Combined Experimental and Theoretical Investigations of Group 6

Jul 25, 2018 - Thermolysis of mono metal carbonyl fragment, [M′(CO)5·thf, M′ = Mo and W, thf = tetrahydrofuran] with an in situ generated interme...
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Combined Experimental and Theoretical Investigations of Group 6 Dimetallaboranes [(Cp*M)2B4H10] (M = Mo and W) Bijan Mondal, Ranjit Bag, and Sundargopal Ghosh* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India

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S Supporting Information *

ABSTRACT: Thermolysis of mono metal carbonyl fragment, [M′(CO)5·thf, M′ = Mo and W, thf = tetrahydrofuran] with an in situ generated intermediate, obtained from the reaction of [Cp*MCl4] (M = Mo and W, Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl) with [LiBH4·thf], yielded dimetallaboranes, 1 and 2. Isolations of [{Cp*M(CO)}2B4H6] (M = Mo (1) and W(2)) provide direct evidence for the existence of saturated molybdaborane and tungstaborane clusters, [(Cp*M)2B4H10]. Our extensive theoretical studies together with the experimental observation suggests that the intermediate may be a saturated cluster [(Cp#M)2B4H10], not unsaturated [(Cp#M)2B4H8] (Cp# = Cp or Cp*), which was proposed earlier by Fehlner. Furthermore, in order to concrete our findings, we isolated and structurally characterized analogous clusters [(Cp*Mo)2(CO)(μ-Cl)B3H4W(CO)4] (3) and [(Cp*WCO)2(μ-H)2B3H3W(CO)4] (4). All the compounds have been characterized by solution-state 1H, 11B, IR, and 13C NMR spectroscopy, mass spectrometry, and the structural architectures of 1, 3, and 4 were unequivocally established by X-ray crystallographic analysis. The density functional theory calculations yielded geometries that are in close agreement with the observed structures. Both the Fenske−Hall and Kohn−Sham molecular orbital analyses showed an increased thermodynamic stability for [(Cp#M)2B4H10] compared to [(Cp#M)2B4H8]. Furthermore, large HOMO−LUMO gap and significant cross cluster M−M bonding have been observed for clusters 1−4.

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Chart 1. Structurally Characterized Dimetallaboranesa

INTRODUCTION

Metallaboranes1 represent a new class of inorganometallic compounds2 that display synergic stabilizations of the boranes and transition metal fragments through nonclassical multicentered bonding.3 The useful connection between cluster geometry and the number of skeletal electron pair (sep) provides a solid foundation rationalizing unusual structure and bonding in cluster chemistry.4 The isolobal analogy allows transition metal and main group moieties to be mapped onto one another based on the frontier orbital energies and occupancies that further augmented the field metallaborane.5 These ideas not only permit to comprehend the cluster shapes in which a transition metal fragment surrogates a {BH} group but also allows conceptual design of the new metallaborane compounds.6 Among them, dimetallaboranes form a larger section, since many of them are reported and structurally characterized.7−9 A few structurally characterized metallaboranes with the M2B4 core are shown in Chart 1.7b,10−12 The density functional theory (DFT) calculations often play pivotal role to understand this apparent systematic rule deviation.7a The nature of the intermetallic interactions in the flattened dimetallaborane compounds (also termed as oblato-nido species) invoke speculations that has been dealt with several times in the literature by molecular orbital (MO) analysis.7a,13−15 The results seem to fully in agreement with © XXXX American Chemical Society

a

The Cp* ligands on the metal atoms are not shown for clarity.

some reports evidencing M−M single bond,7a with some others suggesting internal M−M double bond.13 Received: April 8, 2018

