Transition metal pincer complex
In chemistry, a pincer ligand is a type of chelating agent that binds tightly to three adjacent coplanar sites, usually on a transition metal in a meridional configuration.[1] The inflexibility of the pincer-metal interaction confers high thermal stability to the resulting complexes. This stability is in part ascribed to the constrained geometry of the pincer, which inhibits cyclometallation of the organic substituents on the donor sites at each end. In the absence of this effect, cyclometallation is often a significant deactivation process for complexes, in particular limiting their ability to effect C-H bond activation. The organic substituents also define a hydrophobic pocket around the reactive coordination site. Stoichiometric and catalytic applications of pincer complexes have been studied at an accelerating pace since the mid-1970s. Most pincer ligands contain phosphines.[2] Reactions of metal-pincer complexes are localized at three sites perpendicular to the plane of the pincer ligand, although in some cases one arm is hemi-labile and an additional coordination site is generated transiently. Early examples of pincer ligands (not called such originally) were anionic with a carbanion as the central donor site and flanking phosphine donors; these compounds are referred to as PCP pincers.
Scope of pincer ligands
Although the most common class of pincer ligands features PCP donor sets, variations have been developed where the phosphines are replaced by thioethers and tertiary amines. Many pincer ligands also feature nitrogenous donors at the central coordinating group position (see figure), such as pyridines.[3]
An easily prepared pincer ligand is POCOP. Many tridentate ligands types occupy three contiguous, coplanar coordination sites. The most famous such ligand is terpyridine (“terpy”). Terpy and its relatives lack the steric bulk of the two terminal donor sites found in traditional pincer ligands.
Metal pincer complexes are often prepared through C-H bond activation.[4][5]
History
The original work on PCP ligands arose from studies of the Pt(II) complexes derived from long-chain ditertiary phosphines, species of the type R2P(CH2)nPR2 where n >4 and R = tert-butyl. Platinum metalates one methylene group with release of HCl, giving species such as PtCl(R2P(CH2)2CH(CH2)2PR2).[2]
Pincer complexes catalyze the dehydrogenation of alkanes. Early reports described the dehydrogenation of cyclooctane by an Ir pincer complex with a turnover frequency of 12 min−1 at 200 °C. The complexes are thermally stable at such temperatures for days.[6]
References
- ^ The Chemistry of Pincer Compounds; Morales-Morales, D.; Jensen, C., Eds.; Elsevier Science: Amsterdam, 2007. ISBN 0444531386
- ^ a b Jensen, C. M., "Iridium PCP pincer complexes: highly active and robust catalysts for novel homogeneous aliphatic dehydrogenations", Chemical Communications, 1999, 2443–2449. doi:10.1039/a903573g.
- ^ Gunanathan, C.; Ben-David, Y. and Milstein, D., "Direct Synthesis of Amides from Alcohols and Amines with Liberation of H2", Science, 2007, 317, 790-792.doi:10.1126/science.1145295.
- ^ Selander, Nicklas; j. Szabó, Kálmán (2011). "Catalysis by Palladium Pincer Complexes". Chemical Reviews. 111 (3): 2048. doi:10.1021/cr1002112. PMID 21087012.
- ^ Canty, A. J.; Rodemann, T.; Skelton, B. W.; White, A. H. (2006). "Access to Alkynylpalladium(IV) and -Platinum(IV) Species, Including Triorgano(diphosphine)metal(IV) Complexes and the Structural Study of an Alkynyl(pincer)platinum(IV) Complex, Pt(O2CArF)I(C⋮CSiMe3)(NCN) (ArF= 4-CF3C6H4, NCN = \2,6-(dimethylaminomethyl)phenyl-N,C,N]-)". Organometallics. 25 (16): 3996. doi:10.1021/om0601495.
- ^ Gupta, M.; Hagen, C.; Flesher, R. J.; Kaska, W. C.; Jensen, C. M. (1996). "A highly active alkane dehydrogenation catalyst: Stabilization of dihydrido rhodium and iridium complexes by a PCP pincer ligand". Chemical Communications (17): 2083. doi:10.1039/CC9960002083.