Metallatriazadiphosphorines of zinc(II), cadmium(II) and mercury(II)

Yash Paul and Sushil K. Pandey*Department of Chemistry, University of Jammu, Baba Saheb Ambedkar Road, Jammu 180 006 (J & K), India

Received 24 February 2003; accepted 21 May 2003

Abstract

Metallatriazadiphosphorine complexes corresponding to [{N(PPh2NR)2}M(OAc)] and [{N(PPh2NR)2}2M],(R = Ph or SiMe3; M = Zn, Cd or Hg) have been synthesized under strictly anhydrous and inert conditions bythe reaction of the acyclic bis-silylated phosphazene ligand, [HN(PPh2NSiMe3)2], or the bis-phenylatedphosphazene ligand, [HN(PPh2NPh)2], with Zn, Cd and Hg acetate in 1:1 and 2:1 molar ratios. These complexesare highly soluble in common organic solvents, but unstable hydrolytically as well as thermally, even under reducedpressure. Molecular weight determinations in benzene indicated the monomeric nature of these complexes. Further,they have been characterized on the basis of elemental analysis and spectral studies: i.r. and n.m.r. (1H, 13C and 31P)that plausibly reveal a trigonal planar and tetrahedral geometry around the metal atom in the complexes.

Introduction

Zinc is one of the most biologically important metalsand is apparently essential to all forms of life. The twozinc enzymes, carboxypeptidase and carbonic anhydr-ase, have received highest attention [1]. The carboxy-peptidase catalyses the hydrolysis of the terminalpeptide bond in proteins during digestion and carbonicanhydrase was the first zinc metallaenzyme to bediscovered (1940); it is also widely distributed in plantsand animals in several forms. In addition, zinc findsseveral applications in various industries [2]. On theother hand, cadmium and mercury are among the mosttoxic of elements. Cadmium is extremely toxic [3] andaccumulates in humans, mainly in kidneys and liver.Prolonged intake even of very small amounts leads todysfunction of the kidneys. The history of the toxiceffects of mercury [4] is long and the use of HgCl2 as apoison is well known.A literature survey has revealed the paucity of infor-

mation on these metals particularly with M–N linkages,while in contrast the compounds having M–O and M–Slinkages are well known [5–8]. Studies on cyclic amides ofgroup 12 elements are quite limited [9], however, thesimple amides are well established [7, 10–11]. Thesynthesis of many novel transition-metal containinginorganic ring systems, particularly the P–N ring system,have been reported in the past decade [12] and haveattracted worldwide attention of scientists. Cyclometal-laphosphazene chemistry is supposed to be a highlyactive area of research because of its versatility inphysical properties as well as interest in academia. Theyhave already found applications as potential precursors

in the field of ceramic and inorganic polymer chemistry[13–16]. In the past, we have also reported the synthesisand characterization of several heterometallacyclophos-phazene complexes containing transition and non-tran-sition elements [17–21]. However, only one report isavailable on the insertion of zinc into the phosphazenering system [22]; no phosphazene complexes have beenreported so far with cadmium and mercury [12, 23]. Inview of this, we have extended research into the synthesisof metallatriazadiphosphorines of Zn, Cd and Hg usingtheir acetates as starting materials, knowing that theacetate moiety may show various linkage modes.

Experimental

Materials and methods

Modified Schlenk techniques, a N2 atmosphere andvacuum line were used to carry out all manipulations.All solvents were distilled from Na/K alloy and degassedthree times before use. The bis-phenylated phosphazeneligand (A) and bis-silylated phosphazene ligand (B) wereprepared by literature methods [24, 25]. High purityacetates of Zn, Cd and Hg were procured commerciallyand used as received. Zinc, cadmium and mercury wereestimated, using the gravimetric method, as zinc am-monium phosphate or pyrophosphate, dipyridinecad-mium thiocyanate and mercury(II) sulfide, respectively[26]. I.r. spectra were recorded on a Perkin-Elmer-577spectrophotometer in the 4000–400 cm)1 range usingKBr mulls. 1H- and 13C-n.m.r. spectra were recorded ona Jeol FX 90Q 90 MHz spectrometer using TMS asexternal reference. 31P-n.m.r. spectra were recorded on aBruker DRX 300 (120 MHz) using 80% H3PO4 as* Author for correspondence

Transition Metal Chemistry 29: 19–23, 2004. 19� 2004 Kluwer Academic Publishers. Printed in the Netherlands.

external reference. Molecular weights were determinedcryoscopically in freezing benzene.

