Metal (Hg, Pt, Ru) Bisalkynyl Bridge between Tetrathiafulvalene Electrophores and Electronic Interplay

: A series of metal (Hg, Pt, Ru) bis(alkynyl-tetrathiafulvalene) complexes have been investigated to study the electronic interplay between the metal and the tetrathiafulvalene (TTF), as well as between the two peripheral TTF electrophores along the organometallic bridge. Cyclic voltammetry experiments, together with spectro-electrochemical investigations, have shown the electronic effect of the metal center through the linker on redox properties of the TTF, as well as the inﬂuence of the length of the conjugated organic linker. These data show that the degree of coupling can be modulated from no coupling with mercury to appreciable electronic coupling between different electrophores with ruthenium.


Introduction
Tetrathiafulvalene (TTF) dimers have attracted attention in past decades as potential building blocks for the formation of mixed valence species, (TTF 2 ) +• which is key for the elaboration of conducting materials in charge transfer salts [1][2][3]. Different organic spacer groups, conjugated or non-conjugated, have been used to connect the two TTF moieties and, depending on the nature of the spacer, either through space or through bond interactions, have been observed [1,[4][5][6][7][8][9]. Organic spacer groups are not the only linkers which have been studied. Indeed, the grafting of functional groups on the TTF core allows the formation of TTF ligands. The coordination chemistry of two of these ligands towards the metallic center also generates various dimeric structures [10][11][12]. Most of the interactions detected through organic linkers are weak and coordination chemistry does not allow an increase of electronic interplay [13][14][15]. On the other hand, the use of an organometallic linker such as the trans bis(acetylide) ruthenium one in trans-[Ru(C≡CMe 3 TTF) 2 (dppe) 2 ] mediates strong electronic coupling between the two TTFs [16]. Moreover, within this complex, the TTF and the ruthenium center are also strongly electronically coupled [17,18]. Other trans bis(acetylide) metallic linkers have also been reported in the literature (M = Cr [19][20][21][22], Pt [23]) but these organometallic linkers do not allow an electrochemically measurable electronic interaction between the TTFs. However, the absence of electrochemical evidence for the formation of mixed valence species in the case of the Cr bis(acetylide) linker did not impede on the formation of a mixed valence species in the solid state. In this context, we also focused on another metal center which, compared to Ru and Pt, has been poorly explored so far in TTF chemistry as a part of the organometallic linker, that is mercury [24]. Mercury(II) ion is known to form various complexes with acetylide ligands by forming two-coordinate d 10 mercury(II) species [25][26][27]. Therefore, we investigated the synthesis of a complex where the mercury was linked
Bis(TTF-alkynyl)mercury complexes 5 and 6 were prepared by reacting TTF 3 or 4 with [Hg(OAc) 2 ] in refluxing THF (Scheme 2) [32]. In both cases, a powder that is insoluble in usual organic solvents, was obtained. Nevertheless, an FTIR analysis of these powders demonstrated that the characteristic C-H stretching vibration band observed for the terminal alkynes at ν ≡C-H = 3306 cm −1 and 3278 cm −1 for 3 and 4 respectively had disappeared in the spectra of complexes 5 and 6. Moreover, the stretching vibration band of the C≡C bonds in complex 5 was located at ν C≡C = 2141 cm −1 , which was also consistent with the stretching frequency observed in the analogous ferrocenyl derivative Hg(C≡CFc) 2 (ν C≡C = 2141 cm −1 ) [33]. However, in complex 6, the vibration band was slightly shifted at ν C≡C = 2151 cm −1 . Both ν C≡C values were different from the ones observed in the starting TTF alkynes (ν C≡C = 2190 cm −1 for 3 and 2201 cm −1 for 4), confirming the formation of the mercury complexes. It is worth mentioning that we did not observe a decomposition of complexes 5 and 6 over time, even without purification, as was observed for the ferrocenyl complex Hg(C≡CFc) 2 [34]. In order to purify these complexes, we recrystallized them in pyridine. Single crystals were obtained for 5, allowing an X-ray diffraction study. Bis(TTF-alkynyl)mercury complexes 5 and 6 were prepared by reacting TTF 3 or 4 with [Hg(OAc)2] in refluxing THF (Scheme 2) [32]. In both cases, a powder that is insoluble in usual organic solvents, was obtained. Nevertheless, an FTIR analysis of these powders demonstrated that the characteristic C-H stretching vibration band observed for the terminal alkynes at ν≡C-H = 3306 cm −1 and 3278 cm −1 for 3 and 4 respectively had disappeared in the spectra of complexes 5 and 6. Moreover, the stretching vibration band of the C≡C bonds in complex 5 was located at νC≡C = 2141 cm −1 , which was also consistent with the stretching frequency observed in the analogous ferrocenyl derivative Hg(C≡CFc)2 (νC≡C = 2141 cm −1 ) [33]. However, in complex 6, the vibration band was slightly shifted at νC≡C = 2151 cm −1 . Both νC≡C values were different from the ones observed in the starting TTF alkynes (νC≡C = 2190 cm −1 for 3 and 2201 cm −1 for 4), confirming the formation of the mercury complexes. It is worth mentioning that we did not observe a decomposition of complexes 5 and 6 over time, even without purification, as was observed for the ferrocenyl complex Hg(C≡CFc)2 [34]. In order to purify these complexes, we recrystallized them in pyridine. Single crystals were obtained for 5, allowing an X-ray diffraction study.
