Metal-coumarin complexes: synthesis and characterization of 7-isocyanocoumarin ligands and Mo(CO)4(7-isocyanocoumarin)2 complexes. X-ray crystal structure of Mo(CO)4(7-isocyano-4-trifluoromethylcoumarin)2

Article abstract of DOI:10.1016/S0022-328X(01)01186-X

The preparation and characterization of four different 7-isocyanocoumarin ligands is described. The four ligands are 3,4-dimethyl-7-isocyanocoumarin (Dmic), 7-isocyano-4-methylcoumarin (Mic), 3-chloro-7-isocyano-4-methylcoumarin (Cmic) and 7-isocyano-4-trifluoromethylcoumarin (Tic). Reaction of the four 7-isocyanocoumarin ligands with Mo(CO)₄(pip)₂ (pip=piperidine) gave the Mo(CO)₄L₂ complexes. IR and UV-vis spectra of the complexes indicate that the 7-isocyanocoumarin ligands are significantly stronger π-acids than simple aromatic isocyanide ligands. An intense visible absorption band, assigned to a metal-to-ligand charge transfer (MLCT) transition, is present in the UV-vis spectra of the four Mo(CO)₄L₂ complexes. Excitation into the MLCT band of the Mic, Cmic, and Dmic complexes gave yellow-orange emission at room temperature in methylene chloride solution. Photolysis of Mo(CO)₄(Mic)₂ with P(Ph)₃ in THF solution produced Mo(CO)₃(PPh₃)(Mic)₂.

Full text of DOI:10.1016/S0022-328X(01)01186-X

Journal of Organometallic Chemistry 642 (2002) 97–106
www.elsevier.com/locate/jorganchem
Metal–coumarin complexes: synthesis and characterization of
7-isocyanocoumarin ligands and Mo(CO)4(7-isocyanocoumarin)2
complexes. X-ray crystal structure of
Mo(CO)4(7-isocyano-4-trifluoromethylcoumarin)2
Daniel A. Freedman *, Ivan Keresztes, Ann L. Asbury
Department of Chemistry, State Uni6ersity of New York at New Paltz, New Paltz, NY 12561, USA
Received 11 June 2001; accepted 3 August 2001
Abstract
The preparation and characterization of four different 7-isocyanocoumarin ligands is described. The four ligands are
3,4-dimethyl-7-isocyanocoumarin (Dmic), 7-isocyano-4-methylcoumarin (Mic), 3-chloro-7-isocyano-4-methylcoumarin (Cmic) and
7-isocyano-4-trifluoromethylcoumarin (Tic). Reaction of the four 7-isocyanocoumarin ligands with Mo(CO)4(pip)2 (pip=pipe-
ridine) gave the Mo(CO)4L2 complexes. IR and UV–vis spectra of the complexes indicate that the 7-isocyanocoumarin ligands are
significantly stronger p-acids than simple aromatic isocyanide ligands. An intense visible absorption band, assigned to a
metal-to-ligand charge transfer (MLCT) transition, is present in the UV–vis spectra of the four Mo(CO)4L2 complexes. Excitation
into the MLCT band of the Mic, Cmic, and Dmic complexes gave yellow–orange emission at room temperature in methylene
chloride solution. Photolysis of Mo(CO)4(Mic)2 with P(Ph)3 in THF solution produced Mo(CO)3(PPh3)(Mic)2. © 2002 Elsevier
Science B.V. All rights reserved.
Keywords: Molybdenum; Isocyanide; Coumarin; Photochemistry
1. Introduction
Coumarin and its derivatives have attracted interest
in several fields. Our interest is in the properties of
metal–coumarin complexes, a subject which has re-
The charge-transfer nature of the 7-aminocoumarin
excited state suggests that metal–coumarin complexes
could display interesting excited state properties. Con-
siderable effort in recent years has gone towards the
design of metal–ligand complexes that display photo-
induced charge separation, a key process in designing
artificial photosynthetic systems, chemical sensors, and
molecular level devices [5]. A particular goal is to
prepare complexes which have low energy metal-to-lig-
and charge transfer (MLCT) excited states. There are
only a few examples in the literature of metal–cou-
marin complexes. Koefod and Mann prepared
[CpRu(7-aminocoumarin)]+ complexes where the metal
is coordinated in h6 fashion to the arene portion of the
7-aminocoumarin [6]. An intense visible absorption
band was observed in these complexes assigned to a
ceived relatively little attention. Outside of natural
products chemistry [1], the interest in coumarins is due
mainly to their interesting photochemical and photo-
physical properties [2,3]. The most pertinent example is
the intense tunable, visible wavelength emission dis-
played by 7-aminocoumarins. Emission from 7-
aminocoumarins is thought to occur from the lowest
singlet excited state which has been described as an
intramolecular charge transfer (ICT) transition and can
be represented by resonance structure B shown below,
where charge from the amine has been transferred to
the unsaturated lactone ring [4].
* Corresponding author.
E-mail address: freedman@newpaltz.edu (D.A. Freedman).
0022-328X/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01186-X

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D.A. Freedman et al. / Journal of Organometallic Chemistry 642 (2002) 97–106
Ru“coumarin charge transfer transition. Puddephat
and Aye reported the preparation of Pt(Bpy)(Me)2Br(4-
CH2-7-methoxycoumarin) where the Pt is bonded to the
coumarin methyl group [7]. More recently, Tyson and
Castellano prepared ruthenium complexes of a phenan-
throline ligand with a pendant coumarin group [8]. Our
interest is in complexes with direct conjugation between
the coumarin and the metal, which is lacking the latter
two examples. The presence of charge-transfer ab-
sorbance bands in the [CpRu(7-aminocoumarin)]+
complexes suggests that the ruthenium is strongly con-
jugated with the coumarin moiety and that further
investigation of metal–coumarin complexes is war-
ranted. However, it would be useful to have a more
versatile ligating group on the coumarin then the arene
ring. Our strategy is to prepare isocyanocoumarin
ligands
An isocyanocoumarin should be an excellent ligand.
