Lanthanide complexes with a new luminescent iminophosphonamide ligand bearing phenylbenzothiazole substituents

A new iminophosphonamine Ph2P(HNPbt)(NPbt) (1, HL) bearing chromophore 2-(phen-2'-yl)-1,3-benzothiazole (Pbt) substituents was synthesized and introduced into lanthanide complexes. It was found that salt metathesis reactions between KL (2) generated in situ and LnCl3 lead to the formation of tris-iminophosphonamide complexes [LnL2]L (Ln = Y (3), Sm (4), Gd (5), Dy (6)), regardless of the 2/LnCl3 ratio. Compounds 3-6 consist of a cationic fragment [LnL2]+, where the lanthanide atom is surrounded by two rigidly κ4-coordinated ligands, and an L- anion residing in the outer coordination sphere. Iminophosphonamine 1 shows a rare excitation wavelength-dependent two-band luminescence in the solid state. For compounds containing the deprotonated form, namely potassium salt KL and complexes of Gd and Dy, a single-band luminescence with the color changing from turquoise to orange was observed. The Sm complex reveals a set of a few narrow well-resolved bands corresponding to the f-f transitions against the background of the outer-sphere ligand's emission.

Air-Stable Coordinatively Unsaturated Ruthenium(II) Complex for Ligand Binding and Catalytic Transfer Hydrogenation of Ketones from Ethanol

Coordinatively unsaturated complexes are interesting from a fundamental level for their formally empty coordination site and, in particular, from a catalytic perspective as they provide opportunities for substrate binding and transformation. Here, we describe the synthesis of a novel underligated ruthenium complex [Ru(cym)(N,N')]+, 3, featuring an amide-functionalized pyridylidene amide (PYA) as the N,N'-bidentate coordinating ligand. In contrast to previously investigated underligated complexes, complex 3 offers potential for dynamic modifications, thanks to the flexible donor properties of the PYA ligand. Specifically, they allow both for stabilizing the formally underligated metal center in complex 3 through nitrogen π-donation and for facilitating through π-acidic bonding properties the coordination of a further ligand L to the ruthenium center to yield the formal 18 e- complexes [Ru(cym)(N,N')(L)]+ (4: L = P(OMe)3; 5: L = PPh3; 6: L = N-methylimidazole; 7: L = pyridine) and neutral complex [RuCl(cym)(N,N')] 8. Analysis by 1H NMR and UV-vis spectroscopies reveals an increasing Ru-L bond strength along the sequence pyridine <1-methylimidazole < PPh3 < P(OMe)3 with binding constants varying over 3 orders of magnitude with log(Keq) values between 2.8 and 5.7. The flexibility of the Ru(PYA) unit and the ensuing accessibility of saturated and unsaturated species with one and the same ligand are attractive from a fundamental point of view and also for catalytic applications, as catalytic transformations rely on the availability of transiently vacant coordination sites. Thus, while complex 3 does not form stable adducts with O-donors such as ketones or alcohols, it transiently binds these species, as evidenced by the considerable catalytic activity in the transfer hydrogenation of ketones. Notably, and as one of only a few catalysts, complex 3 is compatible with EtOH as a hydrogen source. Complex 3 shows excellent performance in the transfer hydrogenation of pyridyl-containing substrates, in agreement with the poor coordination strength of this functional group to the ruthenium center in 3.

Versatile Reactivity of Half-Sandwich Rhodium(III) Iminophosphonamide Complexes

Novel 18e̅ and 16e̅ pentamethylcyclopentadienyl rhodium(III) complexes [(η⁵-C₅Me₅)RhX(NPN)] (1a,b, X = Cl; 2a-c, X = PF₆, BAr₄^F) with chelating zwitterionic iminophosphonamide (NPN) ligands (Ph₂P(NR)(NR'); a, R = R' = p-Tol; b, R = p-Tol, R' = Me; c, R = R' = Me) were synthesized and characterized by single-crystal X-ray diffraction. In the 16e̅ complexes 2, the rhodium (Rh) atom is efficiently stabilized by π-donation of unshared N electrons, thus hampering coordination of the external ligands and rendering the 18e̅ complexes labile. Due to low coordination enthalpy, the cationic 18e̅ monocarbonyl and pyridine adducts 2a·L are stable only at low temperatures. At room temperature, 2·CO adducts readily give stable carbonyl-carbamoyl complexes [(η⁵-C₅Me₅)Rh(CO){(CO(NR')Ph₂P(NR)}]⁺ (4) formed as a result of CO insertion into the Rh-N bond, thus showing high nucleophilicity of the N atoms in 18e̅ complexes. High basicity of the Na⁺NPN^- precursors caused side deprotonation of the η⁵-C₅Me₅ ligand during the synthesis of 1 that yields unstable fulvene Rh(I) complexes [(η⁴-C₅Me₄CH₂)Rh{Ph₂P(NR)(NR')₂}] (3a,b). Complex 3a undergoes a facile reaction with isoprene to yield an unusual [(η⁵:η¹-C₅Me₄(CH₂)C(Me)═CHCH₂)Rh(NPN)] complex─the first example of intermolecular 1,4-metallacycloaddition of diene to the Rh-fulvene complex.

Mechanistic diversity in acetophenone transfer hydrogenation catalyzed by ruthenium iminophosphonamide complexes

A series of arene ruthenium iminophosphonamide complexes, [(Arene)RuCl{R₂P(NR')₂}] (1), bearing various arenes and R,R' substituents on the NPN ligand have been investigated as precatalysts in acetophenone transfer hydrogenation in basic and base-free isopropanol. The results clearly demonstrate the presence of two distinct reaction mechanisms, which are controlled by the basicity of the N-atoms. Complexes 1 in which both R' substituents are aryl groups are only active once the neutral hydride complex [(Arene)RuH{R₂P(NR')₂}] (2) is generated in basic isopropanol, the latter being able to reduce a ketone via a stepwise hydride and proton transfer. On the other hand, complexes in which at least one R' group is Me readily catalyze the reaction in the absence of base. In the latter case, the results of kinetic studies and DFT calculations support an outer-sphere concerted asynchronous hydride and proton transfer assisted by the basic N-atom of the NPN ligand, which promotes catalysis via precoordination of an alcohol molecule by hydrogen bonding.