Indian Journal of Chemistry Vol. 42A, February 20m. pp. 255-261

A novel magnetic exchange mechanism in bridged copperCII) coordinations of naturally occurring spin carriers viz. ortho-functionalized p-naphthoquinones

Sandhya Rane*. SUl1ila Gawali & Subhash Padhye

Department of Chemistry. University of Pune. Pune 411 007. India

and

Sadgopal Dale & Pramod Babre

Physical Chemistry Division. National Chemical Laboratory. Pune 411 OOX. India

Received 30 October 2001: revised 13 SeptemiJer 2002

Magnetic exchange and charge transfer mechanisms in dimeric (1) [Cu(LwhH~Ob and monomeric (2). ICu(phth)!1 complexes of lawsone (2-hydroxy-IA-naphthoquinone) and phthiocol (3-methyl-2-hydroxy lA-naphthoquinone) are described. Magnetic susceptibility measurements of ( 1) are fitted to Heisenberg 's isotropic spin pair (S' = 1.1) model and tetranuclear (S' = Y2. '/2• Y2. '/2 ) spins in a two dimensional model. Both models result in ililtradimer antiferromagnetic coupling (.I = -44.4 cm- I

) bctweenICu(NSQ)b units and ferrom;lgnetic coupling (zJ , = 2X.5 cm- I) between one Cu(ll)NSQ

coordinated unit (where NSQ = naphtho-semiquinone). Latter inter:lction leads to a triplet ground state and triplet-triplet intradimer interaction governs the " quintet state" at

room temperature. as detected from X- :lnd Q-band EPR studies. The zero- field split interaction (0.024 cm- I) exhibits

contributions from Dpd (-0.071 cm - I) and combined Do and DOnli (+0.095 cm -I) interactions. The i ntradi mer exchange is due

to a stacked (S' = 1. 1) spin pair with r = 3.04 A. The monomeric complex (2) is devoid of such interactions due to the bulky methyl group at C3 position of quinonoid ring.

Cyclic voltammograms of (1) and (2) show an anodic peak at +0.22 V. assigned to Cu(O) -? Cu(lI) charge transfer.

Only one redox couple at EIi~ = -O.6X7 V due to NSQ (=} CAT (catechol) charge transfer leads to radical coordinatioll in (1) .

However qll<1sireversible NQ (=}NSQ redox couple at EIi~ = -0.525 V together with an irreversible NSQ -? CAT reduction peak in (2) at EII~ = -0.75 V indicate fully the presence of oxidized (NQ) i.e. naphthoquinone ligation in (2).

The electron transfer mechanism associated with copper-quinone coordination is or specific interest to the scientific comlllunity due to oxidative transformation in organic synthesi sl.

2, dioxygen

activation.' and charge distributions in redox isomers catechol/semiqu i none/qu i nones4

.5

. Considerable attention has been devoted to ortho-benzo semi­quinone complexes because they are model intermediates of oxidase and oxygenase enzyme6

.7

.

Although due to the redox abilitl of such complexes [with radical (SQ) coordination 1. meagre reports are fOllnd in the literature for detailing such magnetic exchanges in benzoquinone complexes')'; I .

This report is aimed with reactiviry of redox active o-functional ized-p-naphthoqui none I igand, VIZ.

lawsone (2-hydroxy-I,4-naphthoquinone) and phthiocol (3-methyl-2-hydroxy-I,4-naphthoquinone) on their copper(I I) complexes. Formerly, in 1990 detailing of magnetic exchange interactions in iron(lI) cOlllplexes of lawsone and amino lawsone have been reported from our laboratory' 2". Recently in 2000, ferro/antiferro interactions in Mn2(II. III) dimer of

derivatized lawsone have been reported by usl2b. In the present report. the characteristic excited quintet state at room temperature has been established as result of the triplet-triplet interactions l 1 between the ground state of two [Cu(NSQ)h units. The NSQ chelation profile in the dimeric lawsone complex 0), [Cu(NQ)(NSQ)H:!Oh, is compared with NQ (naphthoquinone) ligation in the monomeric phthiocol complex (2) [Cu(NQh] .

