Homo- and Heteroleptic 8-Quinaldinolate ComplexesFrom Elevated-temperature Rearrangements

Glen B. Deacon,A Peter C. Junk,B,C David R. Turner,A,C

and Julia A. WalkerA

ASchool of Chemistry, Monash University, Clayton, Vic. 3800, Australia.BSchool of Pharmacy and Molecular Sciences, James Cook University,

Townsville, Qld 4811, Australia.CCorresponding authors. Email: david.turner@monash.edu; peter.junk@jcu.edu.au

Three new zinc 8-quinaldinolate complexes have been obtained from rearrangement reactions at elevated temperaturesincluding the first homoleptic zinc 8-quinaldinolate complex. The homoleptic, trinuclear complex [Zn3(MQ)6] (1)

(MQ¼ 8-quinaldinolate) was obtained by the recrystallisation of amorphous Zn(MQ)2 from a 1,2,4,5-tetramethylbenzeneflux at 2708C. The heteroleptic complexes [Zn4Cl4(MQ)4] (2) and [Zn4Cl2(MQ)6] (3) were simultaneously obtained by thereaction between Zn(MQ)2 and anhydrous ZnCl2 under the same conditions. All complexes contain quinaldinolate ligands

in a mixed chelating–bridging coordination mode. The homoleptic complex adopts a V-shaped geometry whereas theheteroleptic complexes adopt closely related cyclic structures.

Manuscript received: 18 April 2013.

Manuscript accepted: 2 May 2013.

Published online: 12 June 2013.

Introduction

The coordination chemistry of 8-hydroxyquinoline (HOQ) and

quinaldine (2-methyl-8-hydroxyquinoline, HMQ) largely stemsfrom the ability of their deprotonated forms to act as strongchelating agents, typically to yield quite insoluble compounds,

hence their early use as agents in gravimetric analysis.[1] Forexample, [Al(OQ)3] is a common component of organic light-emitting diode (with ring substituents altering the luminescenceof the compound).[2] There are also biological applications, with

radiolabelled clioquinol (5-chloro-7-iodo-8-hydroxyquinoline)having been investigated as a biomarker for b-amyloid with theligand targeting zinc and copper ions that aid the formation of

amyloid aggregates.[3] Metal quinolinolate complexes havealso been investigated as active ingredients in antifungaltreatments.[4]

The relatively poor solubility of OQ and MQ complexesleads to great difficulty in being able to structurally characterisecomplexes containing these ligands despite their growing rele-vance in medicinal applications. We recently turned our atten-

tion to monometallic zinc 8-quinaldinolate complexes, forwhich a homoleptic species has never been isolated and struc-turally characterised despite the quinolinolate complex

[Zn4(OQ)8] having been isolated by sublimation and structurallycharacterised in the mid-1980s.[5] Heteroleptic zinc quinaldino-late complexes have been obtained by several different routes.

The mononuclear complex [Zn(MQ)2(H2O)] has been preparedby solution methods using methanol and aqueous ammonia.[6]

The clioquinolate analogue has also been reported to have been

prepared from H2O/THF.[7] These two observations suggest

that the biologically important forms of zinc 8-quinolinolatecomplexes are of the form ML2(H2O), although the

quinolinolate anion has been reported to give the bis-aquacomplex [Zn(OQ)2(H2O)2].

[8] The more soluble, and sterically

demanding, 2-methyl-7-nonyl derivative forms an unsolvated,tetrahedral ML2 complex with zinc whereas its 2-nonyl-7-methyl isomer forms an M2L4 species to extract metal ions into

organic solvents.[9] A tetrahedral ML2 complex is also observedfor a more heavily functionalised ligand that contains chelatingquinolinolate groups.[10]

Several complexes with anionic co-ligands have also been

reported. The charge-separated compound (H2MQ)[Zn(MQ)Cl2],with only one MQ ligand chelating to the distorted tetrahedralmetal centre, has been prepared from both methanol and from

acetonitrile in convection tubes at 608C.[11] The dimeric com-plex [Zn(MQ)(OAc)(MeOH)]2 was prepared using the sameprocedure with a different zinc reagent.[12]

With the limited solubility of OQ and MQ complexes, wehave utilised, to great effect, pseudo-solid-state syntheticmethods(using reactions that are mediated by an inert flux) to synthesisealkali metal, rare earth and transition metal complexes, in

addition to heterometallic complexes.[13] Herein, we apply thissynthetic methodology to synthesise a homoleptic zinc quinal-dinolate complex, [Zn3(MQ)6] (1). We also report the synthesis

and structures of two mixed MQ/Cl complexes, [Zn4Cl4(MQ)4](2) and [Zn4Cl2(MQ)6] (3) (Scheme 1).

