Journal Archive

Johnson Matthey Technol. Rev., 2023, 67, (3), 333
doi: 10.1595/205651323X16765646706676

Moving from Fuel to Feedstock

Selective hydrocarbon activation using rhodium and iridium complexes

  • Paul A. Morton, Stephen M. Mansell*
  • Institute of Chemical Sciences, School of Engineering and Physical Sciences, William Perkin Building, Heriot-Watt University, Edinburgh, EH14 4AS, UK
  • *Email: s.mansell@hw.ac.uk
  •  
  • PEER REVIEWED
    Received 23rd November 2022; Revised 10th February 2023; Accepted 16th February 2023; Online 17th February 2023

Article Synopsis

Carbon-hydrogen bond activations and their subsequent functionalisation have long been an important target in chemistry because C–H bonds are ubiquitous throughout nature, making C–H derivatisation reactions highly desirable. The selective and efficient functionalisation of this bond into many more useful carbon-element bonds (for example, C–B, C–Si, C–O and C–S bonds) would have many uses in pharmaceutical and bulk chemical synthesis. Activation of the C–H bond is, however, challenging due to the high strength and low bond-polarity of this bond rendering its cleavage unfavourable. With the correct choice of reagents and systems, especially those utilising directing groups, kinetically and thermodynamically favourable catalytic processes have been developed. However, a key remaining challenge is the development of undirected, intermolecular reactions using catalysts that are both selective and active enough to make useful processes. In this review, the progress towards optimising Group 9 C–H activation catalysts is discussed, particularly focusing on undirected reactions that are kinetically more difficult, starting with a brief history of C–H activation, identifying the importance of auxiliary ligands including the nature of anionic ligand (for example, cyclopentadienyl, indenyl, fluorenyl and trispyrazolylborate) and neutral ligands (such as phosphines, carbonyl, alkenes and N-heterocyclic carbenes (NHCs)) that contribute towards the stability and reactivity of these metal complexes. The tethering of the anionic ligand to strong σ-donating ligands is also briefly discussed. The focus of this review is primarily on the Group 9 metals rhodium and iridium, however, C–H activation using Group 8 and 10 metals are compared where useful. The most recent advances in this field include the development of C–H borylation of many small hydrocarbon substrates such as arenes, heterocycles and n-alkanes as well as the more challenging substrate methane.

1. Carbon-Hydrogen Activation

The C–H activation and functionalisation of small hydrocarbon molecules that feature relatively non-reactive C–H bonds, such as alkanes and aromatics, has long been of interest in chemistry due to their use as starting materials in many industrial syntheses (1). Hydrocarbons are cheap and highly abundant feedstocks currently derived from the petrochemical industry. Methane is a byproduct of oil extraction and must be burned when it cannot be transported due to its dangerous greenhouse gas potential. The use of hydrocarbons as fuels has unwanted side-effects due to the CO2 released in combustion. If we are to move beyond fossil fuels, part of the solution may be to use these hydrocarbon feedstocks for building more useful and valuable compounds. Thus, if we can establish efficient transformations of C–H bonds, we can better utilise hydrocarbons as chemical building blocks rather than fuels. Additionally, because C–H bonds are ubiquitous in chemistry and biochemistry, these methods will also be applicable to other feedstocks that could be renewable, bio-sourced or generated from waste products, further increasing the sustainability of this chemistry (2). One pertinent example is the hydrocarbons that are produced from Fischer-Tropsch processes (3, 4). These synthetic hydrocarbons could be carbon neutral, if derived from CO2 and hydrogen produced using renewable energy, but will need functionalising if they are to be used as convenient chemical feedstocks.

C–H activation remains difficult due to the strength and lack of polarity in carbon hydrogen bonds, particularly with respect to sp3 carbon centres. This unfavorability in bond breakage can be overcome, however, using a number of approaches to generate thermodynamically and kinetically favourable reactions (Scheme I), even for intermolecular and undirected reactions (5). Strong bases can be used to deprotonate and then subsequently functionalise arenes in stoichiometric reactions, with directed ortho metalation (DoM) a particularly useful reaction (6), and modern research has focused on developing better and more selective, albeit still stoichiometric, reagents and reactions (7).

Scheme I.

Generalisation of metal-based C–H activation pathways

Compared to stoichiometric reactions, catalytic C–H bond functionalisation has the advantage of reducing waste. Focusing on the late transition metals, electrophilic and oxidative addition pathways are commonly proposed, although σ-complex assisted metathesis pathways (σ-CAM) also need to be considered (8, 9). For oxidative addition, using metal complexes with a strongly electron-rich metal centre leads to the formation of strong metal-carbon and metal-hydrogen bonds (Scheme II). The requirement for an electron-rich metal centre makes the late transition metals attractive. To improve reactivity, these metal centres require the coordination of strongly electron-donating ligands to further increase the electron density on the metal centre. This subject has been covered in a plethora of reviews on C–H activation over the last few decades (10–13).