Isocyanides in general act as both s-donors and p-acids
making them capable of stabilizing metals in a wide
range of oxidation states [9]. In addition, metal–
arylisocyanide complexes have been shown to have
interesting photochemical and photophysical properties
indicative of strong conjugation between the metal and
the isocyanide ligand [10–14]. The work reported here
describes the preparation of 7-isocyanocoumarin lig-
ands and Mo(CO)4(7-isocyanocoumarin)2 complexes.
The molybdenum complexes were chosen because they
are readily prepared and there are data available in the
literature on analogous complexes of simple aryl and
alkyl isocyanide complexes [13,14].
shifts are relative to (CH3)4Si. UV–vis spectra were
recorded in non-degassed solutions at room tempera-
ture (r.t.) with a Cary 1 spectrometer. Elemental analy-
ses were performed by MHW Laboratories.
2.2. Emission studies
Solutions of Mo(CO)4(7-isocyanocoumarin)2 com-
plexes were prepared in the dark. Emission spectra were
recorded on an Ocean Optics S2000 CCD Array fiber
optic spectrometer. Excitation at 405 nm was provided
by a 200 W mercury vapor lamp using the appropriate
interference filter. Emission from the sample was fo-
cused on to the fiber optic cable with a fiber-optic
focusing assembly (Oriel). A 475 nm cut-off filter was
placed over the focusing assembly to prevent interfer-
ence from the excitation line. Optically dense solutions
(A\10 in a 1.0 cm cell at the excitation wavelength) of
the complexes were prepared from freshly distilled
methylene chloride and purged with argon for 15 min.
Emission was detected from the front face. The emis-
sion spectra were corrected for grating efficiency and
detector response. The values of bem were determined
from relative peak areas using
a
solution of
[Ru(bpy)3][PF6]2 (bem=0.068) as a standard [18]. Opti-
cally dense solutions of [Ru(bpy)3][PF6]2 were prepared
from freshly distilled acetonitrile and purged with argon
for 15 min. Corrections were made for differences in
the refractive indices of the solvents [18].
2.3. 7-Formamido-4-methylcoumarin (Mfc)
7-Amino-4-methylcoumarin (4.5 g, 0.026 mol), 40 ml
of toluene and 20 ml of 88% formic acid were added to
a round bottom flask equipped with a stir bar, Dean–
Stark trap and a reflux condenser. The mixture was
refluxed for 14 h during which time the water formed in
the reaction was collected in the Dean–Stark trap as
the water–toluene azeotrope. After 14 h, the excess
formic acid was distilled off. The solution was cooled,
precipitating the product which was filtered and washed
with diethyl ether giving 4.22 g (79.9% yield) of off-
2. Experimental
2.1. General considerations
Mo(CO)4(pip)2 [15] (pip=piperidine), Mo(CO)4-
(Dmpi)2 [16], and 7-aminocoumarins [17] were prepared
via literature procedures. Mo(CO)6, ethylacetoacetate,
ethyl 2-methylacetoacetate, ethyl 2-chloroacetoacetate,
and ethyl 4,4,4,-trifluoroacetoacetate were purchased
from Aldrich Chemical Co. 2,6-Dimethylphenyliso-
cyanide was purchased from Fluka. [Ru(bpy)3]Cl2 was
purchased from Aldrich and metathesized to the hex-
aflurophosphate salt by adding NH4PF6 to an aqueous
solution of [Ru(bpy)3]Cl2. The [Ru(bpy)3][PF6]2 was
purified by chromatography (alumina–acetonitrile), re-
crystallized from CH3CN–Et2O and dried in vacuo. All
synthetic procedures were carried out under an inert N2
atmosphere unless otherwise noted. CH2Cl2 was dis-
tilled from P2O5. THF was distilled from Na–ben-
zophenone. Acetonitrile was distilled from P2O5.
Toluene was reagent grade and was used without fur-
1
white powder. H-NMR (90 MHz, d6-DMSO): 2.41 (s,
CH3), 6.26 (s, H3), 7.2–7.8 (m, H5,6,8), 8.39 (s, HCO Z
isomer), 9.01 (d, HCO E isomer, J=10.8 Hz), 10.58 (d,
HN E isomer, J=9.9Hz), 11.02 (s, HN Z isomer).
2.4. 3,4-Dimethyl-7-formamidocoumarin (Dmfc)
3,4-Dimethyl-7-aminocoumarin (4.12 g, 0.0218 mol)
was treated in the same manner as 7-amino-4-methyl-
coumarin giving 2.5 g (53% yield) of off-white powder.
1H-NMR (90 MHz, d6-DMSO): 2.08 (s, CH33), 2.35 (s,
CH43), 7.0–7.8 (m, H5,6,8), 8.36 (s, HCO Z isomer), 9.0
(s, HCO E isomer), 10.50 (s, HN E isomer), 10.76 (s,
HN Z isomer).
1
ther purification. H-NMR spectra were recorded on
JEOL 90FXQ and Varian 300 spectrometers. Chemical

D.A. Freedman et al. / Journal of Organometallic Chemistry 642 (2002) 97–106
99
2.5. 3-Chloro-4-methyl-7-formamidocoumarin (Cmfc)
2.9. 3-Chloro-4-methyl-7-isocyanocoumarin (Cmic)
7-Amino-3-chloro-4-methylcoumarin (2.0 g, 0.0095
mol) was treated in the same manner as 7-amino-4-
methylcoumarin giving 2.14 g (95% yield) of off-white
3-Chloro-7-formamido-4-methylcoumarin (2.0 g,
0.0084 mol) was treated in the same manner as 7-form-
amido-4-methylcoumarin giving 1.1 g (60% yield) of
1
1
powder. H-NMR (90 MHz, d6-DMSO): 2.53 (s, CH3),
white powder. H-NMR (300 MHz, CD2Cl2): 2.63 (s,
7.1–8.0 (m, H5,6,8), 8.38 (s, HCO Z isomer), 9.01 (d,
HCO E isomer, J=9.9 Hz), 10.42 (broad s, HN E
isomer), 10.65 (s, HN Z isomer).
CH3), 7.43 (m, H8), 7.38 (m, H6), 7.69 (m, H5).
