The synthesis of a series of new ruthenium complexes based on the new PONOP ligands 1 and 10 ((C 5 H 3 N-1,3-(OPR 2 ) 2 : 1, R = i Pr; 10, R = t Bu) is presented, including the stable trans-dihydride complexes ( i Pr-PONOP)Ru(H) 2 (PPh 3 ) ( 4) and ( t Bu-PONOP)Ru(H) 2 (CO) (12) and the stable Ru(0) complexes (R-PONOP)Ru(CO) 2 (6, R= i Pr; 15, R= t Bu). A surprisingly stable 16-electron Ru(0) complex ( 13) was formed by deprotonation of 12 with KO t Bu. Complex 13 reacts with H 2 to afford the cis-dihydride complex 12a, which isomerized to the trans-dihydride 12. Complex 13 reacted with CO to afford the saturated Ru(0) complex 15. Reaction of complex 12 with water led to hydrolysis of the phosphinite PONOP ligand and rearrangement to a dimeric product ( 14). Reaction of the transdihydride complex 4 with the electrophiles PhCOCl, MeI, and MeOTf led to abstraction of one of the hydride ligands, forming the monohydride complexes ( i Pr-PONOP)Ru(H)(PPh 3 )(X) (X = Cl (2), I (8a), OTf (8b)) together with benzaldehyde in the case of 2. Similarly, 12 afforded the monohydride complexes ( t Bu-PONOP)Ru(H)(CO)(X) (X = Cl (11), OTf (17), I ( 18)). Reaction of the Ru(0) complexes 6 and 15 with water resulted in hydrolysis of the O-P bond and formation of the zwitterionic complexes 7 and 16. Treatment of 2 and 11 with MeOTf or MeI resulted in abstraction of the chloride ligand rather than the hydride, forming complexes 8a,b and 17, 18, respectively. Additional syntheses of complexes based on ligands 1 and 10 are presented.

Exclusive C-C bond activation involving the new bisphosphinite ligand {C 6 H 3 (CH 3 )[OP( i Pr) 2 ] 2 } (1) was observed at room temperature, upon reaction with the cationic complex [Rh(COE) 2 (THF) 2 ]BF 4 (COE ) cyclooctene) in THF, yielding the Rh(III) complex [(POCOP)Rh(Me)]BF 4 (2) (POCOPNo parallel C-H activation was observed. This preference is assumed to be governed by the better directed phosphinite-bound metal center toward the C-C bond. A single-crystal X-ray diffraction analysis of complex 2 revealed a square pyramidal geometry with the BF 4ion coordinated to the metal center. Complex 2 reacted with H 2 at room temperature in THF to yield the Rh(III)-hydrido complex 3 and methane. Deprotonation of 3 with KO t Bu yielded the Rh(I) dinitrogen complex [(POCOP)Rh] 2 (µ-N 2 ) (4), which upon reaction with 1 equiv of CO or ethylene formed (POCOP)Rh(CO) ( 5) or (POCOP)Rh-(C 2 H 4 ) (6), respectively. Complex 4 readily underwent oxidative addition of MeI, benzyl chloride, and benzyl bromide, forming complexes 7, 8, and 9, respectively. Halide abstraction from complex 9 with AgBF 4 led to the cationic benzyl complex 12, bearing a coordinated BF 4ion as observed by a singlecrystal X-ray diffraction analysis. Finally, we report an apparent R-H elimination from Rh(III)-Me, which takes place upon heating of the C-C activation product 2 at 150 °C in the solid state, yielding the hydride complex 3 and ethylene.

