The catecholate groups in [{Pt(L)}3(μ3-tctq)] (H6tctq = 2,3,6,7,10,11-hexahydroxy-4b,8b,12b,12d-tetramethyltribenzotriquinacene; L = a diphosphine chelate) undergo sequential oxidation to their semiquinonate forms by voltammetry, with ΔE½ = 160–170 mV. The monoradical [{Pt(dppb)}3(μ3-tctq•)]+ is valence-localised, with no evidence for intervalence charge transfer in its near-IR spectrum. This contrasts with previously reported [{Pt(dppb)}3(μ3-ctc•)]+ (H6ctc = cyclotricatechylene), based on the same macrocyclic tris-dioxolene scaffold, which exhibits partly delocalised (class II) mixed valency

Baldansuren, A

Greatorex, S

Halcrow, MA

McInnes, EJL

Patmore, NJ

Sproules, S

Vincent, KB

English

The catecholate groups in [{Pt(L)}3(μ3-tctq)] (H6tctq = 2,3,6,7,10,11-hexahydroxy-4b,8b,12b,12d-tetramethyltribenzotriquinacene; L = a diphosphine chelate) undergo sequential oxidation to their semiquinonate forms by voltammetry, with ΔE1/2 = 160–170 mV. The monoradical [{Pt(dppb)}3(μ3-tctq˙)]+ is valence-localised, with no evidence for intervalence charge transfer in its near-IR spectrum. This contrasts with previously reported [{Pt(dppb)}3(μ3-ctc˙)]+ (H6ctc = cyclotricatechylene), based on the same macrocyclic tris-dioxolene scaffold, which exhibits partly delocalised (class II) mixed valency

Greatorex, Sam

Vincent, Kevin B.

Baldansuren, Amgalanbaatar

McInnes, Eric J.L.

Patmore, Nathan J.

Sproules, Stephen

Halcrow, Malcolm A.

Enlighten: Publications

Rigidification of a macrocyclic tris-catecholate scaffold leads to electronic localisation of its mixed valent redox product

