Regiospecific C(naphthyl)-H bond activation by platinum(II)-isolation, characterization, reactivity and TD-DFT study of the cycloplatinate complexes

Article abstract of DOI:10.1002/ejic.201100468

The regiospecific activation of C(naphthyl)-H bonds in a group of naphthylazo-2′-hydroxyarene ligands (H₂L) has been achieved by platinum(II) compounds under different reaction conditions, and stable cycloplatinates [Pt^IIL(D)] have b

Full text of DOI:10.1002/ejic.201100468

FULL PAPER
DOI: 10.1002/ejic.201100468
Regiospecific C(naphthyl)–H Bond Activation by Platinum(II) –
Isolation, Characterization, Reactivity and TD-DFT Study of the
Cycloplatinate Complexes
Achintesh Narayan Biswas,[a] Purak Das,[a] Vivek Bagchi,[b] Amitava Choudhury,[c] and
Pinaki Bandyopadhyay*[a]
Keywords: C–H activation / Cyclometallation / Platinum / Oxidation / Density functional calculations
The regiospecific activation of C(naphthyl)–H bonds in a
group of naphthylazo-2Ј-hydroxyarene ligands (H2L) has
been achieved by platinum(II) compounds under different re-
action conditions, and stable cycloplatinates [PtIIL(D)] have
been isolated in presence of neutral Lewis bases (D). Struc-
tures of the cycloplatinate complexes of platinum(II) have
been established by single-crystal X-ray crystallography. The
platinum(II) centres are surrounded by a C,N,O-terdentate
ligand frame (L) and Lewis base (D) in a distorted square
planar fashion. These cycloplatinate species have been
found to react with halogens and methyl iodide undergoing
metal-centered two electron oxidation affording platinum(IV)
cycloplatinates with distorted octahedral geometry. In the re-
actions with halogens and methyl iodide, trans oxidative ad-
dition has been found for [Pt(L)D] (D = 4-picoline), whereas
cis addition has been observed for [Pt(L)D] where D is a more
sterically demanding triphenylphosphane. Structures of two
representative platinum(IV) cycloplatinate species have been
determined by single-crystal X-ray crystallography. A time
dependent (TD)-DFT study of representative cycloplatinate
compounds has been performed. Optical absorption spectra
of the cycloplatinate compounds in dichloromethane have
been simulated using the TD-DFT method, and the experi-
mental spectra are in very good agreement.
C(naphthyl)–H bond activation by platinum(II) are rela-
tively few.[13] Moreover, the low electrophilicity of platinum
has been found to facilitate the stabilization of higher oxi-
dation states, which is reflected in their ability to undergo
oxidative addition reactions. A number of studies on oxi-
dative addition reaction of platinum(II) compounds having
monodentate and bidentate phosphorus or nitrogen ligands
have been reported.[14–17] However, cycloplatinate com-
pounds have not enjoyed the same popularity.[12e,16,18]
This work stems from our interest in the selective acti-
vation of different C(naphthyl)–H bonds by transition
metal complexes.[19] Herein, selective C(naphthyl)–H bond
Introduction
Cyclometallation reactions are of considerable interest
because of their role in the selective functionalization of
C–H bonds[1] and numerous applications of the complexes
derived.[2] The topic of C–H bond activation reactions pro-
moted by platinum(II) complexes, as a key step in the func-
tionalization of small molecules,[3] has undergone a rapid
growth. Some representative examples where platinum(II)
complexes have been extensively used includes alkane[3,4]
and arene[5] functionalization, studies of their thermo-
dynamic properties,[6] synthesis of complexes with interest-
ing optical properties (luminescence),[7] C–C coupling reac-
tions,[2c,8] metallomesogen chemistry[9] and anticancer
agents.[10] Since the first report of platinum(II) organome-
tallics containing a Pt–C(aryl) bond,[11] the study of cyclo-
platination reactions has mainly been confined to the do-
main of C(phenyl)–H bond activation.[9,12] Reports on
[a] Department of Chemistry, University of North Bengal, Raja
Rammohunpur,
Siliguri 734013, West Bengal, India
Fax: +91-353-2699001
E-mail: pbchem@rediffmail.com
[b] Department of Chemistry, Indian Institute of Technology,
New Delhi 110016, India
[c] Department of Chemistry, Missouri S & T,
Rolla, MO 65409, USA
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100468.
Scheme 1. Diazene ligands H2L1–H2L3.
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P. Bandyopadhyay et al.
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activation by platinum(II) using a group of naphthylazo-
2Ј-hydroxyarene ligands H2L1–H2L3 as substrates has been
studied. The isolation and characterization of the resulting
cycloplatinate species and rationalization of their structural
and spectroscopic properties are described. Oxidative ad-
dition reactions of square planar divalent cycloplatinate
compounds with halogens and methyl iodide are also re-
ported. Moreover, a TD-DFT study of representative cyclo-
platinate species is presented (Scheme 1).
Results and Discussion
Cycloplatination Reactions
Figure 1. The asymmetric unit of 2a with displacement ellipsoids
drawn at 50% probability. Selected bond lengths [Å] and angles
[°]: Pt1–C2 1.983(5), Pt1–N2 1.931(5), Pt1–O1 2.141(4), Pt1–N3
2.035(6), N1–N2 1.271(7), N1–C1 1.399(8), N2–C11 1.400(8), C2–
Ligands H2L1–H2L3 show a strong dependence on the
platinum precursor and reaction conditions. Attempts to
activate C(naphthyl)–H bonds using K2[PtCl4] as the start- Pt1–N3 100.9(2), N2–Pt1–O1 81.48(19), N2–Pt1–C2 79.5(2), N3–
Pt1–O1 98.17(19).
ing metal complex in ethanol met with failure. Sub-
sequently, it was found that the diazene substrates H2L1–
H2L3 react with the allyl platinum(II) complex [(η3-C4H7)-
Pt(μ-Cl)]2 in the presence of neutral Lewis bases (D) in
chloroform to afford green monomeric cycloplatinate com-
plexes 2–4 at room temperature. Only one cycloplatinate
compound was obtained in each case. Cycloplatinate spe-
cies 2–4 were isolated in their pure forms by preparative
TLC in good yields (ca. 65%). Alternatively, the C(naphth-
yl)–H bond activation by [(η3-C4H7)Pt(μ-Cl)]2 was also
achieved on a solid surface. Cycloplatinate species were ob-
tained in better yields (ca. 70–75%) by heating the reaction
mixture adsorbed on silica gel at 120 °C for 6 h. The ele-
1
mental analytical results and H NMR spectroscopic data
of 2–4 were consistent with their formulations as [PtII-
(L)(D)]. The naphthyl group with the donor bearing sub-
stituents at C1 (H2L1 and H2L2) can offer C2–H or C8–H
for metallation, whereas C1–H and C3–H are the potential
metallation sites when the primary donor is attached to C2.
