Indian Journal of ChemistryVol. 16A, June 1978, pp. 484-487

Aluminium(III) Formate & Its Adducts with Nitrogen BasesRAM C. PAUL, (Miss) TRIPTA PURl & RAMESH KAPOOR

Department of Chemistry, Panjab University, Chandigarh 160014

Received 8 August 1977; accejlted 8 February 1978

Aluminium(III) formate and its complexes with several nitrogen bases have been prep d. . areand characterfzed on the basts of elemental analysis, infrared and conductance data. Theirmode of thermal decomposition has also been investigated. Aluminium(III) formate is formu-lated as Al(OOCH)a.HCOOH instead of the formulation as H+[Al(OOCH).]-.

THE chemistry of aluminium(1II) formate is ofinterest because of its wide use in industry'.I t is expected to be a good Lewis acid and

should form addition compounds with bases. Itmay also act as an acid in the formic acid solvent,i.e. as HCOO- ion acceptor. All the previous reportsdeal with studies On basic aluminium formates! anda study on the properties of normal aluminium(II1)formate is lacking. Only recently Pogodilova et at.have reported the preparation and 1R spectrumof aluminium(II1) formate". We now report analternative method of its preparation and itscomplexes with a few bases.

Materrals and MethodsFormic acid (Riedel, 98-100%) was further purified

by fractional crystallizations. Aluminium(III) chlo-ride and bromide were sublimed before use. Alumi-nium(III) formate was prepared by the reaction ofaluminium(III) chloride or bromide with formicacid. Aluminium(III) chloride and bromide reactvigorously with formic acid with the brisk evolutionof hydrogen halide. The reaction was completedon keeping at reflux for 3-4 hr, when a whitesolid separated out. The precipitate was filtered,washed with dichloromethane and dried in vacuo.The compound gave analysis corresponding toAI(OOCH)a.HCOOH (I). It is non-hygroscopic andis insoluble in common organic solvents, viz. carbontetrachloride, benzene, acetonitrile, nitromethane andchloroform. It is also insoluble in the parentsolvent, i.e. formic acid. It dissolves in wateron warming. The solution on evaporation yieldsthe basic salt, AI2(OH)(HCOO)s·2H20 (II). Thedesolva ted product AI(OOCH)a (III) was obtainedby heating (1) at 110° (10-2 torr) for 3-4 hr whenno further weight loss was observed.

For the preparation of complexes, solution of thebase in dichloromethane was added in excess to anice-cold suspension of (I) in the same solvent.The reactants were stirred for 4-6 hr and voluminouscomplex formed was filtered, washed with dichloro-methane and dried in vacuo. The complex withammonia was prepared by bubbling dry NH3 gasthrough the suspension of (I) and (III) in dichloro-

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methane at room .t~mperature. These complexesare extremely sensitive to moisture and smell of?ase whe~ exposed to moist air. They are allIn~oluble in common organic solvents and formic~cld excep.t for the ethylenediamine complex whichIS soluble In the latter solvent.

Alumin~um was estimated gravimetrically as oxi-na te, Whll~ carbon, hydrogen and nitrogen wereanalysed mlcroanalytically. Infrared spectra of thec0!llpounds ~ere recorded in KBr or in nujol mullsusing a Perkm-~lmer 621 grating spectrophotometer.Thermal a~al~sls .was carried out at a linear heatingrat~ of 5 fmm In static air. A MOM Budapestdenvatograph (type Paulik, Paulik & Erdey) wasuse~ and 110-20 mg of the sample in ceramic orplatlnu.m sample holder was used for these runs,Analytical results of compounds are given inTable 1.

Results and DiscussionThe solubility data of aluminium(III) formate,

AI(OC?CH)a.HCOOH (I) suggest its polymeric nature.The infrared spectra of (I) and (III) agree quiteclosely WIth those reported earlier". Compound (I),shows strong bands at 1728 and 1128 crn! whicha~e characteristic of vC=O and vC-O-H respec-tively", Absence. of these bands in (III) is due tothe loss of coordina tsd formic acid. The presenceof a strong broad band with maxima at 3365 crrr 'also suggests the presence of O-H- - -0 bond in(I). Th~ forrnula tion H+[Al(OOCH)4]- for (I) isnot conSIStent WIth these observations. The vCOOasand vCOOs modes are observed at 1616 and 1380~m-l respectively. The large value of Av (236 cm=),in (I) and (III~ sugg~st~ t~e presence of bridgingforma t.o groups " ThIS IS III accordance with theanalysis of Gngor ev for bidentate acetates". Intensebands present at 500 and 310 crrr" may be assignedto vAI-Oas and vAl-Os modes respectively.

