Synthesis, Structures and Chemistry of the Metallaboranes of Group 4–9 with M2B5 Core Having a Cross Cluster M–M Bond

In an attempt to expand the library of M2B5 bicapped trigonal-bipyramidal clusters with different transition metals, we explored the chemistry of [Cp*WCl4] with metal carbonyls that enabled us to isolate a series of mixed-metal tungstaboranes with an M2{B4M’} {M = W; M’ = Cr(CO)4, Mo(CO)4, W(CO)4} core. The reaction of in situ generated intermediate, obtained from the low temperature reaction of [Cp*WCl4] with an excess of [LiBH4·thf], followed by thermolysis with [M(CO)5·thf] (M = Cr, Mo and W) led to the isolation of the tungstaboranes [(Cp*W)2B4H8M(CO)4], 1–3 (1: M = Cr; 2: M = Mo; 3: M = W). In an attempt to replace one of the BH—vertices in M2B5 with other group metal carbonyls, we performed the reaction with [Fe2(CO)9] that led to the isolation of [(Cp*W)2B4H8Fe(CO)3], 4, where Fe(CO)3 replaces a {BH} core unit instead of the {BH} capped vertex. Further, the reaction of [Cp*MoCl4] and [Cr(CO)5·thf] yielded the mixed-metal molybdaborane cluster [(Cp*Mo)2B4H8Cr(CO)4], 5, thereby completing the series with the missing chromium analogue. With 56 cluster valence electrons (cve), all the compounds obey the cluster electron counting rules. Compounds 1–5 are analogues to the parent [(Cp*M)2B5H9] (M= Mo and W) that seem to have generated by the replacement of one {BH} vertex from [(Cp*W)2B5H9] or [(Cp*Mo)2B5H9] (in case of 5). All of the compounds have been characterized by various spectroscopic analyses and single crystal X-ray diffraction studies.

As part of our continuous interest on metallaborane chemistry of group 4-9 transition metals [23][24][25][26][27][28][29][30][31][32][33][44][45][46][47][48][49][50], we have synthesized a variety of metallaboranes by cluster expansion reactions using transition metal carbonyls or small organic molecules [46,47,51]. Recently, we have reported an account of the chemistry of Mo system with group 6 metal carbonyls that generated several [(Cp*Mo) 2 B 5 H 9 ] derivatives in which one of the {BH} vertices have been replaced by either Cp*M or metal carbonyl fragments [49]. Apart from their significant geometries, these molecules were found to show interesting bonding interactions, as well as electronic structures. Looking at the geometry of the molybdenum systems, tungsten and chromium systems became of interest. Thus, we were  4 ], 3. Besides group 6 metal carbonyls, we were also interested to replace the B-H fragments of B5 core using other metal carbonyl fragments. Thus, we performed the same chemistry using [Fe 2 (CO) 9 ] that led to the isolation of an interesting mixed-metal tungstaborane, [(Cp*W) 2 B 4 H 8 Fe(CO) 3 ], 4. In this report we describe the chemistry of both tungsten and molybdenum derivatives. In addition, we also provide a complete overview of the M 2 B 5 system of group 4-9 metals.  [50] and [(Cp*W) 2 B 5 H 9 ] [40] in moderate yields. The 11 B{ 1 H} nuclear magnetic resonance (NMR) spectrum of compound 1 showed four signals at δ = 70.2, 62.2, 36.5 and 25.0 ppm in 1:1:1:1 ratio indicating the presence of four boron atoms. A similar pattern of 11 B{ 1 H} NMR was also observed for the molybdaborane [(Cp*Mo) 2 (µ-H) 2 (µ 3 -H) 2 B 4 H 4 W(CO) 4 ] [49]. The 1 H NMR spectrum showed four signals in the upfield region, which signifies the presence of four different types of W-H-B protons. Further, the 1 H NMR rationalizes the presence of Cp* ligand. The 11 B{ 1 H} and 1 H NMR of 2 and 3 nearly resemble that of 1, with the four non-equivalent of boron atoms, one chemically equivalent Cp* ligand and four different types of W-H-B protons. The infrared (IR) spectra of 1, 2 and 3 showed the stretching frequencies corresponding to the CO ligands and BH t . Further, the ESI-MS (HR MS) showed a close association among compounds 2 and 3.

