Intramolecular Agostic Interactions and Dynamics of a Methyl Group at a Preorganized Dinickel(II) Site

Abstract

Alkyl nickel intermediates relevant to catalytic processes often feature agostic stabilization, but relatively little is known about the situation in oligonickel systems. The dinickel(I) complex K[LNi^I₂], which is based on a compartmental pyrazolato-bridged ligand L^3- with two β-diketiminato chelate arms, or its masked version, the dihydride complex [KL(Ni^II-H)₂] that readily releases H₂, oxidatively add methyl tosylate to give diamagnetic [LNi^II₂(CH₃)] (1) with d(Ni···Ni) ≈ 3.7 Å. Structural characterization shows that the methyl group in 1 is bound to one Ni^II and exhibits an intramolecular agostic interaction with the more distant Ni^II. This is supported spectroscopically (viz., a ν(C-H) stretch at 2658 cm^-1 and lowered ¹J_C-H of 114 Hz) and by DFT calculations, including topological analysis of the computed electron density for 1. NMR spectroscopy reveals very fast hopping of the CH₃ group between the two Ni^II ions, which according to DFT has a minute barrier of 4 kcal mol^-1 and proceeds via a planar CH₃ moiety in the transition state (Walden-like inversion). The alkylidene group in K[LNi₂(μ-CHSi(Me₃)₃)], obtained from the reaction of [KL(Ni-H)₂] with N₂CHSiMe₃, is symmetrically bridging. This work provides new insight into the stabilization and dynamics of alkyl ligands at dinickel sites with a constrained metal···metal distance.

Scheme 1. Selected Complexes with Agostic Interactions Discussed in this Work (Top), Different Coordination Modes of Bridging CH₃ Groups in Dinuclear Complexes (Middle), and H₂-Releasing Reductive Activation of Small Molecules XY by Dinickel Dihydride Complex [KL(Ni–H)₂] (F^K; Bottom)

Scheme 2. Synthesis of 1 from F^K and H^K

Figure 1

Molecular structure (30% probability of thermal ellipsoids) of 1. Solvent molecules and H atoms, except the ones of the CH₃ group, are omitted for clarity. The positions of the H atoms of the CH₃ group were determined from residual electron density and refined using a DFIX restraint (d(C–H) = 0.98 Å) and should be considered with caution.

Figure 2

Part (3335–1670 cm^–1) of the ATR IR spectra of solid 1 (black) and 1-d₃ (gray) and difference spectrum (orange).

Figure 3

¹H NMR spectrum (300 MHz) of 1 (bottom) and deuterium labeled complex 1-d₃ (middle) in THF-d₈ and ²H NMR spectrum (46 MHz) of 1-d₃ (top) in THF at room temperature.

Figure 4

DFT-optimized geometry (r2SCAN-3c) of 1 with Ni2–H contacts (left); further selected distances are d(Ni1···Ni2) = 3.75 Å, d(Ni1–C40) = 1.99 Å, and d(Ni2–C40) = 2.43 Å. 1^TS (middle) reflects the barrier for Me hopping between Ni1 and Ni2; the imaginary mode of 1^TS corresponds to ν_imag = 556 i cm^–1 (unscaled).

Figure 5

2D plot of ∇²ρ_b for complex 1; left, top view; right, side view. Charge concentration (blue) and depletion (red), bond paths (black lines), bond critical points (blue dots), ring critical points (orange dots). Selected topological characteristics at the bond critical point for the agostic interaction between Ni2 and Ha are given in the gray inset box.

Scheme 3. Synthesis of 2 from F^K and N₂CHSiMe₃

Figure 6

Molecular structure (30% probability thermal ellipsoids) of 2 showing the two molecular entities with three (2a; top) and two (2b; middle) THF coordinated to K⁺ and molecular structure of the anion of 2′ (bottom). Hydrogen atoms except the one connected to alkylidene-C are omitted for clarity.