A

DOI: 10.1021/acs.organomet.8b00204 Organometallics XXXX, XXX, XXX−XXX

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Detailed characterizations of compounds 1 and 2 have been described below. Dimetallaboranes [(Cp*MCO)2(μ-H)2B4H4] (M = Mo (1) and M = W (2)). Compound 1 was isolated as dark green crystals, which were characterized by IR, 1H, 13C, and 11B NMR, mass spectrometry, and single-crystal X-ray crystallography. The 11B{1H} NMR spectrum showed two signals at δ = 15.8 and 63.7 ppm in 1:1 ratio indicating the presence of a pair of equivalent boron atoms. Furthermore, the 1H and 13C NMR spectra showed equivalent Cp* ligands, two equivalent Mo− H−B protons, and CO ligands. The presence of the CO ligands has also been confirmed by IR spectrum with a signal at 1912 and 1869 cm−1, which may be due to its coordination in terminal fashion. Thus, a mixed metallic dimolybdaborane cluster formation was anticipated. However, the mass spectrum showed a molecular ion peak at m/z = 572.1195, which corresponds to C22H36B4O2Mo2. Thus, an unambiguous explanation eluded us until an X-ray structure study revealed the geometry. In order to determine the molecular structure of 1, single crystal X-ray diffraction analysis was undertaken (Figure 1).

In general, the dimetallaborane clusters of the late transition metals often resemble boranes or metal clusters. However, the clusters with early transition metals often depict unusual structural features and often do not obey the Wade−Mingos rules.16,17 For example, the dimetallboranes of group 6 transition metals, in particular the unsaturated 42 electron paramagnetic chromaborane [(Cp*Cr)2B4H8].13a This chromaborane coordinates at ease with CS2 or CO at ambient temperature and lead to the formation of saturated chromaboranes with interesting structural features.13b,18 Notably, analogous molybdaborane compound [(Cp*Mo)2B4H8] (I), could not be structurally characterized; nevertheless, it was proposed by Fehlner et al. as one of the possible intermediate that can be obtained from the reaction of [Cp*MoCl4] with [LiBH4· thf].19a In order to have spectral and structural information on the intermediate [(Cp*M)2B4H10], we performed the reaction of [M′(CO)5·thf] (M′ = Cr, Mo) with the in situ generated intermediate, synthesized from the reaction of [Cp*MoCl4] with [LiBH4·thf] at −78 °C. The reaction led to the isolation of green solid, [(Cp*MoCO)2(μ-H)2B4H4], 1, in good yield (40%).12 Isolation of 1 provide direct evidence for the existence of saturated molybdaborane compound [(Cp*Mo)2B4H10] (II) which was hypothesized earlier as [(Cp*Mo)2B4H8] (I). In addition, compound 1 fills the gap between [(Cp*MoCl)2B3H7] and [(Cp*Mo)2B5H9] clusters.20 Therefore, the tungsten system became of interest to validate the composition of the proposed intermediate. As a result, we explored the tungsten system and isolated [(Cp*WCO)2(μH)2B4H4] (2) that indeed justifies our assumption. In addition, when we performed the reactions with [W(CO)5·thf], under similar reaction conditions it generated [(Cp*Mo)2(CO)(μCl)B3H4W(CO)4] (3) and [(Cp*WCO)2(μ-H)2B3H3W(CO)4] (4), which are indeed analogous to 1 and 2 considering the fact that one of the BH vertices is substituted by isolobal [W(CO)4] fragment. In this article, we describe in detail a combined experimental and theoretical investigations to provide insights into the electronic structures of 1−4 and [(Cp#M)2B4H10] (M = Mo and W).

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Figure 1. Molecular structure and labeling diagram of 1. Selected bond lengths (Å) and bond angles (°): B1−B2 1.613(18), B1−B1 1.71(3), Mo1−Mo_1 2.9262(11), B1−Mo1 2.186(8), B1−Mo_1 2.209(9), B_2−Mo1 2.226(9), B2−Mo_1 2.259(8), Mo1−B2 2.259(8). B2−B1−B_1 121.6(6), Mo1−B2−Mo_1 81.4(3).

RESULTS AND DISCUSSION Thermolysis of in situ prepared intermediates, obtained from the reaction of [Cp*MoCl4]19a or [Cp*WCl4]20 with [LiBH4· thf], in the presence of [M′(CO)5·thf] (M′ = Cr, Mo) yielded clusters 1 and 2 (Scheme 1). Note that one of the major products obtained from this reaction is known [(Cp*Mo)2B5H9]19b or [(Cp*W)2B5H9].20 These compounds were separated by preparative thin-layer chromatography, which allows the characterization of the pure materials.

