Search

The electronic structures of the ground and excited electronic states involved in the oxidative and reductive quenching cycles of 12 relevant ruthenium, iridium, and copper photoredox catalysts (

A dinuclear hafnium complex containing the parent imido ligand [(PN)(PNC)Hf=NH{μ

In this study a detailed scrutiny of the electronic structure changes during the redox events of the oxidative and reductive quenching cycles of the representative homoleptic and heteroleptic octahedral iridium [Ir(bpy)x(ppy)3-x]x+ (x = 0, 1, 2, and 3) and ruthenium [Ru(bpy)x(ppy)3-x]x-1+ (x = 1, 2, and 3) photoredox catalysts is provided through the corresponding electron density difference Δρ(r) distributions. The systematic analysis of the Δρ(r) distributions provides intuitive insights into the details of the metal- and ligand-centered electron transfer processes that take place in the different excited- and ground-state redox steps of classical photoredox catalysis. In addition to the structural metrics, the measured ground-state reduction potentials were also reproduced with great accuracy, typically within 0.15 V, when using the TPSSh functional in combination with the Def2-TZVP basis set coupled to reparameterized implicit solvation model (SMD). We computed the excited-state reduction potentials of these ruthenium and iridium complexes without using TD-DFT, but by directly computing the solution-state Gibbs free energy of the triplet 3MLCT state, giving good agreement with respective experiments. The analyzed Δρ(r) maps reveal the characteristic features of metal- and ligand-centered reductions and oxidations in both ground- and excited states and metal-to-ligand charge transfers (MLCT), sometimes perturbed by additional ligand-to-ligand charge transfer (LLCT) contributions. One of the most interesting features of ligand-centered redox processes is the localization of the accumulated electron density at one redox-active ligand in the case of heteroleptic systems [Ir(bpy)(ppy)2]+ and [Ru(bpy)(ppy)2]0, which is in contrast to the delocalized nature of the ligands-hosted charge in homoleptic photoredox catalysts, such as the classical [Ru(bpy)3]2+ system.

Room temperature photolysis of the bis(azide)cobaltate(II) complex [Na(THF)x][(ketguan)Co(N3)2] (ketguan = [(tBu2CN)C(NDipp)2]−, Dipp = 2,6-diisopropylphenyl) (3a) in THF cleanly forms the binuclea...

The cationic palladium(II) complex 1 of pyridyl-mesoionic carbene ligand catalyzes Markovnikov-selective intermolecular hydroamination between anilines and terminal alkynes into the corresponding imines. The reaction proceeds at room temperature, in the absence of additives, with exquisite selectivity and diverse functional group tolerance. The key intrinsic feature of the catalyst is the pyridine wingtip confined to the proximity of the alkynophilic metal active site, which mimics the function of enzyme-like architectures by assisting entropically favored proton transfers.

In this computational study, the electronic structure changes along the oxidative and reductive quenching cycles of a homoleptic and a heteroleptic prototype Cu(I) photoredox catalyst, namely, [Cu(dmp)2]+ (dmp = 2,9-dimethyl-1,10-phenanthroline) and [Cu(phen)(POP)]+ (POP = bis [2-(diphenylphosphino)phenyl]ether), are scrutinized and characterized using quasi-restricted orbitals (QROs), electron density differences, and spin densities. After validating our density functional theory-based computational protocol, the equilibrium geometries and wavefunctions (using QROs and atom/fragment compositions) of the four states involved in photoredox cycle (S0, T1, Dox, and Dred) are systematically and thoroughly described. The formal ground and excited state ligand- and metal-centered redox events are substantiated by the QRO description of the open-shell triplet metal-to-ligand charge-transfer (3MLCT) (d9L−1), Dox (d9L0), and Dred (d10L−1) species and the corresponding structural changes, e.g., flattening distortion, shortening/elongation of Cu–N/Cu–P bonds, are rationalized in terms of the underlying electronic structure transformations. Among others, we reveal the molecular-scale delocalization of the ligand-centered radical in the 3MLCT (d9L−1) and Dred (d9L−1) states of homoleptic [Cu(dmp)2]+ and its localization to the redox-active phenanthroline ligand in the case of heteroleptic [Cu(phen)(POP)]+.

The mechanism of the gas-phase halogen-exchange reaction between boron- and aluminum-halides (i.e., BX3 + BX3 and AlX3 + AlX3, X = F, Cl, or Br) was discovered using density functional theory. The reaction takes place via a two-step mechanism with the intermediacy of a diamond-core structure analogous to diborane. Good agreement was found between the simulated reaction features and experimental observations, which demonstrate slow kinetics and an equilibrium process for boron species and dimer formation in the case of aluminum-halides. This computational and theoretical study also reveals and quantifies the effect of resonance on the thermodynamic stability of the central intermediate and conceptualizes the extreme stability difference (∼50 kcal mol−1) between boron and aluminum diamond-core bridge structures. Through an interaction energy decomposition analysis in combination with electronic structure analyses, we revealed that, beyond the resonance stabilization in free boron-halides, superior electrostatics in aluminum-halides results in the different reactivities, i.e., dimer formation for the latter species whereas substituent exchange for the former ones.

