Ligand Donor Strength Research Articles

Silicon Nitride Surface Enabled Propane Dehydrogenation Catalyzed by Supported Organozirconium.

Mesoporous silicon nitride (Si3N4) is a nontraditional support for the chemisorption of organometallic complexes with the potential for enhancing catalytic activity through features such as the increased Lewis basicity of nitrogen for heterolytic bond activation, increased ligand donor strength, and metal-ligand orbital overlap. Here, tetrabenzyl zirconium (ZrBn4) was chemisorbed on Si3N4, and the resulting supported organometallic species was characterized by Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), Dynamic Nuclear Polarization-enhanced Solid State Nuclear Magnetic Resonance (DNP-SSNMR), and X-ray Absorption Spectroscopy (XAS). Based on the hypothesis that the nitride might enable facile heterolytic C-H bond activation along the Zr-N bond, this material was found to be a highly active (1.53 molpropene molZr-1 h-1 at 450 °C) and selective (99% to propylene) catalyst for propane dehydrogenation. In contrast, the homologous silica supported complex exhibited negligible activity under these conditions.

Lemieux‐Johnson oxidation with N,N‐bidentate bisPYA ruthenium complexes: insights into reaction conditions and catalyst design

AbstractThe carbonyl group is arguably the most versatile functional group for synthetic purposes. Here, a series of N,N’‐bis(pyridylideneamide) (bisPYA) complexes of ruthenium(II), [Ru(cym)(bisPYA)Cl]+ (cym=cymene), were synthesized and applied as catalysts for Lemieux‐Johnson oxidation of alkenes to access carbonyl functionalities. Complexes bearing para‐ and ortho‐bisPYA ligands induce higher activity and selectivity than their mesoionic meta‐bisPYA analogues. The non‐coordinating counterion of the cationic complexes affects the catalytic performance markedly, with PF6− enhancing the activity compared to trifluoromethanesulfonate, OTf−. Likewise, replacing the anionic ancillary chloride ligand by a neutral and more labile MeCN ligand led to increased activity. Electrochemical and solubility measurements revealed no direct correlation of ligand donor strength to catalytic performance. Variation of the solvent mixture and the ratio of its components provided access to high‐performance catalysts. In H2O/MeCN/CH2Cl2 at 3 : 1:0.5 (v/v) ratio, turnover frequencies up to 10,000 h−1 were obtained for the Lemieux‐Johnson oxidation of styrene with these Ru(bisPYA) complexes, allowing quantitative conversions within 30 min at 0.1 mol % catalyst loadings and corresponding to a 5 times faster reaction than the best catalyst known to date for this transformation.

Vanadyl as a Spectroscopic Probe of Tunable Ligand Donor Strength in Bimetallic Complexes.

Incorporation of secondary metal ions into heterobimetallic complexes has emerged as an attractive strategy for rational tuning of compounds' properties and reactivity, but direct solution-phase spectroscopic interrogation of tuning effects has received less attention than it deserves. Here, we report the assembly and study of a series of heterobimetallic complexes containing the vanadyl ion, [VO]2+, paired with monovalent cations (Cs+, Rb+, K+, Na+, and Li+) and a divalent cation (Ca2+). These complexes, which can be isolated in pure form or generated in situ from a common monometallic vanadyl-containing precursor, enable experimental spectroscopic and electrochemical quantification of the influence of the incorporated cations on the properties of the vanadyl moiety. The data reveal systematic shifts in the V-O stretching frequency, isotropic hyperfine coupling constant for the vanadium center, and V(V)/V(IV) reduction potential in the complexes. These shifts can be interpreted as charge density effects parametrized through the Lewis acidities of the cations, suggesting broad potential for the vanadyl ion to serve as a spectroscopic probe in multimetallic species.

Correlation of Experimental and Calculated Reaction Enthalpies with Ligand Donor Strengths.

