Studies on some reactions of cyclopalladated azobenzenes with N,O-chelators

Article abstract of DOI:10.1016/S0277-5387(98)00216-2

The synthesis and characterization of new planar mixed chelates of the type [Pd(A)(N,O)] (A=ortho-metallated azobenzene or its substituted derivatives, A₁ to A₅, N,O=2-picolinato (pic), quinaldato (qnd), 8-quinolinolato (q) and 2-methyl-5-sulphonic-8-quinolinolato (msq)) are achieved by splitting the halogeno-bridge in [Pd₂(A)₂Cl₂] by aqueous AgNO₃ solution followed by the addition of N,O-chelator. [Pd(A₂)(N,O)] occurs as isomer [Pd(A₃)(N,O)] and the composition have been established using 1H NMR data. The complex composition is supported by elemental analyses and IR data. Cyclic voltammetric studies suggest two successive redox responses correspond to azo reduction.

Full text of DOI:10.1016/S0277-5387(98)00216-2

Polyhedron Vol[ 06\ No[ 12Ð13\ pp[ 3098Ð3003\ 0887
Þ 0887 Elsevier Science Ltd
Pergamon
All rights reserved[ Printed in Great Britain
9166Ð4276:87 ,08[99¦9[99
\
PII] S9166Ð4276"87#99105Ð1
Studies on some reactions of cyclopalladated
azobenzenes with N\O!chelators
K[ Bag\ T[ K[ Misra and C[ Sinhaꢀ
Department of Chemistry\ The University of Burdwan\ Burdwan 602093\ India
"Received 05 January 0887^ accepted 14 May 0887#
Abstract*The synthesis and characterization of new planar mixed chelates of the type ðPd"A#"N\O#Ł "Aꢁortho!
metallated azobenzene or its substituted derivatives\ A0 to A4\ N\Oꢁ1!picolinato "pic#\ quinaldato "qnd#\ 7!
quinolinolato "q# and 1!methyl!4!sulphonic!7!quinolinolato "msq## are achieved by splitting the halogeno!
bridge in ðPd1"A#1Cl1Ł by aqueous AgNO2 solution followed by the addition of N\O!chelator[ ðPd"A1#"N\O#Ł
occurs as isomer ðPd"A2#"N\O#Ł and the composition have been established using 0H NMR data[ The complex
composition is supported by elemental analyses and IR data[ Cyclic voltammetric studies suggest two successive
redox responses correspond to azo reduction[ Þ 0887 Elsevier Science Ltd[ All rights reserved
Keywords] cyclopalladated^ mixed chelates^ N\O!donor^ spectral studies^ electrochemistry[
———————————————————————————————————————————————
INTRODUCTION
RESULTS AND DISCUSSION
Cyclopalladated compounds of transition metals have
attracted a great deal of attention in recent times ð0Ð
7Ł[ Many reviews concerning their synthesis and reac!
tivities appear in literature ð8Ð03Ł[ We have extensively
studied the cyclopalladation reactions of many sub!
stituted azobenzenes and Schi} bases ð04Ð08Ł[ Azo!
benzene becomes palladated at the azo!nitrogen and
ortho!carbon atom and produces bridged dimeric
species ð19Ð11Ł[ The bridge splitting reactions of the
dimer with neutral ligands such as tertiary phosphines
ð12Ð14Ł\ pyridines ð15Ł and anionic ligands such as
acetylacetonate ð16Ł\ arylazooximate ð17Ł gave large
number of planar mixed organopalladium complexes[
In the present paper we report a further new type "1#
of mixed cyclopalladated complexes of azobenzenes
with "N\O#!chelators[ We have chosen monoanionic
N\O!chelator] picolinic acid "Hpic#\ quinaldic acid
"Hqnd#\ 7!quinolinol "Hq# and its derivative 1!methyl!
