perspectives in the selective synthesis of phthalocyanines and related compounds

Perspectives in the selective synthesis of phthalocyaninesand related compounds

TOMAS TORRES*

Departamento de Quı´mica Organica (C-I), Universidad Auto´noma de Madrid, Cantoblanco, E-28049 Madrid, Spain

Accepted 28 October 1999

ABSTRACT: The main aim of this paper is to describe the state of the art and the key problems of the selectivesynthesis of phthalocyanines and related compounds. This non-exhaustive overview comments on the synthesis ofisomerically pure, unsymmetrically substituted and non-centrosymmetric phthalocyanines as well as intrinsicallyunsymmetric phthalocyanine-related compounds. Copyright# 2000 John Wiley & Sons, Ltd.

KEYWORDS: selective synthesis; unsymmetrically substituted Pcs; non-centrosymmetric Pcs; intrinsicallyunsymmetric Pc analogues; chiral Pcs; subphthalocyanines; tribenzoporphyrazines; three-quarter Pcs; hemiporphy-razines

INTRODUCTION

Phthalocyanines (Pcs) [1–7] (Fig 1(a) and 1(b)) are planarmacrocyclic aromatic compounds, isoelectronic with por-phyrins, consisting of four isoindole subunits linkedtogether by nitrogen atoms. A great number of uniqueproperties arise from this electronic delocalization, whichmakes these compounds valuable in different fields oftechnology.

The most usual method to prepare metal complexes ofthese compounds is the metal-templated cyclotetrameriza-tion reaction of phthalonitrile derivatives, whilst 1,3-diiminoisoindolines are employed to obtain the metal-freemacrocycles. Another common method to obtain metal-freephthalocyanines is the treatment of a phthalonitrile withsodium or lithium alkoxide, giving rise to the correspondingalkalimetal Pc which can be subsequently demetallated tothe metal-free phthalocyanine with a mineral acid. In allcases the ring closure is very favourable owing to the highthermodynamic stability of Pcs. The reactions usuallyproceed in one synthetic step through complex reactionpathways which involve the formation of reactive inter-mediates that have only been isolated in a few cases. Adetailed comprehension of the mechanism is difficult toachieve owing to the different reaction conditions (solvent,temperature, etc.) employed in the synthesis of thesecompounds. All these facts have made difficult at presentthe development of controlled stepwise selective reactions[8].

REGIOSELECTIVITY IN THE SYNTHESISOF PHTHALOCYANINES

For all the reasons mentioned before, the preparation ofsubstituted phthalocyanines from non-symmetric precursorsis seldom a regioselective process, and mixtures of allpossible regioisomers are obtained. It occurs for examplewith tetrasubstituted phthalocyanines (Fig. 1(a)) which haveappeared widely in the literature. These compounds,synthesized from 3- and 4-substituted phthalonitriles orother phthalyl derivatives, are obtained as mixtures of fourstructural isomers withD2h, C4h, C2v and Cs symmetriesrespectively. The separation of these isomers is a tediousprocess owing to the strong tendency to aggregation of thePcs in solution, but it was shown to be possible bychromatographic (HPLC) techniques [9]. Neglecting theexistence of electronic and steric effects that can alter thestatistical distribution, a 1:1:2:4 ratio of the respectiveisomers mentioned above is expected to be obtained in thesynthesis of tetrasubstituted Pcs. Thus, for example, in thecase of 4-substituted phthalyl derivatives, whatever thereaction conditions are, neither steric effects of thesubstituents nor electric ones are able to act as drivingforce for a regioselective ring closure [9]. Sometimes,however, the steric and electronic effects of the substituentsin non-peripherally substituted (‘ortho-substituted’) Pcs[10], arising from 3-substituted phthalonitriles, can stronglyaffect the formation of these compounds and the ratio ofisomers, giving rise to a non-statistical distribution ofcompounds and even to the exclusive formation or isolationof the C4h isomer [11–13]. A recent study by Hanack’sgroup has gone deeply into the influence of the substituents(type and position), reaction conditions and central metalson the isomer distribution of tetrasubstituted phthalocya-nines [14].

