Silver(II) complexes of dipyridyl

3958

Notes

J. inorg, nucl. Chem., 1971, Vol. 33, pp. 3958-3962.

Pergamon Press.

Printed in G reat Britain

Silver(ll) complexes of dipyridyl (First received 11 January 1971 ; in revised form 21 January 1971)

SILVER(II) species are known to exist in highly acidic media of mineral acids[l]. As stable compounds they have been usually isolated as silver(II) complexes containing suitable nitrogen-containing heterocycles [2, 3]. Among the latter the bis and tris a, t~'-dipyridyl (dipy) complexes of silver(II) have been described[4]. Although other analogs are unreported, the formulation of various tris(dipyridyl)silver(II) complexes has been widely accepted[2, 5], despite the unfavorable octahedral coordination with neutral ligands expected for silver(I I) as it is for copper(l 1) [5g]. Morgan and Burstall[4] originally isolated tris-(dipyridyl)silver(II) nitrate from the oxidation of bis-(dipyridyl)silver(I) nitrate with peroxydisulfate. However, Barbieri[6] only obtained bis-(dipyridyl)silver(lI) nitrate, which he also prepared by anodic oxidation of bis-(dipyridyl)silver(I) nitrate. These divergent results have not been clear to subsequent workers and the distinction between bis and tris dipyridyl complexes of silver(II) has remained obscure up to the present time. We wish to show that the tris-(dipyridyl)silver(l I) nitrate reported originally [4] was incorrectly characterized and present several simplified procedures for the preparation of bis-(dipyridyl)silver(ll) complexes [Se, f, 6]. At this juncture we have been unable to isolate or otherwise identify tris-(dipyridyl)silver(lI) complexes. EXPERIMENTAL Preparation of Agli(dipy)2(NOa)2 Peroxydisulfate oxidation. Ag~(dipy)2NO3 was prepared by adding an aqueous solution of 0.0858 of reagent grade silver nitrate to a warm solution of 1.79g of c~, ct'-dipyridyl in 20 ml of 50%v aqueous ethanol. The yellow needle-like crystals melted at 154°C after recrystallization from aqueous ethanol and drying. Ag1(dipy)~NO3 was stirred into a cold saturated solution of excess potassium peroxydisulfate according to Morgan and Burstall's procedure [4]. The deep red-brown microcrystalline precipitate formed after 2 hr was triturated with cold nitric acid and extracted with warm water. Subsequent addition of ammonium nitrate to the chilled red-brown solution afforded dark red crystals which decomposed at approximately 174°C (lit. [4], 176°C) depending on the rate of heating. Anal Calcd. for AgII-(dipy)2(NOa)z: Ag, 19.8; dipy, 57.3; NO3-, 22.8; C, 44.11; H, 2.97; N, 15.44. Anal. Calcd. for Agn(dipy)3(NO3)z: Ag, 15.4; dipy, 66.8; NO3-, 17.7; C, 51.42; H, 3.46; N, 16.00. Found: Ag, 20.0; dipy, 54.6; C, 42.5; H, 2-86; N, 15'3. Found (lit.[4]): Ag, 15.5; NO3-, 18.0; dipy, 65.8. The 1. (a) A. A. Noyes, K. S. Pitzer and C. L. Dunn, J. Am. chem. Soc. 57, 1229 (1935); see also 57, 1221 (1935), 59, 1316 (1937); (b) J. B. Kirwin, F. D. Peat, P. J. Proll and L. H. Sutcliffe, J, Phys. Chem. 67, 1617 (1963); (c) G. A. Rechnitz and S. B. Zamochnick, Talanta 11,713, 1645 (1964), 12, 479 (1965); (d) H. N. Po, J. H. Swinehart and T. L. Allen, lnorg. Chem. 7, 244 (1968); (e) D. H. Huchital, N. Sutin and B. Warnqvist, ibid, 6,838 (1967). 2. J. A. McMillan, Chem. Rev. 62, 65 (1962). 3. (a) M. G. B. Drew, G. W. A. Fowles, R. W. Matthews and R. A. Walton, J. Am. Chem. Soc. 91, 7769 (1969);J. chem. Soc. A, 1108 (1968); (b) J. P. Miller, ibid. 1778 (1968). 4. G. T. Morgan and F. H. Burstall,J. chem. Soc. 2594 (1930). 5. See, however, (a) V. Carassiti, G. Condorelli and L. L. Costanzo, Annali di Chim. (Rome) 54, 303 (1964); (b) V. Balzani, A. Bertoluzza, V. Carassiti and A. Malaguti, ibid. 52, 1052 (1962); (c) E. Cervone, ibid. 52, 116 (1962); (d) V. Balzani, A. Bertoluzza and V. Carassiti, Bull Soc. Chim. Belg. 71, 821 (1962); (e) G. A. Barbieri and A. Malaguti, A ttiAccad. Nazi. Lincei, Rend. classe sci., fis., mat e nat. 8, 619 (1950); 9, 349 (1950); 16, 88 (1954); (f) A. Malaguti and Labianca, Gazz. chim. ital. 84, 976 (1954); (g) R. S. Banerjee and S. Basu, J. inorg, nucl. Chem. 26, 821 (1964); (h) M. Bruno and V. Santoro, Ricerca Sci. 26, 3072 (1956); (i) 3. M. Anderson, andJ. K. Kochi, J. Org. Chem.35,986 (1970). 6. G. A. Barbieri,Atti accad. Lincei 6, 44 (1932).

