Search NASA⌕ Search

SEARCH · Search NASA

Results for “Complexes”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,513 records · Page 84
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Determining the Effects of Zero-Field Splitting and Magnetic Exchange in Dimeric Europium(II) Complexes

Related BAP [BAP = bis(acyl)phosphide] and Acac (Acac = β-diketonate) molecules perform as robust supports for both lanthanide and actinide metals. Here, a molecular bimetallic Eu 2+ complex was successfully targeted and isolated by employing sodium bis(mesitoyl)phosphide [Na( mes BAP)] in a salt metathesis with EuI 2 , producing [Eu( mes BAP) 2 (et 2 o)] 2 (et 2 o = metal-coordinated diethyl ether). The corresponding Acac-Eu 2+ complex was targeted using mes Acac– (1,3-dimesityl-1,3-propanedione), generating [Eu( mes Acac) 2 (et 2 o)] 2 . Both complexes were characterized by single-crystal X-ray diffraction, UV–vis, IR, and NMR spectroscopies, and variable-temperature magnetic susceptibility. [Eu( mes BAP) 2 (et 2 o)] 2 was persistent under anaerobic, anhydrous conditions, whereas the analogous [Eu( mes Acac) 2 (et 2 o)] 2 showed evidence of decomposition under identical conditions. Finally, variable-temperature magnetic susceptibility and magnetization studies of [Eu( mes BAP) 2 (et 2 o)] 2 and [Eu( mes Acac) 2 (et 2 o)] 2 were performed, resulting in similar magnetic exchange coupling values of J ex = –0.018 and –0.023 cm –1 and axial zero-field-splitting D values of –0.38 and –0.51 cm –1 , respectively.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

A DFT Comparison of C–C Reductive Coupling from Terminal Cyanido and Cyaphido Complexes of Nickel

The density functional theory study of the thermal C–C reductive coupling from terminal cyanido and hypothetical cyaphido complexes of [Ni(dmpe)] (dmpe = 1,2-bis(dimethylphosphino)ethane) revealed the key reaction intermediate in the reductive C–CP coupling being a σ-CC complex unlike an η 2 -aryl complex in the Ni C–CN system, as already observed in our previous studies. The reaction in THF is endothermic by 4.9 kcal/mol for cyanido with a 32.0 kcal/mol activation barrier and exothermic by 28.5 kcal/mol for cyaphido with an 11.3 kcal/mol activation barrier. To compare our results with the existing experimental data, we chose mesityl as the aryl group and also studied the CP reaction with [Pt(dmpe)] and [Pt(dmpm)] (dmpe = 1,2-bis(dimethylphosphino)methane) fragments. Our findings are consistent with the thermodynamically uphill photolytic C–CP bond activation in phosphaalkynes with Pt and a faster thermal back-reaction with [Pt(dmpe)] compared to that of [Pt(dmpm)]. Furthermore, based on the natural population analysis, when the polarity of the C–C bond is inverted, the sign of ΔG° is also inverted.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Ligand Noninnocence in β-Diketiminate and β-Diketimine Copper Complexes

Metal–ligand cooperative systems have a long precedent in catalysis, with the classification depending on the site of substrate bond cleavage and formation and on redox state changes. Recently, our group reported the participation of a β-diketiminate ligand in chemical bonding to heterocumulenes such as CO 2 and CS 2 by tricopper complexes, leading to cooperative catalysis. Herein, we report the reactivity of these copper clusters, [Cu 3 EL] – (E = S, Se; L = tris(β-diketiminate) cyclophane ligand), toward other electrophiles, viz. alkyl halides and Brønsted acids. We identified a family of ligand-functionalized complexes, Cu 3 EL (R) (R = primary alkyls), and a series of disubstituted products, Cu 3 EL (R) 2 , through single-crystal X-ray diffraction, mass spectrometry, and infrared and UV–visible spectroscopy. Furthermore, as part of mechanistic studies on these alkylation reactions, we evaluated the acid–base reactivity of these complexes and the influence of the backbone substitution on the reduction potential. Implications of these findings for ligand noninnocence and the relevance of the metal core as a cofactor for the ligand’s reactivity are discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Bis( tert -butoxydiphenylsilyl)amide Divalent Lanthanide Complexes

