Extending Metal-Capped Polyynediyl Molecular Wires by Insertion of

Article pubs.acs.org/Organometallics

Extending Metal-Capped Polyynediyl Molecular Wires by Insertion of Inorganic Metal Units Michael I. Bruce,*,† Boris Le Guennic,‡ Nancy Scoleri,† Natasha N. Zaitseva,† and Jean-François Halet*,‡ †

School of Chemistry & Physics, University of Adelaide, Adelaide, South Australia 5005, Australia Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes 1, F-35042 Rennes Cedex, France

‡

S Supporting Information *

ABSTRACT: Several symmetric and asymmetric bis(metalladiynediyl)ruthenium(II) complexes of the general formula trans{LxRu}CCCC{Ru(dppe)2}CCCC{RuL′y} (Lx, L′y = (PPh3)2Cp, (dppe)Cp, (dppe)Cp*), containing Ru(dppe)2 as the central linking group, have been successfully synthesized and characterized spectroscopically. DFT calculations show that their HOMO’s are delocalized over the Ru−C4−Ru−C4−Ru chain, suggesting that there is electronic interaction between the terminal RuLx groups through the C4 chains and the Ru(dppe)2 center. Limited electrochemical measurements reveal that the complexes undergo a series of five stepwise reversible or quasireversible oxidation processes.

■

INTRODUCTION Metal complexes with a π-conjugated carbon bridge1−6 have attracted great attention because of their potential applications in molecular electronics, serving as building units in molecularscale electronic materials and nanotechnological devices.7 Much of this interest has been prompted by their rich redox chemistry and the structural and electronic features of each redox state. The carbon chain length represents one of several fundamental structural variables, and we and others have described various polyynediyl bimetallic redox complexes containing carbon chains that can be extended to over 20 atoms.8 However, previous studies have clearly demonstrated that longer polyynediyl linkers in molecular wires generate (i) poor chemical stability of the oxidized species and (ii) a tendency to decrease the electronic interaction between the termini.8 Introduction of aromatic rings such as benzene, thiophene, and, to a lesser extent, carborane clusters in the polyynediyl spacer constitutes an attractive means to circumvent this instability and eventually to tune their physical properties.2−6,9,10 Compounds containing small carbon atom chains are ideally suited for these studies, and most useful have been the many buta-1,3-diyne-1,4-diyl (C4) complexes {LxM}CCCC{MLx} (where MLx is a redox-active metal−ligand fragment). These have been prepared with different metal−ligand termini containing M = Cr,11 Mo,12,13 W,14,15 Mn,16,17 Re,18,19 Fe,20 Ru,21,22 Os.23 Heterobimetallic examples with M/M′ = Re/ Fe,24 Fe/Ru25 are also known, as are complexes containing different ligand assemblies on Fe, for example.26 Complexes of this type containing bi- or polymetallic end caps have also been described, such as Ru2(ampy)4 (ampy = 2-aminopyridines),27 while in Os3(μ3-η1:η2:η1-FcC2C2Fc)(CO)11 the interaction between the metallocene nuclei through the C4 chain is © 2012 American Chemical Society

modulated by the open-chain metal cluster moiety attached thereto.28 The C4 complexes show stepwise oxidation processes which, depending upon the particular metal−ligand combination, may extend to five interconvertible states [{LxM}C4{MLx}]n+ (n = 0−4). Theoretical studies based on density functional theory (DFT) have shown that the HOMO’s in these complexes are delocalized to a greater or lesser extent over both the metal centers and the C4 bridge.18,22,24,25,29 The nature of M affects the character (carbon vs metal) of the HOMO’s, as shown by calculations on [{M(dpe)Cp}2(μ-C4)]n+ (M = Fe, Ru; dpe = H2PCH2CH2PH2), which for the Fe complexes are more metalcentered while there is a greater weighting from the carbon chain for the Ru derivatives.25 The extremes correspond to “mixed-valence” and “delocalized” systems, electronic interactions occurring between the two metal termini through the conjugated C4 chain in the latter. An alternative approach consists of the synthesis of complexes in which a metal fragment links two metallapolyynediyl groups, which may be the same or different. Various new rigid geometries can then be envisioned for the carbon-rich spacers incorporating such organometallic units, leading to structural variation, which might deeply modify the electronic properties of these molecules. Of interest here is the evaluation of the effect of introducing a central organometallic fragment on the electronic properties and interactions between the terminal MLx groups, i.e., whether the central metal center could be described as acting as a conductor, an amplif ier, or an insulator. For instance, bare transition metals have been inserted Received: February 6, 2012 Published: June 25, 2012 4701

