Spectroscopic Identification of Trifluorosilylphosphinidene and Isomeric Phosphasilene and Silicon Trifluorophosphine Complex

The perfluorinated silylphosphinidene, F3SiP, in the triplet ground state is generated by the reaction of laser-ablated silicon atoms with PF3 in solid neon and argon matrices. The reactions proceed with the initial formation of a silicon trifluorophosphine complex, F3PSi, in the triplet ground state, and a more stable inserted phosphasilene, FPSiF2, in the singlet ground state upon deposition. The trifluorosilylphosphinidene was formed through F-migration reactions of FPSiF2 and F3PSi following a two-state mechanism under irradiation with visible light (λ = 470 nm) and full arc light (λ > 220 nm), respectively. High-level quantum-chemical methods support the identification of F3PSi, FPSiF2, and F3SiP by matrix-isolation IR spectroscopy.


■ INTRODUCTION
Phosphinidenes (R−P) are highly electron-deficient species that feature monovalent phosphorus analogues of nitrenes (R− N) 1 and carbenes (R−C−R′). 2As low-valent phosphorus species, they have been widely used in synthetic chemistry as in situ phosphorus agents 3−5 or as ligands in transition metal complexes. 6−11 In contrast to the free nitrenes and carbenes, which have been extensively investigated, 12 only a handful of uncomplexed phosphinidenes have been experimentally identified (Scheme 1).The parent phosphinidene (H−P) (1) was observed in the argon matrix following the photolysis of phosphaketene, HPCO. 13−16 Methoxyphosphinidene (CH 3 O−P) (4) was generated in a cryogenic neon matrix from the photolysis or flash-vacuum pyrolysis of methoxydiazidophosphine, and its isomeric methylphosphinidene oxide was detected. 17The production of ethynylphosphinidene (HCC− P) (5) from the dehydrogenation of phosphapropyne (CH 3 CP) via the 1-phosphapropadiene, CH 2 �C�PH, and ethynylphosphine, HCCPH 2 , intermediates through UV-lightinduced rearrangement has been reported as well. 18The simplest silylphosphinidene (H 3 Si−P) (6) was formed in an argon matrix experiment via reactions of atomic silicon with phosphane. 19Recently, singlet (phosphino)phosphinidenes (R 2 P�P), 20 stable at room temperature, and transient aminophosphinidenes (R 2 N�P) 21 have been structurally characterized in the solid state and observed by mass spectrometry, respectively.In addition, the singlet diphosphinylidene H 2 P�P was also prepared and characterized by matrix-isolation IR spectroscopy. 22All other directly observed free phosphinidenes have triplet electronic ground states.
It should be stressed that no experimental data have been reported on main-group fluorinated phosphinidenes so far.Trifluorosilylphosphinidene, F 3 Si−P, the perfluorinated silylphosphinidene, has been theoretically studied several times. 23,24It was found that phosphinidene F 3 Si−P is the most stable species of four F 3 PSi isomers in both the singlet and triplet states due to the strong Si−F bond formation.Previous reports have shown that metal phosphides, P�MF 3 (Cr, Mo, W, and U), 25,26 and triplet metal phosphinidenes, F 3 M−P (Ti, Zr, Hf, and Th), 27,28 can be formed from the reactions of laserablated metal atoms with trifluorophosphine.Herein, we report the production and spectroscopic characterization of triplet trifluorosilylphosphinidene via the reaction of laser-ablated silicon atoms with PF 3 in solid neon and argon matrices.We describe that the reactions proceed with the initial formation of a silicon trifluorophosphine complex, F 3 PSi, and the more stable inserted isomer FPSiF 2 upon deposition.The F 3 SiP molecule was synthesized through F-migration reactions of FPSiF 2 and F 3 PSi under irradiation at visible light (λ = 470 nm) and full arc light (λ > 220 nm), respectively.