2.10. 7-Isocyano-4-trifluoromethylcoumarin (Tic)
2.6. 7-Formamido-4-trifluoromethylcoumarin (Tfc)
7-Formamido-4-trifluoromethylcoumarin (2.0 g,
0.0078 mol) was treated in the same manner as 7-form-
amido-4-methylcoumarin giving 1.5 g (80% yield) of
light yellow powder. 1H-NMR (300 MHz, CD2Cl2):
6.87 (q, H3, J3-CF3=0.8 Hz), 7.34 (apparent s, H8), 7.38
(d of d, H6, J6–5=8.6 Hz, J6–8=2.0 Hz), 7.78 (d of q,
H5, J5–6=8.6 Hz, J5-CF3=1.8 Hz).
7-Amino-4-trifluoromethylcoumarin (4.0 g, 0.017
mol) was treated in the same manner as 7-amino-4-
methylcoumarin giving 3.75 g (86% yield) of light yel-
1
low powder. H-NMR (90 MHz, d6-DMSO): 6.92 (s,
H3), 7.2–8.0 (m, H5,6,8 E and Z isomers), 8.42 (s, HCO
Z isomer), 9.07 (d, HCO E isomer, J=10.5 Hz), 10.65
(d, HN E isomer, J=10.4 Hz), 10.80 (s, HN Z isomer).
2.11. Mo(CO)4(Mic)2
2.7. 7-Isocyano-4-methylcoumarin (Mic)
Mo(pip)2(CO)4 (0.100 g, 0.347 mmol) and Mic (0.141
g, 0.760 mmol) were added to a three necked flask
equipped with a reflux condenser and a stir bar. Freshly
distilled methylene chloride (20 ml) was added and the
solution was purged with N2. The solution was refluxed
for 2 h. The resulting orange solution was cooled and
CH3OH was added until the solution turned cloudy.
After 18 h at −15 °C, the resulting precipitate was
filtered and washed with diethyl ether giving an orange,
microcrystalline product (0.095 g, 48% yield). Slow
recrystallization in the dark from CH2Cl2–hexane gave
analytically pure material. Further purification for ab-
sorption–emission spectroscopy studies was accom-
plished by chromatography on silica gel, eluting with
methylene chloride. The pure product eluted first as a
yellow–orange band followed by a red band. The
product was isolated by adding hexanes to the methyl-
ene chloride solution, precipitating the product as a
7-Formamido-4-methylcoumarin (2.8 g, 0.014 mol)
was added to 20 ml of methylene chloride and stirred to
form a slurry. Triethylamine (9.5 ml, 0.069 mol) was
added to the flask through a short column of activated
alumina. Phosphorous oxychloride (1.3 ml, 0.014 mol)
was added dropwise by syringe. The reaction was
stirred for 30 min, during which time the formamide
completely dissolved leaving a clear yellow solution.
The reaction mixture was extracted with 2×10 ml
portions of water and once with 10 ml of saturated
NaCl solution. The methylene chloride layer was dried
with anhydrous MgSO4 and suction filtered through 5
cm of silica gel in a fritted funnel. The solvent was
removed via rotary evaporation giving the product as
an off-white powder which was dried in vacuo (1.8 g,
69% yield). The crude product was suitable for synthe-
sis. Chromatography on silica gel, eluting with methyl-
1
yellow powder. H-NMR (300 MHz, CD2Cl2): 2.43 (d,
1
ene chloride, gave analytically pure material. H-NMR
CH3, JCH3-3=1.0 Hz), 6.30 (apparent d, H3, J3-CF3
=
(300 MHz, CD2Cl2): 2.48 (m, CH3), 6.32 (q, H3, J3-
Me=1.2 Hz), 7.34 (apparent s, H8), 7.33 (d of d, H6,
1.0 Hz), 7.33 (d, H8), 7.32 (d of d, H6, J6–5=9.0 Hz,
6–8=2.0 Hz), 7.66 (d, H5, J5–6=9.0 Hz).
J
J
6–5=9.0 Hz, J6–8=2.0 Hz), 7.65 (d, H5, J5–6=8.1
Hz).
2.12. Mo(CO)4(Dmic)2
2.8. 3,4-Dimethyl-7-isocyanocoumarin (Dmic)
Mo(pip)2(CO)4 (0.0937 g, 0.325 mmol) and Dmic
(0.144 g, 0.723 mmol) were treated in a similar manner
as above to give orange microcrystals (0.121 g, 61.4%
yield). A yellow powder was isolated after chromatog-
raphy. 1H-NMR (300 MHz, CD2Cl2): 2.40 (s, CH43),
2.20 (s, CH33), 7.30 (d, H8, J8–6=2.0 Hz), 7.31 (d of d,
H6, J6–5=9.0 Hz, J6–8=2.0 Hz), 7.65 (d, H5, J5–6=9.3
Hz).
3,4-Dimethyl-7-formamidocoumarin (1.5 g, 0.0069
mol) was treated in the same manner as 7-formamido-
4-methylcoumarin giving 0.90 g (65% yield) of colorless
powder. 1H-NMR (300 MHz, CD2Cl2): 2.40 (s,
CH43), 2.20 (s, CH33), 7.31 (m, H6,8), 7.64 (H5, d, J5-6
=
9.0 Hz).

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D.A. Freedman et al. / Journal of Organometallic Chemistry 642 (2002) 97–106
2.13. Mo(CO)4(Cmic)2
dissolved in CH2Cl2. Adding hexanes precipitated a
dark red powder (60 mg, 57% yield). Slow recrystalliza-
tion from CH2Cl2–hexanes gave analytically pure
product. IR (CH2Cl2); wCO (cm−1)=1905 (s), 1941 (m),
1965 (sh), wCN=2006 (s), 2096 (w); wlactone (cm−1)=
1732 (m), 1606 (s).
