Magnesium(0) Complexes and their Reduction Reactions with Binary Transition Metal Carbonyls

Abstract

Three very bulky β-diketimine protio-ligands (ArNacnacH, {(Ar)N=C(But)}2CH2, Ar = 2,4,6-tricyclohexylphenyl TCHP; 2,6-dicyclohexylphenyl DCHP; TCHP/Dip, Dip = 2,6-diisopropylphenyl) have been synthesised. These have been used to prepare monomeric, three-coordinate β-diketiminato magnesium iodide complexes, [(ArNacnac)MgI] (Ar = TCHP 1, DCHP 3, or TCHP/Dip 5). Sodium metal reduction of 1 afforded the thermally robust anionic magnesium(0) complex [{(TCHPNacnac)Mg}Na]2 6 in good yield. In contrast, reduction of less bulky [(DCHPNacnac)MgI] 3 gave a mixture of unknown products, whilst reduction of [(TCHP/DipNacnac)MgI] 5 yielded the mixed oxidation state, magnesium compound, [{(TCHP/DipNacnac)Mg}2Mg] 7. The related compound [{(TCHPNacnac)Mg}2Mg] 8, was prepared by reduction of a 2:1 mixture of [(TCHPNacnac)MgI] 1 and MgI2. Computational analyses of 6 and 8 reveal their electronic structures to be comparable to those of previously reported analogues of these compounds. Reduction reactions between magnesium(0) compound 6 and the neutral groups 6 and 7 transition metal carbonyls, Cr(CO)6, Mo(CO)6 and Mn2(CO)10, gave a series of complexes [{(TCHPNacnac)Mg}2{μ-M2(CO)n}] (n = 10, M = Cr 9 or Mo 10; n = 8, M = Mn 11) which incorporate metal‒metal single (9 and 10) or double (11) bonded metal carbonyl fragments. In contrast, reduction of Fe(CO)5 with 6 yielded [{(TCHPNacnac)Mg}Na{Fe(CO)4}]2 12 which does not possess an Fe‒Fe bond, and can be viewed as an analogue of Collman’s reagent, Na2[Fe(CO)4], in which one Na+ cation has been replaced by a [(TCHPNacnac)Mg]+ unit.