Edwin Otten

Reversible Metal-Free Carbon Dioxide Binding by Frustrated Lewis Pairs†

Frustrated Lewis pairs comprising phosphine and borane react to reversibly bind and release CO2, offering a rare example of metal-free CO2 sequestration. The mechanism of formation of CO2 derivatives by almost simultaneous P-C and O-B bond formation was characterized by quantum chemical calculations.

Complexation of Nitrous Oxide by Frustrated Lewis Pairs

Frustrated Lewis pairs comprised of a basic yet sterically encumbered phosphine with boron Lewis acids bind nitrous oxide to give intact PNNOB linkages. The synthesis, structure, and bonding of these species are described.

The synthesis and exchange chemistry of frustrated Lewis pair–nitrous oxide complexes

Facile activation of nitrous oxide is achieved using a series of fluoroarylboranes, B(C6F5)3, PhB(C6F5)2, MesB(C6F5)2, (C6F5)2BOC6F5, B(C6F4-p-H)3, B(C6H4-p-F)3 and 1,4-(C6F5)2BC6F4B(C6F5)2 in the presence of the basic, bulky phosphinetBu3P. Room temperature reaction yields mono- and bis-zwitterionic species of the form tBu3P(N2O)B(C6F5)2R (R = C6F51, Ph 2, Mes 3, OC6F54), tBu3P(N2O)BR3 (R = C6F4-p-H 5, C6H4-p-F 6) and tBu3P(N2O)B(C6F5)2C6F4(C6F5)2B(ON2)PtBu37. N2O activation is similarly achieved using Cy3P and B(C6F4-p-H)3 yielding the zwitterionic species Cy3P(N2O)B(C6F4-p-H)38. Reaction of 6 with [Ph3C][B(C6F5)4] results in facile transfer of the robust tBu3P(N2O) fragment to the stronger Lewis acid Ph3C+ generating [tBu3P(N2O)CPh3][B(C6F5)4] 10. Similarly exchange reactions with titanocene and zirconocene cations generate transition metal and phosphine stabilized nitrous oxide salts, of the form [tBu3P(N2O)MCp2Me][MeB(C6F5)3], (M = Zr 11, Ti 12). The alkoxy zirconocene cation [Cp*2Zr(OMe)]+ forms an FLP in the presence of tBu3P which reacts with N2O providing a direct synthetic route to the corresponding salt [tBu3P(N2O)ZrCp*2(OMe)][B(C6F5)4] 13. Kinetic studies of the self-exchange reaction of tBu3P(N2O)B(C6H4-p-F)3 with B(C6H4-p-F)3 were carried out acquiring information regarding the mechanism of exchange.

Versatile coordination of cyclopentadienyl-arene ligands and its role in titanium-catalyzed ethylene trimerization

Cationic titanium(IV) complexes with ansa-(eta(5)-cyclopentadienyl,eta(6)-arene) ligands were synthesized and characterized by X-ray crystallography. The strength of the metal-arene interaction in these systems was studied by variable-temperature NMR spectroscopy. Complexes with a C(1) bridge between the cyclopentadienyl and arene moieties feature hemilabile coordination behavior of the ligand and consequently are active ethylene trimerization catalysts. Reaction of the titanium(IV) dimethyl cations with CO results in conversion to the analogous cationic titanium(II) dicarbonyl species. Metal-to-ligand backdonation in these formally low-valent complexes gives rise to a strongly bonded, partially reduced arene moiety. In contrast to the eta(6)-arene coordination mode observed for titanium, the more electron-rich vanadium(V) cations [cyclopentadienyl-arene]V(N(i)Pr(2))(NC(6)H(4)-4-Me)(+) feature eta(1)-arene binding, as determined by a crystallographic study. The three different metal-arene coordination modes that we experimentally observed model intermediates in the cycle for titanium-catalyzed ethylene trimerization. The nature of the metal-arene interaction in these systems was studied by DFT calculations.

Dual stereocontrol over the Henry reaction using a light- and heat-triggered organocatalyst

Here we present a novel dynamic organocatalyst, based on a first-generation molecular motor core, able to control the stereochemical outcome of the Henry reaction using both light and heat as external stimuli.

