Potassium–telluroether interactions: structural characterisation and computational analysis

Abstract

Dissolution of the potassium complex [K(ATe₂^Tripp2)(dme)₂] (1-Te) in THF, layering with hexanes, and cooling to −30 °C afforded X-ray quality crystals of [K(ATe₂^Tripp2)(THF)₃] (2-Te). The K–TeR₂ distances in 2-Te are substantially shorter than those in 1-Te, and DFT and QTAIM calculations support the presence of K–TeR₂ interactions, providing the first unambiguous examples of s-block–telluroether bonding. Attempts to prepare bulk quantities of 2-Te afforded [K(ATe₂^Tripp2)(THF)₂] (3-Te), and further drying yielded [K(ATe₂^Tripp2)(THF)] (4-Te) and [K(ATe₂^Tripp2)]_x (5-Te). The selenium analogues of 2-Te, 3-Te and 4-Te (2-Se, 3-Se and 4-Se), were also prepared, and 2-Te, 2-Se, 3-Se and 5-Te were crystallographically characterised.

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In the chemistry of hard electropositive metal ions, soft donor ligands have proven valuable for the synthesis of luminescent zero-field single-molecule magnets (SMMs),¹ as a means to promote FLP reactivity,² for preferential complexation of actinide versus lanthanide elements with potential applications in nuclear fuel reprocessing,^3–11 as ligands in metal-containing CVD precursors,^12,13 and to access high nuclearity clusters.¹⁴ Soft donor ligands can also offer benefits in electropositive metal catalysis. For example, neodymium complexes with a pendent phosphine sulfide were substantially more active isoprene polymerization catalysts than phosphine oxide analogues,¹⁵ and group 4 complexes of ligands incorporating phosphine or thioether donors exhibited far higher ethylene polymerization activity than analogues incorporating ether or quinoline donors.¹⁶ Also, in a more general sense, integration of both hard and soft donors into multidentate ligand frameworks has been shown to be a powerful strategy to access electropositive metal complexes with unique properties and reactivity stemming from an atypical electronic environment,^16–24 and an enhanced understanding of the scope and nature of hard metal–soft donor interactions can further these applications.

Interactions between s-block metals and telluroether ligands push the boundaries of hard–soft mismatch, but unambiguous examples of such interactions have thus far proven elusive. For example, the [18]aneO₄Te₂ (1,4,10,13-tetraoxa-7,16-ditellura-cyclooctadecane) macrocycle failed to react with MI₂ (M = Ca or Sr), whereas analogous reactions with [18]aneO₄Se₂ afforded [MI₂([18]aneO₄Se₂)] (M = Ca and Sr).²⁵ Additionally, while the selenoether-ligated alkaline earth dications [M([18]aneO₄Se₂)(MeCN)₂][BAr^F₄]₂ (M = Mg, Ca, Sr), [Ba([18]aneO₄Se₂)(acacH)(MeCN)][BAr^F₄]₂, [Sr(H₂O)₃([18]ane-O₄Se₂)]I₂ and [Mg(κ³-[18]aneO₄Se₂)(H₂O)₂(MeCN)][BAr^F₄]₂²⁶ and the group 1 selenoether complexes [M([18]ane-O₄Se₂)][B{C₆H₃(CF₃)₂-3,5}₄] (M = Na and K)²⁷ have been reported, telluroether analogues are unknown. More broadly, telluroether complexes of electropositive lanthanide or actinide elements are also unknown.

Recently, we reported the lithium and potassium complexes [{Li(ASe₂^Ph2)}₂] and [K(ASe₂^Ar2)(dme)₂] {ASe₂^Ar2; 4,5-bis(arylselenido)-2,7,9,9-tetramethylacridanide; Ar = phenyl or 2,4,6-triisopropylphenyl (1-Se)}, which feature unique or uncommon s-block metal–selenoether interactions.^28,29 The ASe₂^Ar2 ligand in these compounds is a monoanionic SeNSe-donor pincer ligand which encourages κ³-coordination by direct attachment of the selenium donors to a rigid acridanide ligand backbone. We also reported the telluroether analogue, [K(ATe₂^Tripp2)(dme)₂] (1-Te).²⁸ However, the K–TeR₂ distances in the solid-state structure of this compound are approximately 0.39 Å longer than those in the selenoether analogue, even though the covalent radius of tellurium is only 0.18 Å larger than that of selenium.³⁰ Furthermore, DFT and QTAIM calculations on a model of [K(ATe₂^Tripp2)(dme)₂] in which the K–Te distances are constrained to crystallographic values did not yield K–Te bond critical points (BCPs), and other computational metrics suggested minimal interaction between K and Te. Therefore, although a shallow potential energy surface may allow K–Te interactions to form in solution, the solid-state structure of [K(ATe₂^Tripp2)(dme)₂] cannot be considered to feature significant K–TeR₂ interactions, and unambiguous examples of s-block–telluroether compounds remain elusive.

