Coordination-Induced Weakening of a C(sp³)–H Bond: Homolytic and Heterolytic Bond Strength of a CH–Ni Agostic Interaction

Abstract

The scission of a C(sp 3 )−H bond to form a new metal−alkyl bond is a fundamental step in coordination chemistry and catalysis. However, the extent of C−H bond weakening when this moiety interacts with a transition metal is poorly understood and quantifying this phenomenon could provide insights into designing more efficient C−H functionalization catalysts. We present a nickel complex with a robust adamantyl reporter ligand that enables the measurement of C−H acidity (pK a ) and bond dissociation free energy (BDFE) for a C(sp 3 )−H agostic interaction, showing a decrease in pK a by dozens of orders of magnitude and BDFE decrease of about 30 kcal/mol upon coordination. X-ray crystallographic data is provided for all molecules, including a distorted square planar Ni III metalloradical and "doubly agostic" Ni II (κ 2 -CH 2 ) complex.

Accessing Ni(0) to Ni(IV) via nickel–carbon–phosphorus bond reorganization

“…13 The Ni-CH alkyl NMR spectral resonances appear at 1.34 ppm ( 1 H) and 69.6 ppm ( 13 C). Since the Ni-CH alkyl 1 H resonance was buried amongst several the cyclohexyl and adamantyl signals, the chemical shift assignment was dependent on the multiplicity of the alkyl 13 C resonance (triplet; 2 J CP = 10.5 Hz) and heteronuclear 2D NMR spectral techniques ( 1 H-31 P HMBC, 1 H- 13 C HSQC; see ESI †). For comparison, the Ni-CH alkyl peaks appear at 1.81 ppm and 64.6 ppm in PCPNiBr.…”

“…The 31 P NMR spectrum in C 6 D 6 reveals a singlet at 48.8 ppm, which is nearly identical to the chemical shift of PCPNiBr (49.0 ppm). 13 The Ni–CH alkyl NMR spectral resonances appear at 1.34 ppm ( 1 H) and 69.6 ppm ( 13 C). Since the Ni–CH alkyl 1 H resonance was buried amongst several the cyclohexyl and adamantyl signals, the chemical shift assignment was dependent on the multiplicity of the alkyl 13 C resonance (triplet; 2 J CP = 10.5 Hz) and heteronuclear 2D NMR spectral techniques ( 1 H– 31 P HMBC, 1 H– 13 C HSQC; see ESI†).…”

“…For comparison, the Ni-CH alkyl peaks appear at 1.81 ppm and 64.6 ppm in PCPNiBr. 13 X-ray-quality crystals of 1 were obtained from a concentrated pentane solution at room temperature. The solid state structure of 1 shows a slightly distorted square planar geometry at the Scheme 1 Synthesis of complex 1 and complex 2.…”

“…We recently found that the central alkyl moiety of PCPNiBr can be protonated to yield an agostic CH 2 moiety, enabling us to quantify its coordination-induced C(sp 3 )–H bond weakening via p K a measurements and electrochemical analysis of the Ni III/II redox couple. 13 Herein, we describe an unprecedented instance of oxidatively triggered aryl insertion into the pincer phosphine atom of 1 , generating arylphosphonium salt 2 that retains its coordination to nickel through an η 6 -arene bond. Remarkably, this reaction is reversible – the newly formed P–C bond collapses under cathodic conditions to regenerate 1 through a Ni 0 (η 6 -arene) intermediate, as supported by electrochemical measurements and DFT calculations.…”

Capturing Hydrogen Radicals by Neutral Metal Hydroxides

“…Interestingly, the O–H–M interaction in H•M­(OH) 3 is reminiscent of the C–H–M agostic interactions in organometallic compounds, in which a hydrogen atom is bonded simultaneously to both a carbon atom and a metal atom. , A series of complexes with the agostic interactions have been prepared in solution and low-temperature matrices, which substantially contribute to the C–H activation and organometallic catalytic reactions. − However, the bondings in the C–H–M agostic interaction and the hydrogen adduct H•M­(OH) 3 are dramatically different. In the agostic interaction, the C–H bond donates its σ bonding electron to the electron deficient metal d orbital; the C–H bond is slightly longer than the conventional values and is still a strong bond without radical character.…”