Ylide-functionalized phosphines (YPhos) have recently proven to be strongly donating ligands that enable high catalyst activities in gold(I)-mediated transformations. We now report on a calorimetric study dealing with the [Au(YPhos)Cl] system and assess YPhos-Au bond dissociation enthalpies (BDE). Comparison with other commonly used phosphines confirmed the high binding strengths of the YPhos ligands. Furthermore, the values of the reaction enthalpies were shown to correlate with the electronic properties of the ligands measured via the Tolman electronic parameter or the calculated molecular electrostatic potential at phosphorus. Notably, the reaction enthalpies can conveniently be derived by computational methods, thus making these easy-to-obtain descriptors for ligand donor property quantification.

Ylide-Substituted Phosphines: A Platform of Strong Donor Ligands for Gold Catalysis and Palladium-Catalyzed Coupling Reactions.

The development of homogeneous catalysts is strongly connected to the design of new, sophisticated ligands, which resolve limitations of a given reaction protocol by manipulating the electronic properties of the metal and its spatial environment. Phosphines are a privileged class of ligands that find applications in many catalytic transformations, ranging from hydrogenation reactions to hydroformylation and coupling chemistry. For many years, chemists have been trying to improve the efficiency, selectivity, and application of coupling reactions. The use of highly electron-rich and bulky phosphines was often associated with increased selectivity and efficiency and led to the development of a vast variety of electron-rich alkyl-substituted phosphines. However, this concept of increasing the ligand donor strength reaches its limits with the use of trialkyl-substituted phosphines with tri-tert-butylphosphine thus being one of the most active ligands for many years. In the course of our research efforts to use the special donor strength of ylides to stabilize electron-deficient, low-valent main group compounds, we realized that ylide-substituted phosphine (YPhos) ligands possess remarkably strong donor abilities. Moreover, the YPhos ligands are highly tunable by changing the nature of the groups on the phosphonium, phosphine, or central ylidic carbon atom. We thus obtained a ligand platform with donor capabilities ranging from PCy3 to even stronger donor abilities than N-heterocyclic carbenes, while being more sterically demanding than simple phosphines as well as many well-known biarylphosphine ligands.These properties led us to explore the applicability of the YPhos ligands in catalysis. In a series of recent reports, our group applied YPhos ligands in gold and palladium catalyzed reactions at catalytic loadings applicable for medium- to large-scale applications. The increased donor strength and unique architecture allowed for remarkable activities in a series of transformations at mild reactions conditions. For gold(I)-catalyzed reactions, we obtained turnover numbers (TONs) for the hydroamination of phenylacetylene with aniline of over 20 000. Also, more complex reactions were easily catalyzed with efficiencies greater than those of other known gold(I) catalysts. Similar efficacies were found in a series of palladium-catalyzed coupling reactions. In Buchwald-Hartwig aminations, unprecedented activities for the amination of aryl chlorides were reached at room temperature. The speed of formation of the catalytically active mono-YPhos palladium species allowed for some of the amination reactions to be completed in only a few minutes. Adjustment of the ligand design enabled the use of a large variety of different aryl and alkyl amines of different steric demands. Furthermore, the YPhos ligands in general showed high activities and selectivity in the coupling of a variety of carbon nucleophiles with aryl chlorides, bromides, and triflates. This enabled the development of efficient reaction protocols for the α-arylation of unhindered ketones and the coupling of Grignard and zinc reagents as well as the first efficient coupling of chloroarenes with alkyllithium compounds. This Account summarizes the recent development of YPhos ligands and their application in gold and palladium catalysis. We also hope to stimulate further use of this ligand platform in catalysis in the future.

Recent Applications of the Huynh Electronic Parameter (HEP)

Abstract The Huynh electronic parameter (HEP) is a modern alternative method to determine ligand donor strengths by 13C NMR spectroscopy of metal NHC complexes containing the ligand of interest. Using the HEP, the electronic properties of numerous classical Werner-type and organometallic ligands have been evaluated thus far. Moreover, it has been extended to the HEP2, which provides a means to measure bidentate chelators. This Highlight Review covers recent applications of the HEP and its variants from 2016 onwards.

The Stronger the Better: Donor Substituents Push Catalytic Activity of Molecular Chromium Olefin Polymerization Catalysts.

