# Unified Synthesis Platform for 1,2,3-Trisubstituted Cyclopentadienyl Ligands Decouples Sterics from Electronics

## TL;DR

A new method for making cyclopentadienyl ligands allows for more diverse and tunable chemical structures, improving their use in catalysis.

## Contribution

A unified and scalable synthesis platform for 1,2,3-trisubstituted cyclopentadienyl ligands with enhanced functional diversity.

## Key findings

- The method enables incorporation of diverse substituents like halogens and alkynes into cyclopentadienyl ligands.
- The synthesized ligands showed improved performance in catalytic reactions compared to classical Cp ligands.
- A cobalt complex achieved a turnover number of 180 in a benchmark C–H annulation reaction.

## Abstract

Cyclopentadienyl (Cp) ligands are a cornerstone of coordination
chemistry and transition-metal catalysis. Their tuning profoundly
influences the chemical and biological reactivity, the induced selectivity,
and the stability of the corresponding metal complexes. However, compared
to phosphines for instance, the accessible chemical space of Cps is
rather narrow, exhibiting major limitations regarding the nature,
pattern, and number of Cp substituents. A unified synthetic strategy
toward partially substituted Cps bearing diverse functionalities and
closing gaps in chemical space is highly desirable. Herein, we report
a streamlined general strategy to prepare 1,2,3-trisubstituted cyclopentadienes
(1,2,3-Cps) from a central inexpensive precursor. Operationally straightforward
reactions and purifications ensure scalable sequences. The robust
and versatile synthesis platform opens access to underexplored Cp
substitution patterns − including flexible incorporation of
diverse alkyls, aryls, and previously elusive or rare functionalities
such as halogens, chalcogens, and alkynes − with a profound
ligand tunability and decoupling of sterics from electronics. The
complexation ability with a selection of catalytically relevant early
and late transition metals was demonstrated, and parametrization of
the Cps with respect to their stereoelectronic environment was performed
via the corresponding Cp rhodium phosphite species. In exemplary selected
benchmark catalytic transformations, the cobalt and rhodium complexes
directly outperformed classical Cp ligands with respect to individual
reactivity, regioselectivity, and catalyst loading. Regarding catalytic
turnover, a 1,2,3-Cp cobalt complex achieved an attractive turnover
number (TON) of 180 for a benchmark C–H annulation.