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Tethered N-heterocyclic carbene–carboranes: unique ligands that exhibit unprecedented and versatile coordination modes at rhodium

Jordan Holmes a, Christopher M. Pask a, Mark A. Fox b and Charlotte E. Willans *a
aSchool of Chemistry, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK. E-mail: c.e.willans@leeds.ac.uk
bDepartment of Chemistry, Durham University, South Road, Durham, DH1 3LE, UK

Received 23rd February 2016 , Accepted 12th April 2016

First published on 21st April 2016


Abstract

Four brand new hybrid ligands combining an N-heterocyclic carbene tethered with two isomeric nido-dicarbaundecaborane dianions, a neutral closo-dicarbadodecaborane or a closo-dicarbadodecaborane anion are described. Versatile coordination of the ligands to RhI is demonstrated, in which both NHC and carborane moieties covalently coordinate a metal centre.


Ligand design is central to the development of new organometallic and coordination compounds as they control the overall properties, the activities and the reactivities of a metal centre. This leads to potential new applications in materials, biomedicine and catalysis.1–6 Whilst modifying current ligands may induce incremental changes in the chemical behaviour of a metal complex, novel ligand architectures can lead to more diverse variations. N-Heterocyclic carbenes (NHCs) are excellent two electron donor ligands, with their steric and electronic properties being controlled and tuned through alteration of the N-substituents and backbone substituents respectively (Fig. 1A. R1–R4).7,8
image file: c6cc01650b-f1.tif
Fig. 1 Representation of an NHC (A), a neutral ortho-carborane (B), an ortho-carborane anion (C), a nido-carborane dianion (D) and novel hybrid ligands described in this study (E–H).

Carborane anions are also important classes of ligands, having very different properties to neutral NHCs. While there are many known classes of carboranes, the well-known ortho-carborane of type 1,2-R5,R6-1,2-C2B10H10 (Fig. 1B) and the nido-carborane dianion of type [7,8-R5,R6-7,8-C2B9H9]2− (Fig. 1D) are explored here. They can coordinate to a metal through either a boron atom or a carbon atom of a closo-carborane anion (e.g.Fig. 1C),9 or through the open face of a nido-carborane dianion (e.g.Fig. 1D) to form a metallacarborane,10 in analogy to the widely used cyclopentadienyl ligand. Ortho-carborane precursors can be easily modified by substituting the acidic cage hydrogen atom(s) at the cluster carbons with groups that have desirable electronic and steric effects. This includes the addition of tethering substituents such as amino11–13 or cyclopentadienyl14–16 groups. Herein, we report unprecedented tethered NHC–carborane ligands (Fig. 1E–H) in which both the carborane and the NHC moieties are available for metal binding.

Slow addition of one equivalent of tbutylimidazole to bromoethyl-ortho-carborane in toluene was carried out at 75 °C for 18 hours (Scheme 1). The product, isolated in 72% yield, was characterised as an imidazolium salt linked to a closo-carborane via an ethylene tether (1). Addition of excess tbutylimidazole to bromoethyl-ortho-carborane induced deboronation, with the 11B NMR data for the resulting product exhibiting characteristic resonances for a nido-carborane monoanion of type [7,8-R5,R6-7,8-C2B9H10]. The product, isolated in 65% yield, was identified as an unusual imidazolium nido-carborane zwitterion 2. This compound could also be prepared from imidazolium 1 with two equivalents of tbutylimidazole. Compounds 1 and 2 were characterised using multinuclear NMR spectroscopy, mass spectrometry, elemental analysis and X-ray crystallography (see ESI).


image file: c6cc01650b-s1.tif
Scheme 1 Synthesis of new ligand precursors 1 and 2. Deprotonation and variable coordination of the ligands to RhI (unlabelled vertices = BH).

