Rhodium(III) pentamethyl cyclopentadienyl complexes incorporating 1-(4-cyanophenyl)-imidazole: Role of solvent in ligand substitution reactions

Article abstract of DOI:10.1016/j.jorganchem.2004.10.018

Reaction of the dimeric rhodium complex [{(η⁵-C ₅Me₅)Rh(μ-Cl)Cl}₂] with an excess of 1-(4-cyanophenyl)-imidazole in dichloromethane afforded neutral mononuclear complex [(η⁵-C₅Me₅

Full text of DOI:10.1016/j.jorganchem.2004.10.018

Journal of Organometallic Chemistry 690 (2005) 647–652
www.elsevier.com/locate/jorganchem
Rhodium(III) pentamethyl cyclopentadienyl complexes
incorporating 1-(4-cyanophenyl)-imidazole: role of solvent in
ligand substitution reactions
*
Sanjay Kumar Singh, Manoj Trivedi, Manish Chandra, Daya Shankar Pandey
Department of Chemistry, Awadhesh Pratap Singh University, Rewa 486003, M.P., India
Received 26 May 2004; accepted 7 October 2004
Available online 11 November 2004
Abstract
Reaction of the dimeric rhodium complex [{(g5-C5Me5)Rh(l-Cl)Cl}2] with an excess of 1-(4-cyanophenyl)-imidazole in dichlo-
romethane afforded neutral mononuclear complex [(g5-C5Me5)RhCl2(CPI)] (CPI = 1-(4-cyanophenyl)-imidazole) 1. The complex 1
reacted with EPh3 (E = P, As, Sb) and N–N donor bases 2,20-bipyridine and 1,10-phenanthroline in different solvents to give sub-
stitution products wherein, nature of the product was governed by polarity of the solvents employed in the reaction. Resulting com-
plexes have been characterized by elemental analyses, spectral (FAB-MS, IR, 1H,13C, 31P NMR, UV–Vis, Emission) and
electrochemical studies. Coordination of CPI through imidazole nitrogen and the presence of pendant nitrile group have been sup-
ported by spectral studies.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Rhodium; Pentamethyl cyclopentadienyl; 1-(4-cyanophenyl)-imidazole; Piano-stool
1. Introduction
molecule could be a good model to test the possibility
of molecular switching. Bridging ability coupled with
the twisted internal charge transfer (TICT) in CPI, offers
interesting perspectives for the study of intra molecular
metal to metal charge transfer as well as photo-induced
geometrical changes.
Further, it is established that the metal to ligand
bond strength has remarkable importance in coordina-
tion/organometallic synthetic process [4]. Metal–ligand
link can be targeted as strong (metal–phosphine) or
weak (metal–ether or amine) bond. The strength of the
metal–ligand link depends on the nature of transition
metal i.e. its oxidation state and coordination number,
but for a single metal targeted complex the polarity of
the solvent and availability of the counter ion in the
reaction medium becomes determining factor for result-
ing complexes [5]. For neutral approaching ligands, the
polar solvent facilitates the reaction by polarizing
the reactant, but for a non-polar solvent such type of
The dimeric chloro bridged complexes [{(g5-
C5Me5)M(l-Cl)Cl}2] (M = Rh or Ir) have been the sub-
ject of investigation by many research groups as these
are very useful starting materials [1]. The complexes un-
dergo rich variety of chemistry via the intermediacy of
chloro bridge cleavage reactions leading to formation
of a series of interesting neutral and cationic mononu-
clear complexes [2]. Despite extensive studies on the
complex [{(g5-C5Me5)M(l-Cl)Cl]}2], reactivity of this
complex with 1-(4-cyanophenyl)imidazole (CPI) has
yet to be explored. CPI is an interesting molecule which
behaves as bridging ligand as well as exhibit twisted
internal charge transfer [3]. It is expected that the com-
plexes in which, redox sites are bridged by this type of
*
Corresponding author. Tel./fax: +91 766 230684.
E-mail address: dsprewa@yahoo.com (D.S. Pandey).
