Electron Precise Group 5 Dimetallaheteroboranes [{CpV(μ-EPh)}2{μ

Jan 16, 2018 - Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India. Inorg. Chem. , Article ASAP. DOI: 10.1021/acs.in...
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Electron Precise Group 5 Dimetallaheteroboranes [{CpV(μEPh)}2{μ‑η2:η2‑BH3E}] and [{CpNb(μ-EPh)}2{μ‑η2:η2‑B2H4E}] (E = S or Se) Monojit Ghosal Chowdhury, Subrat Kumar Barik, Koushik Saha, Bakthavachalam Kirubakaran, Abhishek Banerjee, Venkatachalam Ramkumar, and Sundargopal Ghosh* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India S Supporting Information *

ABSTRACT: Synthesis and structural elucidation of various electron precise group 5 dimetallaheteroboranes have been described. Room temperature reaction of [Cp2VCl2] with Li[BH3(EPh)], generated from the treatment of LiBH4·THF and Ph2E2 (E = S or Se), for 1 h in toluene, followed by thermolysis, led to the formation of bimetallic complexes [{CpV(μ-EPh)}2{μ-η2:η2-BH3E}], 1 and 2 (1: E = S and 2: E = Se), and [{CpV(μ-SePh)}2{μ-η2:η2-BH(OC4H8)Se}], 3. One of the striking features of these compounds is that they represent a rare class of distorted tetrahedral clusters having bridging hydrogens. Evaluating the skeletal electron pairs and bonding types, compounds 1, 2, and 3 may be considered as isoelectronic with our earlier reported [(CpV)2(B2H6)2]. In an attempt to synthesize the Nb analogues of 1−3, room temperature reactions of [CpNbCl4] and Li[BH3(EPh)] (E = S or Se) were carried out that afforded compounds [{CpNb(μ-EPh)}2{μ-η2:η2-B2H4E}], 4 and 5 (4: E = S and 5: E = Se). The solid-state X-ray structures of both 4 and 5 exemplify electronically saturated [M2B3] systems, and their geometries are analogous to that of [(Cp*MoCl)2B3H7]. For the extension of this work, reaction of [Cp*TaCl4] (Cp* = η5-C5Me5) with Li[BH3(SePh)] was carried out that yielded a tantalaselenaborane cluster [(Cp*Ta)2(μ-Se)B3H6Se(C6H5)] (6). All the new compounds have been characterized using 1H, 11B{1H}, 13C{1H} NMR, UV−vis absorption, and IR spectroscopy, mass spectrometry, and X-ray diffraction studies.

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Cl)}2(B2H6)],14 [Ta2(B2H6)(DTolF)4·2Et2O]15 (DTolF− = [(p-tolyl)NCHN(p-tolyl)]−), and [{(Cp*Ta)(μ-Br)}2(B2H6)].16 In addition, the chemistry of group 5 transition metal boron complexes has produced a great deal of interest owing to their unusual bonding patterns, unique structures, and reactivity. As part of our efforts in synthesizing electron rich/ poor group 5-transition metal boron complexes, we have recently reported various metallaboranes and their derivatives with diverse geometries.13,17 Despite the availability of welldeveloped routes to metallaboranes of group 6−9 metals, to our knowledge, examples of structurally characterized group 5 metallaboranes are rare.13,17 Therefore, there has always been a strong quest for group 5 metallaboranes and their derivatives. As a result, we explored the chemistry of cyclopentadienyl group 5 metal chlorides with Li[BH3(EPh)] (E = S or Se). Herein, we report the isolation and structural elucidation of various chalcogen stabilized group 5 dimetallaheteroboranes. Density functional theory (DFT) studies have been used to have an insight into the bonding nature of these new molecules.

INTRODUCTION Transition metal boron chemistry, comprehending a wide range of structural diversities with higher nuclearity metallaborane clusters to complexes with a single boron, has received significant attention.1−3 In this regard, complexes with varied coordination modes of boron, for example, electron precise borylene,3a σ-borane complexes,4,5 and boryl complexes,3e have been well-established. In particular, the chemistry of transition metal diborane complexes has witnessed substantial growth due to its intriguing bonding aspects and potential catalytic applications.6−8 In 1978, Fehlner and his co-workers reported the first transition metal diboron compound [B2H6Fe2(CO)6]9a and its conjugate base [B2H5Fe2(CO)6]−.9 Later, Shore et al.10 reported the metalladiborane complexes, K2[M(CO)4(η2B2H5)], (M = Fe, Ru or Os) and M′[(η5-C5H5)(CO)2(η2B2H5)], (M′ = Fe or Ru), analogues to metal olefin complexes. The solid-state structures of these complexes illustrate a diborane(6) molecule in which a bridging hydrogen is replaced by an [(η5-C5H5)Fe(CO)2] unit.10 Recently, Shimoi,11 Himmel,12 Braunschweig,3a,c and others have reported various monometallic transition metal diborane(4) complexes with interesting structural aspects. Examples of structurally characterized multidentate diborane complexes, especially with group 5 metals, have been sporadic. Occasional examples in which the B2H6 unit exists in an eclipsed conformation along the (B−B) σ bond are [(Cp*Ta)(B2H6)]2,13 [(CpM)(B2H6)]2 (M = V or Nb),13 [{Cp*Mo(μ© XXXX American Chemical Society

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RESULTS AND DISCUSSION Reactivity of [Cp2VCl2] with Li[BH3(EPh)] (E = S or Se). As shown in the Scheme 1, room temperature reaction of [Cp2VCl2] with Li[BH3(EPh)] (made from the low temperReceived: September 7, 2017

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DOI: 10.1021/acs.inorgchem.7b02305 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthesis of Divanadaborane Complexes (1−3)

