PHOSPHINE DERIVATIVES OF (MU-ETA(2)-METHYLIDYNE)(MU-HYDRIDO)DODECACARBONYLTETRAIRON

A number of phosphine monosubstitution products of the eta-eta(2)-methylidyne tetrairon cluster complex [HFe4(mu-eta(2)-CH)(CO)(12)] (1) have been prepared from 1 and 1 equiv of tertiary phosphine PR(3). In the products [HFe4(mu-eta(2)-CH)(CO)(11)PR(3)] 2a (R = Ph), 2b (Rs = Ph(2)Me), 2c (R(3) = PhMe(2)), and 2d (R = Me), the two hydrogen atoms assume C-H-Fe and Fe-H-Fe bridging positions, as in 1. From 1 and PCy(3) (Cy = cyclo-C6H11) only the salt-libe [HPCy(3)][HFe4C(CO)(12)] was obtained in quantitative yield. The molecular and crystal structure of the phosphine derivative 2a has been determined by single-crystal X-ray diffraction: 2a is monoclinic; space group P2(1)/n; a = 13.136(6), b = 15.084(7), and c = 16.265(4) Angstrom, beta = 100.95(3)degrees; V = 3164(2) Angstrom(3), F(000) = 1616; wR2 = 0.18 [on F-2, all data, 5537 unique reflections]; R(1) = 0.0546 [on F, 2694 reflections with with I < sigma(I)]. The molecular structure of 2a is similar to that of the parent molecule 1 showing a Fee butterfly cluster core with a eta(2)-CH group between the wings and an equatorial PPh(3) ligand bound to a wingtip iron atom. The H(hydride) ligand spans the Fe-4 butterfly hinge. The intermolecular networks of hydrogen bonding interactions established by the mu-eta(2))-methylidyne hydrogen atom and by the bridging hydride with the CO ligands in 2a and 1 have also been investigated. The variable temperature C-13 NMR spectra of isotopically enriched 2a reveal several dynamic processes, which can be explained by intrametal site CO exchange. A slow dynamic process, detectable by H-1 NMR spectroscopy in the T-1 regime, interconverts hydridic and agostic hydrogen sites in the cluster complexes 2. Extended Huckel and Fenske-Hall MO calculations have been performed on 1 and the model complex [HFe4(mu-eta(2)-CH)(CO)(11)PH3] (2e) as well as on the corresponding deprotonated species [1-H](-) and [2e-H](-). The structure of 2 can be rationalized using charge and frontier MO arguments. Fast intramolecular switches of the mu-CH hydrogen between the two wing tips of the iron butterfly framework are predicted by the calculations.