- NSF-PAR ID:
- 10473503
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- ACS Catalysis
- Volume:
- 13
- Issue:
- 20
- ISSN:
- 2155-5435
- Page Range / eLocation ID:
- 13650 to 13662
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
null (Ed.)A series of complexes with low-energy Fe II to Ti IV metal-to-metal charge-transfer (MMCT) transitions, Cp 2 Ti(C 2 Fc) 2 , Cp* 2 Ti(C 2 Fc) 2 , and MeOOC Cp 2 Ti(C 2 Fc) 2 , was investigated using solvatochromism and resonance Raman spectroscopy (RRS) augmented with time-dependent density functional theory (TDDFT) calculations in order to interrogate the nature of the CT transitions. Computational models were benchmarked against the experimental UV-Vis spectra and B3LYP/6-31G(d) was found to most faithfully represent the spectra. The energy of the MMCT transition was measured in 15 different solvents and a multivariate fit to the Catalán solvent parameters – solvent polarizability (SP), solvent dipolarity (SdP), solvent basicity (SB), and solvent acidity (SA) – was performed. The effect of SP indicates a greater degree of electron delocalization in the excited state (ES) than the ground state (GS). The small negative solvatochromism with respect to SdP indicates a smaller dipole moment in the ES than the GS. The effect of SB is consistent with charge-transfer to Ti. Upon excitation into the MMCT absorption band, the RRS data show enhancement of the alkyne stretching modes and of the out-of-plane bending modes of the cyclopentadienyl ring connected to Fe and the alkyne bridge. This is consistent with changes in the oxidation states of Ti and Fe, respectively. The higher-energy transitions (350–450 nm) show enhancement of vibrational modes consistent with ethnylcyclopentadienyl to Ti ligand-to-metal charge transfer (LMCT). The RRS data is consistent with the TDDFT predicted character of these transitions. TDDFT suggests that the lowest-energy transition in Cp 2 Ti(C 2 Fc) 2 CuI, where CuI is coordinated between the alkynes, retains its Fe II to Ti IV MMCT character, in agreement with the RRS data, but that the lowest-energy transitions have significant CuI to Ti character. For Cp 2 Ti(C 2 Fc) 2 CuI, excitation into the low-energy MMCT absorption band results in selective enhancement of the symmetric alkynyl stretching mode.more » « less
-
While synthesizing a series of rhenium–lanthanide triple inverse sandwich complexes, we unexpectedly uncovered evidence for rare examples of end-on lanthanide dinitrogen coordination for certain heavy lanthanide elements as well as for uranium. We begin our report with the synthesis and characterization of a series of trirhenium triple inverse sandwich complexes with the early lanthanides, Ln[(μ-η5:η5-Cp)Re(BDI)]3(THF) (1-Ln, Ln = La, Ce, Pr, Nd, Sm; Cp = cyclopentadienide, BDI = N,N′-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate). However, as we moved across the lanthanide series, we ran into an unexpected result for gadolinium in which we structurally characterized two products for gadolinium, namely, 1-Gd (analogous to 1-Ln) and a diazenido dirhenium double inverse sandwich complex Gd[(μ-η1:η1-N2)Re(η5-Cp)(BDI)][(μ-η5:η5-Cp)Re(BDI)]2(THF)2 (2-Gd). Evidence for analogues of 2-Gd was spectroscopically observed for other heavy lanthanides (2-Ln, Ln = Tb, Dy, Er), and, in the case of 2-Er, structurally authenticated. These complexes represent the first observed examples of heterobimetallic end-on lanthanide dinitrogen coordination. Density functional theory (DFT) calculations were utilized to probe relevant bonding interactions and reveal energetic differences between both the experimental and putative 1-Ln and 2-Ln complexes. We also present additional examples of novel end-on heterobimetallic lanthanide and actinide diazenido moieties in the erbium–rhenium complex (η8-COT)Er[(μ-η1:η1-N2)Re(η5-Cp)(BDI)](THF)(Et2O) (3-Er) and uranium–rhenium complex [Na(2.2.2-cryptand)][(η5-C5H4SiMe3)3U(μ-η1:η1-N2)Re(η5-Cp)(BDI)] (4-U). Finally, we expand the scope of rhenium inverse sandwich coordination by synthesizing divalent double inverse sandwich complex Yb[(μ-η5:η5-Cp)Re(BDI)]2(THF)2 (5-Yb), as well as base-free, homoleptic rhenium–rare earth triple inverse sandwich complex Y[(μ-η5:η5-Cp)Re(BDI)]3 (6-Y).more » « less
