Hybrid density functional theory studies on the magnetic interactions and the weak covalent bonding for the phenalenyl radical dimeric pair

The phenalenyl radical (1) is a prototype of the hydrocarbon radical. Recently, the single crystal of 2,5,8-tri-tert-butylphenalenyl (2) was isolated and showed that the two phenalenyl radicals form a staggered dimeric pair, giving rise to strong antiferromagnetic interactions. The origin of the antiferromagnetic interactions and the nature of the chemical bond for the dimeric pair are challenging issues for chemists. First, spin-polarized hybrid DFT (Becke's half and half LYP (UB2LYP)) and CASSCF calculations were performed for 2 and its simplified model, the staggered-stacking phenalenyl radical dimeric pair (3a), to elucidate the origin of the strong antiferromagnetic coupling and the characteristics of the chemical bond. The calculated results showed that a SOMO-SOMO overlap effect was responsible for the strong antiferromagnetic interactions and weak or intermediate covalent bonding between phenalenyl radicals. The tert-butyl groups introduced at three beta-positions hardly affected the magnetic coupling, mainly causing steric hindrances in the crystalline state. Next, to obtain insight into ferromagnetic stacking, we investigated the stacking effect of staggered (3a)- and eclipsed (3b)-stacking phenalenyl radical dimeric pairs with a change of the SOMO-SOMO overlap on the basis of the extended McConnell model. We found that the stacking mode of the dimeric pair with both a small SOMO-SOMO overlap and a ferromagnetic spin polarization effect provided a ferromagnetic coupling.     

Characterizing the dimerizations of phenalenyl radicals by ab initio calculations and spectroscopy: sigma-bond formation versus resonance pi-stabilization

Electronic-structure calculations for the self-association of phenalenyl radical (P*) predict the formation of dimeric species (sigma-P2) in which both moieties are connected by a sigma-bond with rP-P approximately 1.59 A and bond dissociation enthalpy of DeltaH(D) approximately 16 kcal mol(-1). Such an unusually weak sigma-bond is related to the loss of aromatic stabilization energy of approximately 34 kcal mol(-1) per phenalenyl moiety, largely owing to rehybridization. Ab initio calculations also reveal that the corresponding (one-electron) bond between phenalenyl radical and its closed-shell cation in sigma-P2+* is unstable relative to dissociation. Time-dependent DFT computations indicate the absence of any (strongly allowed) electronic transition in the visible region of the absorption spectrum of phenalenyl sigma-dimer. Such theoretical predictions are supported by experimental (ESR and UV-NIR) spectroscopic studies, in which the availability of a series of sterically hindered phenalenyl radicals allows definitive separations of the sigma-dimerization process from interference by pi-dimerization. As such, the thermodynamic parameters (determined from the temperature dependence of the ESR signals) with DeltaH(D) = 14 kcal mol(-1) and DeltaS(D) = 52 e.u. can be assigned to the formation of the colorless sigma-dimer. Similar results are obtained for all phenalenyl derivatives (provided their substitution patterns allow sigma-bond formation) to confirm the energetic preference of sigma-dimerization over pi-dimerization.     

Reduced and excited states of the intermediates (alpha-diimine)(C5R5)Rh in hydride transfer catalysis schemes: EPR and resonance Raman spectroscopy, and comparative DFT calculations of Co, Rh and Ir analogues

The electronic structures of the highly air-sensitive intermediates (N[caret]N) (C(5)Me(5))Rh, (N[caret]N = 2,2'-bipyridine (bpy), 2,2'-bipyrimidine (bpym), 2,2'-bipyrazine (bpz) and 3,3'-bipyridazine (bpdz)) of hydride transfer catalysis schemes were studied through resonance Raman (rR) spectroscopy and through EPR of the reduced forms [(N[caret]N) (C(5)Me(5))Rh](.-). The rR results are compatible with a predominant MLCT character of the lowest excited states [ (N[caret]N) (C(5)Me(5))Rh]*, and the EPR spectra of the reduced states reveal the presence of anion radical ligands, (N[caret]N) (.-), coordinated by unusually electron rich rhodium(i) centres. The experimental results, including the assignments of electronic transitions, are supported by DFT calculations for the model compounds [(N[caret]N)(C(5)H(5))Rh](o)/(.-), (N[caret]N) = bpy or bpym. The calculations confirm a significant but not complete mixing of metal and ligand orbitals in the lowest unoccupied MO which still retains about 3/4 pi* (N[caret]N) character. DFT calculations on (bpy)(C(5)H(5))M and [(bpy)(C(5)H(5))ClM](+), M = Co, Rh, Ir, agree with the experimental results such as the differences between the homologues, especially the different LUMO characters of the precursor cations in the case of Co-->d(M)) and Rh or Ir (-->pi*(bpy)).     

