Intramolecular donor-stabilized tetra-coordinated germanium(IV) di-cations and their Lewis acidic properties

Abstract

We report the first examples of intramolecular phosphine-stabilized tetra-coordinated germanium(IV) di-cationic compounds: [L^iPr₂Ge][CF₃SO₃]₂3^iPr and [L^Ph₂Ge][CF₃SO₃]₂3^Ph (L^iPr = 6-(diisopropylphosphanyl)-1,2-dihydroacenaphthylene-5-ide; L^Ph = 6-(diphenylphosphanyl)-1,2-dihydroacenaphthylene-5-ide). The step wise synthetic strategy involves the isolation of neutral and mono-cationic Ge(IV) precursors: [L^iPr₂GeCl][X] (X = GeCl₃1^iPr, OTf 2^iPr), [L^Ph₂GeCl₂] 1^Ph and [L^Ph₂GeCl][OTf] 2^Ph. Both 3^iPr and 3^Ph exhibit constrained spiro-geometry. DFT studies reveal the dispersion of di-cationic charges over P–Ge–P sites. Anion or Lewis base binding occurs at the Ge site resulting in relaxed distorted trigonal bipyramidal/tetrahedral geometry. 3^iPr and 3^Ph activate the Si–H bond initially at the P-site. The hydride ultimately migrates to the Ge-site rapidly giving [L^Ph₂GeH][CF₃SO₃] 3^PhH, while sluggishly forming [L^iPr₂GeH][CF₃SO₃] 3^iPrH. Compounds 3^iPr and 3^Ph were tested as catalysts for the hydrosilylation of aromatic aldehydes. While catalytic hydrosilylation proceeded via the initial Et₃Si–H bond activation in the case of 3^iPr, compound 3^Ph as a catalyst showed a masked Frustrated Lewis Pair (FLP) type reactivity in the catalytic cycle.

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Introduction

Germanium(II) di-cations stabilized by neutral Lewis bases have been known for long,¹ possessing inherent electrophilicity² and rarely exhibiting nucleophilic behaviour.³ In contrast there are only very few examples of Ge(IV) di-cations known, which are stabilized by hyper-coordination involving neutral donors: [LGeF₂]²⁺ (L = tris(1-ethyl-benzoimidazol-2-ylmethyl)amine) (Fig. 1A),⁴ [GeF₂(OTf)₂] (OTf = CF₃SO₃) coordinated by two mono-dentate phosphine donors or bi-dentate chelating phosphine donors (Fig. 1B),⁵ and [Me₂Ge(OTf)₂] coordinated by two 4-N,N′-dimethylaminopyridine (DMAP) units or a 2,2′-bipyridine (Fig. 1C).⁶ Alongside in silicon chemistry, terpyridine-stabilized silicon(IV) di-cations (R₂Si²⁺), tri-cations (RSi³⁺),⁷ and silicon tetrakis(trifluoromethanesulfonate)⁸ have been established as Lewis superacids⁹ in recent times. The uses of Ge(IV) di-cations as Lewis acids hold immense potential; however they are yet to be unfolded. In general, while scheming Lewis acidic Ge(IV) polycations, an optimal balance between the stabilization of the polycationic species by neutral donors and accessibility of vacant orbitals is necessary. Therefore, focused investigations on the stabilization of Ge(IV) polycations using suitable donor groups that circumvent hyper-coordination are imperative for their potential Lewis acidity. Furthermore, isolation of such species has remained a worthwhile synthetic target on fundamental grounds.

Fig. 1 Reported examples of Ge(IV) di-cations (A) (N4 = tris(1-ethyl-benzoimidazol-2-ylmethyl)amine) and (B) and (C) (N = 4-N,N′-dimethylaminopyridine); examples of Ge(IV)-based Lewis acids (D) and (E) and the intramolecular phosphine-stabilized Ge(IV) di-cations (F, this work).

As a matter of fact, compared to silicon analogues,¹⁰ there are scarce reports on Ge-based Lewis acids¹¹ both in their neutral or cationic forms. Greb et al. have pioneered neutral bis(perchlorocatecholato)germane as both hard and soft Lewis superacids (Fig. 1D).¹² Bis(catecholato)germane derivatives have been successfully implemented as Lewis acid catalysts.¹³ A significant example of the cationic form of Ge(IV) Lewis acid is [(TPFC)Ge(THF)₂]⁺ (TPFC = tris(pentafluorophenyl)corrole, THF = tetrahydrofuran) (Fig. 1E).¹⁴ Recently, a Ge(II) mono-cationic σ-donor ligand towards Ni(0) has been reported with simultaneous Lewis superacidity.¹⁵ It is worth mentioning that Gabbaï et al. have demonstrated a formally cationic Ge(IV) σ-acceptor site in a dinuclear Pt–Ge(IV) complex.¹⁶

Contemporary research in the field of main-group Lewis acids¹⁷ has introduced the concepts of geometrically constrained Lewis acids,¹⁸ main group Lewis acid/ligand assisted cooperative bond activation,¹⁹ hidden frustrated Lewis pair (FLP) chemistry²⁰ in intra-molecular Lewis base stabilized Lewis acidic fragments.²¹ Taking advantage of the enforced proximity in peri-substituted acenaphthenes,²² herein we have established the unprecedented intramolecularly phosphine-stabilized tetra-coordinated di-cationic Ge(IV) compounds. The aptitude of these geometrically constrained Ge(IV) di-cationic species as Lewis acids has been assessed based on experimental findings and computational analyses. Non-innocent roles of the phosphines have been observed. The proclivity of Ge(IV) di-cationic compounds for Et₃Si–H bond activation coupled with FLP-type reactivity has been realized in a proof-of-concept catalytic hydrosilylation of p-methyl benzaldehyde.

