Zinc amalgam reduction of
tris(acetylacetonato)ruthenium(III),
[Ru(acac)3], in the presence of the
chelating olefinic N- and O-donor ligands (LL‘)
2-vinyl-N,N-dimethylaniline,
o-CH2CHC6H4NMe2
(1),
2-isopropenyl-N,N-dimethylaniline,
o-CH2C(CH3)C6H4NMe2
(2), 3-butenyldimethylamine,
CH2CHCH2CH2NMe2 (3), 2-allylpyridine,
CH2CHCH2C5H4N
(4), isomesityl oxide
(4-methyl-4-penten-2-one),
CH2C(CH3)CH2COCH3
(5), 2-methoxystyrene,
o-CH2CHC6H4OMe
(6), and 3-butenylmethyl ether,
CH2CHCH2CH2OCH3
(7) gives the corresponding
bis(acetylacetonato)ruthenium(II) complexes
[Ru(acac)2(LL‘)] (8−14). These undergo one-electron
oxidation by cyclic voltammetry to the corresponding
cations
[Ru(acac)2(LL‘)]+, the process
being reversible at both room temperature and −60 °C. The
cations were
isolated as deep blue, paramagnetic PF6 or SbF6
salts from the oxidation of the ruthenium(II) precursors
8−12
and 14 with Ag+ or
[FeCp2]+ salts; they are the first stable
alkene complexes of ruthenium(III). At both
oxidation levels, coordination of the prochiral alkene gives rise to a
pair of diastereomers, labeled a, b at
the
Ru(II) level, a
+
,
b
+
at the Ru(III) level, whose
redox potentials E
1/2
(Ru3+/2+) differ by ca. 100 mV. The
equilibrium a/b ratio at the Ru(II) level is
ca. 1:9, although for 8, 10, and 11
this is established only after
several hours at ca. 100 °C, the ratio in the complexes immediately
after isolation being ca. 2:3. Selective
removal of the more easily oxidized diastereomer of the
2-vinyl-N,N-dimethylaniline complex 8a
by treatment
of a 2:3 mixture with ca. 0.5 equiv of Ag+ provides pure
8b, which undergoes reversible one-electron
oxidation
at −60 °C to 8b
+
. Above
−10 °C, 8b
+ isomerizes to an equilibrium
mixture (ca. 85:15) of 8a
+
and
8b
+
, as
shown by UV−visible spectroelectrochemistry. Thus, both
diastereomeric preference and rate of interconversion
are strongly dependent on the oxidation state (number of metal
d-electrons). The metrical parameters pertaining
to alkene coordination in the diastereomers 8a and
8b do not differ significantly, the metal−carbon
distances
being 2.159(4), 2.144(4) Å (8a), 2.142(2),
2.153(3) Å (8b) and the CC distances being
1.383(5) Å (8a) and
1.382(4) Å (8b). The corresponding distances in
the Ru(III) complex
[8a]+[SbF6]-
[Ru−C = 2.239(6), 2.236(7)
Å; CC = 1.355(9) Å] indicate that the alkene is more
weakly bound than in either of its diastereomeric
Ru(II) precursors.