 2 (1). Selected bond distances (A ˚ ) and angles (1): Ni(1)-P(1), 2.2205(8); Ni(1)-P(2), 2.2196(8); P(2)-Ni(1)-P(2), 80.83(2). Thermal ellipsoids are shown at the 50% probability level. For clarity, only the carbons attached to the phosphorous atoms are shown.](https://www.researchgate.net/profile/Simone-Raugei/publication/47396288/figure/fig2/AS:667650627743766@1536191722738/The-crystallographic-structure-of-the-cation-of-NiP-Cy-2-N-t-Bu-2-BF-4-2-1_Q320.jpg)
![Calculated reaction profile for all boat confomer of [Ni(P Cy 2 N Me 2 ) 2 ] 2+ + H 2. Blue lines: energy; red lines: free energy. Dashed lines are only a guide to the eye.](https://www.researchgate.net/profile/Simone-Raugei/publication/47396288/figure/fig3/AS:667650627756042@1536191722863/Calculated-reaction-profile-for-all-boat-confomer-of-NiP-Cy-2-N-Me-2-2-2-H-2_Q320.jpg)
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Hydrogen oxidation catalysis by a nickel diphosphine complex with pendant tert-butyl amines
Abstract and Figures
A bis-diphosphine nickel complex with tert-butyl functionalized pendant amines [Ni(P(Cy)(2)N(t-Bu)(2))(2)](2+) has been synthesized. It is a highly active electrocatalyst for the oxidation of hydrogen in the presence of base. The turnover rate of 50 s(-1) under 1.0 atm H(2) at a potential of -0.77 V vs. the ferrocene couple is 5 times faster than the rate reported heretofore for any other synthetic molecular H(2) oxidation catalyst.
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... In this context, 1 was used for coordination reactions side by side with the already known P 2 cy N 2 tBu for a direct comparison in order to probe the relative influence of aliphatic and aromatic substitution on P. [27] Notably, P 2 cy N 2 tBu exhibits a similar signal pattern as 1 in the 31 P NMR while no structural information from SCXRD studies is available. ...
Based on their general spacial flexibility, their Lewis and Brønsted basicity, and ability to mimic second sphere effects the 1,5‐diaza‐3,7‐diphosphacyclooctane ligand family and their complexes have regained substantial scientific interest. It was now possible to structurally analyze a recently reported member of this family with p‐tolyl and t‐butyl substituents on P and N, respectively, (P2p‐tolN2tBu). Notably, the ligand crystallizes with a ‘twisted’ backbone. This compound is the very first of its kind to have been unambiguously characterized with regard to its chemical and molecular structure as being in this conformation. A temperature‐dependent NMR study provides insight into the molecular dynamics of two isomers in solution, which are most likely also both twisted, as judged by the observed limited reactivity. Despite the in principle unfavorable conformation of the free ligand, it was successfully chelated to tungsten and molybdenum centers in mononuclear carbonyl complexes. The ligand, a derivative thereof and four new complexes were comprehensively characterized and analyzed in comparison. This includes single crystal XRD molecular structures of P2p‐tolN2tBu and all four complexes. P2p‐tolN2tBu, regardless of its twisted conformation, is able to coordinate to metal centers given that enough energy (heat) for a conformational change is provided.
... [14][15] Moreover, these nickel systems demonstrated complementary reactivity for hydrogen oxidation with the correct ligand tuning. [16][17] In addition to the molecular species mentioned above, studies grafting nickel diphosphacycloheptane complexes onto electrode supports have been reported. [18][19][20][21][22][23] Similar investigations have also been carried out with iron polypyridyl, [24] cobalt terpyridine, [25] and cobalt tetraphenylporphyrin catalysts. ...
