Molecular Dihydrogen Activation by (C5Me5)M/N (M=Rh, Ir) Transition Metal Frustrated Lewis Pairs: Reversible Proton Migration to, and Proton Abstraction from, the C5Me5 Ligand

The masked transition-metal frustrated Lewis pairs [Cp*M(κ3N,N',N''-L)][SbF6] (Cp*=η5-C5Me5; M=Ir, 1, Rh, 2; HL=pyridinyl-amidine ligand) reversibly activate H2 under mild conditions rendering the hydrido derivatives [Cp*MH(κ2N,N'-HL)][SbF6] observed as a mixture of the E and Z isomers at the amidine C=N bond (M=Ir, 3Z, 3E; M=Rh, 4Z, 4E). DFT calculations indicate that the formation of the E isomers follows a Grotthuss type mechanism in the presence of water. A mixture of Rh(I) isomers of formula [(Cp*H)Rh(κ2N,N'-HL)][SbF6] (5 a-d) is obtained by reductive elimination of Cp*H from 4. The formation of 5 a-d was elucidated by means of DFT calculations. Finally, when 2 reacts with D2, the Cp* and Cp*H ligands of the resulting rhodium complexes 4 and 5, respectively, are deuterated as a result of a reversible hydrogen abstraction from the Cp* ligand and D2 activation at rhodium.

Controlling Reactivity and Selectivity in the Mizoroki-Heck Reaction: High Throughput Evaluation of 1,5-Diaza-3,7-diphosphacyclooctane Ligands

We report a high throughput evaluation of the Mizoroki-Heck reaction of diverse olefin coupling partners. Comparison of different ligands revealed the 1,5-diaza-3,7-diphosphacyclooctane (P2N2) scaffold to be more broadly applicable than common "gold standard" ligands, demonstrating that this family of readily accessible diphosphines has unrecognized potential in organic synthesis. In particular, two structurally related P2N2 ligands were identified to enable the regiodivergent arylation of styrenes. By simply altering the phosphorus substituent from a phenyl to tert-butyl group, both the linear and branched Mizoroki-Heck products can be obtained in high regioisomeric ratios. Experimental and computational mechanistic studies were performed to further probe the origin of selectivity, which suggests that both ligands coordinate to the metal in a similar manner but that rigid positioning of the phosphorus substituent forces contact with the incoming olefin in a π-π interaction (for P-Ph ligands) or with steric clash (for P-tBu ligands), dictating the regiocontrol.

Molecular Catalysts with Diphosphine Ligands Containing Pendant Amines

Pendant amines play an invaluable role in chemical reactivity, especially for molecular catalysts based on earth-abundant metals. As inspired by [FeFe]-hydrogenases, which contain a pendant amine positioned for cooperative bifunctionality, synthetic catalysts have been developed to emulate this multifunctionality through incorporation of a pendant amine in the second coordination sphere. Cyclic diphosphine ligands containing two amines serve as the basis for a class of catalysts that have been extensively studied and used to demonstrate the impact of a pendant base. These 1,5-diaza-3,7-diphosphacyclooctanes, now often referred to as "P₂N₂" ligands, have profound effects on the reactivity of many catalysts. The resulting [Ni(P^R₂N^R'₂)₂]²⁺ complexes are electrocatalysts for both the oxidation and production of H₂. Achieving the optimal benefit of the pendant amine requires that it has suitable basicity and is properly positioned relative to the metal center. In addition to the catalytic efficacy demonstrated with [Ni(P^R₂N^R'₂)₂]²⁺ complexes for the oxidation and production of H₂, catalysts with diphosphine ligands containing pendant amines have also been demonstrated for several metals for many different reactions, both in solution and immobilized on surfaces. The impact of pendant amines in catalyst design continues to expand.

Splitting of multiple hydrogen molecules by bioinspired diniobium metal complexes: a DFT study

Splitting of molecular hydrogen (H₂) into bridging and terminal hydrides is a common step in transition metal chemistry. Herein, we propose a novel organometallic platform for cleavage of multiple H₂ molecules, which combines metal centers capable of stabilizing multiple oxidation states, and ligands bearing positioned pendant basic groups. Using quantum chemical modeling, we show that low-valent, early transition metal diniobium(ii) complexes with diphosphine ligands featuring pendant amines can favorably uptake up to 8 hydrogen atoms, and that the energetics are favored by the formation of intramolecular dihydrogen bonds. This result suggests new possible strategies for the development of hydrogen scavenger molecules that are able to perform reversible splitting of multiple H₂ molecules.