Figure 1. (a) Molecular structure and labeling diagram of [{CpV(μ-SPh)}2{μ-η2:η2-BH3S}], 1, with ellipsoids drawn at the 30% probability level. Selected bond distances (Å) and angles (deg): V(1)−V(2) 2.6816(12), V(1)−B(1) 2.35(2), V(1)−S(1) 2.4300(16), V(1)−S(3) 2.353(2), B(1)− S(3) 1.684(18); V(1)−B(1)−V(2) 70.2(6), V(1)−S(1)−V(2) 67.14(5). (b) Molecular structure and labeling diagram of [{CpV(μ-SePh)}2{μ-η2:η2BH3Se}], 2, with ellipsoids drawn at the 30% probability level. Selected bond lengths (Å) and angles (deg): V(1)−V(2) 2.751(2), V(1)−B(1) 2.45(6), V(1)−Se(1) 2.5578(18), V(1)−Se(2) 2.538(17), V(1)−Se(3) 2.502(2), B(1)−Se(3) 2.23(5); V(1)−B(1)−V(2) 69.0(16), V(1)−Se(3)− V(2) 66.76(6), V(1)−Se(1)−V(2) 65.16(6). (c) Molecular structure and labeling diagram of [{CpV(μ-SePh)}2{μ-η2:η2-BH(OC4H8)Se}], 3, with ellipsoids drawn at the 30% probability level. Selected interatomic distances (Å) and angles (deg): V(1)−V(2) 2.7562(10), V(1)−B(1) 2.460(5), V(1)−Se(3) 2.5068(9), V(1)−Se(1) 2.5615(8), V(1)−Se(2) 2.5517(10), B(1)−Se(3) 2.064(6), B(1)−O(1) 1.586(7); V(1)−B(1)−V(2) 67.94(14), V(1)−Se(3)−V(2) 66.57(3), V(1)−Se(1)−V(2) 65.00(3), B(1)−Se(3)−V(1) 64.28(14). (Hydrogens on phenyl rings are omitted for clarity. Bridging V-H-B and B-Ht hydrogen atoms for 2 and bridging V-H-B hydrogen atom for 3 were not located in the X-ray diffraction studies.)

ature reaction of LiBH4·THF and Ph2E2 in THF) in toluene for 1 h, followed by thermolysis at 90 °C for 48 h, resulted in the formation of [{CpV(μ-EPh)}2{μ-η2:η2-BHmRnE}] (1: E = S, m = 3 and n = 0; 2: E = Se, m = 3 and n = 0, and 3: E = Se, R = OC4H8, m = 1 and n = 1).18a The 11B{1H} NMR spectra of 1, 2, and 3 show a single resonance appeared each at δ = 33.4, 33.7, and 33.6 ppm, respectively. The 1H and 13C{1H} NMR spectra suggest the presence of two equivalents of Cp ligands for each compound. Also, the 1H NMR spectra of all the compounds show one resonance corresponding to V-H-B protons (1: δ = −9.16, 2: δ = −9.68, and 3: δ = −9.67 ppm). The V-H-B resonance obtained for 3 was weak, and thus, we have tried to get an improved 1H NMR spectrum at low temperature. However, the spectrum obtained was not better than that at room temperature. We believe that the humps that appeared in the 1H NMR spectrum for compound 3 are due to the weak resonance of a single V-H-B proton and that is the reason for poor resolution of the V-H-B resonance. Nonetheless, the low temperature NMR is reasonably better as compared to that of the room temperature one. We have observed weak terminal B−H and V-H-B resonances in the proton spectra for compounds 1 and 2. We found that 11B decoupling in the proton NMR for compounds 1−3 was not fruitful for terminal B-H protons as well as for V-H-B protons. It is important to note that the line width of the protons coupled to 11B for compound 6 (Figure S28) is around 30%− 35% less than the line width (1H NMR - Figure S12: −9.16 for compound 1, and Figure S15: ∼ −9.68 ppm for compound 2) found for compounds 1−3. This could be attributed to very rapid relaxation (especially spin−spin relaxation, T2) of 1H coupled to 11B and 51V. Similarly, due to the same reason as described above, we could not resolve the 1H−11B coupling.

The 11B signal is very poor without 1H decoupling, even with large scans. It is well documented in the literature that the fast relaxation of 11B often mitigated the observation of 11B−X coupling.18b,c We have provided the 1H chemical shift of terminal B-H in the Supporting Information. The reaction of Cp2VCl2 with Li[BH3(SePh)] was monitored by 11B{1H} NMR spectroscopy, both at low temperature and high temperature. At low temperature, the resonance at δ = 1.7 ppm indicates the formation of {(CpV)2(B2H6)2}13 which might have progressively reacted with the in situ generated SePh− fragments and yielded [{CpV(μ-SePh)}2{μ-η2:η2BH3Se}] and [{CpV(μ-SePh)}2{μ-η2:η2-BH(OC4H8)Se}]. At high temperature, the formation of compounds 2 and 3 improved and the same has been noticed in NMR. In order to confirm the spectroscopic assignments and to conclude the solid-state X-ray structures of 1, 2, and 3, the single-crystal Xray diffraction analyses were undertaken (Figure 1). As shown in Figure 1, the solid-state X-ray structures of 1−3 can be seen as [{CpV(μ-EPh)}2{μ-η2:η2-BH3E}] (1: E = S and 2: E = Se) and [{CpV(μ-SePh)}2{μ-η2:η2-BH(OC4H8)Se}] (3) in which each metal is coordinated to a η5-Cp ligand and two μbridging EPh ligands (E = S or Se). One of the interesting structural features of these compounds is the presence of [BH3S], [BH3Se], and [B(OC4H8)HSe] moieties which are perpendicular to the V−V axis. All of these compounds feature a direct metal−metal bond which is comparable to that of [(CpV)2(B2H6)2] (dV−V = 2.6816(12) Å)13 and [(CpV)2(μTePh)2(μ3-Te)BH(OC4H8)] (dV−V = 2.8587(9) Å).17c The observed B−S and B−Se bond lengths (1: dB1−S3 = 1.684(18) Å, 2: dB1−Se3 = 2.23(5) Å, and 3: dB1−Se3 = 2.064(6) Å) are in accord with the sum of their covalent radii of B, S, and Se atoms. Another similarity in all the three molecules is the B

