Edinburgh Research Explorer Preparation and Characterization of P(2)BCh Ring Systems (Ch=S, Se) and Their Reactivity with N-Heterocyclic Carbenes

: Four-membered rings with a P 2 BCh core (Ch = S, Se) have been synthesized via reaction of phosphinidene chalcogenide ( Ar*P=Ch ) and phosphaborene ( Mes*P=BNR 2 ). The mechanistic pathways towards these rings are explained by detailed computational work that confirmed the preference for the formation of P – P, not P – B, bound systems, which seems counterintuitive given that both phosphorus atoms contain bulky ligands. The reactivity of the newly synthesized heterocycles, as well as that of the known (RPCh) n rings (n = 2, 3), was probed by the addition of N-heterocyclic carbenes, which revealed that all investigated compounds can act as sources of low-coordinate phosphorus species.


Introduction
The ability of low-coordinate and low-valent main group compounds to mimic the reactivity of transition metal complexes has pushed research in this area to the top of the inorganic chemistry agenda over the last decade. [1] One strategy to isolate these reactive species is through donor stabilization of the lowvalent main group centre, offering a doorway to explore the chemistry of these elusive intermediates. A common group of two electron σ-donors that have been used exhaustively over the last decade is N-heterocyclic carbenes (NHCs). NHCs have stabilized p-block frameworks in many unusual bonding arrangements, [2] including some noteworthy examples such as the homodiatomic main group allotropes ( Figure 1, A & B), [3][4][5] carbon (0) in carbodicarbenes/bent allenes (C), [6] phosphinidenes (D), [7][8][9][10][11][12][13][14][15][16] phosphaborenes (E), [17] and phosphinidene sulfides (F). [18] While NHCs offer thermodynamic stabilization, they are often used in conjunction with bulky substituents that aid in kinetic stability, rendering the lowcoordinate and low-valent main group compounds "bottleable" and ready for onwards transformations.
While the interest in small strained inorganic rings is important for exploring the structural chemistry of main group heterocycles, these compounds have routinely acted as precursors to low-coordinate main group species that are otherwise difficult to isolate. The approach of using cyclic structures to gain access to otherwise inaccessible species has been successful for P2B2, [19,20] P2N2, [21] P2S2, [18] and B2N [22] rings that upon thermolysis or addition of a Lewis base lead to degradation of their inorganic cores and formation of compounds with low-coordinate PB, PN, PS, and BN moieties (Figure 1, bottom).
In recent years, we have been focused on incorporating pnictogen and chalcogen atoms into unique cyclic structures. While small phosphorus heterocycles with symmetrical cores dominate the literature, [23] there are fewer examples of phosphorus-chalcogen rings. [18,24,25] Moreover, small phosphorus-chalcogen heterocycles that display asymmetry with respect to the 4-membered core are even rarer. These include P-S-P-E (Figure 2, G), [26][27][28] P-Ch-P-C (Ch = S, Se, H), [29][30][31] P-S-C-S (I), [32] and P-S-C-C structures (J), [33] amongst others. [34][35][36][37][38][39] Small ring cores that contain asymmetry in the presence of a P-P bond are by far the rarest, with published examples limited to only three species: P-P-C-N (K), [40] P-P-C-O (L), [37] and P-P-Si-N (M). [41] In this context, we report the synthesis of small main group heterocycles containing a P2BCh core (3Ch; Ch = S, Se) that possess a P-P bond. Compounds 3Ch were prepared [a] through the combination of monomeric phosphinidene chalcogenides (Ar*P=Ch) and