The donor strength of bifunctional pyridine‐cyclopentadienyl ligands was altered systematically by the introduction of donating groups in the para‐position of the pyridine. In the resulting chromium complexes an almost linear correlation between donor strength and the nitrogen‐chromium distance as well as the electronic absorption maximum is experimentally observed. The connection of electron‐donating groups in the ligand backbone leads to an efficient transfer of the electronic influences to the catalytically active metal centre without restricting it through steric effects. Therefore, catalytic olefin polymerization activity, which is already very high for the previously studied catalysts, increase considerably by attaching para‐amino groups to the chelating pyridine or quinoline, respectively. Combining electron‐rich indenyl ligands with para‐amino substituted pyridines lead to the highest catalytic activities observed so far for this class of organo chromium olefin polymerisation catalysts. The resulting polymers are of ultra‐high molecular weight and the ability of the catalysts to incorporate co‐monomers is also very high.

Single monodentate N-donor ligands versus multi-ligand analogues in Pd(II)-catalysed C–C coupling at reduced temperatures

Deployment of reduced operational temperatures is industrially beneficial and use of the highly efficient, phosphine-based precatalysts is limited by their high costs and inaccessible preparation procedures. In order to study of the influence of coordination environments on catalyst reactivities at reduced temperatures, design of palladium(II) complexes bearing single monodentate N-donor ligands was considered necessary. Consequently, dichloridopalladium(II) complexes of 2-(thiophen-2-yl)-1H-imidazole ligands (1–8), 2,4,5-triphenyloxazole (9) and 2-(1H-imidazol-2-yl)pyridine (10) have been prepared, structurally characterized and studied as N-stabilized precatalysts. Ligand donor strengths were spectroscopically estimated by protonation-deprotonation equilibria. The palladium(II) complexes were obtained in three coordination environments; (i) the mono-ligand complexes bearing trans-solvent co-ligands (PdL.acn and PdL.dmf), (ii) the chlorido-bridged dimers µ-(PdL)2 and (iii) the trans-bis-ligand PdL2 complexes. Considering ambient temperature operations, the catalysis outcomes obtained for the monodentate mono-ligand coordination designs represent an improvement in terms of temperature and reaction time relative to previously reported N-stabilized palladium precatalysts. The mono-ligand pre-catalysts efficiently generate living active palladium species from 40 °C while a trans-bis-ligand phosphine-based pre-catalyst analogue PdI2(PPh3)2 displayed no yield under the same temperature conditions. Trans-bis-ligand coordination is observed to utterly hinders catalyst efficiencies at the studied temperatures and preformed mono-ligand complexes of mono-dentate N-donors provided positive ligand effects while in situ catalyst generation failed. Therefore, the use of multiple ligand equivalents should be discouraged.

Synthesis and characterization of heterometallic complexes involving coinage metals and isoelectronic Fe(CO)5, [Mn(CO)5]- and [Fe(CO)4CN]- ligands.

The chemistry of coinage metal ions with Fe(CO)5, [Mn(CO)5]- and [Fe(CO)4CN]- has been explored using Mes3P and N-heterocyclic carbene supporting ligands. A comparison of [(SIPr)Au-Fe(CO)5][SbF6], [(Et2CAAC)Au-Fe(CO)5][SbF6] and [(Mes3P)Au-Fe(CO)5][SbF6] shows that the ligand donor strength towards Au(i) follows the order Mes3P > Et2CAAC > SIPr. These Fe(CO)5 complexes show significant blue shifts in [small nu, Greek, macron]CO bands relative to those observed for free Fe(CO)5 as a result of it serving as a net electron donor to Au(i). Au(i) is a much stronger acceptor in (SIPr)Au-Mn(CO)5 compared to Ag(i) in (SIPr)Ag-Mn(CO)5. The structural details of Mes3PAu-Mn(CO)5 are also presented. [Fe(CO)4CN]- afforded CN bridged coinage metal complexes with (IPr*)Au+, (SIPr)Ag+ and (SIPr)Cu+ moieties, rather than molecules with direct Fe/coinage metal bonds. The computed total interaction energies indicate that both [Mn(CO)5]- and [Fe(CO)4CN]- are stronger donors toward Au(i) than Fe(CO)5. A detailed analysis of the bonding interactions between the coinage metal ions and Fe(CO)5, [Mn(CO)5]- and [Fe(CO)4CN]- suggests that the largest contribution comes from electrostatic attraction, while the covalent component follows the Dewar-Chatt-Duncanson model. The σ-donor interactions of these organometallic ligands with coinage metal ions are considerably stronger than the π-backbonding from the coinage metal ions.

