~ Pergamon Po(vhedron Vol. 16. No. 4, pp. 551 566. 1997

Copyright :t; 1996 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved

P I I : S0277-5387(96)00243-4 0277 5387,'97 $17.00+0.00

P O L Y H E D R O N REPORT NUMBER 64

Some recent coordination chemistry of lead(ll)

Jonathan Parr

Department of Chemistry, Loughborough University. Loughborough, Leics LEI1 3TU, U.K.

1. 2.

C O N T E N T S INTRODUCTION . . . . . . . . . . CONVENTIONAL LIGANDS . . . . . . . . . . . . . . 2.1. Group 14 : donor ligands . . . . . . . . . . . . . . . 2.2. Group 15 : nitrogen ligands . . . . . . . . . . . . 2.3. Group 15 : phosphorous ligands . . . . . . . . . . . . 2.4. Group 16: oxygen ligands . . . . . . . . . . . . . . . 2.5. Group 16: sulfur and selenium ligands . . . . . . . . . . . . 2.6. Miscellaneous . . . . . . . . . . . . . . . . .

551 551 551 552 555 556 557 559

3. MACROCYCLIC LIGANDS . . . . . . . . . . . . . . . . . . 559 3.1. Nitrogen donor macrocyclic ligands . . . . . . . . . . . . . 559 3.2. Nitrogen/oxygen mixed donor macrocyclic ligands . . . . . . . 560 3.3. Nitrogen/sulfur mixed donor macrocyclic ligands . . . . . . . . . . . . 564 3.4. Oxygen donor macrocyclic ligands . . . . . . . . . . . . . . 564

4. CONCLUSION . . . . . . . . . . . . . . . . . . . . . 564

1. I N T R O D U C T I O N

There has been a resurgence of interest in the coordinat ion chemistry of heavy p-block elements in recent years, and this review is concerned with reports of new coordination chemistry of lead(II) that have been published during this renaissance. Since it is a rather large area of chemistry, this report is not comprehensive, but even limiting it to only the most interesting examples has allowed the inclusion of many varied compounds. The expansive literature dealing with macrocyclic ligands is treated in a separate section as it merits being considered as a "special case" of coordination chemistry.

Of all p-block elements, lead(II) has a particular fascination for coordination chemists, as it can adopt many different geometries in its complexes, allowing a degree of tolerance for ligand configurations which is not seen in, for example, d-block elements. Coupled with the ability to bind well to both hard and soft donor atoms, these properties make lead(II) an interesting metal to study, forming as it often does complexes which are different from those which might be conventionally expected,

The toxic nature of lead and its widespread occurrence in the environment, subsequent to its many appli- cations in technology, adds a focus to the search for powerful and selective lead binding materials and ligand systems. For example, there is no oral chelator available for therapeutic uses in the treatment of lead poisoning that has been specifically designed for lead. Certainly much remains to be discovered in the field of lead coordination chemistry, despite the large body of work already published, of which this review covers only a part.

2. C O N V E N T I O N A L LIGANDS

2.1. Group 14 : donor/igands

The family of substituted plumbocenes has been expanded to include examples where 1,2,3,4, l ',2',3",4' = Ph, 5 , 5 ' = 4-(t-Bu)Ph [1], and where 1,2,3,4,5,1',2',3',4',5' = Ph [2] or Me [3]. The half sandwich complexes [Cp*PbX] (where X = C 1 - , [BF4]- or CF~ SO3) were also prepared [8] and their exchange reactions with 2,2" bipyridyl (bipy) and 1,10 phenanthroline (1,10 phen) explored.

The reaction of plumbocene with an excess of ]giCp] yields the fascinating compound [Li(12-C-4)]2

551

552 J. Parr

[(CsHs)gPb4] [(CsHs)sPb2], 1, crystallised on addition of a crown of ether to the reaction mixture. The structure comprises both triple and quintuple decker sandwich moieties [4].

\

~ " " ' ~ ' " ' P b " ' ' ' ~ :Pb . . . . 0 - - P b ~

The two anions present in 1.

