Isolation of an Anionic Dicarbene Embedded Sn2P2 Cluster and Reversible CO2 Uptake

Abstract Decarbonylation of a cyclic bis‐phosphaethynolatostannylene [(ADC)Sn(PCO)]2 based on an anionic dicarbene framework (ADC = PhC{N(Dipp)C}2; Dipp = 2,6‐iPr2C6H3) under UV light results in the formation of a Sn2P2 cluster compound [(ADC)SnP]2 as a green crystalline solid. The electronic structure of [(ADC)SnP]2 is analyzed by quantum‐chemical calculations. At room temperature, [(ADC)SnP]2 reversibly binds with CO2 and forms [(ADC)2{SnOC(O)P}SnP]. [(ADC)SnP]2 enables catalytic hydroboration of CO2 and reacts with elemental selenium and Fe2(CO)9 to afford [(ADC)2{Sn(Se)P2}SnSe] and [(ADC)Sn{Fe(CO)4}P]2, respectively. All compounds are characterized by multinuclear NMR spectroscopy and their solid‐state molecular structures are determined by single‐crystal X‐ray diffraction.


Introduction
Stable compounds with multiple bonds between heavier maingroup elements continue to attract interest owing to their intriguing electronic structures, reactivity profiles, and potential in synthesis. [1]The isolation of the first alkene analogs of phosphorus (i.e., diphosphene) by Yoshifuji [2] and silicon (i.e., disilene) by West [3] in 1981 was an important milestone.The advent of stable carbenes, such as N-heterocyclic carbenes (NHCs, I) [4] and cyclic alkyl amino carbenes (cAACs, II), [5] furnished new tools to tame the reactive, including unsaturated, main-group species (Scheme 1). [6]Early synthesis of the first NHC-stabilized phosphinidene III (R ≐ Ph), [7] monomer of a diphosphene, DOI: 10.1002/advs.202305545emphasized the potential of NHCs as potent Lewis bases. [8]Depending on the acceptor property of carbene, III may be regarded as base-stabilized phosphinidenes (a) or phosphaalkenes (b).
Stable phosphaalkynes RC≡P, [9] the heavier analogs of nitriles (RC≡N), have been known since 1981. [10]The related heavier Group 14 compounds RE≡P (E = Si, Ge, Sn, Pb) remained however rather elusive species because of their low thermodynamic stability and high propensity to oligomerize.This is generally attributed to the reluctance of heavier main-group elements to form multiple covalent bonds. [1]Theoretical calculations predicted that the vinylidene isomers RP═E are thermodynamically more stable than those of alkyne analogs RE≡P. [11]Recently, Inoue [12] and Tan [13] reported the syntheses of Lewis base-stabilized germanium and tin compounds IV (Scheme 1) with a partial E-P double bond.A base-stabilized metallylene-phosphinidene description (a) appears more appropriate for IV as shown by the isolation of its Sn-bound B(C 6 F 5 ) 3 adduct. [12]In 2022, Aldridge and colleagues described the synthesis of phosphino-phosphinidene tethered stannylenes V. [14] In solutions, V exist as P-donor stabilized stannylenes featuring a four-membered SnCP 2 heterocycle.Using sterically very bulky substituents, Aldridge et al. were able to characterize a stannaphosphene VI, featuring a polar Sn═P bond, in the solid state by X-ray diffraction.