Thermoluminescent Antimony-Supported Copper-Iodo Cuboids: Approaching NIR Emission via High Crystallographic Symmetry.

We report the syntheses, structures, and luminescence properties of a series of copper-iodo cuboids supported by L-type antimony ligands. The cuboids are of general formula [(SbR3)4Cu4(I)4] (1-4, 8), where SbR3 is a series of homoleptic and heteroleptic stibines containing both phenyl and a variety of alkyl substituents (R = Cy, iPr, tBu, Ph); triphenyl, iPr2Ph, and Me2Ph stibines resulted in the formation of dimers of type [(SbR3)4(Cu)2(I)2] (5-7). While similar luminescent copper-halide cubes have been studied, the corresponding "heavy-atom" congeners have not been studied, and ligation of such heavy-atom moieties is often associated with long-lived triplet states and low-energy absorption and emission profiles. Overall, two obligate parameters are found to imbue NIR emission: (i) short Cu-Cu bonds and (ii) high crystallographic symmetry; both of these properties are found only in [(SbiPr3)4Cu4(I)4] (1, in I23; λem = 711 nm). The correlation between NIR emission and high crystallographic symmetry (which intrinsically includes high molecular symmetry)-versus only molecular symmetry-is confirmed by the counterexample of the molecularly symmetric tBu-substituted cuboid [(SbtBu3)4Cu4(I)4] (3, λem = 588 nm, in R-3), which crystallizes in the lower symmetry trigonal space group. Despite the indication that the stronger donor strength of the SbtBu3 ligand should red-shift emission beyond that of the SbiPr3-supported cuboid, the emission of 3 is limited to the visible region. To further demonstrate the connection between structural parameters and emission intensity, X-ray structures for 1 and 3 were collected between 100 and 300 K. Lastly, DFT calculations for 1 on its singlet (S0) and excited triplet state (T1) demonstrate two key factors necessary for low-energy NIR emission: (i) a significant contraction of the interconnected Cu4 intermetallic contacts [∼2.45 → 2.35 Å] and (ii) highly delocalized (and therefore low-energy) A1 symmetry HOMO/LUMO orbitals from which the emission occurs. Thus, any molecular or crystallographic distortion of the Cu4 core precludes the formation of highly symmetric (and low-energy) HOMO/LUMO orbitals in T1, thereby inhibiting low-energy NIR emission.

The Thiolate Trans Effect in Heme {FeNO}6 Complexes and Beyond: Insight into the Nature of the Push Effect.

Cyt P450 nitric oxide (NO) reductase (P450nor) is an important enzyme in fungal denitrification, responsible for the large-scale production of the greenhouse gas N2O. In the first step of catalysis, the ferric heme-thiolate active site of P450nor binds NO to produce a ferric heme-nitrosyl or {FeNO}6 intermediate (in the Enemark-Feltham notation). In this paper, we present the low-temperature preparation of six new heme-thiolate {FeNO}6 model complexes, [Fe(TPP)(SPh*)(NO)], using a unique series of electron-poor thiophenolates (SPh*-), and their detailed spectroscopic characterization. Our data show experimentally, for the first time, that a direct correlation exists between the thiolate donor strength and the Fe-NO and N-O bond strengths, evident from the corresponding stretching frequencies. This is due to a σ-trans effect of the thiolate ligand, which manifests itself in the population of an Fe-N-O σ-antibonding (σ*) orbital. Via control of the thiolate donor strength (using hydrogen bonds), nature is therefore able to exactly control the degree of activation of the FeNO unit in P450nor. Vice versa, NO can be used as a sensitive probe to quantify the donor strength of a thiolate ligand in a model system or protein, by simply measuring the Fe-NO and N-O frequencies of the ferric NO adduct and then projecting those data onto the correlation plot established here. Finally, we are able to show that the σ-trans effect of the thiolate is the electronic origin of the "push" effect, which is proposed to mediate O-O bond cleavage and Compound I formation in Cyt P450 monooxygenase catalysis.

