First examples of heteroleptic dipyrrin/η5- pentamethylcyclopentadienyl rhodium/iridium(III) complexes and their catalytic activity

Article abstract of DOI:10.1021/om900349v

Heteroleptic pentamethylcyclopentadienyl rhodium/iridium(III) complexes imparting dipyrrins as co-ligands with the general formulations [(η⁵-C₅Me₅)MCl(L)] [(M = Rh(III) or Ir(III); L = 5-(4-cyanophenyl)dipyrromethene, cydpm; 5-(4-nitrophenyl) dipyrromethene, ndpm; and 5-(4-benzy-loxyphenyl)dipyrromethene, bdpm] have been synthesized. Reactivity of the complexes [(η⁵-C ₅Me₅)MCl(L)] (M = Rh(III), Ir(III); L = ndpm and cydpm) with various species, viz., sodium azide (NaN₃), ammonium thiocyanate (NH4SCN), triphenylphosphine (PPh.3), 4,4′-bipyridine (bpy), and bis(diphenylphosphino)hexane (dpph), has been examined. Resulting complexes have been characterized by elemental analyses and spectral and electrochemical studies. Molecular structures of the representative complexes [(η⁵-C₅Me₅)RhCl(cydpm)], [(η₅-C₅Me₅)RhCl(ndpm)], [(η⁵-C₅Me₅)Rh-(PPh₃)(cydpm)] SO₃CF₃, and [(η⁵-C₅Me ₅)Ir(PPh₃)(ndpm)]SO₃CF₃ have been determined crystal-lographically. The complexes [(η⁵-C ₅Me₅)MCl(L)] [(M = Rh(III) or Ir(III) and L = cydpm, ndpm, or bdpm) effectively catalyze reduction of terephthalaldehyde to 4-hydroxymethybenzaldehyde in the presence of HCOOH/CH₃COONa in water under aerobic conditions. Among these complexes the rhodium complex [(η⁵-C₅Me₅)RhCl(ndpm)] is most effective in this regard.

Full text of DOI:10.1021/om900349v

Organometallics 2009, 28, 4713–4723 4713
DOI: 10.1021/om900349v
First Examples of Heteroleptic Dipyrrin/η5-
Pentamethylcyclopentadienyl Rhodium/Iridium(III) Complexes and
Their Catalytic Activity
Mahendra Yadav, Ashish Kumar Singh, and Daya Shankar Pandey*
Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi-221 005 (U.P.), India
Received May 4, 2009
Heteroleptic pentamethylcyclopentadienyl rhodium/iridium(III) complexes imparting dipyrrins
as co-ligands with the general formulations [(η5-C5Me5)MCl(L)] [(M = Rh(III) or Ir(III); L = 5-
(4-cyanophenyl)dipyrromethene, cydpm; 5-(4-nitrophenyl)dipyrromethene, ndpm; and 5-(4-benzy-
loxyphenyl)dipyrromethene, bdpm] have been synthesized. Reactivity of the complexes [(η5-
C5Me5)MCl(L)] (M = Rh(III), Ir(III); L = ndpm and cydpm) with various species, viz., sodium
azide (NaN3), ammonium thiocyanate (NH4SCN), triphenylphosphine (PPh3), 4,40-bipyridine (bpy),
and bis(diphenylphosphino)hexane (dpph), has been examined. Resulting complexes have been
characterized by elemental analyses and spectral and electrochemical studies. Molecular structures of
the representative complexes [(η5-C5Me5)RhCl(cydpm)], [(η5-C5Me5)RhCl(ndpm)], [(η5-C5Me5)Rh-
(PPh3)(cydpm)]SO3CF3, and [(η5-C5Me5)Ir(PPh3)(ndpm)]SO3CF3 have been determined crystal-
lographically. The complexes [(η5-C5Me5)MCl(L)] [(M=Rh(III) or Ir(III) and L=cydpm, ndpm, or
bdpm) effectively catalyze reduction of terephthalaldehyde to 4-hydroxymethybenzaldehyde in the
presence of HCOOH/CH3COONa in water under aerobic conditions. Among these complexes the
rhodium complex [(η5-C5Me5)RhCl(ndpm)] is most effective in this regard.
Introduction
pyrrole rings in dipyrrins enable them to behave as a better
bidentate nitrogen donor ligand like 2,20-bipyridine or 1,10-
phenanthroline. Further, one can fine-tune properties of the
dipyrrins by incorporating electron-withdrawing/donating
substituents, which have a pronounced effect on the electro-
chemical and optical properties.5 Electron-donating groups
increase electron density on the ligand and redox processes
occur at more negative potentials, while the opposite effect
occurs when electron-withdrawing groups are employed.
While there are a number of examples of dipyrrin complexes
with boron, main group metals, or first-row transition
metals,1,3,6 there is just one report dealing with rhodium
dipyrrinato complexes and not even a single report dealing
with iridium dipyrrinato complexes.7 Further, rhodium/
iridium(III) complexes containing both the dipyrrin and
η5-pentamethyl cyclopentadienyl groups have not yet been
reported.
Enormous current interest has arisen in the coordination
chemistry of dipyrrinato ligands owing to their ease of
synthesis, intense optical absorptions, and propensity to
form stable neutral complexes with a variety of metal ions.1
Taking advantage of these properties, a number of charge-
neutral complexes and supramolecular clusters based on
dipyrrins have been synthesized.2 Much of the interest in
dipyrrin compounds derives from their rich optical proper-
ties. Compounds that incorporate dipyrrins are often deeply
colored and can display photoluminescence.1,3 In this re-
gard, highly conjugated meso-substituted dipyrrins, which
can be easily synthesized from aldehydes via condensation
with pyrrole followed by oxidation, have attracted a great
deal of attention.4 An extensive π-skeleton of the substituted
phenyl ring as meso-substituents and two conjugated rigid
Furthermore, the dimeric chloro-bridged complexes [{(η5-
C5Me5)M(μ-Cl)Cl}2] (M = Rh or Ir) have proved to be
indispensable starting materials in organometallic chemistry.8
*To whom correspondence should be addressed. E-mail: dspbhu@
bhu.ac.in. Phone: þ 91 542 6702480. Fax: þ 91 542 2368174.
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r
2009 American Chemical Society
Published on Web 07/31/2009
pubs.acs.org/Organometallics

4714 Organometallics, Vol. 28, No. 16, 2009
Yadav et al.
The complexes undergo a rich variety of chemistry via inter-
mediacy of the chloro bridge cleavage reactions, leading to the
formation of a series of interesting neutral and cationic mono-
nuclear complexes.9 Because of their potential applications in
many areas, half-sandwich η5-pentamethylcyclopentadienyl
rhodium/iridium(III) complexes have widely been investigated
in the past few years.10 Rhodium/iridium complexes are also
being explored for their medicinal properties as potential
anticancer agents.11
The heteroleptic dipyrrin complexes described to date
result from the formation of complexes based on the Fe(II),
Zn(II), Pd(II), Hg(II), Rh(I), Cr(III), Co(III), and Cu(II)
metal center.1,2,12 With an objective of expanding the chem-
istry of dipyrrins we have synthesized and characterized a
series of new neutral and cationic heteroleptic pentamethyl-
cyclopentadienyl rhodium(III) and iridium(III) complexes
based on meso-substituted dipyrrins. These represent the
first examples of rhodium/iridium(III) complexes containing
both the dipyrrin and η5-pentamethylcyclopentadienyl
groups. In this paper we describe reproducible synthesis,
spectral and structural characterization, reactivity, and cat-
alytic activity of some heteroleptic rhodium/iridium(III)
complexes [(η5-C5Me5)MCl(L)] [(M = Rh(III) or Ir(III)
and L = cydpm, ndpm, or bdpm) based on dipyrrinato
ligands. Also, we present herein the synthesis and character-
ization of some bis(diphenylphosphino)hexane (dpph)-, 4,40-
bipyridine (bpy)-, and azido-bridged dinuclear complexes
based on dipyrrin/η5-pentamethylcyclopentadienyl ligands.
