Coalescence measurements, ligand

VT 13C and 31P NMR studies have shown that the C2 unit undergoes a rapid fluxional process, assigned to a rotation within the Cu4 array with associated flipping of the dppm ligands. Coalescence of the 31P resonances found at 8 —9.6 (P attached to n-bonded Cu) and —12.3 (P attached to a-bonded Cu) occurred at 182 K, while the carbon resonances in the 13C-enriched complex coalesced at 208 K to give a nine-line resonance at 8 149.4 [/(CP) 13.3 Hz]. For the two processes, AG values of ca. 38 and 47 kJ mol-1 were measured. The relatively facile process is indicative of a weak interaction between the C2 ligand and the four copper atoms, which was confirmed by DFT calculations (calc. AG 32.9 kJ mol-1). The UV-vis spectrum contains bands at 262 (dppm intra-ligand transfer), 374 nm (C2 —> Cu4 LMCT), while excitation (X > 350 nm) produces intense... 

Scheme 11.3 provided that the ligands other than the carbene offer an asymmetric environment, which controls the direction of approach of the monomer. If this is not the case, there will not be a favoured direction of approach unless the chain-end effect comes into play. Barriers to rotation about Mt=C have been measured by observation of NMR coalescence temperatures (Guggenberger 1975 Schrock 1978 Kress 1987a). In some cases, these are high enough that epimerization by rotation about Mt=C is unlikely to be important, but in others such a process may be as fast as or faster than the propagation step. 

Synthesis and characterization of [RuH(/ 2-H2)(PP3Cy)]BPh4 (PP3= P(CH2CH2PPh2)3) were carried out in which /(H,D) = 28 Hz was measured and taken as evidence for an intact H—D bond. Structural information was also obtained from /(H,P), indicating that the ( T2-H2) ligand is trans to the central P atom. Variable temperature spectra were obtained in solution and the coalescence of the hydride resonances was observed. In [RuH(tj2-H2)(DMPE)2]" , evidence for the presence of an intact H—H bond was provided by the facile formation of the HD isotopomer and /(H,D) = 32.2 Hz was measured. ... 

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