X-ray crystallographic analyses of the representative cyclo-
platinates have been performed in order to ascertain their
Figure 2. The asymmetric unit of 2b with displacement ellipsoids
exact molecular structures.
drawn at 50% probability. Selected bond lengths [Å] and angles [°]:
Molecular structures of the divalent cyclometallates (2a
Pt1–C2 2.004(5), Pt1–N2 1.997(4), Pt1–O1 2.109(3), Pt1–P1
2.2543(12), N1–N2 1.275(5), N1–C1 1.406(6), N2–C11, 1.390(6),
and 2b) derived from the reaction of H2L1 and [(η3-C4H7)-
C2–Pt1–P1 100.10(14), N2–Pt1–O1 80.92(13), N2–Pt1– C2
79.04(17), P1–Pt1–O1 99.93(8).
Pt(μ-Cl)]2 are shown in Figures 1 and 2, respectively. The
structures show monomeric compounds in which Pt is lo-
cated in a slightly distorted square-planar environment, sur-
rounded by the metallated C(2), N2 of the diazo group and
O1 of the phenolato group of the dianionic terdentate li-
gand, and the fourth coordination site is occupied by N3
Thus, regiospecific cycloplatination has been found to
(2a) or P1 (2b) of the neutral donor molecule. The maxi- occur at the C2(naphthyl)–H bond of H2L1 in contrast to
mum deviations represented by the bite angles C(2)–Pt(1)– the exclusive C8(naphthyl)–H bond activation of H2L1 in
N(3) (2a) and C(2)–Pt(1)–P(1) (2b) are 100.9(2)° and the corresponding cyclopalladation reaction.[19]
100.10(14)°, respectively. The Pt–N(azo) bond length
Regiospecific C2(naphthyl)–H bond activation has also
[1.931(5) Å] is shorter than Pt–N(picoline) [2.035(6) Å] in been observed in H2L2. The molecular geometry of 3a is
2a, presumably due to the greater π-acceptor ability of the presented in Figure 3 with the atom numbering scheme. The
azo chromophore over the picoline moiety.[17] As a conse- platinum(II) centre in 3a is bound to C2(naphthyl) of the
quence, the N(1)–N(2) length of the coordinated azo group 1-azonaphthyl fragment, N2 of the diazene group, O1 of
is elongated to 1.271(7) Å.
the naphtholato group and N3 of picoline.
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Regiospecific C(naphthyl)–H Bond Activation by Platinum(II)
for activation by platinum(II). Structure determination of
4a (Figure 4), obtained from the reaction of H2L3 with [(η3-
C4H7)Pt(μ-Cl)]2 in the presence of 4-picoline, shows that
the regiospecific activation of the C3(naphthyl)–H bond has
occurred. The preferential activation of the C3(naphthyl)–
H bond over the C1(naphthyl)–H bond of H2L3 by plati-
num(II) calls for an explanation. Attempts have been made
to explain it in terms of the SHAB principle. DFT calcula-
tion using the Fukui function[20] for H2L3 shows that
C3(naphthyl) is softer than C1(naphthyl). Thus, bearing in
mind the softness of platinum(II), it seems reasonable that
platinum(II) would preferentially bind C3 rather than C1
in H2L3. However, the possibility of C1(naphthyl)–H bond
activation cannot be ruled out completely in terms of the
SHAB principle alone. More exhaustive mechanistic explo-
ration is, therefore, needed to explain these highly regios-
pecific cycloplatination reactions.
Figure 3. The asymmetric unit of 3a with displacement ellipsoids
drawn at 50% probability. Selected bond lengths [Å] and angles
[°]: Pt1–C2 1.980(5), Pt1–N2 1.941(4), Pt1–O1 2.105(4), Pt1–N3
2.031(5), N1–N2 1.291(6), N1–C1 1.413(6), N2–C11 1.400(6), C2–
Pt1–N3 103.41(19), N2–Pt1–O1 81.39(16), N2–Pt1–C2 80.27(18),
N3–Pt1–O1 95.28(17).
The presence of the primary donor at C1 of the naphth-
alene rings in H2L1 and H2L2 results in regiospecific acti-
vation of the C(2)–H bond by platinum(II) irrespective of
the nature of auxiliary donors (phenolato or naphtholato).
The exclusive C2(naphthyl)–H bond activation can be ra-
tionalized as shown in Scheme 2. It is well known that cy-
clometalation is initiated through the coordination of the
heteroatom to the metal centre. Subsequently the activation
of C(naphthyl)–H bonds is governed by the formation of
either five-membered carboplatinacycle or six-membered
N,O chelate rings (Scheme 2). The formation of a carbopla-
tinacycle ring results in the activation of the C2–H bond
(route i, Scheme 1), whereas initial N,O chelation could lead
to C8–H bond activation (route ii, Scheme 1). The reluc-
tance of soft platinum(II) to bind hard phenolato/naphth-
olato donors in the initial stage excludes the possibility of
C8(naphthyl)–H bond activation.
Figure 4. The asymmetric unit of 4a with displacement ellipsoids
drawn at 50% probability. Selected bond lengths [Å] and angles
[°]: Pt1–C3 1.980(11), Pt1–N2 1.928(6), Pt1–O1 2.111(8), Pt1–N3
2.035(7), N1–N2 1.265(12), N1–C2 1.422(15), N2–C11 1.440(16),
C3–Pt1–N3 101.8(4), N2–Pt1–O1 83.1(4), N2–Pt1–C3 79.4(5), N3–
Pt1–O1 95.7(3).
