Cationic ligands - from monodentate to pincer systems

For a long time, the small group of cationic ligands stood out as obscure systems within the general landscape of coordinative chemistry. However, this situation has started to change rapidly during the last decade, with more and more examples of metal-coordinated cationic species being reported. The growing interest in these systems is not only of purely academic nature, but also driven by accumulating evidence of their high catalytic utility. Overcoming the inherently poor coordinating ability of cationic species often required additional structural stabilization. In numerous cases this was realized by functionalizing them with a pair of chelating side-arms, effectively constructing a pincer-type scaffold. This comprehensive review aims to encompass all cationic ligands possessing such pincer architecture reported to date. Herein every cationic species that has ever been embedded in a pincer framework is described in terms of its electronic structure, followed by an in-depth discussion of its donor/acceptor properties, based on computational studies (DFT) and available experimental data (IR, NMR or CV). We then elaborate on how the positive charge of these ligands affects the spectroscopic and redox properties, as well as the reactivity, of their complexes, compared to those of the structurally related neutral ligands. Among other systems discussed, this review also surveys our own contribution to this field, namely, the introduction of sulfonium-based pincer ligands and their complexes, recently reported by our group.

Dimeric Rh Complexes Supported by a Bridging Phosphido/Bis(Phosphine) PPP Ligand

Rh complexes of a tridentate PPP ligand bearing 1,2-pyrrolediyl linkers have been prepared, including examples with the central P donor being either a phosphine or a phosphide. Three bimetallic Rh complexes containing the diamandoid Rh2P2 core (P = phosphido) have been structurally and spectroscopically characterized. The Rh-Rh interaction in these three dimers was examined by way of structural comparisons and DFT investigations.

Experimental and theoretical investigation of the reactivity of [(BDI*)Ti(Cl){η2-P(SiMe3)-PiPr2}] towards selected ketones

In this work, we report a new type of reactivity of [(BDI*)Ti(Cl){η2-P(SiMe3)-PiPr2}] (1) towards ketones (BDI* = 2,6-diisopropylphenyl-β-methyldiketiminate ligand). In the reaction of 1 with acetone, cyclopentanone or cyclohexanone, a ketone moiety is inserted into Ti-Pphosphanyl or Ti-Pphosphido bonds to form complexes with a new C-P-P moiety, providing [(BDI*)Ti(Cl){η2-P(SiMe3)-PiPr2-C(Me)2O}] (2a), [(BDI*)Ti(Cl){η2-OC(Me)2P(SiMe3)-PiPr2}] (2b), [(BDI*)Ti(Cl){η2-P(SiMe3)-P(iPr)2-{C(CH2)4}O}] (3a), and [(BDI*)Ti(Cl){η2-P(SiMe3)-P(iPr)2-{C(CH2)5}O}] (4a). Starting complex 1 reacts with cyclohexanone, yielding a monocrystalline complex [{(ArN[double bond, length as m-dash]C(Me)CHC(Me)[double bond, length as m-dash]NAr)C(CH2)5O}Ti(Cl){PiPr2-P(SiMe3)C(CH2)5O}] (4d) with the insertion of two ketone molecules. Interestingly, we found that monoinserted complexes 2a and 3a may be oxidized via a reaction with AgCl, leading to elimination of the -SiMe3 group and oxidation of the titanium atom. This reaction led us to isolate the Ti(iv) complex [(BDI*)Ti(Cl){η2-P-P(iPr)2-{C(CH2)5}O}] (5) in crystalline form. To identify the kinds of products that may be formed and determine which products are the most energetically favoured ones, we conducted a thermodynamic DFT study of 1 towards acetone, cyclopentanone and cyclohexanone. Structures 2a, 2b, 3a, 3e, 4a, 4d, and 5 were characterized by X-ray crystallography, and complex 5 was also identified by NMR spectroscopy.

Metal–ligand cooperativity of a Co–P moiety

Metal–ligand cooperative redox reactions and intramolecular group transfer of a P–P containing dicobalt(i) species are shown.

NI(ii) phosphine and phosphide complexes supported by a PNP-pyrrole pincer ligand

The reaction between [(PN^pyrP)NiCl] (1, PN^pyrP = 2,5-bis((di-iso-propylphosphino)-methyl)-1H-pyrrolide) and TlPF₆ in the presence of a monodentate phosphine ligand led to cationic nickel phosphine and phosphite complexes, [(PN^pyrP)Ni(PHPh₂)][PF₆] (2), [(PN^pyrP)Ni(PMe₃)][PF₆] (3), and [(PN^pyrP)Ni{P(OMe)₃}][PF₆] (4). Compound 2 can be deprotonated resulting in the generation of a terminal phosphido complex, [(PN^pyrP)Ni(PPh₂)] (5). When 3 is subjected to a base, a methyl proton of PMe₃ is abstracted to afford [(PN^pyrP)Ni(CH₂PMe₂)] (6), containing a methylene bridge between Ni and the external phosphine. Compounds 2-6 were characterized by single crystal X-ray diffraction in addition to multi-nuclear NMR spectroscopy and elemental analysis.

Palladium(ii) complexes featuring a mixed phosphine–pyridine–iminophosphorane pincer ligand: synthesis and reactivity

Cationic and neutral Pd complexes featuring an original phosphine–pyridine–iminophosphorane (PNN) ligand were synthesised. In particular, an unexpected borylated complex was isolated by reaction with B(C₆F₅)₃.

