Hidden aryl-exchange processes in stable 16e RhIII [RhCp*Ar2] complexes, and their unexpected transmetalation mechanism.

Experiments mixing the stable 16e 5-coordinate complexes [RhCp*Ar2] (Cp* = C5Me5; Ar = C6F5, C6F3Cl2-3,5) uncover fast aryl transmetalations. Unexpectedly, as supported computationally, these exchanges are not spontaneous, but catalyzed by minute amounts of 18e (µ-OH)2[RhCp*Ar]2 as a source of 16e [RhCp*Ar(OH)]. The OH group is an amazingly efficient bridging partner to diminish the activation barrier of transmetalation.

Dramatic mechanistic switch in Sn/Au(I) group exchanges: transmetalation vs. oxidative addition.

The mechanism of Ph/X exchange in reactions involving SnPh(n)Bu3 and [AuXL] complexes switches dramatically from the usual concerted mechanism involving Ar/X mixed bridges when X = Cl, to an unexpected oxidative addition/reductive elimination pathway via an Au(III) intermediate when X = vinyl.

Highly fluorescent complexes with 3-isocyanoperylene and N-(2,5-di-tert-butylphenyl)-9-isocyano-perylene-3,4-dicarboximide.

The perylene derivatives 3-isocyanoperylene (Per-N≡C) (4a) and N-(2,5-di-tert-butylphenyl)-9-isocyano-perylene-3,4-dicarboximide (PMI-N≡C) (4b) were prepared and used to synthesize gold complexes [AuX(CNR)] (X = C6F5 (5a,b), C6F4-O(n)Bu-p (6b)). The reaction of 5b and 6b with HNEt2 led to the carbene complexes [AuX{C(NEt2)(NHR)}] (7b, 8b), respectively. The molecular structure of complexes 7b and 8b have been determined by X-ray diffraction analysis showing intermolecular π-stacking of the perylene groups and C6F5 rings and no Au···Au interactions. The derivative compounds [M(CO)5(CNR)] (M = Cr (9a,b), Mo (10a,b) or W (11a,b)) and trans-[Pd(CNR)2(C6F3Cl2)2] (12a,b) were also prepared. All complexes exhibit fluorescence associated with the perylene fragment with emission quantum yields, in solution at room temperature, in the range 0.05-0.93 and emission lifetimes ~ 4 ns. DFT calculations were performed of the absorption spectra of the ligands Per-N≡C and PMI-N≡C and representative complexes [Au(C6F5)(CNR)], [Cr(CO)5(CNR)], showing a perylene-dominated intraligand π-π* emissive state, from the HOMO and LUMO orbitals of the perylene chromophore, but with significantly different absorption maxima by the influence of the metal fragment, particularly significant in the Per-N≡C derivatives.

Photoisomerizable metallomesogens and soft crystals based on orthopalladated complexes.

With the aim of obtaining light-responsive liquid-crystalline palladium complexes, six palladium complexes derived from an orthometalated imine, bearing one or two azocarboxylato bridges, have been synthesized: [Pd(2)(µ-SC(10)H(21))(µ-O(2)CAzo)(L(1,2))(2)] (7 and 8), [Pd(2)(µ-SC(10)H(21))(µ-O(2)CAzo3C(10))(L(1,2))(2)] (9 and 10), [Pd(µ-O(2)CAzo)L(1)](2) (11), and [Pd(µ-O(2)CAzo3C(10))L(1)](2) (12), in which L(1) = p-H(21)C(10)OC(6)H(3)CH═NC(6)H(4)OC(10)H(21)-p, L(2) = p-H(21)C(10)OC(6)H(3)CH═NC(14)H(29), AzoCO(2)(-) = p-(phenylazo)benzoate, and Azo3C(10)CO(2)(-) = p-(2',3',4'-tris-n-decyloxyphenylazo)benzoate. Three of them (7-9), as well as the precursor Azo3C(10)CO(2)H (3), are thermotropic liquid crystals displaying nematic and smectic A mesophases, while 10-12 have been identified by X-ray diffraction to give rise to "soft" crystal phases. Electronic spectroscopy and (1)H NMR show that all of them undergo a trans-cis isomerization of the azobenzene moiety at λ = 365 nm. The molecular structure determines the photoresponse in solution, which is faster and more stable when the trisubstituted azocarboxylate is present and the motion of the azo group is not hindered by the orthometalated imine. The photoresponse has also been observed in the condensed phases, which change from the ordered phase to the isotropic liquid upon irradiation, except for compound 10, a soft crystal in which a permanent photoalignment highly sensitive to light polarization is produced. The latter is a behavior with potential applications, rather unusual in low-molecular-weight compounds.

