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DOE OSTI2024

Goethite and Hematite Nucleation and Growth from Ferrihydrite: Effects of Oxyanion Surface Complexes

The presence of oxyanions, such as nitrate (NO 3 – ) and phosphate (PO 4 3– ), regulates the nucleation and growth of goethite (Gt) and hematite (Hm) during the transformation of ferrihydrite (Fh). Our previous studies showed that oxyanion surface complexes control the rate and pathway of Fh transformation to Gt and Hm. However, how oxyanion surface complexes control the mechanism of Gt and Hm nucleation and growth during the Fh transformation is still unclear. Here, we used synchrotron scattering methods and cryogenic transmission electron microscopy to investigate the effects of NO 3 – outer-sphere complexes and PO 4 3– inner-sphere complexes on the mechanism of Gt and Hm formation from Fh. Our TEM results indicated that Gt particles form through a two-step model in which Fh particles first transform to Gt nanoparticles and then crystallographically align and grow to larger particles by oriented attachment (OA). In contrast, for the formation of Hm, imaging shows that Fh particles first aggregate and then transform to Hm through interface nucleation. This is consistent with our X-ray scattering results, which demonstrate that NO 3 – outer-sphere and PO 4 3– inner-sphere complexes promote the formation of Gt and Hm, respectively. These results have implications for understanding the coupled interactions of oxyanions and iron oxy-hydroxides in Earth-surface environments.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2017

Complexation and Redox Buffering of Iron(II) by Dissolved Organic Matter

Iron (Fe) bioavailability depends upon its solubility and oxidation state, which are strongly influenced by complexation with natural organic matter (NOM). Despite observations of Fe(II)-NOM associations under conditions favorable for Fe oxidation, the molecular mechanisms by which NOM influences Fe(II) oxidation remain poorly understood. In this study, we used X-ray absorption spectroscopy to determine the coordination environment of Fe(II) associated with NOM (as-received and chemically reduced) at pH 7, and investigated the effect of NOM complexation on Fe(II) redox stability. Linear combination fitting of extended X-ray absorption fine structure (EXAFS) data using reference organic ligands demonstrated that Fe(II) was complexed primarily by carboxyl functional groups in reduced NOM. Functional groups more likely to preserve Fe(II) represent much smaller fractions of NOM-bound Fe(II). Fe(II) added to anoxic solutions of as-received NOM oxidized to Fe(III) and remained organically complexed. Iron oxidation experiments revealed that the presence of reduced NOM limited Fe(II) oxidation, with over 50% of initial Fe(II) remaining after 4 h. Furthermore, these results suggest reduced NOM may preserve Fe(II) by functioning both as redox buffer and complexant, which may help explain the presence of Fe(II) in oxic circumneutral waters.

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

A Single Small-Scale Plutonium Redox Reaction System Yields Three Crystallographically-Characterizable Organoplutonium Complexes

Here, an approach to obtaining substantial amounts of data from a hazardous starting material that can only be obtained and handled in small quantities is demonstrated by the investigation of a single small-scale reaction of cyclooctatetraene, C 8 H 8 , with a solution obtained from the reduction of Cp' 3 Pu (Cp' = C 5 H 4 SiMe 3 ) with potassium graphite. This one reaction coupled with oxidation of a product has provided single-crystal X-ray structural data on three organoplutonium compounds as well as information on redox chemistry thereby demonstrating an efficient route to new reactivity and structural information on this highly radioactive element. The crystal structures were obtained from the reduction of C 8 H 8 by a putative Pu(II) complex, (Cp' 3 Pu II ) 1– , generated in situ, to form the Pu(III) cyclooctatetraenide complex, [K(crypt)][(C 8 H 8 ) 2 Pu III ], 1-Pu , and the tetra(cyclopentadienyl) Pu(III) complex, [K(crypt)][Cp' 4 Pu III ], 2-Pu . Oxidation of the sample of 1-Pu with Ag(I) afforded a third organoplutonium complex that has been structurally characterized for the first time, (C 8 H 8 ) 2 Pu IV , 3-Pu . Complexes 1-Pu and 3-Pu contain Pu sandwiched between parallel (C 8 H 8 ) 2– rings. The (Cp' 4 Pu III ) – anion in 2-Pu features three η 5 -Cp' rings and one η 1 -Cp' ring, which is a rare example of a formal Pu–C η 1 -bond. In addition, this study addresses the challenge of small-scale synthesis imparted by radiological and material availability of transuranium isotopes, in particular that of pure metal samples. A route to an anhydrous Pu(III) starting material from the more readily available Pu IV O 2 was developed to facilitate reproducible syntheses and allow complete spectroscopic analysis of 1-Pu and 2-Pu . Pu IV O 2 was converted to Pu III Br 3 (DME) 2 (DME = CH 3 OCH 2 CH 2 OCH 3 ) and subsequently Pu III Br 3 (THF) x , which was used to independently synthesize 1-Pu , 2-Pu , and 3-Pu .