phosphaborenes (Mes*P=BNR2) via formation of a P-P bond. One could consider this an expected outcome with the phosphorus in Ar*P=Ch as the electrophilic site and the phosphorus in the Mes*P=B-NR2 the nucleophilic one. However, the formation of a P-P bond is counterintuitive when considering the steric demand imposed by both phosphorus atoms as well as the electrophilicity of the boron atom in Mes*P=B-NR2. In agreement with these notions, density functional theory (DFT) calculations showed that the reaction begins with a P→B attack after which the resulting intermediate can follow multiple pathways with the most favourable ones lead to the formation of a P-P bond. The synthesized P2BCh main group rings can be monomerized at elevated temperatures in the presence of NHCs, reforming Ar*P=Ch and Mes*P=BNR2 as well as giving access to NHCstabilized phosphinidenes. [20]

Results and Discussion
The 1:1 stoichiometric reaction of 1Ch and 2 in toluene at 80 °C resulted in the consumption of both starting materials after 2 hours and the appearance of two doublets in the 31 P{ 1 H} NMR spectrum. Removing the volatiles in vacuo and slowly concentrating a pentane solution of the crude reaction mixture produced single crystals suitable for X-ray diffraction, which confirmed the products as 3Ch, four-membered heterocycles formed from the incorporation of monomeric units from 1Ch and 2 (Scheme 1). 31 P{ 1 H} NMR spectra of the redissolved single crystals of 3Ch displayed the same doublets as observed in the crude reaction mixture (3S: δP = -29.8; -13.6, 1 JP-P = 184.0 Hz; 3Se: δP = -33.5, 1 JSe-P = 91.8 Hz, 1 JP-P = 181.7 Hz; -31.5, 1 JP-P = Scheme 1. Synthesis of mixed main group rings 3Ch by combining monomeric Ar*P=Ch and Mes*P=BNR2 units (NR2 = 2,2,6,6tetramethylpiperidine (TMP)). 181.7 Hz), with both doublets shifted upfield from 1Ch and downfield from 2. The 11 B NMR spectra of both 3Ch display similar chemical shifts (3S: δB = 44.2; 3Se: δB = 42.3), as one would expect, and are indicative of three-coordinate boron centers. The solid-state structures of 3Ch confirmed the presence of P-P-B-Ch cores in butterfly conformation with P-Pbond lengths of 2.2371(9) and 2.243(1) Å, and P-B bonds of 1.955(2) and 1.957(2) Å for 3S and 3Se, respectively ( Figure 3). The B-Ch bond lengths in 3S and 3Se are 1.850(3) and 1.985(2) Å, respectively, whereas the corresponding P-Ch bond distances are 2.1301(8) and 2.267(1) Å. The structures of 3Ch revealed the formation of a P-P bond, which is counterintuitive based on the steric demand at both phosphorus atoms and the electronic predisposition to form a P-B bond. In order to ascertain the mechanism of forming 3Ch, we turned to the aid of DFT calculations ( Figure 4). For reasons of computational efficiency, the parent ligands Ar (mdiphenylbenzene) and Mes (2,4,6-trimethylphenyl) were used in the calculations and the formation of 3Ch was assumed to involve only monomeric ArP=S and MesP=B−NMe2 units.
The addition of ArP=S to MesP=B−NMe2 was found to proceed via transition state TS1 with a barrier of only 6 kJ mol -1 (38 kJ mol -1 without dispersion correction). The transition state shows an initial P→B attack that subsequently leads to the formation of the intermediate INT1 with a cyclic P−P−B core. This is understandable as the phosphorus atom in ArP=S is both nucleophilic and electrophilic, whereas the boron and phosphorus atoms in MesP=B−NMe2 are electrophilic and nucleophilic, respectively. The optimized structure of INT1 shows that the Mes and Ar groups reside on opposite side of the plane of the three-membered ring, with the sulfur atom located on the same side of the plane as the Mes ligand.