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points (BCPs) for V1−S3 [ρ(0.083 au) and ∇2ρ(r) (0.205 au)] and B1−S3 [ρ(0.119 au) and ∇2ρ(r) (−0.090 au)] bonds (see Figure 2a, Table S2, and the Supporting Information). The calculated Laplacian of the electron density for 1 also shows the presence of polar V−H−B interactions (Figure 2b, Table S2), which is in agreement with the spectroscopic data.22 Furthermore, the NBO analysis suggests that the boron atom in 1 interacts with two vanadium atoms via two “3-center-2electron” V−H−B bonds with 24% contribution from V, 40% and 36% from H and B, respectively. In addition, the analysis predicts the presence of a σ-bond between B1 and S3 (Figure 2c).23 Wiberg bond indice (WBI) on the natural atomic orbital (NAO) of 1 predicts significant bonding interactions between V and S3 and B1 and S3 (WBIV1−S3 = 0.98, WBIV2−S3 = 0.96, and WBIB1−S3 = 0.76). The WBI value of 1.16, corresponding to the V−V bond, further supports a strong coupling between the V atoms (Figure 2d). To see the role of coordinated THF attached to the boron atom in 3 toward cluster stability, the DFT computed frontier molecular orbitals (PBE0/Def2-TZVP level) of analogous 1−3 were studied and analyzed. Frontier molecular orbital analysis predicts low HOMO−LUMO gaps for both compounds 1 and 2 (1: EHOMO−LUMO = 1.79 eV and 2: EHOMO−LUMO = 1.76 eV, Figure S32) that is consistent with less stability. However, a significant increase in the energy of LUMO in 3 enhances the HOMO−LUMO gap (0.44 and 0.41 eV compared to 1 and 2, respectively). This may be due to the presence of a THF moiety that destabilizes LUMO in 3. The FMOs of 1 and 2 were found to be very similar. The LUMOs of both the compounds are mainly localized on V centers and chalcogen atoms, while the HOMOs are heavily localized on V metal centers. [{CpNb(μ-EPh)}2{μ-η2:η2-B2H4E}] (4: E = S; 5: E = Se). The structurally characterized electron precise group 5 dimetallaheteroboranes are very rare.25 Thus, in an effort to synthesize the analogues of 1−3, we have extended this chemistry to the Nb system. As shown in Scheme 3, the room temperature reaction of [CpNbCl4] with Li[BH3(EPh)] in toluene for 16 h led to the formation of diniobaboranes [{CpNb(μ-EPh)}2{μ-η2:η2-B2H4E}], 4 and 5 (4: E = S and 5: E = Se). They were isolated as green and pink solids, respectively, and characterized by 1H, 1H{11B}, 11B{11H}, and 13 C{1H} NMR spectroscopy and single-crystal X-ray diffraction studies. The 11B{1H} NMR spectra of them show two types of resonances with an intensity ratio of 1:1 (4: δ = 15.6 and −36.1; 5: δ = 18.9 and −26.0 ppm). The 1H{11B} NMR spectra of 4 and 5 revealed a single resonance for Cp protons at δ = 5.74 and 5.70 (for 4) and 6.10 (for 5) ppm, respectively. The

presence of two μ-bridging EPh moieties (E = S, Se), which are bridged between two [CpV] units. The existence of electron precise diborane(6) species of vanadium, [(CpV)2(B2H6)2],13 encouraged us to compare the observed cage frameworks of 1−3 in terms of valence electron counts. For example, in 1, the three connected sulfur atom is a four electron donor and isoelectronic with the BH3 unit. Thus, the [BH3S] unit in 1 may be considered isoelectronic to [B2H6] that carries a 2− charge like [B2H6]2−. The [B2H6]2− unit bridged between two metal centers provides 8 electrons to the two metal centers.9a Considering the neutral electron count approach, similarly, the [BH3S] unit provides six electrons to the V metals. On the other hand, two μ-EPh ligands which are bridged between two vanadium centers contribute six electrons to the V atoms similar to that of the [B2H6] unit. Hence, compound 1 can be compared to [(CpV)2(B2H6)2] and they can be connected (Scheme 2a→b→c) through a hypothetical Scheme 2. Structures of Complexes (a) [{CpV(μ-SPh)}2{μη2:η2-BH3S}], (1); (b) a Hypothetical Complex, [{CpV(μSPh)}2{μ-η2:η2-B2H6}], and (c) [(CpV)2(B2H6)2]

complex, [{CpV(μ-SPh)}2{μ-η2:η2-B2H6}] (Scheme 2b). This analogy clearly demonstrates that compound 1 and [(CpV)2(B2H6)2] are isoelectronic and structurally analogous to each other. Similarly, compounds 2 and 3 can also be associated with [(CpV)2(B2H6)2] having the same electron counts as that of 1. The [BH3E] and [BH(OC4H8)Se] moieties in compounds 1−3 are isoelectronic to that of [B2H6] and comparable to that of [(Cp*TaBr)2B2H6],16 [(Cp*Ta)2(B2H6)2],13 and [(CpM)2(B2H6)2] (M = V or Nb).13 To gain insight into the electronic structure and bonding, the DFT calculations19 were carried out on compounds 1−3 at the PBE0/Def2-TZVP level of theory.20 Coordination of [BH3S] with V metals in 1 is supported by topological analysis as well as by natural bond orbital (NBO) analysis. The results show the forward donation of electron density of the [BH3S] unit to vanadium metal centers via V−S bonding and two “3-center-2electron” V−H−B bonding interactions (Figure 2). The topological analysis21 reveals the existence of bond critical