Vincent, Kevin

McInnes, Eric

Patmore, Nathan

Halcrow, Malcolm

Huddersfield Research Portal

Rigidification of a Macrocyclic Tris-Catecholate Scaffold Leads toElectronic Localisation of its Mixed Valent Redox ProductSam Greatorexa, Kevin B. Vincentb, Amgalanbaatar Baldansurenc, Eric J. L. McInnesc,Nathan J. Patmoreb, Stephen Sproulesd and Malcolm A. Halcrowa,*The catecholate groups in [{Pt(L)}3(μ3-tctq)] (H6tctq = 2,3,6,7,10,11-hexahydroxy-4b,8b,12b,12d-tetramethyltribenzotriquinacene; L =a diphosphine chelate) undergo sequential oxidation to theirsemiquinonate forms by voltammetry, with ΔE½ = 160–170 mV. Themonoradical [{Pt(dppb)}3(μ3-tctq•)]+ is valence-localised, with noevidence for intervalence charge transfer in its near-IR spectrum.This contrasts with previously reported [{Pt(dppb)}3(μ3-ctc•)]+(H6ctc = cyclotricatechylene), based on the same macrocyclic tris-dioxolene scaffold, which exhibits partly delocalised (class II) mixedvalency.Dioxolenes are one of the most versatile and important non-innocent ligands for transition ions. 1 The catecholate (cat)/semiquinonate (sq) redox process occurs at low potential inmetal-bound dioxolenes, which can lead to stable ligand radicalspecies and/or facile charge transfer processes between thedioxolene and coordinated metal ion. This makes catecholateligands useful electron reservoirs for catalysis 2,3 and biologicalredox reactions.3,4 Alternatively, metal-bound sq radicals canact as switchable molecular paramagnets, 5 or show bulkmagnetic ordering when incorporated into coordinationframeworks.6 Lastly, complexes containing multiple dioxolenecentres can exhibit ligand-based mixed-valency 7 with intenseinter-valence charge-transfer absorptions in the near-IR. Thishas been observed in both mononuclear [M(cat) 3] complexes,8and in complexes of more complicated organic scaffoldscontaining two or three linked dioxolene redox sites. 9-11In the latter vein, we recently re-investigated the complexes[{Pt(L)}3(μ3-ctc)] (H6ctc = cyclotricatechylene; L = 1,2-bis{diphenylphosphinobenzene [dppb] or 1,2- bis{diphenyl-phosphinoethane [dppe]; Scheme 1), which were originallysynthesised by Bohle and Stasko. 12 The three catecholategroups in these complexes are oxidised sequentially, leading to[{Pt(L)}3(μ3-ctc•)]+ and [{Pt(L)}3(μ3-ctc••)]2+ radical productsshowing class II mixed valency by UV/vis/NIR spectroscopy, 13despite the absence of direct conjugation between theirdioxolene redox centers.14 The radical products of this workwere too unstable to isolate however, and could only behandled in solution below room temperature. We reasoned thismight reflect a lack of steric protection about the methylenegroups of the oxidised [ctc]n‒ macrocycle, since sq and otherphenoxyl radicals are prone to atom abstraction or couplingreactions at such para substituents.15Scheme 1. The compounds described in this work.Hence, we turned to the related tricatechol 2,3,6,7,10,11-hexahydroxy-4b,8b,12b,12d-tetramethyltribenzotriquinacene(H6tctq; Scheme 1), which is a new variant of the rigidtribenzotriquinacene motif developed by Kuck 16 as a bowl-shaped scaffold for supramolecular architectures 17,18 and softmaterials ¶.19 Since the methylene groups linking the catecholrings in [tctq]6‒ are fully quaternised, we reasoned that radicalsa. School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK.Email: m.a.halcrow@leeds.ac.ukb. Department of Chemical Sciences, University of Huddersfield, HuddersfieldHD1 3DH, UK.c. School of Chemistry and Photon Science Institute, University of Manchester,Oxford Road, Manchester M13 9PL, UK.d. WestCHEM, School of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK.† Electronic Supplementary Information (ESI) available: Experimental proceduresfor the synthesis and physical and computational characterisation of thecompounds in this work; crystallographic Figures and tables; EPR spectra andsimulated parameters; and computed structures, MO manifolds, orbital and spindensity plots. CCDC 1885699. For ESI and crystallographic data in CIF or otherelectronic format see DOI: 10.1039/######.ΐ ĂƚĂƐƵƉƉŽƌƟŶŐƚŚŝƐƐƚƵĚǇĂƌĞĂǀ ĂŝůĂďůĞĂƚŚƩ Ɖ͗ ͬ ͬ ĚŽŝ͘ŽƌŐͬ ϭϬ͘ϱϱϭϴͬ ηηη͘2 |derived by oxidation of [{Pt(L)}3(μ3-tctq)] might be more stableand easier to isolate. In the event that was not observed, butwe report here that sq radicals derived from [{Pt(L)} 3(μ3-ctc)](1a/1b, Scheme 1) and [{Pt(dppb)}3(μ3-tctq)] (2a) displayunexpectedly different electronic properties.2,3,6,7,10,11-Hexamethoxy-4b,8b,12b,12d-tetramethyltri-benzotriquinacene is accessible in nine synthetic steps by aliterature procedure (ESI†).20 H6tctq was obtained by exhaustivedemethylation of this precursor using BBr 3 ¶. Compounds 2aand 2b were prepared by treatment of H 6tctq with 3 equiv ofthe appropriate [PtCl2(L)] reagent in a N,N-dimethylacetamide/methanol solvent mixture, using potassium tert-butoxide as abase. While 2a is soluble in weakly interacting solvents andstable under an inert atmosphere, 2b is much less soluble andapparently less stable, so more limited characterisation of thatcompound was achieved. Similar issues were also encounteredwith 1b.14Differential pulse voltammograms of 2a and 2b inCH2Cl2/0.1M NBu4PF6 resemble those of 1a and 1b, in showingthree closely spaced low-potential oxidations (Fig. 1). 14 Theseare assigned to sequential oxidation of the three cat rings in the[tctq]6‒ ligand to the sq oxidation level (eq 1).[tctq]6‒   [tctq•]5‒   [tctq••]4‒   [tctq•••]3‒ (1)The first two processes for both compounds are chemicallyreversible and occur at similar potentials, but with E½ ca. 50 mVmore positive for 2b than 2a (Table 1). Some differences areseen on the third oxidation wave, however. The[2a••]2+/[2a•••]3+ oxidation is obviously split into two or threeFig. 1 Differential pulse voltammogram of 2a and 2b (CH2Cl2/0.1M NBu4PF6, 293 K).Table 1 Electrochemical data for the complexes in this work (cat = catecholate; sq =semiquinonate; q = quinone). Potentials are referenced against Fc+/0.sq/cat(E½ / V)q/sq(irra, Epa / V)2a –0.34 –0.17 +0.00, 0.08 +0.75, +1.162b –0.28 –0.12 +0.20 +0.82airr = irreversible.components implying some decomposition or deposition of the[2a••]2+ redox product. In contrast the [2b••]2+/[2b•••]3+oxidation occurs at ca. 0.2 V higher potential, and is apparentlyclean. The separation between the first two oxidation potentialsin 2a and 2b, ΔE½ = 0.16–0.17 V, yields the comproportionationconstants Kc = 6-8 × 102 between each oxidation level.10,13 Thatis slightly smaller than for 1a and 1b (ΔE½ = 0.18–0.22 V, Kc = 1–5 × 103),14 which implies electronic communication between thedioxolene rings in coordinated [tctq] n‒ is weaker than for[ctc]n‒. A second series of irreversible processes at +0.75–1.2 Vwas observed for 2a, assignable to further oxidation of thedioxolene rings in [tctq•••]3‒ to the quinone level. These wereless well-defined in 2b, which might reflect its precipitation atthe electrode during the measurement.EPR spectra of [2a•]+ and [2b•]+, generated by in situoxidation of the neutral precursors with 1 equiv [Cp 2Fe]PF6,were measured in frozen CH2Cl2 solution at X- and S-bandfrequencies. The spectra resemble [1a•]+ and other Pt(II)/sqradicals in showing weakly rhombic g-patterns, with hyperfinecoupling to just one 195Pt nucleus (ESI†).14,21,22 That shows theunpaired electrons in [2a•]+ and [2b•]+ are localised on onedioxolene ring under these conditions, on the EPR timescale.Fluid solution EPR spectra in the same solvent were achievedfor [2a•]+, yielding simulated isotropic A{195Pt} and a{31P} valuesthat are 30-50 % larger than for [1a•]+. That is consistent with agreater degree of localisation of the unpaired spin in [2a•]+.The UV/vis spectra of 2a and 2b in CH2Cl2/0.1 M NBu4PF6show just one resolved maximum at 31.7 × 10 3 cm–1.Spectroelectrochemical generation of [2a•]+ from 2a in thissolvent at 253 K proceeds isosbestically, and leads to anincrease in intensity for this peak, coupled to the ingrowth of anew shoulder near 23 × 103 cm–1 (Fig. 2). These changes stronglyresemble those observed during the oxidation of mononuclearPt(II)/cat/diphosphine complexes. 