Results and discussion

Syntheses and characterization of intramolecular phosphine-stabilized Ge(IV) mono- and di-cationic compounds

Intramolecular phosphine-stabilized Ge(IV) di-cationic compounds 3^iPr and 3^Ph were obtained in a step-wise manner as shown in Scheme 1. Initially, L^iPr–Br and L^Ph–Br were synthesized and characterized (see the ESI† for Experimental procedures). Table 1 summarizes the NMR data for all compounds. The reaction between 1.5 equivalents of lithiated L^iPr–Br and GeCl₄ at −78 °C in a tetrahydrofuran (THF) medium, followed by extraction with dichloromethane (DCM), gave 1^iPr (Scheme 1a). Structural elucidation of the single crystals of 1^iPr obtained from acetonitrile (MeCN) reveals the formation of [L^iPr₂GeCl]⁺, interestingly possessing GeCl₃ as a counter anion (Fig. S95, ESI†). The formation of the GeCl₃ anion stems from the reducing ability of phosphines.²³ The mono-cationic compound 1^iPr was characterized by the ³¹P{¹H} NMR chemical shift at −11.1 ppm. An additional peak at −19.5 ppm was observed in the ³¹P{¹H} NMR spectrum of the crude reaction mixture arising from the corresponding P(V) oxidized product (Fig. S7, ESI†).²⁴ Using straightforward lithiated L^iPr–Br and GeCl₄ in a 2 : 1 ratio also led to the formation of 1^iPr. An anion exchange reaction of 1^iPr with one equivalent of trimethylsilyl trifluoromethane sulfonate (TMSOTf) followed by crystallization in DCM resulted in the isolation of 2^iPr (Scheme 1a). 2^iPr was characterized in the solution state using NMR spectroscopy displaying the characteristic ³¹P{¹H} NMR chemical shift at −10.8 ppm.

Scheme 1 Syntheses of the Ge(IV) mono- and di-cationic compounds (see the ESI† for the detailed spectroscopic and experimental procedures of each reaction).

Table 1 NMR data for all compounds

Compounds	^31P{^1H} in ppm (deuterated solvent)	^19F{^1H} in ppm (deuterated solvent)	J values (Hz)	Compounds	^31P{^1H} in ppm (deuterated solvent)	^19F{^1H} in ppm (deuterated solvent)	J values (Hz)
a Chemical shift values at room temperature. b Chemical shift values at low temperature. c In situ NMR. d Measured after 2 days. e Measured after one day.
L^iPrBr	−1.58^a (CDCl3)			3^iPrDMAP	+28.37 (3^iPr), −13.19 (P^iPr2)^b (CD2Cl2)
L^PhBr	−9.40^a (CDCl3)			3^Ph(DMAP)2	−29.16^b (CDCl3)	−79.21^a (CDCl3)
1^iPr	−11.06^a (CDCl3)			3^iPrF	−10.23(d)^a (CDCl3)	−78.12(OTf), −141.03(t)^a (CDCl3)	^2 J P–F = 184.9
1^Ph	−34.79^a (CDCl3)			3^PhF	−17.54(br.)^a, −10.27(m), −23.66(m) (CDCl3)	−150.32(t)^a (CDCl3)	^2 J P–F = 100.6^a, JP–P = 119.8, JP–F = 98.9^b
2^iPr	−10.77^a (CDCl3)	−78.08^a (CDCl3)		3^Ph + Ph3CCl^c	−6.31(3^Ph), −13.46(2^Ph)^a (CDCl3)
2^Ph	−13.20^a, −3.0(d), −20.5(d)^b (CD2Cl2)	−78.84^a (CD2Cl2)	^2 J P–P = 110^b	3^iPrH	−5.15^a (CD2Cl2)	−77.01^a (CD2Cl2)
3^iPr	+28.37^a (CD2Cl2), +26.95^a (CD3CN)	−78.84^a (CD2Cl2)		3^Ph + LiBEt3H^c	−19.07, −19.82, −20.06^a (CDCl3)	−78.35, −78.87^a (CDCl3)	^1H NMR: 6.40(t) (J = 20.2, Ge–H2)
3^Ph	−6.32^a (CDCl3)	−79.74^a (CDCl3)		3^iPr + Et3SiH^c^,^d	+28.51 (3^iPr), +20.28 (P–H), +12.16 (P^iPr2)^a (CD2Cl2)		^1 J P–H = 476
3^iPrOPEt3	+73.4 (bound Et3PO), −12.0 (P^iPr2)^a (CD2Cl2)			3^Ph + Et3SiH^c^,^e	−19.0^a (CDCl3)	−77.43^a (CDCl3)	^2 J P–H = 30.4
3^PhOPEt3	+71.59 (bound Et3PO), −6.30 (3^Ph), −13.38 (PPh2)^a (CDCl3)			3^Ph + aldehyde^c	36.02–32.08, −19.26^a (CDCl3)

