Sterically Stabilized End-On Superoxocopper(II) Complexes and Mechanistic Insights into Their Reactivity with O-H, N-H, and C-H Substrates.

Instability of end-on superoxocopper(II) complexes, with respect to conversion to peroxo-bridged dicopper(II) complexes, has largely constrained their study to very low temperatures. This limits their kinetic capacity to oxidize substrates. In response, we have developed a series of bulky ligands, Ar3-TMPA (Ar = tpb, dpb, dtbpb), and used them to support copper(I) complexes that react with O2 to yield [CuII(η1-O2•-)(Ar3-TMPA)]+ species, which are stable against dimerization at all temperatures. Binding of O2 saturates at subambient temperatures and can be reversed by warming. The onset of oxygenation for the Ar = tpb and dpb systems is observed at 25 °C, and all three [CuII(η1-O2•-)(Ar3-TMPA)]+ complexes are stable against self-decay at temperatures of ≤-20 °C. This provides a wide temperature window for study of these complexes, which was exploited by performing extensive reaction kinetics measurements for [CuII(η1-O2•-)(tpb3-TMPA)]+ using a broad range of O-H, N-H, and C-H bond substrates. This includes correlation of second order rate constants (k2) versus oxidation potentials (Eox) for a range of phenols, construction of Eyring plots, and temperature-dependent kinetic isotope effect (KIE) measurements. The data obtained indicate that reaction with all substrates proceeds via H atom transfer (HAT), reaction with the phenols proceeds with significant charge transfer, and full tunneling of both H and D atoms occurs in the case of 1,2-diphenylhydrazine and 4-methoxy-2,6-di-tert-butylphenol. Oxidation of C-H bonds proved to be kinetically challenging, and whereas [CuII(η1-O2•-)(tpb3-TMPA)]+ can oxidize moderately strong O-H and N-H bonds, it is only able to oxidize very weak C-H bonds.

Formation of Corrugated n = 1 2D Tin Iodide Perovskites and Their Use as Lead-Free Solar Absorbers.

Major strides have been made in the development of materials and devices based around low-dimensional hybrid group 14 metal halide perovskites. Thus far, this work has mostly focused on compounds containing highly toxic Pb, with the analogous less toxic Sn materials being comparatively poorly evolved. In response, the study herein aims to (i) provide insight into the impact of templating cations upon the structure of n = 1 2D tin iodide perovskites (where n refers to the number of contiguous two-dimensional (2D) inorganic layers, i.e., not separated by organic cations) and (ii) examine their potential as light absorbers for photovoltaic (PV) cells. It was discovered through systematic tuning of organic dications that imidazolium rings are able to induce the formation of (110)-oriented materials, including examples of "3 × 3" corrugated Sn-I perovskites. This structural outcome is a consequence of a combination of supramolecular interactions of the two endocyclic N atoms of the imidazolium rings with the Sn-I framework, and the comparatively high tendency of Sn2+ ions to stereochemically express their 5s2 lone pairs . More importantly, the resulting materials feature very short separations between their 2D inorganic layers with iodide-iodide (I···I) contacts as small as 4.174 Å, which is among the shortest ever recorded for 2D tin iodide perovskites. These proximate inorganic distances, combined with the polarizable nature of the imidazolium moiety, eases the separation of photogenerated charge within the materials. This is evident from the measurement of excitonic activation energies as low as 83(10) meV for ImEA[SnI4]. When combined with superior light absorption capabilities relative to their lead congeners, this allowed the fabrication of lead-free solar cells with incident photon-to-current and power conversion efficiencies of up to 70% and 2.26%, respectively, which are among the highest values reported for pure n = 1 2D group 14 metal halide perovskites. In fact, these values are superior to the corresponding lead iodide material, which demonstrates that 2D Sn-based materials have significant potential as less toxic alternatives to their Pb counterparts.

Mechanochemical Synthesis of Tripodal Tris[4-(1,2,3-triazol-5-ylidene)methyl]amine Mesoionic Carbene Ligands and Their Complexation with Silver(I).