The development of new ligand systems to stabilize “nontraditional/ non-classical” divalent lanthanides is key to tuning the chemical and physical properties of their mixed principal quantum number 4f n 5d 1 ground states. The design and study of novel ligand systems which stabilize occupation of differing orbitals within the 5d manifold for these ions constitutes an area ripe for exploration. Our efforts toward the development of redox-innocent bulky silylamide ligands to stabilize pseudo-octahedral coordination geometries for divalent lanthanides have resulted in the synthesis of the bis( tert -butoxydiphenylsilyl) amide ligand, whose coordination complexes with Sm 2+ , Eu 2+ , and Yb 2+ are reported herein. These systems have been fully characterized by single-crystal X-ray diffraction, elemental analysis, cyclic voltammetry, direct-current magnetometry, and infrared, nuclear magnetic resonance, and electronic absorption spectroscopies. Attempts to extend this system to the more reducing Tm 2+ ion resulted in an inseparable mixture of products from which crystals of the analogous Tm 2+ species and a reduced dinitrogen, bimetallic Tm 3+ -Tm 3+ complex bridged by a η 2 -N 2 3− radical could be identified. Though progress toward six-coordinate complexes of reducing “traditional/ classical” divalent ions is noted for these systems, further work is needed to improve the synthetic utility of this ligand framework for the study of “non-traditional/non-classical” divalent lanthanides with a mixed-principal quantum number 4f n 5d 1 ground state.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Elucidation of an Unusually Long Pu–N Bond in a Plutonium(III)-Tetrazolate Complex

Four trivalent, f-element tetrazolate hydrate complexes [M(H 2 O) 9 ](Hdtb) 3 ·nH 2 O (Nd1, n = 7 and Pu1, n = 9; dtb 2– = 1,3-di(tetrazolate-5-yl)benzene) and [M(Hdtb)(H 2 O) 8 ](dtb)·11H 2 O (Nd2 and Pu2) were prepared using metathesis reactions. These complexes contain hydrated M(III) cations, but in the latter complexes, Nd2 and Pu2, one of the water molecules has been displaced by a long interaction between the M(III) cation and a Hdtb – anion. Notably, the Pu(III)–N bond in Pu2, representing the longest IX Pu(III)–N ( IX = nine coordinate) bond reported has a length of 2.8338(15) Å and is slightly shorter than the Nd(III)–N bond length of 2.8425(13) Å in Nd2. Analysis of bond lengths, Wiberg bond indices (WBI), natural localized molecular orbitals (NLMOs), and quantum theory of atoms in molecules (QTAIM) reveals that the metal contribution to the Pu(III)–N bond is marginally greater than that of the Pu(III)–OH 2 bonds in Pu2 and the Nd(III)–N bond in Nd2. Thus, this rather long M–N interaction provides an example where the expectation that An(III) compounds exhibit greater covalency with soft donor ligands compared to harder ligands fails. Furthermore, the absorption spectra of Pu1 and Pu2 further support this observation, highlighting a surprising degree of similarity in their electronic structures.

Covalent bonding↗
DOE OSTI2025

Effects of the Inverse Trans-Influence in a Berkelium(III) Phosphine Oxide Complex

An example of a 249 Bk 3+ phosphine oxide complex has been prepared to examine and quantify the effects of the inverse trans-influence (ITI) on a late actinide complex. This has been accomplished through a comparison of cis and trans Bk-ligand bonds in the meridional berkelium(III) complex, BkBr 3 (OPCy 3 ) 3 (OPCy 3 = tricyclohexylphosphine oxide). A detailed bond metric analysis was completed that includes the shortest published distance for a Bk 3+ −O bond, attributed to the smaller coordination number of Bk 3+ of six in mer-BkBr 3 (OPCy 3 ) 3 . ITI calculations of the trans Bk 3+ −O bond provide an ITI value (98.5(2)%) comparable to that of mer-AmBr 3 (OPCy 3 ) 3 (98.2(2)%) and indicate a small 5f orbital contribution to bonding. This effect is discussed in the context of the isomorphous lanthanide(III) series where the ITI is calculated to be higher for mer-BkBr 3 (OPCy 3 ) 3 than in most Ln 3+ analogues.