Notes

3959

sample prepared in this manner could be purified further by extraction with methylene chloride in which bis-(dipyridyl)silver(1) nitrate is soluble. Electrolytic oxidation. To a mixture containing 0-573g of a, a'-dipyridyl in 60 ml of water and 3 ml of concentrated nitric acid was added 0.313g of silver nitrate dissolved in a small volume of water. The resulting clear solution was transferred to an anodic compartment consisting of a porous porcelain cup (capacity, approximately 100 ml) which was placed in a 400 ml beaker containing 0-2M nitric acid to serve as the cathodic cell. Electrolysis was carried out at 0°C with platinum mesh electrodes using a current of 2 amps. for 8-10rain. The dark red-brown microcrystalline precipitate was collected and dried. After extraction with methylene chloride it melted with decomposition over the range 175-185°C. In order to maximize the yield of silver(II) complex, the filtrate from one run tafter addition of l ml conc. nitric acid) was used as the medium for the conversion of more silver nitrate and dipyridyl. Repeated use of this procedure resulted in conversions as high as 70%. Anal. Calcd. for Ag"(dipy)2(NO:~)2: vide infra Found: Ag, 19-9; dipy, 57-3; NO:~-, 25.5; C, 43.85; H, 3-05; N, 15.18.

Preparation o f Agll(dipy)2(OaSC F:02 AgI(dipyh(O:~SCF3) was prepared from either silver(l) carbonate or oxide. For example, a mixture of 0.92 m-moles of freshly prepared argentous oxide, 3-67 m-moles of a, a'-dipyridyl and excess trifluoromethanesulfonic acid was heated in water until solution was complete. On cooling, yellow needle-like crystals melting at 185-190°C were formed. Anal. Calcd. for Ag~(dipy)2(O3SCF3): Ag, 19.0; dipy, 54.8; C, 44.3; H, 2.8; S, 5.63. Found: Ag, 18.8; dipy, 51-5; C, 44.1 ; H, 3.3; S, 5.74. Approximately 1 g of Agr(dipy)2(O3SCF3) was dissolved in water with the aid of excess trifluoromethanesulfonic acid and the solution electrolyzed as before except dilute trifluoromethanesulfonic acid was used as the catholyte. Dark red crystals were formed on the anode after passing a current of 1 amp. through the solution for 20 min. On extraction with methylene chloride, Agn(dipy)~(QSCF3)2 decomposing at 200-210°C was obtained in 40% yield. Anal. Calcd. for Ag~'(dipyh (O:~SCF:02: Ag, 15-05; dipy, 43.4; C, 36.8; H, 2-2; S, 8.93. Found: Ag, 15.4; dipy, 40.3; C, 37.4: H, 2-9; S, 9.49. Ag"(dipy)2(O3SCF3)2 was also prepared by non-electrolytic routes. Thus, 0.33g of Ag~(dipy)2(O~SCF3) was dissolved in aqueous acetonitrile at 0°C with the aid of trifluoromethanesulfonic acid. When argentic oxide (AGO) was added, the solution turned deep red in less than 1 min [5hi. Rotary evaporation of acetonitrile led to a 30% yield of Ag"(dipy)2(O3SCF3h after purification of methylene chloride. Alternatively, a mixture of dipyridyl, argentic oxide and trifluoromethanesulfonic acid was stirred in aqueous acetonitrile at 0°C [5f]. Work-up of the dark red solution afforded a 74% yield of impure Agn(dipy)2(O,~SCF3)2.

