Md. Tuhinur R. Joya,
Roknuzzamana,
Md. Emdad Hossaina,
Shishir Ghosh*a,
Derek A. Tocherb,
Michael G. Richmondc and
Shariff E. Kabir*a
aDepartment of Chemistry, Jahangirnagar University, Savar, Dhaka 1342, Bangladesh. E-mail: sghosh_006@yahoo.com; skabir_ju@yahoo.com
bDepartment of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK
cDepartment of Chemistry, University of North Texas, 1155 Union Circle, Box 305070, Denton, TX 76203, USA
First published on 19th August 2020
The reaction of the trimetallic clusters [H2Os3(CO)10] and [Ru3(CO)10L2] (L = CO, MeCN) with 2-ethynylpyridine has been investigated. Treatment of [H2Os3(CO)10] with excess 2-ethynylpyridine affords [HOs3(CO)10(μ-C5H4NCH=CH)] (1), [HOs3(CO)9(μ3-C5H4NCCH2)] (2), [HOs3(CO)9(μ3-C5H4NCCCO2)] (3), and [HOs3(CO)10(μ-CHCHC5H4N)] (4) formed through either the direct addition of the Os–H bond across the CC bond or acetylenic C–H bond activation of the 2-ethynylpyridine substrate. In contrast, the dominant pathway for the reaction between [Ru3(CO)12] and 2-ethynylpyridine is C–C bond coupling of the alkyne moiety to furnish the triruthenium clusters [Ru3(CO)7(μ-CO){μ3-C5H4NCCHC(C5H4N)CH}] (5) and [Ru3(CO)7(μ-CO){μ3-C5H4NCCHC(C5H4N)CHCHC(C5H4N)}] (6). Cluster 5 contains a metalated 2-pyridyl-substituted diene while 6 exhibits a metalated 2-pyridyl-substituted triene moiety. The functionalized pyridyl ligands in 5 and 6 derive via the formal C–C bond coupling of two and three 2-ethynylpyridine molecules, respectively, and 5 and 6 provide evidence for facile alkyne insertion at ruthenium clusters. The solid-state structures of 1–3, 5, and 6 have been determined by single-crystal X-ray diffraction analyses, and the bonding in the product clusters has been investigated by DFT. In the case of 1, the computational results reveal a rare thermodynamic preference for a terminal hydride ligand as opposed to a hydride-bridged Os–Os bond (3c,2e Os–Os–H bond).
Chart 1 Known binding modes of non-metalated (a), mono-metalated (b), di-metalated (c), and tri-metalated (d) 2-vinylpyridine ligands at polynuclear transition metal centers. |
In comparison to the large number of reports on the reactivity of 2-vinylpyridine at metal clusters, fewer studies exist on the coordination chemistry of the acetylenic counterpart 2-ethynylpyridine at metal clusters.11,19,20 In 1985, a cooperative investigation on the reaction of [H2Os3(CO)10] with 2-ethynylpyridine was conducted by the groups of Lewis, Hursthouse, and Deeming (LHD). The published report presented data for the existence of two products (Scheme 1), and the structure of one of the products (minor) was unequivocally established by X-ray crystallography. The solid-state structure of the minor product [HOs3(CO)10(μ-C5H4NCHCH)] displays an opened triangular core with the ancillary pyridyl ligand functioning as a 5e donor that includes both nitrogen and (Z)-σ,π-alkenyl contributions.11 The structure of the major isomeric product was formulated to have a dangling (free) pyridyl ligand with an (E)-alkenyl moiety that functions as a traditional σ,π-vinyl donor. The route that affords the major product was ascribed to an addition path involving an Os–H bond and the alkyne unit.
