Indian Journal of ChemistryVol. 23A, June 1984, pp. 470-474

Transition Metal-Organotin Mixed Complexes: First Examples

T S BASU BAUL, T K CHATTOPADHYAY & B MAJEE*Department of Chemistry, University of North Bengal, Darjeeling 734430

Received 12 December 1983; accepted 9 January 1984

The large difference in the chelating properties of the -CooH and phenolic -OH group in appropriate molecularenvironment towards transition metal ions and organotin groups has been exploited to prepare for the first time mixedcomplexes of the type (R3SnL)2M [(M =Cu2 + , Ni2 +, LHH' =o-carbazoxine (I)]. The Iongest wavelength transition of theligand is bathochromically shifted on complexation, the magnitude of the shift is related to the stability of the metal complex.Spectral and magnetic moment data suggest tetrahedral stereochemistry around the metal atom.

A variety of interesting organotin complexes includingbinuclear complexes having tin atoms of differentcoordination numbers have recently been synthesisedusing o-carbazoxine (I, LHH'; Hand H' refer tophenolic and carboxylic protons respectively) asligand 1. This versatile ligand appears to be well suitedfor preparing hitherto unknown organotin-transitionmetal mixed complexes in which the organotin groupand the metal atom are coordinated to the samepolyfunctional ligand. The difficulty in realising suchmixed complexes lies in the facile replacement of theorganotin groups by transition metal ions due togreater stability of the corresponding transition metalcomplexes". The preparation of such complexes is,therefore, possible only with ligands having differentfunctional groups capable of discriminating betweenan organotin group and transition metal ion due todifference in their coordinating abilities.

(1)

Such a situation is obviously realised in I because thecoordinating site on the quinoline moiety can bindtransition metal ions in preference to organotin groupsdue to greater stability of the transition metalquinolinolates. For example, the formation constantof copper 8-quinolinolate is 2.5 x \023 while that ofcopper acetate.' is only 2.0 x 103.The mixed complexesof type (Ill) are thus expected to be formed by reactingthe transition metal quinolinalotes of type (II)prepared as a first step with (R3SnhO.

470

(n)

(Ill)

Materials and Methods

Preparation of metal derivatives of o-carbazoxine:(LH'hM (M = ca. Ni, Mg)

Solution of the appropriate metal acetate (6 mmol)in water (20 ml) containing a few drops of acetic acidwas added dropwise with constant stirring to a hotsolution ofI (13 mmo\) in N/5 aqueous NaOH (130 ml).The mixture was warmed on a water-bath for 30 minand acidified with acetic acid to precipitate the metalcomplex. The crude precipitate was digested on awater-bath, filtered while hot and washed repeatedlywith water to remove excess acid. The product wasredissolved in NaHC03 and reprecipitated byacidification with acetic acid. The solid was digestedwith methanol, filtered and finally washed with ether.

Preparation of organotin-metal mixed complexes:(R3SnLhM (M = Cu, Ni, Mg)

A mixture of (LH}2M (3 mmol) and (Ph3SnhO (2.8mmol) in dry benzene (200 ml) was refluxed for 15 hr,

BASU BAUL et al.: TRANSITION METAL-ORGANOTIN MIXED COMPLEXES

cooled, filtered, and concentrated to yield crystals ofthe desired product. It was washed repeatedly with hotpet. ether, followed by 2% aq. NaHC03 to removeunreacted (LHJ2M, and finally with water. Theproduct was extracted with hot benzene andrecrystallised from benzene-pet. ether.

Results and DiscussionThe preparation of the transition metal-organotin

mixed complexes of I is based on the following twosteps (1) and (2)

2LHH'+MX2->(LHJ2M+2HX ... (1)

Refluxing in(LHJ2M +(R3SnhO d b •ry enzene

(R3SnLhM + H20 ... (2)

Because of the very low solubility of the ligand inwater, an alkaline solution is used. The metalcomplexes are readily obtained in about 90% yield onaddition of equivalent amount of the metal acetate orhalide, followed by acidification. The organotin-metal

mixed complexes are obtained in 50-80% yield byreaction (2).

Although organotin carboxylates are generallyobtained" in good yield by the reaction of NajK or Ag-salts of the carboxylic acid with R3SnX, the reaction of(LNahM or (LAghM with R3SnX, even for aprolonged period, fails to produce any significantamount of(R3SnLhM. Attempts to prepare the mixedcomplexes by the reaction of R3SnLH typecompounds reported earlier! with MX2 invariablyresult in the cleavage of the R3Sn group.