Results and Discussion
Treatment of [(ADC)SnBr] 2 (1) [17] with Na(1,4-dioxane) 2.5 PCO afforded a mixture of [(ADC)Sn(PCO)] 2 (2) and [(ADC)Sn{P(μ- CO)] 2 (3a) in 3:2 ratio as a yellow solid (Scheme 2).Heating a benzene suspension of 2 and 3a at 70 °C for 1 h led to the conversion of 3a into 2, which precipitated out in benzene at room temperature.2 was isolated by filtration in 84% yield as a bright yellow solid.The soluble part contained a mixture of 2 and 3a, which could not be separated because of their comparable sol-ubility (see below).Compound 2 is stable under an inert gas atmosphere but slowly isomerizes in daylight to give the OCbridged species 3a.Therefore, a pure sample of 3a (free from 2) could not be obtained because of the incomplete conversion of 2 into 3a and their almost similar solubility.Thus, NMR data of 3a were extracted from the spectra measured for a sample containing 2 and 3a.In addition to the expected 1 H and 13 C NMR signals for the ADC moiety, the 31 P{ 1 H} (−361 ppm) and 119 Sn{ 1 H} (−165.5 ppm, 1 J P-Sn = 637 Hz) NMR spectra of 2 show a singlet and a doublet, respectively.The 119 Sn{ 1 H} NMR signal for 2 compares well with those of Lewis base stabilized stannylenes with a three-coordinated Sn atom. [18]The IR spectrum of 2 shows a strong band for the unsymmetric stretching frequency of the phosphaketene unit at  asym = 1860 cm −1 . [15a] The 31 P{ 1 H} (singlet at 170.1 ppm) and 119 Sn{ 1 H} (doublet at -148.1 ppm with 1 J P-Sn = 668 Hz) NMR signals for 3a are consistent with those of OC-bridged OCP-containing compounds. [19]uitable crystals for single crystal X-ray diffraction (sc-XRD) were grown by storing a n-hexane layered concentrated THF solution of 2 at −40 °C.Two nearly identical molecules of 2 were found inside the asymmetric unit.Both tin atoms of the central almost planar C 4 Sn 2 ring of 2 (Figure 1) show threefold coordination.The linear PCO substituents at the tin atoms are present in a trans fashion and positioned inwards the C 4 Sn 2 ring.Single crystals of 3a were obtained by storing a benzene solution of a mixture of 2 and 3a at 5 °C and picked up by visual inspection.The crystals of 3a also contained the isomer 3b, both sharing the same positions on two different twofold axes of the space group I2.The asymmetric unit thus contains two half-disordered molecules of 3a and 3b in the ratio 70:30 and 17:83, respectively (see Scheme 2).In solution NMR studies, 3b could not be detected.Thus, the formation of 3b, featuring one four-coordinated and one two-coordinated phosphorus atoms of a dimeric phosphaethynolate species, is likely due to crystal packing effects. [19]15d] As expected, the P-C bond lengths in 3a (1.862(9) Å) and 3b (1.885(9) Å) are longer than those of 2 but are similar to related OC-bridged compounds. [19]t elevated temperature (>80 °C), the solution of 2 begins to decarbonylate, leading to the formation of a new species, i.e., 5 (Scheme 2).The conversion is however rather slow (≈20% af-ter 3 h).The irradiation of an orange fluorobenzene solution of 2 (or a mixture of 2 and 3a) under UV light (360 nm) for 3 h cleanly affords compound [(ADC)SnP] 2 (5) as the sole product (90% isolated yield).Compound 5 is a green solid and stable under an inert gas atmosphere but readily decomposes when exposed to air.Interestingly, 5 is remarkably thermal stable and no change, as monitored by NMR spectroscopy, was observed when a sample of 5 was heated at 150 °C for 3 h.5 exhibits wellresolved 1 H and 13 C{ 1 H} NMR signals for the ADC moieties.The 31 P{ 1 H} NMR spectrum of 5 shows a singlet at −110 ppm with tin satellites.Albeit with a smaller 1 J P-Sn coupling constant (416 Hz), the 31 P NMR signal of 5 (−110 ppm) is consistent with those of diphosphanylstannylene compounds (R 2 P) 2 Sn (−53 to −129 ppm; 1 J P-Sn = 1000-2000 Hz). [20]The 119 Sn{ 1 H} spectrum of 5 exhibits a triplet at 748.1 ppm ( 1 J P-Sn = 416 Hz).These data indicate the presence of Sn-P single bonds and the coupling of tin atoms with two magnetically equivalent 31 P nuclei.The remarkably downfield shifting of the 119 Sn NMR signal  and a rather smaller coupling constant for 5 (748.1 ppm, 1 J P-Sn = 416 Hz) with respect to that of 2 (−165.5 ppm, 643.5 Hz) and 3a (−148.1 ppm, 663.7 Hz) is likely due to the Sn 2 P 2 -ring strain and polar Sn-P bonds (see below). [21]The 119 Sn NMR signal of 5 is comparable with the chemical shifts reported for phosphastannirane (716 ppm) [22] as well as homonuclear tin cluster compounds [Sn(2,6-Mes 2 C 6 H 3 )] 4 (773 ppm) [23] and [Sn 8 (2,6-Mes 2 C 6 H 3 ) 4 ] (750 ppm). [24]Note, the values of 31 P NMR chemical shifts (−91.4-−98.6 ppm) and 1 J P-Sn (539-656 Hz) for the three coordinated phosphorus atom of V (Scheme 1) [14] are comparable to those of 5.While the 1 J P-Sn (1648 Hz) for IV (E = Sn) [12] with a partial Sn≐P bond is larger than that of 5.