The magnetic susceptibility and ErR measurements exploit ferromagnetic exchange within Cu(NSQ) unit of (1) together with intradi Iller anti ferromagnetic exchange between [Cu(NSQ)h units. The columnar

k· d 11-IS t' . d b stac II1g ten ency ' . 0 two untts separate y 3.046 A (calculated from Coffman and Buettner equation4R

) in (1) lead to dipolar interaction and intradimer coupling, however bulky methyl substituent on the quinonoid ring does not permit such interactions in (2};---get-atJ.efr- ·rnagfletic exchange interactions and the electrochemical reaction mechanism is discussed below.

256 INDIAN J CHEM. SEC A, FEI3RUARY 2003

Materials and Methods Complexes (1) and (2) were synthesized at pH 6

using 10% sodium acetate solution, according to literature procedure I6

.17

.

Variable temperature magnetic susceptibility data for the complexes were measured using a Cahn 1000 Faraday electro balance. The absolute accuracy of the temperature measurements was ±0.2 K. The apparatus was cal i brated by mcasu n ng the magnetic susceptibi I ity of mercuric tetrathiocyanato-cobaltate(II)'R. Diamagnetic corrections were applied

. P I' I')O() P I II' d uSing asca s constants '-. 0 ycrysta me power EPR spectra for (I) and (2) were recorded with a Varian V-4502-12 spectrometer working at X- and Q­band frequencies. The field was calibrated with DPPH marker.

Cyclic voltammetry (CV), measurements were performed on a BAS- CV-27 assembly in conjunction with an X-Y recorder using the three electrode systems with direct correction of the uncompensated cell resistance (IR drop). The three electrode cell includes a working electrode as a platinum inlay or glassy carbon (area 0.23 mm2

) referenced with an Ag/ AgCI electrode and with platinum wire as an auxiliary electrode. The glassy carbon working electrodes were activated according to procedure by Thorp ef a/.

21 The solvent methanol used in CV studies were always de-gassed for 10 min before the experiment22

, 0.1 M tetraethyl ammonium perchlorate (TEA P) was used as supporti ng electrolyte. Before each CV run, the solution in cell was carefully deareatcd with purified N2.

Results and Discussion Compositional studies of (1) and (2) were reported

earlier '(,·17.

The temperature dependence of the mverse magnetic susceptibility of (1) in the 297 K - 75.5 K range and of (2) in the 300 K - 5.9 K range are shown in Fig. I (a) and I (c). These plots of (1) follows Curie­Weiss law with e = -15 K. Complex (2) follows Curie law. Variations of fln .M with temperature are shown in Fig. I (b) and 1 (d), for (1) and (2) respectively. For (1) flBM varies from 3.1 to 2.6 B.M., but remains almost constant at -1.73 B.M. for (2).

Failure of the usual Bleaney Bowers treatment to fit data for (l) leads to an unusual type of spi n transfer in (I). Using e = -15 K and g = 2.17 (obtained from ErR mC<fS'l1f'ements) the Heisenberg 's isotropic spin pa ir (S' = 1,1) model 21 (Table I) provides best fits for functions only when the variation of J (as

220 (al (b)

r :: :l:

.f.l00 ~

I 60

20 e=-15K

50 100 150 200250 300 0 50 100 150 200 250300 --T(K)- -- T(K)-

(d)

i 25 al 15 ....,&.---~o ..,. • ....JO:L.

~

I 05 '---'----I._.l.-....J o 100 200 300400

-T(K)- -T(K)--

Fig. I-(a) Variation of IIX", versus temperallire for

[Cu(Lwh(H ,O)b complex; (h) V<lriation of JlB M versus

temperature for ICu(LwHH,O)h complex; (c) Variation of I /X", versus temperallire for [Cu(Phth),1 complex; (d) Variation of ~ln .M .

versus temperature for ICu(Phth h l complex .

Table I-(a) Magnetic parameters from: EPR of ( I) and (2) complexes

Compound Temp. (K) Range g il gl G

(1) 77 X 2.39 2.0X 2 .17

30() X 2.40 2.06 2.17

300 Q 2.21 * 2.15* 2.17

(2) 30() X 2.26 2'()6 2.13

The value ohtained as I g 11/2 and L gl /2.