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2021

Impact of Noncovalent Interactions on the Structural Chemistry of Thorium(IV)-Aquo-Chloro Complexes

Five novel tetravalent thorium (Th) compounds that consist of Th(H 2 O) x Cl y structural units were isolated from acidic aqueous solutions using a series of nitrogen-containing heterocyclic hydrogen (H) bond donors. Taken together with three previously reported phases, the compounds provide a series of monomeric Th IV complexes wherein the effects of noncovalent interactions (and H-bond donor identity) on Th structural chemistry can be examined. Seven distinct structural units of the general formulas [Th(H 2 O) x Cl 8–x ] x-4 (x = 2, 4) and [Th(H 2 O) x Cl 9–x] x-5 (x = 5–7) are described. The complexes range from chloride-deficient [Th(H 2 O) 7 Cl 2 ] 2+ to chloride-rich [Th(H 2 O) 2 Cl 6 ] 2– species, and theory was used to understand the relative energies that separate complexes within this series via the stepwise chloride addition to an aquated Th cation. Electronic structure theory predicted the reaction energies of chloride addition and release of water through a series of transformations, generally highlighting an energetic driving force for chloride complexation. To probe the role of the counterion in the stabilization of these complexes, electrostatic potential (ESP) surfaces were calculated. Finally, the ESP surfaces indicated a dependence of the chloride distribution about the Th metal center on the pK a of the countercation, highlighting the directing effects of noncovalent interactions (e.g., Hbonding) on Th speciation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Synthesis, Characterization, and Water Oxidation Activity of Isomeric Ru Complexes

The synthesis and characterization of the isomeric ruthenium complexes with the general formula cis- and trans-[Ru(trpy)(qc)X] n+ (trpy is 2,2':6',2"-terpyridine, qc is 8-quinolinecarboxylate, cis-1 and trans-1, X = Cl, n = 0; cis-2 and trans-2, X=OH2, n = 1) with respect to the relative disposition of the carboxylate and X ligands are reported. For comparison purposes, another set of ruthenium complexes with general formula cis- and trans-[Ru(trpy)(pic)(OH 2 )]+ (pic is 2-picolinate (cis-3, trans-3)) have been prepared. The complexes with a qc ligand show a more distorted geometry compared to the complexes with a pic ligand. In all of the cases, the trans isomers show lower potential values for all of the redox couples relative to the cis isomers. Furthermore, complexes cis-2 and trans-2 with six-member chelate rings show higher catalytic activity than cis-3 and trans-3. Overall, it was shown that the electronic perturbation to the metal center exerted by different orientation and geometry of the ligands significantly influences both redox properties and catalytic performance.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Multiple N–H and C–H Hydrogen Atom Abstractions Through Coordination-Induced Bond Weakening at Fe-Amine Complexes

