An intramolecular phosphine-oxide stabilized germanium(IV) di-cation with enhanced Lewis acidity and catalytic applications

Abstract

This work reports the intramolecular phosphine-oxide stabilized tetra-coordinated germanium(IV) di-cation on an acenaphthene platform, 1^iPrPO. Computational study shows that the positive charges and the acceptor orbitals are localized on the Ge site. 1^iPrPO is Lewis super acidic, capable of catalysing hydrodefluorination reaction. 1^iPrPO also catalyses hydrosilylation of electron-deficient aldehydes.

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Germanium compounds are less popular as Lewis acids compared to those of silicon¹ analogues due to their inherent higher electronegativity. Contemporary research has shown an upsurge in developing Ge-based Lewis acid catalysts² because of their relatively higher stability and thus handling ease. Prominent examples are the bis(perchlorocatecholato) germane and its derivatives^3,4 exhibiting Lewis superacidity and high catalytic aptitude. Geometric constraint was introduced for Lewis acidity enhancement as observed in the anti-van’t Hoff-Le Bel reactivity of the calix[4]pyrrolato-germane.⁵ Significant increment in the Lewis acidity was noticed by installing positive charges on the Ge centre in the corrole-stabilized planar Ge(IV) mono-cation.⁶ The key factor behind the unusually high electrophilicity is the localization of the cationic charge on the Ge centre. There are a few more examples of germylium ions being used as Lewis acid catalysts.^7–9

Very recently, our group has established the first example of a tetra-coordinated Ge(IV) di-cation as a Lewis acid catalyst (Fig. 1A).¹⁰ The catalytic proficiency of this intramolecular phosphine-stabilized Ge(IV) di-cation was compromised due to the dispersion of the di-positive charges. Therefore, we attempted to localize the acceptor orbitals on the Ge centre of the di-cationic system. A recent study has shown that phosphine oxide serves as an efficient donor towards the cationic sites without significantly hampering their electrophilicity (Fig. 1B and C).^11–14 Inspired by these reports, in our continued efforts for the further advancement of Ge(IV) di-cationic Lewis acids, we have prepared the intramolecular phosphine oxide stabilized Ge(IV) di-cationic compound 1^iPrPO (Fig. 1D), exhibiting an expanded catalytic portfolio. The detail is reported herein.

Fig. 1 (A) 1^iPrP; (B) phosphine-oxide stabilized triaryl carbenium ion; (C) phosphine-oxide stabilized Sb(V) di-cation; and (D) 1^iPrPO (this work).

The intramolecular phosphine-oxide stabilized Ge(IV) di-cationic compound on an acenaphthene platform, 1^iPrPO, was prepared by the simple oxidation of our previously reported compound 1^iPr,¹⁰ with two equivalents of iodosobenzene in dichloromethane (Scheme 1). Colourless single crystals of 1^iPrPO were grown from dichloromethane/pentane layering under room temperature conditions in 75% crystallization yield. Compound 1^iPrPO was characterized in the solution state by multi-nuclear NMR spectroscopy in CD₃CN (Fig. S1–S4, ESI†). The ³¹P{¹H} NMR chemical shifts at +97.2 ppm (in CD₃CN) and +97.6 (in CD₂Cl₂) were assigned to the phosphorus(V) centre present in 1^iPrPO. The similar ³¹P{¹H} NMR chemical shift values obtained invalidate the possible coordination of solvents at the Ge centre.¹⁰ The solid-state structure of 1^iPrPO (Fig. 2 and Table S4, ESI†) shows a spirocyclic geometry with the Ge atom being shared between the two rings. The two triflate (CF₃SO₃ = OTf) counter anions are non-coordinating in nature (closest Ge–O(OTf) contact being 4.3 Å).¹⁵ The average Ge–C bond length in 1^iPrPO (avg. 1.89 Å) is shorter than that observed in the case of 1^iPr (avg. 1.92 Å).¹⁰ The average Ge–O bond length of 1.80 Å lies in the longer range of Ge–O covalent bonds for tetracoordinated Ge(IV) compounds.¹⁶ Correspondingly, the average P–O bond lengths (1.57 Å) exhibit partial double bond character.

Scheme 1 Lewis base binding and hydride and fluoride ion abstraction (see the ESI,† for detailed spectroscopic and experimental procedures of each reaction).

