Article pubs.acs.org/IC
Synthesis, Chemistry, and Electronic Structures of Group 9 Metallaboranes Rosmita Borthakur,† Sourav Kar,† Subrat Kumar Barik,† Somnath Bhattacharya,† Gargi Kundu,† Babu Varghese,‡ and Sundargopal Ghosh*,† †
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India Sophisticated Analytical Instrument Facility, Indian Institute of Technology Madras, Chennai 60036, India
‡
S Supporting Information *
ABSTRACT: Dimetallaoctaborane(12) of Ru, Co, and Rh have been well-characterized by a range of spectroscopic techniques and X-ray diffraction studies. Thus, reinvestigation of the Ir-system became of interest. As a result, a slight modification in the reaction conditions enabled us to isolate the missing Ir analogue of octaborane(12), [(Cp*Ir)2B6H10], 1. Compound 1 adapts a geometry similar to that of its parent octaborane(12) and Ru, Co, and Rh analogues. In [M2B6H10+x](M = Ru, x = 2; M = Co and Rh, x = 0), there exist two M−H−B protons. However, a significant difference observed in [(Cp*Ir)2B6H10] is the presence of two Ir−H instead of Ir−H−B protons that eventually controls the reactivity of this molecule. For example, unlike [M2B6H10](M = Co or Rh), the Ir-analogue does not react with metal carbonyl compounds or [Au(PPh3)Cl]. Along with 1, a closo trimetallic 8vertex iridaborane [(Cp*Ir)3B5H4Cl], 2 was also isolated. Additionally, from another reaction, 12-vertex closo iridaboranes [(Cp*Ir)2B10Hy(OH)z], 3a and 3b (3a: y = 12, z = 0; 3b: y = 8, z = 2), have also been isolated. Further, density functional theory calculations were performed to gain useful insight into the structure and stability of these compounds.
■
INTRODUCTION Boron is the lightest element and covalently bonds to itself, which enables it to build molecules of infinite sizes.1−4 Because boron has only three electrons in its valence shell, it adapts a cluster motif, and this phenomenon gave rise to the rich branch of polyhedral boron chemistry.2,3 It combines with various elements in the periodic table and displays a diversity in its stable architectures.2c,5 Pioneering work by Stock, Lipscomb, Hawthorne, Shore, Kennedy, and Grimes2,6−9on polyhedral boranes introduced a completely new domain of structural chemistry that presented unique structures and novel bonding characteristics. Description of the geometrical arrangement of the polyhedral boranes by Williams,10 followed by the growth of empirical electron counting rules by Wades and Mingos,11,12 provided a firm basis to understand the correlations linking the composition and structure of cluster molecules. Simultaneously, the isolobal principle permitted both transition metals and main group fragments to be plotted upon one another, further facilitating enormous growth of metallaborane chemistry.9,13−15 The replacement of one or two BH vertices by transition-metal units was discovered by Hawthorne and co-workers, which eventually gave new dimensions in borane chemistry.7 The success of cluster-electron counting rules and the isolobal analogy certainly linked boranes, metallaboranes, and metal clusters in a suitable and straightforward manner.3a,11−13 Metallaborane chemistry, as an important part of polyhedral boron chemistry, has been investigated for many years. © XXXX American Chemical Society
However, there are limited numbers of compounds reported in literature. Pyrolysis of cyclopentadienyl metal chlorides of group 4−9 with various borane reagents such as LiBH4·thf, BH3·thf, BHCl2·SMe2, BH2Cl·SMe2 yield various metallaboranes that show different structural varieties.16−22 For instance, early transition metals form moderately stable [M2B5H9+x] (M = Ta, V, x = 2; M = Cr, W, Mo, x = 0),16−20 whereas rhenium forms hypoelectronic closo-rhenaboranes with Re2Bn framework (n = 7−10).21Among the group 9 metals, cobaltaboranes have been studied earlier in a systematic way,23−27 followed by rhodium.28,29 Iridium differs greatly from its group 9 congeners and prefers to appear as monometallic hydrogen-rich clusters.30−32 Many of these compounds are often found in trace yields; however, their existence has facilitated the growth of larger clusters and has sparked great interest. Recently, we have reported isomeric eight-vertex iridaborane clusters.32 In an attempt to isolate the third isomer of [B8H8]2−, we revisited the iridium system. Although the objective of isolating the third isomer was not reached, our experimental efforts enabled us to isolate eight-vertex iridaborane [(Cp*Ir)2B6H10] and a closo trimetallic eight-vertex iridaborane [(Cp*Ir)3B5H4Cl] having a dodecahedron geometry (see both Chart 1 and 2). In addition, we report the isolation of two 12-vertex closo-iridaborane clusters. Received: October 29, 2016
A
DOI: 10.1021/acs.inorgchem.6b02626 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Chart 1. Core Geometry of M2B6H10+x (M = Ru, x = 2; M = Co, Rh, Ir, x = 0) Clusters Showing All the Hydrogensa
a
[(Cp*Ir)2B10Hy(OH)z], 3a and 3b (3a: y = 12, z = 0; 3b: y = 8, z = 2) (Scheme 1b). Although these compounds are produced in a mixture, chromatographic workup using preparative thin-layer chromatography (TLC) allowed us to separate them as pure colorless and yellow solid compounds. Details of spectroscopic and structural characterization of these iridaboranes are discussed below. nido-[(Cp*Ir)2B6H10], 1. Compound 1 was obtained in 15% yield as a colorless solid. The mass spectrometric data exhibit a molecular ion peak at m/z 732.3142 that suggests a molecular formula of C20H40B6Ir2. Room-temperature 11B{1H} NMR of 1 shows two signals at δ 34.4 and −8.27 ppm, and the 1H{11B} NMR displays the signal of a single Cp* at δ 1.94 ppm. This shows the presence of higher symmetry in 1. The 1H NMR shows a singlet at δ −1.45 ppm and a sharp resonance at δ −17.75 ppm apart from the BHt protons. Although the signal at δ −1.45 ppm corresponds to two B−H−B protons, the peak at δ −17.75 ppm may be due to two Ir−H−B or Ir−H protons. The 13C NMR spectrum indicates 1 equiv of Cp* in accord with the molecule being symmetrical. The IR of 1 shows peaks at 2489 and 2513 cm−1 due to terminal B−H stretching frequency. In order to determine the solid-state structure of 1, an X-ray diffraction analysis was performed. Compound 1 crystallizes in triclinic crystal system with space-group P-1. The molecular structure of 1, shown in Chart 1 and Figure 1, has a nido
Cp* ligands at metal centers have been omitted for clarity.