Table I-(b) Exchange coupling parameters from fittings or variable temperallire susceptibility clata or (I) complex

J cm- I Model

J=-43.7 (S'= 1,1)

%.1, = 2X.5 (S' = 1/ 2, 1/ 2• \/2. \(2 )

.I". = -45.9 (S' = \/2. \/2. \/2. \(2)

(I) = [Cu(LwH H20)b (2)= ICu (PhthH

antiferromagnetic coupling constant) from -65.7 to -21. 7 cm- t with temperature is operated [Fig. 2(a)J.

The J value from the above treatment is -43 .7 em-I . An excellent least square fit is obtained with an agreement factor R, 3.03 * 10-7 from the following expression ,

R = I[ (Xm)obs - (Xm) cal12 II (Xm)obs f.

Heisenberg isotropic spin pair (S' = I, I) model-I. originates in the coupling of spins residing in

RANE el (II.: MAGNETIC EXCHANGE MECHANISM IN BRIDGED Cu(ll ) COMPLEXES 257

(b)

160

'E 40 u -..., I 20

\ 0

80

60

1 50

_ ' 40 'E ~ 30 ...,

I

20

120

10

o

(al

50 100 150 200 250300 --'- T(Kl-

160 (el

'E 40 u -..., I

0 \ 20

0 160 200 80 120 160 200

--T(Kl- --T(Kl-

Fi g. 2-Plots of .I versus temperature; (a) Heisenherg ' s isotropic model-I (S' = 1.1 ); (b) Two dimcnsional te tranuc lear mode l-2

with .I = -45 CI11-1 and Z.l, varicd. (c) Two dimensional

telranuc!car l11o(\e l-2 with zJ, = 2R.5 CI11-1 and J varied.

orthogonal magnetic orbitals of the copper(lI) ion and equatorial (NSQ) radical anion . Such ferromagnetic interactions are familiar in nickel(lI)-semiquinone24 or copper(lI) nitroxide25 coordination . As the energy of

n*Pz orbital of the NSQ radical is comparable to that

of non-bonding singly occupied d/-} orbital, for each monomeric set2(" four electrons a~e expected to be parallel in the dimer according to Hund's rule. Magnetic data and infra EPR data suggest that unique 'quintet state' is populated at room temperature which shows anti ferromagnetic exchange (l = -43.7 cm - I) with a triplet ground state at low temperature27.2R.

At a second attempt we tried to fit the Xm data to the

two dimensional tetranuclear model-2 with (S' = Y2, 1 J. I J. 1/. ) . . Th . d . 2<) /2 , 1' 2, /2 Spll1 pair. e equation use IS ,

Ng 2 X2 [F(T)]

Xm = kT - 2zJ I [F(T)]

where

[F(T)] = [6exp(-4J/kT) + 10] . [gexp( -4J/kT) + 2exp( -6J/kT) + 5]

where zJ, is the intercluster exchange term. The best fit was obtained when J was tixed and zJ, was varied and vice versa [Fig. 2(b) and 2(c)]. When J was kept

constant R turns out to be 1.66 * 10-7 and while zJ, is

fixed the R factor becomes 0.498 * 10-7.

Thus Heisenberg's isotropic model is divided into two dimensional exchanges which finally result into the antisymmetric ferromagnetic exchange (zJ,) and symmetric anisotropic anti ferromagnetic exchange

(-1). The best tit values of g, J and zJ, are 2.17, -45.9

cm- I and 28.5 cm- I respectively , which suggest the exact reverse exchange coupled path compared to the

assumed in Heisenberg's (S' = I, I) isotropic model. In the refined two dimensional interaction the intercluster exchange parameter zJ} deals with 'z' the coordination number of the cluster lattice, as suggested by Lines and coworkersJo

•

The best fit can be achieved only when two is substituted for 'z' in the two dimensional equation J 1

,

and hence it leads to an interaction between two copper units, where each monomeric copper unit is

performed by equatorial copper (II)-NSQ interaction

which governs kinetic exchanges (l" . = -45.95 cm- I)

between electron spins residing in non orthogonal

d/ -/ orbital of metal and P, / Py orbital of ligand .