4!sulphonic!7!quinolinol "Hmsq# "H stands for the
phenolic hydrogen# as the capping ligand[ They form
_ve membered chelate ring with cyclopalladated
Pd"C\N# fragment "0#[
The complex "1# is abbreviated as Pd"A#"N\O#
where A represents the azobenzenes[ A0 to A4 are the
speci_c abbreviations of the ligand A depending on
substituents and binding mode as set out in Scheme
0[
The chloro!bridged cyclopalladated azobenzenes
ðPd1"A#1Cl1Ł in acetone were treated with aqueous
AgNO2 and re~uxed for a few hours[ To the _ltrate
"N\O#!chelators were added in equivalent amount "0]0
mole ratio#[ The picolinato ðPd"A#"pic#Ł\ quinaldato
ðPd"A#"qnd#Ł and 7!quinolinolato ðPd"A#"q#Ł and
ðPd"A#"msq#Ł derivatives were precipitated as orange
solid[ The complexes are non!electrolytic in nature
and suggest covalent binding of the chelators[ The
composition of the complexes are supported by
elemental analyses data "Table 0#[
The complexes display spectral characteristics cor!
responding to n"N
N# at 0259Ð0274 cm−0[ The assign!
1
ment of all bands has not been attempted[ However\ a
comparison of the spectra with those of the respective
chloro!bridged dimers and of free "N\O#!chelator
a}ords useful information[ Picolinato ðPd"A#"pic#Ł
and quinaldato ðPd"A#"qnd#Ł derivatives exhibit
strong stretching due to nas"COO# and ns"COO# at
½0549 and 0239 cm−0 respectively which appears in
the free ligand at 0619 cm−0[ The di}erence in stret!
ꢀ Author to whom correspondence should be addressed[
3098

3009
K[ Bag et al[
Table 0[ Microanalyticala and UVÐVIS spectral datab
Complex
Elemental
H")#
UVÐVIS spectral data lmax "nm#
"09−2o\ dm2 mol−0 cm−0
#
C")#
N")#
ðPd"A0#"pic#Ł
ðPd"A1#"pic#Ł
ðPd"A3#"pic#Ł
ðPd"A4#"pic#Ł
ðPd"A0#"qnd#Ł
ðPd"A1#"qnd#Ł
ðPd"A3#"qnd#Ł
ðPd"A4#"qnd#Ł
ðPd"A0#"q#Ł
ðPd"A1#"q#Ł
ðPd"A3#"q#Ł
ðPd"A4#"q#Ł
ðPd"A0#"msq#Ł
ðPd"A1#"msq#Ł
ðPd"A3#"msq#Ł
ðPd"A4#"msq#Ł
41[6 "41[7#
43[9 "42[8#
44[9 "43[8#
34[0 "34[1#
46[7 "46[4#
47[0 "47[2#
48[1 "48[0#
38[7 "49[9#
47[4 "47[3#
48[1 "48[2#
59[1 "59[0#
49[2 "49[3#
49[0 "49[2#
40[2 "40[1#
41[1 "41[9#
33[1 "33[3#
2[9 "2[1#
2[3 "2[4#
2[8 "2[8#
1[2 "1[2#
2[2 "2[2#
2[4 "2[5#
2[8 "2[8#
1[5 "1[4#
2[3 "2[4#
2[8 "2[7#
3[9 "3[0#
1[4 "1[5#
2[2 "2[1#
2[5 "2[4#
2[8 "2[7#
1[2 "1[4#
09[3 "09[2#
8[8 "8[8#
8[4 "8[5#
7[6 "7[7#
8[1 "8[0#
8[9 "7[8#
7[6 "7[5#
6[7 "7[9#
8[5 "8[6#
8[4 "8[3#
8[9 "8[0#
7[4 "7[3#
7[1 "7[9#
6[8 "6[7#
6[3 "6[5#
6[2 "6[0#
367"2879#c\ 337"2259#\ 271"6999#
379"3119#c\ 349"3239#\ 274"6789#
372"3599#c\ 343"4219#\ 276"8709#
374"4739#c\ 341"4979#\ 274"7839#
379"2959#c\ 349"2539#\ 272"6749#
374"4329#c\ 349"5239#\ 279"8459#
373"6134#c\ 343"7479#\ 276"05419#
374"5359#c\ 341"7439#\ 274"09259#
373"2179#c\ 340"3919#\ 275"7139#
379"3129#c\ 349"4115#\ 279"8269#
372"4179#c\ 343"5039#\ 276"00519#
373"3549#c\ 341"4739#\ 279"8139#
377"3379#c\ 337"4559#\ 274"7349#
375"4929#c\ 349"5789#\ 279"8359#
371"3839#c\ 343"5229#\ 274"7919#
374"3679#c\ 349"5159#\ 279"6839#
a Calculated values are in parentheses[
b In CHCl2 except ðPd"A#"msq#Ł derivatives\ those are recorded in DMSO[
c Shoulder[
ching frequency "Dnꢁ189 cm−0# suggests unidentate splitting reaction followed by chelation using N\O!
