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1.
Dalton Trans ; 2021 Jan 18.
Artigo em Inglês | MEDLINE | ID: mdl-33459317

RESUMO

Manganese ([Mn(CO)3]) and rhenium tricarbonyl ([Re(CO)3]) complexes represent a workhorse family of compounds with applications in a variety of fields. Here, the coordination, structural, and electrochemical properties of a family of mono- and bimetallic [Mn(CO)3] and [Re(CO)3] complexes are explored. In particular, a novel heterobimetallic complex featuring both [Mn(CO)3] and [Re(CO)3] units supported by 2,2'-bipyrimidine (bpm) has been synthesized, structurally characterized, and compared to the analogous monomeric and homobimetallic complexes. To enable a comprehensive structural analysis for the series of complexes, we have carried out new single crystal X-ray diffraction studies of seven compounds: Re(CO)3Cl(bpm), anti-[{Re(CO3)Cl}2(bpm)], Mn(CO)3Br(bpz) (bpz = 2,2'-bipyrazine), Mn(CO)3Br(bpm), syn- and anti-[{Mn(CO3)Br}2(bpm)], and syn-[Mn(CO3)Br(bpm)Re(CO)3Br]. Electrochemical studies reveal that the bimetallic complexes are reduced at much more positive potentials (ΔE ≥ 380 mV) compared to their monometallic analogues. This redox behavior is consistent with introduction of the second tricarbonyl unit which inductively withdraws electron density from the bridging, redox-active bpm ligand, resulting in more positive reduction potentials. [Re(CO3)Cl]2(bpm) was reduced with cobaltocene; the electron paramagnetic resonance spectrum of the product exhibits an isotropic signal (near g = 2) characteristic of a ligand-centered bpm radical. Our findings highlight the facile synthesis as well as the structural characteristics and unique electrochemical behavior of this family of complexes.

2.
Inorg Chem ; 60(2): 1107-1115, 2021 Jan 18.
Artigo em Inglês | MEDLINE | ID: mdl-33405902

RESUMO

The behavior of Lewis acidic metal ions in multimetallic systems has become a subject of intense interest in recent years. Parametrizing the behavior of these ions in nonaqueous conditions, commonly used in the field, is challenging due to the lack of direct measures of the Lewis acidity of metal ions in polar organic solvents. Here, we report the use of triphenylphosphine oxide (TPPO) as a 31P nuclear magnetic resonance (NMR) probe to quantify the Lewis acidity of a library of metal triflate salts using the Gutmann-Beckett method. Plots of the pKa values of the corresponding metal-aqua species, [M(H2O)m]n+, measured in H2O vs the 31P NMR shifts of TPPO in the presence of these metals in deuterated acetonitrile (d3-MeCN) and deuterated dichloromethane (CD2Cl2), display tightly colinear relationships, suggesting similar behavior for these ions in water, d3-MeCN, and CD2Cl2. This colinearity reinforces the utility of the common approach of using the aqueous pKa values as a descriptor of Lewis acidity, regardless of the solvent used in the immediate experiments, and provides an insight into the usefulness of this descriptor in wide-ranging applications. Titration studies in d3-MeCN suggest a 1:1 binding of TPPO with monovalent ions, greater than 1:1 binding with divalent ions, and formation of multiple species with the highly Lewis acidic trivalent ions. Together, these data suggest that both aqueous pKa values and other single-measurement descriptors, while useful, provide only a snapshot of the influence of Lewis acidity on multimetallic chemical systems.

