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Rhodium(I) Complexes with π-Coordinated Arylallene. Structures in the Solid State and in Solution and Reaction with Arylallene To Give Rhodacyclopentane

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posted on 17.04.1998, 00:00 authored by Jun-Chul Choi, Susumu Sarai, Take-aki Koizumi, Kohtaro Osakada, Takakazu Yamamoto
Reactions of phenylallene and of (4-fluorophenyl)allene with [Rh(μ-Cl)(PMe3)2]2 (Rh:arylallene = 1:1) give Rh(I) complexes with π-coordinated arylallene, RhCl(η2-CH2CCHAr)(PMe3)2 (1a, Ar = C6H5; 1b, Ar = C6H4F-p). X-ray crystallography of 1a,b has established their square-planar coordination around the rhodium center, which is bonded to the 2,3-double bond of the arylallene. The aryl group of the ligand and the Rh center are situated on the same side of the uncoordinated 1,2-double bond. The position of an ortho hydrogen of the phenyl group in the ligand suggests an agostic interaction between the C−H group and the Rh center. Dissolution of 1a in benzene results in equilibration with its isomer 3a, which does not have a close contact between the ortho C−H group of the ligand and the Rh center. The 1H NMR spectra of an equilibrated mixture in the temperature range 30−55 °C afford thermodynamic parameters for 3a1a of ΔH° = −5.0 kJ mol-1 and ΔS° = −8 J mol-1 K-1. The arylallenes react also with RhCl(PMe3)3 in a 1.2:1 molar ratio to yield pentacoordinated Rh(I)−arylallene complexes, RhCl(η2-CH2CCHAr)(PMe3)3 (2a, Ar = C6H5; 2b, Ar = C6H4F-p). A crystallographic study of 2b shows a distorted trigonal-bipyramidal coordination around the Rh center, which is bonded to equatorial Cl, PMe3, and 2,3-η2-CH2CCHAr ligands and to two apical PMe3 ligands. The 4-fluorophenyl group of the ligand and the Rh center are situated on opposite sides of the uncoordinated CC double bond. Complex 2a is in equilibrium with its isomer 4a, which shows dynamic NMR behavior on the NMR time scale. Reactions in a 3:1 molar ratio give rhodacyclopentanes, Ar = C6H5; 5b, Ar = C6H4F-p) as the main product. Complex 5a is also obtained from reaction of phenylallene with 2a. Reactions of (4-fluorophenyl)allene with RhCl(PEt3)3, RhCl{P(i-Pr)3}2, and RhCl(PPh3)3 proceed smoothly to give the corresponding Rh(I)−(4-fluorophenyl)allene complexes, RhCl(η2-CH2CCHC6H4F-p)(PR3)2 (6, R = Et; 7, R = i-Pr; 8, R = Ph). The 1H NMR spectra of the complexes and X-ray crystallography of 8 indicate square-planar coordination around the Rh center, which is bonded to the 2,3-double bond of the arylallene.

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