Scheme II.

C–H oxidative addition

The history of C–H activation began as long back as 1898 when Dimroth was able to show the mercuriation of benzene using Hg(OAc)2 to give Hg(Ph)OAc. Coincidentally, in 1908 Schorigin also used a mercury compound, HgEt2, in combination with sodium and benzene to generate phenylsodium (7). C–H activation was then extended to aurations in 1931 with Kharasch and coworkers who used gold chloride salts for C–H insertion to form organogold compounds (14). Other early C–H deprotonations could be performed using organolithium and Grignard reagents, however, these reactions generally required nearby functional groups to acidify the C–H bond which both increased reactivity and brought about the selectivity in these reactions, giving rise to DoM.

The metals used in C–H activation moved away from these early C–H activation species to the more electron rich d-block metals and by the 1970s the process of intramolecular C–H activation, termed cyclometallation, was well known in the literature (15) with the first cyclometallations being performed in the late 1950s (16). Often aryl C–H bonds were involved (Scheme III) because aryl C–H bond activation is kinetically favoured over alkyl C–H bond activation due to pre-coordination of the electron rich arene as well as the formation of stronger M-aryl bonds compared to M-alkyl (17). Cyclometallation further favours the C–H activation product due to the high local concentration of C–H bonds present in ligands already attached to the metal centre and the chelate ring that is formed upon cyclometallation (18).

Scheme III.

An example cyclometallation reaction

By the 1960s, Chatt and coworkers were able to demonstrate the more challenging intermolecular C–H activation reaction in the oxidative addition of a naphthalene C–H bond using a ruthenium 1,2-bis(dimethylphosphino)ethane (dmpe) complex, albeit forming a more kinetically and thermodynamically favoured M-aryl bond compared to an M-alkyl bond (19). This reaction was significant as it showed the capability of a low valent d-block complex to C–H activate a molecule that was not already bound to the metal centre. The less reactive tetrahydrofuran (THF) solvent molecule was, however, not C–H activated using this complex and it was not expected at that time that alkanes could be C–H activated (16, 20). Thus, the dual challenge of undirected sp3 C–H activation and the catalytic functionalisation of C–H bonds still remained.

2. Platinum Group Metals

The platinum group metals play a privileged role in catalysis due to their favourable properties including those that derive from their ‘soft’ bonding characteristics, as classified in the hard-soft Lewis acids and bases (HSAB) approach. This classification leads from the high covalency in their bonding, preference for soft ligands, such as those derived from carbon, and tolerance to oxygen and moisture. Additionally, they undergo two electron processes (oxidative addition and reductive elimination) and their properties can be modified easily using the ligands that are attached, which has proven crucial in developments throughout homogeneous catalysis (21).

2.1 A Brief Overview of Palladium and Platinum in Electrophilic Carbon-Hydrogen Activation: Historical Developments to the Modern Day

The Group 10 platinum metals (platinum and palladium) are important metals for C–H activations (22, 23). This has been seen since the late 1960s and early 1970s when Shilov and coworkers were able to develop metal complexes that were able to C–H activate alkanes in an undirected, intermolecular fashion; the so-called Shilov system (Scheme IV) (24). This system made use of a Pt(II) complex as the catalyst and a Pt(IV) complex as a stoichiometric oxidant because, although other oxidants were explored, platinum was shown to be able to deliver the best results. The work by Shilov was mostly unknown to western scientists due to the political split referred to as ‘The Iron Curtain’ at the time. The work by Bercaw and coworkers explored this chemistry further where they were able to follow the progress of Shilov through several international conferences. Bercaw and others were also able to explore the mechanism of this system in greater detail. This brought about knowledge of the active species involved, giving a more detailed answer to whether platinum nanoparticles were the cause of activation. The answer was that while nanoparticles had an effect on H/D exchange, the system was still able to C–H activate the system on its own (25).

Scheme IV.

