Synthesis and Reactivity of Triphosphaallyl Cation Stabilized by N-Heterocyclic Carbenes

Abstract

Reduction of the triphosphaallyl species 6[GaCl₄] with Ga^I[Ga₂Cl₇] affords the imidazoliumyl-substituted (L) bicyclo[2.1.1]-triphosphane 10[Ga₂Cl₇], featuring an unprecedented Ga₂Cl₅-bridged P₃ scaffold. Reactions of 10[Ga₂Cl₇] with nucleophiles (Cl^- or NHC) result in rare, selective P-C bond cleavages, affording Ga₂Cl_y-substituted triphosphiranes (LP₃Ga₂Cl_y, y = 5, 6) via an intramolecular ring closure mechanism. Protonation of 6[GaCl₄] gives rise to a similar ring closure, but without P-C bond cleavage, to afford the L₂P₃H⁺ salt 8[OTfGaCl₃]₂. Additionally, the palladium complex 26[GaCl₄], formed through the reaction of 10[Ga₂Cl₇] with [Pd(PPh₃)₄], presents a novel bicyclic P₃Pd moiety (LP₃Pd(PPh₃)₂[GaCl₄]). Comprehensive DFT calculations have been conducted to elucidate the bonding situation in 26[GaCl₄], uncovering significant metal-to-ligand π-back-donation and a distinctive 3-center-4-electron hyperbonding phenomenon in the P₃Pd framework. These findings offer valuable insights into chemistry of cyclic polyphosphorus compounds and, in particular, the reactivity, structural flexibility, and the coordination properties of cationic triphosphorus species.

Keywords: N‐heterocyclic carbene; P‐C bond activation; cyclic phosphorus scaffolds; gallium‐phosphorus bonding; triphosphaallyl cation.

Figure 1

Selected examples of compounds containing a symmetric or asymmetric σ²‐σ² bonded P‐P unit and resonance structures of triphosphaallyl (I, II) and triphosphapentadienyl (III, IV) compounds.

Scheme 1

Reaction of 5[GaCl₄] with NHC L in a 1:3 ratio to 6[GaCl₄] and 7. C₆H₅F, −40 °C, 5 minutes, −40 °C to rt, 3 hours; 6 (87 %), 7[GaCl₄] 89 %; L = 1,3‐Bis[2,6‐diisopropylphenyl]‐4,5‐dichloro‐1,3‐dihydro‐2H‐imidazol‐2‐ylidene.^{[ 11 ].}

Scheme 2

Reversible protonation (HOTf) and deprotonation (L) of 6 ⁺ and 8 ²⁺. i) +HOTf, CH₂Cl₂, rt, ii) +L, −[LH][OTf], CH₂Cl₂, RT;^{[ 14 ]} Reaction of 6 ⁺ with the Cl⁺‐source [PCl₄][GaCl₄] in the presence of GaCl₃. iii) + [PCl₄][GaCl₄], + 2 GaCl₃, – PCl₃, CH₂Cl₂, rt.

Figure 2

Molecular structure of cation 8 ²⁺ in 8[OTfGaCl₃]₂, hydrogen atoms are omitted for clarity and thermal ellipsoids are displayed at 50% probability; selected bond lengths [Å] and angles [°]: C1–P1 1.853(3), C28–P2 1.851(3), P1–P2 2.221(1), P1–P3 2.206(2), P2–P3 2.214(2), C1–P1–P2 100.66(8), C1–P1–P3 96.06(12), P2–P1–P3 60.02(4), P1–P2–P3 59.65(5), P1–P3–P2 60.33(4), C28–P2–P1 102.25(8), C28–P2–P3 97.05(10).

Scheme 3

Reaction of Ga[Ga₂Cl₇] with 6[GaCl₄] to 10[Ga₂Cl₇]. C₆H₅F, −40 °C to rt; extraction with n‐hexane/CH₂Cl₂ (3:2); 55 %.

