Chapter 7
Interaction of Lewis-bases with Polymeric Metal(II) Oxalates
7.1
xalic acid, (H
Introduction
2ox), HOOC-COOH, can be considered as the
simplest of all dicarboxylic acids. The structure involves
simply two -COOH groups connected directly through the
two carboxyl carbon atoms and has got unique electronic structure
quite different from that of both succinic (H2suc) and malonic
(H2mal) acids. Not only this molecule lacks any spacer function
between the carboxylic groups but also it has got connectivity
through two sp2 hybridized C atoms. Consequently its dianion,
-OOC-COO-, has conjugation extended over all the 6 atoms forming
a perfect planar configuration. This is one main feature that
distinguishes the oxalate moiety from its counterparts mentioned
in earlier chapters. Though, in principle, oxalate moiety can take
part in coordination involving various linking modes referred
earlier, the predominant ligation mode is seen to be mostly bis-
bidentate bridging type (see Chapter 1, Section 1.2, for details).
This makes the oxalate moiety a very strong bridging ligand
through σ-coordination resulting in polymeric metal carboxylates
with rigid 1D, 2D or 3D framework structures. Interesting feature
in these extended structures is the π-electron delocalisation
possible along the whole metal-oxalate framework both through pπ-
pπ and pπ-dπ interactions. Such interactions result in an effective
metal to metal electronic communication via the conjugated oxalate
function leading to interesting magnetic and electronic properties.
O
Many metal oxalate structures are reported in the
literature including those occurring naturally as minerals.(1) The
2D honey-comb(2,3) structure is seen to be the most common for the
oxalates which allows for large variations in molecule type and pore
Chapter 7 312
functionalization. The 1D coordination polymer NH4[Ti(ox)2].2H2O is
seen to contain the cyclic tetranuclear [Ti4O4(ox)8]8- ion units and
are used as basic building blocks.(4) The first example of an
oxalato-uranylate, Na2[(UO2)4(ox)5(H2O)2].8H2O(5) has a zeolite-type
structure. A series of open-framework oxalates containing zirconium
and cadmium or lead along with alkali-metal or ammonium cations
have also been reported.(6) These compounds contain MO8
polyhedral units that are linked through oxalate oxygen atoms to
give 3D open frameworks. In (NH4)2[CdZr(ox)4].3.9H2O and
[H3N(CH2)2NH3][CdZr(ox)4].4.4H2O, right-handed metal-oxalate
helical wires are formed through connectivity between the oxalate
ions and the MO8 polyhedra (M = Cd, Zr).(6a) The oxalate unit serves
to link the chains together to yield a 3D structure with channels.
Tamaki and co-workers have prepared 2D mixed-metal oxalates
with CrIII centers.(7) The [Cr(ox)3]3- ion acts as a building block by
binding to three MII centers (M = Fe, Co, Ni, Cu and Zn) ions through
oxalate-ion bridges, forming an extended network. An interesting
ion-pair [Cu(bipy)2(CH3COO)]+[Cu(bipy)2Cr(ox)3]-.10.5H2O(8) has been
reported as the first compound containing the [Cr(ox)3]3- units as
monodentate ligands. These complexes form examples in the design
and tuning of molecular components in extended structures to
attain desirable properties. Recently Arco et al(9) has reported the
intercalation of [Cr(ox)3]3- complexes in Mg, Al layered double
hydroxides. Mixed-valent oxalates A[MII-MIII(ox)3] (A = monocation;
MII = Mn, Fe, Co, Ni, Cu and Zn; MIII= Cr, Fe and Co) have
distinct magnetic properties, varying from paramagnetic to
ferromagnetic or antiferromagnetic.(10-15) Chiral metal complexes
have also been made use of to form layered magnets with oxalate
networks and 3D anionic networks as in [(ZII(bipy)3)[MIMIII(ox)3]] or
Interaction of Lewis-bases with Polymeric Metal Oxalates 313
[(ZII(bipy)3)(MIIMIII(ox)3)] (bipy = 2,2'-bipyridyl; ZII = Fe, Co, Ni, Ru and
Zn; MI = alkali metal, NH4+; MII = Mn, Fe, Co and Cu; MIII = Cr
and Fe).(16) Other interesting 2D ternary complexes reported are
[Cu(ox)(L)(H2O)].nH2O(17) (L = bipy, phen and n-phen; n = 1, 2),
[M(ox)L]n(18) [L = 4,4'-bipy; M = Co(II), Fe(II), Ni(II) and Zn(II)) and
[M(ox)(L)2].nH2O(19) (n = 4,5; M = Ni, Cu; L = bipy or phen) in
which the oxalate anions are found to function as bidentate
bridging moieties. In [Cu(bipy(ox)].2H2O,(20) the oxalate anions
are centrosymmetric and act as quadridentate bridging ligands
resulting in zigzag polymeric structure whereas
[Cu2(bipy)2(H2O)2(NO3)2(ox)](21) has the unusual ligand arrangement
and the packing of molecules are attributed to intermolecular and
intramolecular hydrogen bonding leading to a zigzag chain which
exhibits antiferromagnetism. In [Cu(en)2][Cu(ox)2],(20a) the oxalate anions
act as tridentate moieties. The 1D complex, {[Cu(bipy)(ox)]2.5H2O}n(22)
consists of columnar stacks of neutral [Cu(ox)(bipy)] units, exhibiting
alternating ferro-antiferromagnetic interactions. The 3D supramolecular
complex, K[Cu(trans[14]dien)][Cr(ox)3](23) has been reported, which
has large helical tunnels that are formed by the oxalate-bridged,
octahedrally coordinated Cr and K centers. In the iron oxalate,
(NH4)2[Fe2O(ox)2Cl2].2H2O,(24) a 3D structure with helical tunnels
occupied by guest species has been reported. In the iron(III)
system, [(acac)2Fe(ox)Fe(acac)2], Fujino et al(25) reported the
stabilization of Fe(III) to Fe(II) mixed valence state by the electronic
delocalisation through the oxalate bridge. A hydrothermally
prepared 1D oxalate, Na2[Co2(ox)3(H2O)2],(26) is known to possesses
a ladder-like topology, which manifests antiferromagnetic
ordering. An interesting sheet like polymeric complex,
[NaCr(bipy)(ox)2(H2O)].2H2O has been reported by Munoz et al(27)
Chapter 7 314
and observed that oxalate ligands function as monodentate as well
as bis-chelating within the chain, showing antiferromagnetic
property. On the other hand, Ba4[{Fe(ox)(OH)}4(ox)Cl2](28) possesses
two distinct channels (concave and convex) formed by the linking
of vertex-sharing FeO6 octahedra through -OH bridges and oxalate
ions in a monodentate fashion. Recently Pointillart et al(29) has
reported an interesting three-dimensional oxalate-based complexes,
{[Ru(bipy)3][Cu2xNi2(1-x)(ox)3]}n (0 ≤ x ≤ 1) where Cu(II) has unusual
tris(bischelated) environment and the compounds are found to be
isostructural, single-phased and at low temperature seen to exhibit
long-range ordered magnetic behaviour. Several cyanogold
complexes react with the binuclear nickel complex,
[{Ni(dien)(H2O)}2(ox)](PF6)2.2H2O(30) resulting in a variety of di- or
polynuclear compounds which show antiferromagnetic behaviour.