■ RESULTS AND DISCUSSION
The F 3 PSi, FPSiF 2 , and F 3 SiP molecules are produced via the reaction of laser-ablated silicon atoms with PF 3 in solid neon and argon matrices.The infrared spectra in the 1000−500 cm −1 region obtained by using a 0.05% PF 3 /Ne sample are demonstrated in Figure 1.After 30 min sample deposition at 5 K, strong absorption bands of PF 3 − (740.4 and 470.9 cm −1 ) 28 are detected, which decrease on annealing and disappear completely upon full arc light excitation.−31 Besides these known absorption bands, new product absorption bands were detected as well.These absorption bands can be classified into three groups according to their identical chemical behaviors (A, B, and C in Figures 1 and 2).The difference infrared spectra showing the photochemical transformation are given in Figure 2. Similar experiments were repeated by using a 0.2% PF 3 /Ar sample.The infrared spectra in the selected region are shown in Figure S1; the corresponding difference infrared spectra are presented in Figure S2.The band positions are summarized in Table 1.
Product A has three absorption bands at 978.5, 933.lower wavenumbers with respect to their positions in the neon matrix due to the larger polarizability of the argon atom.
Considering the previously reported reactions of silicon atoms with PH 3 to form the H 3 PSi, H 2 PSiH, HPSiH 2 , and H 3 SiP molecules, 19 and of transition metal atoms with PF 3 to give PMF 3 complexes, 25−28 four possible isomers F 3 PSi, F 2 PSiF, FPSiF 2 , and F 3 SiP in the electronic singlet and triplet states were studied computationally at the CCSD(T*)-F12a/ aug-cc-pVTZ-F12 level.
Products A and B can be assigned to the phosphasilene isomer, FPSiF 2 , and the silicon trifluorophosphine isomer, F 3 PSi, respectively, by comparison with the computed IR spectra (Table 1 and Table S1).For product A, the first two absorption bands belong to antisymmetric and symmetric stretching vibration modes of the SiF 2 moiety.The two frequencies are very close to that of F 2 Si=S (996, 969 cm −1 , Ar matrix) 32 but lower than that in SiF 2 (864.6,851.0 cm −1 , Ne matrix). 30The band at 765.4 cm −1 can be attributed to the P− F stretching vibration, which is red-shifted compared to FP�S (791.4 cm −1 , Ar matrix), 33 FP�O (811.4 cm −1 , Ar matrix), 34 and FP�NF (826.5 cm −1 , Ar matrix). 35The band positions are in excellent agreement with the computed anharmonic IR frequencies at 977, 933, and 765 cm −1 for the singlet FPSiF 2 molecule.The predicted P−Si stretching vibration at 521 cm −1 is too weak to be observed experimentally.
For product B, the absorption band at 895.9 cm −1 belongs to the antisymmetric vibration mode of PF 3 .This position is very close to the bands of trifluorphosphine and lower than those of transition metal trifluorophosphine complexes. 36The P−Si stretching vibration occurs at 522.1 cm −1 as a weak absorption band.The band position is slightly blue-shifted compared to F 3 SiPH 2 (514 cm −1 , gas phase). 37The calculated anharmonic IR frequencies at 902 and 521 cm −1 of triplet silicon trifluorophores match the experimental values very well.The PF 3 symmetric stretching vibration was predicted at 886 cm −1 as a strong absorption band and could not be observed due to the overlap with the respective band of PF 3 .The other computed vibrations of FPSiF 2 and F 3 PSi are outside the present mid-IR spectral range (4000−450 cm −1 ).