Sequential addition of CO molecules to cationic aryl-hydrido Rh(III) complexes of phosphine-based (PCP) pincer ligands was found to lead first to C-H reductive elimination and then to C-H oxidative addition, thereby demonstrating a dual role of CO. DFT calculations indicate that the oxidative addition reaction is directly promoted by CO, in contrast to the commonly accepted view that CO hinders such reactions. This intriguing effect was traced to repulsive pi interactions along the aryl-Rh-CO axis, which are augmented by the initially added CO ligand (due to antibonding interactions between occupied Rh d(pi) orbitals and occupied pi orbitals of both CO and the arene moiety), but counteracted by the second CO ligand (due to significant pi back-donation). These repulsive interactions were themselves linked to significant weakening of the pi-acceptor character of CO in the positively charged rhodium complexes, which is concurrent with an enhanced sigma-donating capability. Replacement of the phosphine ligands by an analogous phosphinite-based (POCOP) pincer ligand led to significant changes in reactivity, whereby addition of CO did not result in C-H reductive elimination, but yielded relatively stable mono- and dicarbonyl aryl-hydrido POCOP-Rh(III) complexes. DFT calculations showed that the stability of these complexes arises from the higher electrophilicity of the POCOP ligand, relative to PCP, which leads to partial reduction of the excessive pi-electron density along the aryl-Rh-CO axis. Finally, comparison between the effects of CO and acetonitrile on C-H oxidative addition revealed that they exhibit similar reactivity, despite their markedly different electronic properties. However, DFT calculations indicate that the two ligands operate by different mechanisms.

A new, extremely bulky, and electron-rich N-phosphinomethyl-functionalized N-heterocyclic carbene ligand, 5a ( tBuNHCP tBu), and a somewhat less bulky congener, 5b (MesNHCP tBu), forming five-membered chelate rings with metal centers, have been synthesized in four steps starting from the easily accessible di-tert-butyl(hydroxymethyl)phosphine oxide (1). 5a was isolated and fully characterized by spectroscopic methods including UV-photoelectron spectroscopy and X-ray diffraction. The reaction of 5a with [Ru(COD)Cl2] n under hydrogen pressure or with [Ru(p-cymene)Cl2]2 led to the formation of the unsaturated dinuclear complex [Ru( tBuNHCP tBu)(μ-Cl)(Cl)]2 (6), which serves as a precursor for a series of ruthenium carbene complexes (7a–f) using substituted phenyldiazomethanes (p-X-C6H4(CH)N2; X = H (a), Br (b), CF3 (c), NO2, (d), CH3 (e)) and trimethylsilyldiazomethane (f). Treatment of 6 with phosphine or pyridine ligands led to the formation of the mononuclear adducts, [Ru( tBuNHCP tBu)(Cl2)(PR3)] (R = Me (8), Ph (9), Cy (10)) and [Ru( tBuNHCP tBu)(Cl2)(py) n ] (n = 1 (11), 2 (12); if (py)2 = bipy (13)), which were synthesized in order to find alternative precursor complexes because the dimer 6 showed very low solubility in most organic solvents. Complex 7a was obtained analytically pure on a different route via transmetalation from a silver complex bearing tBuNHCP tBu (15) to the first-generation Grubbs catalyst as the ruthenium precursor. Complexes 7a–c and 7e were characterized by X-ray diffraction analysis, revealing a geometry that can be viewed as both a distorted square pyramid and a distorted trigonal bipyramid with the two chloro ligands in a cis configuration. The steric bulk, especially of 5a with its N-tBu moiety, stabilizes 16 VE Ru complexes. In contrast to ligand 5a, the somewhat less bulky MesNHCP tBu ligand 5b has allowed its direct metalation with two ruthenium alkylidene precursors, affording the two new carbene complexes 17 and 18.

A rare case of BAr F anion cleavage (BAr F -) tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) by a metal complex is described. Reaction of the Rh(I) dinitrogen complexes 5a,b and 6a,b, based on the phosphinite pincer ligands {C 6 H 4 [OP( t Bu) 2 ] 2 } (2), with 2 equiv of AgBAr F at room temperature resulted in B-C bond cleavage of one of the BAr F anions and aryl transfer to afford the Rh(III) aryl complexes 7 and 8, respectively. The X-ray structure of 8 revealed a square-pyramidal geometry with a coordinated acetone molecule. The aryl transfer occurred as a result of electrophilic attack by unsaturated Rh(III) on one of the aryl rings of the BAr F anion. Utilizing different solvents yielded the same product, except when CH 3 CN was used, in which case one-electron oxidation took place, yielding complex 9. Treatment of 6a,b with 1 equiv of AgX (X ) BAr F , BF 4 , PF 6 ) resulted in a one-electron oxidation to yield the paramagnetic Rh(II) complexes 9-11, respectively. Complex 11 was characterized by X-ray diffraction, revealing a mononuclear square-planar Rh(II) complex.