Fourth Generation Scorpionates: Coordination Behavior of a New Class of Conformationally Flexible Mixed-Donor (Pyrazol-1-yl)borates

The coordination behavior, the conformational flexibility, and the stability of the novel (pyrazol-1-yl)borate ligands [Ph(pz)B(μ-N(Me))(μ-pz)B(pz)Ph]− ([L1]−), [Ph(pz)B(μ-O)(μ-pz)B(pz)Ph]− ([L2]−), and [Ph(pz)B(μ-O)(μ-OB(Ph)O)B(pz)Ph]2− ([L3]2−) have been experimentally assessed by investigating the following compounds: [Li(thf)L1], [Li(thf)L2], [Mg(Cl)(thf)xL1], [Mg(Cl)(thf)2L2], [Mn(CO)3L2], K(thf)[Mn(CO)3L3], [CoCl2(HL1)], [L1Co(μ-Cl)2CoL1], [Zn(Br)L1], [Cu(Cl)L1], [Cu(Cl)L2]. The L1-complexes were prepared from a mixture of HL1 and the appropriate metal salt by addition of a base. HCl elimination from [CoCl2(HL1)], which gives [L1Co(μ-Cl)2CoL1], does not necessarily require the assistance of a base, but happens spontaneously when a solution of the complex is stored at room temperature for several days. K(thf)[Mn(CO)3L3] was obtained via in situ hydrolysis of HL1/[Mn(CO)5Br] in the presence of K2CO3. In some other cases, formation of the coordination compounds proceeded with decomposition of a part of the ligand molecules and yielded pyrazole (e.g., [Zn(Cl)(Hpz)L2]) or pyrazolide (e.g., [L2Co(μ-Cl)(μ-pz)CoL2]) complexes. As evidenced by the crystal structure analyses of [Zn(Br)L1]/[Mg(Cl)(thf)2L2]/[Mn(CO)3L2] on the one hand and [L1Mg(μ-Cl)2Mg(thf)L1]/[Cu(Cl)L1]/[Cu(Cl)L2]2 on the other, [L1]− and [L2]− are able to adopt both a facial and a meridional conformation. Moreover, while many of the established design principles of scorpionate chemistry are still valid for [L1]−, [L2]−, and [L3]2−, the bonding situation of the central donor moiety (N(Me) in [L1]−; O in [L2]−, [L3]2−) is distinctly different from the way the pyrazolyl rings are attached to the molecule, so that donor scrambling is not an issue in these [N,N,N] and [N,O,N] mixed-donor ligands.