Phosphorus centers of different hybridization in phosphaalkene-substituted phospholes

Abstract

Phosphole-substituted phosphaalkenes (PPAs) of the general formula Mes*P=C(CH(3))-(C(4)H(2)P(Ph))-R 5 a-c (Mes*=2,4,6-tBu(3)Ph; R=2-pyridyl (a), 2-thienyl (b), phenyl (c)) have been prepared from octa-1,7-diyne-substituted phosphaalkenes by utilizing the Fagan-Nugent route. The presence of two differently hybridized phosphorus centers (σ(2) ,λ(3) and σ(3) ,λ(3)) in 5 offers the possibility to selectively tune the HOMO-LUMO gap of the compounds by utilizing the different reactivity of the two phosphorus heteroatoms. Oxidation of 5 a-c by sulfur proceeds exclusively at the σ(3) ,λ(3) -phosphorus atom, thus giving rise to the corresponding thioxophospholes 6 a-c. Similarly, 5 a is selectively coordinated by AuCl at the σ(3),λ(3) -phosphorus atom. Subsequent second AuCl coordination at the σ(2),λ(3) -phosphorus heteroatom results in a dimetallic species that is characterized by a gold-gold interaction that provokes a change in π conjugation. Spectroscopic, electrochemical, and theoretical investigations show that the phosphaalkene and the phosphole both have a sizable impact on the electronic properties of the compounds. The presence of the phosphaalkene unit induces a decrease of the HOMO-LUMO gap relative to reference phosphole-containing π systems that lack a P=C substituent.

Keywords: X-ray diffraction; conjugation; electronic structure; phosphaalkenes; phosphorus.

Scheme 1

Synthesis of octadiyne-phosphaalkenes 4 a–c. i) 1) nBuLi, −130 °C, Trapp mixture, 30 min; 2) MeI, −120 °C, 30 min then to RT, 2 h. Compound 2 93 %. ii) 1,7-octadiynemagnesium bromide, [Pd(dba)₂], PPh₃, THF, reflux, 4–5 h. Compound 3 98 %. iii) 1,7-octadiyne, CuI, [Pd(PPh₃)₂Cl₂], Et₂NH, RT, 3 h. Compound 3 65 %. iv) 1-(2-pyridyl)octa-1,7-diyne, CuI, [Pd(PPh₃)₂Cl₂], Et₂NH, RT, 14 h. Compound 4 a 74 %. v) b) 2-iodothiophene, CuI, [Pd(PPh₃)₂Cl₂], Et₃N, RT, 14 h. Compound 4 b 96 %. c) iodobenzene, CuI, [Pd(PPh₃)₂Cl₂], Et₂NH, RT, 14 h. Compound 4 c 54 %.

Figure 1

ORTEP plots of phosphaalkenes 3 and 4 b,c (ellipsoids are drawn at 50 % probability level). Hydrogen atoms and disorder in the alkyl groups are omitted for clarity. Selected bond lengths [Å]: Compound 3: P1–C1 1.855(2), P1=C19 1.693(2), C21≡C22 1.207(3), C27≡C28 1.194(4); compound 4 b: P1–C1, 1.852(3), P1=C19 1.686(3), C21≡C22 1.194(6), C27≡C28 1.183(6); Compound 4 c: P1–C1 1.8449(16), P1=C19 1.6834(18), C21≡C22 1.198(11), C27≡C28 1.185(10).

Scheme 2

Synthesis of phosphole-phosphaalkenes (PPAs) 5,6 a–c. i) 1) [Cp₂ZrCl₂], nBuLi (2 equiv), THF, −78 °C 1 h, then RT, 14 h; 2) PhPBr₂, −78 °C, then for 5 a; RT, 4 h, 5 b,c; RT, 24 h. Compound 5 a 42 %, 5 b 21 %, 5 c not isolated. ii) sulfur, CH₂Cl₂, RT, 14 h. Compound 6 a 40 %, 6 b 19 % (over 2 steps), 5 c 29 % (over 2 steps).

