| Abstract
| - A high-yield route to the new complexesOsBr2(dppe)2 andtrans-OsHBr(dppe)2 starting from(NH4)2[OsBr6] is described. The new 5-coordinate complexes[OsX(dppe)2]PF6 (X = Cl(8Os) and X = Br (9Os)) areprepared by reaction ofcis-OsX2(dppe)2 withNaPF6. Complexes 8Os and 9Osconsist of distorted trigonal bipyramidalcations with “Y”-shaped equatorial planes. They react inCH2Cl2 with H2 or HD (1 atm)to give complexes trans-[Os(H··H)X(dppe)2]PF6 (X= Cl (1OsPF6), X = Br(2OsPF6)) ortrans-[Os(H··D)X(dppe)2]PF6,respectively. Thelast complexes have J(H,D) = 13.9 and 13.7 Hz,respectively. The BF4- salts of thesecomplexes, 1OsBF4 and2OsBF4, respectively, are prepared by reactingtrans-OsHX(dppe)2 withHBF4·Et2O orDBF4·Et2O. Thesecomplexesare characterized by NMR, IR, and FAB MS. The single-crystal X-rayand neutron diffraction studies of1OsPF6revealed an elongated H··H ligand with dHH= 1.11(6) (X-ray) or 1.22(3) Å (neutron) occupying onesite in anoctahedral complex. The X-ray diffraction study of2OsBF4 produced a similar structure withdHH = 1.13(8) Å.One fluorine of the anion in each structure is positioned near theacidic H2 ligand. A linear relationshipbetweendHH and J(H,D) for many dihydrogencomplexes is used to indicate that complexes 1Os and2Os have H−H distancesof about 1.2 Å in solution. Plots ofln(T1) versus inverse temperature for1Os and 2Os are distorted from theusual“V” shape, suggesting that the rotational frequency of theH2 ligand is near that of the Larmor frequency.Thereforethe dHH for 1Os is between the valuesof 1.04 and 1.31 Å calculated from theT1(min) for fast and slow spinningH2,respectively. The chloride ligand intrans-[Os(H··H)Cl(L)2]+buffers the effect of changing the cis ligands L fromdppe to depe to dcpe so that there is little change in the H−Hdistance. Complexes 1Os and 2Os havepKa valuesof 7.4 and 5.4, respectively, whiletrans-[Os(H2)H(dppe)2]+is much less acidic with a pKa of 13.6.These pKavalues and some E1/2 values are used to showthat 1Os and 2Os are dihydride-like even thoughthey have relativelyshort H−H distances. Properties oftrans-[Os(H··H)Cl(depe)2]BF4(3Os) are also reported.
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