Robert W. Schurko

Metal–organic frameworks with dynamic interlocked components

The dynamics of mechanically interlocked molecules such as rotaxanes and catenanes have been studied in solution as examples of rudimentary molecular switches and machines, but in this medium, the molecules are randomly dispersed and their motion incoherent. As a strategy for achieving a higher level of molecular organization, we have constructed a metal-organic framework material using a [2]rotaxane as the organic linker and binuclear Cu(II) units as the nodes. Activation of the as-synthesized material creates a void space inside the rigid framework that allows the soft macrocyclic ring of the [2]rotaxane to rotate rapidly, unimpeded by neighbouring molecular components. Variable-temperature (13)C and (2)H solid-state NMR experiments are used to characterize the nature and rate of the dynamic processes occurring inside this unique material. These results provide a blueprint for the future creation of solid-state molecular switches and molecular machines based on mechanically interlocked molecules.

Application of solid-state 35Cl NMR to the structural characterization of hydrochloride pharmaceuticals and their polymorphs.

Solid-state (35)Cl NMR (SSNMR) spectroscopy is shown to be a useful probe of structure and polymorphism in HCl pharmaceuticals, which constitute ca. 50% of known pharmaceutical salts. Chlorine NMR spectra, single-crystal and powder X-ray diffraction data, and complementary ab initio calculations are presented for a series of HCl local anesthetic (LA) pharmaceuticals and some of their polymorphs. (35)Cl MAS SSNMR spectra acquired at 21.1 T and spectra of stationary samples at 9.4 and 21.1 T allow for extraction of chlorine electric field gradient (EFG) and chemical shift (CS) parameters. The sensitivity of the (35)Cl EFG and CS tensors to subtle changes in the chlorine environments is reflected in the (35)Cl SSNMR powder patterns. The (35)Cl SSNMR spectra are shown to serve as a rapid fingerprint for identifying and distinguishing polymorphs, as well as a useful tool for structural interpretation. First principles calculations of (35)Cl EFG and CS tensor parameters are in good agreement with the experimental values. The sensitivity of the chlorine NMR interaction tensor parameters to the chlorine chemical environment and the potential for modeling these sites with ab initio calculations hold much promise for application to polymorph screening for a wide variety of HCl pharmaceuticals.

Probing the structural origins of vapochromism of a triarylboron-functionalized platinum(II) acetylide by optical and multinuclear solid-state NMR spectroscopy.

A vapoluminescent triarylboron-functionalized platinum(II) complex that displays a mechanism of vapochromism differing from all previously reported platinum(II) compounds has been synthesized. The luminescence color of 1 switches in response to many volatile organic compounds in the solid state, including hexanes, CH(2)Cl(2), benzene, and methanol. While vapochromism due to changes in Pt-Pt or π-π stacking interactions has been commonly observed, absorption and luminescence studies and single-crystal and powder X-ray diffraction data as well as multinuclear solid-state NMR experiments ((195)Pt, (13)C, (11)B, (2)H, and (1)H) revealed that the vapochromic response of 1 is instead due to changes in the excited-state energy levels resulting from local interactions of solvent molecules with the complex. Furthermore, these interactions result in inversion of the lowest-energy excited states of the complex in some cases, the first observation of this phenomenon in the solid state.

Interaction tensors and local dynamics in common structural motifs of nitrogen: a solid-state 14N NMR and DFT study.

