Protic Ligands Research Articles

Electrocatalytic CO2 reduction to HCO2H by protic NHC-Ir complexes

In electrochemical CO2-conversion strategies, various transition metal-based molecular electrocatalysts are employed to reduce CO2 into products like CO and HCO2H, with H2 as a competitive side product. However, achieving selectivity towards HCO2H remains challenging, and suitable catalysts for the same are limited. Herein we report a normal and an abnormal protic-NHC-based Cp*Ir(III)-half sandwich complexes for catalytic CO2 electroreduction in an aqueous acetonitrile solvent. Both the catalysts predominantly produced HCO2H as the CO2-reduced product at an applied potential of –2.66 V vs Fc+/Fc with 5 % H2O as the proton source; however, the normal protic-NHC-bound complex achieved a Faradic efficiency (FE) of 86±4 %, while the complex with the abnormal protic-NHC ligand furnished FE up to 72±4 %. The protic proton of the protic NHC ligand in these complexes was proposed to participate in a proton relay process, facilitating generation of the crucial Ir–H intermediate, which reacts with CO2 to produce HCO2H through stabilization of the Ir–OCHO intermediate.

Recent Developments in Reactions and Catalysis of Protic Pyrazole Complexes.

Protic pyrazoles (N-unsubstituted pyrazoles) have been versatile ligands in various fields, such as materials chemistry and homogeneous catalysis, owing to their proton-responsive nature. This review provides an overview of the reactivities of protic pyrazole complexes. The coordination chemistry of pincer-type 2,6-bis(1H-pyrazol-3-yl)pyridines is first surveyed as a class of compounds for which significant advances have made in the last decade. The stoichiometric reactivities of protic pyrazole complexes with inorganic nitrogenous compounds are then described, which possibly relates to the inorganic nitrogen cycle in nature. The last part of this article is devoted to outlining the catalytic application of protic pyrazole complexes, emphasizing the mechanistic aspect. The role of the NH group in the protic pyrazole ligand and resulting metal-ligand cooperation in these transformations are discussed.

Asking Ligands to Lend a Hand

In classical transition-metal-catalysts, the ligand is a spectator, while all key transformations such as oxidative addition, β-hydride elimination and reductive elimination occur at the metal center. Herein, the ligand plays no direct role in the catalytic cycle and its function is limited to stabilizing the metal center and tuning its coordination environment. In recent years, participating ligands, which play an active role during the course of reaction, have received greater attention. The use of such ligands has allowed for the development of more active/selective catalysts and opened up new reaction pathways. There are many ways in which a ligand can influence the metal center to accelerate the catalytic transformation. For example, hemilabile ligands allow reversible dissociation of a donor unit to adapt to the stereoelectronic requirements of the reaction intermediates.Redox-non-innocent ligands participate in transferring electrons to/from the metal. There is a class of ligands that directly participate in bond making/breaking and undergo reversible chemical transformation during the catalytic cycle.These modes of ligand cooperativity have been discussed in many excellent reviews including those by Caulton,Milsteinand Grützmacher. Herein, we aim to address two distinct strategies to achieve ‘ligand-driven-chemistry’ – the use of (a) electronically asymmetric ligand, and (b) protic ligands. A brief discussion on the role of ligands in water activation for functionalization of organic compounds is given at the end.

Ruthenium Complexes with Protic Ligands: Influence of the Position of OH Groups and π Expansion on Luminescence and Photocytotoxicity