Scheme 1 LHD major (left) and minor (right) products isolated from the reaction of [H2Os3(CO)10] with 2-ethynylpyridine.11 |
We have investigated the reactivity of different heterocycles containing a vinyl functionality at the ortho position such as 2-vinylpyridine, 2-vinylpyrazine, etc. at polynuclear metal centers over the last few years.16–18,20 Wishing to extend these studies based on alkenyl-substituted heterocyclic ligands, we have investigated the reaction of 2-ethynylpyridine with [Ru3(CO)12], which affords novel Ru3 products based on C–C bond coupling of alkyne carbons of the pyridyl substrate. We have also reinvestigated the reaction of [H2Os3(CO)10] with 2-ethynylpyridine and now report the isolation of two new cluster products 2 and 3 and confirm the unexpected isomerization of the major species found in the LHD study at elevated temperature to the minor isomer having an opened cluster polyhedron.11
Compound | 1 | 2 | 3 | 5 | 6 |
CCDC | 2009586 | 2009587 | 2009588 | 2009591 | 2009592 |
Empirical formula | C17H6NO10Os3 | C16H6NO9Os3 | C17·5H5ClNO11Os3 | C22H10N2O8Ru3 | C29H15N3O8Ru3 |
Formula weight | 954.83 | 926.82 | 1011.27 | 733.53 | 836.65 |
Temperature (K) | 150(1) | 150(1) | 193(1) | 193(1) | 193(1) |
Wavelength (Å) | 0.71073 | 0.71073 | 0.71073 | 0.71073 | 0.71073 |
Crystal system | Triclinic | Monoclinic | Monoclinic | Triclinic | Triclinic |
Space group | P | P21/n | C2/c | P | P |
Unit cell dimensions | |||||
a (Å) | 8.7108(4) | 8.1249(2) | 17.575(4) | 8.187(5) | 11.114(18) |
b (Å) | 9.1324(4) | 15.2910(4) | 8.329(2) | 9.641(6) | 11.221(17) |
c (Å) | 14.1005(6) | 15.3121(4) | 30.267(6) | 15.139(9) | 13.16(2) |
α (°) | 100.068(4) | 100.068(4) | 90 | 97.65(3) | 75.91(5) |
β (°) | 92.632(4) | 91.697(2) | 96.45(3) | 94.716(17) | 73.55(6) |
γ (°) | 111.204(4) | 111.204(4) | 90 | 106.30(2) | 61.99(9) |
Volume (Å3) | 1022.35(8) | 1901.52(8) | 4402.6(17) | 1127.7(12) | 1377(4) |
Z | 2 | 4 | 8 | 2 | 2 |
Density (calculated) (Mg m−3) | 3.102 | 3.237 | 3.051 | 2.160 | 2.017 |
Absorption coefficient (mm−1) | 18.652 | 20.047 | 17.455 | 2.036 | 1.682 |
F(000) | 846 | 1636 | 3600 | 704 | 812 |
Crystal size (mm3) | 0.28 × 0.16 × 0.10 | 0.18 × 0.16 × 0.08 | 0.12 × 0.11 × 0.04 | 0.12 × 0.08 × 0.03 | 0.25 × 0.20 × 0.04 |
2θ range for data collection (°) | 6.854 to 51.998 | 6.336 to 53.996 | 4.664 to 54.482 | 4.462 to 54.31 | 4.472 to 54.436 |
Reflections collected | 15181 | 30729 | 32329 | 32047 | 31001 |
Independent reflections [Rint] | 4006 [Rint = 0.0553] | 4142 [Rint = 0.0623] | 4927 [Rint = 0.0428] | 5010 [Rint = 0.0468] | 6101 [Rint = 0.0720] |
Data/restraints/parameters | 4006/0/281 | 4142/0/270 | 4927/0/312 | 5010/0/316 | 6101/0/388 |
Goodness-of-fit on F2 | 1.017 | 1.103 | 1.135 | 1.039 | 1.052 |
Final R indices [I > 2σ(I)] | R1 = 0.0269, wR2 = 0.0623 | R1 = 0.0211, wR2 = 0.0459 | R1 = 0.0261, wR2 = 0.0534 | R1 = 0.0323, wR2 = 0.0775 | R1 = 0.0687, wR2 = 0.1619 |
R indices (all data) | R1 = 0.0294, wR2 = 0.0641 | R1 = 0.0239, wR2 = 0.0471 | R1 = 0.0318, wR2 = 0.0551 | R1 = 0.0497, wR2 = 0.0854 | R1 = 0.1010, wR2 = 0.1872 |
Largest diff. peak and hole (e Å−3) | 2.26/−1.46 | 1.06/−1.50 | 1.22/−0.99 | 1.24/−0.83 | 3.27/−1.45 |
The reported geometries represent fully optimized ground states (positive eigenvalues) based on the Hessian matrix, and the natural charges (Q) and Wiberg bond indices were computed using Weinhold's natural bond orbital (NBO) program (version 3.1).34,35 The geometry-optimized structures presented here have been drawn with the JIMP2 molecular visualization and manipulation program.36
