Neutral Organometallic Halogen Bond Acceptors: Halogen Bonding in Complexes of PCPPdX (X = Cl, Br, I) with Iodine (I(2)), 1,4-Diiodotetrafluorobenzene (F4DIBz), and 1,4-Diiodooctafluorobutane (F8DIBu)

Artikel i vetenskaplig tidskrift, 2012

The behavior of a sterically crowded neutral pincer {2,6-bis[(di-t-butylphosphino)methyll-phenyl}palladium (PCPPd) halides, PCPPdX (X = Cl, Br or I), as XB acceptors with strong halogen bond (XB) donors, iodine (I(2)), 1,4-diiodotetrafluorobenzene (F4DIBz), and 1,4-dliodooctafluorobutane (F8DIBu) were studied in the solid state. The co-crystallization experiments afforded high-quality single crystals of XB complexes PCPPdCl-I(2) (1a), PCPPdBr-I(2) (2a), PCPPdI-I(2) (3a), PCPPdCl-F4DD3z (lb), PCPPdBr-F4DIBz (2b), and PCPPdBr-F8DIBu (2c). The 1:1 iodine complexes (la, 2a, and 3a) all showed a strong halogen bonding interaction, the reduction of the sum of the van der Waals radii of halogen to iodine being 24.6 (la), 23.9 (2a), and 19.4% (3a) with X center dot center dot center dot I-I angles of 177, 176, and 179 degrees, respectively. While the pincer palladium chloride 1 and bromide 2 were crystallographically isomorphous and showed similar XB behavior, the palladium iodide complex, 3, exhibited markedly different properties, and unlike 1 and 2 it does not, under similar conditions, result in XB complexes with the weaker XB donors F4DIBz and F8D1Bu. The results indicate that PCPPd1 is not nucleophilic enough to have XB interactions with other donors than iodine. However, the weaker XB donors F4DIBz and F8DIBu form XB complexes with the chloride 1 and especially with the bromide 2. The prevalence of the halogen bonding with 2 is probably not only electronic in origin, and it seems to offer the best balance between electron poorness and steric availability. The X13 interactions with F4DIBz and F8DIBu are much weaker than with iodine, the reduction of the sum of the van der Wash radii of halogen to iodine being 13.5, 12.3, and 14.6% with C-I center dot center dot center dot X angles between 163 and 179 degrees for lb, 2b, and 2c, respectively, and results in polymeric (center dot center dot center dot 1 center dot center dot center dot F4DIBz center dot center dot center dot 1 center dot center dot center dot F4DIBz center dot center dot center dot center dot)(w) (center dot center dot center dot center dot 2 center dot center dot center dot center dot F4DIBz center dot center dot center dot center dot 2 center dot center dot center dot center dot F4DIBz center dot center dot center dot center dot)(w) and (center dot center dot center dot center dot 2 center dot center dot center dot center dot F8DIBu center dot center dot center dot center dot 2 center dot center dot center dot center dot F8DIBu center dot center dot center dot center dot)(N) one-dimensional zigzag chains in the solid state.