Regioselective cyclometallation with some platinum group metal ions

Article abstract of DOI:10.1016/j.jorganchem.2014.04.009

Reactions of [Rh(PPh₃)₃Cl], [Ir(PPh₃) ₃Cl], Li₂[PdCl₄] and K₂[PtCl ₄] with the corresponding 4-R-Nê(arylidene)benzohydrazides (H₂Lⁿ; n = 1 and 2 for arylidene = 1-pyrenylidene and R = H and Cl, respectively and n = 3 for arylidene = 1-naphthylidene and R = H) in presence of NaOAc·3H₂O in methanol/toluene provide the complexes with general formula trans-[Rh(Lⁿ)(PPh₃) ₂H] (1 and 2), trans-[Ir(Lⁿ)(PPh₃) ₂H] (3 and 4), [Pd(HLⁿ)Cl] (5) and [Pt(HLⁿ)Cl] (6), respectively. Reactions of 5 and 6 with PPh₃ in acetone produce the complexes [Pd(Lⁿ)(PPh₃)] (7) and [Pt(L ⁿ)(PPh₃)] (8), respectively. All the complexes have been characterized with the help of microanalysis (CHN), spectroscopic (infrared, NMR, electronic and emission) and cyclic voltammetric measurements. Molecular structures of 1-5 and 8 have been confirmed with single crystal X-ray crystallography. In 5 and 6, (HL¹)^- acts as a monobasic CNO-donor ligand, whereas in each of 1-4, 7 and 8, the (Lⁿ) ^2- acts as a dibasic CNO-donor ligand. In 1, 2, 5 and 7, metallation at the peri position of the 1-pyrenyl moiety of the pincer like ligand leads to the formation of 6,5-membered fused chelate rings, whereas in 3, 4, 6 and 8 metallation takes place at ortho position of the 1-pyrenyl/1-naphthalenyl moiety of the ligand with the formation of 5,5-membered fused chelate rings.

Full text of DOI:10.1016/j.jorganchem.2014.04.009

Journal of Organometallic Chemistry 762 (2014) 58e66
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Regioselective cyclometallation with some platinum group metal ions
*
A.R. Balavardhana Rao, Samudranil Pal
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Reactions of [Rh(PPh3)3Cl], [Ir(PPh3)3Cl], Li2[PdCl4] and K2[PtCl4] with the corresponding 4-R-Nʹ-(aryli-
dene)benzohydrazides (H2Ln; n ¼ 1 and 2 for arylidene ¼ 1-pyrenylidene and R ¼ H and Cl, respectively
and n ¼ 3 for arylidene ¼ 1-naphthylidene and R ¼ H) in presence of NaOAc$3H2O in methanol/toluene
provide the complexes with general formula trans-[Rh(Ln)(PPh3)2H] (1 and 2), trans-[Ir(Ln)(PPh3)2H] (3
and 4), [Pd(HLn)Cl] (5) and [Pt(HLn)Cl] (6), respectively. Reactions of 5 and 6 with PPh3 in acetone
produce the complexes [Pd(Ln)(PPh3)] (7) and [Pt(Ln)(PPh3)] (8), respectively. All the complexes have
been characterized with the help of microanalysis (CHN), spectroscopic (infrared, NMR, electronic and
emission) and cyclic voltammetric measurements. Molecular structures of 1e5 and 8 have been
confirmed with single crystal X-ray crystallography. In 5 and 6, (HL1)ꢀ acts as a monobasic CNO-donor
ligand, whereas in each of 1e4, 7 and 8, the (Ln)2ꢀ acts as a dibasic CNO-donor ligand. In 1, 2, 5 and 7,
metallation at the peri position of the 1-pyrenyl moiety of the pincer like ligand leads to the formation of
6,5-membered fused chelate rings, whereas in 3, 4, 6 and 8 metallation takes place at ortho position of
the 1-pyrenyl/1-naphthalenyl moiety of the ligand with the formation of 5,5-membered fused chelate
rings.
Received 24 March 2014
Received in revised form
4 April 2014
Accepted 5 April 2014
Keywords:
Platinum group
Polycyclic aryl
Cyclometallation
Regioselectivity
Crystal structure
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
the bidentate ligands except for few where both ortho- and peri-
metallated products are isolated. However, the specific factors that
Cyclometallation is one of the well-established tools for the
functionalization of CeH bond either in stoichiometric or catalytic
processes [1,2]. Regioselective cyclometallation is of particular in-
terest in connection with the synthesis of tailor-made organic
compounds as it provides the scope for functionalization of a spe-
cific site among various available. Cyclometallation reactions with
Schiff bases and the corresponding cyclometallates are well-known
in literature [3,4]. This type of cyclometallates can be useful in-
termediates in synthesis of various heterocycles [5,6]. In recent
times, several regioselective cyclometallation of the polycyclic aryl
moiety such as 1-naphthalenyl, 9-phenanthryl, 1-pyrenyl, 3-
perilenyl and 3-indolyl in the Schiff bases synthesized from the
corresponding polycyclic aromatic aldehydes with platinum group
metal ions have been reported [6e21]. Generally for tridentate li-
gands either ortho- or peri-metallation occurs in a regioselective
way, while only ortho-metallation takes place for the majority of
control the regioselective cyclometallation processes are yet to be
ascertained. Many parameters such as nature of the metal ion,
steric constraints, reaction temperature, polarity of the solvent etc.
may affect and hence regulate the regioselective metallation of the
substrate [22,23]. A better understanding of these controlling pa-
rameters could lead to the development of new and effective syn-
thetic strategies for designer molecules. In the present work, we
have studied the cyclometallation chemistry of some platinum
group metal (Rh, Ir, Pd and Pt) ions with 4-R-Nʹ-(1-pyrenylidene/1-
naphthylidene)benzohydrazides (H2Ln, 2 ‘H’s represent the amide
and aryl CeH protons) and isolated a new group of cyclometallates
with CNO-pincer like ligands (Scheme 1). The aim was to unravel
whether the regioselective cyclometallation of the polycyclic aryl
group of H2Ln is a metal ion dependent process or not. Indeed in the
present cyclometallates, the 4d metal ions (Rh(III) and Pd(II)) prefer
the peri position, while the 5d metal ions (Ir(III) and Pt(II)) prefer
the ortho position of the polycyclic aryl group and form six- and
five- membered chelate rings, respectively. Herein, we describe the
syntheses, characterization, properties and structures of these
cyclometallates.