0022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.10.018

648
S.K. Singh et al. / Journal of Organometallic Chemistry 690 (2005) 647–652
polarization is not possible, leading to entirely different
products. Because of our interests in the development of
new metallo-ligands based on organometallic system, we
have reacted the dimeric chloro bridged rhodium com-
plex [{(g5-C5Me5)Rh(l-Cl)Cl}2] with CPI [6]. In this
note, we report the synthesis, spectral characterization
of new piano-stool complex [{(g5-C5Me5)RhCl2(CPI)]
1. We also describe herein, the effect of polarity of the
solvents in the ligand substitution reactions of complex
1.
and 1,10-phenanthroline under varying reaction condi-
tions. The substitution chemistry depends mainly on
the lability and inertness of the metal–ligand link. The
strength of the link depends on the nature (acidity or
basicity) of different substituents around the metal center
along with the nature of the metal center itself. In partic-
ular, the strength of a bond is a matter of reaction med-
ium and conditions. Taking these observations in action,
one can use polarity of the solvent as a tool to control the
metal to ligand bond strength, to yield a set of complexes
with tailored charges and substituted ligand.
In the present study, we observed that treatment of
the complex 1 with EPh3 in a polar solvent like metha-
nol afforded cationic mononuclear complexes with the
formulation [(g5-C5Me5)RhCl(CPI)(EPh3)]+ (E = P, 2;
E = As, 3; E = Sb, 4). On the other hand, its reaction
in a non polar solvent like benzene led in the formation
of neutral known complexes [(g5-C5Me5)RhCl2(EPh3)]
(E = P, As and Sb) (Scheme 2) [7]. In the reactions
involving EPh3, substitution of the chloro group, sug-
gested that the metal to CPI bond in the complex 1 is
quite strong. This observation is contrary to our findings
on the analogous arene ruthenium and g3:g3-bis allyl
ruthenium complexes incorporating 4-cyanopyridine
2. Results and discussion
Reactions of the dimeric chloro bridged rhodium
complex [{(g5-C5Me5)Rh(l-Cl)Cl}2] with two equiva-
lents of the bridging ligand CPI in dichloromethane un-
der stirring conditions at room temperature gave a
neutral complex [(g5-C5Me5)RhCl2(CPI)] 1 in excellent
yield (Scheme 1). This orange complex is air stable
and soluble in water and in most of the common organic
solvents.
The complex 1, reacted with various bases viz., EPh3
(E = P, As and Sb) and N–N donor bases 2,20-bipyridine
Rh
N
Dichloromethane
Stirring/ r.t./ 1:2
Cl
Cl
Cl
Cl
N
CPI
+
Rh
Rh
Cl
Cl
C
N
Scheme 1.
+
Rh
Rh
EPh
EPh
EPh
3
3
Cl
3
Cl
Cl
3
N
EPh
Rh
N
With PF
(MeOH)
Without PF
(Benzene)
6
6
B
Cl
N
N
+
Cl
A
C
N
1, 10 Phen
2', 2" Bipyridine
+
With PF
6
(MeOH)
With PF
6
1
Rh
C
(MeOH)
N
Cl
Rh
N
N
Cl
N
N
B
A E = P, As & Sb
E = P;2:As;3:Sb;4
Scheme 2.

S.K. Singh et al. / Journal of Organometallic Chemistry 690 (2005) 647–652
649
Fig. 1. FAB Mass Spectra of the complex 2 with fragmentation pattern.
wherein, the respective base displaces metal bound
4-cyanopyridine [6a,b]. Clearly, polarization of the
Rh–Cl bond in this case is responsible for the formation
of cationic complexes. Since, polarization of the Rh–Cl
bond is not possible in benzene therefore, it leads to sub-
stitution of the coordinated CPI by the respective bases.
In the reactions involving N–N donor bases 2,20-
bipyridine and 1,10-phenanthroline, we could only
isolate the complexes [(g5-C5Me5)RhCl(bipy)]+ and
[(g5-C5Me5)RhCl(phen)]+ already reported in the litera-
ture [9]. Clearly, in these reactions one of the chloro
groups and coordinated CPI is being displaced to form
[(g5-C5Me5)Rh(N–N)Cl]+. Steric stability of the cationic
complexes [(g5-C5Me5)Rh(N–N)Cl]+ over the com-
plexes [(g5-C5Me5)Rh(CPI)(N–N)]2+ favored the forma-
tion of former species.