dx.doi.org/10.1021/om3000949 | Organometallics 2012, 31, 4701−4706

Organometallics

Article

Scheme 1. Synthesis of the trans-Ru{CCCC[Ru(PP)Cp′]}2(dppe)2 Complexes

which the two transition-metal fragments MLx and M′L′y are linked by a trans-(−CCCC)2{M″L″z} moiety. The wellknown redox-active end groups Ru(PPh3)2Cp, Ru(dppe)Cp, and Ru(dppe)Cp* (MLx, M′L′y) were chosen.18,20 Complexes of the general formula trans-{LxM}CCCC{Ru(dppe)2}CCCC{M′L′y} were then synthesized and studied to ascertain the degree of interaction between these remote end groups. Two different routes to these complexes were envisaged: (i) double addition of the transition-metal fragments to the central C4−Ru−C4 group to give symmetrical complexes (i.e., where LxM = M′L′y) or (ii) sequential addition of two different groups to give asymmetric complexes (where LxM ≠ M′L′y). The synthesis of symmetric bis(diynediyl) complexes transRu{CCCC(Ru(PP)Cp′)}2(dppe)2 (where (PP)Cp′ = (dppe)Cp*, (dppe)Cp, (PPh3)2Cp) was achieved by the reactions of trans-Ru(CCCCH)2(dppe)2 (1)39 with 2 equiv of RuCl(PP)Cp′ in the presence of an excess of NEt3 and Na[BPh4] (Scheme 1). The reaction mixture was heated in a refluxing mixture of CH2Cl2 and MeOH for 2 h. The presence of the noncoordinating salt Na[BPh4] facilitates the ionization of the Ru−Cl bond, while NEt3 prevents protonation of the diynyl group to reactive unsaturated carbene complexes (vinylidene or butatrienylidene). The symmetric bis(diynediyl) complex trans-Ru{CCCC[Ru(dppe)Cp*]}2(dppe)2 (2) was obtained as a yellow-green powder in 70% yield, while the complexes trans-Ru{CCCC[Ru(dppe)Cp]}2(dppe)2 (3) and trans-Ru{CCCC[RuCp(PPh3)2]}2(dppe)2 (4) were obtained as brown solids in 75% and 77% yields, respectively (Scheme 1). The three complexes each contain three metal centers bridged by buta-1,3-diyne-1,4-diyl chains. They were fully characterized by 1H, 31P, and 13C NMR, IR, ES-MS and elemental microanalyses. The characteristic peaks for the ligands present in the Ru(dppe)Cp*, Ru(dppe)Cp, and Ru(PPh3)2Cp groups are found in the 1H, 31P, and 13C NMR spectra. In the 31P NMR spectra, the singlets at δ ca. 53−57, assigned to Ru(dppe)2 groups, confirm the trans configuration in each case. The carbon atoms of the C4 chains were not observed in the 13C NMR spectra, on account of the low solubility of the complexes in common NMR solvents. However, the IR spectra of the three complexes contain ν(CC) bands between 1969 and 2124 cm−1. Further characterization was obtained from ES-MS, which contained ions corresponding to M+ and fragment ions [Ru(dppe)2]+ (at m/z 898) and [Ru(dppe)Cp*]+ (m/z 635), [Ru(dppe)Cp]+

into the carbon chain, as exemplified by the complex Hg{C CCC(Ru(dppe)Cp*)}2.30 In this example, DFT calculations suggested that there is no Hg contribution to the HOMO which might be depopulated upon oxidation; i.e., the Hg atom acts as an insulator, with the ruthenium termini acting independently. This was confirmed by cyclic voltammetry (CV), which showed that only one oxidation process occurred. Similar situations were found when Pd, Pt, and Cu were incorporated into the bridging carbon chains. For example, oxidation of trans-Pd{CCCC(Re(NO)(PPh 3 )Cp*)}2(PEt3)2 gives a monocation, for which EPR studies showed that the unpaired electron is localized on a rhenium atom.31 In contrast, bis(alkynyl)ruthenium systems such as {cisRu(CCFc) 2(dppm)2}CuI and trans,trans,trans-Ru(C CFc)2(PBu3)2(CO)(L) (L = CO, pyridine, P(OMe)3) show electronic interactions between the terminal ferrocenyl groups, presumably occurring through the central Ru atom.32 Moreover, cyclic voltammetry of trans-Ru(CCCCFc)2(dppe)2 indicates that electronic communication between the two ferrocenyl moieties occurs via the central Ru(dppe)2 fragment.33 In this complex, the strongly donating Ru(dppe)2 group acts as a conductor (even possibly an amplifier), as a result of the excellent overlap between the central Ru d orbitals and the π orbitals of the C4 fragments. Analogously, voltammetric and spectroelectrochemical studies of the oxidized trans-(FcCCCC)2Ru2(Y-DMBA)4(C CCCFc) complexes (Y-DMBA = N,N′-dimethylbenzamidinate or N,N′-dimethyl-(3-methoxy)benzamidinate) show the exceptional ability of the dimeric Ru2(Y-DMBA)4 unit to mediate electron mobility between ferrocenium and ferrocene groups.34 Other complexes of this type include {(depe)2ClFe}C4{W(dppe)2}C4{W(dppe)2}C4{FeCl(depe)2}35 and the cluster-containing complex {(OC)2(μ-But2P)3Pt3}C2Fc′2C2{Pt3(μPBut2)3(CO)2} (Fc2′ = 1′,1′″-(η-C5H4)Fe(η-C5H4)).36 More recently, an extensive study of {(μ-ampy)4Ru2}C4{Ru2(μampy′)4}C4{Ru2(μ-ampy)4} (ampy, ampy′ = 2-aminopyridines) found voltammetric and spectroscopic evidence for delocalization over the Ru2−C4−Ru2−C4−Ru2 system.37 In the course of this work, CV and nonlinear optics studies of the series 1,3,5{[Cp*(dppe)Fe]CCC6H4CC[Ru(dppe)2]CC}3C6H3, together with related DFT studies, have been reported.38

■

RESULTS AND DISCUSSION Syntheses and Characterization. Following these studies, we decided to explore other examples of such complexes in 4702

dx.doi.org/10.1021/om3000949 | Organometallics 2012, 31, 4701−4706

Organometallics

Article

Figure 1. Optimised geometry of 3-H (distances in Å). Ru, P, C, and H atoms are shown in green, purple, gray, and white, respectively.