* Corresponding author. Tel.: þ91 40 2313 4829; fax: þ91 40 2301 2460.
E-mail addresses: spal@uohyd.ac.in, spscuh2000@yahoo.co.in (S. Pal).
http://dx.doi.org/10.1016/j.jorganchem.2014.04.009
0022-328X/Ó 2014 Elsevier B.V. All rights reserved.

A.R. Balavardhana Rao, S. Pal / Journal of Organometallic Chemistry 762 (2014) 58e66
59
Scheme 1. (i) [Rh(PPh3)3Cl] and CH3COONa$3H2O (1:2) in methanol, refluxing, 8 h; (ii) [Ir(PPh3)3Cl] and CH3COONa$3H2O (1:2) in toluene, refluxing, 48 h; (iii) Li2[PdCl4] and
CH3COONa$3H2O (1:1) in methanol, stirring at room temp., 48 h; (iv) K2[PtCl4] and CH3COONa$3H2O (1:1) in methanol-water, stirring at 60 ꢁC, 12 h; (v) and (vi) PPh3 (2 mole equiv.)
in acetone, stirring at room temp., 24 h.
2. Experimental
298 K under nitrogen atmosphere. Under similar condition the E1/2
value of the ferrocenium/ferrocene (Fcþ/Fc) redox couple was
observed at 0.44 V.
2.1. Materials
2.3. Synthesis of trans-[Rh(L1)(PPh3)2H] (1)
All chemicals used in this work were of analytical grade avail-
able commercially and were used as received. The solvents used
were purified by standard methods [24]. [Rh(PPh3)3Cl] and
[Ir(PPh3)3Cl] were synthesized by literature methods [25,26].
[Rh(PPh3)3Cl] (185 mg, 0.2 mmol) and NaOAc$3H2O (55 mg,
0.4 mmol) were added to a suspension of H2L1 (69 mg, 0.2 mmol) in
methanol (20 ml). The mixture was refluxed under nitrogen at-
mosphere for 8 h. The yellow solid formed was filtered and washed
with cold methanol and dried in air. Yield was 145 mg (74%). Anal.
calcd for C60H45N2OP2Rh: C, 73.92; H, 4.65; N, 2.87. Found: C, 73.81;
H, 4.61; N, 2.78. Selected IR bands (cmꢀ1): nRheH, 2010; nC]N, 1582.
2.2. Physical measurements
Microanalyses (CHN) were carried out with a Thermo Finnigan
Flash EA-1112 elemental analyzer. A Shimadzu LCMS 2010 liquid
chromatograph mass spectrometer was used for the purity verifi-
cation of H2L1eH2L3. Magnetic susceptibility measurements at
room temperature (298 K) were performed with a Sherwood sci-
entific balance. A Thermo Scientific Nicolet 380 FT-IR spectropho-
tometer was used to record the infrared spectra of the complexes in
KBr pellets. Solution electrical conductivities were measured with a
Digisun DI-909 conductivity meter. A Shimadzu UV3600 UVeVise
NIR spectrophotometer was used to collect the electronic spectra.
The emission spectra were recorded on a Horiba Jobin Yvon
Fluoromax-4 spectrofluorometer. The 1H (400 MHz) and 31P
(121 MHz for 1e4 and 7 and 202 MHz for 8) NMR spectra were
recorded with the help of Bruker NMR spectrometers. A CH-
Instruments model 620A electrochemical analyzer was used for
cyclic voltammetric experiments with dichloromethane solutions
of the complexes containing tetrabutylammonium perchlorate
(TBAP) as the supporting electrolyte. The measurements were
carried out with a platinum disk working electrode, a platinum
wire auxiliary electrode and an Ag/AgCl reference electrode at
1H NMR in CDCl3:
d
(ppm) (J(HeH) (Hz)) ¼ 8.26 (br, s, 1H, H11), 7.93
(9) (d, 1H, H2), 7.88 (m, 2H, H6, H8), 7.82 (7) (d, 1H, H3), 7.67 (m, 3H,
H4, H5, H7), 7.53 (br, s, 1H, H9), 7.50 (8) (d, 2H, H14, H18), 7.32e6.91
(m, 33H, H15, H16, H17 and 2 PPh3 protons), ꢀ10.13 (qd, 1H, RheH).
31P NMR in CDCl3:
in CH2Cl2: lmax(nm) (
423 (3.42), 400 (2.51), 318 (6.95), 310sh (6.80), 230 (20.55). Emis-
d
(ppm) (J(RheP) (Hz)) ¼ 39.54 (118) (d). UVeVis
3
(104 Mꢀ1 cmꢀ1)) ¼ 478 (6.29), 448 (5.54),
sion in CH2Cl2: lmax (nm) (lexc (nm)) ¼ 515, 395, 415 (320).
2.4. Synthesis of trans-[Rh(L2)(PPh3)2H] (2)
This complex was prepared in 70% yield by following the same
procedure as described above for 1 using H2L2 instead of H2L1. Anal.
calcd for C60H44N2OP2ClRh: C, 71.40; H, 4.39; N, 2.78. Found: C,
71.32; H, 4.32; N, 2.71. Selected IR bands (cmꢀ1): nRheH, 2016; nC]N
,
1578. 1H NMR in CDCl3:
d
(ppm) (J(HeH) (Hz)) ¼ 8.24 (br, s, 1H, H11),
7.94 (9) (d, 1H, H2), 7.88 (m, 2H, H6, H8), 7.82 (8) (d, 1H, H3), 7.68 (8)
(t, 1H, H7), 7.62 (8) (d, 2H, H4, H5), 7.56 (br, s, 1H, H9), 7.49 (8) (d, 2H,

60
A.R. Balavardhana Rao, S. Pal / Journal of Organometallic Chemistry 762 (2014) 58e66
H14, H18), 7.31e6.91 (m, 32H, H15, H17 and 2 PPh3 protons), ꢀ10.13
2.7. Synthesis of [Pd(HL1)Cl] (5)
(qd, 1H, RheH). 31P NMR in CDCl3:
d
(ppm) (J(RheP) (Hz)) ¼ 39.52
(118) (d). UVeVis in CH2Cl2: lmax(nm) (
(1.91), 451 (1.93), 425 (1.56), 400sh (1.42), 313 (3.19) 230 (9.53).
