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

Antonio J. Martínez-Martínez, Bengt E. Tegner, Alasdair I. McKay, Alexander J. Bukvic, Nicholas H. Rees, Graham J. Tizzard, Simon J. Coles, Mark R. Warren, Stuart Alan Macgregor, Andrew S. Weller

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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 structures, 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)(κ1-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···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.

Original languageEnglish
Pages (from-to)14958–14970
Number of pages13
JournalJournal of the American Chemical Society
Volume140
Issue number44
Early online date10 Oct 2018
DOIs
Publication statusPublished - 7 Nov 2018

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