Probing the electronic factors responsible for the cyclic electron-transfer induced isomerism fac ? mer: Synthesis, electrochemical and spectroscopic studies of fac-[Mn(CO)3(L′-L′)L] 0/+ complexes

Article abstract of DOI:10.1016/j.ica.2005.07.012

Spectroscopic (IR, ³¹P NMR and UV-Vis) and electrochemical studies on fac-[Mn(CO)₃(L′-L′)(L)]^0/+,where L′-L′ = 1,2-bis(diphenylphosphino)ethane (dppe) or 1,10-phenanthroline (phen) and L = bromide, triflate, imidazole (im), isonicotinamide (isn) or N-(2-hydroxyethyl)isonicotinamide (heisn), were undertaken to understand the effect of various ligands on the CO-Mn-L and CO-Mn-(L′-L′) bonding characteristics of these complexes. Crystal structures for L = triflate/L′-L′ = dppe, L = triflate/L′- L′ = phen and L = isn/L′-L′ = phen are reported and they show that the two Mn-O(OSO₂CF₃) and Mn-N(isn) distances are similar. The tricarbonyl complexes exhibit two major bands in the 250-300 and 350-450 nm region of the UV-Vis spectrum. The lowest energy bands have been assigned as a contribution from both the metal-centered (MC) and metal to ligand (d_π → L′-L′) charge transfer (MLCT) transitions. The energy of this maximum absorption decreases in the order Br^- ～ triflate > im > isn ～ heisn. The cyclic four-component mechanism fac-Mn(I)→-efac-Mn(II)→mer-Mn(II)→+emer-Mn(I)→fac-Mn(I) was observed at room temperature by voltammetric techniques for all the cases. On the basis of d metal orbital splitting, an electronic molecular orbital diagram is proposed. In this model, the ligands along the z-axis play a relevant role in the reverse of the HOMO energies of the fac/mer isomers by stabilizing the metal dz2 orbital relative to d_xy in mer-Mn(II).

Full text of DOI:10.1016/j.ica.2005.07.012

Inorganica Chimica Acta 358 (2005) 3735–3744
www.elsevier.com/locate/ica
Probing the electronic factors responsible for the cyclic
electron-transfer induced isomerism fac ¢ mer:
Synthesis, electrochemical and spectroscopic studies of
fac-[Mn(CO)3(L0–L0)L]0/+ complexes
a
a
a
a
a
Cid Pereira , Heber G. Ferreira , Mario S. Schultz , Juliana Milanez , Marta Izidoro ,
a
a
a
Paulo C. Leme , Regina H.A. Santos , Maria Teresa P. Gambardella ,
Eduardo E. Castellano , Benedito S. Lima-Neto *, Rose M. Carlos
b
a,
c,
*
a
Instituto de Quı´mica de Sa˜o Carlos, Universidade de Sa˜o Paulo, P.O. Box 780, CEP 13560-970, Sa˜o Carlos, Sa˜o Paulo, Brazil
b
Instituto de Fı´sica de Sa˜o Carlos, Universidade de Sa˜o Paulo, P.O. Box 369, CEP 13560-970, Sa˜o Carlos, Sa˜o Paulo, Brazil
Departamento de Quı´mica, Universidade Federal de Sa˜o Carlos, P.O. Box 676, CEP 13565-905 Sa˜o Carlos, Sa˜o Paulo, Brazil
c
Received 8 March 2005; received in revised form 10 July 2005; accepted 14 July 2005
Available online 22 August 2005
Abstract
31
Spectroscopic (IR, P NMR and UV–Vis) and electrochemical studies on fac-[Mn(CO)3(L0–L0)(L)]0/+,where L0–L0 = 1,2-
bis(diphenylphosphino)ethane (dppe) or 1,10-phenanthroline (phen) and L = bromide, triflate, imidazole (im), isonicotinamide
(isn) or N-(2-hydroxyethyl)isonicotinamide (heisn), were undertaken to understand the effect of various ligands on the CO–Mn–
L and CO–Mn–(L0–L0) bonding characteristics of these complexes. Crystal structures for L = triflate/L0–L0 = dppe, L = triflate/
L0–L0 = phen and L = isn/L0–L0 = phen are reported and they show that the two Mn–O(OSO2CF3) and Mn–N(isn) distances
are similar. The tricarbonyl complexes exhibit two major bands in the 250–300 and 350–450 nm region of the UV–Vis spectrum.
The lowest energy bands have been assigned as a contribution from both the metal-centered (MC) and metal to ligand
(dp ! L0–L0) charge transfer (MLCT) transitions. The energy of this maximum absorption decreases in the order Brꢀ ꢁ tri-
ꢀe
þe
flate > im > isn ꢁ heisn. The cyclic four-component mechanism fac-MnðIÞ ! fac-MnðIIÞ ! mer-MnðIIÞ ! mer-MnðIÞ
! fac-MnðIÞ was observed at room temperature by voltammetric techniques for all the cases. On the basis of d metal orbital split-
ting, an electronic molecular orbital diagram is proposed. In this model, the ligands along the z-axis play a relevant role in the
reverse of the HOMO energies of the fac/mer isomers by stabilizing the metal dz2 orbital relative to dxy in mer-Mn(II).
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Cyclic induced isomerism; Mn(I); Tricarbonyl complexes; Diphosphine; a-Diimine
1. Introduction
in fac-[Mn(CO)3(phen)(im-Z)](SO3CF3) complexes pro-
duces significant changes in the electrochemical behav-
ior without significantly altering their fundamental
electronic structure [1]. In this study, voltammetry mea-
surements of both facial and meridional isomers at room
temperature gave rise to a cyclic process involving fac-
to mer-oxidation induced isomerization [1]. Although
cyclic processes have been well reported for related tran-
sition metal complexes, such as Cr, Mo and W [2,3], as
Previous reports have shown that substitution of
methyl, phenyl or nitro in the imidazole r-donor ligand
*
Corresponding authors. Tel.: + 55 16 3373 9953; fax: + 55 16 3373
9976.