3
(104 Mꢀ1 cmꢀ1)) ¼ 483
H2L1 (174 mg, 0.5 mmol) and NaOAc$3H2O (68 mg, 0.5 mmol)
were added to a methanol solution (40 ml) of Li2PdCl4 (prepared in
situ using 0.5 mmol of PdCl2 and 1.0 mmol of LiCl). The mixture was
stirred under aerobic condition at room temperature for 48 h. The
product precipitated as a yellow solid was filtered and washed with
cold methanol and dried in air. Yield was 221 mg (90%). Anal.
calcd for PdC24H15N2OCl: C, 58.92; H, 3.09; N, 5.73. Found: C, 58.83;
H, 3.16; N, 5.65. Selected IR bands (cmꢀ1): nNeH, 3144; nC]O, 1643;
Emission in CH2Cl2: lmax (nm) (lexc (nm)) ¼ 515, 415, 395 (310).
2.5. Synthesis of trans-[Ir(L2)(PPh3)2H] (3)
[Ir(PPh3)3Cl] (203 mg, 0.2 mmol) and NaOAc$3H2O (55 mg,
0.4 mmol) were added to a suspension of H2L2 (72 mg, 0.2 mmol) in
toluene (20 ml). The mixture was refluxed under nitrogen atmo-
sphere for 48 h. The orange solution formed was evaporated to
dryness. The solid obtained was dissolved in dichloromethane and
transferred to a silica gel column packed with n-hexane. The elution
was done with n-hexane-acetone mixture. The initial yellow band
was discarded. The following orange band containing 3 was
collected and evaporated to dryness. The yield of 3 thus obtained
was 138 mg (62%). Anal. calcd for C60H44N2OP2ClIr: C, 65.60; H,
4.04; N, 2.55. Found: C, 65.51; H, 4.12; N, 2.48. Selected IR bands
nC]N,1604. 1H NMR in (CD3)2SO:
d
(ppm) (J(HeH) (Hz)) ¼ 11.38 (s,1H,
NH), 9.36 (br, s,1H, H11), 9.15 (s, 1H, H9), 8.50 (8) (d,1H, H3), 8.39 (m,
3H, H2, H6, H8), 8.27 (8) (d, 2H, H4, H5), 8.14 (m, 3H, H7, H14, H18),
7.71 (br, s, 1H, H16), 7.64 (m, 2H, H15, H17). UVeVis in Me2NCHO:
lmax(nm) (
3
(104 Mꢀ1 cmꢀ1)) ¼ 480 (2.39), 450 (2.78), 426 (1.93),
400sh (1.12), 317 (2.57), 307 (2.48), 290sh (1.64). Emission in
Me2NCHO: lmax (nm) (lexc (nm)) ¼ 510, 390, 410 (317).
2.8. Synthesis of [Pt(HL1)Cl (6)
(cmꢀ1): nIreH, 2050; nC]N, 1588. 1H NMR in (CD3)2SO:
d (ppm) (J(He
H2L1 (174 mg, 0.5 mmol) and NaOAc$3H2O (68 mg, 0.5 mmol)
were added to a solution of K2[PtCl4] (208 mg, 0.5 mmol) in a
solvent mixture of methanol (20 ml) and H2O (1 ml). The reaction
mixture was stirred under aerobic condition at 60 ꢁC for 12 h. The
complex [Pt(HL1)Cl] precipitated as a brown solid collected by
filtration, washed with methanol and dried in air. Yield was 235 mg
(80%). Anal. calcd for PtC24H15N2OCl: C, 49.88; H, 2.62; N, 4.85.
H) (Hz)) ¼ 8.29 (br, s, 1H, H11), 8.04 (m, 2H, H6, H8), 7.98 (7) (d, 1H,
H9), 7.85 (m, 2H, H10, H7), 7.72 (9) (d,1H, H5), 7.55 (9) (d, 2H, H14
,
H18), 7.41e7.24 (m, 32H, H15, H17 and 2 PPh3 protons), 7.20 (9) (d,
1H, H4), 7.10 (s, 1H, H3), ꢀ12.56 (td, 1H, IreH). 31P NMR in (CD3)2SO:
3
d
(ppm)
¼
14.94 (s). UVeVis in CH2Cl2: lmax(nm)
(
(104 Mꢀ1 cmꢀ1)) ¼ 510sh (2.23), 476 (2.91), 435 (6.91), 418 (6.71),
395sh (4.44), 356 (4.25), 340 (3.69), 265 (12.71), 229 (19.17).
Found: C, 49.72; H, 2.68; N, 4.75. Selected IR bands (cmꢀ1): nNeH
,
3205; nC]O, 1637; nC]N, 1582. 1H NMR in (CD3)2SO:
d (ppm) (J(HeH)
(Hz)) ¼ 12.06 (s, 1H, NH), 9.54 (s, 1H, H11), 8.84 (m, 1H, H3), 8.60 (8)
(d, 1H, H4), 8.39 (m, 3H, H5, H9, H10), 8.27 (8) (d, 2H, H6, H8), 8.14 (8)
(t, 1H, H7), 8.02 (7) (d, 2H, H14, H18), 7.62 (m, 3H, H15, H16, H17). UVe
2.6. Synthesis of trans-[Ir(L3)(PPh3)2H] (4)
Complex 4 was synthesized in 65% yield by following a proce-
dure similar to that described for 3 by using H2L3 instead of H2L2.
Anal. calcd for C54H43N2OP2Ir: C, 65.51; H, 4.38; N, 2.83. Found: C,
Vis in Me2NCHO: lmax(nm) (
3
(104
M
ꢀ1 cmꢀ1)) ¼ 590sh (0.10), 547
(0.18), 497 (0.81), 464 (1.24), 437 (1.11), 423 (1.06), 400 (1.43),
385sh (1.63), 372 (1.81), 350sh (1.37), 280 (2.42). Emission in
Me2NCHO: lmax (nm) (lexc (nm)) ¼ 600, 645, 405, 390 (350).
