A structural survey of metal⋯π heteroaromatic supramolecular synthons for metal = tellurium, tin,...

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A structural survey of metal/p heteroaromatic supramolecular synthons formetal ¼ tellurium, tin, and gold†

Edward R. T. Tiekink*ab and Julio Zukerman-Schpector*a

Received 26th May 2009, Accepted 24th July 2009

First published as an Advance Article on the web 17th August 2009

DOI: 10.1039/b910209d

A survey of tellurium, tin and gold structures containing metal/p heteroaromatic interactions has

been conducted, and their occurrence correlated with previously described metal/p aryl interactions.

Metal/p heteroaromatic interactions are found for all three metals when the heteroaromatic ring was

a pyridine/pyridine-derivative, and these contacts lead usually to 0-D or 1-D aggregates with rare

examples of 2-D and 3-D architectures. For each of tellurium and tin, metal/p aryl interactions are

more likely to form over metal/p pyridine interactions, while the reverse is true in the case of gold.

These observations are readily correlated with the influence of increasing electron density in the

pyridine compared with an aryl ring. In the case of tellurium and tin, for which the metal/p

interaction involves a metal-based lone pair of electrons being donated to the LUMO of the aryl group,

increasing the electron density of the aromatic ring naturally decreases the probability of forming this

type of interaction. In the case of gold, where the metal/p interaction involves the donation of electron

density from the aromatic ring to the gold centre, increasing the electron density of the aromatic ring

favours such an interaction. At least for the metals surveyed, metal/p interactions are observed in

a significant number of their crystal structures, for example up to 6% of all tellurium structures feature

Te/p aryl interactions as a supramolecular synthon.

Introduction

The pivotal role of hydrogen bonding interactions in stabilising

crystal structures is beyond dispute. This being stated, there are

examples in the crystallographic literature, involving the ubiq-

uitous carboxylic acid functional group, whereby the combina-

tion of a large number of nominally weaker interactions is

sufficient to overcome expected hydrogen bonding interactions.

An example of this phenomenon was reported recently for the

polymorphic 2:1 co-crystal formed between 4-hydroxybenzoic

acid and 2,3,5,6-tetramethylpyrazine.1 In one polymorphic form,

the eight-membered carboxylic acid dimer {/H–O–C]O}2

synthon was formed and molecules were linked into supramo-

lecular chains via hydroxyl-O–H/Npyridine hydrogen bonds. By

contrast, in the second form, a 2-D array was formed comprising

zig-zag rows of 4-hydroxybenzoic acid molecules connected via

carbonyl-O/H–Ohydroxyl hydrogen bonds, with the rows

bridged by 2,3,5,6-tetramethylpyrazine molecules that form

carboxylic acid-O–H/Npyridine hydrogen bonds. Thus, in this

form, the anticipated hierarchy of hydrogen bonding was fol-

lowed but not in former, metastable form.1 Keeping to the

carboxylic theme but in the domain of metallorganic structures,

the polymorphic Cy3PAu(SC5H4CO2H-2) system when

aUniversidade Federal de Sao Carlos, Laborat�orio de Cristalografia,Estereodinamica e Modelagem Molecular, Departamento de Quımica,C.P. 676, Sao Carlos, SP, 13565-905, Brazil. E-mail: [email protected]; Fax: +55 16 3351 8350; Tel: +55 16 3351 8208bSchool of Materials Science And Engineering, Nanyang TechnologicalUniversity, Singapore 639798. E-mail: [email protected];Fax: +65 6790 9801; Tel: +65 6790 6402

† Electronic supplementary information (ESI) available: Structuralinformation on the compounds studied. See DOI: 10.1039/b910209d