Actinides↗
DOE OSTI2025

Unexpected Hydricity of a Bis-Carbene Iridium Complex Provides Insight into Electronic Structure Impacts on Hydride Donor Ability

The hydricity (ΔG° H– ), or hydride donor ability, of a transition metal complex is a thermodynamic parameter which can aid in the design and interpretation of various catalytic reactions that involve hydride transfer as a key step. In an attempt to generate a strong hydride donor, the bis-carbene ligand 3,3′-methylenebis(1-methyl-imidazol-2-ylidene) (“bis-mim”) was installed in an iridium hydride complex, [Cp*Ir(bis-mim)H] + . Experimental and computational studies show that [Cp*Ir(bis-mim)H] + is actually a relatively weak hydride donor, however. To understand why the complex is an unexpectedly weak hydride donor, experimental and computational studies probing the steric and electronic effects on hydricity were conducted. Steric factors had a minimal impact on thermodynamic hydricity but dampened kinetic hydricity. In conclusion, the poor thermodynamic hydride donor ability can be attributed to an electronic structure that results in relatively long Cp*−Ir bonds, an unusually high Ir–H BDFE and pK a values that have an outsized influence on thermochemical cycles for hydricity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Electron transfer reactivity of the aqueous iron(IV)–oxo complex. Outer-sphere vs proton-coupled electron transfer

Here, the kinetics of oxidation of organic and inorganic reductants by aqueous iron(IV) ions, Fe IV (H 2 O) 5 O 2+ (hereafter Fe IV aq O 2+ ), are reported. The substrates examined include several water-soluble ferrocenes, hexachloroiridate(III), polypyridyl complexes M(NN) 3 2+ (M = Os, Fe and Ru; NN = phenanthroline, bipyridine and derivatives), HABTS–/ABTS 2– , phenothiazines, Co II (dmgBF 2 ) 2 , macrocyclic nickel(II) complexes, and aqueous cerium(III). Most of the reductants were oxidized cleanly to the corresponding one-electron oxidation products, with the exception of phenothiazines which produced the corresponding oxides in a single-step reaction, and polypyridyl complexes of Fe(II) and Ru(II) that generated ligand-modified products. Fe IV aq O 2+ oxidizes even Ce(III) (E 0 in 1 M HClO 4 = 1.7 V) with a rate constant greater than 10 4 M –1 s –1 . In 0.10 M aqueous HClO 4 at 25 °C, the reactions of Os(phen) 3 2+ (k = 2.5 × 10 5 M –1 s –1 ), IrCl 6 3– (1.6 × 10 6 ), ABTS 2– (4.7 × 10 7 ), and Fe(cp)(C 5 H 4 CH 2 OH) (6.4 × 10 7 ) appear to take place by outer sphere electron transfer (OSET). The rate constants for the oxidation of Os(phen) 3 2+ and of ferrocenes remained unchanged in the acidity range 0.05 < [H+] < 0.10 M, ruling out prior protonation of Fe IV aq O 2+ and further supporting the OSET assignment. A fit to Marcus cross-relation yielded a composite parameter (log k 22 + E 0 Fe /0.059) = 17.2 ± 0.8, where k 22 and E 0 Fe are the self-exchange rate constant and reduction potential, respectively, for the Fe IV aq O 2+ /Fe III aq O + couple. Comparison with literature work suggests k 22 < 10 –5 M –1 s –1 and thus E 0 (Fe IV aq O 2+ /Fe III aq O + ) > 1.3 V. For proton-coupled electron transfer, the reduction potential is estimated at E 0 (Fe IV aq O 2+ , H + /Fe III aq OH 2+ ) ≥ 1.95 V.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Stereoelectronic Effects in Cl 2 Elimination from Binuclear Pt(III) Complexes