Protic ruthenium complexes using the dihydroxybipyridine (dhbp) ligand combined with a spectator ligand (N,N = bpy, phen, dop, Bphen) have been studied for their potential activity vs. cancer cells and their photophysical luminescent properties. These complexes vary in the extent of π expansion and the use of proximal (6,6'-dhbp) or distal (4,4'-dhbp) hydroxy groups. Eight complexes are studied herein as the acidic (OH bearing) form, [(N,N)2Ru(n,n'-dhbp)]Cl2, or as the doubly deprotonated (O- bearing) form. Thus, the presence of these two protonation states gives 16 complexes that have been isolated and studied. Complex 7A, [(dop)2Ru(4,4'-dhbp)]Cl2, has been recently synthesized and characterized spectroscopically and by X-ray crystallography. The deprotonated forms of three complexes are also reported herein for the first time. The other complexes studied have been synthesized previously. Three complexes are light-activated and exhibit photocytotoxicity. The log(Do/w) values of the complexes are used herein to correlate photocytotoxicity with improved cellular uptake. For Ru complexes 1-4 bearing the 6,6'-dhbp ligand, photoluminescence studies (all in deaerated acetonitrile) have revealed that steric strain leads to photodissociation which tends to reduce photoluminescent lifetimes and quantum yields in both protonation states. For Ru complexes 5-8 bearing the 4,4'-dhbp ligand, the deprotonated Ru complexes (5B-8B) have low photoluminescent lifetimes and quantum yields due to quenching that is proposed to involve the 3LLCT excited state and charge transfer from the [O2-bpy]2- ligand to the N,N spectator ligand. The protonated OH bearing 4,4'-dhbp Ru complexes (5A-8A) have long luminescence lifetimes which increase with increasing π expansion on the N,N spectator ligand. The Bphen complex, 8A, has the longest lifetime of the series at 3.45 μs and a photoluminescence quantum yield of 18.7%. This Ru complex also exhibits the best photocytotoxicity of the series. A long luminescence lifetime is correlated with greater singlet oxygen quantum yields because the triplet excited state is presumably long-lived enough to interact with 3O2 to yield 1O2.

Accessing Unusual Reactivity through Chelation-Promoted Bond Weakening.

Highly reducing Sm(II) reductants and protic ligands were used as a platform to ascertain the relationship between low-valent metal-protic ligand affinity and degree of ligand X-H bond weakening with the goal of forming potent proton-coupled electron transfer (PCET) reductants. Among the Sm(II)-protic ligand reductant systems investigated, the samarium dibromide N-methylethanolamine (SmBr2-NMEA) reagent system displayed the best combination of metal-ligand affinity and stability against H2 evolution. The use of SmBr2-NMEA afforded the reduction of a range of substrates that are typically recalcitrant to single-electron reduction including alkynes, lactones, and arenes as stable as biphenyl. Moreover, the unique role of NMEA as a chelating ligand for Sm(II) was demonstrated by the reductive cyclization of unactivated esters bearing pendant olefins in contrast to the SmBr2-water-amine system. Finally, the SmBr2-NMEA reagent system was found to reduce substrates analogous to key intermediates in the nitrogen fixation process. These results reveal SmBr2-NMEA to be a powerful reductant for a wide range of challenging substrates and demonstrate the potential for the rational design of PCET reagents with exceptionally weak X-H bonds.

Synthesis, structures, and properties of manganese(i) complexes featuring tridentate amidine-toting protic amine ligands

Cationic Mn(CO)3 complexes having a protic amine ligand with cyclic amidine side arms were prepared and their catalytic properties of transfer hydrogenation of acetophenone and electrochemical reduction of CO2 were compared.

Aluminium alkyl complexes supported by imino-phosphanamide ligand as precursors for catalytic guanylation reactions of carbodiimides.

Herein, we report the synthesis, characterisation, and application of three aluminium alkyl complexes, [κ2-{NHIRP(Ph)NDipp}AlMe2] (R = Dipp (2a), Mes (2b); tBu (2c), Dipp = 2,6-diisopropylphenyl, Mes = mesityl, and tBu = tert-butyl), supported by unsymmetrical imino-phosphanamide [NHIRP(Ph)NDipp]− [R = Dipp (1a), Mes (1b), tBu (1c)] ligands as molecular precursors for the catalytic synthesis of guanidines using carbodiimides and primary amines. All the imino-phosphanamide ligands 1a, 1b and 1c were prepared in good yield from the corresponding N-heterocyclic imine (NHI) with phenylchloro-2,6-diisopropylphenylphosphanamine, PhP(Cl)NHDipp. The aluminium alkyl complexes 2a, 2b and 2c were obtained in good yield upon completion of the reaction between trimethyl aluminium and the protic ligands 1a, 1b and 1c in a 1 : 1 molar ratio in toluene via the elimination of methane, respectively. The molecular structures of the protic ligands 1b and 1c and the aluminium complexes 2a, 2b and 2c were established via single-crystal X-ray diffraction analysis. Complexes 2a, 2b and 2c were tested as pre-catalysts for the hydroamination/guanylation reaction of carbodiimides with aryl amines to afford guanidines at ambient temperature. All the aluminium complexes exhibited a high conversion with 1.5 mol% catalyst loading and broad substrate scope with a wide functional group tolerance during the guanylation reaction. We also proposed the most plausible mechanism, involving the formation of catalytically active three-coordinate Al species as the active pre-catalyst.