Thermal or photo-induced decarbonylation of phosphaethynolato-compounds (RE-PCO) is known to generate phosphinidene species (RE-P), [15] which can also be isolated as monomeric compounds. [25]The exact mechanism for the formation of 5 is currently unknown.The decarbonylation of 2 is likely to result in a putative bis-phosphinidene species 4a (Scheme 2), which spontaneously undergoes insertions into the Sn-C bonds to form 5. Our preliminary calculations predict a triplet ground state for 4a (see the Supporting Information), which is energetically 104 kcal mol −1 higher than 5.The exposure of a C 6 D 6 sample of 2 to white light enhances the conversion of 2 into 3a.Therefore, an alternative mechanism in which 3a (via 3b) step-wise undergoes decarbonylation and Sn-C bond insertion (via 4b → 4c → 4d) to ultimately yield 5 cannot be ruled out.In situ NMR monitoring of a sample, however, indicates the presence of 2, 3a, and 5, with no sign of any additional species such as 4c.
To shed further light on the electronic structure 5, we performed quantum chemical calculations (see the Supporting Information for details).The DFT optimized structure of 5 (Figure S60, Supporting Information) at the PBE0-D3BJ/def2-TZVPP level of theory is in good agreement with its X-ray diffraction structure (Figure 2a).
Calculations suggest a closed-shell singlet ground state for 5, which is 26.9 kcal mol −1 lower than the triplet state.The reluctance of heavier main-group elements to participate in bonding interactions is evident in the frequent formation of clusters and cages. [27]Thus, like the main-group phosphanediides with bridging RP 2− ligands, [28] 5 may be regarded as a Sn(II) phosphanediide species (see Figure 2b), in which Sn-P bonds are polarized toward phosphorus atoms.The values of NBO (Natural Bonding Orbital) charges and WBIs (Wiberg Bond Indices) for 5 (Figure 2b) are consistent with this description.The HOMO and HOMO-2 of 5 (Figure 2c) are the -lone-pairs at the tin atoms with a considerable contribution of phosphorus p-orbitals.The LUMO is located mainly on the ligand, while the HOMO-1 resides largely at the phosphorus atoms.The UV-vis spectrum of 5 exhibits three absorption bands at  max = 306, 420, and 688 nm.Based on TD-DFT calculations, they may be assigned to HOMO-2 → LUMO+2, HOMO-1 → LUMO+1, and HOMO → LUMO transitions, respectively (Table S7, Supporting Information).
We also performed fractional occupation number weighted density (FOD) calculations (at the PBE0/def2-TZVPP level of theory) originally introduced by Grimme et al. as a static electron correlation (SEC) diagnostic. [29]FOD analyses provide reliable information on the localization of "hot" (strongly correlated and chemically active) electrons in a molecule.The FOD plot (Figure 2d) and the resulting N FOD number (3.07 e) indicate relatively large electron correlation in 5. To further analyze the static electron correlation in 5, we performed SS-CASSCF (state-specific complete active space self-consistent field) calculations with a CAS(8,8) active space (see the Supporting Information for details).The singlet ground state solution at the CASSCF/def2-TZVPP level with an occupation pattern of "22220000" (91%) and four other double excited configurations (1.1-1.5% each) point to a rather negligible diradical character [30] of 5.
Compounds IV (E = Sn, Scheme 1) with a partial Sn≐P double bond was found to be inert toward H 2 , CO, and CO 2 but underwent the reaction with Ph 2 C═C≐O to form a [2+2]cycloaddition product with a four-membered SnPCO ring. [12]We prompted to reason that the ring strain and highly polarized nature of Sn-P bonds may be attributed to the interesting reactivity of 5. We therefore decided to explore the reactivity of 5 with small molecules. [31]No reaction between 5 and carbon monoxide (1 atm) to give 2 or 3a was observed even at elevated temperatures.Similarly, no reaction between 5 and H 2 (1 atm) was observed.The exposure of a green C 6 D 6 solution of 5 to carbon dioxide (1 atm) at room temperature led to a wine-red solution after 3 h.NMR studies revealed the presence of a mixture of 5 (15%) and 6 (85%) (Scheme 3a).Warming the reaction mixture led to the depletion of 6 and the restoration of 5.In addition, removal of the volatiles under vacuum at room temperature resulted in the complete regeneration of 5.These data collectively suggest reversible CO 2 uptake with 5 (Scheme 3a).31a,32] In contrast, compound 5 irreversibly reacts with CS 2 , yielding a new species that is completely different than 6 according to NMR analyses.Further characterization of this species in currently underway in this laboratory.Reaction of 5 with CO 2 to give 6 is calculated to be thermodynamically favored by 16.9 kcal mol −1 , while the further reaction of 6 with CO 2 to yield 6-CO 2 is endothermic by 3.3 kcal mol −1 (see Figure S66, Supporting Information).Moreover, the comparison of the energy of HOMO of 5 (−4.06 eV) and 6 (−4.24 eV) suggests that the latter is less basic.