The molecular structure given in Figure 1 confirms the formation of the mercurial complex 5 and also indicates the presence of two pyridines in the coordination sphere of the Hg atom, leading to a four coordinate Hg atom with a square planar geometry. Within this complex, denoted trans-[Hg(C≡CMe3TTF)2(pyr)2], the TTF-acetylide are located in a trans arrangement around the Hg Bis(TTF-alkynyl)mercury complexes 5 and 6 were prepared by reacting TTF 3 or 4 with [Hg(OAc)2] in refluxing THF (Scheme 2) [32]. In both cases, a powder that is insoluble in usual organic solvents, was obtained. Nevertheless, an FTIR analysis of these powders demonstrated that the characteristic C-H stretching vibration band observed for the terminal alkynes at ν≡C-H = 3306 cm −1 and 3278 cm −1 for 3 and 4 respectively had disappeared in the spectra of complexes 5 and 6. Moreover, the stretching vibration band of the C≡C bonds in complex 5 was located at νC≡C = 2141 cm −1 , which was also consistent with the stretching frequency observed in the analogous ferrocenyl derivative Hg(C≡CFc)2 (νC≡C = 2141 cm −1 ) [33]. However, in complex 6, the vibration band was slightly shifted at νC≡C = 2151 cm −1 . Both νC≡C values were different from the ones observed in the starting TTF alkynes (νC≡C = 2190 cm −1 for 3 and 2201 cm −1 for 4), confirming the formation of the mercury complexes. It is worth mentioning that we did not observe a decomposition of complexes 5 and 6 over time, even without purification, as was observed for the ferrocenyl complex Hg(C≡CFc)2 [34]. In order to purify these complexes, we recrystallized them in pyridine. Single crystals were obtained for 5, allowing an X-ray diffraction study. The molecular structure given in Figure 1 confirms the formation of the mercurial complex 5 and also indicates the presence of two pyridines in the coordination sphere of the Hg atom, leading to a four coordinate Hg atom with a square planar geometry. Within this complex, denoted trans-[Hg(C≡CMe3TTF)2(pyr)2], the TTF-acetylide are located in a trans arrangement around the Hg atom. For comparison purposes, we collected in Table 1 the bond lengths of the metal (M = Ru, Pt, and Hg) bis(alkynyl) linkers. The Hg-C and Hg-N distances amount to 2.032(6) and 2.850(6) Å respectively and the C≡C bond distance reaches 1.196(9) Å, i.e., slightly shorter than the one found in the ruthenium and the platinum complexes but longer than the starting Me3TTF-alkyne (1.152(8) Å ) ( Table 1) [16]. The TTF units are non-planar and exhibit a boat conformation with the dithiole rings folded along the S···S axis with values of about 26° and 10° and a bond length of the central C=C Scheme 2. Synthesis of mercury alkynyl complexes.
The molecular structure given in Figure 1 confirms the formation of the mercurial complex 5 and also indicates the presence of two pyridines in the coordination sphere of the Hg atom, leading to a four coordinate Hg atom with a square planar geometry. Within this complex, denoted trans-[Hg(C≡CMe 3 TTF) 2 (pyr) 2 ], the TTF-acetylide are located in a trans arrangement around the Hg atom. For comparison purposes, we collected in Table 1 the bond lengths of the metal (M = Ru, Pt, and Hg) bis(alkynyl) linkers. The Hg-C and Hg-N distances amount to 2.032(6) and 2.850(6) Å respectively and the C≡C bond distance reaches 1.196(9) Å, i.e., slightly shorter than the one found in the ruthenium and the platinum complexes but longer than the starting Me 3 TTF-alkyne (1.152(8) Å) ( Table 1) [16]. The TTF units are non-planar and exhibit a boat conformation with the dithiole rings folded along the S···S axis with values of about 26 • and 10 • and a bond length of the central C=C bond consistent with the presence of neutral TTF (1.341 Å). The organometallic linker -C≡C-Hg-C≡C-between the two TTFs is close to linear, with C-Hg-C and Hg-C≡C angles of 176.9(5) and 179.9(2) • respectively. bond consistent with the presence of neutral TTF (1.341 Å ). The organometallic linker -C≡C-Hg-C≡C-between the two TTFs is close to linear, with C-Hg-C and Hg-C≡C angles of 176.9(5) and 179.9(2)° respectively.  