(4-cyanophenyl)dipyrromethane,15a 5-(4-nitrophenyl)dipyrromet-
hane,15b and 5-(4-benzyloxyphenyl)dipyrromethane15c were pre-
pared and purified by reported literature procedures.
General Methods. Elemental analyses for C, H, and N were
performed on an Exeter Analytical Inc. model CE-440 elemental
analyzer. IR spectra were acquired on a Varian 3300 FT-IR
1
spectrometer in the region 4000-400 cm-1. H NMR spectra
were obtained on a JEOL AL 300 FT-NMR spectrometer at rt
using CDCl3 as a solvent and TMS as an internal reference.
Electronic and emission spectra were recorded on Shimadzu
UV-1700 series and LS-45 (Perkin-Elmer) luminescence spec-
trophotometers, respectively. FAB mass spectra were obtained
on a JEOL SX 102/Da-600 mass spectrometer. Cyclic voltam-
metric measurements were performed on a CHI 620c electro-
chemical analyzer. A glassy carbon working electrode, a
platinum wire auxiliary electrode, and Ag/Agþ reference elec-
trode were used in a standard three-electrode configuration.
Tetrabutylammonium perchlorate (TBAP) was used as support-
ing electrolyte, and the solution concentration was ca. 10-3 M.
Synthesis of [(η5-C5Me5)RhCl(cydpm)] (1). DDQ (0.228 g,
1.0 mmol) dissolved in benzene (150 mL) was added slowly (over
an hour) with stirring to a solution of 5-(4-cyanophe-
nyl)dipyrromethane (0.247 g, 1.0 mmol) in CHCl3 (150 mL)
cooled in an ice bath. After TLC examination revealed complete
consumption of the starting material, solvent was evaporated
and the resulting dark residue was redissolved in CHCl3/MeOH
(75 mL; 1:1 v/v). Triethylamine (0.75 mL) was added to this
solution followed by addition of [{(η5-C5Me5)Rh(μ-Cl)Cl}2]
(0.309 g, 0.50 mmol) dissolved in dichloromethane (5 mL).
The dark reaction mixture thus obtained was heated at reflux
temperature overnight and then evaporated to dryness under
vacuum to produce a black solid. The crude product was
charged on a flash column (20ꢀ3 cm, SiO2; CH2Cl2/hexane).
A second, bright orange band was collected and evaporated to
dryness to afford [(η5-C5Me5)RhCl(4-cydpm)]. Yield: 0.268 g,
55%. Anal. Calcd for C26H25ClN3Rh: C, 60.34; H, 4.87; N, 8.12.
Found: C, 60.02; H, 4.75: N, 8.20. IR (KBr pellet, cm-1): 2224,
1533, 1449, 1402, 1374, 1336, 1248, 1203, 1077, 1022, 987, 891,
820, 711, 531, 474. 1H NMR (300 MHz, CDCl3, δ ppm): 1.51 (s,
15H), 6.50 (d, 2H, J=3.6 Hz), 6.54 (d, 2H, J=3.9 Hz), 7.52 (d,
2H, J=7.2 Hz), 7.73 (d, 2H, J=8.1 Hz), 7.82 (s, 2H). 13C NMR
(75.45 MHz, CDCl3, δ ppm): 8.5 (C-CH3), 94.8 (C5Me5), 112.4
(CtN), 118.5, 119.7, 131.2, 132.2, 134.9, 142.7, 143.6, 153.9.
UV-vis (CH2Cl2, λmax nm, ε): 512 (2.36ꢀ104), 436 (1.17ꢀ104),
306 (1.00ꢀ 104), 263 (1.17ꢀ 104), 237 (3.12 ꢀ104).
Experimental Section
Reagents. All the experiments were conducted under a nitro-
gen atmosphere. The solvents were purified rigorously by
standard procedures prior to their use.13 Hydrated rhodium(III)
chloride, hydrated iridium(III) chloride, pentamethylcyclopen-
tadiene, silver trifluoromethanesulfonate, triphenylphosphine,
bis(diphenylphosphino)hexane, sodium azide, ammonium thio-
cyanate, 4,40-bipyridine, 2,3-dicloro-5,6-dicyano-1,4-benzoqui-
none (DDQ), 4-cyanobenzaldehyde, 4-bezyloxybenzaldehyde,
4-nitrobenzaldehyde, and pyrrole (all Aldrich) were used as
received without further purifications. The precursor complexes
[{(η5-C5Me5)M(μ-Cl)Cl}2]14 (M=Rh or Ir) and the ligands 5-
Synthesis of [(η5-C5Me5)RhCl(ndpm)] (2). This complex was
prepared following the above procedure adopted for 1, except
that 5-(4-nitrophenyl)dipyrromethane (0.268 g, 1.0 mmol) was
used in place of 5-(4-cyanophenyl)dipyrromethane. Yield: 0.298 g,
58%. Anal. Calcd for C25H25ClN3O2Rh: C, 55.86, H, 4.69; N,
7.82. Found: C, 56.15; H, 4.78: N, 7.70. IR (KBr pellets, cm-1):
1545, 1374, 1342, 1248, 1199, 1102, 1021, 987, 892, 823, 771, 724,
477. 1H NMR (300 MHz, CDCl3, δ ppm): 1.52 (s, 15H), 6.50 (d,
2H, J=4.8 Hz), 6.54 (d, 2H, J=4.5 Hz), 7.58 (d, 2H, J=9.0 Hz),
7.83 (s, 2H), 8.29 (d, 2H, J=8.4 Hz). 13C NMR (75.45 MHz,
CDCl3, δ ppm): 8.6 (C-CH3), 94.8 (C5Me5), 119.8, 122.6, 132.2,
134.9, 143.2, 144.6, 147.9, 154.0. UV-vis (CH2Cl2, λmax nm, ε):
512 (2.33ꢀ104), 435 (1.18ꢀ104), 305 (1.39ꢀ104), 269 (1.84ꢀ104),
228 (2.35ꢀ104).
(9) (a) Joubran, C.; Grotjahn, D. B.; Hubbard, J. L. Organometallics
1996, 15, 1230. (b) Carmona, E.; Cingolani, A.; Marchetti, F.; Pettinari, C.;
Pettinari, R.; Skelton, B. W.; White, A. H. Organometallics 2003, 22, 2820–
2826. (c) Steinke, T.; Gemel, C.; Cokoja, M.; Winter, M.; Fischer, R. A.
Chem. Commun. 2003, 9, 1066. (d) Ara, I.; Berenguer, J. R.; Eguizabal, E.;
Fornies, J.; Lalinde, E.; Martin, A. Eur. J. Inorg. Chem. 2001, 61, 631.
(10) (a) Crabtree, R. H. The Organometallic Chemistry of the Transi-
tion Metals; John Wiley and Sons: Hoboken, NJ, 2005. (b) For highlights of
important developments in the metal-ligand cooperative activation of sub-
strates, see: Gr€utzmacher, H. Angew. Chem., Int. Ed. 2008, 47, 1814. (c)
Liu, J.; Wu, X.; Iggo, J. A.; Xiao, J. Coord. Chem. Rev. 2008, 252, 782–809.
(d) Hesp, K. D.; McDonald, R.; Ferguson, M. J.; Stradiotto, M. J. Am. Chem.
