A Carbene‐Stabilized Boryl‐Phosphinidene

Abstract Herein, the synthesis and characterization of the carbene‐stabilized boryl phosphinidenes 1–3 are reported. Compounds 1–3 are obtained by reacting Me‐cAAC=PK (Me2‐cAAC=dimethyl cyclic(alkyl)(amino)carbene) and dihaloaryl borane in toluene. All three compounds were characterized by X‐ray crystallography. Quantum mechanical studies indicated that these compounds have two lone pairs on the P center viz., an σ‐type lone pair and a “hidden” π‐type lone pair. Hence, these compounds can act as double Lewis bases, and the basicity of the π‐type lone pair is higher than the σ‐type lone pair.


Introduction
Phosphinidenes (RÀ P¨:) are low-valent phosphorus compounds analogous to carbenes and nitrenes, having a singly coordinated phosphorus atom containing six electrons in its valence level. [1] Due to high reactivity, phosphinidene is a short-lived species. Therefore, several strategies have been employed to stabilize phosphinidenes, including complexation with transition metals, substituting with π-donating and sterically bulky groups, etc. [1b-f] Lappert et al., isolated the first structurally characterized di-coordinated terminal transition-metal-complexed phosphinidene [RÀ P¨:!M(η-C 6 H 5 ) 2 ] (M = Mo, W, R = 2,4,6-tBu 3 -C 6 H 2 ). [2a-b] Several terminal phosphinidene complexes were reported with transition-metal and substituents R on the P atom. [2] Subsequently, metal-free phosphinidenes (RÀ P¨:) were stabilized by    reported the N-heterocyclic carbene supported phosphinidene [(NHC)PÀ Ph, NHC = 1,3,4,5-tetramethylimidazol-2-ylidene]. It was termed as an inversely polarized phosphaalkene and described by three canonical forms A-C (Scheme 1). [3] Form A contains a formal P=C double bond, B is a zwitterionic form with a PÀ C single bond and C shows a dative C!P bond with two lone pairs of electrons on the P atom. Numerous carbene (NHC/cAAC) stabilized phosphinidenes (RÀ P¨:) have been reported, and their structure, electronic properties and reactivity vary with substituent R (Scheme 2, a-g). [4][5] In 2012, Cummins et al. reported a mono-coordinated phosphinidene (iPr 2 NP¨:), which was formed as a transient intermediate while thermolysis of dibenzo-7-phosphanorbornadiene (Scheme 2, h). [1f] Using the thermodynamic and kinetic stability of bulky ligand, Bertrand et al. synthesized a room temperature stable singlet phosphino-phosphinidene compound (Scheme 2, i). The reactivities of phosphino-phosphinidene were studied using several nucleophiles such as CO,isocyanides,carbenes,and phosphines, Recently, Chen et al. reported the synthesis and structure of a terminal scandium boryl phosphinidene complex which showed strong ScÀ P π bonding. [2l] The influence of substitution and the coordination of phosphinidene to carbene/metal play a significant role in bonding and chemical properties. Therefore, the isolation of new phosphinidene derivatives is an exciting area of research. In this communication, we report the synthesis of the carbene stabilized boryl phosphinidenes (Scheme 3, 1-3) which are also characterized as inversely polarized phosphaal-kenes. DFT calculations are employed to obtain further insight into the bonding and structure-property relationship of the newly developed carbene-stabilized boryl phosphinidenes.
The block-shaped yellow crystals of 1, 2, and 3 were obtained in good yield at 0°C from the concentrated toluene solution. Compounds 1-3 were fully characterized by NMR ( 1 H, 13 C, 11 B, and 31 P) and mass spectrometry (LIFDI). All these compounds are highly stable in the solid-state and in solutions at room temperature under an inert atmosphere for months. However, they are unstable under ambient conditions and slowly change their intense yellow color to a colorless form. Single-crystal X-ray diffraction and elemental analysis confirmed the compositions of 1, 2, and 3. 