3.
J Organomet Chem ; 9212020 Aug 15.
Artigo em Inglês | MEDLINE | ID: mdl-32773887

RESUMO

[Cp*Rh] complexes (Cp* = η 5-pentamethylcyclopentadienyl) supported by bidentate chelating ligands are useful in studies of redox chemistry and catalysis, but little information is available for derivatives bearing "hybrid" [P,N] chelates. Here, the preparation, structural characterization, and chemical and electrochemical properties of a [Cp*Rh] complex bearing the κ2-[P,N]-2-[(diphenylphosphino)methyl]pyridine ligand (PN) are reported. Cyclic voltammetry data reveal that [Cp*Rh(PN)Cl]PF6 (1) undergoes a chemically reversible, net two-electron reduction at -1.28 V vs. ferrocenium/ferrocene, resulting in generation of a rhodium(I) complex (3) that is stable on the timescale of the voltammetry. However, 1H and 31P{1H} NMR studies reveal that chemical reduction of 1 generates a mixture of products over a 1 h timescale; this mixture forms as a result of deprotonation of the methylene group of 1 by 3 followed by further reactivity. The analogous complex [Cp*Rh(PQN)Cl]PF6 (2; PQN = κ2-[P,N]-8-(diphenylphosphino)quinoline) does not undergo self-deprotonation or further reactivity upon two-electron reduction, confirming the reactivity of the acidic backbone methylene C-H bonds in the PN complexes. Comparison of the electrochemical properties 1 and 2 also shows that the extended conjugated system of PQN contributes to an additional ligand-centered redox event for 2 that is absent for 1.

4.
Molecules ; 25(14)2020 Jul 13.
Artigo em Inglês | MEDLINE | ID: mdl-32668660

RESUMO

4,5-diazafluorene (daf) and 9,9'-dimethyl-4,5-diazafluorene (Me2daf) are structurally similar to the important ligand 2,2'-bipyridine (bpy), but significantly less is known about the redox and spectroscopic properties of metal complexes containing Me2daf as a ligand than those containing bpy. New complexes Mn(CO)3Br(daf) (2), Mn(CO)3Br(Me2daf) (3), and [Ru(Me2daf)3](PF6)2 (5) have been prepared and fully characterized to understand the influence of the Me2daf framework on their chemical and electrochemical properties. Structural data for 2, 3, and 5 from single-crystal X-ray diffraction analysis reveal a distinctive widening of the daf and Me2daf chelate angles in comparison to the analogous Mn(CO)3(bpy)Br (1) and [Ru(bpy)3]2+ (4) complexes. Electronic absorption data for these complexes confirm the electronic similarity of daf, Me2daf, and bpy, as spectra are dominated in each case by metal-to-ligand charge transfer bands in the visible region. However, the electrochemical properties of 2, 3, and 5 reveal that the redox-active Me2daf framework in 3 and 5 undergoes reduction at a slightly more negative potential than that of bpy in 1 and 4. Taken together, the results indicate that Me2daf could be useful for preparation of a variety of new redox-active compounds, as it retains the useful redox-active nature of bpy but lacks the acidic, benzylic C-H bonds that can induce secondary reactivity in complexes bearing daf.

5.
ChemSusChem ; 13(23): 6338-6345, 2020 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-32196939

RESUMO

Electrochemical studies of CO2 conversion by molecular catalysts are typically carried out in a narrow range of near-ambient CO2 pressures wherein low CO2 solubilities in the liquid phase can limit the rate of CO2 reduction. In this study, five-fold rate enhancements are enabled by pairing CO2 -expanded electrolytes (CXEs), a class of media that accommodate multimolar concentrations of CO2 in organic solvents at modest pressures, with a homogeneous molecular electrocatalyst, [Re(CO)3 (bpy)Cl] (1, bpy=2,2'-bipyridyl). Analysis of cyclic voltammetry data reveals pressure-tunable rate behavior, with first-order kinetics at moderate CO2 pressures giving way to zero-order kinetics at higher pressures. The significant enhancement in the space-time yield of CO demonstrates that CXEs offer a simple yet powerful strategy for unlocking the intrinsic potential of molecular catalysts by mitigating CO2 solubility limitations commonly encountered in conventional liquid electrolytes. Moreover, our findings reveal that 1, a workhorse molecular catalyst, performs with intrinsic kinetic behavior, which is competitive with fast enzymes under optimal conditions in CXEs.

