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(mu-N(Me))(mu-pz)B(pz)Ph (L(1)), Ph(pz)B(mu-O)(mu-pz)B(pz)Ph (L(2)), and Ph(pz)B(mu-O)(mu-OB(Ph)O)B(pz)Ph (L(3)) have been experimentally assessed by investigating the following compounds: [Li(thf)L(1)], [Li(thf)L(2)], [Mg(Cl)(thf)(x)L(1)], [Mg(Cl)(thf)(2)L(2)], [Mn(CO)(3)L(2)], K(thf)[Mn(CO)(3)L(3)], [CoCl(2)(HL(1))], [L(1)Co(mu-Cl)(2)CoL(1)], [Zn(Br)L(1)], [Cu(Cl)L(1)], [Cu(Cl)L(2)]. The L(1)-complexes were prepared from a mixture of HL(1) and the appropriate metal salt by addition of a base. HCl elimination from [CoCl(2)(HL(1))], which gives [L(1)Co(mu-Cl)(2)CoL(1)], 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)(3)L(3)] was obtained via in situ hydrolysis of HL(1)/[Mn(CO)(5)Br] in the presence of K(2)CO(3). 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)L(2)]) or pyrazolide (e.g., [L(2)Co(mu-Cl)(mu-pz)CoL(2)]) complexes. As evidenced by the crystal structure analyses of [Zn(Br)L(1)]/[Mg(Cl)(thf)(2)L(2)]/[Mn(CO)(3)L(2)] on the one hand and [L(1)Mg(mu-Cl)(2)Mg(thf)L(1)]/[Cu(Cl)L(1)]/Cu(Cl)L(2) on the other, L(1) and L(2) 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 L(1), L(2), and L(3), the bonding situation of the central donor moiety (N(Me) in L(1); O in L(2), L(3)) 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.