This journal is ª The Royal Society of Chemistry 2009

re-crystallised from ethylacetate, EtOH, MeOH or DMSO

solutions concomitantly deposited two forms, i.e. one adopting

an extended dimeric structure with the {/H–O–C]O}2 synthon

linking two molecules and another form adopting a more

compact spherical monomeric arrangement with intramolecular

O–H/S hydrogen bonds.2 Here, the expected carboxylate dimer

has been circumvented by intramolecular O–H/S hydrogen

bonds. The principle that eight-membered carboxylic acid dimer

is not always observed in organic crystal structures where it can

potentially form has been thoroughly analysed by Zaworokto

et al.,3 building upon an earlier bibliographic survey.4 It was

reported that the dimer synthon occurred in only 31% of struc-

tures where it can potentially form. However, when structures

with competing hydrogen bonding sites are removed, the

occurrence increases to 93%.3

Another example of similar frustration, at least from the

perspective of the rational design of a crystal structure, is found

in the polymorphic organotellurium system, (p–MeOC6H4)-

Te{C[ ¼ C(H)Cl][c–C(OH)(CH2)5]Cl2.5 Each of the two forms

self-associates into a supramolecular aggregate. In one form,

a dimeric unit is sustained by an eight-membered {Cl/H–O–

Te}2 homosynthon where each cyclohexyl-bound hydroxyl

group forms an O–H/Cl hydrogen bond to one of the tellurium-

bound chloride atoms of a centrosymmetrically related molecule.

In the second form, a six-membered {/H–O–Te/Cl–Te–Cl}

heterosynthon is formed instead, whereby one hydroxyl O–H/Cl hydrogen bond is substituted by a secondary Te/Cl bond so

that a zig-zag supramolecular chain results.5 In each of the

foregoing, identical chemical compositions give rise to quite

distinct packing arrangements sustained by alternate supramo-

lecular synthons and, without the intervention of lattice solvent

molecules, are examples of supramolecular isomers.6 In order for

CrystEngComm, 2009, 11, 2701–2711 | 2701

http://dx.doi.org/10.1039/b910209d

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a ‘‘favoured’’ mode of association to be replaced by another,

ostensibly less favoured synthon, compensatory intermolecular

interactions presumably exist in order to provide a comparable

energy of stabilisation to the overall crystal structure. Such

considerations have given rise to an on-going search of alterna-

tive ‘‘non-conventional’’ supramolecular synthons.

Over the years, investigations in structural macromolecular

chemistry have inspired the discovery and understanding of

several non-conventional non-covalent interactions such as

C–H/O, C–H/p, and cation/p interactions,7–12 which

complement C–H/X, C-X/X and C-X/p (X ¼ F, Cl, Br, and

I) interactions established largely from the crystallographic liter-

ature.13–19 Another non-covalent interaction gaining increasing

attention are lone pair/p interactions, which also emerged from

evaluating protein structures.20–22 Recently, the prevalence of lone

Scheme 1 Geometric parameters defining the intermolecular M/p

interactions covered in this survey with the pyridine ring used as an

exemplar for the heteroaromatic ring.

Scheme 2 Chemical

2702 | CrystEngComm, 2009, 11, 2701–2711

pair/p interactions in molecular compounds have been surveyed

with the observations that these are i) not uncommon in organic

crystal structures, and ii) largely go unrecognised in the primary

literature.23 Lone pair/p interactions whereupon the lone pair

resides on a metal centre, e.g. tellurium24 and tin,25 have also be

reviewed in recent years. Along with the organic examples where

these contacts pertain, lone pair/p interactions arise from the

interaction of the interacting lone pair with the LUMO of an

aromatic ring.22 Perhaps even more intriguing are the recently

reviewed Au/p aryl interactions as supramolecular synthons

where the interaction is between the electron deficient gold atom

and the electron-rich phenyl group.26 In their survey of lone

pair/p interactions, Mooibroek et al. made the point that

increasing the electron density in the rings, e.g. by substituting

a carbon atom with a nitrogen atom, must influence the nature of

the interaction.23 This issue is addressed in the present commu-

nication in the context of tellurium, tin and gold structures.

Herein, the prevalence of metal/p interactions is surveyed

where the metal is tellurium, tin and gold, and where the

p-system is a pyridine ring, or a member of the isomeric series of

di-, tri- and tetra-nitrogen substituted rings, as well as their

oxygen and sulfur analogues, where they exist. The hypothesis

being tested is based on electronic grounds in that enhancing the

electron density of the p-system in the heteroaromatic ring

should diminish the likelihood of forming tellurium/p and

tin/p interactions but, conversely, should increase the likeli-

hood of gold/p heteroaromatic interactions.

structures of 1–6.