We report the use of the reported Fe-phthalocyanine complex, PcFe (1; Pc = 1,4,8,11,15,18,22,25-octaethoxy-phthalocyanine), to generate PcFe–amine complexes 1-(NH 3 ) 2 , 1-(MeNH 2 ) 2 , and 1-(Me 2 NH) 2 . Treatment of 1 or 1-(NH 3 ) 2 to an excess of the stable aryloxide radical, 2,4,6-tritert-butylphenoxyl radical ( t Bu ArO•), under NH 3 resulted in catalytic H atom abstraction (HAA) and C–N coupling to generate the product 4-amino-2,4,6-tritert-butylcyclohexa-2,5-dien-1-one (2) and t Bu ArOH. Exposing 1-(NH 3 ) 2 to an excess of the trityl (CPh 3 ) variant, 2,6-di-tert-butyl-4-tritylphenoxyl radical ( Tr ArO • ), under NH 3 did not lead to catalytic ammonia oxidation as previously reported in a related Ru-porphyrin complex. However, pronounced coordination-induced bond weakening of both α N–H and β C–H in the alkylamine congeners, 1-(MeNH 2 ) 2 and 1-(Me 2 NH) 2 , led to multiple HAA events yielding the unsaturated cyanide complex, 1-(MeNH 2 )(CN), and imine complex, 1-(MeN=CH 2 ) 2 , respectively. Subsequent C–N bond formation was also observed in the latter upon addition of a coordinating ligand. Detailed computational studies support an alternating mechanism involving sequential N–H and C–H HAA to generate these unsaturated products.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Deeper Understanding of Mononuclear Manganese(IV)–Oxo Binding Brønsted and Lewis Acids and the Manganese(IV)–Hydroxide Complex

Binding of Lewis acidic metal ions and Brønsted acid at the metal–oxo group of high-valent metal–oxo complexes enhances their reactivities significantly in oxidation reactions. However, such a binding of Lewis acids and proton at the metal–oxo group has been questioned in several cases and remains to be clarified. In this work, we report the synthesis, characterization, and reactivity studies of a mononuclear manganese(IV)–oxo complex binding triflic acid, {[(dpaq)Mn IV (O)]–HOTf} + (1–HOTf). First, 1–HOTf was synthesized and characterized using various spectroscopic techniques, including resonance Raman (rRaman) and X-ray absorption spectroscopy/extended X-ray absorption fine structure. In particular, in rRaman experiments, we observed a linear correlation between the Mn–O stretching frequencies of 1–HOTf (e.g., ν Mn–O at ~793 cm –1 ) and 1–M n+ (Mn + = Ca 2+ , Zn 2+ , Lu 3+ , Al 3+ , or Sc 3+ ) and the Lewis acidities of H + and M n+ ions, suggesting that H+ and Mn+ bind at the metal–oxo moiety of [(dpaq)Mn IV (O)] + . Interestingly, a single-crystal structure of 1–HOTf was obtained by X-ray diffraction analysis, but the structure was not an expected Mn(IV)–oxo complex but a Mn(IV)–hydroxide complex, [(dpaq)Mn IV (OH)](OTf) 2 (4), with a Mn–O bond distance of 1.8043(19) Å and a Mn–O stretch at 660 cm –1 . More interestingly, 4 reverted to 1–HOTf upon dissolution, demonstrating that 1–HOTf and 4 are interconvertible depending on the physical states, such as 1–HOTf in solution and 4 in isolated solid. The reactivity of 1–HOTf was investigated in hydrogen atom transfer (HAT) and oxygen atom transfer (OAT) reactions and then compared with those of 1–Mn+ complexes; an interesting correlation between the Mn–O stretching frequencies of 1–HOTf and 1–M n+ and their reactivities in the OAT and HAT reactions is reported for the first time in this study.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Synthesis of Trimethyltriazacyclohexane (Me 3 tach) Sandwich Complexes of Uranium, Neptunium, and Plutonium Triiodides: (Me 3 tach) 2 AnI 3