Cationic indium catalysts for ring opening polymerization: tuning reactivity with hemilabile ligands

This is a comprehensive study of the effects of rationally designed hemilabile ligands on the stability, reactivity, and change in catalytic behavior of indium complexes. We report cationic alkyl indium complexes supported by a family of hemi-salen type ligands bearing hemilabile thiophenyl (2a), furfuryl (2b) and pyridyl (2c) pendant donor arms. Shelf-life and stability of these complexes followed the trend 2a < 2b < 2c, showing direct correlation to the affinity of the pendant donor group to the indium center. Reactivity towards polymerization of epichlorohydrin and cyclohexene oxide followed the trend 2a > 2b > 2c with control of polymerization following an inverse relationship to reactivity. Surprisingly, 2c polymerized racemic lactide without an external initiator, likely through an alkyl-initiated coordination-insertion mechanism.

A bioinspired thiolate-bridged dinickel complex with a pendant amine: synthesis, structure and electrocatalytic properties

By employing X(CH₂CH₂S^-)₂ (X = S, tpdt; X = O, opdt; X = NPh, npdt) as bridging ligands, four thiolate-bridged dinickel complexes supported by phosphine ligands, [(dppe)Ni(μ-1SSS':2SS-tpdt)Ni(dppe)][PF₆]₂ (1[PF₆]₂, dppe = Ph₂P(CH₂)₂PPh₂), [(dppe)Ni(μ-1SSN:2SS-npdt)Ni(dppe)][PF₆]₂ (2[PF₆]₂) and [(dppe)Ni(t-Cl)(μ-1SSX:2SS-C₄H₈S₂X)Ni(dppe)][PF₆] (3[PF₆], X = S; 4[PF₆], X = O) were facilely obtained by the salt metathesis reaction. These four thiolate-bridged dinickel complexes have all been fully characterized by spectroscopic methods and X-ray crystallography. In 2[PF₆]₂, elongation of the Ni-N bond distance, possibly caused by steric hindrance, indicates that the pendant nitrogen group shuttles between the two nickel centers in solution, which is evidenced by ³¹P{¹H} NMR spectroscopic results. Furthermore, these thiolate-bridged dinickel complexes have all been proved to be electrocatalysts for proton reduction to hydrogen. Notably, complex 2[PF₆]₂ featuring a pendant amine group in the secondary coordination sphere exhibits the best catalytic activity at a relatively low overpotential.

Bis[diphenylphosphino]methane and its bridge-substituted analogues as chemically non-innocent ligands for H2 activation

Interconversion between coordinated diphosphinomethane and diphosphinomethanide moieties can be used as a new tool for metal–ligand cooperation.

Triple hydrogen atom abstraction from Mn–NH3 complexes results in cyclophosphazenium cations

All three H atoms of the NH₃ ligand of [Mn(depe)₂(CO)(NH₃)]⁺ are abstracted by an organic radical, giving a rare cyclophosphazenium cation; computations suggest that insertion of NH_x into a Mn–P bond provides a strong thermodynamic driving force.

Promoting proton coupled electron transfer in redox catalysts through molecular design

Mini-review on using the secondary coordination sphere to facilitate multi-electron, multi-proton catalysis.

Ligand dynamics and protonation preferences of Rh and Ir complexes bearing an almost, but not quite, pendent base

Pendent nucleophiles can assist transition metals mediate bond rearrangements (e.g. as proton acceptors), but can also act as inhibitory hemilabile ligands. This dual nature has been studied in a series of rhodium and iridium complexes that exhibit disparate nucleophile binding ability in the ground state and in protonation reactions.

Frustration across the periodic table: heterolytic cleavage of dihydrogen by metal complexes

This perspective examines frustrated Lewis pairs (FLPs) in the context of heterolytic cleavage of H2 by transition metal complexes, with an emphasis on molecular complexes bearing an intramolecular Lewis base. FLPs have traditionally been associated with main group compounds, yet many reactions of transition metal complexes support a broader classification of FLPs that includes certain types of transition metal complexes with reactivity resembling main group-based FLPs. This article surveys transition metal complexes that heterolytically cleave H2, which vary in the degree that the Lewis pairs within these systems interact. Many of the examples include complexes bearing a pendant amine functioning as the base with the metal functioning as the hydride acceptor. Consideration of transition metal compounds in the context of FLPs can inspire new innovations and improvements in transition metal catalysis.This article is part of the themed issue 'Frustrated Lewis pair chemistry'.