'.The TG, DTG a~d DTA studies of (I) in air(FIg .. 1). show that It undergoes decomposition intwo distinct steps. The weight loss values (Table 2)correspond very closely with the following decorn-pOSItIOn scheme (Eq. 1):

2Al(00CH)a·HCOOH -+ 2Al(OOCH)a -+ Al203 ••• (1)

PAUL et al.: ADDUCTS OF AI(III) FORMATE

Compound

TABLE 1- ANALYTICAL DATA OF THE COMPLEXES

Found (Calc.), %

Al C H

AI(OOCH)3.HCOOH (I) 12-8 (13-0) 22·9 (23·1) 2·3 (2-4)AI2(OH)(OOCH)5.2H20 (II) 17·0 (16·3) 18·5 (18·4) 3·5 (3·0)AI(OOCH)3 (III) 16·6 (16·7) 22·1 (22·2) 1·7 (1·8)Al(OOCH)s.HCOOH.2en (IV) 7·8 (8·2) 28·8 (29·3) 6·5 (6·4)Al(OOCH)3·HCOOH.3NH3 (V) 10·8 (10·4) 19·2 (18·5) 5·6 (5-4)AI(OOCH)3.3NH3 (VI) 12-4 (12·7) 16·8 (16·9) 5·4 (5·6)Al(OOCH)3·HCOOH.3C.H,oNH (VII) 5·6 (5·8) 48·2 (49·2) 7-6 (8·2)AI(OOCH)3.HCOOH.3C.H8NH (VIII) 6·5 (6·4) 44·2 (45·6) 6·9 (7-6)AI(OOCH)s·HCOOH.3 (C2H5)2NH (IX) 6·2 (6·3) 44·6 (45·0) 8·7 (8·9)AI(OOCH) a-HCOOH.2(CH3)3N (X) 7·8 (8·3) 36·4 (36·8) 7·3 (7·1)AI(OOCH)3·HCOOH. (C2Ho)aN (XI) 8·8 (8·7) 38·0 (38·8) 6·1 (6·5)AIC3H,oa (XII) 16·2 (16·2) 22·6 (21·7) 4·0 (4·2)

N

16·7 (17·1)16·9 (16·2)19-6 (19·7)9-6 (9·1)9·8 (10·0)9·8 (9·8)8·3 (8·6)4-1 (4·5)

TABLE 2 - THERMAL DECOMPOSITION DATA OF (I) AND ITs COMPLEXES

Compound Decomp. temp. (0C) Decomp. product Weight loss (%)

Initial Final Found Calc.

AI(OOCH)3·HCOOH 50 200 Al(OOCH)2 22·0 22·1210 380 Al203 75·3 75·5AI(OOCH) •. HCOOH.3NH3 45 220 AI(OH)(OOCH), 48·3 48·3240 360 Al2O, 80·4 80·3AI(OOCHJa.3NHs 45 220 AI(OH)(OOCH)2 35·5 35·7260 410 Al203 76·1 76·0Al(OOCH)3·HCOOH.3C.H,oNH 45 350 AI(OH)3 82·8 83·1350 700 AI203 89·0 89·0AI(OOCH)a·HCOOH.3C.H8NH 40 320 AI(OH)3 81·0 81·5320 650 Al203 88·0 87·9Al(OOCH)~.HCOOH.3 (C2H5)2NH 45 380 AI(OH)3 81·3 81·7380 600 Al,03 88·0 88·0Al(OOCH)3·HCOOH.2(CH3JaN 45 180 AI(OOCH)3· (CH3)3N 31·2 32·2180 370 Al203 84·0 84·3Al (OOCH)3·HCOOH. (C,Ho)3N 50 220 AI(OOCH)3 46·9 47·6220 400 AI,03 83·5 83·6

0 ..,~,"."

