Synthesis and structure of bis(methylsulfanyl) derivatives of iron bis(dicarbollide)

Dedicated to Professor Narayan Hosmane in recognition of his outstanding contribution in the field of carborane and metallacarborane chemistry and with very best wishes on the occasion of his 70th birthday.
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Highlights

  • A series of methylsulfanyl derivatives of iron bis(dicarbollide) were synthesized.

  • Structures of 8,8’-, 4,4′- and 4,7′-isomers were determined by X-ray diffraction.

  • The ligand rotation is hampered by intramolecular CHcarb···S(Me) hydrogen bonds.

Abstract

Bis(methylsulfanyl) derivatives of iron(II) bis(dicarbollide) [8,8′-(MeS)2-3,3′-Fe(1,2-C2B9H10)2]2- (42−), [4,4′-(MeS)2-3,3′-Fe(1,2-C2B9H10)2]2- (52−), and [4,7′-(MeS)2-3,3′-Fe(1,2-C2B9H10)2]2- (62−) were prepared by the treatment of the corresponding dimethylsulfonium derivatives 13 with potassium butylthiolate. Their oxidation by air in aqueous solution results in the corresponding derivatives of iron(III) bis(dicarbollide) [8,8′-(MeS)2-3,3′-Fe(1,2-C2B9H10)2]- (7-), [4,4′-(MeS)2-3,3′-Fe(1,2-C2B9H10)2]- (8-) and [4,7′-(MeS)2-3,3′-Fe(1,2-C2B9H10)2]- (9-). The structures of (Bu4N)2[8,8′-(MeS)2-3,3′-Fe(1,2-C2B9H10)2] and (Me4N)[4,7′-(MeS)2-3,3′-Fe(1,2-C2B9H10)2] were determined through single-crystal X-ray diffraction. In the solid state, the rotation of the dicarbollide ligands with respect to each other is hampered due to formation of weak intramolecular CHS(Me) hydrogen bonds between the ligands resulting in stabilization of transoid-conformation in the case of 8,8′-isomer and gauche-conformation in the case of 4,4′-and 4,7′-isomers. The synthesized bis(methylsulfanyl) derivatives of iron bis(dicarbollide) can be considered as a versatile platform for design of molecular switches.

Introduction

The decision of the Royal Swedish Academy of Sciences to award Jean-Pierre Sauvage, Sir James Fraser Stoddart and Bernard (Ben) L. Feringa the Nobel Prize in Chemistry 2016 “for the design and synthesis of molecular machines” was natural recognition of foremost importance of this area of research [1]. Moreover, this award brought none of the familiar controversies regarding whether the discovery honored was really chemistry (or, rather, biology or physics) because synthetic molecular machines are completely the creation of chemists which demonstrated imagination and considerable skill in the design and construction of synthetic (supra)molecular systems capable of performing mechanical movements in response to specific stimuli [[2], [3], [4], [5], [6]]. Based on the type of motion, molecular machines can be divided into two main types, that is, molecular motors and molecular switches. Molecular motors are molecular machines that are capable of unidirectional rotation motion by 360° powered by external energy input, whereas molecular switches are molecules or supramolecular complexes that can exist in two or more stable forms that differ in the mutual orientation of their components and can be converted from one state to another by various external stimuli such as heat, light, and chemical reagents, etc. [7]. Molecular switches are the main structural element of any molecular electronics devices, particularly molecular logic gates, where the combination of two or more molecular switches allows molecule to behavior like that of electronic logic gates, suggesting a basis for future nanosize computing devices [8].

