Redox, transmetalation, and stacking properties of tetrathiafulvalene-2,3,6,7-tetrathiolate bridged tin, nickel, and palladium compounds

Abstract

Here we report that capping the molecule TTFtt (TTFtt = tetrathiafulvalene-2,3,6,7-tetrathiolate) with dialkyl tin groups enables the isolation of a stable series of redox congeners and facile transmetalation to Ni and Pd. TTFtt has been proposed as an attractive building block for molecular materials for two decades as it combines the redox chemistry of TTF and dithiolene units. TTFttH₄, however, is inherently unstable and the incorporation of TTFtt units into complexes or materials typically proceeds through the in situ generation of the tetraanion TTFtt⁴⁻. Capping of TTFtt⁴⁻ with Bu₂Sn²⁺ units dramatically improves the stability of the TTFtt moiety and furthermore enables the isolation of a redox series where the TTF core carries the formal charges of 0, +1, and +2. All of these redox congeners show efficient and clean transmetalation to Ni and Pd resulting in an analogous series of bimetallic complexes capped by 1,2-bis(diphenylphosphino)ethane (dppe) ligands. Furthermore, by using the same transmetalation method, we synthesized analogous palladium complexes capped by 1,1′-bis(diphenylphosphino)ferrocene (dppf) which had been previously reported. All of these species have been thoroughly characterized through a systematic survey of chemical and electronic properties by techniques including cyclic voltammetry (CV), ultraviolet-visible-near infrared spectroscopy (UV-vis-NIR), electron paramagnetic resonance spectroscopy (EPR), nuclear magnetic resonance spectroscopy (NMR) and X-ray diffraction (XRD). These detailed synthetic and spectroscopic studies highlight important differences between the transmetalation strategy presented here and previously reported synthetic methods for the installation of TTFtt. In addition, the utility of this stabilization strategy can be illustrated by the observation of unusual TTF radical–radical packing in the solid state and dimerization in the solution state. Theoretical calculations based on variational 2-electron reduced density matrix methods have been used to investigate these unusual interactions and illustrate fundamentally different levels of covalency and overlap depending on the orientations of the TTF cores. Taken together, this work demonstrates that tin-capped TTFtt units are ideal reagents for the installation of redox-tunable TTFtt ligands enabling the generation of entirely new geometric and electronic structures.

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Introduction

Conjugated coordination polymers have attracted recent attention due to promising applications in superconductors,¹ energy storage,² thermoelectrics,³ spintronics,⁴ and other fields.^5–7 However, delocalized metal–organic systems are still rare and most coordination polymers are limited to architectures constructed with nitrogen and oxygen based ligands.^7,8 Some of the most conductive materials⁹ in this area have instead used sulfur based linkers which are perhaps best exemplified by dithiolene units that leverage both a better energy match¹⁰ between sulfur atoms and metal centers and ligand-based redox activity.^4b Nevertheless, stability and controllable synthetic conditions are still significant challenges associated with the incorporation of dithiolene based linkers. Molecular dithiolene complexes have great utility in addressing these challenges as they allow for a detailed understanding of the properties and reactivity of dithiolene units. Furthermore, molecular dithiolene complexes can be used as transmetalating agents to generate materials in a controlled manner.^5f,10c

Of possible dithiolene ligands, TTFtt (TTFtt = tetrathiafulvalene-2,3,6,7-tetrathiolate, Fig. 1) is attractive as it combines the above mentioned properties of dithiolenes with the favorable electronic properties of tetrathiafulvalene (TTF).¹¹ Organic radical salts of TTF and its derivatives are well-known for outstanding electronic properties, such as being components in organic conductors such as TTF-TCNQ and organic superconductors such as [TMTSF]₂[PF₆] (TMTSF: tetramethyl-tetraselenafulvalene).¹² While TTF has been incorporated into coordination polymers to improve conductivity¹³ and enable switching of porosity¹⁴ or magnetism,¹⁵ TTFtt has much less precedent in well-defined complexes or materials. Some conductive TTFtt-transition metal chains were reported by IBM Research Laboratory in 1979 with limited characterization data.^16a In 1995, McCullough and coworkers crystalized the first homobimetallic TTFtt complex and the TTFtt unit was proposed as a promising building block for new magnetic, electronic and optical materials.^17a,b However, there has been little progress towards this end over the following decades. Only one report of installing TTFtt between fullerene supported Co centers using a decarbonylative process at high temperature has been structurally characterized and limited characterization has been reported on molybdocene fragments bridged by TTFtt.¹⁸

