Tris(2-mercapto-1-tert-butylimidazolyl)hydroborato gallium derivatives: synthesis of di- and trigallium compounds in a sulfur-rich coordination environment

Abstract

A series of tris(2-mercapto-1-tert-butylimidazolyl)hydroborato gallium compounds have been synthesized. While GaI₃ and GaCl₃ afford mononuclear {[Tm^But]Ga} compounds, namely {[Tm^But]GaI}I, {[Tm^But]GaCl}[GaCl₄], and [κ²-Tm^But]₂GaI, the reactions of “GaI”, Ga[GaCl₄] and (HGaCl₂)₂ yield compounds that feature Ga–Ga bonds, namely [Tm^But]GaGaI₃, [Tm^But]GaGaCl₃, {[Tm^But]GaGa[Tm^But]}I₂, {[Tm^But]GaGa[Tm^But]}[GaCl₄]₂, {[Tm^But]Ga(GaI₂)Ga[Tm^But]}I and {[Tm^But]GaGa[Tm^But]}{[μ-κ¹,κ²-Tm^But]GaI₂GaI₂GaI}₂. These Ga–Ga bonded compounds may be formally regarded as donor–acceptor adducts between monovalent [Tm^But]Ga and various trivalent moieties; for example, [Tm^But]GaGaI₃ may be described as an adduct of [Tm^But]Ga and GaI₃, while {[Tm^But]Ga(GaI₂)Ga[Tm^But]}⁺ is an adduct between two molecules of [Tm^But]Ga and [GaI₂]⁺. Comparison of the structure of [Tm^But]Ga→B(C₆F₅)₃ with that of [Tm^But]In→B(C₆F₅)₃ indicates that [Tm^But]Ga is a more effective Lewis base than is [Tm^But]In.

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Introduction

Tris(2-mercapto-1-R-imidazolyl)hydroborato ligands, [Tm^R], have found diverse applications with respect to compounds that feature metal centers with sulfur-rich coordination environments.¹ However, despite their widespread use in both main group and transition metal chemistry, there are few studies concerned with the Group 13 metals² and none pertaining to gallium. Here we describe the first application of the [Tm^But] ligand to gallium chemistry, including the synthesis of a series of compounds that feature Ga–Ga bonds which have no precedent in the related indium system.

Results and discussion

Mononuclear gallium compounds may be accessed by treatment of the thallium derivative [Tm^But]Tl³ with either GaI₃ or GaCl₃ (Scheme 1). For example, [Tm^But]Tl reacts with GaI₃ (1 : 1 molar ratio) to give a compound of composition [Tm^But]GaI₂, which has been shown by X-ray diffraction to exist in the solid state as an ion-pair, {[Tm^But]GaI}I, (1-I)I. A related chloride derivative, {[Tm^But]GaCl}[GaCl₄], (1-Cl)[GaCl₄], is likewise obtained via treatment of [Tm^But]Tl with GaCl₃ (1 : 2 molar ratio). The ability of [Tm^But] to bind in a facial manner to gallium via three sulfur donors contrasts with the applications of other [S₃] donors that incorporate additional donors and bind in more encapsulating modes, as illustrated by tetradentate [S₃N]⁴ and hexadentate [S₃N₃]⁵ ligands.

Scheme 1

In addition to {[Tm^But]GaX}⁺ (X = Cl, I), which feature one [Tm^But] ligand per gallium, the 2 : 1 complex [κ²-Tm^But]₂GaI (2), which exhibits κ² coordination of the [Tm^But] ligands, has also been obtained from the reaction of [Tm^But]K with GaI₃ (2 : 1 molar ratio). The ability to isolate {[Tm^But]GaX}⁺ and [κ²-Tm^But]₂GaI provides an interesting comparison with the related indium system. Specifically, the indium counterparts of {[Tm^But]GaX}⁺ have not been reported, while the indium analogue of [κ²-Tm^But]₂GaI exists as an ion pair, {[Tm^But]₂In}I,^2a in which the indium is octahedrally coordinated to the two [Tm^But] ligands in a κ³ manner.⁶

Another notable difference between the gallium and indium systems is that treatment of [Tm^But]M (M = K, Tl) with “GaI”^7,8 does not simply yield monovalent⁹ [Tm^But]Ga, the analogue of [Tm^But]In,^2a but rather gives compounds that feature gallium–gallium bonds, namely (i) dinuclear [Tm^But]GaGaI₃ (3) and {[Tm^But]GaGa[Tm^But]}I₂ (4)[I]₂¹⁰ and (ii) trinuclear {[Tm^But]Ga(GaI₂)Ga[Tm^But]}I (5)[I] (Schemes 2 and 3), depending on the reaction conditions. In addition to these discrete dinuclear and trinuclear compounds, the ion pair {[Tm^But]GaGa[Tm^But]}{[μ-κ¹,κ²-Tm^But]GaI₂GaI₂GaI}₂ that features both dinuclear and trinuclear moieties has been isolated, of which the anionic {[μ-κ¹,κ²-Tm^But]GaI₂GaI₂GaI}⁻ (6)⁻ species features an unprecedented [μ-κ¹,κ²-Tm^But] coordination mode in which the ligand bridges a chain of metal atoms.