E-mail addresses: benedito@iqsc.usp.br (B.S. Lima-Neto), rosem@
dq.ufscar.br (R.M. Carlos).
0020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.07.012

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C. Pereira et al. / Inorganica Chimica Acta 358 (2005) 3735–3744
far as we know, this was the first reported cyclic scheme
for Mn(I) tricarbonyl complex.
The obtained chemical shifts are reported in ppm rela-
tive to high frequency of 85% H3PO4. EPR measure-
ments were carried out at 77 K using a Brucker ESP
300C apparatus (X-band) equipped with a TE102 cavity
and HP 52152A frequency counter. Cyclic voltammetry
was performed using an EG&G Princeton Applied Re-
search model 264A polarographic analyzer/stripping
voltammeter. Voltammograms were obtained in
CH2CL2 (0.1 mol Lꢀ1 TBAP) at RT in a light-protected
voltammetric cell with a platinum for both the working
and the auxiliary electrodes. A silver wire coated with
silver chloride was used as reference electrode, con-
nected to the bulk of the solution by a luggin capillary
filled with the same solvent and electrolyte. The concen-
For studies defining the nature and magnitude of
these processes, the preparation of metal carbonyl deriv-
atives of varying electronic and steric properties is neces-
sary. Therefore, as the p-bonding abilities of the metal
center should be affected by the ligand field strength, it
would be interesting to extend the range of ligands
investigated to those with varying p-acceptor abilities.
In this paper, we report the synthesis, structural char-
acterization and spectroscopic and electrochemical
properties of a series of mononuclear manganese
compounds of fac-[Mn(CO)3(L0–L0)(L)]0/+, (L0–L0 = 1,
10-phenanthroline (phen), 1,2–bis(diphenylphosphino)-
ethane (dppe); L = bromide (Brꢀ), triflate ðSO3CF ꢀÞ,
tration of the complexes was always kept at 1 mmol Lꢀ1
.
3
imidazole (im), isonicotinamide (isn) and N-(2-hydroxy-
ethyl)isonicotinamide (heisn)), aimed especially to estab-
lish both the similarities and differences in the properties
of homologous systems. The basicity of the axial ligand
trans to CO was chosen in such a way as to modulate the
p-acidity of the manganese center. The studies described
here illustrate the interplay between steric and electronic
effects in determining the spectroscopic and electro-
chemical behavior of Mn(I) tricarbonyl complexes.
2.3. Syntheses
All syntheses and preparations for spectroscopic and
electrochemical experiments were carried out under
purified Ar atmosphere using Schlenk techniques. The
solutions were carefully handled in the dark before the
experiments were performed.
31P NMR data for dichloromethane solutions of the
compounds showed no evidence for either free or oxi-
dated phosphine molecule. EPR was used to confirm
the absence of Mn(II) species.
2. Experimental
2.1. Materials
2.3.1. Preparation of fac-[Mn(Br)(CO)3(dppe)]
The halo complex was prepared by reacting
[Mn(CO)5Br] (0.20 g; 0.73 mmol), freshly sublimated,
with dppe (0.10 g; 0.90 mmol) in 40 mL CH2CL2. The
mixture was stirred for 4 h at RT, after which the
complex was precipitated by addition of cold degassed
n-hexane, yielding a yellow-orange powder that was
filtered off and dried under vacuum. Yield = 88%.
Elemental analysis: Calc. C, 56.8; H, 3.9. Found: C,
56.4; H, 3.9%.
[Mn(Br)(CO)5] was from Strem. 1,2-Bis(diphenyl-
phosphine)ethane (dppe), 1,10-phenanthroline (phen),
silver triflate (Ag(SO3CF3), imidazole (im), isonicotina-
mide (isn) and N-(2-hydroxyethyl)isonicotinamide
(heisn) were from Sigma. Methylene chloride, acetoni-
trile, methanol, ethanol and acetone, HPLC grade, were
distilled from the appropriate drying agents immediately
prior to use. Tetrabutylammonium perchlorate (TBAP)
from Eastman Kodak used as supporting electrolyte for
electrochemical experiments was dried overnight under
vacuum at 50 ꢁC and stored under vacuum before use.
2.3.2. Preparation of fac-[Mn(SO3CF3)(CO)3(dppe)]
The complex fac-[Mn(SO3CF3)(CO)3(dppe)] was
prepared by a procedure different from that previously
published for fac-[Mn(SO3CF3)(CO)3(dppe)] [4]. Ag
(SO3CF3) (0.083 g; 0.324 mmol) was added to a solution
of fac-[MnBr(CO)3(dppe)] (0.20 g; 0.324 mmol) in 40
mL CH2Cl2. The mixture was stirred for 1 h at RT. Dur-
ing this time a white precipitate was formed. The reac-
tion mixture was filtered and the resulting solution was
concentrated in a vacuum to ca. 10 mL. Yield = 78%.
Elemental analysis: Calc. C, 52.4; H, 3.6. Found: C,
52.3; H, 3.9%.
2.2. Physical measurements
Unless otherwise specified, all manipulations were
carried out under argon atmosphere at room tempera-
ture (RT; 24 1 ꢁC).
Elemental analyses were performed on an EA 1110
CHNS-O Carlo Erba Instrument. IR spectra were ob-
tained in KBr disks on a Bomem FT-IR MB 102 spec-
trometer in the range of 4000–400 cmꢀ1. UV–Vis
spectra were measured at RT in dichloromethane solu-
tion with a Varian mod. Cary 5G UV–Vis NIR spectro-
photometer using quartz cuvettes (10 mm). 31P{1H}
NMR spectra were obtained in CH2Cl2 solution at
25.0 0.1 ꢁC using a Brucker AC 200 spectrometer.