The Shilov system for C–H activation and functionalisation

An example of more recent developments in platinum chemistry is shown by the work of the Periana group where they were able to synthesise a stable (2,2’-bipyrimidyl)Pt(II) dichloride catalyst, the so-called ‘Periana System’, for the functionalisation of methane to the methanol derivative methyl bisulfate, (CH3)HSO4, in H2SO4 (Scheme V). This high yielding and relatively selective reaction was able to tolerate excess SO3 without forming the undesirable C–S bonded product methanesulfonic acid, CH3SO3H, however, the reaction is limited to a product methanol concentration of ca. 1 M. The authors also noted that the complex was stable for over 300 turnovers before there was any loss in activity when acid concentration is maintained, although further work was required to deduce why this was the case (23). Later work done by the Schüth group in 2016 showed the same reaction using another platinum catalyst, K2PtCl4. Under these reaction conditions the catalyst turnover frequency was 25,000 h–1 making this complex a potentially desirable catalyst for this process industrially. The authors also noted that platinum black was not an issue in these reactions, particularly as oleum in the reaction medium is capable of oxidising platinum black to Pt(IV) (26).

Scheme V.

Periana system for C–H oxidation

Palladium also plays a significant role in C–H activation and there is a plethora of examples of palladium complexes being used in organic syntheses (27). Palladium complexes used in C–H activation typically utilise a concerted metalation-deprotonation (CMD) mechanism, or similar, as opposed to oxidative addition, where the reaction proceeds via the formation of a five- or six‐membered palladacycle. This lowers the activation energy of the species to allow for the cleavage of the C–H bond. Therefore, the selection of directing groups in these reactions is important for achieving high reactivity in C–H bond activation (28, 29).

2.2 Ruthenium

Ruthenium complexes capable of C–H activation have long been known ever since the intermolecular C–H activations mentioned earlier performed by Chatt and coworkers. However, compared to other platinum metals, ruthenium has been relatively under-explored in C–H activation over the last couple of decades. An area where there has been interest in ruthenium C–H activation is in selective meta-C–H functionalisation through ruthenium catalysed σ-activation (30). This was reviewed in 2017 by Leitch and Frost where they were able to explore this reaction in more detail (31). The Frost group previously performed this reaction using a ruthenium para-cymene complex [RuCl2(p‐cymene)]2 which selectively C–H activated the meta-position of phenyl pyridine with an 80% yield in a sulfonation reaction. The high loadings of complex in this reaction were problematic. This was, however, tackled by the Ackermann group who were able to make use of a recyclable heterogeneous ruthenium catalyst on a silica support (32).

Research in ruthenium C–H activation was further expanded in a recent paper by the Baslé group, where they were able to perform phosphorus directed C–H borylation at the ortho-position of arylphosphines using a ruthenium NHC-tethered carboxylate complex. This reaction was highly selective and high yielding for a range of aryl phosphines, where the phosphine acted as a directing group in this reaction. The active site of this species was also brought about by the lability of the arene in the complex (33).

3. Rhodium and Iridium

Rhodium and iridium complexes, the focus of this review, are widely used in C–H activation reactions operating with several different distinct mechanisms depending on oxidation state, choice of ligands and substrate involved. Several of these mechanisms utilise similar ligands sets, so the choice of ligand set and the properties required will be discussed concurrently with the C–H activation processes.

3.1 Ligand Choice and Selection with Respect to Carbon-Hydrogen Activation: Cyclopentadienyl/Pentamethylcyclopentadienyl Complexes

Cyclopentadienyl (Cp, C5H5), which is an aromatic and formally anionic ligand, and pentamethylcyclopentadienyl (Cp*, C5Me5) were the ligands of choice for Group 9 C–H activation dating back to the first well-defined intermolecular C–H oxidative addition reactions in the early 1980s. This is because the ligands bound to a metal centre of a transition metal complex are important for controlling reactivity and Cp/Cp* are useful ligands being able to bind strongly to the metal centre taking up three cis coordination sites. These anionic ligands are not easily dissociated from the metal complex, even at high temperatures, making these ligands excellent anchor groups and spectator ligands (34).

3.2 Rhodium Complexes for Donor-Assisted Carbon-Hydrogen Activation

In general, with donor-assisted C–H activation catalysed by rhodium, two modes of activation are known: chelation-assisted C–H activation (35), particularly useful for the functionalisation of donor-substituted arenes; and the C–H activation of N-heterocycles (36). For chelation assisted C–H activation, a Rh(I) oxidative addition pathway is possible as well as a Rh(III) electrophilic pathway (35). Rh(III) chloride complexes featuring the Cp* ligand are typically used here, with these complexes reacting akin to the palladium complexes mentioned earlier via a directed CMD reaction where the importance of the directing group is fundamental (37). The work of Bergman, Ellman and coworkers in this field is highlighted as these rhodium-catalysed processes are efficient means of C–C bond formation and the functionalisation of N-heterocycles (35, 36, 38). For example, {Cp*RhCl2}2 activated with a silver salt of a weakly coordinating anion in 1,2-dichloroethane (DCE) was used for the C–H activation of benzamide arenes where the amide was the directing group in this reaction (Scheme VI) (39). Amides are more useful motifs in the synthesis of biologically-relevant pharmaceuticals and natural products than earlier α-branched amines that were used.

