Synthesis and Cu(I)/Mo(VI) Reactivity of a Bifunctional Heterodinucleating Ligand on a Xanthene Platform

In an effort to probe the feasibility of a model of Mo-Cu CODH (CODH = carbon monoxide dehydrogenase) lacking a bridging sulfido group, the new heterodinucleating ligand LH₂ was designed and its Cu(I)/Mo(VI) reactivity was investigated. LH₂ ((E)-3-(((5-(bis(pyridin-2-ylmethyl)amino)-2,7-di-tert-butyl-9,9-dimethyl-9H-xanthen-4-yl)imino)methyl)benzene-1,2-diol) features two different chelating positions bridged by a xanthene linker: bis(pyridyl)amine for Cu(I) and catecholate for Mo(VI). LH₂ was synthesized via the initial protection of one of the amine positions, followed by two consecutive alkylations of the second position, deprotection, and condensation to attach the catechol functionality. LH₂ was found to exhibit dynamic cooperativity between two reactive sites mediated by H-bonding of the catechol protons. In the free ligand, catechol protons exhibit H-bonding with imine (intramolecular) and with pyridine (intermolecular in the solid state). The reaction of LH₂ with [Cu(NCMe)₄]⁺ led to the tetradentate coordination of Cu(I) via all nitrogen donors of the ligand, including the imine. Cu(I) complexes were characterized by multinuclear NMR spectroscopy, high-resolution mass spectrometry (HRMS), X-ray crystallography, and DFT calculations. Cu(I) coordination to the imine disrupted H-bonding and caused rotation away from the catechol arm. The reaction of the Cu(I) complex [Cu(LH₂)]⁺ with a variety of monodentate ligands X (PPh₃, Cl^-, SCN^-, CN^-) released the metal from coordination to the imine, thereby restoring imine H-bonding with the catechol proton. The second catechol proton engages in H-bonding with Cu-X (X = Cl, CN, SCN), which can be intermolecular (XRD) or intramolecular (DFT). The reaction of LH₂ with molybdate [MoO₄]^2- led to incorporation of [Mo^VIO₃] at the catecholate position, producing [MoO₃(L)]^2-. Similarly, the reaction of [Cu(LH₂)]⁺ with [MoO₄]^2- formed the heterodinuclear complex [CuMoO₃(L)]^-. Both complexes were characterized by multinuclear NMR, UV-vis, and HRMS. HRMS in both cases confirmed the constitution of the complexes, containing molecular ions with the expected isotopic distribution.

Synthesis and Coordination Ability of a Donor-Stabilised Bis-Phosphinidene

Chelating phosphines have long been a mainstay as efficient directing ligands in transition-metal catalysis. Low-valent derivatives, namely chelating phosphinidenes, are to date unknown, and could lead to chelating complexes containing more than one metal centre due to the intrisic capacity of phosphinidenes to bind two metal fragments at one P-centre. Here we describe the synthesis of the first such chelating bis-phosphinidene ligand, XantP2 (2), generated by the reduction of a diphosphino xanthene derivative, Xant(PH2 )2 (1) with iPr NHC (iPr NHC=[:C{N(iPr)C(H)}2 ]). Initial studies have shown that this novel chelating ligand can act as a bidentate ligand towards element dihalides (i.e. FeCl2 , ZnI2 , GeCl2 , SnBr2 ), forming cationic complexes with the tetryl elements. In contrast, XantP2 demonstrates an ability to bind multiple metal centres in the reaction with CuCl, leading to a cationic Cu3 P3 ring complex, with Cu centres bridged by phosphinidene arms. Density Functional Theory calculations show that 2 indeed holds 4 lone pairs of electrons, shedding further light on the coordination capacity for this novel ligand class through observation of directionality and hybridisation of these electron pairs.