Mo(pip)2(CO)4 (0.100 g, 0.347 mmol) and Cmic
(0.168 g, 0.763 mmol) were treated in a similar manner
as above to give reddish–orange microcrystals (0.137 g,
61.0% yield). An orange powder was isolated after
1
chromatography. H-NMR (300 MHz, CD2Cl2): 2.59
(s, CH3), 7.36 (d, H8, J8–6=2.0 Hz), 7.36 (d of d, H6,
2.16. X-ray crystal structure determination for
Mo(CO)4(Tic)2
J
6–5=9.0 Hz, J6–8=2.0 Hz), 7.70 (d, H5, J5–6=9.0
Hz).
Dark red needles of Mo(CO)4(Tic)2 were grown by
cooling a CH3OH solution of the complex to −15 °C
(Table 4). A crystal of the compound was attached to a
glass fiber and mounted on the Siemens SMART sys-
tem for a data collection at 173(2) K. An initial set of
cell constants was calculated from reflections harvested
from three sets of 30 frames. These initial sets of frames
are oriented such that orthogonal wedges of reciprocal
space were surveyed. This produces orientation ma-
trices determined from 91 reflections. Final cell con-
stants are calculated from a set of 5478 strong
reflections from the actual data collection. The data
were collected by the hemisphere collection method
where a randomly oriented region of reciprocal space is
surveyed to the extent of 1.3 hemispheres to a resolu-
2.14. Mo(CO)4(Tic)2
Mo(pip)2(CO)4 (0.200 g, 0.529 mmol) and Tic (0.278
g, 1.16 mmol) were added to a three necked flask
equipped with a reflux condenser and a stir bar. Freshly
distilled methylene chloride (20 ml) was added and the
solution was purged with N2. The solution was refluxed
for 2 h, changing color from light yellow to dark red.
The solvent was removed via rotary evaporation and
the resulting red solid was dried in vacuo. The solid was
dissolved in CH3OH and cooled at −15 °C for 2
weeks giving an analytically pure mixture of dark red
1
powder and needles (0.060 g, 16.5% yield). H-NMR
(300 MHz, CD2Cl2): 6.84 (s, H3), 7.39 (m, H6,8), 7.77 (d,
,
tion of 0.84 A. Three major swaths of frames are
H5, J5–6=8.4 Hz).
collected with 0.30° steps in ꢀ.
The space group Pnma was determined based on
systematic absences and intensity statistics. A successful
direct-methods solution was calculated which provided
most non-hydrogen atoms from the R-map. Several
full-matrix least squares-difference Fourier cycles were
performed which located the remainder of the non-hy-
drogen atoms. All non-hydrogen atoms were refined
with anisotropic displacement parameters unless stated
otherwise. All hydrogen atoms were placed in ideal
positions and refined as riding atoms with individual
(or group if appropriate) isotropic displacement
parameters.
2.15. Mo(Mic)2(CO)3PPh3
Mo(Mic)2(CO)4 (60.0 mg, 0.130 mmol), triphenyl-
phosphine (34.1 mg, 0.13 mmol) and 10 ml of freshly
distilled THF were added to a Pyrex tube equipped
with a stir bar. The tube was connected to a vacuum
line and the orange solution was freeze-pump-thaw
degassed (three cycles). The solution was photolyzed
for 2 h with a 175 W medium pressure Hg lamp giving
a dark red solution. The THF was removed under
reduced pressure leaving a dark red residue which was
The sample was collected with 30 s frames, because
the sample was small. One half of the complex is in the
asymmetric unit: the Mo atom and two carbonyl lig-
ands are located on the crystallographic mirror.
3. Results
3.1. Ligand synthesis and characterization
7-Aminocoumarins can be prepared via a literature
procedure [17] and provide a convenient starting mate-
rial for the preparation of 7-isocyanocoumarins. The
7-isocyanocoumarins are prepared from the 7-
aminocoumarins as shown in Scheme 1. Refluxing the
7-aminocoumarins in formic acid–toluene produced the
7-formamidocoumarins. The 7-formamidocoumarins
Scheme 1.

D.A. Freedman et al. / Journal of Organometallic Chemistry 642 (2002) 97–106
101
Table 1
Analytical data for 7-formylcoumarins, 7-isocyanocoumarins, and Mo(CO)4(7-isocyanocoumarin)2 complexes
Formula
Calculated (%)
C
Experimental (%)
H
N
C
H
N
Tfc
Mfc
Mic
Dmic
Cmic
Tic
Mo(CO)4(Mic)2
Mo(CO)4(Dmic)2
Mo(CO)4(Cmic)2
Mo(CO)4(Tic)2
Mo(CO)3(Mic)2PPh3
C
11H6F3NO3
51.38
65.02
71.34
72.35
60.16
55.24
54.00
55.46
48.25
45.50
63.56
2.35
4.46
3.82
4.55
2.75
1.69
2.44
2.99
1.87
1.17
3.60
5.45
6.89
7.57
7.03
6.38
5.87
4.84
4.61
4.33
4.08
3.45
51.30
64.78
71.52
72.43
60.32
55.60
54.16
55.62
47.44
45.60
63.80
2.30
4.69
4.00
4.37
2.50
2.12
2.60
3.06
1.96
1.22
3.88
5.53
6.93
7.33
7.05
6.22
5.64
4.60
4.49
4.11
3.83
3.45
C12H11NO3
C11H7NO2
C12H9NO2
C11H6ClNO2
C11H4F3NO2
C26H14MoN2O8
C28H18MoN2O8
C26H12Cl2MoN2O8
C26H8F6MoN2O8
C43H29MoN2O7P
Table 2
IR data (cm−1) for 7-isocyanocoumarin ligands and Mo(CO)4L2 complexes
w(CN)
w(lactone)
Dmic
Mic
Cmic
Tic
2123 (m)
2128 (m)
2127 (m)
2131 (m)
1713 (s), 1614 (m)
1732 (s), 1614 (m)
1738 (s), 1606 (m)
1753 (s), 1616 (m)
w(CN)
w(CO)
D(CN) b
Mo(CO)4(Dmic)2
Mo(CO)4(Mic)2
Mo(CO)4(Cmic)2
Mo(CO)4(Tic)2
2136 (w)
2135 (w)
2134 (w)
2135 (w)
2138 (w)
2079 (m)
2077 (m)
2067 (m)
2067 (m)
2094 (m)
2068(sh)
2059(sh)
2087 (sh)
2014 (m)
1940 (s)
−44
−51
−60
−64
−26
2012(m)
2012 (m)
2012 (m)
2016 (m)
1941 (s)
1943 (s)
1945 (s)
1932 (s)
2051(sh)
a
Mo(CO)4(Dmpi)2
All data were acquired in CH2Cl2 solution.
a Not observed.
b See text for explanation.
