Second-Coordination-Sphere Assisted Selective Colorimetric Turn-on Fluoride Sensing by a Mono-Metallic Co(II) Hexacarboxamide Cryptand Complex.

The preparation of a selective turn-on colorimetric fluoride sensor was achieved through single cobalt(II) ion insertion into a macrobicyclic cryptand. Monometallic [Co(mBDCA-5t-H3)]- (1) and [Zn(mBDCA-5t-H3)]- (2) complexes were prepared in 74 and 84% yields, respectively. Structural characterization of 1 confirmed the presence of a proximal hydrogen-bonding network consisting of carboxamide N-H donors. The reaction of 1 with F- was accompanied by a distinct colorimetric turn-on response in mixed aqueous/organic media, and 1 was capable of selective fluoride sensing in the presence of large quantities of potentially competitive anions. Complex 1 represents a unique example of a fluoride sensor wherein selective F- binding takes place directly at a transition-metal center and induces a color change based upon metal-centered transitions. The metal(II) fluoride complexes [F⊂Co(mBDCA-5t-H3)]2- (3) and [F⊂Zn(mBDCA-5t-H3)]2- (4) were both fully characterized, including single crystal X-ray analyses. Fluoride binding is synergistic involving hydrogen-bond donors from the second-coordination sphere together with metal(II) ion complexation.

Reversible reduction of oxygen to peroxide facilitated by molecular recognition.

Generation of soluble sources of peroxide dianion (O(2)(2-)) is a challenge in dioxygen chemistry. The oxidizing nature of this anion renders its stabilization in organic media difficult. This Report describes the chemically reversible reduction of oxygen (O(2)) to cryptand-encapsulated O(2)(2-). The dianion is stabilized by strong hydrogen bonds to N-H groups from the hexacarboxamide cryptand. Analogous stabilization of peroxide by hydrogen bonding has been invoked recently in crystalline saccharide and protein systems. The present peroxide adducts are stable at room temperature in dimethyl sulfoxide (DMSO) and N,N'-dimethylformamide (DMF). These adducts can be obtained in gram quantities from the cryptand-driven disproportionation reaction of potassium superoxide (KO(2)) at room temperature.

Family of cofacial bimetallic complexes of a hexaanionic carboxamide cryptand.

A series of coordination compounds has been prepared comprising manganese, iron, nickel, and zinc bound by a hexaanionic cryptand where carboxamides are anionic N-donors. The metal complexes have been investigated by X-ray crystallography, and possess metal centers in trigonal monopyramidal geometries with intermetallic distances spanning d(Mn,avg) = 6.080 Å to d(Ni,avg) = 6.495 Å. All complexes featuring trigonal monopyramidal metal(II) ions crystallize in Cc, and feature extended three-dimensional networks composed of cryptate anions linked by bridging potassium countercations. We also report the first solid state structure of the free cryptand ligand, which features no guest in its cavity and which possesses an extended hydrogen-bonding network. SQuID magnetometry data of the metal complexes reveal weak antiferromagnetic coupling of the metal centers. Only the diiron(II) complex exhibits reversible electrochemistry, and correspondingly, its chemical oxidation yields a powder formulated as the diiron(III) congener. The insertion of cyanide into the intermetallic cleft of the diiron(II) complex has been achieved, and comparisons of its solid state structure to the recently reported dicobalt(II) analogue are made. The antiferromagnetic coupling between the diiron(II) and the dicobalt(II) centers when bridged by cyanide does not increase significantly relative to the unbridged congeners. A one-site model satisfactorily fits Mössbauer spectra of unbridged diiron(II) and diiron(III) complexes whereas a two site fit was needed to model the iron(II) centers that are bridged by cyanide.

Cofacial dicobalt complex of a binucleating hexacarboxamide cryptand ligand.

A hexacarboxamide cryptand featuring appended polyether moieties is used as a binucleating ligand for two Co(II) centers, marking the first time cryptands have been used as hexaanionic N donors for metal coordination. A synthesis for the hexacarboxamide cryptand, culminating in a 23% yield high-dilution step and proceeding in 8% overall yield, is reported. The ligand is metalated using cobalt(II) acetate, and a solid-state structure is presented, revealing an intermetallic void over 6.4 A in length. The reactivity of this new type of cryptate is also probed; treatment of the dicobalt cryptate with potassium cyanide at elevated temperature results in a bridging cyanide complex.

A ligand field chemistry of oxygen generation by the oxygen-evolving complex and synthetic active sites.

Oxygen-oxygen bond formation and O2 generation occur from the S4 state of the oxygen-evolving complex (OEC). Several mechanistic possibilities have been proposed for water oxidation, depending on the formal oxidation state of the Mn atoms. All fall under two general classifications: the AB mechanism in which nucleophilic oxygen (base, B) attacks electrophilic oxygen (acid, A) of the Mn4Ca cluster or the RC mechanism in which radical-like oxygen species couple within OEC. The critical intermediate in either mechanism involves a metal oxo, though the nature of this oxo for AB and RC mechanisms is disparate. In the case of the AB mechanism, assembly of an even-electron count, high-valent metal-oxo proximate to a hydroxide is needed whereas, in an RC mechanism, two odd-electron count, high-valent metal oxos are required. Thus the two mechanisms give rise to very different design criteria for functional models of the OEC active site. This discussion presents the electron counts and ligand geometries that support metal oxos for AB and RC O-O bond-forming reactions. The construction of architectures that bring two oxygen functionalities together under the purview of the AB and RC scenarios are described.