Effects of Axial Coordination on the Ru−Ru Single Bond in Diruthenium Paddlewheel Complexes

The 1,8-naphthyridine-based (NP-based) ligands with furyl, thiazolyl, pyridyl, and pyrrolyl attachments at the 2-position
have been synthesized. Reactions of 3-MeNP (3-methyl-1,8-naphthyridine), fuNP (2-(2-furyl)-1,8-naphthyridine),
tzNP (2-(2-thiazolyl)-1,8-naphthyridine), pyNP (2-(2-pyridyl)-1,8-naphthyridine), and prNP-1 (2-(2-pyrrolyl)-1,8-naphthyridine) with [Ru2(CO)4(CH3CN)6]2+ lead to [Ru2(3-MeNP)2(CO)4(OTf)2] (1), [Ru2(fuNP)2(CO)4]2[BF4]2 (2), [Ru2(tzNP)2(CO)4][ClO4]2 (3), [Ru2(pyNP)2(CO)4][OTf]2 (4), and [Ru2(prNP)2(CO)4] (5). The molecular structures of complexes
1−5 have been established by X-ray crystallographic studies. The modulation of the Ru−Ru single-bond distances
with axial donors triflates, furyls, thiazolyls, pyridyls, and pyrrolyls has been examined. A small and gradual increase
in the Ru−Ru distance is measured with various donors of increasing strengths. The shortest Ru−Ru distance of
2.6071(9) Å is observed for the axially coordinated triflates in complex 1, and the longest Ru−Ru distance of
2.6969(10) Å is measured for axial pyrrolyls in complex 5. The Ru−Ru distances in complexes 3 (2.6734(7) Å) and
4 (2.6792(9) Å), having thiazolyls and pyridyls at axial sites respectively, are similar. The Ru−Ru distance for axial
furyls in complex 2 (2.6261(9) Å) is significantly shorter than the corresponding distances in 3, 4, and 5. DFT
calculations provide insight into the interaction of the Ru−Ru σ orbital with axial donors. The Ru−Ru σ orbital is
elevated to a higher energy because of the interaction with axial lone pairs. The degree of destabilization depends
on the nature of axial ligands:  the stronger the ligand, higher the elevation of Ru−Ru σ orbital. The lengthening
of Ru−Ru distances with respect to the axial donors in compounds 1−5 follows along the direction pyrrolyl >
pyridyl ≈ thiazolyl > furyl > triflate, and the trend correlates well with the computed destabilization of the Ru−Ru
σ orbitals.