Toward Metal-Capped One-Dimensional Carbon Allotropes: Wirelike C6−C20 Polyynediyl Chains That Span Two Redox-Active (η5-C5Me5)Re(NO)(PPh3) Endgroups

Modulation of electronic couplings within Ru2-polyyne frameworks

Dimers of [Ru(2)(Xap)(4)] bridged by 1,3,5-hexatriyn-diyl (Xap are 2-anilinopyridinate and its aniline substituted derivatives), [Ru(2)(Xap)(4)](2)(μ-C(6)) (1), were prepared. Compounds 1 reacted with 1 equiv of tetracyanoethene (TCNE) to yield the cyclo-addition/insertion products [Ru(2)(Xap)(4)](2){μ-C≡CC(C(CN)(2))-C(C(CN)(2))C≡C} (2) and 1 equiv of Co(2)(dppm)(CO)(6) to yield the η(2)-Co(2) adducts to the middle C≡C bond, [Ru(2)(Xap)(4)](2)(μ-C(6))(Co(2)(dppm)(CO)(4)) (3). Voltammetric and spectroelectrochemical studies revealed that (i) two Ru(2) termini in 1 are sufficiently coupled with the monoanion (1(-)) as a Robin-Day class II/III mixed valence species; (ii) the coupling between two Ru(2) is still significant but somewhat weakened in 3; and (iii) the coupling between two Ru(2) is completely removed by the insertion of TCNE in 2. The attenuation of electronic couplings in 2 and 3 was further explored with both the X-ray diffraction study of representative compounds and spin-unrestricted DFT calculations.

Electronic communication in dinuclear C(4)-bridged tungsten complexes

The dinuclear tungsten carbyne [X(CO)(2)(dppe)WC(4)W(dppe)(CO)(2)X] (dppe = 1,2-bis(diphenylphosphino)ethane; X = I (3), Cl (7)) complexes were prepared from the bisacetylide precursor Li(2)[(CO)(3)(dppe)WC(4)W(CO)(3)(dppe)] (2) via oxidative replacement of one CO group at each tungsten center with a halide substituent. The iodide ligand in 3 could be substituted with isothiocyanate or triflate resulting in [X(CO)(2)(dppe)WC(4)W(dppe)(CO)(2)X] complexes (X = NCS (8), OTf (9)). Substitution of two and all four CO ligands in 3 was achieved via subsequent photolytic or thermal activation with dppe. The "half-substituted" complex [I(CO)(2)(dppe)WC(4)W(dppe)(2)I] (11) allows reversible one-electron oxidation which results in the monocationic species [I(CO)(2)(dppe)WC(4)W(dppe)(2)I][PF(6)] (11[PF(6)]). The "all-dppe substituted" complex [I(dppe)(2)WC(4)W(dppe)(2)I] (10) possesses two reversible redox states leading to the stable monocationic [I(dppe)(2)WC(4)W(dppe)(2)I][PF(6)] (10[PF(6)]) and the dicationic [I(dppe)(2)WC(4)W(dppe)(2)I][PF(6)](2) (10[PF(6)](2)) compounds. The complexes 2, 3, [W(CO)(3)(dppe)(C[triple bond]CPh)(I)] (4), [X(CO)(2)(dppe)W[triple bond]C-C(Me)=C(Me)-C[triple bond]W(dppe)(CO)(2)X] (X = I (5), Cl (6)), 7, 8, 10, 11 and 11[PF(6)] were characterized by single crystal X-ray diffraction. The electronic properties of complexes 10, 10[PF(6)], 10[PF(6)](2), as well as of compounds 11 and 11[PF(6)], were investigated using cyclic voltammetry (CV), EPR, IR, near-IR spectroscopy, and magnetization measurements. These studies showed that the [W][triple bond]C-C[triple bond]C-C[triple bond][W] canonical form of the bridged system with strong tungsten-carbon interaction contributes significantly to the electronic coupling in the mixed-valent species 10[PF(6)] (comproportionation constant K(c) = 7.5 x 10(4)) and to the strong antiferromagnetic coupling in the dicationic complex 10[PF(6)](2) (exchange integral J = -167 cm(-1)). In addition, the rate for electron transfer between the tungsten centers in 10[PF(6)] was evaluated by near-IR and IR studies.

Synthesis and EPR studies of 2'-deoxyuridines with alkynyl, rodlike linkages

Sonogashira coupling of diacetyl 5-ethynyl-2'-deoxyuridine with diacetyl 5-iodo-2'-deoxyuridine gave the acylated ethynediyl-linked 2'-deoxyuridine dimer (3 b; 63%), which was deprotected with ammonia/methanol to give ethynediyl-linked 2'-deoxyuridines (3 a; 79%). Treatment of 5-ethynyl-2'-deoxyuridine (1 a) with 5-iodo-2'-deoxyuridine gave the furopyrimidine linked to 2'-deoxyuridine (78%). Catalytic oxidative coupling of 1 a (O(2), CuI, Pd/C, N,N-dimethylformamide) gave butadiynediyl-linked 2'-deoxyuridines (4; 84 %). Double Sonogashira coupling of 5-iodo-2'-deoxyuridine with 1,4-diethynylbenzene gave 1,4-phenylenediethynediyl-bridged 2'-deoxyuridines (5; 83%). Cu-catalyzed cycloisomerization of dimers 4 and 5 gave their furopyrimidine derivatives. One-electron addition to 1 a, 3 a, and 4 gave the anion radical, the EPR spectra of which showed that the unpaired electron is largely localized at C6 of one uracil ring (17 G doublet) at 77 K. The EPR spectra of the one-electron-oxidized derivatives of ethynediyl- and butadiynediyl-linked uridines 3 a and 4 at 77 K showed that the unpaired electron is delocalized over both rings. Therefore, structures 3 a and 4 provide an efficient electronic link for hole conduction between the uracil rings. However, for the excess electron, an activation barrier prevents coupling to both rings. These dimeric structures could provide a gate that would separate hole transfer from electron transport between strands in DNA systems. In the crystal structure of acylated dimer 3 b, the bases were found in the anti position relative to each other across the ethynyl link, and similar anti conformation was preserved in the derived furopyrimidine-deoxyuridine dinucleoside.