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1,1′-Bis(ortho-carborane)-based transition metal complexes
Coordination Chemistry Reviews 392 (2019) 146–176
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Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr
Review
1,10 -Bis(ortho-carborane)-based transition metal complexes Igor B. Sivaev ⇑, Vladimir I. Bregadze A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov Str., 119991 Moscow, Russia
a r t i c l e
i n f o
Article history: Received 29 November 2018 Received in revised form 16 April 2019 Accepted 27 April 2019
Dedicated to Professor Armando J. L. Pombeiro on the occasion of his 70th birthday and in recognition of his outstanding contribution in coordination chemistry.
a b s t r a c t The last decade was marked by the growing interest of researchers in the chemistry of 1,10 -bis(orthocarborane) and transition metal complexes based thereof. In this review various types of transition metal complexes with 1,10 -bis(ortho-carborane) based ligands will be considered including those in which it acts as a deprotonated ϭ-ligand, or as a p-ligand formed by decapitation or reduction of the carborane cage as well as complexes based on phosphine derivatives of 1,10 -bis(ortho-carborane). Ó 2019 Elsevier B.V. All rights reserved.
Keywords: Carboranes 1,10 -Bis(ortho-carborane) Transition metal complexes Metallacarboranes
Contents 1. 2.
3. 4. 5. 6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Complexes with 1,10 -bis(ortho-carborane) as p-ligand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Complexes with decapitated carborane ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Complexes with reduced carborane ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Complexes with 1,10 -Bis(ortho-carborane) as r-ligand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1,10 -Bis(ortho-carborane)-Based phosphine ligands and complexes thereof. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Organic derivatives of 1,10 -Bis(ortho-carborane) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Icosahedral carboranes C2B10H12 are a little over 50 years old [1–3], and over those five decades a substantial amount of research has been devoted to them to the point where they are now regarded as an established part of inorganic, bordering on organic, chemistry [4]. Whilst the chemistry of single-cage carboranes is now a mature subject, the chemistry of 1,10 -bis(ortho-carborane) which has practically the same long history is very poorly developed [5]. ⇑ Corresponding author. https://doi.org/10.1016/j.ccr.2019.04.011 0010-8545/Ó 2019 Elsevier B.V. All rights reserved.
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1,10 -Bis(ortho-carborane) (1) is comprised of two orthocarborane units connected by a C–C bond (Fig. 1) [6–8]. In the solid state, 1,10 -bis(ortho-carborane) exists as equal mixture of gaucheand transoid-rotamers with C2-C1-C1A-C2A torsion angles of 108° and 180°, respectively, [8] which according to DFT calculations have near the same energy with the rotation barrier between them being only ca. 5 kcal mol1 [9]. 1,10 -Bis(ortho-carborane) was first prepared more than 50 years ago via insertion of diacetylene into decaborane frameworks [10,11], but its chemistry remained practically unexplored until the last decade, when a new efficient synthetic route based on
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Fig. 1. Crystal structure of 1,10 -bis(ortho-carborane) 1 [8].
copper-mediated coupling of two parent ortho-carborane units was elaborated [7,12,13] (Scheme 1). The lack of convenient methods for the synthesis of 1,10 -bis (ortho-carborane) in the period when the development of organic chemistry of carboranes proceeded rapidly, led to its organic chemistry remaining essentially undeveloped. However, the coordination chemistry of 1,10 -bis(ortho-carborane) is much better developed due to significant interest in its transition metal complexes during the last decade. In this review various types of transition metal complexes based on 1,10 -bis(ortho-carborane) and its derivatives will be considered including those in which it acts as a deprotonated ϭ-ligand, or as a p-ligand formed by decapitation or reduction of the carborane cage as well as complexes based on phosphine derivatives of 1,10 -bis(ortho-carborane). 2. Complexes with 1,10 -bis(ortho-carborane) as p-ligand There are two main types of metallacarborane based on icosahedral carboranes. The first is an icosahedral closo-MC2B9 metallacarborane prepared by removal of one boron atom from the icosahedral carborane cage followed by insertion of a transition metal in its place. The second is supraicosahedral closo-MC2B10 and closo-M2C2B10 metallacarboranes prepared by reduction of the carborane cage followed by insertion of one or two metal atoms [14]. 2.1. Complexes with decapitated carborane ligands The nucleophile-induced removal of one boron atom from the parent ortho-carborane and its derivatives (deboronation or decapitation) resulting in the corresponding nido-carborane is widely used in the preparation of carborane p-ligands in metallacarborane synthesis [14]. In 1,10 -bis(ortho-carborane) both ortho-carborane moieties are susceptible to nucleophilic attack and can be stepwise deboronated under treatment with KOH in refluxing ethanol to give the corresponding closo-nido 2 and nido-nido 3 species (Fig. 2). The deboronation degree depends on the reagent ratio and the reaction time [9,15–17] (Scheme 2). More recently mild
Fig. 2. Structure of the [7-(10 ,20 -closo-C2B10H11-10 -)-7,8-nido-C2B9H11] anion 2 (closo-nido-bis(carborane)) [9].
selective deboronation of 1,10 -bis(ortho-carborane) with water in organic solvents has been discovered [18]. The deboronation of one closo-carborane moiety is strongly facilitated by the presence of the second strongly electron-withdrawing closo-carborane moiety, whereas the deboronation changes the electronic effect of the carborane moiety from electron-withdrawing to electrondonating and the second closo-carborane moiety is left intact (Scheme 2). It should be noted that the decapitation of the closo-carborane cage with its transformation to the nido-structure causes the loss of the plane of symmetry passing through the C–C bond. In the case of asymmetrically C-substituted carboranes, to which 1,10 -bis (ortho-carborane) belongs and which do not have another plane of symmetry, this leads to the formation of a racemic mixture of enantiomers, where the nido-carborane cage plays the role of the chiral center. Thus, that the deboronation of one carborane cage in 1,10 -bis(ortho-carborane) 1 produces a racemic mixture of closo-nido-bis(carborane) enantiomers 2, whereas the deboronation of the second cage results in a mixture of racemic and meso nido-nido-bis(carborane) diasteromers 3 (Scheme 3). Therefore, the subsequent metallations of closo-nido- and nidonido-bis(carboranes) lead to racemic and diastereomeric mixtures of metallacarboranes, respectively. In some cases, the introduction of a metal atom into the carborane cage is accompanied by polyhedral rearrangement, which makes it difficult to identify the compounds formed in this case. It should be noted that the presence of two polyhedra with a different arrangement of metal and carbon atoms makes it difficult or impossible to establish the exact structure of metallacarboranes using 11B-11B COSY NMR spectroscopy due to signal overlapping. Therefore, X-ray diffraction is critical for determination of molecular structures of such
Scheme 1.
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Scheme 2.
Scheme 3.
Scheme 4.
metallacarboranes and in some cases being the only way in which the overall architecture and the isomeric form of the molecule can be established. The first metallacarboranes with the bulky carboranyl group as substituent were synthesized in the 1980 s when it was found that heating salts of labile complex cations [Rh(PEt3)4]+ and [(COD)Ir (PPh3)2]+ with the closo-nido-bis(carborane) anion 2 lead to metal insertion in the nido-carborane cage accompanied by migration of the substituted carbon atom to the lower pentagonal face, resulting in the corresponding 2,1,8-metallacarboranes [8-(10 ,20 closo-C2B10H11-10 -)-2,2-(Et3P)2–2-H-2,1,8-closo-RhC2B9H10] (4) [16] and [8-(10 ,20 -closo-C2B10H11-10 -)-2,2-(Ph3P)2–2-H-2,1,8-closoIrC2B9H10] (5) [19] (Scheme 4). Recently, a series of further metallacarboranes with the carboranyl group as substituent were synthesized and factors affecting their isomerisation were studied. The deprotonation of closonido-bis(carborane) 2 with n-BuLi followed by treatment with
[(p-cymene)RuCl2]2 at ambient temperature results in a mixture of yellow [1-(10 ,20 -closo-C2B10H11-10 -)-3-(p-cymene)-3,1,2-closoRuC2B9H10] (6) and colorless [8-(10 ,20 -closo-C2B10H11-10 -)-2-(pcymene)-2,1,8-closo-RuC2B9H10] (7) ruthenacarboranes (Fig. 3). The 3,1,2-isomer 6 was found to undergo thermal isomerisation to the 2,1,8-isomer 7 in refluxing THF solution for 2 h (Scheme 5) [9]. The deprotonation of closo-nido-bis(carborane) 2 with n-BuLi followed by treatment with [CpCo(CO)I2] results in the orange cobaltacarborane [1-(10 ,20 -closo-C2B10H11-10 -)-3-Cp-3,1,2-closoCoC2B9H10] (8), whereas the similar reaction with CoCl2 and NaCp followed by aerial oxidation gives the isomeric yellow cobaltacarborane [8-(10 ,20 -closo-C2B10H11-10 -)-2-Cp-2,1,8-closoCoC2B9H10] (9) with separated cage carbon atoms (Fig. 4). No thermal isomerization of the 3,1,2-isomer 8 to the 2,1,8-isomer 9 proceeds in refluxing toluene for 5 h, however the isomerization can be achieved by the reduction of 8 with sodium naphthalenide followed by aerial oxidation (Scheme 6). This implies that whilst the 3,1,2-isomer is stable in the case of the Co(III) complex, the Co(II) complex easily isomerizes to the 2,1,8-isomer, and the 3,1,2 ? 2,1,8 isomerization occurs mainly due to electronic rather than steric factors [9]. The rare 8,1,2-isomer [1-(10 ,20 -closo-C2B10H1110 -)-8-Cp-8,1,2-closo-CoC2B9H10] (10) in which both carbon atoms migrated to the lower pentagonal face (Fig. 4c) was obtained as trace product from the reaction of 2-e reduction of 1 followed by metalation with CoCl2 and NaCp (See below) [20]. Similar reaction of the deprotonated closo-nido-bis(carborane) 2 with CoCl2 and NaCp* followed by aerial oxidation gives a mixture of red unisomerised cobaltacarborane [1-(10 ,20 -closo-C2B10H11-10 -)3-Cp*-3,1,2-closo-CoC2B9H10] (11) (Fig. 5a), yellow isomerised cobaltacarborane [8-(10 ,20 -closo-C2B10H11-10 -)-2-Cp*-2,1,8-closoCoC2B9H10] (12) (Fig. 5b) and trace amount of the 13-vertex
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Fig. 3. X-ray structures of [1-(10 ,20 -closo-C2B10H11-10 -)-3-(p-cymene)-3,1,2-closo-RuC2B9H10] (6) (a) and [8-(10 ,20 -closo-C2B10H11-10 -)-2-(p-cymene)-2,1,8-closo-RuC2B9H10] (7) (b).
Scheme 5.
Fig. 4. X-ray structures of [1-(10 ,20 -closo-C2B10H11-10 -)-3-Cp-3,1,2-closo-CoC2B9H10] (8) (a), [8-(10 ,20 -closo-C2B10H11-10 -)-2-Cp-2,1,8-closo-CoC2B9H10] (9) (b) and [1-(10 ,20 closo-C2B10H11-10 -)-8-Cp-8,1,2-closo-CoC2B9H10] (10) (c).
bimetallacarborane/12-vertex carborane species [12-(closo-10 ,20 C2B10H11-10 -)-4,5-Cp*2-closo-4,5,1,12-Co2C2B9H10] (13) (Fig. 5c). As in the case of the CpCo metallacarborane 8, the reduction of [1-(10 ,20 -closo-C2B10H11-10 -)-3-Cp*-3,1,2-closo-CoC2B9H10] (11) with sodium naphthalenide followed by aerial oxidation of the initially formed Co(II) complex produces the corresponding isomerized cobaltacarborane 12 (Scheme 7) [21]. Reactions of the deprotonated closo-nido-bis(carborane) 2 with various nickel phosphine and phosphite complexes were studied. The reaction with [(dppe)NiCl2] results in a mixture of isomeric green [1-(10 ,20 -closo-C2B10H11-10 -)-3-(dppe)-3,1,2-closo-NiC2B9H10] (14) and red–purple [2-(10 ,20 -closo-C2B10H11-10 -)-4-(dppe)-4,1,2-
closo-NiC2B9H10] (15) nickelacarboranes (Fig. 6a,b, Scheme 8). The 4,1,2-isomer can be easily obtained by thermal isomerization of the 3,1,2-isomer in refluxing THF for 2 h. The similar reaction of closo-nido-bis(carborane) with [(dmpe)NiCl2] gives the yelloworange 2,1,8-isomer [8-(10 ,20 -closo-C2B10H11-10 -)-2-(dmpe)-2,1,8closo-NiC2B9H10] (16) (Fig. 6c, Scheme 8) [22]. Based on these results it was supposed that the 3,1,2 ? 4,1,2 isomerisation is caused by steric crowding, whereas the 3,1,2 ? 2,1,8 isomerisation is favored by strongly electron-donating ligands on the metal. Indeed, the reaction of closo-nido-bis (carborane) 2 with [(Ph2MeP)2NiCl2] gives the olive-green 2,1,8isomer nickelaborane [2-(10 ,20 -closo-C2B10H11-10 -)-4,4-(Ph2MeP)2–
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Scheme 6.
Fig. 5. X-ray structures of [1-(10 ,20 -closo-C2B10H11-10 -)-3-Cp*-3,1,2-closo-CoC2B9H10] (11) (a), [8-(10 ,20 -closo-C2B10H11-10 -)-2-Cp*-2,1,8-closo-CoC2B9H10] (12) (b) and [12(closo-10 ,20 -C2B10H11-10 -)-4,5-Cp*2-closo-4,5,1,12-Co2C2B9H10] (13) (c).
Scheme 7.
