Acceptor Orbitals Research Articles

Utilizing Ancillary Ligands to Optimize the Photophysical Properties of 4H‐Imidazole Ruthenium Dyes

Correlations between structural features and photophysical properties offer the possibility to design dyes with tailor-made properties. In this respect, the photophysical properties of a series of 4H-imidazole (4H-im) ruthenium dyes with varying chemical and electronic structures of the complex fragments [(tpy)Ru(4H-im)X] {tpy=4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine; X=Cl(-), NCS(-)} and [(bpy)2Ru(4H-im)](+) {bpy=4,4'-di-tert-butyl-2,2'-bipyridine} were investigated. Variation of the π-donor/acceptor properties of the ancillary ligands offers the possibility to tune the relative energies of the d donor and π* acceptor orbitals of 4H-im, which results in a shift in the Ru→4H-im (1)MLCT (MLCT=metal-to-ligand charge transfer) absorption band in the visible range. Further, the energies of the excited states and also the interactions and mixing of the metal d orbital with the π* 4H-im orbital are sensitive to the chemical and electronic structures of the complex fragment. This causes subtle changes in the photoinduced processes; for example, the rate of the interim population of a planarized state increases with increasing/decreasing π-acceptor/donor character of the ancillary ligand. Although ground-state repopulation in the tpy species follows the energy-gap law, local symmetry-related effects have to be considered to account for the very short lifetime of the excitation in the bpy complex. Additionally, the ultrafast intramolecular relaxation processes leading from the initially excited states to the relaxed triplet states localized on the 4H-im ligand (internal conversion, internal vibrational energy redistribution, intersystem crossing) depend on the nature of the complex fragment. The high excitation-wavelength-dependent rate for the population of the relaxed triplet states in the bpy complex points to additional interligand electron-transfer contributions upon excitation into the higher lying Ru→bpy (1)MLCT states.

N→π* interactions of amides and thioamides: implications for protein stability.

Carbonyl–carbonyl interactions between adjacent backbone amides have been implicated in the conformational stability of proteins. By combining experimental and computational approaches, we show that relevant amidic carbonyl groups associate through an n→π* donor–acceptor interaction with an energy of at least 0.27 kcal/mol. The n→π* interaction between two thioamides is 3-fold stronger than between two oxoamides due to increased overlap and reduced energy difference between the donor and acceptor orbitals. This result suggests that backbone thioamide incorporation could stabilize protein structures. Finally, we demonstrate that intimate carbonyl interactions are described more completely as donor–acceptor orbital interactions rather than dipole–dipole interactions.

New bimetallic [(Cl)3Pt(dpop′)M(dpop′)](PF6) (M = Ru(II) and Os(II); dpop′ = dipyrido (2,3-a:3′,2′-j)phenazine) complexes using dpop′ as a novel mixed denticity bridging ligand

The dpop′ (dipyrido (2,3-a:3′,2′-j)phenazine) ligand structurally displays the possibility of a mixed denticity bridging ligand (BL) with low lying π∗ orbitals. Previously prepared complexes with Fe(II); Ru(II); Os(II) and Re(I) have only demonstrated tridentate coordination to dpop′ and it’s use as a terminal ligand (TL). In this study two new bimetallic complexes [(Cl)3Pt(dpop′)M(dpop′)](PF6) (M=Ru(II) and Os(II)) were synthesized and are the first to use dpop′ as a bridging ligand (BL), with monodentate coordination to Pt(II) and tridentate coordination to Ru(II) and Os(II). The electronic absorption spectra display Ru(II) and Os(II) (dπ)→dpop′ (π∗) (BL) MLCT transitions shifted to lower energy due to stabilization of the bridging dpop′ (π∗) acceptor orbitals. Cyclic voltammetry shows the reversible Ru+2/+3 and Os+2/+3 redox couples are at the same potentials as the respective [M(dpop′)2](PF6)2 complexes while the bridging dpop′0/−1 reversible reductions are shifted to less negative potential by 0.2V. NMR data show downfield shifts for protons bound to the dpop′ (BL) versus dpop′ (TL) ligands.