65.46; H, 4.31; N, 2.89. Selected IR bands (cmꢀ1): nIreH, 2070; nC]N
,
1572. 1H NMR in CDCl3:
d
(ppm) (J(HeH) (Hz)) ¼ 7.75 (br, s, 1H, H9),
7.54e7.07 (m, 39H, H5eH8, H12eH16 and 2 PPh3 protons), 6.69 (8)
(d, 1H, H4), 6.59 (8) (d, 1H, H3), ꢀ12.66 (t, 1H). 31P NMR in CDCl3:
3
2.9. Synthesis of [Pd(L1)(PPh3)] (7)
d
(ppm)
M
¼
16.65 (s). UVeVis in CH2Cl2: lmax(nm)
(
(104
ꢀ1 cmꢀ1)) ¼ 495sh (0.34), 468 (0.41), 434 (0.40), 415 (0.39),
[Pd(HL1)Cl] (98 mg, 0.2 mmol) and PPh3 (105 mg, 0.4 mmol)
were taken in acetone (30 ml) and the mixture was stirred under
395sh (0.38), 360sh (0.64), 346 (0.73).
Table 1
Selected crystal data and structure refinement summary.
Complex
1
2
4
5$Me2NCHO
Chemical formula
Formula weight
Crystal system
Space group
C60H45N2OP2Rh
974.83
Tetragonal
P4 21c
C60H44N2OP2ClRh
1009.27
Monoclinic
P21/n
C54H43N2OP2Ir
990.04
Triclinic
P1
C27H22N3O2ClPd
562.36
Monoclinic
P21/n
Temp. (K)
298
100
298
298
ꢀ
a (A)
24.0598(8)
24.0598(8)
16.2146(7)
90
90
90
9386.2(6), 8
1.380
0.477
12.0869(14)
16.2496(19)
23.833(3)
90
99.508(2)
90
4616.7(9), 4
1.452
0.544
11.821(2)
11.851(2)
18.726(3)
100.320(2)
93.214(3)
119.778(2)
2207.5(7), 2
1.489
10.5779(12)
9.3644(10)
23.157(3)
90
93.803(10)
90
2288.8(4), 4
1.632
0.959
ꢀ
b (A)
ꢀ
c (A)
a
b
g
(o)
(o)
(o)
3
ꢀ
V (A ), Z
r
(g cmꢀ3
)
)
m
(mmꢀ1
3.138
Reflections collected
Reflections unique
89085
8266
32708
8136
21179
7727
9396
4031
Reflections [I ꢂ 2
s(I)]
7074
599
5918
608
7384
544
2950
311
Parameters
R1, wR2 [I ꢂ 2
s
(I)]
0.0527, 0.0974
0.0646, 0.1015
1.142
0.640/ꢀ0.211
0.0421, 0.0824
0.0696, 0.0902
0.980
0.623/ꢀ0.322
0.0199, 0.0499
0.0211, 0.0503
1.063
1.313/ꢀ0.352
0.0415, 0.0809
0.0667, 0.0914
1.024
0.440, ꢀ0.397
R1, wR2 [all data]
GOF on F2
Max./Min. peaks (e A
ꢀꢀ3
)

A.R. Balavardhana Rao, S. Pal / Journal of Organometallic Chemistry 762 (2014) 58e66
61
aerobic condition at room temperature for 24 h. The complex
2.11. X-ray crystallography
precipitated as a yellow solid was filtered, washed with acetone and
dried in air. Yield was 121 mg (85%). Anal. calcd for PdC42H29N2OP:
C, 70.54; H, 4.09; N, 3.92. Found: C, 70.36; H, 4.31; N, 3.85. Selected
Single crystals of 1 and 2 were obtained by slow evaporation of
the corresponding dichloromethane-n-hexane (1:1) solutions,
while single crystals of 4 were grown by slow evaporation of its
dichloromethane-acetonitrile (1:1) solution. Diethyl ether vapour
diffusion into the dimethylformamide solution of 5 provided its
single crystals as 5$Me2NCHO. Unit cell determination and in-
tensity data collection of 1, 2 and 4 were performed on a Brukere
IR bands (cmꢀ1): nC]N, 1561. 1H NMR in (CD3)2SO:
d (ppm) (J(HeH)
(Hz)) ¼ 9.22 (br, s, 1H, H11), 8.58 (8) (d, 1H, H2), 8.41 (8) (d, 1H, H3),
8.24 (m, 4H, H4, H5, H6, H8), 8.11 (br, s, 1H, H7), 7.87e7.32 (m, 20H,
H14, H15, H16, H17, H18 and PPh3 protons), 7.12 (br, s,1H, H9). 31P NMR
in (CD3)2SO:
d
(ppm) ¼ 37.31 (s). UVeVis in Me2NCHO: lmax(nm) (
3
(104 Mꢀ1 cmꢀ1)) ¼ 480 (2.72), 450 (3.17), 426 (2.16), 400sh (1.23),
317 (2.88), 307 (2.81), 290sh (1.80). Emission in Me2NCHO: lmax
(nm) (lexc (nm)) ¼ 510, 390, 410 (317).
Nonius SMART APEX CCD single crystal diffractometer (Mo K
a
ra-
ꢀ
diation,
l
¼ 0.71073 A). An Oxford Diffraction Xcalibur Gemini
ꢀ
single crystal X-ray diffractometer (Mo K
a
radiation,
l
¼ 0.71073 A)
was used to obtain the unit cell parameters and intensity data for
5$Me2NCHO. The SMART and the SAINT-Plus packages [27] were
used for data acquisition and data extraction, respectively for 1, 2
and 4. The absorption corrections were performed with the help of
SADABS program [28]. The CrysAlisPro software [29] was used for
data collection, reduction and absorption correction for
5$Me2NCHO. In each case, the structure was solved by direct
method and refined on F2 by full-matrix least-squares procedures.
