Cr(II) and Cr(I) PCP Pincer Complexes: Synthesis, Structure, and Catalytic Reactivity

Dinitrogen activation at chromium by photochemically induced CrII-C bond homolysis

The synthesis of the organochromium(II) complexes [POCOPtBu]Cr(R) (R = p-Tol, Bn) is reported. Exposure of [POCOPtBu]Cr(Bn) to visible light promoted homolytic Cr-CBn bond cleavage and formed {[POCOPtBu]Cr}2(η1:η1μ-N2) via a putative [POCOPtBu]Cr(I) species.

Cr(II) and Cr(III) NCN pincer complexes: synthesis, structure, and catalytic reactivity

The synthesis, characterization, and reactivity of several new Cr(II) and Cr(III) complexes featuring an NCN pincer ligand with an arene backbone connected to amine donors NEt2 and NiPr2 via CH2-linkers is described. Reacting the in situ lithiated ligand precursor N(C-Br)NCH2-Et with [CrCl3(THF)3] resulted in the formation of the Cr(III) complex trans-[Cr(κ3NCN-NCNCH2-Et)(Cl)2(THF)]. Upon reaction of lithiated N(C-Br)NCH2-iPr with a suspension of anhydrous CrCl2, the Cr(II) complex [Cr(κ2NC-NCNCH2-iPr)2] is formed featuring two NCN ligands bound in κ2NC-fashion. In contrast, when lithiated N(C-Br)NCH2-iPr is reacted with a homogeneous solution of anhydrous CrX2 (X = Cl, Br), complexes [Cr(κ3NCN-NCNCH2-iPr)X] are obtained. Treatment of [Cr(κ3NCN-NCNCH2-iPr)Cl] with 1 equiv of PhCH2MgCl and LiCH2SiMe3 afforded the alkyl complexes [Cr(κ3NCN-NCNCH2-iPr)(CH2Ph)] and [Cr(κ3NCN-NCNCH2-iPr)(CH2SiMe3)]. All Cr(II) complexes exhibit effective magnetic moments in the range of 4.7-4.9 µB which is indicative for d4 high spin systems. If a solution of lithiated N(C-Br)NCH2-iPr is treated with CrCl2, followed by addition of an excess of Na[HB(Et)3], the dimeric complex [Cr(κ2NC-NCNCH2-iPr)(μ2-H)]2 is obtained bearing two bridging hydride ligands. [Cr(κ3NCN-NCNCH2-iPr)(CH2SiMe3)] turned out to be catalytically active for the hydrosilylation of ketones at room temperature with a catalyst loading of 1 mol%. X-ray structures of all complexes are presented.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s00706-023-03128-6.

Synthesis, characterization, and liquid injection field desorption ionization mass spectrometry analysis of pincer ligated group 6 (Cr, Mo, W) carbonyl complexes

We report the synthesis of molybdenum and tungsten bromo dicarbonyl complexes (POCOP^tBu)M^IIBr(CO)₂ (M  =  Mo or W; POCOP^tBu  =  κ³-C₆H₃-1,3-[OP(tBu)₂]₂) supported by an anionic PCP pincer ligand, and the chromium complex (PNP^tBu)Cr⁰(CO)₃ (PNP^tBu  =  2,6-bis(di-tert-butyl-phosphinomethyl)pyridine) bearing a neutral PNP pincer scaffold. The three group six complexes described in this study have been characterized by Liquid Injection Field Desorption Ionization Mass Spectrometry (LIFDI-MS), NMR, and IR spectroscopy. Single crystal X-ray diffraction studies show the Mo^II and W^II complexes adopt a six-coordinate distorted trigonal prismatic geometry, whereas the Cr⁰ complex exhibits a distorted octahedral geometry.

