Fluoro and Trifluoromethyl Derivatives of POCOP-Type Pincer Complexes of Nickel: Preparation and Reactivities in SN2 Fluorination and Direct Benzylation of Unactivated Arenes

Reactions of cyclonickelated complexes with hydroxylamines and TEMPO˙: isolation of new TEMPOH adducts of Ni(II) and their reactivities with nucleophiles and oxidants

This contribution describes a study on the reactivities of the complexes [{κP,κC-(i-Pr)2PO-Ar}Ni(μ-Br)]2, 1a-d (Ar: C6H4, a; 3-Cl-C6H3, b; 3-OMe-C6H3, c; 4-OMe-napthalenyl, d), with hydroxylamines in the presence of TEMPO˙ (TEMPO˙ = (2,2,6,6-tetramethylpiperidinyl-1-yl)oxyl). The results of this study showed that treating 1a-d with a mixture of Et2NOH and TEMPO˙ did not afford the desired oxidation-induced functionalization of the Ni-aryl moiety in 1a-d, giving instead the corresponding κO-TEMPOH adducts [{κP,κC-(i-Pr)2PO-Ar}Ni(Br)(κO-TEMPOH)], 3a-d (TEMPOH = N-hydroxy-2,2,6,6-tetramethylpiperidine). The TEMPOH moiety in these zwitterionic compounds 3 can be displaced by a large excess of acetonitrile (MeCN), 10 equiv. of morpholine, or 1-2 equivalents of imidazole. Although these reactions have given the authenticated products [{κP,κC-(i-Pr)2PO-C6H4}Ni(Br)(NCMe)], 4a, [{κP,κC-(i-Pr)2PO-C6H4}Ni(Br)(morpholine)], 5a, and [{κP,κC-(i-Pr)2PO-C6H4}Ni(imidazole)2]Br, 6a, a few other products were also detected by NMR, indicating that the observed reactivities are far more complex than simple substitution of the TEMPOH moiety. Similarly, treating 3a with AgOC(O)CF3 results in the isolation of [{κP,κC-(i-Pr)2PO-C6H4}Ni{OC(O)CF3}(κO-TEMPOH)], 7a, arising from the substitution of the bromo ligand, whereas spectroscopic evidence suggests further reactivity, possibly including displacement of TEMPOH and oxidative decomposition.

Accessing Ni(0) to Ni(IV) via nickel-carbon-phosphorus bond reorganization

Two-electron oxidation of a NiIIPh(PCP) pincer complex initiates phosphine ligand insertion, generating an η6-arylphosphonium moiety coordinated to NiII. The reaction is fully reversible under reducing conditions. X-ray crystallography, NMR/EPR spectroscopy, electrochemistry, and DFT calculations support the proposed Ni-C-P bond reorganization mechanisms, which access oxidation states from Ni0 to NiIV.

Structures of nickel chloride and thiolate complexes supported by PCN and POCOP pincer ligands and catalytic reactivity of the chloride complexes

Nickel chloride and thiolate complexes supported by benzene-pyridine-based nonsymmetrical PCN pincer ligands, [2-(tBu2PO)-6-(2-pyrindinyl-4-R)-C6H3]NiX (R = H, CH3, CF3; X = Cl, SH, SPh), were synthesized and fully characterized. The structures of these complexes and the catalytic reactivity of the chloride complexes were investigated along with the related POCOP counterparts [2,6-(tBu2PO)2C6H3]NiX (X = Cl, SH). It was found that the composition and substitution of the pincer backbone evidently influence the structures and catalytic reactivity. The Ni-P and Ni-Cipso bond lengths in the PCN complexes are significantly shorter than those in the POCOP complex. The Ni-Cl and Ni-S bond lengths in the PCN complexes are longer than those in the POCOP complexes. An electron rich pyrindinyl ring in the PCN complexes makes the Ni-Cl bond longer. The Ni-N bond length is more sensitive to the auxiliary ligand compared with the Ni-P bond length in the PCN complexes. The PCN chloride complexes were found to be active catalysts for selective hydration of nitriles to primary amides in the presence of NaOH at 80 °C and the catalytic activity increases with the increase of electron richness of the pyridinyl ring. However, the corresponding POCOP counterpart is inactive under the same conditions.

Reactions and catalytic applications of a PNCNP pincer palladium hydride complex

A PNCNP-pincer palladium hydride complex possesses strong deprotonating ability and versatile catalytic activity and its pincer backbone exhibits high water stability.