Figure 2. (a, b) Contour line diagrams of the Laplacian of the electron density, ∇2ρ(r) of 1 in the plane of V1−B1−S3 and V1−H−B1, generated using the Multiwfn program package at the PBE0/Def2-TZVP level of theory.24 (c, d) Illustrations of natural bond orbitals showing σ-type B−S and V−V bonding interactions, respectively. C

DOI: 10.1021/acs.inorgchem.7b02305 Inorg. Chem. XXXX, XXX, XXX−XXX

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and [μ3-E] (E = S or Se) fragments contribute one, two, and four electrons to the cluster bonding framework, respectively; compounds 4 and 5 possesses 6 skeletal electron pairs (sep) ({2(Cp(μ-EPh)Nb) × 1 + (μ3-E) × 4 + 2(BH) × 2 + 2(H) × 1}/2 = 6; E = S or Se). Therefore, they are electronically saturated molecules and obey Wade’s rule. Unlike [(Cp*MoCl)2B3H7], compounds 4 and 5 are electronically saturated due to the presence of bridging EPh ligands attached to two Nb centers. The bonding interaction of the [B2H4S] unit with Nb centers in 4 has also been shown using the Laplacian of the electron density plots (Figure S33). Synthesis of [(Cp*Ta)2(μ-Se)B3H6Se(C6H5)] (6). In an effort to synthesize the analogues of 1−3, we have carried out the reaction of [Cp*TaCl4] with Li[BH3(SePh)]. The reaction led to the formation of [(Cp*Ta)2(μ-Se)B3H6Se(C6H5)], (6), in moderate yield (Scheme 4). The 11B{1H} NMR spectrum of

Scheme 3. Synthesis of Compounds 4 and 5

presence of Cp ligands has also been supported by 13C{1H} NMR spectroscopy. In addition to the Cp protons, the 1H{11B} NMR spectrum reveals the presence of Nb-H-B resonances at δ = −6.24, −6.81 (for 4), and −5.97 ppm (for 5), respectively. Suitable quality single crystals for X-ray diffraction analysis of 4 and 5 were obtained from n-hexane and dichloromethane solution at −5 °C that permitted structural characterization (Figure 3). The geometries of compounds 4 and 5 are

Scheme 4. Synthesis of Compound [(Cp*Ta)2(μSe)B3H6Se(C6H5)], 6

6 shows three resonances that appeared at δ = 46.5, −4.7, and −18.0 ppm. Further, the 1H and 13C{1H} NMR spectra show one type of Cp* ligand. Besides the Cp* protons, the 1H NMR spectrum exhibits broad signals for B-H protons (δ = 3. 51 ppm) and Ta-H-B protons (δ = −7.03 and −10.25 ppm). The spectroscopic data might seem to indicate different chemical compositions as compared to 1−5. In order to confirm the spectroscopic assignments and to determine the solid-state structure of 6, the X-ray structure analysis was carried out. The solid-state X-ray crystal structure of 6, shown in Figure 4, resembles that of a bicapped tetrahedron in which two of the Ta2B faces are capped by Se and BH3 group, respectively. A similar structural interpretation has also been proposed for [(Cp*Ta)2B4H10],29 [(Cp*M)2B4H8]30a,b (M = Cr or Re), [(Cp*Mo)2B4H6(CO)2],30c and [Cp#Co)2B2H2E2]27,28b,31 (when Cp# = Cp; E = S, and when Cp# = Cp*; E = S or Se) in which the tetrahedron core is capped by either 4 electron BH3 or S/Se fragment. The Ta−Ta bond distance of 2.815(1) Å is significantly shorter as compared to that of [(Cp*Ta)2B4H10] (2.8909(4) Å).29 This may be due to the presence of two Se atoms instead of two BH3 fragments.32 Some of the typical structural parameters for 6 have been compared with those for [(Cp*M) 2 (μ-S)B2H2S2]28a,c (M = Mo or W) and other related systems of Mo28a and Co27,28b that exhibit distinct structural differences (Table 1). Deviations of the B−B and Ta−B bond lengths may be attributed due to the larger size of Ta and additional bridging hydrogen atoms. The 1H{11B} NMR shows two upfield resonances at δ = −7.02 and −10.24 ppm with a 1:1 ratio that correspond to four Ta-H-B protons. The 1H chemical shift for the two B-H terminal protons appeared at δ = 3.51

Figure 3. (a) Molecular structure and labeling diagram of [{CpNb(μSPh)}2{μ-η2:η2-B2H4S}], 4, with ellipsoids drawn at the 30% probability level. Selected bond distances (Å) and angles (deg): Nb(1)−Nb(2) 2.7726(12), Nb(1)−S(1) 2.564(3), Nb(1)−B(1) 2.401(12), Nb(1)−B(2) 2.435(12), B(1)−B(2) 1.60(2), B2−S3 1.723(16); Nb(1)−B(1)−Nb(2) 70.7(4), Nb(1)−B(2)−Nb(2) 68.3(3). (b) Molecular structure and labeling diagram of [{CpNb(μSePh)}2{μ-η2:η2-B2H4Se}], 5, with ellipsoids drawn at the 30% probability level. Selected interatomic distances (Å) and angles (deg): Nb(1)−Nb(2) 2.7698(12), Nb(1)−Se(1) 2.7092(14), Nb(1)−B(1) 2.401(11), Nb(1)−B(2) 2.453(11), B(1)−B(2) 1.732(17), B(2)−Se(3) 1.874(13); Nb(1)−B(1)−Nb(2) 70.8(3), Nb(1)−B(2)−Nb(2) 68.3(3). All the bridging Nb-H-B and BHt hydrogen atoms for compounds 4 and 5 were not located in the Xray diffraction studies.

