Indian Journal of ChemistryVol. 19A ,January 1980, pp. 33-36

EPR Studies on Some Copper(Il) Complexes of N ,N-Bis-(z-aminoethy 1)diethylenetriamine

B. A. SASTRY· & M. N. CHARYPhysics Department, Osmania University, Hyderabad 500 007

and

G. PONTICELLI & M. MASSACESIInstituto Chimico PoJicattedra, Universita di Cagliari Via OspedaJe, 09100, Cagliari, Italy

Received 8 March 1979; accepted 25 May 1979

EPR and optical absorption studies have been made on some copper(II) complexes baving tbe generalformula Cu(trenen)X. [trenen = N, N-IJis(2-aminoethyl)diethylenetriamine, X2=CI., Br •• Iz• (N03)z. (CI04)zo(B.,64)2and SO.l in different solvents to study the solute-solvent interactions. metal-ligand bond nature and linewidtbmechanisms. From the value, of 4s contrlbutloa in the 3i-'b grand state of CO,H) it is found that the geometryaround copperflf) in Cu(trcnen) (ClO.). is square planar in pyridine solution. The metal-ligand a-bonding in thecase of Cu(trenen) (CIO.). and Cu(trenen) (Bq>«).is found to be stronger than the corresponding Tt-bonding_ Theisotropic spin-rotational relaxation contribution to the residual linewidth is found to be less indicating the existence ofanisotropic spin-rotational relaxation.

RECENTLY the authors have taken up the EPRand optical absorption studies on polyaminecopper(Il) complexes (i) to study the in-

fluence of 43 character in the ground state of CU(Il)on the spin-Hamiltonian constants and bond para-meters in order to understand the solute-solventinteractions, and (ii) to analyse the hyperfine line-widths with a view to estimating various relaxationmechanisms-". One of the ways of mixing the 4scharacter with the ground state involves interactionbetween the apical ligand orbitals and 43 and 4porbitals of the Cu(II) ion. The presence of 4scharacter in the ground state reduces contact hyper-fine interaction which in turn reduces the hyper-fine line separation. This is also supported by therecent observation that five-coordinate Cu(JI) com-plexes possess low hyperfine separation constants>".The presence of dz2 character in the ground state isalso another reason for 4s mixing with the groundstate and will reduce very much the hyperfine cons-tants. The recent EPR studies on the pseudotetrahe-dral CU(lI) complexes=" support the above fact. It is,thus evident that the estimation of 4s characterin the ground state will throw light on the natureof coordination around Cu(lI) which will be in turnuseful to study the solute- solven t interactions.

Recently some workers10-13estimated the relaxationmechanisms from hyperfine linewidths. However,the linewidth parameters obtained by them werenot accurate due to the fact that they used thelinewidths obtained directly from the experimentalabsorption curve where lot of overlapping of hyper-fine lines was present. In order to obtain the accu-rate linewidth parameters, it is essential to takethis overlapping into consideration.

In the present investigation in order to understand

more about the influence of 43 character in theground state on bond parameters and relaxationmechanisms responsible for the hyperfine linewidth,a series of Cu(II) complexes with generalformula Cu(trenen) X2 [where trenen = N,N-bis-(2-aminoethyl) diethylenetriamine and X2 = C12, Br2•

12, (B4>4)2' (CIO 4)2 and SO4] have been chosen forEPR and optical absorption studies. The penta-dentate ligand N, N-bis (2-aminoethyl) diethylenetri-amine (trenen), CsH23N5' is a branched chainisomer of tetraethylenepentamine (tetren). Withthe help of IR, electronic spectra and molar conduc-tivity data CuttetrenX, have been found to besquare pyramidal formed with five nitrogen atomsof the ligand-", In the series Cu(trenen) X2, onlythe structure of Cuitrenenllsr, has been investigated+and found to be trigonal bipyramidal formed withfive nitrogen atoms of the ligand.

Materials and MethodsThe isomers tetren and trenen are separated care-

fully from the commercially available product. Thecomplexes Cu(trenen)X2 were then prepared usingprocedures similar to that of Ponticelli and Diaz-".The products thus prepared were characterised bychemical analysis. A varian E-4 X-band ESRspectrometer along with E-23l cavity was used torecord the first derivative EPR signals of thesecomplexes in solutions (5 X 1O-3M) both at roomand liquid nitrogen temperatures. The homoge-neity of the magnetic field over the effective samplevolume was ± 30 milligauss. DPPH was used asa field marker. The accuracy in g hyperfine lineseparation and linewidth was about ± 0.001, ± 2gauss and ± 3 gauss respectively. Optical absorp-

33

INDIAN J. CHEM., VOL. 19A, JANUARY 1980

tion spectra in solutions were recorded on a UnicamSP700 spectrophotometer.

