Experimental and DFT Studies of Cationic Imido Titanium Alkyls:  Agostic Interactions and C−H Bond and Solvent Activation Reactions of Isolobal Analogues of Group 4 Metallocenium Cations

Abstract

Synthesis, reactions, and DFT studies of macrocycle-supported imido titanium alkyl cations derived from Ti(N t Bu)(Me 3 [9]aneN 3 )R 2 (R ) Me (1) or CH 2 SiMe 3 (2)) are described (Me 3 [9]aneN 3 ) 1,4,7trimethyltriazacyclononane). Reaction of 1 with 1 equiv of [Ph 3 C][BAr F 4 ] or BAr F 3 (Ar F ) C 6 F 5 ) in C 6 D 5 Br afforded the methyl cation [Ti(N t Bu)(Me 3 [9]aneN 3 )Me] + (6 + ), whereas with half an equivalent of [Ph 3 C][BAr F 4 ] the fluxional methyl-bridged homo-binuclear cation [Ti 2 (N t Bu) 2 (Me 3 [9]aneN 3 ) 2 Me 2 (µ-Me)] + (10 + ) was formed. Reaction of 1 with [Ph 3 C][BAr F 4 ] in CD 2 Cl 2 formed the monochloride cation [Ti(N t Bu)(Me 3 [9]aneN 3 )Cl] + (8 + ), which was also prepared from Ti(N t Bu)(Me 3 [9]aneN 3 )Cl(Me) and [Ph 3 C][BAr F 4 ]. Cation 8 + reacted with pyridine to give the adduct [Ti(N t Bu)(Me 3 [9]aneN 3 )Cl(py)] + (9 + ) and with Ti(N t Bu)(Me 3 [9]aneN 3 )Me 2 to form the chloride-bridged cation [Ti 2 (N t Bu) 2 (Me 3 [9]aneN 3 ) 2 -Me 2 (µ-Cl)] + (11 + ). Reaction of 2 with [Ph 3 C][BAr F 4 ] gave [Ti(N t Bu)(Me 3 [9]aneN 3 )(CH 2 SiMe 3 )] + (7 + ), which is stabilized by a β-Si-C agostic interaction characterized by a high-field-shifted 29 Si NMR resonance. Attempts to generate 7 + by reaction of 2 with [PhNMe 2 H][BAr F 4 ] in CH 2 Cl 2 led to Ti(N t Bu)(Me 3 [9]aneN 3 )Cl 2 and [PhNMe 2 (CH 2 Cl)][BAr F 4 ] (12-BAr F 4) via a series of solvent activation reactions, the details of which have been elucidated. Reaction of 6 + or 7 + with Ph 3 PO afforded the adducts [Ti(N t Bu)(Me 3 [9]aneN 3 )R(Ph 3 PO)] + , whereas with pyridine a C-H bond activation reaction occurred to give [Ti(N t Bu)(Me 3 [9]aneN 3 )(NC 5 H 4 )] + (17 + ) and the corresponding alkane RH. Density functional theory calculations of the isolobal d 0 fragments [Ti(NR)(R′ 3 [9]aneN 3 )] 2+ and [Cp 2 Ti] 2+ found that their frontier orbitals, although broadly similar, featured important differences in their shapes and energies. These account for the absence of any R-C-H agostic interaction in 6 + , whereas [Cp 2 TiMe] + is stabilized by a weak interaction of this type, as judged by DFT-computed geometries. The experimentally observed increase in Ti-Me group average 1 J CH on forming either 6 + from 1 or [Cp 2 TiMe] + from Cp 2 TiMe 2 is reproduced by DFT and attributed to intrinsic global changes in carbon 2s orbital contribution to the Ti-C and C-H bonds upon cation formation. These changes were shown to mask the otherwise expected decrease in average 1 J CH for the R-agostic methyl in [Cp 2 TiMe] + . The difference between the Ti-Me 1 J CH values in 1 (111 Hz) and isolobal Cp 2 TiMe 2 (124 Hz) was also attributed to differences in Ti center electrophilicity. The experimental high-field-shifted 29 Si NMR resonance in 7 + was well reproduced in the DFT-computed β-Si-C agostic structure, and upper and lower limits for the strength of the agostic interaction were estimated. An NBO analysis of the Ti-CH 2 SiMe 3 bonding found several different contributions, including negative hyperconjugation (populat...

Metal‐to‐Ligand Alkyl Migration Inducing Carbon–Sulfur Bond Cleavage in Dialkyl Yttrium Complexes Supported by Thiazole‐Containing Amidopyridinate Ligands: Synthesis, Characterization, and Catalytic Activity in the Intramolecular Hydroamination Reaction

“…Quantitative conversion of 9 to cationic monoalkyl complex 9 + was observed in the 1 H NMR spectrum by the concomitant appearance of signals attributed to the formation of Me 3 SiCH 2 CPh 3 (73.2 % as two isomers in a ca. 2:1 ratio)39 and Ph 3 CH (26.8 %)39 as side products. 1 H NMR (400 MHz, C 6 D 6 /C 6 D 5 Br, 293 K, selected data): δ =0.46 (br s, 2 H, YC H 2 SiMe 3 ), −0.12 (s, Me 3 SiCH 2 CPh 3 of isomer A), −0.04 (s, 9 H, YCH 2 Si Me 3 ), 0.10 (s, Me 3 SiCH 2 CPh 3 of isomer B), 1.20 (m, 12 H, CH(C H 3 ) 2 ), 1.34 (s, 6 H, CH 3 ), 2.03 (s, Me 3 SiC H 2 CPh 3 of isomer B), 2.16 (s, Me 3 SiC H 2 CPh 3 of isomer A), 3.16 (m, 2 H, C H (CH 3 ) 2 ), 5.52 ppm (s, Ph 3 CH).…”

Syntheses, Reactivity and DFT Studies of Group 2 and Group 12 Metal Complexes of Tris(pyrazolyl)methanides Featuring “Free” Pyramidal Carbanions

“…(also in C 6 D 6 ). [49] Since it is known that average 1 J CH values of methyl groups are very sensitive probes of the effective electronegativity of the metal to which they are attached, [50] it can be concluded that the effective charge of the Zn atoms in these directly related zwitterionic complexes is the same and that the metal occupies more or less the same Furthermore, in all instances only k 3 N-coordinated Tpmd ligands have been found. To gain further insight into the structures and reactivity patterns of the new sandwich and half-sandwich compounds and information on experimentally non-observable systems, we carried out a series of DFT calculations (RI-DFT/BP86/def2-TZVP) [53] using the TUR-BOMOLE program (see Supporting Information).…”

Bis(pentalene)dititanium chemistry: C–H, C–X and H–H bond activation

“…It has to be noted that it is slightly shorter than the one found in (3) (2.5610(8) Å)9, and significantly shorter than the ones found in complexes of the general formula {(µ,η 5 :η 5 )Pn † } 2 [Ti(EPh)] 2 (E = S, Se, Te) (2.6519 9, 2.6581 15and 2.6530(9) Å respectively),8 d which also derive from the oxidative addition of PhEEPh across the Ti=Ti double bond in (1). Interestingly, the Ti-N bond distance is significantly shorter than a dative N→Ti bond (N = pyridine, 15,16 Me 3 [ane]N 3 16 ), which is typically found in the range between 2.2-2.4 Å, and falls between the bond lengths found for terminal (longer) and bridging (shorter) Ti amide bonds.…”