Synthesis of arsenic-rich As_n ligand complexes from yellow arsenic

Abstract

The reaction of [{η⁵-Cp′′′Co}₂{μ,η^4:4-toluene}] with yellow arsenic yields the arsenic-rich As_n ligand complexes [{Cp′′′Co(μ,η^2:2-As₂)}₂] (1), [(Cp′′′Co)₄(μ₄,η^4:4:2:2:1:1-As₁₀)] (2) and [(Cp′′′Co)₃(μ₃,η^4:4:2:1-As₁₂)] (3), which were comprehensively characterized. The molecular structure of 1 show a triple-decker complex with two As₂ units forming the middle-deck; compound 2 contains an all-arsenic As₁₀ analogue of dihydrofulvalene in the molecular structure. The As₁₂ ligand in 3 represents the largest As_n ligand complex reported so far.

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Introduction

The synthesis of substituent-free As_n ligand complexes was established in the early 1980s. The first arsenic sources to be used for this purpose were cyclo-arsines such as (MeAs)₅, and (PhAs)₆,¹ which formed the first triple-decker sandwich complex A with a distorted As₃–As₂ middle deck, revealing long As–As contacts of 2.726(3) and 2.752(3) Å.² Subsequently, polyarsenides were introduced into this chemistry, for which the reaction of As₇³⁻ with [Cr(CO)₃(1,3,5-Me₃C₆H₃)] was reported to yield a norbornadiene-like As₇ core in B.³ A similar structural motif was obtained by Goicoechea et al. in [Tl(η²-As₇)]²⁻ by the reaction of As₇³⁻ with TlCl.⁴ The cobalt arsenic cluster [Co₆As₁₂(PEt₂Ph)₆] (C), containing a cyclo-As₆ unit, and [As@Ni₁₂@As₂₀]³⁻ were also synthesized using As₇³⁻ as the starting material.^5,6 Moreover, the neutral nortricyclane derivative, [As₇(SiMe₃)₃], was used as the arsenic source by Fenske et al. in the reaction with [(Cp^RCoCl)₂] (Cp^R = C₅Me₅, C₅Me₄^tBu). Here, the cationic clusters [(Cp^RCo)₃(μ₃,η⁴-As₆)]²⁺ (D), [(Cp^RCo)₃(μ₃,η⁴-As₆)]⁺ and [(Cp^RCo)₂(μ₂,η⁴-As₄)]²⁺ were obtained.⁷ In contrast, the use of yellow arsenic (As₄), the unstable allotrope of arsenic, was introduced by cothermolysis reactions with cyclopentadienyl-containing carbonyl complexes, which led to a variety of cyclo-As₃, cyclo-As₅ and cyclo-As₆ complexes.⁸ For example, the reaction of As₄ with [Cp*Co(CO)₂] (Cp* = C₅Me₅) produced two binuclear complexes, [Cp*Co(μ,η^2:2-As)₂]₂ and [Cp*₂Co₂(μ₂,η^2:2-As₆)], and one trinuclear complex, [Cp*Co(μ,η^2:2-As₂)]₃.⁹ By reacting As₄ with [Cp′′Nb(CO)₄] (Cp′′ = 1,3-^tBu₂C₅H₃), Scherer et al. obtained [Cp′′Nb₂As₈] (E), which contains the largest structurally characterized As_n ligand known to date.¹⁰ In the reaction of [Cp′′Rh(CO)₂] with E₄ (E = P, As) the P₁₀ derivative of F was structurally characterized, whereas the As complex F was only characterized by mass spectrometry.¹¹ Larger structurally characterized arsenic scaffolds are present in the polyarsenide anions (the Zintl ions: As₄²⁻ and As₆⁴⁻ and the cage compounds As₇³⁻, As₁₁³⁻, As₁₄⁴⁻) derived from grey arsenic, with the largest reported polyarsenide anion hitherto being As₂₂⁴⁻.¹² These ionic compounds confer thermodynamic stability through factors such as lattice enthalpy, which does not apply to analogous systems involving neutral As_n ligands.

