Chemistry of Homo-and Heterometallic Bridged-Borylene Complexes

Apr 17, 2013 - Cluster 2 with a M3B core geometry permits a comparison with the true trimetalloborane [(OC)3Mn(μ-CO)2Co(CO)3(μ3-B)Mn(CO)5](7b) and ...
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Chemistry of Homo- and Heterometallic Bridged-Borylene Complexes K. Yuvaraj, Dipak Kumar Roy, K. Geetharani, Bijan Mondal, V. P. Anju, Pritam Shankhari, V. Ramkumar, and Sundargopal Ghosh* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India S Supporting Information *

ABSTRACT: Thermolysis of [(Cp*RuCO)2B2H6] (1; Cp* = η5-C5Me5) with [Ru3(CO)12] yielded the trimetallaborane [(Cp*RuCO)3(μ3-H)BH] (2) and a number of homometallic boride clusters: [Cp*RuCO{Ru(CO)3}4B] (3), [(Cp*Ru)2{Ru2(CO)8}BH] (4), and [(Cp*Ru)2{Ru4(CO)12}BH] (5). Compound 2 is isoelectronic and isostructural with the triply bridged borylene compounds [(μ3-BH)(Cp*RuCO)2(μ-CO){Fe(CO)3}] (6) and [(μ3-BH)(Cp*RuCO)2(μ-H)(μ-CO){Mn(CO)3}] (7), where the [μ3-BH] moiety occupies the apical position. To test if compound 2 undergoes hydroboration reactions with alkynes, as observed with 6, we performed the reaction of 2 with the same set of alkynes under photolytic conditions. However, neither 2 nor 7 undergoes hydroboration to yield a vinyl−borylene complex. On the other hand, thermolysis of 6 with trimethylsilylethylene yielded the novel diruthenacarborane [1,1,7,7,7-(CO)52,3-(Cp*)2-μ-2,3-(CO)-μ3-1,2,3-(CO)-5-(SiMe3)-pileo-1,7,2,3,4,5-Fe2Ru2C2BH] (8). The solid-state X-ray diffraction results suggest that 8 exhibits a pentagonal -bipyramidal geometry with one additional CO capping one of its faces. Cluster 3 is a boride cluster where boron is in the interstitial position of a square-pyramidal geometry, whereas compound 4 can be described as a tetraruthenium boride in which the Ru4 butterfly skeleton has an interstitial boron atom. Electronic structure calculations of compound 2 employing density functional theory (DFT) generate geometries in agreement with the structure determinations. The existence of a large HOMO−LUMO gap in 2 is in agreement with its high stability. Bonding patterns in the structure have been analyzed on the grounds of DFT calculations. Furthermore, the B3LYP-computed 11B and 1H chemical shifts for compound 2 precisely follow the experimentally measured values. All the compounds have been characterized by IR and 1H, 11B, and 13C NMR spectroscopy, and the geometries of the structures were unambiguously established by crystallographic analyses of 2−4 and 8.

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the transfer of the borylene ligand to organic species such as alkynes, for the synthesis of borirenes and diboraheterocyclic complexes.7 In addition to the borylene transfer reactions, borylene complexes show attractive reactivity, such as acidinduced B−N bond cleavage,7a nucleophilic substitution7,8a and addition,9 metal-fragment addition and extrusion,7,10 dimerization,11 cycloaddition,12 and auxiliary ligand exchange.13 Recently in our laboratory we have reported a series of lowboron-content metallaboranes in groups 5−9,14−16 and this turned our attention to synthesizing borylene complexes of both early- and late-transition-metal metallaborane compounds.15 In this connection, we have reported a high-yield synthesis of arachno-[(Cp*RuCO)2B2H6] (1), which in turn yielded numerous bridged-borylene complexes, such as [(μ3BH)(Cp*RuCO) 2 (μ-CO){Fe(CO) 3 }] (6), [(μ 3 -BH)(Cp*RuCO)2(μ-H)(μ-CO){Mn(CO)3}] (7), and [{(μ3-BH)(Cp*Ru)Fe(CO)3}2(μ-CO)], on reaction with metal carbonyls.15a During the investigation of the reactivity of these borylene complexes with alkynes, we found that the triply