From the intermediate INT1, a one-step pathway to 3S was found that proceeds via TS2 with a barrier of 67(71) kJ mol -1 . The reaction involves a straightforward insertion of the dangling sulfur atom into the P-B bond, forming the experimentally observed 2,3-isomer of 3S. Interestingly, no simple one-step pathway connecting INT1 to the 2,4-isomer of 3S was found. Instead, the intermediate three-membered ring first changes conformation from anti to syn via TS2b with a barrier of 53 (56) kJ mol -1 . The resulting intermediate INT2 can then undergo an insertion of the sulfur atom into the P−P bond via TS3 with a barrier of 111(89) kJ mol -1 , giving INT3 that still needs to undergo an additional low-energy syn to anti conformation flip to form 2,4-3S via TS4. The calculations show that, despite the lower steric demand, 2,4-3S is 62(65) kJ mol -1 higher in energy than 2,3-3S, presumably due to the favourable S-B bond in 2,3-3S. This interaction also lowers the energy of the transition state TS2, bringing it significantly below that of TS3, the highest transition state on the path leading to 2,4-3S. Furthermore, while INT2 is an intermediate en route to 2,4-3S, it can also form 2,3-3S via transition state TS3b that is 48(36) kJ mol -1 lower in energy than TS3. Consequently, the formation of 2,4-3S seems highly unlikely when considering the thermodynamic and kinetic favourability of the two competing pathways leading to 2,3-3S. Upon making the unique mixed main group rings 3Ch, we opted to explore their reactivity with strong Lewis bases, namely NHCs. Monitoring the addition of NHC iPr to 3Ch by 31 P{ 1 H} NMR spectroscopy revealed no reaction, even after heating the mixture at 80 °C for 3 days (Scheme 2). We hypothesized that the bulky aryl groups on phosphorus in combination with the sterically encumbered NHC resulted in "molecular frustration" and turned to an NHC with a lower steric demand.
Heating a solution of NHC Me and 3Ch at 80 °C for 16 hours resulted in consumption of the starting material and appearance of two phosphorus-containing products in the 31 P{ 1 H} NMR spectra: two singlets at δP = 32 and 151 for Ch = S, corresponding to 4S Me and 5, and two singlets at δP = -77 and 151 for Ch = Se, corresponding to 6 Me and 5 ( Figure 5). The NHC-stabilized phosphaborene 5 has been reported as the product from the reaction of 2 with NHC Me and was therefore easily identified. [20] Similarly, a derivative of an NHC-stabilized phosphinidene sulfide has also been reported, [18] with 31 P{ 1 H} NMR chemical shifts similar to that observed for 4S Me . [18] While 4S Me is stable and can be observed by NMR spectroscopy in crude reaction mixtures, there is no direct evidence of the analogous selenide 4Se Me ( Figure 5). Instead, we observed the formation of an NHC-stablized phosphinidene 6 Me and selenourea 7 Me , as confirmed by NMR spectroscopy and X-ray crystallography. [43] The inherently weak "P=Se" moiety of Ar*P=Se likely makes 4Se Me an unstable intermediate that easily forms 6 Me and 7 Me in presence of excess NHC (Scheme 2).
[44] Attempting the Lewis acid stabilization of low-coordinate phosphorus species, the addition of AuCl(THT) (THT = tetrahydrothiophene) to 3S resulted in no reaction even after a prolonged period. However, the addition of one stoichiometric equivalent of AuCl(THT) to 3Se resulted in full consumption of the starting material and the appearance of two new doublets in the 31 P{ 1 H} NMR spectrum that were shifted downfield from the starting material (δP = -8.4, 1 JSe-P = 169.9 Hz, 1 JP-P = 246.4 Hz; 10.3, 1 JP-P = 246.4 Hz). Removing the volatiles in vacuo and leaving a concentrated solution in MeCN at -35 °C overnight resulted in single crystals suitable for X-ray diffraction that confirmed the product to be 8Se in which AuCl is coordinated by the P-Se phosphorus site (Scheme 3). The 31 P{ 1 H} NMR spectrum of the