Spectroelectrochemical study of complexes [Mo(CO)23-allyl)(α-diimine)(NCS)] (α-diimine = bis(2,6-dimethylphenyl)-acenaphthenequinonediimine and 2,2′-bipyridine) exhibiting different molecular structure and redox reactivity

Dedicated to Professor Maria José Calhorda on the occasion of her 65th birthday.
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Highlights

  • [Mo(CO)23-allyl)(2,6-xylyl-BIAN)(NCS)] shows an uncommon crystal structure.

  • Reduction path of [Mo(CO)23-allyl)(α-diimine)X] is reported for the first time.

  • Spectator NCS ligand proves very convenient for IR spectroelectrochemistry.

  • Catalytic CO2 reduction with [Mo(CO)23-allyl)(bpy)] in THF is also monitored.

Abstract

The redox properties and reactivity of [Mo(CO)23-allyl)(α-diimine)(NCS)] (α-diimine = bis(2,6-dimethylphenyl)-acenaphthenequinonediimine (2,6-xylyl-BIAN) and 2,2′-bipyridine (bpy)) were studied using cyclic voltammetry and IR/UV–Vis spectroelectrochemistry. [Mo(CO)23-allyl)(2,6-xylyl-BIAN)(NCS)] was shown by X-ray crystallography to have an asymmetric (B-type) conformation. The extended aromatic system of the strong π-acceptor 2,6-xylyl-BIAN ligand stabilises the primary 1e-reduced radical anion, [Mo(CO)23-allyl)(2,6-xylyl-BIAN•−)(NCS)], that can be reduced further to give the solvento anion [Mo(CO)23-allyl)(2,6-xylyl-BIAN)(THF)]. The initial reduction of [Mo(CO)23-allyl)(bpy)(NCS)] in THF at ambient temperature results in the formation of [Mo(CO)23-allyl)(bpy)]2 by reaction of the remaining parent complex with [Mo(CO)23-allyl)(bpy)] produced by dissociation of NCS from [Mo(CO)23-allyl)(bpy•−)(NCS)]. Further reduction of the dimer [Mo(CO)23-allyl)(bpy)]2 restores [Mo(CO)23-allyl)(bpy)]. In PrCN at 183 K, [Mo(CO)23-allyl)(2,6-xylyl-BIAN•−)(NCS)] converts slowly to 2e-reduced [Mo(CO)23-allyl)(2,6-xylyl-BIAN)(PrCN)] and free NCS. At room temperature, the reduction path in PrCN involves mainly the dimer [Mo(CO)23-allyl)(bpy)]2; however, the detailed course of the reduction within the spectroelectrochemical cell is complicated and involves a mixture of several unassigned products. Finally, it has been shown that the five-coordinate anion [Mo(CO)23-allyl)(bpy)] promotes in THF reduction of CO2 to CO and formate via the formation of the intermediate [Mo(CO)23-allyl)(bpy)(O2CH)] and its subsequent reduction.

Graphical abstract

Cathodic electrochemistry of complexes [Mo(CO)23-allyl)(α-diimine)(X)] (X = halide or pseudohalide) has not been investigated so far and reports on their anodic behaviour are scarce. We monitored redox reactions of two electronically and structurally different representatives of this family successfully with UV–Vis–NIR–IR spectroelectrochemistry, and explored their potential to activate carbon dioxide.

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Introduction

There is significant and sustained interest in the catalytic reduction of CO2 to materials that may be used in fuel cells, as chemical fuel sources, or as a feedstock for organic synthesis. A number of different transition-metal complexes have been identified as potential redox catalysts for CO2 reduction, including macrocyclic complexes, phosphine complexes, and complexes with α-diimine ligands [1], [2], [3], [4], [5]. The Group 7 complexes, [Re(CO)3(bpy)Cl] and related species, are known to act as catalyst precursors for efficient electrochemical and photochemical CO2 reduction, with the active catalysts being five-coordinate, 1e reduced radical or 2e reduced anionic species [6], [7], [8]. The manganese analogue, [Mn(CO)3(bpy)Br], containing the cheaper and more abundant first row transition-metal centre, has recently been shown to act as an electrochemical catalyst for the conversion of CO2 to CO in the presence of small concentrations of acid, with an overall 2e reduction to [Mn(CO)3(bpy)] [9], [10]. Much less attention in this regard has been devoted so far to Group 6 metal carbonyl α-diimine complexes. Only recently we have encountered similar catalytic activity towards CO2 reduction exhibited by the structurally related 2e reduced complex [Mo(CO)3(bpy)]2− [11].