The use of metal oxalate complexes of Cr, Fe, Mn, Co and Al as
novel inorganic dopants is well documented.(31) In the present study
we discuss in detail the interaction of Lewis-bases with polymeric
metal oxalates and formation and characterisation of various mixed
ligand complexes.
7.2 Experimental
7.2.1 Preparation of oxalate complexes of divalent metal ions
Metal oxalates of Ni(II), Co(II) and Cu(II) were prepared by
reacting the appropriate metal carbonate with a hot aqueous
solution of oxalic acid in stoichiometric quantities. The complexes
were separated by filtration. These were washed repeatedly with hot
water and methanol and dried under vacuum over P2O5.
Interaction of Lewis-bases with Polymeric Metal Oxalates 315
(a) Nickel(II) oxalate complex, [Ni(H2O)2(ox)]n, 40
To an aqueous solution of oxalic acid (0.13g, 1mM), nickel
carbonate (0.37g, 1mM) was added slowly with stirring. Brisk
effervescence formed indicating the complex formation. The
reaction mixture was boiled on a steam bath for about 30 min. The
amount of nickel carbonate added was slightly less than the
stoichiometric amount of oxalic acid. The light green complex
formed was filtered and washed repeatedly with hot water to
remove any excess acid present and finally with methanol. The
pure complex thus obtained was dried in vacuo. (yield : 90%)
(b) Cobalt(II) oxalate complex, [Co(H2O)2(ox)]n, 41
Cobalt carbonate (0.11g, 1mM) was added in small quantity
with stirring to a hot aqueous solution of oxalic acid (0.13g, 1mM).
The reaction mixture was boiled on a steam bath for about 1h. The
rose complex formed was filtered and washed repeatedly with hot
water to remove any excess acid present and finally with methanol.
The pure complex thus obtained was dried in vacuo. (yield : 90%)
(c) Copper(II) oxalate complex [Cu(ox)]n, 42
The procedure adopted for the preparation of copper(II)
oxalate complex was almost the same as that of nickel(II) or
cobalt(II) complex. About 0.23g (1mM) of cupric carbonate was
added in small quantity to a hot aqueous solution of 0.13g (1mM)
oxalic acid while stirring. The light blue complex formed was
filtered and washed repeatedly with hot water to remove excess
acid and finally with methanol. It was dried under vacuum over
P2O5. (yield : 85%)
Chapter 7 316
7.2.2 Preparation of Lewis-base adducts of nickel(II) oxalate
(a) [Ni(en)(ox)]n, 43
A solution of en (0.19ml, 3mM) in methanol was added
dropwise to a suspension of nickel(II) oxalate (0.18g, 1mM) in
methanol with constant stirring for about 2 h. The complex gets
separated out as rose violet solid. This was filtered and was
repeatedly washed with ether and dried in vacuo. (yield : 80%)
(b) [Ni(bipy)(ox)]n, 44
A solution of 2,2'-bipy (0.47g, 3mM) in methanol was added
dropwise to a suspension of nickel(II) oxalate (0.18g, 1mM) in
methanol. The mixture was kept under reflux for about 2 h. The
complex separated out as light violet solid was filtered. This was
repeatedly washed with ether and dried under vacuum over P2O5.
(yield : 80%)
(c) [(Ni(phen)(ox)).2H2O]n, 45
A solution of 1,10-phen (0.6g, 3mM) in methanol was added
dropwise to a suspension of nickel(II) oxalate (0.18g, 1mM) in
methanol. The mixture was kept under reflux for about 2 h. The
complex separated out as light violet solid was filtered. This was
washed with methanol followed by ether which was then dried in
vacuo. (yield :75%)
(d) [Ni(py)2(ox)]n, 46
An excess of pyridine was added to a nickel(II) oxalate
(0.36g, 1mM) suspension in methanol with constant stirring. The
reaction mixture was refluxed for about 2 h. The complex separated
Interaction of Lewis-bases with Polymeric Metal Oxalates 317
out as light green solid was filtered and repeatedly washed with
methanol followed by ether. This was dried in vacuo. (yield :75%)
(e) [(Ni(pn)(ox)).H2O]n, 47
A solution of pn (0.27ml, 3mM) in methanol was added
dropwise to a suspension of nickel(II) oxalate (0.18g, 1mM) in
methanol with constant stirring for about 2 h. A clear blue solution
was obtained from which solid blue complex was seen to be
separating out soon, making the supernatant liquid colourless. The
solid complex was filtered and repeatedly washed with methanol
followed by acetone. This was dried in vacuo. (yield : 80%)
7.2. 3 Preparation of Lewis-base adducts of cobalt(II) oxalate
(a) [(Co(en)(ox)).H2O]n, 48
A solution of en (0.19ml, 3mM) in methanol was added to a
suspension of cobalt(II) oxalate (0.18g, 1mM) in methanol with
constant stirring followed by refluxing for about 2 h. The complex
separated out as brown solid was filtered and repeatedly washed with
methanol followed by ether. This was dried in vacuo. (yield : 80%)
(b) [Co(bipy)(ox)]n, 49
To a suspension of cobalt(II) oxalate (0.18g, 1mM) in
methanol, a solution of 2,2'-bipy (0.47g, 3mM) in methanol was
added. The reaction mixture was kept under reflux for about
2 h. The complex separated out as orange solid was filtered.
This was repeatedly washed with methanol followed by ether.
This was then dried in vacuo. (yield : 80%)
Chapter 7 318
(c) [(Co(phen)(ox)).H2O]n, 50
A solution of phen (0.6g, 3mM) in methanol was added to a
suspension of cobalt(II) oxalate (0.18g, 1mM) in methanol. The
mixture was kept under reflux for about 2 h. The complex separated
out as orange solid was filtered and repeatedly washed with methanol
followed by ether. This was dried in vacuo. (yield : 85%)
(d) [Co(py)2(ox)]n, 51
An excess of pyridine was added to a cobalt(II) oxalate
(0.36g, 1mM) suspension in methanol with constant stirring. The
reaction mixture was refluxed for about 2 h. The complex
separated out as light pink solid was filtered and repeatedly
washed with methanol followed by diethyl ether. It was dried in
vacuo. (yield : 75%)
(e) [Co(pn)(ox)]n, 52
A solution of pn (0.48ml, 3mM) in methanol was added
dropwise to a suspension of cobalt(II) oxalate (0.36g, 1mM) in
methanol with constant stirring for about 2 h. The complex was
separated out as dark pink solid which was filtered and repeatedly
washed with methanol followed by acetone. This was dried in
vacuo.(yield : 85%)
7.2.4 Preparation of Lewis- base adducts of copper(II) oxalate
(a) [(Cu(en)(ox)).2H2O]n, 53
A solution of en (0.19ml, 3mM) in methanol was added
dropwise to a suspension of copper(II) oxalate (0.15g, 1mM) in
methanol with constant stirring for about 1 h. The complex separated
Interaction of Lewis-bases with Polymeric Metal Oxalates 319
out as violet solid was filtered and repeatedly washed with methanol
followed by ether. This was dried in vacuo. (yield : 80%)
(b) [Cu(bipy)(ox)]n, 54
To a suspension of copper(II) oxalate (0.15g, 1mM) in
methanol , a solution of bipy (0.47g, 3mM) in methanol was added.
