Metal complexes of Co, Ni and Cu with the pincer ligand HN(CH2CH2PiPr2)2: preparation, characterization and electrochemistry

Systematic Variation of 3d Metal Centers in a Redox-Innocent Ligand Environment: Structures, Electrochemical Properties, and Carbon Dioxide Activation

Coordination compounds of earth-abundant 3d transition metals are among the most effective catalysts for the electrochemical reduction of carbon dioxide (CO₂). While the properties of the metal center are crucial for the ability of the complexes to electrochemically activate CO₂, systematic variations of the metal within an identical, redox-innocent ligand backbone remain insufficiently investigated. Here, we report on the synthesis, structural and spectroscopic characterization, and electrochemical investigation of a series of 3d transition-metal complexes [M = Mn(I), Fe(II), Co(II), Ni(II), Cu(I), and Zn(II)] coordinated by a new redox-innocent PNP pincer ligand system. Only the Fe, Co, and Ni complexes reveal distinct metal-centered electrochemical reductions from M(II) down to M(0) and show indications for interaction with CO₂ in their reduced states. The Ni(0) d¹⁰ species associates with CO₂ to form a putative Aresta-type Ni-η²-CO₂ complex, where electron transfer to CO₂ through back-bonding is insufficient to enable electrocatalytic activity. By contrast, the Co(0) d⁹ intermediate binding CO₂ can undergo additional electron uptake into a formal cobalt(I) metallacarboxylate complex able to promote turnover. Our data, together with the few literature precedents, single out that an unsaturated coordination sphere (coordination number = 4 or 5) and a d⁷-to-d⁹ configuration in the reduced low oxidation state (+I or 0) are characteristics that foster electrochemical CO₂ activation for complexes based on redox-innocent ligands.

A series of 3d transition-metal complexes (M = Mn, Fe, Co, Ni, Cu, and Zn) coordinated by a new redox-innocent PNP pincer ligand system were synthesized and structurally as well as electrochemically analyzed to illuminate the role of the metal center in molecular electrochemical carbon dioxide (CO₂) activation.

Hydrogenation reactions catalyzed by HN(CH2CH2PR2)2-ligated copper complexes

Hydrogenation of aldehydes and ketones can be catalyzed by a PNP-ligated copper hydride that is accessible from the copper borohydride or bromide complex or the copper hydride cluster.

From cyclic amines and acetonitrile to amidine zinc(ii) complexes

A seemingly simple combination of [Zn(quin)₂(H₂O)] (quin⁻ = quinaldinate) and a selected secondary cyclic amine, piperidine (pipe), pyrrolidine (pyro) or morpholine (morph), afforded in acetonitrile a number of products: anionic homoleptic quinaldinate, neutral heteroleptic quinaldinate/amine and quinaldinate/amidine complexes. The piperidine and pyrrolidine systems underwent reaction with acetonitrile to give amidines. The in situ formed piperidinoacetamidine (pipeam) or pyrrolidinoacetamidine (pyroam) coordinated to zinc(ii). Reactions with piperidine led to trans-[Zn(quin)₂(pipe)₂]·2CH₃CN (1), [Zn(quin)₂(pipe)]·cis-[Zn(quin)₂(pipe)₂] (2), pipeH[Zn(quin)₃]·CH₃CN (3), [Zn(quin)₂(pipeam)]·CH₃CN (4a), [Zn(quin)₂(pipeam)]·2CHCl₃ (4b), pipeamH[Zn(quin)₃] (5) and pipeamH[Zn(quin)₂(CH₃COO)]·acetamide (6) (pipeH⁺ and pipeamH⁺ denote protonated amine or amidine). By analogy, [Zn(quin)₂(pyro)₂] (7), pyroH[Zn(quin)₃]·CH₃CN (8), pyroH[Zn(quin)₂Cl] (9), [Zn(quin)₂(pyroam)]·CH₃CN·0.5pyroam·0.5H₂O (10a), [Zn(quin)₂(pyroam)]·2CHCl₃ (10b), [Zn(quin)₂(pyroam)]·CH₂Cl₂ (10c) and pyroamH[Zn(quin)₃] (11) were obtained in the pyrrolidine reactions. The morpholine system allowed isolation of only two novel products, trans-[Zn(quin)₂(morph)₂] (12) and morphH[Zn(quin)₃]·CH₃CN (13). Importantly, no amidine could be isolated. Instead, in autoclaves at 105 °C morpholine degraded to ammonia, as confirmed by mass spectrometry of the gas phase. pyroamH[Zn(quin)₃] exists in two polymorphs which differ in the binding modes of quinaldinate ligands. In 11triclinic, the metal ion of [Zn(quin)₃]⁻ features a five-coordinate environment, whereas that in 11monoclinic is surrounded by six donors. Stabilities of the [Zn(quin)₃]⁻ isomers were assessed with DFT calculations. The one with a six-coordinate zinc(ii) ion was found to be more stable than its five-coordinate counterpart. Favorable intermolecular interactions in the solid state stabilize both and reduce the energy difference between them. The calculations show the conversion of the five-coordinate [Zn(quin)₃]⁻ into its coordinatively saturated isomer to be an almost barrierless process.