Formazanate Ligands as Structurally Versatile, Redox-Active Analogues of β-Diketiminates in Zinc Chemistry

A range of tetrahedral bis(formazanate)zinc complexes with different steric and electronic properties of the formazanate ligands were synthesized. The solid-state structures for several of these were determined by X-ray crystallography, which showed that complexes with symmetrical, unhindered ligands prefer coordination to the zinc center via the terminal N atoms of the NNCNN ligand backbone. Steric or electronic modifications can override this preference and give rise to solid-state structures in which the formazanate ligand forms a 5-membered chelate by binding to the metal center via an internal N atom. In solution, these compounds show dynamic equilibria that involve both 5- and 6-membered chelates. All compounds are intensely colored, and the effect of the ligand substitution pattern on the UV-vis absorption spectra was evaluated. In addition, their cyclic voltammetry is reported, which shows that all compounds may be electrochemically reduced to radical anionic (L2Zn(-)) and dianionic (L2Zn(2-)) forms. While unhindered NAr substituents lie in the plane of the ligand backbone (Ar = Ph), the introduction of sterically demanding substituents (Ar = Mes) favors a perpendicular orientation in which the NMes group is no longer in conjugation with the backbone, resulting in hypsochromic shifts in the absorption spectra. The redox potentials in the series of L2Zn compounds may be altered in a straightforward manner over a relatively wide range (∼700 mV) via the introduction of electron-donating or -withdrawing substituents on the formazanate framework.

Reduction of (Formazanate)boron Difluoride Provides Evidence for an N-Heterocyclic B(I) Carbenoid Intermediate

Despite the current interest in structure and reactivity of sub-valent main group compounds, neutral boron analogues of N-heterocyclic carbenes have been elusive due to their high reactivity. Here we provide evidence that 2-electron reduction of a (formazanate)BF2 precursor leads to NaF elimination and formation of an N-heterocyclic boron carbenoid, and describe the formation of a series of unusual BN heterocycles that result from trapping of this fragment. Subsequent chemical oxidation by XeF2 demonstrates that the trapped (formazanate)B fragment retains carbenoid character and regenerates the boron difluoride starting material in good yield. These results indicate that the formazanate ligand framework provides a unique entry into sub-valent boron chemistry.

Locked synchronous rotor motion in a molecular motor

Light-driven rotation of two components in a molecular motor couples synchronously with the motion of a third component. Coupled motion in a light-activated rotor Macroscopic motors rely on gears to keep components in synchrony. Štacko et al. demonstrate an analogous type of coupled motion at the molecular scale (see the Perspective by Baroncini and Credi). They constructed a molecular scaffold in which light absorption drives the rotation of upper and lower fragments around a connecting double bond. At the same time, steric constraints modulate the motion of a third component that is tethered to the top of the rotor, so that it continuously exposes the same face to the bottom. The design paves the way toward more complex synchronized motion in an assembly of molecular machines. Science, this issue p. 964; see also p. 906 Biological molecular motors translate their local directional motion into ordered movement of other parts of the system to empower controlled mechanical functions. The design of analogous geared systems that couple motion in a directional manner, which is pivotal for molecular machinery operating at the nanoscale, remains highly challenging. Here, we report a molecular rotary motor that translates light-driven unidirectional rotary motion to controlled movement of a connected biaryl rotor. Achieving coupled motion of the distinct parts of this multicomponent mechanical system required precise control of multiple kinetic barriers for isomerization and synchronous motion, resulting in sliding and rotation during a full rotary cycle, with the motor always facing the same face of the rotor.

A Metal-Ligand Cooperative Pathway for Intermolecular Oxa-Michael Additions to Unsaturated Nitriles

An unprecedented catalytic pathway for oxa-Michael addition reactions of alcohols to unsaturated nitriles has been revealed using a PNN pincer ruthenium catalyst with a dearomatized pyridine backbone. The isolation of a catalytically competent Ru-dieneamido complex from the reaction between the Ru catalyst and pentenenitrile in combination with DFT calculations supports a mechanism in which activation of the nitrile through metal-ligand cooperativity is a key step. The nitrile-derived Ru-N moiety is sufficiently Brønsted basic to activate the alcohol and initiate conjugate addition of the alkoxide to the α,β-unsaturated fragment. This reaction proceeds in a concerted manner and involves a six-membered transition state. These features allow the reaction to proceed at ambient temperature in the absence of external base.

Ni→B Interactions in Nickel Phosphino-Alkynyl-Borane Complexes

The Ni complexes [{tBu2PC≡CB(C6F5)2}Ni(cod)] and [({tBu2PC≡CB(C6F5)2}Ni(NCMe))2] derived from the reaction between the phosphino-alkynyl-borane tBu2PC≡CB(C6F5)2 and [Ni(cod)2] exhibit an unprecedented metal–alkyne interaction in which the borane substituent bends towards the metal affording a Ni→B dative interaction.