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Results

Preamble

The following study builds on earlier contributions describing

Te/p,24 Sn/p,25 and, more recently, Au/p interactions,26 and

similar protocols are employed herein. The Cambridge Struc-

tural Database (CSD version 5.30, February 2009)27 was

searched using CONQUEST (version 1.11).28 The search was

performed in order to find structures that match the fragment

shown in Scheme 1, or derivatives where the nitrogen is

substituted by oxygen or sulfur atoms. In addition, searches were

conducted for the isomeric two, three and four nitrogen-,

oxygen- or sulfur-containing species. The two restrictions

applied to each search were that i) the distance between the

centroid of the heteroaromatic ring and the metal centre (d) be

Table 1 Geometric descriptors for the intermolecular Te/p pyridineand Sn/p pyridine interactions in 1–6

Compound Number d/�A a/� Ref.

Te/p pyridine1 3.81 17.2 342 3.94 19.5 353 3.98 15.4 36Sn/p pyridine4 3.72 10.1 375 3.87 3.5 386 3.87 16.7 37

Fig. 1 Supramolecular association mediated by Te/p pyridine inter-

actions leading to supramolecular chains with helical (1), zig-zag (2), and

linear (3) topologies, respectively. Colour codes in this and remaining

diagrams: metal, orange; halide, cyan; sulfur, yellow; oxygen, red;

nitrogen, blue; and carbon, grey. The M/p contacts are emphasised as

purple dashed lines. For reasons of clarity, hydrogen atoms are omitted.


equal or less than 4.0 A, and ii) that the angle, a, defined by the

vector perpendicular to the heteroaromatic ring (V1) and the

vector passing through the centroid to the metal atom (V2), be

equal or less than 20�. The 4.0 A cut-off was chosen based on the

sum of the phenyl ring half-thickness of 1.7–1.9 A, being half the

centroid-centroid distance in parallel phenyl rings,29 and the van

der Waals radii of the metal in question, tellurium (2.06 A30), tin

(2.17 A30) and gold (1.85 A31). The generous value of 4.0 A was

calculated to capture all potential metal/p interactions. Manual

sorting was conducted to exclude aggregates sustained by other

supramolecular synthons which take precedence over a supposed

metal/p contact. As with earlier contributions in this area,24–26

the emphasis of the following description is upon identifying

metal/p interactions linking molecules, rather than performing

a detailed analysis of the overall crystal packing. Structure

analyses and interpretation were conducted with the aid of

PLATON32 and original diagrams were drawn with the

DIAMOND programme.33

Fig. 2 Supramolecular association mediated by Sn/p pyridine inter-

actions in (5) leading to a 0-D dimeric aggregate, in (4) associating with

the exposed tin atoms of the polymeric backbone, and in (6) leading to

a 2-D array.

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Te/p heteroaromatic interactions

The original bibliographic review of Te/p aryl interactions

described 48 structures, of which 45 featured intermolecular Te/p aryl interactions leading to 0-, 1- and 2-D supramolecular

Scheme 3 Chemical structures of 7–24; species not particip

2704 | CrystEngComm, 2009, 11, 2701–2711

aggregates.24 Since the appearance of that publication, an addi-

tional 50 structures featuring intermolecular Te/p aryl inter-

actions have appeared in the literature. A search for

intermolecular Te/p heteroaromatic interactions containing at

ating in Au/p pyridine interactions are shown in blue.



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least one nitrogen atom revealed only three structures; no other