Oxidative Addition Reactions
The addition of halogens to organoplatinum(II) com-
plexes is of historical significance.[21] The fundamental
mechanisms of halogen addition to the platinum(II) centre
has been elucidated for complexes with nitrogen-donor li-
gands.[22] Commonly, halogens react with platinum(II) com-
plexes with N-, P-, and As-donor ligands and undergo trans
oxidative addition.[23] In some cases the platinum(IV) prod-
ucts undergo subsequent isomerization to the thermody-
namically stable cis isomers.[24]
Compounds 2a and 2b undergo smooth oxidation with
halogens and methyl iodide in dichloromethane and pro-
duce platinum(IV) cyclometallate species 5a–6c in good
yields. However, methyl iodide failed to react only with 2b.
In all of these cases oxidative addition took place with con-
Scheme 2. Proposed sequential steps for activation of C2(naphth-
yl)–H, Route (i), and C8(naphthyl)–H, Route (ii).
The presence of the primary diazene group at C2 in H2L3 cominant 2e– oxidation of PtII to PtIV (Scheme 3). Bluish
brings an additional factor into consideration. Here, green tetravalent cycloplatinate species of the type
C1(naphthyl)–H and C3(naphthyl)–H bonds are available [PtIV(L)(D)(X2)] containing a PtIV–C(naphthyl) bond have
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P. Bandyopadhyay et al.
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been isolated. Products 5a–6c were characterized by ele-
1
mental analysis and IR and H NMR spectroscopy (vide
infra). Single crystal X-ray diffraction studies of two repre-
sentative PtIV complexes with different Lewis bases 5a (D
= 4-picoline) and 6b (D = PPh3) have been undertaken. The
ORTEP diagrams with the atom numbering schemes are
shown in Figures 5 and 6, respectively. It is noteworthy that
trans oxidative addition of the halogen has resulted in the
case of the cycloplatinate adduct with 4-picoline. The struc-
ture of 5a shows that the platinum(IV) centre is bound to
C2 of the naphthyl ring, N2 of the diazene group, O1 of
the phenolic fragment of the terdentate donor system and
N3 of the 4-picoline donor along with two mutually trans
iodine atoms occupying the vertices of a distorted octahe-
dron (Figure 5). The Pt(1)–I(1) and Pt(1)–I(2) bond lengths
are close to previously reported values.[12a] In contrast, cis
oxidative addition of the halogen is observed in the corre-
sponding cycloplatinate adduct with PPh3 6b (Figure 6).
The coordination geometry around the platinum(IV) centre
is distorted octahedral with the two bromine atoms cis to
each other.
Figure 5. The asymmetric unit of 5a with displacement ellipsoids
drawn at 50% probability. Selected bond lengths [Å] and angles
[°]: Pt1–C2 1.979(16), Pt1–N2 1.985(14), Pt1–O1 2.185(12), Pt1–N3
2.061(13), N1–N2 1.267(19), N1–C1 1.380(20), N2–C11 1.330(2),
Pt1–I1 2.6407(14), Pt1–I2 2.6522(14), C2–Pt1–N3 105.5(6), N2–
Pt1–O1 80.6(5), N2–Pt1–C2 80.0(6), N3–Pt1–O1 93.9(5), C2–Pt1–
I1 87.7(5), C2–Pt1–I2 90.6(5).
Figure 6. The asymmetric unit of 6b·CH2Cl2 with displacement el-
lipsoids drawn at 50% probability. Selected bond lengths [Å] and
angles [°]: Pt1–C2 2.012(7), Pt1–N2 1.965(6), Pt1–O1 2.140(5), Pt1–
P1 2.3285(19), N1–N2 1.289(8), N1–C1 1.396(9), N2–C11 1.409(8),
Pt1–Br1 2.5042(8), Pt1–Br2 2.4709(8), C2–Pt1–P1 94.62(19), N2–
Pt1–O1 81.4(2), N2–Pt1–C2 79.5(3), P1–Pt1–O1 87.22(13), C2–
Pt1–Br1 89.25(19), C2–Pt1–Br2 99.3(2).
Scheme 3. Oxidative addition reactions of divalent cycloplatinates.
The oxidative addition of alkyl halides and halogens by
platinum(II) can proceed by several mechanisms. De-
pending on the metal substrate, alkyl halide, halogen and
2
reaction conditions, either SN or free radical reactions are
possible. A number of studies on oxidative addition of hal-
ogens and alkyl halides to square planar platinum(II) com- [PtII(L1)(4-picoline)X]+X– (X = halogen) in the oxidative re-
plexes have been reported.[25,26] In most cases, reactions of actions by halogens. In the second step, X– attacks the in-
halogens and alkyl halides with square planar d8 plati- termediate trans to the metal bound halogen atom. Con-
num(II) complexes with ligand frames that do not block versely, it is likely that the corresponding PPh3–platinum(II)
2
the vacant axial coordination sites,[27] follow the polar SN
cyclometallate species forms a trans addition product ini-
pathway and afford kinetically controlled trans-plati- tially, followed by rapid rearrangement of the intermediate
num(IV) complexes,[26a] although cis-products may result platinum(IV) complex to the corresponding cis isomer, due
after subsequent isomerism.[28] This case provides a unique to the steric bulk of triphenylphosphane. The formation of
system where two possible types of oxidative addition reac- cis-[PtII(L1)(PPh3)X2] indicates that the cis isomer is ther-
tions have been realized simply by changing the neutral do- modynamically favoured.
2
nor ligand (4-picoline and triphenylphosphane). It is rea-
For methyl iodide, the most likely mechanism for the SN
sonable to believe that the 4-picoline–platinum(II) cyclo- reaction is nucleophilic attack on the platinum centre to
metallate species generates an anion–cation intermediate generate [PtII (L1)(4-picoline)Me]+I–, which rapidly re-
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Regiospecific C(naphthyl)–H Bond Activation by Platinum(II)
arranges to [PtIV (L1)(4-picoline)MeI] (5d). The formation
of the square pyramidal cationic species in related reactions
has already been established,[29] and is thought to be the
key intermediate in reactions of methyl iodide and square
planar platinum(II) complexes. ESI-MS of 5d in dichloro-
methane also gives a similar indication, showing a peak at
563.2274 (100% abundance with correct isotopic pattern)
corresponding to the cationic [PtII(L1)(4-picoline)Me]+
(Supporting Information). Additional information on the
structure of 5d was obtained from NMR spectroscopy (vide
infra).
