Synthesis and Spectroscopic Properties of 1,2-Diiminetricarbonyl-rhenium(I)chloride Complexes with Aliphatic Diimines (or 1,4-Diaza-1,3-butadienes) as Ligands

Article abstract of DOI:10.1515/znb-1999-0308

Re(1,2-diimine)(CO)₃Cl complexes with the aliphatic bidentate ligands diiminosuccinodinitrile (DISN), benzilbisanile (BEAN), bisacetylbisanile (BABA), and benzildiimine (BEDIM) are reported. The compounds show Re(I) to π*(diimine) metal-to-ligand charge transfer (MLCT) absorptions in the visible spectral region. The energy of these MLCT-transitions decreases in the series BABA > BEDIM > BEAN > DISN, depending on the π* acceptor properties of the coordinated diimine ligands. The maxima of these CT bands undergo a solvent-dependent shift (negative solvatochromism), which indicates a partial charge redistribution in the excited state. The compounds are not photoluminescent at room temperature and 77 K.

Full text of DOI:10.1515/znb-1999-0308

Synthesis and Spectroscopic Properties of 1,2-Diiminetricarbonyl-
rhenium(I)chloride Complexes with Aliphatic Diimines
(or 1,4-Diaza-l,3-butadienes) as Ligands
Markus Leirer, Günther Knör*, Amd Vogler*
Institut für Anorganische Chemie, Universität Regensburg, D-93040 Regensburg, Germany
Z. Naturforsch. 54 b, 341-344 (1999); received December 11, 1998
Charge Transfer, Rhenium Complexes, 1,4-Diaza-l ,3-butadienes, Solvatochromism
Re(l,2-diimine)(CO)3Cl complexes with the aliphatic bidentate ligands diiminosuccino-
dinitrile (DISN), benzilbisanile (BEAN), bisacetylbisanile (BABA), and benzildiimine (BE-
DIM) are reported. The compounds show Re(I) to 7r*(diimine) metal-to-ligand charge transfer
(MLCT) absorptions in the visible spectral region. The energy of these MLCT-transitions de-
creases in the series BABA > BEDIM > BEAN > DISN, depending on the 7r* acceptor properties
of the coordinated diimine ligands. The maxima of these CT bands undergo a solvent-dependent
shift (negative solvatochromism), which indicates a partial charge redistribution in the excited
state. The compounds are not photoluminescent at room temperature and 77 K.
1. Introduction
These compounds are accessible by the reac-
tion of Re(CO)5 Cl with the corresponding 1,2-
diimine ligands. In the case of BEDIM, the
1.2-diimine is prepared in situ by reaction of
benzil-bis-trimethylsilylimine with ethanol. Some
data on substituted Re(DAB)(CO)3 X derivatives
with DAB = l,4-diphenyl-2,3-dimethyl-1,4-diaza-
1.3-butadiene or 1,4-bis-/?-tolyl-1,4-diaza-1,3-buta-
diene and X = Cl, Br or trifluoromethanesulfonate
have previously been reported [8].
A variation of the substituents R and R' is ex-
pected to modify the MLCT energies. Such effects
have been observed for Mo(l,2-diimine)(CO)4 [9]
and [Fe(l,2-diimine)3]2+ [10] complexes. Since the
substituents of previously studied aliphatic 1,2-di-
imines were mostly alkyl and aryl groups, only mod-
erate effects were found. In contrast to the results for
these substituents, we expected to observe a more
severe perturbation by introducing R = CN as a
strongly electron withdrawing group. Diiminosuc-
cinodinitrile (DISN) was already employed as lig-
and in other coordination compounds [11],
In contrast to Re(l,2-diimine)(CO)3Cl com-
plexes with polypyridyl ligands (e. g. 2,2'-bipyri-
dine) as diimines [1 -4], the corresponding com-
pounds with delocalized diimines such as substi-
tuted acenaphthenequinone diimine [5] or ortho-
phenylene diimines [6] (orr/zo-benzoquinone di-
imines) as ligands are characterized by the absence
of any luminescence. This lack of emission is appar-
ently related to the low energy of the optical metal-
to-ligand charge transfer (MLCT) transition of these
complexes. Similar properties could be expected for
Re(l,2-diimine)(CO)3Cl compounds with aliphatic
1,2-diimines (1,4-diaza-1,3-butadienes, DAB) as
ligands, because they also provide 7r* orbitals at
rather low energies [7]. We explored this possibility
and selected the following complexes for the present
study:
R'
DAB
R
BABA
BEAN
BEDIM
DISN
c h 3
\
c 6h 5
Re(CO),Cl
2. Experimental
c 6h 5 c 6h 5
X
2.7. Materials and instrumentation
H
R
N
I
c 6h 5
CN
H
R'
Re(CO)sCl and organic starting compounds were com-
mercially available from Aldrich and used without fur-
ther purification. All solvents were spectrograde. UV-Vis
spectra were recorded on a Kontron Uvikon 860 or 960
absorption spectrophotometer. IR spectra were measured
Reprint requests to Dr. G. Knör or Prof. Dr. A. Vogler.