Experimentally, C is produced under the irradiation of products FPSiF 2 (A) and F 3 PSi (B).The reactive intermediates generally rearrange to more stable structural isomers or decompose to stable products upon photolysis.In this case, the absence of FP/SiF 2 and P/SiF 3 species among the photolysis products indicates that the P−Si bond in both products was not cleaved when the matrix samples were subjected to light irradiation.This is in accordance with the large P−Si bond dissociation energies of FPSiF 2 (33.6 kcal mol −1 , CCSD(T*)-F12a/aug-cc-pVTZ-F12) and F 3 SiP (68.3 kcal mol −1 ).Therefore, it is very likely that F-migration happens in matrix-isolated FPSiF 2 (A) and F 3 PSi (B) under irradiation conditions.Accordingly, species C is safely assigned to the triplet trifluorosilylphosphinidene, F 3 SiP, the most stable isomer of F 3 PSi.Its predicted vibrational frequencies (Table 1) are in excellent agreement with the detected IR spectra.The SiF 3 antisymmetric stretching mode appears at 965.6 cm −1 , which is very close to those of SiF 3 (958.6cm −1 , Ne-matrix) 30 and F 3 SiPH 2 (970 cm −1 , gas phase). 37The weak band at 516.0 cm −1 is assigned to the Si−P stretching vibration mode, which is blue-shifted to 522.1 cm −1 in the F 3 PSi complex.However, the computed SiF 3 symmetric vibration at 855 cm −1 is not detected due to its overlap with the PF 3 bands in this region.The calculated vibrational frequencies of the proposed isomer F 2 PSiF (see Table S1) do not match any experimentally observed IR bands.As the agreement between theory and experiment is excellent for the other isomers, we conclude that no significant amount of F 2 PSiF is present at any given time.
The relative energies and structures of F 3 PSi isomers A, B, and C determined by ab initio calculations (CCSD(T*)-F12a/ aug-cc-pVTZ-F12) are shown in Figure 3.The FPSiF 2 (A) species was predicted to have a singlet ground state with a planar structure.The calculated Si−P bond length is 2.104 Å, which is close to the value (2.094 Å) determined by X-ray crystallography on phosphasilenes, but slightly longer than those of HPSi (2.045 Å) 38 and HPSiH 2 (2.084 Å). 19 The analysis using the AdNDP method 39 on FPSiF 2 shows one Si− P σ bond and one Si−P π bond (Figure S3), suggesting the Si−P double bonding character.The calculated Wiberg bond index for the Si−P bond is 1.71, consistent with the AdNDP results.Still, the Si−P bond in FPSiF 2 is weaker than the formal single bond in F 3 SiP (see above).NBO 40 analysis suggests that this is due to a significant population of the Si−P π* bond due to negative hyperconjugation.
Silicon trifluorophosphine complex B, which has not been discussed in earlier theoretical studies, possesses, according to calculations and experiments, a triplet ground state and C 3v symmetry.The energetically higher-lying singlet silicon trifluorophosphine has C 1 symmetry and lies 15.2 kcal mol −1 above B. The computed Si−P bond length (2.212 Å) is larger than in FPSiF 2 (2.119 Å) but shorter than in the H-analogue H 3 PSi (2.356 Å). 19 Similar to other phosphinidenes, such as HP, 13 C 6 H 5 P, 14 CH 3 OP, 17 and H 3 SiP, 19   To unravel the reaction mechanism, the potential energy profile for the reaction of silicon atoms with PF 3 was calculated in both singlet and triplet states at the CCSD(T*)-F12a/augcc-pVTZ-F12//B3LYP/aug-cc-pVTZ level.The results are summarized in Figure 4.The structures of the corresponding intermediates and transition states are displayed in Figure S4.Similar to the reaction of a silicon atom with PH 3 , 19 the first step of the formation of F 3 PSi, starting from a Si atom and trifluorophosphine, is predicted to be exothermic and barrierfree.
If the initial silicon atom has a triplet spin state, we suspect the formation of product B. As further barriers on the triplet surface are high, we expect no further reactions at this point.A singlet Si should lead to the formation of singlet F 3 PSi, which then reacts thermally to product A by fluorine transfer.The intermediately formed F 2 PSiF is not isolated as both barriers (TS1−1 and TS2−1) in the process are of similar height.Inspection of the D 1 diagnostics 41 for the CCSD wave functions of the singlet species shows unsuspicious values between 0.01 and 0.05 for every species but the second transition state (TS2−1).For the latter, D 1 surpasses 0.20, indicating a stronger multireference character.This can be rationalized by the fact that FPSiF 2 (product A) contains a Si− P double bond, while F 2 PSiF does not.The transition state between the two is therefore an intermediate between a Si−P single and double bond, giving rise to the importance of at least two different configurations, thereby suggesting a multireference character.As this will give rise to correlationenergy contributions of TS2−1 beyond what is captured at the CCSD(T*)-F12a level, the calculated barrier shown in Figure 4 is an upper bound.Additional calculations with a cc-pVDZ basis set suggest a further lowering of the barrier at TS2−1 compared to that at CCSD(T) by 2 kcal/mol when applying the higher CCSDT(Q) level.The effect on the other barriers is about one order of magnitude smaller.Possibly, further reductions may be expected at levels with even more static correlation.In any case, this barrier is significantly smaller than those at either TS1−1 or at TS3−1.Keeping in mind that energy dissipation in an Ar matrix is much smaller than for reactions in solution yet significant compared to a gas-phase reaction, 42 this is suggested to explain the sole formation of A in the thermal reaction.That is, we have to assume that upon formation of A, energy dissipation is already so large that the last, higher barrier at TS3−1 cannot be overcome thermally.The computed result is consistent with our experimental observation that only very little product A is formed overall.
