Recent advances in transition metal diborane(6), diborane(4) and diborene(2) chemistry

Coordination Chemistry Reviews 399 (2019) 213021

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Review

Recent advances in transition metal diborane(6), diborane(4) and diborene(2) chemistry Rosmita Borthakur, Koushik Saha, Sourav Kar, Sundargopal Ghosh ⇑ Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, TN, India

a r t i c l e

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Article history: Received 7 June 2019 Received in revised form 30 July 2019 Accepted 4 August 2019 Available online 3 September 2019 Keywords: Boron Metallaborane Diborane Diborene Transition metal complexes

a b s t r a c t Diborane(4) and diborane(6) molecules are demanding renewed interest due to their varied reactivity towards diverse substrates. The chemistry of molecules comprising electron-precise B-B bonds has witnessed swift developments in the recent years. In spite of the continuous interest and extensive efforts in the synthesis of diborane compounds, the formation of boron-boron bonds is still difficult and uncontrolled. On the other hand, the diborene molecules (R-B@B-R0 ; R, R0 = H, phosphine, amine, NHC etc), are also of significant interest owing to their ability to regulate the property of biradicals by changing the substituents of the parent diborene(2), HB@BH. In addition, transition metal diborene species exhibit fascinating photo-physical properties. In this review, we have delivered a background of B-B bond formation and a synopsis of the latest developments in the synthesis of boron-boron single and multiple bonds. The reactivity of diborane/diborene complexes towards various transition metals has also been discussed in detail. Ó 2019 Elsevier B.V. All rights reserved.

Contents 1. 2.

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Introduction and scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Transition metal diborane(6) complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Diborane(6) complexes of iron subgroup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2. Dicobalta diborane(6) complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.3. Diborane(6) complexes of tantalum and niobium. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.4. Diborane(6) complexes of molybdenum and tungsten . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.5. Diborane(6) complexes of manganese and ruthenium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.6. Diborane(6) complexes of copper. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Transition metal diborane(4) complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1. Diborane(4) complexes of nickel, copper and zinc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.2. Diborane(4) complexes of chromium subgroup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.3. Diborane(4) complex of cobalt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.4. Doubly-base stabilized diborane(4) complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.5. Rh-Xantphos supported diborane(4) complex. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.6. Diborane(4) complex of molybdenum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Transition metal diborene(2) complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.1. Diborene complexes of silver and copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.2. Diborene complexes of platinum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.3. Diborene complexes of zinc subgroup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.4. Diborene complexes of gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.5. Diborene complexes of molybdenum and tungsten . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

⇑ Corresponding author. E-mail address: [email protected] (S. Ghosh). https://doi.org/10.1016/j.ccr.2019.213021 0010-8545/Ó 2019 Elsevier B.V. All rights reserved.

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1. Introduction and scope Ever since Alfred Stock reported for the first time an electronprecise diborane(6), a new class of extremely reactive, air and water-sensitive boranes were introduced [1,2]. These highly inflammable boranes, due to difficulty in handling, posed serious problems in understanding their structure and bonding [3]. The simplest member of the series is diborane, B2H6. The structure of B2H6 was proposed with bridging hydrogen atoms which was further supported by spectroscopic and X-ray diffraction studies [4–10]. Later, the concept of three-centre two-electron (3c-2e) bonding, introduced by Lipscomb et al., [9,10] gave an insight about their bonding nature. The two boron atoms were connected through two hydrogen atoms forming a B-H-B bond by sharing a pair of electrons (Fig. 1). Although, the stoichiometry is clearly similar to that of ethane (C2H6), boron has insufficient number of electrons to form two-centre two-electron bonds. According to the IUPAC nomenclature, the B2H6 molecule with two boron atoms and six hydride ligands is called as diborane(6) [11]. The chemistry of diborane(6) occurs mostly via heterolytic or homolytic cleavage of the B-H-B bonds [12,13]. For the broad understanding of the chemical bonding, the structures of diboranes and diborene, B2H6 (I), B2H4 (II), and B2H2 (III), (Fig. 1), have been extensively studied by several research groups [14–19]. Formal removal of two hydrogen atoms from B2H6 leads to B2H4 having a B-B single bond, named diborane(4) due to its four hydride ligands. Cheng et al. observed a novel B2H4 prototype

with two bridging and two terminal hydrogen atoms by irradiation of diborane(6) [20]. Theoretical predictions have proposed three possible forms of the parent diborane(4) – a staggered D2d isomer II, a doubly H-bridged butterfly shaped C2V isomer II0 and a singly bridged Cs isomer II00 [18,21-23]. With two degenerate p orbitals, the diborene (2), III, adopts a linear geometry similar to the acetylene (C2H2). However, unlike in acetylene, the two p orbitals in diborene(2) are accommodated with one electron each [17,24,25]. The first molecule closely related to the diborane(6) was reported by Matsuo and Tamao et al. in 2010, synthesized from a diborane(4) precursor upon exposure to H2 gas (Scheme 1) [17]. This diborane(6), A, was stabilized by bulky aryl groups [26,27]. Alternatively, the reaction of (Eind)BF2 and LiAlH4 in presence of Me3SiCl led to the formation of compound A in good yield. Subsequently, Wagner and co-workers synthesised 1,2-(2,20 -biphenyly lene)diborane(6), B from thermolysis of 9-bromo-9-borafluorene with Et3SiH (Scheme 2) [28]. This compound was also accessible from its isomeric structure, B0 , at high temperature [29]. The boron atoms in B, were linked by two hydrogen atoms as well as by two 2,20 -biphenylylene ligands. Theoretical calculations predicted that the conversion of B0 to B is exothermic by only DrG° = 2.8 kcal mol1, however, they were separated by a considerable barrier (D–G° = 28.6 kcal mol1). Further, the reduction of compound B using excess KC8 led to the formation of another diborane(6) species [K2(THF)4][C], containing two terminal BH protons (Scheme 2). The 11B chemical shift in the upfield region (d = 17.9 ppm) differed significantly from the isomeric compound

Fig. 1. A schematic representation of diboranes, B2H6 (I), B2H4 (II, II0 , II00 ) and diborene, B2H2 (III).

Scheme 1. Synthesis of diborane(6) compound from diborane(4) compound.

Scheme 2. Synthesis of diborane(6) and diborene compounds.

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Table 1 Structures and selected structural parameters of various diborane(4) ligands.

