Journal of Organometallic Chemistry 632 (2001) 197 – 208
www.elsevier.com/locate/jorganchem
Synthesis, bonding and dynamic behavior of
fac -[Mo(II)(CO)
2
(
3
-allyl)] derivatives
J.R. Ascenso
a
, C.G. de Azevedo
a,
*, M.J. Calhorda
b,c
, M.A.A.F. de C.T. Carrondo
b
,
P. Costa
b
, A.R. Dias
a
, M.G.B. Drew
e
, V. Fe ´lix
d
, A.M. Galva ˜o
a
, C.C. Roma ˜o
b
a
Centro de Quı ´mica Estrutural, Complexo I, Instituto Superior Te ´cnico, A. Roisco Pais, 1, 1049 -001 Lisbon, Portugal
b
Instituto de Tecnologia Quı ´mica e Biolo ´gica, A. da Repu ´blica, EAN, Apt 127, 2781 -901 Oeiras, Portugal
c
Departamento de Quı ´mica e Bioquı ´mica, FC, Uniersidade de Lisboa, 1749 -016 Lisbon, Portugal
d
Departamento de Quı ´mica, Uniersidade de Aeiro, 3810 -193 Aeiro, Portugal
e
Department of Chemistry, Uniersity of Reading, Whiteknights, Reading RG62AD, UK
Received 5 March 2001; accepted 10 May 2001
Abstract
Cationic complexes [Mo(
3
-allyl)(CO)
2
(L–L)L]PF
6
, (L–L =C
6
H
5
SCH
2
CH
2
SC
6
H
5
,L =NCCH
3
(1); bipy, NCCH
3
(2); py,
(NCCH
3
)
2
(3); (NCCH
3
)
3
(4); dppe, NCCH
3
(5) and the neutral analogues [Mo(
3
-allyl)(CO)
2
(L–L)X] (L–L =phen (6); bipy (7);
X =Br) were synthesized. Complexes 2, 5, 6 and 7 were characterized by single crystal X-ray diffraction. Depending on the
chelating ligand, these pseudo-octahedral complexes undergo different dynamic processes in solution and NMR spectroscopic
evidence was provided for those studies. The structural trends of the limiting structures depicted by these complexes as well as the
pathways to their inter-conversion were analyzed by ab initio theoretical calculations. Both NMR data and the calculations
showed that for complex 2 the equatorial species predominates at room temperature but that two forms differing only by the
conformation of the allyl coexist. Lowering the temperature leads to the appearance of the equatorial – axial isomer. © 2001
Elsevier Science B.V. All rights reserved.
Keywords: Molybdenum; Allylic complexes; Ab initio calculations; Dynamic NMR; X-ray diffraction
1. Introduction
There is a large number of molybdenum-
3
-allyl-car-
bonyl complexes of general formula [Mo(
3
-C
3
H
5
)-
(CO)
2
L
2
X] and [Mo(
3
-C
3
H
5
)(CO)
2
(L-L)X] in which X
is either a halide or pseudohalide and L is a monoden-
tate ligand and (L–L) a bidentate ligand [1].
These complexes have been found to act as catalysts
for the polymerization of certain dienes [2a,b] (when
X =CF
3
and L–L =CH
3
O(CH
2
)
2
OCH
3
) and the
halide phosphine substituted analogues have been uti-
lized in organic synthesis for allylic alkylations [3a – d].
Therefore, investigations have been carried out to study
the factors that determine their stereochemistry, reactiv-
ity, and possible dynamic processes in solution. There
are studies on the molecular orbital analysis of the
regioselectivity of nucleophilic addition to
3
-allyl com-
plexes and the relative orientation of the
3
-allyl ligand
in [Mo(
3
-C
3
H
5
)(CO)
2
(L–L)L] complexes [4]. The fac -
[Mo(
3
-C
3
H
5
)(CO)
2
] moiety occurs in all
3
-allyldicar-
bonylmolybdenum (II) complexes so far investigated.
The allylic group has its terminal C atoms eclipsing the
carbonyls, which has been demonstrated to be the most
energetically favorable arrangement. Stereochemical
nonrigidity in molybdenum(II) -allyl complexes has
been widely studied [5] and shows that they can adopt,
in the solid state, either a symmetric (A) or a non-sym-
metric (B) structure (Scheme 1).
Although most of the complexes whose structure has
been determined adopt configuration A, and there are
only few examples of structure B [3f], there seems to be
no obvious reason for the choice. In solution, these
complexes exhibit dynamic behavior [6] with intercon-
version of the two species. Several solution dynamic
* Corresponding author. Tel: +351-1-21841-9284; fax: +351-1-
21846-4455/57.
E-mail address: cristina.azevedo@ist.utl.pt (C.G. de Azevedo).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII:S0022-328X(01)00988-3