The reactivity of the novel Re( i ) catalyst [Re( C12 Anth-py 2 )(CO) 3 Br] is modulated by its interactions with the covalent organic framework (COF) TFB-BD. The complex catalyzes either reductive etherification, oxidative esterification, or transfer hydrogenation depending on its local environment (embedded in TFB-BD, in homogeneous solution or co-incubated with TFB-BD, respectively). The results highlight that COFs can drastically modulate the reactivity of homogeneous catalysts.
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Distal scaffold flexibility accelerates ligand substitution kinetics in manganese( i ) tricarbonyls: flexible thianthrene versus rigid anthracene scaffolds
This work investigates the effect of molecular flexibility on fundamental ligand substitution kinetics in a pair of manganese( i ) carbonyls supported by scaffold-based ligands. In previous work, we reported that the planar and rigid, anthracene-based scaffold with two pyridine ‘arms’ ( Anth-py 2 , 2) serves as a bidentate, cis donor set, akin to a strained bipyridine (bpy). In the present work, we have installed a more flexible and dynamic scaffold in the form of thianthrene ( Thianth-py 2 , 1), wherein the scaffold in the free ligand exhibits a ∼130° dihedral angle in the solid state. Thianth-py 2 also exhibits greater flexibility (molecular motion) in solution compared with Anth-py 2 , as evidenced by longer 1 H NMR T 1 times Thianthy-py 2 ( T 1 = 2.97 s) versus Anth-py 2 ( T 1 = 1.91 s). Despite the exchange of rigid Anth-py2 for flexible Thianth-py2 in the complexes [( Anth-py 2 )Mn(CO) 3 Br] (4) and [( Thianth-py 2 )Mn(CO) 3 Br] (3), respectively, nearly identical electronic structures and electron densities were observed at the Mn center: the IR of 3 exhibits features at 2026, 1938 and 1900 cm −1 , nearly identical to the features of the anthracene-based congener (4) at 2027, 1936 and 1888 cm −1 . Most importantly, we assessed the effect of ligand-scaffold flexibility on reactivity and measured the rates of an elementary ligand substitution reaction. For ease of IR study, the corresponding halide-abstracted, nitrile-bound (PhCN) cations [( Thianth-py 2 )Mn(CO) 3 (PhCN)](BF 4 ) (6) and [( Anth-py 2 )Mn(CO) 3 (PhCN)](BF 4 ) (8) were generated in situ , and the PhCN → Br – back-reaction was monitored. The more flexible 3 (thianth-based) exhibited ∼3–4× faster ligand substitution kinetics ( k 25 C = 22 × 10 −2 min −1 , k 0 C = 43 × 10 −3 min −1 ) than the rigid analogue 4 (anth-based: ( k 25 C = 6.0 × 10 −2 min −1 , k 0 C = 9.0 × 10 −3 min −1 ) on all counts. Constrained angle DFT calculations revealed that despite large changes in the thianthrene scaffold dihedral angle, the bond metrics of 3 about the metal center remain unchanged; i.e. the ‘flapping’ motion is strictly a second coordination sphere effect. These results suggest that the local environment of molecular flexibility plays a key role in determining reactivity at the metal center, which has essential implications for understanding the reactivity of organometallic catalysts and metalloenzyme active sites. We propose that this molecular flexibility component of reactivity can be considered a thematic ‘third coordination sphere’ that dictates metal structure and function.
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- Award ID(s):
- 2109175
- PAR ID:
- 10427416
- Publisher / Repository:
- Royal Society of Chemistry
- Date Published:
- Journal Name:
- Dalton Transactions
- Volume:
- 52
- Issue:
- 13
- ISSN:
- 1477-9226
- Page Range / eLocation ID:
- 4028 to 4037
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Herein, we report the synthesis and characterization of two manganese tricarbonyl complexes, MnI(HL)(CO)3Br (
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null (Ed.)4,5-diazafluorene (daf) and 9,9’-dimethyl-4,5-diazafluorene (Me2daf) are structurally similar to the important ligand 2,2’-bipyridine (bpy), but significantly less is known about the redox and spectroscopic properties of metal complexes containing Me2daf as a ligand than those containing bpy. New complexes Mn(CO)3Br(daf) (2), Mn(CO)3Br(Me2daf) (3), and [Ru(Me2daf)3](PF6)2 (5) have been prepared and fully characterized to understand the influence of the Me2daf framework on their chemical and electrochemical properties. Structural data for 2, 3, and 5 from single-crystal X-ray diffraction analysis reveal a distinctive widening of the daf and Me2daf chelate angles in comparison to the analogous Mn(CO)3(bpy)Br (1) and [Ru(bpy)3]2+ (4) complexes. Electronic absorption data for these complexes confirm the electronic similarity of daf, Me2daf, and bpy, as spectra are dominated in each case by metal-to-ligand charge transfer bands in the visible region. However, the electrochemical properties of 2, 3, and 5 reveal that the redox-active Me2daf framework in 3 and 5 undergoes reduction at a slightly more negative potential than that of bpy in 1 and 4. Taken together, the results indicate that Me2daf could be useful for preparation of a variety of new redox-active compounds, as it retains the useful redox-active nature of bpy but lacks the acidic, benzylic C–H bonds that can induce secondary reactivity in complexes bearing daf.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D2DT04048D