Heteroatom analogues of (Cp)- have been used to prepare the plumbocene like complex 2 where R = (t- Bu). The crystal structure of 2 reveals a tilt angle of 142.7 ° indicating that as with other plumbocene type compounds, the remaining lone pair on the lead is stereochemically active [5].

R

Pb I R'< R

2.2. Group 15 : nitroyen liyands

Aminoborane ligands have widespread utility in the coordination chemistry of most metals and lead(II) is no exception. The interaction of polypyrazolylborates with simple lead(lI) salts yields a variety of products, depending upon the substitution of the Tp ligands and the reaction stoichiometry (Scheme 1).

Compounds 3, 4, and 5 were analysed by X-ray crystallography to reveal a distorted trigonal bipyramidal (TB) geometry for 3, and irregular octahedral geometries for both 4 and 5 [6].

The dilithium salt of the aminoborane 6 reacts with lead(II) chloride to give the dimeric compound 6a where in each of the lead(II) centres is tricoordinate, exhibiting two kite shaped [PbNBN] rings linked together through two further Pb-N bonds [7]. A related structural motif is seen in [Pb(NR2)]4 [8].

H H I I tBu

IN-.. IN-.. \

'Bd - - p b ~ . . N I

\ tBu

6 6a

Coordination chemistry of lead(II) 553

+ t~2 + ~-

N .N,

(pz)z-- B ~,. N -N"

+ pb2+

N ..... N,.. /

H - - B , ~ N - - N N N"

>.Pb

Scheme. 1.

The synthesis of [(t-Bu)(Ph)C=N]sPb" Li(THF) from the reaction of benzonitrile and t-butylithium with lead(II) chloride was reported, a further example of tricoordination oflead(II) as well as being the first reported imino complex of lead(II) [9]. The product exists in the solid state as a regular tetrahedron, with the lead(ll) in the apical position and the three imino nitrogens defining the basal triangle. The lithium bridges the three nitrogens on the opposite face of the triangle to the lead and coordinates a molecule of THF.

The versatile ligand triazacyclononane forms 1 : 1 complexes with lead(I1) nitrate and with lead(II) perchlor- ate. In the solid state these both form irregular hexacoordinate complexes with intermolecular bridging through the oxygen atoms of the nitrate or perchlorate counterions to form arrays in the solid state that show considerable similarity despite the apparent steric differences between nitrate and perchlorate ions [10]. The stability of the 1 : 1 complex in aqueous solution has been measured at log K = 10.35 [10].

Complexes formed between lead(II) thiocyanate and (1,10 phen) and 2,2': 6'2" terpyridine (terpy) have been investigated [11,12]. Despite the seeming simplicity of the system a variety of compounds can be prepared by subtle adaption of the reaction stoichiometry, including 7 and 8.

554

c'

N N \ i % ~j'q,

S ~x /

J. Parr

s s, , / \ ~ , /

N N N . ~, \ /

/ ~ c X ~Pb_ C ~ s / s J/I\~-s b ~ / 1 \ i^

s ~ \ L ~ _ .s .... \ / ~ Pb... "~'C "" ...... Pb

s ~ 7 C'" 'N" ~ s ~ _ - --: ~"~'N _

/ \ \ I,< \_yl, N~..>D,/, ........... . , . .~N.~. . . ;p, I

N ....... i t " " - s - - " .... N ....... i \ ~ s

The linear polymer 7 comprises [Pb(terpy)] 2÷ units lined by four bridging thiocyanates to two further [Pb(terpy)] 2÷ units. Compound 8 is again a linear polymer with an octacoordinate lead(II) binding to one (1,10 phen) and six bridging thiocyanates, connecting each lead(II) to four neighbouring lead(II) centres [12].

