Mono- and dinuclear molybdenum complexes with sterically demanding cycloheptatrienyl ligands

Benzoimidazolium-derived dimeric and hydride n-dopants for organic electron-transport materials: impact of substitution on structures, electrochemistry, and reactivity

1,3-Dimethyl-2,3-dihydrobenzo[d]imidazoles, 1H, and 1,1',3,3'-tetramethyl-2,2',3,3'-tetrahydro-2,2'-bibenzo[d]imidazoles, 12, are of interest as n-dopants for organic electron-transport materials. Salts of 2-(4-(dimethylamino)phenyl)-4,7-dimethoxy-, 2-cyclohexyl-4,7-dimethoxy-, and 2-(5-(dimethylamino)thiophen-2-yl)benzo[d]imidazolium (1g-i+, respectively) have been synthesized and reduced with NaBH4 to 1gH, 1hH, and 1iH, and with Na:Hg to 1g2 and 1h2. Their electrochemistry and reactivity were compared to those derived from 2-(4-(dimethylamino)phenyl)- (1b+) and 2-cyclohexylbenzo[d]imidazolium (1e+) salts. E(1+/1•) values for 2-aryl species are less reducing than for 2-alkyl analogues, i.e., the radicals are stabilized more by aryl groups than the cations, while 4,7-dimethoxy substitution leads to more reducing E(1+/1•) values, as well as cathodic shifts in E(12•+/12) and E(1H•+/1H) values. Both the use of 3,4-dimethoxy and 2-aryl substituents accelerates the reaction of the 1H species with PC61BM. Because 2-aryl groups stabilize radicals, 1b2 and 1g2 exhibit weaker bonds than 1e2 and 1h2 and thus react with 6,13-bis(triisopropylsilylethynyl)pentacene (VII) via a "cleavage-first" pathway, while 1e2 and 1h2 react only via "electron-transfer-first". 1h2 exhibits the most cathodic E(12•+/12) value of the dimers considered here and, therefore, reacts more rapidly than any of the other dimers with VII via "electron-transfer-first". Crystal structures show rather long central C-C bonds for 1b2 (1.5899(11) and 1.6194(8) Å) and 1h2 (1.6299(13) Å).

Synthesis, structures, and reactivity of isomers of [RuCp*(1,4-(Me2N)2C6H4)]2

[RuCp*(1,3,5-R₃C₆H₃)]₂ {Cp* = η⁵-pentamethylcyclopentadienyl, R = Me, Et} have previously been found to be moderately air stable, yet highly reducing, with estimated D⁺/0.5D₂ (where D₂ and D⁺ represent the dimer and the corresponding monomeric cation, respectively) redox potentials of ca. -2.0 V vs. FeCp₂^+/0. These properties have led to their use as n-dopants for organic semiconductors. Use of arenes substituted with π-electron donors is anticipated to lead to even more strongly reducing dimers. [RuCp*(1-(Me₂N)-3,5-Me₂C₆H₃)]⁺PF₆^- and [RuCp*(1,4-(Me₂N)₂C₆H₄)]⁺PF₆^- have been synthesized and electrochemically and crystallographically characterized; both exhibit D⁺/D potentials slightly more cathodic than [RuCp*(1,3,5-R₃C₆H₃)]⁺. Reduction of [RuCp*(1,4-(Me₂N)₂C₆H₄)]⁺PF₆^- using silica-supported sodium-potassium alloy leads to a mixture of isomers of [RuCp*(1,4-(Me₂N)₂C₆H₄)]₂, two of which have been crystallographically characterized. One of these isomers has a similar molecular structure to [RuCp*(1,3,5-Et₃C₆H₃)]₂; the central C-C bond is exo,exo, i.e., on the opposite face of both six-membered rings from the metals. A D⁺/0.5D₂ potential of -2.4 V is estimated for this exo,exo dimer, more reducing than that of [RuCp*(1,3,5-R₃C₆H₃)]₂ (-2.0 V). This isomer reacts much more rapidly with both air and electron acceptors than [RuCp*(1,3,5-R₃C₆H₃)]₂ due to a much more cathodic D₂˙⁺/D₂ potential. The other isomer to be crystallographically characterized, along with a third isomer, are both dimerized in an exo,endo fashion, representing the first examples of such dimers. Density functional theory calculations and reactivity studies indicate that the central bonds of these two isomers are weaker than those of the exo,exo isomer, or of [RuCp*(1,3,5-R₃C₆H₃)]₂, leading to estimated D⁺/0.5D₂ potentials of -2.5 and -2.6 V vs. FeCp₂^+/0. At the same time the D₂˙⁺/D₂ potentials for the exo,endo dimers are anodically shifted relative to those of [RuCp*(1,3,5-R₃C₆H₃)]₂, resulting in much greater air stability than for the exo,exo isomer.

Bis(η-cyclo-hepta-trien-yl)tri-μ-hydrido-dimolybdenum(0,I)

In the title compound, [Mo(2)(η(7)-C(7)H(7))(2)(μ-H)(3)], which displays crystallographic mirror symmetry, two (η(7)-C(7)H(7))Mo units are linked along the Mo-Mo axis by three bridging hydride ligands. The Mo-Mo distance is 2.5732 (4) Å. The perpendicular distances of the Mo atoms from the C(7) planes are 1.5827 (8) and 1.5814 (8) Å, with individual Mo-C bond lengths in the range 2.261 (2)-2.2789 (14) Å. Mo-H distances range from 1.77 (3) to 1.85 (4) Å, with Mo-H-Mo angles of 89 (2) and 92 (1)°.

Synthesis and reactivity of functionalized cycloheptatrienyl-cyclopentadienyl sandwich complexes

This feature article provides an overview of the synthesis and reactivity of functionalized cycloheptatrienyl-cyclopentadienyl transition metal sandwich complexes of the type [(eta(7)-C(7)H(7))M(eta(5)-C(5)H(5))] (M = group 4, 5 or 6 metal), which can be used as building blocks for the preparation of metallopolymers and polymetallic complexes. Emphasis is placed on 16-electron group 4 complexes (M = Ti, Zr, Hf) and their reactivity towards sigma-donor/pi-acceptor ligands, which indicates that these complexes bear a close resemblance to Lewis acidic M(+IV) complexes. Based on theoretical calculations, this behavior can be mainly attributed to the strong and appreciably covalent metal-cycloheptatrienyl interaction with the cycloheptatrienyl ring acting more as a -3 ligand than as a +1 ligand in these mixed ring complexes.