The chemistry of tin (II) chelates—I

J. lnorg. Nucl. Chem., 1964, Vol, 26, pp. 767 to 772. Pergamart Press Ltd.

Printed in Northern Ireland

THE CHEMISTRY OF TIN (II) CHELATES--I S O L I D C H E L A T E S O F T I N (II) W I T H E T H Y L E N E D I A M I N E TETRA-ACETIC ACID (EDTA) H. G. LANGER The Dow Chemical Company, Eastern Research Laboratory, Framingham, Massachusetts (Received 21 January 1963 ; in revised form 10 October 1963)

Abstract The preparation and isolation of three new crystalline EDTA chelates Na~Sn(II)Y'2H~O,* Sn(II)2Y'2H20, and CaSn(II)Y'4H20 is reported. Infra-red spectra, and analytical data indicate that Na2Sn(II)Y'2H20 is the sodium salt of a 1 : 1 tin (I1) EDTA chelate, and that Sn(II)2Y'2H..O and CaSn(II)Y.4H~O are binuclear chelates. It is further suggested that tin in these compounds can have a co-ordination number less than six. E D T A CHELATES with divalent m e t a l ions such as c o p p e r a n d nickel h a d been rep o r t e d as early as in 1942311 In 1943, BRINTZINGER et al3 ~1 isolated a slightly soluble 2 : 1 lead (II) E D T A c o m p o u n d a n d suggested the f o r m u l a CH 2 COO /

CH 2 N

Pb CH2

COO /

J ! i

-

(

H20

C H 2 COO

Il

CH2

N/

Pb CH z - C O O

This structure was rejected by TANAKA et al. 13'41 who p o s t u l a t e d the c o m p o s i t i o n Pbe+[PbY]2-'HzO for the 2:1 complex, b a s e d on m e a s u r e m e n t in a q u e o u s electrolyte solutions. SAWYER a n d PAULSEN,ts'6) in an extensive discussion o f solid E D T A chelates a n d their infra-red spectra d i d not assign a definite c o - o r d i n a t i o n n u m b e r for lead in N a 2 P b Y . H 2 0 . A l t h o u g h the tin (II) ion is similar to the lead (II) ion, tin (1I) E D T A complexes are c o n s p i c u o u s l y a b s e n t in all publications. The structure o f a 2:1 S n ( I I ) - E D T A chelate was r e p o r t e d f r o m this l a b o r a t o r y in 1960. tT) P a r t I o f the present p u b l i c a t i o n describes the p r e p a r a t i o n o f three solid tin (II) E D T A c o m p o u n d s , including the 2:1 c o m p l e x , t The difference in structure between the linear 2 : 1 c o m p l e x a n d the cyclic 1 : 1 chelate was established by using * Y = EDTA anion. t Studies on the tin (II)-EDTA system in aqueous solutions will be reported in a subsequent paper. !1> H. BR1NTZINGERand G. HESS,Z. Anorg. Chem. 249, 113 (1942). ~:~ H. BRI~qTZINGER,H. TmELE and U. Mi3LLER,Z. Anorg. Chem., 251, 285 (1943). ~3~N. TANAKA,K. KATOand R. TAMAMUSm,Bull. Chem. Soc. Japan 31, 283 (1958). ~) N. TANAKA,M. KAMATAand G. SAT6, Bull. Chem. Soc. Japan 34, 541 (1961). " D. T. SAWYERand P. J. PAULSEN,7. Amer. Chem. Soc. 80, 1597 (1958). ~';~D. T. SAWYERand P. J. PAULSEN,J. Amer. Chem. Soc. 81,816 (1959). ~:~ H. G. LArqGER,J. Dent. Res. 39. 740 (1960). 767

768

H . G . LANGER

a new c o n c e p t for the i n t e r p r e t a t i o n o f infra-red spectra, involving the c o - o r d i n a t i o n o f water. Differences in o b s e r v e d p r o p e r t i e s are indicative o f different structures which in t u r n require a difference in the m e t a l c o - o r d i n a t i o n . EXPERIMENTAL To obtain reliable and reproducible results, it was essential to exclude oxygen rigorously not only during the preparation and isolation of tin (II)-EDTA complexes but also from all reagent solutions used.