The neutral bidentate ligand tetramethylethylenediamine (tmeda) reacts with lead monoxide in the presence of ammonium iodide to yield the simple chelated complex [Pb(tmeda)212] [13] which has a linear polymer array of [Pb(tmeda)] 2÷ units linked by bridging iodides to give an overall irregular octahedral geometry to the plead(lI) centre. By the same authors, the analogous reaction with ethanolamine replacing tmeda was explored [14] resulting in the slightly more unusual compound [Pb6(Ea)6(HEa)~I6] 9.

Coordination chemistry of lead(lI)

Q ': Q

i

~. ",'"" " o " ~ I " ' " p ~ ' , I \ 1 , - . , 1 ,I, _,O--.-.-.-.-.-.~-,Vb~- . . . . . "~//~1 / I Vb'~i ~ ' - I N - N . ' o . / I \I~ \

Pb ', i

© © ,

i i

i i

v~

OH

H o r N / / i

/ i

6

O=i-

555

9

Nitrilotriacetamide (NTA) has been used to prepare a 1:1 complex [(NTA)Pb(NO3)](NO3) [15]. This decacoordinate structure represents the highest coordination number routinely observed for lead(II), exhibiting in the solid state a configuration which suggests that the remaining lone pair is steroechemically inactive in this compound. Stability constants are reported for the formation of complexes of 5-(4'-amino-2'-azabutane)- 5-methyl-3,7-diazanonane-l,9-diamine, 10, with lead(I1) in water (log K = 9.2) [16]. This tripodal ligand is related to macrocyclic ligands such as cryptands, (log K for (2,2,2-crypt) with lead(II) in MeOH = 12.5) [17].

~ N ~NH2] H J3

10

2.3. Group 15:phosphorus ligands

The first report of a simple compound of empirical formula [Pb(PRz)2] was from Cowley et al [18]. These dimeric neutral complexes form from the reaction of lead(II) chloride with lithium di-t-butylphosphide in stoichiometry 1 : 2, the nature of the product was confirmed by X-ray crystallography [18]. The same reaction in 1:3 stoichiometry gives the trisubstituted anionic lead compound 11 [19].

R R \ /

R ,,P\ R R

11

An excellent and thoroughgoing study of complexes of the phosphorus analogue of the well-known bis- (trimethylsilyl)amide was reported by Buhro et al. [20]. Perhaps unsurprisingly, the complex of empirical formula [Pb(P(SiMe3)2)] is not monomeric, but dimeric. The structural characterisation of this complex shows a puckered (Pb2P2) ring, with a terminal phosphide on each lead(II) centre, the whole crystallising with a cis- configuration [20].

556 J. Parr

Ph2 PPh2 ./Px /

HC Pb -CH \ p / \ PPh2

Ph2

12

A further example of tricoordinate lead(II) is given by Balch and Oran. The lithium salt of deprotonated bis- (diphenylphosphino)methane (dppm) reacts with lead(II) chloride to give 12, and the trimethylsilyl substituted analogue of dppm forms a tetracoordinate species 13 [21]. NMR studies reveal that at ambient temperatures both molecules are fluxional, 12 exchanging between a tri- and a tetracoordinate species, (between r/1 C- bound and ~2 (P)2- bound), 13 exchanging between different tetracoordinate geometries [21].

Ph2 Ph2 / P \ P \ ~ .

Me3Si----C\ P b N /t~--altvle3 P P Ph2 Ph2

13

2.4. Group 16 : oxygen ligands

Tetrameric [Pb4(OH)4](N)3)4 was isolated from a basic aqueous solution of lead(II) nitrate, and the X-ray structure reported [22]. The compound exists in the solid state as a cubane type arrangement of four lead(II) atoms occupying four opposite corners of a cube with four hydroxyl ions completing the array. This remarkable ion is an important addition to the understanding of the speciation of lead(II) ions in aqueous solution, and is unusual of its kind.

A number of compounds have been isolated from the system Pb(C104)2/DMSO, and crystal structures were reported for Pb(DMSO),.(C104)2, where x = 3 or 5 [23]. Two isomers were isolated for the case x = 3 (14) where the bridging oxygen is either from a perchlorate or a DMSO moiety. The report also contains a useful discussion on the potential difficulties of relating the composition of species in solution to the nature of the species which crystallise therefrom.

