Diazadiene Ligands Research Articles

Diazadienes as control ligands in catalysis: Part 12. The nickel-catalyzed cyclotetramerization of propargylic alcohol to substituted cyclooctatetraenes and the structure of 1,3,5,7-tetrakis(hydroxymethyl)cyclooctatetraene

Nickel(O) complexes with diazadiene ligands, RNCR'-CR'NR, (dad), such as Ni(dad) 2 or Ni(dad)(1,5-COD) are very specific low temperature precatalysts for the cyclotetramerization of propargylic alcohol. Ni(dad)(CO) 2 shows an induction period and has to be warmed with the alkynol. Intermediates of the type (dad)Ni(alkyne) or their dimers are detected spectroscopically. Catalysts can also be prepared in situ by reduction of (dad)Ni(II) complexes with sodium borohydride. At low temperatures, cyclotetramers are formed exclusively. Different from propargylic ethers, the main product formed here is the 1,3,5,7-tetrakis(hydroxy-methyl) cyclooctatetraene VIIIa. The substitution pattern is established by a single crystal X-ray diffraction study. VIIIa crystallizes in the orthorhombic space group Pbcn ( a = 1492.8, b = 915.6, c = 1728.4 pm; Z = 8 molecules in the unit cell). Other COT isomers are formed in the catalytic reaction, including probably those with the 1,4,5,8- and the 1,3,6,8-substitution patterns.

Homoleptic diazadiene complexes of titanium, yttrium, and some lanthanoid elements

Cocondensation of titanium vapour with the diazadiene ligands Pridab or Butdab (Rdab = R–NCH–CHN–R) yields, respectively [Ti(σ,σ-N,N′-Pridab)3] and [Ti(σ,σ-N,N′-Butdab)2]; reaction of the vapours of yttrium, neodymium, samarium, or ytterbium with Butdab affords the formally yttrium(0) and lanthanoid(0) complexes [M(σ,σ-N,N′-Butdab)3], M = Y, Nd, Sm, Yb.

Koordination von 1,4-diaza-1,3-dienen (dad) an carbonyldieisen fragmente: IV. Darstellung von (dad)(trialkylphosphit-Fe(1))-pentacarbonyldieisen und mechanismus der intramolekularen photochemischen umlagerung unter scheinbarer wanderung des phosphitliganden von Fe(1) Nach Fe(2)

The binuclear complexes (dad)Fe 2(CO) 6( 1), in which diazadiene ligands bridge the FeFe axis in an unsymmetrical 6 e coordination mode, are found to be unreactive with respect to photochemical CO ligand substitution. Thermally, one CO at Fe(1), the iron atom connected to both dad-N atoms, can be substituted for trialkyl phosphites. Subsequently, with excess phosphite, another CO at Fe(2) may be substituted. On irradiation, the mono-phosphite complexes (dad)Fe 2(CO) 5[P(OMe) 3-Fe(1)] ( 2) and (dad)Fe 2(CO) 5[P(O-i-Pr) 3-Fe(1)] ( 5) of diazabutadienes undergo an intramolecular, photochemically reversible rearrangement to give isomeric complexes (dad)Fe 2(CO) 5[P(OMe) 3Fe(2)] ( 4) and (dad)Fe 2(CO) 5)[P(O-i-Pr) 3Fe(2)] ( 6) in which, seemingly, the phosphite from Fe(1) has exchanged place with a CO ligand at Fe(2). The photostationary equilibrium composition lies far on the side of 4 and 6 and is governed by steric factors, i.e., predominantly by the bulkyness of the phosphite ligand. The rearrangement consists of a valence isomerization of the (dad)Fe 2 frame via a postulated intermediate X in which the dad is symmetrized with respect to the FeFe axis. Out of three different conceivable modes by which the dad from X may change to the unsymmetrical coordination in the rearranged products, the employment of unsymmetrically substituted diazabutadienes shows two of the three rearrangement modes to be operative. The chosen pathway apparently depends on the electronic properties of the respective dad ligand. The rearrangement shows that electronic excitation energy remains localized in the dad ligand, i.e., FeFe or FeCO antibonding states not populated. Thus the missing photoreactivity in complexes 1 can be rationalized.