In the field of metal-metal bonding, the occurrence of stable, multiple bonds between different transition metals is uncommon, and is largely unknown for different first-row metals. Adding to a recently reported iron-chromium complex, three additional M-Cr complexes have been isolated, where the iron site is systematically replaced with other first-row transition metals (Mn, Co, or Ni), while the chromium site is kept invariant. These complexes have been characterized by X-ray crystallography. The Mn-Cr complex has an ultrashort metal-metal bond distance of 1.82 Å, which is consistent with a quintuple bond. The M-Cr bond distances increases across the period from M = Mn to M = Ni, as the formal bond order decreases from 5 to 1. Theoretical calculations reveal that the M-Cr bonds become increasingly polarized across the period. We propose that these trends arise from increasing differences in the energies and/or contraction of the metals' d-orbitals (M vs Cr). The cyclic voltammograms of these heterobimetallic complexes show multiple one-electron transfer processes, from two to four redox events depending on the M-Cr pair.

Synthetic materials that assemble reversibly with polyanions under physiological conditions are of interest for a broad range of biotechnical applications. Cationic polymers are used widely as agents for the condensation of DNA, but polycations also introduce practical limitations in applications for which subsequent dissociation or disassembly of polycation/DNA complexes is desired. The design of cationic polymers that promote the release of associated DNA presents a challenge because it requires the introduction of functionality that is inherently opposed to that required for efficient DNA condensation. Here, we report the synthesis and biophysical characterization of linear poly(ethylenimine) (LPEI) functionalized with methyl ester side chains. The gradual hydrolysis of the ester functionality in these materials under physiologically relevant conditions results in a controlled reduction in cationic charge density and a change in the nature of the electrostatic interactions between the polymers and plasmid DNA, as determined by agarose gel electrophoresis. Using this approach, it is possible to mediate the dissociation of DNA from polymer over a period of hours to days by varying the mole percentage of methyl esters incorporated into the polymer. Polymers having a high degree of substitution (e.g., 80 or 100 mol %) release DNA more rapidly than less-substituted polymers (e.g., 40 and 60 mol % functionalized). Polymers having 20 mol % substitution did not release DNA in these experiments. These charge-shifting materials could provide a basis for the design and fabrication of polyelectrolyte complexes and assemblies that sustain the release of DNA in solution and at interfaces.

The mechanism of reversible alkyne coupling at zirconium was investigated by examination of the kinetics of zirconacyclopentadiene cleavage to produce free alkynes. The zirconacyclopentadiene rings studied possess trimethylsilyl substituents in the alpha-positions, and the ancillary Cp2, Me2C(eta(5)-C5H4)2, and CpCp* (Cp* = eta(5)-C5Me5) bis(cyclopentadienyl) ligand sets were employed. Fragmentation of the zirconacyclopentadiene ring in Cp2Zr[2,5-(Me3Si)2-3,4-Ph2C4] with PMe3, to produce Cp2Zr(eta(2)-PhC[triple bond]CSiMe3)(PMe3) and free PhC[triple bond]CSiMe3, is first-order in initial zirconacycle concentration and zero-order in incoming phosphine (k(obs) = 1.4(2) x 10(-5) s(-1) at 22 degrees C), and the activation parameters determined by an Eyring analysis (DeltaH(double dagger) = 28(2) kcal mol(-1) and DeltaS(double dagger) = 14(4) eu) are consistent with a dissociative mechanism. The analogous reaction of the ansa-bridged complex Me2C(eta(5)-C5H4)2Zr[2,5-(Me3Si)2-3,4-Ph2C4] is 100 times faster than that for the corresponding Cp2 complex, while the corresponding CpCp* complex reacts 20 times slower than the Cp2 derivative. These rates appear to be largely influenced by the steric properties of the ancillary ligands.

