Jenna A. Bilbrey

Exact ligand cone angles.

Many properties of transition‐metal complexes depend on the steric bulk of bound ligands, usually quantified by the Tolman (θ) and solid (θ) cone angles, which have proven utility but suffer from various limitations and coarse approximations. Here, we present an improved, mathematically rigorous method to determine an exact cone angle (θ°) by solving for the most acute right circular cone that contains the entire ligand. The procedure is applicable to any ligand, planar or nonplanar, monodentate or polydentate, bound to any metal center in any environment, and it is ideal for analyzing structures from quantum chemical computations as well as X‐ray crystallography experiments. Exact cone angles were evaluated for a wide array of phosphine and amine ligands bound to palladium, nickel, or platinum by optimizing structures using B3LYP/6‐31G* density functional theory with effective core potentials for the transition metals. The mean absolute deviations of the standard θ and θ parameters from the exact cone angles were 15–25°, mostly caused by distortions from the assumed idealized structures.

Palladium‐Mediated Surface‐Initiated Kumada Catalyst Polycondensation: A Facile Route Towards Oriented Conjugated Polymers

Palladium-mediated surface-initiated Kumada catalyst transfer polycondensation is used to generate poly(3-methyl thiophene) films with controlled thickness up to 100 nm. The palladium initiator density is measured using cyclic voltammetry and a ferrocene-capping agent, where the surface density is found to be 55% (1.1 × 10(14) molecules per cm(2)). UV-Vis spectroscopy and AFM show increased aggregation in palladium-initiated films due to the higher grafting density of palladium initiators on the surface. The anisotropy of the P3MT films is determined using polarized UV-Vis spectroscopy, which indicates a degree of orientation perpendicular to the substrate. Evidence that palladium can maintain π-complexation even at elevated temperatures, is also shown through the exclusive intramolecular coupling of both a phenyl and thiophene-based magnesium bromide with different dihaloarenes.

π-Complexation in Nickel-Catalyzed Cross-Coupling Reactions

The kinetic isotope effect (KIE) is used to experimentally elucidate the first irreversible step in oxidative addition reactions of a zerovalent nickel catalyst to a set of haloarene substrates. Halogenated o-methylbenzene, dimethoxybenzene, and thiophene derivatives undergo intramolecular oxidative addition through irreversible π-complexation. Density functional theory computations at the B3LYP-D3/TZ2P-LANL2TZ(f)-LANL08d level predict η(2)-bound π-complexes are generally stable relative to a solvated catalyst plus free substrate and that ring-walking of the Ni(0) catalyst and intramolecular oxidative addition are facile in these intermediates.

Tuning chelating groups and comonomers in spiropyran-containing copolymer thin films for color-specific metal ion binding

Photochromic molecules can be used to selectively bind divalent metal ions. In order to study the influence of increased chelation in spiropyran-containing copolymers, we synthesized two derivatives: spiropyran methacrylate (SPMA) and spiropyran methacrylate with a methoxy substituent in the 8′ position of the benzopyran ring (MEO). Additionally, the comonomer with which spiropyran was polymerized is also varied between methyl methacrylate (MMA) and 2,2,2-trifluoroethyl methacrylate (TFEMA) to tune the colorimetric response. Fourier transform-infrared (FT-IR) spectroscopy was used to characterize the photoinduced conversion of spiropyran to merocyanine, as well as the merocyanine–metal ion (MC–M2+) interaction. By means of UV-Vis absorption spectroscopy, we demonstrate that each metal ion gives rise to a unique colorimetric response for the various spiropyran-containing copolymers studied.

Rapid electrochemical reduction of Ni(II) generates reactive monolayers for conjugated polymer brushes in one step.

This article reports the development of a robust, one-step electrochemical technique to generate surface-bound conjugated polymers. The electrochemical reduction of arene diazonium salts at the surface of a gold electrode is used to generate tethered bromobenzene monolayers quickly. The oxidative addition of reactive Ni(0) across the aryl halide bond is achieved in situ through a concerted electrochemical reduction of Ni(dppp)Cl2. This technique limits the diffusion of Ni(0) species away from the surface and overcomes the need for solution deposition techniques which often require multiple steps that result in a loss of surface coverage. With this electrochemical technique, the formation of the reactive monolayer resulted in a surface coverage of 1.29 × 10(14) molecules/cm(2), which is a 6-fold increase over previously reported results using solution deposition techniques.

Exact Ligand Solid Angles.

Steric demands of a ligand can be quantified by the area occluded by the ligand on the surface of an encompassing sphere centered at the metal atom. When viewed as solid spheres illuminated by the metal center, the ligand atoms generally cast a very complicated collective shadow onto the encompassing sphere, causing mathematical difficulties in computing the subtended solid angle. Herein, an exact, analytic solution to the ligand solid angle integration problem is presented based on a line integral around the multisegmented perimeter of the ligand shadow. The solution, which is valid for any ligand bound to any metal center, provides an excellent method for analyzing geometric structures from quantum chemical computations or X-ray crystallography. Over 275 structures of various metals bound to diverse mono- and multidentate ligands were optimized using B3LYP density functional theory to exhibit exact solid angle (Ω°) computations. Among the intriguing Ω° solutions, Pd(xantphos) and ferrocene exhibit holes in their ligand shadows, and Fe(EDTA)(2-) has a surprisingly simple shadow defined by only four arcs, despite having a multitude of overlaps among individual shadow cones.

Ligand Steric Descriptors

Abstract Descriptors for the steric size of transition metal-bound ligands suffer from coarse approximations and complicated correction schemes. Common methods such as the Tolman cone angle and solid angle set a universal value for each ligand regardless of the effect of steric environment on ligand geometry. We have developed two approaches to quantify sterics that improve upon the previous descriptors: the exact cone angle (θ°) and the exact solid angle (Θ°). The exact cone angle encapsulates a ligand inside of the smallest possible right circular cone without making any approximations concerning metal–ligand bond length or ligand geometry. The exact solid angle quantifies the area screened by a ligand on the surface of a surrounding sphere centred at the metal atom. While the cone angle only describes a single ligand, the solid angle accounts for the sterics of an entire complex. Herein, the mathematics behind each is described and numerous examples are given. An intriguing case models the change in sterics of first- and second-generation Grubbs’ catalysts over the course of a ligand dissociation reaction.

Ring-Walking of Zerovalent Nickel on Aryl Halides

While ring-walking is a critical step in transition metal catalyzed cross-coupling reactions, the associated metastable intermediates are often difficult to isolate and characterize. In this work, theoretical structures and energetics for ring-walking and oxidative addition of zerovalent nickel with 1-bromo-2-methylbenzene, 2-bromopyridine, 2-bromo-3-methyl-thiophene, and 2-bromopyrrole were computed at the B3LYP-D3/TZ2P-LANL2TZ(f)-LANL08d level. The mechanisms vary qualitatively with substrate ring size and type-the catalyst weaves along the edges of the benzene and pyridine rings, cuts through the interior of the thiophene ring, and arcs along the bond opposite the nitrogen atom in the pyrrole ring. Analogous computations on the ring-walking and oxidative addition of zerovalent palladium with 1-bromo-2-methylbenzene reveal an energetic profile similar to that of Ni but with much weaker overall binding to the arene. In all cases, dispersion corrections are found to be very important for computing accurate metal-substrate binding energies.