Herein, we report the synthesis and solid-state structure of the THF-coordinated analogue of 1-Te, [K(ATe₂^Tripp2)(THF)₃] (2-Te), featuring K–Te distances that are substantially shorter (by ∼0.3 Å) than those in the dme analogue, and quantum chemical calculations which confirm K–Te bonding in 2-Te. Analogues of 2-Te in which potassium is coordinated to 2, 1 or 0 equivalents of THF, and selenoether analogues of these complexes (where potassium is coordinated to 3, 2 or 1 equivalents of THF) are also reported.

Dissolution of dme-coordinated [K(ATe₂^Tripp2)(dme)₂] (1-Te) in THF, layering with hexanes and cooling to −30 °C overnight furnished yellow block-shaped X-ray quality crystals of [K(ATe₂^Tripp2)(THF)₃] (2-Te); Scheme 1. In the solid-state, potassium is κ³TeNTe-coordinated to the ATe₂^Tripp2 ligand as well as three molecules of THF, affording a distorted octahedral geometry (Fig. 1).^31,32 The K–O distances range from 2.584(5) to 2.693(5) Å, and the K–N distance of 2.824(4) Å is comparable to that found in the X-ray structure of [K(ATe₂^Tripp2)(dme)₂] (2.842(3) Å).²⁸ Most interestingly, the K–Te distances in 2-Te are 3.496(2) and 3.639(2) Å, which are 0.312 and 0.277 Å shorter than those in the dme analogue (see Table 1). The substantial difference in the K–Te distances in 1-Te and 2-Te is likely due to a shallow potential energy surface that is readily influenced by crystal packing forces.

Scheme 1 Syntheses of potassium telluroether and selenoether complexes.

Fig. 1 X-ray crystal structure of [K(ATe₂^Tripp2)(THF)₃] (2-Te). One part of a 50 : 50 two-part THF backbone disorder (associated with the THF containing O(2)) is shown. Hydrogen atoms are omitted for clarity. Ellipsoids are drawn at 50% probability.

Table 1 Tabulated K–E (E = Te or Se), K–N and K–O bond distances in the X-ray crystal structures of 1-Te, 2-Te, 5-Te, 1-Se, 2-Se, and 3-Se

Complex	K–E distances (E = Te or Se) (Å)	K–N distances (Å)	K–O distances (Å)
a Two independent molecules are present in the asymmetric unit.
1-Te ^28	3.808(1), 3.916(1)	2.842(3)	2.660(3)–2.865(3)
2-Te	3.496(2), 3.639(2)	2.824(4)	2.584(5)–2.693(5)
5-Te	3.517(4), 3.677(4), 3.680(4)	2.76(1), 2.81(1)	n.a.
1-Se ^28	3.339(2), 3.419(2), 3.484(2), 3.633(2)^a	2.801(4), 2.840(3)^a	2.701(3)–3.13(1)^a
2-Se	3.397(4), 3.472(4)	2.82(1)	2.67(1)–2.79(1)
3-Se	3.347(5), 3.466(5)	2.72(2)	2.70(2), 2.73(2)

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Drying samples of 2-Te under argon, or under vacuum for 10 minutes resulted in loss of one equivalent of THF to afford [K(ATe₂^Tripp2)(THF)₂] (3-Te; Scheme 1), as determined by ¹H NMR integration and combustion elemental analysis. Compound 3-Te was also isolated by deprotonation of H[ATe₂^Tripp2] using KCH₂Ph in THF, followed by evaporation to dryness in vacuo. Further loss of THF from 3-Te was observed after longer exposure (an additional 60 minutes) of solid samples to vacuum, affording [K(ATe₂^Tripp2)(THF)] (4-Te; Scheme 1). Moreover, repetitive dissolution of 4-Te in benzene and removal of volatiles in vacuo afforded THF-free [K(ATe₂^Tripp2)]_x (5-Te; Scheme 1). An X-ray quality crystal of 5-Te was obtained by layering an o-difluorobenzene solution of 4-Te with pentane and cooling to −30 °C for 1 month. In the solid state, 5-Te (Fig. 2) is a 1-dimensional coordination polymer in which potassium bridges between ATe₂^Tripp2 ligands. The K–N distances are 2.76(1) and 2.81(1) Å, and there are three short (3.517(4)–3.680(4) Å) K–Te distances which are only slightly longer than those in 2-Te (vide supra). There is also one longer K–Te distance (K(1)–Te(2) = 4.265(4) Å) that is outside of the range for a K–Te interaction. Interestingly, despite the polymeric structure of 5-Te in the solid state, it is soluble in benzene, indicating that the 1D-chains can easily be disrupted (presumably to form monomers in which potassium is stabilized through interactions with benzene and/or flanking hydrocarbon groups).