A tetranuclear nickel cluster isolated in multiple high-valent states

We report a series of high-valent tetranuclear nickel clusters isolated from the chemical oxidation of an all Ni(ii) ([Ni₄]) neutral cluster. Electrochemical analysis of [Ni₄] reveals three reversible sequential oxidations at 0.248 V (1e^-), 0.678 V (1e^-), and 0.991 V (2e^-) vs. Fc/Fc⁺ corresponding to mono-, di-, and tetra-oxidized species, [Ni₄]⁺, [Ni₄]²⁺, [Ni₄]⁴⁺, respectively. Using spectroscopic, crystallographic, magnetometric, and computational techniques, we assign the primary loci of oxidations to the Ni centers in each case, thus resulting in the isolation of the first tetranuclear all-Ni(iii) cluster, [Ni₄]⁴⁺.

Dual switchable molecular tweezers incorporating anisotropic MnIII-salphen complexes

An alternative strategy for the synthesis of terpyridine based switchable molecular tweezers has been developed to incorporate anisotropic Mn(iii)-salphen complexes. The free ligand was synthesized using a building block strategy based on Sonogashira coupling reactions and was then selectively metalated with manganese in a last step. The conformation of the tweezers was switched from an open 'W' shaped form to a closed 'U' form by Zn(ii) coordination to the terpyridine unit bringing the two Mn-salphen moieties in close spatial proximity as confirmed by X-ray crystallography. An alternate switching mechanism was observed by the intercalation of a bridging cyanide ligand between the two Mn-salphen moieties that resulted in the closing of the tweezers. These dual stimuli are attractive for achieving multiple controls of the mechanical motion of the tweezers. A crystallographic structure of unexpected partially oxidized closed tweezers was also obtained. One of the two Mn-salphen moieties underwent a ligand-centered oxidation of an imino to an amido group allowing an intramolecular Mn-Oamide-Mn linkage. The magnetic properties of the manganese(iii) dimers were investigated to evaluate the magnetic exchange interaction and analyze the single molecule magnet behavior.

Tetra- and dinuclear manganese complexes of xanthene-bridged O,N,O-Schiff bases with 3-hydroxypropyl or 2-hydroxybenzyl groups: ligand substitution at a triply bridging site

Complexation properties of U-shaped ligands, L¹ and L², which are Schiff bases of 5,5'-(9,9-dimethylxanthene-4,5-diyl)bis(salicylaldehyde) (H₂xansal) with 3-amino-1-propanol or 2-hydroxybenzylamine, respectively, were investigated to construct polynuclear manganese complexes. In these ligands, two O,N,O-Schiff bases are bridged by a xanthene backbone. The reactions of H₄L¹ or H₄L² with manganese salts afforded tetra- and dinuclear manganese complexes, including the tetramanganese(ii,ii,iii,iii) complex [Mn₄(L¹)₂(μ-OAc)₂] with a Mn₄O₆ core exhibiting an incomplete double-cubane structure. In the Mn₄O₆ core, phenolate and alkoxide O atoms bridge the manganese ions. Deprotonated 3-hydroxypropyl groups were crucial to the assembly of four manganese ions because the phenolate-bridged dimanganese(iii,iii) complex [Mn₂(H₂L¹)₂]²⁺ was obtained in the absence of a base, and H₄L², which has 2-hydroxybenzyl groups instead of 3-hydroxypropyl groups in H₄L¹, afforded the cyclic dimanganese(iv,iv) complex [Mn₂(L²)₂]. We disclosed that [Mn₄(L¹)₂(μ-OAc)₂] was converted to the oxo-bridged tetramanganese(iii,iii,iii,iii) complex [Mn₄(L¹)(HL¹)(μ₃-O)(μ-OAc)₂]⁺ by treating with NH₄PF₆ or NH₄BF₄: a triply bridging alkoxide was protonated and replaced by an oxide ligand. The cyclic voltammograms of [Mn₄(L¹)(HL¹)(μ₃-O)(μ-OAc)₂]⁺ suggested that the reverse reaction forming [Mn₄(L¹)₂(μ-OAc)₂] occurred in the electrochemical processes and was assisted by protonation.

Dinuclear manganese(iii) complexes with bioinspired coordination and variable linkers showing weak exchange effects: a synthetic, structural, spectroscopic and computation study

High-resolution HFEPR indicates weak exchange interactions between Mn^III ions in agreement with DFT calculations.