Another approach that gives rise to pure substitutedisomers is the use of bisphthalonitriles linked by appro-priately constrained bridging groups which allow the

Journal of Porphyrins and PhthalocyaninesJ. Porphyrins Phthalocyanines4, 325–330 (2000)

Copyright# 2000 John Wiley & Sons, Ltd.

———————*Correspondence to: T. Torres, Departamento de Quı´mica Organica(C-I), Universidad Auto´noma de Madrid, Cantoblanco, E-28049Madrid, Spain.E-mail: [email protected].

Overview

cyclizationto a mononuclearPcandprecludetheformationof isomer mixtures [15, 16]. Little is reported about theselective synthesis of Pcs arising from 3,4- or 3,5-substituted phthalonitriles [17,18]. These types of com-pounds are also formed as mixtures of structural isomerswhose isolation has been achievedin some cases on apreparative scaleby MPLC techniques[17].

THE SEARCH FOR NON-CENTROSYMME TRIC PHTHALOC YANINEDERIVATIVES AND INTRINSIC ALLYUNSYMMETRIC RELATED COMPOUNDS

Currently mucheffort is being madetowardsthepreparationof Pcderivativeslackinga symmetry centre.Approachestobreak the inversion symmetry of the Pc macrocycle itselfhavebeencarriedout. Someexamplesof this tendencyaredescribed below. In all cases, regio- and stereoselectivesynthetic methodsstill needto be developed.

Unsymmetrically Substituted Phthalocyanines

Unsymmetrically substituted phthalocyanines (Fig. 1(b))are interesting targetsfor chemists owing to the striking

features they present, i.e. these compounds can haveimproved self-organizing features[19] and may generatefrequency-doubled waves by SHG processes[20]. Mucheffort has been made to synthesize unsymmetricallysubstitutedphthalocyanines.Despitethevarietyof syntheticroutes developedto prepare these unsymmetric macro-cycles,mixturesof differentPcsareusually obtained whichoften make difficult the purification procedures, thuspreventingthe isolation of the desired product. Differentmethodsare known for the preparation of unsymmetricalphthalocyanines.

The mostwidely usedstrategyto preparea phthalocya-nine which comprisesthreeidentical (A) andonedifferent(B) isoindole subunits is statistical condensation. Thismethodis basedon thestatistical reactionof two differentlysubstituted phthalonitriles or 1,3-diiminoisoindolines.Thestoichiometry can play an important role, and one of thereactantsis usually employed in excess,typically in a 3:1molar ratio, thus favouring the formation of the A3Bphthalocyanine,althoughthemajorcompoundis usuallythesymmetrical phthalocyaninederived from the most abun-dant precursor.The desiredunsymmetric compoundmustthereafter be separated from the reaction mixture bystandard chromatographic techniques. The electroniccharacter or the position of the substituents can alsodetermine the ratio of productsobtained. For this reason

Born in 1951, Prof. Tomas Torres is Vice-Director of the Departmentof OrganicChemistryat theAutonomaUniversityof Madrid (UAM). After hisPh.D.(1978),hespenttwo yearsat theMax-Planck-Institutefor Biochemistry(Munich). Thenhe joined (1981)a privatecompany,Abello S.A. – Merck,SharpandDohme,in Madrid.In 1985hewasappointedto hispresentpositionatUAM. Heis interestedin organic synthesis,supramolecularchemistry and molecular materialsbasedon phthalocyaninederivatives.He is authorof about150 researchpapersandpatents.

Fig. 1. (a)Tetrasubstitutedphthalocyanine(C4h isomer).(b) Unsymmetricallysubstitutedphthalocyanine.(c) Subphthalocyanine(C3isomer).