Fig. 2 Molecular structure of 1^iPrPO in the solid state (thermal ellipsoid 35%, H atoms, triflate counter anions and solvent molecules are omitted for clarity). Selected bond lengths [Å] Ge1–O1 = 1.803(2), Ge1–C1 = 1.895(3), P1–O1 = 1.575(2), Ge1–O2 = 1.797(3), Ge1–C2 = 1.891(3), P2–O2 = 1.570(2); selected bond angles [°]: O1–Ge1–C1 = 103.9(1), O2–Ge1–C2 = 104.1(1), O1–Ge1–O2 = 105.1(1), C1–Ge1–C2 = 121.9(1).

The optimized geometry of 1^iPrPO is in close agreement with the X-ray parameters (see ESI,† for the computational detail). The natural bond orbital (NBO) analyses of the optimized structure reveal donor–acceptor interactions between the lone pair orbitals on the oxygen atoms (from phosphine oxide) to the Ge-centred acceptor orbitals (Table S1, ESI†). The NBO frontier molecular orbitals depicted in Fig. 3A show that the acceptor orbitals are localized on the Ge centre, unlike our previously reported 1^iPrP where the acceptor orbitals comprised Ge–C and Ge–P σ* orbitals.¹⁰ The Wiberg bond order (WBO) calculated for Ge–C (0.79) confirms the single bond between them, while the WBO for Ge–O (0.46) echoes the presence of comparatively weaker bonding interaction between them. The low value of WBO for P–O (0.81) reflects less than double bond character. The electrostatic potential map as shown in Fig. 3B reflects the concentration of the positive charges on the Ge centre in 1^iPrPO, as opposed to the dispersed di-positive charges over the P–Ge–P framework found in 1^iPrP. Overall, compared to our earlier report on 1^iPr, our newly synthesized compound 1^iPrPO has di-cationic charges and orbital vacancies localized^6,17 on the Ge-centre.

Fig. 3 (A) Natural bond orbitals localized on the Ge center; (B) electrostatic potential map with the range of 0.05 (red)–0.40 (blue).

The effective Lewis acidity¹⁸ of 1^iPrPO following the Gutmann–Beckett (GB) method was investigated (see ESI,† for details). A Δδ³¹P value of 25.4 ppm was obtained from the addition of 0.2 equivalents of Et₃PO to 1^iPrPO in CD₃CN (δ for free Et₃PO in CD₃CN is +49.7 ppm; for comparison Δδ³¹P for 1^iPrP = 21.3 ppm) (Fig. S7–S9, ESI†). The formation of the mono-adduct 1^iPrPO·Et₃PO was confirmed from the ³¹P NMR study (δ^iPrPO at +75.57 ppm and δEt₃PO at +75.08 ppm) (Fig. S10–S12, ESI†). The mono-adduct 1^iPrPO·Et₃PO remained in equilibrium with 1^iPrPO, as revealed from the detailed NMR study (Fig. S12, ESI†). We were not successful in obtaining single crystals of the adduct 1^iPrPO·Et₃PO. Rather, single crystals of the adduct formed with 4-N,N′-dimethylaminopyridine (DMAP) 1^iPrPO·DMAP (Scheme 1) were obtained. The molecular structure determined showed the coordination of the Lewis base leading to a distorted trigonal bipyramidal geometry at the Ge centre (Fig. S17, ESI†).