The results reveal the core geometry to be a {Mo2B2} tetrahedron, in which two Mo2B triangular faces are capped by two boron atoms (B2 and B_2). A total skeletal electron pair of 6 sep is also consistent with the structural description of 1. Although, the Mo1−Mo2 bond length of 2.9262(11) Å is in the range observed for Mo−Mo single bond,21,22 is longer than [(Cp#Mo)2B5H5(μ-H)4] (Cp# = Cp or Cp*) by 0.116 Å.21a The solid state structure showed two set of B−B bond distances with 1.613(18) and 1.71(3) Å. The CO ligands are oriented toward the open face of the cluster along with two Mo−H−B bonds in alternate positions. The core geometry can also be visualized as oblato-arachno with the removal of two equatorial vertices from oblato-closo hexagonal bipyramid. Compound 1 is iso-electronic with [(Cp*Re)2B4H8],7b as well as iso-electronic and iso-structural with [(Cp*CrCO)2B4H6].10b Isolation of 1 provides for the first

Scheme 1. Syntheses of 1 and 2

B

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[(Cp#Mo)2B4H10] by symmetrically adding two hydrogen atoms to the optimized [(Cp#Mo)2B4H8], as Mo−H−B as well as Mo−H. The results show possible low-energy geometry for [(Cp#Mo)2B4H10] is the one with terminal Mo−H bonds. For example, the energy of [(Cp*Mo)2B4H10] with two internal Mo−H−B is 40.12 kcal/mol (at BP86/def2-TZVP level) higher than that of [(Cp#Mo)2B4H10] with two terminal Mo− H bonds. Therefore, we conclude the geometry of [(Cp#Mo)2B4H10] is the one with two Mo−H bonds. Similarly, for [(Cp#W)2B4H8] and [(Cp#W)2B4H10] the geometries were used that for the analogous Mo compounds. Fenske−Hall molecular orbital calculations were carried out on the optimized geometries to study the electronic structure using fragment molecular orbital (FMO) approach. In addition, to concrete our hypothesis, we have also looked at the electronic structures of analogous tungsten compounds 2, [(Cp*W)2B4H8] (III), and [(Cp*W)2B4H10] (IV). The MO23 schemes calculated for I′, II′, and 1′ systems (Cp analogues of I, II, and 1, respectively) are shown in Figures 3−5, respectively. For each compound, a large HOMO− LUMO gap has been computed that apparently reveals their thermodynamic stability (Tables 2 and S2). The principal contributor to the net positive overlap in I′, II′, and 1′ is from the singly occupied orbital of the two CpMo or CpMoCO fragments which is essentially a metal dz2 orbital. The positive combination leads to MO 40 in I′, MO 42 in II′, and MO 49 in 1′, which demonstrates the Mo−Mo bonding (Figure S1). However, the negative combination, which is Mo−Mo antibonding, leads to MO 46 in I′, MO 45 in II′ and MO 53 in 1′. The existence of a short Mo−Mo distance of 2.74 Å in I′ splits the Mo−Mo bonding (40) and the antibonding (46) orbitals by 11.61 eV (for II′: 10.44 eV (2.93 Å) and for 1′: 9.05 (2.95) eV, Table S1 and S2). In all the cases, the HOMO shows Mo−Mo bonding interaction with δ symmetry that interacts with boron ring in antibonding fashion (Figure S3). The necessity of an extra pair of electrons in II′ and 1′ relative to I′ can be understood from the comparison of the MO diagrams, shown above, which shows that the LUMO (42) of I′ becomes HOMO−1 (41) of II′ and HOMO (50) of 1′. The additional pair of electrons in II′ and 1′, is thereby accommodated in one of the empty cluster orbitals of I′. Concomitantly an increased Mo−Mo bond distance has been observed from I′ to II′ and 1′ (Table S1). These “two” electrons goes to two additional Mo−H bonding orbitals in MO 41 of II′ and Mo−CO bonding (MO 49) orbitals in 1′. Addition of CO ligand to CpMo fragment strongly splits the Mo 4d based e1(π) and e2(π) sets due to its σ-donor and πacceptor properties (Figure S2). As a result, the empty MO 42 in I′ is substantially stabilized and filled (MO 50) in 1′, which explains the stabilization of the additional cluster orbital. To complement the Fenske−Hall MO results, we have also performed the Kohn−Sham DFT studies on I, II and 1 in the gas phase to elucidate their electronic structures (in order to address the steric effect, as that of experimental finding in 1, we have done the calculations with Cp* ligand. Selected MOs of I, II and 1 are shown in Figure S4). Inspection of the frontier MOs showed almost similar bonding picture to that of Fenske−Hall MO study. In addition the DFT calculated HOMO−LUMO gap showed an increasing order I 0). Blue dots indicate bond critical points (BCPs)].