Classifying donor strengths of dipyrrinato/aza-dipyrrinato ligands.

A parameter is reported by which to use 13C NMR chemical shifts to measure and predict the donor capabilities of N^N dipyrrinato and aza-dipyrrinato ligands chelating in L^X fashion. The results enable the rationalisation of the properties of these ligands and their complexes, as well as enable rational design incorporating both steric and electronic considerations when tuning to effect desired applications. Complexes containing these ligands are prevalent due to their desirable photophysical properties such as high chemical stability, resistance to photodegradation, strong absorbance, and ease of chemical modifiability.

Nitrogen hybridization controls peroxo-oxo equilibrium in ethylenediamine bound binuclear [Cu2O2] complexes

The reaction of O2 with binuclear Cu(I) enzymatic active sites as well as synthetic complexes result in the formation of [Cu2O2]2 + core which can exist either in a η2η2-peroxo dicopper (II) species or bis(μ-oxo) dicopper (III) species. These species are proposed to exhibit different reactivity towards organic substrates. The factors that control the equilibrium between these two isoelectronic electromers have been investigated for decades. Using DFT calculations on [Cu2O2]2 + core supported by ethelenediammine ligands, where the substitutions on the nitrogen are systematically varied, the electronic structure contributions to the relative stabilities of these species is investigated. The calculations show that the CuN bond length varies anomalously with the electron donor ability of the ethylenediamine derivatives reproducing experimental trends. The ground state wavefunctions show that the relative contribution of the 2p and 2s orbitals in the nitrogen-based donor orbitals vary depending on the substituents. Bulky substituents use donor orbitals having distinctly more N2p contribution than the unsubstituted or mildly substituted ammines which use donor orbitals with more N2s contribution. This results in anomalous variation of the CuN distances and relative peroxo-oxo energies with ligand donor strength.

Synthesis, structure and behavior of vanadium(III) diphosphine complexes in the homo- and co-polymerization of ethylene with norbornene: the ligand donor strength and bite angle make the difference

A series of vanadium(III) complexes containing diphosphine ligands of the type R2P(CH2)2PR2 [R = Me (dmpe, 1a), R = Et (depe, 1b), R = Ph (dppe, 1c)] were synthesized by reacting VCl3(THF)3 with 1.3 equiv. of the corresponding ligand. The molecular structures of 1a and 1b were determined by X-ray crystallography, revealing that both are dimers of the type [VCl3(dmpe)]2(μ-P,P′-dmpe) and [VCl3(depe)]2(μ-P,P′-depe), respectively, having one chelating diphosphine ligand, and an additional diphosphine bridging the two vanadium metals. Upon activation with diethylaluminum chloride (Et2AlCl) and in the presence of ethyltrichloroacetate (ETA) the complexes exhibited good activity in the polymerization of ethylene, affording linear, semicrystalline polymers with melting temperatures of about 135 °C. All the complexes (1a–1c) showed from good to excellent activity in the copolymerization of ethylene with norbornene. The resultant copolymers are mainly alternating with a uniform composition and a norbornene incorporation depending on the ligand employed. The substituents at the phosphorus atoms strongly affect the catalytic activity, which increased in the order 1a <1b < 1c with increasing the ligand donor strength, and the comonomer incorporation, while no significant differences were observed in terms of copolymers microstructure and molecular weight. The results are carefully compared with those obtained with the monodentate phosphine VCl3(PMePh2)2 complex and VCl3(THF)3. Overall, we found that the use of diphosphine ligands gave less active catalysts likely because PMePh2 and THF ligands are weakly coordinated to the metal, but (co)polymers with higher molecular weight due to the ability of chelating diphosphines to stabilize the electron-deficient catalytic intermediate, thus reducing the rate of β–hydrogen elimination and subsequent chain-transfer.