Results and Discussion
overview of the M2B5 system of group 4-9 metals. In order to confirm the spectroscopic assignments and to determine the solid-state structures of 1-3, X-ray structure analyses were undertaken. The solid-state structures of 1-3, shown in Figure 1, are consistent with the observed spectroscopic data. The asymmetric unit of 1 contains two independent molecules (one is shown in Figure 1), having similar geometric parameters. Although, the bridging hydride ligands and the B-H terminal hydrogen atoms could not be located in the solid state X-ray structure, their positions were fixed based on 1 H NMR spectroscopy, as shown in Scheme 1. The core geometry of 1-3 is very similar to our recently reported molybdaboranes [(Cp*Mo) 2 (µ-H) 2 [49]. If the classical skeletal electron counting formalism [52][53][54] is applied, the 6-sep clusters 1-3 can be viewed as electron-deficient nido species derived from an 8-vertex oblato-closo hexagonal bipyramidal cluster, with a cross cluster M-M bond [34][35][36]. Alternatively, these 6-sep species can be seen as adopting a trigonal bipyramidal geometry in which the central [ In order to confirm the spectroscopic assignments and to determine the solid-state structures of 1-3, X-ray structure analyses were undertaken. The solid-state structures of 1-3, shown in Figure 1, are consistent with the observed spectroscopic data. The asymmetric unit of 1 contains two independent molecules (one is shown in Figure 1), having similar geometric parameters. Although, the bridging hydride ligands and the B-H terminal hydrogen atoms could not be located in the solid  After successfully isolating clusters 1-3 with group 6 metal carbonyls, we were interested to explore similar chemistry with other metal carbonyls. In this context, we performed the reaction with group 8 metal carbonyls. As shown in Scheme 2, the reaction of in situ generated intermediate, After successfully isolating clusters 1-3 with group 6 metal carbonyls, we were interested to explore similar chemistry with other metal carbonyls. In this context, we performed the reaction with group 8 metal carbonyls. As shown in Scheme 2, the reaction of in situ generated intermediate, The framework geometry of 4 was established by its solid-state structure determination. As shown in Figure 2, it depicts a bicapped trigonal bipyramidal geometry, in which the {W 2 B 2 Fe} trigonal bipyramid core is capped by the boron atoms B42 and B41 at the triangular {W 2 B} and {W 2 Fe} faces similar to compounds 1-3. One of the major difference is the position of the Fe(CO) 3 unit in the molecule. While in 1-3 the metal carbonyl fragment caps one of the {W 2 B} triangular faces, in compound 4 it is part of the triangular faces with tungsten {W 2 Fe}. The geometry of compound 4 resembles that of the tungsten-ruthenium complex [(Cp*W) 2 B 4 H 8 Ru(CO) 3 ] [33] where Ru(CO) 3 occupies the same position as that of Fe(CO) 3 . The W-W bond distance in 4 is comparable to that of 1-3. This type of M 2 B 5 molecule is rare. With 56 cve (6 sep), compound 4 also obeys the electron counting rules and is an analog of [(Cp*W) 2 B 5 H 5 (µ-H) 4 ].

Results and Discussion
The 11 B{ 1 H} NMR of compound 4 displayed four resonances at δ = 79.0, 76.2, 41.0, and 35.8 ppm, that rationalize the presence of four different boron environments. In addition to the resonances for BHt protons, the 1 H NMR spectrum showed a signal at δ = 2.24 ppm corresponding to the Cp* ligands and two upfield signals at δ = −6.97 and −12.92 ppm, that are probably due to W-H-B protons. The presence of CO ligands is confirmed by the IR spectrum. The mass spectrum of 4 showed a molecular ion peak at m/z 853.1456. The framework geometry of 4 was established by its solid-state structure determination. As shown in Figure 2, it depicts a bicapped trigonal bipyramidal geometry, in which the {W2B2Fe} trigonal bipyramid core is capped by the boron atoms B42 and B41 at the triangular {W2B} and {W2Fe} faces similar to compounds 1-3. One of the major difference is the position of the Fe(CO)3 unit in the molecule. While in 1-3 the metal carbonyl fragment caps one of the {W2B} triangular faces, in compound 4 it is part of the triangular faces with tungsten {W2Fe}. The geometry of compound 4 resembles that of the tungsten-ruthenium complex [(Cp*W)2B4H8Ru(CO)3] [33] where Ru(CO)3

Synthesis and Characterization of the Molybdaborane [(Cp*Mo) 2 B 4 H 8 Cr(CO) 4 ], 5
Recently, we were successful in isolating various mixed-metal molybdaboranes [49,50] by treating the intermediate, obtained from the reaction of Cp*MoCl 4 and LiBH 4 , with [W(CO) 5 ·thf]. Following the same procedure we performed the reaction with [Cr(CO) 5 ·thf] and as expected, the reaction enabled us to isolate compound 5 (Scheme 3). Details of its spectroscopic and structural characterization are discussed below.

