SYNTHESIS OF [(ETA(5)-C(5)ME(5))RE(PR(3))(2)(P-N(2)C(6)H(4)OME)][BF4] (R=ME, OME), X-RAY CRYSTAL-STRUCTURE OF [(ETA(5)-C(5)ME(5))RE(CO)(PME(3)) (P-N(2)C(6)H(4)OME)][BF4], AND AN INVESTIGATION OF STEREOCHEMICAL NONRIGIDITY OF A SINGLY-BENT ARYLDIAZENIDO LIGAND

The results of an X-ray structural and variable temperature H-1, C-13{H-1},and (31)p{H-1} NMR study of the half-sandwich complexes [(eta(5)-C(5)Me(5))ReL(1)L(2)(p-N(2)C(6)H(4)OMe)][BF4] (where (a) L(1) = L(2) = CO (1), PMe(3) (10) or P(OMe)(3) (11) and (b) L(1) = CO; L(2) = PMe(3) (3), PEt(3) (4), PPh(3) (5) (where Ph = C6H5), PCy(3) (6) (where Cy = C6H11), P(OMe)(3) (7), or PCage (8) (where PCage = P(OCH2)(3)CMe)) are presented. The new rhenium bis (phosphorus-ligand) complexes 10 and 11 were synthesized by treatment of the cationic bis-acetonitrile complex [(eta(5)-C-5-Me(5))Re(NCMe)(2)(p-N(2)C(6)H(4)OMe)][BF4] (9) with the appropriate phosphine. The (31)p{H-1} NMR spectra of both 10 and 11 showed a single resonance at room temperature which decoalesced into two equal intensity resonances at low temperature, which were assigned to the inequivalent phosphine ligands in the static structure. The (31)p{H-1} variable temperature NMR spectra of 3, 7, and 8 also exhibited a single resonance at room temperature which decoalesced into two unequally populated resonances which were assigned to two stereoisomers. The single-crystal X-ray analysis of 3 revealed that this complex crystallizes in the monoclinic space group Cc with Z = 4, a 12.960(2) Angstrom, b = 13.538(2) Angstrom, c 15.212(3) Angstrom, beta = 108.59(2)degrees, V = 2529.7 Angstrom(3) and the structure was refined to R(F) = 0.019 for 2225 data (I-o greater than or equal to 2.5 sigma(I-o)) and 266 variables. The most important information provided by the X-ray structure of 3 is the specific orientation of the singly-bent aryldiazenido ligand relative to the other co-ligands. The complex has a ground-state structure in which the aryl ring of the aryldiazenido ligand is oriented towards the carbonyl ligand with a torsion angle with respect to the carbonyl co-ligand, defined by C(carbonyl)-Re-N-C(aryl), of -56.0(4)degrees while the torsion angle with respect to the trimethylphosphine co-ligand represented by P-Re-N-C(aryl) is -142.6(5)degrees. Furthermore, both the Re-N-N and the O-C(methoxy) fragments are approximately coplanar with the aryl ring, which suggests a degree of charge delocalization into this ring. Taken together, these results demonstrate that these complexes have ground-state structures in the solid state and in solution in which the aryldiazenido ligand does not orient ''symmetrically'', i.e., with the NNC (aryl) plane bisecting the L(1)ReL(2) angle, but lies ''unsymmetrically'' with the aryl substituent oriented closer to one of the ligands L(1) or L(2)For-the complexes 3, 7, and 8 the major isomer was assigned to the conformer where the aryldiazenido ligand is oriented towards the carbonyl ligand. This assignment is in agreement with the molecular conformation observed in the crystal for 3 and with the observed relative differences in the major:minor isomer ratios for complexes 3, 7, and 8 which are seen to reflect the differing steric properties of the phosphorus ligands. The minor isomer is assigned a structure in which the aryl ring is oriented towards the phosphine ligand. Interconversion between these structures takes place by a conformational isomerization of the aryldiazenido ligand. Thus, compounds of type (a), where L(1) = L(2), i.e., 10 and 11 are 1:1 mixtures of the two possible enantiomers while those of type (b), where L(1) not equal L(2) arthe case of 3, 7, and 8 mixtures of the two possible diastereomers along with their corresponding enantiomers arising from chirality at Re. The barriers to interconversion of the enantiomers in (a) or diastereomers in (b) were measured and the possible mechanisms for the reorientation of the stereochemically non-rigid aryldiazenido ligand in these complexes are discussed.