Third-row Transition Metal Complexes Research Articles

Exploiting the Marcus inverted region for first-row transition metal-based photoredox catalysis.

Second- and third-row transition metal complexes are widely employed in photocatalysis, whereas earth-abundant first-row transition metals have found only limited use because of the prohibitively fast decay of their excited states. We report an unforeseen reactivity mode for productive photocatalysis that uses cobalt polypyridyl complexes as photocatalysts by exploiting Marcus inverted region behavior that couples increases in excited-state energies with increased excited-state lifetimes. These cobalt (III) complexes can engage in bimolecular reactivity by virtue of their strong redox potentials and sufficiently long excited-state lifetimes, catalyzing oxidative C(sp2)-N coupling of aryl amides with challenging sterically hindered aryl boronic acids. More generally, the results imply that chromophores can be designed to increase excited-state lifetimes while simultaneously increasing excited-state energies, providing a pathway for the use of relatively abundant metals as photoredox catalysts.

Transition-metal phosphors with emission peak maximum on and beyond the visible spectral boundaries

Third-row transition-metal complexes displaying efficient true-blue (∼460–470 nm) and near infrared (∼700–1000 nm) emissions were strategically analyzed.

Insights into trans-Ligand and Spin-Orbit Effects on Electronic Structure and Ligand NMR Shifts in Transition-Metal Complexes.

Surprisingly general effects of trans ligands L on the ligand NMR shifts in third-row transition-metal complexes have been found by quasi-relativistic computations, encompassing 5d10 , 5d8 , and to some extent even 5d6 situations. Closer analysis, with emphasis on 1 H shieldings in a series of linear HAuI Lq complexes, reveals a dominance of spin-orbit (SO) effects, which can change sign from appreciably shielding for weak trans ligands to appreciably deshielding for ligands with strong trans influence. This may be traced back to increasing destabilization of a σ-type MO at scalar relativistic level, which translates into very different σ-/π-mixing if SO coupling is included. For the strongest trans ligands, the σ-MO may move above the highest occupied π-type MOs, thereby dramatically reducing strongly shielding contributions from predominantly π-type spinors. The effects of SO-mixing are in turn related to angular momentum admixture from atomic spinors at the metal center. These SO-induced trends hold for other nuclei and may also be used to qualitatively predict shifts in unknown complexes.

ChemInform Abstract: The Mechanism of Enantioselective Ketone Reduction with Noyori and Noyori—Ikariya Bifunctional Catalysts

AbstractReview: 303 refs.

Breaking and Making of Carbon-Carbon Bonds by Lanthanides and Third-Row Transition Metals.

Carbon-atom extrusion from the ipso-position of a halobenzene ring (C6 H5 X; X=F, Cl, Br, I) and its coupling with a methylene ligand to produce acetylene is not confined to [LaCH2 ](+) ; also, the third-row transition-metal complexes [MCH2 ](+) , M=Hf, Ta, W, Re, and Os, bring about this unusual transformation. However, substrates with substituents X=CN, NO2 , OCH3 , and CF3 are either not reactive at all or give rise to different products when reacted with [LaCH2 ](+) . In the thermal gas-phase processes of atomic Ln(+) with C7 H7 Cl substrates, only those lanthanides with a promotion energy small enough to attain a 4f(n) 5d(1) 6s(1) configuration are reactive and form both [LnCl](+) and [LnC5 H5 Cl](+) . Branching ratios and the reaction efficiencies of the various processes seem to correlate with molecular properties, like the bond-dissociation energies of the C-X or M(+) -X bonds or the promotion energies of lanthanides.