Figure 2. Molecular orbital diagram and HOMO plots of 3-H (contour values are ±0.025 (e/bohr3)1/2). The Ru(end)/C/Ru(center) atomic percentages in the HOMO’s are as follows: 54ag (26/61/13), 53ag (20/65/15), 51au (42/58/0), and 50au (34/66/0).

(m/z 565), and [Ru(PPh3)nCp]+ (m/z 691 (n = 2), 429 (n = 1)), respectively. Similar reactions of 1 with only 1 equiv of the appropriate chlororuthenium complex afford the asymmetric bis(diynediyl) complexes trans-Ru(CCCCH){CCCC[Ru(dppe)Cp*]}(dppe)2 (5) and trans-Ru(CCCCH){CCC C[Ru(dppe)Cp]}(dppe)2 (6), which were obtained as green solids in 80% and 85% yields, respectively, and trans-Ru(C CCCH){CCCC[Ru(PPh3)2Cp]} (dppe)2 (7), which was synthesized in 85% yield as a brown powder (Scheme 1). These three complexes were also readily identified from their spectroscopic data and elemental analyses. The characteristic peaks for the Ru(dppe)Cp*, Ru(dppe)Cp, Ru(PPh3)2Cp, and Ru(dppe)2 ligands are present in the various IR and NMR spectra. Singlets for the terminal CH hydrogens at δ 1.44, 1.41, and 1.40 are present in the 1H NMR spectra of 5−7, respectively. The IR spectra of the three complexes each contain two ν(CC) bands at 2022, 1968 cm−1 (for 5) or between 1995 and 1896 cm−1 (6 and 7) and a ν(CH) band at ca. 3055 cm−1. The ES-MS contain strong M+ ions, together with the appropriate [Ru(PP)Cp′]+ fragment ions. The terminal hydrogen atom in the bimetallic complexes trans-Ru(CCCCH){CCCC(Ru(PP)Cp′)}(dppe)2

(where (PP)Cp′ = (dppe)Cp* (5), (dppe)Cp (6), (PPh3)2Cp (7)) can be readily replaced by different end groups, giving a route to asymmetric trinuclear complexes (Scheme 1). Thus, the reaction of 5 or 7 with 1 equiv of RuCl(dppe)Cp in the presence of an excess of NEt3 and Na[BPh4] afforded the asymmetric trimetallic complexes trans-Ru{CCCC[Ru(dppe)Cp*]}{CCCC[Ru(dppe)Cp]}(dppe)2 (8) and trans-Ru{CCCC[Ru(PPh 3 ) 2 Cp]}{CCCC[Ru(dppe)Cp]}(dppe)2 (9) in 77% and 78% yields, respectively. These complexes were fully characterized by 1H, 31P, and 13C NMR, IR, ES-MS, and microanalysis. In the NMR spectra, the characteristic peaks for the Ru(dppe)2, Ru(dppe)Cp*, Ru(dppe)Cp, and Ru(PPh3)2Cp ligands were present for both complexes. The infrared spectra of complexes 8 and 9 show ν(CC) bands at ca. 2015−2020 cm−1 and at 1961 and 1888 cm−1, respectively, but the ν(CH) bands are no longer present. The ES-MS of 8 and 9 show the fragment ions for the different Ru(PP)Cp′ groups present, together with [M − H]+ and [M − Ru(PP)Cp′]+ ions. For 8, only [M − Ru(dppe)Cp]+ (m/z 1629) is found, whereas for 9, both [M − Ru(dppe)Cp]+ (m/z 1685) and [M − Ru(PPh3)2Cp]+ (m/z 1559) are present, suggesting that the C−Ru(dppe)Cp* bond is somewhat stronger than the C−Ru(PP)Cp bonds (PP = dppe, 4703

dx.doi.org/10.1021/om3000949 | Organometallics 2012, 31, 4701−4706

Organometallics

Article

Table 1. Electrochemical Data for 1−4, 8, and 9 and Related Compounds oxidn potential/V (ia/ic)a complex 1 2 3 4 8 9 {Ru(dppe)Cp*}2(μ-C4) {Ru(dppe)Cp}2(μ-C4) {Ru(PPh3)2Cp}2(μ-C4) {Ru(dppe)Cp*}2(μ-C6) {Ru(dppe)Cp*}2(μ-C8)

E1 −0.28 −0.72 −0.70 −0.64 −0.70 −0.52 −0.43 −0.24 −0.23 −0.15 +0.08

(1.0) (1.0) (1.0) (1.0) (1.0)

E2 +0.41 −0.33 −0.25 −0.12 −0.34 −0.14 +0.22 +0.35 +0.41 +0.33 +0.43

(1.0) (1.0) (1.0) (1.0) (1.0)

E3 +1.11 +0.21 +0.47 +0.49 +0.15 +0.46 +1.04 +1.08 +1.03 +1.05 +1.07

E4

(0.85) (0.9) (1.0) (1.0) (1.0)

E5

+1.73 +0.70 (0.6) +0.76 (0.7) +0.79 (0.8) +0.59 (0.8) +0.73 (0.8) +1.54c +1.44 +1.68 +1.33 +1.27