Weakening of Carbide-Platinum Bonds as a Probe for Ligand Donor Strengths.

We report the observation of the weakening of the RuC-Pt single bond in (Cy3P)2Cl2Ru≡C-PtCl2-L (Ru≡C-Pt-L) complexes, leading to the incipient formation of the terminal ruthenium carbide complex, (Cy3P)2Cl2Ru≡C (RuC). In the solid state, elongation of RuC-Pt bonds illustrates the degree of weakening, and in solution, decreasing platinum-carbide coupling constants and increasing carbide chemical shifts reveal weaker interaction through the carbide bridge, as the electron donating ability of L becomes progressively stronger. For the bridging carbide ligands, the chemical shifts and coupling constants to platinum are linearly dependent, and NMR data for parent RuC conform to this relationship, providing a spectroscopic means of determining the strength of the RuC-Pt linkages relative to dissociated RuC. The pliancy of the Ru≡C-Pt-L fragment with regard to the identity of L establishes the carbide-bridged complexes as remarkably wide-ranging and sensitive probes for ligand donor abilities.

The Role of the Secondary Coordination Sphere in a Fungal Polysaccharide Monooxygenase.

Polysaccharide monooxygenases (PMOs) are secreted metalloenzymes that catalyze the oxidative degradation of polysaccharides in a copper-, oxygen-, and reductant-dependent manner. Cellulose-active fungal PMOs degrade cellulosic substrates to be utilized as a carbon source for fungal growth. To gain insight into the PMO mechanism, the role of conserved residues in the copper coordination sphere was investigated. Here, we report active-site hydrogen-bonding motifs in the secondary copper coordination sphere of MtPMO3*, a C1-oxidizing PMO from the ascomycete fungus Myceliophthora thermophila. A series of point substitutions that disrupt this conserved network are used to interrogate its function. Activity assays, in conjunction with EPR spectroscopy, demonstrate that residues H161 and Q167 are involved in stabilizing bound oxygen, and H161 appears to play a role in proton transfer. Additionally, Q167 increases the ligand donor strength of Y169 to the copper via a hydrogen-bonding interaction. Altogether, H161 and Q167 are important for oxygen activation, and the results are suggestive of a copper-oxyl active intermediate.

ChemInform Abstract: Alkenyl Complexes of Platinum Group Metals: A Platform for Diverse Reactivity Patterns

AbstractReview: 132 refs.

Thermochromic and Photoresponsive Cyanometalate Fe/Co Squares: Toward Control of the Electron Transfer Temperature

Two structurally related and photoresponsive cyanide-bridged Fe/Co square complexes, {Fe2Co2}, are reported: {[(Tp(Me))Fe(CN)3]2[Co(bpy)2]2[(Tp(Me))Fe(CN)3]2}·12H2O (2) and {[(Tp(Me))Fe(CN)3]2[Co(bpy)2]2[BPh4]2}·6MeCN (3), where Tp(Me) and bpy are hydridotris(3-methylpyrazol-1-yl)borate and 2,2'-bipyridine, respectively. Through electrochemical and spectroscopic studies, the Tp(Me) ligand appears to be a moderate σ donor in comparison to others in the [NEt4][(Tp(R))Fe(III)(CN)3] series [where Tp(R) = Tp, hydridotris(pyrazol-1-yl)borate; Tp(Me) = hydridotris(3-methylpyrazol-1-yl)borate; pzTp = tetrakis(pyrazol-1-yl)borate; Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate; Tp*(Me) = hydridotris(3,4,5-trimethylpyrazol-1-yl)borate]. The spectroscopic, structural, and magnetic data of the {Fe2Co2} squares indicate that thermally-induced intramolecular electron transfer reversibly converts {Fe(II)LS(μ-CN)Co(III)LS} pairs into {Fe(III)LS(μ-CN)Co(II)HS} units near ca. 230 and 244 K (T1/2) for 2 and 3, respectively (LS: low spin; HS: high spin). These experimental results show that 2 and 3 display light-induced {Fe(III)LS(μ-CN)Co(II)HS} metastable states that relax to thermodynamic {Fe(II)LS(μ-CN)Co(III)LS} ones at ca. 90 K. Ancillary Tp(R) ligand donor strength appears to be the dominant factor for tuning electron transfer properties in these {Fe2Co2} complexes.