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The molecular structure²⁵ of 1^iPr shows that the cationic part has an overall distorted trigonal bipyramidal (TBP) geometry with two pendant ⁱPr₂P groups occupying axial sites. Two Ge–C and one Ge–Cl bond constitute the trigonal plane with the sum of angles at the Ge atom close to 360°. The Ge1–P1 and Ge1–P2 bond lengths are 2.544(1) and 2.654(1) Å, respectively, which are longer than those of [GeF₃(Ph₂PCH₂CH₂PPh₂)(OTf)] (avg. Ge–P = 2.43 Å)⁵ and diaminodiphosphine stabilized bis(chlorogermyliumylidene) (avg. Ge–P = 2.44–2.50 Å).²⁶ The structural parameters of the cationic part in 2^iPr (Fig. 2a) are analogous to 1^iPr. The triflate counter anion in 2^iPr is located far away from the Ge cationic site (closest Ge–O contact being 5.7 Å).

Fig. 2 (a) Molecular structures of (a) 2^iPr and (b) 2^Ph (H atoms, solvent molecule and triflate anion have been removed for clarity; thermal ellipsoid 50%). Selected bond lengths [Å] and angles [°]: (a) Ge1–Cl1 = 2.228(2), Ge1–C1 = 1.988(4), Ge1–P1 = 2.643(1); C1–Ge1–C1 = 147.9(2), Cl1–Ge1–C1 = 106.1(1), Cl1–Ge1–P1 = 86.9(1), P1–Ge1–P1 = 173.9(1); (b) Ge1–C1 = 1.922(3), Ge1–C2 = 1.926(3), Ge1–Cl1 = 2.215(1), Ge1–P1 = 2.357(1); C1–Ge1–C2 120.0(1), C1–Ge1–Cl1 = 104.8(1), C2–Ge1–Cl1 = 104.2(1), C1–Ge1–P1 = 91.1(1), C2–Ge1–P1 = 135.5(1), Cl1–Ge1–P1 = 96.4(1).

The formation of 3^iPr can either proceed as a one-pot reaction between 1^iPr and two equivalents of TMSOTf in DCM or stepwise chloride abstraction from 1^iPrvia the intermediacy of 2^iPr (Scheme 1a). Colourless crystals of 3^iPr were obtained from the concentrated THF solution. The characteristic ³¹P{¹H} NMR chemical shift values obtained for 3^iPr in CD₃CN (+27.0 ppm) and CD₂Cl₂ (+28.4 ppm) are similar, which invalidates possible solvent coordination.

The solid-state structure of 3^iPr (Fig. 3a) shows a spirocyclic geometry^19a with the Ge atom at the nexus and having two non-coordinating triflate counter anions (closest Ge–O contact being 5.8 Å). Due to the presence of dipositive charges in 3^iPr, the Ge–C (avg. 1.92 Å) and Ge–P (avg. 2.36 Å) bond lengths are significantly shorter compared to those of 2^iPr. The strongly electron-donating nature of the two intramolecular ⁱPr₂P groups adequately stabilizes the Ge(IV) di-cation in 3^iPr in an overall tetra-coordinated environment.

Fig. 3 Molecular structures of (a) 3^iPr and (b) 3^Ph (H atoms, solvent molecules and triflate anions have been removed for clarity; thermal ellipsoid 50%). Selected bond lengths [Å] and angles [°]: (a) Ge1–C1 = 1.923(5), Ge1–C2 = 1.923(5), Ge1–P1 = 2.350(1), Ge1–P2 = 2.363(1); C1–Ge1–C2 = 112.5(2), P1–Ge1–P2 = 122.4(1), C1–Ge1–P1 = 90.9(2), C2–Ge1–P2 = 91.4(2), C1–Ge1–P2 = 120.8(2), C2–Ge1–P1 = 121.2(2); (b) Ge1–C1 = 1.924(3), Ge1–C2 = 1.920(3), P1–Ge1 = 2.344(1), Ge1–P2 = 2.338(1); C2–Ge1–C1 = 126.8(1), C2–Ge1–P2 = 91.5(1), C1–Ge1–P2 = 116.1(1), C2–Ge1–P1 = 115.7(1), C1–Ge1–P1 = 91.2(1), P2–Ge1–P1 = 118.1(1).

In the case of the diphenylphosphanyl donor, the neutral compound [L^Ph₂GeCl₂] 1^Ph could be obtained from the reaction between lithiated L^Ph–Br and GeCl₄ taken in a 2 : 1 ratio in a THF medium (Scheme 1b and Fig. S96, see the ESI† for Experimental procedures). Chloride abstractions from 1^Ph using one and two equivalents of TMSOTf in DCM led to the formation of mono-cationic 2^Ph and di-cationic 3^Ph compounds, respectively (Scheme 1b). The characteristic peaks in ³¹P{¹H} NMR spectra for 2^Ph and 3^Ph appear at −13.2 and −6.3 ppm respectively.

