New RuII(arene) Complexes with Halogen‐Substituted Bis‐ and Tris(pyrazol‐1‐yl)borate Ligands

RuCl(arene)(μ‐Cl)]₂ dimers were treated in a 1:2 molar ratio with sodium or thallium salts of bis‐ and tris(pyrazolyl)borate ligands Na(Bp)], Tl(Tp)], and Tl(Tp^{iPr, 4Br})]. Mononuclear neutral complexes RuCl(arene)(κ²‐Bp)] ( 1 : arene=p‐cymene (cym); 2 : arene=hexamethylbenzene (hmb); 3 : arene=benzene (bz)), RuCl(arene)(κ²‐Tp)] ( 4 : arene=cym; 6 : arene=bz), and RuCl(arene)(κ²‐Tp^{iPr, 4Br})] ( 7 : arene=cym, 8 : arene=hmb, 9 : arene=bz) have been always obtained with the exception of the ionic Ru₂(hmb)₂(μ‐Cl)₃]Tp] ( 5′ ), which formed independently of the ratio of reactants and reaction conditions employed. The ionic Ru(CH₃OH)(cym)(κ²‐Bp)]X] ( 10 : X=PF₆, 12 : X=O₃SCF₃) and the neutral Ru(O₂CCF₃)(cym)(κ²‐Bp)] ( 11 ) have been obtained by a metathesis reaction with corresponding silver salts. All complexes 1 – 12 have been characterized by analytical and spectroscopic data (IR, ESI‐MS, ¹H and ¹³C NMR spectroscopy). The structures of the thallium and calcium derivatives of ligand Tp, Tl(Tp)] and Ca(dmso)₆]Tp]₂ ? 2 DMSO, of the complexes 1 , 4 , 5′ , 6 , 11 , and of the decomposition product RuCl(cym)(Hpz^{iPr, 4Br})₂]Cl] ( 7′ ) have been confirmed by using single‐crystal X‐ray diffraction. Electrochemical studies showed that 1 – 9 and 11 undergo a single‐electron Ru^II→Ru^III oxidation at a potential, measured by cyclic voltammetry, which allows comparison of the electron‐donor characters of the bis‐ and tris(pyrazol‐1‐yl)borate and arene ligands, and to estimate, for the first time, the values of the Lever E_L ligand parameter for Bp, Tp, and Tp^{iPr, 4Br}. Theoretical calculations at the DFT level indicated that both oxidation and reduction of the Ru complexes under study are mostly metal‐centered with some involvement of the chloride ligand in the former case, and also demonstrated that the experimental isolation of the μ³‐binuclear complex 5′ (instead of the mononuclear 5 ) is accounted for by the low thermodynamic stability of the latter species due to steric reasons.