Günther Thiele , Markus Balmer and Stefanie Dehnen

Synthesis, structure and electronic configuration of [Rh6Te8(PPh3)6]·4C6H6

De Gruyter | Published online: March 8, 2016

Abstract

[Rh6Te8(PPh3)6]·4C6H6, the first compound with a molecular Chevrel-type [Rh6Te8] cluster core has been synthesized and structurally characterized. By means of quantum chemical calculation, the close relationship of its electronic configuration to that of the lighter homologue has been demonstrated. The different crystal solvent content prevents an isostructural crystallization.

1 Introduction

Reactions of binary anions of main group elements towards transition metal compounds are known to lead to a variety of possible products. Depending on the elemental combination and the reaction conditions, the spectrum ranges from ternary molecular clusters [14] to complicated networks of ternary clusters or neat solid state compounds, which tend to represent zeolite-related – so-called “zeotype” – systems obtained from reactions involving binary anions of Group 14 and 16 elements [58]. Because the intriguing properties of such multinary compounds can be tuned – within certain limits – by choosing the respective elements and also via the choice of precursors and reaction conditions, many working groups all over the world are actively investigating this field.

2 Results and discussion

We are currently investigating the results of reactions involving the binary precursor anion [Pb2Se3]2– within the starting material [K(18-crown-6)]2[Pb2(μ-Se)3]. By reactions of the latter with Wilkinson’s catalyst [Rh(PPh3)3Cl] [9] in ethane-1,2-diamine (ethylenediamine, en) as a solvent, two cluster compounds with a ligand-shielded [Rh3Se2] core, {[K(18-crown-6)][K(en)2]K[Rh3(CN)2(PPh3)4(μ3-Se)2(μ-PbSe)]}2·1.3en (A) and [K([2.2.2]crypt)][Rh3(PPh3)6(μ3-Se)2]·3C6H6 (B) were produced. In A, a {μ-PbSe} unit bridges two of the rhodium atoms, hence representing the second heaviest CO homologue, and the heaviest one known to date [10]. By means of quantum chemical investigations, A was shown to be energetically preferred over a (hypothetic) species with an analogous {μ-CO} bridge. We have been interested in the possibility to generate an example with the corresponding {μ-PbTe} unit, thus the heaviest non-radioactive CO homologue, and therefore carried out an analogous reaction with a corresponding telluride precursor. The synthesis of A indeed also affords the formation of a Chevrel-type cluster compound, [Rh6Se8(PPh3)6]·0.5en (C), as the main reaction product. C has been shown to possess a mixed-valence character [11]. It was thus not unexpected, that in a search for appropriate reaction conditions for the formation of a tellurium homologue of A, the formation of the tellurium homologue of C, namely [Rh6Te8(PPh3)6]·4C6H6 (1) was observed. So far, this compound remained the only identifiable reaction product, which let us conclude that the tellurium homologue of A is less stable than the selenium compound. Herein, we would like to report on compound 1, which represents the first molecular Chevrel-type compound with an [Rh6Te8] core.

Upon layering of a solution of [K(18-crown-6)]2[Pb2Te3] [12] and KCN in en by a saturated benzene solution of [Rh(PPh3)3Cl], a few crystals of 1 were obtained as black blocks. The telluridoplumbate was prepared according to ref. [12], KCN was added to support the formation of the tellurium homologue of A, as reported for the synthesis of the selenium compound. Although the target (minor) component could not be isolated from the reaction mixture, and 1 is not formed in the absence of KCN, we attribute the result to a templating effect of the (coordinating) CN ions.

Compound 1 crystallizes in the triclinic space group P1̅ with two formulae units in the unit cell. Two crystallographically independent Chevrel-type units are present with an inversion center in each of the [Rh6Te8] cores (see Fig. 1). Rh atoms form a [Rh6] octahedron with all faces capped by Te atoms. Each Rh atom is additionally coordinated by one PPh3 ligand. 1 is thus closely related to C, but different types and numbers of solvent molecules exclude an isostructural situation. In accordance with the interatomic distances observed in C (Rh···Rh 3.1069(6)–3.1326(7) Å, Se–Rh 2.4363(7)–2.4990(7) Å, Rh–P 2.2044(15)–2.2118(16) Å, Se···Se 3.3964(9)–3.4383(10) Å), the interatomic distances are within a very narrow range regarding the covalent bonds (Rh–Te 2.5949(8)–2.6367(8); Rh–P 2.204(2)–2.215(2) Å), and vary in a relatively small range for the non-bonding cluster edges (Rh···Rh 3.2831(11)–3.4540(10) Å, Te···Te 3.4996(9)–3.6665(8) Å). The range of the latter is larger than observed for the selenium homologue, which is associated with the fact that two individual clusters are found in the crystal structure of 1, with slightly different distortions of the respective cluster cores, while C comprises only one, rather symmetric, cluster individual.

