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Carbon monoxide is an airborne contaminant that is toxic at low concentration levels and is difficult to detect as it is colorless and odorless. This hazard presents a real danger [1][2][3][4] in urban and indoor air, where concentrations of carbon monoxide can reach quite high levels. Typical anthropogenic sources of this pollutant involve combustion engines and improper burning of other fuels. For healthy adults, CO becomes toxic when it reaches a level higher than 50 ppm with continuous exposure over an eight-hour period. Medium exposure (a CO level between 150 ppm to 300 ppm) produces dizziness, drowsiness, and vomiting, whereas extreme exposure (a CO level of 400 ppm and higher) results in unconsciousness, brain damage, and death. Persons with CO poisoning often overlook the symptoms, and undetected exposure can be fatal. Unintentional CO exposure accounts for an estimated 15 000 emergency department visits and 500 unintentional deaths in the United States alone each year. [5] Most existing CO sensors use semiconducting metal oxides. [6] As an alternative for certain applications, the development of chromogenic sensing systems that allow the presence of CO at poisonous concentrations to be easily detected with the naked eye may be of interest. However, colorimetric chromogenic probes for the detection of this deadly chemical are still rare. Reported chromogenic systems include the use of oxoacetatobridged triruthenium cluster complexes, [7] rhodium complexes, [8] polypyrrole functionalized with iron porphyrin derivatives, [9] hybrid materials incorporating a cobalt(III) corrole complex, [10] and iron compounds of diisopropylphosphinodiaminopyridine.[11] Important drawbacks to some of these reported systems involve poor color modulations, CO sensing in solution but not in air, and relatively large detection limits.Following our interest in the design of novel chromogenic systems, [12] we report herein the development of a colorimetric probe that shows unambiguous color modulations for the sensitive and selective sensing of carbon monoxide.The chromogenic probe involves the use of binuclear rhodium derivatives, and the sensing function is based on the well-known ability of these complexes to bind in axial sites. [13] Furthermore, it was envisioned that a suitable selection of the bridging ligands in these binuclear derivatives may modulate the electron density of the metals in order to facilitate back donation from metal orbitals to the p* molecular orbital of CO. Moreover, it is also known that axial coordination on binuclear rhodium derivatives may result in color shifts. [13] After preparing and testing a number of binuclear rhodium derivatives, we finally selected the compound of the formula cis-[Rh 2 (C 6 H 4 PPh 2 ) 2 (O 2 CCH 3 ) 2 ](HO 2 CCH 3 ) 2 (1·(CH 3 CO 2 H) 2 ) for CO signaling applications (Scheme 1). This product, prepared by Cotton et al. in 1985, was the first dirhodium(II) derivative containing two metalated phosphine ligands in a head-to-tail arrangement. [14] In a preliminary experimen...

Properties of dirhodium catalysts with cyclometalated aryl phosphine ligands have been studied. We report here the study of the acid-base reaction of Rh2(RCO2)2(PC)2(H2O)2 catalysts (PC = cyclometalated aryl phosphine) with different Lewis bases. The determination of the equilibrium constants of these reactions can be used to study to which extent the properties of the axial coordination site of the catalyst, considered the active site, are affected by modification of the metalated phosphines, the carboxylate ligands, or the incoming axial ligand. The trends in the computational density functional theory interaction energies show good agreement with the major trends in the equilibrium constants, thus enabling a further study of the influence of the modification of the ligand core.

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CO‐Fänger: Kohlenmonoxid kann mit dem zweikernigen Rhodiumkomplex cis‐[Rh2(C6H4PPh2)2(O2CCH3)2](HO2CCH3)2 selektiv und empfindlich nachgewiesen werden. Der Komplex, der zwei cyclometallierte Phosphanliganden enthält, koordiniert CO axial und ändert dabei seine Farbe von Violett nach Orangegelb (siehe Bild).

An improved preparation of mixed ortho-metalated phosphine/succinimidato dirhodium(II) complexes, [Rh2(P(C5CH4)Ph2)2(OC4NH4O)2], allowed the isolation and characterization of a new isomeric form having both imidato N donors trans to P, 1', that adds up to the two already known having both, 1, or only one, 2, of the imidato N donors trans to the metalated C (Chart 3). The new complex, 1', isomerizes to the thermodynamically stable complex 2 similarly to what had already been observed for isomer 1. Stoichiometric and kineticomechanistic studies of both isomerization processes have been carried out. The reactions have been shown to occur via an intramolecular dissociatively activated process, despite the involvement of the labile axial Rh2 coordination sites in the formation of intermolecular adducts in solution that do not affect the processes. Density functional theory calculations show two transition states with similar energies for the isomerizations, in very good agreement with the kineticomechanistic measurements. The calculation of the charge generation in the two distinct transition states, TS1 and TS1', indicates an important increase in the N negative charge from the reactants, more pronounced for TS1'. This fact agrees very well with the acceleration observed for the processes in polar solvents, especially for the 1' to 2 reaction, when compared that for the reactions carried out in toluene.

The protonation/demetallation reaction, in CD(3)COOD, of phosphine monometallated triacetato-bridged dirhodium(ii) complexes of general formulae [Rh(2)(μ-OOCCH(3))(3){(RC(5)CH(3))P(RC(6)H(4))(2)}(2)] has been studied from a kinetico-mechanistic perspective. The process has been monitored via the disappearance of the nuclear magnetic resonance signals of the protons present in the non-metallated ortho positions of the phosphine ligand and taking advantage of the relatively fast back metallation process that follows the acidolysis reaction, the sequence behaving as an overall equilibrium reaction. The process has a high associative character with important entropy demands, increasing both for the larger and the more electron withdrawing phosphine ligands. For the complexes with electron withdrawing CF(3) substituents on the cyclometallated phenyl ring, this demand is compensated by very low activation enthalpies. The data agree with an intimate mechanism that corresponds to the reverse of the electrophilic C-H bond activation, which has already been studied for this type of system, and requires the presence of a protonated acetato ligand in the axial position, close to the metallated Rh-C(aromatic) bond. The involvement of external solvent molecules, interacting via hydrogen bonds with the system, is also evident for systems which include CF(3) groups with the correct geometric arrangement.

Like in a collage by Max Ernst, a combination of a known molecular entity (a binuclear rhodium complex) and suitable bridging ligands can take on a new role as a chromogenic chemosensor. R. Martínez‐Máñez and co‐workers describe in their Communication on how the color change these complexes undergo allows them to act as a carbon monoxide sensor.