carboxylato coordination ð18\ 29\ 20Ł[ Additional chelator may produce\ in principle\ two geometric iso!
bands are observed at 759Ð739\ 619Ð699 and 579Ð mers "B and C# for symmetrical and four isomers for
559 cm−0 and support the coordinated picolinate ion[ asymmetrical azobenzenes\ respectively[ The isomers
Similarly quinolinolato ðPd"A#"q#Ł and ðPd"A#"msq#Ł could not be isolated by chromatography on silica gel
exhibit two characteristic band at 0459Ð0479 and or alumina[ There are many bridge splitting reactions
0389Ð0494 cm−0 along with ring deformation at ð12Ð16Ł of cyclopalladated azobenzenes\ Schi} bases
½399 cm−0 supporting the presence of N\O!chelator with pyridines or phosphines where the base occupies
in the cyclopalladated moiety[ All the complexes exhi! trans!position to azo:azomethine!N[ 1!Picolinic acid
bit stretching at 419Ð429 and 249Ð279 cm−0 cor! is also used as bridge splitter in a cyclopalladated
responding to n"PdÐO# and n"PdÐN#\ respectively[
a!naphthylethylamine where heterocyclic!N occupies
The solution electronic spectra of the complexes trans to amine nitrogen[ This may provide a guide
were recorded in CHCl2 and the transitions are to consider single isomer "B# from the symmetrical
assigned on the basis of reported literature ð07Ð11\ 17Ł[ azobenzenes and two isomers from asymmetrical azo!
The transition around 499 nm is assigned to charge benzene[
transfer transition in the metallated fragment ð19Ð11\
17Ł and the higher energy transitions are referred to palladated complexes of symmetrical azobenzenes are
intraligand charge transfer transitions[ the guide lines to the signal assignment[ In cases\
The 0H NMR spectra of free ligands and the cyclo!
For asymmetrical azobenzene "A1:A2# the cyclo! where the metallation of both substituted and unsub!
palladated complex occurs as a mixture of isomers but stituted rings occurs\ the population of the isomer
in no case they are separated by chromatography[ having metallation in the substituted ring is higher\ in
Because of bad solubility of the chloro!bridged species line with the electrophilic nature of the palladation
in common organic solvents direct isomer assessment reaction[ Symmetrical azobenzenes yield single mixed
by 0H NMR is not easily achieved ð21Ł[ Di}erent other chelate complex which is supported by two Me signals
methods ð22\ 23Ł like gas chromatographic analysis of of ðPd"A3#"N\O#Ł in 0H NMR spectra\ in line with the
the products obtained by exhaustive carbonylation of electrophilic nature of the palladation reaction[ Asym!