Analytical data and melting points of thecompounds are given in Table 1.

Infrared spectra

The IR spectral data are summarised in Table 2. Thetransition metal complexes of o-carbazoxine, (LHJ2M,are characterised by a strong absorption at 1700-1720em -1 assignable to VasCOO -. In addition, theappearance of a doublet in some compounds, and abroad band in others, around 560 em -1 ischaracteristic of such complexes. This is probably dueto vM -0, the doublet arising from symmetric and

Table I-Characterisation Data of Transition Metal-Organotin Mixed Complexes

Compound m.p. Found (calc.) (%)°C

(Ph)SnL),Cu(Bu)SnL)2Cu(Pr)SnL)2Cu(Ph)SnL)2Ni(Pr)SnL),Ni(Ph)SnL),Mg(Bu3SnL),Mg

C

60.25(60.67)54.97(54.86)52.60(52.58)60.79(60.89)52.72(52.80)62.28(62.50)57.02(56.68)

235(d)215222

>30028(Xd)185173

H

3.44(3.56)5.69(5.87)5.28(5.25)3.56(3.58)5.19(5.20)3.52(3.67)6.03(6.07)

Sn17.72(17.65)19.40(19.38)21.20(20.80)17.79(17.71)20.83(20.90)18.31(18.18)20.10(20.02)

N

6.20(6.24)6.42(6.85)7.35(7.36)6.16(6.26)7.28(7.30)6.48(6.43)7.12(7.08)

M

4.38(4.72)5.21(5.18)5.62(5.56)4.38(4.37)5.21(5.10)1.85(1.83)2.05(2.02)

Table 2-IR, Electronic Spectral and Magnetic Moment Data of the Complexes

Compound IRa (em -1) Am•• (nm) in !lerr(292.5 K)

va,(OCO) v(M-0) v(Sn- 0) Methanol Benzene (B.M.)

(LH)2CU 1720 525,510 1.79(Ph3SnL),Cu 1620 510< 450 453 460(Bu3SnL)2Cu 1545 510,500 d 452 470 2.06(Pr3SnL),Cu 1545 510,500 430 453 470 1.94(LH)2Ni 1710 490 d 3.04(Ph3SnLhNi 1610b 485< 445 452 440(Pr3SnL),Ni 1610 495< 445 450 440 3.52(Ph3SnL),Mg 1625b 500< d 420 420(Bu3SnL),Mg 1625 500< d 410 415

(a) Due to overlap with strong ring vibrational modes in these region, position of the band maxima may be in error by ± 10 em -1.

(b) Appears as a shoulder of the strong ring vibration modes around 1600 ern -I.

(c) Doublet is not resolved and appears as a broadened absorption band.(d) v(Sn-0) could not be identified.

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INDIAN J. CHEM., VOL. 23A, JUNE 1984

asymmetric combinations of the two vM - 0 modes in(LH'hM. Absorption in this region in transition metalcomplexes of chelating ligands, e.g. oxalic acid,acetylacetone etc. has been attributed+" to a modewhich is predominantly vM - O.

In the mixed complexes, (R3SnLhM, the v.sCOO-is shifted to lower wavenumber and appear in the 1550-1620 em -I region which is usual for organotincarboxylates+ 7,8. The vSn - 0 also appear as a weak tomoderate intensity band around 450 ern -1 (seerefs 7-9).

The low value of va.(OCO) in LHH' and thecorresponding ester LHMel may be attributed to thehydrazone from (IV) in the solid state which isstabilised by intramolecular H-bond involving the-C =0 group!".

On the other hand, substitution of the H-atom of theOH group at C-8 as in the complexes RzSn(LH)z andM(LH)z or the methoxy derivative LH'Me, eliminatesthe possibilitv of hydrazone form and consequently,the intramolecular H-bond involving the carbonyl

IV

group. As a result, I'.JOCO) in such compounds occurin the normal range (- 1700 em -I).

Electronic spectraA CNDOj2 calculation 1 1 -15 on V which may be

regarded as the parent molecule from which LHH' isobtained by substitutions as C-8 and C-2, helps tounderstand the observed electronic spectra of thederivatives of o-carbazoxine.

