Using a monocopper-superoxo complex to prepare multicopper-peroxo species relevant to proposed enzyme intermediates.

With the goal of generating a (peroxo)tricopper species analogous to the Peroxy Intermediate proposed for multicopper oxidases, solutions of the copper-superoxide complex [K(Krypt)][LCuO2] (L = N,N'-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide, Krypt = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) were reacted with the dicopper(I) complex [(TPBN)Cu2(MeCN)2][PF6]2 at -70 °C (TPBN = N,N,N',N'-tetrakis-(2-pyridylmethyl)-1,4-diaminobutane). A metastable intermediate formed, which on the basis of UV-vis, EPR, and resonance Raman spectroscopy was proposed to derive from reaction of two equivalents of the copper-superoxide with one equivalent of the dicopper(I) complex to yield a complex with two (peroxo)dicopper moieties rather than the desired (peroxo)tricopper PI model. A similar intermediate formed upon reaction of [K(Krypt)][LCuO2] with [(BPMA)Cu(MeCN)][PF6] (BPMA = N,N-bis(2-pyridylmethyl)-methyl-amine), which contained the same donor set as provided by TPBN. Comparison of resonance Raman data and consideration of structural preferences for LCuX species led to hypothesis of a µ-η1:η2-peroxo structure for both intermediates.

Formazanate Complexes of Bis-Cyclometalated Iridium.

In this work we describe a series of bis-cyclometalated iridium(III) formazanate complexes, expanding the coordination chemistry of the redox-active formazanate class to iridium. A total of 18 new complexes are described, varying the substituent pattern on the formazanate and the identity of the cyclometalating ligand on iridium. Eight of the new compounds are structurally characterized by single-crystal X-ray diffraction, which along with NMR spectroscopy evinces two binding modes of the formazanate. Two of the compounds are isolated in a C2-symmetric geometry where the formazanate is bound in a six-member chelate "closed" conformation, involving the 1- and 5-positions of the 1,2,4,5-tetraazapentadienyl formazanate core. In most of the examples, the major isomer that forms and is exclusively isolated involves the formazanate bound in a five-member chelate "open" form, coordinating through the 1- and 4-positions of the formazanate core and resulting in C1 point-group symmetry. All complexes are characterized by UV-vis absorption spectroscopy and cyclic voltammetry, with these features depending primarily on the substitution pattern on the formazanate, and to a lesser extent on the identity of the cyclometalating ligand and formazanate binding mode.

Spectroscopic and Electrochemical Properties of Electronically Modified Cycloplatinated Formazanate Complexes.

Formazanates are a class of monoanionic, chromophoric, redox-active chelating ligands, and until recently, the coordination chemistry of formazanates with third-row transition metals was unexplored. Described here is a series of heteroleptic cyclometalated platinum formazanate complexes of the type Pt(C^N)(Fz), where "C^N" is the cyclometalating ligand and "Fz" is the formazanate. This work includes a set of 14 complexes derived from four different cyclometalating ligands and five different formazans, greatly expanding the structural and electronic diversity of this class of complexes. The formazans fall into two major categories: triarylformazans with either electron-donating (Me, OMe) or electron-withdrawing (CN) para substituents, and even more electron-deficient diarylformazans where the center position of the backbone is a cyano group. Seven of the new compounds are characterized by single-crystal X-ray diffraction, and all are characterized by cyclic voltammetry and UV-vis absorption spectroscopy to provide insight into the frontier orbitals. The complexes can be electrochemically reduced by three electrons, with the first two electrons populating formazanate-centered frontier orbitals and the third being added to the Pt-C^N fragment. The formazanate-based reduction potentials are highly sensitive to the substitution pattern of the formazanate and can be tuned over a wide range of ca. 0.8 V by altering substituents. The formazanates also impart these complexes with very strong, low-energy visible absorption bands assigned to HOMO → LUMO transitions, which are greatly perturbed from the free ligand due to interactions of the formazanate frontier orbitals with platinum-centered orbitals. Unlike the redox properties, the formazanate-derived visible absorption band is relatively insensitive to substituent effects. Additional visible absorption bands, attributed to Pt(d) → C^N(π*) MLCT transitions, can be tuned over a wide range by changing the structure of the C^N ligand, giving rise to strong panchromatic absorption for some members of the series.

Heteroleptic Complexes of Cyclometalated Platinum with Triarylformazanate Ligands.

Formazanates are a ligand class featuring a 1,2,4,5-tetraazapentadienyl core, with variable substitution at the 1, 3, and 5 positions. Here we describe a set of four heteroleptic cylcometalated platinum complexes containing triarylformazanate ligands. The complexes are prepared by metathesis reactions of chloro-bridged dimers [Pt(C∧N)(µ-Cl)]2 (C∧N = 2-phenylpyridine or 2-(2,4-difluorophenyl)pyridine) with triarylformazans in the presence of base. X-ray diffraction studies reveal the molecular structures of three such complexes. Cyclic voltammograms and UV-vis absorption spectra of the complexes show features characteristic of both the cyclometalated platinum fragment and the formazanate, with the latter giving rise to two reversible one-electron reductions in the CV and an intense visible π → π* absorption which is red-shifted by >100 nm relative to the free formazan. The electronic structures and redox properties of the complexes were further investigated by UV-vis spectroelectrochemistry and density functional theory calculations. All of the experimental and theoretical work points to a frontier molecular orbital manifold where the formazanate π and π* orbitals are substantially mixed with d-orbitals derived from the platinum center.