A. Preparation (a) NazSn(II)Y'2H20. Two mmoles of solid SnCI~ were added with constant stirring to a solution of 2 mmoles H4Y and 8 mmoles NaOH in 14 ml of O2-free H20. Heating to 60°C produced a clear solution from which pure Na2Sn(II)Y.2H~O crystallized in 60 per cent yield after addition of 150 ml ethanol. Subsequent fractions contained impurities of NaC1 and Sn(IV)--EDTA chelates. All operations were carried out under nitrogen. Found*: C, 25"0; H, 3'3; N, 5"7; Sn~+, 23.68, 24"15. Calc. C, 24"57; H, 3"30; N, 5'73; Sn e+ 24'28~. (b) Sn(II)aY.2H20. Twenty mmoles of solid SnCI2 were added with vigorous stirring to a hot solution of 10 mmoles Na~HzY in 50 ml H20. Upon cooling, Sn(II)~Y-2H~O crystallized in 90 per cent yield. Addition of alcohol or acetone made the yield almost quantitative. A blanket of nitrogen was used to avoid oxidation. The identical product was obtained in 80 per cent yield from 1.25 mmoles H4Y and 2.5 mmoles SnC12 in 200 ml H~O at room temperature. Here, crystallization required two days. Found: C, 21'5; H, 2"7; N, 5"0; Sn, 42"8; Sn~+,43.60, 42.76. Calc. C, 21.39; H, 2.87; N, 4"99; Sn 2+, 42.27%. (c) CaSn(II)Y.4H~O. A slurry of 4 mmoles of Sn(II)~Y.2H~O and 4 mmoles of CaCO3 in 30 ml of oxygen-free water was stirred under nitrogen and heated to 60°C for 30 min. A coffee-brown precipitate of hydrated stannous oxide was removed by filtration. From the clear filtrate, a 90 per cent yield of crystalline CaSn(II)Y'4H20 was obtained after addition of 60 ml ethanol Found: C, 23.6, 23.4; H, 4.0, 4.0; N, 5.4, 5.3; Ash, 40-64, 39.0; Sn 2÷, 22.88, 22.73. Calc. C, 23.14; H, 3.88; N, 5.40; Ash (CaO.SnO~) 39.48 Sn ~+, 22.87%.

B. Dehydration Samples of the three compounds were heated to 160° for 10 days and analysed. Na2Sn(II)Y. Found: C, 23"6; H, 2"6;N, 5'9; Ash, 53'7. Calc. C, 23.87; H, 2'67; N, 6.19; Ash ('Na2COs + SnOa), 56"68%. Sn(II)2Y. Found: C, 22"5; H, 2"0; N, 5'4; Sn, 45"5. Calc. C, 22.85; H, 2'30; N, 5"0;3 Sn, 45"17%). CaSn(II)Y. Found: C, 26.8; H, 2.8; N, 6-1; Ash, 47.3. Calc. C, 26-87; H, 2"71; N, 6.27; Ash (CaO.SnO2), 46"26%.

C. Infra-red spectra Infra-red spectra were recorded as Nujol mulls on a Baird infra-red recording spectrophotometer Model 4-55. The Nujol bands served as internal standards. RESULTS

A, Properties Sn(II)aY.2H~O crystallizes in colourless p r i s m a t i c tablets, i r r e g u l a r , with a n ext i n c t i o n angle o f 33 ° a g a i n s t t h e l o n g edge. The tasteless a n d o d o u r l e s s crystals are * Combustion analyses were carried out by Dr. C. K. Firz, Needham Heights, Massachusetts. Metal contents were calculated from ash; where designated as Sn"J+, tin was determined by iodometric titration.

The chemistry of tin (1I) chelates- I

769

insoluble in all common organic solvents, and the solubility in water is less than 0.05 per cent. The anhydrous compound is stable at 300 ° for several days. The colourless crystals of Na2Sn(II)Y.2HzO are irregular and decompose above 300 ° without melting. They are very soluble in H20, but insoluble in common organic solvents.

o F--

$

4

5

6

7

8

9

Wovelengl"h,

I0

It

17'

13

14

~5

1~5

/a.

FIG. 1

CaSn(II)Y-4H20 was also obtained as colourless a n d irregular crystals. The compound is slightly less soluble in water than the disodium salt, and insoluble in the common organic solvents.

B. Infra-red spectra The spectrum of Na~Sn(II)Y.2H20 is practically identical with that reported for Na2Pb(II)Y.H20. <6~ The main difference between the three tin compounds is a shift of the major carbonyl absorption to lower wave numbers in the order of Na~Sn(II)Y.2H20 , CaSn(II)Y'4H20, and Sn(II)~Y.2H~O. The carbonyl stretching frequencies of both the disodium tin (II), and the calcium tin (II) complex are in the neighbourhood of 1600 cm -1, close to that of tetrasodium EDTA. The major band for Sn(II)zY.2HzO is lower than that of the carboxylate anion, ~n,9>indicating a bond character of less than 1-5 for both carbon to oxygen bonds of the carboxylate groups.