1,3,5-Trimethyl-1,3,5-triazacyclohexane (Me 3 tach) readily complexes uranium triiodide to form (Me 3 tach) 2 UI 3 . Further, the complex is soluble in THF and arenes and can function as a source of UI 3 to form organometallic U(III) complexes. When dissolved in pyridine (py), (Me3tach)2UI3 forms (Me 3 tach)UI 3 (py) 2 . A related complex with the larger 1,4,7-trimethyl-1,4,7-triazacyclononane (Me 3 tacn) ligand, namely (Me 3 tacn)UI 3 (THF), was synthesized for comparison. Since X-ray quality crystals of (Me 3 tach) 2 UI 3 can be synthesized in high yield even with small-scale reactions, the system is ideal for extension to transuranium elements. Accordingly, the neptunium and plutonium complexes (Me 3 tach) 2 NpI 3 and (Me 3 tach) 2 PuI 3 were synthesized in an analogous manner from NpI 3 (THF) 4 and PuI 3 (THF) 4 , respectively.

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

Synthesis and Characterization of Solvated and Base-Free Cerium(III) Mixed-Sandwich Complexes

A series of solvated complexes: [(C8H8)Ln(C5Me4R)(DME)] Ln = La, R = -Me; (La-2) Ln = Ce, R = -Me, -SiMe3, -H; (Ce-2, Ce-3, Ce-4), [(dbCOT)Ln(C5Me5)(DME)] (Ln-6), (dbCOT2- = dibenzocyclooctadienide; Ln = Ce, La), [(dbCOT)Ce(C5Me4H)(DME)] (Ce-7) and [(hdcCOT)Ce(C5Me5)(DME)] (Ce-8) (hdcCOT2- = hexahydrodicyclopentacyclooctatetraenide) and base-free mixed-sandwich complexes [(C8H8)Ce(C5Me5)] (Ce-9), [(C8H8)Ce(C5Me4H) (Ce-10) and [(hdcCOT)Ce(C5Me5)] (Ce-11) of the early lanthanide metals cerium and lanthanum comprising variable cyclopentadienide (Cp-) and cyclooctatetraenide (COT2-) ligands is described. To evaluate the effect of cyclopentadienide and cyclooctatetraenide ligands on the characteristics of these complexes, their solid-state structural, electrochemical, and photophysical properties were studied and accompanied by theoretical calculations. To further evaluate the effect of ligands on the topology of the complexes and the reducing properties of the complexes, syntheses of several base-free congeners were pursued, which led to isolation of the first base-free monomeric and polymeric Ce(III) mixed-sandwich compounds.

Lanthanides↗
DOE OSTI2025

X-ray Spectroscopy Characterization of Electronic Structure and Metal–Metal Bonding in Dicobalt Complexes

Developing multimetallic complexes with tunable metal–metal interactions has long been a target of synthetic inorganic chemistry efforts due to the unique properties that such compounds can exhibit. However, understanding relationships between metal–metal bonding and chemical properties is challenging due to system-dependent factors that influence metal–metal and metal–ligand interactions, including ligand identity, coordination geometry, and metal–metal distance. In this work, we apply X-ray absorption and emission spectroscopy and quantum chemical calculations to describe electronic structure and bonding in a series of dicobalt complexes. The compounds with silane ligands and pseudo-octahedral coordination geometry exhibit Co–Co σ and multicentered bonding character, which we characterize from both the occupied and vacant perspectives via their contributions to the Co X-ray emission and absorption spectra, respectively. In contrast, the dicobalt complexes with a pseudotetrahedral coordination environment do not exhibit Co–Co bonding due to symmetry constraints on orbital overlap. We extend these insights to the theoretical evaluation of related dicobalt complexes to explain how ligand coordination and symmetry dictate the presence or absence of a Co–Co bond. In conclusion, this work highlights how fundamental insights into electronic structure and bonding through X-ray spectroscopy uncover important factors governing metal–metal interactions and guide the rational design of multimetallic complexes with tunable metal–metal bonds.