Scandium, titanium and vanadium complexes supported by PCP-type pincer ligands: synthesis, structure, and styrene polymerization activity

A series of first-row early transition metal dialkyl complexes bearing pincer ligands [(POCOP)M(CH₂SiMe₃)₂] (POCOP: (2,6-(^tBu₂PO)₂-C₆H₃); 1-Sc: M = Sc; 1-Ti: M = Ti; 1-V: M = V) and [(PCP)M(CH₂SiMe₃)₂] (PCP: (2,6-(^tBu₂PCH₂)₂-C₆H₃); 2-Sc: M = Sc; 2-Ti: M = Ti) have been synthesized. These dialkyl complexes were characterized by single-crystal X-ray diffraction, NMR spectroscopy, and solution magnetic susceptibility (Evans method) analyses appropriately. All the complexes exhibited square pyramidal geometries with different extents of distortion. The activities of these complexes were further explored in styrene polymerization, in which combinations of scandium complexes (1-Sc or 2-Sc) with [Ph₃C][B(C₆F₅)₄] were found to be active catalytic systems for highly syndiospecific (>99% rrrr) polymerization of styrene. Meanwhile, the Ti(III) complexes 1-Ti and 2-Ti showed rather low activity in styrene polymerization, which stands in sharp contrast to those in previous reports involving Ti(III) catalysts bearing cyclopentadienyl derivative ligands.

Synthesis and reactivity of titanium ‘POCOP’ pincer complexes

Titanium ‘POCOP’ complexes have been made, and their ability to support further reactivity investigated, giving a rare isolable titanium chlorohydride.

Manganese and iron PCP pincer complexes - the influence of sterics on structure and reactivity

The syntheses of various manganese and iron PCP pincer complexes via a solvothermal oxidative addition methodology is described. Upon reacting [Mn₂(CO)₁₀] with the ligands (P(C-Br)P^CH₂-iPr) (1a) and (P(C-Br)P^O-iPr) (1b), Mn(I) PCP pincer complexes [Mn(PCP^CH₂-iPr)(CO)₃] (2a) and [Mn(-PCP^O-iPr)(CO)₃] (2b) were obtained. Protonation of 2a with HBF₄·Et₂O led to the formation of [Mn(κ³P,CH,P-P(CH)P^CH₂-iPr)(CO)₃]BF₄ (3) featuring an η²-C_aryl-H agostic bond. The solvothermal reaction of 1a with [Fe₂(CO)₉] afforded the Fe(II) PCP pincer complex [Fe(PCP^CH₂-iPr)(CO)₂Br] (4). Treatment of 4 with Li[HBEt₃] afforded the Fe(I) complex [Fe(PCP^CH₂-iPr)(CO)₂] (5a). When using the sterically more demanding ligands (P(C-Br)P^CH₂-tBu) (1c) and (P(C-Br)P^O-tBu)(1d) striking differences in reactivity were observed. While neither 1c nor 1d showed any reactivity towards [Mn₂(CO)₁₀], the reaction with [Fe₂(CO)₉] and [Fe(CO)₅] led to the formation of the Fe(I) complexes [Fe(PCP^CH₂-tBu)(CO)₂] (5b) and [Fe(PCP^O-tBu)(CO)₂] (5c). X-ray structures of representative complexes are provided.

Azanone (HNO): generation, stabilization and detection

HNO (nitroxyl, azanone), joined the 'biologically relevant reactive nitrogen species' family in the 2000s. Azanone is impossible to store due to its high reactivity and inherent low stability. Consequently, its chemistry and effects are studied using donor compounds, which release this molecule in solution and in the gas phase upon stimulation. Researchers have also tried to stabilize this elusive species and its conjugate base by coordination to metal centers using several ligands, like metalloporphyrins and pincer ligands. Given HNO's high reactivity and short lifetime, several different strategies have been proposed for its detection in chemical and biological systems, such as colorimetric methods, EPR, HPLC, mass spectrometry, fluorescent probes, and electrochemical analysis. These approaches are described and critically compared. Finally, in the last ten years, several advances regarding the possibility of endogenous HNO generation were made; some of them are also revised in the present work.

Record High Magnetic Anisotropy in Three-Coordinate MnIII and CrII Complexes: A Theoretical Perspective

Ab initio calculations performed in two three-coordinate complexes [Mn{N(SiMe₃)₂}₃] (1) and [K(18-crown-6) (Et₂O)₂][Cr{N(SiMe₃)₂}₃] (2) reveal record-high magnetic anisotropy with the D values -64 and -15 cm^-1, respectively, enlisting d⁴ ions back in the race for single-ion magnets. A detailed spin-vibrational analysis performed of 1 and 2 suggests dominance under barrier relaxation due to the flexible coordination spheres around the metal ion. Furthermore, several in silico models were constructed by varying the nature of donor atoms based on the X-ray structure of 1 and 2, unveiling much larger anisotropy and robust single-ion magnet (SIM) characteristics for some of the models offering design clues for low-coordinate transition-metal SIMs.