Analytical methods Silver was analyzed by treating a weighed amount of the complex with dilute nitric acid and methanol. The latter was used to reduce silver(II) and did not interfere with the subsequent Volhard titration for silver(l)[7]. Sulfur was analyzed gravimetrically as barium sulfate after oxidation of the complex with sodium peroxide in a nickel crucible [8] and addition of hydrochloric acid to remove silver. Nitrate was analyzed by reduction with ferrous ammonium sulfate followed by back titration with dichromate potentiometrically [9]. Silver(ll) was analyzed either iodometrically [10] or reduction with ferrous ammonium sulfate and back titration with dichromate, a.a'-Dipyridyl was analyzed directly by dissolving a weighed amount of complex in a prescribed volume of aqueous methanol and measuring the absorbance of the resultant solution containing the bis-(dipyridyl)silver(l) complex at 281 nm. The molar absorptivity (~ = 1-4 x 10e) at this wavelength was not affected by silver(1). and both a,a'-dipyridyl or bis-(dipyridyl)-silver(I) complexes were used as calibrants over a wide range of concentrations [ 11 ].

7. 8. 9. 10. 1 I.

A. 1. Vogel, Textbook of Quantitative Inorganic Analysis, 3rd Ed. p. 265. Longmans. London. Ref. [7]. p. 467. I. M. Kolthoff, E. B. Sandell and B. Moscovitz,J.Am. chem. Soc. 55, 1454 (1933). C. P. Lloyd,Analytica chim.Acta 43, 95 (1968). K. Sone. P. Krumholz and H. Stammreich,J.Am. chem. Sot.. 77,777 (1955).

3960

Notes

RESULTS AND DISCUSSION The analytical results showed conclusively that tris-(dipyridyl)silver(lI) nitrate reported by Morgan and Burstall [4] was in fact the bis-dipyridyl complex. The material as isolated by their procedure was somewhat contaminated with bis-(dipyridyl)silver(1) nitrate which was readily removed by extraction with methylene chloride. Aside from this impurity it is not clear why their results were so divergent. We suspect that the other (dipyridyl)silver(lI) complexes reported by them also suffer from similar ambiguities, and we suggest that data reported on these complexes [particularly the tris-(bipyridyl)silver(II) species] be reinterpreted with this in mind [2, 5c, 12, 13]. The visible and u.v. spectra shown in Figs. I and 2 indicated that bis-(dipyridyl)silver(ll) ions derived from the complex nitrate generated by peroxdisulfate or electrolytic oxidation were the same as that derived from bis-(dipyridyl)silver(ll) trifluoromethanesulfonate. Dissociation of the dipyridyl ligands was not apparent since successive dilutions of aqueous solutions of bis-(dipyridyl)silver(II) nitrate using longer path cells showed no deviation from the Beer-Lambert Law down to beyond 10-SM [ 14, 5d]. The determination of the molar absorptivity of the visible band at 455 nm, however, was made difficult by the tail of the u.v. absorption. Attempts to detect the tris-(dipyridyl)silver(lI) cation in solution were unsuccessful. Thus, the addition of excess dipyridyi to a solution of bis(dipyridyl)silver(II) produced an unstable yellow solution which readily turned colorless. The yellow solution could be stabilized by acetonitrile. The magnetic susceptibility of Ag"(dipy)~(NO3)2 measured by the Faraday method was 1858 × 10-a ( g = 2-12B.M.) at 28°C compared to 1851 × 10-6emu obtained by Sugden[13] for the alleged tris-(dipyridyl)silver(ll) nitrate prepared by Morgan and Burstall's method. The measured

0.6

0.4 .g

0.2

600

500 Wavelength,

40o nm

Fig. 1. Visible absorption spectrum of bis-(dipyridyl)silver(II) nitrate: A--5.78 × 10-4M (ex peroxydisulfate); B - 4 . 1 2 × 10-4M (ex electrolytic); and trifluoromethanesulfonate; C - 3 - 2 3 × 10~M; D - 2 . 8 9 × 10-4M; E - 1.93 x 10-4M. 12. J. A. McMillan and B. Smaller, J. chem. Phys. 35, 1698 (1961). 13. S. Sugden, J. chem. Soc. 161 (1932). 14. S. Cabani and E. Scrocco, Annatidi Chim (Rome) 48, 85 (1958).