STEOM-CCSD/aug-cc-pVTZ calculations find a (formally spin-forbidden) singlet−triplet excitation for product A at around 465 nm.The formed triplet state can then cross TS3 on the triplet surface and relax to give the final product C.This explains the experimentally observed transition from A to C upon irradiation with blue light.At the same level, the first triplet−triplet excitation of B is found only at 390 nm with several more excitations present below 220 nm.Upon irradiation with full arc light, several excited-state pathways will therefore be accessible, allowing for the formation of thermodynamically stable product C.

■ CONCLUSIONS
In conclusion, we report the reactions of laser-ablated silicon atoms and PF 3 forming phosphasilene FPSiF 2 (A) and silicon trifluorophosphine complex F 3 PSi (B) in solid neon and argon matrices.The phosphasilene molecule has an electronic singlet ground state, while the silicon trifluorophosphine complex has a triplet ground state.The FPSiF 2 (A) and F 3 PSi (B) molecules rearrange to the more stable trifluorosilylphosphinidene F 3 SiP (C) molecule by one-and three-fluorine atom migration under irradiation at visible light (λ = 470 nm) and full arc light (λ > 220 nm), respectively.The latter molecule is identified to have a triplet electronic ground state.

■ EXPERIMENTAL AND COMPUTATIONAL DETAILS
The experimental method for matrix-isolation infrared spectroscopy has been described in more detail in our previous works. 43Briefly, the 1064 nm fundamental of a Nd:YAG laser (Continuum, Minilite II, 10 Hz repetition rate with 10 ns pulse width) with a pulse energy of up to 50−60 mJ cm −2 was used to ablate a rotating silicon target (abcr, 99.999%) to form Si atoms.The produced Si atoms were codeposited with PF 3 (abcr, 99%) (0.05%) in excess of neon (0.2% in argon) onto a gold-plated mirror cooled to 4K using a closed-cycle helium refrigerator.After 30 min of sample deposition, infrared spectra were recorded on a Bruker Vertex 80 spectrometer at 0.5 cm −1 resolution in the region between 4000 and 450 cm −1 using a liquid nitrogencooled mercury cadmium telluride (MCT) detector.The matrix samples were subjected to irradiation with visible light using a LED light (Oslon 80 4+ PowerStar Circular 4 LED Arrays: λ = 470 ± 20 nm) and a medium-pressure mercury arc streetlamp (λ > 220).No uncommon hazards are noted.
Density functional theory (DFT) calculations were carried out using the Gaussian 16 program package. 44The hybrid functional B3LYP 45−48 was applied in our calculations to obtain molecular structures of minima and transition states.All stationary points were characterized by the appropriate number of imaginary frequencies.All transition states were further analyzed using an intrinsic reaction coordinate calculation and were confirmed to connect the correct minima of the potential energy surface.
The Coupled Cluster calculations with Single, Double, and perturbative Triple substitutions CCSD(T) were carried out in the closed-shell (RHF-CCSD(T)) and partially spin-restricted open-shell (RHF-RCCSD(T)) formalism using default frozen core settings as implemented in the Molpro2022 software package. 49To approach the basis set limit, explicitly correlated calculations were performed using the F12a approximation. 50,51Explicit correlation effects on the perturbative triples were estimated by scaling the (T) contribution as All calculations were performed using aug-cc-pVTZ-F12 basis sets 52 as well as the respective auxiliary basis sets automatically assigned by the MOLPRO program.