Fig. 1 The structure of TTFtt with the TTF core indicated.

The primary challenge with the incorporation of TTFtt into molecules or materials is the sensitivity of this moiety and its synthons. Unprotected TTFttH₄ has not been isolated and characterized, although TTFttLi₄ can be generated transiently as a highly reactive and sensitive solid for metalations as reported in McCullough's work.¹⁷ The conventional synthetic technique for the incorporation of TTFtt involves the in situ deprotection of derivatives such as 2,3,6,7-tetrakis(2′-cyanoethylthio)tetrathiafulvalene, TTFtt(C₂H₄CN)₄.¹⁹ This deprotection typically requires the use of an excess of strong base which limits the choice of solvent and also leads to undesirable side reactions due to the highly basic, nucleophilic, and reducing properties of the TTFtt⁴⁻ tetraanion. Furthermore, the required excess base may also introduce side-reactions. These issues have directly limited the investigation and incorporation of TTFtt.

To alleviate these issues and enable the facile and controlled installation of TTFtt between transition metals, we were inspired by previous work by Donahue and coworkers that demonstrated that capping of dithiolene units such as 1,2,4,5-benzenetetrathiolate with dialkyltin groups enables smooth transmetalation to transition metals.^20,21 In the current work, we successfully employ a similar strategy to stabilize the TTFtt unit and isolate the stable and soluble bis-dibutylstannylated complex TTFtt(SnBu₂)₂, 1. Furthermore, and distinctly from other dithiolene tin agents, complex 1 is redox-active and can be sequentially oxidized to generate stable cation radical and dicationic TTF cores in the isolable complexes [TTFtt(SnBu₂)₂][BAr^F₄], 2 and [TTFtt(SnBu₂)₂][BAr^F₄]₂, 3 ([BAr^F₄] = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate). Compounds 1–3 are efficient transmetalating agents to generate (dppeNi)₂TTFtt, [(dppeNi)₂TTFtt][BAr^F₄] and [(dppeNi)₂TTFtt][BAr^F₄]₂ (dppe = 1,2-bis(diphenylphosphino)ethane, 4–6). To demonstrate the generality of transmetalation with tin precursors to other transition metals, we also prepared the Pd complexes, (dppePd)₂TTFtt, [(dppePd)₂TTFtt][BAr^F₄], (dppfPd)₂TTFtt and [(dppfPd)₂TTFtt][BAr^F₄] (dppf = 1,1′-bis(diphenylphosphino)ferrocene, 7–10). Crystallographic analyses of these materials reveal a variety of stacking arrangements of the TTF cores. Application of cutting-edge variational 2-electron reduced density matrix theory elucidates how the twisting of the TTF–TTF cores affects their electronic structure, a feature which is critical to understand their long-range transport properties. In sum, this work demonstrates that capping TTFtt with dialkyl tin units enables the stabilization and isolation of an unusual redox series of the TTFtt ligand and facile installation of all of these redox congeners onto transition metals. This work now allows for the formation of new materials featuring TTFtt with precise control over synthetic conditions and redox state.