he reactivity of cluster complexes toward alkynes has attracted increasing attention in recent years that reveals the properties of hydrocarbon molecules on a metal surface.1 A substantial number of alkyne-substituted clusters have been prepared, and their reactivities toward acetylide, vinylidene, and alkylidyne complexes have been established.2 In contrast to the extensive research in alkyne cluster chemistry, less research has been carried out on the reactivity of alkynes toward metallaborane and bridged-borylene complexes.3−5 Metallaborane and borylene complexes are interrelated to some extent because they do possess direct metal−boron bonds; however, the nature of these interactions varies. In metallaborane clusters the framework is made up of nonclassical, electron-deficient, multicenter, two-electron bonds, while the borylene ligands (BR) are related to one or more transitionmetal centers by classical, electron-precise, two-center−twoelectron (2c−2e) bonds.6 Since the discovery of various bridged- and terminal-borylene complexes (Chart 1), the chemistry of this sub area of transition-metal complexes of boron has received significant attention from both structure/ bonding and reactivity perspectives.7,8 The most fruitful and synthetically useful reactivity pattern of the borylene system is © XXXX American Chemical Society

Received: March 4, 2013

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Chart 1. Different Coordination Modes of Borylene Ligand: Terminal (I), Heterodinuclear Bridged (II), Semibridging (III), Homometallic Triply Bridged (IV), T-Shaped Borylene (V), Heterometallic Triply Bridged (VI), and Bisborylene (VII)

reproduces the experimental 11B chemical shift value more accurately if we assume that the hydrogen is linked to both B and two Ru atoms in a μ3-H fashion (e.g., δ 88.52 ppm for μ3-H vs δ 74.1 for Ru−H−B; see the Supporting Information, Figures S1 and S2). Further 1H and 13C NMR data revealed the presence of two types of Cp* ligands in a 1:2 ratio. To probe any fluxionality that may be present for the molecule 2 in solution, variable-temperature 1H NMR was recorded. At −60 °C the Cp* resonances remained the same as those observed at room temperature. Similarly, there was no change of the chemical shift of the Ru−H−B proton observed at δ −7.53 ppm. These results clearly support no fluxionality in molecule 2. The IR spectrum shows an intense band at 1743 cm−1, characteristic of bridging carbonyl groups, and a band at 2403 cm−1 due to the terminal B−H stretches. The solid-state structure of 2, shown in Figure 1, confirms the structural inferences made on the basis of spectroscopic results. The study implies that complex 2 has a C3 axis perpendicular to the Ru3 plane. The solid-state structure of 2 confirms the presence of a metal-bound triply bridging borylene ligand. Alternatively, 2 can be viewed as a trigonal pyramid in which the three metals form a tetrahedron with the boron atom. The core geometry of 2 is analogous to those observed for 6 and 7, and the μ3-BH moiety lies 1.425 Å above the Ru1− Ru2−Ru3 plane, which is similar to the case for the other borylene complexes (Chart 2).15a,18,19 The average Ru−Ru bond distance of 2.757 Å is in the range observed for a bond order of 1 but longer than that in the μ3-borylene triruthenium complex [(Cp*Ru)3(μ-H)3(μ3-BOEt)] (2.6770(5) Å).18 The main-group−transition-metal clusters mimic structural aspects of metal clusters, metal hydroborate complexes, and metal−hydrocarbon π complexes. Cluster 2 with a M3B core geometry permits a comparison with the true trimetalloborane [(OC)3Mn(μ-CO)2Co(CO)3(μ3-B)Mn(CO)5]7b and other isoelectronic clusters such as [(Cp*Ru)3(μ-H)3(μ3-BOEt)]18 and [(CpCo)3PPhBPh]·0.5C6H6.19 The structural similarity of 2 with other known triply bridged borylene complexes is shown in Table 1. The main difference between 2 and the triply bridged borylene complex [(Cp*Ru)3(μ-H)3(μ3-BOEt)] is the presence of a longer Ru−Ru distance and a shorter Ru−B distance. In all the cases, low-field 11B chemical shifts, in the range of about δ 87−150 ppm, have been observed. These