How quasi-free holes and electrons are generated in organic photovoltaic interfaces

We introduced a minimal model of the donor-acceptor interface encountered in organic solar cells to explain the efficient generation of free charges in these systems and we investigated the nature of charge transfer states formed by a delocalized exciton in the donor component. Contrarily to the generally accepted view excitons do not generate strongly bound hole-electron pairs, but relatively delocalized charge transfer states with energy very close to the energy of free holes and electrons. These states are kinetically more accessible from the exciton state and very close in structure to the free hole and electron states. The most relevant molecular parameter that affects the rate of exciton dissociation is the electronic coupling between donor orbitals and acceptor orbitals, i.e. the band widths of the donor and acceptor materials. Moreover, and to some surprise, we find that the process of charge separation is mostly a purely electronic process and a very similar physics can be described neglecting the role of nuclear degrees of freedom altogether.

Mechanism of protonation induced changes in Raman spectra of a trisheteroleptic ruthenium complex revealed by DFT calculations

A systematic DFT/(resonance) Raman spectroscopic study of the protonation/deprotonation induced changes in the vibrational spectra of a trisheteroleptic ruthenium complex [(tbbpy)Ru(II)(tmbibzimH2)(dppz)]2+ (tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine, tmbibzim = 5,5′,6,6′-tetra-methyl-2,2′-bibenzimidazole and dppz = dipyrido[3,2-a:2′,3,3′-c]phenazine) is presented. This molecular system combines the pH sensitivity of ruthenium bibenzimidazole systems with the light-switch properties of ruthenium dppz dyes. The results show that the protonation state of the tmbibzim ligand mainly impacts the geometry and the frequencies of the vibrational modes of the tmbibzim ligand itself. Here, band shifts up to 70 cm−1 are observed. The tbbpy and dppz ligand structures remain unaffected. However, the signals of the dppz and tbbpy modes shift upon protonation slightly to higher wavenumbers by 2–3 cm−1, which is assigned to a stronger π-backbonding in the deprotonated form of the complex. The results of the DFT calculations were used to perform a normal mode analysis of the resonance Raman spectra of the differently protonated forms in solution upon excitation into the metal-to-ligand charge transfer band. The resonance Raman spectra lack protonation dependence of their spectral features, which is an indicator that no tmbibzim modes are enhanced in the resonance Raman spectra and no photoinduced metal-to-ligand charge transfer from the central ruthenium ion to tmbibzim localized acceptor orbitals occurs.

Hyperconjugation-Mediated Solvent Effects in Phosphoanhydride Bonds

Density functional theory and natural bond orbital analysis are used to explore the impact of solvent on hyperconjugation in methyl triphosphate, a model for "energy rich" phosphoanhydride bonds, such as found in ATP. As expected, dihedral rotation of a hydroxyl group vicinal to the phosphoanhydride bond reveals that the conformational dependence of the anomeric effect involves modulation of the orbital overlap between the donor and acceptor orbitals. However, a conformational independence was observed in the rotation of a solvent hydrogen bond. As one lone pair orbital rotates away from an optimal antiperiplanar orientation, the overall magnitude of the anomeric effect is compensated approximately by the other lone pair as it becomes more antiperiplanar. Furthermore, solvent modulation of the anomeric effect is not restricted to the antiperiplanar lone pair; hydrogen bonds involving gauche lone pairs also affect the anomeric interaction and the strength of the phosphoanhydride bond. Both gauche and anti solvent hydrogen bonds lengthen nonbridging O-P bonds, increasing the distance between donor and acceptor orbitals and decreasing orbital overlap, which leads to a reduction of the anomeric effect. Solvent effects are additive with greater reduction in the anomeric effect upon increasing water coordination. By controlling the coordination environment of substrates in an active site, kinases, phosphatases, and other enzymes important in metabolism and signaling may have the potential to modulate the stability of individual phosphoanhydride bonds through stereoelectronic effects.