Atoms in Molecule (AIM) Properties and NBO Analyses. Although I, II, and 1 essentially contain the same {Mo2B4} core which is based on tetrahedral geometry, one of the striking features of I is the existence of short Mo−Mo bond (calculated Mo−Mo bond distances of I: 2.739, II: 2.932, and 1: 2.946 Å). In all cases, two different sets of B−B bond distances have been observed. Inside, the tetrahedra core is shorter compared to the other. Therefore, to get some insight into the bonding of I, II, and 1, we have performed the DFT calculations. An inspection of the NBO analysis (Table 2) provided relatively high WBI (Wiberg bond index) value of 1.17 for the Mo−Mo bond in I as compared to II and 1 (WBI

capped by boron (B1 for 3 and B53 for 4) and W1 along the {M2B} triangular faces. As “C4V” {W(CO)4} fragment is isolobal to {BH} and μ2-Cl of 3 donates three electrons to cluster bonding (equivalent to one CO ligand plus one H), compounds 3 and 4 turn out to be iso-electronic to 1 (Figure 8). The M−M bond lengths of 3 and 4 are in agreement with the single bond order.20−22,24 The source of the chloride atom, present in compound 3, could be from LiCl which was generated during the reaction of [Cp*MoCl4] with [LiBH4·thf]. The calculated 11B NMR chemical shifts of 3 and 4, at GIAO−DFT method, corroborated satisfactorily with the experimental values (Table 1). F