2.3.3. Preparation of fac-[Mn(CO)3(dppe)(L)]
(SO3CF3)
The following procedure, described for fac-
[Mn(CO)3(dppe)(im)](SO3CF3), is representative of the

C. Pereira et al. / Inorganica Chimica Acta 358 (2005) 3735–3744
3737
synthesis of the other substituted triflate complexes
(L = isn or heisn). Imidazole (0.02 g; 0.29 mmol) was
added to a solution of fac-[Mn((SO3CF3)(CO)3(dppe)]
(0.20 g; 0.29 mmol) in CH2CL2 (40 mL) and the mixture
was stirred for 24 h at RT. The solution was concen-
trated to ca. 10 mL, mixed with cold ethyl-ether, filtered
and vacuum dried. A yellow crystalline solid was ob-
tained. Elemental analyses: L = im; yield = 85%: Calc.
C, 52.3; H, 3.8; N, 4.8. Found: C, 52.2; H, 3.8; N,
4.7%. L = isn; yield = 85%: Calc. C, 53.5; H, 3.7; N,
3.5. Found: C, 52.2; H, 3.7; N, 3.5%. L = heisn;
yield = 85%: Calc. C, 53.5; H, 4.0; N, 3.3. Found: C,
52.3; H, 4.1; N, 3.5%.
Single crystals of fac-[Mn(CO)3(phen)(isn)](SO3CF3)
(3) were obtained from a dichloromethane solution
maintained in the refrigerator for 7 days, under argon
atmosphere. A yellow crystal (0.10 mm · 0.08 mm
· 0.08 mm) was mounted in a Lindemann capillary
and placed in the beam of an Enraf-Nonius Kappa
CCD diffractometer. Unit cell data and diffraction
intensities were collected at 296K using Mo Ka radia-
tion (k = 0.71073).The optimized cell parameters were
obtained using 25 reflections. The intensity data were
reduced using WinGX [6] and the PSISCAN
absorption correction was applied [7]. The crystal
structure was solved by SIR-92 [8] and refined by
SHELXL-97 [9]. The geometrical analyses were applied
using PLATON [10].
2.3.4. Preparation of fac-[Mn(Br)(CO)3(phen)]
The fac-[Mn(Br)(CO)3(phen)] was prepared from
[Mn(Br)(CO)5], freshly sublimated, by a RT reaction
with 1-10-phenanthroline in CH2CL2 as described
elsewhere [5]. Yield = 85%. Elemental analysis: Calc.
C, 45.1; H, 2.0; N, 7.0. Found: C, 45.9; H, 2.2; N, 7.3%.
The crystal parameters and refinement conditions are
shown in Table 1.
Crystallographic data (excluding structure factors)
for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data
Centre.
2.3.5. Preparation of fac-[Mn(SO3CF3)(CO)3(phen)]
The fac-[Mn(SO3CF3)(CO)3(phen)] complex was
synthesized by the direct reaction of fac-[Mn(Br)(CO)3
(phen)] with Ag(SO3CF3) [1]. Yield = 90% Elemental
analysis: Calc. C, 39.5; H, 2.1; N, 5.8. Found: C, 39.5;
H, 2.2; N, 5.7%.
3. Results and discussion
3.1. Description of structure
The molecular structures of fac-[Mn(SO3CF3)
(CO)3(dppe)], fac-[Mn(SO3CF3)(CO)3(phen)] and fac-
[Mn(CO)3(phen)(isn)](SO3CF3) are shown in Figs. 1
and 2. Selected bond distances and angles are listed in
Tables 2–4.
2.3.6. Preparation of fac-[Mn(CO)3(phen)(L)]
(SO3CF3)
They were prepared by reacting fac-[Mn(SO3CF3)
(CO)3(phen)] (0.20 g; 0.427 mmol) with L = im or isn
(0.29 mmol) in CH2CL2 (40 mL). Elemental analyses:
L = im; yield = 90%: Calc. C, 42.5; H, 2.2; N, 10.4.
Found: C, 43.0; H, 2.2; N, 10.6%. L = isn; yield = 85%:
Calc. C, 44.7; H, 2.4; N, 9.5. Found: C, 43.6; H, 2.5; N,
9.2%. L = heisn; yield = 85%: Calc. C, 45.4; H, 2.9; N,
8.8. Found: C, 45.1; H, 2.8; N, 8.5%.
In fac-[Mn(SO3CF3)(CO)3(dppe)], the metal is in a
distorted octahedral configuration. Phosphorous atoms
from dppe ligand and the carbon atoms from two
carbonyl groups define the equatorial plane. Another
carbonyl carbon and an oxygen atom from a triflate
ligand complete the coordination sphere in apical
positions, forming an angle of 174.8 (3)ꢁ. The three
carbonyls are in facial position and the triflate ligand
is trans to a carbonyl. The Mn–P [2.337(2) and
2.4. Crystal structure analyses
˚
Single crystals of fac-[Mn(SO3CF3)(CO)3(dppe)] (1)
were obtained by the diffusion of cold degassed n-hexane
into a dichloromethane solution. An orange-yellow crys-
tal (0.10 mm · 0.08 mm · 0.05 mm) was mounted and
placed in the beam of a CAD4 diffractometer. Unit cell
data and diffraction intensities were collected at 273K
using Mo Ka radiation (k = 0.71073).
Single crystals of fac-[Mn(SO3CF3)(CO)3(phen)] (2)
were obtained from a dichloromethane solution main-
tained in the refrigerator for 4 days, under argon atmo-
sphere. An orange-yellow crystal (0.15 mm · 0.25
mm · 0.50 mm) was mounted and placed in the beam
of a CAD4 diffractometer. Unit cell data and diffraction
intensities were collected at 263 K using Mo Ka radia-
tion (k = 0.71073).
