Here we report the synthesis and characterization of a BF(2)-azo complex that can be induced to isomerize without the need of deleterious UV light. The complexation of the azo group with BF(2), coupled with the extended conjugation of the N═N π-electrons, increases the energy of the n-π* transitions and introduces new π-nonbonding (π(nb)) to π* transitions that dominate the visible region. The well separated π(nb)-π* transitions of the trans and cis isomers enable the efficient switching of the system by using only visible light. The complexation also leads to a slow cis → trans thermal relaxation rate (t(1/2) = 12.5 h). Theoretical calculations indicate that the absorption bands in the visible range can be tuned using different Lewis acids, opening the way to a conceptually new strategy for the manipulation of azo compounds using only visible light.

Increasing the electron density in BF2-coodinated azo compounds through para-substitution leads to a bathochromic shift in their activation wavelength. When the substituent is dimethyl amine, or the like, the trans/cis isomerization process can be efficiently modulated using near infrared light. The electron donating capability of the substituent also controls the hydrolysis half-life of the switch in aqueous solution, which is drastically longer for the cis isomer, while the BF2-coodination prevents reduction by glutathione.

Imidazole, a subunit of histidine, plays a crucial role in proton-relay processes that are important for various biological activities, such as metal efflux, viral replication and photosynthesis. We show here how an imidazolyl ring incorporated into a rotary switch based on a hydrazone enables a switching cascade that involves proton relay between two different switches. The switching process starts with a single input, zinc(II), that initiates an E/Z isomerization in the hydrazone system through a coordination-coupled proton transfer. The resulting imidazolium ring is unusually acidic and, through proton relay, activates the E/Z isomerization of a non-coordinating pyridine-containing hydrazone switch. We hypothesize that the reduction in the acid dissociation constant of the imidazolium ring results from a combination of electrostatic and conformational effects, the study of which might help elucidate the proton-coupled electron-transfer mechanism in photosynthetic bacteria.

Cyclopentadienes bearing varying length fluorinated “ponytails”, C5H5[(CH2) n (CF2) m F] ([n.m] = [0.8], [0.10], [0.12], [2.6], [2.8], and [2,10]) have been prepared by reaction of the corresponding fluoroalkyl iodides and triphenylphosphine with nickelocene. From these, the 1,1‘-bis[2.6]-, 1,1‘-bis[2.8]-, and 1,1‘-bis[2.10]-disubstituted ferrocenes and [2.10]-substituted [Mn(CO)3L], [Re(CO)3L], and [Co(CO)2L] derivatives (L = η5-C5H4[2.10]) were prepared. Although the anions of the [0.m] compounds were thermally unstable, the corresponding [0.8], [0.10], and [0.12] cobalt dicarbonyl complexes were prepared directly from the cyclopentadienes. The solubilities of these compounds in both saturated fluorocarbon (fluorous) and conventional solvents was found to depend strongly on the length of the fluoroalkyl tail and on the temperature. These compounds all dissolve in fluorous solvents, and the 1,1‘-bis[2.10]ferrocene partitions selectively into the fluorous layer of a biphasic perfluoro(methylcyclohexane)/toluene system. The oxidation potentials of the 1,1‘-, bis[2.6]-, and 1,1‘-bis[2.10]ferrocenes are only very slightly more positive than ferrocene itself, demonstrating that the C2H4 spacer effectively insulates the electron-withdrawing effect of the fluoroalkyl part of the ponytail from the metal. A similar insulating effect is noted in the carbonyl stretching frequencies of [Mn(CO)3L], [Re(CO)3L], and [Co(CO)2L] derivatives (L = η5-C5H4[2.10]), which are slightly to low energy of those in the corresponding parent cyclopentadienyl analogues. In contrast, the corresponding frequencies in [Co(CO)2L] derivatives (L = η5-C5H4[0.10]; η5-C5H4[0.12]) without the hydrocarbon spacer group are at higher energy than the parent compound.

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Ozerov and colleagues describe the synthesis and characterization of linear, twocoordinate, cationic phosphine complexes of monovalent Pd and Pt. Comparison of the structures of these complexes to the neutral, zero-valent analogs revealed significant elongation of the M-P bond upon oxidation. Computational studies offer an explanation for the observed phenomenon, showing that molecularorbital-based arguments alone cannot provide a satisfactory rationalization. HIGHLIGHTS Two-coordinate, cationic complexes of monovalent Pd and Pt are synthesized The monovalent cations possess longer, stronger M-L bonds than their zero-valent analogs Theoretical consideration of electrostatic and Pauli effects offers an explanation MacInnis et al., Chem 1, 902-920 December 8, 2016 ª 2016 Elsevier Inc. http://dx.SUMMARY One-electron oxidation of known ( t Bu 3 P) 2 M (1, M = Pd; 2, M = Pt) with [Ph 3 C] [HCB 11 Cl 11 ] leads to two-coordinate, monovalent cations of the formula [( t Bu 3 P) 2 M][HCB 11 Cl 11 ] (3, M = Pd; 4, M = Pt), which also possess linear geometry but with elongated M-P bonds. Spectroscopic and computational studies consistently show that the unpaired electron of the d 9 configuration of 3 and 4 belongs to largely non-bonding orbitals: the s/d z2 hybrid for Pd and the degenerate d x2-y2 /d xy pair for Pt. We show that molecular-orbital-based arguments alone are incapable of predicting or rationalizing the observed M-P bond lengthening on oxidation; correct prediction and rationalization are achieved only by inclusion of electrostatic and Pauli effects. This emphasizes the dangers of interpreting any perturbative changes in bond metrics solely on the basis of energies and occupancies of molecular orbitals; the inclusion of electrostatic and Pauli components is essential to providing a more complete picture. 7 8 9 10 Scheme 1. Synthesis and Reactivity of Pd(I) and Pt(I) Complexes

Primary perfluoroalkyl compounds of iridium undergo facile two-electron reduction to afford a simple route to difluorocarbene and perfluoroalkylidene complexes, two of which have been crystallographically characterized. Low-temperature protonation reactions illustrate that the thermodynamic site of protonation depends on the nature of the fluorinated carbene; the difluorocarbene complex undergoes protonation at iridium, while its perfluoroethylidene congener undergoes protonation at carbon.