Incorporating design elements from homogeneous catalysts to construct well defined active sites on electrode surfaces is a promising approach for developing next generation electrocatalysts for energy conversion reactions. Furthermore, if functionalities that control the electrode microenvironment could be integrated into these active sites it would be particularly appealing. In this context, a square planar nickel calixpyrrole complex, Ni(DPMDA) (DPMDA=2,2′‐((diphenylmethylene)bis(1H‐pyrrole‐5,2‐diyl))bis(methaneylylidene))bis(azaneylylidene))dianiline) with pendant amine groups is reported that forms a heterogeneous hydrogen evolution catalyst using anilinium tetrafluoroborate as the proton source. The supported Ni(DPMDA) catalyst was surprisingly stable and displayed fast reaction kinetics with turnover frequencies (TOF) up to 25,900 s⁻¹ or 366,000 s⁻¹ cm⁻². Kinetic isotope effect (KIE) studies revealed a KIE of 5.7, and this data, combined with Tafel slope analysis, suggested that a proton‐coupled electron transfer (PCET) process involving the pendant amine groups was rate‐limiting. While evidence of an outer‐sphere reduction of the Ni(DPMDA) catalyst was observed, it is hypothesized that the control over the secondary coordination sphere provided by the pendant amines facilitated such high TOFs and enabled the PCET mechanism. The results reported herein provide insight into heterogeneous catalyst design and approaches for controlling the secondary coordination sphere on electrode surfaces.
... To assess the contribution of the P 2 N 2 ligand to the observed phenomenon, a second set of monodithiolene complexes, 4 and 5, was synthesized using the known P 2 cy N 2 tBu (1 cy ). [15] For these complexes, no such collapse was observed ( Figure S3 and Table 1). In the oxidative regions, the typical 2 e À oxidation is found for each of the four complexes. ...
Heteroleptic molybdenum complexes bearing 1,5‐diaza‐3,7‐diphosphacyclooctane (P2N2) and non‐innocent dithiolene ligands were synthesized and electrochemically characterized. The reduction potentials of the complexes were found to be fine‐tuned by a synergistic effect identified by DFT calculations as ligand‐ligand cooperativity via non‐covalent interactions. This finding is supported by electrochemical studies combined with UV/Vis spectroscopy and temperature‐dependent NMR spectroscopy. The observed behavior is reminiscent of enzymatic redox modulation using second ligand sphere effects.
... This hydrogen transfer to the ligand was observed in a catalytic hydrogen oxidation mechanism using the same type of ligand. [44] This reaction step is sensitive depending on the electronic environment of the nitrogen and thus on the substituents at it (see below). This hydrogen transfer step was observed when a R 1 -I reactant already coordinates to the Ni center to form a tetrahedrally coordinated complex involving the iodide. ...
For decades there were many attempts to dispense with stoichiometric amounts of metal reagents for the synthesis of secondary alcohols. In 2021, the synthetic results of Newman and collaborators pioneered a synthesis still with metals, but not as reactants. Instead, they serverd as catalytic engines. Here we present a description by means of Density Functional Theory calculations of how this process can occur, and an attempt is made to shed light on the mechanism that facilitates the attainment of secondary alcohols, emphasizing the eternal cross‐coupling debate of whether the catalytically active species is Ni(0) or they are really taking shortcuts following the course of Ni(II). Effective Orbital analyses give a clear picture. Furthermore, this paper provides insight not only into the nature of the ligands of the metal catalyst but also the role of the base.
Hydrogen emerges as one of the cleanest and most renewable energy resources for fossil fuel alternatives. Commercial-scale hydrogen production through water-splitting electrocatalysis is considered a cornerstone in which electrocatalysts play a crucial role. The design of active, robust, and cost-effective electrocatalysts is an
indispensable prerequisite to delivering a smart electrocatalyst for mimicking the hydrogen evolution reaction (HER). The recent progress in the design and development of coordination-driven smart electrocatalysts has been
discoursed. New strategies and insights in unveiling the nature of donor sites and metal ions, the primary and secondary structural effect of the ligand, metal–ligand cooperativity, active sites of the electrocatalysts, pH etc., influencing the overpotential, catalytic rate, and stability have been delineated. The mechanistic aspects of the HER in acidic and alkaline media in the light of electrocatalytic, bio-mimetic and computational analysis have been shown. A constructive overview, future perspectives and critical challenges in the future design of robust
and active electrocatalysts for HER have also been outlined in this review.
Catalysts based on first-row transition series complexes have been extensively utilized for electrocatalytic H2 generation and among them are molecular nickel catalysts which have received a flurry of scientific accolades over the past years. They have been under the spotlight because of the peculiarities attached to their dechelation and rechelation protonation procedure leading to multiple catalytic pathways whose intermediates can be probed via advanced theoretical and experimental techniques. Therefore, this review discusses the effects of electronic and structural designs on the electrochemical, electrocatalysis and mechanistic evaluation of saturated, hexacoordinated Ni complexes for electrocatalytic proton reduction to generate H2.