bridged borylene [{(μ3-BH)(Cp*Ru)Fe(CO)3} 2(μ-CO)] yielded a metallacarborane under photolytic conditions.17 In sharp contrast, compound 6 led to the formation of vinyl− borylene complexes under similar reaction conditions (Scheme 1b, vide infra).15c In this report we describe the synthesis of a novel trimetallaborane and various homometallic boride clusters. In addition, the reactivity of some of the heterometallic borylene complexes with alkynes has been studied.

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RESULTS AND DISCUSSION After successfully examining the reactivity of triply bridged heterometallic borylene complex 6 with alkynes,15c,17 we extended this chemistry toward other borylene systems. One of the many accomplishments of the isolobal analogy is the recognition of replacement of the moieties with those having the same number of formal skeletal electrons. Since [Fe(CO)3] fragment is isoelectronic with [Ru(CO)3], we performed the reaction of 1 with [Ru3(CO)12] to generate the homonuclear analogue of 6. Interestingly, the reaction led to the formation of the low-boron-containing trimetalloborane 2 having the isoelectronic moiety [Cp*RuH] instead of a [Ru(CO)3] unit. In parallel to the formation of 2, the reaction also yielded several homometallic boride clusters, such as the interstitial μ5boride [Cp*RuCO{Ru(CO)3}4B] (3) and the μ4-boride [(Cp*Ru)2{Ru2(CO)8}BH] (4) (Scheme 1a). In addition, a M6B type boride cluster, 5, has been isolated in low yield, which has been characterized with limited spectroscopic data and without the aid of an X-ray diffraction study. Although all the compounds have been produced in a mixture, thin-layer chromatographic techniques were efficiently employed to isolate them from the reaction mixture, allowing characterization of pure materials. In the following we give a detailed spectroscopic and structural characterization of clusters 2−5. Synthesis of 2. Compound 2 was isolated in 10% yield as an orange solid. The 11B NMR spectrum displays a singlet at δ 91.3 ppm, which has been shifted significantly downfield with respect to the starting material 1 (δ 21.3 ppm), implying a greater degree of boron−metal interactions. The 1H NMR spectrum shows the presence of one singlet for the BH proton at δ 7.81 ppm and a high-field resonance (δ −7.53 ppm), suggesting the presence of a Ru−H−B bridging hydride. However, the calculated 11B NMR using the gauge-independent atomic orbital density functional theory (GIAO-DFT) method B

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Scheme 1. (a) Synthesis of Homometallic Bridged Borylene 2 and Boride Clusters 3−5 and (b) Summary of the Reactions of Triply Bridged Borylene Complexes with Internal and Terminal Alkynes under Thermolytic and Photolytic Conditionsa

a

Cyclopentadienyl groups are omitted for clarity.

significant deshielding resonances are characteristic of metalrich metallaborane clusters. To understand the electronic structure of 2, quantum chemical calculations were performed with DFT methods. The optimized geometry of 2a (Cp analogue of 2) is in good

agreement with the experimentally determined structure, indicating substantial overlap between B and three Ru atoms (Figure 2). The NBO analysis shows a significant amount of coupling of the bridging hydrogen with two Ru atoms and one B atom in a μ3 fashion (Wiberg bond indices (WBI) of 0.35 and C

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Table 1. Selected Structural Parameters and Chemical Shifts (11B NMR) of 2 and Other Related Clusters compd [{(μ3-BH)(Cp*Ru) Fe(CO)3}2(μ-CO)] 2 6 7 [(Cp*Ru)3(μ-H)3(μ3-BOEt)] [(CpCo)3PPhBPh]·0.5C6H6

11

B NMR δ (ppm)

d(M−M) (Å)a

d(M−B) (Å)b

2.66

2.09

121.3, 110.6

6

2.75 2.75 2.77 2.67 2.47

2.13 2.10 2.19 2.17 2.01

91.3 119.3 89.0 87.7 143.7

6 6 6 4 5

SEP

a

Average metal−metal distance. bDistance between metal to triply bridged atom.

