Tantalum and Titanium Alkylidene Complexes Bearing Phosphinoalkoxide Ligands. Reversible Ortho-Metalation of a Titanium Alkylidene

Early/Late Heterobimetallic Tantalum/Rhodium Species Assembled Through a Novel Bifunctional NHC‐OH Ligand

“…The Ta−C NHC bond distance in 4 (2.355(5) Å) is rather long and falls in the upper range of Ta−C NHC bond distances reported in the literature so far, although slightly shorter than the corresponding distance in 3 (2.447(5) Å). In contrast, the Ta=C21 bond distance is drastically shorter (1.921(5) Å), which reveals considerable multiple bond character and falls in the range of previously reported d 0 tantalum–alkylidene complexes . The large Ta=C21‐C22 angle of 156.8(4)° is indicative of an α‐Ta⋅⋅⋅CH agostic interaction, which is typical for tantalum–neopentylidene complexes and is in agreement with the NMR data reported above.…”

“…In contrast, the Ta=C21 bond distance is drastically shorter (1.921(5) Å), which reveals considerable multiple bond character and falls in the range of previously reported d 0 tantalum–alkylidene complexes . The large Ta=C21‐C22 angle of 156.8(4)° is indicative of an α‐Ta⋅⋅⋅CH agostic interaction, which is typical for tantalum–neopentylidene complexes and is in agreement with the NMR data reported above. The two carbenes in 4 are arranged cis to each other, as in the well‐known Grubbs catalysts.…”

Lability of Ta–NHC adducts as a synthetic route towards heterobimetallic Ta/Rh complexes

“…This bond length is significantly shorter than the other two Ta-C alkyl bonds lengths (2.237(7) Å) and compares well with that of previously reported Ta(V) alkylidene complexes. 23,[36][37][38][39][40] The Ta-CNHC bond distance (Ta1-C1 = 2.261(7) Å) is slightly shorter to that in found in 1 (2.355(5) Å) 23 and falls in the range (2.22-2.40 Å) of the rare examples of tantalum NHC complexes reported to date. 23,[41][42][43][44][45] The Ta-CNHC bond length is thus much longer than that of the Ta=Cneopentylidene which reflects the difference in the metal-carbene interactions (Fischer-type versus Schrock-type carbene).…”

“…50,51 As expected, the Ta=Calkylidene bond length (1.90(1) Å) is significantly shorter than the Ta-Calkyl distances (2.14(1) and 2.156(1) Å), and falls in the expected range. 23,[36][37][38][39][40]52 The Rh centre adopts a square-planar coordination geometry (τ4 = 0.09) which is characteristic for 4-coordinate 16valency electrons Rh(I) species. The Rh-carbene bond length (2.040(1) Å) is comparable to that found in related Rh(I)-NHC species.…”

3d Transition Metals for C–H Activation

“…Titanium is the second-most abundant transition metal in the Earth’s crust, and its low-toxicity renders it particularly attractive for molecular synthesis. − Titanium complexes have shown to be versatile catalysts in several organic transformations including hydroaminations, − nucleophilic additions to carbonyl compounds, − polymerization reactions, , epoxidation of unsaturated compounds, , heterocycle synthesis, , enolate chemistry, and photocatalysis, − among others. , In addition, the titanium catalysis has also been explored for the site-selective inert C–H activation of alkanes and arenes. The stoichiometric studies by the groups of Bercaw, Wolczanski, − Hessen, , Mindiola, − and others − illustrated the power of titanium complexes for C–H activation. In recent years, a significant number of reports on C–H functionalizations using catalytic amounts of titanium complexes have emerged, which will be discussed in this section.…”