Reactions of [Cp*RuCl]4 and [(p-cymene)RuCl2]2 with the Tridentate Ligand [Ph(pz)B(μ-O)(μ-pz)B(pz)Ph]−
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Abstract
The reaction of the tridentate [N,O,N] (pyrazol-1-yl)borate ligand [Ph(pz)B(μ-O)(μ-pz)B(pz)Ph]− ([L1]−) with [Cp*RuCl]4 and [(p-cym)RuCl2]2 gives the RuII complexes [Cp*Ru(L1)] and [(p-cym)Ru(L1)]Cl, respectively (pz = pyrazolyl, Cp* = pentamethylcyclopentadienyl, p-cym = p-cymene). In order to avoid degradation of the [(p-cym)Ru(L1)]+ complex in solution, its Cl− counterion has been exchanged for PF6−, [B(C6F5)4]−, tosylate, and triflate. When the reaction between [L1]− and [(p-cym)RuCl2]2 is carried out in the presence of 4 equiv of TlPF6, the dinuclear pyrazolyl-bridged complex [(p-cym)Ru(μ-Cl)(μ-pz)2Ru(p-cym)]PF6 and the mononuclear species [(p-cym)Ru(L2)] are obtained ([L2]2− = [Ph(pz)B(μ-O)(μ-OB(Ph)O)B(pz)Ph]2−). In a targeted synthesis, the lithium salt of the novel ligand [L2]2− was prepared from 2 equiv of Lipz and 2,4,6-Ph3B3O3 and successfully transformed into [(p-cym)Ru(L2)]. [Cp*Ru(L1)], [(p-cym)Ru(L1)]PF6, and [(p-cym)Ru(L2)] have been characterized by NMR spectroscopy, X-ray crystallography, and (spectro)electrochemistry. One-electron oxidation of [Cp*Ru(L1)] by electrochemical or chemical ([Cp2Fe]PF6) means leads to the RuIII species [Cp*Ru(L1)]PF6, which has been isolated and fully characterized (E1/2(RuII/RuIII) = −0.39 V; CH2Cl2, vs FcH/FcH+). A comparison of the solid-state structures of [Cp*Ru(L1)] and [Cp*Ru(L1)]PF6 reveals that oxidation of the ruthenium center results in a lengthening of the average Ru−Cp* distances and a shortening of all Ru−L1 bond lengths. According to the X-ray data, the angle strain within [(p-cym)Ru(L1)]PF6 is higher than in [(p-cym)Ru(L2)], which could account for the fact that [L1]− is apparently less stable than [L2]2−.
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