Reaction of the acyclic phosphazene ligand, [HN(PPh2-NR)2], with metal acetate, M(OAc)2 in 1:1 and 2:1 mo-lar ratios (R = Ph or SiMe3 and M = Zn, Cd or Hg)

To a PhMe solution of the acyclic bis-phenylatedphosphazene ligand (A) or bis-silylated phosphazeneligand (B) was added dropwise a PhMe solution of themetal acetate, M(OOCMe)2 (M = Zn, Cd and Hg) in a1:1 and 2:1 molar ratio. No change in color wasobserved. The reaction mixture was stirred at roomtemperature for 1 h, then refluxed for 5–6 h. Duringrefluxing, no color change was observed for zinccomplexes but the color changed to light brownish-yellow for cadmium and mercury. The excess of solventwas removed under reduced pressure, yielding solidcompounds in quantitative yield. Finally, the complexeswere washed with CCl4 and dried in vacuo.A similar method was employed for the synthesis of

all other complexes, except that different refluxingperiods were used. These reactions were also carriedout in benzene and similar results were obtained. Thesynthetic and analytical data of all the complexes aregiven in Table 1.

Results and discussion

The bis-phenylated phosphazene ligand, [HN-(PPh2NPh)2], and bis-silylated phosphazene ligand,[HN(PPh2NSiMe3)2], as shown by structures (A) and(B), are the new members of a fundamentally importantclass of nitrogen-containing chelating ligands. Analo-gous acyclic phosphazene ligands were synthesized inthe past and studied extensively because of their uniquegeometric and electronic properties [12, 16]. It wassupposed that the following two ligands (A) and (B)

would produce the results on a par with the previouslyreported acyclic phosphazene ligands, particularly asregards their donor capabilities.

Acetates of zinc, cadmium and mercury, M(OO-CMe)2, were reacted with acyclic phosphazene ligands

(A) and (B) in refluxing PhMe in 1:1 and 2:1 molarratios, resulting in cyclization via the removal of 1 or 2moles of ethanoic acid, respectively. The reactions werefacile and the complexes were obtained in almostquantitative yields. They are soluble in organic solventsand appear to be moisture sensitive, but can be storedfor a long time under a N2 atmosphere without aging.The derivatives of zinc are white but those of cadmiumand mercury are brownish-yellow. The color of the Cdand Hg complexes is due to the greater ease of chargetransfer from ligands to more polarizing cations. There-fore, the color of the complexes deepens as one movesdown the zinc group. Washing with dry CCl4 furtherpurified the complexes for the purpose of analyticalstudies (Scheme 1).Elemental analyses of metal ions, carbon, hydrogen

and nitrogen were found in accordance with theformation of these complexes and the molecular weightdeterminations of a few of the complexes indicated theirmonomeric nature (Table 1).I.r. spectral assignments (4000–400 cm)1) of these

complexes have been carried out on the basis of earlierreports [23–25]. The absence of a strong m(NH) band3350 cm)1 indicated the deprotonation of the ligand andsubsequently the appearance of a sharp new band in the560–540 cm)1 region has been observed which suggestsm(M–N) bond formation. The m(C@O) frequency ap-pears in the 1690–1650 cm)1 region, which is indicativeof the monodentate nature of the acetate moiety whilem(M–O) occurs in the 740–690 cm)1 region. The bandsof characteristic absorption for m(P–N) linkages werefound in the 1090–1400 cm)1 region for these complexesthat are in accordance with the P–N–P ring system(Table 2).The 1H-n.m.r spectra of these complexes (in CDCl3)

gave no signal for the ANH proton in the d4.5–5.0 p.p.m. region (present in the parent ligand) [23–25], which indicated the deprotonation of the ligand.The phenyl protons of APPh2 (1–6) were found as amultiplet in the d6.60–8.25 p.p.m. region where asphenyl protons for APPh2 and ANPh in (7–12) werefound as a double multiplet in the d6.70–7.0 and d7.2–8.85 p.p.m. region. The chemical shift for the ASiMe3protons was observed in the d0.12–0.10 p.p.m. regionwhile the protons of the acetate moiety were found witha slight chemical shift down field in the d2.3–1.9 p.p.m.region, which is indicative of complexation. It isnoteworthy that no chemical shift for the methylprotons of the acetate moiety has been observed in thecases of 2:1 molar ratio products, confirming theformation of a spirocyclic system (Table 3).