Halogen photoelimination is the critical energy-storing step of metal-catalyzed HX-splitting photocycles. Homo- and heterobimetallic Pt(III) complexes display among the highest quantum efficiencies for halogen elimination reactions. Herein, we examine in detail the mechanism and energetics of halogen elimination from a family of binuclear Pt(III) complexes featuring meridionally coordinated Pt(III) trichlorides. Transient absorption spectroscopy, steady-state photocrystallography, and far-infrared vibrational spectroscopy suggest a halogen elimination mechanism that proceeds via two sequential halogen-atom-extrusion steps. Solution-phase calorimetry experiments of the meridional complexes have defined the thermodynamics of halogen elimination, which show a decrease in the photoelimination quantum efficiency with an increase in the thermochemically defined Pt–X bond strength. Conversely, when compared to an isomeric facial Pt(III) trichloride, a much more efficient photoelimination is observed for the fac isomer than would be predicted based on thermochemistry. Here, this difference in the fac vs mer isomer photochemistry highlights the importance of stereochemistry on halogen elimination efficiency and points to a mechanism-based strategy for achieving halogen elimination reactions that are both efficient and energy storing.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Pt–Mg, Pt–Ca, and Pt–Zn lantern complexes and metal-only donor–acceptor interactions [Pt-Mg Pt-Ca and Pt-Zn compounds with metal-only donor-acceptor interactions]

Here, Pt-based heterobimetallic lantern complexes of the form [PtM(SOCR) 4 (L)] have been shown previously to form intermolecular metallophilic interactions and engage in antiferromagnetic coupling between lanterns having M atoms with open shell configurations. In order to understand better the influence of the carboxylate bridge and terminal ligand on the electronic structure, as well as the metal–metal interactions within each lantern unit, a series of diamagnetic lantern complexes, [PtMg(SAc) 4 (OH 2 )] (1), [PtMg(tba) 4 (OH 2 )] (2), [PtCa(tba) 4 (OH 2 )] (3), [PtZn(tba) 4 (OH 2 )] (4), and a mononuclear control (Ph 4 P) 2 [Pt(SAc) 4 ] (5) have been synthesized. Crystallographic data show close Pt–M contacts enforced by the lantern structure in each dinuclear case. 195 Pt-NMR spectroscopy of 1–4, (Ph 4 P) 2 [Pt(SAc) 4 ] (5), and several previously reported lanterns revealed a strong chemical shift dependence on the identity of the second metal (M), mild influence by the thiocarboxylate ligand (SOCR; R = CH 3 (thioacetate, SAc), C 6 H 5 (thiobenzoate, tba)), and modest influence from the terminal ligand (L). Fluorescence spectroscopy has provided evidence for a Pt···Zn metallophilic interaction in [PtZn(SAc) 4 (OH 2 )], and computational studies demonstrate significant dative character. In all of 1–4, the short Pt–M distances suggest that metal-only Lewis donor (Pt)–Lewis acceptor (M) interactions could be present. DFT and NBO calculations, however, show that only the Zn examples have appreciable covalent character, whereas the Mg and Ca complexes are much more ionic.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Synthesis, Structure, and Vibrational Properties of [Ph 4 P] 2 NpO 2 Cl 4 and [Ph 4 P] 2 PuO 2 Cl 4 Complexes

In this paper, the synthesis, structure, and vibrational properties are presented for an isostructural series of Np(VI) and Pu(VI) complexes of the form [Ph 4 P] 2 AnO 2 Cl 4 , where An = Np(VI) or Pu(VI). The reported complexes are readily synthesized in ambient laboratory conditions, and their molecular structures were determined using single crystal X-ray diffraction. Their vibrational spectra were studied using a combination of Raman and FT-IR vibrational spectroscopies. Analysis of the vibrational spectra and force constants highlight the periodic properties associated with the actinide contraction and filling of the 5f electronic shells. Finally and additionally, we have assessed the utility of these complexes as conveniently synthesized starting materials for non-aqueous synthesis of transuranium molecules and materials.