The mixture was kept under reflux for about 2 h. The complex
separated out as light blue solid was filtered and repeatedly washed
with methanol followed by ether. Finally it was dried in vacuo.
(yield : 80%)
(c) [Cu(phen)(ox)]n, 55
The method adopted for the preparation of this compound
was almost same as that employed for the 2,2'-bipy adduct. The
complex was separated out as light blue solid which was then
filtered and repeatedly washed with methanol followed by ether.
This was dried in vacuo. (yield : 80%)
(d) [Cu(py)2(ox)]n, 56
An excess of pyridine was added to a copper(II) oxalate (0.15g,
1mM) suspension in methanol with constant stirring. The reaction
mixture was refluxed for about 2 h. The complex separated out as
light blue solid was filtered and repeatedly washed with methanol
followed by diethyl ether. It was then dried in vacuo. (yield : 80%)
(e) [Cu(pn)(ox)]n, 57
A solution of pn (0.48ml, 3mM) in methanol was added
dropwise to a suspension of copper(II) oxalate (0.30g, 1mM) in
methanol with constant stirring for about 1 h. A clear dark blue
Chapter 7 320
solution was obtained from which solid light blue complex was found
to be separating out soon making the supernatent liquid colourless.
The solid complex was filtered and repeatedly washed with methanol
followed by acetone. This was dried in vacuo. (yield : 80%)
7.3 Results and Discussion
7.3.1 Metal oxalates of Ni(II), Co(II) and Cu(II)
Like succinic acid and malonic acid, oxalic acid also forms
complexes with metals like Ni(II), Co(II) and Cu(II). The preparative
procedures employed for generating these metal oxalates were almost
same as that used for metal succinates and metal malonates which
are described in chapters 3 and 5 respectively. The analytical data
show a strict 1:1 complex (metal : ox) for all the oxalate complexes of
the divalent metal ions. (Table 7.1). The compositions of various
metal(II) oxalates were found to be [M(H2O)2(ox)]n for Ni(II) and Co(II)
and [Cu(ox)]n for the Cu(II) ion. TG analysis could confirm the extent
of water present in the complexes.
The typical carboxylate peaks νas(CO2)and νs(CO2) for the oxalic
acid are seen to occur at 1701 and 1443 cm-1 respectively. For the
present metal oxalates these two peaks are seen to occur at 1636,
1362 cm-1 for [Ni((H2O)2(ox)]n (40), 1635, 1370 cm-1 for [Co(H2O)2(ox)]n
(41) and 1649, 1364 cm-1 for [Cu(ox)]n (42) (Table 7.2). The greater
difference in νΔ of these two bands compared to νΔ (258 cm-1) of
oxalic acid indicates the bis-bidentate bridging character of the
oxalate group.(7,32-38) The C-O stretching frequencies in these
complexes are seen to occur in the range 1316-1321 cm-1 which is
typical of chelating/bridging carboxylate group. Broad peaks in the
range 3235-3250 cm-1 were seen in all the complexes (except for 42)
Interaction of Lewis-bases with Polymeric Metal Oxalates 321
which are characteristic of ν(O-H) band, indicating the presence of
coordinated water in the system. It is interesting to note that νΔ is
consistent with Irving-William series.
Table 7.1 Elemental analytical data of metal(II) oxalates
Elemental content(%) obsvd (calcd) Compound
(Emp. formula) Formula weight
C H M
Colour
[Ni(H2O)2(ox)]n
(NiC2H4O6) 40 182.69 13.17 (13.14)
2.25 (2.21)
32.2 (32.1) Green
[Co(H2O)2(ox)]n
(CoC2H4O6) 41 182.93 13.15 (13.12)
2.22 (2.20)
32.3 (32.2) Rose
[Cu(ox)]n
(CuC2O4) 42 151.54 15.80 (15.84) - 42.0
(41.9) Blue
Table 7.2 Important IR spectral data of metal(II) oxalates (cm-1)
Compound ν(O-H) νas(CO2) νS(CO2) ν(C-O) νΔ ν(O-H)
H2ox 3462 1701 1443 1330 258 1625
40 3250 1636 1362 1316 274 1608
41 3235 1635 1370 1321 265 1610
42 - 1649 1364 1319 285 -
The electronic spectra of various metal(II) oxalates were
recorded in the solid state only because of the high insolubility of
the complexes. The absorption values are given in Table 7.3.
Nickel(II) oxalate (40) gave three peaks at 25310, 15600 and 10,650
Chapter 7 322
cm-1 which are typical of octahedral Ni(II).(39,40) In the case of
cobalt(II) oxalate (41), the absorption peaks were seen at 18450
and 14490 cm-1 which are characteristic of octahedral Co(II)
species.(41,42) The broad absorption band was observed for copper(II)
oxalate (42) at 13850 cm-1 which is expected for square planar
Cu(II) species.(43-45)
Table 7.3 Electronic spectral and magnetic data of metal(II) oxalates
Compound Absor. Max ν (cm-1) Assignments μeff
(BM)
[Ni(H2O)2(ox)]n
40
25310
15600
10650
3A2g(F) → 3T1g(P) ν3
3A2g(F) → 3T1g(F) ν2
3A2g(F) → 3T2g(F) ν1
2.92
[Co(H2O)2(ox)]n
41
18450
14490
4 T1g → 4T1g(P)
4T1g → 4A2g
4.57
[Cu(ox)]n
42 13850 2B1g → 2A1g 1.43
Magnetic measurements showed that all the above oxalate
complexes are paramagnetic in nature. The μeff value of nickel(II)
oxalate complexes was found to be 2.92 BM indicating that the
complex is octahedral. The room temperature μeff value of cobalt(II)
oxalate was seen to be 4.57 BM which is characteristic of
octahedral Co(II) species. The magnetic moment of copper oxalate
complex was found to be 1.43 BM which is noticeably lower than
the value expected of typical Cu(II) complexes. Both spectral and
magnetic data indicated the strict six-coordinated character for
Interaction of Lewis-bases with Polymeric Metal Oxalates 323
Ni(II) and Co(II) complexes while four coordinated character for
Cu(II) complex. A comparatively low value of μeff for 42 indicates
the close proximity of Cu ions in it for a possible Cu-Cu interaction.
Based on the above observations and high insolubility the overall
structure of the metal(II)oxalate complexes could be considered as
polymeric in nature.
