Supplementary data: Selected crystallographic data and optimized coordinates of the DFT calculations are given in the Supporting Information.
Crystallographic data has been deposited at the Cambridge Crystallographic Data Centre with number 1811145. Copies can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax: +44 (0)1223 336033 or ww. ccdc.cam.ac.uk/products/csd/request/].