SYNTHESIS, EXCHANGE-REACTIONS, AND METALLACYCLE FORMATION IN OSMIUM(II) IMIDO SYSTEMS - FORMATION AND CLEAVAGE OF OS-N BONDS

Monomeric, low oxidation state, osmium-imido compounds (eta6-arene)OsN-t-Bu (arene = p-cymene (2a) and C6Me6 (2b)) have been synthesized from [(eta6-arene)OsCl2] (arene = p-cymene (1a) and C6Me6 (1b)) and 4 equiv of LiNH-t-Bu in THF. Further characterization of complex 2a was obtained by N-15 NMR spectrometry on the corresponding N-15-labeled species (2a-N-15). An X-ray crystal structure of complex 2b showed a short Os-N distance (1.737(7) angstrom) and a nearly linear Os-N-C angle (174.1(7)degrees) consistent with osmium to nitrogen multiple bonding. The imide 2a undergoes exchange reactions with H2N(2,6-Me2C6H3) to yield CymOsN(2,6-Me2C6H3) (3) with liberation of H2N-t-Bu. A mechanistic study of this imide/amine exchange showed that the reaction rate is first order in 2a and first order in entering amine, and that k(H)/k(D) = 5.7 when D2N(2,6-Me2C6H3) is used. These data suggest that there action proceeds through a bis(amide) intermediate and that N-H bond cleavage is involved in the rate determining step. Although the bis(amide) complex was not observed by H-1 NMR spectroscopy, the analogous chelated complex CymOs[1,2-(NH)2C6H4] (5) was isolated from the reaction of la with 1,2-(H2N)2C6H4. Imide 3 can also be prepared from 1a and 4.2 equiv of LiNH(2,6-Me2C6H3). The imide CymOsN[2,6-(i-Pr)2Cr6H3] (4) can be prepared using LiNH[2,6-(i-Pr)2Cr6H3]. Compound 2a undergoes cycloaddition reactions with t-BuNCO and RN3 to give the ureylene complex CymOs[(N-t-Bu)2CO] (6) (Cym = eta6-p-Cymene) and tetrazene compounds CymOs[N(t-Bu)N=NN(R)] (R = Ph (7), SiMe3 (8), CPh3 (9), t-Bu (10)). An X-ray crystal structure of phenyl derivative 7 showed a planar OsN4 core with the isopropyl group of the p-cymene ligand tilted away from the large tert-butyl group. Variable temperature NMR studies with 7-9 suggest that the solution conformation of these complexes is dependent on the size of the substituent R.