Several Al-based complexes supported by a bis(imino)aryl NCN pincer ligand have been prepared. Systematic structural and experimental evidence identified a cationic species, supported by a THF molecule, as the best Lewis acid precatalyst for a range of Diels–Alder cycloadditions. This highlighted the importance of fully isolating and characterizing the active species in any catalytic process especially when Lewis acids are used due to the presence of hidden Brønsted acids (HBAs). Finally, the control experiments conducted in order to eliminate HBA activity that involve a bulky pyridine base should be performed with caution as the corresponding protonated pyridinium salt could also serve as a proton source.

The first example of a phosphenium trication has been prepared by using the exceptional nucleophilic properties of a carbodicarbene ligand. According to theoretical investigations the trication contains quite polarized P–C bonds suggesting a substantial contribution from the dative bond model. As one of the resonance forms for the title compound depicted a hypervalent phosphoranide we also showed that phosphoranides, in general, do not contain a hypervalent P centre.

This work expands on the recent synthesis and reactivity of the two-coordinate P(III)-containing dication [((Ph3P)2C)P(NiPr2)]2+ (3a2+). Two additional dications of general formula [((RPh2P)2C)P(NR′2)]2+ (3b2+, R = 1/2 (CH2)3, R′ = iPr; 3c2+, R = Ph, R′ = Cy) have been prepared. Solution and solid-state 31P NMR isotropic shifts for this dication class were observed around 360 ppm. Except for [3a][SbF6]2, the 31P CP/MAS NMR spectra reveal a high degree of structural disorder around the dicationic site in the solid state. Surprisingly, the synthesis of dication 3a2+ was only possible in the presence of AlCl4–, SbF6–, and substituted tetraarylborates, while the use of ClO4–, OTf–, BF4–, PF6–, and BPh4– resulted in complex product mixtures. Single-crystal X-ray analysis of dications 3a2+ and 3c2+ suggested a 4π allyl-like structure for the central CPN fragment, and this has been supported through detailed theoretical investigations. Initial reactivity studies between dications 3a2+/3c2+ and PMe3 show an equilibrium between the free dication and the target adduct. Furthermore, 3a2+ has been shown to be capable of O–H bond activation using water and methanol and polymerizes THF when it is dissolved in this solvent.

C–F bond cleavage by transient phosphorus(III)-based dications [RP(C(PPh3)2)]2+ (4a2+, R = Ph; 4b2+, R = 4-F-Ph) is reported. These dications were generated by reaction of the corresponding monocationic precursors with excess Na[BAr4Cl]. Evidence for the existence of transient dicationic species was obtained by trapping the dication 4a2+ with PMe3. According to theoretical analysis, the low-lying lowest unoccupied molecular orbitals of these species were responsible for the observed activation of C–F bonds.

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.

Ligand substitution is not a common procedure for the preparation of different borenium cations. This work demonstrates that the chloride ligands of several NHC-stabilized dichloroborenium cations [NHC·BCl2][X] (NHC = (R′C)2(NR)2C; 1, R = iPr, R′ = H; 2, R = iPr, R′ = Me; 3, R = tBu, R′ = H; X = AlCl4, B(3,5-Cl2-C6H3)4) could be replaced with a catecholato moiety to produce [NHC·Bcat][X]. According to single-crystal X-ray analyses this particular ligand exchange enhanced the Lewis acidity of the target borocations with respect to the dichloro precursors.

Hypervalent boron centers are proposed to be key intermediates in many stoichiometric and catalytic reactions. However, structurally characterized examples remain rare. We have isolated two new borocations with formal charges of 1+ and 2+. Because the dicationic complex displays evidence of pentacoordination at the boron center, we conclude that the interaction is predominantly electrostatic and is a result of the highly electrophilic dicationic boron atom.

The synthetic viability of several N-heterocyclic carbene stabilized dichloroborenium cations [NHC·BCl2]+ (NHC = (RC)2(NR′)2C; 1, R = R′ = Me; 2, R = H, R′ = iPr; 3, R = Me, R′ = iPr; 4, R = H, R′ = tBu; 5, R = H, R′ = 2,6-iPr2-C6H3) in the presence of Cl–, AlCl4–, OTf– (Tf = O2SCF3), NTf2–, and [BArCl4]− (ArCl = 3,5-Cl2-C6H3) was investigated. None of the target borocations could be synthesized in the presence of Cl–, as only neutral NHC·BCl3 compounds were observed. On the other hand, it was not surprising that all targeted cations were synthetically viable in the presence of AlCl4– but a different degree of interion interaction was evident from 11B NMR experiments. This was confirmed by X-ray analyses of [1·BCl2]+, [2·BCl2]+, and [3·BCl2]+ in the presence of AlCl4– counterions, as the degree of cation–anion interaction was dependent on the steric encumbrance of the corresponding NHCs. Apart from [4·BCl2]+, no borocation was synthetically viable when OTf– and NTf2– were used as the counterions. Finally, we were able to show that only [4·BCl2]+ could be synthetically viable without the counterion stabilization effect(s) as the preparation of [4·BCl2][BArCl4] was achieved. Even though the presence of [3·BCl2][BArCl4] was detected, this compound appeared not to be thermally stable, as it decomposed in solution after 48 h. The thermal stability of [4·BCl2]+ and instability of [3·BCl2]+ in the presence of [BArCl4]− was attributed to the presence and absence, respectively, of very weak intraion (agostic) interactions in these two borocations.

•Several Ru-base complexes bearing various Cp ligands have been prepared.•The most sterically demanding complex was not synthesized due to steric reasons.•All prepared complexes were tested for isomerization of 1-hexene.•Only thermodynamic equilibrium mixtures containing 2- and 3-hexene were obtained.Preparation of a series of cyclopentadienyl- and imidazolyl-phosphine-containing Ru-based complexes bearing a different degree of the Cp-ring methylation has been attempted. According to experimental and structural data the steric factors prevented the formation of the last complex in the series that contains permethylated Cp ring. These complexes were then subjected to alkene isomerization using 1-hexene. The rate of isomerization decreased, in general, with the increase in the Cp-ring methylation suggesting that the initial alkene coordination and/or imidazolyl N decoordination steps are restricted in the overall mechanism.

The synthesis, characterization and X-ray analysis of dichloro- and dibromo-borenium cations stabilized by a 4-membered carbene are reported. The ligand's structural changes, atypical for similar systems, were caused by coordination to electron deficient fragments and its CN2P ring strain.

Several Al-based complexes supported by a bis(imino)aryl NCN pincer ligand have been prepared. Systematic structural and experimental evidence identified a cationic species, supported by a THF molecule, as the best Lewis acid precatalyst for a range of Diels–Alder cycloadditions. This highlighted the importance of fully isolating and characterizing the active species in any catalytic process especially when Lewis acids are used due to the presence of hidden Brønsted acids (HBAs). Finally, the control experiments conducted in order to eliminate HBA activity that involve a bulky pyridine base should be performed with caution as the corresponding protonated pyridinium salt could also serve as a proton source.

The first example of a phosphenium trication has been prepared by using the exceptional nucleophilic properties of a carbodicarbene ligand. According to theoretical investigations the trication contains quite polarized P–C bonds suggesting a substantial contribution from the dative bond model. As one of the resonance forms for the title compound depicted a hypervalent phosphoranide we also showed that phosphoranides, in general, do not contain a hypervalent P centre.