Bis(mu-oxo)dicopper(III) complexes of a homologous series of simple peralkylated 1,2-diamines: steric modulation of structure, stability, and reactivity.

We have synthesized and characterized bis(mu-oxo)dicopper(III) dimers 1b-4b (Os) based on a core family of peralkylated trans-(1R,2R)-cyclohexanediamine (CD) ligands, self-assembled from the corresponding [LCu(MeCN)]CF3SO3 species 1a-4a and O2 at 193 K in aprotic media; additional Os based on peralkylated ethylenediamine and tridentate polyazacyclononane ligands were synthesized analogously for comparative purposes (5b-7b and 8b-9b, respectively). Trigonal-planar [LCu(MeCN)]1+ species are proposed as the active O precursors. The 3-coordinate Cu(I) complexes [(L(TE))Cu(MeCN)]CF3SO3 (4a) and [(L(TB))Cu(MeCN)]CF3SO3 (10a) were structurally characterized; the apparent O2-inertness of 10a correlates with the steric demands of its four benzyl substituents. The rate of O formation, a multistep process that likely proceeds via associative formation of a 1:1 [LCu(O2)]1+ intermediate, exhibits significant dependence upon ligand sterics and solvent: oxygenation of 4a-the slowest-reacting O precursor of the CD series-is first-order with respect to [4a] and proceeds at least 300 times faster in tetrahydrofuran than in CH2Cl2. The EPR, UV-vis, and resonance Raman spectra of 1b-9b are all characteristic of the diamagnetic bis(mu-oxo)dicopper(III) core. The intense ligand-to-metal charge transfer absorption maxima of CD-based Os are red-shifted proportionally with increasing peripheral ligand bulk, an effect ascribed to a slight distortion of the [Cu2O2] rhomb. The well-ordered crystal structure of [(L(ME))2Cu2(mu-O)2](CF3SO3)2.4CH2Cl2 ([3b. 4CH2Cl2]) features the most metrically compact [Cu2O2]2+ core among structurally characterized Os (av Cu-O 1.802(7) A; Cu...Cu 2.744(1) A) and exemplifies the minimal square-planar ligation environment necessary for stabilization of Cu(III). The reported Os are mild oxidants with moderate reactivity toward coordinating substrates, readily oxidizing thiols, certain activated alkoxides, and electron-rich phenols in a net 2e-, 2H+ process. In the absence of substrates, 1b-9b undergo thermally induced autolysis with concomitant degradation of the polyamine ligands. Ligand product distribution and primary kinetic isotope effects (kobsH/kobsD approximately 8, 1b/d24-1b, 293 K) support a unimolecular mechanism involving rate-determining C-H bond cleavage at accessible ligand N-alkyl substituents. Decomposition half-lives span almost 3 orders of magnitude at 293 K, ranging from approximately 2 s for 4b to almost 30 min for d(24)-1b, the most thermally robust dicationic O yet reported. Dealkylation is highly selective where ligand rigidity constrains accessibility; in 3b, the ethyl groups are attacked preferentially. The observed relative thermal stabilities and dealkylation selectivities of 1b-9b are correlated with NC(alpha)-H bond dissociation energies, statistical factors, ligand backbone rigidity, and ligand denticity/axial donor strength. Among the peralkylated amines surveyed, bidentate ligands with oxidatively robust NC(alpha)-H bonds provide optimal stabilization for Os. Fortuitously, the least sterically demanding N-alkyl substituent (methyl) gives rise to the most thermally stable and most physically accessible O core, retaining the potential for exogenous substrate reactivity.

C-H activation by a mononuclear manganese(III) hydroxide complex: synthesis and characterization of a manganese-lipoxygenase mimic?

Lipoxygenases are mononuclear non-heme metalloenzymes that regio- and stereospecifically convert 1,4-pentadiene subunit-containing fatty acids into alkyl peroxides. The rate-determining step is generally accepted to be hydrogen atom abstraction from the pentadiene subunit of the substrate by an active metal(III)-hydroxide species to give a metal(II)-water species and an organic radical. All known plant and animal lipoxygenases contain iron as the active metal; recently, however, manganese was found to be the active metal in a fungal lipoxygenase. Reported here are the synthesis and characterization of a mononuclear Mn(III) complex, [Mn(III)(PY5)(OH)](CF(3)SO(3))(2) (PY5 = 2,6-bis(bis(2-pyridyl)methoxymethane)pyridine), that reacts with hydrocarbon substrates in a manner most consistent with hydrogen atom abstraction and provides chemical precedence for the proposed reaction mechanism. The neutral penta-pyridyl ligation of PY5 endows a strong Lewis acidic character to the metal center allowing the Mn(III) compound to perform this oxidation chemistry. Thermodynamic analysis of [Mn(III)(PY5)(OH)](2+) and the reduced product, [Mn(II)(PY5)(H(2)O)](2+), estimates the strength of the O-H bond in the metal-bound water in the Mn(II) complex to be 82 (+/-2) kcal mol(-)(1), slightly less than that of the O-H bond in the related reduced iron complex, [Fe(II)(PY5)(MeOH)](2+). [Mn(III)(PY5)(OH)](2+) reacts with hydrocarbon substrates at rates comparable to those of the analogous [Fe(III)(PY5)(OMe)](2+) at 323 K. The crystal structure of [Mn(III)(PY5)(OH)](2+) displays Jahn-Teller distortions that are absent in [Mn(II)(PY5)(H(2)O)](2+), notably a compression along the Mn(III)-OH axis. Consequently, a large internal structural reorganization is anticipated for hydrogen atom transfer, which may be correlated to the lessened dependence of the rate of substrate oxidation on the substrate bond dissociation energy as compared to other metal complexes. The results presented here suggest that manganese is a viable metal for lipoxygenase activity and that, with similar coordination spheres, iron and manganese can oxidize substrates through a similar mechanism.

