Abstract
Stannous fluoride (SnF
2
) is an effective fluoride source and antimicrobial agent that is widely used in commercial toothpaste formulations. The antimicrobial activity of SnF
2
is partly ...attributed to the presence of Sn(II) ions. However, it is challenging to directly determine the Sn speciation and oxidation state within commercially available toothpaste products due to the low weight loading of SnF
2
(0.454 wt% SnF
2
, 0.34 wt% Sn) and the amorphous, semi-solid nature of the toothpaste. Here, we show that dynamic nuclear polarization (DNP) enables
119
Sn solid-state NMR experiments that can probe the Sn speciation within commercially available toothpaste. Solid-state NMR experiments on SnF
2
and SnF
4
show that
19
F isotropic chemical shift and
119
Sn chemical shift anisotropy (CSA) are highly sensitive to the Sn oxidation state. DNP-enhanced
119
Sn magic-angle turning (MAT) 2D NMR spectra of toothpastes resolve Sn(II) and Sn(IV) by their
119
Sn chemical shift tensor parameters. Fits of DNP-enhanced 1D
1
H →
119
Sn solid-state NMR spectra allow the populations of Sn(II) and Sn(IV) within the toothpastes to be estimated. This analysis reveals that three of the four commercially available toothpastes contained at least 80% Sn(II), whereas one of the toothpaste contained a significantly higher amount of Sn(IV).
Boron‐containing materials have recently been identified as highly selective catalysts for the oxidative dehydrogenation (ODH) of alkanes to olefins. It has previously been demonstrated by several ...spectroscopic characterization techniques that the surface of these boron‐containing ODH catalysts oxidize and hydrolyze under reaction conditions, forming an amorphous B2(OH)xO(3−x/2) (x=0–6) layer. Yet, the precise nature of the active site(s) remains elusive. In this Communication, we provide a detailed characterization of zeolite MCM‐22 isomorphously substituted with boron (B‐MWW). Using 11B solid‐state NMR spectroscopy, we show that the majority of boron species in B‐MWW exist as isolated BO3 units, fully incorporated into the zeolite framework. However, this material shows no catalytic activity for ODH of propane to propene. The catalytic inactivity of B‐MWW for ODH of propane falsifies the hypothesis that site‐isolated BO3 units are the active site in boron‐based catalysts. This observation is at odds with other traditionally studied catalysts like vanadium‐based catalysts and provides an important piece of the mechanistic puzzle.
Solid‐state NMR spectroscopy of hydrothermally synthesized B‐MWW reveals that boron is fully incorporated into the zeolite framework as trivalent BO3 units. These isolated sites are inactive for oxidative dehydrogenation (ODH) of propane, which refutes the hypothesis that site‐isolated boron species are the active site on boron‐containing ODH catalysts.
Al(OC(CF3)3)(PhF) reacts with silanols present on partially dehydroxylated silica to form well‐defined ≡SiOAl(OC(CF3)3)2(O(Si≡)2) (1). 27Al NMR and DFT calculations with a small cluster model to ...approximate the silica surface show that the aluminum in 1 adopts a distorted trigonal bipyramidal coordination geometry by coordinating to a nearby siloxane bridge and a fluorine from the alkoxide. Fluoride ion affinity (FIA) calculations follow experimental trends and show that 1 is a stronger Lewis acid than B(C6F5)3 and Al(OC(CF3)3)(PhF) but is weaker than Al(OC(CF3)3) and iPr3Si+. Cp2Zr(CH3)2 reacts with 1 to form Cp2ZrCH3≡SiOAl(OC(CF3)3)2(CH3) (3) by methide ion. This reactivity pattern is similar to reactions of organometallics with the proposed strong Lewis acid sites present on Al2O3.
Al(OC(CF3)3)3(PhF) reacts with partially dehydroxylated silica to form very strong Lewis acid sites. The well‐defined aluminum Lewis acids are significantly stronger than common molecular Lewis acids and engage in alkyl ion reactions that are uncommon using well‐defined supported Lewis acids.
Hexagonal boron nitride nanosheets (h-BNNS), the isoelectronic analog to graphene, have received interest over the past decade due to their high thermal oxidative resistance, high bandgap, catalytic ...activity, and low cost. The functional groups that terminate boron and nitrogen zigzag and/or armchair edges directly affect their chemical, physical, and electronic properties. However, an understanding of the molecular edge termination present in h-BNNS is lacking. Here, high-resolution magic-angle spinning (MAS) solid-state NMR (SSNMR) spectroscopy, and plane-wave density-functional theory (DFT) calculations are used to determine the molecular edge termination in exfoliated h-BNNS. 1H → 11B cross-polarization MAS (CPMAS) SSNMR spectra of h-BNNS revealed multiple hydroxyl/oxygen coordinated boron edge sites that were not detectable in direct excitation experiments. A dynamic nuclear polarization (DNP)-enhanced 1H → 15N CPMAS spectrum of h-BNNS displayed four distinct 15N resonances while a 2D 1H{14N} dipolar-HMQC spectrum acquired with fast MAS revealed three distinct 14N environments. Plane-wave DFT calculations were used to construct model edge structures and predict the corresponding 11B, 14N and 15N SSNMR spectra. Comparison of the experimental and predicted SSNMR spectra confirms that zigzag and armchair edges with both amine and boron hydroxide/oxide termination are present. The detailed characterization of h-BNNS molecular edge termination will prove useful for many material science applications. The techniques outlined here should also be applicable to understand the molecular edge terminations in other 2D materials.