1
Dehydration of the formamides to the isocyanides is
readily accomplished with phosphorous oxychloride us-
ing a modification of a literature procedure [20]. Four
different isocyanocoumarins, designated Dmic, Mic,
Cmic, and Tic, were prepared with different sub-
stituents in the three and four positions. Purification by
column chromatography yielded either colorless (Mic,
Dmic, Cmic) or pale yellow (Tic) products which gave
acceptable elemental analyses (Table 1). The 7-iso-
cyanocoumarins were stable when stored at −15 °C,
but exhibited substantial decomposition over several
weeks at room temperature. None of the unpleasant
odor associated with more volatile isocyanides was
observed.
The 7-isocyanocoumarins were fully characterized by
1H-NMR, IR, and UV–vis spectroscopy. The 1H-
NMR spectra are similar to those reported for other
coumarins [17]. The solution IR spectra (Table 2) of the
7-isocyanocoumarins exhibit three peaks of interest.
The peak at ca. 2130 cm−1 is assigned to the CN
stretch of the isocyanide group. Small variations in the
were characterized by H-NMR spectroscopy and by
elemental analysis (Table 1) for Tfc and Mfc. The
1H-NMR spectra are similar for all four formamides.
The resonances for the coumarin ring protons are com-
plex due to small chemical shift differences for the
aromatic hydrogens and the presence of two different
formamide isomers. The presence of two formamide
isomers is typical, and is caused by restricted rotation
about the CꢀN bond [19]. The isomer is designated E
when the NH and formyl hydrogens are trans and Z
when the hydrogens are cis. Although the signals for
the aromatic protons are too complex to readily inter-
pret, two signals each are observed for the formyl and
NH hydrogen resonances in ca. a 3:1 ratio, clearly
showing the presence of the two isomers. Coupling
between the NH and formyl protons is observed only
for the minor isomer. Since the coupling constant
should be greater when the formyl and NH protons are
trans, the predominant isomer is assigned the Z
configuration and the minor the E configuration.

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D.A. Freedman et al. / Journal of Organometallic Chemistry 642 (2002) 97–106
which have been assigned to p–p* transitions [21].
INDO calculations indicate that a forbidden n–p* tran-
sition could also be present at longer wavelengths. This
feature was not observed for coumarin itself, but is
commonly observed in substituted coumarins. The sim-
ilarity between the spectra of the 7-isocyanocoumarins
and coumarin suggest that the four intense, higher
energy features can be assigned to p–p* transitions,
and the weak, low energy shoulder on the ca. 320 nm
band can be assigned to an n–p* transition. The latter
assignment is supported by the consistent shift of this
band to lower energy as the coumarin substituents
become more electron withdrawing.
position of this band are observed for the four different
isocyanides, but there does not appear to be any obvi-
ous correlation with the coumarin substituents. Two
peaks in the 1600–1750 cm−1 region are assigned to
the wCO and wCꢁC stretches of the vinyl lactone ring. The
band assigned to the CꢁC stretch exhibits almost no
variation in position, while the CꢁO stretch shows the
expected correlation with the coumarin substituents,
shifting from 1713 cm−1 (Dmic) to 1753 cm−1 (Tic).
The UV–vis spectra of the 7-isocyanocoumarins
were investigated in methylene chloride solution (Fig.
1). The UV–vis spectra of the four ligands are similar,
exhibiting a peak at about 320 nm which has an intense
high energy shoulder and a weak low energy shoulder,
and two narrower, more intense bands centered at ca.
280 nm. The only significant deviation from this general
pattern occurs for Tic for which the high energy shoul-
der on the 320 nm peak is not observed and the lower
energy of the two narrow, higher energy bands appears
as a shoulder rather than as a separate peak. The
features observed in the spectra of the 7-isocyanocou-
marins have similar energies and relative intensities to
the spectrum of coumarin itself. Coumarin exhibits
intense UV–vis peaks at 310, 285, 274, and 220 nm
3.2. Synthesis and characterization of Mo(CO)4L2
complexes
The Mo(CO)4(7-isocyanocoumarin)2 complexes were
prepared by refluxing Mo(CO)4(pip)2 (pip=piperidine)
with the isocyanide ligand in methylene chloride solu-
tion [15]. The complexes were recrystallized from
CH2Cl2–MeOH to eliminate piperidine then a second
time from CH2Cl2–hexanes to give products with ac-
ceptable elemental analyses (Table 1). An exception to
this was Mo(CO)4(Tic)2 which crystallized out of a cold
methanol solution as analytically pure material, albeit
in poor yield. The four compounds range in color from
yellow (Mic and Dmic complexes) to orange (Cmic
complex) to red (Tic complex). The compounds were
stable as solids and in solution when protected from
light. Further purification for UV–vis absorption and
emission studies was necessary for all compounds ex-
cept for Mo(CO)4(Tic)2. This was accomplished by
chromatography on silica gel, eluting with methylene
chloride, to eliminate small amounts of the Mo(CO)3(7-
isocyanocoumarin)3
contaminants.
complexes
present
as
We were able to obtain needles of Mo(CO)4(Tic)2
suitable for X-ray analysis by cooling a methanol solu-
tion of the complex. The ORTEP (Fig. 2) of a single
molecule shows that the Tic ligands are in the cis
conformation with ca. octahedral geometry about the
molybdenum. There is a mirror plane bisecting the
angle between the isocyanide ligands, giving the
molecule Cs symmetry. The bond angles are quite close
to the expected 90° with the largest deviation being the
angle between the two isocyanide ligands at 86.2°
(Table 5). The isocyanides show only a slight deviation
from linearity with CNC bond angles of 176.2°. The
carbonyl carbons are closer to the metal than the
Fig. 1. UV–vis spectra of Cmic (—); Dmic(– – –); Mic (- - -), and
Tic (– - –) in CH2Cl2 solution.
