Copper(I), silver(I) and gold(I) solvation in nitriles

Copper(I), silver(I) and gold(I) solvation in nitriles

IONIC COPPER(I), SOLVATION-V. SILVER(I) AND GOLD(I) NITRILES A. LEWANDOWSKI* Institute of Chemistry, Technical (Received University 23 May 19...

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of Chemistry,




23 May 1988;



and J. MALINSKA of Poznan,

in revisedform

PL-60 13 July

965 Poznan,



Abstract-The formal standard potentials of the Cu/Cu’, Cu/Cu’+, Ag/Ag+ and Au/Au+ couples have been measured in acctonitrile, propionitrile and benzonitrile in 0.1 mol drne3 tetraethylammonium perchlorate medium at 25°C. The emfdata were used to estimate the Gibbs free energies of Cu*, Cu2*, Ag+ and Au+ transfer from water to nitriles. Determined transfer energies are negative for Cu+, Ag+ and Au+ and positive for Cu’+. The disproportionation constant K,= [Cu2+]/[Cu+]’ has been calculated for all nitriles studied. The results are discussed

in terms of ionic salvation.



Acetonitrile and propionitrile were dried over and distilled from phosphoruspentoxide. Benzonitrile was purified by preliminary treating with a small amount of Cu(ClO,), to oxidize reducing impurities. The purification was followed by drying and vacuum distillation from phosphorus pentoxide. All distillations have been done with the use of a 50 cm length Vigreux column. Copper perchlorates were prepared by oxidation of copper powder with nitrosyl perchlorate[21] in proper nitriles. Copper(R) perchlorates dissolved in AN, PN or BN and treated with copper powder at 5&7O”C were reduced to copper(I) perchlorates. After filtration and cooling, the precipitated white crystals were filtered off and washed with cold nitriles. AgClO, (Ventron) was dried at 60°C in uacuo. Bis(acetonitrile)gold(I) perchlorate was prepared in a way described by Bergerhoff[22] and used as a source of Au(T) ions in all nitriles studied. The formal ‘standard potentials of the Cu/Cu(I), Aa/Aa(I), Au/Au(I) and Cu(I)/Cu(II) couoles were es~abl&hed by me’asuring potentials of the- half-cells (2) and (3) at varying Cu(I), Ag(1) and Au(I) concentrations or [Cu(I)]/[Cu(II)] ratio, respectively:

Copper, silver and gold, the elements of sub-group I, form cations or compounds in which their oxidation state is + 1, + 2 or + 3. Relative stabilities of these various oxidation states depend upon the element, the solvent and the presence of complexing anions. In aqueous solutions copper and silver form stable Cu*+ and Ag+ cations. The hypothetical diaquagold(1) and tetra-aquagold(II1) cations do not exist. It was, however, possible to obtain their standard potentials from potentials of a series of gold(I) -and gold(II1) COIXIDkXeSr1-b The Conner(I) cation is unstable in water and in-the absence of‘hgands stabilizing mono-. valent state it disproportionates to copper and metallic copper (log K, u 6[2]):


“4”[email protected])

+ Cu(0).

In some organic solvents coordinating via “soft” donor atoms (such as oxygen, nitrogen or sulphur), copper(I) and gold(I) are solvated strongly relative to their higher oxidation states and the monovalent state is quite stable or even completely predominating. In dimethysulphoxide (log K, E 0.2[3,4]) Cu(1) is readily oxidized by air, in contrast to solutions in pyridine (log K, = I 14[5]) and in acetonitrile (log K, < - 2Or611, can be used as an oxidizing _ _,. where co~txr/II) agent[7,8], or easily red&&d, which has been utilized for copper regeneration[9]. The formation of the Cu(1) halide and thiocyanate complexes has been studied in acetonitrile (AN), dimethylsulphoxide (DMSO), pyrldine, tetrahydrothiophene and propylene carbonate[3-6, l&14]. It has been found that gold(I) forms stable solutions in AN and DMSO and the formation of its complexes in these solvents has been investigated[1520]. The general aim of this work was to compare gold(I), silver(I) and copper(I) stabilities in acetonitrile, propionitrlle (PN) and in benzonitrile (BN).



