Solid State ¹³C and ²H NMR Investigations of Paramagnetic [Ni(II)(acac)₂L₂] Complexes

Abstract

Nine structurally related paramagnetic acetylacetonato nickel(II) complexes: [Ni(acac)2] and trans-[Ni(acac)2(X)2]nH/D2O, X = H2O, D2O, NH3, MeOH, PMePh2, PMe2Ph, or [dppe]1/2, n = 0 or 1, dppe = 1,2-bis(diphenylphosphino)ethane, as well as cis-[Ni(F6-acac)2(D2O)2], F6-acac = hexafluoroacetylonato, have been characterized by solid state (13)C MAS NMR spectroscopy. (2)H MAS NMR was used to probe the local hydrogen bonding network in [Ni(acac)2(D2O)2]D2O and cis-[Ni(F6-acac)2(D2O)2]. The complexes serve to benchmark the paramagnetic shift, which can be associated with the resonances of atoms of the coordinated ligands. The methine (CH) and methyl (CH3) have characteristic combinations of the isotropic shift (δ) and anisotropy parameters (d, η). The size of the anisotropy (d), which is the sum of the chemical shift anisotropy (CSA) and the paramagnetic electron-nuclei dipolar coupling, is much more descriptive than the isotropic shift. Moreover, the CSA is found to constitute up to one-third of the total anisotropy and should be taken into consideration when (13)C anisotropies are used for structure determination of paramagnetic materials. The (13)C MAS NMR spectra of trans-[Ni(acac)2(PMe2Ph)2], trans-[Ni(acac)2(PMePh2)2], and the noncrystallographically characterized trans-[Ni(acac)2(dppe)]n were assigned using these correlations. The complexes with L = H2O, D2O, NH3, and MeOH can be prepared by a series of solid state desorption and sorption reactions. Crystal structures for trans-[Ni(acac)2(NH3)2] and trans-[Ni(acac)2(PMePh2)2] are reported.

Assignment of solid-state 13C and 1H NMR spectra of paramagnetic Ni(II) acetylacetonate complexes aided by first-principles computations

“…First we discuss the experimental 13 2, and 885(10) ppm, c.f., Table 2 in excellent agreement with earlier reported data for the non-deuterated analogue [31]. (2) and 988(10) ppm should be assigned to CH, C=O, and CH 3 , respectively, c.f., Table 2.…”

“…The third assignment approach uses the longitudinal relaxation times (T 1 , [31]. A complete discussion of the basis-set and electron correlation effects, as well as the influence of the choices of the DFT functional and structure is given in the Supporting Information.…”

“…Thus, the analysis of SSNMR spectra of paramagnetic samples has mainly relied on simple empirical and/or classical models ( Figure 1). In particular, the paramagnetic hyperfine contribution ("Fermi contact shift") to the isotropic shifts ("paramagnetic shifts") has been popular due to its simplicity, with applications to, e.g., battery materials [26,27], iron soil minerals [26][27][28][29], and recently to metal-organic complexes [30][31][32][33][34][35][36][37].…”

Investigation of Organonickel‐Pentafluoroorthotellurates

“…The C–C bond length are similar to distances determined for [NiBr 2 (Hacac) 2 ] of 151 and 152 pm . This excludes an acetylacetonato ligand with C–C bond length of 139.9(1) to 141.3(1) pm . The presence of a CH group and consequently an enol‐like structure (see Scheme ) with alternating single and double bonds can be excluded in view of the experimental structure and those optimized on RI‐B3LYP‐D3/def2‐TZVPP level of theory (see Table ).…”

Magic Angle Spinning NMR Spectroscopy: A Versatile Technique for Structural and Dynamic Analysis of Solid-Phase Systems

“…The applications of MAS NMR to paramagnetic systems encompass analysis of biological systems 11,89,90 as well as numerous inorganic materials. 87,91–93 …”