Photochemical Expulsion of the Neutral Monodentate Ligand L in Ru(Terpy*)(Diimine)(L)²⁺:  A Dramatic Effect of the Steric Properties of the Spectator Diimine Ligand

Abstract

A series of photoreactive complexes of the type Ru(terpy*)(N-N)(L)(2+), where terpy* is 4'-(3,5-ditertiobutylphenyl)-2,2':6',2' '-terpyridine, N-N is the bidentate chelate phen or dmp (phen = 1,10-phenanthroline, dmp = 2,9-dimethyl-1,10-phenanthroline), and L is the monodentate ligand dms, MeBN, or MeOBN (dms = dimethyl sulfide, MeBN = 2,6-dimethyl benzonitrile, MeOBN = 2,6-dimethoxybenzonitrile), has been synthesized and fully characterized by proton NMR spectroscopy, electrospray mass spectrometry, and UV-vis spectroscopy. The X-ray structures of four complexes were also obtained. In neat pyridine, the quantum yields for the photosubsitution of L by pyridine were measured and showed dramatic variations depending on the steric interactions between the spectator bidentate ligand and the leaving monodentate ligand L. The use of dmp instead of phen multiplied the photosubstitution efficiency by a factor of 20-50, depending on L. This effect could be qualitatively correlated to the distortions observed in the X-ray structures of the corresponding complexes. The highly distorted structure of Ru(terpy)(dmp)(dms)(PF(6))(2) showed a very high photosubsitution quantum yield phi = 0.36 in neat pyridine. The high photoreactivity of some of the compounds makes them particularly promising as components of future light-driven molecular machines.

Electronic absorption spectroscopy of [Ru(phen)2(bpy)]2+, [Ru(phen)2(dmbp)]2+, [Ru(tpy)(phen)(CH3CN)]2+ and [Ru(tpy)(dmp)(CH3CN)]2+A theoretical study

“…Moreover, the lowest 3 MC state of this molecule, calculated at 413 nm, is nearly degenerated with the 1 MLCT absorbing states calculated between 428 nm and 416 nm and more easily accessible upon visible irradiation. All these features are in favour of a more efficient photodissociation of CH 3 CN in [Ru(tpy)(dmp)(CH 3 CN)] 2+ than in [Ru(tpy)(phen)(CH 3 CN] 2+ as expected from experiments [7]. …”

“…The first process is based on the presence, in the absorption spectrum, of low-lying metal-to-ligandcharge-transfer (MLCT) states whereas the second process is supposed to involve dissociative metal-centred ( 3 MC) excited states accessible via intersystem crossing. For instance irradiation of [Ru(phen) 2 (dmbp)] 2+ and [Ru(tpy)(phen)(CH 3 CN)] 2+ under visible light leads to the clean photo-expulsion of the dimethylbipyridine [1,5] and acetonitrile [6,7], respectively. Efficient photolabilization of [Ru(phen) 2 (dmbp)] 2+ between 300 nm and 800 nm in acetonitrile was observed with formation of [Ru(phen) 2 (CH 3 CN) 2 ] 2+ in a half reaction time of 28 s. This reaction is supposed to be less efficient in the analogous complex [Ru(phen) 2 (bpy)] 2+ [8].…”

Transition‐Metal‐Complexed Molecular Machine Prototypes

“…A particularly promising feature of the Ru(terpy)-(phen)(L) 2+ series, with respect to future molecular machines and motors, is related to the pronounced effect of steric factors on the photochemical reactivity of the complexes. [24] When the bulkiness of the spectator phenanthroline was increased, the steric congestion of the coordination sphere of the ruthenium complex also increased. This congestion was shown by quantitative and comparative distance and angle measurements on four X-ray crystal structures (see Fig.…”

“…f: photosubstitution quantum yield. Adapted with permission from [24]. Copyright 2004, the American Chemical Society.…”

N‐Acetylmethionine and Biotin as Photocleavable Protective Groups for Ruthenium Polypyridyl Complexes

“…[52] The values found, 0.018(4) for [3] 2 + and 0.011(3) for [4] 2 + , are five to ten times larger than for similar systems with nitrile ligands, [49] which may be attributed to the stronger steric hindrance of thioether ligands. [28] Thus, although cleavage of the Ru À S bond does not take place thermally at human body temperature (see above), photochemical cleavage is selective and efficient. [53] In addition, as shown by 1 H NMR spectroscopy, the photoreleased species [2] 2 + is identical to the product obtained by thermal aquation of the RuÀCl bond when [1]Cl is dissolved in water.…”