Results and discussion

Synthesis of Sn and Ni capped TTFtt redox congeners

All of the reactions involved with Sn and Ni complexes are summarized in Scheme 1. Compound 1 was synthesized via deprotection of TTFtt(C₂H₄CN)₄ with excess sodium methoxide and subsequent reaction with excess Bu₂SnCl₂ in MeOH. In contrast to the high reactivity of the TTFtt⁴⁻ tetraanion, 1 was indefinitely stable as a solid at room temperature and red crystals can be obtained via recrystallization from boiling acetonitrile at 80 °C. The stability of 1 under these conditions suggests that the use of common solvothermal synthetic conditions for coordination polymers should be viable. The cyclic voltammogram (CV) of neutral 1 shows two quasi-reversible features, suggesting that two oxidized species are chemically accessible (Fig. 2A). The reagents [Fc][BAr^F₄] and [Fc^BzO][BAr^F₄] (Fc⁺ = ferrocenium, Fc^BzO = benzoyl ferrocenium) were therefore used to chemically access the singly and doubly oxidized redox congeners 2 and 3. While brown crystals of 2 were obtained which verified the proposed structure of this compound, the oxidation reaction of 1 with 2 equivalents of [Fc^BzO][BAr^F₄] under the same conditions led to the formation of 3·2Fc^BzO where each Sn center is coordinated by an additional Fc^BzO molecule (Fig. S53†). To avoid the formation of these adducts, the reaction and crystallization were both conducted in THF solvent which enabled the isolation of green crystals of 3·2THF.

Scheme 1 The synthesis of Sn and Ni complexes with TTFtt as a bridging ligand.

Fig. 2 Cyclic voltammograms of 1 (A) and 5 (B). Arrow denotes scan direction. Conditions: DCM, 0.1 M [TBA][PF₆], 0.1 V s⁻¹.

The ability of these stannylated species for ligand transmetalation was tested by reactions with 2 equivalents of dppeNiCl₂ in DCM or Et₂O at room temperature. All metalation processes proceed smoothly and provide the three corresponding dinickel complexes, 4–6, in good yield. Complex 4 with a formally neutral TTF core was obtained as an insoluble orange-yellow powder. Compounds 5 and 6, however, were much more soluble enabling crystallization as dark brown and purple crystals respectively. The Ni complexes are also redox-active as indicated by their CV's which show two quasi-reversible oxidations shifted ∼0.4 V more negative than those observed in 1 (Fig. 2B). Compounds 5 and 6 could also be generated by oxidizing 4 with [Fc][BAr^F₄] and [Fc^AcO][BAr^F₄] (Fc^AcO = acetyl ferrocenium) respectively as verified by NMR spectroscopy.

These compounds demonstrate that the stannylation of the reactive and unstable TTFtt⁴⁻ anion is an effective strategy to both stabilize unusual redox series as well as to enable facile transmetalation to transition metals. These tin agents are much more stable than conventional in situ formed TTFtt⁴⁻ anions, allowing for purification, long-term storage, and convenient utility under a wide range of conditions with various solvents. In addition to these advantages, complexes 2 and 3 provide additional synthetic flexibility via controlled redox “doping.” For instance, complex 2, with a TTF radical cation core, enables direct insertion of radical linkers between metal centers. Furthermore, 3 is one of only a few examples of isolable dicationic TTF motifs.^22,23 The facile redox and transmetalation chemistry of 1–3 paves the way for the synthesis of new materials with precisely tuned redox states.