We have prepared 1^iPrPO-F from the addition of one equivalent of KF/18-crown-6 to 1^iPrPO (Scheme 1). The ³¹P{¹H} NMR in CDCl₃ showed a doublet at +77 ppm (³J_P–F = 3.3 Hz) and a corresponding peak at −125.2 ppm in the ¹⁹F{¹H} NMR spectrum (see ESI,† for detail). The molecular structure of 1^iPrPO-F (Fig. 4A and Table S6, ESI†) exhibits a distorted trigonal bipyramidal geometry, analogous to 1^iPrPO·DMAP. The Ge–F bond length is 1.746(2) Å, which is shorter compared to that found in the case of 1^iPrP-F (1.785(2) Å).¹⁰ The Ge–O bond lengths have increased (avg. Ge–O = 1.97 Å) compared to those in 1^iPrPO, with a corresponding decrease in the P–O bond lengths (avg. P–O = 1.53 Å). The calculated gas-phase fluoride ion affinity (FIA)¹⁹ at the Ge site of 1^iPrPO gave a very high value of 865 kJ mol⁻¹ (gas-phase FIA for reference SbF₅ is 497 kJ mol⁻¹) (Tables S2 and S3, see the ESI,† details). Incorporation of the acetonitrile (MeCN) solvated model decreased the calculated FIA value significantly to 142 kJ mol⁻¹ (solvent corrected FIA for reference SbF₅ is 315 kJ mol⁻¹), respectively. This phenomenon of solvent damping is more pronounced with the cationic Lewis acids compared to the neutral ones.^20,21 Nonetheless, 1^iPrPO was found to be capable of abstracting a fluoride ion from AgSbF₆ under heating conditions forming 1^iPrPO-F (Scheme 1). 1^iPrPO also abstracted fluoride from TBABF₄ (TBA = n-tetrabutylammonium), which is a better fluoride ion donor, to give 1^iPrPO-F under room temperature conditions. Thus, 1^iPrPO proved to be Lewis superacidic^22,23 under experimental conditions, despite low calculated values of FIA in solvated models.²⁴

Fig. 4 Molecular structures of (A) 1^iPrPO-F and (B) 1^iPrPO-H in the solid state (thermal ellipsoids 35%, H atoms, triflate counter anions and solvent molecules are omitted for clarity). Selected bond lengths [Å] (a) Ge1–F1 = 1.746(2), Ge1–O1 = 1.971(2), P1–O1 = 1.524(2), Ge1–O2 = 1.990(2), P2–O2 = 1.525(2); (b) Ge1–H1 = 1.436(2), Ge1–O1 = 2.013(2), P1–O1 = 1.520(2), Ge1–O2 = 2.056(2), P2–O2 = 1.517(2).

Gas-phase hydride ion affinity (HIA)¹⁹ calculation (Tables S2 and S3, see ESI† for detail) gave a very high value of 925 kJ mol⁻¹, while incorporation of the solvated model gave low calculated values of 185 KJ mol⁻¹ (MeCN). Notably, the gas phase HIA for reference B(C₆F₅)₃ is 517 kJ mol⁻¹ and the solvent corrected HIA is 244 kJ mol⁻¹ for MeCN. Reaction of 1^iPrPO with NaBH₄ led to the formation of 1^iPrPO-H (Scheme 1). The ³¹P{¹H} NMR spectrum of 1^iPrPO-H displayed a peak at +73.5 ppm. The molecular structure of 1^iPrPO-H closely resembles that of 1^iPrPO-F, possessing a Ge–H bond length of 1.436(2) Å (Fig. 4B and Table S7, ESI†). The inherently hydridophilic Ge center in 1^iPrPO activated the Et₃Si–H bond under room temperature reaction conditions to give 1^iPrPO-H (Scheme 1).^25,26 However, 1^iPrPO does not abstract hydride from Ph₃Si–H²⁷ even upon heating.

Given the experimentally observed Lewis super acidic nature of 1^iPrPO, we have explored the catalytic hydrodefluorination²⁸ of an aliphatic C–F bond. With a catalyst loading of 5 mol% 1^iPrPO and employing Ph₃SiH as the hydride source, we have successfully achieved the hydrodefluorination of 1-adamantyl fluoride (76% conversion) in CD₃CN upon heating overnight at 75 °C (Scheme 2A) (see ESI,† for detail). The in situ catalytic reaction mixture showed NMR signals at δ³¹P{¹H} = +77.8 ppm and δ¹⁹F{¹H} = −125.8 ppm corresponding to the formation of 1^iPrPO-F, indicating the C–F bond activation over the Si–H bond activation (Fig. S40–S45, ESI†). Thus, we have proposed a catalytic pathway (Scheme S1, ESI†) involving the generation of the carbocation, which then abstracts hydride from Ph₃SiH to form the alkane along with the regeneration of the catalyst 1^iPrPO and Ph₃SiF. We do not observe any Ge–H bond formation as an intermediate, discarding the Ph₃Si–H bond activation pathway by 1^iPrPO. This proposed mechanism is in line with that proposed by Müller et al. using the naphthalene-based digermyl hydronium borates as a catalyst.⁷ On the other hand, using Et₃SiH as the hydride source for the same catalytic hydrodefluorination reaction (Scheme 2A) turned out to be less promising owing to the competing Si–H bond activation along with C–F bond activation by 1^iPrPO. Only 40% conversion to the corresponding alkane was possible, the catalyst being transformed into 1^iPrPO-H after 12 hours (Fig. S48–S51, ESI†). Notably, catalytic hydrodefluorination was unachievable using 1^iPrP as the catalyst. As a matter of fact, there are very few reports on catalytic hydrodefluorination reactions achieved using molecular germanium as a catalyst.^3,7,8,17 Our catalyst was ineffective for other C–F bonds. This lack of reactivity may be attributed to the stabilization of the Ge(IV) by the intramolecular P=O donors, thereby compromising the Lewis acidity at the Ge(IV) site.