13 C, 31 P, and 11 B NMR spectroscopy provided a wealth of structural information on the inversely polarized phosphaal-kene. The 13 C{ 1 H} signal of C=P unit of inversely polarized phosphaalkenes appeared at 220.9 (d, J c-p = 91.9 Hz), 220.8 (d, J cp = 94.5 Hz), and 220.7 (d, J c-p = 104.6 Hz) ppm for compounds 1, 2, and 3 respectively. The observed 13 C{ 1 H} NMR signals and J C-P coupling constants for the C=P units are similar to the reported phosphaalkenes. [6b] The 31 P NMR shifts for the compounds are strongly affected by the electronic nature of the substituents and the hybridization of the phosphorus atom. The introduction of an electropositive boryl group provides a deshielding effect on the phosphorus chemical shift. Also, the halogen atoms such as Cl, Br, and I on the boron center typically move the chemical shift significantly downfield. The 31 P{ 1 H} NMR signal appeared at 35.5, 47.1, and 66.9 ppm for compounds 1, 2, and 3, respectively. This observation is in accordance with the increasing order of Lewis acidity of the boron center, which increases when the halogen changes from Cl to I. [6c] Thus, the 31 P{ 1 H} NMR spectroscopy supports the formation of delocalized CÀ PÀ B π-system and the extent of delocalization increases from compound 1 to 3. Note that the 31 P chemical shift values move downfield compared to inversely polarized phosphaalkenes. [6d] For instance, the 31 P chemical shifts for phosphatriafulvenes show inverse polarity around À 23.2 to À 74.1 ppm. [6] The quadrupolar characteristics of the 11 B{ 1 H} nucleus and the asymmetric local fields in compounds usually result in broad 11 B { 1 H} NMR resonances. The 11 B{ 1 H} NMR resonances appear at 72.83, 71.81, and 69.80 ppm for the compounds 1, 2, and 3, respectively. The 31 P and 11 B NMR data indicate that the BÀ P bond has considerable double-bond character and 31 P and 11 B resonances are similar to those reported phosphinidene boranes (RÀ B=PÀ R). [6] The solid-state structures of complexes 1, 2, and 3 consist of a CÀ PÀ B backbone with an almost coplanar halogen (X = Cl, Br, I) atom ( Figure 1). The bond parameters are listed in Table 1. The observed dihedral angles of C carbene À PÀ BÀ X are 18.32 (17)°( 1), 179.02 (6)°(2), and 177.70 (6)°(3). The C=P=B skeleton is significantly bent with a bond angle of 114.68°, 111.94°, 110.78°a nd the dihedral angles between C17À C16À N1 and X1À B1À C1 planes are 25.06°, 21.43°and 31.50°for compounds 1, 2, and 3, respectively.
Quantum mechanical calculations were carried out at the M06-2X/def2-TZVPP//BP86/def2-TZVP level of theory by incorporating Grimme empirical dispersion correction with Becke-Johnson damping (GD3BJ) to explore the bonding and reactivity of the molecules 1-3. [9] The formation of 1-3 are exothermic and the energy increases on changing the halogen from Cl (À 19.20 kcal mol À 1 ) to I (À 38.14 kcal mol À 1 ). The isomer 1', where the BÀ Cl bond is oriented similar to BÀ X bonds in complexes 2 and 3, is less stable by 1.82 kcal mol À 1 . It can be attributed to the shorter PÀ Cl distance (2.97 Å), resulting in strong repulsions between the lone pairs on P and Cl centers ( Figure S20 in the Supporting Information).
The molecular orbital (MO) analysis of 1 reveals that HOMO-2 is a σ-type lone pair orbital with a larger contribution from the P atom ( Figure 2a). The NBO analysis shows a lone pair on the P atom with 54.2 % contribution from 3 s orbital and 45.6 % contribution from 3p orbitals. The lone pair with relatively less %p character can be correlated with a significantly bent CÀ PÀ B backbone. The extent of bending increases as the halogen is changed from Cl to I (see below). HOMO is a π-type MO delocalized over the NÀ CÀ PÀ BÀ X backbone with a major coefficient from the P center. It can be formed by donating a lone pair of electrons from the P center to the antibonding C ! N and B ! X π*-MOs. However, it is seen as a strong secondorder interaction from PÀ C π-NBO to partially filled p-type orbital on B in the NBO analysis. The bond occupancy from the NBO calculations indicates that the P-atom contributes 66 % to the P=C π interaction. This suggests a polarized π bond with a larger contribution on the P atom. The MO and NBO analyses corroborated well with the very low Wiberg bond indices for the C carbene À P (1.28-1.35), PÀ B (1.31-1.41) and C carbene À N (1.29-1.32) bonds. Note that these Wiberg bond indices are lower than the C carbene À N Wiberg bond index of free cAAC (1.52; see below). The difference in electronegativity also induces polarity in the C carbene À P, PÀ B and C carbene À N bonds, reducing the bond index values. These bond index values support the partial double bond character observed in these bonds. The overall high positive group charge on cAAC (0.14-0.18) implies that σdonation from cAAC is higher than the π-back donation from the P center. The NBO analysis shows considerable polarization of the PÀ C cAAC σ-MO and π-MO in opposite directions (Table S8), thus indicating the inversely polarized phosphaalkene nature. However, the net polarization is minimal, as seen from the natural charges on C cAAC (0.10-0.13 e) and P (0.06-0.08 e).