The identity of clusters 1–4 was examined spectroscopically by IR and NMR, and the molecular structures for clusters 1–3 were established by X-ray crystallography. Our product 1 corresponds to the minor product in the LHD report, which incidentally is also labeled as 1 in the original report.11 Given the quality of earlier diffraction data collected for 1 (R = 0.1001), we collected a new data set for 1 and re-determined its solid-state structure (Fig. 1). Surprisingly, our structure is polymorphic to the original LHD structure. As with the original structure, we were unable to locate the terminal hydride ligand on the “spike” Os(CO)4 moiety. The location of the terminal hydride, while not established in the original report containing 1, is supported by the ligand distribution at the Os(CO)4 center in 1. Moreover, the solid-state structure of the related PMe2Ph-substituted derivative [HOs3(CO)9(PMe2Ph)(μ-C5H4NCHCH)], whose hydride shares a common site at the pendent osmium center as 1, was located during data reduction and verified at the “free” site at the Os(CO)3P center.11 Generally speaking, bridging hydrides are preferred at polynuclear clusters.37 The preference of a terminal versus an edge-bridging hydride in 1 was computationally verified, as discussed below.
Since the gross structural features of the two structures for 1 are similar, we will limit our structural discussion to a few highlights. The near-linear Os(1)–Os(2)–Os(3) linkage [159.593(12)°] confirms the polyhedral expansion of the metallic core and is a common phenomenon in trimetallic clusters with a 50e count.38 The C5H4NCHCH ligand bridges the Os(1)–Os(2) edge regiospecifically by μ-C,N coordination with the nitrogen occupying a terminal position on the metallic frame. The metalated C(11) atom and the N(1) donor are located syn to each other at the six-coordinate Os(1) center. The ethenyl moiety produced from the alkyne/OsH addition process functions as a π donor to Os(2) as defined by the C(11)–Os(2) and C(12)–Os(2) bonds, whose mean distance of 2.285 Å is >0.23 Å longer than the Os(1)–C(11) σ bond [2.084(6) Å]. These structural features are typical of clusters containing π,σ-donating ligands. The metalated ethenylpyridine ligand in 1 donates no electrons to the dangling Os(3) center, which also contains a terminal hydride ligand that resides at the “empty” coordination site trans to the O(8)–C(8)–Os(3) linkage. The two Os–Os vectors exhibit a mean distance of 2.8597 Å, with the bridged Os(1)–Os(2) bond [2.8248(4) Å] slightly shorter than the non-bridged Os(2)–Os(3) edge [2.8945(4) Å]. The terminal hydride was not located during data refinement, but its location at the Os(3) atom is supported by the arrangement of carbonyls about the Os(3) center.
The presence of the expected hydride at the Os(3) atom was verified by DFT calculations with the computed structure for A depicted in Fig. 1 alongside the experimental structure. Excellent agreement exists between the experimental and the computed structures. The preference for a terminal hydride in A versus a bridging hydride at the adjacent Os–Os vector was also examined by DFT through a series of step-scan calculations. Reducing the initial Os–Os–H angle of 83° in species A to 25°, well past the typical angle for an edge-bridging hydride of ca. 37°, led to an increase in total energy with no sign of a stable stationary point. All attempts to optimize a structure with a bridging hydride collapsed to species A, and we confidently estimate an energy difference of ≥13 kcal mol−1 in favor of the isomer with a terminal hydride. The natural charges (Q) and Wiberg bond indices were also examined, and these data are reported in Table 2. All three osmium atoms exhibit a negative charge that ranges from −0.96 [Os(1)] to −1.55 [Os(3)], as do the alkenyl carbons C(1) [–0.30] and C(2) [–0.24] and the pyridine N(1) [–0.42] atom. The charge on the terminal H(1) atom is 0.15 and is similar in magnitude to the edge-bridging hydride ligand in species B (cluster 2) and C (cluster 3). The two Os–Os vectors exhibit a mean Wiberg bond index of 0.35, consistent with their single-bond designation, and the metalated alkenyl moiety displays Wiberg indices of 0.46 and 0.47 for the Os(2)–C(1) and Os(2)–C(2) bonds that are a factor of two weaker than the σ-bonding component defined by the Os(1)–C(2) vector (0.87). Finally, the WBI for the Os(3)–H(1) bond is 0.45.