Singlet Oxygen Formation vs Photodissociation for Light-Responsive Protic Ruthenium Anticancer Compounds: The Oxygenated Substituent Determines Which Pathway Dominates.

Ruthenium complexes bearing protic diimine ligands are cytotoxic to certain cancer cells upon irradiation with blue light. Previously reported complexes of the type [(N,N)2Ru(6,6'-dhbp)]Cl2 with 6,6'-dhbp = 6,6'-dihydroxybipyridine and N,N = 2,2'-bipyridine (bipy) (1A), 1,10-phenanthroline (phen) (2A), and 2,3-dihydro-[1,4]dioxino[2,3-f][1,10]phenanthroline (dop) (3A) show EC50 values as low as 4 μM (for 3A) vs breast cancer cells upon blue light irradiation ( Inorg. Chem. 2017, 56, 7519). Herein, subscript A denotes the acidic form of the complex bearing OH groups, and B denotes the basic form bearing O- groups. This photocytotoxicity was originally attributed to photodissociation, but recent results suggest that singlet oxygen formation is a more plausible cause of photocytotoxicity. In particular, bulky methoxy substituents enhance photodissociation but these complexes are nontoxic ( Dalton Trans 2018, 47, 15685). Cellular studies are presented herein that show the formation of reactive oxygen species (ROS) and apoptosis indicators upon treatment of cells with complex 3A and blue light. Singlet oxygen sensor green (SOSG) shows the formation of 1O2 in cell culture for cells treated with 3A and blue light. At physiological pH, complexes 1A-3A are deprotonated to form 1B-3B in situ. Quantum yields for 1O2 (ϕΔ) are 0.87 and 0.48 for 2B and 3B, respectively, and these are an order of magnitude higher than the quantum yields for 2A and 3A. The values for ϕΔ show an increase with 6,6'-dhbp derived substituents as follows: OMe < OH < O-. TD-DFT studies show that the presence of a low lying triplet metal-centered (3MC) state favors photodissociation and disfavors 1O2 formation for 2A and 3A (OH groups). However, upon deprotonation (O- groups), the 3MLCT state is accessible and can readily lead to 1O2 formation, but the dissociative 3MC state is energetically inaccessible. The changes to the energy of the 3MLCT state upon deprotonation have been confirmed by steady state luminescence experiments on 1A-3A and their basic analogs, 1B-3B. This energy landscape favors 1O2 formation for 2B and 3B and leads to enhanced toxicity for these complexes under physiological conditions. The ability to convert readily from OH to O- groups allowed us to investigate an electronic change that is not accompanied by steric changes in this fundamental study.

Preparation of Complexes Bearing N‐Alkylated, Anionic or Protic CAACs Through Oxidative Addition of 2‐Halogenoindole Derivatives