Scheme VI.

Biologically important C–H activation using a Rh(III) complex. Ts = para-tolylsulfonyl

3.3 Alkane Dehydrogenation Studies

In 1979, Crabtree and coworkers utilised the high stability of iridium bonding to unsaturated hydrocarbons to drive the dehydrogenation of alkanes, although a hydrogen acceptor was needed making this reaction a transfer hydrogenation (40, 41). In particular, the high stability of the Ir–Cp fragment drives the dehydrogenation of cyclopentene and cyclopentane to the cyclopentadienyl ligand, thereby demonstrating C–H activation (Scheme VII).

Scheme VII.

Alkane dehydrogenation using iridium

Similar results were also achieved with rhenium hydride complexes [ReH7(PR3)2] (PR3 = PPh3 or PEt2Ph) (42), which could also be made catalytic generating cycloalkenes from cycloalkanes, albeit requiring the use of a hydrogen acceptor (42). Using [IrH5(Pi Pr3)2], n-hexane could be converted into 1-hexene at room temperature (43). In these systems, turnover numbers up to 70 were achieved (44). Crabtree’s system, modified with a chelating trifluoroacetate ligand, could also be used catalytically either thermally with a hydrogen acceptor, or photochemically without (45).

Rhodium phosphine complexes were subsequently found to catalyse the dehydrogenation of cyclohexane to cyclohexene, either photochemically (46) or thermally with a hydrogen acceptor (44, 47). However, the real breakthrough came when using thermally robust pincer ligands in combination with iridium metal centres (Scheme VIII) (48, 49). Subsequent research has driven further progress in this field (48, 50), including the combination of alkane dehydrogenation with alkene metathesis to effect ‘alkane metathesis’ via alkenes as intermediates (51).

Scheme VIII.

Catalytic alkane dehydrogenation using a sacrificial alkene acceptor

3.4 Carbon-Hydrogen Oxidative Addition

Cp* iridium complexes capable of C–H activation were discovered in the early 1980s in seminal work by the Bergman and Graham groups where they were able to use highly electron-rich iridium complexes to stoichiometrically C–H activate neopentane (Scheme IX) (52, 53). The reactive, unsaturated 16 electron intermediate was generated using ultraviolet photolysis. This work showed that it was possible to activate sp3 C–H bonds that were previously considered inert on molecules separate from the metal complex (termed intermolecular C–H activation) not just those bound to the transition metal centre (termed intramolecular C–H activation or cyclometallation).

Scheme IX.

Photolysis driven IrCp* C–H activation

In parallel work, Cp* ligands have also been utilised in C–H activation by σ-bond metathesis (Scheme I). In 1983, shortly after the work published by Graham and Bergman using iridium Cp complexes, Patricia Watson developed rare earth Cp* complexes for intermolecular C–H activation using lutetium as the metal centre (54). A completely different mechanism of reactivity is in use here, termed σ-bond metathesis, enforced by the single accessible oxidation state of lutetium. Watson showed examples of intra- and intermolecular C–H activation of several species under mild conditions (Scheme X). It was also possible to form the intermolecular C–H activation product with tetramethylsilane to give (Cp*)2Lu‐CH2SiMe3. Again, it was the use of Cp* as a stable, supporting ligand that controlled the coordination environment around the metal that led to the successful development of these C–H activation processes.

Scheme X.

Cp* Lutetium complex for intramolecular and intermolecular C–H activation

Returning to Group 9, investigations on the mechanism and thermodynamics of C–H activation using the [RhCp*(PMe3)] fragment were carried out by Jones and coworkers (55, 56). These investigations revealed that [Cp*Rh(H)(Ar)(PMe3)] complexes were stable below 60°C whereas the analogous alkyl complexes displayed irreversible reductive elimination at or about –20°C (56). Thus, photolysis of [Cp*Rh(H)2(PMe3)] in liquid propane at –55°C followed by removal of the solvent at –40°C gave the n-propyl hydride complex (56). Competitive experiments using benzene:propane mixtures showed only a small kinetic preference for benzene over propane (4:1) despite the thermodynamic preference for the formation of the phenyl hydride complex (56). Perutz and coworkers found that it was also possible to use ethene as a leaving group through photolysis of [MCp(PMe3)(C2H4)] (M = rhodium, iridium) to give a similar reactive fragment to that of Bergman et al. (57, 58) The group were also able to explore the C–H activation of methane several years later using photochemical generation of a reactive rhodium fragment, similar to the complex used by the Bergman and Graham groups (Scheme XI) (59).

