A diarsagermylene and a diarsastannylene stabilised by arene⋯Ge/Sn interactions

Abstract

The synthesis and structures of two new diarsatetrylenes {(Dipp)₂As}₂E are presented [E = Ge, Sn; Dipp = 2,6-diisopropylphenyl]. The high barrier to planarisation of As prevents stabilisation by As–E π-interactions; however, arene⋯Ge/Sn interactions stabilise these compounds by up to 181.4 kJ mol⁻¹. This represents a new stabilisation mode for this class of compounds.

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Currently, there is intense interest in low oxidation state main group compounds, particularly with regard to their ability to activate small molecules such as H₂, CO₂ and NH₃.^1,2 This reactivity has been compared to that of transition metal complexes and, consequently, low oxidation state main group compounds have been posited as inexpensive and earth abundant alternatives to precious metal catalysts.³

One of the simplest, and most versatile, classes of low oxidation state main group compounds is the tetrylenes R₂E (E = Si, Ge, Sn, Pb). These electron-deficient compounds possess both a lone pair at the tetrel centre and a vacant p-orbital which lies perpendicular to the plane of the molecule and which act as the donor and acceptor orbitals, respectively, in small molecule activations. The energy difference between these donor and acceptor orbitals plays a key role in the reactivity of tetrylenes and, as a consequence, new methods to modulate this frontier orbital energy gap are of great interest. This energy gap is significantly increased by adjacent heteroatoms (e.g. N, O, S), which stabilise the tetrel lone pair through σ-withdrawing effects and destabilise the acceptor orbital through π-interactions between the vacant p-orbital at the tetrel centre and the heteroatom lone pairs, resulting in lower reactivity.

The chemistry of diaminotetrylenes, (R₂N)₂E (E = Si, Ge, Sn, Pb), the heavier group 14 element analogues of N-heterocyclic carbenes and their acyclic analogues, is well established.⁴ In contrast, far less is known about their phosphorus- and arsenic-substituted homologues, i.e. diphosphatetrylenes, (R₂P)₂E, and diarsatetrylenes (R₂As)₂E.^5–8 Indeed, only three arsatetrylenes have been crystallographically authenticated (1–3), and, of these, compound 3 is the only example of a monomeric diarsatetrylene possessing a two-coordinate tetrel centre.^6a

The low number of reported diphospha- and diarsatetrylenes may, at least in part, be attributed to the high barriers to planarisation of the heavier pnictogens, which prevents stabilisation of the electron-deficient tetrel centre through P–E/As–E π-interactions. Given the foregoing it is perhaps unsurprising that the arsenic centres in 3 adopt a pyramidal configuration with no evidence for As–Sn π-interactions.^6a Nonetheless, it has been calculated that the heavier group 15 elements are as good π-donors as nitrogen if this barrier can be overcome.⁹

The barrier to planarisation of the heavier pnictogens may be lowered by several strategies, including (i) the use of sterically demanding substituents, (ii) the presence of electropositive substituents directly adjacent to the tetrel centre, and (iii) incorporation of the pnictogen centre into a ring.^10,11 In this regard, we recently reported the synthesis, solid state structures and dynamic behaviour of the unusual diphosphatetrylenes 4, in which the phosphorus centres both possess sterically demanding substituents and lie directly adjacent to an electropositive germanium or tin centre^12,13 Compounds 4 adopt a configuration with one planar and one pyramidal phosphorus centre, with a single, stabilising P–E π-interaction.

Based on our successful synthesis of P–E π-stabilised diphosphatetrylenes, we conjectured that the steric and electronic properties of our ligands might be sufficient to enable the synthesis of a similarly stabilised diarsatetrylene. However, we anticipated that the higher barrier to planarisation of the arsenic centres in the target compounds compared to the corresponding phosphorus centres in 4 might have a significant impact on any propensity for As–E π-interactions.¹⁴ We also anticipated that this, combined with the lower electronegativity of arsenic in comparison to phosphorus, would lead to a smaller frontier orbital gap in the resulting diarsatetrylenes and a consequently enhanced reactivity.