A Cobalt Hydride Catalyst for the Hydrogenation of CO2: Pathways for Catalysis and Deactivation

The complex Co(dmpe)2H catalyzes the hydrogenation of CO2 at 1 atm and 21 °C with significant improvement in turnover frequency relative to previously reported second- and third-row transition-metal complexes. New studies are presented to elucidate the catalytic mechanism as well as pathways for catalyst deactivation. The catalytic rate was optimized through the choice of the base to match the pKa of the [Co(dmpe)2(H)2]+ intermediate. With a strong enough base, the catalytic rate has a zeroth-order dependence on the base concentration and the pressure of hydrogen and a first-order dependence on the pressure of CO2. However, for CO2:H2 ratios greater than 1, the catalytically inactive species [(μ-dmpe)(Co(dmpe)2)2]2+ and [Co(dmpe)2CO]+ were observed.

Reduction of Systematic Uncertainty in DFT Redox Potentials of Transition-Metal Complexes

Reliable calculations of redox potentials could provide valuable insight into catalytic mechanisms of electrochemically active transition-metal complexes as well as guidelines for the design of new electrocatalysts. However, the correlation between theoretical and experimental data is often uncertain, since redox properties depend strongly on experimental conditions of electrochemical measurements, including the nature of the solvent, electrolyte, and working electrode. Here, we show that the use of internal references allows for quantitative theoretical predictions of redox potentials with standard deviations σ comparable to typical experimental errors of cyclic voltammetry measurements. Agreement for first-, second-, and third-row transition-metal complexes is demonstrated even at a rather modest level of density functional theory (σ = 64 mV for the UB3LYP/6-311G* level). This is shown for a series of benchmark redox couples, including ([MCp2]0/+ (Cp = η5-cyclopentadienyl), [MCp*2]0/+ (Cp* = η5-1,2,3,4,5-pentamethylcyclopentadienyl), [M(bpy)3]2+/3+ (bpy =2,2′-bipyridine), and [Ir(acac)3]0/+ (acac = acetylacetonate), with M = Fe, Co, Ni, Ru, Os, or Ir) in various nonaqueous solvents [acetonitrile (MeCN), dimethyl sulfoxide (DMSO), and dichloromethane (DCM)].

3+2] Cycloaddition of metal-containing azomethine ylides for highly efficient synthesis of mitosene skeleton

Efficient synthesis of tricyclic indole derivatives bearing a substituent at the 3-position of the indole nucleus was achieved by the [3+2] cycloaddition reaction of transition metal-containing azomethine ylides derived from N-( o-alkynylphenyl)imines with vinyl ethers. Third-row transition metal complexes, especially PtCl 2, turned out to be highly efficient for the reaction of internal alkynes and imidate substrates with wide generality. Moreover, as strong support for the reaction mechanism, the intermediate Pt–carbene complex was found to undergo intramolecular C–H bond insertion reaction to give tetracyclic indoline derivatives when benzyl vinyl ether was employed as a dipolarophile. This protocol provided a facile synthesis of highly functionalized tricyclic indole derivatives found as the basic skeleton of the mitosene family, such as mitomycin C.

Geometries of Third-Row Transition-Metal Complexes from Density-Functional Theory.

A set of 41 metal-ligand bond distances in 25 third-row transition-metal complexes, for which precise structural data are known in the gas phase, is used to assess optimized and zero-point averaged geometries obtained from DFT computations with various exchange-correlation functionals and basis sets. For a given functional (except LSDA) Stuttgart-type quasi-relativistic effective core potentials and an all-electron scalar relativistic approach (ZORA) tend to produce very similar geometries. In contrast to the lighter congeners, LSDA affords reasonably accurate geometries of 5d-metal complexes, as it is among the functionals with the lowest mean and standard deviations from experiment. For this set the ranking of some other popular density functionals, ordered according to decreasing standard deviation, is BLYP > VSXC > BP86 ≈ BPW91 ≈ TPSS ≈ B3LYP ≈ PBE > TPSSh > B3PW91 ≈ B3P86 ≈ PBE hybrid. In this case hybrid functionals are superior to their nonhybrid variants. In addition, we have reinvestigated the previous test sets for 3d- (Bühl M.; Kabrede, H. J. Chem. Theory Comput. 2006, 2, 1282-1290) and 4d- (Waller, M. P.; Bühl, M. J. Comput. Chem. 2007, 28, 1531-1537) transition-metal complexes using all-electron scalar relativistic DFT calculations in addition to the published nonrelativistic and ECP results. For this combined test set comprising first-, second-, and third-row metal complexes, B3P86 and PBE hybrid are indicated to perform best. A remarkably consistent standard deviation of around 2 pm in metal-ligand bond distances is achieved over the entire set of d-block elements.

