The ability of N-heterocyclic carbenes (NHCs) to participate in π-back-bonding interactions was evaluated in a range of transition metal complexes. Rh chloride complexes containing a systematic series of various 1,3-dimethyl-4,5-disubstituted-imidazol-2-ylidenes and either 1,5-cyclooctadiene (cod) or two carbon monoxide ligands were synthesized (i.e., (NHC)RhCl(cod) and (NHC)RhCl(CO)2, respectively) and studied using 1H NMR and IR spectroscopies. In the former series, the 1H NMR chemical shifts of the signals attributable to the olefin trans to the NHC ligand were found to shift downfield by up to 0.17 ppm as the π-acidity of the substituents on the 4,5-positions increased (i.e., H → Cl → CN). Similarly, in the latter series, the IR stretching frequencies of the carbonyl groups trans to the NHC ligands were found to increase by 11 ± 0.5 cm-1 as π-acidity increased over the same series. Using the nitrile group as a diagnostic handle, the CN stretching frequency of (1,3-dimethyl-4,5-dicyanoimidazol-2-ylidene)(cod)RhCl was found to be 4 ± 0.5 cm-1 higher than 1,3-dimethyl-4,5-dicyanoimidazol-2-ylidene)(CO)2RhCl, a more π-acidic analogue. X-ray analysis of the aforementioned series of (NHC)(cod)RhCl complexes indicated changes in N−Ccarbene bond lengths that were consistent with greater π-donation from complexes containing 4,5-dihydroimidazol-2-ylidene relative to the their 4,5-dicyano analogues. Collectively, these results suggest not only that imidazol-2-ylidenes are capable of π-back-bonding but that this interaction may be tuned by changing the π-acidity of the substituents on the imidazole ring.

The diradical 1,1′,5,5′-tetramethyl-6,6′-dioxo-3,3′-biverdazyl, 6, crystallizes in a monoclinic unit cell with a ) 4.224(1), b ) 17.378(2), c ) 7.529(1) Å, ) 96.02(1)°, space group P2 1 /c (No. 14). The crystal structure of 6 shows local D 2h symmetry. ESR measurements in a frozen chloroform solution indicate that isolated 6 has a singlet ground state with a triplet excited state 760 cm -1 (0.094 eV) above the ground state with zerofield splitting parameters D ) 0.038 cm -1 and E ) 0.0016 cm -1 . Semiempirical calculations (AM-1) suggest that 6 is twisted in solution. Crystalline 6 shows a temperature-activated ESR signal with no features characteristic of an isolated triplet. Strong intermolecular π-stacking interactions prevent the analysis of this temperature activation in terms of intermolecular and intramolecular exchange parameters.

A model for the interaction of the water-soluble sapphyrin derivative 1 with a variety of nucleic acid species is presented. Three modes of interaction are described: The first mode, seen with all the nucleic acid species, is that of "phosphate chelation". This mode is exemplified by a solid state structure of the complex formed between the monobasic form of cAMP and the sapphyrin species [2H‚2] 2+ . It involves the specific chelation of the oxyanion of a phosphorylated nucleotide or nucleic acid species with the protonated core of sapphyrin Via Coulombic interactions that include H-bonding interactions. Spectroscopically, this interaction is characterized by a visible absorption at 422 nm and corresponds to complexes formed between the dimeric form of 1 and phosphorylated nucleotides. In the case of double-stranded DNA, this mode of binding shows a preference for the more flexible copolymer [poly-(dA-dT)] 2 over [poly(dG-dC)] 2 . The second mode involves a hydrophobic interaction with the nucleobases present in both monomeric and single-stranded polymeric nucleotides. Spectroscopically, this nucleotide-dependent interaction is characterized by the absorption of the monomeric species of 1 at ca. 450 nm. The third mode involves the highly ordered aggregation of 1 on the surface of certain double-stranded, helical nucleic acids at low phosphate ester to sapphyrin (P/S) ratios and is templated by the higher order structure of these nucleic acid polymers. Spectroscopically, this mode is characterized by a visible absorption at ca. 400 nm and a large, conservative induced CD signal for 1.

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A major theme in the development of the chemical sciences resides in improving the capability to negotiate issues of selectivity in the organization of matter on increasingly greater-length scales. 1 While molecular synthesis has advanced to an art through the precise control of chemo-, regio-, stereo-, and enantioselectivity, the identification and orchestration of persistent noncovalent binding motifs is extending synthetic technology to the nanoscopic regime by allowing the construction of supramolecular architectures through the self-assembly of instructed molecular components under equilibrium conditions. 2 Along these lines, the H-bond-mediated self-assembly of molecular precursors represents a powerful strategy for the logic-driven retrosynthesis and construction of nanoscopic materials in structurally homogeneous form. 2e Nevertheless, the nominal stability of most noncovalent aggregates detracts from their usefulness. To address this deficiency, template-directed syntheses involving the covalent capture of discrete noncovalent superstructures 3 and the polymerization of organized assemblies 4,5 have been described. Robust nanoscale assemblies may also be obtained through the stabilization of kinetically labile systems by the preorganization of binding sites and accumulation of multiple binding interactions. 6 Macromolecular systems are well-suited to this latter strategy, and recently much attention has been given to the preparation of polymers incorporating H-bonding moieties 7 and the self-assembly of dendritic macromolecules 8 and block copolymers. 5a,9,10 As part of a program involving the development of synthetic methodologies for the synthesis of nanostructured materials via self-assembly of macromolecular precursors, we herewith report preliminary studies on the design and de novo synthesis of synthetic polymer strands capable of duplex formation through the "covalent casting" of 1-dimensional H-bonding motifs. Excluding systems that borrow from naturally occurring superstructural motifs (e.g., homo-DNA 11 ), to the best of our knowledge this report represents the first efforts toward unnatural polymer duplexes assembling through the action of inter-strand H-bonds. 12 Cooperative binding via preorganization of multiple complementary binding sites enhances association through the reduction of entropic terms for the formation of discrete objects through receptor-substrate interactions. 2e,13 For the application of this principle to 1-dimensional superstructures, such as the H-bonded tape I, it was hypothesized that preorganization of the molecules comprising the 1-dimensional superstructure could be achieved by introduction of a covalent linker as shown schematically for the partially cast H-bonded tape II. Introduction of a second linking group fully casts the H-bonded tape to afford a covalentnoncovalent ladder material, polymer duplex III (Scheme 1).In accord with this strategy, I should be comprised of molecules capable of functioning as a platform for subsequent elaboration to III. Trichloro-...