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Titanium(IV) Cations With Trigonal Monopyramidal Geometry: Unusual Lewis Acids Supported by a Triaryl Triamidoamine Ligand

Titanium(IV) Cations With Trigonal Monopyramidal Geometry: Unusual Lewis Acids Supported by a Triaryl Triamidoamine Ligand

Priyabrata Ghana, Franziska D van Krüchten, Thomas P Spaniol, Jun Okuda

Chemistry. 2019 Aug 9;25(45):10718-10723.

PMID: 31206822

Abstract:

Protonolysis of the titanium alkyl complex [Ti(CH2 SiMe3 )(Xy-N3 N)] (Xy-N3 N=[{(3,5-Me2 C6 H3 )NCH2 CH2 }3 N]3- ) supported by a triamidoamine ligand, with [NEt3 H][B(3,5-Cl2 C6 H3 )4 ] or [PhNMe2 H][B(C6 F5 )4 ] afforded the cations [Ti(Xy-N3 N)][A] (A- =[B(3,5-Cl2 C6 H3 )4 ]- (1[B(ArCl )4 ]; B(ArCl )4 =tetrakis(3,5-dichlorophenyl)borate); A- =[B(C6 F5 )4 ]- (1[B(ArF )4 ]; B(ArF )4 =tetrakis[3,5-bis(trifluoromethyl)phenyl]borate). These Lewis acidic cations were reacted with coordinating solvents to afford the cations [Ti(L)(Xy-N3 N)][B(C6 F5 )4 ] (2-L; L=Et2 O, pyridine and THF). XRD analysis revealed a trigonal monopyramidal (TMP) geometry for the tetracoordinate cations in 1[B(ArX )4 ] and trigonal bipyramidal (TBP) geometry for the pentacoordinate cations in 2-L. Variable-temperature NMR spectroscopy showed a dynamic equilibrium for 2-Et2 O in solution, involving the dissociation of Et2 O. Coordination to the titanium(IV) center activated the THF molecule, which, in the presence of NEt3 , underwent ring-opening to give the titanium alkoxide [Ti(O(CH2 )4 NEt3 )(Xy-N3 N)][B(3,5-Cl2 C6 H3 )4 ] (3). Hydride abstraction from Cβ,eq of the triamidoamine ligand arm in [Ti(CH2 SiMe3 )(Xy-N3 N)] or [Ti(NMe2 )(Xy-N3 N)] with [Ph3 C][B(3,5-Cl2 C6 H3 )4 ] led to the diamidoamine-imine complex [Ti(R){(Xy-N=CHCH2 )(Xy-NCH2 CH2 )2 N}][B(3,5-Cl2 C6 H3 )4 ] (R=CH2 SiMe3 (4 a); R=NMe2 (4 b)). Hydride addition to 4 b with [Li(THF)][HBPh3 ] gave [Ti(NMe2 )(Xy-N3 N)], whereas KH deprotonated further to give [Ti(NMe2 ){(Xy-NCH=CH)(Xy-NCH2 CH2 )2 N}] (5). XRD on single crystals of 3 and 4 b confirmed the proposed structures.

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