The origin of hindered rotation around the Pt-N bond in platinum amides.

Several platinum amides of formula trans-[PtCl(NHAr)(PEt(3))(2)] (Ar = 3-FC(6)H(4), 2; 4-FC(6)H(4), 3; 4-ClC(6)H(4), 4; 4-IC(6)H(4), 5; 4-Cl,3-NO(2)-C(6)H(3), 6) have been synthesized by reaction of [PtHCl(PEt(3))(2)] with aryl azides. All the complexes feature planar arylamido moieties and hindered rotation around the N-aryl and Pt-N bonds have been detected and separately studied. The X-ray crystal structures of complexes 5 and 6 have been determined. Complex 5 crystallizes in the orthorhombic space group Pnma, with a = 23.806(4) A, b = 15.099(2) A, c = 6.7593(10) A, alpha = beta = gamma = 90 degrees, and Z = 4. Compound 6 shows an N-H...O(NO) hydrogen bond and it crystallizes in the monoclinic space group P2(1)/n, with a = 12.215(3) A, b = 8.078(2) A, c = 13.052(4) A, alpha = gamma = 90 degrees, beta = 90.057(6) degrees, and Z = 2. Except for Ar = 4-Cl,3-NO(2)-C(6)H(3), the activation energies obtained for the complexes indicate that both dynamic processes occur simultaneously with a common barrier which originates in the multiple bond character of the N-aryl bond due to a strong pi-donor behavior of the N atom in the N-aryl bond. The rotation about the Pt-N bond is unfavorable because of steric congestion with the planar amide, which can be overcome only when the aromatic ring can rotate. For the complex trans-[PtCl[NH(4-Cl,3-NO(2)-C(6)H(3))](PEt(3))(2)] the barrier to rotation is mostly due to hydrogen bond interaction between the NO(2) ortho substituent and the amide H atom.

The Pd-catalyzed coupling of allyl halides and tin aryls: why the catalytic reaction works and the stoichiometric reaction does not.

Arylallylpalladium complexes [Pd(5-C6F5-eta3-cyclohexenyl)(C6Cl2F3)(NCMe)] (10) and [Pd2(mu-C6Cl2F3)2(5-C6F5-1,3-eta3-cyclohexenyl)2] (13) have been synthesized. Complex 13 is an example of a rare class of metal complexes with aryl bridges and its X-ray crystal diffraction structure has been determined. These arylallylpalladium complexes are involved in the coupling of Bu3SnRf (1, Rf = dichlorotrifluorophenyl) and [Pd2(mu-Br)2(5-C6F5-1,3-eta3-cyclohexenyl)2] (2); complex 10 has been detected in the course of the stoichiometric coupling reaction in acetonitrile. Decomposition experiments of 10 and 13 in different conditions, and comparison with the reactions of 1 and 2, allow us to determine that reductive elimination does not occur in the absence of additives. p-Benzoquinone coordinates to Pd to give complex 15 and promotes reductive elimination to give the coupling products selectively. The outcome of the coupling reaction is controlled by the reductive elimination step, but the overall rate is controlled by the faster preequilibrium, which determines the concentration of 10 or 13. Palladium-catalyzed coupling of allyl halides and tin aryls works better than the stoichiometric allyl-aryl reductive coupling on isolated allylarylpalladium complexes, because they benefit from the presence in the solution of substrate allylic halides acting as electron-withdrawing olefins and promoting reductive elimination. More efficient allyl-aryl couplings, whether stoichiometric or catalytic, can be achieved upon addition of p-benzoquinone to the reaction mixture in a noncoordinating solvent.

