Orientation and Restricted Rotation of Lopsided N-Donor Heterocyclic Bioligands in Octahedral Ruthenium Complexes

Abstract

In order to analyze the roles of steric and electrostatic factors in determining the orientation and dynamic behavior of benzimidazole‐type ligands in RuII(dimethyl sulfoxide) complexes, compounds of the general formula RuX2(dmso‐S)2−n(CO)n(L)2 [X = Cl, Br; L = 1,5,6‐trimethylbenzimidazole (Me3Bzm), benzimidazole (Bzm); n = 0−2] with cis and trans arrangements of the X ligands, have been prepared and structurally characterized both in solution and in the solid state, by NMR and X‐ray techniques, respectively. New descriptors, based on the torsion angles about the Ru−N bonds, are introduced for a more efficient definition of the head‐to‐head (HH) and head‐to‐tail (HT) orientations of the heterocyclic nitrogen bases. Similar descriptors, based on the torsion angles about the Ru−S bonds, are used to define the orientation of the dimethyl sulfoxide ligands. The experimental molecular conformations are discussed on the basis of molecular mechanics (MM) calculations. The electrostatic interaction between the halide ligands and the positively charged N2CH proton on the imidazole ring has been found to be the essential factor in determining the specific orientation of one Me3Bzm base, both in the solid state and in solution. The different stabilities of the HH and HT base conformations observed in solution, in the solid state, and in the isolated molecules are rationalized in terms of solvation and packing effects. The larger solvent‐accessible surface area in the HH conformation as compared with that in the HT form leads to more effective intermolecular interactions, which are responsible for stabilizing the former geometry in the condensed phase.

The Pivotal Role of Ru‐dmso Compounds in the Discovery of Well‐Behaved Precursors

“…By exploiting the known reactivity of the carbonyl derivatives of 1 , cis,fac ‐[RuCl 2 (CO)(dmso) 3 ] ( 42 ) and cis , cis , cis ‐[RuCl 2 (CO) 2 (dmso) 2 ] ( 44 ), we prepared also the compounds cis , cis , cis ‐[RuCl 2 (dmso‐S)(CO)(Me 3 Bzm) 2 ] ( 71 , only the isomer with CO trans to Cl) and cis , cis , cis ‐[RuCl 2 (CO) 2 (Me 3 Bzm) 2 ] ( 72 ), respectively, that can be thought as formally deriving from 66 by replacing either one or both dmso‐S molecules with CO ligands . For both compounds only one HH atropisomer is preferentially populated in solution, and the same HH rotamer was found in the solid‐state structure of 71 , in which the orientation of Me 3 Bzm “b” is very similar to that found in 66 .…”

“…On the other hand, trans ‐[RuCl 2 (dmso‐S) 4 ] ( 2 ) turned out to be a more well‐behaved precursor, since it selectively replaces two adjacent dmso ligands without geometrical changes (Figure ), i.e. it affords what in supramolecular chemistry is called a cis ‐protected metal fragment , . Thus, for example, when treated with a stoichiometric amount of the linear linker pyrazine (pyz), 2 affords the molecular triangle [{ trans , cis ‐RuCl 2 (dmso‐S) 2 (µ‐pyz)} 3 ] ( 10 ) (instead of the expected molecular square!…”

“…These findings show that changing the nature of even a single spectator ligand dramatically affects the dynamic behavior of the N ligands in this type of compounds. In 71 and 72 we were however unable to assess the torsional barriers for the rotation of the bases about the Ru–N bonds, in other words to establish if in such conformers rotation of the bases is very slow or very fast on the NMR time scale …”

“…Overall, a combined X‐ray structural and dynamic NMR approach allowed us to assess orientation and fluxional motions of two adjacent planar N‐donor heterocyclic ligands in an homogeneous series of mononuclear Ru II ‐dmso compounds. We could establish that the dynamic behavior of the N ligands is strongly influenced not only by steric factors, but also by electrostatic interactions with the adjacent ligands , …”

trans-(2-Acetylpyridine-κ2N,O)dichlorobis(dimethyl sulfoxide-κS)ruthenium(II)

“…The RuÐO [2.084 (3) A Ê ] and RuÐN [2.122 (3) A Ê ] bond lengths in (I) are similar to those of known Ru II to keto-O (Basuli et al, 2000) and pyridine-N bonds (Bellachioma et al, 1998;Sengupta et al, 2001). The RuÐCl distances of 2.3753 (16) and 2.3977 (16) A Ê are unexceptional (Cingi et al, 2000;Alessio et al, 2000).…”