770

H . G . LANGER TABLE 1.--INFRA-RED FREQUENCIES OF TIN

Na~Sn(II)Y'2H~O

CaSn(II)Y'4H20

Sn(II)2Y'2H~O Na2Sn(II)Y

COOMax.

C--N

COO-

3401 s*

1600 b

1116 s 1105 s

927 s 921 sh

1094 s 1109 s

931 s

33553245 b 3150 b

1587 b

3546 s

1569 b

--

1613 b

Sn(I1)2Y

Na,Y.2H~O NavY K4Y

CHELATES

O--H

CaSn(II)Y

H,~Na2Y'2H20

(lI)

3521 s 3396s 3226 b

1605 -1580 b 1672sh 1637 s 1565 b 16341618 b 16131587 b 1579 (s)

1096 s 1086s 1105s 1091 s 1112s 1098sh 1089 s 1098sh 1092s 1105 s I092s

924 sh 928 s 924 sh 930sh 923 s 912sh 933sh 926 s 932sh 926s 917s

1595 tg)

* Abbrev. : s ~ sharp, b = broad, sh = shoulder. Underlined figure represents strongest band in group. DISCUSSION Based on the empirical formulas, as established b y analysis, the chemical p r o p erties, as well as infra-red d a t a clearly indicate a difference in structure b e t w e e n N a 2 S n ( I I ) Y . 2 H 2 0 a n d Sn(II)2Y.2H20. Na2Sn(II)Y.2HzO. T h e structure o f the water-soluble N a z S n ( I I ) Y . 2 H 2 0 is similar to t h a t o f Nag P b ( H ) Y ' H 2 0 as evidenced b y their similar infrared spectra, t6~ The single s h a r p O H b a n d at 3401 c m -1 for the d i h y d r a t e incidates t h a t b o t h w a t e r molecules a r e identical a n d h y d r o g e n b o n d e d to c a r b o n y l groups. A l l d a t a c a n be e x p l a i n e d for tetra- a n d h e x a - c o - o r d i n a t e d tin as s h o w n in T a b l e 2. I f the c h e l a t i n g agent acts as a h e x a d e n t a t e ligand, the c o - o r d i n a t i o n n u m b e r is six. F o r t e t r a d e n t a t e E D T A , two a r r a n g e m e n t s a r e possible. Since the l o c a t i o n o f the w a t e r is uncertain, the c a r b o n y l a b s o r p t i o n n e a r 1600 c m -1 allows no conclusion o n either the degree o f ionic c h a r a c t e r o f the t i n - o x y g e n b o n d o r the c o - o r d i n a t i o n n u m b e r o f tin. T h e b r o a d a n d split c a r b o n y l b a n d s o f the a n h y d r o u s c o m p o u n d , however, p r o v i d e a n a r g u m e n t against the s e c o n d e x a m p l e in T a b l e 2. ~a~D. CHAPMAN,J. Chem. Soc. 1766 (1955). ~9~R. E. SIEVERSand J. C. BAILAR,JR., lnorff. Chem. 1, 174 0962).

The chemistry of tin (II) chelates--I

771

TABLE 2.--LIOAND DISTRIBUTIONIN TIN (II)-EDTA CHELATES

Compound Na2Sn(II)Y.2H~O

Possible co-ordination no. Sn:4

Co-ordinated groups

Frequency

2 Nitrogen atoms 2 - - C O 0 groups (H-bonded)

1600 b

Unco-ordinated groups 2 --COO groups (ionic) 2 H.~O molecules (H-bonded)

Frequency 1600h 3401

or

Sn:6

2 nitrogen atoms 4 --COO- groups (ionic)

2 H.20 molecules (H-bonded) 1600 b

3401s _r

or

Sn:6

Sn(II),,Y.2H,,O

Sn',Sn":4

2 nitrogen atoms 2 - - C O O - groups (H-bonded) 2 H20 molecules (H-bonded) 1 nitrogen atom 1 H~O molecule (H-bonded) 2 - - C O 0 - groups (asym. coord.)

1600 b 3401 s

2-- CO0 groups (ionic) ....

