The p-methoxy-substituted pincer-ligated iridium complexes, (MeO-(tBu)PCP)IrH(4) ((R)PCP = kappa(3)-C(6)H(3)-2,6-(CH(2)PR(2))(2)) and (MeO-(iPr)PCP)IrH(4), are found to be highly effective catalysts for the dehydrogenation of alkanes (both with and without the use of sacrificial hydrogen acceptors). These complexes offer an interesting comparison with the recently reported bis-phosphinite "POCOP" ((R)POCOP = kappa(3)-C(6)H(3)-2,6-(OPR(2))(2)) pincer-ligated catalysts, which also show catalytic activity higher than unsubstituted PCP analogues (Gottker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 1804). On the basis of nu(CO) values of the respective CO adducts, the MeO-PCP complexes appear to be more electron-rich than the parent PCP complexes, whereas the POCOP complexes appear to be more electron-poor. However, the MeO-PCP and POCOP ligands are calculated (DFT) to show effects in the same directions, relative to the parent PCP ligand, for the kinetics and thermodynamics of a broad range of reactions including the addition of C-H and H-H bonds and CO. In general, both ligands favor (relative to unsubstituted PCP) addition to the 14e (pincer)Ir fragments but disfavor addition to the 16e complexes (pincer)IrH(2) or (pincer)Ir(CO). These kinetic and thermodynamic effects are all largely attributable to the same electronic feature: O --> C(aryl) pi-donation, from the methoxy or phosphinito groups of the respective ligands. DFT calculations also indicate that the kinetics (but not the thermodynamics) of C-H addition to (pincer)Ir are favored by sigma-withdrawal from the phosphorus atoms. The high nu(CO) value of (POCOP)Ir(CO) is attributable to electrostatic effects, rather than decreased Ir-CO pi-donation or increased OC-Ir sigma-donation.

Precursors of the pincer-ligated iridium species, (PCP)Ir, react with nitrobenzene or acetophenone at ambient temperature to give O,C-chelated complexes resulting from addition of an aryl C-H bond and coordination of a nitro or acetyl oxygen. The C-H additions appear to be completely regioselective for the position ortho to the functional group; however, structural characterization and low-temperature NMR studies demonstrate that the reaction does not proceed via coordination of the functional group followed by C-H addition. In the case of nitrobenzene, kinetic preference for the para and meta positions is demonstrated at low temperature. Addition occurs more slowly at the ortho position, without assistance by the functional group; the ortho-C-H addition product is then trapped by chelation.

The iridium pincer complexes (PCP)IrH(4) (1; PCP = [kappa(3)-1,3-(CH(2)P(t)Bu(2))(2)C(6)H(3)]) and (POCOP)IrH(4) (2; POCOP = [kappa(3)-1,3-(OP(t)Bu(2))(2)C(6)H(3)]) have proven to be effective catalyst precursors for dehydrogenation of alkanes. The complex (POCOP)IrH(2) has also been applied successfully as a catalyst for release of H(2) from ammonia borane. Investigation of the "tetrahydride" forms of these complexes by solution NMR methods suggests their formulation as dihydrogen/dihydride species. This is in contrast to the solid state structure of 1, determined by neutron diffraction (at 100 K), which indicates a compressed tetrahydride structure with only weak H-H interactions. Complex 1 (C(24)H(47)IrP(2)) crystallizes in the space group P4(2), tetragonal, (Z = 2) with a = 11.7006 (19) A, c = 9.7008(27) A, and V = 1328.1(5) A(3). Electronic structure calculations on 1 and 2 indicate that the global minima on the potential energy surfaces in the gas phase are tetrahydride structures; however, the dihydrogen/dihydride forms are only slightly higher in energy (1-3 kcal/mol). A dihydrogen/dihydride species is calculated to be the global minimum for 2 when in solution. The barriers to interconversion between the tetrahydride and dihydrogen/dihydride species are almost negligible.

Mucosal associated invariant T (MAIT) cells are striking in their abundance and their strict conservation across 150 million years of mammalian evolution, implying they must fulfill critical immunological function(s). MAIT cells are defined by their expression of a semi-invariant αβ TCR which recognizes biosynthetic derivatives of riboflavin synthesis presented on MR1. Initial studies focused on their role in detecting predominantly intracellular bacterial and mycobacterial infections. However, it is now recognized that there are several modes of MAIT cell activation and these are related to activation of distinct transcriptional programmes, each associated with distinct functional roles. In this minireview, we summarize current knowledge from human and animal studies of MAIT cell activation induced (1) in an MR1-TCR dependent manner in the context of inflammatory danger signals and associated with antibacterial host defense; (2) in an MR1-TCR independent manner by the cytokines interleukin(IL)-12/-15/-18 and type I interferon, which is associated with antiviral responses; and (3) a recently-described TCR-dependent "tissue repair" programme which is associated with accelerated wound healing in the context of commensal microbiota. Because of this capability for diverse functional responses in diverse immunological contexts, these intriguing cells now appear to be multifunctional effectors central to the interface of innate and adaptive immunity.