The reaction between amine-protected (iPr₂N)AsCl₂ and two equivalents of DippLi(OEt₂), followed by two equivalents of ethereal HCl gives the secondary chloroarsane (Dipp)₂AsCl (5) (Scheme 1) [Dipp = 2,6-iPr₂C₆H₃]. Treatment of 5 with LiAlH₄ gives the secondary arsane (Dipp)₂AsH (6) in excellent yield. Compound 6 is readily metalated by nBuLi in THF to give the sterically hindered arsanide [(Dipp)₂As]Li(THF)_2.75(Et₂O)_0.25 (7), after recrystallisation from diethyl ether, as a yellow crystalline solid; the constitution of 7 was confirmed by ¹H and ¹³C{¹H} NMR spectroscopy and X-ray crystallography (see ESI†).

Scheme 1 Synthesis of compounds 5–8.

The reaction between two equivalents of in situ generated 7 and GeCl₂(1,4-dioxane) in cold THF gives the diarsagermylene {(Dipp)₂As}₂Ge·PhMe (8Ge) as dark red prisms after crystallisation from toluene (Scheme 1). A similar reaction between 7 and SnCl₂ gives the diarsastannylene {(Dipp)₂As}₂Sn·PhMe (8Sn) as red-purple blocks. Compounds 8Ge and 8Sn are sensitive to air and moisture and solutions of these compounds decompose slowly at room temperature, or more rapidly on exposure to ambient light; in the solid state both 8Ge and 8Sn are reasonably stable at room temperature and can be manipulated without significant decomposition when exposure to light is kept to a minimum.

Compounds 8Ge and 8Sn are isostructural and isomorphous; each compound crystallises as a discrete molecular species with a molecule of toluene in the asymmetric unit (Fig. 1). Compound 8Ge represents the first diarsagermylene to be crystallographically characterised, while 8Sn is only the second monomeric diarsastannylene to be structurally authenticated.

Fig. 1 Molecular structure of 8Ge with H atoms and solvent of crystallisation omitted for clarity [compound 8Sn is isostructural]. Selected bond lengths (Å) and angles (°): 8Ge As(1)–Ge(1) 2.4851(4), As(2)–Ge(1) 2.4771(4), As(1)–C(1) 2.020(3), As(1)–C(13) 2.000(3), As(2)–C(37) 2.023(3), AS(2)–C(37) 2.001(3), Ge(1)⋯C(1) 2.666(3), Ge(1)⋯C(25) 2.710(2), As(1)–Ge–As(2) 90.808(13), Ge(1)–As(1)–C(1) 71.75(7), Ge(1)–As(1)–C(13) 108.70(8), Ge(1)–As(2)–C(25) 73.28(7), Ge(1)–As(2)–C(37) 109.94(8); 8Sn As(1)–Sn(1) 2.6579(4), As(2)–Sn(1) 2.6644(4), As(1)–C(1) 1.994(3), As(1)–C(13) 2.020(3), As(2)–C(37) 1.995(3), AS(2)–C(37) 2.014(3), Sn(1)⋯C(1) 2.858(2), Sn(1)⋯C(37) 2.762(3), As(1)–Sn(1)–As(2) 91.004(11), Sn(1)–As(1)–C(1) 110.57(8), Sn(1)–As(1)–C(13) 73.91(7), Sn(1)–As(2)–C(25) 108.49(8), Sn(1)–As(2)–C(37) 70.79(8).

As expected, the As–Ge distances in 8Ge [2.4851(4) and 2.4771(4) Å] are significantly shorter than the corresponding distances in the tertiary arsane complexes {C₆H₄-1,2-(AsMe₂)₂}GeI₂ [2.6210(11) and 2.6313(11) Å] and [{C₆H₄-1,2-(AsMe₂)₂}GeCl][GeCl₃] [2.5847(5) Å],¹⁵ the only other reported examples of As–Ge(II) bond lengths. Examples of As–Sn(II) bonds are more numerous and the As–Sn distances in 8Sn [2.6579(4) and 2.6644(4) Å] are similar to the As–Sn distances in 3 [average 2.652(1) Å for the two crystallographically independent molecules in the asymmetric unit].^6a The As–E–As angles [90.808(13) and 91.0004(11)° for 8Ge and 8Sn, respectively] suggest negligible hybridisation of the tetrel centres in these compounds and this is supported by DFT calculations (see below); the As–Sn–As angle in 8Sn is slightly smaller than the corresponding angle in 3 [94.64(4)°].