Heteroleptische Molybdänkomplexe mit 1,5‐Diaza‐3,7‐Diphosphacyclooctan (P 2 N 2 ) und non‐innocent (nicht‐unschuldigen) Dithiolenliganden wurden synthetisiert und elektrochemisch charakterisiert. Dabei wurde festgestellt, dass die Reduktionspotentiale der Komplexe durch einen synergistischen Effekt moduliert werden, welcher durch DFT‐Berechnungen als Ligand‐Ligand‐Kooperativität über nicht‐kovalente Wechselwirkungen identifiziert werden konnte. Dieses Ergebnis konnte durch elektrochemische Studien in Kombination mit UV/Vis‐Spektroskopie und temperaturabhängiger NMR‐Spektroskopie bestätigt werden. Das beobachtete Verhalten erinnert an eine enzymatische Redoxmodulation durch Effekte basierend auf Interaktionen in der zweiten Ligandensphäre.
A series of three calixpyrrole ligands (1a-c) with pendant nitrogen-based hydrogen bond donors, R, were synthesized and coordinated to palladium to produce metal complexes (2a-c), where R = NH2 (a); NHC(O)CH3 (b); or NHC(O)OC(CH3)3 (c). The calixpyrrole compounds were generated using a Schiff-base reaction starting with 5,5’-diformyl-2,2’-diphenyldipyrromethane and an aniline precursor. The deprotonated calixpyrrole species were subsequently bound to palladium to generate distorted square planar complexes. The pendant groups were not coordinated to the metal with Pd-N distances of 3.36 to 4.79 Å. The electrochemical properties of the palladium complexes were also explored, and 2a-c displayed two irreversible oxidations above 0.0 V vs. ferrocene/ferrocenium (Fc/Fc⁺), as well as two irreversible reductions below -0.70 V vs. Fc/Fc⁺. Interestingly, the free ligands showed similar electrochemical features, suggesting redox non-innocence. Preliminary reactivity studies indicated the palladium complexes did not activate small molecules, but they did catalyze H2 evolution in the presence of acid. The onset of catalysis for 2a-c was approximately 0.4-0.5 V more positive than a glassy carbon electrode, and the active species could undergo 500 scans without significant changes in activity. The catalysts were found to be heterogeneous in nature and adsorbed onto the working electrode.
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(PR2NR'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(PR2NR'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.
Phosphorus-based Mannich-type reactions (phospha-Mannich reactions) of primary or secondary phosphines with carbonyl compounds and amines (R2NH, RNH2, NH3) provide a powerful synthetic tool in the worldwide search for future ligands. The variety of amines and phosphines available allows for designing and fine-tuning water-soluble, chiral, macrocyclic, pincer, tripodal, photoactive, etc. phosphine ligands with P–C–N linkage(s) (P,N-acetals). Aminized supports and amine-terminated dendrimers can easily be phosphine-functionalized by phospha-Mannich reactions, and used in reusable heterogeneous catalysts. If CH2O is used as a C-component, its stable adducts with phosphines, i.e. RP(CH2OH)2 and R2PCH2OH, are often used in syntheses of P,N-acetals as they are less toxic and non-pyrophoric analogs of the phosphines, whereas other carbonyl compounds are usually converted into imines or iminium salts. In phospha-Mannich reactions, tertiary phosphines form a P+–C–N linkage (P+,N-acetals) that can easily be broken by bases, realizing imine or iminium cations, and are used as imine/iminium donors in organic syntheses.
The cobalt analogue of a highly active nickel electrocatalyst for hydrogen production has been synthesized and characterized as [Co(PPh2NPh2)2(CH3CN)](BF4)2. In the presence of triflic acid in acetonitrile solution, the complex loses one cyclic diphosphine ligand to form [Co(PPh2NPh2)(CH3CN)3](BF4)2, which has been synthesized independently and stucturally characterized. The latter complex serves as an electrocatalyst for hydrogen formation with a turnover frequency of 90 sec-1 and an overpotential of 280 mV using bromoanilinium tetrafluoroborate as the acid. A similar cobalt complex with a related diphosphine ligand that does not contain a pendant base is not catalytically active, confirming an important role for the pendant amine in the catalytic reaction. This work was supported by the Chemical Sciences program of the Office of Basic Energy Sciences of the Department of Energy. The Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy. Invited author, "Energy & Environmental Science", journal article.
