Factors that Influence π- versus η²-Coordination of β-Diketiminato Ligands in Magnesium Complexes

Treatment of N-tert-butyl-4-(tert-butylimino)-2-penten-2-amine with [CpMgMe(Et2O)]2 in diethyl ether afforded CpMg(HC(C(Me)N(tBu))2) (90%). A crystal structure determination of this complex revealed a sandwich structure with a π-coordinated β-diketiminato ligand. CpMg(HC(C(Me)N(tBu))2) did not form isolable adducts with diethyl ether or tetrahydrofuran, but treatment with 4-tert-butylpyridine led to the formation of CpMg(HC(C(Me)N(tBu))2)(4-tBupy) (88%). A crystal structure determination showed that CpMg(HC(C(Me)N(tBu))2)(4-tBupy) possesses a η2-β-diketiminato ligand with some distortion toward π-bonding, but not as much as was found in CpMg(HC(C(Me)N(tBu))2). Similar treatment of N-isopropyl-4-(isopropylimino)-2-penten-2-amine with [CpMgMe(Et2O)]2 and 4-tert-butylpyridine in diethyl ether afforded CpMg(HC(C(Me)N(iPr))2)(tBupy) (73%). The crystal structure of this complex was very similar to that of CpMg(HC(C(Me)N(tBu))2)(4-tBupy). In the absence of 4-tert-butylpyridine, CpMg(HC(C(Me)N(iPr))2)(OEt2) was isolated, but this complex decomposed slowly to mixtures of Cp2Mg and Mg(HC(C(Me)N(iPr))2)2 upon loss of the labile diethyl ether ligand. Treatment of dibutylmagnesium with 2 equiv of N-tert-butyl-4-(tert-butylimino)-2-penten-2-amine or N-isopropyl-4-(isopropylimino)-2-penten-2-amine in hexane afforded Mg(HC(C(Me)N(tBu))2)2 (68%) or Mg(HC(C(Me)N(iPr))2)2 (78%). Crystal structure determinations of Mg(HC(C(Me)N(tBu))2)2 and Mg(HC(C(Me)N(iPr))2)2 revealed η2-β-diketiminato ligands and approximate tetrahedral geometry about the magnesium ions. The thermal stability and volatility of the complexes were determined through preparative sublimation experiments.