Charge-Compensated Derivatives of Nido-Carborane

Compared with organic analogs, onium derivatives of nido-carborane have increased stability due to the stabilizing electron-donor action of the boron cage. Charge-compensated derivatives are considered according to the type of heteroatom bonded to a boron atom.


Introduction
The synthesis of the first polyhedral boranes, carboranes, and metallacarboranes in the early 1960s was one of the major highlights in the development of inorganic chemistry over the last century .The first reports on the synthesis of icosahedral carboranes appeared almost sixty years ago, at the end of 1963 when both the United States and the Soviet Union almost simultaneously declassified documents about their boron fuel projects .A few months later, the nucleophile-promoted removal of one boron atom from the icosahedral ortho-carborane cage to form the 11-vertex nidocarborane cage species (Figure 1) was reported .It was one of the most significant findings in the early years of the development of carborane chemistry, and now, more than five decades later, it remains indispensable for the synthesis of numerous metallacarboranes and hydrophilic functionalized carboranes for medical and other applications.Metallacarboranes based on the dicarbollide ligand [7,8-C B H ] , which is formed upon the deprotonation of nidocarborane with strong bases, resemble the well-known transition metal cyclopentadienyl complexes.However, the dicarbollide ligand differs from the cyclopentadienyl ligand in a number of ways.In addition to its 3D character, the dicarbollide ligand is a significantly stronger donor than the cyclopentadienyl one and has a double charge.The donor nature of the dicarbollide ligand can be largely tuned via the introduction of substituents of various natures.At the same time, the charge of the ligand can be partially compensated by introducing into the dicarbollide ligand the so-called charge-compensating substituents of an onium nature (ammonium, phosphonium, sulfonium, etc.).This significantly brings the properties of the dicarbollide and cyclopentadienyl complexes closer together and causes a high interest in metallacarboranes based on charge-compensated dicarbollide ligands .

Charge-Compensated Derivatives of Nido-Carborane with Boron-Nitrogen Bond
Due to the great diversity of nitrogen chemistry, the charge-compensated derivatives of nido-carborane with the B-N bond are characterized by the greatest variety of forms.The first example of the synthesis of charge-compensated derivatives of nido-carborane with a B-N bond was the reaction of the parent nido-carborane with pyridine in benzene in the presence of anhydrous FeCl , leading to the asymmetrically substituted pyridinium derivative 9-Py-7,8-C B H (Scheme 1) , the structure of which was later supported via a single-crystal X-ray diffraction study (Figure 2) .When FeCl      phosphines and phosphites (Scheme 7, Figure 5) .

Charge-Compensated Derivatives of Nido-Carborane with Boron-Arsenic and Boron-Antimony Bonds
The charge-compensated derivatives of nido-carborane with boron-arsenic and boron-antimony bonds are rare and are limited to a few examples.Similar to the triphenylphosphonium derivative, the asymmetrically substituted triphenylarsonium and tetraphenylstilbonium derivatives 9-Ph X-7,8-Ph -7,8-C B H (X = As, Sb) were prepared via electrocatalyzed oxidative couplings of 7,8-diphenyl-nido-carborane with Ph As and Ph Sb, respectively (Scheme 8, Figure 6) .

Charge-Compensated Derivatives of Nido-Carborane with Boron-Oxygen Bond
Alkyloxonium salts are much less stable than ammonium and phosphonium salts, and some of them are used in organic chemistry as strong alkylating agents.Nevertheless, strong electron-withdrawing of polyhedral boron hydride clusters substituted at boron atoms and, in particular, nido-carborane , is capable of stabilizing their oxonium derivatives .
The first example of such a derivative was obtained very soon after the discovery of nido-carborane via the reaction of the potassium salt of nido-carborane with tetrahydrofuran in the presence of FeCl in benzene.As a result, a mixture of two isomeric tetrahydrofuran derivatives of nido-carborane was obtained.The reaction with the C,C′-dimethyl derivative of nido-carborane proceeds in a similar way (Scheme 10) .derivative of nido-carborane was determined using single-crystal X-ray diffraction (Figure 7) .

Charge-Compensated Derivatives of Nido-Carborane with Boron-Sulfur Bond
Compared with the oxonium derivatives, the sulfonium derivatives of nido-carborane are represented by a wider variety of derivatives and synthetic methods for their preparation.However, the most studied of them are the dimethylsulfonium derivatives of nido-carborane, which are widely used for the synthesis of metallacarboranes .
It should be noted that symmetrically and asymmetrically substituted dimethylsulfonium derivatives of nido-carborane are usually obtained in different ways, which excludes the formation of mixtures of their isomers.The asymmetrically
Hydrogen atoms of organic substituent are omitted for clarity.The asymmetrically substituted dimethylsulfonium derivatives 9-Me S-7,8-Me -7,8-C B H and 9-Me S-7,8-µ-(CH OCH )-7,8-C B H were prepared via the reactions of the corresponding nido-carboranes with dimethylsulfide in the presence of Fe(NO ) in aqueous ethanol .In the case of C-substituted nido-carboranes, such as K[7-Ph-7,8-C B H ], the introduction of a Me S group results in a mixture of 9-Me S-7-Ph-7,8-C B H and 11-Me S-7-Ph-7,8-C B H isomers, which can be separated using column