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The syntheses of trans-[Ru(4,4'-C[triple bond]CC(6)H(4)C[triple bond]CC(6)H(4)NO(2))Cl(dppe)(2)] (19) and the systematically varied complexes trans-[Ru(4,4',4''-C[triple bond]CC(6)H(4)X(2)C(6)H(4)Y(2)C(6)H(4)NO(2))Cl(L(2))(2)] [L(2) = dppe, X(2) = C[triple bond]C, Y(2) = (E)-CH=CH (12), C[triple bond]C (18); L(2) = dppe, X(2) = (E)-CH=CH, Y(2) = C[triple bond]C (14), (E)-CH=CH (16); L(2) = dppm, X(2) = C[triple bond]C, Y(2) = (E)-CH=CH (13); L(2) = dppm, X(2) = (E)-CH=CH, Y(2) = C[triple bond]C (15), (E)-CH=CH (17)] are reported, the latter being donor-bridge-acceptor complexes varying in bridge composition by replacement of yne with E-ene linkages, together with their cyclic voltammetric data, linear optical, and quadratic nonlinear optical response data. Ru(II/III) oxidation potentials increase on replacing yne linkage by E-ene linkage at the phenylene adjacent to the metal center, and on replacing dppe by dppm co-ligands. The low-energy optical absorption maxima occur in the region 20,400-23,300 cm(-1) and are metal-to-ligand charge-transfer (MLCT) in origin; these bands undergo a blue-shift upon pi-bridge lengthening by addition of phenyleneethynylene units, and on replacing E-ene linkages by yne linkages. Time-dependent density functional theory calculations on model complexes have suggested assignments for the low-energy bands. The optical spectra of selected oxidized species contain low-energy ligand-to-metal charge transfer (LMCT) bands centered in the region 9760-11,800 cm(-1). Quadratic molecular nonlinearities from hyper-Rayleigh scattering (HRS) studies at 1064 nm reveal an increase in the two-level-corrected beta(0) value on pi-bridge lengthening, a trend that is not seen with beta values because of the blue-shift in lambda(max) for this structural modification. Replacing yne linkages by E-ene linkage at the phenylene adjacent to the metal center or dppm co-ligand by dppe results in an increase in beta and beta(0) values. In contrast, quadratic molecular nonlinearities by HRS at 1300 nm or electric field-induced second-harmonic generation (EFISH) studies at 1907 nm do not afford clear trends.

Tetrairon(III) single-molecule magnets [Fe4(pPy)2(dpm)6] ( 1) (H3pPy=2-(hydroxymethyl)-2-(pyridin-4-yl)propane-1,3-diol, Hdpm=dipivaloylmethane) have been deliberately organized into supramolecular chains by reaction with RuIIRuII or RuIIRuIII paddlewheel complexes. The products [Fe4(pPy)2(dpm)6][Ru2(OAc)4](BF4)x with x=0 ( 2 a) or x=1 ( 2 b) differ in the electron count on the paramagnetic diruthenium bridges and display hysteresis loops of substantially different shape. Owing to their large easy-plane anisotropy, the s=1 diruthenium(II,II) units in 2 a act as effective seff=0 spins and lead to negligible intrachain communication. By contrast, the mixed-valent bridges ( s=3/2, seff=1/2) in 2 b introduce a significant exchange bias, with concomitant enhancement of the remnant magnetization. Our results suggest the possibility to use electron transfer to tune intermolecular communication in redox-responsive arrays of SMMs. [ABSTRACT FROM AUTHOR]

Ligand design in metal chemistry is a fundamental step when pursuing compounds with specific reactivity. In this paper, the functionalization of the OH group in the lateral chain of the N-heterocyclic-carbene (NHC) ligand bound to a bis-carbonyl cyclopentadienone NHC ruthenium(0) complex allowed the decoration of five generations of poly(propyleneimine) (PPIs) dendrimers with up to 64 organometallic moieties. The coupling was achieved by employing carbonyldiimidazole and the formation of carbamate linkages between dendritic peripheral NH2 and lateral OH groups on ruthenium complexes. The synthetic procedure, chemical purification, and spectroscopic characterization of the five generations of dendrimers (3g1–5) are here described. The ruthenium-modified dendrimers were activated as catalysts in the transfer hydrogenation of the model compound 4-fluoroacetophenone in the presence of cerium ammonium nitrate as their mononuclear congeners. The catalytic activity, being similar for the five generations, shows a decrease if compared to mononuclear complexes. This detrimental effect might be ascribed to the –CH2NH– functionalization, largely present in dendrimer skeleton and that can compete with the hydrogen transfer mechanism, but also partially to a dendritic effect caused by steric encumbrance. [ABSTRACT FROM AUTHOR]