All non-hydrogen atoms were refined anisotropically. The metal
bound hydrogen atoms located in the corresponding difference
maps were refined isotropically for 1 and 2, while for 4 it
was refined with geometric and thermal restraints. In case
of 5$Me2NCHO, the hydrogen atom of the amide fragment (e
NH(C]O)e) of the ligand was located in a difference map and
refined isotropically. The O-atom of the solvent molecule Me2NCHO
is hydrogen bonded with this amide NeH of the ligand (N/O,
2.770(5) A; NeH/O 165(4) ). All the remaining hydrogen atoms in
each structure were included in the structure factor calculation at
idealized positions by using a riding model. The SHELX-97 pro-
grams [30] available in the WinGX package [31] were used for
structure solution and refinement. The Platon [32] and the Mercury
[33] software packages were used for molecular graphics. Selected
crystal data and refinement summary for all four structures are
listed in Table 1.
2.10. Synthesis of [Pt(L1)(PPh3)] (8)
A mixture of [Pt(HL1)Cl] (116 mg, 0.2 mmol) and PPh3 (105 mg,
0.4 mmol) in acetone (30 ml) was stirred under aerobic condition at
room temperature for 24 h. The resulting red solution was evapo-
rated to dryness. The red solid so obtained was dissolved in mini-
mum amount of dichloromethane and transferred to a silica gel
column packed with n-hexane. The yellow band obtained first by
elution with n-hexane-acetone mixture was discarded. The next
red band containing 8 was collected and evaporated to dryness. The
yield of 8 thus obtained was 80 mg (50%). Anal. calcd for
PtC42H29N2OP: C, 62.76; H, 3.64; N, 3.49. Found: C, 62.85; H, 3.72; N,
3.41. Selected IR bands (cmꢀ1): nC]N, 1582. 1H NMR in (CD3)2SO:
d
(ppm) (J(HeH) (Hz)) ¼ 9.05 (10) (d,1H, H11), 8.41 (9) (d,1H, H9), 8.13
ꢁ
ꢀ
(m, 2H, H6, H8), 8.02 (9) (d, 1H, H10), 7.91 (m, 4H, H5, H7, H14, H18),
7.73e7.44 (m, 18H, H15, H16, H17 and PPh3 protons), 6.99 (9) (d, 1H,
H4), 6.67 (br, s, 1H, H3) . 31P NMR in (CD3)2SO:
d
(ppm) ¼ 22.96 (s,
with 195Pt satellites, J(PteP) ¼ 3780 Hz). UVeVis in Me2NCHO:
1
lmax(nm) (
3
(104 Mꢀ1 cmꢀ1)) ¼ 570sh (0.24), 530sh (0.39), 491
(1.76), 462 (3.12), 437 (2.27), 410sh (1.15), 368 (1.61), 350sh (1.13),
317 (2.74), 310 (2.70), 290sh (2.64), 274 (2.91). Emission in Me2N-
CHO: lmax (nm) (lexc (nm)) ¼ 645, 600, 405, 390 (350).
Scheme 2. Possible steps involved in the formation of 1e4.

62
A.R. Balavardhana Rao, S. Pal / Journal of Organometallic Chemistry 762 (2014) 58e66
ratio in methanol (for Pd(II)) or in methanol-water mixture (for
Pt(II)) under aerobic conditions provided complexes 5 and 6 in very
good yields (80e85%). Treatment of each of 5 and 6 with 2 mol
equivalents of PPh3 in acetone produces complexes 7 and 8 in 80
and 50% yields, respectively (Scheme 1). The microanalysis (CHN)
data of 1e8 are consistent with the corresponding general formulas
[M(Ln)(PPh3)2H] (M ¼ Rh(III) (1 and 2) and Ir(III) (3 and 4)), [M(HLn)
Cl] (M ¼ Pd(II) (5) and Pt(II) (6)) and [M(Ln)(PPh3)] (M ¼ Pd(II) (7)
and Pt(II) (8)). In the synthesis reactions, protic methanol was used
as solvent for the rhodium(III) complexes (1 and 2), while a non-
protic solvent toluene was used for the iridium(III) complexes (3
and 4). 1 and 2 can also be synthesized using toluene as solvent and
the same reaction conditions as applied for 3 and 4, but attempts to
synthesize any of 3 and 4 from methanol were unsuccessful.
However, from ethanol instead of 3 an iridium(III) dihydride com-
plex [Ir(HL2)(PPh3)2H2] was isolated. In this complex, (HL2)ꢀ acts as
N,O-chelating ligand. The molecular structure of this complex as
obtained by preliminary X-ray crystallographic experiments is
shown in Fig. S1 (Supplementary Material). Considering the for-
mation of the dihydride complex, synthetic conditions used for 1e4
and the mechanisms reported in literature for the synthesis of
similar cyclometallates and dihydride complexes with thosemi-
carbazones under comparable reaction conditions [34], we propose
the following steps for the formation of 1e4: oxidative addition of
P2
N2
N1
C1
O1
Rh1
P1
H1
(a)
P2
N2
N1
Cl1
C1
O1
Rh1
H1
P1
(b)
Fig. 1. Thermal ellipsoid plots of (a) trans-[Rh(L1)(PPh3)2H] (1) (30% probablity) and (b)
trans-[Rh(L2)(PPh3)2H] (2) (50% probablity). In both plots, for clarity none of the carbon
atoms are labelled except for the metal coordinated one.
(a)
3. Results and discussion
3.1. Synthesis and some properties
The 4-R-Nʹ-(arylidene)benzohydrazides (H2L1eH2L3) were pre-
pared in w90% yields by condensation reactions of equimolar
amounts of 4-R-benzohydrazide and the corresponding polycyclic
aromatic aldehydes in ethanol [16e19]. The identities of these three
compounds were authenticated by microanalysis (CHN) and spec-
troscopic (LC-MS, IR, 1H NMR and UVeVis) measurements. Com-
plexes 1e4 were synthesized in good yields (62e74%) by reacting
the monovalent metal ion starting materials [M(PPh3)3Cl]
(M ¼ Rh(I) and Ir(I)) with H2Ln and NaOAc$3H2O in 1:1:2 mole ratio
in methanol (for Rh(I)) or in toluene (for Ir(I)) under nitrogen at-
mosphere (Scheme 1). On the other hand, reactions of [MCl4]2ꢀ
(M ¼ Pd(II) and Pt(II)) salts, H2L1 and NaOAc$3H2O in 1:1:1 mole
(b)
Fig. 2. Thermal ellipsoid plots of (a) trans-[Ir(L3)(PPh3)2H] (4) (25% probablity) and (b)
[Pd(HL1)Cl] (5) (30% probablity). In the plot of 4, for clarity none of the carbon atoms
are labelled except for the metal coordinated one.