the dimer had been used[ The 0H NMR spectral tech! metrical ligand in ðPd"A1#"N\O#Ł gives closely spaced
nique had been employed in a limited area ð17Ł for the dMe signal[ Originally cyclopalladated complex exists
product analysis obtained by bridge splitting reactions in two isomeric forms and at least two mixed chelates
0
using PPh2\ arylazooximes etc[ We are also using H will appear in the product[ The observed ratio of
NMR technique for the assessment of isomer popu! ðPd"A1#"N\O#Ł]ðPd"A2#"N\O#Ł is 2]1[ The results are in
lation "Table 1#[ For symmetrical azobenzenes "A0\ agreement with those obtained by the bridge splitting
A3\ A4# no such isomerism is observed[ The bridge reaction using PPh2 and pyridine[

Cyclopalladated azobenzenes with N\O!chelators
Table 1[ 0H NMR data of complexes in ðPd"A#"N\O#Ł
3000
Complex
d "ppm#
1!Ha 2!H 3!H 4!H 5!H 6!H 7!H 8!H 09!H 00!H 01!Ha 03\ 07!H 04\ 06!H
Me
ðPd"A0#"pic#Ł
ðPd"A1#"pic#Ł
ðPd"A3#"pic#Ł
ðPd"A4#"pic#Ł
ðPd"A0#"qnd#
ðPd"A1#"qnd#
ðPd"A3#"qnd#
ðPd"A4#"qnd#
ðPd"A0#"q#
−
−
−
−
−
−
−
−
7[54a 7[97b 6[89b 7[14a
7[52a 7[92b 6[75b 7[12a
7[50a 7[99b 6[74b 7[16a
7[57a 7[93b 6[77b 7[14a
−
−
−
−
−
−
−
−
6[53a 6[25a 6[25b 6[7
6[78
6[79
6[6
6[85
6[85
6[71
6[64
7[9
6[47c
6[38c
6[15a
6[57a
6[52c
6[43c
6[20a
6[62a
6[40c
6[34c
6[10a
6[54a
6[35c
6[31c
6[06a
6[37d
6[33d
6[66d
−
−
−
6[98a 6[51
6[93a 6[52
6[47a 6[78
1[33e
1[41\ 1[3
7[67a 7[44b 7[97c 6[70c 6[70c 7[37a 6[56a 6[28b 6[28b 6[73
7[69a 7[49b 7[97c 6[70c 6[70c 7[41a 6[36d
7[69a 7[40b 7[98c 6[68c 6[68c 7[49a 6[37d
7[73a 7[50b 7[02c 6[73c 6[73c 7[41a 6[70d
−
−
−
6[97a 6[48
6[96a 6[55
6[50a 6[82
1[34e
1[37\ 1[3
7[83 7[01b 7[31c 6[58c 6[72c 6[59a
7[81 7[00b 7[23c 6[69c 6[70c 6[59a
7[89 7[97b 7[25c 6[54c 6[79c 6[45a
7[84 7[09b 7[34c 6[61c 6[76c 6[53a
−
−
−
−
−
−
−
6[47a 6[22b 6[22b 6[64
6[7
ðPd"A1#"q#
ðPd"A3#"q#
6[39d
6[26d
6[68d
−
−
−
6[98a 6[53
6[91a 6[5
6[44a 6[74
6[63
6[54
6[80
6[63
6[57
6[5
1[49
1[49\ 1[2
ðPd"A4#"q#
ðPd"A0#"msq#g
ðPd"A1#"msq#g
ðPd"A3#"msq#g\h
−
−
−
6[72a 7[23a
6[79a 7[22a
6[79a 7[29a
−
−
−
7[94a 6[61a
7[92a 6[60a
7[90a 6[58a
6[49a 6[15b 6[15b 6[6
1[73f
6[29d
6[20d
−
−
5[86a 6[47
6[99a 6[45
1[73f\ 1[4
1[71f\ 1[40
a Doublet[
b Triplet[
c Multiplet[
d Singlet[
e Isomeric dMe\ 1[27 ppm[
f dMe of msq[
g Solvent] CDCl2\ except ðPd"A#"msq#Ł where DMSO!d5 is used[
h The concentration of ðPd"A4#"msq#Ł is very low to get well de_ned 0H NMR in DMSO!d5[
The heterocyclic ring protons appear in general at acetonitrile versus saturated calomel electrode "SCE#
the down_eld region relative to arylazo protons[ The
proton numbering pattern is shown in Scheme 0[ The
signals are assigned on the basis of spinÐspin inter!
action and change therein on substitution[ In pic!
olinato derivatives the most down_eld doublet is 2!H
"½7[5 ppm# followed by 5!H "½7[1 ppm# signal[ The
protons 3!H and 4!H appear relatively at up_eld pos!
ition ½7[9 and 6[7 ppm respectively[ The quinaldato
derivatives ðPd"A#"qnd#Ł similarly exhibit resonances
in down_eld region for 2!H to 7!H at 7[6 to 6[7 ppm[
7!Quinolinolato complexes ðPd"A#"q#Ł exhibit 1!H sig!
nal at most down_eld region "½7[8 ppm#^ the pyr!
idinic ring protons appear in down_eld region "1!H
to 3!H\ 7[8 to 7[3 ppm# and phenolic ring protons
appear at relatively up_eld region "4!H to 6!H\ ½6[5
to 6[7 ppm#[ Arylazo ring protons appear relatively in
up_eld position "6[9 to 6[6 ppm# and the signal pos!