Figure 3

Left: ³¹P{H} NMR spectrum (300 K) and VT measurements (insert, 243 to 308 K of 10[Ga₂Cl₇] in CD₂Cl₂); right: molecular structure of cation 10 ⁺ in 10[Ga₂Cl₇]·CH₂Cl₂, hydrogen atoms, anions and solvate molecules are omitted for clarity and thermal ellipsoids are displayed at 50% probability; selected bond lengths [Å] and angles [°]: P1–P2 2.2102(8), P2–P3 2.2500(9), P1–Ga1 2.3932(8), P3–Ga1 2.3371(7), P2–Ga2 2.3571(7), P1–C1 1.829(3), P3–C28 1.839(3), Ga1–Cl1 2.1372(8), Ga1–Cl2 2.3148(7), Ga2–Cl2 2.3816(7), Ga2–Cl3 2.1591(8), Ga2–Cl4 2.1558(7), P1–P2–P3 92.76(3), P1–Ga1–P3 86.09(3), Ga1–Cl2–Ga2 94.14(3), Cl1–Ga1–Cl2 103.93(3), P2–Ga2–Cl2 103.67(3), Cl3–Ga2–Cl4 111.00(3).

Scheme 4

Presumed degradation of 10[Ga₂Cl₇] to 11 and 12 and oxidation of the latter to dication 13 ²⁺.

Figure 4

Molecular structure of 12, hydrogen atoms are omitted for clarity and thermal ellipsoids are displayed at 50 % probability; selected bond lengths [Å] and angles [°]: P1–Ga1 2.3228(7), P1–Ga2 2.3351(7), P1–C1 1.829(3), Ga1–Cl3 2.3562(10), Ga2–Cl3 2.4129(13), Ga1–Cl4 2.1452(9), Ga1–Cl5 2.1636(8), Ga2–Cl6 2.1335(8), Ga2–Cl7 2.1632(10), Ga1–P1–Ga2 84.91(3), Ga1–Cl3–Ga2 82.48(4), P1–Ga1–Cl3 89.83(4), Cl4–Ga1–Cl5 114.58(4), P1–Ga2–Cl3 88.16(3), Cl6–Ga2–Cl7 109.99(4).

Scheme 5

Protonation of 10[Ga₂Cl₇] with 3 eq. of BRØNSTED acids H₂O or HCl.

Figure 5

³¹P{¹H} NMR spectrum of the reaction mixture of 10[Ga₂Cl₇] and one equivalent of water (bottom); the resonance at δ  =  −119.5 ppm belongs to the starting material and that at δ  =  −155.5 ppm is assigned to cation 20 ⁺;^{[ 26 ]} extended signals and splitting diagrams of the product species in the ³¹P and ³¹P{¹H} NMR spectra (middle); potential (faded) and actual (highlighted) isomers and conformers of the reaction products (top); section of the crystal structure of rac‐14 ²⁺ (top, left); selected atoms and counter ions are omitted for clarity; ellipsoids are drawn at 50 % probability level.

Figure 6

Molecular structure of dication R,R‐14 ²⁺ (left) and of the dimer of the anion [(GaCl₃)₂OH]⁻ (top, right); the disorder of the two enantiomeric C₂P₃H₃ sections is depicted in red (S,S‐14 ²⁺) and black (R,R‐14 ²⁺; bottom, right); hydrogen atoms and disorder are omitted for clarity; ellipsoids are drawn at 50% probability level.

Figure 7

Compounds possessing a P₃H₃ structural unit.