Thus, the question arises whether larger As_n units can be generated. Recently, we reported on the activation of white phosphorus by [{Cp′′′Co}₂{μ,η^4:4-toluene}] (Cp′′′ = 1,3,5-^tBu₃C₅H₂),¹³ which led to the formation of extended polyphosphorus scaffolds. Since [{Cp′′′Co}₂{μ,η^4:4-toluene}] dissociates in solution into the unsaturated 14 VE complex [Cp′′′Co], this reactive moiety allows one to work under mild reaction conditions at low temperatures, yielding complexes containing P₁₆ and P₂₄ ligands, respectively, by consuming P₄ moieties for aggregation.¹⁴

This success with phosphorus raised the question of approaching As-rich ligand complexes by a similar methodology using yellow arsenic. However, the poor solubility of yellow arsenic in common solvents combined with its extreme light sensitivity with respect to the formation of grey arsenic, are in strong contrast to the properties of white phosphorus. This complicates the use of As₄ as an arsenic source at ambient temperature and below. Moreover, in the few reports where As₄ has been used in reactions with transition metal compounds at room temperature and below, conversions to relatively small As₁ or As₂ units¹⁵ or to a butterfly As₄²⁻ moiety¹⁶ were described. These observations raise important questions and challenges about whether solutions of As₄ be prepared in sufficiently high concentrations to form extended polyarsenic units that are larger than those currently known. Our findings on this topic are reported herein.

Results and discussion

In contrast to the situation with P₄ solutions (vide supra), the concentration of As₄ in solution is too low at −30 °C. Therefore, the reaction of [{Cp′′′Co}₂{μ,η^4:4-toluene}]¹³ was performed at room temperature, with a saturated arsenic solution in toluene yielding three products (Scheme 1). After column chromatographic workup, 1 was isolated in 54% yield as the main product, followed by the As₁₂ complex 3 (14%) and the As₁₀ complex 2 (8%) (Scheme 1). Complex 1 can be formed selectively when the reaction takes place at 70 °C in good yields (82%) with no evidence for 2 and 3. Similar complexes of 1 have been reported as minor products of the cothermolysis between [Cp^RCo(CO)₂] (Cp^R = C₅Me₅ (Cp*), C₅Me₄Et (Cp′)) and yellow arsenic at 190 °C (for [Cp*CoAs₂]₂ 6% and for [Cp′CoAs₂]₂ 2% yield).⁹

Scheme 1 Synthesis of compounds 1–3.

The ¹H NMR spectra of 1–3 show the corresponding signals for the ^tBu groups and the signals for the aromatic protons. Due to the rotation of the cyclopentadienyl ligands in 2 only broad signals for the ^tBu groups as well as the aromatic protons are observed in the ¹H NMR spectra. In the FDI mass spectra the molecular ion peaks of 1–3 are observed, and in the case of 1 further fragmentation was not detected. For 2 and 3 two fragments, [(Cp′′′Co)₂As₆]⁺ and [(Cp′′′Co)₂As₄]⁺, and in addition for 3 the fragment [(Cp′′′Co)₂As₅]⁺ can be further found in the mass spectra (FDI-MS).

Compound 1 crystalizes from a saturated hexane solution as dark green blocks. The solid state structure (Fig. 1) shows a triple decker complex with two As₂ units as middle deck. The bond length As1–As2 of 2.2795(5) Å is shorter as a single bond determined for As₄ by electron diffraction in the gas phase (2.44 Å (ref. 17) and 2.435(4) Å (ref. 18)) and by DFT calculations (2.437 Å (ref. 15b)), respectively. The As1–As2 distance can be compared with the distance in the diarsene [{(Me₃Si)₃CAs}₂], which contains an arsenic–arsenic double bond of 2.245(1) Å.¹⁹ In contrast, the distance between the two As₂ units of 2.8209(4) Å is beyond what can reasonably be considered a bond, however it is closer than the sum of the van-der-Waals-radii (3.7 Å). The distance between the two As₂ units in [Cp′CoAs₂]₂ was found to be 2.844(1) Å, which is comparable to the distance in 1.

Fig. 1 Molecular structure of 1. H atoms are omitted for clarity. Anisotropic displacement parameters are depicted at 50% probability level. Selected bond lengths [Å] and angles [°]: As1–As2 2.2795(5), As1⋯As2′ 2.8209(4), Co–As1 2.4355(5), Co–As2 2.4211(5), Co–Ccentr. 2.092(3); As2–As1–As2′ 89.747(13).