4,1,2-closo-NiC2B9H10] (17) as the result 3,1,2 ? 4,1,2 isomerisation, whereas the reactions with less sterically hindered nickel complexes [(PhMe2P)2NiCl2] and [(Me3P)2NiCl2] produce uniso-
merised green nickelacarboranes [1-(10 ,20 -closo-C2B10H11-10 -)-3,3(RR’2P)2–3,1,2-closo-NiC2B9H10] (R = R0 = Me (18) (Fig. 7a) and R = Ph, R0 = Me (19) (Fig. 7c)), as well as purple 8-phosphonium-
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Fig. 6. X-ray structures of [1-(10 ,20 -closo-C2B10H11-10 -)-3-(dppe)-3,1,2-closo-NiC2B9H10] (14) (a), [2-(10 ,20 -closo-C2B10H11-10 -)-4-(dppe)-4,1,2-closo-NiC2B9H10] (15) (b), and [8(10 ,20 -closo-C2B10H11-10 -)-2-(dmpe)-2,1,8-closo-NiC2B9H10] (16) (c).
Scheme 8.
Fig. 7. X-ray structures of [1-(10 ,20 -closo-C2B10H11-10 -)-3,3-(Me3P)2–3,1,2-closo-NiC2B9H10] (18) (a), [1-(10 ,20 -closo-C2B10H11-10 -)-3,8-(Me3P)2–3-Cl-3,1,2-closo-NiC2B9H9] (20) (b), and [1-(10 ,20 -closo-C2B10H11-10 -)-3,3-(PhMe2P)2–3,1,2-closo-NiC2B9H10] (19) (c).
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substituted nickelacarboranes [1-(10 ,20 -closo-C2B10H11-10 -)-3,8(RR0 2P)2–3-Cl-3,1,2-closo-NiC2B9H9] (R = R0 = Me (20) (Fig. 7b) and R = Ph, R0 = Me (21)), and the corresponding 10-phosphonium derivatives of closo-nido-bis(carborane) 22 and 23 (Scheme 9) [22]. The reaction of the deprotonated closo-nido-bis(carborane) 2 with [((MeO)3P)2NiBr2] produces the expected purple nickelacarborane [1-(10 ,20 -closo-C2B10H11-10 -)-3,3-((MeO)3P)2–3,1,2closo-NiC2B9H10] (24) (Fig. 8a), whereas the similar reaction with [Ni3(tmeda)3Cl5]Cl followed by addition of P(OMe)3 gives mainly the yellow 2,1,8-isomer [1-(10 ,20 -closo-C2B10H11-10 -)-2,2((MeO)3P)2–2,1,8-closo-NiC2B9H10] (25) (Fig. 8b) in which the unsubstituted carbon atom migrated to the lower pentagonal face as well as some of the 3,1,2-isomer (Scheme 10) [22].
A problem which frequently met in X-ray studies of metallacarborane isomers and should be mentioned there is how to distinguish correctly {BH} and {CH} vertices because the atomic scattering factors of boron and carbon for X-rays are similar due to the adjacency of these elements in the Periodic Table. To solve this problem two effective methods, the Vertex-Centroid Distance (VCD) and Boron-Hydrogen Distance (BHD) methods, have been developed recently [23,24]. The first attempt to obtain bimetallic metallacarboranes based on nido-nido-bis(carborane) also dates to the 1980 s. The reaction of nido-nido-bis(carborane) 3 with [(cod)Rh(PEt3)Cl] in refluxing THF resulted in a mixture of the dark blue complex [3-Et3P-3,1,2RhC2B9H10-1-]2 with a Rh-Rh bond (26) (Fig. 9a) and the red asym-
Scheme 9.
Fig. 8. X-ray structures of [1-(10 ,20 -closo-C2B10H11-10 -)-3,3-((MeO)3P)2–3,1,2-closo-NiC2B9H10] (24) (a) and [1-(10 ,20 -closo-C2B10H11-10 -)-2,2-((MeO)3P)2–2,1,8-closo-NiC2B9H10] (25) (b).
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Scheme 10.
Fig. 9. X-ray structures of [3-Et3P-3,1,2-RhC2B9H10-1-]2 (26) (a) and [2-(20 ,20 -(Et3P)2-20 -H-20 ,10 ,80 -RhC2B9H10-80 -)-3-(g3-C8H13)-3,1,2-RhC2B9H10] (27) (b).
metric complex [2-(20 ,20 -(Et3P)2-20 -H-20 ,10 ,80 -RhC2B9H10-80 -)-3(g3-C8H13)-3,1,2-RhC2B9H10] (27) (Fig. 9b) (Scheme 12) [25,26]. Recently, the planned synthesis of bimetallic metallacarboranes based on nido-nido-bis(carborane) was described. The deprotonation of nido-nido-bis(carborane) 3 with n-BuLi followed by treatment with [(p-cymene)RuCl2]2 at ambient temperature produces in a mixture of yellow isomeric ruthenacarboranes [1-(20 -(p-cymene)-20 ,10 ,80 -closo-RuC2B9H10-80 -)-3-(p-cymene)-3,1, 2-closo-RuC2B9H10] (28) in which one cage has 3,1,2-RuC2B9
architecture whilst the other is 2,1,8-RuC2B9 (Scheme 11, Fig. 10) [17]. Presumably the initial metalation product is the 3,1,2-RuC2B930 ,10 ,20 -RuC2B9 compound (as a diastereomeric mixture), which then undergoes spontaneous isomerisation of one cage to 2,1,8-RuC2B9 architecture as a consequence of untenable steric crowding. In contrast to the singly-metalated species [1-(10 ,20 -closo-C2B10H11-10 -)-3-(p-cymene)-3,1,2-closo-RuC2B9H10] (6) which isomerizes to [8-(10 ,20 -closo-C2B10H11-10 -)-2-(p-cymene)-
Scheme 11.
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Scheme 12.
Fig. 10. X-ray structures of isomeric ruthenacarboranes [1-(20 -(p-cymene)-20 ,10 ,80 -closo-RuC2B9H10-80 -)-3-(p-cymene)-3,1,2-closo-RuC2B9H10] (28).
2,1,8-closo-RuC2B9H10] (7) with mild heating (THF reflux), similar thermolysis of both isomers does not result in the second cage isomerisation. Taking into account that the metallacarborane moiety is slightly larger than the carborane one, it can be assumed that the 3,1,2 ? 2,1,8 cage isomerisation in the case of the singlymetalated species is caused by electronic rather than steric factors. The reaction of the deprotonated the nido-nido-bis(carborane) 3 with [CpCoI2(CO)] in THF at ambient temperature gives orange rac-[1-(30 -Cp-30 ,10 ,20 -closo-CoC2B9H10-10 -)-3-Cp-3,1,2-closoCoC2B9H10] (29) as single isolated product (Scheme 13). An interesting feature of this complex is intramolecular CH. . .HB dihydrogen bonding that results in stabilization of the gauche rotamer (Fig. 11a) [17]. Heating 29 in refluxing 1,2-dimethoxyethane for 1 h results in the 3,1,2 ? 2,1,8 isomerisation of one cobaltacarborane moiety to give [1-(20 -Cp-20 ,10 ,80 -closo-CoC2B9H10-80 -)-3-Cp-3,1,2-closoCoC2B9H10] (30) (Scheme 13). As noted above, the singlymetalated species [1-(10 ,20 -closo-C2B10H11-10 -)-3-Cp-3,1,2-closoCoC2B9H10] (8) isomerises to the 2,1,8-isomer via reduction of the 3,1,2-isomer 9 with sodium naphthalenide followed by aerial oxidation. In a similar way, reduction of the 3,1,2-CoC2B9-30 ,10 ,20 CoC2B9 compound 29 with sodium naphthalenide followed by aerial oxidation of the reduced form produces the doubly isomerized cobaltacarborane [8-(20 -Cp-20 ,10 ,80 -closo-CoC2B9H10-80 -)-2-Cp2,1,8-closo-CoC2B9H10] (31) (Fig. 11b) (Scheme 13) [17]. Reaction of the deprotonated nido-nido-bis(carborane) 3 with with CoCl2 and NaCp in refluxing THF followed by air oxidation gives a mixture of isomeric cobaltacarboranes with one isomerized cage [1-(20 -Cp-20 ,10 ,80 -closo-CoC2B9H10-80 -)-3-Cp-3,1,2-closoCoC2B9H10] (32) (Fig. 12) and two isomerized cages [8-(20 -Cp-20 ,1
0
,80 -closo-CoC2B9H10-80 -)-2-Cp-2,1,8-closo-CoC2B9H10] (31) (Scheme 14). Similarly to rac-[1-(30 -Cp-30 ,10 ,20 -closo-CoC2B9H1010 -)-3-Cp-3,1,2-closo-CoC2B9H10] (29), the reduction of 3,1,2CoC2B9-20 ,10 ,80 -CoC2B9 isomers 32 with sodium naphthalenide followed by aerial oxidation of the reduced form gives the doubly isomerized cobaltacarborane 31 (Scheme 14) [17]. The synthesis of heterometalic metallacarboranes based on nido-nido-bis(carborane) has recently been described. It was found that the carboranyl group in the 2,1,8-metallacarboranes [8-(10 ,20 closo-C2B10H11-10 -)-2-(p-cymene)-2,1,8-closo-RuC2B9H10] (7), [8(10 ,20 -closo-C2B10H11-10 -)-2-Cp-2,1,8-closo-CoC2B9H10] (9) and [8(10 ,20 -closo-C2B10H11-10 -)-2-Cp*-2,1,8-closo-CoC2B9H10] (11) can be selectively deboronated by KF in THF/water at reflux to give diastereomeric mixtures of the corresponding metallacarboranes with a nido-carborane substituent. The deprotonation of [8-(70 ,80 nido-C2B9H11-70 -)-2-(p-cymene)-2,1,8-closo-RuC2B9H10] with nBuLi followed by treatment with CoCl2 and NaCp and aerial oxidation produces a diastereomeric mixture of [8-(30 -Cp-closo-30 ,10 ,20 CoC2B9H10-10 -)-2-(p-cymene)-closo-2,1,8-RuC2B9H10] (33) (Fig. 13, Scheme 15). In contrast to the formation of cobaltacarborane 9, [8-(10 ,20 -closo-C2B10H11-10 -)-2-Cp-2,1,8-closo-CoC2B9H10], no isomerisation of the cobaltacarborane moiety takes place [21]. The deprotonation of [8-(70 ,80 -nido-C2B9H11-70 -)-2-Cp*-2,1,8closo-CoC2B9H10] with n-BuLi followed by treatment with [(p-cymene)RuCl2]2 gives a diastereomeric mixture of [8-(30 -(pcymene)-closo-30 ,10 ,20 -RuC2B9H10-10 -)-2-Cp*-closo-2,1,8-CoC2B9H10] (34), which on thermolysis in refluxing 1,2-dimethoxyethane undergoes isomerisation to a diastereomeric mixture of [8-(20 -(pcymene)-closo-20 ,10 ,80 -RuC2B9H10-20 -)-2-Cp*-closo-2,1,8-CoC2B9H10] (35) (Fig. 14, Scheme 16) [21].
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Scheme 13.
Fig. 11. X-ray structure of rac-[1-(30 -Cp-30 ,10 ,20 -closo-CoC2B9H10-10 -)-3-Cp-3,1,2-closo-CoC2B9H10] (29) (a) and [8-(20 -Cp-20 ,10 ,80 -closo-CoC2B9H10-80 -)-2-Cp-2,1,8-closo-CoC2B9H10] (31) (b).
Fig. 12. X-ray structures of isomeric cobaltacarboranes [1-(20 -Cp-20 ,10 ,80 -closo-CoC2B9H10-80 -)-3-Cp-3,1,2-closo-CoC2B9H10] (32).
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Scheme 14.
Fig. 13. X-ray structures of [8-(30 -Cp-closo-30 ,10 ,20 -CoC2B9H10-10 -)-2-(p-cymene)-closo-2,1,8-RuC2B9H10] (33) diastereomers.
Scheme 15.
2.2. Complexes with reduced carborane ligands The 2-e reduction of 1,10 -bis(ortho-carborane) 1 with sodium naphthalenide in THF results in a [nido-C2B10]2-(closo-C2B10) species, which on the treatment with [(g6-C6H6)RuCl2]2 or [(g6-p-cymene)RuCl2]2 gives the corresponding 13-vertex ruthenacarborane/12-vertex carborane [1-(10 ,20 -closo-C2B10H1110 -)-4-(g6-arene)-4,1,6-closo-RuC2B10H11] (arene = benzene (36) (Fig. 15a); p-cymene (37) (Fig. 15b)). In contrast, the similar reaction with [(g6-1,3,5-Me3C6H3)RuCl2]2 results in a complex in which the 13-vertex ruthenacarborane moiety has isomerized, [8-(10 ,20 -closo-C2B10H11-10 -)-4-(g6-1,3,5-Me3C6H3)-4,1,8-closo-
RuC2B10H11] (38) (Fig. 15c) (Scheme 17). The isomerisation is presumably the result of otherwise untenable steric interactions between the mesitylene and carborane substituents [27]. The reaction of complex 38 with sodium naphthalenide in THF results in reductive opening of the 13-vertex ruthenacarborane rather than the 12-vertex carborane moiety - the subsequent addition of ruthenium complexes [(g6-arene)RuCl2]2 affords in each case two new bimetallic ruthenacarboranes - yellow 14-vertex Ru2C2B10/12-vertex C2B10 [1-(g6-1,3,5-Me3C6H3)-9-(10 , 2 0 - closo-C2B10H11-10 -)-13-(g6-arene)-1,13,2,9-closo-Ru2C2B10H11] (arene = benzene (39); p-cymene (40), mesitylene (41)) as the major product, and red 13-vertex Ru2C2B9/12-vertex C2B10
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Fig. 14. X-ray structure of [8-(20 -(p-cymene)-closo-20 ,10 ,80 -RuC2B9H10-20 -)-2-Cp*closo-2,1,8-CoC2B9H10] (35) (arbitrary diastereomer).