Encouraged by these findings, we sought to investigate the catalytic reduction of CO 2 with 5. Indeed, 5 enables catalytic hydroboration of CO 2 with pinacolborane (HBpin) to form different products (Scheme 3b) depending on the reaction conditions.Catalytic hydroboration of CO 2 (1 atm) was carried out with different loadings of 5 (1 or 5 mol% with respect to HBpin) and HBpin in C 6 D 6 either at room temperature or at 70 °C.The conversion was monitored by 1 H and 11 B NMR spectroscopy (see the Supporting Information).At room temperature, full consumption of HBpin was achieved after 20 h with 5 mol% of 5, giving rise to HCO 2 Bpin as the sole product.At 70 °C, the complete consumption of HBPin required 2 h with 1 mol% of 5 to result in a mixture of HCO 2 Bpin, CH 3 OBpin, CH 2 (OBpin) 2 , and O(Bpin) 2 .The use of 5 mol% of 5 at 70 °C led to the formation of HCO 2 Bpin as the main product only after 10 min.No reaction between CO 2 and HBpin was observed under similar experimental conditions.31a,33] To obtain suitable single crystals of 6 for sc-XRD, a freshly prepared equilibrium solution of 5 (15%) and 6 (85%) in toluene under CO 2 atmosphere (1 atm) was stored at −35 °C for three days.The obtained red crystals were analyzed by sc-XRD.The sc-XRD data revealed that the resulting crystals contain 5 and 6 in 11:89% ratio (see the Supporting Information).Interesting, the drying of crystals under vacuum also led to the removal of CO 2 to yield 5. Thus, a pure sample of 6 for NMR studies could not be obtained.The NMR data for 6 were obtained by analyzing a sample containing 5 and 6 in 15:85 ratio and collected by subtracting the signals due to 5. As expected, the formation of 6 from 5 leads to the lowering of symmetry.Thus, the 1 H NMR spectrum of 6 exhibits a doublet and a septet for each of the isopropyl groups, which is consistent with the 13 C{ 1 H} NMR spectrum of 6.The 31 P{ 1 H} NMR spectrum of 6 shows two distinct doublets at −171.2 and −74.1 ppm ( 2 J P-P = 25.4Hz) accompanied by the corresponding tin satellites.The former is high-field (Δ = 61 ppm) while the latter is downfield (Δ = 36 ppm) shifted with respect to that of 5 (−110.4ppm).The 119 Sn{ 1 H} NMR spectrum of 5 reveals one pseudo triplet at 373.9 ppm ( 1 J Sn-P = 663.2Hz for SnP 2 unit) and a doublet at 466.6 ppm ( 1 J Sn-P = 666.2Hz for OSnP moiety).This is consistent with the presence of two magnetically inequivalent phosphorus as well as tin nuclei in 6.