14 Importantly, theUV/vis/NIR spectrum of [2a•]+ shows no intervalence chargetransfer (IVCT) absorption above 4000 cm ‒1. That implies theelectronic structure of [2a•]+ is class I valence-localised,13 withno delocalisation or migration of the unpaired electron aroundthe [tctq•]5‒ macrocycle. This contrasts with [1a•]+, which showsan IVCT peak at νmax = 7.9 × 103 cm–1 (εmax = 500 M–1 cm–1)indicating class II mixed-valent character. 14 Similar results wereobtained from an oxidative titration of the 2a/[2a•]+ couple with[Cp2Fe]PF6 at room temperature, although the transformationwas not isosbestic under those conditions (ESI†).No crystal structures of 2a or 2b were obtained during thisstudy. However, comparison of metal-free H 6tctq·thf·nMeOH (n≈ 0.15; ESI†) with published structures of H6ctc shows small butconsistent differences in the disposition of their dioxolene rings,which should also be reflected in their complexes. 12,14 TheH6tctq cavitand has a slightly shallower bowl-shapedconformation than H6ctc (Fig. 3), which is reflected in theaverage intramolecular dihedral angle between the catecholrings in H6tctq [62.9(1)°] and H6ctc [typically 68–72°].23,24 Thatmight place the π-systems of the catechol rings in H6tctq furtherapart. Conversely, the closest intramolecular contact betweeneach catechol ring, namely the distance between their ipso Catoms at the base of the bowl-shaped cavity, is 2.513(4)–| 3Fig. 2 The 2a→[2a•]+ oxidation at 253 K in CH2Cl2/0.1 M NBu4PF6, monitored byUV/vis/NIR spectroscopy using an optically transparent electrode. The spectra of thepure starting material and product are highlighted in black while the intermediatespectra are paler. The feature near 12 × 103 cm‒1 is an artefact from a grating change inthe spectrometer.Fig. 3 Overlay of the crystallographic molecular structures of H 6tctq (white, ESI†) andH6ctc (purple, from H6ctc·5dmso24), showing the shallower bowl-shaped conformation ofthe H6tctq macrocycle.2.533(4) Å in H6tctq which is similar to H6ctc (typically 2.53–2.61Å). Hence, there is no simple structural relationship betweenthe dioxolene ligands in [1a•]+ and [2a•]+ that accounts for theirdifferent electronic character.Geometry optimised structures and electronic properties of[2a]0/1+/2+/3+ were calculated at the same level of theory used inour previous study of [1a]0/1+/2+/3+.14 That is, by spin-unrestrictedbroken symmetry DFT calculations at the B3LYP-ZORA level,with the dppb Ph groups replaced with H atoms. The optimizedgeometries of the [Pt(dppb)(dioxolene)] fragments in the twomolecules show only small differences at each oxidation level(ESI†). The greatest differences lie in the Pt‒P bonds which areconsistently 0.012-0.019 Å longer in [2a]z+ than in [1a]z+ at eachoxidation level z; and, the Pt‒O bonds which are 0.014-0.019 Åshorter in [2a•]+ than in [1a•]+ (the Pt‒O bond lengths in the twomolecules are more similar for other z). This may reflect astronger O→Pt -donation interaction in [2a]z+, arising from thestronger inductive effect of its quaternary alkyl dioxolenesubstituents.The average calculated Pt···Pt distances in [2a]z+ and [1a]z+differ by no more than 0.012 Å at each oxidation level, and showthe same trend of gradually increasing with z. Hence, in contrastto the free ligand crystallography (Fig. 3), the greater rigidity ofthe tctq macrocycle has little impact on the structures of itsradical oxidation products at this level of theory. However, thectc and tctq conformations will also be influenced by theinclusion of solvent molecules within their cavities, which istypically observed experimentally (ESI†) but is not accountedfor in the geometry optimisation calculations. 14The calculated Mulliken spin density population (Figure 4)and UV/vis/NIR spectra of [2a•]+ are essentially identical to[1a•]+,14 while the spin populations for the other members ofthe redox series are also identical irrespective of the ligandframework (ESI†). Since the DFT calculations are independentof temperature, that therefore suggests the different mixed-valent character of [1a•]+ and [2a•]+ arises from a temperature-dependent phenomenon. As the electronic structures areidentical for both species, we attribute the spin-localisation in[2a•]+ to the rigidity of the tctq ligand. The flexible secondarymethylene groups in ctc can approach a π radial–σ-bondcoplanar alignment with their bonded dioxolene groups, whichwould activate a through-bond interaction between thedioxolene π-systems via hyperconjugation.25 This orientation isless accessible with the more rigid quaternary linkages in tctq,leading to localisation of the dioxolene spins as observed in[2a•]+.Fig. 4 Mulliken spin population analysis for [2a•]+ (red: α-spin; yellow: β-spin).In conclusion, we have demonstrated that the mixed-valentcharacter of sq radicals derived from complexedcyclotricatechylene macrocycles is sensitive to theirconformational flexibility. Our current work aims to make useof this feature in host:guest systems based on this redox-activemolecular framework.This work was funded by the EPSRC (EP/M506552/1) and aLeverhulme Trust Research Project Grant (RPG-2014-303, toNJP and KBV). Data supporting this study are available in theESIΏĂŶĚĂƚŚƩ Ɖ͗ ͬ ͬ ĚŽŝ͘ŽƌŐͬ ϭϬ͘ϱϱϭϴͬ ηηη͘4 |Conflicts of interestThere are no conflicts to declare.Notes and references¶ A 2,3,6,7,10,11-hexahydroxytribenzotriquinacene tris-catecholrelated to H6tctq, but lacking the methyl substituents on themacrocycle methylene groups, was recently reported.261 C. G. Pierpoint and C. W. Lange, Prog. Inorg. Chem., 1993, 41,331; C. G. Pierpont and J. K. Kelly in The Chemistry of MetalPhenolates Part [1, 2], ed. J. Zabicky, Wiley, Chichester, UK,2014, pp. 669–698.2 D. L. J. Broere, R. Plessius and J. I. van der Vlugt, Chem. Soc.Rev., 2015, 44, 6886 and 2015, 44, 7010 and 7011(corrections).3 W. Kaim and B. Schwederski, Coord. Chem. Rev., 2010, 254,1580; E. M. Shepard and D. M. Dooley, Acc. Chem. Res.,2015, 48, 1218.4 J. Sedó , J. Saiz-Poseu , F. Busqué and D. Ruiz-Molina, Adv.Mater., 2013, 25, 653; J. Yang, M. A. Cohen Stuart andM. Kamperman, Chem. Soc. Rev., 2014, 43, 8271.5 J. S. Miller and K. S. Min, Angew. Chem. Int. Ed., 2009, 48,262; T. Tezgerevska, K. G. Alley and C. Boskovic, Coord.Chem. Rev., 2014, 268, 23.6 J. A. DeGayner, I.