When Molecular Magnetism Meets Supramolecular Chemistry: Multifunctional and Multiresponsive Dicopper(II) Metallacyclophanes As

Review When Molecular Magnetism Meets Supramolecular Chemistry: Multifunctional and Multiresponsive Dicopper(II) Metallacyclophanes as Proof-of-Concept for Single-Molecule Spintronics and Quantum Computing Technologies?

Renato Rabelo 1, Salah-Eddine Stiriba 1 , Danielle Cangussu 2 , Cynthia L. M. Pereira 3 , Nicolás Moliner 1, Rafael Ruiz-García 1,*, Joan Cano 1,*, Juan Faus 1, Yves Journaux 4 and Miguel Julve 1,*

1 Instituto de Ciencia Molecular (ICMol), Universitat de València, Paterna, 46980 València, Spain; [email protected] (R.R.); [email protected] (S.-E.S.); [email protected] (N.M.); [email protected] (J.F.) 2 Instituto de Química, Universidade Federal de Goiás, Goiânia-GO 74001-970, Brazil; [email protected] 3 Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais, Pampulha, Belo Horizonte, Minas Gerais 31270-901, Brazil; [email protected] 4 Institut Parisien de Chimie Moléculaire, Sorbonne Universités UPMC Univ Paris 06, 75005 Paris, France; [email protected] * Correspondence: [email protected] (R.R.-G.); [email protected] (J.C.); [email protected] (M.J.)

Received: 10 November 2020; Accepted: 2 December 2020; Published: 4 December 2020

Abstract: Molecular magnetism has made a long journey, from the fundamental studies on through-ligand electron exchange magnetic interactions in dinuclear metal complexes with extended organic bridges to the more recent exploration of their electron spin transport and quantum coherence properties. Such a ﬁeld has witnessed a renaissance of dinuclear metallacyclic systems as new experimental and theoretical models for single-molecule spintronics and quantum computing, due to the intercrossing between molecular magnetism and metallosupramolecular chemistry. The present review reports a state-of-the-art overview as well as future perspectives on the use of oxamato-based dicopper(II) metallacyclophanes as promising candidates to make multifunctional and multiresponsive, single-molecule magnetic (nano)devices for the physical implementation of quantum information processing (QIP). They incorporate molecular magnetic couplers, transformers, and wires, controlling and facilitating the spin communication, as well as molecular magnetic rectiﬁers, transistors, and switches, exhibiting a bistable (ON/OFF) spin behavior under external stimuli (chemical, electronic, or photonic). Special focus is placed on the extensive research work done by Professor Francesc Lloret, an outstanding chemist, excellent teacher, best friend, and colleague, in recognition of his invaluable contributions to molecular magnetism on the occasion of his 65th birthday.

Keywords: copper complexes; electron exchange; electron transport; ligand design; metallacyclic complexes; metallosupramolecular chemistry; molecular magnetism; molecular spintronics; nanotechnology; quantum computing

1. Introduction and Background: Molecular Magnetism Meets Metallosupramolecular Chemistry for Single-Molecule Spintronics and Quantum Computing The metallosupramolecular chemistry term was coined by Constable in 1994 to describe an emerging research area in the ﬁeld of supramolecular chemistry [1,2], where the advantage of

Magnetochemistry 2020, 6, 69; doi:10.3390/magnetochemistry6040069 www.mdpi.com/journal/magnetochemistry Magnetochemistry 2020, 6, x FOR PEER REVIEW 2 of 24

1. Introduction and Background: Molecular Magnetism Meets Metallosupramolecular Chemistry Magnetochemistryfor Single-Molecule2020, 6, 69 Spintronics and Quantum Computing 2 of 25 The metallosupramolecular chemistry term was coined by Constable in 1994 to describe an coordinationemerging research chemistry area is takenin the to field control of thesupramol metal-directedecular chemistry assembly of [1,2], supramolecular where the systemsadvantage[3–10 of]. Metallosupramolecularcoordination chemistry chemistryis taken to provides control convenientthe metal-directed tools for assembly the current of evolutionsupramolecular from molecular systems magnetism[3–10]. Metallosupramolecular [11–18] toward single-molecule chemistry provides spintronics convenient [19–28 tools] and for quantum the current computing evolution[29 from–45]. Themolecular internal magnetism (steric and[11–18] electronic toward single-molec modulation)ule and spintronics external [19–28] control and (chemo-, quantum electro- computing and photostimulation)[29–45]. The internal of the (steric magnetic and electronic coupling throughmodulation) ligand and design external have control been successfully (chemo-, electro- achieved and in simplephotostimulation) metallosupramolecular of the magnetic complexes, coupling such throug as metalh ligand strings design [46–55 have], rings been [56 –successfully61] and grids achieved [62–74], orin moresimple complex metallosupramolecular supramolecular complexes, aggregates ofsuch polynuclear as metal strings complexes [46–55], which rings behave [56–61] as molecularand grids nanomagnets[62–74], or more [75 –complex92]. They supramolecular include both homo-aggregates and of heterometallic polynuclear ﬂuoridecomplexes wheels which [75 behave–79] and as cyanidemolecular polyhedra nanomagnets [80–85 ],[75–92]. as well They as homo- include and heterovalentboth homo- and metal-oxo heterometallic clusters [ 86fluoride–92]. These wheels speciﬁc [75– characteristics,79] and cyanide combined polyhedra with [80–85], their nanometer as well as size homo and- and easy heterovalent handling, can metal-oxo be used to clusters make the [86–92]. active molecularThese specific components characteristics, of spintronic combined circuits with their and quantumnanometer computers. size and easy In fact,handling, they couldcan be serve used asto encodersmake the ofactive binary molecular information components in molecular-scale of spintronic spin-based circuits quantumand quantum computing computers. devices, In includingfact, they magneticcould serve wires as encoders and switches of binary [49,71 information,88], or magnetic in molecular-scale quantum bits (qubits)spin-based [50 –quantum52,87] and computing quantum cellulardevices, automataincluding (QCA) magnetic [69]. wires and switches [49,71,88], or magnetic quantum bits (qubits) [50– 52,87]Detecting and quantum the response cellular ofautomata the spins (QCA) of a single[69]. magnetic molecule to an external stimulus and, by usingDetecting such a the platform response (in theof the form spins of well-established of a single magnetic stimulus-response molecule to correlations),an external stimulus being able and, to implementby using such quantum a platform logic (in memory the form capabilities, of well-est isablished the key to stimulus–response applications in single-molecule correlations), spintronicsbeing able andto implement quantum quantum computing, logic according memory to capabilities, Sanvito [21 ].is the Two key unique to applications examples ofin divanadium(IV)single-molecule complexesspintronics whichand quantum have been computing, proposed as according prototypes to of molecularSanvito [21]. magnetic Two transistorsunique examples (MTs) and of moleculardivanadium(IV) quantum complexes gates (QGs) which for thehave physical been implementationproposed as prototypes of quantum of information molecular processingmagnetic (QIP)transistors illustrate (MTs) this and idea molecular [93–96]. quantum gates (QGs) for the physical implementation of quantum informationOn the processing one hand, (QIP) a dual illustrate electroswitching this idea [93–96]. (ON/OFF) magnetic behavior upon one-electron metalOn reduction the one hand, and oxidation a dual electroswitching of the trans-diaminomaleonitrile-bridged (ON/OFF) magnetic behavior divanadium(IV) upon one-electron complex metal reduction and oxidation of the trans-diaminomaleonitrile-bridged divanadium(IV) complex of formula of formula V2(µ-C4N4)(CN)4(tmtacn)2 (tmtacn = N,N0,N00-1,4,7-trimethyl-1,4,7-triazacyclononane) hasV2(μ been-C4N reported4)(CN)4(tmtacn) by Long2 (tmtacn et al., as= N shown,N′,N″-1,4,7-trimethyl-1,4,7-triazacyclononane) in Figure1[ 93]. The magnetic bistability has responsible been reported for theby Long MT behavioret al., as shown in this in electron Figure spin-based1 [93]. The systemmagnetic would bistability arise responsibl from the conversione for the MT between behavior the in this electron spin-based system IVwould arise from the conversion between the antiferromagnetically antiferromagnetically coupled V 2 complex with an S = 0 ground state (OFF) and the mixed-valence, coupled VIV2 complex with anIII,IV S = 0 ground state (OFF)IV,V and the mixed-valence, ferromagnetically ferromagnetically coupled V 2 or paramagnetic V 2 species, possessing S = 3/2 and 1/2 ground coupled VIII,IV2 or paramagnetic VIV,V2 species, possessing S = 3/2 and 1/2 ground states (ON),IV respectively states (ON), respectively [94]. The electron exchange (EE) interactions between the V (SV = 1/2) ions [94]. The electron 4exchange (EE) interactions between the VIV (SV = 1/2) ions through the C4NIV44− bridge through the C4N4 − bridge account for the strong antiferromagnetic coupling in the S = 0 V 2 neutral IV moleculeaccount for (Figure the strong1, middle) antiferromagnetic [ 93]. In contrast, coupling the veryin the strong S = 0 V ferromagnetic2 neutral molecule coupling (Figure in the 1, reducedmiddle) [93]. In contrast,III,IV the very strong ferromagnetic coupling in the reduced S = 3/2 VIII,IV2 anionIII results from S = 3/2 V 2 anion results from the double exchange (DE) interactions between the V (SV = 1) and theIV double exchange (DE) interactions between the VIII (SV = 1) and VIV (SV = 1/2)4 ions due to the V (SV = 1/2) ions due to the delocalization of the added electron through the C4N4 − bridge (Figure1, 4− left)delocalization [93]. of the added electron through the C4N4 bridge (Figure 1, left) [93].

Figure 1. Illustration of the electroswitchingelectroswitching of the magnetic coupling in aa transtrans-diaminomaleonitrile--diaminomaleonitrile- bridged dinucleardinuclear vanadium(IV) vanadium(IV) complex complex proposed proposed as a prototypeas a prototype of the molecular of the molecular magnetic transistor.magnetic transistor. On the other hand, the p-phenylenediamidocatecholate-bridged divanadyl(IV) metallacyclic complex of the cyclophane type of formula (Ph4P)4[(VO)2(µ-ppbacat)2] [ppbacat = N,N0-bis(2,3- dihydroxybenzoyl)-1,4-phenylendiamine] reported by Atzori et al. allows for the electron spin-mediated Magnetochemistry 2020, 6, x FOR PEER REVIEW 3 of 24

MagnetochemistryOn the other2020, 6hand,, 69 the p-phenylenediamidocatecholate-bridged divanadyl(IV) metallacyclic3 of 25 complex of the cyclophane type of formula (Ph4P)4[(VO)2(μ-ppbacat)2] [ppbacat = N,N′-bis(2,3- dihydroxybenzoyl)-1,4-phenylendiamine] reported by Atzori et al. allows for the electron spin- mediatedswitching switching of the nuclear of the spins nuclear of each spins VIV ion,of each as shown VIV ion, in Figureas shown2[ 96 in]. Figure The QG 2 behavior[96]. The inQG this behavior nuclear inspin-based this nuclear double spin-based quantum double bit (qubit) quantum system bit results (qubit) from system the fastresults electronic from th spine fast excitations electronic within spin excitationsthe S = 1 state within promoted the S by= 1 the state application promoted of uniformby the application electron paramagnetic of uniform resonanceelectron paramagnetic (EPR) pulses. resonanceThe controlled (EPR) entanglement pulses. The controlled between the entanglement nuclear spin-based between qubits the nuclear in this spin-based very weak qubits magnetically in this IV verycoupled weak V magnetically2 complex is ultimatelycoupled V madeIV2 complex possible is by ultimately the large hyperﬁnemade possible coupling by betweenthe large the hyperfine electron IV couplingand nuclear between spins of the each electron V moiety and nuclear (SV = 1 /spins2 and ofIV each= 7/2), VIV as moiety clearly found(SV = 1/2 in theand parent IV = 7/2), mononuclear as clearly foundvanadyl(IV) in the complexparent mononuclear featuring long vanadyl(IV) spin coherence comple timesx featuring [95]. This long related spin pair coherence of mono- times and [95]. dinuclear This relatedvanadyl(IV) pair complexesof mono- illustrates,and dinuclear in a certainvanadyl(IV) manner, complexes the transition illustrates, from classic in a molecularcertain manner, (Werner) the to transitionmodern supramolecular from classic molecular coordination (Werner) chemistry to mo bydern using supramolecular binucleating aromatic coordination diamidocatecholates chemistry by usingligands binucleating as bridges. aromatic diamidocatecholates ligands as bridges.

