Catalysis by Low-valent Main Group Compounds
Catalysis is, without question, one of the most efficient and powerful strategies for engineering chemical reactions. In this context, the ground-breaking discovery disclosed in 1991; by Arduengo et al. through isolation of the first persistent singlet N-heterocyclic carbene (NHC). These “bottle-able” stable N-heterocyclic carbene led to numerous catalytic applications in organometallic chemistry as well as in organocatalytic field for their nucleophilic behavior. In 2009, Bertrand and co-workers first isolated an abnormal N-heterocyclic carbene (aNHC) by chemically blocking the normal position. The enhanced nucleophilicity of the aNHC can be utilized to develop a variety of homogeneous catalysts.
For example, this aNHC was used in designing transition metal based catalysts using Pd(II), Cu(I), Ni(II) and Fe(0) for different organic transformations reactions. Not only transition metals but also this aNHC stabilized a varied range of main group metals such as K(I), AI(III), for polymerization reaction developed. In addition, we have demonstrated superior nucleophilicity of this aNHC for small molecule activation such as carbon dioxide (CO2), nitrous oxide (N2O), tetrahydrofuran (THF) and thiophene. This aNHC was also established as metal-free catalyst. This aNHC has been established as an efficient catalyst in ring opening polymerization of different cyclic esters at room temperature. Also this aNHC successfully reduced chemically inert carbon dioxide molecule into methanol using isolated aNHC as a metal free catalyst under leading to the highest TON among any homogeneous catalysts for CO2 to methanol conversion under ambient conditions. In addition to these developments, the current research efforts are being made to design novel catalysts using P(I) compounds (such as phosphinidene), C(0) (carbon) and B(I) (borylene) based compounds.
Selected Publications by Mandal and co-workers:
J. Org. Chem. 2018 ASAP, Organometallics 2018 ASAP; Organometallics, 2017, 36, 4753-4758; Inorg. Chem. 2017, 56, 14459-14466; Organometallics 2017, 36, 4753-4758, Angew. Chem. Int. Ed. 2016, 55, 15147–15151Organometallics 2016, 35, 3775–3780; Organometallics 2016, 35, 2930−2937; Adv. Synth. Cat. 2015, 357, 3162 – 3170, J. Org. Chem. 2014, 79, 9150-9160, Chem. Commun. 2012, 48, 555-557; Chem. Commun. 2011, 47, 11972–11974.
Transition Metal Mimicking Catalysis using PLY Ligands
The phenalenyl (PLY) systems with odd alternant hydrocarbons, known from last six decades in designing different smart materials due the accessibility of three different forms, a closed-shell cation, open-shell radical, and closed-shell anion, using its nonbonding molecular orbital (NBMO). Our group is interested in utilizing the reversible redox processes of phenalenyl based moiety to design materials for spin-electronics, catalysis, fuel cell application, and for various important bond activation such as C-H, Si-H or B-H.
In such design, the closed-shell unit of PLY can readily accept free electrons, stabilizing its NBMO upon generation of the open-shell state of the molecule which can initiate a SET (single electron transfer) process. As a result this phenalenyl radical has enormous potential to mimic transition metal based catalysis.
Our approach conceptualizes on the fundamental understanding of organometallic chemistry how typically a metal works during a coupling catalysis and the major steps in such catalysis are termed as oxidative addition (when metal looses electrons) and reductive elimination (when metal gains back its electrons). Our current research capitalizes on this concept using phenalenyl based molecule which can accept electrons, store it and deliver the way a typical transition metal functions in a catalytic reaction. We describe this approach as “Transition Metal Mimicking Catalysis”.
Significant contributions in this area by Mandal and co-workers:
Nature 2013, 493, 509−513; ACS Catal. 2014, 4, 4307−4319; J. Am. Chem. Soc.2015, 137, 5955−5960; Chem. Sci. 2017, 8, 7798-7806; Chem. Sci. 2018, 9, 2817-2825; J. Am. Chem. Soc.2018, 140, 8330-8339.