,
isocyanide carbons by an average of 0.076 A. The
,
MoꢀCO bonds are about 0.030 A shorter for carbonyls
trans to an isocyanide compared to a carbonyl trans to
another carbonyl, consistent with the greater s donat-
ing ability of the isocyanide vs. carbonyl ligands. The
C(22)ꢀMo(1)ꢀC(2)ꢀC(7) dihedral angle is 5.93°, indicat-
Fig. 2. ORTEP view of Mo(CO)4(Tic)2.

D.A. Freedman et al. / Journal of Organometallic Chemistry 642 (2002) 97–106
103
methylene chloride solution reveals four main peaks in
the CO and CN stretching region at ca. 2135, 2070,
2012, and 1940 cm−1. The assignments in Table 2 were
made by comparison with the reported IR spectrum of
Mo(CO)4(Dmpi)2
(Dmpi=2,6-dimethylphenyliso-
cyanide) [16b]. The IR bands generally show only a
small dependence on the isocyanocoumarin ligand. An
exception to this is the lower energy of the two isocya-
nide CN stretches, which shifts from 2067 cm−1 in
Mo(CO)4(Tic)2 to 2079 cm−1 for Mo(CO)4(Dmic)2.
The UV–vis spectra of the four Mo(CO)4L2 com-
plexes were investigated in methylene chloride solution
(Fig. 3; Table 3). All four complexes exhibit four in-
tense absorption bands: two narrow bands between 269
and 296 nm, a band at ca. 335 nm which displays a
high energy shoulder, and a broad, intense absorbance
that ranges between 388 nm for the Dmic complex and
416 nm for the Tic complex. The spectrum of
Mo(CO)4(Tic)2 was also investigated in different sol-
vents; the lowest energy band exhibited considerable
solvatochromism, shifting from 397 nm in acetone solu-
Fig. 3. UV–vis spectra of Mo(CO)4(Dmic)2 (—), Mo(CO)4(Mic)2
(– – –), Mo(CO)4(Cmic)2 (- - -), and Mo(CO)4(Tic)2 (– - –) in
CH2Cl2 solution.
Table 3
UV–vis absorption (umax in nm (m in cm−1
M
−1×104)) and emission
spectral data for 7-isocyanocoumarin and Mo(CO)4(7-isocyanocou-
marin)2 compounds a
Absorption
MLCT
Emission
g
p–p*
umax
bem
Table 4
b
b
b
b
Dmic
Mic
Cmic
Tic
Mo(CO)4(Dmic)2
Mo(CO)4(Mic)2
Mo(CO)4(Cmic)2
Mo(CO)4(Tic)2
Mo(CO)4(Tic)2
Mo(CO)4(Tic)2
318 (1.20)
319 (0.89)
324 (1.39)
320 (0.61)
333 (4.21)
334 (3.67)
337 (3.36)
336 (2.15)
335 (2.70)
338 (2.05)
337
Crystal data and structure refinement parameters for Mo(CO)4(Tic)2
Empirical formula
Formula weight
Temperature (K)
Wavelength (A)
Crystal system
Space group
C13H4F3Mo0.50NO4
343.14
173(2)
0.71073
Orthorhombic
Pnma
388 (3.96)
394 (3.81)
401 (3.69)
416 (3.21)
397 (3.21)
413 (3.13)
423
649
617
626
0.0016
0.0024
0.0048
,
c
Unit cell dimensions
d
,
a (A)
13.5633(7)
31.190(2)
6.0041(3)
90
90
90
2539.9(2)
8
1.795
0.616
1352
0.42×0.16×0.03
1.31–25.10
e
Mo(CO)4(Tic)2
,
b (A)
f
Mo(CO)3(Mic)2PPh3 429
337
,
c (A)
h (°)
i (°)
k (°)
a All data were acquired in methylene chloride solution unless
otherwise noted. Extinction coefficients were determined from the
peak maxima and are not corrected for overlapping bands.
b Not observed.
3
,
V (A )
Z
c Recorded in acetone.
Dcalc (mg m−3
)
d Recorded in toluene.
Absorption coefficient (mm−1
F(000)
)
e Recorded in 10% toluene–hexanes.
f Recorded in THF.
Crystal size (mm)
q range for data collection (°)
g Error for bem values is 910%.
Index ranges
−125h516, −335k537,
−75l57
ing that the coumarin ligand is tilted slightly away from
being parallel with the mirror plane. The other cou-
marin ring is tilted to the same degree with the CF3
groups on the two 7-isocyanocoumarins pointing away
from one another.
Reflections collected
12167
Independent reflections
Absorption correction
Max/min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I\2|(I)]
R indices (all data)
2294 (Rint=0.0660)
Semi-empirical
0.63 and 0.55
Full-matrix least-squares on F2
2293/0/206
The Mo(CO)4(7-isocyanocoumarin)2 complexes were
1.293
1
also characterized by H-NMR, IR, and UV–vis spec-
R1=0.0643 a, wR2=0.1428 b
R1=0.0756, wR2=0.1490
1
troscopy. The H-NMR spectra of the compounds are
Largest difference peak and hole 0.669 and −1.527
very similar to the spectra of the free ligands, showing
almost no chemical shift differences between the signals
for the complexed and free ligands. Examination of the
IR spectra of the Mo(CO)4L2 complexes (Table 2) in
−3
,
(e A
)
a R1=SꢀꢀFoꢀ−ꢀFcꢀꢀ/SꢀFoꢀ.
b wR2=ꢁ([S w(Fo2−Fc2)2/S[wFo]4]).

104
D.A. Freedman et al. / Journal of Organometallic Chemistry 642 (2002) 97–106
Table 5
this band between the free and complexed ligands leads
us to assign this to an intra-ligand transition. It is likely
that the p bonding and antibonding orbitals of the
isocyanide group are involved in the ligand p–p* tran-
sitions, so it is not surprising that complexation of the
ligand to the metal should cause some shift in certain
p–p* transitions. The lowest energy band is clearly
not ligand centered. The wavelength of this band
shows a significant dependence on the coumarin
substitution, following the trend Mo(CO)4(Dmic)2B
Mo(CO)4(Mic)2 BMo(CO)4(Cmic)2 BMo(CO)4(Tic)2.
