Reactivity of tri- and tetra-nuclear ethylidyne cyclopentadienylcobalt clusters towards protonic acids

Abstract

The bis(μ 3-ethylidyne) tricobalt cluster [(CpCo) 3(μ 3-CCH 3) 2] ( 1b) is protonated by trifluoroacetic acid to give the dicobalt edge-protonated cation [H(CpCo) 3(μ 3-CCH 3) 2] + [ lb + H] +. Protonation of the μ 3-ethylidyne tetracobalt cluster hydride [H(CpCo) 4(μ 3-CCH 3)] ( 3) takes place in two consecutive steps. At low temperature [H 2(CpCo) 4(μ 3-CCH 3)] + [ 3 + H] + is formed first, and is then slowly converted into [H 3(CpCo) 4(μ 3-CCH 3)] 2+ [ 3 + 2H] 2+ by an excess of acid. As judged by the 1H NMR data and the crystal structure of [ 3 + X] +[(CF 3COO) 2X] − (X  H or D) the endo hydrogens in [ 3 + H] + and [ 3 + 2H] 2+ occupy μ 3-(Co 3) face capping hydridic positions. The cations [ 1b + H] + and [ 3 + H] + show hydride fluxionality in solution, which in the case of [ 3 + H] + can be frozen out on the NMR timescale at low temperature (ΔG ≠ (203 K) = 40.8 kJ/mol). The structure of [ 3 + X] + [(CF 3COO) 2X] − (X  H or D) was determined by X-ray crystallography. One of the hydrides/deuterides is located on the crystallographic mirror plane, capping a tricobalt face of the cluster cation. The other endo hydrogen atom is believed to be disordered between the other two μ 3-(Co 3) sites, which are related by space group symmetry. Deuteronation of 3 shows a strong normal kinetic deuterium isotope effect. From the temperature independence of the 1H NMR spectrum of [ 3 + 2D] 2+ a non-fluxional solution structure can be inferred. In all the systems studied, hydridic (μ 2- or μ 3-) sites are thermodynamically preferred to possible isomeric agostic CoHC or Co 2HC sites for the endo hydrogens. Agostic interactions cannot, however, be ruled out in transient intermediates during the course of the protonations.