Ni(2,2':6',2''-terpyridine)2: a high-spin octahedral formal Ni(0) complex.

We have synthesised and characterised the complex Ni(tpy)2 (tpy = 2,2':6',2''-terpyridine). This formally Ni(0) complex is paramagnetic both in the solid state and in solution (S = 2). The crystal structure shows an octahedral geometry, with molecules arranged in independent dimers involving π-stacking between pairs of complexes. Magnetic measurementes and DFT calculations suggest the existence of temperature-dependent intermolecular antiferromagnetic coupling in the solid state.

Dinitrogen Binding at a Trititanium Chloride Complex and Its Conversion to Ammonia under Ambient Conditions.

Reaction of [TiCp*Cl3 ] (Cp*=η5 -C5 Me5 ) with one equivalent of magnesium in tetrahydrofuran at room temperature affords the paramagnetic trinuclear complex [{TiCp*(µ-Cl)}3 (µ3 -Cl)], which reacts with dinitrogen under ambient conditions to give the diamagnetic derivative [{TiCp*(µ-Cl)}3 (µ3 -η1 : η2 : η2 -N2 )] and the titanium(III) dimer [{TiCp*Cl(µ-Cl)}2 ]. The structure of the trinuclear mixed-valence complexes has been studied by experimental and theoretical methods and the latter compound represents the first well-defined example of the µ3 -η1 : η2 : η2 coordination mode of the dinitrogen molecule. The reaction of [{TiCp*(µ-Cl)}3 (µ3 -η1 : η2 : η2 -N2 )] with excess HCl in tetrahydrofuran results in clean NH4 Cl formation with regeneration of the starting material [TiCp*Cl3 ]. Therefore, a cyclic ammonia synthesis under ambient conditions can be envisioned by alternating N2 /HCl atmospheres in a [TiCp*Cl3 ]/Mg(excess) reaction mixture in tetrahydrofuran.

Low-Valent Titanium Species Stabilized with Aluminum/Boron Hydride Fragments.

Low-valent titanium species were prepared by reaction of [TiCp*X3 ] (Cp*=η5 -C5 Me5 ; X=Cl, Br, Me) with LiEH4 (E=Al, B) or BH3 (thf), and their structures elucidated by experimental and theoretical methods. The treatment of trihalides [TiCp*X3 ] with LiAlH4 in ethereal solvents (L) leads to the hydride-bridged heterometallic complexes [{TiCp*(µ-H)}2 {(µ-H)2 AlX(L)}2 ] (L=thf, X=Cl, Br; L=OEt2 , X=Cl). Density functional theory (DFT) calculations for those compounds reveal an open-shell singlet ground state with a Ti-Ti bond and can be described as titanium(II) species. The theoretical analyses also show strong interactions between the Ti-Ti bond and the empty s orbitals of the Al atom of the AlH2 XL fragments, which behave as σ-accepting (Z-type) ligands. Analogous reactions of [TiCp*X3 ] with LiBH4 (2 and 3 equiv.) in tetrahydrofuran at room temperature and at 85 °C lead to the titanium(III) compounds [{TiCp*(BH4 )(µ-X)}2 ] (X=Cl, Br) and [{TiCp*(BH4 )(µ-BH4 )}2 ], respectively. The treatment of [TiCp*Me3 ] with 4 and 5 equiv. of BH3 (thf) produces the diamagnetic [{TiCp*(BH3 Me)}2 (µ-B2 H6 )] and paramagnetic [{TiCp*(µ-B2 H6 )}2 ] complexes, respectively.

The Puzzling Monopentamethylcyclopentadienyltitanium(III) Dichloride Reagent: Structure and Properties.

Following the track of the useful titanocene [Ti(η5-C5H5)2Cl] reagent in organic synthesis, the related half-sandwich titanium(III) derivatives [Ti(η5-C5R5)Cl2] are receiving increasing attention in radical chemistry of many catalyzed transformations. However, the structure of the active titanium(III) species remains unknown in the literature. Herein, we describe the synthesis, crystal structure, and electronic structure of titanium(III) aggregates of composition [{Ti(η5-C5Me5)Cl2} n]. The thermolysis of [Ti(η5-C5Me5)Cl2Me] (1) in benzene or hexane at 180 °C results in the clean formation of [{Ti(η5-C5Me5)Cl(µ-Cl)}2] (2), methane, and ethene. The treatment of 1 with excess pinacolborane in hexane at 65 °C leads to a mixture of 2 and the paramagnetic trimer [{Ti(η5-C5Me5)(µ-Cl)2}3] (3). The X-ray crystal structures of compounds 2 and 3 show Ti-Ti distances of 3.267(1) and 3.219(12) Å, respectively. Computational studies (CASPT2//CASSCF and BS DFT methods) for dimer 2 reveal a singlet ground state and a relatively large singlet-triplet energy gap. Nuclear magnetic resonance spectroscopy of 2 in aromatic hydrocarbon solutions and DFT calculations for several [{Ti(η5-C5Me5)Cl2} n] aggregates are consistent with the existence of an equilibrium between the diamagnetic dimer [{Ti(η5-C5Me5)Cl(µ-Cl)}2] and a paramagnetic tetramer [{Ti(η5-C5Me5)(µ-Cl)2}4] in solution. In contrast, complex 2 readily dissolves in tetrahydrofuran to give a green-blue solution from which blue crystals of the mononuclear adduct [Ti(η5-C5Me5)Cl2(thf)] (4) were grown.

Ammonia-Borane Derived BN Fragments Trapped on Bi- and Trimetallic Titanium(III) Systems.

Titanium(III) complexes containing unprecedented (NH2 BH2 NHBH3 )2- and {N(BH3 )3 }3- ligands have been isolated, and their structures elucidated by a combination of experimental and theoretical methods. The treatment of the trimethyl derivative [TiCp*Me3 ] (Cp*=η5 -C5 Me5 ) with NH3 BH3 (3 equiv) at room temperature gives the paramagnetic dinuclear complex [{TiCp*(NH2 BH3 )}2 (µ-NH2 BH2 NHBH3 )], which at 80 °C leads to the trinuclear hydride derivative [{TiCp*(µ-H)}3 {µ3 -N(BH3 )3 }]. The bonding modes of the anionic BN fragments in those complexes, as well as the dimethylaminoborane group trapped on the analogous trinuclear [{TiCp*(µ-H)}3 (µ3 -H)(µ3 -NMe2 BH2 )], have been studied by X-ray crystallography and density functional theory (DFT) calculations.