Second-sphere co-ordination of cationic rhodium complexes [Rh(L)(NH<inf>3</inf>)<inf>2</inf>]⁺ by dibenzo-3n-crown-n ethers [n = 6-12; L = cyclo-octa-1,5-diene (cod) or norbornadiene (nbd)]. Solution ¹H nuclear magnetic resonance spectroscopic studies and X-ray crystal structures of ...

Abstract

full title:Second-sphere co-ordination of cationic rhodium complexes [Rh(L)(NH3)2]+ by dibenzo-3n-crown-n ethers [n = 6-12; L = cyclo-octa-1,5-diene (cod) or norbornadiene (nbd)]. Solution 1H nuclear magnetic resonance spectroscopic studies and X-ray crystal structures of [Rh(cod)(NH3)2·db-21-c-7][PF6], [Rh(cod)(NH3)2·db-24-c-8][PF6], [Rh(cod)(NH3)2·db-30-c-10][PF6], [Rh(nbd)(NH3)2·db-24-c-8][PF6], and [{Rh(cod)(NH3)2}2·db-36-c-12][PF 6]2 Cationic rhodium(I) complexes of the form [Rh(L)(NH3)2][PF6] [L = cyclo-octa-1,5-diene (cod) or norbornadiene (nbd)] are solubilised in halocarbon solvents by the addition of equimolar quantities of crown ethers such as 18-crown-6 (18-c-6) or dibenzo-3n-crown-n (n = 6-12) ethers (db-3n-c-n). Solubilisation is a result of the formation of stable, highly structured, second-sphere co-ordination complexes in which a macrocyclic polyether interacts with the ammine ligands on rhodium via multiple hydrogen-bond formation. 1H N.m.r. spectroscopic investigations in CD2Cl2 show that the resonances of protons associated with the diene ligands (L) undergo significant upfield shifts (Δδ values of up to 0.75 p.p.m.) when certain of the db-3n-c-n ethers (n = 7-10) are used in preference to 18-c-6, suggesting that the dienes are experiencing anisotropic ring-current shielding by the crown ether benzo rings. The close proximity of coordinated diene and db-3n-c-n ether aromatic rings, which this shielding requires, is confirmed by single-crystal X-ray studies of the isolated 1:1 adducts. [Rh(cod)(NH3)2·db-21-c-7][PF6] is monoclinic, space group P21/c, a = 9.121(1), b = 23.405(2), c = 16.705(3) Å, β = 96.30(1)°, Z = 4, R = 0.047. [Rh(cod)(NH3)2·db-24-c-8][PF6] is monoclinic, P21/c, a = 9.1 95(3), b = 29.380(9), c = 13.900(6) Å, β = 93.04(3)°, Z = 4, R = 0.043. [Rh(cod)(NH3)2·db-30-c-10][PF6] is monoclinic, P21/c, a = 9.268(2), b = 34.335(7), c = 13.421(3) Å, β = 93.92(2)°, Z = 4, R = 0.034. [Rh(nbd)(NH3)2·db-24-c-8][PF6] is orthorhombic, Pbca, a = 15.427(3), b = 18.739(3), c = 25.458(5) Å, Z = 8, R = 0.047. In all these supramolecular structures the crown ether adopts a V-shaped conformation with the ammine ligands forming hydrogen bonds to ether oxygens and with the co-ordinated diene sandwiched between the benzo rings of the db-3n-c-n ethers. In addition to electrostatic stabilisation, there are also a large number of other longer range contacts (C ⋯ H, Rh ⋯ C, Rh ⋯ O) at about van der Waals' distances which probably make a small contribution to the binding energy and possibly help to define the structures of the adducts. In contrast to the 1:1 adducts, the 2:1 adduct, [{Rh(cod)(NH3)2}2·db-36-c-12][PF 6]2 [triclinic, P1, a = 9.020(2), b = 12.241(3), c = 14.196(3) Å, α = 100.20(2), β = 99.00(2), γ = 98.11(2)°, Z = 1, R = 0.040] has a relatively flat structure in which a complex ion is hydrogen bonded to each face of the crown ether.