The synthesis of the target platinum complex, trans-[Pt(C≡C-C≡CMe3TTF)2(PPh3)2] 7 was realized, starting from TTF-butadiyne 4 as reported in Scheme 3. The cis-Pt(PPh3)2Cl2 complex in the presence of triethylamine and CuI in dry dichloromethane was reacted with two equivalents of TTF 4 to afford the trans-complex, trans-[Pt(C≡C-C≡CMe3TTF)2(PPh3)2] 7 [35]. The trans configuration was confirmed by the 31 P NMR spectrum of the complex, which only shows one signal at 18.1 ppm, indicating the equivalence of the two phosphorus atoms around the Pt(II) centre. Recently, we reported the synthesis of trans-[Ru(C≡C-C≡CMe3TTF)2(dppe)2] 8 (dppe = Ph2PCH2CH2PPh2) [28] by simply reacting TTF 4 with half an equivalent of cis-[RuCl2(dppe)2] in the presence of NaPF6 and triethylamine in dichloromethane at room temperature under inert atmosphere (Scheme 4). Interestingly, by using the same conditions but one equivalent of cis-[RuCl2(dppe)2], we synthesized the mononuclear ruthenium complex where the Ru is connected to only one TTF through a butadiyne bridge, trans-[RuCl(C≡C-)2Me3TTF(dppe)2] 9 (Scheme 4). Monitoring the reaction by 31 P NMR spectroscopy shows that the signals of the starting complex cis-[RuCl2(dppe)2] at δp = 37.8 ppm and δp = 45.1 ppm gradually disappear in favor of a signal at δP = 47.9 ppm associated with the formation of the monosubsituted complex trans-[RuCl(C≡C-)2Me3TTF  bond consistent with the presence of neutral TTF (1.341 Å ). The organometallic linker -C≡C-Hg-C≡C-between the two TTFs is close to linear, with C-Hg-C and Hg-C≡C angles of 176.9(5) and 179.9(2)° respectively.  The synthesis of the target platinum complex, trans-[Pt(C≡C-C≡CMe3TTF)2(PPh3)2] 7 was realized, starting from TTF-butadiyne 4 as reported in Scheme 3. The cis-Pt(PPh3)2Cl2 complex in the presence of triethylamine and CuI in dry dichloromethane was reacted with two equivalents of TTF 4 to afford the trans-complex, trans-[Pt(C≡C-C≡CMe3TTF)2(PPh3)2] 7 [35]. The trans configuration was confirmed by the 31 P NMR spectrum of the complex, which only shows one signal at 18.1 ppm, indicating the equivalence of the two phosphorus atoms around the Pt(II) centre.

Scheme 3. Synthesis of trans-[Pt
Recently, we reported the synthesis of trans-[Ru(C≡C-C≡CMe3TTF)2(dppe)2] 8 (dppe = Ph2PCH2CH2PPh2) [28] by simply reacting TTF 4 with half an equivalent of cis-[RuCl2(dppe)2] in the presence of NaPF6 and triethylamine in dichloromethane at room temperature under inert atmosphere (Scheme 4). Interestingly, by using the same conditions but one equivalent of cis-[RuCl2(dppe)2], we synthesized the mononuclear ruthenium complex where the Ru is connected to only one TTF through a butadiyne bridge, trans-[RuCl(C≡C-)2Me3TTF(dppe)2] 9 (Scheme 4). Monitoring the reaction by 31 P NMR spectroscopy shows that the signals of the starting complex cis-[RuCl2(dppe)2] at δp = 37.8 ppm and δp = 45.1 ppm gradually disappear in favor of a signal at δP = 47.9 ppm associated with the formation of the monosubsituted complex trans-[RuCl(C≡C-)2Me3TTF The synthesis of the target platinum complex, trans-[Pt(C≡C-C≡CMe 3 TTF) 2 (PPh 3 ) 2 ] 7 was realized, starting from TTF-butadiyne 4 as reported in Scheme 3. The cis-Pt(PPh 3 ) 2 Cl 2 complex in the presence of triethylamine and CuI in dry dichloromethane was reacted with two equivalents of TTF 4 to afford the trans-complex, trans-[Pt(C≡C-C≡CMe 3 TTF) 2 (PPh 3 ) 2 ] 7 [35]. The trans configuration was confirmed by the 31 P NMR spectrum of the complex, which only shows one signal at 18.1 ppm, indicating the equivalence of the two phosphorus atoms around the Pt(II) centre.