redissolved single crystals displayed the same doublets as observed in the crude reaction mixture while a broad singlet was noted in the 11 B NMR spectrum (dB = 44). The solidstate structure revealed that the P2BSe core had remained intact retaining its butterfly conformation with relevant bond distances consistent with the parent structure 3Se ( Figure 6). Compound 8Se is unstable if left in solution, giving a black precipitate along with a wide range of decomposition products, as indicated by 31 P{ 1 H} NMR spectroscopy. This instability is a marked departure from that of the parent ring 3Se that was found to be indefinitely stable, even at higher temperatures. Upon observing that the addition of an NHC to 3Ch resulted in dissociation of the rings and formation of base-stabilized lowcoordinate phosphorus compounds 4S Me and 6 Me , we then wondered if similar reactivity could also be observed with the parent P-Ch rings, 1Ch. Consequently, the 4:1 addition of NHC R (R = Me, iPr) to a THF solution of 1Se at room temperature resulted in the consumption of starting material and the appearance of a singlet in the 31 P{ 1 H} NMR spectrum (dP = -76.7, R = Me; dP = -75.0, R = iPr). Removing the volatiles in vacuo, dissolving the crude reaction mixture in a minimal amount of pentane, and leaving the solutions overnight at -35 °C resulted in single crystals suitable for X-ray diffraction. A subsequent structure analysis confirmed the products as NHC-stabilized phosphinidenes 6 Me and 6 iPr (Figure 6), that is, similar species that were obtained via addition of NHC Me to 3Se.    [45] The 31 P{ 1 H} chemical shifts of 6 R were found to be shifted significantly upfield from the parent 1Se (DdP = 98), indicating the presence of an electron-rich phosphorus site. The 1 H NMR spectra of 6 R display a symmetric environment at phosphorus, indicated by one set of signals for the o-CH3 and p-CH3 groups on the terphenyl ligand, integrating to 12 and 6 hydrogen atoms, respectively, along with one set of signals for the NHC. [46,47] The solid-state structures of 6 R ( Figure 6) display typical features for NHC-stabilized phosphinidenes, with a bent geometry at phosphorus (C Ar -P-C NHC = 101.53(1) and 99.10(17)° for 6 iPr and 6 Me , respectively) and a short P-C NHC bond (1.799(3) and 1.786(4) Å for 6 iPr and 6 Me , respectively). In addition to the formation of 6 R , selenoureas 7 R were concomitantly formed during the reaction and their identities confirmed using X-ray diffraction and NMR spectroscopy. [43] While we have already reported that the reaction of 1S and NHC iPr yields the NHC-stabilized phosphinidene sulfide 4S iPr , [18] we repeated the same chemistry with the more sterically accommodating NHC Me (Scheme 4). In doing so, the 2:1 addition of NHC Me to 1S at -50 °C resulted in the consumption of starting material and appearance of one signal in the crude 31 P{ 1 H} NMR spectrum (dP = 30.0) with a similar chemical shift as found for 4S iPr (dP = 29.0). After removing the volatiles in vacuo, dissolving the crude reaction mixture in toluene, and slowly layering the solution with ether at -35 °C, single crystals suitable for X-ray diffraction were obtained that confirmed the product to be 4S Me (Figure 6). The metrical parameters of 4S Me revealed two essentially identical P-C bond lengths (P-C NHC = 1.852(3) Å and P-C Ar = 1.867(3) Å) and a short P-S bond of 2.0186(15) Å, that is, bond parameters that are similar to those in 4S iPr . The NHC-adduct 4S Me has a strikingly similar structure to the known methylenethioxophosphoranes (10, Scheme 4), [48] however 4S Me contains a longer P-S bond (2.0186(15) Å vs. 1.928 Å) [49] and a significantly longer P-C bond (P-C NHC in 4S Me 1.852(3) Å vs. 1.656 Å). [49] The P-C NHC bond length is longer than expected for P=C, [50][51][52] thus describing 4S Me as containing a multiply bound phosphorus atom is not appropriate.