analogous to that of [(Cp*MoCl)2B3H7].26 The boron−boron bond distance of 1.60(2) Å in 4 is comparatively shorter than that of [(CpNb)2(B2H6)2]13 (1.819(19) Å). Similarly, the B−S bond length of 1.723(16) Å in 4 is shorter than that observed in metallathiaborane clusters, [(Cp*Co)2B2S2H2]27 (dB−S = 1.823(3) Å), and [(Cp*Mo)2(μ3-S)(μ-η1-SPh)2(H2BSPh)]28a (dB−S = 1.852(5) Å). The Nb1−Nb2 bond lengths in 4 and 5 [4: dNb1−Nb2 = 2.7726(12) Å and 5: dNb1−Nb2 = 2.7698(12) Å] are significantly shorter as compared to [(CpNb)2(B2H6)2] (2.9477(16) Å). The contraction of Nb−Nb bond lengths in both clusters 4 and 5 might have arisen due to the presence of μ-bridging EPh (E = S or Se) units attached to metal centers. A comparison of compounds 4 and 5 with that of dimolybda species [(Cp*MoCl)2B3H7]26 with regard to the electron count shows a significant inconsistency. As [CpNb(μ-EPh)], [BH], D

DOI: 10.1021/acs.inorgchem.7b02305 Inorg. Chem. XXXX, XXX, XXX−XXX

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ppm. This may be due the accidental overlap of two B-H resonances. UV/vis Absorption Spectroscopic Study of 1−6. In order to explain the absorption pattern of these colored compounds, the UV−vis spectra were recorded in CH2Cl2. As shown in Figure S2, compound 1 shows three absorption bands at λ = 284, 357, and 463 nm, respectively. The band at 284 nm can be assigned to the π−π* transition of cyclopentadienyl ligands, whereas 357 and 463 nm can be considered for MLCT bands. Compounds 2 and 3 show similar kinds of absorption bands (2: λ = 270, 373, and 484 nm; 3: λ = 277, 372, and 476 nm) (Figures S2 and S3). The shift in the MLCT transition wavelengths in the cases of 1 and 2 is consistent with their corresponding ΔE values. The shift in the charge transfer absorption bands for 2 and 3 may be attributed due to the coordination of the THF moiety in 3. The electronic spectra of compounds 4, 5, and 6 displayed intense charge transfer interactions that explain their color in solution. Compound 4 shows its absorption profiles at λ = 321 and 411 nm, whereas, for 5, the absorption bands are observed at λ = 276, 383, and 483 nm, respectively (Figures S3 and S4). The bands at λ = 411 nm (for compound 4) and at λ = 483 nm (for compound 5) correspond to intramolecular MLCT transitions. For com-

Figure 4. Molecular structure and labeling diagram of [(Cp*Ta)2(μSe)B3H6Se(C6H5)], 6, with ellipsoids drawn at the 10% probability level. Selected bond distances (Å) and angles (deg): Ta(1)−Ta(2) 2.815(1), Ta(1)−B(1) 2.48(2), Ta(1)−B(2) 2.43(2), Ta(1)−B(3) 2.39(3), Ta(2)−B(1) 2.43(2), Ta(2)−B(2) 2.41(3), Ta(2)−B(3) 2.35(3), Ta(1)−Se(1) 2.514(3), Ta(1)−Se(2) 2.677(3), B(1)−B(2) 1.64(3), B(2)−B(3) 1.49(4); B1(1)-B(2)−B(3) 132(2). The bridging Ta-H-B hydrogen atoms were not located in the X-ray diffraction study.

Table 1. Selected Structural Parameters of [(Cp*Ta)2(μ-Se)B3H6Se(C6H5)] and Related Speciesa

a

Av = Average. E

DOI: 10.1021/acs.inorgchem.7b02305 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