6.
Inorg Chem ; 59(4): 2178-2187, 2020 Feb 17.
Artigo em Inglês | MEDLINE | ID: mdl-31990533

RESUMO

Manganese tricarbonyl complexes are promising catalysts for CO2 reduction, but complexes in this family are often photosensitive and decompose rapidly upon exposure to visible light. In this report, synthetic and photochemical studies probe the initial steps of light-driven speciation for Mn(CO)3(Rbpy)Br complexes bearing a range of 4,4'-disubstituted 2,2'-bipyridyl ligands (Rbpy, where R = tBu, H, CF3, NO2). Transient absorption spectroscopy measurements for Mn(CO)3(Rbpy)Br coordination compounds with R = tBu, H, and CF3 in acetonitrile reveal ultrafast loss of a CO ligand on the femtosecond time scale, followed by solvent coordination on the picosecond time scale. The Mn(CO)3(NO2bpy)Br complex is unique among the four compounds in having a longer-lived excited state that does not undergo CO release or subsequent solvent coordination. The kinetics of photolysis and solvent coordination for light-sensitive complexes depend on the electronic properties of the disubstituted bipyridyl ligand. The results indicate that both metal-to-ligand charge-transfer (MLCT) and dissociative ligand-field (d-d) excited states play a role in the ultrafast photochemistry. Taken together, the findings suggest that more robust catalysts could be prepared with appropriately designed complexes that avoid crossing between the excited states that drive photochemical CO loss.

7.
J Am Chem Soc ; 142(6): 3032-3041, 2020 02 12.
Artigo em Inglês | MEDLINE | ID: mdl-31927996

RESUMO

Capture and activation of the water-soluble uranyl dication (UO22+) remains a challenging problem, as few rational approaches are available for modulating the reactivity of this species. Here, we report the divergent synthesis of heterobimetallic complexes in which UO22+ is held in close proximity to a range of redox-inactive metals by a tailored macrocyclic ligand. Crystallographic and spectroscopic studies confirm assembly of homologous UVI(µ-OAr)2Mn+ cores with a range of mono-, di-, and trivalent Lewis acids (Mn+). Cyclic voltammetry data demonstrate that the UVI/UV reduction potential in these complexes is modulated over a span of 600 mV, depending linearly on the Lewis acidity of the redox-inactive metal with a sensitivity of 61 ± 9 mV/pKa unit. These findings suggest that interactions with Lewis acids could be effectively leveraged for rational tuning of the electronic and thermochemical properties of the 5f elements, reminiscent of strategies more commonly employed with 3d transition metals.

8.
Analyst ; 145(2): 466-477, 2020 Jan 20.
Artigo em Inglês | MEDLINE | ID: mdl-31750451

RESUMO

Homoleptic acetonitrile complexes of first-row transition metal ions are a common product of the detrimental speciation of coordination complexes and organometallic compounds. However, the electrochemical properties of such species are mostly unknown, introducing ambiguities into interpretation of electroanalytical data associated with studies of molecular electrocatalysis. Here, we have cataloged the cyclic voltammetric properties of the solvento complexes of Mn(ii), Fe(ii), Co(ii), Ni(ii), Cu(i), and Zn(ii) in acetonitrile electrolyte, providing information on the cathodic electrodeposition and anodic stripping processes occuring with each ion. The electrochemical quartz crystal microbalance (EQCM) has been used to quantify these processes, as well as the rates of the in situ corrosion of electrodeposited materials by the strong organic acid dimethylforamidinium, [DMFH]+. Ex situ X-ray photoelectron spectroscopic results confirm the interpretations of the voltammetric and gravimetric data, and confirm the periodic relationship between the metals. Taken together, the results described here provide an electrochemical roadmap useful in distinguishing currents arising from homogeneous electrocatalysis from currents associated with the redox cycling of secondary heterogeneous materials.