0 0 , /

o / c t , , . _0C103 . - ' 0 \ / ° % /

3ClO ...... P b _ _ 0 ........... ~ ~ ....... 0

/ \% o/---.o 0 0 \ /

CI / \ O O

,o c,jOx ? o o/

T/%o//Vo \ o I o o o ~ x ,

~Cl / Cl(O)2

(oh

O = DMSO Oxygen

PercMorato and DMSO bridged isomers of 14

Lead(II) alkoxides have received much attention particularly due to their potential application as precursors to metal films. In another extensive study by Buhro et al. [24] the alcoholysis of Pb[N(Si(Me)3)]2 with various

Coordination chemistry of lead(ll) 557

alcohols was shown to lead to a series of alkoxides with varying structural features. Sterically undemanding alcohols (isopropanol, 2-methoxyethanol) give linear polymeric compounds [Pb(OR)2]~ where the lead exhibits a distorted trigonal bipyramidal structure, bound to four/~2 alkoxide oxygens. More encumbered alcohols (t- butanol, neopentyl alcohol) form tri-metallic compounds [Pb3(OR)6] (15) [24]. Using an excess of alcohol, or by direct reactions of lead(lI) chloride with an excess of sodium alkoxides, products of the type [Pb4(OR)~,] can be formed [25].

+ +

15

A similar polymeric motif is seen in the product of the reaction of lead monoxide with triethanolamine [N(CH2CH2OH)3] and ammonium iodide [26] which comprises a repeating [Pb414N (CH2CH2)3(EtOH)2(EtOH)] unit, where triethanolamine is a tetradentate ligand, doubly deprotonated. A structurally related compound is reported by Veith et al. [27].

Aminosquaric acid reacts with lead(If) nitrate in methanol solution to give [Pb(C403CH2)'2H20],, where each octacoordinate lead(lI) is bound once to each of four aminosquarate residues and to the oxygens of four /~2 bridging water molecules [28]. Another seemingly simple system, lead(II) acetate and phthalic acid, has been shown by crystallography to give a series of interesting products [29]. The mixed compound lead(ll) acetophthalate comprises three distinct lead(II) centres, each of which exhibit a different coordination geometry.

2.5. Group 16 : sulfhr and selenium ligands

Thiohydroxamates (16) have been show to form simple 2 : 1 complexes with lead where R = Ph, Me. Both complexes exhibit a distorted trigonal bipyramidal geometry in the solid state [30]. Dithiocarbamate complexes have been investigated by vibrational spectroscopy [31], mass spectroscopy, 2°7pb NMR and electrochemical techniques [32]. Whilst it is proposed that [RNCSz] is always present as a bidentate ligand, these ligands are shown in some cases to be labile in experiments measuring the rate of scrambling of ligands in mixtures of the type [[Pb(S2CNR)2] + [Pb(SzCNR')2} [32].

S R~J~'NN.--OH

d

16

The dipotassium salt bis-(2,2-dicyanoethylene-l,l-dithiolato)plumbate was isolated as a hydrate from the reaction of lead(II) acetate with the potassium dithiolate. The anion exhibits a pyramidal geometry in the solid state by X-ray crystallography, a disposition found by calculation to be favoured over a planar arrangement by 25kJmo l 1133].

558 J. Parr

An elegant investigation of the simple yet significant diothiolate complex [Pb(ethane-1,2,-dithiolate)] reveals that the compound exists in the solid state as an extended array, with intermolecular Pb-S interactions giving an overall hexacoordinate configuration to the oligometric lead centres [34]. Less tractable was the [Pb(C4S4)]n tetrathiosquarate complex of lead(II), a high melting insoluble violet polymer [35].

Thiophosphates of various kinds have been shown to form complexes with lead(II) [36-38]. The mixed complexes [Pb(S2P(OEt)2(bipy)]2 and [(Pb(S2P(OEt)2)2(en)], shown in 17 have been prepared and their iso- lation has been used to infer a mechanism for the reaction of simple amines with [lead(II)bis-(O,O) dialkyldithiophosphates] [36].