Solid state structures

Compounds 1–3, 5, and 6 have been crystallographically characterized and their single crystal X-ray diffraction (SXRD) structures are shown in Fig. 3. Their packing patterns are shown in Fig. S48–S52.† Compounds 1, 3, and 5 crystallize in the triclinic space group P1̄, compounds 2 and 6 crystallize in the monoclinic space groups P2₁/c and C2/c, respectively. The geometrical parameters of the TTF cores such as bond lengths and dihedral angles are typically sensitive to the redox state of the TTF unit.^22f Interestingly, in the present Sn capped redox series some of these changes are muted. For instance, planarization of the TTF core is typically observed only upon oxidation, but in 1 the neutral TTF rings are nearly coplanar (Fig. S46†). The trends in the C–C and C–S bond lengths are more informative and are shown in Table 1. As the molecular charge increases, the C–C bond distances in the TTF cores also increase, while the C–S bond lengths generally decrease. These trends are consistent for both the Sn series in 1–3 and the Ni series from 5 to 6. These changes are consistent with previous studies showing similar geometric trends upon oxidation of TTF molecules.^22f Conversely, there is little change or trend in the M–S distances for either the Sn or the Ni complexes, supporting the assignment of primarily TTF-centered redox events.

Fig. 3 Single crystal X-ray diffraction (SXRD) structures of (A) 1·2MeCN, (B) 2·0.5THF·0.5MeCN, (C) 3·4THF, (D) 5, and (E) 6·[BAr^F₄] anions, solvent, H atoms, and disorder are omitted and n-butyl and phenyl groups are shown in wireframe for clarity. The labeling scheme shown for 1 applies for all compounds. Ellipsoids are shown at 50% probability. Selected bond lengths are included in Table 1. Sn is shown in light grey; Ni, green; S, yellow; P, purple; O, red; N, blue; C, dark grey.

Table 1 SXRD metrical parameters for 1–3, 5, and 6

	C3–C4 (Å)	C1,2–C5,6 (Å)	C–S^a (Å)	M–S (Å)	M–S′ (Å)
a C–S bonds includes all C–S bonds in the TTFtt linker. b The two five-membered rings of 2's TTF core are not symmetric.
1	1.333(5)	1.338(4)	1.746(3)–1.760(3)	2.4579(7)	2.5050(7)
2	1.351(16)	1.37(2)	1.72(1)–1.76(1)	2.455(4)	2.446(3)–2.563(3)
3	1.436(18)	1.402(12)	1.681(9)–1.732(8)	2.535(2)	2.502(3)
5	1.385(2)	1.361(2)	1.726(2)–1.740(1)	2.1616(5)	2.1750(8)
6	1.412(5)	1.379(3)	1.704(2)–1.726(2)	2.1684(7)	2.1790(7)

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Most of these compounds also display intermolecular TTF–TTF packing interactions in their SXRD structures, as has been observed extensively in other TTF based systems.¹² Compounds 1, 5 and 6 show extended one dimensional chains via weak side-to-side sulfur–sulfur interactions, although another unusual additional polymorph of 5 was found which will be discussed more thoroughly below. Compound 2 forms dimers in the solid-state via π-stacking. Finally, dicationic 3 shows no significant intermolecular interaction as the TTF core is effectively shielded by the large [BAr^F₄] anions.

Electronic properties of Sn and Ni complexes

The synthetic ease of accessing these series of redox congeners motivates examination of their electronic structure. As discussed above, CV shows two oxidation peaks for 1 at −0.14 V and 0.28 V vs. Fc⁺/Fc. In 5 these features shift to −0.58 V and −0.11 V respectively. The Ni species display an additional irreversible peak at 0.79 V vs. Fc⁺/Fc which is tentatively assigned as a Ni^II to Ni^III oxidation. Redox events at similar potentials were seen for the preliminary study of the complex (dpppNi)₂TTFtt (dppp = 1,3-bis(diphenylphosphino)propane) although limited characterization of this complex is reported.^17c,d It is worth noting that appreciable film deposition at the working electrode surface was observed on repeated scans in our CV studies. We anticipate that this arises from reaction of the oxidized congeners with the [PF₆]⁻ electrolyte anions as has been previously proposed.^21b The CV of 5 with [Na][BAr^F₄] as the electrolyte medium was performed and no obvious degradation was observed over multiple scans. This enhanced stability from fluorinated aryl borates is also reflected in the synthetic chemistry mentioned above. The lack of oxidative features between 0 and 0.6 V suggests that the dicationic species 6 is potentially air-stable. To test this possibility, a CDCl₃ solution of 6 was exposed to air for 12 hours and then analyzed by NMR spectroscopy. Comparison of the ¹H and ³¹P NMR spectra before and after this exposure indicate nearly no decomposition with the exception of a very small amount of oxidized phosphine (<2%, Fig. S23 and S24†). While crude, this initial test indicates that materials composed of typically air-sensitive TTFtt synthons may be made air-stable by tuning the charge state of the TTF core.

