Synthesis and Structure of P‑Halogenated Benzazaphospholes and Their Reactivity toward Pt(0) Sources

Dearomatization of the five-membered ring of N-Dipp (Dipp = 2,6-diisopropylphenyl)-substituted benzazaphosphole 2 with HCl generated 1–Cl, which undergoes substitution with halide sources to provide the remaining members of the P-halogenated series (1–F, 1–Br, and 1–I). These P-heterocycles (1) were characterized by multinuclear [31P­{1H}, 1H, 13C­{1H}, and 19F (if applicable)] NMR spectroscopy, elemental analysis, and X-ray crystallography. 1H and 13C­{1H} NMR spectroscopy revealed that derivatives 1–F, 1–Cl, and 1–Br have C1 symmetry in solution. In contrast, 1–I has effective Cs symmetry in solution due to a rapid, concentration-dependent, inversion at phosphorus, shown by density functional theory (DFT) calculations (B3LYP-D3/6-311G**++) to involve a dimeric iodine-bridged transition structure. In the solid state, 1–F through 1–I all exhibited C1 symmetry with varying degrees of elongation of their P–X bonds. Elongation of the P–X bonds is shown by DFT/natural bond orbital studies to involve NLP → σ*­(P–X) negative hyperconjugation, which increases down the halogen series but is less pronounced than that for the closely related NHP–X counterparts (3–F through 3–I). Treatment of 1–Br/I with [Pt­(P­(t-Bu)3)2] afforded Pt­(I)–Pt­(I) dimers 4–Br/I, which were characterized by 31P­{1H}, 1H, and 13C­{1H} NMR spectroscopy, elemental analysis, and X-ray crystallography. Addition of 1–F to Pt­(PPh3)4 gave Pt­(1–F)­(PPh3)2 (6), a coordination compound in which the P-heterocyclic ligand is bound through its lone pair without P–F bond cleavage. Recrystallization attempts resulted in ligand exchange, furnishing Pt(0) complex 7, which features 1–F and PPh3 donors in a 2:1 ratio. Reactions of 1–Cl with all tested starting materials produced unidentifiable product mixtures by 31P­{1H} NMR spectroscopy, but the combination of Pt­(PPh3)4 or Pt­(PPh3)2(C2H4) and 1–Cl in acetone generated an unisolable Pt complex containing tentatively assigned PtP2 metallacyclopropane structural unit 8.