Scheme 1.

20

Table

1.Synthetic

andanalyticdata

ofcomplexes

ofthetype[{N(PPh2NR) 2}M

and[N

(PPh2NR) 2M(O

Ac)]

Sample

no.

Reactants

Molarratio

(refluxperiod)

Solidproduct

(physicalstate)

Yield

(%)

[m.p.(�C)]

Found

(calcd.)%

Mol.wt.

found(calcd.)

HN(PPh2NR) 2

(g)

M(O

Ac)

2g(m

mol)

MC

HN

(1)

0.65

Zn(O

Ac)

22:1

[{N(PPh2NSiM

e 3) 2} 2Zn]

92

5.4

59.6

6.25

7.0

1173

(1.11mmol)

0.10(0.55)

6h

(white)

[75]

(5.5)

(60.9)

(6.4)

(7.1)

(1181)

(2)

1.06

Zn(O

Ac)

21:1

[N(PPh2NSiM

e 3) 2Zn(O

Ac)]

95

9.3

66.3

6.0

6.1

688

(1.18mmol)

0.34(1.18)

5h

(white)

[35]a

(9.5)

(66.9)

(6.0)

(6.15)

(682)

(3)

0.76

Cd(O

Ac)

22:1

[{N(PPh2NSiM

e 3) 2} 2Cd]

93

9.05

57.7

5.85

6.4

1205

(1.30mmol)

0.15(0.65)

6h

(brownish-yellow)

[86]

(9.1)

(58.6)

(6.2)

(6.8)

(1228)

(4)

0.59

Cd(O

Ac)

21:1

[N(PPh2NSiM

e 3) 2Cd(O

Ac)]

95

15.30

61.6

5.0

5.1

725

(1.00mmol)

0.24(1.00)

5h

(brownish-yellow)

[40]a

(15.4)

(62.55)

(5.6)

(5.8)

(729)

(5)

0.71

Hg(O

Ac)

22:1

[{N(PPh2NSiM

e 3) 2} 2Hg]

92

15.50

54.1

5.05

6.3

(1.20mmol)

0.20(0.60)

6h

(brownish-yellow)

[65]

(15.2)

(54.7)

(5.8)

(6.4)

–

(6)

0.93

Hg(O

Ac)

21:1

[N(PPh2NSiM

e 3) 2Hg(O

Ac)]

97

24.20

54.8

4.9

5.25

–

(1.60mmol)

0.53(1.60)

5h

(brownish-yellow)

[72]

(24.5)

(55.8)

(5.0)

(5.1)

(7)

0.66

Zn(O

Ac)

22:1

[{N(PPh2NPh) 2} 2Zn]

94

5.4

71.1

4.7

7.1

1178

(1.10mmol)

0.10(0.55)

6h

(creamish)

[79]

(5.4)

(72.2)

(5.0)

(7.0)

(1197)

(8)

0.69

Zn(O

Ac)

21:1

[N(PPh2NPh) 2Zn(O

Ac)]

93

9.15

66.15

4.1

6.1

685

(1.21mmol)

0.22(1.21)

6h

(creamish)

[37]a

(9.4)

(66.1)

(4.8)

(6.1)

(690)

(9)

0.83

Cd(O

Ac)

22:1

[{N(PPh2NPh) 2} 2Cd]

97

8.6

68.4

9.65

6.5

(1.40mmol)

0.16(0.70)

6h

(brownish-yellow)

[39]a

(9.0)

(69.45)

(9.8)

(6.75)

–

(10)

0.45

Cd(O

Ac)

21:1

[N(PPh2NPh) 2Cd(O

Ac)]