The conjugate acids of 1,2,3-triazolylidene mesoionic carbenes can be prepared in a straightforward fashion by alkylation of 1-substituted 1,2,3-triazoles. However, this becomes a much more challenging proposition when other nucleophilic centers are present, which has curtailed the development of ligands containing multiple 1,2,3-triazolylidene donors. Herein, methylation of a series of tris[(1-aryl-1,2,3-triazol-4-yl)methyl]amines possessing both electron-rich and electron-deficient aromatic substituents, using Me3OBF4, is shown to proceed with much higher chemoselectivity under mechanochemical conditions than when conducted in solution. This provides a means to reliably access a series of tricationic tris[4-(1,2,3-triazolium)methyl]amines in good yields. DFT calculations suggest that a potential reason for this change in regioselectivity is the difference between the background dielectric of the DCM solution versus the solid state, which is predicted to have a large effect on the relative thermodynamic driving force for alkylation of the tertiary amine center versus the triazole rings. Homoleptic silver complexes of the triazolylidene ligands derived therefrom, of formulas [Ag3(1a-d)2](X)3 (X- = BF4-, TfO-), have been isolated and fully characterized. In the case of the ligand bearing the smallest aryl substituents, 1b, argentophilic interactions yield a triangular Ag3 core. The [Ag3(1a-d)2](X)3 silver salts are viable agents for transmetalation to other transition metals, demonstrated here for cobalt. In the case of 1a, the complex [CoII(1a)(NCMe)](OTf)2 was obtained. Therein, the bulky mesityl substituents enforce a tetrahedral geometry, in which only the triazolylidene donors of 1a coordinate (i.e., it acts as a tridentate ligand). Transmetalation of the less sterically encumbered ligand 1b yields six-coordinate cobalt(III) complexes, [CoIII(1b)(Cl)(NCMe)](OTf)2 and [CoIII(1b)(NCMe)2](OTf)3, in which the ligand coordinates in a tetradentate fashion. These are the first examples of tris(1,2,3-triazolylidene) ligands containing an additional coordinating heteroatom and, more generally, of tetradentate 1,2,3-triazolylidene ligands. Crucially, we believe that the divergent chemoselectivity under mechanochemical conditions (vs conventional solution-based chemistry) demonstrated herein offers a pathway by which other challenging synthetic targets, including further multidentate carbene ligands, can be prepared in superior yields.

Metal Coordination Sphere Deformation Induced Highly Stokes-Shifted, Ultra Broadband Emission in 2D Hybrid Lead-Bromide Perovskites and Investigation of Its Origin.

Published studies of layered (2D) (100)-oriented hybrid lead-bromide perovskites evidence a correlation between increased inter-octahedral (Pb-Br-Pb) distortions and the appearance of broadband white light emission. However, the impact of distortions within their constituent [PbBr6 ]4- octahedra has yet to be assessed. Herein, we report two new (100)-oriented 2D Pb-Br perovskites, whose structures display unusually high intra-octahedral distortions, whilst retaining minimal inter-octahedral distortions. Using a combination of temperature-dependent, power-dependent and time-resolved photoluminescence spectroscopic measurements, we show that increased intra-octahedral distortion induces exciton localization processes and leads to formation of multiple photoinduced emissive colour centres. Ultimately, this leads to highly Stokes-shifted, ultrabroad white light emission at room temperature.

Carbodicarbene Ligand Redox Noninnocence in Highly Oxidized Chromium and Cobalt Complexes.

Carbodicarbenes (CDCs) possess two lone pairs of electrons on their central carbone C atom (Ccarbone). Coordination to a transition metal via a σ bond leaves one pair of electrons with appropriate symmetry for π donation to the metal. However, the high energy of the latter also renders the CDC ligand potentially redox-active. Herein, we explore these alternatives in the redox series [Cr(L)2]n+ and [Co(L)2]n+ (n = 2-5), where L is a tridentate ligand comprised of a central CDC and two flanking pyridine donors. To this end, all members of both redox series were synthesized and their electronic structures were investigated by using a combination of 1H NMR, Evans' NMR, IR, UV-vis, and EPR spectroscopies, SQUID magnetometry, X-ray crystallography, and density functional theory studies. Whereas [CoII(L)2]2+ is a straightforward low-spin (S = 1/2) cobalt(II) complex, the corresponding chromium complex was found to feature an electronic structure that is intermediate between the two limiting resonance forms [CrIII(L•-)(L)]2+ and [CrII(L)2]2+. In the case of the tri-, tetra-, and pentacationic complexes, the qualitatively identical electronic structures [MIII(L)2]3+, [MIII(L•+)(L)]4+, and [MIII(L•+)2]5+ were observed for both metals. Thus, the metal ions retain a 3+ oxidation state throughout, and the higher redox states contain oxidized ligands. The majority of the unpaired spin on the cation radical ligands was calculated to be localized in π-symmetry orbitals on the coordinated Ccarbone atoms. Analogous behavior was previously reported for the corresponding iron redox series and, as such, redox noninnocence in oxidized CDC and, more broadly, carbone complexes is likely widely accessible.

