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The nitrido‐ate complex [(PN)2Ti(N){μ2‐K(OEt2)}]2 (1) reductively couples CO and isocyanides in the presence of DME or cryptand, to form rare, five‐coordinate TiII complexes having a linear cumulene motif, [K(L)][(PN)2Ti(NCE)] (E = O, L = Kryptofix222, (2); E = NAd, L = 3 DME, (3); E = NtBu, L = 3 DME, (4); E = NAd, L = Kryptofix222, (5)). Oxidation of 2‐5 with [Fc][OTf] afforded an isostructural TiIII center containing a neutral cumulene [(PN)2Ti(NCE)] (E = O, (6); E = NAd (7), NtBu (8)). Moreover, 1e‐ reduction of 6 and 7 in the presence of cryptand cleanly reformed corresponding discrete TiII complexes 2 and 5, which were further characterized by solution magnetization measurements and high‐ frequency and ‐field EPR (HFEPR) spectroscopy. Furthermore, oxidation of 7 with [Fc*][B(C6F5)4] resulted in a ligand disproportionated TiIV complex having transoid carbodiimides, [(PN)2Ti(NCNAd)2] (9). Comparison of spectroscopic, structural, and computational data for the divalent, trivalent, and tetravalent systems, including their 15N enriched isotopomers demonstrate these cumulenes to decrease in order of backbonding as TiII→TiIII→TiIV and increasing order of p‐donation as TiII→TiIII→TiIV, thus displaying more covalency in TiIII species. Lastly, we show a synthetic cycle whereby complex 1 can deliver an N‐atom to π‐acids. 

Abstract Described here is a direct entry to two examples of 3d transition metal catalysts that are active for the cyclic polymerization of phenylacetylene, namely, [(BDI)M{κ²‐C,C‐(Me₃SiC₃SiMe₃)}] (2‐M) (BDI=[ArNC(CH₃)]₂CH⁻, Ar=2,6‐ⁱPr₂C₆H₃;M=Ti, V). Catalysts are prepared in one step by the treatment of [(BDI)MCl₂] (1‐M,M=Ti,V) with 1,3‐dilithioallene [Li₂(Me₃SiC₃SiMe₃)]. Complexes2‐Mhave been spectroscopically and structurally characterized and the polymers that are catalytically formed from phenylacetylene were verified to have a cyclic topology based on a combination of size‐exclusion chromatography (SEC) and intrinsic viscosity studies. Two‐electron oxidation of2‐Vwith nitrous oxide (N₂O) cleanly yields a [V^V] alkylidene‐alkynyl oxo complex [(BDI)V(=O){κ¹‐C‐(=C(SiMe₃)CC(SiMe₃))}] (3), which lends support for how this scaffold in2‐Mmight be operating in the polymerization of the terminal alkyne. This work demonstrates how alkylidynes can be circumvented using 1,3‐dianionic allene as a segue into M−C multiple bonds. 

We report the first mononuclear Ti^IIIcomplex possessing a terminal imido ligand.