Combining Solid-state and Solution-based Techniques: Synthesis and Reactivity of Chalcogenidoplumbates(II or IV)

Summary

The synthesis of chalcogenidoplumbates(II,IV) via the in situ reduction of nominal "PbCh₂" (Ch = Chalcogen) and via a solid-state reaction and subsequent solvothermal reactions is presented. Additionally, reactivities of plumbate(II) solutions are portrayed, which yield the heaviest-known CO homolog known to date: the µ-PbSe ligand.

Abstract

The phases of "PbCh₂" (Ch = Se, Te) are obtained from solid-state syntheses (i.e., by the fusion of the elements under inert conditions in silica glass ampules). Reduction of such phases by elemental alkaline metals in amines affords crystalline chalcogenidoplumbate(II) salts comprised of [PbTe₃]^2- or [Pb₂Ch₃]^2- anions, depending upon which sequestering agent for the cations is present: crown ethers, like 18-crown-6, or cryptands, like [2.2.2]crypt. Reactions of solutions of such anions with transition-metal compounds yield (poly-)chalcogenide anions or transition-metal chalcogenide clusters, including one with a µ-PbSe ligand (i.e., the heaviest-known CO homolog).

In contrast, the solid-state synthesis of a phase of the nominal composition "K₂PbSe₂" by successive reactions of the elements and by the subsequent solvothermal treatment in amines yields the first non-oxide/halide inorganic lead(IV) compound: a salt of the ortho-selenidoplumbate(IV) anion [PbSe₄]^4-. This was unexpected due to the redox potentials of Pb(IV) and Se(-II). Such methods can further be applied to other elemental combinations, leading to the formation of solutions with binary [HgTe₂]^2- or [BiSe₃]^3- anions, or to large-scale syntheses of K₂Hg₂Se₃ or K₃BiSe₃ via the solid-state route.

All compounds are characterized by single-crystal X-ray diffraction and elemental analysis; solutions of plumbate salts can be investigated by ²⁰⁵Pb and ⁷⁷Se or ¹²⁷Te NMR techniques. Quantum chemical calculations using density functional theory methods enable energy comparisons. They further allow for insights into the electronic configuration and thus, the bonding situation. Molecular Rh-containing Chevrel-type compounds were found to exhibit delocalized mixed valence, whereas similar telluridopalladate anions are electron-precise; the cluster with the µ-PbSe ligand is energetically favored over a hypothetical CO analog, in line with the unsuccessful attempt at its synthesis. The stability of formal Pb(IV) within the [PbSe₄]^4- anion is mainly due to a suitable stabilization within the crystal lattice.

Introduction

Metal chalcogenides, such as SnSe or CuInSe, are versatile materials with a wide range of applications, for instance, as semiconductor, thermoelectric, or nonlinear optic materials^1-6. Similar elemental compositions are found within chalcogenidometalates, where the metal is in a formally positive oxidation state and coordinated by negative (poly-)chalcogenide ligands to yield an overall anionic species. Different from the abovementioned materials, such metalates are additionally comprised of counter-ions, which are well separated from the anionic substructure. Typical cations are (solvated) alkali or alkaline earth metals, ammonium, or phosphonium ions. Most often, such salts with chalcogenidometalate anions have physical properties that are similar to their parental binary or ternary compounds, such as similar band gaps or photo- and semiconductivity properties. However, due to the broad range of possible anionic architectures within each elemental combination, ranging from isolated molecular species through strands and sheets of interconnected anions to extended three-dimensional frameworks, an even finer tuning of various properties can be achieved, ultimately aiming at the designed synthesis of compounds with the desired properties. Within the concept of dimensional reduction, it has been shown that a relative increase of counter ions per formula unit, which accompanies a reduction from 3D via 2D and 1D to 0D anionic architectures (0D representing molecular species), decreases the observed band gap⁷. Moreover, by the utilization of different (or mixtures of) chalcogenide ligands, it is even possible to achieve an ultra-fine adjustment of the band gap^8,9.

