Indian Journal of Chemistry Vol. 40A, February 2001, pp. 198-201

Isolation and structure of [papH](Cl04) [pap= 2-(phenylazo)pyridine]: Its use in the

synthesis of M-pap complexes

Amrita Saha, Chayan Das & Sreebrata Goswami*·

Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta 700032, India

and

Shie - Ming Peng

Department of Chemistry, National Taiwan University, Taipei, Taiwan, Republic of China

Received 15 September 2000; revised 7 November 2000

The salt, [HL](CI04) [L = 2-(phenylazo)pyrid ine] has been isolated from its conjugate base, L, in almost quantitative yield (>95%). The X-ray structure of the above salt is reported. The geometry of [HLt is trans about the diaza fragment. The N-N bond length in this compound is I .258(5) A which confirms a double bond. This length is elongated in its metal complexes due to extensive dn-pn interactions. The use of [HL] as a reagent for the sy nthesis of M-L complexes from the metal carbonates is explored. In this respect, the syntheses of [Co(L)3](CI04) 2.

[Cu(Lh](CI04), [Ag(Lh](CI04) complexes from the corresponding metal carbonates and [HL](CI04) are described.

The coordination chemistry of 2-(phenylazo)pyridine (L) has had an extensive literature in recent years 1

•4

.

Important phenomena like catalysis2, organic

hydroxylation2, thiolation4 and ami nation processes 1

via C-H activation, is0merisation and oxidation state sequencing2 are some of the current research activities in this area. The ligand is a potent IT-acceptor and strong M-L bonding, therefore, characterizes the complexes of L with low valent metal ions. The effects of strong metal-ligand interactions are observed in vibration frequencies I-J v(N=N), proton dissociation constants5 and, more directll-' 2

, in bond distance data. In almost all the cases, bonding of L to electron-rich metal centres leads to elongation of N-N length. However, the ligand L is a low melting solid and does not give suitable crystals for X-ray structure determination and, hence, N-N length in the free L was not available in the literature. However, the perchlorate salt of its conjugate acid, [HL](Cl04) ,

forms good crystals for X-ray work. We report here

*E-mail: icsg@mahendra.iacs.res.in Fax: 9 I -33-473 2805

synthesis, characterization and X-ray structure determination of [HL](Cl04). This compound acts as a strong acid and has been shown to be suitable reagent for the synthesis of metal-pap complexes directly from metal carbonates.

Experimental Cupric carbonate, silver carbonate and cobalt

carbonate were obtained from SD Fine Chemicals Limited, Bombay. 2-(Arylazo)pyridine was synthesised as reported in the literature 13

.

Tetraethylammonium perchlorate (TEAP) was prepared and recrystallised. Sodium perchlorate was obtained from Fluka, Switzerland. All other chemicals and solvents were of reagent grade and used as received.

A Shimadzu UV-2100 UVIYIS spectrophotometer was used to record electronic spectra. The IR spectra were obtained using a Perk in-Elmer 783 spectrophotometer, 1H NMR spectra were recorded in CDCI3 using a Bruker-A vance DPX300 spectrometer and SiMe4 as internal standard. A Perkin-Elmer 24°C elemental analyser was used to collect microanalytical data (C,H,N). Single crystals of [HL](CI04) , suitable for X-ray diffraction, were grown by slow diffusion of dichloromethane into hexane solution . Diffract ion measurements were carried out on a Nonius CAD-4 fully automated four-cycle diffractometer. The unit cell was determined and refined using setting angles of 25 reflections, with 28 angles in the range 19.00-27.80. Data were collected by 8-28 scans with 28 maximum 50. All the data reduction and structure refinements were performed using NRCV AX packages. The structure was solved by direct method . Significant crystal data are listed below : crystal system, monoclinic; space group, C 2/c; a = 30.116(7) A, b = 5.9 129(19) A, c = 13.833(3) A; f3 = 90.728(25)", V=2463.1(11) A3;_Z=8; crystal stze (mrn), 0.6x 0 .7x 0.7; Rr= 0.046; Rw = 0.058.