Information about the composition of the complexes
and structure and bonding has been derived from ana-
lytical spectral studies. Analytical data of the complexes
conformed well to their respective formulations. More
information about composition of the complexes was
also obtained from FAB MS. Resulting data is recorded
in the experimental section and representative spectrum
of the cationic complex 2 is shown in Fig. 1. The posi-
tion of different peaks and overall fragmentation pattern
in the FAB MS of the respective complexes is consistent
with their formulations.
dazole nucleus towards itself, leading to a decrease of
electron density on the metal center which in turn,
may pull more electron density away from the g5-
C5Me5, leading to deshielding of g5-C5Me5 protons.
The 1H NMR spectral data of neutral complexes of gen-
eral formulation [(g5-C5Me5)RhCl2(EPh3)] are consist-
ent with those already reported in the literature [7,8].
The position and integrated intensity of various reso-
nance supported well the presence of ligand CPI and
formulation of the respective complexes. The nitrile car-
bon of the CPI in the complexes resonated at ꢁ113 ppm,
while the other carbons of CPI resonated in the range
123.01–151.90 ppm [9].
The absorption spectral data of the complexes 1–4 is
recorded in the experimental section along with the
other selected analytical and spectral data of the com-
plexes (Fig. 2). The electronic spectra shows a band at
250–260 nm corresponding to intra-ligand p–p* transi-
tion and another band of lower energy in the region
373–403 nm assignable to MLCT transition [10]. The
MLCT maxima of the complexes shifted towards lower
energy side with an increase in the r donor ability of the
ligand. It is well known that imidazole is a poor accepter
in comparison to the benzonitrile, which leads to a weak
p-back bonding with the metal center, in turn shift in the
The IR spectra of the mononuclear complex 1 and
the cationic complexes 2, 3 and 4 displayed sharp and in-
tense bands around 2226 cmꢀ1 corresponding to
m(C„N). As the position of the m(C„N) remained unal-
tered, it suggested linkage of CPI through its imidazole
nitrogen [3]. It was further supported from the blue
shifts in the position of m(C@C) and m(C–N).
1
The H NMR spectral data of the complexes along
with their assignments are recorded in the experimental
1
section. In the H NMR spectra of the complex 1–4,
C5Me5 protons displayed down field shift as compared
to that in the precursor complex. Downfield shift in
the position of the C5Me5 protons might result from
the change in electron density on the metal center due
to linkage of CPI through its imidazole nitrogen. The
conjugative and electron withdrawing abilities of the –
cyano group pulls electron density away from the imi-
Fig. 2. Electronic absorption spectra of the complexes 1–4.

650
S.K. Singh et al. / Journal of Organometallic Chemistry 690 (2005) 647–652
dium metal center bound to the imidazole and two irre-
versible reduction peaks at ꢀ1.0 and ꢀ1.8 V can be
assigned to the stepwise reduction of the CPI ligand [3].
In this work with the help of analytical and spectral
data we have shown that linkage of CPI in the com-
plexes 1–4 is taking place through its imidazole nitrogen
and the nitrile group is present as a pendant donor
group. Poor quality of crystals of the complex 1 and
its derivatives has restricted us to provide any structural
support at this stage. Due to presence of pendant nitrile
group, the complexes 1–4 have the potential to behave
as metallo-ligands and could be employed in the synthe-
sis of homo/hetero binuclear mixed valence systems hav-
ing interesting properties. The results reported herein
also demonstrated the role of solvent to control substi-
tution chemistry about the metal center and resulting
into a complex with a choice of substituted ligand.
440
450
460
470
480
490
Wavelength/ nm
Fig. 3. Emission spectra of the complex 1 in CH2Cl2 at rt.
position the MLCT to the higher energy side. The latter
band can be assigned to the Rh-imidazole moiety. All
the substituted products (2–4) of the complex 1 showed
higher energy MLCT maxima (1 < 4 < 3 < 2) than the
precursor complex 1.