-R. Jeon, L. Sun, M. Dincă and T. D. Harris,  J. Am. Chem. Soc., 2017, 139, 4175; J. A. DeGayner, K. Wuand T. D. Harris, J. Am. Chem. Soc., 2018, 140, 6550;S. A. Sahadevan, A. Abhervé, N. Monni, C. Sáenz de Pipaón,J. R. Galán-Mascarós, J. C. Waerenborgh, B. J. C. Vieira,P. Auban-Senzier, S. Pillet, E. Bendeif, P. Alemany,E. Canadell, M. L. Mercuri and N. Avarvari, J. Am. Chem. Soc.,2018, 140, 12611.7 J. Hankache and O. S. Wenger, Chem. Rev., 2011, 111, 5138;A. Heckmann and C. Lambert, Angew. Chem., Int. Ed. , 2012,51, 326.8 C. G. Pierpont, Inorg. Chem., 2011, 50, 9766.9 D. A. Shultz, Comments Inorg. Chem. , 2002, 23, 1.10 A. Dei, D. Gatteschi, C. Sangregorio and L. Sorace, Acc. Chem.Res., 2004, 37, 827.11 K. G. Alley, G. Poneti, P. S. D. Robinson, A. Nafady,B. Moubaraki, J. B. Aitken, S. C. Drew, C. Ritchie,B. F. Abrahams, R. K. Hocking, K. S. Murray, A. M. Bond,H. H. Harris, L. Sorace and C. Boskovic, J. Am. Chem. Soc.,2013, 135, 8304.12 D. S. Bohle and D. Stasko, Chem. Commun., 1998, 567.13 K. D. Demadis, C. M. Hartshorn and T. J. Meyer, Chem. Rev.,2001, 101, 2655; D. M. D'Alessandro and F. R. Keene, Chem.Soc. Rev., 2006, 35, 424.14 J. J. Loughrey, N. J. Patmore, A. Baldansuren, A. J. Fielding,E. J. L. McInnes, M. J. Hardie, S. Sproules and M. A. Halcrow,Chem. Sci., 2015, 6, 6935.15 P. Chaudhuri and K. Wieghardt, Prog. Inorg. Chem., 2001, 50,151.16 D. Kuck, Chem. Rev., 2006, 106, 4885.17 See eg J. Strübe, B. Neumann, H.-G. Stammler and D. Kuck,Chem. Eur. J., 2009, 15, 2256; T. Wang, Y.-F. Zhang,Q.-Q. Hou, W.-R. Xu, X.-P. Cao, H.-F. Chow and D. Kuck, J.Org. Chem., 2013, 78, 1062; W.-R. Xu, G.-J. Xia, H.-F. Chow,X.-P. Cao and D. Kuck, Chem. Eur. J., 2015, 21, 12011;Z.-M. Li, D. Hu, J. Wei, Q. Qi, X.-P. Cao, H.-F. Chow and D.Kuck, Synthesis, 2018, 50, 1457.18 B. Bredenkötter, S. Henne and D. Volkmer, Chem. Eur. J.,2007, 13, 9931; B. Bredenkötter, M. Grzywa, M.Alaghemandi, R. Schmid, W. Herrebout, P. Bultinck andD. Volkmer, Chem. Eur. J., 2014, 20, 9100.19 J. Vile, M. Carta, C. G. Bezzu and N. B. McKeown, Polym.Chem., 2011, 2, 2257.20 M. Harig, B. Neumann, H.-G. Stammler and D. Kuck, Eur. J.Org. Chem., 2004, 2381.21 J. A. Weinstein, M. T. Tierney, E. S. Davies, K. Base,A. A. Robeiro and M. W. Grinstaff , Inorg. Chem., 2006, 45,4544; J. Best, I. V. Sazanovich, H. Adams, R. D. Bennett,E. S. Davies, A. J. H. M. Meijer, M. Towrie, S. A. Tikhomirov,O. V. Bouganov, M. D. Ward and J. A. Weinstein, Inorg.Chem., 2010, 49, 10041.22 J. J. Loughrey, S. Sproules, E. J. L. McInnes, M. J. Hardie andM. A. Halcrow, Chem. Eur. J., 2014, 20, 6272.23 J. A. Hyatt, E. N. Duesler, D.Y. Curtin and I. C. Paul, J. Org.Chem., 1980, 45, 5074; J. W. Steed, H. Zhang andJ. L. Atwood, Supramol. Chem., 1996, 7, 37; A. Chakrabarti,H. M. Chawla, G. Hundal and N. Pant, Tetrahedron, 2005, 61,12323; P. Satha, G. Illa, P. 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     Greatorex, S., Vincent, K. B., Baldansuren, A., McInnes, E. J.L., Patmore, N. J., Sproules, S.  and Halcrow, M. A. (2019) Rigidification of a macrocyclic tris-catecholate scaffold leads to electronic localisation of its mixed valent redox product. Chemical Communications, (doi:10.1039/C8CC10122A)  This is the author’s final accepted version.  There may be differences between this version and the published version. You are advised to consult the publisher’s version if you wish to cite from it.  http://eprints.gla.ac.uk/179730/                            Deposited on: 11 February 2019         Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk   Journal Name  COMMUNICATION This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 1  Please do not adjust margins Please do not adjust margins    Rigidification of a Macrocyclic Tris-Catecholate Scaffold Leads to Electronic Localisation of its Mixed Valent Redox Product†‡  Sam Greatorex,a Kevin B. Vincent,b Amgalanbaatar Baldansuren,c Eric J. L. McInnes,c Nathan J. Patmore,b Stephen Sproulesd and Malcolm A. Halcrow*,aThe catecholate groups in [{Pt(L)}3(μ3-tctq)] (H6tctq = 2,3,6,7,10,11-hexahydroxy-4b,8b,12b,12d-tetramethyltribenzotriquinacene; L = a diphosphine chelate) undergo sequential oxidation to their semiquinonate forms by voltammetry, with ΔE1/2 = 160–170 mV. The monoradical [{Pt(dppb)}3(μ3-tctq•)]+ is valence-localised, with no evidence for intervalence charge transfer in its near-IR spectra. This contrasts with previously reported [{Pt(dppb)}3(μ3-ctc•)]+ (H6ctc = cyclotricatechylene), based on the same macrocyclic tris-dioxolene scaffold, which exhibits partly delocalised (class II) mixed valency. Dioxolenes are one of the most versatile and important non-innocent ligands for transition ions.1 The catecholate (cat)/ semiquinonate (sq) redox process occurs at low potential in metal-bound dioxolenes, which can lead to stable ligand radical species and/or facile charge transfer processes between the dioxolene and coordinated metal ion. This makes catecholate ligands useful electron reservoirs for catalysis2,3 and biological redox reactions.3,4 Alternatively, metal-bound sq radicals can act as switchable molecular paramagnets,5 or show bulk magnetic ordering when incorporated into coordination frameworks.6 Lastly, complexes containing multiple dioxolene centres can exhibit ligand-based mixed-valency7 with intense inter-valence charge-transfer absorptions in the near-IR. This has been observed in both mononuclear [M(cat)3] complexes,8 and in complexes of more complicated organic scaffolds containing two or three linked dioxolene redox sites.9-11  In the latter vein, we recently re-investigated the complexes [{Pt(L)}3(μ3-ctc)] (H6ctc = cyclotricatechylene; L = 1,2-bis{diphenyl-phosphinobenzene [dppb] or 1,2-bis{diphenylphosphinoethane [dppe]; Scheme 1), which had been previously synthesised by Bohle and Stasko.12 The three catecholate groups in these complexes are oxidised sequentially, leading to [{Pt(L)}3(μ3-ctc•)]+ and [{Pt(L)}3(μ3-ctc•-•)]2+ radical products which show clear class II mixed valency,13 despite the absence of conjugation between their dioxolene redox centers.14 The radical products of this work were too unstable to isolate however, and could only be handled in solution below room temperature. We reasoned this might reflect a lack of steric protection about the methylene groups of the oxidised [ctc]n‒ macrocycle, since sq and other phenoxyl radicals are prone to atom abstraction or coupling reactions at such para substituents.15   Hence, we turned to the related tricatechol 2,3,6,7,10,11-hexahydroxy-4b,8b,12b,12d-tetramethyltribenzotriquinacene (H6tctq; Scheme 1), which is a new variant of the rigid tribenzotriquinacene motif developed by Kuck16 as a bowl-shaped scaffold for supramolecular architectures17,18 and soft materials.§19 Since the methylene groups linking the catechol rings in [tctq]6‒ are fully quaternised, we reasoned that radicals derived by oxidation of [{Pt(L)}3(μ3-tctq)] might be more stable and easier to isolate.   Scheme 1. The compounds described in this work. In the event that was not observed, but we report here that sq radicals derived from [{Pt(L)}3(μ3-ctc)] (1a/1b, Scheme 1) and [{Pt(dppb)}3(μ3-tctq)] (2a) display unexpectedly different electronic properties.  