The nitride salt [(PN)2Ti≡N{μ2-K(OEt2)}]2 (1) (PN- = (N-(2-PiPr2-4-methylphenyl)-2,4,6-Me3C6H2) can be oxidized with two equiv of I2 or four equiv of ClCPh3 to produce the phosphinimide-halide complexes (NPN')(PN)Ti(X) (X- = I (2), Cl (3); NPN' = N-(2-NPiPr2-4-methylphenyl)-2,4,6-Me3C6H22-), respectively. In the case of 2, H2 was found to be one of the other products; whereas, HCPh3 and Gomberg's dimer were observed upon the formation of 3. Independent studies suggest that the oxidation of 1 could imply the formation of the transient nitridyl species [(PN)2Ti(≡N•)] (A), which can either oxidize the proximal phosphine atom to produce the Ti(III) intermediate [(NPN')(PN)Ti] (B) or, alternatively, engage in H atom abstraction to form the parent imido (PN)2Ti≡NH (4). The latter was independently prepared and was found to photochemically convert to the titanium-hydride, (NPN')(PN)Ti(H) (5). Isotopic labeling studies using (PN)2Ti≡ND (4-d1) as well as reactivity studies of 5 with a hydride abstractor demonstrate the presence of the hydride ligand in 5. An alternative route to putative A was observed via a photochemically promoted incomplete reduction of the azide ligand in (PN)2Ti(N3) (6) to 4. This process was accompanied by some formation of 5. Frozen matrix X-band EPR studies of 6, performed under photolytic conditions, were consistent with species B being formed under these reaction conditions, originating from a low barrier N-insertion into the phosphine group in the putative nitridyl species A. Computational studies were also undertaken to discover the mechanism and plausibility of the divergent pathways (via intermediates A and B) in the formation of 2 and 3, and to characterize the bonding and electronic structure of the elusive nitrogen-centered radical in A.

Room temperature photolysis of the bis(azide)cobaltate(II) complex [Na(THF)

The cationic palladium(II) complex 1 of pyridyl-mesoionic carbene ligand catalyzes Markovnikov-selective intermolecular hydroamination between anilines and terminal alkynes into the corresponding imines. The reaction proceeds at room temperature, in the absence of additives, with exquisite selectivity and diverse functional group tolerance. The key intrinsic feature of the catalyst is the pyridine wingtip confined to the proximity of the alkynophilic metal active site, which mimics the function of enzyme-like architectures by assisting entropically favored proton transfers.

Room temperature photolysis of the bis(azide)cobaltate(II) complex [Na(THF)x][(ketguan)Co(N3)2] (ketguan = [(tBu2CN)C(NDipp)2]–, Dipp = 2,6-diisopropylphenyl) (3a) in THF cleanly forms the binuclear cobalt nitride [Na(THF)4{[(ketguan)Co(N3)]2(μ-N)}]n (1). Compound 1 represents the first example of an isolable, bimetallic cobalt nitride complex, and it has been fully characterized by spectroscopic, magnetic, and computational analyses. Density functional theory supports a CoIII=N=CoIII canonical form with significant π-bonding between the cobalt centers and the nitride atom. Unlike other Group 9 bridging nitride complexes, no radical character is detected at the bridging N-atom of 1. Indeed, 1 is unreactive towards weak C-H donors and even co-crystallizes with a molecule of cyclohexadiene (CHD) in its crystallographic unit cell to give 1·CHD as a room temperature stable product. Notably, addition of pyridine to 1 or photolyzed solutions of [(ketguan)Co(N3)(py)]2 (4a) leads to destabilization via activation of the nitride unit, resulting in the mixed-valent Co(II)/(III) bridged imido species [(ketguan)Co]2(μ-NH)(μ-N3) (5) formed from intermolecular hydrogen atom abstraction (HAA) of strong C-H bonds (BDE ~ 100 kcal/mol). Kinetic rate analysis of the formation of 5 in the presence of C6H12 or C6D12 gives a KIE = 2.5±0.1, supportive of a HAA formation path-way. The reactivity of our system was further probed by photolyzing C6D6/py-d5 solutions of 4a under an H2 atmosphere (150 psi), which leads to the exclusive formation of the bis(imido)[(ketguan)Co(μ-NH)]2 (6) as a result of dihydrogen activa-tion. These results provide unique insights into the chemistry and electronic structure of late 3d-metal nitrides while providing entryway into C-H activation pathways.

The masked Ti(II) synthon (Ketguan)(η6-ImDippN)Ti (1) oxidatively adds across thiophene to give ring-opened (Ketguan)(ImDippN)Ti[κ2-S(CH)3CH] (2). Complex 2 is photosensitive, and upon exposure to light, reductively eliminates thiophene to regenerate 1 – a rare example of early-metal mediated oxidative-addition/reductive-elimination chemistry. DFT calculations indicate strong titanium π-backdonation to the thiophene π*-orbitals leads to the observed thiophene ring opening across titanium, while a proposed photoinduced LMCT promotes the reverse thiophene elimination from 2. Finally, pressurizing solutions of 2 with H2 (150 psi) at 80 °C leads to the hydrodesulphurization of thiophene to give the Ti(IV) sulphide (Ketguan)(ImDippN)Ti(S) (3) and butane.