9.
ChemSusChem ; 12(16): 3761-3768, 2019 Aug 22.
Artigo em Inglês | MEDLINE | ID: mdl-31170315

RESUMO

Multimolar CO2 concentrations are achieved in acetonitrile solutions containing supporting electrolyte at relatively mild CO2 pressures (<5 MPa) and ambient temperature. Such CO2 -rich, electrolyte-containing solutions are termed as CO2 -eXpanded Electrolytes (CXEs) because significant volumetric expansion of the liquid phase accompanies CO2 dissolution. Cathodic polarization of a model polycrystalline gold electrode-catalyst in CXE media enhances CO2 to CO conversion rates by up to an order of magnitude compared with those attainable at near-ambient pressures, without loss of selectivity. The observed catalytic process intensification stems primarily from markedly increased CO2 availability. However, a non-monotonic correlation between the dissolved CO2 concentration and catalytic activity is observed, with an optimum occurring at approximately 5 m CO2 concentration. At the highest applied CO2 pressures, catalysis is significantly attenuated despite higher CO2 concentrations and improved mass-transport characteristics, attributed in part to increased solution resistance. These results reveal that pressure-tunable CXE media can significantly intensify CO2 reduction rates over known electrocatalysts by alleviating substrate starvation, with CO2 pressure as a crucial variable for optimizing the efficiency of electrocatalytic CO2 conversion.

10.
Dalton Trans ; 48(33): 12396-12406, 2019 Sep 07.
Artigo em Inglês | MEDLINE | ID: mdl-31168559

RESUMO

[Cp*Rh] complexes (Cp* = pentamethylcyclopentadienyl) supported by bidentate chelating ligands are a useful class of compounds for studies of redox chemistry and catalysis. Here, we show that the bis(2-pyridyl)methane ligand, also known as dipyridylmethane or dpma, can support [Cp*Rh] complexes in the formally +iii and +ii rhodium oxidation states. Specifically, two new rhodium complexes ([Cp*Rh(dpma)(L)]n+, L = Cl-, CH3CN) have been isolated and structurally characterized, and the properties of the complexes have been compared with those of [Cp*Rh] complexes bearing the related dimethyldipyridylmethane (Me2dpma) ligand. Complex [Cp*Rh(dpma)(NCCH3)]2+ displays a quasireversible rhodium(iii/ii) reduction by cyclic voltammetry; related electron paramagnetic resonance (EPR) spectroscopic studies confirm access to the unusual rhodium(ii) oxidation state. Further reduction to the formally rhodium(i) oxidation state, however, is followed by deprotonation of dpma, as observed in electrochemical studies and chemical reduction experiments. This reactivity can be understood to occur as a consequence of the presence of doubly benzylic protons in the dpma ligand, since use of the analogous Me2dpma enables reduction to rhodium(i) without involvement of ligand deprotonation. These findings highlight the important role of the ligand backbone substitution pattern in influencing the stability of highly-reduced complexes, a key class of metal species for study of electron and proton management in catalysis.

11.
Dalton Trans ; 48(19): 6372-6382, 2019 May 15.
Artigo em Inglês | MEDLINE | ID: mdl-30942228

RESUMO

Understanding the behavior of molecular hydrogen-evolving electrocatalysts is often challenging, because secondary heterogeneous materials can form during reductive electrode polarization and contribute to catalysis. Here, we use an electrochemical quartz crystal microbalance to interrogate secondary heterogeneous materials formed in situ during hydrogen generation with the proton- and difluoroboryl-bridged dimethylglyoxime cobalt catalysts in acetonitrile electrolyte. Detrimental protonation reactions of the molecular precatalysts lead to formation of the transient heterogeneous materials, and the structure of the molecular cobaloxime precursors affects the potential dependence and kinetics of deposition and loss of heterogeneous material from the electrode surface. The strength of the acid in the electrolyte (needed as the proton source for H2 generation) also contributes to the stability and corrosion rate of the heterogeneous materials, as revealed in multistep polarization studies. Overall, these results support involvement of an interplay of electrodeposition, proton-driven (chemical) corrosion, and anodic (electrochemical) stripping in influencing the catalytic activity and the behavior of transient heterogeneous materials that may be generated during catalysis.

