The chemistry of the p-block elements is a huge playground for fundamental and applied work. With their bonding from electron deficient to hypercoordinate and formally hypervalent, the p-block elements represent an area to find terra incognita. Often, the formation of cations that contain p-block elements as central ingredient is desired, for example to make a compound more Lewis acidic for an application or simply to prove an idea. This review has collected the reactive p-block cations (rPBC) with a comprehensive focus on those that have been published since the year 2000, but including the milestones and key citations of earlier work. We include an overview on the weakly coordinating anions (WCAs) used to stabilize the rPBC and give an overview to WCA selection, ionization strategies for rPBC-formation and finally list the rPBC ordered in their respective group from 13 to 18. However, typical, often more organic ion classes that constitute for example ionic liquids (imidazolium, ammonium, etc.) were omitted, as were those that do not fulfill the – naturally subjective – “reactive”-criterion of the rPBC. As a rule, we only included rPBC with crystal structure and only rarely refer to important cations published without crystal structure. This collection is intended for those who are simply interested what has been done or what is possible, as well as those who seek advice on preparative issues, up to people having a certain application in mind, where the knowledge on the existence of a rPBC that might play a role as an intermediate or active center may be useful.

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Reactive p-block cations stabilized by weakly coordinating anions

The imidazolin-2-imino group is an N-heterocyclic imino functionality that derives from the class of compounds known as guanidines. The exocyclic nitrogen atom preferably bonds to electrophiles and its electron-donating character is markedly enhanced by efficient delocalization of cationic charge density into the five-membered imidazoline ring. Thus, this imino group is an excellent choice for thermodynamic stabilization of electron-deficient species. Due to the variety of available imidazoline-based precursors to this ligand, its steric demand can be tailored to meet the requirements for kinetic stabilization of otherwise highly reactive species. Consequently, it does not come as a surprise that the imidazolin-2-iminato ligand has found widespread applications in transition-metal chemistry to furnish pincer complexes or “pogo stick” type compounds. In comparison, the field of main-group metal compounds of this ligand is still in its infancy; however, it has received growing attention in recent years. A considerable number of electron-poor main-group element species have been described today which are stabilized by N-heterocyclic iminato ligands. These include low-valent metal cations and species that are marked by formerly unknown bonding modes. In this article we provide an overview on the present chemistry of main-group element compounds of the imidazolin-2-iminato ligand, as well as selected examples for the related imidazolidin- and benzimidazolin-2-imino system.

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Applications of N-heterocyclic imines in main group chemistry

3,4-dihydro-2H-1,2,4,3-triazaborol-3-yl-lithium 3 was synthesized and fully characterized. The 11B NMR spectrum, X-ray diffraction analysis, and computational studies revealed the ionic nature of the B–Li bond, and indeed 3 displays nucleophilic property which allowed preparation of a series of 1,2,4,3-triazaborol-3-yl-metal complexes (Al; 5, Au; 6, Zn; 7, Mg; 8, Sb; 9, and Bi; 10). 3 reacted with CO (1 atm) and various isonitriles under ambient condition, and mechanistic study suggests that the reactions with CO and aryl isonitriles proceed via an insertion of CO and isonitrile carbon into the B–Li bond followed by isomerization to yield transient carbene species, one of which was confirmed by trapping with S8. With PhNC, compounds 5 and 7·(thf) underwent exchange of THF molecule coordinating to the metal center with isonitrile, whereas insertion of isonitrile carbon occurred at the B–Bi bond in 10 which afforded stable bismuth (boryl)iminomethane 20.

Isolation of 1,2,4,3-Triazaborol-3-yl-metal (Li, Mg, Al, Au, Zn, Sb, Bi) Derivatives and Reactivity toward CO and Isonitriles

Boron-containing aromatic systems exhibit unique electronic properties and reactivities that have been extensively studied for a long time. This review highlights the recent developments in the synthesis of aromatic boron-containing heterocycles. The organization of the contents is based on the sizes of rings and the heteroatoms other than boron. Early work in the field is briefly introduced, but the main focus is on recent reports published during the period of 2008 through 2016. 1 	Introduction 2 	Five-Membered Rings 2.1 	Borole Derivatives 2.2 	B,N-Heterocycles 2.3 	B,O-Heterocycles 3 	Six-Membered Rings 3.1 	Borabenzene Derivatives 3.2 	Boratabenzene Derivatives 3.3 	1,4-Diborabenzene 3.4 	B,N-Heterocycles 3.5 	B,E-Heterocycles (E = O, S, P, Te) 4 	Three-Membered Rings 4.1 	Borirenes 4.2 	Azadiboriridines 4.3 	Triboracyclopropenyl Dianion 5 	Four-Membered Rings 5.1 	bicyclo-Tetraborane(4) 5.2 	1,3-Diborete 5.3 	B,E-Heterocycles (E = N, P, As, Sb, Bi, O, S, Se) 6 	Seven-Membered Rings (Borepines) 7 	Conclusion and Perspective