In order to more firmly assign the redox features observed by CV, UV-vis-NIR investigations were carried out on the Sn compounds 1–3 and on the soluble Ni complexes 5 and 6 (Fig. 4). Compound 1 has an intense feature at 328 nm, assigned as arising mainly from π–π* transitions.²⁴ Upon oxidation to 2 a broad feature emerges at 1053 nm. Appearance of this new low-energy absorption band has been previously interpreted as arising from the formation of π-dimers.²⁵ This absorption band blue-shifts to 941 nm upon further oxidation. Similar spectral features are observed in the Ni complexes 5 and 6 (Fig. 4b). Compared to 2 and 3, the NIR absorption features of 5 and 6 both show a distinct red-shift (Fig. S42 and S43†).

Fig. 4 UV-vis-NIR absorption spectra of 1–3 (A), 5 and 6 (B) in DCM. Concentration: 1, 92 μM; 2, 3, 5 and 6, 50 μM.

In addition to UV-vis-NIR spectra, the signals of the TTF radicals were investigated by EPR spectroscopy. The EPR spectrum of 2 in THF (Fig. S25†) shows an isotropic feature at g = 2.008, consistent with an organic radical. Conversely, anisotropic signals at g = 2.013, 2.007, and 2.003, were observed in the EPR spectrum of 5 (Fig. S26†). Similarly, anisotropic signals have been observed in other TTF radical systems.²⁶ The spectroscopic and structural data for these compounds is very similar to that observed for other TTF systems again suggesting that the redox events of TTFtt are largely localized on the TTF core.

Transmetalation to Pd

To further demonstrate the versatility and generality of transmetalation with these tin precursors to other transition metals, we prepared the Pd complexes, (dppePd)₂TTFtt, [(dppePd)₂TTFtt][BAr^F₄], (dppfPd)₂TTFtt and [(dppfPd)₂TTFtt][BAr^F₄] (7–10; dppf = 1,1′-bis(diphenylphosphino)ferrocene) (Scheme 2). Analogously to the Ni examples above, mixing dppePdCl₂ or dppfPdCl₂ with complexes 1 and 2 results in the new bridged Pd congeners. Complexes 7 and 9 which contain neutral TTF cores were isolated as insoluble pink/orange powders, while complexes 8 and 10 were crystalized as dark brown needles and were characterized by SXRD analysis (Fig. S54 and S55†). The radical species 8 and 10 possess similar geometric parameters, UV-vis-NIR features (Fig. S44†), and EPR signals (Fig. S27 and S28†) to compound 5.

Scheme 2 The synthesis of Pd complexes with TTFtt as a bridging ligand.

Both sets of Pd complexes are also redox-active. As the CV of 10 shows (Fig. S41†), two quasi-reversible peaks assigned to oxidation of the TTF core are observed below 0 V (−0.53 and −0.08 V vs. Fc⁺/Fc). Three additional quasi-reversible features are also observed at 0.72, 0.94 and 1.26 V and are attributed to a two-electron oxidation of both ferrocene units from dppf and two separate one-electron [Pd-dithiolene]^0/1+ processes by comparison to the CV spectrum of (dppfPd)dmit (dmit = 1,3-dithiole-2-thione-4,5-dithiolate).²⁷ Complex 8 shows similar but more negative redox features without the additional ferrocene oxidations (Fig. S40†).