Scheme XI.

RhCp C–H activation

3.5 Indenyl Complexes

Cp and Cp* ligands are difficult to modify further (34). An exception to this is the benzannulated derivatives indenyl and fluorenyl that are readily available. Indenyl, just like Cp, has the ability to bind strongly to the metal centre through an η5-interaction featuring bonding to all five carbon atoms of the five-membered ring. However, it is also possible for indenyl to ‘ring-slip’ to an η3 mode (where only three carbon atoms are coordinated) or even an η1-interaction; this is known as the ‘indenyl effect’ (60). This change in hapticity was first shown by the Mawby group in 1969 where they observed that PPh3 could coordinate to [Mo(C9H7)(Me)(CO)3] after ring slippage of the indenyl from η5 to η3 leaving a vacant site on the metal centre. The species then reverted back to η5 coordination after migratory insertion of the methyl to form [Mo(C9H7)(COMe)(CO)2(PPh3)] (61).

Foo and Bergman were able to use an indenyl phosphine iridium complex for the C–H activation of benzene in 1993 (62). As was hypothesised by the authors, it was found that the indenyl complex was more reactive than the Cp* complex. Since then, there has been other uses of indenyl Group 9 complexes for C–H activation. A recent review (63) highlighted the work by the Rovis group who were able to use a heptamethylindenyl Rh(III) complex to catalyse the C–H activation and diastereoselective benzamidation of cyclopropenes (64). It was also noted that different Cp analogues have differing stereochemical, steric and electronic properties leading to a change in the yields of the desired product.

3.6 Tethered Complexes

Tethered Cp ligands are well established in the literature and combine the advantages of Cp with those of another donor group (65, 66). Tethered species have the advantage of being more thermally stable, but there is also the advantage that the ligand is more rigid allowing for the preparation of an asymmetric catalyst when a chiral centre is present (67). In 1997, Bergman and coworkers were able to show C–H activation using an iridium complex formed from a phosphine tethered to a Cp ligand (Scheme XII). This complex was found to have an advantage over non-tethered complexes because the tethered phosphine could readily dissociate from the metal and then immediately reattach as the Cp is a stable supporting ligand and will not dissociate at high temperatures. The small bite angle effect (68) of the complex is also potentially advantageous, brought about by the rigidity of the ligand structure (69, 70). Contrastingly, a rhodium dihydride complex of the tethered ligand [C5H4SiMe2CH2PPh2]– did not show C–H activation under photochemical conditions (71). The Royo group in 2008 synthesised the first Cp*-type tethered NHC ligand, a synthetically more challenging goal, to give an Ir(III) metal complex (Scheme XII) (72). The work of Danopoulos and coworkers is also highlighted as they were able to synthesise tethered iridium and rhodium indenyl/fluorenyl NHC complexes (73, 74).

Scheme XII.

Synthesis of tethered iridium complexes

3.7 Tp/Tp* – Trispyrazolylborates

Amongst many Cp-mimics, (hydrotris(pyrazolyl)borate) (Tp) ligands are especially useful. These ligands bind to the metal centre as a tridentate ligand through nitrogen donors (75, 76). These ligands were first explored for C–H activation in the early 1990s where Jones and coworkers were able to use a Tp complex, [Rh{HB(3,5‐dimethylpyrazolyl)3}(CNR)(PhN=C=NR)] (R = neopentyl), to C–H activate a range of substrates (Scheme XIII) (77, 78).

Scheme XIII.

C–H activation of a range of substrates using a [HB(3,5-dimethylpyrazolyl)3] Rh(CNR)(PhN=C=NR) (R = neopentyl) complex

Later, these ligands found use in mechanistic studies of C–H activation at room temperature. In 1997, the [RhTp*(CO)2] complex (Tp* = HB–Pz*3, Pz* = 3,5-dimethylpyrazolyl) was used in a collaborative project by the Harris, Bergman and Frei groups (Scheme XIV) (79). The Tp* complex was seen as a better complex to study this reaction compared to Cp analogues because the Tp* complex has a high quantum yield of 30% as opposed to the Cp complex which had a quantum yield of ca. 1% for the formation of the activated intermediate. This allowed the team to use ultrafast spectroscopy with picosecond and femtosecond time resolutions to access the inter/intramolecular processes that take place faster than diffusion. Further work was done in 2002 to report the role of hydrocarbon structure on the process of C–H activation as well as to compare with [RhCp(CO)2] (80). In 2018, the role of sterics and electronics was studied for Tp’ complexes (Tp, Tp* or TptBu = tris(3,5-dimethyl 4-t-butylpyrazolyl)borate) vs. Cp’(Cp or Cp*) Rh(CNR)(carbodiimide) in the C–H activation of cyclic alkanes (81).