structures featuring the heteroaromatic rings specified above

were available in the CSD.27 Chemical structures for the three

‘‘hits’’, i.e. 1–3,34–36 are shown in Scheme 2 and geometric

parameters describing the contacts are given in Table 1. Each of

1–3 self-associates to form supramolecular chains with helical,

zig-zag and linear topologies, respectively, Fig. 1. In the case of

each of 1 and 2, the chains are sustained by one donor- and one

acceptor- Te/p interaction by each molecule, and in dinuclear

3, each molecule forms two donor- and two acceptor- Te/p

interactions. The values of d in 1–3 range from 3.81 to 3.98 A

which correspond to values at the upper end of the comparable

range for Te/p aryl interactions, i.e. 2.98–4.00 A, with a mean

value of 3.73 A.24

Sn/p heteroaromatic interactions

The original compilation of Sn/p aryl interactions described 22

structures leading to 0-, 1- and 2-D aggregation patterns,25 and

since the preparation of that survey, two additional structures

stabilised by Sn/p aryl contacts have appeared in the CSD.27 A

search for intermolecular Sn/p heteroaromatic interactions

again revealed only three structures. Chemical structures for

these, i.e. 4–6,37,38 are shown in Scheme 2, geometric parameters

describing the contacts are given in Table 1, and molecular

diagrams are shown in Fig. 2. Compound 5 exists as a dimeric

aggregate sustained by a pair of donor- and acceptor- interac-

tions. Structures 4 and 6 are based on polymeric chains whereby

the tin atoms are bridged by oxalate ligands. In 4, 2,20-bipyridyl

ligands are associated with the chain via Sn/p pyridine inter-

actions involving one pyridine residue only. In 6, the pyridine

residues participating in the Sn/p contacts are derived from the

Table 2 Geometric descriptors for the intermolecular Au/p pyridineinteractions in 7–24

Compound Number d/�A a/� Motif Ref.

7 3.44 4.7 chain 398 3.46 8.0 dimer 409a 3.46 17.9 dimer 41

3.48 6.6 dimer10 3.48 3.4 chain 4211 3.49 19.1 chain 4312 3.51 17.3 dimer 4413 3.53 9.3 dimer 4514 3.55 11.9 dimer 4615b 3.62 10.2 chain 4716 3.63 19.1 dimer 4817 3.64 16.0 chain 4918 3.66 8.5 dimer 5019 3.69 19.3 monomer 5120 3.70 18.8 dimer 4721 3.73 10.3 3-D 52

3.90 15.23.91 16.83.98 11.0

22 3.73 16.6 chain 5323 3.80 11.8 chain 54

3.88 16.224 4.00 5.5 dimer 55

3.99 10.6

a Two independent molecules in the asymmetric unit. b Molecule has2-fold symmetry.


coordinated 1,10-phenathroline residues. The polymeric chains

interdigitate to form a 2-D array sustained by the Sn/p heter-

oaromatic contacts. The d values for 4–6 range from to

3.72 – 3.87 A, and, as for the tellurium compounds, these are

at the upper end of the range for Sn/p aryl interactions, i.e.

3.25–3.97 A, with a mean value of 3.69 A.25

Au/p heteroaromatic interactions

In stark contrast to the foregoing, there are significantly more

structures featuring Au/p heteroaromatic interactions in the

Fig. 3 Supramolecular association mediated by Au/p pyridine inter-

actions involving gold-containing anions: (19) leading to a 0-D aggregate;

(23) leading to a linear supramolecular chain; (10) leading to 0-D

aggregates that are associated into a linear supramolecular chain via

Au/p interactions where the p-system is a five-membered AuN2CS

chelate; and (7) leading to 0-D aggregates that are associated into a linear

supramolecular chain via additional Au/p interactions where the

p-system is a five-membered AuNC2N chelate.

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literature. There is also a broader representation of p-systems

with 18 structures having Au/p pyridine contacts, four with

Au/p pyrimidine, one with Au/p pyrazine, a sole example of

a structure with a Au/p 1,4-dithiine contact and one with

a Au/p thiopyran contact.

Chemical structures for the species participating in Au/p

pyridine contacts, 7–24,39–55 are shown in Scheme 3 and

geometric parameters characterising these contacts are collated

in Table 2. Seven of the 18 structures feature gold(I) centres and

the remaining have gold(III). For the gold(I) structures, six are

cationic and one anionic, while for the gold(III), neutral (four),

cationic (four) and anionic (three) species are represented. While

it is noted that such variations in oxidation state and charge was

found in the survey of Au/p aryl interactions,26 there is a clear

predominance of cationic species forming Au/p pyridine

contacts. This observation is consistent with the enhanced elec-

tron density in the pyridine versus phenyl rings. What might be

unexpected is the appearance of Au/p pyridine contacts in the

anionic gold(I), 10,42 and anionic gold(III) species, 7,39 1951 and

23.54 In two structures, i.e. 19 and 23, containing gold(III)

anions, the pyridine ring is protonated and so the interactions

may be considered as electrostatic in nature to generate 0-D and

1-D aggregates, respectively, Fig. 3. While a plausible explana-

tion emerges for 19 and 23, the situation is less clear cut for the

gold(I), 10, and gold(III), 7, species. In 10, the Au/p contact

occurs between the gold atom of an [AuCl2]� anion and a pyri-

dine ring incorporated within a ligand bound to a gold(III) centre

within a cation. The structure of 10 also reveals another type of

Au/p contact where the p-system is derived from a five-

Fig. 4 Supramolecular association mediated by Au/p pyridine interaction

aggregate; and (11) leading to a linear supramolecular chain. (9b) Dimeric unit

is a five-membered AuN2CS chelate.