Copyright# 2000JohnWiley & Sons,Ltd. J. PorphyrinsPhthalocyanines4, 325–330(2000)

326 T. TORRES

the stoichiometry of the reactants may be modifiedconsidering the relative reactivity of the precursors. Thusa 9:1 or evenhigher molarratio of A:B canbeusedwhenBis muchmorereactivethanA, andthe ratio could evenbeinverted when B is less reactive than A (e.g. 1:5). Thecondensationof phthalonitriles bearingbulky groupsin the3,6-positions with other differently substituted phthaloni-triles usually leadsto a reduced numberof products in themixtureowing to thesterichindrancebetweenthegroupsinclose vicinity. Moreover, such hindrance facilitates theseparation of the mixture of phthalocyaninesowing to thedecreasingaggregationeffects[21].

A selective approach in the synthesis of unsymmetricA3B phthalocyanines is synthesis on a polymeric supportdeveloped by Leznoff and co-workers,which consistsinattaching a 1,3-diiminoisoindolineor phthalonitrile to aninsoluble polymer,makingit reactwith a large excess of adifferent solublediiminoisoindolineand, after removal ofthe symmetric phthalocyanine, releasing the A3B-Pc fromthe polymer [22,23]. This procedure has not beenextensively used, but the recent developmentof newmethodsof synthesisin the solid phaseopensa promisingway for its renaissance.

Within this context it is worth notingthat thepreparationof unsymmetrical A3B-Pcscanalsobeeffectedthroughringenlargement of subphthalocyanines(SubPcs, Fig. 1(c)) byreaction with substituted diiminoisoindolines. The sub-phthalocyanine methodwasdevelopedat the beginning ofthe last decade by Kobayashi [24]. This strategy wasclaimedto havethreeadvantages overthemixedcondensa-tion methods: relatively good yields, easypurification bycolumn chromatography of theunsymmetric Pcsand,mostimportantly, selectivity, since only one Pc compoundwasobtained in the reaction. However, it has been reportedrecently that the ring enlargement reactionof SubPcsforobtaining substituted phthalocyanines is actually, in ageneral way, a non-selective multistep reaction whichdepends dramatically on severalfactors suchasthesolvent[25]. Openingof theSubPcandsubsequentbreaking of theintermediate often take place, leading to mixtures of allpossible statistical products. However, this ring expansionreaction has been performed on a restricted number ofSubPc substrates[26–29]. Therefore adequately functiona-lized SubPcsableto undergo selectivering openinghavetobepreparedin orderto ascertain thesyntheticinterestof thismethod.

An effective methodto obtain phthalocyanineswith face-to-face(ABAB) identically substitutedisoindoles(Fig.2(a))

is the so-called crossedcondensation of a 1,3-diiminoiso-indoline derivative with 1,3,3-trichloroisoindolenine [30,31]. A phthalocyaninewith this ABAB substitution patterncanalsobe obtained by employing a phthalonitrile or 1,3-diiminoisoindolinewhichdoesnotself-condensateowingtosterical hindrance, namelythe presenceof bulky groupsatthe 3,6-positions of the precursors [32, 33]. Thus onlyphthalocyanineswhich donot presentthesebulky groupsinclose vicinity areformedandABAB derivativescaneasilybe isolated.

Recently,Leznoff andco-workershavedescribeda newapproach, the ‘half-Pc’ intermediate method, to prepareAABB compounds(Fig. 2(b)). It involvestheuseof a ‘half-Pc’ intermediate [34] which can be isolated and subse-quently reacted with anotherphthalonitrile undervery softconditions [35]. No tracesof ABAB phthalocyaninesweredetected. This promising strategy will undoubtedly bedeveloped as a general method to be applied in thepreparation of this particular kind of substituted phthalo-cyanine.

