Synthesis and study of monomeric and dimeric amidine complexes of nickel(II) and platinum(II). Crystal structure of bis(N,N′-diphenylbenzamidino)platinum(II), [Pt{PhNC(Ph)NPh}2]

Abstract

Amidino-complexes, [{M[R′NC(R)NR′]₂}_n](M = Ni or Pt, R,R′= allyl or aryl), have been synthesised from NiCl₂·0.3C₄H₁₀O₂ or [Pt(PhCN)₂Cl₂]. The nickel complexes are dimeric in the solid state, but monomeric with chelating amidino-groups in solution. In contrast, for the platinum complexes, monomeric structures are indicated both in the solid state and solution. This is confirmed by the X-ray crystal structure determination of [Pt{PhNC(Ph)NPh}₂][space group C2/c; unit-cell dimensions a= 25.413(7), b= 6.031(2), c= 20.586(3)Å, β= 109.89(2)°, and Z= 4], which shows both amidino-groups to have electron delocalisation [C–N 1.334(7), 1.340(7)Å] and to be symmetrically σ,σ-bonded through nitrogen atoms [Pt–N 2.040(5) and 2.025(5)Å]. The NCN skeletal angle [106.7(5)°] is similar to the angle in other related complexes. ¹³C N.m.r. studies of selected nickel and platinum complexes in solution are reported, and using the skeletal NCN ¹³C n.m.r. signal as the criterion of bond type, bidentate chelate and not bridging amidino-groups are shown to occur in solution, indicating monomeric species. Interestingly two sets of signals of equal intensity, but differing by only 1 p.p.m. are observed for [Ni₂{PhNC(Ph)NPh}₄] at 25 °C; amidino-structural types in solution are discussed.