Experimental

General Details

Zn(MQ)2 was prepared by mixing aqueous solutions of zincchloride with a stoichiometric amount of Na(MQ), with theprecipitated product recovered by filtration. Infrared spectra

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RESEARCH FRONT

were collected as Nujol mulls between NaCl plates using

a Perkin–Elmer 1600 series spectrometer in the range4000–650 cm�1. X-Ray power diffraction (XRPD) data werecollected using a Phillips 1140 diffractometer, equipped with

Cu-Ka radiation, at room temperature. Calculated powder pat-terns were determined using the program Mercury.[14]

Synthesis

[Zn3(MQ)6]�0.7TMB (1.0.7TMB):Zn(MQ)2 (0.19 g 0.54mmol)

and 1,2,4,5-tetramethylbenzene (TMB) (1.5 g)were placed in anevacuated sealed tube and heated at,2708C for 6 days, resultingin a mixture of red crystals and brown amorphous material.

Isolated yield 0.031 g (19%). nmax(Nujol)/cm�1 1713(m), 1563

(s), 1504(m), 1338(m), 1301(w), 1274(m), 1107(w), 831(m),599(s). XRPD confirms that the only crystalline product that

was present in the bulk sample was [Zn3(MQ)6]�0.7TMB (seeSupplementary Material).

[ZnCl4(MQ)4] (2) and [ZnCl2(MQ)6] (3): Mixtures of

Zn(MQ)2 and anhydrous ZnCl2 were combined with TMB andplaced in an evacuated sealed tube at 2708C for 6 days. Massesof 0.35 g (0.90mmol) and 0.041 g (0.030mmol) were used for

the 3 : 1 reaction, and 0.23 g (0.60mmol) and 0.082 g(0.060mmol) for the 1 : 1 reaction for Zn(MQ)2 and ZnCl2respectively. After cooling, the deep-red crystalline productwas hand-picked from the mixture, giving yields of 0.10 g and

0.07 g for the 3 : 1 and 1 : 1 reactions respectively. Owing to themixture of products, bulk analysis was conducted by PXRD,which showed the presence of 2 and 3 as the only crystalline

products in both cases (see Results and Discussion).

X-Ray Crystallography

Data were collected using the MX1 beamline at the Australian

Synchrotron operating at 17.4 keV (l ,0.7107 A). Data col-lection temperatures were maintained at 100K using an open-flow N2 cryostream. Data collection was carried out using

BluIce.[15] Data indexing and integration were performed usingthe program XDS.[16] All structures were solved by directmethods using SHELXS-97 and refined against F2 usingSHELXL-97.[17] All non-hydrogen atoms were refined using an

anisotropic model except in the case of 1.0.7TMB, whichcontains a partial-occupancy TMB molecule that was refinedusing an isotropic model. All hydrogen atoms were placed in

idealised positions and refined using a riding model. Crystal-lographic data are presented in Table 1.

Results and Discussion

The homoleptic trinuclear complex [Zn3(MQ)6] (1) was isolatedas a TMB co-crystal by recrystallisation of [Zn(MQ)2] in an

evacuated, sealed tube at 2708C using TMB as a flux. [ZnMQ2]is initially prepared as an amorphous solid from treatment ofNaMQ with ZnCl2 in water. Although the bulk empirical for-

mula of this material is readily determined, the actual speciationhas remained elusive, with a hydrated mononuclear complex,[Zn(MQ)2(H2O)] being the closest representation reported to

date.[6] It is interesting to note that the homoleptic MQ complex

N

OH

HMQ

Product

(1)

(2)

(3)

TMB

TMB

270�C

270�C

Reaction

Zn(MQ)2

2 ZnCl2 2 Zn(MQ)2

3 Zn(MQ)2

�

�ZnCl2

[Zn3(MQ)6]

[Zn4Cl4(MQ)4]

[Zn4Cl2(MQ)6]

�

Scheme 1. Reactions employed to isolate the homoleptic zinc

8-quinaldinolate complex 1 and attempts to cleanly isolate the heteroleptic

zinc complexes 2 and 3, which yielded mixed products. All reactions were

conducted in sealed, evacuated glass tubes at 2708C with a 1,2,4,5-

tetramethylbenzene (TMB) flux.