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

Single-Determinant Ground State in Ce 4+ Imidophosphorane Complexes

X-ray spectroscopy techniques are critical in the electronic structure analysis of high-valent lanthanides. The interpretation of multipeaked features at the lanthanide L 3 -edge has remained a challenging question, as it is observed across a range of material classes. A series of structurally related Ce 4+ complexes were prepared to probe the potential ligand field perturbation of the ground state within the series. The tuning of relative 4f and ligand orbital energies in homoleptic and heteroleptic tetravalent Ce imidophosphorane complexes is achieved through ligand derivatization and is clearly demonstrated by UV−vis spectroscopy and electrochemically measured redox potentials. However, Ce L 3 - edge high-energy resolution fluorescence-detected (HERFD) X-ray absorption near-edge structure (XANES) spectra present features at consistent numbers and energies across the range of complexes. Resonant inelastic X-ray scattering (RIXS) is employed to visualize the observed features in the HERFD-XANES spectra. Large complete active space configuration interaction singles and doubles (CASCISD) calculations demonstrate that the ground-state wave function of all complexes can be described employing a single determinant. As a result, the multielectron feature at the L 3 -edge observed in this study for the Ce 4+ imidophosphorane complexes is described as excited-state multiconfigurational behavior that is independent of ligand variation, in systems where the ground state is described using the simple single determinant wave function.

Lanthanides↗
DOE OSTI2026

Tuning Uranium Redox Chemistry in Asymmetric Polyoxometalate Complexes: Access to U(IV), U(V), and Transient U(VI)

We report the synthesis and characterization of asymmetric sandwich-type complexes [TpU IV EW 11 O 39 ] x– (E = P; x = 4, E = Si; x = 5, Tp = trispyrazolylborate). These complexes represent rare examples of the selective formation of asymmetric actinide-polyoxometalate (An-POM) complexes. Oxidation studies show that the U V analogues are stable and accessible, with both complexes isolated. Attempts to access U VI with this framework yield mixed results. Attempts to produce [TpU VI PW 11 O 39 ] 2– only led to decomposition, while electrochemical oxidation experiments gave evidence of transient formation of [TpU VI SiW 11 O 39 ] 3– in solution. Rapid decomposition of [TpU VI SiW 11 O 39 ] 3– inhibits full characterization. These studies indicate that the redox properties and stability of An-POM complexes containing actinides in higher valencies are dependent on the anionic charge of the assembly.

Actinides↗
DOE OSTI2017

Ground State and Excited State Tuning in Ferric Dipyrrin Complexes Promoted by Ancillary Ligand Exchange

Three ferric dipyrromethene complexes featuring different ancillary ligands were synthesized by one electron oxidation of ferrous precursors. Four-coordinate iron complexes of the type ( Ar L)FeX 2 [ Ar L = 1,9-(2,4,6-Ph 3 C 6 H 2 ) 2 -5-mesityldipyrromethene] with X = Cl or t BuO were prepared and found to be high-spin (S = 5/2), as determined by superconducting quantum interference device magnetometry, electron paramagnetic resonance, and 57 Fe Mössbauer spectroscopy. The ancillary ligand substitution was found to affect both ground state and excited properties of the ferric complexes examined. While each ferric complex displays reversible reduction and oxidation events, each alkoxide for chloride substitution results in a nearly 600 mV cathodic shift of the Fe III/II couple. The oxidation event remains largely unaffected by the ancillary ligand substitution and is likely dipyrrin-centered. While the alkoxide substituted ferric species largely retain the color of their ferrous precursors, characteristic of dipyrrin-based ligand-to-ligand charge transfer (LLCT), the dichloride ferric complex loses the prominent dipyrrin chromophore, taking on a deep green color. Time-dependent density functional theory analyses indicate the weaker-field chloride ligands allow substantial configuration mixing of ligand-to-metal charge transfer into the LLCT bands, giving rise to the color changes observed. Furthermore, the higher degree of covalency between the alkoxide ferric centers is manifest in the observed reactivity. Delocalization of spin density onto the tert-butoxide ligand in ( Ar L)FeCl(O t Bu) is evidenced by hydrogen atom abstraction to yield ( Ar L)FeCl and HOtBu in the presence of substrates containing weak C–H bonds, whereas the chloride ( Ar L)FeCl 2 analogue does not react under these conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Cobalt- and Rhodium-Corrole-Triphenylphosphine Complexes Revisited: The Question of a Noninnocent Corrole

