Interaction between a Transition-Metal Fluoride and a Transition-Metal Hydride: Water-Mediated Hydrofluoric Acid Evolution Following Fluoride Solvation

Solution and solid-state studies of hydrogen and halogen bonding with N-heterocyclic carbene supported nickel(II) fluoride complexes

Nickel fluoride complexes of the type [Ni(F)(L)2(ArF)] (L = phosphine, ArF = fluorinated arene) are well-known to form strong halogen and hydrogen bonds in solution and in the solid state. A comprehensive study of such non-covalent interactions using bis(carbene) complexes as acceptors and suitable halogen and hydrogen bond donors is presented. In solution, the complex [Ni(F)(iPr2Im)2(C6F5)] forms halogen and hydrogen bonds with iodopentafluorobenzene and indole, respectively, which have formation constants (K300) an order of magnitude greater than those of structurally related phosphine supported nickel fluorides. Co-crystallisation of this complex and its backbone-methylated analogue [Ni(F)(iPr2Me2Im)2(C6F5)] with 1,4-diiodotetrafluorobenzene produces halogen bonding adducts which were characterised by X-ray analysis and 19F MAS solid state NMR analysis. Differences in the chemical shifts between the nickel fluoride and its halogen bonding adduct are well in line with data that were obtained from titration studies in solution.

Solid-State 19F NMR Chemical Shift in Square-Planar Nickel-Fluoride Complexes Linked by Halogen Bonds

The halogen bond (XB) is a highly directional class of noncovalent interactions widely explored by experimental and computational studies. However, the NMR signature of the XB has attracted limited attention. The prediction and analysis of the solid-state NMR (SSNMR) chemical shift tensor provide useful strategies to better understand XB interactions. In this work, we employ a computational protocol for modeling and analyzing the 19F SSNMR chemical shifts previously measured in a family of square-planar trans NiII-L2-iodoaryl-fluoride (L = PEt3) complexes capable of forming self-complementary networks held by a NiF···I(C) halogen bond [Thangavadivale, V.; Chem. Sci. 2018, 9, 3767-3781]. To understand how the 19F NMR resonances of the nickel-bonded fluoride are affected by the XB, we investigate the origin of the shielding in trans-[NiF(2,3,5,6-C6F4I)(PEt3)2], trans-[NiF(2,3,4,5-C6F4I)(PEt3)2], and trans-[NiF(C6F5)(PEt3)2] in the solid state, where a XB is present in the two former systems but not in the last. We perform the 19F NMR chemical shift calculations both in periodic and molecular models. The results show that the crystal packing has little influence on the NMR signatures of the XB, and the NMR can be modeled successfully with a pair of molecules interacting via the XB. Thus, the observed difference in chemical shift between solid-state and solution NMR can be essentially attributed to the XB interaction. The very high shielding of the fluoride and its driving contributor, the most shielded component of the chemical shift tensor, are well reproduced at the 2c-ZORA level. Analysis of the factors controlling the shielding shows how the highest occupied Ni/F orbitals shield the fluoride in the directions perpendicular to the Ni-F bond and specifically perpendicular to the coordination plane. This shielding arises from the magnetic coupling of the Ni(3d)/F(2p lone pair) orbitals with the vacant σNi-F* orbital, thereby rationalizing the very highly upfield (shielded) resonance of the component (δ33) along this direction. We show that these features are characteristic of square-planar nickel-fluoride complexes. The deshielding of the fluoride in the halogen-bonded systems is attributed to an increase in the energy gap between the occupied and vacant orbitals that are mostly responsible for the paramagnetic terms, notably along the most shielded direction.

Self-complementary nickel halides enable multifaceted comparisons of intermolecular halogen bonds: fluoride ligands vs. other halides

Studies of X–Ni–C₆F₄I···X–Ni–C₆F₄I halogen-bonded networks reveal pronounced differences between fluoride (X = F) and other halides: the ¹⁹F-MAS NMR spectrum is a sensitive probe of the halogen bond.

