On the Stability of a POCsp3OP-Type Pincer Ligand in Nickel(II) Complexes

C(sp)-H, S-H, and Sn-H Bond Activation with a Cobalt(I) Pincer Complex

This study focuses on the stoichiometric reactions of {2,6-(iPr2PO)2C6H3}Co(PMe3)2 with terminal alkynes, thiols, and tin hydrides as part of an effort to develop catalytic, two-electron processes with cobalt. This specific Co(I) pincer complex proves to be effective for cleaving the C(sp)-H, S-H, and Sn-H bonds to give oxidative addition products with the general formula {2,6-(iPr2PO)2C6H3}CoHX(PMe3) (X = alkynyl, thiolate, and stannyl groups) along with the free PMe3. These reactions typically reach completion when the substituents on acetylene, sulfur, and tin are electron-withdrawing groups (e.g., phenyl, pyridyl, and alkenyl groups). In contrast, alkyl-substituted acetylenes, 1-pentanethiol, and tributyltin hydride are partially converted due to the equilibria with the corresponding oxidative addition products. The Co(I) pincer complex is not a hydrothiolation catalyst but capable of catalyzing the hydrostannation of terminal alkynes with Ph3SnH to produce β-(Z)-alkenylstannanes selectively.

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.

Synthesis and catalytic performance of nickel phosphinite pincer complexes in deoxygenative hydroboration of amides

A series of imino-POCN^R, amino-POCN^R2, and bis(phosphinite) POCOP pincer complexes of Ni(II) were prepared and tested in catalytic deoxygenative hydroboration of amides with HBPin to the corresponding amines. In contrast to the deoxygenative hydrosilylation approach, primarily developed for tertiary amides, superior reactivity in Ni-catalyzed deoxygenative hydroboration was demonstrated for secondary carboxamides. The bis(phosphinite) hydride complex (POCOP)NiH proved the most active in these reactions, tolerating potentially reducible functionalities such as internal alkenes, esters, nitriles, heteroaromatic compounds, and tertiary amides. Preferable hydroboration of secondary amides was also demonstrated in the presence of primary amide functionalities. The reactions were conducted at 60-80 °C, representing a rare example of a base-metal catalytic system for selective deoxygenation of secondary amides to the corresponding amines under mild conditions. In contrast to secondary amides, deoxygenative hydroboration of primary amides was demonstrated using an iminophosphinite pre-catalyst (POCN^Dmp)Ni(CH₂TMS) (Dmp = 2,6-Me₂C₆H₃). Deoxygenation reactions were suggested to proceed via a direct C-O bond cleavage mechanism, which is triggered by dehydrogenative N-borylation to access more electrophilic N-borylamides amenable to the addition of HBPin to the carbonyl group.

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.

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.

Roles of Hydrogen Bonding in Proton Transfer to κP,κN,κP-N(CH2CH2P i Pr2)2-Ligated Nickel Pincer Complexes

The nickel PNP pincer complex ( ^{i Pr}PNP)NiPh ( ^{i Pr}PNP = κ^P,κ^N,κ^P-N(CH₂CH₂P ⁱ Pr₂)₂) was prepared by reacting ( ^{i Pr}PNP)NiBr with PhMgCl or deprotonating [( ^{i Pr}PN^HP)NiPh]Y ( ^{i Pr}PN^HP = κ^P,κ^N,κ^P-HN(CH₂CH₂P ⁱ Pr₂)₂; Y = Br, PF₆) with KO ^t Bu. The byproducts of the PhMgCl reaction were identified as [( ^{i Pr}PN^HP)NiPh]Br and ( ^{i Pr}PNP')NiPh ( ^{i Pr}PNP' = κ^P,κ^N,κ^P-N(CH=CHP ⁱ Pr₂)(CH₂CH₂P ⁱ Pr₂)). The methyl analog ( ^{i Pr}PNP)NiMe was synthesized from the reaction of ( ^{i Pr}PNP)NiBr with MeLi, although it was contaminated with ( ^{i Pr}PNP')NiMe due to ligand oxidation. Protonation of ( ^{i Pr}PNP)NiX (X = Br, Ph, Me) with various acids, such as HCl, water, and MeOH, was studied in C₆D₆. Nitrogen protonation was shown to be the most favorable process, producing a cationic species [( ^{i Pr}PN^HP)NiX]⁺ with the NH moiety hydrogen-bonded to the conjugate base (i.e., Cl^-, HO^-, or MeO^-). Protonation of the Ni-C bond was observed at room temperature with ( ^{i Pr}PNP)NiMe, whereas at 70 °C with ( ^{i Pr}PNP)NiPh, both resulting in [( ^{i Pr}PN^HP)NiCl]Cl as the final product. Protonation of ( ^{i Pr}PNP)NiBr was complicated by site exchange between Br^- and the conjugate base and by the degradation of the pincer complexes. Indene, which lacks hydrogen-bonding capability, was unable to protonate ( ^{i Pr}PNP)NiPh and ( ^{i Pr}PNP)NiMe, despite being more acidic than water and MeOH. Neutral and cationic nickel pincer complexes involved in this study, including ( ^{i Pr}PNP')NiBr, ( ^{i Pr}PNP)NiPh, ( ^{i Pr}PNP')NiPh, ( ^{i Pr}PNP)NiMe, [( ^{i Pr}PN^HP)NiPh]Y (Y = Br, PF₆, BPh₄), [( ^{i Pr}PN^HP)NiPh]₂[NiCl₄], [( ^{i Pr}PN^HP)NiMe]Y (Y = Cl, Br, BPh₄), [( ^{i Pr}PN^HP)NiBr]Br, and [( ^{i Pr}PN^HP)NiCl]Cl, were characterized by X-ray crystallography.