Scheme 2 Catalytic applications of 1^iPrPO: (A) hydrodefluorination of 1-adamantyl fluoride and (B) hydrosilylation of electron-deficient aromatic aldehydes.

The hydridophilicity of 1^iPrPO prompted us to explore the substrate scope for the catalytic hydrosilylation reactions (Scheme 2B) of aromatic aldehydes (see ESI,† for detail). In our earlier report, we have shown the catalytic hydrosilylation of p-methyl benzaldehyde using 1^iPrP as a catalyst. However, catalytic hydrosilylation of electron-deficient aromatic aldehydes was not possible using 1^iPrP. In this study, we were successful in converting a variety of electron-deficient aldehydes into the corresponding silyl ethers using 5 mol% of 1^iPrPO under mild heating conditions (Scheme 2B).²⁹

It was observed from the NMR study that the aldehydes did not bind to the Ge-centre of the catalyst. The NMR investigations of the catalytic reaction mixture showed the formation of a Ge–H bond (δ³¹P{¹H} = +73.5 ppm) from Et₃Si–H bond activation, corresponding to the formation of 1^iPrPO-H as an intermediate (Fig. S53, see ESI,† for the NMRs). Therefore, the reaction is likely to proceed by Et₃SiH bond activation by 1^iPrPO followed by Si–H addition across the C=O bond in the carbonyls (Scheme S2, see ESI†). A similar mechanism was observed in the case of 1^iPrP as a catalyst. The ²⁹Si{¹H} NMR spectrum showed the formation of the hydrosilylated products (Fig. S59, ESI†). The formation of Et₃SiOTf as a competent catalyst, though not detected in the NMR spectrum, cannot be completely overruled. The irreversible transformation of the catalyst to 1^iPrPO-H was observed from in situ NMR study of the catalytic reaction mixture, which might be responsible for poor catalytic outcome in certain substrates (see ESI,† for detail).

In conclusion, we have successfully manipulated the frontier orbitals by treating 1^iPrP with an oxidant to form the phosphine-oxide stabilized Ge(IV) di-cation 1^iPrPO. Localization of the positive charges and acceptor orbitals on the Ge-centre has significantly enhanced the Lewis acidity. The well-exposed cationic Ge(IV) site is now capable of abstracting fluoride ions from a hexafluoroantimonate anion, thereby marking its Lewis super acidic nature. The hard and soft Lewis acidic nature of 1^iPrPO have been manifested in catalytic hydrodefluorination of 1-adamantyl fluoride and hydrosilylation of electron-deficient aromatic aldehydes, which were unachievable with our previously reported 1^iPrP. Thus, the localization of the cation charges on the Ge-centre has led to the expansion of the catalytic portfolio. Our group is engaged in investigating further catalytic applications of 1^iPrPO, and extending this catalyst design strategy to other main-group Lewis acids.

M. M. thanks SERB India SPF/2022/000046 and CRG/2022/000673 for financial support. A. K. thanks UGC for the fellowship. We thank Prof. Deepak Chopra from the Indian Institute of Science Education and Research (IISER) Bhopal, India for his help in crystallography.

Data availability

Experimental details, spectroscopic data, X-ray diffraction analysis data, and computational data are available in the ESI.†

Conflicts of interest

There is no conflict to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2416600, 2416604, 2416606 and 2416610. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5cc00250h

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