25 ENERGY STORAGE↗
DOE OSTI2018

Periodic Trends in Actinyl Thio-Crown Ether Complexes

In-cavity complexes and their bonding features between thio-crown (TC) ethers and f-elements are unexplored so far. In this paper, actinyl(VI) (An = U, Np, Pu, Am, and Cm) complexes of TC ethers have been characterized using relativistic density functional theory. The TC ether ligands include tetrathio-12-crown-4 (12TC4), pentathio-15-crown-5 (15TC5), and hexathio-18-crown-6 (18TC6). On the basis of the calculations, it is found that the "double-decker" sandwich structure of AnO 2 (12TC4) 2 2+ and "side-on" structure AnO 2 (12TC4) 2+ are changed to "insertion" structures for AnO 2 (15TC5) 2+ and AnO 2 (18TC6) 2+ due to increased size of the TC ether ligands. The actinyl monocyclic TC ether complexes are found to exhibit conventional conformations, with typical An-O actinyl and An-Sliganddistances and angles. Chemical bonding analyses by Weinhold's natural population analysis (NPA), natural localized molecular orbital (NLMO), and energy decomposed analysis (EDA), show that a typical ionic An-S ligand bond with the extent of covalent interaction between the An and S atoms primarily attributable to the degree of radial distribution of the S 3p atomic orbitals. In conclusion, the similarity and difference of the oxo-crown and TC ethers as ligands for actinide coordination chemistry are discussed. As soft S-donor ligands, TC ethers may be candidate ligands for actinide recognition and extraction.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Influence of Substituents on the Electronic Structure of Mono- and Bis(phosphido) Thorium(IV) Complexes

Here, a series of metallocene thorium complexes with mono- and bis(phosphido) ligands have been investigated with varying hues: (C 5 Me 5 ) 2 Th(Cl)[P(Mes) 2 ] (Mes = mesityl = 2,4,6-(CH 3 ) 3 C 6 H 2 ; dark red-purple), (C 5 Me 5 ) 2 Th[P(Mes)(CH 3 )] 2 (dark red-purple), (C 5 Me 5 ) 2 Th(CH 3 )[P(Mes) 2 ] (dark red-purple), (C 5 Me 5 ) 2 Th(CH 3 )[P(Mes)(SiMe 3 )] (orange), (C 5 Me 5 ) 2 Th(Cl)[P(Mes)(SiMe 3 )] (orange), (C 5 Me 5 ) 2 Th[P(Mes)(SiMe 3 )] 2 (orange), and (C 5 Me 5 ) 2 Th[PH(Mes)] 2 (pale yellow). While all of these complexes bear a mesityl group on phosphorus, the electronic structure observed differs depending on the other substituent (mesityl, methyl, trimethylsilyl, or hydrogen). This sparked an investigation of the electronic structure of these complexes using 31 P NMR and electronic absorption spectroscopy in concert with time-dependent density functional theory calculations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Excited-State Switching between Ligand-Centered and Charge Transfer Modulated by Metal–Carbon Bonds in Cyclopentadienyl Iridium Complexes

Three series of pentamethylcyclopentadienyl (Cp*) Ir(III) complexes with different bidentate ligands were synthesized and structurally characterized, [Cp*Ir(tpy)L] n + (tpy = 2-tolylpyridinato; n = 0 or 1), [Cp*Ir(piq)L] n + (piq = 1-phenylisoquinolinato; n = 0 or 1), and [Cp*Ir(bpy)L] m + (bpy = 2,2'-bipyridine; m = 1 or 2), featuring a range of monodentate carbon-donor ligands within each series [L = 2,6-dimethylphenylisocyanide; 3,5-dimethylimidazol-2-ylidene (NHC); methyl)]. The spectroscopic and photophysical properties of these molecules and those of the photocatalyst [Cp*Ir(bpy)H] + were investigated to establish electronic structure–photophysical property relationships that engender productive photochemical reactivity of this hydride and its methyl analogue. The Ir(III) chromophores containing ancillary CNAr ligands exhibited features anticipated for predominantly ligand-centered (LC) excited states, and analogues bearing the NHC ancillary exhibited properties consistent with LC excited states containing a small admixture of metal-to-ligand charge-transfer (MLCT) character. Yet, the molecules featuring anionic and strongly σ-donating methyl or hydride ligands exhibited photophysical properties consistent with a high degree of CT character. Density functional theory calculations suggest that the lowest energy triplet states in these complexes are composed of a mixture of MLCT and ligand-to-ligand CT originating from both the Cp* and methyl or hydride ancillary ligands. The high degree of CT character in the triplet excited states of methyliridium complexes bearing C^N-cyclometalated ligands offer a striking contrast to the photophysical properties of pseudo-octahedral structures fac -Ir(C^N) 3 or Ir(C^N) 2 (acac) that have lowest-energy triplet excited states characterized as primarily LC character with a more moderate MLCT admixture.

