Review

Frontiers in molecular p-block chemistry: From structure to reactivity

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Science  01 Feb 2019:
Vol. 363, Issue 6426, pp. 479-484
DOI: 10.1126/science.aau5105

Abstract

This year marks the 350th anniversary of the discovery of phosphorus by the alchemist Hennig Brand. However, this element was not included in the p-block of the periodic table until more recently. 2019 also marks the 150th anniversary of the preliminary tabular arrangement of the elements into the periodic system by Mendeleev. Of the 63 elements known in 1869, almost one-third of them belonged to what ultimately became the p-block, and Mendeleev predicted the existence of both gallium and germanium as well. The elements of the p-block have a disparate and varied history. Their chemical structure, reactivity, and properties vary widely. Nevertheless, in recent years, a better understanding of trends in p-block reactivity, particularly the behavior of those elements not typically found in biological systems, has led to a promising array of emerging applications, highlighted herein.

The periodic table now comprises many more than 100 elements, each with its own distinctive chemistry. Among the p-block elements, on the right-hand side, the chemistry of carbon, in conjunction with a small number of other elements, has a preeminent position in the discipline. The importance of organic chemistry cannot be understated, as it defines important aspects of biological chemistry, pharmaceuticals, polymers, and materials science. As a consequence, the exploration of organic chemistry has dominated the efforts of chemists over the past two centuries. During this same time frame, the chemistry of the remaining elements in the p-block has garnered much less attention, often with a mere handful of research groups worldwide doing their best to explore and understand the underlying principles that convert a disparate set of observations into a systematic vision of chemical reactivity. This deficiency was further exacerbated by the fact that, in contrast to carbon, for which the chemistry is dominated by the tetravalent state, many other elements form multiple valencies and oxidation states with distinctive reactivity patterns. This leaves our understanding of the chemistry of the other p-block elements far short of that established for carbon.

The range of nonmetals, metalloids, metals, multiple oxidation states, and valencies within the p-block provides a tremendous wealth of structures ranging from covalent molecules and polymers to metals, alloys, and ionic solids with varied properties. This diversity has both fundamental and practical implications, providing insight into the foundation for life on Earth and finding applications in pharmaceuticals, clothing, plastics, or electronic devices. At the top of the p-block, elements typically follow classical bonding models and are the focus of organic chemists, who study the synthesis, structure, reactivity, and properties of chemical compounds primarily constructed of carbon. Even here there are notable exceptions: Boron forms a large family of hydrides where the apparent deficiency in bonding electrons leads to extensive multicenter bonding, in contrast to their classical hydrocarbon counterparts. The heavier p-block elements, however, are very different from their lighter counterparts, displaying unusual bonding patterns and reactivity. The behavior of the heavier p-block elements has been said to resemble that of the transition metals (1, 2). The diverse and sometimes enigmatic reactivity observed within the p-block is often chaotic, inspiring researchers to gain a deeper understanding of structure, bonding, and reactivity, thereby acquiring an enhanced appreciation of these dynamic elements. An understanding of the reactivity of these elements and their compounds can have important subsequent ramifications in the applications of the p-block elements, whether in materials, synthesis, or catalysis. It is this latter aspect of p-block chemistry that has begun to emerge in the past two decades, generating a renaissance in this field. This review highlights the findings that have resulted from, and continue to inspire, the current renewed interest in the chemistry of p-block elements in molecular chemistry from fundamental studies to small-molecule activation and catalysis. Note that the focus on structure and reactivity presented here leads to a large area of solid-state and molecular materials chemistry that is not described. This subject is by no means of lesser importance but simply falls beyond the scope of the current article.