Understanding the Solution and Solid-State Structures of Pd and Pt PSiP Pincer-Supported Hydrides

The PSiP pincer-supported complex ((Cy)PSiP)PdH [(Cy)PSiP = Si(Me)(2-PCy2-C6H4)2] has been implicated as a crucial intermediate in carboxylation of both allenes and boranes. At this stage, however, there is uncertainty regarding the exact structure of ((Cy)PSiP)PdH, especially in solution. Previously, both a Pd(II) structure with a terminal Pd hydride and a Pd(0) structure featuring an η(2)-silane have been proposed. In this contribution, a range of techniques were used to establish that ((Cy)PSiP)PdH and the related Pt species, ((Cy)PSiP)PtH, are true M(II) hydrides in both the solid state and solution. The single-crystal X-ray structures of ((Cy)PSiP)MH (M = Pd and Pt) and the related species ((iPr)PSiP)PdH [(iPr)PSiP = Si(Me)(2-P(i)Pr2-C6H4)2] are in agreement with the presence of a terminal metal hydride, and the exact geometry of ((Cy)PSiP)PtH was confirmed using neutron diffraction. The (1)H and (29)Si{(1)H}NMR chemical shifts of ((Cy)PSiP)MH (M = Pd and Pt) are consistent with a structure containing a terminal hydride, especially when compared to the chemical shifts of related pincer-supported complexes. In fact, in this work, two general trends relating to the (1)H NMR chemical shifts of group 10 pincer-supported terminal hydrides were elucidated: (i) the hydride shift moves downfield from Ni to Pd to Pt and (ii) the hydride shift moves downfield with more trans-influencing pincer central donors. DFT calculations indicate that structures containing a M(II) hydride are lower in energy than the corresponding η(2)-silane isomers. Furthermore, the calculated NMR chemical shifts of the M(II) hydrides using a relativistic four-component methodology incorporating all significant scalar and spin-orbit corrections are consistent with those observed experimentally. Finally, in situ X-ray absorption spectroscopy (XAS) was used to provide further support that ((Cy)PSiP)MH exist as M(II) hydrides in solution.

Secondary diphosphine and diphosphido ligands: synthesis, characterisation and group 1 coordination compounds

Two types of secondary diphosphines, 1,8-(ArPH)2C14H8 (1a: Ar = Tripp, 2,4,6-triisopropylphenyl; 1b: Ar = Mes, 2,4,6-trimethylphenyl) and 1,3-((t)BuPHCH2)2C6H4 (2), based on rigid 1,8-anthracene and flexible m-xylyl frameworks, respectively, have been synthesized using different strategies. Compounds 1a and 1b were formed by nucleophilic aromatic substitution of a potassium organophosphido salt onto 1,8-difluoroanthracene, while compound 2 was obtained by addition of the Grignard reagent [1,3-(ClMgCH2)2C6H4]x to a dichloroorganophosphine, followed by reduction to the diphosphine. These compounds were isolated as ca. 1 : 1 mixtures of rac and meso diastereomers as determined by multinuclear NMR spectroscopy. Borane and selenide derivatives of 2, 1,3-((t)BuPH(BH3)CH2)2C6H4 (3) and 1,3-((t)BuPH(Se)CH2)2C6H4 (4), were obtained. Preferential crystallization of one diastereomer of 3 and 4 was observed; X-ray crystallographic studies identified this as the rac isomer for diselenide 4. Metallation studies of compounds 1a and 2 yielded several alkali metal salts. The reaction of KH or K metal with 1a yielded the compounds 1,8-(TrippPK)2C14H8·xTHF (5) and 1,8-(TrippPK)2C14H10·xTHF (6), respectively; in complex 6 the central aromatic ring has been reduced to yield a bent dihydroanthracene backbone. A crown ether derivative of 6, [K(18-crown-6)(THF)2]2[1,8-(TrippP)2C14H10] (7), was characterised crystallographically. Double deprotonation of compound 2 with (n)BuLi/TMEDA (TMEDA = N,N,N',N'-tetramethylethylenediamine) afforded the yellow dilithium complex 1,3-((t)BuPLiCH2)2C6H4·TMEDA (8), which crystallized as a dimer featuring lithium-arene π interactions.

Noninnocent behavior of bidentate amidophosphido [NP]2- ligands upon coordination to copper

The synthesis and preliminary coordination chemistry of two new redox-active bidentate ligands containing amido and phosphido donors are described. Treatment of the [(R)NP](2-) (R = Ph, 2,4,6-trimethylphenyl) ligands with CuCl2 and PMe3 results in a dimeric copper(I) P-P coupled product via ligand oxidation. The intermediate of this reaction is proposed to involve a ligand radical generated via oxidation of the [(R)NP](2-) ligand by copper(II), and the existence of such an intermediate is probed using computational methods. Significant radical character on the phosphorus atoms of the alleged [(R)NP](•-)/copper(I) intermediate leads to P-P radical coupling.

Bis{bis-[2-(diisopropyl-phosphan-yl)phen-yl]phosphanido-κ(3) P,P',P''}chloridonickel(II)

In the title compound, [Ni(C₂₄H₃₆P₃)Cl], the Ni^II atom adopts a distorted square-planar geometry with the two neutral P atoms of the tridentate ligand trans to one another. Bond lengths and angles of the phosphide P atom feature a pyramidal geometry of the donor atom, which forms a single bond with the Ni^II atom, retaining a stereochemically active lone pair.

'Hemilabile' silyl pincer ligation: platinum group PSiN complexes and triple C-H activation to form a (PSiC)Ru carbene complex

The first example of PSiN mixed-donor silyl pincer ligation is described. Studies involving platinum group metal complexes of [(2-(t)Bu(2)PC(6)H(4))(2-Me(2)NC(6)H(4))SiMe](-) ((t)Bu-PSiN-Me) confirmed that the ligand amino donor is labile. Within the coordination sphere of Ru, (t)Bu-PSiN-Me is transformed into a PSiC ligand via multiple C-H bond activation events.