The synthesis of (dppfPd)₂TTFtt has previously been reported by using in situ formed TTFtt anions.²⁸ The material from this previous report did not show any redox peaks in its CV with a glassy-carbon electrode. When using a Pt-button working electrode, only three features were found at −0.05, 0.42 and 0.90 V vs. Ag/Ag⁺ assigned as the oxidations of the ferrocene units (−0.05) and TTF core (0.42 and 0.90). Our CV experiment was performed on crystalline 10, whose composition and structure are firmly confirmed by a variety of techniques including SXRD. Furthermore, the redox behaviour of 10 is consistent with the redox behaviour of complexes 5 and 8. We also note that the color and solubility of (dppfPd)₂TTFtt from the previous literature report is quite different than what we have observed for this complex.

These results raise questions about the previous report of the preparation of (dppfPd)₂TTFtt with in situ formed TTFtt⁴⁻. To address these inconsistencies, we repeated the synthesis of (dppfPd)₂TTFtt following the previously reported procedure three times and consistently obtained low yields of 10% or less (versus 66% reported). The small amount of collected product prevented us from detailed characterization of this material. Taken all together, the direct comparison with previous preparations of (dppfPd)₂TTFtt prepared through the conventional method highlights the versatility and efficiency of the TTFtt-tin precursors we report here. This new synthetic protocol enables isolation of new pure complexes and may challenge previous preparations and assignments of these species that suffer from the in situ generation of TTFtt⁴⁻.

Packing and dimerization of TTFtt units

Although the NIR absorptions for radical cations and dications may indicate the presence of π-dimer formation in solution, this interpretation has been questioned.²⁹ To probe the possibility of dimerization in solution, room temperature Evans method experiments on CDCl₃ solutions of 5 were performed. The experimentally measured magnetic moment μ_eff = 1.19 BM is smaller than the predicted spin-only value of 1.73 BM suggesting that some degree of oligomerization is occurring. Additionally, spin quantitation of the EPR spectrum of 5 at 15 K indicates <10% of the expected signal based on the concentration of 5, also supporting some degree of dimerization. Variable-temperature UV-vis-NIR spectroscopic experiments (Fig. S45†) indicate an increase of the absorption peak in the NIR region with cooling, suggesting that the equilibrium shifts to oligomerization as the temperature decreases.²⁹

In addition to these solution studies, we were also interested in examining the effect of the solid-state packing of these molecules. As mentioned, solid-state packing of TTF cores is well-known, and much of the bulk transport properties of TTF based systems arises from their π–π and sulfur–sulfur interactions in the solid state, particularly in single component conductors.^11–13,30 The packing of these compounds has been discussed above and is largely similar to previously reported systems. Solid state magnetic measurements were performed on 5 and indicate a diamagnetic compound, which is also similar to previously reported radical cations of TTF.^18b,31

During the course of these studies, however, we isolated a poorly diffracting alternative polymorph of 5. While the poor quality of this crystal prevented a full structural solution, we have been able to obtain sufficient resolution to observe a stacking interaction which has a twist of the TTF cores by a nearly orthogonal ∼90° (Fig. 5). TTF stacking most commonly has a parallel arrangement, although there are examples of similar twisted interactions, particularly when supported by auxiliary polymeric superstructures.¹³ This structure of 5 is somewhat unusual in that the rotated 1D column of 5 is composed of two elements: trimers with asymmetric orthogonally crossed interactions and dimers with more typical parallel interactions (Fig. 5, and S56–S58†). Higher-quality crystals of complex 8 were obtained and the structure of this species displays very similar chains (Fig. S54†), verifying this unusual structural motif. The strength of TTF–TTF interactions and overlap is dependent on S–S interactions between TTF cores. In these unusual stacks however, the orthogonal and parallel interactions may lead to different overlaps which prompted us to investigate what additional effect the twisting of the TTF–TTF cores has on their interaction.