Unlike 2^iPr, the molecular structure of 2^Ph depicts a distorted tetrahedral geometry (Fig. 2b). Compared to 2^iPr, the Ge1–P1 bond length has noticeably reduced in 2^Ph (2.357(1) Å). Another Ge1⋯P2 bond distance in 2^Ph is 3.012(1) Å. The triflate anion is far away from the Ge atom (closest Ge–O contact being 7.2 Å). The low temperature ³¹P{¹H} NMR for 2^Ph echoes the presence of two inequivalent phosphines exhibiting chemical shift values at −3.0 and −20.5 ppm.

The solid-state structure of 3^Ph exhibits a spiro-geometry with similar bond parameters to those in 3^iPr (Fig. 3b). There are two non-coordinated triflate anions (closest Ge–O contact being 4.2 Å) present in the asymmetric unit. Both 3^iPr and 3^Ph in the solid-state show no sign of decomposition when exposed to air for a day.

The optimized geometries of 2o^iPr, 2o^Ph, 3o^iPr and 3o^Ph at the TPSS-D3(BJ)/def2-TZVPP level of theory are in close agreement with the X-ray parameters (see the ESI† for the detailed theoretical part). On par with the experimental findings, Natural Bond Orbital (NBO) analyses for 2o^iPr suggest significant donor–acceptor (D–A) interactions (avg. 125 kcal mol⁻¹) for and . However, dissimilar bonding situations in 2o^Ph between Ge1–P1 (covalent) and Ge1–P2 (D–A interaction) were confirmed by NBO and AIM (Atoms in Molecules) analyses. The calculated Wiberg bond index (WBI) of Ge1–P2 in 2o^Ph possesses dative nature with a value of 0.183 (0.712 for Ge1–P1). Additionally, no bonding orbital for Ge1–P2 was observed from the NBO analyses. Instead, only one D–A interaction is determined in 2o^Ph with a stabilization energy of 32 kcal mol⁻¹. Furthermore, we have performed AIM analyses. A negative Laplacian value at the BCPs (bond critical points) is associated with shared interactions, indicating covalent bonds, and positive Laplacian values reflect closed shell interactions such as ionic or dative bonds. In the case of 2o^Ph, the higher positive ∇²ρ(r) value on Ge–P2 shows dative bonding character.

The frontier molecular orbitals (FMOs) of 3o^iPr and 3o^Ph reflect similar contributions in terms of Ge–C and Ge–P σ* orbitals (Fig. 4a and b). The orbital scenario is analogous to those observed in the catecholato phosphonium ion^19a or stibonium salt.²⁷ There is no pure vacant p-orbital available²⁸ on the central Ge atom in both cases. WBI calculations predict the values of 0.804 (3o^iPr) and 0.770 (3o^Ph) for Ge–P bonds, displaying a mostly covalent bonding situation. The dispersion of positive charges among the P (+1.161, +1.145 for 3o^iPr; +1.174, +1.174 for 3o^Ph) and Ge (+1.136 for 3o^iPr; +1.157 for 3o^Ph) sites is evident from NBO partial charges and the electrostatic potential map analyses (Fig. 4c and d). Canonical forms describing the dispersion of positive charges in 3o^iPr and 3o^Ph (F) are depicted in Fig. 1.

Fig. 4 (a) Acceptor orbitals in 3o^iPr; (b) LUMO in 3o^Ph; electrostatic potential map for (c) 3o^iPr and (d) 3o^Ph.

Lewis acidic properties of Ge(IV) di-cationic compounds

We have studied the effective Lewis acidity of both 3^iPr and 3^Ph from induced ³¹P NMR shift of triethylphosphine oxide (Et₃PO) in the Gutmann–Beckett (GB) method (see the ESI† for details).²⁹ Et₃PO adducts were targeted by employing varying equivalents of Et₃PO (0.2 to 3.0 equivalents) in deuterated solvents. The addition of 0.2 equivalents of Et₃PO to 3^iPr gave numerous peaks at +75.7, +74.2, −12.3 and −18.2 ppm. The complexity of the ³¹P{¹H} NMR spectra obtained at room temperature impeded the Lewis acidity scaling of 3^iPr by the GB method. However, we have observed peaks appearing at +73.4 and −12.0 ppm in the ³¹P{¹H} NMR spectrum, when a 1 : 1 mixture of 3^iPr and Et₃PO was prepared in CD₂Cl₂ at −78 °C and subsequently raised to room temperature. This gave an induced ³¹P{¹H} NMR chemical shift of Et₃PO for 3^iPr (Δ³¹P = 21.3 ppm) with respect to the free Et₃PO (δ for free Et₃PO in the same solvent is 52.1 ppm).