Figure 1. Molecular structure and labeling diagram for 2. Selected bond lengths (Å) and angles (deg): B1−Ru1 = 2.0970(9), B1−Ru2 = 2.1130(7), B1−Ru3 = 2.2030(8), Ru1−Ru2 = 2.7367(7), Ru1−Ru3 = 2.7757(7); Ru1−B1−Ru2 = 81.1(3), Ru2−B1−Ru3 = 79.5(2), B1− Ru1−Ru3 = 51.5(2), Ru1−Ru2−Ru3 = 60.6(18).

The average Ru−Fe bond distance (2.728 Å) is longer than that found in 6 (2.673 Å). The Ru−B and Fe−B bond distances are slightly shorter than those observed in ruthenium and iron complexes of boron derivatives.15,17 The intermetallic distance of the ruthenium atoms is 2.77(8) Å, which is within the range of usual Ru−Ru single-bond lengths. Consistent with the X-ray results, the 11B NMR spectrum of 8 rationalizes the presence of a single boron resonance at δ 71.8 ppm, suggestive of the boron environment observed in [closo1,2-(Cp*Ru)2(μ-CO)2{Fe2(CO)5}-4-Ph-4,5-C2BH2].15c The 1 H and 13C NMR results suggest the presence of two types of Cp* ligands, and the chemical shift at δ 0.86 ppm indicates the existence of methyl protons of the SiMe3 group. The IR spectrum confirms the presence of both bridging and terminal carbonyl ligands and the terminal BH proton. The reactivity of the bridged-borylene complexes with alkynes is puzzling, and we are not quite clear why the Cp*RuH moiety in compound 2 does not undergo hydroboration or the carborane formation. In our previous studies we have revealed that the computed energy of the reaction of hydroboration as well as carborane formation is facile for 6.15c As a result, we carried out a similar computational study on 2a (Cp analogue of 2). However, the high activation enthalpy for hydroboration or carborane formation (ΔH values are approximately greater than 100 kcal/mol) for the reaction of 2a with alkynes negates the possibility of hydroboration or carborane formation. On the other hand, the NBO analysis shows a decrease in charge over the B atom in 2 as compared to 6 (0.29 vs 0.40, respectively), indicating a decrease in the electrophilic nature of the B atom. Synthesis of Homometallic Boride Clusters 3−5. Metallaboranes are predominantly represented by boron-rich rather than metal-rich clusters.20 The characteristic feature that separates the boride clusters (metal-rich metallaboranes)6,21 from the metallaboranes is the greater number of boron-tometal bonding contacts at the expense of boron−hydrogen bonds. Homometallic boride clusters 3 and 4 were isolated in

