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Bimetallic Organometallic Compounds of σ-gem-Diethynylethene (gem-DEE) Ligands: trans-Ru2(DMBA)4(gem-DEE)2 William P. Forrest, Zhi Cao, Phillip E. Fanwick, Kerry M. Hassell, and Tong Ren* Department of Chemistry, Purdue University, West Lafayette, Indiana 47906, United States
bS Supporting Information ABSTRACT: Reported herein are the preparation and characterization of trans-Ru2(DMBA)4(gem-DEE)2 compounds 1a/1b (gem-DEE = σ-geminal-diethynylethene), the first examples of redox active metal complexes containing gem-DEE ligands. DFT calculations revealed a significant mixing between the π(DEE) and π*/π(Ru2) orbitals in both the HOMO and HOMO-1 of the model compound 10 .
T
ransition-metal compounds containing linear arylacetylide and polyynyl ligands have been studied for several decades with efforts focusing on the linear π-conjugation therein,1 and their promise as molecular electronic wires2,3 and opto-electronic materials.4 Efficient charge transfer has been documented for polyynyl complexes of metal centers such as Fe, Re, and Ru.5 In particular, electrical conductivity at the single- or few-molecule level has been demonstrated using nanojunction techniques.6 In comparison, metal compounds with acetylides of nonlinear π-conjugation have received much less attention. Nevertheless, the rapid progress in the synthesis of nonlinear ene-yne and ene-diyne scaffolds7,8 makes it possible to explore branched metal acetylides based on these scaffolds. Organic compounds based on ene-yne and ene-diyne scaffolds exhibit interesting nonlinear optical properties,9 and those containing the gem-diethynylethene unit (also known as iso-triacetylene and abbreviated as gem-DEE) are interesting to physical organic chemists because of the crossconjugation therein. 8 It has been suggested recently on the basis of theoretical computation that oligomers of gem-DEE may function as molecular wires with conductance modulated by quantum interference effects.10 We hypothesize that the combination of a redox-rich transition metal and an ene-diyne scaffold will facilitate the exploration of the intricate electronic properties such as cross-conjugation and quantum interference effects, and may result in enhanced optoelectronic responses as well. However, transitionmetal compounds with ene-yne or ene-diyne as σ-acetylide ligands are relatively rare, and ones based on gem-DEE are strictly limited to Pt(II) species.11 It is worth noting that Pt(II) and Au(I) complexes of a ligand related to gem-DEE, tetraethynylethene (TEE), have been documented as well.12 Reported in this contribution are the preparation and characterization of the Ru2(DMBA)4 compounds r 2011 American Chemical Society
Scheme 1. Ru2(DMBA)4 Complexes of gem-DEE Ligandsa
a
Conditions: (i) K2CO3, MeOH/THF; (ii) Ru2(DMBA)4(NO3)2, Et2NH, THF, room temperature, 3 h.
(DMBA = N,N0 -dimethylbenzamidinate) 1a,b, containing gemDEE as the axial acetylide ligands (Scheme 1). As shown in Scheme 1, the initial attempt of synthesis started with 1-(trimethylsilyl)-3-((trimethylsilyl)ethynyl)-4-methylpent-3-en-1-yne,13 which was converted to 3-ethynyl-4-methylpent-3-en-1-yne (La) upon desilylation using K2CO3. Subsequently, Ru2(DMBA)4(NO3)214 reacted with La in the presence of Et3N (weakly basic conditions) to yield a red solution that is characteristic of Ru2(DMBA)4(C2R)2 type compounds.15 TLC analysis, however, revealed that in addition to a red species (Rf = 0.72, ethyl acetate/hexanes 1/3 v/v), there is a long reddish brown Received: January 13, 2011 Published: March 29, 2011 2075
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(average 19.2). Two gem-DEE moieties are approximately coplanar with the framework defined by the N1- - -N2- - -N4- - N3 linkage. As shown in Figure 2, compounds 1a,b both undergo a reversible one-electron oxidation and a reversible one-electron reduction. While this behavior is similar to that of Ru2(DMBA)4(C2TIPS)2 (2, a new compound prepared according to the literature procedure for a closely related compound19), the E1/2(A) value (0.50 V) is cathodically shifted by ca. 0.09 V from that of 2 (0.59 V), reflecting the stronger donor nature of gem-DEE. Unlike 2, compounds 1a,b also exhibit an irreversible reduction C at ca. 1.5 V, which is likely due to the partial dissociation of one of the gem-DEE ligands. In order to gain further insight into the