Molecular Engineering of Pure 2D Lead-Iodide Perovskite Solar Absorbers Displaying Reduced Band Gaps and Dielectric Confinement.

Pure 2D lead-iodide perovskites typically demonstrate poor charge transport and compromised visible light absorption, relative to their 3D congeners. This hinders their potential use as solar absorbers. Herein, the systematic tuning of pyridinium-based templating cations is reported to introduce intermolecular interactions that provide access to a series of new 2D lead-iodide perovskites with reduced inter-octahedral distortions (largest Pb-(µ-I)-Pb bond angles of 170-179°) and very short inorganic interlayer separations (shortest I⋅⋅⋅I contacts ≤4.278-4.447 Å). These features manifest in reduced band gaps (2.35-2.46 eV) and relaxed dielectric confinement (excitonic binding energies of 130-200 meV). As a consequence, they demonstrate (more than ten-fold) improved photo- and electrical conductivities relative to conventional 2D lead-iodide perovskites, such as that templated by 2-(1-naphthyl)ethylammonium. Through computational studies, the origin of this behavior was shown to derive from a combination of short iodoplumbate layer separations and the aromaticity of the organic dications.

Observation of Carbodicarbene Ligand Redox Noninnocence in Highly Oxidized Iron Complexes.

To probe the possibility that carbodicarbenes (CDCs) are redox active ligands, all four members of the redox series [Fe(1)2 ]n+ (n=2-5) were synthesized, where 1 is a neutral tridentate CDC. Through a combination of spectroscopy and DFT calculations, the electronic structure of the pentacation is shown to be [FeIII (1.+ )2 ]5+ (S= 1 / 2 ). That of [Fe(1)2 ]4+ is more ambiguous, but it has significant contributions from the open-shell singlet [FeIII (1)(1.+ )]4+ (S=0). The observed spin states derive from antiferromagnetic coupling of their constituent low-spin iron(III) centres and cation radical ligands. This marks the first time redox activity has been observed for carbones and expands the diverse chemical behaviour known for these ligands.

A Highly Reactive Oxoiron(IV) Complex Supported by a Bioinspired N3 O Macrocyclic Ligand.

The sluggish oxidants [FeIV (O)(TMC)(CH3 CN)]2+ (TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and [FeIV (O)(TMCN-d12 )(OTf)]+ (TMCN-d12 =1,4,7,11-tetra(methyl-d3 )-1,4,7,11-tetraazacyclotetradecane) are transformed into the highly reactive oxidant [FeIV (O)(TMCO)(OTf)]+ (1; TMCO=4,8,12-trimethyl-1-oxa-4,8,12-triazacyclotetradecane) upon replacement of an NMe donor in the TMC and TMCN ligands by an O atom. A rate enhancement of five to six orders of magnitude in both H atom and O atom transfer reactions was observed upon oxygen incorporation into the macrocyclic ligand. This finding was explained in terms of the higher electrophilicity of the iron center and the higher availability of the more reactive S=2 state in 1. This rationalizes nature's preference for using O-rich ligand environments for the hydroxylation of strong C-H bonds in enzymatic reactions.

Oxoiron(IV) Tetramethylcyclam Complexes with Axial Carboxylate Ligands: Effect of Tethering the Carboxylate on Reactivity.

Oxoiron(IV) species are implicated as reactive intermediates in nonheme monoiron oxygenases, often acting as the agent for hydrogen-atom transfer from substrate. A histidine is the most likely ligand trans to the oxo unit in most enzymes characterized thus far but is replaced by a carboxylate in the case of isopenicillin N synthase. As the effect of a trans carboxylate ligand on the properties of the oxoiron(IV) unit has not been systematically studied, we have synthesized and characterized four oxoiron(IV) complexes supported by the tetramethylcyclam (TMC) macrocycle and having a carboxylate ligand trans to the oxo unit. Two complexes have acetate or propionate axial ligands, while the other two have the carboxylate functionality tethered to the macrocyclic ligand framework by one or two methylene units. Interestingly, these four complexes exhibit substrate oxidation rates that differ by more than 100-fold, despite having Ep,c values for the reduction of the Fe═O unit that span a range of only 130 mV. Eyring parameters for 1,4-cyclohexadiene oxidation show that reactivity differences originate from differences in activation enthalpy between complexes with tethered carboxylates and those with untethered carboxylates, in agreement with computational results. As noted previously for the initial subset of four complexes, the logarithms of the oxygen atom transfer rates of 11 complexes of the FeIV(O)TMC(X) series increase linearly with the observed Ep,c values, reflecting the electrophilicity of the Fe═O unit. In contrast, no correlation with Ep,c values is observed for the corresponding hydrogen atom transfer (HAT) reaction rates; instead, the HAT rates increase as the computed triplet-quintet spin state gap narrows, consistent with Shaik's two-state-reactivity model. In fact, the two complexes with untethered carboxylates are among the most reactive HAT agents in this series, demonstrating that the axial ligand can play a key role in tuning the HAT reactivity in a nonheme iron enzyme active site.