14 SOLAR ENERGY↗
DOE OSTI2019

Synthetic Model Complex of the Key Intermediate in Cytochrome P450 Nitric Oxide Reductase

Fungal denitrification plays a crucial role in the nitrogen cycle and contributes to the total N 2 O emission from agricultural soils. Here, cytochrome P450 NO reductase (P450nor) reduces two NO to N 2 O using a single heme site. Despite much research, the exact nature of the critical "Intermediate I" responsible for the key N-N coupling step in P450nor is unknown. This species likely corresponds to a Fe-NHOH-type intermediate with an unknown electronic structure. Here we report a new strategy to generate a model system for this intermediate, starting from the iron(III) methylhydroxylamide complex [Fe(3,5-Me-BAFP)(NHOMe)] (1), which was fully characterized by 1 H NMR, UV-vis, electron paramagnetic resonance, and vibrational spectroscopy (rRaman and NRVS). Our data show that 1 is a high-spin ferric complex with an N-bound hydroxylamide ligand that is strongly coordinated (Fe–N distance, 1.918 Å; Fe-NHOMe stretch, 558 cm –1 ). Simple one-electron oxidation of 1 at –80 °C then cleanly generates the first model system for Intermediate I, [Fe(3,5-Me-BAFP)(NHOMe)] + (1 + ). UV–vis, resonance Raman, and Mössbauer spectroscopies, in comparison to the chloro analogue [Fe(3,5-Me-BAFP)(Cl)] + , demonstrate that 1 + is best described as an Fe III -(NHOMe)• complex with a bound NHOMe radical. Further reactivity studies show that 1 + is highly reactive toward NO, a reaction that likely proceeds via N–N bond formation, following a radical–radical-type coupling mechanism. Here our results therefore provide experimental evidence, for the first time, that an Fe III -(NHOMe)• electronic structure is indeed a reasonable electronic description for Intermediate I and that this electronic structure is advantageous for P450nor catalysis because it can greatly facilitate N–N bond formation and, ultimately, N 2 O generation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Molecular and Electronic Structure, and Hydrolytic Reactivity of a Samarium(II) Crown Ether Complex

The reaction of SmI 2 with dibenzo-30-crown-10 (DB30C10), followed by metathesis with [Bu 4 N][BPh 4 ], allows for the isolation of [Sm II (DB30C10)][BPh 4 ] 2 as bright-red crystals in good yield. Moreover, exposure of [Sm(DB30C10)] 2+ to solvents containing trace water results in the conversion to the dinuclear Sm III complex, Sm 2 (DB30C10)(OH) 2 I 4 . Structural analysis of both complexes shows substantial rearrangement of the crown ether from a folded, Pac-Man form with Sm II to a twisted conformation with Sm III . The optical properties of [Sm II (DB30C10)][BPh 4 ] 2 exhibit a strong temperature dependence and change from broad-band absorption features indicative of domination by 5d states to fine features characteristic of 4f → 4f transitions at low temperatures. Here, exploration of the electronic structure of these complexes via ab initio wave function calculations (SO-CASSCF) reflects that the ground state of Sm II in [Sm II (DB30C10)] 2+ is a 4f 6 state with low-lying 4f 5 5d 1 states, where the latter states have been lowered in energy by ~12 000 cm –1 with respect to the free ion. The decacoordination of the Sm II cation by the crown ether is responsible for this alteration in the energies of the excited state and demonstrates the ability to tune the electronic structure of Sm II .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Effect of Redox Active Ligands on the Electrochemical Properties of Manganese Tricarbonyl Complexes

