Energetics and Redox Kinetics of Pure Ferrocene-Terminated N-Heterocyclic Carbene Self-Assembled Monolayers on Gold.

N-heterocyclic carbene (NHC) self-assembled monolayers (SAMs) on gold have received considerable attention, but little is known about the lateral interactions between neighboring NHC molecules, their stability when subjected to aggressive oxidizing/reducing conditions, and their interactions with solution ions, all of which are essential for their use in a wide range of applications. To address these deficiencies, we present a comprehensive investigation of two different ferrocene (Fc)-terminated NHC SAMs with different chain lengths and linking groups. Pure monolayers of Fc-terminated NHCs display only a single, symmetrical pair of redox peaks, implying the formation of a homogeneous SAM structure with uniformly distributed Fc/Fc+ redox centers. By comparison, pure Fc-alkylthiol SAMs exhibit complex and impractical redox chemistry and require surface dilution in order to achieve reproducible properties. The NHC SAMs examined in this study exhibit very fast Fc redox kinetics and comparable or even superior stability against the application of multiple potential cycles or long-time holding at constant potential compared to alkylthiol SAMs. Furthermore, ion pairing of Fc+ and hydrophobic perchlorate and other hydrophilic anions is observed with Fc-NHC SAMs, highlighting conditions favorable for future applications of these monolayers. This study should therefore shed light on the very promising characteristics of redox-active NHC SAMs as an alternative to traditional Fc-alkylthiol SAMs for multiple practical applications, including in sensors and electrocatalysis.

Unique Bora[1]ferrocenophanes with Sterically Protected Boron: A Potential Gateway to Helical Polyferrocenes.

Silicon-bridged [1]ferrocenophanes are a versatile class of monomers to obtain well-defined metallopolymers, however, their boron-bridged analogues are far less utilized despite being significantly higher strained. We assumed that the reactivity of known bora[1]ferrocenophanes towards ring-opening polymerization is hampered by π-donating R2 N groups at the bridging boron atom and therefore prepared the first bora[1]ferrocenophanes lacking such electronic stabilization. The new, isolated ferrocenophane with a 2,4,6-triisopropylphenyl group attached to the bridging boron atom exhibits the most tilted Cp rings among all isolated strained sandwich compounds [α(DFT)=33.3°] with a measured record value of the bathochromic shift (λmax =516 nm). Attempts to purify the mesityl analogue by vacuum sublimation transformed this monomer to a purple-colored polymer that resulted in Cotton effects in circular dichroism spectroscopy. DFT calculations revealed a left-handed helical structure for this polymer. This is the first evidence for a polyferrocene with a chiral secondary structure.

Chiral bora[1]ferrocenophanes: syntheses, mechanistic insights, and ring-opening polymerizations.

A series of new boron-bridged [1]ferrocenophanes ([1]FCPs) was prepared by salt-metathesis reactions between enantiomerically pure dilithioferrocenes and amino(dichloro)boranes (Et2 NBCl2 , iPr2 NBCl2 , or tBu(Me3 Si)NBCl2 ). The dilithioferrocenes were prepared in situ by lithium-bromine exchange from the respective planar-chiral dibromides (Sp ,Sp )-[1-Br-2-(HR2 C)H3 C5 ]2 Fe (R=Me or Et). In most of the cases, mixtures of the targeted [1]FCPs 4 and the unwanted 1,1'-bis(boryl)ferrocenes 5 were formed. The product ratio depends on the bulkiness of the amino group, the speed of addition of the amino(dichloro)borane, the alkyl group on Cp rings, and in particular on the reaction temperature. The formation of strained [1]FCPs is strongly favored by increased reaction temperatures. Secondly, CHEt2 groups at Cp rings favored the formation of the targeted [1]FCPs stronger than CHMe2 groups. These discoveries open up new possibilities to further suppress the formation of unwanted byproducts by a careful choice of the reaction temperature and through tailoring the bulkiness of CHR2 groups on ferrocene. Thermal ring-opening polymerizations of selected boron-bridged [1]FCPs gave metallopolymers with a Mw of 10 kDa (GPC).

N-Heterocyclic carbenes meet toll-like receptors.

Combining the stability of the N-heterocyclic carbenes (NHCs) and broad-spectrum recognition of toll-like receptor (TLR) proteins, we report new electrochemical biosensors for bacteria detection. Instead of traditional thiol-gold chemistry, newly synthesized NHCs are employed as the linker molecules to immobilize TLR bio-recognition elements on gold electrodes. Our proof-of-concept methodology includes testing the fidelity of TLR-based electrochemical sensors with NHC linkers. The performance of the biosensors is demonstrated using whole-cell bacterial cultures.

Strained azabora[2]ferrocenophanes.

Three [2]ferrocenophanes equipped with unsaturated BN moieties at bridging positions were synthesized and structurally characterized. As revealed by DFT calculations, these first examples of azabora[2]ferrocenophanes are similarly strained to the well-known Me2Si-bridged [1]ferrocenophane.

A hydrogen bond scaffold supported synthetic heme Fe(III)-O2(-) adduct.

A hydrogen bonded heme-Fe(III)-O(2)(-) adduct is stabilized and characterized using resonance Raman and EPR spectroscopy. The low O-O vibrations of this complex are quite different from those reported for other heme-Fe(III)-O(2)(-) adducts.