Protodemetalation of (Bipyridyl)Ni(II)-Aryl Complexes Shows Evidence for Five-, Six-, and Seven-Membered Cyclic Pathways

Abstract

Protonation of C-M bonds and its microscopic reverse, metalation of C-H bonds, are fundamental steps in a variety of metal-catalyzed processes. As such, studies on protonation of C-M bonds can shed light on C-H activation. We present here studies on the rate of protodemetalation (PDM) of a suite of arylnickel(II) complexes with various acids that provide evidence for a concerted, cyclic transition state for the PDM of C-Ni bonds and demonstrate that five-, six-, and seven-membered transition states are particularly favorable. Our data show that while the rate of protodemetalation of arylnickel(II) complexes scales with acidity for many acids, several are faster than predicted by pK_a. For example, while acetic acid and acetohydroxamic acid are much less acidic than HCl, they both protodemetalate arylnickel(II) complexes significantly faster than HCl. Our data also show how in the case of acetohydroxamic acid, a seven-membered cyclic transition state (CH₃C(O)NHOH) can be more favorable than a six-membered transition state (CH₃C(O)NHOH). Similarly, five-membered transition states, such as for pyrazole, are highly favorable as well. Comparison of transition state polarization (from density functional theory) compares these new nickel transition states to better-studied precious-metal systems and demonstrates how the base can change the polarization of the transition state giving rise to opposing electronic preferences. Collectively, these studies suggest several new avenues for study in C-H activation as well as approaches to accelerate or slow protodemetalation in nickel catalysis.

Figure 1.

A) Studies on protodemetalation and base-assisted C–H activation are complementary. B) Recent studies have demonstrated both stepwise and concerted mechanisms. C) In this work, we systematically study arylnickel protodemetalation, revealing new possibilities.

Figure 2.

Plot of log of initial rate of protodemetalation of (^tBubpy)Ni^II(o-tolyl)(Br) (2cb, 1 equiv) by various acids (10 equiv) in DMA vs. pK_a values in DMSO. Linear fit performed on selected acids (red data points) excluding outliers and control reaction (blue data points) and HCl studies on the effect of arene identity (faded orange) and ligand electronics (faded yellow) on rates of protodemetalation of various (^Rbpy)Ni^II(aryl)(Br) complexes by HCl_(ethereal). 5-Methyltetrazole and benzenesulfonic acid plotted with estimated pK_a values for related compounds in DMSO. See Supporting Information for details.

Figure 3.

a) Hammett plot for ligand electronics using substituent constants σ ⁻ for the protodemetalation of complexes (^Rbpy)Ni^II(Ar)(Br) (1 equiv) with ethereal HCl_{(1,4-dioxane)} (10 equiv) in DMA. Ar = o-tolyl for R = ^tBu, H, and CO₂Me. Ar = o-cumenyl was used in place of o-tolyl for R = OMe because the o-tolyl analog was unstable in solution (See Supporting Information Section IIE for details). b) Hammett plot for arene electronics using substituent constants σ ⁺ for the protodemetalation of complexes (^tBubpy)Ni^II(o-tolyl-p-R)(Br) (1 equiv) with ethereal HCl_{(1,4-dioxane)} (10 equiv) in DMA. See Supporting Information for details.

Figure 4.

Structures and relative free energies of the nickel-catalyzed concerted cyclic protodemetalation (cCPDM) with acetic acid (AcOH, brown), acetohydroxamic acid (AcNHOH, maroon/orange), HCl (red), and pyrazole (yellow) via cationic nickel intermediate 2. Free energies (kcal/mol, 305.15 K) were computed using UB3LYP-D3(BJ)/def2TZVP-CPCM(dimethylacetamide)//UM06/def2SVP.

Figure 5.

More O’Ferrall Jencks analysis of concerted, base-assisted C–H cleavage transition states using calculated bond indices from literature DFT data originally reported by Carrow and coworkers. Bond indices for acetate-derived transition states are represented by colored circles (●): purple (Fagnou), light blue and lilac (Echavarren),^, red (Roithova), orange (Sanford), magenta (this work), dark blue (Macgregor), blue (Stahl), navy (Fagnou), dark and light green (Yu), teal (Carrow), maroon (Periana), brown (Fagnou), yellow (Larrosa), and peach (Gorelsky and Woo). Bond indices for this study appear as magenta markers for acetate (●), acetohydroxamate (6-membered: ×, 7-membered: ▲), and pyrazolate (⬥) are also depicted.

Chart 1.

Complexes and Acids Evaluated in this Study.

Scheme 1.. Proposed mechanisms for protodemetalation of (bpy)Ni^II(aryl)(Br) complexes.^a

^a HA = acid, A = conjugate base, X = halide, solv = solvent molecule.