E-mail: guenther.knoer@chemie.uni-regensburg.de
0932-0776/99/0300-0341 $ 06.00 (c) 1999 Verlag der Zeitschrift für Naturforschung, Tübingen • www.znaturforsch.com
K
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342
M. Leirer et al. ■Re( 1,2-diimine)(CO)3Cl Complexes with Aliphatic Diimine Ligands
on a Beckman Acculab 6 spectrometer. Mass spectra were
obtained using a Finnigan MAT 95 and Varian MAT 311
equipment. 1H NMR data were recorded on a Bruker AC
250 (250 MHz) spectrometer with TMS as internal stan-
dard. For checking out the emission properties, a Hitachi
850 spectrofluorometer equipped with a Hamamatsu 928
photomultiplier was used.
Table I. Absorption data of the Re(DAB)(CO)3Cl com-
plexes in acetonitrile solution at 298 K.
R
R’
[nm] (s [l/(mol
• cm)])
Amax
543 (6100)
CN
H
296 (6400)
223 (12000)
H
c 6h 5
463 (6300)
474 (4400)
312 sh (5800) 251 sh (9400)
C6H5
c 6h 5
316 s h (4100)
255 sh (11000)
c h 3 c 6h 3
426 (3700) 31 3 sh (2700) 251 sh (7400)
2.2. Synthesis of the Re(DAB)(CO)jCl complexes
Re(DISN)(CO)jCl: Diiminosuccinodinitrile (DISN)
was synthesized by oxidation of diaminosuccinodini-
tril (DAMN) with dichlorodicyano-orf/?obenzoquinone
(DDQ) [12]. A suspension of 181 mg (0.5 mmol) of
Re(CO)5Cl and the stoichiometric amount of DISN was
refluxed for 20 min under argon protection in 10 ml of dry
toluene. After cooling down to r. t., 15 ml of n-hexane was
added. The residue was separated by filtration, washed
twice with ether and dried under vacuum. Yield: 140 mg
(0.34 mmol, 67%).
Analysis for C7H2ClN40 3Re
Calcd C 20.42 H 0.49 N 13.61%,
Found C 20.02 H 0.65 N 13.96%.
MS (FD+, CH2C12): m/z(%): 410 (46.3); 412 (100.0); 414
(19.7). IR(KBr): v{cm“ 1): 3230 (N-H), 2040. 1950, 1915
(CO-stretch,/ac-Re(CO)3Cl-fragment).
wavelength / nm
Re(BEDlM)(CO)}Cl: Benzil-bis-trimethylsilylimine
was prepared from benzil and sodium-bis-trimethylsilyl-
amide following the literature route [13]. A solution of
160 mg (0.5 mmol) of benzil-bis-trimethylsilylimine in
10 ml of toluene was slowly added to a stirred suspen-
sion of 181 mg (0.5 mmol) of Re(CO)5Cl in a mixture
of toluene (10 ml) and ethanol (2 ml). When benzil-bis-
trimethylsilylimine was completely added, the reaction
mixture was heated to reflux for 15 min and then cooled
down to r. t. The metal complex was precipitated by addi-
tion of 50 ml of n-hexane. The crude product was purified
by chromatography over silica gel with acetonitrile as
eluent. Yield: 188 mg (0.37 mmol, 73%).
Fig. 1. Electronic absorption spectra of the Re(DAB)-
(CO)3Cl complexes in acetonitrile; DISN (— ), c = 2.8 x
10 M; BEDIM (------),c = 2.7 x 10~ 4 M; BEAN (— ),
c = 3.6 x 10“ 4 M; BABA (•••), c = 4.6 x 10~ 4 M. 1-cm
cells.
by filtration and dried under reduced pressure. Since
Re(BEAN)(CO)3Cl slowly decomposes in most solvents,
only freshly prepared solutions were investigated. Yield:
246 mg (0.38 mmol. 74%).
Analysis for C29H2oClN203Re
Calcd C 52.29 H 3.03 N4.21%,
Found C 51.54 H 3.40 N 4.07%.
MS (FD+, acetone): m/z (%): 664 (43.3); 665 (16.2); 666
(100.0); 667 (22.4); 668 (25.6); 669 (5.6). 'H NMR (d6-
acetone): <5(ppm) 7.7 - 7.8 (m, CftH?). IR(KBr): P(cm "1):
3061 (C-H stretch); 2019, 1927, 1907 (CO strech, fac-
Re(CO)3Cl-fragment); 1589, 1482 (phenyl rings).
Re(BABA)(CO)jCl: The rhenium bisacetylbisanile
complex Re(BABA)(CO)3Cl was obtained according to
the literature method [9].
Analysis for CnH^ClN^O^Re
Calcd C 45.35 H4.36 N 4.07%,
Found C 45.27 H 3.87 N 4.32%.
MS (FD+, CHCI3): m/z(%): 612 (46.3); 614 (100.0); 616
(22.2). ‘H NMR(CDC13): 6 (ppm) 11.04 (2H. s, NH);
7.54-7.29 (10H, m. C6H5). IR(KBr): ü(cm~'): 2025,
1930. 1905 (CO-stretch,/ac-Re(CO)3Cl-fragment).
