Photocatalytic CO2 Reduction under Visible-Light Irradiation by Ruthenium CNC Pincer Complexes

Abstract

Summary

A photochemical reduction of CO₂ to HCOOH was shown in a catalytic system using ruthenium complexes as the catalyst and the sensitizer in combination with a sacrificial reagent. A TON of 5634 for COOH was reached after 24 h. The experiments were performed using a 450 W high-pressure Hg lamp combined with a K₂CrO₄ solution filter under CO₂ atmosphere (λ≥500 nm).

Advances and special progress

A TON of 5634 for HCOOH with selectivity of 72% was achieved by the photocatalytic reduction under visible light using a ruthenium complex containing a CNC and bpy ligand.

Additional remarks

Four different complexes containing tert-buthyl and methyl substituents on the CNC ligand and CH₃CN and CO ligands on the ruthenium were used as catalysts. Comparison of the TONs of HCOOH after 24 h showed that the bulkier substituent (tBu) prevents access of CO₂ to the metal center, whereas the smaller substituent (Me) allows access of protons as well as CO₂ to the metal center, leading to a higher TON but lower selectivity of HCOOH. The TON was higher for CO-ligands in comparison to the CH₃CN ligands on ruthenium due to the presence of the absorbtion band (λ≥500 nm) in [Ru-NCMe], which reduces the performance of the catalyst.

Content of the published article in detail

The article contains results for the reduction of CO₂ to HCOOH under visible-light catalysis using ruthenium complexes as a catalyst. The catalytic system performs best (referring to the TON of HCOOH production) with the catalyst containing methyl substituents on the CNC ligand and CO ligands on the ruthenium.

Catalyst

[Ru(bpy)(py)-(tBuNHC)2(MeCN)][PF6]2 [Ru(bpy)(py)-(MeNHC)2(MeCN)][PF6]2 [Ru(bpy)(py)-(tBuNHC)2CO][PF6]2 [Ru(bpy)(py)-(MeNHC)2CO][PF6]2

Photosensitizer

[Ru(dmb)3][PF6]2

Investigations

	cat	cat conc [µM]	PS	PS conc [mM]	e-D	e-D conc [M]	solvent A	.	.	.	λexc [nm]	.	TON CO	TON H2	TON HCOOH	.
1.	[Ru(bpy)(py)-(tBuNHC)2(MeCN)][PF6]2	0.01	[Ru(dmb)3][PF6]2	0.05	BI(OH)H	0.1	DMA				450 W Hg lamp, K2CrO4 filter		129	556	3296
2.	[Ru(bpy)(py)-(MeNHC)2(MeCN)][PF6]2	0.01	[Ru(dmb)3][PF6]2	0.05	BI(OH)H	0.1	DMA				450 W Hg lamp, K2CrO4 filter		224	1438	4593
3.	[Ru(bpy)(py)-(tBuNHC)2CO][PF6]2	0.01	[Ru(dmb)3][PF6]2	0.05	BI(OH)H	0.1	DMA				450 W Hg lamp, K2CrO4 filter		129	505	3792
4.	[Ru(bpy)(py)-(MeNHC)2CO][PF6]2	0.01	[Ru(dmb)3][PF6]2	0.05	BI(OH)H	0.1	DMA				450 W Hg lamp, K2CrO4 filter		300	1897	5634
5.			[Ru(dmb)3][PF6]2	0.05	BI(OH)H	0.1	DMA				450 W Hg lamp, K2CrO4 filter
6.	[Ru(bpy)(py)-(MeNHC)2CO][PF6]2	0.01			BI(OH)H	0.1	DMA				450 W Hg lamp, K2CrO4 filter		8.9	87
7.	[Ru(bpy)(py)-(MeNHC)2CO][PF6]2	0.01	[Ru(dmb)3][PF6]2	0.05			DMA				450 W Hg lamp, K2CrO4 filter			20
8.	[Ru(bpy)(py)-(MeNHC)2CO][PF6]2	0.01	[Ru(dmb)3][PF6]2	0.05	BI(OH)H	0.1	DMA				450 W Hg lamp, K2CrO4 filter			307
9.	[Ru(bpy)(py)-(MeNHC)2CO][PF6]2	0.01	[Ru(dmb)3][PF6]2	0.05	BI(OH)H	0.1	DMA				dark

Investigation-Name: Conditions optimizations for photocatalytic reduction of CO2Export

Sacrificial electron donor

In this study, the experiments were done with the sacrificial electron donor BI(OH)H BI(OH)H.

Additives

No additives were used in the described experiments.