Fundamental studies

Many studies concerning heavier p-block element reactivity have been brought about by variously making comparisons to, or contrasting with, well-established carbon chemistry. Through creating conceptual bridges between inorganic and organic chemistry, a deeper understanding of electronic structure and the ability to predict chemical reactivity can be gained. This includes the structure and bonding in cluster compounds, p-block polymers, and multiple bonded systems (35). Over the past few decades, the isolobal analogy has provided a theoretical foundation for rapid experimental developments in the chemistry of the p-block (6). For example, the synthesis of urea in 1828 is often considered the foundation of modern organic chemistry (7). Nonetheless, it was not until recently that a phosphorus-containing urea derivative, H2PC(=O)NH2, was synthesized from the anion [O–C≡P] (Fig. 1A) (8, 9). Indeed, one area of focus has been the synthesis of new p-block reagents such as [O–C≡P]. For example, white phosphorus (P4) is a common industrial reagent for the synthesis of value-added phosphorus-containing compounds. Yet the synthesis of these compounds, which involves the reaction of P4 with chlorine gas (10), is both energy intensive and hazardous. The search for safer, cheaper, and more environmentally friendly alternative reagents has prompted the development of the bis(trichlorosilyl)phosphide anion, [P(SiCl3)2] (Fig. 1A), which can be used in the synthesis of a variety of phosphorus-containing compounds (11). Other reagents for incorporating p-block elements into compounds have included phosphinonitrenes (Fig. 1A). This first isolable nonmetallic nitrene derivative was found to transfer a nitrogen atom to isopropyl isonitrile, affording a carbodiimide (12).

Fig. 1 Fundamental studies.

(A) Novel p-block reagents. (B) Multiple bonds. (C) Heterocycles. (D) Nucleophilic group 13 compounds. Dipp = 2,6-diisopropylphenyl.

The strength of multiple bonding is a common theme of organic chemistry and is facilitated by efficient orbital overlap associated with smaller 2p orbitals. In these systems, the strength of π bonding is comparable to that of σ bonding. On descending the p-block, the more radially expanded nature of the p-orbitals leads to a reduced overlap integral and weaker bonds, and σ bonds become substantially favored over π bonds. The isolation of heavier main-group compounds containing multiple bonds has therefore been of fundamental interest. These compounds are expected to have much-enhanced reactivity (relative to their carbon analogs) arising from the inherent weakness of these multiple bonds. A common strategy to stabilize these multiply bonded species is the use of sterically demanding substituents to protect low-valent, reactive main-group centers. These bulky groups typically prevent polymerization or oligomerization of monomeric units and preclude attack by nucleophiles or electrophiles (3). Following the first report of the tin analog of an alkene in the 1970s (13), other group 14 alkene derivatives, including the landmark disilene (Fig. 1B) (14), were synthesized. Whereas alkenes adopt a planar geometry, the heavier group 14 analogs, R2E=ER2 (R = alkyl or aryl; E = Si, Ge, Sn, or Pb), deviate from planarity, reflecting a change in bonding pattern. Such compounds can be considered as dimers of R2E, where the group 14 R2E unit is amphoteric, possessing both a Lewis basic lone pair and a vacant Lewis acidic orbital. Likewise, the heavier geometrically distorted group 14 derivatives of alkynes, RE≡ER, have also been developed, although more sterically demanding substituents were required (3).

A large sample of compounds containing homonuclear and heteronuclear multiple bonds for heavier p-block elements (principal quantum number n > 2) has now been obtained (3). However, whereas the first-row elements carbon and nitrogen readily form strong triple bonds, boron does not. A recent breakthrough in this area was the stabilization of a boron-boron triple bond. Within the B≡B unit, each boron is Lewis acidic, and addition of a strong donor such as an N-heterocyclic carbene (NHC) generates a L→B≡B←L compound (L = NHC ligand) analogous to an alkyne (Fig. 1B) (15). Such an interpretation has been challenged by the proposition that the π character of the NHC ligand is conjugated with the triple bond to afford an alternative resonance form, R2C=B=B–CR2 (16).

Although aromatic species are ubiquitous in organic chemistry, aromaticity in main-group heterocycles has been more controversial. Borazine (B3N3H6) is considered aromatic but with much lower aromatic stabilization than the isoelectronic benzene (17). The potentially aromatic nature of many planar heavy p-block heterocycles has been discussed, a recent example being the development of the diphosphatriazolate P2N3 heterocyclic anion (Fig. 1C) (18). A similar CNP3 heterocycle was reported more recently, synthesized from the reactions of a cyclotriphosphine (tBuP)3 (tBu = tert-butyl) with nitriles to access 1-aza-2,3,4-triphospholenes (Fig. 1C) (19). The 2-phosphaethynolate anion, [O–C≡P], described earlier has also been found useful as a potential building block for the synthesis of heterocyclic organophosphorus compounds such as phosphinin-2-olates (Fig. 1C) through stepwise cycloaddition reactions (20). The cycloaddition reactions of this anion can be contrasted with the related cycloaddition chemistry of isolobal, [S=N=S]+ (21).