Chiral Phthalocyanines

The non-centrosymmetry soughtfor somepurposes in thepreparation of Pcs has been achieved in symmetricallyoctasubstituted phthalocyaninesby cyclotetramerization ofa chiral precursor [36, 37]. Selectivity towards the C4h

isomer has been observedin the self-condensation of aphthalonitrile bearinga chiral substituent in the 4-position[38]. Pcscontaining eight chainswith a chiral carbon eachself-assemble in a staggeredorientation that leads to anoverall right-handed helical structure [39]. Other recentexamples point out the interestof this kind of compound.Thus chiral molecules in which the cores of copperandnickel octaazaphthalocyanines are fused to four non-racemic [7] heliceneshaverecently given rise to organizedLangmuir–Blodgett filmswith very largesecond-ordernon-linear optical responses[40]. The synthesisof chiral Pcsisnowadays an expanding topic. Thus recent examplesinvolving chiral naphthalocyanines [41,42] and chiraloxovanadiumPcs[42] havebeenoutlined.

Subphthalocyanines

Subphthalocyanines(SubPcs) arethelowesthomologuesofphthalocyanines,composedof threeisoindoleunitscontain-ing boron as central atom (Fig 1(c) and 3) [26]. Thesemacrocyclic complexespresentanaromatic delocalized14-p-electronsystem,andtheunsubstitutedcompoundshaveaC3v cone-shapedstructure. Subphthalocyanineshave re-cently received considerableattention as intermediatesforthe synthesis of unsymmetrically substituted phthalocya-

Fig. 2. (a) ABAB-type substitutedphthalocyanine.(b) AABB-type substitutedphthalocyanine.

Fig. 3. Unsymmetricallysubstitutedsubphthalocyanine.


SELECTIVE SYNTHESISOF PHTHALOCYANINES AND RELATED COMPOUNDS 327

nines, asmentionedbefore,owing to the particular opticaland non-linear optical characteristics these compoundsexhibit [43].

Diff erent substituents have been introduced at theperiphery of thering in orderto raisethesolubility of thesecomplexes and to tune their physical properties. Thesesubstituted derivatives canbe prepared by condensation ofthe corresponding substituted phthalonitriles with a boronderivative,usually aborontrihalide.However, thevarietyoffunctional groups that can be attached is limit ed by thestrongconditionsof the synthetic methodemployed. Mostof the examplesreported deal with subphthalocyaninesarising from 4- or 4,5-substitutedphthalonitrile [26,44,45].

As in thecaseof Pcs,thepreparationof SubPcsfrom 3- or4-substitutedphthalonitriles is not a regioselectivereaction.Trisubstituted Subpcsobtainedby statistical condensationof monosubstituted phthalonitriles are composedof twostructural isomerswith thepointgroupsC3 andC1, typicallyin a 1:3 statistical ratio. Hanack and co-workers haveachieved the separation of theseregioisomers by HPLCtechniques[46]. Very recently,someeffort is being madeonthe preparation of regioisomerically pure trisubstitutedSubPcs[47]. The approachinvolvesthe employmentof 3-substituted precursors, which favours the formation of theless sterically hinderedsubphthalocyanine,in somecasestheC3 derivativebeing themajorcompoundof thereaction.Furthermore, the separation of the isomerscanbe doneinthis case by column chromatography. This substitutionpattern hasalsobeena usefultool for obtaining unsymme-trically substitutedSubPcs(Fig.3) [47]. It is remarkablethatC3 andC1 SubPcregioisomersarechiral compoundswhichhave beenresolvedonly very recently [42,47]. This lastareais beginningto takeshapeasa futurefield of research.

Tr ibenzoporphyraz inesand Three-quarterPhthalocyanines

The formal substitution of only oneisoindolesubunit A in aphthalocyanineby anotherconjugated(hetero)cyclicmoietyrepresentsan interesting way to obtain non-centrosym-metric phthalocyanine analogueswith an A3B pattern, inwhich B is the non-isoindole (hetero)cyclic moiety(benzene,pyridine,pyrrole, thiadiazole,triazole, etc.).

