Preparation of coordination compounds of Cp*2Yb with heterocyclic nitrogen bases:: Examples of antiferromagnetic exchange coupling across bridging ligands

Several adducts are isolated between heterocyclic nitrogen bases and Cp*Yb-2. The adducts fall into several general classes. The membership within the class is related to the reduction potential of the given heterocyclic base relative to that of Cp*Yb-2. Analytically pure 1:1 adducts of the type Cp*Yb-2(L) are formed with pyrazine, quinoxaline, 1,5- and 1,8-naphthyridine, and 4,4'-bipyridine as toluene-insoluble solids. The 1:1 adducts with phthalazine and azobenzene and the 1:2 adduct with pyridazine are soluble in toluene, from which they may be isolated by crystallization. All of the adducts in this class are paramagnetic, and their effective magnetic moments are consistent with the formulation Cp*Yb-2(III)(L.-) (L.- = radical anion), in which spins on the individual units are uncoupled to 5 K. Adducts between Cp*Yb-2 and phenazine, 2,2'-azopyridine, 2,2'-bipyrimidine, 2,2'-azobenzene, and 2,3-bis(2-pyridino)quinoxaline are of 2:1 stoichiometry: (Cp*Yb-2)(2)(mu-L). The crystal structure of (Cp*Yb-2)(2)(mu-bipyrimidine) shows that the two metallocenes are bridged by a planar bipyrimidine ligand, and the other 2:1 adducts are assumed to have a similar structure. The effective magnetic moment of these 2:1 adducts shows that each Cp*Yb-2(III) fragment behaves as an isolated paramagnetic, and the bridging ligand is a diamagnetic dianion at high temperature. At low temperature the last three adducts undergo antiferromagnetic coupling with a Neel temperature of about 20 K. A spin polarization model is advanced to account for the electronic exchange coupling.