2706 | CrystEngComm, 2009, 11, 2701–2711

membered AuN2CS chelate; d ¼ 3.40 A and a ¼ 3.7�. The

resulting supramolecular aggregate is a linear chain that resem-

bles 23, Fig. 3. A search of the CSD revealed a similar interaction

operating in two other structures, one described below, i.e. 9,41

and one other.56 In both examples,41,56 the interacting gold atom

is integrated into the chelate ring, rather than a part of a second

molecule. The gold(III) species, 7, also features an additional

type of Au/p contact, i.e. a five membered AuNC2N chelate

ring; d ¼ 3.62 A and a ¼ 16.4�. The anions are interspersed

between cations to form a supramolecular chain, as shown in

Fig. 3. A search of the CSD revealed an additional seven struc-

tures with similar Au/p AuNC2N contacts, including structure

1749 to be described below.

The advantage of a positively charged gold centre in forming

Au/p pyridine contacts is seen in the structure of [Au(pyr-

idine)2][AuCl2] (8)39 which shows the self-association between

cations to form a dimer, Fig. 4. Similar contacts leading to

dimeric units are seen in the closely related gold(I) structures of

13,45 14,46 16,48 and 18.50 Analogous contacts but extending in

two-dimensions are found in the structure of 11,43 Fig. 4, and in

the ten gold atom highly twisted molecule 21,52 there are four

donor and four acceptor Au/p pyridine contacts with neigh-

bouring molecules that results in a 3-D arrangement.

The remaining cationic gold(III) structures to be described

have additional Au/p interactions in their crystal structures. In

9,41 which comprises two independent molecules in the asym-

metric unit, a pair of centrosymmetrically related molecules form

a dimer as shown in Fig. 4. The second independent molecule,

too, self-associates but via an aurophilic interaction (3.5606(5) A)

s in the structures of: (8) leading to a 0-D aggregate; (9a) leading to 0-D

that is associated via Au/Au and Au/p interactions where the p-system



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supported by Au/p interactions, where the p-system is

a AuN2CS chelate (see above), Fig. 4; d ¼ 3.64 A and a ¼ 17.9�.

Dimers sustained by a single Au/p pyridine contact are found

in the structure of 22.53 An additional link between the molecules

comprising the dimeric unit are provided by two Au/p inter-

actions where the p-system is a five-membered AuC3N ring

{d ¼ 3.76 A and a ¼ 18.6�} and a five membered AuSC2S

ring{d ¼ 3.74 A and a ¼ 15.9�}. The latter interactions are

also formed between dimeric units {d ¼ 3.60 A and a ¼ 5.7�;

d ¼ 3.61 A and a ¼ 6.3�} to result in the formation of a supra-

molecular chain, Fig. 5. The Au/p AuSC2S and Au/p AuC3N

interactions are arranged in a linear fashion through the chain

and are probably the dominant supramolecular synthons.

However, the structure was retained in the analysis to emphasise

the importance of the Au/p chelate interactions. A search of the

CSD revealed an additional five crystal structures with Au/p

AuC3N contacts conforming to the d # 4.00 A and a # 20�

criteria. There were 21 structures featuring the Au/p AuSC2S

interaction. Centrosymmetric dimers are found in the crystal

structure of 17.49 The dimers are connected into a supramolec-

ular chain by Au/p AuNC2N contacts {d ¼ 3.65 A and

a ¼ 17.6�}, discussed above, as well as apparently by Au/p

Fig. 5 Supramolecular association mediated by Au/p pyridine and

additional Au/p interactions leading to supramolecular chains in the

structures of: (22) where the additional p-systems are a five-membered

AuNC3 chelate and a five-membered AuSC2S chelate; and (17) where the

additional p-systems are a five-membered AuNC2N chelate and a four-

membered AuO2Au hetero ring.


