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The 1,4-diaryl-1-azabuta-1,3-diene-catalyzed complexation of cyclohexa-1,3-diene with either nonacarbonyldiiron or pentacarbonyliron is reported to provide high yields of the tricarbonyl(η4-cyclohexa-1,3-diene)iron complex. This procedure enables exploitation of both tricarbonyliron fragments of nonacarbonyldiiron for the complexation of dienes for the first time. Using 12.5 mol % of 1-(4-methoxyphenyl)-4-phenyl-1-azabuta-1,3-diene and optimized reaction conditions (nonacarbonyldiiron, dimethoxyethane, reflux, 16.5 h, or pentacarbonyliron, dioxane, reflux, 45 h), a quantitative catalytic complexation of cyclohexa-1,3-diene is feasible with both reagents. An extensive study with a broad range of 1,4-diaryl-1-azabuta-1,3-dienes shows that the efficiency of the catalysts strongly depends on the substituents of the two aryl rings. Remarkably high activities are found for those catalysts deriving from condensation of cinnamaldehyde and ortho-methoxy-substituted arylamines. A hexacarbonyldiiron complex of 1-(4-methoxyphenyl)-4-phenyl-1-azabuta-1,3-diene is obtained as a byproduct of the catalytic complexation and is structurally confirmed by X-ray crystallography. A mechanism supported by the experimental findings is proposed.

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The racemization/symmetrization of helicate-or meso-helicate-type dinuclear titanium(1V) complexes M,[ (ligand), Ti,] (n = 4: M = Li, Na, K, Cs; n = 2: M = Ca, Sr), which are formed by metal-directed self-assembly of alkyl-bridged bis(catecholate) ligands in the presence of basic alkali-metal or alkaNon-covalent interactions between complementary units lead to the formation of defined supramolecular species with defined properties"]. In metallo-supramolecular chemistry the metals are building blocks which play an active part in the molecular recognition process and thus influence the formation as well its the properties of the obtained aggregatesL21.Dinuclear triple-stranded helicate-or meso-helicate-type coordination compounds are formed in spontaneous and cooperative self-assembly processes from three linear ligand strands, each bearing two chelating units, and two metal ion^ [^-^]. Recently, we introduced alkyl-bridged bis(catecholate) ligands as organic components for such self-assembly processes. We demonstrated that the relative stereochemis- line-earth metal salts, can be monitored by dynamic NMR spectroscopy. The free-energy barrier (AG') of this process is influenced by the nature of the ligand, the solvent, and the counterion. In the solid state as well as in solution, the cryptand-type tetraanionic dinuclear titanium complexes are able to bind one (or more) of the counterions. Therefore, the counterions are not merely "innocent spectators", but are integral parts of the molecular structures and thus should influence the self-assembly process as well as the properties of the formed dinuclear complexes. An important property, which is observed for the catecholate helicates and mesohelicates, is the inversion of the configuration at the metal centers. This process can be monitored by dynamic 'H-NMR spectroscopy by observing the diastereotopic protons of the alkyl s p a~e r [~*~] .In this paper we describe a detailed study of the dynamic behavior of dinuclear titanium(1V) complexes with the linear alkyl-bridged ligands 1-H4-4-H4[81. The influence of the spacer length, the solvent, and of the alkali-metal or alkaline-earth metal counterions on such systems is investigated. Results and Discussion Preparation of the ComplexesThe dinuclear coordination compounds M4[(L)3Ti2] (L = 1-4, M = Li, Na, K; L = 2, M = Cs) were obtained from three equivalents of the ligand l-Hd-4-H~ (L-H4) and two equivalents of [(acac),?TiO] in the presence of two equivalents of alkali-metal carbonate in methanol. Removal of the solvent afforded the dinuclear complexes in good to quantitative yields. When further purification was necessary, this could be achieved by chromatography on Sephadex LH 20 with methanol as solvent. The corresponding salts with alkaline-earth metal cations M'2[(2)3Ti2] (M' = Ca, Sr) were also prepared. The calcium compound was synthesized in a similar manner as the aforementioned alkali-metal salts, while Srz[(2)3Ti2] could only be obtained when DMF was

COMMUNICATIONSis the splitting of the ci1 states of two Nb, clusters (Fig. 2). The combinations of the 2a, orbitals are shown in Scheme 1. / 2 a , \ HOMO Scheme I . The formation of a bonding (HOMO) and an antibonding combination (top) from Zu, orbitals of two Nb, uiii(s.

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