Notes

3961

O.E

0.~,

'

200

G

250

Wavelength,

300 nm

Fig. 2. U.V. absorption spectrum of bis-(dipyridyl)silver(ll) nitrate: F - (2.89 × 10-SM) and trifluoromethanesulfonate; G - (9.64 × 10-rM ). value of the magnetic susceptibility of bis-(dipyridyl)silver(ll) trifluoromethanesulfonate was somewhat higher than expected (×u = 2551 × 10-6 e.m,u.). The electron paramagnetic resonance (EPR) spectrum of an aqueous solution of bis-(dipyridyl)silver(li) nitrate in Fig. 3 showed partially resolved hyperfine interactions. The spectrum shown was almost isotropic but was slightly affected by anisotropic components. The splitting pattern consisted of eleven lines o f approximately equal separation (17 gauss) centered at g = 2"075. A n E P R spectrum o f a powder sample showed no hyperfine splitting but from the shape of the spectrum we infer a tetragonal symmetry for the complex with g± (g~ - g~ = 2.045 and g~b(g3) = 2-129. We tentatively interpret the solution E P R spectrum as consisting of two sets of nine nitrogen hyperline lines. According to this interpretation the magnitude of the splitting due to the silver is approximately twice that due to four equivalent nitrogen nuclei. The silver isotopes l°7Ag and ~°9Ag, which both have I = ½ and nuclear Larmor precession frequencies within 15 per cent, were not resolved. We disfavor an alternative analysis in which the spectrum consisted of thirteen hyperfine lines of which the pair in the "wings" were not seen. Recently, the E P R spectrum of the related Ag~(bipy)2S2Os complex in frozen nitric acid solutions was reported[15]. Axial anisotropy indicated g± = 2,047 and g, = 2.210, and the silver and nitrogen hyperfine tensors [Al~(Ag) = 42.2 gauss,A~(Ag) = 26.0 gauss, Al~(N) = 21.1 gauss and A I ( N ) = 30.1 gauss] were not inconsistent with our results. However, the hyperfine splitting showed quite conclusively that only two nitrogen nuclei were coordinated with silver, which may be 15. T. Halpern, S. M c K o s k e y and J. A. McMillan, J. chem. Phys. 52, 3526 (1970).

J INC- Vol. 33 No. I 1- L

3962

Notes

g -2.075

\

5 0 gauss

Fig. 3. Electron paramagnetic resonance spectrum of bis-(dipyridyl)silver(ll) nitrate in aqueous solution.

possible if one of the dipyridyl ligands was lost by protonation in the acidic medium employed, e.g. [5d, 16, 17] AgI(dipy)2 +2 + H + ~ Agn(dipy) 2+ + dipyH +. In a more direct comparison the isotropic EPR spectrum of a related silver(II) complex has been obtained in which the tetraphenylporphrin ligand constrained the coordination by four nitrogens to a planar array [18]. The splitting in this complex also consisted of eleven almost equidistant lines, the intensities of which indicated two sets of nine lines. The spin Hamiltonian parameters (g) = 2"0585, (aA~) = 43-00 and (an) = 23.48 were similar to our results with bis.(dipyridyl)silver(II) nitrate.

Acknowledgement-We wish to thank the National Science Foundation for financial support and K. Wada for help with the EPR spectra.

Department of Chemistry Indiana University Bloomington, Indiana 47401 U.S.A.

W I L L I A M G. T H O R P E J A Y K. K O C H I

16. T. Halpern, W. D. Phillips andJ. A. McMillan, J. Chem. Phys. 53, 5548 (1970). 17. (a) T. S. Thomas and R. G. Wilkins, Chem. Commun. 1681 (1970); (b) R. F. Pasternak and H. Sigel, J.Am. chem. Sac., 92, 6146 (1970). 18. P. T. Manoharan and M. T. Rogers, Electron Spin Resonance of Metal Complexes, (Edited by T. F. Yen), p. 143. Plenum Press, New York (1969).