The silylium‐like surface species iPr3Si(RFO)3Al−OSi≡) activates (N^N)Pd(CH3)Cl (N^N=Ar−N=CMeMeC=N−Ar, Ar=2,6‐bis(diphenylmethyl)‐4‐methylbenzene) by chloride ion ion to form ...(N^N)Pd−CH3(RFO)3Al−OSi≡) (1). A combination of FTIR, solid‐state NMR spectroscopy, and reactions with CO or vinyl chloride establish that 1 shows similar reactivity patterns as (N^N)Pd(CH3)Cl activated with NaB(ArF)4. Multinuclear 13C{27Al} RESPDOR and 1H{19F} S‐REDOR experiments are consistent with a weakly coordinated ion‐pair between (N^N)Pd−CH3+ and (RFO)3Al−OSi≡). 1 catalyzes the polymerization of ethylene with similar activities as (N^N)Pd−CH3+ in solution and incorporates up to 0.4 % methyl acrylate in copolymerization reactions. 1 produces polymers with significantly higher molecular weight than the solution catalyst, and generates the highest molecular weight polymers currently reported in copolymerization reactions of ethylene and methylacrylate.
A silylium‐like ion supported on silica ionizes a Pd−Cl complex by chloride ion to generate a very active catalyst for (co‐)polymerization reactions.
The silylium-like surface species
Pr
Si(R
O)
Al-OSi≡) activates (N^N)Pd(CH
)Cl (N^N=Ar-N=CMeMeC=N-Ar, Ar=2,6-bis(diphenylmethyl)-4-methylbenzene) by chloride ion abstraction to form (N^N)Pd-CH
(R
O)
...Al-OSi≡) (1). A combination of FTIR, solid-state NMR spectroscopy, and reactions with CO or vinyl chloride establish that 1 shows similar reactivity patterns as (N^N)Pd(CH
)Cl activated with NaB(Ar
)
. Multinuclear
C{
Al} RESPDOR and
H{
F} S-REDOR experiments are consistent with a weakly coordinated ion-pair between (N^N)Pd-CH
and (R
O)
Al-OSi≡). 1 catalyzes the polymerization of ethylene with similar activities as (N^N)Pd-CH
in solution and incorporates up to 0.4 % methyl acrylate in copolymerization reactions. 1 produces polymers with significantly higher molecular weight than the solution catalyst, and generates the highest molecular weight polymers currently reported in copolymerization reactions of ethylene and methylacrylate.
Research of the nontraditional polysaccharide gellan gum (GG) is a growing space for the development of novel drug delivery systems due to its tunable physic-mechanical properties, biocompatibility, ...and stability in a wide range of environments. Unfortunately, high temperature crosslinking is often required, representing a limiting factor for the incorporation of thermosensitive therapeutic agents. Here, we demonstrated that GG can be crosslinked at a low temperature (38 °C) using a simple fabrication process that utilizes trilysine as an alternative to traditional mono- or divalent ion crosslinkers. While elevated temperature mixing is still required to form a clear GG solution, crosslinking of 0.5 – 1 % GG (w/v) in the presence of trilysine (0.03 % - 0.05 % w/v) was achieved at 38 °C resulting in hydrogels with suitable working formulations to facilitate syringe loading. Low injection forces (< 20 N), and biocompatibility was evaluated with normal human dermal fibroblast (cell viability > 90 %). Frequency sweep showed a transition from purely liquid-like behavior to gel-like behavior with increased trilysine concentration. A temperature dependent behavior was lost with higher trilysine concentrations, indicating stable hydrogel formation. NMR results suggest that trilysine participates in gelation via both ionic interactions between the primary amines of trilysine and the carboxylate residues of glucuronic acid and hydrogen bonding. Released studies showed that GG hydrogels can entrap and provide sustained release of IgG in relation to the crosslinker, and antibody concentration used, with a burst release within the first 24 h (∼80 % cumulative released) followed by a sustained released for up to 5 days. Overall, findings demonstrate a promising nontoxic injectable hydrogel that requires lower crosslinking temperatures, is simple to manufacture and serves as a carrier of thermosensitive therapeutic agents.
Display omitted
•Trilysine, a peptide-based molecule, is a viable crosslinker for gellan gum, which crosslinks at 38 °C.•Potential crosslinking mechanism is ionic interaction between trilysine and gellan gum, and hydrogen bonding.•IgG release profile varied in relation to crosslinker concentration and antibody loading concentration.•These nontoxic, injectable gellan gum hydrogels can be used to deliver thermosensitive therapeutic agents.
Differentiation of heterocyclic isomers by solution 1H, 13C, and 15N NMR spectroscopy is often challenging due to similarities in their spectroscopic signatures. Here, 13C{14N} solid-state NMR ...spectroscopy experiments are shown to operate as an “attached nitrogen test”, where heterocyclic isomers are easy to distinguish based on one-dimensional nitrogen-filtered 13C solid-state NMR. We anticipate that these NMR experiments will facilitate the assignment of heterocyclic isomers during synthesis and natural product discovery.
Differentiation of heterocyclic isomers by solution
H,
C, and
N NMR spectroscopy is often challenging due to similarities in their spectroscopic signatures. Here,
C{
N} solid-state NMR spectroscopy ...experiments are shown to operate as an "attached nitrogen test", where heterocyclic isomers are easy to distinguish based on one-dimensional nitrogen-filtered
C solid-state NMR. We anticipate that these NMR experiments will facilitate the assignment of heterocyclic isomers during synthesis and natural product discovery.
PDF
