Synthesis and structural characterization of mono- and dinuclear transition metal complexes containing the tetraaza-[14] annulene ligand

Abstract

Synthetic entries into the area of new mono- and dinuclear transition metal complexes containg the tmtaa macrocyclic ligand (where tmtaa = dianion of 5,7,12,14-tetramethyldibenzo[b,i][1,4,8,11] tetraazacyclotetradecine) are described. The two efficient methods employed in the preparation of these complexes include reactions of transition metal starting materials with: (1) the neutral ligand, H2tmtaa, often under basic reaction conditions (e.g. in the presence of NEt3), or (2) an activated form of the ligand, Li2tmtaa, in tetrahydrofuran. The following mononuclear compounds have been synthesized and structurally characterized: (Cp)V(tmtaa), V(tmtaa)0, Cr(tmtaa)Cl, W(CO)4(tmtaa), Ru(tmtaa) (PMePh2)2 and Co(tmtaa=0) (02CCH3)2. The electronic delocalization of tmtaa stabilizes the compounds of the early transition metals, V and Cr. On the other hand, the ligand's framework flexibility becomes a stabilizing factor in the other mononuclear species. A number of dinuclear compounds synthesized and structurally characterized in this work are: M2(tmtaa)2 where M = Cr, Mo and Rh, Ru(Htmtaa) (02CCH3)3(PhCCPh) and [Rh2(02CCH3)6] [Rh(tmtaa) (PhCCPh) ]2. These metal-metal bonded systems warranted some in depth study due to their formation, despite the steric repulsions of the macrocyclic ligand. The ability of the ligand to displace the metal out of the Na plane and its flexibility to adjust the saddle shape conformation according to the size of the metal and the other M(tmtaa) fragment served to reduce the potentially severe steric interactions of two metal fragments multiply bonded together. Full details about the synthesis, structure and characterization of the above mono- and dinuclear compounds are given. To illustrate the nature of their structure and bonding, they are often discussed in light of other M(tnrtaa)-type compounds, previously reported in tne literature.