Table 1. Refinement details for the X-ray structures of 1]3

TMB, 1,2,4,5-tetramethylbenzene

1 2 3

Compound [Zn3(MQ)6]�0.7TMB [Zn4Cl4(MQ)4] [Zn4Cl2(MQ)6]

Formula C67H57.80N6O6Zn3 C40H32Cl4N4O4Zn4 C60H48Cl2N6O6Zn4M 1239.10 1035.98 1281.42

Cell setting Triclinic Triclinic Triclinic

Space group P-1 P-1 P-1

a [A] 12.846(3) 10.6980(5) 11.040(2)

b [A] 12.973(3) 10.8243(6) 11.837(2)

c [A] 20.208(4) 11.1927(5) 12.227(2)

a [8] 98.24(3) 117.181(1) 61.94(3)

b [8] 102.30(3) 92.394(2) 70.29(3)

g [8] 111.39(3) 106.404(2) 89.01(3)

V [A3] 2972.0(15) 1083.66(9) 1308.1(5)

Z 2 1 1

Refs collected 53701 11947 27120

Unique/observed 13692/9327 6150/4912 7288/6829

Parameters 751 255 346

Rint 0.0798 0.0240 0.0964

R1 (I. 2sI) 0.0675 0.0322 0.0534

wR2 (all data) 0.2028 0.0665 0.1446

B G. B. Deacon et al.

is trinuclear whereas the OQ analogue [Zn4(OQ)8] is tetra-

nuclear,[5] presumably a consequence of the steric bulk providedby the 2-methyl substituent.

The homoleptic complex 1 crystallises in the space groupP-1

with TMBpresent in the lattice. PXRD shows that this is the solecrystalline product that is isolated (see Supplementary Material).The complex contains no internal symmetry and hence the entirecomplex resides within the asymmetric unit (Fig. 1). The tri-

nuclear complex adopts a bent geometry with the central zincatom having a distorted octahedral coordination geometry(N2O4) and the peripheral zinc atoms having distorted square

pyramidal coordination geometries (N2O3). Four of the six MQligands coordinate in m-1k(N,O) : 2k(O) coordination modes,i.e. chelating to one metal with the oxygen further coordinating

to an adjacent metal atom. Two of these ligands chelate to thecentral zinc, with one each on the terminal zinc atoms.The terminal metals also have MQ ligands in a purely chelatingcoordination mode. Overall, this leads to the central zinc being

connected to each of the terminal zinc atoms by two m2 oxygenatoms. The presence of chelating MQ ligands on each metalatom is different to the arrangement observed in the Zn3complex reported by Albrecht et al. containing 7-substitutedOQ, in which chelating ligands are only present on the terminalzinc atoms with the central metal bound only by bridging

phenolate oxygen atoms.[18]

The terminal Zn–O distances in complex 1 are both 1.98 A,with those pertaining to the bridging oxygen atoms in the range

2.01–2.18 A with the shorter Zn–O bond belonging to thechelate ring in all cases (see Table 2). The four bridgingenvironments are similar, with Zn–O–Zn angles in the range

N2

N4

N6

N5N3O6

O4

O5O3

O1

N1Zn2Zn1

Zn3

O2

N

Zn

O

O

O O

O

O

Zn

Zn

N N

N

NN

Fig. 1. The complex [Zn3(MQ)6] from the structure of 1.0.7TMB (top)

(MQ, 8-quinaldinolate; TMB, 1,2,4,5-tetramethylbenzene) and a schematic

representation (bottom). Hydrogen atoms and TMB molecules are omitted

for clarity; selected bond lengths and angles are given in Table 2.

Table 2. Selected bond lengths [A] and angles [8] for compounds 1]3

Symmetry operators used as per Figs 2 and 3

1 [Zn3(MQ)6]

Zn1–O1 1.980(3) Zn2–O4 2.036(3) Zn1–O2–Zn2 103.42(13)

Zn1–N1 2.110(4) Zn2–N4 2.146(4) Zn1–O3–Zn2 102.88(13)

Zn1–O2 2.118(3) Zn2–O5 2.175(4) Zn2–O4–Zn3 106.37(15)

Zn1–O3 2.017(3) Zn3–O4 2.011(4) Zn2–O5–Zn3 100.10(15)

Zn1–N3 2.181(4) Zn3–O5 2.051(4)

Zn2–O2 2.043(3) Zn3–N5 2.166(5)

Zn2–N2 2.193(4) Zn3–O6 1.980(4)

Zn2–O3 2.159(3) Zn3–N6 2.170(5)

2 [Zn4Cl4(MQ)4]

Zn1–O1 1.9929(12) Zn2–Cl1 2.2847(5) O1–Zn1–O2 149.22(6)