Natural charges (Q) | A | B | C |
---|---|---|---|
a Atom numbering for the participant atoms follows the structures depicted below the table caption. | |||
Os1 | −0.96 | −1.01 | −0.96 |
Os2 | −1.18 | −1.24 | −1.13 |
Os3 | −1.55 | −1.21 | −1.29 |
N1 | −0.42 | −0.40 | −0.41 |
C1 | −0.30 | −0.14 | 0.23 |
C2 | −0.24 | −0.39 | −0.15 |
C3 | 0.75 | ||
H1 | 0.15 | 0.14 | 0.13 |
WBI | A | B | C |
---|---|---|---|
Os1–Os2 | 0.38 | 0.42 | 0.37 |
Os1–Os3 | 0.48 | ||
Os2–Os3 | 0.31 | 0.30 | 0.23 |
Os1–N1 | 0.52 | 0.48 | 0.52 |
Os1–C2 | 0.87 | 0.81 | |
Os2–C1 | 0.46 | 0.43 | 0.42 |
Os2–C2 | 0.47 | 0.44 | 0.50 |
Os3–C1 | 0.67 | ||
Os3–C2 | 0.74 | ||
Os3–C3 | 0.77 | ||
O1–C1 | 0.92 | ||
O1–C3 | 0.89 | ||
O2–C3 | 1.81 | ||
Os1–H1 | |||
Os2–H1 | 0.42 | 0.23 | |
Os3–H1 | 0.45 | 0.35 | 0.41 |
Whereas the formation of 1 arises from an anti-Markovnikov Os–H/alkyne addition process, cluster 2 derives from a Markovnikov insertion product where the hydride adds to the terminal alkyne carbon to produce a methylene group. The molecular structure of 2 was established by X-ray crystallography, and the solid-state structure is shown in Fig. 2. The cluster core contains a closed osmium triangle where the Os–Os bond distances range from 2.7913(3) Å [Os(1)–Os(3)] to 2.8581(2) Å [Os(2)–Os(3)] with a mean distance of 2.8357 Å. Each osmium is bound to three carbonyls, and the 5e donor C5H4NCCH2 caps a metallic face through the N(1), C(15), and C(16) atoms. The Os(2) and Os(3) centers bind the latter two atoms of the ligand in a traditional σ,π-vinyl fashion, with the Os(3)–C(15) bond representing the σ component of the metalated alkenyl moiety. The longer Os(2)–C(15) [2.242(4) Å] and Os(2)–C(16) [2.306(5) Å] bonds represent the π component of this ligand, whose Os–C distances are comparable to those bond distances reported in related clusters.11,18,20 The hydride was not located during data reduction but was assumed to span the Os(2)–Os(3) edge based on the disposition of CO ligands about the Os–Os bond. The locus for the hydride was subsequently verified by DFT calculations, and the optimized structure is depicted alongside the experimental structure in Fig. 2. The optimized structure of B closely mirrors the experimental structure with the bridging hydride sharing the Os(2)–Os(3) vector bridged by the alkenyl ligand. The hydride is tipped slightly below the metallic plane opposite the polyhedral face capped by the C5H4NCCH2 ligand. The osmium atoms and the Os-bound N(1), C(1), and C(2) atoms of the activated ligand all display a negative charge, while the edge-bridging hydride exhibits a positive charge of 0.14. The computed WBI for the Os–Os bonds in B range from 0.30 [Os(2)–Os(3)] to 0.48 [Os(1)–Os(2)] with a mean index of 0.40. The Os–N and Os–C WBIs in B associated with the capping C5H4NCCH2 ligand parallel those data reported for A. Finally, the Os–H bond indices for the edge-bridging hydride are comparable in magnitude to the WBIs in related hydride clusters investigated by us.39
The spectroscopic properties of 2 are consistent with the solid-state structure. The 1H NMR spectrum reveals a pair of doublets at δ 8.56 (J 6.0 Hz) and δ 6.92 (J 8.0 Hz) and a pair of multiplets at δ 7.64 and δ 6.98 that are readily ascribed to the ABCD spin system of the pyridyl ring. The two singlets are δ 4.30 and 2.63 are assigned to the hydrogens of the vinyl moiety, with the lower-field singlet confidently assigned to the hydrogen situated syn to the hydride ligand (δ −17.70) based on NOESY 1H NMR experiments.