Three major interactions are responsible for the geometry of the compounds viz., the donation of σ-lone pair on P to B center, the donation of lone pair of electrons from halogen atom to B center, and the donation from PÀ C π-NBO to the B center. The strength of interaction corresponding to the donation of σ-lone pair on P to B center is 5.4 kcal mol À 1 when  (14)  halogen atom is Cl (1), 2.6 kcal mol À 1 for Br (2), and 2.7 kcal mol À 1 for I (3). On the other hand, the energetics corresponding to the donation from halogen to B center is significantly high and decreases as 40.7 kcal mol À 1 for 1, 35.3 kcal mol À 1 for 2, and 30.31 kcal mol À 1 for 3. The strength of the donation from the halogen atom is a deciding factor for the PÀ B and BÀ X bond lengths. The PÀ B bond length is shortest for 3 (1.860 Å); this is in agreement with the weaker donation of I lone pairs to the formally empty p-orbital on B. Note that the strength of the donation from PÀ C π-NBO to the B center is also significant viz., 38.4 kcal mol À 1 for 1, 49.6 kcal mol À 1 for 2, and 56.0 kcal mol À 1 for 3. The balance between these secondorder interactions results in the intermediate dihedral angles between the planes defined by terminal B and C atoms with their corresponding substituents (23.5-33.7°).
Based on the geometrical, MO and NBO analyses, the P center in 1-3 is monovalent, forming an electron sharing bond with B center while accepting a lone pair of electrons from cAAC. The bonding is schematically represented in Figure 2b. As a result, the P center retains two lone pairs of electrons viz. σand π-type. This is a typical description of a carbene stabilized boryl phosphinidene. The singlet boryl phosphinidenes PB(X)(Tipp) (X = Cl, Br, I), with a vacant in-plane p-orbital on the P atom, is more stable than the triplet boryl phosphinidenes by 7.94-8.51 kcal mol À 1 ( Figure S22). Hence, the P atom can act as a coordinating center for accepting electrons from cAAC. The calculated dissociation energy of compounds 1-3 to respective singlet boryl phosphinidene and cAAC was found to be moderately high, indicative of the strong binding between PÀ C centers (55.79 kcal mol À 1 for 1, 54.60 kcal mol À 1 for 2, 53.51 kcal mol À 1 for 3). PÀ C bond dissociation energy values for carbene stabilized phosphinidenes have been calculated to fall in the range 43-75 kcal mol À 1 at BP86/TZ2P level of theory. [10] From the MO analysis, the π -type lone pair on the P center is donated to C ! N and B ! X π*-MOs. Accordingly, double Lewis basic behavior can be expected from the P center.
A bent geometry with an active σ-lone pair and a delocalized π-MO is the prominent feature of carbones and related species. [11] We computed the proton affinity to assess the basicity of the σ-lone pair and delocalized π-MO. The first proton affinity (PA) is quite high (240.7-242.1 kcal mol À 1 ) and close to that of NHCs (254.2 kcal mol À 1 ) and C(PH 3 ) 2 (255.7 kcal mol À 1 ) but less than that in carbodicarbenes, C-(NHC Me ) 2 (294.3 kcal mol À 1 ). [11a] Indeed, the addition of the first proton leads to a tri-coordinated P center with considerable pyramidalization (34.8°-41.9°; Figure S23), thus indicating that the proton has been added to the delocalized π-MO in the molecules 1-3. This suggests that the delocalized π-MO in 1-3 is more basic than the σ-lone pair having a very high scharacter (53.8-54.4 %). Hence, the delocalized π-MO can be considered as a hidden lone pair. Frenking and co-workers have used the term hidden lone pair for a delocalized π-MO in carbodicarbenes and related C(0) compounds, which show very high proton affinity. [11] The PA value is also indicative of its inversely polarized phosphaalkene character.
The first protonation induces an elongation in C carbene À P (0.7-1.3 %) and PÀ B (2.5-2.7 %) bonds and a shortening of the C carbene À N bond (2.4-2.6 %; Figure S23). This corroborated well with increased cAAC group charge (0.49-0.52) and decreased negative natural charge on N (À 0.35) atom. The MO analysis indicates an active lone pair (35.0-38.2 % s character) with a major coefficient on the P atom ( Figure S24). The presence of an active lone pair with relatively low s-character induces the addition of the second proton at the P center. The calculated second proton affinity is also quite high (137.8-141.9 kcal mol À 1 ), indicating that P (I) center in molecules 1-3 can act as a double Lewis base.

Conclusion
In conclusion, we have designed, synthesized, and characterized three cAAC-stabilized boryl-phosphinidenes, which also show inversely polarized phosphaalkene characteristics. Quantum mechanical calculations reveal that compounds 1-3 feature a P(I) center with two lone pairs, viz. a highly reactive "hidden" πlone pair and a σ-lone pair.

Experimental Section
Essential experimental: Crystal data for 1 at 100 (2)  All crystals were selected under cooling by using an X-Temp2 device. [12] The data were integrated with SAINT. [13] A multiscan absorption correction and a 3λ correction [14] in 1 was applied using SADABS. [15] The structures were solved by SHELXT [16] and refined on F 2 using SHELXL [17] in the graphical user interface SHELXLE. [18] Deposition Numbers 2058117 (1), 2058118 (2), and 2058119 (3) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.