Simultaneous gauche and Anomeric Effects in α-Substituted Sulfoxides

α-Substituted sulfoxides can experience both gauche and anomeric effects, since these compounds have the geometric requirements and strong electron donor and acceptor orbitals which are essential to make operative the hyperconjugative nature of these effects. Indeed, the title effects were calculated to take place for 1,3-oxathiane 3-oxide in polar solution, where dipolar effects are absent or at least minimized, while only the gauche effect is present in 2-fluorothiane 1-oxide. Since the fluorine atom is a suitable probe for structural analysis using NMR, the (1)J(CF) dependence on the rotation around the F-C-S═O dihedral angle of (fluoromethyl)methyl sulfoxide was evaluated; differently from 1,2-difluoroethane and fluoro(methoxy)methane, this coupling constant is at least not exclusively dependent on dipolar interactions (or on hyperconjugation). Because of the nonmonotonic behavior of the (1)J(CF) rotational profile, this coupling constant does not appear to be of significant diagnostic value for probing the conformations of α-fluoro sulfoxides.

Theoretical Investigation of Paramagnetic NMR Shifts in Transition Metal Acetylacetonato Complexes: Analysis of Signs, Magnitudes, and the Role of the Covalency of Ligand–Metal Bonding

Ligand chemical shifts are calculated and analyzed for three paramagnetic transition metal tris-acetylacetonato (acac) complexes, namely high-spin Fe(III) and Cr(III), and low-spin Ru(III), using scalar relativistic density functional theory (DFT). The signs and magnitudes of the paramagnetic NMR ligand chemical shifts are directly related to the extent of covalent acac oxygen-to-metal σ donation involving unoccupied metal valence d(σ) acceptor orbitals. The role of delocalization of metal-centered spin density over the ligand atoms plays a minor secondary role. Of particular interest is the origin of the sign and magnitude of the methyl carbon chemical shift in the acac ligands, and the role played by the DFT delocalization error when calculating such shifts. It is found that the α versus β spin balance of oxygen σ donation to metal valence d acceptor orbitals is responsible for the sign and the magnitude of the ligand methyl carbon chemical shift. A problematic case is the methyl carbon shift of Fe(acac)(3). Most functionals produce shifts in excess of 1400 ppm, whereas the experimental shift is approximately 279 ppm. Range-separated hybrid functionals that are optimally tuned for Fe(acac)(3) based on DFT energetic criteria predict a lower limit of about 2000 ppm for the methyl carbon shift of the high-spin electronic configuration. Since the experimental value is based on a very strongly broadened signal it is possibly unreliable.

Fourteen-Electron Ring Model and the Anomalous Magnetic Circular Dichroism of meso-Triarylsubporphyrins

The MCD spectra of meso-triarylsubporphyrins show a sign anomaly which is correlated with the acceptor properties of the aryl substituent. From the spectra, magnetic moments of the excited states are determined. In the context of a simplified orbital model, the sign change is attributed to the quenching of the magnetic moment of the LUMO by acceptor orbitals of the substituent. The actual calculation of this moment presents a major challenge to computational methods. It is shown that wave function techniques based on CASSCF underestimate the covalency effects that are responsible for the quenching. In contrast, a CI method based on DFT orbitals yields excellent results, which fully support the orbital model.

Discontinuities of the exchange-correlation kernel and charge-transfer excitations in time-dependent density-functional theory

We identify the key property that the exchange-correlation (XC) kernel of time-dependent density-functional theory must have in order to describe long-range charge-transfer excitations. We show that the discontinuity of the XC potential as a function of particle number induces a space- and frequency-dependent discontinuity of the XC kernel that diverges as $r\ensuremath{\rightarrow}\ensuremath{\infty}$. In a combined donor-acceptor system, the same discontinuity compensates for the vanishing overlap between the acceptor and donor orbitals, thereby yielding a finite correction to the Kohn-Sham eigenvalue differences. This mechanism is illustrated to first order in the Coulomb interaction.