( O E t ) ~ P ~ . s / b ~ s . ~ P ( O E t ) 2

,S~..~pb/'~ ~P(OEt)2

(OEt)2 (O]Et)2 /

/..N/s, . s I

/ 7 NfO N - ~ s / P ~ ( o E t ) 2

I (OEt)2

The dimeric [Pb(bipyXS 2P(OEt)2)212 The repeating unit of [(Pb(S2P(OEt)2)2(en)]n

17

Inorganic chelate rings have enjoyed much popularity recently and lead(II) has been complexed by tetra- phenyldithiomidophosphinate, which forms a simple neutral 2 : 1 complex with a trigonal bipyramidal geometry in the solid state [37], giving a complex which comprises two [PbSPNPS] rings.

A slightly more sophisticated system reported by Fackler et al. [38] uses an organogold dithiophosphinate 18 to prepare 2 : 1 complex of octahedral geometry exhibiting two Au-Pb interactions of 2.896 and 2.963/k.

Ph Ph I / S = P - - P h ph~P=-S

Pb Au / ( . . . / x

• S=P--Ph P h - - ~ S / I Ph Ph

18

The dianion bis-(2,2-dicyanoethylene-l,l-diselenato)plumbate was prepared from the reaction of lead(II) acetate and the potassium salt of the diselenodicyanoethylene, and isolated as the tetraphenyl arsonium salt.

Coordination chemistry of lead(1 l) 559

The solid stale structure of this by X-ray methods is found to be intermediate between square pyramidal and trigonal bipyramidal [40].

2.6. Miscellaneous

The diphosphine 19 which has a crown ether backbone has been coordinated to an [Ir(C1)(CO)] fragment to yield 19a which can further bind [Pb 2+] in the crown ether function to give the dication 19b. The Pb- l r distance in the crystal structure is 3.117 ,~ [41]. The related complex 20 is reported by the same authors [42] which exhibits shorter Pb-Ir interactions (2.855 and 2.831 A), and is an example of tricoordinate lead(lI). This tricoordination is also observed in a complex reported by Lappert and Power [43] where the sterically encumbered species Pb[N(Si(Me)3)2] acts as a two electron donor ligand complexing to [v/3(allyl)PdC1]. Although this seems an isolated example for lead(IlL both tin(II) and germanium(If) form a range of complexes of this type [43].

Nl-b--b

P~P~ /co/"

?

~ P ~ / c o / Cl ~" ~ ' P P l ~

19 19a 19b

Ph

F Ph2P P Ph2

I / #I ~ .... I ........ irl..~ Vb m------- Ir,~

co"" \ / co Ph2P P Ph2

Ph

20

3. MACROCYCLIC LIGANDS

3.1. Nitrogen donor maeroo'clic ligands

Lead(lI) is a popular metal amongst macrocyclic chemists in part because it exhibits a wide degree of flexibility in the geometry and coordination number of its complexes. A comparative study of complexes of lead(II) with linear and cyclic polyamines reveals an enhancement in stability in smaller cyclic amines over their straight chain analogues, an enhancement which decreases as the size of the polyamine increases [44]. In addition, the larger cyclic polyamines form bi- and tri-nuclear complexes of good stability, and the crystal structure of [30]aneN~0Pb2(C104)4 is also reported. The values for log K for mononuclear complexes do not vary greatly for 22-24, with 21 exhibiting a significantly higher stability. Values for the analogous linear amines 26 and 27 are very close to those seen for 22 and 23, with the log K for 28 greater than log K for 24.