0.47, respectively). Further, the HOMO−LUMO gap in 2 hints at its extra stability over [(μ3-BH)(CpRuCO)2(μ-CO){Ru(CO)3}] (2′) (ca. 3.3 eV for 2 vs 3.0 eV for 2′; Figure S3, Supporting Information). Reactivity of Bridged-Borylene Complexes toward Alkynes. In a preliminary report we have demonstrated that the triply bridged borylene complex [{(μ3-BH)(Cp*Ru)Fe(CO)3}2(μ-CO)] yielded metallacarboranes on photolysis with alkynes.17 Further, the photolysis of 6 in the presence of internal alkynes led to hydroboration, yielding vinyl−borylene compounds.15c To verify whether the isoelectronic trimetallaborane 2 could be a better candidate, the chemistry was expanded by the use of internal alkynes under photolytic and thermal conditions. However, the reaction of 2 with alkynes undergoes neither hydroboration nor metallacarborane formation. In contrast, thermolysis of 6 with ethynyltrimethylsilane in a 1:1 ratio for 6 h yielded the metallacarborane [1,1,7,7,7-(CO) 5 -2,3-(Cp*) 2 -μ-2,3-(CO)-μ 3 -1,2,3-(CO)-5(SiMe3)-pileo-1,7,2,3,4,5-Fe2Ru2C2BH] (8). This reaction also produced other products, which were observed during the chromatographic workup; however, due to their instability and insufficient amounts, isolation and characterization were not possible. Cluster 8 was isolated in 14% yield as a green solid. The solid-state molecular structure of 8 was determined by X-ray structure analysis. As shown in Figure 3, cluster 8 can be viewed as having a pentagonal-bipyramidal geometry with one additional CO capping one of its faces. The equatorial belt of the pentagonal bipyramid consists of Ru1, Ru2, C31, C32, and B1 atoms, whereas Fe1 and Fe2 are at the apical positions. Formally, the SiMe3 group on 8 is a one-electron fragment and it has 8 SEP, appropriate for a normal seven-vertex closo cluster. Chart 2. Different Trimetallic Boron Clusters

D

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Figure 2. (a, b) The μ3-B bonding mode, where the B atom is linked with Ru atoms. (c) The μ3-H binding situation, where H caps the triangular face of 2a.

Figure 4. Molecular structure and labeling diagram for 3 (terminal CO ligands on Ru are omitted for clarity). Selected bond lengths (Å) and angles (deg): Ru1−Ru2 = 2.9065(3), B1−Ru1 = 2.018(3), B1−Ru5 = 2.2680(3), Ru1−Ru2 = 2.9065(3), Ru4−Ru5 = 2.7731(4), Ru3−Ru4 = 2.8785(3); Ru1−B1−Ru3 = 169.4(19), Ru1−B1−Ru4 = 90.0(12), Ru2−Ru5−Ru3 = 60.8(9).

Figure 3. Molecular structure and labeling diagram for 8 (Cp* on Ru and terminal CO groups on Fe are omitted for clarity). Selected bond lengths (Å) and angles (deg): C31a−Ru1a = 2.0930(10), Fe1a−Ru2a = 2.6595(16), Fe2a−Ru2a = 2.8129(16), B1a−Fe2a = 2.2060(12), B1a−Ru2a = 2.1790(12), C31a−C32a = 1.4490(14), C32a−Si1a = 1.8470(10); C31a−C32a−Fe1a = 68.9(5), B1a−C32a−Fe1a = 69.5(5), Ru2a−B1a−Fe2a = 79.8(4).

m/z 1016 corroborating the composition of C23H15B1O13Ru5, 13 major peaks for the loss of CO ligands. The molecular structure of the boride cluster 4 is shown in Figure 5. The cluster 4 contains a butterfly array of four ruthenium atoms, in which the wingtip Ru atoms bear Cp* ligands and the hinge Ru atoms have two terminal carbonyl ligands. The boron atom is nearly centered between the wingtips of the Ru4 array. The bridging hydrogen atom is located between the hinge Ru atoms. The Ru1−B1−Ru1′ angle

