CH/π-stabilization controls the architecture of the PPh3 propeller in transition-metal complexes. CH/π- and Cl/π-interactions determine its orientation within the molecule

Mechanism of Coupling of Methylidene to Ethylene Ligands in Dimetallic Assemblies; Computational Investigation of a Model for a Key Step in Catalytic C1 Chemistry

Methylidene complexes often couple to ethylene complexes, but the mechanistic insight is scant. The path by which two cations [(η⁵-C₅H₅)Re(NO)(PPh₃)(═CH₂)]⁺ (5⁺) transform (CH₂Cl₂/acetonitrile) to [(η⁵-C₅H₅)Re(NO)(PPh₃)(H₂C═CH₂)]⁺ (6⁺) and [(η⁵-C₅H₅)Re(NO)(PPh₃)(NCCH₃)]⁺ is studied by density functional theory. Experiments provide a number of constraints such as the second-order rate in 5⁺; no prior ligand dissociation/exchange; a faster reaction of (S)-5⁺ with (S)-5⁺ than with (R)-5⁺ ("enantiomer self-recognition"). Although dirhenium dications with Re(μ-CH₂)₂Re cores represent energy minima, they are not accessible by 2 + 2 cycloadditions of 5⁺. Transition states leading to ReCH₂CH₂Re linkages are prohibitively high in energy. However, 5⁺ can give non-covalent S_Re/S_Re or S_Re/R_Re dimers with π interactions between the PPh₃ ligands but long ReCH₂···H₂CRe and H₂CRe···H₂CRe distances (3.073-3.095 Å and 3.878-4.529 Å, respectively). In rate-determining steps, these afford [(η⁵-C₅H₅)Re(NO)(PPh₃)(μ-η²:η²-H₂C···CH₂)(Ph₃P)(ON)Re(η⁵-C₅H₅)]²⁺ (13²⁺), in which one rhenium binds the bridging ethylene more tightly than the other (2.115-2.098 vs 2.431-2.486 Å to the centroid). In the S_Re/R_Re adduct, Dewar-Chatt-Duncanson optimization leads to unfavorable PPh₃/PPh₃ contacts. Ligand interactions are further dissected in the preceding transition states via component analyses, and ΔΔG^‡ (1.2 kcal/mol, CH₂Cl₂) favors the S_Re/S_Re pathway, in accordance with the experiment. Acetonitrile then displaces 6⁺ from the more weakly bound rhenium of 13²⁺. The formation of similar μ-H₂C···CH₂ intermediates is found to be rate-determining for varied coordinatively saturated M═CH₂ species [M = Fe(d⁶)/Re(d⁴)/Ta(d²)], establishing generality and enhancing relevancy to catalytic CH₄ and CO/H₂ chemistry.

Trend-Analysis of Solid-State Structures: Low-Energy Conformational 'Reactions' Involving Directed and Coupled Movements in Half-Sandwich Compounds [CpFe(CO){C(=O)R}PPh3]

Trends in solid‐state structures were used to identify preferred intramolecular movements in half‐sandwich compounds [CpFe(CO){C(=O)R}PPh₃]. Three weak interactions were analyzed: 1) the CH/π donor–acceptor interaction of phenyl rings in the PPh₃ ligand, 2) the Ph_PPh3 face‐on Cp stabilization, and 3) the hydrogen bond between the oxygen atom of the acyl group and an ortho‐C−H bond of one of the PPh₃ phenyl rings. Clockwise and counter‐clockwise rotations established directed and coupled movements of the PPh₃ ligand, the acyl group, and the phenyl rings within the PPh₃ ligand.

PPh3 Propeller Diastereomers: Bonding Motif PhPPh3 Face-On π-Ar in Half-Sandwich Compounds [(π-Ar)LL'MPPh3]

Chiral-at-metal compounds (R _Ru,S _C)/(S _Ru,S _C)-[CyRu(1O-2N)PPh₃]PF₆ and (R _Ru,S _C)/(S _Ru,S _C)-[CyRu(2O-1N)PPh₃]PF₆ were prepared using anions 1O-2N^- and 2O-1N^- of the Schiff bases, derived from the hydroxynaphthaldehydes and (S)-1-phenylethylamine. The pure (R _Ru,S _C)-diastereomers were obtained by crystallization. In the unit cell of (R _Ru,S _C)-[CyRu(1O-2N)PPh₃]PF₆, there are three independent molecules, which differ in the propeller sense of the PPh₃ ligand. Molecules [1] and [2] have (M _PPh3 )-configuration and molecule [3] has (P _PPh3 )-PPh₃ configuration. PPh₃ diastereoisomerism is discussed including other pairs of compounds, differing only in the PPh₃ configuration. A conformational analysis reveals an internal stabilization inside the PPh₃ ligand by a system of attractive CH/π interactions and a new bonding motif Ph_PPh3 face-on π-Ar, both characteristic features of [(π-Ar)LL'MPPh₃] compounds. The propeller diastereomers interconvert via a low-energy pathway and a high-energy pathway, corroborated by density functional theory calculations.

Comment on “Conformational analysis of triphenylphosphine ligands in stereogenic monometallic complexes: tools for predicting the preferred configuration of the triphenylphosphine rotor” by J. F. Costello, S. G. Davies, E. T. F. Gould and J. E. Thomson, Dalton Trans., 2015, 44, 5451

The architecture of triphenylphosphine ligands in compounds [Cp^(*)ML¹L²PPh₃] is determined by CH/π interactions.