Search for: All records

Ligands play a central role in dictating the electronic properties of metal complexes to which they are coordinated. A fundamental understanding of changes in ligand properties can be used as design principles for more efficient catalysts. Designing ligands that have multiple protonation states that will change the properties of the coordination complex would be useful as potential ways of controlling catalysis, for example, as an on/off switch where one redox state exists below thermodynamic potential and another exists above. Thus, phenol moieties built into strongly coordinating ligands, like that of tpyPhOH (4′-(4-hydroxyphenyl)-2,2′:6′,2′’-terpyridine) may provide such a handle. Herein, we report the electrochemical and spectral characterization, and the crystallographic and computational analysis of two ruthenium analogs: [Ru(tpy)(tpyPhOH)](PF6)2 and [Ru(tpyPhOH)2] (PF6)2 (tpy =2,2′:6′,2′’-terpyridine). Cyclic voltammetry and differential pulse voltammetry indicate that two redox events occur, one of which is pH independent and we hypothesize that these follow an electrochemical- chemical-electrochemical (ECE) mechanism. XRD results of the ruthenium complexes’ protonated forms are generally consistent with expected bond lengths and angles and are in agreement with computational modeling. The properties are compared to a previously reported analog that contains the –OH group directly connected to terpyridine, [Ru(tpyOH)2](PF6)2, where tpyOH is 4′-hydroxy-2,2′:6′,2′’-terpyridine, with some intriguing differences. Overall, these data indicate that the phenyl-substituent decouples the phenol such that it behaves both as an electron withdrawing substituent and a location for a ligand centered oxidation event to occur. 

Metal–ligand cooperation has proven to be a viable concept for σ bond activation and catalysis, however there are few examples involving phosphorus as an active participant in bond cleavage. The reactivity of E–H bonds (E = S, O) across a metal–phosphorus bond of a cobalt( i ) center ligated by a tridenate N-heterocyclic phosphido (NHP − ) ligand with diphosphine sidearms, (PPP) − , has been explored. Addition of PhOH to (PPP)CoPMe 3 ( 1 ) cleanly affords (PP OPh P)Co(H)PMe 3 ( 2 ), in which the O–H bond was heterolytically cleaved across the M–P NHP bond. Addition of PhSH to 1 first generates (PP H P)Co(SPh)PMe 3 ( 3 ), which undergoes an intermolecular rearrangement to generate (PP SPh P)Co(H)PMe 3 ( 4 ) as the thermodynamic product. A comparison with a related platinum( ii ) system reveals the subtle effects that variations in metal intrinsic properties can have on metal–ligand bifunctional σ bond activation processes.