Roderick E. Wasylishen

A nickel complex incorporating an N₂O ligand with a rare η²‐N,N′‐coordination mode was isolated and characterized by X‐ray crystallography, as well as by IR and solid‐state NMR spectroscopy augmented by ¹⁵N‐labeling experiments. The isoelectronic nickel CO₂ complex reported for comparison features a very similar solid‐state structure. Computational studies revealed that η²‐N₂O binds to nickel slightly stronger than η²‐CO₂ in this case, and comparably to or slightly stronger than η²‐CO₂ to transition metals in general. Comparable transition‐state energies for the formation of isomeric η²‐N,N′‐ and η²‐N,O‐complexes, and a negligible activation barrier for the decomposition of the latter likely account for the limited stability of the N₂O complex.

A nickel complex incorporating an N₂O ligand with a rare η²‐N,N′‐coordination mode was isolated and characterized by X‐ray crystallography, as well as by IR and solid‐state NMR spectroscopy augmented by ¹⁵N‐labeling experiments. The isoelectronic nickel CO₂ complex reported for comparison features a very similar solid‐state structure. Computational studies revealed that η²‐N₂O binds to nickel slightly stronger than η²‐CO₂ in this case, and comparably to or slightly stronger than η²‐CO₂ to transition metals in general. Comparable transition‐state energies for the formation of isomeric η²‐N,N′‐ and η²‐N,O‐complexes, and a negligible activation barrier for the decomposition of the latter likely account for the limited stability of the N₂O complex.

Flexible, chelating bis(NHC) ligand 2, able to accommodate both cis‐ and trans‐coordination modes, was used to synthesize (2)Ni(η²‐cod), 3. In reaction with GeCl₂, it produced (2)NiGeCl₂, 4, featuring a T‐shaped Ni⁰ and a pyramidal Ge center. Complex 4 could also be prepared from [(2)GeCl]Cl, 5, and Ni(cod)₂, in a reaction that formally involved Ni–Ge transmetalation, followed by coordination of the extruded GeCl₂ moiety to Ni. A computational analysis showed that 4 possesses considerable multiconfigurational character and the Ni→Ge bond is formed through σ‐donation from the Ni 4s, 4p, and 3d orbitals to Ge. (NHC)₂Ni(cod) complexes 9 and 10, as well as (NHC)₂GeCl₂ derivative 11, incorporating ligands that cannot accommodate a wide bite angle, failed to produce isolable Ni–Ge complexes. The isolation of (2)Ni(η²‐Py), 12, provides further evidence for the reluctance of the (2)Ni⁰ fragment to act as a σ‐Lewis acid.

Flexible, chelating bis(NHC) ligand 2, able to accommodate both cis‐ and trans‐coordination modes, was used to synthesize (2)Ni(η²‐cod), 3. In reaction with GeCl₂, it produced (2)NiGeCl₂, 4, featuring a T‐shaped Ni⁰ and a pyramidal Ge center. Complex 4 could also be prepared from [(2)GeCl]Cl, 5, and Ni(cod)₂, in a reaction that formally involved Ni–Ge transmetalation, followed by coordination of the extruded GeCl₂ moiety to Ni. A computational analysis showed that 4 possesses considerable multiconfigurational character and the Ni→Ge bond is formed through σ‐donation from the Ni 4s, 4p, and 3d orbitals to Ge. (NHC)₂Ni(cod) complexes 9 and 10, as well as (NHC)₂GeCl₂ derivative 11, incorporating ligands that cannot accommodate a wide bite angle, failed to produce isolable Ni–Ge complexes. The isolation of (2)Ni(η²‐Py), 12, provides further evidence for the reluctance of the (2)Ni⁰ fragment to act as a σ‐Lewis acid.

In light of the intense recent interest in the methylammonium lead halides, CH₃NH₃PbX₃ (X = Cl, Br and I) as sensitizers for photovoltaic cells, the dynamics of the methylammonium (MA) cation in these perovskite salts has been reinvestigated as a function of temperature via ²H, ¹⁴N and ²⁰⁷Pb NMR spectroscopy. In the cubic phase of all three salts, the MA cation undergoes pseudo-isotropic tumbling (ps time scale). For example, the correlation time, τ₂, for the C−N axis of the iodide salt is 0.85 ± 0.30 ps at 330 K. The dynamics of the MA cation are essentially continuous across the cubic ↔ tetragonal phase transition; however ²H and ¹⁴N NMR lineshapes indicate that subtle ordering of the MA cation occurs in the tetragonal phase. The temperature dependence of the cation ordering is rationalized using a six-site model, with two equivalent sites along the c-axis and four equivalent sites either perpendicular or approximately perpendicular to this axis. As the cubic ↔ tetragonal phase transition temperature is approached, the six sites are nearly equally populated. Below the tetragonal ↔ orthorhombic phase transition, ²H NMR lineshapes indicate that the C─N axis is essentially frozen.

We report a combined solid-state (¹H, ²H, ¹³C, ¹⁷O) NMR and plane-wave density functional theory (DFT) computational study of the O···H···O low-barrier hydrogen bonds (LBHBs) in two 1,3-diketone compounds: dibenzoylmethane (1) and curcumin (2). In the solid state, both 1 and 2 exist in the cis-keto–enol tautomeric form, each exhibiting an intramolecular LBHB with a short O···O distance (2.435 Å in 1 and 2.455 Å in 2). Whereas numerous experimental (structural and spectroscopic) and computational studies have been reported for the enol isomers of 1,3-diketones, a unified picture about the proton location within an LBHB is still lacking. This work reports for the first time the solid-state ¹⁷O NMR data for the O···H···O LBHBs in 1,3-diketones. The central conclusion of this work is that detailed information about the probability density distribution of the proton (nuclear zero-point motion) across an LBHB can be obtained from a combination of solid-state NMR and plane-wave DFT computations (both NMR parameter calculations and ab initio molecular dynamics simulations). We propose that the precise proton probability distribution across an LBHB should provide a common basis on which different and sometimes seemingly contradicting experimental results obtained from complementary techniques, such as X-ray diffraction, neutron diffraction, and solid-state NMR, can be reconciled.