PHOTOINDUCED OXIDATIVE ADDITION AT Rh AND Ru CENTRES

Abstract
Ultraviolet irradiation of CpRh(PMe3)(C2H4) in the presence of pentafluoropyridine C5F5N yields an isolable 2-C,C-coordinated pentafluoropyridine complex. This is the first crystallographically characterised example of 2-coordination of pentafluoropyridine to a metal centre. Photolysis of CpRh(PMe3)(C2H4) in the presence of C5F4HN affords cleanly the C-H activated product.
In contrast, photochemical reaction of CpRh(PMe3)(C2H4) in hexane in the presence of OMe- or NMe2- substituted tetrafluoropyridine results in the formation of metallacycles [Cp(PMe3)Rh(2–C,C)CH2N(CH3)C5F3N] and [Cp(PMe3)Rh(2-C,C)CH2OC5F3N], fully characterized by spectroscopic and spectrometric techniques and X-ray crystallography. NMR studies suggest the need of the heteroatom and a primary carbon as in NMe2 and OMe for the ring closure. Elimination of HF provides the driving force for the formation of the cyclometallated species.
The ability of Tp`Rh(PMe3)H2 and Tp`Rh(CNneopentyl)(2-PhN=C=N-R) complexes to activate B-H, Si-H and C-F bonds in photochemical reactions at room temperature in neat substrate (HBpin, Et3SiH/Et2SiH2/PhSiH3, C5F5N/C5F4NH) is presented. The complexes exhibit a remarkable preference for activating these bonds over the aliphatic or aromatic C-H bonds present on the substrates. The thermal stabilities of the products have been tested by heating in C6D6 up to 140ºC. Products Tp`Rh(PMe3)H(Bpin), Tp`Rh(PMe3)H(Et2SiH) and Tp`Rh(PMe3)(pyridyl)(FHF) have been isolated and their molecular structures determined crystallographically. All the structures show characteristic bond lengths for a complete oxidative addition reaction. Competition reactions are performed in the presence of substrates and benzene simultaneously.
The photoreactivity of CpRh(PR3)(C2H4) complexes (R = Me, Ph, Me2Ph) is explored in reactions with (iPr)2NBH2. Oxidative addition of the B-H bond is established for all the reactions to form the [CpRh(PR3)H(BHN(iPr)2)]. These complexes are the first examples of oxidative addition of a B-H bond of aminoboranes to a 16-electron rhodium fragment.
Laser flash photolysis of Ru1, cis-[Ru((R,R)-Me-BPE)2(H)2] and Ru2, cis-[Ru((R,R)-Me-DuPHOS)2(H)2] generates the square planar Ru(PP*)2 transient and allows measurement of reaction rates with different substrates. The data indicate that rate constants are smaller than those of analogues with less hindered achiral bidentate phosphines. The regeneration of the dihydride occurs with a higher rate constant than formation of either the hydrido-silyl or hydrido-boryl complexes. Moreover, the reactions of Ru2 are about ten times slower than those of Ru1.