Triazenide complexes of the heavier alkaline earths:: Synthesis, characterization, and suitability for hydroamination catalysis

A series of triazenide complexes of the heavier alkaline earths, Ca, Sr and Ba, have been synthesized by either protonolysis or salt metathesis routes. Although complexes of the form [{Ar2N3)M(N(SiMe3)(2))(THF)(n)] (M = Ca, n = 2; M = Sr, n = 3; Ar = 2,6-diisopropylphenyl) and [{Ar2N3}Ca(I)(THF)(2)](2) could be isolated and characterized by X-ray crystallography, solution studies revealed the propensity of these species to undergo Schlenk-like redistribution with the formation of [{Ar2N3)(2)M(THF)(n)] (M = Ca, n = 1; M = Sr, n = 2). The latter compounds have been synthesized independently. In the case of the large barium dication, attempts to synthesize the heaviest analogue of the series, [{Ar2N3}(2)Ba(THF)(n)}, failed and led instead to the isolation of the potassium barate complex [K{Ar2N3)Ba{N(SiMe3)(2)}(2)(THF)(4)]. Single crystal X-ray diffraction studies demonstrated that, although in all the aforementioned cases the triazenide ligand binds to the electrophilic group 2 metal centers via symmetrical kappa(2)-N,N-chelates, in the latter compound an unprecedented bridging mode is observed in which the triazenide ligand coordinates through both terminal and internal nitrogen centers. A series of density-functional theory computational experiments have been undertaken to assist in our understanding of this phenomenon. In further experiments, the calcium and strontium amide derivatives [{Ar2N3}M(N(SiMe3)(2)}(THF)(n)] (M = Ca, n = 2; M = Sr, n = 3) proved to be catalytically active for the intramolecular hydroamination of 1-amino-2,2-diphenylpent-4-ene to form 2-methyl-4,4-diphenylpyrrolidine, with the calcium species demonstrating a higher turnover number than the strontium analogue (2a, TOF = 500 h(-1); 2b, TOF = 75 h(-1)). In these instances, because of ambiguities in the structural charcterization of the precatalyst in solution, such quantification holds little value and detailed catalytic studies have not been conducted.