Isolation of an elusive phosphatetrahedrane

Extreme strain eased with P-incorporation.

Tri-tert-butyl cyclopropenyl tetrafluoroborate (18) and HPA (7, A = 9,10-dihydroanthracene-9,10-diyl) (17) were prepared according to literature procedures. Triphenylborane (Strem Chemicals), triphenylphosphine (Sigma-Aldrich), sodium bis(trimethylsilyl)amide (Sigma-Aldrich), lithium 2,2,6,6-tetramethylpiperidide (Sigma-Aldrich), trifluoromethanesulfonic acid (Strem Chemicals), and tungsten hexacarbonyl (Strem Chemicals) were used as received. Lithium 2,2,6,6-tetramethylpiperidide was also prepared according to a literature procedure (34). Pyridine (Sigma-Aldrich) was distilled under air, degassed three times by the freeze-pump-thaw method, and stored over 4Å molecular sieves for 48 h prior to use. Tetramethylammonium fluoride (Sigma-Aldrich) was dried at 100 • C under reduced pressure (50 mTorr) for 48 h prior to use. Tetrabutylammonium chloride (Sigma-Aldrich) was dried at 60 • C under reduced pressure (50 mTorr) for 48 h and crystallized from acetonitrile/pentane prior to use. NMR spectra were obtained on a Jeol ECZ-500 instrument equipped with an Oxford Instruments superconducting magnet, on a Bruker Avance 400 instrument equipped with a Magnex Supplementary Materials Section S1. Synthetic details and characterization of products Scientific or with a SpectroSpin superconducting magnet, or on a Bruker Avance 500 instrument equipped with a Magnex Scientific or with a SpectroSpin superconducting magnet. 1 H and 13 C NMR spectra were referenced internally to residual solvent signals (3 ). 31 P NMR spectra were externally referenced to 85% H 3 PO 4 (0 ppm). 11 B NMR spectra were externally referenced to BF 3 -OEt 2 (0 ppm). 19 F NMR spectra were externally referenced to CFCl 3 (0 ppm). Elemental High resolution mass spectral (HRMS) data were collected using a Jeol AccuTOF 4G LC-Plus mass spectrometer equipped with an Ion-Sense DART source. Data were calibrated to a sample of PEG-600 and were collected in positive-ion mode. Samples were prepared in THF (10 µM concentration) and were briefly exposed to air (<5 s) before being placed in front of the DART source.
Photochemical reactions were performed using a Rayonet photochemical reactor  (Southern New England Ultra Violet Company) loaded with 16 RPR2537A lamps, each emitting ca. 35 W at 253.7 nm.

S1.2 Synthesis of [Na(OEt 2 ) 2 ][Ph 3 BPA] (Na[8])
Aluminum foil was used to limit exposure to ambient light during this experiment. A 250 mL flask charged with a solution of HPA (7, 0.850 g, 4.04 mmol, 1.00 equiv) and triphenylborane (1.10 g, 4.55 mmol, 1.13 equiv) in diethyl ether (60 mL) was frozen in the liquid nitrogen cooled coldwell of the glovebox. Separately, a solution of sodium bis(trimethylsilyl)amide (0.834 g, 4.55 mmol, 1.13 equiv) in diethyl ether (10 mL) was prepared and frozen in the coldwell of the glovebox. Upon thawing, the sodium bis(trimethylsilyl)amide solution was added rapidly to the thawing solution of 7. The solution became white and heterogeneous as it warmed with 5 rapid stirring. After 20 min, the colorless precipitate was collected by vacuum filtration and was washed with Et 2 O (2 × 10 mL). This afforded colorless powder of Na[8] (2.09 g, 3.34 mmol, 83%). The number of Et 2 O molecules in the formula has been determined by X-ray crystal-    S1.3 Synthesis of ( t BuC) 3 PA (9) Aluminum foil was used to limit exposure of the reaction mixture to ambient light during this experiment. A 100 mL flask charged with a solution of Na [8] (1.00 g, 1.61 mmol, 1.00 equiv) in THF (10 mL) and a Teflon R -coated magnetic stir bar was frozen in the liquid nitrogen cooled coldwell of the glovebox. Separately, a solution of tri-tert-butyl cyclopropenyl tetrafluoroborate (18) (0.473 g, 1.61 mmol, 1.00 equiv) in THF (20 mL) was prepared and frozen in the coldwell of the glovebox. Upon thawing, the solution of tri-tert-butyl cyclopropenyl tetrafluoroborate was rapidly added to the thawing solution of Na [8]. The solution became cloudy as it warmed with rapid stirring. After 1 h, the solution was filtered through a coarse sintered frit (15 mL) containing a one-inch plug of Celite R . All volatile materials were removed in vacuo and the resulting white solids were taken up in hexanes (16 mL). Pyridine (ca. 12 drops) was added to the solution, causing precipitation of the triphenylborane adduct of pyridine as a colorless solid (3 ). The reaction mixture was filtered through a coarse sintered frit (15 mL) containing a two-inch plug of charcoal and the plug was washed with hexanes (15 mL). All volatile materials were removed in vacuo from the combined filtrates yielding colorless solids. S1.4 Thermolysis of ( t BuC) 3 PA (9) A 20 mM solution of ( t BuC) 3 PA (9) in toluene-d 8 was prepared and was transferred to a J.
Young tube. The tube was placed in a preheated (110 • C) oil bath for 22 h. Compound 9 was largely unchanged after the heating process and only a trace of ( t BuC) 3 P (1) was observed ( Fig. S10).