The alkyl (aryl)-bishydroxymethylphosphines react with primary amines to give the title compounds. With optically active amines optically active phosphines are formed.
A series of binuclear FeIFeI complexes, (μ-SEt)2[Fe(CO)2L]2
(L = CO (1), PMe3
(1-P)), (μ-SRS)[Fe(CO)2L]2
(R = CH2CH2
(μ-edt): L = CO (2), PMe3
(2-P); R = CH2CH2CH2(μ-pdt): L = CO (3), PMe3
(3-P); and R =
o-CH2C6H4CH2
(μ-o-xyldt): L = CO (4), PMe3
(4-P)), that serve as structural models for the active site of Fe-hydrogenase are shown to be electrocatalysts for H2 production in the presence of acetic acid in acetonitrile. The redox levels for H2 production were established by spectroelectrochemistry to be Fe0Fe0 for the all-CO complexes and FeIFe0 for the PMe3-substituted derivatives. As electrocatalysts, the PMe3 derivatives are more stable and more sensitive to acid concentration than the all-CO complexes. The electrocatalysis is initiated by electrochemical reduction of these diiron complexes, which subsequently, under weak acid conditions, undergo protonation of the reduced iron center to produce H2. An (η2-H2)FeII–Fe0/I intermediate is suggested and probable electrochemical mechanisms are discussed.
A new cyclic 1,5-diaza-3,7-diphosphacyclooctane ligand was prepared with phenyl substituents on phosphorus and (thiophene-3-yl)phenyl substituents on nitrogen. This ligand reacts with [Ni(CH3CN)6][BF4]2 to form the corresponding [Ni(PPh2NAr2)2(NCMe)][BF4]2 complex, 3, which is an active electrocatalyst for H2 production. Kinetic studies indicate that the catalytic rate is first order in catalyst and second order in acid at low concentrations of acid, but at higher acid concentrations the catalytic rate becomes independent of acid concentration. The rate-determining step at high acid concentrations is attributed to the elimination of H2 from a reduced Ni species. The modest overpotential of 280 mV and a turnover frequency of 56 s-1 confirms that high activity observed is a general feature of this class of complexes and not an exception. Oxidation of the pendant thiophene substituents of 3 results in the formation of films on the electrode surface. However these films are not electroactive and electrocatalysis of proton reduction is not observed with these modified electrodes. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
The cyclic diphosphine ligands PtBu2NPh2 and PtBu2NBz2 have been synthesized and used to prepare new complexes of Co(II) and Ni(II) with the formula [M(PtBu2NR2)(CH3CN)n](BF4)2 (n = 2, 3). The products have been characterized by variable-temperature NMR data, X-ray diffraction studies, and cyclic voltammetry, and properties of the new complexes have been compared with those of previously studied complexes containing PPh2NR2 ligands. The variation of either phosphorus or nitrogen substituents in these ligands can result in significant differences in the structure, electrochemistry, and reactivity of the metal complexes. [Co(PtBu2NPh2)(CH3CN)3](BF4)2 is found to be an effective electrocatalyst for the formation of hydrogen using bromoanilinium tetrafluoroborate as the acid, with a turnover frequency of 160 s−1 and an overpotential of 160 mV. These cobalt derivatives are a promising class of catalysts for further study and optimization.
Thermodynamic data for hydride complexes of the general formula [HNi(P2RN2R‘)2](BF4), 3, which have been reported previously to function as effective catalysts for the electrochemical oxidation or production of hydrogen, have been determined. Values of ΔG°H+, ΔG°H•, and ΔG°H- have been determined for 3a where R = Cy, R‘ = Bz, for 3b where R = R‘ = Ph, and for the new complex 3c, R = Ph, R‘ = Bz. In addition, the ΔG values for the heterolytic addition of hydrogen to the Ni(II) precursor complexes [Ni(P2RN2R‘)2](BF4)2, 1a−c, have been determined experimentally or calculated. The data are useful for understanding the factors that control the catalytic activities observed for these complexes and for the design of additional catalysts.
