Metathesis by Partner Interchange in σ-Bond Ligands: Expanding Applications of the σ-CAM Mechanism

Abstract

In 2007 two of us defined the σ-Complex Assisted Metathesis mechanism (Perutz and Sabo-Etienne, Angew. Chem. Int. Ed. 2007, 46, 2578-2592), that is, the σ-CAM concept. This new approach to reaction mechanisms brought together metathesis reactions involving the formation of a variety of metal-element bonds through partner-interchange of σ-bond complexes. The key concept that defines a σ-CAM process is a single transition state for metathesis that is connected by two intermediates that are σ-bond complexes while the oxidation state of the metal remains constant in precursor, intermediates and product. This mechanism is appropriate in situations where σ-bond complexes have been isolated or computed as well-defined minima. Unlike several other mechanisms, it does not define the nature of the transition state. In this review, we highlight advances in the characterization and dynamic rearrangements of σ-bond complexes, most notably alkane and zincane complexes, but also different geometries of silane and borane complexes. We set out a selection of catalytic and stoichiometric examples of the σ-CAM mechanism that are supported by strong experimental and/or computational evidence. We then draw on these examples to demonstrate that the scope of the σ-CAM mechanism has expanded to classes of reaction not envisaged in 2007 (additional σ-bond ligands, agostic complexes, sp² -carbon, surfaces). Finally, we provide a critical comparison to alternative mechanisms for metathesis of metal-element bonds.

Keywords: agostic interactions; homogeneous catalysis; metathesis; organometallic reaction mechanisms; sigma-bond complexes.

Scheme 1

The σ‐CAM mechanism and three other mechanisms for metathesis at transition metal centers (E=H, C, Si, B).

Scheme 2

Possible coordination modes of methane as σ‐bond complexes and comparison to agostic interaction with corresponding nomenclature.

Scheme 3

Dynamic processes of σ‐complexes.

Figure 1

(a) Synthesis of [Rh(Cy₂PCH₂CH₂PCy₂)(η²,η²‐norbornane)][BAr^F ₄]; (b) photochemical synthesis of manganese propane and butane complexes; (c) synthesis of rhodium methane complex by protonation; (d) experimental structure of cation [Rh(Cy₂PCH₂CH₂PCy₂)(η²,η¹‐2‐methylbutane)]⁺ showing coordination at rhodium by the alkane and calculated structure of CpRe(CO)₂(η²‐CH₄) at CCSD(T)/def2‐QZVPP level.

Figure 2

Preferred configurations and binding energies of (a) methane (b) ethane, (c) propane, (d) n‐butane adsorbed on a RuO₂(110) surface as computed by DFT‐D3 methodologies. Reproduced with permission from ref. .

Figure 3

(a) Complexes 2 and 3 with neutron diffraction structure of 2; (b) solid state ¹H‐²⁹Si HETCOR NMR spectrum of 2. Adapted from ref. .

Scheme 4

η²‐Silane and SISHA forms of iron and ruthenium complexes of 1,2‐bis(dimethylsilyl)benzene, adapted from ref. .

Figure 4

Above: Ni⁰ ₂ complex with η²‐H₂ and η²‐SiH‐coordinated P₂SiOSiP₂ ligands. (A) pathway for exchange of H atoms between the η²‐H₂ and η²‐SiH ligands. (B) transition state for interconversion of isomers located by DFT (Ni green; P yellow; Si red; H white). Adapted from ref. .

Figure 5

(a) Molecular structure of [IMe₄‐(Cb)(μ‐H)(HSiEt₃)][B(C₆F₅)₄] (for abbreviations, see text); (b) NBO analysis of the B⋅⋅⋅H−Si interaction. Reproduced with permission from ref. .

Scheme 5

Examples of bonding modes in σ‐bond complexes of 3‐ and 4‐coordinate boranes.

Figure 6

Partial H/D exchange under D₂ and the observation of isotopic perturbation of equilibrium in [Ir(PCy₃)₂(H)₂(η²η²‐H₃B⋅NMe₃)][BAr^F ₄] using ¹H and ²H NMR spectroscopy. Time=0 (¹H NMR spectrum), Time=5, 60 min (²H NMR spectra). The signals close to δ 5 are due to H₂, HD and D₂. Reproduced with permission in part from ref. .

