Stabilization of β‐Diketiminato Nickel(I) with Alkaline Metal Halide Entities for Small Molecule Activation

Abstract

The reduction of the β-diketiminato nickel(II) halide complex [L tBu Ni II Br] [L tBu = CH(CtBuNdipp) 2 -, dipp = 2,6-diisopropylphenyl] with potassium sources proceeds via the initial formation of [(L tBu Ni I ) x (μ-Br) x K x ] aggregates, which could be isolated and characterized for x = ϱ and 6. The KBr equivalents readily give way to external donors or substrates to be activated at the central nickel(I) atoms. To test, in how far the steric bulk induced by the residues at [L tBu ]influences the formation of the KBr adducts β-diketiminato ligands with less steric congestion, namely L Me6 and L Me7 [L Me6 = CH(CMeNdmp) 2 -, L Me7 = CMe(CMeNdmp) 2 -, dmp = 2,6-dimethylphenyl] were employed. Through deprotonation of HL Me6 with * Prof. Dr. Christian Limberg E-Mail: christian.limberg@hu-berlin.de [a]

Electron transfer within β-diketiminato nickel bromide and cobaltocene redox couples activating CO2

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Typically, it is generated by the reaction of a -diketiminato nickel(II) bromido precursor with elemental sodium or potassium, KC 8 or Na/Hg, and subsequently it has three possibilities to reach a stable coordinative saturation: (i) it binds a solvent donor molecule or a donor from the gas phase, like N 2 ; (ii) the alkali metal halide generated concomitantly remains in the coordination sphere; (iii) the aryl ring of a second molecule is coordinated so that a dimerization occurs. 4,9,17 A further option becomes conceivable, if reductants are used that produce a weakly coordinating cation, namely, the preservation of the -diketiminato nickel bromide core. We were interested to examine the behaviour of such a system, especially in a redox regime that is not as harsh as under conditions where alkali metals are used as reductants, bearing in mind that nickel enzymes, which reductively activate CO or CO 2 , also work at comparatively mild potentials.…”

NMR spectroscopic investigations on the successive implementation of nickel and zinc ions to a NacNac‐dibenzofuran‐Br ligand precursor

“…A plausible way to rationalize this finding is to assume, that the desired complex is indeed formed initially but subsequently the organic ligand at the nickel center acts as a base for the NacNacH proton, which is transferred to the arylic position, while the nickel bromide moiety is bound in the NacNac binding pocket, leading to BrNi‐NacNac‐DBF‐H, 1 (Scheme 2). In 1 the nickel center would have a trigonal planar coordination environment or through dimerization a tetrahedral NacNacNi( μ ‐Br) 2 NiNacNac motif could form as observed for similar compounds, thus explaining the paramagnetism [24] …”

“…We started with 6-bromo-4-dibenzofuranamine (H 2 N-DBF-Br) and envisioned the attachment of a binding pocket for one metal center, while the second one was supposed to be introduced by oxidative addition to the CÀ Br bond. As the binding pocket we have chosen the β-diketiminato moiety (NacNac), as this ligand has allowed for the construction of a variety of reactive metal complexes for small molecule activation [15][16][17][18] and unsymmetric variants were utilized in the past for the generation of heterobimetallic compounds. [19] As the metals we selected nickel and zinc ions; nickel, as it can be inserted into the CÀ Br bond starting from the oxidation state zero and subsequent redox chemistry was conceivable, [15][16][17]20] zinc due to its Lewis acidic properties.…”

“…In 1 the nickel center would have a trigonal planar coordination environment or through dimerization a tetrahedral NacNacNi(μ-Br) 2 NiNacNac motif could form as observed for similar compounds, thus explaining the paramagnetism. [24] Hence, in subsequent work zinc was planned to be coordinated first, ideally using precursors with basic ligands capable of deprotonating the NacNacH unit in situ in order to avoid a salt metathesis. Zn(HMDS) 2 (HMDS = hexamethlydisilazide) and Zn(C 6 F 5 ) 2 were thus employed and this indeed quantitatively led to the respective X-Zn-NacNac-dibenzofuran-Br complexes (X = HMDS (2), C 6 F 5 (3)), as shown by multinuclear NMR spectroscopy (Scheme 3).…”

Transformation of Formazanate at Nickel(II) Centers to Give a Singly Reduced Nickel Complex with Azoiminate Radical Ligands and Its Reactivity toward Dioxygen

“…β-Diketiminatonickel­(I) complexes have been shown to reductively activate a wide range of small molecules. − A commonly employed synthetic route to β-diketiminatonickel­(I) species is treatment of the corresponding nickel­(II) halide derivative with KC 8 , or sodium, which reduces the metal center, while the β-diketiminato ligand (Chart , left) is redox-inert. The formazanate ligand (Chart , right) is structurally similar but can be reduced to a dianionic (2−) form .…”