15% and 10% yields as reddish brown and yellow solids, respectively. The solid-state molecular structures of 3 and 4 were determined by X-ray diffraction studies, whereas cluster 5, isolated in low yield, was characterized with limited spectroscopic data. The geometry of cluster 5 is established by comparison of its spectroscopic data22 with those for the octahedral hexaruthenium boride cluster [HRu6CO17B]23 and other related species.20 The single-crystal structure of 3 (Figure 4) confirms a square-based-pyramidal array of ruthenium atoms, with the boron atom lying 0.375 Å below the square face; it is within bonding contact of all five ruthenium atoms. The square-pyramidal framework is unusual in boride chemistry and was found previously in [Ru5(CO)15B{AuPPh3}],24a where five ruthenium atoms arrange in a square-pyramidal fashion and the boron atom lies below the square face. The 1H and 11B NMR spectra are consistent with the solidstate structure of 3 (Figure 4), which rationalizes the presence of a single 11B chemical shift at δ 174.6 ppm, shifted significantly downfield in comparison to the starting material 1. The observed 11B NMR shift of the boride cluster appears to depend only on the number of direct M−B interactions; the 11 B NMR chemical shift of 3 is consistent with other structurally characterized M5B clusters.24a The 1H NMR spectrum showed a single resonance for the Cp* ligand. The mass spectrum shows, in addition to the molecular ion peak at

Figure 5. Molecular structure and labeling diagram for 4. Terminal and bridging CO ligands are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru1−B1 = 2.0085(6), Ru1−Ru2 = 2.8230(4), Ru2−B1 = 2.1820(3); Ru2−Ru1−Ru2′ = 61.7(11), B1−Ru2−Ru1 = 44.8(10), Ru1−B1−Ru2 = 84.5(10). E