Scheme XIV.

TpRh(CO)2 complex for C–H activation

4. Borylation

4.1 Metal Boryls and Stoichiometric Borylation

A major problem with many of the C–H activation reactions listed previously was the difficulty in converting the M–C species generated into new species whilst still yielding reactive metal complexes that can complete a catalytic cycle. The use of metal boryls was found to be an especially useful route forward. Firstly, reactions of C–H bonds (such as those in methane) with tetraalkoxydiboron(4) compounds, B2(OR)4, generate alkylboronate esters (R-B(OR)2) and H-B(OR)2 and are thermodynamically downhill (82, 83). Even the reaction of C–H bonds with HB(OR)2 (the byproduct of the previous reaction) are approximately thermoneutral (82), and so can be driven by release of the H2 gas that is formed. Secondly, boryls are electron donating ligands that facilitate oxidative addition (often favoured in iridium borylation) and σ-CAM pathways (as proposed in rhodium-catalysed borylation) (84) of C–H bonds, with the nominally empty p-orbital on boron also facilitating C–H bond cleavage and C–B bond formation (82, 84). This means that reactions of metal boryls with arenes and alkanes can occur generating a catalytic cycle.

It is also fortunate that the organoboronic esters that are formed from C–H activation of metal boryl complexes are valuable as chemical intermediates in many chemical reactions including Suzuki-Miyaura coupling reactions. Additionally, drugs that contain boron are now being developed successfully, including Bortezomib, a proteasome inhibitor that has activity against a variety of cancers (85). The requirements for a catalyst that can accomplish C–H borylation was highlighted in a review by Hartwig in 2012 (86), identifying the need for catalysts that are both site selective with excellent functional group tolerance and have high turnover numbers. The versatility of these borylated arene intermediates was underscored because they can be further reacted with a wide range of organic reagents producing products that can be used in materials and medicinal applications.

The selective boron-functionalisation of alkanes using transition metals was pioneered by the Hartwig group using transition-metal boryl complexes (87, 88). The group was able to successfully synthesise pentyl-Bcat’ {cat’ = substituted catecholate 1,2-O2C6H2-3,5-(CH3)2} by the stoichiometric reaction of heteroleptic Cp* metal-carbonyl boryl complexes with pentane (Scheme XV). Similar iron and ruthenium complexes were able to form pentyl-Bcat’ in 28% and 40% yield, respectively, with the formation of trace amounts of HBcat’ (~10%).

Scheme XV.

Cp* metal carbonyl complex for C–H borylation

The reaction with the analogous tungsten complex was found to be the best with pentyl-Bcat’ formed in 85% yield (Scheme XVI), therefore, the scope of the tungsten reaction was explored further. The group found that the complex was selective for the borylation of the least hindered sp3 carbon of selected molecules. Ethylcyclohexane was selectively borylated at the terminal sp3 position of the molecule to yield (2-cyclohexyl)-1-ethylboronate ester in 79% yield. The borylation of isopentane was selective for the least hindered terminal site (55% yield) instead of the most sterically hindered terminal carbon (2% yield). Cyclohexane was also borylated, however, the yield for this reaction was only 22%, suggesting that the reaction is selective for sp3 carbons over sp2 carbon centres.

Scheme XVI.

Further early stoichiometric C–H borylations using tungsten

4.2 Catalytic Borylation

At a similar time, Smith and coworkers were also probing the challenge of C–H activation and the borylation of arenes (89). Using an IrCp* complex, they were able to develop a catalytic reaction, albeit under quite severe conditions (Scheme XVII). RhCp* catalysts were quickly shown to be superior, utilising [RhCp*(C6Me6)] as a precursor (Scheme XVII) (90, 91).

Scheme XVII.

C–H borylation of arenes using rhodium and iridium Cp complexes. The rhodium-catalysed reaction generates and uses HBpin to give two equivalents of PhBpin

The first thermally-driven catalytic borylation of alkanes were realised by Hartwig and coworkers in 2000 (Scheme XVIII) (91). This reaction was able to occur, albeit at relatively high temperatures, to give organoboronic esters in good yields using both B2Pin2 and HBPin as a source of boryl fragments.

Scheme XVIII.

