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

Article abstract of DOI:10.1039/c8cc10122a

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 ΔE_1/2 = 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.

Full text of DOI:10.1039/c8cc10122a

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Vincent, A. Baldansuren, E. J. L. McInnes, N. J. Patmore, S. Sproules and M. Halcrow, Chem. Commun.,
2019, DOI: 10.1039/C8CC10122A.
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Rigidification of a Macrocyclic Tris-Catecholate Scaffold Leads to
Electronic Localisation of its Mixed Valent Redox Product
Sam Greatorexa, Kevin B. Vincentb, Amgalanbaatar Baldansurenc, Eric J. L. McInnesc,
Received 00th January 20xx,
Accepted 00th January 20xx
Nathan J. Patmoreb, Stephen Sproulesd and Malcolm A. Halcrowa,*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The catecholate groups in [{Pt(L)}3(μ3-tctq)] (H6tctq = 2,3,6,7,10,11- and in complexes of more complicated organic scaffolds
hexahydroxy-4b,8b,12b,12d-tetramethyltribenzotriquinacene; L = containing two or three linked dioxolene redox sites. 9-11
a diphosphine chelate) undergo sequential oxidation to their
In the latter vein, we recently re-investigated the complexes
cyclotricatechylene; 1,2-
semiquinonate forms by voltammetry, with ΔE = 160–170 mV. The [{Pt(L)}3(μ3-ctc)] (H6ctc
=
L =
½
monoradical [{Pt(dppb)}3(μ3-tctq•)]+ is valence-localised, with no bis{diphenylphosphinobenzene [dppb] or 1,2-bis{diphenyl-
evidence for intervalence charge transfer in its near-IR spectrum. phosphinoethane [dppe]; Scheme 1), which were originally
This contrasts with previously reported [{Pt(dppb)}3(μ3-ctc•)]+ synthesised by Bohle and Stasko.12 The three catecholate
(H6ctc = cyclotricatechylene), based on the same macrocyclic tris- groups in these complexes are oxidised sequentially, leading to
dioxolene scaffold, which exhibits partly delocalised (class II) mixed [{Pt(L)}3(μ3-ctc•)]+ and [{Pt(L)}3(μ3-ctc••)]2+ radical products
valency.
showing class II mixed valency by UV/vis/NIR spectroscopy, 13
despite the absence of direct 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
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
a. School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK.
Email: m.a.halcrow@leeds.ac.uk
Scheme 1. The compounds described in this work.
b. Department of Chemical 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.
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
d. WestCHEM, School of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK.
† 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/######.
ΐ ꢀꢁĂƚĂꢀƐƵƉƉŽƌƟŶŐꢀƚŚŝƐꢀƐƚƵĚLJꢀĂƌĞꢀĂǀ ĂŝůĂďůĞꢀĂƚꢀŚƩ Ɖ͗ͬͬĚŽŝ͘ŽƌŐͬϭϬ͘ϱϱϭϴͬηηη͘
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derived by oxidation of [{Pt(L)}3(μ3-tctq)] might be more stable components implying some decomposition or deposition of the
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•• 2+ ••• 3+
and easier to isolate. In the event that was not observed, but [2a••]2+ redox product. In contrast the [2b ] /[2b
]
DOI: 10.1039/C8CC10122A
we report here that sq radicals derived from [{Pt(L)} 3(μ3-ctc)] oxidation occurs at ca. 0.2 V higher potential, and is apparently
(1a/1b, Scheme 1) and [{Pt(dppb)}3(μ3-tctq)] (2a) display clean. The separation between the first two oxidation potentials
unexpectedly different electronic properties.
in 2a and 2b, ΔE = 0.16–0.17 V, yields the comproportionation
½
2,3,6,7,10,11-Hexamethoxy-4b,8b,12b,12d-tetramethyltri-
constants Kc = 6-8 × 102 between each oxidation level.10,13 That
benzotriquinacene is accessible in nine synthetic steps by a is slightly smaller than for 1a and 1b (ΔE = 0.18–0.22 V, Kc = 1–
½
literature procedure (ESI†).20 H6tctq was obtained by exhaustive 5 × 103),14 which implies electronic communication between the
demethylation of this precursor using BBr3 ¶. Compounds 2a dioxolene rings in coordinated [tctq]n‒ is weaker than for
and 2b were prepared by treatment of H6tctq with 3 equiv of [ctc]n‒. A second series of irreversible processes at +0.75–1.2 V
the appropriate [PtCl2(L)] reagent in a N,N-dimethylacetamide/ was observed for 2a, assignable to further oxidation of the
methanol solvent mixture, using potassium tert-butoxide as a dioxolene rings in [tctq••• 3‒
to the quinone level. These were
]
base. While 2a is soluble in weakly interacting solvents and less well-defined in 2b, which might reflect its precipitation at
stable under an inert atmosphere, 2b is much less soluble and the electrode during the measurement.
