Fusion Position-Dependent Aromatic Transitions of Ligand Backbone Rings for Controlling the Redox Energetics of Photoredox Catalysts.

To reveal, quantify, and rationalize the effect of backbone π-extension on ligand redox activity, we studied the ground- and excited-state reduction potentials of eight ruthenium photoredox catalysts with the formula Ru(ppy)2L (L is the redox-active ligand of the bipyridine family) using density functional theory. Our research underlines the profound importance of the fusion position of backbone aromatic C6 rings on the redox activity of ligands in transition metal photoredox catalysts. Namely, certain fusion positions lead to the dearomatization of C6 rings in ligand-centered electron transfer events, resulting in a thermodynamic penalty equivalent to a half-volt negative shift in the reduction potential. Contrarily, the extent of backbone delocalization shows a minimal impact on redox energetics, which can be explained by the charge concentration at the nitrogen contact atoms in ligand-centered reductions. Grounded in Caulton's conceptual framework, we reaffirm the predictive potency of Lewis structures in ligand-centered redox energetics with qualitative and quantitative data. Our hypothesis regarding the effect of backbone ring dearomatization on redox energetics is further corroborated using magnetic and structure-based aromaticity indicators. Highlighting fusion-dependent dearomatization as a determining factor of ligand-centered electron transfer energetics, our findings hold implications for molecular-level design in advanced electroactive materials and catalysts.

Divalent Titanium via Reductive N-C Coupling of a TiIV Nitrido with π-Acids.

The nitrido-ate complex [(PN)2Ti(N){µ2-K(OEt2)}]2 (1) (PN-=(N-(2-PiPr2-4-methylphenyl)-2,4,6-Me3C6H2) reductively couples CO and isocyanides in the presence of DME or cryptand (Kryptofix222), to form rare, five-coordinate TiII complexes having a linear cumulene motif, [K(L)][(PN)2Ti(NCE)] (E=O, L=Kryptofix222, (2); E=NAd, L=3 DME, (3); E=NtBu, L=3 DME, (4); E=NAd, L=Kryptofix222, (5)). Oxidation of 2-5 with [Fc][OTf] afforded an isostructural TiIII center containing a neutral cumulene, [(PN)2Ti(NCE)] (E=O, (6); E=NAd (7), NtBu (8)) and characterization by CW X-band EPR spectroscopy, revealed unpaired electron to be metal centric. Moreover, 1e- reduction of 6 and 7 in the presence of Kryptofix222cleanly reformed corresponding discrete TiII complexes 2 and 5, which were further characterized by solution magnetization measurements and high-frequency and -field EPR (HFEPR) spectroscopy. Furthermore, oxidation of 7 with [Fc*][B(C6F5)4] resulted in a ligand disproportionated TiIV complex having transoid carbodiimides, [(PN)2Ti(NCNAd)2] (9). Comparison of spectroscopic, structural, and computational data for the divalent, trivalent, and tetravalent systems, including their 15N enriched isotopomers demonstrate these cumulenes to decrease in order of backbonding as TiII→TiIII→TiIV and increasing order of π-donation as TiII→TiIII→TiIV, thus displaying more covalency in TiIII species. Lastly, we show a synthetic cycle whereby complex 1 can deliver an N-atom to CO and CNAd.

Mechanistic Insights into the Oxidative and Reductive Quenching Cycles of Transition Metal Photoredox Catalysts through Effective Oxidation State Analysis.

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 (S0, T1, Dox, and Dred) are characterized using the recently developed effective oxidation state (EOS) analysis, allowing the monitoring of metal and ligand oxidation states (OSs) along the catalytic cycles. The formal oxidation state assignments derived from the EOS analysis are in agreement with those commonly assumed for these complexes in both ground and excited states. Rather clean and separate ligand- and metal-centered redox events along the different quenching cycles are observed in most of the studied molecular systems. The reliability index obtained for the OS assignations can be readily interpreted in terms of the ionic/covalent character of metal-ligand interactions and ligand non-innocent character. In addition, EOS analysis reveals the high-degree localization of the ligand-centered redox event to one or two redox-active ligand(s) in heteroleptic complexes. Ligand- and metal-condensed spin populations were also computed and analyzed for all the open-shell species involved in this study, showing promises for rapid oxidation state assignments in certain systems, especially Ru complexes, however, suffering from severe defects in other cases.

Electronic structure analysis of copper photoredox catalysts using the quasi-restricted orbital approach.

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)]+.

Resonance and electrostatics making the difference in boron- and aluminum-halide structures and exchange reactivity.

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.

Terminal and Super-Basic Parent Imides of Hafnium.

A dinuclear hafnium complex containing the parent imido ligand [(PN)(PNC)Hf=NH{µ2 -K}]2 (2) (PN- =(N-(2-Pi Pr2 -4-methylphenyl)-2,4,6-Me3 C6 H2 ; PNC2- =(N-(2-Pi Pr2 -4-methylphenyl)-2,4,6-CH2 Me2 C6 H2 ), was prepared by reduction of the bisazide trans-[(PN)2 Hf(N3 )2 ] (1) with two equiv of KC8 . Encapsulation of K+ in 2 with crown-ether or cryptand affords the first discrete salt [K(encap)][(PN)(PNC)Hf≡NH] (encap=18-crown-6(THF)2 , 3; 2,2,2-Kryptofix, 4), featuring a terminal parent imide and possessing some of the shortest Hf-N bond lengths known to date. DFT calculations revealed formation of 2 to proceed via an extremely basic monomeric nitrido, [(PN)2 Hf≡N]- (A), having a computed pKBH+ of ∼57 followed by heterolytic splitting of an inert 1,2-CH bond of a benzylic methyl group across the Hf≡N triple bond in A. An electronic structure analysis reveals A to possess a covalent Hf≡N triple bond and of super-basic character. We also showcase reactivity of the Hf≡NH bond with various electrophiles.