AuO2Au contacts {d ¼ 3.56 A and a ¼ 18.5�}. There are no

additional literature examples for such Au/p AuO2Au

contacts.

While the previous discussion has largely focussed upon

charged gold species, this is not the sole criterion for the

formation of Au/p pyridine contacts as neutral gold(III), i.e.

12,45 15,48 20,48 and 24,56 compounds also feature this type of

interaction. Each of the compounds 12, 20 and 24 forms

a centrosymmetric dimer, as the gold atom participates in a single

Au/p pyridine contact. By contrast, in 15, where the gold atoms

lies on a 2-fold axis, each gold atom participates in two Au/p

pyridine contacts resulting in the formation of a supramolecular

chain.

A comparison of the d values for compounds featuring Au/p

aryl or pyridine interactions is instructive. The range of d for all

structures with Au/p aryl interactions was reported to be

3.28–4.00 A (4.00 A was the limit of the search) and this is

comparable to the range of d for Au/p pyridine of 3.46–4.00 A.

However, in the latter case, one example has d ¼ 4.00 A with the

remaining structures having d # 3.80 A. This is reflected in the

average value of d for the Au/p pyridine series of 3.63 A which is

shorter than 3.75 A, the average value of d for the Au/p aryl series.

Four crystal structures were found containing Au/p pyrim-

idine interactions, 25–28,57–59 and chemical structures and

geometric data for these are given in Scheme 4 and Table 3,

respectively. Two of the structures, i.e. gold(I) species 25,57 Fig. 6,

and 28,57 feature a single contact between the gold atom of

a [Au(CN)2]� anion and a pyrimidine ring. As discussed above

for pyridinium systems, a driving force for such interactions

involving the [Au(CN)2]� anion is the protonation of the

pyrimidine ring. Charged species are also found in the remaining

two structures. In 26,58 the gold(III) centre in [AuBr4]� is sand-

wiched between pyrimidine rings to form a supramolecular chain

akin to the structure of 23 shown in Fig. 3. In this structure, the

five-membered azolinium ring is probably precluded from

forming Au/p interactions owing to steric congestion. In the

binuclear compound 27,59 Au/p pyrimidine interactions lead to

a supramolecular chain generated by a crystallographic 4-fold

screw axis, Fig. 6.

In the sole example of a structure featuring a Au/p pyrazine

interaction, 2960 (Scheme 4 and Table 3), also forms a Au/p 1,

4-dithiine contact, again the only example of such an interaction.

A supramolecular chain with a zig-zag topology is formed,

Fig. 7. Each gold atom bridges symmetry independent molecules,

one via a Au/p pyrazine interaction and the other via a Au/p

1,4-dithiine interaction. The final structure to be described is the

only example of a supramolecular aggregate, a dimer in this case

as shown in Fig. 8, sustained by a Au/p thiopyran interaction;

this is the sole example of a structure containing gold and a thi-

opyran ring.

Discussion

In the on-going quest for delineating non-conventional supra-

molecular synthons, metal/p interactions are increasingly being

recognised having some significance in stabilising their crystal

structures. Previous systematic analyses of crystal packing

patterns in tellurium,24 tin25 and gold26 compounds have shown

that some supramolecular aggregates are sustained by metal/p

CrystEngComm, 2009, 11, 2701–2711 | 2707


Scheme 4 Chemical structures of 25–30; species not participating in Au/p heteroaromatic interactions are shown in blue.

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aryl interactions. While lone pair/p interactions operating in

the structures of tellurium and tin has parallels in structures of

organic molecules,23 gold/p aryl interactions do not.26 It is

likely that gold/p aryl interactions arise from electron donation

from an electron-rich heteroaromatic group to an electron defi-

cient gold atom. If true, increasing the electron density of the

aromatic ring, e.g. by introducing a pyridine-nitrogen atom,

might be expected to promote such supramolecular synthons.

Also, when formed, the gold/p pyridine interactions should be

stronger than analogous gold/p aryl contacts.

2708 | CrystEngComm, 2009, 11, 2701–2711

While determining the strength of a gold/p pyridine interaction

is beyond the scope of the present survey, it is noted that the average

d value for gold/p pyridine is shorter than for gold/p aryl inter-

actions (3.63 vs 3.75 A). If this distance criterion is an indicator of the

strength of the interaction, when formed, Au/p pyridine interac-

tions are stronger than Au/p aryl interactions. However, as

a cautionary note see, for example, discussions on the relationship

between distance and strength of weak intermolecular interac-

tions.62–65 As for the previous survey of Au/p aryl interactions,26

there is no apparent correlation between the magnitude of d and a.