Apart from these practical applications and visionary relevancies, chalcogenidometalates are still investigated for fundamental understanding, such as for the generation of novel anionic structure types or the discovery and interpretation of an unusual bonding, as well as for their unprecedented properties. Whereas the lighter congeners (i.e., oxidometalates, commonly referred to as oxometalates) have been extensively studied, in particular for potential catalytic applications, the heavier chalcogenidometalates are far less explored.

Our own interest has been focused on the synthesis, properties, and further reactivity of chalcogenidotetrelates (i.e., the heavier homologs of silicates)^10,11. There is a broad variety of such compounds, ranging from water-stable and soluble binary anions, such as the [SnTe₄]^4- anion¹²; to organic, functionalized, and multinary cluster compounds, such as {[Ir₃(cod)₃(µ₃-S)₂](µ₃-S)SnCl}₂ (cod = cycloocta-1,5-diene)¹³. Our most recent studies deal with chalcogenidoplumbates, with lead as the central metal atom(s). In line with the inert-pair concept for heavy atoms, addressing the stabilization of the 6s orbital due to relativistic effects, lead is usually observed in the formal +II oxidation state. Exceptions like PbO₂ are strong oxidizing agents, and the heavier lead(IV) chalcogenides, "PbCh₂," have not been discovered to date¹⁴. The same holds for the chalcogenidoplumbate(IV) anions, of which only [PbO₄]^4- has been reported¹⁵ until recently (see below).

Apart from a diverse group of structurally investigated oxidoplumbates(II,IV), there have been only few examples of chalcogenidoplumbates(II), namely [PbTe₃]^4-, with a trigonal pyramidal anion¹⁶; and [Pb₂Ch₃]^2-, where Ch = Se or Te, with a trigonal bipyramidal anion¹⁷. These are synthesized by a route that has also been applied for the generation of Zintl ions¹⁸. Upon preparation of multinary intermetallic phases by fusion of the elements at high temperatures, subsequent extraction by solvents in the presence of a sequestering agent affords the desired products in (single-)crystalline form. In the case of the [Pb₂Ch₃]^2- anions, for instance, a phase of the nominal composition "KPbCh" has been extracted with 1,2-diaminoethane (en) in the presence of 4,7,13,16,21,24-Hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane ([2.2.2]crypt). The cryptand is necessary both for crystallization upon increase of the effective cation radius in the {K[2.2.2]crypt}⁺ complex counter ion, to better match the anionic size, and for a shielding of the positive charge that suppresses an electron back-donation from the anion in solution. Such salts with encapsulated cations usually reveal high tendencies for crystallization and thus, fairly good yields when compared to the corresponding salts without sequestration agents. However, a rather cumbersome synthesis or the high prices of cryptands prevent the excessive scaling of such approaches.

In contrast, K₄[PbTe₃]·2en is synthesized via in situ reduction in solution, as has already been used as early as 1891 for the generation of the famous Pb₉^4- anion^19,20. For the latter, elemental alkaline metals were added to suspensions of lead in liquid ammonia at low temperatures, whereas for the telluridoplumbate, an alloy of the nominal composition "PbTe₂" was reduced at room temperature, again by the addition of elemental potassium.

Our first approach towards such metalate species to be presented herein is a combination of both pathways. Here, solid-state synthesis is followed by either reduction in solution in the presence of inexpensive sequestering agents, such as 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6), or via reduction with alkaline metals that are chelated by the solvent itself, without the need for additional sequestering agents, similar to the synthesis of [Na₄(en)₇][Sn₉]²¹. Our second approach also starts with high-temperature synthesis, but it is followed by solvothermal extraction of the resulting phases (i.e., extraction at elevated temperatures and pressures)²². In the following, we will present both synthetic approaches and some of our recent results upon application of these reaction pathways.