560 J. Parr

H

,N H H N

~ N H H N I

H

n F u ] .

n

n = 1, 21, l ogK = 14.13 n = 2, 22, log K = 10.02 n = 3, 23, log K = 10.83 n = 4, 24, log K = 9.77

n = 2, 25, log K = 9.97 n = 3, 26, log K = 9.86 n = 4, 27, log K = 10.37 n = 5, 28, l ogK - 11.32

In a related study, a series of tetraazacycles and oxatriazacycles with various spacing groups were prepared and their stabilities with lead(II) and other metals were investigated, with the highest value of log K reported for the 1:1 complex with lead(ll) of the smallest tetraazacycle ligand, 29, (log K = 15.9) [45]. Tetraazacycle 30, incorporating two pendant amine functions allowing a potentially hexadentate ligand was prepared and its 1 : 1 complex with lead(II) examined by X-ray crystallography [46]. The compound has in the solid state all six amine groups bound to the lead centre and the nonacoordinate geometry is completed by one molecule of water and two oxygens form a perchlorate anion. The log K of this species is reported as 11.8.

rI

N N n \ / n

29

n /----N n

x_____ N N - - - - / n \ / n

30

An unusual result arising from the interaction of lead(II) nitrate and 24(pyridinium)-C-6 is reported by Cramer et al. [47] where the product which is formulated as[Pb(24(pyridinium)-C-6)](NO3)6 is revealed by crystallography to contain a [Pb(NO3)6] 4- ion and [24(pyridinium)-C-6] 4+ with an empty cavity.

3.2. Nitrogen~oxygen mixed donor macrocyclic ligands

A wide variety of N,O donor macrocycles have been reported in coordination studies with lead(II). Ligands of this class derived from Schiff base condensations on pyridine-2,6-dicarboxaldehyde (31-36) have been widely

Coordination chemistry of lead(lI) 561

studied. A 1 : 1 complex of lead(II) with ligand 31 was reported with bidentate nitrates lending an overall decacoordination for the metal [48]. The reduced form of the ligand is also reported with a concomittant change of conformation of its lead(II) complex associated with the increased flexibility of the ligand. The macrocycle 33 contains phenolic and pyridine donor sites, and forms a complex with lead(ll), [Pb2(L)] which is a linear polymer in the solid state [49]. The potentially octadentate macrocycle 34, which comprises alcohol and pyridine donor groups, forms 1 : 1 complexes with lead(ll) [50] despite a similar configuration to 35, which t\~rms both mono- and bi-nuclear complexes [51].

31

32

o. ?

33

34

562 J. Parr

36 and 37 are two- and three-dimensional analogues of the same ligand type. 37 forms a 1 : 1 complex with a heptacoordinate configuration [52] whereas 36 is reported to form an intriguing dimetallic complex, [Pb(fu36)2] (C104)2 [53].

An elegant 13C NMR study on the fluxional processes seen for the lead(II) complex of 38 [54,55] reveals that a hexacoordinate structure is favoured in solution with exchange between the four pendant groups, the macrocyclic donor atoms being bound throughout the exchange process.

. N ~

35

~ N ~

N---(CH2)n--~N : : : ~

n = 2 o r 3

36

N

=:N\ / N

37

HO ~ OH

HO ~ OH

38

Coordination chemistry of lead(II) 563

A series of ligands based upon 39 were prepared and their relative binding efficiency with lead(lI) was investigated to elucidate preference for binding site type, with 1,4,7,10-tetraazacyclododecane-N,N',N",N"'- tetraacetic acid forming the most stable complex of this family of ligands [56]. An expansive series of ligands of general formula 40 were reported and their complexation behaviour with lead(II) investigated [57, 58]. The crystal structure of the complex formed between lead(lI) and 40 where X = S, Y = NH was reported. The highest log K value for this set of ligands is observed for the case where all donor atoms are nitrogens (log K = 9.5), decreasing to a minimum when all X,Y donor sites are thioethers (log K ~ 3). This comprehensive study illuminates the preferences of lead(II) for donor atom type, maintaining as it does a fixed structure and configuration to the ligands, changing only donor atom type.