Recall, one of the four initial products produced in the reaction between [H2Os3(CO)10] and 2-ethynylpyridine is not stable over silica gel, transforming to cluster 3 during chromatographic separation. The hydride signal for the intermediate appears at δ −15.79 and it shifts 0.97 ppm upfield after chromatography. We had to settle on the characterization of this unknown intermediate using isolated 3. NMR analysis of the crude reaction mixture suggests that the silica gel-unstable intermediate does not contain any vinyl hydrogens, leading us to propose the transformation depicted in Scheme 3 as an explanation of the precursor to 3. Here the dimetalated alkenyl moiety of the intermediate undergoes ring expansion through an oxygen capture pathway that is assisted by an ancillary CO ligand at the Os(CO)4 center. No change in the initial product mixture was observed over several hours when exposed to oxygen, suggesting that the stationary support serves as the source of oxygen in this reaction.
Fig. 3 shows the solid-state molecular structure of 3 with selected bond distances and angles reported in the caption. The molecule contains 50e, assuming the face-capping C5H4NCCCO2 ligand functions as a 7e donor, and the opened triosmium core is in concert with the overall electron count.38 The Os(1)–Os(2) [2.8464(9) Å] and Os(2)–Os(3) [2.9561(10) Å] bond distances are consistent with their single-bond designation and the Os–Os bond distances in clusters 1 and 2. Each metal center contains three terminal CO ligands that are situated at mutually cis sites to furnish Os(CO)3 units possessing one axial and two equatorial CO groups. The hydride, which was located crystallographically, spans the Os(2)–Os(3) edge, which is significantly longer (ca. 0.1 Å) than the non-hydride bridged Os(1)–Os(2) vector. The C5H4NCCCO2 ligand may be viewed as a doubly metalated σ,σ,π-alkenyl ligand with the Os(1)–C(11) and Os(3)–C(11) bonds corresponding to the σ components and the π bond represented by the Os(2)–C(11) and Os(2)–C(12) bonds. The mean distance of 2.078 Å for the former two Os–C bonds is ca. 0.16 Å shorter than the mean distance displayed by the latter two Os–C(π) bonds of the alkenyl linkage. The carboxylate ligand, which is defined by the C(10), O(10), and O(11) atoms, exhibits bond distances and angles that are unremarkable and require no comment. The presence of the carboxylate group in 3 was supported by IR spectroscopy based on a low-energy ν(CO) band at 1693 cm−1. The optimized structure of C is depicted alongside the experimental structure, and excellent agreement between the two structures is noted. Apart from the positive charge of 0.23 computed for the C(1) atom of alkenyl ligand, whose adjacent O(1) atom likely serves to withdraw electron density for the metalated carbon center, the charges and WBIs for C mirror the data reported for species A and B.
The slowest moving band isolated from the TLC plate was confirmed as cluster 4, and this material corresponds to the major product reported in the LHD study.11 The initial identity of this product was formulated based on the solution spectroscopic data. The 1H NMR spectrum supported a product with a single hydride and trans alkenyl moiety while the IR spectrum was consistent with a triangular cluster having ten terminal CO ligands. These data supported a product involving the addition of the Os–H bond to the alkyne triple bond. The ancillary pyridyl ligand remains free and functions as a non-coordinated spectator ligand. Accordingly, the product was formulated as possessing a structure similar to the known vinyl cluster [HOs3(CO)10(μ,η2-CHCH2)],40 as depicted by the major product in Scheme 1.