Soc. 2008, 130, 16394–6406. (e) Severin, K. Chem. Commun. 2006, 31,
3869–3867.
(11) (a) Scharwitz, M. A.; Ott, I.; Geldmacher, Y.; Gust, R.; Shel-
drick, W. S. J. Organomet. Chem. 2008, 693, 2299–2309. (b) Dorcier, A.;
Ang, W. H.; Bolao, S.; Gonsalvi, L.; Juillerat-Jeannerat, L.; aurenczy, G.;
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2006, 25, 4090.
(12) (a) King, E. R.; Betley, T. A. Inorg. Chem. 2009, 48, 2361–2363.
(b) Wechsler, J. C.; Ali, A. A.; Chapman, E. E.; Cameron, T. S.; Thompson, A.
Inorg. Chem. 2007, 46, 10947–10949.
Synthesis of [(η5-C5Me5)RhCl(bdpm)] (3). This complex was
prepared following the above procedure for 1, except that 5-
(4-benzyloxyphenyl)dipyrromethane(0.328g, 1.0mmol)wasused
in place of 5-(4-cyanophenyl)dipyrromethane. Yield: 0.298 g,
50%. Anal. Calcd for C32H32N2OClRh: C, 64.17; H, 5.38; N,
4.68. Found: C, 64.46; H, 5.48; N, 4.60. IR (KBr pellet, cm-1):
1602, 1525, 1445, 1374, 1335, 1289, 1247, 1174, 1021, 987, 886,
816, 733, 641. 1H NMR (300 MHz, CDCl3, δ ppm): 1.48 (s, 15H),
5.14 (s, 2H), 6.48 (d, 2H, J=4.8 Hz), 6.73 (d, 2H, J=3.9 Hz), 7.15
(d, 2H, J=9.0 Hz), 7.34-7.47 (m, 7H), 7.73 (s, 2H). 13C NMR
(75.45 MHz, CDCl3, δ ppm): 8.5 (C-CH3), 70.1, 94.5 (C5Me5),
(13) Perrin, D. D.; Armango, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals; Pergamon: Oxford, U.K., 1986.
(14) (a) Kang, W.; Moseley, K.; Maitlis, P. M. J. Am. Chem. Soc.
1969, 91, 5970. (b) Ball, R. G.; Graham, W. A. G.; Heinekey, D. M.; Hoyano,
J. K.; McMaster, A. D.; Mattson, B. M.; Michel, S. T. Inorg. Chem. 1990, 29,
2023. (c) White, C.; Yates, A.; Maitlis, P. M. Inorg. Synth. 1992, 29, 228.

Article
Organometallics, Vol. 28, No. 16, 2009 4715
113.6, 118.8, 127.5, 128.1, 128.6, 130.7, 131.8, 132.7, 136.0,
136.8, 146.5, 152.8, 159.2. UV-vis (CH2Cl2, λmax nm, ε): 507
(2.40ꢀ104), 432 (1.05ꢀ104), 355 (1.11ꢀ104), 268 (7.14ꢀ103), 230
(3.49ꢀ104).
CDCl3, δ ppm): 8.7 (C-CH3), 102.3 (C-CH3), 113.1, 118.5,
121.1, 127.5, 128.1, 129.1, 129.2, 130.5, 130.8, 131.4, 132.5,
133.4, 134.1, 134.6, 141.9, 144.2, 154.0. UV-vis (CH2Cl2, λmax
nm, ε): 501 (2.30ꢀ104), 339 (6.03ꢀ103), 316 (6.88ꢀ103), 267
(1.18ꢀ104), 233 (2.87ꢀ104).
Synthesis of [(η5-C5Me5)IrCl(cydpm)] (4). This complex was
prepared following the above procedure adopted for 1, except
that [{(η5-C5Me5)Ir(μ-Cl)Cl}2] (0.309 g, 0.50 mmol) was used in
place of [{(η5-C5Me5)Rh(μ-Cl)Cl}2]. Yield: 0.298 g, 54%. Anal.
Calcd for C26H25N3ClIr: C, 51.43; H, 4.15; N, 6.92. Found: C,
51.74; H, 4.20; N, 6.85. IR (KBr pellet, cm-1): 2224, 1537, 1452,
Synthesis of [(η5-C5Me5)Rh(N3)(ndpm)] (8). This complex was
prepared following the above procedure for 6, except that NaN3
was used in place of NH4SCN, using complex 2 (0.535 g,
1.0 mmol). Yield: 0.403 g, 75%. Anal. Calcd for
C25H25N6O2Rh: C, 55.16; H, 4.63; N, 15.44. Found: C, 55.42;
H, 4.50: N, 15.52. IR (KBr pellet, cm-1): 2017, 1545, 1523, 1340,
1247, 1199, 1105, 1023, 992, 796, 718, 478. 1H NMR (300 MHz,
CDCl3, δ ppm): 1.54 (s, 15H), 6.55 (d, 2H, J=4.8 Hz), 6.58 (d,
2H, J=4.2 Hz), 7.60 (d, J=8.4), 7.85 (s, 2H), 8.30 (d, 2H, J=
8.4 Hz). 13C NMR (75.45 MHz, CDCl3, δ ppm): 8.3 (C-CH3),
94.6 (C5Me5), 119.8, 122.5, 122.8, 132.2, 135.0, 143.2, 144.6,
147.9, 152.4. UV-vis (CH2Cl2, λmax nm, ε): 504 (2.35ꢀ104), 353
(1.13ꢀ104), 301 (1.71ꢀ104), 271 (2.25ꢀ104), 230 (2.49ꢀ104).
Synthesis of [(η5-C5Me5)Rh(SCN)(ndpm)] (9). This complex
was prepared following the procedure adopted for 6, using
complex 2 (0.535 g, 1.0 mmol) in place of 1. Yield: 0.454 g,
74%. Anal. Calcd for C26H25N4O2SRh: C, 55.72; H, 4.50; N,
10.00. Found: C, 56.03; H, 4.58: N, 9.95. IR (KBr pellet, cm-1):
2094, 1542, 1342, 1246, 1024, 988, 891, 821, 720, 476. 1H NMR
(300 MHz, CDCl3, δ): 1.56 (s, 15H), 6.55 (d, 2H, J=3.9 Hz), 6.60
(d, 2H, J=4.8 Hz), 7.61 (d, J=8.1) 7.73 (s, 2H), 8.31 (d, 2H, J=
8.7 Hz) ppm. 13C NMR (75.45 MHz, CDCl3, δ ppm): 8.3 (C-
CH3), 96.6 (C5Me5), 118.7, 122.3, 122.5, 131.2, 134.9, 143.2,
144.6, 147.9, 153.0. UV-vis (CH2Cl2, λmax nm, ε): 505 (2.35ꢀ
104), 359 (8.23ꢀ103), 306 (1.19ꢀ104), 262 (1.94ꢀ104), 230 (1.83ꢀ
104).
1
1376, 1339, 1248, 1202, 1025, 990, 893, 820, 766, 714, 473. H
NMR (300 MHz, CDCl3, δ ppm): 1.51 (s, 15H), 6.39 (d, 2H, J=
4.5 Hz), 6.47 (d, 2H, J=4.2 Hz), 7.53 (d, 2H, J=8.1 Hz), 7.50 (d,
2H, J=6.6 Hz), 7.74 (s, 2H). 13C NMR (75.45 MHz, CDCl3, δ
ppm): 8.4 (C-CH3), 86.7 (C5Me5), 112.4 (CtN), 117.7, 118.6,
129.1, 131.2, 132.4, 142.4, 144.2, 154.1. UV-vis (CH2Cl2, λmax
nm, ε): 504 (1.86ꢀ104), 457 (2.10ꢀ104), 303 (1.38ꢀ104), 268
(1.36ꢀ104), 233 (2.08ꢀ104).