The paper presents a review of recent studies of complexes of Ni(II) and Fe(II) coordinated by diphosphine ligands with amine bases incorporated into the ligand chelate rings. The role of the bases in the second coordination sphere in mediating rapid intramolecular M–H/N–H exchange as well as intermolecular exchange with protons and in promoting the coupling of proton- and electron-transfer processes has been studied. Factors that favor efficient proton relay properties for the pendant amines have been established and the information has been used to develop efficient electrocatalysts for both hydrogen oxidation and production.
Unsaturated organoiridium complexes were prepared with amidophenolate ligands derived from 2-(2-trifluoromethyl)amino-4,6-di-tert-butylphenol (H2tBAF) and 2-tert-butylamino-4,6-di-tert-butylphenol (H2tBAtBu). The following 16e complexes were characterized: Cp*M(tBAR) with M = Ir (1F and 1t-Bu), Rh (2F), and (cymene)Ru(tBAF) (3F). These complexes undergo two 1e oxidations at potentials of about 0 and −0.25 V vs Cp2Fe0/+. The magnitude of ΔE1/2 is sensitive to the counteranions, and the reversibility is strongly affected by the presence of Lewis bases, which stabilize the oxidized derivatives. Crystallographic measurements indicate that upon oxidation the amidophenolate ligands adopt quinoid character, as indicated by increased alternation of the C−C bond lengths in the phenylene ring backbone and shortened C−N and C−O bonds. Unlike the charge-neutral precursors, the cationic [Cp*M(tBAR)]+ are Lewis acidic and form well-characterized adducts with PR3 (R = Me, Ph), CN−, MeCN (reversibly), and CO. In the absence of competing ligands, the cations oxidize H2. Coulommetry measurements indicate that H2 is oxidized by the monocations [Cp*M(tBAR)]+, not the corresponding dications. Oxidation of H2 is catalytic in the presence of a noncoordinating base at potentials required for the generation of [Cp*M(tBAR)]+. The rate decreases in the order [Cp*M(tBAF)]BArF4 > [Cp*M(tBAF)]PF6 > [Cp*M(tBAt-Bu)]PF6. The reduction of ferrocenium by H2 is catalyzed by Cp*M(tBAR).
Addition of the pendant amine ligand PNRP (PNRP = Ph2PCH2NRCH2PPh2; R = Me, Ph, n-Bu) to Mn(CO)5Br gives fac-Mn(PNRP)(CO)3Br. Photolysis of fac-Mn(PNRP)(CO)3Br with dppm [dppm = 1,2-bis(diphenylphosphino)methane] provides mixed bis(diphosphine) complexes, trans-Mn(PNRP)(dppm)(CO)(Br). Reaction of trans-Mn(PNRP)(dppm)(CO)(Br) with LiAlH4 leads to trans-Mn(PNRP)(dppm)(CO)(H), which has been characterized by crystallography. Mn(P2PhN2Bn)(dppm)(CO)(H) [P2PhN2Bn = 1,5-diphenyl-3,7-dibenzyl-1,5-diaza-3,7-diphosphacyclooctane] can be prepared in a similar manner; its structure has one chelate ring in a chair conformation and the second in a boat conformation. The boat-conformer ring directs the nitrogen of the ring toward the carbonyl ligand, and the N···C distance between one N of the P2PhN2Bn ligand and CO is 3.171(4) Å, indicating a weak interaction between the N of the pendant amine and the CO ligand. Reaction of NaBArF4 (ArF = 3,5-bis(trifluoromethyl)phenyl) with Mn(P−P)(dppm)(CO)(Br) produces the cations [Mn(P−P)(dppm)(CO)]+. The crystal structure of [Mn(PNMeP)(dppm)(CO)][BArF4] shows two very weak agostic interactions between C−H bonds on the phenyl ring and the Mn. The cationic complexes [Mn(P−P)(dppm)(CO)]+ react with H2 to form dihydrogen complexes [Mn(H2)(P−P)(dppm)(CO)]+ (Keq = 1−90 atm−1 in fluorobenzene, for a series of different P−P ligands). Similar equilibria with N2 produce [Mn(N2)(P−P)(dppm)(CO)]+ (Keq generally 1−3.5 atm−1 in fluorobenzene).