Figure 2. Illustration of of the hyperfine hyperﬁne coupling and elec electrontron spin-mediated switching of of the nuclear spin quantum coherence in the mononuclear bis(catecholate)vanadyl(IV) complex ( a) and the related p-phenylenediamidocatecholate-bridged dinuclear vanadyl(IV) complex ( b) proposed as a prototype prototype of the two qubit-based molecular quantum gate.

In addition,addition, two relatedrelated examplesexamples ofof dicopper(II)dicopper(II) andand copper(II)-organiccopper(II)-organic radical complexes are known, featuring a chemo- and photoswitchablephotoswitchable (ON(ON/OFF)/OFF) magnetic behavior between antiferro- or ferromagnetically coupled coupled states states (ON) (ON) and and magnetically isolated isolated ones ones (OFF) (OFF) [97,98]. [97,98]. These These two case studies of ligand-based chemo- and photo-active, bi bistablestable dynamic magnetic systems would constitute suitable candidates candidates for for single-m single-moleculeolecule spintronics spintronics and and quantum quantum computing computing nanotechnologies nanotechnologies [17]. [ 17]. On the one hand, the tweezer-type dicopper(II) complex of formula [Cu (terpytbsalphen)] On the one hand, the tweezer-type dicopper(II) complex of formula [Cu22(terpytbsalphen)] [H terpy-tbsalphen 6,6”-bis(4-ethenyl-N,N -1,2-phenylene-bis(3,5-diterbutysalicylideneimine)-2,2 :6 ,2” [H4terpy-tbsalphen 6,6″-bis(4-ethenyl-0 N,N′-1,2-phenylene-bis(3,5-diterbutysalicylideneimine)-0 0 -terpyridine]2,2′:6′,2″-terpyridine] reported reported by Doisteau by Doisteau et al. provides et al. prov an elegantides an exampleelegant example of the mechanically of the mechanically assisted II assistedchemical chemical switching switching of the magnetic of the magnet couplingic coupling after coordination after coordination of the Znof theion Zn toII ion the to central the central terpy linker, to give the corresponding [ZnCu (terpytbsalphen)]Cl species, as shown in Figure3[ 97]. A terpy linker, to give the corresponding [ZnCu2 2(terpytbsalphen)]Cl2 2 species, as shown in Figure 3 [97]. Aconcomitant concomitant conformational conformational change change of the of terpy–tbsalphen the terpy–tbsalphen bridging bridging ligand occurs, ligand which occurs, is eventuallywhich is eventuallyresponsible responsible for the switching for the betweenswitching the between magnetically the magnetically uncoupled (W-shaped)uncoupled (W-shaped) open isomer open and isomerthe weak and antiferromagnetically the weak antiferromagnetically coupled (U-shaped) coupled (U-shaped) closed isomer. closed The isomer. lack The of through-bond lack of through- EE interactions between the CuII (S = 1/2) ions at such a long intermetallic distance (r = 21.4 Å) accounts bond EE interactions between theCu CuII (SCu = 1/2) ions at such a long intermetallic distance (r = 21.4 Å) accountsfor the negligibly for the negligibly small magnetic small coupling magnetic observed coupling in theobserved open isomer in the (Figureopen isomer3, left). (Figure Otherwise, 3, left). the Otherwise,presence of the direct presence through-space of direct EE through-space interactions between EE interactions the two parallelbetween stacked, the two copper(II)-salphen parallel stacked, copper(II)-salphenfragments at a short fragments intermetallic at a short distance intermetallic (r = 4.03 distance Å) is the (r origin = 4.03 ofÅ) the is the weak origin but of non-negligible the weak but non-negligibleantiferromagnetic antiferromagnetic coupling found coupling in the closed foun isomerd in the (Figure closed3 isomer, right). (Figure 3, right). Magnetochemistry 2020, 6, 69 4 of 25 Magnetochemistry 2020, 6, x FOR PEER REVIEW 4 of 24 Magnetochemistry 2020, 6, x FOR PEER REVIEW 4 of 24

Figure 3. IllustrationIllustration of of the the chemoswitching chemoswitching of of the the magn magneticetic coupling coupling upon upon zinc(II) zinc(II) coordination coordination in a terpyridine-bridgedterpyridine-bridged dicopper(II) dicopper(II) complex complex proposed proposed as a prototype of the two qubit-based molecular quantum gate.

On the other hand, the copper(II)-organic radical complex of formula Cu(hfac)2(phendaeNNO)(phendaeNNO) On the other hand, the copper(II)-organic radical complex of formula Cu(hfac)2(phendaeNNO) (hfac = hexafluoroacetylacetonatehexaﬂuoroacetylacetonate and and phendaeNNO phendaeNNO = =1-[6-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-1-[6-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin- 2-yl)-2-methylbenzothiophen-3-yl]-2-[6-(1,10-ph2-yl)-2-methylbenzothiophen-3-yl]-2-[6-(1,10-phenanthroline-2-yl)-2-methylbenzothiophen-3-yl]-enanthroline-2-yl)-2-methylbenzothiophen-3-yl]- 3,3,4,4,5,5-hexafluorocyclopentene)3,3,4,4,5,5-hexaﬂuorocyclopentene) reported reported by by Takayama et al.al. constitutes constitutes an an example example of of the photoswitching of the magnetic coupling by usin usingg a tailor-made coordinati coordinatingng group-substituted, photoactive nitronyl nitroxide (NNO) radical ligand, as shownshown inin FigureFigure4 [4 [98].98]. The reversible intramolecularintramolecular photocycloaddition of the diarylethe diarylethene-typene-type photochromic linker that occurs after UV UV and visible light irradiation is responsible for the switching between the magnetically uncoupled II open-ring ( (oo)) and and the the presumably presumably weak weak ferromagnetically ferromagnetically coupled coupled closed-ring closed-ring (c) isomers (c) isomers of the of Cu theII- II NNOCu -NNO radical radical species. species. The magnetic The magnetic coupling coupling in the in open the open isomer isomer is very is very weak, weak, if not if notnegligible, negligible, as NNO radical species. The magnetic coupling in the open isomer is very weak, if not negligible, as II expectedas expected because because of the of theabsence absence of th ofrough-bond through-bond EE EEinteraction interaction between between the theCuII Cu (SCu( =S 1/2)= ion1/2) and ion expected because of the absence of through-bond EE interaction between the CuII (SCu = Cu1/2) ion and theand NNO the NNO radical radical (SR = ( S1/2)= (Figure1/2) (Figure 4, left).4, left). Otherwise, Otherwise, the fully the fully conjugated conjugated π-electronπ-electron system system of the of the NNO radical (SR = 1/2)R (Figure 4, left). Otherwise, the fully conjugated π-electron system of the closedthe closed isomer isomer substantially substantially increases increases the the magnitude magnitude of ofthe the EE EE interaction, interaction, giving giving rise rise to to a a weak weak but but non-negligible ferromagnetic coupling coupling due due to to the stri strictct orthogonality of the magnetic orbitals of the IIII CuII ionion and and the the NNO NNO radical radical ligand ligand (Figure (Figure 44,, right).right).

Figure 4. Illustration of the photoswitching of the magnetic coupling upon light irradiation in a Figure 4. Illustration of the photoswitching of the magnetic coupling upon light irradiation in a diarylethene-bridged copper(II)-nitronyl nitroxide radical complex proposed as a prototype of the diarylethene-bridged copper(II)-nitronyl nitroxide radical complex proposed as a prototype of the molecular magnetic transistor. molecular magnetic transistor. The state-of-the-art in metallosupramolecular chemistry concerns the design and synthesis of The state-of-the-art in metallosupramolecular chemistry concerns the design and synthesis of novel classes of chemo-, electro- and photo-active, extended π-conjugated aromatic bridging ligands, novel classes of chemo-, electro- and photo-active, extended π-conjugated aromatic bridging ligands, which should be able to self-assemble with paramagnetic transition metal ions to give new which should be able to self-assemble with paramagnetic transition metal ions to give new multifunctional and multiresponsive metallacyclic complexes [7]. This is nicely exemplified in the work by Lloret and co-workers on the rich metallosupramolecular chemistry of a novel class of Magnetochemistry 2020, 6, 69 5 of 25

Magnetochemistrymultifunctional 2020 and, 6, multiresponsivex FOR PEER REVIEW metallacyclic complexes [7]. This is nicely exemplified in the 5 work of 24 by Lloret and co-workers on the rich metallosupramolecular chemistry of a novel class of ligands bearing ligandstwo oxamato bearing donor two oxamato groups separated donor groups by more separated or less by rigid more non-innocent, or less rigid non-innocent, extended π-conjugated extended πaromatic-conjugated spacers, aromatic which spacers, transmit which both magnetic transmit andboth electronic magnetic coupling and electronic effects between coupling the effects metal betweencenters in the effi metalcient andcenters switchable in efficient ways and [99 switchable,100]. ways [99,100]. In this review, we summarize old and more recent achievements, as as well well as as future future perspectives, perspectives, dealing with the ligand design strategy to contro controll the nature and magnitude of the intramolecular magnetic coupling between distant metal centers, th throughrough both internal and external factors, in a diverse family of oxamato-based di dicopper(II)copper(II) metallacyclophanes, metallacyclophanes, as as illustrated illustrated in in Scheme Scheme 1[ [99,100].99,100]. These double-stranded double-stranded dicopper(II) dicopper(II) metallacyclic metallacyclic complexes complexes of the of thecyclophane cyclophane type, type,resulting resulting from II thefrom self-assembly the self-assembly of dinucleating of dinucleating aromatic aromatic oxamato oxamato ligands with ligands CuII with ions,Cu includeions, a variety include of a potentiallyvariety of potentially chemo-, electro- chemo-, an electro-d/or photoactive, and/or photoactive, extended extendedπ-conjugatedπ-conjugated organic spacers, organic such spacers, as polymethyl-substitutedsuch as polymethyl-substituted m- or p-phenylenesm- or p-phenylenes (Scheme (Scheme1a,b), m-1 a,b),or p-pyridinesm- or p-pyridines (Scheme (Scheme1c,d), oligo-1c,d),p- phenylenesoligo-p-phenylenes or oligo- orp oligo--phenylene-ethynesp-phenylene-ethynes (Scheme (Scheme 1e,f), 1stilbenee,f), stilbene or azobenzene or azobenzene (Scheme (Scheme 1g),1g), o- phenylene-ethyleneso-phenylene-ethylenes (Scheme (Scheme 1h),1h), oligo- oligo-α,αα′-, αor0- oroligo- oligo-β,β′-acenesβ,β0-acenes (Scheme (Scheme 1i,j),1i,j), and and 1,4- 1,4- or 2,6- or anthraquinones2,6-anthraquinones (Scheme (Scheme 1k,l).1k,l). Herein, Herein, we we will will highlight highlight how how this this new new class class of of multifunctional andand multiresponsive metallosupramolecularmetallosupramolecular complexes complexes are are up-and-coming up-and-coming candidates candidates as a proof-of-concept as a proof-of- concept(POC) design (POC) design in the in development the development of molecular of molecular magnetic magnetic devices, devices, such such as as wires wires andand switches, switches, for informationinformation processing processing and and storage storage applications applicatio in thens emergingin the emerging fields of single-moleculefields of single-molecule spintronics spintronicsand quantum and computing. quantum computing.