Protocol

Caution: Always be cautious when working with chemicals. Apply common safety precautions, including the appropriate utilization of gloves, goggles, and a lab coat at all times. In particular, be aware that all discussed compounds containing heavy elements, as well as their elemental sources, are of high toxicity. 1,2-diaminoethane is a corrosive liquid. Alkaline metals and ternary solid-state products may react pyrophorically with air and moisture.

Note: All manipulations are performed in an argon atmosphere using standard Schlenk or glovebox techniques under strict exclusion of air and external moisture. Solids or solutions containing heavy element metalate species or precursors are stored under the exclusion of light by wrapping the respective containers with aluminum foil for inhibition of a light-induced decomposition.

1. Preparation of Solvents and Solutions

1. Add 1 L of freshly purchased 1,2-diaminoethane to 25 g of CaH₂ and stir overnight. Reflux (T_b = 116 °C) until no H₂ is generated (approximately 12 h).
   1. Distill at ambient pressure.
2. Add 1 L of oxolane (THF) to 10 g of NaK alloy and stir overnight. Reflux (T_b = 66 °C) for at least 12 h. Distill at ambient pressure.
3. Make a saturated solution of [Rh(PPh₃)₃Cl] by adding 150 mg of [Rh(PPh₃)₃Cl] to 10 mL of THF. Stir overnight at room temperature (RT) and filtrate with an inert gas filter frit of low porosity.

2. High-temperature Solid-state Reactions

1. Synthesis of PbSe₂
   1. Place 3.81 g of elemental Se in a borosilicate ampule and add 5 g of elemental Pb on top. Heat it with an oxygen/methane burner until optical homogeneity of the melt is achieved (approximately 10 min). Knock the ampule gently with a cork ring throughout the synthesis to detach sublimed Se from the ampule wall, which will then drop back into the reaction mixture.
   2. Allow the ampule to cool down to room temperature. Break the ampule with a pestle in a mortar and manually remove all remaining splinters of the ampule. Pestle the crude PbSe₂ thoroughly.
2. Synthesis of K₂PbSe₂
   1. Place 0.95 g of elemental K and 5 g of elemental Pb in a thick-walled borosilicate ampule. Slowly increase the heat with an oxygen/methane burner until optical homogeneity of the melt is achieved (approximately 20 min).
   2. Carefully add 1.9 g of elemental Se pellets to the molten alloy. Upon the complete addition, increase the temperature until the reaction mixture emits bright yellow/white radiation (approximately 10 min) and hold the temperature for 10 min. Decrease the reaction temperature slightly if the radiation color turns to pure, bright white (a temperature close to the melting point of the ampule).
   3. Allow the reaction mixture to cool down to RT. Break the ampule and manually remove all remaining splinters of the ampule and a regulus of elemental lead. Pestle the crude K₂PbSe₂ thoroughly.

3. In Situ Reduction

1. Synthesis of a solution of [K(18-crown-6)]₂[Pb₂Se₃]
   1. Place 2 g of PbSe₂, 3.1 g of 18-crown-6, 250 mL of 1,2-diaminoethane, and a large stir bar in a round-bottom N₂-flask on a stir plate. Stir rigorously at RT and slowly add 0.45 g of elemental K.
   2. Stir overnight at RT and filter the solution with an inert gas filter frit of low porosity (pore diameters: D3, 16-40 µm or D4, 10-16 µm).

4. Solvothermal Reactions

1. Synthesis of K₄[PbSe₄]·en·NH₃
   1. Place 0.5 g of K₂PbSe₂ and 2 mL of 1,2-diaminoethane in a 10-mL glass vial in a 15-mL polytetrafluorethylene vial in a standard stainless-steel autoclave. Close the autoclave tightly and it heat in an oven to 150 °C for 5 days.
   2. Turn off the oven and leave it for 1 d to slowly cool to RT. Transfer the reaction mixture into paratone oil and manually select crystals of K₄[PbSe₄]·en·NH₃ under a standard light microscope at 15-40X magnification.