Zn1–N1 2.0463(16) Zn2–Cl2 2.1884(5) O2–Zn2–O1* 110.82(5)

Zn1–O2 1.9931(13) Zn2–O1* 1.9635(12) Cl1–Zn2–Cl2 123.52(2)

Zn1–N2 2.0742(16) Zn2–O2 1.9736(13) Zn1–O1–Zn2* 115.85(6)

Zn1–O2–Zn2 112.48(6)

3 [Zn4Cl2(MQ)6]

Zn1–O1 1.9999(17) Zn2–O1# 2.0593(18) O1–Zn1–N1 79.29(7)

Zn1–O2 2.0444(17) Zn2–O2 1.9879(17) O2–Zn2–N2 79.52(7)

Zn1–O3# 2.1992(18) Zn2–O3 2.0981(17) O3–Zn2–N3 77.27(7)

Zn1–N1 2.168(2) Zn2–N3 2.1021(19) Zn1–O1–Zn2# 108.81(8)

Zn1–N2 2.0127(19) Zn2–Cl1 2.2592(9) Zn1–O2–Zn2 107.61(7)

Zn2–O3–Zn1# 100.34(7)

5 7

Compound 3 Calc

Inte

nsity

Compound 2 Calc3 : 1 Expt 1 : 1 Expt

9 11 13 15

2θ [�]17 19 21 23

Fig. 2. Comparisons between the powder X-ray diffraction spectra of the

bulk product from the 1 : 1 and 3 : 1 Zn(MQ)2 : ZnCl2 reactions with the

calculated patterns of 2 and 3 from single-crystal data.

Zinc 8-Quinaldinolate Complexes C

100.1–106.48. One of the interesting features of the complex isthe intramolecular face-to-facep interaction between two of the

MQ ligands. The two interacting ligands are very close toparallel with an angle of 4.88 between the mean planes of thearomatic systems. The shortest C?C distance between the two

ligands is 3.38 A.

Our initial observations of the heteroleptic complexes[Zn4Cl4(MQ)4] (2) and [Zn4Cl2(MQ)6] (3) were from unsuc-

cessful attempts to synthesise a bimetallic complex throughhigh-temperature rearrangement of Sc(MQ)3 and Zn(MQ)2.Dark-red crystals were obtained from two identical reactions

with the samples, from which single-crystal data were collected

O1

O1∗O2

Cl2

Cl1

N1

N2

Zn1

Zn2

N1Zn1

Cl1

Zn2#

Zn2

N2

O2O3

O1

Cl

Cl

ON

O ON N

O N

Zn

Zn Zn

Cl

Cl

Zn

Cl

ON N

O

O

O

NON N

O N

Zn

Zn Zn

Cl

Zn

N3

O1#

(a)

(b)

(c)

Fig. 3. (a) The complex [Zn4Cl4(MQ)4] (2) (symmetry operator used *, 2� x, 2� y, 1� z). (b) The complex [Zn4Cl2(MQ)6]

(3) (symmetry operator used #, 1� x, 2� y, 1� z). (c) Schematic representations of the two complexes. Hydrogen atoms are

omitted for clarity; selected bond lengths and angles are given in Table 2.

D G. B. Deacon et al.

corresponding to one of the complexes each. The source of the

chloride was subsequently found to be an impure Sc(MQ)3precursor. The serendipitous synthesis of these heteroleptic zinccomplexes led us to explore the possibility of deliberately

synthesising the complexes through stoichiometric reactionsbetween Zn(MQ)2 and ZnCl2.

Two reactions were conducted (see Scheme 1) usingdifferent ratios of the Zn(MQ)2 and ZnCl2 starting materials

in order to obtain the desired products, which have MQ : Clratios of 1 : 1 and 3 : 1 (for 2 and 3 respectively). Reactionswere conducted in evacuated, sealed glass tubes using TMB

as an inert flux (see Experimental section for full details).Both reactions yielded dark-red crystalline products, inwhich, by eye, 2 and 3 could not be differentiated. XRPD

was conducted on the bulk material obtained from bothreactions and the results were compared with the calculatedpatterns that were determined from the single-crystal data of2 and 3 (Fig. 2). From these XRPD patterns, it can be seen

that the major product in both cases is that which was beingtargeted, with only very minor signals attributable to thealternative product. The homoleptic species was not detected

in either of these reactions.Both of the heteroleptic complexes 2 and 3 are tetranuclear,

with the four Zn atoms being co-planar and differences arising in

their structures due to the different ratio of ligands that arepresent. Complex 2, [Zn4Cl4(MQ)4], crystallises in P-1 with halfof the complex contained within the asymmetric unit (Fig. 3a).

