Selective Pathway toward Heteroleptic Spin-Crossover Iron(II) Complexes with Pyridine-Based N-Donor Ligands.

A new synthetic pathway is devised to selectively produce previously elusive heteroleptic iron(II) complexes of terpyridine and N,N'-disubstituted bis(pyrazol-3-yl)pyridines that stabilize the opposite spin states of the metal ion. Such a combination of the ligands in a series of the heteroleptic complexes induces the spin-crossover (SCO) not experienced by the homoleptic complexes of these ligands or shifts it to lower/higher temperatures respective to the SCO-active homoleptic complex. The midpoint temperatures of the resulting SCO span from ca. 200 K to the ambient temperature and beyond the highest temperature accessible by NMR spectroscopy and SQUID magnetometry. The proposed "one-pot" approach is applicable to other N-donor ligands to selectively produce heteroleptic complexes─including those inaccessible by alternative synthetic pathways─with highly tunable SCO behaviors for practical applications in sensing, switching, and multifunctional devices.

Reductive Silylation Using a Bis-silylated Diaza-2,5-cyclohexadiene.

1,4-Bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene, 1, was tested as a reagent for the reductive silylation of various unsaturated functionalities, including N-heterocycles, quinones, and other redox-active moieties in addition to deoxygenation of main group oxides. Whereas most reactions tested are thermodynamically favorable, based on DFT calculations, a few do not occur, perhaps giving limited insight on the mechanism of this very attractive reductive process. Of note, reductive silylation reactions show a strong solvent dependence where a polar solvent facilitates conversions.

Multi-electron Reduction Capacity and Multiple Binding Pockets in Metal-Organic Redox Assembly at Surfaces.

Metal-ligand complexation at surfaces utilizing redox-active ligands has been demonstrated to produce uniform single-site metals centers in regular coordination networks. Two key design considerations are the electron storage capacity of the ligand and the metal-coordinating pockets on the ligand. In an effort to move toward greater complexity in the systems, particularly dinuclear metal centers, we designed and synthesized tetraethyltetra-aza-anthraquinone, TAAQ, which has superior electron storage capabilities and four ligating pockets in a diverging geometry. Cyclic voltammetry studies of the free ligand demonstrate its ability to undergo up to a four-electron reduction. Solution-based studies with an analogous ligand, diethyldi-aza-anthraquinone, demonstrate these redox capabilities in a molecular environment. Surface studies conducted on the Au(111) surface demonstrate TAAQ's ability to complex with Fe. This complexation can be observed at different stoichiometric ratios of Fe:TAAQ as Fe 2p core level shifts in X-ray photoelectron spectroscopy. Scanning tunneling microscopy experiments confirmed the formation of metal-organic coordination structures. The striking feature of these structures is their irregularity, which indicates the presence of multiple local binding motifs. Density functional theory calculations confirm several energetically accessible Fe:TAAQ isomers, which accounts for the non-uniformity of the chains.

Detailed electronic structure of a high-spin cobalt(ii) complex determined from NMR and THz-EPR spectroscopy.

Here we report a combined use of THz-EPR and NMR spectroscopy for obtaining a detailed electronic structure of a long-known high-spin complex, cobalt(ii) bis[tris(pyrazolyl)borate]. The lowest inter-Kramers transition was directly measured by THz-EPR spectroscopy, while the energies of higher Kramers doublets were estimated by a recently proposed NMR-based approach. Together, they produced magnetic parameters for a full model that explicitly includes spin-orbit coupling. This approach is applicable to all transition metal ions for which the spin-orbit coupling cannot be treated perturbatively.

A PNNH Pincer Ligand Allows Access to Monovalent Iron.

Reaction of the unsymmetrically armed pincer PNNH (phosphine-pyridyl-pyrazole) ligand with FeCl2 yielded the five-coordinate monomer [(PNNH)FeCl2 ], the NH proton of which captures THF through the formation of a hydrogen bond. Deprotonation of this NH functionality with Li[N(SiMe3 )2 ] did not give the four-coordinate [(PNN)FeCl], but instead retained LiCl to yield [(PNNLi)FeCl2 ], in which the lithium bridges between the pyrazolate ß-nitrogen and one of the chlorides on iron. One-electron reduction of this compound under CO occurred with the loss of LiCl to form the square-pyramidal monovalent iron in [(PNN)Fe(CO)2 ], which was characterized by IR, Mössbauer, and EPR spectroscopy, X-ray diffraction, and DFT calculations. Cyclic voltammetry studies of [(PNN)Fe(CO)2 ] showed a reversible reduction wave and the reduction product was synthesized by using KC8 . The product K[(PNN)Fe(CO)2 ] contains saturated, five-coordinate Fe0 ; the (PNN)Fe subunit is anionic and the K+ cations cluster close to the pyrazolate side of the two CO ligands. Potassium electrophile complete its coordination sphere through interactions with the oxygen atom of a CO of a neighboring unit, thereby creating a polymeric chain. The reaction of [(PNN)Fe(CO)2 ] with HBpin (4,4,5,5-tetramethyl-1,3,2-dioxaborolane) resulted in the reduction of the metal center (by release of H2 ) and borylation of the pyrazole ß-nitrogen atom. This redox-active addition of the H-B bond across the metal-ligand assembly is an unusual example of metal-ligand cooperativity and establishes a ligand that supports iron in three different oxidation states.