Orbital-Resolved Partial Charge Transfer from the Methoxy Groups of Substituted Pyrenes in Complexes with Tetracyanoquinodimethane—A NEXAFS Study

It is demonstrated that the near-edge X-ray absorption fine structure (NEXAFS) provides a powerful local probe of functional groups in novel charge transfer (CT) compounds and their electronic properties. Microcrystals of tetra-/hexamethoxypyrene as donors with the strong acceptor tetracyano-p-quinodimethane (TMP/HMP-TCNQ) were grown by vapor diffusion. The oxygen and nitrogen K-edge spectra are spectroscopic fingerprints of the functional groups in the donor and acceptor moieties, respectively. The orbital selectivity of the NEXAFS pre-edge resonances allows us to precisely elucidate the participation of specific orbitals in the charge transfer process. Upon complex formation, the intensities of several resonances change substantially and a new resonance occurs in the oxygen K-edge spectrum. This gives evidence of a corresponding change of hybridization of specific orbitals in the functional groups of the donor (those derived from the frontier orbitals 2e and 6a(1) of the isolated methoxy group) and acceptor (orbitals b(3g), a(u), b(1g), and b(2u), all located at the cyano group) with π*-orbitals of the ring systems. Along with this intensity effect, the resonance positions associated with the oxygen K-edge (donor) and nitrogen K-edge (acceptor) shift to higher and lower photon energies in the complex, respectively. A calculation based on density functional theory qualitatively explains the experimental results. NEXAFS measurements shine light on the action of the functional groups and elucidate charge transfer on a submolecular level.

Intersystem Crossing Involving Strongly Spin Exchange-Coupled Radical Ion Pairs in Donor–bridge–Acceptor Molecules

Intersystem crossing involving photogenerated strongly spin exchange-coupled radical ion pairs in a series of donor-bridge-acceptor molecules was examined. These molecules have a 3,5-dimethyl-4-(9-anthracenyl)-julolidine (DMJ-An) donor either connected directly or connected by a phenyl bridge (Ph), to pyromellitimide (PI), 1 and 2, respectively, or naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptors, 3 and 4, respectively. Femtosecond transient optical absorption spectroscopy shows that photodriven charge separation produces DMJ(+•)-PI(-•) or DMJ(+•)-NI(-•) quantitatively in 1-4 (τ(CS) ≤ 10 ps), and that charge recombination occurs with τ(CR) = 268 and 158 ps for 1 and 3, respectively, and with τ(CR) = 2.6 and 10 ns for 2 and 4, respectively. Magnetic field effects (MFEs) on the neutral triplet state yield produced by charge recombination were used to measure the exchange coupling (2J) between DMJ(+•) and PI(-•) or NI(-•), giving 2J > 600 mT for 1-3 and 2J = 170 mT for 4. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy revealed that the formation of (3)*An upon charge recombination occurs by spin-orbit charge transfer intersystem crossing (SOCT-ISC) and/or radical-pair intersystem crossing (RP-ISC) mechanisms with the magnitude of 2J determining which triplet formation mechanism dominates. SOCT-ISC is the exclusive triplet formation mechanism in 1-3, whereas both RP-ISC and SOCT-ISC are active for 4. The triplet sublevels populated by SOCT-ISC in 1-4 depend on the donor-acceptor geometry in the charge separated state. This is consistent with the fact that the SOCT-ISC mechanism requires the relevant donor and acceptor orbitals to be nearly perpendicular, so that electron transfer results in a large orbital angular momentum change that must be compensated by a fast spin flip to conserve overall system angular momentum.

Effect of chelate ring and rigidity on Se⋯N interactions: a computational study

The intramolecular Se⋯N interactions between the selenium and the nitrogen atom in three series of o-substituted organoselenium compounds has been studied using density functional theory (DFT). The nature and the strength of these interactions and their dependence on substituents, the chelate ring size as well as the rigidity are predicted using a B3LYP/6-31G(d)/LanL2DZ method. The strength of the Se⋯N interactions is found to be dependent on the nature of Se–X (X = Cl, Br, I, SPh, CH2Ph; Ph: Phenyl) acceptor orbitals and follow the order I > Br > Cl > SPh > CH2Ph. The natural bond orbital (NBO) analysis using DFT methods points to nN→σ*Se–X electron delocalization as the key contributing factor towards Se⋯N nonbonding interactions. Both NBO and atoms in molecules (AIM) methods suggest that the intramolecular interaction in the organoselenium compounds is dominantly covalent in nature. Studies on the effect of solvent on the Se⋯N interaction show that a polar solvent stabilizes the Se⋯N interactions by shortening the Se⋯N distances.