As revealed by the solid-state molecular structure of 6 (Figure 3), one molecule of CO 2 has been inserted into one Sn-P bond of 5.The oxygen atom of the inserted CO 2 unit binds to the tin (Sn-O, 2.137(4) Å) while the carbon atom is attached to the phosphor atom (P-C, 1.853(6) Å).This is in line with the polarity of Sn-P bonds of 5 (see Figure 2b).The Sn-O and P-C bond lengths of 6 compare well with the CO 2 adduct of tin/phosphorus-based frustrated Lewis pair (2.233(2); 1.891(3) reported by Mitzel et al., respectively). [34]Other P-C bond lengths of 6 (1.814(5)-1.853(6)Å) correspond to single bonds (1.86 Å).The peripheral 1,3-imidazole units are slightly less tilted (plane angle of 74.°) than in 5.The Sn-C bonds (2.214(4) and 2.231(1) Å) are in line with those of 5 (2.218(3)-2.245(3)Å).The C-O bond of the carbonyl oxygen (1.217(7) Å) is slightly shorter than that attached to the tin atom (1.297(7) Å).Both C-O bonds are longer than the C-O double bond of gaseous CO 2 (1.16 Å). [35] The sum of the angles at the Sn (Σ = 273.20°,271.77) and P (Σ = 287.48°,287.95) atoms are consistent with the presence of a stereoactive lone-pair.Further reactivity studies of 5 were performed with elemental selenium and Fe 2 (CO) 9 to afford compounds 7 and 8, respectively (Scheme 3a).Compound 7 was isolated as a red solid in 88% yield, which is formally a mixed-valent Sn(II)/Sn(IV) species.Insertion of one selenium atom into a Sn-P bond is akin to the formation of 6.The second tin atom, like stannylenes, [18b] reacts with selenium to result in a formal Sn(IV) center.Expectedly, the 119 Sn{ 1 H} NMR spectrum of 7 features two double doublets at 78.1 ( 1 J Sn-P = 804.2,693.3 Hz) and −66.5 ( 1 J Sn-P = 809.7,72.6 Hz) ppm, which may be assigned to Sn(IV) and Sn(II) nuclei, respectively.The 31 P{ 1 H} NMR spectrum of 7 shows two doublets at −110.1 and −121.5 ( 2 J P-P = 27 Hz) ppm along with tin satellites.The bis-stannylene iron(0) complex 8 was isolated as a red-brown solid in 50% yield.The 119 Sn{ 1 H} NMR spectrum of 8 shows a triplet at 494.9 ppm ( 1 J Sn-P = 493.8Hz), which is high-field shifted relative to that of 5 (748.1 ppm, 1 J P-Sn = 416 Hz).
The solid-state molecular structures of 7 (Figure 3) and 8 (Figure 4) show the expected atom connectivity and corroborate well with their spectroscopic data.The terminal Sn-Se bond length of 7 (2.415(1)Å) is in good agreement with that of a distannabarrelene derivative (2.388(5) Å) based on an ADC ligand [18b] as well as of other tin compounds (2.375(3)−2.394(1)Å) [36] featuring a terminal Sn-Se bond.The bridging Sn-Se bond length of 7 (2.681(1)Å) is slightly longer than the Sn─Se single bond lengths (2.55-2.60Å), [37] probably due to the ring strain.The P-Se bond length of 7 (2.252(1)18b] Consistent with the presence of a stereoactive lone-pair, the sum of the angles at the Sn(II) of 7 amounts to Σ = 273.39°.In 8 (Figure 4), the central Sn 2 P 2 cluster remains intact and each of the tin atoms serves as a twoelectron -donor ligand to bind with a Fe(CO) 4 fragment.Thus, the HOMO and HOMO-2 of 5 (Figure 2c), which essentially de-pict a lone-pair of electrons on the Sn centers, interact with the LUMO of Fe(CO) 4 fragments in resulting 8.The Sn-P (2.584(1)-2.648(1)Å) and C-Sn bond lengths (2.197(3); 2.206(2) Å) of 8 are slightly smaller compared to those of 5.This is in line with the transfer of electron density from the tin to the iron atom.The Sn-Fe bond lengths of 8 (2.505(1) Å, 2.504(1) Å) are comparable to the known Sn(II)-Fe(0) complexes (2.40-2.5 Å). [39] The IR spectrum of 8 exhibits three absorption bands at 2050, 1995, and 1910 cm −1 for CO stretching vibrations.

Conclusion
In conclusion, we have shown the isolation of Sn 2 P 2 cluster compound 5 embedded between two 1,3-imidazole frameworks as green crystals.Reactivity of 5 has been demonstrated with CO 2 , selenium, and Fe 2 (CO) 9 .Compound 5 reversibly uptakes CO 2 to form 6. One CO 2 molecule inserts into the Sn-P bond of 5 to result in a Sn-OC(=O)-P moiety in 6.Under vacuum or at elevated temperature, 6 readily releases CO 2 and regenerates 5. Catalytic hydroboration of CO 2 with 5 is presented.The mixed-valent Sn(IV)/Sn(II) compound 7 and the stannylene-Fe(0) complex 8 have been isolated as stable crystalline solids.All compounds have been characterized by multinuclear NMR spectroscopy and their solid-state molecular structures have been unequivocally established by sc-XRD.