Another strategy to uncover distinctive chemistry involves targeting inverse, or umpolung, reactivity of p-block elements. For example, whereas group 13 elements typically act as Lewis acids, boryl anions can behave as both bases and nucleophiles. For instance, reduction of N,N′-bis(2,6-diisopropylphenyl)-2-bromo-2,3-dihydro-1H-1,3,2-diazaborole leads to a cyclic boryl anion (Fig. 1D) (22). This species, which is isoelectronic with an NHC, exhibits nucleophilic behavior in the reactions with alkyl halides, aldehydes, water, and methyl trifluoromethanesulfonate. Organoboranes incorporating the –BPin (Pin = pinacolate) functionality are widely used intermediates in organic synthesis and in the synthesis of pharmaceuticals, and finding optimal routes to incorporate boron into organic molecules is of paramount importance (23, 24). Though [BPin] functionalities are traditionally electrophilic, examples of nucleophilic sources of the [BPin] moiety are known (25), including the recently reported magnesium boryl derivatives (Fig. 1D) (26). Another area of interest in group 13 chemistry has been approaches to generate triels with low oxidation states. Whereas the inert pair effect makes species such as In+ stable and Lewis basic (nucleophilic), univalent group 13 elements such as B(I) and Al(I) are rare but are expected to offer substantially different reactivity from that of their compounds in the more common Lewis acidic (electrophilic) +3 oxidation state. For example, nucleophilic boron(I) compounds, which are isoelectronic to amines, can be synthesized by reduction of a CAAC→BBr3 [CAAC = cyclic (alkyl)(amino)carbene] adduct generating the borylene (H–B:) (Fig. 1D) (27). The CAACs act as strong π-acceptor ligands to stabilize the lone pair on boron (28). Lewis basicity of the boron lone pair was demonstrated through protonation. As ligands, these species could potentially act as better donors than amines or phosphines, owing to their higher nucleophilicity. Further down group 13, nucleophilic low–oxidation state aluminum species can also be developed, such as the dimethylxanthene-stabilized potassium Al(I) compound (Fig. 1D), which reacts with benzene by C–H oxidative addition (29).

Small-molecule activation

The requirement of energetically accessible occupied and unoccupied orbitals for the activation of small molecules was previously thought to limit such reactivity to the d-block metals. In 2005, the addition of hydrogen to a main-group center was reported: Specifically, the germanium alkyne analog ArGe≡GeAr (Ar = 2,6-(2,6-iPr2C6H3)2C6H3) was shown to undergo oxidative addition with H2, yielding Ar(H)Ge=Ge(H)Ar, Ar(H)2Ge-Ge(H)2Ar, and Ge(H)3Ar (30). Similar studies with other low-valent group 13 and 14 compounds including aluminum, gallium, and tin have subsequently been reported (Fig. 2A) (1). The activation of dihydrogen in these cases involves synergistic bonding and backbonding interactions of the frontier orbitals, whereby the σ orbital of H2 donates to a low-lying vacant orbital on the p-block system, and the highest occupied molecular orbital of the p-block system donates to the H2 σ* orbital (Fig. 2B). Analogous activation of dihydrogen at a single carbon site of a CAAC and an acyclic (alkyl)(amino)carbene species was reported in 2007 (Fig. 2A) (31). This reactivity results from the much more nucleophilic and electrophilic nature of these carbenes relative to the archetypical NHCs (28). In contrast to the transition metal oxidative addition of H2, the carbene behaves as a nucleophile donating to the H–H σ* orbital, prompting hydride delivery to the vacant orbital on the carbon center (Fig. 2B). The distinctive electronic nature of CAACs also allows them to undergo reversible oxidative addition of C–B bonds (32) as well as to activate ammonia (28), a reaction typically not observed with transition metals because of adduct formation.

Fig. 2 Dihydrogen activation.

(A) Activation of dihydrogen with p-block elements [Ar = 2,6-(2,6-iPr2C6H3)2C6H3; iPr = iso-propyl]. (B) Mechanisms of activation. Filled orbitals are shown in blue and donor orbitals in red. M = metal; LA = Lewis acid; LB = Lewis base; E = p-block element.