Manganese K-Edge X-Ray Absorption Spectroscopy as a Probe of the Metal–Ligand Interactions in Coordination Compounds

A series of manganese coordination compounds has been investigated by X-ray absorption spectroscopy (XAS). The K-pre-edge spectra are interpreted with the aid of time-dependent density functional theory (TD-DFT). This method was calibrated for the prediction of manganese K-pre-edges with different functionals. Moreover the nature of all observed features could be identified and classified according to the corresponding set of acceptor orbitals, either 1s to 3d transitions or metal-to-ligand charge transfer (MLCT) bands. The observable MLCT bands are further divided into features that correspond to transitions into empty π* orbitals of π-donor ligands and those of π-acceptor ligands. The ability to computationally reproduce the observed features at the correct relative transition energy is strongly dependent on the nature of the transition. A detailed analysis of the electronic structure of a series of Mn coordination compounds reveals that the different classes of observable transitions provide added insight into metal-ligand bonding interactions.

S K-edge XAS and DFT Calculations on SAM Dependent Pyruvate Formate-Lyase Activating Enzyme: Nature of Interaction between the Fe4S4 Cluster and SAM and its Role in Reactivity

S K-edge X-ray absorption spectroscopy on the resting oxidized and the S-adenosyl-l-methionine (SAM) bound forms of pyruvate formate-lyase activating enzyme are reported. The data show an increase in pre-edge intensity, which is due to additional contributions from sulfide and thiolate of the Fe(4)S(4) cluster into the C-S σ* orbital. This experimentally demonstrates that there is a backbonding interaction between the Fe(4)S(4) cluster and C-S σ* orbitals of SAM in this inner sphere complex. DFT calculations that reproduce the data indicate that this backbonding is enhanced in the reduced form and that this configurational interaction between the donor and acceptor orbitals facilitates the electron transfer from the cluster to the SAM, which otherwise has a large outer sphere electron transfer barrier. The energy of the reductive cleavage of the C-S bond is sensitive to the dielectric of the protein in the immediate vicinity of the site as a high dielectric stabilizes the more charge separated reactant increasing the reaction barrier. This may provide a mechanism for generation of the 5'-deoxyadenosyl radical upon substrate binding.

A novel technique to synthesize nanoparticles of titanium dioxide, cadmium sulfide, and silicon dioxide, capped with 2,2-bipyridine compounds

The dynamics of electron transfer in the molecule–nanoparticle system and charge-transfer intramolecular complexes of TiO2, SiO2, and CdS were compared. Charge-transfer complexes and intramolecular charge-transfer complexes with superfast electron transfer–bipyridine–TiO2 and bipyridine–SiO2– were obtained. Both complexes are characterized by similar charge transfer bands and donor (bipyridine) and acceptor (Ti or Si) orbitals. The electron is localized on titanium or silicon sites in the metal–bipyridine complex but can be localized on the other Ti, Si, and CdS sites in TiO2 and SiO2 nanoparticles. The reverse dynamics of electron transfer from titanium sites to bipyridine ligands in the time interval of 200 fs is observed in the molecular complex. Titanium–cadmium–bipyridine (TCB) and silicon–cadmium–bipyridine (SCB) systems were synthesized and purified by two methods: Blodgett and sol–gel. The synthesized compound was identified by elemental analysis, X-ray powder diffractometry, infrared spectroscopy, scanning electron microscopy, and spectrophotometry in the ultraviolet and visible regions of the spectrum. The optical properties of the samples were investigated.

