Crystal structures and packing of the tricarbonylbis(phosphine)iron(0) complexes trans-Fe(CO)(3)L(2) (L=PPh(2)Me, PPh(3)). Interplay between arene-arene interactions and phosphine conformations

Structural effects of the nature of the phosphines on trans-tricarbonylbis(phosphine)iron()) complexes trans-Fe-(CO)(3)L(2) (L = PPh(2)Me, PPh(3)) are explored. The X-ray crystal structure of trans-Fe(CO)(3)(PPh(2)Me)(2), 1, is reported. Complex 1 crystallizes in the monoclinic space group P2(1)/c with cell parameters a = 15.551(6) Angstrom, b = 9.7024(12) Angstrom, c = 17.320(6) Angstrom, beta = 91.081(11), and Z = 4. Refinement resulted in R = 0.045 and R(W) = 0.054 for 3614 independent reflections with I > 2.0 sigma(I). The structure of 1 is compared to trans-Fe(CO)(3)(PPh(3))(2), 2, and its etherate 2 . OEt(2), 3. Further comparisons are made to the isoelectronic complex trans-[Co(CO)(3)(PPh(3))(2)](+), 4, and the radical cation [Fe(CO)(3)(PPh(3))(2)](.+), 2(.+). Phosphine replacement affects intrinsic structural properties of 1-3 only marginally. In particular, the average Fe-P bond lengths in 1 (2.206 Angstrom), 2 (2.217 Angstrom), and 3 (2.216 Angstrom) all are essentially the same in light of the standard deviations. The structures of the free and complexed phosphines also are essentially identical. Complexes 1-3 show a common motif of distortion from the trigonal planar (pseudo) C-3v bipyramid (P-Fe-P nonlinearity, Fe(CO)(3) C-2v-distortion, and phosphine nonequivalence). Ab initio calculations for trans-Fe(CO)(3)(PH3)(2) at the MP2/LANL1DZ level suggest that these distortions are due to packing. The phosphines are more or less staggered with respect to the carbonyls and the methyl groups are gauche. Co(I) versus Fe(0) replacement retains the trigonal bipyramidal structure while the oxidation of 2 to 2(.+) yields the square pyramidal structure with longer trans Fe-P bonds (2.282 Angstrom). Of special interest are the unexpected phosphine conformations in 1-3 and they are shown to be beneficial for the optimization of intermolecular arene-arene interactions. The crystal packing of 1 reveals displaced face-to-face and displaced T-shape arene-arene contacts that place the two phenyl rings in relative orientations that lead to stabilization in the respective benzene dimers. A rotated displaced T-shape arrangement plays a crucial role in 2. The analyses emphasize the interplay between intermolecular arene-arene packing interactions and the phosphine conformations.