Another family of Group 6 metal carbonyls, of the type [MoII(CO)23-allyl)(L∩L)X] (L∩L = chelating bidentate ligand, X = anionic monodentate ligand), are known to have variable stereochemistry. These complexes usually adopt either a symmetric structure with both L∩L donor atoms trans to equatorial carbonyl ligands (type A), or an asymmetric structure where the η3-allyl ligand and a carbonyl ligand are trans to L∩L (type B) as seen in Chart 1 [12], [13], [14]. Trans-dicarbonyl isomers, although rare, are also observed [15]. For the cis-dicarbonyl complexes the less common B-type structure occurs particularly when L∩L is a diphosphine or a non-rigid bidentate ligand; although, many exceptions are known in the literature [13], [14], [15], and the nature of the X ligand also affects the conformation. For both the A and B structural types, the axial η3-allyl ligand adopts the orientation where the open face is over the two carbonyl groups. Quantum mechanical (DFT, EHMO) calculations have shown that this is the most stable arrangement for both isomers [13], [16].

There are few electrochemical studies of [Mo(CO)23-allyl)(L∩L)X] complexes reported in the literature, and these only consider their anodic behaviour. A single one-electron reversible oxidation is observed at 0.5–0.7 V vs. SCE for a range of [Mo(CO)23-allyl)(L∩L)X] (L∩L = bpy, Ph2PCH2PPh2, Ph2PCH2CH2PPh2, Ph2AsCH2CH2AsPh2; X = Cl, O2CCF3) complexes in dichloromethane (DCM), resulting in the formation of the [Mo(CO)23-allyl)(L∩L)X]+ cation with retention of the stereochemistry. Substitution of the chelating bidentate L∩L ligand for a pair of monodentate ligands results in an electrochemically and chemically irreversible oxidation process [17]. Oxidation of complexes with strongly π-accepting diphosphine and diarsine chelating ligands is less reversible at slower scan rates than for complexes containing α-diimine ligands such as bpy, 1,10-phenanthroline (phen) and N,N′-di-tertbutyl-1,4-diazabuta-1,3-diene (tBu-DAB). IR spectroelectrochemistry of [Mo(CO)23-allyl)(L∩L)X] usually shows the two ν(CO) bands shifted to higher frequencies by more than 100 cm−1 upon oxidation, in line with the dominant metal localization of the HOMO [18].

In this paper, the electrochemical behaviour of [Mo(CO)23-allyl)(bpy)(NCS)], together with its ability to catalyse the reduction of CO2, are investigated. To date, no studies of the reduction of complexes of the type [Mo(CO)23-allyl)(α-diimine)X] have been reported, and predictions of the electrochemical mechanisms can only be based on comparisons with similar complexes such as [M(CO)3(α-diimine)X] (M = Mn, Re) [7], [9], [19], [20]. The related complex, [Mo(CO)23-allyl)(2,6-xylyl-BIAN)(NCS)] (2,6-xylyl-BIAN = bis(2,6-dimethylphenyl)-acenaphthenequinonediimine, Chart 2), is used for comparative electrochemical studies as the extended π-delocalised aromatic system of the 2,6-xylyl-BIAN ligand [21] can stabilise the reduction products.

Section snippets

Materials

All solvents were freshly distilled under a nitrogen atmosphere. Tetrahydrofuran (THF) and hexane were distilled from benzophenone/sodium, acetonitrile (MeCN) over P2O5, and butyronitrile (PrCN) and dichloromethane (DCM) over CaH2. The supporting electrolyte, Bu4NPF6 (TBAH, Aldrich), was recrystallised twice from absolute ethanol and dried under vacuum. [Mo(CO)23-allyl)(MeCN)2(NCS)] [22] and 2,6-xylyl-BIAN [21] were prepared according to literature procedures. The previously reported

Crystal structure of [Mo(CO)23-allyl)(2,6-xylyl-BIAN)(NCS)]

The crystal structure for the green complex [MoII(CO)23-allyl)(2,6-xylyl-BIAN)(NCS)] shows that it has the asymmetric B-type structure with the allyl ligand trans to one of the imino-N coordination sites (Fig. 1). This is in contrast to the structure of the red [MoII(CO)23-allyl)(bpy)(NCS)] complex [24], which is A-type. The bond lengths in the 2,6-xylyl-BIAN complex are similar to those reported for [MoII(CO)23-allyl)(bpy)(NCS)], although the α-diimine and NCS Mo–N and carbonyl M–C bonds

Conclusions

The redox behaviour of [Mo(CO)23-allyl)(bpy)(NCS)] was studied using both cyclic voltammetry and in situ UV–Vis/IR spectroelectrochemistry in THF and PrCN at 293 K and in PrCN at 200–183 K. The previously unreported [Mo(CO)23-allyl)(2,6-xylyl-BIAN)(NCS)] complex was synthesised and characterised by NMR, IR and UV–Vis spectroscopy, and X-ray crystallography. The extended π-aromatic system of the strongly π-accepting 2,6-xylyl-BIAN ligand stabilises the six-coordinate radical anion [Mo(CO)23

Acknowledgements

The authors thank the University of Reading for provision of the Chemical Analysis Facility (the CAF lab). FH is also grateful to the University of Reading for a start-up grant covering this project. Mr Henk Luyten (Purmerend, The Netherlands) is thanked for the maintenance of the spectroelectrochemical cells in the Reading Spectroelectrochemistry Laboratory (RSL) and Mr Nick Spencer for assistance in collection of the single-crystal data in the CAF lab. Dr. Qiang Zeng (University of Reading,

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