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.

Notable Catalytic Activity of Transition Metal Thiolate Complexes against Hydrosilylation and Hydroboration of Carbon-Heteroatom Bonds

Chemists tend to use transition metal hydride complexes rather than thiolate complexes to catalyse chemical transformations because the hydride complexes possess diverse catalytic reactivity, although most of them are air/moisture-sensitive and difficult to prepare. By comparing the catalytic performances of pincer ligated group 10 metal thiolate and hydride complexes in catalysing the hydroboration and hydrosilylation of C=O and C=N bonds, we demonstrate in this review that transition metal thiolate complexes are much better catalysts than the corresponding hydride complexes in catalysing this type of reactions. Many hydroboration and hydrosilylation reactions catalysed by pincer ligated group 10 metal hydride complexes can also be catalysed by the corresponding thiolate complexes and the thiolate systems are far more active. Therefore, the applications of transition metal thiolate complexes in the catalytic hydroboration and hydrosilylation of unsaturated carbon-heteroatom bonds deserve special attention in future work.

Coordination mode and stability of the tetrahydroborate ligand in group 10 metal pincer complexes

The coordination mode of the BH₄^- ligand in transition metal tetrahydroborate complexes is mainly dominated by the nature of the metal centres. However, other factors can also play important roles sometimes. In order to rationalize the coordination modes and the stability of the BH₄^- ligand in group 10 metal tetrahydroborate pincer complexes, [2,6-(^tBu₂PO)₂C₆H₃]Pt(η¹-HBH₃) and [C₆H₄-o-(NCH₂P^tBu₂)₂B]M(η²-H₂BH₂) (M = Ni, Pt) were prepared and characterized. A structural comparison of [2,6-(^tBu₂PCH₂)₂C₆H₃]Ni(BH₄), [2,6-(^tBu₂PO)₂C₆H₃]M(BH₄) and [C₆H₄-o-(NCH₂P^tBu₂)₂B]M(BH₄) (M = Ni, Pd, and Pt) indicates that the M-P bond length, the P-M-P bite angle and the trans-influence of the central atom in the pincer platform also affect the coordination mode of the BH₄^- ligand. The nickel complexes tend to adopt a monodentate coordination mode while the palladium and platinum complexes can adopt either the monodentate or the bidentate mode depending on the structural features of the pincer platforms. Longer M-P bonds and smaller P-M-P bite angles favour the bidentate mode. The stability of the BH₄^- ligand is influenced by both the coordination mode and the nature of the metal centre. The BH₃ species is released more easily from complexes with less electron rich metal centres. Following the series of Ni, Pd, and Pt, complexes with the same pincer ligand more easily lose a BH₃ moiety.

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.

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.

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.

Hydrosulfide complexes of the transition elements: diverse roles in bioinorganic, cluster, coordination, and organometallic chemistry

Molecules containing transition metal hydrosulfide linkages are diverse, spanning a variety of elements, coordination environments, and redox states, and carrying out multiple roles across several fields of chemistry.

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.

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.