7.3.2 Interaction of Lewis-bases with polymeric nickel(II) oxalate
The set of Lewis-bases chosen for interaction and
depolymerisation of nickel(II) oxalate were same as those used for
the succinate and malonate systems. We have carried out the
interaction of the bases with the metal oxalates under various
reaction conditions and stoichiometric proportions as earlier. To
isolate the new structural species, also we have employed some
optimum reaction conditions. These along with some salient
features of the reaction and the nature of products isolated are
presented in Table 7.4. Unlike in the earlier cases the various
Lewis-bases do not seem to depolymerise the metal oxalates very
easily. Only pn seems to fragment the polymer skeleton as evident
from the dissolution observed. All the isolated complexes showed
1:1 (nickel oxalate: Lewis-base) composition except for py adduct
on elemental analysis (Table 7.5). The py adducts isolated in this
system are seen to have 1:2 composition.
Chapter 7 324
Table 7.4 Interaction of metal oxalates with various Lewis-bases (LB)
Metal oxalate +
LB (in MeOH)
Conditions Observation Products Separated
40 + en (1:1) stirring, RT, (2h) no observable change no stoichiometric
compound
40 + en (1:2) stirring, RT, (2h)
slight colour change from green to rose-violet
no characterisable product
40 + en (1:3) stirring, RT, (2h)
colour change from green to rose violet (solution colourless)
[Ni(en)ox]n (43)
40 + bipy (1:1)
stirring under reflux, (2h) no observable change no stoichiometric
compound
40 + bipy (1:2)
stirring under reflux, (2h)
slight colour change from green to light violet
no stoichiometric compound
40 + bipy (1:3)
stirring under reflux, (2h)
colour change from green to light violet (solution colourless)
[Ni(bipy)ox]n (44)
40 + phen (1:1)
stirring under reflux, (2h) no observable change no characterisable
product
40 + phen (1:2)
stirring under reflux, (2h)
slight colour change from green to light violet. (incomplete reaction)
no stoichiometric compound
40 + phen (1:3)
stirring under reflux, (2h)
colour change from green to light violet (solution colourless)
[(Ni(phen) (ox)).2H2O]n (45)
40 + py (1:1)
stirring under reflux, (2h) no observable change no characterisable
product
40 + py (1:2)
stirring under reflux, (2h)
slight colour change from green to light green
no stoichiometric compound
40 + py (excess)
stirring under reflux, (2h)
colour change from green to light green (solution colourless)
[Ni(py)2ox]n (46)
Interaction of Lewis-bases with Polymeric Metal Oxalates 325
40 + pn
(1:1) stirring, RT, (2h) no observable change no characterisable product
40 + pn (1:2) stirring, RT, (2h) partial dissolution
(blue solution) no characterisable product
40 + pn (1:3) stirring, RT, (2h)
complete dissolution followed by blue solid separation
[(Ni(pn)(ox)).H2O]n (47)
41 + en (1:1) stirring, RT, (2h) no observable change no characterisable
product
41 + en (1:2) stirring, RT, (2h) slight colour change
from rose to brown no stoichiometric compound
41 + en (1:3) stirring, RT, (2h)
colour change from green to brown (solution colourless)
[(Co(en)(ox)).H2O]n (48)
41 + bipy (1:1)
stirring under reflux, (2h) no observable change no characterisable
product
41 + bipy (1:2)
stirring under reflux, (2h)
slight colour change from rose to orange
no stoichiometric compound
41 + bipy (1:3)
stirring under reflux, (2h)
colour change from rose to orange (solution colourless)
[Co(bipy)ox]n (49)
41 + phen (1:1)
stirring under reflux, (2h) no observable change no characterisable
product
41 + phen (1:2)
stirring under reflux, (2h)
slight colour change from rose to orange
no stoichiometric compound
41 + phen (1:3)
stirring under reflux, (2h)
colour change from rose to orange (solution colourless)
[(Co(phen)(ox)).H2O]n
(50)
41 + py (1:1)
stirring under reflux, (2h) no observable change no characterisable
product
41+ py (1:2)
stirring under reflux, (1h)
light colour change from rose to pink
no stoichiometric compound
41 + py (excess)
stirring under reflux, (2h)
colour change from rose to light pink (solution colour less)
[Co(py)2(ox)]n (51)
41 + pn (1:1) stirring, RT, (2h) no observable change no characterisable
product
41 + pn (1:2) stirring, RT, (2h) slight colour change
from rose to pink no stoichiometric compound
41 + pn (1:3) stirring, RT, (2h)
colour change from rose to dark pink (solution colour less)
[Co(pn)(ox)]n (52)
Chapter 7 326
42 + en (1:1) stirring, RT, (2h) no observable change no stoichiometric
compound
42 + en (1:2) stirring, RT, (2h) slight colour change
from blue to violet no characterisable product
42 + en (1:3) stirring, RT, (1h)
colour change from blue to violet (solution colour less)
[(Cu(en)(ox)).2H2O]n (53)
42 + bipy (1:1)
stirring under reflux, (2h)
no observable change
no stoichiometric compound
42 + bipy (1:2)
stirring under reflux, (2h)
slight colour change from blue to light blue
no characterisable product
42 + bipy (1:3)
stirring under reflux, (2h)
colour change from blue to light blue (solution colour less)
[Cu(bipy)(ox)]n (54)
42 + phen (1:1)
stirring under reflux, (2h)
no observable change
no stoichiometric compound
42 + phen (1:2)
stirring under reflux, (2h)
slight colour change from blue to light blue
no characterisable product
42 + phen (1:3)
stirring under reflux, (2h)
colour change from blue to light blue (solution colour less)
[Cu(phen)(ox)]n (55)
42 + py (1:1)
stirring under reflux, (2h)
no observable change
no stoichiometric compound
42 + py (1:2)
stirring under reflux, (2h)