warmed to room temperature over 1 h and then heated to 90 °C for 48 h. The solvent was dried and the residue was extracted into nhexane and passed through Celite. After removal of the solvent, the residue was subjected to chromatographic workup by using TLC plates (n-hexane/CH2Cl2, 70:30 v/v) to yield compound 1 as a dark brown solid (0.034 g, 35%), Rf = 0.72. Under similar reaction conditions, the reaction of [Cp2VCl2] (0.100 g, 0.40 mmol) with freshly prepared Li[BH3(SePh)] in THF (10 mL, 1.0 mmol, 0.1 mol/L) resulted in brown compounds 2 (0.036 g, 28%), Rf = 0.72, and 3 (0.045 g, 32%), Rf = 0.69. 1: HRMS (ESI+): m/z calcd for C22H23BS3V2 + Na+: 518.9832 [M + Na+]; found: 518.9815. 1H NMR (500 MHz, CDCl3, 22 °C): δ = 7.63−7.10 (m, 10H; 2 × C6H5), 5.94 (s, 10H; 2 × Cp), 3.44 (br, 1H; BHt), −9.16 ppm (br, 2H; V-H-B); 11B{1H} NMR (160 MHz, CDCl3, 22 °C): δ = 33.4 (br, 1B) ppm; 13C{1H} NMR (125 MHz, CDCl3, 22 °C): δ = 134.7, 132.4, 131.0, 126.5 (s, C6H5), 94.5 ppm (s, C5H5); IR (Hexane): ν̃ = 2530 (BHt), 807 cm−1 (V-S). 2: HRMS (ESI+): m/z calcd for C22H23BSe3V2 + H+: 640.8346 [M + H+]; found: 640.8314. 1H NMR (500 MHz, CDCl3, 22 °C): δ = 7.74−7.18 (m, 10H; 2 × C6H5), 6.05 (s, 10H; 2 × Cp), 3.67 (br, 1H; BHt), −9.68 ppm (br, 2H; V-H-B); 11B{1H} NMR (160 MHz, CDCl3, 22 °C): δ = 33.7 (br, 1B) ppm; 13C{1H} NMR (125 MHz, CDCl3, 22 °C): δ = 136.9, 132.9, 131.9, 125.3 (s, C6H5), 95.6 ppm (s, C5H5); IR (Hexane): ν̃ = 2539 cm−1 (BHt). 3: HRMS (ESI+): m/z calcd for C26H29BOSe3V2 + Na+: 732.8584 [M + Na+]; found: 732.8594. 1H NMR (500 MHz, d8-toluene, −80 °C): δ = 7.45−7.26 (m, 10H; 2 × C6H5), 6.71 (br, 10H; 2 × Cp), 3.58 (m, 4H; C4H8O), 1.42 (m, 4H; C4H8O), −9.67 ppm (br, 1H; V-H-B); 11 1 B{ H} NMR (160 MHz, CDCl3, 22 °C): δ = 33.6 ppm (br, 1B); 13 C{1H} NMR (125 MHz, d8-toluene, −80 °C): δ = 137.4 (s, C6H5), 101.1 (s, C5H5), 67.7 (s, C4H8O), 25.7 ppm (s, C4H8O); IR (Hexane): ν̃ = 1365 cm−1 (B-O). Synthesis of 4 and 5. In a flame-dried Schlenk tube, [CpNbCl4] (0.100 g, 0.33 mmol) was suspended in toluene (6 mL), and the mixture was cooled to −78 °C. Freshly prepared Li[BH3(SPh)] in tetrahydrofuran (8 mL, 0.8 mmol, 0.1 mol/L) was added via syringe, and the solution was allowed to warm slowly to room temperature and stirred for 16 h. The solvent was dried and the residue was extracted into n-hexane and passed through Celite. The solvent was removed, and the residue was subjected to chromatographic workup by using TLC plates (n-hexane/CH2Cl2, 70:30 v/v) to yield green compound 4 (0.0138 g, 14%), Rf = 0.69. Under the similar experimental conditions, treatment of Li[BH3(SePh)] (0.144 g, 0.8 mmol) with [CpNbCl4] (0.100 g, 0.33 mmol) yielded pink 5 (0.046 g, 38%), Rf = 0.62. 4: MS (ESI+): m/z calcd mass for C22H24B2S3Nb2 + H+: 592.9 [M + + H ]; found: 592.6. 1H{11B} NMR (500 MHz, CDCl3, 22 °C): δ = 7.43−7.05 (m, 10H; 2 × C6H5), 5.74, 5.70 (s, 10H; 2 × Cp), 4.62 (br, 1H; BHt), 2.03 (br, 1H; BHt), −6.24, −6.81 ppm (br, 2H; Nb-H-B); 11 1 B{ H} NMR (160 MHz, CDCl3, 22 °C): δ = 15.6 (br, 1B), −36.1 ppm (br, 1B); 13C{1H} NMR (125 MHz, CDCl3, 22 °C): δ = 129.6, 128.5, 126.8, 124.6 (s, C6H5), 98.6 ppm (s, C5H5); IR (Hexane): ν̃ = 2503 cm−1 (BHt). 5: HRMS (ESI+): m/z calcd for C22H24B2Se3Nb2 + H+: 736.7766 [M + H+]; found: 736.7784. 1H NMR (500 MHz, CDCl3, 22 °C): δ = 7.55−7.00 (m, 10H; 2 × C6H5), 6.03 (s, 10H; 2 × Cp), 1.80 (br, 1H; BHt) −5.94 ppm (s, 2H; Nb-H-B); 1H{11B} NMR: δ = 7.67−6.81 (m, 10H; 2 × C6H5), 6.40 (br, 1H; BHt), 6.10 (s, 10H; 2 × Cp), 1.86 (br, 1H; BHt) −5.97 ppm (s, 2H; Nb-H-B); 11B{1H} NMR (160 MHz, CDCl3, 22 °C): δ = 18.9 (br, 1B), −26.0 ppm (br, 1B); 13C{1H} NMR (125 MHz, CDCl3, 22 °C): δ = 131.9, 131.5, 129.9, 125.7 (s, C6H5), 95.9 ppm (s, C5H5); IR (Hexane): ν̃ = 2470 cm−1 (BHt). Synthesis of 6. In a flame-dried Schlenk tube, [Cp*TaCl4] (0.100 g, 0.22 mmol) was dissolved in 10 mL of toluene. The resulting solution was cooled to a temperature of −78 °C. Freshly prepared Li[BH3(SePh)] in THF (6 mL, 0.6 mmol, 0.1 mol/L) was added to the reaction mixture, and the solution was allowed to warm slowly to room temperature. The resulting reaction mixture was then heated to 90 °C for 16 h. The solvent was evaporated in vacuo; the residue was extracted into hexane and passed through Celite. After removal of solvent, the residue was subjected to chromatographic workup using

pound 6, a shoulder peak is observed at λ = 281 nm and the peak at λ = 471 nm signifies the charge transfer interaction (Figure S4).

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CONCLUSION To sum up, this work reports the synthesis and structural characterization of a series of electron precise dimetallaheteroboranes (1−3) of group 5 metals which are isoelectronic to [(CpV)2(B2H6)2]. In addition, the DFT studies further show that the bonding interactions of compounds 1−3 are comparable to those of [(Cp*TaBr)2B2H6], [(Cp*Ta)2(B2H6)2], and [(CpM)2(B2H6)2] (M = V or Nb). The extension of this work to the tantalum system led us to isolate [(Cp*Ta)2(μSe)B3H4Se(C6H5)]. The usage of chalcogenoborate ligands Li[BH3(EPh)] (E = S, Se or Te) for the synthesis of dimetallaheteroboranes of group 5 metals opened a new synthetic route for the expansion of other dimetallaheteroborane compounds.

