O2-dependent aliphatic carbon-carbon bond cleavage reactivity in a Ni(II) enolate complex having a hydrogen bond donor microenvironment; comparison with a hydrophobic analogue

Abstract

A mononuclear Ni(II) complex having an acireductone type ligand, and supported by the bnpapa (N,N-bis((6-neopentylamino-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine) ligand, [(bnpapa)Ni(PhC(O)C(OH)C(O)Ph)]ClO(4) (14), has been prepared and characterized by elemental analysis, (1)H NMR, FTIR, and UV-vis. To gain insight into the (1)H NMR features of 14, the air stable analogue complexes [(bnpapa)Ni(CH(3)C(O)CHC(O)CH(3))]ClO(4) (16) and [(bnpapa)Ni(ONHC(O)CH(3))]ClO(4) (17) were prepared and characterized by X-ray crystallography, (1)H NMR, FTIR, UV-vis, mass spectrometry, and solution conductivity measurements. Compounds 16 and 17 are 1:1 electrolyte species in CH(3)CN. (1)H and (2)H NMR studies of 14, 16, and 17 and deuterated analogues revealed that the complexes having six-membered chelate rings for the exogenous ligand (14 and 16) do not have a plane of symmetry within the solvated cation and thus exhibit more complicated (1)H NMR spectra. Compound 17, as well as other simple Ni(II) complexes of the bnpapa ligand (e.g., [(bnpapa)Ni(ClO(4))(CH(3)CN)]ClO(4) (18) and [(bnpapaNi)(2)(mu-Cl)(2)](ClO(4))(2) (19)), exhibit (1)H NMR spectra consistent with the presence of a plane of symmetry within the cation. Treatment of [(bnpapa)Ni(PhC(O)C(OH)C(O)Ph)]ClO(4) (14) with O(2) results in aliphatic carbon-carbon bond cleavage within the acireductone-type ligand and the formation of [(bnpapa)Ni(O(2)CPh)]ClO(4) (9), benzoic acid, benzil, and CO. Use of (18)O(2) in the reaction gives high levels of incorporation (>80%) of one labeled oxygen atom into 9 and benzoic acid. The product mixture and level of (18)O incorporation in this reaction is different than that exhibited by the analogue supported the hydrophobic 6-Ph(2)TPA ligand, [(6-Ph(2)TPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO(4) (2). We propose that this difference is due to variations in the reactivity of bnpapa- and 6-Ph(2)TPA-ligated Ni(II) complexes with triketone and/or peroxide species produced in the reaction pathway.

Figure 1

Supporting chelate ligands used to isolate Ni(II) enediolate (top) and enolate (bottom) complexes of relevance to Ni(II)-ARD.

Figure 2

Thermal ellipsoid drawings of the cationic portions of 16 and 17. All ellipsoids are drawn at the 50% probability level. Hydrogen atoms, other than the neopentyl amine N-H hydrogen atoms, and the hydroxyamato N-H hydrogen in 17, have been omitted for clarity.

Figure 3

Thermal ellipsoid drawings of the cationic portions of 18 and 19. All ellipsoids are drawn at the 50% probability level. Hydrogen atoms other than the neopentyl amine N-H hydrogen atoms have been omitted for clarity.

Figure 4

Onsager plots of the solution conductivity properties of 9, 16–19, and the 1:1 standard Me₄NClO₄ in CH₃CN at 22(1) °C.

Figure 5

¹H NMR spectral features of (a) 14, (b) 16, and (c) 17 in the region of 20–80 ppm. Spectra obtained in CD₃CN at 22(1) °C.

Figure 6

(a) Proposed structure of [(bnpapa)Ni(PhC(O)C(OH)C(O)Ph)]ClO₄ (14). (b) Structure of [(6-Ph₂TPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO₄(2) as determined by X-ray crystallography.

Scheme 1

Scheme 2

Scheme 3

Scheme 4

Major products identified in reaction of 14 and 2 with O₂; results of reactions performed using ¹⁸O₂.

Scheme 5

Possible reaction pathways for oxidative carbon-carbon bond cleavage in the Ni(II) enolate complexes 2 and 14. The supporting chelate ligands have been omitted for clarity.