ition is highly sensitive to the substituent in the aryl
ring[ In A1:A2\ A3 and A4 singlet resonance is due to
8!H proton[ Metallated ring protons appear in more
down_eld region than non!metallated ring ones[
Other ring protons appear at usual position and the
signal movement is in accordance with the electronic
e}ect of the substituents ð24Ł[
in presence of tetrabutylammonium perchlorate
"TBAP#[ The iR compensation was applied internally
in the instrument[ All the complexes display two redox
responses and the data are collected in Table 2[ The
peak potentials are nearly invariant with scan rate
"49Ð499 mV s−0# and the cathodic peak current "ipc# is
proportional to v0:1 and ipa:ipc lies at 9[8Ð0[91 sug!
gesting reversibility of the redox process[ The current
height measurement shows one!electron redox type
and the peak!to!peak separation "DEp# suggests qua!
sireversible character of the couples[ Palladium"II#
reduction in mixed complexes of cyclopalladated azo!
benzenes were reported at very high potential and a
two!electron quasireversible reduction process fol!
lowed by chemical reaction ð17Ł[ In the present series
of complexes the couples are one!electron reduction
type at relatively lower value and stable on repetitive
cyclic operation[ This excludes the metal reduction[
Ligand reduction is believed to occur primarily at
the azo!function who can accommodate up to two
electrons ð17Ł at the LUMO[ The coulometric experi!
ment for the _rst couple con_rms one!electron
reduction at the azo!centre[ The experiment was per!
formed in one case ðPd"A0#"q#Ł at −0[3 V vs SCE in
MeCN and coulomb count "hꢁQ:Q?ꢁ9[83 where Q
and Q? are observed and calculated coulomb# suggests
just one!electron reduction[ On reduction the solution
Electrochemistry
Cyclic voltammetric data were collected under N1
environment using glassy carbon working electrode in colour has changed from orange red to brown red

3001
K[ Bag et al[
Scheme 0[
and in the UVÐVIS spectra the shoulder at 373 nm is erocyclic coligand is not ruled out which may need
eliminated while new band appears at 409 nm[ The comparatively higher energy to accommodate elec!
ESR spectra of the reduced product shows interesting trons at the antibonding orbital[ There may be a cer!
behaviour^ at room temperature a broad signal is tain degree of involvement of coligand pꢀ!orbital in
observed at `ꢁ1[994 with peak!to!peak separation conjunction with azo!antibonding orbital[$
34 G[ The spectrum at liquid!nitrogen temperature
"66 K# shows hyper_ne splitting pattern[ Detail inves!
tigation is under scrutiny[ The observation supports
EXPERIMENTAL
the literature report ð25\ 26Ł and suggests that the _rst
The solvents used in the reactions were of reagent
reduction is believed to occur at the azo!function of
grade and were dried by reported procedures ð04\ 05Ł[
the ligand[ The e}ect of the substitution on reduction
1!Picolinic acid "Hpic#\ quinaldic acid "Hqnd# "Fluka#\
potential data again supports the assignment[ Change
1!methyl!4!sulfonic!7!quinolinol "Hmsq# "Aldrich#
of substituents "para!to!azo group in aryl ring# in the
and 7!quinolinol "BDH# were recrystallized before use
metallated ring brings about in increasing cathodic
from ethanolÐwater "1]0\ v:v#[ Palladium chloride was
shift of E0:1 in the order Cl×H×Me as expected from
Hammett relation ð27Ł[ The second couple "E0:11# var!
ies irregularly and does not con_rm to the second step
reduction\ of azo!function[ The reduction of het!
$ One of the Referees has suggested in this manner[

Cyclopalladated azobenzenes with N\O!chelators
3002
Table 2[ Cyclic voltammetric data of ðPd"A#"N\O#Ł in ace!