A common method to prepare metallacarboranes involves deprotonation of the nido-carborane of type [7,8-R5,R6-7,8-C2B9H10] using NaH with subsequent coordination of the dianion of type [7,8-R5,R6-7,8-C2B9H9]2− (Fig. 1D) to a metal. As NaH may also be used to generate free carbenes from imidazolium salts, zwitterion 2 was reacted with NaH to deprotonate both sites (Scheme 1). Subsequent treatment with [Rh(COD)Cl]2 for 3 hours at room temperature resulted in the presence of two new Rh–NHC species, 3·MeCN and 4·MeCN, in the 1H NMR spectrum (CD3CN), with the 11B{1H} NMR spectrum showing resonances characteristic of closo MC2B9 metalladicarbaboranes. Crystals grown from the product mixture were identified as complex 3·MeCN (Fig. 2), a bimetallic complex featuring two linked RhI centres in very different chemical environments, formally [RhI(NHC)(COD)(MeCN)]+ and [RhI(carborane)(COD)]. While derivatives of the [RhI(carborane)(COD)] anion have been known for decades,17,18 this is the first solid-state structural elucidation of its kind.


image file: c6cc01650b-f2.tif
Fig. 2 Molecular structure of RhI complexes 3·MeCN (left) and 4·MeCN (right). H atoms are omitted for clarity and ellipsoids are plotted at 50% probability level. Selected bond lengths (Å): 3·MeCN Rh1–C7, 2.061(4); C12–C13, 1.380(6); C16–C17, 1.389(6); Rh2–C1, 2.231(4); Rh2–C2, 2.306(4); C1–C2, 1.608(5); C25–C26, 1.410(5); C22–C29, 1.399(6). 4·MeCN Rh1–C7, 2.048(3); C12–C13, 1.394(4); C16–C17, 1.371(5); Rh2–C1, 2.290(3); C22–C23, 1.404(5); C26–C27, 1.393(5).

Heating of the reaction mixture of 3·MeCN and 4·MeCN in MeCN for 18 hours gave only the thermodynamically stable product 4·MeCN in 67% yield. Similarly to 3·MeCN, crystals of 4·MeCN revealed [RhI(NHC)(COD)(MeCN)]+ and [RhI(carborane)(COD)] moieties, but with the 2,1,8-MC2B9 metalladicarbaborane isomer present, where one cluster carbon atom is not bonded to Rh. The cluster carbon atoms in both 3·MeCN and 4·MeCN were identified using the vertex-to-centroid distance (VCD) method.19 The observed 3,1,2 to 2,1,8 RhC2B9 cluster rearrangement is known, and attributed to relief of steric crowding.20 In addition, Stone and co-workers reported a series of PdII-metalladicarbaboranes, and found that polytopal rearrangement occurs readily in the presence of a COD co-ligand, thought to be promoted by dissociation of the COD.21 Rearrangement of complex 3·MeCN to 4·MeCN occurs under relatively mild conditions, with both steric crowding and the presence of a COD co-ligand being contributing factors.

Recrystallisation of 4·MeCN with a mixed solvent system (MeCN/Et2O/hexane) resulted in the unexpected formation of a dimeric solid-state structure (42) with MeCN ligands absent (Fig. 3). The cationic [RhI(NHC)(COD)]+ moiety appears to be stabilised by the anionic [RhI(carborane)(COD)] moiety of a second molecule via close Rh⋯H–B links and vice versa, resulting in the formation of a dimer. Rhodacarborane clusters containing Rh⋯H–B bridges have previously been reported,22 with the complexes usually bimetallic. The dimer 42 does not appear to persist in solution, as the coordinated acetonitrile ligand is observed in the 1H NMR spectrum (acetone-d6). However, the demonstrated lability of the MeCN ligand and the contrasting RhI environments renders complex 4·MeCN interesting for development in, for example, tandem catalysis without the need for a hetero-bimetallic system.23,24


image file: c6cc01650b-f3.tif
Fig. 3 Molecular structure of 42. H atoms (except H8) are omitted for clarity and ellipsoids are plotted at 30% probability level. Selected bond lengths (Å): Rh1–C7, 2.043(4); Rh2–C1, 2.276(4); Rh2–B8, 2.229(5); Rh1–B8, 2.788.