Results and DiscussionsThe Cu(trenen)CI2, Curtrerienjlsrj, Cuitrenenjlj,

Cu(trenen) (N03)2 and Cuttrenenj Sfr, complexesare found to dissolve only in water. The complexCu(trenen) (Bsb4)2is soluble only in pyridine whileCu(trenen) (CI04)2 is soluble both in water andpyridine. The EPR spectra of all these complexesin solutions of water and pyridine are found toexhibit four spin-dependent linewidths at roomtemperature. The EPR spectra in aqueous frozen(77K) solutions of all these complexes, except thatof Cu(trenen) (CIOJ2' exhibit a single line indicatingsolute segregatiorr'". Therefore for these complexesin aqueous solution it is not possible to get theprincipal g and A values. The go, hyperfine lineseparation (a) and optical absorption values (6.E)of these complexes in aqueous solution are given inTable 1. The EPR spectrum of Cu(trenen) (CI04)

in frozen aqueous solution exhibits a line havingfive points of inflection similar to that of CuS04·5H20(ref. 17). The low field side of the spectrum is moreintense than the high field side (Fig. 1). From thespectrum the gll and g~ values have been calculatedand found to be 2.025 and 2 .154 respective ly. Inthis case the frozen solution behaves like an un-diluted polycrystalline copper complex. It is notclear whether the polycrystalline like frozen solutioncontains magnetically equivalent or inequivalentions rendering it difficult to decide if the g valuesobtained are molecular or not. Similarspectrum has also been observed in the caseof di-f.4-thiocyanato-bis[di(3-aminopropyl)-amine]di-copper(II) perchlorate"; [Cu(dpt) NCS] Cl04. Thefrozen pyridine solution spectra of Cu(trenen)-(CI04)2 and Cu(trenen) (Bsb4)2complexes are charac-teristic of isolated ion spectra 19 with axial symmetry.

TABLE 1 - ESR AND OPTICAL DATA OF CU(TRENEN)XsINAQUEOUSSOLUTION

Substance g, a /:::.EMHz cm-1

Cu (trenen)Cls 2.112 -187.9 15150

Cuttrenenjlsr,(2.102)*

2.111 -189.2 15150Cuttrenenjl, 2.104 15500Cu(trenen)(NO')2 2.108 -200.0 16660

Cu(trenen) (CI0JI(2.100)*

2.114 -187.2 15150Cu(trenen)SO. 2.102 -243.8 18150

*Values obtained at liquid nitrogen temperature.

The room and liquid nitrogen temperature spectra ofCu(trenen) (CI04)2 in pyridine and the frozenaqueous solution spectrum of this complex are shownin Fig. 1. The values of gll and All are obtainedfollowing the usual methodsw. The values of g~

and A~ are obtained from go [gll i 2g~ ] and

[A 1+ 2A.lJ . I' c .a 3 ' Since comp ete information about

spin-Hamiltonian constants is available for Cu(trenen)(CIO4)2 and Cu(trenen)(Bcf>4)2 in pyridine solu-tions, detailed EPR studies have been made onlyfor .these comple~es. .The gll' All, g~ and A ~ andoptical values, grven III Table 2, indicate that theground state of Cu(II) in these complexes is di-V2,

and the geometry around copper(Il) in these com-plexes is either square planar or square pyramidal ordistorted octahedral but not trigonal bipyramidal asobserved in the case of Cuttrenenjlsr, by X-raystructural analysis. In most of the copper com-plexes, the trigonal bipyramidal geometry in solid