Scheme 2

Scheme 3

The formation of each of these dinuclear and trinuclear compounds may be rationalized in terms of disproportion of “GaI” to gallium metal and GaI₃,⁸ of which the latter would react with [Tm^But]K to generate [Tm^But]GaI₂ (vide supra). Thus, dinuclear [Tm^But]GaGaI₃ may be formally regarded as a donor–acceptor adduct between monovalent [Tm^But]Ga and trivalent GaI₃,¹¹ while {[Tm^But]GaGa[Tm^But]}²⁺ may be viewed as a donor–acceptor adduct between [Tm^But]Ga and {[Tm^But]Ga}²⁺. Similarly, trinuclear {[Tm^But]Ga(GaI₂)Ga[Tm^But]}I is formally derived from a 2 : 1 ratio of [Tm^But]Ga and GaI₃. It is also worth noting that the isolation of {[Tm^But]Ga(GaI₂)Ga[Tm^But]}I is best achieved at low temperature (−35 °C), and that solutions in MeCN convert to a mixture comprising [Tm^But]GaGaI₃ and {[Tm^But]GaGa[Tm^But]}²⁺ at room temperature.

The molecular structures of [Tm^But]GaGaI₃, {[Tm^But]GaGa[Tm^But]}I₂, {[Tm^But]Ga(GaI₂)Ga[Tm^But]}I and {[Tm^But]GaGa[Tm^But]}{[μ-κ¹,κ²-Tm^But]GaI₂GaI₂GaI}₂ have been determined by X-ray diffraction, as illustrated respectively in Fig. 1–4. The Ga–Ga bond lengths of [Tm^But]GaGaI₃ [2.4138(4) and 2.4254(3) Å],¹² {[Tm^But]GaGa[Tm^But]}²⁺ [2.412(2) Å], {[Tm^But]Ga(GaI₂)Ga[Tm^But]}⁺ [2.4586(5) Å] and {[μ-κ¹,κ²-Tm^But]GaI₂GaI₂GaI}⁻ [2.406(3) and 2.412(2) Å] are comparable to twice the covalent radius of gallium (2.44 Å,¹³ 2.48 Ź⁴) and are typical for compounds with Ga–Ga single bonds.¹⁵

Fig. 1 Molecular structure of [Tm^But]GaGaI₃ (from benzene).

Fig. 2 Molecular structure of {[Tm^But]GaGa[Tm^But]}I₂ (only the cation is shown).

Fig. 3 Molecular structure of {[Tm^But]Ga(GaI₂)Ga[Tm^But]}⁺.

Fig. 4 Molecular structure of {[μ-κ¹,κ²-Tm^But]GaI₂GaI₂GaI}⁻.

{[Tm^But]GaGa[Tm^But]}²⁺ and {[Tm^But]Ga(GaI₂)Ga[Tm^But]}⁺ are of particular note because there are no other structurally characterized cationic dinuclear and trinuclear gallium compounds listed in the Cambridge Structural Database¹⁶ and catenation is not common for gallium.¹⁷ Furthermore, in view of the analogy between [Tm^R], tris(pyrazolyl)hydroborato [Tp^RR′] and cyclopentadienyl [Cp^R] ligands, it is interesting to note that the {[Tp^RR′]GaGa[Tp^RR′]}²⁺ and {[Cp^R]GaGa[Cp^R]}²⁺ counterparts of {[Tm^But]GaGa[Tm^But]}²⁺ have not been reported, even though such species are isoelectronic with the recently reported neutral zinc complexes Cp^RZnZnCp^R,¹⁸ the first examples of compounds with Zn–Zn bonds.¹⁹

The chloride complexes Ga[GaCl₄] and (HGaCl₂)₂²⁰ may also be used to synthesize compounds with Ga–Ga bonds. For example, Ga[GaCl₄] reacts with [Tm^But]Tl to give [Tm^But]GaGaCl₃ (7) (Scheme 4), while treatment of [Tm^But]K with (HGaCl₂)₂ yields {[Tm^But]GaGa[Tm^But]}[GaCl₄]₂¹⁰ (8) (Scheme 5), both of which have been structurally characterized by X-ray diffraction. While the mechanism for formation of {[Tm^But]GaGa[Tm^But]}²⁺ in the latter reaction is not known, it is pertinent to note that Lewis bases promote reductive dehydrogenation of (HGaCl₂)₂,^20,21 which thereby provides a means to generate {[Tm^But]GaGa[Tm^But]}²⁺.