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this paper were conducted in a Luzchem LZC-4 V photo reactor, with irradiation at 254−350 nm. MALDI-TOF mass spectra were recorded on a Bruker Ultraflextreme using 2,5-dihydroxybenzoic acid as a matrix and a ground steel target plate. The CH analyses were performed on PerkinElmer Instruments series II model 2400. Synthesis of 1 and 2. In a flame-dried Schlenk tube [Cp*MCl4] (0.1 g, 0.27 mmol for Mo and 0.21 mmol for W) in 10 mL of toluene was treated with 5-fold excess of [LiBH4·thf] (0.7 mL, 1.4 mmol) at −70 °C and allowed to stir at room temperature for one hour. After removal of toluene, the residue was extracted into hexane and filtered through a frit using Celite. The hexane extract (brownish-green) was dried and taken in 10 mL of thf. The resulting solution was heated at 65 °C with freshly made [Cr(CO)5·thf] or [Mo(CO)5·thf] for 24 h. The solvent was evaporated in vacuo and residue was extracted into hexane and passed through Celite. After removal of solvent, the residue was subjected to chromatographic workup by silica gel TLC plates. Elution with a hexane/CH2Cl2 (80:20 v/v) mixture yielded 1 (0.06 g, 39%) and 2 (0.057 g, 35%). 1. HRMS (ESI+): m/z calculated for [C22H36O2B4Mo2+Na]+: 595.1093; found: 595.1078. 11B NMR (22 °C, 160 MHz, CDCl3): δ = 63.7 (d, JB−H = 148.8 Hz, 2B), 15.7 (br, 2B). 1H NMR (22 °C, 500 MHz, CDCl3): δ = 6.50 (br, 2H, BHt), 6.13 (br, 2H, BHt) 1.95 (s, 30H, 2Cp*), −9.79 (br, 2H, Mo−H−B). 13C NMR (22 °C, 125 MHz, CDCl3): δ = 185.2 (CO), 102.2 (C5Me5), 10.4 (C5Me5). IR (hexane) ν/cm−1: 2481 (w, BHt), 1912, 1869 (CO). Elemental analysis (%) calcd for C22H36B4O2Mo2: C, 46.54; H, 6.39. Found: C, 46.28; H, 6.12. 2. HRMS (ESI+): m/z calculated for [C22H36O2B4W2+H]+: 745.2184; found: 745.2199. 11B NMR (22 °C, 160 MHz, CDCl3): δ = 60.4 (d, JB−H = 145.6 Hz, 2B), 7.6 (d, JB−H = 128.0 Hz, 2B). 1H NMR (22 °C, 500 MHz, CDCl3): δ = 8.16 (br, 2H, BHt), 7.94 (br, 2H, BHt), 2.08 (s, 30H, 2Cp*), −11.00 (br, 2H, W−H−B). 13C NMR (22 °C, 125 MHz, CDCl3): δ = 201.7, 196.8 (CO), 105.2 (C5Me5), 12.0 (C5Me5). IR (hexane) ν/cm−1: 2527, 2478, 2431 (w, BHt), 1997, 1941, 1891, 1834 (CO). Elemental analysis (%) calcd for C22H36B4O2Mo2: C, 46.54; H, 6.39. Found: C, 46.28; H, 6.12. Synthesis of 3 and 4. In a flame-dried Schlenk tube [Cp*MCl4] (M = Mo and W) (0.1 g, 0.27 mmol for Mo and 0.21 mmol for W) in 10 mL of toluene was treated with 5-fold excess of [LiBH4·thf] (0.7 mL, 1.4 mmol) at −70 °C and allowed to stir at room temperature for 1 h. After toluene removal, the residue was extracted into hexane and filtered through a frit using Celite. The hexane extract was dried, and taken in 10 mL of thf and heated at 65 °C with [W(CO)5·thf] for 24 h. The solvent was evaporated, and residue was extracted into hexane and passed through Celite. After removal of solvent from the filtrate, the residue was subjected to chromatographic workup using silica gel TLC plates. Elution with a hexane/CH2Cl2 (80:10 v/v) mixture yielded brown 3 (0.025 g, 11%) and brown 4 (0.020 g, 9%) Note that compounds 1 and 2 also form in very poor yields. 3. MS (MALDI): m/z calculated for C23H34B3O3Cl1Mo2W1 806.0093 [M − 2CO]+; found 806.0061. 11B NMR (22 °C, 160 MHz, CDCl3): δ = 106.1 (d, JB−H = 157.7 Hz, 1B), 98.9 (d, JB−H = 142.4 Hz, 1B), 74.7 (br, 1B). 1H NMR (22 °C, 500 MHz, CDCl3): δ = 6.54 (br, 1H, BHt), 6.17 (br, 1H, BHt), 5.73 (br, 1H, BHt), 1.85 (s, 15H, 1Cp*), 1.83 (s, 15H, 1Cp*), −8.01 (br, 1H, Mo−H−B). 13C NMR (22 °C, 125 MHz, CDCl3): δ = 193.4, 183.9 (CO), 103.4 (C5Me5), 10.3 (C5Me5). IR (hexane) ν/cm−1: 2403 (w, BHt), 2002, 1894, 1836 (CO); Elemental analysis (%) calcd for C25H34B3O5Cl1Mo2W1: C, 34.98; H, 3.99. Found: C, 35.19; H, 3.79. 4. HRMS (ESI+): m/z calculated for [C26H34O6B3W3+Na]+: 1050.1060; found: 1050.1066. 11B{1H} NMR (22 °C, 160 MHz, CDCl3): δ = 75.9 (br, 1B), 62.0 (br, 1B), 60.5 (br, 1B). 1H NMR (22 °C, 400 MHz, CDCl3): δ = 9.32, 8.91 (br, 3H, BHt), 2.07 (s, 15H, 1Cp*), 2.03 (s, 15H, 1Cp*), −12.62, −12.95 (br, 2H, W−H−B). 13C NMR (22 °C, 100 MHz, CDCl3): δ = 199, 194.7, 188.2 (CO), 109.5 (C5Me5), 14.1 (C5Me5). IR (hexane) ν/cm−1: 2342 (w, BHt), 2008, 1942, 1887 (CO). Elemental analysis (%) calcd for W3C26O6B3H35: C, 30.39; H, 3.43. Found: C, 30.24, H, 3.36. Computational Details. Geometry optimizations and MO calculations were carried out on Gaussian09 (revision C.01) program