1600 b

3546 s 1569 b

or

Sn',Sn" : 6

I nitrogen atom 1 H20 molecule (H-bonded) 2 - - C O 0 - groups (symm. coord.)

3546 s 1569 b

or

Sn':4 Sn" : 4

CaSn(II)Y-4H20 Ca:6

2 nitrogen atoms 2 - - C O O - groups 2 H20 molecules (H-bonded) 2 --COO groups

1569 b 3546 s 1569 b

(Sn: analogous to Sn(II)2Y.2H~O) 1 nitrogen atom 2 - - C O 0 - groups 3 H20 molecules (H-bonded)

S n ( I I ) a Y . 2 H 2 0 . F r o m the s h a r p - - O H a b s o r p t i o n at 3546 c m -1 it is a p p a r e n t t h a t b o t h w a t e r m o l e c u l e s in the crystalline c o m p l e x are identical. T h e b a n d at 1660 c m -1 is t e n t a t i v e l y a s s i g n e d to a n - - O H b e n d i n g f r e q u e n c y r a t h e r t h a n a c a r b o n y l a b s o r p tion. (l°,m Since the m a i n c a r b o n y l f r e q u e n c y at 1569 c m -1 is lower b y 26 c m --x t h a n t h a t o f the c o m p l e t e l y i o n i z e d c a r b o n y l a t e g r o u p o f K4Y, - - w h e r e , a c c o r d i n g to SIEVERS a n d BAILAR, " t h e possibilities for c a r b o x y l a t e r e s o n a n c e are at a m a x i m u m , "(m c~0) I. GAMO,Bull. Chem. Soc. Japan 34, 760 (1961). ~11, I. GAMO, Bull. Chem. Soc. Japan 34, 764 (1961).

772

H.G.

LANGER

---electron withdrawing groups must be attached to the carbonyl oxygen. This is possible by either hydrogen bonding between water and carbonyl groups or by interor intra-molecular participation of both oxygen atoms of a carboxylate group in co-ordination to the metal. The latter condition has been described as symmetrical co-ordination by DUNCANSON e t aL (12)for organoboron compounds and by BEATTIE e t a/.t13) for organotin compounds. For the dehydrated compound Sn(II)2Y a band at 1565 cm -1 is good evidence for a carbon-oxygen bond order of < 1.5 (II) which requires penta-co-ordinated tin, possibly with the lone electron pair in the sixth position. .// II

--C

O---~

% 0--~

The preference for a binuclear structure [H~O-SnYSn-OH~] (first two examples in Table 2) over the salt Sn2+[SnY].2HzO is based on the difference in properties between Sn(II)~Y'2H20 and Na~Sn(II)Y.2H20. Evidence for the existence of the binuclear complex in aqueous solution will be presented in a subsequent paper. CaSn(II)Y'4HzO. Very similar properties of CaSn(II)Y'4H20 and Sn(II)2Y.2H20 suggest analogous structures for the two compounds. It can be expected that the additional two water molecules in the mixed chelate are loosely co-ordinated to the calcium ion. The higher degree of hydration allows increased and stronger hydrogen bonding between water molecules which is expressed by a considerably broader and slightly lower - - O H absorption and a broadening of the carbonyl band at 1587 cm-L In conclusion, the data presented here suggest that Sn(II)~Y.2HzO is a true binuclear chelate and not a tin (II) salt of a 1 : 1 tin (II)-EDTA chelate. An unusually low asymmetric > C = O stretching frequency for the 2:1 complex does not fit into the series of ionic chelates reported by SmVERSand BAILAR(9) but confirms the statement by HOARDe t aL a4) that the stereochemistry of EDTA chelates is "frequently unorthodox." The assumption of SAWYER e t aL cn) that the co-ordination number of lead in NazPb(II)Y.H20 could be less than six also applies to the tin (II)-EDTA system. Acknowledgement--The author is indebted to Mrs. A. H. BLUT for assistance in the preparative work, and to Drs. A. E. MARTELL, F. W. MCLAFFERTY and R. F. BOGUCKI for helpful discussions. tl2) L. A. DUNCANSON, W. GERRARD, M. F. LAPPERT, H. PYSZORA and R. SHAFFERMAN,J. Chem. Soc. 3652 (1958). (la) I. R. BEAT'fIE and T. GILSON, dr. Chem. Soc. 2585 (1961). (14) j. L. HOARD, B. PEDERSEN,S. RICHARDS and J. V. SILVERTON,J. Amer. Chem. Soc. 83, 3533 (1961).