Heterobi- and heterotetranuclear RuRh and RuIr complexes with 2,2′-biimidazolate and 2,2′-bibenzimidazolate anions as bridging ligands

Article abstract of DOI:10.1021/om00004a069

The reaction of the dimer [{(η⁶-p-cymene)RuCl}₂(μ-Cl)₂] (p-cymene = p-isopropylmethylbenzene) with 2,2′-biimidazole (H₂Bim) afforded the chloride complex [(η⁶-p-cymene)Ru-(H₂Bim)Cl]Cl (1) which has also been isolated as the tetrafluoroborate salt [(η⁶-p-cymene)Ru(H₂-Bim)Cl]BF₄ (2) by metathesis with NaBF₄. Complex 1 underwent reaction with [M(acac)-L₂] yielding the corresponding heterobinuclear species [(η⁶-p-cymene)ClRu(μ-HBim)MClL₂] (M = Rh; L₂ = cis,cis-1,5-cyclooctadiene (COD) (3), (CO)₂ (4); M = Ir, L₂ = COD (5)]. Complexes 3-5 exhibit a dynamic behavior which equilibrates the two imidazole rings of the HBim bridging ligand. The reaction of 4 with KOH led to two tetranuclear isomers [(η⁶-p-cymene)ClRu(μ-Bim)Rh(CO)₂]₂ (6) which in solution reached an equilibrium with a K value for the process 6b?6a ≤ 0.11. Reaction of [(η⁶-p-cymene)Ru(acac)Cl] with [Rh(HBim)-(COD)] afforded the heterobinuclear complex [(η⁶-p-cymene)Ru(acac)(μ-HBim)Rh(COD)]A (A = Cl (7), BF₄ (8)). At room temperature the Rh(HBim)(COD) moiety of complexes 7 and 8 rotates rapidly around the Ru-N bond, but this process stops at 223 K. [(η⁶-p-cymene)-Ru(acac)Cl] and [(η⁶-p-cymene)Ru(acac)(PPh₃)]BF₄ reacted with 2,2′-bibenzimidazole (H₂-Bbzim) rendering [(η⁶-p-cymene)Ru(HBbzim)Cl] (9) and [(η⁶-p-cymene)Ru(HBbzim)(PPh₃)]BF₄ (10), respectively. Complex 9 undergoes reaction with [M(acac)L₂] yielding [(η⁶-p-cymene)-ClRu(μ-Bbzim)ML₂]_n (M = Rh; L₂ = COD (n = 1) (11), norborna-2,5-diene (NBD) (n = 2) (12), (CO)₂ (n = 2) (13). M = Ir; L₂ = COD (n = 1) (14)). The molecular structure of complex 12 has been determined. Crystals are triclinic, space group P1, with cell parameters a = 12.998(1) A?, b = 14.035(1) A?, c = 18.081(2) A?, α = 79.864(4)°, β = 78.046(5)°, γ = 73.326(7)°, and Z = 2. The compound is tetranuclear with two Bbzim^2- ligands bridging the metal atoms. Each Bbzim^2- coordinates to the metals in an unsymmetrical tetradentate manner through its four nitrogen atoms, chelated to the Ru atoms and bonded in an unidentate manner to the two Rh atoms. The cationic complex 10 undergo reaction with the acetylacetonates [M(acac)L₂] yielding the cationic binuclear complexes [(η⁶-p-cymene)(PPh₃)-Ru(μ-Bbzim)ML ₂]BF₄ (M = Rh; L₂ = COD (15), NBD (16), (CO)₂ (17). M = Ir; L₂ = COD (18)). Carbonylation of 18 afforded [(η⁶-p-cymene)(PPh₃)Ru(μ-Bbzim)Ir(CO) ₂]BF₄ (19) and complex 17 could analogously be obtained from complexes 15 or 16. The molecular structure of complex 19 has been determined by diffractometric methods. Crystals are monoclinic, space group P2₁/c, with cell parameters a = 11.586(2) A?, b = 20.489(2) A?, c = 17.225(1) A?, β = 95.34(1)°, and Z = 4. In the solid state, two complex cations [(η⁶-p-cymene)(PPh₃)Ru-(μ-Bbzim)Ir(CO)₂] related by an inversion center are joined by an intermetallic Ir-Ir interaction (3.0808(5) A?). The cation consists of a Ru and an Ir atom connected by a Bbzim^2- anion that chelates both metals through four nitrogen atoms. Complexes 11, 14, and 15-19 are active catalysts for the homogeneous hydrogenation of cyclohexene. Kinetic studies showed that the reduction catalyzed by 14 is first-order in catalyst concentration and second-order in hydrogen concentration.