) an interstitial Br–containing Bi₆ octahedron capped by μ₃-coordinated Br^- ligands; (C) a Bi octahedron containing an interstitial Br atom and six η⁵-coordinated Cp* ligands; (D) seven distorted edge-sharing BiBr₆-octahedra connected by μ₂- and μ₃-bridging bromides.

The heptanuclear [Bi₇Br₂₄]^3- (1b) anion is best described as being composed of seven distorted edge-sharing BiBr₆–octahedra connected by μ₂- and μ₃-bridging bromides with bond lengths in the range from 2.8585(13) Å to 2.9346(13) Å and 2.8631(11) Å to 3.3161(12) Å, respectively (Figure 3D), similar to previously reported [Bi₇I₂₄]^3- (Monakhov et al., 2012). The terminal Bi–Br_terminal bond distances amount to 2.6361(13) Å.

The environment of Bi3 in the [Bi₇Br₂₄]^3- anion consists of six μ₃-bridging bromine atoms (Br7), while Br4 is coordinated to two bismuth atoms, and Bi5 and Bi6 are bonded terminal. The Bi-Br distances are similar to those found in [Bi₄Br₁₆]^4- as reported by Rheingold et al. [2.663(4)–2.689(4) Å for terminal and 2.844(4)–3.156(4) Å for bridging bromides] (Rheingold et al., 1983). In the crystal lattice of [Bi₆Cp*₆Br₉][Bi₇Br₂₄] (1), no close contacts between the anions and cations are noticed.

Conclusions

The reaction of [FeCp*(CO)₂]₂ with BiBr₃ was expected to provide [Bi{FeCp*(CO)₂}Br₂] (C). However, unexpectedly the pentamethylcyclopentadienyl substituted bismuth bromido compound [Bi₆Cp*₆Br₉][Bi₇Br₂₄] (1) was isolated. As it was described by Üffing et al., transfer of the Cp* ligand from aluminium to bismuth is possible (Üffing et al., 1999). A similar behavior within our studies was observed, a transfer of the Cp* ligand from iron to bismuth. The crystal structure determination revealed the formation of a cationic cluster [Bi₆Cp*₆Br₉]³⁺ and an anionic cluster [Bi₇Br₂₄]^3-, which do not show significant intermolecular bonding other than electrostatic interaction.

Experimental section