Synthesis, Structures, and Solution Dynamics of Titanium and Zirconium Complexes Carrying a Bis(indolyl) and Two Diethylamido Ligands

Abstract

Indolyl is the anionic species obtained from the deprotonation of the N–H group of indole. Group 4 transition-metal complexes that carry indolyl-based polydentate ligands represent promising homogeneous catalysts for, e.g., olefin polymerization, hydroamination, and nitrogen-fixation reactions due to the weak π-donation and electron-withdrawing properties, as well as the low basicity of indolyl. In this study, we systematically investigated the synthesis and structures of titanium and zirconium complexes that carry deprotonated 2,2′-bis­(indolyl)­methane ligands (henceforth: bis­(indolyl) ligands) and two diethylamido ligands. We found that the coordination geometry of the indolyl nitrogen atom in such bis­(indolyl) ligands is affected by the steric impact of the substituents attached to the central aromatic ring. Moreover, we examined the dynamics of such bis­(indolyl) ligands in solution for the corresponding zirconium complexes, and the mechanism was discussed in conjunction with DFT calculations. The results of this study suggest that bis­(indolyl) ligands may also serve as coordinatively flexible ancillary ligands, and indicate the feasibility of an expansion from bis­(indolyl) to bis­(indolyl)-donor ligands.

Synthesis and Structures of Bis(indolyl)-Coordinated Titanium Dichlorido Complexes and Their Catalytic Application in the Cyclotrimerization of Alkynes

“…The increased π-donation of the indolyls can also stabilize the electron-deficient titanium center. These findings support our recent argument that bis­(indolyl) ligands show flexible coordination features in response to the environment around the metal center . Ligands of this type are expected to serve as flexible ancillary ligands in metal-catalyzed transformation reactions …”

“…In addition to two μ-Cl ligands, each titanium atom is coordinated by a terminal Cl and a bidentate bis­(indolyl) ligand; thus, the titanium centers are both five-coordinated, and their coordination geometries are close to square pyramidal. This geometric arrangement around the titanium center is similar to those of five-coordinate monomeric zirconium complexes that carry a bis­(indolyl), a THF, and two amido ligands . The Ti–N­(indolyl) distances (1.921(2)–1.930(2) Å) of the dimeric structure of 1 are slightly shorter than those of {bis­(indolyl)}­Ti­(NEt 2 ) 2 complexes such as A and B (1.968(3)–1.994(2) Å). ,, These short Ti–N­(indolyl) distances indicate an increased π-donation character from the indolyl ligand to the titanium center induced by the replacement of diethylamido ligands with chlorido ligands.…”

“…This geometric arrangement around the titanium center is similar to those of five-coordinate monomeric zirconium complexes that carry a bis­(indolyl), a THF, and two amido ligands . The Ti–N­(indolyl) distances (1.921(2)–1.930(2) Å) of the dimeric structure of 1 are slightly shorter than those of {bis­(indolyl)}­Ti­(NEt 2 ) 2 complexes such as A and B (1.968(3)–1.994(2) Å). ,, These short Ti–N­(indolyl) distances indicate an increased π-donation character from the indolyl ligand to the titanium center induced by the replacement of diethylamido ligands with chlorido ligands. Moreover, the planar geometries of the N1–N4 atoms in the dimeric structure of 1 (distances from the plane defined by titanium and two carbon atoms bonded to nitrogen atoms: 0.010(2)–0.104(2) Å) also support the increased π-donation character.…”

“…Moreover, the planar geometries of the N1–N4 atoms in the dimeric structure of 1 (distances from the plane defined by titanium and two carbon atoms bonded to nitrogen atoms: 0.010(2)–0.104(2) Å) also support the increased π-donation character. On the other hand, the corresponding N­(indolyl) geometries of {bis­(indolyl)}­Ti­(NEt 2 ) 2 are partially pyramidalized (the corresponding distances: 0.071(2) and 0.186(2) Å). , The Ti1–Cl1 (2.2065(9) Å) and Ti2–Cl4 (2.1991(9) Å) distances in the dimeric structure of 1 are shorter than the Ti–Cl (terminal) distances of previously reported dinuclear titanium chlorido complexes in which two five-coordinated titanium centers are bridged by two chlorido ligands (2.211(1)–2.3116(6) Å) …”

“…Our interest has been focused on the coordination chemistry of 2,2′-bis­(indolyl)­methanes (henceforth: bis­(indolyl)­s) toward group-4 transition metals. Recently, we found that bis­(indolyl) ligands can potentially serve as coordinatively flexible ancillary ligands in catalytic transformation reactions . We have also demonstrated the catalytic performance of {bis­(indolyl)}­Ti­(NEt 2 ) 2 complexes A and B (Scheme ) in alkyne hydroamination and ethylene polymerization reactions .…”