157
[4-(g6-arene)-5-(g6-1,3,5-Me3C6H3)-6-(10 -20 -closo-C2B10H11-10 -)4,5,1,6-closo-Ru2C2B9H10] (arene = benzene (42); p-cymene (43), mesitylene (44)) (Fig. 16), as a minor product (Scheme 18) [27]. The reduction of 1,10 -bis(ortho-carborane) 1 with excess of lithium naphthalenide in THF followed by addition of [(g6-pcymene)RuCl2]2 unexpectedly resulted in the dark red ruthenacarborane [1-(10 ,20 -closo-C2B10H11-10 -)-4-{C10H14Ru(p-cymene)}4,1,6-closo-Ru2C2B10H11] (45) (Fig. 17) as the only isolable product. The molecule is an example of a ‘‘fly-over bridge” compound, in which the p-cymene ligand has been converted into a [l-r,g3: g3,r-C6]2 ligand by reductive cleavage of the C44-C45 bond [28]. The 2-e reduction of 1,10 -bis(ortho-carborane) 1 with lithium in the presence of naphthalene in THF followed by the metalation with CoCl2/NaCp produced only trace amounts of isomerized 13-vertex cobaltacarboranes [8-(10 ,20 -closo-C2B10H11-10 -)-4-Cp4,1,8-closo-CoC2B10H11] (46) and [12-(10 ,20 -closo-C2B10H11-10 -)-4Cp-4,1,12-closo-CoC2B10H11] (47), as well as minor amounts of the 4-e reduction products rac- and meso-[1-(40 -Cp-40 ,10 ,60 -closo-
Scheme 16.
Fig. 15. X-ray structures of [1-(10 ,20 -closo-C2B10H11-10 -)-4-(g6-benzene)-4,1,6-closo-RuC2B10H11] (36) (a), [1-(10 ,20 -closo-C2B10H11-10 -)-4-(g6-p-cymene)-4,1,6-closo-RuC2B10H11] (37) (b) and [8-(10 ,20 -closo-C2B10H11-10 -)-4-(g6-mesitylene)-4,1,8-closo-RuC2B10H11] (38) (c).
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Scheme 17.
Fig. 16. X-ray structures of [4-(g6-p-cymene)-5-(g6-mesitylene)-6-(10 -20 -closo-C2B10H11-10 -)-4,5,1,6-closo-Ru2C2B9H10] (43) (a) and [4,5-(g6-mesitylene)2–6-(10 -20 -closoC2B10H11-10 -)-4,5,1,6-closo-Ru2C2B9H10] (44) (b).
CoC2B10H11-10 -)-4-Cp-4,1,6-closo-CoC2B10H11] (48 and 49, respectively) and the deboronation-metalation product [1-(10 ,20 closo-C2B10H11-10 -)-8-Cp-8,1,2-closo-CoC2B9H10] (10) (Fig. 4c) (Scheme 19) [20]. The 4-e reduction of 1,10 -bis(ortho-carborane) 1 with lithium naphthalenide in tetrahydrofuran produces a diastereomeric
mixture of open 12-vertex nido-nido-bis(carborane). The treatment of this mixture with CoCl2 and NaCp followed by aerial oxidation results in the mixture of red 13-vertex/13-vertex cobaltacarboranes 48 and 49 (Fig. 18, Scheme 20) [29]. Thermolysis of the mixture of 48 and 49 in xylenes at 180 °C for 4 h results in migration of the unsubstituted carbon atoms to the
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Scheme 18.
3. Complexes with 1,10 -Bis(ortho-carborane) as r-ligand
Fig. 17. X-ray structure of [1-(10 ,20 -closo-C2B10H11-10 -)-4-{(l-r,g3:g3,r-C6H4-2Me-4-iPr)Ru(g6-p-cymene)}-4,1,6-closo-Ru2C2B10H11] (45).
lower pentagonal belt giving rac- and meso-[1-(40 -Cp-40 ,10 ,120 closo-CoC2B10H11-10 -)-4-Cp-4,1,12-closo-CoC2B10H11] (50 and 51, respectively, Fig. 19) (Scheme 21) [30]. The 4e-reduction of l-2,20 -(CH2)2–1,10 -bis(ortho-carborane) with sodium in THF followed by addition of [(g6-p-cymene) RuCl2]2 gave a mixture of the 13-vertex RuC2B10/12-vertex C2B10 ruthenacarborane 52, unprecedented 12-vertex RuCB10/12-vertex RuCB10 ruthenacarborane 53 in which two metallacarborane moieties are connected both through ‘‘normal” l-C,C0 -(CH2CH2) bridge and l,g2-B,B0 -(CH@CH) bridge formed by ‘‘extraction” of the CH groups from the both carborane cages, as well as complex 54 in which 1,1’-bis(ortho-carborane) plays the role of a doubly chelating ϭ-ligand (Scheme 22, Fig. 20) [31].
Similar to the parent ortho-carborane, the CH groups in 1,10 -bis (ortho-carborane) 1 have an acidic character and can be easily deprotonated with n-butyllithium to give the corresponding dilithium derivative that readily reacts with electrophilic reagents and metal chlorides resulting in various organic and organometallic derivatives. The first examples of 1,10 -bis(ortho-carborane)based metal complexes were reported more than 45 years ago by Hawthorne, who described reactions of its dilithium derivative with various transition metal chlorides (Scheme 23). Thus, the reaction of the dilithium derivative of 1,10 -bis(orthocarborane) with CuCl2 in diethyl ether results in stable yellow diamagnetic Cu(III) complex [Cu(j2-C,C0 -bCarb)2] (54), where the 1,10 -bis(ortho-carborane) moiety plays the role of five-membered C,C’-chelate ligand. The Cu(III) complex can be reduced to the blue Cu(II) complex [Cu(j2-C,C0 -bCarb)2]2 (55) with Li metal in acetone or Zn metal in dichloromethane [32,33]. The Cu(III) complex was found to have distorted square planar structure (the angle between the chelate ligand planes is 26°), whereas the structure of the Cu(II) complex is intermediate between square planar and tetrahedral (the angle between the ligand planes is 54°) [34] (Fig. 21). The similar reaction of the dilithium derivative of 1,10 -bis(ortho-carborane) with CoCl2 in diethyl ether produces the pale purple Co(II) complex [Co(j2-C,C0 -bCarb)2]2 (56) that can be oxidized to the black diamagnetic Co(III) complex [Co(j3-C,C0 -bCarb)2] (57) with CuCl2 in dichloromethane [33]. In both the Co(II) and Co(III) complexes, the cobalt atom has a tetrahedral environment [34] (Fig. 21), that in the last case is supplemented by an agostic BH ? Co interaction with one of the adjacent BH vertices [35]. The reaction of the dilithium derivative of 1,10 -bis(ortho-carborane) with NiCl2 in diethyl ether produces the orange-red Ni(II) complex [Ni(j2-C,C0 bCarb)2]2 (58) that can be oxidized to the green Ni(III) complex
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Scheme 19.
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Fig. 18. X-ray structures of rac- and meso-[1-(40 -Cp-40 ,10 ,60 -closo-CoC2B10H11-10 -)-4-Cp-4,1,6-closo-CoC2B10H11] 48 and 49.
Scheme 20.
[Ni(j2-C,C0 -bCarb)2] (59) with CuCl2 in dichloromethane [35]. The Ni(II) complex has a distorted square planar structure (the angle between the ligand planes is 26°) due to the steric constraints of the bis(carborane) ligand [34] (Fig. 21). The similar reaction of the dilithium derivative with ZnCl2 gives the white bis(chelate) complex [Zn(j2-C,C0 -bCarb)2]2 (60) [33]. A similar approach was applied recently to the synthesis of mixed ligand complexes of transition metals. Thus, the reactions of the dilithium derivative of 1,10 -bis(ortho-carborane) 1 with [(dmpe)NiCl2] and [(dppe)NiCl2] in THF produce the yellow nearly square planar complexes [(dmpe)Ni(j2-C,C0 -bCarb)] (61) [36] and
[(dppe)Ni(j2-C,C0 -bCarb)] (62) [36,37], respectively (Scheme 24). Similar complexes of platinum [(dppe)Pt(j2-C,C0 -bCarb)] (63) and palladium [(dppe)Pd(j2-C,C0 -bCarb)] (64) were prepared by the reaction of the dilithium derivative with [(dppe)PtCl2] and [(dppe)PdCl2], respectively (Fig. 22, Scheme 24) [37]. The reactions of the dilithium derivative of 1,10 -bis(orthocarborane) 1 with g5-pentamethylcyclopentadienyl complexes of iridium [Cp*IrCl2]2 and rhodium [Cp*RhCl2]2 in THF produce the coordinatively unsaturated 16-electron green complexes [Cp*Ir (j2-C,C0 -bCarb)] (65) and [Cp*Rh(j2-C,C0 -bCarb)] (66), respectively (Fig. 23). The iridium complex is stable in air in the solid state, but
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Fig. 19. X-ray structures of rac- and meso-[1-(40 -Cp-40 ,10 ,120 -closo-CoC2B10H11-10 -)-4-Cp-4,1,12-closo-CoC2B10H11] 50 and 51.
Scheme 21.
reacts readily with CO to give the 18-electron complex [Cp*Ir(CO) (j2-C,C0 -bCarb)] (67) (Scheme 25) [38]. The reaction of the dilithium derivative of 1,10 -bis(orthocarborane) 1 with [(p-cym)RuCl2]2 in THF results in the orange complex [(p-cym)Ru(j3-C,C0 -bCarb)] (68) in which a 16-electron configuration of the metal atom is stabilized by an agostic B(3)H ? Ru interaction (Fig. 24). In solution, four BH units (B(3)H, B (30 )H, B(6)H and B(60 )H) alternatively acts as the agostic BH in rapid exchange with each other. The reaction of 68 with CO affords the expected yellow complex [(p-cym)Ru(CO)(j2-C,C0 -bCarb)] (69) (Scheme 26) [39]. The reaction of complex 68 with 1,2-bis(diphenylphosphino)e thane (dppe) results in facile displacement of the p-cymene ligand by dppe and a change of the ligating mode for 1,10 -bis(orthocarborane) from C,C0 - to C,B0 -giving the yellow complex [(dppe) Ru(j3-C,B0 -bCarb)] (70). In this complex one carborane unit is now bond to the ruthenium atom by B(30 )-Ru bond, whereas the
other is still connected to the metal by a C-Ru bond complemented by a B(3)H ? Ru agostic bond. This species reacts with CO or MeCN to give the corresponding coordinatively saturated octahedral complexes trans-[(dppe)(CO)2Ru(j2-C,B0 -bCarb)] (71) and trans-[(dppe)(MeCN)2Ru(j2-C,B0 -bCarb)] (72) (Fig. 25, Scheme 27) [39]. In a similar way, the reaction of 68 with triphenylphosphine produces the yellow-orange complex [(Ph3P)2Ru(j3-C,B0 -bCarb)] (73). The same complex can be of alternatively prepared by the reaction of doubly-deprotonated 1,10 -bis(ortho-carborane) 1 with [(Ph3P)4RuCl2] or [(Ph3P)3RuCl2] (Scheme 27). The reaction of 68 with CO results in the loss of one PPh3 ligand giving an equimolar mixture of two isomeric octahedral complexes [(Ph3P)(CO)3Ru(j2C,B0 -bCarb)] (74), one with the PPh3 ligand trans to C(2) and the other with PPh3 trans to B(30 ). The reaction with acetonitrile affords a single complex [(Ph3P)(MeCN)3Ru(j2-C,B0 -bCarb)] (75) with the PPh3 ligand trans to C(2) (Fig. 26, Scheme 27) [39].
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Scheme 22.
Fig. 20. X-ray structures of ruthenacarboranes 52 (a) and 51 (b) and complex 53 (c).
Scheme 23.
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Fig. 21. X-ray structures of the [j2-C,C0 -1,10 -bis(ortho-carboranyl)cobalt(II)]2 (56) (a), [j2-C,C0 -1,10 -bis(ortho-carboranyl)nickel(II)]2 (58) (b), [j2-C,C0 -1,10 -bis(orthocarboranyl)copper(III)] (54) (c) and [j2-C,C0 -1,10 -bis(ortho-carboranyl)copper(II)]2 (55) (d) anions.
Scheme 24.
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Fig. 22. X-ray structures of [(dmpe)NiII(j2-C,C0 -bCarb)] (61) (a) [(dppe)NiII(j2-C,C0 -bCarb)] (62) (b), [(dppe)PtII(j2-C,C0 -bCarb)] (63) (c) and [(dppe)PdII(j2-C,C0 -bCarb)] (64) (d).
Fig. 23. X-ray structure of [Cp*Ir(j2-C,C0 -bCarb)] (65).