Fig. 2 X-ray crystal structure of [K(ATe₂^Tripp2)]_x (5-Te). A two-monomer segment of the 1D polymeric structure is shown. Aryl substituents are shown in wireframe and hydrogen atoms and isopropyl groups are omitted for clarity. Ellipsoids are drawn at 50% probability.

Attempts were also made to prepare a selenoether analogue of 2-Te by dissolving [K(ASe₂^Tripp2)(dme)₂] (1-Se) in THF, layering with hexanes, and cooling to −30 °C. This afforded yellow plate-like crystals, several of which were analyzed. One of these crystals could successfully be modelled as [K(ASe₂^Tripp2)(THF)₃]·hexane (2-Se·hexane; Fig. S1, ESI†), whereas another could be modelled as [K(ASe₂^Tripp2)(THF)₂] (3-Se; Fig. 3). However, both crystals, which share the same P2₁/c space group with very similar unit cell a and b axis dimensions,‡ show significant diffuse scattering along the c axis, suggestive of incommensurate structures³³ resulting from intergrowth of 2-Se·hexane with 3-Se.§ As a result, the R-factors are high (17–20%) and C–C bond precision is relatively low (>0.02 Å). Nevertheless, the standard deviations for the K–Se, K–O and K–N distances are sufficiently low to permit meaningful discussion.

Fig. 3 X-ray crystal structure of [K(ASe₂^Tripp2)(THF)₂] (3-Se; with diffuse scattering along the c-axis suggesting intergrowth of 3-Se (major) with 2-Se·hexane (minor). Hydrogen atoms are omitted for clarity. Ellipsoids are drawn at 50% probability.

Potassium is distorted octahedral in 2-Se, and distorted square pyramidal (vacant octahedral) in 3-Se.³² The K–N distances in 2-Se and 3-Se are unremarkable at 2.82(1) and 2.72(2) Å, respectively, and the K–O distances are 2.67(1)–2.79(1) Å in 2-Se and 2.70(2) and 2.73(2) Å in 3-Se. The K–Se distances in 2-Se and 3-Se are similar, at 3.397(4) and 3.472(4) Å in the former, and 3.347(5) and 3.466(5) Å in the latter, and the average K–Se distances of 3.435(4) and 3.407(5) Å in these compounds, respectively, are only slightly shorter than that for dme-coordinated 1-Se (3.469(2) Å; see Table 1).²⁸ It is also notable that the average K–Se distance in 2-Se (3.435(4) Å) is 0.133 Å shorter than the average K–Te distance in 2-Te (3.568(2) Å), which is less than the difference in the covalent radii of selenium and tellurium (0.18 Å).³⁰

Pure 3-Se was obtained by drying samples of 2-Se/3-Se under argon or in vacuo for 10 minutes. However, as observed for the telluroether analogue, additional exposure of 3-Se to vacuum resulted in further loss of THF, affording [K(ASe₂^Tripp2)(THF)] (4-Se; Scheme 1).¶

The ¹H and ¹³C{¹H} NMR spectra of 3-Te–5-Te, 3-Se and 4-Se in C₆D₆ display ligand-based resonances indicative of ligand top-bottom and side-to-side symmetry on the NMR timescale, with chemical shifts that are nearly identical (Δδ¹H < 0.06 ppm, Δδ¹³C < 0.03 ppm) to those of the bis-dme analogues (1-Te or 1-Se). Similarly, the ¹²⁵Te NMR chemical shifts of 3-Te, 4-Te and 5-Te, and the ⁷⁷Se NMR chemical shifts of 3-Se and 4-Se, are within ∼1 ppm of the dme analogues.²⁸ It is also notable that the ¹H and ¹³C NMR signals for THF in compounds 3–4 in C₆D₆ are only very slightly shifted relative to free THF (Δδ¹H < 0.03 ppm, Δδ¹³C < 0.09 ppm), suggestive of substantial (or complete) THF dissociation in solution. This contrasts the situation for 1-Te and 1-Se, wherein notable shifts in the dme ¹H NMR (Δδ 0.13–0.19 ppm) and ¹³C NMR (Δδ 0.02–0.41 ppm) resonances were observed in C₆D₆.²⁸