of I is almost two times than those of II and 1). This led us to conclude I containing sort of MoMo bond. However, neither the FMO nor the NBO analyses showed the presence of parallel overlap along with σ(Mo−Mo) interaction, which can be seen in HOMO−2. Notably, the HOMO of I showed a Mo−Mo bonding interaction with δ symmetry that interacts with boron ring in antibonding fashion. Although the charge density distribution plots of I, II, and 1 show relatively similar charge distribution at the B2Mo plane, a decreasing order in electron density (at the Mo−Mo bond critical point) has been observed in the order I > II > 1. This, together with the negative value of energy density (H(r)) and electron localization function (ELF) values (Figure 9c), suggests that the Mo−Mo interaction in I is more covalent in character than II and 1 (Table 2). Similarly, the B− B bond of the tetrahedral core has more covalent character than that of the outside one, which is reflected in Table 2 (Figure 9d). In addition, the NBO charges showed a lowering of oxidation state of the Mo center from I to II to 1, which may be due to the presence of terminal hydride or carbonyl ligands. Furthermore, the MO, NBO, and AIM analyses on the W systems show similar results to those of Mo analogues.

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CONCLUSIONS In summary, we have described the syntheses and characterization of [(Cp*M)2B4H10] derivatives of molybdenum and tungsten, where either four of the hydrogen atoms are replaced by two carbon monoxide (CO) ligands or one of the BH vertices is substituted by isolobal fragment, [W(CO)4]. The experimental results strongly argue for the existence of “obedient” [(Cp*M)2B4H10] which was proposed earlier by Fehlner19,20 as “disobedient” cluster [(Cp*M)2B4H8]. Our detailed theoretical studies also support in favor of the formation of [(Cp*M)2B4H10], which was further boosted by the isolation of [(Cp*WCO)2B4H6]. Furthermore, isolation of cluster [(Cp*MoCO) 2 B 4 H 6 ] connects well between [(Cp*MoCl)2B3H7] and [(Cp*Mo)2B5H9] which was missing to date. All the synthesized molecules showed crossed cluster M−M bond with large HOMO−LUMO gap signifying their thermodynamic stabilities.

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EXPERIMENTAL SECTION

General Procedures and Instrumentation. All the manipulations were performed under an atmosphere of dry argon or in vacuo using standard Schlenk line or glovebox techniques. Solvents (hexane, toluene, thf) were purified by distillation using suitable drying agents (sodium/benzophenone) under dry argon prior to use. CDCl3 and C6D6 were degassed by three freeze−pump−thaw cycles and stored over molecular sieves. Compounds [Cp*MCl4] (M = Mo and W) and [M′(CO)5·thf] (M′ = Cr, Mo, and W) were prepared according to literature method,25 while other chemicals such as [LiBH4·thf] 2.0 M in THF, Cp*H, n-BuLi, [Cr(CO)6], [Mo(CO)6], and [W(CO)6] were obtained commercially (Aldrich) and used as received. MeI was purchased from Aldrich and distilled prior to use. The external reference for the 11B NMR, [Bu4N(B3H8)], was prepared according to the literature method.26 Preparative thin layer chromatography was performed with Merck 105554 TLC silica gel 60 F254, layer thickness 250 μm on aluminum sheets (20 × 20 cm2). NMR spectra were recorded on 400 and 500 MHz Bruker FT-NMR spectrometers. The residual solvent protons were used as reference (δ, ppm, benzene-d6, 7.16, CDCl3, 7.26), while a sealed tube containing [Bu4N(B3H8)] in benzene-d6 (δB, ppm, −30.07) was used as an external reference for the 11B NMR. The infrared spectra (IR) were recorded on a Nicolet iS10 spectrometer. Electrospray mass (ESI-MS) spectra were recorded on a Qtof Micro YA263. The photoreactions described in G