Bringing Homogeneous Iron Catalysts on the Heterogeneous Side: Solutions for Immobilization

Noble metal catalysts currently dominate the landscape of chemical synthesis, but cheaper and less toxic derivatives are recently emerging as more sustainable solutions. Iron is among the possible alternative metals due to its biocompatibility and exceptional versatility. Nowadays, iron catalysts work essentially in homogeneous conditions, while heterogeneous catalysts would be better performing and more desirable systems for a broad industrial application. In this review, approaches for heterogenization of iron catalysts reported in the literature within the last two decades are summarized, and utility and critical points are discussed. The immobilization on silica of bis(arylimine)pyridyl iron complexes, good catalysts in the polymerization of olefins, is the first useful heterogeneous strategy described. Microporous molecular sieves also proved to be good iron catalyst carriers, able to provide confined geometries where olefin polymerization can occur. Same immobilizing supports (e.g., MCM-41 and MCM-48) are suitable for anchoring iron-based catalysts for styrene, cyclohexene and cyclohexane oxidation. Another excellent example is the anchoring to a Merrifield resin of an FeII-anthranilic acid complex, active in the catalytic reaction of urea with alcohols and amines for the synthesis of carbamates and N-substituted ureas, respectively. A SILP (Supported Ionic Liquid Phase) catalytic system has been successfully employed for the heterogenization of a chemoselective iron catalyst active in aldehyde hydrogenation. Finally, FeIII ions supported on polyvinylpyridine grafted chitosan made a useful heterogeneous catalytic system for C-H bond activation.

POCOP-type cobalt and nickel pincer complexes bearing an appended phosphinite group

The reaction of 1,3,5-(iPr2PO)3C6H3 with Co2(CO)8 leads to the isolation of a POCOP-type mononuclear pincer complex {κP,κC,κP-2,4,6-(iPr2PO)3C6H2}Co(CO)2 (1) or a tetranuclear species {κP-{κP,κC,κP-2,4,6-(iPr2PO)3C6H2}Co(CO)2}2Co2(CO)6 (2), depending on the ligand to cobalt ratio employed. The latter compound can be an impurity during the synthesis of {2,6-(iPr2PO)2-4-Me2N-C6H2}Co(CO)2, when the ligand precursor 5-(dimethylamino)resorcinol is contaminated with phloroglucinol due to incomplete monoamination. Similarly, the reaction of 1,3,5-(iPr2PO)3C6H3 with NiCl2 in the presence of 4-dimethylaminopyridine provides {κP,κC,κP-2,4,6-(iPr2PO)3C6H2}NiCl (3) bearing an appended phosphinite group. Structures 1–3 have been studied by X-ray crystallography.

Synthesis, Characterization, and Catalytic Reactivity of {CoNO}8 PCP Pincer Complexes

The reaction of coordinatively unsaturated Co(II) PCP pincer complexes with nitric oxide leads to the formation of new, air-stable, diamagnetic mono nitrosyl compounds. The synthesis and characterization of five- and four-coordinate Co(III) and Co(I) nitrosyl pincer complexes based on three different ligand scaffolds is described. Passing NO through a solution of [Co(PCP^NMe-iPr)Cl], [Co(PCP^O-iPr)Cl] or [Co(PCP^CH2-iPr)Br] led to the formation of the low-spin complex [Co(PCP-iPr)(NO)X] with a strongly bent NO ligand. Treatment of the latter species with (X = Cl, Br) AgBF₄ led to chloride abstraction and formation of cationic square-planar Co(I) complexes of the type [Co(PCP-iPr)(NO)]⁺ featuring a linear NO group. This reaction could be viewed as a formal two electron reduction of the metal center by the NO radical from Co(III) to Co(I), if NO is counted as NO⁺. Hence, these systems can be described as {CoNO}⁸ according to the Enemark–Feltham convention. X-ray structures, spectroscopic and electrochemical data of all nitrosyl complexes are presented. Preliminary studies show that [Co(PCP^NMe-iPr)(NO)]⁺ catalyzes efficiently the reductive hydroboration of nitriles with pinacolborane (HBpin) forming an intermediate {CoNO}⁸ hydride species.