Quantum chemical calculations (ADF, gas-phase, all-electron, PBE, D3-BJ, TZ2P, ZORA) were carried out to confirm the presence of K–E interactions in 2-Te and 2-Se. These calculations were performed on models of 2-Te and 2-Se in which the 2,4,6-triisopropylphenyl groups have been replaced by 2,6-diisopropylphenyl groups: [K(ATe₂^Dipp2)(THF)₃] (2-Te*) and [K(ASe₂^Dipp2)(THF)₃] (2-Se*). Relative to the solid-state structures, one of the K–Te distances in 2-Te* is overestimated by 0.09 Å while the other is within 0.001 Å of the crystallographic value, and the K–Se distances in 2-Se* are within 0.04 Å of those in 2-Se. The K–E (E = Te or Se) Mayer bond orders in 2-Te* and 2-Se* are 0.08–0.10 and 0.07–0.08, respectively, supporting the presence of K–ER₂ bonding in both complexes, with minimal covalent contributions. Furthermore, Quantum Theory of Atoms in Molecules (QTAIM) bond critical points (BCPs) were located between potassium and both chalcogen donors in 2-Te* and 2-Se*. Small positive values of the total energy density of Cramer and Kraka at the BCP (H_b; 0.0010 au in 2-Te*; 0.0013 au in 2-Se*) and low bond delocalization index (δ) values (0.0678–0.0736 in 2-Te*; 0.0671–0.0673 in 2-Se*) are consistent with primarily electrostatic bonding. Additionally, NBO analysis revealed metal orbital contributions of less than 1.0% in the chalcogen-based NLMO (natural localized molecular orbital) lone pairs in 2-Te* and 2-Se*, consistent with predominantly electrostatic bonding.

In summary, the s-block–chalcogenoether complexes [K(ATe₂^Tripp2)(THF)_x] (x = 0–3) and [K(ASe₂^Tripp2)(THF)_x] (x = 1–3) have been synthesized, and DFT and QTAIM calculations on [K(AE₂^Dipp2)(THF)₃] (E = Te or Se) confirmed the presence of K–ER₂ bonding, with primarily ionic character. [K(ATe₂^Tripp2)(THF)₃] is the first unambiguous example of an s-block telluroether complex, and the K–TeR₂ interactions in this work will provide a valuable point of comparison for other electropositive metal–TeR₂ interactions, such as those involving early transition metals or f-elements.

D. J. H. E. thanks NSERC of Canada for a Discovery Grant, and N. A. G. G. thanks the Government of Ontario for an Ontario Graduate Scholarship (OGS). We are also grateful to Dr Jeffrey S. Price for assistance with X-ray crystallography, and Dr Ignacio Vargas-Baca for helpful discussions on quantum chemical calculations.

Data availability

Data supporting this article is included in the ESI.† Crystallographic data for 2-Te, 2-Se, 3-Se and 5-Te has been deposited at the CCDC with deposition numbers 2408834–2408837, respectively.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2408834–2408837. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4cc06539e

‡ The a, b and c unit cell dimensions are 9.312(5), 18.550(9) and 35.565(16) Å, respectively, in the structure consisting primarily of 2-Se·hexane, and 9.342(4), 18.461(7) and 29.140(8) Å in the structure consisting primarily of 3-Se.

§ Attempts to prepare X-ray quality single crystals of pure 3-Se by dissolving 3-Se in toluene or o-difluorobenzene, layering with hexanes and cooling to −30 °C were unsuccessful.

¶ Attempts to prepare (a) pure [K(ASe₂^Tripp2)(THF)₂] (3-Se) by dissolving [K(ASe₂^Tripp2)(dme)₂] (1-Se) in THF followed by evaporation of the volatiles (×3), or (b) [K(ASe₂^Tripp2)]_x by dissolving [K(ASe₂^Tripp2)(THF)] (4-Se) in benzene followed by evaporation of the volatiles (×2) consistently led to mixtures of the target products (3-Se or [K(ASe₂^Tripp2)]_x) and pro-ligand in an approximate 1 : 0.4 ratio (Fig. S17 and S18, ESI†). Therefore, these reactions were not pursued further.

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