Boron atoms in the closo-dicarbaborane cluster (Fig. 1B) are susceptible to attack by NaH in THF and by NHCs generated from imidazolium salts.25,26 Thus, a milder route to NHC complexes, involving transmetallation from Ag, is successfully applied here without affecting the closo-carbaborane boron atoms. Reaction of imidazolium bromide 1 with [Rh(COD)Cl]2 in the presence of Ag2O in DCM resulted in a RhI–NHC complex (5) in 74% yield (Scheme 1). The carboranyl C–H proton resonates as a broad singlet at 4.17 ppm in the 1H NMR spectrum, and the unsubstituted carborane carbon is at 63.6 ppm in the 13C{1H} NMR spectrum (500 MHz, CDCl3). The solid state structure of 5 displays a square planar RhI centre, coordinating an NHC, a chloride and the two alkenes of a COD ligand (Fig. 4).


image file: c6cc01650b-f4.tif
Fig. 4 Molecular structure of [RhI(NHC)(COD)Cl] 5 (left) and RhI metallacycle 6 (right). H atoms are omitted for clarity and ellipsoids are plotted at 50% probability level. Selected bond lengths (Å): 5 Rh1–C7, 2.036(3); C15–C16, 1.407(6); C12–C19, 1.375(6); C1–C2, 1.653(5). 6 Rh1–C7, 2.028(3); Rh1–C1, 2.156(3); C12–C19, 1.405(4); C15–C16, 1.385(4); C1–C2, 1.738(4).

A surprising result was observed upon further reaction of complex 5 with Ag2O in MeCN to yield complex 6 (Scheme 1). In this case, the ligand chelates the RhI centre, coordinating through the carbenic carbon of the NHC and through a carbon atom of the closo-carborane to form a 7-membered metallacycle. The carboranyl C–H resonance is absent in the 1H NMR spectrum of complex 6, with the 13C{1H} NMR spectrum exhibiting a doublet at 70.6 ppm attributable to the Rh-coordinated carboranyl carbon, with a C–Rh coupling constant of 52.9 Hz. The metallacycle 6 could be formed in 79% yield directly from the imidazolium precursor 1 by conducting the initial [Rh(COD)Cl]2/Ag2O reaction step in MeCN. The solid-state structure of 6 shows a distorted square planar RhI centre, bearing a chelating NHC-closo-carborane ligand coordinating in a cis fashion through the carbenic carbon and through the carbon atom of the carborane cage (Fig. 4). C-cyclometallation of a carborane anion at RhI is rare, with only one other reported example found in the literature.27

While we may expect silver salts such as AgOTf to abstract the chloride anion in 5, leading to a highly reactive RhI cation, the use of Ag2O along with MeCN to generate 6 is more unusual. It is possible that the carborane C–H bond is cleaved to form a Ag–C bond followed by transmetallation with Rh. However, the possibility of an oxidative addition pathway with subsequent reductive elimination of HCl promoted by Ag2O cannot be ruled out.

In conclusion, we have developed new ligand systems which combine soft and hard ligands, namely NHCs and carborane anions. Examination of the ligands has revealed unique and versatile coordination to RhI through both the NHC and either a nido-dicarbaundecaborane dianion or a closo-dicarbadodecaborane anion. The nido-carborane ligands form homo-bimetallic complexes, with two RhI centres in considerably different chemical environments. The closo-carborane ligand forms a remarkable 7-membered metallacycle coordinating through both the NHC and the carbon atom of the carborane. Expansion of these highly tailorable ligand classes through substituent modifications, varying tethers and different metals is currently underway in our laboratory, with the possibilities of extending to pincer ligands28 and introducing chiral centres into the ligand architectures.29 Our novel ligands will be of broad interest for development in catalytic, materials and biomedical applications, with the NHC and carborane moieties working synergistically to tune a particular system.

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Footnote

Electronic supplementary information (ESI) available: Detailed experimental procedures, spectroscopic and crystallographic data. CCDC 1444072–1444078. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6cc01650b

This journal is © The Royal Society of Chemistry 2016