Reversible carbon monoxide reactions of cationic rhodium(I) and -(II) complexes Academic Article uri icon


  • The syntheses, spectral properties, redox chemistry, and structures of complexes obtained from the reaction of carbon monoxide with a rare mononuclear Rh(II) species are described. The ionic compound [Rh(3-TMPP)2][BF4]2 (1) {TMPP = tris(2,4,6-trimethoxyphenyl)phosphine, P(C6H2(OMe)3)3} has been prepared in high yield by reaction of the unbridged Rh24+complex [Rh2(CH3CN)10][BF4]4 with 4 equiv of TMPP in CH3CN. The product crystallizes in the orthorhombic space group Pbcn with unit cell dimensions a = 15.938 (5) , b = 17.916 (7) , c = 21.015 (8) , K= 5376 (3) 3, and Z = 4. Residuals of R = 0.073 and Rw = 0.089 were obtained after least-squares refinement of 369 parameters to convergence and a quality-of-fit of 2.37. The Rh atom, which is situated on a 2-fold axis, is bonded to two ether phosphine groups in a capping, tridentate mode through the phosphorus atom and two o-methoxy substituents. The geometry about the metal is pseudooctahedral, and, surprisingly, the phosphorus atoms lie cis to one another. A Jahn-Teller structural distortion was observed for 1 along the direction of the two trans oxygen atoms, which constitutes the first documentation of this effect in a molecular Rh(II) d7 complex. [Rh(3-TMPP)2][BF4]2 was also characterized by infrared, electronic and EPR spectroscopies, and by elemental analysis. Complex 1 reacts reversibly with carbon monoxide by a series of redox reactions, initiated by a highly unstable Rh(II) dicarbonyl adduct. Products isolated from the reaction under a CO atmosphere are the Rh(III) complex [Rh(3-TMPP)2][BF4]3 (2) and the Rh(l) dicarbonyl complex [Rh(TMPP)2(C0)2][BF4] (3). Compound 3 has been crystallized as its CH2C12 solvate in the space group Pl with unit cell dimensions a = 13.318 (4) , b = 13.404 (2) , c = 18.104 (4) , = 95.908 (3), = 97.037 (3), = 90.711 (3), V = 3200 (2) 3, and Z = 2. Least-squares refinement of 779 parameters gave residuals of R = 0.059 and Rw = 0.084 and a quality-of-fit = 2.316. The Rh(I) cation occupies a general position and exhibits a trans square-planar coordination geometry, with the phosphine ligands acting as monodentate groups. In the absence of a CO atmosphere, 3 readily loses a molecule of CO to form the monocarbonyl species [Rh(2-TMPP)(TMPP)CO] [BF4] (4) which has been structurally characterized as the benzene solvate [Rh(2-TMPP)(TMPP)CO][BF4]2C6H6; triclinic space group Pl with unit cell dimensions of a = 14.898 (5) , b = 18.060 (8) , c = 14.343 (4) , = 96.56 (4),= 113.84 (2), = 104.80 (4), V = 3308 (2) 3, and Z = 2. Final refinement of 757 parameters gave residuals of R = 0.067 and Rw = 0.069 with a quality-of-fit of 2.59. The immediate coordination sphere of the Rh atom consists of one CO ligand and two trans phosphorus atoms with the fourth site being occupied by a pendant methoxy group. The resulting structure is a highly distorted square-planar arrangement as evidenced by the bond angle O(1)Rh(l)C(55) of 150.2 (4). Complex 4 reversibly adds CO, both in the solid state and in solution, to reform 3. Furthermore, [Rh(2-TMPP)(TMPP)CO],+ (4) undergoes a second electron-transfer process with [Rh(3-TMPP)2][BF4]3 (2) to form [Rh(3-TMPP)2]2+ (1) and the unstable Rh(II) complex [Rh(TMPP)2(CO)]2+. The latter can easily lose CO to regenerate the original Rh(II) radical species 1 or add a second CO, thereby continuing the cycle. The quantitative regeneration of the parent Rh(II) complex is ultimately limited by the thermal instability of [Rh(3-TMPP)2]3+, which undergoes a demethylation reaction at room temperature to give the Rh(III) species [Rh(3-TMPP)(P(C6Hr(OMe)2O}{C6H2(OMe)3|2)][BF4]2 (5) with a phenoxide interaction. Compound 5 has been fully characterized by NMR, electrochemistry, and elemental analysis. Complexes 25 have also been prepared in high yield by separate routes and independently characterized. 1991, American Chemical Society. All rights reserved.

published proceedings

  • Journal of the American Chemical Society

author list (cited authors)

  • Haefner, S. C., Dunbar, K. R., & Bender, C.

publication date

  • January 1, 1991 11:11 AM