3D printing of renewable building blocks like cellulose nanocrystals offers an attractive pathway for fabricating sustainable structures. Here, viscoelastic inks composed of anisotropic cellulose nanocrystals (CNC) that enable patterning of 3D objects by direct ink writing are designed and formulated. These concentrated inks are composed of CNC particles suspended in either water or a photopolymerizable monomer solution. The shear-induced alignment of these anisotropic building blocks during printing is quantified by atomic force microscopy, polarized light microscopy, and 2D wide-angle X-ray scattering measurements. Akin to the microreinforcing effect in plant cell walls, the alignment of CNC particles during direct writing yields textured composites with enhanced stiffness along the printing direction. The observations serve as an important step forward toward the development of sustainable materials for 3D printing of cellular architectures with tailored mechanical properties.

N-Heterocyclic carbenes (NHCs) have become very popular ligands in transition metal chemistry predominantly due to their efficiency in improving catalyst activities. 1 Their impact, often superior to that of ubiquitous phosphines, has generally been rationalized by the covalent M-C carbene bond and by the strong donor ability of NHCs. 2 Arduengo-type imidazolylidenes have been used most widely (A, Chart 1), presumably because the free carbene is extensively stabilized by heteroatoms adjacent to the carbene, which makes them easy to handle. 3 Pioneering work by Bertrand and others evidenced that singlet carbenes may be isolated also with less pronounced heteroatom stabilization. 4a For example, carbenes B-D were analyzed by crystallography, 4b-d while more reactive E and F have been analyzed in situ. 4e,f In contrast, abnormal 5 carbenes such as G have not been characterized in their free form, perhaps due to the small contribution of the carbenoic resonance form. 6 First results indicate that such abnormal carbenes are stronger donors than carbenes A-E, 7 which should provide new opportunities for catalyst design. 7b,8 Here we have applied the [3 + 2] cycloaddition of acetylenes with azides ("click" chemistry) as a versatile and flexible method 9 for synthesizing heterocycles that are effective precursors for a new class of abnormal carbenes, thus greatly expanding the family of carbene ligands. Substitution of 1,2,3-triazoles at the 1-and 4-position is virtually unlimited due to the accessibility of a large variety of acetylenes and azides. 10 Copper-mediated cyclization allows for introducing functional groups into the triazole framework for diverse purposes, e.g., for surface functionalization, bioconjugate immobilization, or supramolecular applications. 11 We have used the simple triazoles 1a and 1b for further ligand synthesis (Scheme 1). Alkylation of the 3-position using MeI was regioselective according to nuclear Overhauser experiments and afforded the triazolium salts 2 as abnormal NHC precursors.Since free 1,2,3-triazolylidenes tend to decompose, 12 metalation of the triazolium salts 2 was probed by direct metal insertion via C-H bond activation as well as by using a transmetalation protocol. Direct metalation with Pd(OAc) 2 was performed according to methods that have previously been established for palladation of normal and abnormal NHC ligand precursors (Scheme 1). 13 Thermally induced C-H bond activation afforded the dinuclear monocarbene complex 3 as well as minor quantities of a mononuclear dicarbene species. X-ray structural analyses of 3b unambiguously confirmed the connectivity pattern (Figure 1). The Pd-C bond length is 1.967(13) Å and hence relatively short when compared to other Pd-C NHC bond distances. 13 Characteristic for abnormal carbenes, the triazolylidene (trz) ligand displays a small carbene angle (N1-C1-C2, 103°). The heterocycle is twisted out of the palladium square plane by ∼75°, presumably due to the bulk in the R-position of the carbene carbon. In the 13 C{ 1 H} NMR spec...

A series of new piano-stool iron(II) complexes comprising mono-and bidentate chelating N-heterocyclic carbene ligands [Fe(cp)(CO)(NHC)(L)]X have been prepared and analyzed by spectroscopic, electrochemical, crystallographic, and theoretical methods. Selectively substituting the L site with a series of ligands going from carbene to pyridine to CO suggests that CO is the strongest π acceptor, while the behavior of pyridine and carbene is nearly identical. This suggests that in these complexes comprising an electronrich iron(cp)(carbene) fragment, N-heterocyclic carbenes are not pure σ donors but also moderate π acceptors. Theoretical calculations support this bonding model and indicate charge saturation at the metal as key for π back-bonding to N-heterocyclic carbenes. On the basis of voltammetric measurements, the Lever electrochemical parameter of these carbenes has been determined: E L ) +0.29. Systematic substitution of the wingtip groups of the carbene revealed only subtle changes in the electronic properties of the iron center, thus providing a suitable methodology for ligand-induced fine-tuning of the coordinated metal.

To study the electronic interactions in donor–acceptor (D–A) ensembles, D and A fragments are coupled in a single molecule. Specifically, a tetrathiafulvalene (TTF)‐fused dipyrido[3,2‐a:2′,3′‐c]phenazine (dppz) compound having inherent redox centers has been synthesized and structurally characterized. Its electronic absorption, fluorescence emission, photoinduced intramolecular charge transfer, and electrochemical behavior have been investigated. The observed electronic properties are explained on the basis of density functional theory.

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The metal-mediated (catalytic) activation of strong and typically unreactive bonds under mild conditions requires the development of powerful ligand sets. Two particularly useful strategies have been developed during the last few years. [1] The first relies on electron-deficient systems with high-valent early transition metals, often in a d 0 configuration. [2] Such metal centers have been shown to activate unreactive bonds through agostic interactions and subsequent s-bond metathesis. In a second strategy, electronrich metal centers are used to promote bond activation through oxidative addition, and late transition metals such as the platinum group metals typically accommodate the required electron density for these reactions. [3] The relative basicity can be further increased by using acidic media [4] and by installing strongly donating nontransferable ligands in the metal coordination sphere. [5]

Palladation of N3-alkylated 1,2,3-triazolium salts with Pd(OAc) 2 afforded a μ 2 -I 2 bridged bimetallic complex [Pd(trz)I 2 ] 2 and monometallic bis(carbene) complexes Pd(trz) 2 I 2 as a mixture of trans and cis isomers (trz = 1,2,3-triazol-5-ylidene). Addition of excess halide or modification of the palladation procedure from direct functionalization to a transmetalation sequence involving a silver intermediate allowed for chemoselective formation of the bis(carbene) complex, while subsequent anion metathesis with NaI produced the monometallic bis(carbene) complexes exclusively. Modification of the wingtip group had little influence on the metalation to palladium or rhodium(I) via transmetalation. According to NMR analysis using δ C and 1 J Rh-C , subtle but noticeable tunability of the metal electronic properties was identified. In addition, phenyl wingtip groups as N-substituents in the triazolylidene ligands were susceptible to cyclopalladation in the presence of NaOAc and are thus not chemically inert.

Chiral, facial tris-cyclometalated Ir(III) complexes, fac-Delta-Ir(pppy)(3), fac-Lambda-Ir(pppy)(3), fac-Lambda-IrL (where pppy is (8R,10R)-2-(2'-phenyl)-4,5-pinenopyridine and L is a tripodal ligand comprising three pppy moieties connected through a mesityl spacer) have been synthesized and characterized. In IrL, NMR and CD studies indicate that only one diastereomer is formed, with the Lambda configuration at the metal center, whereas enantiopure pppy yields the fac-Lambda- and the fac-Delta-stereoisomer in a ratio 2:3. fac-Lambda-IrL was structurally characterized using X-ray crystallography. The luminescence properties including CPL, of the three complexes and their sensitivity to dioxygen were examined.