(a = structural data not available) B (d = 10.9 ppm). The X-ray structure of C revealed a trans isomer and the boron-boron bond distance (1.755(4) Å) lies within the B-B single bond distance [29]. On the other hand, reduction of B in presence of excess Li enabled isolation of diborene compound [Li(THF)3]2[D] [30]. The avg. B@B bond distance in D (1.634 Å)

was shorter than that in compound B suggesting a double bond character. In both the cases, excess reducing agents were used to subdue other rearrangement reactions. In addition to their intriguing structural and bonding features, [3,7–11,14–19] diboranes are also an extremely useful and

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versatile class of compounds for various synthetic reactions [19,31,32]. They are efficient borylating agents for the synthesis of boron carbon bonds both in presence and absence of metal [33]. The activation of B-B single bonds usually follows sigma bond metathesis or oxidative addition reactions [34]. Due to the existence of long boron-boron single bond (1.75 Å) and vacant p-orbitals on the boron atoms, diborane(4) exhibits characteristic reactivities [19,33,35]. As a result, much interest has been developed in the chemistry of diborane(4). The metal mediated diboration of unsaturated organic substances is the most significant application of diborane(4) [36,37] and the mechanisms are quite well understood. These compounds also act as potential starting materials for the formation of electron-precise M-B compounds [38]. Unsymmetrical diborane and borylene species can be obtained from diborane(4) reagents [39,40]. Yamashita and coworkers have attractively shown the reactivity of diborane(4) compounds towards compounds with multiple bonds and small molecules, including CO, isocyanides, alkynes, nitriles, pyridine, and H2 [35]. Unsymmetrical diborane(4) compound Mes2B-Bpin and symmetrical diborane(4) compound B2(o-tol)4 (Table 1) exhibit a higher Lewis acidity relative to the most common diborane(4), B2pin2, owing to the overlapping of the vacant p-orbital of the two boron atoms [35]. DFT calculations have clearly demonstrated the coexistence of high electrophilicity of these diborane(4) molecules, which allows coordination of weak nucleophiles. These reac-

tive B-B bonding electrons produce a broad spectrum of interesting reactivity for these kinds of molecules [35]. The addition of Lewis bases to diborane(4) affords sp2-sp3 diborane compounds, which shows adequate nucleophilicity to attack transition metal or organic electrophiles [35,41]. Lately, much progress has been seen in the field of transition-metal boranes which has advanced the studies of chemical bonding and catalysis [42]. The boron-boron double bonds (III) are sparsely explored unlike carbon–carbon double bonds. Robinson, in 2007, reported the first neutral B-B double bond species L(H)B@B(H)L, (L = :C{N(2,6-iPr2C6H3)CH}2) (III0 ) stabilized by a bulky N-heterocyclic carbene (NHC) ligand (Fig. 2) [43]. In the subsequent year, the same group reported another neutral diborene R(H)B@B(H)R (R = :C{N(2,4,6Me3C6H2)CH}2), using a less bulky NHC ligand [44]. Since then, a variety of Lewis base-stabilized B-B multiple bonded species have been developed, using N-heterocyclic carbenes (NHC), cyclic (alkyl)

Fig. 2. Schematic representation of parent diborene, neutral diborene and anionic diborene.

Table 2 Structures and selected structural parameters of various diborene(2) ligands.

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(amino)carbenes (cAAC) etc. [32,45,46]. These diborene molecules display unique reactivity towards light alkali metal cations [47], metal Lewis acids [48-51], oxidants [52,53], small molecules [36,54–59] and homoatomic chalcogen-chalcogen bonds [60,61]. These diborenes are isosteric, isoelectronic as well as isolobal to olefins and are an important entry to the class of compounds comprising main group multiple bonding [62]. In this review, we have focused on diborane(4/6) and diborene(2) complexes stabilized in the coordination sphere of group 5–12 metals. Metal-free diborane(4) and diborene(2) compounds are not discussed here as these have been described more elaborately elsewhere. However, we have provided a summary in the form of Tables 1 and 2, which display a list of metal-free diborane(4) [43,44,49,52,57-59,63-76] and diborene(2) compounds [43,44,49, 53,59,77-82]. 2. Transition metal diborane(6) complexes The field of diborane chemistry with transition metal has developed extensively owing to its unique bonding features and promising applications in catalysis [83–85]. [B2H6] or [B2H5] being isoelectronic to [C2H4], metallaborane compounds comprising [B2H6] or [B2H5]– resemble metal-olefin complexes [86–89]. The first transition metal diborane(6) species was reported by Fehlner and co-workers in 1978 [87,88]. Successively, Shore and coworkers reported diborane complexes of group 8 transition metals analogous to metal olefin complexes [89]. Recently, our group [90,91] and others [33] have synthesised several transition metal diborane(6) complexes. In the following subsections, we have discussed the various synthetic strategies adopted for the synthesis of diborane(6) complexes using different transition metals and their intriguing structural features. 2.1. Diborane(6) complexes of iron subgroup The first transition metal diborane compound [B2H6Fe2(CO)6] [87] (1) was isolated by Fehlner et al. in 1978 from the reaction of [Fe(CO)5] and B5H9 with LiAlH4 followed by addition of HCl. Compound (1) was spectroscopically characterized and was consistent with the proposed structure (Fig. 3). 1H NMR studies displayed a broad resonance at d = -10.3 ppm corresponding to four B-H-Fe protons and the BH terminal protons were observed at d = 0.2 ppm. The 11B NMR studies showed the existence of doublet of triplets; however, the B-H-Fe coupling could not be fixed. Here {Fe2(CO)6}2+ unit was coordinated to [B2H6]2, which donates eight electrons and thus satisfied the 18-electron rule. According to the electron counting rules [92–95], with 12 skeletal electrons, compound (1) has a nido geometry. Compound (1) is isoelectronic to the parent compound [C2H2Co2(CO)6] [96,97] and is the third example of a BnMn cage molecule [98].

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of BH3THF and K2[Fe(CO)4] (Eq. (i)) [99]. Compound (2) is the first example of a diborane(6) complex having a transition metal at the bridging site.

    K2 FeðCOÞ4 þ 3 BH3  THF ! K l  FeðCOÞ4 B2 H5 þ KBH4 þ 3 THF ðiÞ Spectroscopic evidences of compound (2) supported the proposed structure shown in Fig. 4. 11B NMR studies exhibited triplet of doublets with coupling constant JBHt = 112 Hz due to two B-H coupling. The triplet was further splitted into doublets by two BH-B protons. Two resonances at d = 8.20 and 15.17 ppm were observed in the 1H NMR spectrum. The infrared spectrum displayed peaks at 2450 (w) and 2400(w) cm1 for terminal BH and at 1845 (vw) and 1655 (vw) cm1 for the bridging BH protons. The most favourable valence structure predicted for [l-Fe (CO)4B2H5] based on limited spectroscopic characterization is shown in Fig. 4. Here, iron is coordinated to the {B2H3} unit through a B-Fe-B bond with Fe being in dsp3 hybridization mode. Compound (2) is the isoelectronic analogue of [{Fe(CO)4}(g2C2H4)] [100] and [{CpFe(CO)2}(g2-C2H4)][BF4] [101]. This was further examined through Mössbauer spectroscopy.