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.

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.

The synthesis and efficient one-pot catalytic “self-breeding” of asymmetrical NC(sp3)E-hybridised pincer complexes

Asymmetric NC(sp³)E (E = S, O) pincer complexes were examined as catalysts in asymmetric hydrophosphination.

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.

Small molecule activation by POC(sp3)OP-nickel complexes

This contribution describes the reactivities of CO2 , CO, O2 , and ArNC with the pincer-type complexes [(κ(P) ,κ(C) ,κ(P') -POC sp 3OP)NiX] (POC sp 3OP=(R2 POCH2 )2 CH; R=iPr; X=OSiMe3 , NArH; Ar=2,6-iPr2 C6 H3 ). Reaction of the amido derivative with CO2 and CO leads to a simple insertion into the NiN bond to give stable carbamate and carbamoyl derivatives, respectively, the pincer ligand backbone remaining intact in both cases. In contrast, the analogous reactions with the siloxide derivative produced kinetically labile insertion products that either revert to the starting material (in the case of CO2 ) or react further to give the mixed-valent, dinickel species [(POC sp 3OP)Ni(II) {μ,κ(O) ,κ(P) ,κ(P') -OCOCH(CH2 CH2 OPR2 )2 }Ni(0) (CO)2 ]. The zero-valent center in the latter compound is ligated by a new ligand arising from transformation of the POC sp 3OP ligand backbone. The carbonylation and carboxylation of the siloxido derivative also produced minor quantities of a side-product identified as the trinickel species, [{(η(3) -allyl)Ni(μ(O) ,κ(P) -R2 PO)2 }2 Ni], arising from total dismantling of the POC sp 3OP ligand. Similar reactivities were observed with isonitrile, ArNC: reaction with the siloxido derivative resulted in a complex sequence of steps involving initial insertion, a 1,3-hydrogen shift, and an Arbuzov rearrangement to give [Ni(CNAr)4 ] and a methacrylamide based on fragments of the POC sp 3OP ligand. Oxygenation of the amido and siloxido derivatives led to the phosphinate derivative, [(POC sp 3OP)Ni(OP(O)R2 )], arising from oxidative transformation of the original ligand frame; the reaction with the Ni-NHAr derivative also gave ArHNP(O)R2 through a complex NP bond-forming reaction.

Synthesis and characterization of a family of POCOP pincer complexes with nickel: reactivity towards CO2 and phenylacetylene

A cyclohexyl-based POC sp 3OP pincer ligand (POC sp 3OP=cis-1,3-bis(di-tert-butylphosphinito)cyclohexyl) cyclometalates with nickel to generate a series of new POC sp 3OP-supported Ni(II) complexes, including the halide, hydride, methyl, and phenyl species. trans-[NiCl{cis-1,3-bis(di-tert-butylphosphinito)cyclohexane}], [(POC sp 3OP)NiCl] (1 a) and the analogous bromide complex (1 b) were synthesized and fully characterized by NMR spectroscopy and X-ray crystallography. Cyclic voltammetry measurements of 1 a and 1 b alongside their bis(phosphine) analogues [(PC sp 3P)NiCl] (2 a) and [(PC sp 3P)NiCl] (2 a) (PC sp 3P=cis-1,3-bis(di-tert-butylphosphino)cyclohexyl) indicate a reduced electron density at the metal center upon introducing electron-withdrawing oxygen atoms in the pincer arms. The methyl [(POC sp 3OP)NiMe] (3) and phenyl [(POC sp 3OP)NiPh] (4) complexes were formed from 1 a by reaction with the corresponding organolithium reagents. 1 a also reacts with LiAlH4 to give the hydride complex [(POC sp 3OP)NiH] (5). The methyl complex 3 reacts with phenyl acetylene to give the acetylide complex [(POC sp 3OP)NiCCPh] (6). The reactivity of compounds 3-5 towards CO2 was studied. The hydride complex 5 and the methyl complex 3 both underwent CO2 insertion to form the formate species [(POC sp 3OP)NiOCOH] (7) and acetate species [(POC sp 3OP)NiOCOCH3 ] (8), respectively, although with a higher barrier of insertion in the latter case. Compound 4 was unreactive towards CO2 even at elevated temperatures. Complexes 3-8 were all characterized by NMR spectroscopy and X-ray crystallography.