+0.99 (−) +1.19 (−) +1.26 (−) +0.83 (0.6) +1.15 (−-)

ΔE1/2b 0.69 0.38 0.45 0.52 0.35 0.38 0.65 0.59 0.64 0.48 0.35

ref 39 this this this this this 41 42 21 42 42

work work work work work

Recorded with ca. 10−4 M solutions in CH2Cl2 also containing 10−1 M [NBu4]BF4 in a gastight single-compartment three-electrode cell equipped with Pt working, coiled Pt wire auxiliary, and Pt wire pseudo reference electrodes at scan rates of 50−800 mV s−1. Potentials are vs SCE, with internal reference FeCp2/[FeCp2]+ = 0.46 V. bΔE1/2 = E2 − E1. cIrreversible. a

Electrochemistry. Our objective was then to investigate the electrochemical behavior of these new bis{(diynediyl)Ru} complexes by using cyclic voltammetry to test DFT predictions and determine the degree of electronic communication through the Ru(dppe)2 moiety. Cyclic voltammograms of some of the complexes described above were measured under similar conditions in CH2Cl2 (Table 1; see the Supporting Information for details). In most cases, high-potential events were observed close to the solvent front (ca. +1.7 V for CH2Cl2; solvents with higher potentials were not used): in these cases, it was not possible to determine the precise nature of the processes occurring. Up to five waves are found in the CVs of complexes 2−4, 8 and 9, for events which are fully or quasi-reversible (ia/ic ≤ 1), although the intensities of the waves at highest potentials cannot be determined with confidence (Figure 3). They must relate to loss of electrons from their HOMO’s, as suggested by the DFT results discussed above.

(PPh3)2). For this whole series of complexes, all attempts to obtain crystals suitable for X-ray crystallography were unfortunately unsuccessful. Theoretical Studies. The utility of DFT calculations has been previously demonstrated for calculating, with high confidence, geometries of molecules for which X-ray data are not available. DFT geometry optimization was then performed on the model complex trans-Ru{CCCC[Ru(dpe)Cp]}2(dpe)2 (3-H; dpe = H2PCH2CH2PH2) (Figure 1) as representative of complexes 2−4, 8, and 9 (see the Supporting Information for the computational details). The pertinent metric data shown in Figure 1 compare well with those computed or experimentally measured in related compounds.21,23 The electronic properties of 3-H were examined to see if these complexes could be possible models for molecular wires. The HOMO′s (54ag, 53ag, 51au, and 50au) consist of four closely spaced orbitals separated from the LUMO by 1.60 eV and, to a lesser extent, from the rest of the occupied MO’s by ca. 1.0 eV (Figure 2). From the contour plots of these HOMO’s, it can be seen that, in all cases, these orbitals extend across the entire 11-atom Ru−C−C−C−C− Ru−C−C−C−C−Ru arrays in these molecules. The central 9atom chain contributes 74 and 80% to the HOMO and HOMO-1, while the end group metal centers contribute 13 and 10%, respectively, while making up a larger proportion of the lower-lying orbitals. An important consequence of this is that any oxidation process will affect the entire molecule.29,40 This lends credence to the original concept of using complexes of this type as model systems for molecular-scale wires. Moreover, in light of these results, it can be suggested that several oxidation waves should be observed in the CVs of 2−4, 8, and 9 corresponding to the loss of up to a maximum of eight electrons which occupy these HOMO’s in the neutral complex. They are sufficiently isolated from the rest of the occupied MO’s to prevent any MO reordering upon oxidation. Since these MO’s are extensively delocalized, it is not appropriate to assign the individual oxidation processes specifically to the central metal, the terminal metals, or the C4 bridge. Indeed, they will affect the whole backbone, as shown by the optimized geometries of the cation and dication, which indicate some shortening and lengthening of Ru−C and C−C bonds with respect to the neutral molecule (see Table S1 in the Supporting Information).

Figure 3. Cyclic voltammogram of 2.

As anticipated from earlier theoretical studies,21,24,25 the ease of oxidation follows the incorporation of more electrondonating ligands around the terminal Ru atoms: 2 > 3 > 4. Comparison of the electronic properties of 2, for instance, with related binuclear complexes {Cp*(dppe)Ru}2(μ-CC)x (x = 2−4)21,41,42 shows that ΔE1/2 for 2 is between those of {Cp*(dppe)Ru}2(μ-CC)3 (480 mV) and {Cp*(dppe)Ru}2(μ-CC)4 (350 mV). Higher ΔE1/2 values were even measured for 3 (450 mV) and 4 (520 mV). On this basis, it 4704