The synthesis, structural characterization, and electrochemical behavior of the neutral Mn(azpy)(CO) 3 (Br) 4 (azpy = 2-phenylazopyridine) complex is reported and compared with its structural analogue Mn(bipy)(CO) 3 (Br) 1 (bipy = 2,2'-bipyridine). 4 exhibits reversible two-electron reduction at a mild potential (-0.93 V vs Fc +/0 in acetonitrile) in contrast to 1, which exhibits two sequential one-electron reductions at -1.68 V and -1.89 V vs Fc +/0 in acetonitrile. The key electronic structure differences between 1 and 4 that lead to disparate electrochemical properties are investigated using a combination of Mn-K-edge X-ray absorption spectroscopy (XAS), Mn-Kβ X-ray emission spectroscopy (XES), and density functional theory (DFT) on 1, 4, their debrominated analogues, [Mn(L)(CO) 3 (CH 3 CN)][CF 3 SO 3 ] (L = bipy 2, azpy 5), and two-electron reduced counterparts [Mn(bipy)(CO) 3 ][K(18-crown-6)] 3 and [Mn(azpy)(CO)3][Cp2Co] 6. The results reveal differences in the distribution of electrons about the CO and bidentate ligands (bipy and azpy), particularly upon formation of the highly reduced, formally Mn(-1) species. The data show that the degree of ligand noninnocence and resulting redox-activity in Mn(L)(CO) 3 type complexes impacts not only the reducing power of such systems, but the speciation of the reduced complexes via perturbation of the monomer-dimer equilibrium in the singly reduced Mn(0) state. In conclusion, this study highlights the role of redox-active ligands in tuning the reactivity of metal centers involved in electrocatalytic transformations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Insensitivity of Magnetic Coupling to Ligand Substitution in a Series of Tetraoxolene Radical-Bridged Fe 2 Complexes

The elucidation of magnetostructural correlations between bridging ligand substitution and strength of magnetic coupling is essential to the development of high-temperature molecule-based magnetic materials. Toward this end, we report the series of tetraoxolene-bridged Fe II 2 complexes [(Me 3 TPyA) 2 Fe 2 ( R L)] n + (Me 3 TPyA = tris(6-methyl-2-pyridylmethyl)amine; n = 2: OMe LH 2 = 3,6-dimethoxy-2,5-dihydroxo-1,4-benzoquinone, Cl LH 2 = 3,6-dichloro-2,5-dihydroxo-1,4-benzoquinone, Na 2 [ NO 2 L] = sodium 3,6-dinitro-2,5-dihydroxo-1,4-benzoquinone; n = 4: SMe 2 L = 3,6-bis(dimethylsulfonium)-2,5-dihydroxo-1,4-benzoquinone diylide) and their one-electron-reduced analogues. Furthermore, variable-temperature dc magnetic susceptibility data reveal the presence of weak ferromagnetic superexchange between Fe II centers in the oxidized species, with exchange constants of J = +1.2(2) (R = OMe, Cl) and +0.3(1) (R = NO 2 , SMe 2 ) cm –1 . In contrast, X-ray diffraction, cyclic voltammetry, and Mössbauer spectroscopy establish a ligand-centered radical in the reduced complexes. Magnetic measurements for the radical-bridged species reveal the presence of strong antiferromagnetic metal–radical coupling, with J = -57(10), -60(7), -58(6), and -65(8) cm –1 for R = OMe, Cl, NO 2 , and SMe 2 , respectively. The minimal effects of substituents in the 3- and 6-positions of R L x –• on the magnetic coupling strength is understood through electronic structure calculations, which show negligible spin density on the substituents and associated C atoms of the ring. Finally, the radical-bridged complexes are single-molecule magnets, with relaxation barriers of U eff = 50(1), 41(1), 38(1), and 33(1) cm –1 for R = OMe, Cl, NO 2 , and SMe 2 , respectively. Taken together, these results provide the first examination of how bridging ligand substitution influences magnetic coupling in semiquinoid-bridged compounds, and they establish design criteria for the synthesis of semiquinoid-based molecules and materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