The bite angle effect on the rhodium diphosphine catalyzed hydroformylation was investigated in detail. A series of xantphos-type ligands with natural bite angles ranging from 102°to 121°was synthesized, and the effect of the natural bite angle on coordination chemistry and catalytic performance was studied. X-ray crystal structure determinations of the complexes (nixantphos)Rh(CO)H(PPh 3 ) and (benzoxantphos)Rh(CO)H(PPh 3 ) were obtained. In contrast to the natural bite angle calculations, approximately the same diphosphine bite angles were observed in both crystal structures. The solution structures of the (diphosphine)Rh(CO)H(PPh 3 ) and (diphosphine)Rh(CO) 2 H complexes were studied by IR and NMR spectroscopy. The spectroscopic studies showed that all (diphosphine)Rh(CO) 2 H complexes exhibit dynamic equilibria between diequatorial (ee) and equatorial-apical (ea) isomers. The equilibrium compositions could not be correlated with the calculated natural bite angles. In the hydroformylation of 1-octene an increase in selectivity for linear aldehyde formation and activity was observed with increasing natural bite angle. For styrene the same trend in selectivity for the linear aldehyde was found. For the first time CO dissociation rates of (diphosphine)Rh(CO) 2 H complexes were determined using 13 CO labeling in rapidscan high-pressure (HP) IR experiments. The observed CO dissociation rates for three complexes proved to be orders of magnitude higher than the hydroformylation rates and, contrary to the hydroformylation activity, did not reveal a correlation with the natural bite angle. These findings indicate that the bite angle effect on hydroformylation activity is dominated by the rates of reaction of the reactive, unsaturated (diphosphine)Rh(CO)H intermediates with CO and alkene. The bite angle affects the selectivity in the steps of alkene coordination and hydride migration; the structure of the saturated (diphosphine)Rh(CO) 2 H complex has only some circumstantial relevance to the selectivity.
The effects of isoelectronic replacement of a neutral nitrogen donor atom by an anionic carbon atom in terpyridine ruthenium(II) complexes on the electronic and photophysical properties of the resulting N,C,N'- and C,N,N'-cyclometalated aryl ruthenium(II) complexes were investigated. To this end, a series of complexes was prepared either with ligands containing exclusively nitrogen donor atoms, that is, [Ru(R(1)-tpy)(R(2)-tpy)](2+) (R(1), R(2) = H, CO(2)Et), or bearing either one N,C,N'- or C,N,N'-cyclometalated ligand and one tpy ligand, that is, [Ru(R(1)-N(/\)C(/\)N)(R(2)-tpy)](+) and [Ru(R(1)-C(/\)N(/\)N)(R(2)-tpy)](+), respectively. Single-crystal X-ray structure determinations showed that cyclometalation does not significantly alter the overall geometry of the complexes but does change the bond lengths around the ruthenium(II) center, especially the nitrogen-to-ruthenium bond length trans to the carbanion. Substitution of either of the ligands with electron-withdrawing ester functionalities fine-tuned the electronic properties and resulted in the presence of an IR probe. Using trends obtained from redox potentials, emission energies, IR spectroelectrochemical responses, and the character of the lowest unoccupied molecular orbitals from DFT studies, it is shown that the first reduction process and luminescence are associated with the ester-substituted C,N,N'-cyclometalated ligand in [Ru(EtO(2)C-C(/\)N(/\)N)(tpy)](+). Cyclometalation in an N,C,N'-bonding motif changed the energetic order of the ruthenium d(zx), d(yz), and d(xy) orbitals. The red-shifted absorption in the N,C,N'-cyclometalated complexes is assigned to MLCT transitions to the tpy ligand. The red shift observed upon introduction of the ester moiety is associated with an increase in intensity of low-energy transitions, rather than a red shift of the main transition. Cyclometalation in the C,N,N'-binding motif also red-shifts the absorption, but the corresponding transition is associated with both ligand types. Luminescence of the cyclometalated complexes is relatively independent of the mode of cyclometalation, obeying the energy gap law within each individual series.
The rhodium-catalyzed hydroformylation with the diphosphites P[O(2,2‘-(4-X-6-Y-C6H2)2O][O(2,2‘-(4-X-6-Y-C6H2)2)OP][O(4-Z-C6H4]2 (X = Y = tert-butyl, Z = H (1), Z = OMe (2), Z = C6H5 (3), Z = Cl (4); X = OMe, Y = tert-butyl, Z = H (5)), [PO(2,2‘-(4,6-(tert-C4H9)2C6H2)2)O]2R [R = O(CH2)2O (6), R = O(CH2)3O (7)], [PO(2,2‘-(C6H4)2)O]2R, R = O(2,2‘-(4-MeO-6-tert-C4H9C6H2)2O (8), R = O(2,2‘(4,6-tert-C4H9)2C6H2)2O (9)], and [PO(2,2‘-(4,6-(tert-C4H9)2C6H2)2)O]2[O(2,2‘-(C6H4)2)O] (10) as ligands is studied with oct-1-ene and styrene as substrates. For oct-1-ene the highest normal to branched ratio obtained is 48 (5). For styrene the product selectivity depends strongly on the reaction temperature; a branched to normal ratio of 19 is found for 7 when T = 40 °C vs a branched to normal ratio of 0.19 for 1 when T = 120 °C. A bulky and bisequatorially (ee) coordinating diphosphite is required to obtain a high regioselectivity for linear aldehydes, while flexible diphosphites or equatorially−axially (ea) coordinating diphosphites lead to an enhancement of the formation of branched aldehydes. The hydroformylation of oct-1-ene has a first-order dependency in the oct-1-ene concentration, the order in CO is approximately −0.65, and the order in H2 is approximately 0.2. This is consistent with a kinetic scheme in which alkene addition is the rate-determining step. The crystal structures of RhAcac(4) and RhH(CO)2(4) (11), are presented. The hydrido ligand in 11 could not be located. The structure reveals a distorted TBP with 4 ee coordinated, the P(1)−Rh−P(2) angle being 115.95(9)°. The distortion is indicative of a crowded rhodium center which explains the obtained high linearity of the hydroformylation products.
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