The synthesis and study of basic molecular devices are active areas of research aimed toward understanding the fundamental design and functional parameters required for the construction of complex molecular machines. [1] Transition metals have been used extensively in this context in conjunction with elegantly designed ligand frameworks. [2] Typically, transition metals bind to different sets of donors in these frameworks, inducing large molecular motions, as a function of changes in pH, [3] redox processes, [4] excitation by light, [5] heating, [5] introduction of additional ligands, [3] and removal or addition of different metal ions. [6] We report herein strategies based on chloride-mediated Ni-Ni and thermally accessible Ni-P bond cleavage to construct molecular analogues of hinge and mechanical crank mechanisms.A previous report from our group shows that a p-terphenyl platform can act as an axis for rotation of two metals with respect to each other upon reduction of a bonded cofacial Ni I -Ni I moiety to a non-bonded transfacial Ni 0 -Ni 0 species. [7] In that context, the chemical reaction is irreversible; a species that allows better chemical control over molecular motion was sought. Inspired by the ability of related triarylbenzene architectures with pyridine and alkoxide donors to support robust trinuclear transition metal complexes, we developed a triphosphine variant, 1. [8] Addition of a single equivalent each of Ni(COD) 2 (COD = 1,5-cyclooctadiene) and NiCl 2 (dme) (dme = 1,2-dimethoxyethane) to 1 generates a dark brown species, 2 (Figure 1a). A single crystal X-ray diffraction (XRD) study of 2 reveals a dinuclear complex of nickel (Figure 2). One metal center coordinates two phosphines and one chloride and interacts with one carbon (C18) of the central arene ((Ni1-C18) = 2.047(2) Å). Notably, C18 comes out of the plane of the five other carbon atoms of the central ring by ca. 18°. The second nickel center is located on the opposite face of the central ring and binds one phosphine, one chloride and the system of the central ring. The binding of the two nickel centers on opposite faces of the central ring contrasts with the related p-terphenyl and 2',3'-dihydro-p-terphenyl diphosphine dinickel systems in which the dinickel dichloride moiety is cofacially bound by the central ring. [7,9] For compound 2, a similar binding mode would lead to a higher metal coordination number, likely leading to steric repulsions between the chloride and phosphine ligands.Attempting to open a coordination site on nickel in 2, chloride abstraction was performed with TlOTf (OTf = triflate). A new species (3, Figure 1a) was formed based on 1 H and 31 P ** We thank Lawrence M. Henling for assistance with collection of crystallographic data. We are grateful to NSF GRFP (SL), Caltech, SURF office (NCL), and bp for funding. The Bruker KAPPA APEXII X-ray diffractometer was purchased via an NSF CRIF:MU Award to Caltech (CHE-0639094). The 400 MHz NMR spectrometer was purchased via an NIH award, RR027690.