Clearly, as the ligand substituents become more elec-
tron withdrawing the band shifts to longer wavelength.
This suggests that this band is a Mo“coumarin charge
transfer transition. The solvatochromism supports this
assignment. The band shifts to longer wavelengths in
less polar solvents, consistent with the solvatochromic
behavior of the MLCT bands observed in the
analogous molybdenum and tungsten complexes of
mono- and bidentate pyridine ligands [22].
,
Selected bond lengths (A) and angles (°) for Mo(CO)4(Tic)2
Bond lengths
Mo(1)ꢀC(1)
C(1)ꢀN(1)
Mo(1)ꢀC(23)
C(23)ꢀO(23)
Mo(1)ꢀC(22)
C(22)ꢀO(22)
Mo(1)ꢀC(21)
C(21)ꢀO(21)
2.116(5)
1.159(6)
2.023(5)
1.147(6)
2.057(8)
1.133(9)
2.058(8)
1.143(9)
Bond angles
C(23)ꢀ1ꢀMo(1)ꢀC(23)
C(23)ꢀMo(1)ꢀC(22)
C(23)ꢀMo(1)ꢀC(21)
C(23)ꢀMo(1)ꢀC(1)
C(22)ꢀMo(1)ꢀC(1)
C(21)ꢀMo(1)ꢀC(1)
C(1)ꢀ1ꢀMo(1)ꢀC(1)
N(1)ꢀC(1)ꢀMo(1)
88.9(3)
90.7(2)
90.9(2)
92.4(2)
87.4(2)
90.9(2)
86.2(3)
176.4(4)
The emission spectra of the Mo(CO)4L2 complexes
were investigated at room temperature in methylene
chloride solution. The L=Dmic, Mic, and Cmic, com-
plexes exhibited a broad and unstructured emission
profile (Fig. 4; Table 3) upon excitation into the MLCT
absorbance band. No emission was observed from
Mo(CO)4(Tic)2. The emission maxima are 617, 626 and
649 nm for the Mic, Cmic, and Dmic complexes, re-
spectively. Emission quantum yields were determined
for the three complexes and are given in Table 3.
3.3. Photochemistry of Mo(CO)4(Mic)2
Photolysis of a THF solution of Mo(CO)4(Mic)2 and
PPh3 results in a color change from orange to red. A
dark red powder was isolated from the solution which,
after recrystallization from CH2Cl2–hexanes, analyzed
well for Mo(CO)3(PPh3)(Mic)2. The UV–vis spectrum
of this complex shows a broad band at 427 nm and a
sharper band at 337 nm assigned to MLCT and intra-
ligand transitions, respectively. The MLCT transition
is shifted to longer wavelengths compared to
Mo(CO)4(Mic)2 by the increase in electron density at
the metal due to the substitution of a phosphine for
CO. In comparison, the 337 nm band is identical in
position and appearance to the analogous band in
Mo(CO)4(Mic)2, consistent with it being a ligand cen-
tered transition. Photolysis of the Mo(CO)4L2 com-
plexes in methylene chloride in the absence of a ligand
also leads to decomposition. We are currently investi-
gating the products of the reaction, but preliminary
experiments suggest that a ligand disproportionation
reaction is occurring as was observed for
Mo(CO)3(CNCF3)3 [23].
Fig. 4. Corrected emission spectra of Mo(CO)4(Cmic)2 (A);
Mo(CO)4(Mic)2 (B); and Mo(CO)4(Dmic)2 (C) in CH2Cl2 solution.
tion to 423 nm in 10% toluene–hexane. The higher
energy band at ca. 335 nm shows essentially no solvent
dependence. Several of the higher energy features are
readily assigned by comparison with the spectra of the
ligands. With the exception of the Tic complex, the two
sharp, high-energy bands are nearly identical in both
wavelength and general appearance to peaks in the
spectra of the ligands and are thus assigned to intra-lig-
and transitions. These bands in the Tic complex shift
about 10 nm to shorter wavelengths relative to the free
ligand, but retain the same general appearance. The
bands at ca. 335 nm also have a very similar appear-
ance to ligand bands, but are shifted 10–15 nm to
longer wavelengths. The shift is large for a ligand
centered transition, but the lack of any solvatochromic
behavior combined with the similarity in appearance of

D.A. Freedman et al. / Journal of Organometallic Chemistry 642 (2002) 97–106
105
4. Discussion
complexes which do exhibit room temperature emis-
sion typically have MLCT excited states which are
lower in energy than the LF states [24]. One relevant
example here is the series of Mo(CO)4(py)2 complexes
studied by Lees et al. Two main absorbance bands are
observed for these compounds. The higher energy
bands, (350–385 nm) are assigned to LF transitions
and the lower energy bands (395–468 nm) are as-
signed to MLCT transitions. Room temperature emis-
sion is observed for all of these compounds.
Comparison to the isocyanide complexes is interesting.
We were not able to observe LF bands for any of the
isocyanide complexes, but it is likely that they will not
be shifted significantly relative to the pyridine com-
plexes because of the consistent effect of the four
carbonyl ligands. This would place the LF bands for
Mo(CO)4(Dmpi)2 near the MLCT band (358 nm). In
comparison, the MLCT bands for the 7-isocyanocou-
marin complexes are shifted to wavelengths very simi-
lar to the MLCT bands observed for the pyridine
complexes. Since the LF bands should remain largely
unshifted, the MLCT state is now lower in energy and
radiative deactivation from the MLCT state then be-
comes competitive with non-radiative relaxation from
the LF states.
