Mono‐ and Binuclear Arylnickel Complexes of the α‐Diimine Bridging Ligand 2,2′‐Bipyrimidine (bpym)

Abstract

The mono-and binuclear organometallic Ni II complexes [(µ-bpym){Ni(Mes)Br} n ] (bpym = 2,2Ј-bipyrimidine; n = 1 or 2; Mes = mesityl = 2,4,6-trimethylphenyl) were prepared and characterised electrochemically and spectroscopically (NMR, UV/Vis/NIR) in detail. The long-wavelength absorptions for the binuclear complex reveal a marked electronic coupling of the two metal centres over the ligand bridge via their lowlying π*-orbitals. While the mononuclear complex undergoes rapid dissociation of the bromido ligand after one-electron reduction the binuclear derivative exhibits reversible reductive electrochemistry and both of them yield stable radi-

Unsymmetrical N-Aryl-1-(pyridin-2-yl)methanimine Ligands in Organonickel(II) Complexes: More Than a Blend of 2,2′-Bipyridine and N,N-Diaryl-α-diimines?

“…The new complexes all exhibit two long-wavelength absorptions of moderate intensity (2500–4700 M –1 cm –1 ) covering big parts of the visible range (data in Table S4 in the SI). These bands are strongly negative solvatochromic, as Figure illustrates, and can thus be assigned to metal-to-ligand charge-transfer (MLCT) transitions, in line with the assignments of similar complexes. − The intense absorptions in the UV range are due to intraligand π–π* transitions and were also observed in the uncoordinated ligands. A comparison with [(bpy)­Ni­(Mes)­Br] (λ max in CH 2 Cl 2 = 483 nm) , and [( i Pr-DAB)­Ni­(Mes)­Br] (λ max in CH 2 Cl 2 = 569 nm) puts them aside the diazabutadiene complex, and we can thus conclude that the LUMO is more centered on the N -arylimine moiety rather than on the pyridine.…”

“…All complexes were obtained as an air-stable violet microcrystalline material in good yield, and the stabilities of the [(R-PyMA)­Ni­(Mes)­X] complexes in solution very much resemble those of the bpy and related derivatives studied previously (for details, see the SI). − …”

“…Because of the N–Ni–N bite angles with a quite invariable value of about 82° and quite regular X–Ni–C angles of 88–90°, the N–Ni–X and N–Ni–C angles were widened to about 95°, which represents not exactly square-planar coordination. Overall, the Ni–ligand distances were in the range of related complexes. ,,− Remarkably, for the cis isomers, the Ni–N Py distance is markedly shorter (∼1.9 Å) than the Ni–N methanimine distance (∼2 Å), and for the trans isomers, this is more or less exactly reversed, reflecting the trans influence of the Br (weak) versus Mes (strong) coligands but revealing no preference for the cis and trans configurations.…”

“…In the past few years, we have investigated the structures and reactivity on some mononuclear organonickel complexes [(N^N)­Ni­(Mes)­Br] containing symmetrical heteroaromatic or aliphatic α-diimine ligands N^N (Mes = mesityl = 2,4,6-trimethylphenyl) and binuclear complexes [(μ-N^N)­{Ni­(Mes)­Br} 2 ] with unsymmetrical bis­(arylimino)-1,4-pyrazine (bpip) ligands (Chart ) or bridging 2,2′-bipyrimidine (bpym) using various methods. − Especially, their redox chemistry was investigated, in view of the application of such systems in (electro)­catalytical C–C coupling reactions. ,,− …”

“…The most important results of these studies are as follows: (i) Strongly donating solvents like nitriles, N , N -dimethylformamide, or dimethyl sulfoxide slowly replace the bromido ligand (eq ), while in acetone, ethers, or chlorinated solvents, the complexes are completely stable toward such substitution. , ­(ii) Examination of the lowest excited states of [(bpy)­Ni­(Mes)­Br] by UV–vis–near-IR (NIR) absorption spectroscopy and quantum-chemical calculations revealed mixed Ni/Mes/Br contributions to the highest occupied molecular orbital (HOMO) and bpy-based π* lowest unoccupied molecular orbitals (LUMOs). , (iii) The one-electron-reduced complexes [(N^N)­Ni­(Mes)­Br] •– rapidly cleave the bromide coligand (eq ). ,− …”