Table 3 Geometric descriptors for the intermolecular Au/p pyrimidine(25–28), Au/p pyrazine (29), Au/p 1,4-dithiine (29) and Au/p thi-opyran (30) interactions

Compound Number d/�A a/� Motif Ref.

Au/p pyrimidine25 3.66 16.8 monomer 5726 3.67 13.9 chain 58

3.86 15.927 3.73 12.3 chain 5928 3.77 18.4 monomer 57Au/p pyrazine29 3.66 18.8 chain 60Au/p 1,4-dithiine29 3.48 4.9 chain 60Au/p thiopyran30 3.65 10.3 dimer 61

Fig. 6 Supramolecular association mediated by Au/p pyrimidine interactio

chain.


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It is salutatory to consider the prevalence of gold/p inter-

actions. There are 3073 structures containing gold with at least

one aryl ring compared to 565 containing gold and at least one

pyridine ring. Given that gold/p aryl interactions are found in

58 structures and gold/p pyridine in 18 structures, the proba-

bility of forming a gold/p aryl interaction is 53 to 1 compared

to 31 to 1 for gold/p pyridine interactions. The probability for

forming gold/p aryl interactions is most certainly over-

estimated as many gold compounds, for example, contain tri-

phenylphosphine groups so that the number of phenyl groups

available to form gold/p aryl interactions is far greater than

indicated above; i.e. there are 1579 structures having at least one

Au-PPh3 unit. For comparison, similar searches were performed

for tellurium and tin. For tellurium, tellurium/p aryl interac-

tions can potentially occur in 964 structures and in fact occur in

ns: (25) leading to a 0-D aggregate; and (27) leading to a supramolecular

CrystEngComm, 2009, 11, 2701–2711 | 2709


Fig. 7 Two views of the supramolecular chain mediated by Au/p

pyrazine and Au/p 1,4-dithiine interactions in (29).

Fig. 8 Dimer formation mediated by Au/p thiopyran interactions in

(30).

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one out of every 16 structures whereas the probability of forming

tellurium/p pyridine interactions drops significantly to one out

of every 33 structures, there being a total of 56 structures having

both tellurium and at least one pyridine ring. For tin, there are

5049 and 851 structures having at least one aryl or one pyridine

ring, respectively. The probability of forming tin/p interactions

are one out of every 202 structures (aryl) and one out of every 284

structures (pyridine).

From the foregoing, it appears that tellurium and tin

compounds are more likely to form metal/p aryl interactions

over metal/p pyridine interactions, whereas the opposite is true

for gold. Of the three elements, tellurium is more likely to form

tellurium/p aryl interactions, a result possibly correlated with

the presence of two lone pairs of available for forming such

interactions in tellurium(II) compounds.

Conclusions

Supramolecular synthons based on metal/p pyridine interac-

tions are found in tellurium, tin and gold structures leading to 0-

D, 1-D, less commonly 2-D, and rarely 3-D supramolecular

architectures. Metal/p pyridine interactions are more likely to

be found in gold structures compared to tellurium and tin

structures. For gold compounds, gold/p pyridine interactions

are more probably formed compared to gold/p aryl contacts.

2710 | CrystEngComm, 2009, 11, 2701–2711

These trends are consistent with the expected mode of interaction

between the respective metal centre and the (hetero)aromatic

ring. Generally speaking, the formation of metal/p aryl/pyri-

dine interactions appear in a small but significant fraction of

structures, e.g. up to 6% of tellurium structures feature tellu-

rium/p aryl interactions, but, when present, these interactions

clearly contribute to the overall stability of the crystal structure.

Finally, the survey of gold structures herein reveals the possi-

bility of metal/p chelate ring interactions, to complement

C–H/p chelate ring interactions that have emerged recently as

a supramolecular synthon.66–68

Acknowledgements

We thank CNPq, CAPES and FAPESP (Brazil) for support of

this work.

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A structural survey of metal⋯π heteroaromatic supramolecular synthons for metal = tellurium, tin,...

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