The spectroscopic characteristics of the Mo(CO)4L2
complexes of the 7-isocyanocoumarin ligands are qual-
itatively similar to the analogous complexes of simple
arylisocyanide ligands, but we observed spectral shifts
showing that the coumarin moiety enhances the elec-
tron withdrawing ability of the isocyanide ligand. In
the IR this results in enhanced Mo“CNR backbond-
ing as measured by the shift in the CN stretching
frequency between the free and complexed isocyanides
(Table 2). For instance, the decrease in the CN
stretching frequency is only −26 cm−1 for
Mo(CO)4(Dmpi)2
(Dmpi=2,6-dimethylphenyliso-
cyanide). A much larger shift is observed for the 7-iso-
cyanocoumarin ligands, ranging from −44 cm−1 for
Dmic to −64 cm−1 for Tic, showing the expected
correlation with the electron donating–withdrawing
properties of the coumarin substituents. In the UV–vis
spectra, the greater electron accepting ability of the
7-isocyanocoumarin ligands versus simple aryl isocya-
nide ligands is manifested in the significant red shift of
the MLCT transitions for the isocyanocoumarin com-
plexes. The MLCT bands for the 7-isocyanocoumarin
complexes range from 388 nm for the Dmic complex
to 416 nm for the Tic complex. In comparison, the
MLCT bands for the analogous complexes of Dmpi,
p-MeꢀC6H4CN [14], and p-ClꢀC6H4CN [14] are at
358, 360 and 367 nm, respectively.
5. Conclusions
The room temperature emission observed from the
7-isocyanocoumarin complexes is assigned to be of
MLCT origin. The emission is not due to the presence
of small amounts of the free ligands since the ligands
do not exhibit any visible wavelength emission. The
low energy of the emission maxima as well as the
broad, unstructured appearance of the emission bands
are very similar to the MLCT emission observed for
the analogous Mo(CO)4(py)2 (py=substituted pyri-
dine) complexes [22a]. The emission maxima of the
Mic and Cmic complexes are in the expected order
with the presence of the chlorine shifting the emission
to lower energy. Surprisingly, the Dmic complex ex-
hibits the longest wavelength emission of the three
complexes. This suggests that there is a greater degree
of distortion in the excited state of this complex, but it
is not apparent why this should be the case.
In comparison to the intense emission observed
from the 7-isocyanocoumarin complexes, no emission
is observed at room temperature for Mo(CO)4(Dmpi)2,
even though an MLCT band is present in the ab-
sorbance spectrum. This is probably due to efficient
non-radiative decay from LF excited states that are at
energies lower than or similar to the energy of the
MLCT state. Metal–carbonyl complexes typically do
not exhibit room temperature emission due to rapid
CO dissociation from LF excited states. The carbonyl
We have synthesized four different 7-isocyanocou-
marin ligands substituted in the three and four posi-
tions and Mo(CO)4(7-isocyanocoumarin)2 complexes
of the four ligands. The X-ray crystal structure of
Mo(CO)4(Tic)2 demonstrates that the 7-isocyanocou-
marin ligands are in the cis configuration with essen-
tially octahedral geometry about the metal. The
UV–vis and IR data are consistent with the expecta-
tion that the 7-isocyanocoumarin ligands are signifi-
cantly stronger p-acids than simple arylisocyanide
ligands. Presumably this is due to coupling of the low
energy p* orbitals of the coumarin to the molybdenum
through the CN bridge. Comparison of the complexes
of the four different ligands shows that the electronic
properties of the 7-isocyanocoumarin ligands can be
‘tuned’ by varying the coumarin substituents. The
Mo(CO)4L2 complexes of the 7-isocyanocoumarin
complexes exhibit emission at room temperature in
fluid solution. This is a rare property for metal–car-
bonyl complexes and is probably due to the presence
of the relatively low energy MLCT excited states. Our
future work will concentrate on further characterizing
the metal–coumarin excited states for molybdenum
and other metals. Additionally, we plan to investigate
the interaction of metal–coumarin complexes with nu-
cleic acids.

106
D.A. Freedman et al. / Journal of Organometallic Chemistry 642 (2002) 97–106
[7] K. Aye, R.J. Puddephatt, Inorg. Chim. Acta 235 (1995) 307.
6. Supplementary material
[8] Daniel S. Tyson, Felix N. Castellano, Inorg. Chem. 38 (1999)
4382.
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC no. 164518 for compound
Mo(CO)4(Tic)2. Copies of this information may be
obtained free of charge from The Director, CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (Fax: +44-
1223-336033; e-mail: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk).
[9] (a) M.P. Guy, J.T. Guy, D.W. Bennett, J. Mol. Struct. 122
(1985) 95;
(b) L. Malatesta, F. Bonati, Isocyanide Complexes of Metals,
Wiley, New York, 1969;
(c) J.A.S. Howell, J. Saillard, A. Le Beuze, G. Jaouen, J. Chem.
Soc. Dalton (1982) 2533.
[10] (a) K.R. Mann, M. Cimolino, G.L. Geoffrey, G.S. Hammond,
A.A. Orio, G. Albertin, H.B. Gray, Inorg. Chim. Acta 16 (1976)
97;
(b) K.R. Mann, H.B. Gray, G.S. Hammond, J. Am. Chem. Soc.
99 (1977) 306;
(c) H.B. Gray, K.R. Mann, N.S. Lewis, J.A. Thich, R.M.
Richman, Inorganic and organometallic photochemistry, in:
M.S. Wrighton (Ed.), Advances in Chemistry Series 168, Ameri-
can Chemical Society, Washington, DC, 1978, p. 44.
[11] (a) N.E. Stacy, K.A. Connor, D.R. McMillin, R.A. Walton,
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(b) C.J. Cameron, S.M. Tetrick, R.A. Walton, Organometallics 3
(1984) 240.
[12] J.E. Larowe, D.R. McMillin, N.E. Stacy, S.M. Tetrick, R.A.
Walton, Inorg. Chem. 26 (1987) 966.
Acknowledgements
We thank The Research Corporation for their gener-
ous support of this research. We thank Dr Kent Mann
for numerous helpful discussions. We thank Dr Victor
G. Young, Jr. and the University of Minnesota X-ray
Crystallographic Laboratory for solving the crystal
structure of Mo(CO)4(Tic)2.
[13] J.A. Connor, E.M. Jones, G.K. McEwen, M.K. Lloyd, J.A.
McCleverty, J. Chem. Soc. Dalton (1972) 1246.
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