γ-Agostic interactions in (^MesCCC)Fe–Mes(L) complexes

Abstract

γ-Agostic interactions in(MesCCC)Fe–Mes(L) complexes were observed in the crystal structures, NMR data, and DFT calculations.

Iron-Catalyzed Parahydrogen Induced Polarization

“…In contrast, the only other reported N 2 -bridged CCC dimer, [( Mes CCC)FeMes] 2 (μ-N 2 ), remains unambiguously dimeric in solution as determined by the asymmetric ligand environment observed in the 1 H NMR spectrum. 53 The ligand arrangement was confirmed to be unique to 2-Py via the synthesis of the phosphine derivatives ( Mes CCC)Fe- The ATR-IR spectra of 2-PMe 3 and 2-PPh 3 show the N 2 is less activated than in the case of 2-Py, with strong features at 2101 and 2096 cm −1 suggesting the prevalence of the monomeric structure in the solid state (Table 1). Lastly, structural characterization of 2-PR 3 confirmed the identity of the monomeric product.…”

“…Executing the reaction in Scheme under D 2 resulted in an identical 1 H NMR spectrum except for the disappearance of the signal corresponding to the hydride (Figure S9), along with its concomitant appearance in the 2 H NMR spectrum (Figure S10), showing that the formation of 2-Py is derived from the activation of H 2 and extrusion of mesitylene (Scheme ). These complexes highlight the differing reactivity determined by the choice of flanking aryl groups on the CCC scaffold as 2-Py could not be synthesized following the zwitterionic metalation strategy used for the DIPP CCC variant and, conversely, a DIPP CCC analogue of 1-Py is inaccessible via the reported procedure …”

“…These complexes highlight the differing reactivity determined by the choice of flanking aryl groups on the CCC scaffold as 2-Py could not be synthesized following the zwitterionic metalation strategy used for the DIPP CCC variant 58 and, conversely, a DIPP CCC analogue of 1-Py is inaccessible via the reported procedure. 53 Although the spectroscopic evidence supports the assigned formulation of 2-Py, the geometric arrangement of the ligands about the iron center remains dubious. The solid-state structure of the DIPP CCC variant of 2-Py was not reported and was instead proposed by analogy to the structures determined for phosphine derivatives where the L-type and hydride ligands are in a trans disposition and the bound N 2 occupies the position trans to the anionic C Ar of the CCC ligand.…”

Quantum Crystallography and Complementary Bonding Analysis of Agostic Interactions in Titanium Amides

“…Some of the most commonly used criteria for the identification and characterization of the agostic bond involve: 1 ) geometry , elongation of the C−H bond, relatively short H⋅⋅⋅M distances (1.8–2.3 Å), small C−H⋅⋅⋅M angles (90–140°) and increasing C⋅⋅⋅M⋅⋅⋅H angles; [18] 2 ) IR vibrational spectroscopy , a red shift of the C−H stretching vibration [19] related to the C−H bond elongation/weakening and larger values of the C−H compliance constant; [20] 3 ) 1 H‐NMR spectroscopy , low 1 J CH value (50 to 100 Hz) and an upfield shift δ H relative to an uncoordinated CH group; [18,21] 4 ) electron density , a quantum theory of atoms in molecules (QTAIM [22] ) topological electron‐density pattern indicating the presence of a M⋅⋅⋅H interaction [23] as well as a region of local charge depletion at the metal ion core; [24,25] 5 ) bonding descriptors based on natural bond orbitals (NBO [26] ), electron localization and localizability functions (ELF, [27] ELI [28] ) or the non‐covalent interaction (NCI [29] ) index [30–33] …”

“…Some of the most commonly used criteria for the identification and characterization of the agostic bond involve: 1) geometry, elongation of the CÀ H bond, relatively short H•••M distances (1.8 -2.3 Å), small CÀ H•••M angles (90-140°) and increasing C•••M•••H angles; [18] 2) IR vibrational spectroscopy, a red shift of the CÀ H stretching vibration [19] related to the CÀ H bond elongation/weakening and larger values of the CÀ H compliance constant; [20] 3) 1 H-NMR spectroscopy, low 1 J CH value (50 to 100 Hz) and an upfield shift δ H relative to an uncoordinated CH group; [18,21] 4) electron density, a quantum theory of atoms in molecules (QTAIM [22] ) topological electron-density pattern indicating the presence of a M•••H interaction [23] as well as a region of local charge depletion at the metal ion core; [24,25] 5) bonding descriptors based on natural bond orbitals (NBO [26] ), electron localization and localizability functions (ELF, [27] ELI [28] ) or the non-covalent interaction (NCI [29] ) index. [30][31][32][33] From an experimental point of view, there are two major problems or challenges related to the above criteria: 1) All criteria depend critically on the hydrogen atom; and for all but the spectroscopic criteria, no reliable statement can be made without an accurate and precise localization of the hydrogen atom in the agostic bond from diffraction experiments. 2) Electrondensity related criteria of agostic interactions can be assessed through modelling of the experimental charge density based on high-resolution, low-temperature single-crystal X-ray diffraction experiments.…”