Why is Re-Re bond formation/cleavage in [Re(bpy)(CO)3]2 different from that in [Re(CO)5]2? Experimental and theoretical studies on the dimers and fragments

The Re(NN)(CO)3(THF) (NN=bpy=2,2'-bipyridine or dmb=4,4'-dimethyl-2,2'-bipyridine) radical, produced by homolysis of [Re(NN)(CO)3]2 in THF solution by visible irradiation, dimerizes with rate constants kd=20 +/- 3 and 11 +/- 4 M(-1) s(-1) for NN=dmb and bpy, respectively. The dimerization processes are strikingly slow compared to those of typical metal radicals including Re(CO)5 (kd approximately 10(9) M(-1) s(-1)). In order to explain such slow reactions, we have performed B3LYP hybrid DFT and fully ab initio RHF and MP2 calculations on several conformations of [Re(bpy)(CO)3]2 (cis, trans, skewed cis, skewed trans) and [Re(CO)5]2 (staggered) and on their constituent monomer radicals and anions. The calculations show that the most stable geometry of [Re(bpy)(CO)3]2 is skewed cis, and the experimental infrared spectrum and photochemical properties of the [Re(bpy)(CO)3]2 dimer are best described by the calculated properties of the skewed cis conformer in which there is no low-lying unoccupied orbital that is predominantly sigma(MM) in character. The Re(bpy)(CO)3(THF) ligand radical is more stable than the 5-coordinate "17-electron" metal radical, Re(bpy)(CO)3, suggesting that the extremely slow dimerization rate most likely arises from the solvent blocking the binding site (i.e., the estimated fraction of the five-coordinate monomer is 1.6 x 10(-2)). Theoretical results are consistent with our experimental results that the dimerization process proceeds via the Re centered radical, which is involved in a pre-equilibrium favoring the ligand-centered radical. Furthermore, time-dependent DFT calculations on [Re(bpy)(CO)3]2 and [Re(bpy)(CO)3]- identify the origin of UV-vis absorption in THF.     

Sterically crowded bicyclo[1.1.0]butane radical cations

The variability of carbon-carbon single bonds by steric and electronic effects is probed by DFT calculations of sterically crowded bicyclo[1.1.0]butanes and their radical cations. The interplay of sterics and electronics on the gradual weakening and breaking of bonds was studied by investigating bridgehead substitution in 1,3-di-tert-butylbicyclo[1.1.0]butane and 2,2',4,4'-tetramethyl-1,3-di-tert-butylbicyclo[1.1.0]butane and geminal substitution in 2,2'-di-tert-butylbicyclo[1.1.0]butane and 2,2',4,4'-tetra-tert-butylbicyclo[1.1.0]butane. Bridgehead substitution leads to a lengthening of the central bond, whereas bisubstitution on the geminal carbon leads to a shortening of this bond due to a Thorpe-Ingold effect. Although the character of the central bond can be modulated by substitution and electron transfer over a range of 0.35 A, the state forbidden ring planarization does not occur. Sterically crowded bicyclo[1.1.0]butane radical cations are therefore promising candidates for the investigation of extremely long carbon-carbon single bonds.     

