Through the Looking Glass: Using the Lens of [SNS]-Pincer Ligands to Examine First-Row Metal Bifunctional Catalysts

Abstract

Metrics & MoreArticle Recommendations CONSPECTUS: Homogeneous catalysis is at the forefront of global efforts to innovate the synthesis of fine chemicals and achieve carbon-neutrality in energy applications. For decades, the push toward sustainable catalysis has focused on the development of first-row transition metal catalysts to supplant widespread use of precious metals. Metal−ligand cooperativity is an effective strategy to yield high-performing first-row metal molecular catalysts. Despite remarkable progress, state of the art catalysts often employ phosphorus-based ligands which are air-sensitive, potentially toxic, and on occasion offset the cost-savings of the metal. Thus, the development of simple and economical ligands composed of biomimetic donors should be a key focus that cannot be overlooked in the pursuit of sustainable catalyst candidates. This is an Account of our group's efforts to develop first-row transition metal complexes which use [SNS]-pincer ligands for bifunctional catalysis. We have synthesized two potentially tridentate ligands, one bearing an amido and two thioether donors [(S Me NS Me ), L1] and one which includes thiolate, imine, and thioether donors [(SNS Me ), L2], and used them as platforms upon which to explore the reaction pathways of first-row metals. The [SNS] ligand, L1, leads to formation of high-spin paramagnetic metal complexes of the type M(L1) 2 in which the 6-membered ring thioether donor is hemilabile (M = Mn, Fe, Co). This allows Mn(L1) 2 to function as a carbonyl hydroboration catalyst that operates by a novel hydride-free, inner-sphere reaction pathway. Exploring the reactivity of L2 with Fe and Ni revealed unique coordination chemistry and a variety of mono-, di-, tri-, and tetranuclear complexes enabled by bridging thiolates. Further studies showed L2 undergoes selective C aryl −S bond cleavage upon coordination to a metal with electronrich phosphine donors, yielding a new (CNS) 2− pincer ligand. The analogous reaction with L1 afforded a new (CNS Me ) − pincer ligand via both C aryl −S and benzylic C−H bond cleavage. In an attempt to prepare Fe(L2) 2 , we obtained instead an Fe(N 2 S 3 ) complex in which imine C−C bond formation affords a potentially hexadentate redox-active ligand. The Fe(N 2 S 3 ) complex is a selective catalyst for hydroboration of aldehydes and appears to operate through a complicated mechanism. In contrast, a mechanistic study of Mn(L2)(CO) 3 -photocatalyzed dihydroboration of nitriles indicated that both the flexibility of the κ 3 -SNS Me ligand (fac-vs mer-coordination) and ability of Mn to undergo a spin-state change are required to access low energy barriers for this transformation. To effectively compare the reactivity of the thiolate vs amido donor, we prepared two Cu complexes, Cu(L1)(IPr) and Cu(L2)(IPr) [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene],showing that, while both served as carbonyl hydroboration catalysts, only the amido complex was an effective catalyst for carbonyl hydrosilylation. In addition, complexes of the...

Slow Inversion of Coordinated Thioether Groups in SNS‐Type Ruthenium Pincer Complexes

“…In chelating ligands thioethers have raised increasing attention as stabilizing σdonating groups for thiophilic transition metals in lower oxidation states [2][3][4] and as hemilabile groups in metal catalysts. [5,6] In particular the utilization of thioether groups in pincer-type ligands led to active catalysts [7] utilized for the hydrogenation of carbonyl compounds or dehydrogenation of alcohols, [8][9][10][11][12][13][14][15][16][17] ester metathesis, [8] alkane transferhydrogenation, [18] cross coupling, [19] cycloaddition [20] and ethylene oligomerization reactions [21][22][23] in recent years.…”