-
The excited-state dynamics of fac-Co(ppy)3, where ppy = 2-[2-(pyridyl)phenyl], are measured with femtosecond UV-Vis transient absorption spectroscopy. The initial state is confirmed with spectroelectrochemistry to have significant metal-to-ligand charge transfer (MLCT) character, unlike other Co complexes that generally have ligand-to-metal charge transfer or ligand-field transitions in this energy range. Ground-state recovery occurs in 8.65 ps in dichloromethane. Density functional theory (DFT) calculations show that the MLCT state undergoes Jahn-Teller distortion and converts to a 5-coordinate 3MC state in which one Co-N bond is broken. The results highlights a potential pitfall of heteroleptic-bidentate ligands when designing strong-field ligands for transition metal chromophores.more » « less
-
Abstract Biological N2reduction occurs at sulfur‐rich multiiron sites, and an interesting potential pathway is concerted double reduction/ protonation of bridging N2through PCET. Here, we test the feasibility of using synthetic sulfur‐supported diiron complexes to mimic this pathway. Oxidative proton transfer from μ‐η1 : η1‐diazene (HN=NH) is the microscopic reverse of the proposed N2fixation pathway, revealing the energetics of the process. Previously, Sellmann assigned the purple metastable product from two‐electron oxidation of [{Fe2+(PPr3)L1}2(μ‐η1 : η1‐N2H2)] (L1=tetradentate SSSS ligand) at −78 °C as [{Fe2+(PPr3)L1}2(μ‐η1 : η1‐N2)]2+, which would come from double PCET from diazene to sulfur atoms of the supporting ligands. Using resonance Raman, Mössbauer, NMR, and EPR spectroscopies in conjunction with DFT calculations, we show that the product is not an N2complex. Instead, the data are most consistent with the spectroscopically observed species being the mononuclear iron(III) diazene complex [{Fe(PPr3)L1}(η2‐N2H2)]+. Calculations indicate that the proposed double PCET has a barrier that is too high for proton transfer at the reaction temperature. Also, PCET from the bridging diazene is highly exergonic as a result of the high Fe3+/2+redox potential, indicating that the reverse N2protonation would be too endergonic to proceed. This system establishes the “ground rules” for designing reversible N2/N2H2interconversion through PCET, such as tuning the redox potentials of the metal sites.
-
Abstract To explore the structure–function relationships of cobalt complexes in the catalytic hydrogen evolution reaction (HER), we studied the substitution of a tertiary amine with a softer pyridine group and the inclusion of a conjugated bpy unit in a Co complex with a new pentadentate ligand, 6‐[6‐(1,1‐di‐pyridin‐2‐yl‐ethyl)‐pyridin‐2‐ylmethyl]‐[2,2′]bipyridinyl (Py3Me‐Bpy). These modifications resulted in significantly improved stability and activity in both electro‐ and photocatalytic HER in neutral water. [Co(Py3Me‐Bpy)(OH2)](PF6)2catalyzes the electrolytic HER at −1.3 V (vs. SHE) for 20 hours with a turnover number (TON) of 266 300, and photolytic HER for two days with a TON of 15 000 in pH 7 aqueous solutions. The softer ligand scaffold possibly provides increased stability towards the intermediate CoIspecies. DFT calculations demonstrate that HER occurs through a general electron transfer/proton transfer/electron transfer/proton transfer pathway, with H2released from the protonation of CoII−H species.