R = Z Y W Z

O O O O 4– N X XY N II Cu X, Y, Z = H, Me X, Y, Z, W = H, Me O N N O (a) (b) (c) (d)

X R R C C X n n H2C CH2 O N N O n = 1, 2 n = 1, 2 X = CH, N (h) CuII (e) (f) (g) O O O O O O

II 4- [C u 2L 2] n n O n = 1, 2 n = 1, 2 O (i) (j) (k) (l)

Scheme 1.1.General General chemical chemical structure structure for the seriesfor ofthe oxamato-based series of dicopper(II)oxamato-based metallacyclophanes dicopper(II) metallacyclophaneswith different chemo-, with electro-different and chemo-,/or photoactive, electro- and/or extended photoactive,π-conjugated extended aromatic π-conjugated spacers: aromaticpolymethyl-substituted spacers: polymethyl-substitutedm- or p-phenylenes m- or (a ,bp-phenylenes), m- or p-pyridines (a,b), m- (orc, dp),-pyridines oligo-p-phenylenes (c,d), oligo- orp- phenylenesoligo-p-phenylene-ethynes or oligo-p-phenylene-ethynes (e,f), stilbene or (e, azobenzenef), stilbene or (g azobenzene), o-phenylene-ethylenes (g), o-phenylene-ethylenes (h), oligo-α,α0 (-h or), oligo-αβ,,βα0′-acenes- or oligo- (i,βj),,β and′-acenes 1,4- or(i,j 2,6-anthraquinones), and 1,4- or 2,6-anthraquinones (k,l). (k,l). 2. Dinuclear Copper(II) Metallacyclophanes in the Proof-of-Concept (POC) Design of Molecular 2.Magnetic Dinuclear Wires Copper(II) Metallacyclophanes in the Proof-of-Concept (POC) Design of Molecular Magnetic Wires The basic components of a molecular spintronic circuit are molecular magnetic wires (MWs), whichThe would basic facilitate components the magnetic of a molecular communication spintronic between circuit the are spin molecular carriers along magnetic the circuit wires [ 100(MWs),,101]. whichMWs owouldffer a newfacilitate design the concept magnet foric thecommunication transfer of information between the over spin long carriers distances along based the circuit on EE [100,101].interactions MWs and offer without a new current design flowconcept [102 for–115 the], transf in contraster of information to conventional over chargelong distances transport-based based on EEelectronic interactions wires and [116 without–122], as current stated flow by Lloret [102–115], [100]. in Indeed, contrast setting to conventional a long-range charge magnetic transport-based coupling electronicbetween twowires distant [116–122], spin centersas stated connected by Lloret by[100]. a long Indeed, organic setting spacer a long-range that ultimately magnetic extends coupling over betweeninfinite distances two distant (“wire-like spin centers magnetic connected coupling”) by a long is the organic cornerstone. spacer Thatthat beingultimately so, a extends perturbation over infiniteinduced distances by an externally (“wire-like applied magn magneticetic coupling”) field on theis the spin cornerstone. center located That at thebeing beginning so, a perturbation of the wire inducedgenerates by a an change externally on the applied spin center magnetic located field at on the th ende spin of it,center as shown located in at Figure the beginning5. of the wire generates a change on the spin center located at the end of it, as shown in Figure 5. Magnetochemistry 2020, 6, x FOR PEER REVIEW 6 of 24 Magnetochemistry 2020, 6, 69 6 of 25 Magnetochemistry 2020, 6, x FOR PEER REVIEW 6 of 24 (a) (b) (a) (b) H H

Figure 5. Illustration of the transfer of information in a molecular magnetic wire before (a) and after (b) Figure 5. Illustration of the transfer of information in a molecular magnetic wire before (a) and after (b) Figurethe operation 5. Illustration of a perturbation of the transfer induced of information by an extern inal a appliedmolecular magnetic magnetic field. wire Adapted before (witha) and permission after (b) the operation of a perturbation induced by an external applied magnetic field. Adapted with permission thefrom operation Castellano of a et perturbation al. (reprinted induced with permission by an extern fromal applied [100], American magnetic field.Chemical Adapted Society, with 2015). permission from Castellano et al. (reprinted with permission from [100], American Chemical Society, 2015). from Castellano et al. (reprinted with permission from [100], American Chemical Society, 2015). TheThe transmission transmission of of spin spin informationinformation betweenbetween thethe metal centers occurs occurs through through either either σσ- -or or π- π-pathways,pathways,The transmission depending onof spin the naturenature information ofof thethe linker. linker.between TheThe theσ σ-pathway-pathway metal centers is is very very occurs e efficientfficient through over over short eithershort distances distances σ- or π- butpathways,but nearly nearly negligibledepending negligible for foron longthelong nature organicorganic of thespacers. linker. In InThe contrast, contrast, σ-pathway the the π -pathwayisπ -pathwayvery efficient is more is more over efficient short efficient distancesover over long longbutdistances. distances.nearly negligibleIn this In this latter for latter case,long case, ortwoganic two different dispacers.fferent situatio situationsIn contrast,ns can can thebe be envisagedπ-pathway envisaged for is for morelong long organicefficient organic spacers, over spacers, long as asdistances.shown shown in in FigureIn Figure this 6.6latter. case, two different situations can be envisaged for long organic spacers, as shown in Figure 6. (a) (b) (a) (b)

Figure 6. Illustration of the long-range magnetic coupling in a molecular magnetic wire with

non-polyradicalFigure 6. Illustration (a) or polyradical of the long-range (b) spacers. magnetic coupling in a molecular magnetic wire with non- Figurepolyradical 6. Illustration (a) or polyradical of the long-range (b) spacers. magnetic coupling in a molecular magnetic wire with non- Thepolyradical electronic (a) or density polyradical of the (b metal) spacers. centers is partially delocalized, and occasionally polarized, on theThe ligands electronic when they density possess of the a non-polyradical metal centers is character partially (with delocalized, a closed-shell and occasionally singlet ground polarized, state) (Figureon theThe6a). ligands electronic In such when a density case, they the ofpossess double the metal a spin non-polyradica centers polarization is partiallyl mechanism character delocalized, (with through a andclosed-shell the occasionallyπ-electron singlet systempolarized, ground of theonstate) organicthe (Figureligands spacer 6a).when allows In theysuch for possess thea case, transmission thea non-polyradica double of spinspin information polarizationl character over (withmechanism very a closed-shell long through distances singlet the (long-range π -electronground magneticstate)system (Figure of coupling), the organic6a). In provided such spacer a case,allows that the there for double the is transmis a relativelyspin polarizationsion smallof spin highest-occupied informationmechanism overthrough molecularvery the long π -electrondistances orbital (HOMO)system(long-range of/lowest-unoccupied the magnetic organic spacer coupling), allows molecular provided for the orbital transmisthat (LUMO)theresion is a of energyrelatively spin information gap small [108 ].highest-occupied Theover magnetic very long coupling moleculardistances between(long-rangeorbital the(HOMO)/lowest-un magnetic metal centers coupling), isoccupied expected provided molecular to decaythat there continuouslyorbital is a relatively(LUMO) with smallenergy the intermetallichighest-occupied gap [108]. distanceThe molecular magnetic in a moreorbitalcoupling or (HOMO)/lowest-un less between marked the way, metal dependingoccupied centers is molecular onexpected the extended toorbital decay π (LUMO)continuously-conjugated energy naturewith gap the of intermetallic[108]. the organic The magnetic spacer.distance Incouplingin contrast, a more between or a less strong marked the magneticmetal way, centers depending coupling is expected betweenon the to extended decay metal continuously centersπ-conjugated could with nature be the anticipated intermetallic of the organic when distance spacer. the ligandsinIn a contrast, more have or aless a strong unique marked magnetic polyradical way, dependingcoupling character between on the (with extended me antal open-shell centers π-conjugated could singlet be anticipatednature ground of state)the when organic (Figure the spacer.ligands6b). InInhave this contrast, casea unique, the a strong unpairedpolyradical magnetic electrons character coupling of the(with between polyradical an open-shell metal spacer centers singlet act could asground intermediates be anticipatedstate) (Figure in thewhen 6b). transmission theIn this ligands case, ofhavethe spin unpaired a information,unique polyradicalelectrons providing of character the thuspolyradical (with two stronglyan spacer open-shell spin-correlatedact as singlet intermediates ground metal state) in centers. the (Figure transmission This 6b). situation In this of case, spin is reminiscenttheinformation, unpaired of providingelectrons the hopping ofthus the mechanism two polyradical strongly of spin-correlated electricalspacer act conduction as intermediates metal centers. over a in wireThis the situation [ 119transmission]. The is reminiscent magnetic of spin couplinginformation,of the hopping in these providing mechanism polyradical thus of systemstwo electr stronglyical is muchconduction spin-correlated stronger over than a metal wire that [119].centers. found The for This magnetic the situation non-polyradical coupling is reminiscent in ones these (videofpolyradical the infra hopping). More systems mechanism importantly, is much of itstrongerelectr remainsical than conduction more that or lessfound over constant for a wirethe withnon-polyradical [119]. the The intermetallic magnetic ones coupling distance(vide infra in from). theseMore a certainpolyradicalimportantly, length systems ofit theremains organic is much more spacer stronger or less (wire-like constant than that magnetic with found the behavior).for intermetallic the non-polyradical distance from ones a ( videcertain infra length). More of importantly,the organic spacer it remains (wire-like more ormagnetic less constant behavior). with the intermetallic distance from a certain length of 2.1.the Non-Polyradicalorganic spacer Spacers(wire-like magnetic behavior). 2.1.In Non-Polyradical a pioneering Spacers work, Ruiz and Cano demonstrated that the appropriate choice of the 2.1. Non-Polyradical Spacers topologyIn a (substitution pioneering work, pattern) Ruiz and and theCano number demonstrated of methyl that substituentsthe appropriate of thechoice bridging of the topology ligand allows(substitutionIn for a pioneering controlling pattern) work, theand Ruiz naturethe andnumber and Cano magnitude of demonstrated methyl ofsubstituents the that intramolecular the appropriateof the brid magneticging choice ligand of coupling the allows topology on for oxamato-based(substitutioncontrolling the pattern) dicopper(II) nature and and metallacyclophanes themagnitude number ofof themethyl intram with substituents polymethyl-substitutedolecular magnetic of the bridcoupling 1,3-ging and onligand 1,4-phenyleneoxamato-based allows for spacerscontrollingdicopper(II) (Scheme the metallacyclophanes 1naturea,b) [ 123and– 126magnitude]. This with is of appropriately polymethyl-substitutedthe intramolecular expressed magnetic by1,3- the couplingand variation 1,4-phenylene on in oxamato-based the sign spacers and magnitudedicopper(II)(Scheme 1a,b) of metallacyclophanes the [123–126]. magnetic This coupling is appropriately with parameter polymethyl-substituted expr (Jessed) in the by phenomenologicalthe variation 1,3- and in the1,4-phenylene sign spin and Hamiltonian magnitude spacers H(Schemeof= theJ S magnetic 1a,b)S ([123–126].S coupling= S = This Sparameter =is 1appropriately/2)[ (127J) in]. the Density phenomenological expressed functional by the theoryvariation spin Hamiltonian (DFT) in the calculations sign H and = −J magnitudeSA provide · SB (SA = − A · B A B Cu aofS clear-cutB the = S magneticCu = 1/2) answer [127]. coupling to Density the parameter relative functional importance (J) in theory the phenomenological (DFT) of the calculations spin delocalization spin provide Hamiltonian a and clear-cut spin H = polarization−answerJ SA · S Bto (S Athe = mechanismsSrelativeB = SCu =importance 1/2) for [127]. the through-ligand Densityof the spin functional delocalization EE interaction theory (DFT) alongand spin thesecalculations polarization two series provide of mechanisms oxamato-based a clear-cut for answer dicopper(II)the through- to the metallacyclophanesrelativeligand EE importance interaction [ 100of along ].the spin these delocalization two series of oxamato-basedand spin polarization dicoppe mechanismsr(II) metallacyclophanes for the through- [100]. ligand EE interaction along these two series of oxamato-based dicopper(II) metallacyclophanes [100]. Magnetochemistry 2020, 6, 69 7 of 25