dx.doi.org/10.1021/om3000949 | Organometallics 2012, 31, 4701−4706

Organometallics

Article

CH2CH2); 13C NMR (CD2Cl2) δ 136.27−129.37 (m, Ph), 83.85 (s, Cp), 29.09−28.34 (m, CH2CH2); 31P NMR (CDCl3) δ 80.7 (s, Ru(dppe)Cp); 56.5 (s, Ru(dppe)2); ES-MS (MeCN, m/z) 2123 (M+), 2122 ([M − H]+), 898 ([Ru(dppe)2]+), 606 ([Ru(NCMe)(dppe)Cp]+), 565 ([Ru(dppe)Cp]+). Anal. Calcd for C122H106P8Ru3: C, 69.02; H, 5.03. Found: C, 69.05; H, 5.09. trans-Ru{CCC C[Ru(PPh3)2Cp]}2(dppe)2 (4): IR (CH2Cl2, cm−1) ν(CC) 2014 (w), 1979 (w); 1H NMR (CDCl3 δ 7.71−6.93 (m, 110H, Ph), 4.36 (s, 10H, Cp), 2.31−2.19, 1.93−1.80 (2m, 8H, CH2CH2); 13C NMR (CD2Cl2) δ 133.88−127.14 (m, Ph), 83.10 (s, Cp), 30.09−29.56 (m, CH2CH2); 31P NMR (CDCl3) δ 57.0 (s, Ru(dppe)2), 43.0 (s, Ru(PPh3 )2); ES-MS (MeCN, m/z) 2375 (M+), 1685 ([Ru(PPh3)2CpC4Ru(dppe)2C4]+), 898 ([Ru(dppe)2]+), 691 ([Ru(PPh 3 ) 2 Cp] + ), 429 ([Ru(PPh 3 )Cp] + ). Anal. Calcd (for C142H118P8Ru3: C, 71.80; H, 5.01. Found: C, 71.84; H, 5.04. transRu{CCCC[Ru(dppe)Cp*]}{CCCC[Ru(dppe)Cp]}(dppe)2 (8): IR (Nujol, cm−1) ν(CC) 2021 (m), 1961 (m); 1H NMR (CDCl3) δ 7.88−6.90 (m, 80H, Ph), 4.59 (s, 5H, Cp), 2.98− 2.84, 2.24−2.16 (m, 16H, CH2CH2), 1.45 (s, 15H, Cp*); 13C NMR (CD2Cl2) δ 134.82−127.03 (m, Ph), 99.92 (s, C5Me5), 81.76 (s, Cp), 30.82−29.97 (m, CH2CH2), 9.89 (s, C5Me5); 31P NMR (CDCl3) δ 80.7 (s, Ru(dppe)Cp), 76.4 (s, Ru(dppe)Cp*), 55.6 (s, Ru(dppe)2); ES-MS (MeCN, m/z) 2194 ([M + H]+), 1629 ([M − Ru(dppe)Cp]+), 898 ([Ru(dppe)2]+), 635 ([Ru(dppe)Cp*]+), 675 ([Ru(NCMe)(dppe)Cp*]+), 565 ([Ru(dppe)Cp]+), 605 ([Ru(NCMe)(dppe)Cp]+). Anal. Calcd for C127H116P8Ru3: C, 69.45; H, 5.33. Found: C, 68.98; H, 5.33. trans-Ru{CCCC[Ru(PPh3)2Cp]}{C CCC[Ru(dppe)Cp]}(dppe)2 (9): IR (Nujol, cm−1) ν(CC) 2017 (m), 1888 (w); 1H NMR (CDCl3) δ 7.87−6.93 (m, 75H, Ph), 2.71− 2.57, 2.42−2.37 (2m, 12H, CH2CH2), 4.58 (s, 5H, Cp), 4.32 (s, 5H, Cp); 13C NMR (CD2Cl2) δ 135.47−128.56 (m, Ph), 81.25 (s, Cp), 80.93 (s, Cp), 30.10−29.20 (m, CH2CH2); 31P NMR (CDCl3) δ 80.7 (s, Ru(dppe)Cp), 55.9 (s, Ru(dppe)2), 42.9 (s, Ru(PPh3)2); ES-MS (MeCN, m/z) 1685 ([Ru(PPh3)2CpC4Ru(dppe)2C4]+), 1559 ([Ru(dppe)CpC4 Ru(dppe) 2 C4 ] + ), 898 ([Ru(dppe) 2] +), 690 ([Ru(PPh3)2Cp]+), 605 ([Ru(NCMe)(dppe)Cp]+), 565 ([Ru(dppe)Cp]+), 429 ([Ru(PPh3)Cp]+). Anal. Calcd for C132H112P8Ru3: C, 70.39; H, 5.02. Found: C, 69.81; H, 5.37.

may be suggested that insertion of a Ru(dppe)2 unit into the C8 chain reflects the ability of stronger donors to increase the stabilities of the dications. The CVs of 8 and 9 also show five diffusion-controlled oxidation waves at −0.70, −0.34, +0.15, +0.59 and +0.83 V and −0.52, −0.14, +0.46, +0.73 and +1.15 V, respectively. It is not surprising that the oxidation potentials fall between those of the analogous symmetrical complexes, as also found earlier for {Cp(PPh 3 ) 2 Ru}(CCCC){Ru(dppe)Cp} and {Ru(PPh3)2Cp}(CCCC){Ru(dppe)Cp*}, which were found to have E1/2 values comparable to those of {Ru(dppe)Cp*}2(μCCCC), {Ru(dppe)Cp}2(μ-CCCC), and {Ru(PPh3)2Cp}2(μ-CCCC) (Table 1).21 For comparison, the CVs of the asymmetric bis(diynediyl) complexes transRu(CCCCH){CCCC[Ru(dppe)Cp*]}(dppe)2 (5) and trans-Ru(CCCCH){CCCC[Ru(dppe)Cp]}(dppe)2 (6) each show two diffusion-controlled, partially reversible waves. As expected, 5 has a lower oxidation potential than 1 or 2. All complexes described above have ΔE1/2 values between 310 and 670 mV. The electrochemical data reported here support the theoretical conclusion that these molecules have a delocalized electronic structure over the Ru−C4−Ru−C4−Ru chain, with the central C4−Ru−C4 entity as well as the end groups largely determining the nature of the HOMO’s which are emptied upon oxidation. This is somewhat similar to the case for bimetallic systems previously studied, containing Ru−(C C)x−Ru and Os−(CC)x−Os, in which both the carbon chain and the terminal MLx groups strongly contribute to the HOMO’s.21,23 There are several reports in the literature which caution against linking ΔE1/2 values directly with the degree of electronic communication/interaction between metal end groups in carbon-rich ligand-bridged systems.9,43−46 Interpretation of the CV data obtained for the above complexes may be complicated by solvation, ion pairing, and electrostatic effects in addition to the degree of delocalization across the molecule. Further studies of these interesting systems are warranted before final conclusions can be arrived at.