Inorganics 2018, 6, x FOR PEER REVIEW 4 of 12 bond consistent with the presence of neutral TTF (1.341 Å ). The organometallic linker -C≡C-Hg-C≡C-between the two TTFs is close to linear, with C-Hg-C and Hg-C≡C angles of 176.9(5) and 179.9(2)° respectively.  The synthesis of the target platinum complex, trans-[Pt(C≡C-C≡CMe3TTF)2(PPh3)2] 7 was realized, starting from TTF-butadiyne 4 as reported in Scheme 3. The cis-Pt(PPh3)2Cl2 complex in the presence of triethylamine and CuI in dry dichloromethane was reacted with two equivalents of TTF 4 to afford the trans-complex, trans-[Pt(C≡C-C≡CMe3TTF)2(PPh3)2] 7 [35]. The trans configuration was confirmed by the 31 P NMR spectrum of the complex, which only shows one signal at 18.1 ppm, indicating the equivalence of the two phosphorus atoms around the Pt(II) centre.
Recently, we reported the synthesis of trans-[Ru(C≡C-C≡CMe3TTF)2(dppe)2] 8 (dppe = Ph2PCH2CH2PPh2) [28] by simply reacting TTF 4 with half an equivalent of cis-[RuCl2(dppe)2] in the presence of NaPF6 and triethylamine in dichloromethane at room temperature under inert atmosphere (Scheme 4). Interestingly, by using the same conditions but one equivalent of cis-[RuCl2(dppe)2], we synthesized the mononuclear ruthenium complex where the Ru is connected to only one TTF through a butadiyne bridge, trans-[RuCl(C≡C-)2Me3TTF(dppe)2] 9 (Scheme 4). Monitoring the reaction by 31 P NMR spectroscopy shows that the signals of the starting complex Recently, we reported the synthesis of trans-[Ru(C≡C-C≡CMe 3 TTF) 2 (dppe) 2 ] 8 (dppe = Ph 2 PCH 2 CH 2 PPh 2 ) [28] by simply reacting TTF 4 with half an equivalent of cis-[RuCl 2 (dppe) 2 ] in the presence of NaPF 6 and triethylamine in dichloromethane at room temperature under inert atmosphere (Scheme 4). Interestingly, by using the same conditions but one equivalent of cis-[RuCl 2 (dppe) 2 ], we synthesized the mononuclear ruthenium complex where the Ru is connected to only one TTF through a butadiyne bridge, trans-[RuCl(C≡C-) 2 Me 3 TTF(dppe) 2 ] 9 (Scheme 4). Monitoring the reaction by 31 P NMR spectroscopy shows that the signals of the starting complex cis-[RuCl 2 (dppe) 2 ] at δ p = 37.8 ppm and δ p = 45.1 ppm gradually disappear in favor of a signal at δ P = 47.9 ppm associated with the formation of the monosubsituted complex trans-[RuCl(C≡C-) 2 Me 3 TTF (dppe) 2 ] 9. The single peak indicates the equivalence of the phosphorus atoms of the dppe ligands due to the trans-configuration. (dppe)2] 9. The single peak indicates the equivalence of the phosphorus atoms of the dppe ligands due to the trans-configuration. Crystals of the trans-[RuCl(C≡C-)2Me3TTF(dppe)2] complex 9 were obtained by a slow diffusion of pentane into a concentrated solution of the complex in CH2Cl2 under inert atmosphere. This complex crystallizes in the triclinic system, space group 1 P , with one independent molecule and two CH2Cl2 molecules. The molecular structure of 9 is shown in Figure 2. The Ru(II) atom is chelated by two dppe units in the equatorial plane and the octahedral coordination geometry is completed by a chloride ligand trans coordinated to the TTF-butadiyne. The neutral TTF unit (the central C=C bond length amounts to 1.306 Å ) exhibits a boat conformation with the dithiole rings folded along the S···S axis with asymmetric folding angles of 6 and 14°. The -Ru-C≡C-C≡C-organometallic fragment is slightly bent, with angles at -Cα≡Cβ-Cγ, -Cβ-Cγ≡Cδ-and -Cγ≡Cδ-CTTF of 170.32°, 166.84° and 178.25° respectively. It is interesting to compare the bond lengths of the butadiyne rod with those observed in the starting TTFs and the previously reported complex with two TTFs, namely trans-[Ru(-C≡C-C≡C-Me3TTF)2(dppe)2], (Table 2) [28]. Within the mono-TTF substituted complex, the bond lengths for the butadiyne rod indicate a high degree of conjugation compared to the TTF precursors 2 and 4. This higher degree of conjugation can also be visualized on the IR frequencies of the acetylide bonds as for the monosubstituted complex. They are found at a lower energy than in the starting TTFs 2 and 4 ( Table 2). The Ru-Cα distance of 2.075(6) Å stands in the same range as other ruthenium trans coordinated alkyne ligand (Tables 1 and 2) [16,29,36].  Crystals of the trans-[RuCl(C≡C-) 2 Me 3 TTF(dppe) 2 ] complex 9 were obtained by a slow diffusion of pentane into a concentrated solution of the complex in CH 2 Cl 2 under inert atmosphere. This complex crystallizes in the triclinic system, space group P1, with one independent molecule and two CH 2 Cl 2 molecules. The molecular structure of 9 is shown in Figure 2. The Ru(II) atom is chelated by two dppe units in the equatorial plane and the octahedral coordination geometry is completed by a chloride ligand trans coordinated to the TTF-butadiyne. The neutral TTF unit (the central C=C bond length amounts to 1.306 Å) exhibits a boat conformation with the dithiole rings folded along the S···S axis with asymmetric folding angles of 6 and 14 • . The -Ru-C≡C-C≡C-organometallic fragment is slightly bent, with angles at -C α ≡C β -C γ , -C β -C γ ≡C δ -and -C γ ≡C δ -C TTF of 170.32 • , 166.84 • and 178.25 • respectively. It is interesting to compare the bond lengths of the butadiyne rod with those observed in the starting TTFs and the previously reported complex with two TTFs, namely trans-[Ru(-C≡C-C≡C-Me 3 TTF) 2 (dppe) 2 ], (Table 2) [28]. Within the mono-TTF substituted complex, the bond lengths for the butadiyne rod indicate a high degree of conjugation compared to the TTF precursors 2 and 4. This higher degree of conjugation can also be visualized on the IR frequencies of the acetylide bonds as for the monosubstituted complex. They are found at a lower energy than in the starting TTFs 2 and 4 ( Table 2). The Ru-Cα distance of 2.075(6) Å stands in the same range as other ruthenium trans coordinated alkyne ligand (Tables 1 and 2) [16,29,36].