Magnetochemistry 2020, 6, x FOR PEER REVIEW 7 of 24 In the parent oxamato-based dicopper(II) meta- and paracyclophanes (Scheme1a,b, X = YIn= theZ = parentW = H oxamato-based), the distinct naturedicopper(II) of the meta- ground and spinparacyclophanes state can be (Scheme nicely interpreted 1a,b, X = Y based = Z = Won = the H), conceptthe distinct of molecularnature of the ferro- ground and spin antiferromagnetic state can be nicely couplers interpreted (FCs and based ACs), on the as concept illustrated of molecularin Scheme ferro-2. In eachand antiferromagnetic case, the meta- and couplerspara-substituted (FCs and ACs), phenylene as illustrated spacers in act Scheme as FCs 2. and In each ACs, case, the meta- and para-substitutedII phenylene spacers act as FCs and ACs, respectively, between the respectively, between the two Cu ions leading to either a triplet (S = SA + SB = 1) or a singlet two CuII ions leading to either a triplet (S = SA + SB = 1) or a singlet (S = SA − SB = 0) ground spin state (S = SA SB = 0) ground spin state for the corresponding dicopper(II) meta- and paracyclophanes − –1 for(J = the16.8 corresponding and 94 cm–1 ,dicopper(II) respectively meta-)[123 ,125and]. paracyclophanes In both cases, the (J perpendicular= 16.8 and −94 arrangementcm , respectively) of the − [123,125].oxamate donorIn both groups cases, the with perpendicular the central benzenearrangement ring of allows the oxamate a strong donor interaction groups ofwith the the magnetic central benzeneorbitals ofring the allows CuII ions a strong with theinteractionπ-electron of the system magnetic of the orbitals bridging of ligands.the CuII Thisions with leads the to anπ-electron efficient systemspin polarization of the bridging mechanism ligands. which This leads accounts to an for efficient the parallel spin polarization or antiparallel mechanism spin alignments which accounts resulting forfrom the alternating parallel or spinantiparallel densities spin on alignments the m- and resultingp-phenylene from spacers alternating with spin an evendensities or odd on numberthe m- and of pcarbon-phenylene atoms, spacers respectively with [an127 even]. In aor more odd generalnumber way, of carbon this particular atoms, geometricalrespectivelyconfiguration [127]. In a more will generalallow any way, chemical this particular or physical geometrical action occurring configuration in the πwill-electron allow systemany chemical of the bridgingor physical ligands action to occurringhave drastic in repercussionsthe π-electron on system the electron of the spinbridging configuration ligands to of have the Cu drasticII ions, repercussions as we will see on in the the II electronnext section. spin configuration of the Cu ions, as we will see in the next section.

Scheme 2. Proof-of-concept (POC) (POC) design of molecular fe ferro-rro- and antiferromagnetic couplers based on oxamato-based dicopper(II) metacyclophanes metacyclophanes ( (aa)) and and paracyclophanes paracyclophanes ( (bb),), showing showing the the different diﬀerent parallel and antiparallel spin alignments, alignments, respectively respectively,, which arise from the alternating spin densities on the phenylene phenylene spacers. Adapted Adapted with with permission permission fr fromom Castellano et al. (reprinted (reprinted with with permission permission fromfrom [99], [99], Elsevier, 2015).

Likewise, the antiferromagnetic antiferromagnetic coupling in the the series series of of polymethyl-substituted polymethyl-substituted oxamato-based oxamato-based dicopper(II) paracyclophanes continuously increases with the number of methyl substituents (J = 94, dicopper(II) paracyclophanes continuously increases with the number of methyl substituents (J = −94, 124, and 144 cm 1, with x = 0, 1, and 4, respectively) (Scheme1b) [ 125,126]. They act thus as a −124, and −−144 cm−1,− with x = 0, 1, and 4, respectively) (Scheme 1b) [125,126]. They act thus as a kind ofkind adjusting of adjusting screws screws in a inputative a putative molecular molecular anti antiferromagneticferromagnetic transformer transformer (AT). (AT). The The observed observed AT AT behavior points out that the magnitude of of the antiferromagnetic coupling coupling along along this this series series is is mainly mainly governed by by electronic electronic factors factors associated associated with with the the electron electron donor donor properties properties of ofthe the methyl methyl group, group, as supportedas supported by byDFT DFT calculations calculations [126]. [126 ]. InIn subsequent subsequent works, works, Cangussu Cangussu and and Julve, Julve, on on the the one one hand, hand, and and Armentano Armentano and and Lloret, Lloret, on on the the other hand,hand, providedprovided further further support support of theof the occurrence occurrence of a of spin a polarizationspin polarization mechanism mechanism in the relatedin the relatedoxamato-based oxamato-based dicopper(II) dicopper(II) metallacyclophanes metallacyclophanes with 2,6-pyridine with 2,6-pyridine and 1,4-anthraquinone and 1,4-anthraquinone spacers spacers(Scheme 1(Schemec,k) [ 128 –1c,k)131]. [128–131]. In each case, In the each ferro- case, or antiferromagneticthe ferro- or antiferromagnetic nature of the EE interactionnature of isthe likely EE interactionexplained by is thelikelymeta explained- or para-substitution by the meta- pattern or para of-substitution the 2,6-pyridine pattern and of 1,4-anthraquinone the 2,6-pyridine and spacers, 1,4- anthraquinonerespectively, as spacers, illustrated respectively, in Scheme 3as. Inillustrated both cases, in however,Scheme the3. In magnitude both cases, of thehowever, ferro- andthe magnitudeantiferromagnetic of the ferro- coupling and antiferromagnetic for these novel oxamato-based coupling for these dicopper(II) novel oxamato-based metapyridenophanes dicopper(II) and paraanthraquinophanes (J = 7.9 and 84 cm 1, respectively) decreases when compared with their metapyridenophanes and paraanthraquinophanes− − (J = 7.9 and −84 cm−1, respectively) decreases when comparedparent unsubstituted with their dicopper(II)parent unsubstituted meta- and dicopper( paracyclophanes.II) meta- and This paracyclophanes. feature is likely explained This feature by the is likelyreduction explained in the by Lewis the reduction basicity of in the the amidate Lewis basicity donor groups of the amidate from the donor electron-poor groups from 2,6-pyridine the electron- and poor 2,6-pyridine and 1,4-anthraquinone spacers, which causes a decrease in the metal-ligand covalency and thus of the electron spin delocalization and polarization effects on the bridging ligands, as supported by DFT calculations [128,131]. Magnetochemistry 2020, 6, 69 8 of 25

1,4-anthraquinone spacers, which causes a decrease in the metal-ligand covalency and thus of the electron spin delocalization and polarization eﬀects on the bridging ligands, as supported by DFT Magnetochemistrycalculations [128 2020,131, 6, x]. FOR PEER REVIEW 8 of 24

SchemeScheme 3. POCPOC design design of of molecular ferro- ferro- and antiferro antiferromagneticmagnetic couplers based based on oxamato-based dicopper(II)dicopper(II) metapyridenophanes metapyridenophanes ( (aa)) and and paraanthraquinophanes paraanthraquinophanes ( (bb),), showing showing the the different diﬀerent parallel parallel andand antiparallel antiparallel spin spin alignments, alignments, respectively, respectively, which which arise arise from from the the alternating alternating spin spin densities densities on on the the pyridinepyridine and and anthraquinophane anthraquinophane spacers.

OnOn the the other other hand, hand, oxamato-based oxamato-based dicopp dicopper(II)er(II) metallacyclophanes metallacyclophanes with witholigo- oligo-p-phenylenep-phenylene (OP) and(OP) oligo- andp oligo--phenylene-ethynep-phenylene-ethyne (OPE) spacers (OPE) spacershave been have examined been examined by Cano byand Cano Lloret and as Lloretpotential as candidatespotential candidates to obtain molecular to obtain antiferromagneti molecular antiferromagneticc wires (Scheme wires 1e,f) (Scheme [132–134].1e,f) In [fact,132– OP134 ].and In OPE fact, spacersOP and have OPE been spacers demonstrated have been to be demonstrated really effective to in be mediating really eff EEective interactions in mediating between EE paramagnetic interactions metalbetween centers paramagnetic which are metalseparated centers by whichrelatively are long separated distances by relativelyin discrete long metallacyclic distances entities, in discrete as supportedmetallacyclic by DFT entities, calculations as supported [132,134]. by DFT The calculationsEE interaction [132 between,134]. Thethe two EE interactionCuII ions decreases between from the twothe II parentCu ions complexes decreases with from 4,4 the′-diphenylene parent complexes and 4,4′-diphenylene-eth with 4,40-diphenyleneyne spacers and 4,4 (J 0-diphenylene-ethyne= −8.7 and −3.9 cm−1, spacers (J = 8.7 and 3.9 cm 1, respectively) (Scheme1e,f, with n = 1) to the longer homologues respectively) −(Scheme 1e,f,− with− n = 1) to the longer homologues with 4,4′-terphenylene and 1,4-di(4- with 4,4 -terphenylene and 1,4-di(4-phenylethynyl)phenylene ones (J = 1.8 and 0.9 cm 1, phenylethynyl)phenylene0 ones (J = −1.8 and −0.9 cm−1, respectively) (Scheme 1e,f,− with n = 2)− [132,134].− Indeed,respectively) the rather (Scheme low exponential1e,f, with dencay= of2) the [132 antiferromagnetic,134]. Indeed, coupling the rather with low the exponential intermetallic decay distance of alongthe antiferromagnetic both series of oxamato-based coupling with dicopper(II) the intermetallic oligo-p-phenyleno- distance and along oligo- bothp-phenylene-ethynophanes series of oxamato-based indicatesdicopper(II) that oligo- the EEp-phenyleno- interaction through and oligo- thesep-phenylene-ethynophanes rigid rod-like aromatic spacers indicates obeys that a the non-polyradical EE interaction spinthrough polarization these rigid mechanism, rod-like aromatic as illustrated spacers in Scheme obeys a 4. non-polyradical spin polarization mechanism, as illustrated in Scheme4. Interestingly, a much better magnetic communication between very distant metal centers is predicted for the series of oxamato-based dicopper(II) oligophenylethynylenophanes than for the parent dicopper(II) oligophenylenophanes (Scheme4a,b), as reflected by the calculated values of 1 the exponential decay factor (β = 0.31 and 0.35 Å− )[132,134]. This feature clearly indicates that introducing an ethynylene group between the phenylene spacers does not interrupt the magnetic communication between the spins of the metal centers [133]. Instead, a strong orbital overlap occurs between the π-type orbitals of the para-substituted benzene rings across the carbon–carbon triple bonds due to the almost planar configuration of the OPE spacers, when compared to the slightly twisted configuration of the OP spacers. This situation is in agreement with time-dependent density functional theory (TD-DFT) calculations, which point out the linear decay of the π–π* transition energy with the HOMO-LUMO energy gap along this series of dicopper(II) oligophenylethynylenophanes [134].