Figure 7

Molecular structure of Ir(^tBuPOCOP)(H)₂(η²‐HBH₂) as determined by single‐crystal neutron diffraction. Reproduced with permission from ref. .

Scheme 6

Transition metal σ‐bond complexes of η¹,η¹‐H,H‐AlH₂, η¹,η¹‐H,H‐GaH₂ and η²‐Zn‐H.

Scheme 7

Ruthenium η¹,η¹‐H,H zincate cation and its exchange mechanism adapted from ref. .

Figure 8

Structural snapshots of Ga−H bond activation at rhodium in [Rh(bisphosphine){H₂Ga(NacNac)}][BAr^F ₄] with three bisphosphines (a) dppp (Ph₂PCH₂CH₂CH₂PPh₂); (b) dcypp (Cy₂PCH₂CH₂CH₂PCy₂); (c) (PCy₃)₂. [BAr^F ₄]⁻ anions not shown.

Scheme 8

Examples of E−E′ σ‐bond complexes (E,E′=carbon or boron). An=9‐anthryl, SIMes=1,3‐bis(2,4,6‐trimethylphenyl)‐4,5‐ihydroimidazolidin‐2‐ylindene, Dip=2,6‐diisopropylphenyl.

Scheme 9

Protonation via an agostic complex and a σ‐CAM mechanism, adapted from ref. .

Scheme 10

Transformations of agostic complex 6 on reaction with L=DMSO, (adapted from ref. [109]).

Scheme 11

σ‐CAM pathway for gas‐phase rollover of bipyridine employing xenon as the collision gas. ^[111b]

Scheme 12

Cyclometalation of dppe at Ru via σ‐CAM (only phenyl groups involved in the transformation are shown), adapted from ref. .

Figure 9

Free energy diagram (kJ mol⁻¹) for the selective H/D exchange at constant oxidation state Ir^III. Inset shows the calculated transition state structure for D₂/IrH exchange. The entering and leaving ligands are not shown. Adapted from ref.  and with thanks to Dr Marc Reid (University of Strathclyde) for providing the inset structure.

Figure 10

Schematic reaction profiles for the degenerate σ‐CAM reactions of [Ni(CH₃)]⁺ with CH₄ in the ¹A (red) and ³A (blue) states of the cation. Adapted from ref. .

Scheme 13

Hydrogenolysis of an iridium methyl hydride complex with experimental energetics and activation barriers.

Scheme 14

Dissociation of methane from Pt(bpy)(CH₃)]⁺ by CID in gas phase. Note formation of σ‐complex with benzene.

Scheme 15

Formation of ruthenium silyl complex from chlorosilanes and σ‐CAM sequence for D₂ isotopologue.

Scheme 16

SiHBu insertion into a Pt−C bond with σ‐CAM step converting 15 c to 15 d.

Scheme 17

Cobalt σ‐complexes and their σ‐CAM interconversion in initiation of (a) hydrogenation, (b) hydrosilation.

Scheme 18

Kinetic and thermodynamic products in the reaction of [Pt(I^tBuⁱPr′)(I^tBuⁱPr)][BAr^F ₄] with HBpin leading to B−C bond formation.

Scheme 19

Variant of σ‐CAM mechanism with E′ occupying the central position.

Scheme 20

Dehydrogenative benzylic borylation via two sequential σ‐CAM steps.

Figure 11

Computed free energy profile for the degenerate hydrogen exchange between M−H and κ¹,κ¹‐MH₂B in the complexes [M(PCy₃)₂(H)₂(η²η²‐D₃BNMe₃)][BAr^F ₄] (M=Rh or Ir). Dotted lines connect the M=Ir intermediates and transition states, solid M=Rh. Adapted from ref. .

Figure 12

σ‐complexes on Ru nanoparticle surfaces in exchange processes. (A) “Dissociative” and the favored “Associative” mechanisms for H/D exchange between H₂ and D₂. Reproduced with permission from ref. . (B) Calculated C−H activation pathway for cyclopentane activation at Ru₁₃H₁₇. Adapted, with permission, from ref. .

Figure 13

Ligand‐to‐ligand hydrogen transfer (LLHT) mechanism for hydroarylation of an alkyne at nickel showing the TS for LLHT (B3PW91/RECP/6‐31G(d,p)). Adapted with permission from ref. .