slight colour change from blue to light blue
no characterisable product
42 + py (excess)
stirring under reflux, (2h)
colour change from blue to light blue (solution colour less)
[Cu(py)2(ox)]n (56)
42 + pn (1:1)
stirring, RT, (2h)
no observable change
no stoichiometric compound
42 + pn (1:2)
stirring, RT, (2h)
slight colour change from blue to light blue
no characterisable product
42 + pn (1:3)
stirring, RT, (2h)
complete dissolution followed by light blue solid separation
[Cu(pn)(ox)]n (57)
Interaction of Lewis-bases with Polymeric Metal Oxalates 327
Table 7.5 Elemental analytical data of Lewis-base adducts of metal(II) oxalates
Elemental content % obsvd (calcd) Complex
(Emp. formula) Formula Weight
C H N M
Colour
[Ni(en)(ox)]n
NiC4H8N2O4 (43)
206.69 23.20 (23.22)
3.92 (3.90)
13.55 (13.54)
28.4 (28.3)
Rose violet
[Ni(bipy)(ox)]n
NiC12H8N2O4 (44)
302.88 47.57 (47.54)
2.70 (2.66)
9.20 (9,24)
19.4 (19.3)
Light violet
[(Ni(phen)(ox)).2H2O]nNiC14H14N2O7
(45) 380.92 44.15
(44.10) 3.72 (3.70)
7.34 (7.35)
15.5 (15.4)
Light violet
[Ni(py)2(ox)]n
NiC12H10N2O4
(46)
304.89
47.20 (47.23)
3.28 (3.30)
9.20 (9.18)
19.3 (19.2)
Light green
[(Ni(pn)(ox)).H2O]n
NiC5H12N2O5
(47) 238.82 25.10
(25.12) 5.10 (5.06)
11.70 (11.72)
24.6 (24.5) Blue
[(Co(en)(ox)).H2O]n
CoC4H10N2O5
(48) 224.93 21.30
(21.33) 4.50 (4.48)
12.40 (12.44)
26.2 (26.1) Brown
[Co(bipy)(ox)]n
CoC12H12N2O4
(49) 303.12 47.52
(47.50) 2.68 (2.66)
9.25 (9.23)
19.5 (19.4) Orange
[(Co(phen)(ox)).H2O]n
CoC14H12N2O6
(50) 363.16 46.28
(46.26) 3.35 (3.33)
7.70 (7.71)
16.3 (16.2) Orange
[Co(py)2(ox)]n
CoC12H10N2O4
(51) 305.13 47.20
(47.19) 3.31 (3.30)
9.20 (9.17)
19.2 (19.3)
Light pink
Chapter 7 328
[Co(pn)(ox)]n
CoC5H10N2O4
(52) 221.06 27.18
(27.14) 4.50 (4.55)
12.70 (12.66)
26.7 (26.6)
Dark pink
[(Cu(en)(ox)).2H2O]n
CuC4H12N2O6
(53) 247.54 19.40
(19.39) 4.85 (4.88)
33.15 (33.12)
25.7 (25.6) Violet
[Cu(bipy)(ox)]n
CuC12H8N2O4
(54) 307.73 46.82
(46.79) 2.65 (2.62)
9.12 (9.09)
20.7 (20.6)
Light blue
[Cu(phen)(ox)]n
CuC14H10N2O5
(55) 349.77 48.06
(48.03) 2.90 (2.88)
8.02 (8.00)
18.2 (18.1)
Light blue
[Cu(py)2(ox)]n
CuC12H10N2O4
(56) 309.74 46.51
(46.49) 3.20 (3.25)
9.05 (9.03)
20.6 (20.5)
Light blue
[Cu(pn)(ox)]n
CuC5H10N2O4
(57) 225.67 26.60
(26.58) 4.50 (4.46)
12.42 (12.40)
28.2 (28.1)
Light blue
7.3.3 IR spectra of Lewis–base adducts of nickel(II) oxalate
As compared to the IR spectrum of nickel(II) oxalate, its
Lewis-base adducts gave several additional and modified peaks.
The νas(CO2) band occurring at 1636 cm-1 in [Ni(H2O)2(ox])n (40)
were found shifted to 1547-1621 cm-1 in its adducts and the
νs(CO2) band observed at 1362 cm-1 was found lowered to
1335-1380 cm-1 in its adducts. The individual details are discussed
below and important frequencies are given in Table 7.6.
In the IR spectrum of the adduct [Ni(en)(ox)]n, 43, the
νas(NH2) and νs(NH2) bands were observed at 3283 and 3158 cm-1
respectively. The bending mode (δNH2) was seen at 1620 cm-1. A sharp
band observed at 1023 cm-1 could be assigned to ν(C-N) stretching of
Interaction of Lewis-bases with Polymeric Metal Oxalates 329
en. The νas(CO2) and νs(CO2) bands were observed at 1547 and 1335
cm-1 respectively. The νΔ values suggest the bridging character of -
COO- group of oxalate moiety in it.
Table 7.6 IR spectral data of nickel(II) oxalate and its various Lewis-base adducts (cm-1)
Adducts 40 43 44 45 46 47
ν(O-H) 3250 - - 3390 3450
νas(CO2) 1636 1547 1602 1580 1621 1590
νs(CO2) 1362 1335 1352 1345 1363 1380
νΔ 274 212 250 235 258 210
ν(C-O) 1316 1302 1317 1282 1315 1313
C Cν
- - 1633 1646 1605 -
νC N
- 1475 1421 1480 -
ν(C-N) 1023 - - - 1018
ν(C-H) - 771 725 - -
β(C-H) - 1022 - - -
ν(NH2) 3283 3158 - - - 3310 3280
δ(NH2) 1620 - - 1610
(NH2) wag 751 - - 799
M-N stret 526 - - 470
Ring deformation
(outplane)
(inplane)
-
-
498
653
480
639
445
660
-
-
Chapter 7 330
For the adduct [Ni(bipy)(ox)]n, 44, new peaks were observed
at 771, 1022, 1475 and 1633 cm-1 indicating that bipy is
coordinated to Ni through both of its pyridyl nitrogen. The νas(CO2)
and νs(CO2) bands of oxalate moiety were observed at 1602 and
1352 cm-1. The in-plane and out-of-plane ring deformation modes
of bipy were observed to 653 and 499 cm-1 respectively.
The IR spectrum of the complex [(Ni(phen)(ox)).2H2O]n, 45,
gave new peaks at 725,1421 and 1646 cm-1 indicating the presence
of phen. The bands at 1421 and 1646 cm-1 could be assigned to
the ring skeletal vibration of phen. Its ring deformation modes were
observed at 639 and 480 cm-1. The νas(CO2) and νs(CO2) bands of
oxalate group were seen shifted to 1580 and 1345 cm-1 respectively
and the apperance of a broad band at 3390 cm-1 indicates the
presence of lattice water in it.
For the adduct [Ni(py)2(ox)]n, 46, all the characteristic peaks
of pyridine were observed in addition to the peaks due to parent
oxalate. The ring skeletal bands of pyridine were observed at 1605
and 1480 cm-1. The νas(CO2) and νs(CO2) bands of oxalate moiety
were observed at 1621 and 1363 cm-1 respectively. The ring
deformation modes of pyridine were observed at 660 and 445 cm-1 .
The IR spectrum of the adduct [(Ni(pn)(ox)).H2O]n, 47, gave
peaks at 3313 and 3278 cm-1 which are characteristic of νas(NH2)
and νs(NH2) vibrations of coordinated pn. The δ(NH2) band was
observed at 1610 cm-1 and ν(C-N) band was seen at 1018 cm-1. The
νas(CO2) and νs(CO2) bands of oxalate moiety were observed at 1590
and 1380 cm-1 respectively. The appearance of a broad band at
3450 cm-1 indicates the presence of lattice water in it.