The redox properties of the different complexes have been investigated by cyclic voltammetry. The redox potentials are collected in Table 3 together with those of the starting TTFs 3-4 and the previously reported Ru and Pt analogous complexes for comparison. Except for the mercury complexes, which were analyzed in DMF, all the other derivatives were investigated in CH 2 Cl 2 using NBu 4 PF 6 as a supporting electrolyte. Crystals of the trans-[RuCl(C≡C-)2Me3TTF(dppe)2] complex 9 were obtained by a slow diffusion of pentane into a concentrated solution of the complex in CH2Cl2 under inert atmosphere. This complex crystallizes in the triclinic system, space group 1 P , with one independent molecule and two CH2Cl2 molecules. The molecular structure of 9 is shown in Figure 2. The Ru(II) atom is chelated by two dppe units in the equatorial plane and the octahedral coordination geometry is completed by a chloride ligand trans coordinated to the TTF-butadiyne. The neutral TTF unit (the central C=C bond length amounts to 1.306 Å ) exhibits a boat conformation with the dithiole rings folded along the S···S axis with asymmetric folding angles of 6 and 14°. The -Ru-C≡C-C≡C-organometallic fragment is slightly bent, with angles at -Cα≡Cβ-Cγ, -Cβ-Cγ≡Cδ-and -Cγ≡Cδ-CTTF of 170.32°, 166.84° and 178.25° respectively. It is interesting to compare the bond lengths of the butadiyne rod with those observed in the starting TTFs and the previously reported complex with two TTFs, namely trans-[Ru(-C≡C-C≡C-Me3TTF)2(dppe)2], (Table 2) [28]. Within the mono-TTF substituted complex, the bond lengths for the butadiyne rod indicate a high degree of conjugation compared to the TTF precursors 2 and 4. This higher degree of conjugation can also be visualized on the IR frequencies of the acetylide bonds as for the monosubstituted complex. They are found at a lower energy than in the starting TTFs 2 and 4 ( Table 2). The Ru-Cα distance of 2.075(6) Å stands in the same range as other ruthenium trans coordinated alkyne ligand (Tables 1 and 2) [16,29,36].    The redox properties of the different complexes have been investigated by cyclic voltammetry. The redox potentials are collected in Table 3 together with those of the starting TTFs 3-4 and the previously reported Ru and Pt analogous complexes for comparison. Except for the mercury complexes, which were analyzed in DMF, all the other derivatives were investigated in CH2Cl2 using NBu4PF6 as a supporting electrolyte.
