ic970783y_si_001.pdf (386.08 kB)
Trans- and Cis-Water Reactivities in d6 Octahedral Ruthenium(II) Pentaaqua Complexes: Experimental and Density Functional Theory Studies1,2
journal contribution
posted on 1997-12-17, 00:00 authored by Nicolas Aebischer, Elena Sidorenkova, Mauro Ravera, Gábor Laurenczy, Domenico Osella, Jacques Weber, André E. MerbachThe hexaaqua complex of ruthenium(II) represents an ideal
starting material for the synthesis of isostructural
compounds with a
[Ru(H2O-ax)(H2O-eq)4L]2+
general formula. We have studied a series of complexes,
where
L = H2O, MeCN, Me2SO,
H2CCH2, CO, and
F2CCH2. We have evaluated the effect of
L on the cyclic
voltammetric response, on the rate and mechanism of exchange reaction
of the water molecules, and on the
structures calculated with the density functional theory (DFT). As
expected, the formal redox potential,
E°‘(+2/+3), increases with the π-accepting capabilities of the ligands.
For L = N2, the oxidation to Ru(III) is
followed
by a fast substitution of dinitrogen by a solvent molecule, revealing
the poor stability of the Ru(III)−N2
bond.
The water exchange reactions have been followed by 17O
NMR spectroscopy. The variable-pressure and variable-temperature kinetic studies made on selected examples are all in
accordance with a dissociative activation mode
for exchange. The positive activation volumes obtained for the
axial and equatorial water exchange reactions on
[Ru(H2O)5(H2CCH2)]2+
(ΔVax⧧ and
ΔVeq⧧ = +6.5 ± 0.5 and
+6.1 ± 0.2 cm3 mol-1) are the
strongest evidence
of this conclusion. The increasing cis-effect series
was established according to the lability of the
equatorial
water molecules and is as follows: F2CCH2
≅ CO < Me2SO < N2 <
H2CCH2 < MeCN < H2O.
The
increase of the lability is accompanied by a decrease of the
E°‘ values, but no change was found in the
calculated
Ru−H2Oeq bond lengths. The increasing
trans-effect series, established from the lability of the
axial water molecule,
is the following: N2 ≪ MeCN < H2O < CO <
Me2SO < H2CCH2 <
F2CCH2. A variation of the
Ru−H2Oax bond lengths is observed in the calculated
structures. However, the best correlation is found between
the
lability and the calculated Ru−H2Oax bond
energies. It appears, also, that a decrease of the electronic
density
along the Ru−Oax bond and the increase of the lability
can be related to an increase of the π-accepting
capability
of the ligand. For L = N2, the calculations have
shown that the Ru(II)−N2 bond is weak.
Consequently, the
water exchange reaction proceeds through a different mechanism, where
first the N2 ligand is substituted by one
water molecule to produce the hexaaqua complex of Ru(II). The
water exchange takes place on this compound
before re-formation of the
[Ru(H2O)5N2]2+
complex.