A cobalt(ii) iminoiodane complex and its scandium adduct: mechanistic promiscuity in hydrogen atom abstraction reactions.

In addition to oxometal [M(n+)[double bond, length as m-dash]O] and imidometal [M(n+)[double bond, length as m-dash]NR] units, transient metal-iodosylarene [M((n-2)+)-O[double bond, length as m-dash]IPh] and metal-iminoiodane [M((n-2)+)-N(R)[double bond, length as m-dash]IPh] adducts are often invoked as a possible "second oxidant" responsible for the oxo and imido group transfer reactivity. Although a few metal-iodosylarene adducts have been recently isolated and/or spectroscopically characterized, metal-iminoiodane adducts have remained elusive. Herein, we provide UV-Vis, EPR, NMR, XAS and DFT evidence supporting the formation of a metal-iminoiodane complex 2 and its scandium adduct 2-Sc. 2 and 2-Sc are reactive toward substrates in the hydrogen-atom and nitrene transfer reactions, which confirm their potential as active oxidants in metal-catalyzed oxidative transformations. Oxidation of para-substituted 2,6-di-tert-butylphenols by 2 and 2-Sc can occur by both coupled and uncoupled proton and electron transfer mechanisms; the exact mechanism depends on the nature of the para substituent.

Molecular and Electronic Structures of Homoleptic Six-Coordinate Cobalt(I) Complexes of 2,2':6',2″-Terpyridine, 2,2'-Bipyridine, and 1,10-Phenanthroline. An Experimental and Computational Study.

The crystal structures of nine homoleptic, pseudooctahedral cobalt complexes, 1-9, containing either 2,2':6',2″-terpyridine (tpy), 4,4'-di-tert-butyl-2,2'-bipyridine ((t)bpy), or 1,10-phenanthroline (phen) ligands have been determined in three oxidation levels, namely, cobalt(III), cobalt(II), and, for the first time, the corresponding presumed cobalt(I) species. The intraligand bond distances in the complexes [Co(I)(tpy(0))2](+), [Co(I)((t)bpy(0))3](+), and [Co(I)(phen(0))3](+) are identical, within experimental error, not only with those in the corresponding trications and dications but also with the uncoordinated neutral ligands tpy(0), bpy(0), and phen(0). On this basis, a cobalt(I) oxidation state assignment can be inferred for the monocationic complexes. The trications are clearly low-spin Co(III) (S = 0) species, and the dicationic species [Co(II)(tpy(0))2](2+), [Co(II)((t)bpy(0))3](2+), and [Co(II)(phen(0))3](2+) contain high-spin (S = (3)/2) Co(II). Notably, the cobalt(I) complexes do not display any structural indication of significant metal-to-ligand (t2g → π*) π-back-donation effects. Consistent with this proposal, the temperature-dependent molar magnetic susceptibilities of the three cobalt(I) species have been recorded (3-300 K) and a common S = 1 ground state confirmed. In contrast to the corresponding electronic spectra of isoelectronic (and isostructural) [Ni(II)(tpy(0))2](2+), [Ni(II)(bpy(0))3](2+), and [Ni(II)(phen(0))3](2+), which display d → d bands with very small molar extinction coefficients (ε < 60 M(-1) cm(-1)), the spectra of the cobalt(I) species exhibit intense bands (ε > 10(3) M(-1) cm(-1)) in the visible and near-IR regions. Density functional theory (DFT) calculations using the B3LYP functional have validated the experimentally derived electronic structure assignments of the monocations as cobalt(I) complexes with minimal cobalt-to-ligand π-back-bonding. Similar calculations for the six-coordinate neutral complexes [Co(II)(tpy(•))2](0) and [Co(II)(bpy(•))2(bpy(0))](0) point to a common S = (3)/2 ground state, each possessing a central high-spin Co(II) ion and two π-radical anion ligands. In addition, the excited-states and ground state magnetic properties of [Co(I)(tpy(0))2][Co(I-)(CO)4] have been explored by variable-temperature variable-magnetic-field magnetic circular dichroism (MCD) spectroscopy. A series of strong signals associated with the paramagnetic monocation exhibit pronounced C-term behavior indicative of the presence of metal-to-ligand charge-transfer bands [in contrast to d-d transitions of the nickel(II) analogue]. Time-dependent DFT calculations have allowed assignment of these transitions as Co(3d) → π*(tpy) excitations. Metal-to-ligand charge-transfer states intermixing with the Co(d(8)) multiplets explain the remarkably large (and negative) zero-field-splitting parameter D obtained from SQUID and MCD measurements. Ground-state electron- and spin-density distributions of [Co(I)(tpy(0))2](+) have been investigated by multireference electronic structure methods: complete active-space self-consistent field (CASSCF) and N-electron perturbation theory to second order (NEVPT2). Both correlated CASSCF/NEVPT2 and spin-unrestricted B3LYP-based DFT calculations show a significant delocalization of the spin density from the Co(I) dxz,yz orbitals toward the empty π* orbitals located on the two central pyridine fragments in the trans position. This spin density is of an alternating α,ß-spin polarization type (McConnel mechanism I) and is definitely not due to magnetic metal-to-radical coupling. A comparison of these results with those for [Ni(II)(tpy(0))2](2+) (S = 1) is presented.

