Controlling Intramolecular and Intermolecular Electronic Coupling of Radical Ligands in a Series of Cobaltoviologen Complexes.

Controlling electronic coupling between multiple redox sites is of interest for tuning the electronic properties of molecules and materials. While classic mixed-valence (MV) systems are highly tunable, e.g., via the organic bridges connecting the redox sites, metal-bridged MV systems are difficult to control because the electronics of the metal cannot usually be altered independently of redox-active moieties embedded in its ligands. Herein, this limitation was overcome by varying the donor strengths of ancillary ligands in a series of cobalt complexes without directly perturbing the electronics of viologen-like redox sites bridged by the cobalt ions. The cobaltoviologens [1X-Co]n+ feature four 4-X-pyridyl donor groups (X = CO2Me, Cl, H, Me, OMe, NMe2) that provide gradual electronic tuning of the bridging CoII centers, while a related complex [2-Co]n+ with NHC donors supports exclusively CoIII states even upon reduction of the viologen units. Electrochemistry and IVCT band analysis indicate that the MV states of these complexes have electronic structures ranging from fully localized ([2-Co]4+; Robin-Day Class I) to fully delocalized ([1CO2Me-Co]3+; Class III) descriptions, demonstrating unprecedented control over electronic coupling without changing the identity of the redox sites or bridging metal. Additionally, single-crystal XRD characterization of the homovalent complexes [1H-Co]2+ and [1H-Zn]2+ revealed radical-pairing interactions between the viologen ligands of adjacent complexes, representing a type of through-space electronic coupling commonly observed for organic viologen radicals but never before seen in metalloviologens. The extended solid-state packing of these complexes produces 3D networks of radical π-stacking interactions that impart unexpected mechanical flexibility to these crystals.

Uptake, Trapping, and Release of Organometallic Cations by Redox-Active Cationic Hosts.

The host-guest chemistry of metal-organic nanocages is typically driven by thermodynamically favorable interactions with their guests such that uptake and release of guests can be controlled by switching this affinity on or off. Herein, we achieve this effect by reducing porphyrin-walled cationic nanoprisms 1a12+ and 1b12+ to zwitterionic states that rapidly uptake organometallic cations Cp*2Co+ and Cp2Co+, respectively. Cp*2Co+ binds strongly (Ka = 1.3 × 103 M-1) in the neutral state 1a0 of host 1a12+, which has its three porphyrin walls doubly reduced and its six (bipy)Pt2+ linkers singly reduced (bipy = 2,2'-bipyridine). The less-reduced states of the host 1a3+ and 1a9+ also bind Cp*2Co+, though with lower affinities. The smaller Cp2Co+ cation binds strongly (Ka = 1.7 × 103 M-1) in the 3e- reduced state 1b9+ of the (tmeda)Pt2+-linked host 1b12+ (tmeda = N,N,N',N'-tetramethylethylenediamine). Upon reoxidation of the hosts with Ag+, the guests become trapped to provide unprecedented metastable cation-in-cation complexes Cp*2Co+@1a12+ and Cp2Co+@1b12+ that persist for >1 month. Thus, dramatic kinetic effects reveal a way to confine the guests in thermodynamically unfavorable environments. Experimental and DFT studies indicate that PF6- anions kinetically stabilize Cp*2Co+@1a12+ through electrostatic interactions and by influencing conformational changes of the host that open and close its apertures. However, when Cp*2Co+@1a12+ was prepared using ferrocenium (Fc+) instead of Ag+ to reoxidize the host, dissociation was accelerated >200× even though neither Fc+ nor Fc have any observable affinity for 1a12+. This finding shows that metastable host-guest complexes can respond to subtler stimuli than those required to induce guest release from thermodynamically favorable complexes.

Unexpected Formation of Metallofulleroids from Multicomponent Reactions, with Crystallographic and Computational Studies of the Cluster Motion.

New multicomponent reactions involving an isocyanide, terminal or internal alkynes, and endohedral metallofullerene (EMF) Lu3 N@C80 yield metallofulleroids which are characterized by mass-spectrometry, HPLC, and multiple 1D and 2D NMR techniques. Single crystal studies revealed one ketenimine metallofulleroid has ordered Lu3 N cluster which is unusual for EMF monoadducts. Computational analysis, based on crystallographic data, confirm that the endohedral cluster motion is controlled by the position of the exohedral organic appendants. Our findings provide a new functionalization reaction for EMFs, and a potential facile approach to freeze the endohedral cluster motion at relatively high temperatures.

Accessing three oxidation states of cobalt in M6L3 nanoprisms with cobalt-porphyrin walls.

Nanocages with porphyrin walls are common, but studies of such structures hosting redox-active metals are rare. Pt2+-linked M6L3 nanoprisms with cobalt-porphyrin walls were prepared and their redox properties were evaluated electrochemically and chemically, leading to the first time that cobalt-porphyrin nanocages have been characterized in CoI, CoII, and CoIII states.

A delocalized cobaltoviologen with seven reversibly accessible redox states and highly tunable electrochromic behaviour.

CoII mediates electronic coupling between two N-Me-pyridinium-terpyridine ligands that are related to redox-active N,N-dialkyl-4,4'-bipyridinium dications (viologens). Borderline Class II/III electronic delocalization imparts the cobaltoviologen complex with distinct electronic properties (e.g., 7 accessible redox states) relative to those of viologens, leading to enhanced electrochromism.