2,3,6,7,10,11-Hexamethoxy-4b,8b,12b,12d-teramethyltri-benzotriquinacene is accessible in nine synthetic steps by a literature procedure (ESI‡).20 H6tctq was obtained by exhaustive demethylation of this precursor using BBr3.§ Compounds 2a and 2b were prepared by treatment of H6tctq with 3 equiv. of the appropriate [PtCl2(L)] reagent in a N,N-dimethylacet-a. School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK. Email: m.a.halcrow@leeds.ac.uk b. Department of Chemical and Biological Sciences, University of Huddersfield, Huddersfield HD1 3DH, UK. c. School of Chemistry and Photon Science Institute, University of Manchester, Oxford Road, Manchester M13 9PL, UK. d. WestCHEM, School of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK.  † Data supporting this study are available at http://doi.org/10.5518/557 ‡ Electronic Supplementary Information (ESI) available: Experimental procedures for the synthesis and physical and computational characterisation of the compounds in this work; crystallographic figures and tables; EPR spectra and simulated parameters; and computed structures, MO manifolds, orbital and spin density plots. CCDC 1885699. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8cc10122a. COMMUNICATION Journal Name 2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins amide/methanol solvent mixture, using potassium tert-butoxide as a base. While 2a is soluble in weakly interacting solvents and stable under an inert atmosphere, 2b is much less soluble and apparently less stable, so only basic characterisation of that compound was achieved. Similar issues were also encountered with 1b.14  Differential pulse voltammograms of 2a and 2b in CH2Cl2/0.1M NBu4PF6 resemble those of 1a and 1b, in showing three closely spaced low-potential oxidations (Fig. 1).14 These are assigned to sequential oxidation of the three cat rings in the [tctq]6‒ ligand to the sq oxidation level (eq 1).   [tctq]6‒   ⇌    [tctq•]5‒   ⇌    [tctq••]4‒   ⇌    [tctq•••]3‒  (1) The first two processes for both compounds are chemically reversible and occur at similar potentials, but with E1/2 ca. 50 mV more positive for 2b than 2a (Table 1). Some differences are    Fig. 1   Differential pulse voltammogram of 2a and 2b (CH2Cl2/0.1M NBu4PF6, 293 K). seen on the third oxidation wave, however. The [2a••]2+/[2a•••]3+ oxidation is obviously split into two or three components implying some decomposition or deposition of the [2a••]2+ redox product. In contrast the [2b••]2+/[2b•••]3+ oxidation occurs at ca. 0.2 V higher potential, and is apparently clean. The separation between the first two oxidation potentials in 2a and 2b, ΔE1/2 = 0.16–0.17 V, yields the comproportionation constants Kc = 6-8 × 102 between each oxidation level.10,13 That is slightly smaller than for 1a and 1b (ΔE1/2 = 0.18–0.22 V, Kc = 1–5 × 103),14 which implies electronic communication between the dioxolene rings in coordinated [tctq]n‒ is weaker than for [ctc]n‒. A second series of irreversible processes at +0.75–1.2 V was observed for 2a, assignable to further oxidation of the dioxolene rings in [tctq•••]3‒ to the quinone level. These were less well- defined in 2b, which might reflect its precipitation at the electrode during the measurement.EPR spectra of [2a•]+ and [2b•]+, generated by in situ oxidation of the neutral precursors with 1 equiv [Cp2Fe]PF6, were measured in frozen CH2Cl2 solution at X- and S-band Table 1 Electrochemical data for the complexes in this work (cat = catecholate; sq = semiquinonate; q = quinone). Potentials are referenced against Fc+/0.  sq/cat  (E½ / V) q/sq  (irra, Epa / V) 2a –0.34 –0.17 +0.00, 0.08 +0.75, +1.16 2b –0.28 –0.12 +0.20 +0.82 airr = irreversible.  frequencies. The spectra resemble those of [1a•]+ and other Pt(II)/sq radicals in showing weakly rhombic g-patterns, with hyperfine coupling to just one 195Pt nucleus (ESI‡).14,21,22  That shows the unpaired electrons in [2a•]+ and [2b•]+ are localised on one dioxolene ring under these conditions, on the EPR timescale. Fluid solution EPR spectra in the same solvent were achieved for [2a•]+, yielding simulated isotropic A{195Pt} and a{31P} values that are 30-50 % larger than for [1a•]+.   The UV/vis spectra of 2a and 2b in CH2Cl2/0.1 M NBu4PF6 show just one resolved maximum at 31.7 × 103 cm–1. Spectroelectrochemical generation of [2a•]+ from 2a in this solvent at 253 K proceeds isosbestically, and leads to an increase in intsensity for this peak, coupled to the ingrowth of a new shoulder near 23 × 103 cm–1 (Fig. 2).    Fig. 2   The 2a → [2a•]+ oxidation at 253 K in CH2Cl2/0.1 M NBu4PF6, monitored by UV/vis/NIR spectroscopy using an optically transparent electrode. The spectra of the pure starting material and product are highlighted in black while the intermediate spectra are paler. The feature near 12 × 103 cm‒1 is an artefact from a grating change in the spectrometer. These changes strongly resemble those observed during the oxidation of mononuclear Pt(II)/cat/diphosphine complexes.14 Importantly, the UV/vis/NIR spectrum of [2a•]+ shows no evidence for an intervalence charge transfer (IVCT) absorption above 4000 cm‒1. That implies the electronic structure of [2a•]+ is valence-localised, with no delocalisation or migration of the unpaired electron around the [tctq•]5‒ macrocycle. This contrasts with [1a•]+, which shows an IVCT peak at νmax = 7.9 × 103 cm–1 (εmax = 500 M–1 cm–1) indicating class II mixed-valent character.14 Similar results were obtained from an oxidative titration of the 2a/[2a•]+ couple using [Cp2Fe]PF6 at room Journal Name  COMMUNICATION This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3  Please do not adjust margins Please do not adjust margins temperature, although the transformation was not isosbestic under those conditions (ESI‡).  No crystal structures of 2a or 2b were obtained during this study. However, comparison of metal-free H6tctq·thf·-n(pentane) (n ≈ 0.15; ESI‡) with published structures of H6ctc shows small but consistent differences in the disposition of their dioxolene rings, which should also be reflected in their complexes.12,14 The H6tctq cavitand has a slightly shallower bowl-shaped conformation than H6ctc (Fig. 3), which is reflected in the average intramolecular dihedral angle between the catechol rings in H6tctq [62.9(1)°] and H6ctc [typically 68–72°].23,24 That might place the π-systems of the catechol rings in H6tctq further apart. Conversely, the closest intramolecular contact between each catechol ring, namely the distance between their ipso C atoms at the base of the bowl-shaped cavity, is 2.513(4)–2.533(4) Å in H6tctq which is similar to H6ctc (typically 2.53–2.61 Å). Hence, there is no simple structural relationship between the dioxolene ligands in [1a•]+/[1b•]+ and [2a•]+/[2b•]+ that accounts for their different electronic character.   Fig. 3   Overlay of the crystallographic molecular structures of H6tctq (white, ESI‡) and H6ctc (purple, from H6ctc·5dmso24), showing the shallower bowl-shaped conformation of the H6tctq macrocycle.  