No abstract

The Rh(II) mononuclear complexes [(PNPtBu)RhCl][BF4] (2), [(PNPtBu)Rh(OC(O)CF3)][OC(O)CF3] (4), and [(PNPtBu)Rh(acetone)][BF4]2 (6) were synthesized by oxidation of the corresponding Rh(I) analogs with silver salts. On the other hand, treatment of (PNPtBu)RhCl with AgOC(O)CF3 led only to chloride abstraction, with no oxidation. 2 and 6 were characterized by X-ray diffraction, EPR, cyclic voltammetry, and dipole moment measurements. 2 and 6 react with NO gas to give the diamagnetic complexes [(PNPtBu)Rh(NO)Cl][BF4] (7) and [(PNPtBu)Rh(NO)(acetone)][BF4]2 (8) respectively. 6 is reduced to Rh(I) in the presence of phosphines, CO, or isonitriles to give the Rh(I) complexes [(PNPtBu)Rh(PR3)][BF4] (11, 12) (R = Et, Ph), [(PNPtBu)Rh(CO)][BF4] (13) and [(PNPtBu)Rh(L)][BF4] (15, 16) (L = tert-butyl isonitrile or 2,6-dimethylphenyl isonitrile), respectively. On the other hand, 2 disproportionates to Rh(I) and Rh(III) complexes in the presence of acetonitrile, isonitriles, or CO. 2 is also reduced by triethylphosphine and water to Rh(I) complexes [(PNPtBu)RhCl] (1) and [(PNPtBu)Rh(PEt3)][BF4] (11). When triphenylphosphine and water are used, the reduced Rh(I) complex reacts with a proton, which is formed in the redox reaction, to give a Rh(III) complex with a coordinated BF4, [(PNPtBu)Rh(Cl)(H)(BF4)] (9).

In continuation of our studies on bond activation and catalysis by pincer complexes, based on metal−ligand cooperation, we present here a rare example of amine N−H activation by Rh(I) complexes. The novel dearomatized pincer complexes [(PNN*)RhL′] (PNN = 2-(CH 2 -P t Bu 2 )-6-(CH 2 -NEt 2 )C 5 H 3 N, PNN* = deprotonated PNN, L′ = N 2 (5), C 2 H 4 (6)) and [( i PrPNP*)RhL′] ( i PrPNP = 2,6-(CH 2 -P i Pr 2 ) 2 C 5 H 3 N, i PrPNP* = deprotonated i PrPNP, L′ = C 2 H 4 (7), cyclooctene (9)) were prepared and fully characterized by NMR and X-ray analysis. Complexes 5−7 and 9 undergo facile N−H activation of anilines involving aromatization of the pincer ligand without a change in the formal oxidation state of the metal center to form stable anilide complexes [(PNN)Rh(NHAr)] and [( i PrPNP)Rh(NHAr)] (Ar = C 6 H 5 , o-Br-C 6 H 4 , m-Cl-p-Cl-C 6 H 3 , p-NO 2 -C 6 H 4 ). Anilines possessing electron-withdrawing groups accelerate the N−H activation and yield more stable anilide complexes. The pincer and the ancillary ligands also affect the activation rate, which supports an associative mechanism. Spin saturation transfer experiments show chemical exchange between the pyridylic arm of the pincer ligand and the NH− protons of anilines prior to and after the N−H activation. The reverse N−H formation by metal−ligand cooperation from the anilide complexes was observed to give free anilines and dearomatized Rh(I) complexes upon addition of CO or PEt 3 . Deprotonation of complexes [(PNL)Rh(p-NO 2 -NH 2 C 6 H 4 )] (13, P = P t Bu 2 , L = NEt 2 ; 15, P = L = P i Pr 2 ) yields the dearomatized anionic complexes [(PNL*)Rh(p-NO 2 -NH 2 C 6 H 4 )]. An associative mechanism, involving N−H activation of an apically coordinated aniline in a pentacoordinated Rh(I) complex, is suggested.

The chemical content of Sarcophyton glaucum, one of the more abundant soft corals on many coral reefs, collected from many seas, was thoroughly explored, resulting in the discovery of a large number of cembranoids, biscembranoids, sterols, and other secondary metabolites. The presently investigated Kenyan specimens of S. glaucum yielded three new metabolites, i.e., nyalolide (15), a biscembranoid, 16-oxosarcoglaucol acetate (16), a cembranoid, and the sesquiterpene guaiacophine (17). Nyalolide was also isolated from the Kenyan soft coral Sarcophyton elegans. The structures of the new compounds were elucidated by interpretation of their MS and 1D and 2D NMR experiments and, in the case of nyalolide, possessing 11 chiral centers, secured by X-ray diffraction analysis.

A series of cationic, neutral, and anionic Pd(II) and Pt(II) PNP (PNP = 2,6-bis-(di-tert-butylphosphinomethyl)pyridine) complexes were synthesized. The neutral, dearomatized complexes [(PNP*)MX] (PNP* = deprotonated PNP; M = Pd, Pt; X = Cl, Me) were prepared by deprotonation of the PNP methylene group of the corresponding cationic complexes [(PNP)MX][Cl] with 1 equiv of base (KN(SiMe(3))(2) or (t)BuOK), while the anionic complexes [(PNP**)MX](-)Y(+) (PNP** = double-deprotonated PNP; Y = Li, K) were prepared by deprotonation of the two methylene groups of the corresponding cationic complexes with either 2 equiv of KN(SiMe(3))(2) or an excess of MeLi. While the reaction of [(PNP)PtCl][Cl] with an excess of MeLi led only to the anionic complex without chloride substitution, reaction of [(PNP)PdCl][Cl] with an excess of MeLi led to the methylated anionic complex [(PNP**)PdMe](-)Li(+). NMR studies, X-ray structures, and density functional theory (DFT) calculations reveal that the neutral complexes have a "broken" aromatic system with alternating single and double bonds, and the deprotonated arm is bound to the ring by an exocyclic C=C double bond. The anionic complexes are best described as a pi system comprising the ring carbons conjugated with the exocyclic double bonds of the deprotonated "arms". The neutral complexes are reversibly protonated to their cationic analogues by water or methanol. The thermodynamic parameters DeltaH, DeltaS, and DeltaG for the reversible protonation of the neutral complexes by methanol were obtained.