Nonheme oxo-iron(IV) intermediates form an oxyl radical upon approaching the C–H bond activation transition state

Oxo-iron(IV) species are implicated as key intermediates in the catalytic cycles of heme and nonheme oxygen activating iron enzymes that selectively functionalize aliphatic C-H bonds. Ferryl complexes can exist in either quintet or triplet ground states. Density functional theory calculations predict that the quintet oxo-iron(IV) species is more reactive toward C-H bond activation than its corresponding triplet partner, however; the available experimental data on model complexes suggests that both spin multiplicities display comparable reactivities. To clarify this ambiguity, a detailed electronic structure analysis of alkane hydroxylation by an oxo-iron(IV) species on different spin-state potential energy surfaces is performed. According to our results, the lengthening of the Fe-oxo bond in ferryl reactants, which is the part of the reaction coordinate for H-atom abstraction, leads to the formation of oxyl-iron(III) species that then perform actual C-H bond activation. The differential reactivity stems from the fact that the two spin states have different requirements for the optimal angle at which the substrate should approach the (FeO)(2+) core because distinct electron acceptor orbitals are employed on the two surfaces. The H-atom abstraction on the quintet surface favors the "σ-pathway" that requires an essentially linear attack; by contrast a "π-channel" is operative on the triplet surface that leads to an ideal attack angle near 90°. However, the latter is not possible due to steric crowding; thus, the attenuated orbital interaction and the unavoidably increased Pauli repulsion result in the lower reactivity of the triplet oxo-iron(IV) complexes.

Diamagnetic Group 6 Tetrakis(di-tert-butylketimido)metal(IV) Complexes

The addition of 4 equiv of LiN=C-t-Bu(2) to CrCl(3), MoCl(5), and WCl(6) in diethyl ether produced the complexes M(N=C-t-Bu(2))(4) (M = Cr, Mo, W). Single-crystal X-ray diffraction studies revealed that the molecules have flattened tetrahedral geometries with virtual D(2d) symmetry in the solid state. (1)H and (13)C NMR spectra indicated that the complexes are diamagnetic, and a qualitative MO analysis showed that the orthogonal π-donor and -acceptor orbitals of the ketimide ligand cooperatively split the d(xy) and d(z2) orbitals sufficiently to allow spin pairing in the d(xy) orbital. A more sophisticated quantum-mechanical analysis of Cr(N=C-t-Bu(2))(4) using density functional/molecular mechanics methods confirmed the qualitative analysis by showing that the singlet state is 27 kcal/mol more stable than the triplet state.

Effects of Electronic Mixing in Ruthenium(II) Complexes with Two Equivalent Acceptor Ligands. Spectroscopic, Electrochemical, and Computational Studies

