ECE-Type Pincer Complexes of Nickel

Platinum thiolate complexes supported by PBP and POCOP pincer ligands as efficient catalysts for the hydrosilylation of carbonyl compounds

Diphosphino-boryl-based PBP pincer platinum thiolate complexes, [Pt(SR){B(NCH₂P^tBu₂)₂-1,2-C₆H₄}] (R = H, 1a; Ph, 1b), and benzene-based bisphosphinite POCOP pincer platinum thiolate complexes, [Pt(SR)(^tBu₂PO)₂-1,3-C₆H₃] (R = H, 2a; Ph, 2b), were prepared and fully characterized by multinuclear NMR, X-ray crystallography, HRMS and elemental analyses. The application of these complexes in the catalytic hydrosilylation of aldehydes and ketones was investigated. It was found that these platinum thiolate complexes are efficient catalysts for the hydrosilylation of aldehydes and ketones at 65-75 °C. Comparatively, the PBP complexes are more active than the corresponding POCOP complexes. Both phenylsilane and polymethylhydrosiloxane (PMHS) can be used as silyl reagents. The expected alcohols were obtained in good to excellent yields after the basic hydrolysis of the hydrosilylation products and many functional groups were not affected. With turnover frequencies (TOFs) of up to 67 000 h^-1, the present catalytic system represents the most effective platinum catalytic system for the hydrosilylation of carbonyl compounds. The reactions were likely catalysed by the in situ generated platinum hydride species.

The Stability of Diphosphino-Boryl PBP Pincer Backbone: PBP to POP Ligand Hydrolysis

Since moisture may frequently be present in many solvents, it is important to know the reactivity of a catalyst against water for catalytic reactions. In order to explore the stability and understand the transformation process of diphosphino-boryl-based PBP pincer platform, [PdCl{B(NCH₂ P^t Bu₂ )₂ -o-C₆ H₄ }] (1) was treated with PdCl₂ , HB(NCH₂ PPh₂ )₂ -o-C₆ H₄ was reacted with [PdCl₂ (cod)] (cod=cyclo-octa-1,5-diene) and [Pd₂ (dba)₃ ] (dba=dibenzylideneacetone), respectively, in the presence of water. Some novel palladium POP complexes, [Pd₂ Cl₂ (μ-Cl){μ-κ³ -P,O,P-OB(NCH₂ P^t Bu₂ )₂ -o-C₆ H₄ }] (2 a), [Pd₄ (μ-Cl)₂ (μ-O)₂ {μ-κ³ -P,O,P-OB(NCH₂ PPh₂ )₂ -o-C₆ H₄ }₂ ] (2 b), [Pd₂ {μ-κ⁴ -P,P,P,P-O(B(NCH₂ PPh₂ )₂ -o-C₆ H₄ )₂ }{μ-κ² -P,P-(NHCH₂ PPh₂ )₂ -o-C₆ H₄ }] (3), were obtained. It was found that the PBP pincer backbone can easily be converted into a POP backbone in the presence of water. From the crystal structures of the resultant palladium complexes, possible pincer backbone transformation pathways were discussed.

POCOP-type cobalt and nickel pincer complexes bearing an appended phosphinite group

The reaction of 1,3,5-(iPr2PO)3C6H3 with Co2(CO)8 leads to the isolation of a POCOP-type mononuclear pincer complex {κP,κC,κP-2,4,6-(iPr2PO)3C6H2}Co(CO)2 (1) or a tetranuclear species {κP-{κP,κC,κP-2,4,6-(iPr2PO)3C6H2}Co(CO)2}2Co2(CO)6 (2), depending on the ligand to cobalt ratio employed. The latter compound can be an impurity during the synthesis of {2,6-(iPr2PO)2-4-Me2N-C6H2}Co(CO)2, when the ligand precursor 5-(dimethylamino)resorcinol is contaminated with phloroglucinol due to incomplete monoamination. Similarly, the reaction of 1,3,5-(iPr2PO)3C6H3 with NiCl2 in the presence of 4-dimethylaminopyridine provides {κP,κC,κP-2,4,6-(iPr2PO)3C6H2}NiCl (3) bearing an appended phosphinite group. Structures 1–3 have been studied by X-ray crystallography.

Selective metalation of phenol-type proligands for preparative organometallic chemistry

The selective C-metalation of phenol ester derived proligands is a readily applicable addition to state-of-the-art protocols toward cyclometalated structures, in particular of the base metals.