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EXPERIMENTAL SECTION

General Procedures and Instrumentation. All of the operations were conducted under an Ar/N2 atmosphere by using standard Schlenk techniques or in a glovebox. Solvents were distilled prior to use under an Ar atmosphere. [Cp2VCl2], [LiBH4.THF], [Ph2S2], and [Ph 2 Se 2 ] were used as received (Aldrich). [CpNbCl 4 ], 33a [Cp*TaCl4],33b Li[BH3(EPh)]33c (E = S or Se), and the external reference for the 11B{1H} NMR analysis, [Bu4N(B3H8)],34 were synthesized according to the literature methods available. Thin-layer chromatography was carried out on 250 μm diameter aluminum supported silica gel TLC plates (MERCK TLC Plates) to separate the reaction mixtures. The NMR spectra were recorded on a 500 MHz Bruker FT-NMR spectrometer. The 11B decoupled 1H and 1H decoupled 11B spectra were obtained with inverse gated decoupling (zgig) and with power gated decoupling (zgpr) pulse sequences, respectively. All pulse sequences are available in the commercial Bruker spectrometer. 1H decoupled 11B spectra were processed with a backward linear prediction algorithm to remove the broad 11B background signal of the NMR tube.35 We found that 11B decoupling in the proton NMR for compounds (1−3) was not fruitful. This may be attributed to the very short spin−spin (T2) relaxation time of 1H coupled to 11B. However, in the case of compounds 4−6, we observed decoupling. Moreover, the line width of the protons coupled to 11B of compound 6 is around 30%−35% less than the line width found for compounds (1−3). The proton signals (B-H terminal and M-H-B) observed for compounds 1−5 are very weak and broad [especially for M-H-B proton resonances (1H NMR - Figure S12: −9.16 ppm, Figure S15: around ∼ −9.68 ppm)]. The decoupling for vanadaboranes (1− 3) and to some extent for niobaboranes (compounds 4 and 5) did not work as the reason explained above. Thus, we were not able to measure the relative integrated intensities of the bridging and terminal proton resonances for compounds (1−5). However, in the case of compound 6, we have provided the relative integrated intensities for the same. The 1H{11B} NMR spectrum of compound 6 with measured relative integrated intensities has been provided in the Supporting Information. Residual solvent protons were used as reference (CDCl3, δ = 7.26 ppm), while a sealed tube that contained [Bu4N(B3H8)] in [D6]-benzene (δB = −30.07 ppm) was used as an external reference for the 11B{1H} NMR analysis. Mass spectra were recorded with a BrukerMicroTOF-II mass spectrometer in ESI ionization mode. Infrared spectra were recorded with a JASCO FT/IR-4100 spectrometer. Absorption spectra were recorded with Evolution 300 (Thermo Scientific) or Jasco V-650 UV/vis spectrophotometers at 298 K. Synthesis of 1−3. A suspension of [Cp2VCl2] (0.100 g, 0.40 mmol) in 8 mL of toluene at −78 °C was charged dropwise with a freshly prepared solution of Li[BH3(SPh)] in tetrahydrofuran (THF) (10 mL, 1.0 mmol, 0.1 mol/L) over 15 min. The reaction mixture was F

DOI: 10.1021/acs.inorgchem.7b02305 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

analogue compounds, instead of Cp* in order to save computing time. Without any symmetry constraints, all the geometry optimizations were carried out in gaseous state (no solvent effect), using the PBE0 functional37 in combination with the triple-ζ quality basis set Def2TZVP. The projected 11B chemical shielding values, determined at the PBE0/Def2-TZVP level of calculations, were referenced to B2H6 (PBE0/Def2-TZVP, B shielding constant 85.89 ppm), and these chemical shift values (δ) were then converted to the standard BF3· OEt2 scale using the experimental value of +16.6 ppm for B2H6. 1H chemical shifts were referenced to TMS (SiMe4). Computation of the NMR shielding tensors employed gauge-including atomic orbitals (GIAOs),38 using the implementation of Schreckenbach, Wolff, Ziegler, and co-workers.39 Natural bond orbital (NBO) analysis40 within the Gaussian09 package was carried out at the same level of theory. Wiberg bond indexes (WBI)41 values on some selected bonds were obtained on natural bond orbital (NBO) analysis. The ChemCraft package42 was used for the visualizations. The two dimensions electron density and Laplacian electronic distribution plots were generated using Multiwfn package.43