® ®305 K 77K

~~~

, ~DPPH

I I I '.M:: -~-.!..•..!.+.?.

2 2 2 ;:

-H

©77K

DPPH

L--l

100 G

____ H

DPPHFig. 1 - EPR spectra of Cu(trenen) (CIO')2 in (A) pyridineat room temperature (The circles on the experimental curverepresents the absorption values obtained using Eq. (5).(B) frozen (17K) aqueous solution; and (C) frozen (77K)

pyridine solution.

TABLE2-ESR AND OPTICALDATA OF Cu(trenen) (CIOJs AND Cu(trenen) (B"&J. IN PYRIDINE

Complex a All A~ /:::.Ego MHz gll g~ MHz MHz crn=!

Cu(trenen) (CIOJs 2.111 -192.9 2.204 2.065 -472.5 -61.7 15800 ((::.Ex.,)

Cu(trenen) (B'PJ. 2.112 -185.6 2.214 2.061 -451.4 -61.614815 (t::'.Exz)15000 (t:,.Ex.,)15000 (t:,.Ex.)

34

SASTRY et al. : EPR STUDIES ON Cu(II) COMPLEXES

state is found to change to either square pyramidalor distorted octahedral in solution21-23. This maybe due to the fact that in solid state the structureis mostly determined by packing requirements.

The optical absorption in all solutions are foundto be broad and the centre ofthe peak occurs at about15000 cm". Only in the case of Cu(trenen) (CI04)2complex in pyridine solution a weak shoulder isobserved at 14815 em:" (Table 2). Since for mostof the polyamine Cu(II) complexes, the peak at15000 em:" is taken for L.E"y23, we have alsofollowed the same assignment. The weak shoulderin the pyridine for Cuttrenen) (CI04h is assignedas 6.E".. In the spectrum of Cu(trenen) (Bcp4)2in pyridine £::,E"z is taken to be equal to £::,Ex!/.From the values of spin-Hamiltonian constants andoptical absorption data it is evident that them etal-ligand bond is covalent in the pyridine solutions ofCu(trenen) (CI04)2 and Cu(trenen) (Bcp4)2' Takingthe 2s, 2p", 2pll and 2pz orbitals of the four in-planenitrogens into consideration, the metal-ligand anti-bonding molecular orbitals are constructed and theground state of the unpaired electron can then bewritten as in Eq. (l),

B)g = ex.J x2 _y2> - tex.' [- a~l)+ o~2i+ a~3)

.. (1)

where ai = [np! ± (l-n2)1/2sj], i = x, y and zandj = 1,2,3 and 4. The ligand orbitals whichparticipate in a-bonding are taken to be sp2-hybri-dized orbitals. The values of ex.2, ~21 and ~2 whichrepresent o-, in- and out-of plane re-bond coefficientscan be evaluated using the expressions of Maki andMcGarvey 24. These equations can also be used toevaluate isotropic contact term(K), direct dipolarterm and indirect dipolar terms 25. As mentionedearlier it is important to estimate the 4s characterin the ground state to understand solute-solventinteractions. This 4s contribution in the groundstate is not taken into consideration in the MOtreatment of Maki and Mctlarvey>. A term jZwhich represents the fraction of the 3d character inthe 3d-4s ground state has been introduced byKuska et al.26 in the expressions of ex.2 (equations15' and 16' of ref. 26). Following the above ex-pressions we have calculated ex.2, ~L~2.P, K, directdipolar term and indirect dipolar term and are givenin Table 3. It is interesting to note that the value ofp in the case of Cu(trenen)(CI04h is nearly unity,indicating a weak axial bond or a planar geometryaround Cu(lI). Whereas in the case of Cu(trenen)(BrP4)2 the p value is less than that in the above comp-lex indicating stronger axial bond than in the above

complex. When the axial bond is strong, the equi-torial ligands are driven away from the metal ionand hence one can expect an increase in ex.2 (i.e. adecrease in a-bond strength) and gu values and adecrease in All and £::,E,,1I values. This is confirmedby the values of p, a.2, glb All and £::'£."11 of thesecomplexes given in Tables 1 and 2. The in-andout-of-plane 7t-bond strengths in both the complexesare almost the same and are less covalent than thea-bonds.

The hyperfine linewidths of Cu(II) in the presentcomplexes have been analysed using the theory ofWilson and Kivelson 27. Taking the effects ofmotional modulation of anisotropic of g and Atensors on the linewidth into consideration, theyhave developed a theory on hyperfine linewidths indilute paramagnetic solutions where dipolar andexchange interactions are absent. For a hyperfineline having Lorentz shape, the peak to peak widthsare represented by Eq. (2)27

£::,Hpp = a.' + ~M + yM2 + aM3 .. (2)

But the experimental results are not found to fit inthe above equation. The modified equation to fitthe experimental results is as given below :

£::,Hpp = (a.' + a.H) + ~M + yM2 + 8M3 .. (3)