Synthesis of perchlorato-[2-(phenylazo) pyridine}, [HL}(C/04)

To an ice-cold methanolic solution of 2-(phenylazo)pyridine (L) (0.1 00g,0.3527 mmol in I Om! MeOH), a cooled aqueous solution of perchloric acid (ca.S molar) was added drop-wise with constant

NOTES 199

Table !-Selected bond distances of [HL](Cl04) in A N( I )-C(l) 1.352(5) N(3)-C(6) 1.421 (5)

C(l)-C(2) 1.368(8) C(6)-C(7) 1.401(6)

C(2)-C(3) 1.364(7) C(7)-C(8) 1.389(7)

C(3)-C(4) 1.382(7) C(8)-C(9) 1.368(7)

C(4)-C(5) 1.366(6) C(9)-C(IO) 1.383(7)

N(l)-C(5) 1.356(5) C(I 0)-C( II) 1.386(6)

C(5)-N(2) 1.4 17(5) C(6)-C(II) 1.392(6)

N(2)-N(3) 1.258(5)

stirring. The yellow orange microcrystals thus formed were filtered, washed with cold water and dried in vacuo. The compound was recrystallized from dichloromethane-hexane mixture; yield 95%. [Found: C, 46.62; H, 3.43; N, 14.95. Calc. for C 11 H10CIN304: C, 46.56; H, 3.53; N, 14.81%]. IR (KBr): v(C=N) 1602, v(N=N) 1437, v(CI04-) 1115, 627cm-1. Amaxlnm

(E/M-1cm-1) (CH3CN): 440 (700), 320 (19020). 1H NMR (CDCh): 8 = 7.66 (t, 10-H), 8.08 (d, 3-H), 8.13 (d, 4-H), 8.22 (t, 5-H), 8.92 (d, 6-H).

Synthesis of [bis{2-(phenylazo )pyridine]copper( /)] perchlorate, {Cu(L)2](Cl04)

The commercially available salt ,CuC03_Cu(OHh (0.0449g, 0.2032 mmol) was added to an ethanolic solution of the ligand, [HL](CI04) (0.1152g, 0.4063 mmol). The mixture was boiled under reflux for 2.5 h. The resultant violet solution was then evaporated on a water-bath to ca. 10ml when a gummy violet mass was obtained. This was washed thoroughly with (3x10ml) of water. It was then dissolved in methanol and left to crystallize when large red-violet crystals of [Cu(Lh](CI04) deposited. These were collected by filtration and then dried over P40 10; yield 70%. [Found: C, 50.25; H, 4.09; N, 16.11. Calc. For C22H,sCIN604Cu: C, 49.91; H, 3.40; N, 15.88%]. IR (KBr) : v(C=N) 1583, v(N=N) 1373, v(CI04-) 1090, 623 cm-1. Arnaxlnm (EIM 1cm-1)(CH30H): 700 (1590), 560 (6840), 350 (37760), 230 (24350).

Synthesis of [bis{2-(phenylazo) pyridine}silver(l)]­perchlorate, [Ag(Lh}(C/04)

The salt, Ag2C03 (0.1054g, 0.3819 mmol) was added to an ethanolic solution of the ligand, [HL](CI04) (0.4331g, 1.5277mmol). The mixture was boiled under reflux for 2 h.The dark red solution was then evaporated on a water-bath to ca. 10 ml when dark reddish-brown gummy mass was obtained. It was washed thoroughly with water (3x10ml) when a dark coloured compound was produced. It was

recrystallized from methanol-water (1: I) mixture; yield 80%. [Found: C, 46.32; H, 3.66; N, 15.06. Calc. for C22H1sCIN604Ag: C, 46.04; H, 3.14; N, 14.65%]. IR (KBr) : v(C=N) 1582, v(N=N) 1421, v(CI04-) 1113, 627cm-1. Arnax!nm (EIM-1cm-1) (CH3CN): 440 (950), 320 (32100), 230 (21500).

Synthesis of [tris{2-(phenylazo) pyridine}cobalt(/1)] perchlorate, [Co(L)J](Cl04h

The salt, CoC03 (0.0211g, 0.1773 mmol) was added to an ethanolic solution of the ligand, [HL](CI04) (0.1505g, 0.5308 mmol). The mixture was boiled under reflux for 2.5 h. The resultant dark­brown solution was then evaporated on a water-bath to ca. 10 mi. Upon cooling, crystalline [Co(Lh](Cl04h was obtained; yield 80%. [Found: C, 47.92; H, 4.03; N, 14.88. Calc for C33H29ChN909Co: C, 48.00; H, 3.52; N, 15.27%]. IR (KBr): v(C=N) 1603, v(N=N) 1445, v(CI04 -) 1117, 625cm _,_ Arnaxlnm

(EIM-1cm-1)(CH3CN): 370 (10900), 314 (44220).