Piperidine and pyrrolidine, both belonging to a group of secondary cyclic amines, reacted with acetonitrile in the presence of zinc(ii) to give the corresponding amidines that coordinated to the metal through their imine nitrogen.

Carbodiphosphorane-based nickel pincer complexes and their (de)protonated analogues: dimerisation, ligand tautomers and proton affinities

The reactivity patterns of carbodiphosphoranes (CDPs) as ligands are much less explored than those of isoelectronic analogues. In the current manuscript, we investigate the reactivity of the carbodiphosphorane-based PCP nickel(ii) pincer complex [({dppm}2C)NiCl]Cl (1) towards acids and bases, calculate proton affinities, analyse the bonding situation and tautomeric forms with the aim to evaluate whether CDPs can potentially act as cooperative ligands in catalysis (dppm = 1,1-bis(diphenylphosphino)methane). Our investigations show that different tautomeric forms are stable for the coordinated and the uncoordinated ligand. The protonated CDP-based complex 2 represents a rare example of a cationic donor group binding to a cationic metal centre. The continuous arm-deprotonation of 1 leads to the formation of remarkably stable dimers with Ni-C-P-C-metallacycles. In comparison to corresponding boron and amine-based ligands, the coordinated CDP-group exhibits the lowest proton affinity according to DFT calculations, indicating that coordinated CDP ligands can potentially serve as proton relay in cooperative catalysis.

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.

Diamidophosphines with six-membered chelates and their coordination chemistry with group 4 metals: development of a trimethylene-methane-tethered [PN2]-type "molecular claw"

The coordination chemistry of the phosphine-tethered diamidophosphine ligands PhP(CH2CH2CH2NHPh)2 (pr[NPN]H2) and PhP(1,2-CH2-C6H4-NHSiMe3)2 (bn[NPN]H2) featuring six-membered N–C3–P chelates was explored with group 4 metals, which allowed for the consecutive development of a new trimethylene-methane-tethered [PN2] scaffold. In the case of the propylene-linked system pr[NPN]H2, access to the sparingly soluble dibenzyl derivative pr[NPN]ZrBn2 (3-Zr) was gained, while thermally sensitive zirconium and hafnium diiodo complexes bn[NPN]MI2 (5-M, M = Zr, Hf) were isolated in the case of the benzylene-linked derivative bn[NPN]H2. Despite the related phosphine-tethered backbone architectures of both of these ligands, their group 4 complexes were found to exhibit either C1-symmetric (bn[NPN]MX2) or averaged CS-symmetric (pr[NPN]MX2) structures in solution. To restrain the overall flexibility of these systems and thereby control the properties of the resulting complexes without disrupting the six-membered chelates, the new trimethylene-methane-tethered N,N′-di-(tert-butyl)-substituted [PN2]H2 protioligand was designed. This tripodal ligand system was prepared on a gram scale and its CS-symmetric dichloro complexes [PN2]MCl2 (6-M, M = Ti, Zr, Hf) were isolated subsequently. The benzene-soluble dibenzyl derivative [PN2]ZrBn2 (7-Zr) was synthesised as well and characterised by X-ray diffraction. These results are discussed not only in conjunction with the known [NPN]-coordinated group 4 complexes incorporating five-membered chelates, but also in the context of “molecular claws” that are related to the new [PN2] tripod.

A phosphomide based PNP ligand, 2,6-{Ph2PC(O)}2(C5H3N), showing PP, PNP and PNO coordination modes

Coordination chemistry of a versatile phosphomide ligand, 2,6-{Ph₂PC(O)}₂(C₅H₃N), is described.

Selective conversion of glycerol to lactic acid with iron pincer precatalysts

A family of iron complexes of PNP pincer ligands are active catalysts for the conversion of glycerol to lactic acid with high activity and selectivity.

Models for B12-conjugated radiopharmaceuticals. Cobaloxime binding to new fac-[Re(CO)3(Me2bipyridine)(amidine)]BF4 complexes having an exposed pyridyl nitrogen

