OSMIUM IMIDO COMPLEXES - SYNTHESIS, REACTIVITY, AND SCF-X-ALPHA-SW ELECTRONIC-STRUCTURE

Osmium tetraoxide reacts with 3 equiv of 2,6-diisopropylphenyl isocyanate in refluxing heptane over a period of 20 h to afford red-brown crystalline trigonal planar Os(NAr)3 (1; Ar = 2,6-C6H3-i-Pr2) in modest (50%) yield. 1 reacts with relatively small phosphines and phosphites to afford square-planar complexes of the type trans-Os(NAr)2L2 (4; L = PMe2Ph, PMe3, P(OMe)3) in high yield and with trimethylamine oxide to give Os(NAr)3O, which in turn reacts with olefins C2H2R2 to give metallaimidazolidine complexes, Os[N(Ar)CHRCHRN(Ar)](NAr)(O). An X-ray study of the complex made from ethylene (5b; P1BAR, a = 9.607 (2) angstrom, b = 22.294 (6) angstrom, c = 8.997 (2) angstrom, alpha = 100.08 (2)-degrees, beta = 112.44 (2)-degrees, gamma = 90.85 (2)-degrees, V = 1746.6 (8) angstrom 3, Z = 2, rho (calcd) = 1.977 g cm-3, R1 = 0.034, R2 = 0.047) showed it to be a pseudotetrahedral complex in which each nitrogen atom in the ring is trigonal planar. trans-Os(NAr)2(PMe2Ph)2 (4b) reacts with Me3NO to give Os(NAr)2O2, with methyl or ethyl iodide to give complexes of the type Os(NAr)2(R)I(PMe2Ph), and with iodine to give Os(NAr)2(PMe2Ph)I2 (7). An X-ray structure of 7 (P2(1)/n, a = 15.2139 (9) angstrom, b = 14.0999 (9) angstrom, c = 18.158 (1) angstrom, beta = 106.795 (4)-degrees, V = 3729.0 (8) angstrom 3, Z = 4, rho (calcd) = 1.354 g cm-3, R1 = 0.051, R2 = 0.057) showed it to be a trigonal bipyramid with two imido ligands and one iodide in the equatorial plane. 7 reacts with KS2CNEt2 to give Os(NAr)2(S2CNEt2)2, with 2 equiv of AgOAc to give Os(NAr)2(OAc)2(PMe2Ph) (12), and with 2 equiv of MeMgCl to give Os(NAr)2Me2(PMe2Ph). From 12 can be prepared Os(NAr)2(S-t-Bu)2 and Os(NAr)2R2 (R = CH2-t-Bu, CH2SiMe3). An SCF-X-alpha-SW analysis of Os(NH)3 in D3h symmetry agrees with a qualitative MO description. The HOMO (3a1') is essentially an osmium-centered weakly sigma-antibonding orbital and the LUMO (2e") a low-lying pi-antibonding level. The occupied 1a2' orbital, which is largely nitrogen-centered (76% vs 3% osmium), prevents the species from being a true 20-electron complex. An SCF-X-alpha-SW analysis of Os(NH)2(PH3)2 in C2h symmetry also confirmed the qualitative MO description. The two highest occupied MO's are osmium-based (3b(g) (d(xz)) and 7a(g) (d(z)2)) with the 7a(g) approximately 1 eV higher in energy. The LUMO is an in-plane pi* orbital (8a(g)). An occupied nonbonding level (6b(u)) localized almost entirely at nitrogen and phosphorus prevents this species from being a true 20-electron complex.