94

15.8

61.00

4.0

5.1

(0.793mmol)

0.18(0.79)

5h

(brownish-yellow)

[68]

(15.2)

(61.9)

(4.5)

(5.7)

–

(11)

0.70

Hg(O

Ac)

22:1

[{N(PPh2NPh) 2} 2Hg]

97

15.25

64.4

6.05

4.8

1321

(1.23mmol)

0.39(1.23)

6h

(brownish-yellow)

[55]

(15.0)

(65.3)

(6.3)

(4.5)

(1332)

(12)

0.53

Hg(O

Ac)

21:1

[N(PPh2NPh) 2Hg(O

Ac)]

95

25.1

58.5

3.8

5.2

758

(0.93mmol)

0.29(0.93)

5h

(brownish-yellow)

[49]

(26.1)

(59.5)

(4.3)

(5.5)

(766)

WhereM

=Zn,CdorHg;R

=ASiM

e 3(1–6)andAPh(7–12).

aDecomposition.

21

The 13C-n.m.r. spectra of the complexes (in CDCl3) atambient temperature do not differ significantly in theirchemical shift from those of parent acyclic phosphazeneligands and metal acetates. However, the 13C-n.m.r. ofthe complexes in a 1:1 molar ratio shows the presence ofcarbon nuclei of the acetate moiety whereas no carbonnuclei of acetate moiety were observed in the productsof a 2:1 molar ratio.In 31P-n.m.r. spectra, one singlet was found in each

complex of ligand (A) and (B) but with a downfield shiftin the d5–9 or d4–6 p.p.m. range, respectively, comparedto the parent ligands. The chemical shift was observed inthe d12–18 p.p.m. and d14–19 p.p.m. region as a singletfor the complexes with ligands (A) and (B), respectively.

The occurrence of a singlet in these complexes is due tothe equivalence of phosphorus nuclei in the complexthat supports the symmetric nature of the molecules(Table 3).

Structural features

Although, it would not be appropriate to predictprecisely the structure of these complexes, since ourefforts to obtain suitable crystal were unsuccessful, aliterature survey has revealed that of the variousstructural possibilities for these elements, a trigonalplanar and regular tetrahedral is well-known, particu-larly with amine ligands. Furthermore, these ligands

Table 2. I.r. spectral data of complexes of Zn, Cd and Hg (in cm)1)

Sample no. Compound m(C@O) m(P–N–P) m(P@N) m(M–O) m(M–N)

(1) [{N(PPh2NSiMe3)2}2Zn] – 1120–1400 1200,vs – 560,vs

(2) [N(PPh2NSiMe3)2Zn(OAc)] 1650,vs 1120–1280 1250,vs 740,vs 540,vs

(3) [{N(PPh2NSiMe3)2}2Cd] – 1150–1425 1240,vs – 550,vs

(4) [N(PPh2NSiMe3)2Cd(OAc)] 1650,vs 1110–1240 1225,vs 700,vs 540,vs

(5) [{N(PPh2NSiMe3)2}2Hg] – 1120–1240 1230,vs – 550,vs

(6) [N(PPh2NSiMe3)2Hg(OAc)] 1658,vs 1116–1261 1236,vs 746,vs 560,vs

(7) [{N(PPh2NPh)2}2Zn] – 1120–1260 1230,vs – 545,vs

(8) [N(PPh2NPh)2Zn(OAc)] 1690,vs 1100–1250 1170,vs 740,vs 540,vs

(9) [{N(PPh2NPh)2}2Cd] – 1120–1230 1180,vs – 550,vs

(10) [N(PPh2NPh)2Cd(OAc)] 1690,vs 1099–1250 1190,vs 690,vs 560,vs

(11) [{N(PPh2NPh)2}2Hg] – 1120–1240 1170,vs – 540,vs

(12) [N(PPh2NPh)2Hg(OAc)] 1670,vs 1264–1324 1221,vs 696,vs 550,vs

vs = very strong, s = strong and m = medium.

Table 3. 1H-, 13C- and 31P-n.m.r. spectral data of complexes of Zn, Cd and Hg (in d p.p.m.)