■

* Supporting Information

Text, tables, and figures giving full details of syntheses, electrochemical measurements, and DFT computational details and pertinent metric data, bonding energies, and Cartesian coordinates of the geometry-optimized model complex transRu{CCCC[Ru(dHpe)Cp]}2(dHpe)2 (3-H). This material is available free of charge via the Internet at http://pubs.acs. org.

■

CONCLUSION In summary, several symmetric and asymmetric bis(metalladiynediyl)ruthenium(II) complexes containing Ru(dppe)2 as the central linking group have been efficiently synthesized and characterized spectroscopically and theoretically. DFT studies show that these complexes are delocalized along the 11-atom Ru−C4−Ru−C4−Ru chain. Electrochemical studies reveal that they undergo a series of five stepwise oxidation processes, with the stabilities of the dications increasing with the electrondonating power of the end groups.

■

ASSOCIATED CONTENT

S

■

AUTHOR INFORMATION

Corresponding Author

*M.I.B.: tel, +61 8 8303 5939; fax, +61 8 8303 4358; e-mail, [email protected]. J.-F.H.: tel, +33 2 2323 6778; fax, +33 2 2323 6840; e-mail, [email protected]. Notes

EXPERIMENTAL SECTION

The authors declare no competing financial interest.

■

Characterization Data. trans-Ru{CCCC[Ru(dppe)Cp*]}2(dppe)2 (2): IR (CH2Cl2, cm−1) ν(CC) 2012 (m), 1969 (w); 1H NMR (CDCl3) δ 7.72−7.04 (m, 80H, Ph), 2.60−2.55, 1.91− 1.82 (2m, 16H, CH2CH2), 1.46 (s, 30H, C5Me5); 13C NMR (CD2Cl2) δ 135.46−129.61 (m, Ph), 97.80 (s, C5Me5), 31.49−31.15 (m, CH2CH2), 11.53 (s, C5Me5); 31P NMR (CDCl3) δ 76.3 (s, dppeRuCp*), 53.4 (s, Ru(dppe)2); ES-MS (MeOH, m/z) 2266 (M+), 898 ([Ru(dppe) 2 ] + ), 635 ([Ru(dppe)Cp*] + ). Anal. Calcd for C132H126P8Ru3: C, 70.05; H, 5.61. Found: C, 70.01; H, 5.65. transRu{CCCC[Ru(dppe)Cp]}2(dppe)2 (3): IR (CH2Cl2, cm−1) ν(CC) 2124 (m), 2013 (w); 1H NMR (CDCl3) δ 7.67−7.16 (m, 60H, Ph), 4.35 (s, 10H, Cp), 2.23−2.19, 1.91−1.79 (2m, 16H,

ACKNOWLEDGMENTS These studies were facilitated by travel grants (ARC, Australia; CNRS, France). Prof. S. Rigaut (Rennes) is acknowledged for helpful discussions and a reviewer for pertinent comments.

■

REFERENCES

(1) Lang, H. Angew. Chem., Int. Ed. Engl. 1994, 33, 547−550. (2) (a) Paul, F.; Lapinte, C. Coord. Chem. Rev. 1998, 178−180, 427− 505. (b) Paul, F.; Lapinte, C. Magnetic communication in binuclear 4705