As with the earlier LHD study, we were unable to grow X-ray quality crystals of this product and could not unequivocally establish its molecular structure. Accordingly, we examined the LHD-proposed structure of 4 (species A_alt) by electronic structure calculations [Fig. 4]. Both the hydride and alkenyl moiety share the same edge of the Os3 polyhedron. Species A and A_alt are isomers, and the former is computed to be 8.9 kcal mol−1 (ΔG) more stable. Independent control experiments confirmed 4 (kinetic isomer) as the precursor to 1 (thermodynamic isomer). Repeating the reaction between [H2Os3(CO)10] and 2-ethynylpyridine in refluxing hexane furnished cluster 1 at the expense of cluster 4. The yields of 2 and 3 remained unchanged. Heating pure 4 in CDCl3 in a sealed NMR tube at 60 °C led to the conversion to 1 after 2 h, confirming cluster 1 as the thermodynamically preferred isomer. We also examined the transformation of 4 → 1 at 100 °C in toluene-d8, but the reaction was accompanied by visible cluster decomposition. Subsequent studies confirmed that cluster 1 is unstable at 100 °C.
Fig. 4 DFT-optimized structure of major product A_alt from the room temperature reaction of [H2Os3(CO)10] and 2-ethynylpyridine. |
The solid-state molecular structure of 5 is depicted in Fig. 5 along with selected bond distances and angles contained in the figure caption. The triangular Ru3 cluster is ligated by seven terminal carbonyls, an edge-bridging carbonyl, and a μ3-C5H4NCCHC(C5H4N)CH ligand. The Ru–Ru distances range between 2.7868(15) and 2.8747(15) Å with the Ru(2)–Ru(3) edge, which accommodates the bridging carbonyl ligand, being the longest of the three Ru–Ru bonds. The C5H4NCCHC(C5H4N)CH ligand, which is formed via head-to-tail C–C coupling of the ethynyl moieties, functions as an 8e donor ligand that is best viewed as a dimetalated 2-pyridyl-substituted butadiene ligand. The Ru(1)–C(9) and Ru(1)–C(12) vectors, which represent the two metalated bonds associated with the ruthenacyclopentadiene ring defined by the Ru(1)–C(9)–C(10)–C(11)–C(12) atoms, display a mean distance of 2.074 Å. This ligand donates an additional four electrons to the Ru(2) center via the butadiene component of the ruthenacyclopentadiene ring and two electrons through coordination of the pyridyl N(1) moiety to the Ru(3) center. The Ru(3)–N(1) [2.150(4) Å] bond distance and the Ru–C bond distances for the coordinated butadiene moiety [Ru(1)–C(9) 2.081(4), Ru(1)–C(12) 2.066(4), Ru(2)–C(9) 2.279(4), Ru(2)–C(10) 2.241(4), Ru(2)–C(11) 2.250(4), Ru(2)–C(12) 2.233(4)Å] are similar to those distances found in related clusters.17,20 The cluster is electronically saturated based on an electron count of 48e, and the optimized structure of D (Fig. 5) mirrors the experimental structure. Table 3 reports the natural charges and Wiberg bond indices for D. The three rutheniums, the two nitrogens, and the coordinated carbon atoms of the butadiene moiety of the μ3-C5H4NCCHC(C5H4N)CH ligand all exhibit a negative charge. The metalated Ru(3)–C(1) and Ru(3)–C(4) bonds of the ligand display a mean WBI of 0.72 that is twofold stronger than the mean WBI index of 0.36 for the Ru(2)–C(π) bonds involving C(1)–C(4) atoms of the butadiene moiety. Finally, the solution spectroscopic data for 5 also support the solid-state structure. The IR spectrum exhibits seven carbonyl stretching bands between 2068–1937 cm−1 for the terminal carbonyls along with a low-energy absorption at 1792 cm−1 assigned to the bridging carbonyl ligand. The 1H NMR spectrum displays a series of pyridyl multiplets and alkenyl protons consistent with the solid-state structure.