':.~~

20 '.~.,"."

",..... \\~ ,0 r-.~~

I '.''" .,

'" \

0...•

t- 60J:oW~

80

100 200 300 400 7o"c>TEMPERAT URE eC)

Fig. l-TG curves for (A) Al(OOCH)3.HCOOH; (E) AI-(OOCH)3·HCOOH.3NHs; (C) Al(OOCH)s.HCOOH.3Pip.; and

(D) Al(OOCH)3.HCOOH.2(CH3)3N

•.-c

. The tw~ steps give rise to two endothermic peaksIII DTA with the first endotherm being smaller thanthe second endotherm. Any exothermic effects,due to. the. secondary reactions which may involvethe oXId~tIon of the liberated CO and H2 by airpresent III furnace, are not observed. Since thedecomposition temperatures are quite low, we feelthat CO and H2 diffuse out of the sample holderb.efore their conversion into CO2 and H20 respec-tively. These endotherms persisted even whenrelatively small amounts (---20 mg) of the sampleswere .used and enough static air was present for therea~t~ons to take place. The single step decom-PO~ItlO~ of Al(OOCHls into A1203 is surprisinglyqUl~e different from that of aluminium(II1) acetate,whI~h, deco~poses in different steps, viz. the for-mation of diaceta te and monoacetate as the inter-mediates",

The 1R spectrum of (II) shows bands at 348051590s, 1400s, 1385s, 1120s, 1090s, 1000m, 950sh790m, 770s, 720sh, 4605 and 330m crrr+, Thepresence of bands at 1090 and 1000 crrr ' suggestthat hydrolysis of (I) has occurred as these bands

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INDIAN J. CHEM., VOL. t6A, JUNE 1978

HI

are usually assigned to AI-O-Ai bridging groups"Compound (I) when refluxed with large excess ofacetic acid, undergoes acid exchange and aluminium-(III) acetate is obtained. Hydrochlorination of (I)in carbon tetrachloride by bubbling dry HCl gasdoes not give the chloro-substituted products. Re-action of excess potassium formate with the suspen-sion of (I) in formic acid at reflux temperaturedoes not yield clear solution even on prolongedheating. There appears to be no interaction as (I)is recovered unchanged on filtering the contents.It is likely, therefore, that its double salts do notexist. Compound (I) remains unchanged whentreated with bases such as pyridine, Y-picoline,DMSO, DMF, urea, even on warming in dichloro-methane. It is quite surprising that these basescannot replace weakly basic formic acid molecule.This indicates that formic acid is strongly hydrogen-bonded in the AI(OOCH)a network.

Compound (I), however, reacts with ethylene-diamine to form a light pale amorphous solid (IV).A plot of its specific conductivity versus molalityshows that it has conductivity close to that of pureethylenediamine or simple urn-molecular electrolytesin pure formic acid; (IV) may, therefore, beformulated as [enH]t[AI(OOCH),(en)]-.

The IR spectrum of (IV) shows no bands whichwere attributed to coordinated formic acid in (I).A strong band at 1575 cnr? due to "COOas suggeststhat all the three formate groups are equivalent.The "COO. mode, however, appears as a doubletat 1375 and 1340 cm". The splitting of the "COOsb md has been attributed bv Donaldson et al.4 toa change in the mode of coordination of the for-mato group from bidentate to monodentate ligand.The "AI-O bands which appear at 500 and 310crrr? in (I) are absent, but a strong band is observ-ed at 450 cnr+, We suggest a six-coordinated struc-ture for (IV) involving covalently bonded (uniden-tate) formate groups with extensive hydrogen bondingbetween the formate groups and ethylenediamine.