Scheme 4. POC design of molecular magnetic wires based on oxamato-based dicopper(II) oligo-p- phenylenophanes (a) and oligo-p-phenylethynylenophanes (b), showing the non-polyradical spin polarization mechanism responsible for the long-range magnetic coupling. Adapted with permission from Castellano et al. (reprinted with permission from [99], Elsevier, 2015).

Interestingly, a much better magnetic communication between very distant metal centers is predicted for the series of oxamato-based dicopper(II) oligophenylethynylenophanes than for the parent dicopper(II) oligophenylenophanes (Scheme 4a,b), as reflected by the calculated values of the exponential decay factor (β = 0.31 and 0.35 Å−1) [132,134]. This feature clearly indicates that introducing Magnetochemistry 2020, 6, x FOR PEER REVIEW 8 of 24

Scheme 3. POC design of molecular ferro- and antiferromagnetic couplers based on oxamato-based dicopper(II) metapyridenophanes (a) and paraanthraquinophanes (b), showing the different parallel and antiparallel spin alignments, respectively, which arise from the alternating spin densities on the pyridine and anthraquinophane spacers.

On the other hand, oxamato-based dicopper(II) metallacyclophanes with oligo-p-phenylene (OP) and oligo-p-phenylene-ethyne (OPE) spacers have been examined by Cano and Lloret as potential candidates to obtain molecular antiferromagnetic wires (Scheme 1e,f) [132–134]. In fact, OP and OPE spacers have been demonstrated to be really effective in mediating EE interactions between paramagnetic metal centers which are separated by relatively long distances in discrete metallacyclic entities, as supported by DFT calculations [132,134]. The EE interaction between the two CuII ions decreases from the parent complexes with 4,4′-diphenylene and 4,4′-diphenylene-ethyne spacers (J = −8.7 and −3.9 cm−1, respectively) (Scheme 1e,f, with n = 1) to the longer homologues with 4,4′-terphenylene and 1,4-di(4- phenylethynyl)phenylene ones (J = −1.8 and −0.9 cm−1, respectively) (Scheme 1e,f, with n = 2) [132,134]. Indeed, the rather low exponential decay of the antiferromagnetic coupling with the intermetallic distance along both series of oxamato-based dicopper(II) oligo-p-phenyleno- and oligo-p-phenylene-ethynophanes indicates that the EE interaction through these rigid rod-like aromatic spacers obeys a non-polyradical Magnetochemistry 2020, 6, 69 9 of 25 spin polarization mechanism, as illustrated in Scheme 4.

Magnetochemistry 2020, 6, x FOR PEER REVIEW 9 of 24 an ethynylene group between the phenylene spacers does not interrupt the magnetic communication between the spins of the metal centers [133]. Instead, a strong orbital overlap occurs between the π-type orbitals of the para-substituted benzene rings across the carbon–carbon triple bonds due to the almost Scheme 4. POC design of molecular magnetic wires based on oxamato-based dicopper(II) planarScheme configuration 4. POC design of the of OPE molecular spacers, magnetic when comparedwires based to on the oxamato-based slightly twisted dicopper(II) configuration oligo-p of- the oligo-p-phenylenophanes (a) and oligo-p-phenylethynylenophanes (b), showing the non-polyradical OP spacers.phenylenophanes This situation (a) and is inoligo- agreementp-phenylethynylenophanes with time-dependent (b), showingdensity thefunctional non-polyradical theory (TD-DFT) spin polarizationspin polarization mechanism mechanism responsible responsible for the long-ran for the long-rangege magnetic magneticcoupling. coupling.Adapted with Adapted permission with calculations, which point out the linear decay of the π–π* transition energy with the HOMO-LUMO frompermission Castellano from et Castellano al. (reprinted et al. with (reprinted permission with from permission [99], Elsevier, from [99 2015).], Elsevier, 2015). energy gap along this series of dicopper(II) oligophenylethynylenophanes [134]. 2.2. Polyradical Spacers Interestingly, a much better magnetic communication between very distant metal centers is 2.2. Polyradical Spacers predictedOxamato-based for the series dicopper(II) of oxamato-based metallacyclophanes dicopper(II) with oligophenylethynylenophanes ﬂat-like oligo-α,α0- and oligo than-β,β 0for-acene the parent(OA)Oxamato-based spacersdicopper(II) have oligophenylenoph dicopper(II) been investigated metallaanescyclophanes (Scheme by Ruiz 4a,b),with and flat-like as Canoreflected oligo- as byα unique,α the′- and calculated oligo examples-β, βvalues′-acene of of MWs(OA) the spacersexponential(Scheme have1i,j) decay been[135 investigated –factor137]. ( Asβ = such,0.31 by Ruizand they 0.35 and have ÅCano−1 found) [132,134]. as unique a moderately This examples feature strong of clearly MW antiferromagnetics indicates(Scheme 1i,j) that [135–137]. introducing coupling As such,between they the have two found CuII aions moderately separated strong by relatively antiferromagnetic large intermetallic coupling between distances the ( Jtwo= Cu18.6II ions and − separated23.9 cm by1 with relativelyr = 8.3 largande intermetallic 12.5 Å, respectively) distances for(J = the−18.6 former and − members23.9 cm−1 with of these r = 8.3 series and with 12.5 theÅ, − − respectively)shorter 1,8-naphthalene for the former and members 2,6-anthracene of these spacersseries with (Scheme the shorter1i,j, with 1,8-naphthalenen = 1 and 2, and respectively) 2,6-anthracene [137]. spacersThese results(Scheme show 1i,j, with thus n that = 1 and one 2, nanometer respectively) is deﬁnitely[137]. These not results the uppershow thus limit that for one the nanometer observation is definitelyof magnetic not couplingthe upper in limit dicopper(II) for the observation complexes of [ 138magnetic–140]. coupling More importantly, in dicopper(II) they complexes have predicted [138– 140].a wire-like More importantly, magnetic behavior they have for thesepredicted two a series wire-like of oxamato-based magnetic behavior dicopper(II) for these oligo- two αseries,α0- and of oxamato-basedoligo-β,β0-acenophanes, dicopper(II) regardless oligo-α,α of′- theand substitutionoligo-β,β′-acenophanes, pattern, as supportedregardless of by the DFT substitution calculations pattern, [135]. asThis supported unprecedented by DFT wire-likecalculations magnetic [135]. This behavior unpreced arisesented from wire-like the polyradical magnetic character behavior of arises the longer from OAthe polyradicalspacers (n character3), as illustrated of the longer in Scheme OA spacers5. (n ≥ 3), as illustrated in Scheme 5. ≥

SchemeScheme 5. POCPOC design design of of molecular molecular magnetic magnetic wires wires based based on on oxamato-based oxamato-based dicopper(II) dicopper(II) oligo- oligo-αα,,αα′0-- ((a)) and β,β′0-anthracenophanes-anthracenophanes ( (bb),), showing showing the the polyradical polyradical mechan mechanismism that that is is responsible responsible for for the the wire-like magnetic coupling through the longer OA spacers (n 3). Adapted with permission from wire-like magnetic coupling through the longer OA spacers (n ≥ 3). Adapted with permission from CastellanoCastellano et al. (reprinted (reprinted with with permission permission from from [99], [99], Elsevier, 2015).

This wire-like magnetic behavior is accompanied by a change from antiferro- to ferromagnetic This wire-like magnetic behavior is accompanied by a change from antiferro- to ferromagnetic coupling for the latter members of both series of oxamato-based dicopper(II) oligo-α,α - and coupling for the latter members of both series of oxamato-based dicopper(II) oligo-α,α′- and 0oligo- oligo-β,β -acenophanes (Scheme5a,b). Notably, DFT calculations predict a weak but non-negligible β,β′-acenophanes0 (Scheme 5a,b). Notably, DFT calculations predict a weak but non-negligible ferromagnetic coupling (J = 3.0 cm–1) between the two CuII ions separated by a very large intermetallic distance (r ≈ 3 nm) through the decacene spacers in the series of β,β′-disubstituted OA spacers (Scheme 1j, with n = 9) [135]. The current efforts in our group are devoted to the preparation of dicopper(II) metallacyclophanes with longer tetracene and pentacene spacers (Scheme 1i,j, with n = 4 and 5) as unique prototypes of MWs for single-molecule spintronics.

Magnetochemistry 2020, 6, 69 10 of 25 ferromagnetic coupling (J = 3.0 cm–1) between the two CuII ions separated by a very large intermetallic distance (r 3 nm) through the decacene spacers in the series of β,β -disubstituted OA spacers ≈ 0 (Scheme1j, with n = 9) [135]. The current eﬀorts in our group are devoted to the preparation of dicopper(II) metallacyclophanes with longer tetracene and pentacene spacers (Scheme1i,j, with n = 4 andMagnetochemistry 5) as unique 2020 prototypes, 6, x FOR PEER of MWsREVIEW for single-molecule spintronics. 10 of 24

3. Dinuclear Copper(II) Metallacyclophanes in the POC Design of Molecular MagneticMagnetic SwitchesSwitches Molecular magnetic switches (MSs) which would allow for thethe interruptioninterruption and restorationrestoration of thethe magnetic communication between the spin carriers are also basicbasic componentscomponents of aa molecularmolecular spintronicspintronic circuit [[100,141].100,141]. MSs MSs are archetypical examples of bistablebistable dynamic systems presenting twotwo separatelyseparately stable equilibrium states (or two distinctlydistinctly accessible states) having totally didifferentﬀerent magnetic properties, whichwhich can be transformed in in a a reversible reversible manner manner under under some some external external stimuli. stimuli. The The external external stimuli stimuli that thatare responsible are responsible for the for magnetic the magnetic switching switching behavior behavior can be can chemical, be chemical, electronic, electronic, or photonic, or photonic, among amongothers, as others, occurs as in occurs conventional in conventional molecular molecular electronic electronicswitches [142–148]. switches [142–148]. MSs formed by two localizedlocalized spins whosewhose magneticmagnetic communication can be switched by meansmeans of aa chemical,chemical, redox,redox, oror photonicphotonic eventevent (“chemo-,(“chemo-, electro-,electro-, oror photo-switchingphoto-switching magnetic behavior”) constitute thethe simplestsimplest moleculesmolecules toto bebe testedtested [[93,9493,94,,97,98,149–165].97,98,149–165]. InIn principle, thethe spinsspins ofof the metal centers wouldwould be be antiferro- antiferro- or or ferromagnetically ferromagnetically coupled coupled in one in ofone the of states the states (ON), (ON), whereas whereas they would they bewould magnetically be magnetically uncoupled uncoupled in the other in the one other (OFF), one as (OFF), shown as in Figureshown7 .in MSs Figure o ﬀ er7. anMSs alternative offer an designalternative concept design to encodeconcept binary to encode information binary info in theirrmation corresponding in their corresponding ON (“1”) and ON OFF (“1”) (“0”) and states, OFF because(“0”) states, of the because switching of the of switching the magnetic of the coupling magnetic between coupling the between spin carriers, the spin as stated carriers, by Lloretas stated [100 by]. ALloret transistor-like [100]. A transistor-like magnetic behavior magnetic at the behavior molecular at the scale molecular can be achieved,scale can be opening achieved, thus opening the way thus for thethe potentialway for the applications potential applications of MSs in quantum of MSs computingin quantum [ 94computing]. [94].