The underlying principle in chemical reactivity of Lewis acidity and basicity has been known for almost 100 years. Typically, when paired, the Lewis base will donate to the vacant orbital of the Lewis acid to form an adduct. However, in 2006, through the use of sterically demanding groups, unquenched Lewis acidic and basic sites were identified in p-(Mes2P)C6F4(B(C6F5)2) (Mes = mesityl). This compound was shown to activate hydrogen, yielding the phosphonium borate p-(Mes2PH)C6F4(BH(C6F5)2) (Fig. 2A) (33). Notably, this compound also releases dihydrogen above 100°C, regenerating the original phosphine borane. Such reversibility of substrate binding is a key requirement for enabling catalytic turnover (vide infra). This finding spawned the field of frustrated Lewis pair (FLP) chemistry (34). For just more than a decade, the concept of FLPs has altered the way we think about the reactivity of the p-block elements, prompting new reagent design strategies and discovery of ensuing reactivity. Many different FLPs are now known, employing diverse p-block bases commonly derived from the group 15 or 16 elements and p-block Lewis acids usually based on the group 13 or cationic group 13, 14, or 15 elements (34). Although rarer, FLPs containing s- or d-block elements as the Lewis acid or base are also known (35). The mechanism of activation of H2 by FLPs has been the subject of several computational studies in which theory predicts a trimolecular mechanism involving H2 and a Lewis acid and Lewis base (36, 37). The FLP tBu3P/B(C6F5)3 was calculated to proceed through a two-electron diamagnetic mechanism, whereby the Lewis acid and base approach each other, forming a tBu3P···B(C6F5)3 “encounter complex” in which the two species are separated by 4.2 Å. In this complex, the Lewis acid and base have their frontier orbitals correctly aligned for dative bond formation, but adduct formation is precluded on the basis of steric hindrance. In this configuration, the system is perfectly set up for synergic interactions with dihydrogen. Inclusion of dihydrogen in the encounter complex results in simultaneous polarization of H2 through donation of the lone pair of electrons on the Lewis base into the H–H σ* orbital and concurrent donation of the electrons in the H–H σ bond into the boron vacant p orbital (Fig. 2B). This leads to protonation of the phosphine and hydride delivery to the borane. This mechanism, which pertains to many FLP systems, is comparable to that exhibited by bifunctional transition metal complexes (38) and is conceptually similar to the synergic interactions described for other main-group systems and transition metal centers. More recently, an alternative single-electron transfer mechanism for FLP activation of dihydrogen has been suggested on the basis of the Mes3P/B(C6F5)3 FLP (39).

Although the activation of dihydrogen by p-block systems has garnered much attention, the activation of other small gaseous molecules, including acetylene, ethane, CO, CO2, NO, N2O, and SO2, has also been sought (40). These studies have been driven by curiosity about which other molecules reactive p-block elements can activate, as well as by the demand for new practical systems that can bind, sequester, and potentially transform these molecules. The binding of carbon monoxide by transition metals via synergic σ bonding and π backbonding interactions is a key concept in undergraduate inorganic textbooks. However, monoadducts and multicarbonyls [E]-(CO)x outside the d-block are very uncommon. The borane dicarbonyl complex TpB(CO)2 [Tp = 2,6-di(2,4,6-triisopropylphenyl)-phenyl] was prepared only recently via liberation of a borylene ligand from [(OC)5Mo(BTp)] (41). The bonding in this compound, which is isoelectronic with (CAAC)2BH described earlier, is reminiscent of that for metals in which there is extensive backbonding from the filled p-orbital on boron into the C≡O π* orbital (41).

Reactions of p-block elements with CO2 have garnered attention. Activation of CO2 has been achieved via the use of both multiple-bonded compounds and FLPs (1, 2, 34). In the latter systems, activation of CO2 proceeds through a cooperative action of the Lewis base at carbon and the Lewis acid at an oxygen atom (40). Through judicious choice of the Lewis acid and base, hydrogenation of the CO2-bound intermediate affords CH3OH (42). As with FLPs, heteronuclear E=E′ bonds also react with CO2, owing to differences in electronegativity of the elements and polarity of the double bond (1). Conversely, the corresponding reactions of homonuclear E=E bonds are much rarer. However, aromatic diazadiborinine compounds have been shown to undergo [4+2] additions with CO2, and a boron-boron double bond was found to react with CO2 through an initial [2+2] cycloaddition. (43, 44).