Here, a reinvestigation of cobalt-corrole-triphenylphosphine complexes has yielded an unexpectedly subtle picture of their electronic structures. UV-vis absorption spectroscopy, skeletal bond length alternations observed in X-ray structures, and broken-symmetry DFT (B3LYP) calculations suggest partial Co II -corrole •2– character for these complexes. The same methods applied to the analogous rhodium corroles evince no evidence of a noninnocent corrole. X-ray absorption spectroscopic studies showed that the Co K rising edge of Co[TPC](PPh 3 ) (TPC = triphenylcorrole) is redshifted by ~1.8 eV relative to the bona fide Co(III) complexes Co[TPC](py) 2 and Co[TPP](py)Cl (TPP = tetraphenylporphyrin, py = pyridine), consistent with a partial Co II -corrole •2– description for Co[TPC](PPh 3 ). Electrochemical measurements have shown that both the Co and Rh complexes undergo two reversible oxidations and 1-2 irreversible reductions. In particular, the first reduction of the Rh corroles occurs at significantly more negative potentials than that of the Co corroles, reflecting significantly higher stability of the Rh(III) state relative to Co(III). Together, the results presented herein suggest that cobalt-corrole-triphenylphosphine complexes are significantly noninnocent with moderate Co II -corrole •2– character, underscoring – yet again – the ubiquity of ligand noninnocence among first-row transition metal corroles.

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

Diastereomerically Differentiated Excited State Behavior in Ruthenium(II) Hexafluoroacetylacetonate Complexes of Diphenyl Thioindigo Diimine

Mono- and diruthenium hexafluoroacetylacetonate (hfac) complexes of the thioindigo- N , N '-diphenyldiimine chelating ligand 3 have been prepared. The thioindigo diimine ligand binds to ruthenium in a bidentate fashion in the mononuclear compound 2 and serves as a bidentate chelating bridging ligand in the diruthenium complexes 1a and 1b . Compound 2 was isolated as a racemic mixture while the diruthenium complexes were isolated as the meso (ΔΛ) 1a and rac (ΔΔ and ΛΛ) 1b diastereomers. In-depth structural characterization of the compounds was performed, including X-ray crystallography, 1 H, 13 C, and 19 F nuclear magnetic resonance (NMR) spectroscopy, and 2D NMR correlation experiments. Electrochemical properties were evaluated utilizing cyclic voltammetry. Ground state optical properties of the complexes were examined using UV–visible spectroscopy and spectroelectrochemistry. The excited state dynamics of the series were explored by ultrafast transient absorption spectroscopy. Variable temperature NMR experiments demonstrated that the rac diruthenium compound 1b undergoes conformational exchange with a rate constant of 8700 s –1 at 298 K, a behavior that is not observed in the meso diastereomer 1a . The series of complexes possess metal-to-ligand charge transfer (MLCT) absorption bands in the near-infrared (λ max 689–783 nm). The compounds do not display photoluminescence in room temperature solution-phase experiments or in experiments at 77 K. Transient absorption spectroscopy measurements revealed excited states with picosecond lifetimes for 1a , 1b , and 2 , and spectroelectrochemical experiments confirmed assignment of the transient species as arising from MLCT transitions. Surprisingly, the transient absorption measurements revealed disparate time constants for the excited state decay of diastereomers 1a and 1b .

