Synthesis, Structure, and Bonding of Weakly Coordinating Anions Based on CN Adducts

The addition of alkali or silver salts of dicyanoamide (dca), tricyanomethanide (tcm) and tetracyanoborate (tcb) to a solution of B(C6F5)(3) in diethyl ether affords salts containing very voluminous B(C6F5)(3) adduct anions of the type [E(CN)(n)(-)]center dot[B(C6F5)(3)](n):E = N (dca_nb with n = 1, 2; b = B(C6F5)(3)); E = C (tcm_nb with n = 1, 2, 3), and E = B (tcb_nb with n = 1, 2, 3, 4). Salts bearing these anions such as B[(CN)center dot B(C6F5)(3)](4)(-) (= [B(CN)(4)(-)]center dot[B(C6F5)(3)](4)), C[(CN)center dot B(C6F5)(3)](3)(-) (= [C(CN)(3)(-)]center dot[ B(C6F5)(3)](3)), and N[(CN)center dot B(C6F5)(3)](2-) (=[N(CN)(2)(-)]center dot[ B(C6F5)(3)](2)) can be prepared in good yields. They are thermally stable up to over 200 degrees C and dissolve in polar organic solvents. Depending on the stoichiometry mono-, di-, tri-, or tetraadduct formation is observed. The solid state structures of dca_2b, tcm_3b and tcb_4b salts show only long cation center dot center dot center dot anion contacts and thereby weak interactions, large anion volumes and only small distortions of the dca, tcm or tcb core enwrapped between B(C6F5)(3) groups. That is why these anions can be regarded as weakly coordinating anions. On the basis of B3LYP/6-31+G(d) computations the energetics, structural trends and charge transfer of the adduct anion formation were studied. Since tcm_3b and tcb_4b are easily accessible and can also be prepared in large quantities, these anions may be utilized as a true alternative to other widely used weakly coordinating anions. Moreover, for both steric and electronic reasons it seems reasonable to expect that as counterions for cationic early transition metal catalysts such anions may show reduced ion pairing and hence increased catalytic activity.