Solid‐State ¹⁷O NMR Spectroscopy of Paramagnetic Coordination Compounds

Abstract

High-quality solid-state 17 O( I=5/2) NMR spectra can be successfully obtained for paramagnetic coordination compounds in which oxygen atoms are directly bonded to the paramagnetic metal centers.For complexes containing V III (S = 1), Cu II (S = 1/2), and Mn III (S = 2) metal centers,t he 17 O isotropic paramagnetic shifts were found to span ar ange of more than 10 000 ppm. In several cases,h igh-resolution 17 O NMR spectra were recorded under very fast magic-angle spinning (MAS) conditions at 21.1 T. Quantum-chemical computations using density functional theory (DFT) qualitatively reproduced the experimental 17 Ohyperfine shift tensors.NMR signals from paramagnetic substances are generally more difficult to detect than those from diamagnetic compounds.T his is because the hyperfine interactions between magnetic dipoles of unpaired electrons and atomic nuclei are substantially stronger than the typical nuclear spin interactions such as magnetic shielding, nuclear quadrupolar,a nd dipolar couplings.A saresult, the NMR signals from paramagnetic compounds are significantly shifted and broadened compared with diamagnetic compounds.Despite experimental difficulties,s olid-state NMR studies of paramagnetic compounds can be traced back to the early NMR studies on single crystals of CuSO 4 ·5 H 2 O, [1] CuCl 2 ·H 2 O, [2] and MnF 2 . [3] In more recent years,t here have been considerable interest in solution [4] and solid-state [5][6][7][8][9][10][11][12][13][14][15][16] NMR studies of organic and biological systems containing paramagnetic metal ions.T o date,m ost NMR studies of paramagnetic compounds have relied on detection of 1 Ha nd 13 Cn uclei, because hydrogen and carbon atoms are generally remote from the paramagnetic metal centers,t hus experiencing relatively weak hyperfine interactions.Incontrast, as oxygen atoms are often directly bonded to the paramagnetic metal centers, 17 ONMR for paramagnetic coordination compounds is expected to be more challenging than the corresponding 1 Ha nd 13 CNMR studies.F urthermore,t he only NMR-active oxygen isotope, 17 O, has very low natural abundance (0.037 %) and its nuclear spin is quadrupolar (I = 5/2). In light of the recent advances in solid-state 17 ON MR studies of diamagnetic molecules including biological macromolecules, [17] we decided to explore the possibility of using solid-state 17 ON MR to study paramagnetic coordination compounds.Weshould note that solidstate 17 ONMR has been used previously to study ionic high T c superconductors [18] and simple paramagnetic inorganic complexes. [19] Thefocus of our study is on solid-state 17 ONMR of paramagnetic coordination complexes containing organic ligands (that is,containing H, C, N, and Oatoms).We first examined two vanadium(III) (S = 1) complexes: [V([ 17 O 2 ]acac) 3 ]a nd K 3 [V([ 17 O 4 ]oxalate) 3 ]·3 H 2 O. Synthetic details for the preparation of 17 O-labeled paramagnetic coordination compounds are given in the Supporting Information. As seen in Figure 1, the static solid-state 17 ON MR spectrum for [V([ 17...

Toward Relatively General and Accurate Quantum Chemical Predictions of Solid-State 17O NMR Chemical Shifts in Various Biologically Relevant Oxygen-Containing Compounds

“…23 We first tested the use of the X-ray structure without any geometry optimization. Although previously the quantum chemical methods at both the Hartree-Fock and DFT levels 5, 7-9, 11, 13-17, 19, 21, 23-25, 28 were used for NMR chemical shielding calculations, here we focused on evaluating DFT methods, since eventually we will also investigate metal-containing systems. Both the pure DFT method OP86 31, 32 and the hybrid DFT method B3LYP 33 were studied here based on their previous good performance in predicting 17 O NMR chemical shifts in some limited sets of oxygen-containing compounds.…”

“…Both the pure DFT method OP86 31, 32 and the hybrid DFT method B3LYP 33 together with Pople-type basis 6-311++G(d,p) and 6-311++G(2d,2p), and Dunning type basis D95(d,p) 34 and aug-cc-PVDZ 35 for non-metal elements and the effective core potential basis LanL2DZ 36 for metals were studied. Based on our recent work on some oxygen-containing systems 17 and more studies in the Supporting Information, the LanL2DZ basis yields the best NMR shift predictions among a number of metal bases studied. Although the state-of-the-art CCSD(T) calculations shall provide better predictions of absolute NMR shift values than DFT methods, 37 their uses are basically limited to small molecules.…”

“…16 More recently, the solid-state 17 O NMR studies have even been extended to some paramagnetic metal complexes with organic ligands. 17 …”

“…To help understand experimental solid-state 17 O NMR results, a number of theoretical investigations have been reported, such as those on the calculated 17 O NMR shifts of amides 5 , nucleic acid bases 14, 18-20 , amino acids 11-13 , carboxylic compounds 21-23 , peptides 24 as well as a few metal containing compounds 7, 8, 17, 25 . For the diamagnetic systems, regarding the two major types of solid-state 17 O NMR properties, the 17 O NMR chemical shielding/shift tensor and electric field gradient tensor (or related quadrupole coupling constant), previous results show that quantum chemical predictions of the latter one are in most cases satisfactory with errors <10%, while the accurate predictions of the former one are relatively more challenging.…”

“…5 The predictions of δiso ’ s though in some cases can achieve an accuracy of <10-15% error, but also frequently have errors well above 20-30%, especially for metal-containing systems. 5, 7-9, 11-14, 17-25 …”