Among all the investigated complexes, only the Ru complexes display a redox process associated with the metal centre. All the other complexes exhibit two well defined reversible oxidation waves associated with the redox signature of the TTF cores ( Figure 3). With regard to the Hg and Pt complexes, because these complexes include two TTFs, this indicates that either no interaction occurs via the organometallic linker between the two TTF cores or that these interactions are too weak to be observed at the cyclic voltammetry scale. Therefore, the two TTFs oxidize, essentially simultaneously, into the TTF cation radicals and then to the TTF dications. The crystalline complex trans-[Hg(C≡CMe3TTF)2(pyr)2] was also investigated in the same conditions as 5. It is interesting to note that an anodic shift of about 40 mV is observed on the oxidation potentials of the TTFs when two pyridine ligands are coordinated to the mercury atom (Table 3). On the contrary, for the Ru bis(acetylide) complex, trans-[Ru(C≡CMe3TTF)2(dppe)2], each TTF is oxidized sequentially into a TTF radical cation and TTF dication. This indicates that the Ru bis(acetylide) linker is the only organometallic linker which allows the formation of mixed valence species (TTF/TTF +• and TTF +• /TTF 2+ ) within the series [16]. On the other hand, the introduction of a Ru bis(butadiyne) spacer group does not allow the observation of electronic interplay on the first redox process between the two TTF cores but only on the second redox process [28]. This redox behavior is uncommon in the case of TTF dimers and could be tentatively assigned to electrostatic repulsion generated in the TTF +• /TTF 2+ species, as previously observed for the oxidation of the TTF moieties in their dicationic states for the trans-[Ru(C≡CMe3TTF)2(dppe)2] [16].   Among all the investigated complexes, only the Ru complexes display a redox process associated with the metal centre. All the other complexes exhibit two well defined reversible oxidation waves associated with the redox signature of the TTF cores ( Figure 3). With regard to the Hg and Pt complexes, because these complexes include two TTFs, this indicates that either no interaction occurs via the organometallic linker between the two TTF cores or that these interactions are too weak to be observed at the cyclic voltammetry scale. Therefore, the two TTFs oxidize, essentially simultaneously, into the TTF cation radicals and then to the TTF dications. The crystalline complex trans-[Hg(C≡CMe 3 TTF) 2 (pyr) 2 ] was also investigated in the same conditions as 5. It is interesting to note that an anodic shift of about 40 mV is observed on the oxidation potentials of the TTFs when two pyridine ligands are coordinated to the mercury atom (Table 3). On the contrary, for the Ru bis(acetylide) complex, trans-[Ru(C≡CMe 3 TTF) 2 (dppe) 2 ], each TTF is oxidized sequentially into a TTF radical cation and TTF dication. This indicates that the Ru bis(acetylide) linker is the only organometallic linker which allows the formation of mixed valence species (TTF/TTF +• and TTF +• /TTF 2+ ) within the series [16]. On the other hand, the introduction of a Ru bis(butadiyne) spacer group does not allow the observation of electronic interplay on the first redox process between the two TTF cores but only on the second redox process [28]. This redox behavior is uncommon in the case of TTF dimers and could be tentatively assigned to electrostatic repulsion generated in the TTF +• /TTF 2+ species, as previously observed for the oxidation of the TTF moieties in their dicationic states for the trans-[Ru(C≡CMe 3 TTF) 2 (dppe) 2 ] [16].
Another feature is the electronic effect of the metal center through the linker on the redox properties of the TTF. For instance, the presence of the mercury atom does not significantly modify the oxidation potentials of the TTF. This is reminiscent of what was observed in the case of the iron or chromium acetylide-TTF type complex, Cp*(dppe)FeC≡CTTFMe 3 [37,38] and [CrCyclam(C≡CEDTMeTTF) 2 ]OTf [19][20][21][22]. In contrast, a cathodic shift of the redox potentials is observed when a platinum or a ruthenium atom is connected to the TTFs. Interestingly, the cathodic shift observed for the Ru complexes is more pronounced, indicating a large increase of the electron density of the Ru organometallic fragment on the TTF cores through the conjugated linker. It is worth mentioning that this electronic interaction between the ruthenium center and the TTFs is similar in the mono-and the bis-substituted complexes, trans-[RuCl((C≡C-) n Me 3 TTF)(dppe) 2 ] and trans-[Ru((-C≡C) n Me 3 TTF) 2 (dppe) 2 ] (n = 1, 2) respectively. For all the ruthenium complexes, the reversible oxidation of the ruthenium center is ascribed to the last redox process observed on the cyclic voltammogram (Figure 3b). The Ru II/III oxidation is anodically shifted compared to cis-RuCl 2 (dppe) 2 (0.50 V vs. SCE) due to the presence of the dicationic TTF acting as an acceptor in the vicinity of Ru, confirming the electronic interaction between the organic and inorganic electrophores along the conjugated linker. essentially simultaneously, into the TTF cation radicals and then to the TTF dications. The crystalline complex trans-[Hg(C≡CMe3TTF)2(pyr)2] was also investigated in the same conditions as 5. It is interesting to note that an anodic shift of about 40 mV is observed on the oxidation potentials of the TTFs when two pyridine ligands are coordinated to the mercury atom (Table 3). On the contrary, for the Ru bis(acetylide) complex, trans-[Ru(C≡CMe3TTF)2(dppe)2], each TTF is oxidized sequentially into a TTF radical cation and TTF dication. This indicates that the Ru bis(acetylide) linker is the only organometallic linker which allows the formation of mixed valence species (TTF/TTF +• and TTF +• /TTF 2+ ) within the series [16]. On the other hand, the introduction of a Ru bis(butadiyne) spacer group does not allow the observation of electronic interplay on the first redox process between the two TTF cores but only on the second redox process [28]. This redox behavior is uncommon in the case of TTF dimers and could be tentatively assigned to electrostatic repulsion generated in the TTF +• /TTF 2+ species, as previously observed for the oxidation of the TTF moieties in their dicationic states for the trans-[Ru(C≡CMe3TTF)2(dppe)2] [16].  If we concentrate now on the effect of the length of the conjugated organic linker between the TTF and the metal center, the additional acetylenic group modifies the electronic interplay between these two electrophores. Indeed, comparison of the redox potentials of the TTF within the different complexes obtained with Me 3 TTF ethyne 3 and Me 3 TTF butadiyne 4 (Table 3) highlights this influence as the cathodic shifts observed for the complexes with the diacetylenic rod are less pronounced than those with an acetylenic one. This result is in agreement with the work performed by Touchard et al. on trans-[Ru(C≡CFc) 2 (dppe) 2 ] and trans-[Ru(C≡C-C≡CFc) 2 (dppe) 2 ] [29].