Fig. 1: Fragment of the crystal structure of 1 (left) and molecular structure of one of the two independent clusters, with the [Rh6] octahedron emphasized in polyhedral representation (right). Ellipsoids are drawn at the 50 % probability level, C and P atoms are drawn in wire mode. H atoms and solvate benzene molecules are omitted for clarity. Selected structural parameters (Å, deg): Rh···Rh 3.2831(11)–3.4540(10), Rh–Te 2.5949(8)–2.6367(8); Rh–P 2.204(2)–2.215(2), Te···Te 3.4996(9)–3.6665(8); Rh–Te–Rh 77.40(3)–82.47(3), P–Rh–Te 95.57(6)–110.01(7), Te–Rh–Te 83.86(3)–89.41(3).

Fig. 1:

Fragment of the crystal structure of 1 (left) and molecular structure of one of the two independent clusters, with the [Rh6] octahedron emphasized in polyhedral representation (right). Ellipsoids are drawn at the 50 % probability level, C and P atoms are drawn in wire mode. H atoms and solvate benzene molecules are omitted for clarity. Selected structural parameters (Å, deg): Rh···Rh 3.2831(11)–3.4540(10), Rh–Te 2.5949(8)–2.6367(8); Rh–P 2.204(2)–2.215(2), Te···Te 3.4996(9)–3.6665(8); Rh–Te–Rh 77.40(3)–82.47(3), P–Rh–Te 95.57(6)–110.01(7), Te–Rh–Te 83.86(3)–89.41(3).

The electronic structure of 1 was investigated by means of DFT calculations and subsequent Mulliken population analysis [13] of the DFT wave function. The study was carried out with the Turbomole program system [14] (see Experimental section). The results reveal a configuration similar to that found in C: for the three crystallographically independent Rh atoms, one calculates Mulliken charges of +0.270, +0.414, and +0.463, respectively (Fig. 2, top left), and the Mulliken charges at the Te atoms range from –0.344 to –0.517. As the absolute values of the charges do not reflect formal oxidation states, one has to consider relative values, which in this case possess a 1:1.5 or 1:1.7 ratio for the different Rh sites. This is in accordance with a 2:3 ratio of formal oxidation states and therefore corresponds to a mixed valence character of the cluster that formally comprises four times Rh(III) and two times Rh(II). Eight Te(–II) ligands care for an overall charge neutrality. However, despite formally different charges at the Rh atoms, all calculated bond lengths are within a narrow range (Rh–Te 2.65–2.67 Å), in excellent agreement with the experimentally found values (see above and Fig. 1), hence suggesting strong charge delocalization over the 14-atom cluster core, such as found for C. The highest occupied molecular orbital (HOMO) and subsequent orbitals with lower energy are located at Te atoms (Fig. 2, right) which corresponds to both, intuition and the formal –2 charge of the Te atoms. Finally, as found in C, localized molecular orbitals indicate a (very) weak Rh–Rh interaction (Fig. 2, bottom left).

Fig. 2: Representation of the core [Rh6Te8] unit with Mulliken charges for Rh ions (top left), representation of HOMO (black/white), HOMO-1, and HOMO-2 (both light/dark gray; right), and representation of weak Rh···Rh interactions from localized orbitals (bottom left). Note that all representations are aligned in the same direction of view and depict the structure as obtained by means of quantum chemical calculations. The organic periphery (left) or H atoms (right) are omitted for clarity. Orbital contours apply a threshold of ±0.033 a.u.

Fig. 2:

Representation of the core [Rh6Te8] unit with Mulliken charges for Rh ions (top left), representation of HOMO (black/white), HOMO-1, and HOMO-2 (both light/dark gray; right), and representation of weak Rh···Rh interactions from localized orbitals (bottom left). Note that all representations are aligned in the same direction of view and depict the structure as obtained by means of quantum chemical calculations. The organic periphery (left) or H atoms (right) are omitted for clarity. Orbital contours apply a threshold of ±0.033 a.u.

