Syntheses, Characterization, and Structures of (Monomethyl Carbonato)–Nickel(II), –Copper(II), and –Cobalt(II) Complexes with Tetraazacycloalkanes Obtained from CO2 Uptake

A new series of monomethyl carbonato complexes of nickel(II), cobalt(II), and copper(II) containing tetraazacycloalkanes (L) have been synthesized and characterized: Ni(L)(O2COCH3)(ClO4) (L=[14]aneN4 (1); L=[15]aneN4 (2); L=iso-[14]aneN4 (3); L=RSRS-Me4[14]aneN4 (4)), [Ni(RRSS-Me4[14]aneN4)(O2COCH3)2]·HNEt3ClO4 (5), Cu([14]aneN4)(O2COCH3)(ClO4) (6), [Cu(Me4[14]aneN4)(O2COCH3)](ClO4)·CH3OH (7), and Co(L)(O2COCH3)(ClO4) (L=[15]aneN4 (8); L=Me4[14]aneN4 (9)), where [14]aneN4=1,4,8,11-tetraazacyclotetradecane, [15]aneN4=1,4,8,12-tetraazacyclopentadecane, iso-[14]aneN4=1,4,7,11-tetraazacyclotetradecane, Me4[14]aneN4=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, RSRS-=1RS,4SR,8RS,11SR-, and RRSS-=1RS,4RS,8SR,11SR-. These compounds have been obtained through CO2 uptake reaction in basic methanol from the corresponding four-coordinate complex, [M(L)](ClO4)2. X-Ray structures of three compounds, 4, 5, and 6, have been determined. Compound 4 is a five-coordinate complex with the trigonal bipyramidal geometry (Type II), in which CH3OCO2− is coordinated in a unidentate fashion. Compound 5 is a tetragonally distorted sixcoordinate complex with two unidentate CH3OCO2− at axial sites (Type III). Compound 6 is also a tetragonally distorted six-coordinate complex but the CH3OCO2− ligand bridges two neighboring [Cu([14]aneN4)]2+ to form a linear chain structure (Type I). All the monomethyl carbonato complexes synthesized are classified into these three types on the basis of IR and/or electronic spectral data, supplemented with the X-ray results. Efficiency of the CO2 uptake depends strongly on the coordination structure which is controlled by a combination of metal ion and tetraazacycloalkane. Differences in reactivity among the complexes of various metal ions and factors influencing the efficiency have been discussed in connection with the zinc(II) system which undergoes the most effective CO2 uptake of this type.