silica-gel TLC plates. Elution with hexane/CH2Cl2 (70:30 v/v) yielded dark orange 6 (0.038 g, 39%), Rf = 0.79. 6: HRMS (ESI+): m/z calcd for C26H41B3Se2Ta2 + K+: 947.0415 [M + K+]; found: 947.0440. 1H NMR (500 MHz, CDCl3, 22 °C): δ = 7.42−6.95 (m, 5H; 1 × C6H5), 3.47 (br, 2H; BHt), 2.09 (s, 30H; 2 × Cp*), −7.03, −10.26 ppm (br, 4H; Ta-H-B); 1H{11B} NMR: δ = 7.42−6.95 (m, 5H; 1 × C6H5), 3.51 (br, 2H; BHt), 2.09 (s, 30H; 2 × Cp*), −7.02, −10.25 ppm (br, 4H; Ta-H-B); 11B{1H} NMR (160 MHz, CDCl3, 22 °C): δ = 46.5 (br, 1B), −4.7 (br, 1B), −18.0 ppm (br, 1B); 13C{1H} NMR (125, MHz, CDCl3, 22 °C): δ = 131.5, 127.7, 126.8, 124.1 (s, C6H5), 99.1 (s, C5Me5), 10.4 ppm (s, C5Me5); IR (Hexane): ν̃ = 2531 cm−1 (BHt). X-ray Structure Determination. The crystal data for 1 and 4−6 were collected and integrated using a Bruker Axs kappa apex II CCD diffractometer, and crystal data for 2 and 3 were collected using a SuperNova dual CCD diffractometer with graphite-monochromated MoKα (λ = 0.71073 Å) radiation at 296 K. The structures were solved by heavy atom methods using SHELXS-97 or SIR92 and refined using SHELXL-2014.36 Compound 1 has boron and sulfur atoms disordered over two positions with an occupancy factor of 0.81 and 0.19. The structure has been refined for a proper model, and the R = 0.561. Compound 2 has boron and selenium atoms disordered over two positions with an occupancy factor of 0.86 and 0.14 (Figure S1). In the structure of compound 6, the phenyl ring containing carbon C21−C26 is disordered over two positions with an occupancy factor of 0.48 and 0.52. Similarly, the ring containing carbon C47−C52 is also disordered over two positions with an occupancy factor of 0.50 each. The selenium atoms (Se1 and Se2) are disordered over two positions with an occupancy factor of 0.72 and 0.28. Similarly, Se3 and Se4 atoms are also disordered over two positions with an occupancy factor of 0.59 and 0.41. Crystal Data for 1. CCDC 1550451, C22H23BS3V2, Mr = 496.27, Monoclinic, P21/c, a = 15.1673(9) Å, b = 8.1585(4) Å, c = 18.7779(10) Å, α = 90°, β = 111.042(3)°, γ = 90°, V = 2168.7(2) Å3, Z = 4, ρcalcd = 1.520 mg/m3, μ = 1.156 mm−1, F(000) = 1016, R1 = 0.0561, wR2 = 0.1249, Goodness-of-fit on F2 = 1.086, 3551 unique reflections [2θ ≤ 48.8°] and 285 parameters. Crystal Data for 2. CCDC 1550452, C22H20BSe3V2, Mr = 633.96, Monoclinic, P21/c, a = 15.4601(13) Å, b = 8.3268(6) Å, c = 18.7319(14) Å, α = 90°, β = 110.757(9)°, γ = 90°, V = 2254.9(3) Å3, Z = 4, ρcalcd = 1.873 mg/m3, μ = 5.677 mm−1, F(000) = 1228, R1 = 0.0879, wR2 = 0.2655, Goodness-of-fit on F2 = 1.006, 4422 unique reflections [2θ ≤ 51.9°] and 251 parameters. Crystal Data for 3. CCDC 1550453, C26H28BOSe3V2, Mr = 706.05, Triclinic, P1̅, a = 10.1928(4) Å, b = 10.9834(5) Å, c = 13.9085(6) Å, α = 67.308(4)°, β = 86.570(3)°, γ = 68.295(4)°, V = 1328.45(11) Å3, Z = 2, ρcalcd = 1.765 mg/m3, μ = 4.831 mm−1, F(000) = 690, R1 = 0.0540, wR2 = 0.1900, Goodness-of-fit on F2 = 1.052, 8570 unique reflections [2θ ≤ 64.3°] and 298 parameters. Crystal Data for 4. CCDC 1550454, C22H24B2Nb2S3, Mr = 592.03, Monoclinic, P21/c, a = 8.1641(4) Å, b = 17.8803(9) Å, c = 16.4511(8) Å, α = 90°, β = 102.774(3)°, γ = 90°, V = 2342.0(2) Å3, Z = 4, ρcalcd = 1.679 mg/m3, μ = 1.253 mm−1, F(000) = 1184, R1 = 0.0606, wR2 = 0.1505, Goodness-of-fit on F2 = 1.041, 4006 unique reflections [2θ ≤ 49.46°] and 278 parameters. Crystal Data for 5. CCDC 1550455, C22H24B2Nb2Se3, Mr = 732.73, Monoclinic, P21/c, a = 8.2648(8) Å, b = 18.2752(19) Å, c = 16.6983(17) Å, α = 90°, β = 103.688(3)°, γ = 90°, V = 2450.5(4) Å3, Z = 4, ρcalcd = 1.986 mg/m3, μ = 5.398 mm−1, F(000) = 1400, R1 = 0.0539, wR2 = 0.1156, Goodness-of-fit on F2 = 1.025, 4167 unique reflections [2θ ≤ 49.47°] and 274 parameters. Crystal Data for 6. CCDC 1550456, C26H37B3Se2Ta2, Mr = 901.80, Triclinic, P1̅, a = 14.0462(7) Å, b = 14.2532(7) Å, c = 16.1709(8) Å, α = 78.249(3)°, β = 69.823(3)°, γ = 89.796(3)°, V = 2967.3(3) Å3, Z = 4, ρcalcd = 2.019 mg/m3, μ = 9.836 mm−1, F(000) = 1688, R1 = 0.0776, wR2 = 0.2167, Goodness-of-fit on F2 = 1.049, 10377 unique reflections [2θ ≤ 49.98°] and 731 parameters. Computational Details. Quantum chemical calculations were performed on compounds 1−5 using DFT as implemented in the Gaussin09 package.19 The calculations were carried out with the Cp

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02305. Spectroscopic data for 1−6 and other computational details (PDF) Accession Codes

CCDC 1550451−1550456 contain 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 Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91-44-22574230. Fax: +9144-22574202. ORCID

Sundargopal Ghosh: 0000-0001-6089-8244 Funding

This project was supported by the SERB, New Delhi, India (project no. EMR/2015/001274). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to dedicate this paper to Prof. N. Chandrakumar of IIT Madras on the occasion of his 65th birthday, in recognition of his outstanding contributions to the field of NMR spectroscopy. M.G.C. and S.K.B. thank IIT Madras and B.K. thanks SERB-National postdoctoral fellowship (PDF/2016/003998), New Delhi, India, and K.S. thanks CSIR, India, for research fellowships. A.B. thanks SERB, India, for the young scientist award. We thank Dr. Babu Varghese (SAIF, IIT Madras), Dr. R. Jagan (IIT Madras), Dr. Shaikh M. Mobin (IIT Indore), and Anoop Kumar Saini (IIT Indore) for X-ray data analysis. IIT Madras is gratefully acknowledged for computational facilities. G

DOI: 10.1021/acs.inorgchem.7b02305 Inorg. Chem. XXXX, XXX, XXX−XXX

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