Magnetic anisotropy and mechanism of magnetic relaxation in Er(III) single-ion magnets.

Magnetic anisotropy is a key component in the design of single-molecule magnets (SMMs) possessing a large barrier height for magnetization reversal. Lanthanide-based SMMs are the most promising candidates in this arena as they offer a large magnetic anisotropy due to the presence of strong spin-orbit coupling. Among lanthanides, Er(III) complexes are gaining attention in the area of SMMs, because of their intriguing magnetic properties and attractive blocking temperatures. Here, we have undertaken detailed ab initio calculations on four structurally diverse Er(III) SMMs to shed light on how the magnetic anisotropy is influenced by the role of symmetry and structural distortions. The employed CASSCF+RASSI calculations have offered rationale for the observed differences in the estimated Ueff values for the studied complexes and also offered hints to the mechanism of magnetic relaxation. The differences in the mechanism of magnetic relaxations are further analyzed based on the Er-ligand interactions, which is obtained by analyzing the charges, densities, luminescent behavior and the frontier molecular orbitals. Our calculations, for the first time, have highlighted the importance of high symmetry environment and ligand donor strength in obtaining large Ueff values for the Er(III) complexes. We have examined these possibilities by modeling several structures with variable coordination numbers and point group symmetry. These results signify the need of a detailed understanding on the shape of the anisotropy and the point group symmetry in order to achieve large Ueff values in Er(III) single-ion magnets.

Anomalous one-electron processes in the chemistry of uranium nitrogen multiple bonds.

Novel reaction pathways are illustrated in the synthesis of uranium(IV), uranium(V), and uranium(VI) monoimido complexes. In contrast to the straightforward preparation of U(V)(═NSiMe3)[N(SiMe3)2]3 (1), the synthesis of a uranium(V) tritylimido complex, U(V)(═NCPh3)[N(SiMe3)2]3 (4), from U(III)[N(SiMe3)2]3 and Ph3CN3 was found to proceed through multiple one-electron steps. Whereas the oxidation of 1 with copper(II) salts produced the uranium(VI) monoimido complexes U(VI)(═NSiMe3)X[N(SiMe3)2]3 (X = Cl, Br), the reaction of 4 with CuBr2 undergoes sterically induced reduction to form the uranium(VI) monoimido complex U(VI)(═NCPh3)Br2[N(SiMe3)2]2, demonstrating a striking difference in reactivity based on imido substituent. The facile reduction of compounds 1 and 4 with KC8 allowed for the synthesis of the uranium(IV) monoimido derivatives, K[U(IV)(═NSiMe3)[N(SiMe3)2]3] (1-K) and K[U(IV)(═NCPh3)[N(SiMe3)2]3] (4-K), respectively. In contrast, an analogous uranium(IV) monoimido complex, K[U(IV)(═NPh(F))[N(SiMe3)Ph(F)]], Ph(F) = -pentafluorophenyl (6), was prepared through a loss of N(SiMe3)2Ph(F) concomitant with one-electron oxidation of a uranium(III) center. The uranium(IV) monoimido complexes were found to be reactive toward electrophiles, demonstrating N-C and N-Si single bond formation. One-electron reduction of nitrite provided a route to the uranium(VI) oxo/imido complex, [Ph4P][U(VI)O(═NSiMe3)[N(SiMe3)2]3]. The energetics and electrochemical processes involved in the various oxidation reactions are discussed. Finally, comparison of the U(VI)(═NSiMe3)X[N(SiMe3)2]3, X = Cl, Br, complexes with the previously reported U(VI)OX[N(SiMe3)2]3, X = Cl, Br, complexes suggested that the donor strength of the trimethylsilylimido ligand is comparable to the oxo ligand.