tonitrile at 187 K
through G!3 sintered crucible[ To this orange solution
1!picolinic acid "9[97 g\ 9[54 mmol# in acetone "09 cm2#
was added dropwise with continuous stirring[ The
colour of the solution turned to orange!red and the
precipitate appeared slowly on evaporation in air[ The
volume of the solution was reduced to half and the
precipitate was _ltered\ washed with water\ and
recrystallized from chloroformÐhexane via di}usion[
The product was dried over CaCl1[ The yield was 69)[
The yield of other compounds varied between 54
and 79)[
Compounda
Reduction
Potentialsb
−E0:10"DEp#
−E0:11"DEp#
ðPd"A0#"pic#Ł
ðPd"A1#"pic#Ł
ðPd"A3#"pic#Ł
ðPd"A4#"pic#Ł
ðPd"A0#"qnd#Ł
ðPd"A1#"qnd#Ł
ðPd"A3#"qnd#Ł
ðPd"A4#"qnd#Ł
ðPd"A0#"q#Ł
ðPd"A1#"q#Ł
ðPd"A3#"q#Ł
ðPd"A4#"q#Ł
ðPd"A0#"msq#Ł
ðPd"A1#"msq#Ł
ðPd"A3#"msq#Ł
ðPd"A4#"msq#Ł
0[944"099#
0[006"89#
0[04"79#
0[330"059#
0[403"039#
0[474"099#
0[254"039#
0[273"079#
0[324"039#
0[434"029#
0[298"049#
0[449"019#
0[533"039#
0[794"059#
0[165"019#
0[314"009#
0[499"019#
0[509"059#
0[265"019#
9[831"89#
0[049"039#
0[199"099#
0[169"099#
0[930"009#
0[059"039#
0[126"019#
0[205"009#
9[881"89#
0[094"79#
0[054"89#
0[103"019#
9[844"099#
Acknowled`ements*Financial assistance from Department
of Science and Technology\ and University Grants
Commission\ New Delhi are thankfully acknowledged[ One
of the authors "K[ B[# is indebted to Professor B[ R[ De\
Department of Chemistry\ Vidyasagar University\ Mid!
napore for his help\ T[ K[ M[ thanks to the Council of
Scienti_c and Industrial Research\ New Delhi for giving him
a fellowship[
a Meaning of the symbols are as in the text[
b Potentials are 9[4 "Epc¦Epa#^ Epc and Epa are cathodic and
anodic peak potentials respectively\ unit in V and DEp in mV
"reference to SCE#\ scan rate 49 mV s−0\ solute concentration
½09−2 M in MeCN\ ðNBu3\ ClO3Ł is supporting electrolyte
"9[0 M#[
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04[ Chattopadhyay\ P[\ Nayak\ M[ K[\ Bhattacharya\
S[ P[ and Sinha\ C[\ Polyhedron\ 0886\ 05\ 0180[
05[ Basuli\ F[\ Chattopadhyay\ P[ and Sinha\ C[\
Polyhedron\ 0885\ 04\ 1328[
06[ Sinha\ C[\ Polyhedron\ 0882\ 01\ 1252[
07[ Chattopadhyay\ S[\ Sinha\ C[\ Chowdhury\ S[ B[
and Chakravorty\ A[\ J[ Or`anomet[ Chem[\ 0881\
316\ 000[
08[ Chattopadhyay\ S[\ Sinha\ C[\ Basu\ P[ and Chak!
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and references therein[
Synthesis of complexes
Complexes ðPd1"A#1Cl1Ł were prepared according
to reported methods ð19Ð11\ 17Ł[ The mixed ligand
complexes ðPd"A#"N\O#Ł were synthesized by a general
procedure and representative case is detailed below[
1!"Picolinato#"6!phenylazophenyl!C7\N#palladium"II#\
ðPd"A#"pic#Ł
To acetone solution "09 cm2# of ðPd1"A0#1Cl1Ł "9[1 g\
9[20 mmol# was added aqueous AgNO2 "9[00 g\
9[54 mmol# solution[ The mixture was re~uxed for 1 h[
The precipitated AgCl was _ltered at hot condition

3003
K[ Bag et al[
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dyopadhyay\ P[ and Chakravorty\ A[\ Inor`[
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Woollins\ J[ D[\ Polyhedron\ 0877\ 6\ 536[
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gueira\ J[ M[ and Suarez\ A[\ Polyhedron\ 0889\ 8\ 22[ Takahasi\ H[ and Tsuji\ J[\ J[ Or`anomet[ Chem[\
1630[ 0856\ 09\ 400[
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0885\ 04\ 2868[
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Matheis\ W[ and Klein\ A[\ Kaim\ W[\ Inor`[
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224\ 248[ 0854\ 32\ 2072[
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0860\ 13\ 864[
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