Hingeless Negative Linear Compression in the Mechanochromic Gold Complex [(C6F5Au)2(μ-1,4-diisocyanobenzene)]**

The compression of a crystalline material under hydrostatic pressure always results in a reduction in volume of the solid, with the observed reduction typically occurring in all crystallographic cell parameters. However, there are a number of exceptional materials that have been shown to expand in a specific direction upon hydrostatic compression while maintaining a positive volume compression. The phenomenon of unior biaxial expansion under compression is known as negative linear compressibility (NLC). Rare examples of NLC have attracted attention recently owing to the potential applications in a range of materials, including body armor, artificial muscle actuators, and pressure sensors. NLC may be quantified using isothermal compressibility, measured in TPa 1 and defined as the relative rate of change in length with respect to pressure (Kl= (d(In l)/dp)T) which may be used to compare and contrast the compressibility of different materials. Typically a material will display a positive value of compressibility (+ 5–+ 50 TPa ); however, in materials exhibiting NLC the values are negative in certain directions. The largest values of NLC have been reported in framework materials. The metallocyanide framework Ag3[Co(CN)6] exhibits extremely large NLC at low pressures ( 76 TPa 1 from ambient (ambient herein refers to 20 K and 0.0001 GPa) to 0.19 GPa) before transforming to a new highpressure phase that also displays NLC over a greater pressure range ( 5.3 TPa ), while KMn[Ag(CN)2]3, a structural analogue of Ag3[Co(CN)6] has been shown to demonstrate strong persistent NLC ( 12 TPa 1 from ambient to 2.2 GPa). More recently, “giant” NLC has been attributed to a simple zinc gold cyanate complex Zn[Au(CN)2]2. The compound displays a remarkably large NLC effect parallel to the crystallographic c-axis that is the largest of any other material ever reported ( 42(5)TPa 1 between ambient and 1.8 GPa). In all these materials the observation of large NLC is attributed to specific structural motifs where covalent bonds act as hinges allowing the material to behave in a manner that mimics a wine-rack or trellis. 2b,d] In molecular complexes, the largest NLC effect has been reported by Shepherd et al. in the spin crossover complex [Fe(dpp)2(NCS)2]·py (dpp= dipyrido[3,2-a:2’,3’-c]phenazine, py=pyridine). The complex employs a scissoring mechanism in which the angles between the two dpp ligands and the two NCS ligands widen upon compression by pivoting about the central iron atom, resulting in extreme negative linear compressibility ( 10.3(20) TPa 1 from ambient to 2.2 GPa). The wine-rack and scissoring mechanisms are similar except that in the former the pivot is about a covalent bond whereas in the latter it is about an atomic center. Also of relevance to the discussion is the phenomenon of negative thermal expansion (NTE), which is a thermally driven analogue of NLC. It is exhibited by linear dumbbell molecules such as (S,S)-octa-3,5-diyn-2,7-diol, which has exceptionally large positive and negative anisotropic NTE. This phenomenon has been attributed to a combination of their linear geometry and to the presence of strong hydrogen bonds that are able to act as hinges between molecules that facilitate a pivot mechanism similar to that in the metallocyanate frameworks or metal complexes displaying NLC effects. [(C6F5Au)2(m-1,4-diisocyanobenzene)] 1 (Scheme 1) belongs to a family of linear organoauric complexes, several of which display the extraordinary phenomenon of mechanochromism or the alteration of the emission wavelength of a material upon mechanical grinding. The shift in luminescent emission of the material upon grinding has been attributed to its rod-like molecular geometry and the absence