M.A. Beckett et al. / Journal of Organometallic Chemistry 688 (2003) 174ꢂ
/
180
175
of five of these complexes {Mꢀ
/
Mn, Lꢀ
/
P(C6H4Cl-4)3,
the initially formed cis-[MnBr(CO)4L], being thermally
isomerised to mer,trans-[MnBr(CO)3L2]. Our Mn(I)
1/2dppe, 1/2dppf; MꢀRe; Lꢀ
/
/P(C6H4OMe-4)3, 1/
2dppf} are reported. The IR spectra of these complexes
of known stereochemistry are then used as a benchmark
to assign the sterochemistries of the other derivatives on
the basis of their IR spectra.
complexes of monodentate ligands (1ꢂ3), obtained after
/
8 h reflux in CHCl3, are formulated as the mer,trans-
isomers on the basis of spectroscopic evidence and a
crystallographic study of 1.
Spectroscopic evidence for these structural assign-
ments is based on IR and 31P-NMR data. The use of IR
spectra for distinguishing between isomeric metalꢂ
/
2. Results and discussion
carbonyl species based on local symmetry of the CO
groups has been used extensively by organometallic
chemists [4]. However, much early literature on these
fac,cis- and the mer,trans-[MBr(CO)3L2] systems is
confused with two bands sometimes taken (erroneously)
as clear evidence for the fac,cis- product. Using the
point group symmetry of the complexes both the fac,cis
(Cs) and the mer,trans (C2v) isomers, should both show
2.1. Synthesis and stereochemistry of [MBr(CO)3L2]
(MꢀMn, Re) derivatives
/
The complexes [MBr(CO)3L2] {Mꢀ
/
Mn with Lꢀ
/
P(C6H4Cl-4)3 (1), P(C6H4OMe-4)3 (2), P(CH2C6H4)3
(3), 1/2dppe (4), 1/2dppb (5), 1/2dppf (6); Mꢀ
/
Re with
(8),
LꢀP(C6H4Cl-4)3 (7), P(C6H4OMe-4)3
/
3 (2A?ꢁ
bands. Consistent with this, three bands were observed
in the 1850ꢂ 12
2050 cmꢃ1 region for all compounds 1ꢂ
(see Section 3 for data). The relative intensities of these
bands were diagnostic of their structures: 1ꢂ3 showed
two intense bands with a weaker band at higher energy,
whilst 4ꢂ12 showed three bands of approximately equal
/
A?? for fac,cis; 2A1ꢁ
/
B2 for mer,trans) carbonyl
P(CH2C6H4)3 (9), 1/2dppp (10), 1/2dppb (11), 1/2dppf
(12)} were prepared following a literature method [5] by
a stoichiometric reaction in refluxing chloroform for 8 h
(Mn) or 24 h (Re) (Eq. (1)). Satisfactory elemental
analysis was obtained for all compounds and character-
isation data are reported in Section 3. Yields were
moderate to excellent and ranged from 51 to 91% (Mn)
/
/
/
/
intensity but with the absorption at highest energy being
much sharper and slightly more intense. Single-crystal
diffraction studies on representative examples (1, 4, 6, 8,
and 12) confirmed mer,trans- geometries for 1 (and
and 49 to 85% (Re). All compounds except 4 [6ꢂ
/9] are
previously unreported.
[MBr(CO)5] (MꢀMn; Re)ꢁ2L 0[MBr(CO)3L2]
ꢁ2CO
hence 1ꢂ
(and hence 4ꢂ
for 1ꢂ12 are in accord with previous literature formula-
tions for related compounds [6ꢂ9,15ꢂ
18]. 31P-NMR
/3) and fac,cis- structures for 4, 6, 8, and 12
(1)
/12). These general structural assignments
The geometrical configuration about the metal centre
in [MBr(CO)3L2] (MꢀMn, Re) derivatives may be
/
/
/
/
mer,cis-, mer,trans-, or fac,cis-. The [ReBr(CO)3L2]
complexes generally, unless forcing conditions are
employed in the synthesis, have fac,cis- stereochemistry
[10,11], and our products are all formulated as such with
the fac structures of 8 and 12 confirmed by a single-
crystal X-ray studies. The fac,cis- configuration is also
likely to be obtained for Mn(I) derivatives with biden-
spectroscopic evidence was also supportive of the above
formulations with all compounds displaying one signal,
consistent with either the mer,trans or fac,cis config-
uration and inconsistent with the mer,cis configuration
which would be expected to show two signals. Dd(31P)
values (ꢀ/dligand
ꢃ
/
dcomplex/ppm) for complexes 1ꢂ12
/
were generally negative (except 9) demonstrating an
expected high frequency 31P shift upon coordination to
the metal. Generally larger shifts to high frequency are
observed for the Mn(I) derivatives than for the Re(I)
complexes, and this may be due to differences in size an
electron distribution about the metal. The Dd values for
tate organophosphine ligands and complexes 4ꢂ6 are
/
assigned this configuration, with the structures of 4×
/
CHCl3 and 6 confirmed by crystallographic studies. A
recent publication by Pope and Reid [9] has reported the
synthesis of 4 and also describes an X-ray structure of 4×
/
1/2CHCl3. The structures of the [MnBr(CO)3L2] com-
plexes with two monodentate organophosphine ligands
usually have either fac,cis- or mer,trans- stereochem-
istry, and their relative stabilities must be very similar
since Kruger et al. [12] have isolated and crystallogra-
phically characterised both isomers for [MnBr(CO)3{P-
Ph(OMe)2}2]. There are no literature reports of isolation
of the mer,cis- structural type to date, although it
appears that such species have been generated electro-
chemically [13]. It has been argued [5,14] that thermal
substitution of [MnBr(CO)5] proceeds stepwise with the
fac,cis-[MnBr(CO)3L2], obtained from substitution of
complexes 1ꢂ12 fit within the range defined by the
/
extreme values observed for dppm and dppe complexes
[17]. Dd(31P) values for the related complexes
[MnBr(CO)3(PPh3)2]
(Dd
ꢃ
/
59.3),
5.0) were measured and these
and
[Re-
Br(CO)3(PPh3)2] (Dd ꢃ
/
also fit within these dppm/dppe extremes. For Mn(I)
these shifts were not diagnostic of configuration: the
mer,trans-Mn(I)
[MnBr(CO)3(PPh3)2] (Dd ꢃ
the range observed for the fac,cis-Mn(I) complexes 4ꢂ
(Dd ꢃ49.7 to ꢃ82.8).
complexes
1ꢂ
/
3
and
/
54.2 to ꢃ
/
62.0) were within
/6
/
/
Beckett, Michael A.