Geometry optimised structures and electronic properties of [2a]0/1+/2+/3+ were calculated at the same level of theory used in our previous study of [1a]0/1+/2+/3+.14 That is, by spin-unrestricted broken symmetry DFT calculations at the B3LYP-ZORA level, with the dppb Ph groups replaced with H atoms. The optimized geometries of the [Pt(dppb)(dioxolene)] fragments in the two molecules show only small differences at each oxidation level (ESI‡). The greatest differences lie in the Pt‒P bonds which are consistently 0.012-0.019 Å longer in [2a]z+ than in [1a]z+ at each oxidation level z; and, the Pt‒O bonds which are 0.014-0.019 Å shorter in [2a•]+ than in [1a•]+ (the Pt‒O bond lengths in the two molecules are more similar for other z). This may reflect a stronger O→Pt -donation interaction in 2a, arising from the stronger inductive effect of its quaternary alkyl dioxolene substituents.   The average calculated Pt···Pt distances in [2a]z+ and [1a]z+ differ by no more than 0.012 Å at each oxidation level, and show the same trend of gradually increasing with z. Hence, in contrast to the free ligand crystallography (Fig. 3), the greater rigidity of the tctq macrocycle has little impact on the structures of its radical oxidation products at this level of theory. However, the ctc and tctq conformations will also be influenced by the inclusion of solvent molecules within their cavities, which is typically observed experimentally (ESI‡) but is not accounted for in the calculations.14  The Mulliken spin density population of [2a•]+ shown in Fig. 4 is essentially identical to that calculated for [1a•]+.14 The spin population for the other members of the redox series are also identical irrespective of the ligand framework (ESI‡). This is not unexpected as DFT calculations are independent of temperature, and suggests the origin of the different mixed-valent character of [1a•]+ and [2a•]+ is a temperature-dependent phenomenon. As the electronic structures are identical for both species, we can attribute the spin-localisation in [2a•]+ to the rigidity of the tctq ligand. The flexible secondary methylene groups in ctc can approach π radical–σ-bond coplanar alignment with their bonded dioxolene groups, which would activate a through-bond interaction between the dioxolene π-systems via hyperconjugation.25 This orientation is less accessible with the more rigid quaternary linkages in tctq, leading to localization of the dioxolene spins as observed in [2a•]+.  Fig. 4   Mulliken spin population analysis for [2a•]+ (red: α-spin; yellow: β-spin).  In conclusion, we have demonstrated that the mixed-valent character of sq radicals derived from complexed cyclotricatechylene macrocycles is sensitive to their conformational flexibility. Our current work aims to make use of this feature in host:guest systems based on this redox-active molecular framework. This work was funded by the EPSRC (EP/M506552/1) and a Leverhulme Trust Research Project Grant (RPG-2014-303, to NJP and KBV). Conflicts of interest There are no conflicts to declare. COMMUNICATION Journal Name 4 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Notes and references § A 2,3,6,7,10,11-hexahydroxytribenzotriquinacene tris-catechol related to H6tctq, but lacking the methyl substituents on the macrocycle methylene groups, was recently reported.26  1 C. G. Pierpoint and C. W. Lange, Prog. Inorg. Chem., 1993, 41, 331. 2 D. L. J. Broere, R. Plessius and J. I. van der Vlugt, Chem. Soc. Rev., 2015, 44, 6886 and 2015, 44, 7010 and 7011 corrections. 3 W. Kaim and B. Schwederski, Coord. Chem. Rev., 2010, 254, 1580; E. M. Shepard and D. M. Dooley, Acc. Chem. Res., 2015, 48, 1218. 4 J. Sedó , J. Saiz-Poseu , F. Busqué and D. Ruiz-Molina, Adv. Mater., 2013, 25, 653; J. Yang, M. A. Cohen Stuart and M. Kamperman, Chem. Soc. Rev., 2014, 43, 8271. 5 J. S. Miller and K. S. Min, Angew. Chem. Int. Ed., 2009, 48, 262; T. Tezgerevska, K. G. Alley and C. Boskovic, Coord. Chem. Rev., 2014, 268, 23.  6 J. A. DeGayner, I.-R. Jeon, L. Sun, M. Dincă and T. D. Harris, J. Am. Chem. Soc., 2017, 139, 4175; J. A. DeGayner, K. Wu and T. D. Harris, J. Am. Chem. Soc., 2018, 140, 6550.  7 J. Hankache and O. S. Wenger, Chem. Rev., 2011, 111, 5138; A. Heckmann and C. Lambert, Angew. Chem., Int. Ed., 2012, 51, 326. 8 C. G. Pierpont, Inorg. Chem., 2011, 50, 9766. 9 D. A. Shultz, Comments Inorg. Chem., 2002, 23, 1.  10 A. Dei, D. Gatteschi, C. Sangregorio and L. Sorace, Acc. Chem. Res., 2004, 37, 827.  11 K. G. Alley, G. Poneti, P. S. D. Robinson, A. Nafady, B. Moubaraki, J. B. Aitken, S. C. Drew, C. Ritchie, B. F. Abrahams, R. K. Hocking, K. S. Murray, A. M. Bond, H. H. Harris, L. Sorace and C. Boskovic, J. Am. Chem. Soc., 2013, 135, 8304. 12 D. S. Bohle and D. Stasko, Chem. Commun., 1998, 567. 13 K. D. Demadis, C. M. Hartshorn and T. J. Meyer, Chem. Rev., 2001, 101, 2655; D. M. D'Alessandro and F. R. Keene, Chem. Soc. Rev., 2006, 35, 424. 14 J. J. Loughrey, N. J. Patmore, A. Baldansuren, A. J. Fielding, E.J.L. McInnes, M. J. Hardie, S. Sproules and M. A. Halcrow, Chem. Sci., 2015, 6, 6935. 15 P. Chaudhuri and K. Wieghardt, Prog. Inorg. Chem., 2001, 50, 151. 16 D. Kuck, Chem. Rev., 2006, 106, 4885. 17 See e.g.: J. Strübe, B. Neumann, H.-G. Stammler and D. Kuck, Chem. Eur. J., 2009, 15, 2256; T. Wang, Y.-F. Zhang, Q.-Q. Hou, W.-R. Xu, X.-P. Cao, H.-F. Chow and D. Kuck, J. Org. Chem., 2013, 78, 1062−1069; W.-R. Xu, G.-J. Xia, H.-F. Chow, X.-P. Cao and D. Kuck, Chem. Eur. J., 2015, 21, 12011; Z.-M. Li, D. Hu, J. Wei, Q. Qi, X.-P. Cao, H.-F. Chow and D. Kuck, Synthesis, 2018, 50, 1457. 18 B. Bredenkötter, S. Henne and D. Volkmer, Chem. Eur. J., 2007, 13, 9931; B. Bredenkötter, M. Grzywa, M. Alaghemandi, R. Schmid, W. Herrebout, P. Bultinck and D. Volkmer, Chem. Eur. J., 2014, 20, 9100. 19 J. Vile, M. Carta, C. G. Bezzu and N. B. McKeown, Polym. Chem., 2011, 2, 2257. 20 M. Harig, B. Neumann, H.-G. Stammler and D. Kuck, Eur. J. Org. Chem., 2004, 2381. 21 J. A. Weinstein, M. T. Tierney, E. S. Davies, K. Base, A. A. Robeiro and M. W. Grinstaff, Inorg. Chem., 2006, 45, 4544; J. Best, I. V. Sazanovich, H. Adams, R. D. Bennett, E. S. Davies, A. J. H. M. Meijer, M. Towrie, S. A. Tikhomirov, O. V. Bouganov, M. D. Ward and J. A. Weinstein, Inorg. Chem., 2010, 49, 10041. 22 J. J. Loughrey, S. Sproules, E. J. L. McInnes, M. J. Hardie and M. A. Halcrow, Chem. Eur. J., 2014, 20, 6272. 23 J. A. Hyatt, E. N. Duesler, D.Y. Curtin and I. C. Paul, J. Org. Chem., 1980, 45, 5074; J. W. Steed, H. Zhang and J. L. Atwood, Supramol. Chem., 1996, 7, 37; A. Chakrabarti, H. M. Chawla, G. Hundal and N. Pant, Tetrahedron, 2005, 61, 12323; P. Satha, G. Illa, P. Giriteja and S. Chandra, Cryst. Growth Des., 2013, 13, 2636. 24 J. J. Loughrey, C. A. Kilner, M. J. Hardie and M.A. Halcrow, Supramol. Chem., 2012, 24, 2. 25 K. Nunome, K. Toriyama and M. Iwasaki, J. Chem. Phys., 1983, 79, 2499; R. H. Contreras and J. E. Peralta, Prog. Nucl. Magn. Reson., 2000, 37, 321. 26 C.-F. Ng, H.-F. Chow, D. Kuck and T. C. W. Mak, Cryst. Growth Des., 2017, 17, 2822.   Journal Name  COMMUNICATION This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5  Please do not adjust margins Please do not adjust margins TOC Entry   One-electron oxidation of the compound shown (L = a diphosphine chelate) shows no evidence for intervalence charge transfer in the mixed-valent macrocylic ligand radical product. This contrasts with closely related [{Pt(L)}3(μ3-ctc•)]+ (H6ctc = cyclotricatechylene), which exhibits class II mixed valency.  