The masked Ti(II) synthon (Ketguan)(η6-ImDippN)Ti (1) oxidatively adds across thiophene to give ring-opened (Ketguan)(ImDippN)Ti[κ2-S(CH)3CH] (2). Complex 2 is photosensitive, and upon exposure to light, reductively eliminates thiophene to regenerate 1 – a rare example of early-metal mediated oxidative-addition/reductive-elimination chemistry. DFT calculations indicate strong titanium π-backdonation to the thiophene π*-orbitals leads to the observed thiophene ring opening across titanium, while a proposed photoinduced LMCT promotes the reverse thiophene elimination from 2. Finally, pressurizing solutions of 2 with H2 (150 psi) at 80 °C leads to the hydrodesulfurization of thiophene to give the Ti(IV) sulfide (Ketguan)(ImDippN)Ti(S) (3) and butane.

The mononuclear niobium methylidyne [(PNP)(ArO)Nb≡CH] (1; PNP– = N[2-PiPr2-4-methylphenyl]2–, Ar = 2,6-iPr2C6H3) reacts with the isocyanate O═C═NtBu to form a mononuclear niobium oxo species with a rare example of an azaallenyl ligand, namely [(PNP)(ArO)Nb═O(CH═C═NtBu)] (2). When 1 is treated with the phosphaalkyne P≡CAd (Ad = 1-adamantyl), P≡C bond cleavage occurs to form a mononuclear complex where P–P coupling has occurred between the formal phosphaalkyne phosphorus atom and one phosphine arm from the PNP ligand, namely [(PNPP)(ArO)Nb(η2-AdCCH)] (3). Solid-state structural studies and isotopic labeling experiments confirm C–C bond formation of the methylidyne group as well as provide conclusive evidence for the oxo ligand in 2 being terminal and the fate of the phosphorus atom from P≡CAd in complex 3. Computational studies have been applied to understand the pathway involving the P–P bond forming reaction of 1 and P≡CAd.

The first example of the OCPPCO ligand, diisophosphaethynolate, is reported via reductive coupling of a Sc-OCP precursor. Upon reduction with KC8 , isolation of the dinuclear complex, namely [K(OEt2 )]2 [(nacnac)Sc(OAr)]2 (OCPPCO), is observed, leading to a unique motif [OCPPCO]4- , stabilized by two scandium centers. Detailed NMR spectra of all complexes as well as IR and single crystal X-ray studies were obtained to fully elucidate the nature of these complexes in solution as well as in the solid state. Theory is combined to probe the electronic structure and orbitals responsible for the bonding interactions in the Sc-OCPPCO-Sc skeleton but also to compare to the linear mode observed in the precursor.

Using a set of state-of-the-art quantum chemical techniques we scrutinized the characteristically different reactivity of frustrated and classical Lewis pairs towards molecular hydrogen. The mechanisms and reaction profiles computed for the H2 splitting reaction of various Lewis pairs are in good agreement with the experimentally observed feasibility of H2 activation. More importantly, the analysis of activation parameters unambiguously revealed the existence of two reaction pathways through a low-energy and a high-energy transition state. An exhaustive scrutiny of these transition states, including their stability, geometry and electronic structure, reflects that the electronic rearrangement in low-energy transition states is fundamentally different from that of high-energy transition states. Our findings reveal that the widespread consensus mechanism of H2 splitting characterizes activation processes corresponding to high-energy transition states and, accordingly, is not operative for H2-activating systems. One of the criteria of H2-activation, actually, is the availability of a low-energy transition state that represents a different H2 splitting mechanism, in which the electrostatic field generated in the cavity of Lewis pair plays a critical role: to induce a strong polarization of H2 that facilities an efficient end-on acid-H2 interaction and to stabilize the charge separated “H+–H−” moiety in the transition state.

Visible-light photoredox catalysis has become a practical tool in the last years for driving energy-demanding chemical reactions. Owing to their exceptional photoelectrochemical properties, classical octahedral ruthenium and iridium complexes still dominate the field of photoredox catalysis despite their drawbacks, such as sustainability and costs. Luminescent Cu (I) complexes are considered one of the most plausible candidates to replace such traditional Ru- and Ir photoredox catalysts in the near future. This review covers the development of Cu(I) complexes for photoredox catalysis with a special focus on collecting and categorizing the available photophysical and electrochemical information. The principal characteristics, advantages, drawbacks and examples of applications of the distinct classes (homoleptic, heteroleptic, in-situ generated tetra/tri/di coordinated, etc.) of Cu(I) complexes used as photoredox catalysts are presented and discussed in a systematic manner. The strategies that have been implemented to extend the lifetimes and to control the photo/electrochemical properties of Cu(I) complexes are detailed with particular emphasis on the role and potential of using dative carbon contact atoms in different ligand scaffolds. Especially, the incorporation of carbon-donor ligands such as isonitriles and carbenes has been proven to be a promising approach for controlling the photoelectrochemical properties and stability of photoredox catalysts.

In this contribution we present reactivity studies of a rare example of a titanium salt, in the form of [μ 2-K(OEt 2)] 2[(PN) 2Ti≡N] 2 (1) (PN - = N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-trimethylanilide) to produce a series of imide moieties including rare examples such as methylimido, borylimido, phosphonylimido, and a parent imido. For the latter, using various weak acids allowed us to narrow the pK a range of the NH group in (PN) 2Ti≡NH to be between 26-36. Complex 1 could be produced by a reductively promoted elimination of N 2 from the azide precursor (PN) 2TiN 3, whereas reductive splitting of N 2 could not be achieved using the complex (PN) 2Ti=N=N=Ti(PN) 2 (2) and a strong reductant. Complete N-atom transfer reactions could also be observed when 1 was treated with ClC(O) tBu and OCCPh2 to form NC tBu and KNCCPh 2, respectively, along with the terminal oxo complex (PN) 2Ti≡O, which was also characterized. A combination of solid state 15N NMR (MAS) and theoretical studies allowed us to understand the shielding effect of the counter cation in dimer 1, the monomer [K(18-crown-6)][(PN) 2Ti≡N], and the discrete salt [K(2,2,2-Kryptofix)][(PN) 2Ti≡N] as well as the origin of the highly downfield 15N NMR resonance when shifting from dimer to monomer to a terminal nitride (discrete salt). The upfield shift of 15N nitride resonance in the 15N NMR spectrum was found to be linked to the K + induced electronic structural change of the titanium-nitride functionality by using a combination of MO analysis and quantum chemical analysis of the corresponding shielding tensors.