A.R. Balavardhana Rao, S. Pal / Journal of Organometallic Chemistry 762 (2014) 58e66
63
Table 2
evaporation of the corresponding dichloromethane-acetonitrile
(1:1) solutions were very weakly diffracting. As a result, the X-ray
data obtained for the latter two complexes were of very poor
quality. Nonetheless, structures for both could be solved but
refinement failed to reduce the R-factors to a satisfactory level. The
molecular structures of 3 and 8 with the selected crystal data are
provided in Supplementary Material (Fig. S2). In each of the rho-
dium(III) (1 and 2) and the iridium(III) (3 and 4) complexes, the
metal centre is in distorted octahedral CNOHP2 coordination sphere
constituted by the meridional (Ln)2ꢀ, a hydride and two mutually
trans-oriented PPh3 ligands. In the palladium(II) complex (5),
(HL1)ꢀ and the chloride, while in the platinum(II) complex (8),
(Ln)2ꢀ and the PPh3 provide satisfactory CNOCl and CNOP square-
planar coordination geometries, respectively. In 1 and 2, (Ln)2ꢀ
coordinates the metal centre through the 1-pyrenyl peri-C, the
azomethine-N and the amidate-O atoms and forms 6,5-membered
fused chelate rings. Similarly (HL1)ꢀ acts as the 1-pyrenyl peri-C, the
azomethine-N and the amide-O donor to form 6,5-membered fused
chelate rings in 5. On the other hand, (Ln)2ꢀ uses the polycyclic aryl
ortho-C, the azomethine-N and the amidate-O to coordinate the
metal centre in each of 3, 4 and 8 and creates 5,5-membered fused
Selected bond lengths (A) and angles ( ) for trans-[Rh(L1/2)(PPh3)2H] (1 and 2) and
ꢁ
ꢀ
trans-[Ir(L3)(PPh3)2H] (4).
Complex
1 (M ¼ Rh)
2 (M ¼ Rh)
4 (M ¼ Ir)
M(1)eC(1)
M(1)eH(1)
M(1)eN(1)
M(1)eO(1)
M(1)eP(1)
M(1)eP(2)
2.020(4)
1.24(5)
2.030(3)
1.48(3)
2.040(2)
1.575(10)
2.0268(19)
2.1857(18)
2.2982(8)
2.3066(8)
99.3(12)
79.58(9)
154.77(9)
90.25(7)
90.78(7)
174.1(12)
75.19(7)
94.52(6)
100.98(6)
105.7(12)
91.35(5)
94.33(5)
79.6(12)
164.39(2)
84.8(12)
2.049(4)
2.128(3)
2.3235(12)
2.3240(12)
91(2)
93.72(17)
170.50(18)
84.14(12)
89.81(12)
176(2)
2.055(3)
2.115(2)
2.3126(9)
2.3107(9)
94.4(12)
93.77(12)
170.40(11)
88.00(9)
89.39(9)
171.7(12)
76.70(10)
94.24(7)
96.64(7)
95.1(12)
91.49(6)
92.86(6)
87.5(12)
168.95(3)
82.0(12)
C(1)eM(1)eH(1)
C(1)eM(1)eN(1)
C(1)eM(1)eO(1)
C(1)eM(1)eP(1)
C(1)eM(1)eP(2)
N(1)eM(1)eH(1)
N(1)eM(1)eO(1)
N(1)eM(1)eP(1)
N(1)eM(1)eP(2)
O(1)eM(1)eH(1)
O(1)eM(1)eP(1)
O(1)eM(1)eP(2)
P(1)eM(1)eH(1)
P(1)eM(1)eP(2)
P(2)eM(1)eH(1)
77.07(16)
96.79(11)
93.62(11)
99(2)
94.56(9)
93.03(9)
83(2)
168.27(5)
87(2)
ꢀ
chelate rings. The CeO (1.287(4)e1.303(6) A) and the CeN
ꢀ
(1.306(6)e1.323(3) A) bond lengths of the amidate fragment (e
C(Oꢀ)]Nꢀ) of (Ln)2ꢀ in 1, 2 and 4 are consistent with its deproto-
OeH (from the iminol form of azomethine-N coordinated H2Ln)
leading to two electron oxidation of the metal centre, formation of
the five-membered N,O-chelate ring and coordination of the hy-
dride and finally cyclometallation via activation of aryl CeH situ-
ated in close proximity (Scheme 2). Solid state magnetic
susceptibility measurements made at room temperature indicate
the diamagnetic nature of all the complexes. Thus the metal centres
(Rh and Ir) in 1e4 are trivalent and in low-spin state, while that (Pd
and Pt) in 5e8 are bivalent and in square-based coordination ge-
ometry. In chloroform, dichloromethane, dimethylsulfoxide and
dimethylformamide, 1e4 are highly soluble, while 5e8 are
moderately soluble. In solution, each of 1e8 behaves as
nonelectrolyte.
nated state [16e19]. In comparison, the corresponding CeO
ꢀ
ꢀ
(1.246(5) A) and CeN (1.335(5) A) bond lengths are significantly
shorter and longer, respectively due to the protonated form of the
amide functionality of the ligand (HL1)ꢀ in 5. The MeC, MeN, MeO,
MeP and MeH bond lengths in 1e4 are comparable with the cor-
responding bond lengths reported for complexes of trivalent metal
ions having similar coordinating atoms [25,34e36]. The PdeC, Pde
N, PdeO and PdeCl bond lengths in 5 are unexceptional [16,17].
3.3. Spectroscopic characteristics
Infrared spectra of 1e4, 7 and 8 do not display any band
assignable to the NeH and C]O stretches of the free H2Ln [16e19].