Thermally driven catalytic C–H borylations of arenes and alkanes

In 2001, Hartwig, Miyaura and coworkers were able to exemplify the mild, catalysed direct borylation of arenes with B2Pin2 using an iridium catalyst (Scheme XIX) (92). The group made a serendipitous discovery of a very effective catalyst by reacting a commercially available Ir(I) complex with 2,2’-bipyridine (bpy). The catalyst was able to efficiently borylate arene substrates at 80°C using B2Pin2 making use of both Bpin groups. The reaction gave yields of 93% with turnover numbers as high as 8000. The reaction also took place at room temperature using this catalyst, however, the catalyst loading needed to be much higher (92). This finding led to much research into iridium-catalysed C–H borylation using bidentate nitrogen ligands based on bipyridine and phenanthroline (93), and iridium-catalysed arene borylation has even been performed on >70 kg of the borylated product (94). Other metal complexes were inferior to iridium complexes, such as Pt(dba)2/bpy and [RhCl(COD)]2/bpy. The platinum complex was unable to catalyse this reaction and the rhodium complex was only able to form PhBPin in 20% yields at temperatures over 150°C (92). Other ligands were tried, and it was found that 1,10-phenanthroline catalysed the reaction in high yields whereas pyridine, PPh3 and tetramethylethylenediamine (TMEDA) were unable to generate a catalytically active metal complex. Further research showed that with the correct choice of ligand, platinum catalysts can be successful. Work performed by Furawa et al. used an NHC platinum complex for the C–H activation and borylation of sterically congested positions of hydrocarbon substrates (Scheme XX) (95).

Scheme XIX.

Mild catalytic conditions for the C–H borylation of arenes

Scheme XX.

The borylation of arenes using a platinum catalyst. Pin = pinacolato, 1,2-O2C2Me4

The substrate scope of iridium- and rhodium-catalysed C–H borylations has greatly improved since the work of the early 2000s. The Hartwig group have been able to expand the substrate scope of these reactions to ethers, amines, amides and heteroarenes. The scope, regioselectivity and mechanism of the borylation of heteroatoms were explored, in 2014, using an iridium tetramethylphenanthroline complex (96, 97). There have been many more examples of C–H activation and borylation of arenes since then, but the efficient borylation of alkanes remains very difficult (98, 99). A catalyst system involving the 2-methylphenanthroline ligand (5 mol%) with [Ir(mesitylene)(Bpin)3] (2.5 mol%) and B2pin2 allowed the stoichiometric C–H borylation of sp3 C–H bonds to be achieved (cycloalkanes were used as the solvent, primary C–H bonds are favoured, with secondary bonds functionalised only if the primary sites are absent or blocked), an important advance on previous processes that required excess substrate (100). Another recent successful example was demonstrated by the Schley group in 2020 where they were able to use an iridium catalyst with a tuneable ligand scaffold (Scheme XXI) (101).

Scheme XXI.

Tuneable ligand catalyst for the C–H activation of alkanes and arenes

Using the catalyst system designed by the Schley group, it was found that the C–H activation and borylation of n-octane could be performed in near quantitative yields to give octyl-Bpin. The group also found that their ligand system is tolerant to a range of substrates including ethers, esters and tertiary amines. Secondary and branched alkanes, however, showed poor catalytic performance. It was found that although these functional groups were low yielding this reaction could be performed in cyclohexane with negligible competitive solvent borylation. The use of five equivalents of substrate gave high yields in cyclohexane which is useful compared to the typical requirement for neat substrate.

The active species of this reaction is relatively unknown. It was postulated that the ligand binds to the iridium metal centre through the nitrogen atoms and then forms a further bond through the cyclometallation of the phenyl to give an κ3-binding mode, which looks analogous to the η5-Cp* ligand. The other explanation for the enhanced reactivity of this complex is that the complex can install a pinacol group on the sp2 carbon proximal to the metal centre by borylation. This would be similar to the secondary coordination sphere interactions between boryls and substrate in alkane borylation like that proposed by the Hartwig group.

The work of Mansell, Morton, Evans and coworkers has focused primarily on indenyl rhodium NHC complexes for catalytic C–H activation and borylation (Scheme XXII) (102). It was observed that the NHC used in this reaction played a critical role in the rate of these reactions. The saturated NHC 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene (SIDipp) complex was able to perform both arene and alkane borylation in better yields than both the saturated and unsaturated mesityl-substituted NHC complexes. Further work by the group has investigated the scope of these complexes for the borylations of a range of ether, ester and amide substrates. It was anticipated that the tethering of the NHC to a fluorenyl moiety through a C2H4 linker would enable the formation of better catalysts for C–H activation, however, these complexes were both synthetically challenging to achieve in good yields and performed comparatively worse in the C–H borylation of arenes.

Scheme XXII.