Electronic Spectra and Excited Singlet State Calculations
All the cycloplatinate complexes are soluble in common
organic solvents, such as dichloromethane, acetonitrile and
acetone, producing either yellowish green (divalent cyclo-
platinate species) or bluish green solutions (tetravalent cy-
cloplatinate species). Spectral data are presented in Table 1.
The electronic spectra of representative divalent cycloplati-
nate species are shown in Figure 7. The cycloplatinate spe-
cies show several absorptions in the UV/vis region. The
bands appearing in the visible region are generally assigned
to metal to ligand [d(Pt)Ǟπ (azo)] bands.[12e] Absorptions
in the ultraviolet region are attributed to nǞπ* and πǞπ*
transitions occurring within the ligand orbitals. Various
studies by qualitative EHMO calculations have been per-
formed on computer-generated models of related azo com-
plexes.[30] However, to date, no comprehensive theoretical
investigations have been made in this regard. This prompted
us to use a TD-DFT study to provide an insight into the
electronic properties of the platinum(II) compounds. The
compounds 2a and 2b were chosen as models for the study.
Their coordinates were directly imported and single point
calculations were performed.
Figure 7. Electronic spectra of 2a (pink), 2b (blue), 4a (green) and
4b (purple) in dichloromethane.
Orbital Analysis
The divalent cycloplatinate species 2a and 2b have similar
frontier orbitals (Figure 8 and Supporting Information).
The highest occupied molecular orbitals (HOMOs) of 2a
and 2b are mostly ligand based. The largest orbital contri-
butions in the HOMOs arise from the ligand orbitals, re-
sulting from the combination of the carbon and nitrogen p
orbitals with a sizeable percentage of metal d orbitals.
HOMO–1 is similar in shape to the HOMO. The HOMO–
2 of 2a has substantial metal d character (36%), and that
of 2b is almost entirely metal based (82% metal character).
Table 1. Electronic spectroscopic data of 2a–6c in dichloromethane.
λmax /nm (ε/m–1 cm–1)
230 (38,500), 260sh (26,200), 310 (21,700), 346 (11,400), 422
(10,400), 776 (4,100), 890 (3,900).
2a
230 (57,000), 310 (19,500), 346 (13,200), 428 (9,800), 800
(4,200), 900 (3,800).
2b
3a 231 (39,700), 395 (12,100), 395 (2,850).
3b 272 (39,500), 400 (8,800), 664 (7,950), 725 (8,200).
230 (35,800), 342 (28,000), 470 (12,700), 500 (14,100), 660
(4,000).
4a
232 (56,500), 346 (32,500), 478 (13,000), 508 (14,500), 676
(5,100).
4b
230 (34,400), 268 (26,200), 392 (8,400), 628 (4,800), 674
(4,600).
5a
Figure 8. Partial molecular orbital diagram of 2a and 2b.
231 (36,500), 272 (20,500), 402 (9,200), 630 (8,500), 680
5b
(7,500).
The HOMO–3 of 2a is very similar to the HOMO–2 of
2b. The lowest unoccupied molecular orbitals (LUMOs) of
the divalent cycloplatinate species are strongly antibonding
and localized over the chelating ligand. LUMO+1 in both
the platinum(II) complexes is situated over the neutral do-
nors (4-picoline in 2a and triphenylphosphane in 2b).
5c 230 (41,600), 630 (7,100), 676 (7,400).
5d 230 (36,100), 626 (5,900), 658 (6,100).
sh
sh
230 (77,300), 296 (38,800), 408 (14,400), 634 (9,000), 682
(10,100).
6a
6b 232 (85,100), 625 (15,600), 680 (12,700).
6c 230 (95,100), 624 (12,500), 676 (13,900).
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P. Bandyopadhyay et al.
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LUMO+2 of 2a and LUMO+3 of 2b are also localized over could emanate from the mixing of the ligand-to-ligand
the neutral donors. The HOMO–LUMO gaps in 2a and 2b
have been computed to be 1.095 and 0.958 eV, repectively.
A detailed analysis of the highest occupied and lowest un-
occupied molecular orbitals of 2a and 2b is compiled in the
Supporting Information.
charge transfer (LLCT) and metal-to-ligand charge transfer
(MLCT) transition. The transition involving the HOMO–
4ǞLUMO orbital pair in their dominant configurations
appears at 470 nm (sixth excited state) and has been as-
signed to a mixture of MLCT and LLCT transitions. The
other absorptions in the visible region for 2a have common
density redistribution features with a significant amount of
MLCT and more or less π–π* intraligand density redistri-
bution. The most intense transition observed for 2a in the
UV region (ca. 260 nm) involves excitation from 75a
(HOMO), 74a (HOMO–1), 71a (HOMO–4) and 70a
(HOMO–5) to 79a (LUMO+3), 80a (LUMO+4), 84a
(LUMO+8) with predominantly intraligand π–π* character.
For 2b, TD-DFT assigns the band centered at 900 nm to
LLCT transitions with an admixture of MLCT transitions.
The transitions comprising the band at 340 nm are a mix-
ture of intraligand charge transfer (ILCT) and LLCT tran-
sitions but TD-DFT underestimates its oscillator strength
(f).
TD-DFT Singlet Transitions
The calculated UV/Vis spectra of 2a and 2b are in agree-
ment with those obtained experimentally. The absorption
spectra of 2a obtained using TD-DFT methods showed a
number of transitions within the visible and ultraviolet re-
gion and good correlation with the experimentally observed
spectrum is observed (Tables 2 and 3). The results show that
the electronic transition with dominantly contributing con-
figurations involving the HOMOǞLUMO orbital pair oc-
curs at 903 nm (first excited state). The experimental spec-
trum of 2a in dichloromethane shows a broad peak corre-
sponding to this transition. The broadening of this band
Table 2. Selected TD-DFT calculations of excitation energies, wavelengths (λ), oscillator strengths (f) and compositions of the low-lying
singlet states for 2a in dichloromethane.