5. Reactive Layering

1. Synthesis of [(RhPPh₃)₆(µ₃-Se)₈]·0.5en
   1. Place 10 mL of a solution of [K(18-crown-6)]₂[Pb₂Se₃] in a 50-mL flask, add 10 mL of a saturated solution of [Rh(PPh₃)₃Cl] in THF, and stir overnight.
   2. Filter the reaction solution with an inert gas filter frit of low porosity and remove the solvent under dynamic vacuum slowly during 24 h. Transfer the crude reaction product into paratone oil and manually select crystals of [(RhPPh₃)₆(µ₃-Se)₈]·0.5en under a standard light microscope at 15-40X magnification.
2. Synthesis of {[K(18-crown-6)]-[K(en)₂]K[Rh₃(CN)₂(PPh₃)₄(µ₃-Se)₂(µ-PbSe)]}₂·1.3en
   1. Place 10 mL of a solution of [K(18-crown-6)]₂[Pb₂Se₃] in a Schlenk tube and carefully layer it with 10 mL of a saturated solution of [Rh(PPh₃)₃Cl] in THF. Cover the Schlenk tube completely in aluminum foil and leave it undisturbed for 4 weeks.
   2. Transfer the resulting solid into paratone oil and select single crystals quickly under a light microscope.

6. Analysis of the Solutions and Compounds

1. Place 50 mg of "K₂PbSe₂" onto an acrylic glass sample carrier (the compound reacts with elemental Si) and cover it with tape. Place it under ambient conditions in a powder X-ray diffractometer (PXRD) and record the diffraction data within 1 h²³.
2. Place 0.6 mL of a solution of [K(18-crown-6)]₂[Pb₂Se₃] in a nuclear magnetic resonance (NMR) tube and thoroughly seal the latter with protective tape. Transfer it quickly into the NMR probe and record ⁷⁷Se and ²⁰⁵Pb NMR with at least 2,000 and 5,000 pulses, respectively²⁵.
3. Select a single crystal under a light microscope and mount it on the goniometer head of the diffractometer. Measure it with high redundancy to enable the adequate absorption corrections^23-33.
4. Perform a simultaneous optimization of the electronic and geometric structure of [Rh₃(CN)₂(PPh₃)₄(µ₃Se)₂(µPbSe)]^3-. Apply the conductor-like screening model (COSMO) with a 10% increase of the default radii to account for charge compensation²⁸.
   1. Calculate the vibrational frequencies to ensure the energetic minimum²⁸.
   2. Perform Mulliken and/or natural bond orbital (NBO) analyses based on the density functional theory (DFT) wave function to obtain the atomic charges²⁸.
   3. Calculate in-orbital contributions to the "relaxed" and the original structures of a) the complete cluster, b) the CO/PbSe-free cluster, and c) the CO/PbSe ligand, and compare the results²⁸.

Results

The existence of an ortho-selenidoplumbate anion [PbSe₄]^4-23 (see Figure 1, top right) has been confirmed by single crystal diffraction experiments, elemental analysis, and quantum chemical calculations. The crystal structure refinement confirms the almost-perfect tetrahedral coordination geometry, as would be expected for a lead(IV) ion, whereas DFT calculations rationalize the energetically stabilized a_1...

Discussion

The combination of classical high-temperature, solid-state reactions with solution-based methods allows for the generation and isolation of novel compounds that cannot be synthesized by only one of these pathways. Even though, in most cases, a clear identification and full characterization of the intermediate species is difficult or essentially impossible, the general idea is straightforward and can be applied to a variety of elemental combinations. Furthermore, the actual synthetic conditions for the generation of one s...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Ethan-1,2-diamine	Sigma-Aldrich	E26266-2.5L
Calcium hydride	Sigma-Aldrich	213268-100G
Tetrahydrofuran	Sigma-Aldrich	401757-1L
Sodium	Sigma-Aldrich	71172-1KG
Potassium	Sigma-Aldrich	244864-50G
Tris-triphenylphosphine rhodium chloride	Sigma-Aldrich	199982-5G
Lead	Acros	222625000
Selenium	Sigma-Aldrich	209643-50G
18-crown-6	Acros	181561000