Steric modulations in the reversible dimerizations of phenalenyl radicals via unusually weak carbon-centered pi- and sigma-bonds

[reaction: see text] Spontaneous self-associations of various tricyclic phenalenyl radicals lead reversibly to either pi- or sigma-dimers, depending on alkyl-substitution patterns at the alpha- and beta-positions. Thus, the sterically encumbered all-beta-substituted tri-tert-butylphenalenyl radical (2*) affords only the long-bonded pi-dimer in dichloromethane solutions, under conditions in which the parent phenalenyl radical (1*) leads to only the sigma-dimer. Further encumbrances of 1* with a pair of alpha, beta- or beta, beta- tert-butyl substituents and additional methyl and ethyl groups (as in sterically hindered phenalenyl radicals 3* - 6*) do not inhibit sigma-dimerization. ESR spectroscopy is successfully employed to monitor the formation of both diamagnetic (2-electron) dimers; and UV-vis spectroscopy specifically identifies the pi-dimer by its intense near-IR band. The different temperature-dependent spectral (ESR and UV-vis) behaviors of these phenalenyl radicals allow the quantitative evaluation of the bond enthalpy of 12 +/- 2 kcal mol(-1) for sigma-dimers, in which the unusually low value has been theoretically accounted for by the large loss of phenalenyl (aromatic) pi-resonance energy attendant upon such bond formation.     

A comparative study on the photochemistry of two bipyridyl derivatives: [2,2'-bipyridyl]-3,3'-diamine and [2,2'-bipyridyl]-3,3'-diol.

The two isoelectronic bipyridyl derivatives, [2,2'-bipyridyl]-3,3'-diamine and [2,2'-bipyridyl]-3,3'-diol, are experimentally known to undergo very different excited-state double-proton-transfer processes, which result in fluorescence quantum yields that differ by four orders of magnitude. Herein, density functional theory (DFT), time-dependent DFT (TDDFT), and complete active space self-consistent field (CASSCF) calculations are used to study the double-proton-transfer processes in the ground and first singlet pi-->pi* excited state. The quantum-chemistry calculations indicate 1) the existence of only one energy minimum in the ground electronic state corresponding to reactants (thus avoiding the possibility of a fast fluorescent relaxation process from the photoproducts region), 2) an endoergic process of the complete double proton transfer, and 3) the presence of a conical intersection in the excited intermediate region of [2,2'-bipyridyl]-3,3'-diamine. These facts explain the very low fluorescence quantum yield in [2,2'-bipyridyl]-3,3'-diamine compared to [2,2'-bipyridyl]-3,3'-diol.     

Rational design of covalently bridged [Fe(III)₂M(II)O] clusters

We are reporting the first supramolecular dimeric units of basic carboxylates. The neutral [Fe(III)?M(II)O] motif for different 3d M metals is covalently bound through 2,2'-bipyrimidine. We have structurally characterized the hexanuclear clusters and the related trinuclear building blocks. Their magnetic properties have been fully analyzed and DFT calculations have been performed as a supplementary tool. All results evidence a weak antiferromagnetic interaction through the bpym bridge between isolated spin ground states (in some examples) arising from intra-Fe?MO core exchange couplings.     

The Density Functional Theory Account of Interplaying Long-Range Exchange and Dispersion Effects in Supramolecular Assemblies of Aromatic Hydrocarbons with Spin

Aromatic hydrocarbons with fused benzene rings and regular triangular shapes, called n-triangulenes according to the number of rings on one edge, form groundstates with n-1 unpaired spins because of topological reasons. Here, we focus on methodological aspects emerging from the density functional theory (DFT) treatments of dimer models of the n = 2 triangulene (called also phenalenyl), observing that it poses interesting new problems to the issue of long-range corrections. Namely, the interaction comprises simultaneous spincoupling and van der Waals effects, i.e., a technical conjuncture not considered explicitly in the benchmarks calibrating long-range corrections for the DFT account of supramolecular systems. The academic side of considering dimer models for calculations and related analysis is well mirrored in experimental aspects, and synthetic literature revealed many compounds consisting of stacked phenalenyl cores, with intriguing properties, assignable to their long-range spin coupling. Thus, one may speculate that a thorough study assessing the performance of state-of-the-art DFT procedures has relevance for potential applications in spintronics based on organic compounds.     