Figure 2

ORTEP plots of PPAs 5 a and 6 a–c (ellipsoids are drawn at 50 % probability level). Hydrogen atoms and disorder in the alkyl groups are omitted for clarity. Selected bond lengths [Å] and angles [°]: Compound 5 a: P1–C1 1.7984(19), P1–C8 1.811(2), P1–C29 1.828(2), P2=C9 1.705(2), P2–C11 1.8582(19), C1–C2 1.372(3), C2–C7 1.463(3), C7–C8 1.378(3), C8–C9 1.467(3); C1-P1-C8 91.28(9), C9-P2-C11 104.23(9), P2-C9-C8 115.61(15). Compound 6 a; P1–C1 1.802(4), P1–C8 1.823(4), P1–C29 1.821(3), P2=C9 1.706(4), P2–C11 1.839(4), C1–C2 1.367(5), C2–C7 1.482(5), C7–C8 1.377(5), C8–C9 1.455(5), P1–S1 1.9500(12); C1-P1-C8 94.17(17), C9-P2-C11 104.03(18), P2-C9-C8 114.67(3). Compound 6 b; P1–C1 1.813(2), P1–C8 1.814(2), P1–C21 1.822(2), P1–S1 1.9493(11), P2=C13 1.697(2), P2–C15 1.846(2), S2–C12 1.707(3), S2–C9 1.741(2), C1=C2 1.364(3), C1–C13 1.471(3), C2–C7 1.487(3), C7=C8 1.362(3), C9=C10 1.377(3), C10–C11 1.412(3), C11=C12 1.355(4), C13–C14 1.509(3); C13-P2-C15 101.93(10), C1-P1-C8 93.41(10), C1-C13-P2 115.99(16). Compound 6 c: P1=C9 1.6889(12), P1–C11 1.8580(12), P2–C1 1.7999(12), P2–C8 1.8214(12), P2–C29 1.8150(13), C1–C2 1.3487(17), C1–C35 1.4782(16), C2–C7 1.4955(16), C7–C8 1.3571(17), C8–C9 1.4762(16), P2–S1 1.9482(4); C1-P1-C8 94.17(17), C9-P2-C11 104.03(18), P2-C9-C8 114.67(3).

Scheme 3

Coordination of AuCl to phosphole-phosphaalkene (PPA) 5 a. i) [AuCl(tht)] (1 equiv), CH₂Cl₂, RT, 1 h. 7 98 %. ii) [AuCl(tht)] (2 equiv), CH₂Cl₂, RT, 1 h. Compound 8 61 %. ORTEP plot of PPAs 7 and 8 (ellipsoids are drawn at 50 % probability level). Hydrogen atoms and disorder in the alkyl groups are omitted for clarity. Selected bond lengths [Å] and angles [°]: Compound 7: Au1–P1 2.234(2), Au1–Cl1 2.295(2), P1–C1 1.813(7), P1–C22 1.817(9), P1–C8 1.831(7), P2–C9 1.698(8); P1-Au1-Cl1 177.56(7). Compound 8: C1–P9 1.79(2), C9–P7 1.682(18), P7–Au1 2.226(4), P9–Au2 2.228(5), Cl1–Au1 2.280(4), Cl2–Au2 2.280(4), Au1–Au2 3.0692(11); P7-Au1-Cl1 170.85(19), P9-Au2-Cl2 175.6(2).

Figure 3

UV/Vis spectra of PPAs 5 a,b, 6 a–c, and gold complexes 7 and 8 in CH₂Cl₂ at room temperature. Synthesis of phosphole-phosphaalkenes (PPAs) 5,6 a–c.

Figure 4

The relative conformations between the P=C double bond and the orientation of the phosphole. The designation s-cis is used when the torsion angle between the two phosphorus heteroatoms is close to 0° (top left) and anti when the torsion angle between the two phosphorus heteroatoms is close to 180° (top right). The designation s-cis/s-trans is used when the torsion angle between the two phosphorus heteroatoms is close to 0° and the torsion angle between the phosphole phosphorus and the nitrogen (or sulfur for 5,6 b) is close to 180° (bottom left); s-cis/s-cis is used when the torsion angle between the two phosphorus heteroatoms is close to 0° and the torsion angle between the phosphole phosphorus and the nitrogen (or sulfur for 5,6 b) is close to 0° (bottom right).

Figure 5

Calculated HOMO and LUMO with orbital energies [eV] for PPAs 5 and 6 at the DFT B3LYP/6-311G** level of theory; Δ_H–L HOMO–LUMO splitting in eV.

Figure 6

Calculated HOMO and LUMO with orbital energies [eV] for PPAs 7 and 8 at the DFT PBE1PBE/6-311G**/LANL2DZ level of theory; Δ_LUMO–HOMO HOMO–LUMO splitting in eV.