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Organometallics package.27 To estimate the possible impact of functional for the outcome of our DFT investigations, we optimized the structure of 1− 4 with the following combinations. The functionals BP86, B3LYP, and PBE1PBE(PBE0) were used in combination with def2-TZVP basis set.28,29 All combinations returned similar structures close to the experimentally determined structures of 1−3. As expected, the energies of the unoccupied orbitals become larger with increasing the % of HF exchange correlation from BP86 to PBE(0). As the energies of virtual orbitals cannot be accurately determined by theory or experiment, we decided to employ the BP86/def2-TZVP combination, which gives a good description of the geometry determined by X-ray diffraction and is computationally the least expensive.30 The model compounds were optimized in gaseous state (no solvent effect) without imposing symmetry constraints. Vibrational analyses were performed for all the geometries, and the absence of imaginary frequency confirmed that all the structures are at minima on the potential energy hypersurface. The NMR chemical shifts were computed on the BP86/def2-TZVP optimized geometries using hybrid functional B3LYP (Becke−Lee−Yang−Parr). Calculation of the NMR shielding tensors followed gauge-including atomic orbitals (GIAOs) method.31−33 The 11B NMR chemical shifts were calculated with respect to B2H6 (B3LYP B shielding constant 84.23 ppm) and transformed to the usual [BF3·OEt2] scale using the experimental δ(11B) value of B2H6, 16.6 ppm.34 Population analyses were carried out using NBO (natural bond orbital), implemented in Gaussian09.35 Wiberg bond indexes (WBI)36 values were obtained on natural bond orbital analyses. In order to understand the nature of bonding of the synthesized molecules in detail, the topological properties of the resultant electron density, ρ, obtained from the wave functions of all the optimized structures, were analyzed with the quantum theory of atoms in molecules (QTAIM).37 The QTAIM analysis were performed utilizing Multiwfn V.3.4 package,38 whereas the wave functions were generated with Gaussian09 at the same level of theory as was used for geometry optimization. Molecular orbital (MO) analyses were performed using Jimp 2,39 which employs Fenske−Hall calculations and visualization using MOPLOT.40 Fenske−Hall calculations were carried out on the DFT optimized geometries of [(CpM)2B4H8] and [(CpM)2B4H10], 1′ and 2′ (Cp analog). The minimal AO basis set calculations were transformed into a fragment basis set for 2 CpM or 2CpMCO and [B4H8], [B4H10], or [B4H6] fragments. X-ray Structure Determinations of 1, 3, and 4. Suitable X-ray quality crystals of 1, 3, and 4 were grown by slow diffusion of a hexane-CH2Cl2 solution. The crystal data for 1, 3, and 4 were collected and integrated using Bruker Kappa apexII CCD single crystal diffractometer, equipped with graphite monochromated Mo Kα (λ = 0.71078 Å) radiation. Data collection for 1 and 3 were carried out at 296 K, whereas for 4 at 150 K using ω−φ scan modes. Multiscan absorption correction has been performed for the data using SADABS41 program. The structures were solved by heavy atom methods using SHELXS-97 or SIR9242 and refined using SHELXL2014.43

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bridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +91 44-22574202. Tel.: +91 44- 22574230. E-mail: [email protected]. ORCID

Bijan Mondal: 0000-0002-7359-8926 Sundargopal Ghosh: 0000-0001-6089-8244 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the Science and Engineering Research Board (SERB) (Project No. EMR/2015/001274), New Delhi, India, for financial support. B.M. thanks Science and Engineering Research Board (SERB) and R.B. thanks IIT Madras for research fellowship. We acknowledge Dr. Thierry Roisnel, Rennes-Université de Rennes, in solving X-ray data.

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REFERENCES

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* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00204. Spectroscopic data for 1−4; optimized geometries and MO diagrams of 1−4 and I−IV (PDF) Combined optimized molecules (XYZ) Accession Codes

CCDC 1835403 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The CamH

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