However, each copper(II)-NSQ cluster unit shows

ferromagnetic exchange, zJ, = 28.5 cm - I, between two such units which supports our fonner prediction 16

of magnetic exchanges containing bridged 2-oxido-1,4-naphthoquinone coordination and terminal 4-hydroxy-I,2-naphtho-semiquinonato ligation of lawsone per monomeric unit. The effectiveness of the

anti ferromagnetic exchange between copper(ll)-NSQ units to intercluster ferromagnetic exchange is adjudged from J versus T plots [Fig 2(b) and 2(c)]. Thus the exchange coupled interactions, axially anti ferromagnetic and equatorially ferromagnetic between two monomeric units, may operate via two bridged (NSQ) ligands to two copper(ll) centers with two terminal diamagnetic (NQ) mOieties. The probable molecular magnetic framework for the dimeric complex (1) may be similar to tetrameric acetyl acetone-mono (o-hydroxynil) copper(ll)

p complex·~ .

The infra EPR studies lead to D 4h symmetry created by the ligand around copper(lI) ions in dimer and

governs the Cu-O-Cu pathway for ferromagnetic

exchange be tween two copper(ll)-NSQ units only when the tetragonal aXIs on each copper{H) units are

aligned more parallel to each other and the Cu-O-Cu

258 INDIAN 1 CHEM. SEC A, FEBRUARY 2003

approaches nearly to 97°. Its slight distortion to 97.8° may cross overs ferromagnetic to anti ferro­magnetic33J4 behaviour. Hence we conclude, that by using Heisenberg's isotropic (S' = I, I) model, an ~eraging effect of magnetic exchanges is obtained as

J = -43.7 cm-' . A similar result were obtained from the tetranuclear (S' = '/2, 1/2, Y2, Y2) model for two dimensional interaction, viz.

J = 1/3 zJ, + 2/3 Jxy

J = 1/3 (28.50) + 2/3 (45.95)

J = 40.13 cm- I (comparable with J = -43.7 cm- I as • in Heisenberg's isotropic model) [Table I (b)]. The

resultant magnetic interactions from one dimensional and two dimensional models are similar, which suggest the following equilibrium possibilities for their electronic structures (Structure I)

Here the (S', 1/ 2, Y2, 1/2, Y2) pair system will be generated when NSQ spins in [he terminal ligands will transfer through bonds via the d/-} orbitals of metals to the Pz orbital of the oxyge~ atom in a bridged ligands, so that both bridged ligands convert into NSQ magnetic orbitals, orthogonal to the d/- / magnetic orbitals. This results in a "quintet state" at room temperature and "triplet" ground state at liquid nitrogen temperature due to superexchange electron transfer mechanism, as shown in Structure II.

Two types of temperature dependences are normally observed for superexchangeJ5

. In some cases J increases with temperatureJ6

.37 while in some others it decreases with increasing temperatureJ8

-41

. Hence (1) belongs to former class as far as -J variation with temperature is concerned. Two competing factors are involved in the temperature dependence of the exchange parameter. The contraction of the lattice on cooling the crystal should lead to increased overlap of magnetic orbitals. At the same time the overlap integral is modulated by thermal vibration. Due to reduction in the vibrational amplitude upon lowering the temperature the overlap integral should decrease. Hence we conclude that the anti ferromagnetic and ferromagnetic spin coupling components can be sorted only in the two dimensional model by identi fication of speci fic orbital interactions using the accidental orthogonality behaviour of magnetic orbitals - in (1) as suggested by Reed42

. And, in competition with these two coupling mechanisms anti ferromagnetic coupling dominates as seen in Heisenberg's isotropic (S' = I, I) model compared to

«ro-o

2 - oxido - 1,4 -NQ

And

(I)

HH quint~t ,rrn, ,,' ,

(/I)

4- hydroxy-I,2-NSQ

the two dimensional (S' = 1/ 2 , Y2, Y2, Y2) letranuclear model.