(a)

(b)

Figure 7. Illustration of the transistor behavior in a molecular magnetic switch showing the interruption Figure 7. Illustration of the transistor behavior in a molecular magnetic switch showing the interruption and restoration of the parallel (a) or antiparallel (b) spin alignment. Adapted with permission from and restoration of the parallel (a) or antiparallel (b) spin alignment. Adapted with permission from Castellano et al. (reprinted with permission from [100], American Chemical Society, 2015). Castellano et al. (reprinted with permission from [100], American Chemical Society, 2015). Other possibilities exist for the design of an MS, whereby the spins of the magnetic centers would be antiferromagneticallyOther possibilities exist coupled for the in design one of theof an states MS, (OFF)whereby and the ferromagnetically spins of the magnetic coupled centers in the would other onebe antiferromagnetically (ON), as shown in Figure coupled8. In thisin one case, of thethe spinstates alignment (OFF) and can ferromagnetically be switched from coupled antiparallel in the to parallelother one or vice(ON), versa as throughshown in the Figure action of8. aIn switchable this case, magnetic the spin coupler. alignment The ideacan be behind switched this design from conceptantiparallel is to to be parallel able to or invert vice theversa spin through alignment the action of the spin-polarizedof a switchable currentmagnetic along coupler. the molecular The idea circuitbehind by this means design of anconcept electric is potential to be able (”threshold to invert voltagethe spin”), alignment so that a rectifier-like of the spin-polarized magnetic behavior current onalong the the molecular molecular scale circuit can be by achieved means of [166 an –electric168]. potential (‘threshold voltage’), so that a rectifier- like magneticMolecular behavior magnetic on transistors the molecular (MTs) scale and can molecular be achieved magnetic [166–168]. rectifiers (MRs) appear thus as some particular cases of MSs. A large variety of factors can influence over these MSs in a reversible way, such as pH, electrochemical potential, or light irradiation, leading to chemo-, electro- or photo-switching magnetic behaviors, as we will see hereafter.

Figure 8. Illustration of the rectifying behavior in a molecular magnetic switch showing the inversion of the spin alignment from parallel to antiparallel induced by an external applied electric field. Adapted with permission from Castellano et al. (reprinted with permission from [100], American Chemical Society, 2015). Magnetochemistry 2020, 6, x FOR PEER REVIEW 10 of 24

3. Dinuclear Copper(II) Metallacyclophanes in the POC Design of Molecular Magnetic Switches Molecular magnetic switches (MSs) which would allow for the interruption and restoration of the magnetic communication between the spin carriers are also basic components of a molecular spintronic circuit [100,141]. MSs are archetypical examples of bistable dynamic systems presenting two separately stable equilibrium states (or two distinctly accessible states) having totally different magnetic properties, which can be transformed in a reversible manner under some external stimuli. The external stimuli that are responsible for the magnetic switching behavior can be chemical, electronic, or photonic, among others, as occurs in conventional molecular electronic switches [142–148]. MSs formed by two localized spins whose magnetic communication can be switched by means of a chemical, redox, or photonic event (“chemo-, electro-, or photo-switching magnetic behavior”) constitute the simplest molecules to be tested [93,94,97,98,149–165]. In principle, the spins of the metal centers would be antiferro- or ferromagnetically coupled in one of the states (ON), whereas they would be magnetically uncoupled in the other one (OFF), as shown in Figure 7. MSs offer an alternative design concept to encode binary information in their corresponding ON (“1”) and OFF (“0”) states, because of the switching of the magnetic coupling between the spin carriers, as stated by Lloret [100]. A transistor-like magnetic behavior at the molecular scale can be achieved, opening thus the way for the potential applications of MSs in quantum computing [94].

(a)

(b)

Figure 7. Illustration of the transistor behavior in a molecular magnetic switch showing the interruption and restoration of the parallel (a) or antiparallel (b) spin alignment. Adapted with permission from Castellano et al. (reprinted with permission from [100], American Chemical Society, 2015).

Other possibilities exist for the design of an MS, whereby the spins of the magnetic centers would be antiferromagnetically coupled in one of the states (OFF) and ferromagnetically coupled in the other one (ON), as shown in Figure 8. In this case, the spin alignment can be switched from antiparallel to parallel or vice versa through the action of a switchable magnetic coupler. The idea behind this design concept is to be able to invert the spin alignment of the spin-polarized current along the molecular circuit by means of an electric potential (‘threshold voltage’), so that a rectifier- Magnetochemistry 2020, 6, 69 11 of 25 like magnetic behavior on the molecular scale can be achieved [166–168].

Magnetochemistry 2020, 6, x FOR PEER REVIEW 11 of 24

Molecular magnetic transistors (MTs) and molecular magnetic rectifiers (MRs) appear thus as Figure 8. 8. IllustrationIllustration of the of rectifying the rectifying behavior behavior in a molecular in a molecular magnetic magnetic switch showing switch showingthe inversion the some particular cases of MSs. A large variety of factors can influence over these MSs in a reversible inversionof the spin of alignment the spin from alignment parallel from to antiparallel parallel to induced antiparallel by an induced external by applied an external electric applied field. way, such as pH, electrochemical potential, or light irradiation, leading to chemo-, electro- or photo- electricAdapted ﬁeld. with Adapted permission with from permission Castellano from et Castellano al. (reprinted et al. with (reprinted permissi withon permissionfrom [100], fromAmerican [100], switching magnetic behaviors, as we will see hereafter. AmericanChemical Society, Chemical 2015). Society, 2015). 3.1.3.1. Chemoactive Chemoactive Spacers Spacers PereiraPereira and and Julve Julve have recently reported a unique,unique, pH-triggered, stru structuralctural and and magnetic switchingswitching behavior behavior for for a arelated related pair pair of ofoxamato-based oxamato-based dicopper(II) dicopper(II) metallacyclic metallacyclic complexes complexes with with the flexiblethe ﬂexible 4,4′-biphenylethylene 4,40-biphenylethylene spacer spacer (Scheme (Scheme 1h). 1Ith). can It adopt can adopt either either syn (insyn alkaline(in alkaline media) media) or anti or conformationsanti conformations (in slightly (in slightly acidic acidic media) media) depending depending on onthe the protonation protonation degree, degree, as as illustrated illustrated in in SchemeScheme 66[ 169[169–171].–171].

SchemeScheme 6. 6. POC design of aa molecularmolecular ferromagneticferromagnetic switch basedbased onon oxamato-basedoxamato-based permethylatedpermethylated dicopper(II)dicopper(II) 2,2 2,2′-biphenylethylenophanes0-biphenylethylenophanes showing showing the the revers reversibleible chemoswitching chemoswitching magnetic magnetic behavior betweenbetween the the deprotonated deprotonated synsyn andand protonated protonated antianti isomersisomers upon upon ligand ligand protonation protonation of the ofamido the amidodonor groups.donor groups.Adapted Adapted with permission with permission from Do Pim from et al. Do (reprinted Pim et al. with (reprinted permission with from permission [17], Elsevier, from 2017). [17], Elsevier, 2017). A reversible syn–anti conformational change of the ligand occurs in aqueous solution upon protonationA reversible of the syn–antitwo amideconformational groups in the change double-stranded of the ligand dicopper(II) occurs in metallacyclic aqueous solution complex upon of theprotonation cyclophane-type of the two [169]. amide This groups gives rise in theto th double-strandede corresponding dicopper(II)bis(monohydrogenoxamato)-bridged metallacyclic complex of dimerthe cyclophane-type of single-stranded [169 ].copper(II) This gives metallacyclic rise to the corresponding species by free bis(monohydrogenoxamato)-bridged rotation around the central single carbon–carbondimer of single-stranded bond of the copper(II) 2,2′-ethylenediphenylene metallacyclic species spacer. by free This rotation bistable around pair the of centraldicopper(II) single metallacycliccarbon–carbon complexes bond of shows the 2,2 a0 -ethylenediphenyleneswitching from non-interacting spacer. Thisspins bistable (OFF) for pair the of deprotonated dicopper(II) synmetallacyclic isomer, to parallel complexes spin shows alignment a switching (ON) for from the protonated non-interacting anti isomer. spins (OFF) In the for latter the case, deprotonated the weak ferromagneticsyn isomer, to parallelcoupling spin (J = alignment 2.93 cm−1) (ON)is due for to thethe protonated accidental orthoganti isomer.onality In of the the latter magnetic case, the orbitals weak 1 offerromagnetic the two CuII couplingions through (J = the2.93 out-plane cm− ) is dueexchange to the path accidentalway involving orthogonality the axial of the carboxylate magnetic groups. orbitals II Inof the former two Cu case,ions the through extended the non-conjugated out-plane exchange π-pathway pathway of involving the 2,2′-ethylenediphenylene the axial carboxylate spacers groups. connecting the two CuII ions is unable to mediate any significant EE interaction, as supported by DFT calculations [169]. Interestingly, this multifunctional dicopper(II) complex can be easily anchored over niobium oxyhydroxide or adsorbed on hybrid silica-based porous materials [170,171], opening thus the way for future applications as magnetic nanodevices in single-molecule spintronics.

In the former case, the extended non-conjugated π-pathway of the 2,20-ethylenediphenylene spacers connecting the two CuII ions is unable to mediate any signiﬁcant EE interaction, as supported by DFT calculations [169]. Interestingly, this multifunctional dicopper(II) complex can be easily anchored over niobium oxyhydroxide or adsorbed on hybrid silica-based porous materials [170,171], opening thus the way for future applications as magnetic nanodevices in single-molecule spintronics.

3.2. Electroactive Spacers The aforementioned oxamato-based dicopper(II) paracyclophanes can be considered appealing candidates for MSs [125,126]. Because of the redox-active (“non-innocent”) nature of their polymethyl-substituted 1,4-phenylene spacers, the permethylated dicopper(II) paracyclophane exhibits Magnetochemistrya unique redox-triggered 2020, 6, x FOR PEER magnetic REVIEW rectifying behavior [125]. In this case, the magnetic bistability 12 of 24 brings about a change from antiparallel (OFF) to parallel (ON) alignments of the spins of the two theCu IItwoions. Cu ThisII ions. fact This adheres fact adheres to the polarizationto the polarization by the πby-stacked the π-stacked delocalized delocalized monoradical monoradical ligand, ligand,which iswhich generated is upongenerated one-electron upon oxidationone-electron of the oxidation double tetramethyl- of the doublep-phenylenediamidate tetramethyl-p- phenylenediamidatebridging skeleton, as bridging illustrated skelet in Schemeon, as 7illustrated. in Scheme 7.

Scheme 7. POCPOC design design of of a a molecular magnetic rectifier rectiﬁer based on oxamato-based permethylated dicopper(II) paracyclophanesparacyclophanes showing showing the the reversible reversible electroswitching electroswitching magnetic magnetic behavior behavior upon ligandupon ligandoxidation oxidation of the phenylene of the phenylene spacer to spacer the corresponding to the corresponding monoradical monoradical cation. Adapted cation. with Adapted permission with permissionfrom Castellano from etCastellano al. (reprinted et al. with(reprinted permission with permission from [99], Elsevier, from [99], 2015). Elsevier, 2015).