Which Type of Pincer Complex Is Thermodynamically More Stable? Understanding the Structures and Relative Bond Strengths of Group 10 Metal Complexes Supported by Benzene-Based PYCYP Pincer Ligands

The influence of the pincer platform composition and substitution on the reactivity and physical properties of pincer complexes can be easily explored through different experimental techniques. However, the influence of these factors on the molecular structures and thermodynamic stability of pincer complexes is usually very subtle and cannot always be unambiguously established. To rationalize this subtle influence, a survey of crystallographic data from 130 group 10 metal pincer complexes supported by benzene-based PYCYP pincer ligands, [2,6-(R₂PY)₂C₆H_3-nR'_n]MX (Y = CH₂, NH, O, S; M = Ni, Pd, Pt; R = ^tBu, ⁱPr, Ph, Cy, Me; R' = CO₂Me, ^tBu, CF₃, Ac; n = 0-2; X = F, Cl, Br, I, H, SH, SPh, SBn, Ph, Me, N₃, NCS), was carried out. Theoretical calculations for some selected complexes were performed to evaluate the relative bond strength. It was found that the M-C_ipso bond length decreases following the linker series of CH₂ > NH > O and that the relative M-C_ipso bond strength increases following the linker series of CH₂ < NH < O. In most cases, the M-P bond length decreases following the linker series of NH > CH₂ > O. The relative M-P bond strength increases following the linker series of CH₂ < NH < O. A comparison of the thermochemical balance for the isodesmic displacement of the side-arm interactions with PH₃ as a probe ligand indicated that the Ni-P bond in a PCCCP-type pincer complex is far less difficult to break compared with that in a POCOP-type complex. As a result, with the same donor substituents and the same auxiliary ligand, the POCOP-type pincer complexes are thermodynamically more stable than the PCCCP complexes. The influence of other backbone and donor substitutions as well as the pincer platform composition on the M-C_ipso, M-P, and M-X bond lengths, relative bond strengths, and P-M-P bite angles was also discussed.

Photoinduced Trifluoromethylation of Arenes and Heteroarenes Catalyzed by High-Valent Nickel Complexes

We describe a series of air-stable Ni^III complexes supported by a simple, robust naphthyridine-based ligand. Access to the high-valent oxidation state is enabled by the CF₃ ligands on the nickel, while the naphthyridine exhibits either a monodentate or bidentate coordination mode that depends on the oxidation state and sterics, and enables facile aerobic oxidation of Ni^II to Ni^III . These Ni^III complexes act as efficient catalysts for photoinduced C(sp² )-H bond trifluoromethylation reactions of (hetero)arenes using versatile synthetic protocols. This blue LED light-mediated catalytic protocol proceeds via a radical pathway and demonstrates potential in the late-stage functionalization of drug analogs.

Electrochemical Insight into Mechanisms and Metallocyclic Intermediates of C-H Functionalization

Transition metal-catalyzed C-H activation has emerged as a powerful tool in organic synthesis and electrosynthesis as well as in the development of new methodologies for producing fine chemicals. In order to achieve efficient and selective C-H functionalization, different strategies have been used to accelerate the C-H activation step, including the incorporation of directing groups in the substrate that facilitate coordination to the catalyst. In this review, we try to underscore that the understanding the mechanisms of the catalytic cycle and the reactivity or redox activity of the key metal cyclic intermediates in these reactions is the basis for controlling the selectivity of synthesis and electrosynthesis. Combination of the electrosynthesis and voltammetry with traditional synthetic and physico-chemical methods allows one to achieve selective transformation of C-H bonds to functionalized C-C or C-X (X=heteroatom or halogen) bonds which may encourage organic chemists to use it in the future more often. The possibilities and the benefits of electrochemical techniques are analyzed and summarized.

Achiral and chiral NNN-pincer nickel complexes with oxazolinyl backbones: application in transfer hydrogenation of ketones

NNN-based achiral and chiral (oxazolinyl)amido-pincer nickel complexes are developed and employed for the catalytic transfer hydrogenation of ketones.

Cationic tetra- and pentacoordinate complexes of nickel based on POCN- and POCOP-type pincer ligands: synthesis, characterization, and ligand exchange studies

The closely related pincer ligands POCN and POCOP display different electron donating properties and different degrees of resistance to ligand exchange reactions proceeding via cationic reaction intermediates.