3 Conclusion

In conclusion, we have presented the synthesis, crystal structure and electronic configuration of the first compound with a molecular Chevrel-type [Rh6Te8] cluster core, which is both, structurally and electronically closely related to its lighter congener with μ3-Se ligands C. Further attempts towards the synthesis of a corresponding tellurium analog of A are in progress.

4 Experimental section

4.1 General

All manipulations were performed using standard Schlenk or glovebox techniques under exclusion of light. Solvent ethan-1,2-diamine was freshly distilled from CaH2, benzene from Na/K. [Rh(PPh3)3Cl] and KCN (both Sigma Aldrich) were evacuated in dynamic vacuum to a final pressure of p < 1 × 10−3 mbar for 12 h, and stored under argon until use. EDX measurements of 1 confirmed the absence of further metal species, hence excluded the formation of traces of a {μ-PbTe}-bridged species. However, due to the instantaneous decomposition during the sample preparation, no reliable Rh:P:Te ratio could be obtained. NMR investigations were hampered by low yields and mass analysis was prevented by an extreme sensitivity towards light of the dilute solutions.

4.2 Synthesis of 1

10 mL of a solution of [K(18-crown-6)]2[Pb2Te3] in ethane-1,2-diamine, prepared as reported previously [12], was added to 5 mg (0.077 mmol) of KCN, and the mixture was stirred for 10 min. The resulting solution was layered with 10 mL of a saturated solution of [Rh(PPh3)3Cl] in benzene. 1 crystallizes after 3 weeks as a few black blocks, along with amorphous powders and polytellurides [15].

4.3 Crystal structure determination

Single crystal X-ray diffraction data for black, block-shaped crystals (0.07 × 0.03 × 0.02 mm3) of C132H114P6Rh6Te8 (1; Mr = 3524.32 g·mol−1): triclinic, space group P1̅ (no. 2), unit cell parameters at 100(2) K: a = 14.8811(4), b = 17.8674(5), c = 22.2199(7) Å, α = 80.438(2), β = 86.323(2), γ = 85.188(2)°, Z = 2, ρcalc = 2.02 g·cm−3, μ = 2.9 mm−1. MoKα radiation, graphite monochromator (λ = 0.71073 Å), imaging plate detector Stoe IPDS2, θ = 1.375–26.757°, F000 = 3364.0 e, F000′ = 3344.15 e, hkl range: ±18, ±22, –24 → 28; 60843 measured reflections, 24 507 unique reflections (Rint = 0.0933), 15 199 reflections with I > 2 σ(I). Numerical absorption correction including shape optimization with STOE X-Area, Tmin/Tmax = 0.834/0.965. The structure was solved by Direct Methods in WinGX [16] and Olex2 [17] and refined by full-matrix least-squares against F2 in Shelxl-2015 [18, 19]. 1250 refined parameters, 0 restraints, final R and Rw values 0.0870 and 0.1339 (full data set), 0.0538 and 0.1234 (for reflections with I > 2 σ(I)). Used weighting scheme: w = 1/[σ2(Fo2) + (0.0702 P)2] where P = (Fo2 + 2Fc2)/3. Residual electron density (max / min): 1.406/–1.717 eÅ3.

Due to the sensitivities (see above), the selection of suitable crystals for analysis was difficult, resulting in a threefold twinned species as the best choice. Thus the resolution of the refinement is insufficient to correctly model the fourfold disorder (rotational and inclinational) of solvate benzene molecules. The structure of the Chevrel-type compound could unambiguously be elucidated and verified by quantum chemical calculations (see above).

CCDC 1439331 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

4.4 Details on quantum chemical calculations

Calculations were done with the program system Turbomole [14], using the RI approximation [20], the BP86 functional [21, 22], and def2-TZVP basis sets [23, 24] with effective core potentials (ECP-28) for Rh [25] and Te [26] atoms.

Acknowledgments

We thank the Deutsche Forschungsgemeinschaft (DFG) within the framework of SPP 1415 for financial support.

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Received: 2015-11-29
Accepted: 2015-12-19
Published Online: 2016-3-8
Published in Print: 2016-5-1

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