The recently reported 6-membered phosphorus-chalcogen rings 9Ch [53] (Scheme 4, bottom) contain the same general formula as 1Ch (i.e. (RPCh)n) and have long been considered as precursors to phosphinidene chalcogenides, [54] although no experimental evidence has supported this claim. We utilized the same approach employed for 1Ch and 3Ch, and attempted the degradation of these larger P-Ch heterocycles into basestabilized phosphinidene chalcogenides using NHCs. Similar to 3Ch, the addition of NHC iPr to 9Ch at room temperature or elevated temperatures resulted in no reaction, even after prolonged reaction times. The use of a more sterically accommodating carbene NHC Me in 4:1 (Ch = S) or 6:1 (Ch = Se) stoichiometry, however, resulted in the formation of 4S Me (Ch = S) or 6 Me and 7 Me (Ch = Se) even at room temperature, indicating that strong sterically accommodating donors can indeed degrade these larger P-Ch heterocycles into their basestabilized monomers. Upon obtaining NHC-stabilized phosphinidenes 6 R via multiple routes, we opted to test their onwards reactivity towards Lewis acids (Scheme 5). Although reports of such reactivity studies have been previously published, [10][11][12][13][14][15][16] the coordination chemistry of NHC-stabilized phosphinidenes containing bulky terphenyl groups is currently unknown. The addition of 6 iPr to tris(pentafluorophenyl)borane in THF resulted in immediate colour change from yellow to colourless. After concentrating the crude reaction mixture, precipitation with n-pentane gave an offwhite powder that shows a singlet in the 31 P{ 1 H} NMR spectrum (dP = -25), a singlet in the 11 B NMR spectrum (dB = -3), indicating a four-coordinate boron centre, and three resonances in the 19 F NMR spectrum, consistent with the ortho-, meta-, and parafluorine resonances for tris(pentafluorophenyl)borane. An X-ray diffraction experiment on single crystals grown via THF/pentane vapour diffusion confirmed the structure of 11, indicating that a controlled ring-opening of THF by 6 iPr had occurred in the presence of the strongly Lewis acidic B(C6F5)3 ( Figure 7). The ring-opened product 11 had a B-O bond length of 1.495(5) Å and two similar P-C bonds (C Ar* -P = 1.843(4) Å and C NHC -P = 1.839(4) Å), both of which are comparable to corresponding bond distances in 6 iPr .  Direct coordination of a Lewis acidic transition metal was performed by adding two stoichiometric equivalents of AuCl(THT) to a solution of 6 iPr (Scheme 5). After stirring for one hour at room temperature, the starting material was fully consumed and one major product was observed in the 31 P{ 1 H} NMR spectrum (dP = -41.9). After removing the volatiles of the crude reaction mixture in vacuo, a DCM/pentane vapourn diffusion of the crude reaction mixture yielded X-ray quality single crystals of 12. The structure of 12 shows that the phosphorus atom is bound to two AuCl fragments (Figure 7) with a coordination environment reminiscent of other NHC-stabilized phosphinidene compounds with a tetrahedral geometry at phosphorus. [14,55] Significant lengthening of the C NHC -P bond from 1.799(3) Å in 6 iPr to 1.856(4) Å in 12 was observed, which is likely induced by an increase in steric interactions along with complete loss of backbonding with the inclusion of two AuCl units.

Conclusion
In summary, we have demonstrated the synthesis of unique inorganic heterocycles containing group 13, 15, and 16 elements as 4-membered P2BCh rings (3Ch). The synthesis of 3Ch was accomplished via reaction between two highly reactive monomers: phosphinidene chalcogenide and phosphaborene. The P2BCh rings 3Ch were found to be a source for lowcoordinate phosphorus products, phosphinidene sulfides 4S and phosphinidenes 6, from their reaction with NHCs at elevated temperatures. Similar reactivity was also observed for the 4-and 6-membered (RPCh)n rings 1Ch and 9Ch. The coordination chemistry of NHC-stabilized phosphinidenes 6 was also explored, demonstrating their ability to ring-open THF and coordinate to Lewis acids via both lone pairs at phosphorus. Consequently, the controlled reactivity of monomeric Ar*P=Ch and Mes*P=BNR2 units, accessed from their parent dimers 1Ch and 2, has led to unique p-block rings that have eluded characterization until now. This method of forming novel heterocycles should allow for the synthesis of more asymmetric rings as well as the production of new low-coordinate main group compounds with possibly unforeseen structures and reactivity.

Experimental Section
All manipulations were performed under an inert atmosphere either in a nitrogen-filled MBraun Labmaster 130 glovebox or on a Schlenk line. 1Ch, [18] 2, [19] 9Ch, [53] AuCl(THT), [56] and NHCs [57] were made following literature methods. Trispentafluorophenylborane was purchased from Strem Chemicals and sublimed overnight prior to use (50 °C, 0.01 mmHg, -10 °C cold finger). Solvents were obtained from Caledon and dried using an MBraun solvent purification system. Dried solvents were collected under vacuum in a flame dried Straus flask and stored over 4 Å molecular sieves. Solvents for Nuclear Magnetic Resonance (NMR) spectroscopy (CDCl3, C6D6, and THF-d8) were stored in the glovebox over 4 Å molecular sieves. NMR spectra were recorded on a Varian INOVA 400 MHz ( 1 H 400.09 MHz, 31 11 B, and 19 F NMR spectroscopy were referenced externally to phosphoric acid, trimethyl borate, and trifluorotoluene, respectively. Mass spectrometry was recorded in house in positive-and negativeion modes using electrospray ionization Micromass LCT spectrometer. Melting or decomposition points were determined by flame-sealing the sample in capillaries and heating using a Gallenkamp variable heater. Elemental analysis was performed at the University of Montreal and is reported as an average of two samples weighed under air and combusted immediate thereafter.