Oxoiron(IV) Complex of the Ethylene-Bridged Dialkylcyclam Ligand Me2EBC.

We report herein the first example of an oxoiron(IV) complex of an ethylene-bridged dialkylcyclam ligand, [Fe(IV)(O)(Me2EBC)(NCMe)](2+) (2; Me2EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane). Complex 2 has been characterized by UV-vis, (1)H NMR, resonance Raman, Mössbauer, and X-ray absorption spectroscopy as well as electrospray ionization mass spectrometry, and its properties have been compared with those of the closely related [Fe(IV)(O)(TMC)(NCMe)](2+) (3; TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), the intensively studied prototypical oxoiron(IV) complex of the macrocyclic tetramethylcyclam ligand. Me2EBC has an N4 donor set nearly identical with that of TMC but possesses an ethylene bridge in place of the 1- and 8-methyl groups of TMC. As a consequence, Me2EBC is forced to deviate from the trans-I configuration typically found for Fe(IV)(O)(TMC) complexes and instead adopts a folded cis-V stereochemistry that requires the MeCN ligand to coordinate cis to the Fe(IV)═O unit in 2 rather than in the trans arrangement found in 3. However, switching from the trans geometry of 3 to the cis geometry of 2 did not significantly affect their ground-state electronic structures, although a decrease in ν(Fe═O) was observed for 2. Remarkably, despite having comparable Fe(IV/III) reduction potentials, 2 was found to be significantly more reactive than 3 in both oxygen-atom-transfer (OAT) and hydrogen-atom-transfer (HAT) reactions. A careful analysis of density functional theory calculations on the HAT reactivity of 2 and 3 revealed the root cause to be the higher oxyl character of 2, leading to a stronger O---H bond specifically in the quintet transition state.

Solution Dynamics of Redox Noninnocent Nitrosoarene Ligands: Mapping the Electronic Criteria for the Formation of Persistent Metal-Coordinated Nitroxide Radicals.

The redox-noninnocence of metal-coordinated C-organo nitrosoarenes has been established on the basis of solid-state characterization techniques, but the solution-phase properties of this class of metal-coordinated radicals have been relatively underexplored. In this report, the solution-phase properties and dynamics of the bis-nitrosobenzene diradical complex trans-Pd(κ(1)-N-PhNO)2(CNAr(Dipp2))2 are presented. This complex, which is best described as containing singly reduced phenylnitroxide radical ligands, is shown to undergo facile nitrosobenzene dissociation in solution to form the metalloxaziridine Pd(η(2)-N,O-PhNO)(CNAr(Dipp2))2 and thus is not a persistent species in solution. An equilibrium between trans-Pd(κ(1)-N-PhNO)2(CNAr(Dipp2))2, Pd(η(2)-N,O-PhNO)(CNAr(Dipp2))2, and free nitrosobenzene is established in solution, with the metalloxaziridine being predominantly favored. Efforts to perturb this equilibrium by the addition of excess nitrosobenzene reveal that the formation of trans-Pd(κ(1)-N-PhNO)2(CNAr(Dipp2))2 is in competition with insertion-type chemistry of Pd(η(2)-N,O-PhNO)(CNAr(Dipp2))2 and is therefore not a viable strategy for the production of a kinetically persistent bis-nitroxide radical complex. Electronic modification of the nitrosoarene framework was explored as a means to generate a persistent trans-Pd(κ(1)-N-ArNO)2(CNAr(Dipp2))2 complex. While most substitution schemes failed to significantly perturb the kinetic lability of the nitrosoarene ligands in the corresponding trans-Pd(κ(1)-N-ArNO)2(CNAr(Dipp2))2 complexes, utilization of para-formyl or para-cyano nitrosobenzene produced bis-nitroxide diradical complexes that display kinetic persistence in solution. The origin of this persistence is rationalized by the ability of para-formyl- and para-cyano-aryl groups to both attenuate the trans effect of the corresponding nitrosoarene and, more importantly, delocalize spin density away from the aryl-nitroxide NO unit. The results presented here highlight the inherent instability of metal-coordinated nitroxide radicals and suggest a general synthetic strategy for kinetically stabilizing these species in solution.