S1
.5 Characterization of melted ( t BuC) 3 PA (9) ( t BuC) 3 PA (9, 2 mg) was loaded into a flame sealed glass capillary and was melted at 130 • C. No discoloration of the material was observed. The melted material was extracted from the capillary with benzene-d 6 (0.7 mL) and the homogeneous solution was transferred to an NMR tube.
The solution was irradiated with 254 nm light. 31 P{ 1 H} NMR spectra were collected after 10 min and 45 min. While traces of ( t BuC) 3 P (1) were observed after 10 min of irradiation ( Fig. S13), this signal disappeared after 45 min of irradiation (Fig. S14). Photolysis also generates a new 31 P NMR signal at −47.96 ppm, which we tentatively assign to a [2+2] dimer of tri-tert-butyl phosphacyclobutadiene (refer to section S1.13 for more information). Fig. S14. 31 P{ 1 H} NMR (162 MHz, hexanes, 25°C) spectrum of 9 in hexanes after being exposed to 254-nm light for 45 min. S1.7 Synthesis of ( t BuC) 3 P (1) To a 20 mL scintillation vial charged with a solution of ( t BuC) 3 PA (9, 0.300 g, 0.719 mmol, 1.00 equiv) in THF (2 mL) and a Teflon R -coated magnetic stir bar, was added a solution of trifluoromethanesulfonic acid (0.108 g, 0.719 mmol, 1.00 equiv) in THF (2 mL). After 20 min, a slurry of tetramethylammonium fluoride (0.067 g, 0.719 mmol, 1.00 equiv) in THF (1 mL) was added dropwise. After the addition, the colorless heterogeneous solution was stirred for 30 min. All volatile materials were then removed in vacuo from the solution, resulting in a colorless residue. This material was slurried in pentane (5 mL) and the solution was filtered through a coarse sintered frit (15 mL) containing a one-inch plug of charcoal. The plug was washed with pentane (10 mL). All volatile materials were then removed in vacuo from the combined filtrates, resulting in a colorless oil. This oil was taken up in THF (2 mL) to give a solution of ( t BuC) 3 P(F)H (10) that was frozen in the liquid nitrogen cooled coldwell of the glovebox. Separately, a solution of lithium tetramethylpiperidide (0.106 g, 0.719 mmol, 1.00 equiv) in THF (2 mL) was prepared and frozen in the liquid nitrogen cooled coldwell of the glovebox. Upon thawing, the lithium tetramethylpiperidide solution was added dropwise to the thawing solution of 10. After 20 min, all volatile materials were removed in vacuo, yielding colorless solids. The solids were taken up in pentane (2 mL) and the solution was filtered through a coarse sintered frit (15 mL) containing a one-inch plug of acidic alumina. The plug was subsequently washed with pentane (1 mL). Under reduced pressure, all volatile materials were removed from the combined filtrates, yielding a pale yellow oil (87 mg, see Fig. S26 and Fig. S27 for NMR characterization). This oil was transferred to a Teflon R -sealed trap-to-trap distillation apparatus, which was removed from the glovebox, connected to a Schlenk line, and placed under static vacuum (50 mTorr). One trap was kept at 23 • C by using an oil bath, while the other trap was cooled to −78 • C by using a mixture of dry ice and acetone (see Fig. S15).
After 2 h, colorless oil collected in the −78 • C trap while a yellow gel formed in the 23 • C trap.
The apparatus was removed from the two baths, backfilled with nitrogen, and brought into the glovebox. The colorless oil was taken up in pentane (0.5 mL) and the solution was filtered through a glass fiber filter paper plugged pipette containing a two-inch plug of silica. The plug was subsequently washed with pentane (1.5 mL). All volatile materials were removed from the combined filtrates under reduced pressure, yielding colorless solids (33 mg, 0.138 mmol, 19% S1.7.1 Carbon-13 NMR satellites of ( t BuC) 3 P (1) Collection of 31 P{ 1 H} NMR data on a 162 MHz instrument led to the observation of two sets of isotope shifted satellites associated with carbon-13 incorporation in the tetrahedrane core (green) and in the tert-butyl groups (blue) (Fig. S29 and Fig. S30). However, the corresponding signal of each set was obscured by the main, unsubstituted phosphatetrahedrane 31 P NMR signal (orange). Collection of 31 P{ 1 H} NMR data on a 202 MHz instrument resolved the 13 C satellites associated with carbon-13 incorporation in the tetrahedrane core (green). Also note that the 1 J PC coupling constant of this set of satellites is 37.9 Hz, identical to what is measured in the 13 C{ 1 H} NMR spectrum (Fig. S18). Integration of the natural abundance 13 C satellites is consistent with three equivalent carbon atoms bonded to a single phosphorus atom.   S1.7.2 Raman spectrum of ( t BuC) 3 P (1) In a glovebox, ( t BuC) 3 P (1, 2 mg, 0.008 mmol) was loaded into a quartz capillary, and the opening was sealed with vacuum grease. The capillary was placed in the instrument and analyzed using 538 nm excitation. Density functional theory (DFT) calculations were carried out as described in section S3.1. A band was observed in the experimental spectrum S32 at 1580 cm −1 , corresponding to the totally symmetric breathing mode (a 1 ), according to pseudo-C 3v symmetry.