Our group is currently investigating a novel series of oxamato-based dicopper(II) metallacyclophanes with flat-like ﬂat-like electroactive 1,4- or or 2,6-anthraqu 2,6-anthraquinoneinone (OAQ) (OAQ) spacers spacers (Scheme (Scheme 11k,l)k,l) as new examples of MSs. In In this this latter latter case, case, a a complete electrochemical electrochemical reversibility may be reached upon fourfour proton proton/electron-coupled/electron-coupled reduction reduct andion oxidation and inoxidation the dicopper(II) in the 2,6-anthraquinophane, dicopper(II) 2,6- anthraquinophane,as reported earlier as for reported the related earlier dicopper(II) for the rela 1,4-anthraquinophane,ted dicopper(II) 1,4-anthra whichquinophane, acts as a which prototype acts II asof a a prototype molecular of magnetica molecular capacitor magnetic (MC) capacitor [130,131 (M].C) [130,131]. In the former In the case,former the case, spins the spins of the of Cu the CuionsII ions would would be antiferromagneticallybe antiferromagnetically coupled coupled through through the the extended extendedπ π-conjugated,-conjugated, fullyfully reduced dihydroanthraquinolate (ON), (ON), wher whereaseas they are magnetically uncoupled across the non-conjugated π-pathway of the 2,6-anthraquinone spacers (OFF), asas illustratedillustrated inin SchemeScheme8 8..

Scheme 8. POC design of a molecular antiferromagnetic switch in oxamato-based dicopper(II) 2,6- anthraquinophanes showing the electroswitching magnetic behavior upon ligand reduction of the anthraquinone spacers.

Magnetochemistry 2020, 6, x FOR PEER REVIEW 12 of 24 the two CuII ions. This fact adheres to the polarization by the π-stacked delocalized monoradical ligand, which is generated upon one-electron oxidation of the double tetramethyl-p- phenylenediamidate bridging skeleton, as illustrated in Scheme 7.

Scheme 7. POC design of a molecular magnetic rectifier based on oxamato-based permethylated dicopper(II) paracyclophanes showing the reversible electroswitching magnetic behavior upon ligand oxidation of the phenylene spacer to the corresponding monoradical cation. Adapted with permission from Castellano et al. (reprinted with permission from [99], Elsevier, 2015).

Our group is currently investigating a novel series of oxamato-based dicopper(II) metallacyclophanes with flat-like electroactive 1,4- or 2,6-anthraquinone (OAQ) spacers (Scheme 1k,l) as new examples of MSs. In this latter case, a complete electrochemical reversibility may be reached upon four proton/electron-coupled reduction and oxidation in the dicopper(II) 2,6- anthraquinophane, as reported earlier for the related dicopper(II) 1,4-anthraquinophane, which acts as a prototype of a molecular magnetic capacitor (MC) [130,131]. In the former case, the spins of the CuII ions would be antiferromagnetically coupled through the extended π-conjugated, fully reduced dihydroanthraquinolate (ON), whereas they are magnetically uncoupled across the non-conjugated Magnetochemistry 2020, 6, 69 13 of 25 π-pathway of the 2,6-anthraquinone spacers (OFF), as illustrated in Scheme 8.

SchemeScheme 8. 8. POCPOC design design of ofa molecular a molecular antiferromagnetic antiferromagnetic switch switch in oxamato-based in oxamato-based dicopper(II) dicopper(II) 2,6- anthraquinophanes2,6-anthraquinophanes showing showing the theelec electroswitchingtroswitching magnetic magnetic behavior behavior upon upon ligand ligand reduction reduction of of the the anthraquinoneanthraquinone spacers. 3.3. Photoactive Spacers

Oxamato-based dicopper(II) oligoacenophanes are appealing candidates as MSs because of the photoactive (“non-innocent”) nature of the OA spacers [135–137]. As such, the aforementioned dicopper(II) 1,5-naphthalenophane and 2,6-anthracenophane exhibit a photo-triggered magnetic transistor behavior [136,137]. The reported photomagnetic bistability arises from the more or less complete and thermally reversible conversion of the weak antiferromagnetically coupled dicopper(II) oligocenophane (ON) to the corresponding magnetically uncoupled dicopper(II) photodimer product (OFF) resulting from the intramolecular [4 + 4] photocycloaddition reaction of the two facing oligoacene spacers under UV light irradiation and heating, as illustrated in Scheme9. This intramolecular (“pseudo-bimolecular”) photocycloaddition reaction constitutes a unique example of coordination-driven self-assembly for the supramolecular control of photochemical reactivity and photophysical properties in the solid state [99]. The lower photochemical efficiency and thermal reversibility of the dicopper(II) 1,5-naphthalenophane when compared to those of the 2,6-anthracenophane derivative agree with the different reactivity toward the photodimerization of anthracene and naphthalene themselves [137]. However, their photochemical efficiency and thermal reversibility are comparable to those reported for the intramolecular [4 + 4] photocycloaddition of purely organic naphthalenophane and anthracenophane analogues, reflecting thus the importance of the entropic effects associated with the cyclic or metallacyclic structures. A novel series of oxamato-based dicopper(II) metallacyclophanes with rod-like photoactive oligo(p-phenylenevinylene)-(OPV) and oligo(p-phenyleneazo) (OPA) spacers, such as 4,40-stilbene or 4,40-azobenzene (Scheme1g), is currently under investigation by some of us. As such, a complete and totally reversible photochemical transformation may be reached after successive irradiation, with UV and visible light, of the dicopper(II) 4,40-stilbenophane and 4,40-azobenzenophane featuring a cis–trans geometric isomerization, as illustrated in Scheme 10. The spins of the CuII ions are antiferromagnetically coupled through the planar trans-azobenzene spacers (ON state) in the former case, whereas they are magnetically uncoupled across the non-planar cis-azobenzene spacers (OFF state) in the latter one because of the steric repulsion between the ortho hydrogen atoms from the two benzene rings. Magnetochemistry 2020, 6, x FOR PEER REVIEW 13 of 24

3.3. Photoactive Spacers Oxamato-based dicopper(II) oligoacenophanes are appealing candidates as MSs because of the photoactive (“non-innocent”) nature of the OA spacers [135–137]. As such, the aforementioned dicopper(II) 1,5-naphthalenophane and 2,6-anthracenophane exhibit a photo-triggered magnetic transistor behavior [136,137]. The reported photomagnetic bistability arises from the more or less complete and thermally reversible conversion of the weak antiferromagnetically coupled dicopper(II) oligocenophane (ON) to the corresponding magnetically uncoupled dicopper(II) photodimer product (OFF) resulting from the intramolecular [4 + 4] photocycloaddition reaction of the two facing Magnetochemistry 2020, 6, 69 14 of 25 oligoacene spacers under UV light irradiation and heating, as illustrated in Scheme 9.

Magnetochemistry 2020, 6, x FOR PEER REVIEW 14 of 24

and visible light, of the dicopper(II) 4,4′-stilbenophane and 4,4′-azobenzenophane featuring a cis–trans Scheme 9. POC design of a molecular antiferromagnetic switch based on oxamato-based geometricScheme isomerization, 9. POC design as of illustrated a molecular in antiferromagnetic Scheme 10. The switchspins of based the Cuon IIoxamato-based ions are antiferromagnetically dicopper(II) dicopper(II) 1,5-naphthalenophanes (a) and 2,6-anthracenophanes (b), showing the thermally reversible coupled1,5-naphthalenophanes through the planar ( atrans) and-azobenzene 2,6-anthracenophanes spacers (ON ( bstate)), showing in the formerthe thermally case, whereas reversible they are photoswitchingphotoswitching magnetic magnetic behavior behavior upon upon ligand ligand ph photocycloadditionotocycloaddition of of the the two two facing facing naphthalene naphthalene magnetically uncoupled across the non-planar cis-azobenzene spacers (OFF state) in the latter one because andand anthracene anthracene spacers. spacers. Adapted Adapted with with permission permission fr fromom Castellano Castellano et et al. al. (reprinted (reprinted with with permission permission of the steric repulsion between the ortho hydrogen atoms from the two benzene rings. fromfrom [99], [99], Elsevier, Elsevier, 2015). 2015).

This intramolecular (“pseudo-bimolecular”) photocycloaddition reaction constitutes a unique example of coordination-driven self-assembly for the supramolecular control of photochemical reactivity and photophysical properties in the solid state [99]. The lower photochemical efficiency and thermal reversibility of the dicopper(II) 1,5-naphthalenophane when compared to those of the 2,6-anthracenophane derivative agree with the different reactivity toward the photodimerization of anthracene and naphthalene themselves [137]. However, their photochemical efficiency and thermal reversibility are comparable to those reported for the intramolecular [4 + 4] photocycloaddition of purely organic naphthalenophane and anthracenophane analogues, reflecting thus the importance of the entropic effects associated with the cyclic or metallacyclic structures. A novel series of oxamato-based dicopper(II) metallacyclophanes with rod-like photoactive oligo(p-phenylenevinylene)-(OPV) and oligo(p-phenyleneazo) (OPA) spacers, such as 4,4′-stilbene or 4,4′-azobenzene (Scheme 1g), is currently under investigation by some of us. As such, a complete and totally reversible photochemical transformation may be reached after successive irradiation, with UV

Scheme 10. 10. POCPOC design design of ofa molecular a molecular magnetic magnetic switch switch in oxamato-based in oxamato-based dicopper(II) dicopper(II) 4,4′- stilbenophanes4,40-stilbenophanes (a) and (a) and 4.4′ 4.4-azobenzenophanes0-azobenzenophanes (b) ( bshowing) showing the the photoswitching photoswitching magnetic magnetic behavior behavior upon cis–trans geometrical photoisomerization of the aromatic spacers.

4. Conclusions and Outlook: Metallosupramolecular Chemistry Acts as a Rail from Single- Molecule Spintronics to Quantum Computing Metallosupramolecular complexes are particularly attractive systems for the construction of molecular magnetic devices for highly integrated molecular spintronic circuits. The chemical, electro- and photo-chemical reactivities of the metal centers and/or the ligand spacers, their ability to respond to changes in chemical and electrochemical potential or photoexcitation, and the geometrical features that allow positioning of substituent groups, allow for the exploration of a vast amount of molecular magnetic wires (MWs) and switches (MSs) [19–28]. The extension of the control of charge localization (or current flow) through MWs and MSs in response to one or several input signals is mandatory in the development of molecule-based logic circuits for quantum computing. The controlled entanglement through a very weak (but non-negligible) switchable magnetic interaction between a pair of highly coherent spin carriers acting as individual quantum bits (qubits) is crucial for the physical implementation of quantum information processing (QIP) through quantum gates. In contrast to current computational methodology based on classical bits, QIP may offer a new paradigm for performing quantum logic operations that exploits the quantum coherence properties of electron spin- based qubits in forming part of a quantum gate in future quantum computers [29–45]. The present review provides a brief overview on the evolution of the field of oxamato-based dicopper(II) metallacyclophanes, from the simple systems with ferro- and antiferromagnetic interactions, depending on the substitution pattern of the m- and p-phenylene, 2,6-pyridine or 1,4- anthraquinophane spacers, to a plethora of multifunctional magnetic systems, playing with the chemical, electro- and photo-chemical properties of the oligo-o-phenylethylene, oligo-p-phenylene or p- phenylethyne, and oligo-α,α′- or β,β′-acene spacers (see Scheme 1). Oxamato-based dicopper(II) metallacyclophanes constitute thus a unique class of metalosupramolecular complexes because they combine the inherent magnetic properties of the intervening metal centers with the chemical, electro- and photo-chemical reactivity of the organic bridging ligands. Besides their use as ground tests for the Magnetochemistry 2020, 6, 69 15 of 25