Fig. 4. The structure of [l-M(CO)4B2H5] (M = Fe (2), Ru = (5) and Os (6)).

In 1979 Shore and co-workers reported the first neutral metalla-diborane(6) complex [CpFe(CO)2(g2-B2H5)], (3) (Fig. 5) [102] from the reaction of nucleophilic anion [CpFe(CO)2] with [BH3Me2O] (Eq. (ii)). However, this compound could only be characterized spectroscopically. Complex (3) is unstable in air and decomposes at elevated temperature under vacuum. On the other hand the ruthenium complex [CpRu(CO)2(g2-B2H5)], (4) was reported to be unstable at elevated temperature.   
 K M0 ðCOÞ2 þ 3BH3  Me2 O ! M0 ðCOÞ2 g2  B2 H5 þ K½BH4  þ 3Me2 O M0 ¼ CpFe ð3Þ;CpRuð4Þ ðiiÞ

Fig. 5. The proposed structure of [CpFe(CO)2(g2-B2H5)] (3).

Fig. 3. The proposed structure of [B2H6Fe2(CO)6] (1).

On the other hand, in the same year Shore et al., reported another diborane species, K[l-Fe(CO)4B2H5] (2), from the reaction

In 1989 the X-ray diffraction analysis of (3) (Fig. 6) was established [89]. Compounds (3) and (4) are analogs of a metal-olefin complex [CpFe(CO)2(g2-C2H4)]+[BF4]–, [101]. The single crystal Xray diffraction analysis revealed a diborane(6) species where

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{CpFe(CO)2} fragment replaces one of the bridging hydrogen atoms. The 1H and 11B NMR spectra also supported diborane like structure. The B-B bond distance (1.773(8) Å) in (3) is comparable to that in [B2H6] (1.776 Å) [103]. The presence of three-centered BFe-B bonds was confirmed by Mössbauer spectroscopy. The bonding in (3) can be related to Dewar-Chatt-Duncanson model of metal-olefin bonding, where the B-B bond donates electron to the vacant d-orbital of metal with slight back donation of electron [104,105]. Theoretical calculations supported the presence of {B2H5} fragment as a p-bonded ligand where [B2H4]2 bind to hydrogen in the p lobe opposite to the metal.

Scheme 3. Synthesis of diborane compounds of ruthenium (7) and molybdenum (8).

2.2. Dicobalta diborane(6) complex

Fig. 6. ORTEP drawing of the [CpFe(CO)2(g2-B2H5)] (3). Reprinted with permission from American Chemical Society.

With the establishment of the solid-state structure of (3), these complexes further gained interest. Several metal carbonylate reaction were performed, for example, [CpRu(CO)2] with [BH3L] (L = Me2O, THF) and [M(CO)4]2 (M = Fe, Ru or Os) that led to the isolation of metalladiborane complexes (2 and 5–6) (Fig. 4, Eqs. (ii) and (iii)) similar to analogous metal olefin complexes [101,110].

  
 K2 M0 ðCOÞ4 þ 3BH3  THF ! K M0 ðCOÞ4 g2  B2 H5 þ KBH4 þ 3THF M0 ¼ Fe ð2Þ;Ruð5Þ;Osð6Þ

ðiiiÞ Green and coworkers in 1988 reported the ruthenium diborane [(Cp*Ru)(PMe3)(g2-B2H7)] (7), the metallaborane derivative of the parent neutral borane [B3H9] (7) [106]. Compound (7) was obtained from the reaction of [Cp*Ru(PMe3)Cl2] with an excess of NaBH4 (Scheme 3a). The single-crystal X-ray structure showed that the central unit comprised of a {(Ru)(l-H)2(B2H5)} unit. Compound (7) had an g2-B2H7 bonding geometry and the first example of its kind in a three-vertex metallaborane of the type [M-(l-H)2B2H5]. The occurrence of Ru-H-B bridges was indicated by Ru-H (1.61 Å) and B-H (1.43 Å) distances. The 11B NMR studies displayed triplet of triplets at d = -21.4 ppm corresponding to BH2, which splits owing to the presence of adjacent bridging hydrogen atoms. On the other hand, compound (8), [Cp2MoH(g2-B2H5)] [106] was synthesized following two different routes – (i) from the reaction of [Cp2MoH2] with [BH3THF] under photolytic condition (Scheme 3b); (ii) from the reaction of [Cp2MoCl2] and [NaB3H8]. 11B NMR showed signals at d = -9.6 and 11.1 ppm. X-ray crystallography clearly showed the binding of {B2H5} fragment to the [Cp2MoH] unit. Compound (8) was found to be isoelectronic with [M (CO)n(g2-C2H4)] (M = Cr, Mo and W; n = 5 and M = Fe; n = 4) [101,106-108].

In 1988 Fehlner and co-workers isolated and structurally characterized the cobaltaborane, [(CpCo)2(l-PPh2)B2H5] (9) that consist of a {B2H5} unit comparable to a r-p allyl ligand [109] (Fig. 7). Compound (9) was obtained from the reaction of [CpCo(PPh3)2] with BH3THF. X-ray diffraction analysis exhibited a {(CpCo)2} fragment bridged by a {B2H5} ligand in an asymmetric manner. The cobalt atoms were bridged by PPh2 ligand on one side. Both the boron atoms were connected to the cobalt centres by Co-H-B interactions. One of the boron atoms was coordinated to both the metal centres while the other one is coordinated to only one cobalt centre. Fig. 7 displayed a proper bonding pattern with the BH bonds donating two electrons to the metal centres. The boron and cobalt connected through a three-centred bond, is expected to have a B-M distance of 2.2 Å. However in case of compound (9), the Co-B distances were found to be shorter than 2.2 Å. A difference in the Co-B bond distances indicated a multicentre cluster bonding in compound (9).

Fig. 7. Representation of the bonding of [(CpCo)2(m-PPh2)B2H5] (9).