State
Energy
[eV]
λcal
[nm]
λexp
[nm]
f
Composition
Character
S1
S6
S10
S13
1.3735
2.6382
3.2894
3.5623
903
470
377
348
890
422
–
0.047
0.115
0.137
0.215
HOMOǞLUMO (97.2%)
HOMO–4ǞLUMO (79.7%)
HOMO–6ǞLUMO (84%)
ILCT+MLCT
MLCT+LLCT
ILCT+MLCT
ILCT+MLCT
–
HOMOǞLUMO+4 (63.4%)
HOMO–8ǞLUMO (11.3%)
HOMO–4ǞLUMO+1 (40.4%)
HOMO–2ǞLUMO+1 (25.9%)
HOMO–8ǞLUMO (18.1%)
HOMO–6ǞLUMO+1 (31.8%)
HOMO–11ǞLUMO (19.7%)
HOMO–1ǞLUMO+4 (17.7%)
HOMO–5ǞLUMO+1 (12.9%)
HOMO–1ǞLUMO+4 (18.7%)
HOMOǞLUMO+8 (15.3%)
HOMO–4ǞLUMO+4 (9.3%)
HOMO–5ǞLUMO+4 (8.5%)
HOMO–4ǞLUMO+3 (8.4%)
HOMO–11ǞLUMO+1 (36%)
HOMO–3ǞLUMO+5 (23.2%)
HOMO–5ǞLUMO+4 (9.3%)
S19
3.7263
333
310
0.073
MLCT+ILCT
S32
4.344
285
–
0.059
ILCT+LLCT+MLCT
S45
4.7549
5.3104
260
233
260
230
0.158
0.067
LLCT+ILCT+ MLCT
LLCT+MLCT
S69
Table 3. Selected TD-DFT calculations of excitation energies, wavelengths (λ), oscillator strengths (f) and compositions of the low-lying
singlet states for 2b in dichloromethane.
State
Energy
[eV]
λcal
[nm]
λexp
[nm]
f
Composition
Character
S1
S5
1.2060
2.4974
1028
495
900
–
0.136
0.069
HOMOǞLUMO (97.4%)
ILCT+MLCT
MLCT+LLCT
HOMO–4ǞLUMO (64.1%)
HOMO–5ǞLUMO (19.2%)
HOMOǞLUMO+6 (65.7%)
HOMO–13ǞLUMO (14.2%)
HOMOǞLUMO+2 (93.3%)
HOMO–5ǞLUMO+2 (42.0%)
HOMO–5ǞLUMO+3 (18.3%)
HOMO–7ǞLUMO+2 (11.8%)
S19
3.3103
375
428
0.072
ILCT+MLCT
S24
S59
3.6446
4.5332
340
270
346
–
0.020
0.012
ILCT+LLCT
LLCT +ILCT MLCT
3744
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 3739–3748

Regiospecific C(naphthyl)–H Bond Activation by Platinum(II)
7.40–7.65 (m, 6 H), 8.25 (d, 1 H, J = 8.1 Hz), 8.81 (d, 1 H, J =
8.6 Hz), 16.30 (s, 1 H, OH) ppm.
Conclusions
This study shows that naphthylazo-2Ј-hydroxyarene li-
gands (1) undergo regiospecific cyclometallation reactions
mediated by the allyl platinum(II) complex [(η3-C4H7)Pt (μ-
Cl)]2. The position of the primary donor (diazene group)
was found to dictate the sites of metallation. Exclusive
C2(naphthyl)–H bond activation has been achieved when
the diazene group is at C1 and the auxiliary donor is phenol
or naphthol. On the other hand, exclusive C3(naphthyl)–H
bond activation has been observed in H2L3, where the pri-
mary donor is attached to the C2 atom. The regiospecificity
in these cases can be explained in terms of five-membered
carboplatinacycle ring formation followed by N,O-chela-
tion. The square planar bivalent cycloplatinate species are
shown to undergo oxidative addition reactions with hal-
ogens and methyl iodide affording air-stable platinum(IV)
compounds. The influence of the steric bulk of the donor
molecules (D) on the stereochemistry of the oxidized prod-
ucts has been observed. Here both cis and trans addition of
the electrophiles have been achieved using different D. The
electronic and UV/Vis spectroscopic properties of the repre-
sentative platinum(II) complexes in dichloromethane have
been investigated by TD-DFT calculations.
[Pt(L1)(4-picoline)] (2a): H2L1 (0.025 g, 0.0954 mmol) was added to
a solution of [(η3-C4H7)Pt(μ-Cl)]2 (0.027 g, 0.0477 mmol) in chloro-
form (15 mL). Compound 2a was isolated following the two work
up procedures outlined below.
(i) The reaction mixture was stirred magnetically for 8 h and a solu-
tion of 4-picoline (0.088 g, 0.954 mmol) in chloroform (5 mL) was
added to the solution. The greenish solution was stirred for a fur-
ther 2 h. The solution was then filtered, washed with chloroform
(4ϫ5 mL) and the solvent evaporated. The crude product was dis-
solved in CH2Cl2 and preparative TLC was performed on a silica
gel plate with petroleum ether (60–80 °C)/ethyl acetate (4:1, v/v) as
eluent. The green band was isolated and extracted with acetone.
Removal of acetone gave 2a; yield 75%. C23H19N2OPt (534.23):
calcd. C 50.36, H 3.49, N 7.66; found C 50.49, H 3.32, N 7.58%.
1H NMR (CDCl3):δ: 2.34 (s, 3 H, aryl CH3), 2.42 (s, 3 H, CH3 of
4-picoline); aromatic protons: 6.95–8.00. IR (KBr): ν = 1410
˜
(–N=N–) cm–1. MS: m/z 548 [M]+.
(ii) The reaction mixture was layered on a preparative silica gel
plate. The plate was then heated in an air-oven at 120 °C for 6
hours. The desired product was the isolated by eluting the silica
plates with 10% 4-picoline in petroleum ether (60–80 °C)/ethyl
acetate (4:1, v/v); yield 70%.