The reaction of the dilithium derivative of 1,10 -bis(orthocarborane) 1 with [(4,40 -tBu2bpy)PtCl2] in THF produces a mixture of the C,C0 - to C,B0 -bonded complexes [(4,40 -tBu2bpy)Pt(j2-C,C0 -
bCarb)] (76) and [(4,40 -tBu2bpy)Pt(j2-C,B0 -bCarb)] (77) (Fig. 27) [37]. It was found that the isomer ratio varies widely depending on the reaction time and temperature, as well as the n-BuLi batch. The C,C0 -isomer 76 can be prepared selectively using potassium bis (trimethylsilyl)amide as an alternative to n-BuLi, whereas the C,B0 -isomer 77 was obtained by the reaction of the dipotassium derivative of 1,10 -bis(ortho-carborane) 1 with [(4,40 -tBu2bpy)PtCl2] pretreated with MeLi (Scheme 28) [40]. A similar reaction of the dilithium derivative of 1,10 -bis(9,12Et2-ortho-carborane) with [(4,40 -tBu2bpy)PtCl2] gives mainly the C,B0 -bound isomer 78 (Fig. 28, Scheme 29) [37]. The 1,10 -bis(ortho-carborane)-based Pt(II) complexes with 4,40 di-tert-butyl-2,20 -bipyridine ligand were found to display blue phosphorescent emission dominated by MLCT from the Pt(II) center to the 4,40 -tBu2bpy ligand. Furthermore, since the 1,10 -bis (ortho-carboranyl) ligand introduces sufficient steric bulk above and below the square plane of the metal center, it effectively shuts down undesired intramolecular Pt(II). . .Pt(II) interactions in the solid state, which commonly lead to luminescence quenching [37]. Compared to common bidentate ligands, the introduction of bulky carboranes enormously changes the electronic structures, electroluminescence properties (including the phosphorescence
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Scheme 25.
Fig. 24. X-ray structures of [(p-cym)Ru(j3-C,C0 -bCarb)] (68) (a) and [(p-cym)Ru(CO)(j2-C,C0 -bCarb)] (69) (b).
Scheme 26.
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Fig. 25. X-ray structures of [(dppe)Ru(j3-C,B0 -bCarb)] (68) (a), trans-[(dppe)(CO)2Ru(j2-C,B0 -bCarb)] (71) (b) and trans-[(dppe)(MeCN)2Ru(j2-C,B0 -bCarb)] (72) (c).
Scheme 27.
emission band), radiative and non-radiative decay processes. The different binding modes between the Pt atom and the 1,10 -bis (ortho-carboranyl) ligand have a minor effect on the radiative decay rate but a clear influence on the non-radiative decay process [41]. There are only two examples of a non-chelated complexes with the 1,10 -bis(ortho-carborane) ligand. The first one was obtained from the reaction of its dilithium derivative with [(Ph3P)AuCl] in tetrahydrofuran. In the solid state, complex [{(Ph3P)Au}2(j2-C,C0 bCarb)] (79) has a syn-conformation due to an aurophilic Au. . .Au interaction of 3.119 Å (Fig. 29) [42]. The second one is the toluene
solvated dicopper derivative [{(g3-toluene)Cu}2(j2-C,C0 -bCarb)] (80) prepared by the reaction of the dilithium derivative of orthocarborane with CuCl in toluene (Fig. 29) [7]. In addition to transition metals, the 1,10 -bis(ortho-carboranyl) ligand was found to form complexes with some non-transition metals. The reactions of 1,10 -bis(ortho-carborane) and 1,10 -bis (9,10,11,12-Me4-ortho-carborane) with (n-Bu)2Mg in 1,2dimethoxyethane result in the j2-C,C0 -magnesium complexes (Fig. 30) which on the treatment with MeSnCl2 give the corresponding 1,10 -bis(ortho-carboranyl)stannyl derivatives 81 and 82 (Scheme 30) [43].
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Fig. 26. X-ray structures of [(Ph3P)2Ru(j3-C,B0 -bCarb)] (73) (a), [(Ph3P)(MeCN)3Ru(j2-C,B0 -bCarb)] (74) (b) and [(Ph3P)(CO)3Ru(j2-C,B0 -bCarb)] (75) (c).
Fig. 27. X-ray structures of [(4,40 -tBu2bpy)Pt(j2-C,C0 -bCarb)] (76) (a) and [(4,40 -tBu2bpy)Pt(j2-C,B0 -bCarb)] (77) (b).
4. 1,10 -Bis(ortho-carborane)-Based phosphine ligands and complexes thereof Synthesis of the first 1,10 -bis(ortho-carborane)-based phosphine l-2,20 -PhP-1,10 -bis(ortho-carborane) (83) was reported 40 years ago, however its characterization was incomplete [44]. Subsequently synthesis of l-2,20 -ClP-1,10 -bis(ortho-carborane) (84) and its transformation into l-2,20 -FP-1,10 -bis(ortho-carborane) (85) was also reported [45]. Very recently, the synthesis of cyclic phosphines l-2,20 -EtP-1,10 -bis(ortho-carborane) (86) and l-2,20 -PhP-1, 10 -bis(ortho-carborane) (83) (Fig. 31) by the reaction of the dilithium derivative of 1,10 -bis(ortho-carborane) 1 with EtPCl2 and PhPCl2 as well as their complete characterization was described (Scheme 31) [46]. The reactions of phosphines 86 and 83 with [(tht)AuCl] in dichloromethane affords the corresponding gold complexes [ClAu
{l-2,20 -EtP-1,10 -bis(ortho-carborane)}] (87) and [ClAu{l-2,20 -PhP1,10 -bis(ortho-carborane)}] (88) (Scheme 32, Fig. 32). The Tolman cone angles for the ethylphosphine and phenylphosphine complexes were calculated to be 171.6° and 172.5° for 87 and 88, respectively. Although Ph is a larger substituent than Et, the similarity of the cone angles reflects the fact that the vast majority of the steric bulk in l-2,20 -EtP- and l-2,20 -PhP-1,10 -bis(orthocarborane) comes from the common bis(ortho-carborane) fragment. In terms of their size the synthesized phosphines are the most comparable to (but slightly larger than) tricyclohexylphosphine, whereas they are much less basic than PCy3 and the least basic of any carboranylphosphines so far reported [46]. The reaction of 1,10 -bis(ortho-carborane) 1 with 1 equiv. MeLi and 1 equiv. (iPr)2PCl in THF produces the expected phosphino derivative 2-iPr2P-1,10 -bis(ortho-carborane) (89) (Fig. 33a), whereas the reactions with 2 equiv. MeLi and 2 equiv. R2PCl
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Scheme 28.
(R = i-Pr, Ph, N(i-Pr)2) result in unexpected products 90–92 with the formation of one open 12-vertex nido-carborane cluster following the cleavage C10 -C20 bond. One of boron atoms in the nido-carborane, adjacent to the C10 carbon atom of the initial closo-carborane, underwent a B-H activation with the formation of the C-P-B bridge between the closo-and nidocarborane cages (Fig. 33b). The deprotonation of 89 with MeLi and subsequent reaction with (iPr)2PCl results in the formation of closo-C2B10/nido-C2B10 diphosphonium bis(carborane) 90 (Scheme 33) [47]. 5. Organic derivatives of 1,10 -Bis(ortho-carborane)
Fig. 28. X-ray structure of [(4,40 -tBu2bpy)Pt(j2-C,B0 -bCarb-9,90 ,12,120 -Et4)] (78).
Since some of the transition metal complexes described above were derived from derivatives of 1,10 -bis(ortho-carborane), we thought it appropriate to devote one of the parts of this review to organic derivatives of 1,10 -bis(ortho-carborane). At first of all, it should be noted that organic chemistry of 1,10 -bis(orthocarborane) is much less explored then its organometallic chemistry. The above mentioned 9,90 ,12,120 -tetraethyl-1,10 -bis(orthocarborane) (93) and 8,80 ,9,90 ,10,100 ,12,120 -octamethyl-1,10 -bis (ortho-carborane) (94) were prepared by copper-mediated coupling of 9,12-diethyl-ortho-carborane [37] and methylation of 1,10 -bis(ortho-carborane) with excess of MeI in the presence of AlCl3 [48], respectively (Scheme 34).
Scheme 29.
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Fig. 29. X-ray structures of [{(Ph3P)Au}2(j2-C,C0 -bCarb)] (79) (a) and [{(g3-toluene)Cu}2(j2-C,C0 -bCarb)] (80) (b).
Fig. 30. X-ray structure of [(1,2-DME)2Mg(j2-C,C0 -bCarb)].
Despite the fact that the CH groups of 1,10 -bis(ortho-carborane) are easily metalated, only a very limited number of its organic derivatives were obtained using this way. The C,C0 -dimethyl derivative 2,20 -Me2-1,10 -bis(ortho-carborane) (95) was prepared by the reaction of the dilithium derivative of 1 with methyl iodide [10], whereas the similar reaction with 1,2-dibromoethane produces the ethylene-bridged derivative 2,20 -l-CH2CH2-1,10 -bis (ortho-carborane) (96) [31]. The bis(hydroxymethyl) derivative 2,20 -(HOCH2)2-1,10 -bis(ortho-carborane) (97) was prepared by the treatment of the dilithium derivative with formaldehyde in ether-benzene [49] (Scheme 35). The treatment of the dilithium derivative of 1 with nitrosyl chloride NOCl in ether produces unstable blue bis(nitroso) derivative 2,20 -(NO)2–1,10 -bis(ortho-carborane) which was not isolated but can be trapped with 1,3-cyclohexadiene to give the products Diels-Alder cycloaddition as a mixture of meso- and rac-isomers 2,20 -(NOC6H8)2–1,10 -bis(ortho-carborane) (98) (Scheme 36) [50]. The similar reaction of the monolithium derivative of 1 with NOCl gave the nitroso derivative 2-NO-1,10 -bis(ortho-carborane) (99) which on an aqueous treatment is reduced to the corresponding
Scheme 30.
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Fig. 31. X-ray structures of l-2,20 -EtP-1,10 -bis(ortho-carborane) (86) (a) and l-2,20 -PhP-1,10 -bis(ortho-carborane) (83) (b).
Scheme 31.
Scheme 32.
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Fig. 32. X-ray structures of [ClAu{l-2,20 -EtP-1,10 -bis(ortho-carborane)}] (87) (a) and [ClAu{l-2,20 -PhP-1,10 -bis(ortho-carborane)}] (88) (b).
Fig. 33. X-ray structures of 2-iPr2P-1,10 -bis(ortho-carborane) (89) (a) and l-2,30 -iPr2P-1,10 -(1,2-C2B10H10)(nido- CB10H9CPH(iPr)2) (90) (b).
hydroxylamine 2-HONH-1,10 -bis(ortho-carborane) (100) (Scheme 37) [50]. Of particular interest is the synthesis of five-membered cyclic derivatives of 1,10 -bis(ortho-carborane). The closure of the fivemembered cycle easily proceeds when the dilithium derivative of 1,10 -bis(ortho-carborane) reacts with transition metal chlorides, as well as with chlorides of elements of the third (arsenic [51]) and fourth (magnesium [43], phosphorus [44–46]) periods of the Periodic System, whose atoms have large diameters. At the same time, the closure of the five-membered cycle with the participation of small elements of the second period is a rather nontrivial task. For example, the lithium and magnesium derivatives of 1,10 -bis (ortho-carborane) 1 readily react with methyl or ethyl formate to give the bis(carborane) analogue of 9-fluorenol 2,20 -l-HOCH-1,10 bis(ortho-carborane) (101) [52,53], whereas the dilithium deriva-
tive does not react with dihalogenmethanes CH2X2 (X = Cl, Br, I) at all. Nevertheless, the bis(carborane) analogue of fluorene 2,20 l-CH2-1,10 -bis(ortho-carborane) (102) (Fig. 34a) was successfully synthesized by the reaction of the dicopper derivative of 1 with CH2I2 [54,55] (Scheme 38). All attempts to prepare 9-borafluorene analogue of 1,10 -bis (ortho-carborane) using its lithium, magnesium and tin derivatives were unsuccessful. Finally, the desired heterocycle 2,20 -l-iPr2NB1,10 -bis(ortho-carborane) (103) (Fig. 34b) and their methylated analogue 2,20 -l-iPr2NB-1,10 -bis(ortho-carborane-8,9,10,12-Me4) (104) were synthesized by the reaction of dipotassium derivatives of 1 and 94 with iPr2NBCl2, respectively (Scheme 39). It should be noted that all numerous attempts to prepare the 9-borafluorene analogues using BX3 (X = F, Cl, Br, I) or ArBCl2 (Ar = Ph, 4-MeC6H4, 2,4,6-Me3C6H2, 2,4,6-iPr3C6H2) failed as well [56].
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Scheme 33.
Scheme 34.
6. Conclusions In general, 1,10 -bis(ortho-carborane) has the capacity to undergo similar reactions to single-cage carboranes but with two important differences, specifically 1) reactivity at a single cage is significantly influenced by the presence of a bulky electronwithdrawing substituent and 2) the reactions could occur at both cages and such reactivity could be either isolated or cooperative. Both these differences play important roles in the chemistry of transition metal complexes with 1,10 -bis(ortho-carborane)-based ligands. In particular, the large size and strong electronwithdrawing effect of ortho-carborane cage as substituent has
strong impact on the stability of certain isomers of metallacarboranes based on decapitated 1,10 -bis(ortho-carborane), whereas cooperative reactivity of two ortho-carborane fragments presents the basis for use 1,10 -bis(ortho-carborane) as a chelating ligand with the possibility of intramolecular activation of a B-H bond without the presence of special directing groups. Finally, both these differences should be taken into account in design and synthesis of 1,10 -bis(ortho-carborane)-based ligands with Lewis base centers, such as phosphine groups. All this makes the chemistry of 1,10 -bis(ortho-carborane) and their transition metal complexes extremely exciting and one which attracts the growing interest of researchers.
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Scheme 35.
Scheme 36.
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Scheme 37.
Fig. 34. X-ray structures of 2,20 -l-CH2-1,10 -bis(ortho-carborane) (102) (a) and 2,20 -l-iPr2NB-1,10 -bis(ortho-carborane) (103) (b).
Scheme 38.
Scheme 39.
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Acknowledgments The authors thank Prof. Alan J. Welch (Heriot-Watt University, Edinburgh) for helpful discussion and Russian Foundation for Basic Research (15-03-05822) and the Ministry of Science and Higher Education of the Russian Federation for financial support.