Transmetallation studies with the phosphaethynolate ion, [OCP]-, have largely resulted in coordination according to classical Lewis acid-base theory. That is, for harder early transition metal ions, O-bound coordination has been observed, whereas in the case of softer late transition metal ions, P-bound coordination predominates. Herein, we report the use of a V(iii) complex, namely [(nacnac)VCl(OAr)] (1) (nacnac- = [ArNC(CH3)]2CH; Ar = 2,6-iPr2C6H3), to transmetallate [OCP]- and bind via the P-atom as [(nacnac)V(OAr)(PCO)] (2), the first example of a 3d early transition metal that binds [OCP]-via the P-atom. Full characterization studies of this molecule including HFEPR spectroscopy, SQuID measurements, and theoretical studies are presented.

In this work, we prepared, isolated, and structurally characterized a zirconium complex having a terminally bound imide motif, (PN)2Zr≡NH (PN– = (N-(2-iPr2P-4-methylphenyl)-2,4,6-trimethylanilide)), along with the zirconium nitride complex {(PN)2Zr≡N[μ2-Li(THF)]}2. (PN)2Zr≡NH was prepared by reduction of trans-(PN)2Zr(N3)2 with KC8. Isotopic labeling and spectroscopic studies were conducted using the respective 15N enriched isotopologues, whereas solid-state structural studies confirmed some of the shortest Zr≡N distances known to date (Zr≡NH, 1.830(3) A; Zr≡N–, 1.822(2) A). It was found that the nitride in {(PN)2Zr≡N[μ2-Li(THF)]}2 is super basic and in the range of −36 to −43 pKb units. Computational studies have been applied to probe the bonding and structure for this new class of zirconium–nitrogen multiple bonds.

The particle-particle random phase approximation (pp-RPA) has been deployed to study the spin-state energetics of transition metal (TM) complexes for the first time in this work. Namely, we designed and implemented a non-canonical reference pp-RPA protocol that is capable of capturing the singlet low-spin (LS) – triplet intermediate-spin (IS) excitation process of iron(II) complexes; herein we applied this method to iron-porphyrin related heme derivatives with clearly defined LS and IS electronic states. Coupled to the CAM-B3LYP functional and to Dunning-type basis sets, we utilized both the active-space and Davidson methods to solve the pp-RPA equation effectively to obtain vertical singlet–triplet excitation energies. Correcting these vertical metrics with a structural relaxation factor for each species, we evaluated the relative stability of LS and IS electronic states. Comparison of the pp-RPA results to established ab initio data revealed that pp-RPA describes well excitation energies and related relative spin state stabilities if the transition is based on non-bonding d-orbitals, such as complexes without an axial ligand in the investigated set of molecules. But it notably overestimates the stability of the singlet LS state to the triplet IS state in complexes, where the d-orbitals at which the excitation is centered have bonding or antibonding character.

The most relevant manifestations of ligand noninnocence of quinone and bipyridine derivatives are thoroughly scrutinized and discussed through an extensive and systematic set of octahedral ruthenium complexes, [(en)2RuL]z, in four oxidation states (z = +3, +2, +1, and 0). The characteristic structural deformation of ligands upon coordination/noninnocence is put into context with the underlying electronic structure of the complexes and its change upon reduction. In addition, by means of decomposing the corresponding reductions into electron transfer and structural relaxation subprocesses, the energetic contribution of these structural deformations to the redox energetics is revealed. The change of molecular electron density upon metal- and ligand-centered reductions is also visualized and shown to provide novel insights into the corresponding redox processes. Moreover, the charge distribution of the π-subspace is straightforwardly examined and used as indicator of ligand noninnocence in the distinct oxidation states of the complexes. The aromatization/dearomatization processes of ligand backbones are also monitored using magnetic (NICS) and electronic (PDI) indicators of aromaticity, and the consequences to noninnocent behavior are discussed. Finally, the recently developed effective oxidation state (EOS) analysis is utilized, on the one hand, to test its applicability for complexes containing noninnocent ligands, and, on the other hand, to provide new insights into the magnitude of state mixings in the investigated complexes. The effect of ligand substitution, nature of donor atom, ligand frame modification on these manifestations, and measures is discussed in an intuitive and pedagogical manner M.G. and P.S. are thankful for financial support from the FEDER grant UNGI10-4E-801 and MINECO grant CTQ2014-59212-P/BQU