Fig. 5 Stacking diagram for twisted polymorph of 5 with phenyl groups, hydrogen atoms, and anions removed for clarity. Ni is shown in green, S in yellow, P in purple, and C in grey. The computationally examined parallel dimer, orthogonal dimer, and orthogonal trimer are indicated.

Computational analysis of TTFtt–TTFtt interactions

We then undertook calculations on 5 as a model for the effect of the twisted TTF–TTF interactions. Understanding the interactions in the dimer and trimer units in detail and how the twisting of the TTF–TTF interaction affects electronic structure requires large scale CASSCF calculations with extensive active spaces, leading to prohibitively high computational costs with conventional methods. Instead, we employed variational 2-electron reduced density matrix (V2RDM) techniques,³² which have previously been demonstrated to successfully describe the electronic structure of a variety of strongly correlated large molecules.³³ V2RDM calculations were carried out as implemented in the Maple Quantum Chemistry Package.³⁴ The phenyl ligands were replaced with methyl groups and [18,20] active space V2RDM calculations with the 3-21G basis set were performed for both geometries providing the data shown in Table S7.†³⁵ The electronic structures of both arrangements show significant degrees of correlation as demonstrated by partial occupancies in their frontier natural orbitals (NOs). The orthogonal arrangement shows more radical character, with frontier orbital natural occupation numbers (NON) of 1.224860 and 0.771141 suggesting significant bi-radical character, compared to 1.49923 and 0.51331 in the parallel arrangement. Mulliken charges show an effective charge of +1/2 for the Ni centers in both geometries, with a slightly higher cumulative charge of 1.94745 in the parallel arrangement compared to 1.70585 in the orthogonal system.

Frontier orbital densities, occupations and splittings for the parallel dimer and orthogonal dimer and trimer are shown diagrammatically in Fig. 6A and B respectively. All frontier NOs are localized on the bridging ligand with no involvement of the Ni centers, consistent with experimental results. There are significant differences in the orbital configurations elucidating the variation in frontier NON across the two arrangements. The larger splitting of the NO occupancy in the parallel arrangement clearly arises from better orbital overlap between the two monomers, allowing for greater energetic orbital splitting into NO 384 with significant antibonding character, showing no overlap between the two monomers, and NON 383 with significant bonding character and orbital overlap. In contrast, the orthogonal dimer shows two frontier NOs with very similar densities, both showing significant bonding character and overlap between the two monomers, yielding a smaller splitting and correspondingly greater bi-radical character.

Fig. 6 Frontier NO occupations and densities for (A) the parallel dimer, showing the splitting into bonding and antibonding orbitals. Data and densities obtained via [18,20] V2RDM calculations with a 3-21G basis set. (B) For the orthogonal dimer and trimer. Good overlap and correspondingly small splitting in the orthogonal dimer give way to a clear splitting into bonding, non-bonding and antibonding frontier NOs upon transitioning to the orthogonal trimer. Data and densities obtained via V2RDM calculations with a 3-21G basis set and [18,20] and [17,20] active spaces for the dimer and trimer respectively.

As the orthogonal dimeric arrangement is actually part of a larger asymmetrically stacked unit, a trimeric unit was run separately in V2RDM using a [17,20] active space and the 3-21G basis set, giving a SCF calculation with 1308 orbitals. Data are shown in Table S8.† Similar to the dimeric case, the trimer unit exhibits clear radical character and three partially occupied NOs with NON of 1.32748, 0.97218 and 0.64935. Mulliken charges in this arrangement are particularly symmetric with each nickel showing a charge of 0.43 to 0.45 with very little variation between the individual centers. Transitioning from a dimeric to a trimeric unit gives rise to splittings and symmetries in line with a classic Hückel picture with the orbitals splitting into bonding, non-bonding and antibonding. The bonding and antibonding orbitals NO 574 and 576 both show roughly equal distribution of the electron density across all three units within the trimer. NO 574 has good matching of the phases between the orbitals localized on each of the units in the trimer leading to overlap between the orbitals on all units and giving rise to significant bonding character and a NO occupancy of 1.32748. Constituent orbitals of NO 576 in contrast constitute a worse matching of the phases, reducing overlap between the individual units and leading to an overall antibonding interaction and a NO occupancy of 0.64935. Singly occupied non-bonding NO 575 is localized on the top and bottom molecules with a nodal plane and negligible density on the central unit, leading to an electron entangled across the two isolated top and bottom units within the trimer.