UV-Vis-NIR spectroelectrochemical investigations were carried out on trans-[Hg(C≡CMe 3 TTF) 2 (dppe) 2 ] 5, trans-[Pt(C≡C-C≡CMe 3 TTF) 2 (PPh 3 ) 2 ] 7 and trans-[RuCl(C≡C-C≡CMe 3 TTF)(dppe) 2 ] 9 complexes. Due to the low solubility of the mercury complex 5, the experiment was carried out in DMF while it was performed in CH 2 Cl 2 for the more soluble complexes 7 and 9. The changes in the UV-Vis-NIR spectra upon oxidation of 5 and 7 are shown in Figure 4. The absorption spectra show that the neutral complexes 5 and 7 absorb in the UV-Vis region but not beyond 400 nm. In both cases, upon gradual oxidation, only the spectral signature of the TTF radical cation species at 460 nm and 680 nm is observed for 5 and at 440 nm and 810 nm for 7 with no evidence of intervalence charge-transfer (IVCT) bands at lower energies. Similar evolution of the UV-Vis-NIR spectra were observed for the Pt complex with a shorter linkage, trans-[Pt(C≡CMe 3 TTF) 2 (PPh 3 ) 2 ] [23].
Concerning the investigation carried out on the complex 9, the evolution of the spectra upon oxidation is totally different ( Figure 5). The neutral complex exhibits absorption bands in the UV-visible range at λ max values of 259, 320 and 400 nm, but not beyond. Gradual oxidation from neutral to mono-oxidized species leads to a diminution of UV visible absorption bands at λ = 320 and 400 nm and the new bands growth at λ = 355, 445, 640 nm and λ = 1300 nm (0.95 eV), the last one being a broad absorption band. Previous studies on the ethyne analogue complex trans-[RuCl(C≡CMe 3 TTF)(dppe) 2 ] have shown a similar evolution of the spectra upon oxidation with the same kind of broad band in the NIR range at 1399 nm (0.88 eV) due to a SOMO-LUMO transition [17]. That is consistent with an electronic communication between the TTF and ruthenium moieties existing in both complexes. nm and the new bands growth at λ = 355, 445, 640 nm and λ = 1300 nm (0.95 eV), the last one being a broad absorption band. Previous studies on the ethyne analogue complex trans-[RuCl(C≡CMe3TTF)(dppe)2] have shown a similar evolution of the spectra upon oxidation with the same kind of broad band in the NIR range at 1399 nm (0.88 eV) due to a SOMO-LUMO transition [17]. That is consistent with an electronic communication between the TTF and ruthenium moieties existing in both complexes.

Materials and Methods
General Procedures. All the reactions were performed under an argon atmosphere using standard Schlenk techniques. The solvents were purified and dried by standard methods. The cis-RuCl2(dppe)2 and trimethylsilylprotected TTF 1 were synthesized according to procedures laid out in the literature [16][17][18]. NMR spectra were recorded on a AV300III spectrometer (Bruker, Billerica, MA, USA). Chemical shifts were reported in ppm and 1 H NMR spectra were referenced to residual CHCl3 (7.26 ppm). 31   nm and the new bands growth at λ = 355, 445, 640 nm and λ = 1300 nm (0.95 eV), the last one being a broad absorption band. Previous studies on the ethyne analogue complex trans-[RuCl(C≡CMe3TTF)(dppe)2] have shown a similar evolution of the spectra upon oxidation with the same kind of broad band in the NIR range at 1399 nm (0.88 eV) due to a SOMO-LUMO transition [17]. That is consistent with an electronic communication between the TTF and ruthenium moieties existing in both complexes.

