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][PF6]2

Abstract

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][PF₆]₂[triclinic, P, 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.