Absence of these bands indicates the deprotonated state of the
amide functionality in the corresponding ligands. In contrast, both
5 and 6 display a broad band at w3175 cmꢀ1
(
nNeH) and a moder-
3.2. Description of crystal structures
ately strong band at w1640 cmꢀ1
(
nC]O) due to the protonated
The molecular structures of 1, 2, 4 and 5 have been determined
by single crystal X-ray crystallography. The structures of the rho-
dium(III) complexes (1 and 2) are depicted in Fig. 1 and the struc-
tures of the iridium(III) (4) and the palladium(II) (5) complexes are
illustrated in Fig. 2. The bond parameters associated with the metal
centres in 1, 2 and 4 are listed in Table 2 and those for 5 are sum-
marized in Table 3. Attempts to grow single crystals of the
remaining complexes were partially successful. Crystals of 6 and 7
could not be grown, while that of 3 and 8 obtained by slow
amide functionality of the ligands in them. A medium intensity
band for the MeH stretch appears at 2010 and 2016 cmꢀ1 for the
rhodium(III) complexes 1 and 2, respectively, while that appears at
2050 and 2070 cmꢀ1 for the iridium(III) complexes 3 and 4,
respectively. These MeH stretching band positions are comparable
with the literature values [25,34,35]. A moderate to strong band in
the range 1570e1604 cmꢀ1 observed for all the complexes is
attributed to the metal coordinated azomethine C]N stretch [16e
19]. Three strong bands in the ranges 740e755, 690e696 and 510e
525 cmꢀ1 in the spectra of 1e4, 7 and 8 correspond to the coordi-
nated PPh3 in each of them [18,19,34,36].
Table 3
The NMR spectra of all the complexes except 1, 2 and 4 were
recorded in (CD3)2SO. The spectra of 1, 2 and 4 were recorded in
CDCl3. In the 1H NMR spectra of both 1 and 2, the hydride resonates
Selected bond lengths (A) and angles (ꢁ) for [Pd(HL1)Cl]$
ꢀ
Me2NCHO (5$Me2NCHO).
Complex
5$Me2NCHO
as a quartet of doublets centred at
d
ꢀ10.12 (Fig. 3). The quartet
indicates coupling of the hydride with both 103Rh and 31P nuclei
Pd(1)eC(1)
Pd(1)eN(1)
Pd(1)eO(1)
Pd(1)eCl(1)
C(1)ePd(1)eN(1)
C(1)ePd(1)eO(1)
C(1)ePd(1)eCl(1)
N(1)ePd(1)eO(1)
N(1)ePd(1)eCl(1)
O(1)ePd(1)eCl(1)
2.001(4)
1.961(3)
2.145(3)
2.3227(12)
92.45(15)
170.97(14)
98.42(12)
79.41(12)
168.04(10)
90.05(9)
1
2
and comparable J(RheH) and J(PeRheH/cis) coupling constants
(12.8 Hz) [37]. Further splitting of each peak to doublet is very likely
due to weak coupling of the hydride with a nearby ligand H-atom
(J(HeH) ¼ 3.1 Hz for 1 and 3.4 Hz for 2). On the other hand, the metal
coordinated hydride in the iridium(III) complexes 3 and 4 appears
as a triplet at
d
ꢀ12.56 (2J(PeIreH/cis) ¼ 18.6 Hz) and ꢀ12.66 (2J(PeIreH/
¼ 18.2 Hz), respectively (Fig. 3). Interestingly for 3 each peak of
cis)
the triplet again splits into a doublet due to weak HeH coupling

64
A.R. Balavardhana Rao, S. Pal / Journal of Organometallic Chemistry 762 (2014) 58e66
Fig. 3. Metal hydride 1H NMR resonances in (a) trans-[Rh(L2)(PPh3)2H] (2), (b) trans-[Ir(L1)(PPh3)2H] (3) and (c) trans-[Ir(L3)(PPh3)2H] (4) and (d) CeH/HeM interaction in 1e3.
(J(HeH) z 1.2 Hz) as observed for the rhodium(III) complexes, but for
4 there is no such splitting (Fig. 3). Appearance of triplet with no
further splitting suggests that coupling of hydride occurs only with
the two mutually cis oriented 31P nuclei in 4. The X-ray structures
(vide supra) of both 1 and 2 show that the rhodium coordinated
the rhodium bound hydride (Fig. 3). Such CeH/HeRh hydrogen
bonding and similar NMR behaviour of metal bound hydride have
been observed before [38]. In the iridium(III) species, the pyrenyl-H
ꢀ
(in 3) or the naphthalenyl-H (in 4) is at a longer distance (w2.9 A)
from the metal coordinated hydride compared to the correspond-
ing distances in the rhodium(III) complexes (1 and 2). Perhaps for
this reason CeH/HeIr hydrogen bonding and hence the HeH NMR
coupling is weaker in 3 than in 1 and 2 and the coupling could not
be observed in 4 at all. The amide functionality NeH proton of
ꢀ
hydride is within 2.1e2.4 A from one pyrenyl CeH (ortho to the
metallated-C) and two phosphine phenyl-H atoms. Between these
three H-atoms, it is expected that due to the rigidity of the tri-
dentate ligand only the pyrenyl-H can not alter its relative position
with respect to the hydride. Thus, it is very likely that in solution
the pyrenyl-H is involved in a hydrogen bonding interaction with
(HL1)ꢀ in 5 and 6 appears as a singlet at
d 11.38 and 11.95, respec-
tively. As expected no such resonance is observed for 1e4, 7 and 8
Fig. 4. Electronic spectra of trans-[Ir(L2)(PPh3)2H] (3) (————), [Pd(HL1)Cl] (5) (-$-$-$-)
and [Pt(L1)(PPh3)] (8) (d).
Fig. 5. Emission spectra of trans-[Rh(L1)(PPh3)2H] (1) (———) and [Pt(L1)(PPh3)] (8)
(d).