Compound 2 crystallizes from a saturated dichlormethane solution and constitutes the all-pnictogen analogue As₁₀ of dihydrofulvalene, which acts formally as a 16-electron donor ligand. The main feature of the structure of 2 (Fig. 2) is an As₁₀ ligand consisting of two As₅ units bonded by an As–As bond. In each of the As₅ rings four arsenic atoms coordinate to a [Cp′′′Co] fragment whereas a second [Cp′′′Co] fragment is coordinated by two arsenic atoms of one As₅ ring and one arsenic atom of the second As₅ ring. Accordingly, there are two types of [Cp′′′Co] fragments, one coordinates via π bonds to four As atoms (av. As–Co 2.456(2) Å) and the other [Cp′′′Co] fragment is coordinated formally by its lone pair to the atom Co1 (As5–Co1 2.273(2) Å), and side-on to an As–As bond (As1–Co1, As2–Co1 2.350(3) Å). Viewing the bond distance alternations, the shorter lone pair donation leads obviously to a longer π-type bond inclusive of the longer As–As bond of the linking As atoms (vide infra). However, formal coordination of the arsenic lone pair to cobalt occurs in the range of 2.326(1)–2.350(2) Å in compounds such as [Co₂{μ-(C₂(CO₂Me)₂)}{μ-(AsMe₂)₂S}(CO)₄] and [Co₂(R′CCR′′){μ-(AsPh₂)₂S}(CO)₄] (R′, R′′ = CO₂Me, Ph).²⁰ The As–As bond lengths of 2 are in the range characteristic of single bonds (for details see ESI‡), and only two distances between As1–As2 (As1′–As2′) are longer 2.705(2) Å (Fig. 2). However, these elongated distances are shorter than the As–As distances found in [(Cp*Fe)₂(Cp*Co)As₆], where two As₃ triangles are connected by As–As bonds with distances of 2.800(2) to 2.871(1) Å.²¹ In the triple-decker sandwich complex A there are As–As distances in the range of 2.726(3) and 2.752(3) Å which are regarded as being bonds with the bond order of 0.5.² Therefore, one can speculate of a weak interaction between the atoms As1 and As2.

Fig. 2 Molecular structure of 2·4CH₂Cl₂. The H atoms and the solvent molecules are omitted for clarity. Anisotropic displacement parameters are depicted at 50% probability level.

Single crystals of 3 were obtained from a saturated hexane solution as black needles. The structure of 3 (Fig. 3) can be derived from that of 2 in which one As₅(CoCp′′′)₂ unit is replaced by a norbornane-like As₇CoCp′′′ fragment. All bond lengths are in the range of As–As single bonds, with the exception of the linking distance of the two As_n moieties at the atoms As1–As2 (2.6684(5) Å). This distance is comparable with the corresponding bond length in 2 (2.705(2) Å). The elongation of the As–As bond through the coordination of two [Cp′′′Co] fragments is comparable with the phosphorus analogue of 3, where a similar behavior is observed.¹⁶ Also in 3 the formal lone-pair coordination of Co to As (Co2–As6 2.2628(6) Å) is slightly shorter than the other Co–As distances (average 2.4314(6) Å). The As₁₂ ligand, which is the largest yet obtained, can be described as a 12-electron donor.

Fig. 3 Molecular structure of 3. H atoms are omitted for clarity. Anisotropic displacement parameters are depicted at 50% probability level.

Conclusions

In summary, it has been shown that the use of the Co complex [{Cp′′′Co}₂{μ,η^4:4-toluene}] can initiate mild activation of yellow arsenic. Using this method, arsenic-rich As_n ligand complexes could be synthesized. Complexes 2 and 3 contain As₁₀ and As₁₂ ligands, which are the largest substituent-free polyarsenic ligands yet observed in transition metal complexes, and have been unambiguously characterized by X-ray crystallography for the first time.

Acknowledgements

This work was supported by the Deutsche Forschungsgemeinschaft.

Notes and references

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Footnotes

† Dedicated to Professor Martin Jansen on the occasion of his 70's birthday.

‡ Electronic supplementary information (ESI) available: Experimental and X-ray data. CCDC 1002519–1002521. For ESI and crystallographic data in CIF or other electronic format. see DOI: 10.1039/c4sc03543g

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