Abstract
A homologous family of low‐coordinate complexes of the formulation trans‐[M(2,2′‐biphenyl)(PR3)2][BArF 4] (M=Rh, Ir; R=Ph, Cy, iPr, iBu) has been prepared and extensively structurally characterised. Enabled through a comprehensive set of solution phase (VT 1H and 31P NMR spectroscopy) and solid‐state (single crystal X‐ray diffraction) data, and analysis in silico (DFT‐based NBO and QTAIM analysis), the structural features of the constituent agostic interactions have been systematically interrogated. The combined data substantiates the adoption of stronger agostic interactions for the IrIII compared to RhIII complexes and, with respect to the phosphine ligands, in the order PiBu3>PCy3>PiPr3>PPh3. In addition to these structure–property relationships, the effect of crystal packing on the agostic interactions was investigated in the tricyclohexylphosphine complexes. Compression of the associated cations, through inclusion of a more bulky solvent molecule (1,2‐difluorobenzene vs. CH2Cl2) in the lattice or collection of data at very low temperature (25 vs. 150 K), lead to small but statistically significant shortening of the M−H−C distances.
Highlights
The coordination chemistry of CÀH bonds is an important facet of contemporary organometallic chemistry.[1,2,3] Adoption of 3-centre-2-electron MÀHÀC bonds can help stabilise otherwise reactive low-coordinate metal complexes that are implicated in many catalytic reactions, and from a fundamental perspective represent an opportunity to gain insight into transition-metal-mediated CÀH bond activation reactions.[4]
The 1H NMR spectrum of 1 a recorded at 185 K shows a significantly upfield shifted ortho-phenyl 3H signal at d 6.02 that exhibits a strong NOE interaction with the 6,6’-biphenyl resonances indicating that they are pointing downwards towards the metal. Based on this data we suggest that coordination of the solvent is not significant under the range of temperatures we have studied, and instead there is a very weak bonding interaction between the phenyl ring of one of the phosphine ligands and the metal www.chemeurj.org
The combined data substantiates the adoption of stronger agostic interactions for the IrIII compared to RhIII complexes and, with respect to the phosphine ligands, in the order PiBu3 > PCy3 > PiPr3 > PPh3
Summary
The coordination chemistry of CÀH bonds is an important facet of contemporary organometallic chemistry.[1,2,3] Adoption of 3-centre-2-electron MÀHÀC bonds can help stabilise otherwise reactive low-coordinate metal complexes that are implicated in many catalytic reactions, and from a fundamental perspective represent an opportunity to gain insight into transition-metal-mediated CÀH bond activation reactions.[4]. As first articulated by Brookhart and Green, the consistent interactions are termed “agostic” and typified by MÀHÀC contacts of < 3 .[1,5] The characterisation of alkane complexes is significantly more experimentally demanding, but has been achieved in solution using time-resolved spectroscopic meth-. A. Ohlin Department of Chemistry, Umeå University Linneausvag 6, 907 34 Umeå (Sweden) Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/ chem.201705990.
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