In the case of 3^Ph, we have noticed the induced ³¹P{¹H} NMR chemical shift from the addition of 0.2 equivalents of Et₃PO in CDCl₃ (Δ³¹P = 21.6 ppm, δ for free Et₃PO in the same solvent is 50.0 ppm); without any complexity observed in the NMR spectrum at room temperature. These Δ³¹P values are close to the values reported for the Si(IV) di-cations (Δ³¹P = 23.5 ppm for [Ph₂Si(terpy)]²⁺).⁷ Both 3^iPr and 3^Ph exhibit similar effective Lewis acidity as obtained following the GB method. The peak at +71.6 ppm in the ³¹P{¹H} NMR spectrum assigned to the bound Et₃PO is shifted up-field upon increasing the amount of Et₃PO. This obvious shift is due to the expected weak acceptor ability upon further Et₃PO coordination at the cationic site.^10f We have crystallized the mono-adduct 3^PhOPEt₃ (Fig. S97†) under low temperature conditions from the reaction between 3^Ph and Et₃PO. The molecular structure of 3^PhOPEt₃ shows a penta-coordinated Ge site. Interestingly, the solution-state NMR study of 3^PhOPEt₃ shows characteristic ³¹P{¹H} NMR chemical shifts at +68.5 and −20.5 ppm along with a peak at −6.3 ppm assigned to the in situ generated free 3^Ph. This observation reflects the presence of the equilibrium in the solution-state.

Despite the dispersion of dipositive charges over the P–Ge–P sites in 3^iPr and 3^Ph, we have observed the preferential binding of Lewis bases such as Et₃PO and 4-N,N′-dimethylaminopyridine (DMAP) at the Ge site (Scheme 2). The reaction between 3^iPr and DMAP taken in a 1 : 1 ratio gave 3^iPrDMAP, which was crystallized in small amounts from DCM/hexane layering at room temperature (Scheme 2). Single crystals of dimethylamino pyridium triflate were obtained in large amounts, which were separated from the 3^iPrDMAP crystals. The molecular structure of 3^iPrDMAP from X-ray analysis reveals an overall TBP geometry with the DMAP coordination at one of the trigonal planar equatorial sites of the Ge atom (Fig. S98†). However, 3^iPrDMAP is unstable under room temperature conditions in the solution state. The ³¹P{¹H} NMR of in situ generated 3^iPrDMAP at −40 °C in CD₂Cl₂ shows a peak at −13.2 ppm in addition to the uncoordinated 3^iPr peak (+28.4 ppm) present in the reaction medium. These peaks gradually disappeared upon warming to room temperature, ultimately giving new peaks at +75.7 ppm and −18.2 ppm (vide supra). We have observed a peak at +12.9 ppm in the ¹H NMR spectrum corresponding to the formation of the conjugate acid dimethylamino pyridium triflate. Similar side-reactions might be involved in the reaction between 3^iPr and donors (Et₃PO and DMAP), as was reported earlier by Stephan et al.³⁰

Scheme 2 Anion/Lewis base binding and bond activation by the Ge(IV) di-cationic compounds (see the ESI† for detailed spectroscopic and experimental procedures of each reaction).

A dynamic process^13a between mono- and bis-adduct formation was observed upon reacting 3^Ph with one equivalent of DMAP at room temperature. The NMR scale reaction in CDCl₃ shows a peak at −18.5 ppm and a broad peak centered at −27.0 ppm in the ³¹P{¹H} NMR spectrum. The gradual decrease in temperature up to −50 °C resulted in a complete disappearance of the peak at −18.5 ppm. Concurrently two close sharp peaks at ∼−29 ppm appeared along with the generation of the peak for 3^Ph at −6.3 ppm. Inferring from this NMR study, a bis-adduct was isolated (corresponding peak at ∼−29 ppm) from the reaction between DMAP and 3^Ph at −35 °C. The crystallization from DCM/pentane layering at the same temperature gave single crystals of trans bis-adduct 3^Ph(DMAP)₂ (Fig. S99†). As a matter of fact, in this case we find the spontaneous formation of the more favourable hexa-coordinated bis-adduct over the mono-adduct at lower temperatures.^11d The octahedral coordination type at the Ge atom of 3^Ph(DMAP)₂ comprises two L^Ph in the equatorial plane and two DMAPs occupying the axial sites. Thus, dynamic processes were observed for the adducts formed between 3^Ph and donors.

The preferential binding of donor groups at the Ge site is associated with the relief in constraint on going from a strained spiro-geometry in 3^iPr and 3^Ph to the relaxed TBP (3^iPrDMAP) and distorted tetrahedral geometry (3^PhOPEt₃) in the adducts, respectively. Notably, this results in a handful of penta-coordinated Ge compounds obtained having TBP geometry which are uncommon in Ge chemistry.^11d The viable expansion of the peri-distances²² in the two intramolecular P–Ge bonds in 3^iPr and 3^Ph allows smooth structural rearrangements with the addition of nucleophiles.

