FIXATION OF CO2 BY A SERIES OF ETHYNYL-BRIDGED POLYNUCLEAR ALUMINUM-MAGNESIUM COMPLEXES - SYNTHESIS, CHARACTERIZATION, AND CRYSTAL-STRUCTURES OF [ME(2)AL(MU-I-PR2N)(2)MG(MU-C-CR)](2) (R=C6H5, C6H4-P-CH3, T-BU,SIME(3)), [ME(2)AL(MU-ET(2)N)(2)MG(MU-C-CC6H5)](2), ((ME(2)AL)(2)[MU-OOC(I-PR2N)](2)), AND ((ME(2)AL)(2)[(MU-OOC(I-PR2N))(2)]MG-2)

A series of ethynyl-bridged polynuclear aluminum-magnesium complexes, [Me(2)Al(mu-R'N)(2)Mg(mu-C=CR)](2) (R = C6H5 (1), C6H4-p-CH3 (2), t-Bu (3), SiMe(3) (4), C6H5 (5), C6H4-p-CH3 (6); R' = i-Pr (1-4), Et for ( 5, 6)), were prepared by reaction of the aluminum-magnesium tetramer [ Me(2)Al(mu-i-Pr2N)(2)Mg(mu-Me)](4) (A) and the dimer [Me(2)Al(mu-Et(2) N)(2)Mg(mu-Me)](2) (B) with various substituted acetylenes. Subsequent reaction of complexes 1 and 2 with CO2 gives selective insertion prod ucts, viz. {[Me(2)Al(mu-i-Pr2N)(2)Mg[mu-OOC(C=CR)]}(2) (R = C6H5 (7), C6H4-p-CH3 (8)), and the insertion occurs at the Mg-C bond. Reaction o f compound A with CO2 in diethyl ether gives two different insertion p roducts, {(Me(2)Al)(2)[mu-OOC(i-Pr2N)](2)} (9), a dialuminum carbamato complex, and {(Me(2)Al)(2)[(mu-OOC(i-Pr2N))(2)](2)}-Mg (10), a mixed- metal aluminum-magnesium carbamato complex. Reaction of compound B wit h CO2 gives {(Me(2)Al)(2)[(mu-OOC(Et(2)N))(2)]Mg-2} (11), and sublimat ion of 11 gives compound 12, {(Me(2)Al)(2)[mu-OOC(Et(2)N)](2)}. The cr ystal structures of 1-5, 9, and 10 were determined by single-crystal X -ray diffraction techniques. Complexes 1-5 all consist of three four-m embered rings made of two tetrahedrally coordinated Al and Mg atoms wi th two bridging amido groups. The ethynyl bridging groups are almost p erpendicular to the Mg-Mg vector. The pertinent bonding features (d(C- alpha=C-beta) = 1.183(6)-1.204(10) Angstrom, angle Mg-C-alpha=C-beta = 142.1(7)-165.9(6)degrees, angle Mg-C-alpha=C-beta = 107.6(5)-131.8(7 )degrees) indicate no substantial pi-interaction between the magnesium and the ethynyl groups, yet a tendency toward such interactions is no ticed. The dialuminum and aluminum-magnesium mixed-metal carbamato com plexes 9 and 10 result from the insertion of CO2 into the metal-N bond at the bridging amido ligands. Their syntheses and structures are dis cussed.