Sample no. Compound 1H-n.m.r.a 13C-n.m.r.a 31P-n.m.r.a

(1) [{N(PPh2NSiMe3)2}2Zn] 0.12,s, 36H (SiMe3) 3.5 (ASiMe3) 15.06,s

6.80–8.20,m, 40H (APh) 125–134 (AC6H5)

(2) [N(PPh2NSiMe3)2Zn(OAc)] 0.10,s, 18H (ASiMe3) 3.5 (ASiMe3) 15.75,s

1.96,s, 3H (ACH3) 25.5 (ACH3)

6.75–8.08,m, 20H (APh) 122–135 (AC6H5), 170 (ACO)

(3) [{N(PPh2NSiMe3)2}2Cd] 0.9,s, 36H (ASiMe3) 3.4 (ASiMe3) 17.09,s

6.80–8.25,m, 40H (APPh2) 122–135 (AC6H5)

(4) [N(PPh2NSiMe3)2Cd(OAc)] 0.18,s, 18H (ASiMe3) 3.4 (ASiMe3) 16.96,s

2.14,s, 3H (ACH3) 3.5 (ACH3)

6.60–8.20,m, 20H (APh) 123–134 (AC6H5), 171 (ACO)

(5) [{N(PPh2NSiMe3)2}2Hg] 0.12,s, 36H (ASiMe3) 3.3 (ASiMe3) 16.69,s

6.70–8.20,m, 40H (APPh2) 123–134 (AC6H5)

(6) [N(PPh2NSiMe3)2Hg(OAc)] 0.10,s, 18H (ASiMe3) 3.3 (ASiMe3) 18.98,s

6.70–8.20,m, 40H (APPh2) 25.5 (ACH3)

(7) [{N(PPh2NPh)2}2Zn] 6.20–7.40,m, 20H (ANPh) 123–134 (AC6H5) 16.50,s

7.50–8.20,m, 40H (APPh2) 122–134 (AC6H5)

(8) [N(PPh2NPh)2Zn(OAc)] 1.9,s, 3H (ACH3) 25.3 (ACH3) 16.95,s

6.30–8.20,m, 30H (APh) 123–135 (AC6H5)

(9) [{N(PPh2NPh)2}2Cd] 6.20–7.25,m, 30H (ANPh) 123–134 (AC6H5) 16.94,s

7.40–8.25,m, 30H (APPh2)

(10) [N(PPh2NPh)2Cd(OAc)] 2.15,s, 3H (ACH3) 25.3 (ACH3) 15.08,s

6.60–8.35,m, 60 (APh) 125–134 (AC6H5)

(11) [{N(PPh2NPh)2}2Hg] 6.30–7.40,m, 20H (ANPh) 170 (ACO) 15.45,s

7.50–8.30,m, 40H(APPh2) 128–132 (AC6H5)

(12) [N(PPh2NPh)2Hg(OAc)] 1.9,8, 3H (ACH3) 25.3 (ACH3) 16.10,s

6.30–7.40,m, 10H (ANPh) 127–132 (AC6H5)

7.50–8.25,m, 20H (APPh2) 170 (ACO)

s = singlet and m = multiplet.a Chemical shift.

22

commonly depict the bidentate mode of bonding [19,22–23]. Therefore, in conjunction with molecularweight, elemental analysis, spectral studies like i.r. andn.m.r. (1H, 13C and 31P) and literature reports [22], atrigonal planar geometry for the complexes [{N(PPh2-NR)2}M(OAc)] and tetrahedral (th) geometry [27] forthe complexes [{N(PPh2NR)2}2M] may tentatively beproposed in which the phosphazene ligand has abidentate mode of bonding with the central metal atom(Figure 1a and b).

Acknowledgements

We gratefully acknowledge financial support from theDST, New Delhi. The authors thank the RRL Jammuand Department of Chemistry, University of Rajasthan,Jaipur for spectral studies. One of the authors (Y.P.) isgrateful to the DST, New Delhi for a JRF.

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TMCH 5618

Fig. 1. (a) Trigonal planar geometry of [N{PPh2NR}2M(OAc)] and (b) tetrahedral geometry of [{N(PPh2NR)2}2M] (R = Ph or SiMe3 and

M = Zn, Cd or Hg).

23