Molecular and Electronic Structures of Mononuclear and Dinuclear Titanium Complexes Containing π-Radical Anions of 2,2'-Bipyridine and 1,10-Phenanthroline: An Experimental and DFT Computational Study.

Whereas reaction of [(η(5)-Cp*)Ti(IV)Cl3](0) (1) with 2 equiv of neutral 2,2'-bipyridine (bpy) and 1.5 equiv of magnesium in tetrahydrofuran affords the mononuclear complex [(η(5)-Cp*)Ti(III)(bpy(•))2](0) (2), performing the same reaction with only 1 equiv each of magnesium and bpy provides the dinuclear complex [{(η(5)-Cp*)Ti(µ-Cl)(bpy(•))}2](0) (3). Conducting the latter reaction using 1,10-phenanthroline (phen) in place of bpy resulted in formation of dinuclear [{(η(5)-Cp*)Ti(µ-Cl)(phen(•))}2](0) (4). The structures of 2, 3, and 4 have all been determined by high-resolution X-ray crystallography at 153 K; the Cpy-Cpy distances of 1.420(3) and 1.431(4) Å in the N,N'-coordinated bpy ligands of 2 and 3, respectively, are indicative of the presence of (bpy(•))(1-) ligands, rather than neutral (bpy(0)). The electronic spectra (300-1600 nm) of these two complexes are similar in form, and contain intense π → π* transitions associated with the (bpy(•))(1-) radical anion. Temperature dependent magnetic susceptibility measurements (4-300 K) show that mononuclear 2 possesses a temperature independent magnetic moment of 1.73 µB, which is indicative of an S = (1)/2 ground state. Broken symmetry density functional theory (BS-DFT) calculations yield a picture consistent with the experimental findings, in which the central Ti atom possesses a +3 oxidation state and is coordinated by a η(5)-Cp* ligand and two (bpy(•))(1-). Strong intramolecular antiferromagnetic coupling of these three unpaired spins, one each on the Ti(III) center and on the two (bpy(•))(1-) ligands, affords the experimentally observed doublet ground state. The magnetic susceptibility measurements for dinuclear 3 and 4 display weak but significant ferromagnetic coupling, and indicate that these complexes possess S = 1 ground states. The mechanism of the spin coupling phenomenon that yields the observed behavior was analyzed using BS-DFT calculations, and it was discovered that the tight π-stacking of the N,N'-coordinated (bpy(•))(1-)/(phen(•))(1-) ligands in these two complexes results from direct overlap of their SOMOs and formation of a two-electron multicentered bond. This yields a diamagnetic {(bpy)2}(2-)/{(phen)2}(2-) bridging unit whose doubly occupied HOMO is spread equally over both ligands. The two remaining unpaired electrons, one at each Ti(III) center, couple weakly in a ferromagnetic fashion to yield the experimentally observed S = 1 ground states.

An Ultra-Stable Oxoiron(IV) Complex and Its Blue Conjugate Base.

