SYNTHESIS OF RUTHENIUM PHOSPHINE COMPLEXES WITH HETERODIENE LIGANDS

Monoazadienes (RC(H)=C(H)C(H)=NR'; MAD) react with Ru3(CO)9(PPh3)3 (1) at 115-degrees-C in heptane to give the mononuclear complex Ru(CO)2(PPh3){eta4-RC(H)=C(H)C(H)=NR'} (2a, R, R, = Ph, (i)Pr; 2b, R, R' = Ph, p-Tol). The X-ray crystal structure of 2b has been determined, showing a distorted square pyramidal geometry with three facial sites occupied by the MAD ligand. Crystals of 2b are monoclinic, space group P2(1)/n, with a = 15.688(3) angstrom, b = 12.815(1) angstrom, c = 16.618(3) angstrom, beta = 99.62(1)-degrees, and Z = 4. Convergence was reached at R = 0.026 for 5522 reflections with I > 2.5 sigma(I). In the case of MAD c (R, R' = Me, (i)Pr) cyclometalation and subsequent C-H activation within the R-group takes place, which leads to the formation of Ru2(CO)4(PPh3)2{C(H)2CC-(H)C(H)=N(i)Pr} (3). The thermal fragmentation of 1 has also been examined for other heterodienes, viz. (i)PrN=C(H)C(H)=N(i)Pr ((i)Pr-DAB) and (i)PrN=C(Ph)C(Ph)=O (AOD). In the case of (i)Pr-DAB quantitative formation of Ru(CO)2(PPh3)(sigma-N,sigma-N-(i)Pr-DAB) (4) occurs, whereas the product (5) in the case of the AOD ligand is too air-sensitive to allow characterization but is probably analogous to complex 4. Complex 1 appears to be a convenient starting complex for the synthesis of mononuclear Ru complexes, which are generally poorly accessible. Mononuclear phosphine complexes containing cyclometalated MAD-yl ligands are prepared via opening of the halide bridge in [Ru(CO)2Cl{RC=C(H)C(H)=N(i)Pr}]2 (a, R = Ph; c, R = Me) by PPh3 affording two isomers of Ru(CO)2(PPh3)Cl{RC=C(H)C(H)=N(i)Pr} (6). Complex 6a reacts with NaBH4 to give complex 2a in high yield, revealing that cyclometalation of MAD on Ru(CO)2(PPh3) fragments is thermodynamically unfavorable.