Aryl compounds of rhodium: syntheses and X-ray crystal structures

Abstract

The interactions of fac-RhCl₃(tht)₃, tht = tetrahydrothiophene, with aryl-lithium or -magnesium bromide reagents, aryl = 2,4,6-R₃C₆H₂(R = Me or Prⁱ), 2,6-Me₂C₆H₃, or 2-MeC₆H₄, and also Mg(PhCH₂)Br, have been studied. With Li(2,4,6-Prⁱ₃C₆H₂)(Et₂O) only the unique, square, paramagnetic rhodium(II) compound trans-Rh(2,4,6-Prⁱ₃C₆H₂)₂(tht)₂ is obtained. This reacts with CO reversibly to give labile carbonyl species and with Bu^tNC to give the rhodium(I)σ-bonded iminoacyl Rh(CNBu^t)₃(2,4,6-Prⁱ₃C₆H₂CNBu^t). A similar mesityl (mes) compound is obtained from Rh(mes)₃ and Bu^tNC. Homoleptic rhodium(III) aryls Rh(aryl)₃ have been obtained for aryl = 2,6-Me₂C₆H₃ and 2,4,6-Me₃C₆H₂. The mesityl reacts with CO to give a carbonyl species that has a bridged acyl and a bridge in which a mesityl group σ-bonded to one Rh atom is bound η⁶ to the other. It also reacts with PMe₂Ph to give Rh(mes)(PMe₂Ph)₃. The interaction of RhCl₃(tht)₃ with Mg(PhCH₂)Br gives only the unsymmetric bridged species (tht)(PhCH₂)₂Rh(µ-Cl)₃Rh(CH₂Ph)(tht)₂. The crystal structures of six compounds have been determined: trans-Rh(2,4,6-Prⁱ₃C₆H₂)₂(tht)₂ is square with a two-fold axis of symmetry perpendicular to the molecular plane [Rh–C and Rh–S 2.11(1) and 2.291(5)Å]. The compounds Rh(CNBu^t)₃(σ-RCNBu^t), (R = 2,4,6-Me₃C₆H₂ or 2,4,6-Prⁱ₃C₆H₂) have similar structures in which the rhodium(I) centres are square [Rh–C (isocyanide) and Rh–C (imino) 1.88–1.94(2) and 2.08 and 2.06(2)Å]. The compound Rh₂(2,4,6-Me₃C₆H₂CO)(2,4,6-Me₃C₆H₂(CO)₃ contains two rhodium centres with quite different co-ordination: Rh(1) is square with bonds to two carbonyls [Rh–C 1.81(1) and 1.90(1)Å], an aryl [Rh–C 2.07(1)Å] and an oxygen from the aroyl ligand [Rh–O 2.08(1)Å]; Rh(2) is trigonal bipyramidal with the aryl σ-bonded to Rh(1) now η⁶ bonded over two equatorial [Rh–C 2.33(1) and 2.34(1)Å] and one axial [Rh–C 2.44(1)Å] Sites, the remaining axial and equatorial sites being occupied by the aroyl carbon [Rh–C 1.94(1)Å] and a carbonyl group [Rh–C 1.84(1)Å]. The compound Rh(2,4,6-Me₃C₆H₂)(PMe₂Ph)₃ has square rhodium(I) geometry [Rh–C 2.09(1) and Rh–P 2.304(4)Å(trans to the aryl) and 2.276(4) and 2.287(4)Åtrans to each other]. The dimer Rh₂(PhCH₂)₃Cl₃(tht)₃ has a confacial bioctahedral structure with Rh–Cl, Rh–C and Rh–S distances showing considerable variations due to the differing trans influences of the three ligand types.