Treatment of [FeII(L)](OTf)2 (4), (where L = 1,4,8-Me3cyclam-11-CH2C(O)NMe2) with iodosylbenzene yielded the corresponding S = 1 oxoiron(IV) complex [FeIV(O(L)](OTf)2 (5) in nearly quantitative yield. The remarkably high stability of 5 (t1/2 ≈ 5 days at 25 °C) facilitated its characterization by X-ray crystallography and a raft of spectroscopic techniques. Treatment of 5 with strong base was found to generate a distinct, significantly less stable S = 1 oxoiron(IV) complex, 6 (t1/2 ~ 1.5 hrs. at 0 °C), which could be converted back to 5 by addition of a strong acid; these observations indicate that 5 and 6 represent a conjugate acid-base pair. That 6 can be formulated as [FeIV(O)(L-H)](OTf) was further supported by ESI mass spectrometry, spectroscopic and electrochemical studies, and DFT calculations. The close structural similarity of 5 and 6 provided a unique opportunity to probe the influence of the donor trans to the FeIV=O unit upon its reactivity in H-atom transfer (HAT) and O-atom transfer (OAT), and 5 was found to display greater reactivity than 6 in both OAT and HAT. While the greater OAT reactivity of 5 is expected on the basis of its higher redox potential, its higher HAT reactivity does not follow the anti-electrophilic trend reported for a series of [FeIV(O)(TMC)(X)] complexes (TMC = tetramethylcyclam) and thus appears to be inconsistent with the Two-State Reactivity rationale that is the prevailing explanation for the relative facility of oxoiron(IV) complexes to undergo HAT.

Molecular and electronic structures of the members of the electron transfer series [Mn(bpy)3]n (n = 2+, 1+, 0, 1-) and [Mn(tpy)2]m (m = 4+, 3+, 2+, 1+, 0). An experimental and density functional theory study.

The members of the electron transfer series [Mn(bpy)3](n) (n = 2+, 1+, 0, 1-) and [Mn(tpy)2](m) (m = 2+, 1+, 0) have been investigated using a combination of magnetochemistry, electrochemistry, and UV-vis-NIR spectroscopy; and X-ray crystal structures of [Mn(II)((Me)bpy(•))2((Me)bpy(0))](0), [Li(THF)4][Mn(II)(bpy(•))3], and [Mn(II)(tpy(•))2](0) have been obtained (bpy = 2,2'-bipyridine; (Me)bpy = 4,4'-dimethyl-2,2'-bipyridine; tpy = 2,2':6,2″-terpyridine; THF = tetrahydrofuran). It is the first time that the latter complex has been isolated and characterized. Through these studies, the electronic structures of each member of both series of complexes have been elucidated, and their molecular and electronic structures further corroborated by broken symmetry (BS) density functional theoretical (DFT) calculations. It is shown that all one-electron reductions that comprise the aforementioned redox series are ligand-based. Hence, all species contain a central high-spin Mn(II) ion (SMn = 5/2). In contrast, the analogous series of Tc(II) and Re(II) complexes possess low-spin electron configurations.

Coordination equilibria between seven- and five-coordinate iron(II) complexes.

Octahedral, tetrahedral, and square planar geometries are the most often encountered coordination geometries for transition metal complexes. In certain cases, coordination equilibria can exist between different geometries, such as between six- and four-coordinate geometries in nickel(II) complexes, which were discovered half a century ago. Here, we present the first examples of a seven-five coordination equilibrium. Extensive spectroscopic studies in solution have provided evidence for a dynamic equilibrium between two iron(II) complexes, one with a seven-coordinate pentagonal bipyramidal geometry and one with a five-coordinate trigonal bipyramidal geometry.

Molecular and electronic structures of six-coordinate "low-valent" [M((Me)bpy)3]0 (M = Ti, V, Cr, Mo) and [M(tpy)2]0 (M = Ti, V, Cr), and seven-coordinate [MoF((Me)bpy)3](PF6) and [MX(tpy)2](PF6) (M = Mo, X = Cl and M = W, X = F).

The electronic structures of a series of so-called "low-valent" transition metal complexes [M((Me)bpy)3](0) and [M(tpy)2](0) ((Me)bpy = 4,4'-dimethyl-2,2'-bipyridine and tpy = 2,2',6',2″-terpyridine) have been determined using a combination of X-ray crystallography, magnetochemistry, and UV-vis-NIR spectroscopy. More specifically, the crystal structures of the long-known complexes [Ti(IV)(tpy(2-))2](0) (S = 0, 6), [V(IV)(tpy(2-))2] (S = 1/2, 7), [Ti(III)((Me)bpy(•))3](0) (S = 0, 1), [V(II)((Me)bpy(•))2((Me)bpy(0))](0) (S = 1/2, 2), and [Mo(III)((Me)bpy(•))3](0) (S = 0, 4) have been determined for the first time. In all cases, the experimental results confirm the electronic structure assignments that we ourselves have recently proposed. Additionally, the six-coordinate complex [Mo(III)(bpy(0))2Cl2]Cl·2.5CH3OH (S = 3/2, 13), and seven-coordinate species [Mo(IV)F((Me)bpy(•))2((Me)bpy(0))](PF6) (S = 0, 5), [Mo(IV)Cl(tpy(•))2](PF6)·CH2Cl2 (S = 0, 11), and [W(V)F(tpy(•))(tpy(2-))](PF6)·CH2Cl2 (S = 0, 12) have been synthesized and, for the first time, crystallographically characterized. Using the resulting data, plus that from previously published high-resolution X-ray structures of analogous compounds, it is shown that there is a linear correlation between the average C(py)-C'(py) bond distances in these complexes and the total charge (n) of the ligands, {(bpy)3}(n) and {(tpy)2}(n). Hence, an assignment of the total charge of coordinated bpy or tpy ligands and, by extension, the oxidation state of the central metal ion can reliably be made on the basis of X-ray crystallography alone. In this study, the oxidation states of the metal ions range from +II to +V and in no case has an oxidation state of zero been validated. It is, therefore, highly misleading to use the term "low-valent" to describe any of the aforementioned neutral complexes.

