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Modulation of σ-alkane interactions in [Rh(L2)(alkane)]+ solid-state molecular organometallic (SMOM) systems by variation of the chelating phosphine and alkane: Access to η2,η2-σ-alkane Rh(I), η1-σ-alkane Rh(III) complexes, and alkane encapsulation

DOI: 10.1021/jacs.8b09364 DOI Help

Authors: Antonio J Martinez-Martinez (University of Oxford) , Bengt E. Tegner (Heriot Watt University) , Alasdair I. Mckay (University of Oxford) , Alexander J. Bukvic (University of Oxford) , Nicholas H. Rees (University of Oxford) , Graham J. Tizzard (University of Southampton) , Simon J. Coles (University of Southampton) , Mark R. Warren (Diamond Light Source) , Stuart A. Macgregor (Heriot Watt University) , Andrew S. Weller (University of Oxford)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Journal Of The American Chemical Society

State: Published (Approved)
Published: October 2018
Diamond Proposal Number(s): 17308

Abstract: Solid/gas Single–Crystal to Single–Crystal (SC–SC) hydrogenation of appropriate diene precursors forms the corresponding σ–alkane complexes [Rh(Cy2P(CH2)nPCy2)(L)][BArF4] (n = 3, 4) and [RhH(Cy2P(CH2)2(CH)(CH2)2PCy2)(L)][BArF4] (n = 5, L = norbornane, NBA; cyclooctane, COA). Their struc-tures, as determined by single–crystal X-ray diffraction, have cations exhibiting Rh···H–C σ–interactions which are modulated by both the chelating ligand and the identity of the alkane, while all sit in an octahedral anion–microenvironment. These range from chelating η2,η2 Rh···H–C (e.g. [Rh(Cy2P(CH2)nPCy2)(η2η2–NBA)][BArF4], n = 3 and 4), through to more weakly bound η1 Rh··H–C in which C–H activation of the chelate backbone has also occurred (e.g. [RhH(Cy2P(CH2)2(CH)(CH2)2PCy2)(η1–COA)][BArF4]) and ultimately to systems where the alkane is not ligated with the metal center, but sits encapsulated in the supporting anion microenvironment – [Rh(Cy2P(CH2)3PCy2)][COA⊂BArF4] – in which the metal center instead forms two intramolecular agostic η1 Rh···H–C interactions with the phosphine cyclohexyl groups. CH2Cl2 adducts formed by displacement of the η1–alkanes in solution (n = 5; L = NBA, COA), [RhH(Cy2P(CH2)2(CH)(CH2)2PCy2)(η1–ClCH2Cl)][BArF4], are characterized crystallographically. Analyses via periodic DFT, QTAIM, NBO and NCI calculations, alongside variable temperature solid–state NMR spectroscopy, provide snapshots marking the onset of Rh σ-alkane interactions along a C···H activation trajectory. These are negligible in [Rh(Cy2P(CH2)3PCy2)][COA⊂BArF4]; in [RhH(Cy2P(CH2)2(CH)(CH2)2PCy2)(η1–COA)][BArF4] σC–H→Rh σ-donation is supported by Rh→σ*C-H ‘pregostic’ donation; and in [Rh(Cy2P(CH2)nPCy2)(η2η2-NBA)][BArF4] (n = 2-4) σ-donation dominates, supported by classical Rh(dπ)→σ*C-H π-back donation. Dispersive interactions with the [BArF4]- anions and Cy substituents further stabilize the alkanes within the binding pocket.

Journal Keywords: Anions; Ligands; Group 15 compounds; Nuclear magnetic resonance spectroscopy; Cations

Subject Areas: Chemistry

Instruments: I19-Small Molecule Single Crystal Diffraction

Added On: 15/10/2018 11:33


Discipline Tags:

Chemistry Organometallic Chemistry

Technical Tags:

Diffraction Single Crystal X-ray Diffraction (SXRD)