12.
Inorg Chem ; 58(6): 3606-3615, 2019 Mar 18.
Artigo em Inglês | MEDLINE | ID: mdl-30256096

RESUMO

Monomeric half-sandwich rhodium hydride complexes are often proposed as intermediates in catalytic cycles, but relatively few such compounds have been isolated and studied, limiting understanding of their properties. Here, we report preparation and isolation of a monomeric rhodium(III) hydride complex bearing the pentamethylcyclopentadienyl (Cp*) and bis(diphenylphosphino)benzene (dppb) ligands. The hydride complex is formed rapidly upon addition of weak acid to a reduced precursor complex, Cp*Rh(dppb). Single-crystal X-ray diffraction data for the [Cp*Rh] hydride, which were previously unavailable for this class of compounds, provide evidence of the direct Rh-H interaction. Complementary infrared spectra show the Rh-H stretching frequency at 1986 cm-1. In contrast to results with other [Cp*Rh] complexes bearing diimine ligands, treatment of the isolated hydride with strong acid does not result in H2 evolution. Electrochemical studies reveal that the hydride complex can be reduced only at very negative potentials (ca. -2.5 V vs ferrocenium/ferrocene), resulting in Rh-H bond cleavage and H2 generation. These results are discussed in the context of catalytic H2 generation, and development of design rules for improved catalysts bearing the [Cp*] ligand.

13.
Molecules ; 23(11)2018 Nov 02.
Artigo em Inglês | MEDLINE | ID: mdl-30400193

RESUMO

[Cp*Rh] complexes (Cp* = pentamethylcyclopentadienyl) are attracting renewed interest in coordination chemistry and catalysis, but these useful compounds often undergo net two-electron redox cycling that precludes observation of individual one-electron reduction events. Here, we show that a [Cp*Rh] complex bearing the 4,4'-dinitro-2,2'-bipyridyl ligand (dnbpy) (3) can access a distinctive manifold of five oxidation states in organic electrolytes, contrasting with prior work that found no accessible reductions in aqueous electrolyte. These states are readily generated from a newly isolated and fully characterized rhodium(III) precursor complex 3, formulated as [Cp*Rh(dnbpy)Cl]PF6. Single-crystal X-ray diffraction (XRD) data, previously unavailable for the dnbpy ligand bound to the [Cp*Rh] platform, confirm the presence of both [η5-Cp*] and [κ²-dnbpy]. Four individual one-electron reductions of 3 are observed, contrasting sharply with the single two-electron reductions of other [Cp*Rh] complexes. Chemical preparation and the study of the singly reduced species with electronic absorption and electron paramagnetic resonance spectroscopies indicate that the first reduction is predominantly centered on the dnbpy ligand. Comparative cyclic voltammetry studies with [NBu4][PF6] and [NBu4][Cl] as supporting electrolytes indicate that the chloride ligand can be lost from 3 by ligand exchange upon reduction. Spectroelectrochemical studies with ultraviolet (UV)-visible detection reveal isosbestic behavior, confirming the clean interconversion of the reduced forms of 3 inferred from the voltammetry with [NBu4][PF6] as supporting electrolyte. Electrochemical reduction in the presence of triethylammonium results in an irreversible response, but does not give rise to catalytic H2 evolution, contrasting with the reactivity patterns observed in [Cp*Rh] complexes bearing bipyridyl ligands with less electron-withdrawing substituents.


Assuntos
2,2'-Dipiridil/química , Cicloparafinas/química , Nitrocompostos/química , Ródio/química , Eletroquímica , Difração de Raios X
14.
Chem Commun (Camb) ; 54(14): 1694-1697, 2018 Feb 13.
Artigo em Inglês | MEDLINE | ID: mdl-29303524

RESUMO

The ligand 2,2'-bipyridine (bpy) can support metal centers in low formal oxidation states by delocalization of electron density into its π-system. We show that, in a model rhodium complex supported by the pentamethylcyclopentadienyl ligand (Cp*), the analogous dimethyldipyridylmethane ligand (Me2dpma) enforces a bpy-like coordination environment but disrupts the inter-ring conjugation responsible for charge delocalization upon metal reduction. As a result, reduction proceeds in discrete one-electron steps (Rh(iii) to Rh(ii) to Rh(i)), contrasting with the 2e- chemistry engendered by bpy. Upon reduction to Rh(i), the Me2dpma ligand rearranges to activate strong π-backbonding via facial coordination of one pyridine motif. Structural and spectroscopic studies confirm stabilization of the Rh(i) center in this compound, revealing a mode of metal-ligand cooperation that represents a useful counterpoint to charge delocalization in conjugated poly(pyridyl) ligands.

