1. School of Chemistry, University of Bristol Bristol BS8 1TS, U.K. X-ray Crystallographic Unit, School of Chemistry University of Bristol, Bristol, BS8 1TS, U.K. X-ray Crystallographic Unit, School of Chemistry University of Bath, Bath, BA2 7AY, U.K. Zeneca Pharmaceuticals, Mereside, Alderley Park Macclesfield SK10 4TG, U.K.