(breathing mode)
A scaling factor of 0.955 was applied to the predicted vibrational frequencies of 1 (38), as discussed in S3.1.   S33. Visualization of the totally symmetric breathing mode (a 1 ), according to pseudo-C 3v symmetry, of 1.

S1.7.4 Stability of [( t BuC) 3 P(H)A][OTf] in solution
To a 20 mL scintillation vial charged with a solution of ( t BuC) 3 PA (9, 0.050 g, 0.12 mmol, 1.00 equiv) in THF (1 mL) was added a solution of trifluoromethanesulfonic acid (0.018 g, 0.12 mmol, 1.00 eq) in THF (1 mL). After stirring for 20 min, the solution was then filtered through Celite R . An aliquot (0.5 mL) of the solution was transferred to an NMR tube. To this NMR tube was added a glass capillary containing a 0.67 M solution of Ph 3 P in benzene-d 6 .
This reaction was monitored by 31 P{ 1 H} NMR (see Fig. S40 and Fig. S41 S1.7.5 Generation of ( t BuC) 3 P(F)H (10) To a 20 mL scintillation vial charged with a solution of ( t BuC) 3 PA (9, 0.050 g, 0.12 mmol, 1.0 equiv) in THF (1 mL) was added a solution of trifluoromethanesulfonic acid (0.018 g, 0.12 mmol, 1.00 eq) in THF (1 mL). After stirring for 20 min, a slurry of tetramethylammonium fluoride (0.011 g, 0.12 mmol, 1.0 equiv) in THF (1 mL) was added dropwise. After stirring for 30 min, all volatile materials were then removed in vacuo from the solution, resulting in a colorless residue. This material was slurried in pentane (2 mL) and the solution was filtered through a glass fiber filter paper plugged pipette (2 mL) containing a one-inch plug of charcoal.
All volatile materials were then removed in vacuo, resulting in a colorless oil (22 mg). While        S1.7.6 Stability of ( t BuC) 3 P(F)H (10) in solution A solution of ( t BuC) 3 P(F)H (10) in benzene-d 6 was transferred to an NMR tube. To this NMR tube was added a glass capillary containing a 0.67 M solution of Ph 3 P in benzene-d 6 . This sample was monitored by 31 P{ 1 H} NMR spectroscopy over a period of 48 h (see Fig. S48 and     To a 20 mL scintillation vial charged with a solution of ( t BuC) 3 PA (9, 0.050 g, 0.12 mmol, 1.0 equiv) in THF (1 mL) was added a solution of trifluoromethanesulfonic acid (0.018 g, 0.012 mmol, 1.00 eq) in THF (1 mL). After stirring for 20 min, the heterogeneous solution was added to a vial containing tetrabutylammonium chloride (0.033 g, 0.12 mmol, 1.00 eq).
After vigorously stirring for 20 min, the reaction mixture became homogeneous. This solution was filtered through a glass fiber filter paper plugged pipette (2 mL) containing a one-inch plug      S1.8 Air stability of ( t BuC) 3 P (1) in solution A 0.007 M solution of 1 in benzene-d 6 was prepared and transferred to an NMR tube. To this tube was added a glass capillary containing a 0.67 M solution of Ph 3 P in benzene-d 6 and an initial 31 P{ 1 H} NMR spectrum was collected (Fig. S54). The cap of the tube was removed outside of the glovebox for a period of 30 min. After this period, a 31 P{ 1 H} NMR spectrum was collected (Fig. S55), revealing no consumption of 1. An additional 31 P{ 1 H} NMR spectrum was collected after being exposed to air for 12 h (Fig. S56  S1.9 Thermal