3. Experimental
All the synthetic manipulations were performed under
oxygen free nitrogen atmosphere. The solvents were
dried and distilled before use following the standard
procedures. Pentamethylcyclopentadiene, triphenyl-
phosphine, triphenylarsine, 2,20-bipyridine, 1,10-
phenanthroline, hydrated rhodium(III) chloride,
ammonium hexafluorophosphate (all Aldrich) were used
as received. The ligand 1-(4-cyanophenyl)-imidazole [3]
and the precursor complex [{(g5-C5Me5)Rh(l-Cl)Cl}2]
were prepared and purified following the literature pro-
cedures [11]. Elemental analyses, spectral and electro-
chemical data of the complexes were obtained as
described elsewhere [12].
The fluorescence spectrum (Fig. 3) of the complex
[(g5-C5Me5)RhCl2(CPI)] 1 shows emission maxima at
467 nm upon excitation at 403 nm with a quantum yield
abs
max
(U) of 0.014. k
were taken as excitation kmax for
recording the emission spectra. Emission spectrum was
obtained in dichloromethane at rt. Upon excitation at
403 nm, corresponding to metal to cyano phenyl imidaz-
ole (CPI) charge transfer, emission is intense and could
not be attributed to the free ligand. The emission maxi-
ma and quantum yields for complexes 2, 3 and 4 are 438
nm (0.018), 442 nm (0.021) and 449 nm (0.023), respec-
tively. The position and shape of the emission band indi-
cated that it is due to MLCT states.
3.1. Preparation of complexes
Cyclic voltammogram (Fig. 4) of the complex 1 in
acetonitrile exhibits an irreversible oxidation peak at
1.30 V corresponding to RhIV/RhIII oxidation of rho-
3.1.1. Preparation of [(g5-C5Me5)RhCl2(CPI)] (1)
To a suspension of [{(g5-C5Me5)Rh(l-Cl)Cl}2] (0.310
g, 0.5 mmol) in 25 ml of dichloromethane, CPI (0.169 g,
1.0 mmol) was added and contents of the flask were stir-
red at rt for 5 h. The resulting yellow-orange solution was
filtered through celite to remove any solid impurities. The
filtrate was concentrated under vacuum to dryness and
washed several times with diethyl ether. The solid mass
was extracted with dichloromethane, filtered and was
precipitated by addition of petroleum ether (60–80°).
The resulting crystalline product was filtered, washed
twice with methanol and petroleum ether (60–80°). Yield
(0.182 g, 76%) (Found: C, 50.27; H, 4.62; N, 8.69% M 478
C20Cl2H22N3Rh: C, 50.21; H, 4.60; N, 8.79%). IR (cmꢀ1
,
nujol): 2221.8. kmax/nm, CH2Cl2 (e/dm3 molꢀ1 cmꢀ1):
403(2060), 260(190325), 238(179768). 1H NMR (300
MHz; CDCl3, SiMe4, J Hz): d 1.68(s, 15H, g5-C5Me5),
8.48(s, 1H), 7.77(d, 1H, 9.0Hz), 7.80(d, 1H, 9.1Hz),
7.47(d, 2H, 6.0Hz), 7.41(d, 2H, 9.0Hz). 13C{1H} NMR
Fig. 4. Cyclic voltammogram of the complex 1.

S.K. Singh et al. / Journal of Organometallic Chemistry 690 (2005) 647–652
651
(CDCl3): 151.40(aC–Imd), 148.60(a0C–Imd), 132.90
(aC–CNPh), 129.40(cC–CNPh), 127.21(bC–CNPh),
3.1.4. Preparation of [(g5-C5Me5)RhCl(CPI)(SbPh3)]-
PF6 (4)
124.43(dC–CNPh),
124.30(bC–Imd),
112.12(CN–
It was prepared by following the above procedure (2)
except that SbPh3 was used in place of PPh3. It was iso-
lated as orange crystalline solid. Yield (0.641 g, 68%)
CNPh), 93.46(CC–Cp*), 9.49(CH3–Cp*). FAB-MS m/z
479(478), [M]+; 443(442), [M]+–Cl; 274(273), [M]+–Cl–
CPI; 238(236) [M]+–Cl–CPI–Cl.