In marked contrast to the phosphorus centres in 4, both arsenic centres in 8Ge and 8Sn adopt a pyramidal geometry [sum of angles at As for 8Ge = 285.86 and 287.76°; 8Sn = 285.96 and 289.58°], consistent with the high barrier to planarisation of arsenic compared to phosphorus.¹⁴ The pyramidal geometries of the arsenic centres necessarily preclude any stabilisation of the tetrel centres by As–Ge/Sn π-interactions. However, in both 8Ge and 8Sn, the ipso-carbon of one phenyl ring from each arsanide ligand lies in close proximity to the tetrel centre [Ge⋯C_ipso 2.710(2) and 2.666(3) Å; Sn⋯C_ipso 2.762(3) and 2.858(2) Å]. These distances are well within the sums of the van der Waals radii of Ge and C (3.81 Å) and of Sn and C (3.87 Å),¹⁶ suggesting a significant interaction between the electron density at these ipso-carbon atoms and the vacant p-orbital at the tetrel centre. Consistent with this, the short Ge/Sn⋯C_ipso distances are associated with rather acute C_ipso–As–Ge/Sn angles [8Ge 71.75(7) and 73.28(7)°; 8Sn 70.79(8) and 73.91(7)°].

Group 14 element⋯arene interactions have been observed in a small number of Si(IV)-containing cations,¹⁷ and in a limited number of compounds in which the tetrel centre is in the +2 oxidation state (e.g.9–12).^18–21 The Ge⋯C and Sn⋯C distances in 8Ge and 8Sn are similar to the corresponding distances in 11 [Ge⋯C 2.642(2) and 2.661(2) Å] and 12 [Sn⋯C 2.821(2) and 2.827(2) Å];²¹ for comparison, the closest Sn⋯C distances in 9 are 3.27 and 3.35 Å,¹⁹ suggesting a much weaker interaction in this case.

The room temperature ¹H NMR spectra of 8Ge and 8Sn consist of a single set of extremely broad signals due to the iPr and aromatic protons of the ligands, indicating dynamic exchange between the aromatic rings engaged in the E⋯C_ipso interactions and those that are not. At −60 °C the iPr peaks decoalesce into eight methyl signals (spanning the range 0.48–1.42 ppm for 8Ge and 0.51–1.52 ppm for 8Sn) and four methine signals (spanning the range 2.51–4.86 ppm for 8Ge and 2.80–4.93 ppm for 8Sn), consistent with the C₂ symmetry of the solid-state structures. The large spread of methyl and methine signals in these spectra is due to the proximity (or not) of these protons to the aromatic rings or the arsenic lone pairs of the ligands.

We noted previously that, due to their inherently broad nature, the ¹¹⁹Sn signals for 4Sn and its more substituted analogue {(2,4,6-iPr₃C₆H₂)₂P}₂Sn (13) could not be located at room temperature and only a very broad signal (FWHM ca 2000 Hz) could be observed for 13 even at −95 °C.^12,13 This line-broadening is likely to be exacerbated by the presence of two adjacent quadrupolar ⁷⁵As nuclei in 8Sn [⁷⁵As natural abundance 100%, I = 3/2], however, while we could not observe a signal at room temperature, in part due to the thermal instability of this compound in solution, at −60 °C we were able to locate a broad ¹¹⁹Sn signal for 8Sn at 547 ppm (FWHM 500 Hz). Although no ¹¹⁹Sn NMR data are available for 2 and 3,^6a,8b the ¹¹⁹Sn chemical shift of 8Sn is similar to that observed for 1 (475 and 671 ppm for the cis and trans isomers, respectively)^8a and 13 (440 ppm).¹⁵

The presence of significant Ge/Sn⋯C_ipso interactions in 8Ge and 8Sn is supported by DFT calculations. Geometry optimisations at the B97D/6-311G(2d,p) [LANL2DZ for Sn] level of theory give structures (8Ge_pyr and 8Sn_pyr) which correlate well with those obtained by X-ray crystallography (see Fig. 2 and ESI†). In particular, the calculated structures replicate the short E⋯C_ipso distances rather nicely [Ge⋯C_ipso 2.757/2.644 Å, Sn⋯C_ipso 2.849/2.848 Å].