Synthesis of [(η5-C5Me5)IrCl(ndpm)] (5). This complex was
prepared following the above procedure adopted for 1, except
that [{(η5-C5Me5)Ir(μ-Cl)Cl}2] (0.309 g, 0.50 mmol) was used
in place of [{(η5-C5Me5)Rh(μ-Cl)Cl}2] and 5-(4-nitrophe-
nyl)dipyrromethane (0.268 g, 1.0 mmol) in place of 5-(4-cyano-
phenyl)dipyrromethane. Yield: 0.298 g, 55%. Anal. Calcd for
C25H25ClO2N3Ir: C, 47.88; H, 4.02; N, 6.70. Found: C, 47.88; H,
4.10: N, 6.60. IR (KBr pellets, cm-1): 1546, 1455, 1377, 1343,
1249, 1202, 1102, 1024, 990, 893, 823, 723, 477. 1HNMR (300MHz,
CDCl3, δ ppm): 1.52 (s, 15H), 6.39 (d, 2H, J=4.2 Hz), 6.48 (d, 2H,
J=3.9 Hz), 7.59 (d, 2H, J=8.4 Hz), 7.70 (s, 2H), 8.30 (d, 2H, J=
8.4 Hz). 13C NMR (75.45 MHz, CDCl3, δ ppm): 8.4 (C-CH3),
86.8 (C5Me5), 118.7, 122.6, 131.2, 132.3, 143.8, 144.2, 147.9, 154.3.
UV-vis (CH2Cl2, λmax nm, ε): 502 (2.02ꢀ104), 459 (2.11ꢀ104),
302 (1.83ꢀ104), 271 (2.27ꢀ104), 229 (2.25ꢀ104).
Synthesis of [(η5-C5Me5)Rh(PPh3)(ndpm)]SO3CF3 (10). It
was prepared following the method employed for 7 starting
from complex 2 (0.535 g, 1.0 mmol). Yield: 0.637 g, 72%. Anal.
Calcd for C44H40N3O5F3PSRh: C, 57.84; H, 4.41; N, 4.60.
Found: C, 57.46; H, 4.32; N, 4.71. IR (KBr pellet, cm-1):
1553, 1521, 1436, 1378, 1344, 1267, 1150, 1095, 1030, 990, 892,
Synthesis of [(η5-C5Me5)Rh(SCN)(cydpm)] (6). Complex 1
(0.515 g, 1.0 mmol) was treated with NH4SCN (0.076 g,
1.0 mmol) in dry acetone (20 mL), and the suspension was
stirred at room temperature for 3 h. It was concentrated to
dryness under vacuum, and the residue was extracted with
dichloromethane (10 mL) and filtered to remove solid ammo-
nium chloride. The filtrate was concentrated to ∼2 mL, and an
excess of hexane was added to assist precipitation. The orange-
colored product thus obtained was washed with diethyl ether
and dried under vacuum. Yield: 0.412 g, 77%. Anal. Calcd for
C27H25N4SRh: C, 60.00; H, 4.66; N, 10.37. Found: C, 60.38; H,
4.52 ; N, 10. 28. IR (KBr pellet, cm-1): 2229, 2099, 1545, 1441,
1376, 1341, 1250, 1024, 991, 812, 772, 723, 673, 476. 1H NMR
(300 MHz, CDCl3, δ ppm): 1.55 (s, 15H), 6.52 (d, 2H, J=3.9
Hz), 6.58 (d, 2H, J=3.9 Hz), 7.54 (d, 2H, J=8.1 Hz), 7.71 (s,
2H), 7.74 (d, 2H, J=8.1 Hz). 13C NMR (75.45 MHz, CDCl3,
δ ppm): 8.3 (C-CH3), 96.6 (C5Me5), 112.6, 118.9, 120.0, 131.2,
132.7, 134.7, 142.5, 143.9, 152.9. UV-vis (CH2Cl2, λmax nm, ε):
504 (2.31ꢀ104), 427 (7.14ꢀ103), 357 (6.78ꢀ103), 308 (9.15ꢀ103),
259 (1.49ꢀ104), 234 (2.91ꢀ104).
1
821, 747, 697, 637, 572, 522. H NMR (300 MHz, CDCl3, δ
ppm): 1.40 (s, 15H), 6.36 (d, 2H, J=3.6 Hz), 6.53 (d, 2H, J=4.5
Hz), 7.16 (s, 2H), 7.37-7.57 (m, 17H), 8.34 (d, 2H, J=8.7). 13
C
NMR (75.45 MHz, CDCl3, δ ppm): 8.7 (C-CH3), 102.4
(C5Me5), 121.2, 122.9, 123.0, 127.6, 128.2, 129.2, 130.7, 131.0,
133.4, 134.1, 134.5, 143.7, 154.1. UV-vis (CH2Cl2, λmax nm, ε):
501 (2.37ꢀ104), 349 (7.69ꢀ103), 305 (9.24ꢀ103), 262 (1.99ꢀ104),
229 (2.37ꢀ104).
Synthesis of [(η5-C5Me5)Ir(PPh3)(ndpm)]SO3CF3 (11). It was
prepared following the method employed for 7 starting from
complex 5 (0.535 g, 1.0 mmol). Yield: 0.637 g, 74%. Anal. Calcd
for C44H40N3SF3O5PIr: C, 52.69; H, 4.02; N, 4.19. Found: C,
53.03; H, 4.12: N, 4.25. IR (KBr pellet, cm-1): 1633, 1576, 1348,
1265, 1143, 1096, 1028, 992, 819, 747, 695, 636, 519. 1H NMR
(300 MHz, CDCl3, δ ppm): 1.42 (s, 15H), 6.34 (d, 2H, J=4.5
Hz), 6.41 (d, 2H, J=4.5 Hz), 7.22 (s, 2H), 7.38-7.60 (m, 17H),
8.34 (t, 2H). 13C NMR (75.45 MHz, CDCl3, δ ppm): 8.3 (C-
CH3), 96.4 (C5Me5), 120.4, 122.9, 123.0, 127.2, 127.9, 129.2,
130.6, 131.1, 131.6, 133.6, 143.2, 144.1, 148.3, 155.0. UV-
vis (CH2Cl2, λmax nm, ε): 500 (2.14ꢀ104), 427 (6.03ꢀ 103), 302
(1.16 ꢀ104), 266 (1.65ꢀ104), 230 (2.47ꢀ104).
Synthesis of [(η5-C5Me5)Rh(PPh3)(cydpm)]SO3CF3 (7). Com-
pound 1 (0.515 g, 1.0 mmol) was treated with AgSO3CF3
(0.257 g, 1 mmol) in dry acetone (30 mL) and stirred for 2 h at
rt. It was filtered through Celite to remove silver chloride.
Triphenylphosphine (0.262 g, 1 mmol) was added to the filtrate
and stirred at rt for 4 h. The solvent was removed in vacuo and
residue extracted with dichloromethane (5 mL) and filtered. An
excess of hexane was added to the filtrate to assist precipitation.
A yellow-colored precipitate thus obtained was separated by
filtration, washed with diethyl ether, and dried under vacuum.
