Cationic Terminal Aminoborylene Complexes: Controlled Stepwise Insertion into MB and BN Double Bonds

Nucleophilic reactivity of the gold atom in a diarylborylgold(i) complex toward polar multiple bonds

A di(o-tolyl)borylgold complex was synthesized via the metathesis reaction of a gold alkoxide with tetra(o-tolyl)diborane(4). The resulting diarylborylgold complex exhibited a Lewis acidic boron center and a characteristic visible absorption that arises from its HOMO-LUMO excitation, which is narrower than that of a previously reported dioxyborylgold complex. The diarylborylgold complex reacted with isocyanide in a stepwise fashion to afford single- and double-insertion products and a C-C coupled product. Reactions of this diarylborylgold complex with C[double bond, length as m-dash]O/N double bond species furnished addition products under concomitant formation of Au-C and B-O/N bonds, which suggests nucleophilic reactivity of the gold metal center. DFT calculations provided details of the underlying reaction mechanism, which involves an initial coordination of the C[double bond, length as m-dash]O/N bond to the boron vacant p-orbital of the diarylboryl ligand followed by a migration of the gold atom from the tetracoordinate sp3-hybridized boron center, which is analogous to the reactivity of the conventional sp3-hybridized borate species. The DFT calculations also suggested a stepwise mechanism for the reaction of this diarylborylgold complex with isocyanide, which afforded three different reaction products depending on the applied reaction conditions.

Isolation of ligand-centered borocations in molybdenum complexes containing a triaminoborane-bridged diphosphorus ligand

Metal complexes that form isolable, ligand-centered borenium ions (i.e. reactive three-coordinate boron cations) are rare, especially with highly-versatile diphosphorus ligands. Here we report the first structurally-characterized examples of ligand-centered borocations in a class of diphosphorus ligands derived from the bicyclic triaminoborane 1,8,10,9-triazaboradecalin (TBD). Treating (PhTBDPhos)Mo(CO)4 (1) with HOTf or HNTf2 resulted in protonation of the bridgehead α-nitrogen on the TBD backbone and formation of ligand-centered borenium ions (1-HOTf and 1-HNTf2, respectively), whereas reaction of 1 with HBF4·Et2O resulted in protonation of two α-nitrogen atoms and fluoride abstraction from BF4- to form a four-coordinate borocation in [1-H2F][BF4]. Single-crystal XRD data confirmed the formation of the borocations, and multinuclear NMR and IR spectroscopy studies were used to interrogate the electronic environment at molybdenum and boron. The 11B NMR resonances for the borenium ions in 1-HOTf and 1-HNTf2 (δ 29.5 and 29.6 ppm) were more deshielded than the resonance for 1 (δ 25.9 ppm), consistent with the decreased electron density at boron. The 31P NMR data revealed similar trends in response to increasing protonation on the TBD backbone, and aligned well with small, stepwise increases in Mo-CO stretching frequencies that followed the order borane (1) < borenium (1-HOTf and 1-HNTf2) < boronium ([1-H2F][BF4]). Density functional theory calculations conducted on 1, 1-HOTf, and [1-H2F][BF4] revealed subtle changes in boron and nitrogen atomic charges consistent with those calculated for more well-established borenium ions in metal-free systems. Overall, the results confirm previous observations of latent borenium ion reactivity in TBDPhos complexes.

Interactions of Isonitriles with Metal-Boron Bonds: Insertions, Coupling, Ring Formation, and Liberation of Monovalent Boron

Boryl, borylene, and base-stabilized borylene complexes of manganese and iron undergo a range of different reactions when treated with isonitriles including single, double, and partial isonitrile insertions into metal-boron bonds, ring formation, isonitrile coupling, and the liberation of new monovalent boron species. Two of the resulting cyclic species have also been found to react selectively with anhydrous HCl to form ring-opened products. The diverse isonitrile-promoted reactivity of transition-metal-boron compounds has been explored computationally.

Reactive p-block cations stabilized by weakly coordinating anions

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.

Iminoborylene complexes: evaluation of synthetic routes towards BN-allenylidenes and unexpected reactivity towards carbodiimides

The iminoborylene complex [CpFe(PCy₃)(CO)(BNCMes₂)]⁺ undergoes MB metathesis reactivity with carbodiimides, resulting in FeB cleavage and the formation of isonitrile complexes.

Activation of boryl-, borylene and metalloborylene complexes by isonitriles

One or several isonitriles can insert into metalloboryl and metalloborylene complexes to afford cyclic or spiro compounds with bonding controlled by the electron count at the metal and in the ring.

Reductive borylene-CO coupling with a bulky arylborylene complex

Partial metal-boron bond cleavage and coupling of a borylene with two CO ligands was observed upon reduction of a new bulky arylborylene complex. Both the borylene precursor and dianionic product were structurally and spectroscopically characterized. In contrast, reduction of an aminoborylene complex led to complete loss of the borylene ligand and classical Hieber reduction. A rationale for these differences based on DFT methods is presented.

Transition metal borylene complexes: boron analogues of classical organometallic systems

In the ten years since the first examples of terminal borylene complexes were reported, rapid advances in the chemistry of this ligand class have been made, and parallels emerged with classical organometallic ligand systems (such as carbenes, vinylidenes and even CO) as well as with other subvalent main group ligand systems. This article reviews key developments in synthetic, structural and reaction chemistry, with particular attention focusing on recent discoveries in the field.

Insertion reactions of dicyclohexylcarbodiimide with aminoboranes, -boryls and -borylenes

Insertion reactions of dicyclohexylcarbodiimide with aminoboranes and with aminoboryl and -borylene transition metal complexes have been examined as potential routes to new boron-containing ligand systems. Reactions with systems containing two-coordinate boron centres are found to be significantly more facile than those with three-coordinate substrates. Thus, reaction of (dicyclohexylamino)boron dichloride () with dicyclohexylcarbodiimide over 36 h at 50 degrees C generates the (structurally authenticated) guanidinate complex Cy(2)NC(NCy)(2)BCl(2) () via insertion into the BN bond. By contrast, the corresponding reaction with the cationic aminoborylene complex [CpFe(CO)(2)(BNCy(2))](+)[BAr(f)(4)](-) () proceeds rapidly at ca.-30 degrees C, via initial insertion into the FeB bond to give [CpFe(CO)(2)C(NCy)(2)BNCy(2)](+)[BAr(f)(4)](-) (). Consistent with related studies, a key factor in facilitating such insertion chemistry is thought to be the formation of an initial donor/acceptor complex between the diimide and the group 13 centre. Thus, DFT studies suggest that [CpFe(CO)(2)B(NCy(2))(CyNCNCy)](+)[BAr(f)(4)](-) is a potential intermediate in the reaction of with CyNCNCy, and that further reaction to give the observed product, , is strongly exergic (-183 kJ mol(-1)). By contrast, DFT calculations for the alternative isomer [CpFe(CO)(2)B(CyN)(2)CNCy(2)](+)[BAr(f)(4)](-) (), formed by BN insertion, suggest that it is 112 kJ mol(-1) less stable than . Such experimental and computational findings imply that under reaction conditions where a suitable isomerisation pathway is available, cationic complexes such as , which contain a four-membered boron-donor heterocycle are likely to be disfavoured with respect to alternative C-bound isomers.