15.
Chemistry ; 24(1): 141-149, 2018 Jan 02.
Artigo em Inglês | MEDLINE | ID: mdl-29024095

RESUMO

Assembly of heterobimetallic complexes is synthetically challenging due to the propensity of ditopic ligands to bind metals unselectively. Here, we employ a novel divergent approach for selective preparation of a variety of bimetallic complexes within a ditopic macrocyclic ligand platform. In our approach, nickel is readily coordinated to a Schiff base cavity, and then a range of redox-inactive cations (M=Na+ , Ca2+ , Nd3+ , and Y3+ ) are installed in a pendant crown-ether-like site. This modular strategy allows access to complexes with the highly Lewis acidic trivalent cations Nd3+ and Y3+ , a class of compounds that were previously inaccessible. Spectroscopic and electrochemical studies reveal wide variations in properties that are governed most strongly by the trivalent cations. Exposure to dimethylformamide drives loss of Nd3+ and Y3+ from the pendant crown-ether site, suggesting solvent effects must be carefully considered in future applications involving use of highly Lewis acidic metals.

16.
ChemSusChem ; 10(22): 4589-4598, 2017 11 23.
Artigo em Inglês | MEDLINE | ID: mdl-29024563

RESUMO

We demonstrate that [Cp*Rh] complexes bearing substituted 2,2'-bipyridyl ligands are effective hydrogen evolution catalysts (Cp*=η5 -pentamethylcyclopentadienyl). Disubstitution (at the 4 and 4' positions) of the bipyridyl ligand (namely -tBu, -H, and -CF3 ) modulates the catalytic overpotential, in part due to involvement of the reduced ligand character in formally rhodium(I) intermediates. These reduced species are synthesized and isolated here; protonation results in formation of complexes bearing the unusual η4 -pentamethylcyclopentadiene ligand, and the properties of these protonated intermediates further govern the catalytic performance. Electrochemical studies suggest that multiple mechanistic pathways are accessible, and that the operative pathway depends on the applied potential and solution conditions. Taken together, these results suggest synergy in metal-ligand cooperation that modulates the mechanisms of fuel-forming catalysis with organometallic compounds bearing multiple non-innocent ligands.


Assuntos
Hidrogênio/química , Ródio/química , 2,2'-Dipiridil/química , Catálise , Técnicas Eletroquímicas , Fontes Geradoras de Energia , Ligantes , Compostos Organometálicos/química
17.
Inorg Chem ; 56(18): 11375-11386, 2017 Sep 18.
Artigo em Inglês | MEDLINE | ID: mdl-28862433

RESUMO

Recent work has shown that Cp*Rh(bpy) [Cp* = pentamethylcyclopentadienyl, bpy = 2,2'- bipyridine] undergoes endo protonation at the [Cp*] ligand in the presence of weak acid (Et3NH+; pKa = 18.8 in MeCN). Upon exposure to stronger acid (e.g., DMFH+; pKa = 6.1), hydrogen is evolved with unity yield. Here, we study the mechanisms by which this catalyst evolves dihydrogen using density functional theory (M06) with polarizable continuum solvation. The calculations show that the complex can be protonated by weak acid first at the metal center with a barrier of 3.2 kcal/mol; this proton then migrates to the ring to form the detected intermediate, a rhodium(I) compound bearing endo η4-Cp*H. Stronger acid is required to evolve hydrogen, which calculations show happens via a concerted mechanism. The acid approaches and protonates the metal, while the second proton simultaneously migrates from the ring with a barrier of ∼12 kcal/mol. Under strongly acidic conditions, we find that hydrogen evolution can proceed through a traditional metal-hydride species; protonation of the initial hydride to form an H-H bond occurs before migration of the hydride (in the form of a proton) to the [Cp*] ring (i.e., H-H bond formation is faster than hydride-proton tautomerization). This work demonstrates the role of acid strength in accessing different mechanisms of hydrogen evolution. Calculations also predict that modification of the bpy ligand by a variety of functional groups does not affect the preference for [Cp*] protonation, although the driving force for protonation changes. However, we predict that exchange of bpy for a bidentate phosphine ligand will stabilize a rhodium(III) hydride, reversing the preference for bound [Cp*H] found in all computed bpy derivatives and offering an appealing alternative ligand platform for future experimental and computational mechanistic studies of H2 evolution.