Macrocyclic tetrakis-phosphines and their copper(I) complexes

A series of mono-, di- and tetranuclear copper(I) complexes with macrocyclic 14-, 16-, 18- and 20-membered tetrakis-phosphine ligands (P4N2) was obtained and fully characterized including single crystal X-ray diffraction. The 14-membered P4N2 macrocycles form very stable cationic mononuclear tetraphospha-coordinated copper(I) complexes, whereas their higher 16-, 18- and 20-membered homologues give neutral dinuclear [Cu2I2L] complexes under the same conditions. An unusual tetranuclear complex with two three- and two four-coordinated copper(I) atoms was obtained from 16-PMes4NCH2CH2Py-22 and copper(I) iodide. This is the first example of N-coordination of cyclic aminomethylphosphines towards “soft” copper(I).

Electrocatalytic generation of H2 from neutral water in acetonitrile using manganese polypyridyl complexes with ligand assistance

The earth-abundant element, manganese can be employed in electrocatalytically active complexes for H₂ generation from neutral water added to acetonitrile solutions.

Molecular electrocatalysts for oxidation of hydrogen using earth-abundant metals: shoving protons around with proton relays

Sustainable, carbon-neutral energy is needed to supplant the worldwide reliance on fossil fuels in order to address the persistent problem of increasing emissions of CO2. Solar and wind energy are intermittent, highlighting the need to develop energy storage on a huge scale. Electrocatalysts provide a way to convert between electrical energy generated by renewable energy sources and chemical energy in the form of chemical bonds. Oxidation of hydrogen to give two electrons and two protons is carried out in fuel cells, but the typical catalyst is platinum, a precious metal of low earth abundance and high cost. In nature, hydrogenases based on iron or iron/nickel reversibly oxidize hydrogen with remarkable efficiencies and rates. Functional models of these enzymes have been synthesized with the goal of achieving electrocatalytic H2 oxidation using inexpensive, earth-abundant metals along with a key feature identified in the [FeFe]-hydrogenase: an amine base positioned near the metal. The diphosphine ligands P(R)2N(R')2 (1,5-diaza-3,7-diphosphacyclooctane with alkyl or aryl groups on the P and N atoms) are used as ligands in Ni, Fe, and Mn complexes. The pendant amines facilitate binding and heterolytic cleavage of H2, placing the hydride on the metal and the proton on the amine. The pendant amines also serve as proton relays, accelerating intramolecular and intermolecular proton transfers. Electrochemical oxidations and deprotonations by an exogeneous amine base lead to catalytic cycles for oxidation of H2 (1 atm) at room temperature for catalysts derived from [Ni(P(Cy)2N(R')2)2](2+), Cp(C6F5)Fe(P(tBu)2N(Bn)2)H, and MnH(P(Ph)2N(Bn)2)(bppm)(CO) [bppm = (PAr(F)2)2CH2]. In the oxidation of H2 catalyzed by [Ni(P(Cy)2N(R')2)2](2+), the initial product observed experimentally is a Ni(0) complex in which two of the pendant amines are protonated. Two different pathways can occur from this intermediate; deprotonation followed by oxidation occurs with a lower overpotential than the alternate pathway involving oxidation followed by deprotonation. The Mn cation [Mn(P(Ph)2N(Bn)2)(bppm)(CO)](+) mediates the rapid (>10(4) s(-1) at -95 °C), reversible heterolytic cleavage of H2. Obtaining the optimal benefit of pendant amines incorporated into the ligand requires that the pendant amine be properly positioned to interact with a M-H or M(H2) bond. In addition, ligands are ideally selected such that the hydride-acceptor ability of the metal and the basicity of a pendant are tuned to give low barriers for heterolytic cleavage of the H-H bond and subsequent proton transfer reactions. Using these principles allows the rational design of electrocatalysts for H2 oxidation using earth-abundant metals.