Natural charges (Q) | D | E |
---|---|---|
a Atom numbering for the participant atoms follows the structures depicted below the table caption. | ||
Ru1 | −0.85 | −1.09 |
Ru2 | −0.98 | −0.84 |
Ru3 | −1.27 | −1.11 |
N1 | −0.41 | −0.41 |
N2 | −0.49 | −0.48 |
N3 | −0.48 | |
C1 | −0.07 | −0.16 |
C2 | −0.16 | −0.27 |
C3 | −0.04 | −0.06 |
C4 | −0.18 | −0.12 |
C5 | −0.12 | |
C6 | −0.09 |
WBI | D | E |
---|---|---|
Ru1–Ru2 | 0.29 | 0.32 |
Ru1–Ru3 | 0.24 | |
Ru2–Ru3 | 0.30 | 0.40 |
Ru1–N1 | 0.43 | 0.47 |
Ru1–C6 | 0.71 | |
Ru2–C1 | 0.38 | 0.48 |
Ru2–C2 | 0.31 | |
Ru2–C3 | 0.28 | 0.65 |
Ru2–C4 | 0.41 | |
Ru2–C5 | 0.22 | |
Ru2–C6 | 0.37 | |
Ru3–C1 | 0.68 | |
Ru3–C2 | 0.50 | |
Ru3–C3 | 0.53 | |
Ru3–C4 | 0.75 | 0.30 |
Fig. 6 shows the solid-state molecular structure of 6 and the caption contains selected bond distances and angles. The cluster contains 50-valence electrons and exhibits an expanded triruthenium core that possesses two formal Ru–Ru bonds. The coordination sphere of 6 contains eight carbonyl groups and a C5H4NCCHC(C5H4N)CHCHC(C5H4N) ligand, the latter which is formed via alkyne C–C bond coupling of three 2-ethynylpyridine ligands. The alkyne-based ligand functions as a 10e donor and may be considered as a dimetalated tris(2-pyridyl-substituted) conjugated triene ligand. The two Ru–Ru bonds defined by the Ru(1)–Ru(2) and Ru(1)–Ru(3) vectors exhibit a mean distance of 2.769 Å that is similar in magnitude to the shortest Ru–Ru bond distance observed in 5. Each terminal ruthenium is bound to three carbonyls, while the central ruthenium is bound to two carbonyls, one of which is nominally semibridging in nature based on a bond angle of 159.5(9)° for the Ru(1)–C(1)–O(1) linkage. The dimetalated tris(pyridyl-substituted) triene ligand coordinates all three ruthenium atoms using the alkenyl carbons (σ and π fashion) and one of the pyridyl nitrogens [Ru(3)–N(2)]. The triene moiety displays an altered C–C backbone [C(9)–C(10) 1.417(13), C(10)–C(11) 1.428(13), C(11)–C(12) 1.465(13), C(12)–C(13) 1.480(12), C(13)–C(14) 1.405(14) Å] that effectively transforms the three CC π units into a discrete pair of 3e donating allyl ligands that bind the Ru(1) and Ru(2) atoms. The metalated components of the ligand are represented by the Ru(1)–C(9) and Ru(3)–C(14) vectors, which display a mean bond distance of 2.086 Å. The Ru(3)–N(2) [2.138(9) Å] and Ru–C(triene) [Ru(1)–C(9) 2.093(10), Ru(1)–C(12) 2.146(9), Ru(1)–C(13) 2.270(9), Ru(1)–C(14) 2.291(9), Ru(2)–C(9) 2.154(9), Ru(2)–C(10) 2.189(9), Ru(2)–C(11) 2.249(10), Ru(3)–C(14) 2.079(10) Å] bond distances involving this ligand are similar to the bond distances in 5 and related ruthenium clusters.17,20 The optimized structure of E is shown alongside the crystallographic structure in Fig. 6. The computed charges and bond indices for E parallel the data reported for D.
Footnote |
† Electronic supplementary information (ESI) available: IR and 1H NMR spectral data on clusters 2, 3, 5, and 6. Atomic coordinates and energies for all DFT-optimized structures are available upon request (MGR). CCDC 2009586 (1), 2009587 (2), 2009588 (3), 2009591 (5), and 2009592 (6). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0ra05393g |
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