Modulation of σ-Alkane interactions in [Rh(L2)(alkane)]+ solid-state 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

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 [tiebar above startRhH(Cy2P(CH2)2(tiebar above endCH)(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., [tiebar above startRhH(Cy2P(CH2)2(tiebar above endCH)(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), [tiebar above startRhH(Cy2P(CH2)2(tiebar above endCH)(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 [tiebar above startRhH(Cy2P(CH2)2(tiebar above endCH)(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.

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Identifiers

ORCID for Graham J. Tizzard: orcid.org/0000-0002-1577-5779

ORCID for Simon J. Coles: orcid.org/0000-0001-8414-9272

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