Anharmonic vibrational frequencies were calculated at the VCISDTQ56 level of theory 53,54 allowing up to five excitations within one mode.VCI calculations were performed on a polynomial fit 55 of a multilevel surface 56,57 using CCSD(T*)-F12a/aug-cc-pVDZ-F12 energies for the two-body and CCSD-F12a/aug-cc-pVDZ-F12 energies for the three-body terms.Intensities were computed using dipole moments at the HF level of theory.
Bonding analyses were carried out by using the adaptive natural density partitioning (AdNDP) method.The color-mapped AdNDP isosurface (0.5 au) graphs were rendered by the VMD 1.9.3 program. 58Additionally, natural-bond orbital (NBO) analyses were used.Optical excitation energies were obtained at the STEOM-CCSD 59,60 level.Calculations were performed using the ORCA program, 61 Version 5.0.2.All calculations employed the RIJCOSX approximation, 62 the aug-cc-pVTZ basis sets, 63 and the appropriate auxiliary basis sets. 64,65In every case, the first 10 excitation energies were obtained.Additional CCSDT(Q) calculations were performed using the MRCC program package and the cc-pVDZ basis set. 66ASSOCIATED CONTENT * sı Supporting Information The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.4c00135.
Infrared spectra from co-deposition of silicon atoms with PF 3 in argon; difference infrared spectra from codeposition of Si atoms with PF 3 in argon; AdNDP chemical bonding and nonbonding pattern of FPSiF 2 ; calculated structural parameters of the higher-energy isomers and the transition states; calculated total energies and IR frequencies for various F 3 PSi isomers; and the Cartesian coordinates of the studied complexes (PDF) ■ AUTHOR INFORMATION 7, and 765.4 cm −1 .They are observed right after deposition and almost do not change when annealing to 10 K but completely disappear under blue LED (λ = 470 nm) light irradiation.Two absorption bands at 891.0 and 520.4 cm −1 are observed for product B. Both increase slightly under blue LED (λ = 470 nm) light irradiation but disappear upon full arc (λ > 220 nm) irradiation.Product C has two absorption bands at 965.5 and 516.0 cm −1 .These absorption bands almost do not change upon annealing but remarkably increase under blue LED (λ = 470 nm) light and full arc (λ > 220 nm) irradiation at the expense of the absorption bands of products A and B, respectively.The product absorption bands in the argon matrix are located at 974.2, 933.1, 759.4 cm −1 (A), 891.0, 520.4 cm −1 (B), and 959.7, 513.6 cm −1 (C).All bands are red-shifted to
which have been characterized by a triplet ground state, the trifluorosilylphosphinidene (C) is predicted to have a 3 A 1 ground state with C 3v symmetry.The calculated Si−P bond length is 2.230 Å, which is larger than the values of FPSiF 2 (2.119 Å) but close to that of the F 3 PSi (2.250 Å) complex, calculated at the same level of theory.Si−P multiple bonding is absent.The calculated singlet−triplet energy gap Δ(E ST ) of trifluorosilylphosphinidene is 25.0 kcal mol −1 .As the most stable isomer of F 3 PSi, the triplet trifluorosilylphosphinidene, F 3 SiP (C), lies −34.8, −60.6, and −84.8 kcal mol −1 below FPSiF 2 (A), F 2 PSiF, and F 3 PSi (B), respectively.

Table 1 .
Experimentally Observed and Calculated IR Frequencies (cm −1 ) of FPSiF 2 , F 3 PSi, and F 3 SiP Molecules (Absorption Bands above 400 cm −1 Are Listed) d asym.SiF 3 str.aAnharmonicfrequenciescalculatedat the CCSD(T*)-F12a/aug-cc-pVTZ-F12 level; the complete sets of vibrational frequencies are provided in Supporting Information TableS1.Intensities in parentheses were obtained from HF dipole moments.bAssignmentsbased on calculated vibrational displacement vectors.cAbsorptionband not observed due to small intensity.dThecomponent of the formally degenerate vibration with higher intensity is given.e Absorption band not observed due to overlap with the PF 3 band.