In Eq. (3) a',~, y and a represent linewidth para-meters which can be calculated using the expressionof Wilson and Kivelson'". a.W represents the residuallinewidth which can not be attributed to anisotropiesin molecular magnetic parameters modulated bymolecular reorientation. The spin-rotational relaxa-tion is found to contribute to some extent to theresidual linewidth and isgi ven by Eq. (4) derivedby Wilson and Kivelson 27 on the basis of Hubbardtheory 28.

211 KT ( 2 2 2) 4a.RS = 43go~o . 12'1;r31) L.g

lI+ 6,g 1- .. ( )

This formula is derived only by considering sphericaldiffusional spin-rotational relaxation. As a first stepin the calculations of the linewidth parameters, thetrue linewidths are obtained from the experimentalabsorption curve by fitting it in the Eq. (5)

21+1df(l!l = ~ ~ (6.H~/2) (llo - M'Ja - H)

dH 2/+1 f::l[(l::.Hi/2)2+(Ho-M'Ja-H)2]2

.. (5)

where Mv = v-I - I and other constants havetheir usual meanings". In order to get the 'true'values of the peak-to-peak linewidth from the half-width at half power points, the relation (l::.Hi/2)

TABLE3 - BOND PARAMETERSOBTAINEDIN PYRIDINE SOLUTIONOF Cu(trenen) (CIO.). AND Cu(trenen) (Bc;6.).

Isotropic contact Direct dipolar Indirect dipolarComplex (II 'I ~I ~'1 contribution (KG) contribution contribution

(MHz) (MHz) (MHz)

Cu(trenen) (ClO.). 0.66 0.999 0.92 0.77 -347.2 -150.9 -450.2 +324.3Cu(trenen) (Bcp.). 0.67 ·0.990 0.87 0.77 -348.7 -151.6 -459.9 +357.4

35

INDIAN J. CHEM., VOL. 19A. JANUARY 1980

a! a.' + fl." a.'

TABLE4 - EXPERIMENTALAND THEORETICALVALUESOF LINEWIDTHPARAMETERSIN PYRmINE

"RSSubstance

Exp. Cal. Cal. Exp. Cal.Exp. Cal.Exp.

42.07 54.37 12.30 9.5 9.58 2.50 1.82 -1.6757.44 61.75 4.31 22.75 17.98 4.50 2.97 -3.00

0.020.03

22.7512.00

Cu(trenen) (ClOJaCu(trenen)(Bs6Ja

'OO~-------,-,'\

80 "- " -, -,<,

<,<,

<,

c,c,I

-c 60

40,'----",--.L, ------{l,-2 -2" +"2 +z

M

Fig. 2 - Variation of !::,.H'D'D with nuclear spin quantumDumber (M) observed and calculated in pyridine solutions of(a) Cu(trenen)(Bs6.). and (b) Cu(trenen) (CIO.)a [(a) x-x-xexperimental - - - calculated; (b) @-@ -@- experi mental,

--- calculated]

..f3/2(6Hp;) is substituted in Eq. (5). Substitutingthis 'true' values of linewidths (t~H~) in Equationpp(3), the values of the linewidth parameters areobtained. Now the value of the hydrodynamicalradius in the equations of ~ and y of Wilson andKivelsonst is adjusted in such a way that the cal-culated values of ~ and yare as much near aspossible to the corresponding experimentally ob-tained values. The values of ta' and a are calculatedusing this value of the hydrodynamical radius in theexpressions of rJ.' and a. The value of a" is obtainedby subtracting the value of a' obtained from thesecalculations from the experimentally derived valueof (cx.' + rJ."). The calculated and experimentalvalues, cx.', cx.",~, y, a and rJ.RS are given in Table 4.Using the calculated values of ~, y and a and thevalue (rJ.' + rJ.") obtained from the true linewidths,the linewidths are recalculated. The variation ofthese calculated linewidths and experimental linewid-ths with nuclear spin quantum number(M) are shownin Fig. 2. As is evident from Fig. 2, the agreementbetween theory and the experiment is not as good asit is in the case of vanadyl and copper acetylaceto-nates-". This is due to the fact that the theory holdsgood for complexes having smaller values of I b/wo I[b=2/3 (Ail - Al.) rad/sec and Wo = microwavefrequency] and I 6y/y I [,6y = ~oLlg/1i; 6g =gll-gJ.]. These values for the present complexes arefound to be more compared to the above acetylaceto-nate complexes. In the present studies the signs ofAll and AJ. are taken to be negative. To decidethe M values for the four hyperfine lines (Eq. 6)equa tion is used 27

al[(I -+ 1) 1- M2]go~oH

.. (6)

Fig. 1 shows the M values for different hyperfine lines.In the present studies the values of rJ.RS are found

36

to be very much less compared to the residual line-width. This type of difference between the residuallinewidth and rJ.RS is also recently observed in vanadylacetylacetonate dissolved in different alcohols. Thisdiscrepancy is attributed to non-diffusional aniso-tropic spin-rotational contributions".

Acknow iedgementThe authors are grateful to Prof. K. V. Krishna

Rao, for inspiration and constant encouragementand to Prof. T. Navaneeth Rao for permission torecord the optical absorption spectra.

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