Chart 2. Structures of Isomeric Eight-Vertex Iridaboranes
■
RESULTS AND DISCUSSION Reaction of [Cp*IrCl2]2 with [BH3·thf] and [LiBH4·thf]. As shown in Scheme 1a, compounds 1 and 2 were obtained in moderate yields from the thermolysis of [Cp*IrCl2]2 with an excess of [BH3·thf] at 130 °C. In parallel to the formation of compounds 1 and 2, known iridaboranes (nido[(Cp*Ir)2B8H12], isomeric isocloso [(Cp*Ir)2B8H8], and isocloso [(Cp*Ir)2B9H9])32 were also obtained in moderate yields. Applying different reaction conditions, for example, the reaction of [Cp*IrCl2]2 with [LiBH4·thf] followed by the addition of excess [BH3·thf] and thermolysis at 130 °C enabled us to isolate h igher-ver tex iridaborane clusters
Figure 1. Molecular structure and labeling diagram for 1 (Cp* ligands are omitted for clarity). Selected bond lengths (Å) and angles [°]: B1− Ir1 2.239(13), B1−Ir2 2.221(15), B1−B4 1.88(2); B4−B−Ir2 64.5(7), B4−B1−Ir1 65.5(6), Ir2−B1−Ir1 119.4(7).
structure based on a closo tricapped trigonal prism geometry with one vertex missing and 1 resembles that of nido-[B8H12],33 where two BH units in B8H12 have been replaced by Cp*Ir
Scheme 1. (a) Synthesis of Eight-Vertex Iridaboranes, 1 and 2; (b) Synthesis of Twelve-Vertex Iridaboranes, 3a and 3b
B
DOI: 10.1021/acs.inorgchem.6b02626 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry fragments. Compound 1 has degree three, four, and five vertices, in which two iridium atoms occupy degree four vertices, two boron atoms occupy degree five, and the other four boron atoms reside in degree three in the open face. The average Ir−B bond length of 2.18 Å is slightly longer (0.2 Å) than those reported in literature,30−32 whereas the B−B bond lengths are in agreement with other eight-vertex iridaboranes.32 All the boron atoms are linked to the terminal hydrogen atoms. The extra bridging hydrogen atoms are anticipated to be positioned on the open six-membered face. Furthermore, to validate assignment of hydrogen atoms in 1, a 2D 1H{11B}/11B{1H} HSQC experiment (Figure S1) was carried out that confirms the absence of interaction between the proton at δ −17.75 ppm with any of the boron atoms in the ring. The variable-temperature 1H{11B} and 11B{1H} NMR of 1 shows no fluxional behavior (Figure S2), which supports the absence of M−H−B in 1. Therefore, based on all these spectroscopic evidence, it appears that compound 1 does not have any Ir−H−B protons instead it has two Ir−H protons. To confirm these assignments, density functional theory (DFT) calculations were carried out, which indeed confirm the presence of Ir−H protons. To understand the bonding situation in the adjacent H−Ir−B motif, natural bond orbital (NBO) analysis at PBE0/Def2-TZVP level was performed. The NBO analysis predicts the existence of a 2-center-2-electron σbond between Ir and H atoms instead of a 3-center-2-electron Ir−H−B bond. The σIr−H bond {σIr−H = 0.839 (spndn)Ir + 0.543(s)H} is formed by the overlap of spndn-hybrid orbitals of Ir and 1s orbital of H atom (Ir: s(10.09%), p(4.68%) and d(85.19%) and H: s(99.94%) p(0.06%). Further, the molecular orbital analysis shows σ-type overlap of dz2-orbital of Ir with 1s orbital of H that is illustrated in HOMO-17 (Figure 2a).
the skeleton electron count of [B8H12]. To further understand the electronic structure and thermodynamic stability, DFT studies were carried out on compound 1′ (Cp analogue of 1) at the PBE0/Def2-TZVP level of theory (Table S2). Frontier molecular orbital analysis of compound 1′ (Figure S3 and Table S3) showed significant HOMO−LUMO gap (ca. 5.52 eV), which is consistent with its higher stability. The HOMO− LUMO gap for known Ru, Co, and Rh analogues, (1a′, 1b′, and 1c′, respectively) are comparable with 1′. The results show slightly higher HOMO−LUMO gap for 1′ as compared to other systems, and the energy gap increases in the order 1a′