Two hitherto unknown organometallic compounds with antitumor activity, [Ru(η6-2-(1-propenyl)anisole)(en)(Cl)]PF6 (3) and [Ru(η6-2-(1-propenyl)anisole)(en)(l)]PF6 (4), where en is ethylenediamine, were synthesized and completely characterized using standard techniques (1H and 13C NMR, high-resolution MS and elemental analysis). The lipophilicity and hydrolysis rate kinetics were assessed and compared to the previously reported [Ru(η6-4-(1-propenyl)anisole)(en)(halogen)]PF6 derivatives (4-(1-propenyl)anisole or anethole), where the halogen is Cl (1) or I (2). Based on the obtained rate constants, the coordination of (1-propenyl)anisole to the Ru(en) moiety yielded organometallic compounds that are as active as compounds that bind p-cymene as the arene ligand. Consistent with previously reported kinetic data, our density functional theory-based computational study revealed that an associative interchange mechanism predominates in the hydrolysis of this type of compound, and only small variations (∼1 kcal mol−1) were observed between the stabilities of the transition states corresponding to different derivatives. In vitro analyses of the anti-proliferative activity revealed that compounds 1 to 3 generally exhibit better cytotoxicity and selectivity (tumor versus non tumor cells) toward the gastric tumor cell lines AGS and SNU-1, compared to the parent [Ru(η6-p-cymene)(en)X]PF6 (X: Cl and I) systems. Compound 3 showed similar cytotoxicity to compound 1 toward the AGS cell line, indicating that the change in the substitution pattern of the coordinated arene from 4-(1-propenyl)anisole to 2-(1-propenyl)anisole did not prominently affect the biological behavior. Compound 2 remained the most promising candidate to treat gastric cancer.

Utilizing the bulky guanidinate ligand [LAr*]− (LAr* = (Ar*N)2C(R), Ar* = 2,6-bis(diphenylmethyl)-4-tert-butylphenyl, R = NCtBu2) for kinetic stabilization, the synthesis of a rare terminal Fe(IV) nitride complex is reported. UV irradiation of a pyridine solution of the Fe(II) azide [LAr*]FeN3(py) (3-py) at 0 °C cleanly generates the Fe(IV) nitride [LAr*]FeN(py) (1). The 15N NMR spectrum of the 115N (50% Fe≡15N) isotopomer shows a resonance at 1016 ppm (vs externally referenced CH3NO2 at 380 ppm), comparable to that known for other terminal iron nitrides. Notably, the computed structure of 1 reveals an iron center with distorted tetrahedral geometry, τ4 = 0.72, featuring a short Fe≡N bond (1.52 Å). Inspection of the frontier orbital ordering of 1 shows a relatively small HOMO/LUMO gap with the LUMO comprised by Fe(dxz,yz)N(px,y) π*-orbitals, a splitting that is manifested in the electronic absorption spectrum of 1 (λ = 610 nm, ε = 1375 L·mol–1·cm–1; λ = 613 nm (calcd)). Complex 1 persists in low-temperature solutions of pyridine but becomes unstable at room temperature, gradually converting to the Fe(II) hydrazide product [κ2-(tBu2CN)C(η6-NAr*)(N-NAr*)]Fe (4) upon standing via intramolecular N-atom insertion. This reactivity of the Fe≡N moiety was assessed through molecular orbital analysis, which suggests electrophilic character at the nitride functionality. Accordingly, treatment of 1 with the nucleophiles PMe2Ph and Ar–N≡C (Ar = 2,6-dimethylphenyl) leads to partial N-atom transfer and formation of the Fe(II) addition products [LAr*]Fe(N═PMe2Ph)(py) (5) and [LAr*]Fe(N═C═NAr)(py) (6). Similarly, 1 reacts with PhSiH3 to give [LAr*]Fe[N(H)(SiH2Ph)](py) (7) which Fukui analysis shows to proceed via electrophilic insertion of the nitride into the Si–H bond.

The first example of the OCPPCO ligand, diisophosphaethynolate, is reported via reductive coupling of a Sc-OCP precursor. Upon reduction with KC

In this study, an intuitive concept is derived, which explains the characteristic dependence of spin-state energetics on the exact exchange admixture of DFT functionals in the case of octahedral transition metal complexes. The change in electron density distributions upon varying the admixture, c 3, in the B3LYP functional is analyzed for archetype ionic and covalent systems as well as for the Fe 2+ ion in an ideal octahedral field. An understanding of how the DFT description of the electronic structure of octahedral complexes changes as a function of c 3 is sought. A systematic spin-state energy analysis of 50 octahedral complexes of various metals and ligands with consistent experimental data is presented, allowing the derivation, in theory, of an optimal c 3 value for each system. The notion that the admixture dependence of DFT spin-state energetics stems from the treatment of nondynamic electrons arising from the mixing of (M–L z2) 0(d z2) 2 and (M–L x2-y2 0(d( x2-y2)) 2 configurations into the dominant (M–L x2-y2) 2(d x2-y2) 0 and (M–L x2-y2) 2(d x2-y2) 0 ones in the low(er) spin states is put forward. That is, in the effort to mimic such electron–electron interactions, E x LDA overestimates, whereas exact exchange downplays the contribution of this type of electron correlation to the stability of low(er) spin states, leading to the widespread practical observation that the higher the exact exchange admixture, the more stable the high-spin-state configuration.