Macrocyclic compounds have attracted considerable attention in numerous applications, including host-guest chemistry, chemical sensing, catalysis, and materials science. A major obstacle, however, is the limited number of convenient, versatile, and high-yielding synthetic routes to functionalized macrocycles. Macrocyclic compounds have been typically synthesized by ring-closing or condensation reactions, but many of these procedures produce mixtures of oligomers and cyclic compounds. As a result, macrocycle syntheses are often associated with difficult separations and low yields. Some successful approaches that circumvent these problems are based on "self-assembly" processes utilizing reversible bond-forming reactions, but for many applications, it is essential that the resulting macrocycle be built with a strong covalent bond network. In this Account, we describe how zirconocene-mediated reductive couplings of alkynes can provide reversible carbon-carbon bond-forming reactions well-suited for this purpose. Zirconocene coupling of alkenes and alkynes has been used extensively as a source of novel, versatile pathways to functionalized organic compounds. Here, we describe the development of zirconocene-mediated reductive couplings as a highly efficient method for the preparation of macrocycles and cages with diverse compositions, sizes, and shapes. This methodology is based on the reversible, regioselective coupling of alkynes with bulky substituents. In particular, silyl substituents provide regioselective, reversible couplings that place them into the α-positions of the resulting zirconacyclopentadiene rings. According to density functional theory (DFT) calculations and kinetic studies, the mechanism of this coupling involves a stepwise process, whereby an insertion of the second alkyne influences regiochemistry through both steric and electronic factors. Zirconocene coupling of diynes that incorporate silyl substituents generates predictable macrocyclic products in very high yields. In the absence of significant steric repulsion, the macrocyclization appears to be entropically driven, thereby providing the smallest strain-free macrocyclic structure. The scope of the reaction has been explored by variation of the spacer group between the alkynyl substituents and by incorporation of functional and chiral groups into the macrocycle. The size and shape of the resulting macrocycles are largely determined by the length and geometry of the dialkyne spacer, especially in the case of terminal trimethylsilyl-substituted diynes. For example, linear, rigid diynes with four or fewer phenylene rings lead to trimeric macrocycles, whereas bent or flexible diynes produce dimers. Depending on the reaction conditions, functional groups (such as N-heterocycles and imines) are tolerated in zirconocene coupling reactions, and in selected cases, they can be used to influence the shape of the final macrocyclic product. More recently, Cp(2)Zr(pyr)(Me(3)SiC≡CSiMe(3)) has been employed as a more general zirconocene synthon; it affords hig...

The reaction of one equiv of 1-alkynylphosphines, R2PCCR′ (R = Et, i Pr, or Ph and R′ = Ph or Mes), with Cp2Zr(pyr)(η2-Me3SiCCSiMe3) resulted in formation of monoalkyne complexes. In the case where R = Et, i Pr, or Ph and R′ = Ph, a “ligand free” zirconacyclopropene complex is produced. These complexes are stabilized by intermolecular donation of the phosphorus lone-pair in the dimeric complexes [Cp2Zr(η2-R2PCCPh)]2 (R = Et, i Pr, or Ph). However, with R = Ph and R′ = Mes, the zirconocyclopropene-pyridine complex Cp2Zr(pyr)(η2-Ph2PCCMes) is formed. Homocoupling of the 1-alkynylphosphines was demonstrated by reaction of a second equiv of Ph2PCCPh with [Cp2Zr(η2-Ph2PCCPh)]2 to give the diphosphinozirconacyclopentadiene Cp2Zr[2,5-(Ph2P)2-3,4-Ph2C4] with high regioselectivity (77%). The zirconacyclopropene complexes also react with one equiv of PhCCPh or EtCCEt to give zirconacyclopentadienes in which the phosphino substituent preferentially adopts the 2-position (α) of the zirconacyclopentadiene ring. These unsymmetrical zirconacyclopentadienes undergo substitution of the R2PCCR′ moiety with the less bulky alkynes PhCCPh or EtCCEt. The substituents on the 1-alkynylphosphines significantly influence the rates of alkyne substitution such that sterically more demanding substituents in either the α- (-P i Pr2) or β- (-Mes) position of the zirconacycle lead to faster exchange. The α-phosphinozirconacyclopentadienes were readily converted to 1-phosphinobutadienes via reaction with benzoic acid. The zirconacyclopentadiene Cp2Zr[2-Ph2P-3,4,5-Ph3C4] was converted to the corresponding thiophene oxide by the oxo-transfer reaction with sulfur dioxide. In the case of ((1Z,3E)-3-ethyl-2-phenylhexa-1,3-dienyl)diphenylphosphine and (3,4,5-triphenylthiophen-2-yl oxide)diphenylphosphine, the molecules were isolated as their phosphine oxides. Reactions of the zirconacyclopropene complexes [Cp2Zr(η2-Ph2PCCPh)]2 and Cp2Zr(pyr)(η2-Ph2PCCMes) with the diyne (F5C6)CC−1,4-C6H4−CC(C6F5) gave bis(zirconacycle)s terminated with phosphino groups. These bis(zirconacycle)s were converted to the corresponding phosphino-terminated oligomers by protonolysis with hydrochloric acid. In addition, the Ph2PCCMes moiety of Cp2Zr[2-Ph2P-3-Mes-4-(C6F5)C4]−1,4-C6H4−Cp2Zr[2-Ph2P-3-Mes-4-(C6F5)C4] was exchanged with PhCCPh to give the phenylene(zirconacyclopentadiene) Cp2Zr[2,3-Ph2-4-(C6F5)C4]−1,4-C6H4−Cp2Zr[2,3-Ph2-4-(C6F5)C4].