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reference (δ, ppm: [D6]-benzene, 7.16), while a sealed tube containing [Bu4N(B3H8)] in [D6]-benzene (δB, ppm: −30.07) was used as an external reference for 11B NMR. Infrared spectra were recorded on a Nicolet iS10 spectrometer. The mass spectra were recorded on a Bruker MicroTOF-II mass spectrometer. Synthesis of 2−5. In a flame-dried Schlenk tube, a yellow solution of 1 (0.07 g, 0.13 mmol) and Ru3(CO)12 (0.270 g, 0.39 mmol, 3 equiv) in toluene (15 mL) was thermolyzed for 30 h at 100 °C. The volatile components were removed under vacuum, and the remaining residue was extracted into hexane and the extract passed through Celite. After removal of solvent, the residue was subjected to chromatographic workup using silica gel TLC plates. Elution with hexane/CH2Cl2 (80/20 v/v) yielded orange 2 (0.010 g, 10%), reddish brown 3 (0.019 g, 15%), yellow 4 (0.044 g, 10%), and red 5 (0.007, 4%). 2: MS (ESI+) 807; calcd mass for 12C331H4711B116O3101Ru3 805.7552, obsd 807.0729; 11B NMR (22 °C, 128 MHz, [D6]-benzene) δ 91.3 (br, 1B); 1H NMR (22 °C, 400 MHz, [D6]-benzene) δ 7.81 (b, 1H, BHt), 1.91 (s, 15H, Cp*), 1.77 (s, 30H, 2Cp*), −7.53 (s, 1H, Ru−H−B); 13C NMR (22 °C, 100 MHz, [D6]-benzene) δ 188.3, 183 −1 ) 2403 (B−Ht), 1799, (CO), 97.0 (C5Me5), 14.1 (C5Me5); IR (ν/cm ̅ + 1743 (CO). 3: MS (ESI ) 1016; calcd mass for 12 C231H1511B116O13101Ru5 1015.51, obsd 1016.0729; 11B NMR (22 °C, 128 MHz, [D6]-benzene) δ 174.6 (s, 1B); 1H NMR (22 °C, 400 MHz, [D6]-benzene) δ 2.08 (s, 15H, Cp*); 13C NMR (22 °C, 100 MHz, [D6]-benzene) δ 198.18, (CO), 98.4 (C5Me5), 14.2 (C5Me5); IR −1 ) 2078, 2039, 2018 (CO). 4: 11B NMR (22 °C, 128 MHz, (ν/cm ̅ [D6]-benzene) δ 207.7 (s, 1B); 1H NMR (22 °C, 400 MHz, [D6]benzene) δ 2.08 (s, 30H, 2Cp*), −22.4 (s, 1H, Ru−H−Ru); 13C NMR (22 °C, 100 MHz, [D6]-benzene) δ 184, 180 (CO), 99.2 −1 ) 2042, 1978, 1973 (CO). 5: 11B (C5Me5), 13.4 (C5Me5).; IR (ν/cm ̅ NMR (22 °C, 128 MHz, [D6]-benzene) δ 191.8 (s, 1B); 1H NMR (22 °C, 400 MHz, [D6]-benzene) δ 2.07 (s, 15H, Cp*), 1.83 (s, 15H, Cp*), −21.6 (s, 1H, Ru−H−Ru); 13C NMR (22 °C, 100 MHz, [D6]−1 ) benzene) δ 192, 179 (CO), 95.7 (C5Me5), 12.8 (C5Me5); IR (ν/cm ̅ 2008, 1983, 1976 (CO). Synthesis of 8. In a flame-dried Schlenk tube, a red-brown solution of 6 (0.07 g, 0.1 mmol) and ethynyltrimethylsilane (0.009 g, 0.1 mmol) in toluene (15 mL) was thermolyzed for 6 h at 90 °C. The volatile components were removed under vacuum, and the remaining residue was extracted into hexane and the extract passed through Celite. After removal of solvent, the residue was subjected to chromatographic workup using silica gel TLC plates. Elution with a hexane/CH2Cl2 (90/10 v/v) mixture yielded green 8 (0.012 g, 14%). 8: MS (ESI+) 892; calcd mass for 12C321H4111B116O7 56Fe228Si1101Ru2 890.39, obsd 892.0729; 11B NMR (22 °C, 128 MHz, [D6]-benzene) δ 71.8 (br, 1B); 1H NMR (22 °C, 400 MHz, [D6]-benzene) δ 10.27 (b, B−Ht), 3.78 (C−H), 1.84 (s, 15H, Cp*), 1.72 (s, 15H, Cp*), 0.86 (s, 9H, SiMe3); 13C NMR (22 °C, 100 MHz, [D6]-benzene) δ 195.1 −1 ) 2503 (CO), 93.8 (C5Me5), 13.1 (C5Me5) 2.52 (Si-Me); IR (ν/cm ̅ (B−Ht), 2020, 1976, 1923 (CO). X-ray Structure Determination. The crystal data for 2, 3, and 8 were collected and integrated using a Bruker AXS kappa apex2 CCD diffractometer, with graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation at 173 K. Crystal data for 4 were collected and integrated using an Oxford Diffraction Super Nova CCD system equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) radiation at 150 K.The structures were solved by heavy-atom methods using SHELXS-97 or SIR9226 and refined using SHELXL-97.27 DFT Calculations. All the DFT calculations were performed in the gas phase using the Gaussian09 computational package.28 The molecules were optimized without any symmetry constraints using the hybrid density functional B3LYP.29 We used Stuttgart/Dresden double-ζ (SDD) effective core potentials (ECPs)30 for the Ru metal center and the 6-31g* basis set for all nonmetallic atoms (H, B, C, and O). Further, the nature of the stationary points on the potential energy surface were checked by performing harmonic frequency calculations at the same level of theory on the optimized geometry and obtained thermal corrections to the energy values reported herein. This level of theory gave good results that matched very well with the crystallographic measurements. NMR chemical shifts were also calculated at