The ethylene complex (PNP)Ti(η(2)-H2C═CH2)(CH2(t)Bu) or (PNP)Ti═CH(t)Bu(CH2(t)Bu) (PNP(-) = N[2-P(CHMe2)2-4-methylphenyl]2) reacts with H2CPPh3 to form the κ(2)-phosphinoalkylidene (PNP)Ti═CHPPh2(Ph) (1). Compound 1 activates benzene via the transient intermediate [(PNP)Ti≡CPPh2] (C). By treatment of (PNP)Ti═CH(t)Bu(OTf) with LiCH2PPh2, 1 or its isotopologue (PNP)Ti═CDPPh2(C6D5) (1-d6) can be produced by an independent route involving intermediate C, which activates benzene or benzene-d6 and dehydrogenates cyclohexane-d12. Addition of MeOTf to 1 results in elimination of benzene concomitant with the formation of the phosphonioalkylidyne complex, [(PNP)Ti≡CPPh2Me(OTf) (2). Theoretical studies of 2 suggest a resonance structure having dominant Ti-C triple-bond character with some contribution also from a C-P multiple bond.

The nature and origin of ion-π and ion-σ interactions has been systematically investigated using dispersion-corrected density functional theory and the recently developed noncovalent interaction (NCI) method. A detailed analysis of these interactions is performed with the aim to identify the requirements that have to be fulfilled by the molecular system for strong ion-ligand interactions. Interestingly, our results indicate that aliphatic systems, such as cyclohexane, can interact as strong as aromatic ones with both cations and anions, despite of having a negligible quadrupole moment. In fact, cyclohexane binds anions stronger than benzene itself but slightly weaker that hexafluorobenzene. The NCI method reveals that the interaction between the ions and three C–H bonds of the saturated fragment are responsible for the surprisingly strong ion-σ interaction. A weakening of the ion-σ interactions is observed in the order: Li + > F − > Na + > Cl − > Br − ≈ K + . In addition, a complete Ziegler–Rauk type energy decomposition analysis has been carried out in order to reveal the origins of the thermodynamic driving force for complex formations. The electron density deformation upon complex formation has been scrutinized with a complementary NOCV analysis allowing the identification of molecular orbital interaction contributions to the stabilization. Based on these analysis, it is shown that the formally anion-π interaction is rather an anion-σ ∗ interaction.

Metal and ligand-based reductions have been modeled in octahedral ruthenium complexes revealing metal–ligand interactions as the profound driving force for the redox-active behaviour of orthoquinoid-type ligands. Through an extensive investigation of redox-active ligands we revealed the most critical factors that facilitate or suppress redox-activity of ligands in metal complexes, from which basic rules for designing non-innocent/redox-active ligands can be put forward. These rules also allow rational redox-leveling, i.e. the moderation of redox potentials of ligand-centred electron transfer processes, potentially leading to catalysts with low overpotential in multielectron activation processes.

Inspection of the electrostatic potential of C2F3X (X = F, Cl, Br, and I) revealed a second electropositive region in the immediate vicinity of the C═C double bond apart from the σ hole of chlorine, bromine, and iodine, leading to C(sp(2))-X···Y halogen bonding, through which complexes stabilized by so-called lone pair···π interactions can be formed. Consequently, the experimental studies for the complexes of dimethyl ether with C2F3X (X = F, Cl, Br, and I) not only allowed one to experimentally characterize and rationalize the effects of hybridization on halogen bonding but, for the first time, also allowed the competition of C-X···Y halogen bonding and lone pair···π interactions to be studied at thermodynamic equilibrium. Analysis of the infrared and Raman spectra reveals that in the cryosolutions of dimethyl ether and C2F3I, solely the halogen-bonded complex is present, whereas C2F3Br and C2F3Cl give rise to a lone pair···π bonded complex as well as a halogen-bonded complex. Mixtures of dimethyl ether with C2F4 solely yield a lone pair···π bonded complex. The experimentally derived complexation enthalpies for the halogen bonded complexes are found to be -14.2(5) kJ mol(-1) for C2F3I·DME and -9.3(5) kJ mol(-1) for C2F3Br·DME. For the complexes of C2F3Cl with dimethyl ether, no experimental complexation enthalpy could be obtained, whereas the C2F4·DME complex has a complexation enthalpy of -5.5(3) kJ mol(-1). The observed trends have been rationalized with the aid of an interaction energy decomposition analysis (EDA) coupled to a Natural Orbital for Chemical Valence (NOCV) analysis and also using the noncovalent interaction index method.

The chalcogen bond has been acknowledged as an influential noncovalent interaction (NCI) between an electron-deficient chalcogen (donor) and a Lewis base (acceptor). This work explores the main features of chalcogen bonding through a large-scale computational study on a series of donors and acceptors spanning a wide range in strength and character of this type of bond: (benzo)chalcogenadiazoles (with Ch = Te/Se/S) versus halides and neutral Lewis bases with O, N, and C as donor atoms. We start from Pearson's hard and soft acids and bases (HSAB) principle, where the hard nature of the chalcogen bond is quantified through the molecular electrostatic potential and the soft nature through the Fukui function. The σ-holes are more pronounced when going down in the periodic table and their directionality matches the structural orientation of donors and acceptors in the complexes. The Fukui functions point toward an n→σ*-type interaction. The initial conjectures are further scrutinized using quantum mechanical methods, mostly relating to the systems' electron density. A Ziegler–Rauk energy decomposition analysis shows that electrostatics plays a distinctly larger role for the soft halides than for the hard, uncharged acceptors, associated with the softness matching within the HSAB principle. The natural orbital for chemical valence analysis confirms the n→σ* electron donation mechanism. Finally, the electron density and local density energy at the bond critical point in the quantum theory of atoms in molecules study and the position of the spikes in the reduced density gradient versus density plot in the NCI theory situate the chalcogen bond in the same range as strong hydrogen bonds. © 2017 Wiley Periodicals, Inc.

