Isolation of a planar π-aromatic Bi5– ring in a cobalt-based inverse sandwich-type complex

Monocyclic aromatic molecules are well-known in organic chemistry and for coordination chemistry. In contrast, isoelectronic cycles comprising (semi)metal atoms only are much rarer. We present Bi5−, the so far elusive heaviest analogue of (C5H5)− which we trapped in an inverse-sandwich-type complex.
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Bi5- trapped in [{(IMes)Co}2Bi5]

Aromaticity and the associated properties of aromatic molecules, for example their stability and unique reactivity patterns, is a foundation of organic chemistry. Bridging the perceived boundary between organic and inorganic chemistry are organometallic compounds, where the aromatic cyclopentadienide anion, (C5H5), is arguably one of the most well-used functional ligands. As a planar, five-membered ring, this molecule, or its derivatives, has been used as a ligand in complexes of almost every metal and semi-metal of the periodic table. By the isoelectronic concept, it is possible to substitute the CH fragment with a group 15 element (pnictogen, Pn), thus opening up the possibility to five-membered rings including one to five atoms of the pnictogen elements. While Pn5 rings have been realized for N, P, As and Sb, that of the heaviest congener, Bi, has long been missing from this series. By taking advantage of the unique reactivity of polybismuthides, it has been possible to finally access the elusive Bi5, which has been trapped and isolated in an inverse-sandwich compound. Here, we describe our approach.

Negatively charged molecules made up entirely of bismuth are known as polybithmuthides. Currently, only Bi22, Bi42, Bi73 and Bi113 have been synthetically realized, though the existence of a species with the composition and charge of Bi53− was postulated by Eduard Zintl in the 1930s by the potentiometric titration of a solution of Na in liquid ammonia with BiI3. This species could not yet be verified in an isolated compound though. Many decades after Zintl’s seminal work, other polybismuthide anions were formed and crystallized as salts, for instance, Bi73− or Bi113−. Other polybismuthide units have been observed in heteroatomic cluster molecules upon reaction of Bi-based Zintl anions with d- or f-block metal complexes, with the current largest one being the {Bi18} unit found in [{Ru(cod)}4Bi18]4−. Therefore, it stands to reason that a {Bi5} species should be equally achievable, especially as it is often found in electrospray ionization mass spectrometry measurements of polybismuthide reaction solutions, and because five-membered ring motifs plays a significant role in the structural chemistry in group 15 elements.

The accessible polybismuthides, in particular Bi22, have been seen as versatile reagents in the formation of larger bismuth-based structural frameworks. However, one important limiting factor is the choice of solvent. Bi22 in the salt [K(crypt-222)]2Bi2 (crypt-222 = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane), for example, has a high charge density rendering it soluble in only highly polar solvents. Ethane-1,2-diamine has long been the solvent of choice in polybismuthide chemistry, and anionic p-block cluster chemistry in general, though comes with significant drawbacks. It is a reactive solvent that behaves both as a Lewis and Brønsted base and, additionally, can be deprotonated. This has often limited the choice of suitable d- and f-block complexes for investigating the reactivity of polybismuthides. 

We have found that ortho-diflourobenzene (o-DFB) is suitable as a solvent for reactions involving polybismuthides. It has a similar polarity to the classically used solvents and is therefore able to dissolve polybismuthides, but at the same time lacks the reactivity of ethane-1,2-diamine, which means that a wider range d- or f-block metal complexes are stable in this solvent. A reaction between [IMes2CoCl] (IMes = bis{1,3-(2,4,6-trimethylbenzene)}imidazole-2-ylidene) and the Bi-based binary Zintl anion (TlBi3)2−, as a source of polybismuthide units upon release of elemental Tl, in o-DFB led to the isolation of crystals of [(IMesCo)2Bi5], an inverse sandwich-type compound with an {IMesCo} fragment above and below an almost planar five-membered Bi5 ring. While the exact formation pathway for this structure is unknown, in-situ mass spectrometry measurements of the rection solution reveals the presence of {Bi5} and {(IMesCo)2Bi5}.  While {Bi5} has been observed in mass spectrometric measurements, including in a simple reaction between BiI3 and two equivalents of Bi22 in ethane-1,2-diamine, it has never been isolated in the solid state before. Therefore, the choice of the metal complex plays a significant role in catching and isolating the {Bi5} unit.

Similar to the aromatic organic cyclopentadienide anion, the {Bi5} ring in [(IMesCo)2Bi5] shows equivalent Bi−Bi bonds, indicative of a delocalization of the bonding within the five-membered ring. Overall, the heterometallic compound is neutral, a unique situation as the chemistry of polybismuthides typically results in anionic clusters. The lack of charge results in the case where the Co atoms must be mixed valent, adopting formal oxidation states of 0 and +I in [(IMesCo)2Bi5]. This results in the complex being open shell and thus has the potential for unique magnetic properties. Magnetic measurements confirmed the paramagnetic nature of this compound with a J = S = 1/2 being the most likely situation when taking into account the saturation of measured M(H) curves and fitting of the paramagnetic Brillouin functions. Additionally, a mild intramolecular interaction across the Co−Co distance of 2.516 Å could be observed. Quantum chemical calculations supported and explained the findings of the experimental measurements of [(IMesCo)2Bi5].

 The utilization of o-DFB as a solvent in polybismuthide chemistry has opened the door to numerous new possibilities. Before, the use of amine-based solvents led to a limitation in the nature of reagents for investigating the reaction behavior of polybismuthides. Now, many more classes of d- or f-block metal compounds are available for corresponding studies, which has resulted in the trapping of the long sought after Bi5 anion.   

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