Pursuit of an Electron Deficient Titanium Nitride.

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.

Phosphorus and Arsenic Atom Transfer to Isocyanides to Form π-Backbonding Cyanophosphide and Cyanoarsenide Titanium Complexes.

Decarbonylation along with E atom transfer from Na(OCE) (E=P, As) to an isocyanide coordinated to the tetrahedral TiII complex [(TptBu,Me )TiCl], yielded the [(TptBu,Me )Ti(η3 -ECNAd)] species (Ad=1-adamantyl, TptBu,Me- =hydrotris(3-tert-butyl-5-methylpyrazol-1-yl)borate). In the case of E=P, the cyanophosphide ligand displays nucleophilic reactivity toward Al(CH3 )3 ; moreover, its bent geometry hints to a reduced Ad-NCP3- resonance contributor. The analogous and rarer mono-substituted cyanoarsenide ligand, Ad-NCAs3- , shows the same unprecedented coordination mode but with shortening of the N=C bond. As opposed to TiII , VII fails to promote P atom transfer to AdNC, yielding instead [(TptBu,Me )V(OCP)(CNAd)]. Theoretical studies revealed the rare ECNAd moieties to be stabilized by π-backbonding interactions with the former TiII ion, and their assembly to most likely involve a concerted E atom transfer between Ti-bound OCE- to AdNC ligands when studying the reaction coordinate for E=P.

Isolation of a Bimetallic Cobalt(III) Nitride and Examination of Its Hydrogen Atom Abstraction Chemistry and Reactivity toward H2.

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)} (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 toward weak C-H donors and even cocrystallizes 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)/Co(III) bridged imido species [(ketguan)Co(py)][(ketguan)Co](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 pathway. The reactivity of our system was further probed by photolyzing benzene/pyridine solutions of 4a under H2 and D2 atmospheres (150 psi), which leads to the exclusive formation of the bis(imido) complexes [(ketguan)Co(µ-NH)]2 (6) and [(ketguan)Co(µ-ND)]2 (6-D), respectively, as a result of dihydrogen activation. These results provide unique insights into the chemistry and electronic structure of late 3d metal nitrides while providing entryway into C-H activation pathways.

Electron Density Difference Analysis on the Oxidative and Reductive Quenching Cycles of Classical Iridium and Ruthenium Photoredox Catalysts.

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.

Molecular Zirconium Nitride Super Base from a Mononuclear Parent Imide.

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) Å; Zr≡N-, 1.822(2) Å). 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 p Kb units. Computational studies have been applied to probe the bonding and structure for this new class of zirconium-nitrogen multiple bonds.

Characterization of chalcogen bonding interactions via an in-depth conceptual quantum chemical analysis.

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.

Conceptual Insights into DFT Spin-State Energetics of Octahedral Transition-Metal Complexes through a Density Difference Analysis.

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, c3 , in the B3LYP functional is analyzed for archetype ionic and covalent systems as well as for the Fe2+ 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 c3 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 c3 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-Lz2 )0 (dz2 )2 and (M-Lx2-y2 )0 (dx2-y2 )2 configurations into the dominant (M-Lx2-y2 )2 (dx2-y2 )0 and (M-Lx2-y2 )2 (dx2-y2 )0 ones in the low(er) spin states is put forward. That is, in the effort to mimic such electron-electron interactions, ExLDA 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.

A Scandium-Stabilized Diisophosphaethynolate Ligand: [OCPPCO]4.

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.

A Terminal Iron(IV) Nitride Supported by a Super Bulky Guanidinate Ligand and Examination of Its Electronic Structure and Reactivity.

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.

A Planar Ti2 P2 Core Assembled by Reductive Decarbonylation of - O-C≡P and P-P Radical Coupling.

The complex [(nacnac)Ti(OAr)]2 (µ2 :η2 ,η2 -P2 ) (1) is formed via reductive decarbonylation of the phosphaethynolate ion - [OCP], which serves as a P atom source. Complex 1 is the first structurally characterized Group 4 transition metal P2 complex and its structure reveals the rhombic Ti2 P2 core is essentially planar with short bond lengths suggesting some degree of multiple bonding character between the Ti-P and P-P sites. Computational studies of 1 provide an understanding of the Ti2 P2 core as well as the origin of the highly downfield 31 P NMR spectroscopic signal.

Influence of Solvation and Dynamics on the Mechanism and Kinetics of Nucleophilic Aromatic Substitution Reactions in Liquid Ammonia.

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.

Scrutinizing the Noninnocence of Quinone Ligands in Ruthenium Complexes: Insights from Structural, Electronic, Energy, and Effective Oxidation State Analyses.

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.

Mononuclear and Terminally Bound Titanium Nitrides.

The Ti(III) azido complex (PN)2Ti(N3) (PN(-) = (N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-trimethylanilide), can be reduced with KC8 to afford the nitride salt [µ2-K(OEt2)]2[(PN)2Ti≡N]2 in excellent yield. While treatment of the dimer with 18-crown-6 yields a mononuclear nitride, complete encapsulation of the alkali metal with cryptand provides the terminally bound nitride as a discrete salt [K(2,2,2-Kryptofix)][(PN)2Ti≡N]. All complexes reported here have been structurally confirmed and also spectroscopically, and the Ti-Nnitride bonding has been probed theoretically via DFT-based methods.

Phosphinoalkylidene and -alkylidyne Complexes of Titanium: Intermolecular C-H Bond Activation and Dehydrogenation Reactions.

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.