Light Emitting Materials for Organic Electronics

This paper presents the case studies on the syntheses and characterization of three third-row transition metal complexes, all of them are associated with at least one functionalized 2-pyridyl pyrazolate chelates. Each of these complexes possesses distinctive central metal atom and shows highly efficient phosphorescent emission ranging from red, green and blue of the visible spectrum. Their photophysical properties are briefly discussed, including the assignment of their fundamental absorption and emission characteristics. Finally, performance data on fabrication of phosphorescent OLEDs using these complexes as emitters are presented.

Why [(η5-C5MenH5-n)2Ti]2(μ2,η2,η2-N2) Can Not Add a H2 Molecule to the Side-On-Coordinated N2 while Its Zr and Hf Analogues Can? Insights from Computational Studies

The potential energy surface of the reaction [(eta5-C5MenH5-n)2M]2(micro2,eta2,eta2-N2) + H2 --> [(eta5-C5MenH5-n)2M][(eta5-C5MenH5-n)2MH](micro2,eta2,eta2-NNH) at low-lying singlet and triplet electronic states of the reactants was investigated using density functional methods, for n = 0 and 4, and M = Ti, Zr, and Hf. Ground electronic states of the Ti complexes are found to be triplet states, while that for the corresponding Zr and Hf complexes are singlet states. In their singlet state, all these complexes satisfy known necessary conditions (they have a side-on-coordinated N2 molecule and appropriate frontier orbitals) for successful addition of an H2 molecule to the coordinated N2, and consequently, add of an H2 molecule with a reasonable energy barrier. Hf complexes show slightly higher reactivity than corresponding Zr complexes, and in turn, both are more reactive than their singlet-state Ti counterparts. The calculated trend in reactivity of Zr and Hf complexes is consistent with the latest experimental data (see refs 13 and 16). However, Ti complexes have the ground triplet state that lacks in appropriate frontier orbitals. As a result, H2 addition to the Ti complexes at their triplet ground states requires a larger activation barrier than the singlet state and is endothermic (lacks of driven force for reaction). On the basis of these results, we predict that the [(eta5-C5Me4H)2M]2(micro2,eta2,eta2-N2) and [(eta5-C5H5)2M]2(micro2,eta2,eta2-N2) complexes cannot react with an H2 molecule for M = Ti, while those for M = Zr and Hf can. It was shown that the difference in the B3LYP (hybrid) and PBE (nonhybrid) calculated energy gaps between the lowest closed-shell singlet and triplet states of the present complexes reduces via first- > second- > third-row transition metals; both hybrid and nonhybrid density functionals can be safely used to describe reactivity of the low-lying low-spin and high-spin states of second- and third-row transition metal complexes.

Dual Room-Temperature Fluorescent and Phosphorescent Emission in 8-Quinolinolate Osmium(II) Carbonyl Complexes: Rationalization and Generalization of Intersystem Crossing Dynamics

A new series of quinolinolate osmium carbonyl complexes were synthesized and characterized by spectroscopic methods. Single-crystal X-ray diffraction studies indicate that these complexes consist of an octahedral ligand arrangement with one chelating quinolinolate, one tfa or halide ligand, and three mutually orthogonal terminal CO ligands. Variation of the substituents on quinolinolate ligands imposes obvious electronic or structural effects, while changing the tfa ligand to an electron-donating iodide slightly increases the charge density on the central osmium atom. These Os(II) complexes show salient dual emissions consisting of fluorescence and phosphorescence, the spectral properties and relaxation dynamics of which have been studied comprehensively. The results, in combination with the theoretical approaches, lead us to propose that the emission mainly originates from the quinolinolate pi pi* state. Both experimental and theoretical approaches generalize various types of intersystem crossing versus those of the tris(quinolinolate) iridium Ir(Q)3, and their relative efficiencies were accessed on the basis of the associated frontier orbital configurations. Our results suggest that [1d(pi)pi* absolute value(H(so))3 pi pi*] (or [3d(pi)pi* absolute value(H(so))1 pi pi*]) in combination with a smaller deltaE(S1-T1) gap (i.e., increasing the MLCT (d(pi)pi*) character) is the main driving force to induce the ultrafast S1 --> T1 intersystem crossing in the third-row transition metal complexes, giving the strong phosphorescent emission.