dx.doi.org/10.1021/om3000949 | Organometallics 2012, 31, 4701−4706

Organometallics

Article

(22) Gao, L.-B.; Zhang, L.-Y.; Shi, L.-X.; Chen, Z.-N. Organometallics 2005, 24, 1678. (23) (a) Bruce, M. I.; Costuas, K.; Davin, T.; Halet, J.-F.; Kramarczuk, K. A.; Low, P. J.; Nicholson, B. K.; Perkins, G. J.; Roberts, R. L.; Skelton, B. W.; Smith, M. E.; White, A. H. Dalton Trans. 2007, 5387−5399. (b) Bruce, M. I.; Kramarczuk, K. A.; Skelton, B. W.; White, A. H. J. Organomet. Chem. 2010, 695, 469−473. (24) (a) Paul, F.; Meyer, W. E.; Toupet, L.; Jiao, H.; Gladysz, J. A.; Lapinte, C. J. Am. Chem. Soc. 2000, 122, 9405−9414. (b) Jiao, H.; Costuas, K.; Gladysz, J. A.; Halet, J.-F.; Guillemot, M.; Toupet, L.; Paul, F.; Lapinte, C. J. Am. Chem. Soc. 2003, 125, 9511−9522. (25) Bruce, M. I.; Costuas, K.; Davin, T.; Ellis, B. G.; Halet, J.-F.; Lapinte, C.; Low, P. J.; Smith, M. E.; Skelton, B. W.; Toupet, L.; White, A. H. Organometallics 2005, 24, 3864−3881. (26) Coat, F.; Guillevic, M.-A.; Toupet, L.; Paul, F.; Lapinte, C. Organometallics 1997, 16, 5988−5998. (27) Ren, T.; G. Zou, G.; Alvarez, J. C. Chem. Commun. 2000, 1197− 1198. (28) (a) Adams, R. D.; Qu, B. Organometallics 2000, 19, 2411−2413. (b) Adams, R. D.; Qu, B.; Smith, M. D.; Albright, T. A. Organometallics 2002, 21, 2970−2978. (29) Costuas, K.; Rigaut, S. Dalton Trans. 2011, 40, 5643−5658. (30) Bruce, M. I.; Halet, J.-F.; Le Guennic, B.; Skelton, B. W.; Smith, M. E.; White, A. H. Inorg. Chim. Acta 2003, 350, 175−181. (31) Weng, W.; Bartik, T.; Brady, M.; Bartik, B.; Ramsden, J. A.; Arif, A. M.; Gladysz, J. A. J. Am. Chem. Soc. 1995, 117, 11922−11931. (32) Zhu, Y.; Clot, O.; Wolf, M. O.; Yap, G. P. A. J. Am. Chem. Soc. 1998, 120, 1812−1821. (33) Lebreton, C.; Touchard, D.; Le Pichon, L.; Daridor, A.; Toupet, L.; Dixneuf, P. H. Inorg. Chim. Acta 1998, 272, 188−196. (34) Xu, G.-L.; Crutchley, R. J.; De Rosa, M. C.; Pan, Q.-J.; Zhang, H.-X.; Wang, X. T.; Ren, T. J. Am. Chem. Soc. 2005, 127, 13354− 13363. (35) Semenov, S. N.; Taghipourian, S. F.; Blacque, O.; Fox, T.; Venkatesan, K.; Berke, H. J. Am. Chem. Soc. 2010, 132, 7584−7585. (36) Albinati, A.; Balzano, F.; Fabrizi de Biani, F.; Leoni, P.; Manca, G.; Marchetti, L.; Rizzato, S.; Barretta, G. U.; Zanello, P. Inorg. Chem. 2010, 49, 3714−3720. (37) Ying, J.-W.; Liu, I. P.-C.; Xi, B.; Song, Y.; Campana, C.; Zuo, J.L.; Ren, T. Angew. Chem., Int. Ed. 2010, 49, 954−957. (38) Gauthier, N.; Argouarch, G.; Paul, F.; Toupet, L.; Ladjarafi, A.; Costuas, K.; Halet, J.-F.; Samoc, M.; Cifuentes, M.; Corkery, T. C.; Humphrey, M. G. Chem. Eur. J. 2011, 17, 5561−5577. (39) (a) Rigaut, S.; Perruchon, J.; Le Pichon, L.; Touchard, D.; Dixneuf, P. H. J. Organomet. Chem. 2003, 670, 37−44. (b) Rigaut, S. Personal communication. (40) Olivier, C.; Costuas, K.; Choua, S.; Maurel, V.; Turek, P.; Saillard, J.-Y.; Touchard, D.; Rigaut, S. J. Am. Chem. Soc. 2010, 132, 5638−5651. (41) Bruce, M. I.; Ellis, B. G.; Low, P. J.; Skelton, B. W.; White, A. H. Organometallics 2003, 22, 3184−3198. (42) Bruce, M. I.; Ellis, B. G.; Smith, M. E. Unpublished results. (43) (a) Barrière, F.; Geiger, W. E. J. Am. Chem. Soc. 2006, 128, 3980−3989. (b) Geiger, W. E.; Barrière, F. Acc. Chem. Res. 2010, 43, 1030−1039. (44) (a) Low, P. J.; Brown, N. J. J. Cluster Sci. 2010, 21, 235−278. (b) Schauer, P. A.; Low, P. J. Eur. J. Inorg. Chem. 2012, 390−411. (45) D’Alessandro, D. M.; Keene, F. R. Chem. Soc. Rev. 2006, 35, 424−440. (46) Evans, D. Chem. Rev. 2008, 108, 2113−2144.