Yield: 0.606 g, 70%. Anal. Calcd for C45H40N3SF3O3PRh: C,
60.47; H, 4.51; N, 4.70. Found: C, 60.11; H, 4.51: N, 4.81. IR
(KBr pellet, cm-1): 2230, 1625, 1555, 1436, 1379, 1341, 1270,
Synthesis of [{(η5-C5Me5)Ir(ndpm)}2(μ-dpph)](SO3CF3)2
(12). Complex 12 was prepared following the above procedure
starting from complex 5 (0.535 g, 1.0 mmol) and dpph (0.192 g,
0.50 mmol). Yield: 0.521 g, 64%. Anal. Calcd for
C82H82N6O10F6P2S2Ir2: C, 50.87; H, 4.27; N, 4.34. Found: C,
51.05; H, 4.35; N, 4.23. IR (KBr pellet, cm-1): 1556, 1520, 1381,
1346, 1265, 1150, 1102, 1031, 994, 895, 823, 746, 719, 636, 517.
1H NMR (300 MHz, CDCl3, δ ppm): 1.54 (s, 30H), 2.17-2.35
(m, 12H), 6.32 (d, 4H, J=3.9 Hz), 6.49 (d, 4H, J=3.9 Hz), 7.06-
7.49 (m, 28H), 8.26 (t, 4H). 13C NMR (75.45 MHz, CDCl3, δ
ppm): 8.4 (C-CH3), 24.3, 25.5, 31.5, 96.3(C5Me5), 120.7, 123.0,
123.1, 127.4, 128.1, 129.2, 131.0, 133.4, 134.0, 134.5, 143.4,
1
1141, 1097, 1031, 990, 891, 812, 750, 696, 634, 516. H NMR
(300 MHz, CDCl3, δ ppm): 1.39 (s, 15H), 6.35 (d, 2H, J =
3.6 Hz), 6.52 (d, 2H, J=4.5 Hz), 7.14 (s, 2H), 7.33-7.70 (m,
17H), 7.77 (d, 2H, J= 8.1 Hz) ppm. 13C NMR (75.45 MHz,

4716 Organometallics, Vol. 28, No. 16, 2009
Yadav et al.
Scheme 1
144.1, 148.3, 155.1. UV-vis (CH2Cl2, λmax nm, ε): 496 (2.15ꢀ
104), 430 (6.18ꢀ103), 302 (1.10ꢀ104), 268 (1.56ꢀ104), 226 (2.55ꢀ
104).
were collected on an Oxford Diffraction X CAUBER-S, while
for 1 and 2 on a Bruker SMART APEX diffractometer using
graphite-monochromatized Mo KR radiation. The structure
was solved by direct methods and refined using SHELX 97.16
The non-hydrogen atoms were refined with anisotropic thermal
parameters. The H atoms attached to carbon were included as a
fixed contribution and were geometrically calculated and re-
fined using the SHELX riding model.
Synthesis of [{(η5-C5Me5)Ir(ndpm)}2(μ-bpy)](SO3CF3)2 (13).
Complex 13 was prepared following the above procedure start-
ing from complex 5 (0.535 g, 1.0 mmol) and 4,40-bpy (0.078 g,
0.50 mmol). Yield: 0.472 g, 65%. Anal. Calcd for
C62H58N8O10F6S2Ir2: C, 45.47; H, 3.57; N, 6.84. Found: C,
45.16; H, 3.43; N, 6.95. IR (KBr pellet, cm-1): 1649, 1553, 1520,
1380, 1344, 1253, 1151, 1030, 994, 822, 721, 635. 1H NMR
(300 MHz, CDCl3, δ ppm): 1.56 (s, 30H), 6.53 (d, 4H, J=4.5
Hz), 6.69 (d, 4H, J=3.6 Hz), 7.57 (m, 4H), 8.02 (s, 4H), 8.12 (d,
4H, J=6.3 Hz) 8.28 (t, 4H), 8.53 (d, 4H, J=6.0 Hz). 13C NMR
(75.45 MHz, CDCl3, δ ppm): 8.5 (C-CH3), 89.5 (C5Me5), 120.7,
122.6, 122.9, 125.2, 130.9, 131.4, 132.6, 143.2, 144.7, 145.2,
148.2, 152.0, 153.1. UV-vis (CH2Cl2, λmax nm, ε): 496 (2.10ꢀ
104), 427 (6.03ꢀ103), 302 (1.06ꢀ104), 267 (1.5ꢀ104), 227 (2.45ꢀ
104).
Complex 1. Formula=C26H25ClN3Rh, Mr=517.86, mono-
˚
clinic space group C2/c, a=16.047(4) A, b=16.610(4) A, c=
˚
3
˚
˚
19.215(4) A, R=90.00°, β=99.38°, γ=90.00°, V = 5053(2) A ,
Z=8, Dc (Mg m-3)=1.454, μ=0.749, T(K)=293(2), λ=0.71073
˚
A, R(all)=0.0380, R[I>2σ(I)]=0.0322, wR2=0.0784, wR2
[I>2σ(I)]=0.0760, GooF=1.121.
Complex 2. Formula = C25H25ClN3O2Rh, Mr = 537.85,
˚
orthorhombic space group Pbca, a = 13.2123(11) A, b =
3
˚
˚
16.6245(14)v, c=23.926(2) A, β=90.00, V=5255.3(8) A , Z=
8, Dc (Mg m-3)=1.530, μ=0.964, T(K)=293(2), λ=0.71073 A,
˚
Synthesis of [{(η5-C5Me5)Rh(cydpm)}2(μ-N3)]Cl (14). A sus-
pension of the complex 1 (0.515 g, 1.0 mmol) and sodium
azide, NaN3 (0.033 g, 0.5 mmol), were stirred in dry acetone
for 3 h at room temperature. The solvent was removed
in vacuo and the residue extracted with dichloromethane
(10 mL) and filtered to remove solid sodium chloride. The
filtrate was concentrated to ∼2 mL, and an excess of hexane
was added to assist precipitation. The orange-colored pro-
duct separated, which was filtered, washed with diethyl
ether, and dried under vacuum. Yield: 0.425 g 73%. Anal.
Calcd for C52H50ClN9Rh2: C, 59.93; H, 4.84; N, 12.10.
Found: C, 59.60; H, 4.71; N, 12.20. IR (KBr pellet, cm-1):
2226, 2016, 1615, 1545, 1446, 1375, 1338, 1248, 1023, 989,
891, 813, 769, 719, 474. 1H NMR (300 MHz, CDCl3, δ ppm):
1.53 (s, 30H), 6.50-6.58 (m, 8H), 7.52 (m, 4H), 7.73 (d, 4H,
J=7.2 Hz), 7.84 (s, 4H). 13C NMR (75.45 MHz, CDCl3, δ
ppm): 8.5 (C-CH3), 96.5 (C5Me5), 112.5, 118.7, 119.7, 131.3,
132.3, 134.6, 142.4, 143.8, 152.2. UV-vis (CH2Cl2, λmax nm, ε):
493 (2.38ꢀ104), 430 (8.22ꢀ103), 303 (1.43ꢀ103), 266 (2.78 ꢀ
104), 225 (3.46ꢀ104).
R(all)=0.0440, R[I > 2σ(I)]=0.0385, wR2=0.1013, wR2 [I >
2σ(I)]=0.0969, GooF=1.017.
Complex 7. Formula = C45H40N3SF3O3PRh, Mr = 893.77,
˚
orthorhombic space group Pnma, a = 15.3601(14) A, b =
3
13.4016(11) A, c = 19.6671(12) A, V = 4048.5(6) A , Z = 4,
˚
˚
˚
Dc (Mg m-3)=1.474, μ=0.572, T(K)=150(2), λ=0.71073 A,
˚
R(all)=0.0734, R[I > 2σ(I)]=0.0634, wR2=0.1421, wR2 [I>
2σ(I)]=0.1391, GooF=1.235.