Coordination modes of 2-hydroxynicotinic acid in second- and third-row transition metal complexes

The new Pd(II), Pt(II), Re(V), Mo(VI) and W(VI) complexes of 2-hydroxynicotinic acid (H 2nicO), trans-[PdCl(HnicO)(PPh 3) 2]·0.75CH 3CN ( 1), K[PdCl(HnicO) 2]·H 2O ( 2), [Pd(HnicO) 2(bipy)] ( 3), cis-[PtCl(HnicO)(PPh 3) 2]·0.75CH 3OH·0.5H 2O ( 4), [PtCl(HnicO)(bipy)] ( 5), cis-[ReOI 2(HnicO)(PPh 3)] ( 6), Na 2[Mo 2O 6(HnicO) 2]·5H 2O ( 7), Na 2[Mo 4O 12(HnicO) 2]·2H 2O ( 8) and Na 2[W 2O 6(HnicO) 2]·5H 2O ( 9) have been prepared. The crystal structures of 1 and 4, were determined by X-ray diffraction and show the HnicO − ligand coordinated to palladium or platinum through the nitrogen atom only. Infrared, Raman, 1H and 13C{ 1H} NMR spectroscopic data for the complexes are presented and are in agreement with the crystallographic results.

Regular and inverse secondary kinetic enthalpy effects (KHE) for the rate of inversion of thioether and 1,1'-biisoquinoline complexes of ruthenium and osmium.

Thioether complexes with the formula Delta/Lambda-chloro(thioether)bis(2,2'-bipyridine)metal(II) (M = Ru, Os; thioether = dimethyl sulfide (3a(+)), diethyl sulfide (3b(+)), and tetrahydrothiophene (3c(+))) have been synthesized. The rates of inversion at the sulfur atom of the thioether ligands have been measured by spin-inversion transfer and line-shape NMR methods. In every case, the ruthenium derivative exhibits a faster inversion frequency at a given temperature than the corresponding osmium derivative. In contrast, similar complexes with the formula chloro(delta/lambda-1,1'-biisoquinoline)(2,2':6',2"-terpyridine)metal(II), 4(M=Ru,Os)(+), undergo atropisomerization of the misdirected 1,1'-biisoquinoline (1,1'-biiq) ligand with rates that are faster for osmium than ruthenium. As a result of the lanthanide contraction effect and the similar metric parameters associated with the structures of second-row and third-row transition metal derivatives, steric factors associated with the isomerizations are presumably similar for the Ru and Os derivatives of these compounds. Since third-row transition metal complexes tend to have larger bond dissociation enthalpies (BDE) than their second-row congeners, we conclude the difference in reactivities of 3(M=Ru)(+) versus 3(M=Os)(+) and 4(M=Ru)(+) versus 4(M=Os)(+) are attributed to electronic effects. For 3, the S3p lone pair of the thioether, the principal sigma donor orbital, is orthogonal to the metal sigma acceptor orbital in the transition state of inversion at sulfur and the S 3s orbital is an ineffective sigma donor. Thus, a regular relationship between the kinetic and thermodynamic stabilities of 3(M=Ru)(+) and 3(M=Os)(+) is observed for the directed <==> [misdirected] <==> directed (DMD) isomerization (the more thermodynamically stable bond is less reactive). In contrast, atropisomerization of 4(+) involves redirecting (strengthening) the M-N bonds of the misdirected 1,1'-biiq ligand in the transition state. Therefore, an inverse relationship between the kinetic and thermodynamic stabilities of 4(M=Ru)(+) and 4(M=Os)(+) is observed for the misdirected <==> [directed] <==> misdirected (MDM) isomerization (the more thermodynamically stable bond is more reactive). The rates obtained for 4(+) are consistent with the rates of atropisomerization of Delta/Lambda-(delta/lambda-1,1'-biisoquinoline)bis(2,2'-bipyridine)metal(II), 1(M=Ru,Os)(2+), and (eta(6)-benzene) Delta/Lambda-(delta/lambda-1,1'-biisoquinoline)halometal(II), 2(M=Ru,Os;halo=Cl,I)(+), that we reported previously. We term the relative rates of reaction of second-row versus third-row transition metal derivatives kinetic element effects (KEE = k(second)/k(third)). While the KEE appears to be generally useful when comparing reactions of isostructural species (e.g. the relative rates of 1(M=Ru)(2+), 1(M=Os)(2+), and 1(M=Ir)(3+)), different temperature dependencies of reactions prevent the comparison of related reactions between species that have different structures (e.g., the 1,1'-biiq atropisomerization reactions of 1(M=Ru,Os)(2+) versus 2(M=Ru,Os;halo=Cl,I)(+) versus 4(M=Ru,Os)(+)). This problem is overcome by comparing entropies of activation and kinetic enthalpy effects (KHE = DeltaH(third)/DeltaH(second)). For a given class of 1,1'-biiq complexes, we observe a structure/reactivity relationship between DeltaH and the torsional twist of the 1,1'-biiq ligands that are measured in the solid state.