Palladium(ii) complexes supported by PBP and POCOP pincer ligands: a comparison of their structure, properties and catalytic activity

A Pd(ii) chloride complex supported by a Yamashita-Nozaki PBP pincer ligand, [C6H4-1,2-(NCH2PtBu2)2B]PdCl (1a), was synthesized. The structure, properties and catalytic activity of complex 1a were compared with those of the corresponding POCOP pincer complex [C6H3-2,6-(OPtBu2)2]PdCl (2a). It was found that the Pd centre in complex 1a is more electron rich and easier to be oxidized than that in complex 2a; complex 1a is a much better catalyst for Suzuki-Miyaura cross-coupling reactions than complex 2a. Starting from complexes 1a and 2a, two series of Pd(ii) pincer complexes bearing a SH, BH4, N[combining low line]CS, N[combining low line]CSe or N3 covalent ligand, [C6H4-1,2-(NCH2PtBu2)2B]PdY (Y = SH, 1b; BH4; 1c; N[combining low line]CS, 1d; N[combining low line]CSe, 1e; and N3, 1f) and [C6H3-2,6-(OPtBu2)2]PdY (Y = SH, 2b; BH4, 2c; N[combining low line]CS, 2d; N[combining low line]CSe, 2e; and N3, 2f), were synthesized and fully characterized. Single crystal X-ray diffraction analysis indicated that the Pd centre is less tightly chelated in PBP pincer complexes. The strong σ-donor ability of the PBP pincer ligand has little influence on the structure of the covalent ligand possessing both σ-donor and π-acceptor properties. However, the stretching vibrational frequencies of N[combining low line]CS, N[combining low line]CSe and N3 ligands and the coordination mode of the BH4 ligand are significantly different in these two types of palladium pincer complexes.

The stability of group 10 metal POCOP pincer complexes: decomposition/reconstruction pathways of the pincer backbone

Organometallic chemists usually like to use the word robust to describe pincer ligand frameworks in metal pincer complexes. Although most transition metal pincer complexes are thermally stable, the pincer backbone frameworks can still decompose under certain circumstances. In order to explore the stability of the bis(phosphinite) (POCOP) pincer backbone in transition metal pincer complexes, group 10 metal POCOP pincer complexes were exposed to different nucleophilic and electrophilic conditions, respectively. It was found that the POCOP pincer backbone is stable under intermolecular nucleophilic conditions but cannot survive intramolecular nucleophilic attack; the POCOP pincer backbone is also stable under weak electrophilic conditions but the backbone can be completely destroyed by strong Lewis acids such as AlCl3. Possible decomposition/reconstruction pathways of the POCOP pincer ligand framework were proposed.

Unsymmetrical NCN-pincer mononuclear and dinuclear nickel(ii) complexes of N-heterocyclic carbene (NHC): synthesis, structure and catalysis for Suzuki-Miyaura cross-coupling

This report describes the synthesis and characterization of a family of unsymmetrical NCN pincer Ni(ii) complexes 2-8 with NHC-triazole arms. All of these complexes have been fully characterized by X-ray single crystal analysis, NMR spectroscopy, and elemental analysis. Complexes 2 and 4-6 were square planar with a chloride trans to the carbene carbon atoms. Complex 3 was a paramagnetic octahedral complex with its central metal surrounded by two NCN pincer ligands. Complexes 7 and 8 contain [(NHC)₂Ni₂-OH] moieties bearing a OH bridge. Both the [(NHC)₂Ni₂-OH] complexes 7 and 8 and [(NC_NHCN)Ni-Cl] complexes 2 and 4-6 were synthesized similarly via the reactions of the in situ formed Ag-NHCs from the corresponding imidazolium salts with [NiCl₂(PPh₃)₂]. The catalytic activities of all complexes for Suzuki-Miyaura cross-coupling were examined. Under the optimized conditions, complex 4 was active in the Suzuki-Miyaura cross-coupling reactions of aryl iodides and aryl bromides at 110 °C. Aryl chlorides were successfully coupled in the presence of triphenylphosphine as an additive.

Synthesis of quinolinyl-based pincer copper(ii) complexes: an efficient catalyst system for Kumada coupling of alkyl chlorides and bromides with alkyl Grignard reagents

Well-defined quinolinamide-based pincer copper complexes have been developed and demonstrated in the Kumada coupling reaction of nonactivated alkyl chlorides and bromides with alkyl magnesium chloride.

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.

