Dinuclear [RNi(oxam)NiR] Complexes (oxam = N¹,N²-bis(2-pyridylmethyl)-N³,N⁴-bis(2,4,6-trimethylphenyl)oxalamidinate; R = Me, Ph, C⋮CH, C⋮CPh):  Reactions of the Methyl Complex and Formation of [Li(THF)]₂Li₂Ni₂Me₈ and [Li(THF)]₄Ni₂Me₈^†

The reaction between 1 equiv of [{(acac)Ni}₂(A)] and 2 equiv of LiR in THF afforded the binuclear organometallic complexes [(RNi)₂(A)] (A:  N¹,N²-bis(2-pyridylmethyl)-N³,N⁴-bis(2,4,6-trimethylphenyl)oxalamidinate; 1:  R = Me; 2:  R = Ph). Analogously, 3b (R = C⋮C−H was formed from [{(acac)Ni}₂(A)] and sodium acetylide. The reaction of 1 with phenylacetylene resulted in the formation of complex 3a (R = C⋮C−Ph), accompanied by the evolution of methane. Compounds 1−3 were characterized by ¹H and ¹³C NMR spectroscopy, elemental analysis, and mass spectroscopy. In addition, the molecular structures of 1 and 3b were determined by X-ray crystallography. The NMR spectra of the complexes showed very simple patterns, thus indicating that only one isomer is present in solution containing a highly symmetrical structure with planar Ni(II) centers. X-ray studies confirmed that two Ni(II) centers are connected via the oxalamidinato bridge A, which coordinates in a tridentate fashion at each Ni atom. The fourth position at the metal center is occupied by the organic groups R. Compound 1 reacts with an excess of LiMe in THF, yielding [Li(THF)]₂Li₂Ni₂Me₈ (5). Upon recrystallization, the THF-enriched complex [Li(THF)]₄Ni₂Me₈ (6) was isolated. Both complexes were characterized by X-ray crystallography and ¹H/ ¹³C NMR spectroscopy. Both 5 and 6 are highly symmetrical compounds in which each planar Ni(II) center is surrounded by four methyl groups. The Ni···Ni separations are relatively long (3.167(1) and 3.142(1) Å, respectively), reflecting the absence of a metal−metal bond between those two metal centers. Furthermore, agostic Li−H interactions stabilize the structure in a way similar to that in the corresponding Cr(II) compound [Li₄(THF)₄Cr₂Me₈], in which very short contacts have been found (Cr···Cr distance 1.968(2) Å).