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Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr
Review
1,10 -Bis(ortho-carborane)-based transition metal complexes Igor B. Sivaev ⇑, Vladimir I. Bregadze A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov Str., 119991 Moscow, Russia
a r t i c l e
i n f o
Article history: Received 29 November 2018 Received in revised form 16 April 2019 Accepted 27 April 2019
Dedicated to Professor Armando J. L. Pombeiro on the occasion of his 70th birthday and in recognition of his outstanding contribution in coordination chemistry.
a b s t r a c t The last decade was marked by the growing interest of researchers in the chemistry of 1,10 -bis(orthocarborane) and transition metal complexes based thereof. In this review various types of transition metal complexes with 1,10 -bis(ortho-carborane) based ligands will be considered including those in which it acts as a deprotonated ϭ-ligand, or as a p-ligand formed by decapitation or reduction of the carborane cage as well as complexes based on phosphine derivatives of 1,10 -bis(ortho-carborane). Ó 2019 Elsevier B.V. All rights reserved.
Keywords: Carboranes 1,10 -Bis(ortho-carborane) Transition metal complexes Metallacarboranes
Contents 1. 2.
3. 4. 5. 6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Complexes with 1,10 -bis(ortho-carborane) as p-ligand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Complexes with decapitated carborane ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Complexes with reduced carborane ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Complexes with 1,10 -Bis(ortho-carborane) as r-ligand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1,10 -Bis(ortho-carborane)-Based phosphine ligands and complexes thereof. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Organic derivatives of 1,10 -Bis(ortho-carborane) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Icosahedral carboranes C2B10H12 are a little over 50 years old [1–3], and over those five decades a substantial amount of research has been devoted to them to the point where they are now regarded as an established part of inorganic, bordering on organic, chemistry [4]. Whilst the chemistry of single-cage carboranes is now a mature subject, the chemistry of 1,10 -bis(ortho-carborane) which has practically the same long history is very poorly developed [5]. ⇑ Corresponding author. https://doi.org/10.1016/j.ccr.2019.04.011 0010-8545/Ó 2019 Elsevier B.V. All rights reserved.
146 147 147 156 159 168 169 173 176 176
1,10 -Bis(ortho-carborane) (1) is comprised of two orthocarborane units connected by a C–C bond (Fig. 1) [6–8]. In the solid state, 1,10 -bis(ortho-carborane) exists as equal mixture of gaucheand transoid-rotamers with C2-C1-C1A-C2A torsion angles of 108° and 180°, respectively, [8] which according to DFT calculations have near the same energy with the rotation barrier between them being only ca. 5 kcal mol1 [9]. 1,10 -Bis(ortho-carborane) was first prepared more than 50 years ago via insertion of diacetylene into decaborane frameworks [10,11], but its chemistry remained practically unexplored until the last decade, when a new efficient synthetic route based on
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Fig. 1. Crystal structure of 1,10 -bis(ortho-carborane) 1 [8].
copper-mediated coupling of two parent ortho-carborane units was elaborated [7,12,13] (Scheme 1). The lack of convenient methods for the synthesis of 1,10 -bis (ortho-carborane) in the period when the development of organic chemistry of carboranes proceeded rapidly, led to its organic chemistry remaining essentially undeveloped. However, the coordination chemistry of 1,10 -bis(ortho-carborane) is much better developed due to significant interest in its transition metal complexes during the last decade. In this review various types of transition metal complexes based on 1,10 -bis(ortho-carborane) and its derivatives will be considered including those in which it acts as a deprotonated ϭ-ligand, or as a p-ligand formed by decapitation or reduction of the carborane cage as well as complexes based on phosphine derivatives of 1,10 -bis(ortho-carborane). 2. Complexes with 1,10 -bis(ortho-carborane) as p-ligand There are two main types of metallacarborane based on icosahedral carboranes. The first is an icosahedral closo-MC2B9 metallacarborane prepared by removal of one boron atom from the icosahedral carborane cage followed by insertion of a transition metal in its place. The second is supraicosahedral closo-MC2B10 and closo-M2C2B10 metallacarboranes prepared by reduction of the carborane cage followed by insertion of one or two metal atoms [14]. 2.1. Complexes with decapitated carborane ligands The nucleophile-induced removal of one boron atom from the parent ortho-carborane and its derivatives (deboronation or decapitation) resulting in the corresponding nido-carborane is widely used in the preparation of carborane p-ligands in metallacarborane synthesis [14]. In 1,10 -bis(ortho-carborane) both ortho-carborane moieties are susceptible to nucleophilic attack and can be stepwise deboronated under treatment with KOH in refluxing ethanol to give the corresponding closo-nido 2 and nido-nido 3 species (Fig. 2). The deboronation degree depends on the reagent ratio and the reaction time [9,15–17] (Scheme 2). More recently mild
Fig. 2. Structure of the [7-(10 ,20 -closo-C2B10H11-10 -)-7,8-nido-C2B9H11] anion 2 (closo-nido-bis(carborane)) [9].
selective deboronation of 1,10 -bis(ortho-carborane) with water in organic solvents has been discovered [18]. The deboronation of one closo-carborane moiety is strongly facilitated by the presence of the second strongly electron-withdrawing closo-carborane moiety, whereas the deboronation changes the electronic effect of the carborane moiety from electron-withdrawing to electrondonating and the second closo-carborane moiety is left intact (Scheme 2). It should be noted that the decapitation of the closo-carborane cage with its transformation to the nido-structure causes the loss of the plane of symmetry passing through the C–C bond. In the case of asymmetrically C-substituted carboranes, to which 1,10 -bis (ortho-carborane) belongs and which do not have another plane of symmetry, this leads to the formation of a racemic mixture of enantiomers, where the nido-carborane cage plays the role of the chiral center. Thus, that the deboronation of one carborane cage in 1,10 -bis(ortho-carborane) 1 produces a racemic mixture of closo-nido-bis(carborane) enantiomers 2, whereas the deboronation of the second cage results in a mixture of racemic and meso nido-nido-bis(carborane) diasteromers 3 (Scheme 3). Therefore, the subsequent metallations of closo-nido- and nidonido-bis(carboranes) lead to racemic and diastereomeric mixtures of metallacarboranes, respectively. In some cases, the introduction of a metal atom into the carborane cage is accompanied by polyhedral rearrangement, which makes it difficult to identify the compounds formed in this case. It should be noted that the presence of two polyhedra with a different arrangement of metal and carbon atoms makes it difficult or impossible to establish the exact structure of metallacarboranes using 11B-11B COSY NMR spectroscopy due to signal overlapping. Therefore, X-ray diffraction is critical for determination of molecular structures of such
Scheme 1.
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Scheme 2.
Scheme 3.
Scheme 4.
metallacarboranes and in some cases being the only way in which the overall architecture and the isomeric form of the molecule can be established. The first metallacarboranes with the bulky carboranyl group as substituent were synthesized in the 1980 s when it was found that heating salts of labile complex cations [Rh(PEt3)4]+ and [(COD)Ir (PPh3)2]+ with the closo-nido-bis(carborane) anion 2 lead to metal insertion in the nido-carborane cage accompanied by migration of the substituted carbon atom to the lower pentagonal face, resulting in the corresponding 2,1,8-metallacarboranes [8-(10 ,20 closo-C2B10H11-10 -)-2,2-(Et3P)2–2-H-2,1,8-closo-RhC2B9H10] (4) [16] and [8-(10 ,20 -closo-C2B10H11-10 -)-2,2-(Ph3P)2–2-H-2,1,8-closoIrC2B9H10] (5) [19] (Scheme 4). Recently, a series of further metallacarboranes with the carboranyl group as substituent were synthesized and factors affecting their isomerisation were studied. The deprotonation of closonido-bis(carborane) 2 with n-BuLi followed by treatment with
[(p-cymene)RuCl2]2 at ambient temperature results in a mixture of yellow [1-(10 ,20 -closo-C2B10H11-10 -)-3-(p-cymene)-3,1,2-closoRuC2B9H10] (6) and colorless [8-(10 ,20 -closo-C2B10H11-10 -)-2-(pcymene)-2,1,8-closo-RuC2B9H10] (7) ruthenacarboranes (Fig. 3). The 3,1,2-isomer 6 was found to undergo thermal isomerisation to the 2,1,8-isomer 7 in refluxing THF solution for 2 h (Scheme 5) [9]. The deprotonation of closo-nido-bis(carborane) 2 with n-BuLi followed by treatment with [CpCo(CO)I2] results in the orange cobaltacarborane [1-(10 ,20 -closo-C2B10H11-10 -)-3-Cp-3,1,2-closoCoC2B9H10] (8), whereas the similar reaction with CoCl2 and NaCp followed by aerial oxidation gives the isomeric yellow cobaltacarborane [8-(10 ,20 -closo-C2B10H11-10 -)-2-Cp-2,1,8-closoCoC2B9H10] (9) with separated cage carbon atoms (Fig. 4). No thermal isomerization of the 3,1,2-isomer 8 to the 2,1,8-isomer 9 proceeds in refluxing toluene for 5 h, however the isomerization can be achieved by the reduction of 8 with sodium naphthalenide followed by aerial oxidation (Scheme 6). This implies that whilst the 3,1,2-isomer is stable in the case of the Co(III) complex, the Co(II) complex easily isomerizes to the 2,1,8-isomer, and the 3,1,2 ? 2,1,8 isomerization occurs mainly due to electronic rather than steric factors [9]. The rare 8,1,2-isomer [1-(10 ,20 -closo-C2B10H1110 -)-8-Cp-8,1,2-closo-CoC2B9H10] (10) in which both carbon atoms migrated to the lower pentagonal face (Fig. 4c) was obtained as trace product from the reaction of 2-e reduction of 1 followed by metalation with CoCl2 and NaCp (See below) [20]. Similar reaction of the deprotonated closo-nido-bis(carborane) 2 with CoCl2 and NaCp* followed by aerial oxidation gives a mixture of red unisomerised cobaltacarborane [1-(10 ,20 -closo-C2B10H11-10 -)3-Cp*-3,1,2-closo-CoC2B9H10] (11) (Fig. 5a), yellow isomerised cobaltacarborane [8-(10 ,20 -closo-C2B10H11-10 -)-2-Cp*-2,1,8-closoCoC2B9H10] (12) (Fig. 5b) and trace amount of the 13-vertex
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Fig. 3. X-ray structures of [1-(10 ,20 -closo-C2B10H11-10 -)-3-(p-cymene)-3,1,2-closo-RuC2B9H10] (6) (a) and [8-(10 ,20 -closo-C2B10H11-10 -)-2-(p-cymene)-2,1,8-closo-RuC2B9H10] (7) (b).
Scheme 5.
Fig. 4. X-ray structures of [1-(10 ,20 -closo-C2B10H11-10 -)-3-Cp-3,1,2-closo-CoC2B9H10] (8) (a), [8-(10 ,20 -closo-C2B10H11-10 -)-2-Cp-2,1,8-closo-CoC2B9H10] (9) (b) and [1-(10 ,20 closo-C2B10H11-10 -)-8-Cp-8,1,2-closo-CoC2B9H10] (10) (c).
bimetallacarborane/12-vertex carborane species [12-(closo-10 ,20 C2B10H11-10 -)-4,5-Cp*2-closo-4,5,1,12-Co2C2B9H10] (13) (Fig. 5c). As in the case of the CpCo metallacarborane 8, the reduction of [1-(10 ,20 -closo-C2B10H11-10 -)-3-Cp*-3,1,2-closo-CoC2B9H10] (11) with sodium naphthalenide followed by aerial oxidation of the initially formed Co(II) complex produces the corresponding isomerized cobaltacarborane 12 (Scheme 7) [21]. Reactions of the deprotonated closo-nido-bis(carborane) 2 with various nickel phosphine and phosphite complexes were studied. The reaction with [(dppe)NiCl2] results in a mixture of isomeric green [1-(10 ,20 -closo-C2B10H11-10 -)-3-(dppe)-3,1,2-closo-NiC2B9H10] (14) and red–purple [2-(10 ,20 -closo-C2B10H11-10 -)-4-(dppe)-4,1,2-
closo-NiC2B9H10] (15) nickelacarboranes (Fig. 6a,b, Scheme 8). The 4,1,2-isomer can be easily obtained by thermal isomerization of the 3,1,2-isomer in refluxing THF for 2 h. The similar reaction of closo-nido-bis(carborane) with [(dmpe)NiCl2] gives the yelloworange 2,1,8-isomer [8-(10 ,20 -closo-C2B10H11-10 -)-2-(dmpe)-2,1,8closo-NiC2B9H10] (16) (Fig. 6c, Scheme 8) [22]. Based on these results it was supposed that the 3,1,2 ? 4,1,2 isomerisation is caused by steric crowding, whereas the 3,1,2 ? 2,1,8 isomerisation is favored by strongly electron-donating ligands on the metal. Indeed, the reaction of closo-nido-bis (carborane) 2 with [(Ph2MeP)2NiCl2] gives the olive-green 2,1,8isomer nickelaborane [2-(10 ,20 -closo-C2B10H11-10 -)-4,4-(Ph2MeP)2–
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Scheme 6.
Fig. 5. X-ray structures of [1-(10 ,20 -closo-C2B10H11-10 -)-3-Cp*-3,1,2-closo-CoC2B9H10] (11) (a), [8-(10 ,20 -closo-C2B10H11-10 -)-2-Cp*-2,1,8-closo-CoC2B9H10] (12) (b) and [12(closo-10 ,20 -C2B10H11-10 -)-4,5-Cp*2-closo-4,5,1,12-Co2C2B9H10] (13) (c).
Scheme 7.