Complex 11. Formula=C22H16N4RuS2, Mr=501.60, orthor-
˚
hombic space group P212121, a=13.0911(11) A, b=14.3862(13)
A, c=21.2380(18) A, V=3999.8(6) A , Z=4, Dc (Mg m-3)=
3
˚
˚
˚
˚
1.666, μ=3.495, T(K)=293(2), λ=0.71073 A, R(all)=0.0403,
R[I > 2σ(I)]=0.0336, wR2=0.0719, wR2 [I > 2σ(I)]=0.0684,
GooF=0.977.
Results and Discussion
Synthesis of the Complexes. The dipyrromethanes em-
ployed in this work were synthesized following the literature
procedures,15 and the dipyrrins were obtained by oxidation
of the corrresponding dipyrromethane using 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone (DDQ) in a chloroform/ben-
zene solution. Resulting dipyrrins were not isolated, but used
Catalytic Experiments. A solution of 1.0 equiv of terephthalal-
dehyde, HCOOH (13 mmol), CH3COONa (13 mmol), and
1 mol % of the catalyst (few drops of acetonitrile were added
for dissolution of catalyst) in water (5.5 mL) was stirred at 50 °C
under aerobic conditions. After full conversion had been achieved
the reaction was filtered through silica and MgSO4, washed
(20% EtOAc/80% hexane), and concentrated under vacuum to
give the reduction product. The residue was purified by flash
column chromatography.
Crystallographic Studies. Crystals suitable for single-crystal
X-ray diffraction analyses for 1, 2, 7, and 11 were grown from
dichloromethane and diethyl ether at room temperature using
the diffusion technique. Preliminary data on the space group
and unit cell dimensions as well as intensity data for 7 and 11
(16) (a) Mackay, S.; Dong, W.; Edwards, C.; Henderson, A.; Gilmox,
C.; Stewart, N.; Shankland, K.; Donald, A. MAXUS; University of
Glasgow: Scotland, 1999. (b) Sheldrick, G. M. SHELX-97: Programme for
refinement of crystal structures; University of Gottingen: Gottingen,
Germany, 1997. (c) PLATON. Spek, A. L. Acta Crystallogr. A 1990, 46,
C31.
(15) (a) Rao, P. D.; Dhanalekshmi, S.; Littler, B. J.; Lindsey, J. S. J.
Org. Chem. 2000, 65, 7323–7344. (b) Littler, B. J.; Miller, M. A.; Hung, C.
H.; Wagner, R. W.; O'Shea, D. F.; Boyle, P. D.; Lindsey, J. S. J. Org. Chem.
1999, 64, 1391–1396. (c) Lee, C. H.; Lindsey, J. S. Tetrahedron 1994, 50,
11427–11440.

Article
Organometallics, Vol. 28, No. 16, 2009 4717
Scheme 2. Synthesis of Mononuclear Complexes (i) NaN3/Acetone, (ii) NH4SCN/Acetone, and (iii) AgSO3CF3/PPh3/Acetone
Scheme 3. Synthesis of Binuclear Complexes (i) NaN3/Acetone and (ii) AgSO3CF3/L-L/Acetone
in situ to generate the desired metal complexes.17-20 Synth-
eses of the neutral heteroleptic metal complexes [(η5-
C5Me5)RhCl(cydpm)] (1), [(η5-C5Me5)RhCl(ndpm)] (2),
[(η5-C5Me5)RhCl(bdpm)] (3), [(η5-C5Me5)IrCl(cydpm)] (4),
and [(η5-C5Me5)IrCl(ndpm)] (5) were achieved by treatment
of a solution of the chloro-bridged dimeric precursor com-
plexes [{(η5-C5Me5)M(μ-Cl)Cl}2] (M=Rh or Ir) in dichlor-
omethane in the presence of triethylamine to a solution
containing respective dipyrrins. A simple scheme showing
the syntheses of neutral heteroleptic metal complexes is
depicted in Scheme 1. The complexes were formed under
refluxing conditions, after which the products were purified
by column chromatography over silica. In general, the
complexes obtained are bright orange-red compounds that
appeared lustrous green upon removal of the solvent. Com-
plexes 1-5 are air-stable solids, and these do not show any
signs of decomposition in solution upon exposure to air for
several days. These are soluble in polar organic solvents such
as chloroform, methanol, ethanol, dichloromethane, aceto-
nitrile, dimethylformamide, and dimethylsulfoxide and in-
soluble in ether, benzene, and hexane.
and 2 with N-3 or SCN- in acetone afforded neutral com-
plexes [(η5-C5Me5)Rh(SCN)(cydpm)] (6), [(η5-C5Me5)Rh-
(N3)(ndpm)] (8), and [(η5-C5Me5)Rh(SCN)(ndpm)] (9),
wherein Cl- was replaced by N-3 or SCN-. On the other
hand, reactions of 1, 2, and 5 with PPh3 or exobidantate
ligands dpph and 4,40-bipyridine in the presence of AgSO3CF3
afforded cationic mononuclear complexes [(η5-C5Me5)-
Rh(PPh3)(cydpm)]SO3CF3 (7), [(η5-C5Me5)Rh(PPh3)(ndpm)-
SO3CF3 (10), and [(η5-C5Me5)Ir(PPh3)(ndpm)SO3CF3
(11) and dinuclear complexes [{(η5-C5Me5)Ir(ndpm)}2-
(μ-dpph](SO3CF3)2 (12) and [{(η5-C5Me5)Ir(ndpm)}2(μ-
bpy)](SO3CF3)2 (13).
Further, it was observed that the reaction of 1 with NaN3
afforded the binuclear complex [{(η5-C5Me5)Rh(cydpm)}2-
(μ-N3)]Cl (14). A simple scheme showing the synthesis of
derivatives of 1, 2, and 5 is depicted in Schemes 2 and 3.
Characterization. The complexes were characterized by
elemental analyses, FAB-MS, IR, NMR (1H and 13C), and
electronic spectral and electrochemical studies. Analytical
data of the complexes (recorded in the Experimental Section)
supported the proposed formulations. Information about
composition of the binuclear complexes 12 and 14 was also
obtained from FAB-MS spectral studies, and resulting spec-
tra along with their assignments are shown in Figures 1 and 2.
Fragmentation patterns of these complexes provided
valuable information about the relative stability of the
various moieties bonded to Ir(III)/Rh(III). Complex 12 in
its FAB mass spectrum displayed peaks at m/z 1787(1786.46,
66.67%), 1046(1046.35, 37.50%), and 592(592.16,
100.00%), assignable to [Mþ - SO3CF3], [Mþ - (SO3CF3 þ
IrCp*(ndpm)], and [Mþ-(SO3CF3 þ IrCp*(ndpm) þ dpph)],
Reactivities of the complexes [(η5-C5Me5)RhCl(4-cydpm)]
(1), [(η5-C5Me5)RhCl(ndpm)] (2), and [(η5-C5Me5)IrCl-
(ndpm)] (5) were examined with some nitrogen and phos-
phorus donor ligands. It was observed that the reactions of 1
€
(17) Bruckner, C.; Zhang, Y.; Rettig, S. J.; Dolphin, D. Inorg. Chim.
Acta 1997, 263, 279–286.
€
(18) Bruckner, C.; Karunaratne, V.; Rettig, S. J.; Dolphin, D. Can. J.
Chem. 1996, 74, 2182–2193.
(19) Cohen, S. M.; Halper, S. R. Inorg. Chim. Acta 2002, 341, 12–16.
(20) Halper, S. R.; Cohen, S. M. Chem.;Eur. J. 2003, 9, 4661–4669.

4718 Organometallics, Vol. 28, No. 16, 2009
Yadav et al.
Figure 1. FAB mass spectrum of complex 12.
Figure 2. FAB mass spectrum of complex 14.
respectively, while complex 14 exhibited prominent peaks
at m/z 1007(1006.23, 57.00%) and 482(482.11, 100.00%).