The two unique zinc centres have different coordinationenvironments. One zinc atom (Zn2) adopts an almost idealtetrahedral coordination geometry with two chloride ligandsand two m2-oxygen atoms from quinaldinolate (MQ) ligands.

The other zinc atom (Zn1) is coordinated by two almost perpen-dicular (81.78), N,O-chelatingMQ ligands and adopts a distortedtetrahedral geometry that appears to be influenced by a long

interaction to one of the chloride ligands. Overall, the complexhas a cyclic form with the four zinc atoms being bridged by the8-quinaldinolate oxygen atoms. Given that the bridging mode

between the Zn atoms is similar, the Zn?Zn distances betweenadjacentmetal atoms are also similar, at 3.30 and 3.35 A.With thediffering geometries around the zinc centres, the cyclic motif iscontracted in one direction. The distance between the O2Cl2 zinc

atoms is 5.08 A whereas the distance between the N2O2 zincatoms is only 4.30 A. All four of theMQ ligands have m-1k(N,O) :2k(O) coordination modes. The distorted coordination environ-

ment of the N2O2 coordinated zinc is most notable whencomparing the O–Zn–O angles around the two metal centres(Table 2). An angle of 1498 is subtended at the distorted zinc

atom comparedwith amore standard tetrahedral angle of 1108 atthe O2Cl2-bound zinc atom. The distortion is partly due to thebite angle of the quinaldinolate ligands (O–Zn–N angle of

,838). In complex 2, there is no fifth bond, yet there is a longnon-bonding Zn1?Cl1 ‘interaction’ of 2.80 A (which liesoutside the sum of the van der Waals radii 2.41 A)[19] that mayplay a steric role. In complex 3 (below), the tight MQ bite angle

allows the metal centres to be five-coordinate.Complex 3, [Zn4Cl2(MQ)6], crystallises in the triclinic space

group P-1 with the asymmetric unit containing half of the

complex (Fig. 3b). The two zinc atoms both have distortedsquare pyramidal geometries with different ligands in theircoordination spheres. Zn1 has a coordination sphere of ClNO3

and is chelated by one MQ ligand with two Zn–O interactionsfrom MQ ligands chelated to adjacent metal atoms and oneterminal chloride ligand. Zn2 has an N2O3 coordination sphere

with two chelating MQ ligands and a Zn–O interaction from a

neighbouring ligand. All three MQ ligands adopt a m-1k(N,O) :2k(O) coordination mode. The structure of the complex issimilar to that of [Zn4(OQ)6(OAc)2], which contains monoden-

tate acetate ligands in place of the chloride ligands in 3.[20]

However, in [Zn4(OQ)6(OAc)2], two of the OQ ligands have m3bridging oxygen atoms to form a double pseudo-cubane motifrather than a cyclic structure. In the structure of 3, there is a non-

bonding Zn?O distance of 2.53 A (Zn1?O3; see Fig. 3),which is required to be coordinated in order to form thepseudo-cubane motif.

The chelate angles of the three crystallographically uniqueligands are similar to each other, as are the geometries at thebridging phenolate oxygen atoms (Table 2). The four zinc atoms

are coplanar with the bridging oxygen atoms lying in the range0.85–1.15 A above or below this plane. Around the complex,there are two pairs of MQ ligands that form an intramolecular,face-to-face p?p interaction. The shortest distance between

two ring centroids in this interaction is 3.55 A between thephenolate ring of one ligand and the pyridyl ring of the other.Adjacent complexes in the crystal structure are aligned such that

these intramolecular motifs are next to each other and form acontinuous intermolecular p–p stack.

Conclusions

The high-temperature synthesis and structure of the homolepticcomplex [Zn3(MQ)6] is reported. This is the first example of a

homoleptic zinc complex of the 8-quinaldinolate ligand anddiffers from the previously reported 8-quinolinolate analogue.We have also isolated two mixed MQ/Cl complexes and shown

that their synthesis can be driven towards the desired product byaltering the reaction stoichiometry.

Supplementary Material

Crystallographic data (CIF) for all structures and supplementaryXRPD data are available on the Journal’s website.

Acknowledgements

Part of this work was conducted using the MX1 beamline at the Australian

Synchrotron, Victoria, Australia. GBD and PCJ acknowledge the Australian

ResearchCouncil for funding.DRT acknowledges theAustralian Institute of

Nuclear Science and Engineering (AINSE) for a research fellowship.

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