Scheme 4

Scheme 5

Since [Tm^But]GaGaX₃ (X = Cl, I) and {[Tm^But]GaGa[Tm^But]}²⁺ may be formally regarded as donor–acceptor adducts between monovalent [Tm^But]Ga and the respective trivalent moiety, GaI₃ and {[Tm^But]Ga}²⁺, we sought evidence for the existence of [Tm^But]Ga in this system.²² In this regard, support for the accessibility of [Tm^But]Ga is provided by the observation that the simple adduct [Tm^But]Ga→B(C₆F₅)₃ (9) may be isolated via the reaction of [Tm^But]K and (HGaCl₂)₂ in the presence of B(C₆F₅)₃ (Scheme 5). Furthermore, the bridging sulfido complex [Tm^But]GaSGaCl₃ (10) may be obtained from the reaction of [Tm^But]K with (HGaCl₂)₂ in the presence of sulfur (Scheme 5). In view of the fact that [Tp^But₂]Ga reacts with elemental chalcogens to give the terminal chalcogenido complexes [Tp^But₂]GaE (E = S, Se, Te),²³ the formation of [Tm^But]GaSGaCl₃ is consistent with a sequence in which in situ generated [Tm^But]Ga reacts with sulfur to give the terminal sulfido complex [Tm^But]GaS, which is subsequently trapped by GaCl₃, although other mechanisms are possible.²⁴

The molecular structure of [Tm^But]GaSGaCl₃ has been determined by X-ray diffraction (Fig. 5), thereby demonstrating that the Ga–S bonds for the bridging sulfido ligand [2.2024(6) and 2.2153(6) Å] are shorter than those for the [Tm^But] ligand (2.30–2.31 Å), an observation which is in accord with the L₂X nature of [Tm^But],²⁵ such that the Ga–[Tm^But] interaction is appropriately described as a composite of covalent and dative covalent bonds. The gallium sulfido bond lengths are, nevertheless, comparable to the values for other bridging sulfido complexes,²⁶ but are considerably longer than that for the terminal sulfido complex [Tp^But₂]GaS [2.093(2) Å].^23a The similarity of the two gallium–sulfido bond lengths is significant in view of the fact that the gallium centers in the isolated fragments, namely monovalent [Tm^But]Ga and trivalent GaCl₃, are electronically distinct. However, in view of the zwitterionic nature of the bonding within [Tm^But]Ga⁽⁺⁾–S–Ga⁽⁻⁾Cl₃, both gallium centers in the sulfido complex are trivalent and the Ga–S bond lengths are comparable.

Fig. 5 Molecular structure of [Tm^But]GaSGaCl₃.

Comparison of the structure of [Tm^But]Ga→B(C₆F₅)₃ (Fig. 6) with that of the indium counterpart provides a means to assess the relative abilities of the [Tm^But]Ga and [Tm^But]In moieties to serve as a Lewis base. Specifically, the deviation of the B(C₆F₅)₃ ligand from planarity, as indicated by the magnitude of Σ(C–B–C) provides a simple indication of the strength of the interaction with the Lewis base.²⁷ On this basis, Σ(C–B–C) for [Tm^But]Ga→B(C₆F₅)₃ [342.2°]²⁸ is smaller than that for [Tm^But]In→B(C₆F₅)₃ [347.9°],^2a thereby indicating that the lone pair on gallium is more available for bonding than is that on indium, an observation which is in accord with the so-called “inert pair effect” being more prevalent for the heavier element.²⁹ Support for the greater Lewis basicity of [Tm^But]Ga, as implied by the structural differences between [Tm^But]Ga→B(C₆F₅)₃ and [Tm^But]In→B(C₆F₅)₃, is provided by DFT calculations which indicate that coordination of B(C₆F₅)₃ to [Tm^But]Ga (−23.7 kcal mol⁻¹) is more exothermic than is coordination to [Tm^But]In (−19.3 kcal mol⁻¹).

Fig. 6 Molecular structure of [Tm^But]GaB(C₆F₅)₃.

It is also pertinent to note that the 5.7° difference in Σ(C–B–C) for [Tm^But]Ga→B(C₆F₅)₃ and [Tm^But]In→B(C₆F₅)₃ is distinctly larger than the corresponding difference for ArGa→B(C₆F₅)₃ and ArIn→B(C₆F₅)₃ counterparts; for example, Σ(C–B–C) for (2,6-C₆H₃Trip₂)Ga→B(C₆F₅)₃ (337.5°)^27b and (2,6-C₆H₃Trip₂)In→B(C₆F₅)₃ (339.3°)³⁰ differ by only 1.9°. As such, this observation demonstrates that supporting ligands can exert differential effects on the relative Lewis basicities of monovalent gallium and indium centers.

Conclusions

In summary, the tris(2-mercapto-1-tert-butylimidazolyl)hydroborato ligand has been employed to synthesize the first cationic digallium and trigallium compounds, {[Tm^But]GaGa[Tm^But]}²⁺ and {[Tm^But]Ga(GaI₂)Ga[Tm^But]}⁺, while comparisons between [Tm^But]Ga→B(C₆F₅)₃ and [Tm^But]In→B(C₆F₅)₃ indicate that [Tm^But]Ga is a more effective Lewis base than is [Tm^But]In.

Acknowledgements

We thank the National Science Foundation (CHE-0749674) for support of this research and for the acquisition of an X-ray diffractometer (CHE-0619638).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and crystallographic data. CCDC reference numbers 763457–763470. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0sc00145g

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