We have validated the above understanding from the deformation energy^9,29 values obtained in the spiro- to TBP geometric transformation. We have prepared 3^iPrF from the addition of one equivalent of KF to 3^iPr (Scheme 2), which exhibits a distorted TBP geometry (Fig. 5a) like 2^iPr in the solid state. DFT calculations reveal that the fluoride ion binding to 3^iPr involves a low deformation energy (E_def = 126 kJ mol⁻¹), falling within the range reported in the literature.^10g,19a,293^iPrF was characterised by NMR spectroscopy in the solution state showing a characteristic triplet at −141.0 ppm (²J_F–P = 185 Hz) in the ¹⁹F{¹H} NMR spectrum and a corresponding doublet in the ³¹P{¹H} NMR spectrum (−10.2 ppm).

Fig. 5 Molecular structures of (a) 3^iPrF and (b) 3^PhF (H atoms, solvent molecule and triflate anion have been removed for clarity; thermal ellipsoid 50%). Selected bond lengths [Å] and angles [°]: (a) Ge1–C1 = 1.951(2), Ge1–F1 = 1.785(2), Ge1–P1 = 2.565(1); C1–Ge1–C1 = 145.1(1), F1–Ge1–C1 = 107.5(1), P1–Ge1–P1 = 172.8(1), P1–Ge1–F1 = 86.4(1); (b) Ge1–F1 = 1.797(2), Ge1–C2 = 1.920(3), Ge1–C1 = 1.927(3), Ge1–P1 = 2.368(1), Ge1–P2 = 3.043(1); F1–Ge1–C2 = 102.1(1), F1–Ge1–C1 = 102.8(1), C2–Ge1–C1 = 120.6(1), F1–Ge1–P1 = 97.3(1), C2–Ge1–P1 = 137.3(1), C1–Ge1–P1 = 91.2(1).

Like 3^iPr, the addition of one equivalent of KF led to the formation of 3^PhF (Scheme 2) showing similar characteristic peaks in NMR spectra (δ¹⁹F{¹H}: −150.3 ppm, triplet, and ²J_P–F = 100 Hz; δ³¹P{¹H}: −17.5 ppm). The molecular structure of 3^PhF (Fig. 5b) resembles that of 2^Ph. The low temperature ³¹P{¹H} NMR spectrum reveals the presence of inequivalent phosphines in 3^PhF. In this case, the release of the structural constraint occurs on going from spirocyclic 3^Ph to the relaxed geometry in 3^PhF involving a deformation energy value of 148 kJ mol⁻¹.^10g,19a,29

The calculations on the gas-phase fluoride ion affinity (FIA)³¹ at the Ge sites give very high values of 873 kJ mol⁻¹ and 859 kJ mol⁻¹ for 3^iPr and 3^Ph respectively (gas-phase FIA for reference SbF₅ is 497 kJmol⁻¹) (see the ESI† for details). However, the calculated FIA values in DCM solvated models decreased significantly giving the values of 194 kJ mol⁻¹ and 190 kJ mol⁻¹ for 3^iPr and 3^Ph, respectively (solvent corrected FIA for reference SbF₅ is 331 kJmol⁻¹). This is a common phenomenon observed due to solvation damping, being more pronounced with a cationic Lewis acid.^7,19a Both 3^iPr and 3^Ph did not abstract the fluoride anion when reacted with [PPh₄][SbF₆] and hence are not Lewis superacids. The chloride ion affinity study using trityl chloride shows that only 3^Ph abstracts chloride with characteristic coloration^10g and the NMR data reveal the formation of 2^Ph (Scheme 2 and Fig. S62, ESI†).

Gas-phase hydride ion affinity (HIA)³¹ calculations also give very high values of 949 kJ mol⁻¹ and 928 kJ mol⁻¹ for 3^iPr and 3^Ph, respectively (gas-phase HIA for reference B(C₆F₅)₃ is 517 kJ mol⁻¹) (see the ESI† for details). Reasonable HIA values are obtained after considering the solvation in DCM: 254 kJ mol⁻¹ and 269 kJ mol⁻¹ for 3^iPr and 3^Ph, respectively (solvent corrected HIA for reference B(C₆F₅)₃ is 270 kJmol⁻¹). Thus, both 3^iPr and 3^Ph are hydrophilic in nature. 3^iPr efficiently abstracts the hydride from LiBEt₃H at room temperature leading to the formation of 3^iPrH (Scheme 2). The ¹H NMR spectrum of 3^iPrH reveals the Ge–H resonance as a triplet (²J_P–H = 36 Hz) at +7.2 ppm; the chemical shift value falls within the range of reported Ge-hydrides.¹⁵ The molecular structure of 3^iPrH attains a distorted TBP geometry (Fig. S100†) analogous to 2^iPr. The Ge–H bond length of 1.540 Å in the optimized geometry 3o^iPrH agrees well with that found in the molecular structure of 3^iPrH (1.54(3) Å) (see the ESI† for details). The reaction of 3^Ph with two equivalents of LiBEt₃H gave the neutral 3^PhH₂ (Scheme 2). The molecular structure of 3^PhH₂ is shown in Fig. S101.† The Ge⋯P distances in 3^PhH₂ are close to 3 Å; however they fall within the sum of van der Waal's radii ΣvdW(Ge–P) = 3.91 Å.³² The ¹H NMR spectrum of 3^PhH₂ reveals a Ge–H₂ resonance at 6.4 ppm as a triplet (J_P–H = 20 Hz). The reaction between 3^Ph and one equivalent of LiBEt₃H followed by crystallization from different solvent combinations gives the single crystals of 3^PhEt and 3^PhH(Et) (Scheme 2 and Fig. S102, S103, ESI†), confirming B–C bond activation.³³ Thus, preliminary reactivity studies identify 3^Ph as a highly reactive di-cation.