Reactions of a pincer proligand with copper iodide: bridging instead of C–H metalation

Various precursors of divalent copper have been treated with the meta-disubstituted phenylene-based proligand POC(H)OP (1,3-(i-Pr2PO)2C6H4) with the objective of preparing classical pincer complexes (POCOP)CuX. However, in no case was such species obtained, presumably owing to the difficult C–H metallation step. Analogous reactions of monovalent precursors were also unsuccessful, whereas reaction of POC(H)OP with CuI under different conditions gave the non-metallated adducts {(μ, κP, [Formula: see text]-POC(H)OP)Cu(μ-Ι)}2, 1, {(μ, κP, [Formula: see text]-POC(H)OP)Cu2(μ-Ι)2(DMAP)2}, 2 (DMAP = 4-dimethylaminopyridine), and {(μ, κP, [Formula: see text]-POC(H)OP)Cu2(μ3-Ι)2}2, 3. Treating 1 with DMAP gave the adduct 2, whereas 3 could be obtained by treating 1 with BuLi or by sublimation of 1. The solid state structures of these complexes revealed the tetrahedral geometry that might be anticipated for the d10 Cu(I) centers, in addition to fairly close I–H distances; on the other hand, no C–H interaction (agostic or otherwise) was observed with the Cu centers in any of these structures. The unsuccessful metallation of the C(2)–H moiety is thought to be a result of the strong preference of monovalent copper center to form bridging interactions with iodide and the POC(H)OP ligand; this appears to prevent the approach of the central carbon of the ligand to the Cu centers.

Nickel-Mediated Trifluoromethylation of Phenol Derivatives by Aryl C-O Bond Activation

The increasing pharmaceutical importance of trifluoromethylarenes has stimulated the development of more efficient trifluoromethylation reactions. Tremendous efforts have focused on copper- and palladium-mediated/catalyzed trifluoromethylation of aryl halides. In contrast, no general method exists for the conversion of widely available inert electrophiles, such as phenol derivatives, into the corresponding trifluoromethylated arenes. Reported herein is a practical nickel-mediated trifluoromethylation of phenol derivatives with readily available trimethyl(trifluoromethyl)silane (TMSCF₃ ). The strategy relies on PMe₃ -promoted oxidative addition and transmetalation, and CCl₃ CN-induced reductive elimination. The broad utility of this transformation has been demonstrated through the direct incorporation of trifluoromethyl into aromatic and heteroaromatic systems, including biorelevant compounds.

A Comparative Study on the Thermodynamics of Halogen Bonding of Group 10 Pincer Fluoride Complexes

The thermodynamics of halogen bonding of a series of isostructural Group 10 metal pincer fluoride complexes of the type [(3,5‐R₂‐^tBuPOCOP^tBu)MF] (3,5‐R₂‐^tBuPOCOP^tBu=κ³‐C₆HR₂‐2,6‐(OPtBu₂)₂ with R=H, tBu, COOMe; M=Ni, Pd, Pt) and iodopentafluorobenzene was investigated. Based on NMR experiments at different temperatures, all complexes 1‐tBu (R=tBu, M=Ni), 2‐H (R=H, M=Pd), 2‐tBu (R=tBu, M=Pd), 2‐COOMe (R=COOMe, M=Pd) and 3‐tBu (R=tBu, M=Pt) form strong halogen bonds with Pd complexes showing significantly stronger binding to iodopentafluorobenzene. Structural and computational analysis of a model adduct of complex 2‐tBu with 1,4‐diiodotetrafluorobenzene as well as of structures of iodopentafluorobenzene in toluene solution shows that formation of a type I contact occurs.