The introduction of a new heterocyclic moiety B canbedone by either preserving the porphyrazine core

(tribenzoporphyrazines, Fig. 4(a)) or disrupting it (three-quarter phthalocyanines, Fig. 4(b)). The non-substitutedmetal-free tribenzoporphyrazine was synthesizedfor thefirst time by ElvidgeandLindsteadby statistical condensa-tion of thediiminoisoindolineandthediaminopyrrole [48].Later on, it was demonstrated that tribenzoporphyrazines(Fig. 4(a)), also designatedas norphthalocyanines, werealsoavailable by an expansion reaction of the subphthalo-cyanine with succinoimidine [24]. Other differently sub-stituted norphthalocyanines have been prepared bystatistical condensation betweendicyanobenzeneand theappropriatemalodinitrile [49,50]. Recently, Cook andco-workers have published the preparation of thiopheno-tribenzoporphyrazines [51] and pyridino[3,4]tribenzopor-phyrazines[52].

Somethree-quarter phthalocyanineanalogueshavebeenreported. The most striking examples so far are thetriisoindole-benzeneand triisoindole-pyridine macrocyclesdescribedby Elvidge and Bamfield respectively [53,54].More recently,an importantnew family of phthalocyanineanalogueshas emerged showing interesting propertiesaspotentialbuilding blocks for molecular organicmaterials:the triazolephthalocyanines (Fig. 4(b)) [55]. In thesecompoundsone isoindole of the phthalocyaninecore hasbeenformally replaced by a1,2,4-triazole,leadingto an18-p-electron fully conjugated system isoelectronic to thephthalocyanine one. Two synthetic methodologies havebeenappliedin the preparation of triazolephthalocyanines.The first one involves a non-regioselective statisticalcondensation betweenthe corresponding 1,3-diiminoisoin-dolineand3,5-diamino-1,2,4-triazolein a3:1molarratio inthe presenceof a metal salt. A two-step regioselectivesyntheticmethodhasbeendeveloped in order to achieveunsymmetrically substitutedtriazolephthalocyanines.In thefirst step a three-unit compound is prepared; this issubsequently condensed with an adequately substituted1,3-diiminoisoindoline in thepresenceof ametalsaltactingas template [56]. This two-step strategy has also beenappliedin thesynthesisof thiadiazolephthalocyanines[57].

Hemiporphyraz ines

When a (hetero)aromatic diamine (A) is reactedwith adiiminoisoindoline or a phthalonitrile (B), ABAB-typemacrocycles are obtained, in which two isoindole unitsandtwo (hetero)aromaticmoieties arebridgedthroughazaatoms in an alternate manner.Such systems are calledhemiporphyrazines [58] (Fig. 5). Mixtures of isomers areobtained by the above-mentioned statistical syntheticmethod when at least one of the two reagents isunsymmetrically substituted. Recently, a regioselectiveapproachhas been described for the synthesis of sometriazolehemiporphyrazines.The proposed stepwise metho-dologyinvolvesthecondensationof a regiochemically pure

Fig. 4. (a) Tribenzoporphyrazineor norphthalocyanine.(b) Athree-quarterphthalocyanine:triazolephthalocyanine.

Fig. 5. Unsymmetricallysubstitutedhemiporphyrazine.


328 T. TORRES

three-unit compound with a diiminoisoindoline [59].Despite the structural similarities with phthalocyanines,metallohemiporphyrazines and related compounds havehardly beeninvestigated, most probablyowing to the factthat these compounds are ‘non-Huckel’ systems. Moreattention shouldbe paid to this family of compoundsfromthe synthetic point of view.

OUTLOOK

Thesynthesisof adequately functionalizedphthalocyaninesandrelated compoundsdesignedfor particular purposesisanexpandingtopic. Thesearch for betterregiocontrol in theformation of Pcsand related compounds, which implies athorough understanding of the reaction mechanisms,deserves much more attention in the next few years.Thiseffort will allow us to obtain new, specially designedcompoundswith particular properties for many applica-tions, profiting from the electronic richness of the Pcsystems that, in some way, remainsunexploited.

Acknowledgements

I thankDr Gemadela Torre andDr ChristianClaessensforhelpful discussionon the subjectof this overview.

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330 T. TORRES

perspectives in the selective synthesis of phthalocyanines and related compounds

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