18.
Inorg Chem ; 56(17): 10824-10831, 2017 Sep 05.
Artigo em Inglês | MEDLINE | ID: mdl-28832122

RESUMO

There are few examples of the isolation of analogous metal complexes bearing [η5-Cp*] and [η4-Cp*H] (Cp* = pentamethylcyclopentadienyl) complexes within the same metal/ligand framework, despite the relevance of such structures to catalytic applications. Recently, protonation of Cp*Rh(bpy) (bpy = 2,2'-bipyridyl) has been shown to yield a complex bearing the uncommon [η4-Cp*H] ligand, rather than generating a [RhIII-H] complex. We now report the purification and isolation of this protonated species, as well as characterization of analogous complexes of 1,10-phenanthroline (phen). Specifically, reaction of Cp*Rh(bpy) or Cp*Rh(phen) with 1 equiv of Et3NH+Br- affords rhodium compounds bearing endo-η4-pentamethylcyclopentadiene (η4-Cp*H) as a ligand. NMR spectroscopy and single-crystal X-ray diffraction studies confirm protonation of the Cp* ligand, rather than formation of metal hydride complexes. Analysis of new structural data and electronic spectra suggests that phen is significantly reduced in Cp*Rh(phen), similar to the case of Cp*Rh(bpy). Backbonding interactions with olefinic motifs are activated by formation of [η4-Cp*H]; protonation of [Cp*] stabilizes the low-valent metal center and results in loss of reduced character on the diimine ligands. In accord with these changes in electronic structure, electrochemical studies reveal a distinct manifold of redox processes that are accessible in the [Cp*H] complexes in comparison with their [Cp*] analogues; these processes suggest new applications in catalysis for the complexes bearing endo-η4-Cp*H.

19.
Dalton Trans ; 46(35): 11779-11789, 2017 Sep 12.
Artigo em Inglês | MEDLINE | ID: mdl-28829062

RESUMO

A new tripodal ligand featuring three appended pyrene moieties has been prepared for studies of noncovalent binding of lanthanide metal complexes to graphitic carbon surfaces. The ligand tightly binds lanthanide(iii) ions by encapsulation within a heptadentate coordination environment; the cerium, neodymium, samarium, and europium complexes (all with formal metal oxidation state of +3) have been synthesized and characterized. These compounds are readily immobilized on graphitic electrodes-this is driven by the presence of the pyrene moieties, as complexes of an analogous ligand without pyrene groups are not stably adherent to the surface. X-ray photoelectron spectra confirm the molecular identity of the pyrene-appended metal complexes upon immobilization, with unique signals appearing for the metal centers as well as all for all symmetry-related nitrogen atoms on the ligand. Consistent with these surface characterization data and studies of a soluble model compound, the surface-immobilized cerium(iii) complex is reversibly oxidized to cerium(iv) near -0.34 V vs. Fc+/0. Based on electrochemical data, this complex is stable for minutes to hours on the surface.

20.
Chem Commun (Camb) ; 53(53): 7286-7289, 2017 Jun 29.
Artigo em Inglês | MEDLINE | ID: mdl-28426091

RESUMO

Molecular proton-reduction catalysts can decompose under the acidic conditions required for hydrogen evolution, resulting in formation of secondary metal-containing materials. Using an electrochemical quartz crystal microbalance (EQCM), we report a new method for probing electrodeposition of catalytically active heterogeneous material from molecular precursors. The data collected suggest that EQCM can provide a direct measure of the homogeneity of molecular proton-reduction catalysts over short timescales.

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