4,1,2-closo-NiC2B9H10] (17) as the result 3,1,2 ? 4,1,2 isomerisation, whereas the reactions with less sterically hindered nickel complexes [(PhMe2P)2NiCl2] and [(Me3P)2NiCl2] produce uniso-
merised green nickelacarboranes [1-(10 ,20 -closo-C2B10H11-10 -)-3,3(RR’2P)2–3,1,2-closo-NiC2B9H10] (R = R0 = Me (18) (Fig. 7a) and R = Ph, R0 = Me (19) (Fig. 7c)), as well as purple 8-phosphonium-
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Fig. 6. X-ray structures of [1-(10 ,20 -closo-C2B10H11-10 -)-3-(dppe)-3,1,2-closo-NiC2B9H10] (14) (a), [2-(10 ,20 -closo-C2B10H11-10 -)-4-(dppe)-4,1,2-closo-NiC2B9H10] (15) (b), and [8(10 ,20 -closo-C2B10H11-10 -)-2-(dmpe)-2,1,8-closo-NiC2B9H10] (16) (c).
Scheme 8.
Fig. 7. X-ray structures of [1-(10 ,20 -closo-C2B10H11-10 -)-3,3-(Me3P)2–3,1,2-closo-NiC2B9H10] (18) (a), [1-(10 ,20 -closo-C2B10H11-10 -)-3,8-(Me3P)2–3-Cl-3,1,2-closo-NiC2B9H9] (20) (b), and [1-(10 ,20 -closo-C2B10H11-10 -)-3,3-(PhMe2P)2–3,1,2-closo-NiC2B9H10] (19) (c).
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substituted nickelacarboranes [1-(10 ,20 -closo-C2B10H11-10 -)-3,8(RR0 2P)2–3-Cl-3,1,2-closo-NiC2B9H9] (R = R0 = Me (20) (Fig. 7b) and R = Ph, R0 = Me (21)), and the corresponding 10-phosphonium derivatives of closo-nido-bis(carborane) 22 and 23 (Scheme 9) [22]. The reaction of the deprotonated closo-nido-bis(carborane) 2 with [((MeO)3P)2NiBr2] produces the expected purple nickelacarborane [1-(10 ,20 -closo-C2B10H11-10 -)-3,3-((MeO)3P)2–3,1,2closo-NiC2B9H10] (24) (Fig. 8a), whereas the similar reaction with [Ni3(tmeda)3Cl5]Cl followed by addition of P(OMe)3 gives mainly the yellow 2,1,8-isomer [1-(10 ,20 -closo-C2B10H11-10 -)-2,2((MeO)3P)2–2,1,8-closo-NiC2B9H10] (25) (Fig. 8b) in which the unsubstituted carbon atom migrated to the lower pentagonal face as well as some of the 3,1,2-isomer (Scheme 10) [22].
A problem which frequently met in X-ray studies of metallacarborane isomers and should be mentioned there is how to distinguish correctly {BH} and {CH} vertices because the atomic scattering factors of boron and carbon for X-rays are similar due to the adjacency of these elements in the Periodic Table. To solve this problem two effective methods, the Vertex-Centroid Distance (VCD) and Boron-Hydrogen Distance (BHD) methods, have been developed recently [23,24]. The first attempt to obtain bimetallic metallacarboranes based on nido-nido-bis(carborane) also dates to the 1980 s. The reaction of nido-nido-bis(carborane) 3 with [(cod)Rh(PEt3)Cl] in refluxing THF resulted in a mixture of the dark blue complex [3-Et3P-3,1,2RhC2B9H10-1-]2 with a Rh-Rh bond (26) (Fig. 9a) and the red asym-
Scheme 9.
Fig. 8. X-ray structures of [1-(10 ,20 -closo-C2B10H11-10 -)-3,3-((MeO)3P)2–3,1,2-closo-NiC2B9H10] (24) (a) and [1-(10 ,20 -closo-C2B10H11-10 -)-2,2-((MeO)3P)2–2,1,8-closo-NiC2B9H10] (25) (b).
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Scheme 10.
Fig. 9. X-ray structures of [3-Et3P-3,1,2-RhC2B9H10-1-]2 (26) (a) and [2-(20 ,20 -(Et3P)2-20 -H-20 ,10 ,80 -RhC2B9H10-80 -)-3-(g3-C8H13)-3,1,2-RhC2B9H10] (27) (b).
metric complex [2-(20 ,20 -(Et3P)2-20 -H-20 ,10 ,80 -RhC2B9H10-80 -)-3(g3-C8H13)-3,1,2-RhC2B9H10] (27) (Fig. 9b) (Scheme 12) [25,26]. Recently, the planned synthesis of bimetallic metallacarboranes based on nido-nido-bis(carborane) was described. The deprotonation of nido-nido-bis(carborane) 3 with n-BuLi followed by treatment with [(p-cymene)RuCl2]2 at ambient temperature produces in a mixture of yellow isomeric ruthenacarboranes [1-(20 -(p-cymene)-20 ,10 ,80 -closo-RuC2B9H10-80 -)-3-(p-cymene)-3,1, 2-closo-RuC2B9H10] (28) in which one cage has 3,1,2-RuC2B9
architecture whilst the other is 2,1,8-RuC2B9 (Scheme 11, Fig. 10) [17]. Presumably the initial metalation product is the 3,1,2-RuC2B930 ,10 ,20 -RuC2B9 compound (as a diastereomeric mixture), which then undergoes spontaneous isomerisation of one cage to 2,1,8-RuC2B9 architecture as a consequence of untenable steric crowding. In contrast to the singly-metalated species [1-(10 ,20 -closo-C2B10H11-10 -)-3-(p-cymene)-3,1,2-closo-RuC2B9H10] (6) which isomerizes to [8-(10 ,20 -closo-C2B10H11-10 -)-2-(p-cymene)-
Scheme 11.
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Scheme 12.
Fig. 10. X-ray structures of isomeric ruthenacarboranes [1-(20 -(p-cymene)-20 ,10 ,80 -closo-RuC2B9H10-80 -)-3-(p-cymene)-3,1,2-closo-RuC2B9H10] (28).
2,1,8-closo-RuC2B9H10] (7) with mild heating (THF reflux), similar thermolysis of both isomers does not result in the second cage isomerisation. Taking into account that the metallacarborane moiety is slightly larger than the carborane one, it can be assumed that the 3,1,2 ? 2,1,8 cage isomerisation in the case of the singlymetalated species is caused by electronic rather than steric factors. The reaction of the deprotonated the nido-nido-bis(carborane) 3 with [CpCoI2(CO)] in THF at ambient temperature gives orange rac-[1-(30 -Cp-30 ,10 ,20 -closo-CoC2B9H10-10 -)-3-Cp-3,1,2-closoCoC2B9H10] (29) as single isolated product (Scheme 13). An interesting feature of this complex is intramolecular CH. . .HB dihydrogen bonding that results in stabilization of the gauche rotamer (Fig. 11a) [17]. Heating 29 in refluxing 1,2-dimethoxyethane for 1 h results in the 3,1,2 ? 2,1,8 isomerisation of one cobaltacarborane moiety to give [1-(20 -Cp-20 ,10 ,80 -closo-CoC2B9H10-80 -)-3-Cp-3,1,2-closoCoC2B9H10] (30) (Scheme 13). As noted above, the singlymetalated species [1-(10 ,20 -closo-C2B10H11-10 -)-3-Cp-3,1,2-closoCoC2B9H10] (8) isomerises to the 2,1,8-isomer via reduction of the 3,1,2-isomer 9 with sodium naphthalenide followed by aerial oxidation. In a similar way, reduction of the 3,1,2-CoC2B9-30 ,10 ,20 CoC2B9 compound 29 with sodium naphthalenide followed by aerial oxidation of the reduced form produces the doubly isomerized cobaltacarborane [8-(20 -Cp-20 ,10 ,80 -closo-CoC2B9H10-80 -)-2-Cp2,1,8-closo-CoC2B9H10] (31) (Fig. 11b) (Scheme 13) [17]. Reaction of the deprotonated nido-nido-bis(carborane) 3 with with CoCl2 and NaCp in refluxing THF followed by air oxidation gives a mixture of isomeric cobaltacarboranes with one isomerized cage [1-(20 -Cp-20 ,10 ,80 -closo-CoC2B9H10-80 -)-3-Cp-3,1,2-closoCoC2B9H10] (32) (Fig. 12) and two isomerized cages [8-(20 -Cp-20 ,1
0
,80 -closo-CoC2B9H10-80 -)-2-Cp-2,1,8-closo-CoC2B9H10] (31) (Scheme 14). Similarly to rac-[1-(30 -Cp-30 ,10 ,20 -closo-CoC2B9H1010 -)-3-Cp-3,1,2-closo-CoC2B9H10] (29), the reduction of 3,1,2CoC2B9-20 ,10 ,80 -CoC2B9 isomers 32 with sodium naphthalenide followed by aerial oxidation of the reduced form gives the doubly isomerized cobaltacarborane 31 (Scheme 14) [17]. The synthesis of heterometalic metallacarboranes based on nido-nido-bis(carborane) has recently been described. It was found that the carboranyl group in the 2,1,8-metallacarboranes [8-(10 ,20 closo-C2B10H11-10 -)-2-(p-cymene)-2,1,8-closo-RuC2B9H10] (7), [8(10 ,20 -closo-C2B10H11-10 -)-2-Cp-2,1,8-closo-CoC2B9H10] (9) and [8(10 ,20 -closo-C2B10H11-10 -)-2-Cp*-2,1,8-closo-CoC2B9H10] (11) can be selectively deboronated by KF in THF/water at reflux to give diastereomeric mixtures of the corresponding metallacarboranes with a nido-carborane substituent. The deprotonation of [8-(70 ,80 nido-C2B9H11-70 -)-2-(p-cymene)-2,1,8-closo-RuC2B9H10] with nBuLi followed by treatment with CoCl2 and NaCp and aerial oxidation produces a diastereomeric mixture of [8-(30 -Cp-closo-30 ,10 ,20 CoC2B9H10-10 -)-2-(p-cymene)-closo-2,1,8-RuC2B9H10] (33) (Fig. 13, Scheme 15). In contrast to the formation of cobaltacarborane 9, [8-(10 ,20 -closo-C2B10H11-10 -)-2-Cp-2,1,8-closo-CoC2B9H10], no isomerisation of the cobaltacarborane moiety takes place [21]. The deprotonation of [8-(70 ,80 -nido-C2B9H11-70 -)-2-Cp*-2,1,8closo-CoC2B9H10] with n-BuLi followed by treatment with [(p-cymene)RuCl2]2 gives a diastereomeric mixture of [8-(30 -(pcymene)-closo-30 ,10 ,20 -RuC2B9H10-10 -)-2-Cp*-closo-2,1,8-CoC2B9H10] (34), which on thermolysis in refluxing 1,2-dimethoxyethane undergoes isomerisation to a diastereomeric mixture of [8-(20 -(pcymene)-closo-20 ,10 ,80 -RuC2B9H10-20 -)-2-Cp*-closo-2,1,8-CoC2B9H10] (35) (Fig. 14, Scheme 16) [21].
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Scheme 13.
Fig. 11. X-ray structure of rac-[1-(30 -Cp-30 ,10 ,20 -closo-CoC2B9H10-10 -)-3-Cp-3,1,2-closo-CoC2B9H10] (29) (a) and [8-(20 -Cp-20 ,10 ,80 -closo-CoC2B9H10-80 -)-2-Cp-2,1,8-closo-CoC2B9H10] (31) (b).
Fig. 12. X-ray structures of isomeric cobaltacarboranes [1-(20 -Cp-20 ,10 ,80 -closo-CoC2B9H10-80 -)-3-Cp-3,1,2-closo-CoC2B9H10] (32).
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Scheme 14.
Fig. 13. X-ray structures of [8-(30 -Cp-closo-30 ,10 ,20 -CoC2B9H10-10 -)-2-(p-cymene)-closo-2,1,8-RuC2B9H10] (33) diastereomers.
Scheme 15.
2.2. Complexes with reduced carborane ligands The 2-e reduction of 1,10 -bis(ortho-carborane) 1 with sodium naphthalenide in THF results in a [nido-C2B10]2-(closo-C2B10) species, which on the treatment with [(g6-C6H6)RuCl2]2 or [(g6-p-cymene)RuCl2]2 gives the corresponding 13-vertex ruthenacarborane/12-vertex carborane [1-(10 ,20 -closo-C2B10H1110 -)-4-(g6-arene)-4,1,6-closo-RuC2B10H11] (arene = benzene (36) (Fig. 15a); p-cymene (37) (Fig. 15b)). In contrast, the similar reaction with [(g6-1,3,5-Me3C6H3)RuCl2]2 results in a complex in which the 13-vertex ruthenacarborane moiety has isomerized, [8-(10 ,20 -closo-C2B10H11-10 -)-4-(g6-1,3,5-Me3C6H3)-4,1,8-closo-
RuC2B10H11] (38) (Fig. 15c) (Scheme 17). The isomerisation is presumably the result of otherwise untenable steric interactions between the mesitylene and carborane substituents [27]. The reaction of complex 38 with sodium naphthalenide in THF results in reductive opening of the 13-vertex ruthenacarborane rather than the 12-vertex carborane moiety - the subsequent addition of ruthenium complexes [(g6-arene)RuCl2]2 affords in each case two new bimetallic ruthenacarboranes - yellow 14-vertex Ru2C2B10/12-vertex C2B10 [1-(g6-1,3,5-Me3C6H3)-9-(10 , 2 0 - closo-C2B10H11-10 -)-13-(g6-arene)-1,13,2,9-closo-Ru2C2B10H11] (arene = benzene (39); p-cymene (40), mesitylene (41)) as the major product, and red 13-vertex Ru2C2B9/12-vertex C2B10
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Fig. 14. X-ray structure of [8-(20 -(p-cymene)-closo-20 ,10 ,80 -RuC2B9H10-20 -)-2-Cp*closo-2,1,8-CoC2B9H10] (35) (arbitrary diastereomer).
