Synthesis and Characterization. Aiming for the simultaneous removal of lithium chloride and trimethylsilyl chloride (Me3SiCl), we first added an equimolar amount of our previously reported lithium NHB-phosphide 1 (31P NMR: −286.4 ppm, 11B NMR: 38.3 ppm)23 to a THF solution of GeCl2(dioxane) at − 40°C (Scheme 1). Contrary to our expectations, species 2 (31P NMR: −98.9 ppm) with a P2Ge2 core was readily produced after workup, akin to Power’s C, and the anticipated monomeric phosphagermyenylidene remained undetected. This outcome suggests that the NHB group does not provide sufficient steric bulk to prevent dimerization, consistent with our theoretical calculations that show dimerization to be energetically favored by 43.1 kcal/mol (Figure S4-1).
Pursuing the possibility that additional Lewis bases might disrupt the dimeric nature of 2,13–14 we reacted it with a small NHC, 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (Me2IPr).26 However, this reaction gave 7 (31P NMR: 50.8 ppm), as corroborated by single-crystal X-ray diffraction (Figure S3-4). This product formation was a straightforward result of Lewis base-acid interaction, with no subsequent de-dimerization occurring even in the presence of an excess of Me2IPr. Alternatively, heating a solution of Me2IPr-ligated phosphinochlorogermylene 6 (31P NMR: −208.2 ppm; See Figure S3-3 for its X-ray structure), derived from the salt metathesis of 1 and Me2IPr→GeCl2, in toluene at 75°C for three hours, also resulted in 7.
Gratifyingly, when 2 was treated with a more nucleophilic and bulkier cyclic (alkyl)(amino)carbene (CAAC)27 in toluene, and the temperature was gradually raised from − 40°C to ambient, de-dimerization of 2 occurred, leading to the complete formation of phosphagermyenylidene 3 in the coordination sphere of CAAC (Scheme 1). Compound 3 could also be obtained as a dark red solid with a superior overall yield of 70% by promoting Me3SiCl elimination via heating 5 (31P NMR: −180.3 ppm), synthesized through salt metathesis of 1 with CAAC→GeCl2 (4). The 11B NMR resonances of the NHB groups in 5 and 3 were noted at 36.4 and 40.6 ppm, respectively. The 31P NMR spectrum of 3 presented a sharp singlet at 306.8 ppm, which is within the expected range for typical phosphagermenes but shows a significant downfield shift compared to those of NHB − P = B − NHI (− 291.9 ppm; NHI = N-heterocyclic imino),23 NHB − P = C = NR (− 262.6 ppm for R = Mes; −256.2 ppm for R = tBu)28, and NHB − P = Sc(Me2NNacnac)ClK(18-C-6) (19.6 ppm; Me2NNacnac = (DippN)[(CMe)HC(CMe)](NCH2CH2NMe2), 18-C-6 = 18-Crown-6).29
The molecular architectures of 3 and 5 were unambiguously confirmed by X-ray crystallography (Fig. 2), verifying the formation of monomeric phosphagermyenylidene and phosphinochlorogermylene, respectively, each within the coordination environment of CAAC. In the solid-state structure of 3, both P(1) and Ge(1) atoms exhibit di-coordination, with the NHB and CAAC moieties effectively shielding the central P(1)Ge(1) core. The P(1) − Ge(1) bond length is found to be 2.1859(4) Å, considerably shorter than its precursor 5, which has lengths of 2.3769(9) and 2.3981(10) Å in two crystallographically independent molecules in the unit cell, and also slightly less than those seen in A (2.2364(6) Å)13 and B (2.242(2) Å).14 This length approaches those of phosphagermene Mes*P = GeMes2 (2.138(3) Å)30 and Mes*P = Ge(SiMetBu2)2 (2.1748(14) Å) (Mes* = 2,4,6-tBu3C6H2),31 reinforcing its double bond character. The C(1) − Ge(1) bond length at 2.0520(13) Å is marginally longer than that in the diarylgermylene of (Ter)2Ge (2.033(4) Å, Ter = 2,6-(2,4,6-Me3C6H2)2C6H3)32 and the bis-CAAC-stabilized germylone (CAAC)2Ge, which ranges from 1.9386(18) to 1.965(3) Å,33 both of which possess a prominent C − Ge − C π-bond system, as well as being slightly longer than Pyykkö’s standard value for a C − Ge single bond (1.96 Å).34 Notably, the bond angles of P(1) − Ge(1) − C(1) and B(1) − P(1) − Ge(1) are acutely set at 95.90(4)° and 93.11(5)°, respectively. These angles suggest a considerable involvement of p-orbitals from Ge(1) and P(1) in forming bonds with adjacent atoms. For comparison, the TerGeP(SiMe3)2 features a more obtuse P − Ge − C angle of 106.6(1)°.35
The UV-visible spectrum of 3, recorded in toluene solution at room temperature, exhibits a pronounced broad absorption at 380 nm and a less intense broad absorption at 475 nm (Figure S2-39). These absorption features are predominantly ascribed to transitions from HOMO-1 and HOMO to LUMO and LUMO + 1, based on time-dependent DFT calculations (Figure S4-4).