2.3. Diborane(6) complexes of tantalum and niobium In 1989, Messerle and co-workers reported early transition metal coordinated diborane species [(Cp*Ta)2(m-X)2(B2H6)] (10a and 11a: X = Cl; 10b and 11b: X = Br) from the reaction of LiBH4 with [(Cp*Ta)2(m-X)4] (X = Cl or Br) (Scheme 4) [110]. Spectroscopic

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Scheme 4. Synthesis of [(Cp*Ta)2(l-X)2(B2H6)] (X = Cl, Br) (10 and 11).

analysis supported the formation of an unsymmetrical [B2H6]2 complex and their presence in the solution in two isomeric forms. One isomer contain one B-H-B, three Ta-H-B and two terminal B-H hydrogens (10a-b), whereas, the other isomer possessed four Ta-HB and two B-Ht terminal hydrogens (11a-b). The X-ray diffraction of (10b) matched with the proposed structure having a Ta@Ta bond (2.839(1) Å) bridged by two bromine atoms as well as an unsymmetrical {B2H6} group containing long BB separation (1.88(3) Å). The Ta-B bond lengths clearly showed the presence of three instead of four Ta-H-B bridges and one Ta-B bond. The Ta@Ta bond distance was within single bond distance, however a r2d2 configuration double bond was anticipated for [B2H6]2 bonded to a {(Cp*Ta)2(m-X)2}2+ fragment. The area of early transition metal boron compounds are less explored and complex (10b) is a novel example of this class. Successively, Cotton and co-workers isolated and structurally characterized compounds (12) and (13), comprising {l2,g4B2H6}2 ligand, from the reaction of [TaCl5] with LiDTolF and LiDPhF, respectively [111]. These molecules adapted closely a C2 symmetry where the C2 axis passed through the center of the Ta@Ta and B-B bonds (Fig. 8). The most intriguing aspect of these compounds is the central {B2H6} unit similar to ethane, which symmetrically coordinates with the tantalum atoms. This type of structures has been previously discussed only on the basis of spectroscopic data; their complete characterization was never achieved prior to this [110,112,113]. If one looks very carefully, there is a formal resemblance between [Ta2(l2,g4-B2H6)] unit and the [B4H10] (Fig. 9). However, the central B-B distance in the Ta species is significantly shorter (1.70 Å) as compared to B4H10 (1.75 Å). The electronic

Fig. 8. A view of the central portion of the molecules in (12) and (13) with a labeling scheme. Reprinted with permission from American Chemical Society.

structure of these compounds demonstrated that the tantalum atoms were assigned oxidation state of +3 as (12) possess a B2H2 ion (isoelectronic to ethane) and four formamidinium 6 anions. The metal–metal bond distances of these two compounds (12 and 13) were in the range of 2.6–2.8 Å and thus, were considered to have metal–metal double bond. Based on the Ta-Ta bond length (2.73 Å) and the presence of diamagnetic character, compounds (12) and (13) were proposed to have Ta@Ta bond. Wachter and co-workers synthesized [(Cp+Nb)2(m-Cl)2(B2H6)], (14) using a

Fig. 9. A schematic comparison of borohydrides B4H10, B2H2 6 , and related nido-metallaboranes (12 and 13).

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Fig. 10. Structure of [(Cp*Nb)2(m-Cl)2(B2H6)], (14).

different synthetic approach [114]. Treatment of NbCl5 with NaBH4, and LiCp+ (Cp+ = EtMe4C5) yielded [(Cp+Nb)2(m-Cl)2(B2H6)] (14) (Fig. 10). 2.4. Diborane(6) complexes of molybdenum and tungsten Fehlner et al. reported the first group 6 metal diborane(6) complex [(Cp*MoCl)2B2H6] (15) [115] as an intermediate from the reaction [Cp*MoCl4] with BH3THF (Scheme 5). 1H NMR spectrum displayed a single resonance for two Cp* units, one M-H-B resonance for four protons and one BH resonance for two protons. The 11B NMR spectrum showed single resonance. Note that similar spectral pattern was also observed for [(Cp*Ta)2(l-Br)2B2H6] (11). X-ray crystallographic analysis confirmed the presence of two {Cp*Mo} units connected by a pair of bridging chloride ligands

and a {B2H6} unit in a side-on fashion. The B-B bond distance of 1.63(3) Å was found to be slightly shorter as compared to other diborane species [110,112]. Cluster valence electron (cve) count of 40 or 6 skeletal electron pairs (sep) count further supported M2B2 tetrahedron core for compound (15) having a Mo-Mo bond with an interatomic distance of 2.696(2) Å. Compound (15) can also be considered as a diborane complex where the two molybdenum centers were bridged by the [B2H6]2 ligand. The [B2H6]2 ligand acting as an eight electron donor, which when considered alongside the Mo-Mo single bond helped the molybdenum centers attain an 18-electron configuration. Group 5 complexes [(Cp*TaBr)2B2H6] [110] and [(C5Me4EtNbCl)2B2H6] [111] exhibited similar structures. Following this, the same group reported four different types of diborane(6) species of tungsten (Fig. 11) [116]. Tungstaborane [(Cp*WCl)2B2H6] (16) was isolated from the reaction of [Cp*WCl4] or [Cp*WCl2]2 with BH3THF. First step was the generation of [Cp*WCl2] by reducing [Cp*WCl4] using BH3THF. The 11B NMR showed a peak at d = 47.8 ppm. 1H NMR displayed a peak corresponding to B-H-W protons at d = 13.8 ppm. Unlike molybdenum, depending on the rate of addition of borane and the oxidation of the starting material, tungsten produces different metallaborane compounds. Depending on the addition of LiBH4 to [Cp*WCl4], compounds [(Cp*WH3)2B2H6] (17) and [(Cp*W)2HCl (B2H6)], (18a and 18b) were also produced as co-products along with nido-2-[(Cp*WH3)B4H8]. Compound [(Cp*W)2HCl(B2H6)] (18) was isolated as two different isomers, where one form has two terminal B-H and four W-H-B hydrogens and the other form has two B-H-B, two W-H-B and two B-H terminal hydrogens (Fig. 11). The isomeric complexes (18a and 18b) have a W„W bond. 2.5. Diborane(6) complexes of manganese and ruthenium

Scheme 5. Synthesis of [(Cp*MoCl)2B2H6] (15).

The group 7 metal diborane(6) complexes are very rare as compared to other transition metals. Dahl and co-workers in 1965 reported the first diborane(6) complex of manganese (19) [117]. This compound was isolated as a side product in the synthesis of [HMn(CO)4]3 and could be characterized with limited spectroscopic data. Later in 2014, our group synthesized this compound as a side product from a different reaction (Scheme 6) [91]. The

Fig. 11. Structures of tungsten diborane(6) species (16–18).

Scheme 6. Synthesis of [{(OC)4Mn}(g6-B2H6){Mn(CO)3}2(l-H)] (19).