Interaction of Lewis-bases with Polymeric Metal Oxalates 331
As in the case of succinates and malonates we have made
attempts to look at the gradation of νΔ in the nickel(II) oxalate and
its adducts also. While in parent nickel(II) oxalate the νΔ is
274 cm-1, in its adducts the difference is seen to be decreasing
progressively in the order 40 > 46 > 44 > 45 > 43 > 47. This is
consistent with the fact that pn and en are totally σ-donors (with
no π-accepting character) while phen, bipy and py have both σ-
donor and π-accepting nature. The σ-donor ligands would enhance
the electron density on the metal which would in turn make the
metal release more electron to the π* orbitals of the oxalate moiety.
The effect of such back donation would be to decrease the O-C-C-O
bond order in the oxalate which gets reflected in νΔ also. In the
case of phen, bipy and py the Lewis-bases have on their own some
π-acceptor property also and hence will not try to accumulate
much electron density on the metal. Consequently the back
donation to oxalate π* orbital would be less. The trend observed
agrees well with the expectation.
7.3.4 Electronic spectra and magnetic data of Lewis-base adducts of nickel(II) oxalate
The electronic spectra of various adducts of nickel(II) oxalate
isolated were recorded in solid state. Some of the spectra are
reproduced in Fig.7.1. The characteristic special features in the
bands of parent nickel(II) oxalate are seen disappearing and new
set of absorptions are seen emerging in its adducts. The absorption
bands were observed in the region 25910-27620 cm-1, 16260-
18380 cm-1 and 10730-11670 cm-1 in all the adducts. These bands
could be assigned to 3A2g(F) → 3T1g(P) (ν3); 3A2g(F) → 3T1g(F) (ν2) and 3A2g(F) → 3T2g(F) (ν1) respectively in agreement with typical Ni(II)
Chapter 7 332
octahedral complex.(39,40) We have evaluated the ligand-field
parameters Dq, for the various adducts employing the methods as
discussed earlier (Chapter 3). The Dq values among the various
adducts were seen to decrease in the order 43 > 46 > 44 > 45 >
47. The trend is seen to be more or less dependent on the pka
value of the various Lewis-bases. Eventhough the pKa value of pn
is greater than that of en, the Dq value of pn adduct (47) is found
to be less than that of en adduct (43). This may be because of the
relatively less chelating stability of pn as compared to en.
The μeff values evaluated for all the Ni(II) adducts are found
to be in the range 3.01-3.18 BM which are in agreement with the
values expected for octahedral complexes. The electronic
transitions, their assignments and magnetic moment data are given
in Table 7.7.
Interaction of Lewis-bases with Polymeric Metal Oxalates 333
(a) [Ni(H2O)2(ox)]n, 40
(b) [Ni(en)(ox)]n, 43
(c) [Ni(bipy)(ox)]n, 44
(d) [(Ni(phen)(ox)).2H2O]n, 45
(e) [Ni(py)2(ox)]n, 46
(f) [(Ni(pn)(ox)).H2O]n, 47
Fig. 7.1 Electronic spectra of Lewis-base adducts of nickel(II) oxalate
Chapter 7 334
Table 7.7 Electronic spectral and magnetic moment data of Lewis-base adducts of nickel(II) oxalate
Complex Absor. max ν (cm-1)
Assignments*
Dq (cm-1) β B
(cm-1) ν2/ν1μeff
(BM)
40
25310
15600
10650
ν3
ν2
ν1
1065 0.60 597 1.46 2.92
43
27620
18380
11670
ν3
ν2
ν1
1167 0.65 773 1.57 3.07
44
26110
17850
10950
ν3
ν2
ν1
1095 0.63 741 1.63 3.15
45
25910
17950
10800
ν3
ν2
ν1
1080 0.64 764 1.66 3.18
46
26250
16260
11130
ν3
ν2
ν1
1113 0.61 608 1.47 3.01
47
26040
16610
10730
ν3
ν2
ν1
1073 0.66 697 1.55 3.05
*3A2g(F) → 3T2g(F) (ν1); 3A2g(F) → 3T1g(F) (ν2); 3A2g → 3T1g(P) (ν3)
Interaction of Lewis-bases with Polymeric Metal Oxalates 335
7.3.5 Interaction of Lewis-bases with polymeric cobalt(II) oxalates
Like nickel(II) oxalate, cobalt(II) oxalate also can be expected to
have an extended polymeric structure as mentioned in section 7.3.1
based on its spectral properties. The set of Lewis-bases chosen for the
adduct formation with the cobalt(II) oxalate were same as those used
for nickel(II) oxalate. Interaction with various Lewis-bases was carried
out in different reaction conditions as before. The details are presented
in Table 7.4. We could not find any indication of depolymerisation
being initiated as none of the Lewis-bases tend to bring the metal
oxalate in solution. The analytical data shown in Table. 7.5 confirm the
1:1 composition (cobalt oxalate : Lewis-base) for all the adducts except
with pyridine. The pyridine adduct, however, has 1:2 composition.
7.3.6 IR spectra of Lewis-base adducts of cobalt(II) oxalate
The presence of Lewis-bases in all the adducts could be
confirmed by their characteristic peaks in the IR spectra. The peaks
due to oxalate moiety were also present with some shift in their
position as compared to the values in the parent metal oxalates. The
asymmetric stretching νas(CO2) band observed at 1635 cm-1 in parent
cobalt(II) oxalate was found shifted to 1600-1622 cm-1 in its adducts.
Similarly the νs(CO2) band observed at 1370 cm-1 was found shifted
to 1350-1388 cm-1 in them. The spectra of individual adducts are
discussed below in detail.
In the IR spectrum of [(Co(en)(ox)).H2O]n, 48, the νas(NH2) and
νs(NH2) band were seen at 3340 and 3204 cm-1 showing the
coordination of en. The νas(CO2) and νs(CO2) bands of the oxalate
group are observed in the adduct at 1600 and 1365 cm-1 respectively.
The δ(NH2) and ν(C-N) bands of en were seen at 1630 and 1061 cm-1
Chapter 7 336
respectively and the appearance of a broad band at 3440 cm-1
indicates the presence of lattice water in it. In the case of
[Co(bipy)(ox)]n adduct, 49, new peaks were observed at 771, 1013,
1455 and 1674 cm-1 indicating that bipy is coordinated to Co through
both of its pyridyl Ns. The C-H out-of-plane skeletal vibration of bipy
(νC-H) at 771 cm-1 gets split into 798 and 737 cm-1 in the adduct
indicating the bidentate nature of 2,2'-bipyridyl. The ring deformation
modes of bipy moiety were seen at 660 and 488 cm-1 in the adduct.
The peaks at 1607 and 1355 cm-1 could be assigned to the
asymmetric and symmetric stretching bands of the –COO- group of
oxalate moiety in the adduct.
The spectrum of [(Co(phen)(ox)).H2O]n, 50, showed new bands
at 730, 1425 and 1640 cm-1 showing the chelating nature of phen.