The complex [(nacnac)Ti(OAr)]

The first structural elucidation of the first group 4 transition metal P2 complex, namely [(nacnac)Ti(OAr)]2(μ2:η2,η2-P2) (1), is reported. Complex 1 is formed via reductive decarbonylation of the phosphaethynolate ion -[OCP], which serves as a P atom source. The rhombic Ti2P2 core is essentially planar with short bond lengths suggesting some degree of multiple bonding character between the Ti-P and P-P sites. Also observed in 1 was the rather downfield shifted 31P NMR spectroscopic signal, appearing as a broadened resonance (Δν1/2 ~ 4034 Hz) at 907 ppm. Computational studies of 1 provide an understanding of the Ti2P2 core as well as the origin of the highly downfield 31P NMR spectroscopic signal.

Treatment of [TaCl2(CH3)3] with 2 equiv of NaOAr′ (OAr′ = 2,6-bis(diphenylmethyl)-4-tert-butylphenoxide) yields cleanly the bis-aryloxide trimethyl complex [(Ar′O)2Ta(CH3)3] (1), which is isolated ...

The alkyne complexes [(PNP)Ti(η2-HC≡CH)(CH2tBu)] (2) and [(PNP)Ti(η2-HC≡CMe)(CH2tBu)] (3) have been prepared by treatment of [(PNP)Ti═CHtBu(OTf)] (1) with the Grignard reagents H2C═CHMgCl and MeHC═CHMgBr, respectively. Complex 3 can be also prepared using the Grignard H2C═C(Me)MgBr and 1. The 2-butyne complex [(PNP)Ti(η2-MeC≡CMe)(CH2tBu)] (4) can be similarly prepared from 1 and MeHC═C(Me)MgBr. Complexes 2 and 3 have been characterized with a battery of multidimensional and multinuclear (1H, 13C, and 31P) NMR spectroscopic experiments, including selectively 31P decoupled 1H{31P}, 1H–31P HMBC, 1H–31P HOESY, and 31P EXSY. Variable-temperature 1H and 31P{1H} NMR spectroscopy reveals that the acetylene ligand in 2 exhibits a rotational barrier of 11 kcal mol–1, and such a process has been corroborated by theoretical studies. Formation of the titanium alkyne ligand in complexes 2 and 3 proceeds via the vinyl intermediate [(PNP)Ti═CHtBu(CH═CHR)] followed by a concerted, metal-mediated β-hydrogen abstraction ste...

The intrinsic electrophilic reactivity of linear acenes up to octacene was investigated by using interaction energy potentials and their components, namely Pauli repulsion, orbital interaction, and electrostatic interaction potentials using the Ziegler–Rauk decomposition scheme. We found that the shared regions above C1 and C2 of the outer ring are slightly more attractive towards an incoming electrophile than the inner ring secondary carbon atoms, mostly due to a more advantageous electrostatic interaction. This result agrees with the stability order of prereactive π complexes formed between anthracene and the electrophilic HCl. Decomposition potentials determined in the bond formation regime indicate that longer acenes become more reactive because of the increasing orbital interaction. In addition, the orbital interaction potentials, which represent the overall effect of the high lying reactive orbital, exhibit straightforward patterns, predicting strongly preferred inner ring secondary carbon atoms over outer ring carbon atoms. Such an orbital-interaction controlled regioselectivity shows up in the total interaction energy potentials when the electrostatic interaction is scaled down, predicting the functionalization of acenes with electrophiles on the inner secondary carbon atoms, in agreement with the experimental regioselectivity. Upon analyzing the stability of transition states for the addition of HCl to anthracene, the Pauli repulsion shows up to be an important factor in controlling the stability of the transition states and favoring functionalization along the central C9 and C10 positions to outer ring carbon atoms. The resonance stabilization concept, which is generally accepted to account for the regioselectivity of higher acenes, could not be evidenced by our analysis.

The metal-free reaction of terminal arylacetylenes with α,α-dichloroaldimines in 1,1,1,3,3,3-hexafluoro-2-propanol as the sole solvent results in the rapid and selective formation of γ,γ-dichloro-β-amino ketones. In this solvent the expected dichlorinated propargylamines and/or allylic amines are not formed. The dichloromethylene moiety of the aldimine acts as an activating group and is essential to accomplish this transformation. Electron-rich acetylenes lead to the best results and work well with all imines (with or without α′-H at the nitrogen substituent), while electron-deficient acetylenes only reacted with N-tert-butylaldimines (no α′-H). The mechanistic pathway showed 1,1,1,3,3,3-hexafluoro-2-propanol to protonate the aldimine, which in the rate-determining step will react with the arylacetylene to form a resonance-stabilized allene cation, which is trapped by a HFIP molecule giving rise to an enol ether, which promptly hydrolyzes to furnish exclusively the β-amino ketones. Using DFT techniques we found that the first CC bond forming step is the rate-determining step and is associated with a barrier of about 21 kcal mol−1.