[Pt(L1)(PPh3)] (2b): Complex 2b was synthesized a method similar
to that used for 2a using triphenylphosphane instead of 4-picoline;
yield 70%. C35H27N2OPPt (717.67): calcd. C 58.58, H 3.79, N 3.90;
1
found C 58.71, H 3.82, N 4.01. H NMR (CDCl3): δ = 2.18 (s, 3
Experimental Section
H, aryl CH3), aromatic protons: 6.10–6.14 (dd, J = 8.4 Hz, 1 H),
6.35 (d, J = 8.7 Hz, 1 H), 6.76–6.80 (dd, J = 8.7 Hz, 1 H), 6.87 (d,
J = 8.4 Hz, 1 H), 7.03 (s, 1 H), 7.23–7.32 (m, 5 H), 7.38–7.50 (m,
8 H), 7.60–7.73 (m, 5 H), 8.40 (d, J = 8.1 Hz, 1 H) ppm. IR (KBr):
General Remarks: C, H and N elemental analyses were performed
with either Perkin–Elmer (Model 240C) or Heraeus Carlo–Erba
1108 elemental analyzers. IR spectra were obtained with a Jasco
5300 FT-IR. Visible and ultraviolet spectra were recorded with a
Jasco V-530 spectrophotometer. 1H NMR spectra were measured
in CDCl3 with a Bruker Avance DPX 300 NMR spectrometer with
SiMe4 as an internal standard. The FAB mass spectra were re-
corded with a JEOL SX 102/DA-6000 Mass Spectrometer/Data
System using Argon/Xenon (6 kV, 10 mA) as the FAB gas. ESI
mass spectra were recorded with a Micromass Quattro II triple
quadrupole mass spectrometer. K2[PtCl4] was obtained from
Arora-Matthey, Kolkata. [(η3-C4H7)Pt (μ-Cl)]2 was prepared fol-
lowing a reported method.[31] Solvents and chemicals were of ana-
lytical grade. H2L1–H2L3 were prepared according to a published
procedure.[32]
ν = 1405 (–N=N–) cm–1. MS: m/z 718 [M]+.
˜
The cycloplatinates 3a, 3b, 4a and 4b were synthesized following
the method used for 2a and 2b.
[Pt(L2)(4-picoline)] (3a): Yield 65%. C26H19N3OPt (584.553): δ =
2.52 (s, 3 H, CH3 of 4-picoline); aromatic protons: 6.80 (d, J =
8.7 Hz, 1 H), 7.12 (d, J = 9.3 Hz, 1 H), 7.25 (d, J = 10.2 Hz, 2 H),
7.30–7.45 (m, 5 H), 7.48–7.60 (m, 4 H), 7.81 (m, 2 H), 8.6 (d, J =
8.6 Hz, 1 H) ppm. IR (KBr): ν = 1410 (–N=N–) cm–1. MS: m/z
˜
585 [M]+.
[Pt(L2)(PPh3)] (3b): Yield 62%. C38H27N2OPPt (753.71): calcd. C
60.56, H 3.61, N 3.72; found C 60.42, H 3.65, N 3.84. 1H NMR
(CDCl3): δ = aromatic protons: 6.78 (d, J = 8.7 Hz, 1 H), 7.05 (d,
J = 9.0 Hz, 1 H), 7.22–7.56 (m, 15 H), 7.71–7.85 (m, 6 H), 7.92
(m, 2 H), 8.06 (m, 2 H), 8.6 (d, J = 9.0 Hz, 1 H) ppm. IR (KBr):
1-(2Ј-Hydroxy-5Ј-methylphenylazo)naphthalene (H2L1): Yield 65%.
C17H15N2O (263.12): calcd. C 77.84, H 5.38, N 10.68; found C
1
79.95, H 4.52, N 9.31. H NMR (300 MHz, CDCl3): δ = 6.90 (d,
ν = 1412 (–N=N–) cm–1. MS: m/z 753 [M]+.
1 H, J = 9.4 Hz), 7.40 (t, 1 H), 7.55–7.66 (m, 5 H), 7.73–7.82 (m,
2 H), 7.90 (d, 1 H, J = 8.1 Hz), 8.19 (d, 1 H, J = 7.5 Hz), 8.32 (d,
1 H, J = 8.3 Hz), 8.62 (d, 1 H, J = 8.1 Hz), 17.23 (s, 1 H, OH)
ppm.
˜
[Pt(L3)(4-picoline)] (4a): Yield 68%. C23H19N2OPt (534.51): calcd.
1
C 50.36, H 3.49, N 7.66; found C 50.41, H 3.56, N 7.64. H NMR
(CDCl3): δ = 2.14 (s, 3 H, aryl CH3), 2.50 (s, 3 H, CH3 of 4-pic-
oline); aromatic protons: 6.62–6.69 (t, 2 H), 6.98 (d, J = 8.7 Hz, 1
H), 7.26–7.42 (m, 6 H), 7.62 (d, J = 8.3 Hz, 1 H), 7.82 (s, 1 H),
1-(2Ј-Hydroxynaphthylazo)naphthalene
(H2L2): Yield 75%.
C20H14N2O (298.34): calcd. C 80.52, H 4.73, N 9.39; found C
1
8.77 (d, 2 H) ppm. IR (KBr): ν = 1405 (–N=N–) cm–1. MS: m/z
80.25, H 4.52, N 9.47. H NMR (300 MHz, CDCl3): δ = 6.89 (d,
˜
548 [M]+.
1 H, J = 9.3 Hz), 7.37–7.62 (m, 5 H), 7.70 (d, 1 H, J = 9.3 Hz),
7.83–8.06 (m, 5 H), 8.62 (d, 1 H, J = 8.0 Hz), 16.34 (s, 1 H, OH)
ppm.
[Pt(L3)(PPh3)] (4b): Yield 65%. C38H27N2OPPt (753.71): calcd. C
60.56, H 3.61, N 3.72; found C 60.55, H 3.51, N 3.81. 1H NMR
(CDCl3): δ = 2.17 (s, 3 H, aryl CH3); aromatic protons: 6.54 (d, J
= 8.7 Hz, 1 H), 6.71 (d, J = 8.7 Hz, 1 H), 6.93 (d, J = 8.7 Hz, 1
H), 7.07–7.20 (m, 3 H), 7.39–7.54 (m, 10 H), 7.65–7.72 (m, 8 H)
2-(2Ј-Hydroxy-5Ј-methylphenylazo)naphthalene (H2L3): Yield 55%.
C17H15N2O (263.12): calcd. C 77.84, H 5.38, N 10.68; found C
1
78.02, H 5.51, N 10.41. H NMR (300 MHz, CDCl3): δ = 1.54 (s,
3 H, 5Ј-Me) 6.90 (d, 1 H, J = 8.6 Hz), 7.26 (d, 1 H, J = 8.1 Hz), ppm. IR (KBr): ν = 1415 (–N=N–) cm–1. MS: m/z 718 [M]+.