AbstractCAAC precursors 2‐chloro‐3,3‐dimethylindole 1 and 2‐chloro‐1‐ethyl‐3,3‐dimethylindolium tetrafluoroborate 2BF4 have been prepared and oxidatively added to [M(PPh3)4] (M=Pd, Pt). Salt 2BF4 reacts with [Pd(PPh3)4] in toluene at 25 °C over 4 days to yield complex cis‐[3]BF4 featuring an N‐ethyl substituted CAAC, two cis‐arranged phosphines and a chloro ligand. Compound trans‐[3]BF4 was obtained from the same reaction at 80 °C over 1 day. Salt 2BF4 reacts with [Pt(PPh3)4] to give cis‐[4]BF4. The neutral indole derivative 1 adds oxidatively to [Pt(PPh3)4] to give trans‐[5] featuring a CAAC ligand with an unsubstituted ring‐nitrogen atom. This nitrogen atom has been protonated with py⋅HBF4 to give trans‐[6]BF4 bearing a protic CAAC ligand. The PdII complex trans‐[7]BF4 bearing a protic CAAC ligand was obtained in a one‐pot reaction from 1 and [Pd(PPh3)4] in the presence of py⋅HBF4.

Preparation of Complexes Bearing N-Alkylated, Anionic or Protic CAACs Through Oxidative Addition of 2-Halogenoindole Derivatives.

CAAC precursors 2‐chloro‐3,3‐dimethylindole 1 and 2‐chloro‐1‐ethyl‐3,3‐dimethylindolium tetrafluoroborate 2BF4 have been prepared and oxidatively added to [M(PPh3)4] (M=Pd, Pt). Salt 2BF4 reacts with [Pd(PPh3)4] in toluene at 25 °C over 4 days to yield complex cis‐[3]BF4 featuring an N‐ethyl substituted CAAC, two cis‐arranged phosphines and a chloro ligand. Compound trans‐[3]BF4 was obtained from the same reaction at 80 °C over 1 day. Salt 2BF4 reacts with [Pt(PPh3)4] to give cis‐[4]BF4. The neutral indole derivative 1 adds oxidatively to [Pt(PPh3)4] to give trans‐[5] featuring a CAAC ligand with an unsubstituted ring‐nitrogen atom. This nitrogen atom has been protonated with py⋅HBF4 to give trans‐[6]BF4 bearing a protic CAAC ligand. The PdII complex trans‐[7]BF4 bearing a protic CAAC ligand was obtained in a one‐pot reaction from 1 and [Pd(PPh3)4] in the presence of py⋅HBF4.

Zinc Complexes of β‐Ketoiminato Ligands as Efficient Catalysts for the Synthesis of α‐Amino Nitriles via Strecker Reaction

AbstractWe report the synthesis of α‐aminonitriles by one‐pot coupling reaction of aldehyde, amines and trimethylsilyl cyanide (TMSCN) using β‐ketoiminato zinc complexes as a pre‐catalyst in very good yield under mild reaction condition. Homoleptic zinc complex [{κ2‐(2,4,6‐Me3C6H2NC(Me)=CHC(Me)=O)}2Zn] (1 a) was synthesized by the treatment of protic ligand [(2,4,6‐Me3C6H2NHC(Me)=CHC(Me)=O)] (L1H) with potassium hydride and anhydrous zinc diiodide in 2 : 2 : 1 molar ratio in THF. Analogous reactions using [(2,6‐iPr2C6H3NHC(Me)=CHC(Me)=O)] (L2H) and [(Ph2CHNHC(Me)=CHC(Me)=O)] (L3H) as protic ligands, dinuclear zinc complexes [{κ3‐(2,6‐iPr2C6H3NC(Me)=CHC(Me)=O)}ZnI]2 (1 b) and [Zn(Ph2CHNHC−(Me)=CHC(Me)=O)ZnI2] (1 c) were obtained in good yield. Molecular structures of ligands L1H, L3H, and zinc complexes 1 a, 1 b, and 1 c were established by single‐crystal x‐ray diffraction analysis. Dinuclear zinc complexes 1 b, and 1 c exhibited high transformation, greater selectivity and broad substrate scope for the synthesis of α‐aminonitrile under mild condition. A most plausible mechanism for synthesis α‐aminonitrile is proposed based on several controlled reactions.

