Photo–Thermal Haptotropism in Cyclopentadienylcobalt Complexes of Linear Phenylenes: Intercyclobutadiene Metal Migration

Abstract

An attractive strategy for achieving solar-thermal energy conversion is to harvest sunlight in the form of activated chemical bonds through photoisomerization of a suitable molecule that can release, on demand, such stored and transportable energy by thermal reversal to its original form.[1] When such reversible isomerizations entail significant topological alterations, they provide blueprints for eliciting further functions, for example in switches, machines, datastorage, sensors, and other devices. [1,2] Because of their expanded tunability and generally favourable electronic absorption regimes, organometallic complexes are advantageous in this respect, yet have remained relatively unexplored.[3] Among them, topologically simplest are metallohaptotropic arrays in which a single metal moiety photomigrates, thermally reversibly, to a higher-energy position along a fused p framework, without the assistance of additives.[4] Only two such systems are known, [Mo(PMe 3 ) 3 ] complexes of indole and quinazoline, discovered as part of a study focusing on catalytic hydrogenations of heterocycles. [5] We report 1) the photothermal reversibility of {CpCo} complexes of linear phenylenes [6] by a novel mode of haptotropism, namely, h 4 :h 4 from one cyclobutadiene ring to another (Scheme 1); 2) the first X-ray structures of metalated linear phenylenes, illustrating the aromatization of the ligand on complexation; 3) mechanistic aspects of the isomerization cycles; and 4) a DFT study providing a detailed picture of how the {CpCo} unit moves across the arene separating the two cyclobutadiene rings.The new complexes, 2, 5, and 6-8, were made from the respective 2,3,6,9-tetraethynyl linear [3]phenylene, 1,2,4,5-tetraethynylbenzene, and 2,3-diethynylbiphenylene ([D 2 ] for 8) synthons and their cocyclization with bis(trimethylsilyl)-acetylene (BTMSA) in the presence of stoichiometric [CpCo(CO) 2 ] (MeCp for 7), a strategy employed previously Scheme 1. Photo-thermal cycles of CpCo-phenylene complexes.

{Rh(PiBu3)2}+ Fragments Ligated to Arenes: From Benzene to Polyaromatic Hydrocarbons, Part I – An Experimental Approach

“…This observation has motivated anumber of gas‐phase studies on metal ion ligation to extended π‐systems 24. The incorporation of transition metal centers onto the surface of PAH systems can moderate their reactivity,25 as well as the photophysical26 and electronic19 properties of the hydrocarbon moiety. Transition metal functionalized PAHs also represent important models27 for the coordination of metals to graphene and other extended carbon surfaces 28…”

Electronic structure and coordination chemistry of phenanthridine ligand in first‐row transition metal complexes: A DFT study

“…For the first time, in 1983, Albright et al7 Proposed a general qualitative rationalization of the haptotropic migration based on orbital interactions and electron counts using the semiempirical extended Hückel theory method. Recently, the density functional theory (DFT) and the correlated ab initio studies have confirmed these earlier qualitative results 8–16…”

Ring-whizzing in polyene-PtL2 complexes revisited

“…A more inclusive term is haptotropic rearrangement [ 3 ] wherein a metal atom changes its hapticity along the reaction path. Haptotropic rearrangements in ML 3 and MCp complexes are numerous [ 4 – 9 ] and have found use in synthetic strategies [ 10 ], switching devices [ 11 – 13 ] and energy storage [ 14 – 15 ]. Much less is known about the polyene–ML 2 analogs.…”