Synthesis and Characterization of the Molybdaborane [(Cp*Mo)2B4H8Cr(CO)4], 5
Recently, we were successful in isolating various mixed-metal molybdaboranes [49,50]  To confirm the spectroscopic assignments, an X-ray crystallographic analysis was undertaken. The core geometry of 5 is similar to the recently reported molybdaboranes [(Cp*Mo)2(μ-H)2(μ3-H)2B4H4W(CO)4] and [(Cp*Mo)2(μ-H)2B4H4W(CO)5] [49]. The asymmetric unit of 5 contains two independent molecules, which differ slightly in terms of geometric parameters (one is shown in Figure 3). Although, the bridging hydride ligands could not be located in the solid-state X-ray structure, their positions were fixed based on 1    of 5 contains two independent molecules, which differ slightly in terms of geometric parameters (one is shown in Figure 3). Although, the bridging hydride ligands could not be located in the solid-state X-ray structure, their positions were fixed based on 1   In metallaborane chemistry, the mutually synergistic interactions of metal and organic ligands can generate molecules with interesting and diverse geometries. As evident in Table 2 [34][35][36], Mo [37,38], and W [40]), as well as for [(Cp*ReH)2B5Cl5] [39]; in each case, the trigonal bipyramidal M2B3 unit is capped by two BH3 fragments over the M2B faces. A comparison of the structural parameters and chemical shifts of these analogues is shown in Table 2. Another interesting molecule from group 5 is [(Cp*Ta)2B5H11Cl2] [23] having a contrasting geometry. It has a nido structure based on a closo dodecahedron by removing one five-connected vertex. Apart from these M2B5 molecules, group 8 and 9 metals generate monometallic clusters viz. [(η 5 -C5H5)FeB5H10] [57], [Cp*RuB5H10] [58], [(η 5 -C5H5)CoB5H9] [59] and [(η 5 -C5Me5)IrB5H9] [60] showing interesting sandwich structures mimicking their organometallic counterpart ferrocene. These molecules provided an additional bridge between metallaborane and organometallic chemistry. In metallaborane chemistry, the mutually synergistic interactions of metal and organic ligands can generate molecules with interesting and diverse geometries. As evident in Table 2 [60] showing interesting sandwich structures mimicking their organometallic counterpart ferrocene. These molecules provided an additional bridge between metallaborane and organometallic chemistry.

General Procedures and Instrumentation
All the manipulations were conducted under an argon atmosphere using standard Schlenk techniques. Solvents were distilled prior to use under an argon atmosphere.

Synthesis of 4:
In a flame-dried Schlenk tube [Cp*WCl 4 ], (0.1 g, 0.22 mmol) in 15 mL of toluene was treated with a 5-fold excess of [LiBH 4 ·thf] (0.55 mL, 1.1 mmol) at −78 • 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 brownish-green hexane extract was dried in vacuo, and taken in 15 mL of THF and heated at 65 • C with Fe 2 (CO) 9 for 22 h. The solvent was evaporated in vacuo 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/CH 2 Cl 2 (75:15 v/v) mixture yielded green 4 (0.020 g, 11%) along with known [(Cp*W) 2 B 5 H 9 ] (0.040 g, 26%).

Synthesis of 5:
In a flame-dried Schlenk tube [Cp*MoCl 4 ], (0.1 g, 0.27 mmol) in 10 mL of toluene was treated with a 5-fold excess of [LiBH 4 ·thf] (0.7 mL, 1.4 mmol) at −78 • 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 brownish-green hexane extract was dried in vacuo, and taken in 10 mL of THF and heated at 65 • C with [Cr(CO) 5 ·thf] for 18 hours. The solvent was evaporated in vacuo 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/CH 2 Cl 2 (80:10 v/v) mixture yielded dark brown 5 (0.026 g, 14%) along with known [(Cp*Mo) 2 B 5 H 9 ] (0.042 g, 29%).

Conclusions
In summary, we have described the synthesis of various homo and heterometallic tungstaborane clusters [(Cp*W) 2