Molybdenum carbonyl assisted reductive tetramerization of CO by activated magnesium(i) compounds: squarate dianion vs. metallo-ketene formation.

Reactions of a dimagnesium(i) compound, [{( Dip Nacnac)Mg} 2 ] ( Dip Nacnac = [HC(MeCNDip) 2 ] - , Dip = 2,6-diisopropylphenyl), pre-activated by coordination with simple Lewis bases (4-dimethylaminopyridine, DMAP; or TMC, :C(MeNCMe) 2 ), with 1 atmosphere of CO in the presence of one equivalent of Mo(CO) 6 at room temperature, led to the reductive tetramerisation of the diatomic molecule. When the reactions were carried out at room temperature, there is an apparent competition between the formation of magnesium squarate, [{( Dip Nacnac)Mg}{ cyclo -(κ 4 -C 4 O 4 )}{μ-Mg( Dip Nacnac)}] 2 , and magnesium metallo-ketene products, [{( Dip Nacnac)Mg}[μ-O[double bond, length as m-dash]CC{Mo(CO) 5 }C(O)CO 2 ]{Mg(D)( Dip Nacnac)}], which are not inter-convertible. Repeating the reactions at 80 °C led to the selective formation of the magnesium squarate, implying that this is the thermodynamic product. In an analogous reaction, in which THF is the Lewis base, only the metallo-ketene complex, [{( Dip Nacnac)Mg}[μ-O[double bond, length as m-dash]CC{Mo(CO) 5 }C(O)CO 2 ]{Mg(THF)( Dip Nacnac)}] is formed at room temperature, while a complex product mixture is obtained at elevated temperature. In contrast, treatment of a 1 : 1 mixture of the guanidinato magnesium(i) complex, [(Priso)Mg-Mg(Priso)] (Priso = [Pr i 2 NC(NDip) 2 ] - ), and Mo(CO) 6 , with CO gas in a benzene/THF solution, gave a low yield of the squarate complex, [{(Priso)(THF)Mg}{ cyclo -(κ 4 -C 4 O 4 )}{μ-Mg(THF)(Priso)}] 2 , at 80 °C. Computational analyses of the electronic structure of squarate and metallo-ketene product types corroborate the bonding proposed from experimental data, for the C 4 O 4 fragments of these systems.