A Heteroleptic Dirhodium Catalyst for Asymmetric Cyclopropanation with α‐Stannyl α‐Diazoacetate. “Stereoretentive” Stille Coupling with Formation of Chiral Quarternary Carbon Centers

AbstractThe heteroleptic dirhodium paddlewheel catalyst 7 with a chiral carboxylate/acetamidate ligand sphere is uniquely effective in asymmetric [2+1] cycloadditions with α‐diazo‐α‐trimethylstannyl (silyl, germyl) acetate. Originally discovered as a trace impurity in a sample of the homoleptic parent complex [Rh2((R)‐TPCP)4] (5), it is shown that the protic acetamidate ligand is quintessential for rendering 7 highly enantioselective. The ‐NH group is thought to lock the ensuing metal carbene in place via interligand hydrogen bonding. The resulting stannylated cyclopropanes undergo “stereoretentive” cross coupling, which shows for the first time that even chiral quarternary carbon centers can be made by the Stille–Migita reaction.

A Heteroleptic Dirhodium Catalyst for Asymmetric Cyclopropanation with α-Stannyl α-Diazoacetate. "Stereoretentive" Stille Coupling with Formation of Chiral Quarternary Carbon Centers.

The heteroleptic dirhodium paddlewheel catalyst 7 with a chiral carboxylate/acetamidate ligand sphere is uniquely effective in asymmetric [2+1] cycloadditions with α‐diazo‐α‐trimethylstannyl (silyl, germyl) acetate. Originally discovered as a trace impurity in a sample of the homoleptic parent complex [Rh2((R)‐TPCP)4] (5), it is shown that the protic acetamidate ligand is quintessential for rendering 7 highly enantioselective. The ‐NH group is thought to lock the ensuing metal carbene in place via interligand hydrogen bonding. The resulting stannylated cyclopropanes undergo “stereoretentive” cross coupling, which shows for the first time that even chiral quarternary carbon centers can be made by the Stille–Migita reaction.

Iridium catalysts featuring amine-containing ligands for the dehydrogenation of formic acid

The use of protic ligands to generate metal-ligand bifunctional catalysts has proved an excellent strategy to enhance the catalytic activity in formic acid dehydrogenation. We present here synthetic routes for complexes of general formula [Ir(H)2(IPr)(PR3)(CH3CN)2]BF4 (1a-c), [Ir (8-AQ)(H)2(IPr)(PR3)]BF4 (2a-c and 3), and [Ir(IPr)(C-N)(8-AQ)H]BF4 (5a-c) (8-AQ = 8-aminoquinoline; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; C-N = 1-phenylpiridine-1H, 1-phenylpirazol-1H or 2-(p-tolyl-1H)pyridine-1H). Complexes 2a-c, 3, 5a and c were evaluated as catalysts for the dehydrogenation of formic acid in water. The nature of the phosphane in complexes 2a-c and 3 affects drastically the catalytic activity, with more basic and less encumbered phosphanes bringing about better performances. DFT calculations suggest that the coordinated NH2 moiety of the 8-AQ ligand plays a crucial role throughout the catalytic cycle. Two key steps are the protonation of the hydride ligand directed by the NH2 moiety (H2 formation), and the hydride abstraction to regenerate the dihydride active species (CO2 formation). Complexes 5a and 5c do not improve the results obtained with 2a-c and 3, probably due to the fact that a preactivation step is required. This may be attributable to an isomerization step that places the NH2, initially trans to the hydride ligand, in cis position.

Highly Chemoselective Hydroboration of Alkynes and Nitriles Catalyzed by Group 4 Metal Amidophosphine-Borane Complexes.