The results from V2RDM CASSCF calculations help rationalize the appearance and stability of the different morphologies in the TTFtt stacks. Packing geometries in both the parallel and orthogonal arrangement allow for good orbital overlap between the individual units. Both morphologies show the frontier natural orbitals form via π–π stacking utilizing orbitals localized on the TTF linkers. The resulting NOs differ slightly between the different morphologies with overlap in the parallel geometry allowing for better splitting into clear bonding and antibonding frontier orbital pairs, reducing radical character. The splitting is less pronounced in the orthogonal dimer; however, as the chain size increases clear splitting into bonding, non-bonding and antibonding frontier orbital pairs is recovered in the trimer. In all cases partial occupations in the frontier NOs is retained, allowing for radical chain development and electron entanglement across multiple units.

In 1985, Hoffman and coworkers predicted possible stacking structures of metal bisdithiolenes based on qualitative molecular orbital and band structure calculations.³⁶ Soon afterwards, in 1988, a LAXS (Large Angle X-ray Scattering) and EXAFS (Extended X-ray Absorption Fine Structure) investigation was performed on amorphous nickel tetrathiolate polymers, proposing two types of polymers with hexagonal (honeycomb) and tetragonal packings for small and large cations, respectively.³⁷ However, the stackings of the TTFtt radicals in 5 and 8 highlight the key role of strong intermolecular interactions between radicals in the control of morphology. In sum, the stabilization and synthetic access provided by the Sn capped compounds reported here enables the observation of a variety of solid-state interactions of the TTF core. We anticipate that the redox flexibility of these synthons will enable the observation of novel interactions and electronic structures in TTFtt based coordination polymers.

Conclusions

TTFtt is an attractive building block for redox-switchable and highly conjugated metal–organic materials. The work presented here demonstrates that capping TTFtt with dialkyl Sn groups stabilizes this ligand and facilitates the use of redox-active TTFtt moieties. Furthermore, the redox flexibility of these synthons helps to precisely control doping, charge, and crystallinity via homogeneous molecular reactions. The synthesis and characterization of the corresponding dinickel and dipalladium complexes validates the ease of transmetalation as a synthetic strategy. We have also observed an unusual “twisted” geometry in the solid state which impacts the electronic structure of the TTF–TTF interaction, effectively demonstrating the utility of these new synthons. Overall, this work demonstrates the usefulness of molecular TTFtt compounds and offers exciting promise for the design and synthesis of multi-functional TTFtt-based coordination polymers.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank Prof. Jing-lin Zuo for the gift of an initial TTFttPG sample. We thank Dr Ethan Hill for assistance with EPR spectroscopy and Kate Jesse for the assistance with UV-vis-NIR spectroscopy. Parts of this work were carried out at the Soft Matter Characterization Facility of the University of Chicago. J. S. A. and D. A. M. gratefully acknowledge support for this work from the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0019215. D. A. M. gratefully acknowledges the National Science Foundation (NSF) Grant No. CHE-1152425, and the United States Army Research Office (ARO) Grant No. W911NF-16-1-0152. This work was supported by the Chicago MRSEC, which is funded by NSF through Grant DMR-1420709. The University of Chicago is thanked for startup funds. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357, and we thank Dr Yu-Sheng Chen for assistance with SXRD data acquisition at beamline 15-ID-B, C, and D.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and CIF files. CCDC 1950301–1950306, 1971219, 1971220. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9sc04381k

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