2.348(2) A] and Mn–C(CO) distances [1.814(9) and
˚
1.815(8) A] in the equatorial plane are in agreement with
those in the trans OC–Mn–P moiety in the bromide
complex fac-[Mn(Br)(CO)3(dppe)], [2.314(3) and
˚
˚
2.327(3) A; 1.81(1) and 1.83(1) A] [11] and in the aqua
complex fac-[Mn(CO)3(dppe)(H2O)](BF4), [2.347(1)
˚
˚
and 2.354(2) A; 1.835(2) and 1.836(2) A], respectively
[4]. The P–Mn–P bite angle of 83.14(8)o agrees with
the values of 84.14(10)ꢁ for fac-[Mn(Br)(CO)3(dppe)]
[11], 84.6(1)ꢁ for fac-[Mn(CO)3(dppe)(OC(O)OCH3)]
[12] and 83.73(3)ꢁ for fac-[Mn(CO)3(dppe)(H2O)](BF4)
[4].
In fac-[Mn(SO3CF3)(CO)3(phen)], the metal is in an
approximate octahedral configuration similar to that
in fac-[Mn(SO3CF3)(CO)3(dppe)]. The phen ligand and

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C. Pereira et al. / Inorganica Chimica Acta 358 (2005) 3735–3744
Table 1
Crystal data and X-ray experimental parameters for fac-[Mn(SO3CF3)(CO)3(dppe)] (1), fac-[Mn(SO3CF3)(CO)3(phen)] (2), and fac-[Mn(CO)3(phen)
(isn)](SO3CF3) (3)
Complex
1
2
3
Empirical formula
Formula weight
Crystal system
Space group
C30H24F3MnO6P2S
686.43
monoclinic
P21/c
15.646(4),
10.031(8)
C16H8F3MnN2O6S
468.25
monoclinic
P21/n
6.917(2)
13.137(3)
C22H14F3MnN4O7S
590.37
monoclinic
P21/n
11.508(1)
14.006(1)
˚
a (A)
˚
b (A)
˚
c (A)
19.324(3)
19.867(5)
15.186(1)
b (ꢁ)
V (A )
102.219(10)
2964.1(9)
99.55
1780.3(8)
99.464(2)
2414.38(8)
3
˚
Z
2
4
4
Dcalc (Mg mꢀ3
)
1.538
0.686
1400
1.747
0.928
936
1.624
0.708
1192
Absorption coefficient, l (mmꢀ1
F(000)
)
Crystal size (mm)
T (K)
0.10 · 0.08 · 0.05
273(2)
0.15 · 0.25 · 0.50
263(2)
0.10 · 0.08 · 0.08
296(2)
Limiting indices
0 6 h 6 20, 0 6 k 6 13,
ꢀ25 6 l 6 24
2.28–27.63
7139/6896 (0.0886)
100.0
0.9665 and 0.9346
full-matrix least-squares on F2
6896/0/388
0.0339, 0.1710, 0.895
0.583 and ꢀ0.767
0 6 h 6 8, 0 6 k 6 16,
ꢀ25 6 l 6 25
2.59–26.99.0
4377/3868 (0.0648)
99.9
0.7391 and 0.8016
full-matrix least-squares on F2
3868/0/262
0.0534, 0.143, 0.945
0.345 and ꢀ0.379
ꢀ13 6 h 6 13, ꢀ16 6 k 6 16,
ꢀ17 6 l 6 18
1.99–25.00
33008/4245 (0.1263)
99.9
0.9442 and 0.9326
full-matrix least-squares on F2
4245/0/352
0.0441, 0.1140
0.322 and ꢀ0.480
h Range for minimum to maximum (ꢁ)
Reflection collected/unique (Rint
Completeness to h maximum (%)
Maximum and minimum transmission
Refinement method
Data/restraints/parameters
R1, wR2, S
Largest difference in peak and hole (e A
)
ꢀ3
˚
)
two carbonyl groups are in the equatorial plane, with
the triflate ligand and a carbonyl forming an angle of
176.6 (3)ꢁ in apical positions.
In the triflate coordinated complexes, the Mn–
N(phen) distances of 2.062(4) and 2.053(4) A are shorter
than the Mn–P in the dppe complex (2.337(2) and
˚
0
0
˚
In fac-[Mn(CO)3(phen)(isn)](SO3CF3), the Mn atom
also exhibits octahedral coordination with the two
N(phen) atoms and two terminal carbonyl groups in
the equatorial positions, and one of the terminal C
atom of the carbonyl group and the N(isn) occupy
the apical positions forming an angle of 178.27(10)ꢁ.
The distances and angles are the expected ones for
this kind of compounds as [{fac-Mn(CO)3(phen)}2
(l-CN)]PF6 [13] or fac-[Mn(CO)3(phen)(1,8-naphthy-
ridine)]ClO4 [14].
2.348(2) A). Further, the angle L -Mn–L in the case
with phen (79.9(2)ꢁ) is smaller than in the case with dppe
(83.14(8)ꢁ), reflecting the small hole size of the phenan-
throline. This comparison is also similar for the isn
derivative (79.85(8)ꢁ) and for other fac-Mn(I) complexes
with phen [13,14].
Relative to bonds involving sp2 nitrogen atoms, the
Mn–N(im) distance in fac-[Mn(CO)3(dppe)(im)]
(SO3CF3) is expected to be close to that found in fac-
[Mn(CO)3(phen)(isn)](SO3CF3) (Mn–N(isn), 2.107(2)
Whereas for fac-[Mn(SO3CF3)(CO)3(L0–L0)], the
mean interatomic distances Mn–CO and Mn–
O(SO3CF3) are not sensitive to the substitution of phen
for dppe (see Tables 2 and 3), the Mn–L distance in fac-
A) since this value is close to that found in fac-
˚
˚
[Mn(CO)3(dppe)(H2O)](PF6) (Mn–O(H2O), 2.075(2) A
[4]), a weaker r-donor ligand than im and isn. Conse-
quently, it can be inferred that the Mn–L distances
follow the sequence Brꢀ > trif ꢁ im ꢁ isn ꢁ heisn, so
that variations in bonding between the three latter
ligands are not structurally dominant.