Compound (I) reacts with ammonia to form awhite crystalline product (V). The IR spectrum of(V) is similar to that of (IV). The bands due tocoordinated formic acid are absent in its spectrum.The "COOas band appears at 1575 em"! and the"COOs band is observed as a doublet at 1380 and1320 crrr". The vAI-O band is again observedat 450 cnr". Thus, (V) may be represented asNH~[AI(00CH),(NH3)2J-·

The ammonia complex of (III) has also beenprepared which is analysed to be AI(OOCHh3NH3(VI). The "COOas and "COOs frequencies in thespectrum of (VI) differ very little from the corres-ponding frequencies in (V). This indicat~s that thenature of bonding of the formato groups IS the samein both these compounds. It is likely that theformato groups exhibit unidentate c<?ordin~tion inthe ammine complexes as Al atom IS unlikely toshow coordination number higher than six. Thelow value of "COOas band may be attributed tostrong hydrogen bonding between the forma~ogroups and NHa molecules. These results are in

486

agreement with the earlier investigations on thetriammine complex of aluminium(III) acetates. TheN-H band is significantly lowered (maxima between3260 and 3100 crrr+) in comparison with the vN-Hfrequencies in the free ammonia which are observedat 3414 and 3336 cnr".

Unlike its reactions with tertiary aromatic bases,compound (I) reacts with secondary bases (BH)(piperidine, pyrrolidine and diethylamine) at roomtemperature to form white amorphous solids (VII),(VIII) and (IX) respectively. The composition ineach cass corresponds to Al(00CHh.HCOOH.3BH.These compounds do not exhibit any band whichmay be assigned to coordinated formic acid. Theyexhibit a series of bands in the region 2800-1800crrr+, generally associated with -NH; stretchingvibra.tions=w. The deformation mode of the -NH2group could not be ascertained unambiguouslydue to the presence of "COOas band (1590-1605 cnr<)in the same region. The "AI-O band is observedbetween 450 and 460 crrr+. The vCOOs is observedas a doublet at 1380 and 1320 crrr". In the lightof these observations, compounds VIII-IX may beassigned the structure, BH;[AI(00CH),(BH)2J-.

Compound (I) reacts with trimethylamine andtriethylamine to give white crystalline products(X) and (XI) respectively. They are insoluble incommon organic solvents. The IR spectrum of (X)shows a broad absorption at 2700 crrr+ along witha host of other bands between 2600 and 2000 crrr-'.These bands have been assigned to =NH+ stretchingvibra tions-I. The bands due to coordinated formicacid are missing. The IR spectrum of (XI) retainsall the characteristic vibrations of (I) which arereadily identified. However, it shows two additionalabsorptions centred at 2500 and 2605 cm+, thelatter being more broad and intense. These bandsare assigned to NH stretching vibrations. Thelarge displacement of the band from its normalposition is a ttributed to a strong bond of the type+ -N -H ... O-C-H. Similar observations have earlier

IIobeen. made in the case of triethylamine hydro-chloridew, We feel that (XI) is an adduct oftriethylamine with solvated formic acid, and theAI(OOCH)3 c2ge is not perturbed to any significantextent.

An attempt has also been made to prepare somemixed methoxide derivatives of (I) on the lines ofwork carried out for the preparation of iron(II1)methoxides by the methanolysis of the forma tesor acetates-". A compound (XII), conforming tothe analysis AICaH706, is obtained which can beassigned the structures AI(OCHa)(OOCH)2.H20 orAl(OCH3)(OH)(00CH).HCOOH. Its IR spectrumshows bands at 1065, 790, 725, 552 and 355 em"!in addition to those observed for (I). The band at1065 em+ is assigned to AI-O-CH3 vibrations.This is in conformity with the assignments ofGuertin et al.u and Bell et al.u for AI-O-Cvibrations in several other aluminium alkoxides.The low frequency bands are attributed to "AI-Ofre9.uency of AI-OCH3 moiety in agreement with theassignments of Barraclough et al.I6. The presence

PAUL et al.: ADDUCTS OF AI(III) FORMATE

-of strong bands at 1730 and 1126 cm-1 suggests theformulation Al(OCH)a(OH)(OOCH).HCOOH, but thestructure Al(OCHa)(OOCH)2.H20 cannot be ruledout, if un identate coordination for one of theforma to group is considered.

The thermal decomposition of compounds of (I)with bases have also been studied to establishtheir mode of decomposition. These compounds arethermally unstable and lose base on heating.This was also confirmed by heating these compoundsbelow 50-60° in vacuo and collecting the evolvedbases in separate experiments. The final residue ineach case is A120a. Weight loss values correspondingto various intermediate steps of decomposition aregiven in Table 2. A few representative ToG curvesare given in Fig. 1.

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