Unexpectedly lengthened N-H•••Co hydrogen bonds?

Low-temperature crystal structures of QuinH(+)Co(CO)(4)(-), 1 (QuinH(+) = quinuclidinium), (DABCO)H+Co(CO)(3)P-(p-tolyl)(3)(-), 2, and (DABCO)H+Co(CO)(3)PPh2(p-tolyl)(-), 3 (DABCO = 1,4-diazabicyclooctane), 2 and 3 as their acetonitrile solvates, demonstrate that these salts exhibit intermolecular N-H ... Co hydrogen bonding between the cation and anion components. NMR and IR data demonstrate the persistence of these interactions in toluene solution. Such solution-state data, which examine solvated ion pairs, suggest little difference between these salts and the corresponding previously reported salts (DABCO)H+Co(CO)(3)L- (4, L = CO; 5, L = PPh3). However, in the solid state, the N-H ... Co hydrogen bonds in 1-3 are some 0.1-0.15 Angstrom longer than would be predicted from consideration of the structures of 4 and 5 and the aforementioned similarity to 4 and 5 in solution. In previous reports we have shown that major steric or electronic changes to the anion or cation have resulted in substantial changes (0.15-0.3 Angstrom) in the N ... Co [H ... Co] separation for N-H ... Co hydrogen bonds in related R3NH+Co(CO)(3)L- (L = CO, PR3) salts. In this report, we present examples in which small changes are made, to the anion or cation remote from the N-H ... Co hydrogen bond. In the solid state, the effect of these small changes on this hydrogen bond is subsumed by the effect of changes in the supramolecular structure. This clearly indicates the sensitivity of the geometry of these hydrogen bonds to the overall balance of intermolecular interactions in the solid state and as such is pertinent to current interest in weak (intermolecular) interactions for which characterization by X-ray crystallography is important.