(14)N solid-state NMR powder patterns have been obtained at high field (21.1 T) using broadband, frequency-swept pulses and a piecewise acquisition method. This approach allowed the electric field gradient (EFG) tensor parameters to be obtained from model organic and inorganic systems featuring spherically asymmetric nitrogen environments (C(Q) values of up to ca. 4 MHz). The advantages and limitations of this experimental approach are discussed, and the observation of (14)N T(2) relaxation anisotropy in certain systems is also reported, which can shed light on dynamic processes, allowing motional geometries and jump rates to be probed. In particular, we show that observable effects of dynamics on (14)N spectra can be mediated by modulation of either the EFG tensor or heteronuclear dipolar couplings. It is demonstrated that the QCPMG protocol can be used to selectively enhance certain types of nitrogen environments on the basis of differences in T(2). We also present the results of extensive density functional theory calculations on these systems, which show remarkably good correlation with the experimental results and allow the prediction of tensor orientations, assignment of parameters to crystallographic sites, and a rationalization of the origin of the EFG tensors in terms of contributions from individual molecular orbitals. This work demonstrates that ultra-wideline (14)N solid-state NMR can, under favorable circumstances, be a straightforward, useful, and informative probe of molecular structure and dynamics.

Signal enhancement in NMR spectra of half-integer quadrupolar nuclei via DFS–QCPMG and RAPT–QCPMG pulse sequences

Abstract Two signal enhancement schemes capable of providing large signal-to-noise gains in static and magic-angle spinning NMR spectra of the central transition of half-integer quadrupolar nuclei are presented. Amplitude-modulated double-frequency sweeps (AM-DFS) and rotor-assisted population transfer (RAPT) pulse sequences are used as preparatory sequences in combination with the quadrupolar Carr–Purcell Meiboom–Gill (QCPMG) pulse sequence to obtain large signal enhancements ranging from one to two orders of magnitude. Specific applications to two spin-3/2 nuclei ( 87 Rb and 39 K) and one spin-5/2 nucleus ( 85 Rb) are presented.

Metal–Organic Frameworks with Mechanically Interlocked Pillars: Controlling Ring Dynamics in the Solid-State via a Reversible Phase Change

Metal-organic framework (MOF) materials have been prepared that contain a mechanically interlocked molecule (MIM) as the pillaring strut between two periodic Zn-carboxylate layers. The MIM linker is a [2]rotaxane with a [24]crown-6 (24C6) macrocycle and an aniline-based axle with terminal pyridine donor groups. The single-crystal X-ray structures of MOFs UWDM-2 (1,4-diazophenyl-dicarboxylate) and UWDM-3 (1,4-biphenyl-dicarboxylate) show that both frameworks are large enough to contain the free volume required for rotation of the interlocked 24C6 macrocycle, but the frameworks are interpenetrated (UWDM-2, three-fold, and UWDM-3, two-fold). In particular, for UWDM-3 the 24C6 rings of the pillaring MIM are positioned directly inside the square openings of neighboring zinc dicarboxylate layers. Variable-temperature (VT) (2)H SSNMR demonstrated that the 24C6 macrocycles in UWDM-2 and UWDM-3 can only undergo restricted motions related to ring flexibility or partial rotation but are incapable of undergoing free rotation. VT-powder X-ray diffraction studies showed that upon activation of UWDM-3, by removing solvent, a phase change occurs. The new β-phase of UWDM-3 retained crystallinity, and (2)H SSNMR demonstrated that the 24C6 macrocyclic ring of the pillared MIM strut is now free enough to undergo full rotation. Most importantly, the phase change is reversible; the β version of the MOF can be reverted to the original α state by resolvation, thus demonstrating, for the first time, that the dynamics of a MIM inside a solid material can be controlled by a reversible phase change.

Solid-state chlorine NMR of group IV transition metal organometallic complexes.