˚
[Mn(SO3CF3)(CO)3(dppe)] (2.109(5) A) is significantly
shorter than those found in fac-[Mn(Br)(CO)3(dppe)]
˚
(2.504(2) A [11]) and fac-[Mn(I)(CO3)(bpy)] (2.724(2)
˚
A [15]) in which the coordinated halides are a substan-
tially better p donors than triflate.
3.2. 31P and 13C NMR spectroscopy
The Mn–C(CO) distance in the axial plane for fac-
˚
[Mn(CO)3(phen)(isn)](SO3CF3), 1.812(3) A, is some-
The compounds fac-[Mn(CO)3(dppe)(L)](SO3CF3)
show a single resonance in their 31P NMR spectra, con-
sistent with the facial configuration (Table 5). The small
31P NMR downfield shift when compared to the neutral
bromo precursor complex is attributed to charge migra-
tion from the axial ligand via r-donation or p-back-
what longer than that found in fac-[Mn(SO3CF3)
(CO)3(phen)] (1.783(7) A) and fac-[Mn(SO3CF3)(CO)3
(dppe)] (1.797(8) A). This may be attributed to the
moderated p-acceptor nature of the isn increasing the
p-electron competition.
˚
˚

C. Pereira et al. / Inorganica Chimica Acta 358 (2005) 3735–3744
3739
Table 2
˚
Selected bond distances (A) and angles (ꢁ) for fac-[Mn(SO3CF3)
(CO)3(dppe)] (1)
˚
Bond distances (A)
Mn–C1
Mn–C2
1.814(9)
1.815(8)
1.797(8)
2.109(5)
2.337(2)
2.348(2)
1.150(8)
1.157(8)
1.133(8)
Mn–C(3)
Mn–O(4)
Mn–P(1)
Mn–P(2)
C(1)–O(1)
C(2)–O(2)
C(3)–O(3)
Bond angles (ꢁ)
P(1)–Mn–P(2)
C(1)–Mn–P(1)
C(3)–Mn–P(1)
C(2)–Mn–P(1)
O(4)–Mn–P(1)
C(1)–Mn–P(2)
C(2)–Mn–P(2)
C(3)–Mn–P(2)
O(4)–Mn–P(2)
C(3)–Mn–O(4)
83.14(8)
173.7(2)
90.3(2)
93.3(2)
86.5(1)
90.6(2)
176.0(2)
94.3(2)
81.2(1)
174.8(3)
Table 3
˚
Selected bond distances (A) and angles (ꢁ) for fac-[Mn(SO3CF3)(CO)3
(phen)] (2)
˚
Bond distances (A)
Mn–C(1a)
Mn–C(2a)
Mn–C(3a)
Mn–O(3)
Mn–N(1)
Mn–N(2)
O(1a)–C(1a)
O(2a)–C(2a)
O(3a)–C(3a)
N(2)–C(11)
C(11)–C(12)
S–O(1)
1.783(7)
1.813(6)
1.805(6)
2.096(4)
2.062(4)
2.053(4)
1.141(8)
1.141(8)
1.159(8)
1.372(7)
1.413(7)
1.423(4)
1.422(4)
1.459(4)
1.805(7)
Fig. 1. ORTEP projections of fac-[Mn(SO3CF3)(CO)3(dppe)] (1) and
fac-[Mn(SO3CF3)(CO)3(phen)] (2).
S–O(2)
S–O(3)
S–C(13)
Bond angles (ꢁ)
O(3)–Mn–N(1)
O(3)–Mn–N(2)
O(3)–Mn–C(1)a)
O(3)–Mn–C(2a)
O(3)–Mn–C(3a)
N(1)–Mn–N(2)
N(1)–Mn–C(1a)
N(1)–Mn–C(2a)
N(1)–Mn–C(3a)
N(2)–Mn–C(1a)
N(2)–Mn–C(2a)
N(2)–Mn–C(3a)
C(1a)–Mn–C(2a)
C(1a)–Mn–C(3a)
C(2a))–Mn–C(3a))
O(1)–S–O(2)
84.1(2)
82.1(2)
176.3(2)
91.7(2)
95.4(2)
79.9(2)
93.9(2)
172.5(2)
96.9(2)
94.6(2)
93.4(2)
176.0(2)
89.9(3)
87.9(3)
89.8(3)
117.9(3)
132.7(2)
Mn–O(3)–S
Fig. 2. ORTEP projection of fac-[Mn(CO)3(phen)(isn)](SO3CF3).

3740
C. Pereira et al. / Inorganica Chimica Acta 358 (2005) 3735–3744
Table 4
structure derived from the X-ray analyses as well as ob-
served for similar fac-Mn(I) complexes with diphos-
phines [16,17] . Two bands arise from the symmetric
and asymmetric stretching vibrations of the two equiva-
lents cis-COÕs Cs symmetry) [16,17]. The remaining band
at lowest frequency is assigned to the stretching mode of
the CO trans to L since the greatest degree of Mn@CO p
bonding is expected for fac-[Mn(Br)(L0–L0)(CO)3],
where Brꢀ is a good p-donor. The increase of the lowest
energy CO frequency in going from Brꢀ to the isn ligand
(Brꢀ, triflate, im, heisn, isn) is consistent with the trans-
influence of the axial ligands in agreement with the
increased p-electron competition.