The presence of molecular ion peaks at m/z 1787(1786.46)
and 1007(1006.23) in the FAB-MS of 12 and 14, respec-
tively, strongly supported formation of the binuclear
complexes.
downfield shift as compared to that in the respective dipyr-
romethanes. In general, R-pyrrolic protons (∼δ 6.76) dis-
played a significant deshielding (∼δ 7.83), and the β-protons
in 1, 2, 4, and 5, where the meso-susbstituent contains an
electron-withdrawing group (-CN or NO2) appeared as two
doublets (closely spaced). On the other hand, in complex 3, in
which meso-susbstituent contains an electron-releasing
group (-OCH2Ph), these appeared as two well-resolved
doublets (Figure S1). In the iridium complexes 4 and 5
separation between the doublets is somewhat larger as
compared to that in the rhodium complexes 1 and 2. For
example, the 1H NMR spectra of complex 2 exhibited
1H and 13C NMR spectral data of the complexes are in
good agreement with their respective formulations. To facil-
1
itate assignment of various resonances, H-1H COSY ex-
periments were performed, and the resulting spectrum for
complex 2 is shown in Figure 3. Upon complexation with the
metal center, the dipyrrin protons exhibited an appreciable

Article
Organometallics, Vol. 28, No. 16, 2009 4719
Figure 3. 1H-1H COSY spectrum of complex 2 in CDCl3.
Figure 4. UV-visible spectra of complexes 1-5 in dichloromethane.
resonances at 6.50(d, 2H, J=4.8 Hz), 6.54(d, 2H, J=4.5 Hz),
7.58(d, 2H, J=9.0 Hz), 7.83(s, 2H), and 8.29(d, 2H, J=8.4
Hz) ppm assignable to pyrrolic and phenyl protons of the
dipyrrin (ndpm). This region of the spectrum integrated for
10 protons, as expected for the coordinated ndpm ligand.
The protons due to pentamethylcyclopentadienyl resonated
at δ 1.52(s, 15H) ppm. The position and integrated intensity
of various signals conformed well to the proposed formula-
tion of the complex. Similar trends have been observed in the
1H NMR spectra of other complexes. In the 13C{1H} NMR
spectra of complex 2, the ndpm carbons resonated at 119.8,
122.6, 132.2, 134.9, 143.2, 144.6, 147.9, and 154.0, whereas
pentamethylcyclopentadienyl21 carbons resonated at
8.55(C-CH3) and 94.78(C5Me5) ppm. In general, it is ob-
served that the number of signals in the 13C NMR spectra of
the complexes was not in agreement with the number
of expected signals. This discrepancy may be attributed to
the resonance of various types of carbons at the same
position.
(21) Herberich, G. E.; Ganter, B. Inorg. Chem. Commun. 2001, 4,
100–103.

4720 Organometallics, Vol. 28, No. 16, 2009
Yadav et al.
Figure 5. ORTEP views of 1, 2, and 11 at 30% probability. Hydrogen atoms and the trifluoromethanesulfonate anion are omitted for
clarity.
Comparative electronic spectra of the mononuclear com-
plexes 1-5 in dichloromethane are depicted in Figure 4.
Electronic spectra of complexes 1-5 showed moderately
intense bands in the region ∼512, 435, 304, 269, and 228
nm (see Experimental Section). By analogy with the earlier
reports the intense low-energy bands (512/435 nm) have been
assigned to highly conjugated dipyrrin-based S0 f S1 (π f
π*) transitions22 along with metal-dipyrrin charge transfer
transitions in the visible region.17,20,22c The higher energy
bands may be ascribed to the dipyrrin-based π f π*/n f π*
transitions.17,20,22 In complex 3, the bands at 355 nm ex-
hibited a red shift in comparison to the complexes 1, 2, 4,
and 5 (∼304 nm). This shift may be attributed to the presence
of a benzyloxy chromophore. The pentamethylcyclopenta-
dienyl (Cp*) mainly shows low-intensity π f π* transition
bands in the UV region overlapping with dipyrrin-based
π f π*/n f π* transitions.23 The absorption bands for the
iridium complexes are blue-shifted in comparison to their
rhodium counterparts; this may be attributed to the greater
sepration between t2g and eg sets of orbitals in Ir(III).
Emission experiments on the complexes were carried out in
DCM with 1.0 mM solutions at rt. Upon excitation at their
respective lowest energy bands, these complexes did not
exhibit any emission.
Molecular Structures. Molecular structures of the repre-
sentative complexes 1, 2, 7, and 11 have been determined
crystallographically. Details about the data collection, solu-
tion, and refinement are summarized in the Experimental
Section. Molecular structures with atom-numbering
schemes are shown in Figure 5, and important geometrical
parameters are gathered in Table S1.
(23) (a) Chandra, M.; Sahay, A. N.; Mobin, S. M.; Pandey, D. S. J.
Organomet. Chem. 2002, 658, 43–49. (b) Singh, S. K.; Chandra, M.; Dubey,
S. K.; Pandey, D. S. Eur. J. Inorg. Chem. 2006, 3954–3961. (c) Singh, S. K.;
Trivedi, M.; Chandra, M.; Pandey, D. S. J. Organomet. Chem. 2005, 690,
647–652.
(22) (a) Halper, S. R.; Malachowski, M. R.; Delaney, H. M; Cohen,
S. M. Inorg. Chem. 2004, 43, 1242–1249. (b) Smalley, S. J.; Waterland, M.
R.; Telfer, S. G. Inorg. Chem. 2009, 48, 13–15. (c) Halper, S. R.; Stork, J. R.;
Cohen, S. M. Dalton Trans. 2007, 1067–1074.

Article
Organometallics, Vol. 28, No. 16, 2009 4721
Figure 6. Linear chain resulting from C-H Cl interaction along the b-axis in complex 2.
3 3 3
Figure 7. H-bonding interactions along the a-axis in complex 11.
˚
˚
A common structural feature of the rhodium complexes 1,
2, 7, and iridium complex 11 is the arrangement of various
groups about the metal centers rhodium and iridium. In
these complexes the rhodium (1, 2) metal center is coordi-
nated to the dipyrrin nitrogen N(1) and N(2), chloro group
Cl(1), and pentamethylcyclopentadienyl ring in a η5-manner,
while in 7 and 11 the rhodium (7) or iridium (11) metal center
is coordinated through P(1) of the PPh3 in place of Cl(1) [one
of the phenyl rings of the coordinated PPh3 ligand in 7 is
highly disordered]. Considering the pentamethylcyclopenta-
dienyl ring as a single coordination site, the overall coordi-
nation geometry about the rhodium or iridium metal center
might be described as a piano-stool geometry. Average C-C
distances in the pentamethylcyclopentadienyl rings are 1.410
2.082(3), 2.066(3) A in 2, and 2.089(4), 2.089(4) A in 7, which
are comparable to Rh-N distances reported in the literature.
The dipyrrinato bite angles N(1)-Rh(1)-N(2) are 85.02(9)°,
85.49(10)°, and 87.6(2)°, respectively in 1, 2, and 7. The Rh-P
˚
distance in 7 is 2.335(2) A, which is again comparable to
the Rh-P distances in other rhodium complexes. As expected,
the meso-phenyl substituents are twisted out of plane
from the dipyrrin π-system by -75.4° in 1, -113.8° in 2,
-90.8° in 7, and -88.0° in 11.
The iridium to pyrrolic nitrogen of dipyrrin distances in 11
[Ir(1)-N(1), Ir(1)-N(2)] are comarable and are 2.074(5) and
2.090(5) A. The pentamethylcyclopentadienyl ring in this
complex is almost planar, and iridium is displaced from the
˚
˚
centroid of the pentamethylcyclopentadienyl ring by 1.843 A.