apparently less stable, so more limited characterisation of that
compound was achieved. Similar issues were also encountered oxidation of the neutral precursors with 1 equiv [Cp 2Fe]PF6,
with 1b.14
were measured in frozen CH2Cl2 solution at X- and S-band
EPR spectra of [2a•]+ and [2b•]+, generated by in situ
Differential pulse voltammograms of 2a and 2b in frequencies. The spectra resemble [1a•]+ and other Pt(II)/sq
CH2Cl2/0.1M NBu4PF6 resemble those of 1a and 1b, in showing radicals in showing weakly rhombic g-patterns, with hyperfine
three closely spaced low-potential oxidations (Fig. 1). 14 These coupling to just one 195Pt nucleus (ESI†).14,21,22 That shows the
are assigned to sequential oxidation of the three cat rings in the unpaired electrons in [2a•]+ and [2b•]+ are localised on one
[tctq]6‒ ligand to the sq oxidation level (eq 1).
[tctq]6‒ [tctq•]5‒ [tctq••]4‒
dioxolene ring under these conditions, on the EPR timescale.
Fluid solution EPR spectra in the same solvent were achieved
[tctq••• 3‒
]
(1)
The first two processes for both compounds are chemically for [2a•]+, yielding simulated isotropic A{195Pt} and a{31P} values
reversible and occur at similar potentials, but with E ca. 50 mV that are 30-50 % larger than for [1a•]+. That is consistent with a
more positive for 2b than 2a (Table 1). Some differences are greater degree of localisation of the unpaired spin in [2a•]+.
½
seen on the third oxidation wave, however. The
[2a••]2+/[2a••• 3+ oxidation is obviously split into two or three
The UV/vis spectra of 2a and 2b in CH2Cl2/0.1 M NBu4PF6
show just one resolved maximum at 31.7 × 10 3 cm–1.
Spectroelectrochemical generation of [2a•]+ from 2a in this
solvent at 253 K proceeds isosbestically, and leads to an
increase in intensity for this peak, coupled to the ingrowth of a
new shoulder near 23 × 103 cm–1 (Fig. 2). 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 intervalence charge
transfer (IVCT) absorption above 4000 cm‒1. That implies the
electronic structure of [2a•]+ is class I valence-localised,13 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 with
[Cp2Fe]PF6 at room 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·nMeOH (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
Fig. 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 =
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)–
semiquinonate; q = quinone). Potentials are referenced against Fc+/0
.
sq/cat
q/sq
(E / V)
(irra, Epa / V)
+0.75, +1.16
+0.82
½
2a
2b
–0.34
–0.28
–0.17
–0.12
+0.00, 0.08
+0.20
airr = irreversible.
2 | J. Name., 2012, 00, 1-3
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the same trend of gradually increasing with z. Hence, in contrast
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to the free ligand crystallography (Fig. 3),DtOheI: 1g0r.e10a3t9e/rCr8iCgiCd1i0ty12o2Af
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 geometry optimisation calculations.14
The 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 of
the redox series are also identical irrespective of the ligand
framework (ESI†). Since the DFT calculations are independent
of temperature, that therefore suggests the different mixed-
valent character of [1a•]+ and [2a•]+ arises from a temperature-
dependent phenomenon. As the electronic structures are
identical for both species, we attribute the spin-localisation in
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 [2a•]+ to the rigidity of the tctq ligand. The flexible secondary
spectra are paler. The feature near 12 × 103 cm‒1 is an artefact from a grating change in
methylene groups in ctc can approach a
π radial–σ-bond
the spectrometer.
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 localisation of the dioxolene spins as observed in
[2a•]+.
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.
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•]+ and [2a•]+ that accounts for their
different electronic character.
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
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). Data supporting this study are available in the
ESIΏꢀĂŶĚꢀĂƚꢀŚƩ Ɖ͗ͬͬĚŽŝ͘ŽƌŐͬϭϬ͘ϱϱϭϴͬηηη͘
(ESI†). The greatest differences lie in the Pt
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]z+, arising from the
‒P bonds which are
‒
‒
→
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
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19 J. Vile, M. Carta, C. G. Bezzu and N. B. McKeown, Polym.
Conflicts of interest
There are no conflicts to declare.
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