157
[4-(g6-arene)-5-(g6-1,3,5-Me3C6H3)-6-(10 -20 -closo-C2B10H11-10 -)4,5,1,6-closo-Ru2C2B9H10] (arene = benzene (42); p-cymene (43), mesitylene (44)) (Fig. 16), as a minor product (Scheme 18) [27]. The reduction of 1,10 -bis(ortho-carborane) 1 with excess of lithium naphthalenide in THF followed by addition of [(g6-pcymene)RuCl2]2 unexpectedly resulted in the dark red ruthenacarborane [1-(10 ,20 -closo-C2B10H11-10 -)-4-{C10H14Ru(p-cymene)}4,1,6-closo-Ru2C2B10H11] (45) (Fig. 17) as the only isolable product. The molecule is an example of a ‘‘fly-over bridge” compound, in which the p-cymene ligand has been converted into a [l-r,g3: g3,r-C6]2 ligand by reductive cleavage of the C44-C45 bond [28]. The 2-e reduction of 1,10 -bis(ortho-carborane) 1 with lithium in the presence of naphthalene in THF followed by the metalation with CoCl2/NaCp produced only trace amounts of isomerized 13-vertex cobaltacarboranes [8-(10 ,20 -closo-C2B10H11-10 -)-4-Cp4,1,8-closo-CoC2B10H11] (46) and [12-(10 ,20 -closo-C2B10H11-10 -)-4Cp-4,1,12-closo-CoC2B10H11] (47), as well as minor amounts of the 4-e reduction products rac- and meso-[1-(40 -Cp-40 ,10 ,60 -closo-
Scheme 16.
Fig. 15. X-ray structures of [1-(10 ,20 -closo-C2B10H11-10 -)-4-(g6-benzene)-4,1,6-closo-RuC2B10H11] (36) (a), [1-(10 ,20 -closo-C2B10H11-10 -)-4-(g6-p-cymene)-4,1,6-closo-RuC2B10H11] (37) (b) and [8-(10 ,20 -closo-C2B10H11-10 -)-4-(g6-mesitylene)-4,1,8-closo-RuC2B10H11] (38) (c).
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Scheme 17.
Fig. 16. X-ray structures of [4-(g6-p-cymene)-5-(g6-mesitylene)-6-(10 -20 -closo-C2B10H11-10 -)-4,5,1,6-closo-Ru2C2B9H10] (43) (a) and [4,5-(g6-mesitylene)2–6-(10 -20 -closoC2B10H11-10 -)-4,5,1,6-closo-Ru2C2B9H10] (44) (b).
CoC2B10H11-10 -)-4-Cp-4,1,6-closo-CoC2B10H11] (48 and 49, respectively) and the deboronation-metalation product [1-(10 ,20 closo-C2B10H11-10 -)-8-Cp-8,1,2-closo-CoC2B9H10] (10) (Fig. 4c) (Scheme 19) [20]. The 4-e reduction of 1,10 -bis(ortho-carborane) 1 with lithium naphthalenide in tetrahydrofuran produces a diastereomeric
mixture of open 12-vertex nido-nido-bis(carborane). The treatment of this mixture with CoCl2 and NaCp followed by aerial oxidation results in the mixture of red 13-vertex/13-vertex cobaltacarboranes 48 and 49 (Fig. 18, Scheme 20) [29]. Thermolysis of the mixture of 48 and 49 in xylenes at 180 °C for 4 h results in migration of the unsubstituted carbon atoms to the
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Scheme 18.
3. Complexes with 1,10 -Bis(ortho-carborane) as r-ligand
Fig. 17. X-ray structure of [1-(10 ,20 -closo-C2B10H11-10 -)-4-{(l-r,g3:g3,r-C6H4-2Me-4-iPr)Ru(g6-p-cymene)}-4,1,6-closo-Ru2C2B10H11] (45).
lower pentagonal belt giving rac- and meso-[1-(40 -Cp-40 ,10 ,120 closo-CoC2B10H11-10 -)-4-Cp-4,1,12-closo-CoC2B10H11] (50 and 51, respectively, Fig. 19) (Scheme 21) [30]. The 4e-reduction of l-2,20 -(CH2)2–1,10 -bis(ortho-carborane) with sodium in THF followed by addition of [(g6-p-cymene) RuCl2]2 gave a mixture of the 13-vertex RuC2B10/12-vertex C2B10 ruthenacarborane 52, unprecedented 12-vertex RuCB10/12-vertex RuCB10 ruthenacarborane 53 in which two metallacarborane moieties are connected both through ‘‘normal” l-C,C0 -(CH2CH2) bridge and l,g2-B,B0 -(CH@CH) bridge formed by ‘‘extraction” of the CH groups from the both carborane cages, as well as complex 54 in which 1,1’-bis(ortho-carborane) plays the role of a doubly chelating ϭ-ligand (Scheme 22, Fig. 20) [31].
Similar to the parent ortho-carborane, the CH groups in 1,10 -bis (ortho-carborane) 1 have an acidic character and can be easily deprotonated with n-butyllithium to give the corresponding dilithium derivative that readily reacts with electrophilic reagents and metal chlorides resulting in various organic and organometallic derivatives. The first examples of 1,10 -bis(ortho-carborane)based metal complexes were reported more than 45 years ago by Hawthorne, who described reactions of its dilithium derivative with various transition metal chlorides (Scheme 23). Thus, the reaction of the dilithium derivative of 1,10 -bis(orthocarborane) with CuCl2 in diethyl ether results in stable yellow diamagnetic Cu(III) complex [Cu(j2-C,C0 -bCarb)2] (54), where the 1,10 -bis(ortho-carborane) moiety plays the role of five-membered C,C’-chelate ligand. The Cu(III) complex can be reduced to the blue Cu(II) complex [Cu(j2-C,C0 -bCarb)2]2 (55) with Li metal in acetone or Zn metal in dichloromethane [32,33]. The Cu(III) complex was found to have distorted square planar structure (the angle between the chelate ligand planes is 26°), whereas the structure of the Cu(II) complex is intermediate between square planar and tetrahedral (the angle between the ligand planes is 54°) [34] (Fig. 21). The similar reaction of the dilithium derivative of 1,10 -bis(ortho-carborane) with CoCl2 in diethyl ether produces the pale purple Co(II) complex [Co(j2-C,C0 -bCarb)2]2 (56) that can be oxidized to the black diamagnetic Co(III) complex [Co(j3-C,C0 -bCarb)2] (57) with CuCl2 in dichloromethane [33]. In both the Co(II) and Co(III) complexes, the cobalt atom has a tetrahedral environment [34] (Fig. 21), that in the last case is supplemented by an agostic BH ? Co interaction with one of the adjacent BH vertices [35]. The reaction of the dilithium derivative of 1,10 -bis(ortho-carborane) with NiCl2 in diethyl ether produces the orange-red Ni(II) complex [Ni(j2-C,C0 bCarb)2]2 (58) that can be oxidized to the green Ni(III) complex
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Scheme 19.
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Fig. 18. X-ray structures of rac- and meso-[1-(40 -Cp-40 ,10 ,60 -closo-CoC2B10H11-10 -)-4-Cp-4,1,6-closo-CoC2B10H11] 48 and 49.
Scheme 20.
[Ni(j2-C,C0 -bCarb)2] (59) with CuCl2 in dichloromethane [35]. The Ni(II) complex has a distorted square planar structure (the angle between the ligand planes is 26°) due to the steric constraints of the bis(carborane) ligand [34] (Fig. 21). The similar reaction of the dilithium derivative with ZnCl2 gives the white bis(chelate) complex [Zn(j2-C,C0 -bCarb)2]2 (60) [33]. A similar approach was applied recently to the synthesis of mixed ligand complexes of transition metals. Thus, the reactions of the dilithium derivative of 1,10 -bis(ortho-carborane) 1 with [(dmpe)NiCl2] and [(dppe)NiCl2] in THF produce the yellow nearly square planar complexes [(dmpe)Ni(j2-C,C0 -bCarb)] (61) [36] and
[(dppe)Ni(j2-C,C0 -bCarb)] (62) [36,37], respectively (Scheme 24). Similar complexes of platinum [(dppe)Pt(j2-C,C0 -bCarb)] (63) and palladium [(dppe)Pd(j2-C,C0 -bCarb)] (64) were prepared by the reaction of the dilithium derivative with [(dppe)PtCl2] and [(dppe)PdCl2], respectively (Fig. 22, Scheme 24) [37]. The reactions of the dilithium derivative of 1,10 -bis(orthocarborane) 1 with g5-pentamethylcyclopentadienyl complexes of iridium [Cp*IrCl2]2 and rhodium [Cp*RhCl2]2 in THF produce the coordinatively unsaturated 16-electron green complexes [Cp*Ir (j2-C,C0 -bCarb)] (65) and [Cp*Rh(j2-C,C0 -bCarb)] (66), respectively (Fig. 23). The iridium complex is stable in air in the solid state, but
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Fig. 19. X-ray structures of rac- and meso-[1-(40 -Cp-40 ,10 ,120 -closo-CoC2B10H11-10 -)-4-Cp-4,1,12-closo-CoC2B10H11] 50 and 51.
Scheme 21.
reacts readily with CO to give the 18-electron complex [Cp*Ir(CO) (j2-C,C0 -bCarb)] (67) (Scheme 25) [38]. The reaction of the dilithium derivative of 1,10 -bis(orthocarborane) 1 with [(p-cym)RuCl2]2 in THF results in the orange complex [(p-cym)Ru(j3-C,C0 -bCarb)] (68) in which a 16-electron configuration of the metal atom is stabilized by an agostic B(3)H ? Ru interaction (Fig. 24). In solution, four BH units (B(3)H, B (30 )H, B(6)H and B(60 )H) alternatively acts as the agostic BH in rapid exchange with each other. The reaction of 68 with CO affords the expected yellow complex [(p-cym)Ru(CO)(j2-C,C0 -bCarb)] (69) (Scheme 26) [39]. The reaction of complex 68 with 1,2-bis(diphenylphosphino)e thane (dppe) results in facile displacement of the p-cymene ligand by dppe and a change of the ligating mode for 1,10 -bis(orthocarborane) from C,C0 - to C,B0 -giving the yellow complex [(dppe) Ru(j3-C,B0 -bCarb)] (70). In this complex one carborane unit is now bond to the ruthenium atom by B(30 )-Ru bond, whereas the
other is still connected to the metal by a C-Ru bond complemented by a B(3)H ? Ru agostic bond. This species reacts with CO or MeCN to give the corresponding coordinatively saturated octahedral complexes trans-[(dppe)(CO)2Ru(j2-C,B0 -bCarb)] (71) and trans-[(dppe)(MeCN)2Ru(j2-C,B0 -bCarb)] (72) (Fig. 25, Scheme 27) [39]. In a similar way, the reaction of 68 with triphenylphosphine produces the yellow-orange complex [(Ph3P)2Ru(j3-C,B0 -bCarb)] (73). The same complex can be of alternatively prepared by the reaction of doubly-deprotonated 1,10 -bis(ortho-carborane) 1 with [(Ph3P)4RuCl2] or [(Ph3P)3RuCl2] (Scheme 27). The reaction of 68 with CO results in the loss of one PPh3 ligand giving an equimolar mixture of two isomeric octahedral complexes [(Ph3P)(CO)3Ru(j2C,B0 -bCarb)] (74), one with the PPh3 ligand trans to C(2) and the other with PPh3 trans to B(30 ). The reaction with acetonitrile affords a single complex [(Ph3P)(MeCN)3Ru(j2-C,B0 -bCarb)] (75) with the PPh3 ligand trans to C(2) (Fig. 26, Scheme 27) [39].
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Scheme 22.
Fig. 20. X-ray structures of ruthenacarboranes 52 (a) and 51 (b) and complex 53 (c).
Scheme 23.
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Fig. 21. X-ray structures of the [j2-C,C0 -1,10 -bis(ortho-carboranyl)cobalt(II)]2 (56) (a), [j2-C,C0 -1,10 -bis(ortho-carboranyl)nickel(II)]2 (58) (b), [j2-C,C0 -1,10 -bis(orthocarboranyl)copper(III)] (54) (c) and [j2-C,C0 -1,10 -bis(ortho-carboranyl)copper(II)]2 (55) (d) anions.
Scheme 24.
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Fig. 22. X-ray structures of [(dmpe)NiII(j2-C,C0 -bCarb)] (61) (a) [(dppe)NiII(j2-C,C0 -bCarb)] (62) (b), [(dppe)PtII(j2-C,C0 -bCarb)] (63) (c) and [(dppe)PdII(j2-C,C0 -bCarb)] (64) (d).
Fig. 23. X-ray structure of [Cp*Ir(j2-C,C0 -bCarb)] (65).