New mononuclear amidine complexes, fac-[Re(CO)3(Me2bipy)(HNC(CH3)(pyppz))]BF4 [(4,4'-Me2bipy (1), 5,5'-Me2bipy (2), and 6,6'-Me2bipy (3)] (bipy = 2,2'-bipyridine), were synthesized by treating the parent fac-[Re(I)(CO)3(Me2bipy)(CH3CN)]BF4 complex with the C2-symmetrical amine 1-(4-pyridyl)piperazine (pyppzH). The axial amidine ligand has an exposed, highly basic pyridyl nitrogen. The reaction of complexes 1-3 with a B12 model, (py)Co(DH)2Cl (DH = monoanion of dimethylglyoxime), in CH2Cl2 yielded the respective dinuclear complexes, namely, fac-[Re(CO)3(Me2bipy)(μ-(HNC(CH3)(pyppz)))Co(DH)2Cl]BF4 [(4,4'-Me2bipy (4), 5,5'-Me2bipy (5), and 6,6'-Me2bipy (6)]. (1)H NMR spectroscopic analysis of all compounds and single-crystal X-ray crystallographic data for 2, 3, 5, and 6 established that the amidine had only the E configuration in both the solid and solution states and that the pyridyl group is bound to Co in 4-6. Comparison of the NMR spectra of 1-3 with spectra of 4-6 reveals an unusually large "wrong-way" upfield shift for the pyridyl H2/6 signal for 4-6. The wrong-way H2/6 shift of (4-Xpy)Co(DH)2Cl (4-Xpy = 4-substituted pyridine) complexes increased with increasing basicity of the 4-Xpy derivative, a finding attributed to the influence of the magnetic anisotropy of the cobalt center on the shifts of the (1)H NMR signals of the pyridyl protons closest to Co. Our method of employing a coordinate bond for conjugating the fac-[Re(I)(CO)3] core to a vitamin B12 model could be extended to natural B12 derivatives. Because B12 compounds are known to accumulate in cancer cells, such an approach is a very attractive method for the development of (99m)Tc and (186/188)Re radiopharmaceuticals for targeted tumor imaging and therapy.

Chemical implications of incompatible ligand versus metal coordination geometry preferences

Binding an electron deficient pincer ligand which strongly dictates planar, mer stereochemistry, to a metal which prefers tetrahedral structure, e.g., d(10) CuCl, is explored for possible intramolecular redox chemistry. Experiment shows that the pincer ligand 2,2'-bis-tetrazinyl pyridine, btzp, forms a complex (btzp)CuCl which is a chloride-bridged polymer in the solid state, hence with 20 valence electrons around copper. DFT calculations show that even the monomer has nonplanar copper with the tetrazinyl nitrogen lone pairs somewhat misdirected away from copper, with long Cu/N bonds, in a singlet ground state; 13.9 kcal/mol less stable is a triplet, whose electronic structure shows one electron from the ground state Cu(I) has been transferred to a pincer π* orbital. Outer sphere electron transfer to (btzp)CuCl yields (btzp)Cu where the added electron has gone into the pincer, to leave a ligand-centered radical, characterized by EPR, chemical reactivity, and X-ray photoelectron spectroscopy.

Synthesis and characterization of coordinatively unsaturated nickel(II) and manganese(II) alkyl complexes supported by the hydrotris(3-phenyl-5-methylpyrazolyl)borate (Tp(Ph,Me)) ligand

The synthesis of several nickel(II) and manganese(II) alkyl complexes supported by hydrotris(3-phenyl-5-methylpyrazolyl)borate (Tp(Ph,Me)) ligand is reported. The metal halide complexes Tp(Ph,Me)MnCl(CH3CN) (1) and Tp(Ph,Me)NiCl (4) were used as precursors for synthesis of Tp(Ph,Me)MnCH2Si(Me)3 (2), Tp(Ph,Me)MnCH2Ph (3), Tp(Ph,Me)NiCH2Si(Me)3 (5) and Tp(Ph,Me)NiCH2Ph (6). The resulting Mn(II) and Ni(II) alkyl complexes, 2-3 and 5-6, were characterized by X-ray crystallography, NMR spectroscopy, and FT-IR spectroscopy. X-ray crystallographic analysis revealed distorted tetrahedral geometries for complexes 2-3 and 5 with a κ(3)-Tp(Ph,Me). Complex 6, on the other hand, was found to have a distorted square planar geometry with κ(2)-Tp(Ph,Me) and an η(3)-benzyl ligand. Transformations of 4 and Tp(Ph,Me)CoCl (10) via treatment with NaN3 to yield Tp(Ph,Me)NiN3 (11), Tp(Ph,Me)CoN3 (12), along with the synthesis of (Tp(Ph,Me))2Ni (8) and Tp(Ph,Me)NiCl(3-Ph-5MepzH) (9) are also reported.

Nickel(II) complexes containing a pyrrole-diphosphine pincer ligand

A new pincer ligand, (P(2)(Ph)Pyr)(-), based on the anion of 2,5-bis[(diphenylphosphino)methyl]pyrrole has been prepared in four steps from pyrrole. The ligand undergoes oxidation to diphosphine oxide under ambient conditions and was therefore isolated as its borane adduct, H(P(2)(Ph)Pyr)·2BH(3) (2). Delivery of the ligand to nickel(II) was accomplished by the direct reaction of NiCl(2) with 2 in the presence of Et(2)NH to afford [NiCl(P(2)(Ph)Pyr)]. Salt metathesis reactions of the chloro complex afford new compounds including [Ni(CH(3))(P(2)(Ph)Pyr)] and [Ni(NCCH(3))(P(2)(Ph)Pyr)](OTf). In all cases, the ligand gives rise to diamagnetic square-planar complexes, which have been fully characterized in solution and the solid state. All complexes examined display an irreversible oxidation to nickel(III) according to cyclic voltammetry. Reduction of the chloro complex in dichloromethane results in an electrocatalytic process, whereas reduction in tetrahydrofuran leads to the irreversible formation of a nickel(I) species.