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B NMR studies exhibited a chemical shift at d = 40.2 ppm and H NMR exhibited upfield signals at d = 9.87 and 12.32 ppm in 1:2 ratio corresponding to the Mn-H-B protons. X-ray diffraction studies confirmed a diborane complex where the {B2H6} unit adopts an eclipsed conformation. Similar situation was also observed for other diborane complexes of niobium and tantalum [110,112,115,118]. The average Mn-B distance (2.271 Å) was longer compared to similar r-borane complexes [119]. Compound (19) exhibited a shorter B-B bond (1.68 Å). Theoretical calculations confirmed a weak B-BMn and a polar B-H  Mn interaction in (19). In compound (19) the basal {Mn(CO)3} and the apical {Mn(CO)4} were coordinated by a [B2H6]2 unit, which by donating eight electrons, satisfied the 18-electron rule around the metal centres. Our group has earlier reported a ruthenium derivative of diborane(6) [90]. The reaction of nido-[(Cp*Ru)2B3H9] and 2-mbz (2mbz = 2-mercapto-benzothiazole) yielded [Cp*RuB2H3(L)2] (20) (L = C7H4NS2) as a by-product (Fig. 12). The X-ray diffraction analysis showed that both the boron atoms having a B-B distance of 1.776(4) Å and symmetrically coordinated to ruthenium centre with an average Ru-B single bond distance (2.201 Å). One of the B-H-B hydrogen atom in compound (20) get replaced by a {Cp*Ru} unit. The \B-Ru-B bond angle (47.53(11)°) was comparable with an analogous species [CpFe(CO)2(g2-B2H5)] [89,102]. In addition, the structure of (20) resembles that of [(CO)2(PCy3)FeB2N(SiMe3)2Dur], [120] [(PEt3)2PtB2(Dur)2] [121] (Dur = 2,3,5,6-tetra-methylphenyl), and metalladiborane(4) complexes [122,123]. 1

2.6. Diborane(6) complexes of copper A series of copper-diborane(6) complexes were recently reported by Braunschweig and co-workers from the room temperature reaction of neutral and base-stabilized diborane(5) with CuX2 in benzene (Scheme 7) [34]. These complexes adopted the structure similar to that of doubly-bridged diborane(6) species, in which one copper unit replaced one of the B-H-B hydrogens. The B-B bonds in (21–26) (1.762(2)-1.795(3) Å) were considerably elongated compared to that of the free ligands (1.682(3) Å and 1.670(3) Å) and the planarity of the {C4B2} unit was effectively retained in these complexes. As a result the covalent character increases in these complexes. With the larger electrophilic copper centres a stronger metal-diboron orbital interaction was observed, which was further supported by longer B-B and shorter B-Cu bond lengths. 3. Transition metal diborane(4) complexes Although a significant development has been witnessed in the chemistry of hydrogen substituted diborane compounds [33,90,121–126], quest for bimetallic transition metal diborane(4) compounds has seen very less success [29,80,121–125,127]. The simplest diborane(4), B2H4, has rarely been used as ligand unlike other derivatives of diborane(4). Several theoretical studies predicted the energy difference among various isomers of B2H4 to be

Fig. 12. Ruthenium diborane(6) [Cp*RuB2H3(L)2] (20).

Scheme 7. Synthesis of copper-diborane(6) complexes (21–26). (An = 9-anthryl; Mes = 2,4,6-trimethylphenyl; SIMes = 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazolidin-2-ylidene; Otf = trifluoro-methanesulfonate and Pf = pentafluorophenyl).

less than 5.0 kcal.mol1 (Fig. 1) [18,21,22,128]. These different diborane(4) species having small energy differences can be stabilized using transition metal fragments [18]. Diborane(4) compounds have been extensively used for catalytic borylation of olefins [19]. In recent years, various boryl and diboranyl(4) complexes were synthesized [84,129–132]. The metal-diborane bonding is usually dominated by M-H-B bridges [122,133,134], however, there are reports where the metal-ligand bonding is due to the contribution of electrons from the B-B bond instead of B-H bond. The two metalligand bonding modes are outlined in Fig. 13. 3.1. Diborane(4) complexes of nickel, copper and zinc Kodama and co-workers first synthesized and proposed the structure of [{Ni(CO)2}(B2H42PMe3)] (27) (Scheme 8) [124]. The 1 H NMR studies exhibited resonances at d = 1.26, 0.21 and 2.42 ppm for the methyl, BHt and Ni-H-B protons respectively. The NMR spectroscopic data supported the molecular structure of (27), in which the {B2H42PMe3} ligand was coordinated in bidentate-bridging mode similar to the previously characterized Zn(II) complex [ZnCl2{B2H42PMe3}] (28) (Scheme 9) [124]. Note that compound (28) is stable at room temperature. It was isolated from the room temperature reaction of ZnCl2 and {B2H42PMe3}.

Fig. 13. Metal-ligand bonding modes in diborane(4) complexes.

Scheme 8. Synthesis of [{Ni(CO)2}(B2H42PMe3)] (27).

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in compound (29) bounded to the copper centre through Cu-H-B bonds and the Cu-H bond lengths lie in the single bond region (1.91(7) and 1.96(7) Å) as observed in similar complexes [(B10H10){Cu(PPh3)}2] (1.84(6)–2.09(7)Å) [137] and [(B3H8){Cu (PPh3)2}] (1.83(5)–1.85(5) Å). The Cu-B bond distances (2.179 (9) and 2.184(10) Å) were significantly shorter as compared to [(PPh3)2Cu(B3H8)] (2.30(1) Å) and [{(PPh3)2Cu}2(B10H10)] (2.2(1)2.32(1) Å). Scheme 9. Synthesis of [ZnCl2{B2H42PMe3}] (28).

Complex (28) showed a doublet at d = 43.1 ppm in the 11B NMR spectrum. The 1H NMR spectrum revealed chemical shifts corresponding to BHt protons at d = 0.43 and 0.33 ppm. Presence of the two proton signals corresponding to terminal BH protons ruled out the possibility of any fluxionality in the molecule with respect to terminal and bridging hydrogens. The B-B bond length in (28) (B1-B2 1.814 (6) Å) was in good agreement with a single bond distance. The first example of monomeric copper(I) species coordinated tetrahedrally through hydrogen atoms was reported by Ogino et al. (Scheme 10) [136]. Compounds (29) and (30) were regarded as commo-class metallapentaborane derivative. The single crystal X-ray crystallographic study revealed that two {B2H42PMe3} ligand is coordinated to the copper centre in tetrahedral fashion (avg. \H-Cu-H = 108.5°) in compound (29). The borane moieties

Scheme 10. Synthesis of monomeric copper(I) complex, [Cu{B2H42PMe3}2]X (29: X = Cl; 30: X = I).