From NAD+ to Nickel Pincer Complex: A Significant Cofactor Evolution Presented by Lactate Racemase

Lactate racemase (LarA), a new nickel enzyme discovered recently, catalyzes the racemization between d- and l-lactates with a novel nickel pincer cofactor (Ni-PTTMN) derived from nicotinic acid. In this study, by using DFT and a 200-atom active-site model, LarA is revealed to employ a modified proton-coupled hydride-transfer mechanism in which a hydride is transferred to a cofactor pyridine carbon from the substrate α-carbon along with proton transfer from the substrate hydroxy group to a histidine, and then moved back from the opposite side. Tyr294 and Lys298 provide significant acceleration effects by orientating substrates and stabilizing the negative charge developing at the substrate hydroxy oxygen. The barrier was determined to be 12.0 kcal mol^-1 , which reveals enhanced racemase activity relative to the LarA reaction using NAD⁺ -like cofactors. Compared with NAD⁺ , Ni-PTTMN has a stronger hydride-addition reactivity in moderate and high environmental polarity and may fit perfectly the moderately polar active site of LarA.

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.

Oxidative C-C Bond Formation Reactivity of Organometallic Ni(II), Ni(III), and Ni(IV) Complexes

The use of the tridentate ligand 1,4,7-trimethyl-1,4,7-triazacyclononane (Me₃tacn) and the cyclic alkyl/aryl C-donor ligand -CH₂CMe₂-o-C₆H₄- (cycloneophyl) allows for the synthesis of isolable organometallic Ni^II, Ni^III, and Ni^IV complexes. Surprisingly, the five-coordinate Ni^III complex is stable both in solution and the solid state, and exhibits limited C-C bond formation reactivity. Oxidation by one electron of this Ni^III species generates a six-coordinate Ni^IV complex, with an acetonitrile molecule bound to Ni. Interestingly, illumination of the Ni^IV complex with blue LEDs results in rapid formation of the cyclic C-C product at room temperature. This reactivity has important implications for the recently developed dual Ni/photoredox catalytic systems proposed to involve high-valent organometallic Ni intermediates. Additional reactivity studies show the corresponding Ni^II species undergoes oxidative addition with alkyl halides, as well as rapid oxidation by O₂, to generate detectable Ni^III and/or Ni^IV intermediates and followed by C-C bond formation.

Ligand effects on the properties of Ni(iii) complexes: aerobically-induced aromatic cyanation at room temperature

Ligand effect studies on (^RN3C)Ni^IIIL₂ complexes reveal aerobically-induced aromatic cyanation with tBuNC at room temperature for the (^NpN3C)Ni system.

METALLOPROTEINS. A tethered niacin-derived pincer complex with a nickel-carbon bond in lactate racemase

Lactic acid racemization is involved in lactate metabolism and cell wall assembly of many microorganisms. Lactate racemase (Lar) requires nickel, but the nickel-binding site and the role of three accessory proteins required for its activation remain enigmatic. We combined mass spectrometry and x-ray crystallography to show that Lar from Lactobacillus plantarum possesses an organometallic nickel-containing prosthetic group. A nicotinic acid mononucleotide derivative is tethered to Lys(184) and forms a tridentate pincer complex that coordinates nickel through one metal-carbon and two metal-sulfur bonds, with His(200) as another ligand. Although similar complexes have been previously synthesized, there was no prior evidence for the existence of pincer cofactors in enzymes. The wide distribution of the accessory proteins without Lar suggests that it may play a role in other enzymes.

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.

From bis(silylene) and bis(germylene) pincer-type nickel(II) complexes to isolable intermediates of the nickel-catalyzed Sonogashira cross-coupling reaction

The first [ECE]Ni(II) pincer complexes with E = Si(II) and E = Ge(II) metallylene donor arms were synthesized via C-X (X = H, Br) oxidative addition, starting from the corresponding [EC(X)E] ligands. These novel complexes were fully characterized (NMR, MS, and XRD) and used as catalyst for Ni-catalyzed Sonogashira reactions. These catalysts allowed detailed information on the elementary steps of this catalytic reaction (transmetalation → oxidative addition → reductive elimination), resulting in the isolation and characterization of an unexpected intermediate in the transmetalation step. This complex, {[ECE]Ni acetylide → CuBr} contains both nickel and copper, with the copper bound to the alkyne π-system. Consistent with these unusual structural features, DFT calculations of the {[ECE]Ni acetylide → CuBr} intermediates revealed an unusual E-Cu-Ni three-center-two-electron bonding scheme. The results reveal a general reaction mechanism for the Ni-based Sonogashira coupling and broaden the application of metallylenes as strong σ-donor ligands for catalytic transformations.