Spin-State Effects on the Thermal Dihydrogen Release from Solid-State [MH(η²-H₂)dppe₂]⁺ (M = Fe, Ru, Os) Organometallic Complexes for Hydrogen Storage Applications

Abstract

Mössbauer spectroscopy, experimental thermodynamic measurements, and computational studies were performed to investigate the properties of molecular hydrogen binding to the organometallic fragments [MHdppe2]+ (M = Fe, Ru, Os; dppe =1,2-bis(diphenylphosphino)ethane) to form the dihydrogen complex fragments [MH(η2-H2)dppe2]+. Mössbauer spectroscopy showed that the dehydrogenated complex [FeHdppe2]+ adopts a geometry consistent with the triplet spin state, transitioning to a singlet state complex upon addition of the dihydrogen molecule in a manner similar to the previously studied dinitrogen complexes. From simulations, this spin transition behavior was found to be responsible for the strong binding behavior experimentally observed in the iron complex. Spin-singlet to spin-singlet transitions were found to exhibit thermodynamics consistent with the 5d > 3d > 4d binding trend observed for other transition metal dihydrogen complexes. Finally, the method for distinguishing between dihydrogen and dihydride complexes based on partial quadrupole splittings observed in Mössbauer spectra was confirmed, providing a tool for further characterization of these unique species for Mössbauer active compounds.

Dihydrogen Complexation

“…The chemistry of ’ignoble’ metals is receiving increased attention, but this is an area where spin state problems and paramagnetism are more likely. For example, the reaction of [MH­(dppe) 2 ] + (M = Fe, Ru, Os; dppe =1,2-bis­(diphenylphosphino)­ethane) with H 2 to give trans -[MH­(H 2 )­(dppe) 2 ] + was probed by Fultz and co-workers who documented a triplet to singlet spin state transition on binding that accounted for the strong binding affinity for M = Fe. A number of Ni­(II)­(H 2 ) complexes have also been reported recently. , In paramagnetic (H 2 ) complexes, EPR data can provide useful information.…”

Real-time noise reduction for Mössbauer spectroscopy through online implementation of a modified Kalman filter

“…Smoothing algorithms such as Savitzky-Golay [12] and Whittaker [13] smoothers are common for extracting peaks from noisy data, and are available in commercial spectrum processing software [14]. However, such techniques are often discouraged for Mössbauer spectroscopy due to an undesirable broadening of the peaks caused by these smoothers and the common presence of highly overlapping peaks within the spectra [15,16]. Advancements in processing power and memory management, however, have enabled researchers to revisit software algorithms as a means of improving spectrum characteristics, leading to improvements in the speed and operations of Mössbauer spectroscopy beyond what was available from hardware improvements alone.…”

High-Frequency Fe–H and Fe–H2 Modes in a trans-Fe(η2-H2)(H) Complex: A Speed Record for Nuclear Resonance Vibrational Spectroscopy

“…Nuclear resonance vibrational spectroscopy (NRVS) − has become a popular technique for elucidating the element-selective normal modes of appropriate Mössbauer isotopes. − In previous work, we have shown that 57 Fe-NRVS can be utilized to observe Fe–H bending and Fe–H–Fe­(Ni) wagging modes in various model compounds as well as intermediate species of [NiFe] and [FeFe] hydrogenases. − However, to date, the only reported Fe–H stretching modes were the bands around 1230/1430 cm –1 for a doubly bridging Fe­(μ-H) 2 Fe complex and at 1468/1532 cm –1 for a singly bridging Ni­(μ-H)Fe complex . To better gauge the feasibility for observing even higher frequency stretching modes expected for an iron-terminal hydride and dihydrogen, we have applied NRVS to examine a well-studied trans -[ 57 Fe­(η 2 -H 2 )­(H)­(dppe) 2 ]­[BPh 4 ] [dppe = 1,2-bis­(diphenylphosphino)­ethane] complex, , hereafter called 1 H H2 . As shown in Figure , our density functional theory (DFT) modeling of 1 H H2 produced an optimized structure that overlaps well with the X-ray diffraction result .…”

“…21−23 However, to date, the only reported Fe−H stretching modes were the bands around 1230/1430 cm −1 for a doubly bridging Fe(μ-H) 2 Fe complex 24 and at 1468/1532 cm −1 for a singly bridging Ni(μ-H)Fe complex. 20 To better gauge the feasibility for observing even higher frequency stretching modes expected for an iron-terminal hydride and dihydrogen, we have applied NRVS to examine a well-studied trans-[ 57 Fe(η 2 -H 2 )(H)(dppe) 2 ][BPh 4 ] [dppe = 1,2-bis(diphenylphosphino)ethane] complex, 25,26 hereafter called 1 H…”