Theoretical Investigation. To gain insights into the electronic structure of 3, we carried out DFT calculations at the M06-2X/def2-SVP level of theory. The HOMO at − 5.81 eV and the LUMO at − 0.58 eV predominantly characterize the π and π* orbitals of the P = Ge double bond, respectively (Figure S4-3). In-depth analysis of the bonding situation was further elucidated through intrinsic bond orbital (IBO)36 analysis. Compound 3 exhibits σ-bonding orbitals for B(1) − P(1), P(1) − Ge(1), and Ge(1) − C(1) (Figs. 3a, e, and b). Notably, the Ge(1) − C(1) orbital is considerably polarized towards C(1), accounting for 70.8% at C(1) and 27.3% at Ge(1). Each of P(1) and Ge(1) maintains a predominantly non-bonding lone pair orbital within the B − P = Ge − C plane—96.1% localized at P(1) and 1.5% at B(1) (Fig. 3c); 95.2% at Ge(1) and 2.4% at C(1) (Fig. 3d)—and these are oriented in opposite directions to each other, contributing to a counter double bent geometry. This aligns with the electron localization function (ELF)37 calculations, which show substantial localized electron density around the Ge(1) and P(1) atoms (Figure S4-5). In addition, a significant out-of-plane π-bonding orbital involves P(1) (63.3%) and Ge(1) (31.8%), with a slight contribution of 2.3% from B(1) (Fig. 3f). For comparison, in the absence of CAAC, the resulting free phosphagermyenylidene, NHB-P = Ge, presents a similar bonding picture with a conventional P = Ge double bond (Figure S4-16). Furthermore, the nature of the P = Ge bonding in 3 was further probed through energy decomposition analysis - natural orbitals for chemical valence (EDA-NOCV).6d, 38 The absolute value of |ΔEorb| between the triplet fragments (CAAC − Ge and NHB − P) is significantly smaller (156.04 kcal/mol) compared to that between the singlet fragments (204.59 kcal/mol). This difference indicates the presence of conventional electron-sharing σ and π bonds be-tween P(1) and Ge(1) (Figures S4-12 − S4-14 and Tables S4-5). Overall, these findings underscore a P = Ge double bond system in 3, complemented by the presence of lone pairs at both the P and Ge atoms. This structure is effectively stabilized by the inclusion of NHB and CAAC ligands, serving as robust anchors for the molecular framework of a phosphagermyenylidene.
The Wiberg bond indices (WBIs) obtained from natural bond orbital calculations for the bonds P(1) − Ge(1), B(1) − P(1), and Ge(1) − C(1) are 1.63, 1.13, and 0.69, respectively. Notably, the WBI for the P(1) − Ge(1) bond is higher than that calculated for the P − Ge bond in A (1.30), suggesting a more pronounced double-bond character for the P(1) − Ge(1) bond in 3. Natural population analysis reveals considerable charge separation, with B(1) (0.79 a.u.) and Ge(1) (0.36 a.u.) carrying positive charges, P(1) (− 0.51 a.u.) bearing a negative charge, and C(1) (0.07 a.u.) being nearly neutral. Furthermore, there is a marked net electron transfer of 0.20 a.u. from the CAAC moiety to the Ge(1) = P(1) − NHB fragment (Figure S4-6), aligning with the dative nature of the C(1) − Ge(1) bond.39 Valence shell charge concentration (VSCC) profiles, derived from quantum theory of atoms in molecules,40 for the C(1)–Ge(1) bond reveal a distinct VSCC minimum near the electronegative carbon atom and a weak shoulder, corresponding to the second VSCC near Ge(1), indicative of dative bonding.39 These features, typical of dative bonds, are further substantiated by EDA-NOCV(Figures S4-8 − S4-10 and Tables S4-3 − S4-4).6d, 38
Reactivity. To provide experimental evidence of the dative bond between C(1) and Ge(1), we explored the reactivity of 3 with Lewis acids. Remarkably, GeCl2 emerged as the most effective Lewis acid in disrupting the coordination of CAAC. Treating 3 with GeCl2(dioxane) in THF at room temperature yielded CAAC→GeCl2 (4) and 2 (Scheme 1), both of which were isolated and characterized by multinuclear NMR spectroscopy. The formation of 2 and 4 suggests the transient existence of a free phosphagermyenylidene, which rapidly dimerizes, indicating the potential of base-stabilized phosphagermyenylidenes as a transfer agent for generating free phosphagermyenylidenes.