The mixed valent radical obtained by oxidation of the platinum complex of a new tris-catecholate macrocycle has a class I, electronically localised structure

Halcrow, Malcolm A

Vincent, Kevin B

McInnes, Eric J L

Patmore, Nathan J

Research Data Leeds Repository (University of Leeds)

Microanalysis of 2a (code SG140)Microanalysis of 2b

Data to support study of Rigidification of a Macrocyclic Tris-Catecholate Scaffold Leads to Electronic Localisation of its Mixed Valent Redox Product

White Rose Research Online

This is a repository copy of Rigidification of a macrocyclic tris-catecholate scaffold leads toelectronic localisation of its mixed valent redox product.White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/141857/Version: Accepted VersionArticle:Greatorex, S, Vincent, KB, Baldansuren, A et al. (4 more authors) (2019) Rigidification of amacrocyclic tris-catecholate scaffold leads to electronic localisation of its mixed valent redox product. Chemical Communications, 55 (16). pp. 2281-2284. ISSN 1359-7345 https://doi.org/10.1039/C8CC10122A© The Royal Society of Chemistry 2019. This is an author produced version of a paper published in Chemical Communications. Uploaded in accordance with the publisher's self-archiving policy. Further re-use and further distribution is restricted. eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse Items deposited in White Rose Research Online are protected by copyright, with all rights reserved unless indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by national copyright laws. The publisher or other rights holders may allow further reproduction and re-use of the full text version. This is indicated by the licence information on the White Rose Research Online record for the item. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. Rigidification of a Macrocyclic Tris-Catecholate Scaffold Leads toElectronic Localisation of its Mixed Valent Redox ProductSam Greatorexa, Kevin B. Vincentb, Amgalanbaatar Baldansurenc, Eric J. L. McInnesc,Nathan J. Patmoreb, Stephen Sproulesd and Malcolm A. Halcrowa,*The catecholate groups in [{Pt(L)}3(ʅ3-tctq)] (H6tctq = 2,3,6,7,10,11-hexahydroxy-4b,8b,12b,12d-tetramethyltribenzotriquinacene; L =a diphosphine chelate) undergo sequential oxidation to theirゲWﾏｷケ┌ｷﾐﾗﾐ;デW aﾗヴﾏゲ H┞ ┗ﾗﾉデ;ﾏﾏWデヴ┞が ┘ｷデｴ らE½ = 160–170mV. Themonoradical [{Pt(dppb)}3(ʅ3-tctq)]+ is valence-localised, with noevidence for intervalence charge transfer in its near-IR spectrum.This contrasts with previously reported [{Pt(dppb)}3(ʅ3-ctc)]+(H6ctc = cyclotricatechylene), based on the same macrocyclic tris-dioxolene scaffold, which exhibits partly delocalised (class II) mixedvalency.Dioxolenes are one of the most versatile and important non-innocent ligands for transition ions. 1 The catecholate (cat)/semiquinonate (sq) redox process occurs at low potential inmetal-bound dioxolenes, which can lead to stable ligand radicalspecies and/or facile charge transfer processes between thedioxolene and coordinated metal ion. This makes catecholateligands useful electron reservoirs for catalysis 2,3 and biologicalredox reactions.3,4 Alternatively, metal-bound sq radicals canact as switchable molecular paramagnets, 5 or show bulkmagnetic ordering when incorporated into coordinationframeworks.6 Lastly, complexes containing multiple dioxolenecentres can exhibit ligand-based mixed-valency 7 with intenseinter-valence charge-transfer absorptions in the near-IR. Thishas been observed in both mononuclear [M(cat) 3] complexes,8and in complexes of more complicated organic scaffoldscontaining two or three linked dioxolene redox sites. 9-11In the latter vein, we recently re-investigated the complexes[{Pt(L)}3(ʅ3-ctc)] (H6ctc = cyclotricatechylene; L = 1,2-bis{diphenylphosphinobenzene [dppb] or 1,2-bis{diphenyl-phosphinoethane [dppe]; Scheme 1), which were originallysynthesised by Bohle and Stasko. 12 The three catecholategroups in these complexes are oxidised sequentially, leading to[{Pt(L)}3(ʅ3-ctc)]+ and [{Pt(L)}3(ʅ3-ctc)]2+ radical productsshowing class II mixed valency by UV/vis/NIR spectroscopy, 13despite the absence of direct conjugation between theirdioxolene redox centers.14 The radical products of this workwere too unstable to isolate however, and could only behandled in solution below room temperature. We reasoned thismight reflect a lack of steric protection about the methylenegroups of the oxidised [ctc]n函 macrocycle, since sq and otherphenoxyl radicals are prone to atom abstraction or couplingreactions at such para substituents.15Scheme 1. The compounds described in this work.Hence, we turned to the related tricatechol 2,3,6,7,10,11-hexahydroxy-4b,8b,12b,12d-tetramethyltribenzotriquinacene(H6tctq; Scheme 1), which is a new variant of the rigidtribenzotriquinacene motif developed by Kuck 16 as a bowl-shaped scaffold for supramolecular architectures 17,18 and softmaterials ¶.19 Since the methylene groups linking the catecholrings in [tctq]ヶ函 are fully quaternised, we reasoned that radicalsa.School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK.Email: m.a.halcrow@leeds.ac.ukb.Department of Chemical Sciences, University of Huddersfield, HuddersfieldHD1 3DH, UK.c. School of Chemistry and Photon Science Institute, University of Manchester,Oxford Road, Manchester M13 9PL, UK.d.WestCHEM, School of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK.† Electronic Supplementary Information (ESI) available: Experimental proceduresfor the synthesis and physical and computational characterisation of thecompounds in this work; crystallographic Figures and tables; EPR spectra andsimulated parameters; and computed structures, MO manifolds, orbital and spindensity plots. CCDC 1885699. For ESI and crystallographic data in CIF or otherelectronic format see DOI: 10.1039/######.ょ;デ;ゲ┌ヮヮﾗヴピﾐｪデｴｷゲゲデ┌S┞;ヴW;┗;ｷﾉ;HﾉW;デｴ─ヮぎっっSﾗｷくﾗヴｪっヱヰくヵヵヱΒっセセセく2 |derived by oxidation of [{Pt(L)}3(ʅ3-tctq)] might be more stableand easier to isolate. In the event that was not observed, butwe report here that sq radicals derived from [{Pt(L)} 3(ʅ3-ctc)](1a/1b, Scheme 1) and [{Pt(dppb)}3(ʅ3-tctq)] (2a) displayunexpectedly different electronic properties.2,3,6,7,10,11-Hexamethoxy-4b,8b,12b,12d-tetramethyltri-benzotriquinacene is accessible in nine synthetic steps by aliterature procedure (ESI†).20 H6tctq was obtained by exhaustivedemethylation of this precursor using BBr 3 ¶. Compounds 2aand 2b were prepared by treatment of H6tctq with 3 equiv ofthe appropriate [PtCl2(L)] reagent in a N,N-dimethylacetamide/methanol solvent mixture, using potassium tert-butoxide as abase. While 2a is soluble in weakly interacting solvents andstable under an inert atmosphere, 2b is much less soluble andapparently less stable, so more limited characterisation of thatcompound was achieved. Similar issues were also encounteredwith 1b.14Differential pulse voltammograms of 2a and 2b inCH2Cl2/0.1M NBu4PF6 resemble those of 1a and 1b, in showingthree closely spaced low-potential oxidations (Fig. 1). 14 Theseare assigned to sequential oxidation of the three cat rings in the[tctq]ヶ函 ligand to the sq oxidation level (eq 1).[tctq]ヶ函   [tctq]ヵ函   [tctq]ヴ函   [tctq]ン函 (1)The first two processes for both compounds are chemicallyreversible and occur at similar potentials, but with E½ ca. 50 mVmore positive for 2b than 2a (Table 1). Some differences areseen on the third oxidation wave, however. The[2a]2+/[2a]3+ oxidation is obviously split into two or threeFig. 