The syntheses of three series of complexes designed with self-complementary motifs for formation of halogen bonds between an iodotetrafluorophenyl ligand and a halide ligand at square-planar nickel are reported, allowing structural comparisons of halogen bonding between all four halides C₆F₄I···X–Ni (X = F, Cl, Br, I). In the series trans-[NiX(2,3,5,6-C₆F₄I)(PEt₃)₂] 1pX and trans-[NiX(2,3,4,5-C₆F₄I)(PEt₃)₂] (X = F, Cl, Br, I) 1oX, the iodine substituent on the benzene ring was positioned para and ortho to the metal, respectively. The phosphine substituents were varied in the series, trans-[NiX(2,3,5,6-C₆F₄I)(PEt₂Ph)₂] (X = F, I) 2pX. Crystal structures were obtained for the complete series 1pX, and for 1oF, 1oCl, 1oI and 2pI. All these complexes exhibited halogen bonds in the solid state, of which 1pF exhibited unique characteristics with a linear chain, the shortest halogen bond d(C₆F₄I···F–Ni) = 2.655(5) Å and the greatest reduction in halogen bond distance (I···F) compared to the sum of the Bondi van der Waals radii, 23%. The remaining complexes form zig-zag chains of halogen bonds with distances also reduced with respect to the sum of the van der Waals radii. The magnitude of the reductions follow the pattern F > Cl ∼ Br > I, 1pX > 1oX, consistent with the halogen bond strength following the same order. The variation in the I···X–Ni angles is consistent with the anisotropic charge distribution of the halide ligand. The temperature dependence of the X-ray structure of 1pF revealed a reduction in halogen bond distance of 0.055(7) Å on cooling from 240 to 111 K. Comparison of three polymorphs of 1oI shows that the halogen bond geometry may be altered significantly by the crystalline environment. The effect of the halogen bond on the ¹⁹F NMR chemical shift in the solid state is demonstrated by comparison of the magic-angle spinning NMR spectra of 1pF and 1oF with that of a complex incapable of halogen bond formation, trans-[NiF(C₆F₅)(PEt₃)₂] 3F. Halogen bonding causes deshielding of δ_iso in the component of the tensor perpendicular to the nickel coordination plane. The results demonstrate the potential of fluoride ligands for formation of halogen bonds in supramolecular structures.

Lactate Racemase Nickel-Pincer Cofactor Operates by a Proton-Coupled Hydride Transfer Mechanism

Lactate racemase (LarA) of Lactobacillus plantarum contains a novel organometallic cofactor with nickel coordinated to a covalently tethered pincer ligand, pyridinium-3-thioamide-5-thiocarboxylic acid mononucleotide, but its function in the enzyme mechanism has not been elucidated. This study presents direct evidence that the nickel-pincer cofactor facilitates a proton-coupled hydride transfer (PCHT) mechanism during LarA-catalyzed lactate racemization. No signal was detected by electron paramagnetic resonance spectroscopy for LarA in the absence or presence of substrate, consistent with a +2 metal oxidation state and inconsistent with a previously proposed proton-coupled electron transfer mechanism. Pyruvate, the predicted intermediate for a PCHT mechanism, was observed in quenched solutions of LarA. A normal substrate kinetic isotope effect ( k_H/ k_D of 3.11 ± 0.17) was established using 2-α-²H-lactate, further supporting a PCHT mechanism. UV-visible spectroscopy revealed a lactate-induced perturbation of the cofactor spectrum, notably increasing the absorbance at 340 nm, and demonstrated an interaction of the cofactor with the inhibitor sulfite. A crystal structure of LarA provided greater resolution (2.4 Å) than previously reported and revealed sulfite binding to the pyridinium C4 atom of the reduced pincer cofactor, mimicking hydride reduction during a PCHT catalytic cycle. Finally, computational modeling supports hydride transfer to the cofactor at the C4 position or to the nickel atom, but with formation of a nickel-hydride species requiring dissociation of the His200 metal ligand. In aggregate, these studies provide compelling evidence that the nickel-pincer cofactor acts by a PCHT mechanism.

Non-precious metal complexes with an anionic PCP pincer architecture

This perspective article provides an overview of the advancements in the field of non-precious metal complexes featuring anionic PCP pincer ligands with the inclusion of aliphatic systems. It covers research from the beginning in 1976 until late 2015 and provides a summary of key developments in this area, which is, to date, limited to the metals nickel, cobalt, iron, and molybdenum. While the research in nickel PCP complexes is already quite extensive, the chemistry of cobalt, iron, and molybdenum PCP complexes is comparatively sparse. With other non-precious metals such as copper, manganese, chromium or vanadium no PCP complexes are known as yet. In the case of nickel PCP complexes already many catalytic applications such as Suzuki-Miyaura coupling, C-S cross coupling, Kharasch and Michael additions, hydrosilylation of aldehydes and ketones, cyanomethylation of aldehydes, and hydroamination of nitriles were reported. While iron PCP complexes were found to be active catalysts for the hydrosilylation of aldehydes and ketones as well as the dehydrogenation of ammonia-borane, cobalt PCP complexes were not applied to any catalytic reactions. Surprisingly, only one molybdenum PCP complex is reported, which was capable of cleaving dinitrogen to give a nitride complex. This perspective underlines that the combination of cheap and abundant metals such as nickel, cobalt, and iron with PCP pincer ligands may result in the development of novel, versatile, and efficient catalysts for atom-efficient catalytic reactions.