Rhodium(I) complexes with π-coordinated arylallene.: Structures in the solid state and in solution and reaction with arylallene to give rhodacyclopentane

Reactions of phenylallene and of (4-fluorophenyl)allene with [Rh(mu-Cl)(PMe3)(2)](2) (Rh: arylallene 1:1) give Rh(I) complexes with pi-coordinated arylallene, RhCl(eta(2)-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 H-1 NMR spectra of an equilibrated mixture in the temperature range 30-55 degrees C afford thermodynamic parameters for 3a <----> 1a of Delta H degrees = -5.0 kJ mol(-1) and Delta S degrees = -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(eta(2)-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-eta(2)-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, mer-<(Rh[Ch(2)C(=CHAr)C(=CHArC)over bar>H-2]Cl(PMe3)(3) (5a, 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(eta(2)CH(2)=C=CHC6H4F-p)(PR3)(2) (6, R = Et; 7, R = i-Pr; 8, R = Ph). The H-1 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.