Third-order nonlinear optical properties of open-shell supermolecular systems composed of acetylene linked phenalenyl radicals

The third-order nonlinear optical (NLO) properties, at the molecular level, the static second hyperpolarizabilities, γ, of supermolecular systems composed of phenalenyl and pyrene rings linked by acetylene units are investigated by employing the long-range corrected spin-unrestricted density functional theory, LC-UBLYP, method. The phenalenyl based superethylene, superallyl, and superbutadiene in their lowest spin states have intermediate diradical characters and exhibit larger γ values than the closed-shell pyrene based superpolyene systems. The introduction of a positive charge into the phenalenyl based superallyl radical changes the sign of γ and enhances its amplitude by a factor of 35. Although such sign inversion is also observed in the allyl radical and cation systems in their ground state equilibrium geometries, the relative amplitude of γ is much different, that is, |γ(regular allyl cation)/γ(regular allyl radical)| = 0.61 versus |γ(phenalenyl based superallyl cation)/γ(phenalenyl based superallyl radical)| = 35. In contrast, the model ethylene, allyl radical/cation, and butadiene systems with stretched carbon-carbon bond lengths (2.0 ?), having intermediate diradical characters, exhibit similar γ features to those of the phenalenyl based superpolyene systems. This exemplifies that the size dependence of γ as well as its sign change by introducing a positive charge on the phenalenyl based superpolyene systems originate from their intermediate diradical characters. In addition, the change from the lowest to the highest π-electron spin states significantly reduces the γ amplitudes of the neutral phenalenyl based superpolyene systems. For phenalenyl based superallyl cation, the sign inversion of γ (from negative to positive) is observed upon switching between the singlet and triplet states, which is predicted to be associated with a modification of the balance between the positive and negative contributions to γ. The present study paves the way toward designing a variety of open-shell NLO supermolecular systems composed of phenalenyl radical building blocks.     

Insertion reaction of carbon dioxide into Sn-OR bond. Synthesis, structure and DFT calculations of di- and tetranuclear isopropylcarbonato tin(IV) complexes

The reaction of carbon dioxide with the stannane nBu2Sn(OiPr)2 and distannoxane [nBu2(iPrO)Sn]2O leads to the selective insertion into one Sn-OiPr bond generating the corresponding nBu2Sn(OiPr)(OCO2(i)Pr) and nBu2(iPrO)SnOSn(OCO2(i)Pr)nBu2 species. Both compounds are characterised by multinuclear NMR, FT-IR and single-crystal X-ray crystallography. In the solid state, they adopt a dimeric arrangement with bridging isopropoxy and terminal isopropylcarbonato ligands. The X-ray crystal structure of the dinuclear stannane shows that the Sn2O2 ring and the two Sn-OCO2C fragments are nearby coplanar. The same holds for the ladder-type tetranuclear distannoxane. The dimeric structures are also evidenced by solution NMR in non-coordinating solvents. Interestingly, the assignment of the exo and endo tin resonances of the dimeric distannoxane is unambiguous using a labeled 13CO2 experiment. The stability of the dimeric association has been probed in the stannane series on the basis of DFT calculations.     

Anodic preparation of [Re2Cp2(CO)6]2+: a dimeric dication that provides the powerful one-electron oxidant [ReCp(CO)3]+

The half-sandwich piano-stool compounds Re(eta5-C5R5)(CO)3 (1, R = H; or 2, R = Me) are oxidized to the corresponding 17-electron Re(II) cations at glassy carbon anodes in CH2Cl2/[NBu4][B(C6F5)4]. Despite the very strongly positive E1/2 values of the couples (1.16 V for 1/1+ and 0.91 V for 2/2+ vs ferrocene/ferrocenium), the radical cations are persistent in this medium and exist in equilibrium with the corresponding dimeric dications, which may be cathodically reduced back to the neutral starting material. DFT calculations show that the dimer of 1+ achieves its stability through formation of a single long (almost 3.3 A) Re-Re bond made possible when the HOMO in 1 is rehybridized away from the metal in the one-electron oxidation process. The pure salts [1][B(C6F5)4]2 and [2][B(C6F5)4]2 were isolated by preparative anodic electrochemistry. The former may be used for storage of the very strong one-electron oxidant 1+, which was used to prepare a number of oxidation products as their [B(C6F5)4]- salts.     

2,6,7-trithiabicyclo[2.2.2]octanes as promising photolabile tags for combinatorial encoding

The adducts of trithiabicyclo[2.2.2]octane (TTBO) and carbonyl compounds undergo efficient photoinduced fragmentation with quantum yields comparable to that of dithiane adducts. The effect of the third sulfur on the stability of the respective radical cations and radicals is examined computationally and experimentally in a laser flash photolysis study. A straightforward synthetic approach to a variety of 4-substituted trithiabicyclo[2.2.2]octanes from 3-bromo-2,2-bis(bromomethyl)propanol is developed, making a diverse set of mass-differentiated photolabile tags readily available for combinatorial encoding.     