The attempt to grow a single crystal of (1) in coordinating solvent breaks the super exchange pathways43. Normal rCu(II)(NQhJ type of ligations is concluded in (2) from its average and constant magnetic moment (1.73 B.M.). Stereospecificity and inductive effect of CH1 substituent at C3 position in phthiocol leads to innocent coordinations in (2) .

X and Q band powder spectra of (1) at 298 K are presented in Fig. 3, which are typical of axial symmetry44 with elongated tetragonal or square planar distortions (gil> g-L and g-L > 2.04) respectively as shown in Table 1 a. Refined magnetic parameters from these spectra provide clear indication of its field dependence. The characteristics four line feature of Q-band spectrum arises from the two non-equivalent sites occupied by the same paramagnetic species and supports the unique population of "quintet state,,45 at room temperature. The resonance fields due to the ~ms= ± I transition between the quintet energy levels are given by following expressions,

HII (-2 ~ -I) = Ho + 3y + 8

H-L (-I ~ 0) = Ho + Y + £

H-L (0 ~ I) = Ho - Y + £

HII (1 ~ 2) = Ho - 3y + 8

•

RANE ef (II.: MAGNETIC EXCHANGE MECHANISM IN BRIDGED Cu(ll) COMPLEXES 259

I

Scan ,.n9" : 2000G Fi.ld owl : 3300 G Gain: 3.2 MOdulalion amptitud : 0 ·63G T ; m~ constant: 0·3 SQ.C

Scan timq : 4 min Mit rowavQ frczqUfiZ nc)'~ 9-319 GHz Tcrmpcrratun. : RT

2400 2600 2800 3000 3200 3400 3600 3800 4000 4200

Scan '."9'" 2000 G Fiqld •• 1 : 12000 G Gain' 400 Modulation amplitudCl: ZG Timer constanl : ().128sllc Sc6n tim<l : 4 min MicrOW'~vcr frcz:qullncy;a00065.01 TemperalyrlZ: RT

OPPH

9681 10081 10481 10881 11281 11681 12031 12481 12881 13281

Fi g. 3- X and Q-hand polycrystalline powder EI~R spectra of (1) .

where y = DZ7 - 1/ 2 (D" + Dyy) and Ho = hu / g~, is the

first order term and 8 and {:: are the second order terms. The pairs of parallel and perpendicular

resonating fields correspond to the transitions I ± 2 > ~ I ± I > and I ± 1 > ~ 10 > respectively. The experimental zero field split (zfs) can be measured from these separations since they are the first order terms and have simple expressions. The second ordered terms are approximated for axial symmetry . The average zero field , (D) parameter is found out to

be 0 .02457 cm -I (ref. 45). D consists of pseudodipolar contribution which is according to the simple theory put forward by B1eaney & Bowers46 and is estimated by the following equation:

to be 0 .07 Cill -i. It is interesting to note that this

matches with the values observed for the "quintet slale,,47 found in tri-<j>-dicarbene radical molecule

containing S' = 1,1 spin pair rather than the "triplet

stale" of the di-<j>-carbene having S' = 1"2, 1"2 spin pair. Such compartmentalization of zero field splitting parameter finally results in the dipolar contribution

Fig. 4-CY plots of (a) Cu(CI-13COOh.H~O : 1. IClI(Lwh( H ~O)h:

2. IClI(PhthhJ ; (b) Scan rate variation plots.

towards D as 0 .095 cm- I. Estimated J and D

parameters suggest that the magnetic exchange interactions are dominant compared to zfs interaction. As a result for the dinuclear molecule like (1) the spins are essentially Heisenberg in nature and hence the effective spin hamiltonian is operative as23

,

The distance between the two copper-naphthosemiquinone units is calculated as 3.046 A from the Coffman Buettner48 expression of long range magnetic exchanges . Thus above novel spi n transfer mechanism in (1) due to short intradimer distance with 3.046 A units is suggestive of its probable use as catalyst in oxidative processes in copper containing

. 4') protetns .