We report a series of titanium and zirconium complexes supported by dianionic amidophosphine–borane ligands, synthesized by amine elimination and salt metathesis reactions. The TiIV complex [{Ph2P(BH3)N}2C6H4Ti(NMe2)2] (1) was obtained by the reaction between tetrakis-(dimethylamido)titanium(IV) and the protic aminophosphine–borane ligand [{Ph2P(BH3)NH}2C6H4] (LH2) at ambient temperature. Both the heteroleptic zirconium complexes—[η5-(C5H5)2Zr{Ph2P(BH3)N}2C6H4] (2) and [[{Ph2P(BH3)N}2C6H4]ZrCl2] (3)—and the homoleptic zirconium complex [[{Ph2P(BH3)N}2C6H4]2Zr] (4) were obtained in good yield by the salt metathesis reaction of either zirconocene dichloride [η5-(C5H5)2ZrCl2] or zirconium tetrachloride with the dilithium salt of the ligand [{Ph2P(BH3)NLi}2C6H4] (LLi2), which was prepared in situ. The molecular structures of the complexes 1, 2, and 4 in their solid states were confirmed by single-crystal X-ray diffraction analysis. Of these complexes, only titanium complex 1 acts as an effective catalyst for the facile hydroboration of terminal alkynes, yielding exclusive E-isomers. The hydroboration of organic nitriles yielded diborylamines with a broad substrate scope, including broad functional group compatibility. The mechanism of hydroboration occurs through the formation of titanium hydride as an active species.

Cellular uptake of protic ruthenium complexes is influenced by pH dependent passive diffusion and energy dependent efflux

The lipophilic vs. hydrophilic properties of three protic ruthenium compounds were studied as a function of pH. Specifically, we measured Log(Do/w) values for [(N,N)2Ru(6,6′-dhbp)]2+ complexes (where N,N = 2,2′-bipyridine (1A), 1,10-phenanthroline (2A), 2,3-dihydro-[1,4]dioxino[2,3-f][1,10]phenanthroline (3A) and 6,6′-dhbp is the diprotic 6,6′-dihydroxy-2,2′-bipyridine ligand) from pH 4.0 to 8.0. This study allowed us to demonstrate that as the ligand is deprotonated at higher pH values the resulting neutral charge on the complex improves its lipophilic properties. Thus, improved uptake by passive diffusion is expected with protic ligands on Ru(II). Furthermore, cellular studies have demonstrated that passive diffusion is the dominant pathway for cellular uptake. However, metabolic inhibition has also shown that energy dependent efflux reduces the amount of the ruthenium complex (as measured by mean fluorescence intensity) in the cells. These compounds have been shown by fluorescence microscopy to accumulate in the nuclei of cancer cells (MCF7, MDA-MB-231, and HeLa). Taken together, this data shows that uptake is required for toxicity but uptake alone is not sufficient. The greatest light activated toxicity appears to occur in breast cancer cell lines with relatively moderate uptake (MCF7 and MDA-MB-231) rather than the cell line with the greatest uptake of complex 3A (normal breast cell line MCF-10A).

Synthesis of Iron(0) Complexes Bearing Protic NHC Ligands: Synthesis and Catalytic Activity

Preparation of iron(0) pNHC complexes by oxidative addition of N-phosphine-tethered azoles to [Fe3(CO)12] is described. Phosphine-tethered imidazoles 1 and 2 react with [Fe3(CO)12] to give mononucl...

Impact of Protic Ligands in the Ir-Catalyzed Dehydrogenation of Formic Acid in Water

New Ir–IPr complexes featuring a variety of chelate ligands, some of them featuring NH moieties, were synthesized. These complexes proved to be efficient catalysts for the dehydrogenation of formic acid in DMF and H2O. The dehydrogenation rates were dependent on the nature of the ligands that accompany IPr in the coordination sphere of the metal. In fact, complexes that contain protic ligands, namely, [Ir(8-aminoquinoline)(H)2(IPr)(PPhMe2)]BF4 and [Ir(CH3CN)(2-phenylimidazole)(H)(IPr)(PPhMe2)]BF4, showed the best activities in H2O. The former, which presents an NH2 group bound to the Ir center, is the most active (in H2O) of the catalysts presented in this work and can be recycled up to 10 times without an apparent loss of activity. This behavior may be attributed to the robust ligand system and, likely, to its ability to establish outer-sphere interactions. This postulation is supported by the fact that complex [Ir(8-dimethylaminoquinoline)(H)2(IPr)(PPhMe2)]BF4, which features an −NMe2 instead of an −NH2...