3.2. Diborane(4) complexes of chromium subgroup Shimoi and co-workers reported several diborane(4) species of group 6 metals [125,135]. The photolysis reaction of {B2H42PMe3} and M(CO)6 (M = Cr, Mo or W) led to the formation of complexes (31–35) (Scheme 11). These were the first examples of {B2H42PMe3}-coordinated group 6 metal complexes where the {B2H42PMe3} ligand acted as a monodentate ligand (complexes (31) and (32)) as well as a bidentate ligand (complexes (33–35)). The X-ray structure showed that the {B2H42PMe3} ligand in complex (31) adopted an eclipsed conformation. This was also observed in case of [ZnCl2(B2H42PMe3)] [124] and [Cu{B2H42PMe3}]X (29: X = Cl; 30: X = I) [133] complexes that consisted of a bidentate {B2H42PMe3} ligand. The chromium diborane(4) species [{Cr(CO)4}2(g4-H,H0 ,H00 ,H000 BH2-BH2(PMe2)2CH2)] (36) was obtained by treating Cr(CO)6 with {BH3PMe2CH2PMe2BH3} (Scheme 12) [134]. The 1H NMR spectrum revealed a broad quartet at d = 8.5 ppm corresponds to the Cr-H-B protons. The IR spectrum showed appearance of bands due to symmetrical and unsymmetrical CO stretching of the two {Cr(CO)4} units. This type of IR spectrum is expected for molecules having a C2V local symmetry. A single crystal X-ray diffraction study showed that the {B2H.4PMe2CH2PMe2} ligand coordinated to the {Cr(CO)4} units by Cr-H-B bonds. As a result, {B2H4PMe2CH2PMe2} acted as a tetradentate ligand and with the two {BH2Cr(CO)4} units, it formed a butterfly framework. The Cr-H bond distances were 1.83(7) and 1.84(6) Å, whereas, the B-H were 1.23(7) and 1.27(6) Å. The B-B distance (1.733(13) Å) was within the boron-boron single bond distance. The chromium centres were seen to have distorted geometry from that of a regular octahedron.

Scheme 11. Synthesis of compounds (31–35).

Scheme 12. Synthesis of chromium diborane(4), [{(OC)4Cr}2(g4-H,H0 ,H0 0 ,H0 0 0 -BH2-BH2PMe2CH2PMe2)] (36).

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3.3. Diborane(4) complex of cobalt The diborane(4) coordinated species of cobalt, [Co2(CO)6B2H4] (37) was first isolated by Fehlner et al. in 1995 from the reaction of Co2(CO)8 and BH3SMe2 (Scheme 13) [138]. Although compound (37) could not be characterized crystallographically, its structure was evidently elucidated by spectroscopic techniques. The 11B NMR spectrum of (37) showed a broad doublet at d = 0.9 ppm. The 1H NMR studies exhibited broad resonances at d = 3.3, 0.6, and 13.7 ppm in the intensity ratio of 2:1:1 that thermally decouples at lower temperatures and these were assigned to terminal BH, B-H-Co and B-H-B protons, respectively. 3.4. Doubly-base stabilized diborane(4) complexes Himmel et al. isolated the zinc complexes of doubly basestabilized neutral diborane(4) (38–40) from the room temperature

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reaction of [ZnX2] and [HB(l-hpp)]2 in dichloromethane/diethyl ether solvent (hpp = 1,3,4,6,7,8-hexahydro-2H-pyimidino-[1,2-a]p yrmidinate, Scheme 14) [139]. The 1H NMR spectrum of (38) showed a single resonance at d = 8.61 ppm attributed to Zn-H-B bridged protons. X-ray diffraction analysis finally identified compound (38) as [ZnCl2{HB(l-hpp)}2]. The Zn-B distance in (38) measures 2.200(2) Å in one and 2.215(2) Å in the other molecule within the unit cell. The B-B bond distances of 1.841(4) and 1.834(4) Å in (38) are considerably lengthened compared to free ligand {HB(lhpp)}2 (1.772(3) Å) [75]. Subsequently, the same group reported the synthesis of similar diborane(4) species with group 6 metals compounds (41–43) from the photolysis of M(CO)6 and {HB(l-hpp)}2 (Scheme 15) [139]. On the other hand, reaction of [M(cod)(m-X)]2 (M = Rh or Ir; X = Cl, Br or I) and {HB(l-hpp)}2 yielded complexes (44–48) (Scheme 15). The B-B bond distance (dB-B = 1.811(6) Å) in [RhCl(cod){HB(lhpp)}2] (44) was shorter than that in the iodide complex (46) (dB-B = 1.781(7) Å) but longer than the isothiocyanate compound (47) (dB-B = 1.815(4) Å). The spectroscopic, structural as well as the computational data for group 6 and 9 species were found to be diverse. All spectroscopic evidences accessible for [M(cod)(mCl)]2 (M = Rh or Ir) specified a dominant coordination through boron-boron bond electrons. 3.5. Rh-Xantphos supported diborane(4) complex

Scheme 13. Synthesis of diborane(4) complex of cobalt, [Co2(CO)6B2H4] (37).

Weller et al. reported the first rhodium(I)-diborane(4) species (49) (Scheme 16) [140]. X-ray crystal structure analysis demonstrated the homocoupling of two {H3BNMe3} molecules on the rhodium center to form the diborane {B2H42NMe3}. This occured

Scheme 14. Synthesis of zinc-diborane(4) complexes (38–40).

Scheme 15. Synthesis of compounds (41–48).

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Scheme 16. Synthesis of rhodium(I)-diborane(4) species [Rh(j2P,P-Xantphos)(g2-H4B22NMe3)][BArF4] (49).

through vicinal Rh-H-B bonds leaving the diborane in an eclipsed conformation. The B-B single bond distance (1.678(7) Å) was shorter than that of [{(CO)4Cr}(H4B22PMe3)] (1.748(11) Å) [134] and [Cu(H4B22PMe3)2]I (1.80(2) Å) [136]. The Rh-B distances (2.405(4) and 2.411(5) Å) lie in between that observed in [Rh(j2P, 2 t P-Xantphos)(g -Me3NH2BCH2CH2 Bu)][BArF4]. The NMR data of (49) were in agreement with the X-ray structure. The 31P{1H} NMR (d = 26.2 ppm, JRhP = 172 Hz) displayed only one type of resonance; while the 1H NMR showed two types of BH protons (d = 1.51 and 8.47 ppm), one Xantphos methyl and one NMe3. This suggested that at room temperature the molecule exhibits fluxional behaviour that allows the phosphorus and the {BH2NMe3} groups equivalent. 3.6. Diborane(4) complex of molybdenum Our group has lately, reported the first structurally characterized bimetallic diborane(4) complex [141]. Similar type of diborane(4) species [Co2(CO)6(B2H4)], was reported earlier by Fehlner et al., however there was no structural characterization available for this molecule [138]. The room temperature reaction of [(Cp*Mo)2(m-Cl)2(B2H6)] and CO resulted the formation of complex (50) (Scheme 17). Although room-temperature 11B and 1H NMR spectra did not give much information, low temperature (40 °C) 1H NMR spectrum showed peaks for Mo-H-B hydrogen atom at d = 11.3 ppm, B-H-B and BHt protons at d = 0.86 and 3.28 ppm respectively. On the other hand, low temperature 11B {1H} NMR studies showed signals at d = 27.3 and 31.1 ppm. Compound (50) is a singly bridged bimetallic diborane(4) complex with a Cs symmetry, the first example of this kind.

Fig. 14. Molecular structure of molybdenum diborane(4) species (50). Reprinted with permission from Wiley.