˚
Selected bond distances (A) and angles (ꢁ) for fac-[Mn(CO)3
(phen)(isn)](SO3CF3) (3)
˚
Bond distances (A)
Mn–C(21)
Mn–C(20)
Mn–C(19)
Mn–N(1)
Mn–N(2)
Mn–N(3)
O(15)–C(18)
O(19)–C(19)
O(20)–C(20)
O(21)–C(21)
1.812(3)
1.819(3)
1.810(3)
2.051(2)
2.059(2)
2.107(2)
1.227(3)
1.145(3)
1.142(4)
1.141(3)
Bond angles (ꢁ)
C(21)–Mn–C(20)
C(21)–Mn–C(19)
C(20)–Mn–C(19)
C(21)–Mn–N(1)
C(19)–Mn–N(2)
C(19)–Mn–N(1)
C(21)–Mn–N(1)
C(20)–Mn–N(1)
C(19)–Mn–N(1)
N(1)–Mn–N(2)
C(21)–Mn–N(3)
87.83(13)
90.34(12)
88.01(13)
90.91(10)
173.71(10)
94.35(10)
90.91(10)
177.33(11)
94.35(10)
79.85(8)
3.4. Electronic absorption spectra
Electronic absorption spectra for fac-[Mn(CO)3-
(L0–L0)(L)]0/+ complexes in CH2Cl2 solution are shown
in Figs. 3 and 4. Table 6 summarizes the absorption
positions and absorptivities for a number of complexes
relevant to this investigation. In addition to ligand
localized absorption bands (<300 nm), the electronic
spectra of these complexes are dominated by relatively
intense bands in the visible region (350–460 nm).
178.27(10)
The spectrum of the fac-[Mn(Br)(CO)3(dppe)]
complex is characterized by one low energy absorption
band in the visible region kmax = 388 nm, e = 1300 molꢀ1
L cmꢀ1). This wavelength and intensity are comparable to
those of MC in [Mn(Br)(CO)5] [18], and [Mn(Br)(CO)3-
(CH3CN)2] [19] (Table 6); thus, a similar assignment
would be sound for that band. This maximum absorption
becomes a strong band in the corresponding triflate com-
plex. In this case, the high e value is most likely an MLCT
band due to the Mn(dp) ! dppe transition, as will be
shown below. For L = im, the maximum absorption is
shifted to higher energy as compared to the neutral pre-
cursor, as it should due to the greater ligand field strength
Table 5
Selected IR and NMR spectral data for fac-[Mn(CO)3(L0–L0)(L)]0/+
complexes
31P{1H} NMRb
13C NMRc
a
L0–L0
dppe
L
mCO
Brꢀ
2014; 1944; 1911 70.0
triflate 2027; 1959; 1932 72.0
im
heisn
isn
2026; 1955; 1936 76.1
2029; 1957; 1940 76.3
2029; 1965; 1947 76.4
phen
Brꢀ
2019; 1944; 1919
153.4
153.3
153.4
triflate 2042; 1953; 1922
im
heisn
isn
2037; 1941; 1921
2042; 1952; 1936
2042; 1951; 1943
a
In KBr; m in cmꢀ1
In CD2Cl2; in ppm; free dppe = 40.0 ppm.
.
b
c
Ref. [1]; in CD2Cl2; in ppm; free imine = 150.3 ppm.
bonding without much compensation by the dishielding
effect of r-donation at the phosphorous ligand. Similar
behavior is observed for the corresponding 13C NMR of
immine ligand phenanthroline substituted complexes
(Table 5), which show that the molecule is fairly rigid
and therefore consistent with the facial configuration.
3.3. Infrared spectroscopy
The infrared spectrum of fac-[Mn(CO)3(L0–L0)(L)]0/+
complexes in the terminal carbonyl region shows three
strong bands between 2050 and 1950 cmꢀ1 (Table 5).
This is consistent with the facial arrangement of the
three COÕs in the coordination sphere shown from the
Fig. 3. Electronic absorption spectra of fac-[Mn(CO)3(dppe)L]0/+
complexes in CH2Cl2. (a) L = Brꢀ; (b) L = triflate; (c) L = im; (d)
L = isn; (e) L = heisn.

C. Pereira et al. / Inorganica Chimica Acta 358 (2005) 3735–3744
3741
bution. In fact, the position and intensity of the higher
energy absorption in fac-[Mn(CO)3(dppe)L]0/+ are sen-
sitive to the nature of L, but are not solvent dependent
(CH3CN, CH3OH, CH2CL2, CHCl3 or DMSO). These
data are best rationalized by considering the r-charge
densities on the metal center. Going from L = Brꢀ to tri-
flate and im, more charge will be donated to the metal,
thus increasing the metal contribution to the highest
filled orbital. As a result, the intensity of the lowest en-
ergy absorption band will increase. The energy of the
transition between 390 and 320 nm decreases in the or-
der Brꢀ ꢁ triflate > im > isn ꢁ heisn in accord to in-
creases in the spectrochemical series of ligands. The
energetic dependence of the first band in dppe with re-
spect to L is as found in [Re(CO)5X] [21].
Changing the equatorial ligand from dppe to phen re-
sults in a shift of the absorption maxima to longer wave-
lengths in all cases, indicating that the transitions are
directed to the p* (phen) orbital. This may be related
to the well-defined structural organization of the phen
ligand and its small hole size compared to that of the
phosphine, which is a non-rigid molecule. This behavior
suggests an MLCT band in the visible region.
Fig. 4. Electronic absorption spectra of fac-[Mn(CO)3(phen)L]0/+
complexes in CH2Cl2. (a) L = Brꢀ; (b) L = triflate; (c) L = im; (d)
L = isn; (e) L = heisn.
Table 6
Electronic absorption data for a series of fac-Mn(I) complexes
Complex
k )
max/nm (e/molꢀ1 L cmꢀ1
By analogy with the fac-[Mn(CO)3(dppe)L]+ ion
complexes, the two well-separated bands in fac-
[Mn(Br)(CO)5]a
385 (1.700)
[Mn(Br)(CO)3(CH3CN)2]b
[Mn(Br)(CO)3(dppe)]
[Mn(SO3CF3)(CO)3(dppe)]
[Mn(CO)3(dppe)(im)]+
[Mn(CO)3(dppe)(isn)]+
[Mn(CO)3(dppe)(heisn)]+
[Mn(Br)(CO)3(phen)]
[Mn(SO3CF3)(CO)3(phen)]
[Mn(CO)3(phen)(im)]+
[Mn(CO)3(phen)(isn)]+
[Mn(CO)3(phen)(heisn)]+
375 (1.500)
388 (1.300)
390 (3.050)
356 (2.700)
329 (4.500) 360sh (3.500)
329 (3.200) 360sh (2.100)
380 (3.300) 425 (3.400)
356 (3.700) 405 (4.000)
376 (4.200)
350sh (2.500) 400sh (1.800)
400sh (2.100)
[Mn(Br)(CO)3(phen)]
and
fac-[Mn(SO3CF3)(CO)3
(phen)] are assigned to MLCT and MC transitions.
