Synthesis, characterization, substitution, and atom-transfer reactions of (.eta.2-alkyne)(tetratolylporphyrinato)titanium(II). X-ray structure of trans-bis(4-picoline)(tetratolylporphyrinato)titanium(II)

Synthetic Approaches to (smif)2Ti (smif = 1,3-di-(2-pyridyl)-2-azaallyl) Reveal Redox Non-Innocence and C–C Bond-Formation

“…One wonders whether bis -( tris -pyrazolylborate)Ti 27 and (porphyrin)Ti(alkynes) 26 actually have related electronic features that would better fit a Ti(III) description, although the latter compounds have metalacyclopropene character, and are thus “oxidized” by p-backbonding.…”

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“…A crystal structure analysis ‡ reveals the expected bis(k 3 -Tp) structure, with approximate molecular D 3d symmetry and octahedral microsymmetry about the Ti(II) center; the average Ti-N bond distances § are comparable to values previously observed for Ti(II)-N(aromatic) contacts. [5][6][7] Compound 1 is paramagnetic and, as a polycrystalline solid, obeys the Curie-Weiss law (30-300 K) for an S = 1 state (m eff = 2.62 m B ). Under rigorously anaerobic, aprotic conditions, the complex is soluble, stable, and well-defined in a variety of solvents.…”

“…Stable, isolable complexes of divalent titanium typically require the presence of p-acceptors and/or soft donor ligands to support the reduced metal ion; 9 Ti(II) centers ligated solely by hard donors (as in 1) are rare [4][5][6][7] and often undergo progressive, spontaneous decomposition in solution. 4 We ascribe the exceptional stability of complex 1, which remains unchanged in toluene solution even after prolonged heating (90 °C, 1 week, dinitrogen atmosphere), to the effectiveness of the bis(k 3 -Tp) ligation in maintaining a coordinatively saturated metal center.…”

Generation of TiII Alkyne Trimerization Catalysts in the Absence of Strong Metal Reductants

“…7 Formally, Ti II coordination complexes are fairly uncommon because of the extremely high thermodynamic stability of the Ti IV oxidation state. Nevertheless, discrete Ti II complexes have been isolated and characterized across diverse ligand sets: cyclopentadienyl, 2c,8 calixarene, 4,9 isocyanide, 10 porphyrin, 7a,11 pyridine, 7b and 1,2-bis(dimethylphosphino)ethane. 12 Similarly, “masked” Ti II complexes have also been reported in which the low-valent state is stabilized by strong π -acceptors such as alkyne, 13 alkene, 14a–c and carbonyl.…”

“…Given that they are highly reducing, Ti II reagents have typically been formed from the reaction of Ti IV halide precursors with powerful reductants: KC 8 , , LiAlH 4 , Na/Hg, and Mg . Recently, we have reported several catalytic oxidative C–N bond forming reactions that proceed through a formal Ti II /Ti IV redox couple (Scheme ).…”