Of the 83 MOs obtained by linear combination ofthe valence AOs of the atoms, 18 are of zr-type. TheHOMO and the LUMO are n-type showing that thelongest wavelength intense transition in the azoxines 1

is a tt-n" transition. The net atomic charges (a + zt] are

H H *HZ'3~N " 0 H/j 4

N • 5

H H

H

H H(V)

472

shown in Fig. I while the AO coefficients of theHOMO (LUMO) are shown in Fig. 2.The transition density (charge migration) of the first rt-n* transition, shown in Fig. 3, indicates that thelongest wavelength transition involves mostly atransfer of an electron from the quinoline ring, moreparticularly, Nand C-8 atoms to the azo-group, the f3-N atom getting somewhat greater share of the electrontransfer.

Substitution at C-8 by an electron releasing grouplike -OH or -OR would obviously lower thetransition energy of the first n +tt" transition becauseit involves transfer of an electron from C-8 to the azogroup. In the organotin 1 and the metal complexes, theO-atom acquires large negative charge due to highpolarity of the M - 0 bond thereby lowering the rr - rr*transition energy.

+1

-5 +4

+4 -6

Fig. 1-- Electron density in units of 10 -3

(0.3786)-0.4155(0.0128)

-0.0286(-02839)

0.3147 N

01673 -0.0613(0.1731) ( 0.0544)

0.1566 -0.2341(0.1927) (0.2081)

Fig.2 .. AO coefficient of HOMO (LUMO)

-2 +12

-2 + I

-35~O)N +1+lr~1!

-2 + I

-13 + 12

Fig. ~ Charge migration. in units of 10 J in the first tt - n"transition

BASU BAUL et al.: TRANSITION METAL-ORGANOTlN MIXED COMPLEXES

Since the 2' -position is not greatly affected in thefirst 11:-11:* transition, substitution at 2' -position isexpected to have very little effect on this transitionunless the substituent has some intra- or inter-molecular interaction with the other part of themolecule, particularly, the azo group. Any interactionwith the azo N-atoms, either through the H-bond as inLHH' (VI) or in the organotin derivatives with N -vSnbonds (VII), is expected to lead to a bathochromic shiftrelative to that in the non-interacting system (VIII) 10

because withdrawal of charge from the azo groupwould facilitate the flow of electron from the quinolinering to the azo group, thereby, lowering the transitionenergy.

( VI)

(VII )

(V1l1)

The transition metal-organotin mixed complexes,(R3SnLhM, are expected to be structurally quitesimilar to (R3SnLhSnR~ type complexesl. Thecompounds show a single intense absorption in thevisible region both in pro tic and aprotic solvents(Table 2). However, (R3SnL)2M type compounds (M=Cu, Ni) show a larger bathochromic shift ascompared to (R3SnLhSnR' 2' Interestingly, the

magnitude of the shift in (R3SnLhM (M =Cu, Ni, Mg)follows the order of the stability of their oxinatecomplexes, which decreases in the sequence M = Cu>Ni >Mg. This is presumably due to the fact thatCu2+ and Ni2+ have energetically favourable d-orbitals which can interact with the 1I:-MOs of theligand through the oxygen pn-orbital. Since suchinteraction will stabilise the LUMO to a greater extentthen the HOMO of the ligand, a large bathochromicshift of the longest wavelength 11:- n* transition isexpected 16.1 7. On the other hand, energeticallyfavourable d-orbitals are not available either in themagnesium complex or in the organotin complexes.Since the relative stabilities of the complexes will berelated to the extent of metal-ligand interactions, themagnitude of the bathochromic shift of the transitionof the ligand is expected to follow the order of thestabilities of the corresponding metal complexes.

Magnetic momentsThe magnetic moment data, corrected for

diamagnetic contributions, are given in Table 2. Acomparison with the known complexes of Cu2+ andNi2+ having tetrahedral stereochemical environmentaround the metal ions 18 suggests the mixed complexesof the type (R3SnLhM to have a similarstereochemistry around the metal ions. It is, however,interesting to note that stannylation of the carboxylgroup results in an increase of Perr. The increase ispresumably due to increase in the dihedral anglesbetween the planes of the two ligand moleculestowards tetrahedral orientation to avoid stericcrowding of the bulky R3Sn group.

The complexes should, therefore, be formulated asin (IX), the geometry around M being tetrahedral.

AcknowledgementOne of the authors (T S B B) thanks the UGC, New

Delhi, for the award of a junior research fellowship.The authors thank the Director, CDRI, Lucknow foranalytical data and spectral measurements and Prof AK Chandra, Indian Institute of Science, Bangalore forthe CNDO/2 calculation.

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