Dinuclear azolato-bridged complexes of the nickel group metals with haloaryl ligands: a reinvestigation of their behavior in solution.

A reinvestigation of the NMR spectra of the complexes (NBu4)2[M2(mu-LL)2R4] (M = Pd, Ni, Pt, LL = pyrazolate (pz), 3,5-dimethylpyrazolate (dmpz), 3-methylpyrazolate (mpz), indazolate (indz), R = C6F5; M = Pd, LL = pz, dmpz, mpz, indz, R = 2,4,6-C6F3H2) shows that the boat-shaped dimeric structures of their anions are quite stable in solution, and the previously proposed fast equilibria or dissociations to give species such as [R2M(N-N)(acetone)]-, [R2M(acetone)2] + 2dmpz-, or [R2M(N1-N2)(acetone)]- + [R2M(N2-N1)(acetone)]- in no case occur. A mixture of the two diastereoisomers (head-to-head, HH, and head-to-tail, HT) is present for the asymmetrically substituted azolates (mpz and indz), in a ratio ranging from 1:7 to 1:30 for the different complexes. Strong through-space coupling between the endo ortho fluorine nuclei of different MR2 fragments is observed in the 19F NMR spectra of these diastereoisomers whose boatlike structures place these atoms at short distances.

Rhodium complexes with the chelating and binucleating ligands P(CH2CH2Py)nPh3-n (Py = 2-pyridyl; n = 1,2): structures and fluxional behavior.

Several rhodium(I) complexes of the type [RhX(CO)(PePy2)], [Rh(diene)(PePy)]+, and [Rh(diene)(PePy2)]+ (PePyn = P(CH2CH2Py)nPh3-n; Py = 2-pyridyl; n = 1, 2) have been prepared. The two former are square planar; the latter are pentacoordinated for diene = tetrafluorobenzobarrelene or norbornadiene (confirmed by X-ray diffraction), but an equilibrium of 4- and 5-coordinate isomers exists in solution for diene = 1,5-cyclooctadiene. The fluxional behavior of all these complexes is studied by NMR spectroscopy. The complex [Rh(NBD)(PePy2)]PF6.Cl2CH2 crystallizes in the monoclinic space group P21/n with a = 8.455(1) A, b = 18.068(3) A, c = 19.729(3) A, beta = 99.658(3)degrees, and Z = 4. The complexes [Rh(diene)(PePy2)]+ react with CO to give the dimeric complex [Rh2(CO)2[P(CH2CH2Py)2Ph]2](BF4)2 with the pyridylphosphine acting as P,N-chelating and P,N-bridging.

Mesogenic palladium complexes with pincer ligands derived from dipicolinic acid.

4-Substituted pyridine-2,6-dicarboxylic acids, (E)-dipicH2, and 4-substituted pyridine-2,6-bis(thiocarboxylic) acids, (E)-pdtcH2, (E = OCnH2n + 1, SCnH2n + 1) have been synthesized and used as O,N,O- and S,N,S-pincer ligands with palladium. In the fourth coordination site the complexes bear 4-decyloxy-4'-stilbazole (L1), 4-decyloxy-N-(4-pyridylmethylene)anilines (L2), decyl 4-pyridinecarboxylate (L3), 4-(4'-decyloxyphenyl)pyridinecarboxylate (L4), 4-(3',4',5'-tridecyloxybenzyl)pyridinecarboxylate (L5), 4-isocyano-1-decyloxybenzene (L6), or 4-isocyano-4'-decyloxybiphenyl (L7). Thermotropic mesomorphism is observed for the (E)-dipic complexes with L5 and n = 12, which display columnar phases. The complexes with S,N,S-pincers show an important depression in the melting point compared to their O,N,O homologues and this change gives rise to mesomorphic materials (SC). However, in the case of L5 the mesomorphic behavior observed for the O,N,O derivative is lost in its S,N,S analogue. The alkylsulfanyl compounds exhibit lower transition temperatures and wider mesophase ranges than their alkoxy analogues.