Temperature-dependent elongation of the H-H bond in dihydrogen complexes of Ru(II) bearing an NHC ligand: Effect of the NHC and trans ligands

Ruthenium hydride complexes bearing an N-heterocyclic carbene ligand RuHCl(CO)(IMes)(PPh3)(L/L')] (L = py, 2; 4Mepy, 3; L' = MeCN, 4; Me3CCN, 5) have been synthesized in high-yields via reaction of RuHCl(CO)(IMes)(PPh3)] (1) with pyridyl ligands L (L = py and 4Mepy) or nitrile ligands L' (L' = MeCN and Me3CCN). The ligands L/L' are labile in all the ruthenium hydride complexes; they can be easily replaced by Lewis bases. The X-ray structures of complexes 2 and 3 show intramolecular pi-pi interactions between the aromatic ring of PPh3, IMes, and pyridyl ligands. The protonation reaction of 2-5 gives the corresponding dihydrogen complexes of the type RuCl(eta(2)-H-2)(CO)(IMes)(PPh3)(L/L')]OTf] complexes (L = py, 6; 4Mepy, 7; L' = MeCN, 8; Me3CCN, 9). In all the dihydrogen complexes, H-H bond distances of eta(2)-H-2 ligand is temperature-dependent 0.98 angstrom to 0.93 angstrom in the temperature range of 183-233 K. Attempts to synthesize analogous ruthenium hydride complexes bearing phosphine ligands resulted in a mixture of cis and trans- RuHCl(CO)(PPh3)(2)(L)] L = py, 10/11 (trans(HCl)/cis(HCl)); 4Mepy, 12/13 (trans(HCl)/cis(HCl))] complexes. A comparative study has been done to get an insight into the temperature-dependent H-H bond distances in complexes 6-8 by synthesizing analogous ruthenium dihydrogen complexes, RuCl(eta(2)-H-2)(CO)(PPh3)(2) (L)](OTf) (L = py, 15; 4Mepy, 17). All the complexes have been characterized using NMR spectroscopy. The X-ray crystal structures of complexes 2, 3, and 12 have also been determined.