Homolysis of weak Ti-O bonds: experimental and theoretical studies of titanium oxygen bonds derived from stable nitroxyl radicals.

Titanium-oxygen bonds derived from stable nitroxyl radicals are remarkably weak and can be homolyzed at 60 degrees C. The strength of these bonds depends sensitively on the ancillary ligation at titanium. Direct measurements of the rate of Ti-O bond homolysis in Ti-TEMPO complexes Cp2TiCl(TEMPO) (3) and Cp2TiCl(4-MeO-TEMPO) (4) (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl, 4-MeO-TEMPO = 2,2,6,6-tetramethyl-4-methoxypiperidine-N-oxyl) were conducted by nitroxyl radical exchange experiments. Eyring plots gave the activation parameters, deltaH++ = 27(+/- 1) kcal/mol, deltaS++ = 6.9(+/- 2.3) eu for 3 and deltaH++ = 28(+/- 1) kcal/mol, deltaS++ = 9.0(+/- 3.0) eu for 4, consistent with a process involving the homolysis of a weak Ti-O bond to generate the transient Cp2Ti(III)Cl and the nitroxyl radical. Thermolysis of the titanocene TEMPO complexes in the presence of epoxides leads to the Cp2Ti(III)Cl-mediated ring-opening of the epoxide followed by trapping by the nitroxyl radical. The X-ray crystal structure of the Ti-TEMPO derivative, Cp2TiCl(4-MeO-TEMPO) (4), is reported. DFT (B3LYP/6-31G*) calculations and experimental studies reveal that the strength of the Ti-O bond decreases dramatically with the number of cyclopentadienyl groups on titanium. The calculated Ti-O bond strength of the monocyclopentadienyl complex 2 is 43 kcal/mol, whereas that of the biscyclopentadienyl complex 3 is 17 kcal/mol, a difference of 26 kcal/mol. These studies reveal that the strength of these Ti-O bonds can be tuned over an interesting and experimentally accessible temperature range by appropriate ligation on titanium.

X-ray absorption spectroscopic investigation of the spin-transition character in a series of single-site perturbed iron(II) complexes.

Select ferrous spin-transition complexes with the pentadentate ligand 2,6-bis(bis(2-pyridyl)methoxymethane)pyridine (PY5) were examined using variable-temperature solution solid-state magnetic susceptibility, crystallography, X-ray absorption spectroscopy (XAS), and UV/vis absorption spectroscopy. Altering the single exogeneous ligand, X, of [Fe(PY5)(X)]n)+ is sufficient to change the spin-state of the complexes. When X is the weak-field ligand Cl-, the resultant Fe complex is high-spin from 4 to 300 K, whereas the stronger-field ligand MeCN generates a low-spin complex over this temperature range. With intermediate-strength exogenous ligands (X = N3-, MeOH), the complexes undergo a spin-transition. [Fe(PY5)(N3)]+, as a crystalline solid, transitions gradually from a high-spin to a low-spin complex as the temperature is decreased, as evidenced by X-ray crystallography and solid-state magnetic susceptibility measurements. The spin-transition is also evident from changes in the pre-edge and EXAFS regions of the XAS Fe K-edge spectra on a ground crystalline sample. The spin-transition observed with [Fe(PY5)(MeOH)]2+ appears abrupt by solid-state magnetic susceptibility measurements, but gradual by XAS analysis, differences attributed to sample preparation. This research highlights the strengths of XAS in determining the electronic and geometric structure of such spin-transition complexes and underscores the importance of identical sample preparation in the investigation of these physical properties.

A spectrochemical walk: single-site perturbation within a series of six-coordinate ferrous complexes.

A series of ferrous complexes with the pentadentate ligand 2,6-(bis-(bis-2-pyridyl)methoxymethane)pyridine (PY5) was prepared and examined. PY5 binds ferrous iron in a square-pyramidal geometry, leaving a single coordination site accessible for complexation of a wide range of monodentate exogenous ligands: [Fe(II)(PY5)(X)](n+), X = MeOH, H(2)O, MeCN, pyridine, Cl-, OBz-, N(3)-, MeO-, PhO-, and CN-. The spin-states of these ferrous complexes are extremely sensitive to the nature of the single exogenous ligand; the spectroscopic and structural properties correlate with their high-spin (hs) or low-spin (ls) electronic ground state. Systematic metrical trends within six crystallographic structures clearly indicate a preferred conformational binding mode of the PY5 ligand. The relative binding affinities of the exogenous ligands in MeOH indicate that exogenous ligand charge is the primary determinant of the binding affinity; the [Fe(II)(PY5)](2+) unit preferentially binds anionic ligands over neutral ligands. At parity of charge, strong-field ligands are preferentially bound over weak-field ligands. In MeOH, the pK(a) of the exogenously ligated MeOH in [Fe(PY5)(MeOH)](2+) (9.1) limits the scope of exogenous ligands, as strongly basic ligands preferentially deprotonate [Fe(PY5)(MeOH)](2+) to yield [Fe(PY5)(OMe)](1+) rather than ligate to the ferrous center. Exogenous ligation by a strongly basic ligand, however, can be achieved in polar aprotic solvents.

Synthesis and molecular structure of titanium complexes containing a reduced TEMPO radical.

Two titanium compounds containing monoanionic ligands derived from TEMPO were synthesized and structurally characterized, demonstrating the flexibility of the coordination modes adopted by the ligand.