2-electron mixed-valence complexes small molecule activation and photocatalytic hydrogen production

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2001.Vita.Includes bibliographical references.Two-electron mixed-valence complexes of rhodium may be supported by diphosphazane ligands (RN(PY2)2). Whereas diphosphazane ligands with strongly [pi]-acidic phosphine groups (Y = F, OCH2CF3) react with RhI starting materials to afford binuclear Rh20,II complexes, Rh20,II(MeN(PY)2)3X2(L) or Rh2"II(MeN(PY),)3X2 (X = Cl, Br), diphosphazane ligands with weaker [pi]-acid phosphines (Y = OMe, OPh) give only valence-symmetric Rh2I,I complexes, Rh2I,I(MeN(PY2)2X2(L)2. Moreover, the formation of Rh20,II(tfepma)3C12 and [ClRh'(tfepx)]2([mu]-tfepx) (tfepma = MeN[P(OCH2CF3)2]2; tfepx = (3,5-Me2C6H3)N[P(OCH2CF3)2] suggest that delocalization of the nitrogen lone pair into the [pi]-system of the aryl group defeats formation of the two-electron mixed-valence species. These results are interpreted in terms of a polarizable nitrogen lone pair mediating the [pi]-acid properties of the PY2 groups to induce disproportionation of valence-symmetric dirhodium cores. Application of this knowledge to iridium provided the first example of a two-electron mixed-valence complex for this metal, Ir20,II(tfepma)3C12. X-ray diffraction studies reveal a coordination environment with two bridging tfepma ligands and a third tfepma chelating the Ir0 center. Trigonal bipyramidal geometry at the Ir0 is completed by a metal-metal bond to a square pyramidal IrII containing cis-disposed chloride ligands. Ir20-II(tfepma)3C12 is Lewis-acidic, readily accepting donor ligands to form 36 e- complexes. Oxidative addition is also rapid: chlorine and hydrogen chloride react to afford Ir2I-III(tfepma)3C14 and Ir2I,III(tfepma)3HC13, respectively. Hydrogen adds reversibly to Ir20,II(tfepma)3C12, providing the first example of such an addition across a preserved metal-metal single bond.(cont.) Two electron oxidation and reduction reactions also are facile for Rh20,II(dfpma)X2(L), affording Rh2II-II(dfpma)X4 and Rh20,0(dfpma)(L)2 complexes, respectively. These three species form a homologous series of metal-metal bonded complexes with well characterized trigonal bipyramidal Rh0 and octahedral RhII centers. Preservation of the metal-metal bond across the series supports the multi-electron reactivity of the system, as evidenced by photo-induced halogen elimination. We obtained the mixed-valence complex, Rh20-II(dfpma)X2(L), quantitatively when solutions of Rh2(dfpma)3X4 containing excess L were photolyzed in the presence of a halogen-atom trap such as THF. Further irradiation of the Rh20,II(dfpma)X2(L) photoproduct resulted in a second 2e- elimination reaction to give Rh20,0(dfpma)(L)2 in quantitative yield. In the overall transformation, the two-electron mixed-valence LRh0--RhIIX2 compound sustains the multi-electron photoreactivity of the system by coupling the 2e- M-X chemistry of the individual Rh centers. M-X photoactivation from this two-electron mixed-valence platform provides the basis for the photocatalytic production of H2 from HX in homogeneous solution ...by Alan F. Heyduk.Ph.D