The lowest energy metal to ligand charge transfer (MLCT) absorption bands found in ambient solutions of [Ru(NH(3))(4)(Y-py)(2)](2+) and [Ru(L)(2)(bpy)(2)](+) complexes (Y-py a pyridine ligand and (L)(n) a substituted acetonylacetonate, halide, am(m)ine, etc.) consist of two partly resolved absorption envelopes, MLCT(lo) and MLCT(hi). The lower energy absorption envelope, MLCT(lo), in these spectra has the larger amplitude for the bis-(Y-py) complexes, but the smaller amplitude for the bis-bpy the complexes. Time-dependent density functional theory (TD-DFT) approaches have been used to model 14 bis-bpy, three bis-(Y-py), and three mono-bpy complexes. The modeling indicates that the lowest unoccupied molecular orbital (LUMO) of each bis-(Y-py) complex corresponds to the antisymmetric combination of individual Y-py acceptor orbitals and that the transition involving the highest occupied molecular orbital (HOMO) and LUMO (HOMO-->LUMO) is the dominant contribution to MLCT(lo) in this class of complexes. The LUMO of each bis-bpy complex that contains a C(2) symmetry axis also corresponds largely to the antisymmetric combination of individual ligand acceptor orbitals, while the LUMOs are more complex when there is no C(2) axis; furthermore, the energy difference between the HOMO-->LUMO and HOMO-->LUMO+1 transitions is too small (<1000 cm(-1)) to resolve in the spectra of the bis-bpy complexes in ambient solutions. Relatively weak MLCT(lo) absorption contributions are found for all of the [Ru(L)(2)(bpy)(2)](m+) complexes examined, but they are experimentally best defined in the spectra of the (L)(2) = X-acac complexes. TD-DFT modeling of the HOMO-->LUMO transition of [Ru(L)(4)bpy](m+) complexes indicates that it is too weak to be detected and occurs at significantly lower energy (about 3000-5000 cm(-1)) than the observed MLCT absorptions. Since the chemical properties of MLCT excited states are generally correlated with the HOMO and/or LUMO properties of the complexes, such very weak HOMO-->LUMO transitions can complicate the use of spectroscopic information in their assessment. As an example, it is observed that the correlation lines between the absorption energy maxima and the differences in ground state oxidation and reduction potentials (DeltaE(1/2)) have much smaller slopes for the bis-bpy than the mono-bpy complexes. However, the observed MLCT(lo) and the calculated HOMO-->LUMO transitions of bis-bpy complexes correlate very similarly with DeltaE(1/2) and this indicates that it is the low energy and small amplitude component of the lowest energy MLCT absorption band that is most appropriately correlated with excited state chemistry, not the absorption maximum as is often assumed.

A Comparison of 4f vs5fMetal−Metal Bonds in (CpSiMe3)3M−ECp* (M = Nd, U; E = Al, Ga; Cp* = C5Me5): Synthesis, Thermodynamics, Magnetism, and Electronic Structure

Reaction of (CpSiMe(3))(3)U or (CpSiMe(3))(3)Nd with (Cp*Al)(4) or Cp*Ga (Cp* = C(5)Me(5)) afforded the isostructural complexes (CpSiMe(3))(3)M-ECp* (M = U, E = Al (1); M = U, E = Ga (2); M = Nd, E = Al (3); M = Nd, E = Ga (4)). In the case of 1 and 2 the complexes were isolated in 39 and 90% yields, respectively, as crystalline solids and were characterized by single-crystal X-ray diffraction, variable-temperature (1)H NMR spectroscopy, elemental analysis, variable-temperature magnetic susceptibility, and UV-visible-NIR spectroscopy. In the case of 3 and 4, the complexes were observed by variable-temperature (1)H NMR spectroscopy but were not isolated as pure materials. Comparison of the equilibrium constants and thermodynamic parameters DeltaH and DeltaS obtained by (1)H NMR titration methods revealed a much stronger U-Ga interaction in 2 than the Nd-Ga interaction in 4. Competition reactions between (CpSiMe(3))(3)U and (CpSiMe(3))(3)Nd indicate that Cp*Ga selectively binds U over Nd in a 93:7 ratio at 19 degrees C and 96:4 at -33 degrees C. For 1 and 3, comparison of (1)H NMR peak intensities suggests that Cp*Al also achieves excellent U(III)/Nd(III) selectivity at 21 degrees C. The solution electronic spectra and solid-state temperature-dependent magnetic susceptibilities of 1 and 2, in addition to X-ray absorption near-edge structure (XANES) measurements from scanning transmission X-ray microscopy (STXM) of 1, are consistent with those observed for other U(III) coordination complexes. DFT calculations using five different functionals were performed on the model complexes Cp(3)M-ECp (M = Nd, U; E = Al, Ga), and empirical fitting of the values for Cp(3)M-ECp allowed the prediction of binding energy estimates for Cp*Al compounds 1 and 3. NBO/NLMO bonding analyses on Cp(3)U-ECp indicate that the bonding consists predominantly of a E-->U sigma-interaction arising from favorable overlap between the diffuse ligand lone pair and the primarily 7s/6d acceptor orbitals on U(III), with negligible U-->E pi-donation. The overall experimental and computational bonding analysis suggests that Cp*Al and Cp*Ga behave as good sigma-donors in these systems.