A Multifunctional Pincer Ligand Supports Unsaturated Cobalt: Five Functionalities in One Pincer.

A pyridyl pincer ligand was developed to incorporate steric bulk, through a PtBu2 arm, and proton responsivity, through a pyrazole pincer ligand arm, together with reactivity at benzylic hydrogen and redox activity within a 1,4 diazabutadiene moiety. Binding it to CoCl2 yielded square-pyramidal [(PNNH)CoCl2 ], which was deprotonated by Li[N(SiMe3 )2 ] to form [{Li(THF)2 PNN}CoCl2 ]. Reduction of this LiCl adduct with KC8 under CO atmosphere led to formation of CoI mono- and dicarbonyl complexes, which can be protonated but also further deprotonated at the benzylic CH group to give a dearomatized pyridyl group. The ligand was characterized in its neutral, monoanionic, and dianionic forms, and the anions were shown to exist as intimate ion pairs with Li+ bound to pyrazolate N and chloride bound to Lewis acidic cobalt. X-ray photoelectron spectroscopy was used to assay both Li content and cobalt oxidation states. The general character of binding of LiCl to a metal complex acidic at metal and nucleophilic at ligand (pyrazolate Nß) is discussed, as are potential catalytic applications of the concept.

Ligand Design toward Multifunctional Substrate Reductive Transformations.

The synthesis of bis(N1-phenyl-5-hydroxypyrazol-3-yl)pyridines ("L") is described, and these are silylated to achieve analogues ("Si2L") without the variable of the hydroxyl proton mobility. One hydroxyl example is characterized in its bis-pincer iron(II) complex, which shows every OH proton involved in hydrogen bonding. The steric bulk of the silylated N-phenyl-substituted ligands allows the synthesis and characterization of paramagnetic (Si2L)FeCl2 complexes, and one of these is reduced, under CO, to give the diamagnetic (Si2L)Fe(CO)2 species. Structural comparison and density functional theory calculations of the dichloride and dicarbonyl species show that much, but not all, of the reduction occurs at both the ligand pyridine and pyrazole rings, and thus this ligand type is more resistant to reduction than the simpler bis(iminopyridines). The OSiR3 substituent offers a useful diagnostic of reduction at pyrazole via the degree of π-donation to pyrazole by the oxygen lone pairs, and the stereoelectronic features of the NPh moiety are analyzed. The X-ray photoelectron spectroscopy binding energies of both iron and nitrogen are analyzed to show details of the locus of reduction.

Tetrazine Assists Reduction of Water by Phosphines: Application in the Mitsunobu Reaction.

Reaction of 3,6-disubstituted-1,2,4,5-tetrazines with water and PEt3 forms the corresponding 1,4-dihydrotetrazine and OPEt3 . Thus PEt3 , as a stoichiometric reductant, reduces water, and the resulting two reducing equivalents serve to doubly hydrogenate the tetrazine. A variety of possible initial interactions between electron-deficient tetrazine and electron-rich PR3 , including a charge transfer complex, were evaluated by density functional calculations which revealed that the energy of all these make them spectroscopically undetectable at equilibrium, but one of these is nevertheless suggested as the intermediate in the observed redox reaction. The relationship of this to the Mitsunobu reaction, which absorbs the components of water evolved in the conversion of alcohol and carboxylic acid to ester, with desirable inversion at the alcohol carbon, is discussed. This enables a modified Mitsunobu reaction, with tetrazine replacing EtO2 CN=NCO2 Et (DEAD), which has the advantage that dihydrotetrazine can be recycled to tetrazine by oxidation with O2 , something impossible with the hydrogenated DEAD. For this tetrazine version, a betaine-like intermediate is undetectable, but its protonated form is characterized, including by X-ray structure and NMR spectroscopy.

Two- and Three-Electron Oxidation of Single-Site Vanadium Centers at Surfaces by Ligand Design.

Rational, systematic tuning of single-site metal centers on surfaces offers a new approach to increase selectivity in heterogeneous catalysis reactions. Although such metal centers of uniform oxidation states have been achieved, the ability to control their oxidation states through the use of carefully designed ligands had not been shown. To this end, tetrazine ligands functionalized by two pyridinyl or pyrimidinyl substituents were deposited, along with vanadium metal, on the Au(100) surface. The greater oxidizing power of the bis-pyrimidinyltetrazine facilitates the on-surface redox formation of V(3+), compared to V(2+) when paired with the bis-pyridinyltetrazine, as determined by X-ray photoelectron spectroscopy. This demonstrates the ability to control metal oxidation states in surface coordination architectures by altering the redox properties of organic ligands. The metal-ligand complexes take the form of one-dimensional polymeric chains, resolved by scanning tunneling microscopy. The chain structures in the first layer are very uniform and are based on the same quasi-square-planar coordination geometry around single-site V with either ligand. Formation of a different, dimer structure is observed in the early stages of the second layer formation. These systems offer new opportunities in controlling the oxidation state of single-site transition metal atoms at a surface for new advances in heterogeneous catalysts.