Ruthenium(II) and osmium(II) complexes bearing bipyridine and the N-heterocyclic carbene-based C

Ruthenium(II) and osmium(II) complexes [M(C^N^C)(N^N)L](n+) (L = Cl(-), n = 1; L = CH3CN, t-BuNC, n = 2) containing a neutral tridentate N-heterocyclic carbene (NHC)-based pincer ligand, either 2,6-bis(1-butylimidazol-2-ylidene)pyridine (C(1)^N^C(1)) or 2,6-bis(3-butylbenzimidazol-2-ylidene)pyridine (C(2)^N^C(2)), and a neutral 2,2'-bipyridine-type aromatic diimine have been prepared. Investigations into the effects of varying M (Ru and Os), C^N^C, N^N, and L on the structural, electrochemical, absorption, and emission characteristics associated with [M(C^N^C)(N^N)L](n+) are presented. Interestingly, spectroscopic findings and time-dependent density functional theory (TD-DFT) calculations in this work support a dπ(Ru(II)/Os(II)) → π*(N^N) metal-to-ligand charge transfer (MLCT) assignment for the lowest-energy transition in [M(C^N^C)(N^N)L](n+) and not a dπ(Ru(II)/Os(II)) → π*(C^N^C) MLCT assignment. This is in stark contrast to [Ru(tpy)(bpy)Cl](+) and [Os(tpy)(bpy)Cl](+) (tpy = 2,2':6',2″-terpyridine, bpy = 2,2'-bipyridine) for which the lowest-energy transitions are assigned as dπ(Ru/Os) → π*(tpy) MLCT transitions. [Ru(II)(C^N^C)(N^N)L](n+) is emissive with emission maxima of around 600-700 nm observed upon photoexcitation of their dπ(Ru(II)) → π*(N^N) MLCT bands. The electronic structures for [Ru(C^N^C)(N^N)Cl](0) have also been probed by spectroelectrochemistry, electron paramagnetic resonance (EPR) spectroscopy, and DFT calculations, which reveal that the lowest unoccupied molecular orbitals (LUMOs) for [Ru(C^N^C)(N^N)Cl](+) are N^N-based.

2,2'-Bipyridine compounds of group 14 elements: a density functional theory study.

The molecular and electronic structures of the 2,2'-bipyridine containing series of group 14 compounds (a) [MF4(bpy)](0); (b) [MCl2(bpy)2](2+/0) (c) [MCl2(bpy)](0); (d) [M(bpy)2](2+/0); (e) [Si(bpy)3](1+,0,1-,2-); and (f) [M(bpy)3](0) (M = C, Si, Ge, Sn, Pb) have been calculated using density functional theory (DFT). Where possible, geometry optimized structures are compared with their experimentally determined structures. In general, good to excellent agreement is observed. It is shown that the three successive one-electron reductions within the experimentally known series [Si(bpy)3](1+,0,1-,2-) are ligand-based and the Si center has a +IV oxidation state throughout. Hence, these species have the electronic structures [Si(IV)(bpy(•))3](+) (S = 1/2), [Si(IV)(bpy(•))2(bpy(2-))](0) (S = 0), [Si(IV)(bpy(•))(bpy(2-))2](-) (S = 1/2), and [Si(IV)(bpy(2-))3](2-) (S = 0). Similarly, it is shown that the crystallographically characterized compound [Si(bpy)2](0) (S = 0) possesses the electronic structure [Si(IV)(bpy(2-))2](0), which contains a tetravalent Si ion and two (bpy(2-))(2-) dianions. It should not be described as [Si(0)(bpy(0))2](0). For the heavier Ge, Sn, and Pb congeners the divalent state, characterized by a stereochemically active electron pair, becomes increasingly significant and dominates in 4-coordinate Sn and Pb species.