1 Differential pulse voltammogram of 2a and 2b (CH2Cl2/0.1M NBu4PF6, 293 K).Table 1 Electrochemical data for the complexes in this work (cat = catecholate; sq =semiquinonate; q = quinone). Potentials are referenced against Fc+/0.sq/cat(E½ / V)q/sq(irra, Epa / V)2a –0.34 –0.17 +0.00, 0.08 +0.75, +1.162b –0.28 –0.12 +0.20 +0.82airr = irreversible.components implying some decomposition or deposition of the[2a]2+ redox product. In contrast the [2b]2+/[2b]3+oxidation occurs at ca. 0.2 V higher potential, and is apparentlyclean. The separation between the first two oxidation potentialsin 2a and 2bが らE½ = 0.16–0.17 V, yields the comproportionationconstants Kc = 6-8 × 102 between each oxidation level.10,13 Thatis slightly smaller than for 1a and 1b ふらE½ = 0.18–0.22 V, Kc = 1–5 × 103),14which implies electronic communication between thedioxolene rings in coordinated [tctq] n函 is weaker than for[ctc]n函. A second series of irreversible processes at +0.75–1.2 Vwas observed for 2a, assignable to further oxidation of thedioxolene rings in [tctq]ン函 to the quinone level. These wereless well-defined in 2b, which might reflect its precipitation atthe electrode during the measurement.EPR spectra of [2a]+ and [2b]+, generated by in situoxidation of the neutral precursors with 1 equiv [Cp 2Fe]PF6,were measured in frozen CH2Cl2 solution at X- and S-bandfrequencies. The spectra resemble [1a]+ and other Pt(II)/sqradicals in showing weakly rhombic g-patterns, with hyperfinecoupling to just one 195Pt nucleus (ESI†).14,21,22 That shows theunpaired electrons in [2a]+ and [2b]+ are localised on onedioxolene ring under these conditions, on the EPR timescale.Fluid solution EPR spectra in the same solvent were achievedfor [2a]+, yielding simulated isotropic A{195Pt} and a{31P} valuesthat are 30-50 % larger than for [1a]+. That is consistent with agreater degree of localisation of the unpaired spin in [2a]+.The UV/vis spectra of 2a and 2b in CH2Cl2/0.1 M NBu4PF6show just one resolved maximum at 31.7 × 10 3 cm–1.Spectroelectrochemical generation of [2a]+ from 2a in thissolvent at 253 K proceeds isosbestically, and leads to anincrease in intensity for this peak, coupled to the ingrowth of anew shoulder near 23 × 103 cm–1 (Fig. 2). These changes stronglyresemble those observed during the oxidation of mononuclearPt(II)/cat/diphosphine complexes. 14 Importantly, theUV/vis/NIR spectrum of [2a]+ shows no intervalence chargetransfer (IVCT) absorption above 4000 cm ௅1. That implies theelectronic structure of [2a]+ is class I valence-localised,13 withno delocalisation or migration of the unpaired electron aroundthe [tctq]ヵ函 macrocycle. This contrasts with [1a]+, which showsan IVCT peak at ʆmax = 7.9 × 103 cm–1 (ɸmax = 500 M–1 cm–1)indicating class II mixed-valent character. 14 Similar results wereobtained from an oxidative titration of the 2a/[2a]+ couple with[Cp2Fe]PF6 at room temperature, although the transformationwas not isosbestic under those conditions (ESI†).No crystal structures of 2a or 2b were obtained during thisstudy. However, comparison of metal-free H 6tctq·thf·nMeOH (nЯ ヰくヱヵき ESIゆぶ ┘ｷデｴ ヮ┌HﾉｷゲｴWS ゲデヴ┌Iデ┌ヴWゲ ﾗa H6ctc shows small butconsistent differences in the disposition of their dioxolene rings,which should also be reflected in their complexes. 12,14 TheH6tctq cavitand has a slightly shallower bowl-shapedconformation than H6ctc (Fig. 3), which is reflected in theaverage intramolecular dihedral angle between the catecholrings in H6tctq [62.9(1)°] and H6ctc [typically 68–72°].23,24 Thatmight place the ʋ-systems of the catechol rings in H6tctq furtherapart. Conversely, the closest intramolecular contact betweeneach catechol ring, namely the distance between their ipso Catoms at the base of the bowl-shaped cavity, is 2.513(4)–| 3Fig. 2 The 2aĺ[2a]+ oxidation at 253 K in CH2Cl2/0.1 M NBu4PF6, monitored byUV/vis/NIR spectroscopy using an optically transparent electrode. The spectra of thepure starting material and product are highlighted in black while the intermediatespectra are paler. The feature near 12 × 103 cm௅1 is an artefact from a grating change inthe spectrometer.Fig. 3 Overlay of the crystallographic molecular structures of H6tctq (white, ESI†) andH6ctc (purple, fromH6ctc·5dmso24), showing the shallower bowl-shaped conformation ofthe H6tctq macrocycle.2.533(4) Å in H6tctq which is similar to H6ctc (typically 2.53–2.61Å). Hence, there is no simple structural relationship betweenthe dioxolene ligands in [1a]+ and [2a]+ that accounts for theirdifferent electronic character.Geometry optimised structures and electronic properties of[2a]0/1+/2+/3+ were calculated at the same level of theory used inour previous study of [1a]0/1+/2+/3+.14 That is, by spin-unrestrictedbroken symmetry DFT calculations at the B3LYP-ZORA level,with the dppb Ph groups replaced with H atoms. The optimizedgeometries of the [Pt(dppb)(dioxolene)] fragments in the twomolecules show only small differences at each oxidation level(ESI†). The greatest differences lie in the Pt௅P bonds which areconsistently 0.012-0.019 Å longer in [2a]z+ than in [1a]z+ at eachoxidation level z; and, the Pt௅O bonds which are 0.014-0.019 Åshorter in [2a]+ than in [1a]+ (the Pt௅O bond lengths in the twomolecules are more similar for other z). This may reflect astronger OĺPt -donation interaction in [2a]z+, arising from thestronger inductive effect of its quaternary alkyl dioxolenesubstituents.The average calculated Pt···Pt distances in [2a]z+ and [1a]z+differ by nomore than 0.012 Å at each oxidation level, and showthe same trend of gradually increasing with z. Hence, in contrastto the free ligand crystallography (Fig. 3), the greater rigidity ofthe tctq macrocycle has little impact on the structures of itsradical oxidation products at this level of theory. However, thectc and tctq conformations will also be influenced by theinclusion of solvent molecules within their cavities, which istypically observed experimentally (ESI†) but is not accountedfor in the geometry optimisation calculations. 14The calculated Mulliken spin density population (Figure 4)and UV/vis/NIR spectra of [2a]+ are essentially identical to[1a]+,14 while the spin populations for the other members ofthe redox series are also identical irrespective of the ligandframework (ESI†). Since the DFT calculations are independentof temperature, that therefore suggests the different mixed-valent character of [1a]+ and [2a]+ arises from a temperature-dependent phenomenon. As the electronic structures areidentical for both species, we attribute the spin-localisation in[2a]+ to the rigidity of the tctq ligand. The flexible secondarymethylene groups in ctc can approach a ʌ radial–ʍ-bondcoplanar alignment with their bonded dioxolene groups, whichwould activate a through-bond interaction between thedioxolene ʌ-systems via hyperconjugation.25 This orientation isless accessible with the more rigid quaternary linkages in tctq,leading to localisation of the dioxolene spins as observed in[2a]+.Fig. 4 Mulliken spin population analysis for [2a]+ (red: ɲ-spin; yellow: ɴ-spin).In conclusion, we have demonstrated that the mixed-valentcharacter of sq radicals derived from complexedcyclotricatechylene macrocycles is sensitive to theirconformational flexibility. Our current work aims to make useof this feature in host:guest systems based on this redox-activemolecular framework.This work was funded by the EPSRC (EP/M506552/1) and aLeverhulme Trust Research Project Grant (RPG-2014-303, toNJP and KBV). Data supporting this study are available in theESIゆ;ﾐS;デｴ─ヮぎっっSﾗｷくﾗヴｪっヱヰくヵヵヱΒっセセセく4 |Conflicts of interestThere are no conflicts to declare.Notes and references¶ A 2,3,6,7,10,11-hexahydroxytribenzotriquinacene tris-catecholrelated to H6tctq, but lacking the methyl substituents on themacrocycle methylene groups, was recently reported.261 C. G. Pierpoint and C. W. Lange, Prog. Inorg. Chem., 1993, 41,331; C. G. Pierpont and J. K. Kelly in The Chemistry of MetalPhenolates Part [1, 2], ed. J. Zabicky, Wiley, Chichester, UK,2014, pp. 669–698.2 D. L. J. Broere, R. Plessius and J. I. van der Vlugt, Chem. Soc.Rev., 2015, 44, 6886 and 2015, 44, 7010 and 7011(corrections).3 W. Kaim and B. Schwederski, Coord. Chem. Rev., 2010, 254,1580; E. M. Shepard and D. M. Dooley, Acc. Chem. Res.,2015, 48, 1218.4 J. Sedó , J. Saiz-Poseu , F. Busqué and D. Ruiz-Molina, Adv.Mater., 2013, 25, 653; J. Yang, M. A. Cohen Stuart andM. Kamperman, Chem. Soc. Rev., 2014, 43, 8271.5 J. S. Miller and K. S. Min, Angew. 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 The catecholate groups in [{Pt(L)} 3 (μ 3 -tctq)] (H 6 tctq = 2,3,6,7,10,11-hexahydroxy-4b,8b,12b,12d-tetramethyltribenzotriquinacene; L = a diphosphine chelate) undergo sequential oxidation to their semiquinonate forms by voltammetry, with ΔE 1/2 = 160-170 mV. The monoradical [{Pt(dppb)} 3 (μ 3 -tctq)] + is valence-localised, with no evidence for intervalence charge transfer in its near-IR spectrum. This contrasts with previously reported [{Pt(dppb)} 3 (μ 3 -ctc)] + (H 6 ctc = cyclotricatechylene), based on the same macrocyclic tris-dioxolene scaffold, which exhibits partly delocalised (class II) mixed valency. </p

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Rigidification of a macrocyclic tris-catecholate scaffold leads to electronic localisation of its mixed valent redox product

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