Re(BEAN)(CO)iCl: Benzilbisanile was obtained ac-
cording to published procedures [14], A suspension of
181 mg (0.5 mmol) of Re(CO)5Cl and 180 mg (0.5 mmol)
of BEAN in 10 ml of toluene was heated to reflux
for 10 min. After cooling down to r. t., 50 ml of n-
hexane was added. The brown precipitate was isolated
3. Results and Discussion
The compounds Re(DAB)(CO)3 Cl with DAB =
BABA, BEAN, BEDIM, and DISN are character-
ized by an intense absorption in the visible spectral
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M. Leirer et al. • Re(l,2-diimine)(CO)3Cl Complexes with Aliphatic Diimine Ligands
343
Table II. Solvent dependence of the charge transfer ab-
sorption band of the Re(DAB)(COhCl complexes at
298 K.
region (Fig. 1, Table I). Depending on the ligand and
the solvent used, a shoulder appears occasionally on
the long-wavelength side of this band. In analogy
to many other R e(l,2 -diimine)(CO)3Cl complexes,
the chromophoric absorption is assigned to a MLCT
transition terminating at 7r* orbitals of the diimine
ligand. The MLCT energies of Re(DAB)(CO)3Cl
decrease in the series BABA > BEDIM > BEAN
> DISN. This variation can be rationalized by the
following considerations: Substitution at the car-
bon atoms of the imino groups has the strongest
influence. The electron-donating nature of CH3 de-
creases the 7r-accepting strength of the diimine. Ac-
cordingly, Re(BABA)(CO)3Cl shows its MLCT ab-
sorption at rather short wavelength. On the con-
trary, the electron-withdrawing nitrile group shifts
the MLCT band of Re(DISN)(CO)3Cl to much
longer wavelength. The MLCT absorptions of
Re(DAB)(CO)3Cl with DAB = BEDIM and BEAN,
which carry phenyl substituents at the imino carbon
atom, appear at intermediate wavelengths. The sub-
stitution of hydrogen (BEDIM) at the imino nitro-
gen by phenyl (BEAN) has a relatively small effect
on the MLCT energy. However, these phenyl groups
are expected to have a nearly orthogonal orientation
to the 1,4-diaza-1,3-butadiene plane. It follows that
they do not contribute significantly to the 7r-conju-
gation of the diimine moiety.
[nm]
BEDIM BABA
Solvent
Amax
Emlct
[15] DISN BEAN
_
556
0.42 542
0.59 540
509
508
497
496
494
462
474
502
502
497
474
491
454
463
470
468
459
447
425
421
426
Toluene
Benzene
Chloroform
THF
0.30
0.34
Dichloromethane 0.67 543
—
DMF
Acetonitrile
0.95
0.98 543
25000
-
24000
I 23000
Ö 22000
i 21000
s
1 20000
£
19000
18000
0.2
0.4
0.6
0.8
1.0
solvent parameter E* LCT
Generally, the metal-to-ligand charge transfer
(MLCT) absorption of Re(diimine)(CO)3Cl com-
plexes undergoes a red shift with decreasing sol-
vent polarity. This negative solvatochromism be-
comes smaller with increasing 7r-acceptor strength
of the diimine owing to a reduced charge transfer
character in the MLCT transition. This is a con-
sequence of stronger mixing of d7r and 7r*(DAB)
orbitals by ir back bonding. Indeed, the extent of
charge transfer in the MLCT transition decreases
in the series Re(DAB)(CO)3Cl with DAB = BABA
> BEDIM > BEAN > DISN, as indicated by the
variation of the solvatochromic behaviour (Fig. 2,
Table II). The slope of the plots displayed in Fig. 2
decreases in the same order from 3430 to 2860,2500
and 490 cm-1, respectively. Since the d7r donor
and 7r*(diimine) acceptor orbitals are apparently al-
most equally delocalized between metal and ligand,
Re(DISN)(CO)3Cl is only slightly solvatochromic
with only minor charge transfer character.
Fig. 2. Correlation between the MLCT energies of the
Re(DAB)(CO)3Cl complexes in different solvents and
the Emict solvent parameter [15] (• DISN, ♦ BABA,
« bedim ! T BEAN).
The complexes Re(DAB)(CO)3Cl with DAB =
BABA, BEDIM, BEAN, and DISN are not lumi-
nescent in contrast to those containing 2 ,2 '-bipyridyl
and related diimine ligands. The reasons for the ab-
sence of this luminescence are not quite clear yet,
but most probably associated with the low energy
of the corresponding MLCT states, which facilitates
radiationless deactivation to the electronic ground
state [16].
Acknowledgements
This work was supported by the Deutsche Forschungs-
gemeinschaft and the Fonds der Chemischen Industrie.
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M. Leirer et al. ■Re(l,2-diimine)(CO)3Cl Complexes with Aliphatic Diimine Ligands
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