We report that pincer-ligated iridium complexes undergo oxidative addition of the strained C-C bond of biphenylene. The sterically crowded species ((tBu)PCP)Ir ((R)PCP = κ(3)-1,3-C6H3(CH2PR2)2) initially reacts with biphenylene to selectively add the C(1)-H bond, to give a relatively stable aryl hydride complex. Upon heating at 125 °C for 24 h, full conversion to the C-C addition product, ((tBu)PCP)Ir(2,2'-biphenyl), is observed. The much less crowded ((iPr)PCP)Ir undergoes relatively rapid C-C addition at room temperature. The large difference in the apparent barriers to C-C addition is notable in view of the fact that the addition products are not particularly crowded, since the planar biphenyl unit adopts an orientation perpendicular to the plane of the (R)PCP ligands. Based on DFT calculations the large difference in the barriers to C-C addition can be explained in terms of a "tilted" transition state. In the transition state the biphenylene cyclobutadiene core is calculated to be strongly tilted (ca. 50°-60°) relative to its orientation in the product in the plane perpendicular to that of the PCP ligand; this tilt results in very short, unfavorable, non-bonding contacts with the t-butyl groups in the case of the (tBu)PCP ligand. The conclusions of the biphenylene studies are applied to interpret computational results for cleavage of the unstrained C-C bond of biphenyl by ((R)PCP)Ir.

Experimental: General methods and CompoundsGeneral Experimental Methods. All reactions, recrystallizations, and routine manipulations were performed at ambient temperature under argon atmosphere using an argon--filled glove box or by using standard Schlenk techniques. Reagent grade solvents were distilled from sodium/potassium alloy and degassed with argon. Norbornene was purified by sublimation. All deuterated NMR solvents were dried, vacuum--transferred, and stored in an argon--filled glovebox. 1 H and 31 P{ 1 H} NMR spectra were obtained on 400--MHz or 500 MHz Varian spectrometers. 1 H NMR chemical shifts are reported in ppm downfield from tetramethylsilane and were referenced to residual protiated ( 1 H) or deuterated solvent. 31 P{ 1 H} NMR chemical shifts are reported in ppm and were referenced to 85% H3PO4 or a capillary tube containing the external standard Me3P in mesitylene--d12 solvent (δ --62.0 ppm). The compounds tBu4 PCPIrH2 and iPr4 PCPIr(C2H4) were prepared as described in the literature. 14--18 ( tBu4 PCP)Ir(H)(C14H9) (1) tBu4 PCPIrH2 (5.9 mg, 0.010 mmol) was dissolved in 0.5 mL of mesitylene--d12 in a J--Young NMR tube at room temperature, and 2 equivalents of norbornene were added from a stock solution in p--xylene. Phenanthrene (1.1 eq; 0.011 mmol) was added to the resulting solution; after stirring for one minute, the dark red--orange solution turned bright orange. NMR spectra indicate the presence of multiple isomers/rotamers resulting from C--H oxidative addition. 31

No abstract

No abstract

The crystal structure of the title compound, C8H10N2O3S, displays predominant C—H...O hydrogen-bonding and π–π stacking interactions. The hydrogen bonds are between the O atoms of the sulfonyl group and H atoms on methyl groups. The π–π stacking interactions occur between adjacent aromatic rings, with a centroid–centroid distance of 3.868 (11) Å. These interactions lead to the formation of chains parallel to (101).