Electrochemical studies illustrate [Fig. 4(a)] cyclic voltammograms (CY) for methanolic solutions of eu (CH j COOh,H 20 and complexes (1) and (2) respectively . The scan rate dependence of reversible or quasi reversible redox couples are presellted in: Fig. 4(b) for respective complexes. The metal based and ligand based charge transfers of (1) and (2) are

260 . INDIAN J CHEM , SEC A. FEBRUARY 2003

Table 2-Redox reaction of Cu(OAc)].H 20 nnd 0). (2) complexes by cv

(a) Cu(OAch.H~O

Metal hased :

Cu(O) +CU7)V ) Cu(II)+2e-

Cu(O) +II .• ~ .\ )Cu(I)+le -

(b) Lw NQ -<I.~XV ) NSQ -<1.75\'

Ligand based :

NQ - fU 7V) NSQ

) CAT

(NQ concentration is not predominant) -CUi\'

Coupled NSQ <==~ CAT (Ell' = - O.687Y) - 057V -

L'lEp = 250 mY

Metal based:

Cu(O)

ClI(O)

) Cu(ll ) + ClI(lI) + 2c ­

) Cu(l ) + Ic-

(d) Phth: NQ - HAV ) NSQ - i).XV ) CAT

(e) (2) rCu(Phthhl

Meta l based: ClI(O) ) ClI(lI) + 2c-

Li gand based: NQ ) NSQ

L'lEp = 50 mY. E ln = -0.525 Y

(Conceillration of NQ is predominant)

- O.RW

NSQ <==~ CAT (E ll' = - 0.75) - ().()7V -

compared with Cu(CH,COOhH 20 and their respective ligands50

. The redox reactions are summarized in Table 2. In lawsone two redox processes can be reasonably attributed to irreversible

redox sequence such as naphthoquinone NQ ---t NSQ,

and reversible redox sequence as NSQ <=> CAT. The electrochcmical behaviour of (1) is in agreement with effectively ligand centred one electron processes for

NSQ <=> CAT species. Existence of Cu(l\), Cu(l) species is also detected. Here copper ions provide facile electron transfer path for their interactions through NSQ radical species. However, from CV of (2) complex, existence of only Cu(") is detected

which indicates absence of metal-ligand interaction in (2) conipared to (1).

In casc of (2), scan rate variation shows NQ <=> NSQ redox process at -0.55 V has typical features of reversible one electron charge transfer as the ratio

ipc/ipa being equal to one and ratio of ipc/u"2 being constant. Further more the peak to peak separation

approaches the value close to - 60 m V at low scan rate51 . Using same criteria of reversibility in case of

(1), second redox process NSQ <=> CAT (E 1I2 = -0.67 V) is effectively reversible having .6.Ep - 250 mV

corresponding to four electron transfer at 200 Vs- I

scan rate. The four electron process is attributed to presence of four (NSQ) species in solution at a time. It accounts for species generated from two terminal (NQ) reductions in first step and two equatorial (NSQ) species which are already present in (1) [refer

Xm fitting tetranuclear (S' = 1/2, Y2, 1/2, 1/2) model-2] . As far as pure metal based electrochemical reactions in Cu(CH,COOh.H20 for concerned Cu(") and Cu(!) oxidation peaks are observed at +0. 175 V and +0.425 V respectively . Similarly the oxidation peaks for Cu(lI), CuO) are observed at +0.22 V and +0 .52 V respectively in compound (1).

More positive shift in redox potentials of two steps

in (1) NQ ---t NSQ <=> CAT confirms the effective

metaHigand interactions through a-bond mediated by central field of metal ion and it also leads changes in spin-pairing energies on ligation5~. This is in accordance with spin transfer mechanism developed in supra magnetic studies. Very low ipc value in NQ

---t NSQ step compared to NSQ <=> CAT step in (1) is also the result of enormous stabilization of [Cu(NSQ)h species23.24.52.

In case of complex (2) CV shows no shift in reduction potentials (Ere> as compared to (ree ligand leads to conclusion that there is no effective charge transfers between metal and ligand. Finally we conclude from magnetic and electrochemical studies exchange mechanisms of (l) may lead it as a good catalyst in oxidative transformations in bioprocess compared to (2) .

Acknowledgement One of us (SDG) is thankful to the UGC , SYR IS

grateful to the CSIR, New Delhi, India (OI(J686)/OO/EMR-II) for the grant.

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