Catalytic hydrosilylation of an aromatic aldehyde by the Ge(IV) di-cationic compounds

Given the hydrophilicity of the soft Ge-based Lewis acids 3^iPr and 3^Ph, we targeted the activation of the Si–H bond in Et₃SiH (see the ESI† for details).³⁴ Monitoring the reaction between 3^iPr and excess Et₃SiH at room temperature using ³¹P NMR displays the formation of two new peaks at +20.3 ppm and +12.2 ppm along with retention of 3^iPr. The peak at +20.3 ppm showed a coupling of J_PH ≈ 476 Hz. Furthermore, ¹H–³¹P 1D and 2D Heteronuclear Single Quantum Coherence (HSQC) NMR measurements (CNST2 optimized at 480 Hz) confirm that ¹J_PH = 476 Hz.³⁵ Thus, the Si–H bond activation and hydride binding occur preferentially at the P-site over the Ge-site giving 3^iPr–PH. This is in contrast to 3^iPrH formed by hydride abstraction from LiBEt₃H (vide supra). Notably, although Si–H bond activation occurs at the P-site of 3^iPr, standing of the reaction mixture for 14 days ultimately led to Ge–H bond formation (t, ²J_P–H = 36 Hz at +7.2 ppm) giving 3^iPrH.

In contrast, 3^Ph activates the Si–H bond in Et₃SiH giving 3^PhH (Scheme 3a) as the product within 24 hours (see the ESI† for details). However, the time dependent ³¹P NMR monitoring of the initial reaction mixture displays a minor peak at +10.0 ppm with a coupling of ¹J_PH = 534 Hz corresponding to the P–H formation, which subsequently disappeared giving 3^PhH within 24 hours. The ¹H NMR spectrum of 3^PhH discloses the Ge–H resonance at +7.0 ppm as a triplet (²J_P–H = 30 Hz) and a corresponding doublet at −19.0 ppm in the ³¹P{¹H} NMR spectrum. The ²⁹Si{¹H} NMR of the reaction mixture shows the corresponding formation of Et₃SiOTf (δ = +45.0 ppm).

Scheme 3 (a) Si–H bond activation by 3^Ph leading to 3^PhH within 24 hours; (b) hydrosilylation of p-methyl benzaldehyde using 3^iPr or 3^Ph as the catalyst (see the ESI† for the detailed spectroscopic and experimental procedures of each reaction).

The above-mentioned observations suggest that the hydride binding initially occurs at the P-site due to the dispersion of dipositive charges over P–Ge–P units in these di-cationic species. Computational investigations done on the hydride migration from P to Ge for both 3^iPr and 3^Ph reveal the involvement of a relatively higher transition state energy barrier in the case of the former (Fig. S112, ESI†). The hydride migration mechanism proposed based on the DFT calculations performed on the cationic part of the compounds proves only the trends of the hydride shifts from P to Ge. Nonetheless, the very low energy barriers obtained are inadequate to support the longer time requirements for hydride migrations observed experimentally. Probably, the inclusions of the solvent models and counter anions to the systems would be able to resolve the disagreement at an associated extremely high computational cost. Thus, the hydride migration mechanism proposed herein is rather based on the experimentally observed data. Additionally, the strong electron donating nature of ⁱPr₂P compared to Ph₂P pushes electron density towards the Ge-site in 3^iPr thereby making the P-site appropriate for hydride. The ultimate formation of 3^iPrH or 3^PhH stems from the geometric constraint empowered Lewis acidity at the Ge site of 3^iPr or 3^Ph respectively (vide supra).

Despite the differences observed in the reaction with Et₃SiH, the intrinsic hydrophilicity of 3^iPr and 3^Ph prompted us to perform a test reaction on the catalytic hydrosilylation³⁶ of p-methyl benzaldehyde with Et₃SiH at room temperature (Scheme 3b, see the ESI† for details). Using 5 mol% of 3^iPr led to the complete conversion to the silyl ether within 24 hours. It is observed from the ¹H NMR study that the aldehyde does not bind to the catalyst, unlike most Lewis acid catalysts.^10e Thus, the reaction is likely to proceed via silane activation by 3^iPr (Scheme 4).³⁷ The NMR investigations of the catalytic reaction mixture show P-mediated Si–H bond activation (¹H NMR = +8.5 ppm; ³¹P NMR = +20.2 ppm). The ²⁹Si{¹H} NMR spectrum shows the formation of a hydrosilylated product and the presence of Et₃SiH with the corresponding peaks at +8.9 ppm and +0.3 ppm, respectively. The ²⁹Si{¹H} NMR spectrum did not show the presence of Et₃SiOTf which is a competent catalyst. Nonetheless, the latter's presence in trace amounts and participation in the catalytic cycle cannot be completely neglected. The control experiment shows no hydrosilylated product being formed, as confirmed from the ¹H NMR study of the reaction mixture containing p-methyl benzaldehyde and Et₃SiH (Fig. S94, ESI†).