0.1 M, nitrile R or water/, (2)

and: Pt/Cu(I),


Et.+NClO& 0.1 M, nitrile R/, (3)

where M =Cu(Hg), Ag(rod) or Au(foi1) and M(I) = Cu(I), Ag(I) or Au(I), respectively. The half-cells (2) and (3) were connected with the reference half-cell (4): /Et,NClO,

0.1 M, R/Et,NClO,

Ag(1) 0.01 M, R or AN/Ag. 333

0.1 M, (4)





All solutions were deaerated with argon. The temperature was maintained at 25°C with the use of a water circulating thermostat Potentials were measured with a high impedance digital volatmeter (Meratronik, Type V541, Poland) with an accuracy of *O.l mV.

RESULTS The potential


of the half-cell

DISCUSSION (2) is given by:



where Eb (M/Mf)=E,(M/M+)+(2.303RT/F) logy, and E,(M/M ‘) is the standard potential of the M/M+ couple. The activity coefficient y can be estimated from the extended Debye-Huckel equation: logy = - Izl*A&(

1 + aEJi).


At the constant ionic strength medium used, the linear extrapolation of potentials measured at various M(I) concentrations to log[M(I)] =0 is used to give the Eb value (Fig. 1). The potential of the half-cell (3) is related to the Cu(I) and Cu(I1) concentrations by the equation: E(Cu + jcu *+)=Eb(CU+/CUZ+) + (2.303RT/F)


+]/[Cu ‘I),


where Eb(Cu+/Cu~+)=E,(Cu+/Cu~+) +(2.303RT/F)

- 600

1 .


. -2.5

-, -,

calculated -WW’WU)




= CE&W’WI))



Fig. 1. Determination of the .b&(Cu/Cu(I), BN). The potentials were measured vs Ag/Ag(I) 0.01 M, Et,NCIO, 0.1 M, BN/reference. T=25”C. Medium: Et,NCIO, 0.1 M.


&,(CWYWIU)1/2, (8)

and values of the disproportionation (Reaction (1)) from the equation: log K, = [Eo(Cu/Cu(I))

- E,(Cu(I)/Cu(II))/59

constant mV. (9)

Standard electrode potentials and disproportionation constants listed in Table 1 can be used for the estimation of the free energy of the transfer of single ions from water to nitriles, neglecting the liquid junction potential between the solutions in solvents being compared: AG,(M” + , W-+R)=nF[E,(M”+,


The experimental value of the potential at [Cu”]/[Cu+] = 1 corresponds to Eb(Cu+/Cu*+) (Fig. 2). The standard electrode potentials, corrected to zero ionic strength, were obtained using activity coefficients calculated from Equation (6) with a = 4A. Corrections for ion-pair formation were not made as there are no data available for all systems studied here. Furthermore, the literature indicates that for AgCIO, and CuCIO, in AN the ion association is negligible[23]. The standard potential of the CL&U(H) couple was


Fig. 2. Determination of the Eb(Cu(I)/Cu(II), BN). The potentials were measured vs Ag/Ag(I) 0.01 M, Et,NClO, 0.1 M, BN/reference T=25”C. Medium: Et,NClO, 0.1 M.