Room-Temperature Spin Crossover in a Solution of Iron(II) Complexes with N,N'-Disubstituted Bis(pyrazol-3-yl)pyridines.

Here, we report a combined study of the effects of two chemical modifications to an N,N'-disubstituted bis(pyrazol-3-yl)pyridine (3-bpp) and of different solvents on the spin-crossover (SCO) behavior in otherwise high-spin iron(II) complexes by solution NMR spectroscopy. The observed stabilization of the low-spin state by electron-withdrawing substituents in the two positions of the ligand that induce opposite electronic effects in SCO-active iron(II) complexes of isomeric bis(pyrazol-1-yl)pyridines (1-bpp) was previously hidden by NH functionalities in 3-bpp precluding the molecular design of SCO compounds with this family of ligands. With the recent SCO-assisting substituent design, the uncovered trends converged toward the first iron(II) complex of N,N'-disubstituted 3-bpp to undergo an almost complete SCO centered at room temperature in a less polar solvent of a high hydrogen-bond acceptor ability.

Residual heavy metals in industrial chitosan: State of distribution.

A series of industrial chitosans were analyzed on the presence of residual heavy metals. For the first time, optical microscopy data showed that chitosan solution retained a huge number of insoluble microparticles while transmittance electron microscopy revealed that insoluble fibrous microparticles were incrusted by crystalline nanoparticles with the sizes 5-50 nm. A series of filters used for chitosan solution filtration was analyzed on the presence of retained heavy metal and other residuals by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDXS) and mass-spectrometry with inductively coupled plasma (ICP-MS) methods. The SEM-EDXS analysis revealed the presence of Fe residuals together with Si, Al, N and S in the particles found on the filters. ICP-MS analysis found the presence of heavy metals (mainly Fe, Cr and Ni) both on the filter surfaces and in the effluent chitosan solution passed though the filters. This study draws attention to the necessity of a careful selection of industrially manufactured chitosan in order to avoid hidden undesirable effects of chitosan on pharmaceuticals and biomaterials and gives a warning of inapplicability of a stainless-steel made apparatus as a reactor susceptible to caustic soda corrosion for chitin deacetylation and production of medical and food grade chitosan.

A substituent-tolerant synthetic approach to N/P-"loaded" heteroarenes.

Tetrazines react with OCP-1 through a reverse electron demand Diels-Alder process to produce 3,6-disubstituted-1,2,4-diazaphosphinin-5-olates. DFT calculations reveal that both Diels-Alder and subsequent aromatization barriers are low for both EWG and ED tetrazine substituents. The structure of the solid sodium salt shows the interaction of Na+ with aryloxide and also both nitrogens of a neighboring anion, leading to coordination polymer character. 1,2,4-Diazaphosphinin-5-olates react as nucleophiles towards MeI and R3SiCl, respectively, and were installed on the (Ph3P)2Ru(CO)H fragment to investigate their properties as ligands.

Redox-active ligand controlled selectivity of vanadium oxidation on Au(100).

Metal-organic coordination networks at surfaces, formed by on-surface redox assembly, are of interest for designing specific and selective chemical function at surfaces for heterogeneous catalysts and other applications. The chemical reactivity of single-site transition metals in on-surface coordination networks, which is essential to these applications, has not previously been fully characterized. Here, we demonstrate with a surface-supported, single-site V system that not only are these sites active toward dioxygen activation, but the products of that reaction show much higher selectivity than traditional vanadium nanoparticles, leading to only one V-oxo product. We have studied the chemical reactivity of one-dimensional metal-organic vanadium - 3,6-di(2-pyridyl)-1,2,4,5-tetrazine (DPTZ) chains with O2. The electron-rich chains self-assemble through an on-surface redox process on the Au(100) surface and are characterized by X-ray photoelectron spectroscopy, scanning tunneling microscopy, high-resolution electron energy loss spectroscopy, and density functional theory. Reaction of V-DPTZ chains with O2 causes an increase in V oxidation state from VII to VIV, resulting in a single strongly bonded (DPTZ2-)VIVO product and spillover of O to the Au surface. DFT calculations confirm these products and also suggest new candidate intermediate states, providing mechanistic insight into this on-surface reaction. In contrast, the oxidation of ligand-free V is less complete and results in multiple oxygen-bound products. This demonstrates the high chemical selectivity of single-site metal centers in metal-ligand complexes at surfaces compared to metal nanoislands.