General method for synthesis of 3Ch
A mixture of 1Ch and 2 in benzene was heated at 80 °C for 2 hours after which the crude 31 P{ 1 H} NMR spectrum showed complete consumption of starting materials and appearance of two doublets. The volatiles were removed in vacuo and the crude reaction mixture was redissolved using 3 mL pentane and placed at -35 °C for 1 hour, leading to precipitation of a yellow powder.  126.1, 128.4, 129.8, 130.0, 136.3, 136.9, 137.0, 137.2,   137.5, 139.7, 148.6, 148.7, 148.8, 150.1, 155.1, 155.2

General method for synthesis of 6 R
A solution of NHC R in THF was added to a solution of 1Se or 9Se in THF. The mixture was left to stir for 15 minutes after which the initial orange solution had changed to dark red. The volatiles were removed in vacuo and the crude reaction mixture was dissolved in 5 mL pentane and left at -35 °C overnight, giving orange crystals.

Synthesis of 11:
A solution of 6 iPr (50 mg, 0.0953 mmol) in 3 mL THF was added to a solution of trispentafluorophenylborane (49 mg, 0.0953 mmol) in 3 mL THF. After 30 minutes, the initial orange solution had turned colourless and the volatiles were removed in vacuo. The crude powder was washed with pentane (3 x 3 mL) and the resulting white solid was collected. Yield: 94 mg (89 %). Single crystals suitable for X-ray diffraction were obtained by pentane/THF vapour diffusion at - 35

CCDC
1573801-1573808 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre. The single crystal X-ray diffraction studies were performed at the Western University X-Ray facility. Samples were mounted on a Mitegen polyimide micromount with a small amount of Paratone N oil, at a temperature of 110 K for data to be collected on a Bruker Apex II detector using Mo-Kα radiation (λ = 0.71073 Å) or Cu-Kα radiation (λ = 1.54178 Å). The Bruker and Nonius instrument operate SMART [58] and COLLECT [59] software, respectively. The data collection strategy was a number of ϕ and ω scans which collected data up to 2q. The frame integration was performed using SAINT. [60] The resulting raw data was scaled and absorption corrected using a multiscan averaging of symmetry equivalent data using SADABS. [61] The structure was solved by using a dual space methodology using the SHELXT program. [62] All non-hydrogen atoms were obtained from the initial solution. The hydrogen atoms were introduced at idealized positions and were allowed to ride on the parent atom. The structural model was fit to the data using full matrix least-squares based on F. The calculated structure factors included corrections for anomalous dispersion from the usual tabulation. The structures were refined using the SHELXL program from the SHELXTL suite of crystallographic software. [61,63] For all compounds reported (3Ch, 4S Me , 6 R , 8Se, 11, and 12), the non-hydrogen atoms were well ordered and refined with anisotropic thermal parameters. In the case of 8Se, a MeCN solvate was found in the asymmetric unit that could not be refined anisotropically but isotropically.

Computational Details
Calculations were performed using the PBE1PBE [64][65][66][67] density functional with the Gaussian09 [68] program package. Ahlrichs' TZVP basis sets were used for all atoms. [69] The importance of dispersion interactions was examined by performing optimizations both with and without Grimme's GD3BJ empirical dispersion correction. [70,71] The bulky Ar* and Mes* groups were replaced with Ar (m-diphenylbenzene) and Mes (2,4,6trimethylphenyl), respectively, to speed up the calculations. Similarly, 2,2,6,6-tetramethylpiperidine was replaced with dimethylamine throughout calculations. 631483) and the University of Edinburgh. JV and HMT are grateful to the University of Jyväskylä for funding and acknowledge grants of computer capacity from the Finnish Grid and Cloud Infrastructure (persistent identifier urn:nbn:fi:research-infras-2016072533).