Results and discussion

Isolation and characterisation The cationic compound [HLt was isolated as

crystalline perchlorate salt from a concentrated aqueous HC104 solution of orange L ( Scheme I ) in almost quantitative yield (95%). This salt is more soluble in polar solvents. The cationic compound [HLt showed resolved 1H NMR spectrum in CDCh. The pyridyl proton resonances, in general, appeared 14

at lower frequencies than the corresponding phenyl proton resonances. It may be worthwhile to compare the 1H NMR spectrum of [HLt with that of L. Interestingly, downfield shift of the pyridyl proton resonances were observed in moving from L to [HLt. This is as expected since the pyridyl nitrogen in L, being a stronger base than the diazo nitrogen, is preferentially protonated. The c+ompound [HL](Cl04) formed suitable crystals for single crystal

e

@ HCI04(aq.) :@H

(CI04) 3 N=N N=N uQ)s © 11 10

L [HL](CI04 )

Scheme I

200 INDIAN J CHEM, SEC. A, FEBRUARY 2001

C2

03

01

02 ca

C3 C4

Fig. I-ORTEP and atom numbering scheme of the salt, [Hpap](CI04). Hydrogen atoms are not shown.

C9

X-ray structure determination . The ORTEP and atom numbering scheme for the above are shown in Fig. I and bond lengths are collected in Table 1. The geometry of this ligand is trans about the diaza

fragment. The N-N distance is 1.258(5) A, indicative of a double bond. For comparison, this distance in other diaza ligands, PhN=NPh15, MeN=NMe 16

MeC(=NOH)N=NPh 17 and PyN=NPh(p-N(H)Ph) 18 i~ 1.253(3), 1.254(3), 1.256(2), and 1.264(3) A respectively.

It may be interesting to compare the N-N distance in [HL](Cl04) with the N-N distances observed in some M-L complexes (Table 2). In almost all cases there has been considerable elongation of N-N bond in the metal complexes of L. The rhodium(lll) complex [RhCh(L)2r is the only exception 12

, where

almost negligible drc-prc interaction is expected. The N-N length in this rhodium compound is similar to that observed in [HL](Cl04). All other bond distances in [HL](CI04) are within the range of normally expected values.

Use of [HL](C/04) in the synthesis of M-L complexes The fact that the compound [HL](CI04) is highly

ac idic and loses H+ readily in solution prompted us to explore its reactions with transition metal carbonates. It was anticipated that the liberated H+ from [HLt would decompose the metal carbonates and should be a suitable reagent for the direct synthesis of M-L type of complexes of Cu2+, Ag+ and Co2+. It may be noted here that though the above metal salts are cheap, yet are totally unreactive towards L even under very

Table 2-N-N length in [HL](CI04) and in some M-L complexes

Compound N-N (A) Reference

[HL](CI04) 1.258 This work

RuCiiLh 1.279- I .300 6

Ru(N3h(Lh I .288- I .299 7

[Ru(biimH2)(Lh](CI04h 1.27 I II

OsBr2(Lh 1.3 IO 8

CrCI2(L)2 1.282- 1.3 I 4 9

ReCI 2(NPh)(L) 1.289 IO

[RhCI2(Lh](CI04 ) I .255- I .258 12

biimH2 = 2,i- biimidazole

stron~ reaction con+diti~ns. In line with our strategy, react1ons of [HL] w1th the corresponding metal carbonates proceeded very smoothly. The compounds were isolated in their low-valent states as [Cu(L)2](CI04) , [Ag(Lh](CI04) and [Co(L)3](Cl04h, respectively. These were characterised based on their elemental analysis and physicochemical data. The above three compounds were known 19-21 in the literature and have very rich chemical properties. For example, the use of silver-pap complexes in trans­metallation reactions '-22 , particularly for the synthesis of mixed chelates, has now been established. The cobalt-pap complexes have been found to be o-ood mediator1'12 for the aromatic ring amination proce~ses. The spectral properties of the samples obtained from our synthetic procedure exactly corresponded to those of the authentic samples. The synthetic methodology reported here is one pot with many advantages. The synthesis of the cuprous compound, [Cu(Lh](Cl04), in this respect, needs special mention. It was reported 19 that prolonged boiling of a mixture of Cu(Cl04h and L in methanol-water solvent mixture produced [Cu(Lh](Cl04). In comparison, our synthetic procedure is much faster than the reported one and the yield is also very high . It may be relevant to add here that the examples of synthesis of a cuprous compound directly from a cupric salt are very

scarce in the literature. Strong rc-acceptor ability of pap is responsible for the stabilisation of cuprous state. Finally, we have been working further to use our strategy to synthesise the compounds of L which otherwise are inaccessible.

Acknowledgement Financial assistance received from the Council of

Scientific and Industrial Research , New Delhi is gratefully acknowledged .

NOTES 201

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