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(Et 2 O)] 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 η 2 -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(Et 2 O)] 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 )(OEt 2 ) was isolated, but this complex decomposed slowly to mixtures of Cp 2 Mg 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 η 2 -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.