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

Abstract

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 LH 2 was designed and its Cu(I)/Mo(VI) reactivity was investigated. LH 2 ((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). LH 2 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. LH 2 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 LH 2 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(LH 2 )] + with a variety of monodentate ligands X (PPh 3 , 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 LH 2 with molybdate [MoO 4 ] 2− led to incorporation of [Mo VI O 3 ] at the catecholate position, producing [MoO 3 (L)] 2− . Similarly, the reaction of [Cu(LH 2 )] + with [MoO 4 ] 2− formed the heterodinuclear complex [CuMoO 3 (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.

Combining [MoVIO3] and [M0(CO)3] (M = Mo, Cr) Fragments within the Same Complex: Synthesis and Reactivity of the Single Oxo-Bridged Heterobimetallics Supported by Xanthene-Based Heterodinucleating Ligands

“…The observation of the catechol protons at 13.98 and 6.11 ppm implies the lack of metal coordination to the catecholate site. A highly downfield signal at 13.98 ppm further indicates strong intramolecular H-bonding to the nearby imine, suggesting syn conformation of the iminocatechol group. , 13 C NMR spectra exhibit one CO peak at 229.95 ppm. A single C O resonance can be rationalized by the effective C s symmetry (on the NMR time scale) combined with a weak bond to the central amine (see below), which leads to exchange of the carbonyls.…”

“…We postulated that our inability to isolate the desired pure dinuclear Mo(0)/Mo(VI) complex with L 2 is due to the conformational lability of the two sides of the complex, which could negatively affect its stability and crystallinity, as was previously observed for the Cu(I)/Mo(VI) complex with the same ligand. 28 Tridentate coordination of dipicolylamine results in a coordinatively saturated octahedral [Mo 0 (CO) 3 (dipicolylamine)] site along with the highly nucleophilic [Mo VI O 3 (catecholate)] 2− site. As a result, a reactive conformationally labile complex ensues, which lacks a stabilizing interaction between Mo(0) and (highly nucleophilic) Mo(VI) sites, which could lead to subsequent decomposition.…”

“…In contrast, L 1 features an iminopyridine that is expected to bind bidentate to Mo(0), instead of the tridentate chelate for L 2 . 27,28 We hypothesized that the use of L 1 instead of L 2 would leave one labile position at Mo-(CO) 3 (iminopyridine), which could be used by a bridging ligand.…”

“…Replacement of the iminopyridine chelate by the more robust dipicolylamine (L 2 , Scheme ) enabled formation of a metastable bimetallic Mo(VI)/Cu(I) species directly observable by NMR spectroscopy and high-resolution mass spectroscopy (HRMS); the related tetrametallic [Mo 2 Cu 2 ] complex was recently structurally characterized . However, no oxidation of the coordinated CNR (isocyanide, a close CO analogue) was observed by the heterobimetallic complex [Cu I Mo VI O 3 (L 2 )] − , and only fleeting CO coordination was observed with the corresponding Cu(I) complex [Cu I (L 2 H 2 )] + . As a possible solution to this problem, we decided to replace it with a π-basic metal known to coordinate the CO strongly.…”

Some Aromatic Schiff Bases and Their Metal Complexes

“…codes OWUXUB [18] and RUTYIP [19]), as well as in three Cu(I) complexes (Ref. codes VAHHUL, VAHJIB, and VAHKIC) [20] and one silicone complex (Ref. code XOSMOJ) in which the coordination is realized through the residue of the imine ligand precursor [21].…”