Reactions of p-block elements such as selenium, iodine, and boron with C–C π bonds have received much attention for applications in organic synthesis. However, reactions with gaseous ethane and ethylene are much less studied, yet multiple bonds of aluminum and tin have been shown to react with alkenes, yielding formal [2+2] addition products (2, 45, 46). Another application that exploits the reactions of p-block elements with C–C π bonds is in gas separation. The separation of saturated and unsaturated hydrocarbons is critical to industries such as petroleum refining. FLPs are unreactive with alkanes but undergo addition to alkenes. Using segmented gas–liquid microfluidic flow, the tBu3P/B(C6F5)3 FLP was shown to be an efficient reagent to separate ethylene and ethane mixtures (Fig. 3A). This work may pave the way to effective methods to separate gases, provided that routes to subsequently release the ethylene and recycle the FLP can be found (47).

Fig. 3 Small-molecule activation.

(A) Separation of ethylene and ethane using FLPs. (B) Activation of N2 using borylenes through bonding and backbonding interactions and the N2 activated complex (crystal structure).

Of all small molecules, N2 is one of the most difficult to activate, even for transition metals. The Haber-Bosch process has been in use for more than a century, and although the process is energy intensive, viable alternatives remain elusive. Very recently, p-block elements have been shown to assist d-block elements in the activation and functionalization of N2. Specifically, the strong Lewis acid B(C6F5)3 has been shown to weaken the N–N bond in M–N2 (M = Fe, Mo, W) complexes to allow for subsequent reaction through protonation, hydroboration, or hydrosilylation of the N2 unit (48, 49). In a truly seminal finding, the CAAC stabilized borylene [(CAAC)BDur] (Dur = 2,3,5,6-tetramethylphenyl) was very recently found to activate N2 in the absence of transition metals (Fig. 3B) (50). The product displays a substantially elongated N–N bond as the two boron centers act in a “push-pull” fashion to weaken the N≡N bond. This discovery represents an exciting leap forward in the chemistry of the p-block. It is likely that in the future other p-block systems will be targeted for N2 activation and conversion.

Catalysis

The development of novel catalysts and improvements in catalytic performance have a direct influence on society and play a pivotal role in maintaining the quality of everyday human life. The d-block elements have dominated catalytic processes; however, many catalysts rely on scarce, often toxic precious metals (51). This prompts interest in the potential of p-block catalysts to replace metals, reducing purification costs and lowering toxicity while simultaneously offering distinctive selectivity or reactivity. Organocatalysis is now well established (52) but, with the exception of simple Lewis acid catalysis (using, e.g., AlCl3), the use of other p-block elements in homogeneous catalysis is a more recent area of exploration. The small-molecule activation studies described earlier were crucial in the realization that catalytic processes could be developed with p-block elements. Notably, the studies that demonstrated the use of a perfluorinated aryl borane catalyst in hydrosilylation foreshadowed the broader use of p-block elements in catalysis (53). Indeed, the advent of FLP activation of hydrogen prompted metal-free hydrogenation catalysis. In the first study, the intramolecular FLPs (R2P)C6F4(B(C6F5)2) (R = Mes, tBu) were found to catalyze the reduction of imines to amines (Fig. 4A). (54) This study led to a range of FLPs being developed and applied in metal-free hydrogenation of a variety of substrates. Key developments in this regard include the FLP-catalyzed hydrogenation of ketones using ethereal solvents as the Lewis base (55, 56) as well as the use of FLPs to catalyze the highly chemo- and stereoselective hydrogenation alkynes to cis-alkenes (57). For applications in organic synthesis, chiral FLP catalysts have been targeted for application to asymmetric reductions. For example, chiral bis-borane (58) and chiral aminoborane (59) catalysts have been found to give enantioselectivities of up to 90% in the FLP reduction of prochiral imines. In addition to hydrogenation catalysis, FLPs have also proven effective in C–H dehydrogenative borylation of heteroarenes (60), a reaction typically performed by transition metal catalysis (24).

Fig. 4 p-Block catalysis.

(A) FLP-catalyzed hydrogenation of imines and alkynes. (B) Catalytic CO2 reduction. (C) Phosphonium catalysis. Et = ethyl. (D) PIII/PV redox cycling. (E) Stereoselective hypervalent iodine catalysis.

