Asymmetric catalysis with phosphine free ligands is a rapidly developing field where N-heterocyclic carbenes (NHCs) are making increasingly more impact. Herein, we report the synthesis and characterization of carbene-oxazoline containing proligands that exhibit two chiral centers. The corresponding rhodium(I) complexes show excellent activity toward the asymmetric hydroboration of styrenes, which occurs with good regio– and enantioselectivity (up to 96% ee). The resulting boronic esters were subsequently oxidized, and the corresponding chiral alcohols were isolated in moderate to good yields (up to 75%). Overall, the hydroboration reaction occurs under ambient conditions (CH2Cl2; 25 °C), is compatible with a variety of functional groups, and provides easy access to chiral secondary alcohols from the corresponding alkenes.
This paper details a next-generation Co(I)-based hydrogen energy carrier (H2EC) that can readily extract one electron from hydrogen, store the electron for more than 3 months, and use the electron elsewhere when needed. It is noteworthy that the ability of the Co(I) complex to perform reductive coupling is much higher than that of a similar Ni(I) complex that we recently reported. Three X-ray structures are presented: the Co(II) complex before reaction with H2, the Co(I) complex after reaction with H2 (i.e., the H2EC), and a Co(III) chloromethyl complex formed by oxidative addition of dichloromethane to the Co(I) complex.
A cyclometalated Pt(II) complex was synthesized by appending a pyridyl ligand containing a benzothienobenzothiophene (BTBT) unit, which is a well-known small organic molecule semiconductor. The target complex [Pt(ppy)(BTBT-py)Cl] (2) was characterized by 1H NMR, steady-state UV–vis absorption, and photoluminescence spectroscopies in comparison with [Pt(ppy)(ppyH)Cl] (1) and BTBT-py (where ppy = 2-phenylpyridine and BTBT-py = 2-(3-pyridyl)-[1]benzothieno[3,2-b][1]benzothiophene). Complex 2 exhibited absorption bands derived from the ppy-centered π–π* transition and intramolecular charge transfer (ICT) from BTBT to pyridine moieties, indicating that 2 inherited the combined properties of the parent complex 1 and the parent ligand BTBT-py. Moreover, DFT calculations showed that 2 inherited HOMO and LUMO characteristics from BTBT-py as well as HOMO–1 and LUMO+1 characteristics from 1. While BTBT-py showed fluorescence from the 1ICT state, the corresponding fluorescence was quenched in 2 because the Pt center, a heavy atom, enhanced the intersystem crossing. The ancillary ligand-centered 3ICT, rather than the ppy-centered 3π–π*, state was successfully monitored in 2 using femtosecond transient absorption spectroscopy. Replacing the phenyl group with the BTBT unit in the cyclometalated Pt(II) complex restructured the electronic structure of the lowest triplet excited state, shifting the centered ligand from ppy to BTBT-py.
Phosphine–imidate-supported nickel catalysts were synthesized and tested for ethylene/acrylate copolymerization activity. Phosphine–amide metalation studies with (pyridine)2Ni(CH2Si(CH3)3)2 (py2Ni(CH2TMS)2) suggest that the metalation proceeds via an intermediate phosphine–amide complex ([P,O–amide]Ni(CH2TMS)2). Subsequent deprotonation of the amide N–H gives access to the phosphine–imidate-supported nickel complex ([P,O–imidate]pyNi(CH2TMS)). The imidate form of the ligand proved to be critical for polymerization activity. The phosphine–imidate nickel catalysts described herein were capable of producing ethylene/acrylate copolymers with molecular weights (Mw) from 700 to 120,000 g/mol and with acrylate incorporation >2.6 mol % in some cases. It was determined that hemilabile alkoxy groups on the phosphine aryl groups were necessary to achieve high Mw and acrylate incorporation. These results demonstrate that phosphine–amides, a well-studied ligand class, can be utilized in the imidate form to access highly active neutral nickel polymerization catalysts.
This tutorial on transition metal chemistry─sans the lanthanides and actinides─is intended as preliminary information for a high-level undergraduate (UG) inorganic, organometallics, or a comparable first-year graduate (G) course. It features ionization energies (IEs) and comparative nd and (n+1)s orbital energies, electron affinities (EAs), electronegativities (ENs), and covalent radii (cr). Redox properties of appropriate aquo complexes (ΔE°red = E°(red)) are also included due to their impact on critical properties and reactivity, as are pertinent aspects of the main group elements.
In order to establish design criteria for Rh C–H borylation catalysts, analogues of the successful catalyst [Rh(Ind)(SIDipp)(COE)] (Ind = η5-indenyl, SIDipp = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene, and COE = cis-cyclooctene) were synthesized by changing the indenyl and carbene ligands. [RhCp(SIDipp)(COE)] (1) formed alongside the C–C activated, cyclometalated byproduct [RhCp(κ2CAr,Ccarbene-SIDipp′)(iPr)] (rac-2; SIDipp′ = 1-(6-isopropylphenyl)-3-(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene). Computational modeling of COE dissociation showed that both C–C and C–H activation of the SIDipp aryl group is thermally attainable and reversible under experimental conditions, with the C–C activation products being the more thermodynamically stable species. Oxidative addition of 1 with SiH(OEt)3 gave the Rh silyl hydride [RhCp(H){Si(OEt)3}(SIDipp)] (rac-3). [Rh(Ind)(IDipp)(COE)] (4; IDipp = 1,3-bis(2,6-diisopropylphenyl)-imidazole-2-ylidene), the carbonyl analogue [Rh(Ind)(IDipp)(CO)] (5; νCO = 1940 cm–1, cf. 1944 cm–1 for [Rh(Ind)(SIDipp)(COE)]), and [Rh(Ind)(IMe4)(COE)] (6; IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene) were also characterized, but attempts to synthesize Rh carbene complexes with fluorenyl or 1,2,3,4-tetrahydrofluorenyl ligands were not successful. For the catalytic C–H borylation of benzene using B2pin2, 1 was inactive at 80 °C, and [Rh(Ind)(SIDipp)(COE)] was superior to all other complexes tested due to the shortest induction period. However, the addition of HBpin to precatalyst 4 eliminated the induction period. Catalytic n-alkane C–H borylation using [Rh(Ind)(NHC)(COE)] gave yields of up to 21% alkylBpin, but [RhCp*(C2H4)2] was the better catalyst.
The synthesis and structural characterization of a series of oxime-derived palladium complexes are reported. Coordination of 2-pyridyl-1-ketoxime to palladium as a neutral ligand has been achieved. Palladation of 1,1′-(1,3-phenylene)bis(pentan-1-one) dioxime (BuPhdOxH2) resulted in the formation of an unexpected N,C,O-pincer complex through a Beckman rearrangement of one oxime group to an amide. This complex is the first metalation of C2 of a 1,1′-(1,3-phenylene)bis(1-alkanone) dioximine. In contrast, the analogous 1,1′-(pyridine-2,6-diyl)bis(1-pentanone) dioxime (BuPydOxH2) gives the expected N,N,N-pincer complex without ligand rearrangement. Initial mechanistic studies suggest that the Beckman rearrangement of BuPhdOxH2 occurs by a process involving palladium.
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