is 166°, and the boron lies 0.3 Å from the line joining the wingtip ruthenium atoms. The average Ru−Ru edge distance of 2.844 Å in 4 is similar to those found in Ru4B clusters.24b,c Regarding the ruthenium−boron skeletal distances in 4, the pair Ru1−B1 and Ru1′−B1 averages to 2.008 Å and the other pair Ru2−B1 and Ru2′−B1 to 2.182 Å. The dihedral angle defined by the Ru1−Ru2−Ru2′ and Ru2−Ru2′−Ru1′ planes is 110°, which is the same as that found in the ruthenium boride cluster [HRu4(CO)12BH2].24b Despite the presence of bridging hydrogen, the Ru2−Ru2′ distance is not much different from the other Ru−Ru separations.24b,c The 11B NMR spectrum of 4 shows a single resonance at δ 207.7 ppm, consistent with a boride cluster. The 1H NMR shows a singlet for the two Cp* ligands at δ 2.07 ppm and a singlet at higher field for the Ru− H−Ru proton at δ −22.4 ppm. The core geometries of 3 and 4 are analogous to those of other known boride clusters.21,24 Two parameters that are used to define the nature of the boride clusters presented in this work and related butterfly clusters are the internal dihedral angle of the M4 framework and the height of the boron atom above the Mwingtip−Mwingtip axis; these are given in Table 2. Table 2. Selected Structural Parameters and Chemical Shifts (11B NMR) of 3, 4, and Other Related Clusters 11

B NMR δ (ppm)

d(M−M) (Å)a

d(M−B) (Å)b

3

2.73

2.09

174.6

[Ru5(CO)15B{AuPPh3}] 4

2.89 2.83

2.16 2.18

172.5 207.7

[HRu4CO12BH2] [HFe4CO12BH2]

2.82 2.66

2.11 1.97

113.5 191

compd

ref this work 23a this work 23b 23b

a

Average metal−metal distance. bDistance between metal and boron (boride boron).

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CONCLUSION The results described herein demonstrate the achievements and perspectives of our extended work on triply bridged borylene complexes. We have illustrated the formation of a trimetallaborane cluster along with a series of homometallic boride clusters. This observation provides a basis for the development of borylene molecules in other systems and the unique structures to be envisaged. The experimental results were complemented and rationalized by means of DFT studies, which reveal geometries in agreement with the structure determinations. Furthermore, the DFT method was proven to be helpful in predicting the stability, geometry, electronic structure, and assignment of the 1H and 11B NMR chemical shift values for compound 2.

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EXPERIMENTAL SECTION

General Procedures and Instrumentation. All syntheses were carried out under an argon atmosphere with standard Schlenk and glovebox techniques. Solvents were dried by common methods and distilled under N2 before use. Compounds 1, 6, and 7 were prepared15a according to the literature methods, while other chemicals were obtained commercially and used as received. The external reference for 11B NMR, [Bu4N(B3H8)], was synthesized by the literature method.25 Thin-layer chromatography was carried out on 250 mm diameter aluminum-supported silica gel TLC plates (Merck TLC plates). The NMR spectra were recorded on a 400 MHz Bruker FT-NMR spectrometer. Residual solvent protons were used as F

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the aforementioned level of theory. Computation of the NMR shielding tensors employed gauge-including atomic orbitals (GIAOs), using the implementation of Schreckenbach, Wolff, Ziegler, and coworkers.31−34,35a The 11B chemical shifts have been calculated relative to B2H6 (B3LYP/6-31g* B shielding constant, 93.5 ppm) and converted to the usual BF3·OEt2 scale using the experimental δ(11B) value of B2H6, 16.6 ppm.35b TMS (SiMe4) was used as an internal standard for the 1H NMR chemical shift calculations. Finally the NBO calculations were performed using the NBO routine as implemented in the Gaussian09 package.

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ASSOCIATED CONTENT

S Supporting Information *

CIF files giving crystallographic data for 2−4 and 8 and tables and figures giving details of the computations and spectroscopic data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for S.G.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Department of Science and Technology, DST (Project No SR/SI/IC-13/2011), New Delhi, India. K.Y., D.K.R., and G.R. are grateful to the Council of Scientific and Industrial Research (CSIR), India, for fellowships. B.M. and V.P.A. are grateful to the Indian Institute of Technology Madras, India, for Research Fellowships. We also thank the Center for Environmental Science and Technology, University of Notre Dame, for the mass analysis, supported by the NSF under CHE-0741793.

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