Ruthenium carbonyl 1,4-diaza-1,3-butadiene (R-DAB) complexes. Reactivity of Ru2(CO)5(R-DAB) toward molecular hydrogen. Molecular structure of H2Ru4(CO)8(R-DAB)2, containing an unique linear chain of four ruthenium atoms

Article abstract of DOI:10.1021/om00130a014

Ru₂(CO)₅(R-DAB) (R-DAB = 1,4-disubstituted 1,4-diaza-1,3-butadiene = RN=CHCH=NR;² R = i-Pr, c-Hx, c-HxCH₂) reacts smoothly at 90°C with molecular hydrogen to yield the oxidative addition product H₂Ru₂(CO)₅(R-DAB). The same product can be obtained when Ru₂(CO)₆(R-DAB) (R = i-Pr, c-Hx, t-Bu) is used as starting material. In the products, which are characterized by FD-mass, IR, and ¹H NMR spectroscopy, the R-DAB ligand is in a 6e-bonding mode. The ¹H NMR spectra indicate that the two hydride atoms are bonded (presumably terminally) to the same Ru atom. Ru₃(CO)₈(neo-Pent-DAB) reacts with H₂ to yield both known H₄Ru₄(CO)₁₂ and novel H₂Ru₄(CO)₈(neo-Pent-DAB)₂. The latter compound can also be obtained from the reaction of Ru₂(CO)₅(neo-Pent-DAB) with H₂. Possible reaction routes are discussed. An X-ray crystal structure determination established the molecular geometry of this tetranuclear Ru-α-diimine complex. Crystals of H₂Ru₄(CO)₈(neo-Pent-DAB)₂ are monoclinic of space group C2/c with cell constants a = 18.399 (7) A?, b = 8.725 (3) A?, c = 29.177 (8) A?, β = 110.43 (4)°, and Z = 4. A total of 1318 reflections have been used in the refinement resulting in a final R value of 0.038 (R_w = 0.064). The molecule possesses a pseudo-C_2h symmetry. Surprisingly, the four Ru atoms are located in a linear chain arrangement which is very unusual. The Ru(1)-Ru(2)-Ru(2)* angle is 176.48 (6)°. The Ru-Ru distances are in the range expected for single Ru-Ru bond lengths (Ru(1)-Ru(2) = 2.806 (2) A?, Ru(2)-Ru(2)* = 2.745 (2) A?). Ru(1) is coordinated by two terminal carbonyls and Ru(2) by one. The central Ru(2)-Ru(2)* bond is symmetrically bridged by two carbonyls. The neo-Pent-DAB ligand is σ-N,σ-N′-bonded to Ru(1) with equal Ru(1)-N bond lengths of 2.09 (1) A? and η²-C=N,η²-C′=N′ bonded to Ru(2) with an average Ru(2)-ligand atom distance of 2.30 (5) A?. Likewise, the C-N and C-C bond lengths within the α-diimine part are equal (1.36(3) A?) which is consistent with an 8e-bonding mode. The hydride ligands could not be located directly, however; the geometry around Ru(1) and the ¹H NMR data strongly indicate a terminal (on Ru(1)) bond position, which is quite unusual in a polynuclear metal carbonyl complex. Electronically, this species can be considered as electron precise, i.e., a 66e complex having three metal-metal bonds.