The νas(CO2) and νs(CO2) bands of the oxalate group were seen at 1605
and 1350 cm-1 respectively. The ring deformation modes of phen
moiety appear at 643 and 428 cm-1 respectively. The presence of
lattice water is confirmed by the appearance of a broad band at 3407
cm-1. In the case of [Co(py)2(ox)]n adduct, 51, new peaks were
observed at 1610 and 1478 cm-1 due to ν(C-C) and ν(C-N) ring
stretching skeletal vibrations of pyridine. The νas(CO2) and νs(CO2)
peaks of oxalate group were seen shifted to 1622 and 1361 cm-1
respectively. The in-plane and out-of-plane ring deformation modes of
pyridine moiety were observed at 632 and 492 cm-1.
In the pn adduct, [Co(pn)(ox)]n, 52, peaks at 3360 and
3240 cm-1 characteristic of NH2 stretching of coordinated pn are seen.
The bands at 1654 and 1048 cm-1 could be assigned as δ(NH2) and
ν(C-N) vibrations of pn. The νas(CO2) and νs(CO2) bands of –COO¯
group of oxalate moiety were observed at 1601 and 1388 cm-1
Interaction of Lewis-bases with Polymeric Metal Oxalates 337
respectively. All the other important IR absorption frequencies of
cobalt(II) oxalate and its Lewis-base adducts are given in Table 7.9.
The νΔ found for the adducts are in the order
41 > 51 > 50 > 49 > 48 > 52 which is in the expected order when
we consider the σ-donor ability of pn and en and σ-donor and
π-acceptor property of phen, bipy and py.
Table 7.9 IR spectral data of Lewis-base adducts of cobalt(II) oxalate (cm-1)
Adducts 41 48 49 50 51 52
ν(O-H) 3235 3440 - 3407 - -
νas(CO2) 1635 1600 1607 1605 1622 1601
νs(CO2) 1370 1365 1355 1350 1361 1388
νΔ 265 235 252 255 261 213
ν(C-O) 1321 1317 1310 1312 1315 1304
C Cν
- - 1674 1640 1610 -
νC N
- 1455 1425 1478 -
ν(C-N) 1061 - - - 1048
ν(C-H) - 771 730 - -
β(C-H) - 1013 - - -
ν(NH2) 3290 3204 - - - 3310
3240
δ(NH2) 1630 - - 1654
(NH2) wag 778 - - 767
M-N stret 575 - - 551
Ring deformation
(outplane)
(inplane)
-
-
488
660
481
643
492
632
-
-
Chapter 7 338
7.3.7 Electronic spectra and magnetic data of Lewis-base adducts of cobalt(II) oxalate
The electronic spectra of all the adducts of cobalt(II) oxalate
were recorded in solid-state because of their insolubility. Some of
the spectra are reproduced in Fig. 7.2. All the adducts show the
set of bands in the region 15480-15870 and 19530-21550 cm-1.
These are very characteristic of octahedral Co(II) and could be
assigned to 4T1g→4A2g and 4T1g→4T1g(P) transitions respectively.(41,42)
The data are tabulated in Table 7.10.
Table 7.10 Electronic spectral and magnetic data of Lewis-base adducts of cobalt(II) oxalate
Adducts Absor. max ν (cm-1) Assignments
μeff
(BM)
41 18450
14490
4T1g → 4T1g(P)
4T1g → 4A2g
4.57
48 21550
15870
4T1g → 4T1g(P)
4T1g → 4A2g
4.71
49 21010
15580
4T1g → 4T1g(P)
4T1g → 4A2g
4.74
50 21270
15530
4T1g → 4T1g(P)
4T1g → 4A2g
4.77
51 19530
15480
4T1g → 4T1g(P)
4T1g → 4A2g
4.65
52 21320
15600
4T1g → 4T1g(P)
4T1g → 4A2g
4.69
Interaction of Lewis-bases with Polymeric Metal Oxalates 339
Fig.7.2 . Electronic spectra of cobalt(II) oxalate and its Lewis-base adducts
(a) [Co(H2O)2(ox)]n, 41 (d) [(Co(phen)(ox)).H2O]n, 50
(b) [(Co(en)(ox)).H2O]n, 48 (e) [Co(py)2(ox)]n, 51
(c) [Co(bipy)(ox)]n, 49 (f) [Co(pn)(ox)]n, 52
Chapter 7 340
The room temperature magnetic susceptibility measurements
were carried out and μeff values evaluated for all the adducts. These
were found to be in the range 4.65-4.77 BM. No definite trend in μeff
values could be seen among these adducts. The high spin rather
than low spin state manifested in the μeff values for all the Lewis-base
adducts indicate the weak ligation property of the Lewis-bases with
the parent cobalt(II) oxalate.
7.3.8 Interaction of Lewis-bases with copper(II) oxalate
Just as in the case of nickel(II) and cobalt(II) oxalates, adduct
formation studies were investigated with polymeric copper(II)oxalate
also with the same set of ligands. The interaction was studied in
various reaction conditions and also by changing the stoichiometric
ratio of the parent oxalate and Lewis-bases. These along with some
salient features of the reactions and products isolated are presented
in Table 7.4. The analytical data shown in Table 7.5 confirm the 1:1
composition (metal oxalate: Lewis-base) for all the Lewis-base adducts
isolated except that with pyridine.
7.3.9 IR spectra of Lewis-base adducts of copper(II) oxalate
Like in the case of nickel(II) and cobalt(II) oxalate, copper(II)
oxalate and its Lewis-base adducts also show characteristic peaks in
their infrared spectra. The νas(CO2) observed at 1649 cm-1 in parent
copper(II) oxalate was found shifted to 1580-1624 cm-1 in its adducts
and the νs(CO2) band observed at 1364 cm-1 to 1345-1390 cm-1 in
them. The individual details are discussed below.
In the IR spectrum of [(Cu(en)(ox)).2H2O]n, 53, new bands were
observed at 3306 and 3219 cm-1 which could be assigned as νas(NH2) and
νs(NH2) vibrations respectively. The δ(NH2) band was seen at 1590 cm-1
and the ν(C-N) band was observed at 1044 cm-1. The νas(CO2) and
Interaction of Lewis-bases with Polymeric Metal Oxalates 341
νs(CO2) bands of –COO¯ group of oxalate moiety were observed at 1596
and 1390 cm-1 respectively. The presence of lattice water is indicated
by the appearance of a broad band at 3390 cm-1.
For the adduct [Cu(bipy)(ox)]n, 54, new bands were observed
at 775, 1019, 1472 and 1661 cm-1 indicating that bipy is coordinated
to Cu through both of its pyridyl N. The C-H out-of plane skeletal
vibration of bipy (νC-H) at 775 cm-1 gets split into 782 and 731 cm-1 in
the adduct. This reflects the bidentate nature of 2,2'-bipyridyl.
Moreover νas(CO2) and νs(CO2) bands of oxalate moiety were seen
shifted to 1597 and 1360 cm-1. The ring deformation modes of bipy
appear at 662 and 486 cm-1 respectively.
In the IR spectrum of [Cu(phen)(ox)]n, 55, new peaks were seen
at 723, 1427 and 1663 cm-1 showing that phen is coordinated through
both of its pyridyl N. The νas(CO2) and νs(CO2) bands of oxalate moiety
were observed at 1603 and 1345 cm-1. The ring deformation modes of
phen moiety appear at 646 and 482 cm-1 respectively.