The increased intensity obtained for L = triflate with re-
spect to that of the bromo complex is explained by the
increased metal character in the metal-triflate bonding
combination. Going from triflate to im, a better r-do-
nor ligand, it will increase the metal contribution to
the highest filled orbital and consequently the intensity
of the first band will increase overlapping the MC band.
It is interesting to note that for all the complexes
studied, MC and MLCT bands dominated the UV–Vis
absorption spectrum in the 350–460 nm region, while a
very small and solvatochromic behavior was only ob-
served for fac-[Mn(CO)3(phen)(im)]+. This finding leads
to the conclusion that the energy of the MLCT transi-
tion is almost entirely due to overlap stabilization and
in going from L = Brꢀ to heisn, the MLCT state moves
to higher energy by a greater amount than do the MC
states. Thus, in the case of bromide, triflate and imida-
zole, the MC and MLCT transitions are overlapping as
in the isoelectronic [Mo(CO)5L] [21] but for [Mn(CO)3
(L0–L0)L]+, L0–L0 = dppe or phen and L = isn or heisn,
the MLCT transitions are well below the MC bands.
a
Ref. [20].
Ref. [21].
b
of im versus that of Brꢀ and to the concomitant rise in en-
ergy of the metal d orbitals. Consequently, the MC
absorptions in fac-[Mn(CO)3(dppe)(L)]0/+
,
where
L = triflate or im, are obscured by the more intense
Mn(dp) ! dppe CT absorption. But, since the immediate
coordination sphere is the same for all complexes, it is as-
sumed that the MC states are at nearly the same energy in
all cases. Curiously, for L = isn or heisn, the 330 nm max-
imum appears to be split into two components, with a
weaker component appearing at ꢁ2700 cmꢀ1 towards
the red of the actual maximum. Considering that the
isn/heisn groups may act as r-donor and p-acceptor,
these results suggest that the transition around 360 nm
has a MLCT (Mn ! dppe) character and, therefore, the
330 nm absorption is assigned as MC transition. Similar
behavior has been observed for the MLCT transition in
[W(CO)4(phen)] [20].
3.5. Cyclic voltammetry at a Pt electrode
Cyclic voltammograms for complexes fac-[Mn(SO3
CF3)(CO)3(dppe)] and fac-[Mn(Br)(CO)3 (phen)] are
depicted in Fig. 5. The electrochemical behavior ob-
served for these two complexes is representative of the
other Mn(I)-(L0–L0) complexes substituted with ligands
In these complexes, the large intensity of the MC
transition is attributed to the high metal–ligand contri-

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C. Pereira et al. / Inorganica Chimica Acta 358 (2005) 3735–3744
Table 7
Cyclic voltammogram data (V) for oxidation of fac-[Mn(CO)3(L0–
L0)(L)]0/+ complexesa
L0–L0
L
Ea
(I)
Ec
(I0)
E1/2
(I/I0)
Ea
(II)
Ec
(II0)
E1/2
(II/II0)
dppe
Brꢀ
triflate
im
1.26
1.22
1.32
1.42
1.16
1.12
1.16
1.20
1.21
1.17
1.24
1.31
0.86
0.82
0.87
0.67
0.79
0.74
0.79
0.57
0.83
0.78
0.83
0.62
isn
phen
Brꢀ
triflate
imb
1.18
1.29
1.22
1.40
1.00
1.26
1.00
1.18
1.09
1.27
1.11
1.29
0.97
1.06
0.66
0.68
0.71
1.02
0.28
0.60
0.84
1.04
0.47
0.64
isn
a
At a platinum electrode at room temperature in CH2Cl2 (0.1 mol
Lꢀ1 TBPA) vs. Ag/AgCl.
b
Ref. [1].
and II/II0 to happen in a cyclic way, even after repeated
scans, indicating that these species are never exhausted
and do not cause changes in the coordination sphere
compositions. On the basis of these data, processes I/I0
and II/II0 are all associated with electrochemical and
interconversion reactions of the isomeric species fac-
Mn(I/II) and mer-Mn(I/II). The overall reaction scheme
is shown in Fig. 6.
The only other characterized example of a similar in-
duced isomerization observed for Mn(I) tricarbonyl
x2-y2
x2-y2
z2
xy
z2
(0)
- e
xy
(2)
(2)
Fig. 5. Cyclic voltammograms for the oxidation of fac-[Mn(SO3CF3)
(CO)3(dppe)] (a) and fac-[Mn(Br)(CO)3(phen)] (b) in CH2CL2 (0.1 mol
Lꢀ1 TBPA); vs. Ag/AgCl, scan rate 100 mV/s.
xz, yz
(2)
xz, yz
fac-Mn(I)
fac-Mn(II)
L. Similar electrochemical behavior of the fac-
[Mn(CO)3(phen)(L)]+, where L = im or triflate, has been
previously reported [1]. The potential values are summa-
rized in Table 7.
x2-y2
xy
x2-y2
Taking the voltammogram of fac-[Mn(SO3CF3)
(CO)3(dppe)] as example for discussion (Fig. 5(a)), it is
observed that it displays on the first oxidative scan the
process I and on the reverse scan the cathodic processes
I0 and II0. On the second and subsequent scans, the
redox couples II/II0 and I/I0 are observed and the cyclic
voltammogram attains a steady state response. These
observations indicate that the cathodic process observed
at ꢁ0.8 V (process II0) results from the reduction of a
Mn(II) species produced in the first oxidation step.
