Journal of Organometallic Chemistry 632 (2001) 49 – 57
www.elsevier.com/locate/jorganchem
Mechanism of the electrochemical reduction of
[Fe(
5
-C
6
H
7
)(CO)
3
][PF
6
] — a theoretical approach to the
intermediates
M. Fernanda N.N. Carvalho
a,
*, M. Ame ´lia N.D.A. Lemos
a
, Luı ´s F. Veiros
a
,
G. Richard Stephenson
b
a
Centro de Quı ´mica Estrutural, Complexo I, Instituto Superior Te ´cnico, A. Roisco Pais, 1049 -001 Lisbon, Portugal
b
School of Chemical Sciences, Uniersity of East Anglia, Norwich NR47TJ, UK
Received 22 January 2001; accepted 5 March 2001
Abstract
A mechanism is proposed for the electrochemical reduction of [Fe(
5
-C
6
H
7
)(CO)
3
][PF
6
] based on cyclic voltammetry and
simulation techniques. In [NBu
4
][X]/CH
3
CN (X =BF
4
or ClO
4
) but not in [NBu
4
][BF
4
]/CH
2
Cl
2
, a rapid equilibrium prior to the
electron transfer process is identified between [Fe(
5
-C
6
H
7
)(CO)
3
][PF
6
] and a species formulated as [Fe(
3
-C
6
H
7
)(CO)
3
(NCMe)]
+
.
The formation of the species under equilibrium involves solvent coordination and
5
to
3
ring slippage of the cyclohexadienyl
ligand as the response of the system to the high electron count. Electrochemical electron transfer to [Fe(
3
-C
6
H
7
)(CO)
3
(NCMe)]
+
affords a highly reactive 19-electron intermediate exhibiting chemical reactivity (ECE mechanism) that leads to the formation of
dimer-type species. A ‘father – son’ type mechanism is proposed for the formation of the products of the electrochemical reduction
of [Fe(
5
-C
6
H
7
)(CO)
3
][PF
6
]. All the species involved in the mechanism were analysed by theoretical means and are proposed on
the basis of calculations made with the B3LYP HF/DFT hybrid functional. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Mechanism; Cyclohexadienyl; Electrochemistry; Ring slippage; Hapticity change; DFT calculations
1. Introduction
The high reactivity of the cyclohexadienyl ligand
coordinated at chromium, iron or manganese in com-
plexes of the type [M(
5
-cyclohexadienyl)(CO)
3
]
n
(n =0
or +1) has been successfully used for regio- and
stereospecific organic syntheses of alkaloids, steroids
and terpenes [1 – 5]. The processes involve nucleophilic
attack at the coordinated six-membered ring directed
by its substituents [1,3a] or the entering nucleophile [6].
The involvement of the metal in the process that leads
to attack to the -ligand was proposed in the mecha-
nism of the addition of PMe
3
to the cyclopentadienyl
ligand coordinated at [Ru(
5
-C
5
H
5
)(
4
-C
5
H
4
O)L]
+
. In
a such case the nucleophilic attack occurs at the metal
thus forming a 20-electron intermediate that undergoes
5
to
3
ring slippage followed by migration of the
phosphine to the ring [7].
Ring slippage from
5
to
3
or
5
to
1
induced by
ligand addition or electrochemical reduction [8 – 10] at
cyclopentadienyl or indenyl ligands has been evidenced
by structural analysis, but as far as we know, no such
evidence exists in the case of the cyclohexadienyl ligand.
The
3
coordination of cyclohexadienyl was proposed
in a few cases [11–13] and it was studied theoretically
in a process involving the addition of phosphine to
[Mn(
5
-C
6
H
7
)(CO)
3
] [14].
The theoretical results obtained within this work
indicate that
5
to
3
haptotropic shifts occur at several
stages of the process associated to the electrochemical
reduction of [Fe(
5
-C
6
H
7
)(CO)
3
][PF
6
].
2. Results and discussion
In [NBu
4
][BF
4
]/CH
3
CN, [Fe(
5
-C
6
H
7
)(CO)
3
][PF
6
](1)
displays by cyclic voltammetry (at scan rates between
0.05 and 0.80 V s
-1
) one irreversible reduction process * Corresponding author.
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII:S0022-328X(01)00802-6