The Reactivity of Mercapto Groups against Boron Hydrides in Pincer Ligated Nickel Mercapto Complexes

Several pincer ligated nickel mercapto complexes, [2,6-(R₂ PCH₂ )₂ C₆ H₃ ]NiSH (R=tBu, 1 a; iPr, 1 b), [2,6-(R₂ PO)₂ C₆ H₃ ]NiSH (R=tBu, 2 a; iPr, 2 b) and [4-MeOCO-2,6-(tBu₂ PO)₂ C₆ H₂ ]NiSH (3 a), were synthesized and fully characterized. The reactivity of the mercapto groups against boron hydrides and organic bases was investigated. It was found that the mercapto groups are difficult to be deprotonated by boron hydrides or organic bases. The treatment of complex 2 a or 2 b with an excess amount of catecholborane (HBcat) afforded the corresponding pincer ligated nickel borohydride complexes and the HBcat degradation product. The treatment of complex 1 a, 2 a or 2 b with an excess amount of BH₃ ⋅THF produced the corresponding nickel borohydride species and the S-bridged triborane species THF⋅BH₂ -μ₂ -S(B₂ H₅ ) (5). No reactions between these complexes and organic bases were observed. DFT calculations were carried out to understand this reactivity and get mechanistic insights into the reactions.

Application of POCOP Pincer Nickel Complexes to the Catalytic Hydroboration of Carbon Dioxide

The reduction of CO2 is of great importance. In this paper, different types of bis(phosphinite) (POCOP) pincer nickel complexes, [2,6-(R2PO)2C6H3]NiX (R = tBu, iPr, Ph; X = SH, N3, NCS), were applied to the catalytic hydroboration of CO2 with catecholborane (HBcat). It was found that pincer complexes with tBu2P or iPr2P phosphine arms are active catalysts for this reaction in which CO2 was successfully reduced to a methanol derivative (CH3OBcat) with a maximum turnover frequency of 1908 h−1 at room temperature under an atmospheric pressure of CO2. However, complexes with phenyl-substituted phosphine arms failed to catalyze this reaction—the catalysts decomposed under the catalytic conditions. Complexes with iPr2P phosphine arms are more active catalysts compared with the corresponding complexes with tBu2P phosphine arms. For complexes with the same phosphine arms, the catalytic activity follows the series of mercapto complex (X = SH) ≈ azido complex (X = N3) >> isothiocyanato complex (X = NCS). It is believed that all of these catalytic active complexes are catalyst precursors which generate the nickel hydride complex [2,6-(R2PO)2C6H3]NiH in situ, and the nickel hydride complex is the active species to catalyze this reaction.

Nickel-Alkyl Complexes with a Reactive PNC-Pincer Ligand

Based on previous work related to the design and application of rigid tridentate phosphine-pyridine-phenyl coordination offered by a PNC-pincer ligand upon cyclometalation to nickel, the synthesis, spectroscopic and solid state characterization and redox-reactivity of two NiII(PNC) complexes featuring either a methyl (2CH3 ) or CF3 co-ligand (2CF3 ) are described. One-electron oxidation is proposed to furnish C-C reductive elimination, as deduced from a combined chemical, electrochemical, spectroscopic and computational study. One-electron reduction results in a ligand-centered radical anion, as supported by electrochemistry, UV spectroelectrochemistry, EPR spectroscopy, and DFT calculations. This further attenuates the breadth of chemical reactivity offered by such PNC-pincer ligands.

A reaction of [2,6-(tBu2PO)2C6H3]NiSCH2Ph with BH3·THF: borane mediated C–S bond cleavage

C–S bond cleavage of the thiolato ligand of [2,6-(^tBu₂PO)₂C₆H₃]NiSCH₂Ph mediated by BH₃ as a suitable model for the transition metal catalyzed C–S bond activation of mercaptans.

C–H nickellation of phenol-derived phosphinites: regioselectivity and structures of cyclonickellated complexes

Cyclonickellation of aryl phosphinites derived from substituted phenols is regioselective: high sensitivity to sterics favors nickellation at the less hindered C–H.

Highly efficient reduction of carbon dioxide with a borane catalyzed by bis(phosphinite) pincer ligated palladium thiolate complexes

New palladium catalysts developed for highly efficient hydroboration of CO₂ under very mild conditions with turnover frequencies up to 1780 h⁻¹.

Synthesis of unsymmetrical 5,6-POCOP′-type pincer complexes of nickel(ii): impact of nickelacycle size on structures and spectroscopic properties

Different structures and reactivities observed with the unsymmetrical (5,6-POCOP) and symmetrical (5,5-POCOP) pincer complexes of Ni(ii) underline the importance of the metallacycle ring size and its degree of flexibility.

Reactions of phenylacetylene with nickel POCOP-pincer hydride complexes resulting in different outcomes from their palladium analogues

Nickel POCOP-pincer hydride complexes react with phenylacetylene to afford alkenyl complexes whereas the palladium analogues give alkynyl complexes.