Despite the significant progress in the synthesis and study of molecular switches, there are some problems caused by relatively low stability of many organic materials to atmospheric oxygen and moisture that stimulates the search for new types of compounds that can be used as versatile platforms in design of effective molecular switches. Therefore, there is growing interest in stable molecular switches based on metallocene complexes such as ferrocene or transition metal bis(dicarbollides) [3,3′-M(1,2-C2B9H11)2]n− [[9], [10], [11], [12]]. Unlike the cyclopentadienyl ligands, the dicarbollide ligands contain two carbon atoms and three boron atoms in its open pentagonal face that results in nonequivalence in energy of different rotamers. Preferability of varying conformations depends on the nature of the metal and its oxidation state. For examples, the transoid conformation with two pairs of carbon vertices reflected through a centre of symmetry is the most preferable for nickel(III) bis(dicarbollide), whereas nickel(IV) bis(dicarbollide) prefers cisoid conformation (rotation angle 36°) [13]. The interconversion of the transoid- and cisoid-geometries via controlled the change of the nickel oxidation state provides the basis for the controlled rotation of the dicarbollide ligands and makes the nickel bis(dicarbollide) moiety a promising module for design of rotation molecular switchers [10,11]. However, rather low rotation barrier (∼8 kJ/mol) between the transoid- and gauche-rotamers reduces the efficiency of the redox molecular switches based on the nickel bis(dicarbollide) moiety [14,15]. Therefore, an additional stabilization of individual rotamers of transition metal bis(dicarbollide) complexes is very important. Such stabilization can be achieved through the introduction of substituents which are able to form intramolecular hydrogen bonds between the dicarbollide ligands. It was found that the effective stabilization of the transoid conformation due to formation of the intramolecular hydrogen bonds CcarbH···XB (X = Cl, Br, I) between the ligands, can be achieved by introduction of halogen atoms at positions 8 and 8′ of cobalt [[16], [17], [18]] and iron [19,20] bis(dicarbollides). Recently we demonstrated that stabilization of definite rotamers of cobalt bis(dicarbollides) can be reached via introduction of the methylsulfanyl substituents that results in formation of the intramolecular CcarbH···S(Me)B hydrogen bonds [21]. The addition of an external complexing metal M* results in the rupture of the weak hydrogen bonds in favor of formation of stronger M*···S(Me)B coordination bonds leading to the turn of the dicarbollide ligands [22]. Therefore, the methylsulfanyl derivatives of cobalt bis(dicarbollide) can be considered as a versatile platform for design of molecular switches.

In this contribution we report synthesis of a series of the B-methylsulfanyl derivatives of iron bis(dicarbollide) [X,Y’-(MeS)2-3,3′-Fe(1,2-C2B9H10)2]n−.

Section snippets

Results and discussion

In contrast cobalt bis(dicarbollide) [21], our attempt to prepare methylsulfanyl derivatives of iron bis(dicarbollide) by direct reactions of iron salts with the corresponding deprotonated methylsulfanyl derivatives of nido-carborane were unsuccessful. Therefore, we proposed another synthetic scheme based on demethylation of known dimethylsulfonium derivatives of iron(II) bis(dicarbollide) (Fig. 1).

The symmetrically substituted 8,8′-dimethylsulfonium derivative of iron(II) bis-(dicarbollide)

Conclusions

A series of isomeric derivatives of iron bis(dicarbollide) containing methylsulfanyl substituents at the boron atoms in the pentagonal face of the dicarbollide ligands, [8,8′-(MeS)2-3,3′-Fe(1,2-C2B9H10)2]2-/1-, [4,4′-(MeS)2-3,3′-Fe(1,2-C2B9H10)2]2-/1-, and [4,7′-(MeS)2-3,3′-Fe(1,2-C2B9H10)2]2-/1-, were synthesized. In the solid state, the rotation of the two dicarbollide ligands with respect to each other is hampered by the formation of intramolecular CHcarb···S(Me) hydrogen bonds between the

Materials and methods

Dimethylsulfonium derivatives of nido-carborane 9-Me2S-7,8-C2B9H11 [24] and 10-Me2S-7,8-C2B9H11 [41] were synthesized as described in the literature. Anhydrous FeCl2 (98%) was purchased from Aldrich. THF was distilled under sodium. All other chemicals were reagent grade and received from commercial vendors. The 1H, 11B, 11B{1H} and 13C NMR spectra were recorded on Bruker Avance-400 spectrometer. 1H chemical shifts were referenced to residual protons in the lock solvents. 11B chemical shifts

Acknowledgments

This work was supported by the Russian Science Foundation (16-13-10331). The NMR spectroscopic and X-ray diffraction data were obtained using the equipment of the Centre for Molecular Structure Studies at the A. N. Nesmeyanov Institute of Organoelement Compounds.

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