Figure 8

(top) Proposed reaction of 10[Ga₂Cl₇] with brønsted acids (protonation) to cations 14[X]₂, nucleophile‐induced ring closure to cyclo‐triphosphirane‐2,3‐diide 21 and subsequent nucleophilic attack to yield either Et₃NH[23] or 24; i) a) + 3 eq. H₂O, rt, CH₂Cl₂, [X]⁻ = [(GaCl₃)₂OH]⁻, b) + 3 eq. HCl, rt, CH₂Cl₂, [X]⁻ = [GaCl₄]⁻; ii) Nu = Cl⁻: a) + 1 eq. HDMAP[Cl], ‐LGaCl₃, ‐HDMAP[Ga₂Cl₇], CH₂Cl₂, rt; b) + Et₄N[Cl], ‐LGaCl₃, – Et₄N[Ga₂Cl₇], CH₂Cl₂, rt, 16%; Nu = NHC L: c) + NHC, – 2 LGaCl₃, – GaCl₃, CH₂Cl₂, rt; iii) + NHC, CH₂Cl₂, rt; iv): from 10[Ga₂Cl₇] in one step: + 3 Et₃NH[Cl], – 2 Et₃NH [GaCl₄], CH₂Cl₂, rt, 20%; (bottom) molecular structures of 22, 23 ⁺ in Et₃NH[23]·C₆H₅F and 24, hydrogen atoms, anions, and solvate molecules are omitted for clarity and thermal ellipsoids are displayed at 50% probability; selected bond lengths [Å] and angles [°]: 22: P1‐P2: 2.1937(13), P2‐P3: 2.2158(15), P1‐P3: 2.2005(15), P2‐Ga2: 2.3479(12), P3‐Ga1: 2.3351(12), P1‐C1: 1.855(4), P1‐P2‐Ga2: 91.12(5), P1‐P3‐Ga1: 97.32(5); 23 ⁻: P1‐P2: 2.1791(9), P2‐P3: 2.1959(13), P1‐P3: 2.1690(9), P2‐Ga1: 2.3464(9), P1‐C: 1.855(2), P1‐P2‐Ga1: 100.52(3); 24: P1‐P2 2.196(3), P1‐P3 2.185(2), P2‐P3 2.213(2), P2‐Ga2 2.3603(18), P3‐Ga1 2.3469(16), Ga2‐Cl1 2.1970(17), Ga2‐Cl2 2.176(2).

Scheme 6

Mechanism of the formation of 22 through the chloride‐induced rearrangement of 10⁺ . Reaction of 10[Ga₂Cl₇] with 1 eq. of NHC L to yields compound 22 through in‐situ generation of chloride from the [Ga₂Cl₇]⁻ counterion. a) + 1 eq. HDMAP[Cl], ‐LGaCl₃, ‐HDMAP[Ga₂Cl₇], CH₂Cl₂, rt; b) + Et₄N[Cl], ‐LGaCl₃, – Et₄N[Ga₂Cl₇], CH₂Cl₂, rt, 16 %; c)+L, – 2 LGaCl₃, rt, CH₂Cl₂.

Scheme 7

Nucleophilic opening of 22 with NHC L to yield 24 or with Et₃NH[Cl] to yield Et₃NH[23].

Scheme 8

Reaction of 10[Ga₂Cl₇] with Pd(PPh₃)₄. i)‐ L–GaCl₃, – 2 PPh₃, – Ga[GaCl₄], CH₂Cl₂, rt, 5 minutes, 40 %.

Scheme 9

Proposed mechanism for the Pd(0)‐initiated back reaction of 6 ⁺ to 26 ⁺.

Figure 9

Molecular structure of 25 ⁺ in 25[GaCl₄]·CH₂Cl₂, hydrogen atoms are omitted for clarity and thermal ellipsoids are displayed at 50% probability; selected bond lengths [Å]: P1‐P2: 2.2033(8), P2‐P3: 2.1319(8), P1‐P3: 2.2053(8), P1‐Pd: 2.9269(6), Pd‐P2: 2.3874(6), Pd‐P3: 2.3781(6), P1‐C: 1.861(2), Pd‐P4: 2.3323(5), Pd‐P5: 2.3557(5).

Figure 10

Contour plots of complementary NOCVs of complex 25 ⁺ (surface isovalue = 0.04) showing donor (green) and acceptor (red) together with the corresponding orbital interaction energies. Arrows depict the flux of electrons. Hydrogen atoms are omitted for clarity.

Figure 11

Visualization of ∇²ρ(r) (P2–P3–Pd plane, green points: BCP, red points: RCP) and the molecular graph of compound 25⁺ with atom numbering. Charge accumulations (∇²ρ(r) < 0) are printed in red, charge depletion (∇²ρ(r) > 0) in blue. 2,6‐Dimethylphenyl and phenyl moieties were omitted for clarity.