No abstract

We report the ethylene homopolymerization and ethylene/methyl-acrylate (MA) and ethylene/acrylic-acid (AA) copolymerization behavior of a series of (N,N′-diaryl-α-diimine)­Pd catalysts that contain secondary amide (−CONHMe) or tertiary amide (−CONMe2) substituents on the N-aryl rings, including the “first-generation” catalysts {(2,6- i Pr2-Ph)­NCMeCMeN­(2-CONHMe-6- i Pr-Ph)}­PdMeCl (1a,a′) and {(2,6- i Pr2-Ph)­NCMeCMeN­(2-CONMe2-6- i Pr-Ph)}­PdMeCl (1b,b′) and the “second-generation” catalysts [{2,6-(CHPh2)2-4-Me-Ph}­NCMeCMeN­(2-CONHMe-6- i Pr-Ph)]­PdMeCl (1d,d′) and [{2,6-(CHPh2)2-4-Me-Ph}­NCMeCMeN­(2-CONMe2-6- i Pr-Ph)]­PdMeCl (1e,e′). Activation of 1d,d′ and 1e,e′ by NaB­{3,5-(CF3)2C6H3}4 generates active ethylene polymerization catalysts that produce highly branched (77–81 br/1000 C) polyethylenes with number-average molecular weights (M ns) in the range 26–60 kDa. The replacement of two isopropyl units in 1a,a′ and 1b,b′ with benzhydryl groups in 1d,d′ and 1e,e′ leads to a significant improvement in ethylene homopolymerization performance. The secondary amide-functionalized catalyst 1d,d′ incorporates ca. twice as much MA and ca. three times as much AA as the i Pr-substituted catalyst [{2,6-(CHPh2)2-4-Me-Ph}­NCMeCMeN­(2,6- i Pr2-Ph)]­PdMeCl (1f,f′) in copolymerization with ethylene. The reactions of 1a,a′ and 1b,b′ with metal salts that contain weakly coordinating anions lead to extrusion of CH4 and the formation of [{(μ-κ2-N,N′,κ-O-α-diimine)­Pd}2(μ-CH2)]2+ complexes, in which the amide carbonyl O atoms coordinate to Pd centers.

In multidimensional spectroscopy, dynamics of coherences between excited states report on the interactions between electronic states and their environment. The prolonged coherence lifetimes revealed through beating signals in the spectra of some systems may result from vibronic coupling between nearly degenerate excited states, and recent observations confirm the existence of such coupling in both model systems and photosynthetic complexes. Understanding the origin of beating signals in the spectra of photosynthetic complexes has been given considerable attention; however, strategies to generate them in artificial systems that would allow us to test the hypotheses in detail are still lacking. Here we demonstrate control over the presence of quantum-beating signals by packing structurally flexible synthetic heterodimers on single-walled carbon nanotubes, and thereby restrict the motions of chromophores. Using two-dimensional electronic spectroscopy, we find that both limiting the relative rotation of chromophores and tuning the energy difference between the two electronic transitions in the dimer to match a vibrational mode of the lower-energy monomer are necessary to enhance the observed quantum-beating signals.

The reaction between Mo(O)(CHAro)(ORF6)2(PMe3) (Aro = ortho-methoxyphenyl, ORF6 = OCMe(CF3)2) and two equivalents of LiOHMT (OHMT = O-2,6-(2,4,6-Me3C6H2)2C6H3) leads to Mo(O)(CHAro)(OHMT)2, an X-ray structure of which shows it to be a trigonal bipyramidal anti benzylidene complex in which the o-methoxy oxygen is coordinated to the metal trans to the apical oxo ligand. Addition of one equivalent of water (in THF) to the benzylidyne complex, Mo(CArp)(OR)3(THF)2 (Arp = para-methoxyphenyl, OR = ORF6 or OC(CF3)3 (ORF9)) leads to formation of {Mo(CArp)(OR)2(μ-OH)(THF)}2(μ-THF) complexes. Addition of one equivalent of a phosphine (L) to Mo(CArp)(ORF9)3(THF)2 in THF, followed by addition of one equivalent of water, all at room temperature, yields Mo(O)(CHArp)(ORF9)2(L) complexes in good yields for several phosphines (e.g., PMe2Ph (69% by NMR), PMePh2 (59%), PEt3 (69%), or P(i-Pr)3 (65%)). The reaction between Mo(O)(CHArp)(ORF9)2(PEt3) and two equivalents of LiOHMT proceeds smoothly at 90 °C in toluene to give Mo(O)(CHArp)(OHMT)2, a four-coordinate syn alkylidene complex. Mo(O)(CHArp)(OHMT)2 reacts with ethylene (1 atm in C6D6) to give (in solution) a mixture of Mo(O)(CHArp)(OHMT)2, Mo(O)(CH2)(OHMT)2, and an unsubstituted square pyramidal metallacyclobutane complex, Mo(O)(CH2CH2CH2)(OHMT)2, along with ethylene and ArpCH=CH2. Mo(O)(CHArp)(OHMT)2 also reacts with 2,3-dicarbomethoxynorbornadiene to yield syn and anti isomers of the “first-insertion” products that contain a cis C=C bond.