A.R. Balavardhana Rao, S. Pal / Journal of Organometallic Chemistry 762 (2014) 58e66
65
nature except the iridium(III) complexes (3 and 4). Below 450 nm,
the emission spectral profiles of all the complexes (lexc ¼ 315 nm for
1, 2, 5 and 7 and lexc ¼ 350 nm for 6 and 8) are very similar and
display two closely spaced bands at w410 and w390 nm (Fig. 5). In
addition to these two bands, rhodium(III) complexes (1 and 2)
display a third relatively stronger band at w515 nm. This same third
band is also observed on excitation at 470 nm. On the other hand, the
palladium(II) (5 and 7) complexes displaya similar band at w510 nm
only on excitation at 400 nm. Considering the electronic spectro-
scopic characteristics of the free H2Ln and the corresponding com-
plexes all these emission bands are assigned to predominantly
ligand centred emissive states [12,13,17,45]. Unlike the other com-
plexes, the platinum(II) (6 and 8) complexes display not one but two
additional very weak bands above 450 nm at a comparatively much
longer wavelengths (w600 and w645 nm) (Fig. 5). These two bands
can also be seen on excitation at w570 nm. Perhaps these weak
emissions are due to the spin-forbidden optical transitions observed
for the platinum(II) complexes (6 and 8) [42e44].
3.4. Redox properties
Fig. 6. Cyclic voltammograms of trans-[Rh(L1)(PPh3)2H] (1) (———) and trans-
[Ir(L3)(PPh3)2H] (4) (d) in dichloromethane (0.1 M TBAP) at 298 K.
Redox characteristics of all the complexes in dichloromethane
solutions have been examined by cyclic voltammetric measure-
ments at room temperature. The rhodium(III) and the iridium(III)
complexes (1e4) display an oxidation response on the anode side of
the Ag/AgCl reference electrode. The one-electron nature of this
oxidation is proposed by comparing the current heights with the
current heights of known one-electron redox processes under
similar conditions [18,19,45]. Representative voltammograms are
illustrated in Fig. 6. The response for each of 1 and 2 is irreversible
and the Epa value is 0.95(ꢃ1) V. On the other hand, quasi-reversible
responses are observed for both 3 and 4 at E1/2 values of 0.92 and
due to the deprotonation of the amide functionality in the corre-
sponding ligands. The azomethine (CH]N) proton resonates in the
ranges d 7.75e8.29 for 1e4 and d 9.05e9.54 for 5e8. It appears as a
rather broad singlet for all the complexes except 8 where it is
observed as a doublet. Similar doublet reported for analogous
complexes has been suggested to be due to the coupling of the
azomethine proton with the phosphorus atom of the PPh3 [39,40].
The 31P NMR spectra of all the triphenylphosphine coordinated
complexes have been recorded. The rhodium(III) complexes display
a doublet due to 103Rh and 31P coupling at
d
w39.53 (1J(Rhe
0.82 V, respectively. The peak-to-peak separations (DEp ¼ Epa ꢀ Epc
)
¼ 118 Hz for 1 and 2). In contrast, a singlet was observed in the
are 240 and 260 mV for 3 and 4, respectively. Absence of any such
oxidation response in the cyclic voltammograms of the palladiu-
m(II) and the platinum(II) complexes (5e8) indicates that the oxi-
dations observed for 1e4 are metal centred.
P)
spectra of the iridium(III) complexes (at
4, respectively) and the palladium(II) and platinum(II) complexes
(at 37.31 and 22.96 for 7 and 8, respectively).
The room temperature electronic absorption spectra of the
d 14.94 and 16.65 for 3 and
d
rhodium(III) and the iridium(III) complexes (1e4) were collected in
dichloromethane, while that of the palladium(II) and the plati-
num(II) complexes (5e8) were recorded in dimethylformamide.
Each complex shows multiple strong to very strong absorption
bands in the visible and ultraviolet regions. Representative spectra
are illustrated in Fig. 4. H2L1 and H2L2 display two strong bands at
w370 and w285 nm [18], while H2L3 shows only one strong band at
w340 nm [17]. Each of these bands is flanked by several shoulders.
This type of spectra are very similar to that of pure polycyclic aro-
matic hydrocarbon except for the red-shift and poorly resolved
vibrational structure [12,13,17e19,41]. The spectra of 1e8 are
somewhat comparable and display a group of absorptions in the
visible region (within w390e510 nm) and one or two bands in the
ultra-violet region (<390 nm). In general the spectral profiles
particularly in the visible range are similar to the profiles of free
H2Ln except for the further red-shift and more pronounced vibra-
tional structure. Thus the absorption bands displayed by 1e8 are
attributed to the transitions primarily centred at the polycyclic aryl
group of the corresponding ligands ((HLn)ꢀ or (Ln)2ꢀ) [12,13,17e19].
Above 510 nm only the platinum(II) complexes (6 and 8) show two
weak shoulders at w535 and w580 nm at the tail of the adjoining
shorter wavelength strong band. It is very likely that these shoul-
ders are due to spin-forbidden transitions involving the metal
centre having high spin-orbit coupling constant [42e44].
4. Conclusions
Synthesis, characterization and spectroscopic properties of one
pair of each of rhodium(III), iridium(III), palladium(II) and plati-
num(II) cyclometallates with tridentate 4-R-Nʹ-(1-pyrenylidene/
naphthylidene)benzohydrazides are reported. Electronic spectral
features correspond to primarily ligand centred transitions. Except
for the iridium(III) complexes, the remaining complexes are emis-
sive. The rhodium(III) and the iridium(III) complexes are redox
active and show a metal centred oxidation. The molecular struc-
tures of the rhodium(III) and iridium(III) complexes and one each
from the palladium(II) pair and the platinum(II) pair of complexes
are determined by X-ray crystallography. The similarities in the
physical properties of each pair of bivalent metal ion complexes
suggest that the molecular structures in the corresponding pairs
are also analogous. The structures reveal that the polycyclic aryl
group of the tridentate ligand undergoes peri-metallation in case of
the 4d metal ions, while the same undergoes ortho-metallation in
case of the 5d metal ions. With the present CNO-donor ligand
system peri-metallation leads to 6,5-membered fused chelate rings,
while ortho-metallation results in the formation of 5,5-membered
fused chelate rings.
Acknowledgements
Room temperature emission spectral measurements were per-
formed with dichloromethane solutions of 1e4 and dime-
thylformamide solutions of 5e8. All the complexes are emissive in
A. R. B. Rao thanks the Council of Scientific and Industrial
Research for a research fellowship. We gratefully acknowledge the

66
A.R. Balavardhana Rao, S. Pal / Journal of Organometallic Chemistry 762 (2014) 58e66
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Appendix A. Supplementary material
CCDC 991421e991424 contain the supplementary crystallo-
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Appendix B. Supplementary information
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