Given that both Ge(1) and P(1) possess a lone pair of electrons, the coordination behavior of 3 was investigated. Reaction of 3 with silver triflate in THF, from − 40°C to room temperature, resulted in the formation of complex 8 (Scheme 2). The 31P NMR spectrum indicated a preference for the Ag(1) coordination to P(1) over Ge(1), as evidenced by a high-field resonance at 150.4 ppm appearing as a doublet with a one-bond phosphorus-silver coupling constant of 611.2 Hz. This selectivity is attributed to the enhanced nucleophilicity at P(1), as indicated by dual descriptor (DD) values obtained from conceptual DFT calculations,41 which yield DD values of − 0.29 for P(1) and − 0.21 for Ge(1), respectively (Figure S4-15). Indeed, attempts to coordinate silver to the Ge(1) center were unsuccessful, with no reaction between 8 and further silver triflate, possibly due to the energetically low-lying lone pair orbital on Ge(1). X-ray diffraction analysis of 8 showed marginally extended P(1) − Ge(1) (2.2063(15) Å) and Ge(1) − C(1) (2.073(5) Å) bond lengths (Fig. 4a). The bond length between P(1) and Ag(1) was determined to be 2.3278(14) Å, consistent with bond lengths found in conventional silver-phosphine complexes.42
The reactivity of the double bond in 3 was further explored (Scheme 2). Despite its inertness towards elemental tellurium, reaction with tributylphosphine telluride (nBu3PTe) led to the formation of a novel species, 9 (31P NMR: −227.1 ppm). This was attributed to a formal [1 + 2] cycloaddition across the Ge(1) − P(1) double bond, leaving the lone pairs on Ge(1) and P(1) unaffected. In the solid-state structure of 9, a distinctive P(1)Ge(1)Te(1) triangle is observed, constituting the first example of a GePTe three-membered ring (Fig. 4b).43 This structure showcases a P(1) − Ge(1) bond length of 2.3827(15) Å, while the bond distances between Te(1) − Ge(1) and Te(1) − P(1) are determined to be 2.6882(8) Å and 2.4967(15) Å, respectively.
The regioselective addition of methyl iodide (MeI) to the Ge = P double bond in 3 gave product 10 (31P NMR: −118.6 ppm) (Scheme 2 and Fig. 4c). Intriguingly, treating 3 with ammonia (NH3) resulted in the complete cleavage of both the P(1) = Ge(1) double bond and the C(1) − Ge(1) dative bond, producing 1144 and NHB-PH2.23 Compounds 11 and NHB − PH2 were characterized by multinuclear NMR spectroscopies after an unidentified white amorphous precipitate (hypothesized to be [GeNH]n) was removed by filtration.
In conclusion, we have reported the synthesis and characterization of a phosphagermyenylidene, stabilized through a synergy between a bulky electropositive NHB and a strong σ-donating CAAC. This species represents the first instance of an isolable phospho-rus/germanium analog of isonitrile, characterized by a Ge = P double bond and lone pairs of electrons on both the Ge and P atoms. This compound, featuring multiple reactive sites, showcases behaviors such as the dissociation of the coordinating CAAC, the ability to coordinate to silver via the P lone pair, and the (cyclo)addition and cleavage of the Ge = P double bond. Investigations into its further reactivity and a deeper understanding of the reaction mechanisms remain active areas of our research.