Thus, after the first oxidative process there are two
Mn(II) carbonyl species in solution and, probably, they
have the same stability on the electrochemical time scale.
In general, when fac-Mn(I) complexes are oxidized at
the electrode surface they cause electrode processes I/I0
(0)
z2
xy
z2
(1)
+ e
yz
xz
(2)
(3)
yz
xz
(2)
(3)
mer-Mn(I)
mer-Mn(II)
Fig. 6. Qualitative diagram showing the energy level of the d orbitals
for the Mn(I/II) tricarbonyl isomers for the electrochemical–chemical–
electrochemical–chemical process. The numbers in parentheses show
the numbers of CO ligands that can interact with a given metal dp
orbital (adapted from [23]). For clarity, only the subscripts of the d
orbitals are shown.

C. Pereira et al. / Inorganica Chimica Acta 358 (2005) 3735–3744
3743
complexes is that of fac-[Mn(Cl)(CO)3(dppm)], which
differs from our example by exhibiting a thermodynam-
ically stable oxidized mer-Mn(Cl)(CO)3(dppm)]+ isomer
which was isolated [22].
be expected if we consider that the dp orbitals lie
between the ligands and they are not so sensitive to
changes in M-L distances as the X-ray results showed.
Furthermore, compared to the fac-[Mn(CO)3(dppe)(L)]
(SO3CF3) complexes series, the redox potentials in fac-
[Mn(CO)3(phen)(L)](SO3CF3) complexes are only
slightly shifted, reflecting the small difference in donor
strength of the dppe and phen ligands. Then, the equa-
torial ligand substitutions affect reduction potential less
than the changes in axial ligands, a finding that is consis-
tent with electron transfer to dz2 in forming mer-Mn(II).
It is interesting to note that light irradiation of the
MLCT/MC manifold in fac-[Mn(CO)3(phen)(im)]
(SO3CF3), which weaken metal–ligand bonds, has the
effect of removing an electron from a dp level, decreasing
p-bonding stabilization [24]. In a sense, the excited state
from which fac- to mer-photoisomerization occurs
resembles the oxidized fac-Mn(II) isomer. Similar fac-
tors may govern the isomeric distributions in both the
oxidative and the photoisomerization reaction.
In the present case of the fac-Mn(II/I) complexes, the
factors responsible for determining the thermodynami-
cally preferred forms of the isomers are not entirely
clear. But it is clear that upon oxidation to fac-Mn(II),
the mer-Mn(II) isomer becomes thermodynamically
favored. This could happen if steric repulsion between
ligands is important or if the fac-Mn(II) complex under-
goes a Jahn–Teller distortion as has been suggested for
other isomerization reactions [2]. The former effect, if
it exists, would be small since no steric repulsion has
been observed when dppe is replaced by phen in fac-
Mn(I) as shown from the crystal data discussed in
Section 3.1. Thus, the cyclic scheme behavior can be
interpreted qualitatively by considering the ligand field
splitting of the dp (dxy, dxz and dyz) and dr ðdx2ꢀy2
and dz2 Þ manifold orbitals involved in the change in oxi-
dation states, which leads to the prediction of an elec-
tronic preference in each species (Fig. 6).
Notice that the present facial isomers are character-
ized by three good p-acceptor ligands (the CO
molecules) that are well bonded to the filled dp orbitals.
Further, in each case, the arrangement of the molecules
along the z-axis presents ligands exhibiting different r/p-
donor/acceptor abilities in sight of a synergic L !
Mn ! CO bonding effect.
4. Conclusion
Comparison of the series of complexes fac-Mn(I) re-
vealed similar bonding properties and redox reactivity
independent of the ligand field strength of the axial or
bidentate equatorial ligandsꢀ. eThe electrochemical model
proposed for fac-MnðIÞ ! fac-MnðIIÞ !mer-MnðIIÞ
þe
According to the MO diagrams (Fig. 6), after the
oxidative process, the high oxidation state of the metal
and its small size favor a shortness along the x–y-axes
and an elongation along the z-axis, reducing the energies
of both dxz and dyz orbitals with respect to the dxy. The
dr set will also be reorganized: the dz2 will be stabilized
and the dx2ꢀy2 destabilized. Hence, if the ligands along
the z are further from the metal, the dz2 orbital will be
stabilized relative to the dxy. Dealing with an inert six-
coordinated complex, on populating the fifth d electron
in dz2 it will show a tendency to weaken the bonds along
the z-axis and so it will also favor rearrangements which
places a weaker r-composite axis along z, i.e., CO–Mn–
CO, favoring the mer-Mn(II) isomer. By the same token,
upon reduction to an 18 eꢀ system, which temporarily
places 2 eꢀ in dz2 , increasing a distortion due to the great
r-labilizing effect, the dxy orbital will be stabilized
relative to the dz2 . Thus, the filled dp orbitals favor
rearrangement back to the fac-Mn(I) isomer when they
present equal number of ligands CO that can interact
with the metal dp orbitals [23]. Furthermore, the differ-
ent number of ligands CO that can interact with the
metal dp orbitals explains the lower anodic potential of
the mer species in relation to the fac ones.
! mer-MnðIÞ ! fac-MnðIÞ cyclic-induced isomerism,
to the light of several spectroscopic, crystallographic
and voltammetric measurements, is consistent with the
ligand rearrangement in the Mingos model [25]. More-
over, the present study shows that the electron-transfer
induced isomerization process in fac-[Mn(CO)3(L0–
L0)(L)]0/+ is governed by the relative energy order of
the dz2 orbital; that is, electron population along the
z-axis leads to ligand repulsion and a switch to a weaker
r-field in that direction for the oxidation step.
Acknowledgments
The authors thank CNPq, CAPES and FAPESP for
financial supports. The authors are also indebted to
one of the referees on occasion of the publication of
the paper cited in [1] who stimulated part of our
conclusion.
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