(CO)2}2{m-g2:g2-C2R2}] (R = H, Et or Ph; 2.971(1) Å) and considerably shorter than that of [Mo2(CO)10]2 (3.213(7) Å) [141–145]. Theoretical studies showed that (50) mimics dimolybdenumalkyne complex synthesised by Cotton [{CpMo(CO)2}2C2H2] [141]. Compound (50) is isoelectronic with the molybdenum-acetylene complex, [{CpMo(CO)2}2C2H2]. 4. Transition metal diborene(2) complexes Ever since Robinson and Braunschweig demonstrated the use of Lewis bases as stabilizing ligands to isolate neutral diborenes [43,44,62,146,147], the B-B multiple bonded compounds has attracted considerable attention [32,148,149]. Considering the isoelectronic and isolobal relationship between diborene and alkene, the diborenes in the coordination sphere of metals has been a topic of significance [45,150]. However, despite its fundamental importance and potential in application, the coordination chemistry of organoboranes featuring B-B multiple bonded species are largely unexplored compared to olefin metal p-complexes.

Scheme 17. Synthesis of bimetallic diborane(4) species (50).

X-ray structure analyses completely supported the low temperature 1H and 11B{1H} NMR assignments. The single crystal X-ray diffraction analysis showed that the {B2H4} moiety was connected to two {Cp*Mo(CO)2} units and the B-B as well as Mo-Mo bonds lied perpendicular to each other (Fig. 14). The B-B single bond length (1.700(12) Å) was found to be considerably shorter with respect to base stabilized diborane(4) complexes [34,122–125]. The Mo-Mo distance (3.0617(8) Å) was found to be comparable to [{CpMo(CO)2}2{m-g2:g2-P2}] (3.022(1) Å) and [{CpMo

4.1. Diborene complexes of silver and copper Transition metal diborene complexes were first reported by Braunschweig et al. in 2012 [49]. A neutral p-complex of Lewis base-stabilized diborene (51) was obtained from the reaction of [L:(Dur)B@B(Dur):L] (L = NHC) and [AgCl] under ambient conditions in THF solution (Scheme 18). Later, under similar reaction conditions the copper complex (52) was isolated from the benzene solution (Scheme 18) [48]. The 11B NMR signals for both (51) and (52) somewhat shifted upfield compared to the free ligand. The solid-state structures of compounds (51) and (52) closely resembled each other and confirmed the g2-coordination mode of the

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Scheme 19. Synthesis of platinum p-diborene complex [(Et3P)2Pt(B2Dur2)] (53). Scheme 18. Synthesis of the p-diborene complexes of Ag (51) and Cu (52).

diborene ligands. The B-B bond was slightly elongated due to the perpendicular orientation of the Cu-Cl fragment to the diborene ligand. Unexpectedly, these complexes were found to show unusual photophysical properties with high fluorescence quantum yields in solution. Braunschweig and co-workers have studied in detail the absorption, emission and excitation spectroscopy of the diborene, [L:(Dur)B@B(Dur):L] (L = NHC) and its silver and copper complexes (51) and (52) respectively [49]. The diborene species, having an intense red color, exhibited a strong absorption band at kabs = 541 nm. While a weak, broad band was observed at higher energy that overlaps with other bands both in ether and toluene medium. The diborene has a very weak emission (kem = 657 nm) and a small Stokes shift of ca. 3300 cm1. On the otherhand, on coordination with Cu or Ag, the 541 nm absorption band shifts to higher energy region and both the complexes showed a band at 417 nm along with other bands in the region of 290–350 nm. The copper complex (52), upon excitation at 420 nm, displayed an intense yellow luminescence (kem = 578 nm) with a huge Stokes shift of 6700 cm1, while the silver complex (51) exhibited a lower wavelength luminescence (kem = 519 nm). The quantum yield (ɸ = 0.18) and lifetime (s = 2.47 ns) confirmed the fluorescence process (S1 ? S0) in complex (52). No phosphorescence was observed even at 77 K. The large Stokes shift was associated with the prominent charge-transfer (CT) properties or any major geometrical variation in the emitting singlet excited state S1. On the other hand, both the quantum yield (ɸfluo = 0.77) and the fluorescence lifetime (sfluo = 6.18 ns) of the silver complex (51) were found to be noticeably higher as compared to (52). In more polar solvent, the quantum yield and the fluorescence lifetime values were reduced in case of (52), while a higher quantum yield was observed in case of (51). Highly energetic HOMO of diborenes, that get stabilized upon coordination with the metal, are primarily responsible for the phenomenon observed in these complexes. Apart from the slight r donation and p back donation, the M-B2 bonding is dominated by electrostatic interactions. As a result the phosphorescence is suppressed in these complexes. This type of photophysical phenomenon was not observed in case of olefins. It was interesting to see how the replacement of carbon by boron in metal olefin complexes of coinage metals could generate highly luminescent molecules which were otherwise non-emissive. These results opened new avenues for B-B multiple bonded compounds into the material chemistry indicating their potential application in photoactive materials. 4.2. Diborene complexes of platinum Following the previous work, Braunschweig group reported the synthesis of a platinum p-diborane complex [(Et3P)2Pt(B2Dur2)] (53) from the reduction of bromodiduryldiboran(4)yl using [Mg (MesNacnac)]2 (MesNacnac = (MesNCMe)2CH) (Scheme 19) [29]. Compound (53) was stable in the solid state at inert atmosphere but unstable in the solution state. 11B NMR displayed a resonance at d = 129.9 ppm and a single 31P NMR resonance at d = 48.6 ppm,