C–H borylation using indenyl rhodium NHC complexes, both tethered and non-tethered

4.2 Methane Borylation

Methane functionalisation is a key challenge due to the high abundance and therefore low cost of methane but represents a particularly challenging goal for C–H activation. A lot of methane is flared at the point of extraction if it cannot be readily transported by pipeline (common in areas far from end users) to avoid releasing methane into the atmosphere due to methane’s high global warming potential. However, this is a waste of a valuable resource, so facile and economic conversion into a liquid product would be very useful. Methane is a more challenging substrate to borylate compared to arenes due to its non-polar nature with unreactive, strong sp3 C–H bonds. Methane is also poorly soluble in many polar and non-polar solvents. The solvents must also be less reactive to C–H activation than methane. The most significant issue with the borylation of methane is the chemoselectivity where any new borane species formed will be potentially more reactive than the bonds in methane, leading to poor selectivity such as overfunctionalisation and overoxidation.

The Mindiola and Sanford groups were able to independently identify catalysts for the C–H activation and borylation of methane (100, 103, 104). The Mindiola group utilised an iridium catalyst with the softer Lewis basic dmpe (Me2PC2H4PMe2)2 ligand (Scheme XXIII). The group were able to overcome the chemoselectivity problems associated with methane borylation to synthesise methyl-BPin with a yield of 52% and a turnover number of 104.

Scheme XXIII.

C–H borylation of methane by the Mindiola group

The Sanford group made use of [RhCp*(C6Me6)] to borylate methane (Scheme XXIV). This work followed density functional theory calculations done by Hall and coworkers in 2005 where they suggested that Cp*Rh complexes were capable of performing this reaction (84).

Scheme XXIV.

C–H borylation of methane by the Sanford group

5. Conclusions

Platinum group metals have long been associated with C–H activation, from the early days of the Shilov system where platinum was used for the catalytic functionalisation of alkanes, to the complexes of the early 1980s where well defined intermolecular C–H activations were performed using rhodium and iridium cyclopentadienyl complexes. Research has now progressed into the realm of catalytic C–H activation and functionalisation, with noble metal complexes still being used and developed. The correct choice of ligands is very important to the design and functionality of complexes for C–H activation, with anionic ligands being purposefully chosen and partnered with the correct neutral co-ligands to create the desired properties. An efficient catalyst for C–H functionalisation brings about the possibility of numerous applications. The products of borylation, boronate esters, are useful chemical intermediates in many medicinal and material syntheses, and progress towards the synthesis of these through C–H borylation is well underway with the selectivity, scope and scale of these reactions increasing. Progress towards the borylation of methane is still a challenge and since the seminal works of both Sanford and Mindiola there have been few additional examples of methane borylation (105).

The drive towards greater sustainability has been seen in many parts of society and is of no less importance here for C–H activation, particularly due to the cost and scarcity of these noble metals. Therefore, it is easy to see that the next challenge in C–H activation will be to perform these reactions in a more sustainable way. This would require the synthesis of much better and longer-lived catalysts or complexes that can be easily reused. Alternatively, C–H activation could be performed using less scarce metals such as iron, the second most abundant element in the earth’s crust. Iron has begun to show promise in recent years for a number of C–H activation and functionalisation reactions (106). There are also a few rare examples of metal-free pathways for C–H activation, however, these reactions remain underexplored (107, 108). It can easily be envisioned that platinum metals will continue to be used as the metals of choice for C–H activation due to their reliability and their great diversity of use within these reactions. However, work should continue to make these reactions more sustainable by designing more robust catalyst systems.

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Acknowledgements

The authors thank the Engineering and Physical Sciences Research Council (EPSRC, PhD studentship to Paul A. Morton), the Scottish Funding Council (Saltire Emerging Researcher Scheme, travel funding for Paul A. Morton) and the Royal Society of Chemistry (Research Fund grant R21-6824221494) for funding. The authors would also like to thank Johnson Matthey plc for the award of platinum group metal materials used in their research.

The Authors

Paul Morton is currently a PhD student at Heriot-Watt University, UK, under the supervision of Stephen Mansell. His research interests focus on the development of unconventional tethered-NHC rhodium complexes, as well as the synthesis of other non-tethered NHC rhodium systems. These complexes are applied to catalytic C–H functionalisation reactions, particularly C–H borylation.

Stephen Mansell is an Assistant Professor in the Institute of Chemical Sciences at Heriot-Watt University. The research interests of the Mansell group focus on harnessing main group and transition metal chemistry in unison in order to develop better catalysts. Specific targets include the application of transition metal phosphinine complexes as catalysts for hydrogen atom processes (hydrogen borrowing, acceptorless dehydrogenation) and rhodium NHC complexes for C–H bond activation and borylation.

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