The reaction of the dilithium derivative of 1,10 -bis(orthocarborane) 1 with [(4,40 -tBu2bpy)PtCl2] in THF produces a mixture of the C,C0 - to C,B0 -bonded complexes [(4,40 -tBu2bpy)Pt(j2-C,C0 -
bCarb)] (76) and [(4,40 -tBu2bpy)Pt(j2-C,B0 -bCarb)] (77) (Fig. 27) [37]. It was found that the isomer ratio varies widely depending on the reaction time and temperature, as well as the n-BuLi batch. The C,C0 -isomer 76 can be prepared selectively using potassium bis (trimethylsilyl)amide as an alternative to n-BuLi, whereas the C,B0 -isomer 77 was obtained by the reaction of the dipotassium derivative of 1,10 -bis(ortho-carborane) 1 with [(4,40 -tBu2bpy)PtCl2] pretreated with MeLi (Scheme 28) [40]. A similar reaction of the dilithium derivative of 1,10 -bis(9,12Et2-ortho-carborane) with [(4,40 -tBu2bpy)PtCl2] gives mainly the C,B0 -bound isomer 78 (Fig. 28, Scheme 29) [37]. The 1,10 -bis(ortho-carborane)-based Pt(II) complexes with 4,40 di-tert-butyl-2,20 -bipyridine ligand were found to display blue phosphorescent emission dominated by MLCT from the Pt(II) center to the 4,40 -tBu2bpy ligand. Furthermore, since the 1,10 -bis (ortho-carboranyl) ligand introduces sufficient steric bulk above and below the square plane of the metal center, it effectively shuts down undesired intramolecular Pt(II). . .Pt(II) interactions in the solid state, which commonly lead to luminescence quenching [37]. Compared to common bidentate ligands, the introduction of bulky carboranes enormously changes the electronic structures, electroluminescence properties (including the phosphorescence
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Scheme 25.
Fig. 24. X-ray structures of [(p-cym)Ru(j3-C,C0 -bCarb)] (68) (a) and [(p-cym)Ru(CO)(j2-C,C0 -bCarb)] (69) (b).
Scheme 26.
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Fig. 25. X-ray structures of [(dppe)Ru(j3-C,B0 -bCarb)] (68) (a), trans-[(dppe)(CO)2Ru(j2-C,B0 -bCarb)] (71) (b) and trans-[(dppe)(MeCN)2Ru(j2-C,B0 -bCarb)] (72) (c).
Scheme 27.
emission band), radiative and non-radiative decay processes. The different binding modes between the Pt atom and the 1,10 -bis (ortho-carboranyl) ligand have a minor effect on the radiative decay rate but a clear influence on the non-radiative decay process [41]. There are only two examples of a non-chelated complexes with the 1,10 -bis(ortho-carborane) ligand. The first one was obtained from the reaction of its dilithium derivative with [(Ph3P)AuCl] in tetrahydrofuran. In the solid state, complex [{(Ph3P)Au}2(j2-C,C0 bCarb)] (79) has a syn-conformation due to an aurophilic Au. . .Au interaction of 3.119 Å (Fig. 29) [42]. The second one is the toluene
solvated dicopper derivative [{(g3-toluene)Cu}2(j2-C,C0 -bCarb)] (80) prepared by the reaction of the dilithium derivative of orthocarborane with CuCl in toluene (Fig. 29) [7]. In addition to transition metals, the 1,10 -bis(ortho-carboranyl) ligand was found to form complexes with some non-transition metals. The reactions of 1,10 -bis(ortho-carborane) and 1,10 -bis (9,10,11,12-Me4-ortho-carborane) with (n-Bu)2Mg in 1,2dimethoxyethane result in the j2-C,C0 -magnesium complexes (Fig. 30) which on the treatment with MeSnCl2 give the corresponding 1,10 -bis(ortho-carboranyl)stannyl derivatives 81 and 82 (Scheme 30) [43].
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Fig. 26. X-ray structures of [(Ph3P)2Ru(j3-C,B0 -bCarb)] (73) (a), [(Ph3P)(MeCN)3Ru(j2-C,B0 -bCarb)] (74) (b) and [(Ph3P)(CO)3Ru(j2-C,B0 -bCarb)] (75) (c).
Fig. 27. X-ray structures of [(4,40 -tBu2bpy)Pt(j2-C,C0 -bCarb)] (76) (a) and [(4,40 -tBu2bpy)Pt(j2-C,B0 -bCarb)] (77) (b).
4. 1,10 -Bis(ortho-carborane)-Based phosphine ligands and complexes thereof Synthesis of the first 1,10 -bis(ortho-carborane)-based phosphine l-2,20 -PhP-1,10 -bis(ortho-carborane) (83) was reported 40 years ago, however its characterization was incomplete [44]. Subsequently synthesis of l-2,20 -ClP-1,10 -bis(ortho-carborane) (84) and its transformation into l-2,20 -FP-1,10 -bis(ortho-carborane) (85) was also reported [45]. Very recently, the synthesis of cyclic phosphines l-2,20 -EtP-1,10 -bis(ortho-carborane) (86) and l-2,20 -PhP-1, 10 -bis(ortho-carborane) (83) (Fig. 31) by the reaction of the dilithium derivative of 1,10 -bis(ortho-carborane) 1 with EtPCl2 and PhPCl2 as well as their complete characterization was described (Scheme 31) [46]. The reactions of phosphines 86 and 83 with [(tht)AuCl] in dichloromethane affords the corresponding gold complexes [ClAu
{l-2,20 -EtP-1,10 -bis(ortho-carborane)}] (87) and [ClAu{l-2,20 -PhP1,10 -bis(ortho-carborane)}] (88) (Scheme 32, Fig. 32). The Tolman cone angles for the ethylphosphine and phenylphosphine complexes were calculated to be 171.6° and 172.5° for 87 and 88, respectively. Although Ph is a larger substituent than Et, the similarity of the cone angles reflects the fact that the vast majority of the steric bulk in l-2,20 -EtP- and l-2,20 -PhP-1,10 -bis(orthocarborane) comes from the common bis(ortho-carborane) fragment. In terms of their size the synthesized phosphines are the most comparable to (but slightly larger than) tricyclohexylphosphine, whereas they are much less basic than PCy3 and the least basic of any carboranylphosphines so far reported [46]. The reaction of 1,10 -bis(ortho-carborane) 1 with 1 equiv. MeLi and 1 equiv. (iPr)2PCl in THF produces the expected phosphino derivative 2-iPr2P-1,10 -bis(ortho-carborane) (89) (Fig. 33a), whereas the reactions with 2 equiv. MeLi and 2 equiv. R2PCl
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Scheme 28.
(R = i-Pr, Ph, N(i-Pr)2) result in unexpected products 90–92 with the formation of one open 12-vertex nido-carborane cluster following the cleavage C10 -C20 bond. One of boron atoms in the nido-carborane, adjacent to the C10 carbon atom of the initial closo-carborane, underwent a B-H activation with the formation of the C-P-B bridge between the closo-and nidocarborane cages (Fig. 33b). The deprotonation of 89 with MeLi and subsequent reaction with (iPr)2PCl results in the formation of closo-C2B10/nido-C2B10 diphosphonium bis(carborane) 90 (Scheme 33) [47]. 5. Organic derivatives of 1,10 -Bis(ortho-carborane)
Fig. 28. X-ray structure of [(4,40 -tBu2bpy)Pt(j2-C,B0 -bCarb-9,90 ,12,120 -Et4)] (78).
Since some of the transition metal complexes described above were derived from derivatives of 1,10 -bis(ortho-carborane), we thought it appropriate to devote one of the parts of this review to organic derivatives of 1,10 -bis(ortho-carborane). At first of all, it should be noted that organic chemistry of 1,10 -bis(orthocarborane) is much less explored then its organometallic chemistry. The above mentioned 9,90 ,12,120 -tetraethyl-1,10 -bis(orthocarborane) (93) and 8,80 ,9,90 ,10,100 ,12,120 -octamethyl-1,10 -bis (ortho-carborane) (94) were prepared by copper-mediated coupling of 9,12-diethyl-ortho-carborane [37] and methylation of 1,10 -bis(ortho-carborane) with excess of MeI in the presence of AlCl3 [48], respectively (Scheme 34).
Scheme 29.
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Fig. 29. X-ray structures of [{(Ph3P)Au}2(j2-C,C0 -bCarb)] (79) (a) and [{(g3-toluene)Cu}2(j2-C,C0 -bCarb)] (80) (b).
Fig. 30. X-ray structure of [(1,2-DME)2Mg(j2-C,C0 -bCarb)].
Despite the fact that the CH groups of 1,10 -bis(ortho-carborane) are easily metalated, only a very limited number of its organic derivatives were obtained using this way. The C,C0 -dimethyl derivative 2,20 -Me2-1,10 -bis(ortho-carborane) (95) was prepared by the reaction of the dilithium derivative of 1 with methyl iodide [10], whereas the similar reaction with 1,2-dibromoethane produces the ethylene-bridged derivative 2,20 -l-CH2CH2-1,10 -bis (ortho-carborane) (96) [31]. The bis(hydroxymethyl) derivative 2,20 -(HOCH2)2-1,10 -bis(ortho-carborane) (97) was prepared by the treatment of the dilithium derivative with formaldehyde in ether-benzene [49] (Scheme 35). The treatment of the dilithium derivative of 1 with nitrosyl chloride NOCl in ether produces unstable blue bis(nitroso) derivative 2,20 -(NO)2–1,10 -bis(ortho-carborane) which was not isolated but can be trapped with 1,3-cyclohexadiene to give the products Diels-Alder cycloaddition as a mixture of meso- and rac-isomers 2,20 -(NOC6H8)2–1,10 -bis(ortho-carborane) (98) (Scheme 36) [50]. The similar reaction of the monolithium derivative of 1 with NOCl gave the nitroso derivative 2-NO-1,10 -bis(ortho-carborane) (99) which on an aqueous treatment is reduced to the corresponding
Scheme 30.
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Fig. 31. X-ray structures of l-2,20 -EtP-1,10 -bis(ortho-carborane) (86) (a) and l-2,20 -PhP-1,10 -bis(ortho-carborane) (83) (b).
Scheme 31.
Scheme 32.
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Fig. 32. X-ray structures of [ClAu{l-2,20 -EtP-1,10 -bis(ortho-carborane)}] (87) (a) and [ClAu{l-2,20 -PhP-1,10 -bis(ortho-carborane)}] (88) (b).
Fig. 33. X-ray structures of 2-iPr2P-1,10 -bis(ortho-carborane) (89) (a) and l-2,30 -iPr2P-1,10 -(1,2-C2B10H10)(nido- CB10H9CPH(iPr)2) (90) (b).
hydroxylamine 2-HONH-1,10 -bis(ortho-carborane) (100) (Scheme 37) [50]. Of particular interest is the synthesis of five-membered cyclic derivatives of 1,10 -bis(ortho-carborane). The closure of the fivemembered cycle easily proceeds when the dilithium derivative of 1,10 -bis(ortho-carborane) reacts with transition metal chlorides, as well as with chlorides of elements of the third (arsenic [51]) and fourth (magnesium [43], phosphorus [44–46]) periods of the Periodic System, whose atoms have large diameters. At the same time, the closure of the five-membered cycle with the participation of small elements of the second period is a rather nontrivial task. For example, the lithium and magnesium derivatives of 1,10 -bis (ortho-carborane) 1 readily react with methyl or ethyl formate to give the bis(carborane) analogue of 9-fluorenol 2,20 -l-HOCH-1,10 bis(ortho-carborane) (101) [52,53], whereas the dilithium deriva-
tive does not react with dihalogenmethanes CH2X2 (X = Cl, Br, I) at all. Nevertheless, the bis(carborane) analogue of fluorene 2,20 l-CH2-1,10 -bis(ortho-carborane) (102) (Fig. 34a) was successfully synthesized by the reaction of the dicopper derivative of 1 with CH2I2 [54,55] (Scheme 38). All attempts to prepare 9-borafluorene analogue of 1,10 -bis (ortho-carborane) using its lithium, magnesium and tin derivatives were unsuccessful. Finally, the desired heterocycle 2,20 -l-iPr2NB1,10 -bis(ortho-carborane) (103) (Fig. 34b) and their methylated analogue 2,20 -l-iPr2NB-1,10 -bis(ortho-carborane-8,9,10,12-Me4) (104) were synthesized by the reaction of dipotassium derivatives of 1 and 94 with iPr2NBCl2, respectively (Scheme 39). It should be noted that all numerous attempts to prepare the 9-borafluorene analogues using BX3 (X = F, Cl, Br, I) or ArBCl2 (Ar = Ph, 4-MeC6H4, 2,4,6-Me3C6H2, 2,4,6-iPr3C6H2) failed as well [56].
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Scheme 33.
Scheme 34.
6. Conclusions In general, 1,10 -bis(ortho-carborane) has the capacity to undergo similar reactions to single-cage carboranes but with two important differences, specifically 1) reactivity at a single cage is significantly influenced by the presence of a bulky electronwithdrawing substituent and 2) the reactions could occur at both cages and such reactivity could be either isolated or cooperative. Both these differences play important roles in the chemistry of transition metal complexes with 1,10 -bis(ortho-carborane)-based ligands. In particular, the large size and strong electronwithdrawing effect of ortho-carborane cage as substituent has
strong impact on the stability of certain isomers of metallacarboranes based on decapitated 1,10 -bis(ortho-carborane), whereas cooperative reactivity of two ortho-carborane fragments presents the basis for use 1,10 -bis(ortho-carborane) as a chelating ligand with the possibility of intramolecular activation of a B-H bond without the presence of special directing groups. Finally, both these differences should be taken into account in design and synthesis of 1,10 -bis(ortho-carborane)-based ligands with Lewis base centers, such as phosphine groups. All this makes the chemistry of 1,10 -bis(ortho-carborane) and their transition metal complexes extremely exciting and one which attracts the growing interest of researchers.
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Scheme 35.
Scheme 36.
I.B. Sivaev, V.I. Bregadze / Coordination Chemistry Reviews 392 (2019) 146–176
Scheme 37.
Fig. 34. X-ray structures of 2,20 -l-CH2-1,10 -bis(ortho-carborane) (102) (a) and 2,20 -l-iPr2NB-1,10 -bis(ortho-carborane) (103) (b).
Scheme 38.
Scheme 39.
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Acknowledgments The authors thank Prof. Alan J. Welch (Heriot-Watt University, Edinburgh) for helpful discussion and Russian Foundation for Basic Research (15-03-05822) and the Ministry of Science and Higher Education of the Russian Federation for financial support.
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