H O 2 C / ~ N / - - - ~ N ~ C O 2 H

/ ~(CH2)n (cn2)n X =

f HO2C-~

Nc~N~

LCO2 H

39

(CH2)a

/ (CH2)c

\ 7 Y

where a = l-4, b = c = 2 x=(~a ) , o, s v = 0 ~ , o , s

411

\ /

0 H N

(o o) o..9

41 42

The azacrown ethers 1,4,7,10-tetraoxa-13-azacyclopentadecane (41) and 1,4,7,10,13-pentaoxa-16-aza- cyclooctadecane (42) have been show to form more stable complexes with lead(I1) than their oxygen analogues 15-C-5 and 18-C-6, (vide infra) with log K [Pb(41)] 2+ = 6.0 (cf 3.92) [59] [Pb(42)] 2+ = 8.4 (cf 6.99) [60]. Structural characterisation of [Pb(41)(NO3)2] shows an asymmetric structure with the pentadentate macrocycle bound on one side of the lead(II) with two bidentate nitrate groups bound on the other face, giving a significantly distorted coordination sphere, whereas [Pb(42)(NO3)z] has a planar equatorial disposition of the hexadentate macrocycle, with the two bidentate nitrate functions disposed on opposite sides, giving an alto- gether more symmetrical decacoordinate structure [61].

564 J. Parr

3.3. Nitrogen/sulfi~r mixed donor macrocyclic ligands

A comparison of ring and chain configurations of the ligands [9]aneN2S and atam (43) and their complexes with lead(II) was given by Herman et al. [62]. The relatively low values for log K (6.9 and 5.3, respectively) compare well with the result reported by McPartlin et al. [57, 58] where inclusion of thioether functions reduced ligand efficiency relative to the nitrogen analogue. The neutral mixed S,N donor ligands. 44a, 44b and their complexes with lead(II) were reported by Murphy et al. [63]. The crystal structure of the complex [Pb (44a) (MeOH) (H20)] (CIO4)2 was also given.

n / - - h H N N H H

[9]aneN2S 43

N N

atam

S $

44a 44b

3.4. Oxygen donor macrocyclic ligands

The structure of the complex [Pb(15-C-5)(SCN)2] has been shown in the crystal structure to comprise two distinct types of coordinated lead(II). In one, the lead is octacoordinate binding five oxygens from the crown ether and three thiocyanato N atoms, the second being similarly bound to the crown ether and one thiocyanato N and two thiocyanato S atoms [64]. IR studies of the prepared complex in the solid state show bands consistent with this change in going from S- to N- bound thiocyanates [64]. Single crystal X-ray structures of the complexes [Pbl8-C-6(SCN)2], and [Pb(cis-anti-cisdicyclohexyl 18-C-6(SCN)2 ] were reported along with spectroscopically characterised [Pb(cis-syn-cisdicyclohexyl 18-C-6(NCS)2] [65]. In both structures the lead(II) exists in a hexagonal bipyramidal array, with the macrocycle describing the median plane. For [Pb(cis-anti- cisdicyclohexyl 18-C-6(SCN)2] both thiocyanate groups are monodentate S bound, whereas in [PbI8-C- 6(SCN)2] one is monodentate S bound, the other monodentate N bound [65].

The complex formed from lead(II) acetate and 18-C-6 has also been investigated in the solid state, revealing a somewhat different configuration [66]. The decacoordinate [PbI8-C-6(CH3CO2)2] has both bidentate acetate groups disposed cis on one side of the lead(I1) and the hexacoordinate crown ether on the opposite face (cf [Pb(42) (NO3) 2] (vide supra).

Sulfur donor macrocycles based upon trithiacycladecane have been shown to complex lead(II) with moderate stability, although no critical stability data has been reported [67].

4. CONCLUSION

The conclusion that is drawn from the foregoing collection of reported research is that there is a wealth of lead(II) coordination chemistry. It is not bounded by the constraints of a requirement for a particular ligand

Coordination chemistry of lead(I1) 565

conformation, nor by a requirement for a specific donor atom type, forming complexes with coordination number ranging from three to ten, with many donor atom types. The significance that lead has as an environmental contaminant and as a poison to humans ensures its future as a metal of interest to coordination chemists.

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