Synthesis,Structure, and Catalytic Activity of Cyclometalated Iridium Complexes with a Bidentate POC Ligand

Synthesis, characterization and catalytic activity of cyclometalated iridium complexes with a bidentate POC ligand is presented. Metalation of POC-H (di-tert-butyl(phenoxy)phosphane) with [Ir(COD)Cl]₂ proceeded rapidly at room temperature and afforded mixture of (POC)(POC-H)IrHCl ( 1 a ) and (POC)(COD)IrHCl ( 1 b ), from which complexes (POC)(L)IrHCl where L=PPh₃ ( 1 c ), bipyridine ( 1 d ) and [2,2′-bipyridine]-6,6′-diol ( 1 e ) were prepared through ligand exchange. The compounds were tested in acceptorless dehydrogenation of 1-phenylethanol and transfer dehydrogenation of ethanol in a context of comparison with pincer counterparts (POCOP)IrHCl and (PCN)IrHCl. An attempt to prepare a dihydride complex from 1 e led to dimeric complex [(POC)(bipy-diol−)IrH]₂ ( 3 ) that could explain the low activity of 1 e . DFT studies provided insight into POC-H vs POCOP-H metalation mechanism.     

[2+2]-Cycloadditionen und -Cycloreversionen in Radikalanionen. Eine ESR.-spektroskopische Beschreibung am Beispiel 2,2′-disubstituierter Biphenyle

[2+2]-Cycloadditions and -cycloreversions in radical anions. An ESR. spectroscopic study for 2,2′-disubstituted diphenyl derivatives The radical anions derived from the polycyclic olefins 1, 2 and 3 are shown by ESR. spectroscopy to undergo [_π2+_π2]-cycloaddition reactions even at low temperatures. Similarly, facile cleavage by [_σ2+_σ2]-cycloreversion processes is observed for the radical anions of the corresponding cyclobutane species. This reactivity, which is in marked contrast with the thermal stability of the neutral parent compounds, is discussed taking into account the molecular geometry and the spin density distribution.     

Group 3 metal complexes of radical-anionic 2,2'-bipyridyl ligands

A new method for generating group 3 metal complexes containing radical-anionic 2,2'-bipyridyl (bipy) ligands is described that relies on hydrogen-atom abstraction from dearomatized biheterocyclic complexes. This method does not involve electron transfer to neutral 2,2'-bipyridyl or salt metathesis between the lithium salt of the 2,2'-bipyridyl radical anion and group 3 metal halides. The new metal complexes were characterized by single-crystal X-ray diffraction, electron paramagnetic resonance, and absorption spectroscopy. Density functional theory (DFT) calculations were used to probe the electronic structure of these compounds. All these methods support the radical-anionic character of bipy in all bipy compounds presented.     

Dipyridylketone as a versatile ligand precursor for new cationic heteroleptic cyclometalated iridium complexes

Three new bis-cyclometalated iridium(III) complexes, of general formula [Ir(2-phenylpyridine)(2)(L)](+), are reported. The compounds contain a dipyridine-type ligand (L) derived from di-2-pyridylketone (dipyridin-2-ylmethanol, 2,2'-(hydrazonomethylene)dipyridine and 3-hydroxy-3,3-di(pyridine-2-yl)propanenitrile) and were synthesized through two different reaction pathways. The alternative synthetic pathway herein proposed, namely the direct reactions on the complex [Ir(2-phenylpyridine)(2)(2,2'-dipyridylketone)](+), overcame the inconveniences encountered with the standard reaction between the dimeric precursor [Ir(2-phenylpyridine)(2)(μ-Cl)](2) and the ancillary ligands (L). The photophysical characterization of the iridium complexes reveals that modifications on the ancillary ligand introduce large changes in the photophysical behaviour of the complexes. High emission quantum yield is associated with the presence of a saturated carbon between the two pyridyl moieties: [Ir(2-phenylpyridine)(2)(2,2'-dipyridylketone)](+) and [Ir(2-phenylpyridine)(2)(2,2'-(hydrazonomethylene)dipyridine)](+) are extremely low emissive, while [Ir(2-phenylpyridine)(2)(dipyridin-2-ylmethanol)](+) and [Ir(2-phenylpyridine)(2)(3-hydroxy-3,3-di(pyridine-2-yl)propanenitrile)](+) are good photoemitters. DFT and TD-DFT calculations confirmed the mixed LC/MLCT character of the excited states involved in the absorption and emission processes and highlighted the role of the π-conjugation between the two subunits of the ancillary ligand in determining the nature of the LUMO.     