W)], (10)

and: AG,(Cu+)=0.5{AG,(Cu2+) +2.303RZlog[KD(R)/Ko(W)]}


The estimates of the aqua gold(I) electrode potential range from 1.67 to 2.12V[l]; the value of 1.83V reported by Skibsted and Bjerrum[ l] has been accepted for the AG,(Au+) calculations. Taking into account that the value of the Cu/Cu(II),,, couple standard potential equals 0.337 V[24] and measured us the reference (4) (R = AN) equals - 0.152 V[ 141, the standard potential of the Au/Au(I),, couple vs the latter reference can be estimated by the value of 1.34 V. Calculated copper(I) and (II), silver(I) and gold(I) transfer energies are listed in Table 2. The log K,(AN) value, given in Table 1, agrees well with the literature data, which range from -20 to -21.1[6]. There is also good agreement of AG,(Cu+, W-AN) and AG,(Cu2+, W-AN) values estimated here with those of previous investigations[25,27]. Returning now to Tables 1 and 2 and considering copper(I) and (II) transfer energies, relatively small changes in disproportionation constant are not unexpected. The copper(I) is much more stable in nitriles than in aqueous solutions due to both very poor copper(I1) solvation and to strong copper(I) salvation. The solvating ability of nitriles towards both the copper oxidation states follows the same order (AN > PN z- BN) and the decrease in Cu(1) solvation,

Ionic salvation-V


Table 1. Standard formal electrode potentials (mV) measured us Ag/Ag(I) 0.01 M, Et,NCIO, 0.1 M, AN/ reference together with the K~=[C~(II)]/[CU(I)]~ values. T.=25”C. Medium: EtoNCIO, 0.1 M. Standard deviations are given in parentheses. Au/Au+ W AN PN BN W AN PN BN





log Ko

1062( 5) 1105(6) 1102(5)

302(2) 117(l) 171(3) 221(3)

Experimental values* -152’ -475(2) 122(5) 142(7) -404(4) -363(l) 196(4)

719(3) 698(3) 755(3)

- 20.240.2) - 18.68(0.2) - 18.95(0.2)

1084 1132 1132

309 139 198 252

Zero ionic strength’ - 145 -453 166 -377 201 -333 256

785 779 845

- 20.98 - 19.59 - 19.97

*The potential when the concentrations quotient, not the activities quotient, is unity. ‘[14]. t Corrected to zero ionic strength using activity coefficients given by the Debye-Huckel theory with D= 4.&. Table 2. Free energies (kJ mol - ‘) of single ion transfer from water to nitriles Au(I) *





- 26.8 - 22.7 -.23.0

~ 17.9” - 12.6” -7.Sf’

-48.4n -42.0 -37.5

52.9** 56.7 67.2


-24.7 -20.1 -20.1

Zero ionic strength - 16.4 -46.9 - 10.7 -40.1 - 7.8 -35.3

59.6 66.4 67.2

*From E,(Au/Au(I), R) listed in Table 1 and from E,(Au/Au(I),, = 1.34 V us the same reference. ‘From E,(Ag/Ag(I), R) listed in Table 1. ‘From G,(Cu(II)) and assuming that log K,(W)= 6. *From E,(Cu/Cu(II), R) listed in Table 1. “Literature value: - 17.6 kJmol_’ [25] (negligible liquid junction potential (NLJP) assumption). ++Literature value: -8.4 kJ mol-‘[26] (AG,(AgCl;) = AG,(AgBr;) assumption). “Literature value: - 18.2 kJmol_‘[29] (based on the free energy of Ag(1) complexing by cryptand 222 in BN and PC). nLiterature value: -48.2 kJmol-‘[25] (NLJP assumption). **Literature values: 55.3 kJ mol-‘[25] and 59.5 kJmol-’ [27] (NLJP assumption). when the system is transferred from AN to PN or BN, is largely compensed by the decrease in Cu(I1) solvation. Negative gold(I) and silver(I) transfer energies reflect their stronger solvation in nitriles than in water. While the values of the Au/Au(III),,[l] and Ag(I),,/Ag(II),,[28] couples standard potentials have been reported, the corresponding quantities for nonaqueous media do not appear to be known, which makes it impossible to estimate silver(I) and gold(I) stabilities relative to their higher oxidation states.

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