C−H Bond Trifluoromethylation of Arenes Enabled by a Robust, High‐Valent Nickel(IV) Complex

Abstract

The robust, high-valent Ni complex [(Py) Ni F (CF ) ] (Py=pyridine) was synthesized and fully characterized by NMR spectroscopy, X-ray diffraction, and elemental analysis. It reacts with aromatic compounds at 25 °C to form the corresponding benzotrifluorides in nearly quantitative yield. The monomeric and dimeric Ni CF complexes 2⋅Py and 2 were identified as key intermediates, and their structures were unambiguously determined by EPR spectroscopy and X-ray diffraction. Preliminary kinetic studies in combination with the isolation of reaction intermediates confirmed that the C-H bond-breaking/C-CF bond-forming sequence can occur both at Ni CF and Ni CF centers.

C−H Bond Trifluoromethylation of Arenes Enabled by a Robust, High‐Valent Nickel(IV) Complex

“…As apparent by EPR spectroscopy,amixture of two Ni III CF 3 complexes, 2 and 2·Py (see below), was obtained when 1equiv of XeF 2 was used (see Figure 1b). [14] These Ni III dimers are diamagnetic (singlet spin state) or paramagnetic (triplet spin state) depending on the magnitude of the antiferromagnetic coupling between the two Ni III centers.D FT calculations fully support 2 to be paramagnetic as the triplet dimer was calculated to be 43.5 kcal mol À1 lower in energy than the corresponding singlet, which is in agreement with the fact that complex 2 is NMR-silent but seen by EPR spectroscopy.T herefore,t he bridging fluoride ligands do not favor antiferromagnetic coupling between the two Ni III centers in 2.T he frozen-solution EPR spectrum (Figure 1b [20] Thermal ellipsoids set at 50 %probability.b)EPR spectrum (blue line) for the mixture of 2 and 2·Py in 3:1PrCN/DCBat1 20 Kand the spectrum (red line) simulated using the following parameters: 2: g x = 2.237, g y = 2.172, g z = 2.018 (A N = 18.4 G, A F = 220 G); 2·Py: g x = 2.232, g y = 2.166, g z = 2.018 (A 2N = 19.6 G). XRD analysis showed that complex 2 crystallizes as adimer with two distinct Ni centers, which are linked by two bridging fluoride ligands.I nterestingly,t he coordination environment around each Ni unit is different, resulting in ahighly unusual, unsymmetric structure for 2.O ne of the Ni centers exhibits an octahedral environment, whereas the second one is in as quare-pyramidal environment.…”

“…[14] These Ni III dimers are diamagnetic (singlet spin state) or paramagnetic (triplet spin state) depending on the magnitude of the antiferromagnetic coupling between the two Ni III centers.D FT calculations fully support 2 to be paramagnetic as the triplet dimer was calculated to be 43.5 kcal mol À1 lower in energy than the corresponding singlet, which is in agreement with the fact that complex 2 is NMR-silent but seen by EPR spectroscopy.T herefore,t he bridging fluoride ligands do not favor antiferromagnetic coupling between the two Ni III centers in 2.T he frozen-solution EPR spectrum (Figure 1b . [20] Thermal ellipsoids set at 50 %probability.b)EPR spectrum (blue line) for the mixture of 2 and 2·Py in 3:1PrCN/DCBat1 20 Kand the spectrum (red line) simulated using the following parameters: 2: g x = 2.237, g y = 2.172, g z = 2.018 (A N = 18.4 G, A F = 220 G); 2·Py: g x = 2.232, g y = 2.166, g z = 2.018 (A 2N = 19.6 G).…”

Aerobic C–C and C–O bond formation reactions mediated by high-valent nickel species

“…[4][5][6][7][8][9] The most common oxidation states involved in these catalytic transformations are Ni 0 , Ni I , and Ni II , although more recent studies show that Ni III and Ni IV oxidation states can also play a role in C-C bond formation. [10][11][12][13][14][15][16][17][18][19][20] By comparison, stoichiometric and catalytic Ni-mediated C-heteroatom bond formation reactions have been developed mostly in the past two decades. [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] In the past several years we have employed tetradentate pyridinophane ligands to stabilize uncommon organometallic Pd III/IV and Ni III/IV complexes, [37][38][39][40][41][42][43][44] which can undergo C-C and C-heteroatom bond formation reactions.…”

Photoinduced Cobalt(III)−Trifluoromethyl Bond Activation Enables Arene C−H Trifluoromethylation

“…Accordingly, new methods to activate M−CF 3 intermediates are being pursued to access the full power of organometallic C−H activation in selective fluoroalkylations. For instance, C−CF 3 reductive elimination from high‐valent electrophilic Group 10–11 metals continues to garner significant interest . Baker and co‐workers are using fluoride abstraction from Co−CF 3 complexes to circumvent the stability of Co perfluoroalkyl complexes …”