Dinuclear Cobalt(II) Complexes of Pyrazole Ligands with Chelating Side Arms

A series of pyrazole‐based potential ligands bearing polydentate amine substituents in the 3‐ and 5‐positions of the heterocycle has been synthesized [3,5‐bis(R2NCH2)‐pyzH R2N = Me2N(CH2)3NMe (2aH), [Me2N(CH2)3]2N (2bH), (Et2NCH2CH2)2N (2cH)]. Upon reaction with two equivalents of CoCl2 they form complexes LCo2Cl3 (3a–c; L = 2a–c, respectively) which are shown crystallographically to contain a dinuclear metal core bridged by both the pyrazolate unit and a chlorine atom, with each cobalt center carrying a further terminal chlorine atom. Two of the ligand side arms in 3b, c are dangling, thus leading to five‐coordination of the cobalt(II) centers in all cases. Addition of two equivalents of NaBPh4 to solutions of 3b, c induced coordination of the formerly dangling side arms to the metal centers by substitution of the terminal chlorine atoms. The resulting compounds [LCo2Cl](BPh4)2 (4b, c, respectively) were characterized by X‐ray structure analyses. They can be viewed as dinuclear linked versions of tran‐type complexes [(tran = tris(aminoalkyl)amine] with distorted trigonal‐bipyramidal coordination spheres around cobalt(II). Conformational analyses employing force‐field calculations were carried out for 4b, c in order to rationalize the conformations observed in the solid state with regard to the accessible conformational space.