The effect of reduction on rhenium(I) complexes with binaphthyridine and biquinoline ligands: a spectroscopic and computational study

A number of rhenium complexes with binaphthyridine and biquinoline ligands have been synthesized and studied. These are [Re(L)(CO)3Cl] where L = 3,3'-dimethylene-2,2'-bi-1,8-naphthyridine (dbn), 2,2'-bi-1,8-naphthyridine (bn), 3,3'-dimethylene-2,2'-biquinoline (dbq), and 3,3'-dimethyl-2,2'-biquinoline (diq). This series represents ligands in which the electronic properties and steric preferences are tuned. These complexes are modeled using density functional theory (DFT). An analysis of the resonance Raman spectra for these complexes, in concert with the vibrational assignments, reveals that the accepting molecular orbital (MO) in the metal-to-ligand charge transfer (MLCT) transition is the LUMO and causes bonding changes at the inter-ring section of the ligand. The electronic absorption spectroelectrochemistry for the reduced complexes of [Re(dbn)(CO)3Cl], [Re(dbq)(CO)3Cl], and [Re(diq)(CO)3Cl] suggest that the singly occupied MO is delocalized over the entire ligand structure despite the nonplanar nature of the diq ligand in [Re(diq)(CO)3Cl]. The IR spectroelectrochemistry for [Re(dbn)(CO)3Cl], [Re(dbq)(CO)3Cl], and [Re(bn)(CO)3Cl] reveal that reduction lowers the CO ligand vibrational frequencies to a similar extent in all three complexes. The substitution of naphthyridine for quinoline has little effect on the nature of the singly occupied MO. These data are supported by DFT calculations on the reduced complexes, which reveal that the ligands are flattened out by reduction: This may explain the similarity in the properties of the reduced complexes.     

Electronic Structures and Chiroptical Properties of Post‐functionalized Helicene Quinones

The synthesis of redox‐active p‐ and o‐quinones 2‐phenylamino‐4‐phenylimino[6]helicene‐1‐one 1 , 2‐phenylamino[6]‐helicene‐1,4‐dione 2 , and 4‐phenyl[6]helicene‐1,2‐dione 3 in their enantiopure forms by post‐functionalization of (P)‐ and (M)‐1,2‐dimethoxy[6]helicene is presented. Structural characterization in solution and in the solid state was accomplished by 2D NMR spectroscopy methods and X‐ray diffraction analysis, respectively. Interpretation of electrochemical redox data was accompanied by a detailed orbital picture, derived from DFT calculations. The electronic structures of compounds 1 – 3 were investigated by UV/Vis and electronic circular dichroism (ECD) spectroscopy, complemented by TD‐DFT calculations. Quinones 1 – 3 were chemically reduced to study the EPR signatures of their respective radical anions. DFT methods were used for the atom assignment of the hyperfine coupling constants. The results are discussed within the context of electrochromic chiral switches and molecular recognition.     

Biradicaloid character of phenalenyl-based aromatic compounds with a small HOMO–LUMO gap

Biradicaloid character of three Kekulé aromatic compounds containing two phenalenyl moieties is discussed on the basis of the theoretical and experimental results. DFT calculation of the compounds reveals a small HOMO–LUMO gap with a large spatial overlap between them, leading to a singlet biradical character in a ground state and an excited triplet biradical state with a small ΔE_S–T. Singlet biradical character for 1 (see Fig. 1) is indicated by the X-ray crystallographic analysis, which shows dimeric pairs with substantially short non-bonding contacts of ∼3.1 Å. The ESR measurements for 1 and 3 give typical spectra for triplet species and the temperature dependence of the half-field signal indicates the thermal excitation to the triplet states.