JPtP = 1,726 Hz. The X-ray diffraction analysis established compound (53) as the p-diborene complex of platinum. Compound (53) was reported to have the shortest B-B distance (1.510(14) Å) ever observed and lies between the base-stabilized diborenes [43,151] and the triply bonded diboryne [152]. Platinum diborene complex [(Cy3P)3Pt3(g2:l2-B2Dur2)], (54) [126] was obtained from mild thermolysis of [Pt(PCy3)2] and H2BDur (Dur = 2,3,5,6-tetramethylphenyl) in 1:1 ratio (Scheme 20). The solid-state structure showed that the two platinum centres of {Pt3(PCy3)3} unit were ligated by the diborene ligand and three {Pt(PCy3)} moieties formed a close equilateral triangle. The B-B single bond length (1.614(6) Å) was significantly longer compared to [(Et3P)2Pt(g2-B2Dur2)] (1.510(14)Å) [121], [(IDip)2B2H2] (1.561(18) Å), [43], [(IDip)2B2Br2] (1.546(6) Å) [151] and [(cAAC)2B2] (1.489 Å; cAAC = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetra-methylpyrrolidin-2 -ylidene) [152]. In compound (54), the electron goes from the metal centre to the empty bonding p-orbital of the diborene ligand and this complex is an example of violation of the Dewar-ChattDuncanson model. On the other hand, when [Pt(PCy3)2] was treated with H2BDur in 1:2 molar ratio at 68 °C, within 6 min another compound (55) was isolated (Scheme 20) [126]. Solid state structure showed presence of a tetrahedral Pt2B2 core. The B-B and Pt-Pt bonds were bridged by hydride atoms and both the Pt atoms were connected with two terminal Pt-H bonds. Compound (55) was found to have a hypercloso cluster. The B-B distance (1.648(7) Å) was found to be comparable to that in compound (54) (1.614(6) Å), but longer than that in [(Et3P)2Pt(g2-B2Dur2)] (1.51(1) Å) [121]. The 11B NMR resonance observed at d = 11.7 ppm in (55) was upfield shifted compared to (53) [121]. 4.3. Diborene complexes of zinc subgroup In 2017, Braunschweig et al., reported p-diborene complexes of group 12 metals (56–60) from the reaction of diborane ligands with anhydrous MX2 (M = Zn/Cd; X = Cl/Br) in benzene (Scheme 21) [50]. The reaction of 9-anthryl diborene with [ZnCl2] generated a zinc-diborene complex (56). The 31P{1H} NMR signal showed a broad resonance at d = 14.0 ppm and 11B NMR displayed a peak at d = 27.4 ppm. Subsequently a series of Zn and Cd diborene complexes (56–60) were isolated using different diborene ligands (Scheme 21). The X-ray structures showed that in all the complexes the B-B bond lengths were slightly elongated compared to the diborane ligand (cf. 1.524(6) Å). Similar tendency was also observed in case of coinage metal diborene complexes [48,49]. In all the cases, the metal center symmetrically interacted with both the boron atoms. The metal centers adopted a trigonal planar geometry. The orientational flexibility of the B@B bond restricted the p-back bonding in these complexes. 4.4. Diborene complexes of gold Although the diborene complexes of Cu, Ag, Pt, Cd and Zn were accessible through different synthetic routes, the examples of group 11 diborene complexes were not known until recently. In 2018, Kinjo and co-workers reported the cationic B2-Au complexes

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Scheme 20. Synthesis of platinum diborene complexes [(Cy3P)3Pt3(g2:l2-B2Dur2)] (54) and [{(Cy3P)HPt}2 (l-H){l:g2-B2Dur2H}] (55).

Scheme 21. Synthesis of Zn and Cd diborenes (56–60).

Scheme 22. Synthesis of cationic B2-gold complex (61).

[51]. Dimeric gold complex (61) was obtained from the reaction of the asymmetric diborene ligand with 0.5 equivalent of [PPh3. AuOTf] (Scheme 22). The 31P NMR spectrum showed a singlet at d = 53.5 ppm corresponding to AuPPh3 and a broad singlet at d = 8.9 ppm for B(PMe3)2. The X-ray structure displayed a Au-pdiborene complex where the gold(I) centre is coordinated with the two diborane units. The C@B@B units in compound (61) were nearly linear (avg. 176.8(3)°) and the B@B bond lengths (avg. 1.585 Å) were well within the double bond distances.

oughly. Diborene complex [{Cp*Mo(CO)2}2(B2H2){W(CO)4}] (62) was synthesised from the reaction of compound (50) with [W (CO)5THF] (Scheme 23) [141]. Two hydrogen atoms in compound (50) got replaced by {W(CO)4} unit, which is a 2e donor. The {B2H2}

4.5. Diborene complexes of molybdenum and tungsten The limited examples of diborene complexes of early transition metals prompted us to explore the reactivity of compound (50). In this regard, the potentiality of compound (50) in the synthesis of diborene(2) complexes of molybdenum was investigated thor-

Scheme 23. Synthesis of diborene complex of molybdenum (62).

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unit present in compound (62) has shorter B-B bond distance (1.624(4) Å). The diborene complexes (63) and (64) were also isolated from the thermolysis of [Cp*WCl4] in presence of LiBH4THF followed by [M(CO)5THF] (M = Mo or W) (Scheme 24).

Scheme 24. Synthesis of diborene complexes of tungsten (63) and (64).

5. Conclusion Diboranes are an exceptionally useful and versatile class of compounds for various synthetic reactions. As a result, the field of diborane chemistry has seen renewed interest in recent times. The progress in the field of transition-metal borane chemistry has advanced the studies of chemical bonding and catalysis. The diborane(4) species were found to be reactive towards compounds with multiple bonds and small molecules. The exciting area of boron-boron multiple bonds have been gaining ground ever since the discovery of first neutral boron-boron double bond compound in 2007. Prior to this discovery the limited number of compounds were known in this category due to the uncontrolled and unpredictable reaction conditions. Subsequently, several research groups around the globe have developed strategies used for the construction of boron-boron multiple bond systems. The coordination chemistry of B-B multiple bond compounds has met with very little success compared to its olefin counterparts. Nevertheless, with the improved synthetic strategies, this area lately has witnessed substantial growth with some exciting discoveries. Diborene metal complexes were found to exhibit interesting fluorescence and phosphorescence properties. In this review, we have tried to accumulate an account on transition metal diborane and diborene species and their early and recent developments carried out by others and us. Acknowledgements This work has been supported by the Indo-French Centre for the Promotion of Advanced Research (IFCPAR-CEFIPRA), Grant No. 5905-1, New Delhi, India. K.S. thanks CSIR, India and S.K. thanks IIT Madras for research fellowship. References [1] A. Stock, C. Massenez, Borwasserstoffe, Berichte Der Dtsch. Chem. Gesellschaft. 45 (1912) 3539–3568, https://doi.org/10.1002/cber.191204503113. [2] A. Stock, Hydrides of Boron and Silicon, Cornell University Press, Ithaca, NY, 1933. [3] A.N. Alexandrova, A.I. Boldyrev, H.-J. Zhai, L.-S. Wang, Coord. Chem. Rev. 250 (2006) 2811–2866, https://doi.org/10.1016/J.CCR.2006.03.032. [4] W. Dilthey, Angew. Chem. 34 (1921) 595–596, https://doi.org/10.1002/ ange.19210349508. [5] W.C. Price, J. Chem. Phys. 15 (1947) 614, https://doi.org/10.1063/1.1746611. [6] W.C. Price, J. Chem. Phys. 16 (1948) 894–902, https://doi.org/10.1063/ 1.1747028. [7] R.P. Bell, H.C. Longuet-Higgins, Proc. R. Soc. Lond. A. Math. Phys. Sci. 183 (1945) 357–374. http://www.jstor.org/stable/97820. [8] K. Hedberg, V. Schomaker, J. Am. Chem. Soc. 73 (1951) 1482–1487, https:// doi.org/10.1021/ja01148a022. [9] J.M.E. Goldschmidt, Boron Hydrides, John Wiley & Sons Ltd, New York, 1963. [10] W.H. Eberhardt, B. Crawford, W.N. Lipscomb, J. Chem. Phys. 22 (1954) 989– 1001, https://doi.org/10.1063/1.1740320.

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