A series of 3,4-disubstituted (hex-3-ene-1,5-diyne-1,6-diyl)diron complexes with FeCp*(dppe) ( Fe ) end caps, Fe −C⋮CC(R)C(R)C⋮C− Fe (R = H, C⋮C−SiMe3, C⋮C−H, C6H5, p-C6H4CF3), and the related (octa-3,5-diene-1,7-diyne-1,8-diyl)diron complex, Fe −C⋮C−C(H)C(H)C(H)C(H)C⋮C− Fe , has been prepared, and their performance as molecular wires has been evaluated. The enyne complexes have been synthesized via vinylidene intermediates, [ Fe CC(H)C(R)C(R)C(H)C Fe ]2+, derived from the corresponding terminal alkynes or the Me3Si-protected precursors (XC⋮CCC(R)C(R)C⋮C−X; X = H, SiMe3), and the products have been characterized spectroscopically and crystallographically. The performance of the obtained dinuclear enyne complexes as molecular wires has been evaluated on the basis of the comproportionation constants (K C) obtained by electrochemical measurements and the V ab values obtained from the spectral parameters of the intervalence charge transfer bands of the isolated monocationic radical species appearing in the near-IR region. As a result, the C6-enediyne complexes turn out to be excellent molecular wires, with K C values larger than 108 as well as V ab values larger than 0.35, belonging to class III compounds according to the Robin and Day classification and being comparable to the related polyynediyl complexes as well. It is notable that, in the enyne system, the performance can be readily tuned by introduction of appropriate substituents onto the olefinic part. Thus, diiron complexes containing an enyne spacer can be regarded as tunable molecular wires.

The dirhodium-tetracarbonyl complex with the PNNP ligand, [Rh 2 (PNNP)(CO) 4 ]BF 4 , 1‚BF 4 [PNNP ) 3,5-bis(diphenylphosphinomethyl)pyrazolato], serves as a precursor for the active species [Rh 2 (PNNP)(CO) 2 ] + , A, with cis-divacant coordination sites, which can accommodate a donor species with up to four donor electrons. Interaction of 1‚BF 4 with LiCtC-R, Li-CHdCH 2 , PPN[M(CO) n ], CN-Cy, SMe 2 , and Et-CtC-Et leads to the formation of the corresponding dinuclear adducts [(X)Rh 2 (PNNP)(CO) 2 ] n+ (BF 4 ) n [X/n ) µ-η 1 :η 2 -CtC-R/0 (2), µ-η 1 :η 2 -CHdCH 2 /0 (3), µ-Co(CO) 4 /0 (4a), µ-Mn(CO) 5 /0 (4b), (η 1 -CtN-Cy) 2 /1 (5‚BF 4 ), µ-η 1 :η 1 -SMe 2 /1 (6‚BF 4 ), µ-η 2 :η 2 -Et-CtC-Et/1 (7‚BF 4 )], respectively. Selective replacement of the inner CO ligands trans to the P atoms is verified by spectroscopic and crystallographic characterizations of the adducts. The µ-acetylide (2) and µ-vinyl complexes (3) show dynamic behavior via the conventional windshield wiper motion of the unsaturated hydrocarbyl ligand. Systematic structural analysis of a series of the µ-acetylide complexes 2 (R ) H, SiMe 3 , n-Bu, Ph, p-tol) reveals three typical conformations for the Rh 2 (PNNP)(CO) 2 backbone: C s -(type I), C 2 -(type II), and C 2v -symmetrical ones (type III). On the other hand, interaction of 1‚BF 4 with 1-alkyne, hydrosilanes, and hydroxo anion produces tetranuclear adducts resulting from 1 (donor):2 [Rh 2 (PNNP)(CO) 2 ] coupling reactions. The µ 4 -η 1 (C R ):η 2 (C R tC )-acetylide complexes, [(µ 4 -CtC-R)Rh 4 (PNNP) 2 (CO) 4 ]BF 4 , 8‚BF 4 , consisting of an acyclic, folded Z-shaped Rh 4 linkage are fluxional via a combination of windshield wiper-like motions between the wingtip Rh centers and between the wingtip and hinge Rh centers, which involve reversible M-M bond scission and recombination processes. The ethynyl complex 8a‚BF 4 (R ) H) is readily deprotonated by the action of a base to give the µ 4 -dicarbide complex (µ-η 1 :η 1 :η 2 :η 2 -CtC)Rh 4 (PNNP)(CO) 4 , 9, via cleavage of three Rh-Rh bonds, and this process is reversed upon protonation of 9 with HBF 4 . The reaction of 1‚BF 4 with hydrosilanes and hydroxo anion furnishes the isostructural tetranuclear complexes [(µ 4 -X)Rh 4 (PNNP)(CO) 4 ] n+ -(BF 4 ) n [X/n ) H/1 (10‚BF 4 ), O/0 (11)], in which the four Rh atoms arranged in a tetrahedral array are connected by the µ 4 -bridging ligand (X), as characterized by X-ray crystallography. The tetranuclear structures are retained mainly by Rh-X interactions even in the case of the electron-deficient hydride complex 10‚BF 4 with 58 valence electrons (cf. 64e for a coordinatively saturated species), where the filled, spherically distributed 1s orbital of the hydride ligand interacts with the four Rh centers arranged in a tetrahedral array. Depending on the size of the bridging ligand X, the structure of the tetranuclear complexes varies from the encapsulated structure (10‚BF 4 and 11) to the one with a folded Z-shaped metal linkage (8‚BF 4 ). The present study reveals (i) the high reactivity (electr...

Tetrarhodium complexes with a PNNP auxiliary, [(μ4-X)Rh4(CO)4(PNNP)2] n + (X/n = RC⋮C/1 (acetylide), C2/0 (dicarbide); PNNP = 3,5-bis((diphenylphosphino)methyl)pyrazolato), are integrated mainly through M−X interactions and exhibit unique dynamic behavior associated with reversible metal−metal bond cleavage and recombination processes.