# Inorganic Cobalt Sandwich Complex [(η5‑P5)Co(η3‑P3)]−

## TL;DR

Scientists created a new cobalt sandwich complex using phosphorus rings, expanding inorganic chemistry beyond traditional carbon-based structures.

## Contribution

A novel inorganic cobalt sandwich complex with all-phosphorus ligands was synthesized and characterized.

## Key findings

- The complex [(η5-P5)Co(η3-P3)]− was synthesized via a stepwise P4 activation/unmasking sequence.
- The dianion [(η4-P5)Co(η3-P3)]2− was isolated and confirmed using X-ray diffraction and EPR spectroscopy.
- The study demonstrates a strategy for creating carbon-free metallocene structures with cobalt in multiple oxidation states.

## Abstract

Sandwich compounds
are foundational to organometallic chemistry,
yet carbon-free analogs remain exceptionally rare. We report the all-phosphorus
heteroleptic cobalt sandwich anion [(η5-P5)­Co­(η3-P3)]− (4), obtained via a stepwise P4 activation/unmasking
sequence in which (nacnac′)­SiP4 serves as a controllable,
silicon-protected P4 synthon. Reaction with the cobalt­(−I)
complex [K­(THF)0.2]­[Co­(η2:η2-cod)2] furnishes a bis­(sila­tetra­phospha­cyclo­penta­dienyl)
cobaltate anion (1), and successive cleavage of the Si­(nacnac′)
units delivers the “naked” cyclo-P5/cyclo-P3 sandwich framework via
monosilylated intermediates 2 and 3. The
cryptate salts [M­(crypt-222)]4 (M = Na, K) were characterized
by 31P­{1H} NMR spectroscopy, ESI-MS, and single-crystal
X-ray diffraction (scXRD) for [Na­(crypt-222)]4. In solution, 4 undergoes slow disproportionation to give the paramagnetic
dianion [(η4-P5)­Co­(η3-P3)]2– (5), isolated as
[K­(crypt-222)]2
5 and authenticated by scXRD
and EPR spectroscopy. Complexes 4 and 5 establish
a deliberate strategy for accessing inorganic sandwich architectures
and extend carbon-free metallocene chemistry to cobalt in two different
oxidation states.