electronic interaction between the gem-DEE ligands and the Ru2 core, density functional calculations at the B3LYP/LanL2DZ level (Gaussian03 program)20 were performed on the model compound 10 , where the silyl and methyl groups of gem-DEE were replaced by H atoms and DMBA was simplified to (HNC(H)NH).21 While most of the optimized bond lengths and angles (see the Supporting Information) are in good agreement with the crystal structure of 1b, the optimized RuRu bond (2.611 Å) is significantly longer than the experimental value, which is likely attributed to the underestimation of weak metal metal interactions by the DFT (B3LYP) method.22 The emphasis of the current study lies in the orbital interactions in 10 , and the computed energies and counter plots of the most relevant MOs are given in Figure 3. It is worth mentioning that the electronic interactions between gem-DEE and several divalent 3d metal ions were analyzed by us recently using DFT.23 It is clear from Figure 3 that the four highest energy MOs, namely HOMO-3 to HOMO, are the combinations of π(Ru2) and π(gem-DEE). The HOMO-3 is the antibonding combination of πxz(RuRu) and two π (DEE) (in-plane π orbital of gem-DEE), while the HOMO-2 is the antibonding combination of πxz*(RuRu) and two π (DEE). It is noteworthy that although it is formally antibonding, the πxz*(RuRu) component in the HOMO-2 displays significant σ-type overlap due to the severe distortion of the equatorial coordination sphere, as noted in a prior study from our laboratory.24 HOMO-1 and HOMO are the antibonding combinations of two π^(DEE) (out-of-plane π orbital )
band not far from the baseline. The red species was isolated via recrystallization in 20% yield and unambiguously identified as trans-Ru2(DMBA)4(La)2 (1a) through 1H NMR, nESI-MS, and HR-MS.16 We surmised that the appearance of very polar byproducts in the synthesis of 1a is likely due to the formation of oligomerized species attributed to the presence of the free ethynyl group (CtCH) in 1a, which may replace a coordinated La to yield a dimer bridged by μ(C,C0 )-gem-DEE, and subsequently trimer etc. To eliminate the possibility of oligomerization, our attention turned to 1-(triisopropylsilyl)-3-((trimethylsilyl)ethynyl)4-methylpent-3-en-1-yne,13 which retains the triisopropylsilyl (TIPS) protection group to yield 1-(triisopropylsilyl)-3-ethynyl4-methylpent-3-en-1-yne (Lb) upon treatment with K2CO3. The ensuing reaction between Ru2(DMBA)4(NO3)2 and Lb under weakly basic conditions yielded a red compound as the sole product (isolated yield 77%), which was identified as transRu2(DMBA)4(Lb)2 (1b) through 1H NMR, nESI-MS, and HR-MS.17 Further confirmation of the identity of this type of compound came from the single-crystal X-ray diffraction study of 1b, and the resultant molecular structure is shown in Figure 1.18 It is clear from the structural plot that the two gem-DEE ligands are approximately coplanar, while the two TIPS-ethynyl fragments are oriented in opposite directions. The geometric parameters around the Ru2 core are comparable to those of Ru2(DMBA)4(C2R)2 type compounds.3 The RuRu bond length is 2.4624(9) Å in 1b, while those of the latter are in the range 2.4412.476 Å. The RuC distances in 1b are approximately 1.98 Å and are in agreement with those determined for other Ru2(DMBA)4(C2R)2 compounds (1.952.00 Å). These data indicate that the gem-DEE ligand behaves similarly to simple acetylides in bonding to the Ru2 core. Typical of Ru2(III,III) compounds bearing strong axial donor ligands, the arrangement of the equatorial N donor atoms deviates significantly from the eclipsed D4h symmetry. The eight N atoms fall into two distinct groups, those of elongated RuN bonds and those of shortened RuN0 bonds, which also span the large torsional angle NRuRuN0
Figure 2. Cyclic (CV, gray) and differential pulse voltammograms (DPV, black) recorded for compounds 1a,b in 0.20 M THF solution of Bu4NPF6 alongOK with the CV and DPV of Ru2(DMBA)4(C2TIPS)2 (2).
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Figure 1. Molecular structure of 1b. Hydrogen atoms were omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru1Ru2, 2.4624(9); Ru1C1, 1.982(9); Ru2C7, 1.979(9); C1C2, 1.198(11); C7C8, 1.215(11); C3C4, 1.321(14); C9C10, 1.343(10); C5C6, 1.209(12); C11C12, 1.190(11); RuN(av), 2.102[6]; RuN0 (av), 1.989[6]; Ru2Ru1C1, 165.3(3); Ru1Ru2C7, 163.8(2); NRu1Ru2N0 (av), 19.2[2].