Scheme 4 Proposed catalytic cycle for the hydrosilylation of aromatic aldehydes using 3^iPr and 3^Ph as catalysts.

Full conversion of p-methyl benzaldehyde to the silyl ether was achieved within 24 hours under room temperature conditions by using a lower catalyst loading to 1 mol% of 3^Ph (see the ESI† for details). Importantly, the aldehyde (in the absence of any other reactant) binds to 3^Ph as evident from the NMR study. The ³¹P{¹H} NMR spectrum depicts a peak at −19.3 ppm and multiple peaks at around +36 ppm in addition to the peak for free 3^Ph (−6.3 ppm). Correspondingly, we have obtained single crystals from a THF solution of 3^Ph and excess aldehyde. The crystal structure shows the insertion of an aldehyde into the P–Ge bond in 3^Ph (Fig. S104, ESI†). Thus, a masked frustrated Lewis pair (FLP) type behavior^21,38 of 3^Ph was observed, which is unsurprising due to the adaptability of the P–Ge bond in peri-systems. The NMR spectrum recorded immediately after the addition of 3^Ph to the reaction mixture of the aldehyde and Et₃SiH also displays similar chemical shift values (−19.3 and around +36 ppm in the ³¹P{¹H} NMR study). The NMR study did not show any peak arising from Si–H bond activation by 3^Ph. This suggests the formation of the carbonyl-FLP insertion product taking place at the initial stage of the catalytic cycle. Notably, no Et₃SiOTf formation is detected from the ²⁹Si{¹H} NMR monitoring (Fig. S93, ESI†). Thus, the catalytic hydrosilylation is likely to proceed via the carbonyl insertion followed by the R₃Si–H addition across the carbonyl functional group (Scheme 4). The catalyst 3^Ph is clearly observed after complete consumption of the reactants.

Thus, both 3^iPr and 3^Ph show different pathways catalysing the hydrosilylation of the aldehyde. Based on our experimental findings, while the former proceeds through the Si–H bond activation at the Lewis acidic site, the latter utilizes the masked FLP for the aldehyde insertion followed by Et₃Si–H addition to the C=O bond.

Conclusion

In this study, we have established the first examples of tetra-coordinated Ge(IV) di-cationic compounds 3^iPr and 3^Ph. This was achieved through the intramolecular stabilization of the Ge di-cationic site using a phosphine donor in peri-substituted acenaphthene. The strong P–Ge bonds led to the dispersion of di-cationic charges over Ge and P sites. The simple anion binding or Lewis base coordination occurs exclusively at the Ge site owing to the release of the geometric constraint on going from the spiro-geometry of the di-cation to the distorted TBP/tetrahedral geometry of the resultant species. Although the Si–H bond activation occurs initially at the P site for both the di-cationic species due to their dispersed di-positive charges, the migration of hydrides to the Ge site occurs promptly in the case of 3^Ph compared to 3ⁱPr. Preliminary tests on the catalytic hydrosilylation of the benzaldehyde demonstrate the involvement of two different catalytic pathways for the two catalysts 3^iPr and 3^Ph. In the case of 3^iPr, the catalytic hydrosilylation was initiated with Et3Si–H bond activation. On the other hand, masked Frustrated Lewis Pair (FLP) type reactivity led to hydrosilylated product formation for 3^Ph as the catalyst. Fine tuning the donor group properties in such charged peri species is being explored in our group for their applicability as catalysts in diverse organic transformations.

Data availability

All data associated with this publication are provided in the ESI.†

Author contributions

Investigation, methodology and formal analysis: Balakrishna Peddi and Souvik Khan. X-ray crystallography: Rajesh G. Gonnade. Computational study, comment and editing draft: Cem B. Yildiz. Conceptualization, funding acquisition, supervision, and writing-original draft: Moumita Majumdar.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank SERB India (SPF/2022/000046, CRG/2022/000673) for their financial support. The numerical calculations reported in this paper were partially performed at the TUBITAK ULAKBIM, High Performance and Grid Computing Center (TRUBA resources) and Param Brahma supercomputing facility at IISER Pune. The authors acknowledge the ‘Chemical Biology and Molecular Imaging Group, Department of Chemical Sciences, Tata Institute of Fundamental Sciences’ along with Prof. Ankona Datta and Ms. Deepika Rao for elemental analyses. The authors thank Mr Hritwik Haldar, a 5th year BS–MS student, for assisting in computational studies. The authors thank all the reviewers for their valuable comments in improving the manuscript.

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, spectroscopic analyses and theoretical details of all compounds. CCDC 2223776, 2223777, 2223780–2223782, 2223784, 2256454–2256458, 2256463, 2256466–2256468 and 2256567. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3sc03717g

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