Of particular interest have been catalytic methods to transform CO2 into functional hydrocarbons using p-block elements. Whereas most reported systems are typically heterogeneous metal-based catalysts, the stoichiometric fixation of CO2 by FLPs suggests their potential in the catalytic reduction of CO2. The o-phenylene bridged catalyst 1-BCat-2-PPh2-C6H4 (BCat = catecholboryl; Ph = phenyl) was capable of catalyzing the hydroboration of CO2 to methoxy boranes with excellent turnover frequencies and turnover numbers (Fig. 4B) (61). The catalytic N-formylation of amines with CO2 has also been achieved in a metal-free manner to yield synthetically useful formamides via an aromatic diazadiborinine catalyst (Fig. 4B) (62).

In the search for other p-block Lewis acid catalysts, the salt [(C6F5)3PF][B(C6F5)4] was developed (63). The cation was found to have an extremely high Lewis acidity owing to the four electron withdrawing substituents, resulting in a low-lying P–F σ* orbital. This high electrophilicity of the cation enables activation of C(sp3)–F bonds. Approaches to degrade fluorocarbons or to functionalize C–F bonds are generally challenged by the high dissociation energy. In addition to other reactions, this phosphonium catalyst has been found to catalyze hydrodefluorination reactions as well as hetero-dehydrocoupling reactions between p-block E–H and E′–H bonds (E = O, N, S; E′ = Si) (Fig. 4C) (64).

In contrast to metal-based catalysis, catalytic reactions using p-block elements do not generally involve a change in oxidation state. Recently, however, phosphorus(III) compounds have been used in C–C coupling reactions reminiscent of that in Pd-catalyzed process, which involves a phosphorus(V) intermediate (65). This reaction, however, is stoichiometric as the starting phosphine cannot be regenerated. Nonetheless, catalytic systems based on a PIII/PV redox couple have been developed and employed (66). Similarly, behaving as biphilic reagents, small-ring phosphacycles (phosphetanes) have been shown to be active in PIII/PV=O redox cycling in oxygen atom–transfer catalysts. The four-membered 1,2,2,3,4,4-hexamethylphosphetane P-oxide was found to be an excellent catalyst for N−N bond-forming heterocyclization reactions in the Cadogan indazole synthesis (67), as well as for intramolecular C–N heterocyclization of o-nitrobiaryl and -styrenyl compounds to access carbazole and indole products (Fig. 4D) (68).

Although hypervalent iodine compounds have long been applied in organic synthesis, these compounds are typically used in stoichiometric quantities (69). Nevertheless, examples of catalytic reactions are emerging with the use of stoichiometric oxidants to regenerate the hypervalent iodine reagent (Fig. 4E). The choice of oxidant in these reactions is crucial because the oxidant must selectively oxidize the iodine species and not the substrate. For example, meta-chloroperbenzoic acid (mCPBA) can be used as the oxidant to regenerate hypervalent iodine(III) reagents for the spirocyclization of phenol derivatives. Via this method, only 5 mole % of iodotoluene is needed to generate the lactone products (70). Similar conditions have been used to enable the α-oxygenation of carbonyl compounds using a catalytic amount of an iodoarene with mCPBA as the oxidant (71). An improvement to this reaction, which frequently relies on harsh conditions and generates meta-chlorobenzoic acid (mCBA) waste, was later reported whereby the oxidative coupling reactions of carbonyl compounds with carboxylic acids could be achieved using catalytic amounts of tetra-butylammonium (hypo)iodite and an environmentally benign oxidant (either hydrogen peroxide or tert-butyl hydroperoxide) (72). Hydrogen peroxide was also used as the oxidant to oxidize chiral quaternary ammonium iodide salts in situ to yield the active (hypo)iodite, which could be employed as a catalyst for the asymmetric oxidative cycloetherification of ketophenols to yield biologically relevant 2-acyl-2,3-dihydrobenzofuran derivatives (Fig. 4E) (73).

Outlook

It is clear from this short review that the renaissance of main-group chemistry is accelerating as fundamentally new principles and reactivity continue to emerge with our developing understanding of the diverse reactivity of the p-block. Moreover, p-block chemists will use insights gleaned from the broad base of knowledge surrounding transition metal chemistry to design and uncover new applications in synthetic chemistry. Though p-block catalysts will undoubtedly present their own challenges, the promise of new reactivity and the potential for commercial applications keep the chemistry of the p-block elements perpetually exciting.

References and Notes

Acknowledgments: Funding: R.L.M. is grateful to the EPSRC for an Early Career Research Fellowship (EP/R026912/1). Competing interests: The author declares no competing interests.

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