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giving X-ray crystallographic data for the structure determination of compound 1b. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We gratefully acknowledge financial support from the National Science Foundation (No. CHE 0715404) and Purdue University. ’ REFERENCES
Figure 3. Molecular orbital diagrams for 10 obtained from DFT calculations.
of gem-DEE) with πyz(RuRu) and πyz*(RuRu), respectively. The LUMO is dominated by δ*(RuRu) and contains no contribution from gem-DEE due to the orbital orthogonality. The LUMOþ1 is mostly the contribution of two σ*(RuC) bonds, where two Ru dz2 orbitals have an appearance of σ-type bonding. The computed HOMOLUMO gap is about 2.04 eV, which is comparable to that of trans-Ru2(DMBA)4(C2Fc)2 (1.84 eV)22 and slightly larger than the electrochemical HOMO LUMO gap (1.66 V). The visnear-IR absorption spectra of compounds 1a,b feature peaks at ca. 510 (ε ≈ 1.5 104) and 890 nm (ε ≈ 3 103; see the Supporting Information). The lowenergy transition is attributed to the dipole-allowed HOMO (πyz*(RuRu))LUMO (δ*(RuRu)) transition, while the highenergy transition is likely due to the LMCT from the amidinate π(N) orbital (HOMO-7) to the δ*(RuRu) (LUMO). Because of our interest in molecular wires based on the Ru2-gem-DEE combination, it is worth noting that the π-delocalizations are most extensive in HOMO and HOMO-1, where both the vinyl and free ethynyl groups of the gem-DEE ligand also contribute significantly to the π^ orbital that mixes intimately with the dyz orbitals of the Ru centers. Consequently, the hole transfer mechanism will likely dominate the charge delocalization in Ru2-gem-DEE based wires. In summary, the work described here demonstrated the feasibility of preparing gem-DEE complexes of transition-metal ions other than Pt(II) under very mild conditions. Extensive conjugation in the [M]-gem-DEE construct is possible and most likely realized through the mixing of dπ and π^(DEE) orbitals, which resulted in both the rise of the E(HOMO) value and the reduction of the optical HOMOLUMO gap in compounds 1a, b. Further exploration of gem-DEE metalation chemistry with other transition-metal synthons is under way in our laboratory.
’ ASSOCIATED CONTENT
bS
Supporting Information. Text, figures, and tables giving synthesis details and characterization data for compounds 1 and 2 and DFT calculations for model compound 10 and a CIF file
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(3H, Me (gem-DEE)), 2.05 (3H, Me (gem-DEE)); nESI-MS (m/e, based on 101Ru) 998, corresponding to [M þ H]þ. (17) The reaction of 0.151 g (0.165 mmol) of Ru2(DMBA)4(NO3)2 and 0.094 g (0.363 mmol) of Lb in 30 mL of THF and 1 mL of Et2NH resulted in a red solution in 3 h. After solvent removal, the residue was recrystallized from 1/4 THF/MeOH to afford 1b as a red microcrystalline material (0.166 g, 77% based on Ru). Data for 1b: 1H NMR (CDCl3, δ) 7.457.33 (12H, Ph), 6.996.96 (8H, Ph), 3.28 (24H, NMe), 2.14 (3H, Me (gem-DEE)), 2.04 (3H, Me (gem-DEE)), 0.99 (21H, iPr3Si); nESI-MS (m/e, based on 101Ru) 1310, corresponding to [M þ H]þ. (18) X-ray diffraction data for 1b were collected on a Rigaku Rapid II image plate diffractometer using Cu KR radiation (λ = 1.541 84 Å) at 150 K. Crystal data for 1b 3 THF: C74H106N8ORu2Si2, fw 1382.03, triclinic, P1, a = 10.626(2) Å, b = 16.654(3) Å, c = 21.360(3) Å, R = 80.51(8), β = 79.91(10), γ = 87.79(12), V = 3671(1) Å3, Z = 2, Dcalcd = 1.250 g cm3, R1 = 0.078, wR2 = 0.218. (19) Xu, G.-L.; Jablonski, C. G.; Ren, T. J. Organomet. Chem. 2003, 683, 388. (20) Frisch, M. J., et al. Gaussian 03, Revision D.02; Gaussian, Inc., Wallingford, CT, 2003. (21) The structure of model compound 10 in the ground state was fully optimized using the density functional method, B3LYP (Becke’s three-parameter hybrid functional using the LeeYangParr correlation functional). In the calculations, quasi-relativistic pseudopotentials of the 16 valence electrons of Ru are employed and the LanL2DZ basis sets associated with the pseudopotential are adopted. All the calculations were performed using the Gaussian03 program package. (22) Xu, G.-L.; Crutchley, R. J.; DeRosa, M. C.; Pan, Q.-J.; Zhang, H.-X.; Wang, X.; Ren, T. J. Am. Chem. Soc. 2005, 127, 13354. (23) Cao, Z.; Ren, T. Organometallics 2011, 30, 245. (24) Liu, I. P.-C.; Ren, T. Inorg. Chem. 2009, 48, 5608.
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