The role of the solvent and the influence of dynamics on the kinetics and mechanism of the SNAr reaction of several halonitrobenzenes in liquid ammonia, using both static calculations and dynamic ab initio molecular dynamics simulations, are investigated. A combination of metadynamics and committor analysis methods reveals how this reaction can change from a concerted, one-step mechanism in gas phase to a stepwise pathway, involving a metastable Meisenheimer complex, in liquid ammonia. This clearly establishes, among others, the important role of the solvent and highlights the fact that accurately treating solvation is of crucial importance to correctly unravel the reaction mechanism. It is indeed shown that H-bond formation of the reacting NH3 with the solvent drastically reduces the barrier of NH3 addition. The halide elimination step, however, is greatly facilitated by proton transfer from the reacting NH3 to the solvent. Furthermore, the free energy surface strongly depends on the halide substituent and the number of electron-withdrawing nitro substituents.

The metal-free reaction of terminal arylacetylenes with α,α-dichloroaldimines in 1,1,1,3,3,3-hexafluoro-2-propanol as the sole solvent results in the rapid and selective formation of γ,γ-dichloro-β-amino ketones. In this solvent the expected dichlorinated propargylamines and/or allylic amines are not formed. The dichloromethylene moiety of the aldimine acts as an activating group and is essential to accomplish this transformation. Electron-rich acetylenes lead to the best results and work well with all imines (with or without α′-H at the nitrogen substituent), while electron-deficient acetylenes only reacted with N-tert-butylaldimines (no α′-H). The mechanistic pathway showed 1,1,1,3,3,3-hexafluoro-2-propanol to protonate the aldimine, which in the rate-determining step will react with the arylacetylene to form a resonance-stabilized allene cation, which is trapped by a HFIP molecule giving rise to an enol ether, which promptly hydrolyzes to furnish exclusively the β-amino ketones. Using DFT techniques we found that the first C[BOND]C bond forming step is the rate-determining step and is associated with a barrier of about 21 kcal mol−1.

The argentate trinuclear cluster Ag3(μ2-3,5-(CF3)2PyrPy)3 (3,5-(CF3)2PyrPy = 2,2′-pyridylpyrrolide– ligand) catalytically promotes the insertion of the carbene of ethyl diazoacetate at room temperature into the C–H bond of a series of alkanes ranging from ethane to hexane, as well as branched and cyclic hydrocarbons. In addition to experimental studies, we also present theoretical studies elucidating the mechanism to C–H activation and functionalization by the transient silver carbene monomer (3,5-(CF3)2PyrPy)Ag(CHCO2Et). On the basis of DFT studies, formation of the silver carbene complex was found to be rate-determining for alkane substrates such as ethane and propane. On the other hand, DFT studies on methane, a substrate that we failed to activate, revealed that carbene insertion into the C–H bond was overall rate-determining. Theoretical analysis of charge flow also shows that the change from separated reagents to the TS involves charge flow from alkane to the silver carbene carbon with the bridging ...

We report the first mononuclear three-coordinate vanadium(II) complex [(nacnac)V(ODiiP)] and its activation of N2 to form an end-on bridging dinitrogen complex with a topologically linear V(III)N2V(III) core and where each vanadium center antiferromagnetically couples to give a ground state singlet with an accessible triplet state as inferred by HFEPR spectroscopy. In addition to investigating the conversion of N2 to the terminal nitride (as well as the microscopic reverse process), we discuss its similarities and contrasts to the isovalent d(3) system, [Mo(N[(t)Bu]Ar)3], and the S = 1 system [(Ar[(t)Bu]N)3Mo]2(μ2-η(1):η(1)-N2).

The synthesis and characterization of two high-valent vanadium-cyclo-P3 complexes, (nacnac)V(cyclo-P3)(Ntolyl2) (1) and (nacnac)V(cyclo-P3)(OAr) (2), and an inverted sandwich derivative, [(nacnac)V(Ntolyl2)]2(μ2-η(3):η(2)-cyclo-P3) (3), are presented. These novel complexes are prepared by activating white phosphorus (P4) with three-coordinate vanadium(II) precursors. Structural metrics, redox behavior, and DFT electronic structure analysis indicate that a [cyclo-P3](3-) ligand is bound to a V(V) center in monomeric species 1 and 2. A salient feature of these new cyclo-P3 complexes is their significantly downfield shifted (by ∼300 ppm) (31)P NMR resonances, which is highly unusual compared to related complexes such as (Ar[(i)Pr]N)3Mo(cyclo-P3) (4) and other cyclo-P3 complexes that display significantly upfield shifted resonances. This NMR spectroscopic signature was thus far thought to be a diagnostic property for the cyclo-P3 ligand related to its acute endocyclic angle. Using DFT calculations, we scrutinized and conceptualized the origin of the unusual chemical shifts seen in this new class of complexes. Our analysis provides an intuitive rational paradigm for understanding the experimental (31)P NMR spectroscopic signature by relating the nuclear magnetic shielding with the electronic structure of the molecule, especially with the characteristics of metal-cyclo-P3 bonding.

The transient titanium alkylidyne, (PNP)Ti≡C(t)Bu (PNP = N[2-P(i)Pr(2)-4-methylphenyl](2)(-)), activates a C-H bond of ethane at room temperature, and a β-hydrogen of the resulting ethyl ligand is subsequently transferred to the adjacent alkylidene ligand to form an ethylene adduct of titanium. Treatment of the ethylene complex with two-electron oxidants such as organic azides results in extrusion of ethene concomitant with formation of a mononuclear titanium imido complex.