For the adduct [Cu(py)2(ox)]n, 56, new bands are seen at 1600
and 1487 cm-1 which could be assigned to ν(C-C) and ν(C-N) ring
stretching skeletal bands of pyridine. The peaks at 1624 and 1362
cm-1 are due to νas(CO2) and νs(CO2) modes of –COO¯ group of oxalate
moiety. The ring deformation modes of pyridine moiety were observed
at 687 and 443 cm-1 respectively.
In the IR spectrum of the adduct [Cu(pn)(ox)]n, 57, the νas(NH2)
and νs(NH2) vibrations of coordinated pn were observed at 3290 and
3250 cm-1. The νas(CO2) and νs(CO2) bands of oxalate moiety were
observed at 1580 and 1378 cm-1 respectively. All other relevant IR
Chapter 7 342
absorption frequencies of copper(II) oxalate and its Lewis-base
adducts are shown in Table 7.11.
In the system also we have monitored the trend in νΔ among
the various adducts. The order was found to be 42 > 56 > 55 > 54 >
53 > 57.
Table 7.11 IR spectral data of Lewis-base adducts of copper(II) oxalate (cm-1)
Adducts 42 53 54 55 56 57
ν(O-H) - 3390 - - - -
νas(CO2) 1649 1596 1597 1603 1624 1580
νs(CO2) 1364 1390 1360 1345 1362 1378
νΔ 285 206 237 258 262 202
ν(C-O) 1319 1310 1294 1298 1316 1269
C Cν
- - 1661 1663 1600 -
νC N
- 1472 1427 1487 -
ν(C-N) - 1044 - - - 1038
ν(C-H) - 775 723 - -
β(C-H) - 1019 - - -
ν(NH2) 3306
3219 - - -
3290
3250
δ(NH2) 1590 - - 1626
(NH2) wag 714 - - 785
(M-N)stret 525 - - 498
Ring deformation
(outplane)
(inplane)
-
-
486
662
482
646
443
686
-
-
Interaction of Lewis-bases with Polymeric Metal Oxalates 343
7.3.10 Electronic spectra and magnetic data of Lewis-base
adducts of copper(II) oxalate
The electronic spectra of various adducts of copper(II)
oxalate were recorded in solid state. Some of these are reproduced
in Fig.7.3. The data are presented in Table 7.12. The authenticity
of all the peaks was confirmed by recording the spectrum by
repeated sample preparation.
In all the adducts generated in the present study a broad
band occur in the region 14180-18650 cm-1. This is indicative of
tetragonal configuration around copper(II) ion.(46,47) Moreover both
n→π* and π→π* bands were found to be blue shifted and appearing
in the region 3330-33670 cm-1 and 40160-41150 cm-1 respectively,
compared to that of the parent complex. Further, the electronic
spectra of all the complexes exhibit an intense absorption band in
the region 26450-28980 cm-1 which could be assigned due to
charge transfer transitions.
Chapter 7 344
Table 7.12 Electronic spectral data and magnetic moments of Lewis-base adducts of copper(II) oxalate
Adducts Absorption max
ν (cm-1) Assignments
μeff
(BM)
42
40000 33220 26040 13850
π→π*
n→π*
Charge Transfer 2B1g → 2A1g
1.43
53
40160 33330 28980 18650
π→π*
n→π*
Charge Transfer 2B1g → 2A1g
1.62
54
40480 33670 28090 14490
π→π*
n→π*
Charge Transfer 2B1g → 2A1g
1.67
55
41150 33440 27700 14180
π→π*
n→π*
Charge Transfer 2B1g → 2A1g
1.68
56
40320 33550 26450 15190
π→π*
n→π*
Charge Transfer 2B1g → 2A1g
1.57
57
40810 33330 28190 15870
π→π*
n→π*
Charge Transfer 2B1g → 2A1g
1.59
Interaction of Lewis-bases with Polymeric Metal Oxalates 345
(a) [Cu(ox)]n, 42
(b) [(Cu(en)(ox)).2H2O]n, 53
(c) [Cu(bipy)(ox)]n, 54
(d) [Cu(phen)(ox)]n, 55
(e) [Cu(py)2(ox)]n, 56
(f) [Cu(pn)(ox)]n, 57
Fig.7.3 Electronic spectra of Lewis-base adducts of copper(II) oxalate.
The magnetic moment values of the present copper(II)
complexes vary in the range 1.57-1.68 BM (Table 7.12). The
comparatively lower magnetic moment values seen for all these
polymeric adducts indicates the possibility of structures with close
Cu-Cu proximity.(48)
Chapter 7 346
7.3.11 EPR spectra of Lewis-base adducts of copper(II) oxalate
Compared to the adducts of copper(II) succinate and
copper(II) malonate, the adducts of copper(II) oxalate are
insoluble in most of the solvents. Their polymeric nature is also
evident from the electronic and IR spectral data. Owing to this, the
EPR spectra of this compounds could be recorded in solid state
only at room temperature. Fig 7.4 gives the spectra of en adduct
53, bipy adduct 54, py adduct 56 and pn adduct 57. Eventhough
the spectra were not well resolved, their features appear almost the
same in all the cases. Comparing with some of the known spectra,
the g⎜⎜ and g⊥ values for this adducts could be evaluated. These are
given in Table 7.13. The trend g⎜⎜>g⊥>2.0023 seen in all the adducts
indicate dx2-y2 ground state in them.(49) The parameters giso and G
could be calculated for the adducts by using the equations
mentioned earlier. The axial symmetry parameter, G, which
indicate the nature of exchange interaction between the copper
centres for the adducts has value below 4.(50) Significant exchange
interaction can therefore be expected from all these adducts. The
nature of the EPR spectra of the pn adduct 57 and its G value
(1.60) indicate that the structural features of 57 could be much
different from those of others.
Interaction of Lewis-bases with Polymeric Metal Oxalates 347
(a) [(Cu(en)(ox)).2H2O]n, 53
(b) [Cu(bipy)(ox)]n, 54
(c) [Cu(py)2(ox)]n, 56
(d) [Cu(pn)(ox)]n, 57
Fig. 7.5 The EPR spectra of the polymeric adducts of copper(II) oxalate.
Table 7.13 The various EPR parameters of the polymeric adducts of copper(II) oxalate
Adducts g⎜⎜ g⊥ giso G
53 2.18 2.06 2.10 3.00
54 2.21 2.08 2.12 2.63
56 2.16 2.07 2.10 2.29
57 2.14 2.09 2.11 1.60
Chapter 7 348
All the analytical, spectral (IR, electronic, EPR), and
magnetic moment data of the adducts of metal(II) oxalate clearly
indicate that all of them have a polymeric octahedral structure with
bis-bidentate chelating oxalate functions. The high insoluble nature
of all the adducts in various organic solvents also gave a clear
evidence for their polymeric structure.
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