Reaction of the ligand 1,3-bis((di-tert-butylphosphino)methyl)benzene (1a) with the [RhCO] + fragment in THF resulted in clean formation of the crystallographically characterized bis-chelated complex 2a which contains an η 2 agostic Rh C-H bond. Both the NMR data and the X-ray crystal structure show strong interaction between the metal center and the agostic C-H bond, which results in high acidity of the agostic proton. Reaction of 2a with a weak organic base (NEt 3 or collidine) affords the known cyclometalated complex 3. Reaction of the new ligand 1,3-bis((di-tert-butylphosphino)methyl)-4,5,6-trimethoxybenzene (1b) with the [RhCO] + fragment in THF gives the analogous to 2a agostic complex 2b. Analysis of the NMR data and the reactivity of both 2a and 2b showed that there is very little, if any, contribution of a metal arenium structure. This interpretation is supported by B3LYP/LANL2DZ density functional calculations on model compounds. Thus, deprotonation of η 2 aromatic C-H agostic complexes can be proposed as an alternative route to electrophilic metalation of aromatic compounds.

The diphosphine 1,3-bis[(di-tert-butylphosphino)methyl]-2,4,6-trimethylbenzene (1a) upon reacting with the rhodium and iridium olefin complexes M2(olefin)4Cl2 (M = Rh, Ir) undergoes rapid, selective metal insertion into the strong unstrained aryl−methyl bond under very mild conditions (room temperature), yielding ClM(CH3)[C6H(CH3)2(CH2P(t-Bu)2)2] (M = Rh (4a), Ir (7a)). The carbon−carbon bond activation is competitive with a parallel C−H activation process, which results in formation of complexes ClMH(L)[CH2C6H(CH3)2(CH2P(t-Bu)2)2] (M = Rh (3a), Ir (6a); L = cyclooctene in the case of 6a and is absent in 3a). Complexes 3a and 6a undergo facile C−H reductive elimination (at room temperature (3a) or upon moderate heating (6a)), followed by C−C oxidative addition, resulting in clean formation of 4a and 7a, respectively. The C−C bond activation products are stable under the reaction conditions, demonstrating that this process is the thermodynamically favorable one. X-ray single-crystal analysis of 4a demonstrates that the rhodium atom is located in the center of a square pyramid, with the methyl group occupying the position trans to the vacant coordination site. Direct kinetic comparison of the C−C and C−H activation processes shows thatin contrast to theoretical calculationsmetal insertion into the carbon−carbon bond in this system is not only thermodynamically but also kinetically preferred over the competing insertion into the carbon−hydrogen bond. When the ligand 1,3-bis[(di-tert-butylphosphino)methyl]-2,4,6-trimethyl-5-methoxybenzene (1b), bearing the strong electron-donating methoxy group in the position trans to the Ar−CH3 bond to be cleaved, was used instead of 1a, no effect on the reaction rate or on the ratio between the C−H and C−C activation products was observed. Our observations indicate that the C−C oxidative addition proceeds via a three-centered mechanism involving a nonpolar transition state, similar to the one proposed for C−H activation of hydrocarbons. An η2-arene complex is not involved in the C−C activation process.

The new rhodium−dinitrogen complex Rh(N2)[HC(CH2CH2P(t-Bu)2)2] (2) was prepared by elimination of HCl with sodium hydride from the hydrido chloride HRh(Cl)[HC(CH2CH2P(t-Bu)2)2] (1). Complex 2 reacts with various small gaseous molecules, giving rise to the new complexes Rh(X)[HC(CH2CH2P(t-Bu)2)2], X = H2 (3), C2H4 (4), CO2 (5). The first 16 electron rhodium−carbon dioxide complex 5 is spectroscopically characterized. All the transformations are reversible in the presence of free nitrogen. The thermodynamic parameters for these equilibria reactions have been evaluated. It is found that at 25 °C formation of the dihydrogen complex 3 is about 1.24 kcal/mol more favorable than formation of its dinitrogen analogue 2, whereas formation of the carbon dioxide (5) and, surprisingly, ethylene (4) complexes is less favorable than 2 by 2.97 and 1.57 kcal/mol, respectively, yielding the ligating ability to the Rh(I) T-shaped core L = H2 > N2 > C2H4 > (CO2). The new hydrido formate HRh(O2CH)[HC(CH2CH2P(t-Bu)2)2] (7) can be obtained either by reaction of the dihydrogen complex 3 with CO2 or from the carbon dioxide complex 5 with hydrogen.

Reaction of the new aromatic aminophosphine ligand 1-((diethylamino)methyl)-3-((di-tert-butylphosphino)methyl)-2,4,6-trimethylbenzene (4) with [Rh(COE)2Cl]2 (COE = cyclooctene) or with [Rh(ethylene)2Cl]2 at room temperature results in selective carbon−carbon bond activation, yielding the complex ClRh(CH3)[C6H(CH3)2(CH2N(C2H5)2)(CH2P(t-Bu)2)] (5). No competing C−H activation was observed during the course of the reaction. When 4 was reacted with (COD)PtCl2 (COD = cyclooctadiene), selective C−H activation of the methyl group situated between the phosphine and amine groups took place, with concomitant intramolecular H transfer to the amine ligand, resulting in the zwitterionic Pt(II) complex Cl2PtCH2(C6H(CH3)2(CH2NH(C2H5)2)(CH2P(t-Bu)2) (11).

This Account presents an overview of current research activities that focus on novel types of interactions between cationic transition metal complexes and arene systems and on unprecedented quinonoid complexes which result from such interactions. When a negatively charged phenoxy group is present in a position para to the metal in a high oxidation state, intramolecular charge transfer occurs, giving the corresponding metallaquinones or quinone methide complexes. In addition, two types of interactions involving low-valent metal compounds have been observed: methylene arenium complexes which result from positive charge transfer to the aromatic ring and sigma-bonded C-H and C-C agostic complexes of cationic metals. These sigma-complexes are proposed as intermediates in metal-based bond activation processes.