A quantum chemical investigation of the metal-to-ligand-charge-transfer photochemistry

According to a number of recent experiments reported for a class of α-diimine mono- and di-nuclear transition metal carbonyls, these molecules may either photodissociate leading to highly reactive intermediates or manifest the photophysics of metal-to-ligand-charge-transfer (MLCT) complexes. The duality between these two extremes of behavior may be used to promote different applications like catalytic activity or energy/electrons transfers. The potential energy surfaces (PESs) associated with the low-lying singlet and triplet excited states of Mn(H)(CO) 3(DAB) (DAB=1,4-diaza-1,3-butadiene) calculated for the Mn–H bond homolysis and for the dissociation of an axial carbonyl ligand illustrate the complexity and the richness of the photochemistry and photophysics of this class of molecules. It is shown that the presence of significant energy barriers on the reaction path corresponding to the homolysis of the Mn–H bond are responsible for the low efficiency of this process in related complexes. The simulation of the dynamics of the 1MLCT excited state, calculated at 26 000 cm −1 and contributing mainly to the visible absorption spectrum, indicates that the most probable primary reaction is an ultrafast dissociation (<200 fs) of the carbonyl ligand. The relative positions of the quasi-bound MLCT states with respect to the sigma bond-to-ligand-charge-transfer (SBLCT) excited state corresponding to the σ Mn–H →π ∗ DAB excitation will govern the photoreactivity of the molecules. A comparative study of the sequence, the nature of the low-lying excited states and of the associated state correlation diagrams for the hydride complex and the ethyl analog Mn(Et)(CO) 3(DAB) points to a better reactivity of the ethyl-substituted complex in agreement with the experimental data. This is a consequence of a lowering of the excited states, an increase in the density of states in the visible energy domain and a mixing of the triplet SBLCT and MLCT states on going from the hydride to the ethyl complex. The influence of the metal center on going from the manganese to the rhenium compound is expressed by an important mixing of the MLCT and SBLCT states and a lowering of the excited states due to a relativistic destabilization of the d shells of the metal center and an indirect stabilization of the π ∗ DAB orbital through its interaction with the 6p of the rhenium. These heavy atom effects should induce a better photoreactivity for the third-row transition metal complexes than for the first-row analogs.