Carbene complexes are characterized by presence of a formally double bond M=CR₂. In electrophilic carbenes of Fischer type, the M^δ– = C^δ+ polarity of this bond is typical. An extended study of mononuclear complexes (CO)_nM = C(NR’)R showed that there are two rather independent redox active centers present: oxidation is aimed on the metal atom, reduction on the carbene moiety.[1] This picture is in agreement with their HOMO and LUMO composition. [2]

Studied biscarbenes: (CO)₅ Cr = C(X) - bridge - C(X) = Cr (CO)₅ ; bridges: 1,3- or 1,4-phenylene, bis-, tris-, tetrakis-2,5-thienylene and their combinations; X: dialkylamino- or alkoxy-

Biscarbenes belong to molecules with multiple redox centres as they comprise two oxidizable metal atoms together with two reducible carbene moieties connected by a bridge. Electronic communication manifests itself in the course of redox process depending on the electronic properties of the bridge. It is evaluable by analysis of electrochemical behavior of the compounds in the frame of homologous series and with respect to their mononuclear analogs.

For oxidation, the extent of this communication can be identified by occurrence of two subsequent oxidation signals whereas in absence of the electronic communication both the metals are oxidized at the same potential. Potential of the first oxidation step of biscarbenes does not differ from that of monocarbenes indicating lack of the communication.

The reduction process of biscarbenes is influenced by the reducibility of the bridge itself, and by its participation in the LUMO. Since the reduction of biscarbenes is substantially easier than that of monocarbenes, the bridge is involved in the delocalization system and the amino or alkoxy substituent X plays important role in tuning of the reduction potential.

Recently, a new modular synthesis of biscarbenes [3] has open a way to molecules with longer bridging groups with various intramolecular conductivity.

Acknowledgement: Financial support from GACR 17-21770S and from specific university research (MSMT No 20/2017) is gratefully acknowledged.

References:

[1] Ludvík, J.; Hoskovcová, I.; in Advances in Organometallic Chemistry and Catalysis: The Silver/Gold Jubilee Int. Conf. on Organomet. Chem. Celebration Book; Pombeiro, A. J. L., Ed., Wiley 2014, pp. 653

[2] Kvapilová, H.; Hoskovcová, I.; Ludvík, J. ; Záliš, S. Organometallics 2014, 33, 4964,

[3] Váňová, H.; Tobrman, T.; Hoskovcová I.; Dvořák D. Organometallics 2016, 35, 2999