˜
Eur. J. Inorg. Chem. 2011, 3739–3748
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3745

P. Bandyopadhyay et al.
FULL PAPER
General Procedure for the Oxidative Addition of Electrophiles to Di-
valent Cycloplatinates: A dichloromethane solution of the halogen
or methyl iodide (1%) was added dropwise to a dichloromethane
solution (10 mL) of the PtII complex (0.055 mmol) with stirring
until the yellowish green colour of the reaction mixture changed to
bluish green. The mixture was stirred for a further 1 h to ensure
the complete disappearance of the starting materials. The solvent
was evaporated and the crude product was dissolved in a minimum
quantity of dichloromethane and subjected to purification by TLC
on silica gel with petroleum ether (60–80 °C)/ethyl acetate (10:1,
v/v) as eluent to afford 5a–6c.
NMR (CDCl3): δ = 2.26 (s, 3 H, aryl CH3), 2.66 (s, 3 H, CH3 of
4-picoline); aromatic protons: 6.4–9.3 (br) ppm. IR (KBr): ν = 1405
˜
(–N=N–) cm–1. MS: m/z 619 [M]+, 548 [M – 2Cl]+.
[Pt(L1)(4-picoline)(CH3)(I)] (5d): Yield 72%. C24H22IN3OPt
(690.45): calcd. C 41.7, H 3.2, N 6.1; found C 41.8, H 3.3, N 6.1.
1H NMR (CDCl3): δ = 1.8 (t, JPt-H = 63 Hz, 3 H, Pt-CH3), 2.33
(s, 3 H, aryl CH3), 2.55 (s, 3 H, CH3 of 4-picoline); aromatic pro-
tons: 6.8 (d, J = 9.0 Hz, 1 H), 7.0 (d, J = 9.0 Hz, 1 H), 7.2 (d, J =
9.0 Hz, 1 H), 7.4–7.7 (m, 5 H), 7.8 (d, J = 8.1 Hz, 1 H), 8.6 (d, J
= 8.4 Hz, 2 H), 9.1 (m, 2 H) ppm. IR (KBr): ν = 1410 (–N=N–)
˜
cm–1. MS: m/z 690 [M]+, 564 [M – I]+. ESI-MS: 563.2274 [M – I]
[Pt(L1)(4-picoline)I2] (5a): Yield 66%. C23H19I2N3OPt (802.32):
(calculated mass: 563.1411).
1
[Pt(L1)(PPh3)I2] (6a): Yield 70%. C35H27I2N2OPPt (971.48): calcd.
calcd. C 34.43, H 2.39, N 5.24; found C 34.28, H 2.40, N 5.56. H
1
NMR (CDCl3): δ = 2.34 (s, 3 H, aryl CH3), 2.55 (s, 3 H, CH3 of
4-picoline); aromatic protons: 6.84 (d, J = 9 Hz, 1 H), 7.08 (d, J =
9.3 Hz, 1 H), 7.17 (d, J = 10.2 Hz, 1 H), 7.20–7.77 (m, 5 H), 7.80
(d, J = 7.8 Hz, 1 H), 7.88 (d, J = 8.1 Hz, 1 H), 8.62 (d, J = 7.8 Hz,
C 43.27, H 2.80, N 2.88; found C 43.16, H 2.59, N 2.90. H NMR
(CDCl3): δ = 2.12 (s,3 H, aryl CH3); aromatic protons: 6.34–8.48
ppm. IR (KBr): ν = 1410 (–N=N–) cm–1. MS: m/z 971 [M]+, 718
˜
[M – 2I]+.
1 H), 9.36 (m, 2 H) ppm. IR (KBr): ν = 1400 (–N=N–) cm–1. MS:
˜
[Pt(L1)(PPh3)Br2] (6b): Yield 75%. C35H27Br2N2OPPt (877.48):
m/z 802 [M]+, 548 [M – 2I]+.
1
calcd. C 47.91, H 3.10, N 3.19; found C 48.05, H 2.98, N 3.15. H
[Pt(L1)(4-picoline)Br2] (5b): Yield 70%. C23H19Br2N3OPt (708.32):
NMR (CDCl3): δ = 2.18 (s, 3 H, aryl CH3); aromatic protons: 6.42
(d, J = 9.0 Hz, 1 H), 6.53 (d, J = 8.7 Hz, 1 H), 6.84 (m, 2 H), 7.10
(s, 1 H), 7.17–7.48 (m, 15 H), 7.58 (dd, 1 H), 8.39 (d, J = 7.8 Hz,
1
calcd. C 39.00, H 2.70, N 5.93; found C 38.95, H 2.81, N 5.84. H
NMR (CDCl3): δ = 2.31 (s, 3 H, aryl CH3), 2.58 (s, 3 H, CH3 of
4-picoline); aromatic protons: 7.10 (d, J = 8.7 Hz, 1 H), 7.36–7.49
(m, 6 H), 7.55–7.59 (m, 1 H), 7.74 (d, J = 8.6 Hz, 1 H), 7.85 (d, J
= 8.2 Hz, 1 H), 8.62 (d, J = 8.3 Hz, 1 H), 9.17 (m, 2 H) ppm. IR
1 H), 8.69 (d, J = 8.7 Hz, 1 H) ppm. IR (KBr): ν = 1406 (–N=N–
˜
) cm–1. MS: m/z 878 [M]+, 718 [M – 2Br]+.
[Pt(L1)(PPh3)Cl2] (6c): Yield 68%. C35H27Cl2N2OPPt (788.58):
(KBr): ν = 1402 (–N=N–) cm–1. MS: m/z 706 [M]+, 548 [M –
˜
1
calcd. C 53.31, H 3.45, N 3.55; found C 53.42, H 3.41, N 3.70. H
2Br]+.
NMR (CDCl3): δ = 2.17 (s,3 H, aryl CH3); aromatic protons: 6.65–
[Pt(L1)(4-picoline)Cl2] (5c): Yield 62%. C23H19Cl2N3OPt (619.42):
8.61 (br) ppm. IR (KBr): ν = 1412 (–N=N–) cm–1. MS: m/z 788
˜
1
calcd. C 44.60, H 3.09, N 6.78; found C 44.51, H 3.21, N 7.01. H
[M]+, 718 [M – 2Cl]+.
