Cationic Rh complexes with novel spiro tetraarylpentaborate anions prepared from arylboronic acids and aryloxorhodium complexes

Abstract

Ar–B(OH)₂ (1a: Ar = C₆H₄OMe-4, 1b: Ar = C₆H₃Me₂-2,6) react immediately with Rh(OC₆H₄Me-4)(PMe₃)₃ (2) in 5 ∶ 1 molar ratio at room temperature to generate [Rh(PMe₃)₄]⁺[B₅O₆Ar₄]⁻ (3a: Ar = C₆H₄OMe-4, 3b: Ar = C₆H₃Me₂-2,6). p-Cresol (92%/Rh), anisole (80%/Rh) and H₂O (364%/Rh) are formed from 1a and 2. The reaction of 1a with 2 for 24 h produces [Rh(PMe₃)₄]⁺[B₅O₆(OH)₄] ⁻ (4) as a yellow solid. This is attributed to hydrolytic dearylation of once formed 3a because the direct reaction of 3a with excess H₂O forms 4. An equimolar reaction of 2 with phenylboroxine (PhBO)₃ causes transfer of the 4-methylphenoxo ligand from rhodium to boron to produce [Rh(PMe₃)₄]⁺[B₃O₃Ph₃(OC₆H₄Me-4)]⁻ (5). Arylboronic acids 1a and 1b react with Rh(OC₆H₄Me-4)(PR₃)₃ (6: R = Et, 8: R = Ph) and with Rh(OC₆H₄Me-4)(cod)(PR₃) (11: R = ⁱPr, 12: R = Ph) to form [Rh(PR₃)₄]⁺[B₅O₆Ar₄]⁻ (7a: R = Et, Ar = C₆H₄OMe-4, 7b: R = Et, Ar = C₆H₃Me₂-2,6, 9a: R = Ph, Ar = C₆H₃Me₂-2,6) and [Rh(cod)(PR₃)(L)]⁺[B₅O₆Ar₄]⁻ (13b: R = ⁱPr, L = acetone, Ar = C₆H₃Me₂-2,6, 14a: R = Ph, L = PPh₃, Ar = C₆H₄OMe-4, 14b: R = Ph, L = PPh₃, Ar = C₆H₃Me₂-2,6), respectively. Hydrolysis of 14a yields [Rh(cod)(PPh₃)₂]⁺[B₅O₆(OH)₄]⁻ (15) quantitatively.