Monomeric, Dimeric, and Trimeric Calcium Compounds Containing Substituted Pyrrolyl and Ketiminate Ligands: Synthesis and Structural Characterization

A series of monomeric, dimeric, and trimeric calcium compounds containing substituted pyrrolyl or ketiminate ligands were synthesized, and characterized by NMR spectroscopy and single crystal X-ray diffractometry. The reaction of Ca[N(SiMe3)(2)](2)(THF)(2) with 1 equiv of [C4H3NH(2-CH2NEt2)] in toluene generates the dimeric complex, [Ca{N(SiMe3)}[mu-eta(1):eta(5)-{C4H3N(2-CH2NEt2)}]](2) (1) in which two substituted pyrrolyl ligands bind two Ca centers in a eta(1) and eta(5) fashion. The reaction between Ca[N(SiMe3)(2)](2)(THF)(2) and 2 equiv of [C4H3NH(2-CH2NEt2)] in THF yields a monomeric calcium compound Ca[C4H3N(2-CH2NEt2)](2)(THF)(2) (2) that exhibits a facial octahedral geometry on the central Ca atom. Similarly, the reactions of Ca[N(SiMe3)(2)](2)(THF)(2) With 1 and 2 equiv of OCMeCHCMeNHAr (Ar = 2,6-diisopropylphenyl) generate [Ca(OCMeCHCMeNAr){N(SiMe3)(2)}](2) (3) and [Ca(mu-OCMeCHCMeNAr)-(OCMeCHCMeNAr)](2) (4), respectively. In 3, the Ca atom possesses a distorted tetrahedral geometry where as in 4, a square plane is developed by the two calcium atoms with the bridging participation of two oxygen atoms from two ketiminate ligands. The in situ reaction of OCMeCHCMeNHAr, Ca[N(SiMe3)(2)](2)(THF)(2), and isopropyl alcohol results in a trimeric calcium alkoxide compound Ca-3(mu-OCMeCHCMeNAr)(2)(OCMeCHCMeNAr)(mu(3)-O-Pr-i)(2)(mu(2)-O-Pr-i) (5). Compounds 1, 2, and 5 showed good catalytic activity in the ling-opening polymerization of E-caprolactone and L-lactide.