Table 4. Crystallographic data and experimental details for 2a, 2b, 3a, 4a, 5a and 6b.
2a
2b
3a
4a
5b
6b
Empirical for-
mula
C23H19N3OPt
C35H27N2OPPt
C26H19N3OPt
C23H19N3OPt
C23H19I2N3OPt
C36H29N2 Br2OPPt
Formula weight
Temperature
Crystal system
Space group
548.50
293(2) K
monoclinic
P21/c
717.67
584.54
293(2) K
monoclinic
P21/c
548.50
293(2) K
monoclinic
P21
802.30
293(2) K
monoclinic
P21/c
962.39
293(2) K
monoclinic
C2/c
293(2) K
triclinic
¯
P1
a = 15.723(19) Å
b = 7.9134(9) Å
c = 17.345(2) Å
α = 90°
a = 9.7817(11) Å
b = 10.6325(12) Å
c = 14.3017(16) Å
α = 78.877(2)°
β = 83.8270(10)°
γ = 68.5090(10)°
1356.9(3) A3
2, 1.757
a = 15.173(5) Å
b = 7.445(2) Å
c = 18.396(6) Å
α = 90°
a = 10.1763(9) Å
b = 7.4642(6) Å
c = 13.2483(11) Å
α = 90°
a = 13.488(3) Å
b = 14.095(3) Å
c = 14.236(3) Å
α = 90°
a = 33.230(3) Å
b = 1.5207(10) Å
c = 22.438(2) Å
α = 90°
Unit cell
dimensions
β = 112.081(2)°
γ = 90°
β = 101.528(5)°
γ = 90°
β = 102.3970(10)°
γ = 90°
β = 111.218(4)°
γ = 90
β = 128.435(2)°
γ = 90°
Volume
1999.9(4) A3
4, 1.822
2036.0(11) A3
4, 1.907
982.85(14) A3
2, 1.853
2522.9(10) A3
4, 2.112
6728.8(11) A3
8, 1.900
Z, dcalc [Mg/m3]
Abs. coefficient
F(000)
7.034 mm–1
1056
5.262 mm–1
6.916 mm–1
1128
7.156 mm–1
528
8.027 mm–1
1480
6.785 mm–1
3712
704
Crystal size
Theta range for
data collection
Limiting
0.42ϫ0.22ϫ0.11
1.40 to 25.00°
–18 Յ h Յ 18
–9 Յ k Յ 9
–20 Յ l Յ 20
18179/3520
[R(int) = 0.0277]
3520/0/255
0.45ϫ0.36ϫ0.23
2.24 to 25.00°
–11 Յ h Յ 11
–11 Յ k Յ 12
–16 Յ l Յ 16
7551/4714
0.25ϫ0.18ϫ0.10
1.37 to 25.00°
–18 Յ h Յ 18
–8 Յ k Յ 8
–21 Յ l Յ 21
18304/3567
[R(int) = 0.0452]
3567/0/281
0.44ϫ0.18ϫ0.04
1.57 to 24.98°
–12 Յ h Յ 12
–8 Յ k Յ 8
–15 Յ l Յ 15
9416/3436
0.36ϫ0.25ϫ0.19
2.11 to 25.00°
–16 Յ h Յ 16
–16 Յ k Յ 16
–16 Յ l Յ 16
22325/4338
[R(int) = 0.0695]
4338/0/268
0.32ϫ0.24ϫ0.18
1.93 to 26.00°
–20 Յ h Յ 40
–14 Յ k Յ 14
–27 Յ l Յ 19
18761/6609
[R(int) = 0.0607]
6609/0/402
indices
Reflections
collected/unique
Data/restraints/
parameters
Goof on F2
Final R indices
[IϾ2σ(I)]
[R(int) = 0.0288]
4714/0/362
[R(int) = 0.0267]
3436/1/255
1.114
1.022
1.069
1.077
1.190
1.042
R1 = 0.0411
wR2 = 0.1218
R1 = 0.0460
wR2 = 0.1295
R1 = 0.0295
wR2 = 0.0661
R1 = 0.0326
wR2 = 0.0673
1.582 and –0.742
R1 = 0.0314
wR2 = 0.0655
R1 = 0.0396
wR2 = 0.0686
0.786 and –0.519
R1 = 0.0356
wR2 = 0.0958
R1 = 0.0376
wR2 = 0.0974
0.891 and –0.475
R1 = 0.0846
wR2 = 0.1732
R1 = 0.1042
wR2 = 0.1816
4.217 and –2.860
R1 = 0.0470
wR2 = 0.0921
R1 = 0.0647
wR2 = 0.0972
1.544 and –1.731
R indices
(all data)
Largest diff. peak 4.130 and –1.044
and hole (e·A–3)
3746
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 3739–3748

Regiospecific C(naphthyl)–H Bond Activation by Platinum(II)
X-ray Crystallography: Crystallographic data and experimental de-
tails for 2a, 2b, 3a, 4a, 5a and 6b are summarized in Table 4. Data
were collected with a Bruker SMART 1000 CCD area-detector dif-
fractometer using graphite monochromated Mo-Kα (λ = 0.71073 Å)
radiation by ω scan. The structures were solved by direct methods
using SHELXS-97[33] and difference Fourier syntheses and refined
with SHELXL-97 incorporated in WinGX 1.64 crystallographic
collective package.[34] All the hydrogen positions for the compound
were initially located in the difference Fourier map, and for the
final refinement, the hydrogen atoms were placed geometrically and
held in the riding mode. The last cycles of refinement included
atomic positions for all the atoms, anisotropic thermal parameters
for all non-hydrogen atoms and isotropic thermal parameters for
all the hydrogen atoms. Full-matrix-least-squares structure refine-
ment against |F2 |. Molecular geometry calculations were per-
formed with PLATON,[35] and molecular graphics were prepared
using ORTEP-3.[36]
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CCDC-823076 (for 2a), -823077 (for 2b), -823078 (for 3a), -823079
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Computational Details: Calculations were performed with the TD-
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Acknowledgments
Financial assistance received from the Department of Science and
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Received: May 3, 2011
Published Online: August 5, 2011
3748
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 3739–3748