stability of ( t BuC) 3 P (1) in solution (75 • C) A 0.02 M solution of 1 in benzene-d 6 was prepared and transferred to an J. Young tube. To this tube was added a glass capillary containing a 0.67 M solution of Ph 3 P in benzene-d 6 and an initial 31 P{ 1 H} NMR spectrum was collected (Fig. S57). This tube was then placed in a preheated 75 • C oil bath for 45 min. After this period a 31 P{ 1 H} NMR spectrum was collected ( Fig. S58), revealing no consumption of 1.

S1
.10 Thermal stability of ( t BuC) 3 P (1) in solution (130 • C) A 0.06 M solution of 1 in toluene-d 8 was prepared and transferred to a flame-sealed NMR tube. An initial 31 P{ 1 H} NMR spectrum was collected (Fig. S59). This tube was transferred to a preheated 130 • C oil bath. After 3 h, a 31 P{ 1 H} NMR spectrum was collected (Fig. S60), revealing formation of a diphosphahousene previously reported by Slootweg et al. (14) and a small amount of what we tentatively assign as phosphacyclobutadiene dimer (refer to section S1.13 for more information).

S1
.11 Photolysis of ( t BuC) 3 P (1) A 0.020 M solution of ( t BuC) 3 P (1) in pentane was prepared and transferred to a quartz NMR tube. The solution was irradiated with 254 nm light for five minutes. 31 P{ 1 H} NMR spectra were collected before (Fig. S61) and after irradiation (Fig. S62). Irradiation of 1 produced a number of unidentified species, a diphosphene previously reported by Slootweg et al. (14), and what we tentatively assign as phosphacyclobutadiene dimer (refer to section S1.13 for more information).   ) spectrum of ( t BuC) 3 P in pentane after being exposed to 254-nm light for 5 min. S1.12 Treatment of ( t BuC) 3 P (1) with W(THF)(CO) 5 W(THF)(CO) 5 was prepared by irradiating a solution of W(CO) 6 (THF, 0.1 M) with 254 nm light for 1 h at 23 • C (40). The bright yellow homogeneous solution of W(THF)(CO) 5 was brought into the glovebox and used immediately. To a vial containing ( t BuC) 3 P (1, 10 mg, 0.04 mmol) was added 0.5 mL (0.05 mmol, 1.25 equiv) of the freshly prepared solution. After stirring for 24 h, the solution was transferred to an NMR tube and a 31 P{ 1 H} NMR spectrum was collected (Fig. S63). This reaction produces a number of unknown species; however, the major product of this reaction is diphosphahousene 5.  To a stirring solution of ( t BuC) 3 P (1, 0.040 g, 0.04 mmol, 1 equiv) in benzene (1 mL) was added triphenylborane (0.004 g, 0.004 mmol, 0.1 equiv). After 10 min, all volatile materials were removed from the reaction mixture under reduced pressure. The colorless solids were taken up in pentane (2 mL) and one drop of pyridine was added to the solution. The cloudy solution was filtered through a glass fiber filter paper plugged pipette, and the all volatile materials were removed from the filtrate in vacuo. The colorless solids were taken up in benzene-d 6 and trans-   Fig. S46) δ −47.60 ppm. A preliminary X-ray structure determination that is consistent with our assignment has also been obtained. Further characterization of this dimer is in progress. This reaction also produces a small amount of diphosphahousene 5.