Materials and Methods
General Procedures. All the reactions were performed under an argon atmosphere using standard Schlenk techniques. The solvents were purified and dried by standard methods. The cis-RuCl2(dppe)2 and trimethylsilylprotected TTF 1 were synthesized according to procedures laid out in the literature [16][17][18]. NMR spectra were recorded on a AV300III spectrometer (Bruker, Billerica, MA, USA). Chemical shifts were reported in ppm and 1 H NMR spectra were referenced to residual CHCl3 (7.26 ppm). 31 P NMR spectra were referenced to H3PO4. The 13 C NMR spectra of complexes trans-[Hg(-C≡C-Me3TTF)2] and trans-[Hg(-C≡C-C≡C-Me3TTF)2] could not be obtained due to their low solubility. Mass spectra were recorded by the Centre Régional de Mesures Physiques de l'Ouest, Rennes. Cyclic voltammetry investigations were carried out on a 10 −3 M solution containing 0.1 M NBu4PF6 as a supporting electrolyte. Voltammograms were recorded at 0.1 V·s −1 on a platinum disk electrode. The potentials were measured against the Saturated Calomel Electrode (SCE). The spectroelectrochemical setup was performed in a solution containing NBu4PF6 0.2 M, using a Pt grid as the working electrode, a Pt wire as the counter electrode, and SCE reference electrode. A 3600 spectrophotometer (Shimatzu, Kyoto, Japan) was employed to record the UV-Vis-NIR spectra.

Materials and Methods
General Procedures. All the reactions were performed under an argon atmosphere using standard Schlenk techniques. The solvents were purified and dried by standard methods. The cis-RuCl 2 (dppe) 2 and trimethylsilylprotected TTF 1 were synthesized according to procedures laid out in the literature [16][17][18]. NMR spectra were recorded on a AV300III spectrometer (Bruker, Billerica, MA, USA). Chemical shifts were reported in ppm and 1 H NMR spectra were referenced to residual CHCl 3 (7.26 ppm). 31 P NMR spectra were referenced to H 3 PO 4 . The 13 C NMR spectra of complexes trans-[Hg(-C≡C-Me 3 TTF) 2 ] and trans-[Hg(-C≡C-C≡C-Me 3 TTF) 2 ] could not be obtained due to their low solubility. Mass spectra were recorded by the Centre Régional de Mesures Physiques de l'Ouest, Rennes. Cyclic voltammetry investigations were carried out on a 10 −3 M solution containing 0.1 M NBu 4 PF 6 as a supporting electrolyte. Voltammograms were recorded at 0.1 V·s −1 on a platinum disk electrode. The potentials were measured against the Saturated Calomel Electrode (SCE). The spectroelectrochemical setup was performed in a solution containing NBu 4 PF 6 0.2 M, using a Pt grid as the working electrode, a Pt wire as the counter electrode, and SCE reference electrode. A 3600 spectrophotometer (Shimatzu, Kyoto, Japan) was employed to record the UV-Vis-NIR spectra.

Crystallography
Data were collected on an APEXII, Bruker-AXS diffractometer, for complexes 5 and 9, Mo Kα radiation (λ = 0.71073 Å). The structures were solved by direct methods using the SIR97 program [39], and then refined with full-matrix least-square methods based on F 2 (SHELXL-97) [40] with the aid of the WINGX program [41]. All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. H-atoms were finally included in their calculated positions. Crystallographic data on X-ray data collection and structure refinements are given in Table 4. The X-ray crystallographic data for structural analysis reported in this article have been deposited in the Cambridge Crystallographic Data Center, under the deposition numbers CCDC 1834591-1834592. These data can be obtained free of charge from the Cambridge Crystallographic Data Center via http://www.ccdc.cam.ac.uk/data_ request/cif.

Conclusions
In summary, we have synthesized four new complexes where either two TTFs are connected by an organometallic linkage (M = Hg, Pt) or one TTF is linked through a butadiynyl linker to a RuCl(dppe) 2 fragment.
These four complexes, trans-[Hg(C≡CMe 3 TTF) 2 ], trans-[Hg(C≡C-C≡CMe 3 TTF) 2 ], trans-[Pt(C≡C-C≡CMe 3 TTF) 2 (PPh 3 ) 2 ] and trans-[RuCl(C≡C-C≡CMe 3 TTF)(dppe) 2 ] complete a series of previously reported metal (Pt, Ru) bis(alkynyl-tetrathiafulvalene) complexes. The electrochemical and spectroelectrochemical investigations carried out on these novel complexes have been analyzed and compared. Cyclic voltammetry experiments allowed us to evaluate the extent of the interaction between the TTFs depending on the nature of the bridging metal (Hg, Pt, Ru) and the length of the conjugated organic linker. No modification of the redox potentials of the TTFs within the mercury complexes has been observed. This indicates that the two TTFs behave as if they were independent. Concerning the platinum complexes, no evidence of electronic communication between the two peripheral TTFs was shown. However, within the platinum complexes, the organometallic fragment through the conjugated linker induces an increase of the electron density on the TTF cores due to the electronic interaction between the metal center and the TTFs. The ruthenium complexes are the only ones where mutual strong electronic interactions between the TTF and the Ru atom have been demonstrated, together with electronic coupling between the two covalently linked TTFs, and the extent of the interaction decreasing with the length of the conjugated bridge. Further investigations on these complexes will be devoted to the elaboration of the oxidized species by electrocrystallization.