˚
and 1.431 A in 1 and 2. The rhodium to carbon distances are
The metal to centroid of the pentamethylcyclopentadie-
nyl ring distance in 11 is normal and comparable to the
values reported in other iridium complexes.24,25 The Ir-P
almost equal, with an average Rh-C distance of 2.148 in 1
and 2.161 A in 2 and the rhodium metal center is displaced
from the centroid of the pentamethylcyclopentadienyl ring
˚
˚
distance in 11 is 2.3167(13) A, which is again comparable
˚
by 1.785, 1.786, and 1.844 A respectively in 1, 2, and 7, which
to the Ir-P distances in other iridium complexes. The
bond angle between iridium and N(1) and N(2) nitrogens
of dipyrrin is 86.64(16)° (Table S1).
are comparable to the values reported in other rhodium
complexes.24 The metal to centroid of the pentamethylcy-
clopentadienyl ring distance in 7 is longer than that in its
precursor complex 1. The Rh-Cl bond distances are
Crystal packing in complexes 1, 2, 7, and 11 is stabilized by
C-H X type (X=Cl, F, N, O) intermolecular hydrogen
3 3 3
˚
˚
2.3949(9) and 2.4091(8) A in 1 and 2, respectively, which
bonding. The C-H Cl (C(25)-H(25) Cl(1) = 2.82 A)
3 3 3 3 3 3
are normal and comparable to those reported in other related
systems.24 The rhodium to pyrrolic nitrogen distances
interaction in complex 2 leads to linear structural motifs shown
in Figure 6. The C-H F, C-H N, and C-H O weak
bonding interactions are present in complex 11. A simple view
3 3 3
3 3 3
3 3 3
˚
[Rh(1)-N(1), Rh(1)-N(2)] are 2.065(2), 2.068(2) A in 1,
(24) Govindaswamy, P.; Mozharivskyj, Y. A.; Kollipara, M. R.
Polyhedron 2007, 26, 5039–5044.
(25) Govindaswamy, P.; Mozharivskyj, Y. A.; Kollipara, M. R.
Polyhedron 2005, 24, 1710.

4722 Organometallics, Vol. 28, No. 16, 2009
Yadav et al.
Table 1. Electrochemical Data of the Complexes 1-5
Scheme 4. Transfer Hydrogenation of Terephthalaldehyde with
Catalysts 1-5 in Water and Aerobic Conditionsa
complex
E1/2, V (ΔE, mV)
Ep,a, V
1
2
3
4
5
0.83(74)
0.86(76)
0.78(78)
0.92(83)
0.94(71)
0.70
0.72
0.66
0.75
075
a S/C ratio is 100.
current intensity at a potential of ca. 0.66-0.75 V. This may
be attributed to the oxidation of respective dipyrrins; this
value falls within the range for dipyrrinato complexes.1,27,28
Catalytic Studies. Transfer hydrogenation reactions of
aldehyde were catalyzed by a number of half-sandwich
Rh/Ir complexes. However, the reaction necessitated high
temperature and inert atmosphere.29 The selective reduction
of terephthalaldehyde was carried out either with NaBH4 or
via catalytic hydrogenation using Pd/C catalysis or metallo-
cenes; in all cases conversion yield is not good and also needs
an organic solvent system.30 To evaluate the selectivity and
efficiency of complexes 1-5 as catalysts toward reduction of
the diformyl group, terephthalaldehyde was used as a model
substrate. Transfer hydrogenation was initiated by introdu-
cing formic acid-sodium acetate and terephthalaldehyde
(1.0 mmol) in water (a few drops of acetonitrile was used to
dissolve the catalyst) and air at 50 °C with 1% catalyst. All
the complexes were found to catalyze transfer hydrogenation
of terephthalaldehyde to produce 4-hydroxymethybenzalde-
hyde in aqueous solution (Scheme 4). It was observed that in
all the cases only one formyl group of terephthalaldehyde
was selectively reduced. Further, we found that the rhodium
complexes 1 and 2 led to almost complete conversion of
terephthalaldehyde into 4-hydroxymethylbenzaldehyde in 1 h.
Analogous iridium complexes 4 and 5 were less active as
compared to the rhodium complexes 1 and 2, and using these,
complete conversion of the substrate takes place in 2 h.
However, complex 3 required 3 h for the same reduction
and 3 is least effective in this regard. Among the rhodium
complexes dipyrrin containing electron withdrawing meso-
substituents (CN, NO2) are more effective than its electron-
releasing counterparts. This may be due to the inductive effect
of the substituents on the metal center.
Figure 8. Cyclic voltammogram of complex 2 (vs Ag/Agþ) in
acetonitrile.
resulting from these interactions is shown in Figure 7. Matrixes
for weak hydrogen bonding interactions in these complexes are
shown in the Supporting Information Table S2.
Electrochemistry. The redox properties of compounds 1-5
were followed by cyclic voltammetry (CV) using 0.1 M tert-
butylammoniumperchlorate (TBAP) in acetonitrile as sup-
porting electrolyte. The potential of the Fc/Fcþ couple under
the experimental conditions was 0.10 V (80 mV) vs Ag/Agþ.
The electrochemical data are presented in Table 1. The cyclic
voltammogram (CV) for compound 2 is depicted in Figure 8.
The neutral complexes 1-5 exhibited one-electron reversible
oxidation corresponding to the M(IV/III) (M=Rh, Ir) redox
ox
couple at the half-wave oxidation potential values (E in V
1/2
vs Ag/Agþ) in the range 0.78-0.94 V.26 Although the ipa/ipc
ratio is less than 1 for metal-based oxidation, it may be due to
deposition of the electrogenerated species at the electrode
surface. The complexes 1-3 showed that the oxidation
potentials are affected by the electron-withdrawing/donat-
ing ability of the meso-substituents of the dipyrrinato li-
gands. Electron-donating groups (benzyloxy 3) increase the
electron density on the ligand and also on the rhodium
center, which in turn leads to the redox process occurring
at more negative potentials, while the opposite effect occurs
when electron-withdrawing groups are used. The iridium
complexes 4 and 5 exhibited more anodic redox potential
compared to that in rhodium complexes 1-3. All the com-
plexes exhibited an additional oxidation wave with lower
Conclusion
Through this work an attempt has been made to synthe-
size and characterize a series of heteroleptic piano-stool
rhodium/iridium(III) complexes imparting dipyrrin as
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Article
Organometallics, Vol. 28, No. 16, 2009 4723
co-ligands. The complexes represent the first example of
heteroleptic complexes of rhodium/iridium containing both
dipyrrin and pentamethylcyclopentadienyl ligands. Further-
more, it has been shown that complexes 1-5 effectively
catalyze the reduction of terephthalaldehyde, and among
these, the rhodium complexes 1 and 2 are more effective
transfer hydrogenating catalysts for use in water and air.
SAIF, Central Drug Research Institute, Lucknow, for
providing spectral facilities, and National Single Crystal
X-ray Diffraction Laboratory, Indian Institute of Tech-
nology, Powai, Mumbai, for X-ray facilities. Special
thanks are due to Prof. P. Mathur, Department of
Chemistry, IIT, Mumbai, for encouragement.
Supporting Information Available: Figures S1-S16, full
crystallographic data for the structure determination of 1,
2, 7, and 11 in CIF format and tables S1-S2. This material is
available free of charge via the Internet at http://pubs.acs.
org.
Acknowledgment. Thanks are due to the Council of
Scientific and Industrial Research, New Delhi, India, for
providing financial assistance through the scheme
HRDG 01(2074)/06/EMR-II. We also thank the Head,