(Found: C, 48.50; H, 3.96; N, 4.60%
M 942
C38ClF6H37N3PRhSb: requires C, 48.41; H, 3.93; N,
4.46%). IR (cmꢀ1, nujol): 2224.1. kmax/nm, CH2Cl2
(e/dm3 molꢀ1 cmꢀ1): 386(3336), 258(204321), 236(181260).
1H NMR (300 MHz; CDCl3, SiMe4, J Hz): d 1.64 (s,
15H, g5-C5Me5), 7.27–7.61(br. m, aromatic protons of
AsPh3), 8.21(s, 1H), 7.33(d, 1H, 9.1), 7.85(d, 2H,
8.9Hz), 7.70(d, 2H, 8.9Hz), 7.69(d, 2H, 9.0Hz), 31P
NMR (300 MHz): ꢀ149.16(sep., PF6). 13C{1H} NMR
(CDCl3): 151.21(aC–Imd), 148.54(a0C–Imd), 132–
136.67(m, Ph), 130.52(cC–CNPh), 129.51(aC–CNPh),
128.21(bC–CNPh), 124.62(bC–Imd), 121.50(dC–CNPh),
120.80(CN–CNPh), 90.31(CC–Cp*), 8.82(CH3–Cp*).
FAB-MS m/z 797(796), [M]+; 628(629), [M]+–CPI;
592(591), [M]+–CPI–Cl; 238(238) [M]+–CPI–Cl–SbPh3.
3.1.2. Preparation of [(g5-C5Me5)RhCl(CPI)(PPh3)]-
PF6 (2)
To a suspension of [(g5-C5Me5)RhCl2(CPI)] (0.478 g,
1.0 mmol) in 25 ml of methanol, PPh3 (0.262 g, 1.0 mmol)
was added and contents of the flask were heated under re-
flux for 1 h. The resulting dark red solution was cooled to
roomtemperatureandfilteredthroughcelite. Ammonium
hexafluorophosphate dissolved in 10 ml of methanol was
added to the filtrate, whereupon a red crystalline solid sep-
arated. It was filtered and washed twice with methanol
and diethyl ether. Yield (0.613 g, 72%) (Found: C, 53.61;
H, 4.62; N, 5.19% M 849 C38ClF6H37N3P2Rh: C, 53.71;
H, 4.36; N, 4.95%). IR (cmꢀ1, nujol): 2230.0. kmax/nm,
CH2Cl2 (e/dm3 molꢀ1 cmꢀ1): 373(4777), 251(248329),
232(189671). 1H NMR (300 MHz; CDCl3, SiMe4, J Hz):
d 1.47 (s, 15H, g5-C5Me5), 7.21–7.45 (br. m, aromatic pro-
tons of PPh3), 7.98(s, 1H), 7.18(d, 1H, 8.8Hz), 7.77(d, 2H,
8.4Hz), 7.44(d, 2H, 8.5Hz), 7.60(d, 2H, 8.7Hz), 31P NMR
(300 MHz): 10.85(s, PPh3) ꢀ149.52 (sep., PF6). 13C{1H}
NMR (CDCl3): 151.90(aC–Imd), 148.30 (a0C–Imd),
132–138.67(m, Ph), 131.50(aC–CNPh), 129.01(cC–
Acknowledgments
We gratefully thank the Department of Science and
Technology for financial assistance (SP/S1/F-04/2000),
Sophisticated Analytical Instrument Facility, Central
Drug Research Institute, Lucknow, for analytical data
and the Head, Department of Chemistry, A.P.S. Univer-
sity, Rewa, (M.P), India, for extending laboratory
facilities.
CNPh),
128.20(bC–CNPh),
125.43(dC–CNPh),
123.01(bC–Imd), 113.11(CN–CNPh), 90.34(CC–Cp*),
8.68(CH3–Cp*). FAB-MS m/z 704(706), [M]+; 535(535),
[M]+–CPI; 500(500) [M]+–CPI–Cl; 237(237) [M]+–CPI–
Cl–PPh3.
3.1.3. Preparation of [(g5-C5Me5)RhCl(CPI)(AsPh3)]-
PF6 (3)
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