14 SOLAR ENERGY↗
DOE OSTI2018

Redox-Active vs Redox-Innocent: A Comparison of Uranium Complexes Containing Diamine Ligands

Here uranium complexes ( Mes DAE) 2 U(THF) ( 1-DAE ) and Cp 2 U( Mes DAE) ( 2-DAE ) ( Mes DAE = [ArN-CH 2 CH 2 -NAr]; Ar = 2,4,6-trimethylphenyl (Mes)), bearing redox-innocent diamide ligands, have been synthesized and characterized for a full comparison with previously published, redox-active diimine complexes, ( Mes DAB Me ) 2 U(THF) ( 1-DAB ) and Cp 2 U( Mes DAB Me ) ( 2-DAB ) ( Mes DAB Me = [ArN=C(Me)C(Me)=NAr]; Ar = Mes). These redox-innocent analogues maintain an analogous steric environment to their redox-active ligand counterparts to facilitate a study aimed at determining the differing electronic behavior around the uranium center. Structural analysis by X-ray crystallography showed 1-DAE and 2-DAE have a structural environment very similar to 1-DAB and 2-DAB , respectively. The main difference occurs with coordination of the ene-backbone to the uranium center in the latter species. Electronic absorption spectroscopy reveals these new DAE complexes are nearly identical to each other. X-ray absorption spectroscopy suggests all four species contain +4 uranium ions. The data also indicates that there is an electronic difference between the bis(diamide)-THF uranium complexes as opposed to those that only contain one diamide and two cyclopentadienyl rings. Finally, magnetic measurements reveal that all complexes display temperature-dependent behavior consistent with uranium(IV) ions that do not include ligand radicals. Overall, this study determines that there is no significant bonding difference between the redox-innocent and redox-active ligand frameworks on uranium. Furthermore, there are no data to suggest covalent bonding character using the latter ligand framework on uranium, despite what is known for transition metals.

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

Experimental and Theoretical Comparison of Transition-Metal and Actinide Tetravalent Schiff Base Coordination Complexes

A series of homoleptic tetravalent transition-metal and actinide Schiff-base coordination complexes, ML 2 {M = Zr, Hf, Th, U; L = N,N'-bis[(4,4'-diethylamino)salicylidene]-1,2-phenylenediamine}, have been synthesized that feature a rigid phenyl backbone. These complexes create the opportunity for comparing a series of complexes containing metal cations in the formal IV+ oxidation state by structural, spectroscopic, and theoretical analysis that also incorporate the previously reported Ce(IV) and Pu(IV) analogues. X-ray crystallographic analysis reveals that all complexes are isomorphous and feature a co-facial ligand geometry. TD-DFT and other quantum mechanical methods were used to explore bonding differences across between the complexes, and resulting calculated absorbance spectra for ML 2 are in good agreement with the experimental data. The computational results also suggest that U(IV) and Pu(IV) analogs have more covalent character in their bonding than found with the other metal cations reported here.

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

Intriguing Effects of Halogen Substitution on the Photophysical Properties of 2,9-(Bis)halo-Substituted Phenanthrolinecopper(I) Complexes

Three new copper(I) complexes [Cu-(L-x)(2)](+)(PF6-) (where L-x stands for 2,9-dihalo-1,10-phenanthroline and X = Cl, Br, and I) have been synthesized in order to study the impact of halogen substituents tethered in the alpha position of the chelating nitrogen atoms on their physical properties. The photophysical properties of these new complexes (hereafter named Cu-X) were characterized in both their ground and excited states. Femtosecond ultrafast spectroscopy revealed that early photoinduced processes are faster for Cu-I than for Cu-Cl or Cu-Br, both showing similar behaviors. Their electronic absorption and electrochemical properties are comparable to benchmark [Cu(dmp)(2)](+) (where dmp stands for 2,9-dimethyl-1,10-phenanthroline); furthermore, their optical features were fully reproduced by time-dependent density functional theory and ab initio molecular dynamics calculations. All three complexes are luminescent at room temperature, showing that halogen atoms bound to positions 2 and 9 of phenanthroline are sufficiently bulky to prevent strong interactions between the excited Cu complexes and solvent molecules in the coordination sphere. Their behavior in the excited state, more specifically the extent of the photoluminescence efficiency and its dependence on the temperature, is, however, strongly dependent on the nature of the halogen. A combination of ultrafast transient absorption spectroscopy, temperature-dependent steady-state fluorescence spectroscopy, and computational chemistry allows one to gain a deeper understanding of the behavior of all three complexes in their excited state.

Brown-Xu, Samantha↗