organometallic complexes mediated by carbon-rich bridges. In Unusual Structures and Physical Properties in Organometallic Chemistry; Gielen, M., Willem, R., Wrackmeyer, B., Eds.; Wiley: London, 2002; pp 220− 291. (3) Szafert, S.; Gladysz, J. A. Chem. Rev. 2003, 103, 4175−4206; Chem. Rev. 2006, 106, PR1−PR33. (4) Long, N. J.; Williams, C. K. Angew. Chem., Int. Ed. 2003, 42, 2586−2617. (5) (a) Bruce, M. I.; Low, P. J. Adv. Organomet. Chem. 2004, 50, 179−444. (b) Bruce, M. I. Coord. Chem. Rev. 2004, 248, 1603−1625. (6) Ren, T. Organometallics 2005, 24, 4854−4870. (7) (a) Low, P. J. Dalton Trans. 2005, 2821−2824. (b) Tao, N. J. Nature Nanotechnol. 2006, 1, 173−181. (8) See for example: (a) Bartik, T.; Bartik, B.; Brady, M.; Dembinski, R.; Gladysz, J. A. Angew. Chem., Int. Ed. Engl. 1996, 35, 414−417. (b) Sakurai, A.; Akita, M.; Moro-oka, Y. Organometallics 1999, 18, 3241−3244. (c) Rigaut, S.; Perruchon, J.; Le Pichon, L.; Touchard, D.; Dixneuf, P. H. J. Organomet. Chem. 2003, 670, 37−44. (d) Xu, G.-L.; Zou, G.; Ni, Y.-H.; De Ross, M. C.; Crutchley, R. J.; Ren, T. J. Am. Chem. Soc. 2003, 125, 10057−10065. (e) Antonova, A. B.; Bruce, M. I.; Ellis, B. G.; Gaudio, M.; Humphrey, P. A.; Jevric, M.; Melino, G.; Nicholson, B. K.; Perkins, G. J.; Skelton, B. W.; Stapleton, B.; White, A. H.; Zaitseva, N. N. Chem. Commun. 2004, 960−961. (f) Qi, H.; Gupta, A.; Noll, B. C.; Snider, G. L.; Lu, Y.; Lent, C.; Fehlner, T. P. J. Am. Chem. Soc. 2005, 127, 15218−15227. (9) Lapinte, C. J. Organomet. Chem. 2008, 693, 793−801. (10) See for example: (a) Fox, M. A.; Roberts, R. L.; Baines, T. E.; Le Guennic, B.; Halet, J.-F.; Hartl, F.; Yufit, D. S.; Albesa-Jové, D.; Howard, J. A. K.; Low, P. J. J. Am. Chem. Soc. 2008, 130, 3566−3578. (b) Khairul, W. M.; Fox, M. A.; Schauer, P. A.; Yufit, D. S.; AlbesaJové, D.; Howard, J. A. K.; Low, P. J. Dalton Trans. 2010, 39, 11605− 11615. (c) Fitzgerald, E. C.; Ladjarafi, A.; Brown, N. J.; Collison, D.; Costuas, K.; Edge, R.; Halet, J.-F.; Justaud, F.; Low, P. J.; Meghezzi, H.; Roisnel, T.; Whiteley, M. W.; Lapinte, C. Organometallics 2011, 30, 4180−4195. (d) Fox, M. A.; Le Guennic, B.; Roberts, R. L.; Brue, D. A.; Yufit, D. S.; Howard, J. A. K.; Manca, G.; Halet, J.-F.; Hartl, F.; Low, P. J. J. Am. Chem. Soc. 2011, 133, 18433−18446. (e) Costuas, K.; Cador, O.; Justaud, F.; Le Stang, S.; Paul, F.; Monari, A.; Evangelisti, S.; Toupet, T.; Lapinte, C.; Halet, J.-F. Inorg. Chem. 2011, 50, 12601− 12622. (11) Egler-Lucas, C.; Blacque, O.; Venkatesan, K.; López-Hernández, A.; Berke, H. Eur. J. Inorg. Chem. 2012, 1536−1545. (12) Woodworth, B. E.; White, P. S.; Templeton, J. L. J. Am. Chem. Soc. 1997, 119, 828−829. (13) Fitzgerald, E. C.; Brown, N. J.; Edge, R.; Helliwell, M.; Roberts, H. R.; Tuna, F.; Beeby, A.; Collison, D.; Low, P. J.; Whiteley, M. W. Organometallics 2012, 31, 157−169. (14) Roberts, R. L.; Puschmann, H.; Howard, J. A. K.; Yamamoto, J.; Carty, A. J.; Low, P. J. Dalton Trans. 2003, 1099−1105. (15) Semenov, S. N.; Blacque, O.; Fox, T.; Venkatesan, K.; Berke, H. J. Am. Chem. Soc. 2010, 132, 3115−3127. (16) Kheradmandan, S.; Heinze, K.; Schmalle, H. W.; Berke, H. Angew. Chem., Int. Ed. 1999, 38, 2270−2273. (17) Venkatesan, K.; Fox, T.; Schmalle, H. W.; Berke, H. Organometallics 2005, 24, 2834−2847. (18) (a) Zhou, Y.; Sewyler, J. W.; Weng, W.; Arif, A. M.; Gladysz, J. A. J. Am. Chem. Soc. 1993, 115, 8509. (b) Brady, M.; Weng, W.; Zhou, Y.; Seyler, J. W.; Amoroso, A. J.; Arif, A. M.; M. Böhme, M.; Frenking, G.; Gladysz, J. A. J. Am. Chem. Soc. 1997, 119, 775−788. (19) Yam, V.W.-W.; Lau, V.C.-Y.; Cheung, K.-K. Organometallics 1996, 15, 1740−1748. (20) (a) Le Narvor, N.; Toupet, L.; Lapinte, J. J. Am. Chem. Soc. 1995, 117, 7129−7138. (b) Akita, M.; Chung, M.-C.; Sakurai, A.; Sugimoto, S.; Terada, M.; Tanaka, M.; Moro-oka, Y. Organometallics 1997, 16, 4882−4888. (21) (a) Bruce, M. I.; Low, P. J.; Costuas, K.; Halet, J.-F.; Best, S. P.; Heath, G. A. J. Am. Chem. Soc. 2000, 122, 1949−1962. (b) Bruce, M. I.; Ellis, B. G.; Skelton, B. W.; White, A. H. J. Organomet. Chem. 2005, 690, 792−801. 4706

dx.doi.org/10.1021/om3000949 | Organometallics 2012, 31, 4701−4706