Static solid-state (35)Cl (I = (3)/(2)) NMR spectra of the organometallic compounds Cp(2)TiCl(2), CpTiCl(3), Cp(2)ZrCl(2), Cp(2)HfCl(2), Cp*(2)ZrCl(2), CpZrCl(3), Cp*ZrCl(3), Cp(2)ZrMeCl, (Cp(2)ZrCl)(2)mu-O, and Cp(2)ZrHCl (Schwartzs reagent) have been acquired at 9.4 T with the quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) sequence in a piecewise manner. Spectra of several samples have also been acquired at 21.1 T. The electric field gradient (EFG) tensor parameters, the quadrupolar coupling constant (C(Q)) and quadrupolar asymmetry parameter (eta(Q)), are readily extracted from analytical simulations of the spectra. The (35)Cl EFG and chemical-shift tensor parameters are demonstrated to be sensitive probes of metallocene structure and allow for differentiation of monomeric and oligomeric structures. First-principles calculations of the (35)Cl EFG parameters successfully reproduce the experimental values and trends. The origin of the observed values of C(Q)((35)Cl) are further examined with natural localized molecular orbital (NLMO) analyses. The combination of experimental and theoretical methods applied to the model compounds are employed to structurally characterize Schwartzs reagent (Cp(2)ZrHCl), for which a crystal structure is unavailable. Aside from a few select examples of single-crystal NMR spectra, this is the first reported application of solid-state (35)Cl NMR spectroscopy to molecules with covalently bound chlorine atoms. It is anticipated that the methodology outlined herein will find application in the structural characterization of a wide variety of chlorine-containing transition-metal and main-group systems.

Fast and simple acquisition of solid-state 14N NMR spectra with signal enhancement via population transfer.

A new approach for the acquisition of static, wideline (14)N NMR powder patterns is outlined. The method involves the use of frequency-swept pulses which serve two simultaneous functions: (1) broad-band excitation of magnetization and (2) signal enhancement via population transfer. The signal enhancement mechanism is described using numerical simulations and confirmed experimentally. This approach, which we call DEISM (Direct Enhancement of Integer Spin Magnetization), allows high-quality (14)N spectra to be acquired at intermediate field strengths in an uncomplicated way and in a fraction of the time required for previously reported methods.

Static solid-state 14N NMR and computational studies of nitrogen EFG tensors in some crystalline amino acids

The recently reported direct enhancement of integer spin magnetization (DEISM) methodology for signal enhancement in solid-state NMR of integer spins has been used to obtain static (14)N powder patterns from alpha-glycine, L-leucine and L-proline in relatively short experimental times at 9.4 T, allowing accurate determination of the quadrupolar parameters. Proton decoupling and deuteration of the nitrogen sites were used to reduce the (1)H-(14)N dipolar contribution to the transverse relaxation time allowing more echoes to be acquired per scan. In addition, ab initio calculations using molecular clusters (Gaussian 03) and the full crystal lattice (CASTEP) have been employed to confirm these results, to obtain the orientation of the electric field gradient (EFG) tensors in the molecular frame, and also to correctly assign the two sets of parameters for L-leucine. The (14)N EFG tensor is shown to be highly sensitive to the surrounding environment, particularly to nearby hydrogen bonding.

Broadband adiabatic inversion pulses for cross polarization in wideline solid-state NMR spectroscopy.

Efficient acquisition of ultra-wideline solid-state NMR powder patterns is a continuing challenge. In particular, when the breadth of the powder pattern is much larger than the cross-polarization (CP) excitation bandwidth, transfer efficiencies suffer and experimental times are greatly increased. Presented herein is a CP pulse sequence with an excitation bandwidth that is up to ten times greater than that available from a conventional spin-locked CP pulse sequence. The pulse sequence, broadband adiabatic inversion CP (BRAIN-CP), makes use of the broad, uniformly large frequency profiles of chirped inversion pulses, to provide these same characteristics to the polarization transfer process. A detailed theoretical analysis is given, providing insight into the polarization transfer process involved in BRAIN-CP. Experiments on spin-1/2 nuclei including (119)Sn, (199)Hg and (195)Pt nuclei are presented, and the large bandwidth improvements possible with BRAIN-CP are demonstrated. Furthermore, it is shown that BRAIN-CP can be combined with broadband frequency-swept versions of the Carr-Purcell-Meiboom-Gill experiment (for instance with WURST-CPMG, or WCPMG for brevity); the combined BRAIN-CP/WCPMG experiment then provides multiplicative signal enhancements of both CP and multiple-echo acquisition over a broad frequency region.