Section S2. X-ray diffraction studies
Low-temperature diffraction data were collected on a Bruker-AXS X8 Kappa Duo diffractometer with IµS micro-sources, coupled to a Photon 3 CPAD detector using Cu K α radiation (λ = 1.54178Å) for the structure of 1 and a Smart APEX2 CCD detector using Mo K α radiation (λ = 0.71073Å) for the structures of Na[8] and 9, performing φand ω-scans. The structures were solved by dual-space methods using SHELXT (41) and refined against F 2 on all data by fullmatrix least squares with SHELXL-2017 (41) following established refinement strategies (42).
All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were included into the model at geometrically calculated positions and refined using a riding model. The isotropic displacement parameters of all hydrogen atoms were fixed to 1.2 times the U-value of the atoms they are linked to (1.5 times for methyl groups). Details of the data quality and a summary of the residual values of the refinement are listed in tables S1, S3, and S5. Tables S2, S4, and S6 give all bond lengths and angles for the structures.
Compound Na[8] crystallizes in the monoclinic centrosymmetric space group P 2 1 /c with two molecules of Na[8] and one half molecule of diethyl ether per asymmetric unit. The additional half molecule of Et 2 O is located on a crystallographic inversion center and disordered accordingly. This was addressed by reducing the occupancy of all ether atoms to 50% while suppressing entries into the connectivity array of bonds between those atoms and their symmetry equivalents. In addition, both diethyl ether molecules coordinating to sodium cations in one of the two crystallographically independent molecules are disordered over two positions.
Disorders were refined with the help of similarity restraints on 1−2 and 1−3 distances and displacement parameters. The disorder ratios of the coordinated ether molecules were refined freely and converged at 0.709(8) and 0.776(8), respectively. The crystal at hand was found to be twinned by pseudo-merohedry. The twin law corresponds to a 180 • rotation about the crystallographic a-axis and the twin ratio refined to 0.4303(8). As is often needed with twinned structures and always for disorders, similarity restraints on displacement parameters as well as rigid bond restraints for anisotropic displacement parameters were applied to all atoms to stabilize the refinement. The thermal ellipsoid plot is shown in Fig. S68.
Compound 9 crystallizes in the triclinic centrosymmetric space group P1 with one molecule of 9 per asymmetric unit. The crystal quality was only mediocre; however structure determination was straightforward and no restraints were applied. The thermal ellipsoid plot is shown in Compound 1 crystallizes in the monoclinic centrosymmetric space group P 2 1 /n with one molecule of 1 per asymmetric unit. In spite of the low crystal and data quality, structure determination was surprisingly straightforward. To stabilize the refinement and to counteract the low data-to-parameter ratio resulting from the low resolution of the data, similarity restraints on displacement parameters as well as rigid bond restraints for anisotropic displacement parameters were applied to all atoms. The thermal ellipsoid plot is shown in Fig. S70 Goodness-of-fit a 1.053 Table S1. Crystallographic data for Na[8].        Goodness-of-fit a 1.080 Table S3. Crystallographic data for 9.

S3.5 Generation of the molecular graph for ( t BuC) 3 P (1)
The electron density is from a GAMESS run using the following input file: In the GUI that pops up the desired quantities were added to the graphic window and a molecular graph graphic was generated.

S3.6 Natural bond orbital analysis
Examination of the natural bond orbitals (27) indicates that there is no significant change in the lone pair composition from P 4 to phosphatetrahedrane (ca. 80% s in character), and likewise the atomic orbital contributions to the bonds in these molecules changes almost not at all. The central bonds of the tetrahedral cores of the molecules shown in Fig. ??B are very high in porbital content, moreso for P than for C. The C atom directs an external hybrid orbital that is a rich 40% in s character which, as discussed by Wiberg and Bader (31), is the origin of the high strain energy for tetrahedrane (C is stabilized relative to the carbon in the standard methine group, but H is destabilized by an even greater amount).