The problems of detecting hydrides in metal carbonyl clusters by 1H NMR: the case study of [H4−nNi22(C2)4(CO)28(CdBr)2]n− (n = 2–4)

Abstract

The new tetra-acetylide carbonyl clusters [H_4−nNi₂₂(C₂)₄(CO)₂₈(CdBr)₂]ⁿ⁻ (n = 2–4) have been prepared by reacting [Ni₁₀C₂(CO)₁₅]²⁻ with a large excess of CdBr₂·xH₂O and the molecular structure of the di-anion [H₂Ni₂₂(C₂)₄(CO)₂₈(CdBr)₂]²⁻ has been fully elucidated by means of X-ray crystallography. The corresponding [HNi₂₂(C₂)₄(CO)₂₈(CdBr)₂]³⁻ and [Ni₂₂(C₂)₄(CO)₂₈(CdBr)₂]⁴⁻ conjugated bases are quantitatively obtained upon dissolution of [H₂Ni₂₂(C₂)₄(CO)₂₈(CdBr)₂]²⁻ salts in more basic solvents such as acetonitrile and DMSO, respectively. The hydride nature of both [H₂Ni₂₂(C₂)₄(CO)₂₈(CdBr)₂]²⁻ and [HNi₂₂(C₂)₄(CO)₂₈(CdBr)₂]³⁻ has been directly proved by ¹H NMR spectroscopy. Their resonances are very broad under all experimental conditions and their chemical shift greatly depends on solvent as well as temperature. Observation of the hydride resonances in [H_4−nNi₂₂(C₂)₄(CO)₂₈(CdBr)₂]ⁿ⁻ (n = 2, 3) makes these clusters a case study of the phenomena behind the loss of any NMR signal in higher-nuclearity metal carbonyl cluster anions (MCCA). In the attempt to obtain a better insight on this experimental spectroscopic behaviour, solutions of [NMe₄]₃[HNi₂₂(C₂)₄(CO)₂₈(CdBr)₂] have been investigated by dynamic light scattering (DLS) at various concentrations. The DLS experiments point out the presence in solution of a distribution of particles with nominal hydrodynamic diameters enormously greater than those of the free cluster ions resulting, probably, from aggregation in solution. This could formally justify the observed NMR behaviour, even if the present observations are preliminary and their quantitative assessment requires further systematic studies on MCCA aggregation in solution.