Construction of Boron-Containing Aromatic Heterocycles

The reaction of the bulky bis(imidazolin-2-iminato) ligand precursor (1,2-(L(Mes)NH)2-C2H4)[OTs]2 (1(2+)  2[OTs](-); L(Mes) = 1,3-dimesityl imidazolin-2-ylidene, OTs = p-toluenesulfonate) with lithium borohydride yields the boronium dihydride cation (1,2-(L(Mes)N)2-C2H4)BH2[OTs] (2(+)  [OTs](-)). The boronium cation 2(+)  [OTs](-) reacts with elemental sulfur to give the thioxoborane salt (1,2-(L(Mes)N)2-C2H4)BS[OTs] (3(+)  [OTs](-)). The hitherto unknown compounds 1(2+)  2[OTs](-), 2(+)  [OTs](-), and 3(+)  [OTs](-) were fully characterized by spectroscopic methods and single-crystal X-ray diffraction. Moreover, DFT calculations were carried out to elucidate the bonding situation in 2(+) and 3(+). The theoretical, as well as crystallographic studies reveal that 3(+) is the first example for a stable cationic complex of three-coordinate boron that bears a B=S double bond.

Isolation of a three-coordinate boron cation with a boron-sulfur double bond.

We disclose the synthesis and structural characterization of the first acid-free anionic oxoborane, [K(2.2.2-crypt)][(HCDippN)2BO] (1) (Dipp = 2,6-iPr2C6H3), which is isoelectronic with classical c...

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An Acid-Free Anionic Oxoborane Isoelectronic with Carbonyl: Facile Access and Transfer of a Terminal B═O Double Bond.

1,2,4,3-Triazaborole-based neutral oxoborane stabilized by a Lewis acid

Carbonyl compounds (R 2 C=O) are cornerstones of organic chemistry. In sharp contrast, main group carbonyl analogues (R 2 E=O) derived from p -block elements from groups 13 to 15, have long been considered to be elusive species. Employment of chemical tricks such as acid/base stabilization protocols granted access to these transient species in their masked forms. However, electronic and steric effects inevitably perturb their chemical reactivity and distinguish them from classical carbonyl compounds. A new era was marked by the recent isolation of acid-base (i.e. acid and base) free main group carbonyl analogues, ranging from a lighter boracarbonyl to the heavier silacarbonyls, phosphacarbonyls and a germacarbonyl. Most importantly, the rich chemistry of these unperturbed main group carbonyl compounds has begun to emerge, spanning the vista from classical organic carbonyl-type reactions to transition metal-like oxide ion transfer chemistry. The anionic nature of boracarbonyl can be employed to make a strong donor ligand while the cationic phosphacarbonyl exhibits potent Lewis acidity. On the other hand, neutral sila/germacarbonyls display more ambiphilic behaviour. In this review, we survey the strategies used for the isolation of such systems and document their emerging reactivity profiles, with a view to providing fundamental comparisons both with carbon and transition metal oxo species. This highlights the emerging opportunities for exciting 'crossover' reactivity offered by these derivatives of the p -block elements.

https://onlinelibrary.wiley.com/doi/epdf/10.1002/anie.202008174

Acid-Base Free Main Group Carbonyl Analogues.

Introduced here is a new type of strongly donating N-heterocyclic boryloxy (NHBO) ligand, [(HCDippN)2 BO]- (Dipp=2,6-diisopropylphenyl), which is isoelectronic with the well-known N-heterocyclic iminato (NHI) donor class. This 1,3,2-diazaborole functionalized oxy ligand has been used to stabilize the first acyclic two-coordinate dioxysilylene and its Ge, Sn, and Pb congeners, thereby presenting the first complete series of heavier group 14 dioxycarbene analogues. All four compounds have been characterized by X-ray crystallography and density-functional theory, enabling analysis of periodic trends: the potential for the [(HCDippN)2 BO]- ligand to subtly vary its electronic-donor capabilities is revealed by snapshots showing the gradual evolution of arene π coordination on going from Si to Pb.

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An N‐Heterocyclic Boryloxy Ligand Isoelectronic with N‐Heterocyclic Imines: Access to an Acyclic Dioxysilylene and its Heavier Congeners

A novel dicationic system containing a PN fragment has been synthesized and structurally characterized. According to the solid-state analysis and theoretical investigation, the dicationic iminophosphane resonance form is the most appropriate description for the dication. However, the contribution from the phosphorus mononitride resonance form is not negligible.

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Ying Kai Loh

Papers

An Acid-Free Anionic Oxoborane Isoelectronic with Carbonyl: Facile Access and Transfer of a Terminal B═O Double Bond.

1,2,4,3-Triazaborole-based neutral oxoborane stabilized by a Lewis acid

Acid-Base Free Main Group Carbonyl Analogues.

An N‐Heterocyclic Boryloxy Ligand Isoelectronic with N‐Heterocyclic Imines: Access to an Acyclic Dioxysilylene and its Heavier Congeners

A dicationic iminophosphane.