Fig. 2 Optimised geometries for the pyramidal (8Ge_pyr and 8Sn_pyr) and planar (8Ge_plan and 8Sn_plan) forms of {(Dipp)₂As}₂E (E = Ge, Sn) [B97D/6-311G(2d,p) [LANL2DZ for Sn]].

The HOMO and LUMO for both 8Ge_pyr and 8Sn_pyr are largely based on the tetrel lone pair and the vacant p-orbital at the tetrel centre, respectively, although the LUMO has a significant component on the two nearby aromatic rings in each case (see ESI†). Natural Bond Orbital (NBO) analysis reveals that the tetrel lone pair in each case has >90% s-character, while the “vacant” orbital has essentially pure p-character. NBO analysis also reveals that this “vacant” orbital has an occupancy of 0.21 electrons in 8Ge_pyr and 0.18 electrons in 8Sn_pyr. This is associated with significant delocalisation of the As–C_ipso σ-bonding electron density into this vacant orbital. Second order perturbation theory analysis shows that the approximate stabilisations afforded by these interactions (the E(2) energies) are 77.2 and 65.8 kJ mol⁻¹ for 8Ge_pyr and 40.2 and 40.3 kJ mol⁻¹ for 8Sn_pyr. Consistent with this, the Wiberg Bond Indices (WBIs) for the Ge⋯C interactions are 0.114 and 0.099, while the WBIs for the Sn⋯C interactions are both 0.094. Although small, these WBIs indicate a significant bonding interaction in each case (cf. WBIs of 0.763 and 0.771 for the As–Ge bonds in 8Ge_pyr and 0.726 for the two As–Sn bonds in 8Sn_pyr). The foregoing clearly indicates that these C_ipso⋯E interactions stabilise the arsatetrylenes to a significant extent. In support of this, NBO deletions (in which the energy of the system is recalculated in the absence of the specified interactions) indicate that these interactions stabilise 8Ge_pyr and 8Sn_pyr by 181.4 and 106.2 kJ mol⁻¹, respectively.

For comparison, we have located minimum energy geometries for both 8Ge and 8Sn in which one of the arsenic centres is planar (i.e. the direct analogues of 4, possessing Ge/Sn–As π-interactions), 8Ge_plan and 8Sn_plan. These lie 18.0 and 32.1 kJ mol⁻¹, respectively, higher in free energy than the ground state forms 8Ge_pyr and 8Sn_pyr.

It has been suggested that the energy gap between the singlet and triplet states (Δ_S–T) of tetrylenes correlates well with their reactivities, with smaller gaps indicating higher reactivities.²² For 8Ge_pyr and 8Sn_pyrΔ_S–T is calculated to be 79.8 and 90.0 kJ mol⁻¹, respectively, suggesting that these compounds should exhibit interesting reactivities [for comparison, Δ_S–T for the prototypical N-heterocyclic germylene {CHNH}₂Ge has been calculated to be 195.0 kJ mol⁻¹,^22c consistent with the low reactivity of N-heterocyclic germylenes].

In summary, we have shown that the high barrier to planarisation of arsenic prevents the stabilisation of the diarsatetrylenes {(Dipp)₂As}₂E through As–E π-interactions. However, canting of the aromatic rings in these compounds leads to a significant interaction between the ipso carbon atoms of the rings and the tetrel centres which stabilises the tetrylenes by up to 181.4 kJ mol⁻¹.

This work was partly funded by the U.K. Engineering and Physical Sciences Research Council (EPSRC; Grant EP/L5048281).

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthesis and characterisation of all compounds. Details of the crystal structure of 7 and crystallographic details for 7, 8Ge, and 8Sn. NMR spectra for all compounds. Details of DFT studies. CCDC 1810821–1810823. Data supporting this publication is openly available under an ‘Open Data Commons Open Database License’. Additional metadata are available at: http://dx.doi.org/10.17634/154300-77. Please contact Newcastle Research Data Service at rdm@ncl.ac.uk for access instructions. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7cc09564c

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