4. Conclusions and Outlook: Metallosupramolecular Chemistry Acts as a Rail from Single-Molecule Spintronics to Quantum Computing Metallosupramolecular complexes are particularly attractive systems for the construction of molecular magnetic devices for highly integrated molecular spintronic circuits. The chemical, electro- and photo-chemical reactivities of the metal centers and/or the ligand spacers, their ability to respond to changes in chemical and electrochemical potential or photoexcitation, and the geometrical features that allow positioning of substituent groups, allow for the exploration of a vast amount of molecular magnetic wires (MWs) and switches (MSs) [19–28]. The extension of the control of charge localization (or current flow) through MWs and MSs in response to one or several input signals is mandatory in the development of molecule-based logic circuits for quantum computing. The controlled entanglement through a very weak (but non-negligible) switchable magnetic interaction between a pair of highly coherent spin carriers acting as individual quantum bits (qubits) is crucial for the physical implementation of quantum information processing (QIP) through quantum gates. In contrast to current computational methodology based on classical bits, QIP may offer a new paradigm for performing quantum logic operations that exploits the quantum coherence properties of electron spin-based qubits in forming part of a quantum gate in future quantum computers [29–45]. The present review provides a brief overview on the evolution of the field of oxamato-based dicopper(II) metallacyclophanes, from the simple systems with ferro- and antiferromagnetic interactions, depending on the substitution pattern of the m- and p-phenylene, 2,6-pyridine or 1,4-anthraquinophane spacers, to a plethora of multifunctional magnetic systems, playing with the chemical, electro- and photo-chemical properties of the oligo-o-phenylethylene, oligo-p-phenylene or p-phenylethyne, and oligo-α,α0- or β,β0-acene spacers (see Scheme1). Oxamato-based dicopper(II) metallacyclophanes constitute thus a unique class of metalosupramolecular complexes because they Magnetochemistrycombine the inherent 2020, 6, x magnetic FOR PEER properties REVIEW of the intervening metal centers with the chemical, electro- 15 of and 24 photo-chemical reactivity of the organic bridging ligands. Besides their use as ground tests for the fundamental research on electron exchange ( (EE)EE) magnetic interactions through through extended extended ππ-conjugated-conjugated aromatic ligands (both experimentally and and theoreti theoretically),cally), they they have have emerged as ideal model systems for the proof-of-concept design of molecule-based multifunctional magnetic devices in the emerging fieldsfields of single-molecule single-molecule spintronics spintronics and and quan quantumtum computing, computing, as as shown shown in in Figure Figure 99 [99,100].[99,100].

Figure 9. Oxamato-based dicopper(II) metallacyclophanes as multifunctional molecular magnetic devices for electrical ( a) and and logic logic circuits circuits ( (b)) of of single-molecule single-molecule spintronics spintronics and and quantum quantum computing. computing.

Oxamato-based dicopper(II)dicopper(II) metallacyclophanes metallacyclophanes appear appear thus thus as suitableas suitable candidates candidates for the for study the studyof spin-dependent of spin-dependent electron electron transport transport (ET) across (ET) single-molecule across single-molecule junctions junctions when they when are connected they are connectedto two gold to electrodestwo gold electrodes through their through carboxylate-oxygen their carboxylate-oxygen donor atoms donor (Figure atoms9a), (Figure as proposed 9a), as proposedearlier [99 ].earlier A topological [99]. A topological control of the control ET can of be the envisaged ET can inbe dicopper(II) envisaged meta-in dicopper(II) and paracyclophanes meta- and paracyclophanes(Scheme1a,b). The (Scheme parallel spin1a,b). alignment The parallel observed spin alignment in the former observed case would in the lead former to a greatercase would electrical lead toconductance a greater thanelectrical the antiparallel conductance one foundthan the in the anti latterparallel case [one172 ,173found]. Alternatively, in the latter the case application [172,173]. of Alternatively,a moderate magnetic the application field that reversesof a moderate the spin magnet alignmentic field from that antiparallel reverses tothe parallel spin alignment would allow from for antiparallela magnetic rectification to parallel of ETwould in dicopper(II) allow for oligophenylenophanes a magnetic rectification and oligophenylethynylenophanes of ET in dicopper(II) oligophenylenophanes(Scheme1e,f). In both cases, and the oligophe energynylethynylenophanes di fference between the (Scheme parallel (triplet)1e,f). In andboth antiparallel cases, the (singlet)energy differencespin states between can be chemicallythe parallel tuned(triplet) by and the antiparallel length of the (singlet) spacer spin [119 states,120]. can Likewise, be chemically the electrical tuned by the length of the spacer [119,120]. Likewise, the electrical rectification of the ET seems feasible in dicopper(II) anthraquinophanes (Scheme 1k,l). A change of the spin alignment from antiparallel to parallel is predicted after the successive reduction of the two anthraquinone spacers under an applied voltage of the electrical current [166]. Another possibility is to get an optical switching of the ET through molecular wires in dicopper(II) oligoacenophanes (Scheme 1i,j). In this case, the photocycloaddition of the oligoacene spacers by UV light irradiation would lead to the interruption of the electrical conductance, and thermal relaxation by heating would restore it [174–177]. One further step in this area will be the use of oxamato-based dicopper(II) metallacyclophanes as basic logic units (quantum gates) for the future generation of quantum computers (Figure 9b). The recent work by Rüffer and Kataev on the quantum coherence (QC) properties of related mononuclear oxamato- containing copper(II) complexes is particularly encouraging, as it shows that sufficiently long spin coherence times for quantum gate operations can be achieved in these simple systems [178]. However, this is not an easy task because it implies the rational design of very weakly interacting (“entangled”) and potentially switchable molecules before applications could be envisaged, as illustrated by the impressive work of Winpenny on supramolecular arrays of metal rings [75–79]. In this respect, the two novel classes of electro- and photo-active, oxamato-based dicopper(II) 4,4′-azobenzenophanes and 2,6-anthraquinophanes (Scheme 1g,l) are particularly interesting as prototypes of double qubit-based quantum gates. In both cases, each CuII ion (SCu = ½) constitutes a two-state magnetic quantum system represented by the pair of mS = ±1/2 states, which may be viewed as a basic unit of quantum information or a “single” qubit, the quantum analogue of the classical bit with “0” and “1” values. That being so, the two potentially switchable CuII ions may function as a “double” qubit with up to four degenerated |00>, |01>, |10>, and |11> quantum states. In fact, a completely reversible electro- or photo-magnetic switching could be achieved in the resulting dicopper(II) 2,6-anthraquinophane and 4,4′-azobenzenophane after redox cycling or under irradiation with UV and visible light (Schemes 8 and 10a, respectively). The spins of the CuII ions are antiferromagnetically coupled through the extended π-conjugated, 2,6-anthraquinolate and trans- Magnetochemistry 2020, 6, 69 16 of 25 rectification of the ET seems feasible in dicopper(II) anthraquinophanes (Scheme1k,l). A change of the spin alignment from antiparallel to parallel is predicted after the successive reduction of the two anthraquinone spacers under an applied voltage of the electrical current [166]. Another possibility is to get an optical switching of the ET through molecular wires in dicopper(II) oligoacenophanes (Scheme1i,j). In this case, the photocycloaddition of the oligoacene spacers by UV light irradiation would lead to the interruption of the electrical conductance, and thermal relaxation by heating would restore it [174–177]. One further step in this area will be the use of oxamato-based dicopper(II) metallacyclophanes as basic logic units (quantum gates) for the future generation of quantum computers (Figure9b). The recent work by Rüffer and Kataev on the quantum coherence (QC) properties of related mononuclear oxamato-containing copper(II) complexes is particularly encouraging, as it shows that sufficiently long spin coherence times for quantum gate operations can be achieved in these simple systems [178]. However, this is not an easy task because it implies the rational design of very weakly interacting (“entangled”) and potentially switchable molecules before applications could be envisaged, as illustrated by the impressive work of Winpenny on supramolecular arrays of metal rings [75–79]. In this respect, the two novel classes of electro- and photo-active, oxamato-based dicopper(II) 4,40-azobenzenophanes and 2,6-anthraquinophanes (Scheme1g,l) are particularly interesting as II 1 prototypes of double qubit-based quantum gates. In both cases, each Cu ion (SCu = 2 ) constitutes a two-state magnetic quantum system represented by the pair of m = 1/2 states, which may be S ± viewed as a basic unit of quantum information or a “single” qubit, the quantum analogue of the classical bit with “0” and “1” values. That being so, the two potentially switchable CuII ions may function as a “double” qubit with up to four degenerated |00>, |01>, |10>, and |11> quantum states. In fact, a completely reversible electro- or photo-magnetic switching could be achieved in the resulting dicopper(II) 2,6-anthraquinophane and 4,40-azobenzenophane after redox cycling or under irradiation with UV and visible light (Schemes8 and 10a, respectively). The spins of the Cu II ions are antiferromagnetically coupled through the extended π-conjugated, 2,6-anthraquinolate and trans-azobenzene spacers (“coupled” state), whereas they very weakly interact across the non-extended 2,6-anthraquinone and non-planar cis-azobenzene spacers (“entangled” state). Indeed, this review on the use of oxamato-based dicopper(II) metallacyclophanes as prototypes of multifunctional and multiresponsive molecular magnetic devices for single-molecule spintronics and quantum computing is more a proposal than a real achievement. This is rightly expressed in the voice of the most famous Portuguese poet, Fernando Pessoa: “Não sou nada./Nunca serei nada./Não posso querer ser nada./À parte isso, tenho em mim todos os sonhos do mundo” (extracted from the poem Tabacaria by Álvaro de Campos). In the years to come, we would like to accomplish some of these dream worlds concerning the physical realization of QIP in future quantum computers. In this respect, this review is also meant to be a tribute letter to Professor Francesc Lloret, who introduced us to the fantastic experience of molecular magnetism, on the occasion of his 65th birthday, and hoping that he will continue to pursue his dreams, from both professional and personal viewpoints.

Author Contributions: R.R.-G., J.C. and M.J. conceived and wrote the manuscript; R.R. and N.M. did bibliographic research and graphical editing of the manuscript; S.-E.S., D.C., C.L.M.P., J.F. and Y.J. proof-read, reviewed, and corrected the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Spanish MINECO (Projects PID2019-109735GB-I00, CTQ2016-75068P, MAT2017-89203-R, and Unidad de Excelencia María de Maeztu MDM-2015-0538). Acknowledgments: We acknowledge all people who have collaborated in this work and whose names appear in the reference list below, and particularly our colleagues from the groups of E. Pardo and J. Ferrando-Soria (Universitat de València), C. Ruiz-Pérez and J. Pasán (Universidad de La Laguna), and G. De Munno and D. Armentano (Università della Calabria). Thanks are also extended to M. Castellano, M. Viciano-Chumillas, and J.M. Carbonell-Vilar (Universitat de València), X. Ottenwaëlder (University of Concordia), R. Lescouëzec (Université Piere et Marie Curie), M.-C. Dul (Université d’Angers), and W.P. Barros (University of Campinas), whose contributions and enthusiasm have been essential to the birth and further development of the research on oxamato-based dicopper(II) metallacyclophanes as prototypes of multifunctional and multiresponsive magnetic devices for single-molecule spintronics and quantum computing. Magnetochemistry 2020, 6, 69 17 of 25

Conﬂicts of Interest: The authors declare no conﬂict of interest.

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