Hydroboration, Cyanosilylation, and Sequential Cyanosilylation and Hydroboration of Carbonyl Compounds in the Presence of a TiIV Amido Complex as an Efficient Catalyst

Catalytic hydroboration and cyanosilylation of a wide range of aldehydes and ketones, with pinacolborane (4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolane) and trimethylsilyl cyanide, respectively, in the presence of titanium(IV) complexes [κ2‐{(C5H4N)CH2N‐P(=Se)Ph2}Ti(NMe2)3] (1) and [{κ2‐(C6H4F)NP(=Se)Ph2}Ti(NMe2)3] (2) are reported. The catalytic hydroboration and cyanosilylation processes are high‐yielding and general, and result in efficient conversion of carbonyl compounds into the corresponding products under mild conditions. Titanium catalyst 1 exhibits high tolerance toward several functional groups. First‐order kinetics with respect to the catalyst and each of the substrates were observed for the titanium‐catalyzed hydroboration of carbonyl compounds. First‐principles calculations were performed to explore the catalytic hydroboration reaction mechanism. In addition, a combination of both reactions and sequential cyanosilylation/hydroboration is reported. TiIV complexes 1 and 2 were prepared by aminolysis of [Ti(NMe2)4] with protic ligands [(C5H4N)CH2NHP(=Se)Ph2] (L1H) and [(C6H4F)NHP(=Se)Ph2] (L2H), respectively. The third titanium complex [κ4‐{Ph2P(=Se)NCH(C5H4N)CH(C5H4N)P(=Se)Ph2}‐Ti(NMe2)2] (3) was obtained from the reaction between [Ti(NMe2)4] and ligand L1H in a 1:2 molar ratio at room temperature. The molecular structures of complexes 1 and 3 in the solid state are reported.

Ring Opening Polymerization and Copolymerization of Cyclic Esters Catalyzed by Group 2 Metal Complexes Supported by Functionalized P-N Ligands.

We report the preparation of alkali and alkaline earth (Ae) metal complexes supported by 2-picolylamino-diphenylphosphane chalcogenide [(Ph2P(=E)NHCH2(C5H4N)] [E = S (1-H); Se (2-H)] ligands. The treatment of the protic ligand, 1-H or 2-H, with alkali metal hexamethyldisilazides at room temperature afforded the corresponding alkali metal salts [M(THF)2(Ph2P(=E)NCH2(C5H4N)] [M = Li, E = S (3a), Se (3b)] and [{M(THF) n(Ph2P(=E)NCH2(C5H4N)}2] [M = Na, E = S (4a), Se (4b); M = K, E = Se (5b)] in good yield. The homoleptic Ae metal complexes [κ2-(Ph2P(=Se)NCH2(C5H4N)Mg(THF)] (6b) and [κ3-{(Ph2P(=Se)NCH2(C5H4N)}2M(THF) n] (M = Ca (7b), Sr (8b), Ba (9b)] were synthesized by the one-pot reaction of 2-H with [KN(SiMe3)2] and MI2 in a 2:2:1 molar ratio at room temperature. The molecular structures of the protic-ligands 1-H and 2-H, as well as complexes 3a,b-5a,b and 6b-9b were established using single-crystal X-ray analysis. The Ae metal complexes 6b-9b were tested for ring-opening polymerization (ROP) of racemic lactide ( rac-LA) and copolymerization of rac-LA and ε-caprolactone (ε-CL) at room temperature. In the ROP of rac-LA, the calcium complex 7b exhibited high isoselectivity, with Pi = 0.89, whereas both the barium and strontium complexes showed lower isoselectivity with Pi = 0.78-0.62. In the copolymerization of rac-LA and ε-CL, both barium and strontium complexes proved to be efficient precatalysts for the formation of the block copolymer rac-LA-CL, but the reactivity of 9b was found to be better than that of 8b. All the polymers were fully characterized using differential scanning calorimetry, thermogravimetric analysis, and gel permeation chromatography analyses. Kinetic studies on the ROP reaction of LA confirmed that the rate of polymerization followed the order Ba ≫ Sr ≈ Ca.